UNIVERSITY OF NOVA GORICA GRADUATE SCHOOL

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MASTER'S THESIS

Tine Oblak

Mentor: Asst. prof. Tomaž Skapin

Ljubljana, 2009

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Metal fluorides have interesting optical, electrical, magnetic, surface and

catalytic properties. Their use is still concentrated only on specific fields of

applications. Novel fluoride sol-gel and oxidative decomposition of fluorometalates

methods for the preparation of metal fluorides with extraordinary high surface areas

opens new possibilities for wider use of these materials. Focus of this thesis was on

the application of these two methods to the preparation of gallium(III) fluoride, GaF3,

with improved properties, high surface area, high Lewis acidity and as a result of

those higher catalytic activity. Pure GaF3 was not yet produced using these two

preparation approaches. Majority of the research performed so far was done on the

preparation and characterization of HS-AlF3 prepared via fluoride sol-gel, starting

from the commercially available low-cost aluminium isopropoxide. In the present

work, we evaluated the preparation of GaF3 via fluoride sol-gel against the basic

parameters, which were defined during the preparation of HS-AlF3. Most important

parameters of synthesis were temperature, concentration and ratio of reactants and

type of fluorinating agent. GaF3 synthesized by this route is amorphous material with

high Lewis acidity. It has also been found that such GaF3 catalyzes reaction of

dismutation of dichlorodifluoromethane, CCl2F2, although to a lesser extent than HS-

AlF3. GaF3 was also prepared via oxidative decomposition which is an alternative

inorganic route to metal fluorides. Material prepared in this way had higher purity but

lower surface area, was weakly crystalline and catalytically inactive. The final goal

was to characterize these two materials prepared via different methods and compare

them to well characterized HS-AlF3. All newly prepared materials were characterized

by determination of specific surface area (BET), elemental analysis, scanning

electron microscopy, photoacoustic IR spectroscopy and X-ray powder diffraction. In

addition, the materials were also characterized by two indirect methods,

determination of acidity by pyridine adsorption and measurement of catalytic activity

in test reaction of dismutation of dichlorodifluoromethane, CCl2F2. Further

optimization of the procedure for preparation of GaF3 will allow us to proceed with

research on this kind of materials and their possible applications.

Keywords: gallium fluoride, sol-gel, oxidative decomposition, catalysis, Lewis acidity

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Statement I am the original author of this work. Date: 30. 6. 2009 Tine Oblak

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TT aa bb ll ee oo ff CC oo nn tt ee nn tt ss:: I LIST OF FIGURES..........................................................................................- 5 - II LIST OF TABLES............................................................................................- 6 - III LIST OF SCHEMES........................................................................................- 6 - IV LIST OF ABBREVIATIONS, ACRONYMS AND SYMBOLS..........................- 7 - V ACKNOWLEDGEMENTS...............................................................................- 8 - VI PREGLED MAGISTRSKE NALOGE

(SUMMARY IN SLOVENIAN LANGUAGE).....................................................- 9 -

1 INTRODUCTION........................................................................................... - 14 -

2 THEORETICAL BACKGROUND AND GOALS .......................................... - 17 -

2.1 INORGANIC METAL FLUORIDES.............................................................................................- 17 - INTRODUCTION TO INORGANIC METAL FLUORIDES.....................................................................- 17 - GALLIUM(III) FLUORIDE ........................................................................................................................- 19 - HIGH SURFACE METAL FLUORIDES ...................................................................................................- 21 -

2.2 PREPARATION TECHNIQUES TO OBTAIN HIGH SURFACE METAL FLUORIDES.....- 23 - INTRODUCTION TO SYNTHESIS STRATEGIES ..................................................................................- 23 - GENERAL PRINCIPLE OF SOL-GEL PREPARATION ..........................................................................- 23 - FLUORIDE SOL-GEL PREPARATION ....................................................................................................- 26 - COMPARISON BETWEEN THE CLASSICAL SOL-GEL PROCEDURE AND FLUORIDE SOL-GEL ROUTE.......................................................................................................................................- 28 - OXIDATIVE DECOMPOSITION OF HYDRAZINIUM FLUOROMETALATES ..................................- 29 -

2.3 EXAMPLE OF HS-AlF3 PREPARED WITH APPLICATION OF FLUORIDE SOL-GEL AND OXIDATIVE DECOMPOSITION PROCEDURES....................................................................................- 31 -

HS-AlF3 PREPARED VIA FLUORIDE SOL-GEL PROCEDURE............................................................- 31 - HS-AlF3 PREPARED VIA OXIDATIVE DECOMPOSITION ..................................................................- 34 -

2.4 SCOPE AND OUTLINE..................................................................................................................- 36 -

3 EXPERIMENTAL WORK.............................................................................. - 37 -

3.1 BASIC EQUIPMENT.......................................................................................................................- 37 - VACUUM SYSTEM ...................................................................................................................................- 37 - DRY BOX....................................................................................................................................................- 38 - REACTION VESSELS................................................................................................................................- 38 - REACTORS, OVENS AND AUTOCLAVES.............................................................................................- 39 - SCALES.......................................................................................................................................................- 39 - TEMPERATURE CONTROLLER AND MASS FLOW CONTROLLER.................................................- 40 -

3.2 PREPARATION OF GaF3 VIA FLUORIDE SOL-GEL..............................................................- 41 - INTRODUCTION........................................................................................................................................- 41 - PURIFICATION OF GALLIUM(III) ISOPROPOXIDE ............................................................................- 42 - PREPARATION OF GaF3 PRECURSOR...................................................................................................- 43 - POST-FLUORINATION OF GaF3 PRECURSOR......................................................................................- 45 - ADDITIONAL POST-FLUORINATION TESTS.......................................................................................- 47 -

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3.3 PREPARATION OF GaF3 VIA OXIDATIVE DECOMPOSITION...........................................- 49 - INTRODUCTION........................................................................................................................................- 49 - PREPARATION PROCEDURE..................................................................................................................- 50 -

3.4 INSTRUMENTATION AND CHARACTERIZATION METHODS..........................................- 53 - PHOTOACOUSTIC INFRARED SPECTROSCOPY.................................................................................- 53 - ELEMENTAL ANALYSIS .........................................................................................................................- 54 - SPECIFIC SURFACE AREA......................................................................................................................- 54 - GAS CHROMATOGRAPHY......................................................................................................................- 55 - X-RAY POWDER DIFFRACTION ............................................................................................................- 55 - SCANNING ELECTRON MICROSCOPY.................................................................................................- 56 -

3.5 RESEARCH OF CATALYTIC AND ACIDIC PROPERTIES ...................................................- 57 - DETERMINATION OF CATALYTIC ACTIVITY....................................................................................- 57 - RESEARCH OF ACIDIC PROPERTIES....................................................................................................- 57 -

3.6 USED CHEMICALS ........................................................................................................................- 59 -

4 RESULTS AND DISCUSSION...................................................................... - 60 -

4.1 INTRODUCTION ............................................................................................................................- 60 -

4.2 MATERIALS PREPARED VIA SOL-GEL...................................................................................- 61 - SUBLIMATION OF Ga(OiPr)3 ...................................................................................................................- 61 - CONCENTRATION OF SUBLIMATED Ga(OiPr)3 IN SOLVENT..........................................................- 61 - EFFECT OF Ga(OiPr)3 : HF RATIO...........................................................................................................- 62 - ELEMENTAL ANALYSIS OF PRECURSOR...........................................................................................- 63 - POST-FLUORINATION OF PRECURSOR WITH CCl2F2 AND aHF......................................................- 64 - COMPARISON OF PHOTOACOUSTIC IR SPECRA...............................................................................- 65 - RESULTS OF ADDITIONAL POST-FLUORINATION TESTS ..............................................................- 67 -

4.3 MATERIALS PREPARED VIA OXIDATIVE DECOMPOSITION..........................................- 70 - ELEMENTAL ANALYSIS AND SURFACE AREA.................................................................................- 70 - PHOTOACOUSTIC IR SPECRA................................................................................................................- 71 -

4.4 X-RAY POWDER DIFFRACTION ...............................................................................................- 72 -

4.5 DETERMINATION OF CATALYTIC ACTIVITY .....................................................................- 73 -

4.6 RESEARCH OF ACIDIC PROPERTIES......................................................................................- 74 -

4.7 SCANNING ELECTRON MICROSCOPY - SEM .......................................................................- 76 -

5 CONCLUSIONS............................................................................................ - 77 -

6 REFERENCES.............................................................................................. - 81 -

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LL ii ss tt oo ff FF ii gg uu rr ee ss:: Figure 1: 700 ml PCTFE reaction vessel for oxidative decomposition of N2H6AlF5 with F2 in aHF

medium (350 ml). ...................................................................................................................... - 34 - Figure 2: Vacuum system.................................................................................................................- 37 - Figure 3: FEP reaction vessels. Capacity of 30 mL (left) and 300 mL (right). ................................. - 38 - Figure 4: Flow reactor in an electric furnace. ...................................................................................- 39 - Figure 5: Nickel autoclave (up) and oval oven (down). ....................................................................- 39 - Figure 6: Digi-Sense Temperature controller. .................................................................................. - 40 - Figure 7: MKS mass flow controller.................................................................................................. - 40 - Figure 8: Sublimation of Ga(OiPr)3. Furnace (left) and Ice bath (below). ........................................ - 43 - Figure 9: Vessel with purified Ga(OiPr)3. ......................................................................................... - 43 - Figure 10: Condensation of solvent (isopropanol) into the reaction vessel on vacuum

system....................................................................................................................................... - 44 - Figure 11: Addition of aHF on vacuum system. FEP reaction vessel (1) and aHF in thin

vessel (2). ................................................................................................................................. - 45 - Figure 12: Wet gel (up) and dry gel (down) after pumping on vacuum system. .............................. - 45 - Figure 13: Precursor (dry gel)...........................................................................................................- 45 - Figure 14: Flow reactor in an electric furnace. .................................................................................- 46 - Figure 15: Addition of aHF to reaction vessels on the vacuum line. ................................................ - 50 - Figure 16: Reaction vessel with N2H6GaF5

.H2O in aHF. .................................................................. - 50 - Figure 17: Reaction between N2H6GaF5

.H2O and fluorine. .............................................................. - 51 - Figure 18: Mtec model 300 photoacoustic detector. ........................................................................ - 53 - Figure 19: Scattering of surface areas for materials obtained at different Ga(OiPr)3 : HF ratios..... - 63 - Figure 20: PAS spectra of Ga(OiPr)3 and other fluorinated materials.............................................. - 66 - Figure 21: PAS spectra of GaF3 post-fluorinated with CCl2F2 (down) and the same material

which was conditioned on 250°C (vacuum line) before post-fluorination (up). ........................ - 67 - Figure 22: PAS spectra of GaF3 post-fluorinated (16 hours) with CCl2F2 (down) and

aHF (up).................................................................................................................................... - 68 - Figure 23: Coke deposited on final material..................................................................................... - 69 - Figure 24: GaF3 after preparation.....................................................................................................- 70 - Figure 25: PAS spectra showing the presence of NH4

+ in GaF3, HS-AlF3 prepared by oxidative decomposition and NH4AlF4 (reference)................................................................................... - 71 -

Figure 26: PAS spectra of GaF3 post-fluorinated with CCl2F2. (1-after, 2-before) pyridine adsorption. GaF3 post-fluorinated with aHF. (3-after, 4-before) pyridine adsorption................ - 74 -

Figure 27: PAS spectra of GaF3 prepared via oxidative decomposition before (down) and after (up) pyridine adsorption. Inset (right) shows the difference spectrum. ............................ - 75 -

Figure 28: Comparison of PAS spectra between different HS metal fluorides after pyridine adsorption. ................................................................................................................................ - 75 -

Figure 29: SEM of precursor (140 m2/g). .........................................................................................- 76 - Figure 30: SEM of final material post-fluorinated with CCl2F2 (40 m2/g). .........................................- 76 - Figure 31: SEM of final material prepared via oxidative decomposition (19 m2/g). ......................... - 76 - Figure 32: Comparison of purities between GaF3 and AlF3 materials.............................................. - 78 - Figure 33: Comparison of surface areas between GaF3 and AlF3 materials. .................................. - 78 -

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LL ii ss tt oo ff TT aa bb ll ee ss:: Table 1: Comparison of surface areas of precursors prepared from non-sublimed and sublimed

Ga(OiPr)3 at stoichiometric molar ratio of reactants. ................................................................ - 61 - Table 2: Comparison of surface areas of precursors prepared from different concentrations of

Ga(OiPr)3 in isopropanol at stoichiometric molar ratio of reactants.......................................... - 62 - Table 3: Effect of Ga(OiPr)3 : HF molar ratio on precursor surface properties................................. - 63 - Table 4: Surface area and elemental analysis of precursor and precursor additionally threaded

at 250°C in vacuum. ................................................................................................................. - 64 - Table 5: Elemental analysis and surface areas of materials post-fluorinated with CCl2F2. .............. - 64 - Table 6: Elemental analysis and surface areas of materials post-fluorinated with aHF. .................. - 65 - Table 7: Comparison of surface areas and elemental analysis of GaF3 post-fluorinated with

CCl2F2 and the same material which was conditioned on 250°C (vacuum line) before post-fluorination. ....................................................................................................................... - 67 -

Table 8: Comparison of surface areas and elemental analysis of GaF3 prepared with extended (16 hours) post-fluorination time (CCl2F2 and aHF). ................................................................ - 68 -

Table 9: Comparison of catalytic activity of materials in probe reaction of dismutation of CCl2F2 ... - 73 - Table 10: Comparison between GaF3 materials prepared via fluoride sol-gel and oxidative

decomposition (o.d.). ................................................................................................................ - 77 - Table 11: Comparison between HS-AlF3 materials prepared via fluoride sol-gel and oxidative

decomposition (o.d.). ................................................................................................................ - 78 - Table 12: Comparison between fluoride sol-gel and oxidative decomposition preparation method.- 80 -

LL ii ss tt oo ff SS cc hh ee mm ee ss:: Scheme 1: Overview of the sol-gel process and their main applications. ........................................ - 25 - Scheme 2: Metal fluoride preparation via non-aqueous fluoride sol–gel route. ............................... - 27 - Scheme 3: Metal fluoride preparation via oxidative decomposition of hydrazinium

fluorometalates in liquid aHF medium. ..................................................................................... - 30 - Scheme 4: (A) Al4(OiPr)12 (aluminium isopropoxide), (B) partially fluorinated, (C) Al(OR, F)6

intermediate, and (D) Al alkoxide fluoride (precursor)13 ........................................................... - 32 - Scheme 5: Structure of gallium(III) isopropoxide.............................................................................. - 42 - Scheme 6: Pyridine interaction with Brønsted acid sites (B). Pyridine coordinated to

Lewis (L) acid sites. .................................................................................................................. - 58 -

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LL ii ss tt oo ff AA bb bb rr ee vv ii aa tt ii oo nn ss,, AA cc rr oo nn yy mm ss aa nn dd,, SS yy mm bb oo ll ss:: Å angstrom a.u. arbitrary unit BET Brunauer-Emmett-Teller surface area °C degree celsius FTIR fourier transformation infrared FEP tetrafluoroethylene-hexafluoropropylene GC gas chromatography HS high surface h hour IR-PAS infrared photoacoustic spectroscopy mg milligram μl microliter ml milliliter mm millimeter mol mole ppm part per million SEM scanning electron microscopy TA temperature (°C) of activation TEM transmission electron microscopy TPD temperature-programmed desorption XRD X-ray diffraction

LL ii ss tt oo ff CC hh ee mm ii cc aa ll ss:: AlF3 aluminium fluoride Al(OiPr)3 aluminium(III) isopropoxide ACF aluminium chloride fluoride aHF anhydrous hydrogen fluoride Ar argon CCl2FCClF2 1,1,2-trichlorotrifluoroethane CCl2F2 dichlorodifluoromethane (CH3)2CHOH 2-Propanol, anhydrous, 99.5+% C5H5N Pyridine (Py) CFC chlorofluorocarbons GaF3 gallium(III) fluoride Ga(OiPr)3 gallium(III) isopropoxide H2 hydrogen He helium HF hydrogen fluoride HFC hydrofluorocarbons iPrOH 2-propanol N2 nitrogen N2H6AlF5 hydrazinium(+2) fluoroaluminate(III) N2H6GaF5 hydrazinium(+2) fluorogallate(III) F2 fluorine SbF5 antimony pentafluoride

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I wish to express my deep gratitude to my mentor and advisor asst. prof.

Tomaž Skapin who introduced me into an interesting research theme. I also

appreciate his valuable time spent on deliberating and discussing this project with me

and for following the work closely.

I am very grateful to dr. Gašper Tavčar for much helpful discussion, ideas, and

advices throughout the study. He has shown me some of the grandeur and

enthusiasm involved in the quest to obtain good experimental results.

I would like to thank asst. prof. Maja Ponikvar–Svet and Mira Zupančič for

elemental analysis and for their precise interpretation of results.

I wish to express my gratitude to all co-workers at Department of Inorganic

Chemistry and Technology at Jožef Stefan Institute for their assistance and moral

support during my research work. The discussions with them and their suggestions

were always very helpful.

I wish to extend my special thanks also to co-workers from other departments

that were involved in my work, especially to Sebastjan Glinšek who made all my SEM

measurements.

I also thank Slovenian agency for research and development, ARRS and

European Commission Project FUNFLUOS (6th F.W.) for the financial support.

And last, I would like to thank all my friends in the University of Nova Gorica

and my friends outside the University. Special thanks for my family for their support

and caring all the time.

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(SUMMARY IN SLOVENIAN LANGUAGE)

NOVI POSTOPKI PRIPRAVE GALIJEVEGA(III) FLUORIDA S SPECIFIČNIMI LASTNOSTMI

UVOD

Uporaba kovinskih fluoridov, kljub zanimivim optičnim, električnim, magnetnim,

površinskim in katalitskim lastnostim ni tako razširjena, kot je uporaba kovinskih oksidov. Glavni vzrok za to je pomanjkanje ustreznih sintetskih postopkov za pripravo kovinskih fluoridov z natančno definiranimi lastnostmi. Klasični postopki priprave kovinskih fluoridov navadno potekajo pri višjih temperaturah, pogosto pri povišanem tlaku, kot fluorirno sredstvo se najpogosteje uporabljata fluor ali vodikov fluorid. Produkti, dobljeni pri teh relativno ostrih reakcijskih pogojih, so delno ali popolno kristalizirani kovinski fluoridi. Klasični metodi priprave galijevega(III) fluorida sta razklop amonijevega heksafluorogalata, (NH4)3GaF6, v toku F2 pri 400 °C (673 K) ali reakcija med kovinskim galijem in plinastim HF pri 550 °C (823 K). V zadnjih letih je viden precejšen napredek pri iskanju alternativnih sinteznih poti za pripravo fluoridov s specifičnimi lastnostmi. Opaznejši dosežek na tem področju je razvoj dveh postopkov za direktno pripravo fluoridov (Poglavje 2.2). To sta fluoridni sol-gel postopek in oksidativen razpad hidrazinijevih fluorometalatov. Glavni namen magistrske naloge je bil uporaba teh dveh postopkov za pripravo GaF3 s specifičnimi lastnostmi, karakterizacija končnih produktov in primerjava novo pridobljenih materialov z dobro raziskanim aluminijevim(III) fluoridom z veliko površino, HS-AlF3. Pričakovane nove lastnosti so velika specifična površina, močna Lewisova kislost in kot posledica velika katalitska aktivnost. Čisti GaF3 še ni bil pripravljen po novih postopkih priprave, čeprav je Kemnitz s sodelavci že pripravil katalitsko aktiven MgF2 dopiran z GaF3 z veliko površino. Sam MgF2 pripravljen po fluoridnem sol-gel postopku ni katalitsko aktiven.

Priprava amorfnih kovinskih fluoridov z veliko površino po modificiranem fluoridnem sol-gel postopku poteka v dveh stopnjah (Shema 2, stran 27). V prvi stopnji raztopini kovinskega alkoksida v suhem alkoholu ali drugem brezvodnem organskem topilu dodamo stehiometrično količino brezvodnega vodikovega fluorida (aHF) pri sobni temperaturi ter dobimo moker fluoridni gel. S sušenjem mokrega fluoridnega gela dobimo amorfen prašnat produkt (prekurzor). Prekurzor poleg kovinskega fluorida, vsebuje še znatne količine organskih nečistot. Organske ostanke

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odstranimo v drugi fazi sinteze z dodatnim fluoriranjem s plinastim fluorirnim sredstvom pri povišani temperaturi. Kot fluorirno sredstvo v tej fazi lahko uporabimo različne delno ali popolno halogenirane fluoro(kloro)ogljikovodike ali aHF. V primeru katalitsko neaktivnega prekurzorja na osnovi aluminijevega(III) fluorida, AlF3, s tako obdelavo dobimo katalitsko zelo aktiven amorfen AlF3 z veliko površino (HS-AlF3), zato to fazo pogosto označujejo kot aktivacijo. Zagotavljanje zadovoljive ponovljivosti postopka v fazi dodatnega fluoriranja prekurzorja (intermediata) je navadno težavno, zaradi narave reakcij, ki pri tem potekajo. To predstavlja glavno pomanjkljivost fluoridnega sol-gel postopka.

Zaradi omenjenih težav pri odstranjevanju organskega ostanka je bil razvit alternativni sintetski pristop, oksidativnega razpada hidrazinijevih fluorometalatov z elementarnim fluorom, ki prestavlja popolnoma novo anorgansko sintezno pot do kovinskih fluoridov s specifičnimi lastnostmi (Shema 3, stran 30). Postopek poteka tako, da v reakcijsko posodo najprej odtehtamo hidrazinijev fluorometalat ter nato dodamo tekoči brezvodni vodikov fluorid (aHF), ki se uporablja kot reakcijski medij. Na vakuumskem sistemu v posodo dodajamo točno določene količine elementarnega fluora. Reakcija je zelo eksotermna, zato je potrebno med reakcijo zagotoviti, dovolj intenzivno mešanje (mehansko mešalo), da preprečimo usedanje trdne faze in zagotovimo učinkovito odvajanje toplote. Pri popolnem oksidativnem razpadu anorganskih prekurzorjev nastane kovinski fluorid, kot edini trden produkt.

Večina raziskav na področju sinteze amorfnih kovinskih fluoridov z visoko površino je bila narejena na pripravi in karakterizaciji HS-AlF3, pripravljenem po fluoridnem sol-gel postopku (Poglavje 2.3). Mehanizme fluoridnih sol-gel procesov kot tudi optimizacijo priprave HS-AlF3 je intenzivno preučeval Kemnitz s sodelavci. HS-AlF3 je material z izredno visoko Lewisovo kislostjo, primerljivo s kislostjo antimonovega pentafluorida, SbF5. Zaradi tega HS-AlF3 katalizira nekatere reakcije halogeniranih ogljikovodikov, ki potekajo le v navzočnosti najmočnejših Lewisovih kislin.

V naših raziskavah smo se osredotočili na pripravo amorfnega GaF3, ki naj bi po podatkih iz literature imel visoko Lewisovo kislost. Material pripravljen po novih postopkih priprave naj bi imel izboljšane kislinske lastnosti, podobno kot HS-AlF3, ki ima višjo kislost kot AlF3, pripravljen z običajnimi postopki.

EKSPERIMENTALNI DEL

Pri pripravi GaF3 po fluoridnem sol-gel postopku (Poglavje 3) smo komercialni

galijev izopropoksid, Ga(OiPr)3, zaradi velike količine prisotnih nečistot prečistili s sublimacijo v dinamičnem vakuumu pri 136 °C (409 K). V reakcijsko posodo iz fluorirane plastike (FEP) smo dodali približno 1 g predhodno sublimiranega Ga(OiPr)3 in z dodatkom izopropanola pripravili približno 20 %(m/m) raztopino. V reakcijsko

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posodo smo na vakuumskem sistemu dodali stehiometrično količino aHF. Moker gel smo evakuirali 16 ur in dobili suh gel (prekurzor). Prekurzor smo dodatno fluorirali oziroma aktivirali z mešanico dušika in CCl2F2 (280 °C) ali z aHF (120 °C). Organske komponente, ki so izhajale med fluoriranjem prekurzorja, smo zasledovali s plinskim kromatografom.

GaF3 smo pripravili tudi po postopku oksidativnega razpada (Poglavje 3). V reakcijsko posodo, kjer je bilo približno 1–5 g hidrazinijevega(II) akvapentafluorogalata (N2H6GaF5

.H2O), smo dodali tekoči brezvodni vodikov fluorid (aHF), ter nato na vakuumskem sistemu v reakcijsko posodo dodajali elementarni fluor v odmerjenih količinah. Po končani reakciji smo odčrpali topilo in preostale pline v reakcijski posodi ter dobili bel prašnat produkt.

Dobljene fluoride smo karakterizitali (Poglavje 4) z določitvijo specifične površine po metodi BET, elementno analizo, visokoločljivo elektronsko mikroskopijo, fotoakustično IR spektroskopijo in rentgensko praškovno analizo. Končne material smo karakterizirali tudi s posrednima metodama, določitvijo kislosti z adsorpcijo piridina in merjenjem katalitske aktivnosti v testni reakciji dismutacije difluorodiklorometana, CCl2F2. To reakcijo katalizirajo le srednje močne Lewisove kisline. Produkte dismutacije, CClF3, CCl3F in CCl4, smo kvantificirali s plinsko kromatografijo.

REZULTATI IN RAZPRAVA

Pri pripravi GaF3 po fluoridnem sol-gel postopku smo se opirali na osnovne

parametre sinteze, definirane pri pripravi HS-AlF3. Prvi poskusi so pokazali, da imajo materiali pripravljeni iz prečiščenega (sublimiranega) Ga(OiPr)3 bistveno večjo površino, kot tisti pripravljeni iz komercialnega Ga(OiPr)3. Drugi preučevan parameter je bila koncentracija Ga(OiPr)3 v topilu. Kot najbolj optimalno koncentracijo za pripravo gela smo izbrali 20 %(m/m) raztopino Ga(OiPr)3 v izopropanolu. Nadaljnji poskusi so bili namenjeni preučevanju vpliva razmerja reaktantov na površino prekurzorja. Za aluminijeve sisteme v literaturi navajajo kot optimalno razmerje, ki je enako stehiometričnemu, Al : HF = 1 : 3. Razmerja nad stehimetrijo so v primeru Al privedla do bistveno nižjih površin zaradi urejanja strukture. V nasprotju s temi rezultati so naši poskusi pokazali, da pri različnih razmerjih Ga(OiPr)3 : aHF = 1 : 2–10 dobimo približno enako površino prekurzorja od 135 do 195 m2/g. Galijevi sistemi so v primerjavi z aluminijevimi bolj stabilni, saj se amorfna struktura gela ohranja tudi pri visokih presežkih aHF. Amorfnost prekurzorja in končnega materiala smo dokazali z rentgensko praškovno analizo. Največjo površino (40 m2/g) je imel material, ki je bil dodatno fluoriran s CCl2F2. Iz elementne analize je razvidno, da smo tudi v najboljšem primeru pri tem materialu dobili razmerje Ga : F = 1 : 2,55 in čistoto 91,9 %(m/m) . Visok delež fluora v prekurzorju in materialu,

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dodatno fluoriranem z aHF, lahko pripišemo prostemu HF, ki ostaja v materialu nezreagiran. Če prekurzor sušimo na 250 °C (523 K), vidimo, da se razmerje bistveno zniža.

V primeru bolje raziskanega HS-AlF3 ima prekurzor površino približno 400 m2/g, vsebuje okoli 30 %(m/m) organskega ostanka, molsko razmerje Al : F je 1 : 2. Končni material ima površino 200 m2/g in le 0,5 %(m/m) organskih nečistoč, z molskim razmerjem Al : F = 1 : 3. Iz tega je razvidno, da smo predvsem za naš končni material dobili precej slabše rezultate. Možno je, da dodatno fluoriranje prekurzorja ni poteklo do konca kljub temu, da s plinskim kromatografom izhajajočih organskih komponent nismo več zaznali. To potrjujejo tudi fotoakustični IR spektri produktov, dodatno fluoriranih s CCl2F2, v katerih so še opazni trakovi, ki jih lahko pripišemo preostalim izopropoksidnim skupinam. Poleg tega se na teh materialih nabira obloga ogljika, ki najverjetneje nastane pri reakciji prekurzorja s CCl2F2. V materialih, ki jih segrevamo do istih temperatur v toku dušika, karbonizacije ne opazimo. Pojav karbonizacije so opazili tudi pri dodatnem fluoriranju prekurzorja pri pripravi HS-AlF3, vendar v manjšem obsegu in pri ostrejših pogojih aktivacije. Prekurzor smo poskušali dodatno fluorirati tudi s plinastim aHF, ker smo se želeli izogniti izločanju ogljika. HF je bolj aktivno fluorirno sredstvo kot CCl2F2, zato je potrebno z njim delati pri milejših pogojih. Kljub temu je bila površina končnega materiala nižja, ostanek nečistot pa celo nekoliko večji kot pri dodatnem fluoriranju s CCl2F2. Karbonizacija v tem primeru ni potekala.

Postopek dodatnega fluoriranja smo poskušali izboljšati z nekaterimi dodatnimi postopki, kot so predhodna obdelava pred dodatnim fluoriranjem, podaljšan čas dodatnega fluoriranja, uporaba elementarnega fluora, kot fluorirnega sredsta ali dodatek kisika, k fluorirni mešanici, vendar nam kljub temu ni uspelo dobiti materialov z boljšimi lastnostmi.

Končni material, pripravljen po postopku oksidativnega razpada, je bil bistveno bolj čist (96,6 %(m/m)), z molskim razmerjem Ga : F = 1 : 3. Za razliko od materiala pripravljenega po fluoridnem sol-gel postopku je imel nižjo površino (19 m2/g) in je bil delno kristaliničen. Pri pripravi materiala je potrebno biti posebej pazljiv v začetni stopnji priprave. Elementaren fluor je potrebno dodajati v manjših odmerkih, da ne pride do pregretja in posledično poškodbe reakcijske posode. Med pripravo sta nam pregoreli kar dve posodi od petih.

Primerjava SEM slik prekurzorja in končnega materiala, pridobljenega po fluoridnem sol-gel postopku ter obdelanega s CCl2F2, prikazuje razliko v poroznosti in morfologiji obeh materialov. Prekurzor sestavljajo porozni aglomerati 30 nm kroglastih osnovnih delcev, kar se ujema z večjo površino tega materiala. Dodatno fluoriranje s CCl2F2 bistveno spremeni strukturo materiala. Opazna sta izguba izhodne strukture in poroznosti ter rast večjih zrn, kar vodi k znatnemu znižanju

- 13 -

površine. SEM slika materiala, pripravljenega po postopku oksidativnega razpada jasno prikazuje skupke 30–100 nm velikih kristalov.

Fotoakustični IR spektri adsorbiranega piridina v vseh končnih materialih kažejo prisotnost Lewisovih in Brønstedovih kislih centrov. Prevladujejo Lewisovi kisli centri relativno visoke jakosti. Brønstedovi kisli centri so najverjetneje posledica disociativne adsorpcije vode med manipulacijo vzorca.

Katalitski test dismutacije CCl2F2 smo opravili na različnih materialih: na GaF3 pripravljenim po fluoridnem sol-gel postopku in dodatno fluoriranim s CCl2F2 in aHF, na GaF3 pripravljenim s postopkom oksidativnega razpada, na komercialnem GaF3 ter slepi probi. Najbolj je bil katalitsko aktiven GaF3 pripravljen po fluoridnem sol-gel postopku in dodatno fluoriranim s CCl2F2. Konverzija CCl2F2 v produkte dismutacije je bila pri 300 °C (573 K) 0,8 %, pri 350 °C (623 K) pa 4 %. GaF3 pripravljen po postopku oksidativnega razpada ni bil katalitsko aktiven, ravno tako ni bil aktiven komercialen GaF3. Katalitska aktivnost HS-GaF3 je bila v primerjavi z HS-AlF3 nizka. Za HS-AlF3 je bila konverzija pri 300 °C (573 K) skoraj 96 %. Nižjo katalitsko aktivnost GaF3 lahko v prvi vrsti pripišemo nižji Lewisovi kislosti Ga3+ v primerjavi z Al3+. Dodaten faktor, ki lahko vpliva na nižjo katalitsko aktivnost, je visok delež relativno stabilnega organskega ostanka v naših vzorcih. Razpadni produkti tega ostanka lahko blokirajo aktivne kislinske centre, tako da se nanje fizično ali kemijsko vežejo.

ZAKLJUČEK

Nova postopka, fluoridni sol-gel in oksidatvni razpad hidrazinijevih fluorometalatov, sta bila uspešno uporabljena za pripravo GaF3 s specifičnimi lastnostmi. Materiali, pridobljeni po različnih metodah priprave, se med seboj nekoliko razlikujejo, povzetki rezultatov karakterizacije so v Tabeli 10 in 11 (Poglavje 5)

Bistvena razlika v pripravi GaF3 in HS-AlF3 po fluoridnem sol-gel postopku je v tem, da je v našem sistemu propoksid bistveno stabilnejši v prvem delu sinteze, saj na lastnosti prekurzorja ne vplivajo presežki aHF. To se odraža tudi v drugem delu sinteze, kjer dobimo GaF3 s skoraj 8 %(m/m) vsebnostjo nečistot in ne čistega produkta, kot v primeru HS-AlF3. Bistvena razlika med GaF3 in HS-AlF3, pripravljenim po postopku oksidativnega razpada, je v tem, da ima GaF3 bistveno manjšo površino ter, da ni katalitsko aktiven.

Nadaljnje raziskave na tem področju bodo omogočile boljše razumevanje mehanizma pri pripravi kovinskih fluoridov po fluoridnem sol-gel postopku in postopku oksidativnega razpada. Znanja, pridobljena pri pripravi GaF3 ter ostalih kovinskih fluoridov, bomo lahko uporabili za pripravo novih materialov. To bo lahko kasneje vodilo tudi v raziskave za njihovo uporabo na najrazličnejših področjih.

- 14 -

11 IINNTTRROODDUUCCTTIIOONN

Metal fluorides are not as widely used as metal oxides, although they often

exhibit properties superior to metal oxides. Metal fluorides have interesting optical,

magnetic, electrical and catalytic properties, making them useful in a broad range of

applications1-6.

Several preparation routes to solid metal fluorides have been developed and

are used today. Classical fluorination processes are prevalently performed at high

temperature and/or high pressure with fluorine, hydrogen fluoride or other fluorinating

agents. These relatively rigorous conditions favour the growth of larger particles and

formation of crystallized products. Fluoride products obtained by these procedures

are partly or fully crystallized materials with relatively low specific surface areas of

1–20 m2/g. Lack of adequate preparation procedures to obtain fluorides with higher

surface areas is one of the main reasons why metal fluorides are not as widely used

as oxides. In the last years an important progress in the preparation of solid metal

fluorides with high specific area was achieved. Two new chemical preparation

approaches were developed, (i) non-aqueous fluoride sol-gel procedure7 and (ii)

oxidative decomposition of fluorometalates8. The products of both procedures are

usually amorphous fluorides with high specific surface areas of 100–300 m2/g and

particle size in range of 5–20 nm. Amorphous structure, high surface and nano sized

particles are preserved in materials due to milder preparation conditions. Fluoride sol-

gel method is a type of classical sol-gel procedure. This is inexpensive and easy way

to produce high surface metal oxides which made them very popular for variety of

uses in the last few decades. Oxidative decomposition of fluorometalates using

elemental fluorine is a new inorganic technique for preparation of high surface metal

fluorides and can be used as alternative to the fluoride sol-gel method.

Most of the research of this two preparation techniques have been done under

project FUNFLUOS (6th Framework programme of the EU). A variety of metal

fluorides has been prepared during this project and research has also focused on

their properties and possible applications9-11. High surface materials are interesting

especially in the area of heterogeneous catalysis where the activity of the material

- 15 -

depends on its surface area and acidity. One of the most investigated metal fluoride

was HS-AlF3 which exhibits high surface area and an extremely strong Lewis acidity,

comparable with some of the strongest known Lewis acids such as SbF5 and

ACF12-14.

The main objective of this thesis was preparation of GaF3 with specific

properties by the application of two novel preparation techniques, fluoride sol-gel

procedure and oxidative decomposition of fluorometalate, hydrazinium(II)

aquapentafluorogallate(III) (N2H6GaF5.H2O)15. These properties are high surface

area, high Lewis acidity and as a result of those higher catalytic activity. GaF3 is

potential new high surface material with high Lewis acidity. Primar objective of the

work was to define the key parameters of the preparation and characterization of

newly prepared materials. The materials were characterized by determination of

specific surface area (BET), elemental analysis, scanning electron microscopy,

photoacoustic IR spectroscopy and X-ray powder diffraction. In addition, the

materials were also characterized by two indirect methods, determination of acidity

by pyridine adsorption14 and by measurement of catalytic activity in the test reaction

of CCl2F2 dismutation16. This reaction can be catalyzed by medium strong Lewis

acids.

Theoretical quantitative scale for Lewis acidity, published by Christe and

coworkers17 indicates that GaF3 could exhibit high Lewis acidity, slightly lower than

AlF3. Pure GaF3 was not yet produced using novel preparation techniques although

Kemnitz and coworkers18 studied mixed metal fluorides as doped Lewis acid

catalysts which also included a detailed investigation of the MgF2/GaF3 system.

Catalytically inactive MgF2 served as a host for an active metal trifluorides which

were used to adjust the Lewis acidity of the solid catalyst. In this case MF3 dopant

improves Lewis acidity due to generation of excess positive charge at the metal(III)

site which results in increased electron acceptor ability. MgF2/GaF3 doped system

was a high surface material (BET = 100 m2/g) which became more amorphous with

increasing dopant concentrations and achieved its maximum catalytic activity at a

concentration of about 20 wt.%. Due to good results from

Ga-doped system and other metal fluorides (HS-AlF3) prepared via novel preparation

methods, it was expected that our pure GaF3 would be amorphous, high surface and

catalytically active solid material with high Lewis acidity.

- 16 -

Majority of the research performed so far was done on the preparation and

characterization of HS-AlF3 starting from the commercially available low-cost

aluminium isopropoxide. Influence of many parameters has been studied in detail to

optimize this process12. Preparation of GaF3 was evaluated against the basic

parameters, defined during the preparation of HS-AlF3. Characterization results of

different steps of GaF3 preparation were compared with those obtained during HS-

AlF3 preparation.

New fluoride sol-gel method should give amorphous GaF3 with improved

acidic properties, prepared at much lower temperatures than with standard

preparation methods.

- 17 -

22 TTHHEEOORREETTIICCAALL BBAACCKKGGRROOUUNNDD

AANNDD GGOOAALLSS

22..11 IINNOORRGGAANNIICC MMEETTAALL FFLLUUOORRIIDDEESS

INTRODUCTION TO INORGANIC METAL FLUORIDES

The use of natural inorganic fluorides can be traced back to Greek and Roman

times when large crystals of fluorspar were used to make dishes, vases and other

ornaments. Nowadays main sources of fluorides as raw materials in industrial

production are: natural cryolite (AlF3·3NaF), fluorspar (CaF2) and fluorapatite

(CaF2·3Ca3(PO4)2). Besides metallurgical uses, the production of considerable

tonnages of inorganic fluorides for industrial use is relatively recent. The industrial

production of aluminium started in the late 19th century with the process of electrolytic

preparation of aluminium by reducing its oxide, which is dissolved in a bath of molten

cryolite. The production of hydrogen fluoride reached industrial stage in 1930 with

creation of two production units in U.S.A. and France. Rapid development of

chemical industry such as varied refrigerants, plastics, petrochemical industry,

aerosol propellants, surfactants, space industry, health service, electrical industries

and nuclear industry significally increased production of this acid from fluorspar.

Intensive research related to the development of nuclear energy for military and

peaceful purposes started in 1950s. It can be said that atomic era is era of fluorine as

the preparation of large amounts of uranium hexafluoride was necessary for isotopic

separation plants. This research has contributed to a better knowledge and

properties of numerous fluorides, leading to many advances and new applications.

Metal fluorides are known to have superior properties over metal oxides for

several applications. Many future developments can be predicted in certain fields of

use related to specific features such as for example optical (low refractive index),

- 18 -

electrical (high ionic conductivity), magnetic (high-spin configurations), catalysis

(super acidity) and other interesting applicable properties19. Their use is limited

especially due to the problems with processing. This new interest in fluorides justifies

research of new appropriate and low-cost synthetic techniques. In general, we have

to perform various types of fluorination procedures which may also involve high

pressure or high temperature reactions with fluorine, hydrogen fluoride or other

fluorinating agents such as chlorine trifluoride, sulfur tetrafluoride, arsenic fluoride,

ammonium difluoride, hydrazinium fluoride and others. Because of their strong

oxidizing power, fluorine and some other highly reactive derivates react

spontaneously with many compounds and give very exothermic reactions. Methods

such as decomposition of fluorides and hydrates or reducing process are also used

but are often done at high temperatures20.

There are three general reaction types to synthesize fluorides. First are gas-

solid reactions where, gaseous fluorine or other gaseous fluorinating agents react

with solid metals or metallic salts. F2 has very strong oxidizing power and this leads

in most cases to the fluoride with highest oxidation state for the metallic cation. Other

fluorinating agents give results similar to those obtained with F2. The reaction

conditions vary from ambient temperatures to more than 1000 °C. Second are liquid-

solid reactions (wet way) where chemical reactions are done in liquid medium, for

example in HF. Oxides, halides, carbonates and acetates can be used as starting

agents. Solid state reactions are third reaction type, where sample preparation

almost always requires dry boxes, gold or platinum tubes and inert atmospheres. A

perfect example of solid state reaction is preparation of K3Fe5F15, where

stoichiometric quantities of KF, FeF2 and FeF3 are mixed under dry argon, sealed in

gold tube and left in the oven at approximately 800 °C for 15 days3.

Described methods are quite demanding and require serious precautions or

procedures can be extremely dangerous at some points because of high

temperatures, high pressures and use of reactive or corrosive reagents. In addidtion

most of fluorides are moisture sensitive and some of them are highly corrosive. They

have to be handled in dry boxes in inert atmosphere and any direct human contact

has to be avoided.

- 19 -

GALLIUM(III) FLUORIDE

Since main focus of this thesis is gallium(III) fluoride it is important to mention

few basic properties, known standard preparation techniques, current known

applications and point out some possible future uses. Gallium(III) fluoride is a

chemical compound made from gallium, metal with exceptional liquid range (30 °C to

2000°C) and fluorine, most electronegative, highly reactive element. Gallium(III)

fluoride is white powder with molecular formula GaF3 and molecular mass of 126.718

g/mol. It has boiling point is around 1000 °C and it is almost insoluble in water.

Anhydrous gallium(III) fluoride can be prepared by three known methods21-23 and in

all three cases final product is partly or fully crystalline material.

First procedure is the thermal decomposition of ammonium hexafluorogallate,

(NH4)3GaF6 in a stream of F2 at 400 °C (reaction 1). Procedure is carried out in nickel

tube reactor. Extreme precaution is needed when working with fluorine at such high

temperatures. (NH4)3GaF6 is produced from Ga(OH)3 using HF and NH4F as

reagents.

Second procedure is reaction between metallic gallium in a stream of HF-gas

at 550°C (reaction 2). We must use nickel tube with a nickel boat. HF-gas can be

mixed with N2. Reaction is slow and may last few hours. GaF3 is formed on the

surface of Ga metal.

Third procedure consists of two steps. In first step GaF3·3H2O (trihydrate) is

obtained by dissolving gallium hydroxide, gallium oxide or gallium metal in 40 wt.%

hydrofluoric acid so as to leave small excess of acid. Evaporation to dryness at

100 °C at air gives GaF3·3H2O which is a white crystalline powder. Anhydrous GaF3

is obtained by drying the hydrate in vacuum at 200 °C and after that it has to be

additionally treated in a stream of F2.

The crystal structure of gallium(III) fluoride23 consists of layers of more or less

close packed fluoride ions with gallium ions occupying certain octahedral holes

- 20 -

between them. Each gallium atom has six fluorine neighbors at a distance of 1.89 Ǻ

(Sum of ionic radii = 1.95 Ǻ). Each fluorine atom has four neighbors in its own layer

at 2.69 Ǻ and four others in adjacent planes at 2.67 Ǻ. Each fluorine atom lies

equidistant from two gallium atoms but slightly displaced from the line joining them.

The Ga–F–Ga angle is 145° and the closest approach of gallium atoms to one

another is 3.61 Ǻ. The unit cell is a bimolecular rhombohedron with space group

symmetry R3c. The unit cell dimensions are: αR = 5.2 + 0.01 Ǻ; αR =57.5°. The

structure is similar to the structures of all trifluorides of the first transition series. GaF3

is isostructural with FeF3.

Gallium compounds have currently only one major application and this is as

materials for semiconductors24. More than 90 % of such devices are based on silicon,

because this element is more common and is much cheaper to produce. Silicon

however does not posses some the of most outstanding electronic properties as for

example GaAs (gallium arsenide) or other complex gallium containing compounds

that also exhibit semiconducting properties. GaAs devices have an advantage in

speed, better thermal stability and lower power consumption. The limiting factor for

the extensive use of GaAs is the fact that it does not have a passivating layer and,

therefore, devices containing GaAs can not be satisfactorily produced. GaF3 finds a

role in perfecting and improving this application. Some research using GaF3 as

suitable insulator that can be grown on GaAs layer has been done in the past25. The

problem was partially solved by providing a layer of indium gallium fluoride on the

surface of the gallium arsenide substrate. This resulted in semiconductor device with

improved insulating/passivating layer of indium gallium fluoride (InGaF), which is

produced by mixing together GaF3 with InF3 in specific ratio to get crystallographically

lattice matching layer on the surface of the GaAs substrate26.

Most of the research on GaF3 as a compound is focused in various optical

systems27, 28, which may find its applications in optical communication technologies.

Fluorides have several specific features such as small refractive index, wide

transmission wavelength range, etc. GaF3 and some other fluoride based glasses are

promising for applications which need wider IR transmission range and lower phonon

energy. For example GaF3 was used as compound for planar lightwave circuits which

have a valuable advantage (integrated in a very small area) compared to well known

silica-based glass optical fibers29.

- 21 -

HIGH SURFACE METAL FLUORIDES

Metal fluorides can be superior to metal oxides in several possible applications

but, due to the problems in processing, their use is limited. New preparation routes

are needed to open new fields of applications for metal fluorides. One of the most

promising and investigated route is non-aqueous fluoride sol-gel preparation which is

good alternative and low-cost method for preparation of metal fluorides with specific

properties. Metal fluorides prepared by sol – gel method, such as AlF3, MgF2 and

other complex metal fluorides, have extraordinary high surface areas up to 500 m2/g

with pores in the range of 10 – 40 Ǻ as a result of their morphological characteristics

and nanosized and mesoporous structures. If technologic borders regarding the

preparation of metal fluorides are overcome this may open a new fields for their

application.

Metal fluorides with strong Lewis acidity can be used as acid catalysts.

For example amorphous, HS-AlF3 with high Lewis acidity comparable to that of SbF56

and ACF30 is a potential candidate for several Lewis acid catalyzed reactions.

HS-AlF3 has under-coordinated surface Al sites that exhibit very strong acidity. It is

the first example for a direct sol-gel preparation of nano-metal fluorides resulting in

materials with exciting properties which differ remarkably from those exhibited by

classically prepared AlF3 phases. AlF3 is one of the most important catalysts for

halogen exchange and related reactions31, 32. This type of reactions involving

halofluorocarbons are of interest for the production of HFCs and for CFCs

transformations33. High catalytic activity of HS-AlF3 and other new metal fluorides can

be used in a wide range of reactions such as isomerisation of alkanes,

polymerization, carbonylation, Friedel-Crafts acylation and in general for the

preparation of new fluorinated organic compounds to be used in pharmaceutics. AlF3

may also be used as reactive coating on metal surfaces of micro-reactors to initiate

surface catalytic reactions. In some cases extraordinary strong sites in HS-AlF3 need

to be adjusted in reactivity to fit to the specific reactant molecule. Fine-tuning of a

catalyst system is achieved by doping metal fluoride matrices with the three-valent

metals18 such as FeIII, CrIII and GaIII. There are also other known fine-tuning

techniques like anion doping34 or applications of metal fluorides as supports35.

- 22 -

Metal fluorides exhibit very interesting optical properties as low refraction

indexes and very good UV transparency. For example MgF236 has lower refraction

index than SiO2 (n500 = 1.38) and may be used as anti-reflective coating in optical

systems. Fluoride sol-gel method is cheap, low temperature preparation technique

that can be used to produce metal fluoride coatings from diluted sol solutions of

known concentrations giving reproducible films of 15–50 nm thickness. Thin film

coating may serve in many known applications such as coating for: sensitive metal

surfaces, temperature-sensitive components like polymers, improvement of surface

properties of electronic parts, hydrophobicity and medical sterility. Most metal

fluorides obtained through sol-gel route are nano-sized and X-ray amorphous

powders. Cold pressed powders like MgF2, AlF3, Na3AlF6 and others give transparent

ceramic plates which may open an alternative route to transparent glassy metal

fluorides. Complex fluorometalates37 are accessible by fluoride sol-gel method at

room temperature. Their known optical, magnetic and ferroelectric properties are of

great interest. An unexplored field of fluoride sol-gel method is the functionalization of

nano-fluorides by leaving a defined number of OR-groups unconverted to use them

as chemical anchors to bind other compounds on it. This gives access to metal

fluoride inorganic or organic hybrid systems as it has been developed in the metal

oxide systems.

Based on these novel preparation strategies and new properties resulting from

them, metal fluorides could experience wider field of applications in the next future.

- 23 -

22..22 PPRREEPPAARRAATTIIOONN TTEECCHHNNIIQQUUEESS TTOO OOBBTTAAIINN HHIIGGHH SSUURRFFAACCEE MMEETTAALL FFLLUUOORRIIDDEESS

INTRODUCTION TO SYNTHESIS STRATEGIES

Several chemical and physical methods have been developed and used to

prepare high surface and/or nano-sized metal fluorides7. Some common physical

methods are condensation from a suitable vapor phase (e.g. PbF2)38, laser deposition

(e.g. NaF)39, molecular-beam epitaxy for coatings (e.g. LaF3)40, and mechanical

milling, which was also used to prepare nanostructured GaF3 powder41. Chemical

methods have been also intensively researched and developed, especially non-

aqueous fluoride sol-gel method7 and oxidative decomposition of fluorometalates8, 42.

These two methods will be reviewed in more details since the focus of this thesis is

preparation and characterization of GaF3 by the application of these two methods.

Also two other synthetic routes were widely investigated, but were not used to

prepare GaF3. The first is post-fluorination of metal oxide prepared via sol-gel route43

and the second is formation of thermal decomposition of metal trifluoroacetates44.

GENERAL PRINCIPLE OF SOL-GEL PREPARATION

The sol-gel process is a wet-chemical technique used for preparation of

versatile materials at lower temperatures. Typical materials prepared with classical

sol-gel are pure or multicomponent metal oxides or related hybrid inorganic-organic

materials. The technique is cheap and can be used to prepare diverse materials such

as light aerogles or tough dense ceramics. It also allows a fine control of the

product’s chemical composition and it can yield very pure materials.

The method is based on the hydrolysis and condensation reactions. Most

typical precursors are metal alkoxides, M(OR)x. The first step of reactions is

- 24 -

hydrolysis (reaction 3)45, the nucleophilic substitution of the alkoxides groups (OR) by

hydroxyl groups (OH).

Hydrolysis:

Second step is condensation reaction which leads to the formation of the

extended 3D network (gel) by the formation of M-O-M links or M-O(H)-M link with the

evolution of water or alcohol. The gel state is described as a viscoelastic material

composed of interpenetrating solid and liquid phases. Main condensation reactions

are alcoxolation/oxolation (reactions 4 and 5) and olation (reactions 6 and 7).

Examples of condensation reactions45:

Alcoxolation:

Oxolation:

Olation:

Solution or stable suspension of colloidal particles (sol) produces porous 3D

continuous solid networks entrapping a continuous liquid phase (wet gel). Next step

is removal of the solvent. In the case of evaporation, xerogel (dried gel) is obtained.

When the removal of the solvent is done by supercritical extraction aerogels are

obtained. Aerogels are very porous gels having high surface area which were dried

so that the pore and network structure of the wet gel are maintained. It is also

possible to use wet gel directly and deposit it on a substrate to form a film (dip or spin

coating) or threat it (heating) to produce ceramic fibers, glasses, etc.45 (Scheme 1).

- 25 -

Sol-gel transition could be explained in few steps. At the beginning sol

particles aggregate to give the gel network and small oligomeric species are formed,

bearing OH groups on the surface (nucleation seed). This process continues with the

agglomeration of the particles to larger polymeric structures and increase of the

viscosity which leads to formation of a continuous network, elastic gel (gel point). In

some cases we also use gel ageing which leads to further stabilization of the

inorganic network by further condensation of the remaining precursor (monomer).

One of the results of gel ageing is shrinkage of the gel (syneresis).

Scheme 1: Overview of the sol-gel process and their main applications45.

There are many parameters affecting the sol-gel process and the properties of

the final products. The most important are nature and concentration of the

precursors, reactant ratio (M(OR)x : H2O), effect of pH on hydrolysis and

condensation, reaction temperature, nature of the solvent, drying conditions and

presence of stabilizers/additives.

To summarize all advantages of sol-gel process we can say that it is versatile

process which allows fine modulation of the final features of the materials by a careful

- 26 -

tuning of different experimental parameters. It gives good control of the sample

morphology, chemical composition, homogeneity, porosity, crosslinking density and

special functionalities. As mentioned before it is possible to prepare materials in a

wide variety of forms: ultra-fine powders, fibers, solids, xerogels, aerogels, thin films

and bulk materials at relatively low or even ambient temperatures. This synthetic

route is an important direction for the preparation of a wide range of novel materials

with applications especially in the areas of photonics and chemical sensors, catalysis,

ceramics, coatings and glasses. Ambient processing conditions also enable

encapsulation of numerous organic, organometallic and biological molecules within

these sol-gel derived inorganic matrices and vice versa.

There are also some draw backs of sol-gel process like availability of

appropriate alkoxides, high cost of some raw precursors (alkoxides), high shrinkage

during gelation, dynamic nature of the process involved and difficulties in fine tuning

some of the preparation parameters.

FLUORIDE SOL-GEL PREPARATION

The non-aqueous fluoride sol-gel preparation of inorganic metal fluorides is a

relatively new method. Most of the efforts in past have been made to study sol-gel

processes for the preparation of inorganic oxides for various applications. Since

fluorides have potential importance in similar or even new applications as oxides

there is an interest in developing new synthetic routes for the preparation of fluorides.

The foundation and first basic research for non-aqueous sol-gel preparation route

was done by Kemnitz and co-workers9. Modification of conventional sol-gel

preparation was developed with the aim to prepare amorphous metal fluorides.

Fluoride sol-gel is a two step preparation (Scheme 2) of amorphous highly

distorted metal fluorides with unusually high surface area caused by a mesoporous

morphology and surface texture10.

- 27 -

Metal alkoxide+

Water free solvent

Anhydrous HF+

Organic solvent

Mixing

Wet fluoride gel Dry fluoride gel

Removal ofsolvent

Metal fluoride

Additionalfluorination/activation

Step 1

Step 2

Scheme 2: Metal fluoride preparation via non-aqueous fluoride sol–gel route9.

In first step metal alkoxide is dissolved in water free alcohol or other

non-aqueous solvent. Calculated amount of aHF in organic solvent is added to the

mixture under stirring at ambient temperature. A wet gel forms immediately and could

be either used directly for coatings or dried to yield solid metal fluorides. The sol-gel

fluorination reaction results in complex development of a polymer-like network of a

gel. Depending on the type of metal and on concentrations used a wet gel of metal

fluoride and alcohol is formed. After removing all volatile material under vacuum a dry

gel (xerogel) is formed which is also referred as precursor. It is amorphous solid with

the content of some organic material. Because of organic part present in material

second step of post-fluorination is needed to yield final product, pure amorphous high

surface metal fluoride. Precursor (dry gel) is usually treated with a fluorinating agent,

such as fluorocarbon compounds (e.g. CCl2F2) or gaseous HF. This process is also

named activation because after such treatment the catalytically inactive dry Al–F-gel

becomes catalytically active. Not all fluorocarbon compounds are good enough for

post-fluorination, because some of them cause extensive coke formation on the

fluoride material. HF can be used only at rather low temperatures because of its

higher fluorinating activity which can affect the final properties of material such as

surface area or Lewis acidity.

Original fluoride sol-gel procedure from Kemnitz was later modified at

Department of Inorganic Chemistry and Technology, "Jožef Stefan" Institut.

Reactions were carried out in FEP (HF resistant plastic) and all operations were

- 28 -

performed in a closed vacuum system avoiding any contact with ambient air.

Advantages of improved method are that aHF can be dosed more accurately, pure

aHF without organic solvent can be added and it is safer due to limited exposure to

HF. Both methods yield materials with comparable properties.

Main disadvantage of both fluoride sol-gel procedures is their low

reproducibility in the second step of fluorination due to the nature of heterogeneous

reactions involved.

COMPARISON BETWEEN THE CLASSICAL SOL-GEL PROCEDURE AND FLUORIDE SOL-GEL ROUTE

Novel fluoride sol-gel method has few parallels with classical sol-gel method

because it follows similar principles. In classical sol-gel, reaction is started by adding

exact amount of water to metal alkoxide which result in hydrolysis reaction followed

by condensation and finally ends in formation of gel. Actually this means that OH

groups, which appear in first step of reaction (hydrolysis) react with OR and OH

groups in second step of reaction (condensation) which leads to formation of oxide

gel. All structure of the gel is made from metal – oxygen bonds, –M–O–M–. The main

difference between fluoride and classical sol-gel method is in the use aHF instead of

water. Addition of aHF to solution of metal alkoxide starts a nucleophilic substitution

of OR groups by fluoride ion and may be considered as fluorolysis. This step

resembles to a great extent the hydrolysis step in classical sol-gel reaction for metal

oxide preparation. Fluoride ion is not able to continue reaction as condensation

known in classical sol-gel, so coordinative covalent bond (dative bond) is formed with

second metal atom which helps to stabilize the gel. Besides similarities in the first

step of the process mechanism of fluoride sol-gel is therefore quite different from the

classical sol-gel process. Comparison between two synthesis mechanisms of

classical (reaction 8) and fluoride (reaction 9) sol-gel is shown below12.

- 29 -

OXIDATIVE DECOMPOSITION OF HYDRAZINIUM FLUOROMETALATES

Oxidative decomposition of hydrazinium fluorometalates with elemental

fluorine is novel inorganic one-step route to high surface area metal fluorides8, 42, 46.

The preparation is based on milder synthetic procedures which result in amorphous

and nano-structurized final materials. Metal fluoride is the only solid product after

oxidative decomposition of hydrazinium fluorometalates. This method was developed

as inorganic alternative to fluoride sol-gel preparation because of its known problems

with removal of organic residue in the second step of preparation. Oxidative

decomposition was first used for the preparation of aluminum and chromium

fluorides, two of the catalytically most relevant fluorides.

Preparation procedure of materials was studied under heterogeneous

(gas-solid) conditions and in liquid aHF medium. Results showed that material

prepared in liquid aHF medium yielded amorphous materials with surface areas up to

300 m2/g (Scheme 3).

- 30 -

Scheme 3: Metal fluoride preparation via oxidative decomposition of hydrazinium fluorometalates in

liquid aHF medium.

- 31 -

22..33 EEXXAAMMPPLLEE OOFF HHSS--AAllFF33 PPRREEPPAARREEDD WWIITTHH AAPPPPLLIICCAATTIIOONN OOFF FFLLUUOORRIIDDEE SSOOLL--GGEELL AANNDD OOXXIIDDAATTIIVVEE DDEECCOOMMPPOOSSIITTIIOONN PPRROOCCEEDDUURREESS

HS-AlF3 PREPARED VIA FLUORIDE SOL-GEL PROCEDURE7, 12

The majority of the research was performed on the preparation and

characterization of HS-AlF3 starting from aluminium isopropoxide. HS-AlF3 is

amorphous material with very high Lewis acidity. It was prepared as described in

Scheme 2 by non-aqueous fluoride sol-gel route. Slightly modified process was

applied for the preparation of gallium fluoride gels. Fluoride sol-gel procedure is

therefore exemplified by the preparation and characterization of HS-AlF3.

Al(OiPr)3 was dissolved in dry iPrOH (aprox. 100 g/L) at room temperature.

Stoichiometric amount of aHF also dissolved in dry iPrOH was added to the mixture

within a few minutes during continuous stirring. The stable transparent gel (wet gel)

was formed in within some minutes. The overall reaction of aluminium alkoxide with

HF can be formulated as shown in reaction 10:

The precursor had specific surface area 450 m2/g. Post-fluorination of dry

Al-F-gel was carried out in a flow reactor consisting of a Ni tube (5 mm i.d.) located

vertically in an electric furnace. Dry gel (1–2 g) was placed on a silver wool plug in

the centre of the reactor. Precursor was treated with a fluorinating agent (CCl2F2)

diluted by N2 at 300 °C. The goal was to remove the organic component by replacing

it fully by fluorine. The gases from the exhaust of reactor were analyzed by on-line

GC. With CCl2F2 as fluorinating agent the progress of activation was followed up by

monitoring the dismutation of the reactant and the decrease of other organic residue

compounds concentrations. When dismutation reached about 98 % the activation

process was complete. The final product had a ratio of fluorine to aluminum of 3 : 1

- 32 -

and the carbon content was less than 0.5 wt.%. HS-AlF3 had a specific surface area

of about 200–300 m2/g.

The research to fully understand the mechanism of the sol and gel formation

of direct fluoride sol-gel method is still in progress, but to some extent mechanism

has already been clarified13. A proton goes to the leaving OR group during the

fluorination reaction. Following step is the protonation of the alkoxy groups, an

alcohol splits off and Al–F bonds form. As the number of Al–F bonds increases, they

bridge with each other and form a network. Different steps of sol-gel process and

formation of random network with successive replacement of oxygen atoms by

fluorine in bridging bonds are shown in Scheme 4.

Scheme 4: (A) Al4(OiPr)12 (aluminium isopropoxide), (B) partially fluorinated, (C) Al(OR, F)6

intermediate, and (D) Al alkoxide fluoride (precursor)13

The influence of many parameters on the HS-AlF3 preparation has been

studied in detail aiming optimization of the process. It was found, that concentration

of metal alkoxide has no important effect on the properties of the final metal fluoride.

Al3+: HF ratio had distinct effect on the properties of final material. Over-

stoichiometric excess of HF than Al3+ : HF equal to 1 : 3 resulted in decrease of

surface area and in formation of partly crystalline AlF3. Under-stoichiometric amounts

- 33 -

of HF could be balanced by the second fluorination step. The most ideal was

stoichiometric amount of Al3+ : HF = 1 : 3 because less post-fluorination is needed. A

dry gel of aluminium fluoride (precursor) contains always substantial amounts of

alcohol and/or alkoxide groups mounting up to an analytical organic content of

30 wt.%. In going from AlF3 to MgF247 and to NaF48 the organic content of the dry gel

goes down to less than 1 wt.%. This shows that the Lewis acidity of the metal cation

in its fluoridic environment is responsible for the degree of solvent binding. At the

same time the organic constituents are covering and blocking preferably the

strongest Lewis acid sites. The removal of organic constituents from aluminium

fluoride dry gel, with aim to prepare extremely highly Lewis acidic HS-AlF3 is done in

a flow of a gaseous fluorinating agent. Simple heating of the dry gel in an inert gas

was not successful. This process is named activation because the catalytically

inactive dry gel of aluminium fluoride becomes active catalyst. Fluorocarbon

compounds and HF have been tested as fluorinating agents. CCl2F2 have been

successfully employed, whereas CHClF2 and gaseous HF were less effective.

CHClF2 caused extensive coke formation because of lower stability due to its

hydrogen content and gaseous HF can be only used at rather low temperatures (120

°C) because of its higher fluorinating capability. HS-AlF3/HF material prepared this

way had lower Lewis acidic characteristics.

Measurements of catalytic activity which depends specifically on the action of

Lewis acidic catalysts have been done. Dismutation reaction of CCl2F2 can be only

catalyzed by medium strong Lewis acids. Conversion reached 96 % using HS-AlF3

as catalysts at 300 °C. HS-AlF3 proved as a successful catalyst also in the

isomerization reaction of CCl2FCF2Cl which can be only catalyzed by very strong

Lewis acids. Photoacoustic IR spectroscopy was used for indirect determination of

Lewis acidity by pyridine adsorption test. Results of these tests show the presence of

almost exclusively Lewis acid sites with relatively high strength.

The development of HS-AlF3 having high surface area and high Lewis acidity

may be considered as a breakthrough in metal fluoride preparation.

- 34 -

HS-AlF3 PREPARED VIA OXIDATIVE DECOMPOSITION8, 46

Preparation of HS-AlF3 was the first studied procedure. Starting material for

preparation was hydrazinium(II) fluoroaluminate, N2H6AlF5. Complete decomposition

of N2H6AlF5 with elemental fluorine should yield pure AlF3 (reaction 11). The reaction

was done under heterogeneous (gas-solid) conditions and in liquid anhydrous

hydrogen fluoride (aHF) medium.

Initial experiments showed that oxidative decomposition under heterogeneous

conditions (gas-solid) is not suitable for preparation of metal fluorides with high

surface areas. Material is over-heated due to intense heterogeneous exothermic

reaction. This leads to partly crystallized materials with low surface area.

On the contrary liquid aHF proved as

excellent reaction medium for this kind of

reactions. Final material prepared under

gas-solid conditions yielded partly crystallized

AlF3 with surface areas of 25 m2/g, while

material prepared in liquid aHF medium

yielded amorphous material with surface

areas of 200–300 m2/g. Continuous stirring is

needed during reaction to prevent subsiding

of the solid phase and to ensure efficient

conveying off the heat. Scale-up of the

oxidative decomposition of N2H6AlF5 with F2 in

aHF medium is possible. The scaled-up

preparation could be safely reproducible

(Figure 1), which allowed preparation of

batches with product capacity of up to 10 g.

Figure 1: 700 ml PCTFE reaction vessel for oxidative decomposition of N2H6AlF5 with F2 in aHF medium (350 ml).

Elemental analysis showed that HS-AlF3 contain approximately 95 wt.% of

AlF3 and other residues which could not be quantified due to the nature of the

- 35 -

samples. IR and XPS spectra proved that the residue consist of nitrogen and oxygen

compounds. Oxygen compounds, most likely H3O+ and OH–, originate from the

residual water in the starting material, N2H6AlF5. Nitrogen compounds, mostly NH4+

appear due to incomplete decomposition. Research of catalytic activity showed that it

is directly connected to the amount of NH4+ in the material. Amounts higher than

0,5 wt.% of NH4+ resulted in materials with no catalytic activity in selected probe

reactions43, isomerization of CClF2CCl2F and dismutation of CCl2F2. Amount of NH4+

in HS-AlF3 can be reduced by decanting the liquid aHF phase, additional extraction of

HS-AlF3 with liquid aHF or treatment with gaseous fluorinating agent such as CCl2F2.

It results in catalytically active material with unchanged high surface area. Catalytic

activity of HS-AlF3 prepared with oxidative decomposition is comparable to HS-AlF3

prepared with fluoride sol-gel process.

- 36 -

22..44 SSCCOOPPEE AANNDD OOUUTTLLIINNEE

Primary purpose of this thesis was preparation of GaF3, with improved specific properties by the application of:

Fluoride sol-gel procedure using metal alkoxide, gallium(III) isopropoxide, as

starting material.

Oxidative decomposition of fluorometalate, hydrazinium(II)

aquapentafluorogallate(III), N2H6GaF5·H2O.

The focus of preparation was to define the key parameters of the synthesis such

as concentration of reactants, type of solvent, type of fluorinating agent, etc.

All materials were characterized by:

Determination of specific surface area (BET method),

Elemental analysis (Ga : F molar ratio, residues)

Scanning electron microscopy (morphology)

Photoacoustic IR spectroscopy (comparison of materials in different stages of

preparation)

Gas chromatography (identifying the organic reaction products from catalytic

experiments)

X-ray powder diffraction, (amorphous?)

In addition the materials were also characterized by two indirect methods:

Determination of acidity by pyridine adsorption14

Measurement of catalytic activity in test reaction of CCl2F2 dismutation16.

.

- 37 -

33 EEXXPPEERRIIMMEENNTTAALL WWOORRKK

33..11 BBAASSIICC EEQQUUIIPPMMEENNTT::

VACUUM SYSTEM

Most of reactants used to synthesize our materials and also final products are very

reactive and toxic. Exposure to ambient air is most problematic and results in quick

reaction with water that ruins our materials in matter of minutes. Reactions were

carried out in closed vacuum system (Figure 2) to prevent degradation of our

reactants and products.

Figure 2: Vacuum system

This vacuum system is a modified version of vacuum system, developed at Jožef

Stefan Institute at Department of Inorganic Chemistry and Technology49. System is

- 38 -

made from nickel, glass and plastic pipes and appropriate valves connected to oil

rotational and mercury diffusion vacuum pump. Working vacuum from 10–1 to 10–3 Pa

is achieved using mercury diffusion pump and the vacuum of 1 Pa needed for

diffusion pump operation is achieved by oil rotational pump.

DRY BOX

The manipulation of all non-volatile solids is accomplished in dry argon

atmosphere within a dry-box (M. Braun, Germany). The residual water in the

atmosphere within the dry-box should never exceed 2 ppm.

REACTION VESSELS

HF resistant plastic has to be used for

reaction vessel instead of glass, which

reacts with HF. All reactions are carried out

in FEP (tetrafluoroethylene-

hexafluoropropylene; Polytetra, Germany)

reaction vessels (Figure 3). FEP has similar

properties like PTFE (Teflon) but is more

suitable for reactions since it is transparent

and vessels allow visual monitoring of the

processes.

Figure 3: FEP reaction vessels. Capacity of

30 mL (left) and 300 mL (right).

- 39 -

REACTORS, OVENS AND AUTOCLAVES

Flow reactor consisting of a Ni tube (5 mm i.d.) located vertically in an electric

furnace (Figure 4) was used for post-fluorination of precursors and for selected probe

reactions. Nickel autoclave was used together with oval oven to dry precursors and

final materials (Figure 5). The whole system was additionally connected to a vacuum

system and heated up to 250 °C.

Figure 4: Flow reactor in an electric furnace.

Figure 5: Nickel autoclave (up) and oval oven (down).

SCALES

Scale with accuracy of +0,1 mg (Mettler 0 – 1000 g) was used for accurate

weighting of reactants and final materials in closed reaction vessels. Scale with

accuracy of +1 mg (Tehtnica 100M-300C, Železniki) was used for apprpximate

weighting of reactants and final materials in dry box.

- 40 -

TEMPERATURE CONTROLLER AND MASS FLOW CONTROLLER

Programable temperature controller (Digi-Sense R/S) was used to control the

temperature program in electric furnace (flow reactor) during post-fluorination,

catalysis test and pyridine adsorption test (Figure 6).

Mass flow controller MKS (PR 4000 - console, 179B - measuring module) was

used to accurately measure and control the mass flow of gases into flow reactor

(Figure 7).

Figure 6: Digi-Sense Temperature controller.

Figure 7: MKS mass flow controller.

- 41 -

33..22 PPRREEPPAARRAATTIIOONN OOFF GGaaFF33 VVIIAA FFLLUUOORRIIDDEE SSOOLL--GGEELL::

INTRODUCTION

The preparation of GaF3 is affected by several parameters. Our focus was to

determine the key parameters of preparation; temperature, purity of alkoxide

precursor, concentration of reactants and type of fluorinating agent. Since majority of

properties of our material depends on its surface, measurements of specific surface

area according to the BET method were performed preferentially on all newly

prepared precursors and final materials. Elemental analysis has been done on final

materials after fluorination and some precursors to compare Ga : F ratios and to

quantify residues in prepared materials. FT-IR spectroscopy with photoacoustic

detection was used to obtain IR spectra of precursors and final materials. This

method was used to compare our materials in different stages of preparation. These

measurements were performed before other characterization steps described in

chapter 6.

Non-aqueous sol-gel fluoride preparation route for GaF3 was investigated on

the modified equipment as described in the previous chapter with HS-AlF3

preparation and characterization. Preparation of GaF3 can be divided in three main

steps:

1. Purification of gallium(III) isopropoxide

2. Preparation of GaF3 precursor (dry gel)

3. Post-fluorination of GaF3 precursor to yield the final material

- 42 -

PURIFICATION OF GALLIUM(III) ISOPROPOXIDE

Starting material was a

commercially available Ga(OiPr)3

(gallium(III) isopropoxide – Alfa

Aesar). We succeeded to prepare

single crystal for crystal structure

determination by leaving small

amount of Ga(OiPr)3 in glass tube for

two months in dry box. Crystal

structure of Ga(OiPr)3 was

determined. As expected, it is

isostructural with aluminium(III)

isopropoxide, and crystallizes in

monoclinic P 21/c space group with a

= 10.3(1) b = 20.9(2), c = 23.6(4) and

β = 95.1(1).

Scheme 5: Structure of gallium(III) isopropoxide

The basic building block is tetramer where four gallium atoms are connected

through two oxygen bridging atoms from isopropoxide groups. The central gallium

atom is octahedrally surrounded by six oxygen atoms. Other three gallium atoms are

surrounded by 4 oxygen atoms from isopropoxide groups in tetrahedral arrangement

(Scheme 5).

- 43 -

Dissolution of the commercial

Ga(OiPr)3 in isopropanol was incomplete.

Obtained solutions were opaque indicating

the presence of insoluble impurities. The

impurities probably originate from the

hydrolysis of Ga(OiPr)3 since original

package is not closed tight enough. This

was the reason that Ga(OiPr)3 was

additionally purified by sublimation50.

Sublimation system for commercial

Ga(OiPr)3 was constructed from two oval

glass vessel. First vessel was used to hold

commercial Ga(OiPr)3 in furnace and the

second one was immersed in ice bath to

collect the purified product (Figure 8).

Sublimation was performed at 140 °C

under dynamic vacuum for 12 hours. Pure

Ga(OiPr)3 is transparent honey like

material (Figure 9), and the yield of the

purified Ga(OiPr)3 was 50 – 70 %.

Residue was a white-yelow solid, probably

oxide from hydrolysis of Ga(OiPr)3.

Figure 8: Sublimation of Ga(OiPr)3. Furnace (left) and Ice bath (below).

Figure 9: Vessel with purified Ga(OiPr)3.

Its composition was not defined. With the purified Ga(OiPr)3 clear solutions

with up to 30 wt.% of alkoxide in isopropanol could be prepared.

PREPARATION OF GaF3 PRECURSOR

Next step was preparation of precursor (dry Ga-F-gel), an amorphous solid,

which still contains some remaining alkoxide groups and solvent. Closed vacuum

system was used for all operations to avoid any contact with ambient air. The overall

reaction of gallium alkoxide with HF can be formulated as shown in reaction 12:

- 44 -

Pure Ga(OiPr)3 (1 g) was added in reaction vessel from HF resistant plastic

(fluoropolymers) mounted with teflon valve in metallic frame. Reaction vessel

(immersed in liquid nitrogen) and flask with dry isopropanol (with molecular sieves

Ǻ3) were connected to a vacuum line. Dry isopropanol was condensed into the

reaction vessel and approximately 20 wt.% solution of the alkoxide was prepared

(Figure 10).

Figure 10: Condensation of solvent (isopropanol) into the reaction vessel on vacuum system.

Reaction vessel was heated to ambient temperature and content was stirred

for approximately 15 minutes till all Ga(OiPr)3 dissolved.

- 45 -

Anhydrous HF was

added to the mixture within a

few minutes during

continuous stirring at

ambient temperature. aHF as

a gaseous reactant was

added with high accuracy

using vacuum system

(Figure 11). A stable

transparent viscous gel (wet

gel) was formed immediately

after the addition of the aHF

(Figure 12).

Figure 11: Addition of aHF on vacuum system. FEP reaction vessel (1) and aHF in thin vessel (2).

Figure 12: Wet gel (up) and dry gel (down) after pumping on vacuum system.

Reaction vessel was again connected

to vacuum system after the synthesis,

followed by removal of the solvent yielding a

dry red sugar like (Figure 13) Ga-F-gel

(xerogel). This material was precursor for

our final material GaF3.

Figure 13: Precursor (dry gel).

POST-FLUORINATION OF GaF3 PRECURSOR

Precursor needs additional fluorination in order to obtain the final material,

amorphous GaF3 (reaction 13). Influence of fluorinating agents CCl2F2 and gaseous

aHF on properties of final material was studied on precursor prepared from the

stoichiometric ratio of reactants in 20 wt.% solution of Ga(OiPr)3 in isopropanol.

- 46 -

Post-fluorination of precursor was carried out in a

flow reactor consisting of a Ni tube (5 mm i.d.) located

vertically in an electric furnace (Figure 14). Precursor

(approximately 1 g) was placed on a silver wool plug

which was fixed in the centre of the reactor.

Two fluorinating agents were predominately

tested at different temperatures:

CCl2F2 (CCl2F2 : N2 = 1 : 1 vol.)

Temperatures from 200°C to 300°C

Gaseous aHF (HF : N2 = 1 : 20 vol.)

Temperatures from 120°C to 200°C

Diluent N2 was added to fluorinating agents as a

carrier gas and to moderate the fluorinating activity. All

gases were introduced into reactor via teflon tubes.

Figure 14: Flow reactor in an electric furnace.

With CCl2F2 as a fluorinating agent the progress of post-fluorination was

followed up by monitoring the exhaust from the reactor. Exhaust gases were

analyzed on-line by GC. The flow of gaseous CCl2F2 and the diluent N2 entering

reactor were measured by mass-flow meters. The process of post-fluorination with

CCl2F2 is finished when we do not detect any residue organic compounds like

isopropanol on our chromatogram (Approximately 4 - 6 hours). Clear indication that

activation is finished is also the appearance of dismutation products of CCl2F2 which

appear when material has its active sites free.

- 47 -

Gaseous aHF was also used as fluorinating agent and since its fluorinating

activity is higher than that of fluorocarbon compounds post-fluorination was done at

lower temperatures. Progress of post-fluorination with aHF was not followed and

analyzed by GC due to the nature of the fluorination agent. Fluorinating time was set

to 4 hours, according to the result observed by the fluorination of HS-AlF3 with aHF51.

ADDITIONAL POST-FLUORINATION TESTS

As already mentioned before, main disadvantage of fluoride sol-gel procedure

is to assure satisfactory reproducibility involved in this process of precursor

fluorination due to the nature of the reactions. Two main problems were residues still

present in final material and coke formation during post-fluorination. This is why some

additional tests with the goal to improve the post-fluorination step were done.

1. Test: Conditioning of precursor before post-fluorination

The goal of this test was to remove majority of the residual organic

compounds from precursors before post-fluorination with CCl2F2. In this way the

chances of coke formation during post-fluorination would be minimized. Precursor

was heated at 250 °C and connected to vacuum for 8 hours before post-fluorination.

It was expected that in this way solvent (isopropanol) and other organic compounds

formed during the synthesis would be removed from the dry gel.

2. Test: Extended post-fluorination time

The goal of this test was to fluorinate the material at longer times. This could

affect the material by improving its purity by reducing the amount of the organic

residue. Precursor was post-fluorinated with aHF and CCl2F2. The post-fluorination

took 16 hours instead of only 4 to 6 hours as used in previous tests. The expected

drawback of this procedure was lower surface area and as a result of this also lower

catalytic activity.

- 48 -

3. Test: Using elemental fluorine as fluorinating agent

Elemental fluorine was also tested as fluorinating agent. Dambouret and

coworkers52 reported successful low-temperature fluorination of AlF3 precursor with

gaseous F2. Post-fluorinated material was catalytically active HS-AlF3 with surface

area SBET = 330 m2/g. According to these results, GaF3 precursor was post-

fluorinated at room temperature with a mixture of fluorine and argon (Ar : F2 = 1 : 1)

under static conditions in a nickel autoclave.

4. Test: Addition of oxygen to fluorinating mixture

The problem occurring during post-fluorination was formation of coke on material.

Deactivation of the catalyst due to coke formation is a well known industrial

problem53. Coke deposition on the catalyst surface significantly deteriorates its

catalytic activity. Coked catalysts are usually regenerated by combustion in a stream

of diluted oxygen or air. Upon combustion, coke is converted to CO2 and H2O but in

the absence of oxygen, CO may also be formed. Combustion of coke is exothermic

and this may accelerate phase changes or sintering in catalysts. To avoid these

effects the gas stream contains small amounts of oxygen (2–5 %) and the

concentration of oxygen is only gradually increased in order to complete coke

burnoff. The same amount of oxygen was added to our fluorinating mixture. The bulk

of coke is removed by oxidation at temperatures higher than 450 °C. These high

temperatures can significantly affect our materials so we tried this procedure at

300 °C.

- 49 -

33..33 PPRREEPPAARRAATTIIOONN OOFF GGaaFF33 VVIIAA OOXXIIDDAATTIIVVEE DDEECCOOMMPPOOSSIITTIIOONN::

INTRODUCTION

GaF3 was prepared by oxidative decomposition of hydrazinium(II)

aquapentafluorogallate(III), N2H6GaF5.H2O, with elemental fluorine. Full

decomposition should yield pure GaF3 (reaction 14) as the only solid product.

Reaction was carried out in liquid anhydrous hydrogen fluoride (aHF) medium by

similar procedure as HS-AlF3 described in Chapter 2.3.

Extreme caution is needed during preparation of GaF3 due to exothermal nature of

the reaction. Slow stirring was used in order to avoid overheating and possible

deposition of unreacted particles of the starting material on the walls of reaction

vessel out of the solvent.

- 50 -

PREPARATION PROCEDURE

N2H6GaF5.H2O (3–5 g) was weighted in FEP reaction vessel (30 mL) with a

magnetic stir bar. Anhydrous HF (aHF) was added as a solvent (at least 3 cm above

N2H6GaF5.H2O) on the vacuum line by condensation. Starting material was not

soluble in aHF (Figure 15 and Figure 16).

Figure 15: Addition of aHF to reaction vessels on the vacuum line.

Figure 16: Reaction vessel with

N2H6GaF5.H2O in aHF.

Next step was addition of fluorine in small portions, 130 kPa of fluorine in

reaction vessel. Fluorine was added to reaction vessel at -196 °C and then the

reaction mixture was carefully stirred at room temperature (Figure 17). Large quantity

of solvent (aHF) was needed to increase the warmth capacity of the reaction mixture.

This was necessary because the reaction is strongly exothermic. Liquid aHF has two

main functions: it serves as a dispersing medium for the solid reactant and dissipates

the heat released during the exothermic decomposition.

- 51 -

After each F2 addition the course of

reaction was monitored by cooling the

reaction vessel to liquid nitrogen

temperature again and by measuring the

pressure inside the vessel. When the

pressure was different from that of elemental

fluorine at -196 °C, the reaction was still not

completed. After the measurement of the

pressure, gasses were pumped out at

-196 °C and new portion of F2 was added.

Later F2 was added in larger quantities, but

no larger than 660 kPa. Reaction vessel was

left stirring for two days before new fluorine

addition. Approximately 30 % excess of F2

was added according to the stoichiometric

ratio. After the reaction was completed, the

remaining gasses and aHF were pumped

out at room temperature. Example of

preparation is shown on next page.

Figure 17: Reaction between N2H6GaF5

.H2O and fluorine.

Preparation has been done in five different reaction vessels. Preparation in

two batches was not successful since reaction vessel burned through due to

overheating.

The weight of the final product was higher than stoichiometrically calculated

value due to residual HF being present in the material. Product was dried for 20

hours at 250 °C on the vacuum line in a nickel autoclave and later stored in dry-box.

- 52 -

Example of preparation:

Stating material (N2H6GaF5·H2O) = 3.67 g

Addition of fluorine:

Theoretical = 5100 kPa (F2 at 25°C) Practical (30 % excess) = 7200 kPa (F2 at 25°C)

Addition number Amount added T = 25 °C (298 K)

Cumulative Pressure of residual gasses*

1. addition 130 kPa 130 kPa /

2. addition 130 kPa 260 kPa /

3. addition 130 kPa 390 kPa /

4. addition 660 kPa 1050 kPa 66 kPa

5. addition 660 kPa 1710 kPa 80 kPa

6. addition 660 kPa 2370 kPa 66 kPa

7. addition 660 kPa 3030 kPa 66 kPa

8. addition 660 kPa 3690 kPa 60 kPa

9. addition 660 kPa 4350 kPa 60 kPa

10. addition 660 kPa 5010 kPa 50 kPa

11. addition 660 kPa 5670 kPa 43 kPa

12. addition 730 kPa 6400 kPa 33 kPa

13. addition 800 kPa 7200 kPa 33 kPa

* Residue gases were measured in reaction vessel at -196 °C (77 K). Vapor pressure of fluorine at -196 °C (77 K) is 33 KPa.

- 53 -

33..44 IINNSSTTRRUUMMEENNTTAATTIIOONN AANNDD CCHHAARRAACCTTEERRIIZZAATTIIOONN MMEETTHHOODDSS::

PHOTOACOUSTIC INFRARED SPECTROSCOPY

Photoacoustic spectroscopy (PAS)54 is unique

as a sampling technique, because it does not require

that the sample be transmitting, has low sensitivity to

surface condition, and can probe over a range of

selectable sampling depths from several micrometers

to more than 100 μm. PAS has these capabilities

because it directly measures infrared (IR) absorption

by sensing absorption-induced heating of the sample

within an experimentally controllable sampling depth

below the sample’s surface. Heat generated within

this depth transfers to the surrounding gas at the

sample surface, producing a thermal-expansion-driven

pressurization in the gas, known as the PAS signal,

which is detected by a microphone.

Figure 18: Mtec model 300 photoacoustic detector.

PAS signal generation is initiated when the FT-IR beam, which oscillates in

intensity, is absorbed by the sample resulting in the absorption-induced heating in the

sample and oscillation of the sample temperature.

FT-IR Perkin-Elmer Spectrum GX with MTEC model 300 photoacoustic

detector (Figure 18) was used to analyze our samples. Approximately 50 mg of

sample was put into sample holder and inserted into MTEC detector. All process was

monitored and operated via Perkin-Elmer “Spectrum” software. IR spectra were used

as reference to follow our materials in different step of preparation, to compare newly

prepared materials with old ones and to detect adsorbed pyridine in final materials

with goal of determination of the type of acid sites on the surface of our materials.

- 54 -

ELEMENTAL ANALYSIS

The purpose of elemental analysis is to determine the amounts of individual

components in a certain compound. It is important that chemical analysis is

accomplished with highly accurate and precise analytical methods that assure a total

error below 1 %.

Since synthesized compounds are moisture sensitive and rapidly hydrolyse in

the presence of moist air all work is carried out in a dry-box. After weighing the

sample (synthesis product) into an airtight Teflon container, the container is cooled

with liquid nitrogen to moderate the reaction during the subsequent hydrolysis in

strong alkaline solution. Different weights of samples are used for the determination

of total fluorine, gallium and in some samples also total chlorine.

The individual samples are evaporated to dryness on a sand bath and then fused

with NaKCO3. The melt is transferred into a volumetric flask, dissolved by addition of

sulphuric acid in the case of fluoride, with HNO3 in the case of determination of

chloride and with HCl in the case of determination of gallium. The amount of fluoride

is determined by direct potentiometry using fluoride ion selective electrode, the

amount of chloride by precipitation titration with Hg(NO3)2 and the amount of gallium

with complexometric titration with EDTA.

All elemental analyses were performed in the analytical laboratory of the

Department of Inorganic Chemistry and Technology, Jožef Stefan Institute, Ljubljana.

SPECIFIC SURFACE AREA

The sample was degassed in vacuum at 250 °C for 10 h before the

measurements to remove the physically adsorbed water, HF and propanol from the

pores. The specific surface areas were determined by N2 adsorption at liquid N2

temperature, according to the BET method55. Method is based on the physical

adsorption of gas molecules on a solid surface and serves as the basis for an

important analysis technique for the measurement of the specific surface area of a

material.

- 55 -

Samples (100 mg) were measured with Micromeritics Flowsorb 2300 by

determining the quantity of a gas that adsorbs as a single layer of molecules

(monolayer) on a sample. The area of the sample is thus directly calculated from the

number of adsorbed molecules. Mixture of 30 volume % of nitrogen in helium was

used for the formation of a monolayer of adsorbed nitrogen at atmospheric pressure

and at temperature of liquid nitrogen.

.

GAS CHROMATOGRAPHY

For identifying the reaction products from catalytic experiments, a Perkin

Elmer 8700 gas chromatograph with flame ionization detector (FID) was used.

Products were separated with a 3 m x 2.1 mm nickel column, filled with

4,8 % Fomblin YR 1200 perfluoropolyether fluid (Montefluos) on 60/80 mesh

Carbopack B (Supleco)56 with He as carrier gas. Heating program was applied. The

initial column temperature was 50 °C and remained constant for 1 min followed by an

increase of the temperature (10 °C/min) to 150 °C, which was held at this

temperature for 10 min. The retention times for the organic products, CClF3, CCl2F2,

CCl3F, and CCl4 were determined as 2.2, 5.1, 9.5, and 15.0 min, respectively.

X-RAY POWDER DIFFRACTION

X–Ray powder diffraction (XRD) patterns of all the samples were obtained

using Seiffert ID-3000 diffractometer with CuKα (λ = 1.5418 Ǻ) radiation (Ni filter).

Debye-Scherrer camera 114 mm was used. Samples were placed into the quartz

capillaries with inner diameter of 0.3 mm and were filled in dry-box sealed with

halocarbon grease and later melted in flame of oxygen and hydrogen. Lines (d-

values) on the films were visually checked. Only information that was needed from

this method was if our samples were amorphous or not.

- 56 -

SCANNING ELECTRON MICROSCOPY

Morphology of precursors and final materials were compared on SEM pictures

made with field emission scanning electron microscope FE-SEM SUPRA 35 VP (Carl

Zeiss). The powder samples were mounted on double-sided adhesive carbon tape in

dry-box and then inserted to sample holder in SEM.

- 57 -

33..55 RREESSEEAARRCCHH OOFF CCAATTAALLYYTTIICC AANNDD AACCIIDDIICC PPRROOPPEERRTTIIEESS::

Final materials would be best characterized by determination of Lewis acid

strength, but unfortunately, no universal criteria exist to determine its strength like the

pKa value for Brønsted acids. The Lewis acidity of the solids is a property of the

surface and not of the bulk. Several indirect characterization methods have been

developed. We used two of theses methods to characterize Lewis acidity in our

materials, measurement of catalytic activity in test reaction of dismutation of

dichlorodifluoromethane, CCl2F216, and determination of acidity by pyridine

adsorption14.

DETERMINATION OF CATALYTIC ACTIVITY

Measurement of catalytic activity which depends specifically on the strength of

Lewis acid sites was used to determine catalytic activity of our final materials. This is

reaction of dismutation of CCl2F2 (reaction 15)16 that can be only catalyzed by

medium strong Lewis acids. Reaction is monitored by gas chromatography where we

follow the ratio between CCl2F2 and its dismutation products.

RESEARCH OF ACIDIC PROPERTIES

Method for indirect determination of the type of acid sites on the surface of our

materials and strength of Lewis acidity is the adsorption of a probe basic molecule,

for example pyridine14. Pyridine bonds selectively to different acid sites and acts as a

- 58 -

probe for the identification of Lewis or Brønsted acid sites. The lone-pair electron of

the nitrogen atom of pyridine can either bind coordinately to Lewis acid sites or

interact with acidic OH groups to form pyridinium cations and is adsorbed via

hydrogen bonds (Brønsted acid sites).

Scheme 6: Pyridine interaction with Brønsted acid sites (B). Pyridine coordinated to Lewis (L) acid

sites.

Photoacoustic IR spectra of adsorbed pyridine in final material confirm the

presence of strong Lewis acidic centers. Interaction of pyridine via nitrogen lone-pair

electrons, with aprotic (Lewis) and protonic (Brønsted) acid sites can be detected by

monitoring the ring vibration modes 8a, 8b, 19a and 19b, following the nomenclature

introduced by Wilson57. Wavenumbers of specific bands are characteristic for acidic

materials with adsorbed pyridine14, 58. For GaF3 Bands at approximately 1456 cm–1

(ν19b), 1579 cm–1 (ν8b) and 1641 cm–1 (ν8a) are typical vibrations of pyridine

coordinated to strong Lewis centres. Band at 1491 cm–1 (ν19a) is typical for both

Lewis and Brønsted acidic centres. Band at 1620 cm–1 (ν8a) is typical vibration of

pyridine coordinately bonded to weaker Lewis acid site. Band at approximately 1545

cm–1 (ν19b) is caused by adsorbed pyridinium ions and is typical for Brønsted acidic

centres. They may arise from very small amount of water that is taken up by the solid

during the transfer into the measuring cell. However, the catalytic activity is not

influenced significantly by such small amounts of water. Difference in the strength of

Lewis acidity is determined by the shift of bands because of the change in vibrations

in pyridine bonded to Lewis acid sites. It is always good to compare spectra from

other similar Lewis acids.

- 59 -

33..66 UUSSEEDD CCHHEEMMIICCAALLSS::

Solids: Ga(OiPr)3 Gallium(III) isopropoxide, mixture of oligomers, 99 % Alfa Aesar

GaF3 Gallium(III) fluoride, powder, anhydrous, 99,99+%, Alfa Aesar

Liquids: (CH3)2CHOH 2-Propanol, anhydrous, 99.5+% Merck

C5H5N Pyridine (Py) Merck

HF Hydrofluoric acid 40 % Merck

Gases: N2 Nitrogen (5.0) Messer

Ar Argon (5.0) Messer

He Helium (5.0) Messer

Air Synthetic air (CH free) Messer

H2 Hydrogen (5.0) Messer

CCl2F2 Dichlorodifluoromethane Solvay Fluor

F2 Fluorine Solvay Fluor

aHF Anhydrous hydrogenfluoride Merck

- 60 -

44 RREESSUULLTTSS AANNDD DDIISSCCUUSSSSIIOONN

44..11 IINNTTRROODDUUCCTTIIOONN::

Measurements of specific surface area, elemental analysis and comparisons

of photoacoustic spectra have been done from materials prepared via sol-gel

procedure and oxidative decomposition. Main goal of characterization was to

determinate the Lewis acidity and catalytic behavior in a selected test reaction.

The most interesting properties of this metal fluoride which were in the focus of

this work were for catalytic applications. Catalytic activity test was done in reaction of

dismutation of dichlorodifluoromethane, CCl2F216 and determination of acidity test

was done by pyridine adsorption14. FT-IR spectrometer with photoacoustic detector

was used to detect pyridine bonded to final materials in the test of determination of

Lewis acidity.

Two more techniques were used to additionally characterize our materials.

X-ray powder diffraction was done on standard Debye-Scherrer camera to prove

amorphous or crystalline structure of our materials. Field emission scanning electron

microscope (FE-SEM) was used to determine the morphology (particle size) of

precursors and final materials.

- 61 -

44..22 MMAATTEERRIIAALLSS PPRREEPPAARREEDD VVIIAA SSOOLL--GGEELL::

SUBLIMATION OF Ga(OiPr)3

Comparison of surface areas of precursor made from sublimated or non-

sublimated Ga(OiPr)3 was done. Stoichiometric amount of aHF and approximately

15 wt.% solution of Ga(OiPr)3 in isopropanol was used for this experiment. It showed

(Table 1) that precursor prepared from sublimated Ga(OiPr)3 had significantly higher

surface areas (130 – 160 m2/g) than precursors prepared from non-sublimated

Ga(OiPr)3, which did not have surface areas higher than 60 m2/g. Impurities in

commercial Ga(OiPr)3 are most probably products of hydrolysis which are not formed

under controlled conditions. These particles are larger than those formed during the

sol-gel procedure. Presence of impurities with larger particles and consequently

lower surface areas can explain lower surface areas of precursors prepared with

non-sublimated Ga(OiPr)3.

Table 1: Comparison of surface areas of precursors prepared from non-sublimed and sublimed Ga(OiPr)3 at stoichiometric molar ratio of reactants. Non-sublimated Sublimated Sample number: 1n 2n 3n 1s 2s 3s Surface area, SBET (m2/g) 25 60 40 130 160 140

CONCENTRATION OF SUBLIMATED Ga(OiPr)3 IN SOLVENT

Next parameter investigated was the concentration of Ga(OiPr)3 in

isopropanol. It is known from previous research on HS-AlF3 that concentration of

metal alkoxide should have no decisive effect on the properties of the final metal

fluoride12.

- 62 -

Stoichiometric amount of aHF was used in the molar ratio Ga(OiPr)3 : aHF =

1 : 3. Results show that compact gel are formed at concentrations of Ga(OiPr)3

higher than 12 wt.%. At lower concentrations only viscous liquids are formed. With

concentrations from 12–30 wt.%, precursors with a random distribution of surface

areas from 144 to 160 m2/g were obtained. In all cases reaction between Ga(OiPr)3

and aHF occurs instantaneously. There were no differences in gel formation time

between the four samples that formed gels. All gels formed immediately. Precursors

prepared from viscous liquid with concentration below 12 wt.% did not exceed

surface area of 80 m2/g. Sample with 10 wt.% solution of Ga(OiPr)3 in isopropanol

was left for three days, but no changes were observed, viscous liquid did not change

to gel. 20 wt.% solution of Ga(OiPr)3 in isopropanol was chosen as optimal

concentration. Results are shown in Table 2.

Table 2: Comparison of surface areas of precursors prepared from different concentrations of Ga(OiPr)3 in isopropanol at stoichiometric molar ratio of reactants. Sample number: 15 27,5 310 412,5 515 620 730 Concentration of Ga(OiPr)3 in isopropanol [wt.%]

5 7,5 10 12,5 15 20 30

Surface area, SBET [m2/g] 40 75 80 155 140 160 145 Form Viscous liquid Compact gel

EFFECT OF Ga(OiPr)3 : HF RATIO

For aluminum system it is known that optimal ratio between Al-isopropoxide

and HF is very close to the stoichiometric amount, Al(OiPr)3 : HF = 1 : 3. Higher

molar ratios lead to a significant decrease of surface area. Our results show that in

gallium systems molar ratios above stoichiometry did not have significant effect on

surface area. 20 wt.% solution of Ga(OiPr)3 in isopropanol was used at molar ratios

of Ga(OiPr)3 : HF = 1 : 2–10. Precursor had random surface areas from 135 to 195

m2/g (Table 3, Figure 19). It appears that gallium precursor is much more stable than

aluminum one and amorphous structure of the gel is not destroyed even at very high

concentrations of aHF. Stoichiometric amount of aHF was chosen as optimal

- 63 -

concentration for further experiments, since there was no need for excess of aHF in

this step.

Table 3: Effect of Ga(OiPr)3 : HF molar ratio on precursor surface properties. Sample number: 1 2 3 4 5 6 7 8 9 10 11 12

HF / Ga(OiPr)3 Ratio: 1,99 2,21 2,78 2,86 2,88 2,95 3,00 3,28 3,78 4,09 4,53 10,86

Surface area, SBET (m2/g) 136 177 161 144 157 160 140 171 176 135 195 151

Figure 19: Scattering of surface areas for materials obtained at different Ga(OiPr)3 : HF ratios.

ELEMENTAL ANALYSIS OF PRECURSOR

Elemental analysis (Table 4) shows Ga : F ratio and amount of organic residue

in our precursor. Precursor was additionally threaded at 250 °C in vacuum, since

there are still some volatile compounds present. The reason for high ratio of fluorine

in precursor is most probably due to the presence of non-reacted HF residue in our

material. Results from additionally treated precursor give a clearer picture of its

composition. Analyzed precursor has a ratio of Ga : F = 1 : 2 and results of elemental

analysis indicate that there is still 23,6 % of organic residue in the precursor.

- 64 -

Table 4: Surface area and elemental analysis of precursor and precursor additionally threaded at 250°C in vacuum. Sample: Precursor Precursor (250°C) Elemental analysis:

Ga : F = 1 : 3,0 Ga + F = 55,8 wt. %

Ga : F = 1 : 2,0 Ga + F = 76,4 wt. %

Surface area, SBET (m2/g)

/ 140

POST-FLUORINATION OF PRECURSOR WITH CCl2F2 AND aHF

Precursor fluorinated with CCl2F2 was treated at temperatures from 200 °C to

300°C (Table 5). Most optimal post-fluorinating temperature was 280 °C which is also

in agreement with literature51. Results on other metal fluorides have shown that

fluorination at higher temperatures result in excessive decrease of surface area while

at lower temperatures fluorination is not effective. The main problem observed during

fluorination was the formation of coke on our final materials9. Materials were

bright–red and powdery, except the one prepared at 300 °C which was dark black.

Material prepared at 280 °C is most suitable candidate for catalytic test reaction due

to its high surface area and relatively high purity compared to other materials.

Table 5: Elemental analysis and surface areas of materials post-fluorinated with CCl2F2. Sample number:

Temperature [°C]:

Elemental analysis:

Surface area, SBET (m2/g):

C1 200 Ga : F = 1 : 2,24 Ga + F = 84,5 wt. % /

C2 250 Ga : F = 1 : 2,35 Ga + F = 86,3 wt. % /

C3 280 Ga : F = 1 : 2,55 Ga + F = 91,9 wt. % 40

C4 300* Ga : F = 1 : 2,35 Ga + F = 87,6 wt. % 21

* For the material post-fluorinated at 300 °C an extensive coke formation was observed.

The post-fluorinating reaction with aHF was done at lower temperatures from

120 °C to 200 °C (Table 6) with the aim to preserve the highly disordered structure

and high surface area of the materials. Advantage of aHF use is that, unlike

- 65 -

fluorocarbons, HF can not form coke. Experiments of preparation of other high

surface metal fluorides36 showed that fluorination starts at 120 °C and that

concentrations higher than HF : N2 = 1 : 20 lead to high decrease of surface area in

final material.

Table 6: Elemental analysis and surface areas of materials post-fluorinated with aHF. Sample number:

Temperature [°C]:

Elemental analysis:

Surface area, SBET (m2/g):

H1 120 Ga : F = 1 : 2,86 Ga + F = 89,0 wt.% 24

H2 160 Ga : F = 1 : 2,88 Ga + F = 89,7 wt.% 17

H3 200 Ga : F = 1 : 2,90 Ga + F = 90,3 wt.% 9

Materials prepared at 120 °C and 160 °C were bright-red powdery materials,

except the one prepared at 200°C which was dark brown. Although material prepared

at 200 °C has the highest Ga : F molar ratio and is most pure, its surface area is

significantly lower than materials prepared at 120 °C and 160 °C. Material prepared

at 120 °C is most suitable candidate for catalytic tests due to its high surface area.

Commercial GaF3 is a white crystalline powder with Ga : F = 1 : 3 ratio and

surface area of only 3 m2/g, which is significantly lower than our materials. Most of

our final materials were bright-red powdery materials and not white. This might be

due to high content of residues in final materials.

GaF3 post-fluorinated with CCl2F2 has the highest surface area (Table 5 –

sample C3). Material contains 8,1 wt.% of organic residues which is the lowest value

obtained for the fluoride sol-gel method. A small amount of chlorine has also been

detected in this material. Elemental analysis also shows that the final material post-

fluorinated with aHF has the highest Ga : F molar ratio but has much lower surface

area than material post-fluorinated with CCl2F2.

COMPARISON OF PHOTOACOUSTIC IR SPECRA

IR spectroscopy was used to characterize our materials in different

preparation step and to compare newly prepared materials with old ones. Figure 20

- 66 -

shows photoacoustic IR spectra of starting material, Ga(OiPr)3 and other fluorinated

materials in different preparation steps.

Broad bands from 3300

to 3700 cm-1, which are seen in

precursor and other fluorinated

materials, are from solvating

isopropanol (OH stretching). It

could be also attributed to

adsorption of water, which

bonds to our material during

manipulation due to high Lewis

acidity. CH3 and CH absorption

bands at 2800 to 3000 cm-1 are

characteristic for the alkoxide.

Figure 20: PAS spectra of Ga(OiPr)3 and other fluorinated materials.

Spectra contain many peaks in the area below 1500 cm-1 but these are harder

to assign to specific vibration. C–C frame absorptions of the organic constituents are

scattered in this area. Bands from 950 to 1300 cm-1 are usually from C-O vibrations.

Valence vibrations from 350 to 800 cm-1 are indicative of the metal-oxygen and

metal-fluorine bonds59.

Spectra (Figure 20) show that our final materials still contain some residues

after activation. Spectrum of GaF3 activated with CCl2F2 indicates the presence of the

starting material, Ga(OiPr)3, or precursor, GaFx(OiPr)3-x. GaF3 activated with aHF

shows only traces of the residual propoxide, so organic residues can not be

confirmed as the main residue in this final material. Residues in final materials are

also confirmed with elemental analyses which indicate approximately 10 wt.% of

residues in both of our final materials. It appears from IR spectra that residues in

GaF3 activated with HF are not organic. With available methods we were not able to

unequivocally indentify the residues present.

- 67 -

RESULTS OF ADDITIONAL POST-FLUORINATION TESTS

Results from 1. Test: Conditioning of precursor before post-fluorination

Spectra (Figure 21)

show comparison between

conditioned precursor on

250 °C (vaccum line) before

post-fluorination with CCl2F2

and non-conditioned.

Conditioning resulted in final

material without CH3 and CH

absorption bands at 2800 to

3000 cm-1 that are

characteristic for the

alkoxide. This means that

we do not have starting

material present in GaF3.

Figure 21: PAS spectra of GaF3 post-fluorinated with CCl2F2 (down) and the same material which was conditioned on 250°C (vacuum line) before post-fluorination (up).

On the contrary elemental analysis (Table 7) showed that purity of final

material was not improved and that residues are still present. We did not characterize

residues in conditioned material but spectra indicate that these residues should not

be organic. Table 7: Comparison of surface areas and elemental analysis of GaF3 post-fluorinated with CCl2F2 and the same material which was conditioned on 250°C (vacuum line) before post-fluorination. Sample: GaF3 (CCl2F2) GaF3 (CCl2F2) Cond.250°C Elemental analysis:

Ga : F : Cl = 1 : 2,55 : 0,12 Ga + F + Cl = 91,9 wt.%

Ga : F = 1 : 2,40 : 0,17 Ga + F+ Cl = 90,3 wt.%

Surface area, SBET (m2/g) 40 29

- 68 -

The goal of this test to purify the final material was not achieved. Although in

the newly prepared material alkoxide could not be detected the material had lower

Ga : F molar ratio, lower surface area and approximately the same residues.

Results from 2. Test: Extended post-fluorination time

Extended post-

fluorination time (16 hours) did

not significantly improve the

properties of final materials.

Some changes that were noted

are: bands characteristic for the

alkoxide were not visible

(Figure 22), Ga : F molar ratio

was improved in case of aHF

post-fluorination, purity of final

materials remained almost the

same with around 10 % of

residues.

Figure 22: PAS spectra of GaF3 post-fluorinated (16 hours) with CCl2F2 (down) and aHF (up).

As expected surface areas of both materials were affected (Table 8) and are

lower than from materials prepared at 4 – 6 hours.

Table 8: Comparison of surface areas and elemental analysis of GaF3 prepared with extended (16 hours) post-fluorination time (CCl2F2 and aHF). Sample: GaF3 (CCl2F2) – 16 hours GaF3 (aHF) – 16 hours Elemental analysis:

Ga : F : Cl = 1 : 2,45 : 0,16 Ga + F + Cl = 91,2 wt.%

Ga : F = 1 : 3,00 Ga + F = 91,8 wt.%

Surface area, SBET (m2/g) 22 19

- 69 -

Results from 3. Test: Using elemental fluorine as fluorinating agent

Fluorine is a very strong oxidizing/fluorinating agent so extreme caution is

needed during direct fluorination of this kind of metal fluoride precursors. Although

there were some reports52 of successful usage of elemental fluorine in such

fluorinationt, our experiment showed that fluorine is not adequate for post-treatment

of precursor due to its too high reactivity. Experimental fluorination with F2 resulted in

immediate ignition of the precursor.

Results from 4. Test: Addition of oxygen to fluorinating mixture

Adding oxygen to fluorinating

mixture did not have desired effect.

Instead of removal of coke we got a

reverse effect. Coke deposited even

more to our material than before (Figure

23). The main reason for this might be

because we were not able to expose our

material to such high temperatures as in

classical catalyst regeneration

procedures (for oxides) which start at

around 450 °C and up. Heating of our

materials to this high temperature would

significantly affect its properties (surface

area) and accelerate phase changes or

sintering.

Figure 23: Coke deposited on final material.

- 70 -

44..33 MMAATTEERRIIAALLSS PPRREEPPAARREEDD VVIIAA OOXXIIDDAATTIIVVEE DDEECCOOMMPPOOSSIITTIIOONN::

ELEMENTAL ANALYSIS AND SURFACE AREA

Final material, GaF3 was additionally threaded at 250 °C in vacuum before

analysis since there are still some volatile compounds present, mainly HF.

Theoretical [%] Found [%] Ga 55,0 53,4 Ft

- 45,0 43,2 NH4

+ 0 0,6

Molar ratio: Ga : F = 1 : 3,00 Ga + F = 100,0 wt.%

Ga : F = 1 : 2,97 Ga + F = 96,6 wt.%

GaF3 prepared via oxidative

decomposition method is relatively pure

and the Ga : F ratio indicates the

formation of GaF3. Elemental analysis

showed that final material contains

0,6 wt.% of NH4+. X-ray powder

diffraction showed that compound which

contain NH4+ is (NH4)3GaF6. The final

material is a white powdery material

(Figure 24) and has a relatively low

surface area of 19 m2/g.

Figure 24: GaF3 after preparation.

- 71 -

PHOTOACOUSTIC IR SPECRA

The presence of NH4+ was proved by comparison with previous results done

on Al compounds. NH4+ vibrations can clearly be seen on comparison spectra. It is

known from literature46 that NH4+ is present in HS-AlF3 prepared via oxidative

decomposition. NH4AlF4 which is a possible side product in the preparation of HS-

AlF3 is shown as a reference with evident vibration band of NH4+ at approximately

1450 cm-1 (Figure 25).

Previous research

on HS-AlF3 preparation

showed that NH4+

presence in high surface

materials has significant

effect on catalytic activity

in selected probe

reactions. Test of catalytic

activity of GaF3 and

possible influence of NH4+

on it is described in

chapter 6. Figure 25: PAS spectra showing the presence of NH4

+ in GaF3, HS-AlF3 prepared by oxidative decomposition and NH4AlF4 (reference).

- 72 -

44..44 XX--RRAAYY PPOOWWDDEERR DDIIFFFFRRAACCTTIIOONN::

X-Ray powder diffraction was done on materials prepared via fluoride sol-gel

(precursor and final materials) and final material prepared via oxidative

decomposition.

Results of materials prepared via fluoride sol-gel:

• Precursor (amorphous)

• GaF3 – post-fluorinated with CCl2F2 (amorphous)

• GaF3 – post-fluorinated with aHF (amorphous)

Results of material prepared via oxidative decomposition:

• GaF3 (weakly crystalline)

Results of X-ray powder diffraction indicate small quantities of (NH4)3GaF6

present in final material prepared via oxidative decomposition.

- 73 -

44..55 DDEETTEERRMMIINNAATTIIOONN OOFF CCAATTAALLYYTTIICC AACCTTIIVVIITTYY::

Catalytic activity test of CCl2F2 dismutation was done on final materials

prepared via fluoride sol-gel and post-fluorinated with either CCl2F2 or HF, final

material prepared via oxidative decomposition, commercial crystalline GaF3 and

blank. Table 9 shows all results of catalytic activity of tested materials. Besides

temperatures shown in Table 9 catalytic tests have also been done at 200 °C and

250 °C but none of the samples converted CCl2F2 to its dismutation products at these

temperatures.

Table 9: Comparison of catalytic activity of materials in probe reaction of dismutation of CCl2F2

Dismutation of CCl2F2 [%] Temperature [°C] GaF3

(CCl2F2) GaF3 (aHF)

GaF3 (ox. decom.)

GaF3 (commercial)

Blank

300 0,8 0 0 0 0 350 4 0,4 0 0 0

Catalytic activity of GaF3 in this test reaction was very low. Maximal

conversion of CCl2F2 to its dismutation products was only 0,8 % at 300 °C and 4 % at

350 °C using material prepared via fluoride sol-gel and activated with CCl2F2. For

example conversion of CCl2F2 using HS-AlF3 was 96 % at 300 °C. Reason for lower

catalytic activity of GaF3 could be lower Lewis acidity of Ga3+ compared to Al3+ 8.

Additional factors that may have an influence on low catalytic activity are lower

surface area and high amounts of relatively stable organic residues in our final

materials prepared via the fluoride sol-gel process. Organic residues and their

decomposed products may physically or chemically block the active acid sites.

Catalytic inactivity of GaF3 prepared via oxidative decomposition is most

probably connected to the presence of NH4+ in material (0,6 wt.%). Previous research

on HS-AlF3 obtained by oxidative decomposition showed that amounts higher than

0,5 wt.% resulted in materials with no catalytic activity in selected probe reactions46.

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44..66 RREESSEEAARRCCHH OOFF AACCIIDDIICC PPRROOPPEERRTTIIEESS::

Photoacoustic IR spectra

(Figure 26, spectra 1 and 3) show

presence of Lewis and Brønsted

acid sites on materials prepared via

fluoride sol-gel process. Lewis acid

sites with relatively high strength

are prevalent. Brønsted acid sites

are probably the result of

dissociative adsorption of water.

Both materials post-fluorinated with

CCl2F2 and aHF have similar acidic

properties. Peaks indicating

pyridine bonding to Lewis and

Brønsted acid sites are located at

same positions. Intensity of peaks is

lower in material post-fluorinated

with aHF due to higher amounts of

adsorbed water in this material

(lower temperature during

preparation) and lower surface

area. Detailed descriptions of peak

positions are in chapter 3.

Figure 26: PAS spectra of GaF3 post-fluorinated with CCl2F2. (1-after, 2-before) pyridine adsorption. GaF3 post-fluorinated with aHF. (3-after, 4-before) pyridine adsorption.

Comparison of spectra (Figure 27) of pyridine adsorption test show that GaF3

prepared via oxidative decomposition has the same peak positions than materials

prepared via fluoride sol-gel method. Peaks are not that clear due to interferences

from other peaks in that area specific for material prepared via oxidative

decomposition. Peaks from starting material (N2H6GaF5.H2O), NH4

+ vibrations and

adsorbed water are present in that area. Refined spectra obtained by using the

- 75 -

spectra difference function clearly indicate the presence of peaks characteristic of

Lewis acid sites with relatively high strength.

Figure 27: PAS spectra of GaF3 prepared via oxidative decomposition before (down) and after (up) pyridine adsorption. Inset (right) shows the difference spectrum.

Comparison of spectra (Figure 28) of different HS metal fluorides after pyridine

adsorption shows that GaF3 has very similar peak positions than HS-AlF3. There are

small differences but in general we can confirm that GaF3 has lower Lewis acidity as

HS-AlF3. Direct quantification is not possible due to method limitations.

Peak intensity depends on

sample type, its color and its

surface area while peak positions

depend on the type of acid centers

and acidity strength. For example

HS-MgF3 has much weaker Lewis

acidity than HS-AlF3 and

comparison of these two spectra

clearly shows that all peaks

responsible for bonding to Lewis

sites are shifted to lower

wavelengths.

Figure 28: Comparison of PAS spectra between different HS metal fluorides after pyridine adsorption.

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44..77 SSCCAANNNNIINNGG EELLEECCTTRROONN MMIICCRROOSSCCOOPPYY -- SSEEMM::

Morphology comparison of

precursor and GaF3 post-fluorinated with

CCl2F2 has been made with electron

microscopy. Images were made with field

emission scanning electron microscope.

Precursor is composed of 30 nm

particles (Figure 29) which is also in

agreement with higher surface area of

this material. Additional post-fluorination

of precursor leads to significant changes

of morphology. Sintering and growth of

larger particles is observed (Figure 30).

These processes result in decrease of

surface area.

SEM image of GaF3 prepared via

oxidative decomposition has also been

made (Figure 31) for comparison with

material prepared via fluoride sol-gel.

Clear difference can be seen between

these two materials. Material prepared

with fluoride sol-gel has amorphous

porous structure while material prepared

with oxidative decomposition has clear

crystalline structure composed of small

30 – 100 nm crystallites. Crystalline

nature of this material has also been

proved with X-ray powder diffraction.

Figure 29: SEM of precursor (140 m2/g).

Figure 30: SEM of final material post-fluorinated with CCl2F2 (40 m2/g).

Figure 31: SEM of final material prepared via oxidative decomposition (19 m2/g).

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55 CCOONNCCLLUUSSIIOONNSS

Novel preparation methods, fluoride sol-gel and oxidative decomposition of

fluorometalates, have been successfully applied for preparation of GaF3. General

data and comparison between GaF3 and AlF3 materials are summarized in Table 10

and Table 11. Additional graphical comparison of purity (wt %) and surface areas

(m2/g) between these materials can be seen in Figure 32 and Figure 33.

Main difference between GaF3 and HS-AlF3 prepared via fluoride sol-gel

procedure is in the stability of isopropoxide in the first step of the preparation.

Ga(OiPr)3 is significantly more stable since high excess of HF does not have impact

on the properties of precursor. This is also reflected in second step of the preparation

where the purest prepared final material post-fluorinated with CCl2F2 still contains

8,1 wt.% of residues. This is much higher than in HS-AlF3 which has only

0,5 wt.% of residues in final material (Table 11). It is evident from the Table 10 that

our final material has under stoichiometric Ga : F ratio, impurities (residues) and

much lower surface area than HS-AlF3. All these factors and considering the fact that

GaF3 theoretically exhibits lower Lewis acidity than AlF3 might be the main reason for

lower catalytic activity of GaF3. Table 10: Comparison between GaF3 materials prepared via fluoride sol-gel and oxidative decomposition (o.d.).

GaF3 (Precursor)

GaF3 (aHF)

GaF3 (CCl2F2)

GaF3 (o.d.)

Surface area, SBET (m2/g) 140 24 40 19

Elemental analysis (ratio)

Ga : F = 1 : 2,00

Ga : F = 1 : 2,86

Ga : F = 1 : 2,55

Ga : F = 1 : 2,97

Residue (wt. %) 23,6 11,0 8,1 3,4 Catalytic activity (CCl2F2 dismutation) / YESI YESII NO

ICatalytic activity is very low (0,4 %). IICatalytic activity is low (4 %).

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Table 11: Comparison between HS-AlF3 materials prepared via fluoride sol-gel and oxidative decomposition (o.d.).

HS-AlF3 (sol-gel)12, 51 HS-AlF3 (o.d.)60 Precursor Final material Final material Surface area, SBET (m2/g) 400 200 210

Elemental analysis (ratio): Al : F = 1 : 2 Al : F = 1 : 3 Al : F = 1 : 3

Residues (wt. %) 30 0,5 5 Catalytic activity (CCl2FCF2Cl isomerization) / YESI YESII ICatalytic activity is very high (~ 98 %). IICatalytic activity depends on the amount of NH4

+ in material. Amounts higher than 0,5 % (wt.%) resulted in materials with no catalytic activity.

. Figure 32: Comparison of purities between GaF3 and AlF3 materials.

Figure 33: Comparison of surface areas between GaF3 and AlF3 materials.

It was expected that GaF3 prepared via fluoride sol-gel would have higher

surface area and catalytic activity due to results from MgF2/GaF3 doped system

prepared by Kemnitz and coworkers. Practical experiments showed that our GaF3

has much lower surface area and lower catalytic activity than other potential metal

fluoride catalysts (HS-AlF3, MgF2/GaF3) prepared via the fluoride sol-gel route.

- 79 -

It is possible that post-fluorination of precursor was not finished entirely in

spite of the fact that volatile organic components were not detected with gas

chromatography any more. Even at elongated post-fluorination times final materials

still contained almost the same amounts of residue. Another problematic effect is

coke formation during post-fluorination of precursor with CCl2F2. For example no

coke formation was detected on materials which are just heated to the same

temperatures in the flow of nitrogen. The formation of coke was also noticed during

HS-AlF3 preparation but at much lower extent and at harder activation conditions.

Considering reproducibility, preparation of GaF3 was most problematic in the

post-fluorination step due to the nature of these reactions. Few additional tests were

done to improve this step but no significant improvements could be obtained.

Elemental fluorine was tested as fluorination agent, but the test was not successful

resulting in the ignition of the sample. Additional tests were abandoned due to

excessive risk. Proces of post-fluorination was also modified with addition of oxygen

to the existing fluorinating agent (CCl2F2). Mixture but did not result in any

improvement in the temperature range used and coke removal could not be

achieved.

GaF3 prepared via oxidative decomposition of hydrazinium(II)

aquapentafluorogallate(III), N2H6GaF5.H2O, with elemental fluorine was also

successful. Material was very pure (96.6 wt.%) compared to other GaF3 and AlF3

materials but had significantly lower surface area than HS-AlF3 prepared with the

same method. Low surface area, crystalline structure and the fact that it contained

some NH4+ (0,6 wt.%) might be the main reasons that it was not catalytically active.

Despite the fact that material was successfully prepared extreme caution is needed

during preparation due to the very exothermic nature of the reaction.

Materials from both preparation methods have their advantages and

disadvantages. Table 12 shows some of major differences between these two

methods.

- 80 -

Table 12: Comparison between fluoride sol-gel and oxidative decomposition preparation method.

Fluoride sol-gel Oxidative decomposition Two step preparation procedure, more parameters affect the preparation than in the case with oxidative decomposition. Caution is needed during preparation due to highly corrosive and toxic aHF.

One step preparation with highly reactive elemental fluorine. Extreme caution is needed, but in general it is simple procedure.

Most problematic is second step of preparation due to problems with reproducibility.

Most problematic is beginning of preparation due to highly exothermal reaction.

All advantages of sol-gel (different forms) could be used (thin films, ceramics, powders, etc...).

Produces only powdery materials.

Could be used for industrial or laboratory use. It has similar application options as classical sol-gel.

Industrial use could be problematic due to very exothermal nature of reaction. It is more suitable for smaller quantities on laboratory scale (~10g ).

Produces amorphous materials. Produces weakly crystalline or crystalline materials.

Further development on this area will enable even better understanding of the

preparation mechanisms of metal fluorides with fluoride sol-gel and oxidative

decomposition procedures. The present thesis developed an alternative route

towards GaF3 preparation with specific properties, which is the basic for possible

technical applications of this new kind of material. Knowledge gathered during GaF3

preparation and other metal fluorides could be used to prepare new materials with

improved properties.

- 81 -

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PARTS OF THIS THESIS HAVE BEEN PRESENTED ON SCIENTIFIC MEETING:

Tine Oblak, Gašper Tavčar in Tomaž Skapin, Preparation and characterization of gallium(III) fluoride with specific properties, Slovenski kemijski dnevi 2008, Maribor, 25. and 26. September 2008