a study of carbonaceous char

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ELSEVIER PII: SOO16-2361(97)00157-9Fuel Vol. 77, No. 6, 563-569, 1998p.

0 1998 Elsevier Science Ltd. All rights reservedPrinted in Great Britain

0016-2361/98 $19.00+0.043

A study of carbonaceous charoxidation in air by semi-quantitativeFTIR spectroscopyAlain Kochan *, Andrtej Krzton b, Giskle Finqueneisel a,Olivier Heintza, Jean-Victor We bera and Thierry ZimnyaallJT Dhpartement Chimie, rue V. Demange, 57500, Saint-Avold, Franceb/nstitute of Coal Chemistry, Polish Academy of Science, Sowinskiego 5, 44-700, Gliwice, Poland(Received 6 February 1997; revised 12 May 19971

The aim of this study was to describe the methodology of semi-quantitative characterization of carbonaceousmaterial oxidized un der mild thermal conditions. Infrared spectroscopic analysis was applied to the determinationof chemical changes induced by oxidation. A curve fitting model has thus been developed to evidence theevolution of specific chem ical groups during treatment. The possibility to follow mechanisms and kinetics ofcarbonaceous char oxidation through data obtained by this model has been demonstrated. 0 1998 Elsevier ScienceLtd. All rights reserved(Keywords: chars; oxidation; FTIR; spectroscopy; curve fitting; modelling)

Carbonac eous materials are of great interest because oftheir use as filtering media or catalyst support 1-4. Related tothis, particular attention is accord ed to low cost substancesthat can be converted into active carbon with specificproperties. The applications using these materials requirespecific features (adsorptive potentialities, specific area...)that underline the need to develop adequate preparationmetho ds and to use selected raw materials 5-11.

For the study of active chars preparation method s, adevice designed for the treatment of samples in a gasflow has been developed. The parameters that can becontrolled on this apparatus are the temperatu re, theduration of the treatment and the nature of the gas. Thepresent paper foc uses on the development of a curve fittingmodel that will permit the identification of individualchemical functional groups and their evolution during anoxidation process.

For this purpose, Fourier transformed infrared spec tro-scopy (FTIR) in the diffuse reflectance mode w as used. Onthe basis of the spectrosco pic data and those obtained inthe literature 12- .21-26, it has been po ssible to deconvolutethe area specific to carbonyl vibrations into several bandswith the help of mathem atical tools for spectral datatreatment. Thoug h this metho d is in its preliminary stage,it has proven its ability to visualize the transformationsundergone by chars through variations of individualchemical groups. T hus, the possibility to access anunderstanding of the mechanism s and kinetics o f theoxidation of chars by means of infrared analysis has beenshown.

* Corresponding author.

EXPERIMENTALThe samples used were coal chars from the carbonization ofa law rank co al in a pilot scale rotary kiln (length: 6 m,diameter: 0.6 m). Carbonization was achieved in air in twostages. In the first, coal was heated from ambienttemperature to 700C (coal tempera ture) in 20 min. In thesecond, coal w as kept at 700C for 4 min. After this, thechars were cooled with water. The main features of the coalsand relevant chars are given in Table 1.

Chars were also characterized by thermogravimetricanalysis (TGA) in the range 20 -900C.

Before thermal treatment, chars were crushed and sievedto a particle size ranging from 200 to 630 pm. This specificparticle size was needed for other experiments. For thethermo-oxidative treatment, the chars were inserted in aglass column enclosed in a vertical tubular o ven. The glasscolumn (diameter: 35 mm, length: 5 00 mm) was designed topermit the sample to be in a gas flow to simulate a movingbed device (see Figure 1 for more details). Becau se of thistreatment in a gas flow, too small a particle size had to beavoided to prevent sample loss.

The thermal treatment can be decom posed into threesteps. During the first step, the temperatu re was increased infour ramps until the treatment tempera ture was attained.Through out this step, the sample remains in a nitrogen flow.Four ramps with decreasing heating rates were used in orderto avoid overstepping the treatment temperatu re due toinertia in the heating control.

When the oxidation temperatu re was reache d, nitrogenwas replaced by air. During this second step, thetempera ture was kept constant for a given durationdepending on experimental procedu re. After the secondstep, heating is stopped and nitrogen replaced the air. The

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Carbonaceous char oxidation in air: A. Koch et al.

Table 1 Features of the coal and relevant chars used as samplefor the oxidation treatmentcoal Char

Volatile matter (wt%) 42 11.9Ash content (wt%) 2.1 2.9Vitrinite reflectance (%) 0.56 -Specific area (BET N2) (m g-) < 20 14 3

ml-l

temperaturecontroller H

Ejj fJ=-w~e#flowmeterOVEN \

d Y irIAl NitrogenFigure 1 Schem atic drawing of the laboratory device used forsample oxidation.

sample remained during this third step in the column untilambient temperature was reached. Du ring these steps, gasflows were kept constant a t a value of 150 ml min-. Thisflow rate was determined by taking into account that highervalues could provoke partial combustion of the sampleduring treatment with air as was observed with othercarbonaceous material. Temperature and duration of thesecond step (oxidation step) were used to determine thetreatment parameters.

Initially, eight experiments were realized by graduallyvarying the temperature of oxidative treatment from 260 to330C in 10C steps, each experiment duration being 140 min.A further, six oxidative treatments were carried at 280C withdurations varying from 150 to 250 min in steps of 20 min.Infrared spectrometric measurem ents were achieved on aspectrometer equipped with a high sensitive mercury-cadmium -telluride (MCT ) detector. Sam ples were ana-lyzed in diffuse reflectance (DRIFT) mode by using theGraseby Specac Selector accessory in an off-axis opticalgeometry. Spectra were recorded by co-adding 750 scans inthe range 4000-700 cm- at a resolution of 2 cm-. Theanalysis chambe r was purged continuously with dry air andinfrared scan ning began only 10 min after sample insertion.Potassium bromide (KBr) ground to an average particle sizeof 10 pm was used as sample matrix a nd reference material.The sample, ground separately to a particle size rangingfrom 40 to 100 pm was mixed with potassium bromide inthe proportions 5/95 (% by weight) respectively. Tolinearize the relationship between con centration andspectral response the Kubelka-M unk function flRm) wasapplied to the spectral data

f(R m = (1 -Rc$ _ 2.303~2R , swhere: R, represents the ratio of the single beam reflectancespectrum of the sample to the single beam reflectance spec-trum of a non absorbing standard (here KBr), E is the molarabsorptivity, c is the concentration of the sam ple and s is ascattering coefficient. The Kubelka-M unk function isknown to be applicable only if the scattering coefficientremains constant. This implies among other parametersthat particle size and packing density of the samples rn:ssJ& kept constant and that an off axis geometry is needed

. Hence, attention was paid to standardization of samplepreparations and spectra recording. To develop a curve fit-ting model that could be used to follow the evolution ofoxidation, attention was focused on infrared absorptionbands specific to carbon-oxygen linkages.

I ,I I I I I I3500 3000 2500 2000 1500 1000

Wavenumber (cm-l)Figure 2 Example of DRIFT spectrum recorded on an oxidated char. A-raw spectrum, B-smoothed spectrum.

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Nonethe less, the part of the spectrum corresponding toetheric C-O vibrations could not be used as it was poorlyresolved and bands due to mineral matter componen ts thatcould interfere were present. Th erefore, the wavelengthsdomain 1850-1620 cm-, specific to carbonyl C=Ostretching v ibrations, was preferred.

As carbonaceous samples such as chars give rise to weakinfrared signal in absorbance, it is necessary to avoid thepresence of bands due to atmospheric water. Such bands arevisible even if water is present at very low concentration andcan therefore induce errors in the interpretation of spectraespecially in the domain 1850-1 620 cm-. Thus, thespectra w ere recorde d as single beams a t different periodscorresponding to different drying steps of the samplechamb er. By this means, it was possible to select thespectrum wher e the rotational-vibrational bands of watervapor were so low and sharp that they could be removed asnoise by smoothing. An examp le of a spectrum before andafter smoothing is given Figure 2.

Curve fitting wa s realized for the region 1890-1 505cm-. For this operation, the concerned wavelengths zonewas truncated from the spectrum and submitted to a linearbaseline correction that could be repeated on each sample byusing identical points as baseline models were developed.To determine the number of bands presen t in the region andevaluate their width at half height, mathem atical treatmentssuch as first and second derivatives * were us ed as well asreports of work w ith the same objective and data concerningidentified vibrations in the infrared 12,15,17,22 -26. Theseparam eters could be averaged as fifteen samplesrepresenting fifteen different steps in oxidation were used.Once the number, positions and width at half height of thebands determined, curve fittings were achieved with thehelp of the Bio-Rad Win-IR release 3.01~ programimplemented with such a calculation module The algorithmused adjusts iteratively every variable for each peak in anattempt to minimize the statistical variable K* (chi squared)

which is a weighte d difference between fitted and measure ddata.

RESULTS AND DISCUSSIONThermogravimetric analysis of the non treated chars

Three weight losses of different magnitude were found(Figure 3). The first, at a temperature of about lOO C,corresponds to the water loss due to moisture. The secondone, very weak (approximately 2% of the total weight),appea red in the temperatu re range situated between 220 and420C. In this latter temperature range, oxidations werecarried out. The last weight loss had its maximum (12% oftotal weight) at a temperature of 700C.Infrared spectrometric measurements

The 1890-1505 cm- region of the spectra obtainedafter truncation and baseline correction are represented inFigure 4A for samples treated at 280C with differentdurations and in Figure 4B for samples treated for 140 minat different temperatu res. Care must be taken becausescaling of Figure 4A and B are different and traces w eredrawn to prevent spectrum parts being hidden. Becaus e o fthis, the ascending order of temperature s is not respecte d onFigure 4B and semi-quantitative comparison of data cannotbe done here. These figures sh ow that a normalization ofthese spectra by a convenient quantity is necessary for thesemi-quantitative evolution of bands and thereby theinfluence of oxidation param eters.

As shown in Figure 5 (example of a sample treated at260C fo r 140 min), 12 bands have been identified with thehelp of the minima of second derivative curves in the zone1890-1 505 cm- applied to the fifteen spectra recorded. Inthis examp le, the position of the band at 1748 cm- wasdetermined on other spectra and, for non treated ch ars, thebands at 1846 and 1809 cm- were not present. To obtain

Temperature (C) - ( DTG(%/min) - - - - . . . . . TG (%)- 1000

.- 800 ?? . . .

. I2.5

0.0

-2.5

Figure 3 Data obtained by thermogravimetric analysis (TGA ) of the non treated chars.

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l&O do0 I{50 do0 Id50 Id00 Id50 1850 Ii00 1750 I?00 l&50 do0 Ii50Wavenumber (cm-l) Wavenumber (cm-l)

Figure 4 DRIFT spectra of the region 1890-1505 cm- after truncation and baseline co rrection. A -sample oxidated at 280C withincreasing durations (express ed in minutes), B-sam ple ox idated during 1 40 min with increasing tem peratures (express ed in C).

Table 2 Imposed intervals for positions and widths at half heightof the bandset used for the curve fittin g model in the region 1890-1505 cm-Wavenumber intervals (cm-) M aximum width at half height (cm-)1842-1848 261809-1816 331773-1781 371742-1750 401708-1716 451682-1690 321655-1663 411629-1636 351601-1608 471572-1580 401545-1552 351526-1530 22

these 12 bands, the most representative ones, which werealso common to all spectra, were selected. They have beenchosen by taking into account that most of the sharp peaksarising in a second derivative curve were related toremaining water bands. It is also to be noted that addingsupplementary bands should result during fitting in multiplesolutions in terms of band height and width for adjacentband groups in a same spectrum. For this reason and inorder to avoid aleatory variations, the minimal bandnumber necessary to obtain a satisfactory fit was selected.For the development of the curve fitting model, ban dpositions and widths at half height were forced to vary inspecific intervals that were determined through the analysisof second derivative curves (see Table 2). Band shapeswere set to pure Gaussian or to a mix between Gaussianand Lorentzian (usually very weak percentage oflorentzian; mostly 1 X 10e6%). Only band heights couldvary freely.


After each spectrum was split into an elementary bandsetby applying the curve fitting model, norm alization of the1890-1505 cm- region was achieved by dividing theintensity of each point (calculated through the Kubelka-Munk relation) by the area of the band in the 1601-1608cm-i zone. This important band, is assiaromatic ring stretching vibrations 8ned in this case to13*2 25.This referencechoice can be justified by referring to the structure of thesamp les. Cha rs issued from a pyrolysis up to 700C can beconsidered as essentially constituted of large polyaromaticlayers. Thus, ring stretching vibrations can only be observedat the edges and at the level of intraplanar defects due to thepresence of heteroatoms or aliphatic bridges. In addition, thereactive sites of polynuclear structures are known to besituated a t the layer edges 27328. aking this into account, itis assumed that the amount of aromatic ring stretchingvibrations can be connected with the number of reactivesites likely to be oxidized. Thus, after normalization, it ispossible to show the evolutions of individual carbonylgroups in the 1890-1580 cm- region by examination of thefitted curves.Band assignement

Th e individual bands resulting from the curve fitting ofthe 1890-1505 cm- zone were attributed to specificinfrared stretching vibrations as follows (refer to Figure 5):1860-l 750 cm- : 4 bands attributed to lactones andanhydrides1750-1680 cm- : 2 bands due to ketones, carboxyls andesters1680-1620 cm- : 2 bands due to conjugated ketones andquinonesl620-15IO cm- : 4 bands that were attributed toaromatic ring stretching modes 13V23.


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I I I I I I I I1850 1800 1750 1700 1650 1600 1550Wavenumber (cm-l)

Figure 5 Exam ple o f a bandset issued from the curve fitting of a DRIFT spectrum in rhe region 1890-1 505 cm-. Upper trace correspondsto the second derivative of the initial spectrum (thick line).

tempe3

re (C)

ki sa3Fpr 7 t-

wavenumber (cm-l)Figure 6 Three dimensional epresentationof the evolution of carbonyl bands versus the duration of the oxidative treatment.

Evolution of carbonyl bands versus oxidation parametersFo r the visualization of evolutions undergone by thecarbonyl bandset issued from the curve fitting of this zone,graphical constructions were elaborated for the parameterstemperature and time. On these constructions, aromatic ringstretching modes are not represented in order to clarify thedrawings.

Samples oxidized for 140 min with increasing temperaturesOn the three dimensional representation of Figure 6

(value 20C for non oxidized char), it appea rs that areas oflactones and anhydrides bands (numbered 1-4) as well asthose of the ketones, carboxyls and esters band (5) increase

with temperature and this in a way that can be considered ascontinuous. The parallel variations of these carbonyl groupssuggests the existence of an equilibrium between anhy-drides and carboxyls. For conjugated ketones and quinonebands (6, 7) evolutions are not clearly shown. The area ofband 7 (quinones) varies around a value appreciably higherthan that of the non treated chars. Band 6 (conjugatedketones) show s variations intermediate between ba nd 7 andthe bands l-5. This latter observation suggests that band 6is a mixed band in which several carbonyl groupsparticipate. Variations of bands 6 and 7 seem to occur byjumps as if the formation of the relevant chemical groupswould be dependent of energy quanta. This is suggested by



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time (n

Figure 7 Three dimensional representation of the evolution of carbonyl bands versus the temperature of the oxidative treatment.

the fact that band areas fall to values that are approximatelyequal to the initial one and that during the heating ramp,samples were under an inert nitrogen atmosphere.Samples oxidi zed at 280C w ith increasing duration

As seen with variable oxidation temperature, resultsgiven in Figure 7 (value 0 for non oxidized char) show threedistinct evolution trends. For lactones, anhydrides, ketones,carboxyls an d esters (bands l-5), after a strong increase dueto temperature, areas seem to be stabilized arou nd a constantvalue. All variations for these 5 bands are quasi parallel. Theband attributed to conjugated ketones (6) follows approxi-mately the evolution of bands l-5. This confirms theobservations made previously about its mixed nature. Forthe quinones band (7), an increase tha t can be considered ascontinuous is observed. This trend is not well understoodyet. A relation between increasing duration and enhance-ment of reactive sites cannot be invoked here. Such arelation would be valuable only if an increase in the area ofother bands could be observed.CONCLUSIONFor the study of chars and especially for investigatingchemical transformations undergone by superficial chemi-cal groups during oxidation, infrared spectroscopy can beconsidered as a well adapted tool. The present study hasshown that the access to kinetics and mechanisms ofoxidation reactions is made possible with the help ofadequate m athema tical treatments such as curve fitting.Being in a preliminary stage, this work has nonethelesspointed out that complex regions of spectra obtained in theinfrared DRIFT mode can be decomposed into individualbands which evolution is a key to the understanding ofoxidation. Thereby, the influence of treatment parameterscan be followed and results can be used for optimization ofprocesses.


ACKNOWLEDGEMENTSThe authors would like to thank the Agence Nationale pourla Valorization de la Recherche (ANVA R-Lorraine) andthe EUREKA program EU 1436 Cheap Adsorbents forfinancial support.

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a study of carbonaceous char

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