Ž .Applied Surface Science 140 1999 176–181

ž /AlPO -5 molecular sieve modified with Cr acac4 3

Yuri V. Plyuto a,), Igor V. Babich a, Roger A. Sheldon b

a Laboratory for Inorganic Surface Chemistry, Institute of Surface Chemistry, National Academy of Sciences of Ukraine, Pr. Nauki 31,252022 KieÕ, Ukraine

b Laboratory of Organic Chemistry and Catalysis, Delft UniÕersity of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands

Received 15 May 1998; accepted 30 September 1998

Abstract

Ž .AlPO -5 molecular sieve was modified by gas phase reaction with Cr acac affording CrrAlPO -5 material with a4 3 4

chromium content not exceeding 0.2 wt.%. Attachment of Cr species to the external surface of AlPO -5 is suggested to4Ž .result from interaction of surface hydroxyl groups with the acac ligands of Cr acac , which is too bulky to enter the3

Ž .AlPO -5 pores. It was shown that the Cr acac species which retained their acac ligands predominate on AlPO -5 surface.4 3 4Ž .These ligands are removed by subsequent thermal treatment in air which simultaneously oxidizes Cr III ions to give

Ž .surface-bound Cr VI oxo-species. q 1999 Elsevier Science B.V. All rights reserved.

Keywords: Molecular sieves; AlPO ; Chromium acetylacetonate4

1. Introduction

Chromium containing molecular sieves of theAlPO family have attracted particular attention due4

to potential applications in various industrially im-w xportant catalytic processes 1–5 . In this context, a

number of articles has been devoted recently to thew xsynthesis and characterisation of CrAPOs 6–11 .

The direct incorporation of chromium ions in theAlPO framework via substitution of aluminium or4

phosphorus atoms during hydrothermal synthesis wasproposed since metal ions cannot be introduced intothese molecular sieves by ion exchange owing to the

w xneutral character of their framework 12 .In the present study, an alternative approach to

the synthesis of a chromium containing AlPO -54

) Corresponding author. Tel.: q380-44-2656765; Fax: q380-44-2640446; E-mail: user@surfchem.freenet.kiev.ua

molecular sieve was examined which was designedto result in metal localisation on the external surface.For this purpose we choose a reaction of the transi-tion metal precursor with surface hydroxyl groupsterminating ALPO lattice. To our knowledge, thisapproach has not been previously followed probablybecause it is not clear if they are reactive enough tobe used for the anchoring of transition metal precur-

w xsors. In the case of AlPO -5 13,14 , the outer sur-4

face terminal P–OH and Al–OH groups were foundto be stable up to 9008C. In order to attach chromiumŽ . Ž .III and chromium VI species to the externalsurface of AlPO -5 the interaction of its terminal4

hydroxyl groups with a volatile chromium precursor,which is too bulky to enter the AlPO -5 pores, can4

Ž .be used. For this purpose Cr acac , the molecular3Ž . w xdiameter of which 0.90 nm 15 exceeds the free

diameter of the AlPO -5 unidimensional channels4Ž . w xclose to 0.80 nm 12 was selected.

0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0169-4332 98 00586-8

( )Y.V. Plyuto et al.rApplied Surface Science 140 1999 176–181 177

2. Experimental

A sample of AlPO -5 was prepared from a gel of4

the following composition 1.5 Pr N:1.0 Al O :3 2 3Ž1.05 P O :50 H O. Pseudoboehmite Pural SB, Con-2 5 2

.dea Chemie, 75.0% Al O was used as the alu-2 3

minium source. It was combined with one third ofthe calculated amount of water and the resultingslurry was subjected to overnight mechanical shak-ing. Subsequently, orthophosphoric acid diluted withtwo thirds of the calculated amount of water wasadded to the slurry resulting in the formation of avery viscous gel. After vigorous homogenizing byhand for 1 h and mechanical shaking for 1 h, the gelbecame more fluid. Afterwards, tripropylamine wasadded and the resulting mixture was subjected tomechanical shaking for 2 h. Crystallisation was car-ried out statically at 1508C for 4 h in a teflon-linedautoclave placed into a preheated forced convection

w xoven 16 . Crystals were recovered by decantation,washed with water several times, filtered and driedovernight at 1108C. The template was removed bycalcination of the as-synthesized material in air at5008C for 20 h.

The deposition procedure included preliminaryŽ .dehydration of the AlPO -5 sample 4.0 g in a4

w xvertical Pyrex tube reactor 17 under a flow of drynitrogen at 5008C for 2 h. Subsequently, 0.1 g ofŽ .Cr acac was introduced into the cooled reactor3

where the modifier and the AlPO -5 sample were4

separated by a porous plate. During deposition ofŽ .Cr acac on AlPO -5 surface the temperature was3 4

kept at 2008C for 3 h to ensure complete sublimationŽ .of Cr acac and its passing through the AlPO -53 4

sample. Condensation of the modifier near the outletof the reactor indicated that a sufficient excess of themodifier was used and when deposition was com-plete. The sample was subsequently subjected to aflow of dry nitrogen at the same temperature for 3 hin order to remove all unbound modifier from theAlPO -5 sample and the reactor was cooled to room4

temperature.X-ray powder diffractograms were measured on a

Philips PW 1840 diffractometer using Cu Ka radia-Ž .tion. Thermogravimetric analysis TG accompanied

Ž .by differential scanning calorimetry DSC was car-Žried out with a STA-1500 H thermobalance PL

.Thermal Sciences at a heating rate of 108Crmin in

Ž .an oxidizing air flow atmosphere. Diffuse re-flectance UV–VIS and FTIR spectra were measuredwith a Cary 3 UV–VIS and a Magna FTIR spectro-photometer, respectively. Temperature programmed

Ž .reduction TPR was carried out in a conventionalapparatus equipped with a thermal conductivity de-

Ž .tector TCD to monitor hydrogen consumption froma 5.0% H rAr mixture. The total flow rate was 252

mlrmin. Approximately 100 mg of the sample wasloaded into a quartz reactor and subsequently mixedwith powdered SiC to ensure proper heat transfer.The temperature of the sample was increased linearlyat a heating rate of 108Crmin. The chromium con-tent in the modified sample was determined by iodo-metric titration after its calcination at 5008C for 2 hin air and subsequent treatment in an aqueous KOH–

w xH O solution 18 .2 2

3. Results and discussion

The synthesized AlPO -5 exhibited an X-ray4Ž Ž ..diffraction pattern Fig. 1 1 characteristic of thew xAFI-type structure 12 . No changes were observed

when the sample was subjected to modification withŽ . Ž Ž ..Cr acac Fig. 1 2 and subsequent thermal treat-3

Ž Ž ..ment in air at 5008C Fig. 1 3 . This means that the

Ž .Fig. 1. XRD patterns of the initial AlPO -5 1 , after deposition of4Ž . Ž .Cr acac at 2008C 2 and consequent calcination in air at 5008C3

Ž .3 .

( )Y.V. Plyuto et al.rApplied Surface Science 140 1999 176–181178

initial AlPO -5 molecular sieve retains its crys-4Ž .tallinity upon Cr acac deposition and subsequent3

calcination.The as-synthesized CrrAlPO -5 material had a4

greyish green colour which changed to yellow aftercalcination in air at 5008C. Chromium loading, deter-mined by chemical analysis, was found to be close to0.2 wt.%.

Ž .The presence of the surface Cr III species in theŽ .as-synthesized and Cr VI oxo-species in the cal-

cined samples was established by diffuse reflectanceUV–VIS spectroscopy. Spectra in Fig. 2 are given inreflectance mode and the minima correspond to the

Ž .absorption bands. Strong Cr III d–d bands wereobserved at 390 and 580 nm in the as-synthesized

Ž Ž .. Ž .sample Fig. 2 1 that, by analogy with Cr acac ,3

can be attributed to 4A ™4 T and 4A ™

4 T2g 1g 2g 2gw xtransitions 19 , respectively, in the octahedrally co-

Ž .ordinated chromium III ions. Calcination of theas-synthesized sample resulted in the complete dis-

Ž Ž ..appearance of these d–d bands Fig. 2 2 owing toŽ . Ž .Cr III oxidation to Cr VI ions. The latter are

Ž .characterised by a low intensity oxygen to Cr VIcharge transfer band at 450 nm 1t ™2e which is1

Fig. 2. Diffuse reflectance UV–VIS spectra of AlPO -5 molecular4Ž . Ž .sieve after deposition of Cr acac at 2008C 1 and consequent3Ž .calcination in air at 5008C 2 .

Fig. 3. Diffuse reflectance FTIR spectra of the initial AlPO -54Ž . Ž . Ž .molecular sieve 1 , after deposition of Cr acac at 2008C 2 and3

Ž .consequent calcination in air at 5008C 3 .

symmetry forbidden and two symmetry allowed highintensity bands at 350 nm 1t ™2e and 280 nm16 w xt ™2e 20 .2

Unfortunately, the spectroscopic data do not allowus to distinguish between monochromate and dichro-

Ž .mate polychromate species on AlPO -5 surface.4

The assignment of the 445 nm band in the diffusereflectance spectra of supported chromium oxide sys-

w xtems has been widely discussed in the literature 21 .The low intensity of the shoulder around 450 nm in

Ž Ž ..the spectrum Fig. 2 2 and the yellow colour of thew xsample, that is typical for monochromates 22 , sug-

gest that these species predominate on the surface.IR studies confirm the presence of the acac lig-

Ž .ands coordinated to Cr III ions in the surfacespecies. The diffuse reflectance FTIR spectrum of

Ž .the as-synthesized sample KBr contains distincty1 Ž Ž ..bands at 1380 and 1528 cm Fig. 3 1 which

Ž y1 .correspond to C–O 1383 cm and C PPPasŽ y1 . w xC PPP C 1522 cm vibrations 23 of the acety-as

Ž .lacetonate ligands in Cr acac . Their complete dis-3

appearance is observed upon sample calcination atŽ Ž ..5008C in air Fig. 3 2 . This means that removal of

Ž .the acac ligands from the chromium III acac sur-face species occurs under these conditions.

( )Y.V. Plyuto et al.rApplied Surface Science 140 1999 176–181 179

Fig. 4. TGrDSC profiles of AlPO -5 molecular sieve with the4Ž .deposited Cr acac .3

Displacement of the acetylacetonate ligands fromŽ .chromium III surface species starts at 220 and is

complete at 3808C. This is clearly seen from the TGŽ .curve of the modified sample recorded in air Fig. 4 .

Disappearance of the acetylacetonate ligands is ac-companied by their oxidative destruction, which isreflected by two exothermic peaks in the DSC curve.

The chromium loading in the as-synthesized sam-ple and the observed weight loss associated with theremoval of the acac ligands allow calculation of theacacrCr ratio in the chromium surface species. Thefollowing stoichiometry of the red-ox transforma-tions was used:

Cr acac ™CrO qvolatilesŽ . 3 3

Ž .The ratio was close to 2.6 meaning that Cr acac 3

species which retained acac ligands predominate on

Fig. 5. TPR pattern of AlPO -5 molecular sieve after deposition of4Ž .Cr acac at 2008C and calcination in air at 5008C.3

the AlPO -5 surface. The same observation was made4

in the case of modification of silica and aluminaŽ .supports with Cr acac from the gas phase3

w x17,24,25 .A reasonable explanation for the interaction ofŽ .Cr acac with the AlPO -5 surface is via the forma-3 4

tion of hydrogen bonds between AlPO -5 terminal4Ž .hydroxyl groups and two acac ligands of Cr acac .3Ž .Hence, the acac ligands in the attached Cr acac 3

molecule are not equivalent: two interact with sur-face hydroxyl groups while the third acac ligand is

Ž .oriented away from the surface see Scheme 1 .Thermally activated removal of these not equivalentacac ligands occurs separately and two distinct peaks

Ž .are observed in the DSC curve Fig. 4 consistentwith two types of acac ligands. A stepwise removal

Scheme 1.

( )Y.V. Plyuto et al.rApplied Surface Science 140 1999 176–181180

Ž .of the acac ligands from Cr acac adsorbed on silica3w xand alumina surfaces was previously observed 17,25

and a low temperature pathway was associated withthe proton assisted elimination of the hydrogenbonded acac ligands while a high temperature one

w xwas attributed to their thermal destruction 26 .The proton assisted substitution of the acac lig-

Ž .ands in surface Cr acac species starts at 2008C and3Ž .proceeds intensively around 2508C Fig. 4 . There-

fore, it becomes clear that the calculated acacrCrratio in the chromium surface species reflects apossibility of partial elimination of the acac ligandseven upon the synthesis.

In oxidizing atmosphere, the removal of the acacligands is accompanied by simultaneous oxidation of

Ž . Ž .Cr III ions to surface Cr VI oxo-species. Aproposed sequence of the observed coordination and

Ž .red-ox transformations of Cr acac on the surface of3

AlPO -5 support is illustrated by Scheme 1.4Ž .The TPR pattern of the oxidized sample Fig. 5

revealed one distinct peak at 4008C and a broadmaximum around 8008C. The low-temperature peakcorresponds to that observed at 3918C for the reduc-

Ž .tion of Cr VI oxo-species dispersed on a silica-w xalumina surface 27 . Hence, the maximum at 4008C

Ž . Ž .can be attributed to the Cr VI to Cr III reductionŽ .step of Cr VI oxo-species localized on the external

surface of AlPO -5 molecular sieve. The symmetri-4

cal shape of the observed low-temperature maximummeans that the reduction occurs within a narrowtemperature interval and is consistent with the reduc-

Ž .tion of a single type Cr VI oxo-species. Thisconstitutes additional evidence in support of thepresence of monochromates which was concludedfrom the diffuse reflectance UV–VIS spectrum ofthe oxidized material. The second maximum, around

Ž .8008C, most likely corresponds to a further Cr IIIŽ .to Cr II reduction which requires a high tempera-

ture.

4. Conclusions

Modification of AlPO -5 molecular sieve with4Ž .Cr acac was carried out from the gas phase. This3

Ž .resulted in deposition of Cr acac on the external3

surface of AlPO -5 molecular sieve, presumably via4

the formation of hydrogen bonds between the acac

ligands and hydroxyl groups which terminate theAlPO -5 lattice. The acac ligands are removed on4

Ž .calcination, with simultaneous oxidation of Cr IIIŽ .ions to surface Cr VI oxo-species. The catalytic

properties of the resulting material are currently un-der investigation.

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