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Catalysis Communications 6 (2005) 627–632

Uranyl-based heterogeneous catalyst for the selective oxidationof benzylic alcohols to form corresponding carbonyl compounds

Dharmesh Kumar a, R.P. Bhat b, S.D. Samant b, N.M. Gupta a,*

a Applied Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, Indiab Applied Chemistry Division, University Institute of Chemical Technology, Mumbai 400 019, India

Received 11 October 2004; accepted 8 June 2005Available online 28 July 2005

Abstract

Uranyl ions (O@U@O)2+ immobilized in mesoporous MCM-41 are found to serve as highly active heterogeneous catalyst for theformation of benzaldehyde in liquid–solid-phase oxidation of benzylic alcohols, using tert-butyl hydrogen peroxide (t-BuOOH) asan oxidant. The benzoic acid was formed on subsequent oxidation of benzaldehyde and therefore its yield increased with the increas-ing contact time. The solvent employed as a reaction medium influenced the catalyst activity considerably, and the best results wereobtained using ethyl acetoacetate as the solvent. In the repeated cycles of activity measurements, fairly reproducible results wereachieved after an initial decrease in the activity because of the partial leaching of uranyl ions.� 2005 Elsevier B.V. All rights reserved.

Keywords: UO2-MCM-41; Liquid-phase oxidation; Benzylic alcohols; Benzaldehyde; Benzoic acid

1. Introduction

The oxides of uranium are found to serve as activecatalysts, both in the bulk and dispersed forms, forthermal oxidation of CO, NO and volatile organiccompounds [1–7]. In some respects, the catalytic prop-erties of uranium oxides are known to be similar tothose of the oxides of a number of transition metalssuch as Fe, Cr, Mo, W, etc. As discussed in [1], thissimilarity is due to the [Rn] 5f36d17s2 electronic struc-ture of uranium and the 5f electrons are responsible tothe catalytic activity because of their being more delo-calized than the 4f electrons. Nevertheless, the ura-nium-based catalysts are yet to find many practicalapplications, mainly because of their inherent naturalradioactivity. In view of these considerations, we havedeveloped a number of highly dispersed uranium cat-

1566-7367/$ - see front matter � 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.catcom.2005.06.005

* Corresponding author. Tel.: +91 22 2550 5146; fax: +91 22 25505151/2551 9613.

E-mail address: nmgupta@magnum.barc.ernet.in (N.M. Gupta).

alysts. In these catalysts, the uranyl groups and nano-size crystallites of a-U3O8 were anchored/entrappedwithin the channels of mesoporous materials in orderto provide them a protective cover [8–12]. These hy-brid host–guest systems (i.e., UOx/MCM) are foundto serve as thermally stable and highly efficientcatalysts/photocatalysts for the low-temperature gas–solid-phase oxidation of organics, such as CH3OH,CH4, C2H4, C6H6 and C6H12, etc. [13–16]. The choiceof MCM-41 as host matrix was based on the largepore size of these materials that helped in encapsula-tion of the UOx moieties in the pore system and atthe same time made them accessible to the reactantmolecules. Also, MCM-41 consists of a large numberof uncondensed hydroxyl groups (�35–40%), existingas Si–OH groups within their pore system. These hy-droxy groups in turn help in the immobilization ofuranyl ions via formation of SiO–UO2–OSi linkages[14]. In addition, the large surface area of MCM ma-trix helps in better dispersion of uranyl ions, thusenhancing the number of active reaction sites.

628 D. Kumar et al. / Catalysis Communications 6 (2005) 627–632

In continuation, we have now evaluated the catalyticbehavior of UOx/MCM-41 sample for liquid–solid-phasereactions, and the model reaction chosen for this pur-pose was the oxidation of benzylic alcohol and its substi-tuted compounds. The brief highlights of these resultsare presented in this paper.

Fig. 1. DR UV–vis spectra of: (a) uncalcined IUM; (b) IUM calcinedat 400 �C; (c) uranyl acetate samples.

2. Experimental

2.1. Sample preparation and characterization

The immobilization of uranyl ions within the poresof MCM-41 was achieved by wet-impregnation routeand the method adopted for this purpose is reportedin our earlier publications [9–11]. In brief, theUO2þ

2 =MCM samples were obtained by contactingabout 1.0 g of calcined MCM-41 with a 0.1 molaruranyl acetate solution for �0.5 h. The mass obtainedwas filtered, washed thoroughly and dried at roomtemperature under vacuum. The sample was calcinedat a relatively low temperature of 400 �C, so thatthe occluded uranium existed predominantly as uranylions and the formation of secondary phase U3O8 wasminimized [9]. This sample, referred to as IUM in thetext for the sake of brevity, consisted of around14 wt% of U. The uranium content of the fresh sam-ple and also of the catalyst samples recovered afterdifferent reaction cycles was estimated by spectropho-tometry using colorimetric method. Hydrogen perox-ide was used as a complexing agent for this purpose.The physico-chemical properties of these host–guestsamples were monitored with the help of various tech-niques such as XRD, N2-sorption, DR UV–vis, TEM,etc., the details of which are given elsewhere [9].

2.2. Reaction procedure

The oxidation of benzylic alcohols was carried out inliquid-phase and at atmospheric pressure, using the pris-tine MCM-41 and IUM samples. In the typical experi-ments, a mixture containing 8 mmol of substrate and5 ml of solvent was taken in a round bottom flask,placed over a magnetic stirrer. To this was added50 mg of a catalyst sample and 16 mmol of oxidant [ter-tiary butyl hydrogen peroxide (t-BuOOH), 70%]. Theresulting mixture was then stirred constantly for thetime periods varying between 0.25 and 24 h, while main-taining the bed temperature in the range of 70–110 �C.After the reaction, the products were separated by filtra-tion and analyzed by gas chromatography (Chemito-8510) using an OV-17 column. The products formedwere further identified using a GC–MS (Hewlett–Pac-kered-model G1800A), equipped with a HP-5 capillarycolumn. The reaction conditions mentioned above wereoptimized on the basis of the experiments conducted

using different solvents and oxidants and also for vari-ous oxidant-to-substrate ratios. The experiments wererepeated a number of times using the same lot of a cat-alyst, so as to monitor its recyclability.

3. Results and discussion

3.1. Physico-chemical properties of IUM sample

Fig. 1 presents the DR UV–vis spectra of IUM sam-ple in 200–600 nm region, recorded prior to (curve a)and after calcination at 400 �C (curve b). The spectrumof uranyl acetate is also shown in curve (c) of this figurefor comparison, which represents the characteristicstructure due to electron–vibration interaction in a ura-nyl ion [17]. A comparison of spectral features in Fig. 1thus confirms the presence of uranyl species (UO2þ

2 ) inthe synthesized samples, and also since spectra (a) and(b) are almost identical in their absorbance value, it isinferred that no significant UO2þ

2 ! U3O8 transforma-tion occurred during the adopted calcination tempera-ture. The broadening of UV–vis spectrum in curves (a)and (b) and the blue shift in their position with respectto uranyl acetate (curve c) are attributable to the bindingof the uranyl groups to the silicate walls of MCM-41host material. A detailed description of these aspects isgiven in our earlier publications [8,9].

The XRD patterns of U-free MCM-41 and IUMsamples confirmed that the long range ordering of hostmatrix was affected only marginally as a result of ura-nium loading. At the same time, a decrease was ob-served in the unit cell parameter, the typical value ofa0 being 40.5 A for IUM as compared to 43.5 A in the

Fig. 2. Effect of reaction time on oxidation of benzyl alcohol on IUMsample: curve: (a) % conversion of benzyl alcohol; (b) % selectivity ofbenzaldehyde; (c) % selectivity of benzoic acid. Reaction conditions:temperature = 100 �C, oxidant (t-BuOOH):substrate ratio = 2:1(16:8 mmol), solvent = ethyl acetoacetate.

D. Kumar et al. / Catalysis Communications 6 (2005) 627–632 629

case of uranium free MCM-41 sample. Shrinkage in theframework structure of MCM-41 is ascribed to stronginteraction between the uranyl groups and the silicawalls. Such interaction, leading to the cross-linking ofthe silanol groups and hence in the contraction of theoverall host structure, has been discussed earlier indetail [8,9].

3.2. Oxidation of benzyl alcohols

Oxidation of benzyl alcohols was carried out undervarious reaction conditions, using tertiary butyl hydro-gen peroxide (t-BuOOH) or hydrogen peroxide as anoxidant. The use of t-BuOOH gave rise to at least threetimes higher conversion of benzyl alcohol as comparedto H2O2 under the parallel reaction conditions, andwas therefore adopted as an oxidant for the detailedstudies reported here. The experiments were conductedas a function of various parameters, such as: reactiontime, temperature, amount of the catalyst employed ina test run, oxidant to substrate ratio and the nature ofthe solvent employed as a medium. The highlights ofthese studies are given below.

3.2.1. Effect of solvent

Oxidation of benzyl alcohol with tertiary butylhydrogen peroxide (t-BuOOH) gave rise to formationof benzaldehyde as the main reaction product at theearly stages of the reaction, i.e., till about 30–40 minof time. The overall conversion of benzyl alcohol de-pended upon the solvent used as reaction medium andonly a negligible conversion of benzyl alcohol to prod-ucts was observed in the absence of a solvent. Table 1gives these results for a typical reaction temperature of100 �C. Among various solvents used in the presentstudy, ethyl acetoacetate showed the best results(�80% conversion of benzyl alcohol to benzaldehyde).Other researchers have also reported the influence of asolvent on the progress of certain oxidation reactions[18].

It is important to mention that no measurable reac-tion products were detected when similar studies were

Table 1Effect of solvent on oxidation reaction of benzyl alcohol

Solvent % Conversion % Selectivity

Benzaldehyde Benzoic acid

No solvent 1.8 100 0Acetone 23.1 92.6 7.4Acetonitrile 32.0 98.0 2.0Acetylacetone 15.6 97.5 2.5Chlorobenzene 30.7 99.0 1.0Ethyl acetoacetate 80.0 83.1 16.9Methanol 31.4 95.0 5.0

Reaction conditions: time = 1 h, temperature = 100 �C, oxidant:sub-strate ratio = 2:1 (16:8 mmol).

conducted on uranium-free MCM-41, thus confirmingthe catalytic role of the uranyl groups.

3.2.2. Effect of reaction time

Fig. 2 depicts the oxidation of benzyl alcohol overIUM sample at a reaction temperature of 100 �C andas a function of time. The ethyl acetoacetate was usedas a solvent for these experiments. As seen in this fig-ure, the conversion took place at a fast rate during theinitial stages of the reaction and a saturation stagewas reached in �2 h. The benzaldehyde was the majorreaction product during around first 30 min of reac-tion time, benzoic acid being the other product. Theyield of benzoic acid increased progressively whenthe reaction was continued for prolonged periods oftime. The change in the relative concentration of benz-aldehyde and benzoic acid as a function of reactiontime is plotted in curves (b) and (c) of Fig. 2. Theseresults clearly indicate that the formation of benzoicacid occurred due to further oxidation ofbenzaldehyde.

The following steps may thus represent the overallreaction:

CH2OH

+ t-BuOOHcatalyst

Solvent,

CHO

+ t-BuOH H2O+ (i)

Primary step:

Benzyl alcohol Benzaldehydet-Butyl hydrogen t- Butanol

peroxide

630 D. Kumar et al. / Catalysis Communications 6 (2005) 627–632

Secondary step:

CHO

+ t-BuOOHcatalyst

Solvent,

COOH

+ t-BuOH (ii)

Benzoic acid

It is therefore imperative that the formation of benz-

aldehyde will be affected considerably by the controlparameters adopted for the reaction.

3.2.3. Effect of reaction temperature

Fig. 3 shows the influence of temperature on the reac-tion products formed after oxidation of benzyl alcoholat the lapse of 1 h time. The reaction onset temperaturewas �70 �C, the conversion at this temperature being�20%. No measurable amounts of reaction productswere observed at room temperature. With further risein temperature, conversion of benzyl alcohol was foundto increase, reaching a saturation stage at �100 �C (Fig.3(a)). The formation of benzoic acid also increased pro-gressively with the rise in reaction temperature (Fig.3(c), reaction ii).

3.2.4. Effect of oxidant to substrate ratio

The conversion was found to increase considerablywith the increasing oxidant-to-substrate ratio and themaximum conversions were obtained for the ratio of �3and above. For an oxidant:substrate (TBHP:Benzyl alco-hol) ratio of 3:1, conversion of�90% of benzyl alcohol toproducts was observed as compared to conversion ofabout 80% for a ratio of 2:1 (Fig. 2). These results thusindicate that the oxidant is not utilized completely at thisreaction temperature, probably due to its decomposition,and therefore the reaction attains a saturation stage after

Fig. 3. Effect of reaction temperature on oxidation of benzyl alcoholon IUM sample: curve: (a) % conversion of benzyl alcohol; (b) %selectivity of benzaldehyde; (c) % selectivity of benzoic acid. Reactionconditions: time = 1 h, oxidant (t-BuOOH):substrate ratio = 2:1(16:8 mmol), solvent = ethyl acetoacetate.

�2 h. The results obtained after 1 h of reaction at 100 �Cin the presence of ethyl acetoacetate medium are pre-sented in Fig. 4. The yield of benzoic acid was also foundto increase with the increase in oxidant amount (Fig. 4(c))while that of the benzaldehyde decreased (Fig. 4(b)), indi-cating that the secondary step reaction of benzaldehyde isalso promoted by the oxidant.

3.2.5. Recycling studies

In order to check for the possible leaching of uraniumoxide species and its influence on catalytic activity, thereaction of benzyl alcohol was conducted repeatedlyusing the same lot of a catalyst sample. After each reac-tion cycle, the catalyst was washed with acetone, driedand heated in oxygen at 200 �C for 3 h before using itfor the next cycle of reaction. Around 15% decreasewas observed in the benzyl alcohol conversion after firstuse of the catalyst. The activity was affected only to asmall extent during the subsequent cycles of reaction,the reaction conditions remaining the same. The typicalresults obtained for four successive reaction cycles aregiven in Table 2, along with the U-content of the cata-lyst sample after each cycle. These data indicate thatsubsequent to initial leaching of active uranyl speciesduring the first one or two cycles, perhaps from theexternal surface of the host, the catalyst exhibited an al-most reproducible behavior.

In order to ascertain the role of uranyl ions leachedout in liquid phase during the reaction, the catalyst masswas separated after 50% completion of the reaction in anexperiment, and a fresh lot of oxidant was added to the

Fig. 4. Effect of oxidant (t-BuOOH):substrate ratio on oxidation ofbenzyl alcohol on IUM sample curve: (a) % conversion of benzylalcohol; (b) % selectivity of benzaldehyde; (c) % selectivity of benzoicacid. Reaction conditions: temperature = 100 �C, time = 1 h, sol-vent = ethyl acetoacetate.

Table 2Recycling studies on oxidation of benzyl alcohol over IUM sample

Catalyst U contentin catalystafter areactioncycle (wt%)

%Conversion

% Selectivity

Benzaldehyde Benzoicacid

Calcined IUM 14.2 80.0 83.1 16.9First cycle 12.5 66.2 89.1 10.9Second cycle 11.0 63.6 92.3 7.7Third cycle 10.7 63.4 92.0 8.0

D. Kumar et al. / Catalysis Communications 6 (2005) 627–632 631

filtrate. The progress of the reaction was monitored for�2 h and no further increase was observed in the prod-uct yield. This confirmed that the reaction occurred inthe heterogeneous mode and the catalytic action was in-deed due to the uranyl ions anchored on the MCM sup-port matrix.

3.2.6. Oxidation of various substituted alcoholsThe catalytic activity of IUM sample for oxidation

of various substituted benzylic alcohols was evaluatedunder the reaction conditions mentioned above. Thesubstituted alcohols with an aliphatic alcoholic sidechain were found to oxidize in the similar manneras benzyl alcohol, so as to produce correspondingaldehydes and ketones. The results of these studiesare summarized in Table 3. While chloro benzylic

Table 3Catalytic oxidation reactions of benzylic alcohols using IUM catalyst

Substrate Produc

CH OHCl 2 Cl

CH OHCOH23

COH3

CHOH CH3CO

CHOH CHCOH 33 COH3

CHOH CHNO 32NO2

Reaction conditions: time = 1 h, temperature = 100 �C, oxidant:substrate ra

alcohol showed maximum conversion of about 49%for the formation of carbonyl counterpart, the nitroand methoxy a-methyl benzylic alcohol showed highselectivity for the formation of carbonyl compound.

4. Conclusions

Our studies have shown that the uranyl ions immobi-lized in the mesoporous silicates serve as highly activecatalyst for the liquid/solid-phase partial oxidation ofbenzylic alcohols and their substituted homologues. Cor-responding aldehydes, ketones and acids were the mainreaction products, which serve as important raw materi-als in chemical industry. The activity and selectivity ofthese catalysts depended upon experimental conditions,i.e., the nature of the solvent, contact time, and the reac-tion temperature. It is observed that by choosing anappropriate combination of solvent and oxidant for thereaction, it is possible to achieve not only highconversion of benzyl alcohol (�87%) but also the highselectivity for aldehyde formation (�85%). In this re-spect, the performance of UOx/MCM sample appearsto be better than that of certain supported transition met-als and the metal complex catalysts [19,20]. Further stud-ies are being taken up to substantiate this observation.

t %

Conversion

after 1 h

%

Selectivity

CHO 49.2 76

CHO 44.2 83

CH3 46.3 98

CHCO 3 42.3 99

CHCO 3 16.5 100

tio = 2:1 (16:8 mmol).

632 D. Kumar et al. / Catalysis Communications 6 (2005) 627–632

Acknowledgements

Authors thank Dr. P.K. Sharma, Analytical Chemis-try Division, for the spectrophotometric analysis of ura-nium in different catalyst samples.

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