Indian Journal of Chem istry Vol. 40A. July 200 1, pp. 704-707

Acylation of alcohols over solid acid catalysts

V Umamaheswari, M Palanichamy, Banumathi Arabindoo & V Murugesan*

Department of Chemi stry, Anna University, Chennai- 600025, Indi a

Received 24 lilly 2000; revised 20 April 2001

Acy lat ion of l -met hoxy-2-propanol and I-butyl alcohol has been in vestigated over MCM -4 1, AI-MCM-4 1 (40), HPA­MCM-41 and HPA-AI-MCM-4 I (400) /H JPW 1P 40: 12H20 supported MCM-41 and AI-MCM-41 (400)1 in the liquid phase, at appropriate temperatures. Though HPA-M CM-41 and HPA-A I-MCM -4 1 (400) showed high alcohol conversion, leaching of HPA from the catalyst surface is the major proble m. Though the attainment of equi librium conversion is little de layed over AI-MCM -4 1 (40) compared to HPA-MCM-41 it could still be a better catalys t, as it gives high conversion as HPA­MCM -41. T he reaction has been found to occur largely wi thin the channe ls of the molecular sieves compared to the surface. T he reaction has also been studied over H ~ zeolite (Si/AI=3. 14) for compari son and the results are discussed.

Acylation of alcohols with acetic anhydride is an industrially important reaction as the products are extensively used as solvents for cellulose, oils, gums, resins etc., and as plasticizers I. They also serve as raw material s for the production of artificial flavours and fragrances 2. l-Methoxy-2-propyl acetate and 1-

butyl acetate, the products of the reactions of the present investigation, are finding applications as thinner in paint formulation and as the raw materi al for the production of anti-rheumatic drugs respec­ti vely 3. Liquid- phase acid catalysts such as sulphuric acid, p-toluenesulphonic acid, methanesulphonic acid, hydrochloric acid etc have also been used. They, however, produce waste products as well as undesired by-products such as ethers and olefins. For certain esterifications, non-acidic tetrabutyl titanates and zirconates produce lower amounts of byproducts than the liquid acids but their activity is much lower than that of the Bronsted acids 4. In most of the industri al processes AlCl3 and ZnCb are the commonly used catalysts in stoichiometric amounts fo r this reaction. Due to economic as well as waste disposal problem with these catalysts, there has been conti nued interest to develop easily separable and recyc lable solid ac id catalysts. The more popular among them are zeolites. As zeolites are microporous materials, all the Bronsted ac id sites cannot be put into activity at any instant as there is restriction for the simultaneous entry of large number of reactant molecules into the zeolite pores . Since the recently discovered MCM-4l

*E- 11lail: v _11lurugu@hotmail.c011l , Fax : 9 1-044-2200660

materials, particularly, AI-MCM-4l materi al has large pore diameter with widely scattered distribution of mild Bronsted acid sites, they are ex ected to be more suitable for acy lation of alcohol s. Al-MCM-41 molecular sieves were proved to have less hydro­thermal stability based on the observation of structural collapse in aqueous solutions during treatment for ion exchange 5-7. Since acy lation of alcohols with aceti c acid would produce water as the byproduct, use of acetic ac id as the acylating agent may also lead to the structural collapse of AI-MCM-41 molecular sieves. Hence, in the present investigation the acy lation of 1-methoxyl-2-propanol and t-butyl alcohol has been carried out with acetic anhydride, as the byproduct produced here is only acetic acid . Though acetic ac id thus produced could acylate, acetic anhydride is far more reactive than acetic acid. As a result water either may not be produced or even if produced it would be in too meagre an amou nt to produce structural degradation in AI-MCM-4I.

Materials and Methods The siliceous MCM-41 was synthes ised fo llowing

the procedure given by Beck el al h. The procedure was slightly modified for the synthesis of Al-MCM-4 1 mesoporous molecular sieves wirh Si/AI ratio (40) and (400). In a typical synthesis, 10.6 g of sodium silicate nanohydrate (Merck) in water was combined with appropriate amount of aluminiu m sulphate (Merck) in water and then acidified with 1M sulphuric acid to bring the pH= 10.5 under constant stirring. After 60 min of stirring an aqueous solution of cetytri­methylammonium bromide (CTA 8 ) (Merck) was

UMAMAHESWARI el al.: ACYLATION OF ALCOHOLS 705

added and the surfactant silicate mixture was stilTed for further 30 min at room tempL:rature. The molar composition of the resultant mixture was Si02 :

xAl20 3 : 0.2 CT AB : 0.89 H2S04 : 120 H20 . The resulting gel was autoclaved for 12 h at 170°C. The

. solid obtained was filtered, dried at 80 °C and calcined at 550°C in air for 5 h in a muffle furnace.

The supported catalysts were prepared by impregnation of H3PW 1204o.12H20 on MCM-41 and Al-MCM-41 (400) following the procedure of Kozhevnikov et al. 9

. MCM-41 (lg) was suspended in 30 ml of an aqueous solution of H3PW 1204o.12H20 so as to obtain a loading of 30 wt%. The suspension was stirred overnight at room temperature, the solvent was evaporated to get HPA-MCM-41 and the same procedure was adopted to get HPA-Al-MCM-41 (400) . The materials were dried, calcined at l30°C and stored in a desiccator until use.

Zeolite NaS (S ilAI=3 .14) obtained from United Catalysts India Limited was successively ion exchanged with 1M NH4Cl solution at 80°C to obtain its protonic form. X-ray diffractograms were recorded on a Scintag XDS 2000 diffractometer equipped with a liquid nitrogen cooled germanium solid state detector and Cu Ka radiation source.

For acylation of alcohols, the alcohol (0.1 mol), acetic anhydride (0.1 mole) and catalyst (0.2 g) were taken in a 25 ml round bottomed flask fitted with a reflux condenser. The flask with its contents was heated in a constant temperature oil bath and stirred magnetically . To monitor the progress of the reaction, aliquots of the hot mixture were withdrawn at regul ar intervals and centrifuged. The clean centrifugate was analysed by Hewlett Packard 5890A Gas chromato­graph equipped with Carbowax column and FID detector.

Results and Discussion The XRD patterns of the calcined MCM-41, AI­

MCM-41 (40) and AI-MCM-41 (400) shown in Fig 1 confirm the mesoporous nature of the materials . The samples exhibit a very strong peak at about 2° (28) and weak peaks between 2.8 and 4°(28). These peaks can be indexed on a hexagonal unit cell.

Acy lation of 1-methoxy-2-propanol with acetic anhydride was studied over MCM-4 1 and Al-MCM-41 (40) at 80°C. Similarly the reaction was also studied over HS zeolite, HPA-MCM-41 and HPA-AI­MCM-41 (400). The results obtained are presented in Table l. Ester was the only product observed in th is

reaction. When the reaction was calTied at 80°C, in the absence of the catalyst acylation was not observed. But at lOO°C the reaction occurred with 50.2% alcohol conversion under equilibrium. It is assumed that the conversion was not due to presence of water or acetic acid impurity in the reactants, as only purified reagents were only used in the reaction. Hence, high conversion in the absence of any external catalyst necessitates proposing a mechanism as shown in Scheme (la).

There could be hydrogen bonding interaction between the alcoholic O-H and carbonyl oxygen of acetic anhydride which may increase the nucleo­philicity of the former thus aiding nucleophilic attack on the adjacent carbonyl carbon of the same acetic anhydride. The acetic acid produced in this reaction could also enter and speed up the react ion auto­catalytically to attain fast equilibrium in thi s reaction. In order to verify the autocatalytic role of the acetic acid in this reaction , the reaction was also performed in the presence of acetic acid. At 80°C the reaction gave equilibrium conversion of 55%. Since the reaction was catalysed by weak acid such as acetic acid, it is expected that Al-MCM-41 with in-built mild acidity could also be a convenient catalyst for this reaction . Hence the reaction was carried out at 80°C with Al-MCM-41 (40). The reaction attained equilibrium at the end of 2 h with 97% conversion. The reaction may proceed through the mechanism as shown in Scheme (lb).

6

Vl 5240 -.II) C ::J 0 3930 u .... 0

'" c -' 2620

(e )

( b)

(a)

o 2 3 4 5 6 2 a/Omega

Fig. I-XRD patterns of (a) MCM-4I , (b) AI -MCM-41 (40), (c) AI-MCM-41 (400)

706 INDIAN J CHEM. SEC. A, JULY 200 1

Table I-Results of acy lation of l-methoxy-2-propanol

Catalyst Temp. Conversion (%)

(0C) 2h 4h 6h

MCM-41 100 50.8 58.73

AI-MCM-4 1 (40) 80 96.6 96.6 96.6

HPA-MCM-4 1 80 96.8 96.8 96.8

HPA-A I-MCM-41 (400) 80 89.33 98.60 98.60

H~ 80 79.89 86.53 86.53

Acetic anhydride is protonated with the bridged hydroxyl of the catalyst. The acy l cation thus produced is attacked by I-methoxy-2-propanol to yield I-methoxy-2-propylacetate. Studies with MCM-41 revealed absence of alcohol conversion at 80°C, but at 100°C there was 58.4% conversion which is only 8% more than the reaction studied in the absence of the catalyst. This little excess conversion may be a tributed to the Bronsted ac id sites of uncondensed silanol O-H groups on the wall surface. When the reaction was studied over H~ zeolite, it gave 87.4% equilibrium conversion. MCM-41 supported HPA catalysts were tested for esteri ficatio n of hexanoic acid and acetic acid with I-propanol and [-buty l alcohol respectively at reflux temperature. The catalyst was proved to be active for esterification 10.

Xin Chen et al. II also reported esterification of acrylic acid with t-butanol over heteropolyacids. Hence in the present study acy lation of I-methoxy-2-propanol and t-bu tyl alcohol were studied over MCM-41, AI­MCM-4 1 (40), HPA-MCM-41 and HPA-Al-MCM-41(400). Over HPA-MCM-41 the reaction was very fast ill ustrati ng the importance of density and strength of Bronsted acid sites of the catalysts to catalyse this rcaction. When the act ivity of AI-MCM-4 1 (40) and HPA-MCM-41 are compared, both of them gave the same equilibrium conversions. This illustrates that the quantity of the catalysts used in both is very large.

The only observation which stands tricky is less equ ilibrium conversion over H~ zeolite whose Bronsted acidity is more than that of AI-MCM-41 (40). The little decrease might be attributed to its less pore diameter than Al -MCM-41 rather than its ac id ity and acid strength. As the pore diameter in zeolite is less, the reactant molecules may not freely diffuse as in MCM-41 due to blocking of pores by the reactants as well as by the produc t. Simi larly the study over HPA-AI-MCM-41 (400) revealed hi gh conversion even at less time thus proving similar activity as that of HPA-MCM-41. In order to understand whether the reaction occurred prefe rentially within the channel or the outer surface, the reaction was studied over uncalcined AI-MCM-4 1 (40) molecular sieves. The conversion was on ly 68% thll s proving that the

~,

H,C-O-CH2-~H2 + (CH,CO), O ---.-..

W"O

. H)C-C=O t~- a-0i2- eM

" / O,j bH /S\ / A\

t 10 )

Scheme J

... !::-c=c

reaction was catalysed largely within the channels. But the value is little larger than the calcined MCM-41. The little excess activity might therefore be due to the presence of more O-H groups on the surface of the catalyst. Hence the study of acylat ion of I-methoxy-2-propanol with acetic anhydride reveals that A!­MCM-41 could be a convenient catalyst in industrial manufacturing processes . Since the process is ecofriendly and does not require any solvent, [his cou ld also be a convenient alternative to the conven­tional mineral acid catalysts.

Acylation of t-butyl alcohol with acetic anhydride was studied in the absence and presence of catalysts at 100 and 80°C respectively . The reaction was observed to be fast with the yie ld of ester as the on ly product. The results are presented in the Table 2. The data clearly indicate the susceptibility of the reacti on to the catalytic influence. In the absence of catalyst the reaction required high temperature . nd low conver­sion of 24 % at the end of 6 h. Over AI-MCM-41 the temperature was brought down to 80°C and the conversion increased to 82.5%. Hence, the react ion is' proved to be ac id catalysed involving nucleophi lic attack of alcohol on acy l cation produced by the protonation of acetic anhydride. MCM-41 without Bronsted ac id sites showed less act ivi ty, req uiring high temperature (lOO°C). The act ivity of MCM-41 could be ascribed to the ex istence of Bronsted ac id sites of crystal imperfection. The reacti on was also studied over H~ zeolite for comparison. H~ zeolite was found to be more active than AI-MCM-41 , though it is microporous permitting very controlled

UMAMAHESWARI el 01.: ACYLATION OF ALCOHOLS 707

Table 2- Results of acylation of I-butyl alcohol

Catalyst Temp. Conversion (%)

(0C) 2 h 4h 6h

MCM-41 100 72.20 78.50

AI-MCM:41 (40) 80 72.60 75.45 82.50

HPA-MCM-41 80 80.50 86.40 86.40

HF'A-AI-MCM-41 (400) 80 86.90 96.00 96 .00

H~ 80 8&. 10 88.50 94 .80

access for the reactant molecules to enter the channel where the active sites are present.

But the acylation of I-methoxy-2-propanol over H~ zeolite was fo und to be less active than AI-MCM-41. Since {-butyl alcohol is more hydrophobic the con'esponding t-butyl acetate must be still more hydrophobic. Hence as and when the ester is formed, it may be released out of the pores of H~ zeolite, making the pores available for the fresh reactants to enter a ll the ti me. The reverse is true for I-methoxy-2-propanol. I-Methoxy-2-propanol is more polar and hence the ester would be largely retained in the pores of H~ zeolites. Hence, there cou ld be large restriction for the entry of free reactants.

As the reacti on can proceed with mild acid sites the decrease in conversion over AI-MCM-41 (40) compared to H~ might also be due to its feeble number of Bronsted acid sites available in the channel s. Hence many of the sites are expected to be buried in the bulk of the mesopore wall. Though the reaction could be very well catalysed by strongly acidic Bronsted acid sites, the decrease in conversion in HPA-MCM-41 might be due to poor loading of HPA. In HPA-AI-MCM-41 the conversion was found to be 96% at the end of 4 h, which might certainly be due to high load ing of HPA. The entire activity of the catalyst migh t be attributed to only HPA, as the Bronsted acid sites of AI-MCM-41 are expected to be masked by the film of HPA. Hence compared to the uncatalysed path, the heterogeneous catalytic process is better for acylation of {-butyl alcohol.

When acylation of I-methoxy-2-propanol and (­butyl alcohol was compared, the former was found to be more readily acylated than the latter. As the alcohols are to react only with acyl cation which ' is linear and steric free, the steric crowding around the alcoholic O-H will not have much influence on the rate of alcohol conversion. Hence the rate of alcohol conversion is expected to depend on the ease with which it can enter the catalyst channel and react with acyl cation, which stays at the intra framework channels as the counter ions. Since I-methoxy-2-propanol has more oxidic sites than t-butyl alcohol , it can more easily enter and diffuse through th~ channel

by hydrogen bonding interaction with the Bronsted acid sites and react with the acyl cation.

Conclusion Study of acylation of alcohols with acetic

anhydride over MCM-41, AI-MCM-41, HPA-MCM-41, HPA-AI-MCM-4l and H~ catalysts indicates hi gh conversion than the reaction performed in the absence of the catalyst. The catalyst aided reaction is also observed to require less temperature and less time to reach equilibrium conversion. The act ivity of the catalyst for acylation is observed to be in the order HPA-AI-MCM-41 (400) > HPA-MCM-41 > AI­MCM-41 (40) > H~ > MCM-41 for I-methoxy-2-propanol and for I-butyl alcohol the order is HPA-Al­MCM-41 (400) > H~ > HPA-MCM-41 > Al-MCM-41 (40) > MCM-41 , the major problem with regard to the use of HPA supported catalyst is due to leaching of the HPA. lt is concluded that AI-MCM-4l is found to be more active and ecofriendly catalyst for the title reaction.

Acknowledgement The authors gratefully acknowledge the financia l

support from the Department of Science and Technology, Government of Indi a, New Delhi for thi s major project (Project No. SP/S IIH-23/96). The authors are thankful to Prof A. Kalanidhi Vice Chancellor, Anna University , Chennai, for his constant encouragement and providing all the facilities to carry out the work.

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