Indian Journal of Chemistry Vol. 31 A, October 1992, pp. 803-805

Use of tungstate in acid catalysis: The ion exchange property of a-zirconium Some esterification reactions phosphate, Zr(HP04hHzO has been examined in detail5-7 and in the hydrogen form shown to be an Pramod Patel, Anjali Shivanekar & Uma Chudasama· acidic catalyst8,9. It has been shown by us earlier, Department of Chemistry, Faculty of Science, M S University of the potential use of zirconium molytxlate as a Baroda, Baroda 390 002 Bronsted acid catalystlO• The synthesis and ion ex• change properties of zirconium tungstate has been Received 9 December 1991; revised 10 March 1992; accepted studied earlierll-13• So far no work has been done 1 May 1992 to examine the catalytic properties of these com• pounds. The present work consists of synthesis, Amorphous zirconium tungstate, an inorganic cation characterisation and study of catalytic behaviour exchanger has been prepared. The protons of the hy• (Bronsted acid catalyst) of amorphous zirconium droxyl groups are the active sites. Such a material indi• tungstate (ZW) using esterification as a mOdel cates good potential for application in Bronsted cataly• reaction. sis. Esterification has been studied as a model reaction wherein ethyl acetate has been synthesized. The variation of Experimental several parameters has established the use of zirconium tungstate as a Bronsted acid catalyst. Preparation of the catalyst Zirconium tungstate was prepared by adding sodium· tungstate to an aqueous solution of zirco• Acid salts of tetravalent metals are usually ob• nium oxychloride till complete precipitation. The tained as amorphous materials I, but of late they gel was digested at room for several have been obtained as crystalline compoundsz. An hrs, washed with conductivity water till the remo• extensive literature on their synthesis, structure val of any excess chloride, filtered and dried at and properties has accumulated3-6• These com• 100°C. The dried material was broken down to pounds have general formula M(IVXHX04hnHzO the desired particle size (30-60 mesh) and sized where M(IV) = Ti, Zr, Ce, Th etc. and X = P, As, W, by sieving. It was converted to the hydrogen form Mo. They can be prepared in the form of granules in a by immersion in 1M HCI, the acid being intermitt• range of mesh sizes, by simply combining tetrava• ently replaced. It was then washed several times lent metals and polybasic acids to obtain insoluble with conductivity water, filtered and dried at gelatinous precipitates at room temperature, with 100°C. ZW prepared is in the form of white hard or without refluxing. The compounds show a great granules. promise in preparative reproducibility, and stabil• ity,both thermal and chemical. Acid catalysis (Esterification reactions) The hydroxyl protons of the acid salts can be ea• The esterification reactions were carried out sily replaced by other cations. Owing to their pec• varying particl~.size, amount of the catalyst and uliar properties, such as selectivity for certain ions, mole ratio of alcohol and acid. high stability in strongly acidic and/or oxidising For the prew;lration of the ester, acetic acid and solutions, high thermal stability and resistance to ethyl alcohol were combined in a refluxing assem• radiation, acid salts are attractive inorganic ion ex• bly. The catalyst was then added in the required changers, for particular uses, especially when or• amount. The mixture was heated at 60°C for 2 hrs ganic resins cannot be employed because of their and ester formed was distilled over slowly. The degradability3.7. The presence of acid sites on such amount of ester formed was monitored on gas materials indicates good potential for application chromatograph. in Bronsted catalysis. It has also been observed that amorphous materials possess greater number of acid sites as compared to crystalline materialsR• Catalyst characterisation Thus amorphous materials are catalytically more The sample ZW was analysed for zirconium active as corp.pared to their analogous crystalline and . Zirconium wa<;determined gravimet• varieties. rically as zirconium . Tungsten was deter-

-. -. j, ....." JI 804 INDIAN J CHEM. SEe. A. OCTOBER 1992

42.8054.0064.3657.0034.3038.0029.50Yield0.229.800.40.459.000.20.227.700.456.000.20.20.20.20.452.000.2AcidYieldAmountofAlcoholAcid0.502.001.00 f Table I-lPercentageAlcohol(M)(%)('X,)catalyst(M) yield of ethyl acetate varying amount of Table 2-Percentagecatalyst yield of ethyl acetate for regenerated (M)0.2 I (g) I catalyst0.2 and mole ratio of acetic acid

mined avimetrically as barium tungstate. Chemi• mmetric and symmetric hydroxo-OH and aquo-OH cal anal sis indicated the composition of zirconi• stretches. A sharp medium band at - 1620 cm - I um tun state, Zr:W to be in 1:2 ratio. As already was attributed to - (H - 0- H) bending. indicate the hydrogen ions of the OH groups are the acti protons. Results and discussion No c ange in colour or form was observed on Esterification of carboxylic acid is a reaction If. heating ith water. The material was stable in dif• subject to general Bionsted acid catalysis and the ferent ineral acids like HCI, HZS04' HNO] and reaction of ethyl alcohol with acetic acid was se• .~I , bases Ii e NaOH, KOH, NaZCO] etc. upto - 4 M lected as a model reaction. tI ( t: I concent ation. w t I Abou 1 g of the material was soaked in con• .I CH3COOH + C2H,OH ~ CH3COOC2Hs + H20 I l ductivit water for a few hrs till constant pH was H' obtaine . During this time protons from the mate• rial wer released and pH of the solution dec• As observed from the data in Table 1, the yields reased. he solution was then titrated against of ethyl acetate can be increased, with increase in standar alkali till - pH 12. Comparison of the the amount of catalyst. The yields can also be in• curve . h the standard acid curve, indicated that creased by increasing [acid]. However, maximum ZW beh ved as a weak acid. The surface area of yields are obtained when amount of catalyst taken material was measured by the nitrogen absorption is 2 g and alcohol to acid proportion is 1:2. With BET me hod. The surface area of ZW was 17 m2/ g. change in mesh size there is no change observed in the yields. Ther ogravimetric analysis of the sample was The catalyst was regenerated by washing several perform d on Shimadzu Thermal Analyzer mod• times with water and finally treating it with 1M el, DT 3 at a heating rate of 10°C/min. TGA of HCI (as per method suggested in experimental ZW indi ated the presence of about 20% hydrat• section). In the regenerated samples the yields de• ed wate which was lost very slowly within the crease by 4% in the case where excess acetic acid temperat re range of 80-18()°C. There was no is taken while 10% in the' case where the alcohol evidence of the decomposition of the solid within to acid proportion is 1:1 (Table 2). the temp rature range of 30°C-1000°C . During the course of reaction there is a change Differ ntial scanning calorimetric analysis of in the colour of the catalyst from white to grey. ZW was obtained at a heating rate of 20cC/min. The IR spectra of this sample gives additional on Perk n Elmer (range up to 500°C). OSC bands in the region - 1400 cm - I which may be showed nly one exothermic peak at 120°C indi• attributed to aliphatic - CH bending vibrations. cating th presence of water. There was no en• This gives an indication that during the course of dothermi peak up to 500°C, indicating that there the reaction the reacting molecules come on to the was no p ase change. surface of the catalyst. Some of them enter into IR spe tra of the samples were obtained on Shi• reaction to give the ester but a few of them get ad• madzu I -408 spectrometer using KBr discs. The sorbed on the surface which is marked by change spectrum of ZW showed broad bands in the re• in colour of the catalyst. The fact that the adsorp• gion - 3 00 cm - I which were attributed to asy- tion is weak is observed from the fact that the ca-

I I II:. I. I I I~ NOTES 805 talyst regains its original colour when treated with References acid for regeneration. The possibility of molecules 1 Amphlett C B, Inorganic ion exchangers, (Elservier, Am• entering interstices cannot be ruled out. This is sterdam) (1964) 2 Clearfield A & Stynes J A, J inorg nuc/ chem, 26 (1964) observed from the fact that the yields go down by 117. 5% after every regeneration leading to deactiva~ 3 Ion exchange and solvent extraction, edited by J A Marins- tion of the catalyst. ky & Y Marcus (Dekker, New York) 5 (1973)Ch.1. 4 Alberti G & Costantino U, J chrom, 102 (1974) 5. The above results establish the use of zirconium 5 Alberti G, Acc chem Res, II (1978) 16J. tungstate as a Bronsted acid catalyst. Conventional 6 Inorganic ion exchange materials, edited by A aearfied (eRC press, Boca Raton, Florida)Ch. 1-3 (1982). methods use H2S04 as a catalyst while preparing 7 Alberti G, Layered acid salts of tetravalent metilis and their esters. The yields are high but traces of H2S04 are organic derivatives as ion exchangers, intercalation agents difficult to remove. The use of solid acid catalyst and molecular sieves, in Recent developments in ion ex• such as zirconium tungstate is definitely advanta• change, edited by P A Williams & M J Hudson (Elsevier, geous, since the ester· formed can be simply dis• London) (1987) 233. 8 Clearfield A & Thakur D S, J Cata/, 65 (1980) 185. tilled over and there is no catalyst contamination; 9 Hattori T, Ishiguro A & Murakami Y, J inorg nucl diem, besides the catalyst can be regenerated and 40 (1978) 1107. reused. 10 Shivanekar A S & Chudasama U V, Recent developments in catalysis, edited by B Vishwanathan & C H Pillai, (l990) Acknowledgement 489. 11 Qureshi M, Gupta J P & Sharma V, Talanto, 21 (1974) Thanks are due to the Head, Department of 102. Chemistry, for providing necessary laboratory faci~ 12 Srivastava R K, Pal B, Raizada B B & Pandya K B, J Indi• lities. One of the authors (A S) is also thankful to an chem Soc, Vol VU(1980) 391. the CSIR, New Delhi for the award of a fellow• 13 Shankar B & Basu S, Indian J Chem, Vol 25A, (1989) ship. 346.