Indian Journal of Chemical Technology Vol. 1 0, January 2003, pp. 44-47

Articles

Metal(IV)tungstates as solid acid catalysts for the synthesis of phthalate diester- dioctyl phthalate (DOP)

Alpana Parikh & Uma Chudasama*

Department of Applied Chemistry, Faculty of Technology and Engineering, M S University of Baroda, Baroda 390 00 1 , India

Received 9 Novemher 2000; revised received J J October 2002; accepted J 5 October 2002

The study involves synthesis and characterization of amorphous M(IV) tungstates [M (IV) = Sn, Ti or Zr] which are inorganic ion exchangers of the class of tetravalent metal acid (tma) salts. The presence of structural hydroxyl protons on such materials indicates good potential for Bronsted acid catalysis. The catalytic activity of the materials abbreviated as SnW, TiW and ZrW has been explored by synthesizing a phthalate diester-dioctyl phthalate (DOP).

Dioctyl phthalate (DOP), an i ndustrial plasticizer i s prepared by treating 2-ethyl hexanol with phthalic anhydride in the l iquid phase ei ther with a monoester intermediate stage or by a direct route l . For esterification of phthalic anhydride with 2-ethyl hexanol, normally, sulphuric acid is used. However, it gives rise to many problems such as corrosion of the reactors, difficulties in product separation and catalyst recovery, pollution by acidic waste water and a poor quality of DOP because of sulphites in the product. Other catalysts used are p-toluene sui phonic aci d (PTSA), methane sui phonic acid (MSA), hydrochloric acid, phosphoric acid, etc . Further, these catalysts are known to colour the product due to the formation of byproducts and cannot be reused.

In v iew of the environmental concern, there is a global effort to replace the conventional l iquid acid catalysts by solid acids which are less toxic, easily separable from the products, easy to handle and reusable. It has been observed that i n recent times, DOP has been synthesized by using a variety of solid acid catalysts.

DOP formation has been catalyzed by metall ic oxides and hydroxides (or hydrates) of magnesium, zinc, t i tanium, zirconium, etc. as well as metal oxide

o 1 complexes-. Yudao et al: has reported the use of zeoli te for the synthesis of DOP. DOP has also been catalyzed by soli d super acids4.5 and heteropolyacids5.6. Suter7 has reported a non-catalytic process for the manufacture of DOP, at very high temperatures, at which autocatalysis occurs.

A number of monoesters have been synthesized i n this laboratory using inorganic ion exchangers o f the

* For correspondence

class of tetravalent metal acid (tma) salts8- 1 1 • These materials possess the general composi tion M(lV) (HX04h nH20 where M(IV) = Zr, Ti , Th, Ce, Sn, etc and X = P, Mo, W, As, Sb, etc. These materials can be obtained i n both crystall ine '2 and amorphous IJ forms. They behave as cation exchangers due to the presence of exchangeable protons contained in the structural hydroxyl groups. These materials are granular in nature and can be obtained in a range of mesh sizes. Besides, these compounds offer great promise i n preparative reproducibil i ty and stabili ty both chemical and thermal. The presence of acid sites on such materials i nd icates good potential for application i n Bronsted catalysis.

In the present endeavour, amorphous inorganic ion exchangers of the class of tma salts have been synthesized, namely the tungstates of tetravalent tin, titanium and zirconium abbreviated as SnW, TiW and ZrW respectively. The catalysts have been characterized for elemental analysis, thermogravimetric analysis (TGA), X-ray, spectral analysis (FfIR) and surface area measurement (BET method). The Na+

exchange capacity has been determined and effect of heating on ion exchange capacity studied. The catalytic behaviour (Bronsted catalysis) of the synthesized materials has been explored by synthesizing dioctyl phthalate, an i mportant plasticizer.

Experimental Procedure

Synthesis of catalysts (Sn W, TiW and ZrW) All three catalysts, SnW, TiW and ZrW were

prepared by adding dropwise, aqueous solution of sodium tungstate (0.2 M , 1 00 mL) to aqueous solution of metal salt (0. 1 M, 1 00 mL) with continuous stirring. The pH of the resulting solution, along with

Parikh & Chudasama : Synthesis of dioctyl phthalate (DOP) using metal(IV) tungstates as catalysts Articles

the gel obtained was maintained at - 2 pH. The solution along with gel was allowed to stand for at least 24 h, then filtered, washed with conductivity water till free of chloride ions and dried at room temperature. The dried material was brought to the desired particle size (60- 1 00 mesh) and sorted by sieving. The materials was finally converted to the acid form by immersing in 1 M HN03, the acid being intermittently replaced, washed with conductivity water till free from acid and again dried at room temperature.

Synthesis of DOP A three necked round bottom flask (RBF) of

1 00mL capacity was used. A stirring rod containing teflon cork and teflon blade which was driven mechanically was fitted to the middle neck of the RBF. Dean and Stark apparatus with a condenser was also fitted to one of the side neck of the RBF in order to remove water formed during the reaction and drive the reaction towards the product side. Nitrogen was bubbled to create an inert atmosphere and prevent colouration of DOP. The temperature of the system was controlled by means of thermostatic heating mantle in which the above set up was placed.

The monoester was first prepared by taking phthalic anhydride (S.92g, 0.04 mol) and 2-ethyl hexanol (7.S mL, 0.048 mol) together in the reactor and stirring the mixture at l 70°C for about IO- I S min. The anhydride gets completely converted to the monoester, so that the acid concentration at this stage can be taken as the initial concentration. Further, the diester formation starts only after the addition of a second mole of alcohol (7.S mL, 0.048 mol + 2.S mL excess) and required amount of the catalyst. The mixture was refluxed for Sh, during which, the acid concentration kept on decreasing. The reaction could, therefore, be monitored, on the basis of number of acid groups esterified, by titrating the reaction mixture against alcoholic KOH, using universal indicator. The yields of DOP obtained in every case was calculated according to the acid value. In one case each (0.2Sg catalyst) of the product was isolated and analysed by GC and the yields were found comparable to that obtained by titration method as acid value. In all cases, the reactions were carried out by varying the amount of the catalyst.

Results and Discussion Chemical analysis of SnW, TiW and ZrW indicates

the metal(lV) to tungstate ratio to be 1 :2. TGA of SnW, TiW and ZrW indicates a gradual weight loss

within the temperature range of 1 ()()" 1 80°C, corresponding to the loss of external water molecules, after which a gradual weight loss is further observed till 600°C. This may be attributed to the condensation of structural hydroxyl groups.

FTIR spectra of Sn W, TiW and Zrw show a broad band in the region - 3400 cm- I attributed 10 asymmetric and symmetric hydroxo-OH and aqua­OH stretches. A sharp medium band at - 1 620 cm- I is attributed to aquo (H-O-H) bending. The bands arc indicative of presence of external water molecules and structural hydroxyl groups in the material.

XRD spectra of SnW, TiW and Zrw do not exhibit peaks, which is indicative of the noncrystalline nature of the materials.

Surface area of Sn W, TiW a'ld Zrw have been found to be 1 3.04, 1 4.00, and 1 09.00 m'2/g respectively.

The Na+ exchange capacities (icc) for SnW, TiW and ZrW were found to be I .S2, I .9S, and 1 .32 meq/g respectively. The effect of heating on iec reveals that iec decreases as the temperature of heating increases in all the three cases.

For synthesis of DOP, the reaction pathway is as exhibited in Fig. I . The esterification of phthalic anhydride with 2-ethyl hexanol involves two steps. The first reaction giving the monoester is very fa<;t and is completed in a short time without any catalyst. The diester formation in the second stage of the reaction takes place only after the addition of second mole of alcohol and catalyst, where the catalyst ha<; a significant influence. Since this is an equilibrium reaction, it is essential to remove the water formed during the course of the reaction continuously to drive the reaction towards the product side (diester). Water forms an azeotrope with 2-ethyl hexanol which can be distilled off. % yield of DOP with varying amount of catalysts is presented in Table 1 .

As observed from Table I , the yield of DOP increases with increasing amount of the catalysts. The yields of DOP is higher in case of Sn W as compared to TiW and ZrW. Also in case of TiW and ZrW. there is no marked change in the yields of DOP with increa'iing amount of the catalyst.

In all the cases, the catalysts were regenerated by washing it several times with water and finally treating it with 1 M HNOJ to bring it to the acid form (as per the method suggested in experimental section)_ It is observed that in the regenerated catalysts, the yield of esters obtained decreases by -5%.

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Articles Indian J. Chern. Techno\., January 2002

Reaction pathway First step

1 7� ..

10-15 min

Phthalic anhydride monoester

Second step

+

monoester

1 700c j Solid Acid Catalyst 5h M(IV) tungstates

M(IV) = Sn. Ti. Zr

DioctyI phthalate

Table I - % Yield of DOP varying amount of catalyst

% Yield of DOP Amount of SnW TiW ZrW catalyst (g)

0.25 89.27 90.02 90.87 0.50 93. 1 3 9 1 .09 92.49 0.75 94. 1 0 92.80 92.8 1

During the course of the reaction. the colour of the catalyst changes from white to brown in case of Sn W, from yellow to greyish yellow in case of TiW and from white to yellow in case of ZXW. This gives an indication that during the course of the reaction, the reacting molecules come onto the surface of the catalyst. Most of

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them enter into reaction to give the diester, but a few of them get adsorbed on the surface which is marked by change in the colour of the catalyst. The fact that the adsorption is weak. can be concluded on the basis of the fact that the catalyst almost regains its original colour and form when treated with acid (during regeneration). The possibility of molecules entering interstices cannot be ruled out. This is observed from the fact that the yields go down after regeneration.

Conclusion The esterification of phthalic anhydride with 2-

ethyl hexanol to form dioctyl phthalate IS efficiently catalysed by SnW, TiW and ZrW.

Parikh & Chudasama : Synthesis of dioctyl phthalate (DOP) using meta1(IV) tungstates as catalysts Articles

The reaction reaches completion at a relatively moderate temperature ( 1 70°C) and completed within a short time (5h). Further, good yields of the diester, DOP is obtained i n all the three cases. The study establishes the use of M(lV) tungstates as promising solid acid catalysts i n the synthesis of DOP.

Acknowledgements Thanks are due to the Head, Department of

Applied Chemistry, Faculty of Technology & Engineering, M.S. University of Baroda, Vadodara, for providing necessary laboratory facilities.

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