Transesterification is a crucial step in several industrial processes such as (i) production of higher acrylates from methylmethacrylate (for applications in resins and paints), (ii) polyethylene terephthalate (PET) from dimethyl terephthalate (DMT) and ethylene glycol (in polyester manufacturing), (iii) intramolecular transesterifications leading to lactones and macrocycles, (iv) alkoxy esters (biodiesel) from vegetable oils, and (v) co-synthesis of dimethyl carbonate (an alkylating agent, octane booster and precursor for polycarbonates) and ethylene glycol from ethylene carbonate and methanol (U. Schuchardt et al., J. Braz. Chem. Soc. 9 (1998) 199). Acids and bases are known to accelerate the rate of transesterifications (J. Otera Chem. Rev. 93 (1993) 1449). Other than mineral acids and bases, compounds like metal alkoxides (aluminum isopropoxide, tetraalkoxytitanium, (RO)Cu(PPh.sub.3).sub.n, PdMe(OCHCF.sub.3Ph(dpe)), organotin alkoxides etc.), non-ionic bases (amines, dimethylaminopyridine, guanidines etc.) and lipase enzymes also catalyze these transformations (J. Otera Chem. Rev. 93 (1993) 1449).
Alkaline metal alkoxides (as CH.sub.3ONa for the methanolysis) are the most active catalysts, since they give very high yields (>98%) of fatty acid alkyl esters in transesterification of triglycerides with alcohols in short reaction times (30 mm) even if they are applied at low molar concentrations (0.5 mol %) (J. Food Composition and Analysis Year 2000, Vol. 13, pages 337-343). However, they require high quality oil and the absence of water, which makes them inappropriate for typical industrial processes (J. Braz. Chem. Soc. Vol. 9, No. 1, Year 1998, pages 199-210). Alkaline metal hydroxides (NaOH and KOH) are cheaper than metal alkoxides but require increasing catalyst concentration (1-2 mol %). NaOH is more superior to KOH as the latter and other alkali hydroxides yield more saponified products than the biodiesel.
Recently, enzymatic transesterification using lipase has become more attractive for transesterification of triglycerides, since the glycerol produced as a by-product can easily be recovered and the purification of fatty acid esters is simple to accomplish. (J. Mol. Catal. B: Enzymatic Vol. 17, Year 2002, pages 133-142).
Use of immobilized lipases in the synthesis of fatty acid methyl esters from sunflower and soybean oils were reported by Soumanou and Bornscheuer and Watanabe et al (Enzy. Microbiol. Tech. Vol. 33, Year 2003, page 97; J. Mol. Catal. B: Enzymatic Vol. 17, Year 202, pages 151-155). They found that the immobilized enzyme is active at least for 120 h during five batch runs without significant loss of activity. Among the various lipases investigated the enzyme from Pseudomonas fluorescens (Amano AK) exhibited the highest conversion of oil. Khare and Nakajima (Food Chem. Vol. 68, Year 2000, pages 153-157) also reported the use of immobilized lipase enzyme.
U.S. Pat. No. 5,713,965 describes the production of biodiesel, lubricants and fuel and lubricant additives by transesterification of triglycerides with short chain alcohols in the presence of an organic solvent such as an alkane, arene, chlorinated solvent, or petroleum ether using Mucor miehei or Candida Antarctica-derived lipase catalyst. Patents Nos. WO 00/05327 A1, WO 02/28811 A1, WO 2004/048311 A1, WO 2005/021697 A1 and WO 2005/016560 A1 and U.S. Pat. Nos. 5,578,090; 6,855,838; 6,822,105; 6,768,015; 6,712,867; 6,642,399; 6,399,800; 6,398,707; 6,015,440 also teach us the production fatty acid alkyl esters using either lipase catalysts or metal ion catalysts. Patent No. WO 2004/085583 A1 describes transesterification of fats with methanol and ethanol in the presence of a solid acid catalyst having ultrastrong-acid properties in a short time at around ordinary pressure.
Replacement of homogeneous catalyst by a solid catalyst eliminates the processing costs. At the end of the reaction, the solid catalyst can be recovered by simple filtration from the product mixture and reused. Leclercq et al. (J. Am. Oil. Chem. Soc. Vol 78, Year 2001, page 1161) studied the transesterification of rapeseed oil in the presence of Cs-exchanged NaX and commercial hydrotalcite (KW2200) catalysts. At a high methanol to oil ratio of 275 and 22 h reaction time at methanol reflux, the Cs-exchanged NaX gave a conversion of 70% whereas 34% conversion was obtained over hydrotalcite. ETS-4 and ETS-10 catalysts gave conversions of 85.7% and 52.7%, respectively at 220° C. and 1.5 h reaction time (U.S. Pat. No. 5,508,457). Suppes et al (J. Am. Oil. Chem. Soc. Vol. 78, Year 2001, page 139) achieved a conversion of 78% at 240° C. and >95% at 160° C. using calcium carbonate rock as catalyst. Of late, Suppes et al reported the use of Na, K and Cs exchanged zeolite X, ETS-10, NaX occluded with NaOx and sodium azide in the transesterification of soybean oil with methanol (Appl. Catal. A: Gen. Vol. 257, Year 2004, page 213). Furuta et al (Catal. Commun. Vol. 5, Year 2004, pages 721-723) tell transesterification of soybean oil with methanol at 200-300° C. using solid superacid catalysts of sulfated tin and zirconium oxides. Use of tin complexes immobilized in ionic liquids for vegetable oil alcoholysis was reported by Abreu et al (J. Mol. Catal. A: Chem. Vol. 227, Year 2005, pages 263-267; J. Mol. Catal. A: Chem. Vol. 209, Year 2004, pages 29-33). Kim et al reported the use of heterogeneous base catalysts (Na/NaOH/Al2O3) for the methanolysis of vegetable oils. More efficient reusable solid catalyst for transesterifications is highly desirable.
The present invention eliminates the drawbacks of the prior-art processes. It deals with preparation of a transesterification catalyst which comprises reacting an aqueous ZnCl2 solution, an aqueous K4Fe(CN)6 solution and a tri-block copolymer poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (E020-PO70-EO20; molecular weight of about 5800) dissolved in tert.-butanol at ambient conditions, separating the solid catalyst and drying. Co-existence of Zn and Fe in the active site linking through cyano bridges makes it efficient for transesterification reactions. The catalyst could be separated easily by centrifugation or by simple filtration and reused. Unlike the prior-art catalysts no leaching of metal ions into the reaction mixture was observed with the catalysts of the present invention. Most importantly, the catalyst is highly efficient and only a small amount (1 wt % of oil) is needed to carryout the reaction at moderate conditions.