Transesterification is an important process in the exchange of organic residues in several industrial processes. It is for example used in the large scale synthesis of polyesters. In this application diesters undergo transesterification with diols to form macromolecules. Another example is in the production of biodiesel (fatty acid methyl ester, FAME) through transesterification of vegetable oils or animal fats with short-chain aliphatic alcohols (typically methanol or ethanol). But also in other industrial processes such as (i) intramolecular transesterications leading to lactones and macrocycles, (ii) production of (intermediates of) specific active pharmaceutical ingredients (API's), (iii) production of polylactic acid (PLA) from lactide, (iv) co-synthesis of dimethyl carbonate and ethylene glycol from ethylene carbonate and methanol, transesterification is a crucial step.
Transesterification reactions normally are carried out in the presence of a catalyst, including among others mineral acids, metal hydroxides, metal oxides, metal alkoxides (aluminum isopropoxide, tetraalkoxytitanium, organotin alkoxides etc.), non-ionic bases (amines, dimethylaminopyridine, guanidines etc.) and lipase enzymes. (J. Otera and J. Nishikido, Esterification, p. 52-99, Wiley 2010). Activity of these conventional catalysts however is possibly hampered in the presence of unsaturated bonds, amines, additional hydroxy groups or other functional groups in the respective ester or alcohol reactants. A strong mineral acid such as sulfuric acid or methane sulfonic acid, for example, usually leads to slow reaction rates and the resulting transester product is typically accompanied by the formation of high concentrations of side-products. The latter usually result from dehydration of the alcohols to yield olefins and ethers which eventually contaminate the product. In case of acrylic esters, also Michael-addition products (addition of alcohol to C═C double bond) and substantial amounts of polymeric products are found in the final reaction mixture.
Similar to acid catalysts, alkali metal alkoxide catalysts (for example, sodium methoxide or potassium tert-butoxide) promote undesirable side reactions and, moreover, they are deactivated by the presence of water in the reaction solution. Therefore, catalyst should by continuously added to the reaction mixture, while it must subsequently be removed to avoid alkoxide-promoted polymerization or degradation during distillation or other thermal treatment of the products, especially if the products are unsaturated esters such as acrylic esters.
Titanium (Ti) and tin alkoxides generally have higher selectivity but suffer from specific drawbacks. Titanate catalysts are particularly sensitive to water (generally losing activity in mixtures containing greater than 500 ppm water), thus necessitating the same need to add more catalyst to the reaction. In addition, Ti compounds can lead to undesired discoloration (yellowing) during storage of the resulting product, which is caused by factors including the presence of Ti(III) compounds in addition to Ti(IV) compounds and/or by the tendency of titanium to form complexes. It is also recognized that tin compounds are potential carcinogens to humans, hence their presence in the final products is undesired. Rigorous removal is thus essential and residues should be efficiently disposed of.
Because of these problems with conventional catalysts, the need exists for an improved transesterification catalyst of high activity and selectivity in presence of other functional groups and with reduced sensitivity to water.
Previous steps toward meeting this need have been undertaken in the art by using metal acetylacetonates catalysts for the production of a variety of ester compounds. Examples of esters thus prepared include (meth)acrylic esters (U.S. Pat. No. 4,202,990, U.S. Pat. No. 7,071,351, US2004/0249191) or more specific allyl methacrylate (WO2009003746), prenyl(meth)acrylates esters (DE102008043810), ethylthioethanyl methacrylates (FR2707290). Also the production of aliphatic oligocarbonate polyols (U.S. Pat. No. 7,060,778, US2006/0052572, U.S. Pat. No. 6,350,895), alpha-ketocarboxylic esters (U.S. Pat. No. 6,222,063), wax monomers (U.S. Pat. No. 5,856,611), bis(3-hydroxypropyl)terephthalate monomer (U.S. Pat. No. 5,840,957), and polyethylene terephthalate resin (U.S. Pat. No. 3,528,946, U.S. Pat. No. 3,528,945) have been described.
The preferred metal is predominantly zirconium (U.S. Pat. No. 4,202,990, WO2009003746, U.S. Pat. No. 7,071,351, US2004/0249191, U.S. Pat. No. 5,856,611, FR2707290), but also other metal such as ytterbium(III) (U.S. Pat. No. 7,060,778, US2005/006539), yttrium/samarium compounds (U.S. Pat. No. 6,350,895), lanthanum (U.S. Pat. No. 5,840,957, EP1064247), hafnium (IV) (U.S. Pat. No. 5,037,978), cerium and lead (U.S. Pat. No. 3,532,671) have been described.
The use of Zn (II) or Fe (III) acetylacetonates is only occasionally mentioned. An example is the method for producing 2-methyl-2-hydroxy-1-propyl(meth)acrylate by reacting a (meth)acrylate with 2-methyl-2-hydroxy-1-propyl alcohol in the presence of a Zn-acetylacetonate catalyst (JP2005/132790). The yield was high (95%) while other frequently used catalysts such as tetra isopropoxy titanate gave lower yield (57%). Other examples include the manufacturing of dialkylaminoalkyl(meth)acrylates (JP 02017155) and the preparation of higher alkyl(meth)acrcylate esters starting from the lower alkyl esters (JP 53105417, EP 236994). Fe(III) acetylacetonate is described in synthesis of biodegradable glycolide/L-lactide copolymers (Polymer (2002), 43(9), 2595-2601)
We have found that the combination of a Zn or Fe 1,3-dicarbonyl complex and an inorganic salt shows an unexpectedly high activity. Thus, it is possible to achieve a higher conversion rate of transesterified ester products of a lower alkyl ester with an appropriate alcohol in the presence of mixture of salts consisting of a metal 1,3-dicarbonyl complex, in particular a Zn or Fe 1,3-dicarbonyl complex, more in particular a Zn (II) or Fe (III) 1,3-dicarbonyl complex and an inorganic salt.