Titanium-catalyzed transesterification

Figure 2.7 Mechanism for the titanium-catalyzed transesterification in PET, as proposed by Weingart [39]...

The use of supported metal complexes in transesterification reactions of TGs is not new. An earlier patent claimed that supported metals in a hydroxylated solid could effectively catalyze transesterification. The catalyst preparation used an inert hydrocarbon solvent to attach transition metal alkoxide species to the support surface. The reaction, however, was carried out in the presence of water. The author claimed that water was essential in preparing materials with good catalytic activity. Among the metals employed, titanium catalysts showed the best activity. However, it was not clear from the preparation method if reproducibility could be easily achieved, an important requirement if such catalysts were to be commercially exploited. 

The following transformations demonstrate the characteristie feature of titanium alk-oxide-catalyzed transesterification. The isolated double bond does not enter into conjugation with the active methylene moiety (Eq. 218) [515]. Removal of a sterically demanding chiral auxiliary was possible without affecting the ketoester moiety (Eq. 219) [516,517]. Preparation of an allyl ester was achieved in good yield with retention of the nitrone moiety essential for subsequent cycloaddition (Eq. 220) [518]. 

The d-block or transition metal alkoxides make the third category. Titanium alkoxides (methoxide, ethoxide, propoxide, isopropoxide, butoxide and isobutoxide) have been able to catalyze transesterification reaction of fatty acids (reaction 7.13) [64]. The results of Nawaratna et aV.s researches showed that the catalytic behavior of titanium alkoxides in transesterification of fatty acids depended on their steric effects meaningfully and lower steric hindrance increased the yield and selectivity of the produced ester [64]. 

Biodiesel is a mixture of methyl esters of fatty acids and is produced from vegetable oils by transesterification with methanol (Fig. 10.1). For every three moles of methyl esters one mole of glycerol is produced as a by-product, which is roughly 10 wt.% of the total product. Transesterification is usually catalyzed with base catalysts but there are also processes with acid catalysts. The base catalysts are the hydroxides and alkoxides of alkaline and alkaline earth metals. The acid catalysts are hydrochloride, sulfuric or sulfonic acid. Some metal-based catalysts can also be exploited, such as titanium alcoholates or oxides of tin, magnesium and zinc. All these catalyst acts as homogeneous catalysts and need to be removed from the product [16, 17]. The advantages of biodiesel as fuel are transportability, heat content (80% of diesel fuel), ready availability and renewability. The... 

In the case of the esterification of the diacid, the reaction is self-catalyzed as the terephthalic acid acts as its own acid catalyst. The reverse reaction, the formation of TPA and EG from BHET is catalytic with regard to the usual metal oxides used to make PET, but is enhanced by either the presence of hydroxyl groups or protons. In the case of transesterification of dimethyl terephthalate with ethylene glycol, the reaction is catalytic, with a metal oxide needed to bring the reaction rate to commercial potential. The catalysts used to produce BHET are the same as those needed to depolymerize both the polymer to BHET and BHET to its simpler esters. Typically, titanium, manganese and zinc oxides are used for catalysts. 

However the T s did not correspond to those of pure components and in addition, there was a slight depression in the melting point of PBT. A melt-phase reaction was hypothesized to take place. Subsequent smdies showed that the melt phase transesterification reaction between PBT and PC can indeed take place, at these high temperatures (T > 260°C), as followed by NMR spectral changes as a function of melt residence time [Deveux et al., 1982]. The presence of titanium catalyst normally present in PBT also catalyzed this reaction. 

The transesterification reaction is typically carried out in a CSTR by continuously feeding a paste of premixed DMT/diol into a diol-ester prefllled reactor operating between 160 and 240 °C. Typically DMT/diol molar ratios of 1 2 to 1 2.2 are used for PET and 1 1.1 to 1 1.5 for PBT. The reaction for PET is catalyzed by a metal acetate. Zinc acetate is commonly used, but antimony, barium, calcium, and magnesium acetates may also be used. For PBT titanium orthoesters, such as tetra-butyl titanate, are employed as catalysts. The CSTR operates at low pressures, 0.1 to... 

Ti (Zr), Si = tetravalent titanium, zirconium, or silicon. The Ti—O (or Zr—O) bond is capable of disassociation allowing transesterification, transalkylation, and other catalyzed reactions such as repolymerization, while the Si—C bond is more stable and thus umeactive,... 

Fig. 2.3 Scheme illustrating the polycondensation through transesterification catalyzed by titanium isopropoxide... 

The synthesis of tricyclic compound 120 was first reported by Tamura and co-workers using a tandem transesterification-cycloaddition reaction of nitrone methyl ester 118 and cyclohexen-3-ol (Scheme 16.17). This reaction was catalyzed by titanium tetrachloride to make nitrone cyclohexenyl ester 119. A subsequent 1,3-dipolar cycloaddition reaction proceeded presumably through a transition state involving the i-cis-like conformation of the ester C—O bond and the E configuration of the nitrone C=N bond (Scheme 16.17). 

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