Catalysts in transesterification

At low temperatures, the activity of acid catalysts in transesterification is normally fairly low and to obtain a sufficient reaction rate it is necessary to increase the reaction temperature to >170 °C. Therefore, sulfonic acid resins can be used in esterification reactions where they perform well at temperatures <120 °C and particularly in the pretreatment of acidic oils. Under these reaction conditions, acidic resins are stable. Poly(styrenesulfonic add), for example, has been used in the esterification of a by-product of a vegetable oil refinery with a 38.1 wt% acidity at 90-120 ° C and 3-6 atm. It was not deactivated after the first batch and maintained a steady catalytic performance in the next seven batches [22]. 

Dialkyltin dicarboxylates, e g. Bu2Sn(02CCu 1123)2, are useful catalysts for polyurethane foam formation and as cross-linking agents for room-temperature vulcanization. Monobutyltins have been used as catalysts in transesterification and esterification reactions, e.g. in the reaction between phthalic anhydride and 2-ethylhexanol. 

H. Deleuze, X. Schultze, D. C. Sherrington, Reactivity of some polymer-supported titanium catalysts in transesterification and epoxidation reactions, /. Mol. Catal. A 159 (2000) 257. 

Methyl esters can be obtained directly from triglycerides by transesterification with methanol traditional catalysts in transesterification to form methyl esters have been acids (e.g., sulfuric) and sodium methoxide. However, if the sodium methoxide contains any sodium hydroxide, some of the triglyceride will be hydrolyzed to the carboxylate salt, which cannot be esterified. On this account, we/o-trifluoromethyl-phenyltrimethylammonium hydroxide (w-CF3-QH4NMe3 OH ) was offered as a transesterification catalyst [208] methanolysis is reported to occur rapidly at room temperature. 

The metal alkoxides affect substantially these reactions. Thus Ti(OPr )4 and OV(OPr )3, cocondensates act as catalysts in transesterification of tetramethoxy- and tetraethoxysilanes (TMOS and TEOS, respectively) during sol-gel nanocomposite preparation. ... 

Alkali catalysts are commonly used in transesterification reactions because of their relatively low cost and ease of handling (Atadashi et al., 2012). In addition, transesterification reactions can be performed at low temperatures and pressures with a very high conversion yield, reaching 98% in a short time (Fukuda et al., 2001). Sodium hydroxide (NaOH), potassium hydroxide (KOH), and sodium methoxide (CHjONa) are the most common homogeneous alkali catalysts employed (Demirbas, 2009a,b Helwani et al., 2009 Sharma et al., 2008). Many researchers have studied the use of these catalysts in transesterification reactions with oils of different FFAs content, ranging from 5 wt% to 15 wt%, as listed in Table 6.4. However, the use of... 

Thermo-oxidation of PET is catalysed by the metal ions (Zn, Mn etc.) used as catalysts in transesterification of dimethyl terephthalate. Compounds which form complexes with these metal ions inhibit their catalytic action in general and improve the stability of PET towards oxidation. These compounds are mainly esters of phosphoric acid, e.g. triphenyl phosphate (see Table 1). 

Enzymes can also be used as catalysts in transesterification. The enzymatic route can provide advantages such as elimination of organic solvents, chemical catalyst and waste water (Ranganathan 2008). On the other hand, it has several disadvantages such as high enzyme cost, low yield and longer reaction time. 

Albuquerque, M. C. G., Santamaria-Gonzalez, J., Merida-Robles, J. M., Moreno-Tost, R., Rodnguez-CasteUon, E., Jimenez-Lopez, A., Azevedo, D. C. S., Cavalcante Jr., C. L., and Maireles-Torres, P. MgM (M = A1 and Ca) oxides as basic catalysts in transesterification processes. Applied Catal A General 347,162-168 (2008). 

Stannous oxalate is used as an esterification and transesterification catalyst for the preparation of alkyds, esters, and polyesters (172,173). In esterification reactions, it limits the undeskable side reactions responsible for the degradation of esters at preparation temperatures. The U.S. Bureau of Mines conducted research on the use of stannous oxalate as a catalyst in the hydrogenation of coal (174) (see Coal). 

Since the catalytical behaviour of titanium derivatives depends on their degree of condensation, these catalysts are used under conditions where the water concentration is very low, for instance in transesterifications or in the last steps of esterifications. However, the amounts of water required to hydrolyse these compounds are so low (less than 0.5 ppm for Ti(OBu)4) that hydrolysis is probably the determining phenomenon in most studies. It seems that before Fradet and Marshal230 the contribution of this side effect has not been taken into consideration. 

Unactivated esters, typically alkyl esters, often show low reactivity toward lipase catalyst for transesterifications. In the case of the lipase-catalyzed polycondensation of dialkyl esters with glycols, the polymer of high molecular weight was not obtained. The molecular weight improved when vacuum conditions were used Mw reached more than 2 x 104 in the combination of diethyl sebacate and 1,4-butanediol catalyzed by lipase MM [30]. 

The transesterification of sucrose has been performed with a fatty acid ester of a volatile alcohol in the presence of an alkaline catalyst in a dipolar, aprotic solvent.142 The reaction of sucrose (293 mmoles) with methyl dodecanoate (293 mmoles) in A/,N-dimethylformamide in the presence of sodium methoxide in a pressure bomb for 8 h at 130° gave, after solvent extraction and crystallization, sucrose mono(dodecanoate) (m.p. 72-80° [a]D+52°) in 50% yield.142 Commercialization of these sucrose esters has so far been limited, in part because of the use of expensive solvents, and, in part, because solvent remaining in the product makes it unsuitable for use as a food emulsifier. In view of this situation, methods have been developed in which the use of toxic and expensive solvents has been avoided. 

Yoda [28] investigated the activity of 20 catalysts in the transesterification reaction of PET and poly(ethylene isophthalate) (PEI) and found the same order of reactivity as for the transesterification of DMT with EG. The most effective catalysts were the acetates of Zn, Pb(n) and Hg(n), together with Co(m) acety-lacetonate and Sb203. Titanium catalysts were not included in Yoda s study, but are known to be effective catalysts in PET blending [46],... 

The SSP plant for repelletized PET is similar to a virgin SSP plant. It is usually smaller, because of smaller feed stock availability, and should provide the flexibility to adapt to changing product requirements. Another difference results from the IV increase rate of recycled PET, which tends to be lower. This is attributed to a lower activity of the transesterification catalyst in recycled PET. 

As shown in previous sections, NHCs promote acyl transfer in transesterification reactions. In a similar manner, O C acyl transfer can be achieved with substrates such as 351 in the presence of 0.9 mol% of triazolium pre-catalyst 353 and KHMDS (Scheme 53). Moderate yields are obtained by varying substitution of the oxazole from R = Me, Ph, t-Bu, and t-Pr [171], Deprotonation of the triazolium salt followed by nucleophilic addition to the carbonate moiety of the oxazole results in enolate intermediate LXXXIII and activated carboxylate LXXXIV. Enolate addition and regeneration of the active catalyst provides quaternary stereocenters 352. 

Tranesterification involves the conversion of one ester into another. In this process, a less volatile alcohol replaces a more volatile alcohol. For example, heating an excess of ethanol with a methyl ester while rapidly removing the more volatile methanol as it forms results in transesterification. An acid catalyst facilitates the reaction, which is illustrated in Figure 12-24. To produce a propyl ester, the action of propanol on either a methyl ester or an ethyl ester would work. 

Basic catalysts are often employed in transesterification reactions probably to increase the nucleophilicity of the alcohol through formation of secondary bonding to the alcohol-proton, resulting in the alcohol being more nucleophilic (4.44) ... 

Many types of solid catalysts have been tested in esterification and transesterification reactions of fatty acids, TG feedstock and simple esters. Nonetheless, it is possible to group most catalysts in three general categories metal, base, and acid catalysts. The following sections deal with these three groups accordingly. 

Many catalysts do not use metals in their pure reduced metallic forms. Anchored organometallic complexes are often analogs of homogenous catalysts fixed on a solid support. In particular, titanate complexes both in solution and in supported form have been found to be especially active in transesterifications of simple esters.It was proposed that titanates catalyze the transesterification reaction through a Lewis acid mechanism where the reactant ester and metal form a Lewis complex activating the carbonyl groups for a nucleophilic attack by the reactant alcohol. The tetrahedral intermediate that is formed breaks down into the product alcohol and an ester-metal Lewis... 

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. 

Alkaline earth metal oxides and hydroxides have also been tested in transesterification reactions. Ca(OH)2 did not show significant catalytic activity in the transesterification of rapeseed oil with methanol at conditions normally used to prepare biodiesel.Peterson et al. reported relative alcoholysis activities of a series of supported CaO catalysts under near reflux conditions of methanol-rapeseed oil mixtures at 6 1 molar ratios.Among the catalysts tested, the most active was CaO (9.2 wt% CaO) on MgO. For instance, in a 12 h reaction the total oil conversion using this catalyst was over 95%, similar to... 

Transesterification Reactions. The heterogeneous acid-catalyzed transesterification of TGs has not been investigated as much as its counterpart, the base-catalyzed reaction. Various solids are available with sufficient acid strength to be effective catalysts for the named reaction. Among the solid acids available are functionalized polymers, such as the acid forms of some resins, as well as inorganic materials, such as zeolites, modified oxides, clays, and others. Some of these solids have already been found to be effective in transesterification reactions of simple esters and (3-ketoesters. 

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