Acid catalysis transesterification

The traditional catalyst used for esterification of acids to methyl esters is sulfuric acid. Homogeneous sulfuric acid catalysis has many downsides. When using sulfuric acid, much capital expense is required for Hastalloy and/or other specialty metals of construction. Homogeneous catalysis results in the contamination of the product by sulfur containing species. Therefore, neutralization and removal of acid is required to meet biodiesel specifications and to protect the downstream transesterification reactor. Inevitably, when using sulfuric acid, organic sulfur compounds will be produced. These products will cause the resultant biodiesel to fail specification tests. 

Perhaps, a more important reason for the little research in this particular area is the slow reaction rate associated with acid catalysis in general. However, the ability of solid acids to catalyze both esterification and transesterification reactions simultaneously and the possibility for employing catalysts that are reusable and green, meaning that they do not pose a great environmental threat, are attractive aspects that make the study of these materials imperative. 

Because of the difficulty encountered in acetylation of the complexed alcohol, it was of interest to see if the ester complex behaves in a normal fashion. Refluxing (HaO) [Cr(AcO-A)2] in methanol or ethanol caused methyl or ethyl acetate to be formed, while refluxing in ethyl propionate formed ethyl acetate. When the potassium salt was used in place of the oxonium salt no transesterification was observed this could be due to the necessity of acid catalysis or a difference in solubility in these essentially heterogeneous systems. The oxonium salt, (H30) [Cr(AcO-A)2], appears to have typical ester reactivity. 

Alcohols can also be prepared from support-bound carbon nucleophiles and carbonyl compounds (Table 7.4). Few examples have been reported of the a-alkylation of resin-bound esters with aldehydes or ketones. This reaction is complicated by the thermal instability of some ester enolates, which can undergo elimination of alkoxide to yield ketenes. Traces of water or alcohols can, furthermore, lead to saponification or transesterification and release of the substrate into solution. Less prone to base-induced cleavage are support-bound imides (Entry 2, Table 7.4 see also Entry 3, Table 13.8 [42]). Alternatively, support-bound thiol esters can be converted into stable silyl ketene acetals, which react with aldehydes under Lewis-acid catalysis (Entries 3 and 4, Table 7.4). 

The transesterification reaction is usually conducted with alkali catalysts (sodium or potassium hydroxide or methoxide). Alkali catalysis is much more rapid than acid catalysis in the transesterification reaction (Canakci Van Gerpen, 1999 Freedman... 

For these last reactions, acid catalysis gives a better conversion rate than basic catalysis, which promotes transesterification and the formation of side products. The reaction of epoxidized rapeseed methyl esters with heptanol and PTSA (100°C, 12 h, 1 bar) led to fatty ethers with an oxirane value of 0.0 and a saponification number of 130 methyl heptyloxy-hydroxystearate was the main reaction product. 

Show how acid catalysis is used to synthesize acid derivatives, as in the Fischer esterification and in transesterification. Propose mechanisms for these reactions. 

Fatty acids. Fatty acids can be ethoxylated in what amounts to a two-step process where the first mole of EO adds slowly to the dry, precatalyzed acid to yield the hydroxyethyl ester. Because of the presence of carboxylic acid, the reaction runs slowly under general acid catalysis and does not produce substantial quantities of polyethoxylated product. Once the free carboxylic acids are completely capped, the pH of the system becomes alkaline and the reaction proceeds in a comparable manner to any primary alcohol ethoxylation. Some competing transesterification occurs at temperatures above 120°C, leading to a product distribution of PEG monoesters, diesters, and free PEG. 

An alcoholysis reaction, in which an ester reacts with an alcohol to form a new ester and a new alcohol, is called a transesterification reaction. An example of this alcoholysis reaction is given in Figure 19.5. Transesteiification reactions are slow reactions because alcohols are poor nucleophiles and esters have very basic leaving groups hence, they occur under support of acid catalysis (Bmice, 2004). Similar to esterifications, transesteiification reactions are equihbiium reactions (Hoydonckx, Vos, Chavan, Jacobs, 2004) hence, an excess of alcohol and a direct extraction of products is needed to shift the equilibrium. 

Transesterification of triglycerides can be achieved via either acid catalysis or base catalysis to produce biodiesel. 

Acid-catalyzed transesterification this transformation also works with base catalysis R 0... 

Acid Catalysis Add catalysis transesterification includes the combination of three reversible reactions (Fig. 22.4). The high conversion of the acid catalyzed transesterification procedure is due to the capacity to transesterify fatty acids and fatty acid salts present in the system. The acids employed are HCl, H SO, BFj, and sulfonic acids [15, 22-24]. Generally, acid catdysis is many times slower than basic catalysis. The rate of the biodiesel production reaction... 

FIGURE 22.4 Mechanism of transesterification according to acid catalysis. 

In the case of acid catalysis or transesterification, it is possible to push the reaction to the right by removal of water (azeotropic distillation) or the volatile methanol. Alternatively, preparation of the tert-butyl ester confers resistance to alkali, but allows removal of the protecting group under mildly acidic conditions. The ester, which for steric reasons cannot be prepared from fert-butanol, may be synthesized from isobutylene gas in the presence of acid, or by transesterification with ferf-butyl acetate. 

Methylphenol is converted to 6-/ f2 -butyl-2-methylphenol [2219-82-1] by alkylation with isobutylene under aluminum catalysis. A number of phenoHc anti-oxidants used to stabilize mbber and plastics against thermal oxidative degradation are based on this compound. The condensation of 6-/ f2 -butyl-2-methylphenol with formaldehyde yields 4,4 -methylenebis(2-methyl-6-/ f2 butylphenol) [96-65-17, reaction with sulfur dichloride yields 4,4 -thiobis(2-methyl-6-/ f2 butylphenol) [96-66-2] and reaction with methyl acrylate under base catalysis yields the corresponding hydrocinnamate. Transesterification of the hydrocinnamate with triethylene glycol yields triethylene glycol-bis[3-(3-/ f2 -butyl-5-methyl-4-hydroxyphenyl)propionate] [36443-68-2] (39). 2-Methylphenol is also a component of cresyHc acids, blends of phenol, cresols, and xylenols. CresyHc acids are used as solvents in a number of coating appHcations (see Table 3). 

Generally, the above transesterification reactions are catalyzed by strong acids or alkalis [1, 2]. In the homogeneous catalytic process by acids or alkalis, neutralization is required of the product. This post-treatment produces waste water, and increases equipment investment and production cost. Recently, more attention has been paid to the heterogeneous catalysis process [3] for an easier production process and to reduce pollution of the environment. 

Optically active 3-hydroxybutanoic acid and its methyl ester were first prepared by McKenzie, Magnus-Levy, and Emil Fischer.3 The biopolymer PHB and mixed polymers containing (R)-3-hydroxybutanoate and (R)-3-hydroxypentanoate were also discovered long ago,4 5 and are now produced on an industrial scale. .7 As described here, depolymerization by transesterification [H+ or Ti(OR)4 catalysis], or by hydrolysis, produces8-9 the corresponding monomeric (R)-esters and (R)-acids 1. The 3-hydroxybutanoic acid can also be prepared by hydrolysis of the ester.2-10... 

Fatty acid esters of sugars are also very important biodegradable and biocompatible surfactants that are prepared either by transesterification of methyl ester with sugar on basic catalysts or by esterification of fatty acids with sugar on acidic catalysts. Liquid acids and bases have been replaced by enzymatic catalysis with lipase, giving a higher yield of monoester [43, 44], but solid catalysts have not been used extensively so far. 

One of the best examples of the utility of enzymatic synthesis in catalyzing reactions that cannot be accomplished by any other route is the synthesis of substituted oxazolidine diesters. The oxazolidine ring is extremely water sensitive, the oxazolidine rapidly reverting back to the alkanolamine and aldehyde in the presence of water. Bis-oxazolidines have been used as hardeners for polymer coatings but the diester based on the hydroxyethyl oxazolidine and adipic acid cannot be synthesized directly with chemical catalysis because of the rapid rate of reaction of the oxazolidine ring with either the water from the esterification or the alcohol from transesterification. ... 

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