Catalysts used in esterifications

Inventory of the Main Catalysts Used in Esterifications and Polyesterifications 65... 

Inventory of the main catalysts used in esterifications and polyesterifications, Chap. 4. 

The main catalysts used in esterification or polyesterification are listed in Table 1. We reported each catalyst as it is defined in the corresponding reference, without any consideration about its behaviour and any assumption on the effective catalytic species. This will be discussed in Sect. 4. 

The solid superacid (SO42" /MxOy) is a new type of catalyst used in esterification (Jiang et al., 2004). MxOy are usually some transition metal oxides such as Zr02 and Ti02. 

Acidic Cation-Exchange Resins. Brmnsted acid catalytic activity is responsible for the successful use of acidic cation-exchange resins, which are also soHd acids. Cation-exchange catalysts are used in esterification, acetal synthesis, ester alcoholysis, acetal alcoholysis, alcohol dehydration, ester hydrolysis, and sucrose inversion. The soHd acid type permits simplified procedures when high boiling and viscous compounds are involved because the catalyst can be separated from the products by simple filtration. Unsaturated acids and alcohols that can polymerise in the presence of proton acids can thus be esterified directiy and without polymerisation. 

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. 

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]. 

The physical properties of most acids (esters) and alcohols allow the reaction to be carried out either in the liquid or in the vapour phase. In the liquid phase, the effects of solvents and of transport phenomena may play a more important role than in the vapour phase. On the other hand, the side reactions (mainly the ether and/or olefin formation from the alco- TABLE 20 Reactants and inorganic catalysts used in kinetic studies of esterification (transesterification) ... 

The reactants and inorganic catalysts used in kinetic studies of heterogeneous catalytic esterifications (transesterifications) are summarised in Table 20. As can be seen, no systematic comparative study with more than one catalyst (with the exception of paper [406]) has been performed by any one worker. The greatest attention was paid to silica gel [407— 411]. The reactants were usually low molecular weight acids and alcohols a typical pair of reactants is acetic acid—ethanol. Only in one study [126] was the structure of the reactants systematically varied in order to establish the effect on the reactivity. 

One of the greatest difficulties with direct esterification processes has been the tendency for the olefins to polymerize at the temperatures and in the presence of the catalysts used in the process. Energetic reagents such as anhydrous zinc chloride, ferric chloride, sulfur chloride, aluminum chloride or bromide such substances as fullers earth forms of energy as light, heat, silent electric discharge and the alkali or alkali earth metals are all known to affect greatly the polymerization of olefins. 

Polymeric counterparts of 4-dimethylaminopyridine have been prepared by several groups. Reilly Industries sells one as Reillex PolyDMAP (5.32a).141 When used in esterifications, the polymeric catalyst offers easy separation, reduced toxicity and the ability to use it in excess. It can be reused. The monomeric reagent is highly toxic. The two other polymeric analogues of 4-dimethylaminopyridine (5.33) are based on other chemistry.142 The first is a polyamide, the second, a polyurethane. In the second case, glycerol was also sometimes added to produce an insoluble product. 

The traditional batch process follows the route in Figure 2.8. The catalyst, used in catalytic proportions, can be washed out at the end of the process. The excess methyl ester and methanol formed as the esterification proceeds is distilled off as a mixture. Separation of this mixture is usually not straightforward and recycle of the excess methyl ester is therefore difficult and expensive. Byproduct formation, leading to undesirable impurities which have to be removed, is also likely as reagents and end products, in the presence of a catalyst, exist together for extended periods at elevated temperatures. 

Since the Bart s research [1] was limited to one selected catalyst, it seemed reasonable to widen the work to other catalyst used in industry. The goal of this work was to learn about the kinetics of esterification of levulinic acid with 2-ethylhexanol using different catalysts, based on wide range of experiments in a semibatch reactor. 

Table 1. Catalysts used in epoxy-carboxy esterifications and polyesterifications...

First of all, we collected the main techniques used in the kinetic studies of the reaction (reaction procedures, analytical methods, treatment of experimental data). In fact, unambiguous results can be obtained only if some basic conditions are observed. The survey of the literature shows a great diversity of the catalysts used in epoxy-carboxy esterifications or polyesterifications however, at least in the case of kinetic studies, tertiary amines and ammonium salts are largely predominant. From the fundamental studies carried out with organic models, and mostly in solution, several mechanisms were proposed involving the formation of a complex which can be cyclic or not. 

This chapter discussed esterification mechanisms, and evaluated the kinetics objectively and quantitatively, which provided a most effective way to select catalyst and design reactor for different esterification systems. It is discovered that some new catalysts (such as lipases, room temperature ionic liquids) have being used in esterification nevertheless, there are few research on the case. Herein, it is worthy to be investigated deeply. 

Esterification reactions proceed with or without a catalyst. In the absence of a catalyst, the reaction is, however, extremely slow, since its rate depends on the autoprotolysis of the carboxylic acid. Therefore, esterification is carried out in the presence of an acid catalyst, which acts as a proton donor to the carboxylic acid. Different catalysts used for esterification reactions are listed in Table 1.2. 

The conventional copolymerization pathways to PDMS and PET copolymers are paved with difficulties due to both physical incompatibility and chemical convertibility issues with regard to the catalysts and temperatures used for esterification and transesterification reactions [22]. In particular, the strong acids typically used in esterification or transesterification reactions will break the siloxane bonds Si-O-Si unless great care is taken. In order to address this problem, a facile enzymatic synthesis of silicone aromatic polyester (SAPE) and silicone aromatic polyamide (SAPA) in toluene under mild reaction conditions has been reported [26, 27], as shown in Schemes 2.4 and 2.5. 

It is used as a catalyst in esterification, dehydration, polymerization and alkylation reactions. Converted by e.g., ihionyl chloride, to melhanesulphonyl chloride (mesyl chloride) which is useful for characterizing alcohols, amines, etc. as melhanesulphonyl (mesyl) derivatives. 

In each case the configuration around the boron changes from trigonal planar to tetrahedral on adduct formation. Because of this ability to form additional compounds, boron trifluoride is an important catalyst and is used in many organic reactions, notably polymerisation, esterification, and Friedel-Crafts acylation and alkylations. 

Esters. Most acryhc acid is used in the form of its methyl, ethyl, and butyl esters. Specialty monomeric esters with a hydroxyl, amino, or other functional group are used to provide adhesion, latent cross-linking capabihty, or different solubihty characteristics. The principal routes to esters are direct esterification with alcohols in the presence of a strong acid catalyst such as sulfuric acid, a soluble sulfonic acid, or sulfonic acid resins addition to alkylene oxides to give hydroxyalkyl acryhc esters and addition to the double bond of olefins in the presence of strong acid catalyst (19,20) to give ethyl or secondary alkyl acrylates. 

Formic acid forms esters with primary, secondary, and tertiary alcohols. The high acidity of formic acid makes use of the usual mineral acid catalysts unnecessary in simple esterifications (17). Formic acid reacts with most amines to form formylamino compounds. With certain diamines imida2ole formation occurs, a reaction that has synthetic utiHty (18) ... 

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). 

Highly cross-linked polyol polytitanates can be prepared by reaction of a tetraaLkyl titanate with a polyol, such as pentaerythritol, followed by removal of the by-product alcohol (77). The isolated soHds are high activity catalysts suitable for use in the preparation of plasticizers by esterification and/or transesterification reactions. The insoluble nature of these complexes faciUtates their... 

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