Water in esterification

Mensah, P. Gainer, J. L. Carta, G. Adsorptive Control of Water in Esterification with Immobilized Enzymes I. Batch Reactor Behavior. Biotechnol. Bioeng. 1998, 60 (4), 434-444. 

Separation of products from the reaction mixture In situ product removal from enzymatic reactor via a nanofiltration or ultrafiltration membrane Removal of selected enantiomer via a liquid membrane Removal of water in esterification reactions via a pervaporation membrane... 

Mensah, P., Gainer, J. L., and Carta, G., Adsorptive control of water in esterification with immohilized enzymes. U. Fixed-hed reactor behavior, Biotechnol. Bioeng., 60, 445 53, 1998. 

Mensah P, Carta G (1999) Adsorptive control of water in esterification with immobilized enzymes. Continuous operation in a periodic counter-current reactor. Biotechnol Bioeng 66 137-146... 

Acryflc acid, alcohol, and the catalyst, eg, sulfuric acid, together with the recycle streams are fed to the glass-lined ester reactor fitted with an external reboiler and a distillation column. Acrylate ester, excess alcohol, and water of esterification are taken overhead from the distillation column. The process is operated to give only traces of acryflc acid in the distillate. The bulk of the organic distillate is sent to the wash column for removal of alcohol and acryflc acid a portion is returned to the top of the distillation column. If required, some base may be added during the washing operation to remove traces of acryflc acid. 

The methyl a-hydroxyisobutyrate produced is dehydrated to MMA and water in two stages. First, the methyl a-hydroxyisobutyrate is vaporized and passed over a modified zeoHte catalyst at ca 240°C. A second reactor containing phosphoric acid is operated at ca 150°C to promote esterification of any methacrylic acid (MAA) formed in the first reactor (74,75). Methanol is co-fed to improve selectivity in each stage. Conversions of methyl a-hydroxyisobutyrate are greater than 99%, with selectivities to MMA near 96%. The reactor effluent is extracted with water to remove methanol and yield cmde MMA. This process has not yet been used on a commercial scale. 

Depending on the requirements of the chemical procedure, the processing method may be varied with different mechanical arrangements to remove the by-product, water, in order to drive the esterification reaction toward completion. 

Use of Desiccants and Chemical Means to Remove Water. Another means to remove the water of esterification is calcium carbide supported in a thimble of a continuous extractor through which the condensed vapor from the esterification mixture is percolated (41) (see Carbides). A column of activated bauxite (Elorite) mounted over the reaction vessel has been used to remove the water of reaction from the vapor by adsorption (42). 

When yohimbine is heated with potash solution it is eonverted into potassium yohimbate, from which yohimbic acid (the forms yohimboie and yohimboaic are also used and noryohimbine), C20H24O3N2. H2O, is liberated by acetie acid it crystallises from water in lustrous prisms, m.p. 269° or 299° (dry, dec.), [ajo 138-8° (pyridine), and, on esterification with methyl alcohol and its homologues, reproduces yohimbine and its homologues, analysis of which by Field confirmed the view that yohimbine is methyl yohimbate, and has the formula assigned to it by Fourneau and Page. ... 

Fischer esterification (Section 15.8) Alcohols and carboxylic acids yield an ester and water in the presence of an acid catalyst. 

Ester hydrolysis is the most studied and best understood of all nucleophilic acyl substitutions. Esters are fairly stable in neutral aqueous media but are cleaved when heated with water in the presence of strong acids or bases. The hydrolysis of esters in dilute aqueous acid is the reverse of the Eischer esterification (Sections 15.8 and 19.14) ... 

Ethyl -aminobenzoate has been prepared by the esterification of p-aminobcnzoic acid1 and by the reduction of ethyl /i-nitrobenzoate with ammonium sulfide.2 Although commer-dally the reagent used is usually iron and water in presence of a little acid, in the laboratory the catalytic reduction as described in the procedure is by far the most convenient. 

An example of a reversible reaction in the liquid phase is afforded by the esterification reaction between ethanol and acetic (ethanoic) acid forming ethyl acetate and water. Since, however, ethyl acetate undergoes conversion to acetic acid and ethanol when heated with water, the esterification reaction never proceeds to completion. 

The volume of the water removed in esterifications through azeotropic distillation287-289 and also in polyesterifications is determined6,19,264,292). This method is difficult to carry out because it is necessary to quantitatively collect the water which is released and to determine its volume or its weight. It is important to estimate as accurately as possible the amount of water remaining in the distillation column. 

Semibatch or fully continuous operation with continuous removal of a by-product gas is also common. It is an important technique for relieving an equilibrium limitation, e.g., by-product water in an esterification. The pressure in the vapor space can be reduced or a dry, inert gas can be sparged to increase Ai and lower a, thereby increasing mass transfer and lowering u/ so that the forward reaction can proceed. 

Another mode of semibatch operation involves the use of a purge stream to remove continuously one or more of the products of a reversible reaction. For example, water may be removed in esterification reactions by the use of a purge stream or by distillation of the reacting mixture. Continuous removal of product(s) increases the net reaction rate by slowing down the reverse reaction. 

Consider the esterification of ethyl alcohol with formic acid to give ethyl formate (and water) in a mixed alcohol-water solvent, such that the alcohol and water are present in large excess. Assume that this is pseudo-first-order in both esterification (forward) and hydrolysis (reverse) directions ... 

In a lipase-catalyzed reaction, the acyl group of the ester is transferred to the hydroxyl group of the serine residue to form the acylated enzyme. The acyl group is then transferred to an external nucleophile with the return of the enzyme to its preacylated state to restart the catalytic cycle. A variety of nucleophiles can participate in this process. For example, reaction in the presence of water results in hydrolysis, reaction in alcohol results in esterification or transesterification, and reaction in amine results in amination. Kirchner et al.3 reported that it was possible to use hydrolytic enzymes under conditions of limited moisture to catalyze the formation of esters, and this is now becoming very popular for the resolution of alcohols.4... 

Analysing volatile acids in aqueous systems, resulting mainly from the presence of water, have been reported [19]. The volatile acids high polarity as well as their tendency to associate and to be adsorbed firmly on the column require esterification prior to gas chromatographic determination. The presence of water interferes in esterification so that complex drying techniques and isolation of the acids by extraction, liquid solid chromatography, distillation, and even ion exchangers had to be used [20-23],... 

After this brief characterisation of reversibility, we may use the example of esterification to consider next the question how the limitation of the reaction is to be explained. To the extent that acid and alcohol interact, and their reaction products, ester and water, are formed, the reverse reaction (ester + water = acid + alcohol) also gains in extent. A point is eventually reached at which just as many molecules of add and alcohol react to form ester as molecules of ester and water are decomposed to form acid and alcohol. The two reactions balance each other, and it would seem as if the reacting system had come to a state of rest. But this apparent rest is simulated by the fact that, in unit time, equal numbers of ester molecules are formed and decomposed. A state of equilibrium has been attained, and, as the above considerations indicate, this state would also have been reached had the reaction proceeded at the outset from the opposite side between equimolecular amounts of ester and water. In the latter case the hydrolysis of the ester would likewise have been balanced sooner or later, according to the conditions prevailing, by the opposing esterification—in this case when 33-3 per cent of the ester had been decomposed. The equilibrium is therefore the same, no matter from which side it is approached on this depends its exact experimental investigation, both here and in many other reactions. 

Currently, the most successful methodology for the optimization of an SMB s performance is the so-called triangle theory, which was recently also applied to the SMBR [158]. The analysis was based on a mathematical model describing the esterification of acetic acid and ethanol into ethyl acetate and water in a fixed-bed chromatographic reactor [159]. A mixture of ethanol and acetic acid is intro-... 

All reactions involved in polymer chain growth are equilibrium reactions and consequently, their reverse reactions lead to chain degradation. The equilibrium constants are rather small and thus, the low-molecular-weight by-products have to be removed efficiently to shift the reaction to the product side. In industrial reactors, the overall esterification, as well as the polycondensation rate, is controlled by mass transport. Limitations of the latter arise mainly from the low solubility of TPA in EG, the diffusion of EG and water in the molten polymer and the mass transfer at the phase boundary between molten polymer and the gas phase. The importance of diffusion for the overall reaction rate has been demonstrated in experiments with thin polymer films [10]. 

With values between 13 and 16, the equilibrium constant is still high enough to regard the formation of DEG from EG to be irreversible in an open industrial system. DEG formation is not only an important side reaction during esterification, polycondensation and glycolysis, but also during distillation of EG and water in the process columns. In particular, the residence time in the bottom reboiler of the last separation column is critical, where the polycondensation catalyst and... 

The vapour pressures of the main volatile compounds involved in esterification and polycondensation are summarized in Figure 2.25. Besides EG and water, these are the etherification products DEG and dioxane, together with acetaldehyde as the main volatile product of thermal PET degradation. Acetaldehyde, water and dioxane all possess a high vapour pressure and diffuse rapidly, and so will evaporate quickly under reaction conditions. EG and DEG have lower vapour pressures but will still evaporate from the reaction mixture easily. 

The hydrolysis of an ester to alcohol and acid (1) and the esterification of a carboxylic acid with an alcohol (2) are shown here as an example of the Sn2 mechanism. Both reactions are made easier by the marked polarity of the C=0 double bond. In the form of ester hydrolysis shown here, a proton is removed from a water molecule by the catalytic effect of the base B. The resulting strongly nucleophilic OH ion attacks the positively charged carbonyl C of the ester (la), and an unstable sp -hybridized transition state is produced. From this, either water is eliminated (2b) and the ester re-forms, or the alcohol ROH is eliminated (1b) and the free acid results. In esterification (2), the same steps take place in reverse. 

Yet another possiblity for the nonlinearity in the low conversion region is the decrease in the volume of the reaction mixture with conversion due to loss of one of the products of reaction (water in the case of esterification). This presents no problem if concentration is plotted against time as in Eq. 2-20. However, a plot of 1/(1 — p j2 against time (Eq. 2-22) has an inherent error since the formulation of Eq. 2-21 assumes a constant reaction volume (and mass) [Szabo-Rethy, 1971]. Elias [1985] derived the kinetics of step polymerization with correction for loss of water, but the results have not been tested. It is unclear whether this effect alone can account for the nonlinearity in the low conversion region of esterification and polyesterification. 

Lipases are enzymes that catalyze the in vivo hydrolysis of lipids such as triacylglycerols. Lipases are not used in biological systems for ester synthesis, presumably because the large amounts of water present preclude ester formation due to the law of mass action which favors hydrolysis. A different pathway (using the coenzyme A thioester of a carboxylic acid and the enzyme synthase [Blei and Odian, 2000]) is present in biological systems for ester formation. However, lipases do catalyze the in vitro esterification reaction and have been used to synthesize polyesters. The reaction between alcohols and carboxylic acids occurs in organic solvents where the absence of water favors esterification. However, water is a by-product and must be removed efficiently to maximize conversions and molecular weights. 

In addition to its direct influence via the water activity in the system, the amount of water also influences the activity coefficients of the other components in the mixture, and therefore equiUbrium constants like K0 can vary with the water activity in the system (Table 1.5) [29, 63, 64]. This can be seen as a solvent effect on the equilibrium constant The tendency in esterification reactions is that increases with decreasing water activity, which means that it is favorable to use low water activity because of both the direct effect of water activity on the equilibrium and the influence of water on K0. 

Solvent effects on enzymatic reactions have been most thoroughly studied for esterification reactions. It has been observed that those reactions are favorably carried out in relatively hydrophobic solvents, while the equilibrium position is less favorable for esterification in more hydrophilic solvents. Correlations between equilibrium constants and solvent parameters have been evaluated. It was shown that the solubility of water in the solvent (Sw/0) gave better correlation with esterification equilibrium constants than log P and other simple solvent descriptors [61]. 

Although enzymes function admirably in water (their natural medium), some reactions cannot easily be performed in water owing to insolubility of hydrophobic reactants or equilibrium limitations, e.g., in esterifications and amidations. 

A series of sorbitol-based nonionic surfactants are used in foods as water-in-oil emulsifiers and defoamers. They are produced by reaction of fatty acids with sorbitol. During reaction, cyclic dehydration as well as esterification (primary hydroxyl group) occurs so that the hydrophilic portion is not only sorbitol but also its mono- and dianhydride. The product known as sorhitan monostearate [1338 -1 -6], for example, is a mixture of partial stearic and palmitic acid esters (sorbitanmonopalmitate [26266-57-9]) of sorbitol, 1,5-anhydro-D-glucitol [154-58-8], 1,4-sorbitan [27299-12-3], and isosorbide [652-67-5], Sorbitan esters, such as the foregoing and also sorbitan monolaurate [1338-39-2] and sorhitan monooleate [1338-43-8], can be further modified by reaction with ethylene oxide to produce ethoxylated sorbitan esters, also nonionic deteigents FDA approved for food use. 

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