Transesterification comparison

In the reaction, it was essential to use an IL as a co-solvent. Lozano, Iborra and co-workers recently reported an interesting stabilizing effect of two types of water-immiscible ILs ([emim][TFSI] and [BuMe3N][TFSI]) for CAL-B-catalyzed transesterification of vinyl butyrate. The synthetic activity and the stability of the enzyme in these IL solvent systems were markedly enhanced as compared to those in hexane. CAL-B maintained its activity higher than 75% after 4 days of incubation in [emim][TFSI] solvent, while it showed an activity of only 25% when incubated in both water and hexane media at 50°C. Comparison of the ratio of a-helix and (3-sheet by CD spectra showed the activity was closely related with a-helix content which reduced to 31% immediately after lipase was added to hexane and had reached only 2% after 4 days in hexane. On the contrary, no significant reduction of a-helix content was... 

Figure 14-5. Side by side comparison of the two-dimensional potential energy surface for the transesterification reaction computed with B3LYP (left) and M06-2X (right)...

The catalysis afforded by the La3 + system for the transesterifications of paraoxon in ethanol and methanol is quite spectacular relative to the background reactions that are assumed to be promoted by the lyoxide. The reaction rate constant of ethoxide with paraoxon in ethanol at 5.1 x 10-3 dm3 mol-1 s-133 is roughly a factor of two lower than the rate constant of methoxide with paraoxon in methanol (1.1 x 10 2dm3mol 1 s-1).17a However a solution 2mmoldm-3 in total [La3 + ], which contains 1 mmol dm-3 of Lal+, has a maximum rate constant of 7 x 10-4s-1 for decomposition of 1 in ethanol at pH of 7.3, and accelerates the rate of ethanolysis of paraoxon by a factor of 4.4 x 10n-fold relative to the ethoxide reaction at the same pH.34 By way of comparison, the acceleration afforded by a 1 mmol dm-3 solution of the La + dimer catalyzing the methanolysis of 1 at the maximal pH of 8.3 (kobs = 0.0175 s 1) is 109-fold greater than its background methoxide reaction. On this simple basis La2+ in ethanol appears to be catalytically superior to La2+ in methanol, but this stems almost exclusively from the pH values... 

Fig. 4. a rac-Phenylethylamine (PEA) and 2-methoxy-J T-[(li )-l-phenylethyl]-acetamide (MET) are substrate and product in a lipase catalyzed transesterification, b Linear relationship between relative signal intensities in MALDI-MS and the relative concentrations of PEA/d5-labeled PEA. c Comparison of decrease in PEA during enzymatic conversion as measured by quantitative MALDI-MS and gas chromatography (GC)... 

When bromide 14, in molar excess, was reacted with the selectively blocked disaccharide 9 in dichloromethane in the presence of mercury(II) cyanide as catalyst and 4A molecular sieve as the acid acceptor, a 90% yield of the fully blocked tetrasaccharide was obtained after column chromatography. Removal of blocking groups was accomplished by the hydrogenolysls of the benzyl ether and benzylidene acetal groups In acetic acid, followed by transesterification. The deblocked tetrasaccharide 19 had NMR parameters in agreement with its structure. This could be confirmed by comparison with chemical shift data for the... 

By comparison, the catalyzed transesterification reaction between ethylene carbonate and methanol (Equation 7.3) offers an alternative for greening DMC production. In this Asahi Kasei process [27], the preferred catalyst is based on an anion-exchange resin operating under catalytic distillation conditions between 333-353 K. This reactor design shifts the thermodynamic equilibrium towards complete conversion of ethylene carbonate, such that both the yield and selectivity for DMC and monoethylene glycol are 99.5%. The process is capable of supplying monoethylene glycol to the market, and DMC for captive use to produce DPC. 

Besides the stabilization of charged transition states, the effect as entropy traps is decisive for the effect as a catalyst. Entropy trap refers to the ability of a catalyst to bind substrates in a favorable orientation and thus to freeze out translational and rotational degrees of freedom of the substrate (Chapter 2, Section 2.2.3). The result corresponds to a drastic increase in the effective molarity of the substrate, to a concentration level which can never be reached in free solution. The transesterification reaction in Figure 18.5 (Schultz, 1993) would be difficult to conduct in water under ordinary circumstances as the hydrolysis side reaction is aided by a water concentration of 55.5 m. A comparison of kcM/KM [(m s) x] with the uncatalyzed reaction in water [s-1] (strictly speaking, [H20] at 55.5 m has to be considered as well, so a dimension of [(m s) 11 again results) yields an effective molarity of 106 m. 

We conclude that a commercial immobilized lipase from C. antarctica (Novozym 435) was stable in SCC02 for all experimental conditions investigated. Based on the results obtained here and comparison of them with the results obtained by other investigators, it can be concluded that the magnitude of pressure, temperature, decompression rate, and exposure time needed to inactivate the enzyme strongly depends on the nature and the source of enzyme and, primarily, whether the enzyme is in its native or immobilized form. For the purpose of using this enzyme to catalyze the transesterification reaction of vegetable oils in order to produce esters, the results obtained herein are relevant, because the immobilized lipase can be used with low activity loss at typical conditions of temperature and pressure employed in many biotransformations of raw materials. 

Fig. 5. Comparison in yield of methyl esters between transesterification of triglycerides and methyl esterification of fatty acids by supercritical methanol treatment at various temperatures. , Transesterification at 270°C A, transesterification at 300°C , transesterification at 350°C O, methyl transesterification at 270°C A, methyl transesterification at 300°C , methyl transesterification at 350°C.

Figure 5 shows a comparison of the yields of methyl esters between transesterification of triglycerides (rapeseed oil) and methyl esterification of fatty acids by supercritical methanol at various temperatures. At 350°C, both reactions could produce very similar results. At 300°C, transesterification produced about 90% methyl esters at 12 min of treatment, whereas methyl esterification resulted in a complete conversion. When triglycerides were transesterified at 270°C, a plateau was reached at about 40 min of treatment with a yield of about 76%. However, much higher yield could be achieved by methyl esterification at 20 min of treatment. These results, therefore, indicate that the reaction rate in methyl esterification is higher than that in transesterification. 

Dembiras, A., Comparison of transesterification methods for production of biodiesel, Energy Conver. Manag., 2007... 

It is generally stated that biocatalysis in organic solvents refers to those systems in which the enzymes are suspended (or, sometimes, dissolved) in neat organic solvents in the presence of enough aqueous buffer (less than 5%) to ensure enzymatic activity. However, in the case of hydrolases water is also a substrate and it might be critical to find the water activity (a ) value to which the synthetic reaction (e.g. ester formation) can be optimized. Vahvety et al. [5] found that, in some cases, the activity of Candida rugosa lipase immobihzed on different supports showed the same activity profile versus o but a different absolute rate. With hpase from Burkholderia cepacia (lipase BC), previously known as lipase from Pseudomonas cepacia, and Candida antarctica lipase B (CALB) it was found that the enzyme activity profile versus o and even more the specific activity were dependent on the way the enzyme was freeze dried or immobihzed [6, 7]. A comparison of the transesterification activity of different forms of hpase BC or CALB can be observed in Tables 5.1 and 5.2, respectively. 

B. Campou and A. N. Klibanov, Comparison of different strategies for the lipase-catalysed preparative resolution of racemic acids and alcohols Asymmetric hydrolysis esterification and transesterification. Biotech. Bioeng., 26 1449 (1984). 

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