Esters heterogeneous catalysis

Experiments showed that high methyl ester yields can be achieved with solid bases and super acids under moderate reaction conditions. The solid bases were more effective catalysts than the solid super acids. High stability can be achieved by an ordinary inexpensive preparation process, and the catalyst can be separated easily from the reaction products in the heterogeneous catalysis process. The costly catalyst removal process can be avoided compared with the homogeneous process. Therefore, the heterogeneous process using a solid catalyst should be more economical for biodiesel production. 

One of the exciting results to come out of heterogeneous catalysis research since the early 1980s is the discovery and development of catalysts that employ hydrogen peroxide to selectively oxidize organic compounds at low temperatures in the liquid phase. These catalysts are based on titanium, and the important discovery was a way to isolate titanium in framework locations of the inner cavities of zeolites (molecular sieves). Thus, mild oxidations may be run in water or water-soluble solvents. Practicing organic chemists now have a way to catalytically oxidize benzene to phenols alkanes to alcohols and ketones primary alcohols to aldehydes, acids, esters, and acetals secondary alcohols to ketones primary amines to oximes secondary amines to hydroxyl-amines and tertiary amines to amine oxides. 

Mineral surfaces may accelerate the rate of ester hydrolysis (Stone, 1989 Hoffmann, 1990 Torrents and Stone, 1991). One plausible scheme for this heterogeneous catalysis assumes a nucleophilic addition of the ester to the surface functional group, e.g., in case of a carboxylic acid ester... 

Hydrogenolysis of esters to aldehydes or alcohols is difficult to attain either by homogeneous or heterogeneous catalysis. In fact, high temperatures and high pressures are required to achieve the reaction, leading to a non-selective hydrogenolysis with formation of acids, alcohols, CO2, CO and hydrocarbons. 

Stereoelectronic Influences of Phosphite Ester Ligands in Homogeneous and Heterogeneous Catalysis... 

The transformations that use asymmetric heterogeneous catalysis will be highlighted P-keto esters and diketone reductions by Raney nickel catalyst modified with R,R-tartaric acid and NaBr. a-Keto acid reductions with cinchona modified Pt catalysts are discussed in Chapter 18. 

Hence, manufacturing several fatty esters in a reactive distillation setup by using heterogeneous catalysis is possible. The equipment is simple and efficient in operation. Making use of a fast catalyst remains the key aspect in the technology. 

Thus the peculiar titration results were due to the heterogeneous catalysis, not of the original reaction, but of a new reaction that had arisen from the analytical procedure. Precipitates formed by the reagents themselves can also prove catalytically active. For instance, the catalysis by rare earth metal salts of the hydrolysis of acid phosphonate esters was actually due to the development of metal hydroxide gels [159], A quite different example concerns reactions of mercury salts where the disproportionation... 

Catalytic activity of rare earth elements (i.e., lanthanides, symbol Ln) in homogeneous catalysis was mentioned as early as 1922 when CeCls was tested as a true catalyst for the preparation of diethylacetal from ethanol and acetaldehyde [1]. Solutions of inorganic Ln salts were subsequently reported to catalyze the hydrolysis of carbon and phosphorous acid esters [2], the decarboxylation of acids [3], and the formation of 4-substituted 2,6-dimethylpyrimidines from acetonitrile and secondary amines [4]. In the meantime, the efficiency of rare earth metals in heterogeneous catalysis, e. g., as promoters in lanthanide (element mixtures)-... 

Olah, G. A., Arvanaghi, M., Krishnamurthy, V. V. Heterogeneous catalysis by solid superacids. 17. Polymeric perfluorinated resin sulfonic acid (Nafion-H) catalyzed Fries rearrangement of aryl esters. J. Org. Chem. 1983,48, 3359-3360. 

Heterogeneous catalysis should be a better way to produce ethyl esters, provided the catalyst improvement looked for in the case of methyl ester is successful. In any case, the process will be less competitive and the product more expensive. 

The immobilized catalyst provided good enantioselectivity and activity in the heterogeneous catalysis of the oxidative kinetic resolution of secondary alcohols and can be recovered and recycled for four times without obvious loss of enantioselectivity and activity. Oxidative kinetic resolutions of meso-diols, hydroxyl esters, and primary alcohols were also studied using this catalytic system. 

Catalysis. Many reactions are catalyzed, i.e., increased in rate, by a compound in solution (homogeneous catalysis) or a group at the surface of a particle (heterogeneous catalysis), where the catalyst is not consumed itself. Examples are various hydrolyzing reactions, like the ester hydrolysis mentioned above, that are catalyzed by H+ as well as OH ions. In such a case the reaction rate greatly depends on pH, though the ions themselves do not appear as reactants in the overall reaction scheme. Ubiquitous in natural foods are enzyme-catalyzed reactions. The simplest case leads to Michaelis-Menten kinetics, but several complications may arise. 

Unusual reaction orders are found in product-promoted or reactant-inhibited ("autocatalytic") reactions, the former with positive apparent order with respect to a product, the latter with negative apparent order with respect to a reactant (see Section 8.9). An example of a product-promoted reaction is acid-catalyzed ester hydrolysis. An example of a reactant-inhibited reaction has already been encountered, namely, olefin hydroformylation, whose order with respect to CO is negative (see eqn 6.12 in Section 6.3). Such behavior is also not uncommon in heterogeneous catalysis (see Section 9.3.2) and enzyme catalysis ("substrate-inhibited" reactions in biochemistry lingo, Section 8.3). A reaction having an order with respect to a silent partner—CO in a homogeneous hydrogenation—will be examined in some detail later in this chapter (see Examples 7.3 and 7.4). 

An interesting possible further extension is the functionalization of bispidine ligands with hydrophobic groups, for example, for metal ion selective extractions (69, 339). biopolymers for nuclear medicinal applications (340), solids for heterogeneous catalysis and sensors, or additional coordination sites for the synthesis of heterodinuclear complexes with applications in biomimetic chemistry, catalysis, and as luminescence sensors. There is a variety of possible sites for ligand modification. Of particular interest is the C9 position, which has been selectively and stereospecifically reduced to an alcohol (190), and the two hydrolyzed C1,C5 ester groups (167). 

The Fries reaction consists of the catalyzed rearrangement of phenyl esters with the production of hydroxy aryl ketones (Scheme 5.11). From analysis of a great number of papers dealing with the Fries reaction promoted xmder both homogeneous and heterogeneous catalysis, it becomes clear that mechanistic problems are more and more entwined. In order to help the reader draw general conclusions from the wealth of sometimes... 

Collier P.J., Goulding T., Iggo J.A. and Whyman R. (1995) Studies of the Pt-cinchona alkaloid catalyst for enantioselective alpha-keto esters hydrogenation, in Jannes G. and Dubois V. (eds.). Chiral Reactions in Heterogeneous Catalysis, Plenum Press, N.Y. p. 105-110. 

The key structural feature of POST-1 - the presence of dangling pyridine groups in the channels - affords a unique opportunity to perform asymmetric heterogeneous catalysis. Thus, potentially, any base catalyzed reactions (e.g., esterification or hydrolysis) can be performed with POST-1. Moreover, chiral pores should induce a degree of enantioselectivity in the final product mixture. The catalytic activity of POST-1 in the transesterification reaction was examined. Although the reaction of 16 and ethanol in the presence of POST-1 in carbon tetrachloride produced ethyl acetate in 11% yield, little or no transesterification occured without POST-1 or with the iV-methylated POST-1 (Sect. 2.2). The post chemical modification of the pyridine groups in POST-1 proves the role of free pyridine moiety in transesterification reaction. Transesterification of ester 16 with bulkier alcohols such as isobutanol, neopentanol, and 3,3,3-triphenyl-l-propanol occurs at a much slower rate under otherwise identical reaction conditions. Such size selectivity suggests that catalysis mainly occurs in the channels. 

Recently, a new approach was pursued combining organocatalysis with heterogeneous catalysis [11]. In this case, molecular hydrogen is used as hydrogen source and supported noble metal catalysts for the activation of hydrogen. The implementation of chirahty is affected by a suitable modifier such as chiral phosphoric add esters (P-add) ]55]. 

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