Maximum Catalytic Efficiency Research Articles

Electrocatalytic reduction of CO 2 on electrodes modified by fac-Re(2,2'-bipyridine)(CO) 3Cl complexes bonded to polypyrrole films

Carbon dioxide reduction on platinum and carbon felt electrodes coated with polymeric films obtained by oxidative electropolymerization of pyrrole-containing Re(L)(CO) 3Cl complexes, where L is a 2,2'-bipyridine, has been investigated. Catalytic activities of modified electrodes prepared by polymerization of differently substituted complexes have been compared, on the basis of cyclic voltammetry and preparative-scale experiments. Electrocatalytic reduction of CO 2 to CO takes place with high current yields, and maximum catalytic efficiency corresponds to an optimum film thickness.

Hydrophobic interactions in catalysis by imidazole‐containing polymers

AbstractThe catalytic activity of imidazole‐containing polymers in the hydrolyses of substrates with poor leaving groups was examined. Hydrolyses of p‐methoxyphenyl esters (Sn) catalyzed by poly[4(5)‐vinylimidazole] (pvIm) in relation to imidazole (Im) indicates that both cooperative and hydrophobic interactions are operative. Hydrolyses of 3‐methoxy‐4‐acyloxybenzoic acid substrates (S) catalyzed by pvIm and a water‐soluble copolymer, copoly[1‐methyl‐4‐vinylimidazole/4(5)‐vinylimidazole], exhibit many characteristics of enzyme‐catalyzed reactions like saturation kinetics, bellshaped pH‐rate profiles, and nonproductive binding. The importance of general‐base, cooperative interactions for substrates with poor leaving groups and hydrophobic interactions in the formation of a stable catalyst‐substrate complex in the case of long‐chain esters is demonstrated. The interesting similarity between the Michaelis constants Km for the two polymer catalysts, pvIm and the copolymer, suggests that the common underlying principle involved in binding is hydrophobic interactions and the presence of NCH3 group in the copolymer does not increase the hydrophobicity significantly. The pH‐rate profiles for the hydrolyses of S by pvIm show that the optimum pH is around neutrality, which indicated that the presence of neutral Im units to serve as nucleophiles and protonated Im units to serve as electrostatic binding sites is essential for maximum catalytic efficiency. The rate enhancement as a function of acyl chain length at different pH leads to the conclusion that a cooperative effect between electrostatic and hydrophobic interactions would reduce the nonspecificity of hydrophobic interactions and result in their better realization. An approximate calculation shows that the binding of S, in relation to S, with the copolymer in aqueous solution corresponds to the additional interactions of two methylene units.Our recent studies1–10 in the field of catalysis by polymeric imidazoles are oriented mainly toward the recognition of the significance of hydrophobic interactions. The role of hydrophobic interactions in enzymatic catalysis11–13 and synthetic macromolecular catalysts1–10,14–29 has been understood only recently. Hydrophobic interactions describe the tendency of nonpolar groups to associate themselves in aqueous solution30,31. Because the catalytic mechanism generally involves the prior complexation of the catalyst with the substrate and the catalyst‐substrate complex is apolar, the influence of hydrophobic interactions in catalysis is conceivable. The favorable free energy of formation of hydrophobic interaction leads to better binding and better catalysis.In the present study we examined the hydrolytic reactions of the following substrates with poor leaving groups, magnified image catalyzed by imidazole‐containing vinyl polymers, poly[4(5)‐vinylimidazole] (pvIm), and a water‐soluble copolymer, copoly[1‐methyl‐4‐vinylimidazole/4(5)‐vinylimidazole], ∼(1:1)M, magnified image Esters of varying acyl chainlength are chosen to determine the influence of hydrophobic interactions.

Catalyses by Polymer Complexes. VI. Catalysis of Proton Abstraction from α-Ketols by Hydroxamate Ions Bound to Polymer Micelles

Hydroxamate anions bound to partially laurylated poly(2-ethyl-1-vinylimidazole) were found to serve as excellent catalysis for proton abstraction from α-ketols. Proton removal from benzoin by N-methylmyristohydroxamate anion in the presence of the polymer (lauryl group content, 29 mol%) was 2.1×104 times faster than the corresponding reaction in the absence of the polymer, and 330 times faster than that by hydroxide ion. The maximum catalytic efficiency existed at ca. 30-mol% lauryl group content. Addition of KCl and ethanol significantly suppressed the reaction. Thus, hydroxamate—polymer complexes would act generally as excellent base catalysts for proton abstraction to form the carbanion intermediates.

Slowdown of oxyhydrogen detonation by surface catalysis

The propagation velocities of stoichiometric oxyhydrogen detonations were measured in thin tubes of different wall materials. From 4 to 6% velocity differences were observed consistently for pyrex and γ-alumina walls. By changing the tube geometries, it was confirmed that this velocity difference can be explained only by the catalysis of the wall material. To elucidate this result, one-dimensional gasdynamic analysis was performed, including various possible wall reactions. The solutions of this eigenvalue problem gave the relation of each reaction and its catalytic efficiency to the calculated detonation velocity. The results showed that the observed velocity difference can be explained only if we assume that H+OH→H 2 O or H→1/2 H 2 occurs with the maximum catalytic efficiency, i.e., unity. After some speculations, H+OH→ H 2 O was considered more probable, the catalytic efficiency of γ-alumina being nearly unit for this reaction at 300 K.