Bronsted acid catalysis homogeneous

Bhugun I, Lexa D, Saveant JM (1994) Ultraeffident selective homogeneous catalysis of the electrochemical reduction of carbon dioxide by an iron(O) porphyrin associated with a weak bronsted acid cocatalyst. J Am Chem Soc 116 5015-5016... 

It must be noted that the phenol/aldehyde reaction can be catalyzed by Bronsted acids (protonation of the carbonyl oxygen) as well as by Lewis acids (coordination of the carbonyl oxygen). In the latter case one Lewis centre (e.g. Al ) can accommodate and activate both the phenol and the aldehyde (cq. the benzyl alcohol, in the consecutive reaction). As a consequence, ortho-substitution is favoured [14,15]. The high 2,2 -dihydroxydiphenylmethane selectivity we obtained with homogeneous Al " -catalysis and with 7-alumina is consistent with these data. Additionally, the finding that the H - US - Y catalyzed toluene/formaldehyde-condensation gives a low 2,2 -selectivity, 19% [16], compared to the 32% we obtained with phenol, also indicates the hydroxyl-group plays a role. However, transalkylation, reported to lead to ortho-substitution in condensations of phenol with methanol on both zeolite- and non-zeolite Bronsted acid catalysts [17], can t be ruled out. 

Before we discuss characteristic features of a metal sulfate catalyst, it is to be noted that the model reaction should be one which has as straightforward a mechanism as possible, preferably in a homogeneously catalyzed reaction. This is the only way we can critically evaluate the efficiency of the present solid catalyst system. The depolymerization of paraldehyde was most extensively studied in view of the foregoing criterion. For the homogeneous acid catalysis of the depolymerization of paraldehyde, there are ample data given by Bell and his associates in nonaqueous solvent (by proton acid as well as Lewis acid) and also in aqueous solution (55,56). Since most Hammett indicators change their color when adsorbed on the surface acids of both Bronsted and Lewis type, it is fortunate that this depolymerization proceeds easily by acids of both types. Evidently the dotted line in Fig. 2 shows excellent... 

CO and two of H2O and produces two moles of CO2. Homogeneous catalysis of this reaction can be accomplished with an iron carbonyl in conjunction with a Bronsted acid or base. Thus it appears that conditions might be found where the shift reaction itself can be eflFected homogeneously using metal complex catalysts. To this end, we have been examining the activity of various homogeneous catalysts for the shift reaction, and our investigations of metal carbonyl cluster complexes are summarized here. 

The nature of catalysis in homogeneous systems has been the subject of a considerable amount of research. A catalyst is any substance which affects the rate of reaction but is not consumed in the overall reaction. From thermodynamic principles we know that the equilibrium constant for the overall reaction must be independent of the mechanism, so that a catalyst for the forward reaction must also be one for the reverse reaction. In aqueous solution, a large number of reactions are catalyzed by acids and bases for our purposes we shall employ the Bronsted definition of acids and bases as proton donors and acceptors, respectively. Catalysis by acids and bases involves proton transfer either to or from the substrate. For example, the dehydration of acetaldehyde hydrate is subject to acid catalysis [20], probably by the mechanism (II). 

Catalysis by solid acids is of paramount importance in industrial chmnistry, namely due to its application in catalytic cracking, one of the most important processes in the world. However, despite its enormous importance, only recently have practical and quantitative relationship between the acidity of the catalyst and its catalytic activity began to appear, unlike homogeneous acid catalysis, which has made use of the Brdnsted relations for many years. The difficulties to be overcome are of various nature but it was found by some of the authors that Bronsted-like relationships also apply to solid acid catalysts [1]. 

Fig. 8.16 Reaction network for tandem reaction of glucose conversion to HMF by CrCI3 and HCI in aqueous phase (FA- furfural, Lewis-catalyzed reaction network is shown by dashed line). (Modified from T. D. Swift, H. Nguyen, A. Anderko, V. Nikolalds, D.G. Vlachos, Tandem Lewis/Bronsted homogeneous acid catalysis conversion of glucose to 5-hydoxymethylfurfural in an aqueous chromium (III) chloride and hydrochloric acid solution. Green Chemistry 17 (2015) 4725—4735. Copyright 2015 Royal Society of Chemistry).

Homogeneous Chemical Catalysis of the Reduction of Carbon Dioxide. Synergistic Effect of Bronsted and Lewis Acids... 

Catalysis by proton transfer is significant in the snbsnrface and associated environment and is common in homogeneous reactions. The strength of the acid or base is determined by the ionization constant, while its efficiency as a catalyst is controlled by the reaction rate. This relation, known as the Bronsted catalysis law, is expressed as... 

NaY commercial zeolite can be utilized to prepare catalysts with different SAR framework. Y(18) is the best catalyst in the benzoylahon of anisole with benzoic acids (BACs). Synthetic results are reported in Table 4.4. The activity of Y zeolite increases with its Lewis acidity, showing that, in this case, the Lewis acid sites are more active than the Bronsted ones. The mechanism of the reaction is found to be similar to that observed under homogeneous catalysis. 

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