Acid catalysis, bifunctional

B. Q. Xu, T. Yamaguchi, and K. Tanabe, Acid-base bifunctional catalysis in the decomposition of alkylamines, Appl Catal. 75, 75-86 (1991). 

Acid and Bifunctional Metal/Acid Catalysis Active Sites... 

Under the operating conditions, the reaction intermediates (w-hexenes and i-hexenes in n-hexane isomerization) are thermodynamically very adverse, hence appear only as traces in the products. These intermediates (which are generally olefinic) are highly reactive in acid catalysis, which explains that the rates of bifunctional catalysis transformations are relatively high. The activity, stability, and selectivity of bifunctional zeolite catalysts depend mainly on three parameters the zeolite pore structure, the balance between hydrogenating and acid functions, and their intimacy. In most of the commercial processes, the balance is in favor of the hydrogenation function, that is, the transformations are limited by the acid function. 

This reaction encompasses a number of interesting features (general Brpnsted acid/ Brpnsted base catalysis, bifunctional catalysis, enantioselective organocatalysis, very short hydrogen bonds, similarity to serine protease mechanism, oxyanion hole), and we were able to obtain a complete set of DFT based data for the entire reaction path, from the starting catalyst-substrate complex to the product complex. 

Bell-shaped pH-rate profiles are obtained in lysozyme reactions (Rupley et al., 1967) which are consistent with direct involvement of two groups in the reaction. However, bell-shaped pH-rate constant profiles are also observed in the hydrolysis of benzaldehyde disalicyl acetals, and in the case of p-nitrobenzaldehyde o-carboxyphenyl p-carboxyphenyl acetal only one carboxyl group can participate. One should then take care in postulating bifunctional catalysis in the lysozyme reaction, since the observed kinetics and the rate enhancements are explicable in terms of a chemically simpler mechanism (general acid catalysis by glutamic acid-35 along with release of ground state... 

Noble metal zeolite catalysts are used in various processes, most of them occurring through bifunctional hydrogenating/acid catalysis. One exception, however, is the selective aromatization of n-alkanes (e.g. n-hexane into benzene) proceeding through monofunctional metal catalysis. Indeed the PtLTL catalyst used commercially does not present any protonic sites. 

A versatile synthesis of pyrazoles from benzophenone hydrazones was demonstrated with a variety of 1,3-bifunctional substrates under acidic conditions <2002TL2171>. Hydrazones 668 and 671, prepared from palladium-catalyzed heteroaryl halides with benzophenone hydrazone, reacted with 1,3-bifunctional substrates 669 and 672 under acidic conditions to yield pyrazoles 670 and 673, respectively (Equations 139 and 140) <2004TL5935>. Enolates of a-chloro-/3-oxoaldehydes react with arylhydrazines in the presence of acetic acid or / -oxo-a-chlorenamines react with arylhydrazines to produce osazones of carbonyl-substituted glyoxals, which under conditions of acid catalysis undergo intramolecular heterocyclization to the corresponding functionalized pyrazoles <1998CHE167>. 

The esterification of carboxylic acids can be provided, also, by synthetic equivalents of alcanols acetals RCH(0R )2 (with acid catalysis), ortho-esters RC(0R )3 (acid catalysis), and dialkylcarbonates C0(0R)2 (base catalysis). The series of bifunctional reagents of this type [dimethylformamide dialkylac-etals (CH3)2N-CH(0R)2] is commercially available. Besides the esterification of carboxyl groups, these compounds react with primary amino groups and, thus, are used for GC analysis of amino acids (see the entry Derivatization of Amines, Amino Acids, and Related Compounds for GC Analysis ) ... 

Bass JD, Solovyov A, Pascal AJ, Katz A (2006) Acid-base bifunctional and dielectric outer-sphere effects in heterogeneous catalysis A comparative investigation of model primary amine catalysts. J Am Chem Soc 128 3737... 

However, due to the amphoteric character of alumina, acid catalysis due to OH surface groups or to A1 atoms can take place in a bifunctional catalysis mechanism [Eq. (13)] [24]. 

In benzene solution mutarotation is catalysed by a range of bifunctional proton-transfer catalysts, including carboxylic acids and the nucleoside cytosine. Ab initio calculations on formic acid catalysis of the mutarotation of 2-hydroxytetrahydropyran indicate strong coupling of the double proton transfer to endocyclic C-O bond cleavage. ... 

Considering the potential interest of this method and its generic character, the IFP decided to apply it and validate it for nearly all the major refining processes involving acid catalysis, alone and/or bifunctional. Examples are given in Fig. 42. Specific adaptations are required each time to take into account the characteristics of the transformations but also of the complex hydrocarbon feedstocks involved for each of these processes. 

The catalysts on which these reactions occur are bifunctional that is, they possess two different types of catalytic activity. In addition to catalyzing hydrogenation and dehydrogenation reactions, they also catalyze hydrocarbon rearrangements of a type commonly observed in acid catalysis. These two distinct types of activity are associated with two different components of the catalyst, at least under conditions typical of catalytic reforming. 

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