Acids, acid strength catalysis, alcohol

Shape selective catalysis as typically demonstrated by zeolites is of great interest from scientific as well as industrial viewpoint [17], However, the application of zeolites to organic reactions in a liquid-solid system is very limited, because of insufficient acid strength and slow diffusion of reactant molecules in small pores. We reported preliminarily that the microporous Cs salts of H3PW12O40 exhibit shape selectivity in a liquid-solid system [18]. Here we studied in more detail the acidity, micropore structure and catal3rtic activity of the Cs salts and wish to report that the acidic Cs salts exhibit efficient shape selective catalysis toward decomposition of esters, dehydration of alcohol, and alkylation of aromatic compound in liquid-solid system. The results were discussed in relation to the shape selective adsorption and the acidic properties. 

Formation of the tetrahedral intermediate carbinolamine and subsequent elimination of water are amenable to acid-base catalysis and do not require a metal surface. The relative rates of adduct formation and subsequent dehydration to imine or enamine depend on the structure of alcohol and amine, and on the nature and strength of acidic and basic sites on the catalyst surface. It must be stressed that several side-reactions (e. g. dimerization and oligomerization, dehydration) are also acid or base-catalyzed, and good selectivity for the desired product requires proper tuning of the redox and acid-base properties of the catalyst. This is crucial in catalyst development when choosing a suitable support, additive, or modifier. Even traces of impurities remaining on the surface from the catalyst precursor can strongly influence product distribution [10]. 

In the range of the study of new processes to improve the atom efficiency, the discovering versatility of a catalyst as multifunctional catalyst in a transformation is important. On the strength of this concept. Jiao and coworkers reported an enantioselective reduction and alkylation reaction of a,p-unsaturated aldehydes with alcohols, in which the ammonium salt catalyst performed three kinds of catalytic functions, namely, iminium catalysis, enamine catalysis, and acid catalysis [90]. 

The reverse reaction, i.e., the decomposition of diacetone alcohol to acetone, was used by Jurczyk and Kania [233]. They explored Fe-, Cr-, Ni-, Mo-, and Mg-modified aluminas as catalysts. Similarly, Na-X, Na-Y, H,Na-X, and H,Na-Y were employed by Przystaiko et al. [234] as a test reaction for sites with basic character. As the critical ability of the catalyst is to polarize bonds, acidic and basic sites play a role in the catalysis. However, the strength of the basic site will determine the effectiveness of the catalyst. 

The Zn ion, among the series of transition metals, is a cofactor which is not involved in redox reactions under physiological conditions. As a Lewis acid similar in strength to Mg , Zn participates in similar reactions. Hence, substituting the Zn ion for the Mg ion in some enzymes is possible without loss of enzyme activity. Both metal ions can function as stabilizers of enzyme conformation and their direct participation in catalysis is readily revealed in the case of alcohol dehydrogenase. This enzyme isolated from horse liver consists of two identical polypeptide chains, each with one active site. Two of the four Zn ions in the enzyme readily dissociate. Although this dissociation has no effect on the quaternary structure, the enzyme activity is lost. As described under section 2.3.1.1, both of these Zn ions are involved in the formation of the active site. In catalysis they polarize the substrate s C—O linkage and, thus, facilitate the transfer of hydride ions from or to the cosubstrate. Unlike the dissociable ions, removal of the two residual Zn ions is possible only under drastic conditions, namely disruption of the enzyme s quaternary structure which is maintained by these two ions. 

---