Single catalytic active phases

The structure of the single phase bismuth-iron molybdate compound of composition Bl3FeMo20.2 related to the scheellte structure of Bi2Mo30-2( ). It is reported(, ) that the catalytic activity and selectivity of bismuth-iron molybdate for propylene oxidation and ammoxidatlon is not greater than that of bismuth molybdate. 

The syntheses of a variety of "multi-site" phase-transfer catalysts (PTCs) and the determination of their catalytic activity toward some simple Sn2 reactions and some weak nucleophile-weak electrophile SnAr reactions are described. In general, at the same molar ratio, the "multi-site" PTCs are as or more effective than similar "single-site" PTCs. Thus, the "multi-site" PTCs offer an economy of scale compared to "single-site" PTCs. 

A characteristic of catalysis processes is that a variety of compounds may catalyse a particular reaction, but only one or two of these catalysts show enough selectivity, activity and stability to warrant use in an industrial process. Selectivity is the ability of a catalyst to increase the relative rate of formation of a desired product when two or more competing reactions may occur. For modification of the direction of a reaction, mixed catalysts consisting of two compounds both with moderate to good catalytic activity have been developed. For example, the vapour phase oxidation of alcohols to aldehydes and ketones involves a mixed a- Fe203/ M0O3 catalyst rather than a single oxide. 

Preparation and characterization of two-dimensional zirconium phosphonate derivatives in either crystalline or amorphous forms have been investigated. Two composite zirconium phosphonates in single crystal phase have also been investigated and characterized by XRD, i c-, and 3ip-MASNMR. The catalytic performance over zirconium phosphonates are evaluated by hydrolysis of ethylacetate in aqueous solution. When the composite zirconium phosphonate is composed with an acidic function and with a hydrophobic function in single crystal phase, the catalytic activity in aqueous medium showed higher activity than that of single acidic zirconium phosphonate. The composite materials become accessible to any reactant molecule and improve hydnq>hobicity. 

The acidic function of single zirconium phosphonate showed rather poor catalytic activities for hydrolysis of ethylacetate in aqueous solutions. In addition, over Zr(03PCH2S03H)2 catalyst, the reaction proceeds as a homogeneous reaction, even though the catalytic activity is higher than other acidic zirconium phosphonates. The objective of this study is to explore the role of a second phosphonate function in single crystal phase on the catalytic performance of acidic function and hydrophobic function of zirconium phosphonates and to learn how to exploit this second function to achieve a catalytic advantage in certain applications. 

Impressive theoretical progress has been made in the prediction of fundamental modifications of catalyst surface structures and compositions as a function of the chemical potential of the environment in relatively simple cases (N0rskov et al., 2006 Reuter and Scheffler, 2002 Stampfl et al., 2002). This level of dynamic analysis with either single crystals or realistic polycrystalline catalyst materials has not yet been attained experimentally and certainly not in experiments with XRD under reaction conditions. There are no investigations that provide quantitative links between the phases and texture of a catalyst with its performance. All investigations discussed here can at best provide evidence relating the catalytic activity with a phase or a defect structure of a phase. 

True lipases show the interfacial activation phenomenon in their catalytic activity pattern. At low concentration of water-insoluble substrates, lipases are almost inactive, and the hydrolytic activity does not increase linearly. At a certain substrate concentration, however, the hydrolytic activity of lipases increases rapidly and the lipase kinetics resembles normal enzyme kinetics. This boost in activity is related to the formation of water-insoluble substrate aggregates such as micelles or another second phase. Only when this second phase is present, do lipases become fully active. This interfacial activation is caused by a large conformational change in the 3D structure of the lipases. In their water-soluble form, the active site is covered by a lid, which prevents the substrates from reaching it. At the lipidAvater interface, the lid is opened and the active site is accessible to the substrates. In addition, the now accessible area is mainly hydrophobic, which gives the open-form lipase the shape and behavior of conventional surfactant molecules with a hydrophilic and a hydrophobic moiety in one single molecule. 

Figures 1.1 a-c show the relationships between the catalytic activity and the degree of development that have been studied in the hydrogenation of cyclohexanone, naphthalene, and benzene over single phase NiAl3 and Co2A19 alloys. The rates of hydrogenation peak at around 82-86% degrees of development with both the alloys, and tend to decrease markedly with further development, irrespective of the compounds hydrogenated. It is noted that the cobalt catalyst from Co2A19 is...

Carbonyl addition reactions include hydration, reduction and oxidation, the al-dol reaction, formation of hemiacetals and acetals (ketals), cyanohydrins, imines (Schiff bases), and enamines [54]. In all these reactions, some activation of the carbonyl bond is required, despite the polar nature of the C=0 bond. A general feature in hydration and acetal formation in solution is that the reactions have a minimum rate for intermediate values of the pH, and that they are subject to general acid and general base catalysis [121-123]. There has been some discussion on how this should be interpreted mechanistically, but quantum chemical calculations have demonstrated the bifunctional catalytic activity of a chain of water molecules (also including other molecules) in formaldehyde hydration [124-128]. In this picture the idealised situation of the gas phase addition of a single water molecule to protonated formaldehyde (first step of Fig. 5) represents the extreme low pH behaviour. 

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