Adsorption base catalysis

Volume 20 Catalysis by Iteids and Bases. Proceedings of an International Symposium, Villeurbanne (Lyon), September 25-27, 1984 edited by B. Imelik, C. Naccache, G. Coudurier,Y. Ben Taarit and J.C.Vedrine Volume 21 Adsorption and Catalysis on Oxide Surfaces. Proceedings of a Symposium,... 

The hydrolysis is industrially conducted by acid or base catalysis (temperatures 210°C-260°C) or by enzymatic hydrolysis in the case of sensitive fatty acids. The resulting crude product mixture is separated and purified mainly by means of distillation or crystallization, and more rarely by adsorption or extraction. 

Considering adsorption as the first step in catalysis, both physical and chemical adsorption has been observed with intercalates. Watanabe et a/. have shown that selective adsorption based on molecular size occurs with potassium-, rubidium-, and caesium-graphite. The molecular sieve properties are not as distinctive as in zeolites, as a result of the comparatively easy expandability of the layers. Once the gas enters the layers, both chemisorption and chemical reaction is possible. Therefore, for example, adsorption... 

The present finding shows the critical importance of the atomic-scale design of the surface of a material. The arrangements of Ti4 + ions and O2- ions create new catalytic functions for desired chemical processes. The active sites can be produced in situ under the catalytic reaction conditions, even if there are no active sites at the surface before the catalysis. The surface is also modified by adsorption of a reactant to form a new surface with a different add-base character from the intrinsic property. The dynamic acid-base aspect at a catalyst surface is the key issue to regulate the acid-base catalysis, which may provide a new strategy for creation of acid-base catalysts. 

The use of nitromethane as a probe of basicity of zeolites (NaX, CsX, CsX 9Cs) and mixed oxides, Mg(Al)0, is discussed. Various species (physisorbed nitromethane, aci-anion nitromethane, and methazonate salt analogue) formed upon nitromethane adsorption were characterized by C MAS NMR spectroscopy. Heterogeneous base catalysis of the Michael addition of nitromethane on cyclohex-2-en-1-one was also studied. Low rates were obtained for catalysts showing only nitromethane physisorption. Formation of aci-anion nitromethane was observed for solids of medium efficiency correlation of the chemical shift with the initial rate was established. Finally, the decrease of Lewis acidity and concomitant increase of basicity led to methazonate formation and to the more efficient catalysts. 

It is now widely recognized that this important result of systematic surface research constitutes a firm base of Taylor s idea, expressed in 1925, of active sites and stracture sensitivity in heterogeneous catalysis [25Tay], Moreover, atomic steps are not only crystallographic features but also electronic defects since their local electronic stmcture, characterized by the local electron charge density, differs from that of terrace sites. This was experimentally demonstrated by work function measurements of stepped surfaces which led to the assigmnent of a dipole moment to the step [77Bes]. In this context the so-called stractural effect of adsorption (and catalysis) is at the same time also an electronic effect. 

In the transmission method, the /rt component of radiation that is regularly transmitted through the sample is detected (Fig. 1.22). Measurements of this component do not require any special device—the peUet pressed from the powder is directly positioned in the holder in the sample compartment so as to be in the beam focus. This means that studies of adsorption and catalysis in situ are quite straightforward, based on reactive chambers with a fairly simple optical scheme [111]. Theoretical and practical aspects of the IR transmission spectroscopy of molecules adsorbed on powders have been treated in excellent monographs [108, 109] and will not be repeated here, except to list the basic requirements of the sample. 

The solid/gas interface was traditionally studied with respect to adsorption and catalysis. Here the assertion that the Bronsted definition of acidity is a particular case of the Lewis definition is neither obvious nor even helpful. It suffices to say that many reactions in heterogeneous catalysis require specifically the presence of either Bronsted or Lewis acidic (or basic) sites, and the reaction mechanisms depend on the nature of the surface site. A long-term goal of surface studies for the characterization of solid catalysts was to distinguish and quantify the number of Bronsted or Lewis sites with potential catalytic activity for gas-phase reactants. For that reason, when discussing the acid-base behavior of solid surfaces, it is no longer possible, nor desirable, to adopt the viewpoint that subsumes Bronsted acid-base properties in the more general Lewis definition. 

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