Catalytic Selectivity of Catalyst Performance Parameters
Catalytic selectivity is one of the important parameters of catalyst performance. Catalytic selectivity refers to the fraction of the consumed raw material that is converted into the target product, and the measure of selectivity is generally the selectivity factor σ (the ratio of the reaction rate constants). Catalytic selectivity illustrates the relative magnitude of the degree of the primary and secondary reactions; it mainly includes the reaction selectivity, the substrate selectivity and the product selectivity.
(1) Reaction selectivity: the degree of reactants along a certain pathway, compared with the degree of reaction along other pathways, that is, the selectivity of the catalyst for a reaction. If a mixture of two substances A1 and A2, catalyzed by the same catalyst, produces products B1 and B2, respectively, the reaction formula (usually a competitive reaction):
(2) Substrate selectivity: for a class of catalytic substrate reaction and other substrates do not react, which is very typical of a selective selectivity, commonly found in molecular sieves, MOF, COF and CMP and other porous catalysts, due to the size of the substrate molecules are not the same, the catalytic site in the pore, the substrate molecules are too large, can not get into the pore, and therefore can not occur in the catalytic reaction, the realization of the selectivity of the substrate. Such as COF type catalytic:
(3) Product selectivity: there is only one raw material, but there are different reaction directions, thus realizing substrate selectivity.
On the reasons for the emergence of catalytic selectivity:
(1) Selectivity due to different reaction mechanisms, different catalysts are selective for specific reaction systems, called mechanism selectivity. For example, dehydration and dehydrogenation of ethanol over oxide catalysts:
(2) Catalysts are selective due to different catalyst structures, called diffusion selectivity, and selectivity can be realized when the diffusion process of reactants in the catalyst pores is a tachycritical step.
(3) Thermodynamic causes, called thermodynamic selectivity. For example, the selective hydrogenation of ethylene and acetylene, the hydrogenation rate of ethylene is greater than that of acetylene, but when ethylene and acetylene co-exist, the hydrogenation rate of ethylene is slower than that of acetylene, mainly due to the adsorption of acetylene in the active center, which is thermodynamically superior to that of ethylene, so that ethylene can not be adsorbed at all under the reaction conditions, thus realizing a better substrate selectivity for acetylene.
For any kind of industrial catalyst, it is necessary to have high selectivity, and when catalytic activity and selectivity cannot be realized at the same time, activity is often sacrificed for selectivity; thus the importance of selectivity can be seen. For example, in the ethylene polymerization reaction, if the raw material ethylene contains a trace amount of acetylene (e.g., 10 ppm), then the resulting polymer will contain a small amount of triple bonding, generating colored impurity polymers, so that the quality of the product is significantly reduced. A good way to remove acetylene is to selectively hydrogenate the active triple bond, but in general when hydrogenated, often part of the double bond of ethylene is also hydrogenated, so to hydrogenate the triple bond without involving the double bond at the same time, it is necessary to use a highly selective catalyst. At present, there is one such catalyst in the industry-Pd/Al2O3, which can replace the commonly used platinum (Pt) and nickel (Ni) catalysts, and the price of Pd is only one-fourth of the price of Pt, which solves the problem of removing impurity acetylene from ethylene.
