Characterization of heterogeneous catalysts through model reactions

Catalysis is omnipresent in our technological society. It is estimated that over 95% of all molecules synthesized have seen a homogeneous or heterogeneous catalyst at least once. Catalysts, and in particular solids (heterogeneous catalysis), are essential for the selective manufacture of a wide range of intermediates and chemicals under optimum conditions, as well as for the pollution control of plant effluents and exhaust gases. Heterogeneous catalysis, along with biocatalysis to a lesser extent, is the keystone of the energy transition and the solution to many of our society's environmental challenges. This multi-disciplinary field, which is both scientifically and economically exciting, requires knowledge of catalytic materials, from their preparation to their characterization (in particular of their surface), through to an understanding of reactions (kinetics and mechanisms), not forgetting the optimization of their implementation.

Physico-chemical methods are frequently used to characterize heterogeneous catalysts. However, the operating conditions for analysis inherent in each characterization technique are often far removed from those of catalytic reactions: for example, is the information obtained for a catalyst studied under vacuum sufficient to understand the properties of a catalyst operating under pressure? This discrepancy is limited or non-existent when we characterize the catalyst using model reactions carried out under conditions that are close to industrial conditions. Based on in-depth knowledge of their mechanism and/or reaction intermediates and products, the use of model reactions makes it possible to study, under operating conditions close to the application, the characteristics of the active sites: nature (acid, base, acid-base, redox), strength, density, environment and their effect on the reaction rate. Meanwhile, textural properties such as external surface, microporous and mesoporous volumes, crystal size and shape, etc., remain almost exclusively accessible via ex-situ analysis methods.

This article therefore aims to list the contribution of model reactions in characterizing the major families of heterogeneous catalysts, such as oxides (SiO ₂ -Al ₂ O ₃ , Al ₂ O ₃ , MgO, etc.), zeolites, supported metals and sulfide catalysts. Particular attention will be paid to the conditions under which this characterization technique is implemented, in order to obtain usable activity and selectivity data. As the reactions adapted to each of these major catalyst families are independent, the reader will be able to restrict his or...