A perspective on catalysis in solid acids

Abstract

Solid Bronsted acids continue to be important for a large number of catalytic processes, and it is important to have reliable methods to characterize both the site concentrations and strengths. Solid Bronsted acids are very different from homogeneous acids and attempts to equate the two should be discouraged. Some of the most commonly used techniques for measuring site densities and strengths, such as NH3 TPD, are unreliable. Site densities can be determined from TPR measurements of alkylamines or IR measurements of adsorbed pyridine but determining site strengths remains a challenge. Reaction rates are the most reliable means for defining site strengths; however, the presence of co-catalysts (in the case of alkane activation), adsorption geometry, surface coverage, and diffusion can all influence rates and product selectivities, so that higher reaction rates do not always imply “stronger” sites.