R. Ramos Pinto

The Acidity of Zeolites: Concepts, Measurements and Relation to Catalysis: A Review on Experimental and Theoretical Methods for the Study of Zeolite Acidity

In this article, we considered all aspects of acidity (nature of acid sites, strength, density, etc.) in solid catalysts and in zeolites in particular. After reminding the definition of acidity in liquid and solid acids, we emphasized acidity characterization by the most used physical techniques, such as Hammetts indicator titration, microcalorimetry of adsorbed probe molecules (ammonia, pyridine or other amines for acidity characterization and CO2 or SO2 for basicity characterization), ammonia or any amine thermodesorption, IR spectroscopy of hydroxyl groups and of several probe molecules adsorbed (ammonia, pyridine, piperidine, amines, CO, H2, etc.), MAS-NMR of 27Al, 29Si, 1H elements and of 1H, 13C, 31P, etc. of adsorbed probe molecules, and model catalytic reactions. Modeling the way the acid features of zeolites influence the catalytic activity of these catalysts toward acid-catalyzed reactions (relation between ammonia desorption activation energy values and catalytic activities, reaction mechanism, and kinetics) completes the general analysis of acidity and zeolite chemistry.

Analysis and Modelling of Multi-Site Acid Catalysts

Acid catalysis plays a major role in industrial applications of catalytic processes. In particular, the use of microporous solid acid catalysts has an enormous impact in the petroleum refining industry, where large-scale processes, such as the catalytic cracking of heavy oil fractions, are carried out over very active zeolite catalysts. In this paper we will try to give an overview of the way one can characterise the acidity of a solid catalyst and used this information to predict the activity of the catalyst for a given acid catalysed reaction.

Acidity, Activity and Micro-Kinetics Studies in an H-ZSM5

An H-ZSM5 zeolite has been characterized by TPD of ammonia and its performance was tested with several olefins (ethylene and propylene) and n-hexane. The catalytic tests were performed at a wide range of temperatures (300 – 450°C) and molar fractions of reagent in the feed. The results from the TPD have been deconvoluted in order to obtain a distribution of the strength of the acidity of the active sites. A micro-kinetic model and a simplified kinetic equation have been developed and fitted to results for the conversion of n-hexane and the olefins, respectively.