Theo L. M. Maesen

Molecular Simulations of Zeolites: Adsorption, Diffusion, and Shape Selectivity

Note: Chevron, Energy Technology Company, 100 Chevron Way, Richmond, California 94802-0627 Reference EPFL-ARTICLE-200588doi:10.1021/cr8002642 URL: http://pubs3.acs.org/acs/journals/doilookup?in_doi=10.1021/cr8002642 URL: http://pubs.acs.org/doi/pdfplus/10.1021/cr8002642 Record created on 2014-08-14, modified on 2017-05-12

Towards a molecular understanding of shape selectivity

Shape selectivity is a simple concept: the transformation of reactants into products depends on how the processed molecules fit the active site of the catalyst. Nature makes abundant use of this concept, in that enzymes usually process only very few molecules, which fit their active sites. Industry has also exploited shape selectivity in zeolite catalysis for almost 50 years, yet our mechanistic understanding remains rather limited. Here we review shape selectivity in zeolite catalysis, and argue that a simple thermodynamic analysis of the molecules adsorbed inside the zeolite pores can explain which products form and guide the identification of zeolite structures that are particularly suitable for desired catalytic applications.

Shape selectivity through entropy

Based on a comparison between measured and simulated adsorption properties, we demonstrate that a decrease in the Gibbs free energy of formation and adsorption—due to higher adsorption entropy—satisfactorily explains the selective production and adsorption of the most compact, branched paraffins in n-hexadecane hydroconversion in molecular sieves with pore diameters of ∼ 0.75 nm. Adsorption entropy is important because the pores are saturated with reactant, and because the adsorbed phase is not at gas-phase chemical equilibrium. This explanation supplants the traditional kinetic explanation involving changes in the Gibbs free energy of formation of the relevant transition states. Instead, we attribute the effect of molecular sieve structure on the branched paraffin yield to a redirection of the hydroisomerization reactions away from the gas-phase chemical equilibrium distribution, commensurate with the Gibbs free energy of adsorption of the isomers inside the pores. These shape-selective changes to the reaction rates appear to be as ubiquitous as those originating from steric constraints imposed on intracrystalline diffusion and reaction rates. This would make adsorption-induced changes in the Gibbs free energy of formation of reactants, intermediates, and products a missing cornerstone in traditional shape selectivity theory.  2003 Elsevier Science (USA). All rights reserved.

Understanding Zeolite Catalysis: Inverse Shape Selectivity Revised

Reference EPFL-ARTICLE-200562doi:10.1002/1521-3773(20020715)41:14 3.0.CO;2-T Record created on 2014-08-14, modified on 2017-12-10

Hydrocracker Pretreat Catalyst Development

This paper reviews the history of Chevron Technology Marketings hydrocracker pretreat catalyst development. The excellent hydrodenitrogenation (HDN) performance on recently developed catalysts is explained by the theory that more Type II active sites lead to better HDN/HDS (hydrodesulfurization) performance.