The kinetics of hydrocarbon conversion on ZSM-5 zeolites
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The Kinetics of Hydrocarbon Conversion on ZSM-5 Zeolites M. A. Botavina, N. V. Nekrasov, and S. L. Kiperman t Zelinskii Institute of Organic Chemistry, Russian Academy of Sciences, Moscow, 117913 Russia ReceivedMay 12, 1999 Abstract--The complete oxidation of hydrocarbons with various structures (methane, n- and iso-pentanes, cyclohexane, and benzene) on copper-modified ZSM-5 zeolite and the aromatization of propane on H-ZSM-5, are studied to analyze diffusion effects on the reaction kinetics. Comparison of the kinetic parameters of complete oxidation shows that the process rate and the rate law strongly depend on the reactant structure. Apparently, n-pentane oxidation is controlled by kinetics; the kinetics for other hydrocarbons depends on diffusion inside zeolite channels to different degrees. In the case of propane aromatization, its dehydrogenation and cracking, as well as the formation of aromatics from olefins, occurs on different active sites. Propane conversion is supposed to occur inside the zeolite channels, and the aromatization of olefins occurs on the outer surface of the zeolite crystal. INTRODUCTION Zeolite catalysts are applied to various processes. For example, ZSM-5 modified with metals is a good catalyst for the processes of complete hydrocarbon oxidation [1-5]. However, most of the studies of reactions on zeolites provide incomplete kinetic data, because it is difficult to determine how diffusion affects reaction rates. Diffusion was shown to alter the numerical values of the rate constants and the process selectivity [6-8]. To solve the problem, we need to study the conversion of related compounds with different structures in the same reaction. It is desirable to exclude the influence of product diffusion on the reaction rate. The complete oxidation of hydrocarbons leads to the formation of carbon dioxide and water and thus suits the goal. We studied the kinetics of the complete oxidation of hydrocarbons that have different structures (methane, n- and iso-pentanes, cyclohexane, and benzene) on ZSM-5 zeolite modified with copper. We chose the reaction of propane aromatization on H-ZSM-5 to study the effect of product diffusion on reactant conversion. The products of this reaction have lower diffusion coefficients than the reactants. We choose ZSM zeolites for our study for the following reasons. The selective cracking of normal alkanes is possible on 4A and 5A zeolites [9]; however, the cracking of isoalkanes does not proceed because of the diffusion constraints, which do not exist for the cracking of both kinds of hydrocarbons on 10X zeolite. The pore size in ZSM-5 zeolite (5.1-5.8 A) is intermediate between those of A zeolites (3.8-4.3/~) and X zeolites (7.5-9.5 A,). This is why we could expect some influence of diffusion constraints on the process rate for the reaction on ZSM-5. tDeceased.
The kinetic study of complete methane, n-pentane, cyclohexane, and benzene oxidation, as well as propane aromatization on ZSM-5-based catalysts. The results of the study are presented below. EXPERIMENTAL
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