Rudolph M. Lago

Transport and reactivity of hydrocarbon molecules in a shape-selective zeolite

For the catalytic cracking of C6 to C9 hydrocarbons on ZSM-5, we demonstrate quantitatively the contributions of each of two mechanisms for molecular shape selectivity. Using crystallites of different sizes and activities, and classical methods for evaluating diffusion inhibition of the reaction rate, we separate the effects of mass-transport-induced selectivity from that created by steric inhibition by the size of a reaction complex. The selective cracking of n-paraffins compared to monomethyl paraffins (from C6 to C9) is due to a higher intrinsic rate constant of the n-paraffin, with diffusional mass transport playing no appreciable role. In contrast, dimethyl paraffin cracking is strongly diffusion-inhibited. The methyl paraffin/n-paraffin discrimination is a result of steric constraint on the sizeable methyl paraffin/carbonium ion reaction complex. This structural selectivity is shown to be absent for the corresponding olefins where such complexes do not arise. The diffusivities at reaction conditions have been determined. For the linear hydrocarbon, diffusivity notably exceeds that expected from the Knudsen model. This reminds us to review assumptions of conventional concepts of mass transport. The availability of zeolites now allows us to probe many basic phenomena in catalysis, molecular configuration and dynamics, including mass transport.

The Kinetics of the Cracking of Cumene by Silica-Alumina Catalysts

Publisher Summary This chapter discusses the kinetics of the cracking of cumene by silica-alumina catalysts. The cracking of cumene has received considerable attention as a reaction typical of one class of cracking reactions—namely, dealkylation of aromatics. The reaction is particularly simple, yielding only benzene and propylene. An integral reactor is one that yields data already containing an integration of the kinetics over a changing concentration of reactant in the catalyst bed—that is, a reactor operated so that the conversion of reactant to products is relatively high. The rate of gas production can be considered essentially identical with the rate of cumene cracking. The rate of production of gas is measured by noting the pressure rise on a water manometer in a given interval of time. The calibration of the system necessary to convert the pressure reading to moles of propylene produced is accomplished by means of duplicate readings made when a known volume is added to the system.

54 The Inhibition of Cumene Cracking on Silica-Alumina by Various Substances

In the elucidation of the kinetics of the cracking of cumene on silicaalumina catalyst, the actions of inhibitors (poisons) on the reaction were studied. These inhibitors compete with cumene for cracking sites. Theoretical analysis leads to an expression from which the equilibrium constant for adsorption of inhibitors on cracking sites can be calculated. The kinetics are given for both the “differential” (Schwab) reactor, in which the reactant concentration is essentially constant over the whole catalyst bed, and the “integral” reactor, in which the reactant concentration decreases significantly as it passes over the bed. The equilibrium constants for adsorption on cracking sites are given for some pure hydrocarbons and some oxygen, sulfur, and nitrogen compounds. Several of the calculations are made from data in the literature. For some compounds studied, the equilibrium constants were determined at different temperatures. Heats and entropies of adsorption on cracking sites are calculated.