Date of Award

1990

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical Engineering

First Advisor

Kerry M. Dooley

Abstract

The diffusivities of molecules whose sizes approach the intracrystalline pore sizes of zeolites have been studied at and near conditions prevailing during catalysis, by both desorption and kinetics techniques. For the desorption approach, a series of unary isothermal desorption experiments (following an almost instantaneous initial temperature rise) were conducted for toluene or benzene with five different zeolites. Diffusion and readsorption are shown to control the process. The intracrystalline diffusivities (D$\sb{\rm e}$) were derived by fitting the desorption curves with four models of varying complexity; only models which take into account readsorption (either at equilibrium or not) can both fit the data adequately and give reasonable estimations of D$\sb{\rm e}$. A nonequilibrium readsorption model is probably superior to an equilibrium readsorption model, because: (1) it can fit both the rising and falling portions of isothermal desorption curves and (2) the D$\sb{\rm e}$'s predicted by such a model are comparable to those reported in the literature by other techniques (chromatographic, adsorptive uptake) under similar conditions. But these D$\sb{\rm e}$'s are still at least three orders of magnitude smaller than those predicted by the Pulse Field Gradient (NMR) technique. For the kinetics approach, the disproportionation of toluene, a well understood reaction, was studied using the same zeolites at intraparticle-diffusion-limited conditions in order to estimate the product of effectiveness factors and rate constants. The D$\sb{\rm e}$ for toluene in counterdiffusion was estimated by fitting the rate data to generalized effectiveness factor expressions, using literature results for the rate expression. The rate constants obtained in this study are comparable to the second-order intrinsic rate constants for zeolite catalysts obtained from the literature; however, the effective binary D$\sb{\rm e}$'s for toluene computed from this approach are at least two to three orders of magnitude smaller than the D$\sb{\rm e}$'s derived from the desorption curves. This result suggests a reduction of molecular mobility resulting from restricted passage of counterdiffusing reactants in zeolite windows during catalysis.

Pages

326

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