Rate Parameter Distributions for Isobutane Dehydrogenation and Isobutene Dimerization and Desorption over HZSM-5

Abstract

Deconvolution of the evolved isobutene data obtained from temperature-programmed, low-pressure steady-state conversion of isobutane over HZSM-5 has yielded apparent activation energies for isobutane dehydrogenation, isobutene dimerization and desorption. Intrinsic activation energies and associated isobutane collision frequencies are also estimated. A combination of wavelet shrinkage denoising, followed by time-varying flexible least squares of the evolved mass-spectral abundance data over the temperature range 150 to 450 °C, provides accurate, temperature-dependent, apparent rate parameters. Intrinsic activation energies for isobutane dehydrogenation range from 86 to 235.2 kJ mol⁻¹ (average = 150 ± 42 kJ mol⁻¹) for isobutene dimerization from 48.3 to 267 kJ mol⁻¹ (average = 112 ± 74 kJ mol⁻¹) and for isobutene desorption from 64.4 to 97.8 kJ mol⁻¹ (average = 77 ± 12 kJ mol⁻¹). These wide ranges reflect the heterogeneity and acidity of the zeolite surface and structure. Seven distinct locations and sites, including Lewis and Brønsted acid sites can be identified in the profiles. Isobutane collision frequencies range from 10⁻⁰˙⁴ to 10²²˙²s⁻¹ and are proportional to the accessibility of active sites, within the HZSM-5 micropores or on the external surface.