Neopentane Conversion Catalyzed by Pd in L-Zeolite: Effects of Protons, Ions, and Zeolite Structure

Abstract

The conversion of 2,2-dimethylpropane (neopentane) has been studied over zeolite-L-supported Pd catalysts; the results are compared with data for Pd on zeolite Y. Variation of the charge compensating ion (Li+, K+, or Ca2+) in L has only a moderate effect on the catalytic parameters, due either to variations in the heat of physisorption or to "electronic" effects on Pd. The reduction temperature, TR, of Pd/L samples that have previously been calcined at 250°C, strongly affects the catalytic performance: after TR = 250°C, sufficient NH3 ligands of the ion exchanged Pd(NH3)2+4 ions survive to neutralize the protons that are formed during reduction; catalytic activity then increases during reaction while NH+4 ions decompose and Pd-proton adducts of high catalytic activity are formed. At TR = 400°C, the NH3 ligands are destroyed and this catalyst is stable and very active. Activity, selectivity, and kinetic parameters depend on the structure of the support; apparent activation energies, Ea, are in the range of 45-55 kcal/mol for Pd/Y, but 75-90 kcal/mol for Pd/L. A linear relation between Ea and log of the preexponential factor is obtained. While classical pore-diffusion operates in Y, single-file diffusion appears more likely in L; i.e., channels tend to be plugged with physisorbed molecules at low temperature. The difficult of primary reaction products to escape from the pores correlates with a higher fraction of secondary products observed for Pd/L vis-á-vis Pd/SiO2. It is conceivable that different diffusion types also contribute to the different Ea values for zeolites with three-dimensional or unidimensional channel systems.