A Unified Single-Event Microkinetic Model for Alkane Hydroconversion in Different Aggregation States on Pt/H−USY-Zeolites

Abstract

A single-event microkinetic model for the catalytic hydroconversion of hydrocarbons on Pt/H-US-Y bifunctional zeolite catalysts developed for low-pressure vapor phase conditions was extended to cover high-pressure vapor phase and liquid phase conditions. The effect of the density of the bulk hydrocarbon phase on the physisorption as well as on the protonation steps of the reaction network was accounted for explicitly and can be interpreted in terms of "compression" of the hydrocarbon sorbate inside the zeolite pores and "solvation" of the catalyst framework by the dense bulk hydrocarbon phase. The bulk phase density effect on the physisorbed state is described via a single excess free enthalpy of physisorption. A dense bulk hydrocarbon phase destabilizes the sorbate molecules inside the catalyst pores. An expression of the excess free enthalpy of physisorption involving the fugacity coefficient and a zeolite dependent factor allows description of physisorption data. Typical excess free enthalpy values are in the range 1.5-5.1 kJ mol(-1) increasing with carbon number in the series of C5-C16 alkanes. At high-pressure vapor phase and liquid phase conditions, the excess standard protonation enthalpy is estimated at -7.8 kJ mol(-1) leading to relatively more stable carbenium ions at dense bulk phase conditions. As a result of the excess physisorption and protonation properties, the lightest hydrocarbons in mixtures are more competitive at dense phase conditions and their conversion is enhanced compared to low-density conditions.