Kinetic modeling of pore mouth catalysis in the hydroconversion of n-octane on Pt-H-ZSM-22

A microkinetic model based on the single-event theory was developed to describe the hydroconversion of n-octane on Pt-H-ZSM-22 zeolite. In the model, alkene protonation and subsequent acid-catalyzed reactions, i.e., skeletal isomerization and cracking, occur in the pore mouths of ZSM-22. In contrast to USY, in these pore mouths, the molecules are physisorbed according to different modes, each with a specific enthalpy and entropy loss. Moreover after protonation of the physisorbed alkenes, the stability of the resulting carbenium ions increases with the number of carbon atoms entering the pore mouth. The reaction network is based on transformations of alkylcarbenium ions that can be obtained by protonation of physisorbed alkenes in pore mouths. Reactions that are sterically hindered or that lead to alkylcarbenium ions with charged carbon atoms outside the pore were discarded. In particular, methyl shifts as well as branching and β-scission reactions involving tertiary alkylcarbenium ions were excluded. The composite activation energy, ΔEactcomp, i.e., the sum of the protonation enthalpy and the real activation energy for the allowed reactions, was taken from data on a reference ultrastable Y zeolite (CBV760). This pore mouth catalysis model adequately describes the conversion of n-octane on Pt-H-ZSM-22. The difference in composite activation energy ΔEactcomp of −8.9 (±0.3) kJ mol−1 between the reference USY zeolite and the ZSM-22 corresponds to the higher average acid strength in ZSM-22. Further, the yield pattern of individual isomers of octane is described well using three parameters: (i) hcd with a value of −1.1 (±0.2) kJ mol−1 accounting for differences in carbenium ion stability proportional to the number of carbon atoms entering the pore mouth; (ii) ΔEpcp; and (iii) ΔEβ with values of −4.0 (±0.2) and −16.7 (±2.1) kJ mol−1.