Transient and Steady-State Kinetic Studies of Formaldehyde Alkylation of Benzene to Form Diphenylmethane on HZSM-5 Catalysts

A combined steady-state and transient kinetic study on the mechanism of diphenylmethane (DPM) formation in HZSM-5 from HCHO and benzene revealed two kinetically relevant steps - the alkylation of benzene by HCHO and the deprotonation of a diphenylmethane benzenium ion (DPM+) to form DPM. The functional dependence of the rate of each of these reactions was determined by observing the transient rate after a step-change in reactant partial pressures, specifically whether the rate was discontinuous through a step-change in partial pressure. Steady-state isotopic switching experiments revealed that a persistent surface intermediate with two aromatic rings, likely DPM+, is formed and has a fractional coverage ranging from sparse (near-zero) to complete (near-one) that varies with process conditions. Reaction orders obtained from steady-state rate measurements suggest that HCHO, C6H6, and H2O competitively adsorb on acid sites and that DPM+ deprotonation is first-order in C6H6, implying that deprotonation is assisted by the presence of aromatics. Herein, we propose a complete mechanism for the condensation between HCHO and C6H6 to form DPM, and from this mechanism, a six-parameter (three kinetic/thermodynamic parameters, three apparent activation/thermodynamic energies) kinetic model is derived that quantitatively describes the transient and steady-state rates and steady-state fractional coverages of DPM+.