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+.
Bibliographical noteFunding Information:
We thank Dr. Dandan Xu for synthesizing the SPP-HZSM-5 catalyst, and B.L.F. thanks Dr. Thomas Chen, Professor Matthew Neurock, Dr. Andrew Hwang, Professor Praveen Bollini, Professor James Harris, Dr. Anurag Kumar, Dr. Linh Bui, Neil Razdan, Matthew Simons, Jacob Miller, Zhichen Shi, Dr. Sukaran Arora, and Dr. Udit Gupta for their helpful technical discussions. This material is based upon the work supported by the National Science Foundation Graduate Research Fellowship under Grant no. 00039202. We also acknowledge the National Science Foundation (CBET 1701534) for financial support.
Copyright © 2020 American Chemical Society.
- catalyst deactivation
- chemical transients
- kinetic modeling
- methanol-to-hydrocarbons conversion