Kinetics and Mechanism of Benzene, Toluene, and Xylene Methylation over H‑MFI

The methylation of benzene, toluene, para-xylene, and ortho-xylene over MFI structured H-ZSM-5 and mesoporous self-pillared pentasil (H-SPP) with dimethyl ether (DME) at low conversions (<0.1%) and high DME:aromatic ratios (>30:1) showed linear rate dependencies on aromatic pressure and zero dependence on DME pressure for benzene and toluene. These results are consistent with studies performed for olefin methylation, and are indicative of a zeolite surface covered in DME-derived species reacting with benzene or toluene in the rate-determining step. Saturation in the reaction rate was observed in xylene pressure dependence experiments (at 473 K, <5 kPa xylene); however, enhancement in the reaction rate was not observed when comparing ∼1 μm crystallite H-ZSM-5 and 2–7 nm mesopore H-SPP, indicating that xylene methylation proceeds in the absence of diffusion limitations. Simultaneous zero-order rate dependencies on xylene and DME pressures are described by a model based on adsorption of xylene onto a surface methylating species. This model is consistent with observed secondary kinetic isotope effects (k_H/k_D = 1.25–1.35) and extents of d₀, d₃, and d₆ DME formation in the effluent because of isotopic scrambling between unlabeled and d₆ DME when co-fed with aromatics over H-ZSM-5. Post-reaction titration of surface species with water after desorption of physisorbed intermediates showed a 1:1 evolution of methanol to Al present in the catalyst, indicating the presence and involvement of surface methoxides during steady-state methylation of aromatics species.