Monodentate Formate-Mediated Toluene Methylation to Xylene at Low Temperature Using Formic Acid and Hydrogen

The industrial methanol-toluene methylation process for xylene production faces a challenge of lowering the reaction temperature while suppressing the side reaction of methanol to hydrocarbons. This depends on the low-temperature formation of the methoxy intermediate required for methylation. Here, we establish a route for methoxy formation in which formic acid (FA) hydrogenates via a metastable monodentate formate intermediate. With the GaZrO_x-ZSM-5 catalyst, Ga-H bonds act as a "placeholder", enabling FA adsorption as a monodentate formate rather than a stable bridged formate. At 300 °C and 3.0 MPa, toluene conversion reaches 13.9% with 77.6% xylene selectivity, outperforming established routes. Kinetic isotope effect experiments revealed that Ga-based catalysts show a secondary isotope effect, whereas non-Ga catalysts exhibit an inverse isotope effect. Density functional theory calculations indicated that the monodentate formate was more susceptible to hydrogenation which then facilitated methylation. This work provides a competitive route for xylene production from toluene under mild conditions using sustainable FA and H₂.