Silanol nests-enriched silicalite-1 zeolite catalyst for efficient toluene alkylation with methanol to para-xylene

Controllable and stable toluene alkylation with methanol to p-xylene is an important but challenging research target. Conventional Al-containing ZSM-5 catalysts suffer from low alkylation efficiency and quick coke deposition due to strong methanol self-reaction and microporous diffusion limitation. Herein, we report Al-free silanol nests-enriched silicalite-1 zeolite synthesized through simple seeds induction, which present outstanding alkylation performance. 1H MAS NMR, OH-IR and TEM etc. results indicated that crystallization temperature, time and seed amount strongly determined silanol nests formation and final crystal size. Alkylation performance showed that the weak acidity of silanol nests suppressed methanol self-reaction and improved alkylation efficiency to above 50.0% even at a high toluene conversion (∼20.0%), and no significant deactivation was found due to the reduced coke deposition. Inhibited p-xylene isomerization on the inert external surface increased PX selectivity in xylene to 55.9% for silicalite-1 with 1.6 μm crystal size. Silica layer was coated on the external surface of large-sized silicalite-1 to increase the diffusion resistance, which further increased PX selectivity to 70.6%. Surprisingly, significantly improved mass diffusion at 100 nm crystal size of silicalite-1 obviously increased toluene conversion rate to 39.1% with a quite high alkylation efficiency of 94.7%, although the shape-selection ability for PX was weakened. Moreover, concerted conversion mechanism for toluene and methanol on the silanol nests was confirmed as a probable mechanism based on in situ DRIFTS. This work provides more options for designing silanol nests-enriched silicalite-1 catalysts to modulate the alkylation efficiency and catalytic stability for toluene alkylation.