Targeted Modification of Zeolites for Exceptionally Active and Selective Generation of PX and Light Olefins from Methanol–Toluene Co-Conversion

Abstract

External surface modification is an effective means of achieving selective production in the acid-catalyzed process over the zeolite. However, catalyst modification, including external surface modification, often fails to break the seesaw effect between the reaction activity and selectivity. In the present work, an acid site-targeted chemical adsorption deposition (acid site-chemical adsorption–deposition (ASCAD)) method is applied to precisely control the deposition of silica. The modified ZSM-5 (ZSM-5-ASCAD) shows notable improvements in shape selectivity and catalytic activity in methanol–toluene coconversion. The total selectivity of light olefins and paraxylene (PX) reaches 94%, and the proportion of PX among xylene isomers is 99.5%. Meanwhile, toluene conversion is maintained at 43%, which is much higher than that over ZSM-5 modified by the conventional chemical liquid deposition (CLD) method (18%). Applying multiple techniques, including time of flight secondary ion mass spectrometry for depth profiling, the zero length column method combined with infrared microscopy (IRM) for diffusion evaluations and isotope labeling technology to reveal the mechanism and reaction pathway, we confirm that the ASCAD method achieves a minimized silica deposition that precisely shields the acid sites on the external surface while introducing only a slight impact on the diffusion compared to the severe diffusion depression of the CLD method. ASCAD modification effectively suppresses unwanted and uncontrollable side reactions and maintains high reactant conversion simultaneously. This unique modification method minimizes the disparity in mass transfer capability between the reactant methanol and toluene, which has not been achieved with other modification methods before, leading to enhanced methanol–toluene coconversion within the ZSM-5 crystal and exhibiting promoted ethene production and super high PX selectivity at the same time. Targeted modification of the zeolite surface provides an effective approach to simultaneously enhancing the activity and shape selectivity of zeolite-catalyzed reactions.

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