Relay and Feedback Conversion Effect of Hydrocarbons between ZSM-5 Grains on Product Selectivity and Coke Formation during Methanol Aromatization

The catalytic conversion of hydrocarbon molecules over an acidic silica-aluminum molecular sieve is the core of the petrochemical and coal chemical industry. Finely controlling the reaction process between catalyst grains for catalytic stability and product selectivity can be achieved by configuring zeolites with different acid properties, but the details continue to be indefinable. Herein, mixed ZSM-5 with different acid configurations was used as the catalytic platform to study the inter-catalytic behavior of the grains using methanol to aromatics as the model reaction. It was found that the obvious multiple reactions of hydrocarbon intermediates occurred between ZSM-5 grains. The typical C3–C5 hydrocarbons produced from Zn-free ZSM-5 with a low SiO2/Al2O3 ratio could be aromatized on adjacent Zn60 grains. Although the external coke deposition of Zn-free ZSM-5 grains was promoted due to the secondary conversion of returned aromatics, the dealkylation reaction was enhanced on external strong acid sites, which significantly increased the light aromatic selectivity. Decreasing the SiO2/Al2O3 ratio of Zn-free ZSM-5 to 120 could significantly promote the heavy aromatic de-alkylation and inhibited external coke deposition due to the sufficient external strong acid sites. The lifetime of such a mixed catalyst was prolonged nearly 3 times than that of the previously reported olefin-aromatic doubled catalytic system, and the heavy aromatic selectivity was reduced by 30%. This work helps to clarify the catalytic behavior of acidic molecular sieve catalysts in the hydrocarbon conversion reaction and provides an effective path to improve the catalytic performance by configurating different molecular sieve catalysts.