Modification of highly acid ZSM-5 composed of nanocrystal stacks for reforming of lignite pyrolysis volatiles to light aromatics

Abstract

The catalytic conversion of pyrolytic volatile matter from lignite represents an effective strategy for the high-value utilization of low-rank coal. Highly acidic (Si/Al = 11) ZSM-5 zeolites consisting of nanocrystalline stacks were synthesized in a controlled manner and treated with ZSM-5 using an ethanol-triethylamine-water buffer solution system, and the acidity and structure of the zeolites were modulated by varying the treatment time. The series catalysts were employed in the conversion of lignite pyrolysis volatiles to light aromatics (LAs). The pore size of the highly acidic ZSM-5 zeolite was increased by the treatment of the slow-release solution, the strong acid content was reduced and the medium and weak acid content was increased. This one-step modification combines the processes of pore size enlargement and acid center optimization, thus reducing the rate of catalyst deactivation and increasing light aromatic yields. HZ-168 (treated with an alkaline solution for 168 h) gave LAs yielded up to 27.1 mg/g, characterized by the highest amount of B/L acid, the largest pore volume and mesoporous surface area. Furthermore, the activity remained stable after five cycles, indicating the potential for sustained performance. The increase in the number of nanocrystalline interstitial pores and the enlargement of the average pore size promoted the mass transfer efficiency of the reactants as well as the accessibility of the catalyst acid sites with the treatment time.