Insights into coke location of catalyst deactivation during in-situ catalytic reforming of lignite pyrolysis volatiles over cobalt-modified zeolites

TLDR: In this paper, the influence of hierarchical pores and metallic sites on the deposition and nature of coke and on its location in the channels or/and the outer surface of zeolites was investigated.

Abstract: HZSM-5, Co-incorporated HZSM-5 (3Co/Z5) and Co-incorporated hierarchical HZSM-5 (3Co/DZ5) were employed in catalytic reforming of volatiles from lignite pyrolysis in a drop bed reactor aiming at high yields of light aromatics. The co-effect of hierarchical pores and metallic sites on 3Co/DZ5 promoted the formation of benzene, toluene, xylene and naphthalene, and improved the service lifespan by decreasing coke formation. The influence of hierarchical pores and metallic sites on the deposition and nature of coke and on its location in the channels or/and the outer surface of zeolites was investigated. For 3Co/DZ5, the hierarchical pores retarded its deactivation by preventing internal coke formation while facilitating the external coke formation, owing to the reduced diffusion resistance of coke precursors (aromatic species) out of channels. The introduction of metallic sites brought the suppression of O-containing species depositing on zeolite. After continuous operation, the skeleton structures of both spent and regenerated catalysts were seriously damaged by coke deposition and sintering. The main causes of catalyst deactivation were assigned to two deactivating carbonaceous species with aromaticity and aliphaticity located on the inner pores and outer surface. The soluble long-chain aliphatics from coke extracts located on the outer surface were attributed to the decomposition of inherent aliphatic chain structure in lignite. The residual coke trended to graphite-like structure. The coking behavior was revealed by means of some technologies that the internal coke mainly had impact on the catalyst performance, especially for the insoluble aromatics with high polycondensation, whereas the external coke consumes the surface active sites, resulting in uninterrupted accumulation of coke precursors, eventually blocking the accessibility to channels.