Effect of addition of zeolite to iron-based activated-carbon-supported catalyst for Fischer–Tropsch synthesis in separate beds and mixed beds

Abstract

In-reactor upgrading of the products of Fischer–Tropsch (FT) reactions using synthesis gas was studied by adding zeolite ZSM-5 to a supported iron-based FT catalyst. The effects of ZSM-5 amounts and bed arrangements on the product distribution were investigated. Product profiles were analyzed for changing the ZSM-5 weight from 50 to 200% of the FTS amount. Oligomerization, aromatization, isomerization, and cracking reactions are enhanced with increasing ZSM-5 weight. The base catalyst showed no selectivity toward aromatic compounds, but addition of ZSM-5 formed C7–10 aromatic compounds. Long-chain hydrocarbon molecules (C13+) formed by the base catalyst are effectively cracked, and the products are isomerized on the ZSM-5 catalyst surface. Two catalyst-bed configurations were used: separate-bed and mixed-bed. Both arrangements show conversion of alcohols to hydrocarbons and water. The separate-bed arrangement gave higher conversions of CO and H2, and higher yields of liquid hydrocarbons. The relative decline in FT activity for the mixed bed is probably due to metal migration from FT catalyst to ZSM-5, as confirmed by energy-dispersive X-ray spectroscopy of the spent ZSM-5. The maximum amount of i-C5–12 plus aromatics (a measure of high-octane gasoline-range hydrocarbons) is formed in the separate bed using equal amounts of zeolite and base catalyst.