Coking characteristics and deactivation mechanism of the HZSM-5 zeolite employed in the upgrading of biomass-derived vapors

HZSM-5 zeolite was employed in the upgrading of biomass pyrolysis vapors in this study. The coking characteristics were investigated by means of TG, FT-IR, XRD, N2 adsorption–desorption, NH3-TPD, Py-IR, SEM and TEM methods. When the zeolite was used for three times about 120 min, its activity had been lost and the amount of coke was 12.15% composed of 9.90% of I type filamentous coke and 2.25% of II type graphite-like coke. The skeleton structure of zeolite was basically intact, which had not been seriously destroyed by coke. The surface area, pore volume and acidity of zeolite were all deteriorated vary degrees with the usage time. The zeolite granules became larger after deactivated, and the filamentous coke can be observed both on the surface and in the pore. The compositions of the coke precursors were analyzed by FT-IR and GC/MS. The precursors of coke deposited in the pore were mainly aromatic hydrocarbons, while the species of the precursors deposited on the outer surface was more including many long-chain saturated hydrocarbons. The deactivation of HZSM-5 zeolite began from inner, large molecular substances blocked the pores which resulted in the zeolite deactivation eventually.