Polyethylene-derived radical mediated hydrogen atom transfer synergizes with conjugated dual size effect of core-shell particles to enhance BTEX formation

Abstract

Aromatic hydrocarbons (AHs) production from in-situ catalytic co-pyrolysis (CCP) of biomass and plastics is a promising technology. However, the combined effects of the mixing mode and proportion of raw materials in catalytic co-pyrolysis and the double-size effect of micro-mesoporous core-shell catalysts on AHs production remain questionable. The formation characteristics and mechanism of AHs in CCP process were studied from mass transfer and free radial direction. The results showed that the yield of BTEX (benzene, toluene, ethylbenzene, and xylene) was 20.44 wt% when the mass ratio of cellulose/polyethylene/cellulose (C-P-C) was 0.5:1:0.5 at 500 ℃. The free radicals generated during the breaking of polyethylene bonds can promote the transfer of hydrogen atoms to cellulose pyrolysis intermediates, which results in mass transfer effect. The C-P-C loading mode makes hydrogen atoms always pass through the cellulose without escaping, and the increase of polyethylene ratio makes the number of hydrogen free radicals increase significantly, both of which jointly regulate the hydrogen free radical mass transfer efficiency. However, the high concentration of hydrogen free radicals can quench the pyrolysis of cellulose. The conjugated double size effect of ZSM-5 @MCM-41 leads to their greater catalytic capacity for free radical carrying co-pyrolysis products compared to ZSM-5. With the increase of mesoporous shell, the yield of BTEX increases first and then decreases, but the yield of coke is opposite. The BTEX yield of 1.0SH-Z5 @ M41 is 3.15 times that of ZSM-5, and the coke yield is only 14.70% that of ZSM-5. The reaction pathway of promoting cellulose pyrolysis by polyethylene and the mechanism of hydrogen free radical - mass transfer co-pyrolysis involved were put forward.