Aromatic And Olefin Production By Catalytic Co-pyrolysis Of Lignocellulose And Plastic Wastes

Petrochemicals derived from petroleum are the building-blocks for a broad range of useful products(e.g. plastics, fibres, solvents and medicines). While the market demands for petrochemicals are increasing rapidly worldwide, petroleum reserves on the earth are depleting at a rapid rate. Producing renewable petrochemicals from renewable resouces, especially biomass feedstocks, has therefore gained increasing interests. Catalytic fast pyrolysis(CFP) with zeolite catalysts is a promising technology that can convert solid biomass feedstocks(e.g., agricultural residues, industrial pulping wastes, and office paper wastes) directly into valuable aromatic and olefin hydrocarbons(two of the most important classes of petrochemicals).Catalytic fast pyrolysis(CFP) of Kraft ligninswith HZSM-5 zeolite for producing aromatics was investigated using analytical pyrolysis methods. Two Kraft lignins were fast pyrolyzed in the absence and presence of HZSM-5 in a Curie-point pyrolyzer. Without the catalyst, fast pyrolysis of lignin predominantly produced phenols and guaiacols that were derived from the subunits of lignin. However, the presence of HZSM-5 changed the product distribution dramatically. As the Si O2 /Al2O3 ratio of HZSM-5 decreased from 200 to 25 and thecatalyst-to-lignin ratio increased from 1 to 20, the lignin-derived oxygenates progressively decreased to trace and the aromatics increased substantially. The aromatic yield increased considerably as the pyrolysis temperature increased from 500 to 650 °C, but then decreased with yet further increase of pyrolysis temperature.Although CFP of lignocellulose can produce important petrochemicals, it also produces large amounts of solid residue, i.e., char and coke(carbon yields usually above 30%). The formation of solid residue decreases the carbon efficiency of biomass to petrochemical products. Moreover, coke would cause catalyst deactivation, leading to significant loss in the conversion efficiency of biomass and high catalyst regeneration costs. It is therefore crucial to decrease the formation of solid residues in CFP of biomass.Co-feed CFP of a series of biomass(cellulose, lignin, and pinew ood) with plastics(low-density polyethylene(LDPE), polyethylene(PP), and polystyrene(PS)) with ZSM-5 zeolite were investigated. The results show ed that biomass and plastics has a significant synergy that enhances the production of valuable petrochemicals and decreases the undesired coke in CFP. For example, co-feed CFP of cellulose and LDPE mixtures(mixing ratio of 2) produced 55.6% petrochemicals compared to 35.6% and 41.0% from CFP of cellulose and LDPE alone. The coke yield also decreased from 34.0% for CFP of cellulose alone to 18.1% for co-feed CFP of cellulose with LDPE. Similar results could be observed in co-feed CFP of Pinewood and LDPE mixtures. However, this synergy was less pronounced for the other combinations of biomass and plastics(cellulose/PP, cellulose/PS, and lignin/LDPE) tested in our study.Reaction mechanism analysis reveals that the synergy between biomass and plastics is mainly attributed to the Diels-Alder reactions of biomass-derived furans with LDPE-derived linear α-olefins, which produce aromatics in the presence of ZSM-5 zeolites. This synergy consists of one-step Diels-Alder cycloaddtion between furans(act as diene) and olefins(act as dienophile) and the subsequent dehydration of the cycloa dduct into aromatics and water. Generally, cellulose content accounts for 40~50 wt.% of natural biomass. Co-feeding LDPE with natural biomss would improve the product distribution and decrease the coke formation.