On-line deoxygenation of cellulose pyrolysis vapors in a staged autothermal reactor

Abstract

We demonstrate the coupling of oxidative cellulose pyrolysis with zeolite-catalyzed upgrading of pyrolysis vapors in a staged autothermal reactor over bifunctional combinations of HZSM-5, HFER, or USY with Pt/γ-Al₂O₃. The pyrolysis vapors formed from oxidative decomposition of cellulose passed through a downstream bifunctional zeolite stage to form a mixture of aromatics, C₂–C₄ olefins and paraffins, CO, CO₂, and CH₄. A comparison of the bifunctional staged reaction with that of monofunctional zeolite catalysts revealed a ∼13% increase in conversion of pyrolysis vapors to deoxygenated products. Of the pyrolysis vapor entering the zeolite catalyst, 24% was converted to aromatics while an additional 20% was converted to C₂–C₄ olefins and paraffins. Complete conversion of cellulose to CO, CO₂, light hydrocarbons and aromatics was obtained at 500 °C upon addition of Pt/γ-Al₂O₃ to HZSM-5. Deactivation of the catalysts reduced the conversion of pyrolysis vapors to deoxygenated products as indicated by an increase in the oxygenate yield with time-on-stream. Oxygenate yield measured after the zeolite stage increased to 6% after 30 minutes on stream over bifunctional HZSM-5/Pt, and increased from 13% to 31% at the same time-on-stream over monofunctional HZSM-5. Pt likely promoted hydrogenation of the coke precursors produced in the staged reactor while the 10-MR pore size of HZSM-5 limited the formation of polyaromatic species. A 6% increase in CO₂ yield and 5% increase in CH₄ yield was observed when Pt/γ-Al₂O₃ was added to each zeolite, indicating the possible presence of water–gas-shift and methanation reactions, respectively, over Pt sites. The small-scale high-throughput autothermal reactor that we describe here is capable of carrying out cellulose depolymerization and upgrading in a single-pipe process.