A Perspective on Catalytic Strategies for Deoxygenation in Biomass Pyrolysis

Abstract

AbstractBiomass fast pyrolysis is a technology that is able to convert biomass, a low‐density solid, into primarily a liquid bio‐oil product that is denser and more easily storable and transportable. However, utilizing raw bio‐oil is difficult, mainly due to its high oxygen content. Bio‐oil must be upgraded and deoxygenated first before it can be co‐fed into a petroleum refinery. One method of upgrading involves condensing the pyrolysis vapors into bio‐oil and then hydrotreating the bio‐oil. Significant deoxygenation can be achieved; however, the hydrotreatment is typically performed under high pressure (>6900 kPa) and with multiple temperature stages. Also, lignin‐derived phenolic compounds can be prone to polymerize and plug reactors. Generally, the catalysts that have been tested are expensive noble metals or transition metal sulfides. Another upgrading approach is to react the pyrolysis vapors over a catalyst to produce a deoxygenated liquid product upon condensation. Common petroleum refining catalysts, such as zeolites, have been found to produce hydrocarbons although their yields are generally low to moderate whereas the yields of solids (char/coke) and gases (CO and CO2) can be high. In addition, zeolites can suffer rapid deactivation through coking and loss of acidity. Other, new catalysts that have been tested for vapor‐phase deoxygenation include noble‐metal and transition‐metal catalysts. Noble metals are active but are also prone to saturating aromatic rings. More cost‐effective catalysts, that is, transition metal‐based catalysts, have also been tested for hydrodeoxygenation efficacy. Particularly, two molybdenum‐based catalysts, MoO3 and Mo2C, were found to deoxygenate biomass model compounds under low pressure and moderate temperatures. In addition, MoO3 has been shown to deoxygenate pyrolysis vapors obtained from complete biomass. From our perspective, the most feasible upgrading strategy would likely involve catalytically upgrading pyrolysis vapors under low pressure, moderate temperature, and with minimal hydrogen consumption to produce unsaturated alkenes and aromatics, which can then be further processed and refined in a traditional petroleum refinery.