Biomass catalytic fast pyrolysis over hierarchical ZSM-5 and Beta zeolites modified with Mg and Zn oxides

Hierarchical ZSM-5 and Beta zeolites, loaded with MgO and ZnO, have been explored for the catalytic fast-pyrolysis of eucalyptus woodchips. These materials exhibit a high dispersion of the MgO or ZnO phases, which is probably due to the presence of a hierarchical porosity with both micro- and mesopores in the zeolitic supports. The incorporation of these metal oxides led to a significant reduction in the textural properties and to changes in the acidic properties of the parent zeolites. Thus, a decrease in the concentration of Brønsted acid sites was observed, which was accompanied by the generation of additional Lewis acid sites with medium strength. In addition, the incorporation of metal oxide promotes the formation of significant amount of basic sites, especially for the samples loaded with MgO. Catalytic fast pyrolysis experiments of eucalyptus woodchips were performed in a fixed bed reactor at 500 °C and atmospheric pressure under a nitrogen flow. In comparison with non-catalytic fast pyrolysis, the use of zeolitic catalysts caused a decrease in the bio-oil* (water free basis bio-oil) production due to enhanced formation of gases, as well as coke deposition on the catalyst. However, the quality of the bio-oil* was enhanced since the catalysts were able to decrease its oxygen content. In this way, h-ZSM-5-based catalysts showed a clearly deeper deoxygenation degree compared to those having h-Beta as support, with very low content of anhydro sugars and the formation of a significant amount of aromatics. Regarding the effect of the metal oxide phase, MgO-loaded samples provided bio-oil* with enhanced energy yields and lower oxygen content, probably due to the adequate balance of Lewis acid and basic sites. Likewise, significant differences were observed among the catalysts regarding the deoxygenation pathways and the compounds families present in the bio-oil*.