Catalytic conversion of acetol over HZSM-5 catalysts – influence of Si/Al ratio and introduction of mesoporosity

Crude bio-oil from fast pyrolysis of biomass has a high oxygen content and acidity, leading to problems with corrosion and polymerization/instability. In this work, the catalytic vapor phase upgrading of acetol, a compound that is present in high concentrations in pyrolysis vapors, was investigated over three microporous HZSM-5 and four mesoporous HZSM-5 catalysts with different levels of acidity and mesoporosity. All catalysts were steam-treated prior to testing in order to attribute deactivation unambiguously to coking rather than simultaneously occurring dealumination, and catalysts were characterized by Argon and N₂ physisorption, NH3-TPD, Pyridine-IR, TEM, and XRD. With increasing temperature, the gas yield increased between 400 and 500 °C while the coke yields decreased, along with increased acetol conversion and increased selectivities to oxygen-free hydrocarbons in the liquid phase. Catalyst screening was conducted at 450 °C by studying the conversion of acetol as a function of increased amounts of fed reactant. CO and ethene were the major reaction products in the gas phase. At high conversions, the condensed liquid contained olefins such as iso-butene and BTX monoaromatics. With increasing amounts of fed acetol, the selectivity to oxygen-free hydrocarbons decreased while the concentration of oxygenated compounds such as ethers, ketones, and furans increased. Simultaneously, coke formation rapidly decreased the weak and strong acid sites of the zeolites. A maximum monoaromatics yield of ∼5 wt.% of fed acetol was achieved in the initial upgrading phase for the purely microporous zeolites with Si/Al = 15 and 29, while mesoporous HZSM-5 showed slightly lower yields of monoaromatics. While the coking propensity increased after introduction of mesopores, all mesoporous catalysts showed an improved capacity for converting acetol compared to their microporous versions. Mesoporous HSZM-5 prepared via desilication of parent HZSM-5 (Si/Al = 15) with a mixture of NaOH and a pore-directing agent showed the highest tolerance towards deactivation, which is attributed to a favorable combination of acid strength and preservation of crystallinity and microporous volume with well distributed mesopores improving the accessibility of reactants to active sites.