Catalytic fast pyrolysis (CFP) offers a simple and robust route to convert raw lignocellulosic biomass to aromatic hydrocarbons. During CFP, cellulose, hemicellulose, and lignin are first thermally decomposed to bio-oil vapors that are further converted to aromatics in the presence of a ZSM-5 zeolite catalyst. The high temperatures required for CFP also favor coke formation, an undesired byproduct, through condensation of the oxygenated intermediates on ZSM-5′s outer surface and/or secondary reactions inside its micropores. Introducing mesopores through desilication represents a possible strategy to enhance mass transport and intracrystalline diffusion, and consequently favor aromatic production over undesired coke formation. Here, we study the effect of desilication on the structure, acidity, and performance of aluminum-rich ZSM-5. Detailed characterization of the obtained zeolite catalysts indicates that mild desilication conditions do not significantly affect the elemental composition, crystallographic structure, microporosity, and distribution of aluminum atoms in framework and extraframework sites. However, the number of accessible Brønsted acid sites increased by ∼50% as a result of the enhanced mesoporosity. Desilication increased the aromatic yields obtained for red oak pyrolysis (27.9%) compared to the parent zeolite (23.9%), without impacting the liquid product distribution (67.4% selectivity to benzene, toluene, and xylene). Our results suggest the catalytic performance could be further improved by enlarging the mouth of ink bottle shaped mesopores in order to further enhance mass transport between the gas phase and the zeolite's micropore network.

Zeolite catalysts used for the conversion of carbohydrates to renewable platform chemicals in the condensed phase are shown to be sensitive to the presence of inorganic salts which alter the zeolite surface chemistry. The presence of NaCl (0.07−37 wt %) enhances the hydrolysis of Si−O−Al bridges and the release of Al 3+ species that catalyze the conversion of glucose through homogeneous catalytic processes and obscure the apparent reactivity of the zeolite catalyst.T he production of renewable chemicals from biomassderived carbohydrates commonly takes places under hydrothermal conditions at temperatures between 100 and 200°C. 1−3 Typical oxide catalysts and catalyst supports, including silica, alumina, and zeolites, undergo phase transitions and partial dissolution under these severe conditions. 4−11 The hydrothermal breakdown of mesoporous silica is dramatic as evidenced from the 90% loss of its surface area within 10 h at 200°C. 4 Hydrothermal degradation of γ-alumina is also rapid and is evident from the phase transition to hydrated boehmite under the same conditions. 5 Y and β zeolites degrade through leaching and amorphization. 7−12 In the case of binary oxides (e.g., silica), dissolution occurs until reaching an equilibrium concentration of inorganic species in solution. 13 At equilibrium, the rates for dissolution and deposition are equal, and both reactions take place simultaneously. Oxides can then undergo major changes in crystal size and structure under relatively mild conditions through dissolution−deposition. Small-angle neutron scattering (SANS) demonstrated that dissolution of SBA-15 in water at 115°C starts in areas of positive curvature (e.g., at the pore mouth), and the dissolved silica diffuses deeper into the micropores where it is redeposited. 6,13 Although not often used in catalysis, models that explain these phenomena have been developed by geochemists, and the key parameters that influence these transformations are known. 13−16 The critical factors governing hydrothermal breakdown are the elemental composition of the oxide, its crystallographic structure, temperature, pH, and ionic strength of the solution. 13−16 Stability tests performed on oxide catalysts and catalyst supports were typically carried out in hot liquid water. 1,4−7,10−13 Although this medium is relevant for the liquid-phase conversion of biomass, deionized water does not accurately model real reaction conditions, especially (i) for acid−base catalyzed reactions, (ii) when organic acids are formed under reaction conditions, (iii) when salts are present in the biomass feedstock or added to the process. Salts only represent about 1.5 wt % of dry biomass. 17 Therefore, the effects of these inorganic compounds on heterogeneous catalysts is often overlooked, and experiments are performed under idealized conditions.Here, we studied the effect of pH and salts on the activity and stability of ZSM-5 (MFI structure), the only zeolite that has been previously demonstrated to be stable in deionized water at 150 and 200°C for more...

The production of aromatic hydrocarbons from cellulose by zeolite-catalyzed fast pyrolysis involves a complex reaction network sensitive to the zeolite structure, crystallinity, elemental composition, porosity, and acidity. The interplay of these parameters under the reaction conditions represents a major roadblock that has hampered significant improvement in catalyst design for over a decade. Here, we studied commercial and laboratory-synthesized ZSM-5 zeolites and combined data from 10 complementary characterization techniques in an attempt to identify parameters common to high-performance catalysts. Crystallinity and framework aluminum site accessibility were found to be critical to achieve high aromatic yields. These findings enabled us to synthesize a ZSM-5 catalyst with enhanced activity, which offers the highest aromatic hydrocarbon yield reported to date.

The thermal stability and unique shape selectivity of the ZSM-5 structure have made this zeolite a popular choice for gas-phase petrochemical and biorenewable conversions. However, with processes such as catalytic fast pyrolysis and methane aromatization being studied at temperatures between 500 and 800 °C, it is imperative to better understand the dynamic changes that may occur under these conditions. Here, we study the chemistry of high aluminum content commercial ZSM-5 zeolites in the high-temperature regime. Our results suggest these catalysts thermally degrade within hours above 600 °C, emphasizing the importance of operating conditions on long-term catalyst performance. Detailed characterization of the thermally treated zeolites indicates that they retained the desired MFI crystallographic structure but displayed significant changes in Brønsted and Lewis acid site densities due to extensive dealumination. Depending on temperature, up to 50% of the aluminum initially present in the zeolite structure was lost to form extra-framework species that restrict the diffusion of reactants and products inside the catalyst particles. These alterations led to a 70% drop in performance for the catalyzed fast pyrolysis of cellulose. Low aluminum content ZSM-5 zeolites were more stable, suggesting a compromise must be found between reaction temperature and catalyst features to achieve high activity and long-term stability.