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...

N and S dual-doped carbon materials, N-S-CMK-3, are fabricated with >1000 m2 g−1 surface area and uniform mesoporous and macroporous structures, and exhibit outstanding ORR activity and durability in both half cell and direct biorenewable alcohol fuel cell tests.

Catalyst stability is one of the greatest challenges faced for the utilization of heterogeneous catalysts in the development of biomass conversion to chemicals and fuels. As many biomass transformations are performed in water, hydrothermal stability of supported metal catalysts is especially critical. This Review aims to increase attention on the hydrothermal stability of supported metal catalysts by looking at the stability of common catalyst supports, deactivation modes, and strategies to improve their durability. While common oxides such as silica, alumina, zeolite, and zirconia are not stable to hydrolytic attack, carbon, and titania show promising resistance. In addition to catalyst support leaching, amorphization, and collapse caused by hydrothermal conditions, supported metal catalysts can deactivate by sintering, leaching, poisoning, carbon deposition, and restructuring of the active metal sites. Several strategies are discussed to improve stability of supported metal catalysts: coating on the oxide, overcoating on the supported metal catalyst, metal−support interaction, embedding metal particles, bimetallic catalysts, reactor design and process optimization, and other methods. A fundamental understanding of liquid−solid interactions and deactivation mechanisms, as well as strategies to improve the catalyst durability will help to develop robust catalytic materials for the scale-up and further application of aqueous-phase biomass conversion processes.