(277f) Controlled Synthesis of Fluidizable Ni Catalysts and Their Performance for Biomass Steam Gasification

This study reports a new tunable La₂O₃ promoted Ni/γ-Al₂O₃ catalyst. Catalysts are prepared using a specially designed incipient wetness technique: a multi-step impregnation with direct reduction of metal precursors after each impregnation in fluidized bed conditions. Morphology of the Ni catalyst is tailored by adjusting La₂O₃ loading and catalyst reduction conditions. Catalytic steam gasification of biomass surrogates (glucose and 2-methoxy-4-methylphenol representing the biomass cellulose and lignin content, respectively) are performed in a CREC Riser Simulator under the expected conditions of a twin circulating fluidized bed gasifier.

Catalyst characterization shows that the addition of La₂O₃ up to 5 wt% improves surface area, CO₂ adsorption capacity, Ni reducibility and metal dispersion, as well as reduces support acidity. As the lanthanum content is increased from 5 to 10 wt%, a diminution in dry gas yield and an increase in coking were observed. The formation of undesirable LaAlO₃ on the Ni catalyst containing 10 wt% La₂O₃, as found by XRD, is responsible for its poor gasification performance. TPR results showed that excess La₂O₃ content causes: i) the suppression of some of the active nickel and ii) favors agglomeration of surface Ni crystallites which are susceptible to coking.

XRD show undesirable LaAlO₃ formation when catalysts with more than 10 wt% La₂O₃ are calcined above 1000 °C. This result points toward the increase of local catalysts bed temperatures during the exothermic reduction of metal nitrates. Higher reduction gas flow rates can control the rise of catalyst bed temperature by effectively removing the generated heat. This controlled reduction helps to minimize sintering/dehydroxylation of the metastable γ-Al₂O₃, with octahedral and tetrahedral site ratio being used as an indicator of γ-Al₂O₃ dehydroxylation. The octahedral and tetrahedral site ratios are estimated using H₂ TPR and NH₃-TPD.

Gasification performance of the prepared catalyst is found to be a function of Ni dispersion and support basicity/acidity ratio. It is hypothesized that acid sites of γ-Al₂O₃ are responsible for coke deposition via hydrocarbon cracking, whereas basic sites facilitate coke reforming. Based on these data, a 20% Ni/5% La₂O₃-γAl₂O₃ is developed, in this study, optimizing catalyst formulation and preparation conditions. This catalyst yields a 97% carbon conversion of glucose to permanent gases with no tar formation at 650 °C. In the case of 2-methoxy-4-methylphenol gasification, a 85.5% carbon conversion with only 8.3% tar formation is achieved.