Steam reforming of fast pyrolysis-derived aqueous phase oxygenates over Co, Ni, and Rh metals supported on MgAl2O4

Abstract

In this study we examine the feasibility of steam reforming the mixed oxygenate aqueous fraction derived from fast pyrolysis bio-oils. Catalysts selective towards hydrogen formation and resistant to carbon formation utilizing feeds with relatively low steam-to-carbon (S/C) ratios are desired. Rh (5wt%), Pt (5wt%), Ru (5wt%), Ir (5wt%), Ni (15wt%), and Co (15wt%) metals supported on MgAl2O4 were evaluated for catalytic performance at 500°C and 1atm using a complex feed mixture comprising acids, polyols, cycloalkanes, and phenolic compounds. The Rh catalyst was found to be the most active and resistant to carbon formation. The Ni and Co catalysts were found to be more active than the other noble metal catalysts investigated (Pt, Ru, and Ir). However, Ni was found to form significantly more carbon (coke) on the catalyst surface than Co. Evaluating the effect of temperature on stability for the Rh catalyst we found that catalyst stability was best when operated at 500°C as compared to the higher temperatures investigated (700°C, 800°C). When operating at 700°C, significantly more graphitic carbon was observed on the spent catalyst surface. Operating at 800°C resulted in significant carbon deposition, resulting in reactor plugging as a result of thermal decomposition of the reactants. Thus, a concept analogous to the petroleum industries' use of a pre-reformer, operated at approximately 500°C for steam reforming of the heavier naphtha components, followed by a high temperature methane reformer operated in the 600–850°C temperature range, could be applied in the case of steam reforming biomass derived oxygenates. Additional stability evaluations performed over the Rh, Ni, and Co catalysts at 500°C and 1atm, under similar initial conversions, reveal the Co catalyst to be the most stable and selective towards H2 production. However, deposition of carbon on the surface was observed. High resolution TEM on the spent catalysts revealed the formation of graphitic carbon on the Rh catalyst, and filamentous carbon formation on both the Ni and Co catalysts, albeit less pronounced on Co. Conversion and selectivity to CH4 over Co remained relatively stable at approximately 80% and 1.2%, respectively. By contrast, the Rh and Ni catalysts CH4 selectivity's were approximately 7–8%. The low selectivity to CH4 and enhanced resistance to coke formation suggests the Co catalyst may be a desirable economic alternative for the steam reforming of biomass-derived oxygenates compared to the more conventional Ni and Rh-type steam reforming catalysts.