Atomic Layer Deposition for Improved Stability of Catalysts for the Conversion of Biomass to Chemicals and Fuels
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Atomic Layer Deposition for Improved Stability of Catalysts for the Conversion of Biomass to Chemicals and Fuels Monika K. Wiedmann, Yomaira J. Pagan-Torres, Mark H. Tucker, James A. Dumesic, T. F. Kuech Department of Chemical and Biological Engineering, University of Wisconsin-Madison, Madison, WI 53706 ABSTRACT Atomic layer deposition (ALD) has been used to coat SBA-15 and functionalized SBA15 with various metal oxides. Use of SBA-15 coated with 4-10 ALD cycles of titania, alumina, niobia, or zirconia in the acid-catalyzed dehydration of fructose to 5-hydroxymethylfurfural (HMF) resulted in 24-57% conversion, with 0-22% selectivity, at 130 °C with 2 wt % fructose in 4:1 THF:H2O. Propylsulfonic acid functionalized SBA-15 (SBA-15-PrSO3H) had a 25% conversion and 48% selectivity for HMF under the same conditions. SBA-15-PrSO3H was also coated with 2 ALD cycles of titania followed by 8 ALD cycles silica. The deactivation rate constant for SBA-15-PrSO3H was 2.7 x 10-2 h-1 for the dehydration of fructose to HMF in a flow reactor at 130 °C with a feed of 2 wt % fructose in 4:1 THF:H2O. In comparison, the deactivation rate constant for the ALD coated SBA-15-PrSO3H-ALD was 7.9 x 10-3 h-1. INTRODUCTION Growing concerns of global climate change and diminishing fossil fuel resources have led to significant interest in the transformation of biomass-derived carbohydrates to fuels and chemicals. Biomass, an abundant renewable source, has gained importance in gradually shifting our dependence away from petroleum. Specifically, the acid-catalyzed dehydration of carbohydrates for the production of furan derivatives such as 5-hydroxymethylfurfural (HMF) and furfural is an important step, since these furan derivatives are key substances that can serve as platform molecules for the production of fine chemicals and biomass-derived polymers [1]. However, a major challenge of aqueous phase reactions such as the dehydration of carbohydrates for the production of biorenewable platform chemicals is developing heterogeneous catalysts that withstand liquid water at high temperatures, while retaining surface area and catalytic activity. Mesoporous silica materials have high surface areas and have the potential for use as catalyst supports for biomass processing. However, they are not stable in aqueous conditions at high temperatures and pressures, leading to significant loss of surface area and hence catalytic activity. Recently SBA-15, a mesoporous silica with hexagonally ordered pores, has been studied as a catalyst after atomic layer deposition (ALD) of Nb2O5 within the pores of the scaffold [2]. The deposition of niobia on SBA-15 provided stability to the framework of SBA-15 in liquid water at elevated temperatures. This enhanced stability of SBA-15 can contribute to future catalytic uses of SBA-15, a material that currently has limited utility in liquid water at high temperatures owing to its weak hydrothermal stability [3-4]. Furthermore, functionalized SBA15 containing organic moieties exhibits significant catalyst deactivation under hy
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