(50c) Investigating the Role of Solid Acid Sites in Surface Reactions of a Model Sugar

Sugars derived from biomass possess a high degree of functionality and offer promise as a potential feedstock for production of commodity chemicals. Erythrose is a cyclic C4 sugar with similar functionality to glucose, which makes it an attractive model feedstock to study. Heterogeneous catalysts, such as metal oxides, have been shown to convert biomass-derived sugars to range of chemicals via acid catalyzed reactions. This work analyzes the interactions of erythrose with a variety of metal oxides that possess different types of acid sites.

¹³C MAS NMR spectroscopy with a direct polarization (DP) pulse sequence was used to identify surface species when the catalysts were impregnated with erythrose at room temperature. The following catalysts were tested due to varying degrees of Brønsted and Lewis acidity: γ-Al₂O₃, CeO₂, Nb₂O₅, SiO₂, SnO₂, TiO₂, and ZrO₂. CeO₂ and Nb₂O₅ promoted ring opening as evidenced by the appearance of two peaks in the carbonyl region of the spectrum. Nb₂O₅ also produced a significant amount of surface species containing aliphatic carbon suggesting further reaction to other species. SnO₂, TiO₂, and ZrO₂ showed the most drastic reactions when the temperature was increased from 25 ⁰C to 50 ⁰C. Both carbonyl and aliphatic carbon became significantly more abundant at elevated temperature. In addition, species on TiO₂ had the sharpest peaks suggesting it is weakly bound to the active sites due to the moderately acidic nature of TiO₂.

Contact time experiments were carried out to determine the mobility of specific functional groups within the surface species present at 25 ⁰C and 50 ⁰C. This was done by comparing the relaxation times of different moieties. Some groups were greatly affected by the temperature increase, while others were immune. This is a significant consideration because one can discriminate which moieties are preventing the molecules release into the bulk phase.