(270d) Functionalization of 5-Hydroxymethylfurfural By Selective Etherification
Several catalysts were evaluated including, H-FAU, H-MFI, H-BEA, H-MOR, amorphous SiO2-Al2O3, Amberlyst-15, hydrotalcite, tungsten oxide, and Î³-Al2O3. It was found that catalysts with BrÃ¸nsted acid sites (i.e., H-FAU, H-MFI, H-MOR, SiO2-Al2O3, and H-BEA Zeolites, and Amberlyst-15) showed the highest selectivities to the ether product at 70-80% HMF conversion. These same catalysts also showed high production rates, most notably Amberlyst-15 and H-BEA. Catalysts possessing either basic sites (e.g., hydrotalcite) or Lewis acid sites (e.g., WO3 and Î³-Al2O3) were an order of magnitude less active and less selective than those possessing BrÃ¸nsted sites. The ether production rates using H-BEA and Amberlyst-15 are very similar; however, H-BEA is ~40% more selective than Amberlyst-15. The improved selectivity observed for H-BEA-Zeolite, compared with Amberlyst-15, is hypothesized to be due to the ability of the zeolite to stabilize reaction intermediates. To further understand the capability of H-BEA several different SiO2:Al2O3 ratios were evaluated for the etherification, revealing that the turnover frequency for HMF etherification is approximately constant and suggesting the absence of strong transport limitations with this zeolite morphology.
In the second portion of this study ethanol, butanol, phenol, and cyclohexanol were used to evaluate the production of a variety of ether products. Increasing the size of the alcohol (e.g., cyclohexanol or phenol vs. ethanol or butanol) was found to have little effect on the ether production rate, suggesting that this rate is governed by the interaction between HMF and the catalyst. Based on these results, we hypothesize that the structure of H-BEA zeolite leads to improved activity for etherification, possibly due to the size compatibility between HMF and the pores of H-BEA. We suggest that Brønsted acidity is needed for etherification, and catalyst morphology may be highly important for obtaining high selectivity.