(270d) Functionalization of 5-Hydroxymethylfurfural By Selective Etherification

There is a desire to use biomass to replace oil as the feedstock for modern chemicals and products. In this regard, polymers represent an attractive class of potential biobased materials. In this study, we seek to create tunable polymers using 5-hydroxymethylfurfural (HMF) as a renewable platform chemical. The properties of the final polymer can be tuned by modifying the side-chain of the monomer, which is added by the etherification of HMF with an alcohol. The R-group of the alcohol is then carried through the subsequent reactions to become the side-chain of the monomer. Here the etherification reaction was investigated first, by evaluating several catalysts with a variety of active sites and pore structures for their activity in this reaction and second by evaluating the interplay of R-group identity and active site structure on the etherification kinetics.

Several catalysts were evaluated including, H-FAU, H-MFI, H-BEA, H-MOR, amorphous SiO₂-Al₂O₃, Amberlyst-15, hydrotalcite, tungsten oxide, and γ-Al₂O₃. It was found that catalysts with Brønsted 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 γ-Al₂O₃) were an order of magnitude less active and less selective than those possessing Brønsted 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.