A Continuous Flow Strategy for the Coupled Transfer Hydrogenation and Etherification of 5‐(Hydroxymethyl)furfural using Lewis Acid Zeolites

Abstract

Hf-, Zr- and Sn-Beta zeolites effectively catalyze the coupled transfer hydrogenation and etherification of 5-(hydroxymethyl)furfural with primary and secondary alcohols into 2,5-bis(alkoxymethyl)furans, thus making it possible to generate renewable fuel additives without the use of external hydrogen sources or precious metals. Continuous flow experiments reveal nonuniform changes in the relative deactivation rates of the transfer hydrogenation and etherification reactions, which impact the observed product distribution over time. We found that the catalysts undergo a drastic deactivation for the etherification step while maintaining catalytic activity for the transfer hydrogenation step. (119) Sn and (29) Si magic angle spinning (MAS) NMR studies show that this deactivation can be attributed to changes in the local environment of the metal sites. Additional insights were gained by studying effects of various alcohols and water concentration on the catalytic reactivity.

Lewis Acid Zeolites for Biomass Conversion: Perspectives and Challenges on Reactivity, Synthesis, and Stability

“…They found that increasing the hydrophobicity of the material via silylation allowed the material to retain 50% of its activity even with 10 wt% water in the feed. For the coupled MPV reduction and etherification of HMF in ethanol, the addition of 0.2 wt% water decreases the etherification activity of Hf-Beta by 50%, whereas in 2-butanol, the same amount of water has negligible effect on the activity (25).…”

“…For example, the MPV reduction of cyclohexanone has the highest rates over Sn-Beta by a factor of 4.5, but if the reactant is switched to benzaldehyde, Zr-Beta has the highest rates (10). Similarly, for the combined MPV reduction and etherification of 5-(hydroxymethyl)furfural (HMF) to 2,5-bis(ethoxymethyl)furan (25) and the cross-aldol condensation of aromatic aldehydes and acetone (26), both featuring a dual-binding transition state, we observe higher reactivity over Hf-and Zr-Beta and lower reactivity over Sn-Beta. Table 1 shows that Ti-zeolites are most active for epoxidations; Sn-zeolites are most active for BV oxidation and intramolecular carbon and hydride shifts; and Zr-and Hf-zeolites are equally or more active for intermolecular hydride shifts, etherifications, and aldol condensations.…”

“…Some notable examples are liquid-phase flow studies of Hf-, Zr-, and Sn-Beta zeolites for the transfer hydrogenation and etherification of HMF to produce 2,5-bis(alkoxymethyl)furans (25) and for the MPV reduction of furfural to furfuryl alcohol (107); of Sn-Beta for the conversion of DHA and formaldehyde to HBL (22); of Sn-MFI synthesized by alkali-assisted metalation for the production of glycolic acid from glyoxal (84); and of Sn-MFI, MOR, BEA, and FAU for the isomerization of dihydroxyacetone and xylose (108). The flow study of hydrogenation and etherification of HMF reveals important trends in deactivation that could not have been seen with simple batch reactions (25). Specifically, for reactions using Sn-Beta in ethanol solvent at 393 K, initial transient deactivation from 80% to 60% conversion in the first 10 h time on stream (TOS) is followed by an apparent steady state up to 60 h TOS, which then leads to significant deactivation to below 10% conversion after 100 h TOS (Figure 5b).…”

“…Note that isomerization rates are two times higher in methanol than in water, indicating that the latter solvent is a stronger inhibitor than the former (35,73). M-Beta zeolites have been shown to retain activity in the presence of water for several reactions, including BV oxidations, MPV reductions, etherifications, and aldol additions (14,23,25,26,33,109,110). Water tolerance is particularly beneficial for cascade reactions, such as the one-pot conversion of furfural to GVL using Zr-Beta and Al-MFI-ns, where certain steps in the process require or produce water.…”

“…It has been shown that the presence of silanol defect sites is the most significant factor contributing to crystallinity loss during hydrothermal treatments and that hydrophobic materials tend to be more stable (113)(114)(115)(116). The structural integrity of Lewis acid zeolites is commonly justified with powder X-ray diffraction (PXRD) patterns that are unchanged after reaction, showing that long-range crystal order is preserved (22,25,29,83). However, PXRD is not able to detect partial pore collapse, which can be quantitatively measured using nitrogen or argon adsorption/desorption.…”