Tuning the Selectivity of the Hydrogenation/Hydrogenolysis of 5‐Hydroxymethylfurfural under Batch Multiphase and Continuous‐Flow Conditions

Abstract

The hydrogenation/hydrogenolysis of 5-hydroxymethylfurfural (HMF) has been carried out either under single (aqueous) phase or batch multiphase (MP) conditions using mutually immiscible aqueous/hydrocarbon phases, 5 % Ru/C as a catalyst, and both with and without the use of trioctylmethyl phosphonium bis-(trifluoro methane) sulfonimide ([P 8881 ][NTf 2 ]) as an ionic liquid (IL). Alternatively, the hydrogenation of HMF was explored in the continuous-flow (CF) mode with the same catalyst. By changing reaction parameters, experiments were optimized towards the formation of three products: 2,5-bis(hydroxy methyl)furan (BHMF), 2,5-bis(hydroxymethyl)tetrahydrofuran (BHMTHF), and 1-hydroxyhexane-2,5-dione (HHD), which were obtained in up to 92, 90, and 99 % selectivity, respectively, at quantitative conversion. In particular, the single (aqueous) phase reaction of HMF (0.2 m) carried out for 18 h at 60 °C under 30 bar of H 2 , allowed the exclusive synthesis of BHMF from the partial (carbonyl) hydrogenation of HMF, while the MP reaction run at a higher T and p (100 °C and 50 bar) proved excellent to achieve only HHD derived from a sequence of hydrogenation/ hydrogenolysis. It is worth noting that under MP conditions, the catalyst was perfectly segregated in the IL, where it could be recycled without any leaching in the aqueous/hydrocarbon phases. Finally, the hydrogenation of HMF was explored in a H-Cube® flow reactor in the presence of different solvents, such as ethyl acetate, tetrahydrofuran, and ethanol. At 100 °C, 50 bar H 2 , and a flow rate of 0.1 mL min À 1 , the process was optimized towards the formation of the full hydrogenation product BHMTHF. Ethyl acetate proved the best solvent.

High-Yield Synthesis of 1-Hydroxyhexane-2,5-dione via Hydrogenation/Hydrolysis of 5-Hydroxymethyl-furfural in Ionic Liquid-Assisted Multi-Phase Systems

“…For these reasons and with the aim to design protocols as accessible as possible, commercial 5% Ru/C was exclusively used throughout this study. This catalyst was characterized for its structural, morphological, and acidic properties in recent papers by our group. ,,, …”

“…Based on our previous investigations, if not otherwise indicated, MP experiments for the synthesis of HHD were run in a stainless-steel autoclave charged with an aqueous solution of HMF (HMF: 63 mg; H 2 O: 5 mL), 5% Ru/C (5 mol % with respect to HMF, 50 mg), isooctane (5 mL), and an IL (500 mg) selected among the above-quoted six onium salts. The reactor was heated at the desired temperature and pressurized with H 2 as described in the following sections.…”

“…These results were consistent with the partition of the hydrophobic catalyst in isooctane in the water/isooctane biphase (Figure , left). Such confinement of Ru/C out of the aqueous solution where the reactants and the products were soluble implied that an interfacial hydrogenation took place with slower kinetics compared to the reaction in water alone . Accordingly, both a slightly lower conversion (90%) of HMF and a higher amount of the partially hydrogenated product, BHMF, were observed in the water/isooctane system.…”

“…Based on our long-standing expertise on the chemical upgradation of bio-based molecules through multiphase (MP)-based protocols for the hydrogenation and reductive amination of levulinic acid, , hydrogenation of sugars and sugar-like substrates, and oxidation of HMF, we have recently demonstrated that the same MP conditions could be successfully used for the synthesis of HHD from HMF. At 100 °C and 50 bar of H 2 , in an MP system comprising water, isooctane, methyltrioctylphosphonium bis­(trifluoromethyl) sulfonimide ([P 8881 ]­[NTf 2 ]) as an ionic liquid (IL), and commercial 5 wt % Ru/C as the catalyst, HHD was achieved from HMF with 99% selectivity and 75% isolated yield . No additional acids were necessary.…”

Linear diketones as next-generation biomass-derived platform molecules: from heterogeneous catalytic synthesis to supply of high-end chemicals

“…Apart from biphasic solvents, a multiphase solvent consisting of immiscible aqueous/hydrocarbon phases and ionic liquid phase ([P 8881 ][NTf 2 ]) was also developed for the Ru/C-prompted HMF conversion to HHD. 96 This multiphase solvent afforded HHD as the only product with up to 99% selectivity, while DHMF and MF were detected as by-products in the single-phase water solvent. Also, the Ru/C catalyst could be completely separated from the water/hydrocarbon phases in the ionic liquid, as shown in Fig.…”