Towards sustainable hydrogenation of 5-(hydroxymethyl)furfural: a two-stage continuous process in aqueous media over RANEY® catalysts

The hydrogenation of 5-(hydroxymethyl) furfural (HMF) to 2,5-bis(hydroxymethyl) tetrahydrofuran (DHMTHF) in aqueous media under relatively mild reaction conditions has been investigated over heterogeneous RANEY (R) Cu and Ni catalysts using a continuous-flow hydrogenation reactor. These RANEY (R) catalysts were selected following a screening of several catalysts including precious metals supported on carbon for the hydrogenation of HMF. A single-stage versus a two-stage process for the hydrogenation of HMF into DHMTHF, i.e. via 2,5-dihydroxymethylfuran (DHMF) has been evaluated. The best result with an average selectivity of 98% for DHMTHF was obtained using a two-stage process; RANEY (R) Cu was used as a catalyst for the highly selective hydrogenation of HMF to DHMF (92 mol%) in the first stage and this product was used without further purification for in a second-stage selective hydrogenation of DHMF into DHMTHF using RANEY (R) Ni as a catalyst. The influence of the HMF concentration in the feeding solution (1-3 wt%), flow rate (0.05-0.25 mL min(-1)) and total pressure (2090 bar) were investigated for the first-stage hydrogenation of HMF into DHMF over RANEY (R) Cu. HMF was found to exert an inhibiting effect on the conversion due to strong adsorption. The RANEY (R) Ni catalyst used in the second stage gradually deactivated. A procedure for in situ regeneration of the partially deactivated RANEY (R) Ni catalyst using acetic acid washing was investigated with limited success.