Tunable and selective hydrogenation of HMF to furan diols in flow reactor using Ru/C

5-hydroxymethylfurfural (HMF) is a key renewable platform-chemical and precursor of several valuable products. In particular, 2,5-bis(hydroxymethyl)furan (BHMF) and 2,5- bis(hydroxymethyl)tetrahydrofuran (BHMTHF) are very important monomers for the synthesis of bio- polymers and precursors of other relevant monomers, such as caprolactame and 1,6-hexanediol. BHMF and BHMTHF derive from the hydrogenation of the aldehydic group and also of the furanic ring of HMF, respectively. The two steps are catalysed by noble metals and Ru/C has already resulted particularly active and selective for the synthesis of each diol performing the reaction in batch in water medium. However, for industrial application the employment of a flow reactor set-up is more suitable because it allows a continuous production and improves the energy efficiency, mixing control and heat transfer. The employment of the flow reactor is fundamental in particular for three phase reactions, as the hydrogenation of HMF. In fact, it allows the increase of the interfacial area between the phases, leading to more efficient heat and mass transfers. Although the advantages of the flow reactor, only recently in the literature it has been used for the HMF hydrogenation to BHMF and BHMTHF. The reaction has been generally carried out in organic solvents, in the presence of ad hoc synthesised catalysts, producing only one of the two diols. In this context, the present work proposes, for the first time, the optimization of the selective synthesis of each diol, BHMF and BHMTHF, starting from pure aqueous HMF solution in the flow reactor employing the same commercial catalyst (5 wt% Ru/C), by simply tuning the reaction parameters.