Selective Hydrogenation of 5-Hydroxymethylfurfural to 5-hydroxymethyltetrahydrofuran: Study of the reaction with commercial Pd-based catalysts

The transition from fossil resources to renewable ones is highly required to develop bio-based routes for fuels and chemicals synthesis. Among the available renewable molecules, 5-hydroxymethylfurfural (HMF) is a really promising platform compound, because of its multifunctional structure, which enables the production of diverse value-added intermediates. In particular, 5-hydroxymethyltetrahydrofuran (HMTHF) is an attractive intermediate for the synthesis of pharmaceuticals, polymers, and fine chemicals. It can be obtained by the selective hydrogenation of the HMF furanic ring, while preserving the aldehyde group. However, its selective production remains challenging, due to the competitive hydrogenation of furanic ring to 2,5-bis(hydroxymethyl)furan (BHMF), as well as the full reduction to 2,5-bis(hydroxymethyl)tetrahydrofuran (BHMTHF). In the field of heterogeneous catalysis, Pd supported systems are attractive for improving the HMTHF selectivity. On this basis, in this work, the influence of key reaction parameters, including catalyst loading, Pd loading, type of catalyst support and hydrogen pressure, was studied, to achieve the selective hydrogenation of HMF to HMTHF, adopting EtOAc as the solvent. Noteworthy, commercial Pd-based catalysts have been preferred over the ad-hoc synthesized ones, being easily scalable at industrial scale. The preliminary results highlighted that an excessive catalyst activity led to the predominant formation of BHMTHF, rather than HMTHF. Therefore, lower catalyst amount, lower Pd dispersion and lower hydrogen pressure enhanced the selectivity towards the desired product. In the perspective of working at low hydrogen pressure, the reaction was also successfully performed at atmospheric pressure, under continuous hydrogen flow, and the role of the solvent was specifically considered. Our findings indicated that primary alcohols (MeOH, EtOH, 1-PrOH and 1-BuOH) led to higher HMF conversion than EtOAc but unfortunately also to lower HMTHF selectivity, thus promoting the carbonyl hydrogenation. However, branched alcohol, such as 2-PrOH, enabled the complete HMF conversion, at the same time keeping high the BHMTHF selectivity. To get more insight into possible synergistic effects of base–metal interactions on this reaction, solid basic co-catalyst (CaO, MgO or ZrO2) was also included in the reaction mixture, as physical mixture with 5% Pd/C (in different ratios), employing also a mechanochemical approach to improve the interactions between the two catalysts. In conclusion, this work shows that it is possible to improve the HMTHF synthesis, preferring sustainable reaction conditions, and properly tuning the solvent-catalyst characteristics.