Supported Metal Catalysts for Total Hydrogenation of Furfural and 5-Hydroxymethylfurfural

The use of biomass-derived compounds as substitutes of pet roleum has received much a t tent ion1)~4). Because of the complex nature of biomass, the conversions of biomass to chemicals usually start with the production of small molecules called platform chemicals by thermal, chemical or biological processing5). Among these platform chemicals, furfural6) and 5hydroxymethylfurfural (HMF)7) are especially important ones, which are produced from pentoses and hexoses, respectively, by acid-catalyzed dehydration. Production of furfural has been already commercialized. Various methods of furfural or HMF conversions to useful chemicals have been reported including reduction, oxidation and condensation8),9). Because of the high unsaturation, reduction with hydrogen is one of the most important conversion methods10). However, there are a number of potential products from furfural or HMF reduction: products from furfural include furfuryl alcohol, tetrahydrofurfuryl alcohol, 1,2,5-pentanetriol, pentanediols, pentanols, cyclopentanone, cyclopentanol, 2-methylfuran, and 2-methyltetrahydrofuran (Fig. 1). More kinds of products can be formed from HMF which has more complex structure than furfural. In addition, polymerization of furfural, HMF and furfuryl alcohol to brown solid, which is called humin, is frequently observed, especially in aqueous conditions. High selectivity to one product is sometimes difficult to obtain, while selective hydrogenation of furfural to furfuryl alcohol has been well established11). Total hydrogenation reaction of furfural and HMF gives te t rahydrofur fury l a lcohol (THFA) and 2,5-bis(hydroxymethyl)tetrahydrofuran (BHTHF), respectively. In addition to the utilization of THFA as solvent, the utilization of THFA and BHTHF as a source of polymer has attracted much attention since a selective hydrogenolysis system of THFA to 1,5pentanediol was discovered by our group in 200912). We and other research groups have intensively investigated the hydrogenolysis of THFA and BHTHF to α,ωdiol (1,5-pentanediol and 1,6-hexanediol, respectively), which can be used as monomers of polyurethane or polyester resin (Fig. 2)13)~16). The hydrogenolysis systems dissociate the C_O bond neighboring CH2OH group. The effective catalysts are the bimetallic ones with the combination of group 9 noble metal (Rh or Ir) and group 5-7 transition metal such as V, Mo, W and Re, and water solvent is particularly effective. The yield of 1,5-pentanediol from THFA can reach 8090 %. Therefore, the importance of total hydrogenation of furfural and HMF is growing. Aqueous phase reaction is preferred to the reactions in other solvents in view of the total process into α,ω-diol; however water solvent is disadvantageous to obtain high selectivity in 1 Journal of the Japan Petroleum Institute, 60, (1), 1-9 (2017)