Transfer Hydrogenation of Furfural with Ethanol: Rediscovering the Role of Homogeneous Zr- and Al-Based Catalysts

The production of chemicals and fuels from lignocellulosic biomass, as a sustainable alternative to fossil reserves, is attracting increasing attention. In this context, furfural (FF) is one of the principal and widely commercialized biomass-derived compounds, typically produced through the acid-catalyzed hydrolysis of C5 monosaccharides. Currently, over 60% of the annual output of FF (~300 kton) is hydrogenated into furfuryl alcohol (FA), a key derivative due to its strategic industrial application in the production of biofuels, resins, pharmaceuticals, flavourings and solvents. Industrially, the commercial processes for FA production involve the employment of molecular hydrogen, thus showing several drawbacks, including the use of high-pressure hydrogen, which increases the operational risk and equipment costs, high energy consumption, and the use of copper chromite as catalyst, which pose environmental concerns due to the release of toxic Cr(VI) species. Furthermore, commercial hydrogen is still predominantly derived from fossil reserves, making the process not fully dependent on renewable resources. Thus, catalytic transfer hydrogenation (CTH) is emerging as a greener alternative to conventional hydrogenation processes due to the adoption of milder reaction conditions and a hydrogen donor, such as primary and secondary alcohols, which replace the use of molecular hydrogen. In the present study, the CHT of FF to FA was investigated, giving particular attention to two main key aspects: the choice of hydrogen donor and the catalyst. To enhance the sustainability of the process, ethanol was selected as the hydrogen donor, despite the primary alcohols are less efficient than the secondary ones in CHT, as it can be sourced from biomass. On the other hand, recent CHT studies have been dominated by noble metal-based homogeneous and heterogeneous catalysts, often requiring harsh conditions and energy-intensive preparation, the use of earth-abundant metal-based homogeneous systems represents a promising alternative. In this regard, the present study initially focused on the use of cheap and commercially available aluminium and zirconium catalysts, particularly promising for their ability to operate via the Meerwein–Ponndorf–Verley (MPV) mechanism, which is known for its high selectivity toward carbonyl groups reduction. Remarkably, under the best reaction conditions, the highest FA yield of 83 mol% was ascertained. In conclusion, the present work investigated the CHT of FF through MPV reaction by using renewable hydrogen donor, as ethanol, and economic catalysts based on Zr and Al, thus contributing to the development of a more sustainable synthesis of FA.