(559g) Hydrogenation and Hydrodeoxygenation of Oxygenates and Nitroarenes at Ambient Conditions Via a Two-Step Redox Cycle

Hydrogenation is a critical reaction in the chemical industry. The chemoselective hydrogenation of biomass-derived oxygenates¹ and aromatic nitro compounds^{2, 3} holds significant scientific and technological value for producing crucial intermediates of a variety of products, including renewable fuel additives, pharmaceuticals, agrochemicals, dyes, pigments, and others. However, conventional hydrogenation typically requires pressurized hydrogen, high temperatures and pay severe energetic debt. Therefore, cost-effective and sustainable catalytic processes for efficient upgrading is urgently needed.

Herein we report a two-step solar thermochemical hydrogenation process, sourcing hydrogen directly from water and concentrated solar radiation for bio-oils upgrading and nitroarenes hydrogenation. A metal or reduced metal oxide, obtainable from solar thermal or electrochemical reduction, provides the active sites for reactants adsorption and H₂O dissociation. Fundamentally different from conventional hydrogenation, the in situ generated reactive H atoms hydrogenate biomass-derived oxygenates and nitroarenes, eliminating the barriers for H₂ dissolution and mass transfer in the reaction medium and the subsequent dissociation at the catalyst surface.

Materials and Methods

Zn, Fe, Sn, Al, Al alloy, Mg, Mn were purchased from Sigma-Aldrich and used without further treatment. CeO₂ was purchased from Noah Technologies Corporation. Prior to the experiment, the as-received CeO₂ was reduced in 10% H₂/Ar (100 mL/min) at 800 ^oC for 2 h. The supported catalysts loaded on CeO₂ with nominal Ru loadings of 1 wt% was prepared via the incipient wetness impregnation method.

Results and Discussion

We proposed and validated a two-step solar thermochemical/electrochemical hydrogenation (ST/ECH) process, sourcing hydrogen directly from water and concentrated solar radiation for biomass-derived oxygenates and nitroarenes upgrading. Fig. 1a illustrates simplified schematic of the two-step ST/ECH process. During the upgrading step, a metal or oxygen deprived metal oxide provides the active sites for furan and aromatic rings adsorption as well as a bulk source of reducing potential to dissociate H₂O for in situ hydrogen generation. The reactive H atoms are highly effective for upgrading biomass-derived oxygenates and nitroarenes to hydrogenated products. During the regeneration step, the lattice oxygen is abstracted from the reoxidized metal oxide at elevated temperatures with concentrated solar energy or through well-established electrochemical reduction using renewable electricity. The oxophilic zero-valent metals including Zn, Sn, Fe, Al, Mg and Mn as well as a non-stoichiometric CeO_2-δ redox catalyst were employed to investigate the feasibility. Notably, efficient upgrading of furfural and nitrobenzene were achieved at room temperature and atmospheric pressure, with H₂ utilization efficiency 1-2 orders of magnitude higher than molecular-hydrogen-based HDO processes (Fig. 1c, d). Using suitable metal/reduced metal oxide catalysts with different oxophilicity, the reducing potential for H₂O dissociation and the subsequent hydrogenation can be manipulated. Mechanistic investigations indicate that zero-valent metals or surface oxygen vacancies are active redox centers for in-situ generation of H atoms from H₂O, via a reverse Mars–van Krevelen mechanism (Fig. 1e, f). The efficient STCH process was also extended to converting other biomass derivatives and substituted nitroarenes with similar functional groups, exhibiting superior catalytic performance.

Significance

This work provides an efficient and versatile strategy for bio-oils and nitroarenes upgrading and a promising pathway for renewable energy storage. The general approach can be extended to catalytic upgrading of other platform compounds with similar functional groups.

References

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