Catalytic hydrodeoxygenation (HDO) is a fundamental process for bio-resources upgrading to produce transportation fuels or added value chemicals. The bottleneck of this technology to be implemented at commercial scale is its dependence on high pressure hydrogen, an expensive resource which utilization also poses safety concerns. In this scenario, the development of hydrogen-free alternatives to facilitate oxygen removal in biomass derived compounds is a major challenge for catalysis science but at the same time it could revolutionize biomass processing technologies. In this review we have analysed several novel approaches, including catalytic transfer hydrogenation (CTH), combined reforming and hydrodeoxygenation, metal hydrolysis and subsequent hydrodeoxygenation along with non-thermal plasma (NTP) to avoid the supply of external H 2 . The knowledge accumulated from traditional HDO sets the grounds for catalysts and processes development among the hydrogen alternatives. In this sense, mechanistic aspects for HDO and the proposed alternatives are carefully analysed in this work. Biomass model compounds are selected aiming to provide an in-depth description of the different processes and stablish solid correlations catalysts composition-catalytic performance which can be further extrapolated to more complex biomass feedstocks. Moreover, the current challenges and research trends of novel hydrodeoxygenation strategies are also presented aiming to spark inspiration among the broad community of scientists working towards a low carbon society where bio-resources will play a major role. Figure 1. Three basic phenylpropane monomers: (1) p-coumaryl alcohol; (2) coniferyl alcohol; (3) sinapyl alcohol.

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Ni-foam-supported Co(OH)F and Co–P nanoarrays were developed for energy-efficient hydrogen production with the assistance of urea electrolysis.

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