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.

Carbon formation and sintering remain the main culprits regarding catalyst deactivation in the dry and bi-reforming of methane reactions (DRM and BRM, respectively). Nickel based catalysts (10 wt.%) supported on alumina (Al2O3) have shown no exception in this study, but can be improved by the addition of tin and ceria. The effect of two different Sn loadings on this base have been examined for the DRM reaction over 20 h, before selecting the most appropriate Sn/Ni ratio and promoting the alumina base with 20 wt.% of CeO2. This catalyst then underwent activity measurements over a range of temperatures and space velocities, before undergoing experimentation in BRM. It not only showed good levels of conversions for DRM, but exhibited stable conversions towards BRM, reaching an equilibrium H2/CO product ratio in the process. In fact, this work reveals how multicomponent Ni catalysts can be effectively utilised to produce flexible syngas streams from CO2/CH4 mixtures as an efficient route for CO2 utilisation

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In the context of Carbon Capture and Utilisation (CCU), the catalytic reduction of CO 2 to CO via reverse watergas shift (RWGS) reaction is a desirable route for CO 2 valorisation. Herein, we have developed highly effective Ni-based catalysts for this reaction. Our study reveals that CeO 2 -Al 2 O 3 is an excellent support for this process helping to achieve high degrees of CO 2 conversions. Interestingly, FeO x and CrO x , which are well-known active components for the forward shift reaction, have opposite effects when used as promoters in the RWGS reaction. The use of iron remarkably boosts the activity, selectivity and stability of the Ni-based catalysts, while adding chromium results detrimental to the overall catalytic performance. In fact, the iron-doped material was tested under extreme conditions (in terms of space velocity) displaying fairly good activity/stability results. This indicates that this sort of catalysts could be potentially used to design compact RWGS reactors for flexible CO 2 utilisation units.

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Abstract:In this comprehensive review, the main aspects of using Au/CeO 2 catalysts in oxidation reactions are considered. The influence of the preparation methods and synthetic parameters, as well as the characteristics of the ceria support (presence of doping cations, oxygen vacancies concentration, surface area, redox properties, etc.) in the dispersion and chemical state of gold are revised. The proposed review provides a detailed analysis of the literature data concerning the state of the art and the applications of gold-ceria systems in oxidation reactions.

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