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This review article focuses on the challenges and opportunities of biomass-based chemical looping technologies and explores fundamentals, recent developments and future perspectives.

Development of efficient catalysts for the direct hydrogenation of CO2 to methanol is essential for the valorization of this abundant feedstock. Here we show that a silica-supported Cu/Mo2CTx (MXene) catalyst achieves a higher intrinsic methanol formation rate per mass Cu than the reference Cu/SiO2 catalyst with a similar Cu loading. The Cu/Mo2CTx interface can be engineered owing to the higher affinity of metallic Cu for the partially reduced MXene surface (in preference to the SiO2 surface) and the mobility of Cu under H2 at 500 C.Increasing the reduction time, the Cu/Mo2CTx interface becomes more Lewis acidic due to the higher amount of Cu + sites dispersed onto the reduced Mo2CTx and this correlates with an 2 increased rate of CO2 hydrogenation to methanol. The critical role of the interface between Cu and Mo2CTx is further highlighted by DFT calculations that identify formate and methoxy species as stable reaction intermediates.

Early transitional metal carbides are promising catalysts for hydrogenation of CO2. Here, a two-dimensional (2D) multilayered 2D-Mo2C material is prepared from Mo2CTx of the MXene family. Surface termination groups Tx (O, OH, and F) are reductively de-functionalized in Mo2CTx (500 °C, pure H2) avoiding the formation of a 3D carbide structure. CO2 hydrogenation studies show that the activity and product selectivity (CO, CH4, C2–C5 alkanes, methanol, and dimethyl ether) of Mo2CTx and 2D-Mo2C are controlled by the surface coverage of Tx groups that are tunable by the H2 pretreatment conditions. 2D-Mo2C contains no Tx groups and outperforms Mo2CTx, β-Mo2C, or the industrial Cu-ZnO-Al2O3 catalyst in CO2 hydrogenation (evaluated by CO weight time yield at 430 °C and 1 bar). We show that the lack of surface termination groups drives the selectivity and activity of Mo-terminated carbidic surfaces in CO2 hydrogenation.

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Fluorocarbon-containing hydrophobically associating polymers have been synthesized by copolymerization of acrylic acid with a small amount of C8 fluorocarbon-containing methacrylate. The association behavior of the fluorocarbon-modified poly(acrylic acid) (FA) over a broad pH range has been investigated by a fluorescent probe technique and viscosity measurements. The copolymer has the strongest intermolecular association and maximum viscosity at the acidic condition of pH 5.5. Both pyrene and fluorocarbon-substituted pyrene (PyCORf) are usable to detect this strong association and its dependences on both the fluorocarbon content and polymer concentration. Less acidic pH causes progressive disruptions of hydrophobic association, leading to a dramatic decrease in viscosity. At pH > 7, the stretched polymer chains reach a viscosity plateau much lower than the maximum viscosity but still higher than the viscosity of the poly(acrylic acid) homopolymer. This indicates that relatively weak associations are present. PyCORf, due to its high affinity to the fluorocarbon domains, is effective in monitoring the formation of this kind of weak association while pyrene fails to do so.

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