Solid acid catalysts alone or in combination with redox metals play a pivotal role in biomass valorization to obtain alternative fuels and chemicals. In acid-catalyzed biomass conversions, water is a key reagent/byproduct that can induce leaching/poisoning of catalyst’s acid species, a major problem toward catalyst recyclability and product purification. Thus, developing efficient water-tolerant solid acid catalysts is vital for viable biomass valorization. TiO2 is considered to be a promising water-tolerant solid acid catalyst for biomass conversions because of the presence of coordinatively unsaturated Ti4+ sites, which are robust and less prone to leaching in the aqueous medium. Besides, the synergistic combination of TiO2 with redox metals (Ru, Pd, Ni, Cu, etc.) provides abundant bifunctional acid-redox sites, which exhibit a favorable catalytic role in the deoxygenation of biomass molecules to practically useful hydrocarbons. Therefore, this review provides an overview of recent progress toward TiO2-based water-tolerant acid catalysis for biomass conversion, with a focus on hydrothermal stability of TiO2, its acidity, and catalysts’ synthesis methods. Various biomass conversions over TiO2-based catalysts, where water-tolerant acid sites or acid-redox dual sites show a significant catalytic effect, were discussed. Structure–activity relationships based on water-tolerant Lewis acidity of TiO2 were emphasized. We believe that this review will provide valuable information for developing efficient water-tolerant solid acid catalysts not only for biomass valorization but also for other challenging reactions in the aqueous medium.

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Two series of LaNi x Al 1−x O 3 catalysts (0 ≤ x ≤ 1) were prepared by hydrothermal and sol-gel methods and characterized by X-ray diffraction (XRD), BET surface area, Temperature programmed reduction (TPR) and Fourier-transform infrared spectroscopy (FT-IR) techniques. The performance of these catalysts was studied for CO 2 reforming of methane (also called dry reforming of methane, DRM) at atmospheric pressure and in the temperature range of 600−800 • C, maintaining a space velocity of 28,800 h −1. Catalysts containing trimetallic perovskite showed higher CH 4 and CO 2 conversions than the bimetallic perovskite, due to the strong interaction of Ni with the former. Strong interaction increased the reduction temperature of the active species and reduced the sintering of metallic particles. At 800 • C, the sol-gel catalysts reached their maximum activity in terms of both CH 4 and CO 2 conversions at x = 0.3, whereas the same for hydrothermal catalysts required a Ni ratio x = 0.6. The trimetallic perovskite formation was responsible for the catalyst stability. A comparison of the best catalysts from the two series revealed that the hydrothermal catalysts exhibited a slightly better performance during the time on stream analysis. The results are interpreted in terms of changes in the physicochemical properties of the catalysts.

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La-Ni x -Ce 1Àx mixed oxide catalysts were prepared by a sol-gel method varying the Ni composition (0 # x # 1). The catalysts were characterized by X-ray diffraction (XRD), inductively coupled plasma optical emission spectroscopy (ICP-OES), BET surface area, X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR), H 2 chemisorption and Fourier transform infrared spectroscopy (FT-IR) techniques. CO 2 reforming of methane was carried out at atmospheric pressure and 800 C, maintaining a reactant CO 2 /CH 4 /N 2 ratio of 80/80/80 (total flow rate ¼ 240 ml min À1 , GHSV of 28 800 h À1 ). The catalysts offered higher activity even at lower Ni compositions. LaNi 0.4 Ce 0.6 O 3 . showed the highest conversion of CH 4 and CO 2 . The H 2 /CO ratio in the syngas was stable at 0.85 AE 0.02. The performance of the sol-gel catalysts was compared with that of the hydrothermally prepared catalysts, reported earlier. High surface area and better Ni dispersion were found to be the reasons for superior activity of the sol-gel catalysts.