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Catalytic hydrotreating of guaiacol as a model compound was investigated using bifunctional catalysts constituted of Ni supported on chemically modified zeolites with increased mesoporosity. In the reaction conditions, the hydrogen required for the process was generated in situ by the Zn–H2O redox system, which represents a promising green alternative to the use of gaseous hydrogen. The guaiacol hydrotreating conversion using as support zeolites with increased mesoporosity, is largely higher than that obtained with the original ones. The introduction of mesopores through desilication treatment with NaOH and TBAOH significantly increased the mass transfer of guaiacol and improved the accessibility of the active sites, accordingly enhancing the catalytic performance. The alkaline treatment notably increased the mesopore volume of Ni/HZSM-5 and Ni/HBeta by 5.6 and 3.8 times, respectively. Ni supported on desilicated HBeta zeolite displayed high hydrodeoxygenation and hydrodearomatization efficiencies with values of 69.37% and 62.82%, respectively. The reusability of this catalyst was investigated, showing a decrease in the performance after three consecutive runs due to the oxidation of Ni active site, coking and zinc oxide contamination. The main of guaiacol conversion products are cyclohexane, cyclohexanone, cyclohexanol, benzene and phenol. A reaction pathway of guaiacol hydrotreating using Ni-zeolites catalysts is proposed.

The biomass conversion into more valuable fuels represents one of the most viable routes for the exploitation of this material. Hydrothermal liquefaction is currently considered one of the most efficient processes to convert wet biomass into a bio-crude, which however requires expensive upgrading treatments to be used as biofuel. The use of catalysts able to directly improve bio-crude yield and quality during the reaction is of fundamental importance to increase the overall process efficiency. Homogeneous alkaline catalysts are the most studied, but they are not recoverable at the end of the process and so cannot be reused. The use of heterogeneous catalysts allows to overcome this issue making the recovery and reuse possible, maintaining anyway high activity and selectivity in the bio-crude production. The aim of this review is to critically summarize the effect of heterogenous catalyst addition on the hydrothermal liquefaction of lignocellulosic biomass, looking specifically at the improvement in bio-crude yield and quality. On the basis of literature data about the effect of heterogeneous catalyst addition on bio-crude yield and quality in the hydrothermal liquefaction of lignocellulosic biomass, a common catalytic action was identified allowing to group the several catalysts into four classes (alkaline metal oxides, transition metals, lanthanides oxides and zeolites). The hydrodeoxygenation activity of the catalysts, their effect on bio-crude yield and quality and the operating conditions used are highlighted. The highest bio-crude yields are reported using transition metals and lanthanide oxides which are able to guarantee, at the same time, a high-quality bio-crude.