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.

将生物质转化为更有价值的燃料是利用这种材料的最可行途径之一。目前，水热液化被认为是将湿生物质转化为生物原油的最有效方法之一，然而，这需要昂贵的提质处理才能用作生物燃料。在反应过程中使用能够直接提高生物粗品收率和质量的催化剂，对于提高整体工艺效率至关重要。对均相碱性催化剂的研究最多，但它们在处理结束时无法回收，因此无法重复使用。使用非均相催化剂可以克服这一问题，使回收和再利用成为可能，并始终保持生物原油生产中的高活性和选择性。

根据有关添加异质催化剂对木质纤维素生物质水热液化过程中生物粗品收率和质量的影响的文献数据，确定了一种常见的催化作用，可以将几种催化剂分为四类（碱性金属氧化物，过渡金属） ，镧系元素氧化物和沸石）。突出了催化剂的加氢脱氧活性，它们对生物粗品收率和质量的影响以及所用的操作条件。据报道，使用过渡金属和镧系元素氧化物可同时保证高质量的生物粗品，生物粗品产量最高。