Unsupported Ni metal catalyst in hydrothermal liquefaction of oak wood: Effect of catalyst surface modification.

Hydrothermal liquefaction of oak wood was carried out in tubular micro reactors at different temperatures (280-330 °C), reaction times (10-30 min), and catalyst loads (10-50 wt%) using metallic Ni catalysts. For the first time, to enhance the catalytic activity of Ni particles, a coating technique producing a nanostructured surface was used, maintaining anyway the micrometric dimension of the catalyst, necessary for an easier recovery. The optimum conditions for non-catalytic liquefaction tests were determined to be 330 °C and 10 min with the bio-crude yield of 32.88%. The addition of metallic Ni catalysts (Commercial Ni powder and nanostructured surface-modified Ni particle) increased the oil yield and inhibited the char formation through hydrogenation action. Nano modified Ni catalyst resulted in a better catalytic activity in terms of bio-crude yield (36.63%), thanks to the higher surface area due to the presence of flower-like superficial nanostructures. Also, bio-crude quality resulted improved with the use of the two catalysts, with a decrease of C/H ratio and a corresponding increase of the high heating value (HHV). The magnetic recovery of the catalysts and their reusability was also investigated with good results.