Evaluation of kraft and hydrolysis lignin hydroconversion over unsupported NiMoS catalyst

Catalytic hydroconversion of Kraft and hydrolysis lignins was for the first time compared in a batch reactor over an unsupported NiMoS-SBA catalyst. We also report the effect of key reaction parameters on the yields and properties of the products. The results obtained at 20 wt% catalyst loading for hydrolysis lignin showed the highest monomer yield of 76.0 wt%, which consisted of 39 wt% aromatics with the lowest alkylphenolics yield of 10.1 wt%. Identical operating conditions, 400 degrees C, 80 bar, 5 h at 10 wt% catalyst loading, were used to compare both lignins and the highest monomer yield (64.3 wt%) was found for the hydrolysis lignin, consisting of 16.0 wt % alkylphenolics and 20.1 wt% aromatic compounds. These values are considerably higher than those for Kraft lignin with its 47.0 wt% monomer yield. We suggest that the reason for high yields of monomeric units from hydrolysis lignin is that it is more reactive due to its lower ash and sulfur contents and the chemical structural differences compared to the Kraft lignin. More precisely, the bio-oil from hydrolysis lignin contained higher yields of small molecules, sourced from ring-opening of cellulose in the hydrolysis lignin, which could stabilize the reactive oligomeric groups. These yields were two to seven times higher from kraft and hydrolysis lignin, respectively, compared to those obtained without catalyst. The results showed that the NiMoS-SBA catalyst is a promising catalyst for reductive depolymerization of lignin and in addition that the regenerated catalyst had good stability for multiple reaction cycles.

Automated summary

The catalytic hydroconversion of Kraft and hydrolysis lignins was compared for the first time using an unsupported NiMoS-SBA catalyst in a batch reactor. The study investigated the effect of key reaction parameters on product yields and properties. The results showed that hydrolysis lignin had higher monomer yields compared to Kraft lignin due to its lower ash and sulfur contents, as well as chemical structural differences. The hydrolysis lignin also produced more small molecules from ring-opening of cellulose, which stabilized the reactive oligomeric groups. The NiMoS-SBA catalyst showed promise for the depolymerization of lignin and had good stability for multiple reaction cycles.