Hydrogenolysis of Lignine in Supercritical Ethanol. The Influence of Nature of Catalyst Active Sites and Reducing Agent
Lignin depolymerization into lignin monomers has fascinated researchers for decades as a pathway to valorize lignin biopolymer . However, there are a number of obstacles that need to be overcome before lignin processing becomes economically viable . Here, we report the study of the catalytic hydrogenolysis of grass, soft and hard wood lignins (corncob and birch wood ethanol-lignin , pine wood hydrolytic lignin) in the supercritical ethanol over solid bifunctional catalysts, which contain acidic and metallic sites. The hydrogenolysis process was conducted at 260 ºС and 60-80 bar during 6 hours. The products of reaction were studied using GC-MS and GC.
The influence of catalyst support nature was studied using three different acid-basic materials, namely: sepiolite, zeolite ZSM-5 supported on Al2O3 and Sibunit (Sib) graphite-like carbon promoted via oxidation. The Sib material was found to produce highest yield of liquid products due to the highest amount of surface acid sites . The increase of acid sites concentration of Sib led to formation of bigger amount of ethanolysis products resulted from lignin depolymerization. The bifunctional Ni- and Ru-containing catalysts based on oxidized Sib support were prepared and characterized . Applying metal-containing Sib catalyst in lignin depolymerization process allowed us to decrease the coke formation and to increase the amount of aromatic monomers and the total yield of liquid products up to 81 % (36 % were monomeric components) in case of 3% Ru/Sib catalyst. According to GC-MS, the use of Ni-contained catalysts provided formation of C-C bond cracking products of alkyl fragments of lignin. The increase of Ni content led to a decrease in the amount of cracking products of the aliphatic substituent and to increase in the yields of the hydrogenolysis products (total yield of liquid products was 72 %, the monomeric products content was 25 %). The result obtained over 3% Ni/Sib catalyst was inferior to the result obtained in presence of 1% Ru/Sib catalyst. The use of Ru-contained catalysts led to the formation of deoxygenation products. The increase of Ru content resulted in intensification of hydrogenation processes. We found that reducing agents significantly affected process: using H2 / i-PrOH / HCOOH led to increasing of liquid phase content up to 89 % (39 % is monomeric components) and decrease coke formation (less than 4 %). Moreover, the choice of reducing agent changed main product components distribution: in case of H2 and i-PrOH it was 4-propilsyringol, when HCOOH used it was 4-allylsyringol. The origin of lignin (biomass nature and separation technique) significantly affected depolymerization process. Well soluble ethanol-lignins provided high yields of liquid products (70-90 %). Nevertheless, ethanol-lignin obtained from corncob biomass provided low monomeric products yield (15 %). The hydrolytic lignin of pinewood provided high char formation. The liquid products took only 52 % (36,5 % was monomeric products). Aromatic monomers (p-allylsyringol, p-propyl syringol, p-propylgivacol, etc.) obtained in catalytic hydrogenolysis process of lignin are valuable raw materials for the chemical and pharmaceutical industries, and also can be used for the production of polymers and fuel additives.