Understanding Low-Pressure Hydropyrolysis of Lignin Using Deuterated Sodium Formate

In the present study, hydropyrolysis of lignin was investigated by copyrolyzing lignin with sodium formate as the hydrogen donor in order to generate reactive hydrogen atoms under atmospheric pressure. It was found that the free hydrogen atoms released from thermal decomposition of sodium formate promotes the production of phenol, guaiacol, syringol, methyl and ethyl phenols, while reducing the yield of the phenolics with vinyl, propenyl and carbonyl groups. Acetic acid was eliminated by reacting with sodium carbonate byproduct. In comparison to pyrolyzing lignin alone, the pyrolysis-oil produced from copyrolysis of lignin and sodium formate contained a higher fraction of phenolic monomers and a lower fraction of phenolic oligomers both before and after storage. Deuterated sodium formate was copyrolyzed with lignin to investigate the mechanism of hydrogen transfer during low-pressure hydropyrolysis of lignin. Deuterium atoms were found in all of the GC/MS detectable phenolic compounds. As the amount of sodium formate increased, the fraction of deuterated molecules in total number of the compound molecules increased, as did the number of deuterium atoms in the individual deuterated molecules regardless if the compound yield increased or decreased. Among the products, phenol, and nonmethoxy methyl, and ethyl phenols were deuterated most significantly. The cleavage of side chain C-C and C-O bonds of lignin polymer by pyrolysis generates radical intermediates. The free hydrogen atoms in the vicinity not only promoted the bond cleavages but also capped the radical intermediates to avoid the coupling reactions among lignin radicals. Secondary reactions of primary pyrolysis products of lignin were also investigated by pyrolyzing phenolic monomers with deuterated sodium formate. It was found that in addition to thermal cracking, the reactive monomers likely polymerize and then further decompose during pyrolysis. The external hydrogen promotes the secondary reactions to produce more stable secondary products.