Effect of highly selective oxypropylation of phenolic hydroxyl groups on subsequent lignin pyrolysis: Toward the lignin valorization

Abstract For the valorization of lignin, it is important and a great challenge to prevent the negative impact of phenolic hydroxyl (Ph-OH) on lignin pyrolysis by blocking the Ph-OH. Herein, lignin samples with different oxypropylation degree are prepared to investigate the effect of masking of Ph-OH on subsequent pyrolysis of softwood kraft lignin (SKL). It is found that the propylene oxide shows high selectivity (~94%) towards the oxypropylation of Ph-OH, which enhances the hydrogen to carbon effective ratio (H/Ceff) of SKL. In addition, the acquired oxypropylated lignins (OPLs) display a slight change in the structure compared to the original lignin except for their Ph-OH content. Moreover, the maximum degradation rate of lignin during heating significantly increases with the increase of oxypropylation degree. Noticeably, the relative content of low-oxygen aromatic compounds derived from lignin pyrolysis shows a positive correlation with the oxypropylation degree. More importantly, the oxypropylation of Ph-OH has a great contribution to improving the bio-oil yield from lignin pyrolysis. The reveal of the relationship between specific OPLs and pyrolysis behavior is of great significance for the highly selective conversion of lignin into high value-added specific compounds.