Comparison of different cyclic organic carbonates in the oxyalkylation of various types of lignin
The synthetic versatility and the low toxicity of cyclic alkyl carbonates has driven a wider interest in their use to access new biobased and environmentally friendly polymer building blocks, especially as alkylating reagents. Here, we report for the first time, the synthesis of biobased polyols from lignins of different sources (hardwood, softwood, annual plants) and pulping processes (organosolv, kraft, soda) by oxyalkylation with ethylene (EC), propylene (PC), butylene (BC) and glycerol carbonate (GC). The synthesis of lignin polyols was performed using optimized conditions regarding catalysts, catalyst-to-biomass ratio, reaction time, temperature and amount of cyclic carbonates. To better understand the oxyalkylation of the various types of lignins using different cyclic organic carbonates, the generated polyols were comprehensively characterized by FTIR, 1H, 13C, and 31P NMR spectroscopy as well as size exclusion chromatography (SEC). Emphasis was set on the hydroxyl amount, the degree of substitution (DS) and the alkyl chain. This oxyalkylation strongly facilitates the generation of a uniform lignin polyol equipped with two alkyl units on the aliphatic and phenolic hydroxyl groups. Higher degrees of substitution were achieved using cyclic organic carbonates in the following order EC > GC > PC > BC, reaching a maximum of DS 0.96 for a hardwood kraft lignin. Additionally, the influence of ash, sulfur and polysaccharide impurities as well as lignins molar masses was elucidated.