Wet explosion pretreatment of loblolly pine leads to an increase in methoxylation of the lignin

Background: In biorefineries, various pretreatments traditionally employ hazardous chemicals (ammonia, sulfuric acid, sulfite, etc.) for opening the softwood structure and to facilitate easy accessibility of the cellulose for further downstream processing. The resultant lignin (known as technical lignin) after extraction of the carbohydrate fraction as sugars has been either burned as fuel or used in biochemical or biofuel production. It has been observed that the technical lignin after such biomass pretreatments is often more condensed and, hence, cannot be easily used to produce fine chemicals of high value. In this study, we examine lignin after wet explosion pretreatment where the biomass in subjected to oxygen to understand how these interactions will affect lignin utilization for biochemical production. Results: In this study, the structural transformations within the softwood lignin as a response to wet explosion (WEx) pretreatment of loblolly pine at different experimental conditions (165–175 °C, 18–30 min) were examined using GC/MS and NMR spectroscopy. The results showed that the H-type structures within the lignin molecule decreased while S-type structures increased after pretreatment. Since S-type lignin sub-units have a higher degree of methoxylation compared to H units, the potential of S-type lignin to undergo re-condensation at lower temperatures (after pretreatment), by forming bonds with other lignin sub-units, is lower due to stearic hindrance, resulting in the generation of lignin with a lower tendency to form new complex lignin bindings (high-quality biorefinery lignin). Conclusions: The less condensed biorefinery lignin generated after WEx pretreatment was found to provide a platform for production of lignin polymer with more labile β-O-4 linkages. This type of lignin could potentially be superior for the production of high-value bio-products compared to re-condensed lignin after acidic and other types of chemical pretreatments.