COMPARISON OF THERMAL AND HYDROTHERMAL LIGNIN DEGRADATION – REACTION MECHANISMS AND A STRATEGY FOR LIGNIN RECOVERY

摘要

Lignin is generated in huge amounts of about 50 Mio t from wood pulping as by-product and enormousrnefforts in research were made to develop a process for sustainable phenol production from lignin. To understand thatrnproblem and to identify the bottlenecks we compare the different reaction mechanisms of thermal and hydrothermalrnlignin depolymerisation. Whereas the thermal lignin degradation is a radical chain reaction starting with the cleavagernof weak ether bonds, consecutive -scissions, hydrogen abstractions, termination reactions, the hydrothermal ligninrndepolymerisation is influenced by the competitive hydrolysis reaction in the aqueous medium and thermalrndegradation reactions. Hydrotreating with organic sovents and catalyst influence the radical chain process ofrndegradation reactions in the way that radicals are captured and hydrogenated and functional groups are eliminated.rnDue to the special properties of water near the critical point and in situ hydrogenation via the water gas shift reaction,rnthe hydrothermal lignin depolymerisation is more selective and favors the formation of catechols. The hydrothermalrndegradation of lignin starts with a fast hydrolysis of weak ether bonds and lead to methoxy phenols and reactivernoligomers as primary products. Nevertheless the amount of methoxy phenols is very limited, which is the main reasonrnfor the insufficient amount of monocycles from lignin degradation. The reactive oligomers further degrade torncatechols, stable dimers and perform crosslinking reaction to chars which are comparable to chars from pyrolysis.rnThe oligomers are monitored by GPC and the monocyclic compounds are quantified by GC-FID. On that basis thernreaction mechanism for hydrothermal lignin depolymerisation is developed and validated. A strategy for a possiblernlignin recovery could be to stop the hydrothermal lignin depolymerisation at an early point of reaction and to take outrnthe methoxy phenols and catechols in the water phase and to extract the solvable reactive oligomers from the solidrnresidues and to recycle the unreacted lignin back to the process.