Mechanistic Study of Heterogenously Catalyzed Lignin Depolymerization
Lignin is a low valued waste stream from pulping (paper making) that currently is burnt to generate process heat. Through catalytic upgrading, this waste stream has the potential to become an important renewable carbon feed-stock for chemical synthesis, and the production of biofuel. Currently, the catalytic upgrading of lignin is performed with an excess of hydrogen through a hydrogenolysis reaction generating an alkanyl ether as intermediate in an initial step. This intermediate requires a high activation energy for the C-O bond cleavage that is responsible for the depolymerization. Because of the harsh reaction conditions, such methodologies have not been commercialized. New experimental studies of ours have identified a unique reaction mechanism, proceeding through a low energy barrier with a transfer dehydrogenation reaction as an initial step that generates a ketone intermediate (DOI:10.1002/cssc.201500117). This reaction can be performed at 80 degrees Celsius without the addition of hydrogen gas, as compared to the previous procedures that required above 200 degrees Celsius and high hydrogen pressure. The ketone intermediate has a calculated bond dissociation energy (BDE) that is 10 kcal/mol lower in energy than the corresponding alkenyl ether for the subsequent C-O bond cleavage (depolymerization). The difference in the calculated BDE:s for the two different pathways can only explain the different requirements in reaction conditions (80 vs 200 degrees Celsius) to a degree. This was also confirmed by measuring activation energies. We propose that the formation of the ketone intermediate, that is a key species in the low energy pathway, tautomerize to its enol form and this enol adsorbs with its C=C bond because of its higher affinity to the chemically soft palladium surface. We have observed the ketone intermediate in solution by NMR spectroscopy, which supports the proposed reaction mechanism. However, the ketone intermediate could also be generated as a side reaction through another reaction mechanism. The enol tautomer has not been observed and this is not expected. Thereby, experiments are not sufficient to determine the reaction mechanism in these complex transformations and need to be supplemented by a theoretical one.