Mechanistic Study of Heterogenously Catalyzed Lignin Depolymerization
Abstract
Lignin is a low-valued and a waste stream from pulping (paper making) that currently is burnt up to generate process heat. By catalytic upgrading, this waste stream has the potential to become an important renewable carbon feed-stock for chemical synthesis as well as a 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 high activation energy for the C-O bond cleavage that is responsible for the depolymerization. Because of the harsh reaction conditions, these 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 as an initial step that generates a ketone intermediate (DOI:10.1002/cssc.201500117). The current reaction can be performed at 80 degrees Celsius without addition of hydrogen gas, as compared to the previous procedures that require 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 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 with a higher affinity to the soft palladium surface. We have observed the ketone intermediate in solution by NMR spectroscopy and this supports the proposed reaction mechanism. However, the ketone intermediate could also be generated as a side reaction in another reaction mechanism. The 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.
We intend to use density functional theory calculations to study palladium-catalyzed depolymerization of lignin to supplement the experiments. Computations regarding heterogeneous catalysis on lignin has never been performed beyond calculations of BDE:s. The current study will therefore be important not only to the lignin transformation community, but also within the fields of heterogeneous catalysis, computational chemistry, green chemistry, and physical (organic) chemistry.
We intend to study how the different species (starting material, intermediate, and alkanyl ether) adsorbs to the palladium surface. Good results from the adsorption studies will already be publishable. We will calculate the activation energies for the transitions states between the intermediates. Finally, we will calculate the steps that lead to a C-O bond cleavage that is responsible for the depolymerization of lignin.
This is an interdisciplinary research project:
-Joseph Samec’s group (experimental part)
-Peter Broqvist’s group (computational and inorganic chemistry)
We have promising initial results from calculations (See: Resource Usage).
