1a. Objectives (from AD-416):
To develop a method for the catalytic depolymerization of pyrolytic lignin into chemical feedstocks. Design of new catalysts with high conversion and high selectivity for C-O bond cleavage in lignin for production of chemical intermediates and subsequent production of alkane based fuels.
1b. Approach (from AD-416):
Catalysts will be synthesized and screened using small molecule analogs of lignin as well as samples of pyrolytic lignin from biomass pyrolysis using reactor systems developed at ARS (process development unit, PDU). Catalysts will be evaluated in terms of rate, selectivity, and lifetime; the best catalyst candidates will be synthesized and screened using small molecule analogs of lignin scaled up and incorporated into the ARS PDU for further testing. The best candidates will also be further studied at the bench scale using lignin analogs to develop an understanding of their reaction mechanisms so that the catalysts can be further optimized using an intelligent design.
3. Progress Report:
The goal of this project is to formulate and produce new catalysts for the depolymerization of biomass derived lignin into valuable chemical feedstocks. We have synthesized a variety of transition metal catalysts based on nickel, ruthenium and vanadium (Ni, Ru, and V) which are designed to cleave aryl ether C-O bonds, such as those in lignin. We have synthesized several variants of each system and are investigating the effect that ligand modifications have on their catalytic reactivity. Catalysts screening was conducted at USciences using both lignin model compounds and pyrolyzed lignin supplied by ARS. We have synthesized several small molecule models of the different C-O linkages that are present in lignin so that these can be used to test the reactivity and selectivity of our metal catalysts in a more controlled manner than using actual lignin samples. We have used analytical and chemical testing of pyrolyzed Etek lignin to provide information on how the structure and composition of the lignin changes during pyrolysis. This characterization is necessary so that we can tailor our catalysts to the different structures that will be encountered following pyrolysis and to anticipate what compounds should be targeted. Deconstruction reactions utilizing our metal catalysts with both the model compounds and pyrolytic lignin samples have been conducted simultaneously. In addition to the Etek lignin we have also used pyrolytic lignin isolated from switchgrass; chemical and analytical characterization of this lignin is currently underway. The Ni carbene class of catalysts has shown the greatest ability to cleave a variety of C-O bonds in model compounds, yet have not shown great results with the lignin samples and tend to lead to more decomposition with lignin. The Ru and V catalysts both generally cleave only B-O-4 type C-O bonds, but these systems have fared better with lignin samples, leading to a reduction in the molecular weight of the bulk sample and to the appearance of aromatic aldehydes in the NMR spectra. Two peer-reviewed journal articles were published “Structure–Property Characteristics of Pyrolytic Lignins Derived from Fast Pyrolysis of a Lignin Rich Biomass Extract.” In ACS Sustainable Chemistry and Engineering and “Structure and Radical Scavenging Activity Relationships of Pyrolytic Lignins” in the Journal of Agricultural and Food Chemistry.