Collaborative Research: Photoelectrosynthetic Aminoxyl Catalyzed Alcohol Oxidation

Lignocellulosic biomass offers a carbon-neutral alternative source of fuels and chemicals which are currently sourced from petroleum and other fossil fuels. Converting lignin, a largely underutilized yet chemically rich byproduct of the biorefining industry, to valuable aromatic platform chemicals is an energy-intensive process that has made this approach cost prohibitive. Recently, dye-sensitized photoelectrosynthetic cells (similar to photovoltaic cells) have emerged as a low-cost and environmentally friendly technology for converting solar energy into chemical fuels or electricity. These photoelectrochemical cells offer a means of using renewable solar energy to drive energy-intensive chemical conversions at ambient temperature and pressure. Here, the collaborative fundamental research project will study how a dye-sensitized photoanode can chemoselectively oxidize lignin with a suitable catalyst as a first step toward a complete light-driven lignin depolymerization process. This approach will expand on the use of heterogeneous catalysis for the oxidation of primary and secondary alcohols to produce carbonyl or carboxyl compounds for the fine chemical and pharmaceutical industries. This work represents a new application for dye-sensitized photoelectrosynthetic cells, and the research findings from the project will be disseminated to the public through research publications, conference presentations, and by organizing and hosting educational outreach programs for future professionals in the STEM field. The PIs will also actively recruit and support underrepresented minority students through the outreach program.

Organic oxidation reactions are important in organic synthesis or lignocellulosic biomass processing. Chemoselective oxidation of the aliphatic and/or benzylic alcohol moieties in lignin is a good target for controlling the degradation of lignin to generate desired small molecular products. This project aims to elucidate a photoelectrosynthetic chemoselective oxidation of alcohol moieties in lignin by combining the use of aminoxyl mediators with a dye-sensitized photoanode (DSP) at room temperature. Essential to this approach is the use of a dye-sensitized electrode interface to activate a nitroxyl mediator via light-induced charge separation. This presents both a new approach for driving the photochemical oxidation of the secondary benzylic alcohol and the primary aliphatic alcohol functional groups found in lignin, as well as a new photocatalytic application for dye-sensitized photoelectrosynthetic cells, which have traditionally focused on solar water splitting. The approach will involve (1) the synthesis of photoactive polymeric catalysts and the elucidation of their underlying photochemical electron transfer properties for activating nitroxyl mediators, (2) the fabrication and evaluation of mesoporous semiconductor-based electrodes specifically designed for the chemoselective oxidation of lignin dimer model compounds with a series of nitroxyl mediators, and (3) the elucidation of mechanistic pathways for the light-driven oxidation of 2o benzylic and 1o aliphatic alcohols using a DSP and address practical challenges presented by the use of oligomer model compounds and technical lignin. This research is significant as a first test case of a DSP to carry out the selective oxidation of real lignin at room temperature as a first step toward light-driven biomass conversion to value-added chemicals.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

Award Number: 2624835
Principal Investigator: Gyu Leem
Funds Obligated: $55,181
State: TX