(172a) Cobalt Zeolitic Imidazolate Frameworks for the Oxidation of Lignin and Lignin-Related Model Compounds

Lignin is a renewable resource that has the potential to serve as a feedstock for the production of high-value aromatic chemicals. The development of catalysts capable of valorizing lignin has therefore received increased attention, and several homogeneous metal catalysts have been demonstrated to oxidize lignin and related model compounds. In an effort to circumvent the catalyst recovery issues associated with homogeneous catalysts while maintaining high oxidation activity, in this work a series of cobalt zeolitic imidazolate frameworks (Co-ZIF) were synthesized and investigated for their ability to serve as heterogeneous oxidation catalysts for the valorization of lignin. Similarly to the homogeneous systems, these Co-ZIF catalysts were found to efficiently oxidize several lignin model compounds in the presence of molecular oxygen, and the catalysts were readily recovered at the end of the reaction. An exemplary reaction conducted with the Co-ZIF system is the oxidation of veratryl alcohol, which is a simple non-phenolic lignin model compound, in which the benzylic functionality was selectively oxidized to form veratrylaldehyde. Infrared and x-ray diffraction analysis of the catalysts was conducted for structural characterization and purity both before and after reactions, and the importance of the Co-ZIF structure on the lignin and lignin-related model compounds will be addressed. The stability of these catalysts were also investigated under a wide range of conditions and using several different solvents, and we discuss the extension of the catalysts for the valorization of actual lignin streams.