An oxidant to replace nitrobenzene in lignin analysis

Abstract

Four metal organic frameworks (MOFs) are being evaluated as possible catalysts for alkaline lignin oxidation. One aspect of the screening process is the oxidation of in-situ hardwood lignin by a high mole ratio of Cu, Fe, or Cu + Fe contained in the MOFs. The MOF’s were prepared in a microwave-assisted synthesis. One of the MOFs, a benzenetricar-boxylic acid complex of Cu (II) and Fe (III), converted the phenylpropane (C9) units in in-situ poplar lignin to approximately 50% monomers. The ratio of syringyl (S) to guiacyl (G) monomers was similar to the S:G ratio obtained from nitrobenzene oxidation (NBO). The S:G ratio from NBO was then compared to that from MOF oxidation (MOFO) for two other poplars. The S:G ratios for the three poplars by MOFO were 1.20, 1.51 and 1.67, respectively while the corresponding NBO values were 1.25, 1.45 and 1.68. These initial results indicate that MOFO could be developed into a credible replacement for NBO, because it affords a higher yield of monomers when a large MOF dose is used. Some preliminary results are also presented for MOF acting as a catalyst (100 ppm Cu (II) plus 81 ppm Fe (III) in NaOH) for oxygenation of the hardwoods. The products from O2 + MOF are identical to MOF only, but the ratios amongst them are different.