Chemists at the University of Illinois report the development of a nickel-based homogeneous catalyst that could more effectively support the production of fuels and chemical feedstocks from biomass as well as improved liquefaction of coal. A paper on their work is published in the current issue of Science.
The catalyst enables the selective hydrogenolysis of aromatic carbon-oxygen (C-O) bonds under mild conditions. This would break down lignin—a recalcitrant biomass polymer that consists of a network of nine hydrocarbon units connected by C-O bonds. Currently, the hydrogenolysis of aromatic C-O bonds requires heterogeneous catalysts that operate at high temperature and pressure (more than 250°C and 30 bar) and with poor selectivities, note Alexey Sergeev and John Hartwig.
Selective hydrogenolysis of aromatic carbon-oxygen (C-O) bonds (scission by reaction with hydrogen to form CH and OH bonds in their place) is challenging because of the strength and stability of these linkages; yet, this process is important for the conversion of oxygen-rich lignocellulosic plant biomass to deoxygenated fuels and commercial chemicals.
Whereas the exclusively aliphatic C-O bonds in cellulose can be cleaved with hydrolysis and dehydration, the aromatic C-O bonds in lignin cannot undergo these processes and have resisted selective cleavage by hydrogen. In addition, brown coal’s polymeric network contains aromatic C-O bonds inherited from lignocellulosic biomass, and the liquefaction of these bonds could facilitate the liquefaction of this carbon source and its conversion to arene feedstocks.
...To realize a selective hydrogenolysis of aromatic C-O bonds, we envisioned a reaction that involves insertion of a discrete transition metal complex into the aromatic C-O bond and reaction of the resulting intermediate with hydrogen to yield arene and alcohol. We anticipated that the low reactivity of homogeneous catalysts toward hydrogenation of aromatic rings would prevent competitive formation of cycloalkanes and cycloalkanols from such a process. Nickel complexes known to activate aromatic C-O bonds in the presence of aliphatic C-O bonds were the starting point for our catalyst development.—Sergeev and Hartwig
In their paper, Sergeev and Hartwig report the selective hydrogenolysis of aromatic C-O bonds in aromatic ethers that form exclusively arenes and alcohols. This process is catalyzed by a soluble nickel carbene complex under just 1 bar of hydrogen at temperatures of 80 to 120°C.
Although...mechanistic issues are yet to be resolved, and turnover numbers of the catalyst must be improved, the results from this work have demonstrated that the selective cleavage of aromatic C-O bonds in the presence of other C-O bonds can be conducted without reduction of the arene units by using a widely available metal and the cheap, mild, and atom-economical reductant hydrogen.—Sergeev and Hartwig
Alexey G. Sergeev and John F. Hartwig (2011) Selective, Nickel-Catalyzed Hydrogenolysis of Aryl Ethers. Science Vol. 332 no. 6028 pp. 439-443 doi: 10.1126/science.1200437