“You can make anything from lignin” . . . a selective oxidation for lignin from researchers at the University of Wisconsin

Lignin is one of the main components of plant biomass (~40% of the energy content) and is a gooey aromatic polymer that encases the sugar polymers in plant structures. It is usually an impediment to productive uses of biomass. It hinders digestion of grass by animals (and may make it taste gross!), its removal from wood is a major step in many papermaking processes, and it is a barrier to the enzymatic breakdown of biomass for cellulosic fuels. Most old and wise wood scientists like to say “You can make anything from lignin except money!”

Recently, Shannon Stahl, John Ralph, and coworkers at the University of Wisconsin-Madison published a selective method to oxidize lignin as a first step toward breaking it down into useful pieces ("Chemoselective Metal-Free Aerobic Alcohol Oxidation in Lignin"). Non-selective oxidation has already been used for making valuable products from lignin; vanillin, the source of vanilla flavor, can be produced in low yield by oxidation of the lignin liquor waste from sulfite pulping of wood. Nonetheless, the poor vanillin yield and large amounts of waste produced in this process have made it unattractive and led to its demise in the US and Canada (for this interesting story, see this paper by Martin Hocking). Noting the low selectivity of previous methods of lignin oxidation, Stahl and coworkers performed a careful screen of a number of different oxidation methods on a lignin model compound and found that TEMPO-based oxidants and co-catalysts performed well, achieving up to 94% isolated yield for oxidation of a benzylic secondary alcohol to a ketone (see scheme below).

Here, oxygen is used as the terminal oxidant and the nitrogen oxides (from HNO3) and TEMPO derivative serve as co-catalysts. The oxidative chemistry performed well with a range of other lignin model compounds. However, it is common for attempts at lignin chemistry to be quite successful with model compounds but fail miserably with real lignin. To their credit, the Wisconsin team tested their chemistry with an isolated aspen lignin and examined its conversion by 2D NMR. They found that the benzylic alcohols were oxidized to ketones on essentially all of the guaiacyl units and the majority of the syringyl units according to the NMR. The mass recovery in the lignin reaction was slightly more than 80%, which could indicate that some of the lignin was broken down into soluble organics as well.

There are some important limitations to this small scale lignin study, though. According to the supplemental procedures and references, the particular lignin sample used in this work was prepared by ball milling of wood, enzymatic removal of the associated cellulose and hemicellulose, and extraction of the lignin into dioxane. This process leads to a soluble and easy to handle form of lignin. Practical processes using lignin are likely to have to work with poorly soluble, impure, and intractable lignins.

As a final note, Stahl and coworkers showed that the main oxidized lignin model compound which they produced could be broken into vanillic acid and guaiacol with alkaline hydrogen peroxide in moderate yield. It is unfortunate, though, that the control reaction of the non-oxidized lignin model compound was not also included in the discussion. Alkaline hydrogen peroxide is a widely used condition for breaking down lignin, and it has been previously reported that this condition can be used to cleave a lignin model compound very similar to the one used by Stahl and coworkers. Is there a clear difference in lignin deconstruction due to the selective oxidation? Hopefully this topic as well as the depolymerization of real lignin will be addressed in future publications. It is exciting to see lignin chemistry receiving more attention from chemists like Stahl, and this may someday lead to chemistry for making money from lignin.