(760e) Mechanocatalytic Hydrogenolysis of a Model Lignin Ether

Converting lignin to value-added chemicals will be essential as the world shifts to renewable sources of carbon. Recalcitrance of the lignin polymer during fractionation further limits the feasibility of many of these strategies, so unconventional valorization methods are necessary to overcome these issues. Mechanocatalysis is a promising novel approach to lignin upgrading because costly and environmentally harmful solvents can be avoided. These reactions use mechanical energy, typically in the form of collisions or impacts, to drive catalytic reactions in place of thermal energy. Here, the mechanocatalytic hydrogenolysis of benzyl phenyl ether (BPE), a lignin model compound, is studied over supported nickel catalysts.

The milling of BPE in a hydrogen environment with a commercial nickel (53 wt%) on silica-alumina catalyst resulted in 99% conversion within 120 min. Hydrogenolysis occurred at the benzyl ether bond, resulting in the primary products of toluene, phenol, and cyclohexanol. A maximum in phenol yield around 60 min indicated aromatic ring hydrogenation occurred after hydrogenolysis. The toluene volatilized and left the reactor before significant hydrogenation occurred. The mechanical action during also liberated fresh nickel surfaces, avoiding a high-temperature pretreatment step. Recycle experiments show that polyaromatic coke forms during milling, resulting in a steady deactivation.

Hydrogenolysis reactions were also conducted using three 5 wt% nickel catalysts to explore the influence of the support properties. These catalysts with less nickel were synthesized by dry impregnation using high surface area amorphous silica-alumina, high surface area silica gel, and low surface area silica as supports. Contrary to expectation, the low surface area catalyst had the best performance with the highest toluene yield and the lowest carbon loss. The high surface area silica supported catalyst did not show evidence of polyaromatic coke, despite having large carbon losses. Instead, the milling creates reactive sites on the silica that strongly binds the products.