Hydrogenation of different carbon substrates into light hydrocarbons by ball milling

Abstract

The conversion of carbon-based solids, like non-recyclable plastics, biomass, and coal, into small molecules appears attractive from different points of view. However, the strong carbon–carbon bonds in these substances pose a severe obstacle, and thus—if such reactions are possible at all—high temperatures are required1–5. The Bergius process for coal conversion to hydrocarbons requires temperatures above 450 °C6, pyrolysis of different polymers to pyrolysis oil is also typically carried out at similar temperatures7,8. We have now discovered that efficient hydrogenation of different solid substrates with the carbon-based backbone to light hydrocarbons can be achieved at room temperature by ball milling. This mechanocatalytic method is surprisingly effective for a broad range of different carbon substrates, including even diamond. The reaction is found to proceed via a radical mechanism, as demonstrated by reactions in the presence of radical scavengers. This finding also adds to the currently limited knowledge in understanding mechanisms of reactions induced by ball milling. The results, guided by the insight into the mechanism, could induce more extended exploration to broaden the application scope and help to address the problem of plastic waste by a mechanocatalytic approach.

Mild Catalytic Degradation of Crystalline Polyethylene Units in a Solid State Assisted by Carboxylic Acid Groups

“…After the milling, a slight decrease in molecular weight was detected by SEC analysis (from M n = 8.3 kg/mol, D̵ = 2.6 to M n = 6.5 kg/mol, D̵ = 2.3, Figure a). Although the mechanochemical promotion of the radical decarboxylation process may be anticipated, this should have a limited effect since a simple PE is also known to generate alkyl radicals that cause degradation under ball milling conditions . Furthermore, a significant molecular weight loss was observed after the irradiation without a solvent at 80 °C (down to M n = 1.0 kg/mol, D̵ = 6.3 after 24 h, as shown in Table S4, and M n = 0.3 kg/mol, D̵ = 5.6 after 96 h in Figure b).…”