Two key challenges facing the chemical industry are tackling climate change and improving sustainability. Improved understanding of the catalytic activity of carbon would assist the development of carbonaceous catalysts sourced from renewable materials, replacing increasingly-scarce metals. The phenomenon of catalytically active coke could also be exploited to improve the efficiency of industrial reactions.
Biochars are by-products of biomass pyrolysis and are attracting increasing interest as catalytic materials. Studies often overlook the impact of feedstock choice, graphitic carbon and trace metal content on catalytic performance. In the present work, the catalytic activity of biochars from four feedstocks before and after surface treatments, alongside a commercial activated carbon, is studied in two reactions utilising waste products as feedstocks, to gain insights into properties influencing catalytic activity.
The catalytic activity of untreated biochar was demonstrated for the first time in the conversion of methanol to dimethoxymethane (DMM) and in the upgrading of glycerol with carbon dioxide (CO2) to glycerol carbonate and acetins. Potassium content was a key influence: removal of potassium enhanced DMM production, whilst the yields of glycerol carbonate were reduced up to 100 times over demineralised biochar ashes. In addition, methanol conversion over biochar catalysts led to the production of 1,1-dimethoxyethane (1,1-DME), which has not previously been noted in the literature. Production of 1,1-DME may be correlated with more graphitic carbon.
Following development of a novel liquid phase tar impregnation method, pyrolysis of the tar over biochar led to increased surface carbon content. The graphiticity of the surface carbon increased over biochars with higher potassium content, and decreased over biochars with lower potassium content. Over demineralised samples, this trend reversed, implying potassium may influence the formation of graphitic carbon during pyrolysis.
Future studies should consider an application-centred approach to biomass feedstock selection, to realise the full potential of biochars in catalysis.
