Catalytic conversion of toluene over a biochar bed under an inert atmosphere – The comparison of chars from different types of wood and the role of selected metals

In this paper, toluene, a tar-representing compound, was used in pyrolytic conversion tests with a wood-derived char catalyst. The decomposition mainly occurred via two competing heterogeneous reactions; the favoured reaction led to coke formation on the catalyst bed. With char saturation of the carbon deposit, a second demethylation reaction gained significance, until selectivity towards benzene formation reached 12–16%. By-products of the secondary reaction (xylenes, ethylbenzene and styrene) were also detected; however, no gas-phase toluene decomposition occurred. The principles of toluene conversion were the same for all of the examined chars from the different tree species. However, a quantitative difference was observed between the chars from the wood from coniferous and deciduous trees. Pine yielded char with less mesopores and active sites and thus demonstrated a lower efficiency as a catalyst for toluene decomposition. An assessment of alkali and alkaline earth metals importance revealed that, while their presence in the char strongly enhances its oxidation rate, they did not affect the toluene pyrolysis.