Catalytic conversion of toluene by biochar modified with KMnO4

•KMnO4 is suitable for the modification of oak char.•KMnO4 increases oxygen-containing groups and improves biochar activity.•H2 and biphenyl derivatives are the main gas and liquid products, respectively.•Longer residence time damages pore structure and consumes oxygen-containing groups.•Moderate water vapor is beneficial to the toluene catalytic conversion.