Effect of preoxidation treatment with H2O2 on ruthenium ion-catalyzed oxidation of Yima long flame coal

•PO increases the yields of BCAs, ADAs, and AAs sharply.•PO is more effective to afford BCAs with more carboxylic groups.•PO provides more active sites to significantly raise the yield of AAs and ADAs.•PO promotes the formation of –COOH from other oxygen-containing groups or bonds.•The formation of –COOH is the critical to increase the yield of oxidation products.