The K+ ion of KOH could have been inserted in the vacant sites of ZrO2 which accelerated dissociative dispersion and decomposition of KOH to form basic sites in the activation process. The more, KOH was loaded on ZrO2, the more free vacant sites of ZrO2 decreases. This results in the surface enrichment of potassium species which probably was the active sites of the catalyst [21,39].