Catalysis of water molecules acting as Bronsted acids at Lewis acid sites on niobium oxide

The effects of water addition on cumene cracking over niobium oxide and H-ZSM-5 are investigated in order to clarify the behavior of water molecules on solid acid catalysts. The activity of niobium oxide was improved in the presence of water vapor although H-ZSM-5 showed constant activity. When heavy water was introduced into the reaction over niobium oxide, the hydrogen atoms were substituted with the deuterium atoms in about 85 % of the produced benzene. The heavy water molecule on niobium oxide acts as the catalyst for cumene cracking, resulting in the formation of benzene containing D atoms. In addition, the Lewis acid on the niobium oxide was observed to change reversibly to the Bronsted acid in the presence of water using IR of adsorbed pyridine. We propose a mechanism that water behaves as Bronsted acid, which was clarified by repeating adsorption and desorption on Lewis acid sites of niobium oxide.