THE EFFECT OF ALKALI PROMOTERS ON CU-NA-ZSM-5 CATALYSTS IN THE OXIDATION OF BENZYL ALCOHOL

The oxidation activities of alkali-added Cu-Na-ZSM-5 zeolites in the g as-phase catalytic oxidation of benzyl alcohol were studied. The main products were benzaldehyde and carbon oxides (carbon dioxide + carbon monoxide). The partial oxidation activity was found to markedly increa se when an alkali-metal-added Cu-Na-ZSM-5 was used, in comparison with that obtained when the Cu-Na-ZSM-5 zeolite itself was used. On the ot her hand, the increase in yield of deep oxidation products (CO2 + CO) catalyzed by the alkali-added Cu-Na-ZSM-5 was rather low. The alkali a dded to the Cu-Na-ZSM-5 zeolite was thus found to selectively promote catalytic activity for the partial oxidation of benzyl alcohol. Certai n types of alkali salts were used as the additive with the results tha t Na+, K+, and Rb+, but not Li+, had similar behavior for benzyl alcoh ol oxidation. An alkali oxide such as Na2O, rather than the alkali sal t itself, was suggested to substantially function as an active promote r for the oxidation of benzyl alcohol as judged from the pretreatment condition of the alkali-added Cu-Na-ZSM-5 zeolite and its similar cata lytic activity in the oxidation of alkali salts with the same alkali c ation but different counteranions. The added alkali was found to act m ore effectively as a promoter for oxidation when alkali and Cu ions we re accessible to each other; therefore the alkali was suggested to int eract directly with the Cu ion to promote the oxidation of benzyl alco hol, rather than this promotion being a separate function of the alkal i itself. One of the roles of the alkali added to the Cu-Na-ZSM-5 zeol ite was promoting the dissociation of the oxygen molecule, as evidence d by the results of the exchange reaction of the oxygen molecule. The alkali was also suggested to neutralize the acid sites in the Cu-Na-ZS M-5 zeolite and prevent its deactivation by retarding the formation of coke-like materials on the zeolite. (C) 1995 Academic Press, Inc.