Strength, Type and Catalytic Activity of Acid Sites in ZSM5 Zeolites

ZSM5 zeolites with two Si/Al ratios and modifications with HC1 and NaOH were studied by i.r. spectroscopy, temperature programmed desorption (t.p.d.) and reactions with methanol.Three types of sites were found: Bronsted acid sites and two types of Lewis acid sites. The stronger Lewis acid sites are accessible cations. The Al₃₊ cations are well shielded by surrounding oxygen, as indicated by the wavenumber of the 19b band (1450-1453cm^-1).Three of the adsorption states were detected by i.r. spectroscopy and t.p.d. (physisorption sites, adsorption on Na⁺ and on Bransted acid sites), a fourth (adsorption on Al³⁺) only by i.r. spectroscopy. These adsorption states are identical for pyridine and ammonia, suggesting that both molecules are able to adsorb on the same sites. The amount of water adsorbed increases with the density of accessible metal cations. While BrØnsted acid sites were found to be indispensable for formation of hydrocarbons from methanol, Lewis acid sites seem not to participate in that reaction. A model for the location of BrØnsted acid sites is discussed.