Catalytic and infrared spectroscopic investigations of the molecular sieve types SAPO-34 and SAPO-11
The molecular sieves SAPO-34 and SAPO-11 have been synthesized by means of hydrothermal procedures. The acidic properties of the samples are investigated by infrared spectroscopy and thermodesorption of ammonia. SAPO-34 contains significantly more Brønsted acid sites than SAPO-11 and is able to transform methanol to alkenes with high selectivity. In situ infrared investigations on SAPO-34 show the existence of two forms of adsorbed methanol. At 493 K, dimethyl ether (DME) is formed predominantly, probably by a reaction between physisorbed methanol and methoxy groups. Up to 563 K, the more strongly adsorbed species react to form alkenic intermediates and the formation of coke begins, which proceeds very rapidly at 623 K. The spectrum of the coke is similar to that of aromatic compounds with long aliphatic side chains.