Catalytic and infrared spectroscopic investigations of the molecular sieve types SAPO-34 and SAPO-11

Abstract

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 Brsnsted 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.

Potassium poisoning on Cu-SAPO-34 catalyst for selective catalytic reduction of NOx with ammonia

“…Al-(OH)-Si (3620 and 3597 cm -1 ), respectively [21,22]. In the N-H stretching mode region, bands at 3342, 3272 cm -1 are assigned to the ammonium ions, while the band at 3176 cm -1 can be assigned to NH 3 -Cu + species [23].…”

Crystallization and Si incorporation mechanisms of SAPO-34

“…The Brønsted acid sites of SAPO-34 are attributed to the introduction of Si atoms into the neutral AlPO 4 -34 framework. From the viewpoints of the references [12][13][14][15][16], it is proposed that Si atoms incorporate into the AlPO 4 -34 structure by two different substitution mechanisms: the first, denoted as SM1, is that the Si substitution for phosphorus form Si(4Al) entities, which gives rise to negative charges for forming Brønsted acid sites; the second mechanism (SM2) is the double substitution of neighboring aluminum and phosphorus by two silicon atoms to form Si(nAl) (n ¼ 3-0) structures, which leads to the formation of stronger Brønsted acid sites. The structure, acidity and catalytic property of SAPO-34 depend on the number and distribution of Si in the framework [16][17][18], which is closely related to the synthesis process, i.e., to the crystallization and Si incorporation mechanisms.…”

High-throughput characterization of heterogeneous catalysts by temperature-programmed analysis method

“…This may be caused by influences of the individual pore width and structure of the void volume of the different types of molecular sieves on the diffusion of the desorbing ammonia molecules [18]. Indeed, the shifting of the high-temperature peak for small pore SAPOs (SAPO-11 and SAPO-34) had been observed by other authors [19,20].…”