Pore system, SAPO molecular sieves

In the early 1980s researchers at Union Carbide discovered that small-pore size silicoaluminophosphate (SAPO) molecular sieves were effective for converting methanol to ethylene and propylene. The best performances were obtained with SAPO-34 and SAPO-17 catalysts (6). SAPO-34 has the CHA structure with a three dimensional pore system consisting of large cavities (about 9.4 A in diameter) separated by small windows (3.8 x 3.8 A). 

Catalyst Characterization. The three SAPO molecular sieves employed in this study represents the three pore sizes of molecular sieves, ranging tom 0.4 nm to 0.8 nm. While SAPO-5 and SAPO-11 have unidimensional pores, SAPO-34 has a multidimensional pore system with supercages. The chemical composition and total ammonia uptake of the tifiree SAPO molecular sieves are listed in Table I. 

The MeAPSO family further extends the structural diversity and compositional variation found in the SAPO and MeAPO molecular sieves. These quaternary frameworks have Me, Al, P and Si as framework species [27]. The MeAPSO structure types include framework topologies observed in the binary AIPO4 and ternary (SAPO, MeAPO) compositional systems and the novel structure -46 with a 0.7 nm pore. The structure of -46 has been determined [34]. 

On the other hand, the primary carbenium ions ethyl+, propyF, and butyF, whose formation involves the oxonium methyl ylide, can be deprotonated into the corresponding olefins, but they can also undergo methylation and oligomerization into higher carbenium ions, leading to paraffins formation via H-transfer reactions and to aromatic via cyclization [119]. The latter transformation leads to polyring aromatics, which are too bulky to leave the catalyst because of the structure of SAPO-34. They cover sites and block pores and thus deactivate the catalyst. However, the catalyst should be partially deactivated to a primary formed propylene which could escape the pore system of the molecular sieve. 

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