Mechanistic Comparison of the Dealumination in SSZ-13 and the Desilication in SAPO-34

Abstract

With the purpose of understanding the behavior of aluminosilicate zeolites and silicoaluminophosphates (SAPOs) in the presence of steam, we carried out a computational density functional theory (DFT) study on the desilication of SAPO-34. The mechanism studied was a stepwise hydrolysis of the four bonds to the Si heteroatom. An analogous process to the desilication of SAPO-34 is the dealumination of SSZ-13. To investigate possible mechanistic differences between the two processes, we compared the results of this study with the results of a previous study on dealumination in SSZ-13. We found that the intermediates along the dealumination path of SSZ-13 have one of the protons bonded to a bridging oxygen atom. In the corresponding intermediates of the desilication path in SAPO-34, the same proton prefers to be part of an aqua ligand coordinated to an Al atom. The principal factor determining the different proton locations is the electronic requirement of the atoms surrounding the proton. The different proton locations in SSZ-13 and SAPO-34 put clear conditions on possible mechanisms, thus causing them to be different for the two materials. We expect the principles determining the proton location also to be valid for other mechanisms of dealumination in SSZ-13 and desilication in

Conversion of methanol to olefins: Stabilization of nanosized SAPO-34 by hydrothermal treatment

“…Recently, Aramburo et al [25] reported no difference with the purely thermally treated samples [31,34,35], i.e., a reduction in acidity with migration of silicon species to silicon islands. However, in an earlier contribution Park et al [37] showed that SAPO-34 thermally treated at 750ºC loses more crystallinity after reaction Theoretical calculations on the role of steaming on clusters of isomorphous zeolite SSZ-13 and SAPO-34 [38] also to support our hypothesis on the effect of steaming on SAPO-34 stability.…”

“…In this case, steaming at temperatures of 750ºC or higher and high partial pressures of water are used for simulating an equilibrium catalyst. However, this kind of hydrothermal stability test does not seem suitable for SAPO-34 since the mechanism of dealumination for zeolites is different from the desilication on silicoaluminophosphates [38]. In fact, while steaming at high temperature generates wide dealumination in ZSM-5 crystals with severe loss of acidity [47][48][49], in the case of standard SAPO-34, both, textural and acidity properties are preserved (see Table 1 and ref.…”

A comparison of hydrothermal aging effects on NH3-SCR of NO over Cu-SSZ-13 and Cu-SAPO-34 catalysts

“…The protons are connected to Al atoms as part of a ligand. The different locations of the protons may lead to possible mechanistic differences in hydrothermal aging [33]. Indeed, it was reported that the crystal structure of SAPO-34 can be maintained up to 1000-1200 • C even in the presence of moisture [34].…”

Improvement of low-temperature hydrothermal stability of Cu/SAPO-34 catalysts by Cu 2+ species

“…This rapid loss of structural integrity and acidity has become a major drawback for the practical application of SAPO-34. Several research groups have investigated this deactivation phenomenon and proposed potential deactivation mechanisms, such as Si-O-Al bonds opening [3][4][5] and P-O-Al bonds opening [6,7]. In addition, Janssen et al [8] proposed methods to inhibit the skeleton collapse of SAPO molecular sieve by using templates and carbonaceous material to shield the catalytic sites from contacting with H 2 O molecules.…”