Reversible modification of the Si environment in template-free SAPO-34 structure upon hydration-dehydration cycles below ca. 400 K

Abstract

On hydration template-free SAPO-34 shows the opening of some Si-0-Al bonds as suggested by 29Si MAS NMR and XRD results, the effect being reversible upon dehydration; the hydrated distorted structure is stable for months with no further modifications and the ordered material can be regenerated by removal of water at 423 K.

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

“…3a presents the 27 Al MAS NMR spectra of dehydrated Cu/ SAPO-34 samples. For the F-Samples, the spectra show a single signal with a chemical shift of 32 ppm, which could be assigned to the 4 ] in the aluminophosphate regions of framework and no signal was observed due to phosphorus species coordinated to Si. This indicates that Si selectively replaces P in the aluminophosphate framework to create the Brønsted acid sites.…”

“…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.…”

Correlation of the changes in the framework and active Cu sites for typical Cu/CHA zeolites (SSZ-13 and SAPO-34) during hydrothermal aging

“…The dominant peak at -89 ppm in 29 Si MAS NMR spectra represented the Si(4Al) species, indicating the highly dispersion of Si atoms in SAPO-34 framework. [17][18][19] The rough ratio of Si(4Al) species decreased from 1 to 0.6, and both the ratio of Si(3Al) and Si(2Al) species increased to 0.2. [17][18][19] The rough ratio of Si(4Al) species decreased from 1 to 0.6, and both the ratio of Si(3Al) and Si(2Al) species increased to 0.2.…”

Cooperation of hierarchical pores with strong Brønsted acid sites on SAPO-34 catalysts for the glycerol dehydration to acrolein

“…In addition to the contribution of enhanced accessibility of active sites to reaction activity, diffusion intensification on hierarchical SAPO-34 can help shorten the residence time of intermediates/products and thus inhibit the further coke formation. Direct synthesis with templates [44][45][46][47][48] or the indirect post-treatments [41][42][43][49][50][51][52] can be used to construct the hierarchically porous SAPO-34 zeolites. In contrast to a low industrialization chance of direct synthesis, the post-synthetic method with HCl, TEAOH, or HF-NH4F or hydration-dehydration reversible modification [52], is easier to operate on a large-scale and more promising for the wide application.…”

“…Direct synthesis with templates [44][45][46][47][48] or the indirect post-treatments [41][42][43][49][50][51][52] can be used to construct the hierarchically porous SAPO-34 zeolites. In contrast to a low industrialization chance of direct synthesis, the post-synthetic method with HCl, TEAOH, or HF-NH4F or hydration-dehydration reversible modification [52], is easier to operate on a large-scale and more promising for the wide application. However, so far, in the case of SAPO-34, it needs the careful tuning of the synthesis conditions to get the hierarchical structure owing to the relatively weaker framework stability of SAPO-34 in acid or alkaline compared to silica-alumina zeolites [50].…”