Investigation of hydroxyl groups in crystalline silicoaluminophosphate SAPO-34 by diffuse reflectance infrared spectroscopy

Abstract

Hydroxyl groups in crystalline silicoaluminophosphate SAPO-34 have been investigated by diffuse reflectance IR spectroscopy. By carefully comparing the intensities of stretching vibrations with the appropriate combination and overtone bands the two types of bridged Si-OH-AI group were assigned to the fundamental stretching vibrations at 3625 and 3600 cm-'. Both types of group exhibited approximately the same acidic properties but differed in their localization. The bridged groups with v = 3600 cm-' were assumed to be localized in the hexagonal prisms, forming an H-bond with adjacent oxygen anions of the framework. In-plane and out-of-plane bending vibrations were observed for the first time in SAPOs.

Theoretical Simulations Elucidate the Role of Naphthalenic Species during Methanol Conversion within H‐SAPO‐34

“…In the literature there are two ways for labeling the oxygen atoms within the chabazite topology. [50,51] We use the labeling applied in references [52][53][54][55] as illustrated in Figure 1. Three sites are part of an eight-membered ring connecting two cages, whereas the fourth oxygen atom, O(3), is attached to a six-membered ring and is only in contact with one cage.…”

New route for light olefins production from chloromethane over HSAPO-34 molecular sieve

“…Two peaks at 3625 and 3598 cm À1 can be attributed to two types of Si(OH)Al groups differed in their localization. The bridged group with y = 3598 cm À1 is assumed to be localized in the hexagonal prism, forming an H-bond with adjacent oxygen atoms of the framework while the isolated bridged OH groups pointing towards the center of the elliptical cages give the vibration frequency at 3625 cm À1 [15][16][17][18]. These two types of OH groups have been considered to have similar acidic properties and to be the active sites for acid-catalyzing reaction [19,20].…”

A study of the acidity of SAPO-34 by solid-state NMR spectroscopy

“…If we assigned the former peak to acid sites, the total acid sites calculated from our NH 3 -TPD data were 1.6 and 0.9 mmol/g for SAPO-34_H and SAPO-34_L, respectively, which are comparable to those obtained by 1 H MAS NMR. Studies on the basis of IR experiments [15][16][17][18] showed two distinct Brønsted acid sites. In Refs.…”

“…In Refs. [15,16], the high-frequency component of the OH stretching peaks was assumed to come from bridged OH groups in the hexagonal prism, forming an H-bond with adjacent oxygen atoms of the framework, and the low-frequency component was from isolated bridging OH groups pointing toward the center of the elliptical cage. However, in Ref.…”

“…The acid strength is independent on the content of Si atoms, the SAPO-34 particle size and the metal incorporation [12,13], and the number of the strong acid sites did not change significantly with the silicon content of the synthesis gels [14]. However, two distinct Brønsted acid sites corresponding to protons residing on inequivalent oxygen atoms have been revealed on the basis of FTIR and neutron diffraction experiments [15][16][17][18]. Using multipeak curve-fitting approach, a third acid site assigned to the bridged hydroxyls at the border of Si islands was even proposed [19], and its acid strength is comparable to that in zeolites.…”