SAPO‐34 Framework Contraction on Adsorption of Ammonia: A Neutron Scattering Study

Abstract

Neutron scattering data was recorded from SAPO‐34 using the OSIRIS instrument before and after repeated ammonia adsorption at pressures up to 8 bar. Coherent scattering from the zeolite framework provides the neutron powder diffraction pattern and gave evidence for anisotropic contraction on ammonia dosing. Incoherent quasielastic scattering from the hydrogen of the ammonia showed that mobile ammonia was present in the framework. The quasielastic data was fitted to a model where the ammonia was confined within the chabazite cage in the c direction of the crystal lattice, with diffusion solely occurring through the perpendicular 8‐membered rings. The calculated diffusion constant reached a maximum of 6.3×10−8 m2 s−1 at 5 bar.

Induced Active Sites by Adsorbate in Zeotype Materials

“…31,32 Both quantitative and qualitative information with high resolution and sensitivity can also be achieved. Before the adsorption experiment using 13 C-2-acetone, 1 H, 27 Al, 29 Si, and 31 P SSNMR (detailed parameters of SSNMR experiments are given in the SI) characterizations were performed (Figures S3−S9), which show the presence of a typical BAS in the pristine samples with values in agreement with the literature. 31,33,34 The curve in Figure 1a shows the 13 C crosspolarization (CP) magic-angle spinning (MAS) NMR spectra of H-SAPO-34 zeolite adsorbed with 13 C-2-acetone.…”

“…Interestingly, two peaks at 217 ppm (type-I) and 225 ppm (type-II) are clearly observed. According to the previous literature, 35 the major peak at 217 ppm is attributed to 13 C-2-acetone adsorbed on the BAS. However, the other 13 C resonance at 225 ppm persistently appears in repeated measurements but remained ambiguous.…”

“…12 Thus, the formation of Lewis acidic and basic sites from these structures during molecular adsorption/catalysis is anticipated to take place more readily than aluminosilicate zeolites. By combining in situ synchrotron-radiation X-ray powder diffraction and density functional theory (DFT) calculations, Arstad et al 7 demonstrated that the structure of SAPO-37 can collapse after adsorbing water molecules under 345 K. A similar observation was reported by Sliverwood et al, 13 who showed that the SAPO-34 framework contracts following ammonia adsorption by using a neutron scatting technique. Using solid-state nuclear magnetic resonance (SSNMR) and DFT calculations, Morris and Ashbrook et al 14,15 confirmed that framework Al−O bonds in zeolites can be reversibly broken through the adsorption and desorption of water at room temperature.…”

Adsorbate-induced Active Site in Zeolite

“…9 Thus, the formation of Lewis acidic and basic sites from these structures during molecular adsorption/catalysis is not yet clear. By combining in-situ synchrotron-radiation X-ray powder diffraction and DFT calculations, Arstad et al 5 demonstrated that the structure of SAPO-37 can collapse after adsorbing water molecules under 345 K. A similar observation was reported by Sliverwood et al 10 who showed that the SAPO-34 framework contracts following ammonia adsorption by using a neutron scatting technique. Using SSNMR and DFT calculations, Morris et al 11,12 con rmed that framework Al-O bonds in zeolites can be reversibly broken through the adsorption and desorption of water at room temperature.…”

Thermal Alteration in Adsorption Sites over SAPO‐34 Zeolite

“…Nowadays, there is a growing interest in the study of the “flexible framework” of zeolites which have long been treated as rigid and robust inorganic porous solid materials [14] . The reversible changes of their structures and properties in response to external stimuli have been reported in a large number of works during the past time [14f–j] . In particular, the framework flexibility accounted for the newly created active sites in zeolites has been well recognized.…”