Mapping Al Distributions in SSZ-13 Zeolites from ²³Na Solid-State NMR Spectroscopy and DFT Calculations

Abstract

Al distributions and locations of corresponding nonframework cations in SSZ-13 zeolites have significant impact on their catalytic and adsorption properties. Herein, we demonstrate the feasibility of elucidating Na+ locations and correlated Al distributions in a series of Na-SSZ-13 zeolites with Si/Al ratios ranging from 4 to 48 by combining 23Na MQ MAS NMR spectroscopy and DFT calculations. As only one crystallographically inequivalent T site exists in CHA cages, the isolated Al and the Al pair (Al–O–Si–O–Si–O–Al) and (Al–O–Si–O–Al) in 6-membered ring (MR) distributions and the corresponding Na+ locations are clearly discriminated. In the high-silica Na-SSZ-13 zeolite, Na+ ions are mainly located at the 6-MR SIIa0 site corresponding to one Al substitution in the CHA cage. However, a small amount of the Al pair in 6-MR is still observable even with an Si/Al ratio of 48, corresponding to two Na+ ions located at 6-MR SIIa1 and 8-MR SIII’a1 sites, respectively. With more Al substitutions in the CHA cage, a portion of Na+ ions with high mobility among different SII sites may result in the SIII’b site. In Al-rich SSZ-13 zeolite (Si/Al = 4), the presence of Si­(2Al) and Si­(3Al) may lead to Na+ being located at 6-MR SIIa2, with 23Na NMR signals exhibiting relatively large quadrupolar interactions, and at 8-MR SIII’a2, with NMR signals overlapped with that of the SIII’a1 site. This approach may expand its application in discrimination of Al distributions in other zeolites to gain deeper understanding of the relationship between the structure and the property for their applications in catalysis and adsorption.

Tuning the Aluminum Distribution in Zeolites to Increase their Performance in Acid‐Catalyzed Reactions

“…SSZ‐13 has recently emerged as a material from industry, and it shows enormous potential in catalysis (i.e., in NO x abatement from diesel exhausts) as the first available Si‐rich small‐pore zeolite. Catalytic results indicate variability in the Al distribution . Tuning of the Al distribution by varying the SDA and the Al source results in dramatic variation in the content of single Al atoms in the zeolite (Si/Al=14–18).…”

Carbonylation of Dimethyl Ether to Methyl Acetate over SSZ-13

“…There are three primary positions for monovalent, extraframework cations to site in SSZ-13 ( Figure 7) associated with the crystallographically-distinct O atoms. 25,47,49 Cations in site SI nest in the D6MR, in site SII sit above the plane of the 6MR, and in site SIII' orient at the 8MR. While small cations (e.g., H + ) can fit in the D6MR, it is not considered an active center because it is inaccessible by the gas-phase reactants involved in the carbonylation reaction.…”

“…The framework Si/Al not only impacts the density of Brønsted acid sites (and, commensurately, the density of the surface-bound methyl groups that replace them during the induction period), but also the distribution of those methoxys within different confining environments. 25 In this way, we systematically alter the number density of the framework positions where the transition state can form during the RDS, and determine the impact on the MA production activity. These studies facilitate the identification of the most reactive methoxy group-the one located at the 8MR window-for DME carbonylation to MA in CHA-type zeolites.…”

Local anisotropy in single crystals of zeotypes with the MFI framework structure evidenced by polarised Raman spectroscopy

“…The hierarchical and crystalline structure of zeolites has been studied by high resolution methods like TEM and XRD, [1][2][3] while the challenging question related to the position and distribution of Al atoms in the framework structure has been addressed by NMR and DFT methods. [4][5][6] High-quality TEM images of zeolites require advanced sample preparation, while both XRD and solid state NMR methods are applied to samples in powder form whereby the local orientation characteristic of single crystals cannot be probed. Infrared spectroscopy is an established method for quantitative studies of chemical reactions occurring inside the pores of zeolites, but vibrational spectroscopy in general (including Raman spectroscopy, infrared spectroscopy and inelastic neutron scattering) has been sparingly used to characterise the framework structure in zeolites.…”