Well-Defined Silanols in the Structure of the Calcined High-Silica Zeolite SSZ-70: New Understanding of a Successful Catalytic Material

Abstract

The structure of the calcined form of the high-silica zeolite SSZ-70 has been elucidated by combining synchrotron X-ray powder diffraction (XRPD), high-resolution transmission electron microscopy (HRTEM), and two-dimensional (2D) dynamic nuclear polarization (DNP)-enhanced NMR techniques. The framework structure of SSZ-70 is a polytype of MWW and can be viewed as a disordered ABC-type stacking of MWW-layers. HRTEM and XRPD simulations show that the stacking sequence is almost random, with each layer being shifted by ±1/3 along the ⟨110⟩ direction with respect to the previous one. However, a small preponderance of ABAB stacking could be discerned. DNP-enhanced 2D Si{Si} J-mediated NMR analyses of calcined Si-SSZ-70 at natural Si isotopic abundance (4.7%) establish the through-covalent-bondSi-O-Si connectivities of distinct Si sites in the framework. The DNP-NMR results corroborate the presence of MWW layers and, more importantly, identify two distinct types of Q silanol species at the surfaces of the interlayer regions. In the first, an isolated silanol group protrudes into the interlayer space pointing toward the pocket in the adjacent layer. In the second, the surrounding topology is the same, but the isolated -SiOH group is missing, leaving a nest of three Si-O-H groups in place of the three Si-O-Si linkages. The analyses clarify the structure of this complicated material, including features that do not exhibit long-range order. With these insights, the novel catalytic behavior of SSZ-70 can be better understood and opportunities for enhancement recognized.

Preferential Siting of Aluminum Heteroatoms in the Zeolite Catalyst Al‐SSZ‐70

“…[32] Acquisition of such spectra has been exceptionally challenging in the past, but is enabled here by the use of low-temperature (< 100 K) measurement conditions,w hich lead to improved NMR signal sensitivity (ca. Fore xample,t he 2D 27 Al{ 29 Si} J-HMQC spectrum of calcined Al-SSZ-70 in Figure 2a shows correlated signal intensities at 27 Al shifts of 60 ppm, 55 ppm, and 49 ppm that are correlated with 29 Si signals in the chemical shift range À105 ppm to À108 ppm (red band in Figure 2a) from different Q 4 (1Al) 29 Si species.T he 27 Al signal at 55 ppm is additionally correlated with 29 Si signals in the À99 to À101 ppm range (blue band), which also arise from Q 4 (1Al) species.T he different correlated 27 Al- 29 Si signals in the 2D 27 Al{ 29 Si} J-mediated spectrum in Figure 2a are assigned based on comparisons with previous 2D 29 Si{ 29 Si}and 29 Si{ 1 H} NMR analyses of calcined Si-SSZ-70, [20] analyses of 2D 27 Al MQMAS spectra of Al-SSZ-70 ( Figures S4, S5, Tables S1, S2), well-established semi-empirical correlations relating the isotropic 27 Al and 29 Si chemical shift values to the -T-O-Tbond angles, [33][34][35] and prior literature. Fore xample,t he 2D 27 Al{ 29 Si} J-HMQC spectrum of calcined Al-SSZ-70 in Figure 2a shows correlated signal intensities at 27 Al shifts of 60 ppm, 55 ppm, and 49 ppm that are correlated with 29 Si signals in the chemical shift range À105 ppm to À108 ppm (red band in Figure 2a) from different Q 4 (1Al) 29 Si species.T he 27 Al signal at 55 ppm is additionally correlated with 29 Si signals in the À99 to À101 ppm range (blue band), which also arise from Q 4 (1Al) species.T he different correlated 27 Al- 29 Si signals in the 2D 27 Al{ 29 Si} J-mediated spectrum in Figure 2a are assigned based on comparisons with previous 2D 29 Si{ 29 Si}and 29 Si{ 1 H} NMR analyses of calci...…”

“…Thes pectrum manifests correlated 27 Al{ 1 H} signal intensities from 27 Al-1 Hn uclear spin pairs that are dipole-dipole coupled through space,b eing principally sensitive to interactions over distances of < 5 . [38] Analyses of 1D 1 Hand 2D 29 Si{ 1 H} HETCOR NMR spectra of calcined Si-and Al-SSZ-70 ( Figures S3, S8) show that both materials possess isolated and strongly H-bonded interlayer -OH species.T he 2D 27 Al{ 1 H} HETCOR spectrum of calcined Al-SSZ-70 in Figure 2b shows correlated intensities at 55 and 60 ppm in the 27 Al dimension and at 2.8-4.0 ppm (green band, Figure 2b) and 5.1-5.6 ppm (gray band, Figure 2b)inthe 1 Hdimension, which arise from tetrahedrally coordinated 27 Al species proximate to isolated interlayer -OH groups [20] and nanopore-adsorbed water, [39] respectively.T hese signals are consistent with the assignment of the 27 Al signals at 55 and 60 ppm to fully crosslinked Al2a/b and Al3 sites,respectively. By comparison, the 27 Al signal at À5ppm is correlated with ab road, continuous distribution of 1 Hs ignals from 2.8-14.3 ppm.…”

“…Thelayered zeolite SSZ-70 has recently emerged as apromising platform for new catalysts because it can be exfoliated to generate high-surface-area active materials that exhibit enhanced catalysis rates for alkylation reactions, [18] while calcined Al-SSZ-70 exhibits high catalytic activity for hydrocarbon cracking. [20] SSZ-70 crystallizes into hexagonal flakes (Figure 1a)a nd has two types of 2-dimensional channel systems with effective cross-sectional pore openings of 4.4 5.9 2 (within the MWW-type layers) and 4.0 11.5 2 (between the MWW-type layers). [20] SSZ-70 crystallizes into hexagonal flakes (Figure 1a)a nd has two types of 2-dimensional channel systems with effective cross-sectional pore openings of 4.4 5.9 2 (within the MWW-type layers) and 4.0 11.5 2 (between the MWW-type layers).…”

Preferential Siting of Aluminum Heteroatoms in the Zeolite Catalyst Al‐SSZ‐70

“…[19] Thestructure of calcined SSZ-70 has 14 crystallographically distinct Tsites. [20] SSZ-70 crystallizes into hexagonal flakes (Figure 1a)a nd has two types of 2-dimensional channel systems with effective cross-sectional pore openings of 4.4 5.9 2 (within the MWW-type layers) and 4.0 11.5 2 (between the MWW-type layers). However, in contrast to other MWW-type zeolite catalysts,t he catalytic selectivity of calcined Al-SSZ-70 for hydrocarbon cracking does not change substantially with time on stream, [19] characteristic of reactions occurring in as ingle type of nanochannel system.…”

“…Analyses of the 1D 29 Si MAS NMR spectra ( Figure S1 in the Supporting Information) show that the local environments of framework 29 Si atoms in calcined SSZ-70 are almost identical in the siliceous and aluminosilicate forms.B ecause of the small absolute quantity of Al in the material (Si/Al = 25, Ref. [19]) and overlapping 29 Si NMR signals from different Q 4 (0Al) and Q 3 (0Al) species in SSZ-70, [20] the 29 Si signals from Q 4 (1Al) species cannot be resolved or quantified from the 1D 29 Si MAS NMR spectra alone. (The Q m (nAl) notation refers to at etrahedrally coordinated 29 Si atom that is covalently linked through bridging Oa toms to m other Si or Al atoms,ofw hich n are Al.…”