Article pubs.acs.org/JPCC
New Insights into the Short-Range Structures of Microporous Titanosilicates As Revealed by 47/49Ti, 23Na, 39K, and 29Si Solid-State NMR Spectroscopy Jun Xu,† Bryan E. G. Lucier,† Zhi Lin,‡ Andre Sutrisno,† Victor V. Terskikh,§ and Yining Huang*,† †
Department of Chemistry, University of Western Ontario, London, Ontario, Canada N6A 5B7 Department of Chemistry, CICECO, University of Aveiro, 3810 Aveiro, Portugal § Department of Chemistry, University of Ottawa, Ottawa, Ontario, Canada K1N 6N5 ‡
S Supporting Information *
ABSTRACT: Seven prototypical microporous titanosilicates have been studied by multinuclear solid-state NMR (SSNMR) spectroscopy, representing four typical Ti environments: square-pyramidal TiO5 units (natisite, AM-1, ETS-4), edge-shared brookite-type TiO6 chains (AM-4), cubane-type Ti4O16 clusters (sitinakite, GTS-1), and corner-shared TiO6 chains (ETS-10, ETS-4). 47/49Ti SSNMR spectra at 21.1 T are related to the coordination, crystal symmetry, and local environment of Ti. Distortions in Ti−O bond lengths and O−Ti−O coordination angles are reflected via CQ(47/49Ti) values that range from 8 to 16 MHz. Several titanosilicates feature axially symmetric 47/49Ti electric field gradient (EFG) tensors that permit facile spectral assignment and detection of deviations in local symmetry. This study uses 29Si NMR experiments to assess phase purity and crystallinity. 23Na NMR is used to probe the location and mobility of the sodium ions in the framework. The potential of 39K SSNMR for investigation of extra-framework counter cations is demonstrated by ETS-10, with increased spectral resolution and enhanced sensitivity to changes in local environment versus 23Na experiments. Plane-wave DFT calculations predicted 47/49Ti NMR parameters assisting in spectral assignments and help correlate 23Na and 29Si NMR resonances to crystallographic sites. The approach described in this work should promote further SSNMR investigations of microporous solids, such as titanosilicates, with unknown or poorly defined structures.
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INTRODUCTION Titanium compounds have realized many practical applications in fields such as catalysis1 and electrochemistry.2 In particular, a variety of titanosilicate materials are under extensive study due to their industrial importance and potential applications in the areas of ion exchange,3−7 catalysis,8−12 adsorption,13−16 and water purification.17 Medium-pore microporous titanosilicates, such as sitinakite18 and GTS-1,19,20 have been suggested as possible materials for the removal of radioactive 137Cs+ and 90 2+ Sr from nuclear waste solutions because of their high ion selectivity and radiation stability. Microporous titanosilicates featuring large pores have also gained attention, with several studies devoted to two well-known members featuring 12member rings (12-rings), ETS-10 and ETS-4.21 The structures of microporous titanosilicates are distinct from classical aluminosilicate zeolites in terms of coordination environments of framework atoms:14 whereas zeolites typically feature four-coordinate Al3+, titanosilicates may possess fiveand/or six-coordinated Ti4+, permitting the formation of TiO5 and TiO 6 polyhedra. Moreover, these polyhedra can interconnect via Ti−O−Ti bonds, forming one-dimensional edge- or corner-shared chains or clusters, whereas direct Al− O−Al connectivity in natural zeolites is energetically unfavorable and rarely observed.22 These unique structural features are © XXXX American Chemical Society
considered to be responsible for the novel properties of microporous titanosilicates.14 Charge-balancing extra-framework countercations, such as Na+ and K+, also play an important role in these compounds.13,14,18−20,23 The nature of the countercation employed, its size, and its affinity to the negatively charged framework often influence the atomic position of the cation and the overall structural morphology. Thorough structural characterization of microporous titanosilicates is essential to understand the relationship between the molecular level structure, properties, and applications of these intriguing materials. For many microporous compounds, it is difficult to obtain suitable crystals for single-crystal X-ray diffraction (XRD) experiments. In such cases, less informative powder XRD (pXRD) data are used to solve the crystal structure. The situation is further complicated for microporous titanosilicates with nonstoichiometric compositions, disordered/mobile extra-framework countercations, or frameworks of low crystallinity. Solid-state nuclear magnetic resonance (SSNMR) spectroscopy, as a technique complementary to XRD, is often applied to characterize crystalline and disordered Received: August 1, 2014 Revised: November 6, 2014
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and natisite feature five-coordinate Ti4+ and AM-4, sitinakite, and GTS-1 contain six-coordinate Ti4+, whereas ETS-10 (sixcoordinate Ti4+) and ETS-4 (both five- and six-coordinate Ti4+) have disordered frameworks. Extracted 47/49Ti EFG and chemical shift (CS) tensor parameters from simulations of experimental SSNMR spectra permit extensive analysis of the relationship between Ti NMR parameters and the Ti4+ coordination environment. Furthermore, 23Na (at 9.4 T) and 39 K (at 21.1 T) SSNMR spectra also provide key structural information regarding the extra-framework countercations, whereas 29Si SSNMR spectra at 9.4 T offer insight into the interior of the framework and the purity of the compounds.
phases.24 SSNMR spectroscopy provides detailed structural information around the nucleus of interest and is well-suited for the investigation of microporous titanosilicates: 47/49Ti and 29Si SSNMR experiments can be used to probe the coordination environment of Ti and Si within the framework, whereas 23Na and 39K SSNMR experiments allow observation of the extraframework cations. 47/49 Ti and 39K SSNMR experiments have the potential to reveal a wealth of structural information25−36 but are very challenging to successfully perform at low and moderate magnetic fields because these are unreceptive quadrupolar nuclei (spin I > 1/2) with very low gyromagnetic ratios (γ). In addition, both 47Ti and 49Ti have low natural abundances (7.28 and 5.51%, respectively).37 The low γ and natural abundances of these nuclei lead to very low intrinsic detection sensitivities. Typically, the central transition (CT, spin 1/2 ↔ −1/2) powder pattern is acquired in SSNMR experiments involving quadrupolar nuclei. Unfortunately, the CT is typically broadened across tens to thousands of kilohertz by the second-order quadrupolar interaction (SOQI) of the nuclear quadrupole moment (Q) with nonspherical electric field gradients (EFGs). These EFGs arise from nuclear surroundings, such as the number and nature of bound ligands, as well as the symmetry of the local environment. Both 47/49Ti and 39K have relatively large values of Q,38 giving rise to broad SSNMR spectra that exhibit low signal-to-noise ratios (S/N) and rendering high-quality spectral acquisition difficult. The two NMR-active isotopes of titanium, 47Ti (I = 5/2) and 49 Ti (I = 7/2), have very similar gyromagnetic ratios. The difference in Larmor frequencies for the two isotopes is only ca. 10 kHz at a magnetic field of 21.1 T. Both 47Ti and 49Ti resonances from the same site are typically observed simultaneously in the same NMR spectrum, complicating spectral interpretation. The 47Ti isotope possesses a larger Q (Q(47Ti)/Q(49Ti) = 1.22), and the broadening of 47Ti due to the SOQI is ca. 3.5 times larger than that of 49Ti, so the narrower and stronger component of the spectrum is due to 49 Ti, whereas the broader and weaker component is attributed to 47Ti. New advances in hardware, pulse sequences, and ultrahigh magnetic field NMR spectrometers have facilitated SSNMR studies of unreceptive quadrupolar nuclei.39 Broadening due to the SOQI is inversely proportional to the magnitude of the external magnetic field, meaning that powder pattern breadths are significantly reduced at higher fields. The intrinsic detection sensitivity also increases at high magnetic fields. The sensitivity can be further enhanced by using quadrupolar Carr−Purcell− Meiboom−Gill (QCPMG)40,41 techniques combined with the adiabatic wide-band uniform-rate smooth truncation (WURST) pulse sequence,42−44 dubbed the WURST-QCPMG pulse sequence. Recently, the local structure around Ti within several titanium phosphates has been examined via their 47/49Ti static NMR spectra,45 and the structure of titanium alkoxide and chloride precursors immobilized by grafting or tether procedures has also been probed via 47/49Ti SSNMR.46 Titanocene chlorides have recently been studied via 47/49Ti MAS and static NMR to relate the NMR tensor parameters to their molecular and electronic structures.47 In this work, several prototypical titanosilicates including natisite, AM-1, AM-4, sitinakite, GTS-1, ETS-10, and ETS-4 were studied by natural abundance 47/49Ti SSNMR spectroscopy at an ultrahigh magnetic field of 21.1 T. Titanosilicate samples possess several Ti4+ coordination environments: AM-1
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EXPERIMENTAL SECTION Sample Preparation and Characterization. The hydrothermal syntheses of AM-1,48 AM-4,48 sitinakite,49 GTS-1,3 ETS-10,3,50 and ETS-44 were performed according to literature procedures. Natisite was synthesized by mixing 15.00 g of sodium silicate solution (27% m/m SiO2, 8% m/m Na2O, Merck), 11.90 g of NaOH (99.9% m/m), and 2.70 g of anatase (98% m/m, Merck) in Teflon-lined autoclaves, followed by treatment at 230 °C for 3 days under autogenous pressure without agitation. The product was filtered off, washed at room temperature with distilled water, and dried at 70 °C overnight. The identity and crystallinity of titanosilicate samples were confirmed by pXRD experiments (Figure S1 in the Supporting Information). The experimentally obtained pXRD patterns closely resemble those simulated from reported crystal structures, and multinuclear (47/49Ti, 23Na, 29Si, and 39K) SSNMR powder patterns are also devoid of impurity peaks, indicating a high degree of purity within these compounds. The presence of a minor impurity in AM-1 is noted (vide infra). All pXRD measurements were conducted on a Rigaku rotating anode diffractometer using graphite-monochromated Co Kα radiation (λ = 1.7902 Å). Experimental pXRD patterns were recorded for 2θ values between 5° and 65°, using a step size of 0.02°. Simulated powder XRD patterns were generated using Mercury software. The Si/Ti ratios of ETS-10 and ETS-4, as well as the Na/K ratios of GTS-1 and ETS-10, were determined using energy dispersive X-ray spectroscopy (EDS) and are provided under Results and Discussion. The water content of sitinakite was checked by thermogravimetric analysis (TGA). During TGA experiments, the sample was heated under N2 atmosphere on a Mettler Toledo TGA/DTA851e instrument from 25 to 500 °C at a constant heating rate of 10 °C/min. 47/49 Ti SSNMR Experiments. Natural abundance 47/49Ti SSNMR spectra were acquired at 21.1 T (ν0(49Ti) = 50.67 MHz) using a 7 mm home-built single-channel NMR probe with a Bruker Avance II spectrometer at the National Ultrahigh-Field NMR Facility for Solids in Ottawa, Canada. A 49 Ti CT-selective 90° pulse width of 1.8 μs was used. Titanium spectra were referenced by setting the 49Ti resonance of a concentrated Cp2TiCl2/CH2Cl2 solution to −773 ppm relative to neat TiCl4.51 For natisite, AM-1, AM-4, and GTS-1, 47/49Ti static (nonspinning) WURST-QCPMG spectra were collected,44 which significantly increase the S/N of the spectra by concentrating the signal into equally spaced spikelets whose manifold traces out the overall powder pattern. A spikelet separation of 2 kHz was utilized. For these samples, 128 refocused echoes were acquired per scan using a 50 μs WURST pulse. The overall broad 47/49Ti spectra were acquired piecewise B
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Figure 1. (a) Structure of natisite (Na−O bonds are omitted for clarity). (b) Experimental (WURST-QCPMG) and simulated natural abundance 47/49 Ti static spectra of natisite at 21.1 T.
Simulations of SSNMR Spectra. 47/49Ti static SSNMR spectra were simulated using the Ti47−49 model integrated in the DMFIT package,54 which can simultaneously produce both 47 Ti and 49Ti powder patterns. 39K and 23Na MAS SSNMR spectra were simulated using the QuadFit package.55 The experimental error for each measured parameter was estimated through bidirectional variation and visual comparison of experimental spectra with those simulated. To provide a rough estimate of the experimental error, the parameter of concern was varied bidirectionally from the best-fit value, while other parameters were kept constant, until noticeable differences between the two spectra were observed. The parameter of interest was then varied bidirectionally from its maximum and minimum estimated values while all other parameters were varied through their individual estimated ranges to provide a final estimate of experimental error bounds. First-Principles Calculations. Plane-wave DFT methods were selected for the calculation of NMR tensor parameters for these titanosilicate compounds, because these types of calculations have successfully predicted NMR parameters in studies involving a wide variety of systems and nuclei.56 Gaugeincluding projector augmented-wave (GIPAW) quantum chemical calculations were conducted using the CASTEP code57,58 (version 4.4, Accelrys Materials Studio) running on a HP xw4400 workstation with a single Intel dual-core 2.67 GHz processor and 8 GB DDR RAM. The NMR module59,60 was used to calculate EFG tensors. Unit cell parameters and atomic coordinates were taken from the corresponding crystal structures,18,20,61−63 and full geometry optimization of all crystal structures was performed prior to EFG calculations. The hydrogen atoms of AM-4 were not included with the crystal structure and were later added without the reduction of the symmetry of the space group. For sitinakite, H2O moieties and the disordered Na2 site were removed prior to calculation. The calculations were performed using ultrasoft pseudopotentials generated from the on-the-fly method within CASTEP. The generalized gradient approximation (GGA) along with the Perdew, Burke, and Ernzerhof (PBE) functional was used.64 Various plane-wave cutoff energies (450, 500, and 550 eV for coarse, medium, and fine basis set accuracy, respectively) were applied depending on the size of the unit cell. A complete listing of all calculated NMR parameters, along with a brief discussion, may be found in appendix A of the Supporting Information.
from the assembly of multiple subspectra in the frequency domain, using the variable offset cumulative spectra (VOCS) method.52 47/49Ti static SSNMR spectra were also acquired by using the 90°−τ−90° quadrupolar echo sequence.53 An interpulse delay (τ) of 50 μs was used. The free induction decay (FID) was recorded prior to the echo maximum and leftshifted to ensure that the FID used in Fourier transformation began exactly at the echo maximum. 47/49 Ti SSNMR experimental parameters are summarized in Table S1 of the Supporting Information. Additional WURST echo and quadrupolar echo spectra not pictured in the main text are shown in Figure S2. 23 Na SSNMR Experiments. 23Na SSNMR experiments were carried out at 9.4 T (ν0(23Na) = 105.67 MHz) using a Varian/Chemagnetics 3.2 mm HX magic angle spinning (MAS) probe. MAS spectra were acquired using the one-pulse sequence at a spinning speed of 20 kHz. The measured solution 90° pulse width on a 1.0 M NaCl (aq) solution (δiso = 0 ppm) was 1.8 μs, corresponding to a 0.9 μs solid 90° pulse. The spectra were acquired using a 30° pulse of 0.3 μs. 23Na 3QMAS experiments were also performed. Please see sections S1 and S2 of the Supporting Information for complete MAS and 3QMAS experimental details, along with spectra not featured in the main text. Experimental and calculated 23Na NMR parameters are tabulated in appendix A of the Supporting Information. 39 K SSNMR Experiments. For GTS-1 and ETS-10, 39K SSNMR spectra were collected at 21.1 T ((ν0(39K) = 41.99 MHz) using a Bruker 7 mm HX MAS probe. MAS experiments were conducted at a spinning speed of 5 kHz using the rotorsynchronized 90°−τ−180° echo sequence53 with continuouswave 1H decoupling. The 39K 90° pulse width using 1.0 M KCl (aq) solution (δiso = 0 ppm) was 12 μs, corresponding to a CTselective 90° pulse of 6 μs. Pulse delays of 1 and 5 s were used for acquisition of GTS-1, whereas 2 s pulse delays were employed for ETS-10. The spectrum of GTS-1 required 8192 scans, whereas 22528 scans were needed for ETS-10. 29 Si SSNMR experiments. 29Si MAS experiments were performed on all applicable samples at a magnetic field of 9.4 T (ν0(29Si) = 79.36 MHz). Please see sections S1 and S3 of the Supporting Information for complete experimental details and MAS spectra and appendix A for experimental and calculated 29 Si NMR parameters. C
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Table 1. Experimental and Calculated 49Ti NMR Parametersa,b site
coordination
CQ (MHz)
ηQ
Ω (ppm)
κ
δiso (ppm)
exptl calcd
1 1
5 5
10.7(1) 9.88
0.05(5) 0.00
500(50) 644.94
1.00(10) 1.00
−740(30) −769.5
exptl calcd
1 1
5 5
13.4(1) 17.05
0.05(5) 0.00
500(50) 705.39
1.00(10) 1.00
−783(30) −874.1
1 1
6 6
8.2(2) 11.13
1.00(10) 0.92
127.35
0.55
−850(30) −916.7
1 1
6 6
13 ± 11 8.70
0.85 ± 0.15 0.45
97.83
0.68
−850 −922.8
2 1 2
6 6 6
11.0(5) 39.25 10.13
0.40(20) 0.00 0.52
52.44 129.56
1.00 0.20
−900(40) −951.0 −921.1
1
6
16.5(2)
0.85(10)
sample natisite
AM-1
AM-4 exptl calcd sitinakite exptl calcd GTS-1 exptlc calcdd ETS-10e exptl
−1100(50)
Chemical shifts (δiso) were calculated by using the formula δiso = −0.894σiso − 1035.3, which is the line of best fit on a graph of experimental δiso vs calculated σiso. Several additional similar Ti compounds were also included in this graph for increased accuracy, see appendix A of the Supporting Information. bThe CS tensor parameters Ω and κ could only be quantified in some spectra due to the dominance of the EFG tensor on spectra appearance. Where CS tensor parameters are reported, the EFG and CS tensors are considered to share a coincident orientation in the molecular frame. cThe experimental spectrum was simulated using a Gaussian distribution of Ti sites, and this powder pattern corresponds to the Ti2 site within NaGTS-1. See main text for details. dCalculated values are based on the crystal structure of NaGTS-1. See main text for details. eCalculations were not performed on ETS-10 due to the intrinsic disorder of the framework and uncertain atomic coordinates. a
neighboring symmetry-related SiO4 tetrahedra, whereas the apical O2 forms a terminal bond with Ti and presents on both sides of the titanosilicate layer. The bond length between Ti and O2 (1.695 Å) is significantly shorter than those between Ti and O1 (1.990 Å), implying that the former has significant TiO double bond character, whereas the latter appear to be TiO single bonds. The natural abundance static WURST-QCPMG 47/49Ti SSNMR spectrum of natisite at 21.1 T (Figure 1b) presents a 49Ti powder pattern of relatively high intensity from ca. −400 to −1400 ppm (ca. 50 kHz), convoluted with a relatively weaker and broader 47Ti pattern that ranges from ca. 0 to −2600 ppm (ca. 140 kHz). Both the 47Ti and 49Ti signals exhibit typical well-defined second-order quadrupolar lineshapes, indicative of a single crystallographic Ti site with a high degree of local order. Attempts to simulate our experimental 47/49 Ti spectrum on the basis of quadrupolar interactions alone were not successful, as illustrated in Figure 1b, and inclusion of CS tensor parameters was required to achieve an acceptable fit to the experimental spectrum. WURST and quadrupolar echo spectra are shown in Figure S2 of the Supporting Information. Our complete measured 49Ti NMR parameters of natisite are CQ(49Ti) = 10.7(1) MHz, ηQ = 0.05(5), δiso = −740(30) ppm, Ω = 500(50) ppm, κ = 1.00(10), and (α, β, γ) = (0°, 0°, 0°) (Table 1). The square-pyramidal geometry about Ti is slightly distorted, indicated by a CQ value that is somewhat large for Ticontaining minerals but lies in the general range for 5coordinated Ti sites.25,66 The high Ti site symmetry of C4v within this system gives rise to a near-zero asymmetry parameter (ηQ) of 0.05(5), consistent with a single Ti site with rotational symmetry ≥C3 and in agreement with the proposed XRD crystal structure.63 Although ηQ is near zero, it is nevertheless nonzero and, paired with the moderate CQ value, once again indicates that a minor distortion of the TiO5 square
The EFG tensor may be modeled via three orthogonal components, V11, V22, and V33, which are ordered such that |V11| ≤ |V22| ≤ |V33|. The quadrupolar coupling constant (CQ) and asymmetry parameter (ηQ) describe the EFG tensor and are defined as follows: CQ = (eV33Q/h) × 9.7177 × 1021 and ηQ = (V11 − V22)/V33, where e is the electric charge, Q is the nuclear quadrupole moment,38 and h is Planck’s constant. A conversion factor of 9.7177 × 1021 V m−2 is used during the calculation of CQ to convert from atomic units to hertz. 47Ti NMR parameters are generally not shown, because they can be calculated from 49 Ti NMR parameters using the nuclear quadrupole moments and gyromagnetic ratios of 47Ti and 49Ti.38 The chemical shift (CS) tensor may be defined by three orthogonal components, δ11, δ22, and δ33, which are ordered such that δ11 ≥ δ22 ≥ δ33. The span of the CS tensor, Ω, is defined as Ω = δ11 − δ33. The skew of the CS tensor, κ, is defined as κ = 3(δ22 − δiso)/Ω. Values for κ range from −1 to +1, with either limit representing an axially symmetric CS tensor. To describe the relative orientations of the EFG and CS tensors in the molecular frame, a set of three Euler angle (α, β, γ) is used, defined by the Rose convention.65
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RESULTS AND DISCUSSION Square-Pyramidal TiO5 Units: Natisite and AM-1. Both natisite and AM-1 feature local square-pyramidal TiO5 environments. Natisite, Na2TiOSiO4 or Na2TiSiO5, crystallizes in the tetragonal space group P4/nmm (no. 129).63 The structure is composed of anionic titanosilicate layers constructed via corner-sharing TiO5 and SiO4 polyhedra, which are separated by interlayer charge-balancing Na+ cations (Figure 1a). There is one crystallographically unique Ti4+ center, located on the C4 axis of a square pyramid, that consists of four basal oxygen atoms (denoted O1) and one apical oxygen atom (O2). The four basal O1 atoms are shared with four D
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Figure 2. (a) Structure of AM-1 (hydrogen atoms and Na−O bonds are omitted for clarity). (b) Experimental (WURST-QCPMG) and simulated natural abundance 47/49Ti static spectra of AM-1 at 21.1 T. The offset of the transmitter frequency is indicated on each subspectrum.
natisite, with four long TiO bonds (1.951 Å) and one short TiO bond (1.689 Å). The Ti center resides on a C4 rotation axis in a square-pyramidal local environment, and each TiO5 unit connects to four symmetry-related SiO4 tetrahedra via corner-sharing basal O2 atoms. TiO5 units are isolated from each other in this system, with the apical O3 atoms of TiO5 square pyramids pointing toward the interlayer space. The titanosilicate layers confer an anionic charge, which is balanced by interlamellar Na+ cations that are located over the 6-rings and effectively block the entrance to the intralamellar space. A layer of water molecules is sandwiched between two layers of Na+ cations. The degree of TiO5 distortion in AM-1 is larger than that within natisite, which should result in a relatively larger CQ(49Ti). In addition, the local C4 rotational symmetry about Ti within the TiO5 unit should give rise to an ηQ value of zero, corresponding to an axially symmetric EFG tensor. The natural abundance WURST-QCPMG 47/49Ti static spectrum of AM-1 at 21.1 T was assembled from three individual subspectra and is shown in Figure 2b, whereas the WURST-echo spectrum is shown in Figure S2 of the Supporting Information. This spectrum is indicative of a single crystallographic Ti site, consisting of a strong, narrow 49Ti powder pattern from ca. −200 to −1700 ppm superimposed on a weak, broad 47Ti pattern from ca. 0 to −2600 ppm. Spectral simulations required consideration of both the EFG and CS tensors to properly fit the experimental spectrum. The 49Ti NMR parameters are CQ(49Ti) = 13.4(1) MHz, ηQ = 0.05(5), δiso = −783(30) ppm, Ω = 500(50) ppm, κ = 1.00(10), and (α, β, γ) = (0°, 0°, 0°). Much like within natisite, the EFG and CS tensors in AM-1 are coincident and axially symmetric. The observed CQ(49Ti) value of AM-1 (13.4 MHz) is somewhat larger than that of natisite (10.7 MHz), reflecting the increased degree of TiO5 distortion within AM-1 and again illustrating
pyramid exists. Although very close, Ti site symmetry may not be exactly C4v as suggested in the proposed structure. This result shows that 47/49Ti SSNMR experiments serve as very sensitive probes of local symmetry in this instance. The lack of Euler angles, accompanied by the near-zero ηQ value and +1.0 κ value, indicates that the EFG and CS tensors are both axially symmetric with their primary components (V33, δ33) oriented coincident along the C4 axis within this system. δiso compares well to other Ti-containing minerals.66,67 Although there is a previous 47/49Ti SSNMR study of natisite at 14.1 T,66 the authors reported only 49Ti NMR parameters (CQ = 18.2 MHz, ηQ = 0.00, δiso = −673 ppm) without depicting the SSNMR spectrum. It is noteworthy that the CQ(49Ti) value obtained in this work is markedly dissimilar from the value reported in the literature. The higher intrinsic sensitivity of 47/49Ti NMR at 21.1 T and our corresponding high-quality spectra, as opposed to the lower sensitivity afforded by a 14.1 T field and the unknown quality of the previously reported 49Ti spectrum, likely contribute to the large difference in reported values. Another titanosilicate featuring square-pyramidal TiO5 units is AM-1,62 (Na4Ti2Si8O22·4H2O, also referred to as JDF-L1), which is an unusual noncentrosymmetric layered titanosilicate that crystallizes in the tetragonal space group of P4212 (no. 90). The pXRD pattern of our AM-1 sample revealed a significant degree of long-range crystallinity, with a single minor impurity reflection at ca. 2θ = 26° (Supporting Information Figure S1). AM-1 is composed of [Ti2Si8O22]4− layers along the a axis, featuring five-member rings (5-rings) in the crystallographic bc plane built up of four SiO4 tetrahedra and one TiO5 square pyramid, along with 6-rings in the ac plane built up of four SiO4 tetrahedra and two TiO5 units (Figure 2a). The coordination environment of the single Ti site of AM-1 is similar to that of E
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Figure 3. (a) Structure of AM-4 (hydrogen atoms and Na−O bonds are omitted for clarity). (b) Experimental (WURST-QCPMG) and simulated natural abundance 47/49Ti static spectra of AM-4 at 21.1 T.
the sensitivity of 47/49Ti SSNMR experiments to subtle changes in the local environment. The δiso value is also in the normal range of Ti-containing minerals.25,66,67 Calculations of 47/49Ti NMR parameters can be challenging, originating from the calculation conditions (i.e., basis set or functional employed) and/or subtle differences in local structure between the reported and actual crystal structures.45,47 Plane-wave DFT calculations (Table 1) accurately predict the 47/49 Ti EFG and CS tensor parameters of natisite, particularly CQ, ηQ, and κ. Plane-wave DFT calculations were also successful at predicting the experimental 47/49Ti NMR EFG tensor parameters of AM-1, although the experimental value of CQ was overestimated by ca. 25%. For both natisite and AM-1, the CS tensor was correctly predicted to be axially symmetric, and the calculated values of Ω in both compounds are also reasonably accurate. Because the EFG tensor typically dictates spectral breadth and appearance in these systems, plane-wave DFT calculations are well-suited for calculation of 47/49Ti NMR tensor parameters within natisite and AM-1 and, perhaps, titanosilicates in general; however, these calculations do not enjoy the same high degree of accuracy associated with calculations of NMR parameters for other nuclei such as 17O.68 The interlayer charge-balancing Na+ cations in natisite may serve as powerful probes of local structure. To the best of our knowledge, this is the first report of 23Na SSNMR experiments involving natisite. The 23Na MAS SSNMR spectrum of natisite at 9.4 T (Supporting Information Figure S3) consists of a narrow resonance at ca. 0 ppm overlapping with a broad resonance ranging from ca. 20 to −110 ppm. The two Na sites observed in the MAS spectrum of natisite are clearly resolved in the 2D 3QMAS spectrum (Figure S4). The narrow resonance has a relative intensity of 16% and corresponds to CQ = 1.4(1) MHz, ηQ = 0.00(10), and δiso = 3.0(6) ppm, whereas the broad resonance accounts for the remaining 84% of intensity and may be fit using CQ = 4.5(1) MHz, ηQ = 0.80(10), and δiso = 18.6(2) ppm. The broad resonance is assigned to the single crystallographically unique Na site due to its high relative intensity and large CQ value; the coordination sphere of Na+ in natisite is highly distorted, with Na−O bond lengths ranging from 2.307 to2.584 Å, and is expected to give rise to a relatively broad powder pattern, in good agreement with plane-wave DFT calculations (Supporting Information appendix A, Table A2). The narrow resonance is likely due to a small amount of
sodium-rich impurities, the origin of which was investigated via 29 Si SSNMR experiments. The 29Si MAS SSNMR spectrum of natisite is shown in Figure S5. A single, sharp 29Si resonance at −78.4 ppm was observed, consistent with the single crystallographic Si site within natisite. The 29Si chemical shift is typical of Si sites in Si(4Ti) local environments69,70 and is predicted reasonably well by plane-wave DFT calculations (Supporting Information Table A1). The lack of any additional 29Si resonances indicates that any sodium-rich impurities are unlikely to be associated with Si. A previously reported 29Si SSNMR spectrum of natisite69 was of low S/N and exhibited four resonances at −61.7, −82.1, −89.1, and −93.6 ppm. The peak at −82.1 ppm was assigned to the single Si site in natisite, and the other peaks were determined to arise from impurities. It is likely that our sample has higher crystallinity and purity than the sample used to obtain the previously reported spectrum, explaining the single 29Si resonance in our spectra. The 23Na and 29Si MAS spectra of AM-1 closely resemble those described in previous papers and are described in sections S2 and S3 of the Supporting Information. In agreement with the small amount of impurity observed in the pXRD pattern (Figure S1), the 29Si spectrum of AM-1 also has a minor peak that may be assigned to the impurity (Supporting Information section S3). Plane-wave DFT calculations yield accurate estimates of CQ(23Na), ηQ(23Na), and δiso(29Si) (Supporting Information appendix A). Brookite-Type TiO6 Chains: AM-4. The space group of layered AM-4 (Na3(Na,H)Ti2O2[Si2O6]2·2H2O) was determined as A2/a (no. 15) from powder XRD data, and the coordinates of atoms were refined by ab initio calculations in a previous paper.61 The structure is built from interconnecting brookite-type71 TiO6 octahedra and SiO4 octahedra, forming layers perpendicular to the c axis (Figure 3a). Each layer can be viewed as a five-tier sandwich of SiO4:TiO6:SiO4:TiO6:SiO4 polyhedra. The TiO6 octahedra are linked to each other via edge-sharing, forming brookite-type zigzag chains. In contrast, the SiO4 tetrahedra are linked via corner-sharing, forming pyroxene-type chains. Both the titanate and silicate chains run along the a axis and interconnect to form layers in the ab plane. Ti resides off-center within the TiO6 octahedra due to Ti4+··· Ti4+ cationic repulsion within the titanate chains, resulting in Ti−O distances that vary from 1.844 to 2.138 Å. The chargebalancing Na+ cations are found in two locations, either in small F
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Figure 4. (a) Structure of sitinakite (hydrogen atoms and Na−O bonds are omitted for clarity). (b) Experimental (echo) and simulated natural abundance 47/49Ti static spectra of sitinakite at 21.1 T. (c) Experimental 23Na MAS spectrum of sitinakite at 9.4 T.
The resonance at −90.5 ppm is assigned to Si1 of AM-4 in a Si(2Ti) environment, whereas the signal at −94.2 ppm corresponds to Si2 in a Si(1Ti, 1Na) environment. Planewave DFT calculations of δiso(29Si) are of high accuracy and confirm spectral assignment (Supporting Information Table A1). Ti 4O 16 Clusters: Sitinakite and GTS-1. Sitinakite (Na2Ti2SiO7·xH2O, TGA indicates x ≈ 2.3 for our sample), a promising ion exchanger for the removal of radioactive nuclides,5,6 is a microporous titanosilicate featuring cubanelike Ti4O16 clusters composed of four TiO6 octahedra linked to each other by four 3-coordinated oxygens. These Ti4O16 clusters interconnect via four bridging oxygens along the c axis and are connected by tetrahedrally coordinated silicon atoms through oxygen in the other two dimensions, forming a 3D framework (Figure 4a) that contains a single crystallographically unique 6-coordinated Ti site and features tunnels along the c axis. There are two types of Na+ in sitinakite, located in the channels or coordinated by four oxygens of the framework and two water molecules. The space group of sitinakite was determined as P42/mcm (no. 132) by both pXRD and neutron diffraction experiments.18,73 Thorogood et al. reported that although the crystal symmetry of sitinakite is unchanged in the temperature range from 20 to 298 K, the degree of TiO6 octahedral distortion indeed changes: the Ti−O bond lengths at 20 K are between 1.832 and 2.073 Å, whereas at 298 K they are between 1.869 and 2.043 Å.73 Natural abundance 47/49Ti static WURST-QCPMG experiments at 21.1 T were attempted, but no signal was observed due to a very short 47/49Ti spin−spin relaxation time (T2). CPMG-type experiments require relatively long T2 values; the failure of these experiments is likely an indication of local disorder about Ti centers within sitinakite. In these situations, Hahn echo or quadrupolar echo experiments may be applied to characterize both disordered and crystalline systems, although they exhibit much lower sensitivity than QCPMG experiments. The natural abundance quadrupolar echo 47/49Ti static spectrum of sitinakite (Figure 4b) displays a narrow, featureless peak centered at ca. −900 ppm. This peak is asymmetrically broadened and tails off to the low-frequency side, hinting at a distribution of 47/49Ti NMR parameters due to a range of slightly different Ti local environments (i.e., local disorder). The most obvious source of this disorder is mobile water
cages formed by TiO6 and SiO4 within the layers or associated with water molecules in the interlayer space. In contrast to natisite and AM-1, AM-4 features sixcoordinate Ti centers that present a distinct 47/49Ti SSNMR powder pattern. The natural abundance WURST-QCPMG 47/49 Ti static spectrum of AM-4 at 21.1 T is shown in Figure 3b, and the quadrupolar echo spectrum is shown in Figure S2 of the Supporting Information. Simulations yielded CQ(49Ti) = 8.2(2) MHz, ηQ = 1.00(10), and δiso = −850(30) ppm. This compound gives rise to an EFG-dominated line shape with ηQ ≈ 1, in sharp contrast to the ηQ ≈ 0 47/49Ti lineshapes of natisite and AM-1, again illustrating the value of 47/49Ti SSNMR experiments as sensitive probes of local structure in titanosilicates. The high η Q value indicates an axially asymmetric EFG tensor and relatively low (i.e., ≤ C2) rotational symmetry about Ti. There are no obvious spectral contributions from the CS tensor in this system other than δiso. The AM-4 line shape is somewhat reminiscent of brookite,71 consistent with the structural similarities between the two materials. The CQ(49Ti) value of AM-4 is larger than that of brookite (6.7 MHz) due to increased distortion of the TiO6 octahedra: the Ti−O bond lengths vary from 1.844 to 2.138 Å in AM-4 but measure from 1.910 to 1.992 Å in brookite.72 The δiso(49Ti) of AM-4 is also close to that of brookite (−853 ppm). There is a lack of well-defined features in the quadrupolar echo spectrum of AM-4 (Figure S2), hinting at the presence of limited local disorder. AM-4 further illustrates the utility of plane-wave DFT calculations of 47/49Ti NMR parameters (Table 1), which correctly predict a nonaxially symmetric EFG tensor, although the calculated CQ is somewhat overestimated. The 23Na MAS and 3QMAS spectra of AM-4 (Figures S3 and S4) closely resemble previous literature descriptions; however, the 29Si MAS spectrum (Figure S5) is dissimilar to a prior study.48 The chemical shifts of the two resonances observed in our 29Si MAS spectrum at −90.5 and −94.2 ppm are similar to the earlier paper, whereas the third resonance described in the literature at −92.1 ppm was not evident in our spectra. The origin of this third peak was not explained by the authors and will not be discussed further. Deconvolution of our spectrum yielded an intensity ratio of 54% (−90.5 ppm) to 46% (−94.2 ppm), consistent with the 1:1 ratio of two crystallographically distinct Si sites present in the crystal structure.61 G
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Figure 5. (a) Structure of KGTS-1 (K−O bonds and protons are omitted for clarity). (b) Experimental (WURST-QCPMG and echo) and simulated natural abundance 47/49Ti static spectra of GTS-1 at 21.1 T. The # symbol represents an impurity phase.
molecules within sitinakite. To extract the 47/49Ti NMR parameters of sitinakite, spectral simulations incorporating a Gaussian distribution of CQ and ηQ values (using the QuadFit package)55 were performed, yielding CQ(49Ti) = 13 ± 11 MHz, ηQ(49Ti) = 0.85 ± 0.15, and δiso(49Ti) = −850 ppm. These 47/49 Ti SSNMR results imply that the local Ti4+ environment in sitinakite is disordered at room temperature, although the overall structure exhibits significant long-range order (pXRD patterns, Supporting Information Figure S1). The low accuracy of plane-wave DFT calculations of 47/49Ti EFG tensor parameters (Table 1), combined with the prior demonstrated success of these types of calculations on titanosilicates, indicates that the reported crystal structure of sitinakite may not completely describe the Ti local environment; this is most likely due to the significant local disorder about Ti, owing to the distributions of Na+ cations and/or H2O molecules at higher temperature, which has been postulated.73 To further investigate the crystallinity of sitinakite, the local Na+ environments were studied by 23Na MAS SSNMR experiments at 9.4 T. The 23Na MAS spectrum of sitinakite (Figure 4c) exhibits a single featureless peak centered at −9 ppm that ranges from ca. 10 to −30 ppm, similar to a previous study (Supporting Information Table A2).74 The lack of fine detail or resemblance to a typical SOQI-dominated line shape in this 23Na SSNMR spectrum is further evidence that the local Na+ environments are indeed disordered at room temperature. It appears that the disordering in distribution of Na ions contributes, at least partially, to the disordered Ti local environment. The GTS-1 pair of microporous titanosilicates also feature cubane-type Ti4O16 clusters and are synthetic analogues of the recently discovered mineral ivanyukite75 that exhibit an identical framework topology but incorporate different extraframework cations (K+ or Na+). The framework structure of GTS-1 (Figure 5a) is very similar to sitinakite in that both structures involve interconnection of Ti4O16 cubane-like clusters with isolated SiO4 tetrahedra. Unlike sitinakite, Ti4O16 clusters connect with SiO4 tetrahedra in all three crystallographic directions within GTS-1, resulting in a 3D channel system based on 8-rings.
The structure of GTS-1 with K+ as the countercation (KGTS-1: HK3Ti4O4(SiO4)3·4H2O) was previously resolved from pXRD data;19 KGTS-1 crystallizes in the cubic space group P−43m (no. 215) and has one crystallographically unique Ti site with C3v local symmetry. The single K+ site is located in the center of the channels, coordinated to four water molecules (K−O: 3.170 Å) and eight framework oxygens (K− O: 3.233 Å). The structure of the Na form of GTS-1, NaGTS-1 (Na4Ti4O4(SiO4)3·6H2O), has also been determined from pXRD data. NaGTS-1 has a reduced crystal symmetry of R3m (no. 160, a subgroup of P−43m)20 because Na+ is not positioned in the center of the channels. Within NaGTS-1 there are two Ti sites, Ti1 and Ti2, that exist in a 1:3 ratio. The symmetry of Ti1 in NaGTS-1 is C3v, the same as the Ti site of KGTS-1, but the local symmetry of Ti2 is reduced to Cs. NaGTS-1 has two Na sites: one is four-coordinate (Na1, site symmetry C3v), and the other one is six-coordinate (Na2, site symmetry Cs). Our GTS-1 sample is a mixture of KGTS-1 and NaGTS-1 (see pXRD pattern in Figure S1 of the Supporting Information). The estimated phase ratio of NaGTS-1 to KGTS-1 in our sample is ca. 0.56 from pXRD data, whereas the measured Na/K ratio is ca. 0.65 from EDS experiments. The natural abundance 47/49Ti WURST-QCPMG static spectrum of this sample at 21.1 T is illustrated in Figure 5b. The spectrum consists of a 49Ti signal between ca. −400 and −1400 ppm convoluted with a weaker and broader 47Ti pattern that ranges from ca. 0 to −3000 ppm. The line shape of this powder pattern indicates some axial asymmetry of the 47/49Ti EFG tensor. The crystallographic site associated with this resonance is Ti2 of NaGTS-1, because both the single Ti site of KGTS-1 and the Ti1 site of NaGTS-1 reside in environments of high local symmetry (i.e., ≥ C3) and thus should correspond to axially symmetric EFG tensors (vide infra). Simulation of the WURST-QCPMG spectrum yields the following 49Ti NMR parameters for Ti2 of NaGTS-1: CQ(49Ti) = 11.0(5) MHz, ηQ(49Ti) = 0.4(2), δiso(49Ti) = −900(40) ppm. There is a negligible contribution to spectral appearance from the CS tensor. H
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Figure 6. (a) Experimental and simulated 23Na MAS spectrum of GTS-1 at 9.4 T as a function of pulse delay. (b) Experimental and simulated 39K MAS spectrum of GTS-1 at 9.4 T as a function of pulse delay. ∗ denotes spinning sidebands; ◊ denotes a transmitter artifact.
calculations of 23Na NMR parameters in GTS-1 predict distinct CQ and ηQ values for each site and support our spectral assignment (Supporting Information Table A2). 23Na 3QMAS experiments were also performed on our sample of GTS-1, but were unable to fully resolve the individual sodium resonances (Figure S4). 39 K MAS experiments at 21.1 T were performed on our GTS-1 sample to probe the potassium local environments in KGTS-1. The results, difference spectrum, and corresponding spectral simulations are shown in Figure 6b. Similar to 23Na NMR experimental observations (vide supra), two 39K resonances are apparent, with each linked to a separate 39K EFG tensor and a unique T1 value. The 39K resonance corresponding to an axially symmetric EFG tensor reflects a high degree of local symmetry and may be described with CQ(39K) = 1.3(1) MHz, ηQ(39K) = 0.1(1), δiso = 3(4) ppm. In contrast, the 39K site with an axially asymmetric EFG tensor required simulations that modeled a distribution of CQ and ηQ, indicating a significant degree of local disorder and yielding CQ(39K) = 1.3 ± 0.4 MHz, ηQ(39K) = 0.9 ± 0.1, δiso = −2 ppm. Direct observation of two 39K resonances is surprising, given that the crystal structure of KGTS-1 indicates a sole crystallographic K site. The 39K resonance with the welldefined quadrupolar powder pattern corresponds to the known crystallographic potassium site; however, the origins of the disordered potassium site are not clear. It is possible that the solid-state ion exchange takes place to some degree at room temperature, leading to some K+ ions migrating from KGTS-1 to NaGST-1 crystallites without well-defined positions. Corner-Shared TiO6 Chains: ETS-10 and ETS-4. The local structure within compounds featuring corner-shared TiO6 chains was explored via multinuclear SSNMR experiments on ETS-10 and ETS-4. A high degree of local disorder and distortion in ETS-4 limited the utility of SSNMR experiments;76 a detailed discussion of 47/49Ti and 23Na SSNMR experiments is given in the Supporting Information (section S4), and ETS-4 is not discussed further in the main text.
Ti1 of NaGTS-1 is not observed, likely due to a very large CQ(49Ti), which gives rise to a pattern too broad to be measured even at 21.1 T: the degree of distortion within (Ti1)O6 units is significantly larger (Ti1−O bond lengths: 1.828−2.107 Å) than in (Ti2)O6 units (Ti2−O bond lengths: 1.983−2.100 Å). Plane-wave DFT calculations support this spectral assignment (Table 1); of the two Ti sites within NaGTS-1, Ti1 was calculated to have a CQ value of 39.25 MHz and ηQ = 0.00, whereas Ti2 is predicted to resemble experimental observations with a much smaller CQ of 10.13 MHz and ηQ = 0.52. The 47/49Ti signal corresponding to crystalline KGTS-1 is not observed, likely also due to a high degree of TiO6 distortion (Ti−O bond lengths: 1.851−2.048 Å), which likely results in a very large CQ value reminiscent of the Ti1 site within NaGTS-1. An additional narrow feature centered at about −900 ppm is observed in the echo spectrum and is attributed to an impurity phase. To confirm that the crystalline phase observed in 47/49Ti SSNMR experiments is NaGTS-1, 23Na MAS experiments were performed on our GTS-1 sample at 9.4 T. At first glance, the 23 Na MAS spectrum (Figure 6a) appears to consist of a single resonance with two horns, typical of SOQI-dominated powder patterns. However, further investigation of 23Na MAS spectra as a function of pulse delay and examination of difference spectra unambiguously validated the existence of two 23Na resonances: one with an axially symmetric 23Na EFG tensor (ηQ ≈ 0) with a relatively long spin−lattice relaxation time (T1), whereas the second resonance corresponds to an axially asymmetric (ηQ ≈ 1) 23Na EFG tensor with a relatively short T1. The observed ηQ values agree well with the reported crystal structure of NaGTS1: Na1 has a high site symmetry of C3v and corresponds to the axially symmetric EFG tensor, whereas Na2 exhibits a low site symmetry of Cs and is linked to the axially asymmetric EFG tensor. The extracted 23Na NMR parameters for both sites are as follows: Na1, CQ(23Na) = 1.8(1) MHz, ηQ(23Na) = 0.1(1), δiso(23Na) = 6.0(10) ppm; and Na2, CQ(23Na) = 2.2(1) MHz, ηQ(23Na) = 1.0(1), δiso(23Na) = 8.3(10) ppm. Plane-wave DFT I
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Figure 7. (a) Structure of ETS-10. The bridging O atoms are omitted to show the connectivity between Si/Si and Si/Ti. The indicated distances a ≈ b cause the stacking faults. The dashed lines present the corner-shared TiO6 chains. The five possible Na sites are also shown, indicated by Roman numerals. (b) Experimental (echo) and simulated natural abundance 47/49Ti static spectra of ETS-10 at 21.1 T. (c) Experimental 23Na MAS spectrum of ETS-10 at 9.4 T. (d) Experimental and simulated 39K MAS spectra of ETS-10 at 21.1 T. ∗ represents spinning sidebands.
ETS-10 ((Na2−xKx)TiSi5O13·4H2O, Na/K ≈ 4.5 and Si/Ti ≈ 4.5 from EDS) is a large-pore microporous titanosilicate with a framework containing TiO6 chains (Figure 7a), which extend along two orthogonal directions, surrounded by tetrahedral silicate units.77,78 The structure consists of 12-, 7-, 5-, and 3rings forming a 3D large-pore channel system, which makes ETS-10 promising for many applications such as catalysis. There is intrinsic disorder of the framework of ETS-10 arising from structural faulting: 12-rings alternate between being unoccupied or occupied by Ti, but adjacent empty 12-rings may merge to create an even larger ring and corresponding void space. It is possible to describe the structure of ETS-10 in terms of an intergrowth of two ordered polymorphs with tetragonal (polymorph A) and monoclinic (polymorph B) symmetry, respectively (Supporting Information Figure S6).77,78 The charge-balancing extra-framework cations in our sample of ETS-10 are Na+ and K+. The location of the five Na sites in this structure have been previously determined,79 suggesting Na(I) and Na(II) are near the TiO6 chains and close to the 5-ring apex on either side of the 12-ring; Na(III) is also near the TiO6 chains but at the top (or bottom) of the 12-ring, Na(IV) resides in the segregated 7-ring portion of the 12-ring filled with Ti between two orthogonal TiO6 chains, and Na(V) resides nearer the center of the void 12-ring. In principle, K+ could occupy all of the aforementioned five Na sites; however, previous studies have shown that Na(IV) and Na(V) are the most favored sites for K+ substitution.79,80 The 47/49Ti echo spectrum of our ETS-10 sample at 21.1 T (Figure 7b) exhibits a broad powder pattern from ca. 1800 to −4800 ppm that arises from a single Ti site, as expected from the crystal structure, and can be simulated reasonably well using CQ(49Ti) = 16.5(2) MHz, ηQ(49Ti) = 0.85(10), and δiso(49Ti) = −1100(50) ppm. The CQ(49Ti) of ETS-10 is quite large for 6coordinated Ti,25,67 reflecting the high distortion of TiO6 units evident in the crystal structure. A previous natural abundance 47/49 Ti static quadrupolar echo spectrum of ETS-10 was
reported by Nakata et al.,81 but it was recorded at a low magnetic field of 9.4 T, suffered from rather poor S/N, and was not simulated or interpreted. Due to the intrinsic disorder within this system and the uncertainty of the crystal structure, plane-wave DFT calculations were not performed. The 23Na MAS spectrum of ETS-10 consists of a single narrow resonance (Figure 7c), despite the five crystallographic Na sites in this structure. The lack of resolution stems from the composition of ETS-10: incorporated H2O promotes cation mobility, and Na+ ions are likely quite mobile and undergoing rapid site exchange, giving rise to a sole 23Na resonance. In contrast, two 39K resonances can be clearly resolved in the 39K MAS spectrum at 21.1 T (Figure 7d). One resonance corresponds to an axially symmetric 39K EFG tensor and NMR parameters of CQ(39K) = 1.8(1) MHz, ηQ(39K) = 0.1(1), and δiso(39K) = −18(4) ppm, whereas the other resonance corresponds to a less axially symmetric 39K EFG tensor with NMR parameters of CQ(39K) = 0.95(10) MHz, ηQ(39K) = 0.6(1), and δiso(39K) = −7(5) ppm. Only two 39K resonances are observed, rather than five, indicating preferential substitution of K+ for Na+ at the Na(IV) and Na(V) sites.79,80 The former 39K resonance (CQ(39K) = 1.8(1) MHz) is assigned to K+ that occupies the Na(IV) site, and the latter resonance (CQ(39K) = 0.95(10) MHz) is assigned to K+ that occupies the Na(V) site, hereafter referred to as K(IV) and K(V), respectively. Because K(IV) resides within smaller 7-rings whereas K(V) is inside larger 12-rings, K+ is associated with stronger interactions with the framework at the K(IV) site, resulting in a considerable distortion of its coordination sphere and a relatively larger CQ value. Conversely, the coordination sphere of K+ ions positioned at the K(V) site exhibits a smaller degree of distortion due to decreased interaction with the framework, giving rise to a smaller CQ value. Our results reveal that nearly exclusive, not preferential, substitution of K into the Na(IV) and Na(V) sites occurs. J
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ETS-10 is an excellent illustration of the potential 39K SSNMR spectroscopy has for the investigation of extraframework countercations and their substitution patterns within microporous titanosilicates. Both 23Na and 39K are spin-3/2 quadrupolar nuclei, but the higher Q associated with 39K gives rise to a Sternheimer antishielding factor that is ca. 4 times larger than that of 23Na,82 producing powder patterns dispersed across a larger frequency range that offer increased resolution of spectral features and enhanced sensitivity to subtle changes in local environment. In this instance, the larger Q of 39K allows for reliable differentiation of individual K+ environments and cation sites, whereas the smaller Q of 23Na prohibits such analysis. Of course, the relatively lower Q(23Na) and higher γ(23Na) values mean that 23Na SSNMR experiments generally require far shorter experimental times than experiments involving the challenging low-γ 39K nucleus.
investigation of local environments within microporous titanosilicates, particularly those with partially described structures. Because microporous titanosilicates necessarily incorporate a significant proportion of NMR-active nuclei, SSNMR has been, and will continue to be, an invaluable experimental tool for comprehensive characterization of these unique materials.
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ASSOCIATED CONTENT
S Supporting Information *
Powder XRD patterns, SSNMR measurement conditions, additional 23Na and 29Si SSNMR spectra, structures of two polymorphs of ETS-10, 47/49Ti and 23Na SSNMR spectra of ETS-4, and tabulated values for all experimental and calculated NMR parameters. This material is available free of charge via the Internet at http://pubs.acs.org.
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CONCLUSIONS In this work, natural abundance 47/49Ti SSNMR spectra of several prototypical titanosilicates representing typical Ti local environments were acquired at a magnetic field of 21.1 T, with these spectra exhibiting distinct features that are directly related to the immediate Ti environment. The observed 47/49Ti NMR parameters of these titanosilicates correspond to unique structural features, such as TiO5 square-pyramidal units within natisite and AM-1 and relate to the short-range symmetry and local ordering about Ti. Accurate measurements of EFG tensor parameters allow for clear differentiation between each compound and individual Ti coordination environment and, paired with plane-wave DFT calculations, permit assignment of SSNMR resonances to crystallographic sites (e.g., GTS-1) as well as detection of local deviations in symmetry. One of the striking observations is that the local Ti environments of sitinakite and KGTS-1 are significantly disordered, although pXRD experiments indicate both compounds exhibit a high degree of long-range order, illustrating the utility of SSNMR for examination of short-range structure within titanosilicates. 23 Na MAS and 3QMAS SSNMR experiments at 9.4 T were used to probe the local environment of countercations, assess sample purity, and determine local ordering about Na; we also report the first 23Na MAS and 3QMAS experiments on natisite. 39 K SSNMR experiments at 21.1 T were able to differentiate multiple cation sites within ETS-10 that were not resolved in 23 Na SSNMR experiments and confirmed preferential substitution of K+ into two specific Na+ crystallographic sites. The potential of 39K SSNMR spectroscopy for investigation of extraframework countercations in microporous titanosilicates was illustrated by ETS-10, which gives rise to powder patterns dispersed across a relatively large frequency range that offer increased resolution of spectral features and enhanced sensitivity to subtle changes in local environment versus 23Na experiments. 39K NMR also indicates the possibility of solidstate ion exchange in a mixture of NaGST-1 and KGST-1. 29Si SSNMR spectra were performed on all applicable titanosilicates to confirm sample purity and investigate anomalous resonances reported in prior studies. We have recently demonstrated that SSNMR experiments targeting the organic ligands,83 metal centers,84 and guest species85 within a variety of microporous materials are useful for molecular level characterization. The experimental approaches described and the results presented in this work clearly illustrate the value of SSNMR experiments for the
AUTHOR INFORMATION
Corresponding Author
*(Y.H.) Phone: (519) 661-2111, ext. 86384. E-mail: yhuang@ uwo.ca. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Y.H. thanks the Natural Sciences and Engineering Research Council of Canada (NSERC) for a Discovery Grant and a DAS award. Funding from the Canada Research Chair program is also gratefully acknowledged. Access to the 900 MHz NMR spectrometer and CASTEP software for select calculations was provided by the National Ultrahigh-Field NMR Facility for Solids (Ottawa, Canada), a national research facility funded by the Canada Foundation for Innovation, the Ontario Innovation Trust, Recherche Québec, the National Research Council Canada, and Bruker BioSpin and managed by the University of Ottawa (http://nmr900.ca).
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