8707
J. Phys. Chem. 1995,99, 8707-8716.
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Dipolar-Based 27Al Sieve Frameworks
29SiSolid-state NMR Connectivity Experiments in Zeolite Molecular
C. A. Fyfe,* K. C. Wong-Moon, Y. Huang, H. Grondey, and K. T. Muellert Department of Chemistry, University of Britisn Columbia, 2036 Main Mall, Vancouver, B.C. V6T 1Z1, Canada Received: June 14, 1994; In Final Form: March 2, 1995@
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In this work we present the results of a systematic investigation of a set of dipolar-based 27Al 29Siconnectivity experiments for a series of representative zeolite molecular sieve frameworks: zeolites A, X, Y, and 52. Coherence transfer from quadrupolar 27Al (I = 5 / 2 ) to 29Si (I = '/2) nuclei through cross polarization and transferred-echo double-resonance (TEDOR) experiments is observed, and an experimental investigation of the parameters controlling the efficiency of both experiments is presented. Rotational-echo double-resonance (REDOR) and dipolar-dephasing difference experiments are also demonstrated. These experiments utilize the heteronuclear dipolar couplings, whose magnitudes are proportional to the inverse third power of the internuclear distance and are therefore extremely sensitive to the separation of the coupled spins. In addition, we demonstrate that for these dipolar-based experiments, there is a relative enhancement of a given resonance which is approximately linear with the number of aluminum atoms in the neighboring tetrahedral sites. Twodimensional extensions of the cross-polarization and TEDOR experiments are also possible and will be of use for complex mixtures or where several resonances are observable in the one-dimensional spectra of the two nuclei.
Introduction Molecular sieve systems are characteristically open-framework structures containing internal cavities which are accessible through one-, two-, or three-dimensional pore systems. These channels are of molecular dimensions, and this gives the structures their molecular sieving characteristics. Because of this chemical selectivity, they are widely used as sorbents and catalysts particularly in the petroleum industry. The most common sieve systems are the zeolites which are aluminosilicates of general formula (1) made up of comer and edge-
sharing Si04 and A104 tetrahedra. Because of the difference in charge between Si and Al, a balancing positive charge must be present for each aluminum in the structure.'%2The ratio of Si/Al can range from 1.0 to infinity and, in general, the distribution of Si and A1 over the tetrahedral sites (T sites) is disordered. Recent years have seen the growth of a second class of molecular sieves, containing A1 and P called Alp04 materials3 In this case, the Al/P ratio is always 1.0 according to eq 2, and
the system is neutral with no charge-balancing extraframework ions present. Further, there is exact altemation of the A104 and PO4 tetrahedra giving, in this case, a perfectly ordered structure. In general, these molecular sieve materials are highly crystalline, but they are almost always microcrystalline with crystal dimensions on the order of microns. This usually precludes the use of single-crystal X-ray diffraction and recourse must be made to the much more limited powder diffraction techniques. Even with the use of synchrotron radiation sources4
* To whom correspondence
should be addressed. address: Department of Chemistry, The Pennsylvania State University, 152 Davey Laboratory, University Park, PA 16802. Abstract published in Advance ACS Abstracrs, May 1, 1995.
' Present @
and Rietveld refinement procedure^,^ it is very difficult to determine their framework structures. In addition, in the case of zeolites, the lack of Si, Al ordering means that only average structures will be obtained from diffraction measurements. Over the past decade, high-resolution solid-state NMR spectroscopy6 has emerged as a powerful complementary technique for structural investigations of these material^.^,^ In the case of low SUA1 ratio zeolites, the 29Simagic angle spinning (MAS) NMR spectra of the simplest systems show five resonances corresponding to the five local silicon environments: Si[4Al], Si[3Al, Si], Si[2A1, 2Si], Si[Al, 3Si] and Si[4Si]. Because of the lack of Si, A1 ordering the resonances are relatively broad, about 5 ppm. The A1 atoms in the framework are always in tetrahedral sites and the 27AlMAS NMR spectra show single resonances characteristically at 40-60 ppm. Extraframework octahedrally coordinated aluminum gives rise to signals at -0 ppm. Removal of all of the aluminum from zeolites produces completely siliceous structures which are now perfectly ordered and a direct link can be made between the NMR and diffraction methods.g The 29SiMAS NMR spectra now show very sharp resonances ( A Y I / ~ 1 ppm) whose number correspond to the number of crystallographicallyinequivalent sites in the unit cell and whose relative intensities reflect the populations of these sites. In these ordered systems it is now possible to establish the threedimensional 29Si-O-29Si bonding connectivities in the framework by two-dimensional 29Sihomonuclear correlation experiments such as COSY and INADEQUATE, and a number of studies have demonstrated their reliability.8 Quadrupolar nuclei, particularly 27Al, are present in many molecular sieve systems. These nuclei present a more complex situation for NMR experiments. First, it is only the central transition of the 27Alwhich is normally observed in MAS NMR, and second, MAS alone does not completely average the secondorder quadrupolar b r ~ a d e n i n g . ~Complete ,'~ averaging can be achieved by spinning around more than one axis as in the dynamic angle spinning (DAS)''.'2 and double rotation @OR)13,14 techniques, but often MAS alone gives sufficient resolution to clearly identify the signals. A more difficult problem is whether
0022-3654/95/2099-8707$09.00/0 0 1995 American Chemical Society
8708 J. Phys. Chem., Vol. 99, No. 21, 1995
the central transition in quadrupolar nuclei can be treated as analogous to a spin l/2 nucleus to establish heteronuclear connectivities based on the dipolar interaction. VegaI5.l6has shown that for quadrupolar nuclei, the spin locking of the central transition in an MAS experiment is a complex process due to the time dependence of the first-order quadrupolar interaction, and thus its efficiency is severely limited. Direct CP experiments will therefore have limited efficiency. REDOR and TEDOR experiments, which also utilize the dipolar coupling to establish connectivities, are a newer class of double-resonance experiments introduced by Schaefer and co-workersI7.l* for measuring internuclear distances between isolated pairs of spin l/2 heteronuclei. In previous work we have extended each of these connectivity experiments to include quadrupolar nu~1ei.l~ Specifically, we demonstrated that cross-polarization, the dipolar-dephasing experiment,20and the REDOR and TEDOR techniques could be used with success in the Alp04 systems VPI-5 and AlP04-8 to establish 27Al-O-31P connectivities, and that two-dimensional CP and TEDOR experiments could provide the threedimensional heteronuclear bonding scheme. A related twodimensional NMR experiment has also been recently reported by van Eck and Veeman,2' and these authors obtained very similar results for the VPI-5 system. The Alp04 materials are particularly attractive cases for these experiments: Both 27Aland 31Pare 100% abundant isotopes and have large magnetogyric ratios giving substantial magnetizations and sensitivity. In addition, the large magnetogyric ratios give relatively large 27Al/31Pdipolar couplings. Furthermore, as noted above, they are perfectly ordered systems and hence have narrow resonances enhancing sensitivity as well as resolution. Low SUA1 ratio zeolites are more typical of molecular sieve systems but the experiments will predictably be much more difficult, mainly due to reduced sensitivity. The 29Sinucleus is only 4.7% abundant, and its lower magnetogyric ratio also means that the dipolar interaction in the 27Al-O29Siunit will be smaller (approximately 210 Hz for 27Al-O29Si where r = 3.1 Furthermore, the zeolite systems are usually not ordered, giving rise to a number of local environments whose resonances will each be relatively broad due to disordering over the outer T sites, again reducing the sensitivity of the experiment. In the present paper we present the results of a systematic investigation of a set of dipolar-based 27Al 29Siconnectivity experiments for a series of representative zeolites. We demonstrate the success and utility of the experiments and the optimum experimental conditions and report the approximate experimental times which are involved for the different systems.
A).
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Experimental Section Materials. Zeolite A (Si/AI = 1) was prepared by hydrothermal synthesis from a mixture of molar ratio 2.0 Si02:A1203: 5.0 Na20:150 H20 heated at 70 "C for 3.5 h. Zeolite X (Si/AI = 1.75), kindly provided by Dr. W. Schwieger, was prepared by hydrothermal synthesis from a mixture of molar ratio 10 Si02:A1203:6.5 Na20:0.27 Cs20:280 H20 heated at 95 "C for 5 days. Zeolite Y (Si/AI = 2.47) was obtained from Linde (LZY52). Zeolite omega (Si/AI = 4.2) was obtained from Union Carbide (ELZ-Q-5). In each case, the sample crystallinity and purity were checked by powder X-ray diffraction measurements (Rigaku rotating anode powder diffractometer).
Fyfe et al.
(90)
(90)
SpinLock
Spin Lock
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Figure 1. (a) Cross-polarization experiment used for coherence transfer from I (*'Al) spins to S (29Si)spins. (b) Two-dimensional heteronuclear correlation experiment with cross polarization. The cI time period (Ispin evolution) is varied for a series of experiments with acquisition of the S-spin free induction decay during the rz period.
NMR Experiments. All experiments were carried out using a Bruker MSL 400 spectrometer modified to include a third radio frequency channel. NMR resonance frequencies were 104.264 MHz for 27Aland 79.495 MHz for 29Si. Initial studies were performed using a home-built double-tuned probe incorporating a 10 mm high-speed (turbo) spinner supplied by Doty Scientific Inc. However, these experiments were limited by the S / N obtainable in reasonable time periods, and all of the work presented here was carried out using a home-built double-tuned probe incorporating a 14 mm "pencil spinner'' with an internal volume of 2.8 mL supplied by Chemagnetics. Samples were spun at 2.2-2.9 kHz with the spinning rate monitored by an optical sensor unit. The 29Si 90" pulse and the 27Al90" pulse (central transition only) were 15-21 ,us. The four experiments investigated all involved observation of 29Si, as the reverse experiments (27Al observation) are impractical due to the low natural abundance and long T I values of the 29Sinuclei. The pulse sequences are indicated in Figures 1-3. The CP MAS experiments (Figure la) used the standard spin-lock sequence22with spin temperature inversion to suppress artifacts. The Hartmann-Hahn matching ~ondition?~ modified to take into account of the excitation of only the central 27Al transition (eq 3), was established experimentally by setting the Y S i B I ,Si
= 3YAlB
I ,AI
(3)
pulse power for both channels so that the 90" pulse times were equal. In the case of zeolite A, the S / N ratio was good enough to observe an FID from one or a small number of scans making experimental optimization possible. Null experiments where one or more pulses were removed from the sequence gave no observable magnetization. Two-dimensional CP correlation experiments (Figure 1b) were carried out by encoding the aluminum evolution frequencies after the initial 90" pulse in an incremented (tl)time period. The 27Alspin polarization was then transferred to the 29Sinuclei during the Hartmann-Hahn matched spin-lock period and subsequently detected in t 2 . The spin-lock selects only one orthogonal component of the 27Al magnetization, and pureabsorption phase line shapes in both final frequency dimensions are obtained by cycling the phase of the initial 90" pulse by time proportional phase incrementation (TPPI).24.2s
J. Phys. Chem., Vol. 99, No. 21, 1995 8709
Zeolite Molecular Sieve Frameworks
S
Figure 2. Dipolar-dephasing difference experiment under MAS conditions with a rotational period of tr.A signal from a spin echo is recorded first ([acq]+) without S spin decoupling (due to 10 MHz
resonance offset). The experiment is immediately repeated with subtraction ([acql-) of the spin echo signal obtained with on-resonance decoupling of the S spins during the first half of the experiment. Dipolar-dephasing difference experiments measure the difference between spin echoes for I nuclei (in the present case 29Si)obtained with and without irradiation of the S nuclei (27Al) during part of the sequence.20 The difference spectrum is thus a measure of the heteronuclear dipolar interactions between the spins. In the present work, these experiments were carried out using the sequence presented in Figure 2. This differs from that originally proposed by Veeman and co-workers20 in two ways: Rather than conducting two separate experiments, one with and one without irradiation of the S spins and then subtracting the spectra after Fourier transformation, the sequence was changed between successive scans and the signals alternately added and subtracted to give the FID corresponding to the difference spectrum directly. It was found that S/N improved considerably, presumably by the minimization of the effects of small variations in experimental conditions. Second, to eliminate artifacts from the effect of small switching times within the spectrometer, the experiment was performed with pulses applied to the S nuclei in both steps of the composite sequence at exactly the same time during the echo formation. The desired effect of decoupling or not decoupling the S nuclei was obtained by switching the frequency of this channel on and off resonance using a frequency offset list. The resonance offset used was 10 MHz and a variety of negative experiments confirmed that the dipolar-dephasing difference spectra originated from the heteronuclear dipolar interaction as proposed. Additionally, any effects caused by Bloch-Siegert shifts of the 29Sicoherences are minimized. Figure 3a shows the basic sequence used for the REDOR experiments originally proposed by Schaefer and co-workers. The format of the experiment is similar to that of the dipolardephasing difference experiment in that a spin-echo sequence is used to form a signal for the I spins (29Si), and two experiments are carried out with and without perturbation of the S spins, the difference between the two signals reflecting the heteronuclear dipolar couplings. The difference between the two is that the application of the pulses in the REDOR sequence is gated with the spinning rate and the pulses applied to the S spins are at specific points in a rotor cycle and their effects are exactly understood, making the sequence one which, at least for an isolated spin pair, can be described theoretically and the results interpreted quantitatively. In the first experiment,
(c)
1
S
Figure 3. (a) REDOR experiment in which, in the first experiment, a conventional spin echo acquired after IZ rotor cycles provides the reference signal SO,and no pulses are applied on the S channel. In the second experiment, 180" pulses are applied to the S channel twice per rotor cycle, and the modified signal Sf, is recorded. (b) TEDOR experiment in which 180" pulses are applied for n rotor cycles to the S spins, shown here at t44 and 3tJ4 in each rotor cycle. Simultaneous 90" pulses then transfer the spin coherence from the I spins to the S spins, and further evolution for m rotor cycles under dipolar-dephasing produces observable signal, ST.(c) Two-dimensional heteronuclear correlation TEDOR experiment with preliminary evolution of I-spin magnetization during the t l time period, and detection of S-spin magnetization during the t2 time period.
the I spin evolution is initiated by a 90" pulse and after an integral number of rotor cycles (t = nzr/2 with n even and zr the rotor period) a 180" refocusing pulse is applied to the I spins producing an echo centered at t = nrr. The heteronuclear dipolar coupling is eliminated and causes no dephasing, and the frequency signal from this experiment is designated SO. In a second experiment, the I spins are treated exactly as previously, but the S spins are subjected to 180" pulses which change the sign of the dipolar coupling at half-integral values of a rotor cycle, with the exception of t = nzJ2 when the 180" refocusing pulse is being applied to the I spins. The echo formed is now smaller due to the dephasing effect of the nonzero average heteronuclear dipolar coupling, and the signal obtained after Fourier transformation is designated Sf.By subtraction of this signal from SO,the difference signal AS = SO - Sfis obtained which is due only to I spins involved in I-S dipolar couplings. The experimental data can be obtained as a function of the number of rotor cycles and the function AS/&) used to describe the signal intensities. As in the dipolar-dephasing experiment, a direct difference experiment was used in the present work to obtain AS and a separate experiment performed to obtain So. Again, S spin pulses were used in both parts of the composite difference experiment to avoid timing errors and the frequency switched alternately on and off resonance using a frequency offset list. Null experiments again confirmed the origin of the difference signals as being the heteronuclear dipolar interaction.
8710 J. Phys. Chem., Vol. 99, No. 21, 1995
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I
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I
I
I
-90 -100 -110 Frquency @pm from T M S )
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Fyfe et al.
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Figure 4. 27A1 29SiTEDOR experiment for zeolite A (structure shown inset), with n = 2 and m = 1. A total of 4133 scans were acquired with a recycle delay of 0.1 s, with a total experimental time of 7 min. Line broadening of 20 Hz was applied.
In the first part of the TEDOR experiment (Figure 3b), the I spins (27A1in the present work) are subjected to a 90"-z 180"-t spin-echo sequence as before with tau equal to an integral number of rotor cycles.I8 The 180" pulses on the S channel which reverse the sign of the dipolar coupling are now applied at the l/4 and 3/4 points of each cycle and 180' pulses are applied to both nuclei after t = n t J 2 in order to preserve the sign of the dipolar coupling in the formation of the spin echo. The major difference from the REDOR is that the application of simultaneous 90" pulses to both spins at the echo maximum gives coherence transfer of the I spin anti-phase magnetization to the S spins. After a further m rotor cycles with continued dephasing pulses, the TEDOR signal is acquired. In the present work, these dephasing pulses were applied to the S spins (29Si) to minimize the number of pulses applied to the quadrupolar I spin nuclei (27Al). Null experiments involving the selective removal of various pulses again confirmed the origin of the TEDOR signal as being the 27AlP9Si heteronuclear dipolar interactions. Because of the coherence transfer step, the initially excited nucleus is not that finally observed. A two-dimensional version of the TEDOR experiment may be constructed by the addition of a frequency encoding tl period after the initial 90" pulse and the timing of the rest of the sequence begins at this point (Figure 3c). Unlike the two-dimensional CP experiment where one orthogonal component is selected by the spin locking pulse, this selection must be done by the application of a 90" I pulse in the two-dimensional TEDOR experiment at the end of the tl period in order to use TPPI.
Results and Discussion A variety of zeolite molecular sieves have been investigated in the present work to evaluate the general applicability and relative efficiences of the various experiments and the measurement times involved. The results will be presented in order of increasing difficulty of the experiments. Zeolite A. Zeolite A, whose structure26 is shown inset in Figure 4, is the most straightforward case. The Si/A1 ratio is unity, giving the highest concentration of AI possible for these systems. Further, there is only one lattice site in the structure and there is exact altemation of Si and Al giving a 29Sispectrum consisting of a single sharp resonance. This system is optimum for these experiments and good spectra can be obtained in xery short periods of time, as illustrated by the 29SiTEDOR spectrum in Figure 4 which was obtained by magnetization transfer from 27A1to 29Siin a total measuring time of 7 min. For this reason,
zeolite A was used to investigate in detail the performance of the different experiments described in the Experimental Section to confirm the nature of the interactions involved from their general adherence to the theoretically predicted behaviors. For comparison, the S/N obtained in a fixed period of time (17 min) from the different experiments is given in Table 1 together with the experimental conditions. To aid in setting up the experiments and in the interpretation of the resulting data, the 27Al and 29Si relaxation parameters in zeolite A were measured. 27A1: TI = 4.27 ms, T2 = 1.23 ms, T I , = 213 ps. 29Si: T I = 24.3 s, T2 = 6.95 ms, T I , = 405 ms (77%), 224 ms (23%). (In the case of 29Si, there were two Tte relaxation times, perhaps due to variations in trace impurity levels within the sample.) Figure 5 shows the variation of the 29Si signal intensity vs the 29Si spin-locking field strength for 27Al 29Si crosspolarization measured at a contact time of 2 ms, which as will be seen in Figure 6 corresponds approximately to the maximum in the signal intensity as a function of the contact time. Figure 5 shows that, in accordance with the expected behaviour in the limit of fast spinning and weak radio frequency (rf) field strength, the Hartmann-Hahn match condition is shifted by the spinning frequency,I6 as shown in eq 4 for the present case.
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(4) The rf field strength 71B1.1 is determined experimentally (in frequency units) by taking the reciprocal of the 360" pulse time for the Inuclei. The 27A190" pulse length of 17.0ps (measured for the central transition) corresponds to an rf field strength of 3YA]B],AI= 14.7 1
