J . Phys. Chem. 1990, 94, 4195-4198
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NMR Spectroscopy of Xenon Sorbed in Pentasil Zeolites: Silicalites Chihji Tsiao,+,*David R. Corbin,! Vincent Durante,” Darrell Walker,ll and Cecil Dybowski*qt Center for Catalytic Science and Technology, Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, Central Research and Development Department.1 E . I . du Pont de Nemours and Company, Inc.. Experimental Station P.O. Box 80262, Wilmington, Delaware 19880-0262, and Sun Refining and Marketing Company, Marcus Hook, Pennsylvania 19061 (Received: February 22, 1989; In Final Form: December 1 1 1989)
129XeNMR spectroscopy of xenon sorbed in silicalite samples shows a complex line shape. We conclude that there are at least two regions in these samples, one of which is a macroscopic region free of occlusions and the other is a region (or regions) containing occlusions. We suggest that these occlusions are residual template molecules from the preparation of the material. Analysis of commercial preparations of silicalite and one prepared in this laboratory indicates that xenon NMR spectroscopy is a simple, straightforward means of examining residual template or other macroscopic occlusions in similar microporous materials.
Introduction Silicalite is essentially pure crystalline silica, with a structure consisting of a three-dimensional system of interconnecting straight and sinusoidal channels, the cross-sectional dimensions of which are 5.8 X 5.2 A and 5.5 X 5.1 A, respectively. Thus, silicalite has the pentad structure, similar to ZSM-5. Because of the low concentration of charge centers, it is hydrophobic and organophilic. Otherwise, it sorbs materials in a manner analogous to other molecular sieves.’-) Because of its selective sorption of organic materials, silicalite offers catalytic reactivity rather different from the aluminosilicates. It has been investigated structurally by techniques such as X-ray diffraction4 and 29SiNMR ~pectroscopy.~ Highly crystalline silicalite contains 24 distinct tetrahedral locations, mainly populated by silicon atoms, but, in some preparations, same residual aluminum is presenLsS6 In the preparation of silicalite, the structure is developed by the addition of a template that remains in the material if not removed by subsequent treatment.’ 129XeNMR spectroscopy is a sensitive technique for indirectly investigating properties of microporous materials, such as crystallinity,’ location of cati0ns,8+~ metal-particle size, and distribution of chemisorbed phases.’*12 In this investigation, we focus on 129XeN M R spectroscopy of the gas sorbed in several silicalite samples that show complex line shapes. The complex ’29XeNMR line shapes originate from macroscopic inhomogeneities in the sample. By comparison with I3C CPMAS N M R spectroscopy of the same samples, we conclude that these occlusions arise from incomplete removal of template. These occlusions affect the lz9Xe N M R spectroscopy in a manner that allows one to calculate the amount of material in each of the two areas. We find that different preparations of silicalite have varying amounts and distributions of the template through analysis of the 129XeN M R line shapes. Experimental Section The four silicalite samples, their designations, and their origins are summarized in Table I. SIS and SID are materials obtained from Union Carbide that are nominally the same material, but made in different batches, whereas SIM is a material made in the Applied Research Laboratory of Sun Refining and Marketing Co. The fourth sample, SIT, was prepared from SIM by exposing it to a 120 mL/min flow of air at 753 K for 4 h to remove template molecules by combustion. *To whom correspondence should be addressed. University of Delaware. *Present address: Chemistry Division, Argonne National Laboratory, Ar onne, IL 60439. !E. I . du Pont de Nemours and Company, Inc. 11 Sun Refining and Marketing Company. Contribution No. 5035.
0022-3654/90/2094-4195$02.50/0
TABLE I: Silicate Samples and Their Origin
sample SIS
origin commercial silicalite, Union Carbide S-115 Lot No. 96188306001 1-3-3
SID
SIM SIT
commercial silicalite, Union Carbide S-115 Lot No. 835002 silicalite prepared at Sun Refining and Marketing Co. silicalite prepared by treatment of SIM in flowing air at 753 K for 4 h
All samples are pretreated for xenon analysis by the following procedure: Approximately 0.3 g of material is placed in a resealable N M R cell having a coaxial stopcock.!) The sample is Torr at 295 K using a grease-free glass outgassed to 1.0 X manifold and then slowly heated to 673 K over 6 h, after which it is maintained at that temperature for 12 h. After cooling to 295 K, xenon gas (Air Products, 99.99%) is introduced into the sample to the desired pressure, and the sample is allowed to equilibriate at 295 K. Spectra are recorded after equilibration with xenon gas at pressures between 100 and 700 Torr. Xenon adsorption is measured at 295 K during the preparation of the N M R samples. The isotherps may be fit to the Langmuir form over the pressure range investigated. IZ9XeN M R spectra are measured with a Bruker WM-250 Fourier transform NMR spectrometer at 69.19 MHz. All spectra result from coadding 500 transients, with a relaxation delay of 0.7 s. Because relaxation times are short, this delay is adequate for complete repolarization of the xenon magnetization. All spectra are measured relative to a secondary standard of xenon ~
~~~~~
(1) Flanigen, E.; Bennett, J.; Grose, R.; Cohen, J.; Patten, R.; Kircher, R.; Smith. J. Nature 1978. 271. 9. (2)’Price, G.D.; Pluth, J. J.; Smith, J. V.; Bennett, J. M.; Patton, R. L. J . Am. Chem. SOC.1982, 104, 5971. (3) Price; G.D.; Pluth, J. J.; Smith, J. V.; Araki, T.; Bennett, J. M. Nature 1981, 292, 818. (4) Pluth, J. J.; Smith, J. V. In Intrazeolite Chemistry; American Chemical Society: Washington, DC, 1983; p 119. (5) Fyfe, C.; Gobbi, G.; Klinowski, J.; Thomas, J.; Ramdas, S. Nature 1982, 296, 530. (6) Hope, A,; Catlow, C.; Leng, C.; Adams, C. In Zeolites; Drazj, B., Hocevar, S., Pejovnik, S., Eds.; Elsevier: Amsterdam, 1985; p 297. (7) Springuel-Huet, M.; Ito, T.; Fraissard, J. In Proceedings of the Con-gress on Structure and Reactiuity. of- Modified Zeolites; Elsevier: Amsterdam, 1984; pp 13-21. (8) Ito, T.; Fraissard, J. J . Chem. Soc., Faraday Trans. I 1987.83, 451. (9) Bansal. N.: Dvbowski. C. J . Phvs. Chem. 1988, 92, 2333. (10) De Menorvai, L. C.; Ito, T.; Fraissard, J. J . Chem. Soc., Faraday Trans. I 1985, 81, 2855. (1 1) De Menorval, L. C.; Ito, T.; Fraissard, J. J . Chem. Soc., Faraday Trans. 1 1982, 78, 403. (12) Fraissard, J.; Ito, T.; De Menorval, L. C. In Proceedings ofrhe 8th International Congress on Catalysis; Weinheim: Berlin, 1984; pp 25-35. (13) Mesaros, D.; Dybowski, C. Appl. Spectrosc. 1987, 41, 610.
0 1990 American Chemical Society
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The Journal of Ph,ysical Chemistry, Vol. 94, No. 10, 1990
TABLE 11: Average Fraction of Xenon Exhibiting a Lorentzian NMR Line (f,) and Weight Fractions in This Phase (F,) for Silicalite
?
sample SIS SID
B
SIM SIT
c
E
Z
w
x
L-J
I
'00
200
300
400
500
600
XENON PRESSURE (torr)
Figure 1 . Isotherms of total xenon uptake on silicalite samples. 0,SID: e. SIM: 0. SIT
I,"
1 .oo 0.83 f 0.03 0.80 f 0.03 0.87 f 0.03
Fr 1 .OOb
0.83 f 0.04 0.72 f 0.04 0.82 f 0.05
"Errors are quoted using 95% confidence limits. bAssumed to be free of template.
1-
o
?
Tsiao et al.
.,
700
SIS;
to this model (the solid line). The dashed lines indicate the Gaussian and Lorentzian subspectra. Addition of more functions does not significantly improve the quality of the fit, and sums of either two Lorentzians or two Gaussians give quantitatively poorer fits than the sum of a Lorentzian and a Gaussian. From the calculated Gaussian and Lorentzian subspectra, one estimates the relative fraction of xenon atoms in each of the two environments, fL, assuming one detects all xenon atoms: (1)
sorbed in a Na-Y zeolite in equilibrium with the gas at 590 Torr, and shifts are reported relative to the resonance position of bulk xenon gas extraporated to zero pressure.14 Positive shifts are to higher frequency. Solid-state I3C CPMAS NMR spectra were recorded at 25.013 MHz on a Chemagnetics mlOOS spectrometer. The particular parameters used are an accumulation of 2000 transients, a relaxation delay of 2 s, a 90" pulse length of 6.5 ps, and a contact time of 8 ms. The I3C chemical shifts are reported with respect to external TMS. Results and Discussion Figure I gives the isotherms of xenon sorption in the various silicalite samples at 295 K. All isotherms are of similar form; only very small differences in capacity among the samples are suggested by the analyses. In Figure 2 are spectra of xenon in these samples as a function of pressure. Despite the fact that the isotherms show that the samples take up xenon in very similar ways, systematic variations in the xenon N M R spectra demonstrate that the local environments seen by xenon gas vary from sample to sample. For example, SIS exhibits a single Lorentzian line shape at all pressures, whereas SID shows a single Lorentzian line at low pressures and a composite resonance for Pxe> 377 Torr. For S I M and SIT the spectra are composite at pressures as low as 160 Torr, although the relative proportion of the two spectral components changes upon treatment. There must be ut least two different environments for xenon in these latter three materials, even though the materials by sorption and X-ray analysis are nominally of the same structure and pore volume. To understand the xenon N M R results, we examined the samples by I3C CPMAS spectroscopy. A typical CPMAS I3C N M R spectrum (Figure 3) indicates the existence of tetrapropylammonium ions in the silicalite, in this case SIM.15J6 SIT is derived from SIM by a high-temperature combustion, and we note that. because of the partial elimination of template, the Lorentzian component observed with xenon spectroscopy increases from 71.6% to 82.3% upon combustion for 4 h. A loss of I3C NMR intensity is also observed for this sample. Thus, the loss of template seen by I3C N M R spectroscopy is accompanied by an increase in the Lorentzian component as seen by '29XeNMR spectroscopy. To specify the Iz9Xechemical shift variation as completely as possible, each spectrum is decomposed into the sum of a Lorentzian and a Gaussian function using a Simplex algorithm. Figure 4 shows a typical spectrum (the points), together with the best fit (14) Scharpf, E.; Crecely, R.; Gates, 9.; Dybowski, C. J . Phys. Chem. 1986, 90, 9 ( 1 5 ) Baxhoorn, G.; van Santen, R. A,; van Erp, W. A,; Hays, G. R.; Huis, R.J. J . Chem. SOC.,Chem. Commun. 1982, 264. (16) Tsiao, C.-J.; Dybowski, C. R.; Durante, V.; Walker, D.; Corbin, D. R. Langmuir 1988, 4. 1219.
where A , and AG are the areas of the best fit subspectra. The values offL vary slightly with pressure, but we do not believe the variation is statistically meaningful. The average value offL for each sample is given in the second column of Table 11. The NMR chemical shift of xenon adsorbed in a microporous material is given by Fraissard's eq~ation:"-'~ 6(d) = 60
+ 6,d + 6,d-1
where d is the density of xenon adsorbed, in atoms per gram of dry material, 6,,is the density-independent term, 6,is the first-order coefficient in the expansion of the xenon chemical shift in the xenon density, and bE is the electrostatic (or adsorption) term. 6o is composed of two factors, the wall-collision effect and a second due to collision with material sorbed in the silicalite. For silicalite, 3 0), ~ and we neglect there should be no electrostatic effect ( ~ = the third term in eq 2.' For the cases that give rise to the spectra of Figure 2, a xenon atom finds itself in one of two macroscopic regions. From each isotherm, one knows only the ouerall uptake for a sample at a given pressure: (3)
where N x e is the total number of xenon atoms taken up by the silicalite and W is the total dry weight of silicalite. The xenon atoms, however, partition between two NMR-distinguishable regions. The fractional area for the Lorentzian component in eq 1 is then the fraction of xenon atoms adsorbed in the Lorentzian structure in equilibrium with that pressure of xenon gas. One obtains isotherms for xenon in each region: using the N M R parameter,f;, defined in eq 1. F, is the fraction of dry weight corresponding to regions where xenon will resonate in the ith component of the N M R spectrum. This equation contains two unknown quantities, F, and d,,,. To proceed, we make the assumption that dXe,Lis the same as that for a pure silicalite (SIS), which we know shows no Gaussian phase. The I3C N M R spectrum of this material is consistent with this assumption since we fail to detect a I3C resonance for this sample. In this manner we calculate FL, the weight fraction of silicalite that exhibits the Lorentzian component for each of the other samples via eq 4. These results are tabulated in Table 11. The values indicate that the three samples-SID, SIM, and SITcontain substantial material (20-25%) where xenon resonates as part of the Gaussian component. We emphasize that this pro(17) Fraissard, J.; 110, T.; Springuel-Huet, M . ; Demarquay, J. Stud. Surf. Sci. Catal. 1986, 28, 393. ( 1 8 ) Ito, T.; Fraissard, J. J . Chem. Phvs. 1982, 76, 5225. (19) Ita, T.; Fraissard. J. In Proceedings of the 5th International Conference on Zeolires; Heyden London, 1980; pp 5 10-5 1 5 .
N M R of Xenon Sorbed in Silicalites
The Journal of Physical Chemistry, Vol. 94, No. 10, 1990 4191
1
1
140ppm
I,
11Oppm 14Oppm
110ppm140~~m
110ppm140ppm
I
1
1
1 1Oppm
Figure 2. '29XeN M R spectra of xenon sorbed in silicalites as a function of xenon pressure. The number by each spectrum indicates the equilibrium pressure of xenon (in Torr).
I
BE
79
70
62
53
44
35
1E
26
9
I0
CHEMICAL SHIFT (ppm) Figure 3. Carbon-I3 CPMAS NMR spectrum of SIM. T,, = 8 ms.
120
1
CHEMICAL SHIFT
7
(PPM)
Figure 4. Example of analysis of composite resonance as a sum of a Lorentzian component and a Gaussian component. The dots are data. The solid line is the sum of the two subspectra, and the dashed lines are the Lorentzian and Gaussian subspectra, respectively.
cedure counts all regions in which the xenon shift contains additional effects. Because collisions with template molecules make spectroscopy of xenon in regions containing template different from the spectroscopy of xenon in channels without the template, one may infer from the xenon N M R spectra that there exist at least two different regions-one in which the xenon does not experience collisions with templates and another in which it experiences some collisions with template that are detected as a shift in the resonance. In actuality, the Gaussian component most likely represents a distribution of environments, the common factor being the presence of some template molecules. Since xenon gas moves over many microscopic regions during the course of an N M R experiment,*O this shows that the Lorentzian regions are macroscopic in extent. Determination of FLand fL for each sample permits one to specify calculated isotherms for xenon sorption in each region of each sample. Comparison of these isotherms (not shown) indicates that, on a weight basis, the Gaussian regions take up slightly less (about 8-10%) than the Lorentzian regions. This produces a 1-272 difference in total uptake, which is not noticeable in total isotherms such as those of Figure 1 . The fact thatf, is roughly independent of the pressure shows that the template's presence does not substantially alter the sorption process but merely slightly reduces the uptake in the regions with template. It is possible to fit the datafor each region of each sample to Fraissard's equation. Figure 5 shows plots of the chemical shift as a function of dx,,i in these phases for these samples. The parameters 6o and 6, obtained from Fraissard's equation are listed in Table 111 for each region of each sample. From the parameters of Table 111, one can see that the local environments of xenon atoms in all of these samples are quite similar. The first-order coefficient, Q,, which measures the effect of xenon-xenon collisions, is essentially the same for all the materials. There may be some slight difference between the materials SIT and SIM and the materials SID and SIS, as shown by the values of the intercepts. It seems, by comparison of Gaussian and Lorentzian resonances for each sample, that when the template is present in the material, it slightly changes the intercept but does not affect the xenon-xenon interactions, as (20) Shoemaker, R.; Apple, T. M. J . Phys. Chem. 1987, 91, 4024.
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Tsiao et al.
The Journal of Physical Chemistry, Vol. 94, No. 10, 1990
I
$G 1 110
901
0
,
,
I
'
,
,
, ,
1
2
3
4
5
6
7
8
, ,
,
1
, , I
9 1 0 1 1 1 2 1 3 1 4 1 5
XENON DENSITY (atom/gram x
.,
Plots of xenon chemical shift versus xenon density in the Gaussian and Lorentzian regions of SIS, SID,SIM, and SIT. Gaussian regions of SID; 0,Lorentzian regions of SIS and SID;0 , Gaussian regions of S l M and SIT; 0,Lorentzian regions of SlM and Figure 5.
SIT. TABLE 111: Parameters Derived from Fraissard's Equation for the Silicalite Samples"
sample
SIS SID SIM SIT
SID SIM SIT
60, ppm 100 f 3 100 f 3 96 f 2
96 i 2 101 f 2 103 f 2 103 f 2
1O%,, ppm g/atom
A,*
A
A. Lorentzian 3.1 f 0.3 2.9 f 0.2
3.1 f 0.3 3.0 f 0.1 3.0 f 0.1
2.9 f 0.2 3.1 f 0.1 3.1 f 0.1
B. Gaussian 3.2 f 0.3 2.9 f 0.1 3.1 f 0.4 2.8 f 0.2 3.1 f 0.4 2.8 f 0.2
Dc,eff.*A 7.3 7.3 7.5 7.5
f 0.3 f 0.3 f 0.2 f 0.2
7.3 f 0.2 7.2 f 0.4 7.2 f 0.4
"Errors are quoted using 95% confidence limits. *Calculated from eq 6.
represented by the constancy of the slope of chemical shift versus uptake from sample region to sample region. One means of describing this effect is by a slight change in size of regions in which xenon finds itself when template is present, as has been given by Demarquay and Fraissard.21 They have found that they can empirically specify the dependence of 6, on a parameter they call the mean free path, A, of xenon in the zeolite by the following equation: 60 = 243[2.054/(2.054 + A)] (5) The mean free path they define is a measure of the frequency of collision with the framework and hence measures the free volume available to the xenon atoms.22 Considering silicalite to be a system of interconnecting straight channels, the mean free path, can be related to the effective diameter of the channel by2' (21) Demarquay, J.: Fraissard, J. Chem. Phys. Letl. 1987, 136, 314 (22) Cheung, T. T. P.;Fu, C. M. J . Phys. Chem. 1989, 93, 3740.
where Dc,effis the effective diameter of channel and Dxe is the diameter of xenon atom (4.4 A). The calculated mean free paths, A, and the effective channel diameters for the various silicalite samples are summarized in Table 111. As one can see by comparison to X-ray results (5.5 8, < D < 5.8 A), the model of Demarquay and Fraissard overestimates diameters in this material by about 1.5-2 A. This discrepancy could be accounted for by using covalent radii when interpretting pore dimensions from the X-ray structure. The shapes and uptake dependences of '29Xechemical shifts for all of the materials are different from each other when considered as a function of the total uptake, although the spectra are qualitatively similar in the sense that they each contain two components. When considered as a function of the uptake in each region, one sees that the N M R parameters are very similar, indicating that the structure and its effect on the xenon NMR spectroscopy are not very different from sample to sample. Were one to consider that the Gaussian resonance represents the response of xenon atoms in a variety of regions, some having many template molecules with which to interact, some having only a few, the differences in the width of the Gaussian resonance quite likely represents something about the specific distribution of templates in a material. We do not know how to compare these data to other measures of the distribution of template, but the fact that xenon N M R spectra are sensitive to this feature indicates that other experiments, perhaps low-temperature xenon NMR spectroscopy,22 could be utilized to give more information about the distribution of template. Conclusions
The sensitivity of xenon-129 N M R spectroscopy makes it possible to distinguish two different macroscopic environments for xenon sorbed in silicalite. Using this effect one can estimate the amount of material with template remaining in the channels from the xenon N M R spectrum. The Lorentzian regions have been associated with regions without template in the channels. Using the xenon N M R results and the overall isotherms, we predict the uptake in each region and the Fraissard parameters for xenon in each environment. The dependence of the xenon N M R parameters on the uptake in each of the regions is quite similar for all the resonances. We find that application of the relationship of Demarquay and Fraissard to our data gives a predicted size of the channels that is not in agreement with X-ray data. The results could be rationalized by changes in the parameters of the Demarquay-Fraissard equation to bring our results into agreement with X-ray diffraction results or vice versa. Although the results must be viewed as semiquantitative, we show how xenon N M R spectroscopy can play a unique and sensitive role in the analysis of materials that appear to be similar by other techniques.
Acknowledgment. This work was supported by the sponsors of the Center for Catalytic Science and Technology of the University of Delaware; the donors of the Petroleum Research Fund, administered by the American Chemical Society; and Sun Marketing and Refining Company. Registry No. SiO,, 7631-86-9; Xe, 7440-63-3.
