J . Phys. Chem. 1990, 94. 3718-3721
3718
to-Mo points to position a instead of b. It should be noticed that the local surrounding of the Co atoms, as is shown in Figure 10, is incomplete, since the Co atom is expected to be coordinated to more than 2 (or 4, respectively) sulfur atoms. I n fact, the Co atom in the sulfided Co-Mo/C catalyst appears to be coordinated by at least five sulfur atoms, as we have recently reported.49 Figure IO, in this respect, is restricted to the local surrounding of the Mo atom, in which the extra sulfur atoms surrounding the Co promoter atom are not included.
Conclusions Detailed information regarding the structure of the sulfide particles in carbon-supported Mo and Co-Mo sulfide catalysts have been obtained. 1. The catalysts probably consist of very small MoS, particles which contain on average 5-6 Mo ions (Mo/C) and 7-8 Mo ions (49) Bouwens, S. M. A. M.; Koningsberger, D. C.; de Beer, V. H. J.; Prins, R. CataI. Lett. 1988, I , 5 5 . (50) Stern, E. A. In X-Ray Absorption, Principles. Applications, Techniques of EXAFS, SEXAFS, and X A N E S Koningsberger, D.C . , and Prins. R., Eds.; Wiley: New York, 1988; p 40.
(Co-Mo/C), respectively. Comparison with literature data shows that the average local ordered structure in these carbon-supported catalysts is very much the same as their alumina-supported counterparts. 2. A detailed EXAFS data analysis shows the presence of carbon neighbors next to the Mo atoms at a distance of 2.1 f 0.1 A. This short distance could imply an intimate interaction between the active phase and the carbon support, which may be the cause of the high dispersion of the active phase. From the value of 2.1 8, it can be inferred that the Mo-C coordination is restricted to the exposed Mo atoms and that a support carbon atom takes the place of a sulfur vacancy. 3. The EXAFS spectra show evidence for the existence of cobalt neighbors next to the Mo atoms at a distance of 2.8 f 0.1 A. Acknowledgment. We acknowledge the technical assistance of the SRS staff and the design of the H,S resistant in situ EXAFS cell by Dr. F. W. H. Kampers. The information included in this paper is partly derived from a contract (EN3V-O009/NL) concluded with the European Economic Community. This study was supported by the Netherlands Organization for Scientific Research (NWO). Registry No. Co, 7440-48-4; MoS2, 13 17-33-5; C, 7440-44-0.
Crystal Structure of Silica-ZSM-12 by the Combined Use of Hlgh-Resolution Solid-state MAS NMR Spectroscopy and Synchrotron X-ray Powder Diffraction C. A. Fyfe,*.+H. Gies,**tG. T. Kokotailo,+B. Marler,t and D. E. Cox5 Department of Chemistry, UBC Vancouuer, Vancouuer, British Columbia, Canada, Mineralogisches Institut der CAU, 23 Kiel, FRG, and Brookhaven National Laboratory, Upton, New York 1 1 973 (Received: July 13, 1989: I n Final Form: Nouember 15, 1989)
The crystal structure of the synthetic zeolite silica-ZSM-12, 56 SO2, has been solved by the combined use of high-resolution solid-state MAS NMR spectroscopy and high-resolution synchrotron X-ray powder diffraction. ZSM- 12 crystallizes in the monoclinic space group C2/c with a. = 24.863 A, bo = 5.012 A, c, = 24.328 A, and 0 = 107.7'. The zeolite host structure is built from corner-linked Si04 tetrahedra to give a three-dimensional 4-connected net. The pores of the structure are one-dimensional channels that do not intersect, with 12-membered ring pore openings of approximately 5.6 X 7.7 A. The structure of ZSM-12 is frequently twinned with (100) as the twin plane, which indicates a new zeolite structure type.
Introduction High-silica zeolites have attracted much attention due to their unique properties in catalysis' and as molecular sieves2and more recently as potential host structures for quantum-size particles, quantum dots, etc3 Since it is the crystal structure, or more precisely the topology of the silica host framework, that determines the properties of these systems, it is therefore essential to have precise information available on both the local and long-range order of zeolite structures. ZSM-12 is a high-silica zeolite first synthesized by Rosinski and Rubin.4 LaPierre et ai. proposed the host framework topology from electron and X-ray powder diffraction data combined with model b ~ i l d i n g . ~We report here on the first crystal structure refinement of silica-ZSM- 12, where the structural investigations have been carried out by use of a combination of two different techniques, high-resolution solid-state NMR and synchrotron X-ray powder diffraction. These are sensitive to short- and long-range order, respectively, and are therefore complementary in nature for the characterization of solid-state structures. Only with a combination of the two techniques was it possible to arrive
'* Mineralogisches UBC Vancouver. Institut der CAU iBrookhaven
National Laboratory.
0022-3654/90/2094-37 18$02.50/0
at a complete description of the structure that involves a subtle pseudosymmetry problem not recognized in previous structural studies.
Experimental Section Synthesis. Highly crystalline ZSM-12 was obtained by hydrothermal synthesis6 and dealuminated by steaming the sample at elevated temperatures.' The same material was used for both the NMR and XRD investigations. Single crystals of silicaZSM-I2 were synthesized in sealed silica tubes at 200 O C from 1 M aqueous solutions of silicic acid in the presence of 4,4'-trimethylenedipiperidine as template. N M R Experiments. 29SiNMR spectra were recorded with a Bruker MSL-400 spectrometer at 79.6 MHz (proton frequency (1)
232.
Holderrich, W.; Hesse, M.; Naumann, F. Angew. Chem. 1988, 100.
( 2 ) Barrer, R. M. Zeolites and Clay Minerals as Sorbents and Molecular Sieues; Academic Press: London, 1978. ( 3 ) Ozin, G. A.; Kuperman, A,; Stein, A. Angew. Chem. 1989, 100. 373. (4) Rosinski, E. J.; Rubin, M. K. US Patent 3 832 449, 1974. ( 5 ) LaPierre, R. B.; Rohrman, A. C.; Schlenker, J. L.; Wood. J. D.; Rubin, M. K.; Rohrbaugh, W. J. Zeolites 1985, 5. 346. (6) Fyfe, C. A.; Strobl, H.; Kokotailo, G. T.; Pasztor, C. T.; Barlow, G. E.; Bradley, S. Zeolites 1988, 8, 132. (7) Fyfe, C. A.; Gobbi, G. C.; Kennedy, G. J. J. Phys. Chem. 1984,88,
3248.
0 1990 American Chemical Society
Crystal Structure of Silica-ZSM-12
The Journal of Physical Chemistry, Vol. 94, No. 9, 1990 3719
16000. I
.I
12000-
8000.
4000.
0. 1200--
01-
- 1200,
;i
-
- -
Io
I
0
10
20
30
40
50
60 2- THETA ['I
Figure 1. Observed, calculated, and difference X-ray powder diffraction diagrams of silica-ZSM-12. The insert shows a part of the indexed X-ray powder diffraction diagram showing superstructure reflections. The very minor intensity of the extra reflections is still clearly above the background of the diffractogram.
400 MHZ) using a standard Bruker MAS probe. In order to average the chemical shift anisotropy, samples were spun about the magic angle at approximately 3.5 kHz in alumina spinners.* The setting of the magic angle was optimized by using the 79Br resonance of KBr as described by Frye and MacieL9 X-ray Experiments. Single crystals of silica-ZSM-I2 of approximately 12 X 30 X 150 pm were used for Weissenberg studies. The small crystallite size and, in addition, twinning of the crystal with (100) as the twin plane, precluded structural studies on a conventional four-circle diffractometer. X-ray powder diffraction data from a flat plate sample of silica-ZSM-12 were recorded on beam line X13A at the NSLS at the Brookhaven National Laboratory. A Ge( 111) incident beam monochromator scattering in the horizontal plane and a Ge(220) analyzer crystal were used to obtain a monochromatic beam wavelength of 1.3186 A. Details of the diffractometer have been reported elsewhere.I0 Intensity data were recorded by step scanning in the vertical plane over a 20 range 5-65' in steps of 0.01'. For more details see Table I. Refinement Procedure. A Rietveld refinement of the synchrotron powder data was performed with a locally modified version of the Rietveld-Hewat program." As a starting model for the refinement, the framework topology proposed by LaPierre et aL5 was used. The initial set of atomic positions was determined by distance least-squares procedures (DLS-76)I2 in space group C2/c with a Si-0 bond length of 1.602 A and Si-0-Si angles of 15 1 .O'. These are mean values taken from the structure re(8) Schaeffer, J.; Stejskal, E. 0. J . A m . Chem. S o t . 1976, 98, 1031. (9) Frye, J. S.; Maciel, G. E. J . Magn. Reson. 1982, 48, 125. (IO) Cox, D. E.; Hastings, J. B.; Cardoso, J. P.; Finger, L. W. In High Resolution Powder Dijfraetion; Catlow, C. R. A., Ed.; Materials Science Forum 9; Technomic: Lancaster, PA, 1986; p 89. (1 1) Hewat, A. W.Atomic Energy Research Establishment, Harwell, Report No. R7350, 1973. (12) Baerlocher, Ch.; Hepp, A.; Meier, W. M. DLS-76, A Program for the Simulation of Crystal Structures by Geometric refinement; Zurich, Switzerland, 1977.
TABLE I: Summary of Experimental Parameters of the Synchrotron Experiment and the Rietveld Structure Refinement wavelength, A 1.3186 profile range used, deg (28) 5.0-65.0 step size, deg (28) 0.01 peak range in fwhm 12.0 number of observations 6000 number of contributing reflections 889 number of profile parameters 13 number of structural parameters 65 Residuals"
ao, A
Refined Cell Parameters
bo, A co, 8, P, deg
24.8633 (3) 5.01238 (7) 24.3275 (7) 107.7215 (6)
" I = integrated intensity; y = intensity at given 28; N = number of observations; P = number of least-squares parameters; W = weight. finement of silica-Theta-]" which is closely related in the topology of its subunits of the silica framework to those of the silica
framework of ZSM-12. For the refinement, no geometric constraints were put on the framework atoms and an overall thermal parameter for Si and 0 was used. Since the sample had been calcined, only the framework Si and 0 atoms were included for the refinement. The peak shape was approximated with a pseudevoigt profile function. As the refinement proceeded, it was necessary to include a pre(13) Marler, B. Zeolites 1987, 7, 393.
Fyfe et al.
3720 The Journal of Physical Chemistry, Vol. 94, No. 9, 1990 TABLE 11: Fractional Coordinates and Selected Bond Angles and Distances of Silica-ZSM- 12 (ESD's in parentheses) atom Y Z X 0.4129 (3) 0.4402 (3) 0.5319 (20) Sil" 0.0678 (2) -0.0708 (19) 0.4589 (3) Si2 0.3754 (2) 0.0320 (20) 0.3609 (3) Si3 0.42 18 (20) 0.4484 (3) 0.1338 (2) Si4 0.4275 (3) 0.0822 (21) 0.2836 (2) Si5 0.5853 (19) 0.3832 (3) 0.2139 (3) Si6 0.2463 (3) 0.2869 (2) 0.0100 (20) Si7 0.5053 (35) 0.4730 (5) 0.4280 (4) 01 0.4725 (35) 0.4225 (4) 0.5034 (4) 02 0.3872 (4) 0.4245 (4) 0.8220 (36) 03 -0.0045 (34) 0.3956 (5) 0.3301 (5) 04 -0.3624 (36) 0.4513 (5) 0.0841 (5) 05 0.2972 (5) -0.0218 (40) 0.3452 (4) 06 0.2441 (40) 0.2629 (5) 0.2504 (5) 07 0.4078 (5) 0.3716 (36) 0.2602 (5) 08 0.3911 (4) 0.5101 (39) 0.1554 (4) 09 0.5026 (4) 0.4480 (35) 0.1853 (4) 010 0.1886 (4) 0.2997 (4) 0.0905 (37) 011 0.3702 (5) 0.3370 (37) 0.3995 (5) 012 0.4394 (5) 0.1372 (36) 0.1069 (4) 013 0.4093 (5) 0.2343 (5) 0.8828 (36) 014 Sil-Si2 Sil-Si2 Sil-Si3 Si3-Si5 Si2-Si4 Si3-Si7 Si747 Si6-Si5 Si6-Si4 Si4-Si5 Si7-Si6 Si 1-Si3 Si4-Si2 Si5-Si6
3.19 ( I ) 3.07 (1) 3.04 ( I ) 3.19 ( I ) 3.08 (1) 2.98 ( I ) 3.14 ( I ) (2X) 3.06 (1) 3.01 ( I ) 3.09 ( I ) 3.17 ( I ) 3.04 ( I ) 3.02 ( I ) 3.04 ( 1 )
Si I-01-Si2 Sil-0242 Sil-03453 Si3-0445 Si2-05-Si4 Si3-06-Si7 Si7-0747 Si6-08-Si5 Si6-09-Si4 Si4-010-Si5 Si7-01 ]-Si6 Si 1-0 12-Si3 Si4-013-Si2 Si 5-0 14-Si6
159 ( I ) 156 ( I ) 146 ( I ) 158 (1) 148 ( I ) 145 ( I ) 157 ( I ) (2X) 156 ( I ) 134 ( I ) 152 ( I ) 153 ( I ) 145 (1) 146 (1) 146 ( I )
K
l
mean values
in the range of
1.526 (12)1.697 (14) 1.597
2.511 (25)2.712 (18) 2.608
103.4 (6)114.3 (9) 109.5
"Bovcrd, = 0.01 (3).
ferred orientation correction for the needlelike crystals, which tend to be packed with their [OlO] axis in the plane of the sample holder. The final set of coordinates was obtained by including all parameters and observations for the refinement which converged to residuals of R, = 6.9% ( R , = 18.1%) with an expected value of Re = 5.8% (for more details see Table I). Final atomic positions and selected angles and distances are summarized in Table 11, and the observed and calculated powder profiles are shown in Figure I .
Results and Discussion Structure Determination. In contrast to the model proposed by LaPierre et al.,s Weissenberg photographs of silica-ZSM- 12 clearly showed that the co parameter had to be doubled (Figure 2). Careful inspection of the synchrotron data set revealed extra reflections which also could only be indexed with a doubled co parameter (see insert in Figure 1). Although the intensities of the extra reflections due to the superstructure were very weak, the refinement would only succeed when all of these were included. The new cell dimensions as refined from the powder XRD are a. = 24.863 (1) A, bo = 5.012 (1) A, co = 24.328 (1) A, and /3 = 107.7 ( 1 ) O . The Weissenberg photographs showed that the crystals were twinned with (100) as the twin plane. Therefore all hkO reflections of the two crystals coincide and have intensity values of the two individuals. All other reflections could be indexed individually for one or the other of the twin crystals. Systematic extinctions for hkl, h + k = 2n; h01, h, I = 2n led to space groups Cc or C2/c. which have respectively 14 and 7 symmetrically
K
K
Figure 2. Projection of the silica host framework of the structure of silica-ZSM-12 along bo. The unit cell is indicated by solid lines with a doubled c, parameter as compared to the proposed structure by LaPierre et aL5
- 105
1
I
I
f
1
PPM
1
1
FROM
1
1
TMS
I
~
I
- I10
Figure 3. ?Si MAS NMR spectrum of silica-ZSM-12. Seven resonance signals of equal intensity indicate seven symmetrically inequivalent Si atoms of equal multiplicity per unit cell of the structure.
Figure 4. The 12-membered ring channel seen perpendicular to the channel axis. The net of six-membered rings building up to the channel is shown. The limited pore opening of the six-membered rings (aproximately 2.5 A) precludes diffusion of the guest molecules out of the channel.
inequivalent Si sites of equal multiplicity for the model assumed. However, the 29SiMAS N M R spectrum of the same sample as used for the XRD data collection shows seven signals of equal intensity, unambiguously leading to the space group symmetry C2/c (Figure 3). Description of the Framework. The silica host framework of ZSM-I 2 is a three-dimensional 4-connected net built from corner-linked [SO4] tetrahedra. In the projection of the structure along bo the honeycomb-like silica framework is shown (Figure 2). It is composed of four-, five-, and six-membered rings of [SiO,] tetrahedra. This is the backbone of the porous structure, resulting in 12-membered ring channels along bo which do not intersect. Along bo the channels are composed of only six-membered rings with pore openings of less than 2.8 8, (Figure 4). This is too small
~
~
The Journal of Physical Chemistry, Vol. 94, No. 9, 1990 3721
Crystal Structure of Silica-ZSM-12
b
I
1
a
probably due to uncertainties introduced because of the pronounced preferred orientation (cf. Table 11). In order to assign NMR signals to a particular Si atom in these structures, empirical correlations of chemical shift vs mean Si-Si distances are attractive. However, sin‘ce minor changes in the local environment of the individual Si atom are reflected in systematic changes of their chemical shifts (0.1 ppm corresponds to 0.01 8,16), and the errors in the diffraction data are quite large, one must be very careful in using geometrical data to correlate with the chemical shift values for the individual Si atoms from the N M R experiments, especially in cases like the present structure where there is no differentiation of the T sites based on intensity data with which to cross-check the assignments. In fact the assignment obtained in this way from the presented data, which is based on high-quality data and refines to low residuals, is not in agreement with the unambigous assignment recently presented from twodimensional N M R experiments on the same sample.” Polytypism. Regular twinning of ZSM-12 with (100) as the twin plane would lead to another zeolite structure type with one-dimensional 12-membered ring channels (Figure 5) where the relationship between the structures is similar to the relationship between those of Theta-1 and ZSM-23. Whereas the material synthesized according to the patent literature showed regular twinning in larger domains (e.g., ref 5), the material synthesized with 4,4’-trimethylenedipiperidineas template was very clean ZSM-12. There were only a few twin boundaries randomly distributed across the crystals. As already reported,s the cell dimensions for the orthorhombic structure would be a. = 24.30 A, bo = 23.70 A, and co = 5.01 8,in space group Pmcn. Variation in synthesis conditions and templating agents might lead to the new structure type. Conclusion
Figure 5. Projection of the structure of the (a) twinned and (b) regularly twinned form of ZSM-12. The new structure type also has 12-membered ring channels in one dimension that do not intersect.
for the guest species in the pores to migrate from one channel to another, and this produces a one-dimensional channel system. In the as-synthesized form the channels contain the organic guest molecules which act as templates for the pores and determine their size and dimen~ionality.’~The 12-membered ring pores have openings of approximately 5.6 X 7.7 A in the calcined form and are considerably larger than the 10-membered rings of related zeolites, e.g., ZSM-5 (5.3 X 5.6 AI5). The angles (L(Si-0-Si)) and distances (&A and cispsi) of the refined silica host framework are within the range expected for silica frameworks. However, there is considerable scatter about the overall average values of the structure, some of which is (14) Gies, H. Inclusion Compounds; Atwood, J. L., Davies, J. E. D., McNicol, D. D., Eds.; Oxford University Press: Oxford; Vol. 5 , in press. (15) Meier, W. M.; Olson, D. H. Atlas ofzeolite strucrure types; International Zeolite Association. 1987.
By applying N M R and XRD techniques, which are complementary probes of short- and long-range order in crystalline solids, respectively, the proposed topology of the host framework of ZSM-12 has been confirmed and a detailed structure from a refinement in a new space group determined. The very high resolution of the synchrotron X-ray powder data revealed the pseudosymmetry of the structure and made the refinement possible. High-resolution solid-state MAS N M R spectra led to an unambigous assignment of the space group symmetry. The successful demonstration of the combination of X-ray powder diffraction and high-resolution solid-state MAS N M R for the structural characterization of crystalline material should stimulate further use of these techniques together to provide a more complete picture of the solid state. Acknowledgment. The work at Brookhaven is supported by the Division of Materials Sciences, U S . Department of Energy, under Contract no. DE-AC02-76CH00016. H.G. acknowledges the finanical assistance of the Alexander von Humboldt Foundation. (16) Engelhardt, G.;Michel, D. High Resolution Solid State N M R of Silicates und Zeolites; Wiley: Chichester, 1987; p 131 and references herein. (17) Fyfe, C. A,; Gies, H.; Feng, Y.; Kokotailo, G.T. Nature 1989, 341, 223.
