The Journal of
Physical Chemistry
Q Copyright, 1987, by the American Chemical Society
VOLUME 91, NUMBER 9 APRIL 23, 1987
LETTERS Weakly Acidic Bridging Hydroxyl Groups and Nonframework Aluminum Species in Zeollte D-RHO Shallow-Bed Calcined In Steam Reinhard X. Fischer,? Central Research & Development Department,$E. I. du Pont de Nemours & Company, Experimental Station, Wilmington, Delaware 19898
Werner H. Baur,* Institut fur Kristallographie und Mineralogie der Johann Wolfgang Goethe- Universitat Frankfurt, Senckenberganlage 30, 0-6000 Frankfurt am Main 1 , Bundesrepublik Deutschland
Robert D. ShaMOn, and Ralph H. Staley Central Research & Development Department, E. I. du Pont de Nemours & Company, Experimental Station. Wilmington. Delaware 19898 (Received: December 5, 1986)
A deuteriated, dehydrated zeolite RHO sample, shallow-bed calcined in steam at 773 K, with the composition D&~.,A&Sb20w and five nonframework AI atoms, was studied by time-of-flight neutron wder diffraction and IR spectroscopy. The analysis of the neutron diffraction data (space group Imgm, a = 15.0620 ( 3 ) ) reveals the presence of a bridging hydroxyl group, with an 0-D distance 0.96 (2) A, and approximate planarity of the two adjoining T (=Si, AI) sites, the bridging oxygen atom, and the D atom. Near the single six-ring of the aluminosilicate framework an A10 group has been located in which the nonframework AI atom is in a very distorted tetrahedral coordination. It cannot be decided whether the A10 group is a formally charged AIO+molecule, charge balanced by an equal number of AI02-moieties, or part of neutral AI2O3. Ammonia desorption studies show the OH group to be weakly acidic; this represents the first known example of a weakly acidic, bridging hydroxyl group in a zeolite.
r
Introduction When the NH4+ form of a silica-rich zeolite is calcined at temperatures between 700 and 1000 K a highly acidic hydrogen form of the zeolite is usually obtained. At the same time the tetrahedral aluminosilicate framework [TOz, where T is (Si, Al)] of the zeolite is partly dealuminated.' The resultant H-zeolites 'Now at Mineralogisches Institut der Universitgt Wiirzburg, Am Hubland, D-8700 Wilrzburg, Bundesrepublik Deutschland. *Contribution No. 4148.
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have increased catalytic activity: perhaps related to nonframework Al (NFA). On the basis of quantum chemical studies the H atoms are assumed to reside either in hydroxyl groups, bridging between A1 and Si, or in terminal hydroxyl groups in defect positions of (1) Kerr, G. T . In Molecular Sieues; Meier, W . M., Uytterhoeven, J. B., Eds.; American Chemical Society: Washington, DC, 1973; Adv. Chem. Ser. No. 121, p 219. (2) Flockhart, B. D.; Megarry, M. C.; Pink, R. C. In Molecular Sieues; Meier, W. M., Uytterhoeven, J. B., Eds.; American Chemical Society: Washington, DC, 1973; Adv. Chem. Ser. No. 121, p 509.
0 1987 American Chemical Society
2228 The Journal of Physical Chemistry, Vol. 91, No. 9, 1987
Letters
TABLE I: Positional Parameters in Fractional Coordinates and Isotropic Temperature Factors [A2], Site Symmetry, Wyckoff Positions, and Occupancies’ site no. of atoms atom X Y Z B sYm Wyckoff position in unit cell 0.1029 (2) ‘12 Y 1.0 (1) ..2 48(i) 42/6b Si/Al(T) ‘I4 0.3841 (2) 2.5 (1) m.. 486) 48 0 0.2173 (3) O(1) 0.1667 (2) X 0.3760 (2) 2.9 (1) ..m 48(k) 48 O(2) D 0 0.381 (1) 0.154 (1) 3.8 (7) ..m 486) 8.4 ( 5 ) cs 0.452 (4) 0 0 5.OC 4m.m 12(e) 1.7 (2) 0.208 (2) X 0.263 (4) 5.oc ..m 48(k) 5.5 ( 5 ) Wf) 0.303 (1) X 0.375 (2) 5.0‘ ..m 48(k) 4.9 (3) “This D-RHO crystallizes at 623 K in space group Im3m with a cell constant a = 15.0620 (3) A. The final R factors are R(1) = 9.4%, R(wp) = 3.7%. We used 251 Bragg reflections with d values ranging from 0.952 to 5.37 A in the refinement; eight profile parameters and 22 structural parameters were varied; the number of data points in the profile was 2256. bSi/Al ratio from MASNMR. cHeld constant in refinement.
the Direct experimental diffraction evidence for the location of the H atoms is tentative (see the neutron powder diffraction study of an H-Y-zeolite5), and little is known about the location of the A1 after it has left the framework. This NFA has been reported to reside in zeolites Y and A in the sodalite Shallow-bed calcination of NH,-RHO under flowing nitrogen or under vacuum at 773 K gives H-RHO with a strong, sharp infrared band a t 3610 cm-I due to stretching of bridging hydroxyls.”’ Infrared studies of the effects of calcination conditions on the hydroxyl-stretching bands show that higher temperature, steam, and/or deepbed conditions produce a decrease in this band with the appearance of bands at higher wavenumbers.12 We have identified a particular range of conditions (773-873 K in steam) that cleanly convert the 3610-cm-’ band to a strong, sharp band at 3640 cm-’, suggesting the generation of a homogeneous population of framework hydroxyls different from those of the original H-RHO. Ammonia desorption studies show that the acidity of the 3640-cm-’ hydroxyls is substantially lower than that of the 3610-cm-’ species. H-RHO samples with a sharp 3640-an-’ band thus afford an opportunity to identify the structure of NFA since such A1 may be expected to exist in a relatively homogeneous population of sites in these samples. To our knowledge this is the first example of zeolitic bridging hydroxyl groups that are not highly acidic.
Experimental Section Na,Cs-RHO was prepared by using the method of Robson et al.l2J3 Scanning electron micrographs showed dodecahedral crystals of zeolite R H O -0.5 pm in diameter and of spherical chabazite aggregates (10 vol ’7%) of similar size. The chemical composition of the NH,+-exchanged material was (NH4)10,7C S ~ ,1,3Si36,7096-43H20. ~ A ~ ~ After shallow-bed calcination for 4 h at 773 K in steam (612 Torr of H 2 0under a N z atmosphere) and after deuteriation and dehydration, the composition was D5,3Cs,,7Al&4z096 (D-RHO-SS773, for nomenclature see ref 12) with 4 AI atoms per unit cell outside of the framework. X-ray diffraction patterns showed that the chabazite phase was amorphitized during the conversion to H-RHO. The Si/Al ratio of (3) Mortier, W. J.; Sauer, J.; Lercher, J. A,; Noller, H. J . Phys. Chem. 1984. 88. 905. (4) Geerlings, P.; Tariel, N.; Botrel, A.; Lissillour, R.; Mortier, W. J. J . Phys. Chem. 1984, 88, 5752. ( 5 ) Jirik, Z.; Vratislav, S.; Bosficek, V. J . Phys. Chem. Solids 1980, 41, I 0x9.
(6) Maher, P. K.; Hunter, F. D.; Scherzer, J. In Molecular Sieve Zeolites I; Flanigen, E. M., Sand, L. B., Eds.; American Chemical Society: Washington, DC, 1971; Adv. Chem. Ser. No. 101, p 266. (7) Parise, J. B.; Corbin, D. R.; Abrams, L.; Cox, D. E. Acta Crystallogr. C40, 1493. ( 8 ) Pluth, J. J.; Smith, J. V. J . Am. Chem. SOC.1982, 104, 6977. (9) Flank, W. H. Molecular Sieues II; Katzer, J. R., Ed.; American Chemical Society: Washington, DC, 1977; ACS Symp. Ser. No. 40, p 43. (10) Baur, W. H.; Fischer R. X.;Shannon, R. D.; Staley, R. H.; Vega, A. J.; Jorgensen, J. D.Z . Kristallogr., in press. (11) Jacobs, P. A,; Mortier, W. J. Zeolites 1982, 2, 226. (12) Fischer, R. X.; Baur, W. H.; Shannon, R. D.; Staley, R. H.; Vega, A. J.; Abrams, L.; Prince, E. J . Phys. Chem. 1986, 90, 4414. (13) Robson, H. E.; Shoemaker, D. P.; Ogilvie, R. A.; Manor, P. C. In Molecular Sieues; Meier, W. M., Uytterhoeven, J. B., Eds.; American Chemical Society: Washington, DC, 1973; Adv. Chem. Ser. No. 121, p 106.
10
12
14
16
18
d-SPACING
20
(A)
22
2L
Figure 1. Observed (crosses) and calculated (line) profiles for D-RHO, with difference plots underneath, showing the deviations between observed and calculated intensities. Tick marks at the bottom line of the profiles indicate peak positions. Only the most crowded part of the profile is shown. Twelve more peaks at d > 2.6 A are omitted from the plot (but were included in the refinement).
the calcined material was 7.0 as determined from a well-resolved 29Si magic angle spinning N M R spectrum with relative peak (A. J. Vega, intensities of 8:41:51 for Si(2A1):Si(lA1):Si(OAl)14 personal communication). The deuterium content was assumed by difference to the Cs content and the A1 content of the framework; it could be off by 1 or 2 D per unit cell. Infrared spectra were taken as described in ref 12. The hydroxyl group stretching frequency was at 3640 cm-’. Ammonia adsorbed at 323 K and 1 Torr was found to desorb from hydroxyls of HRHO-SS773 (v = 3640 cm-’) at ~ 4 2 K 3 whereas desorption was found to occur in H-RHO4773 (dry shallow-bed calcined, v = 3610 cm-’) at 623 K. Time-of-flight neutron diffraction data were collected at 623 K on the special environment powder diffractometer at the Intense Pulsed Neutron Source of Argonne National Lab0rat0ry.l~ The structure refinements were performed with a local versionL6of Rietveld’s program” adapted to the time-of-flight method. The crystal structure illustration was drawn with the plot program STRUPL084.I8
Results and Discussion A broad diffuse peak between d = 0.95 and 1.4 8, is present in the diffraction pattern and is apparently caused by amorphitized chabazite. It could not be fitted by the standard background expressions in our version of the Rietveld code, and therefore the programs were modified to fit the background with fifth-degree (14) Klinowski, J., Progr. NMR Spectrosc. 1984, 16, 237. (1 5 ) Jorgensen, J. D.; Faber, J. In Proceedings of the 6th Meeting of the International Collaboration on Advanced Neutron Sources, Carpenter, J., Ed.; Argonne National Laboratory: Argonne, IL, 1982; ANL-82-80, p 105. (16) Rotella, F. J. Users Manual for Rietveld Analysis of Time-of-Flight Neutron Powder Data at IPNS, revised 1984; Intense Pulsed Neutron Source, Argonne National Laboratory, Argonne, IL, 1982. (17) Rietveld, H. M., J. Appl. Crystallogr. 1969, 2, 65. (18) Fischer, R. X., J . Appl. Crystallogr. 1985, 18, 2 5 8 .
Letters TABLE 11: Selected Interatomic Distances (A) and Angles (deg) 1.612 (4) 2x Si/Al-O( 2) 2x lx 1.638 ( 5 ) Si-A1-O( 1) 2x lx mean 1.625 ( 5 ) 2x 1.622" estim mean lx 1.61 (5) Al(nf)-O(nf) lx lx 1.92 (5) Al(nf)-O(2) lx Ix Al(nf)-O(2) 2.51 (4) 2x 2x Al(nf)-Si/Al 2.64 ( 5 ) 2x lx 2.98 (4) Al(nf)-O(2) 2x Ix 2.81 (2) 2x O(nf)-0(2) lx 2.90 (3) lx O(nf)-0(2) 2x D-O( 1) lx 0.96 (2) D-Si/Al 2.13 (1) 2x
The Journal of Physical Chemistry, Vol. 91, No. 9, 1987 2229
0(1)-0(2)
2.627 2.658 2.669 2.670
.oilj-o(i j 0(2)-0(2) O(1)-0(2) O(nf)-Al(nf)-0 (2) O(nf)-Al(nf)-0 (2) O(nf)-Al(nf)-0 (2) O(2)-Al(nf)-0 (2) 0(2)-Al(nf)-O (2) Si/Al-O(2)-Si/Al Si/Al-O( 1)-Si/Al Si/Al-O( 1)-D sum of angles around O(1)
(3) (4j (4) (4)
107.8 (1) 108.5 (3j 111.8 (2) 110.5 (2) 105 (2) 143 (2) 87 (2) 73 (2) 126 (2) 152.8 (3) 142.2 (3) 107.8 (2) 357.8
*See text.
polynomial^.^^ Fourier and difference Fourier syntheses did not yield evidence for possible nonframework positions. Such positions were found only when the asymmetric unit was searched systematically on a grid with a step size of 0.3 A.19This immediately revealed the position of a deuterium atom at 0.40,0.14,0 which refined to the coordinates given in Table I. It is located in an eight-ring with an 0-D distance of 0.96 (2) A. Its occupancy factor was 8.4 ( 5 ) atoms/unit cell, in approximate agreement with the seven protons expected from N M R (A. J. Vega, personal communication). The search map showed eight more peaks, which were interpreted on the basis of the following assumptions: (a) each position is occupied by only one nonframework species; (b) each nonframework atom is bonded to the framework by a short bond; (c) the lengths of these bonds must be within a few esd's of the bond lengths estimated from the sum of their effective ionic radii;20(d) the overall formal charge of the nonframework species must be zero, because charge balance is already attained by the deuterium ions; (e) the nonframework species cannot contain OD groups, because 'H NMR (on a protonated sample of D-RHO) shows only as many H atoms as were already located in the D sites. According to these criteria only two of the peaks close to the six-ring were acceptable and remained stable in the refinements, Al(nf) and O(nf) (Tables I and 11). The observed and calculated powder patterns are shown in Figure 1. The mean T-O distance of 1.625 8, compares favorably with the (T-O)",, estimated for a R H O framework with 6 Al/unit ce11.12 A single-crystal diffraction study of an aluminosilicate with a bridging OH group has never been performed. However, a neutron powder diffraction study of a zeolite H-Y, Hs2NaCaA1s5Si1370384~ has shown that the arrangement of the T atoms and the H atom around the bridging 0 atom is approximately planar. The precision of that refinement is so low that the validity of this finding is questionable. Our refinement of D-RHO, however, gives virtually the same result, but with a much higher precision. The sum of the angles T-0-T and T-0-D (twice) is close to 360O. The mean values for 0-D,T-0-D and T-D observed here are close to a quantum-chemical e ~ t i m a t e .The ~ position of the D atoms of the bridging OD groups corresponds to what we would find if we calculated it on the basis of electrostatic energy minima.21 It is unlikely that what we see as a bridging OD group is in fact a statistical overlap of terminal OD groups, because if this were the case the T-O( 1) distance should be shorter than the T-O(2) distance, whereas the opposite is true. Also we would expect the temperature factor of atom O(1) to be large, but it is not. The Al(nf) atom has as next neighbors O(nf) at 1.61 ( 5 ) A and O(2) at 1.92 ( 5 ) A. A distorted tetrahedral coordination is completed by two more O(2) atoms at 2.51 (4) A. The mean bond length Al(nf)-O (2.14 A) is reasonable if we recall that the Al(nf) site is not fully occupied and that the coordination polyhedron of oxygen atoms around it is distorted. Both these effects tend
"
I
Y
Y
"
*
Y
Y
Figure 2. Projection of the crystal structure of D-RHO from -0.5 tb 1.5 in all crystallographic directions in polyhedral representation. The outline of the unit cell is indicated; its origin is inthe upper left corner, x pointing down, y pointing right, and z pointing up. The smallest circles represent D atoms bonded to O(l), the medium-sized circles correspond to the Al(nf) atoms, and the largest circles to the oxygen atoms, O(nf). We show a possible distribution of the nonframework atoms. The positions of six deuterium atoms and six A10 groups in the unit cell have been chosen to give sufficiently long interatomic distances from each other (more than 4 A). Except for this criterion the choice is arbitrary.
to increase the apparent mean cation-anion distance.20 The A10 molecule itself has in the gaseous state an Al-O separation of about 1.6 A.22 We cannot distinguish if the A10+ molecule observed here is isolated or if it is part of an A102- group or even of A1203. Low-condensed A10 species have not been proved to occur in the solid state; however, the formation of an A10+ molecule has been proposed for Cu-Y zeolite.23 Though the refined positions for Al(nf) and O(nf) match the criteria a to e, we cannot be certain about the validity of this result. The grid search method yields a large number of peaks; some of them are spurious, some are clearly valid, and some are borderline. In cases where a peak remains stable in the least-squares refinement, but with a small occupancy factor, it is difficult to decide on its validity: is it a strong ghost or a valid peak with a small occupancy factor? In this study we have shown that the infrared band at 3640 cm-' must be 'assigned to the precisely determined bridging hydroxyl group. Furthermore we found indications for the presence of an A1 species formed upon dealumination, but these results are tentative because the Al(nf) and O(nf) positions have low occupancy factors. Nevertheless, we know that the overall charge of the species must be neutral, because all the available D atoms are attached to the hydroxyl bridges. The weak acidity of this bridging OH is surprising in light of previous work on the relationship of acidity to bridging vs. nonAt the present time we believe bridging character of OH group~.~P
(19) Baur, W. H.; Fischer, R. X. Adu. X-Ray Anal. 1986, 29, 131. (20) Shannon, R. D. Acta Crystallogr. 1976, A32, 75 1. (21) Baur, W. H. Acta Crystallogr. 1965, 19, 909.
(22) Srivastava, R. D.; Uy, 0. M.; Farber, M. J. Chem. Soc., Faraday Trans. 2 1972,68, 1388. (23) Jacobs, P. A.; Beyer, H. K. J. Phys. Chem. 1979,83, 1174.
J. Phys. Chem. 1987, 91, 2230-2232
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that the shift of the O H frequency from 3610 cm-’ in H-RHOS773 to 3640 cm-’ in H - R H O S 7 7 3 and the weak acidity accompanying the presence of the 3640-cm-’ band in H-RHO-SS773 is related to the occurrence of the NFA in the six-ring.
Acknowledgment. We thank Dr. A. J. Vega for communicating the NMR results, Dr. Vega and Dr. J. D. Jorgensen for discus-
sions, the North Atlantic Treaty Organization for Research Grant No. 149/84 and the Computer Center of the University of Illinois at Chicago, where part of the work was done, for computer time. R.X.F. and W.H.B. thank duPont de Nemours & Co. for a grant. The work at the Intense Pulsed Neutron Source was supported by the U S . Department of Energy, BES-Materials Science, under contract W-31-109-ENG-38.
Comparlson of Benzene Adsorption on Ni( 111) and Ni( 100) A. K. Myers, G. R. Schoofs, and J. B. Benziger* Department of Chemical Engineering, Princeton University, Princeton, New Jersey 08544 (Received: January 12, 1987)
The adsorption of benzene on the Ni( 100) and the Ni( 111) crystal faces was compared in order to investigate the effect of crystallographic orientation on the interaction of benzene with nickel. Temperature programmed reaction (TPR) was used to characterize adsorption bond strengths and determine product distributions. Benzene was found to adsorb 44 kJ/mol less strongly on the Ni( 111) plane than on the Ni( 100) surface. Di-hydrogen evolution formed after decomposition of benzene was similar for both surfaces. Benzene chemisorption was modeled by using extended Hilckel theory (EHT), a semiempirical molecular orbital method. The calculations predict bonding of benzene over a threefold hollow site on Ni( 111). Multicenter bonding of the benzene carbon atoms with the nickel atoms is indicated by the calculations. The binding strength of benzene is controlled by the degree of overlap of the carbon x orbitals with the nickel atom orbitals. Benzene binds more strongly to the Ni( 100) surface because the carbon x orbitals can overlap with four nickel atoms on the fourfold hollow site, whereas on Ni(ll1) the carbon atoms are closely associated with only three nickel atoms on the threefold hollow site.
It is known that the crystallographic orientation of the surface of metal catalysts may be an important factor in determining catalytic activity. Thus, it is important to understand how sensitive the chemisorption and reaction of a particular molecule is to the arrangement of surface metal atoms. In this study, we have investigated the influence of surface structure on the chemisorption of benzene on nickel. Adsorption on the (100) and (1 11) crystal faces of nickel was compared by using both experiment, temperature programmed reaction (TPR), and a molecular orbital model, extended Huckel theory (EHT). Experiments were performed on clean Ni( 100) and Ni( 11 1) crystals in a stainless steel ultrahigh vacuum chamber described previously.1,2 The crystallographic orientations were verified by low-energy electron diffraction (LEED). Benzene (99%) was obtained from Mallinckrodt and was used as received. Prior to each experiment the cleanliness of the crystal was verified by Auger electron spectroscopy (AES). Adsorption was carried out a t 250 K or below; exposure was sufficient to ensure saturation coverage. TPR was used to determine product distributions. After adsorption, the crystal temperature was linearly ramped to 600 K, and the desorbing gases were monitored by a quadrupole mass spectrometer located 6 cm directly in front of the crystal face. Five mass-to-charge ratios ( m / q ) were recorded simultaneously by a microcomputer. Adsorbed carbon remaining on the surface after an experiment was measured by AES. Figure 1 compares the TPR spectra for benzene and di-hydrogen desorption from Ni(100) and Ni(ll1) following saturation coverage at 200 K. On the (100) face, benzene desorbed in a peak at 475 K with a long tail trailing to lower temperatures. Competing with molecular desorption is the thermal decomposition of benzene, which produces H2desorption between 350 and 500 K with the primary H2 peak at 500 K. After heating to 600 K, AES indicates a surface contaminated with carbidic carbon at a coverage of 0.20 monolayers. The benzene TPR spectrum following adsorption at 250 K on the (1 11) nickel crystal exhibited a sharp peak at 300 K with a broad shoulder trailing down to 450 K. Di-hydrogen is evolved in a broad peak centered at 450 K. *Author to whom inquiries should be addressed.
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After the temperature was raised to 600 K carbon coverage was 25% of a monolayer. Excluding benzene fragments formed in the mass spectrometer, no carbon-containing reaction products were detected from either surface. On both crystal planes of nickel, benzene decomposition, as evidenced by the di-hydrogen evolution and formation of adsorbed carbon, competed with molecular desorption. H2 was produced from benzene decomposition at roughly the same temperature for reaction of benzene on both the Ni( 11 1) and the Ni( 100) face. However, the TPD data show that, while molecular benzene desorbed at about 475 K on Ni(100), desorption of benzene on Ni( 1 11) occurred a t much lower temperatures (the main peak a t 300 K). Thus we may estimate a difference in molecular benzene binding energy’ on the two nickel crystal faces of about 44 kJ/mol. Extended Huckel calculations with a correction for repulsion have been performed previously by us for benzene on a 17-atom nickel cluster with (100) symmetry! Those results indicated that benzene bonds flat on the metal surface, most probably over a fourfold hollow site (see Figure 2a). In this study we have extended the EHT calculations to benzene on Ni( 11 1 ) in order to explain the difference in adsorption strength on the (1 1 1) and (100) surfaces. In most applications of EHT, the total energy of the system is given as the sum of the energies of the occupied molecular orbitals, Eh. Total energies calculated simply as the sum of EHT orbital energies omit internuclear repulsion and a u n t electron-electron repulsion twice and are only valid if the internuclear repulsion and the electron-electron repulsion contributions cancel one another. In general, and particularly for small internuclear distances, this is not the case. In our calculations, the total energy of the molecule adsorbed on the cluster is given as E, + E,, where E, is a repulsive term to compensate for the neglect of the difference in energy between the internuclear and interelectronic repulsion terms.5 A more detailed discussion of the (1) Schoofs, G.R. Ph.D. Thesis, Princeton University, 1986. (2) Benziger, J. B.; Preston, R. E. J . Phys. Chem. 1985, 89, 5002. (3) Redhead, P. A. Vacuum 1962, 12, 203. (4) Myers, A. K.; Benziger, J. B. Langmuir, in press. ( 5 ) Anderson, A. B. J. Chem. Phys. 1975,62,1187.
0 1987 American Chemical Societv
