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J. Phys. Chem. 1082, 86, 2698-2700
the composition of the chloride does not change. Earlier, Scott and colleagues3 suggested that "use of solvents in which the mixed valence phases have greater solubilities should allow the direct photochemical preparation of compounds having higher T"F+/TTF" ratios". It is also interesting that TTFCb.se does not dissociate in solution as shown by the fact that the UV spectrum of TTFBr0.71
is different from that of TTFCb.,. In ethanol solution the principal bands for TTFCb.BBare at 340,436, and 575 nm while those for TTFBr0,,, are at 345, 447, and 605 nm.
Acknowledgment. We acknowledge with thanks the support by the National Science Foundation MRL Program under grant No. DMR 76-80994.
Oxygen-I8 Exchange between Zeollte ZSM-5 and Water R. von BaHmooe and W. M. Meier' InstlM fiir Krlstalbgraphhs und Petfographie, ETH ZMch, 8092 Zijrkh, Swltzerlend (Received:December 1, 7987; I n Flnal Form: Merch 8, 1982)
Porotektosilicate frameworks of zeolites, including high silica materials, are considerably more reactive than we tend to think. This has been demonstrated by studiea of '80 exchange between high-purity samples of synthetic zeolite ZSM-5 in the ammonium form and liquid water at 95 "C. The investigation of the '80-exchange kinetics revealed a two-step reaction which can be interpreted in terms of hydroxyl groups exchangingrelatively rapidly (primary exchange) and oxygen atoms from T-0-T bridgea exchanging at a rate about 40 times slower (secondary exchange). Since the T sites in ZSMd contain mostly Si and only minor amounts of Al, the reported findings imply that not only S i U A l but also Si-0-Si bridges are cleaved under relatively mild conditions in the presence of water.
Introduction The work reported here was to provide information on oxygen exchange between water and the important high silica zeolite ZSM-5, including its Al-free end member. By combining l8O exchange with IR,' thermal gravimetric analysis (TGA), and temperature-programmed desorption (TPD) of ammonia, the number of hydroxyl groups from Br0nsted acid sites and from terminal silanol groups could be determined quantitatively. The primary exchange capacity of gel and low-crystallinity materials has been noted to exceed greatly that of pure zeolite samples.15 Thus, exchange reactions with '80 enriched water are also a sensitive means for detecting small amounts of these common impurities in samples of synthetic zeolites. The surfaces of a number of oxide catalysts have been studied by exchange a t elevated temperatures,2 but no exchange between '802and zeolites X, Y, and mordenite was observed below 600 0C.3 Adsorbed C'Q2 was reported to exchange l80only on heating the samples to temperatures of 200-500 0C.4 The rate of exchange at 300 "C could be correlated with the C7-cracking activity of the zeolite. More recently, extensive l8O exchange was reported to occur at 200-300 "C between C1802and zeolite X, while zeolite Y was found to exchange considerably leas.5 No Si-0-Si bonds were cleaved in these reactions since the exchange mechanism involved the formation of a surface carbonate species associated with the charge-balancing Na+ cations of the A104- tetrahedra. Exchange reactions between H2180 and silica glass: feldspars,' and X-ray amorphous catalyst materials8 have (1) Jacobs, P. A,; von Ballmoos, R. J. Phys. Chem. In press. (2) Novakova, J. Cat. Rev. 1970, 4, 77. (3) Antoshin, G. V.; Minachev, Kh. M.; Sevastjanov, E. N.; Kondratjev, D. A.; Newy, C. Z. Adu. Chem. Ser. 1971, No. 101,514. (4) Peri, J. B.J.Phys. Chem. 1975, 79, 1582. (5) Gensse, C.; Anderson, T. F.; Fripiat, J. J. J. Phys. Chem. 1980,84, 3562. (6) Roberta, J. P.; Moulson, A. J. Nature (London) 1958, 182, 200. 0022-3654l82/2086-2698$O1.2510
been studied before. Silica glass and feldspars were found to exchange '80 in the temperature range 360-1100 "C and pressures of up to 600 bars. The mobility of framework atoms in feldspars was thus dem~nstrated.~ Amorphous aluminosilicate cracking catalysts were observed to exchange l8O in two steps at 450 0C.8 The first was attributed to exchange of hydroxyl oxygen, and the second (slower) step to exchange of oxygen from T-O-T (T = Si, Al) bridges. ZSM-5 with Si/A1 ratios ranging from -10 to >4O0O9 has been synthesized from highly siliceous systems containing tetrapropylammonium (TPA) and sodium ions.1° The crystal structure and the structure-related properties of the porosilicate framework of ZSM-5 have been described Experimental Section High-purity ZSM-5 samples were crystallized from dilute systems containing around 15-20 mg of solid per cm3 of s~lution.'~Typical molar compositions of the synthesis mixtures were as follows: 21-36 Si02:A1(N03)3:8.5-10.5 NaOH:57-72 TPA(0H):lO-11 NH40H:2600-3300 H20425-533 C3H5(OH),. Crystallizationswere carried out at 200 "C for 5-6 days in 1-L autoclaves equipped with ~
~
(7) Wyart, J.; Sabatier, G.; Curien, H.; Ducheylard, G.; SGverin, M.
Bull. Sac. Fr. Minbral. Criatallogr. 1969,82, 387. (8) Oblad, A. G.;Hindin, S. G.; Mills, G.A. J. Am. Chem. SOC.1953, 75, 4096. Lago, R. M. J. Cotal. 1980, 61, 390. (9) Olson, D. H.;Haag, W. 0.; (10) Argauer, R. J.; Landolt, G. R. US. Patent 3 702 886, 1972. (11) Kokotailo, G.T.; Lawton, S.L.; Olson, D. H.; Meier, W. M. Nature (London)1978,272,437. (12) Kokotailo, G. T.; Meier, W. M. Chem. SOC.Spec. Publ. 1980,33, 133. (13) Olson,D. H.;Kokotailo, G.T.; Lawton, S. L.; Meier, W. M. J. Phys. Chem. 1981,85, 2238. (14) Wu,E. L.; Lawton, S. L.; Olson, D. H.; Rohrman, A. C.; Kokotailo, G.T. J. Phys. Chem. 1979,83,2777. (15) von Ballmoos,R. Ph.D. Thesis, ETH, Zurich, 1981. "The lSO-
Exchange Method in Zeolite Chemistry: Synthesis,Characterization and Dealumination of High Silica Zeolites", in "Texte zur Chemie und Chemietechnik"; Salle & Sauerlinder: Frankfurt, 1981.
0 1982 American Chemical Society
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Exchange between ZSM-5 and Water
The Journal of Physical Chemistry, Vol. 86, No. 14, 1982
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-Y
d
d
%
04 0
1
2
!
-
3
Lo
5
4
Al/uc
Flgure 2. "0 exchange of gel-free ammonium ZSMQ and water at 95 OC in 2 days. The slope amounts to 0.95 '*Oox per Ai while the surface exchange capacity of Ai-free samples corresponds to about 0.7 per unit cell. (Dotted line refers to 1 leOBxper AI.)
TABLE I: I8O Exchange, Infrared (IR), Temperature Programmed Desorption (TPD) of Ammonia, and Thermal Gravimetric Analysis (TGA) Data of ZSM-5Containing Different Amounts of A1 AI I unit cell
a
"Oexl unit cell (i0.15)
0 0.63,0.90 2.5 3.16 2.5 3.30 3.5 3.87 3.6 3.91 4.1 4.83 4.1 4.37 4.7 4.52,4.59 Reference 19.
OH groups per unit cell based on
TPDa 0
TGA
2.5 3.4 3.8 4.1
2.3 3.5 3.8 4.0
2.1 3.1 4.2
4.7
5.0
3.9
IRa
0
0
faster primary exchange reaction involves terminal hydroxyl groups (at the crystal surface) in addition to oxygens of B r ~ n s t e dacid sites in the hydrolyzed ammonium zeolite.ls The slower reaction (secondary exchange) must be due to exchange of bridging oxygens and therefore involves breaking of Si-0-Si(A1) bonds. The two reaction steps could be separated and apparent first-order rate s-l for the primary and 5 X constants of 2 X for the secondary exchange at 95 OC were determined. The corresponding half-times of these exchange reactions are 10 (f0.2) and 470 (h40) h, respectively. The products of the exchange reactions were characterized by Guinier type XRD and SEM. No alterations with respect to the parent samples could be detected. Subsequent 180-exchangestudies were carried out with ZSM-5 samples containing different amounts of aluminum and suitable reaction conditions were chosen such that the observed exchange could be essentially assigned to primary exchange capacities. Overlap of the two reaction steps was considered to be minimal at 95 OC and reaction times of 2 days. Thus, samples of ammonium ZSM-5 containing no detectable amount of gel by conventional methods were found to exchange oxygen to the extent of one per Al atom, plus an additional 0.3 to 0.9 exchange sites per unit cell
(16) von Ballmoos, R.; Meier, W. M. Nature (London) 1981,289,782. (t7) D2180 was used because D20 has a higher '*Ostarting concentration than H 2 0 and thus facilitates l80enrichment.
(18) The hydrolysis of the NH4+cation should be regarded as a transition state only because the free NH, concentrationwas found to be very low. Protons (or D+) exchanging positions in the AIOl tetrahedra are therefore trapped by NH, in the channels. For structural reasons the reconstituted NH,+ leads, when hydrolyzed, to an OH configuration on the same Al-0-Si bond again. We thus observed only one 180,.per A1 in NH4-ZSM-5. In contrast, experimente with H-ZSM-5 (same samples and conditions) yield the expected 4 1 8 0 e = per Al. (19) Jacobs, P. A., personal communication.
(Figure 2) attributed to sites on the external surface. These latter figures compare well with calculated amounts of surface hydroxyl groups which would about equal a maximum of 0.5 to 1exchange site per unit cell for crystals in the 5-15-pm size range. These 180-exchangemeasurements were supplemented by IR,TPD, and TGA studies in order to examine whether or not the one exchangeable oxygen per Al could be related to framework hydroxyl groups, and the additional exchange to silanol groups on the crystal surface. The results are summarized in Table I. IR measurements (based on integrated absorbance at 3600 cm-I), TPD experiments (based on cumulative NH3 evolution of temperature programmed desorption), and TGA (recording amount of chemical water formed between 500 and 1150 "C) all supply substantial evidence for a one-to-one relationship between Brcansted acid centers and Al content in highpurity ZSM-5. Reasonably good agreement can also be noted in Table I with respect to l80exchange if the data are corrected for surface exchange (by substracting 0.7 l8oeX per unit cell). Comparable l80-exchangeexperiments were also carried out with (ordinary) ZSM-5 samples containing some gel and with X-ray amorphous silica. The former were found to exchange over five times the amount of l80compared to gel-free samples of ZSM-5. Chromatography gel exchanged 39% and Aerosill5% of the oxygen in 2 days at
95 "C. This demonstrates the sensitivity of the 180-exchange method to detect gel impurities in zeolite samples. The reported 180-exchangestudies of ammonium ZSM-5 and water supply ample evidence that not only Si-0-A1 but also Si-0-Si bridges in zeolites are cleaved at measurable rates even under mild conditions at temperatures as low as 95 "C (see Figure 1). This finding is very significant for an improved understanding of reactions of zeolite frameworks at moderate temperatures and the role of water in such procewa. Under steaming conditions (600 "C and 1bar of water vapor) Al-free ZSM-5 was found to exchange 40% of its framework oxygen atoms in 1h, while 70% exchange was recorded in the case of a sample of ZSM-5 containing 4.1 A1 per unit cell. This throws new light on the reactivity of Si-0-Si bridging oxygens which (at least in zeolite chemistry) have all too frequently been assumed to be "inert". On the other hand, point defects (vacancies) which are bound to occur in silicate frameworks could well turn out to be crucial in determining the mobility of framework atoms.
Acknowledgment. We thank Professor P. Baertschi for his continued interest and practical support of this work. Thanks are also due to Mr. A. Rub for carrying out TGA analyses, and to Dr. P. A. Jacobs and Dr. Lynne McCusker for helpful discussions. This study was supported by funds from the Swiss National Science Foundation.
An Infrared Reflection Spectroscopy Study of Oriented Cadmium Arachidate Monolayer Films on Evaporated Silver D. L. Allara' Bell Laboratories, Murray Hili, New Jersey 07974
and J. D. Swalen IBM Research Laboratories. Sen Jose, California 95193 (Received: December 3, 1981)
The infrared spectra of one to ten monolayer assembliesof cadmium arachidate have been measured by glancing angle reflection spectroscopy. No frequency shifts were noted for the bands with increasing number of layers and the intensities scaled almost linearly. The observed and assigned bands exhibited a very different intensity pattern from calculated spectra based on bulk cadmium arachidate optical constants. We explain our results on the basis of oriented monolayer assemblies with the alphatic chains normal to the surface. Our data allow for a small angle of tilt of the chains away from the surface normal and suggest some twisting of the first few methylene groups adjacent to the carboxylate group.
Introduction Organized monolayer assemblies have been extensively studied since the first reports by Langmuir and Blodgett.'+ A variety of techniques has been used to characterize the structures of these films, in particular, assemblies of multilayers many monolayers thick. However, there are very few reports of characterization of films one to several monolayers in thickness. X-ray photoelectron spectroscopy (XPS) studies have been reported recently for single
monolayers of cadmium arachidate (Cd(CH3(CH2)&02H),)on silver! gold,' indium! and glass8 substrates. An infrared spectrum for a monolayer of cadmium arachidate on a glass substrate in contact with an internal reflection element has been reported.8 It is generally assumed from known values of the molecular packing density that the first monolayer of cadmium arachidate on silver is oriented with the alkyl chain axis perpendicular to the substrate surface plane. The XPS studies are consistent with this
(1)I. Langmuir, J. Am. Chem. Soc., 39,1848 (1917). (2)K.B. Blodgett, J. Am. Chem. SOC., 57, 1007 (1953). (3)K.B. Blodgett, Phys. Reu., 55,391 (1939). (4)I. Langmuir, Proc. R. SOC.(London),Ser. A, 170,15(1939). (5)See G. Gaines, 'Insoluble Monolayers at Liquid Gas Interfaces", Wiley, New York, 1966,for more references.
(6)C.R. Brundle. H. Howter, and J. D. Swalen, J. Chem. Phys., 70, 5190 (1979). (7)D.T.Clark.and Y. C. T. Fok. J. Electron Spectrosc. Relat. Phenon., 22, 173 (1981). (8)T.Ohnisi, A. Iahitani, H. Ishida, N. Yamamoto, and H. Tsubomum, J. Phys. Chem., 82, 1989 (1978).
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0 1982 American Chemical Society
