9448
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two C-H bonds, then for purposes of comparison with the methoxide spectra the relative intensity of the u a mode decreases to 3.3 times that of the uCH2mode. Clearly, the uco mode does not dominate the spectrum in the same way that it does in the methoxides on Cu surfaces where the CO bond is oriented close to the surface normal. This observation supports the model in which the C-C bond of ethoxide is normal to the surface, resulting in the C - 0 bond being tilted at 70°. The shift in the uCH frequency on formation of the fluorinated alkoxides occurs for all three molecules. This could be due either to reorientation of the C-H bonds such that they are interacting with the surface or to a change in C-H bond strength resulting from the formation of the alkoxide. While the HREEL spectra of trifluoroethanol reveal structural change on formation of the trifluoro ethoxide, the spectra of the longer chain fluorinated alcohols are not as easily analyzed for structural information. The surface chemistry that we expect on the basis of desorption measurements is clearly observed for both the fluoropropanol and fluorobutanol. Other than loss of the O-H modes, however, the changes on going from the alcohol to the alkoxide are minor, and we must assume that there is no change in orientation of the alkyl chain with respect to the surface. This is consistent with our observation for the hydrocarbon alcohols on the Ag( 1 IO) surface. Although the transformation from methanol to methoxide results in a clear increase in the CO stretch intensity, the longer chain hydrocarbon alcohols exhibit little change in the spectra on going from the alcohol to alkoxide. Quantitative XPS measurements have demonstrated that the alkyl chains of the hydrocarbon alcohols are interacting with the surface, and we believe that this is also the case for the alkoxide~.~ The XPS measurements were made by adsorbing saturated monolayers of the hydrocarbon alcohols on the clean Ag( 1 10) surfaces and measuring the C( 1s) and O( 1s) photoemission intensities. The relative intensities were then compared for ethanol through pentanol with the predictions of two structural models for the monolayer. In one the alcohols formed an assembled layer with
the alkyl chains oriented away from the surface while in the other the alkyl chains were oriented parallel to the surface. Adsorbate spacings were predicted from molecular dimensions. The dependence of the carbon and oxygen coverages on chain length clearly matched the predictions of the model having the alkyl chains oriented parallel to the surface. As in the case of the longer chain fluorocarbon alcohols, there is very little change in the vibrational spectrum of the hydrocarbon alcohols on formation of the alkoxide.6 It appears that in the cases of the longer chain fluorinated alcohols deposited from the vapor phase the alkyl chains of both the alcohols and alkoxides are interacting with the surface rather than orienting themselves away from the surface as would be the case in Langmuir-Blodgett films. Conclusions
The basic chemistry of the fluorinated alcohols (C,,F2,+,CH20H) on the Ag(1 IO) surface is the same as that of the hydrocarbon alcohols. At monolayer coverages they are adsorbed reversibly on the clean surface but deprotonate on the preoxidized surface to form adsorbed alkoxides. The alkoxides then decompose during heating by hydride loss at the @-carbon atom, producing the aldehyde. From the point of view of their potential tribological applications, the important property of these materials is that fluorination of the alkyl chain increases the kinetic barrier to hydride loss, making the fluorinated alkoxide much more stable on the surface than its hydrocarbon analogue. Trifluoroethanol is adsorbed on the clean Ag( 1 IO) surface with its C-C bond tilted away from the surface normal. Formation of the alkoxide on the preoxidized surface results in reorientation of the molecule such that its C-C bond is aligned along the surface normal.
Acknowledgment. This work was supported by the Air Force Office of Scientific Research under Grant AFOSR 89-0278. A.J.G. holds a fellowship from the David and Lucile Packard Foundation and is a Sloan Foundation research fellow.
I ntrazeolite Semiconductors: 23NaMAS NMR, TI+ Luminescence Quenching, and Far-IR Studies of Acid-Base Precursor Chemistry in Zeolite Y Lisa McMurray, Andrew J. Holmes, Alex Kuperman, Geoffrey A. Ozin,* and Saim Ozkart Lash Miller Chemical Laboratories, 80 St. George Street, University of Toronto, Toronto, Ontario, Canada M5S IAl (Received: May 20, 1991; In Final Form: June 28, 1991)
Proton-loaded zeolites, prepared from fully dehydrated zeolites and gaseous, anhydrous Bransted acids, represent an important step in the synthesis of intrazeolite semiconductor quantum supralattices. Adsorption-induced 23Na MAS NMR chemical shifts, far-1R Na+ and TI+ translatory mode frequency shifts, and TI+ luminescence quenching effects were chosen as probes of cation-anion interaction in these materials. Samples of zeolite Y with various loadings of TI+ were prepared via aqueous ion-exchange techniques. The samples were characterized by powder X-ray diffraction and far-IR spectroscopy. Luminescence measurements revealed TI+ excitation and emission bands in the UV spectral region. Exposure of thallium TI(1) zeolite Y to anhydrous HBr quenched the luminescence intensity. The intensity quenching followed Stern-Volmer quenching kinetics. Preliminary luminescence lifetime studies of this system supported a static ion pair quenching model. Compelling additional evidence in favor of cation-anion pair formation comes from the observation of a-cage site-specific *)Na MAS NMR chemical shifts in HBr/Na%Y compared to virgin NaFY. The relevance of these observations for proton-loaded zeolite Y to the acid-base precursor chemistry involved in the synthesis of semiconductor nanostructures encapsulated in zeolite Y is critically discussed.
Preamble By taking advantage of the nanometer dimension spatial constraints of the cavity and channel reaction chambers of different zeolite structure types, it has recently proven possible to organize, one atom a t a time, uniform arrays of single-size quantum and dielectrically confined aggregates of the constituents that normally
make up bulk semiconductors.’ When isolated and electronically decoupled from one another, these nanostructures are considered to be zero- and one-dimensional quantum-confined dots and wires, respectively (QD’s, QW’s). Coupling these quantized objects together in the form of an ordered superstructure effectively creates a quantum supralattice (an “expanded s e m h ” t o r ” in the
‘On leave of absence from the Chemistry Department, Technical University Ankara, Ankara, Turkey.
(1) Ozin, G. A.; Stein, A.; Kuperman, A. Angew. Chem. Ado. Mater. 1989, 101. 373. Stucky, G. D.; MacDougall, J. Science 1990, 247, 669.
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original terminology of Matt*) of great current interest in quantum electronics and nonlinear optic^.^ The act of essentially dismembering a traditional, close-packed semiconductor lattice and reassembling its atomic constituents in the form of tiny, monodispersed aggregates organized entirely on the inside surface of a crystalline microporous host lattice, such as a zeolite, can be considered to be a form of t ~ p o t a x y . ~By viewing the process of creating intrazeolite quantum dots, wires, and supralattices in this way, one can begin to appreciate the similarity of this kind of topotaxy to the procedure of epitaxial growth, whereby quantum-confined semiconductor layers and superlattices are deposited on, and matched to, the atomic grid pattern found on the surface of two-dimensional planar ~ubstrates.~ Materials of the latter type are most often accessed through what are called planar engineering methods, typified by CVD, MBE, and MOCVD.6 Such two-dimensional quantum-confined layer structures and superlattices thereof can subsequently be “machined” into one- and zero-dimensional quantum-confined wire and dot patterns, respectively, using nanolithographic lateral engineering techniques employing, for example, finely focused electron and ion beams. State-of-the-art nanolithography can produce such patterns having dimensions of the order of 300 A, with a size dispersion of about 50 A.6 By contrast, intrazeolite topotactic synthetic methods can produce high population densities of “single-size” quantized nanostructures, locked into ordered arrays, within a dielectric host lattice. Chemical approaches that are currently employed to encapsulate the components of bulk semiconductors inside the void spaces of zeolites include aqueous and melt ion exchange (CdS, AgX), metal organic chemical vapor deposition (Gap), vapor-phase impregnation (Se), and metal carbonyl phototopotaxy (W03).19394 Imbibing the constituents of bulk semiconductors inside zeolite cavities and channels via the so-called “ion-exchange” route appears superficially to be a rather straightforward process.’ Apparently, all that appears to be involved is a single interchange of, for example, mundane Na+ cations of the “as-synthesized” zeolite with more interesting M*+cations like Zn2+, Cd2+, and Pb2+. By analogy with aqueous acid-base chemical equilibrium methodology, these extraframework cations are contacted with reagents like H2S and H,Se, the idea being to create spatially, quantum, and dielectrically confined aggregates of the respective semiconductor components, namely, (MS), and (MSe),. At this level of understanding the process is deceptively simple. In reality a complex sequence of intrazeolite transformations are involved in the passage from cationic and anionic precursors to cluster product. This kind of “intrazeolite acid-base” chemistry aimed a t encapsulating the components of bulk semiconductors inside zeolite host lattices has a very interesting history, which probably is best traced back to the work of Barrer and co-workers8 with “occluded salts”. The forces that drove Barrer’s work in this area were quite different from those currently aimed at advanced zeolite materials science. These endeavors were usually directed at zeolite catalysis and gas separation science, as well as application areas where controlled release of zeolite-encapsulated chemicals was a desirable attribute.B Gas, aqueous, and melt ion-exchange methods were all employed and quite a wide range of zeolite-imbibed salts were (2) New Directions in Solid Slate Chemistry; Rao, C. N. R., Gopalakrishnan, Eds.; Cambridge University Press: New York, 1986. (3) Ozin, G . A.; Kirkby, S.;Meszaros, M.; Ozkar, S.; Stein, A.; Stucky, G. D. Intrazeolite Semiconductor Quantum Dots and Quantum Supralattices: New Materials for Nonlinear Optical Applications. In Marerials for Nonlinear Optics; A C S Symposium Series; American Chemical Society: Washington, DC, 1990; p 554 and references cited therein. (4) Ozin, G.A.; Ozkar, S. J . Pfiys. Cfiem. 1990, 94, 7556. Ozin, G. A.; Ozkar, S. Adu. Mater., in press. Ozin, G . A.; Gil, C. Cfiem.Reu. 1989, 89, 1749. (5) Ploog, K. Angew. Cfiem.,Int. Ed. Engl. 1988, 27, 593. (6) Nanosrrucrure Physics and Fabrication; Reed, M. A., Kirk, W. P., Eds.; Academic Press: New York, 1989. (7) Wang, Y.; Herron, N.; Mahler, W. Inorg. Cfiem. 1989,28,2914 and
references cited therein. (8) Barrer, R. M. Hydrothermal Chemistry of Zeolites; Academic Press: London, 1982.
subject to Barrer’s early studies.8 Many of the nanostructures produced in the 1960s by these methods were considered by Barrer at that time to be ‘curiosities which might find some as yet unknown uses in the future”. What foresight! This kind of occluded-salt zeolite chemistry is really the “forerunner” of present day attempts to create intrazeolite QDs, Q W s , and QS’s built from the constituents of bulk semiconductors. The driving forces are different, namely, quantum electronics and nonlinear optics,’~~ but the chemistry is similar. If one is to target reduced-dimension quantum nanostructures of this type for applications in semiconductor physics, then one must begin to quantify the chemistry and learn how to control the assembly process to at least the specifications which have come to be the industrial norm for quantum wells and superlattices.5*6 Some clues from early work which assists in the understanding of intrazeolite acid-base chemistry related to the formation of occluded salts are listed below: (a) The observation of irreversible isotherms for the adsorption-desorption of anhydrous hydrogen halides in various zeolite types (Barrer).9 (b) The dehydrohalogenation of n-propyl chloride in various zeolite types to yield encapsulated propylene and hydrogen chloride (Angell).lo (c) The Si/AI ratio dependence of H2Sadsorption into a range of sodium faujasites, to yield either molecularly adsorbed H2S (Si/AI > 2.5) or ionized H2S (Bransted acid and hydrosulfide sites, Si/AI < 2.5) (Karge).” (d) The in situ generation of Bransted acid and telluride dianion sites from the vapor phase impregnation of H2 and Te (effectively H,Te) into Na,Y (Olson).I2
Introduction With this as background knowledge and the renewed interest in assembling, one atom at a time, intrazeolite quantum-confined nanostructures out of the components of bulk semicondu~tors,~J we set out on a program of research focused a t a step-by-step comprehension of intrazeolite acid-base precursor chemistry. Our initial studies in this area dealt with the spectroscopy, diffraction, and kinetics of the adsorption-desorption, polarization, ionization, charge separation, solvation, and structural properties of anhydrous HX (X = C1, Br, 1)13 and H2SI4in the archetype (fully dehydrated) systems Na5,+,H,Y ( n = 0-56), MS6Y ( M = Li, Na, K, Rb, Cs), Si02-Y, and ALPO-5. Our results for the acid-base chemistry of HX in Na#Y (denoted NaOZ) can be succinctly delineated in the following reaction sequence: mHX, nNaOZ 9 m(HX),,nNaOZ a (1) (11) mX-NaOZ,mH+OZ,(n - m)NaOZ a (111) m(NaX),mHOZ,(n - m)NaOZ
+
f,TW ’ I
The occluded-salt denoted m(NaX) in stage IV of the above reaction scheme represents the “final product” of at least three preceding stages, most simply described as vapor-phase impregnation (I), adsorption (II), and polarization-ionization-charge separation (111). An analogous scheme is found for H2S, except that because of the dibasic nature of H2S, an extra stage beyond HS- can be introduced which involves S2-according to mHS-NaOZ,mH+OZ,(n - m)NaOZ G (IIIA) mS2-NaOZ,2mH+OZ,(n- m)NaOZ (IIIB)
(9) Barrer, R. M.; Kanellopoulos, A. G. J . Chem. SOC.A 1970, 766. Barrer, R. M.; Kanellopoulos, A. G. J . Cfiem. Soc. A 1970, 775. (IO) Angell, C. L.; Schaffer, P. C. J. Pfiys. Cfiem. 1965,69, 3463. Angell, C. L.; Howell, M. V . J . Pfiys. Cfiem. 1970, 74, 2737. Angell, C. L.; Schaffer, P. C. J . Phys. Chem. 1966, 70, 1413. ( I 1) Karge, H. G.;Rasko, J. J . Colloid Interface Sci. 1978, 64, 522. (12) Olson, D. H.; Mikovsky, R.J.; Shipman, G.F.; Dempsey, E. J. Catal. 1972, 24, 161. (13) Ozin,,,G. A.; Ozkar, S.; Stucky, G. D. J . Pfiys.Chem. 1990.94.7562. Ozin, G.A.; Ozkar, S.; McMurray, L. J . Phys. Chem. 1990,94,8289, 8297.
9450 The Journal of Physical Chemistry, Vol. 95, No. 23, 1991
McMurray et al.
Figure 1. Proposed model for adsorption, ionization, and charge separation of anhydrous HX in dehydrated zeolite Y, HX/Na5,Y (ref 13).
AE
The thermodynamic and kinetic factors at work in these systems are only just beginning to be u n d e r ~ t o o d . ~ ~The - ' ~ extent of ionization-charge separation appears to bear a close relationship to the acid strength of the adsorbate (e.g., HBr versus H2S versus PH3) and the magnitude of the local electric fields associated with "accessible" extraframework cations. The latter is found to intimately depend on the framework oxygen charge density and the ionic potential of the cations. In the present day context of intrazeolite semiconductors, an especially pertinent transformation in the above scheme concerns the "charge-balanced" interconversion of "pairs" of extraframework cations and anions (111) into an occluded salt (IV). Discovering intimate details of the (Ill)-( 1V) process involves delving into structure bonding and energetic dynamical questions concerning the association of coexisting intrazeolite cations, protons, and anions. All that we know so far is that the "additional" protons that result from ionization of HX in Nasty, are cy- and 0-cage located and behave like normal Br~lnstedacid sites (e.g., bonding, solvation, acidity, spectroscopy).13 The extraframework cations appear to tenaciously adhere to their original sites of occupancy in the pristine zeolite. Only circumstantial evidence suggests that the extraframework anions are loosely interacting with cy-cage cations, as "intimate cation-anion pairs" (far-IR adsorption induced cation translatory mode frequency shiftsI3 and blocking of specific molecule adsorptionI0). This proposal appears quite reasonable in view of Olson's X-ray structure analysis of the Na5,Y/H2/Te system, which depicts the presence of neighboring Na+Te2- ion pairs.I2 Clearly a better understanding of the (111)-(IV) interconversion would be greatly assisted if one could precisely elucidate the nature and location of anion sites within the zeolite. In what follows we describe two new experiments which serve to directly achieve this goal. The first involves the observation of 23Na MAS N M R adsorption induced chemical shifts in the fully anhydrous HBr/Nas6Y system. The second involves the observation of TI+steady-state and time-resolved luminescence quenching in the fully anhydrous HBr/T1,Nas6-,Y system. Thallium(1) Zeolite Y, Luminescence Probe. Proton loading of zeolite Y is accomplished by treatment of the fully dehydrated zeolite with the vapor of an anhydrous Bronsted acid, such as a hydrogen halide HX, where X = CI, Br, I.I3 This results in protonation of the aluminosilicate framework such that the protons so formed are essentially indistinguishable from normal extraframework charge-balancing protons. The cation distribution remains essentially unchanged. The proposed mechanism for this process, shown in Figure 1, involves absorption of HX into the zeolite, ionization of H X induced by the large electric field associated with the "half-naked" cation (which is coordinated by framework oxygens on only one side), and separation of the two oppositely charged H+/X- species. However, previous evidence concerning the location of the halide anion in the archetypal NaS6Y/HX proton-loaded zeolite system was not definiti~e.'~ Rietveld refinement of powder X-ray diffraction (PXRD) patterns, mid-infrared (IR) probing of OH stretching modes, and far-IR probing of cation vibrational mode^^^,'^ have all been consistent with a contact ion pair model but have failed to determine conclusively that the anion of the acid HX is associated with the extraframework cation and not somehow with the framework. As described earlier, knowledge of cation-anion interactions in this system is a crucial step toward an understanding of the mechanism (14) Ozin, G. A,; Ozkar, S.; Bowes, C.; Steele, M., unpublished work.
(IS) Ozin,G. A.; Orkar, S.: Stucky, B. D.; Harrison, W., unpublished work.
Figure 2. Energy level summary scheme of TI+ spectroscopy (ref 18).
of intrazeolite semiconductor nanocrystallite formation. Therefore it was considered necessary to develop alternative probes of these interactions. One of the methods chosen for this purpose is the luminescence of thallium(1). A luminescence technique was chosen, for example, over Mossbauer spectroscopy (e.g., Sn2+,Fez+) and electron spin resonance (e.g., Cu2+, Mn2+) because of the desirability and availability of a suitable "clone" monovalent cation; hence the results obtained from the probe system are believed to be comparable to those from the archetypal Nas6Y/HBr system. Thallium(1) also has several practical advantages: its spectroscopy is relatively simple (the ground state has a 6s2 electronic configuration), it is easily ion-exchanged into zeolite Y without any complications such as dealumination or metal hydroxide formation during dehydration,I6 and it has the potential to be used as a magic-angle spinning N M R probe since the 205Tl(spin I = nucleus has a high natural abundance, high sensitivity, and suitable relaxation times." An energy level scheme appropriate for TI+ is shown in Figure 2. Thallium is a heavy atom (Z = 81), exhibiting strong spin-orbit coupling, and hence L and S are no longer g o d quantum numbers. Therefore, the selection rules AL = 0, f l and AS = 0 break down, and only the selection rule AJ = 0, f l (except J = 0 to J = 0) applies. All of the transitions shown are parity-allowed. By labeling the states as the correlated Russell-Saunders states, one determines that the 3P, 'So and the 'Pi 'So excitations are allowed, while 3P0 'So and 3P2 'So are forbidden. In the free ion, the 3PI ISoabsorption occurs in the ultraviolet at 181 nm.18 As a probe of extraframework cation-anion interactions in protonated zeolite Y, the luminescence of thallium(1) was expected to behave in one of two ways. First, the energy levels of dIos2ions such as T1+,PbZ+,and Bi3+ are well documented to be very sensitive to their chemical en~ir0nment.I~This is due to the nephelauxetic effect, Le., outer shell expansion due to electron density donation. Heuristically speaking, due to the greater spatial extent of s orbitals with respect to p orbitals, the 6s2 configuration expands more on electron donation than does the 6sp, and the former is therefore more highly screened from the n ~ c l e u s . ' Both ~ the ground- and excited-state levels are destabilized; however, the 6s2 increases more in energy than does the 6sp. The result is a net decrease in the energy difference between the ground state and the excited state, and therefore a red-shift in both absorption and emission is expected upon increased electron donation to a dIos2 ion. Extensive use has been made of such nephelauxetic shifts in probing the extent of electron donation in glasses with TI+,Pb2+, and Bi3+.Iw2* The
- -
-
(16) Breck, D. W. Zeolite Molecular Sieves; Wiley: New York, 1984. (17) Engelhardt, G.; Michel, D. High Resolurion Solid-State NMR of Silicates and Zeolites; Wiley: New York, 1987. (18) Teegarden, K. In Luminescence of Inorganic Solids; Goldbcrg, P., Ed.; Academic Press: New York, 1966; and references cited therein. (19) Alexander, M. N.; Onorato, P.0. K.; Struck, C. W.; Tasker, G. W.; Uhlmann, D. R. J. Non-Crysr. Solids 1987, 91, 63. (20) Alexander, M. N.; Onorato, P. 1. K.; Struck, C. W.; Rozen, J. R.; Tasker, G. W.; Uhlmann, D. R. J. Non-Crysr. Solids 1986, 79, 137.
The Journal of Physical Chemistry, Vol. 95, No. 23, 1991 9451
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-
3P, ISoabsorption in these materials generally occurs in the range 200-270 nm.I9 Therefore, electron donation from the halide anion in question to extraframework TI+ in a proton-loaded zeolite system might be expected to produce halide-dependent TI+ excitation and emission red shifts. Alternatively, one might expect quenching of the TI+ luminescence to occur; if so, luminescence intensity data combined with lifetime measurements would provide useful information on the nature of the quenching species.23 In the first part of this study, the question of extraframework cationanion interaction in proton-loaded zeolite Y has been addressed by using luminescence spectroscopy, together with intensity and lifetime quenching experiments involving the charge-balancing cation thallium(1) as a sodium cation Wonen. Sodium(1) Zeolite Y, 23NaMAS NMR Probe. The 23Na nucleus has spin I = 3/2, 100% natural abundance, and a large quadrupole moment. Even at the highest magnetic field strengths and under MAS conditions, the second-order quadrupolar line shape of Z3Na, although narrowed, is not reduced to a single In dehydrated NaS6Y, four distinct cation sites have been observed by X-ray crystallographyz5and far-IR spectroscopy:6 II(29.4-32.2), 1 (7.1-7.7), I' (13.4-19.5), 111 (MS),where the numbers in parenthesis represent the X-ray calculated population range of Na+ cations determined by different authors.25 In the case of fully dehydrated NaS6Yone observes in the 23Na MAS NMR spectrum an intense asymmetrically shaped 23Na resonance centered around -12 ppm, which arises from a convolution of signals due to Na+ cations mainly in sites 11, I, 1', and Ill (see later).27 The most prominent narrow component centred a t -12 ppm has been ascribed to the hexagonal prism Na+ site (six coordinate, essentially Ohsymmetry, smallest second-order quadrupole line broadening). All other Na+ sites have lower symmetries (C,,, Ch). They are expected to exhibit second-order quadrupole line broadening and appear to fall within the high-field shoulders (-22 and -38 ppm) of the main 23Na re~onance.~' The outcome of absorbing various species, like PMe3 and W(CO)6, into Na5,Y on the aforementioned 23Na MAS N M R spectrum has also previously been studied.z7 In essence one finds that one of the components of the high-field shoulder (around -22 ppm) always shows the greatest response to the presence of adsorbates in the a-cage. Thus charge transfer from the adsorbate to an extraframework a-cage Na+ cation may serve to (a) increase the shielding of those Na+ cations accessible to the act of adsorption, (b) decrease the second-order quadrupole line broadening, and (c) cut down or eliminate Na+ site exchange processes. The "primary" effects of adsorption are then expected to be a shift of the a-cage 23Na N M R resonances to higher fields with a concomitant enhancement of their intensity. This essentially appears to be the situation observed in practice.27 In the second part of this study, the question of extraframework cation-anion interaction in proton-loaded zeolite Y has been addressed using 23Na NMR spectroscopy with the objective of searching for halide anion induced z3Na+chemical shifts and intensity changes of the above type. Experimental Section Materials. Zeolite Y, lot number 12074-33, was obtained from Union Carbide, Tarrytown, NY. Elemental analysis of this Iron material gave the composition Na~(A10z)M(Si02)lx~xHz0. was found to be present as an impurity at a concentration of 0.14 dry wt % Fe20,. Thallium nitrate was obtained from the Aldrich Chemical Co., Inc., Milwaukee, WI, and was of 99.9%purity. Anhydrous hydrogen bromide was of research grade and was obtained from Canadian Liquid Air. It was used without any attempt to purify it further. ~~
(21) Ghosh,A. K. J . Chem. Phys. 1966.41. 535. (22) Duffy. J. A.; Ingram, M. D. J . Chem. Phys. 1952, 20, 124. (23) Demas. J . N. ExcitedSrare Liferime Memuremenrs; Academic Pms: New York, 1983; and references cited therein. (24) Samoson,A.; Lippmaa, E.; Pines, A. Mol. Phys. 1988.65, 1013 and references cited therein. (25) Fitch, A. N.; Jobic, H.; Renouprez, A. J. Phys. Chem. 1986,90, 131 I . (26) Ozin, G . A.; Godber, J. J . Phys. Chem. 1988, 92. 2841. (27) Ozin, G . A.; 6zkar, S.; Macdonald, P. J . Phys. Chem. 1990, 94,6939.
Preparation of TI,Nas,Y. Samples of zeolite Y with nominal compositions TI,NaS6,Y (n = 1, 8, 16, 32, 40, 48, 56) were prepared by standard aqueous ion-exchange methods.16 For all preparations, the solution volume to zeolite mass ratio was 200 mL:l g. Assuming complete exchange, the correct number of equivalents of TIN03 for the desired zeolite concentration was dissolved in the required volume of deionized water. After it was stirred overnight at room temperature, the mixture was filtered and the zeolite washed with deionized water until no trace of TI+ could be detected in the filtrate by the addition of a few drops of 0.1 M KI solution [Tl+(aq) I-(aq) TlI(s) (yellow)]. A sample of nominal composition n = 40 was prepared similarly but with an excess of TIN03 (9X the required number of equivalents) in solution. A high-exchange sample, nominally n = 48, was prepared using a similar volume to mass ratio, but with a very concentrated TIN03 solution (1.5 M) and at an elevated temperature (100 "C). This procedure was carried out twice in order to achieve a fully exchanged sample, nominally n = 56. Elemental analysis of the T1,Na5&,Y samples, henceforth denoted TI,Y, was carried out at Galbraith Laboratories, Inc., Knoxville, TN. The nominal and observed values are summarized below
-
+
nominal ( n ) 1 8
obsd (n) 0.49
nominal ( n )
5.4
48 56
16
10.0
32
19.2
40
&sd (n) 42.8 52.3 55.6
From these figures one determines that the extent of TI+ ion exchange was rather less than the nominal value in the cases where the precise number of TIN03 equivalents were supplied. All samples will henceforth be referred to by their observed compositions, rounded off in the interest of simplicity. Cells. In situ mid-far-IR, mid-IR/UV-vis, and luminescence lifetime cells used in this study were identical to those described previously from this laboratory.2* Pretreatment of Zeolites. Self-supporting zeolite wafers were prepared by grinding 40-60 mg of zeolite and pressing it at 7 tons for a few seconds. Wafers thus prepared and powders were dehydrated with the following schedule, using an Omega Series CN-2010 temperature controller: 0.5 h from 25 to 100 OC, 1 h at 100 OC, 3 h from 100 to 450 OC, and 1 h at 450 OC. Spechvwcopy and Lifetime Measurements. Mid-IR spectroscopy was performed on a Nicolet 20 SXB FT instrument. A Nicolet 20F FTIR instrument was used for measurements in the far-IR region. Powder X-ray diffraction data were obtained on a SCINTAG PADX X-ray diffractometer. Cu K a radiation was used, with a scan rate of 2 deg/min and a resolution of 0.03O. Silicon powder was used as the internal reference. UV-vis absorption spectra were taken on a Perkin-Elmer 330 spectrophotometer. Steady-state luminescence spectra were obtained on a Perkin-Elmer MPF-44B fluorimeter. Lifetime measurements were taken on an apparatus employing a frequency-doubled dye laser pumped by a nitrogen laser and a multichannel analyzer (MCA) in the multichannel scaling (MCS) mode. A Photochemical Research Associates (PRA) system 4000 comprised the counting electronics. This apparatus has been described in detail previously.z8
Results and Discussion Preparation and Characterization of TI,Y. Ion Exchange. The series of thallium zeolites TI,Y (n = 0.5, 5, 10, 19, 43, 52, 56) was prepared with the rationale that while dilute samples are desirable for luminescence so that self-quenching is minimized, a fully exchanged sample or at least one with all of the a-cages filled was necessary to ensure that HBr was split on the TI+ cation at room temperature and not on the Na+ cations. Note that HBr is too large to pass through the 2.2-A window to the @-cagesat room temperature. The present luminescence quenching study (28) Ozin, G . A.; Gcdber, J. J . Phys. Chem. 1989, 93, 878. Ozin, G . A,; McCaffrey, J. G.J . Chem. Phys. 1988, 89, 1844.
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n
I
351
296
241
186
131
76
21
WAVE NUMB E R
Figure 3. FT-far-IR spectra of fully dehydrated TI,Na-Y,
for nominal and 56. The loading dependence of the site 11, I, I f , 111 Na+ and 11, I11 TI+translatory mode frequencies and intensities are indicated with arrows. Framework bands are denoted F. n = 0, 8, 16. 32,40, 48,
focuses attention on T143Yand TI,,Y; the others are left for future investigation. The ion exchange behavior of the Na+/TI+ system is well-known.I6 TI+exchanges readily for Na+ up to a limit of 70%, corresponding to full exchan e of the a-cages. TI+, with is too large at room temionic diameter on the order of 3.2 perature to enter the @-cage. However, it has been found30 that fully exchanged samples of Rb,Y and CsMYcan be obtained via aqueous ion exchange of Rb+ and Cs+ with Na,Y despite their respective ionic diametersz9of 3.4 and 3.6 A, by employing very concentrated aqueous solutions under reflux conditions. By analogy, the TI,Y sample was prepared in this way. Far-IR Studies. The degree of ion exchange is very conveniently probed by observing cation translatory modes in the far-infrared region. It has been shownt6that there are normal modes of the charge-balancing cations which can be considered to be essentially decoupled from the lattice vibrations. Each cation site has a mode with a characteristic frequency in the far-infrared.16 Since the cation-oxygen bond stretching force constant depends mainly on mass and charge and since TI+ is monovalent but considerably heavier than Na+, its modes are expected to occur at much lower frequencies than those of Na+. The far-IR spectra of several of the TI+-exchanged samples are shown in Figure 3. The four Na+ cation translatory modes (for sites 1, I', 11, and 111) decrease in intensity with increasing TI+ concentration. Concurrently, two lower frequency modes grow in at 69 and 49 cm-' in TI,Y. These are beyond any reasonable doubt TI+ cation translatory modes. The peak at 69 cm-' is most likely the a-cage site 11 TI+cation mode, which is expected to be the highest frequency component by analogy with NaS6Y. Residual Na+ site 11, 1, I f , and 111 bands remain in the nominally n = 56 material, which by analysis is actually n = 55.6. Some of the Na+ and TI+ cation modes appear as broader bands in the mixed than in the fully exchanged samples; this is likely due to the increased inhomogeneity of microenvironments in the former, since in a mixed system not all cations experience the same set of cationic neighbors. The growth and decay of Na+ and TI+ cation modes in this series of spectra (Figure 3) are consistent with the elemental analysis results. Not unexpectedly, cooperative
1,l9
(29) Cotton, F. A.: Wilkinson, G. W. Advanced Inorganic Chemistry, 5th ed.; Wiley: New York, 1988. (30) Ozin, G. A.; Godber, J.; Baker, M.D. J . Phys. Chem. 1989,93,1409.
-
-50 -100 PP m Figure 4. 23NaMAS NMR spectra of fully dehydrated TI,Na*,Y, for nominal n = 0, 8, 16, 32, and 40. Note that the spectral gain increases down the series. 50
0
effects3' on the frequencies of the Na+ and TI+ cation translatory modes are apparent in the far-IR spectra of the mixed TI,Na-Y samples (Figure 3). 23NaMAS NMR Studies. In the case of fully dehydrated Na,Y, the 23Na MAS N M R spectrum displays an intense asymmetrically shaped resonance centered around -1 2 ppm (Figure 4), which as described earlier has been ascribed to a convolution of signals due to Na+ cations mainly in sites 11, I, I', and III.27 The corresponding 23Na MAS N M R spectra for fully dehydrated TI,Na5&,Y samples, where nominal n = 0, 8, 16, 32, and 40, are also shown in Figure 4. The main effect observed on passing from n = 0 to 32 is an overall reduction in the 23Na signal intensity concomitant with a gradual loss of the intensity of the broad high-field shoulders around -22 and -38 ppm with respect to the narrow resonance peaked centered at -12 ppm (Figure 4). These spectral alterations are rather similar to those previously observed for fully dehydrated H,Na5+,Y samples on passing from n = 0 to 16.27 By reference to these data as well as the far-IR spectra for T1,Nas6,Y shown in Figure 3, one can associate the loss of intensity of the high-field 23Na NMR resonance with the depletion of Na+ cations mainly from the a- and @-cagesites (11, 111, and I'), with respect to those in the hexagonal prism sites (I). Within the limitations of an ionic bonding description for the extraframework Na+ cations, this assignment seems reasonable on the grounds the quadrupole broadening effects are expected to be most pronounced and shielding effects greatest (Lamb diamagnetic term) for the low symmetry and low oxygen coordination number sites 11, I11 and 1'. In this way one can associate the narrower resonance a t -12 ppm with Na+ site I, while the broader ones at -22 and -38 ppm are best associated with a convolution of resonances from sites 11,111, and 1'. Interestingly, although the *"a MAS N M R spectrum of TId3Nal3Yis about a factor of 40 down in intensity compared to NaS,Y, the line shapes (31) Ozin, G. A.; Baker, M. D.; Godber, J.; Gil, C. J. J . Phys. Chem. 1989, 93,2899. Ozin, G. A.; Baker, M.D.; Godber, J. Card. Rev.-Sci. Eng. 1985. 27, 591.
The Journal of Physical Chemistry, Vol. 95, No. 23, 1991 9453
Intrazeolite Semiconductors I
I
1
I
I
I
3
I,
I
I
I
I
I
20 WAVELENGTH /nm 260
C
I
Figure 5. Steady-stateluminescence spectra of dehydrated TIMY,hx= 267 nm, &,, = 370 nm.
are themselves rather similar (Figure 4). The implication is that the residual Na+ cations in T143Na,3Ysamples have roughly the same site distribution and relative site populations as those found in the parent system NaS6Y. The far-IR spectra of these same samples support this proposal (Figure 3). Luminescence of Tl,Y. Steady-StateExperiments. Dehydrated T143Yand Tl%Y were transferred under an argon atmosphere into air-tight copper holders with suprasil quartz windows. Their steady-state luminescence emission and excitation spectra are shown in Figure 5. No difference in luminescence was found between the samples under Ar and the samples under vacuum. Their emission and excitation energies occur in the expected spectral region, although at relatively low energy compared to those in thallium-doped aluminosilicate g l a s s e ~ . ' This ~ ~ ~is~ not surprising, however, since the highly coordinating environment of TI+ in a densely packed glass is very different from the "halfnaked" one-sided open coordination found in an intrazeolitic environment. An attempt to isolate the emission of different TI+ sites in the zeolites by site-selective emission and excitation spectroscopy was unsuccessful. Since there must be more than one site in T15,Y (there are only 32 site 11 positions in a unit cell), it is likely that the individual site emission bands are too broad to be resolved. Note that laser-induced luminescence also produced the same result (this study). There are several broadening mechanisms that could be operative for TI+ in addition to the natural broadening of any transition due to the uncertainty principle. One, already mentioned in the context of the TI+ far-infrared spectra, is inhomogeneous broadening due to the "glassy" nature of the cation distribution in zeolites. Also, the C3,symmetry of site I1 TI+ and the C , symmetry of site 111 TI+ cause the )PIstate to be split into an A and a doubly degenerate E level: the E is Jahn-Teller active and the degeneracy can be further removed by vibronic coupling. Given the resulting complexity of the TI+ excited-state potential energy surface in zeolite Y, it is not surprising that resolution of the luminescence from different sites was not possible in this study. Proton Loading: Mid-IR Data. Having determined that intrazeolite TI+does indeed luminesce, it was important to determine that TIS6Yand T b Y could be proton loaded, Le., that TI+ could split HBr at room temperature. Figure 6 shows the mid-infrared spectra of the virgin T143Yand of T143Ywith successive additions of 22 pmol of HBr. The bands a t 3620 and 3550 cm-l are characteristic of uOH, and vOH,, respectively;13the broad bands around 2940 and 2400 cm-I have been attributed to solvation of the Bransted acid site protons by molecular HBr.13 TIS6Ygave a similar result! The appearance of vOH, and vOH, bands shows that TI+ as a charge-balancing cation in zeolite Y can split HBr. Therefore TI+ is a valid "clone" for Na+ in zeolite Y and it can be used as an informative probe of the cation-anion interaction. Far-IR Data. Having determined that TI+ splits HBr at room temperature in zeolite Y, it was necessary to confirm that no intrazeolite (TIBr), clusters were formed; this was known for Nas6YI3but not for its thallium-exchanged analogue. Barrersp9 showed that the reaction 111-IV described earlier occurred at room temperature for some zeolites, but only on prolonged heating at 300-400 ' C for NaS6Y. The far-IR cation translatory modes turn out to be very useful for investigating questions of this type, since a small frequency shift is expected for interactions in which the
-JWb a
I
I I I I 3200 2800 2400 2000 WAVE NU MBER Figure 6. Proton loading of fully dehydrated TI,,Y with, (a)-(h), increasing amounts of anhydrous HBr. See ref 13 for detailed discussion of the H,, H,, C, and D bands of proton-loaded zeolite Y, HX/NaS6Y. 4000
I
3600
Mid-infrared
WAVENUMBER
For-infrarea
WAVENUMBER
Figure 7. Far-IR and mid-IR spectra of (a) dehydrated T156Yand TI,Y (b) exposed to 14 pmol of HBr, (c) exposed to 135 pmol of HBr, (d) pumped at 450 OC for 1 h, and (e) heated at 350 OC for 1 h in the presence of 135 pmol of HBr.
cation remains associated with its site, while if it is removed altogether into a (TlBr), cluster the mode should disappear entirely. Figure 7a,b shows the mid- and far-IR spectra of T156Y before and after exposure to 14 pmol of HBr. Note that 14 pmol of HBr is significantly substoichiometric; in a typical wafer there are 120 pmol of TI+. Although protonation and solvation have occurred (mid-IR Figure 7b,c), the only manifestation of this in the far-IR is a slight broadening of the T1+ site I1 translatory mode. Therefore the far-IR spectrum of TIS6Yis insensitive to HBr a t this concentration, an important consideration later in this work. An excess of free HBr did produce a frequency shift in the main far-IR TI+ translatory mode; this is illustrated in Figure 7c. The shift from 69 to 120 cm-' is likely due to solvation of the T1+ by molecular HBr. A model of this interaction is proposed in Figure 8. If this is the case, the solvated TI+ mode should be strongly coupled to the H-bonding deformation mode (in Figure 8), to which the band at 2940 cm-' in the mid-IR has been attributed." Therefore the C-band anharmonic progression should have a Au of about 120 cm-I, and Figure 7c shows that this is probably the case. Thus we conclude that the translatory mode frequency shift is probably due to solvation of TI+ by HBr rather than a cation-anion interaction. Furthermore, the reaction can be reversed by pumping the sample for 1 h at 450 "C (Figure 7d). This implies that the shift is not due to (TIBr), cluster formation, which is
McMurray et al.
9454 The Journal of Physical Chemistry, Vol. 95, No. 23, 1991
. .6+
Figure 8. Solvation model for TI,,Y by excess HBr. 4.4
0
expected to be irre~ersible.**~ In order to confirm this, another experiment was performed in which the goal was to 'intentionally" produce (TIBr), clusters. These data appear in Figure 7e. After a TIs6Y sample was treated with 135 pmol of HBr and heated with the HBr in a closed cell for 1 h at 350 O C , the cation mode disappeared. Furthermore, the protons ZOH, and ZOH, remained. The broad absorption in the mid-IR between 2800 and 3200 cm-l is likely due to protons solvated by occluded (TIBr), clusters. From these results it appears that the intrazeolite reaction described earlier in steps 111-IV has occurred to produce occluded (TIBr), clusters. Thus the likelihood of (TIBr), cluster formation on exposing dehydrated TI,Y to anhydrous HBr at room temperature is considered from these studies to be negligible. Luminescence Quenching: Intensity Quenching Experiments. Having ascertained that T I ' splits HBr and that no (TIBr), clusters were formed at room temperature, the effect of HBr on the intrinsic intrazeolite TI+ luminescence could be investigated. No nephelauxetic shift on the TI+ emission/excitation energies was observed upon addition of HBr to TI,,Y; however, the luminescence intensity was quenched. This is not surprising, since quenching by ion-pair form?tion is well-doc~mented.~~ The quenching of the luminescence of T143Ywith HBr is shown in Figure 9A. The excitation spectrum, being sharper than the emission spectrum, is more useful for showing the quenching phenomenon. A similar result was obtained for TIs6Y. It is interesting to note that the amount of HBr required to quench the TI+ fluorescence completely is significantly substoichiometric, that is, 14 lmol of HBr to about 120 pmol of TI'. It is not easy to interpret this result without further work; however, it is possible to speculate on the reasons for the "substoichiometric quenching". The system is highly concentrated in TI+,and it is likely that concentration self-quenching is significant. Therefore it may not be necessary to quench the fluorescence of all of the TI+ cations to quench the observed signal. It may also be possible that, due to the crowding of the a-cage by many large TI' cations (by comparison with RbS6Y/CsS6Y,greater than 90% a-cage TI+ implies 4Tlll+ and 2TIIII+),only one bromide anion is required to quench several cations. The CHEM-X representation of HBr/TI,,Y vividly illustrates this point (Figure IO). Rietveld PXRD profile analysis has failed to locate the bromide anion in the NaS,Y/HBr system, but it has not been established whether this is due to static or dynamic d i ~ 0 r d e r . lPossibly, ~ then, the bromide is sufficiently mobile to quench the fluorescence of several TI+ cations, as can be easily imagined from inspection of Figure 10. In any case, further work is necessary to clarify this point; the question of bromide mobility is addressed later in this paper. It is also interesting that 1.4 fimol of HBr quenches the luminescence to approximately 10% of its original intensity, when this amount has very little effect on the far-IR TI' cation translatory mode. Since the model for quenching is bimolecular ZOTI
+ ZO + HBr s ZOTI.-Br- + ZOH'
it was postulated that Stern-Volmer quenching kinetics might be followed. The Stern-Volmer law, a relationship between luminescence
4.8
E NE RGY /eV
5.2
56
ilLzl
2, -
-20o
a2
os
OA
a8
0.1
1.2
MOLES HBR ADOED ( p W )
Figure 9. (A) Intensity quenching of the luminescence of dehydrated T14,Y by anhydrous HBr. Excitation spectrum: &,, = 414 nm (B)
Stem-Volmer intensity quenching plot of the luminescence of dehydrated T156Y by anhydrous HBr. intensity and quencher concentration for bimolecular quenching, is easily derived.23 Briefly stated, if we have the following situation D
0
+h
*D
v d *D
kr 4
D
+ hu'
k,
*D---,D+A *D
-+ kP
D
products
+
+ +
*D Q A D Q A k = k , + k , + k, + kZ[Q] and we consider diffusional quenching only, then upon applying the steady-state approximation d[*D]/dt = 0 - k[*D] the Stern-Volmer law is obtained:
Io/[ - 1 = kz~o[QI= L [ Q 1 In the above expression, Io is the unquenched emission intensity, I is the emission intensity, T~ is the unquenched lifetime l/(kr + k, k,), and K,, = k p O . A lifetime form of the law can, for exponential decay, be derived as follows: 1 / 7 = ( k , + k , + k,) + WQ1
+
1/7 = 1 / T O TO/T
+4Q1
- 1 = kz~o[QI= K J Q 1
It is possible also to have static quenching, in which D reacts with Q to give some nonemitting species DQ but Q does not quench *D by diffusion as above, that is D+Q%DQ DQ + hu DQ + A In simple static quenching IO/[- 1 = &[Q1
-.
and TO/T
-1=0
Intrazeolite Semiconductors
The Journal of Physical Chemistry, Vol. 95, No. 23, 1991 9455
Y Y 9,
0
Ih
AI
J
I
I
I
I
t
I
I
I
I
n 9,
0
J
w 9,
0 -J U
0
10203040
Figure 11. Stern-Volmer lifetime quenching of the luminescence of (A) virgin TIMYexposed to (B) 0.54 pmol of HBr and (C) 1.4 pmol of HBr, A,, = 267 nm, &,,, = 414 nm.
since, with no diffusion, k2 = 0. If both diffusional and static quenching are present, it can be shown23that
and
Figure 10. Chem-X representations of (top) TI,,Y containing 7TI+ per a-cage (4Tlll++ 3TIll,+) and (middle and bottom) 8HBr,TIJ,Y containing one Br- per a-cage. The framework atoms and H+ are omitted for clarity, while the cation and bromide van der Waals radii are displayed. Chem-X developed and distributed by Chemical Design Ltd., Oxford, England.
Therefore, by combining intensity and lifetime quenching measurements, it is in principle possible to determine some details of the quenching mechanism. The Stern-Volmer intensity plot for TI,Y is shown in Figure 9B. The coefficient of determination was 9 = 0.999; that is, 99.9% of the variation in the l o / I - 1 values is explained by the SternVolmer model. A similar result was obtained with Tl,,Y; the slopes were not employed to make quantitative kinetic measurements, as the precise masses of the T1,Y wafers were unknown. The linear Stern-Volmer plot implies either diffusional or static quenching of TI+ in zeolite Y, but not both. It remains, however, to distinguish between simple diffusional and simple static quenching. Lifetime Quenching Data. Decay curves were determined at approximately 20-nm intervals from 300 to 530 nm. A typical result recorded at 414 nm is shown in Figure 11A. No attempt was made to extract quantitative lifetimes from these data due to complications believed to originate from energy transfer to Fe3+, self-quenching, and delayed fluorescence; this is currently the object of further photophysical investigations, particularly focussing on samples with low T1+ concentrations. The results of the HBr/Tld3Y lifetime quenching experiment are shown in Figure 1 1B,C. As mentioned above quantitative lifetimes were not extracted from the data; the observation of either "some change or no change" in the TI+luminescence lifetime was the desired goal of the experiment. Although the relative intensities of the short- and long-lived emission components differ from spectrum to spectrum, analysis of the spectra reveals that the slopes of the two luminescent decays are indistinguishable. It is likely,
9456 The Journal of Physical Chemistry, Vol. 95, No. 23, 1991
McMurray et al. This essentially appears to be the case in practice (Figure 12) and provides compelling additional evidence in favor of an HBr ionizabion-charge separation model in Na%Y involving framework oxygen protonation and concomitant creation of cationanion tight ion pairs.
Conclusion This study rovides an impressive demonstration of the power of combined Na MAS NMR and Na+/Tl+ far-IR spectroscopy in conjunction with T1+ luminescence quenching studies, for providing DIRECT information on the adsorption, ionization, and charge separation processes that abound in proton-loaded zeolites.13 Adsorption-inducedalterations in the 23NaNMR chemical shifts and line shapes for HBr/NawY yield compelling evidence for the existence of sodium cation-bromide anion tight ion pairs located in the supercage of zeolite Y. This result is consistent with previous observations.l3,I5 Since chargebalancing TI+ cations in zeolite Y have been found to luminesce, it also proved possible to use T1+ as a probe of cation-anion interactions in proton-loaded zeolite Y. The luminescence intensity and lifetime quenching data obtained in this study, in conjunction with the far-infrared data, support a static contact ion pair mode of cation-anion interaction in the protonloaded zeolite Y/hydrogen bromide system. This interpretation brings internal consistency with the above results as well as earlier observation^.'^ From a purely basic scientific point of view, the intrazeolitic spectroscopy of TI+,while complex, is very interesting. The details of the potential energy surfaces and energy-transfer processes that exist in this system could lead to new insights into the spectroscopy of species with ground-state configuration ns2. More practically, the luminescence of TI+ ion-exchanged zeolite Y materials is extraordinarily sensitive to the presence of adsorbed hydrogen bromide. There exists therefore promising applications of these materials for probing the interactions of other adsorbents with the charge-balancing T1+ in zeolite Y, with possible extensions to the field of size- and shape-selective chemical sensing.
P
I 50
I
I
25
0
I -25
I -50
I -75
I -100
PPm
Figure 12. *lNa MAS NMR spectra of (A) dehydrated Na56Yand (B) 16HBr,Na5,Y,
then, since the TI+ lifetime does NOT change with concentration of the HBr quencher, that the quenching is static. Therefore, at this stage of experimentation, the luminescence quenching data support a static ion pair interaction model for proton-loaded zeolite Y. Adsorption-InducedChemical Shifts in the 23NaMAS NMR Spectra of 16HBr,NaS6Y. The outcome of absorbing HBr into fully dehydrated Nas6Y, under loading conditions that are conducive to protonation but not to solvation,” has been investigated by 23NaMAS NMR spectroscopy. A representative set of spectra is depicted in Figure 12. In essence one finds that only the 23Na resonance around -22 ppm shows a primary adsorption-induced shift to higher fields (shielding) with respect to the hexagonal prism Na+ site I resonance centered around -12 ppm. A simple model that can be used to explain the HBr adsorption-induced spectral changes in the Z3NaMAS NMR spectrum on passing from NaS6Y to 16HBr,Na5,Y considers Coulombic effects between the Na+ cation and its oxygen nearest neighbors in the framework. Charge transfer from the 4p lone-pair valence electron density of a Br- anion (the result of HBr ionization-charge separation) to an extraframework a-cage Na+ cation may serve to (a) increase the shielding of those 23Na+cations accessible to interaction with Br- anions, (b) decrease the 23Na+second-order quadrupole broadening, and (c) slow down or eliminate cation site exchange processes. The primary effects of the interaction of Br- anions with accessible extraframework Na+ cations are then envisaged to be a shift of the a-cage 23Na+NMR resonances to higher field with a concomitant enhancement of their intensitv.
-
Acknowledgment. We acknowledge the Natural Sciences and Engineering Research Council of Canada’s Operating and Strategic Grants Programmes for generous financial support of this work. S.O. expresses his gratitude to the Middle East Technical University for granting him an extended leave of absence to conduct his research at the University of Toronto. Supplies of high-quality zeolites from Dr. Edith Flanigen at Union Carbide, UOP, Tarrytown, NY, are gratefully appreciated. Technical assistance from, and valuable scientific discussions with, Dr. Galen Stucky, Dr. Bill Harrison, and Dr. Jim MacDougall of the University of Santa Barbara on powder XRD and Rietveld powder profile refinement and Dr. Peter Macdonald on 23NaMAS-NMR is deeply appreciated. We also thank all of our co-workers at Toronto for many stimulating and enlightening discussions during the course of this work. Registry No. TI, 7440-28-0; HBr, 10035-10-6.
