Raman spectroscopy on zeolites

MICROPOROUS MATERIALS ELSEVIER

Microporous Materials 8 (1997) 3-17

Review

Raman spectroscopy on zeolites’ Peter-Paul Knops-Gerrits, Dirk E. De Vos *, Eddy J.P. Feijen, Peter A. Jacobs Center for Surface Science and Catalysis, Katholieke

Vniversiteit Leaven, Kardinaal Belgium

Received 7 August 1996; accepted 8 August

Mercierlaan

92, B-3001 Heverlee,

1996

AbStTflCt The zeolite Raman literature is reviewed,with an emphasison zeolite structure and synthesis,adsorptionand metal complexformation in zeolites Keywor&:

Raman spectroscopy;

Zeolite synthesis; Intrazeolite

metal complexes

Content3 1. Introduction .............................................................................................................................................................................4 2. Framework vibrations .............................................................................................................................................................. 4 2.1. The structure sensitive band(s) between 300 and 600 cm-i ............................................................................................. 4 2.2. High-frequency Raman bands (850-1210 cm-i) ............................................................................................................. 5 2.3. Other framework vibrations ............................................................................................................................................. 6 2.4. Raman studies of xeoli te. synthesis ................................................................................................................................... 6 2.5. Template observation via Raman ..................................................................................................................................... 7 2.6. Isomorphous framework substitution .............................................................................................................................. 7 2.7. Extra-framework ions ....................................................................................................................................................... 7 2.8. Raman of aluminophosphate and gallophosphate frameworks ....................................................................................... 7 3. Raman spectroscopy on zeolite-adsorbed molecules ............................................................................................................... 8 3.1. Adsorption of hydrocarbons ............................................................................................................................................ 8 3.2. Reaction of surface sites with organic bases .................................................................................................................... 8 3.3. Polymerisation of organ& in zeolites .............................................................................................................................. 9 3.4. Raman on oriented adsorbed molecules .......................................................................................................................... 9 3.5. Adsorption of inorganic molecules ................................................................................................................................... 9 4. Raman on zeolite-confhied metal complexes ........................................................................................................................... 9 4.1. Metal-carbonyl complexes ............................................................................................................................................... 9 4.2. Co-polyamine complexes .................................................................................................................................................10 4.3. Ru-2,2’-bipyridine (bpy) and related complexes .............................................................................................................. 10 4.4. Other complexes ...............................................................................................................................................................11 * Corresponding author. ‘Dedicated to Dr. Helhnut Karge on the occasion of his 65th birthday. 0927-6513/97/%17.00 Copyright Q 1997 Elsevier Science B.V. All rights reserved PIZ SO927-6513(96)00088-O

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5. Advanced Raman techniques ...................................................................................................................................................11 5.1. Resonance Raman spectroscopy (RR) ............................................................................................................................. 11 5.2. Surface-enhanced Raman spectroscopy (SERS) .............................................................................................................. 11 6. Examples of FT-Raman spectra of zeolitc samples ................................................................................................................. 12 6.1. ScbifT base complexes in zeolites ......................................................................................................................................12 6.2. Bipyridine complexes in zeolites .......................................................................................................................................13 6.3. Crown-ether templates in the syntheses of cubic and hexagonal faujasites ..................................................................... 14 Acknowledgement .........................................................................................................................................................................15 References ......................................................................................................................................................................................15

1. Introduction In the development of zeolite science, infrared spectroscopy has been one of the major tools for structure and reactivity characterization [ 1,2]. In comparison with this vast amount of IR work, the field of zeolite Raman spectroscopy seems rather underdeveloped. A major reason for this is that there is considerable difficulty in obtaining Raman spectra with acceptable signal-to-noise ratios from highly dispersed materials such as zeolites. The Raman effect is intrinsically a weak phenomenon, and Raman spectra of zeolites are often obscured by a broad fluorescence. The nature of this background has been the subject of detailed investigations, in which for instance the excitation wavelength has been varied [ 3-61. Two major causes for fluorescence have been identified. First, small amounts of aromatic, strongly luminescent molecules might be present in the samples. High temperature treatment under O2 often reduces this problem. In some cases it was remarked that the heating treatment actually increased the background [7]. This may be due to the transformation of simple organic molecules into fluorescent species at high temperature, possibly under the influence of acid sites. Secondly, the presence of Fe impurities in the lattice is known to cause luminescence. The latter problem can be overcome via highpurity synthesis, starting e.g. from metallic Al. The advent of Fourier transform Raman (FT-Raman) spectroscopy with excitation in the near infrared (NIR) domain offers new perspectives in reducing background fluorescence. This paper is intended to give an overview of the existing Raman literature on zeolite framework vibrations, zeolite synthesis, adsorption on zeolites and metal complex chemistry in zeolites. Finally the relevance of some advanced Raman techniques

to zeolite chemistry is discussed, and the application of FT-Raman in the study of zeolite-con6ned metal species is illustrated by a number of examples.

2. Framework vibrations The examination of a large number of framework topologies and compositions has enabled to derive structural information from experimentally observed zeolite Raman spectra [8,9]. In recent years signiticant progress has also been made in the calculation of vibrational modes of frameworks and prediction of experimental IR and Raman spectra. Details of the applied computational methods can be found in literature [ 10-161. 2.1. The structure sensitive band(s) between 300 and 600 cm - ’ The most intense band in the Raman spectrum of a zeolite generally occurs between 300 and 600 cm- ‘. This band has been assigned to the motion of an oxygen atom in a plane perpendicular to the T-O-T bonds [8,9,17]. In an early report, it was already observed that in this domain, the spectrum of mordenite, with a band at 395 cm-‘, was substantially different from that of most other zeolites like faujasite and chabazite, which have a Raman band between 480 and 520 cm-’ [ 71. Extension to many other topologies later allowed to derive a correlation between the frequency vsooT) of this Raman band and the occurrence of smaller building units in the zeolite structure [9]. In general, smaller rings correspond to higher frequencies. Zeolites containing exclusively evennumbered rings (4MR, 6MR, 8MR, 1OMR or 12MR) have the band around 500 cm- ‘. Examples

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include aluminosilicates with CHA, FAU, LTA, LTL and OFF topologies. The presence of fivemembered rings brings the main frequency down to between 390 and 460 cm-‘, and the precise band pattern depends on the type of rings present. Ferrierites, with 5,6,8 and lOMR, display a band at 430 cm- ‘. The absence of an 8MR (as in TON) results in a band at 450 cm-‘. MFI and MOR altinosilicates contain 4, 5, 6, 8, 10 or 12MR, and have Raman frequencies of 390 and 460 cm- ‘. It has been predicted that the presence of 3MR would increase the frequency of this band above the value usually observed for 4MR containing zeolites. However, it seems that in the expected domain (550-800 cm-‘), 3MR zeolites such as VPI-7 and ZSM-18 do not display common vibrational bands indicative of the presence of 3MR, even when materials with analogous chemical compositions are compared [ 181. Within the above-mentioned groups of zeolites, the VS(TOT) frequency further depends on the value of the average T-O-T angle. A higher T-O-T angle results in a decreased bending force constant, hence in a lower VS(TOT) frequency. Thus for NaX the angle is on the average 139.2” and v = 515 cm-‘; for NaA the angle increases to 148.3”, while the frequency decreases to 490 cm- ’ [ 81. If within one zeolite structure the individual T-O-T angles are sufficiently different, a splitting of the V~TOT, band can be observed. This is the case for the K-ZK-5 zeolite (KFI topology), in which four crystallographically distinguishable 0 sites give rise to a split spectrum [ 191. While this effect of the T-O-T angle is obvious, the influence of the Si/Al ratio on the vS(TOT) Raman band is less clear. For the LTA topology (A, ZK-4), the frequency increases with an increasing Si/Al ratio [20], while in the FAU group (X, Y, siliceous faujasite), the frequency rather tends to decrease [21]. More importantly, the widths of the VS(TOT) bands in faujasite spectra can be rationalized based on the ordering in the structure [21,22]. Thus zeolite X and especially purely siliceous FAU have narrower bands than the faujasites with intermediate Al content, which have more substitutional disorder in the structure. Effects of exchanged cations may especially be prominent for zeolites with a high cation exchange

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capacity, such as zeolite A. In the series TlA-KANaA-LiA, the @TOT) frequency increases from 482 to 497 cm-’ [23,24]. The small Li+ cation is indeed expected to induce the strongest framework deformations, with small T-O-T angles and a higher frequency. However, it has been pointed out that in the same series all frequencies of framework vibrations increase [24]. This might be an electronic effect, as smaller monovalent cations are expected to cause a stronger polarisation in the T-O bonds. A special case is the NH:-exchanged A zeolite, in which the frequency is unexpectedly high because of hydrogen bond formation [ 241. 2.2. High-frequency cm-‘)

Raman bar&

(850-1210

Whereas the IR spectra show intense absorptions in this domain, only bands of low to moderate intensity are observable in the corresponding Raman spectra. The bands are generally ascribed to the asymmetric stretching vibration of the Si-0 bond. In zeolite NaA, the different frequencies of the Si-0 s&etchings have been related to the three crystallographically different oxygen positions in the lattice [24,25]. The vibrational coupling between the adjacent SiO, and AlO tetrahedra in the NaA framework is expected to be small, and this allows to consider the vibrations as being perturbed from those of the individual tetrahedra. For a larger T-O-T angle (and a shorter Si-0 bond length), the coupling between adjacent Si04 and AlO tetrahedra increases, giving an increase of the Si-0 stretch frequency and a decrease of the Al-O frequency. Thus for the oxygen atom shared by the 6MR and 8MR, the largest T-O-T angle has been observed in Rietveld refinements, and this position has been associated with the highest Raman Si-0 frequency. The two bands at lower frequency then correspond to the vibrations involving the Ol and O3 atoms. These assignments were conGrmed by studying the effect of gradual exchange with other monovalent cations. The site preferences for mixed cation populations have been studied well, and relate satisfactorily to the observed changes of the Raman stretching bands.

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In the particular case of LiA, observed band splittings may arise from a symmetry lowering in this zeolite in comparison with NaA, and some Raman bands are even coincident with IR bands. For A zeolites, a correlation of the bands in this domain with the Si/Al ratio is not evident [20]. In X type zeolites (Si/Al = 1.2), there are few Si-0-Si bonds, and the Si-0 stretch vibrations can again be treated as those of individual Si04 tetrahedra, with weak vibrational coupling to the adjacent AlO, units. In analogy with the assignment in the zeolite A case, four bands in the Raman spectrum of zeolite X (between 954 and 1066 cm-‘) can be associated with four different lattice 0 atoms, even if the differences are less marked than for A zeolites [21]. An increase of the faujasite Si/Al ratio increases the number of adjacent SiO, tetrahedra. Consequently the four distinct framework sites are no longer distinguishable in the spectrum, and merging of peaks is observed. Concurrently a large shift towards higher frequencies results from the much stronger coupling between the adjacent Si04 tetrahedra. For fully siliceous faujasite, the Si-0 stretches are eventually shifted up to 1186 and 1209 cm-’ [21]. 2.3. Other framework vibrations The bands between 650 and 750 cm-’ in the zeolite A spectrum were originally thought to originate from the asymmetric Al-O stretching [25]. Later this assignment was revoked; the band was considered to be typical for the presence of double 4 rings (D4R) [20]. In ZSM-5 spectra, symmetric Si-0 stretchings are clearly observed between 800 and 900 cm- ’ [26,27]. In many other zeolites, framework bands between 600 and 850 cm-’ and at low frequency have been described, but they seem to be of little diagnostic value. 2.4. Raman studies of zeolite synthesis Just like IR spectroscopy [ 281, Raman can detect small, X-ray amorphous zeolite particles. Therefore Raman has been used to examine both the liquid and the solid phase of zeolite synthesis mixtures. Ex situ methods (with separation of solid and liquid) and in situ methods have been applied.

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In studying the liquid phase, one should remember that (i) minimum concentrations for detection of spontaneous Raman from liquids are typically 0.05-0.1 M [27,29], (ii) that the cross-section of the Al(OH); species is much stronger than e.g. for silicate or aluminosilicate anions [30]. Thus species which are present in low concentration or with variable structures may easily be overlooked in Raman spectra of the synthesis liquors. When zeolites are synthesized starting from a silica sol, the Raman spectrum of the initially formed solid phase resembles that of vitreous silica, with for instance a broad band around 450-460 cm-’ [27,29,31,32]. For zeolites A, X and Y, aluminate is available in solution, as shown by the characteristic Raman band at 620 cm- ’ [3&34]. Al can be rapidly incorporated into the solid phase. In particular for zeolites X and Y, this process is accompanied by the appearance in solution of monomeric silicate species, such as SiO,(OH)$(780 cm- ‘) or dimeric silicates with characteristic frequencies [30,32,34]. There is no Raman evidence for the presence of aluminosilicate ions in solution. Analysis of the 400-550 cm-’ spectrum during heating or eventual aging of the synthesis gel provides information on the building blocks present in the still relatively disordered solid phase. Again this analysis is based on the correlation between ring SIZJZ and VS(TOT) frequency. Thus for zeolites A and X, the band close to 500 cm-’ indicates the presence of 4MR at the beginning of the gel heating [31]. In zeolite Y synthesis, the gel aging causes a shift towards lower frequencies (440, 361 cm-‘), showing the formation of 6MR. During the subsequent heating, sodalite units with 4MR are formed, and the 500 cm-’ band gains intensity again [32]. Syntheses from Si sources other than colloidal silica have also been studied 1341. Mordenite and ZSM-5 are synthesized at lower pH values, and it is not surprising that in these conditions silicate species are not usually observed in solution [ 27,291. In both cases, the solid initially resembles vitreous silica. In the case of ZSM-5, new vibrations in the solid phase are only observed when ZSM-5 crystals appear [27]. More information concerning the intermediate synthesis steps

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can be retrieved from mordenite synthesis spectra [29]. In mordenite synthesis, a 495 cm-’ band is observed at an early stage in the spectrum of the solid fraction of the gel. In analogy to what is observed for A and X zeolites, this band is ascribed to 4MR aluminosilicate units. Only at a later stage, broader bands appear at 402 and 465 cm-‘, indicative of the formation of still rather disordered 5MR mordenite-like units. These bands eventually sharpen into the characteristic framework vibrations of mordenite.

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[ 38-401. Deo et al. have attributed this band to the Si-0 stretching vibration of a Si04 tetrahedron with one non-bridging oxygen atom [38]. Moreover, Raman is highly sensitive in identifying anatase, which is an undesired TiOz impurity in TS-1. Anatase has a characteristic intense Raman band at 140 cm-‘. It should be noted that other forms of TiOz are in low concentration essentially Raman silent. The effect of isomorphous substitution of the MFI framework with other elements (e.g., Ge) has also been investigated via Raman [411.

2.5. Template observation via Raman 2.7. Extra-framework Observation of the organic template against the weak zeolite background is highly facilitated in Raman in comparison with IR. In some particular cases, the incorporation and conformation of the organic template can be followed. In ZSM-5 synthesis, the tetrapropylammonium ion (TPA’) is initially incorporated into the solid amorphous aluminosilicate in the all trans configuration, which it also possesses in solution. The eventual ZSM-5 crystals however contain a high-energy TPA+ conformer, in which one of the N-C bonds is rotated [27,35]. The strong non-bonded interactions between -CH,- groups of different propyl groups are evidenced by the changed rocking and wagging modes. This conformation has also been proved in XRD studies. In syntheses of LTA zeolites, the tetramethylammonium ion (TMA+) can be incorporated in the sodalite cages. The tightness of this fit induces a frequency increase in the symmetric C-N stretching vibration of the template [33,36]. Raman studies indicate that at a fixed pH, organic cations (e.g. TMA+) and alkali ions stabilize different silicate species [37]. This clarifies the role of TMA+ in synthesizing LTA zeolites with high Si/Al ratio, e.g., ZK-4. 2.6, Isomorphous framework substitution The 960 cm - ’ band in the IR vibrational spectrum of Ti-Silicalite-1 (TS-1) is closely related to the unique catalytic activity of the Ti site. A Raman band is observed in the same domain, and its intensity increases with an increasing Ti content

ions

The translational motion of charge-compensating cations causes low-frequency Raman bands [4,22,42]. The study of complex exchanged ions such as UO$+ is much easier in Raman than in IR, because of the low intensity of the Raman zeolite framework vibrations. The frequencies for symmetric stretching of the linear UOg+ ion vary between 830 and 880 cm- ’ as a function of the degree of hydrolysis. These effects were used to probe the influence of exchange conditions and Al content on the U speciation in faujasites [43]. The entrapment of another 0x0 ion, MnO;, in the cages of sodalite, has been studied by resonance Raman in combination with IR [44]. 2.8. Raman of aluminophosphate gallophosphate frameworks

and

Relations have been proposed between the Raman vibrational spectrum of AlPOs and their framework architecture [45]. An essential parameter is the average Al-O-P angle. For the whole lattice, or around the unidimensional channel, this average angle decreases with increasing pore diameter in the series AlPO-11, AlPO-5, AlPO-8 and VPI-5. In the Raman spectra, the stretching frequencies (between 1100 and 1300 cm- ‘) increase with increasing Al-O-P angle, while the bending frequencies (between 550 and 650 cm- ‘) show the inverse behavior. The frequency of the ring-breathing mode (v) increases with pore size from 262 cm-’ (AlPO-11) to 305 cn-’ (VPI-5). This trend is described quantitatively by the linear relation

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between v* and cos 8, with 8 the average Al-O-P angle around the channel. In the Raman spectrum of the gallophosphate cloverite, the vibrations of the Ga-O-P lattice occur at lower frequency than those of Al-O-P microporous frameworks. However, the most intense lines in the spectrum of the as-synthesized cloverite are those of the quinuclidinium template. It is clear from the spectra that the protonated base dominates over the free base quinuclidine form in the synthesis product [46]. 3. Raman spectroscopy molecules

on zeolite-adsorbed

Adsorption of molecules from gas or liquid phase on zeolites and subsequent study via vibrational spectroscopy mostly shows that the adsorbed molecule rather resembles the solid or liquid form of the component than the gaseous state. Generally molecules can be physisorbed or chemisorbed. Evacuation, eventually as a function of temperature, is frequently necessary in order to discriminate between the different states of sorption. 3.1. Adsorption

of hydrocarbons

Vibrational spectroscopy is particularly helpful in identifying the geometry of sorbate-sorbent complexes. In the assignment of different cotigurations, comparisons are frequently made with data from X-ray diffraction, IR or W-visible spectroscopy. In the case of acetylene sorption on a series of A and X zeolites, Raman data favor the side-on over the end-on binding of the organic molecule [47,48]. This is evidenced by the lowering of the Raman-active vc=c frequency below that of the molecule in the crystalline state. The vc=c frequency shift displays an inverse dependence on the polarizing power of the cations. Thus in KA, the perturbation of the acetylene 7cbonds, as reflected in the frequency lowering, is larger than in NaA or LiA. This dependence has been explained based on the distribution of the cations over the zeolite lattice sites and the degree of exposure of these sites. When for example Li+ occupies the center

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of an LTA 6MR (site S,,), the cation is relatively well-shielded by the oxidic framework. A larger cation (e.g. K’) that does not fit as well into the S,, site, protrudes significantly into the u-cage (site S$) and hence is much more exposed for interaction with an adsorbate. As a result of the symmetry lowering from Dab to C,,, the Raman-allowed vczc vibration of acetylene also acquires some intensity in the IR, where it is normally forbidden. In the case of dimethylacetylene, the methyl groups impede a favorable interaction between the zeolite cations and the K system, and hardly any frequency shifts are observed [48]. For benzene on A and X zeolites, a similar side-on interaction between cation and rc system is proposed as for acetylene [49]. The most sensitive vibration in this case is the intense v1 ring breathing mode, which corresponds to a simultaneous stretching or shortening of all C-C bonds. Adsorption of benzene on CsX gives a much larger vi frequency downshift than for NaX. Such differences were again explained on steric grounds in terms of cation exposure at different sites. For the sorption of cyclopropane on cationexchanged X, the frequency shifts were found to be consistent with an edge-on adsorption (C,, symmetry) and not with the side-on model (C,, symmetry) [50]. In this case a direct polarizing power dependence is observed, i.e. shifts are larger for LiX and NaX than for CsX. Other Raman studies of zeolite-hydrocarbon complexes concern propylene [7], 1-hexene [51], xylenes [52], adamantane and hexamethylbenzene [ 531. 3.2. Reaction of surface sites with organic bases

Organic bases such as pyridine can adsorb (i) on Lewis acid sites, (ii) on Bronsted acid sites, with formation of pyridinium ions, (iii) via hydrogen bonding, without proton transfer, (iv) on alkali or earth alkali cations or (v) via simple physisorption. For over 30 years IR has been applied to distinguish between these sorption modes [ 21. In Raman spectroscopy, the most sensitive vibration in the spectrum of adsorbed pyridine is the v1 ring-breathing mode. Based on the solution spectra of pyridine in different environments, the following classification has been proposed: physisorption (991 cm-‘), hydrogen bonding

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(994-1008 cm-‘), Brernsted (1009-1014 cm-‘) and Lewis acidity (1016-1025 cm-l) [5457]. For alkali (earth) cations, the observed frequency increase of v1 is proportional with the polarizing power of the cation [54,55]. This implies that pyridine binds to the cations via the nitrogen lone pair and not in a side-on fashion like benzene. Analogous binding has been observed for pyrazine on X zeolites [58]. In this case, the symmetry lowering leads to a planar C-H bending mode which is coincident in IR and Raman. 3.3. Polymerisation

of organics in zeolites

Polyacetylene can be formed upon prolonged contact of acetylene with various zeolites, e.g. KX, CsX, COY, NiY or Cs mordenite [ 59-631. In all cases the spectra of the polyene chains allow to assign an exclusive tram conformation to the polyacetylene. The Raman spectra of this colored polymer are resonance enhanced. Consequently, polymer amounts which are not detected in IR are easily observed in Raman [ 601. Upon hydrogenation [61] or oxidation [62] of the polyene, the conjugation is lost, and the Raman intensity decreases. The vczc vibration positions of the polyacetylene gradually decrease with increasing chain length. Moreover fragments with different length have different visible absorption maxima. Thus fragments with varying numbers of C=C units can be observed selectively via excitation with different laser frequencies. This approach allows to estimate the length of the conjugated chain [60-621. Reported chain lengths in zeolites are between 6 and 30, which is considerably shorter than for the polymers obtained in a Ziegler-Natta catalyzed reaction. Whereas in the case of polyacetylene a substantial fraction of the polymer might be outside the micropores, polypyrrole can be confined to the micropore volume by intrazeolite oxidation of pyrrole with zeolite-exchanged, oxidizing metal ions (Fe 3+ , Cuzl) [64]. Further oxidation of the polypyrrole yields radical cation centers, which cause a local relaxation of the aromatic polymer structure of neutral polypyrrole to a quinoidal structure. These two arrangements are distinguishable by Raman. Using this criterion, it was demonstrated that polypyrrole in Cu-ZSM-5 and Cu-Y

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is more oxidized than in Cu-Mor [65]. Bein and co-workers have also employed Raman in the characterization of polymers in the mesoporous molecular sieve MCM41, such as polyaniline and polyacrylonitrile and its pyrolysis products [6668]. 3.4. Raman on oriented adsorbed molecuies

Caro and colleagues have pioneered the spectroscopy of large AlPO-5 crystals containing oriented molecules with strong dipoles. For p-nitroaniline in a single AlPO-5 crystal, strong Raman is only observed when the exciting light is polarized along the z length axis of the AlPO-5 crystal, and the scattered light is detected with the same z polarization. Via comparison of the intensities detected under z and x polarization, the maximum deviation in the alignment of the p-nitroaniline molecules was found to be 9” [69]. 3.5. Ahorption

of inorganic molecules

The separation of air by preferential adsorption of N, on a 4A zeolite can be followed by in situ Raman spectroscopy [ 701. In the disproportionation reactions of NO2 on zeolite supports, Raman can be complementary to IR, as it allows the observation of the symmetric stretching modes in e.g. NO, and NO: [71]. Products of the NOz dismutation are NO; and NO+. NO; can be entrapped in sodalite cages where its vibration frequencies are altered. For NO+, resonance enhancement is observed in the UV. Resonance Raman (RR) at various wavelengths was also applied to distinguish between products of chemisorption of SOz in X zeolites, e.g. S;, S3 or S; [72]. The study of the chemisorption of Br, and I, on A ad X zeolites is facilitated in Raman by the observation of low-frequency modes of Br,, I*, Br; , I; or Br-Olatticc [ 73-751. 4. Raman on zeoiite-confined metal complexes 4. I. Metal-carbonyi complexes

Whereas IR is a common technique for the study of intrazeolite carbonyl complexes [76],

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Raman has been much less applied. This is primarily due to the sensitivity of the M(CO), compounds to light- and heat-induced decomposition [77]. Nevertheless Raman spectra can be recorded when appropriate precautions are taken, e.g. limitation of laser power, sample spinning and cooling at liquid Nz temperature [ 78,791. Raman on intrazeolite Mo(CO), shows that the spectroscopic features of this complex in siliceous faujasite and in solution are highly similar. At contrast, in faujasites with high Al content, a symmetry lowering of Mo(CO), (to C,, or C,,) indicates that the complexes are docked to cationic surface sites [79]. At high loadings of the M(CO), complexes, dipolar intermolecular interactions occur between pairs of carbonyl complexes. This was demonstrated by co-adsorption of Cr(CO), and W(CO)6 into Nay. The C-0 stretching frequencies of these complexes are normally well-separated. The co-adsorption results in the observation of bands of intermediate frequency, which are ascribed to the intermolecular vibrational coupling [ 781. 4.2. Co-polyamine

complexes

Exposure of Co exchanged faujasites to ethylenediamine (en) or tetraethylenepentamine (tetren) leads to intrazeolite formation of [Co(en)$+ and [Co(tetren)]‘+ [24]. These complexes are rather weak Raman scatterers. However, exposure to oxygen yields colored oxygen adducts, and these display resonance Raman enhancement upon irradiation with visible lasers. O2 can be coordinated in different fashions: (i) in a mononuclear Co”’ *0; superoxo adduct, (ii) in a dinuclear Co”‘. OS- * Co”’ peroxo adduct, and (iii) in a dinuclear Co”’ * 0; . Co”’ superoxo complex. These different chelates all exhibit different electronic charge transfer transitions and Raman vibrations. This enables one to monitor the aggregation of the Co complexes in various molecular sieve topologies [80,81]. 4.3. Ru-2,2’-bipyridine complexes

(bpy) and related

These compounds have been extensively investigated in view of the utilization of solar energy.

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Illumination of [Ru”(bpy)#+ in the metal-toligand charge-transfer band promotes an electron from the metal to one of the bpy ligands, yielding Ru”’ and a bpy- ’ radical anion. Stabilization of the charge-separated 3MLCT state has been pursued by modification of the environment of the light-capturing Ru center. The zeolite-immobilized Ru complexes have been studied by a combination of spectroscopies such as electronic absorption, emission, ground-state resonance Raman, timeresolved resonance Raman of the excited state and by measurements of excited state lifetimes [82891. Moreover, the architecture of the complexes has been varied considerably, by partial or total replacement of bpy with 2,2’-bipyrazine (bpz), 4,4’dimethyl-2,2’-bipyridine (dmb), 4-methyl-2,2’bipyridine and diazafluorene [ 82-841. Due to resonance enhancement by the visible laser light, Raman spectra of the entrapped FWbwM2 + can be obtained with excellent signal-to-noise ratios. For heteroleptic complexes, the Raman spectrum allows to identify the correct complexation stoichiometry. Thus co-adsorption of bpy and dmb on the Ru zeolite yields PWb9z@W12+ and not [Ru(bpy)(dmb),]‘+ [83]. In general, there are only small differences in the vibration frequencies of solution and zeoliteentrapped complexes. The largest shift in the ground state resonance Raman spectrum is observed for the Ru-N bonds (x 10 cm-‘). The distortion of the Ru-N bond in the zeolite species seems to impede elongation of this bond and population of the metal-localized triplet excited state (3dd). As thermal population of this 3dd state is one of the major pathways for decay of the 3MLCT state, lifetimes of the 3MLCT state for the entrapped complexes are often considerably longer than for the complex in solution [83,84]. Time-resolved resonance Raman spectroscopy with UV excitation allows one to look at the spectrum of the excited state [83,86,87]. The new spectral features which are superimposed on those of the ground state, can be ascribed to bpy -. , which confhms that in the zeolite the excited electron is also located in a single ligand molecule. Moreover, for the heteroleptic complexes, the excited electron occupies the same ligand molecule Thus photo-excitation of as in solution.

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[ Ru( bpy),( bp2)j2 +-Nay results in the selective formation of bpz- ’ [83]. The longest lifetimes of the 3MLCT state are measured in hydrated zeolites with low Ru concentrations and in the absence of 0,. The effects of an increased loading and dehydration can be probed in the Raman spectroscopy of the systems. At high complex loadings (e.g., 1 complex per zeolite supercage), bpy ligands of adjacent PWwyM2 + come in close contact. In the Raman spectra, these contacts are evidenced by major shifts of those vibrations that are localized at the periphery of the ligand, e.g. the C-H wagging modes. These neighbor interactions result in fast decay of the 3MLCT state [86]. Dehydration of the zeolite causes a destabilization of the 3MLCT state, probably because of the electrostatic repulsion between the bpy - . anion and the negatively charged zeolite wall. The consequence of this decreased stability is that in the time-resolved resonance Raman spectra, the bpy -. can no longer be observed [87].

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Raman spectra have also been reported for zeolite-immobilized [Cu( NH3)4]2 + [ 901, [pt(NH3)4i2+ [911, Db0412+ [921, 1Co(C~),i+ [ 931 and [ Fe(Cp),] + [94]. Vibrations in these complexes appear to depend slightly on the hydration state of the zeolite, as was demonstrated for the N-H bending in [Cu(NH,),]‘+ and for the C-H stretching in [Co(Cp),]+. Relations between such changes and the (de)stabilization of the entrapped complexes have been proposed. In the case of zeolite-entrapped [Co(cy~lam)]~+ and [Ni(cyclam)12+, the low-frequency Me-N vibrations are helpful in determining the conformation (cis or trans) of the ligands around the metal ion [95].

5. Advanced Raman techniques (RR)

For molecules containing a chromophore, the Raman intensities can be strongly enhanced when

11

3-17

the frequency of the incident laser light is in resonance with the chromophore. The result is a large increase in sensitivity, which is of great importance as signals from zeolite samples tend to be weak. Examples in zeolite chemistry are faujasite-entrapped [Ru(bpy)J2’ and its photo-excitation products [82-891, and the oxygen adducts of Co polyamine complexes (see above) [ 80,8 11. When different molecules with different absorption spectra are present in the sample, the Raman lines of the individual molecules can be selectively enhanced by irradiation at different wavelengths. Technically this approach is facilitated by the availability of an increasing number of laser lines. In zeolite-catalyzed polymerizations, the polymer chain length can be estimated by use of different visible excitation frequencies [60-621. Moreover, selective RR enhancement with excitation in the UV domain has been used to discriminate between protonated and free-base forms of aromatic amines in NaY [96,97]. 5.2. Surface-enhanced (SERS)

4.4. Other complexes

5.1. Resonance Raman spectroscopy

Materials

Raman spectroscopy

In the early seventies, it was discovered that intensities of adsorbed molecules on roughened Ag electrode surfaces were enhanced about lo4 times. The nature of this effect is partially electromagnetic, but the roughness of the electrode and the related high surface area are also important [98]. This surface-enhancement effect was later demonstrated to occur also with other conducting substrates (e.g. Cu, Au, graphite) and on colloidal particles [ 99,100]. In early work on a Ag+-exchanged X zeolite, Trotter noted that the Raman intensities of adsorbed 1-hexene were anomalously strong [ 5 11. Later the feasibility of SERS on zeolites was investigated more systematically. By reduction of a Ag+ faujasite with hydrazine, metallic Ag clusters were formed on the outer zeolite surface [ 1011. The Raman spectrum of riboflavin mononucleotide, adsorbed on the outer surface of the reduced silver zeolite is clearly surface-enhanced. The detection limit of this signal was estimated to be in the nanogram range. For molecules which are small enough to enter the inner zeolite volume (e.g.,

12

P.-P. ffiops-Gerriis

et al.jMicroporous

Materials

8 (1997)

3-17

TMA+), the presence of external silver clusters however reduces the Raman intensity. The latter phenomenon has been ascribed to the highly reflective nature of the Ag coated surface. A more generalized methodology for SERS in zeolite media will require well-controllable procedures for intrazeolite formation of small metallic clusters, or controlled deposit of zeolite crystals on a conducting surface.

6. Examples of F’T-Raman spectra of zeolite samples The following three examples illustrate the use of FT-NIR-Raman spectroscopy in the study of zeolite-co&-red complexes. The synthesis of the different samples has been reported previously [ 102- 1041. As explained earlier, the main advantage of the NIR excitation is that fluorescence is largely avoided. As a reference, spectra of crystalline reference materials have been included in Figs. 1 and 2. It is immediately obvious from these plots that the signal to noise ratios of the zeolite spectra are not problematically lower than those of the spectra of the corresponding pure compounds.

1700

Wavenumber

(cm-‘)

1000

6.1. Sch# base complexes in zeoIites

Fig. 1. FT-Raman spectra of (a) [Mn(salen)(OH)], (b) [Mn(salen)]+-Nay, (c) salenH,, adsorbed on zeolite NaY. For details on sample synthesis, see Ref. [ 1021. Raman spectra were recorded with a Bruker IFS 100 spectrometer, with Nd’+:YAG excitation at 1064mn and a 180” backscattering collection geometry. A liquid nitrogencooled Ge device was used for detection. For each sample, about 500 scans were averaged, and measurements were performed twice to discriminate between noise and weak peaks. The spectral resolution was 4 en-‘.

Zeolite-confined Me-salen complexes and related compounds have been studied for applications in dioxygen binding (Me=Co) [105,106], in oxidation catalysis (Me=Mn) [ 102,107], or in hydrogenation catalysis (Me = Pd) [ 108,109]. Spectroscopic studies with UV-visible diffuse reflectance and especially ESR have revealed that there are often subtle differences between the complexation reactions in solution and in the zeolite [ 1061. For these low-symmetry complexes, vibrational spectroscopy usually does not distinguish between different complex conformations (e.g., cis or trans), but at least IR and Raman allow to check whether the binding of the metal ion to the ligand donor atoms is similar for free and zeolitecormned species. Binding of a metal ion in a ligand of the salen type is primarily expected to cause shifts in the

imine stretching vibration. Secondly, the complexation is accompanied by deprotonation of the phenol groups and corresponding vibrational changes. The usual procedure for synthesis of Me-salen complexes in zeolites comprises a final extraction procedure to remove excess ligand. This extraction and subsequent handling of the material e.g. in catalytic experiments, often result in the presence of some water on the samples. This is a serious obstacle in observing the imine stretching vibration in IR, as this band overlaps with the bending vibration of water (dH20. Evacuation of the sample under heating (e.g. at 373 K) is thus necessary to observe the ~c=~ vibration [log]. Moreover the hydrogen bonding between the salen ligand and the zeolite surface causes several IR bands to be broad.

P.-P. Knops-Gerrits

1800

1500

Raman

1200

et al,~Microporou.s

900

8 (1997)

(bottom) [Mn(bpy),(NO,)J.

These problems are considerably reduced in FT-Raman mode. The L&o band is very weak in Raman, and the sharp lines of the C=C and C=N stretchings are well observable between 1450 and 1650 cm-l (Fig. 1). The band with the highest frequency in this domain is usually ascribed to the imine stretching [ 1 lo]. Whereas the ~c=~ stretching occurs at 1638 cm- ’ in free, protonated salenH,, the band is downshifted to 1628-1630 cm-’ in crystalline [Mn(salen)]+ and in the zeoliteencaged complex. In general there is excellent agreement between the bands of free and entrapped [Mn(salen)]+; for instance, both species display an intense band around 1335 cm-‘, which is absent in the spectrum of the zeolite-adsorbed salenH, ligand. 6.2. Bipyridine complexes in zeolites As discussed before, the properties of zeolite entrapped [ Ru( bpy),12 + and related complexes have been studied by resonance Raman spectro-

3-17

13

500

900

Raman

shift (cm-l)

Fig. 2. FT-Raman spectra of (top) [Mn(bpy)#-NaY, configuration as for Fig. 1.

Materials

shift (cm-l)

Sample synthesis as in Ref. [103]; spectrometer

scopy [82-891. We now demonstrate that for similar compounds the Fourier transform technique allows one to obtain spectra with excellent signal to noise ratios, even if the signals are not enhanced by a resonance effect. Fig. 2 shows the FT-Raman spectra with 1064 run excitation for [Mn(bpy)d2’-NaY and for the related crystalline

compound

Pfn@w)2WW21. WUwy)d2+-

NaY was recently discovered to display an unprecactivity in epoxidation catalysis edented [103,111,112]. The Raman spectra of these bpy complexes are usually interpreted based on a local symmetry, i.e. a single bipyridine ligand is considered instead of the whole complex. While such an approach does not allow for geometry effects in the overall complex (e.g., cis vs. truns conformation in a bis bpy complex), it facilitates the analysis of the normal modes [ 1131. Crystalline bpy has a C,, symmetry, with the nitrogen atoms in trans with respect to the inter-ring C,-C$ bond. For bpy adsorbed on NaY [ 1141, for [Mn(bpy)2]2+-NaY and for

14

P.-P. Knops-Gewits

et al./Mcroporous

[Mn(bpy),(NO,),], the bpy molecule is in the cis form, with C,, symmetry. As a result, highly similar spectra are obtained for the latter three materials. There is however a clear difference between the spectra of bpy containing NaY and the [ Mn( bpy),]‘+-NaY zeolite. For bpy adsorbed on Nay, the intense symmetric ring-breathing mode v1 occurs at 1001 cm-’ [114]. In the [ Mn(bpy)d2+-NaY zeolite, this band shifts to 1015 cm-‘, due to the interaction of the lone pairs in the bpy ligand with the metal ion. This type of binding is obviously similar to the complexation of pyridine on exchanged cations (see above). For [Mn(bpy),12+--NaY, the 1001 cm-l band is only a weak shoulder on the main 1015 cm-’ band. This implies that the large majority of the bpy molecules in the [Mn(bpy)J2+-NaY catalyst is interacting with Mn’+. Further differences between the spectra of zeolite-entrapped and crystalline [ Mn( bpy),12 + are due to the presence of different charge-compensating anions. In the zeolite spectrum, the band at 503 cm- ’ is characteristic for the 4MR in the faujasite structure. The NO; anion in the crystalline complex shows a strong band of the symmetric stretching vibration at 1041-1033 cm-’ [ 1151. 6.3. Crown-ether templates in the syntheses of cubic and hexagonal faujasites The Na+ forms of the cyclic ethers 18-crown-6 ( 18C6) and 15-crown-5 (15C5) can be used to direct the synthesis of faujasites towards the cubic FAU or the hexagonal EMT topology [104,116,117]. The FT-Raman spectra of as-synthesized FAU, EMT and a structural intergrowth of both topologies (MIX) are shown in Fig. 3. FAU was synthesized with 15C5; EMT with pure 18C6, and the intergrowth with a 3:l 18C6: 15C5 mixture. The FT-Raman technique yields superior spectra compared to visible excitation, as is clear by comparison of our data with other Raman reports on this system [ 118,119]. In the spectra, features of the framework and the crown ether template are superimposed. The intense band at 503 cm- ’ is the framework symmetric bending vibration; most other bands are crown-ether vibrations. There are a few differences

Materials

8 (1997)

3-17

1

1730

5CO

1250

IO00 La”e”~“t~r

750 cm-’

500

250

:I20

Fig. 3. FT-Raman spectra of as-synthesized faujasites with 18C6 and 1X5: (a) a FAU-EMT intergrowth, containing approximately 3 Na+-18C6 per Na+-15C5, (b) EMT, containing only Na+-18C6, (c) FAU, containing only Na+-15C5. Sample synthesis as in Ref. [104]; spectrometer configuration as for Fig. 1.

between the spectra of Na+-15C5 in FAU and Naf-18C6 in EMT. Both ethers have a characteristic intense band between 800 and 900 cm-‘, which has C-O stretching and CH, rocking character. The frequencies of this vibration are 869 cm-’ and 860 cm- ’ for EMT and FAU, respectively, which agrees fully with previously reported frequencies for Na+-18C6 and Na+-15C5 [120]. Moreover, a sharp band is observed at 1001 cm-’ in FAU and to a lesser extent in the intergrowth. This band is lacking in as-synthesized pure EMT. Especially for 18-crown-6 and its complexes, the relation between the Raman spectra and the symmetry of the compound (&, Ci or Cl) has been studied in depth [ 12&122]. Typical examples are Kf-18C6, which always assumes D3d symmetry, and crystalline (Na+-18C6)(SCN), in which the

P.-P. Knops-Gerrits

et al./Microporous

crown ether has an envelope-like C1 structure. For dissolved Naf-18C6, the conformation and the corresponding Raman spectrum are less well defined, and the geometry may actually fluctuate between different conformers (OS6 or C,) depending on the solvent. While Na+-18C6 is expected to undergo some distortion in the EMT hypocage, the EMT hypercage probably provides sufhcient space for a more relaxed crown ether conformation, similar to the solution structures. Based on the structural assignments by Takeuchi et al. [ 1201 and Ozutstmri et al. [122] the peak at 807 cm-’ may be considered as indicative for a C1 symmetry, but Raman spectroscopy alone is probably insufficient to make conclusive conformation assignments for these samples.

Acknowledgment DEDV and PPKG thank the Belgian Science Foundation (NFWO) for positions as post-doctoral researcher and research assistant, respectively. This work was supported by the Belgian Federal Government in the frame of an IUAP project. We thank X.Y. Li (Hong Kong University for Science and Technology) for the use of a Raman spectrometer.

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