J. Phys. Chem. 1995, 99, 11167- 11177
11167
N2 Adsorption at 77 K on H-Mordenite and Alkali-Metal-Exchanged Mordenites: An IR Study F. Geobaldo: C. Lamberti,' G. Ricchiardi, S. Bordiga, and A. Zecchina" Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Universith di Torino, I-10125 Via P. Giuria 7, Torino, Italy
G. Turnes Palomino and C. Otero Arehn Departamento de Quimica, Universidad de las Mas Baleares, 07071 Palma de Mallorca, Spain Received: December 23, 1994; In Final Form: April 7, 1995@
N2 interacts with positive Mf centers (M+ = H+,Li+, Na+, K+, Rb+, and Cs') present in Mf-mordenites, with activation in the IR of the N E N stretching vibration. From the analysis of the IR spectra and from the examination of the available space present in the main channels and in the lateral pockets, it is concluded that NZcan interact with smallest ions located in both positions and that only when the biggest ions are involved (Rb+, Cs+), the penetration of N2 into the lateral pockets is not possible. The V(N=N) frequency is found to depend linearly upon the l/(Rx (R, = cation radius): this is considered a clear proof of the predominant role of the electrostatic forces. When the perturbed V(NEN) stretching frequency is compared with the 9( N s N ) of N2 perturbed by dispersion forces only (as for N?in the cages of rare gas matrices), it is concluded that the effect of the perturbation induced by the positive ions is always to shift the frequency toward higher values (as found for CO in M+-mordenite and Mf-ZSM5): this result strongly suggests an end-on interaction of N2 with all alkali-metal extraframework cations. Indications that NZ behaves as an hindered rotator (especially in Cs+-mordenite) were also obtained and discussed.
+
1. Introduction Mordenite is a zeolite with orthorhombic structure which has a set of parallel channels running along the [OOl] direction. These channels have an elliptical cross section of about 0.70 x 0.65 nm in diameter, defined by 12-membered rings of TO4 tetrahedra.'.2 The channel has side pockets, in the [OlO] direction, defined by eight-membered rings and which have a free window of about 0.39 nm. Mordenite framework has a net negative charge equal to the number of aluminum atoms, balanced by protons (in the acidic form) or by metal cations (in the exchanged form). The presence of counterions, either protons or metal cations, is particularly relevant for many applications of this kind of material since their presence strongly influences the electric field inside the channels, this being a function of cation charge and size. This has strong bearings on the usage of zeolites as components for host-guest composites, a proposed kind of advanced materia13-6 where large pore zeolites act as a guest for encapsulating and organizing molecules, supramolecular entities, and crystalline nanophases inside their pores. Space confinement leading to quantum size effects and host-guest (electrostatic) interaction can thus be engineered for application in a number of technological fields, such as photochemistry, optoelectronics, and chemical sensing. For this purpose precise knowledge of the electric field inside this kind of porous material is required. It is well-known that vibrational spectroscopy of adsorbed molecules such as CO, H2,0 2 , C h , N20, or N2 (see, for example, refs 7-13), is very useful to probe the electrostatic fields on the internal surface of the zeolites. In this work, in
* Author to whom correspondence should be addressed. E-mail:
[email protected] ' Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, 1-10129 C.so Duca degli Abruzzi 24, Torino, Italy. Also with I "Sezione di Torino, Torino, Italy. Abstract published in Advance ACS Abstracts. June 1, 1995. 7
@
complement to previous work on CO adsorptiont4we present a vibrational study of the adsorption at low temperature of dinitrogen, a homopolar molecule isoelectronic with carbon monoxide, on a series of alkali-metal-exchangedmordenites and on Hf-Mord. The adsorption of NZis very interesting, as it is representative of the unperturbed diatomic homonuclear molecules which are more stably adsorbed in the end-on forml5 and which are Raman but not infrared active. When this kind of molecule is subjected to an external electric field exerted by the adsorption sites (counterions inside the channels and their negatively charged nearest neighbors), they become polarized: as a consequence, relaxation of selection rules occurs with subsequent activation in the IR of the stretching vibration of the perturbed molecule. Dinitrogen, when used as a molecular probe, is chemically inactive and therefore does not form chemical bonds with the adsorption sites. Consequently, its perturbation can be considered as due only to electrostatic and van der Waals interactions. This assumption allows us to gain further knowledge on the local environment of the adsorption sites and in particular on the magnitude of electric fields inside the zeolite channels. An important advantage of such molecular probes is the absence of the adsorption bands for unperturbed molecules. This sometimes makes it possible to easily detect even very small amounts of specifically adsorbed species. More important, as the intensity is expected to be dramatically dependent upon the strength of the field?,t6the study of N2 spectroscopy will profit not only from the change of stretching frequency induced by the polarizing field but also from the change of the specific intensities. This aspect differentiates N2 from CO: in fact in the latter case it is known that the intensity is nearly independent of the strength of the perturbing field."
2. Experimental Section Mordenite in the ammonium form with SUA1 = 5 was supplied by Enichem Laboratories Bollate (Milano). From this
0022-365419512099-11167$09.00/0 0 1995 American Chemical Society
11168 J. Phys. Chem., Vol. 99, No. 28, 1995
Figure 1. Mordenite structure showing cation sites. Dots outline Conaolly surfaces obtained using a probe molecule with a radius of 1.4 A. Side pockets connected to the main channels are clearly evident.
zeolite sample, the cationic forms M-Mord (M = Li+, Na+, K+, Rb+, and Cs+) were prepared by standard ion-exchange procedures using aqueous solutions of the corresponding alkalimetal halides. In the case of lithium, only a partial exchange was achieved (see below), while for all the other alkali-metal cations homoionic forms of the exchanged zeolite were obtained. The protonic form was obtained by thermal treatment of the ammonium form for 1 h at 673 K under dynamic vacuum (ca. Pa). For IR measurements, thin self-supporting wafers of each zeolite were prepared and activated in vacuum at 673 K for 1 h, inside an IR cell allowing in situ thermal treatments, gas dosage, and low-temperature measurements to be made. The spectra were collected with a resolution of 2 cm-’ on a Bruker FTIR 66 equipped with a MCT cryodetector. Although the IR cell was permanently cooled with liquid nitrogen, the actual sample temperature (under the IR beam) was likely to be ca. 100-110 K. As the zeolite is an insulating solid and because the contact between the pellet and the sample holder (which is at 77 K) can be slightly different for the various samples, the effective temperature is supposed to vary in a 10 K range on passing from one sample to the other. For the weakly adsorbed N2, this temperature uncertainty can induce a dramatic change in the amount of adsorbed N2 for any given pressure with subsequent uncertainty increase in the calculated IR specific intensity. For this reason calculations based on peak intensity cannot be safely used for fully quantitative conclusions. The modelization of the available internal space of the mordenite has been obtained using commercial programs by Biosym Technologies Inc.
3. Results and Discussion 3.1. Mordenite Structure and Cation Locations. A representation of the geometric structure of mordenite along the [OOl] direction is reported in Figure 1. The atom connections are represented by sticks, while the accessible space (internal surfaces) is evidenced by Connolly surfaces (radius of the probe molecule r = 1.4 A).18.19 This complex structure is constituted by a pore network formed by straight channels (cross section 0.70 x 0.65 nm) interconnected with small channels elongated in the [OlO] direction (cross section 0.39 nm). The Connolly surface is particularly useful, as it makes evident the shape of
Geobaldo et al. the small channels, which are obstructed by very narrow bottlenecks located midway from the intersections with the main channels. The openings of these narrow windows (cross section of 0.26 nm;see the arrow in Figure 1) are too small to allow the penetration of diatomic molecules (like N2); however, they are ideal locations for cations. The high occupancy of these sites causes the nearby complete occlusion of the small opening: for this reason the lateral channel is normally called a “side pocket”. Cations located at the bottom of side pocket (site A’ in Figure 1) are in a highly coordinated situation, and hence they do not represent preferential sites for molecule adsorption and polarization. In other words, their presence cannot be revealed by using the usual weakly interacting probe molecules such as CO and N2 because the local field is not strong enough to polarize the molecule. For this reason they will not be considered anymore. Besides site A’, the other counterions are located in the sites labeled in Figure 1 as follow: A (on the walls of the side pockets); B (at the access to the side pockets); C (on the walls of the main channels). These positions are all highly occupied because mordenite is an aluminum-rich zeolite and hence contains a large concentration of cations. The relative population of the A, B, and C sites is dictated by several factors, among which we mention (i) the negative charge distribution on the skeleton; (ii) the charge-radius ratio of the cations, which determines their polarization power; and (iii) the cation-cation repulsion, which prevents the simultaneous occupation of A and B sites. When small cations are involved, the simultaneous presence of all these factors results in a nearly uniform occupancy of sites A, B, and C. When bigger cations are involved (Rb, Cs), sites A and B become hardly distinguishable. Figure 2 shows the results of computer modeling of N2 molecules interacting with extraframework cations. With small cations (Li+ and Na+) and also with protons, nitrogen adducts can be formed in the main channels and in the side pockets (Figure 2a). For larger cations (Figure 2b) only sites in the main channel are accessible. The case of K+-Mord (not shown in Figure 2) is borderline. 3.2. Comparison of the IR Spectra (3800-3450 cm-’ Region) of H+- and Li+-, Na+-, K+-, Rb+- and Cs+Exchanged Mordenites. Spectra a-g in Figure 3 correspond to samples all thermally treated in vacuo at the same temperature (673 K) for the same time. The quantity of sample (mg/cm2) in the pellets of the various samples was approximately constant, but not identical. No specific efforts have been made to exactly equalize the weight of the pellets. Although the zeolite powder has been pressed at the same pressure in all cases (to obtain the disk), the scattering profile, which strongly depends upon local inhomogeneities in the sample, was slightly different on passing from one sample to the other. We notice that the general features of the spectrum in the 3747-3450 cm-I range are entirely similar to those reported in the literaturei2.20-23for H+-Mord with the SUA1 ratio in the 5-8 interval. The weak peak at 3747 cm-’ is the Y(OH) of nonacidic, free, -SiOH groups mainly located on the external surface. The second, stronger, and broad band centered at 3609 cm-’ is the Y(0H) of free framework Brgnsted sites of the zeolite.
Both peaks show a tail in the low-frequency side. These lowfrequency tails are plausibly associated with groups perturbed by hydrogen bonding by adjacent acceptor species. The large
J. Phys. Chem., Vol. 99, No. 28, 1995 11169
N2 Adsorption at 77 K on H-Mordenite
1.o
! 3800
1
0.0
3700
3600
3500
3400
3300
wavenumber cm-'
Figure 3. Infrared spectra in the OH stretching region of H+-Mord and alkali-exchanged mordenites outgassed at 673 K: (a) H+-Mord; (b) Li+Mord after the first exchange; (c) LP-Mord after the second exchange; (d) Na+-Mord; (e) K+-Mord; (f) Rb+-Mord; (8) Cs+Mord.
Figure 2. Computer modeling of N2-cation adducts: (a, top) Na+Nz; (b, bottom) Cs+-N?.
half-width of the 3609 cm-' band is also likely implying that we are dealing with a family more than with a single species. This heterogeneity is not unexpected, as the Brgnsted framework groups are located in different and distinguishable positions in the lattice. In fact, as mordenite possesses two types of channels of distinctly different radii (Figure l), a plausible hypothesis is that the frequency of .li(OH) of Bronsted groups located in the side pockets is slightly different from the frequency of the F(OH) located in the main channel, and this is the main source of heterogeneity and, hence, of broadening. This is in agreement with the general opinion*I that the V(0H) frequency of Brgnsted groups in the narrower lateral pockets is different (lower) with respect to that of groups in the main channels because of a small hydrogen bond interaction with the oxygen of the walls. We shall return to this point when the interaction with N:! is discussed in greater detail (vide infra). The substitution of H+ with Li+ is never complete even after repeated exchange procedures (spectra b and c). This can be ascribed to the fact that the ionic radius of solvated Li+ (0.38 nm) is too big to allow the penetration of Li+ in the lateral pockets. This keeps the exchange percentage far from 100%. Of course we have to keep in mind that. while the situation
after the wet exchange procedure (i.e. when the zeolite cations are totally solvated by water) is plausibly characterized by incomplete exchange of the protonic sites in the side pockets, the same does not necessarily hold for the sample subjected to thermal treatments in vacuo. In fact upon H20 eliminationfrom the coordination sphere, the Li+ ions are expected to move toward new positions in order to maximize their electrostatic interactions with the negative charges of the framework. This tendency will be particularly relevant for Li+, because the cations possessing high positive chargehadius ratio should show the highest tendency to move toward more shielded positions. We thus expect that after activation in vacuo at 673 K the fraction of Li+ cations that are localized in the side pockets is increased, while that localized in straight channels is decreased. One of the interesting points in spectra b and c is that the T(OH) of residual (unexchanged) Brgnsted groups is observed at 3622 cm-I, i.e. at a frequency distinctly different from that of the maximum in H+-Mord (spectrum a). This is in favor of the hypothesis that the band at 3609 cm-' is due to a family of similar species which exchange with a different rate. A second interesting feature deserving a comment is the clear shoulder at -3675 cm-I, which becomes more and more evident as the exchange proceeds. The band at -3675 cm-I cannot be clearly observed in H+-Mord, because it is likely overshadowed by the stronger band at 3609 cm-I. This peak belongs to the .li(OH) of -Al-OH groups in a partial extralattice p o ~ i t i o n . * ~ * ~ ~ Since they are less acidic than the normal Brgnsted groups, their tendency to exchange is less pronounced. On the same line we understand how silanol groups (which are very weakly acidic) show no tendency at all to exchange with Li+ (and also with the other cations). 3.3. IR Spectra of N2 Adsorbed on H+-Mord Activated at 673 and 973 K. 3.3.1. OH Stretching Region (3800-3300 cm-I). Figure 4 shows the spectra of H-Mord in the 38003300 cm-I range before and after N2 dosage. In the spectrum taken before N2 admission (dotted line), a weak peak centered
Geobaldo et al.
11170 J. Phys. Chem., Vol. 99, No. 28, 1995
38bO
3700
3600 35bq wavenumber cm'
3400
33bO
Figure 4. OH stretching region of H+-Mord outgassed at 673 K (dotted line) and the effect of increasing doses of adsorbed N? (full lines): equilibrium pressure from ca. lo-? Torr up to 50 Torr (1 Torr = 133.3 Pa).
a
I
3700
3600
3500
3400
3 300
wavenumber cm"
Figure 5. (a) Computer deconvolution of the 3609 cm-' 0-H stretching band: solid lines, experimentally observed spectrum and resolved components; dotted line, fit. (b) Computer deconvolution of the 3500 cm-' band (O-H...N? adducts).
at 3747 cm-' and a much stronger maximum centered at 3609 cm-' are observed. Zholobenko et al.*O and Wakabayashi et a1.I2 have speculated (by using a conventional band deconvolution program) that the asymmetric shape of 3609 cm-' band is due to the superimposition of two components centered at 3616-3612 and 3590-3585 cm-l. In Figure 5a, the result of such deconvolution (made with the program described in refs 24 and 25) is illustrated, for the sake of confirming that the hypothesis of the presence of two bands in that region is indeed very reasonable. The two components were assigned to the
stretching vibration of the acidic hydroxyls located in the main channel (high-frequency component) and in the side pockets (low-frequency component). Although the two bands are associated with OH oscillators having identical "local" Si(0H)A1 structures, nevertheless they have different frequencies and bandwidths (Bronsted groups in the smallest pores absorb at lower frequencies and have larger bandwidths). This can be understood if the electrostatic perturbation, due to the pore walls, is considered: in fact, OH groups vibrating in small pores (and cages) can form weak hydrogen bonds with the oxygens of the opposite walls and thus undergo a bathochromic shift (with subsequent increase of the bandwidth26-28),as compared to those located in larger cavities.21 We must also recall that this could not be the sole source of broadening, as even the high-frequency component (Le. that associated with OH groups in the larger channels and hence less perturbed and broadened by wall effects) has a half-width of about 32 cm-I, which is still considerable (if compared with the half-width of oscillators located in external positions). Progressive NZ dosage apparently causes the preferential erosion of the high-frequency component at 3616-3612 cm-' (Figure 4, full line spectra) and the parallel formation of a new band at about 3500 cm-I (fwhm 60 cm-I, AV = - 109 cm-I). As a decrease of the frequency accompanied by a simultaneous and proportional increase of the bandwidth is the usual result of a hydrogen-bonding type perturbation,26-28this proves that hydrogen-bonded >OH..*NZ species l 2 are formed. At high coverages, NZadsorption also causes the partial erosion of the silanols band at 3747 cm-' with concomitant development of a broad IR absorption at ca. 3710 cm-I: this is again the IR manifestation of a very weak hydrogen-bond type interaction between silanols and N?. In Figure 4 it is also clearly visible that while for high N2 coverages the high-frequency component (3616-3612 cm-I) seems completely consumed, the low-frequency one (35903585 cm-') on the contrary appears to be scarcely perturbed. Wakabayashi et a1.I2 have already noticed this fact and have concluded that Bronsted sites in side pockets are not accessible to the molecular nitrogen and that only those located in the main channels can interact with N?. We think that their conclusions are too drastic and simplified. To discuss this point, let us report the effect of CO adsorption on the V(OH) band at 3609 cm-' measured on the same sample, Figure 6 (the spectra have been already documented in ref 14; however, they are reported here because a direct comparison with those obtained with N2 is crucial). It is evident from Figure 6 that while the highfrequency component (3616-3612 cm-') is eroded first (in agreement with the NZ results), also the second peak (35903585 cm-I) is consumed (contrary to that observed for Nz), although only at the highest filling condition. We have interpreted such a behavior in terms of higher accessibility of the OH groups in the main channels with respect to OH groups in side pockets. However as, at the end, all OH groups were consumed with formation of OH-CO adducts, we have concluded that also the Bronsted groups located in side pockets can be engaged in hydrogen-bonding interaction with CO. This result contrasts with that hypothesized for N? by Wakabayashi et a1.I2 The difference is remarkable, especially if we consider that, as CO and NZmolecules have about the same dimension, it is difficult to understand why side pockets are not accessible at all to molecular nitrogen while they are not precluded to CO. Because of this contradiction, we have reconsidered in detail the band deconvolution analysis, especially in the presence of N?. As shown in Figure 5b, we can notice that at the highest N2 coverages the lower frequency component is not left
J. Phys. Chem., Vol. 99, No. 28, 1995 11171
N? Adsorption at 77 K on H-Mordenite 1
I
I
0.7:
0.50
.-aC L
a
P)
z
j 3 0.25
0.0 I
3800
I
3800
I
3600
3400
3200
wavenumber cm.'
Figure 6. OH stretching region of H+-Mord: effect of increasing doses of adsorbed CO.
unperturbed; in fact, the peak shifted to low frequency by about 14 cm-l (which is not a negligible shift), while the bandwidth has distinctly increased. Of course in the hypothesis that dinitrogen is not penetrating at all in the side pockets, this OH peak should remain unperturbed both in frequency and bandwidth (only a much smaller perturbation common to both N? and CO caused by long distance dielectric effects being expected). The observation that also the low-frequency band is perturbed by N? encouraged us to hypothesize that, as observed for CO, all Br~nstedsites of mordenite (with the exception of A') are acessible to N2 molecules. Having established this important point, we return to the debated problem of the attribution of the shoulder at ~ ~ 3 5 cm-I. 7 5 Two limiting hypotheses could be advanced: (i) The band at ~ 3 5 7 5cm-' is nothing else than the 35903585 cm-' component (attributed to the stretching mode of Br~nstedsites in the side pockets), shifted by about - 14 cm-l as a consequence of interaction with N2. Such a small shift (compared with the -109 cm-' characteristic of the linear >OH.-"? adducts formed in the main channels) could be justified only if the N2 molecules could not maximize their interaction with the Br~nstedsites (with formation of linear species) because of spatial restriction associated with the reduced dimension of the pocket. Inspection of Figure 2a shows clearly that for the H+ and Li+ cations in the A position (and Na+ as well) this is not the case. In favor of this conclusion let us remark that a -14 cm-' shift of the OH stretching peak should cause a change of the line width smaller than that experimentally observed. (ii) A second explanation can be that all the >OH..*N2 species formed with A, B, and C cations in straight channels and pockets are linear, they are all contributing to the band at ~ 3 5 0 0cm-' (as for CO adductsI4), and the shoulder centered at about 3575 cm-' observed after N2 admission has a different origin. This new point of view can be considered as reasonable only if a new explanation of the component at 3590 cm-l can be put forward. To do this, it is useful to compare the spectra
3700
I
3600
I
3500 wavenumber cm-'
1
3400
3300
Figure 7. OH stretching region of H+-Mord outgassed at 973 K: effect of increasing doses of adsorbed Nz; equilibrium pressure from ca. lo-? Torr up to 50 Torr ( 1 Torr = 133.3 Pa).
of H+-Mord activated at 673 K and at 973 K (Figures 4 and 7) both before and after Nz admission. From this comparison we notice that in the spectra of the sample activated at the highest temperature, the intensity of the peak at 3609 cm-' is greatly decreased (as is well-known, the high-temperature treatment causes partial dealumination with migration of Ai3+ to extralattice or partially extralattice positions) and that a new band in the 3747-3609 cm-l interval (which was probably already present also on the spectra of the 673 K activated sample but partially overshadowed by the major component at 3609 cm-I) emerges. This band is the same as that already noticed in the Li+, Na+-, K+-, Rb+-, and Cs+-exchanged Mord (Figure 3) and is the V(0H) of -AI-OH groups in partially extralattice positions. The effect of thermal treatments is well-known, and many authors such as Karge,22B a r t h ~ m e u f and , ~ ~Miller29have interpreted the changes in the above mentioned spectral region by hypothesizing the presence of many defective OH centers whose formation is ascribed to the relatively low stability of framework aluminum. From Figure 7 inspection, it is also seen that the -Al-OH groups absorbing at 3675 cm-l are engaged in hydrogen bonding with N?, with formation of a clear peak at 3580-3575 cm-' (Le. the same frequency of the shoulder observed for low-temperature-activated H+-Mord!). The formation of these adducts is better evidenced on 973 K activated samples because the spectroscopic manifestations are not overshadowed by those of the normal >OH**-N2adducts. It is evident that these findings substantiate the previously advanced hypothesis on the different origin of the 3575 cm-' shoulder. Further light will come from the inspection of the 2370-2310 cm-' region, where the 3(N=N) stretch of adsorbed N? is occurring. 3.3.2. N E N Stretching Region. Adsorbed N? on H+-Mord outgassed at 673 K gives two groups of bands at 2357-2350 cm-' (first group) and 2340-2325 cm-' (second group), Figure 8. The first group is associated with species irreversibly adsorbed at 77 K (that is, the decrement of the equilibrium pressure does not affect the intensity of this group of bands). The V(NEN) is considerably upward shifted with respect to the V(NEN) of the gas phase (9 = 2330.7 cm-I) or to the V(NEN)
Geobaldo et al.
11172 J. Phys. Chem., Vol. 99, No. 28, I995 I
,
I
0.070
v1 *
'4 0
3
f
0035
2 d
2370
I
2360
,
2350
I
2340
2330
,
2320
2: 3
wavenumber cm.'
Figure 8. IR spectra of N? adsorbed at 77 K and increasing N? equilibrium pressures (from ca. lo-' Torr up to 50 Torr), on H+Mord outgassed at 673 K. All spectra were background subtracted.
2370
2360
2350
2340
2330
2320
2310
wavenumber cm" Figure 9. As in Figure 8 but with H+-Mord outgassed at 973 K.
SCHEME 1 3710 C m ' (AT
- 37 C m ' )
I
F
of N2 in the condensed state (2327 cm-I) or in rare gas matrices (Ar, 2325.9, Kr, 2324.3; and Xe, 2322.0).30 If an average V (NEN)," value of 2324 cm-I is used as reference (vide infra), then AV = 33-26 cm-I. We assign these peaks to the V(N'N) of N2 end-on adsorbed on two types of A13+cusions in partially extralattice positions. The -A13+- *N2 bond is presumably mainly electrostatic in nature. The high electrostatic field of A13+ partially anchored to the framework has been already documented in the case of H+-ZSM5 outgassed at high temperature, where it has been shown that it is responsible for the formation of -A13+. * C O adducts characterized by an unusually high CO stretching f r e q ~ e n c y . ~The ' . ~ ~presence of -A13+. * ON?adducts even on samples treated at 673 K indicates that some dealumination occurs also at the lowest activation temperatures. The second group of bands belongs to species reversibly adsorbed at 77 K. Three main and heavily superimposed components can be singled out at -2325, 2330, and 2333 cm-I. On pure silicalite the interaction with internal and external silanols gives at least two bands in the 2328-2324 cm-' interval. In particular N? adsorbed on isolated external silanols gives a narrow peak (fwhm = 2 cm-l) at 2324 cm-I, while N2 adsorbed on silanols localized in internal nests gives a slightly broader peak (fwhm = 3.5 cm-I) at 2328 cm-'.33 The adsorption in the 2325-2320 cm-I range is thus assigned
to N2 interacting with -SiOH; in particular, the shoulder at 2325 cm-' is assigned to the V(NEN) of NZadsorbed on extemal silanols. This is also in agreement with the fact that it is observed only at the highest N? pressures in connection with the erosion of the silanol peak and the formation of the corresponding band at 3710 cm-I. The structure and the spectroscopic properties of the "extemal" complexes are summarized in Scheme 1. The peaks at 2334-2330 cm-I, absent on purely siliceous zeolites, are associated with the V(N"N) modes of N? adsorbed on two types of Brcdnsted sites whose polarizing power is on the order 2334-2330 cm-I. It is tempting to hypothesize that the two peaks are due to the Bronsted acid sites of the main channel (B and C: highest polarizing power) and of the side pockets (A: lower polarizing power). The slightly higher stability of the second species could be due to nonspecific van der Waals interactions with the walls of the narrow pockets, which contribute an extra stabilization energy to the complex. The structure and the main spectroscopic properties of these complexes are summarized in Scheme 2. Figure 9 shows the spectra of N? adsorbed at 77 K on H+Mord activated at 973 K in dynamic vacuum. These spectra are characterized by the high intensity of the peak at 2350 cm-' ascribed to the N2 complexes with partially extraframeworkAi3+ (Scheme 3), as expected because these high-temperature treatments are associated with extensive framework dealumination. It is interesting to notice that only the concentration of one of the A13+ species already present in the previous sample has been increased by the thermal treatments. Parallel to the increase of partially extraframework A13+, a strong decrease of the V(N=N) peak of the Bransted groups***N?
Nz Adsorption at 77 K on H-Mordenite
J. Phys. Chem.,Vol. 99,No. 28, I995 11173
0.12
3
J
8
3d
0.06
d
2 70
I
2360
2350
2340
23'30
2i20
2: 0
wavenumber cm"
Figure 10. As in Figure 8: Li+-Mord, after the second exchange.
Figure 11. As in Figure 8: Na+-Mord.
SCHEME 4
Very few Lewis acid sites (A13* partially extralattice) are formed upon activation; in fact the D(NIN) bands of the A13+.-*N2adducts have a very low intensity. 3.4.2. Na*-ExchangedMord. As Br~nstedsites' manifestations (peak at 3609 cm-') are no longer observable, this means that the absorption in the 2350-2320 cm-' range must be completely ascribed to Naf* complexes formed in the side pockets and in the main channels (A, B, and C sites). In fact the Na+ cation is not large enough to prevent penetration of the N2 in the side pockets (Figure 2a). From Figure 11, we see that the V(N'N) of the complexes (see Scheme 4) falls at 2334 cm-l, i.e. a frequency lower than that observed on Li*exchanged Mord. This is a direct consequence of the smaller polarizing power of Na* (vide infra). The band is quite complex, as shoulders at 2337 and 2327 cm-' are clearly observable at intermediate coverages. Moreover, the main peak shows a distinct tendency to shift to lower frequency when the filling conditions approach the maximum values; a similar effect has been already described for the Na+* C O peaks in the Na-ZSMWO system36and has been ascribed to changes in the dielectric constant of the medium inside the pores as liquid-like CO fills the open spaces of the channels. Besides the shift to lower frequency, the peak is also distinctly broadened and largely asymmetric on the low-frequency side. For the time being it is not possible to advance a reasonable justification of all these facts. We do not exclude (but we cannot demonstrate it either) that the two shoulders at 2327 and 2337 cm-I are the lateral branches of a hindered rotator (we shall further retum to this problem when the results conceming all the other exchanged zeolites are compared, vide infra). Also in the present case we observe the complete absence of A13+-*.N2 complexes: this means that the tendency of A1 to migrate into extraframework positions is smaller on alkaliexchanged Mord than on H+-Mord. Finally let us notice that the total intensity of the V(N3N) absorption is decreased (with respect to Li+-Mord) by about 33% (as expected, because the polarizing effect of Naf is smaller than that of Li+). 3.4.3. K+-,Rb+-, and Cs+-ExchangedMord. At the highest equilibrium pressures, the main peak associated with K+* "2
N
Ill
YM'
-
adducts is observed (as expected), while the fraction -SiOH.**N2 is unaffected. Notice how the peak at 2330 cm-' (tentatively assigned to the N? adducts of Br~nstedgroups in side pockets) is no longer observable. 3.4. IR Spectra of NZAdsorbed on Exchanged Mordenite. 3.4.1. Li+-ExchangedMord. As shown in curve c of Figure 3, a large fraction of cations is not exchanged. Hence, N? always finds a situation characterized by the simultaneous presence of H+ and Lif sites. The effect of N? interaction is shown in Figure 10. We notice that the new and strong peak at 2341 cm-' (slightly tailed on the low-frequency side) is undoubtedly the 3(N=N) stretch of the complex shown in Scheme 4 formed in both linear channels and in side pockets. The IR absorptions in the 2335-2320 cm-I range with clear shoulders at 2333 and 2325 cm-' are due to N2 adsorbed on unexchanged Bronsted groups and on silanols, respectively. The intensity of the whole absorption in the 2345-2325 cm-' range is much higher than that of the purely protonic complexes in H-Mord. This means that the extinction coefficient of the 5(NZN) mode at 2341 cm-l has increased because of the perturbing effect of the polarizing field. This is not unexpected because the effect of the field is to polarize the NEN molecule with subsequent appearance of a dipole moment and increment of the dipole moment d e r i ~ a t i v e . ~ This ~ result greatly differentiates Nz from CO: in fact, in the latter case, the specific intensity of the V ( C 5 0 ) stretch, being dictated mainly by the original properties of the molecule itself (possessing a dipole moment and an appreciable dipole moment derivative), is influenced to a much lesser extent by the presence of a polarizing field.35
Geobaldo et al.
11174 J. Phys. Chem., Vol. 99, No. 28, 1995 I
I
I
I
0.08
0 07
-
I
I +
'3
'$
8
004
f
%B
0035
s:
n -2
d
000
2370
2360
2350
2340
2330
2320
2
wavenumber cm" Figure 12. As in Figure 8: K+-Mord.
adducts is now observed (Figure 12) at 2329 cm-I, i.e. at a frequency even lower than that of the corresponding Na+- *Nz adducts (and a fortiori of the Li+. *N2 complexes). The band is quite complex as a shoulder at 2327 cm-I, and two broad tails at 2335 and 2321 cm-' can be seen especially at the lowest coverages. The intensity is also somewhat lower. As on Na+-Mord, the main v(NZN) peak undergoes a (smaller) downward shift with coverage. Strange enough, the broadening effects observed on Na+-Mord are absent. Moreover, the fwhm of the peak is smaller and does not change with the dosage. We assign the main peak to the V(NSN) of the structure, shown in Scheme 4,located in the main channel. The shoulder at slightly lower frequencies (2327 cm-') could be assigned to complexes formed in the side pockets (as hypothesized for H'exchanged zeolite). This attribution is debatable, as the K+ radius is now large enough to possibly cause steric effects. The two broad tails at higher and lower frequencies could be the residual rotational envelope of hindered rotators. As in the Naf case, the hypothesis cannot be demonstrated yet. As far as Rb+- and Cs+-exchanged Mord are concerned, the spectra are reported in Figures 13 and 14. They look very similar to those of the K-Mord/Nz system, only the frequency of the maxima is gradually displaced toward lower V . Also in this case the main peak is ascribed to the stretching vibration of the Rbf. -N? and Cs+. * -N?complexes in the main channels. This is justified by the fact that when these big ions occupy the lateral pockets, they leave no space for N? adsorption (Figure 2b). In the case of the Csf-.*N? system (Figure 14), we distinctly notice that the main absorption is accompanied by high-frequency and low-frequency branches in the 2345-2335 and 2321-2315 cm-' intervals. Due to the high intensity of these lateral absorptions, we think that the only plausible explanation is that they are the branches of the hindered rotators located in the main channel. The fact that the Nz molecules in Mf..*N2 adducts appear to acquire more and more rotational freedom on passing from Li+ to Cs+ is associated with the gradual diminution of the polarizing field of the cation, which approximately decreases with the second inverse power of the cationic radius (vide infra).
2370
2360
2350
2340
2330
2320
2310
2320
2 0
wavenumber cm"
Figure 13. As in Figure 8: Rb+-Mord.
0.0
3
.g 8
s
e
00
w
n 4
2370
2360
2350
2340
2330
wavenumber cm.' Figure 14. As in Figure 8: Cs+-Mord. The two vertical arrows indicate the rotational branches of the hindered rotational motion of the N? molecules confined in the zeolite channels (see text).
3.5. Effect of the Cationic Radius on the N2 Stretching Frequency: General Considerations. In two recent p a p e r ~ l ~ . ~ ' concerning the interaction of CO with alkali-exchanged ZSM-5 and Mord, we have reported on the linear dependence of the V ( C 0 )of M+**COadducts (main band) vs U(RX RCO)~, where R, is the ionic radius of Na+, K+, Rb', and Cs+ (taken from ref 38) and RCO represents the radius of the carbon monoxide molecule (Rco = 2.1 A from ref 37). This linear relation has been considered as a clear proof of the predominant role played by electrostatic forces in polarization of the CO molecules at the cationic sites in zeolites. In refs 14 and 17 the strength of the perturbing electric field at the center of
+
J. Phys. Chem., Vol. 99, No. 28, 1995 11175
N2 Adsorption at 77 K on H-Mordenite
TABLE 1: Frequency (cm-') of the Main IR Absorption Band of Nz and of COI4 Adsorbed at 77 K on Alkali-Metal-Exchanged Mordenites" CO stretching N? stretching N? integrated cation frequency (cm-I) Vsnm-l frequency (cm-') intensity (au) Li+
2188 2177 2 163 2159 2155
Na' K+ Rb+
cs+
234 1 2334 2329 2328 2326.5
8.2 6.1 3.5 2.7 2.0
N, frequencies
CO frequencies
1 0.797
0.608 0.508 0.304
The local electric field at the cationic sitesIJ and the normalized D(N=N) integrated area (measured at the same N? pressure) are also reported. the CO molecule was also inferred by using the AD-electric field relationship calculated by Pacchioni et al.39 As it can be safely supposed that the local electric field generated by each cationic site is not dependent upon the probing molecule, we infer that the electric fields reported in ref 14 are those sensed also by the Nz probe. The experimental stretching frequencies of the M+.--CO and Mf*-*N2adducts formed in the mordenite main channel are compared in Table 1 together with the local electric field strength generated by the corresponding site. In a recent c o n t r i b ~ t i o n ,we ~ ~ have shown how the experimental electric field probed by CO in cation-exchanged ZSM-5 and mordenites can be theoretically reproduced on the basis of a simple electrostatic model by taking into account the positive contribution of the cation itself and the negative contributions due to the surrounding oxygen atoms and to the polarized zeolitic framework. The data reported in Table 1 are important because they represent an experimental relationship between the stretching frequency of a polarized N? molecule and the strength of the electric field causing the polarization. The predominant role of electrostatic forces is also illustrated by Figure 15, where it is shown that the same linear dependence of F(C0) and V ( N N ) upon l/(Rx R ~ Oand ) ~l/(Rx R N N is ) ~verified. The linearity is clearly documented for Na+, K+, Rb+, and Cs', while the case of Li+ (and that of H+) is slightly anomalous, as the induced shift is lower than expected on the basis of the simple assumption that Li+ ions possess the whole f l charge. This fact has been a t t r i b ~ t e d lto ~ .the ~ ~reduced positive charge present on lithium ions (f0.76)el instead of +lie[) caused by a partial covalent character of the O.**Libond in the structure shown in Scheme 4. The partial covalency of the Li'Z- (Z- = zeolite skeleton) is associated with the high polarizing power of the small Li+ cation.I4.l7 The absolute frequency change induced by the polarizing fields on the F ( N N ) is, however, approximately one-third of that observed for CO, thus showing that N? is less influenced by an external electric field than CO is (this is still an expected phenomenon, CO having its own dipole moment). The fact that both V(NN) and V(C0) show a linear dependence on U(Rx RN?,co)*is especially remarkable, as it strongly suggests an end-on interaction of N? with extraframework cations (as in the CO case). This conclusion, based on experimental data only, is in agreement with theoretical results recently reported by several g ~ o u p s who ~ - ~have ~ performed ab initio calculations on the M+. *N2 (M+ = Li+, Na+, Cr', and Co+) clusters using different basis sets and levels, and they have concluded that, whatever the cation M+, the basis set, and the level, the most stable adducts are in a linear configuration. Koubi et aL4* have also reported on the NZN stretching frequencies of Mf. .N? adducts obtained from their ab initio calculations. These data can allow, in principle, a direct
+
+
+
-.-- . *
v nm"
I
I
I
I
8
10
12
14
.
----
l/(Rx + Rads)' (nm-')
+
Figure 15. Comparison between ?(CEO) us l/(& R,,)? and D(NIN) us l/(Rx RN$; R, = alkali cation radius; on the left side D(CmO), on the right side D(N=N). The vertical scale in Vnm-I (left-hand side)
+
shows the electric field created by cations, as deduced from interaction with CO (ref 14).
comparison with the present experimental data. However, such comparison is not as direct as it appears because the important role played by the negative zeolitic framework surrounding the cation (and already recognized by the authors of ref 42) is neglected. The importance of the negatively charged framework in determining the stretching frequency of an adsorbed diatomic molecule has been recently shown in ref 13, where the interactions of H2 and CO with sodium forms of the zeolites ZSM-5, MOR, faujasite-type Y and X, and Linde-4A have been investigated by means FTIR spectroscopy. In this work it has been shown that the V(C0) and V ( H H ) stretching frequencies are greatly influenced by the surrounding zeolitic framework. A similar trend has been found by Makarova et al.,"3 who have studied the H?adsorption on different zeolites in the protonic form. From these data, it is thus reasonable to suggest that also the F ( N N ) frequency will be influenced by the framework surrounding the cation site. On the basis of these considerations, a quantitative comparison of the experimental results and of ab initio calculations on simple M+.*CO adducts must be done with care. There is an important point which now merits a specific discussion. From Table 1 we notice that the V(") of dinitrogen adsorbed on K+, Rb+, and Cs+ is lower than that of gaseous nitrogen (2330.7 cm-I). Consequently, although the overall function dependence of the P(NN) upon l/(Rx RNJ* is the same as observed for CO, the shifts (calculated using the gas frequency as reference) are positive for Li+- and Naf-Mord and negative for K+-, Rb+-, and Cs+-Mord; in contrast, CO always shows hypsochromic shift of its stretching frequency when interacting with all the alkali cations both in ZSM-5 and in M ~ r d . ' ~ .This ~ ' fact has been evidenced also by Yamazaky et al.,@who have investigated the IR spectra of M+***N2adducts in alkali-metal-exchanged ZSM-5. The authors have explained this result in terms of space limitations inducing different
+
Geobaldo et al.
11176 J. Phys. Chem., VoE. 99, No. 28, 1995 orientations of the N? molecules with respect to the pore surface (end-on in the case of Li’- and Na+-ZSM-5 and tilted in the case of K+, Rb+-, and Cs+-ZSM-5 because of space restrictions; see Figure 3 of that work). In other words, only in the case of Li+ and Naf cations could the N? molecule freely orient itself to maximize the interaction with the electric field, while this would not be possible (because of space restrictions) when the large Kf, Rb+, and Csf cations are involved. However, if this is the explanation, why are all the experimental frequencies showing the same dependence upon l / ( R x-I-R N J ~(which is a typical Coulombic term in an end-on structure)? And why is the same holding also for CO adducts in both alkali-exchanged ZSM-5 and mordenites (which have distinctly different internal geometries)? In this respect, let us remark that the use of the shift instead of the frequency could be misleading. In fact, while the frequencies are an experimental observable, the 2330.7 cm-’ value (P(NN) of the gas) assumed as reference for the shift evaluation is not observable in the present experiment, and it is strictly correct only for the M+*..N? adducts in the gas phase. The use of this reference value can consequently be arbitrary in the present case, where the adducts are formed inside narrow channels. To illustrate this concept, let us consider the forces which are influencing the frequency of adsorbed N?. They can be described schematically as follows: (i) dispersion forces (mainly due to the siliceous framework acting as a solvent). These forces could influence the frequency of N? as does an inert matrix surrounding and encaging a N2 molecule in a matrix isolation experiment. These forces can be considered, to a first approximation, as characteristic of the framework and independent of the ions present in the zeolite; (ii) electrostatic forces exerted by the electric field set up by extraframework cations and surrounding (negatively charged) zeolite framework. Only the forces described in (ii) can shift the F ( N N ) toward higher values; in contrast, forces described in (i) usually have an opposite (although smaller) effect. From this consideration it is evident that the best internal “reference frequency” is that of N? under the effect of dispersion forces only, and that only the deviations from this reference value can be ascribed to field effects. The B(NN) of gaseous Nr is consequently not the best reference state for the present system. A better reference could be Y(NN) in the solid (2327 cm-’) or V ( N N ) in rare gas matrices (Ar, 2325.9; Kr, 2324.3; Xe, 2322.0).30 The Y(N=N) of N? in Xe should be preferred because it has a refractive index and dielectric constant similar to those of the guest zeolite. Of course the Y(N=N) of nitrogen physically adsorbed in silicalite (a fully siliceous material with channels of nearly identical radii) should be preferred. Unfortunately, unlike the CO case, for which a peak at 2138 cm-’ (i.e. 5 cm-’ lower than the frequency of CO gas) has been clearly observed on silicalite and alkaliexchanged ZSM-5 and ascribed to physically adsorbed species, the G(N‘N) of physically adsorbed NZin zeolites and silicalites cannot be observed in the IR. In the absence of this IR key figure, we can use as reference the Raman frequency V(N=N) = 2324 cm-’ reported in ref 45. On the basis of this value, it is inferred that (i) the effect of the cations is always to increase the V(N”N) frequencies and (ii) the explanation based on tilting43is not strictly necessary (although it is not excluded to give a minor contribution to the observed frequency shift). Of course as the cation radius increases, its polarizing effect decreases, and the adsorption enthalpy and the shift do the same. The N2 molecule will thus progressively tend to acquire rotational freedom (when the available space is sufficient, as in the main channels) around the direction of maximum field, and this will be evidenced by the appearance of rotational branches of hindered On this basis we start to understand
the reason for the presence of low- and high-frequency tails in the spectrum of adsorbed N:, (especially evident in Cs+-Mord). More experimental data (at different temperatures) are, however, needed for a more conclusive analysis. As a final remark, we will remember that a linear dependence is expected between the integrated area A of the N2 absorption band and the square of the external electric field E.16.48 To investigate this point, in Table 1 the (relative) integrated intensities of the M+.*.N? adsorption bands (evaluated at the same N2 pressure) are reported. The area of the Y(NEN) band of the Li+*-*N?adduct has been normalized to unity. In agreement with t h e ~ r y ’an ~ .increment ~~ of the integrated area with the increase of the external field is documented, which approximately follows the A = I? relationship. This result is not associated with a lack of general validity of the A = I? relationship: it simply means that our intensity data are not sufficiently accurate for the experimental reasons detailed in section 2.
4. Conclusions The present study has shown that dinitrogen specifically interacts (at 77 K) with protons and cations located in the main channels and side pockets of M-mordenites (M = Hf, Lif, Na+). For Rb+-Mord and Cs+-Mord only cations located in the main channels are involved in the interaction. The case of Kf-Mord is borderline. The positive electric field associated with extraframework cations (and protons) causes polarization of the N? molecule, giving rise to an induced dipole moment and a corresponding dipole moment derivative, the latter being responsible for the appearance of an R-active (NEN) stretching mode. For dinitrogen interacting with cations present in the main channels, Y(NEN) was found to change linearly with l / ( R x R N ? ) ~where , R , is the cation radius. This strongly suggests formation of end-on adducts, as in the case of CO adsorption, for which an analogous linear dependence of ?(CEO) has been reported. When the appropriate reference is used, frequency shifts for the N=N stretching vibration of adsorbed N2 were always found to be positive, as expected. The presence of distinct tails on both sides of the main IR absorption band, especially noticeable for N?/Cs+-Mord, suggests that N? molecules (inside the zeolite channels) behave as hindered rotators. The rotational freedom decreases with decreasing cation radius because of a parallel increase in polarizing power.
+
Acknowledgment. This work as been supported by the Spanish DGICYT, F’royecto Pb93-0425, and by the Italian MURST and CNR (Progetto strategic0 Chimica e Tecnologia). References and Notes ( 11 Szostak. R. M. Molecular Sieves: Van Nostrand Reinhold: New York. 1989. (2) Meier, W. M.: Olson, D. H. Atlas of Zeolite Structure Types; Butterworths: London, 1987. (3) Meier. W. M. Z . Kristallogr. 1961, 115, 439. (4) Schlenker. J. L.; Pluth. J. J.; Smith.J. V. Mater. Res. Bull. 1978, 13. 901. ( 5 ) Schlenker. J. L.: Pluth, J. J.: Smith, J. V. Mater. Res. Bull. 1979, 14, 751. (6) Mortier. W. J. Compilation of Extra Framework Sites in Zeolites; Buttenvorths: London, 1982. (7) Choen de Lara. E.; Kahn, R.: Seloudoux. R. J . P h y . Chem. 1985, 83. 2646. (8) Barrachin, B.: Cohen de Lara. E.: Delaval. Y . J . Chem. Soc., Faraday Trans. 2 1986, 82. 1953. (9) Cohen de Lara, E. Mol. Phvs. 1989, 66, 479. (10) Kustov. L. M.: Kazansky. V . B. J . Chem. Soc., Faraday Trans. 1991. 87. 2675.
J. Phys. Chem., Vol. 99, No. 28, 1995 11177
Nz Adsorption at 77 K on H-Mordenite (11) Bordiga, S.; Scarano, D.; Spoto, G.; Lamberti, C.; Zecchina, A,; Otero, Arean, C. Vib. Spcctrosc. 1993, 4, 273. (12) Wakabayashi, F.; Kondo, J.: Wada, A,; Domen, K.; Hirose, C. J. Phys. Chem. 1993, 97, 10761. (13) Bordiga, S . ; Garrone, E.; Lamberti, C.; Zecchina, A.; Otero Arein, C.; Kazansky, V. B.: J . Chem. Soc., Faraday Trans. 1994, 90, 3367. (14) Bordiga, S.; Lamberti, C.; Geobaldo, F.; Zecchina, A,; Tumes Palomino, G.; Otero Arean, C. Langmuir 1995. 11, 527. (15) Staemmler. V. Chem. Phvs. 1975, 7, 17. (16) Cohen de Lara. E.; Delaial, Y. J. Chem. Soc., Faradav Trans. 2 1978. 74, 790. (17) Zecchina, A,; Bordiga, S.; Lamberti, C.; Spoto, G.; Carnelli, L.; Otero Arean. C. J . Phvs. Chem. 1994, 98, 9577. (18) Connolly, M. L. Science 1983, 221, 709. (19) Bordiga, S . : Ricchiardi, G.; Spoto, G.; Scarano, D.; Carnelli, L.; Zecchina, A.; Otero Arean. C. J . Chem. Soc., Faraday Trans. 1993, 89, 1843. (20) Zholobenko. V. L.; Makarova, M. A,; Dwyer, J. J. Phys. Chem. 1993, 97, 5962. (21) Jacobs, P. A.; Mortier, W. J. Zeolites 1982, 2, 226. (22) Karge, H.G. Z. Phys. Chem. Neue Folge 1980, 122, 103. (23) Ha. B. H.; Barthomeuf. D. J. Chem. Soc.. Faraday Trans. I 1979, 75. 2366. (24) Lamberti. C.: Bordiga, S . ; Cerrato, G.; Morterra, C.; Scarano, D.; Spoto, G.: Zecchina. A. Comput. Phys. Commun. 1993, 74, 119. (25) Lamberti, C.: Morterra, C.; Bordiga. S . ; Cerrato, G.; Scarano, D. Vib. Spectrosc. 1993. 4. 273. (26) Pimentel. G. C.; McClellan, A. L. The Hydrogen Bond; W. H. Freeman: San Francisco. 1960; Chapter 3. (27) Buzzoni, R.; Bordiga, S.;Ricchiardi, G.; Spoto. G.; Zecchina, A. J. Phys. Chem., in press. (28) Hadzi. D.; Bratos, S. The Hydrogen Bond; Schuster, P., Zundel, G., Sandorfy, C., Eds.: North Holland: Amsterdam, 1976; Vol. 11, pp 565611. (29) Miller, J. T.: Hopkins. P. D.; Meyers, B. L.; Ray, G. J.; Roginski, R. T.; Zajac, G. W.; Rosenbaum N. H. J. Catal. 1992, 138, 115. (30) Lowen, H. W.; Jodl. H. J.; Lowenschuss. A,; Daufer, H. Can. J . Phys. 1988. 66. 308.
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