8120
J. Phys. Chem. 1992, 96, 8120-8125
NMR Investigation of the Distributton of Benzene in NaX and UaY Zeolites: Influence of Cation Location and Adsorbate Concentrationt Sbang-Bin Lib* Long-Ja Ma, May-Whei Lin, Institute of Atomic and Molecular Sciences, Academia Sinica, P.O. Box 23-166, Taipei, Taiwan 10764, ROC
Jin-Fu Wu,and Tun-Li Chen Department of Chemistry, Tamkang University, Tamsui, Taiwan 25137, ROC (Received: March 24, 1992; In Final Form: June 10, 1992)
The adsorption of benzene on dehydrated NaX (Si/AI = 1.23) and NaY (Si/AI = 2.49,2.70) zeolites has been studied directly by 'Hand I3C NMR measurements of the adsorbed benzene and probed indirectly by the combination of lz9XeNMR and isotham measurements of the coadsorbed xenon. Powdered zeolite samples with varied adsorbate concentrations were investigated. Detailed macrascopic and microscopic adsorption phenomena of benzene on faujasite-type zeolites, including the loading capacity, mobility, and sites of adsorption, are presented in terms of measurements of NMR line widths and chemical shifts.
Introduction The adsorptive properties of benzene on zeolites have drawn much research attention in recent decades due to the related role of aromatics in zeolitic catalysts for alkylation and cracking reactions. In particular, the system of benzene adsorbed on synthetic faujasite-type zeolites has been a subject of major interest; for instance, its mobility and thermodynamic properties have been studied by conventional diffusion'" and adsorption7+ techniques. Moreover, the adsorbate-zeolite interactions and related motion and location of the adsorbate molecules within the zeolite cavities and have been investigated by many theoretical calc~lations'"~~ by various spectral methods such as UV,I6J7IR,17-26neutror1,2"~ R a m ~ n and , ~ ~NMR.3242 However, despite the fact that the system has been extensively studied, little attention has been devoted to the equilibrium state of adsorption and the distribution of adsorbate within zeolite cavities. Sorption diffusivities, which characterize transport processes, are commonly inconsistent when obtained from different technique~?~* The possible origins of the discrepancies arising from transport r e s i ~ t a n c e s ~and ~ - ~from ~ adsorbateadsorbent charact e r i s t i ~ s ' ~ ,have ~ ~ Jbeen ~ - ~discussed. ~ When benzene molecules are adsorbed on dehydrated faujasite-type zeolites, the common assumption is that the molecules adsorb within the zeolite supercage and disperse uniformly. This assumption was first questioned by Pines and c o - ~ o r k e r s ~who, ~ * ~by * using 129Xeand 'H multiplequantumNMR, showed sample activation dependence on the distribution of organic adsorbates within the supercages of the powdered NaY zeolite. This observation was later reexamined and confirmed by Liu and co-worker~'~@' who studied samples having varied preparatory and experimental conditions using Iz9Xe, 'H,and I3C NMR; they concluded that thermal treatment of the benzene-zeolite sample at elevated temperature (250 "C) for an extended period of several hours is indispensable to ensure a uniform adsorbate distribution within the supercages of NaX and NaY zeolites. Moreover, this effect depends not only on the Si/Al ratio of the adsorbent samples and related samplebed configurations but also on the adsorbent and adsorbate content^?^ Following the pioneering works of Ito and Fraissard6I and Ripmeester,Q '%e NMR of xenon adsorbed on zeolite has proven to be a sensitive probe of its local environment due to its chemical inertness and excellent ~ensitivity.6~ In this work, we used 'H and I3CNMR measurements of the adsorbed benzene in conjunction with lZ9XeNMR and adsorption isotherm measurements of the coadsorbed xenon to study the "homogeneous" adsorption behavior of benzene on faujasite-type zeolites with various Si/Al ratios. Author to whom correspondence should be addressed. Presented, in part, at National Meeting of the American Chemical Society, New York City, Aug 1991.
Detailed macroscopic and microscopic adsorption states of the benzene in various NaX and NaY zeolites are discussed in terms of NMR line widths and chemical shifts and are compared with the results obtained from other studies. ExperimeaW Section
Powdered, binderless NaX (Si/Al = 1.23) and NaY (Si/Al = 2.49, 2.70) zeolites (Strem Chemicals, Inc.), each having an average crystalline diameter of ca. 1 bm, were used as adsorbents. The crystalline framework structure of these materials was confmed by X-ray diffraction (XRD). Their chemical compositions were determined by ICP-AES and the corresponding Si/Al ratios were further a " e d by "si MAS NMR measurements. Before adsorption of guest molecules, a known amount (typically ca. 1 g) of hydrated zeolite in the sample tube was dehydrated by gradual heating to 400 OC in vacuum Torr; 1 Torr = 133.32 Pa) and was then maintained at this temperature for at least 15 h. The sample tube configuration was designed so that a 1 0 " standard NMR tube joined to a vacuum valve could be conveniently set up for adsorption or desorption of adsorbate molecules on a vacuum apparatus and for isolation of the sample from the atmosphere during NMR experiments. The zeolite sample (ca.1 g) had a pacing height of about 35 mm in the NMR sample tube, which had an overall length of about 250 mm. After dehydration, benzene guest molecules (adsorbates) were introduced into the samples of the host zeolites (adsorbents) by vapor transport at room temperature (22 "C). The exact amount of benzene coverage was determined gravimetrically; the average number of benzene molecules in each zeolite supercage is hereafter denoted 6. Prior to the experiments, samples with various benzene concentrations were subjected to thermal treatment at 250 OC for 10 h to render uniform adsorbate distribution within the zeolite ~ a v i t i e s . ~ ~Gaseous 9~ xenon was coadsorbed into the samples on the vacuum apparatus; the xenon equilibrium pressure was measured by an absolute-pressuretransducer ( M I S Baratron). The adsorption isotherms of the coadsorbed xenon in the samples were measured volumetrically at 22 OC. All NMR experiments were done at 22 OC on a spectrometer (Bruker MSL-300) using a broad-band NMR probe with proton-decoupling option. The field strength of the wide-bore superconducting magnet was 7.05 T, corresponding to resonance frequencies of 300.13, 75.47, and 83.01 MHz for nuclei of 'H, "C, and lZ9Xe,respectively. For the lz9XeNMR experiments, free induction decays (FID) were recorded at room temperature following a single radio-frequency pulse (ca. 30') at 0.3-s intervals. We followed this procedure because the adsorbed xenon has a large spin-lattice relaxation time (Tl 2 3 s). In order to ensure adequate signal-to-noise (SIN) ratios, we accumulated typically 1OCMk 240 OOO FID depending on the amount of xenon adsorbed on the
0022-3654/92/2096-8 120S03.OO/O 0 1992 American Chemical Society
Benzene Distribution in NaX and NaY
The Journal of Physical Chemistry, Vol. 96, No. 20, 1992 8121 n 00 05
IO 15
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Xenon loading /atom Pressure /torr FIgm 1. Room-temperature (22 " C ) adsorption isotherms of xenon coadsorbed with various amounts of benzene in various zeolites: (a) NaX, Si/AI = 1.23; (b) Nay, Si/AI = 2.49; (c) Nay, Si/Al = 2.70.
Figure 2. Variation of 129Xechemical shift with the number of xenon
atoms coadsorbai per supercage for various benzcntzeolite systems: (a) NaX, Si/Al = 1.23; (b) Nay, Si/Al = 2.49; (c) Nay, Si/Al = 2.70. 250
sample. The chemical shift of IBXe was referred to that of gaseous xenon at zero density according to the equation given by Jameson.64 All resonance signals of %e adsorbed on the samples were shifted to higher frequency relative to the reference, which we define to be the positive direction. The 'HNMR spectra were also obtained by the singlepulse sequence using a 2-s recycle delay. One-hundred twenty accumulated FID were typically adequate to generate spectra with an adequate SIN ratio. The I3CNMR spectra were obtained with proton decoupling; 6000 scans were typically averaged every 0.6 s. The reference for the chemical shifts of both 'Hand 13Cresonances was liquid benzene (adjusted to tetramethyhilane, TMS, as standard) at the same spectrometer settings.
R d t s and Discussion Figure 1 displays xenon adsorption isotherms at room temperature (22 "C) of the coakbed xenon for three zeolite samples loaded with varied amounts of benzene. A consistent decrease of adsorption with increasing 8 was found for each b e m e z e o l i t e system. By comparing the slope at low xenon pressures, i.e., in the Henry's law region, we obtained for the adsorption strength NaX(1.23) > NaY(2.49) 1 NaY(2.70). Moreover, extrapolation along the asymptotic 8 valua to null adsorption in Figure 1 enabled the estimation of the saturation concmtration of benzene adsorbed on zeolites with varied Si/Al ratios; they follow the relation NaX(1.23) < NaY(2.49) < NaY(2.70). The dependence on xenon pressure of the measured '%e NMR chemical shift (6) and line width (Aw) was also recorded for each adsorbattadsorbent sample. In combination with the adsorption data in Figure 1, the dependence of the lz9Xechemical shift on the density of adsorbed xenon is presented in Figure 2. For a given 8, the observed 6 increases with increasing xenon pressure due to increasing interactions among xenon atoms. In contrast, for a given xenon loading, the increase of 6 with increasing 8
,cage- I
r
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0 NaX(123)
NaY (2 48)
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2
e
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Hgwe 3. Dependence of (a) '29Xechemical shift at zero xenon load(6,) and (b) slope (ux,~,), both obtained from Figure 2, on benzene
coverage (e) for various benzenezeolite systems.
characterizes a decrease in the internal void space of the benzentzeolite samples, hence, a decrease in the mean free path of xenon.61 The measured xenon chemical shift is described as the sum of three terms: 6 = 60 + 4 + ~xxcxlxe (1) Here 6o = 0 is the reference; 6, corresponds to the shift at zero xenon loading which represents in turn the sum of two contributions, 6, = 6,(0) + &(e), and can be obtained by extrapolation
Liu et al.
8122 The Journal of Physical Chemistry, Vol. 96, No. 20, 1992 TABLE I: N d Didbution md Related Experimental Results for Bename Adsorbed in Different Zeolites
no. of site I1 Na+/supcrcage no. of site 111 Na+/supcrcage fitting parameters'
W)
zeolite(Si/Al) NaX(1.23) NaY(2.49) NaY(2.70) 4.0 3.9 3.5 3.0
0.0
0.0
65.64
51.36 19.59
55.58
-5.52 2.24 9.1
-5.30
27.80
AI A2 A3
U'Wb 4&('3C)b benzene saturation coverageC
130.9 4.0-4.2
19.11 -5.30 1.40
1.56
I
2
4
3
I
9.3 131.1
9.5
4.9-5.2
5.1-5.3
5
1
132.3
"Obtained by regressional fitting from eq 2; 6,(0) in ppm, A l in ppm.(molecules/cage)-', A2 in ppm(molecules/cage)-2, A3 in p p m (m~leculcs/cage)-~.bChemicalshifts of benzene obtained by 'H and I3C NMR below saturation benzene coverages; both in ppm (k0.l). cEstimated from Xe adsorption isotherm, Iz9Xe,'H,and 13CNMR results. of the straight lines (for various 8 ) in Figure 2; and the results are presented in Figure 3a. The term 6,(0) arises from the interactions between xenon and the zeolite walls in the absence of benzene (Le., 8 = 0) and hence should be independent of 8. The term 6,(8) arises from the interactions between xenon and the adsorbed benzene. The last term in eq 1, being characteristic of xenon-xenon interactions in which binary collisions of xenon atoms dominate, is proportional to the density of the adsorbed xenon. The variations of uxxtxe with 8, in Figure 3b, which were obtained from the slopes of the straight limes in Figure 2, therefore reflect changes of the microscopic environmentsof the adsorbates within the zeolite supercages. A nonlinear increase of 6, upon increasing 8 is evident for all the adsorbawdsorbent systems (Figure 3a); the curvatures reflect many-body interactions between a single xenon atom with benzene molecules within the zeolite supercage. Regressional fitting of the experimental values to a third-degree polynomial produces
6, = 6,(0)
+ Ale + A2e2 + ~
~
8
3
1
2
3
5
I
J 2
1
3
4
8 /molecule
5
cage'
Figure 4. Variation of '29XeNMR line width (h) with respect to benzene coverage (e) for various benzene-zeolite systems: (a) NaX, Si/Al = 1.23; (b) Nay, Si/Al = 2.49; (c) Nay, Si/Al = 2.70.
(2)
in which 8 is the benzene loading per supercage; the results for 6,(0), A I ,A*, and A3 are listed in Table I. At constant benzene coverage, the offset of 6, values among the three different adsorbateadsorbent systems (Figure 3a) depends on the number of Na+ cations present in the respective zeolite supercage. The Na+ cation distributions in the zeolite samples are included in Table I; they were deduced from the XRD results of Mortier and ~xjworkers.6~ The difference between dehydrated NaX and NaY zeolites, with regard to benzene adsorption, is the number of Na+ cations at sites I1 and I11 within the supercage.66 The values of 6,(0),i.e., at 8 = 0, also increase with decreasing Si/Al ratio of the adsorbents, consistent with the greater number of cations in the NaX vs NaY zeolite supercage. This observation contradicts the previous results obtained by It0 and Fraissard61who concluded that 6,(0) is independent of the Si/Al ratio in faujasite-type zeolites. For benzene adsorbed on faujasite-type zeolites, benzene adone corresponds to the site sorption sites are of two types;23-30*39*40 I1 Na+ cations within the NaY supcage, and the other is centered in the window formed by the 12-membered oxygen ring between adjacent supercages. The adsorption state depends not only on the benzene coverage22-M*39~40 but also on the properties of zeolite adsorbents such as the nature of the cation, the Si/AI ratio, and the adsorption of a basic or of an acidic compound.zs-26From Figure 3b, that the values of uxcxc for 8 I1.5 are nearly independent of 8 indicates a weak influence of adsorbed benzene within the supercages of different zeolites sensed by the coadsorbed xenon. For 8 > 1.5, several anomalous apexes at various values of 8 for different adsorbateadsorbent systems are observed which can be a priori related to the microscopic adsorption state of benzene, as we discuss below in conjunction with the corresponding 129Xe line-width measurements.
4
I
0.0 A A
% :
NaX (1.23) NaY (2.49) NaY (270)
& \
I
1 '
0
05
Xenon loading
10
15
/atom cage- 1
Figure 5. Linear fit between the location of maximum lz9XeNMR line width ) ,e( and xenon loading for various benzene-zeolite systems.
The variation of lz9XeNMR line width (b, measured by full width at half-maximum; FWHM) with respect to 8 is illustrated in Figure 4. For the benzene-NaX( 1.23) system in Figure 4a, two line-width maxima at 8 Y 2.0 and 3.0 were observed, the locations of the line-width maxima (denoted Om=) are found to decrease linearly with increasing xenon loading (Figure 5). The xenon loadings specified in Figure 5 were obtained in conjunction with xenon adsorption isotherms (cf. Figure 1) at various benzene coverages. At 8 Y 2.0 and 3.0, two anomalous axxtxcapexes are found in Figure 3b for the benzentNaX(1.23) system. Similar statements also apply for the benzentNaY(2.49) and benzene NaY(2.70) systems except that in these systems only one linewidth maximum is observable because site I11 Na+ cations are absent. The occurrences of the line-width and uxe-xCmaxima therefore reflect the same microscopic phenomena originatiug from detailed changes of the benzene adsorption states. At small 8, the '%e line width i n m a w with increasing 6 that the uxexcvalues are nearly independent of 8 and the Si/Al ratio
Benzene Distribution in NaX and NaY
The Journal of Physical Chemistry, Vol. 96, No. 20, I992 8123
80 of the adsorbents indicates that xenon atoms are relatively free to move rapidly between zeolite supercages and that the benzene molecules, although they may still migrate from one adsorption 60 site to another, are bound to the site I1 cations and confined to P the supercages for 8 I1, in a m r d with previous ~ o r k . ' ~ ~ * ~ J ' ~ ~ ' \ From the 8 dependence of uxtxc and Au in Figure 3b and Figure 4, the first anomalous peak occurring near 8 e 2.0 for benzene-NaX( 1.23) is associated with the existence of site I11 Na+ ions in the supercage,41whereas the anomalous peak that occurs at 2.5 < 8 < 4.0 for all three benzene-adsorbent systems is associated with the site I1 cations. The linear dependence of the linewidth maximum (e-) with xenon loading displayed in Figure 0 2 4 6 0 2 4 6 5 therefore reflects a competition or mutual hindrance between 0 /molecule. c a g e ' the xenon and benzene molecules when both speciesare atadsorbed in the supercage. This behavior is ascribed to delocalizationand/or F i e 6. Dependence of (a) 'Hand (b) 13CNMR line widths (Au)on rearrangement of benzcne molecules within the zeolite supercage. benzene coverage (e) for various benzentzeolite systems. Recent results from molecular dynamics calculations'2 based on the system of two benzene molecules adsorbed per supercage on also start to occupy less favorable window sites after all site I1 NaY zeolite (Si/AI = 3.0) illustrated that the adsorbed benzene Na+ cations have been occupied. The decrease of uxtxe in molecules, which have highly anisotropic motions, are confined benzeneNaY(2.49) and the increase in benzentNaY(2.70) at largely to the cage walls. The results from neutron s t ~ d i e s ~ ~ .B~>~4.0 may be a priori ascribed to the variation in the geometric using NaY zeolites with Si/AI = 2.78 and 2.43 have also shown arrangement of benzene-Na+ that, for 8 2 2.5, benzene molecules tend to aggregate and migrate As described earlier, for 2.5 < 8 < 4.0, benzene molecules tend toward the center of the supercage. From their low-temperature to aggregate and migrate toward the center of the supercage (4 K) results, the authors concluded that the average distance possibly either due to the charge distribution of site I1 Na+ between benzene and a site I1 Na+ cation decreases from 0.270 cations67and the related charge balance scheme upon increasing nm at low benzene coverage to 0.264 nm at high benzene coverage, benzene coverage or due to the formation of benzentNa+ comwhereas the site I1 Na+ cation, while binding to the benzene plexes. From measurements of the heat adsorption, Neddenriep@ molecule, displad ca. 0.01 nm toward the center of the supercage. estimated an effective Na+ cation charge in NaY zeolite, about Subsequent experiments performed at room temperature (300 K) two-thiids of a unit charge. If there are four site I1 Na+ cations showed that the benzene molecules, while mobile or highly disper NaY supercage, the number of effective "active sites" for the ordered, are still located in the supercage close to their position formation of benzene-Na+ complexes amounts to 4(2/3) = 2.7. at 4 K the distance between benzene and site I1 Na' also deAccording to this simple argument, we explain the occurrence of creased from 0.28 nm at low coverage to 0.27 nm at high coverage. 8- shown in F w e 5. Due to the deficiency of site I1 Na+ cations The decrease in distance is due to migration of Na+ by 0.01 nm.% (or a decrease in aluminum content) in the benzentNaY(2.70) However, theoretical calculati~ns~~ and 'H NMR second-moment system, the geometrical arrangement of the benzentNa' commeasurements3' made at 77 K indicated that the most favorable plexes within the zeolite supercage for 8 1 4.0 is less symmetrical distance between a benzene molecule and an Na+ cation is 0.32 than that in benzene-NaY(2.49) and is responsible for the increase nm. Moreover, site-specific IR adsorption data24for benzene of uXtXe found above B = 4.0. When combined with xenon adsorbed on NaY zeolite (Si/AI = 2.5) have indicated that, for adsorption isotherms, the '29XeNMR results also provide ma8 1 2.5, although benzene molecules are sorbed preferentially at croscopic information related to adsorption capacities in various site I1 Na+ cations, the possibility of adsorption in the window adsorbate-adsorbent systems. The saturation benzene concensite increases with increasing benzene concentration. Hence, the trations estimated from the above results are listed in Table I for results obtained from our work agree with published data. the three systems examined, the results were confiied by 'H and For benzentNaX(1.23) with 8 < 2.5, the first line-width "C NMR. maximum gives a range for e,, from 2.3 at zero xenon loading In order to confirm the 129XeNMR results obtained indirectly to about 1.7 at the greatest observable xenon loading, hence from the a d s o r b e d xenon, we performed separate NMR exproviding an onset of benzene rearrangement due to the presence periments on the adsorbed benzene. Specifically, the 'H and "C of Na+ cations at site 111, although, as have been demonstrated NMR spectra of benzene were measured with the same samples by IR measurements of benzene adsorbed on CsNaX m l i t e ~ , ~ 2 ~ ~ in the lZ9XeNMR experiments but without xenon. The used the possibility of benzene first occupying the window sites may variations of the 'H and 13CNMR FWHM line widths (Au)with not be excluded based on the present 129XeNMR results. benzene coverage (8) are summarized in Figure 6. The results However, it is well-known that the sizable Cs+ ions (diameter 0.34 obtained from the two different nuclei give a similar profile; for nm) can be located only in the supercage of CsNaX zeolites and 8 < 4.0, the observed 'H and I3C line widths were independent hence cannot be readily compared with the present adsorbateof 8, but for 8 > 4.0, abrupt increases of the line width with adsorbent system. For 2.5 < 8 < 4.0, the overlapped straight lines incteasing 8 were observed. The above observations are explained in the observable range of 8- for benzene-NaX(1.23) (from 3.0 by a progressive occupancy of Na+ cations by the benzene to 3.3) and benzentNaY(2.49) (from 2.8 to 3.3) in Figure 5 molecules within the zeolite supercage. For 8 < 4.0, the benzene provide a direct indication of nearly the same number of site I1 molecules are adsorbed preferentially at site I1 Na+ cations and cations existing in the two adsorbents (Table I); the fact that the less favorably at the window site;24while still confined in the latter has a greater range is consistent with the absence of site supercage, they undergo vigorous molecular motions which efI11 cations. The greater values of 8 , for benzene-NaY(2.70), fectively average the 'H and 13C resonances. For 8 2 4.0, in ranging from 3.4 to 3.6, therefore signify the deficiency of site addition to the increasing probability of window site ~ c c u p a n c y ~ ~ I1 cations. For benzentNaX(1.23) systems at large benzene cooperative interactions or mutual hindrance among adjacent benzene molecules inside the limited space of the supercage results coverages, the decrease in both uxcxc(Figure3b) and '%e NMR line widths (Figure 4a) upon increasing 8 is due to the interaction in a decrease of benzene mobility and hence the observed line between xenon and the benzene-Na+ complexes, in accord with broadening. At the same benzene coverage, the increase of overall the notion that the nonlocalizable site I11 cations arc responsible 'H and 13CNMR line widths with increasing Si/AI ratio of the for the greater benzene mobility in NaX than in NaY adsorbents is consistent with the decrease in benzene mobility.7,a~36.4~ For benzene adsorbed on NaY(2.49) and NaY(2.70) at 1 4.0, the observed Iz9XeNMR line widths remain almost constant at For the benzentNaY(2.49) system, samples with benzene first but increase with increasing 8 when the benzene molecules coverages exceeding saturation coverage were also examined. Both
Liu et al.
8124 The Journal of Physical Chemistry, Vol. 96, No. 20, 1992
h
A Jz :: h 1 -
.
40
.
J
0
L
-40
4
250
5
2
150
L
50
Chemical shift / P P ~ Figure 7. Room-temperature (a) 'H and (b) "C N M R spectra of benzene adsorbed on N a Y (Si/AI = 2.49) zeolite at various benzene coverages (e).
IH and I3C spectra for samples with benzene coverage exceeding saturation show a narrow line superimposed on a broad line, depicted in Figure 7; although the line width of the narrow line decreased with increasing 0, the line width of the broad component remained almost unchanged. Similar behavior has been demonstrated for 13CNMR (Si/Al ratio not specified) by Borovkov and ~o-workers~~ and for 'H NMR (Si/Al = 1.18,2.50 and 2.71) by Lechert and c o - ~ o r k e r sbut ~ ~ with - ~ ~ somewhat greater line widths. All these authors have elucidated that a decrease in benzene mobility due to mutual disturbance of the molecules inside the limited space of the supercages, which also favors an increase in molecular dipoledipole interaction, is responsible for the increase in IH and I3Cline widths at 0 > 4.0. The Occurrence of the superimposed narrow line has been regarded as resulting from mobile benzene molecules (i.e., those not paired with Na+ cations) inside the zeolite supercage. The latter argument is, however, that the superimposed narrow in contrast to our recent line arises from excess (free) benzene sorbed on the external surfaces of the zeolite crystallites. Therefore, the appearance of the superimposed narrow line may be considered the onset of saturation coverage and, in conjunction with the Iz9XeNMR results, may also be used60 to estimate the saturation benzene coverage in different zeolite adsorbates. The values of the saturation benzene coverages listed in Table I agree well with those p u b l i ~ h e dand ~ ~ were , ~ ~ found to increase with increasing Si/Al ratio of the adsorbents. Below saturation benzene coverage, the observed 'H and proton-docoupled I3C chemical shifts, denoted 6,&(H) and 6,,(C), were both found to increase slightly with Si/Al ratio of the adsorbents but were independent of adsorbate coverage (Table I). = 7.24 These values being greater than the well-known ppm and 6,(C) = 128.7 ppm (with proton decoupling) values for the bulk liquid benzene have been ascribed to corrections for medium effects.70 However, such a downfield shift was not obw e d in previous 13CNMR experiments on similar sy~tems.'2'~,~~ Although no 'H chemical shift data are available for comparison, a small (
