The Host−Guest Interactions Investigated by Multinuclear NMR

J. Phys. Chem. C 2008, 112, 16961–16967

16961

On the Use of CHClF2 as a Probe of Basic Sites in Zeolites: The Host-Guest Interactions Investigated by Multinuclear NMR Manuel Sa´nchez-Sa´nchez,†,‡ Teresa Blasco,*,† and Avelino Corma† Instituto de Tecnologı´a Quı´mica (UPV-CSIC), AVda. Los Naranjos, s/n, 46022 Valencia, Spain, and Department of Chemical and EnVironmental Technology, ESCET, UniVersidad Rey Juan Carlos, C/ Tulipa´n s/n, 28933 Mo´stoles, Madrid, Spain ReceiVed: June 10, 2008; ReVised Manuscript ReceiVed: July 30, 2008

The use of chlorodifluoromethane (CHClF2) as a probe molecule of zeolites basicity has been investigated by using infrared and multinuclear NMR spectroscopies and a series of alkali-exchanged faujasite with different Si/Al ratios (X and Y) and compensating cations of different nature. The 1H NMR peak of adsorbed CHClF2 shifts to low fields and the CH stretching frequency (νCH) shifts to low wavenumbers as the zeolite basicity determined by the Sanderson method increases. Poorer linear correlation is observed for the νCH band suggesting the occurrence of extra interactions of the fluorine atoms of the adsorbed chlorodifluoromethane with the nonframework cations. This interaction is evidenced for the first time by the spectroscopic modification of the probe molecule itself; 19F NMR chemical shifts and 1J(C,F) spin-spin coupling constants are determined by the nature of the extraframework atoms and not by the framework basicity. The occurrence of the interactions between the fluoride atoms and the compensating cation does not allow quantifying the number of sites of similar basicity. Introduction The industrial and academic interest of zeolite materials has prompted a continuous effort to find structures with new pore dimensions, topologies, and compositions.1,2 Regarding their properties, acid zeolites are widely used as heterogeneous catalysts in petrochemical industry. However, the use of basic zeolites has been limited so far,3-5 although there is increasing interest nowadays because of their potential applications in catalysis and also in the highly demanded adsorptions of halocarbons6-12 and hydrogen.13-16 Zeolite basicity resides on the negative charge density on the framework oxygen atoms,17 which depends on the framework topology and chemical composition. For a given structure, basicity increases as the Si/Al ratio decreases and the electropositive character of the compensating cations increases; for instance, NaX (Si/Al e 1.3) is more basic than NaY (Si/Al ) 2.4), whereas partially Cs-exchanged NaY is more basic than NaY. Several methods, both theoretical and experimental, have been developed to measure zeolite basicity as well as a methodology based on the catalyst activity using reactants of different acidity.18 The theoretical Sanderson electronegativity equalization method19,20 gives average values of the negative charge over oxygen atoms that are exclusively based on the zeolite chemical composition and not on the structure. Alternatively, experimental methods are based on the use of acidic probe molecules with the most successful being those using infrared (IR) spectroscopy3,5 probably because of easy handling and general availability. Among the tested molecules, chloroform21-29 and above all pyrrole30-35 have been the subject of several publications. The N-H and C-H groups of pyrrole and chloroform, respectively, form hydrogen bonds with framework oxygen * To whom correspondence should be addressed. Telephone: +34963877812; fax: +34-963877809; e-mail: [email protected]. † Instituto de Tecnologı´a Quı´mica. ‡ Universidad Rey Juan Carlos.

atoms producing a red shift of the IR stretching frequencies and the displacement to low fields of the 1H NMR resonances in an extent which depends on the zeolite basicity. Studying a series of zeolites with the same structure, the basicity obtained by both techniques correlates with that calculated by the Sanderson electronegativity method. With respect to site heterogeneity, the identification of several contributions to the broad IR NH band of pyrrole in faujasite zeolites has been assigned to the existence of different adsorption sites.34,35 A complete characterization of zeolite basicity would require the identification and quantification of sites of different strength, while the correlation with the catalyst performance would allow discriminating the active and selective sites for a specific reaction. This knowledge is of key importance to progress on the ultimate goal of developing a tailor-made catalyst for a given application. A valid approach to identify sites of different basic strengths is the use of a series of probe molecules with similar structures possessing a proton of varying acid strengths. A possibility is to use the halocarbon family of general composition CHCl3-xFx for which the proton acidity decreases as the number of fluorine atoms increases requiring stronger basic sites for adsorption. However, the occurrence of additional interactions of the halocarbons with the compensating alkaline cations in faujasite (FAU) type zeolites questions their validity as probe molecules to measure the zeolite basicity and, even more, to differentiate and quantify sites of different basicity. Indeed, there are situations where cations can migrate to the supercage to satisfy a particular probe molecule/cation stoichiometry,35-39 and the models proposed for the adsorption of halocarbons on FAU type zeolites involve the alkali cations. NMR shows that chloroform interacts with alkaline cations,28,29 and theoretical calculations suggest that chloride interacts electrostatically with the accessible supercage cations but also with framework oxygen, while the hydrogen atom points toward the zeolite oxygen.27 In the model proposed for the adsorption of CHF3 on FAU zeolites derived from theoretical calculation, the

10.1021/jp805100t CCC: $40.75  2008 American Chemical Society Published on Web 10/02/2008

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TABLE 1: Chemical Composition and the Average Negative Charge on the Framework Oxygen Atoms of the Commercial and Partially-Exchanged Zeolites as well as the C-H Bond Stretching Frequency (νCH) and the 1H Chemical Shift (δ 1H) of Adsorbed CHClF2 δ 1H(CHClF2)/ppm zeolite

Na/u.c.b

M/u.c.b

-δO

LiY NaY KY CsY LiX NaX KX CsX adsorbate

30 55 13 17 29 85 20 33

24 0 41 37 57 0 66 53

0.347 0.346 0.372 0.387 0.407 0.411 0.445 0.459

a

c

-1

νCH CHClF2/cm 3043 3039 3037 3041 3034 3030 3018 3020 302442

30 molec./u.c. 7.50 7.70 7.80 8.05 8.10 8.25 8.55 8.40 6.70

chemisorbed at RT

chemisorbed CHClF2 molecules at RT/u.c.

7.80 8.05 8.15 8.35 8.50 8.35

22 45 24 22 48 28

a The Si/Al ratio of the zeolites Y was 2.50 except 2.40 for NaY; for zeolites X, the Si/Al ) 1.23 as determined from 29Si MAS NMR spectra. b Number of Na+ and exchange cation M+ per unit cell. c Average negative charge over framework oxygen atoms calculated following the method of Sanderson.19

hydrogen atom is directed to the six-member rings, and the fluorine points toward the alkaline cation.40 These results suggest that the H-bonding with network oxygen and the halogen-cation interaction dominate the adsorption of CHCl3 and CHF3, respectively. There is now no longer any doubt about the participation of alkali cations on the adsorption of pyrrole and halocarbons on FAU type zeolite; however, although they cannot be ignored, experimental IR and NMR results show that both molecules can be used as probes for zeolite basicity. Then, the choice of an appropriate molecule to measure zeolite basicity requires evaluating the relative contribution of the basic framework oxygen atoms and the acidic compensating cations on the adsorption process and, ultimately, a complete understanding of the adsorbate-zeolite system. In this work, we investigate the adequacy of chlorodifluoromethane (CHClF2) with acidic character and dipolar moment intermediate between CHCl3 and CHF3 to characterize zeolite basicity strongly dependent on the interaction prevailing upon the adsorption. If this would be a valid probe, then its combined use with the more acidic chloroform should allow to identify and to estimate the amount of basic sites of different strengths. Here, we have used infrared and multinuclear NMR spectroscopies to evaluate the adsorbate-cation interactions and the validity of using CHCl3-xFx as probes for zeolite basicity. Experimental Section Materials. Zeolites NaX (13X) and NaY (CBV 100) were commercially available from Aldrich and PQ Zeolite, respectively. Partially cation-exchanged zeolites LiX, KX, and CsX and LiY, KY, and CsY were prepared from commercial NaX and NaY, respectively, by conventional chemical exchange using 1 M aqueous solution of the corresponding alkaline chloride and by refluxing at 353 K for 1 h. Two consecutive exchange processes were carried out with intermediate and final filtering and washing until total absence of chloride and drying at 353 K. Chemical composition of all zeolites, summarized in Table 1, was determined by atomic absorption and by inductively coupled plasma spectroscopy (ICP) for Li. Samples are denoted using the name of the alkali-exchanged cation followed by X or Y depending on the commercial zeolite used as starting material omitting Na although its exchange was not complete in any of the samples, for instance, LiY or KX instead of the LiNaY or KNaX designation. 29Si magic-angle spinning (MAS) NMR confirmed the Si/Al ratio, whereas 27Al MAS NMR

certified the absence of extraframework aluminum species. CHClF2 (>98%) was supplied by Prestogaz in a 1 L gas cylinder. Experimental Methods. For NMR, approximately 20 mg of the zeolite was introduced into a glass insert and was heated under dynamic vacuum at 673 K for 12 h until a final pressure below 10-5 kPa was reached. The adsorption was carried out at room temperature by contacting the evacuated sample with a CHClF2 pressure of 16 kPa (the vapor pressure of chloroform at 298 K) for 5 min and with subsequent desorption for 30 min. Hereafter, the molecules remaining adsorbed onto the zeolite after evacuating at room temperature will be referred to as “chemisorbed” CHClF2. In other sets of experiments, a given amount of CHClF2, 30 molecules per unit cell of the zeolite, measured using a calibrated volume, was introduced into the sample by submerging the glass insert in a Dewar vessel containing liquid nitrogen until a pressure below 0.01 kPa was reached, and the glass tube was then sealed (without any further evacuation). This alternative experimental procedure was carried out to compare all zeolites, including LiY and NaY which do not chemisorb CHClF2 at room temperature, at the same loading. The amount of CHClF2 adsorbed (30 molecules per unit cell) was chosen to ensure that all the gas fit into the available free space in all zeolites taking into account that 27 molecules of bulkier CHCl3 can be logged into zeolite CsX, which is the zeolite with less free space.28 1H, 13C, and 19F MAS NMR spectra were recorded at room temperature with a Varian VXR-S 400-WB spectrometer at 399.9, 100.6, and 376.3 MHz, respectively. An RT CP/MAS Varian probe with 7 mm silicon nitride rotors spinning at 5 KHz was used for all nuclei. To acquire the 1H NMR spectra, a 90° pulse of 6 µs and a recycle delay of 5 s were used. The 1H chemical shifts were referenced to the signal of D O (4.7 2 ppm) added into the rotor but outside the glass insert estimating an error of less than (0.05 ppm. Quantification of the proton species was carried out by integration of the NMR peaks using a known amount of chloroform as external reference. The 13C spectra were acquired using a conventional single pulse sequence with high power 1H decoupling using a 90° pulse of 7 µs and a recycle delay of 5 s. 19F NMR spectra were recorded by applying 90° pulses of 7 µs and a recycle delay of 5 s. The 13C and 19F chemical shifts were referenced to TMS and CFCl3. Infrared measurements were performed with a Nicolet 710 FT spectrophotometer. The digital resolution was 4 cm-1 when using self-supporting wafers of 10 mg cm-2 and a Pyrex vacuum IR cell. The samples were heated under dynamic vacuum at

CHClF2 as a Probe of Basic Sites in Zeolites

J. Phys. Chem. C, Vol. 112, No. 43, 2008 16963

Figure 1. C-H region of the FT-IR spectra of zeolites NaY (left) and CsX (right) evacuated at 673 K (bottom) under a pressure of 16 kPa of CHClF2 (middle) and subsequent desorption at room temperature (top).

673 K for 12 h until a final pressure of ca. 10-5 kPa was reached, and then they were contacted with 16 kPa of CHClF2 at room temperature and subsequently were desorbed at the same temperature for 30 min to measure the chemisorbed halocarbon molecules. In some experiments, the infrared spectra were recorded without the final desorption step. Results and Discussion A. Hydrogen Bonding of CHClF2 with Zeolite Framework Oxygen Atoms. As mentioned in the Introduction, the use of CHClF2 as a probe for zeolite basicity is based on the expected correlation between the red-shift of the νCH band and the low field displacement of the 1H NMR signal with the zeolite basicity. Accordingly, we have studied and used both techniques to measure the zeolite basicity using CHClF2 as a probe. IR Spectroscopy. To check the validity of CHClF2 to act as a probe for basic sites using infrared spectroscopy, zeolites NaY and CsX were degassed at 673 K, were contacted with 16 kPa of CHClF2, and subsequently were evacuated at room temperature. The CH stretching region of the corresponding infrared spectra are displayed in Figure 1; there, it can be seen that only those recorded under CHClF2 pressure show an intense band at ca. 3020-3040 cm-1 assigned to the stretching vibration frequency of the C-H bond (νCH). After evacuation at room temperature, a very weak band remains only in zeolite CsX indicating that chemisorbed CHClF2 can be used as a probe molecule only for more basic zeolites. However, in the spectra measured under pressure of CHClF2, the νCH band is red-shifted for CsX according to its stronger basicity. Indeed, the spectra recorded for all zeolites under similar conditions, depicted in Figure 2, show that the νCH band shifts to lower frequency as the zeolite basicity increases suggesting that physisorbed CHClF2 can be used as a probe for zeolite basicity. Figure 3 shows that the νCH frequency increases linearly with the negative charge density on the oxygen atoms calculated by the Sanderson method (Table 1, Figure 3),19,20 although the correlation is worse than that reported for chloroform28 and pyrrole35 in this range of zeolite composition suggesting that other contributions besides zeolite basicity play a role on the C-H vibration frequency of CHClF2. MAS NMR Spectroscopy. To test the validity of CHClF2 as a probe for zeolite basicity by means of NMR spectroscopy, we have recorded the 1H and 13C spectra of this molecule adsorbed on alkali-exchanged zeolites. Figure 4 shows the 1H NMR spectra of the zeolites loaded with 30 molecules of CHClF2 (Figure 4A) and after adsorption of CHClF2 and subsequent desorption at room temperature in where only chemisorbed molecules are present (Figure 4B). All spectra

Figure 2. C-H frequency region of the FT-IR spectra recorded under a pressure of 16 kPa of CHClF2 on the zeolites indicated in the figure.

Figure 3. CH band stretching frequencies (νCH) of CHClF2 adsorbed over the zeolites indicated in the graphic vs the mean negative charge over the framework oxygen -δO calculated by the Sanderson method.

Figure 4. 1H MAS NMR spectra of the alkali-exchanged FAU type zeolites recorded at room temperature (A) after adsorption of 30 molecules of CHClF2 per unit cell and (B) after being in contact with 16 kPa of CHClF2 and subsequent desorption at room temperature (zeolites LiY and NaY do not adsorb any chlorodifluoromethane).

consist of a triplet of relative intensity 1:2:1 because of the spin-spin coupling of 1H with the two equivalent 19F nuclei through two bonds (H-C-F), 2J(H,F). Only the less basic zeolites NaY and LiY do not chemisorb any CHClF2 at room temperature (spectra not included in Figure 4B). Table 1 shows the amount of CHClF2 chemisorbed at room temperature on the alkali-exchanged zeolites determined by the

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Figure 5. Correlations of δ 1H of CHClF2 and CHCl3 (from ref 28) adsorbed on ion-exchanged faujasite type zeolites versus the mean negative charge over the framework oxygen atom -δO.

integration of the 1H NMR spectra of Figure 4B. Zeolite CsX, with the smallest free space available because of the high cation population and large volume of Cs+, chemisorbs 28 molecules of chlorodifluoromethane per unit cell (Table 1). Therefore, all zeolites must have enough free space to fit 30 molecules of halocarbon, which is the loading used to record the spectra of Figure 4A. According to the amount of CHClF2 chemisorbed in all zeolites, summarized in Table 1, different interactions with the halocarbon must occur under the experimental conditions used to record the spectra of Figure 4A. Thus, all CHClF2 molecules (30 per unit cell) must be chemisorbed on CsY and KX and mostly on CsX; a part will be physisorbed on KY, LiX, and NaX; and all will be physisorbed on LiY and CsY. As expected by the hydrogen bonding with the zeolite host, the 1H NMR signal of chemisorbed CHClF (zeolite CsY, KX, and 2 CsX) must be low field shifted with respect to the gas phase (see Figure 4A). However, also the 1H chemical shift of CHClF2 physisorbed in zeolites LiY and NaY is largely shifted to low field with respect to that of the gas molecules indicating that although weaker, they form hydrogen bonding with the zeolite oxygen atoms (Figure 4A). Physisorbed and chemisorbed molecules that are present when 30 molecules of CHClF2 are loaded in zeolites KY, LiX, and NaX exchange rapidly compared with the NMR time scale and then inhibit the detection of individual lines for both kinds of species, and only an average single line is observed. Moreover, there are only small differences in the 1H chemical shifts of the chlorodifluoromethane on the same zeolites in the spectra of Figure 4A (30 molecules/u.c.) and 4B (chemisorbed molecules). Figure 5 represents the evolution 1H NMR chemical shifts of CHClF2 (30 molecules/u.c.) adsorbed on different alkaliexchanged zeolites with the negative charge on the framework oxygen atom calculated by the Sanderson method (Table 1), showing a linear correlation, in agreement with the results previously reported for CHCl3.28,41 This result suggests that CHClF2 is an adequate 1H NMR probe molecule to measure the average zeolite basicity. Figure 6 shows the 13C MAS NMR spectra of CHClF2 chemisorbed on cation-exchanged zeolites (adsorption followed by desorption at room temperature). As for 1H, the 13C spectra consist of a triplet from the spin-spin coupling through one bond, 1J(C, F), with the 19F nuclei (1H decoupling was used during 13C signal acquisition, so 13C-1H spin-spin coupling is not detected). The adsorption of CHClF2 on basic zeolites shifts the position of the 13C peak to lower fields as the basicity of the zeolite increases suggesting that also 13C NMR can be used as a measure of the solid basicity. Nevertheless, the faster longitu-

Figure 6. 13C MAS NMR spectra of the alkali-exchanged FAU type zeolites after adsorption and subsequent desorption of CHClF2 at room temperature. The spectrum of NaY was acquired upon adsorption of 30 molecules of CHClF2 per unit cell without further desorption.

dinal relaxation of 1H and their much higher NMR sensitivity makes this nucleus much more adequate to estimate the zeolite basicity. B. The Interaction of CHClF2 with the Compensating Zeolite Alkaline Cations. For CHClF2, electrostatic interactions between the halogen and the compensating cations are expected to be stronger for the more electronegative fluorine atom. This additional base-acid interaction of Lewis type, in which the halocarbon acts as a base and the alkaline cation as an acid, questions the validity of these molecules as probes of the zeolite basicity. Taking advantage of the nuclear properties of 19F (I ) 1/ , 100% natural abundance, high sensitivity), we have studied 2 the CHClF2-zeolite system by 19F NMR spectroscopy. 19F NMR Chemical Shifts. Figure 7 shows the 19F MAS NMR spectra of the alkali-exchanged FAU zeolites, containing 30 molecules of CHClF2 per unit cell, displayed in an increasing order of basicity from bottom to top. All spectra consist of a doublet from the spin-spin coupling of 19F with the 1H through two chemical bonds 2J(F,H). When the two series of alkaliexchanged zeolites Y and X are considered separately, the 19F peaks shift to lower field as the zeolite basicity increases, and therefore, it could be thought to depend on the zeolite basicity. However, when the complete series of zeolites is considered, it is evident at a first glance that the position of the 19F resonance is strongly dependent on the nature of the exchanged cations as all zeolites contain a fraction of Na+ because of incomplete exchange (see Table 1). The distribution of the compensating cations in the extraframework sites has been previously published for dehydrated zeolites of series Y used here.35,38,43 According to this and other data found in the bibliography,44 it is observed that bulkier cations occupy supercage positions accessible to the guest molecules. Moreover, adsorption may provoke migration of cations toward the supercage to interact with the guest molecules, and usually, there is preferential adsorption of the acidic probe molecule on more basic sites associated with more electropositive cations.35,39,43 Chlorodifluoromethane cannot enter the small cages of FAU type zeolites and, taking into

CHClF2 as a Probe of Basic Sites in Zeolites

J. Phys. Chem. C, Vol. 112, No. 43, 2008 16965 TABLE 2: 1J(C,F), 2J(H,F), and 2J(F,H) Coupling Constants Measured in the Spectra of Figures 4, 6, and 7 zeolite CHClF2 gas LiY NaY KY CsY LiX NaX KX CsX

1

J(C,F)/Hza

284.7 289.0 290.8 283.9 283.9 289.9 290.7

2

J(H,F)/Hzb 46.3 63.5 61.4 60.7 62.2 63.6 60.4 60.9 62.6

2

J(F,H)/Hzc 63.4 61.7 61.5 62.6 63.8 61.3 61.5

a Values directly measured from the spectra of Figure 6. b Values determined by simulating the spectra of Figure 4. c Values determined by simulating the spectra of Figure 7.

Figure 7. 19F MAS NMR spectra of alkali-exchanged FAU type zeolites with 30 molecules of CHClF2 per unit cell.

account these considerations, we can assume that this molecule will preferentially be adsorbed on the sites associated with the more electropositive cations in the partially exchanged zeolites mostly located in supercage positions. Although we cannot establish the exact site for CHClF2 adsorption, the results reported below are consistent with these general trends. As it is observed in Figure 7, the 19F chemical shift of CHClF2 on zeolites containing bulky Cs+ and K+ cations is low field shifted with respect to the neat halocarbon, but it is independent of the Si/Al ratio. Moreover, δ 19F is similar for KX and KY and for CsX and CsY suggesting that fluorine is mostly interacting with the bulkier cations present in the zeolite (K+ and Cs+). The 19F deshielding, that is, the decrease of the electron density on 19F, is consistent with the higher electron withdraw by the electropositive Cs+ and K+ alkaline cations. Different results are observed for Li+ and Na+ exchanged zeolites. For these, inspection of Figure 7 leads to the following main conclusions: (1) The Si/Al ratio dictates the 19F chemical shift, which differs for Y and X samples, suggesting a weak fluorine-cation interaction. (2) The 19F chemical shift of CHClF2 is similar for the two Y (LiY and NaY) and the two X (LiX and NaX) samples, indicating the preferential interaction with the bulkier Na+, in agreement with previous results for pyrrole adsorbed on LiX in where the probe molecule interacts exclusively with Na+ ions.38 (3) The δ 19F of CHClF2 over the more basic LiX and NaX zeolites is similar to that of the neat halocarbon indicating that the interaction of fluorine with the cations is negligible and that the adsorption process is probably dominated by the hydrogen bonding with framework oxygen. Meanwhile, despite that zeolites LiY and NaY are unable to chemisorb CHClF2 at room temperature (Figure 4B), δ 19F is low field shifted with respect to the gas molecules indicating some interaction of fluorine with the cations. Therefore, physisorbed chlorodifluoromethane interacts through the fluorine atoms with the alkaline cations (preferentially Na+) besides the hydrogen bonding with framework oxygen atoms deduced from the 1H NMR spectra (Figure 4). To summarize, 19F NMR spectroscopy suggests that the interaction of CHClF2 with faujasites depends on the relative

strength of the zeolite acid-base pair formed by the alkaline cation-framework oxygen. The results shown in Figure 7 suggest the occurrence of strong fluorine-cation interaction upon adsorption on zeolites containing bulkier cations (K+ and Cs+). Meanwhile, fluorine-cation interactions are negligible in LiX and NaX zeolites; these are significant in LiY (with Na+) and NaY, but they are not basic enough to chemisorb the halocarbon at room temperature. Spin-Spin Coupling Constants. According to the results obtained for 19F chemical shifts, it is logical to think that spin-spin coupling constants of the CHClF2 nuclei will depend on their interactions with the zeolite cations; fluoride-cation will determine the 1J(13C,19F), whereas also the hydrogen bonding is expected to influence 2J(1H,19F) (or 2J(19F,1H)). To check this, we have measured 1J(C,F) directly from the spectra in Figure 6 and the equivalent 2J(H,F) and 2J(F,H) from Figures 4 and 7. For determining 2J values, the spectra have been simulated using individual lines separated by the coupling constant to overcome the peak overlapping. The results are summarized in Table 2. Figure 8 shows the evolution of the 1J(13C,19F) and 2J(1H,19F) (or 2J(19F,1H)) values for FAU type zeolites with increasing the electropositive character of the exchanged cations from left to right. Figure 8A shows that the 13C-19F coupling (1J(C,F)) increases as the electropositive character of the exchanged cation does, irrespective of the Si/Al ratio of the zeolite, indicating that, effectively, 1J(13C,19F) is dictated by the cation-halogen interaction. Moreover, the results reported in Figure 8A indicate that F atoms interact more strongly with bulkier cations in good agreement with what has been suggested from 19F chemical shifts. The similarity of 1J(C,F) for zeolites LiX and NaX can be explained by the selective adsorption of CHClF2 over sites involving the more electropositive Na+ cations. The order of δ 19F values (Figure 7) coincides with the order given by 1J(C,F) values (Table 2). As for 1J(C,F), 2J(H,F) does not depend on the Si/Al molar ratio of the zeolite again indicating that the F-cation interaction determines the coupling constant value and, according to the results in Figure 8A, one would expect a progressive increase in 2J(H,F) with the volume of the exchange cation. However, Figure 8 shows that 2J(H,F) decreases as the electropositive character of the compensating cation increases and grows only for Cs+ suggesting the contribution of interactions of opposite sign. Indeed, the acidic hydrogen atom bearing positive charge density will bind stronger to more basic framework oxygen associated with bulkier cations then decreasing its coupling with fluorine. The contribution of both opposite effects results in a minimum 1H-C-19F coupling constant for Na+-K+.

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Figure 9. Number of CHCl3 (from ref 28) and CHClF2 molecules per unit cell chemisorbed at room temperature on the zeolites indicated in the graphic (after exposure to 16 kPa of CHCl3 or CHClF2 at room temperature and subsequent desorption at the same temperature).

Figure 8. Spin-spin coupling constants of adsorbed CHClF2 versus the nature of the alkali-exchanged cations in NaY and NaX commercial zeolites (A) 1J(C,F) and (B) 2J(H,F) (average of 2J(F,H) and 2J(H,F) values shown in Table 2).

C. CHClF2 as a Probe of Zeolite Basicity. The frequency of the IR νCH band of adsorbed CHClF2 correlates linearly with the zeolite basicity estimated by the Sanderson method (Figure 3). However, this correlation is worse than that reported previously of CHCl3 suggesting that the νCH band position of CHClF2 is also affected by the interaction of the fluorine atom with the zeolite cation even if the C-H bond vibration is not directly involved. Regarding NMR spectroscopy, δ 1H of adsorbed CHClF2 correlates linearly with the zeolite basicity (Figure 5) suggesting that chemical shift is mainly determined by the interaction with the framework oxygen atoms. Meanwhile, the firmest evidence of the cation-halogen interaction in the halocarbon-zeolite adsorption system is evidenced by 19F NMR; chemical shifts and coupling constant involving this nucleus 1J(C,F) are almost exclusively dependent on the nature of the compensating cation. Therefore, multinuclear NMR of the CHClF2/alkali-zeolite system allows to independently evaluate the two acid-base interactions: that of the acidic proton of the CHClF2 molecule with the basic framework oxygen by 1H and 13C NMR and that of the electronegative fluoride with the acidic alkaline cation by 19F NMR. To completely evaluate the possibilities of the combined use of CHClF2 and CHCl3 as probes of zeolite basicity, we have compared the amounts chemisorbed at room temperature determined by 1H NMR spectroscopy. The results obtained are depicted in Figure 9. According to the lower acidity of the proton of CHClF2 compared to that of CHCl3 and the requirement of stronger basic sites for chemisorption, one would expect lower CHClF2 uptake in all samples. This is true for zeolites containing less electropositive Li+ and Na+ cations and for KY; LiY and NaY chemisorb CHCl3 but they are not basic enough to chemisorb CHClF2 at room temperature. The amount of CHClF2 adsorbed by zeolites KY, LiX, and NaX is lower than that of CHCl3. However, zeolites containing more electropositive Cs+ cations, that is, zeolites CsY and CsX, as well as KX, adsorb more CHClF2 than CHCl3 according to the higher affinity of

the F atom of the guest molecule probably filling the whole void volume of these zeolites, The experimental results reported here indicate that the halogen-cation interaction increases with the electropositive character of the cation and, in agreement with theoretical calculations,11,40 with the electronegativity of the halogen atom. Accordingly, this is weaker for chloride than for fluoride, and then, the hydrogen bonding with the zeolite oxygen dominates the νCH and δ 1H of CHCl3 validating its use as infrared and NMR probes of zeolite basicity. Our results indicate that CHClF2 can be also used to estimate the mean zeolite basicity by using 1H NMR spectroscopy and, although with worse correlation, infrared spectroscopy. However, the adsorption of CHClF2 cannot be used to quantify the number of basic sites. Then, in general, halocarbons can be used as probes of basic sites in zeolites when they are constituted by large halogen atoms (Cl or larger) and when the zeolite contains small compensating cation (the smaller the better). However, the halogen-cation interaction has smaller effects on the δ 1H NMR than on the νCH frequency making the former spectroscopy more adequate to measure the mean zeolite basicity by using halocarbon as probe molecules. Conclusions We have investigated the use of CHClF2 as a probe for basic sites of a series of alkali-exchanged FAU type zeolites by means of IR and NMR spectroscopies. The stretching frequency of the CH bond, νCH, correlates with zeolite basicity estimated by the Sanderson method. The CHClF2-zeolite system has also been investigated by multinuclear NMR spectroscopy. The hydrogen bonding of CHClF2 with the zeolite oxygen has been studied by 1H and 13C chemical shifts. δ 1H shows a good correlation with the average negative charge over the zeolite oxygen estimated by the Sanderson method indicating that it can be used to estimate the mean zeolite basicity. Meanwhile, the interaction of the fluorine atoms of the halocarbon molecule with the zeolite cations has been evidenced by 19F NMR. δ 19F results are particularly sensitive to the nature of the alkali cation and are almost independent of the Si/Al ratio of zeolites, specially for bulkier cations. Only for LiX and NaX zeolites, with less electropositive cations and relatively strong basic sites because of the low Si/Al ratio, the fluorine-cation interaction is negligible suggesting that hydrogen bonding of CHClF2 dominates the adsorption. The fluorine-cation interaction is further supported by the NMR coupling constants 1J(13C,19F) and

CHClF2 as a Probe of Basic Sites in Zeolites 2J(1H,19F),

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