1H-NMR study of heterogeneous adsorbent–adsorbate (water, methanol) equilibria at 4 K: application to the acid strength of solids

Colloids and Surfaces A: Physicochemical and Engineering Aspects 158 (1999) 211 – 220 www.elsevier.nl/locate/colsurfa

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H-NMR study of heterogeneous adsorbent–adsorbate (water, methanol) equilibria at 4 K: application to the acid strength of solids Jacques Fraissard *, P. Batamack Laboratoire de Chimie des Surfaces, Associe´ au CNRS, ESA 7069, Uni6ersite´ P. et M. Curie (Paris 6), Case 196, 4 place Jussieu, 75252 Paris Cedex 05, France

Abstract 1

H NMR, performed on sealed samples, provides various methods for studying the acidity of solids. Among high resolution NMR techniques, magic angle spinning (MAS) is the best developed. It inquires into the nature and concentration of OH groups of different types in fully desorbed samples, identifying the bridging acidic sites and framework defects, as well as extraframework species. The chemical shift of a fully desorbed solid alone, however, is not enough to measure its acid strength; interaction with a base is necessary. Under such conditions, chemical exchange between H atoms, from the acidic site and from the base, causes coalescence of these H atom resonance positions and no useful measurement can be made. Broad-line NMR in ‘rigid lattice’ conditions (4 K) can be used to identify and quantify the quenched species formed when a base (containing a small number of H atoms) and a zeolite Brønsted acid site interact. A scale of Brønsted acid strength has been proposed, which is discussed in terms of the nature, the composition and defects of solids. © 1999 Elsevier Science B.V. All rights reserved. Keywords: 1H-NMR; Heterogeneous equilibria; Water; Methanol; Acidity

1. Introduction After a few rare applications of NMR to the study of adsorbed phases in about 1960–1970 [1,2] there was a veritable explosion of this technique for the characterisation of adsorbate–adsorbent interactions with the arrival of Fourier transform technology and high resolution tech-

* Corresponding author. Tel.: +33-1-442-76013; fax: +331-442-75536. E-mail address: [email protected] (J. Fraissard)

niques for solids. Several cases can be distinguished depending on the nature of the adsorption and the solid.

1.1. Physical adsorption The interaction is of the van der Waals type. The molecules are very mobile at the surface of the solid and are generally associated with a single, relatively narrow signal corresponding to the average of the interactions to which they are subjected. The best example of this is the 129Xe NMR of xenon used as a proble [3].

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1.2. Chemical adsorption 1.2.1. On diamagnetic solids Generally the motion of chemisorbed species is slow on the NMR time-scale. The result is a characteristic broad unresolved spectrum. Nevertheless, if the broadening is not too great, solid NMR techniques (MAS, CRAMPS) usually make it possible to obtain well resolved spectra. However, it is difficult to rotate samples in sealed tubes at more than 5 kHz. This is why the spectral components are often too broad to be narrowed by these techniques. In this case, if there is only one type of chemisorbed species, one can use the exchange of this species with molecules physisorbed in large excess. This exchange, which is fast because the required activation energy is generally small, makes it possible to detect a relatively narrow coalescence signal, whose dependence on the concentration of the physisorbed phase [4] can be used to work back to the NMR characteristics of the chemisorbed species. If, on the other hand, the chemisorbed phase includes different types of species, the rapid exchange technique cannot be used to determine the characteristics of each one of them. One then has to lower the temperature of the sample, in order to prevent such fast site exchange, which usually leads to too much broadening for high resolution spectra to be obtained. The only solution then is to use dipolar interactions in the rigid lattice. 1.2.2. On paramagnetic solids In this case the lines are usually very broad at 25°C. Only the technique of fast exchange with an excess of physical adsorption can be used to get back to the characteristics of one chemisorbed phase [5,6]. 1.2.3. On metals The adsorbate signals are, of course, broad but their chemical shift (Knight shift) is also very large (a few percent) and can therefore generally be measured despite this broadness [7,8]. As one can see, the title proposed by the organising committee ‘NMR of adsorbed molecules’ is very broad. We had to make a choice. We have decided to treat the NMR of a nucleus which is

used less and less, 1H, and in particular an almost completely forgotten technique, NMR in the rigid lattice, for studying heterogeneous adsorbate–adsorbent equilibria, choosing as example the interaction of H2O or CH3OH with more or less acidic solids.

2. 1H isotropic chemical shift

2.1. 1H isotropic chemical shift in ‘anhydrous’ samples The isotropic chemical shift, diso, of H atoms belonging to the SOH group of solids, as well the IR frequencies, nOH, of the corresponding stretching vibrations to which it can be related, depend not only on the OH bond polarisation but also on hydrogen bond formation, which is particularly likely with neighbouring O atoms of the framework [9,10]. diso and nOH can also be related to the calculated deprotonation energy of the solid [11]. This seems logical since these three quantities are related to the polarisation of the OH bond. However, to relate these quantities to the acid strength seems to us incorrect, since the acid strength depends not only on the polarisation of the OH bond but also on its polarisability. Consequently, to measure the acidity, the interaction of a solid hydroxyl group with a base is necessary. 1 H high resolution NMR of anhydrous solids is not absolutely indispensable for the NMR study at 4 K. It is, however, very useful since it makes it possible to distinguish quantitatively the various types of OH groups (OH acid or not, with stronger or weaker hydrogen bonding, etc.) present at the surface of the solid, which facilitates the simulation of the spectra at 4 K and confirms its validity. diso covers a range of about 10 ppm depending on the solid [12]. For sulfated zirconia calcined at 570 K we found 5.8 ppm (OHa) and a small signal at 1.4 ppm [13]. Niobic acid H2Nb6O16 also presents two signals: one at 2.9 ppm; and a small, broad shoulder at around 8 ppm (OHa) [14]. In dried NafionH, diso, (OHa)= 10.6 ppm [15]. Pfeifer and Freude et al. [16–24] have been the most active in the determination and use of diso, obtained for HY as

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follows: (a) 1.3 – 2.3 ppm: silanol groups, SiOH; (b) 3.8–4.4 ppm: bridging SOHa groups pointing towards the supercages; (c) ca. 5 ppm: bridging S-OHa groups pointing towards the sodalite cavities; and (d) 6.5 – 7 ppm: possibly remaining ammonium ions. After partial dealumination some Al OH groups can be detected at around 2.6–3.6 ppm. In the case of zeolites, a signal at about 6.8–7 ppm can also be detected, characteristic of d-OH bridged [25 – 30] or non-acidic [27,31–33] but both hydrogen-bonded to the framework O atoms. These two types can be distinguished by using 1H{27Al} dipolar dephasing experiments [27].

2.2. Interaction of solid acidic hydroxyl groups with a base Bases such as pyridine, ammonia, methanol, trirnethylphosphine, carbone monoxide, paraffins and water have been used to study the acidity of solids [4,12,34 – 36]. After base adsorption, 1H MAS NMR spectra recorded at room temperature cannot be used to identify and quantify the oxy-hydrogenated species formed by its interaction with the hydroxyl groups of a solid, because of the superposition of several effects, in particular chemical exchange [4,23]. This latter can be eliminated by running the experiment at low temperature, but it results in too much line-broadening for it to be possible to obtain by HR-NMR techniques a resolved spectrum and the components characteristic of each species for samples in sealed tubes. Water and methanol adsorption will be developed in the broad-line NMR section. However, taking adsorbed water as an example we show the interest of 1H NMR experiments at ambient temperature. In the case of water adsorption in zeolites, a first result is that the signals of silanols are unchanged both in chemical shift and absolute intensity: these groups do not interact with water [23,37]. The diso value of the single signal, which is due to the resonance of the oxyprotonated species formed from the protons of SOHa and water in fast exchange, increases from d(OHa) to 5–7 ppm when the concentration ratio (interacting

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H2O):SOHa is 1; it decreases later on, being always greater than 4.8 ppm, which is the value for liquid water. The 1H MAS spectra of partially dealuminated zeolite samples allow the identification and quantification of a type of Lewis acid site [23,30,38]. After adsorption of water at a concentration which is small compared to that of acidic SOHa, a signal at 6.5 ppm corresponds to the resonance of the H atoms of water molecules coordinately bound to the Lewis sites. The nature of these Lewis sites was discussed [23,38]; they are probably tricoordinate Al atoms still bound to the zeolite framework [27,38]. In conclusion, the above examples demonstrate clearly the interest of 1H MAS NMR on sealed samples for determining the nature and concentration of different OH groups in anhydrous solid samples and those in interaction with the adsorbed base.

3. Broad-line NMR in ‘rigid lattice’ conditions

3.1. Adsorption of water or methanol (CD3OH) Water was chosen first as the adsorbed base for the following reasons: (i) it is small enough to gain access to all acidic sites; (ii) it contains only two protons, which means that either the hydrogen-bonded complex or the ionic one contains only three protons, which allows spectrum simulation without too much calculation; and (iii) from the results, its basic strength and hardness have appeared convenient. The interaction of water (as a base) with more or less acidic OH groups can give the following equilibria: Solid-OH+ H2O= Solid-OH…OH2 = H3O+ + Solid-O −

(1)

The question of the formation of ions or the hydrogen-bonded complexes in zeolites is currently much debated. For example, recent ab initio calculations [39–42] on clusters such as (H3SiO)2Al(OH)OSiH3 containing up to 28 T (T=Si, with the exception of one being Al) [43] lead to the conclusion that, in the case of ad-

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sorbed water, a hydrogen-bonded complex is the more stable, when the H2O:OHa ratio is 1. However, the stability difference between this complex and the hydroxonium ion is always a few kJ/mol, which is small. Nortier [44] showed that this result can be reversed by a slight change in the partial charge distribution in the lattice, which must be the case when each of several OH groups interact with a water molecule. For example, Smith et al. [45], using combined neutron diffraction and IR spectroscopy, have shown the formation of a hydroxonium ion on a Brønsted site of the molecular sieve H-SAPO-34, when a hydrogen-bonded complex is formed on another neighbour Brønsted site. Using 1H-NMR Batamack et al. [46,47] found the same result on HY zeolite. Jobic et al. [48] concluded from inelastic neutron scattering (INS) that the two forms of the adsorbate were formed, and then on the basis of the same spectra adopted the theoretical conclusions of Sauer et al. [41]. The interactions of CD3OH with OH groups should depend also on their acid strength: Solid-OH + CD3OH = Solid-OH…O(H)-CD3 =Solid-O − +(CD3OH2)+ (2)

3.2. Recording and simulating broad-line spectra The proton is a convenient nucleus for investigating catalysts using broad-line NMR at 4 K because its spin is 12 and the range of its chemical shifts is small enough to be negligible compared to the main effect which is, as mentioned above, the dipolar interaction between spins. When this effect is studied, two important points must be borne in mind: (i) the experimental temperature must be low enough to prevent diffusional and rotational motion of molecules or groups of atoms within the sample: this is the so-called ‘rigid-lattice condition’; and (ii) the interaction between two spins distance r apart is proportional to r − 3 [49]. Consequently, closest neighbour spins will give the greatest effect. The number of near neighbours and the geometry of their configuration are then responsible for the shape of the components of the spectrum.

After adsorption at room temperature the samples are quenched at 4 K, and the spectra are generally recorded as the derivative of the absorption relative to the static magnetic field, at 60 MHz, using a probe matched and tuned at the experiment temperature. Details on the experimental conditions can be found in Ref. [13]. From the standpoint of broad-line NMR, each of the chemical species in Eqs. (1) and (2) is a magnetic configuration of protons, characteried by their number and the geometry of this configuration. The interaction between neighbouring protons belonging to distinct configurations (and/or that of configuration protons with other nuclei of nonzero spin) is approximated by Gaussian broadening of the spectra of each configuration. The parameter of the Gaussian, b, for each configuration, can often be related to the shortest distances, x, between protons of distinct configurations. The spectrum of each chemical species in powder samples is calculated by using the following magnetic configurations: (i) H2O, CD3-(OH2)+ or OH…O(H)CD3: the two-spin configuration calculated by Pake [49]; (ii) H2O…HO-Solid: the threespin isosceles configuration by Andrew and Finch [50], revised by Dore´mieux-Morin [51]; (iii) H3O+: the three-spin equilateral configuration revised by Richards and Smith [52], after Andrew and Bersohn’s paper [53], or the three-spin isosceles configuration, if the ion is assumed to be deformed; and (iv) OH: either a Gaussian and/or a Lorentzian curve for the OH groups considered together, or a much broadened two-spin configuration [54]. The H2O, CD3-(OH2)+, OH…O(H)CD3 and H3O+ (assumed to be undeformed) species are characterized by a single intra-configuration H–H distance parameter (r), to which corresponds a magnetic field (a); two parameters are required to characterize H2O…HO and distorted H3O+, (distances r and r%, the base r being the shorter). The concentrations of the species are calculated from the weights of the corresponding contributions to the principal spectrum. The area under the experimental absorption is normalised to the total number of H atoms in the sample per chosen unit (uc or SOHa unit). The concentrations of the following species must therefore be known: (i)

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adsorbed water or methanol (measured gravimetrically); (ii) H atoms of each type in the ‘anhydrous’ zeolite determined by 1H MAS NMR experiments. There is no doubt as to the identification of the species, due to the particular shape of the spectrum of each magnetic configuration and in particular the position of maxima and minima and, consequently, no further doubt about the distribution of the adsorbed molecules relative to the OH groups of the solid (Fig. 1). The concentrations are usually measured to within 9 10%. Finally, we should point out right now that the hydrogen bonding interactions of these species with the solid surfaces cannot be quantified since oxygen has no nuclear spin (except for 17O). Further

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Fig. 2. Half the derivative 1H NMR broad-line spectrum of Nafion-H with 1.02 H2O/SO3. (…) experimental spectrum; ( — ) simulated spectrum; (H3O+ contribution).

studies with 17O-enriched solids would doubtless enable us to pinpoint the position of the adsorbed species on these surfaces.

4. Results

4.1. Adsorption of H2O

Fig. 1. Influence of the type of magnetic configuration on the shape of the experimental half-spectrum (as derivative of the absorption).

The concentrations of species of Eq. (1) or Eq. (2) are obtained from the simulation of the broadline spectra. As examples, three experimental spectra and their simulations are presented. Fig. 2 shows the experimental and the calculated spectra of 1:1 Nafion-H/H2O. Only hydroxonium ions are formed and ionisation is complete. These hydronium ions are distorted probably due to hydrogen bonding with neighbouring O atoms. They are approximately simulated by a three-spin isosceles triangle configuration. Shown in Fig. 3 are the experimental and simulated spectra of a HNAY sample (Si/Al = 2.4 with 48 SOHa/uc) without framework defects [46,47] and especially without silanol groups (this example is denoted Y2 in Table 1). For an (adsorbed H2O)/SOHa ratio of 1:1 only two components are summed to give the simulated spectrum. These results show that: (i) all SOHa groups and all water molecules interact; and (ii) 20% initial SOHa gave hydroxonium ions; 80% initial SOHa gave hydrogen-bonded complexes.

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be 1:1. The ‘excess’ adsorbed water, 3/SOHa, gives a spectrum component typical of water [49]. Moreover, the sample contained silanol groups found quantitatively without interaction with water and typically ‘far’ from others. In all cases, the 1H MAS result [23,47], that zeolite silanol OH groups do not interact with water [30,37,55], was confirmed by the broad-line experiments.

Fig. 3. Half the derivative 1H NMR broad-line spectrum of a HNa-Y (Y2) with 48.5 Brønsted acid sites (ZOH) per uc and with the same concentration of adsorbed water molecules. (…) experimental spectrum; (—) simulated spectrum; (---) H3O+ contribution; (–· –) H2O…OH contribution.

Fig. 4 shows an experimental spectrum corresponding to a typical general case. It is that of a H-ZSM-5 zeolite with 4 Al atoms/uc [37] and four adsorbed water molecules/SOHa. Again, the (interacting water molecule)/SOH ratio is found to

4.2. Definition of an acidity coefficient: acid strength We have seen that NMR at 4 K can be used to define the nature and the concentration of the protonated species which occur at the surface of solids after adsorption at 25°C of a well determined quantity of water. These species are defined by their characteristic interproton distances (r, r%) obtained when the spectra are simulated. In the absence of any means of measuring or calculating the pKa of solids we have chosen as a criterion of

Table 1 Experimental values of the acidity coefficient, xa, and the strength of the hydrogen bond in terms of O O distance in OH…H2O species for various solidsa Sample

Zeolite silanols Superficial OH groups on amorphous TiO2 H2AlP3O10·2H2O H-Y H2Nb6O16·2H20 HNa-Y H-ZSM-5 H-ZSM-5 H-Y H-mordenite H-mordenite H-Y H-Y H2Sb4O11·2H2O H-Y H-ZSM-5 H-ZSM-5 Sulfated zirconia Nafion-H·1H2O a

Sample symbol

Zeolite (Si/Al)XRD

Zeolite (Si/Al)XRF

xa

0 0

Y1

9.2

2.8

Y2 Z23 Z39 Y3 M1 M2 Y4 Y5

2.4 23 39 10.6 9.4 10.3 10.6 11.5

2.4

Y6 Z74 Z180 SZr

11.5 74 180

7.8

11.4 9.3 6.9 –

0 0.19 0.20 0.20 0.20 0.20 0.23 0.33 0.33 0.34 0.37 0.4 0.41 0.77 0.98 1.0 1.0

Zeolite Si/Al ratios as determined by XRD (framework value) and XRF (overall value).

O O distance (pm)

285 242

Reference

56 56 [56] [55] [14] [47] [59] [59] [55] [30] [30] [55] [55] [56] [55] [60] [60] [13] [15]

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Fig. 4. Half the derivative 1H NMR broad-line spectrum of a H-ZSM-5 (Z23) zeolite with four Brønsted acid sites (ZOH) per uc and with four adsorbed water molecules/ZOH; the attribution of the various components to the simulated spectrum are given in the figure.

acid strength the ‘acidity coefficient, xa’ representing the H3O+ concentration when each OHa interacts with one water molecule. In the case of solids too weakly acidic to give rise to H3O+ but only to hydrogen-bonded complexes, we have proposed [56,57] to use the strength of this hydrogen bond as a measure of the acid strength of the OH groups. By simulation the distances r (base of H2O) and r% (length of the two identical sides, assuming C2v symmetry of the OH…OH2 groups) can be determined and, consequently, the distance between the two O atoms calculated.

4.2.1. Scale of Brønsted acidity The acidity coefficient values, xa, and the strength of the hydrogen bond (when xa =0) measured for different solids are shown in Table 1, the samples being ranked from the least to the greatest value. 4.2.1.1. Zeolites. In the case of zeolites, it has been assumed that the maximum acid strength is reached for H-faujasite, H-mordenite and HZSM-5 when the minimum framework (Si/ Al)XRD = 5.8, 9.4 and 9.5, respectively, assuming

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that the Al atom distribution is homogeneous in the sample frameworks [58]. H-faujasites. The xa value has been measured for six H-faujasite samples [38,47,55] with Si/Al ratio from 2.4 to about 11.5 (denoted Yn where n is their rank, in Table 1). Their framework Si/Al ratios, (Si/Al)XRD, are listed in Table 1. All these samples, with the exception of Y2, were steamed under the same conditions. The steamed samples, with the exception of Y1, were washed under different conditions after the steaming. xa is the same, 0.37 90.04, for Y4, Y5 and Y6 [55] for which 10.6 5(Si/Al)XRD 5 11.5; this is much greater than that of Y2 (xa = 0.2) which has a (Si/Al)XRD B 5.8. These results are in agreement with Barthomeuf’s proposal [58]. The authors noted that for Y4, Y5 and Y6 the crystallinity is good (from 74 to 90%) and the chemical value of Si/Al, measured using X-ray fluorescence, (Si/ Al)XRF, is greater than 5.8, as is (Si/Al)XRD. The situation is less clear for two other samples: (i) Y1 has a low xa value of only 0.19 [55], though its (Si/Al)XRD is 9.2, while its (Si/Al)XRF is only 2.8: therefore, it contains much amorphous debris, strongly aluminic, and likely to disturb the measurement; (ii) the xa value of Y3 is low too at 0.23, [55] though its (Si/Al)XRD is 10.6 and its (Si/Al)XRF is 7.8; X-ray photoelectron spectroscopy (XPS) experiments on this sample, however, showed that the distribution of Al atoms is highly heterogeneous, the surface Si/Al ratio being larger than 20. For the interior of the crystallites, therefore, the local framework Si/Al ratio should still be small, probably less than 5.8. H-mordenites. The two mordenite samples [30], M1 and M2, have similar (Si/Al)XRD: 9.4 and 10.3, respectively, equal to and greater than the limiting value (9.4) calculated by Barthomeuf [58]. The two samples give the same, relatively large xa, 0.33, as expected. H-ZSM-5. It is well known that all ZSM-5 zeolites have generally (Si/Al)XRD greater than about 10. The SOHa sites, therefore, should all show the same value of the acidity coefficient, but this is not the case. The xa value depends strongly on the (Si/Al)XRD ratio [59,60]. It is 0.20, 0.20, 0.77, and 0.98 for (Si/Al)XRD equal to 23, 39, 74 and 180, respectively (Table 1). There is no simple

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Fig. 5. Geometries of methanol complexes with zeolite fragment.

On the other side, the species formed when water molecules interact with superficial OH groups on amorphous titania or with H2AlP3O10 are only hydrogen-bonded: H2O…HO. Therefore, for these hydroxyl groups xa = 0. The shorter the O O distance, the stronger the acid. Therefore, H2AlP3O10 is more acidic than amorphous TiO2. Zeolitic silanols do not interact with water molecules. They are not acidic at all.

4.3. CD3OH as base explanation for this result. The samples have not undergone dealumination treatment. Heeribout et al. [60] have proposed, at least for samples with (Si/Al)XRD equal to 74 and 180, that it could be related to a heterogeneous distribution of the Al atoms in the framework. The concentration per uc near the Z74 crystal surface is found to be 6.591 by XPS, five times larger than 1.3, the mean chemical value; the presence of silanol groups whose concentration is close to that of the SOHa and for which the distance between neighbouring H atoms (about 250 pm) would favour the existence of geminal silanols [61,62]. Indeed, quantum chemical calculations have shown that SOH groups close to a terminal OH group are somewhat more acidic [63] or active [64] than when these are surrounded by typical framework tetrahedra only.

4.2.1.2. Other solids. Both Brønsted sites in niobic acid interact with water molecules in H2Nb6O16·2H2O to give either hydroxonium ions or hydrogen-bonded complexes. The xa value of 0.2 of this acid is equal to that of zeolites Y2, Z23 and Z39. Antimonic acid dihydrate, H2Sb4O11·2H2O, is more acidic than niobic acid. In both diacids, both Brønsted acid sites interact with water. This is not the case of sulfuric acid: the first Brønsted acid is very strong whereas the second is too weak to interact with water molecules up to 4H2O/H2SO4 [65]. Nafion-H, as well as the first Brønsted site of sulfated zirconia (SZr) is so strong an acid that proton transfer from the acid to the base is complete. Water is too strong a base to differentiate these compounds.

Methanol is a stronger base than water [66]. One should expect proton transfer with acids like zeolites. Although it is now accepted that there is no proton transfer from the zeolite acidic OHa groups to methanol, previous studies concluded that methanol was protonated [67,68]. The calculated H–H distance in CD3OH+ 2 is 1589 2 pm [69]. Calculations by Kubelkova et al. [33,70] showed that the H–H distance in the ion, 143 pm, is shorter than that in the ‘neutral’ complex, 192 pm (Fig. 5) [33]. Since the dipolar effect studied is proportional to r − 3, where r is the distance between two interacting nuclei, 1H broad-line NMR at low temperature can answer the question of the proton transfer from an acid to methanol.

Fig. 6. Half the derivative 1H NMR broad-line spectrum of 1:1 CF3SO3H/CD3OH. (…) experimental spectrum; ( — ) simulated spectrum; (---) H3O+ contribution; ( – ·– ) CD3OH+ 2 contribution.

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We studied the interaction of deuteriated methanol with H-ZMS-5, Nafion-H and the superacid CF3SO3H. The broad-line spectra of CF3SO3H/CD3OH and Nafion-H/CD3OH 1:1 (Fig. 6) reveal the formation of methoxonium ions with a H–H distance of 1709 2 pm. This is about 10 pm more than the calculated value, which can be attributed to hydrogen bond interactions between methoxonium ions and their surroundings. The species formed during the interaction for 1:1 H-ZMS-5/CD3OH has a H – H distance of 199 92 pm characteristic of ‘neutral’ complex species.

5. Conclusion 1

H Broad-line NMR under H-ZMS-5/CD3OH ‘rigid lattice’ conditions allows the study of certain heterogeneous adsorbate – adsorbant equilibria, as shown by the examples of water and methanol adsorbed on more or less acidic solids. The acidity of mild Brønsted acid solids is conveniently studied in the presence of small amounts of water molecules as base using 1H broad-line NMR under ‘rigid lattice’ conditions. The acidity coefficient, xa, deduced from this study is an effective parameter for differentiating solids. Zeolites with different Si/Al ratios have been compared. The acid strength of these materials depends mostly on the treatments the samples undergo after the synthesis. The maximum xa value is 0.35 – 0.40 and 0.33, for faujasites and mordenites, respectively. For the ZSM-5 studied, xa increases strongly with the Si/Al ratio, reaching 1 when (Si/Al)XRD is equal to 180. For strong acids such as Nafion-H and sulfated zirconia for which xa = 1, a weaker base is required. Methanol is protonated in the presence of Nafion-H and trifluoromethanesulfonic acid. With H-ZSM-5 zeolite, a H-bonded species is observed. In cases such as zeolites, MAS NMR is especially suitable for the identification and quantification of the Brønsted sites but the strength of the acid sites is not accessible by this technique. However, although 1H HR NMR is not absolutely necessary, the results it provides make it easier to simulate the broad-band spectra. Correlation be-

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tween the acid strength obtained using the broadline technique and catalytic results has been found [71].

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