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Characterization of protonic sites in H3PW12O40 and Cs1.9H1.1PW12O40: a solid-state 1H,2H,31P MAS-NMR and inelastic neutron scattering study on samples prepared under standard reaction conditions
Applied Catalysis A: General 194 –195 (2000) 109–122
Characterization of protonic sites in H3 PW12 O40 and Cs1.9 H1.1 PW12 O40 : a solid-state 1 H, 2 H, 31 P MAS-NMR and inelastic neutron scattering study on samples prepared under standard reaction conditions N. Essayem ∗ , Y.Y. Tong1 , H. Jobic, J.C. Vedrine2 Institut de Recherches sur la Catalyse, CNRS, 2 avenue Albert Einstein, F-69626 Villeurbanne Cedex, France Accepted 14 June 1999
Abstract Spectroscopic techniques in controlled atmosphere, such as solid-state 1 H, 2 H, and 31 P magic angle spinning nuclear magnetic resonance (MAS-NMR) and inelastic neutron scattering (INS) spectroscopies, have been used to investigate the effect of dehydration on structural modifications and acidic properties of solid 12-tungstophosphoric acid H3 PW12 O40 and its cesium salt Cs1.9 H1.1 PW12 O40 . Thermogravimetric analysis and XRD experiments gave complementary informations about proton/water contents and structure of the samples. 1 H, 2 H, and 31 P MAS-NMR spectra were recorded as a function of the degree of dehydration/rehydration and allowed one to characterize the protonic species present in the samples, such as OH groups and protonated clusters H+ (H2 O)n . INS spectra, recorded at 4 K on samples dehydrated at 473 K, suggested the presence of hydroxonium ion H3 O+ in bulk H3 PW12 O40 and of hydroxyl type species in the porous cesium salt Cs1.9 H1.1 PW12 O40 . After dehydration at a higher temperature, 573 K, the INS spectra showed the presence of hydroxyl groups in both samples. These four techniques provided a detailed description of the acidic features (nature, strength and number of the acid sites) of H3 PW12 O40 and Cs1.9 H1.1 PW12 O40 samples in relation with their structure and hydration state. ©2000 Elsevier Science B.V. All rights reserved. Keywords: Heteropolyanion; 12-Tungstophosphoric; 1 H MAS-NMR; INS; Hydroxonium ion
1. Introduction Strong Brønsted acidity of heteropolyacids (HPA) such as 12-tungstophosphoric acid (H3 PW12 O40 ) and particularly of its Cs salts (Cs3−x Hx PW12 O40 ) has ∗ Corresponding author. Fax: +33-4-72-44-5399. E-mail address: [email protected] (N. Essayem). 1 Present address: Department of Chemistry, University of Illinois at Urbana-Champaign Urbana, IL 61801, USA. 2 Present address: The Leverhulme Centre for Innovative Catalysis, Department of Chemistry, The University of Liverpool, Liverpool, L69 7ZD, UK.
attracted great attention recently due to their high catalytic activity in acid-type reactions such as n-butane isomerization or isobutane alkylation by butenes [1–3]. An important step towards a better understanding of their catalytic properties is the characterization of their acidic features, including the nature, strength and number of acid sites, which are closely related to the molecularity of the materials and their hydration level. The nature of the protonic species in the hexahydrate H3 PW12 O40 ·6H2 O and its crystallographic structure have been described in the literature [4] as H5 O2 + clusters and a cubic structure. However, for
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more or less dehydrated 12-tungstophosphoric acids, the description of their protonic species, in relation to the HPA structure, has not yet been carefully established. Solid and liquid NMR spectroscopies are powerful methods to investigate the primary structure of heteropolycompounds. For instance 29 Si, 31 P and 183 W nuclei were studied in solutions since a long time [5] and correlations were established between 31 P chemical shift and the structure of the heteropolyacids. More recently, investigations of solid HPA by multinuclear solid-state NMR have also appeared in the literature. For supported HPA, solid state MAS-NMR has shown that strong interactions exist between heteropolyacids and its support, depending on nature of the support and on dispersion of the HPA [6–10]. Although for bulk heteropolyacids, most studies were centered on the characterization of their structural evolution with thermal treatments, they are limited to the investigations of the thermolysis of 12-molybdophosphoric and of 12-tungstophosphoric acids by 31 P MAS-NMR [11,12]. Concerning the structural investigation of porous heteropolycompounds as Mx H3−x PW12 O40 (with M = Cs, K), 31 P MAS-NMR studies have also shown the presence of several peaks in the 31 P spectra depending on the chemical composition of the materials and on their hydration state [2,13,14] but different assignments of these resonance lines were given. Brønsted acid sites in solid acids can be characterized by 1 H NMR spectroscopy. For zeolites, correlations between 1 H chemical shift and acid strength could be established but a generalisation was not possible. The basic idea is that a more acidic proton has less electron in its vicinity, therefore is less shielded and, subsequently, its NMR chemical shift ␦H , will be more positive (lower-field shift). However, other contributions, such as hydrogen bonding, may also influence ␦H values, which may make its relationship with acidity strength ambiguous [15,16]. Acidity of different heteropolyacids has already been investigated by 1 H MAS-NMR [17,18] and for bulk 12-tungstophosphoric acid treated under vacuum, chemical shift values as high as 9 ppm (with respect to TMS) were reported. 17 O MAS-NMR has also been used to characterize protonic sites in solid HPA [8,19], since terminal W=Ot bonds were thought to be the dominant proton sites in dehydrated H3 PW12 O40 and Cs3 PW12 O40 .
Previous INS investigations of the bulk 12-tungstophosphoric acid have revealed the presence of various protonic species, such as H5 O2 + , H3 O+ , ‘lone’ proton or hydroxyl groups, depending on its hydration level [20]. However, to our best knowledge, the protonic sites in acidic Cs salts have not been investigated either by 1 H MAS-NMR or by INS so far. By combining multi-nuclear (1 H, 2 H and 31 P) solid-state MAS-NMR, INS, thermogravimetry (TG), and X-ray diffraction (XRD) techniques, we present in this paper a detailed characterization of Brønsted sites in H3 PW12 O40 ·nH2 O and Cs1.9 H1.1 PW12 O40 ·nH2 O samples, as a function of their dehydration state obtained under dynamic conditions at different temperatures, i.e. in conditions similar to those used as pretreatment for catalytic testings.
2. Experimental 2.1. Materials Pure heteropolyacid H3 PW12 O40 sample was prepared according to the classical method including the synthesis of the sodium form, the extraction of H3 PW12 O40 by diethyl ether, and its purification by recrystallization in water. It’s BET surface area was equal to 7 m2 g−1 . The preparation of the cesium salt was achieved by addition of a Cs chloride solution (5 M) to an aqueous solution of H3 PW12 O40 (0.1 M), using a molar ratio Cs/P = 2. The suspension was then kept under stirring for 24 h. The precipitate was washed twice with distilled water and separated from the liquid phase by centrifugation. It’s chemical composition was deduced from chemical analysis of Cs and W content and corresponds to Cs1.9 H1.1 PW12 O40 . It’s BET surface area was equal to 71 m2 g−1 . Deuterated 12-tungstophosphoric acid was prepared as follows: H3 PW12 O40 ·nH2 O was dissolved in D2 O (99% D). The solution was kept in a dessicator under Ar atmosphere to avoid any further exchange with atmospheric water protons. No recrystallization occurred after several days. Then the deuterated HPA was recovered after complete evaporation of D2 O and its Keggin structure was checked by IR. Obviously, the deuteration was expected not to be complete since the starting HPA contained protons.
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2.2. Thermogravimetric experiments Thermogravimetric and differential thermal analyses were carried out under flowing dry N2 using a Setaram 92–12 microbalance. The temperature increase programme included a linear ramp (1 h) up to a plateau maintained for 2 h at a chosen temperature and a final increase up to 1023 K at 5 K min−1 . This temperature programme was applied to all samples before all analyses. The weight losses measured during the first temperature rise, the isothermal step and the last temperature rise up to 1023 K gave data on the water/proton contents at each stage.
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conditions given above but at only 473 and 573 K. The samples (20–30 g) were contained in glass ampoules sealed off from the preparation lines and then transferred to the sample holders in the glove box flushed with dry argon. The INS experiments were performed on the spectrometer IN1BeF at the Institut Laue-Langevin in Grenoble, France. The INS spectra were recorded at 4 K using Cu(200), (220) and (331) monochromator planes. With the beryllium filter, the instrumental resolution varies from 25 cm−1 at low energy transfers, to 50 cm−1 at large energy transfers. The frequency values given in the text have been corrected from a systematic shift due to the beryllium filter. The estimated absolute accuracy is ca. 20 cm−1 .
2.3. Multinuclear solid-state MAS-NMR and XRD studies in controlled atmosphere 3. Results and discussion For 1 H, 2 H, 31 P MAS-NMR and XRD analyses, the samples were dehydrated under flowing dry N2 for 2 h at 323, 373, 473, 573, 673, and 773 K. The dehydrated samples were first transferred to the glass ampoules connected to the dynamic reactor and were sealed off, then opened in a glove box flushed with dry Ar in order to avoid any moisture contamination and finally transferred in a rotor for NMR experiments or in a sample holder closed with a capton film for XRD analyses. All NMR experiments were carried out using a Bruker DSX 400 spectrometer at room temperature with a spinning rate of 12 kHz, repetition time of 60 s for 31 P and 1 s for 1 H. TMS, D2 O and H3 PO4 (85% solution) were taken as external references for 1 H, 2 H and 31 P, respectively. 2.4. Inelastic neutron scattering spectroscopy For INS experiments, H3 PW12 O40 and Cs1.9 H1.1 PW12 O40 samples were dehydrated in the same
3.1. Dehydration of H3 PW12 O40 under flowing N2 3.1.1. Thermogravimetric (TG) and differential thermal analysis (DTA) In addition to the conventional determination by TGA of the number of crystallization water molecules, it is possible to determine the number of remaining acidic protons (e.g., three in the case of H3 PW12 O40 ) from the amount of water eluded stemming from acidic protons and oxygen atoms of the anion in a defined temperature range [2,21]. TGA data, summarized in Table 1, are expressed in number of water molecules released before, during and after the isothermal step. For example, the TGA and DTA curves, including an isothermal step at 473 K, are reported in Fig. 1a. Considering the fact that for the 12-tungstophosphoric acid, the number of protonic sites should be of 3H+ per Keggin unit (KU) for assuring the electroneutrality of the system, it is
Table 1 Hydration state of H3 PW12 O40 after thermal treatments involving an isothermal step for 2 h under N2 flow calculated from TGA data Isothermal step T (K)
373 473 573 673 773
Water losses [mol H2 O/KU]
HPA state at the thermal step
Before the thermal step
During the thermal step (plateau)
From the thermal step up to 1023 K
Crystalisation water in [mol H2 O/KU]
H+ /KU
15.6 19.6 20.6 21.6 22.5
0.1 0.7 0 0.7 0.04
7 1.6 1.3 0.6 0
5.5 0.1 0 0 0
3 3 2.6 1.2 ∼0
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Fig. 1. TG-DT analyses of (a) H3 PW12 O40 .21H2 O and (b) Cs1.9 H1.1 PW12 O40 ·10H2 O samples with the following heating rate temperature programme: (1) Linear increase of the temperature at 5 K min−1 rate from 288 up to 473 K under a dry N2 flow of 1.2 dm3 h−1 . (2) Step of 2 h at 473 K under the same N2 flow. (3) Linear increase of the tempearture from 473 to 1023 K, rate: 5 K min−1 under the same N2 flow.
then possible to calculate the number of remaining crystallization water molecules and of protonic sites, respectively.
3.1.2. XRD and 31 P MAS-NMR studies XRD and NMR are complementary techniques for the characterization of material structure since XRD provides informations related to long-range order while NMR is very sensitive to local structure modifications. For all characterizations the samples were dehydrated as described above. For convenience, we designated a, b, c, d, e, and f the samples dehydrated under dry nitrogen for 2 h at 323, 373, 473, 573, 673 and 773 K, respectively. XRD spectra were recorded to follow the crystalline structure of these six intermediate hydrates with the capton film technique to prevent rehydration. The XRD patterns of Samples a
and b showed clearly the features corresponding to a cubic structure, which is the well accepted secondary structure for H3 PW12 O40 ·6H2 O. These features are still present on Sample c, which was thermally treated at 473 K, indicating that the structure was still cubic, although with a reduced cell parameter value: 1.213, 1.208 and 1.190 nm for Samples a–c, respectively. This finding, which was reproducible, is somehow surprising since a completely anhydrous phase is expected to have a tetragonal structure [21]. We believe that the residual crystallization water, (Table 1), have preserved such a long-range cubic symmetry found for Sample c. Thermal treatment at 673 K produced nearly anhydride HPA, and the corresponding XRD spectrum showed the features characteristic of a tetragonal structure. These tetragonal features are still distinguishable, although broadened, for Sample f, indicating a lower crystallization state. It is
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Fig. 2. Room temperature 31 P MAS-NMR spectra of 12-tungstophosphoric acid dehydrated at: a 323 K, b 373 K, c 473 K, d 573 K, e 673 K and f 773 K under dry N2 flow for 2 h.
worthnoting that for Sample f, XRD pattern did not show any features due to WO3 , in agreement with the DTA isothermal step of 2 h at 773 K during which no exothermic event due to the crystallization of WO3 was observed. However, an exothermic event was observed when the temperature rose from 773 to 1023 K. 31 P MAS-NMR study of the sample upon increasing dehydration showed that the chemical shift of 31 P is very sensitive to the thermal treatment. Three subgroups can be distinguished among the Samples: a and b; c–e and f (Fig. 2). For Samples a and b a narrow peak at −15.6 ppm was obtained as expected for an
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hydrated form. Dehydration up to 673 K broadened and shifted the peak to around −11 ppm, Spectra c–e. The peak broadening is most remarkable for the sample dehydrated at 473 K, and is almost four times that of the other two samples treated at higher temperatures. While the broadness of peak 2c was reproducible, its lineshape differed slightly from one treatment to another one, indicating subtle structural changes at this temperature (see discussion below). Further dehydration to 773 K resulted in a broad peak, ranging from −6 to −14 ppm, Spectrum 2f, suggesting some major local structural changes. Note that the broadening and lowfield shift of 31 P NMR peak upon thermal dehydration is consistent with previous investigations [12,22–24]. It is also interesting to note that even though the 31 P chemical shift and peak width is dependent on water content, it is impossible to establish a 1 H–31 P cross polarization. This is consistent with the fact that P was geometrically at the center of the Keggin anion, i.e. far away from the proton, and that the Keggin anion remained intact, even though some local distortions had occurred. Several important structural observations can be made. First, there exist two major secondary structures, i.e., the cubic and tetragonal structures for hydrated and completely anhydrous or anhydride HPA, respectively, as shown by XRD results. Correspondingly, there also exist two primary structures due most probably to some subtle local structure distortions of Keggin anion, as suggested by 31 P NMR spectra a, b and d, e. Second, the low-field shift of 31 P NMR peak of the sample dehydrated at 473 K indicates that the local structure of Keggin anion has already changed while the XDR results show that a cubic secondary structure is still dominant. This suggests that there exists a transition between the two well defined major structures at around 473 K. During this transition, two forces compete: the first one acts to adapt the local Keggin structure to the presence of almost bare H+ s in HPA, which seems to prefer a tetragonal secondary structure, while the second one acts to preserve the cubic secondary structure, which is more stable in the presence of crystallization water. Such a competition produces local structural diversity as demonstrated by the remarkable broadening of 31 P NMR line, Fig. 2c. Third, the decrease of cubic cell parameter at 473 K may suggest that the electrostatic field of a bare H+ (i.e., without hydration water) attracts the nearest
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Keggin anions closer, pulling electrons away from the central P atom, and therefore shifting the 31 P NMR peak toward low-field. 3.1.3. 1 H MAS-NMR As mentioned in the introduction, the chemical shift of 1 H can be considered as an indicator of the acidity of a proton. Since all our samples corresponded to the same Keggin anion to which the proton is attached and the Keggin anion remained stable during thermal treatments, the evolution of the proton resonance peak as a function of the degree of dehydration was therefore a reasonable indicator of the strength of the proton acidity. Fig. 3 shows the evolution of 1 H MAS-NMR spectrum when the HPA sample was thermally pretreated at 323 (3a), 373 (3b), 473 (3c), 573 (3d), 673 (3e) and 773 K (3f), respectively as for 31 P MAS-NMR experiments. Sample a showed a narrow (ca. 400 Hz) and symmetrical 1 H peak at 7 ppm. Sample b showed a main peak at 7.5 ppm and a shoulder at c.a. 8.5 ppm, the linewidth of the main peak being very close to that of Fig. 3a. Sample c had a dominant peak at 10.5 ppm with a small but well separated shoulder at 8.9 ppm. Samples d and e showed a strong resonance peak at 8.8 ppm, with a shoulder at about 7.5 ppm, more pronounced and better separated for Sample e. For Sample f, a broad peak was observed at ca. 7 ppm with a shoulder extended up to 8.7 ppm. It is important to precise that all 1 H spectra given in Fig. 3 were normalized to give the same maximum amplitude, while a progressive decrease of the 1 H signal intensity was observed upon dehydration. In contrast to 31 P NMR results where Samples a and b gave the same spectra, the 1 H peak shifted from 7.0 ppm for Sample a to 7.5 ppm for Sample b with an additional shoulder at 8.5 ppm. From TGA data (Table 1), we knew that the only difference between these two samples was the amount of crystallization water. The highest amount of water retained by sample a leads to the presence of large protonated water clusters as H+ (H2 O)n . No separated water peak was observed, which means that on the scale of 1 s, the protons exhibit a fast exchange with all hydrogens of the cluster water molecules resulting in the narrow 1 H peak in Fig. 3a. TGA results showed that Sample b corresponds to the nearly hexahydrated compound, with the stoichiometry H3 PW12 O40 ·5.5H2 O. It is thus
Fig. 3. Room temperature 1 H MAS-NMR spectra of 12-tungstophosphoric acid dehydrated at: a 323 K, b 373 K, c 473 K, d 573 K, e 673 K and f 773 K under dry N2 flow for 2 h.
reasonable to believe that the dominant peak at 7.5 ppm is due to H+ (H2 O)2 species and the shoulder signal at 8.5 ppm to H3 O+ species with hydrogen bonding between H2 O and H3 O+ . Sample c showed some peculiar features with the dominant peak exhibiting the largest chemical shift (10.5 ppm). Following the chemical shift–acidity relationship, this indicates the presence of strongly acidic protons. From TGA data (Table 1), these protons must be bare H+ s, while the small shoulder at 8.9 ppm may be from some residual hydrated form, most probably in the form of hydroxonium ion, H3 O+ .
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The high-field shift of 1 H peaks for dehydrations at T > 473 K, which reveals a decrease of the acid strength, may be rationalized with the appearance of tetragonal structure where protons find more electrons in their neighborhood and may be responsible for the peaks at ca. 8.8 ppm and below. Indeed, INS results (see Section 3.5) showed a dominant presence of hydroxyl groups for samples dehydrated at 573 K. It is also known that progressive ‘deprotonation’, which occurs at increasing dehydration temperatures, leads to a modification of the anion composition with a loss of its external oxygen atoms. The peak at ca. 7.5–7.0 ppm certainly arises from remaining protons attached to these structures. 3.1.4. 31 P, 1 H and 2 H MAS-NMR of deuterated 12-tungstophosphoric acid. 31 P, 1 H and 2 H NMR study of a deuterated bulk heteropolyacid has also been explored. The aim was to get more insights on the relation between primary/secondary structure changes and on the different protonic sites. It was observed (Fig. 4) that the 31 P NMR spectra were not appreciably affected by deuteration. After treatment at 323 K, the 31 P peak was obtained at −15.5 ppm (Fig. 4a) as in Fig. 2a. After dehydration at 473 K a broad 31 P peak was observed at ca. −11.8 ppm against −11 ppm for the starting acid
Fig. 4. Room temperature 31 P MAS-NMR spectra of deuterated 12-tungstophosphoric acid: D3−x Hx PW12 O40 treated at: a 323 K and c 473 K.
Fig. 5. Room temperature 2 H MAS-NMR spectra of deuterated 12-tungstophosphoric acid treated at: a 323 K and c 473 K.
(Fig. 2c). This slight chemical shift is presumably related to a slight difference in the amount of residual crystallization water. As it was expected, the deuteration precedure was not complete and a 1 H NMR investigation of the sample treated at 323 K showed a peak at −7.6 ppm which is consistent with the presence of protonated water clusters. The 2 H NMR spectra obtained for two samples treated at 323 K and 473 K were relatively narrow for a quadrupolar nucleus (Fig. 5). This indicates a high mobility and/or high local symmetry of deuterium in HPA. The 2 H spectrum obtained after the treatment at 323 K showed a very sharp peak at −2.5 ppm and a broader one at 2.0 ppm. Since no major change in 31 P and 1 H chemical shifts were observed with respect to those observed after thermal treatment of H3 PW12 O40 , the broad peak at 2.0 ppm may be assigned to D+ (D2 O)2 species, and the narrow peak at −2.5 ppm to larger clusters as D+ (D2 O)n , assuming the mobility of deuterons in larger clusters D+ (D2 O)n is higher than in D+ (D2 O)2 . The rise of the dehydration temperature lead to a broadening of the main peak at 2 ppm, which may correspond to loss of mobility for the remaining deuterons by elimination of cristallization water.
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3.2. Rehydration mechanism of H3 PW12 O40 3.2.1. 31 P MAS-NMR and XRD studies after rehydration under ambient atmosphere The previous six dehydrated samples (a–f) were rehydrated by exposing them to ambient atmosphere at room temperature for 48 h. The rehydration restored the peak at ca. −15.0 to −15.5 ppm for all samples (Fig. 6). For Samples a–c, a narrow peak at −15.5 ppm, and cubic XRD pattern with a high crystallinity, were observed showing a complete recovery. For Samples d and e, a slightly broader peak at ca −15.2 ppm was observed, with a very weak peak at −12 ppm for Sample e. The XRD pattern con-
Fig. 6. Room temperature 31 P NMR spectra of Samples a–f after exposure in ambient atmosphere for 48 h.
firmed the cubic structure for both samples, though the diffraction lines were rather broad and less intense. For Sample f, XRD pattern showed a tetragonal structure, showing that the 773 K dehydrated sample was only partly rehydrated. The 31 P NMR spectrum of Sample f is complicated with a sharp and weak peak at −15.5 ppm, a narrow peak at −15 ppm, and a broad peak centered at ca. −12 ppm. The sharp peak at −15.5 ppm may be assigned to a hydrated phase H3 PW12 O40 ·nH2 O with high crystallinity and the peak at −15 ppm may arise from incomplete rehydration of tetragonal structure observed in XRD. The broad band centered at ca. −12 ppm can be attributed to some phases intermediate between the anhydride and anhydrous phases of the heteropolyacid rather than to an amorphous P2 O5 oxide, although a completely decomposed 12-tungstophosphoric acid is know to give a broad peak centered at −11.5 ppm [25]. The rehydration of samples treated at the lowest temperatures (T < 473 K) occurs readily while after a treatment at higher temperature, a complete rehydration requires much more time. Particularly, it can be concluded that the H3 PW12 O40 sample dehydrated under dry nitrogen flow at 673 K for 2 h can be rehydrated almost completely to its original state after 48 h, while dehydration at 773 K gives rise to the tetragonal structure and to only partial recovery of the initial state upon rehydration. 3.2.2. 1 H MAS-NMR after exposure to ambient atmosphere After exposure to ambient atmosphere for 48 h, Samples a–c showed a single peak at ca. 7 ppm previously assigned to H+ (H2 O)n , Samples d and e showed a broader peak at ca. 7.5 ppm (previously ascribed to H+ (H2 O)2 ) and Sample f showed a peak at 6.6 ppm. Sample c was studied more carefully during its rehydration by recording NMR spectra after a few minutes, 24 and 48 h of exposure to air. A single isotropic peak at 9.7 ppm was observed after few minutes (Fig. 7a), an asymmetrical one at 7 ppm after 24 h (Fig. 7b) and finally the symmetrical peak at 7 ppm after 48 h. We think that the appearance of this narrow peak at 9.7 ppm, an intermediate position between the weakly bonded acidic proton of the anhydrous phase at 10.5 ppm and the H+ (H2 O)2 of the nearly hexahydrate phase at 7.5 ppm, should correspond to the formation of the hydroxonium ion H3 O+ , with three
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573 K. The weight losses converted in mol H2 O per KU are given in Table 2. Assuming that deprotonation occured only after crystallization water was eluded, one could calculate the amount of remaining protonic sites. It was observed that the sample treated at 473 K corresponded to a fully protonic sample without crystallization water and with a ratio H+ /P = 1, close to the initial true value H+ /P = 1.1 (measured by chemical analysis of W and P contents). On the other hand, the treatment at higher temperature, 573 K, leads to a partly deprotonated sample with a remaining H+ /P ratio equals to 0.56. These data reported in Table 2 also indicated that the Cs salt contained less crystallization water (or physisorbed water) than the acid form although its surface area was much higher and that deprotonation at 573 K was larger (44 against 13%). As an example, a curve corresponding to the isothermal step a 473 K is reported in Fig. 1b. Fig. 7. Room temperature 1 H NMR spectra of sample c after exposure in ambient atmosphere for few minutes (a) and for 24 h (b).
equivalent protons, in absence of hydrogen bonds with neutral water molecules. This shows the progressive rehydration of bare protons (at 10.5 ppm) to H3 O+ (9.7 ppm), H5 O2 + at 7.5 ppm and H+ (H2 O)n clusters at 7 ppm. This assignment is different to that proposed by Misono [24] who attributed a peak at ca. 9 ppm observed after evacuation of the HPA at 473 to the acidic ‘free’ proton and, surprisingly, reported the same 1 H chemical shift for both H3 PW12 O40 ·0H2 O and H3 PW12 O40 ·2.2H2 O samples. 3.3. Dehydration of Cs1.9 H1.1 PW12 O40 under flowing N2 3.3.1. ATG-ATD TG and TD analyses were performed at two different temperatures with isothermal plateaus at 473 and
3.3.2. 31 P MAS-NMR 31 P MAS-NMR spectra of Cs H PW O after 1.9 1.1 12 40 pretreatment at 323, 373, 473, 573, 673, and 773 K are given in Fig. 8 (Samples a0 –f 0 ). All six samples showed a dominant peak at −15 ppm, as for neutral salt Cs3 PW12 O40 [2]. A small peak at −13.5 ppm appeared after treatment at 373 K (Spectrum 8b0 ), which was stable until thermal treatment at 673 K when a broad band appeared at ca. −10.7 ppm (Spectrum 8e0 ) at the expense of the −13.5 ppm peak. We have previously suggested that for Cs3−x Hx PW12 O40 , prepared as described in the experimental part (precipitation and washing of the Cs salt), the sample was actually composed of the acid form H3 PW12 O40 deposited on the Cs3 P salt rather than an acid salt as Cs3−x Hx PW12 O40 suggesting a true solid solution, which should be characterized by a single 31 P peak with an intermediate position between the pure acid and the neutral Cs salt. If the model of a H3 P trapped on Cs3 P holds true, the results above
Table 2 Hydration state of Cs1.9 H1.1 PW12 O40 after different thermal treatments as calculated from TGA experiments Isothermal step T (K)
Water losses [mol H2 O/KU]
473 573
Before the thermal step 9.9 10.3
During the thermal step (plateau) 0 0
Protons content after the thermal step From the thermal step up to 1023 K 0.5 0.28
H+ /KU 1 0.56
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as expected by thermal treatment. After dehydration at 323 K, a 1 H peak at ca. 6.7 ppm (Fig. 9a0 ), was observed with a position and line width quite similar to those obtained for the bulk acid after the same treatment (Fig. 3a). Thus it is reasonable to assign the peak at ca. 6.7 ppm to H+ (H2 O)n species. Pretreatments at 373 and 473 K gave peaks at ca. 9 and 8.5 ppm, respectively (Fig. 9b0 and c0 ). A significant broadening of the linewidth was observed for Samples e0 and f 0 , while a broad 31 P peak at −11 ppm appeared (Fig. 8e0 and f0 ). It is interesting to note that the most positive 1 H shift occurred at 373K when the 31 P peak at −13.5 ppm appeared. The concerted change of 31 P and 1 H peaks,
Fig. 8. Room temperature 31 P MAS-NMR spectra of Cs1.9 H1.1 PW12 O40 sample treated at: a0 323 K, b0 373 K, c0 473 K, d0 573 K, e0 673 K and f 0 773 K.
indicate that there is a strong interaction between the acid and the Cs3 P salt resulting in a small shift in 31 P peak versus dehydration as observed in Fig. 8. 3.3.3. 1 H MAS-NMR The evolution of the 1 H spectrum for Cs2 HP sample as a function of the degree of dehydration (Fig. 9), was different from that observed for the bulk acid (Fig. 3). No matter what pretreatment temperature was, only a single peak was detected. The spectra were again normalized in Fig. 9 to give the same maximum amplitude and a strong decrease of the 1 H signal was indeed observed which is obviously due to the loss of H content
Fig. 9. Room temperature 1 H MAS-NMR spectra of Cs1.9 H1.1 PW12 O40 treated at a0 323 K, b0 373 K, c0 473 K, d0 573 K, e0 673 K and f 0 773 K.
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observed after dehydration at 373 K, suggests that the Cs salt has already reached a defined dehydrated state at that temperature, while for 12-tungstophosphoric acid, the removal of the crystallization water happened at 473 K rather than at 373 K. The high field shift of the 1 H peak for samples treated above 573 K may indicate the decrease of their acidity strength. 3.4. Rehydration mechanism of Cs1.9 H1.1 PW12 O40 Rehydration by 48 h exposure to ambient amosphere tends to reverse completely the effect of thermal treatment by restoring the initial 31 P MAS-NMR spectra at −15 ppm except for Sample f 0 which exhibited a weak background ranging from −10 to −14 ppm indicating that the rehydration was not complete. The 1 H MAS-NMR spectra (Fig. 10) appear to be more complicated than 31 P NMR ones. Sample treated at 323 K and rehydrated showed two narrow peaks at 7 and 5.4 ppm (Fig. 10a0 ). While the peak at 7 ppm may again be assigned to H+ (H2 O)n species, the one at 5.4 ppm is more troublesome. It might correspond to neutral water in the structure as the sample was not outgassed again. The sample pretreated at 373 K showed only one peak at 7 ppm (Fig. 10b0 ), but that pretreated at 473 K showed two peaks: one dominant peak at 5.8 ppm and a small one at 5.5 ppm (Fig. 10c0 ). For samples pretreated at temperatures above 573 K, only one peak at 6 ppm was observed, with a linewidth increasing with temperature rising (Fig. 10d0 –f0 ). So, it appeared, for the Cs salt, that 1 H MAS-NMR is more sensitive to the history of thermal treatment and rehydration than 31 P MAS-NMR.
Fig. 10. Room temperature 1 H MAS-NMR spectra of Cs1.9 H1.1 PW12 O40 Samples a0 , b0 , c0 , d0 , e0 and f 0 after exposure in ambient atmosphere for 48 h.
3.5. INS study INS spectroscopy is a powerful vibrational technique for the investigation of H-containing materials since only H atoms give signal with significant intensity [26]. The dehydration process of 12-tungstophosphoric acid has been already investigated by INS. Spectra assigned to H5 O2 + species, H3 O+ and ‘lone’ proton or OH groups have been reported [20]. Comparison of the 12-tungstophosphoric acid and its Cs salt when dehydrated at 473 and 573 K under N2 flow was investigated, as it is known from previous catalytic studies that these pretreat-
ment temperatures gave different catalytic behaviours in an highly demanding acidic type reaction, as the n-butane isomerization [27]. The INS spectrum obtained for H3 PW12 O40 treated at 473 K is consistent with the presence of H3 O+ with characteristic vibrations at 331, 468, 678 and 1072 cm−1 (Fig. 11c). Upon heating at 573 K, the spectrum showed strong modifications with bands at 504, 610 and 1141 cm−1 which can be assigned to hydroxyls groups (Fig. 11d). Our spectra are similar to those obtained by Mioc et al. [20].
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Fig. 11. 4 K INS spectra of H3 PW12 O40 dehydrated under dry N2 flow at c 473 K and at d 573 K.
The spectra obtained for the Cs salts presented a lower intensity than those obtained for the pure acid, which is obviously due to lower content in protonic sites. Upon preheating at 473 or at 573 K, the two spectra were almost similar and showed essentially the characteristic vibrational bands of hydroxyl groups at 506 and 1075 cm−1 (Fig. 12). Thus, the evolution of spectra obtained by INS spectroscopy is consistent with the presence of several protonic species for bulk 12-tungstophosphoric acid. Netherveless, some discrepancies appeared since the
evolution of 1 H MAS-NMR signal with the degree of dehydration showed the coexistence of H3 O+ species and strongly acidic protons (‘bare proton’) after treatment at 473 K (Fig. 3c), while INS spectroscopy detected mainly the presence of H3 O+ ions. For higher treatments, at 573 K, 1 H NMR interpretations and INS spectra were in agreement, and both indicated the presence of hydroxyl groups. For the Cs salt, 1 H NMR revealed that the same protonic species are formed after treatment at 473 or 573 K. INS spectroscopy confirmed this observation
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Fig. 12. 4 K INS spectra of Cs1.9 H1.1 PW12 O40 dehydrated under N2 flow at c0 473 K and d0 573 K.
and showed only the presence of hydroxyl groups. A recent FTIR study of the H3 PW12 O40 sample, treated at 473 K under N2 flow showed the presence of a broad band in the 2700–3600 cm−1 region and a band at 1710 cm−1 ascribed to protonated water clusters, the distinction between H3 O+ and H+ (H2 O)n being difficult by IR [25]. Note also that such materials appeared to be very sensitive to water traces, which makes difficult comparison of data from several techniques. The great advantage of the present study was to be performed after activation conditions as close as possible
to those chosen for catalytic studies [27] which are known to be very sensitive to water [25,27,28].
4. Conclusions The four techniques used: TGA, XRD, MAS-NMR and INS in addition to previous FTIR data have provided a detailed description of the changes in the acidic features of H3 PW12 O40 and Cs1.9 H1.1 PW12 O40 samples and in their structural modifications when
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submitted to thermal treatments. The following main conclusions may be drawn: • For thermal treatment under dry nitrogen flow at low temperatures, (T ≤ 373 K for H3 P and T ≤ 323 K for Cs1.9 H1.1 P) the solids are characterized by the presence of protonic clusters H+ (H2 O)n . • The strongest acidity corresponds to the formation of a ‘bare’ proton and coincides with the departure of the crystallization water. This state is obtained after treatment of H3 P at 473 K and of Cs1.9 H1.1 P at 373 K, as demonstrated by 1 H and 31 P NMR data. Chemical shift as large as 10.5 ppm was observed for 1 H of H3 P sample, and 9 ppm for Cs1.9 H1.1 P sample. • Finally, thermal treatments at higher temperatures lead to a decrease of the strength of acidity ascribed to modifications of the primary and secondary structures of the heteropolycompounds which modify the electron density around the proton. • 1 H MAS-NMR shows that for H3 P various protonic species are present and their relative amounts change with the extent of dehydration, which does not hold true for the Cs salt. This result is partly confirmed by INS study which gave spectra ascribed to H3 O+ species for H3 P treated at 473 K and to hydroxyl groups for H3 P treated at 573 K and its Cs salts treated at 473 and 573 K. Nevertheless some disagreements emerged between the different techniques about the nature of the protonic species found on H3 P after dehydration at 473 K. 1 H NMR showed that a ‘bare’ proton and a small proportion of H3 O+ ions should coexist, in good agreement with TGA data, while INS spectrum is rather consistent with the major presence of H3 O+ . This disagreement could be due to small differences in the final hydration states reached by the two samples, a much larger quantity of sample being treated for INS experiments and/or to the very low temperature for INS measurements (4 K). Because of the high sensitivity of heteropolyacids to moisture, we believe also that it is rather difficult to keep a given hydrated state, particularly after the treatment at 473 K which corresponds to a rather disordered structure. Acknowledgements We thank M.T. Gimenez for XRD-data acquisition and V. Martin for thermogravimetric experiments.
References [1] K. Na, T. Okuhara, M. Misono, J. Chem. Soc., Faraday Trans. 91 (1995) 367. [2] N. Essayem, G. Coudurier, M. Fournier, J.C. Védrine, Catal. Lett. 34 (1995) 223. [3] N. Essayem, S. Kieger, G. Coudurier, J.C. Védrine, Stud. Surf. Sci. Catal. 101A (1996) 591. [4] G.M. Brown, M.R. Noe-Spirley, W.R. Busing, A. Levy, Acta Cryst. B33 (1977) 1038. [5] R. Massart, R. Contant, J.M. Fruchart, J.P. Ciabrini, M. Fournier, Inorg. Chem. 16 (1977) 2916. [6] V.M. Mastikhin, S.M. Kulikov, A.V. Nosov, I.L. Kozhevnikov, I.L. Mudrakovsky, M.N. Timofeeva, J. Mol. Catal. 60 (1990) 65. [7] R. Thouvenot, M. Fournier, C. Rocchiccioli-Deltcheff, J. Chem. Soc., Faraday Trans. 87 (17) (1991) 2829. [8] I.V. Kozhevnikov, A. Sinnema, R.J.J. Jansen, H. Van Bekkum, Catal. Lett. 27 (1994) 187. [9] V.M. Mastikhin, V.V. Terskikh, O.P. Timofeeva, O.P. Krivoruchko, J. Mol. Catal. A: Chemical 95 (1995) 135. [10] J.C. Edwards, C.Y. Thiel, B. Benac, J.F. Knifton, Catal. Lett. 51 (1998) 77. [11] V.M. Bondareva, T.V. Andrushkevich, R.I. Maksimovskaya, L.M. Plyasova, A.V. Ziborov, G.S. Litvak, L.G. Detusheva, Kinet.i Katal. 35 (1994) 114. [12] R.I. Maksimovskaya, Kinet.i Katal. 36 (1995) 836. [13] J.B. Black, N.J. Clayden, P.L. Gai, J.D. Scott, E.M. Serwicka, J.B. Goodenough, J. Catal. 106 (1887) 1. [14] T. Okuhara, T. Nishimura, H. Watanabe, M. Misono, J. Mol. Catal. 74 (1992) 247. [15] V.M. Mastikhin, I.L. Mudrakovsky, A.V. Nosov, Prog. NMR Spectrosc. 23 (1991) 259. [16] M. Hunger, Catal. Rev.- Sci. Eng. 39 (4) (1997) 345. [17] E. Brunner, Catal. Today 38 (1997) 361. [18] T. Baba, Y. Hasada, M. Nomura, Y.I. Ohno, Y. Ono, J. Mol. Catal. A: Chemical 114 (1996) 247. [19] I.V. Kozhevnikov, A. Sinnema, H. Van Bekkum, Catal. Lett. 34 (1995) 213. [20] U.B. Mioc, Ph. Colomban, M. Davidovic, J. Tomkinson, J. Mol. Struct. 326 (1994) 99. [21] M. Fournier, C. Feumi-Jantou, C. Rabia, G. Hervé, S. Launay, J. Mater. Chem. 2 (1992) 971. [22] Y. Kand, K.Y. Lee, S. Nakata, S. Asaoka, M. Misono, Chem. Lett. 139 (1988). [23] Y. Hirano, K. Inumaru, T. Okuhara, M. Misono, Chem. Lett. 1111 (1996). [24] S. Uchida, K. Inumaru, J.M. Dereppe, M. Misono, Chem. Lett. 643 (1998). [25] N. Essayem, G. Coudurier, J.C. Vedrine, D. Habermacher, J. Sommer, J. Catal. 183 (1999) 292. [26] H. Jobic, in: B. Imelik, J.C. Védrine (Eds.), Catalyst Characterization, Plenum Press, New York, 1994, p. 347. [27] N. Essayem, G. Coudurier, J.C. Vedrine, Proceedings of the DGMK Conference Catalysis on Solid Acids and Bases DGMK Tagungsbericht, 1996, p. 111. [28] A.K. Ghost, J.B. Moffat, J. Catal. 101 (1986) 288.
Characterization of protonic sites in H3 PW12 O40 and Cs1.9 H1.1 PW12 O40 : a solid-state 1 H, 2 H, 31 P MAS-NMR and inelastic neutron scattering study on samples prepared under standard reaction conditions N. Essayem ∗ , Y.Y. Tong1 , H. Jobic, J.C. Vedrine2 Institut de Recherches sur la Catalyse, CNRS, 2 avenue Albert Einstein, F-69626 Villeurbanne Cedex, France Accepted 14 June 1999
Abstract Spectroscopic techniques in controlled atmosphere, such as solid-state 1 H, 2 H, and 31 P magic angle spinning nuclear magnetic resonance (MAS-NMR) and inelastic neutron scattering (INS) spectroscopies, have been used to investigate the effect of dehydration on structural modifications and acidic properties of solid 12-tungstophosphoric acid H3 PW12 O40 and its cesium salt Cs1.9 H1.1 PW12 O40 . Thermogravimetric analysis and XRD experiments gave complementary informations about proton/water contents and structure of the samples. 1 H, 2 H, and 31 P MAS-NMR spectra were recorded as a function of the degree of dehydration/rehydration and allowed one to characterize the protonic species present in the samples, such as OH groups and protonated clusters H+ (H2 O)n . INS spectra, recorded at 4 K on samples dehydrated at 473 K, suggested the presence of hydroxonium ion H3 O+ in bulk H3 PW12 O40 and of hydroxyl type species in the porous cesium salt Cs1.9 H1.1 PW12 O40 . After dehydration at a higher temperature, 573 K, the INS spectra showed the presence of hydroxyl groups in both samples. These four techniques provided a detailed description of the acidic features (nature, strength and number of the acid sites) of H3 PW12 O40 and Cs1.9 H1.1 PW12 O40 samples in relation with their structure and hydration state. ©2000 Elsevier Science B.V. All rights reserved. Keywords: Heteropolyanion; 12-Tungstophosphoric; 1 H MAS-NMR; INS; Hydroxonium ion
1. Introduction Strong Brønsted acidity of heteropolyacids (HPA) such as 12-tungstophosphoric acid (H3 PW12 O40 ) and particularly of its Cs salts (Cs3−x Hx PW12 O40 ) has ∗ Corresponding author. Fax: +33-4-72-44-5399. E-mail address: [email protected] (N. Essayem). 1 Present address: Department of Chemistry, University of Illinois at Urbana-Champaign Urbana, IL 61801, USA. 2 Present address: The Leverhulme Centre for Innovative Catalysis, Department of Chemistry, The University of Liverpool, Liverpool, L69 7ZD, UK.
attracted great attention recently due to their high catalytic activity in acid-type reactions such as n-butane isomerization or isobutane alkylation by butenes [1–3]. An important step towards a better understanding of their catalytic properties is the characterization of their acidic features, including the nature, strength and number of acid sites, which are closely related to the molecularity of the materials and their hydration level. The nature of the protonic species in the hexahydrate H3 PW12 O40 ·6H2 O and its crystallographic structure have been described in the literature [4] as H5 O2 + clusters and a cubic structure. However, for
0926-860X/00/$ – see front matter ©2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 9 9 ) 0 0 3 5 9 - 2
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more or less dehydrated 12-tungstophosphoric acids, the description of their protonic species, in relation to the HPA structure, has not yet been carefully established. Solid and liquid NMR spectroscopies are powerful methods to investigate the primary structure of heteropolycompounds. For instance 29 Si, 31 P and 183 W nuclei were studied in solutions since a long time [5] and correlations were established between 31 P chemical shift and the structure of the heteropolyacids. More recently, investigations of solid HPA by multinuclear solid-state NMR have also appeared in the literature. For supported HPA, solid state MAS-NMR has shown that strong interactions exist between heteropolyacids and its support, depending on nature of the support and on dispersion of the HPA [6–10]. Although for bulk heteropolyacids, most studies were centered on the characterization of their structural evolution with thermal treatments, they are limited to the investigations of the thermolysis of 12-molybdophosphoric and of 12-tungstophosphoric acids by 31 P MAS-NMR [11,12]. Concerning the structural investigation of porous heteropolycompounds as Mx H3−x PW12 O40 (with M = Cs, K), 31 P MAS-NMR studies have also shown the presence of several peaks in the 31 P spectra depending on the chemical composition of the materials and on their hydration state [2,13,14] but different assignments of these resonance lines were given. Brønsted acid sites in solid acids can be characterized by 1 H NMR spectroscopy. For zeolites, correlations between 1 H chemical shift and acid strength could be established but a generalisation was not possible. The basic idea is that a more acidic proton has less electron in its vicinity, therefore is less shielded and, subsequently, its NMR chemical shift ␦H , will be more positive (lower-field shift). However, other contributions, such as hydrogen bonding, may also influence ␦H values, which may make its relationship with acidity strength ambiguous [15,16]. Acidity of different heteropolyacids has already been investigated by 1 H MAS-NMR [17,18] and for bulk 12-tungstophosphoric acid treated under vacuum, chemical shift values as high as 9 ppm (with respect to TMS) were reported. 17 O MAS-NMR has also been used to characterize protonic sites in solid HPA [8,19], since terminal W=Ot bonds were thought to be the dominant proton sites in dehydrated H3 PW12 O40 and Cs3 PW12 O40 .
Previous INS investigations of the bulk 12-tungstophosphoric acid have revealed the presence of various protonic species, such as H5 O2 + , H3 O+ , ‘lone’ proton or hydroxyl groups, depending on its hydration level [20]. However, to our best knowledge, the protonic sites in acidic Cs salts have not been investigated either by 1 H MAS-NMR or by INS so far. By combining multi-nuclear (1 H, 2 H and 31 P) solid-state MAS-NMR, INS, thermogravimetry (TG), and X-ray diffraction (XRD) techniques, we present in this paper a detailed characterization of Brønsted sites in H3 PW12 O40 ·nH2 O and Cs1.9 H1.1 PW12 O40 ·nH2 O samples, as a function of their dehydration state obtained under dynamic conditions at different temperatures, i.e. in conditions similar to those used as pretreatment for catalytic testings.
2. Experimental 2.1. Materials Pure heteropolyacid H3 PW12 O40 sample was prepared according to the classical method including the synthesis of the sodium form, the extraction of H3 PW12 O40 by diethyl ether, and its purification by recrystallization in water. It’s BET surface area was equal to 7 m2 g−1 . The preparation of the cesium salt was achieved by addition of a Cs chloride solution (5 M) to an aqueous solution of H3 PW12 O40 (0.1 M), using a molar ratio Cs/P = 2. The suspension was then kept under stirring for 24 h. The precipitate was washed twice with distilled water and separated from the liquid phase by centrifugation. It’s chemical composition was deduced from chemical analysis of Cs and W content and corresponds to Cs1.9 H1.1 PW12 O40 . It’s BET surface area was equal to 71 m2 g−1 . Deuterated 12-tungstophosphoric acid was prepared as follows: H3 PW12 O40 ·nH2 O was dissolved in D2 O (99% D). The solution was kept in a dessicator under Ar atmosphere to avoid any further exchange with atmospheric water protons. No recrystallization occurred after several days. Then the deuterated HPA was recovered after complete evaporation of D2 O and its Keggin structure was checked by IR. Obviously, the deuteration was expected not to be complete since the starting HPA contained protons.
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2.2. Thermogravimetric experiments Thermogravimetric and differential thermal analyses were carried out under flowing dry N2 using a Setaram 92–12 microbalance. The temperature increase programme included a linear ramp (1 h) up to a plateau maintained for 2 h at a chosen temperature and a final increase up to 1023 K at 5 K min−1 . This temperature programme was applied to all samples before all analyses. The weight losses measured during the first temperature rise, the isothermal step and the last temperature rise up to 1023 K gave data on the water/proton contents at each stage.
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conditions given above but at only 473 and 573 K. The samples (20–30 g) were contained in glass ampoules sealed off from the preparation lines and then transferred to the sample holders in the glove box flushed with dry argon. The INS experiments were performed on the spectrometer IN1BeF at the Institut Laue-Langevin in Grenoble, France. The INS spectra were recorded at 4 K using Cu(200), (220) and (331) monochromator planes. With the beryllium filter, the instrumental resolution varies from 25 cm−1 at low energy transfers, to 50 cm−1 at large energy transfers. The frequency values given in the text have been corrected from a systematic shift due to the beryllium filter. The estimated absolute accuracy is ca. 20 cm−1 .
2.3. Multinuclear solid-state MAS-NMR and XRD studies in controlled atmosphere 3. Results and discussion For 1 H, 2 H, 31 P MAS-NMR and XRD analyses, the samples were dehydrated under flowing dry N2 for 2 h at 323, 373, 473, 573, 673, and 773 K. The dehydrated samples were first transferred to the glass ampoules connected to the dynamic reactor and were sealed off, then opened in a glove box flushed with dry Ar in order to avoid any moisture contamination and finally transferred in a rotor for NMR experiments or in a sample holder closed with a capton film for XRD analyses. All NMR experiments were carried out using a Bruker DSX 400 spectrometer at room temperature with a spinning rate of 12 kHz, repetition time of 60 s for 31 P and 1 s for 1 H. TMS, D2 O and H3 PO4 (85% solution) were taken as external references for 1 H, 2 H and 31 P, respectively. 2.4. Inelastic neutron scattering spectroscopy For INS experiments, H3 PW12 O40 and Cs1.9 H1.1 PW12 O40 samples were dehydrated in the same
3.1. Dehydration of H3 PW12 O40 under flowing N2 3.1.1. Thermogravimetric (TG) and differential thermal analysis (DTA) In addition to the conventional determination by TGA of the number of crystallization water molecules, it is possible to determine the number of remaining acidic protons (e.g., three in the case of H3 PW12 O40 ) from the amount of water eluded stemming from acidic protons and oxygen atoms of the anion in a defined temperature range [2,21]. TGA data, summarized in Table 1, are expressed in number of water molecules released before, during and after the isothermal step. For example, the TGA and DTA curves, including an isothermal step at 473 K, are reported in Fig. 1a. Considering the fact that for the 12-tungstophosphoric acid, the number of protonic sites should be of 3H+ per Keggin unit (KU) for assuring the electroneutrality of the system, it is
Table 1 Hydration state of H3 PW12 O40 after thermal treatments involving an isothermal step for 2 h under N2 flow calculated from TGA data Isothermal step T (K)
373 473 573 673 773
Water losses [mol H2 O/KU]
HPA state at the thermal step
Before the thermal step
During the thermal step (plateau)
From the thermal step up to 1023 K
Crystalisation water in [mol H2 O/KU]
H+ /KU
15.6 19.6 20.6 21.6 22.5
0.1 0.7 0 0.7 0.04
7 1.6 1.3 0.6 0
5.5 0.1 0 0 0
3 3 2.6 1.2 ∼0
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Fig. 1. TG-DT analyses of (a) H3 PW12 O40 .21H2 O and (b) Cs1.9 H1.1 PW12 O40 ·10H2 O samples with the following heating rate temperature programme: (1) Linear increase of the temperature at 5 K min−1 rate from 288 up to 473 K under a dry N2 flow of 1.2 dm3 h−1 . (2) Step of 2 h at 473 K under the same N2 flow. (3) Linear increase of the tempearture from 473 to 1023 K, rate: 5 K min−1 under the same N2 flow.
then possible to calculate the number of remaining crystallization water molecules and of protonic sites, respectively.
3.1.2. XRD and 31 P MAS-NMR studies XRD and NMR are complementary techniques for the characterization of material structure since XRD provides informations related to long-range order while NMR is very sensitive to local structure modifications. For all characterizations the samples were dehydrated as described above. For convenience, we designated a, b, c, d, e, and f the samples dehydrated under dry nitrogen for 2 h at 323, 373, 473, 573, 673 and 773 K, respectively. XRD spectra were recorded to follow the crystalline structure of these six intermediate hydrates with the capton film technique to prevent rehydration. The XRD patterns of Samples a
and b showed clearly the features corresponding to a cubic structure, which is the well accepted secondary structure for H3 PW12 O40 ·6H2 O. These features are still present on Sample c, which was thermally treated at 473 K, indicating that the structure was still cubic, although with a reduced cell parameter value: 1.213, 1.208 and 1.190 nm for Samples a–c, respectively. This finding, which was reproducible, is somehow surprising since a completely anhydrous phase is expected to have a tetragonal structure [21]. We believe that the residual crystallization water, (Table 1), have preserved such a long-range cubic symmetry found for Sample c. Thermal treatment at 673 K produced nearly anhydride HPA, and the corresponding XRD spectrum showed the features characteristic of a tetragonal structure. These tetragonal features are still distinguishable, although broadened, for Sample f, indicating a lower crystallization state. It is
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Fig. 2. Room temperature 31 P MAS-NMR spectra of 12-tungstophosphoric acid dehydrated at: a 323 K, b 373 K, c 473 K, d 573 K, e 673 K and f 773 K under dry N2 flow for 2 h.
worthnoting that for Sample f, XRD pattern did not show any features due to WO3 , in agreement with the DTA isothermal step of 2 h at 773 K during which no exothermic event due to the crystallization of WO3 was observed. However, an exothermic event was observed when the temperature rose from 773 to 1023 K. 31 P MAS-NMR study of the sample upon increasing dehydration showed that the chemical shift of 31 P is very sensitive to the thermal treatment. Three subgroups can be distinguished among the Samples: a and b; c–e and f (Fig. 2). For Samples a and b a narrow peak at −15.6 ppm was obtained as expected for an
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hydrated form. Dehydration up to 673 K broadened and shifted the peak to around −11 ppm, Spectra c–e. The peak broadening is most remarkable for the sample dehydrated at 473 K, and is almost four times that of the other two samples treated at higher temperatures. While the broadness of peak 2c was reproducible, its lineshape differed slightly from one treatment to another one, indicating subtle structural changes at this temperature (see discussion below). Further dehydration to 773 K resulted in a broad peak, ranging from −6 to −14 ppm, Spectrum 2f, suggesting some major local structural changes. Note that the broadening and lowfield shift of 31 P NMR peak upon thermal dehydration is consistent with previous investigations [12,22–24]. It is also interesting to note that even though the 31 P chemical shift and peak width is dependent on water content, it is impossible to establish a 1 H–31 P cross polarization. This is consistent with the fact that P was geometrically at the center of the Keggin anion, i.e. far away from the proton, and that the Keggin anion remained intact, even though some local distortions had occurred. Several important structural observations can be made. First, there exist two major secondary structures, i.e., the cubic and tetragonal structures for hydrated and completely anhydrous or anhydride HPA, respectively, as shown by XRD results. Correspondingly, there also exist two primary structures due most probably to some subtle local structure distortions of Keggin anion, as suggested by 31 P NMR spectra a, b and d, e. Second, the low-field shift of 31 P NMR peak of the sample dehydrated at 473 K indicates that the local structure of Keggin anion has already changed while the XDR results show that a cubic secondary structure is still dominant. This suggests that there exists a transition between the two well defined major structures at around 473 K. During this transition, two forces compete: the first one acts to adapt the local Keggin structure to the presence of almost bare H+ s in HPA, which seems to prefer a tetragonal secondary structure, while the second one acts to preserve the cubic secondary structure, which is more stable in the presence of crystallization water. Such a competition produces local structural diversity as demonstrated by the remarkable broadening of 31 P NMR line, Fig. 2c. Third, the decrease of cubic cell parameter at 473 K may suggest that the electrostatic field of a bare H+ (i.e., without hydration water) attracts the nearest
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Keggin anions closer, pulling electrons away from the central P atom, and therefore shifting the 31 P NMR peak toward low-field. 3.1.3. 1 H MAS-NMR As mentioned in the introduction, the chemical shift of 1 H can be considered as an indicator of the acidity of a proton. Since all our samples corresponded to the same Keggin anion to which the proton is attached and the Keggin anion remained stable during thermal treatments, the evolution of the proton resonance peak as a function of the degree of dehydration was therefore a reasonable indicator of the strength of the proton acidity. Fig. 3 shows the evolution of 1 H MAS-NMR spectrum when the HPA sample was thermally pretreated at 323 (3a), 373 (3b), 473 (3c), 573 (3d), 673 (3e) and 773 K (3f), respectively as for 31 P MAS-NMR experiments. Sample a showed a narrow (ca. 400 Hz) and symmetrical 1 H peak at 7 ppm. Sample b showed a main peak at 7.5 ppm and a shoulder at c.a. 8.5 ppm, the linewidth of the main peak being very close to that of Fig. 3a. Sample c had a dominant peak at 10.5 ppm with a small but well separated shoulder at 8.9 ppm. Samples d and e showed a strong resonance peak at 8.8 ppm, with a shoulder at about 7.5 ppm, more pronounced and better separated for Sample e. For Sample f, a broad peak was observed at ca. 7 ppm with a shoulder extended up to 8.7 ppm. It is important to precise that all 1 H spectra given in Fig. 3 were normalized to give the same maximum amplitude, while a progressive decrease of the 1 H signal intensity was observed upon dehydration. In contrast to 31 P NMR results where Samples a and b gave the same spectra, the 1 H peak shifted from 7.0 ppm for Sample a to 7.5 ppm for Sample b with an additional shoulder at 8.5 ppm. From TGA data (Table 1), we knew that the only difference between these two samples was the amount of crystallization water. The highest amount of water retained by sample a leads to the presence of large protonated water clusters as H+ (H2 O)n . No separated water peak was observed, which means that on the scale of 1 s, the protons exhibit a fast exchange with all hydrogens of the cluster water molecules resulting in the narrow 1 H peak in Fig. 3a. TGA results showed that Sample b corresponds to the nearly hexahydrated compound, with the stoichiometry H3 PW12 O40 ·5.5H2 O. It is thus
Fig. 3. Room temperature 1 H MAS-NMR spectra of 12-tungstophosphoric acid dehydrated at: a 323 K, b 373 K, c 473 K, d 573 K, e 673 K and f 773 K under dry N2 flow for 2 h.
reasonable to believe that the dominant peak at 7.5 ppm is due to H+ (H2 O)2 species and the shoulder signal at 8.5 ppm to H3 O+ species with hydrogen bonding between H2 O and H3 O+ . Sample c showed some peculiar features with the dominant peak exhibiting the largest chemical shift (10.5 ppm). Following the chemical shift–acidity relationship, this indicates the presence of strongly acidic protons. From TGA data (Table 1), these protons must be bare H+ s, while the small shoulder at 8.9 ppm may be from some residual hydrated form, most probably in the form of hydroxonium ion, H3 O+ .
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The high-field shift of 1 H peaks for dehydrations at T > 473 K, which reveals a decrease of the acid strength, may be rationalized with the appearance of tetragonal structure where protons find more electrons in their neighborhood and may be responsible for the peaks at ca. 8.8 ppm and below. Indeed, INS results (see Section 3.5) showed a dominant presence of hydroxyl groups for samples dehydrated at 573 K. It is also known that progressive ‘deprotonation’, which occurs at increasing dehydration temperatures, leads to a modification of the anion composition with a loss of its external oxygen atoms. The peak at ca. 7.5–7.0 ppm certainly arises from remaining protons attached to these structures. 3.1.4. 31 P, 1 H and 2 H MAS-NMR of deuterated 12-tungstophosphoric acid. 31 P, 1 H and 2 H NMR study of a deuterated bulk heteropolyacid has also been explored. The aim was to get more insights on the relation between primary/secondary structure changes and on the different protonic sites. It was observed (Fig. 4) that the 31 P NMR spectra were not appreciably affected by deuteration. After treatment at 323 K, the 31 P peak was obtained at −15.5 ppm (Fig. 4a) as in Fig. 2a. After dehydration at 473 K a broad 31 P peak was observed at ca. −11.8 ppm against −11 ppm for the starting acid
Fig. 4. Room temperature 31 P MAS-NMR spectra of deuterated 12-tungstophosphoric acid: D3−x Hx PW12 O40 treated at: a 323 K and c 473 K.
Fig. 5. Room temperature 2 H MAS-NMR spectra of deuterated 12-tungstophosphoric acid treated at: a 323 K and c 473 K.
(Fig. 2c). This slight chemical shift is presumably related to a slight difference in the amount of residual crystallization water. As it was expected, the deuteration precedure was not complete and a 1 H NMR investigation of the sample treated at 323 K showed a peak at −7.6 ppm which is consistent with the presence of protonated water clusters. The 2 H NMR spectra obtained for two samples treated at 323 K and 473 K were relatively narrow for a quadrupolar nucleus (Fig. 5). This indicates a high mobility and/or high local symmetry of deuterium in HPA. The 2 H spectrum obtained after the treatment at 323 K showed a very sharp peak at −2.5 ppm and a broader one at 2.0 ppm. Since no major change in 31 P and 1 H chemical shifts were observed with respect to those observed after thermal treatment of H3 PW12 O40 , the broad peak at 2.0 ppm may be assigned to D+ (D2 O)2 species, and the narrow peak at −2.5 ppm to larger clusters as D+ (D2 O)n , assuming the mobility of deuterons in larger clusters D+ (D2 O)n is higher than in D+ (D2 O)2 . The rise of the dehydration temperature lead to a broadening of the main peak at 2 ppm, which may correspond to loss of mobility for the remaining deuterons by elimination of cristallization water.
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3.2. Rehydration mechanism of H3 PW12 O40 3.2.1. 31 P MAS-NMR and XRD studies after rehydration under ambient atmosphere The previous six dehydrated samples (a–f) were rehydrated by exposing them to ambient atmosphere at room temperature for 48 h. The rehydration restored the peak at ca. −15.0 to −15.5 ppm for all samples (Fig. 6). For Samples a–c, a narrow peak at −15.5 ppm, and cubic XRD pattern with a high crystallinity, were observed showing a complete recovery. For Samples d and e, a slightly broader peak at ca −15.2 ppm was observed, with a very weak peak at −12 ppm for Sample e. The XRD pattern con-
Fig. 6. Room temperature 31 P NMR spectra of Samples a–f after exposure in ambient atmosphere for 48 h.
firmed the cubic structure for both samples, though the diffraction lines were rather broad and less intense. For Sample f, XRD pattern showed a tetragonal structure, showing that the 773 K dehydrated sample was only partly rehydrated. The 31 P NMR spectrum of Sample f is complicated with a sharp and weak peak at −15.5 ppm, a narrow peak at −15 ppm, and a broad peak centered at ca. −12 ppm. The sharp peak at −15.5 ppm may be assigned to a hydrated phase H3 PW12 O40 ·nH2 O with high crystallinity and the peak at −15 ppm may arise from incomplete rehydration of tetragonal structure observed in XRD. The broad band centered at ca. −12 ppm can be attributed to some phases intermediate between the anhydride and anhydrous phases of the heteropolyacid rather than to an amorphous P2 O5 oxide, although a completely decomposed 12-tungstophosphoric acid is know to give a broad peak centered at −11.5 ppm [25]. The rehydration of samples treated at the lowest temperatures (T < 473 K) occurs readily while after a treatment at higher temperature, a complete rehydration requires much more time. Particularly, it can be concluded that the H3 PW12 O40 sample dehydrated under dry nitrogen flow at 673 K for 2 h can be rehydrated almost completely to its original state after 48 h, while dehydration at 773 K gives rise to the tetragonal structure and to only partial recovery of the initial state upon rehydration. 3.2.2. 1 H MAS-NMR after exposure to ambient atmosphere After exposure to ambient atmosphere for 48 h, Samples a–c showed a single peak at ca. 7 ppm previously assigned to H+ (H2 O)n , Samples d and e showed a broader peak at ca. 7.5 ppm (previously ascribed to H+ (H2 O)2 ) and Sample f showed a peak at 6.6 ppm. Sample c was studied more carefully during its rehydration by recording NMR spectra after a few minutes, 24 and 48 h of exposure to air. A single isotropic peak at 9.7 ppm was observed after few minutes (Fig. 7a), an asymmetrical one at 7 ppm after 24 h (Fig. 7b) and finally the symmetrical peak at 7 ppm after 48 h. We think that the appearance of this narrow peak at 9.7 ppm, an intermediate position between the weakly bonded acidic proton of the anhydrous phase at 10.5 ppm and the H+ (H2 O)2 of the nearly hexahydrate phase at 7.5 ppm, should correspond to the formation of the hydroxonium ion H3 O+ , with three
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573 K. The weight losses converted in mol H2 O per KU are given in Table 2. Assuming that deprotonation occured only after crystallization water was eluded, one could calculate the amount of remaining protonic sites. It was observed that the sample treated at 473 K corresponded to a fully protonic sample without crystallization water and with a ratio H+ /P = 1, close to the initial true value H+ /P = 1.1 (measured by chemical analysis of W and P contents). On the other hand, the treatment at higher temperature, 573 K, leads to a partly deprotonated sample with a remaining H+ /P ratio equals to 0.56. These data reported in Table 2 also indicated that the Cs salt contained less crystallization water (or physisorbed water) than the acid form although its surface area was much higher and that deprotonation at 573 K was larger (44 against 13%). As an example, a curve corresponding to the isothermal step a 473 K is reported in Fig. 1b. Fig. 7. Room temperature 1 H NMR spectra of sample c after exposure in ambient atmosphere for few minutes (a) and for 24 h (b).
equivalent protons, in absence of hydrogen bonds with neutral water molecules. This shows the progressive rehydration of bare protons (at 10.5 ppm) to H3 O+ (9.7 ppm), H5 O2 + at 7.5 ppm and H+ (H2 O)n clusters at 7 ppm. This assignment is different to that proposed by Misono [24] who attributed a peak at ca. 9 ppm observed after evacuation of the HPA at 473 to the acidic ‘free’ proton and, surprisingly, reported the same 1 H chemical shift for both H3 PW12 O40 ·0H2 O and H3 PW12 O40 ·2.2H2 O samples. 3.3. Dehydration of Cs1.9 H1.1 PW12 O40 under flowing N2 3.3.1. ATG-ATD TG and TD analyses were performed at two different temperatures with isothermal plateaus at 473 and
3.3.2. 31 P MAS-NMR 31 P MAS-NMR spectra of Cs H PW O after 1.9 1.1 12 40 pretreatment at 323, 373, 473, 573, 673, and 773 K are given in Fig. 8 (Samples a0 –f 0 ). All six samples showed a dominant peak at −15 ppm, as for neutral salt Cs3 PW12 O40 [2]. A small peak at −13.5 ppm appeared after treatment at 373 K (Spectrum 8b0 ), which was stable until thermal treatment at 673 K when a broad band appeared at ca. −10.7 ppm (Spectrum 8e0 ) at the expense of the −13.5 ppm peak. We have previously suggested that for Cs3−x Hx PW12 O40 , prepared as described in the experimental part (precipitation and washing of the Cs salt), the sample was actually composed of the acid form H3 PW12 O40 deposited on the Cs3 P salt rather than an acid salt as Cs3−x Hx PW12 O40 suggesting a true solid solution, which should be characterized by a single 31 P peak with an intermediate position between the pure acid and the neutral Cs salt. If the model of a H3 P trapped on Cs3 P holds true, the results above
Table 2 Hydration state of Cs1.9 H1.1 PW12 O40 after different thermal treatments as calculated from TGA experiments Isothermal step T (K)
Water losses [mol H2 O/KU]
473 573
Before the thermal step 9.9 10.3
During the thermal step (plateau) 0 0
Protons content after the thermal step From the thermal step up to 1023 K 0.5 0.28
H+ /KU 1 0.56
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as expected by thermal treatment. After dehydration at 323 K, a 1 H peak at ca. 6.7 ppm (Fig. 9a0 ), was observed with a position and line width quite similar to those obtained for the bulk acid after the same treatment (Fig. 3a). Thus it is reasonable to assign the peak at ca. 6.7 ppm to H+ (H2 O)n species. Pretreatments at 373 and 473 K gave peaks at ca. 9 and 8.5 ppm, respectively (Fig. 9b0 and c0 ). A significant broadening of the linewidth was observed for Samples e0 and f 0 , while a broad 31 P peak at −11 ppm appeared (Fig. 8e0 and f0 ). It is interesting to note that the most positive 1 H shift occurred at 373K when the 31 P peak at −13.5 ppm appeared. The concerted change of 31 P and 1 H peaks,
Fig. 8. Room temperature 31 P MAS-NMR spectra of Cs1.9 H1.1 PW12 O40 sample treated at: a0 323 K, b0 373 K, c0 473 K, d0 573 K, e0 673 K and f 0 773 K.
indicate that there is a strong interaction between the acid and the Cs3 P salt resulting in a small shift in 31 P peak versus dehydration as observed in Fig. 8. 3.3.3. 1 H MAS-NMR The evolution of the 1 H spectrum for Cs2 HP sample as a function of the degree of dehydration (Fig. 9), was different from that observed for the bulk acid (Fig. 3). No matter what pretreatment temperature was, only a single peak was detected. The spectra were again normalized in Fig. 9 to give the same maximum amplitude and a strong decrease of the 1 H signal was indeed observed which is obviously due to the loss of H content
Fig. 9. Room temperature 1 H MAS-NMR spectra of Cs1.9 H1.1 PW12 O40 treated at a0 323 K, b0 373 K, c0 473 K, d0 573 K, e0 673 K and f 0 773 K.
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observed after dehydration at 373 K, suggests that the Cs salt has already reached a defined dehydrated state at that temperature, while for 12-tungstophosphoric acid, the removal of the crystallization water happened at 473 K rather than at 373 K. The high field shift of the 1 H peak for samples treated above 573 K may indicate the decrease of their acidity strength. 3.4. Rehydration mechanism of Cs1.9 H1.1 PW12 O40 Rehydration by 48 h exposure to ambient amosphere tends to reverse completely the effect of thermal treatment by restoring the initial 31 P MAS-NMR spectra at −15 ppm except for Sample f 0 which exhibited a weak background ranging from −10 to −14 ppm indicating that the rehydration was not complete. The 1 H MAS-NMR spectra (Fig. 10) appear to be more complicated than 31 P NMR ones. Sample treated at 323 K and rehydrated showed two narrow peaks at 7 and 5.4 ppm (Fig. 10a0 ). While the peak at 7 ppm may again be assigned to H+ (H2 O)n species, the one at 5.4 ppm is more troublesome. It might correspond to neutral water in the structure as the sample was not outgassed again. The sample pretreated at 373 K showed only one peak at 7 ppm (Fig. 10b0 ), but that pretreated at 473 K showed two peaks: one dominant peak at 5.8 ppm and a small one at 5.5 ppm (Fig. 10c0 ). For samples pretreated at temperatures above 573 K, only one peak at 6 ppm was observed, with a linewidth increasing with temperature rising (Fig. 10d0 –f0 ). So, it appeared, for the Cs salt, that 1 H MAS-NMR is more sensitive to the history of thermal treatment and rehydration than 31 P MAS-NMR.
Fig. 10. Room temperature 1 H MAS-NMR spectra of Cs1.9 H1.1 PW12 O40 Samples a0 , b0 , c0 , d0 , e0 and f 0 after exposure in ambient atmosphere for 48 h.
3.5. INS study INS spectroscopy is a powerful vibrational technique for the investigation of H-containing materials since only H atoms give signal with significant intensity [26]. The dehydration process of 12-tungstophosphoric acid has been already investigated by INS. Spectra assigned to H5 O2 + species, H3 O+ and ‘lone’ proton or OH groups have been reported [20]. Comparison of the 12-tungstophosphoric acid and its Cs salt when dehydrated at 473 and 573 K under N2 flow was investigated, as it is known from previous catalytic studies that these pretreat-
ment temperatures gave different catalytic behaviours in an highly demanding acidic type reaction, as the n-butane isomerization [27]. The INS spectrum obtained for H3 PW12 O40 treated at 473 K is consistent with the presence of H3 O+ with characteristic vibrations at 331, 468, 678 and 1072 cm−1 (Fig. 11c). Upon heating at 573 K, the spectrum showed strong modifications with bands at 504, 610 and 1141 cm−1 which can be assigned to hydroxyls groups (Fig. 11d). Our spectra are similar to those obtained by Mioc et al. [20].
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Fig. 11. 4 K INS spectra of H3 PW12 O40 dehydrated under dry N2 flow at c 473 K and at d 573 K.
The spectra obtained for the Cs salts presented a lower intensity than those obtained for the pure acid, which is obviously due to lower content in protonic sites. Upon preheating at 473 or at 573 K, the two spectra were almost similar and showed essentially the characteristic vibrational bands of hydroxyl groups at 506 and 1075 cm−1 (Fig. 12). Thus, the evolution of spectra obtained by INS spectroscopy is consistent with the presence of several protonic species for bulk 12-tungstophosphoric acid. Netherveless, some discrepancies appeared since the
evolution of 1 H MAS-NMR signal with the degree of dehydration showed the coexistence of H3 O+ species and strongly acidic protons (‘bare proton’) after treatment at 473 K (Fig. 3c), while INS spectroscopy detected mainly the presence of H3 O+ ions. For higher treatments, at 573 K, 1 H NMR interpretations and INS spectra were in agreement, and both indicated the presence of hydroxyl groups. For the Cs salt, 1 H NMR revealed that the same protonic species are formed after treatment at 473 or 573 K. INS spectroscopy confirmed this observation
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Fig. 12. 4 K INS spectra of Cs1.9 H1.1 PW12 O40 dehydrated under N2 flow at c0 473 K and d0 573 K.
and showed only the presence of hydroxyl groups. A recent FTIR study of the H3 PW12 O40 sample, treated at 473 K under N2 flow showed the presence of a broad band in the 2700–3600 cm−1 region and a band at 1710 cm−1 ascribed to protonated water clusters, the distinction between H3 O+ and H+ (H2 O)n being difficult by IR [25]. Note also that such materials appeared to be very sensitive to water traces, which makes difficult comparison of data from several techniques. The great advantage of the present study was to be performed after activation conditions as close as possible
to those chosen for catalytic studies [27] which are known to be very sensitive to water [25,27,28].
4. Conclusions The four techniques used: TGA, XRD, MAS-NMR and INS in addition to previous FTIR data have provided a detailed description of the changes in the acidic features of H3 PW12 O40 and Cs1.9 H1.1 PW12 O40 samples and in their structural modifications when
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submitted to thermal treatments. The following main conclusions may be drawn: • For thermal treatment under dry nitrogen flow at low temperatures, (T ≤ 373 K for H3 P and T ≤ 323 K for Cs1.9 H1.1 P) the solids are characterized by the presence of protonic clusters H+ (H2 O)n . • The strongest acidity corresponds to the formation of a ‘bare’ proton and coincides with the departure of the crystallization water. This state is obtained after treatment of H3 P at 473 K and of Cs1.9 H1.1 P at 373 K, as demonstrated by 1 H and 31 P NMR data. Chemical shift as large as 10.5 ppm was observed for 1 H of H3 P sample, and 9 ppm for Cs1.9 H1.1 P sample. • Finally, thermal treatments at higher temperatures lead to a decrease of the strength of acidity ascribed to modifications of the primary and secondary structures of the heteropolycompounds which modify the electron density around the proton. • 1 H MAS-NMR shows that for H3 P various protonic species are present and their relative amounts change with the extent of dehydration, which does not hold true for the Cs salt. This result is partly confirmed by INS study which gave spectra ascribed to H3 O+ species for H3 P treated at 473 K and to hydroxyl groups for H3 P treated at 573 K and its Cs salts treated at 473 and 573 K. Nevertheless some disagreements emerged between the different techniques about the nature of the protonic species found on H3 P after dehydration at 473 K. 1 H NMR showed that a ‘bare’ proton and a small proportion of H3 O+ ions should coexist, in good agreement with TGA data, while INS spectrum is rather consistent with the major presence of H3 O+ . This disagreement could be due to small differences in the final hydration states reached by the two samples, a much larger quantity of sample being treated for INS experiments and/or to the very low temperature for INS measurements (4 K). Because of the high sensitivity of heteropolyacids to moisture, we believe also that it is rather difficult to keep a given hydrated state, particularly after the treatment at 473 K which corresponds to a rather disordered structure. Acknowledgements We thank M.T. Gimenez for XRD-data acquisition and V. Martin for thermogravimetric experiments.
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