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

doi:10.1016/S0926-860X(99)00359-2

Applied Catalysis A: General

Volumes 194–195, 13 March 2000, Pages 109-122

https://doi.org/10.1016/S0926-860X(99)00359-2 Get rights and content

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 H₃PW₁₂O₄₀ and its cesium salt Cs_1.9H_1.1PW₁₂O₄₀. 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⁺(H₂O)_n. INS spectra, recorded at 4 K on samples dehydrated at 473 K, suggested the presence of hydroxonium ion H₃O⁺ in bulk H₃PW₁₂O₄₀ and of hydroxyl type species in the porous cesium salt Cs_1.9H_1.1PW₁₂O₄₀. 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 H₃PW₁₂O₄₀ and Cs_1.9H_1.1PW₁₂O₄₀ samples in relation with their structure and hydration state.

Introduction

Strong Brønsted acidity of heteropolyacids (HPA) such as 12-tungstophosphoric acid (H₃PW₁₂O₄₀) and particularly of its Cs salts (Cs_3−xH_xPW₁₂O₄₀) has 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], [2], [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 H₃PW₁₂O₄₀·6H₂O and its crystallographic structure have been described in the literature [4] as H₅O₂⁺ clusters and a cubic structure. However, for 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], [7], [8], [9], [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 M_xH_3−xPW₁₂O₄₀ (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 δ_H, will be more positive (lower-field shift). However, other contributions, such as hydrogen bonding, may also influence δ_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 H₃PW₁₂O₄₀ and Cs₃PW₁₂O₄₀.

Previous INS investigations of the bulk 12-tungstophosphoric acid have revealed the presence of various protonic species, such as H₅O₂⁺, H₃O⁺, ‘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 H₃PW₁₂O₄₀·nH₂O and Cs_1.9H_1.1PW₁₂O₄₀·nH₂O 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.

Section snippets

Materials

Pure heteropolyacid H₃PW₁₂O₄₀ sample was prepared according to the classical method including the synthesis of the sodium form, the extraction of H₃PW₁₂O₄₀ 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 H₃PW₁₂O₄₀ (0.1 M), using a molar ratio Cs/P = 2. The suspension was then kept under stirring for 24 h. The

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 H₃PW₁₂O₄₀) 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

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 H₃PW₁₂O₄₀ and Cs_1.9H_1.1PW₁₂O₄₀ samples and in their structural modifications when 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 H₃P and T ≤ 323 K for Cs_1.9H_1.1P) the solids are characterized by the presence of protonic clusters H

Acknowledgements

We thank M.T. Gimenez for XRD-data acquisition and V. Martin for thermogravimetric experiments.

References (28)

N. Essayem et al.
Stud. Surf. Sci. Catal.
(1996)
V.M. Mastikhin et al.
J. Mol. Catal.
(1990)
V.M. Mastikhin et al.
J. Mol. Catal. A: Chemical
(1995)
T. Okuhara et al.
J. Mol. Catal.
(1992)
V.M. Mastikhin et al.
Prog. NMR Spectrosc.
(1991)
E. Brunner
Catal. Today
(1997)
T. Baba et al.
J. Mol. Catal. A: Chemical
(1996)
U.B. Mioc et al.
J. Mol. Struct.
(1994)
N. Essayem et al.
J. Catal.
(1999)
K. Na et al.
J. Chem. Soc., Faraday Trans.
(1995)

N. Essayem et al.

Catal. Lett.

(1995)

G.M. Brown et al.

Acta Cryst.

(1977)

R. Massart et al.

Inorg. Chem.

(1977)

R. Thouvenot et al.

J. Chem. Soc., Faraday Trans.

(1991)

Cited by (85)

Synthesis and proton-conduction properties of Cs<inf>3</inf>PW<inf>12</inf>O<inf>40</inf> composites
2024, Solid State Ionics
Cs₃PW₁₂O₄₀ (Cs3WPA) composites with CsHSO₄ (CHS) were prepared, and their effects on proton conductivity were investigated. These composites comprise CHS, H₂SO₄, and Cs3WPA, where the Cs3WPA served as the matrix element. The CHS in the composites became amorphous, and it was caused by the strong acidic catalytic properties and a large surface area of Cs3WPA. Amorphous-CHS provides an effective proton carrier, and the conductivity increases with increasing amorphous-CHS content. CHS-H₂SO₄-Cs3WPA ternary system was prepared based on the three aforementioned components of the composites. It was found that the activation energy of proton conductivity for the ternary system is lower than that of a composite with an identical composition. The difference of activation energy between composite and ternary system results from the difference of the quantity of incorporated water molecules.
Synthesis and structural characterization of HPW-doped niobium pillared Brazilian clay
2024, Microporous and Mesoporous Materials
Natural clay lamellar structure can be controlled through various treatments, such as pillaring with metallic oxides. In this work, lamellar-structured Brazilian montmorillonite was a raw material for developing a new Nb-pillared clay dopped with phosphotungstic acid (HPW). The pristine clay was exchanged with CTAB to obtain an organophilic intermediate, which was further submitted to an Nb source treatment, followed by HPW doping and calcination. XRD, textural and thermal analysis, electronic scanning spectroscopy (SEM) with EDS microscopy, ²⁷Al and ³¹P solid-state nuclear magnetic resonance spectroscopy, and infrared of pyridine adsorption-desorption acidity properties were employed in the characterization of this new class of pillared clay. The XRD analyses showed the maintenance of the lamellar structure after the calcination step at 400 °C, as planes 001, 110, 004, and 130 are visible. The ³¹P MAS NMR spectrum of the ARGPil-Nb-HPW sample showed a single signal centered at −14.9 ppm, the same chemical shift of the phosphotungstic acid. The absence of additional peaks confirmed the maintenance of the Keggin hetero-polyacid structure in the Nb pillared clay. The pillarization process led to a significant increase in the specific area from 58 m²/g to 111 m²/g in ARGPil-Nb-HPW, suggesting that niobium acts as a pillar of Nb₂O₅, covalently binding to the tetrahedral sheets, as evidenced by the FTIR spectra, and keeping them separated, which may favor catalytic processes that require more significant space for molecules to access the internal surfaces of materials and, therefore, to the Brønsted sites and especially to the strong Lewis sites. The catalysts showed strong Lewis sites by retaining chemically adsorbed pyridine after heating at 250 °C. The pillared catalyst showed activity in the esterification of mandelic acid with methanol at ambient temperature, which was maintained after three cycles.
Confinement and surface sites control methanol adsorbate stability on MFI zeolites, SBA-15, and a silica-supported heteropoly acid
2022, Catalysis Science and Technology
We herein investigate methanol adsorbates on a variety of heterogeneous catalysts. We quantitatively desorb methanol from saturated MFI zeolites, SBA-15 materials and silicotungstic acid (STA) supported on silica, all in the respective siliceous, Na- and H-forms. Surface species are identified by ¹H and ¹³C MAS NMR and DRIFTS. On saturated surfaces, we find liquid-like methanol in weak surface interaction. For siliceous materials, adsorption on silanol Si(OH) groups is dominant, especially on materials with amorphous pore walls like SBA-15. Weak methanol binding on microporous silicalite is caused by a repulsive effect due to micropore confinement. For Na-form materials, methanol complexes at Na⁺ counter ions dominate the adsorption of methanol in Na-ZSM-5 micropores. The strong confinement leads to stronger methanol adsorption compared to less confined systems. Without confinement, no complex at Na⁺ is observed and Si(OH) groups dominate adsorption. On H-form materials, methanol complexes at acid sites form in a higher quantity under confinement in H-ZSM-5. After treatment at 423 K, the formation of dimethyl ether (DME) was evidenced by IR and ¹³C MAS NMR spectroscopy and the acid site proton peak found at δ_¹H = 14.4 ppm. Si(OH) groups bind methanol stronger than counter ions and acid sites (Na⁺ and H⁺). This explains why defects and the Si(OH) density influence heterogeneous reactions. Our findings show that confinement in micropores is crucial for the stabilization of methanol complexes at counter ions Na⁺ and acid sites H⁺.
Synthesis and characterization of nanoparticulate silica with organized multimodal porous structure impregnated with 12-phosphotungstic acid for its use in heterogeneous catalysis
2020, Molecular Catalysis
Citation Excerpt :
The 31P magic angle spinning-nuclear magnetic resonance spectra of MESSI-2PTA20, MESSI-2PTA30, and MESSI-2PTA40 samples (Fig. 7a–c, respectively) display a line at -14.8 ppm, which was attributed to [HXPW12O40](3−X)− species [43,44]. Moreover, the 31P NMR spectrum of MESSI-2PTA40 sample shows a second and less intense line at -15.1 ppm ascribed to the [PW12O40]3− Keggin anion [45] interacting with H+(H2O)2 species [46,47]. In the case of hydrated PTA (H3PW12O40.6H2O) this line appears at -15.3 ppm.
Mesoporous silica (MESSI) nanoparticles with ordered mesoporous frameworks were successfully obtained using the triblock copolymer P123 as the mesoporous template and agar as micropore former. The agar/TEOS ratio influences the specific surface area (S_BET) and the micropore area (S_MIC). The S_BET values increase with the increment of the agar/TEOS ratio used in the synthesis. SAXS (small angle X-ray scattering) patterns and TEM (transmission electron microscopy) images of MESSI samples reveal the presence of a 2D hexagonal mesopore array. The study of all the materials impregnated with PTA by ³¹P NMR and FT-IR confirmed the existence of undegraded [H_3-XPW₁₂O₄₀]^(3−X)− and [PW₁₂O₄₀]³⁻ anions interacting with the ≡Si-OH₂⁺ groups. The potentiometric titration results show that the MESSI-2PTA solids present very strong acid sites and that both the acid strength and the number of sites increase with the increment of the PTA amount in the samples.
The performance of MESSI-2PTA materials as catalysts in the solvent-free synthesis of a series of substituted 3H-1,5-benzodiazepines was evaluated. The yields achieved were high, without formation of by-products resulting from competitive reactions or decomposition products, so the prepared materials are highly selective and reusable catalysts.
Effect of mixed alkali metal ions in highly proton conductive K/Cs-hydrogen sulfate-phosphotungstic acid composites prepared by mechanical milling
2019, Solid State Ionics
Citation Excerpt :
This corroborates the formation of new materials and their higher chemical shift positions indicates the emergence of higher acidic protons, hence their higher proton conductivities than the pristine precursor materials. The chemical shift of the pristine WPA·6H2O is a narrow peak around 7.5 ppm; this is attributed to protons from water clusters of very fast exchange among all available hydrogens in the water clusters [33]. The peak position of 11.1 ppm of CHS is attributed to the phase III of the monoclinic symmetry of CHS [34].
Fast proton transport materials are of high interest for their application in diverse fields and devices like electrochemical sensors, fuel cells, capacitors, electrochromic devices and electrochemical reactors. This work shows inorganic solid proton conducting composites prepared with phosphotungstic (WPA) heteropolyacid base material and MHSO₄ (MHS (MK and Cs)) inorganic solid acids via mechanical planetary ball milling. Very fast proton conducting irreversible new phase composites were obtained, with 90 (50KHS-50CHS)-10WPA (mol %) mixed metal ions composite exhibiting the highest performance with conductivity from 4.9 × 10⁻² to 1.4 × 10⁻³ S cm⁻¹ at 160 to 40 °C, respectively; compared to those of the precursor materials (CHS and MHS) on the order of 10⁻³ to 10⁻⁸ under the same temperature regime. The XRD, FT-Raman, ¹H-MAS-NMR and HR-TEM results of the composites revealed the formation of an amorphous nano-interface proton-conducting pathway shell around an M-substituted WPA crystalline core, resulting in a nanoionics effect leading to the high conductivities observed; in addition to preferred sites of alkali metal ions substitution into the WPA with different stabilities and conductivities with respect to type of ion. These results will be of significant interest to the applicable fields and inspire further research.
Highly selective production of C<inf>5</inf>-C<inf>12</inf> hydrocarbons over efficient Ru/heteropoly-acid catalysts
2019, Fuel
Citation Excerpt :
31P MAS NMR spectrum provides further evidence to confirm the interaction between Ru and HPW. As shown in Fig. 6B and Fig. 6C, the 31P MAS spectrum of a bulk HPW sample exhibits a narrow peak at −15.8 ppm as obtained in literature [26], which can be attributed to an intact Keggin type HPW structure [27]. In contrast, two peaks in the 31P MAS spectrum can be observed for Ru-50PA sample: the narrow one at −13.5 ppm and the broader one at −11.4 ppm.
It is a great challenge in catalysis to increase selectivity without sacrificing activity. Furthermore, the highly selective production of C₅-C₁₂ hydrocarbons, which are main components of gasoline fuel, from syngas is important for Fischer-Tropsch synthesis (FTS), particularly for its industrial process. Herein, we demonstrate an approach to increase selectivity for C₅-C₁₂ hydrocarbons, namely, the 145% increase in selectivity of C₅-C₁₂ hydrocarbons without sacrificing high CO conversion was obtained by tuning Al₂O₃ support with tungstophosphoric acid (HPW) for Ru-based catalysts. This finding offers a simple and novel approach to develop efficient catalysts for FTS.

View all citing articles on Scopus

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.

View full text

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

Abstract

Introduction

Section snippets

Materials

Thermogravimetric (TG) and differential thermal analysis (DTA)

Conclusions

Acknowledgements

Stud. Surf. Sci. Catal.

J. Mol. Catal.

J. Mol. Catal. A: Chemical

J. Mol. Catal.

Prog. NMR Spectrosc.

Catal. Today

J. Mol. Catal. A: Chemical

J. Mol. Struct.

J. Catal.

J. Chem. Soc., Faraday Trans.

Catal. Lett.

Acta Cryst.

Inorg. Chem.

J. Chem. Soc., Faraday Trans.

Characterization of protonic sites in H₃PW₁₂O₄₀ and Cs_1.9H_1.1PW₁₂O₄₀: a solid-state $^{1} H,^{2} H,^{31} P$ MAS-NMR and inelastic neutron scattering study on samples prepared under standard reaction conditions