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J. Phys. Chem. C 2010, 114, 19024–19034
Controlled Interactions between Anhydrous Keggin-Type Heteropolyacids and Silica Support: Preparation and Characterization of Well-Defined Silica-Supported Polyoxometalate Species Eva Grinenval,† Xavier Rozanska,§ Anne Baudouin,† Elise Berrier,‡ Franc¸oise Delbecq,§ Philippe Sautet,§ Jean-Marie Basset,†,| and Fre´de´ric Lefebvre*,† UniVersite´ de Lyon, ICL, C2P2 UMR 5265 (CNRS - CPE - UniVersite´ Lyon 1), LCOMS - CPE Lyon, 43 BouleVard du 11 NoVembre 1918, F-69616, Villeurbanne, France, UniVersite´ de Lyon 1, ICL, Ecole Normale Supe´rieure de Lyon, Laboratoire de Chimie, UMR 5182 (CNRS - ENS de Lyon), 46 Alle´e d’Italie, 69007 Lyon, France, Ecole Nationale Supe´rieure de Chimie de Lille, CNRS, UMR 8181, Unite´ de Catalyse et de Chimie du Solide, Baˆt. C7, BP 90108, 59652 VilleneuVe d’Ascq Cedex, France ReceiVed: August 4, 2010; ReVised Manuscript ReceiVed: October 6, 2010
Anhydrous Keggin-type phosphorus heteropolyacids were deposited on partially dehydroxylated silica by using the surface organometallic chemistry (SOMC) strategy. The resulting solids were characterized by a combination of physicochemical methods including IR, Raman, 1D and 2D 1H, and 31P MAS NMR, electron microscopy experiments and density functional theory (DFT) calculations. It is shown that the main surface species is [tSi(OH...H+)]2[H+]1[PM12O403-] where the polyoxometalate is linked to the support by proton interaction with two silanols. Two other minor species (10% each) are formed by coordination of the polyoxometalate to the surface via the interaction between all three protons with three silanol groups or via three covalent bonds formed by dehydroxylation of the above species. Comparison of the reactivity of these solids and of compounds prepared by a classical way shows that the samples prepared by the SOMC approach contain ca. 7 times more acid sites. Introduction Polyoxometalates (POMs) are constituted by transition metals in their highest oxidation states and oxo anions. This class of inorganic compounds is of increasing interest owing to its wide range of properties and applications, for example, in catalysis, medicine, and material science.1,2 POMs reveal indeed an important molecular, structural, and electronic diversity of nanosized species. Their structures are based on (MOp) and (XOq) polyhedra, where M is a transition metal and X a cation, sharing usually vertices or edges. POMs have received much attention in the area of catalysis because their chemical properties such as redox potentials, acidities, and solubilities can be tuned easily by choosing the constituent elements.3-6 They are catalysts in oxidation and acid-base reactions and exhibit catalytic activity in the conversion of methane into oxygenated compounds7-14 or the production of ethyl acetate from ethylene and acetic acid, known as the AVADA process, for instance.15-17 Both heterogeneous18 and homogeneous19 catalytic systems have been investigated. Heterogeneous systems should be preferred for environmental and economical reasons. Unfortunately, POM powders (with the exception of some alkaline salts) have limited surface specific areas (1-5 m2 · g-1), and therefore their dispersion on a suitable support is essential to enhance the availability of active sites per surface unit. The most studied strategy consists in dispersing POM units onto oxide surfaces using a classical impregnation technique. Among the supports described in the literature, silica has the advantage * To whom correspondence should be addressed. † Universite´ de Lyon, C2P2. ‡ Ecole Nationale Supe´rieure de Chimie de Lille. § Ecole Normale Supe´rieure de Lyon. | Present address: KCC, King Abdullah University of Science and Technology, Thuwal, Saudi Arabia.
of keeping the POM molecular structure,20 meanwhile the protons of the POM interact with surface silanols, [tSi(OH...H)+], as described by Lefebvre.21 This results in a change of the properties of POMs. For example, impregnation of phospho- and silicotungstic acids on silica corresponds indeed to the consumption of three acidic protons.22,23
3[tSiOH] + HnXW12O40 f [tSi(OH...H+)]3[Hn-3XW12O403-], with (X ) P, n ) 3; × ) Si, n ) 4). However, impregnation techniques can lead to different situations. The samples of silica-supported POMs contain both highly dispersed and aggregated surface species (Scheme 1).24,25 The existence of these two phases has been highlighted and quantified using spin-lattice relaxation NMR techniques.26 The relative proportions of these two phases are difficult to reproduce because of the lack of strategies to carefully prepare well-defined surface species. The choice of the POM, the impregnation solvent, and the evaporation procedures are also critical and have already been carefully discussed.27-29 From an atomistic point of view, the interactions are not of the same type in the aggregated and dispersed surface phases. POMs interact together via their protons in aggregated phase and individually with the support through POMs’ proton-surface hydroxyl interactions in the dispersed phase. Up to now, the role of the silanols as a function of their surface density on the chemisorption process of the POMs has not been considered to a great extent. Little is known about the means to control the interplay between the POMs and silica surfaces. A reason for this situation is that silica pretreated at room temperature used in conventional
10.1021/jp107317s 2010 American Chemical Society Published on Web 10/20/2010
Well-Defined Silica-Supported Polyoxometalate Species
J. Phys. Chem. C, Vol. 114, No. 44, 2010 19025
SCHEME 1: POMs Species on Silica-Supported Samples Prepared by a Classical Way
impregnation are covered both by H-bonded hydroxyl groups and physisorbed water molecules. This results in ill-defined highly dispersed POM surfaces due to the presence of water. We use herein the concepts and strategy of surface organometallic chemistry (SOMC)30-33 to circumvent the problems associated with conventional impregnation. The main principle consists in carefully controlling the impregnation of anhydrous heteropolyacids on partially dehydroxylated silica. The silicasupported POMs thus obtained are characterized by IR, Raman, solid-state NMR techniques, transmission electron microscopy (TEM), and the structures and energies are discussed on the basis of density functional theory (DFT) calculations. By comparing samples prepared by SOMC and conventional impregnation, we show that the acidity and dispersion of POMs can be controlled onto the silica surface. The strategy of impregnation and structural information drawn from this study provides new insights into silica-supported POMs. Results and Discussion Anhydrous H3PMo12O40 was obtained by thermal treatment under vacuum (2 h, 200 °C, 10-5 Torr) and checked by 1H MAS NMR spectroscopy. The NMR data are in agreement with those reported by Ganapathy et al.34 Solubility tests of anhydrous POMs revealed that only polar aprotic solvents such as DMSO and acetonitrile are suitable for the preparation of well-defined supported species (absence of water). However, the physical properties of DMSO (low vapor pressure, high boiling point) resulted in uneasy procedures and acetonitrile was preferred in most experiments. A typical experimental protocol to graft anhydrous POMs on silica begins with a quantitative addition of the dehydrated POM in acetonitrile to partially dehydroxylated silica at 500 °C (SiO2-(500), 1.4 OH · nm-2, 0.465 mmol OH · g-1 35). The impregnation is followed by filtration and drying steps. The conditions were optimized so that the isolated silanols (as monitored by in situ IR) are fully consumed. The resulting yellow-green solid, referred as [HPMo/SiO2-(500)] (1), contained 0.8 HPMo per surface silanol, to compare with the theoretical 1.0 POM/[tSiOH] for a complete coverage of silica. The structure of the surface POM species 1 has been characterized by IR, Raman, and solid-state NMR spectroscopies. First, the interaction of anhydrous H3[PMo12O40] with SiO2-(500) was monitored by in situ infrared spectroscopy (Figure 1). Upon impregnation, a quasi-complete disappearance of the isolated silanol groups (free ν(SiOH) at 3746 cm-1) and a concomitant apparition of a broad band centered at 3200 cm-1, characteristic
Figure 1. Impregnation of H3PMo12O40 on a silica partially dehydroxylated at 500 °C monitored by in situ IR spectroscopy: (a) SiO2(500) pellet (17 mg); (b) sample after impregnation of anhydrous H3PMo12O40 (1 g) in DMSO (10 mL) (12 h, 25 °C), followed by two washings (10 min, 25 °C) in acetonitrile and a drying step under vacuum (10-5 Torr, 4 h, 25 °C).
Figure 2. Raman spectra of: (a) anhydrous H3PMo12O40 and (b) Silicasupported POM 1.
of the H-bonded hydroxyl species, were observed.36-38 In analogy with classical experiments in heterogeneous catalysis, pyridine adsorption monitored by infrared spectroscopy was used to characterize the acid sites of the anhydrous H3PMo12O40 before and after impregnation. However, for the silica-supported POM, this probe molecule was not suitable to permit distinction between different environments of Brønsted acid sites. Indeed, pyridine would react similarly with the available protons in aggregated and dispersed POM and with the protons interacting with silanols, resulting in the formation of a pyridinium salt of the POM on the silica surface. The Raman spectrum of anhydrous H3[PMo12O40] displays bands at 998, 980, 911, 608, and 252 cm-1, which have already been assigned to νs(Mo-Od), νas(Mo-Od), νas(Mo-Ob-Mo), νs(Mo-Oc-Mo), and νs(Mo-Oa), respectively (Figure 2) where Oa, Ob, Oc, and Od correspond to the oxygen atoms linked to phosphorus, to oxygen atoms bridging two molybdenum (from two different triads for Ob and from the same triad for Oc), and to the terminal oxygen ModO, respectively.39 After impregnation on the partially dehydroxylated silica, the Raman spectrum of 1 was unchanged, confirming the conservation of the Keggin structure. Furthermore, the absence of a significant band shift in the spectra indicates that the environment of the Keggin unit is similar in the bulk and supported POMs: it has been shown that the positions of the bands of the POM are weakly dependent on the cation.39,40
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Figure 4. MAS.
31
P MAS NMR spectra of 1: (a) 31P{1H} and (b) 31P CP-
Figure 3. MAS NMR spectra of 1: (a) 1H and (b) 31P{1H}.
Figure 3 shows the proton and phosphorus one-dimensional (1D) magic-angle spinning (MAS) NMR spectra of 1. The proton spectrum displays three types of resonances: one sharp signal at 1.8 ppm due to isolated silanols; some broad resonances between 2.5 and 8.0 ppm, and one peak at 11.5 ppm. The broad signals between 2.5 and 8 ppm are ascribed to H-bonded silanol groups and/or protons of POM in interaction with surface hydroxyl groups. These surface species correspond to interaction between POM protons and silica and are described in the following by the general formula [tSi (OH...H+)]21 rather than [tSi (OH2+)] as the presence of a broad band indicates that these protons are in different local environments (see below). These chemical shifts are within the expected spectral region for silanol/protonated silanol given the literature precedents on solution analogues: proton chemical shifts of 3.12 and 8.04 ppm are assigned to t-Bu3SiOH and t-Bu3Si(OH2+), respectively.41 The last resonance at 11.5 ppm can be assigned to the POM protons of POM in analogy with the data on anhydrous POM.34 The 31P{1H} MAS NMR spectrum shows three resonances centered at ca. -1.5, -3.0, -4.5 ppm corresponding to species thereafter called (1a), (1b), and (1c), respectively. A direct quantification of phosphorus species is possible because the spectrum was recorded at full relaxation of 31P nuclei. A Dmfit program42 fit provided the following integrations: 10/80/10 for (1a)/(1b)/(1c), respectively. The phosphorus cross-polarization (CP) MAS spectrum of 1 (Figure 4) exhibited the same resonances than those observed in MAS NMR. However, the relative intensity of species (1a) was weaker suggesting that there are less protons in the vicinity of (1a) than of (1b) and (1c). The 2D HETCOR spectrum, which appears when 1H and 31P spins are spatially close, displayed five distinct correlations involving only (1b) and (1c) phosphorus species (Figure 5a). Note that no heteronuclear correlation is observed for the (1a) phosphorus species suggesting that there is no proton next to
Figure 5. 2D MAS NMR spectra of 1: (a) 1H-31P heteronuclear (HetCor) and (b) 1H double-quantum (DQ).
phosphorus. This can be explained by the formation of covalent bonds between the POM and the surface together with water removal.22 (1a) is therefore ascribed to [tSiO]3[PMo12O37]. Observed correlations for (1c) phosphorus species involve only some protonated surface silanols (1H broad resonances at ca. 3.0 and 6.0 ppm). This minor species corresponds to the consumption of three strong acidities and thus (1c), similarly to the species observed in a conventional impregnation, is assigned to [tSi(OH...H+)]3[PMo12O403-]. However, correlations observed for the main (1b) phosphorus species at -3.0
Well-Defined Silica-Supported Polyoxometalate Species ppm involve both protonated surface silanols and strong acidic protons (1H at 11.5 ppm). These correlations are then associated with surface species containing these two groups [tSi(OH...H+)]3+ 3x[H ]y[PMo12O40 ]. It is not possible to conclude from these data to the presence (in this case x ) 2 and y ) 1) or not (x ) y ) 1 or 2) of a covalent bonding between the POM and the support. The identification of the protonated surface silanols and remaining acidic protons of (1b) and (1c) was further analyzed by 2D proton double quantum NMR spectroscopy.43-47 The 2D proton double-quantum (DQ) single-quantum (SQ) correlations spectrum of Figure 5b was recorded using the following steps: excitation of DQ coherences, t1 evolution under proton homonuclear decoupling of double quantum coherences, reconversion of these coherences into observable magnetization, Z-filter, and detection. The corresponding 2D map yields (in the ω1 and ω2 dimensions) correlations between pairs of dipolar coupled protons (i.e., spatially close). The DQ frequency in the indirect ω1 dimension corresponds to the sum of the two SQ frequencies of the two coupled protons and correlates in the ω2 dimension with the two individual proton resonances. Therefore, the observation of a DQ peak implies a close proximity between the two considered protons. Furthermore, two equivalent protons will give an autocorrelation peak located on the ω1 ) 2 ω2 line of the 2D map. Conversely, single spins will not give rise to diagonal peaks. In the DQ spectrum of Figure 5b, as expected for protons in a close environment, the [tSi(OH...H+)] resonance resulting from silica protonation present in the proton spectrum between 2.5 and 8.0 ppm displays strong autocorrelation peaks along the ω1 ) 2 ω2 line (5 to 16 ppm in the ω1 dimension). These protons have slightly different chemical shifts (from 2.5 to 8.0 ppm) due to a different local environment. This is confirmed by the presence of a weaker correlation band between one of these protons peak [tSi OH...H+] (at 5.8 ppm) and the resonance of [tSiOH] at 2.0 ppm (ca. 8 ppm in the ω1 dimension). Conversely, the proton resonance at 11.5 ppm does not display an autocorrelation DQ peak at 23 ppm in the ω1 dimension (blue dotted circle), suggesting that the main species 1b contains only one acidic proton and thus can be assigned to [tSi(OH...H)+]2[H+][PMo12O403-] or to [tSi(OH...H)+][H+][tSiO][PMo12O392-]. Moreover, the strong POM acidity H+ correlates with the [tSiOH...H+] resonance at ca. 3 ppm (peak at ca. 15 ppm in the ω1 dimension; the signal at ω2 ) 3 ppm and ω1 ) 15 ppm is not seen, probably due to the broadening of the peak) involving a close proximity between these both kinds of protons. To further characterize the structure of the main species 1b, a temperature-programmed desorption (TPD) experiment under inert atmosphere was performed on 1 (see Supporting Information Figure S1). A total of 2.6 water molecule is released in the TPD. Water is the only molecule to desorb. Upon heating, 1a should not result in any water desorption, and 1c should result in three water molecules desorption because of the formation of the three covalent bonds between the support and the POM. The amount of 1a, 1b, and 1c is 10, 80, and 10%, respectively, as deduced from 31P NMR. Then, 1c should lead to the release of 0.3 water molecules, which implies that 1b releases 2.3 water molecules. This corresponds to 2.87 water molecules per species 1b. This allows one to discriminate the two possible structures for 1b. Indeed the expected amount of water for [tSi(OH...H)+][H+][tSiO][PMo12O392-] should be one molecule from the protonated silanol and 0.5 (if there is combination
J. Phys. Chem. C, Vol. 114, No. 44, 2010 19027 TABLE 1: Proportions, 1H, and 31P NMR Chemical Shifts of Surface POM Species 31 P proportions NMR δ, entry (%) ppm
1a 1b 1c
10 80 10
-1.5 -3.0 -4.5
1 H NMR δ, ppm
assignment
[tSiO]3[PMo12O37] 11.5; 2.5-8 [tSi(OH...H+)]2[H+][PMo12O403-] 2.5-8 [tSi(OH...H+)]3[PMo12O403-]
between two POMs) or one molecule (if there is reaction with a silanol) from the acidic proton. So in this case the total amount should be 1.5 or 2.0. In the case of [tSi(OH...H)+]2[H+][PMo12O403-] the total amount should be 2.5 or 3.0. The agreement between the experimental and expected values is reasonably good for the later case as more than two water molecules are obtained; 1b can then be attributed to [tSi(OH...H+)]2[H+][PMo12O403-]. In summary, the combination of 2D HETCOR, proton DQ, and of TPD experiments allowed a complete assignment of the 1 H and 31P NMR spectra of silica-supported 1. The exhaustive range of techniques was crucial to characterize the surface polyoxometalate species (Table 1). Scheme 2 gives a schematic representation of the surface species obtained by the SOMC impregnation way. This first application of the SOMC strategy to surface polyoxometalates leads to surface species where most of the grafted POM (entry 1b) preserves a protonic acidity in the Keggin structure. Conformational differences in the surface POMs through the formation of 1a and 1c is quite unexpected with regard to the starting silanols density of partially dehydroxylated silica (data are detailed hereinafter). Three surface silanols are indeed involved for these two species and to further investigate these surface POM species, an NMR spectroscopic study including 2D proton quantum and triple quantum NMR spectroscopy is performed on silica samples partially dehydroxylated at 200, 500, and 700 °C (referred to as SiO2-(200), SiO2-(500), and SiO2-(700), respectively) and is then correlated with DFT calculations. The 1H MAS NMR spectrum of SiO2-(200) exhibits two signals: an intense peak at 1.8 ppm characteristic of free silanols and a broad resonance centered at 3.2 ppm which corresponds to hydrogen bonded silanol groups. Its 1H DQ MAS spectrum (Figure 6a) shows a broad autocorrelation peak for the proton resonances between 1.8 and 3.2 ppm (3.6 to 6.4 ppm in the ω1 dimension). This broad signal includes three possible correlations: (i) between two isolated silanols at 3.6 ppm in the ω1 dimension, (ii) between a hydrogen-bonded silanol and an isolated silanol at 5.0 ppm in the ω1 dimension, and (iii) between two hydrogenbonded silanols groups at 6.4 ppm in the ω1 dimension. Chemical determination and 1H NMR show that there are 3.5 OH · nm-2 on SiO2-(200),35 which corresponds, assuming a uniform distribution, to an average distance of 5.4 Å between two hydroxyl groups. Autocorrelation peaks of the 2D map suggest that some surface silanols are in a close environment. This observation has also been proved for silica samples dehydroxylated at higher temperatures. Indeed the 1H MAS NMR spectra of SiO2-(500) and SiO2-(700) exhibit only one peak at 1.8 ppm characteristic of free silanols. These isolated hydroxyl groups display one sharp autocorrelation peak in 1H DQ MAS NMR (Figure 6b,c). Dehydroxylation at 500 or 700 °C leads to a silanol density of 1.4 and 0.7 OH · nm-2 and corresponds to an average distance of ca. 8-12 Å between two isolated silanols. The occurrence of diagonal signals indicates unambiguously a relative closeness between surface silanols. Different local
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SCHEME 2: POM Species on the Silica-Supported Samples Prepared by SOMC Strategy
environments of silica surface can explain these NMR results given the literature precedents on hydroxyls clusters. Indeed Gerstein et al.48,49 reported, on the basis of N-coherence spectrometry, the formation of small clusters of hydroxyls on silica surface and the decrease of the cluster size upon thermal treatment. Such phenomena were assumed for precipitated silica only owing to a complex surface and large amounts of hydroxyls but proton DQ experiments prove that even flame silica displays some of these hydroxyl clusters. The dehydroxylation process gives rise to a partially heterogeneous distribution of the surface silanols and this has been confirmed using proton TQ correlations. It is nevertheless worth to note that this phenomenon concerns only a fraction of the hydroxyl groups. In analogy with the DQ-SQ correlation experiment, the TQ frequency in the ω1 dimension corresponds to the sum of the three SQ frequencies of the three coupled protons and correlates in the ω2 dimension with the three individual proton resonances.50-53 Three equivalent protons will give an autocorrelation peak along the ω1 ) 3ω2 line of the 2D map. Conversely, groups of less than three equivalent spins will not give rise to diagonal signals in the spectrum. Two-dimensional DQ and TQ correlation experiments can thus be applied to determine in a reliable way the number of protons in a close environment. The 1H TQ MAS spectrum of SiO2-(500) (Figure 7) shows an intense autocorrelation peak observed for the isolated silanol resonance at 1.8 ppm (5.4 ppm in the ω1 dimension). This diagonal signal reveals that three protons are coupled together in a spatially confined neighborhood and strengthens therefore the hypothesis of a partially random distribution. NMR results of SiO2-(500) combined with literature precedents suggest that this environment can be a silica cluster of three hydroxyl groups. The occurrence of such clusters on partially dehydroxylated silica even at high temperature can explain the formation of the surface POM species (1c) and to a lower extent (1a). To check further the validity of our experimental results interpretation, periodic density functional theory calculations were performed using the VASP code.54-57 Such calculations have been shown to give insightful results in other SOMC systems.58 After optimization of the {101} and {111} surfaces of β-cristobalite, H3PMo12O40 (POM) was adsorbed onto these model surfaces yielding the POM•{101} and POM•{111} species (Figure 8). In the adsorption process, POM protons have been moved from their gas phase optimized positions34,59 to a position maximizing the number of hydrogen bonds between the POM and the surfaces. The chemisorbed states both with and without proton transfers from the POM to the surfaces have been considered, but after energy minimization, the geometries always resulted in proton positioned onto the POM yet showing very strong hydrogen bonds with the surface hydroxyl groups, in agreement with a [tSi(OH...H+)] formulation. In the structures of the POM chemisorbed onto {101} and {111} surfaces, the shortest distances between the three POM’s protons and surface oxygen atoms are 152, 155, and 206 and 150, 151,
and 183 pm, respectively. At the same time, hydrogen atoms in the surface hydroxyl groups form also hydrogen bonds with oxygen in the POM. The shortest distances between hydroxyl group hydrogen atoms and POM’s oxygen atoms are 177, 193, 199, and 216 pm for the {101} surface and 186, 186, and 213 pm for the {111} surface. This variety of distances is in agreement with the numerous peaks for these species in the 1H MAS NMR spectrum (see the signals between ca. 2 and 8 ppm in Figure 3a). The interaction energies of the POM onto {101} and {111} surfaces are -97 and -106 kJ · mol-1, respectively. These energies are calculated using the equation
Eint(S) ) EPOM•S - EPOM(g) - ES
(1)
where S is the surface {101} or {111}. Furthermore, the geometry of the POM has been optimized when it forms three covalent bonds with the support. The protons of the POM can be transferred to the surface hydroxyl groups resulting in the formation of three water molecules that desorb from the surface. Covalent bonds are then formed between the POM and three silicon atoms on the surface and the structures noted POMt{101} or POMt{111} are formed. Such a reaction is endothermic but becomes favorable, as determined by its Gibbs free energy, because of the large entropic gain associated with releasing water in the gas phase. The energies of reaction for POM•{101}fPOMt{101} + 3H2O(g) and POM•{111}f POMt{111}+ 3H2O(g) are 145 and 106 kJ · mol-1, respectively. To calculate the Gibbs free energies, some assumptions have been made. First, the translation and rotation partition functions of POM•{101}, POMt{101}, POM•{111}, and POMt{111} are neglected (the support is considered immobile), second, it has been considered that the frequencies of vibration remain practically unchanged in the covalent and hydrogen-bonded chemisorbed states, which allows us to neglect the contribution of the zero point energies of vibration (ZPE) and of the partition functions of vibration (qv). Therefore, the following equation has been used to obtain the Gibbs free energy in the reaction POM•SfPOMtS + 3H2O(g), where S is the support {101} or {111}
∆rGc-p ) EPOMtS + 3EH2O - EPOM•S -
(
( ))
3RT ln(qrH OqtH O) - ln 2
2
PH2O P°
(2)
where EPOMtS, EPOM•S, and EH2O are the electronic energies of the three-covalently bonded POM, the physisorbed POM onto the surface, and H2O, respectively, qrH2O and qtH2O are the partition functions of rotation and translation of H2O, respectively, and PH2O is the partial pressure of H2O(g). Similar approximations
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Figure 7.
Figure 6. 1H DQ MAS NMR spectra of (a) SiO2-(200) (the small peak at 1 ppm seen on the projection is due to an impurity in the sample); (b) SiO2-(500) and (c) SiO2-(700).
have been used before60 and show reasonable agreement with Gibbs free energies of reaction when vibrational contributions (ZPE and qv) are taken into account. The resulting Gibbs free reaction energies are reported at various temperatures for a partial pressure PH2O ) P° and 10-5 Torr in Table 2. As can be seen in Table 2, the Gibbs free energies are close to zero or slightly negative at ambient temperature and negative
1
H TQ MAS NMR spectrum of SiO2-(500).
at higher temperature. The calculations explain well the formation of species 1a in the conditions of the classical impregnation preparation (drying at T ) 473 K and P ) 10-5 Torr for t ) 2 h) since in these conditions the dehydration reaction is strongly exothermic and should then happen fast. In the conditions of the SOMC strategy preparation, the calculated reaction energies are only weakly exothermic (and they are perfomed in the most favorable case with optimal OH positions). Calculations also suggest that thermodynamic equilibrium is not reached after 1 h. The calculations suppose an optimal position of three OH groups on the silica surface. Any deviation from this optimal situation would make the Gibbs free reaction energy more positive. Values of Table 2 are hence lower limits. This hence explains why a mixture of hydrated and dehydrated species is formed on the heterogeneous sites of the silica surface. The stable structure of the POM onto the surfaces is the three-covalently bonded complex when the hydroxyl groups topology allows it: surface hydroxyl group local coverage must be above 3 on 0.8 nm2 to permit the formation of the covalent bonds between the POM, which has a diameter of ca. 1 nm, and the surface oxygen atoms. Such heterogeneity in the hydroxyl group’s distribution onto silica surface is indeed experimentally evidenced by our NMR results. Furthermore, the topology of the surface hydroxyls must as well show a good fit with the topology of the POM to lead to the formation of covalent bonds (Figure 8, bottom). For steric reasons, only the terminal oxygen atoms in the POM can get involved in the formation of covalent bonds with the surface. The moderate temperature also reduces the surface reconstructions which should lower the availability of three surface hydroxyl groups presenting ideal match for covalent bond formation between the silica surface and the POM. Therefore, the presence of 1b and 1c in the SOMC approach should stem from kinetic blocking of both dehydration and silica surface reconstruction. We have attempted to account for the many local surface hydroxyl topologies that are representative of silica surfaces by using different silica surface models ({101} and {111}). Obviously, many more possibilities exist but they cannot reasonably all be explored at an atomistic scale. Similar results (conservation of protonic acidity in the Keggin unit environment and preparation of well-defined surface POM species in agreement with the SOMC strategy) were also obtained after impregnation of anhydrous H3PW12O40 on silica dehydroxylated at 500 °C (species 2). In addition, TEM characterization of 2 has been investigated as tungsten displays a better contrast than molybdenum in
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Grinenval et al. TABLE 2: Gibbs Free Reaction Energies of POM•SdPOM≡S + 3H2O(g)a surface (S) PH2O ) P°
10-5 Torr
T
{101}
{111}
{101}
{111}
298 473
6 -95
-33 -134
-33 -157
-72 -196
a
T in K and energies in kJ · mol-1.
Figure 9. Characterization of 2 by TEM image with scaling at 10 nm.
Figure 8. Details of the top and side views of H3PMo12O40 physisorbed (top) and chemisorbed through three covalent bonds associated with three water molecules elimination (bottom) onto {101} (left) and {111} (right) β-cristobalite surfaces.
microscopy. The TEM image of 2 is shown in Figure 9 and corresponds to silica edge. The dark particles indicated by the red dotted circles are POMs of mean-size around 1-2 nm. Note that this is the first observation of silica-supported POMs at this scale and the small size of particles allows us to suggest that the Keggin units are mainly isolated on the oxide support. The TEM characterization provides a first evidence of the distribution of POM units on silica and, to further investigate the dispersion of [PW12O403-] in the solid 2, in situ Raman study as a function of temperature and reactivity of tetramethyl tin
Figure 10. Raman spectra of silica-supported phosphotungstic acid recorded at different temperatures prepared by (a) SOMC strategy (solid 2, [HPW/SiO2-(500)]) and (b) classical impregnation technique (solid 3, [HPW/SiO2]).
have been performed and compared to samples prepared by conventional impregnation. Samples 2 ([HPW/SiO2-(500)], SOMC strategy) and 3 ([HPW/ SiO2], classical impregnation technique) were heated under oxygen atmosphere and their Raman spectra recorded in situ from room temperature to 400 °C. The results are depicted in Figure 10a,b. Note first that spectra obtained for both samples at room temperature display exactly the same vibrational bands. The stretching frequencies observed at 1015, 990, 935, 890, 525, and 225 cm-1 are assigned according to literature data to
Well-Defined Silica-Supported Polyoxometalate Species νs(W-Od), νas(W-Od), ν(P-Oa), νas(W-Ob-W), νs(WOc-W) and νs(W-Oa). These bands are similar to those of the bulk phosphotungstic acid H3[PW12O40]22,61 and indicate consequently that the Keggin structure has been conserved upon both impregnation methods. Upon thermal treatment of solid 3, the main vibrational differences noticed in Figure 10b concern the width and intensity of peaks as well as small shifts of stretching frequencies. The partial disappearance of the peaks at 525 and 890 cm-1 can be assigned to a slight broadening of νas(W-Ob-W) and νs(W-Oc-W) signals because of a partial symmetry loss of the POM (from tetrahedral symmetry to axial symmetry order 3) due to the formation of the [Si(OH2+)] countercation.21 Literature on infrared and Raman spectroscopic data indicates indeed that the structure of Keggin-type POMs is greatly dependent on the nature of counter-cations.39,40 Upon heating, 3 displays almost the same behavior from room temperature to ca. 250 °C. However, above ca. 300 °C, a small broadening of the νs(W-Od) and νas(W-Od) vibrational bands indicates a modification of terminal oxygen atoms. In addition, this change seems irreversible because even back to room temperature, the sample displays the same ν(W-Od) features. These data indicate that the Keggin structure is kept upon heating even at high temperatures (300 °C), in agreement with literature,22 and thus suggests that the dehydration process took place within small aggregates of Keggin units. Similar conclusions have been drawn before39 and are therefore consistent with a heterogeneous dispersion of POM units on silica. The in situ Raman study performed on solid 3 confirms previously mentioned properties for a conventional impregnation technique.24-26 A different behavior is observed upon thermal treatment of 2 (Figure 10a). Until 200 °C, the differences are similar to those previously pointed out: a small broadening of the νas(WOb-W) and νs(W-Oc-W) vibrational frequencies is indeed noticed. However, their intensity is more strongly affected than in the previous sample. The Keggin structure within the surface POM species 2 suffers therefore from the same conformational constraints up to 200 °C. From 250 °C, stretching modes corresponding to the terminal oxygen atoms ν(W-Od) at 1015 and 990 cm-1 are drastically modified; the occurrence of an important broadening does not allow to distinguish the two bands anymore. Furthermore, note that upon thermal treatment of surface POM species 2, spectra display only frequencies of symmetrical and antisymmetrical W-O vibrations of the POM (at 990 and 1015 cm-1). No vibrational frequency at ca. 800 cm-1 has been found for the formation of WO3 stable species. This compound could have been expected if the phosphotungstic acid had been decomposed.62,63 This suggests that the observed decrease in intensity is not due to the decomposition but rather to the symmetry loss of the Keggin structure in agreement with previous literature results.64 Upon thermal treatment of surface POM species 2, the dehydration process does not occur within the POM units and the in situ Raman study is consistent with a drastic change of their environment. This drastic modification in the POM symmetry is assigned to a direct bond of the POM unit with the oxide support (by removal of water molecules upon thermal treatment) in agreement with literature.22 As a result, the comparison of both in situ Raman experiments show that there is a better dispersion of the Keggin units in the solid 2 in agreement with the above results. In summary, the in situ Raman study of silica-supported POM 2 performed at different temperatures provides a second good evidence of the POM distribution on silica using the SOMC strategy. This result is confirmed by comparison with a silica-
J. Phys. Chem. C, Vol. 114, No. 44, 2010 19031 supported POM sample prepared by a conventional impregnation technique and shows in addition that the SOMC strategy allows the preparation of well-defined and well-distributed POM units on silica. Finally, to further confirm and summarize the SOMC strategy interest, the samples of silica-supported POMs have been tested in a reaction with tetramethyl tin. This reaction reported recently in the literature by Legagneux et al.65 for silica-supported silicotungstic acid can be decomposed into two steps: (1) reaction of tetramethyl tin with the acidic proton of the tungstic supported POM, as observed when doing the reaction in solution,66 followed by (2) a migration of the trimethyl tin moiety on the silica support and reformation of the acidic proton on the tungstic heteropolyacid
[tSiOH2]3[HSiW12O40] + SnMe4 f [tSiOH2]3[SiW12O40][SnMe3] + CH4 [tSiOH2]3[SiW12O40][SnMe3] + tSiOH f tSi-O-SnMe3 + [tSiOH2]3[HSiW12O40] and the silica-supported trimethyl tin fragment can further react, leading to bigrafted species by the same mechanism tSi-O-SnMe3 + [tSiOH2]3[HSiW12O40] f [tSiOH2]3[SiW12O40][tSi-OSnMe2] + CH4 [tSiOH2]3[SiW12O40][tSi-O SnMe2] + tSi-OH f [tSiOH2]3[HSiW12O40] + [tSi-O]2SnMe2
The first step depends only on the number of acid sites, while the migration on the support will depend mainly on the POM dispersion on the silica support. Typically, if the POMs are not well-dispersed on the surface but rather present as aggregates and even if these aggregates have the same number of acidic protons than isolated species, the second step will be affected by this heterogeneous distribution because the migration on the silica surface will occur more slowly than on a well-dispersed catalyst, the mean distance between hydroxyl groups and POMs being higher. The reactivity of tetramethyl tin is therefore an elegant way to characterize both acidity and dispersion of silicasupported POMs and has been used as a tool to compare both impregnation approaches. As soon as SnMe4 is introduced in the infrared cell containing the sample of silica-supported POM, the gas phase analysis reveals a release of methane that is quantified by integration of the 3100-3125 cm-1 spectral region. The results obtained for the silica-supported phosphotungstic (3) and phosphomolybdic (4) acid samples prepared by a classical impregnation (and further dehydroxylated under vacuum at 200 °C) or by the SOMC strategy (without any treatment, samples (2) and (1) respectively) are depicted in Figure 11 and Table 3. For all samples, the kinetic curves can be decomposed into two components including a first rapid increase of the methane evolution followed by a slower increase. The initial rates and the final amount of evolved methane are given in Table 3 for the four samples. As shown above, the initial rates are proportional to the amount of available strong acid sites. It is evident from the data of Table 3 that the samples prepared by the SOMC strategy contain ca. 7 times more acid sites than those prepared by a classical way. It is also interesting to point out that the molybdic
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Figure 11. Methane release (µmol · g-1sample) during the reactivity of tetramethyl tin with silica-supported phosphotungstic acid samples prepared by SOMC (2) and classical (3) impregnations. The inset shows an expansion of the curves for the beginning of the reaction.
TABLE 3: Initial Rates and Amount of Methane Formed in the Reaction of Tetramethyl Tin with Silica-Supported POMs initial rate entry
sample
1 2 3 4
(2), [HPW/SiO2-(500)] (1), [HPMo/SiO2-(500)] (3), [HPW/SiO2] (4), [HPMo/SiO2]
formed CH4
(µmolCH4.g-1sample.min-1) (mmolCH4.g-1sample)
200 174 34 24
2.4 2.0 0.7 0.3
and tungstic heteropolyacids give quite the same result while it is generally admitted that the tungstic ones have a stronger acidity. We can conclude that the acidity of the anhydrous phosphomolybdic acid is sufficient to activate the Sn-C bond. Finally, the amount of released methane gives an indication on the POM dispersion on the surface. Clearly the SOMC sample provides a larger amount of methane and thus suggests a better dispersion of POM units on the silica support. Conclusion We have reported a new strategy to prepare silica-supported POMs that consists in controlling the formation of surface species. This approach based on the SOMC was performed using anhydrous POMs with partially dehydroxylated silica. Contrary to a classical impregnation method, this strategy enabled us to obtain well-distributed and well-defined surface species described by various techniques, the main ones being [tSi (OH...H+)]2[H+][PM12O40]3- (M ) Mo, W). Calculations show that the protons of the POM are displaced around the POM to optimize the interaction with the silica support, that they establish strong H-bond with available OH groups on silica, but that they are not transferred to the OH group and remain on the POM, tSi-OH · · · H+-O-Mo structures are hence formed. Dehydration can take place forming a covalent tSi-O-Mo bond. This reaction is highly exothermic in classical preparation conditions at high T, explaining the main formation of the dehydrated species 1a. The big advantage of the SOMC approach is to remain at room temperature where the dehydration reaction is only thermoneutral or weakly exothermic (depending on the nature of the silica surface), hence permitting to maintain hydroxylated species and a high acidity. The analysis of these samples has evidenced a partial heterogeneity in the distribution of surface hydroxyl groups upon dehydroxylation process, characterized by 3 OH spread over 6
silica rings. Their formation and dimension explain the possible covalent grafting of Keggin units onto the surface. The key added value of our synthetic strategy is a better control of the POM-surface interaction, limiting the dehydration process and covalent grafting, hence enabling to keep the strong acidity of the POM upon grafting. Experimental Section General Procedure for the Preparation of Starting Materials. Phosphomolybdic acid H3PMo12O40 · xH2O (99+%, Aldrich), phosphotungstic acid H3PW12O40 · xH2O (99+%, Aldrich), and triethylphosphine oxide (99%, Aldrich) were used as received. Tetramethyltin Sn(CH3)4 (95%, Aldrich) was stored under argon and degassed prior to use. All experiments were carried out by using standard air free methodology in argon filled Vacuum Atmospheres glovebox, on a Schlenk line, or in a Schlenk-type apparatus interfaced to a high vacuum line (10-5 Torr). All solvents (pentane, acetonitrile, DMSO) and pyridine were purified according to published procedures, stored under argon and degassed prior to use.67 Elemental analyses were performed at the CNRS Central Analysis Department of Solaize or at the laboratory ICMUB of the University of Bourgogne in Dijon. Infrared spectra were recorded on a Nicolet 5700-FT spectrometer by using an infrared cell equipped with CaF2 windows, allowing in situ studies. Typically, 32 scans were accumulated for each spectrum (resolution 1 cm-1). Raman spectra of solid samples were recorded with a LabRam HR spectrometer (Jobin Yvon). The laser was focused on the solids with a ×100 objective at a power of 100 µW. The 1H MAS and 31P CP-MAS NMR spectra were recorded on a Bruker DSX-300 or a Bruker Avance 500 spectrometers equipped with a standard 4 mm double-bearing probehead. The samples were introduced under argon in a zirconia rotor, which was then tightly closed. The spinning rate was typically 10 kHz. A typical cross-polarization sequence was used with 5 ms contact time and a recycle delay of 1 to 4 s to allow the complete relaxation of the 1H nuclei. All chemical shifts are given with respect to TMS, as an external reference. The 2D 31P-1H dipolar HETCOR solid state NMR spectrum of 1 was acquired under TPPM-15 1H decoupling. The contact time for CP and the recycle delay were 5 ms and 4 s respectively. A total of 128 t1 points with 64 scans each were
Well-Defined Silica-Supported Polyoxometalate Species collected. The processing was done with a 50 Hz line broadening in both dimensions and three zero fillings in the ω1 dimension. The proton DQ experiments were performed as follows: rf field strength was 70 kHz (7*ωR) during the excitation and reconversion periods that were chosen equal to 200 µs (corresponding to 7 post C7 basic elements). For the SiO2-(θ) samples, 512 increments of 21 pC7 blocks with 24 scans each were collected with 4 s recycle delay that gave a total experiment time of 14 h. For 1, 256 increments of 14 pC7 blocks with 16 scans each were collected with 4 s recycle delay that gave a total experiment time of 4.5 h. The processing was done with a 50 Hz line broadening in both dimensions and one zero filling in the ω1 dimension. Assignment of the spinning side bands was ensured by running the same experiment at spinning speed of 12 kHz (rf field strength 84 kHz during the C7 cycles). Sample for TEM analysis was prepared from powder sample dispersed in dried and degassed pentane in a glovebox and was transferred inside a special vacuum transfer holder under inert atmosphere to a Philips CM200 Transmission Electron Microscope. The acceleration voltage was 200 kV. Preparation of Surface Oxide. Preparation of SiO2-(θ). The oxide support was silica Aerosil from Degussa that has a specific area of 200 m2 · g-1. It was compacted with distilled water, dried at 140 °C for at least 5 days, sieved, calcined at 500 °C in air for 4-8 h and partially dehydroxylated at 500 °C for 12 h under high vacuum (10-5 Torr). The solid was referred as SiO2-(500) (in SiO2-(θ) samples, θ corresponds to the temperature of dehydroxylation). After this treatment the specific area was about 200 m2 · g-1 with an OH density of 1.4 OH · nm-2 (0.465 mmolOH · g-1). IR (cm-1): 3745 (s) ν(O-H). Solid state MAS 1 H NMR (δ ppm): 1.8 (tSiOH). Preparation of Silica-Supported POMs. Preparation of Silica-Supported POMs by Classical Impregnation [POM/ SiO2]. A mixture of the heteropolyacid dissolved in acetonitrile (10 mL) and silica (typically 2 g of compacted and sieved silica) was performed to obtain a monolayer on the oxide support: 37.5 wt % for H3PW12O40 and 27.5 wt % for H3PMo12O40. The dispersion was stirred and evaporated to dryness. The resulting powder was then treated during 2 h at 200 °C under high vacuum (10-5 Torr) and stored under argon. Preparation of Silica-Supported POMs by the SOMC Strategy [POM/SiO2-(500)]. A mixture of anhydrous POM (1.34 g of H3PW12O40, 0.465 mmol or 0.85 g of H3PMo12O40, 0.465 mmol) in acetonitrile (10 mL) and SiO2-(500) (1 g, 0.465 mmol of tSiOH) was stirred at 25 °C for 12 h under argon. After removal of the solution containing the unreacted POM, the resulting powder was dried 1 h at room temperature under high vacuum (10-5 Torr) and stored under argon. Reactivity of Silica-Supported POMs. Monitoring of 1 by in Situ Infrared Spectroscopy. Silica (20-25 mg) was pressed into a 18 mm self-supporting disk, adjusted in a sample holder, and put into a sealed glass high vacuum reactor equipped with CaF2 windows. After calcination at 500 °C under air for 2-4 h, the silica disk was treated under high vacuum (10-5 Torr) at 500 °C for 12 h. The silica support thus obtained, referred to as SiO2-(500), (20-25 mg, 9-12 µmol tSiOH) was then immersed into a DMSO (10 mL) solution of anhydrous POM (500 mg) at 25 °C for 12 h, followed by a drying step under high vacuum (10-5 Torr) at 25 °C for 2 h. ReactiWity of Pyridine on 1 and Desorption Followed by in Situ Infrared Spectroscopy. A vapor pressure of dry pyridine was introduced at room temperature into a sealed glass high vacuum reactor equipped with CaF2 windows and containing a [POM/SiO2-(500)] pellet prepared as described above. The sample
J. Phys. Chem. C, Vol. 114, No. 44, 2010 19033 was treated 2 h at room temperature under high vacuum (10-5 Torr). The thermodesorption of pyridine was realized by steps of 50 °C from 50 to 400 °C. Temperature-Programmed Desorption. The samples [POM/ SiO2-(500)] and [POM/SiO2] were loaded (typically 500 mg of powder) under argon in a continuous tubular reactor. The studied sample was heated at a rate of 50 °C · h-1 from 40 to 500 °C under helium flow (5 mL · min-1). The temperature was carefully recorded as a function of time and the composition of the evolved gases was determined online by gaseous chromatography. ReactiWity of Tetramethyl Tin (Sn(CH3)4) on Silica-Supported POMs. Silica-supported POMs (250 mg to 1 g) were introduced into a sealed glass high vacuum reactor of known volume equipped with CaF2 windows according to the following procedure: - Samples prepared by SOMC were loaded into a glovebox then argon was removed under high vacuum (10-5 Torr). - Samples prepared by classical impregnation were loaded and treated 2 h at 200 °C under high vacuum (10-5 Torr). After that, a large excess of tetramethyl tin (0.2 mL) dried over molecular sieves and degassed prior to use was introduced in the reactor. The reaction was monitored by infrared spectroscopic analysis of the gas phase at room temperature. Infrared spectra were recorded each two minutes during 760 min. The appearance of methane was quantified by integration of the region between 3126.10 and 3101.51 cm-1 (it was first checked that this surface area was proportional to the methane pressure). Periodic DFT Calculations. The periodic density functional theory (DFT) calculations have been done using VASP code.54-57 The PW91 functional was used.68,69 The planewave basis set expansion is set up to an energy cutoff of 400 eV. The ultrasoft pseudopotentials as provided in VASP were used.57 The pseudopotential outmost cutoff radii of Si, O, H, P, and Mo are 2.48, 1.55, 1.25, 2.33, and 2.75 Å, respectively. Because of the relatively large unit cells employed, the Brillouin zone sampling was restricted to the Γ-point. β-cristobalite was chosen as model for silica. All atomic positions and unit cell size were optimized for the β-cristobalite bulk using the quasi-Newton algorithm; the geometry was considered converged when all forces were below 0.5 eV · nm-1. The initial geometry of the system was taken from ref 70. The optimized unit cell, which contains 8 Si and 16 O, has a geometry defined by a ) 7.34 Å (experimentally, 7.16 Å) in the cubic space group, which was forced in the optimization. The surfaces {101} and {111} were built based on the optimized β-cristobalite bulk using Material Studio software; the number of Si layers was 3 and 4 for the {101} and {111} surfaces, respectively, resulting in slab thickness of ca. 8.00 Å. A sufficiently large empty space between the two surfaces of the slab was chosen in the c-direction (c ) 3 nm) to avoid periodic artifact resulting from interactions between the top and bottom surfaces and between the bottom surface and the polyoxometalate after the later is adsorbed onto the top surface. Apart from this, the unit cell vectors as obtained from the bulk were chosen and kept fixed to build the surfaces but extended in the a- and b-directions to minimize the self-contacts between the POM periodic images (the POM has a diameter of ca. 1 nm). For the {101} surface, the unit cell geometry is defined by a ) 20.77, and b ) 14.68 Å in the orthorhombic space group, and for {111} surface, a ) 20.77 Å in the hexagonal space group. The {101} system contains 48 Si, 112 O, and 32 H atoms and the {111} system, 64 Si, 144 O, and 32 H atoms, which is increased by 1 P, 32 Mo, 40 O, and 3 H atoms after the POM is adsorbed. These surfaces have an hydroxyl group coverage of 5.24 and 4.29 nm-2 for {101} and
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