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1H and 31P solid-state NMR of trimethylphosphine adsorbed on heteropolytungstate supported on silica
Applied Surface Science 255 (2009) 4897–4901
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
1
H and 31P solid-state NMR of trimethylphosphine adsorbed on heteropolytungstate supported on silica
J. Deleplanque, R. Hubaut, P. Bodart, M. Fournier, A. Rives * Unite´ de Catalyse et de Chimie du Solide, U.M.R.-C.N.R.S. 8181, Universite´ des Sciences et Technologies de Lille, 59655 Villeneuve d’Ascq, France
A R T I C L E I N F O
A B S T R A C T
Article history: Received 21 July 2008 Received in revised form 11 December 2008 Accepted 11 December 2008 Available online 24 December 2008
Trimethylphosphine (TMP) has been used as an NMR probe in order to determine the acidity of Keggintype 12-tungstophosphoric heteropolyacid (HPW), pure and supported on silica, dehydrated at 473 K. Adsorption of TMP on pure dehydrated HPW leads to the formation of trimethylphosphonium ions (TMPH+) characteristic of the presence of strong Bro¨nsted acid sites. TMP replaces the water molecules lost by dryness and allows the Keggin secondary structure to recover. Silica interacts with TMP by two kinds of acid sites: with weak acid support sites through the isolated silanol groups and with strong Bro¨nsted acid, which lead to the formation of TMPH+, through the hydrogen-bonded silanol groups. Silica only interacts with HPW through its isolated silanol groups. ß 2008 Elsevier B.V. All rights reserved.
PACS: 82.56Dj 68.43.h 82.65.+r Keywords: Solid-state NMR Chemisorption Bro¨nsted acidity Polyoxometalates Silica
1. Introduction Keggin-type heteropolyacids (HPAs) have attracted much interest for solid acid catalysts because their strong acidity can be easily tuned through their salts. 12-Tungstophosphoric (HPW) possesses pure Bro¨nsted acidity strongest than many conventional solid acids such as SiO2–Al2O3, H3PO4/SiO2, and HY zeolithe. HPW has already been used in liquid phase at moderate temperature, for reactions like cracking, isomerization or alkylation of alkanes, as well as oxidation reactions in both homogeneous and heterogeneous systems [1,2]. The relative strength acidity of the HPAs has already been estimated by conventional methods as ammonia desorption [3] which can be delicate owing to the presence of protons in several environments (primary structure, secondary structure and water). Trimethylphosphine (TMP, (CH3)3P), which is a weak base (pK aTMPHþ =TMP ¼ 8:65) is an interesting molecule probe for acidity because large NMR shifts exist between TMP species, depending on the environment. Table 1 summarizes 31P and 1H NMR chemical
* Corresponding author at: Unite´ de Catalyse et de Chimie du Solide, U.M.R.C.N.R.S. 8181, Batiment C3, Universite´ des Sciences et Technologies de Lille, bvd Langevin, 59655 Villeneuve d’Ascq, France. Tel.: +33 3 20 33 77 33; fax: +33 3 20 43 65 61. E-mail address: [email protected] (A. Rives). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.12.031
shift for trimethylphosphine, trimethylphosphonium ion and trimethylphosphine oxide in liquid phase, obtained in the literature [4,5]. Large NMR shifts exist between liquid TMP (62.2 ppm) and TMPH+ (3.2 ppm). TMP is also moisturesensitive given trimethylphosphine oxide (TMPO) species with a 36 ppm 31P NMR shift and could be used as oxidation molecule probe as well. Adsorption of TMP has been previously used as NMR probe to explore the acidity of zeolithe [6–11], alumina and silica–alumina [12,13], zirconia and tungstated zirconia [14] or and more recently on HPAs [15]. Table 2 gives 31P NMR chemical shift of trimethylphosphine adsorbed on several solid acids observed in the literature [6,8– 11,14]. Signals between ca. 60 ppm and 40 ppm are principally attributed to the interaction of TMP with Lewis acid sites. However, few authors found signals concerning weaker Lewis acid sites between 28 ppm and 12 ppm [8,10]. Some authors also assigned the signal observed at 62 ppm to an interaction with an acid site of Lewis type [11] but it seems that this latter signal should better correspond to physisorbed TMP species as shown in Table 1. Interactions between TMP and Bro¨nsted acid sites are usually observed with a 31P NMR shift included between 5 ppm and 2 ppm with a dipolar coupling of 500 Hz corresponding to the chemical shift observed for TMPH+ species.
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Table 1 31 P and 1H NMR chemical shift (ppm) of trimethylphosphine, trimethylphosphonium ion and trimethylphosphine oxide in liquid phase according to Refs. [4,5].
Liquid TMP TMPH+ TMPO
diso (ppm) 31P
diso (ppm) 1H
62.2 [4] 3.2 [4] 36 [5]
1.14 [4] 6.36 [4] –
In this study, 1H and 31P NMR have been performed to determine acid–base interactions between TMP and pure HPW, TMP and silica, HPW-species and silica and TMP with HPW supported on silica. 2. Experimental 2.1. Sample preparation H3PW12O4029H2O has been synthesized according to the method described by Pope [16]. Silica (Aerosil 380 from DEGUSSA) was firstly suspended in a 0.1 M nitric acid solution, dried and calcined at 673 K. The surface area of the silica after treatment is 321 m2 g1. Silica-supported HPW samples were prepared by the wet impregnation method. Silica and HPW were first dissolved in the minimum amount of water in different batch with the desired weight ratio. After mixture, the slurry of silica and impregnation solutions was constantly stirred at 323 K for the evaporation of water, dried at 383 K under vacuum and then finally put into desiccators to protect the samples from atmospheric moisture. The final catalysts are named X% HPW/SiO2 where X is the weight percentage of HPW. Adsorption of trimethylphosphine had been performed in a microflow reactor. 300 mg of each sample was firstly dehydrated at 473 K under nitrogen. TMP (Strem, 99%) was distilled under vacuum before using and then adsorbed on the solid from the vapor phase (10 Torr) at 323 K up to saturation and outgassed 15 min at the same temperature under nitrogen to remove all physisorbed species from the surface. Important cares were made to prevent oxidation of TMP. The sample were put on a glove box under argon, before to be inserted into a 4 mm ZrO2 NMR rotor, which was closed, and transferred to the spectrometer without exposure to air. 2.2. Characterization The Raman spectra of the samples were recorded using a Raman microprobe (Infinity from Jobin-Yvon) equipped with a photodiode array detector. The exciting laser source was the 532 nm line of an Nd-YAG laser with a power of the beam of 0.23 mW at the focal point. The wavenumber accuracy was 2 cm1.
Fig. 1. Raman spectra of fresh H3PW12O40 (a) and 20% HPW/SiO2 (b).
TG measurements were carried out in a SDT 2960 DSC-TGA instrument by thermal advantage. The 20 mg samples were performed with a flowing rate of 50 ml min1 in nitrogen from room temperature to 773 K at a heating rate of 5 K min1. NMR spectra were recorded at room temperature on a Bruker Avance ASX-400 spectrometer with a WVT triple resonance probe. Larmor frequencies used for 1H and 31P were 400.13 MHz and 161.97 MHz, respectively. Tetramethylsilane (TMS) and orthophosphoric acid (H3PO4) were used as references for 1H and 31P chemical shifts, respectively. The spinning speed (14 kHz) was stabilized to within 2 Hz by using a Bruker spinner control unit. 3. Results and discussion Polyoxometalates are sensitive to the hydration ratio. Thus, purity of HPW as a Keggin structure and 20% HPW/SiO2 have been checked Raman spectroscopy and spectra of both samples are shown in Fig. 1. The typical symmetric and antisymmetric W O vibration bands, respectively at 1009 cm1 and 992 cm1 were observed in accordance with the literature [1,17]. Fig. 2 displays the thermogravimetric profile of HPW under nitrogen. At room temperature, before dehydration, H3PW12 O4029H2O is a stable species. Two dehydration stages (well put in evidence by heat flow curve) correspond to the first weight losses observed and H3PW12O406H2O is obtained between 323 K and 400 K. A temperature up to 500 K should be applied to be sure to reach the anhydrous state in accordance with the literature [18,19].
Table 2 31 P NMR chemical shift of interaction between trimethylphosphine and physisorption, Lewis and Bro¨nsted sites, and JP–H dipolar coupling of the trimethylphosphonium ion with several catalysts obtained from the literature. Catalyst
H-ZSM-5 H-Y H-Y Dealuminated H-Y H-b H-b H-mordenite H-mordenite Zirconia W zirconia
NMR chemical shift (ppm) Physisorption sites
Lewis sites
Bro¨nsted sites
– 67 – 67 – – – – – 57
45
3.5 2 4.9 4.5 5.0 5.0 2.2 3.7 – 4 to 2
– – – – – – 48 to 23 55 to 20
JP–H (Hz)
References
480 550 520 520 487 409 493 417 – –
[8] [6] [8] [8] [9] [10] [10] [11] [14] [14]
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Fig. 2. Thermogravimetric profile of H3PW12O40 under nitrogen (50 ml min1). Fig. 4. 31
31
P NMR spectra of TMP adsorbed on dehydrated silica at 473 K.
Fig. 3 displays 31P MAS-NMR spectra of fresh and dehydrated heteropolyacid, as well as TMP adsorbed on dehydrated HPW. NMR spectra of the fresh and dehydrated HPW were previously described [18–21]. The fresh solid (13H2O) is characterized by two sharp peaks at 15.4 ppm and 15.1 ppm assigned to well crystallized polyanionic species in its secondary structure. The presence of two peaks can be explained by the existence of a heterogeneous distribution of crystallites sizes [15]. Dehydration at 473 K leads to shift and broadening of peaks at, respectively, 12.5 ppm and 11.1 ppm, corresponding to amorphous phases of the HPA, already described by Ganapathy et al. [18] and Essayem et al. [19]. However, dehydration is not complete at this temperature and thus, a weak resonance line at 15 ppm is still observable. Adsorption of the trimethylphosphine gives rise to a larger peak at 15.5 ppm and to two new types of peaks at 4.6 ppm and 1.7 ppm. These two latter broad signals are characteristic of the acidic proton of trimethylphosphonium ion (TMPH+) as described by several authors [6–14]. Indeed, it has already been reported that TMP interacts with the Bro¨nsted acid sites to form a protoned species trimethylphosphonium ((CH3)3P–H)+, with a chemical shift at ca. 4 ppm and a JP–H coupling of ca. 500 Hz. The shift obtained in our experiments and the JP–H coupling observed (525 Hz) confirm the interaction of the TMP with the strong Bro¨nsted acid sites existing on bulk H3PW12O40.
On the other hand, the presence of a signal at 15.5 ppm shows that the well crystallized Keggin unit is recovered, probably because TMP replaces the molecules of water lost during the dehydration process and thus reforms the secondary structure of the heteropolycompound. Moreover, this NMR signal becomes larger than in the case of the fresh HPW, involving that this structure may be less mobile than the initial one, due to a sterical effect. Fig. 4 displays the 31P spectrum of TMP adsorbed on dehydrated silica. Clearly, some traces of TMPH+ species can be observed (4.5 ppm), but the main contributions of phosphorus are the peak at 20 ppm. This signal can be assigned to the interaction between TMP and some weak acid sites of silica and will be discussed later in this paper. We can note that there is no signal corresponding to physisorbed TMP for all compounds. Fig. 5 presents the 31P spectra of dehydrated 20% HPW/SiO2 (a) and TMP adsorbed on it (b). A large distribution of peaks between 10 ppm and 16 ppm can be observed. This distribution can be attributed to both dehydrated HPA and heterogenous distribution of crystallite size, as already explained for Fig. 3. Due to the amount of HPA on silica (20%), the intensity of peak is relatively low. Anyway, there is no difference between intensity of peaks at ca. 10 ppm to 12 ppm (dehydrated HPW) and 15 ppm/16 ppm (fully hydrated – well crystallized secondary Keggin unit). Compared to pure and bulk dehydrated HPW whose NMR spectrum presented a very weak contribution at 15 ppm, that
Fig. 3. 31P NMR spectra of fresh H3PW12O40 (a), H3PW12O40 at 473 K (b) and TMP adsorbed on dehydrated H3PW12O40 at 473 K (c).
Fig. 5. 31P NMR spectra of dehydrated 20% HPW/SiO2 at 473 K (a) and TMP adsorbed on dehydrated 20% HPW/SiO2 (b).
3.1.
P NMR spectra
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observation should indicate that strong interactions exist between silica and HPW which can protect HPW from dehydration. Adsorption of TMP on that solid leads to the formation of a broad peak between 5 ppm and 0 ppm characteristic of TMPH+. This observation indicates the presence of strong Bro¨nsted acid sites, mainly brought by the acidity of HPW. Anyway, there is still a contribution at ca. 20 ppm, already explained as a weak acidity of the silica (Fig. 4). Once again, the sharp peak at 15.5 ppm points out the recovering of the secondary structure of HPW. These observations on the formation of TMPH+ are consistent with the ones obtained by Kim et al. [15] in 31P CP MAS-NMR on HPW supported on silica. They also observed a NMR peak at 45 ppm but no explanation was given about this contribution. If we make a comparison with the results obtained on our silica (31P contribution at 20 ppm), we can suppose that this contribution at 45 ppm is due to a weak acidity of the silica they used. Indeed, synthesis methods for silica are different. In our case, silica was washed with nitric acid and thus may be more acidic than the one prepared with silicon precursor by Kim et al. Those observations can be surprising due to the fact silica is very weakly acid, generally known as neutral, but this technique is fairly sensitive and those phenomena can be explained by the presence of different silanol groups. Moreover, Bro¨nsted acidity has already been characterized with help of probe molecule in the literature, such as pyridine [22] or CO [23]. Some authors [24] could also detect unusual Bro¨nsted acidity due to free and H-bonded silanol on silica, with cyclohexanone as molecule probe. However, in all cases, only hydrogen bond interactions have been observed. 31P chemical shift is thus very sensitive to distinguish differences of acidity between silica. 3.2. 1H NMR spectra The 1H MAS-NMR spectrum of the dehydrated H3PW12O40 (Fig. 6a) shows an intense resonance at +10.4 ppm, characteristic of the protons linked to the most basic bridged oxygen atoms of the primary structure of H3PW12O40 [19]. A small shoulder is also present around 9.1 ppm and is attributed to H3O+ species [19], showing that HPW is not completely dehydrated. In the presence of TMP (Fig. 6b), a peak due to the interaction between TMP and H3PW12O40, appears at +2.35 ppm, attributed to the protons of the methyl groups of TMP. Simultaneously, signal at +10.4 ppm vanishes indicating that free H+ are trapped by TMP, to form TMPH+. We can thus observe on this spectrum a very weak resonance at +6.36 ppm relative to the protons belonging to TMPH+ [25,26]. H3O+ species still exist at +9.15 ppm and hardly interact with TMP.
Fig. 6. 1H NMR spectra of dehydrated HPW (a) and TMP adsorbed on dehydrated HPW (b).
Fig. 7. 1H NMR spectra of dehydrated silica (a) and TMP adsorbed on dehydrated silica (b).
Fig. 7 displays the 1H MAS-NMR spectrum of the dehydrated silica (a) and the spectrum of the TMP adsorbed on the dehydrated silica (b). Fig. 8 schematizes two kinds of silanol groups observed in silicas. Two distinct resonances at +1.85 ppm and +3.10 ppm are observed (Fig. 7a). The first one, at +1.85 ppm, can be assigned to hydrogen of isolated silanols [27] (Fig. 8a) while the other one is characteristic of hydrogen linking some silanols groups [27] (Fig. 8b). In Fig. 7b, if the signal of the isolated silanols is still present, the one coming from the groups linked by hydrogen bonds seems to be vanished. Thus, the interaction between TMP and silica seems to exist through these particular silanols. Moreover, a new peak at 1.4 ppm appears, due to the methyl groups of the TMP. Results obtained with 31P NMR allow refining the phenomena. The stronger acidic sites of silica which form TMPH+ with a low density (signal at 4.5 ppm) are the hydrogen-linked silanols. Indeed, protons are more mobile in that configuration. The weaker ones, with a 31P NMR signal at 20 ppm are the isolated silanols. In this case, the proton transfer is not complete: hydrogen bond between TMP and the hydrogen of the isolated silanol gives rise to the d d species TMP–H +–O . 1 The H MAS-NMR spectrum of the 20% H3PW12O40/SiO2 dehydrated sample (Fig. 9a) shows two distinct broad signals at +3.10 ppm and included between +7.5 ppm and +10.4 ppm. For the HPW, with a weak signal at +10.4 ppm (H+), a broad signal at +8.65 ppm (H3O+) and a small shoulder at +7.5 ppm (H5O2+), it seems that HPW on silica is not as dehydrated as pure HPW. Thus silica prevents HPW from dehydration. Concerning the contribution of silica, silanols groups linked by hydrogen bonds seem to be still present (broad peak at +3.10 ppm) whereas the absence of the peak at +1.85 ppm shows that the isolated silanol groups do no longer exist. It is difficult to compare our results with those obtain by Kozhevnikov et al. [2,28,29]. Same conclusions can thus be given about the ‘‘interacting’’ species (SiOH2+)(H2PW12O40), but in our study, we can specify that those interactions exist through the isolated silanol groups. However, same conclusions [29] upon the role of silica on HPW are reported herein: supporting H3PW12O40 on silica leads to a slight modification of the secondary structure of the Keggin unit with a shift of 31P NMR signal from ca. 15.5 ppm to 14 ppm and a decrease of the acidity of the heteropolycompound’s protons. Nevertheless, silica fairly prevents H3PW12O40 from dehydration as the intensity of 31P NMR chemical shift at 15 ppm is still present. Those conclusions are consistent with general observations given in the literature about the thermal stability of heteropolyacids supported on silica. In the presence of TMP (Fig. 9b), a peak at 2.35 ppm is observed, representative of the interaction between TMP and H3PW12O40/
J. Deleplanque et al. / Applied Surface Science 255 (2009) 4897–4901
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Fig. 8. Scheme of isolated (a) and bridged silanols (b).
supported on silica, clearly possesses strong Bro¨nsted acid sites. However, silica tends to decrease acidic strength of those acid sites. Availability and quantification of protons belonging to the H3PW12O40 supported on silica will be published elsewhere. Acknowledgements The authors are grateful to B. Revel for his help in the acquisition of the NMR spectra and the Ministe`re de l’Education Nationale, de l’Enseignement Supe´rieur et de la Recherche for its financial support. References
Fig. 9. 1H NMR spectra of dehydrated 20% HPW/SiO2 (a) and TMP adsorbed on dehydrated 20% HPW/SiO2 (b).
SiO2 (hydrogen of the methyl group of TMP), while the peak at +10.4 ppm vanishes and peak at +8.65 ppm drastically decreases. In the same time, the resonance of the hydrogen bonds which link silanol (+3.10 ppm) also disappears confirming the interaction of the TMP with the weak acid available sites of the silica. 4. Conclusions 31 P MAS-NMR spectroscopy of the adsorption of TMP on solids allows distinguishing different strength and types of acid sites. Silica and HPW present different behaviors: bulk HPW only possesses strong Bro¨nsted acid sites. Silica has two kinds of weak acid sites as described in Fig. 8: the ‘‘strongest’’ sites correspond to the hydrogen-bonded silanols and gives TMPH+, whereas the weaker, inducing more moderate interactions with TMP, are the isolated silanol groups. These assertions can be confirmed by the 1 H NMR spectra. The 1H and 31P solid-state NMR spectra of silica, HPW on silica alone or in the presence of trimethylphosphine clearly demonstrate that H3PW12O40 and TMP interact with silica by different ways: H3PW12O40 is linked to silica with the help of isolated silanols which have basic character whereas TMP is adsorbed on silica through the hydrogen-linked silanols sites. H3PW12O40, even
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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
1
H and 31P solid-state NMR of trimethylphosphine adsorbed on heteropolytungstate supported on silica
J. Deleplanque, R. Hubaut, P. Bodart, M. Fournier, A. Rives * Unite´ de Catalyse et de Chimie du Solide, U.M.R.-C.N.R.S. 8181, Universite´ des Sciences et Technologies de Lille, 59655 Villeneuve d’Ascq, France
A R T I C L E I N F O
A B S T R A C T
Article history: Received 21 July 2008 Received in revised form 11 December 2008 Accepted 11 December 2008 Available online 24 December 2008
Trimethylphosphine (TMP) has been used as an NMR probe in order to determine the acidity of Keggintype 12-tungstophosphoric heteropolyacid (HPW), pure and supported on silica, dehydrated at 473 K. Adsorption of TMP on pure dehydrated HPW leads to the formation of trimethylphosphonium ions (TMPH+) characteristic of the presence of strong Bro¨nsted acid sites. TMP replaces the water molecules lost by dryness and allows the Keggin secondary structure to recover. Silica interacts with TMP by two kinds of acid sites: with weak acid support sites through the isolated silanol groups and with strong Bro¨nsted acid, which lead to the formation of TMPH+, through the hydrogen-bonded silanol groups. Silica only interacts with HPW through its isolated silanol groups. ß 2008 Elsevier B.V. All rights reserved.
PACS: 82.56Dj 68.43.h 82.65.+r Keywords: Solid-state NMR Chemisorption Bro¨nsted acidity Polyoxometalates Silica
1. Introduction Keggin-type heteropolyacids (HPAs) have attracted much interest for solid acid catalysts because their strong acidity can be easily tuned through their salts. 12-Tungstophosphoric (HPW) possesses pure Bro¨nsted acidity strongest than many conventional solid acids such as SiO2–Al2O3, H3PO4/SiO2, and HY zeolithe. HPW has already been used in liquid phase at moderate temperature, for reactions like cracking, isomerization or alkylation of alkanes, as well as oxidation reactions in both homogeneous and heterogeneous systems [1,2]. The relative strength acidity of the HPAs has already been estimated by conventional methods as ammonia desorption [3] which can be delicate owing to the presence of protons in several environments (primary structure, secondary structure and water). Trimethylphosphine (TMP, (CH3)3P), which is a weak base (pK aTMPHþ =TMP ¼ 8:65) is an interesting molecule probe for acidity because large NMR shifts exist between TMP species, depending on the environment. Table 1 summarizes 31P and 1H NMR chemical
* Corresponding author at: Unite´ de Catalyse et de Chimie du Solide, U.M.R.C.N.R.S. 8181, Batiment C3, Universite´ des Sciences et Technologies de Lille, bvd Langevin, 59655 Villeneuve d’Ascq, France. Tel.: +33 3 20 33 77 33; fax: +33 3 20 43 65 61. E-mail address: [email protected] (A. Rives). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.12.031
shift for trimethylphosphine, trimethylphosphonium ion and trimethylphosphine oxide in liquid phase, obtained in the literature [4,5]. Large NMR shifts exist between liquid TMP (62.2 ppm) and TMPH+ (3.2 ppm). TMP is also moisturesensitive given trimethylphosphine oxide (TMPO) species with a 36 ppm 31P NMR shift and could be used as oxidation molecule probe as well. Adsorption of TMP has been previously used as NMR probe to explore the acidity of zeolithe [6–11], alumina and silica–alumina [12,13], zirconia and tungstated zirconia [14] or and more recently on HPAs [15]. Table 2 gives 31P NMR chemical shift of trimethylphosphine adsorbed on several solid acids observed in the literature [6,8– 11,14]. Signals between ca. 60 ppm and 40 ppm are principally attributed to the interaction of TMP with Lewis acid sites. However, few authors found signals concerning weaker Lewis acid sites between 28 ppm and 12 ppm [8,10]. Some authors also assigned the signal observed at 62 ppm to an interaction with an acid site of Lewis type [11] but it seems that this latter signal should better correspond to physisorbed TMP species as shown in Table 1. Interactions between TMP and Bro¨nsted acid sites are usually observed with a 31P NMR shift included between 5 ppm and 2 ppm with a dipolar coupling of 500 Hz corresponding to the chemical shift observed for TMPH+ species.
J. Deleplanque et al. / Applied Surface Science 255 (2009) 4897–4901
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Table 1 31 P and 1H NMR chemical shift (ppm) of trimethylphosphine, trimethylphosphonium ion and trimethylphosphine oxide in liquid phase according to Refs. [4,5].
Liquid TMP TMPH+ TMPO
diso (ppm) 31P
diso (ppm) 1H
62.2 [4] 3.2 [4] 36 [5]
1.14 [4] 6.36 [4] –
In this study, 1H and 31P NMR have been performed to determine acid–base interactions between TMP and pure HPW, TMP and silica, HPW-species and silica and TMP with HPW supported on silica. 2. Experimental 2.1. Sample preparation H3PW12O4029H2O has been synthesized according to the method described by Pope [16]. Silica (Aerosil 380 from DEGUSSA) was firstly suspended in a 0.1 M nitric acid solution, dried and calcined at 673 K. The surface area of the silica after treatment is 321 m2 g1. Silica-supported HPW samples were prepared by the wet impregnation method. Silica and HPW were first dissolved in the minimum amount of water in different batch with the desired weight ratio. After mixture, the slurry of silica and impregnation solutions was constantly stirred at 323 K for the evaporation of water, dried at 383 K under vacuum and then finally put into desiccators to protect the samples from atmospheric moisture. The final catalysts are named X% HPW/SiO2 where X is the weight percentage of HPW. Adsorption of trimethylphosphine had been performed in a microflow reactor. 300 mg of each sample was firstly dehydrated at 473 K under nitrogen. TMP (Strem, 99%) was distilled under vacuum before using and then adsorbed on the solid from the vapor phase (10 Torr) at 323 K up to saturation and outgassed 15 min at the same temperature under nitrogen to remove all physisorbed species from the surface. Important cares were made to prevent oxidation of TMP. The sample were put on a glove box under argon, before to be inserted into a 4 mm ZrO2 NMR rotor, which was closed, and transferred to the spectrometer without exposure to air. 2.2. Characterization The Raman spectra of the samples were recorded using a Raman microprobe (Infinity from Jobin-Yvon) equipped with a photodiode array detector. The exciting laser source was the 532 nm line of an Nd-YAG laser with a power of the beam of 0.23 mW at the focal point. The wavenumber accuracy was 2 cm1.
Fig. 1. Raman spectra of fresh H3PW12O40 (a) and 20% HPW/SiO2 (b).
TG measurements were carried out in a SDT 2960 DSC-TGA instrument by thermal advantage. The 20 mg samples were performed with a flowing rate of 50 ml min1 in nitrogen from room temperature to 773 K at a heating rate of 5 K min1. NMR spectra were recorded at room temperature on a Bruker Avance ASX-400 spectrometer with a WVT triple resonance probe. Larmor frequencies used for 1H and 31P were 400.13 MHz and 161.97 MHz, respectively. Tetramethylsilane (TMS) and orthophosphoric acid (H3PO4) were used as references for 1H and 31P chemical shifts, respectively. The spinning speed (14 kHz) was stabilized to within 2 Hz by using a Bruker spinner control unit. 3. Results and discussion Polyoxometalates are sensitive to the hydration ratio. Thus, purity of HPW as a Keggin structure and 20% HPW/SiO2 have been checked Raman spectroscopy and spectra of both samples are shown in Fig. 1. The typical symmetric and antisymmetric W O vibration bands, respectively at 1009 cm1 and 992 cm1 were observed in accordance with the literature [1,17]. Fig. 2 displays the thermogravimetric profile of HPW under nitrogen. At room temperature, before dehydration, H3PW12 O4029H2O is a stable species. Two dehydration stages (well put in evidence by heat flow curve) correspond to the first weight losses observed and H3PW12O406H2O is obtained between 323 K and 400 K. A temperature up to 500 K should be applied to be sure to reach the anhydrous state in accordance with the literature [18,19].
Table 2 31 P NMR chemical shift of interaction between trimethylphosphine and physisorption, Lewis and Bro¨nsted sites, and JP–H dipolar coupling of the trimethylphosphonium ion with several catalysts obtained from the literature. Catalyst
H-ZSM-5 H-Y H-Y Dealuminated H-Y H-b H-b H-mordenite H-mordenite Zirconia W zirconia
NMR chemical shift (ppm) Physisorption sites
Lewis sites
Bro¨nsted sites
– 67 – 67 – – – – – 57
45
3.5 2 4.9 4.5 5.0 5.0 2.2 3.7 – 4 to 2
– – – – – – 48 to 23 55 to 20
JP–H (Hz)
References
480 550 520 520 487 409 493 417 – –
[8] [6] [8] [8] [9] [10] [10] [11] [14] [14]
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Fig. 2. Thermogravimetric profile of H3PW12O40 under nitrogen (50 ml min1). Fig. 4. 31
31
P NMR spectra of TMP adsorbed on dehydrated silica at 473 K.
Fig. 3 displays 31P MAS-NMR spectra of fresh and dehydrated heteropolyacid, as well as TMP adsorbed on dehydrated HPW. NMR spectra of the fresh and dehydrated HPW were previously described [18–21]. The fresh solid (13H2O) is characterized by two sharp peaks at 15.4 ppm and 15.1 ppm assigned to well crystallized polyanionic species in its secondary structure. The presence of two peaks can be explained by the existence of a heterogeneous distribution of crystallites sizes [15]. Dehydration at 473 K leads to shift and broadening of peaks at, respectively, 12.5 ppm and 11.1 ppm, corresponding to amorphous phases of the HPA, already described by Ganapathy et al. [18] and Essayem et al. [19]. However, dehydration is not complete at this temperature and thus, a weak resonance line at 15 ppm is still observable. Adsorption of the trimethylphosphine gives rise to a larger peak at 15.5 ppm and to two new types of peaks at 4.6 ppm and 1.7 ppm. These two latter broad signals are characteristic of the acidic proton of trimethylphosphonium ion (TMPH+) as described by several authors [6–14]. Indeed, it has already been reported that TMP interacts with the Bro¨nsted acid sites to form a protoned species trimethylphosphonium ((CH3)3P–H)+, with a chemical shift at ca. 4 ppm and a JP–H coupling of ca. 500 Hz. The shift obtained in our experiments and the JP–H coupling observed (525 Hz) confirm the interaction of the TMP with the strong Bro¨nsted acid sites existing on bulk H3PW12O40.
On the other hand, the presence of a signal at 15.5 ppm shows that the well crystallized Keggin unit is recovered, probably because TMP replaces the molecules of water lost during the dehydration process and thus reforms the secondary structure of the heteropolycompound. Moreover, this NMR signal becomes larger than in the case of the fresh HPW, involving that this structure may be less mobile than the initial one, due to a sterical effect. Fig. 4 displays the 31P spectrum of TMP adsorbed on dehydrated silica. Clearly, some traces of TMPH+ species can be observed (4.5 ppm), but the main contributions of phosphorus are the peak at 20 ppm. This signal can be assigned to the interaction between TMP and some weak acid sites of silica and will be discussed later in this paper. We can note that there is no signal corresponding to physisorbed TMP for all compounds. Fig. 5 presents the 31P spectra of dehydrated 20% HPW/SiO2 (a) and TMP adsorbed on it (b). A large distribution of peaks between 10 ppm and 16 ppm can be observed. This distribution can be attributed to both dehydrated HPA and heterogenous distribution of crystallite size, as already explained for Fig. 3. Due to the amount of HPA on silica (20%), the intensity of peak is relatively low. Anyway, there is no difference between intensity of peaks at ca. 10 ppm to 12 ppm (dehydrated HPW) and 15 ppm/16 ppm (fully hydrated – well crystallized secondary Keggin unit). Compared to pure and bulk dehydrated HPW whose NMR spectrum presented a very weak contribution at 15 ppm, that
Fig. 3. 31P NMR spectra of fresh H3PW12O40 (a), H3PW12O40 at 473 K (b) and TMP adsorbed on dehydrated H3PW12O40 at 473 K (c).
Fig. 5. 31P NMR spectra of dehydrated 20% HPW/SiO2 at 473 K (a) and TMP adsorbed on dehydrated 20% HPW/SiO2 (b).
3.1.
P NMR spectra
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observation should indicate that strong interactions exist between silica and HPW which can protect HPW from dehydration. Adsorption of TMP on that solid leads to the formation of a broad peak between 5 ppm and 0 ppm characteristic of TMPH+. This observation indicates the presence of strong Bro¨nsted acid sites, mainly brought by the acidity of HPW. Anyway, there is still a contribution at ca. 20 ppm, already explained as a weak acidity of the silica (Fig. 4). Once again, the sharp peak at 15.5 ppm points out the recovering of the secondary structure of HPW. These observations on the formation of TMPH+ are consistent with the ones obtained by Kim et al. [15] in 31P CP MAS-NMR on HPW supported on silica. They also observed a NMR peak at 45 ppm but no explanation was given about this contribution. If we make a comparison with the results obtained on our silica (31P contribution at 20 ppm), we can suppose that this contribution at 45 ppm is due to a weak acidity of the silica they used. Indeed, synthesis methods for silica are different. In our case, silica was washed with nitric acid and thus may be more acidic than the one prepared with silicon precursor by Kim et al. Those observations can be surprising due to the fact silica is very weakly acid, generally known as neutral, but this technique is fairly sensitive and those phenomena can be explained by the presence of different silanol groups. Moreover, Bro¨nsted acidity has already been characterized with help of probe molecule in the literature, such as pyridine [22] or CO [23]. Some authors [24] could also detect unusual Bro¨nsted acidity due to free and H-bonded silanol on silica, with cyclohexanone as molecule probe. However, in all cases, only hydrogen bond interactions have been observed. 31P chemical shift is thus very sensitive to distinguish differences of acidity between silica. 3.2. 1H NMR spectra The 1H MAS-NMR spectrum of the dehydrated H3PW12O40 (Fig. 6a) shows an intense resonance at +10.4 ppm, characteristic of the protons linked to the most basic bridged oxygen atoms of the primary structure of H3PW12O40 [19]. A small shoulder is also present around 9.1 ppm and is attributed to H3O+ species [19], showing that HPW is not completely dehydrated. In the presence of TMP (Fig. 6b), a peak due to the interaction between TMP and H3PW12O40, appears at +2.35 ppm, attributed to the protons of the methyl groups of TMP. Simultaneously, signal at +10.4 ppm vanishes indicating that free H+ are trapped by TMP, to form TMPH+. We can thus observe on this spectrum a very weak resonance at +6.36 ppm relative to the protons belonging to TMPH+ [25,26]. H3O+ species still exist at +9.15 ppm and hardly interact with TMP.
Fig. 6. 1H NMR spectra of dehydrated HPW (a) and TMP adsorbed on dehydrated HPW (b).
Fig. 7. 1H NMR spectra of dehydrated silica (a) and TMP adsorbed on dehydrated silica (b).
Fig. 7 displays the 1H MAS-NMR spectrum of the dehydrated silica (a) and the spectrum of the TMP adsorbed on the dehydrated silica (b). Fig. 8 schematizes two kinds of silanol groups observed in silicas. Two distinct resonances at +1.85 ppm and +3.10 ppm are observed (Fig. 7a). The first one, at +1.85 ppm, can be assigned to hydrogen of isolated silanols [27] (Fig. 8a) while the other one is characteristic of hydrogen linking some silanols groups [27] (Fig. 8b). In Fig. 7b, if the signal of the isolated silanols is still present, the one coming from the groups linked by hydrogen bonds seems to be vanished. Thus, the interaction between TMP and silica seems to exist through these particular silanols. Moreover, a new peak at 1.4 ppm appears, due to the methyl groups of the TMP. Results obtained with 31P NMR allow refining the phenomena. The stronger acidic sites of silica which form TMPH+ with a low density (signal at 4.5 ppm) are the hydrogen-linked silanols. Indeed, protons are more mobile in that configuration. The weaker ones, with a 31P NMR signal at 20 ppm are the isolated silanols. In this case, the proton transfer is not complete: hydrogen bond between TMP and the hydrogen of the isolated silanol gives rise to the d d species TMP–H +–O . 1 The H MAS-NMR spectrum of the 20% H3PW12O40/SiO2 dehydrated sample (Fig. 9a) shows two distinct broad signals at +3.10 ppm and included between +7.5 ppm and +10.4 ppm. For the HPW, with a weak signal at +10.4 ppm (H+), a broad signal at +8.65 ppm (H3O+) and a small shoulder at +7.5 ppm (H5O2+), it seems that HPW on silica is not as dehydrated as pure HPW. Thus silica prevents HPW from dehydration. Concerning the contribution of silica, silanols groups linked by hydrogen bonds seem to be still present (broad peak at +3.10 ppm) whereas the absence of the peak at +1.85 ppm shows that the isolated silanol groups do no longer exist. It is difficult to compare our results with those obtain by Kozhevnikov et al. [2,28,29]. Same conclusions can thus be given about the ‘‘interacting’’ species (SiOH2+)(H2PW12O40), but in our study, we can specify that those interactions exist through the isolated silanol groups. However, same conclusions [29] upon the role of silica on HPW are reported herein: supporting H3PW12O40 on silica leads to a slight modification of the secondary structure of the Keggin unit with a shift of 31P NMR signal from ca. 15.5 ppm to 14 ppm and a decrease of the acidity of the heteropolycompound’s protons. Nevertheless, silica fairly prevents H3PW12O40 from dehydration as the intensity of 31P NMR chemical shift at 15 ppm is still present. Those conclusions are consistent with general observations given in the literature about the thermal stability of heteropolyacids supported on silica. In the presence of TMP (Fig. 9b), a peak at 2.35 ppm is observed, representative of the interaction between TMP and H3PW12O40/
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Fig. 8. Scheme of isolated (a) and bridged silanols (b).
supported on silica, clearly possesses strong Bro¨nsted acid sites. However, silica tends to decrease acidic strength of those acid sites. Availability and quantification of protons belonging to the H3PW12O40 supported on silica will be published elsewhere. Acknowledgements The authors are grateful to B. Revel for his help in the acquisition of the NMR spectra and the Ministe`re de l’Education Nationale, de l’Enseignement Supe´rieur et de la Recherche for its financial support. References
Fig. 9. 1H NMR spectra of dehydrated 20% HPW/SiO2 (a) and TMP adsorbed on dehydrated 20% HPW/SiO2 (b).
SiO2 (hydrogen of the methyl group of TMP), while the peak at +10.4 ppm vanishes and peak at +8.65 ppm drastically decreases. In the same time, the resonance of the hydrogen bonds which link silanol (+3.10 ppm) also disappears confirming the interaction of the TMP with the weak acid available sites of the silica. 4. Conclusions 31 P MAS-NMR spectroscopy of the adsorption of TMP on solids allows distinguishing different strength and types of acid sites. Silica and HPW present different behaviors: bulk HPW only possesses strong Bro¨nsted acid sites. Silica has two kinds of weak acid sites as described in Fig. 8: the ‘‘strongest’’ sites correspond to the hydrogen-bonded silanols and gives TMPH+, whereas the weaker, inducing more moderate interactions with TMP, are the isolated silanol groups. These assertions can be confirmed by the 1 H NMR spectra. The 1H and 31P solid-state NMR spectra of silica, HPW on silica alone or in the presence of trimethylphosphine clearly demonstrate that H3PW12O40 and TMP interact with silica by different ways: H3PW12O40 is linked to silica with the help of isolated silanols which have basic character whereas TMP is adsorbed on silica through the hydrogen-linked silanols sites. H3PW12O40, even
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