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Synthesis and structural characterization of MSU-type silica–tin molecular sieves: Post-synthesis grafting of tin chlorides
Microporous and Mesoporous Materials 91 (2006) 261–267 www.elsevier.com/locate/micromeso
Synthesis and structural characterization of MSU-type silica–tin molecular sieves: Post-synthesis grafting of tin chlorides Manuela Casagrande a, Elisa Moretti a, Loretta Storaro a, Maurizio Lenarda Jacopo Gersich a, Lorenzo Stievano b, Friedrich E. Wagner c b
a,*
,
a INSTM UdR di Venezia, Dipartimento di Chimica, Universita` Ca’ Foscari, Via Torino 155b, 30172 Mestre Venezia, Italy Laboratoire de Re´activite´ de Surface UMR 7609, Universite´ Pierre et Marie Curie – Paris VI, Casier 178, 4, Place Jussieu, 75252 Paris Cedex 05, France c Physik-Department E15, Technische Universita¨t Mu¨nchen, D-85747 Garching, Germany
Received 1 September 2005; received in revised form 28 November 2005; accepted 7 December 2005 Available online 25 January 2006
Abstract Supermicroporous Sn-incorporated MSU-type silica molecular sieves, with different amounts of tin, have been synthesized using divalent and tetravalent Sn chlorides as grafting agents. The structural and chemical properties of the prepared materials, have been studied using a variety of techniques such as X-ray diffraction, N2 physisorption, FT-IR spectroscopy of chemisorbed pyridine, temperature programmed desorption of ammonia (NH3-TPD) and 119Sn Mo¨ssbauer spectroscopy. The introduction of tin produced purely Lewis acid sites on the silica surface. The amount of grafted tin was found to vary depending on the Sn precursor salt. The grafting procedure was more effective when anhydrous SnCl4 was used. A consistent part of the tin present in the MSU-Sn samples, that resulted accessible to incoming molecules, was found to reversibly change coordination number upon dehydration in He at 673 K and re-exposition to air at room temperature. 2005 Elsevier Inc. All rights reserved. Keywords: Tin–silica molecular sieves; Supermicroporous; Sn-MSU; Grafting; Surface acidity
1. Introduction The synthesis of mesoporous silicas MCM-41 [1], HMS [2] and MSU [3] by cooperative assembly of silica and surfactants, has attracted a great deal of research interest in recent years. These mesoporous materials have a large pore size (1.5–10 nm) and a high internal surface area (up to 1200 m2/g) [4,5]. MCM-41 and HMS mesostructured silicas, doped with heteroatoms, have been studied for their potential interest as catalysts and materials of this type, containing Mn, Al, B, Ga, Fe or Ti, V, Sn, Zr, have been reported in the literature [6–8]. A small number of these studies deals with pure siliceous MSU mesostructures or ones partially substituted with het-
*
Corresponding author. E-mail address: [email protected] (M. Lenarda).
1387-1811/$ - see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.12.001
eroatoms, in spite of their high thermal and hydrothermal stability [9,10]. Tin substituted MCM-41 and HMS have been reported [11–13]. Partial substitution of Si atoms with Sn is expected to impart Lewis-type acidic properties to these materials which makes them potentially useful for catalytic purposes. Some of us have recently prepared supermicroporous Sn–Si molecular sieves, with different amounts of tin, using poly-(ethylene oxide)-type surfactants as structure directing agents and a silicon alkoxide as precursor [10]. This research work continued with the preparation and characterization of MSU type silica–tin mesostructures with a narrow pore distribution, via post-synthesis grafting with Sn(II) or Sn(IV) chlorides, used both in anhydrous and hydrous form. It is known that the incorporation of heteroelements into pure mesoporous silicas can also be carried out by ‘‘post-synthesis’’ chemical grafting [7]. Nevertheless it was
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found that the resulting materials are quite different from those prepared by ‘‘in synthesis’’ methods [7]. The ‘‘in synthesis’’ method, in fact, was found to generate mesostructures where the active sites are present not only within the channels, and therefore accessible to incoming molecules, but can also be located within the wall and therefore result inaccessible. On the other hand, when the grafting procedure is used, the grafted species are expected to lay on the surface of the mesopores and therefore be accessible, in principle, to any guest molecule diffusing in the pores. 2. Experimental and synthesis
flow = 15 l/min, auxiliary argon flow = 0.5 ml/min, argon flow to the nebulizer = 0.65 ml/min and RF power = 1400 W. 2.2.2. Nitrogen adsorption/desorption Adsorption–desorption experiments using N2 were carried out at 77 K on a Micromeritics ASAP 2010. Before each measurement the samples were first outgassed at 423 K for 12 h at 0.6 Pa and then at room temperature for 2 h at 5 · 10 7 Pa. The N2 isotherms were used to determine both the specific surface areas (S.A.) using the BET equation and the specific pore volume (Vs). Vs was calculated at p/p0 = 0.98.
2.1. Synthesis of MSU-Sn samples Pure siliceous MSU(I) was prepared according to the literature [14,15]. Grafting, to prepare MSU-Sn materials, was carried out both by using divalent (SnCl2 98%, Aldrich or SnCl2 Æ 2H2O > 98%, Acros) and tetravalent (SnCl4 99%, Aldrich or SnCl4 Æ 5H2O 99%, Aldrich) tin chloride precursors. 2.1.1. Grafting with Sn(II) chloride Different portions of siliceous MSU(I) were first treated at 383 K overnight and then suspended under argon in absolute ethanol (1 g/100 ml). Anhydrous SnCl2 or SnCl2 Æ 2H2O (27 mmol/g of support in ethanol) were added and the mixtures were refluxed for 1 h in argon [16] to obtain the MSU-Sn2a and MSU-Sn2 samples, respectively. The white powders obtained were centrifuged, washed with ethanol dried in air at 343 K for 12 h and ground to a fine powder (40 mesh). The materials were then calcined in air up to 473 K (3 K/min) and kept at this temperature for 6 h, and subsequently up to 893 K (3 K/min) and kept at this temperature for a further 6 h. 2.1.2. Grafting with Sn(IV) chloride Different portions of siliceous MSU(I) were first treated at 383 K overnight and then suspended in isopropanol under argon (1 g/100 ml). Anhydrous SnCl4 or SnCl4 Æ 5H2O (27 mmol/g of support in isopropanol for both) were used for the grafting [17] and the mixtures were refluxed for 7 h in argon to obtain the MSU-Sn4a and MSU-Sn4 samples, respectively. The white powders obtained were centrifuged, washed with isopropanol, dried in air at 343 K for 12 h and ground to a fine powder (40 mesh). The materials were then calcined in air first up to 473 K (3 K/min) and kept at this temperature for 6 h, and then subsequently up to 893 K (3 K/min) and kept at this temperature for another 6 h. 2.2. Characterization methods 2.2.1. Elemental analysis by optic emission spectrometer (ICP-OES) The analyses were performed with a Perkin–Elmer Optima 3100 XL. The operative parameters were: argon
2.2.3. FT-IR spectroscopy measurements The IR spectra were recorded on a Nicolet Magna 750 FT Instrument, using pressed discs of pure catalyst powders. The FT-IR spectra of adsorbed pyridine were carried out in an evacuable Pyrex cell with CaF2 windows. Pyridine adsorption experiments and calculations were carried out as described in the literature [18]. The samples were ground to a fine powder. A total of 30 mg of the sample were pressed at 4 ton, in order to get a self-supporting wafer. The wafers were mounted in the holder of the IR cell and degassed by heating at 673 K and 1.3 · 10 2 Pa. After cooling to room temperature, pyridine was adsorbed on the samples. The spectra were recorded after degassing the wafers under vacuum at 423 K and 1.3 · 10 2 Pa for 2 h. 2.2.4. (NH3-TPD) temperature programmed desorption of ammonia The total acidity of the samples was determined by temperature-programmed desorption of ammonia (NH3-TPD). Measurements were carried out by an AUTOCHEM 2910 automatic temperature programmed desorption apparatus (Micromeritics). About 100 mg of sample were treated at 773 K in helium flow for 90 min. The temperature was reduced to 393 K and the sample was kept in a flow of NH3 (1% in helium) for 30 min and then in helium flow (40 ml/min) for 60 min. The amount of desorbed NH3 was determined by heating the sample at 25 K/min up to 923 K in helium flow (40 ml/min). 2.2.5. X-ray diffraction spectra X-ray diffraction spectra were measured with a Philips diffractometer using Cu Ka radiation (Cu Ka k = ˚ ). The samples were disc shaped pressed powders. 1.54184 A 2.2.6.
119
Sn Mo¨ssbauer spectroscopy Sn Mo¨ssbauer measurements were carried out at 4.2 K with both the sample and the 119Sn source (CaSnO3) in a liquid helium bath cryostat. For the in situ measurements, the samples were reduced in special glass reactors and then transferred to cylindrical glass ampoules attached to the reactors. Before sealing off the ampoules the atmosphere was changed to helium in order to ensure that temperature equilibrium was rapidly established when the 119
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samples were cooled down to 4.2 K for the Mo¨ssbauer measurement. A NaI scintillation detector was used for the detection of c-rays. The spectra were analyzed by a least-square fitting routine using the MOS-90 computer program [19]. A Gaussian broadening of the Lorentzian lines into Voigt profiles was allowed, in order to take in account a Gaussian distribution of the quadrupole splitting. The resulting spectral parameters, i.e., the mean quadrupole splitting (QS), the isomer shift (IS), and the relative resonance areas (Area) of the different spectral components are listed in Table 4. The isomer shifts are given relative to the CaSnO3 source.
263
a
Intensity (a.u.)
b
c
d
3. Results and discussion 3.1. Material characterization
2
3.1.1. Chemical composition Tin content of all four samples, determined by elemental analysis with an ICP-Optical Emission Spectrometer, is reported in Table 1 as the Sn/Si % molar ratio. From the ICP values, it appears that grafting on the siliceous matrix with Sn(II) salts is more efficient than with the Sn(IV) ones. No chlorides are detected in the sample, indicating their complete removal, by washing. 3.1.2. X-ray diffraction Typical powder X-ray diffraction patterns are shown in Fig. 1. The XRD profile is characteristic of MSU type mesostructured silicas for the purely siliceous starting MSU(I) sample (Fig. 1a) with the position of the peak indicating a d100 spacing of 4.2 nm. The XRD patterns of the MSU-Sn grafted samples (Fig. 1b–d) also show a similar single peak corresponding to a d100 spacing of 4.2 nm (see Table 2). The X-ray powder diffraction patterns of all the samples are nearly identical, and only a slight decrease of the intensity of the d100 profile compared to the purely siliceous sample is observed when the aqueous salts of the heteroatom is employed for the grafting. The d100 distance appears to slightly decrease in the case of the MSU-Sn4 and -Sn4a samples. 3.1.3. Nitrogen adsorption/desorption N2 adsorption–desorption isotherms of the MSU-Sn samples are similar to those of the pure siliceous MSU(I)
Table 1 Composition of some MSU-Sn selected samples determined by ICP-OES Sample
Tin precursor
(Sn/Si)ICP [mol%]a
MSU-Sn2 MSU-Sn2a MSU-Sn4 MSU-Sn4a
SnCl2 Æ 2H2O SnCl2 SnCl4 Æ 5H2O SnCl4
2.21 1.64 0.83 0.80
a Tin content is calculated supposing that all the silicon and tin were present as SiO2 and SnO2.
3
4
2θ
Fig. 1. Powder X-ray diffraction patterns of samples: (a) MSU(I), (b) MSU-Sn4a, (c) MSU-Sn2, (d) MSU-Sn4.
Table 2 Textural properties of MSU-Sn materials Sample
S.A. [m2/g]
Vs [cm3/g]
d100 [nm]
MSU(I) MSU-Sn2 MSU-Sn2a MSU-Sn4 MSU-Sn4a
1090 948 909 895 918
0.56 0.44 0.44 0.41 0.42
4.2 4.2 4.2 4.2 4.1
S.A. = BET specific surface area. Vs = pore specific volume at p/p0 = 0.98. d100 = interplanar distance.
sample and can be classified as a mixture of Type I isotherm, typical of a microporous adsorbent, and a Type IV, typical of a mesoporous material [10]. Similar features were observed by Boissie`re et al. and are characteristic of a microporous material with pores ranging between 1.5 and 2.0 nm that are usually classified as ‘‘supermicroporous’’ [14]. The absence of hysteresis in the N2 desorption isotherms of all samples suggests that the architecture of the individual pores is uniform and does not show bottlenecking. The textural properties of the synthesized materials are summarized in Table 2. Both the specific surface area and the specific pore volume appear to slightly decrease after grafting and the phenomenon seems independent from the tin species used. 3.1.4. Acidity evaluation by NH3-TPD and FT-IR spectroscopy of adsorbed pyridine The relative amount and the nature of the acid sites of both pure silica precursors and Sn grafted samples were studied by NH3-TPD and FT-IR spectroscopy of adsorbed pyridine. Ammonia is a strong base (pKb 5) that reacts even with extremely weak acid sites which therefore makes NH3-TPD a useful technique both for evaluating the
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relative amount of acid sites present on a surface as well as to roughly estimate their acid strength. This method, however, cannot discriminate Lewis from Bro¨nsted acid sites [20]. On the other hand, pyridine is a significantly weaker base (pKb 9), that does not react with some of the weak sites able to react with ammonia, but the study of the IR spectrum of chemisorbed pyridine is very useful to distinguish Lewis from Bro¨nsted acid sites [18]. Fig. 2 shows the NH3-TPD profile of both an all silica Si-MSU(I) sample and of the grafted MSU-Sn2 one. After deconvolution, both curves resulted composed of two peaks, one at a lower (LT peak, 478–498 K) and the other at a higher temperature (HT peak, 573 K). The total amount of desorbed ammonia and the amounts ascribable to the ammonia desorption at the high (HT) and low temperatures (LT), respectively, are summarized in Table 3. First of all, it can be noticed that the addition of tin influences the total peak area, i.e., the total amount of acid sites, the strongest increase being observed for the LT peak, while the HT peak increases only moderately. No correlation is found, however, between the total peak area and the tin loading. The HT peak is usually attributed to the desorption of ammonia from strong Bro¨nsted and Lewis acid sites. The attribution of the LT peak, on the contrary, is still the subject of much debate in literature [21]. In these types of materials, however, it can be ascribed to the weak Lewis acid sites. The presence of Lewis acid sites, in fact, is detected in the infrared spectra of adsorbed pyridine.
Fig. 2. TPD profile of desorbed ammonia from samples Si-MSU(I) (bottom) and MSU-Sn2 (top).
Table 3 Calculated amount of total acid sites and desorbing at low (LT) and high (HT) temperature
MSU(I) MSU-Sn2 MSU-Sn2a MSU-Sn4 MSU-Sn4a
Acid sites (lmol/g)
LT peak (lmol/g)
HT peak (lmol/g)
50 129 124 128 140
13 80 62 68 74
37 49 62 60 66
0.2 MSU-Sn2
Assorbance (a.u.)
264
MSU-Sn2a
MSU-Sn4
MSU-Sn4a
1650 1600 1550 1500 1450 1400
Wavenumber (cm -1) Fig. 3. FT-IR spectra of pyridine adsorbed on the MSU-Sn samples after evacuation at 423 K.
The infrared spectra of pyridine adsorbed on tin grafted MSU samples, after evacuation at 423 K, are shown in Fig. 3. As shown in a previous paper the spectrum of the purely siliceous MSU(I) support shows two bands at 1445 cm 1 and 1596 cm 1, attributed to pyridine physisorbed through hydrogen bond [21,13] and a band at 1577 cm 1 ascribed either to the chemisorption of the pyridine on weak Lewis acid centers [22] or to its interaction with the hydroxyls on the MSU(I) surface [23]. The described bands are present only after evacuation at 298 K but disappear upon evacuation at 423 K. The bands characteristic of pyridine adsorbed on Lewis acid sites (1453 and 1614 cm 1), however, were found in the spectra of all the tin grafted MSU samples even after evacuation at 423 K, as shown in Fig. 3. This observation is in agreement with the possible presence of tin, either tetra-coordinated substituting for silicon in the SiO2 structure or occupying surface defect sites. Actually, both these situations produce coordinatively insaturated adsorption sites. These bands are more intense in the spectra of the MSU-Sn4 and MSU-Sn4a samples, obtained by grafting by Sn4+ salts and suggest the presence of a higher number of acid sites when compared to the samples obtained by grafting with Sn2+ precursors. This result is quite surprising, if one considers that, as reported in Table 1, samples prepared by grafting with Sn4+ salts showed lower Sn/Si values. Moreover, for both divalent
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and tetravalent tin precursors, these bands result more intense in samples grafted by anhydrous salts than in those prepared from hydrated salts.
100
3.1.5. 119Sn Mo¨ssbauer spectroscopy The 119Sn Mo¨ssbauer spectra of all the as-prepared samples (Table 4 and Fig. 4) consist of a single narrow quadrupole doublet close to zero velocity corresponding to the presence of tetravalent tin. The hyperfine parameters of the quadrupole doublet, however, are slightly different from the values observed for crystalline SnO2 (QS = 0.49 mm/s, IS = 0.02 mm/s), allowing one to group the samples in pairs:
Dehydration in He flow at 673 K induces a noticeable change in the Mo¨ssbauer spectra, and an additional quadrupole doublet with an IS of 3 mm/s and a QS of 2 mm/s appears (Fig. 4), giving evidence of the rather surprising reduction of a noticeable fraction of Sn(IV) into Sn(II). This reduction is associated to a perceptible change in the color of the samples from white to light brown. The hyperfine parameters of divalent tin are similar to those observed for Sn(II) in glasses [24–26]. Moreover, this treatment produces both a substantial increase in the IS of Sn(IV), which becomes positive for all samples, as well as a slight increase in QS (Fig. 5). The greatest variation on the hyperfine parameters is seen in the samples prepared from tetravalent tin chlorides. Exposition of the dehydrated samples to air at room temperature for a few hours again changes the Mo¨ssbauer Table 4 119 Sn Mo¨ssbauer parameters at 4.2 K for the MSU-Sn samples Sample
Treatment
QS [mm/s]
MSU-Sn2
As made He/673 K
0.61(1) 0.90(1) 2.05(2)
MSU-Sn2a
As made He/673 K
MSU-Sn4
MSU-Sn4a
a
ISa [mm/s]
Area [%]
Chemical state of tin
0.07(1) 0.04(1) 3.21(1)
100 92(1) 8(1)
Sn(IV) Sn(IV) Sn(II)
0.62(1) 0.70(1) 2.14(2)
0.08(1) 0.01(1) 3.09(1)
100 82(1) 18(1)
Sn(IV) Sn(IV) Sn(II)
As made He/673 K
0.65(1) 0.65(1) 2.45(1)
0.20(1) 0.04(1) 3.02(2)
100 73(1) 27(1)
Sn(IV) Sn(IV) Sn(II)
As made He/673 K
0.64(1) 0.64(1) 2.60(1)
0.21(1) 0.10(1) 2.99(2)
100 46(1) 54(1)
Sn(IV) Sn(IV) Sn(II)
Isomer shift referred to the CaSnO3 source.
99
(a) 100.0
99.5
Relative transmission [%]
(I) The two samples prepared from divalent tin chloride, either hydrated or anhydrous, have a QS of 0.6 mm/s and an average IS of 0.07 mm/s, both relatively close to those of pure SnO2. (II) The two samples prepared from tetravalent tin chloride, which have a QS of 0.7 mm/s and an average IS of 0.20 mm/s, the latter significantly more negative than that of SnO2.
265
(b) 100
98
(c) 100
(d) 98
-6
-3
0
3
6
Velocity [mm/s] Fig. 4. 119Sn Mo¨ssbauer spectra at 4.2 K of sample MSU-Sn4 as-prepared (a), after dehydration in situ in He flow at 673 K (b), after exposition to air for 24 h (c) and after exposition to air for 12 days (d). A similar chemical behavior is observed for all studied samples.
spectrum (Fig. 4): on the one hand, QS and IS of the quadrupole doublet of tetravalent tin decrease slowly; on the other hand, the component representing divalent tin decreases in relative intensity, indicating that Sn(II) oxidizes back to Sn(IV). After a week in air at room temperature (Fig. 4), all the samples give Mo¨ssbauer spectra which are practically the same as those observed for the as-prepared samples, showing that the transformations induced by the treatment in He at 673 K are completely reversible upon exposition to air at room temperature. As discussed elsewhere [10], the variation of the IS of the Sn(IV) component upon dehydration in He and rehydration in air agrees with an average lowering of the
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3.2. Structure of the supermicroporous MSU-Sn materials
1.0
0.9
QS [mm/s]
0.8
0.7
0.6
SnO 2
0.5
SnCl 2 SnCl 4
0.4 -0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
IS [mm/s] Fig. 5. 119Sn Mo¨ssbauer quadrupole splitting vs. isomer shift the catalyst samples. The hyperfine parameters for the catalysts samples both asprepared (empty symbols) and dehydrated in He flow at 673 K (filled symbols), grouped by the oxidation state of the tin precursor, are reported.
coordination number of tetravalent tin upon dehydration of the samples and the subsequent restoring of the initial conditions once the samples is exposed to air. It is remarkable that this effect is the most pronounced in the samples prepared from tetravalent tin chloride, where the amount of divalent tin produced is the highest (cf. Table 4 and Fig. 4). These observations suggest that a consistent part of the tin in mesoporous MSU-Sn prepared from Sn(IV) precursors is accessible on the surface and can reversibly vary its coordination number upon dehydration and re-exposition to air at room temperature. In this picture, the reversible reduction of a large part of Sn(IV) to Sn(II) could be related to a particularly stable form of divalent tin in a low-coordination state, probably similar to that observed in silica glasses where the coordination number of Sn(II) is known to take values as low as three [24,25]. Conversely, the small variation of the IS of the Sn(IV) component in the samples prepared from divalent tin chlorides might be in line with the presence of low amounts of surface Sn(IV). In this case, the small variation of the IS and QS is probably due to the presence of small SnO2 particles precipitated on the support in addition to the grafted surface tin component. This hypothesis is also supported by the formation, upon dehydration in He, of smaller quantities of Sn(II) than for the samples prepared from tetravalent tin chloride. Due to the very similar parameters of the surface Sn(IV) and of the SnO2 spectral components, the two components cannot be discerned in the fit and the parameters of the single hyperfine doublet used for the fit are weight averaged between the two components.
The characterization of the materials by XRD and N2 physisorption confirm that the addition of tin to a purely siliceous MSU(I), either as Sn(II) or as Sn(IV) chlorides, appears to only slightly influence the size of the pores, and only a slight decrease of the porous volume is observed after grafting. These variations do not seem to be correlated to the oxidation state or to the presence of hydration water molecules in the tin precursor salts used. The supermicroporous structure of these materials are thus not substantially different from that of the Sn-MSU materials prepared by the introduction of tin as isopropoxide in the initial gel [10]. In the materials prepared by grafting, however, the tin is present mainly on the surface, whereas tin addition in the gel synthesis can result in its partial burial within internal regions where it will be inaccessible to reagents. The amounts of grafted tin, obtained using Sn(II) chlorides, is more than twice that those obtained starting from Sn(IV) chlorides and this difference can be related to the different reactivity of the two cationic forms of tin with the silica surface. FT-IR of adsorbed pyridine clearly shows that the introduction of tin produces purely Lewis acid sites on the surface of the silica. On the other hand, the final amount of Lewis acid sites, measured by NH3 desorption, is virtually the same regardless of the nature of the starting Sn chloride precursor. This result indicates that, when Sn(II) chlorides are used, a higher level of tin incorporation is obtained, but a substantial part of the tin does not originate Lewis acid sites. This lack of correlation between the amount of grafted tin and the number of Lewis acid centers can be well explained by the results of the in situ 119Sn Mo¨ssbauer spectroscopy investigation. In all the materials, the tin is clearly present as Sn(IV) after calcination at 893 K. On the one hand, for the samples prepared from Sn(IV) chlorides, a strong decrease in coordination is observed during dehydration at 673 K in He accompanied by a consequent reduction of the Sn(IV) to Sn(II). The initial conditions are restored by simple expose of the sample to air at room temperature. This behavior, which indicates a high reactivity of the tin species, is in agreement with the presence of isolated grafted tin ions, which, in the dehydrated form, can be regarded as active Lewis acid sites. On the other hand, minor changes in coordination are observed upon dehydration of the samples prepared from Sn(II) chlorides, where a conspicuous amount of tin appears to be present as precipitated tin oxide (SnO2). The latter is largely unreactive and does not contribute to the Lewis acidity of the material. 4. Conclusions Supermicroporous MSU-Sn materials with purely Lewis-type acid sites are effectively obtained by grafting of divalent and tetravalent tin chlorides. Even if larger tin loadings are obtained when Sn(II) salts are used, the
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grafting procedure is more effective when anhydrous SnCl4 is employed, in agreement with the intensity of the IR bands of adsorbed pyridine. In fact, when Sn(II) chlorides are used only part of the tin generates active Lewis acid centers whereas the rest is most probably present as precipitated tin oxide species. Moreover, as resulted from 119Sn Mo¨ssbauer spectral data, a consistent part of the tin in the mesoporous MSU-Sn sample, prepared from Sn(IV) precursors, appears to be accessible to incoming molecules and can reversibly vary its coordination number upon dehydration in He at 673 K and re-exposition to air at room temperature. Acknowledgements Financial support of the MIUR (Ministero dell’Istruzione, Universita` e Ricerca) is gratefully acknowledged. The authors thank A. Talon for the analytical determinations. References [1] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [2] P.T. Tanev, T.J. Pinnavaia, Science 267 (1995) 865. [3] E. Prouzet, T.J. Pinnavaia, Angew. Chem. Int. Ed. Engl. 36 (1997) 516. [4] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schimitt, C.T.-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834. [5] T. Maschmeyer, F. Rey, G. Sankar, J.M. Thomas, Nature 378 (1995) 159.
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[6] A. Tuel, Micropor. Mesopor. Mater. 27 (1999) 151. [7] S.A. Bagshaw, F. Di Renzo, F. Fajula, Chem. Commun. (1996) 2209. [8] S.A. Bagshaw, T. Kemmitt, N.B. Milestone, Micropor. Mesopor. Mater. 22 (1998) 419. [9] M. Casagrande, L. Storaro, M. Lenarda, J. Gersich, L. Stievano, F.E. Wagner, T. Montanari, J. Mater. Chem. 14 (2004) 1010. [10] T.K. Das, K. Chaudhari, A.J. Chandwadkar, S. Sivasanker, J. Chem. Soc. Chem. Commun. (1995) 2495. [11] K. Chaudhari, T.K. Das, P.R. Rajmohanan, K. La´za´r, S. Sivasanker, A.J. Chandwadkar, J. Catal. 183 (1999) 281. [12] T.M. Abdel-Fattah, T.J. Pinnavaia, Chem. Commun. (1996) 665. [13] E.P. Parry, J. Catal. 2 (1963) 371. [14] G. Grosse, MOS-90, Version 2.2, Technical University of Munich, 1992. [15] C. Boissie`re, A. Larbot, A. van der Lee, P.J. Kooyman, E. Prouzet, Chem. Mater. 12 (2000) 2902. [16] C. Boissie`re, A. Larbot, C. Bourgaux, E. Prouzet, C.A. Bunton, Chem. Mater. 13 (2001) 3580. [17] R. Ryoo, S. Jun, J.M. Kim, M.J. Kim, Chem. Commun. (1997) 2225. [18] Y. Teraoka, S. Ishida, A. Yamasaki, N. Tomonaga, A. Yasutake, J. Izumi, I. Moriguchi, S. Kagawa, Micropor. Mesopor. Mater. 48 (2001) 151. [19] P.A. Webb, C. Orr, Analytical Methods in Fine Particle Technology, Micromeritics Instrument Corporation, Norcross, USA, 1997. [20] F. Lo´nyi, J. Valyon, Micropor. Mesopor. Mater. 47 (2001) 293. [21] B. Chakraborty, B. Viswanathan, Catal. Today 49 (1999) 253. [22] S. Damyanova, M.A. Centeno, L. Petrov, P. Grange, Spectrochim. Acta Part A 57 (2001) 2495. [23] J.A. Johnson, C.E. Johnson, D. Holland, A. Sears, J.F. Bent, P.G. Appleyard, M.F. Thomas, A.C. Hannon, J. Phys.: Condens. Matter 12 (2000) 213. [24] J.F. Bent, A.C. Hannon, D. Holland, M.M.A. Karim, J. Non-Cryst. Solids 232–234 (1998) 300. [25] D. Benne, C. Ru¨ssel, O. Lehmann, M. Menzel, D. Niemeier, K.D. Becker, J. Non-Cryst. Solids 318 (2003) 202. [26] K.F.E. Williams, C.E. Johnson, O. Nikolov, M.F. Thomas, J.A. Johnson, J. Greengrass, J. Non-Cryst. Solids 242 (1998) 183.
Synthesis and structural characterization of MSU-type silica–tin molecular sieves: Post-synthesis grafting of tin chlorides Manuela Casagrande a, Elisa Moretti a, Loretta Storaro a, Maurizio Lenarda Jacopo Gersich a, Lorenzo Stievano b, Friedrich E. Wagner c b
a,*
,
a INSTM UdR di Venezia, Dipartimento di Chimica, Universita` Ca’ Foscari, Via Torino 155b, 30172 Mestre Venezia, Italy Laboratoire de Re´activite´ de Surface UMR 7609, Universite´ Pierre et Marie Curie – Paris VI, Casier 178, 4, Place Jussieu, 75252 Paris Cedex 05, France c Physik-Department E15, Technische Universita¨t Mu¨nchen, D-85747 Garching, Germany
Received 1 September 2005; received in revised form 28 November 2005; accepted 7 December 2005 Available online 25 January 2006
Abstract Supermicroporous Sn-incorporated MSU-type silica molecular sieves, with different amounts of tin, have been synthesized using divalent and tetravalent Sn chlorides as grafting agents. The structural and chemical properties of the prepared materials, have been studied using a variety of techniques such as X-ray diffraction, N2 physisorption, FT-IR spectroscopy of chemisorbed pyridine, temperature programmed desorption of ammonia (NH3-TPD) and 119Sn Mo¨ssbauer spectroscopy. The introduction of tin produced purely Lewis acid sites on the silica surface. The amount of grafted tin was found to vary depending on the Sn precursor salt. The grafting procedure was more effective when anhydrous SnCl4 was used. A consistent part of the tin present in the MSU-Sn samples, that resulted accessible to incoming molecules, was found to reversibly change coordination number upon dehydration in He at 673 K and re-exposition to air at room temperature. 2005 Elsevier Inc. All rights reserved. Keywords: Tin–silica molecular sieves; Supermicroporous; Sn-MSU; Grafting; Surface acidity
1. Introduction The synthesis of mesoporous silicas MCM-41 [1], HMS [2] and MSU [3] by cooperative assembly of silica and surfactants, has attracted a great deal of research interest in recent years. These mesoporous materials have a large pore size (1.5–10 nm) and a high internal surface area (up to 1200 m2/g) [4,5]. MCM-41 and HMS mesostructured silicas, doped with heteroatoms, have been studied for their potential interest as catalysts and materials of this type, containing Mn, Al, B, Ga, Fe or Ti, V, Sn, Zr, have been reported in the literature [6–8]. A small number of these studies deals with pure siliceous MSU mesostructures or ones partially substituted with het-
*
Corresponding author. E-mail address: [email protected] (M. Lenarda).
1387-1811/$ - see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.12.001
eroatoms, in spite of their high thermal and hydrothermal stability [9,10]. Tin substituted MCM-41 and HMS have been reported [11–13]. Partial substitution of Si atoms with Sn is expected to impart Lewis-type acidic properties to these materials which makes them potentially useful for catalytic purposes. Some of us have recently prepared supermicroporous Sn–Si molecular sieves, with different amounts of tin, using poly-(ethylene oxide)-type surfactants as structure directing agents and a silicon alkoxide as precursor [10]. This research work continued with the preparation and characterization of MSU type silica–tin mesostructures with a narrow pore distribution, via post-synthesis grafting with Sn(II) or Sn(IV) chlorides, used both in anhydrous and hydrous form. It is known that the incorporation of heteroelements into pure mesoporous silicas can also be carried out by ‘‘post-synthesis’’ chemical grafting [7]. Nevertheless it was
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found that the resulting materials are quite different from those prepared by ‘‘in synthesis’’ methods [7]. The ‘‘in synthesis’’ method, in fact, was found to generate mesostructures where the active sites are present not only within the channels, and therefore accessible to incoming molecules, but can also be located within the wall and therefore result inaccessible. On the other hand, when the grafting procedure is used, the grafted species are expected to lay on the surface of the mesopores and therefore be accessible, in principle, to any guest molecule diffusing in the pores. 2. Experimental and synthesis
flow = 15 l/min, auxiliary argon flow = 0.5 ml/min, argon flow to the nebulizer = 0.65 ml/min and RF power = 1400 W. 2.2.2. Nitrogen adsorption/desorption Adsorption–desorption experiments using N2 were carried out at 77 K on a Micromeritics ASAP 2010. Before each measurement the samples were first outgassed at 423 K for 12 h at 0.6 Pa and then at room temperature for 2 h at 5 · 10 7 Pa. The N2 isotherms were used to determine both the specific surface areas (S.A.) using the BET equation and the specific pore volume (Vs). Vs was calculated at p/p0 = 0.98.
2.1. Synthesis of MSU-Sn samples Pure siliceous MSU(I) was prepared according to the literature [14,15]. Grafting, to prepare MSU-Sn materials, was carried out both by using divalent (SnCl2 98%, Aldrich or SnCl2 Æ 2H2O > 98%, Acros) and tetravalent (SnCl4 99%, Aldrich or SnCl4 Æ 5H2O 99%, Aldrich) tin chloride precursors. 2.1.1. Grafting with Sn(II) chloride Different portions of siliceous MSU(I) were first treated at 383 K overnight and then suspended under argon in absolute ethanol (1 g/100 ml). Anhydrous SnCl2 or SnCl2 Æ 2H2O (27 mmol/g of support in ethanol) were added and the mixtures were refluxed for 1 h in argon [16] to obtain the MSU-Sn2a and MSU-Sn2 samples, respectively. The white powders obtained were centrifuged, washed with ethanol dried in air at 343 K for 12 h and ground to a fine powder (40 mesh). The materials were then calcined in air up to 473 K (3 K/min) and kept at this temperature for 6 h, and subsequently up to 893 K (3 K/min) and kept at this temperature for a further 6 h. 2.1.2. Grafting with Sn(IV) chloride Different portions of siliceous MSU(I) were first treated at 383 K overnight and then suspended in isopropanol under argon (1 g/100 ml). Anhydrous SnCl4 or SnCl4 Æ 5H2O (27 mmol/g of support in isopropanol for both) were used for the grafting [17] and the mixtures were refluxed for 7 h in argon to obtain the MSU-Sn4a and MSU-Sn4 samples, respectively. The white powders obtained were centrifuged, washed with isopropanol, dried in air at 343 K for 12 h and ground to a fine powder (40 mesh). The materials were then calcined in air first up to 473 K (3 K/min) and kept at this temperature for 6 h, and then subsequently up to 893 K (3 K/min) and kept at this temperature for another 6 h. 2.2. Characterization methods 2.2.1. Elemental analysis by optic emission spectrometer (ICP-OES) The analyses were performed with a Perkin–Elmer Optima 3100 XL. The operative parameters were: argon
2.2.3. FT-IR spectroscopy measurements The IR spectra were recorded on a Nicolet Magna 750 FT Instrument, using pressed discs of pure catalyst powders. The FT-IR spectra of adsorbed pyridine were carried out in an evacuable Pyrex cell with CaF2 windows. Pyridine adsorption experiments and calculations were carried out as described in the literature [18]. The samples were ground to a fine powder. A total of 30 mg of the sample were pressed at 4 ton, in order to get a self-supporting wafer. The wafers were mounted in the holder of the IR cell and degassed by heating at 673 K and 1.3 · 10 2 Pa. After cooling to room temperature, pyridine was adsorbed on the samples. The spectra were recorded after degassing the wafers under vacuum at 423 K and 1.3 · 10 2 Pa for 2 h. 2.2.4. (NH3-TPD) temperature programmed desorption of ammonia The total acidity of the samples was determined by temperature-programmed desorption of ammonia (NH3-TPD). Measurements were carried out by an AUTOCHEM 2910 automatic temperature programmed desorption apparatus (Micromeritics). About 100 mg of sample were treated at 773 K in helium flow for 90 min. The temperature was reduced to 393 K and the sample was kept in a flow of NH3 (1% in helium) for 30 min and then in helium flow (40 ml/min) for 60 min. The amount of desorbed NH3 was determined by heating the sample at 25 K/min up to 923 K in helium flow (40 ml/min). 2.2.5. X-ray diffraction spectra X-ray diffraction spectra were measured with a Philips diffractometer using Cu Ka radiation (Cu Ka k = ˚ ). The samples were disc shaped pressed powders. 1.54184 A 2.2.6.
119
Sn Mo¨ssbauer spectroscopy Sn Mo¨ssbauer measurements were carried out at 4.2 K with both the sample and the 119Sn source (CaSnO3) in a liquid helium bath cryostat. For the in situ measurements, the samples were reduced in special glass reactors and then transferred to cylindrical glass ampoules attached to the reactors. Before sealing off the ampoules the atmosphere was changed to helium in order to ensure that temperature equilibrium was rapidly established when the 119
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samples were cooled down to 4.2 K for the Mo¨ssbauer measurement. A NaI scintillation detector was used for the detection of c-rays. The spectra were analyzed by a least-square fitting routine using the MOS-90 computer program [19]. A Gaussian broadening of the Lorentzian lines into Voigt profiles was allowed, in order to take in account a Gaussian distribution of the quadrupole splitting. The resulting spectral parameters, i.e., the mean quadrupole splitting (QS), the isomer shift (IS), and the relative resonance areas (Area) of the different spectral components are listed in Table 4. The isomer shifts are given relative to the CaSnO3 source.
263
a
Intensity (a.u.)
b
c
d
3. Results and discussion 3.1. Material characterization
2
3.1.1. Chemical composition Tin content of all four samples, determined by elemental analysis with an ICP-Optical Emission Spectrometer, is reported in Table 1 as the Sn/Si % molar ratio. From the ICP values, it appears that grafting on the siliceous matrix with Sn(II) salts is more efficient than with the Sn(IV) ones. No chlorides are detected in the sample, indicating their complete removal, by washing. 3.1.2. X-ray diffraction Typical powder X-ray diffraction patterns are shown in Fig. 1. The XRD profile is characteristic of MSU type mesostructured silicas for the purely siliceous starting MSU(I) sample (Fig. 1a) with the position of the peak indicating a d100 spacing of 4.2 nm. The XRD patterns of the MSU-Sn grafted samples (Fig. 1b–d) also show a similar single peak corresponding to a d100 spacing of 4.2 nm (see Table 2). The X-ray powder diffraction patterns of all the samples are nearly identical, and only a slight decrease of the intensity of the d100 profile compared to the purely siliceous sample is observed when the aqueous salts of the heteroatom is employed for the grafting. The d100 distance appears to slightly decrease in the case of the MSU-Sn4 and -Sn4a samples. 3.1.3. Nitrogen adsorption/desorption N2 adsorption–desorption isotherms of the MSU-Sn samples are similar to those of the pure siliceous MSU(I)
Table 1 Composition of some MSU-Sn selected samples determined by ICP-OES Sample
Tin precursor
(Sn/Si)ICP [mol%]a
MSU-Sn2 MSU-Sn2a MSU-Sn4 MSU-Sn4a
SnCl2 Æ 2H2O SnCl2 SnCl4 Æ 5H2O SnCl4
2.21 1.64 0.83 0.80
a Tin content is calculated supposing that all the silicon and tin were present as SiO2 and SnO2.
3
4
2θ
Fig. 1. Powder X-ray diffraction patterns of samples: (a) MSU(I), (b) MSU-Sn4a, (c) MSU-Sn2, (d) MSU-Sn4.
Table 2 Textural properties of MSU-Sn materials Sample
S.A. [m2/g]
Vs [cm3/g]
d100 [nm]
MSU(I) MSU-Sn2 MSU-Sn2a MSU-Sn4 MSU-Sn4a
1090 948 909 895 918
0.56 0.44 0.44 0.41 0.42
4.2 4.2 4.2 4.2 4.1
S.A. = BET specific surface area. Vs = pore specific volume at p/p0 = 0.98. d100 = interplanar distance.
sample and can be classified as a mixture of Type I isotherm, typical of a microporous adsorbent, and a Type IV, typical of a mesoporous material [10]. Similar features were observed by Boissie`re et al. and are characteristic of a microporous material with pores ranging between 1.5 and 2.0 nm that are usually classified as ‘‘supermicroporous’’ [14]. The absence of hysteresis in the N2 desorption isotherms of all samples suggests that the architecture of the individual pores is uniform and does not show bottlenecking. The textural properties of the synthesized materials are summarized in Table 2. Both the specific surface area and the specific pore volume appear to slightly decrease after grafting and the phenomenon seems independent from the tin species used. 3.1.4. Acidity evaluation by NH3-TPD and FT-IR spectroscopy of adsorbed pyridine The relative amount and the nature of the acid sites of both pure silica precursors and Sn grafted samples were studied by NH3-TPD and FT-IR spectroscopy of adsorbed pyridine. Ammonia is a strong base (pKb 5) that reacts even with extremely weak acid sites which therefore makes NH3-TPD a useful technique both for evaluating the
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relative amount of acid sites present on a surface as well as to roughly estimate their acid strength. This method, however, cannot discriminate Lewis from Bro¨nsted acid sites [20]. On the other hand, pyridine is a significantly weaker base (pKb 9), that does not react with some of the weak sites able to react with ammonia, but the study of the IR spectrum of chemisorbed pyridine is very useful to distinguish Lewis from Bro¨nsted acid sites [18]. Fig. 2 shows the NH3-TPD profile of both an all silica Si-MSU(I) sample and of the grafted MSU-Sn2 one. After deconvolution, both curves resulted composed of two peaks, one at a lower (LT peak, 478–498 K) and the other at a higher temperature (HT peak, 573 K). The total amount of desorbed ammonia and the amounts ascribable to the ammonia desorption at the high (HT) and low temperatures (LT), respectively, are summarized in Table 3. First of all, it can be noticed that the addition of tin influences the total peak area, i.e., the total amount of acid sites, the strongest increase being observed for the LT peak, while the HT peak increases only moderately. No correlation is found, however, between the total peak area and the tin loading. The HT peak is usually attributed to the desorption of ammonia from strong Bro¨nsted and Lewis acid sites. The attribution of the LT peak, on the contrary, is still the subject of much debate in literature [21]. In these types of materials, however, it can be ascribed to the weak Lewis acid sites. The presence of Lewis acid sites, in fact, is detected in the infrared spectra of adsorbed pyridine.
Fig. 2. TPD profile of desorbed ammonia from samples Si-MSU(I) (bottom) and MSU-Sn2 (top).
Table 3 Calculated amount of total acid sites and desorbing at low (LT) and high (HT) temperature
MSU(I) MSU-Sn2 MSU-Sn2a MSU-Sn4 MSU-Sn4a
Acid sites (lmol/g)
LT peak (lmol/g)
HT peak (lmol/g)
50 129 124 128 140
13 80 62 68 74
37 49 62 60 66
0.2 MSU-Sn2
Assorbance (a.u.)
264
MSU-Sn2a
MSU-Sn4
MSU-Sn4a
1650 1600 1550 1500 1450 1400
Wavenumber (cm -1) Fig. 3. FT-IR spectra of pyridine adsorbed on the MSU-Sn samples after evacuation at 423 K.
The infrared spectra of pyridine adsorbed on tin grafted MSU samples, after evacuation at 423 K, are shown in Fig. 3. As shown in a previous paper the spectrum of the purely siliceous MSU(I) support shows two bands at 1445 cm 1 and 1596 cm 1, attributed to pyridine physisorbed through hydrogen bond [21,13] and a band at 1577 cm 1 ascribed either to the chemisorption of the pyridine on weak Lewis acid centers [22] or to its interaction with the hydroxyls on the MSU(I) surface [23]. The described bands are present only after evacuation at 298 K but disappear upon evacuation at 423 K. The bands characteristic of pyridine adsorbed on Lewis acid sites (1453 and 1614 cm 1), however, were found in the spectra of all the tin grafted MSU samples even after evacuation at 423 K, as shown in Fig. 3. This observation is in agreement with the possible presence of tin, either tetra-coordinated substituting for silicon in the SiO2 structure or occupying surface defect sites. Actually, both these situations produce coordinatively insaturated adsorption sites. These bands are more intense in the spectra of the MSU-Sn4 and MSU-Sn4a samples, obtained by grafting by Sn4+ salts and suggest the presence of a higher number of acid sites when compared to the samples obtained by grafting with Sn2+ precursors. This result is quite surprising, if one considers that, as reported in Table 1, samples prepared by grafting with Sn4+ salts showed lower Sn/Si values. Moreover, for both divalent
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and tetravalent tin precursors, these bands result more intense in samples grafted by anhydrous salts than in those prepared from hydrated salts.
100
3.1.5. 119Sn Mo¨ssbauer spectroscopy The 119Sn Mo¨ssbauer spectra of all the as-prepared samples (Table 4 and Fig. 4) consist of a single narrow quadrupole doublet close to zero velocity corresponding to the presence of tetravalent tin. The hyperfine parameters of the quadrupole doublet, however, are slightly different from the values observed for crystalline SnO2 (QS = 0.49 mm/s, IS = 0.02 mm/s), allowing one to group the samples in pairs:
Dehydration in He flow at 673 K induces a noticeable change in the Mo¨ssbauer spectra, and an additional quadrupole doublet with an IS of 3 mm/s and a QS of 2 mm/s appears (Fig. 4), giving evidence of the rather surprising reduction of a noticeable fraction of Sn(IV) into Sn(II). This reduction is associated to a perceptible change in the color of the samples from white to light brown. The hyperfine parameters of divalent tin are similar to those observed for Sn(II) in glasses [24–26]. Moreover, this treatment produces both a substantial increase in the IS of Sn(IV), which becomes positive for all samples, as well as a slight increase in QS (Fig. 5). The greatest variation on the hyperfine parameters is seen in the samples prepared from tetravalent tin chlorides. Exposition of the dehydrated samples to air at room temperature for a few hours again changes the Mo¨ssbauer Table 4 119 Sn Mo¨ssbauer parameters at 4.2 K for the MSU-Sn samples Sample
Treatment
QS [mm/s]
MSU-Sn2
As made He/673 K
0.61(1) 0.90(1) 2.05(2)
MSU-Sn2a
As made He/673 K
MSU-Sn4
MSU-Sn4a
a
ISa [mm/s]
Area [%]
Chemical state of tin
0.07(1) 0.04(1) 3.21(1)
100 92(1) 8(1)
Sn(IV) Sn(IV) Sn(II)
0.62(1) 0.70(1) 2.14(2)
0.08(1) 0.01(1) 3.09(1)
100 82(1) 18(1)
Sn(IV) Sn(IV) Sn(II)
As made He/673 K
0.65(1) 0.65(1) 2.45(1)
0.20(1) 0.04(1) 3.02(2)
100 73(1) 27(1)
Sn(IV) Sn(IV) Sn(II)
As made He/673 K
0.64(1) 0.64(1) 2.60(1)
0.21(1) 0.10(1) 2.99(2)
100 46(1) 54(1)
Sn(IV) Sn(IV) Sn(II)
Isomer shift referred to the CaSnO3 source.
99
(a) 100.0
99.5
Relative transmission [%]
(I) The two samples prepared from divalent tin chloride, either hydrated or anhydrous, have a QS of 0.6 mm/s and an average IS of 0.07 mm/s, both relatively close to those of pure SnO2. (II) The two samples prepared from tetravalent tin chloride, which have a QS of 0.7 mm/s and an average IS of 0.20 mm/s, the latter significantly more negative than that of SnO2.
265
(b) 100
98
(c) 100
(d) 98
-6
-3
0
3
6
Velocity [mm/s] Fig. 4. 119Sn Mo¨ssbauer spectra at 4.2 K of sample MSU-Sn4 as-prepared (a), after dehydration in situ in He flow at 673 K (b), after exposition to air for 24 h (c) and after exposition to air for 12 days (d). A similar chemical behavior is observed for all studied samples.
spectrum (Fig. 4): on the one hand, QS and IS of the quadrupole doublet of tetravalent tin decrease slowly; on the other hand, the component representing divalent tin decreases in relative intensity, indicating that Sn(II) oxidizes back to Sn(IV). After a week in air at room temperature (Fig. 4), all the samples give Mo¨ssbauer spectra which are practically the same as those observed for the as-prepared samples, showing that the transformations induced by the treatment in He at 673 K are completely reversible upon exposition to air at room temperature. As discussed elsewhere [10], the variation of the IS of the Sn(IV) component upon dehydration in He and rehydration in air agrees with an average lowering of the
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3.2. Structure of the supermicroporous MSU-Sn materials
1.0
0.9
QS [mm/s]
0.8
0.7
0.6
SnO 2
0.5
SnCl 2 SnCl 4
0.4 -0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
IS [mm/s] Fig. 5. 119Sn Mo¨ssbauer quadrupole splitting vs. isomer shift the catalyst samples. The hyperfine parameters for the catalysts samples both asprepared (empty symbols) and dehydrated in He flow at 673 K (filled symbols), grouped by the oxidation state of the tin precursor, are reported.
coordination number of tetravalent tin upon dehydration of the samples and the subsequent restoring of the initial conditions once the samples is exposed to air. It is remarkable that this effect is the most pronounced in the samples prepared from tetravalent tin chloride, where the amount of divalent tin produced is the highest (cf. Table 4 and Fig. 4). These observations suggest that a consistent part of the tin in mesoporous MSU-Sn prepared from Sn(IV) precursors is accessible on the surface and can reversibly vary its coordination number upon dehydration and re-exposition to air at room temperature. In this picture, the reversible reduction of a large part of Sn(IV) to Sn(II) could be related to a particularly stable form of divalent tin in a low-coordination state, probably similar to that observed in silica glasses where the coordination number of Sn(II) is known to take values as low as three [24,25]. Conversely, the small variation of the IS of the Sn(IV) component in the samples prepared from divalent tin chlorides might be in line with the presence of low amounts of surface Sn(IV). In this case, the small variation of the IS and QS is probably due to the presence of small SnO2 particles precipitated on the support in addition to the grafted surface tin component. This hypothesis is also supported by the formation, upon dehydration in He, of smaller quantities of Sn(II) than for the samples prepared from tetravalent tin chloride. Due to the very similar parameters of the surface Sn(IV) and of the SnO2 spectral components, the two components cannot be discerned in the fit and the parameters of the single hyperfine doublet used for the fit are weight averaged between the two components.
The characterization of the materials by XRD and N2 physisorption confirm that the addition of tin to a purely siliceous MSU(I), either as Sn(II) or as Sn(IV) chlorides, appears to only slightly influence the size of the pores, and only a slight decrease of the porous volume is observed after grafting. These variations do not seem to be correlated to the oxidation state or to the presence of hydration water molecules in the tin precursor salts used. The supermicroporous structure of these materials are thus not substantially different from that of the Sn-MSU materials prepared by the introduction of tin as isopropoxide in the initial gel [10]. In the materials prepared by grafting, however, the tin is present mainly on the surface, whereas tin addition in the gel synthesis can result in its partial burial within internal regions where it will be inaccessible to reagents. The amounts of grafted tin, obtained using Sn(II) chlorides, is more than twice that those obtained starting from Sn(IV) chlorides and this difference can be related to the different reactivity of the two cationic forms of tin with the silica surface. FT-IR of adsorbed pyridine clearly shows that the introduction of tin produces purely Lewis acid sites on the surface of the silica. On the other hand, the final amount of Lewis acid sites, measured by NH3 desorption, is virtually the same regardless of the nature of the starting Sn chloride precursor. This result indicates that, when Sn(II) chlorides are used, a higher level of tin incorporation is obtained, but a substantial part of the tin does not originate Lewis acid sites. This lack of correlation between the amount of grafted tin and the number of Lewis acid centers can be well explained by the results of the in situ 119Sn Mo¨ssbauer spectroscopy investigation. In all the materials, the tin is clearly present as Sn(IV) after calcination at 893 K. On the one hand, for the samples prepared from Sn(IV) chlorides, a strong decrease in coordination is observed during dehydration at 673 K in He accompanied by a consequent reduction of the Sn(IV) to Sn(II). The initial conditions are restored by simple expose of the sample to air at room temperature. This behavior, which indicates a high reactivity of the tin species, is in agreement with the presence of isolated grafted tin ions, which, in the dehydrated form, can be regarded as active Lewis acid sites. On the other hand, minor changes in coordination are observed upon dehydration of the samples prepared from Sn(II) chlorides, where a conspicuous amount of tin appears to be present as precipitated tin oxide (SnO2). The latter is largely unreactive and does not contribute to the Lewis acidity of the material. 4. Conclusions Supermicroporous MSU-Sn materials with purely Lewis-type acid sites are effectively obtained by grafting of divalent and tetravalent tin chlorides. Even if larger tin loadings are obtained when Sn(II) salts are used, the
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