- Email: [email protected]
Zr molar ratio on the synthesis of Zr-based mesoporous molecular sieves
Materials Chemistry and Physics 114 (2009) 139–144
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Effect of the Si/Zr molar ratio on the synthesis of Zr-based mesoporous molecular sieves P. Salas a,∗ , J.A. Wang b , H. Armendariz c , C. Angeles-Chavez c , L.F. Chen b a
Centro de Física Aplicada y Tecnología Avanzada, Universidad Nacional Autónoma de México, Apartado Postal 1-1010, Querétaro 76000, Mexico Escuela Superior de Ingeniería Química e Industrias Extractivas, Instituto Politécnico Nacional, Av. Politécnico S/N, Col. Zacatenco, 07738 México D.F., Mexico c Instituto Mexicano del Petróleo, Eje Lázaro Cárdenas 152, 07730 México D.F., Mexico b
a r t i c l e
i n f o
Article history: Received 18 June 2007 Received in revised form 26 August 2008 Accepted 29 August 2008 Keywords: Zr-MCM-41 Mesoporous molecular sieves Zirconium FTIR Acidity
a b s t r a c t Highly ordered Zr-based mesoporous molecular sieves were synthesized via a surfactant-templated method and the effect of the Si/Zr molar ratio on the crystalline structure, textural properties and surface acidity were studied by XRD, FTIR, TEM and 29 Si MAS-NMR techniques. FTIR spectra show that the intensity of the band around 890 cm−1 which corresponds to the vibration of Si–O–Zr bond was increased with increasing of the zirconium content, therefore, this band may be used as an indicator of the degree of the zirconium incorporation into the Si-framework. When the zirconium content increased in the materials, the Q3 /Q4 value obtained from 29 Si MAS-NMR was linearly increased, whereas, the intensity the XRD peaks was gradually reduced; as a result, the pore wall thickness of the resultant materials was gradually increased, the surface area and the structural regularity were diminished. In order to obtain Zr-MCM-41 with highly ordered mesostructure and large surface area, proper Si/Zr molar ratio is a key factor, e.g., Si/Zr should be no less than 10. It was also found that the Brønsted acid sites which resulted from charge unbalance or local structure deformation due to the Zr4+ incorporation into the vicinity of the hydroxyls carrying silicon were created on the surface of the Zr-MCM-41 solids; strong Brønsted acidity could be formed on the solid with high zirconium content. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In the history of the porous materials development, a breakthrough took place in 1992 when a new family of mesoporous molecular sieves called as M41S was synthesized by the scientists in Mobil company using cationic or anionic surfactants as synthesis template [1]; this family of mesoporous materials includes three members, Si-MCM-41, Si-MCM-48 and Si-MCM-50, and all of these have highly ordered pore channel system together with high surface area, big pore diameter and large pore volume. This synthesis idea directly results in the discoveries of a wide range of mesoporous materials, for example, mesostructured metals [2–4], carbon [5,6], rare metal oxide [7], transition metal oxides [8–13], mixed metal oxides [14] and sulfides [15]. Since the pure Si-MCM-41 possesses a neutral framework, which limits its applications as catalyst or catalyst support. Consequently, isomorphous substitution of silicon with other transition metals has been found to be an effective strategy in creating catalytically active sites and anchoring sites for active molecules in
∗ Corresponding author. Tel.: +52 55 56234163. E-mail address: [email protected] (P. Salas). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.08.086
the design of new heterogeneous catalysts. Various transition metals such as Al3+ [16,17], Fe3+ [16,18], Ga3+ [19], Ti4+ [20], Zr4+ [21] and V5+ [22] incorporated mesoporous molecular sieves have been synthesized. Among these materials, zirconium incorporated mesoporous molecular sieves are particularly interest because of the important applications of Zr-based materials in a wide variety of areas in heterocatalysis, petroleum processing, biotechnology, etc. In the synthesis, the parameters of pH value, reaction temperature, type and concentration of synthetic template, metal precursors and the others strongly influence the properties of the resultant Zr-containing mesoporous materials. Depending on the synthetic conditions, different types of Zr-based nanomaterials could be obtained where zirconium ions are usually in a tetrahedral coordination with a different proportion between surface and bulk [23–25]. For example, ZrO2 –SiO2 mesocomposites with worm-like pores could be obtained via a sol–gel method [25]. In the present work we are synthesized zirconium containing mesoporous silicas with high zirconium content and mesoporous hexagonal-type structure. The Si/Zr molar ratio ranged from 40 to 5. More attention will be focused, mainly in the samples with higher Zr content, on maintain of mesoporous structural regularity and textural properties. In order to enhance the surface acidity of the resultant
140
P. Salas et al. / Materials Chemistry and Physics 114 (2009) 139–144
Zr-MCM-41 mesoporous materials, the tetrahedral ZrO4 groups were “in situ” sulfated” adding a desired amount of SO4 2− in the synthesis media. 2. Experimental 2.1. Synthesis of Zr-Si-MCM-41 The Zr-MCM-41 solids were prepared using tetraethyl orthosilicate (TEOS) as Si precursor and zirconium-n-propoxide (70% in propanol) as Zr source, along with cetyltrimethylammonium bromide (CTAB) as synthesis template. The typical preparation procedure of a Zr-MCM-41 sample with a molar ratio of Si/Zr = 20 is as follows: First of all, two solutions were prepared: the first solution was made by adding 2.2 ml of zirconium-n-propoxide and 22 ml of TEOS with stirring; the second solution was made by adding 11.4 g of CTAB and 4.7 g of (NH4 )2 SO4 into hot water (around 50 ◦ C) with stirring, followed by the addition of 145 ml of NH3 ·H2 O (28 wt.%). Then, the first solution was added, drop by drop, into the second solution. During the addition, the mixture was vigorously agitated for about 2 h, until a gel was formed. The resultant gel was loaded into a stoppered Teflon bottle without stirring and heated at 100 ◦ C for 72 h. After cooling to room temperature, the resulting solid product was recovered by filtration and was washed for four times with 500 ml deionized water. The white solid obtained was dried in air at 100 ◦ C for 24 h. Finally, the sample was calcined at 600 ◦ C for 6 h in air. The heating rate was 1 ◦ C min−1 . 2.2. X-ray diffraction (XRD) analysis The low angle X-ray diffraction patterns of the samples were measured in a D-500 SIEMENS diffractometer with a graphite secondary beam monochromator to obtain a monochromatic Cu K␣1 radiation, and the evaluation of the diffractograms was made by DIFFRAC/AT software. The scanning was made from 1.5 to 10, 2 step size of 0.02 and step time of 2 s. In order to avoid the problem of illuminated area at low 2 angle, all the samples were measured using the same sample holder. In this way the hexagonal reflection (1 0 0) positions as well as the intensities are directly comparative. Position correction was made using the NIST standard reference material 675.
Fig. 1. XRD patterns of the as-made Zr-MCM-41 solids. The number on the curves indicates the SiO2 /ZrO2 molar ratio composition of the solids.
3. Results and discussion
2.3. N2 adsorption measurement
3.1. XRD analysis
The specific surface area, pore volume and pore size distribution of the samples were measured in a Digisorb 2600 equipment by using low temperature N2 physisorption isotherms. Before the measurement the sample was evacuated at 350 ◦ C under vacuum condition. The surface area was calculated using the BET method based on the adsorption data in the partial pressure P/P0 range from 0.01 to 1. The mesopore volume was determined from the N2 adsorbed at a P/P0 = 0.4.
Fig. 1 shows the XRD patterns of the as-made zirconiumcontaining solids. All the samples have four well-defined diffraction peaks, which correspond to (1 0 0), (1 1 0) (2 0 0) and (2 1 0) reflections of the solid, respectively. The XRD patterns clearly indicate that the long-range order of mesoporous molecular sieves with hexagonal framework was formed in the materials with low zirconium content or high Si/Zr molar ratio. These XRD patterns are very similar to the one shown in the pure Si-MCM-41 sample, indicating that the small amount zirconium incorporated into the Si-MCM-41 framework does not strongly modify the structure of Si-MCM-41. However, it is observed that as the SiO2 /ZrO2 molar ratio decreases, or zirconium content increases, intensities of the XRD peaks gradually diminish, showing reduction of the structural ordering. For the sample containing high zirconium content, e.g., Si/Zr = 5, the peaks related to (1 1 0), (2 0 0) and (2 1 0) disappeared completely, which indicates that the hexagonal lattice structure is partially collapsed, forming wormhole-like pore system. As shown in Fig. 2, when the as-made samples were calcined at 600 ◦ C, all the diffraction peaks became sharper compared to that of the as-synthesized samples, even for the sample with high zirconium content, SiO2 /ZrO2 = 5, the diffraction peaks at high 2 positions reappeared, this indicates that the structural regularity was improved after calcination. It seems that formation of the Zr-MCM-41 framework can be extended from the hydrothermal treatment procedure into the calcination step. Calcination may be favourable to the structure reconstruction, thus the structural ordering of the calcined solid is further improved.
2.4. Solid state nuclear magnetic resonance (29 Si MAS-NMR) Solid state 29 Si MAS-NMR spectra were recorded on a Bruker 400 MHz spectrometer at a frequency of 79.49 MHz spinning 7.5 kHz, using pulses at 90 s intervals and 4 mm zirconia rotors. The number of accumulations was 500. All the measurements were carried out at room temperature. For the analysis of 29 Si, tetramethylsilane (0 ppm) was used as standard reference to obtain the chemical shift of the solid materials. 2.5. FTIR spectroscopy of pyridine adsorption To evaluate and analyze the strength and types of acid sites, pyridine adsorption on the samples were performed on a 170-SX Fourier transform infrared (FTIR) spectrometer in the temperature ranging from 25 to 400 ◦ C. Before pyridine adsorption, the sample in a vacuum was heated to 300 ◦ C in order to eliminate the adsorbed water or impurities on the surface, and then cooled to room temperature. Afterwards, the solid wafer was exposed to pyridine, by breaking, in the spectrometer cell, a capillary containing 50 l liquid pyridine. The IR spectra were recorded in various conditions by increasing cell temperature from 25 to 400 ◦ C. 2.6. Transmission electron microscopy Transmission electron microscopy, TEM was performed in JEM-2200FS transmission electron microscope with accelerating voltage of 200 kV. The microscope is equipped with a Schottky-type field emission gun and an ultra high resolution (UHR) configuration (Cs = 0.5 mm; Cc 1.1 mm; point to point resolution, 0.19 nm) and in-column energy filter omega-type. The samples were grounded, suspended in isopropanol at room temperature, and dispersed with ultrasonic agitation; then, an aliquot of the solution was dropped on a 3 mm diameter lacey carbon copper grid.
3.2. Textural properties Fig. 3 shows the loops of the N2 adsorption–desorption isotherms as well as the pore diameter distribution of the samples.
P. Salas et al. / Materials Chemistry and Physics 114 (2009) 139–144
Fig. 2. XRD patterns of the Zr-MCM-41 solids calcined at 600 ◦ C. The number on the curves indicates the SiO2 /ZrO2 molar ratio composition of the solids.
Fig. 3. Loops of the N2 adsorption–desorption isotherms (left) and the pore diameter distribution of the samples (right). The SiO2 /ZrO2 molar ratio composition of the samples is shown on curves.
141
There are clearly defined four stages in the loops of the N2 adsorption–desorption isotherms, which are typical of MCM-41type mesoporous materials. Based on the IUPAC classification [26], the loops of the N2 adsorption–desorption isotherms belong to type IV profiles. In all the samples, the presence of framework-confined mesoporous is indicated by the adsorption step centered in the relative pressure p/p0 region from 0.3 to 0.35. The sharp step at this region reveals the highly ordered mesoporous systems. It is found that the shape and the narrow sharpness of the loops practically do not vary with the zirconium content. As the zirconium content increases in the Zr-MCM-41 materials, only the nitrogen adsorbed volume, associated to capillary condensation within mesopores, gradually decreases. It is worthwhile to note that although is clear the decrease of the height of capillary condensation (pore filling) step in the isotherm of the calcined Zr-MCM-41 sample with the higher Zr content (SiO2 /ZrO2 = 5), the narrow sharpness in the relative pressure (P/P0 ) range from 0.3 to 0.35 is maintained. It is a measure of the presence of a good pore size uniformity of the remained hexagonal-type mesoporous. The pore diameter distributions of the samples calculated from desorption branch of the isotherm by using the BJH method is also shown in Fig. 3. In all the samples a bimodal pore diameter distribution was observed: the main pores presented a diameter at approximately 2.7 nm and a small percentage of pores show larger diameter around 3.6 nm. The peak position of the main peaks do not significantly vary with the Si/Zr molar ratio, while, the fraction of the pores with larger diameter is slightly increased as the Si/Zr molar ratio decreases. Departures from a sharp and clearly defined pore-filling step are usually an indication of increase in pore size heterogeneity (i.e., widening of pore size distribution). The samples with higher zirconium content (SiO2 /ZrO2 = 5 and 10) exhibit a well-defined bimodal pore distribution with a very narrow diameter size distribution. This is be related to the mesoporous structure regularity in the Zr-MCM-41 solids. In accord to N2 adsorption–desorption isotherms as well as XRD spectra, it seems that increasing zirconium content in the Zr-MCM-41 sample decrease the mesoporous hexagonal-type density, developing another narrow pores family centered at about 3.6 nm. Because the structural hexagonal order is not destroyed with zirconium content but its population decrease, it seem that zirconium would not be uniformly incorporated in the framework of mesoporous silica. Another possibility would be that zirconia is incorporated as very small crystals of ZrO2 , generating the second family of porous. However, this zirconium oxide phase was not detected by XRD and not clearly observed by HR-TEM either. The surface area decreases from 604 m2 g−1 to 594, 484, 277 m2 g−1 as the Si/Zr molar ratio decreases from 40, to 20, 10, 5. Among these four samples, the solid with SiO2 /ZrO2 = 5 ratio has the largest lattice cell dimension and smallest surface area. These observations show that zirconium incorporation strongly affects the textural properties of the resultant materials. High zirconium content not only leads to diminution of the long-range order of the mesostructure and surface area, but also increases the population of the pores with large diameter. The related data are reported in Table 1. From these data it can be observed that as zirconium loading is increased the d-spacing in calcined samples is lightly increased. From 3.83 nm in SiO2 /ZrO2 = 40 sample to 3.89 nm fin SiO2 /ZrO2 = 5 sample. In this same sense, a small increase in the wall thickness was also observed. The wall thickness increases from 1.13 to 1.20 and 1.16 nm in SiO2 /ZrO2 = 40, 10 and 5 samples, respectively. This behavior is associated to isomorphous substitution of silicon with zirconium. If Si4+ , with an ionic radius of 0.41 nm, is being substituted with Zr4+ , with a larger ionic radius (0.84 nm) a wall thickness must be observed.
142
P. Salas et al. / Materials Chemistry and Physics 114 (2009) 139–144
Table 1 Physical properties of mesoporous zirconia-silicas SiO2 /ZrO2 (mole ratio)
d100 (nm)
5 10 20 40
3.89 3.82 3.84 3.83 √ = d100 − Dp ; ao = 2d100 / 3.
ao (nm)
Dp (nm)
(nm)
S.A.BET (m2 g−1 )
4.49 4.41 4.43 4.42
2.73 2.72 2.64 2.70
1.16 1.10 1.20 1.13
277 484 594 604
3.3. FTIR characterization In order to verify the incorporation of zirconium ion into the framework, the calcined samples were characterized with FTIR technique. Usually, the fundamental vibrations of the Zr–O–Si and Si–O–Si bonds in the framework can be seen below 1300 cm−1 . As shown in Fig. 4, several bands around 500, 800, 976, 1086, 1240, 1630 cm−1 are observed. The band at 1630 cm−1 is due to the hydroxyls groups in the solids, while, the other bands are related to the vibrations of the M–O bonds. The band at 460 cm−1 is assigned to rocking vibration of the Si–O–Si bond. The band indicating Si–O–Si asymmetric stretching vibration, which is found at 1100 cm−1 for pure silica, is observed at 1086 cm−1 . The shift in stretching frequency is due to deteriorating silica framework after insertion of Zr-atom [23]. It is noteworthy that as the SiO2 /ZrO2 molar ratio decreases, the relative intensity of the bands around 980 and 1086 cm−1 is increased. For instance, in the sample with the higher Zr content (SiO2 /ZrO2 = 5) the relative intensity of the bands at 1086 and 976 cm−1 increased about a 20% respect to relative intensity presented in SiO2 /ZrO2 = 20 sample. Generally, as SiO2 /ZrO2 mole ratio decreased, more silicon ions might be replaced by zirconium ion in the framework, which leads to reduction of the population of the Si–O–Si bonds and an increment of the number of the Zr–O–Si linkage. Therefore, the band at 980 cm−1 is responsible for the vibration of Zr–O–Si bond, which might be used as an indicator of the formation of the Zr–O–Si bond or its intensity as a measure of the degree of zirconium ion substitution in the framework. Also, it is observed that the bands positions slightly vary with Si/Zr mole ratio. All the bands show red-shift in the case of high zirconium content. As the Si/Zr mole ratio increases from 40,
Fig. 4. FTIR spectra of the samples. The number on the curves indicates the SiO2 /ZrO2 molar ratio composition of the samples.
Fig. 5. The 29 Si MAS-NMR spectra of the Zr-MCM-41 calcined samples. The number on the curves indicates the SiO2 /ZrO2 molar ratio composition of the samples.
to 20, 10 and to 5, for example, the band around 500 cm−1 shifts its position from 500 cm−1 to 490, 485 and 482 cm−1 . These changes strongly indicate that the incorporation of zirconium ions into the framework of Si-MCM-41 is enhanced as the zirconium content increases. It would appear that the Si–O–Zr bond gives rise to a chemical shift similar to that of Si–O–H bond. 3.4.
29 Si
MAS-NMR analysis
Figs. 5 and 6 show the 29 Si MAS-NMR 29 Si-1 H MAS-NMR spectra of the Zr-modified samples. The 29 Si MAS-NMR spectrum presents a broad signal between −60 and −125 ppm, which can be deconvoluted to three main components with chemical shifts at ca. −92,
Fig. 6. 29 Si-1 H MAS-NMR spectra of the Zr-MCM-41 calcined samples. The number on the curves indicates the SiO2 /ZrO2 molar ratio composition of the samples.
P. Salas et al. / Materials Chemistry and Physics 114 (2009) 139–144 Table 2 Zirconia effect on silicon species (SO4 2− /(ZrO2 –SiO2 ) = 0.3) SiO2 /ZrO2 (molar ratio)
5 10 20 40 a b
29
One pulse (%)
Si-1 H CP (%)
Q4
Q3
Q3 /Q4
Q4
Q3
Q2
(Q3 + Q2 )/Q4
59.6 58.7 62.2 70.9
40.4 41.3 37.8 29.1
0.68 0.70 0.61 0.41
29.7 28.0 29.2 29.2
52.3 58.9 62.8 57.0
11.3 + 6.7a 13.1 8.6 13.8
2.37b 2.57 2.42 2.42
Q1 . (Q1 + Q3 + Q2 )/Q4 .
−100 and −115 ppm. These signals resulted from Q2 (−92 ppm), Q3 (−100 ppm) and Q4 (−115 ppm) silicon nuclei, where the Qx corresponds to silicon nuclei with x siloxane linkages, i.e., Q2 to disilanol Si–(O–Si)2 (–O–X)2 , where X is H or Zr; Q3 to silanol (X–O)–Si–(O–Si)3 and Q4 to Si–(O–Si)4 in the framework [27]. The fraction of Q2 , Q3 and Q4 silicons from both cross-polarization and single-pulse experiments and their relative value derived from Fig. 5 are reported in Table 2. Usually, the shape and peak width of the spectra are strongly dependent on the phase composition of the sample. One may see that all the samples with high zirco-
143
nium content show a larger population of the Q3 coordination, and accordingly, a bigger value of Q3 /Q4 , that strongly indicates more Si ions being replaced by Zr ions in the framework. It can be seen that the value of Q3 /Q4 steadily increases with the decrease of the SiO2 /ZrO2 mole ratio until to 10, confirming that the degree of Si replacement by Zr is enhanced or the number of the Si–O–Zr linkages are increased in the solid containing high zirconium content. At the solid with SiO2 /ZrO2 = 5, the value of Q3 /Q4 from one-pulse measurement or (Q1 + Q2 + Q3 )/Q4 is from the cross-polarization measurement slightly diminished in comparison with other samples, which is probably indicative that too high zirconium might lead to segregation of zirconia from the Si–Zr–O mesoporous materials. 3.5. Morphological features The morphological features of the samples calcined at 600 ◦ C were studied by TEM technique. As shown in Fig. 7, these solids have mesopores with hexagonal shape in the whole crystals investigated. The arrangement of these mesopores is rather ordered, showing a long-range ordering of the mesostructure (Fig. 7a–c). In the sample with high zirconium content Si/Zr = 5, in the boundary region of
Fig. 7. Bright field TEM images of the Zr-MCM-41 calcined samples prepared with different SiO2 /ZrO2 ratios. (a) SiO2 /ZrO2 = 40; (b) SiO2 /ZrO2 = 20; (c) SiO2 /ZrO2 = 10; and (d) SiO2 /ZrO2 = 5.
144
P. Salas et al. / Materials Chemistry and Physics 114 (2009) 139–144
pore system and surface acidity has great potential to be used as catalysts or catalyst support. 4. Conclusions (1) The Si/Zr molar ratio is a key factor influencing the textural properties, surface acidity and structural regularity of the Zr-containing mesoporous molecular sieves. High zirconium content, i.e., Si/Zr ≤ 5, is unfavourable to the formation of highly ordered Zr-MCM-41. (2) Incorporation zirconium ions into the framework of Si-MCM-41 solids may result in an increase in the pore wall thickness and surface Brønsted acidity, but it leads to reduction of the surface area and structural regularity of the mesoporous materials. (3) The Q3 /Q4 value obtained from 29 Si MAS-NMR and the intensity of the IR band at around 890 cm−1 in the materials vary with the Si/Zr molar ratio, which may be used as indicator of the Zr–O–Si bond formation and thus as a measure of the incorporation degree of zirconium ions into the framework of Si-MCM-41 solid. Fig. 8. FTIR spectra of pyridine adsorption on the Zr-MCM-41 calcined samples prepared with different SiO2 /ZrO2 ratios.
the crystal studied, the mesostructure is highly ordered, however, in the center area, the arrangement of the channels or pore system is rather disordered (Fig. 7d). Therefore, high content of zirconium in the solid may result in diminishing of the structural regularity of the solid, which is in a good agreement with the results of the XRD analysis and textural measurement. 3.6. Surface acidity The surface acidity of the samples was measured by in situ FTIR of pyridine adsorption method. As shown in Fig. 8, both, Lewis (L) and Brønsted (B) acid sites, are formed on these samples as indicated by the bands at 1450 cm−1 (L), 1590 cm−1 (L), 1540 cm−1 (B) and 1640 cm−1 (B) [28]. In addition, the band at 1490 cm−1 indicates the formation of the adjacent Lewis (L) and Brønsted (B) acid sites. The number of the Lewis acid sites is generally predominant. The sample with low SiO2 /ZrO2 molar ratio presents more Brønsted acid sites. Because the pure Si-MCM-41 solid lacks Brønsted acid sites, therefore, the incorporation of zirconium ions into the framework of the Si-MCM-41 is responsible for the formation of these Brønsted acid sites. It is well known that the Zr4+ ions diameter (0.084 nm) is much larger than that of Si4+ ion (0.026 nm); when the smaller Si4+ ions are replaced by the larger Zr4+ ions in the framework of the solid, the bond length of Zr–O–Si clearly differs from the one of Si–O–Si, and this must lead to structural microstrain within the lattice cell. Changes in the electron density around Si, due to charge unbalance, or differences in electronegativity or local structure deformation resulting from the introduction of the Zr4+ ion into the vicinity of the hydroxyls carrying silicon, may weaken the SiO–H bond [29]; this is one of the possible origins giving rise to the Brønsted acid sites on the Zr-modified mesoporous materials. These Zr-based mesoporous molecular sieves with highly ordered
References [1] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartulli, J.S. Beck, Nature 359 (1992) 710. [2] G.S. Attard, C.G. Göltner, J.M. Corker, S. Henke, R.H. Templer, Angew. Chem. Int. Ed. 36 (1997) 1315. [3] M. Antonietti, C.G. Göltner, Adv. Mater. 9 (1997) 431. [4] H. Kang, Y.W. Jun, J.I. Park, K.B. Lee, J. Cheon, Chem. Mater. 12 (2000) 3530. [5] R. Ryoo, S.H. Joo, S. Jun, J. Phys. Chem. B 103 (1999) 7743. [6] B. Sakintuna, Y. Yürüm, Ind. Eng. Chem. Res. 44 (2005) 2893. [7] J.A. Wang, J.M. Domínguez, A. Montoya, S. Castillo, J. Navarrete, M. MoránPineda, J. Reyes-Gasaga, X. Bokhimi, Chem. Mater. 14 (2002) 4676. [8] S.A. Bagshaw, T.J. Pinnavaia, Angew. Chem. Int. Ed. 35 (1996) 1102. [9] M. Yada, M. Ohya, M. Machida, T. Kijima, Chem. Commun. (1998) 1941. [10] P. Yang, D. Zhao, D.I. Margolese, B.F. Chmeleka, G.D. Stucky, Chem. Mater. 111 (1999) 2813. [11] F. Schüth, U. Ciesla, S. Schacht, M. Thieme, Q. Huo, G.D. Stucky, Mater. Res. Bull. 34 (1999) 483. [12] X. He, D.M. Antonelli, M.L. Treudeau, Adv. Mater. 12 (2000) 1036. [13] Q. Huo, D. Margolese, U. Ciesla, P. feng, T. Gier, P. Sieger, R. Leon, P. Petroff, F. Schüth, G.D. Stucky, Nature 368 (1994) 317. ˜ J.A. Toledo, S. Castillo, M. Moran[14] L.F. Chen, G. Gonzalez, J.A. Wang, L. Norena, Pieda, Appl. Surf. Sci. 243 (2005) 319. [15] Y.D. Li, X.L. Li, R.R. He, J. Zhu, Z.X. Deng, J. Am. Chem. Soc. 121 (7) (2002) 1411. [16] S. Udayakumar, S. Ajaikumar, A. Pandurangan, Appl. Catal. A: Gen. 307 (2) (2006) 245. [17] G. Calleja, J. Aguado, A. Carrero, J. Moreno, Catal. Commun. 6 (2) (2005) 153. [18] A. De Stefanis, S. Kaciulis, L. Pandolfi, Micropor. Mesopor. Mater. 99 (2007) 140. [19] N.S. Nesterenko, O.A. Ponomoreva, V.V. Yuschenko, I.I. Ivanova, F. Testa, F. Di Renzo, F. Fajula, Appl. Catal. A: Gen. 254 (2) (2003) 261. [20] P. Wu, M. Iwamoto, J. Chem. Soc., Faraday Trans. 94 (1998) 2871. [21] S. Contier, A. Tue, Appl. Catal. A: Gen. 143 (1996) 125. [22] G. Du, Y. Yang, W. Qiu, S. Lim, L. Pfefferle, G.L. Haller, Appl. Catal. A: Gen. 313 (1) (2006) 1. [23] K. Chaudhari, R. Bai, T.Kr. Das, A. Chandwadkar, D. Srinivas, S. Sivasamker, J. Phys. Chem. B 104 (2000) 11066. [24] A. Tarafdar, A.B. Panda, P. Pramanik, Micropor. Mesopor. Mater. 84 (2005) 223. [25] X.X. Wang, L. Veyre, F. Lefebvre, J. Patarin, J.M. Basset, Micropor. Mesopor. Mater. 66 (2003) 169. [26] K.S. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquérol, T. Siemoeniewsha, Pure Appl. Chem. 57 (1985) 603. [27] D.J. Rosenberg, F. Coloma, J.A. Anderson, J. Catal. 210 (2002) 218. [28] D. Srinivas, R. Srivastava, P. Ratnasamy, Catal. Today 96 (2004) 127. [29] J.A. Anderson, C. Fergusson, I. Rodriguez Ramos, A. Guerrero Riuz, J. Catal. 192 (2000) 344.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Effect of the Si/Zr molar ratio on the synthesis of Zr-based mesoporous molecular sieves P. Salas a,∗ , J.A. Wang b , H. Armendariz c , C. Angeles-Chavez c , L.F. Chen b a
Centro de Física Aplicada y Tecnología Avanzada, Universidad Nacional Autónoma de México, Apartado Postal 1-1010, Querétaro 76000, Mexico Escuela Superior de Ingeniería Química e Industrias Extractivas, Instituto Politécnico Nacional, Av. Politécnico S/N, Col. Zacatenco, 07738 México D.F., Mexico c Instituto Mexicano del Petróleo, Eje Lázaro Cárdenas 152, 07730 México D.F., Mexico b
a r t i c l e
i n f o
Article history: Received 18 June 2007 Received in revised form 26 August 2008 Accepted 29 August 2008 Keywords: Zr-MCM-41 Mesoporous molecular sieves Zirconium FTIR Acidity
a b s t r a c t Highly ordered Zr-based mesoporous molecular sieves were synthesized via a surfactant-templated method and the effect of the Si/Zr molar ratio on the crystalline structure, textural properties and surface acidity were studied by XRD, FTIR, TEM and 29 Si MAS-NMR techniques. FTIR spectra show that the intensity of the band around 890 cm−1 which corresponds to the vibration of Si–O–Zr bond was increased with increasing of the zirconium content, therefore, this band may be used as an indicator of the degree of the zirconium incorporation into the Si-framework. When the zirconium content increased in the materials, the Q3 /Q4 value obtained from 29 Si MAS-NMR was linearly increased, whereas, the intensity the XRD peaks was gradually reduced; as a result, the pore wall thickness of the resultant materials was gradually increased, the surface area and the structural regularity were diminished. In order to obtain Zr-MCM-41 with highly ordered mesostructure and large surface area, proper Si/Zr molar ratio is a key factor, e.g., Si/Zr should be no less than 10. It was also found that the Brønsted acid sites which resulted from charge unbalance or local structure deformation due to the Zr4+ incorporation into the vicinity of the hydroxyls carrying silicon were created on the surface of the Zr-MCM-41 solids; strong Brønsted acidity could be formed on the solid with high zirconium content. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In the history of the porous materials development, a breakthrough took place in 1992 when a new family of mesoporous molecular sieves called as M41S was synthesized by the scientists in Mobil company using cationic or anionic surfactants as synthesis template [1]; this family of mesoporous materials includes three members, Si-MCM-41, Si-MCM-48 and Si-MCM-50, and all of these have highly ordered pore channel system together with high surface area, big pore diameter and large pore volume. This synthesis idea directly results in the discoveries of a wide range of mesoporous materials, for example, mesostructured metals [2–4], carbon [5,6], rare metal oxide [7], transition metal oxides [8–13], mixed metal oxides [14] and sulfides [15]. Since the pure Si-MCM-41 possesses a neutral framework, which limits its applications as catalyst or catalyst support. Consequently, isomorphous substitution of silicon with other transition metals has been found to be an effective strategy in creating catalytically active sites and anchoring sites for active molecules in
∗ Corresponding author. Tel.: +52 55 56234163. E-mail address: [email protected] (P. Salas). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.08.086
the design of new heterogeneous catalysts. Various transition metals such as Al3+ [16,17], Fe3+ [16,18], Ga3+ [19], Ti4+ [20], Zr4+ [21] and V5+ [22] incorporated mesoporous molecular sieves have been synthesized. Among these materials, zirconium incorporated mesoporous molecular sieves are particularly interest because of the important applications of Zr-based materials in a wide variety of areas in heterocatalysis, petroleum processing, biotechnology, etc. In the synthesis, the parameters of pH value, reaction temperature, type and concentration of synthetic template, metal precursors and the others strongly influence the properties of the resultant Zr-containing mesoporous materials. Depending on the synthetic conditions, different types of Zr-based nanomaterials could be obtained where zirconium ions are usually in a tetrahedral coordination with a different proportion between surface and bulk [23–25]. For example, ZrO2 –SiO2 mesocomposites with worm-like pores could be obtained via a sol–gel method [25]. In the present work we are synthesized zirconium containing mesoporous silicas with high zirconium content and mesoporous hexagonal-type structure. The Si/Zr molar ratio ranged from 40 to 5. More attention will be focused, mainly in the samples with higher Zr content, on maintain of mesoporous structural regularity and textural properties. In order to enhance the surface acidity of the resultant
140
P. Salas et al. / Materials Chemistry and Physics 114 (2009) 139–144
Zr-MCM-41 mesoporous materials, the tetrahedral ZrO4 groups were “in situ” sulfated” adding a desired amount of SO4 2− in the synthesis media. 2. Experimental 2.1. Synthesis of Zr-Si-MCM-41 The Zr-MCM-41 solids were prepared using tetraethyl orthosilicate (TEOS) as Si precursor and zirconium-n-propoxide (70% in propanol) as Zr source, along with cetyltrimethylammonium bromide (CTAB) as synthesis template. The typical preparation procedure of a Zr-MCM-41 sample with a molar ratio of Si/Zr = 20 is as follows: First of all, two solutions were prepared: the first solution was made by adding 2.2 ml of zirconium-n-propoxide and 22 ml of TEOS with stirring; the second solution was made by adding 11.4 g of CTAB and 4.7 g of (NH4 )2 SO4 into hot water (around 50 ◦ C) with stirring, followed by the addition of 145 ml of NH3 ·H2 O (28 wt.%). Then, the first solution was added, drop by drop, into the second solution. During the addition, the mixture was vigorously agitated for about 2 h, until a gel was formed. The resultant gel was loaded into a stoppered Teflon bottle without stirring and heated at 100 ◦ C for 72 h. After cooling to room temperature, the resulting solid product was recovered by filtration and was washed for four times with 500 ml deionized water. The white solid obtained was dried in air at 100 ◦ C for 24 h. Finally, the sample was calcined at 600 ◦ C for 6 h in air. The heating rate was 1 ◦ C min−1 . 2.2. X-ray diffraction (XRD) analysis The low angle X-ray diffraction patterns of the samples were measured in a D-500 SIEMENS diffractometer with a graphite secondary beam monochromator to obtain a monochromatic Cu K␣1 radiation, and the evaluation of the diffractograms was made by DIFFRAC/AT software. The scanning was made from 1.5 to 10, 2 step size of 0.02 and step time of 2 s. In order to avoid the problem of illuminated area at low 2 angle, all the samples were measured using the same sample holder. In this way the hexagonal reflection (1 0 0) positions as well as the intensities are directly comparative. Position correction was made using the NIST standard reference material 675.
Fig. 1. XRD patterns of the as-made Zr-MCM-41 solids. The number on the curves indicates the SiO2 /ZrO2 molar ratio composition of the solids.
3. Results and discussion
2.3. N2 adsorption measurement
3.1. XRD analysis
The specific surface area, pore volume and pore size distribution of the samples were measured in a Digisorb 2600 equipment by using low temperature N2 physisorption isotherms. Before the measurement the sample was evacuated at 350 ◦ C under vacuum condition. The surface area was calculated using the BET method based on the adsorption data in the partial pressure P/P0 range from 0.01 to 1. The mesopore volume was determined from the N2 adsorbed at a P/P0 = 0.4.
Fig. 1 shows the XRD patterns of the as-made zirconiumcontaining solids. All the samples have four well-defined diffraction peaks, which correspond to (1 0 0), (1 1 0) (2 0 0) and (2 1 0) reflections of the solid, respectively. The XRD patterns clearly indicate that the long-range order of mesoporous molecular sieves with hexagonal framework was formed in the materials with low zirconium content or high Si/Zr molar ratio. These XRD patterns are very similar to the one shown in the pure Si-MCM-41 sample, indicating that the small amount zirconium incorporated into the Si-MCM-41 framework does not strongly modify the structure of Si-MCM-41. However, it is observed that as the SiO2 /ZrO2 molar ratio decreases, or zirconium content increases, intensities of the XRD peaks gradually diminish, showing reduction of the structural ordering. For the sample containing high zirconium content, e.g., Si/Zr = 5, the peaks related to (1 1 0), (2 0 0) and (2 1 0) disappeared completely, which indicates that the hexagonal lattice structure is partially collapsed, forming wormhole-like pore system. As shown in Fig. 2, when the as-made samples were calcined at 600 ◦ C, all the diffraction peaks became sharper compared to that of the as-synthesized samples, even for the sample with high zirconium content, SiO2 /ZrO2 = 5, the diffraction peaks at high 2 positions reappeared, this indicates that the structural regularity was improved after calcination. It seems that formation of the Zr-MCM-41 framework can be extended from the hydrothermal treatment procedure into the calcination step. Calcination may be favourable to the structure reconstruction, thus the structural ordering of the calcined solid is further improved.
2.4. Solid state nuclear magnetic resonance (29 Si MAS-NMR) Solid state 29 Si MAS-NMR spectra were recorded on a Bruker 400 MHz spectrometer at a frequency of 79.49 MHz spinning 7.5 kHz, using pulses at 90 s intervals and 4 mm zirconia rotors. The number of accumulations was 500. All the measurements were carried out at room temperature. For the analysis of 29 Si, tetramethylsilane (0 ppm) was used as standard reference to obtain the chemical shift of the solid materials. 2.5. FTIR spectroscopy of pyridine adsorption To evaluate and analyze the strength and types of acid sites, pyridine adsorption on the samples were performed on a 170-SX Fourier transform infrared (FTIR) spectrometer in the temperature ranging from 25 to 400 ◦ C. Before pyridine adsorption, the sample in a vacuum was heated to 300 ◦ C in order to eliminate the adsorbed water or impurities on the surface, and then cooled to room temperature. Afterwards, the solid wafer was exposed to pyridine, by breaking, in the spectrometer cell, a capillary containing 50 l liquid pyridine. The IR spectra were recorded in various conditions by increasing cell temperature from 25 to 400 ◦ C. 2.6. Transmission electron microscopy Transmission electron microscopy, TEM was performed in JEM-2200FS transmission electron microscope with accelerating voltage of 200 kV. The microscope is equipped with a Schottky-type field emission gun and an ultra high resolution (UHR) configuration (Cs = 0.5 mm; Cc 1.1 mm; point to point resolution, 0.19 nm) and in-column energy filter omega-type. The samples were grounded, suspended in isopropanol at room temperature, and dispersed with ultrasonic agitation; then, an aliquot of the solution was dropped on a 3 mm diameter lacey carbon copper grid.
3.2. Textural properties Fig. 3 shows the loops of the N2 adsorption–desorption isotherms as well as the pore diameter distribution of the samples.
P. Salas et al. / Materials Chemistry and Physics 114 (2009) 139–144
Fig. 2. XRD patterns of the Zr-MCM-41 solids calcined at 600 ◦ C. The number on the curves indicates the SiO2 /ZrO2 molar ratio composition of the solids.
Fig. 3. Loops of the N2 adsorption–desorption isotherms (left) and the pore diameter distribution of the samples (right). The SiO2 /ZrO2 molar ratio composition of the samples is shown on curves.
141
There are clearly defined four stages in the loops of the N2 adsorption–desorption isotherms, which are typical of MCM-41type mesoporous materials. Based on the IUPAC classification [26], the loops of the N2 adsorption–desorption isotherms belong to type IV profiles. In all the samples, the presence of framework-confined mesoporous is indicated by the adsorption step centered in the relative pressure p/p0 region from 0.3 to 0.35. The sharp step at this region reveals the highly ordered mesoporous systems. It is found that the shape and the narrow sharpness of the loops practically do not vary with the zirconium content. As the zirconium content increases in the Zr-MCM-41 materials, only the nitrogen adsorbed volume, associated to capillary condensation within mesopores, gradually decreases. It is worthwhile to note that although is clear the decrease of the height of capillary condensation (pore filling) step in the isotherm of the calcined Zr-MCM-41 sample with the higher Zr content (SiO2 /ZrO2 = 5), the narrow sharpness in the relative pressure (P/P0 ) range from 0.3 to 0.35 is maintained. It is a measure of the presence of a good pore size uniformity of the remained hexagonal-type mesoporous. The pore diameter distributions of the samples calculated from desorption branch of the isotherm by using the BJH method is also shown in Fig. 3. In all the samples a bimodal pore diameter distribution was observed: the main pores presented a diameter at approximately 2.7 nm and a small percentage of pores show larger diameter around 3.6 nm. The peak position of the main peaks do not significantly vary with the Si/Zr molar ratio, while, the fraction of the pores with larger diameter is slightly increased as the Si/Zr molar ratio decreases. Departures from a sharp and clearly defined pore-filling step are usually an indication of increase in pore size heterogeneity (i.e., widening of pore size distribution). The samples with higher zirconium content (SiO2 /ZrO2 = 5 and 10) exhibit a well-defined bimodal pore distribution with a very narrow diameter size distribution. This is be related to the mesoporous structure regularity in the Zr-MCM-41 solids. In accord to N2 adsorption–desorption isotherms as well as XRD spectra, it seems that increasing zirconium content in the Zr-MCM-41 sample decrease the mesoporous hexagonal-type density, developing another narrow pores family centered at about 3.6 nm. Because the structural hexagonal order is not destroyed with zirconium content but its population decrease, it seem that zirconium would not be uniformly incorporated in the framework of mesoporous silica. Another possibility would be that zirconia is incorporated as very small crystals of ZrO2 , generating the second family of porous. However, this zirconium oxide phase was not detected by XRD and not clearly observed by HR-TEM either. The surface area decreases from 604 m2 g−1 to 594, 484, 277 m2 g−1 as the Si/Zr molar ratio decreases from 40, to 20, 10, 5. Among these four samples, the solid with SiO2 /ZrO2 = 5 ratio has the largest lattice cell dimension and smallest surface area. These observations show that zirconium incorporation strongly affects the textural properties of the resultant materials. High zirconium content not only leads to diminution of the long-range order of the mesostructure and surface area, but also increases the population of the pores with large diameter. The related data are reported in Table 1. From these data it can be observed that as zirconium loading is increased the d-spacing in calcined samples is lightly increased. From 3.83 nm in SiO2 /ZrO2 = 40 sample to 3.89 nm fin SiO2 /ZrO2 = 5 sample. In this same sense, a small increase in the wall thickness was also observed. The wall thickness increases from 1.13 to 1.20 and 1.16 nm in SiO2 /ZrO2 = 40, 10 and 5 samples, respectively. This behavior is associated to isomorphous substitution of silicon with zirconium. If Si4+ , with an ionic radius of 0.41 nm, is being substituted with Zr4+ , with a larger ionic radius (0.84 nm) a wall thickness must be observed.
142
P. Salas et al. / Materials Chemistry and Physics 114 (2009) 139–144
Table 1 Physical properties of mesoporous zirconia-silicas SiO2 /ZrO2 (mole ratio)
d100 (nm)
5 10 20 40
3.89 3.82 3.84 3.83 √ = d100 − Dp ; ao = 2d100 / 3.
ao (nm)
Dp (nm)
(nm)
S.A.BET (m2 g−1 )
4.49 4.41 4.43 4.42
2.73 2.72 2.64 2.70
1.16 1.10 1.20 1.13
277 484 594 604
3.3. FTIR characterization In order to verify the incorporation of zirconium ion into the framework, the calcined samples were characterized with FTIR technique. Usually, the fundamental vibrations of the Zr–O–Si and Si–O–Si bonds in the framework can be seen below 1300 cm−1 . As shown in Fig. 4, several bands around 500, 800, 976, 1086, 1240, 1630 cm−1 are observed. The band at 1630 cm−1 is due to the hydroxyls groups in the solids, while, the other bands are related to the vibrations of the M–O bonds. The band at 460 cm−1 is assigned to rocking vibration of the Si–O–Si bond. The band indicating Si–O–Si asymmetric stretching vibration, which is found at 1100 cm−1 for pure silica, is observed at 1086 cm−1 . The shift in stretching frequency is due to deteriorating silica framework after insertion of Zr-atom [23]. It is noteworthy that as the SiO2 /ZrO2 molar ratio decreases, the relative intensity of the bands around 980 and 1086 cm−1 is increased. For instance, in the sample with the higher Zr content (SiO2 /ZrO2 = 5) the relative intensity of the bands at 1086 and 976 cm−1 increased about a 20% respect to relative intensity presented in SiO2 /ZrO2 = 20 sample. Generally, as SiO2 /ZrO2 mole ratio decreased, more silicon ions might be replaced by zirconium ion in the framework, which leads to reduction of the population of the Si–O–Si bonds and an increment of the number of the Zr–O–Si linkage. Therefore, the band at 980 cm−1 is responsible for the vibration of Zr–O–Si bond, which might be used as an indicator of the formation of the Zr–O–Si bond or its intensity as a measure of the degree of zirconium ion substitution in the framework. Also, it is observed that the bands positions slightly vary with Si/Zr mole ratio. All the bands show red-shift in the case of high zirconium content. As the Si/Zr mole ratio increases from 40,
Fig. 4. FTIR spectra of the samples. The number on the curves indicates the SiO2 /ZrO2 molar ratio composition of the samples.
Fig. 5. The 29 Si MAS-NMR spectra of the Zr-MCM-41 calcined samples. The number on the curves indicates the SiO2 /ZrO2 molar ratio composition of the samples.
to 20, 10 and to 5, for example, the band around 500 cm−1 shifts its position from 500 cm−1 to 490, 485 and 482 cm−1 . These changes strongly indicate that the incorporation of zirconium ions into the framework of Si-MCM-41 is enhanced as the zirconium content increases. It would appear that the Si–O–Zr bond gives rise to a chemical shift similar to that of Si–O–H bond. 3.4.
29 Si
MAS-NMR analysis
Figs. 5 and 6 show the 29 Si MAS-NMR 29 Si-1 H MAS-NMR spectra of the Zr-modified samples. The 29 Si MAS-NMR spectrum presents a broad signal between −60 and −125 ppm, which can be deconvoluted to three main components with chemical shifts at ca. −92,
Fig. 6. 29 Si-1 H MAS-NMR spectra of the Zr-MCM-41 calcined samples. The number on the curves indicates the SiO2 /ZrO2 molar ratio composition of the samples.
P. Salas et al. / Materials Chemistry and Physics 114 (2009) 139–144 Table 2 Zirconia effect on silicon species (SO4 2− /(ZrO2 –SiO2 ) = 0.3) SiO2 /ZrO2 (molar ratio)
5 10 20 40 a b
29
One pulse (%)
Si-1 H CP (%)
Q4
Q3
Q3 /Q4
Q4
Q3
Q2
(Q3 + Q2 )/Q4
59.6 58.7 62.2 70.9
40.4 41.3 37.8 29.1
0.68 0.70 0.61 0.41
29.7 28.0 29.2 29.2
52.3 58.9 62.8 57.0
11.3 + 6.7a 13.1 8.6 13.8
2.37b 2.57 2.42 2.42
Q1 . (Q1 + Q3 + Q2 )/Q4 .
−100 and −115 ppm. These signals resulted from Q2 (−92 ppm), Q3 (−100 ppm) and Q4 (−115 ppm) silicon nuclei, where the Qx corresponds to silicon nuclei with x siloxane linkages, i.e., Q2 to disilanol Si–(O–Si)2 (–O–X)2 , where X is H or Zr; Q3 to silanol (X–O)–Si–(O–Si)3 and Q4 to Si–(O–Si)4 in the framework [27]. The fraction of Q2 , Q3 and Q4 silicons from both cross-polarization and single-pulse experiments and their relative value derived from Fig. 5 are reported in Table 2. Usually, the shape and peak width of the spectra are strongly dependent on the phase composition of the sample. One may see that all the samples with high zirco-
143
nium content show a larger population of the Q3 coordination, and accordingly, a bigger value of Q3 /Q4 , that strongly indicates more Si ions being replaced by Zr ions in the framework. It can be seen that the value of Q3 /Q4 steadily increases with the decrease of the SiO2 /ZrO2 mole ratio until to 10, confirming that the degree of Si replacement by Zr is enhanced or the number of the Si–O–Zr linkages are increased in the solid containing high zirconium content. At the solid with SiO2 /ZrO2 = 5, the value of Q3 /Q4 from one-pulse measurement or (Q1 + Q2 + Q3 )/Q4 is from the cross-polarization measurement slightly diminished in comparison with other samples, which is probably indicative that too high zirconium might lead to segregation of zirconia from the Si–Zr–O mesoporous materials. 3.5. Morphological features The morphological features of the samples calcined at 600 ◦ C were studied by TEM technique. As shown in Fig. 7, these solids have mesopores with hexagonal shape in the whole crystals investigated. The arrangement of these mesopores is rather ordered, showing a long-range ordering of the mesostructure (Fig. 7a–c). In the sample with high zirconium content Si/Zr = 5, in the boundary region of
Fig. 7. Bright field TEM images of the Zr-MCM-41 calcined samples prepared with different SiO2 /ZrO2 ratios. (a) SiO2 /ZrO2 = 40; (b) SiO2 /ZrO2 = 20; (c) SiO2 /ZrO2 = 10; and (d) SiO2 /ZrO2 = 5.
144
P. Salas et al. / Materials Chemistry and Physics 114 (2009) 139–144
pore system and surface acidity has great potential to be used as catalysts or catalyst support. 4. Conclusions (1) The Si/Zr molar ratio is a key factor influencing the textural properties, surface acidity and structural regularity of the Zr-containing mesoporous molecular sieves. High zirconium content, i.e., Si/Zr ≤ 5, is unfavourable to the formation of highly ordered Zr-MCM-41. (2) Incorporation zirconium ions into the framework of Si-MCM-41 solids may result in an increase in the pore wall thickness and surface Brønsted acidity, but it leads to reduction of the surface area and structural regularity of the mesoporous materials. (3) The Q3 /Q4 value obtained from 29 Si MAS-NMR and the intensity of the IR band at around 890 cm−1 in the materials vary with the Si/Zr molar ratio, which may be used as indicator of the Zr–O–Si bond formation and thus as a measure of the incorporation degree of zirconium ions into the framework of Si-MCM-41 solid. Fig. 8. FTIR spectra of pyridine adsorption on the Zr-MCM-41 calcined samples prepared with different SiO2 /ZrO2 ratios.
the crystal studied, the mesostructure is highly ordered, however, in the center area, the arrangement of the channels or pore system is rather disordered (Fig. 7d). Therefore, high content of zirconium in the solid may result in diminishing of the structural regularity of the solid, which is in a good agreement with the results of the XRD analysis and textural measurement. 3.6. Surface acidity The surface acidity of the samples was measured by in situ FTIR of pyridine adsorption method. As shown in Fig. 8, both, Lewis (L) and Brønsted (B) acid sites, are formed on these samples as indicated by the bands at 1450 cm−1 (L), 1590 cm−1 (L), 1540 cm−1 (B) and 1640 cm−1 (B) [28]. In addition, the band at 1490 cm−1 indicates the formation of the adjacent Lewis (L) and Brønsted (B) acid sites. The number of the Lewis acid sites is generally predominant. The sample with low SiO2 /ZrO2 molar ratio presents more Brønsted acid sites. Because the pure Si-MCM-41 solid lacks Brønsted acid sites, therefore, the incorporation of zirconium ions into the framework of the Si-MCM-41 is responsible for the formation of these Brønsted acid sites. It is well known that the Zr4+ ions diameter (0.084 nm) is much larger than that of Si4+ ion (0.026 nm); when the smaller Si4+ ions are replaced by the larger Zr4+ ions in the framework of the solid, the bond length of Zr–O–Si clearly differs from the one of Si–O–Si, and this must lead to structural microstrain within the lattice cell. Changes in the electron density around Si, due to charge unbalance, or differences in electronegativity or local structure deformation resulting from the introduction of the Zr4+ ion into the vicinity of the hydroxyls carrying silicon, may weaken the SiO–H bond [29]; this is one of the possible origins giving rise to the Brønsted acid sites on the Zr-modified mesoporous materials. These Zr-based mesoporous molecular sieves with highly ordered
References [1] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartulli, J.S. Beck, Nature 359 (1992) 710. [2] G.S. Attard, C.G. Göltner, J.M. Corker, S. Henke, R.H. Templer, Angew. Chem. Int. Ed. 36 (1997) 1315. [3] M. Antonietti, C.G. Göltner, Adv. Mater. 9 (1997) 431. [4] H. Kang, Y.W. Jun, J.I. Park, K.B. Lee, J. Cheon, Chem. Mater. 12 (2000) 3530. [5] R. Ryoo, S.H. Joo, S. Jun, J. Phys. Chem. B 103 (1999) 7743. [6] B. Sakintuna, Y. Yürüm, Ind. Eng. Chem. Res. 44 (2005) 2893. [7] J.A. Wang, J.M. Domínguez, A. Montoya, S. Castillo, J. Navarrete, M. MoránPineda, J. Reyes-Gasaga, X. Bokhimi, Chem. Mater. 14 (2002) 4676. [8] S.A. Bagshaw, T.J. Pinnavaia, Angew. Chem. Int. Ed. 35 (1996) 1102. [9] M. Yada, M. Ohya, M. Machida, T. Kijima, Chem. Commun. (1998) 1941. [10] P. Yang, D. Zhao, D.I. Margolese, B.F. Chmeleka, G.D. Stucky, Chem. Mater. 111 (1999) 2813. [11] F. Schüth, U. Ciesla, S. Schacht, M. Thieme, Q. Huo, G.D. Stucky, Mater. Res. Bull. 34 (1999) 483. [12] X. He, D.M. Antonelli, M.L. Treudeau, Adv. Mater. 12 (2000) 1036. [13] Q. Huo, D. Margolese, U. Ciesla, P. feng, T. Gier, P. Sieger, R. Leon, P. Petroff, F. Schüth, G.D. Stucky, Nature 368 (1994) 317. ˜ J.A. Toledo, S. Castillo, M. Moran[14] L.F. Chen, G. Gonzalez, J.A. Wang, L. Norena, Pieda, Appl. Surf. Sci. 243 (2005) 319. [15] Y.D. Li, X.L. Li, R.R. He, J. Zhu, Z.X. Deng, J. Am. Chem. Soc. 121 (7) (2002) 1411. [16] S. Udayakumar, S. Ajaikumar, A. Pandurangan, Appl. Catal. A: Gen. 307 (2) (2006) 245. [17] G. Calleja, J. Aguado, A. Carrero, J. Moreno, Catal. Commun. 6 (2) (2005) 153. [18] A. De Stefanis, S. Kaciulis, L. Pandolfi, Micropor. Mesopor. Mater. 99 (2007) 140. [19] N.S. Nesterenko, O.A. Ponomoreva, V.V. Yuschenko, I.I. Ivanova, F. Testa, F. Di Renzo, F. Fajula, Appl. Catal. A: Gen. 254 (2) (2003) 261. [20] P. Wu, M. Iwamoto, J. Chem. Soc., Faraday Trans. 94 (1998) 2871. [21] S. Contier, A. Tue, Appl. Catal. A: Gen. 143 (1996) 125. [22] G. Du, Y. Yang, W. Qiu, S. Lim, L. Pfefferle, G.L. Haller, Appl. Catal. A: Gen. 313 (1) (2006) 1. [23] K. Chaudhari, R. Bai, T.Kr. Das, A. Chandwadkar, D. Srinivas, S. Sivasamker, J. Phys. Chem. B 104 (2000) 11066. [24] A. Tarafdar, A.B. Panda, P. Pramanik, Micropor. Mesopor. Mater. 84 (2005) 223. [25] X.X. Wang, L. Veyre, F. Lefebvre, J. Patarin, J.M. Basset, Micropor. Mesopor. Mater. 66 (2003) 169. [26] K.S. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquérol, T. Siemoeniewsha, Pure Appl. Chem. 57 (1985) 603. [27] D.J. Rosenberg, F. Coloma, J.A. Anderson, J. Catal. 210 (2002) 218. [28] D. Srinivas, R. Srivastava, P. Ratnasamy, Catal. Today 96 (2004) 127. [29] J.A. Anderson, C. Fergusson, I. Rodriguez Ramos, A. Guerrero Riuz, J. Catal. 192 (2000) 344.