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Characterization and investigation on the difference of hydrothermal stability for ordered mesoporous aluminosilicate sieves
Advanced Powder Technology 22 (2011) 20–25
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Original Research Paper
Characterization and investigation on the difference of hydrothermal stability for ordered mesoporous aluminosilicate sieves Caiyun Han a, Hua Wang b, Liuyi Zhang a, Rongtao Li a, Yanyan Zhang a, Yongming Luo a,b,*, Xiaoming Zheng c,* a b c
College of Environmental Science and Engineering, Kunming University of Science and Technology, Kunming 650093, PR China College of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, PR China Institute of Catalysis, Department of Chemistry, Zhejiang University (Xixi campus), Hangzhou 310028, PR China
a r t i c l e
i n f o
Article history: Received 11 December 2009 Received in revised form 28 February 2010 Accepted 1 March 2010 Available online 9 March 2010 Keywords: Ordered mesoporous sieves Synthesis Hydrothermal stability Characterization
a b s t r a c t Ordered hexagonal mesoporous aluminosilicates molecular sieves, designated as MSAMS-2A, MSAMS-2G and MSAMS-2, have been synthesized via re-crystallization of mesoporous SBA-15 within the diluted solution of aluminosilicate sol–gel, glycerol, and both of them, respectively. The three materials have been characterized by XRD, N2 adsorption–desorption, FT-IR, FE-SEM, 27Al MAS NMR and 29Si MAS NMR, and the corresponding hydrothermal stability of these three sieves is in the order of MSAMS2 > MSAMS-2G MSAMS-2A. The hydrothermal stability difference between MSAMS-2 and MSAMS2G might be attributed to the synergistic effect of the higher condensation of silanol groups and insertion of all non-framework Al atoms into the framework of MSAMS-2. The hydrothermal stability of MSAMS2G is higher than that of MSAMS-2A, which is likely because the high viscosity of glycerol will be in favor of the silanol groups interacting with zeolite-like subunits and moreover glycerol can act as a stabilizing guest molecule. Ó 2010 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Ordered mesoporous molecular sieves (OMMSs) with high surface area, well-defined and tunable pore diameters of 2–50 nm have attracted much attention for potential applications as versatile catalysts and catalysts supports, template, adsorption and separation materials [1–9]. However, compared with microporous aluminosilicate zeolites, the acidity and hydrothermal stability of OMMSs are relatively low, which severely limit them practical applications as catalytic support and/or catalysts in petroleum refining and fine chemical synthesis [1,2,6–9]. In general, both weak acidity and low hydrothermal stability of OMMSs could be attributed to the amorphous nature of their mesoporous pore wall. Therefore, in the past decade years, many efforts have been focused on improving both the hydrothermal stability and the acidity of OMMSs [9–19]. One might expect to improve both the stability and acidity of these materials if zeolite-like order could be introduced into the mesopore walls. It is well-documented that the amorphous walls of the mesostructure could indeed be converted
* Corresponding authors. Address: College of Environmental Science and Engineering, Kunming University of Science and Technology, Kunming 650093, PR China. Tel.: +86 871 5170905; fax: +86 871 5170906 (Y. Luo). E-mail addresses: [email protected], [email protected] (Y. Luo).
to a partially zeolitic product. For instance, van Bekkum et al. reported that the walls of Al-HMS and Al-MCM-41 could be transformed into partially zeolitic structures by ion-exchanged them with a microporous zeolites structure template such as TPA+[10]. Hydrothermally stable mesoporous aluminosilicate molecular sieves have been synthesized via assembly of zeolite seeds of ZSM-5, BEA, TS-1 and FAU by use of cetyltrimethyl ammonium bromide (CTAB) or HO(CH2CH2O)20(CH2CH(CH3)O)70(CH2CH2O)20H (P123) as a structure-directing agent [11–17]. A coated route was used to synthesis of ultrastable mesoporous aluminosilicate molecular sieves through ion-exchanged Al-SBA-15 and Al-MCF with ZSM-5 and FAU seeds, respectively [18,19]. More recently, SBA-16 and MCM-41 type mesoporous aluminosilicates with high hydrothermal stability have been synthesized by the use of the dissolution of zeolite Na-A [20,21] and the dissolved ZSM-5 [22] zeolite as a single-source of silicon and aluminum. In this paper, three well-ordered hexagonal mesoporous aluminosilicates molecular sieves, designated as MSAMS-2A, MSAMS-2G and MSAMS-2, have been successfully synthesized via re-crystallization of SBA-15 within the diluted solution of aluminosilicate sol–gel, glycerol, and both of them, respectively. XRD results clearly exhibit that all the three ordered mesoporous aluminosilicate molecular sieves exhibit higher hydrothermal stability than that of SBA-15, and the corresponding hydrothermal stability of them is in the order of MSAMS-2 > MSAMS-2G MSAMS-2A.
0921-8831/$ - see front matter Ó 2010 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. doi:10.1016/j.apt.2010.03.001
C. Han et al. / Advanced Powder Technology 22 (2011) 20–25
Furthermore, the difference of hydrothermal stability among the three ordered mesoporous Aluminosilicate molecular sieves was investigated and discussed in detail via the use of FT-IR, FE-SEM, 27 Al and 29Si MAS NMR characterization. 2. Experimental 2.1. Synthesis of materials 2.1.1. Synthesis of SBA-15 Hexagonal ordered mesoporous SBA-15 molecular sieve was synthesized by using triblock copolymer HO(CH2CH2O)20(CH2CH(CH3)O)70(CH2CH2O)20H (Pluronic P123, Aldrich) as a structuredirecting agent, and tetraethyl orthosilicate (TEOS) as a silica source. The chemical composition of the reaction mixture was 1 mol P123: 60 mol TEOS: 390 mol HCl: 8536 mol H2O. Hydrolysis and crystallization temperature is 38 °C and 96 °C, and the corresponding time is 20 and 24 h, respectively. Then, the reaction products were filtered, washed and dried at 45 °C for 48 h. Finally, the samples were calcined at 550 °C in air for 8 h. 2.1.2. Preparation of diluted solution of aluminosilicate sol–gel Tetrapropylammonium bromide (7.2 g) (27.0 mmol TPA+) was dissolved in 28.8 ml of deionized water to form a solution. Next to, 13.5 g of TEOS (64.8 mmol), 0.18–0.93 g of sodium aluminate (41.1wt% Al2O3 and 25.9 wt% Na2O) and 75 ml of H2O (4166.7 mmol) were added into the solution. Subsequently, the resulting mixture was stirred at 25 °C overnight. Finally, the reaction product was diluted with denionized water to yield 450 ml of aluminosilicate sol–gel solution. 2.1.3. Synthesis of MSAMS-2A Calcined SBA-15 (5 g) was added into the diluted solution of aluminosilicate sol–gel (150 ml) under vigorous stirring at room temperature (RT) for 2 h. Next to, the reaction product was transferred into Teflon-lined stainless autoclaves and crystallized at 110–130 °C for 12–36 h. After that, followed by filtered, washed and dried at 80 °C for 24 h. At last, the product was calcined at 550 °C in air for 8 h. 2.1.4. Synthesis of MSAMS-2G Calcined SBA-15 (5 g) was added into the diluted solution of aluminosilicate sol–gel (150 ml) under vigorous stirring at RT for 2 h. Subsequently, the reaction product was filtered, washed and dried at 80 °C for 24 h. After that, the product was suspended in 12.5 ml of glycerol and crystallized at 130–150 °C for 48 h. Finally, the product was filtered, washed, dried, and calcined at 550 °C in air for 8 h.
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300 mA. BET surface area and N2 adsorption–desorption isotherms were carried out on an Omnisorp - 100CX apparatus at 196 °C. All samples were degassed at 250 °C for 2 h prior to analysis. The BET specific surface area was calculated from adsorption data in the relative pressure range from 0.05 to 0.25. The pore size distribution (PSD) was calculated by employing an algorithm based on the concept of Barrett–Joyner–Halenda and using the adsorption branch data of the N2 adsorption–desorption isotherms. TEM images were obtained on a JEM-2010 HR transmission electron microscope, operating with an accelerate voltage of 200 kV. The 27Al and 29Si MAS NMR spectra were recorded on a Bruker DSX 300 spectrometer with a spinning frequency of 12 kHz and 4 kHz, respectively. FT-IR spectra of the samples in the form of KBr pellets were recorded by using a Nicolet 560 IR spectrometer. 2.3. Hydrothermal stability tests The hydrothermal stability of the synthesized samples was investigated by mixing ca. 0.5 g of the calcined sample with 50 g of deionised water and heating in a closed bottle at 100 °C under static conditions for different time periods. 3. Results and discussion Fig. 1 illustrates N2 adsorption–desorption isotherms and the corresponding XRD patterns of SBA-15, MSAMS-2A, MSAMS-2G and MSAMS-2. According to the IUPAC classification, the N2 adsorption–desorption isotherms of MSAMS-2A, MSAMS-2G and MSAMS-2 are founded to be of type IV, which are similar to that of SBA-15. Moreover, a clear type-H1 hysteresis loop is observed for each of them. These results indicate that MSAMS-2A, MSAMS2G and MSAMS-2 display the typical behavior of mesoporous materials (Fig. 1A). The relative pressure (P/P0) position of the inflection point is related to a diameter in the mesopore range, and the sharpness of these steps indicates the uniformity of the pore size [24,25]. As shown in Fig. 1A, the absorption branches of the isotherms for MSAMS-2A, MSAMS-2G and MSAMS-2 show the inflection point at lower relative pressure value than that of SBA-15, indicating that the mean pore size of them is smaller than that of SBA-15. A well resolved pattern with a prominent peak (1 0 0) and two weak peaks (1 1 0) and (2 0 0) are detected in the 2h region of 0.5–2° for all the four samples, which is an indication of good long-range hexagonal ordering. As can be seen by comparing with SBA-15 (Fig. 1B), scarcely any changes in the basal spacing (d1 0 0) and the diffraction intensity of (1 0 0) reflection for MSAMS-2A, MSAMS-2G and MSAMS-2 are observed, indicating that the unit cell dimensions of them are the same as that of SBA-15. On the basis of the characterization results from N2
2.1.5. Synthesis of MSAMS-2 Calcined SBA-15 (5 g) was added into the diluted solution of aluminosilicate sol–gel (150 ml) under vigorous stirring at RT for 2 h. After that, the reaction product was transferred into Teflonlined stainless autoclaves and crystallized at 110–130 °C for 12–36 h, followed by filtration, washed and dried at 80 °C for 24 h. Subsequently, the product was suspended in 12.5 ml of glycerol and crystallized at 130–150 °C for 48 h. Finally, the product was filtered, washed, dried, and calcined at 550 °C in air for 8 h. For comparison purposes, Al-SBA-15 was synthesized according to the procedure reported in Ref. [23]. 2.2. Characterization of materials Powder XRD patterns were performed on a Rigaku D/max 2550 PC diffractometer using Cu Ka radiation, operating at 40 kV and
Fig. 1. (A) N2 adsorption–desorption isotherms and (B) XRD patterns of (a) SBA-15, (b) MSAMS-2A, (c) MSAMS-2G and (d) MSAMS-2. The isotherms of (b), (c) and (d) are offset by 300, 500 and 750 cm3g1 for clarity, respectively.
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adsorption–desorption and XRD, the smaller pore size along with the unchanging unit cell dimension is an indication of thicker pore wall for MSAMS-2A, MSAMS-2G and MSAMS-2. The thicker pore wall is expected to provide significantly improved hydrothermal stability to them [8], which might be attributed to the five-membered ring subunits overgrow within the channels of SBA-15 with the aid of microporous zeolites structure template TPA+. Fig. 2 provides the XRD patterns of SBA-15, MSAMS-2A, MSAMS-2G and MSAMS-2 before and after hydrothermal stability tests. As shown in Fig. 2A, SBA-15 has maintained a well-ordered hexagonal symmetry mesostructure (p6mm) no more than 120 h, which is in excellent agreement with the previous reports [8,26]. MSAMS-2A retains XRD order after treated with 100 °C water over 120 h, but the mesostructure was almost destroyed after this treatment for 360 h (Fig. 2B). XRD results clearly exhibit that MSAMS2G retains ordered mesostructure after treated with 100 °C water for 360 h. However, with the treatment time increasing to 434 h, the mesostructure of MSAMS-2G was completely lost (Fig. 2C). As for MSAMS-2, an intense (1 0 0) diffraction peak and two well resolved higher order (1 1 0) and (2 0 0) peaks are still observed in spite of the treatment time increasing to 434 h (Fig. 2D). Furthermore, there were no significant changes in the basal spacing (d1 0 0), the linewidths and (1 0 0) reflection diffraction intensity of MSAMS-2 resulted from the hydrothermal stability treatment. However, it is noted that the ratio of the diffraction intensity of (200) to (110) decreases with the treatment time increasing, suggesting that the corresponding pore wall thickness becomes thinner [27–31]. The N2 adsorption–desorption isotherms and corresponding BJH pore size distribution curves for MSAMS-2 before and after hydrothermal stability tests are displayed in Fig. 3. All the samples after hydrothermal treatment still exhibit type IV isotherms with H1 hysteresis loops (Fig. 3A). Furthermore, a narrow pore size distribution is observed for each of them (Fig. 3B). Combining the results of XRD characterization which show that MSAMS-2 retains XRD order, it can be concluded that the mesoporous channels of MSAMS-2 with two-dimensional hexagonally ordered
Fig. 2. XRD patterns of (A) SBA-15, (B) MSAMS-2A, (C) MSAMS-2G and (D) MSAMS2 before and after treated with 100 °C water.
Fig. 3. (A) N2 adsorption–desorption isotherms and (B) BJH pore size distribution curves of MSAMS-2 before and after hydrothermal stability test. (a) Before, (b) after treated in 100 °C water for 120 h, and (c) after treated in 100 °C water for 434 h. The isotherms of (b) and (c) are offset by 200 and 400 cm3g1 for clarity, respectively.
arrays were perfectly maintained after these hydrothermal treatments. Table 1 gives the corresponding physicochemical properties of MSAMS-2 and compares them with those for SBA-15, MSAMS-2A and MSAMS-2G under equivalent hydrothermal treatment conditions. It is very interesting to note that the surface area and mesoporous volume of MSAMS-2 increase slightly even after treated with 100 °C water for 434 h, while the surface area and total pore volume of SBA-15, MSAMS-2A and MSAMS-2G reduce with the hydrothermal treatment time increasing. In light of these results, the hydrothermal stability of the four samples is in the order of MSAMS-2 > MSAMS-2G MSAMS-2A > SBA-15. In following of this paper, other characterization methods are used to investigate the difference of hydrothermal stability for SBA-15, MSAMS-2A, MSAMS-2G and MSAMS-2 in detail. The band centered in the 550–600 cm1 region is well-expressed in the spectra of MSAMS-2, MSAMS-2A and MSAMS-2G but not for the SBA-15 (Fig. 4), which was assigned to characteristic vibration of five-membered ring subunits [32–34]. The band presence indicates that zeolite-like subunits comprised of AlO4 and SiO4 tetrahedra were formed within the framework of these ordered mesoporous materials, which should play an important role in improving their hydrothermal stability [12]. A distinct IR vibration centered at 960 cm1 was detected for the four samples, which is the characteristic of non-condensed silanol groups [35,36]. However, the intensity of the band for MSAMS-2, MSAMS-2A and MSAMS-2G is much weaker than that of SBA-15, suggesting that the condensation degree of silanol groups of the three samples was far higher than that of SBA-15, as further confirmed by 29Si NMR characterization result in the following of the paper. Fig. 5 illustrates 27Al MAS NMR spectra of MSAMS-2A, MSAMS2G and MSAMS-2. It is clearly that only a single resonance centered near 53 ppm is detected for the 27Al MAS NMR spectrum of MSAMS-2, which has been attributed to tetrahedral aluminum center (framework Al) and indicated that all the Al atoms were incorporated into the mesoporous framework of MSAMS-2. An intense along with a weak resonance, centered near 53 and 0 ppm, are observed for the 27Al MAS NMR spectra of MSAMS-2A and MSAMS-2G, and the latter resonance is characteristic of octahedrally coordinated Al (non-framework Al, d = 0 ppm) [37,38]. The 27Al NMR results indicate that non-framework Al atoms can be inserted into the framework of MSAMS-2 by re-crystallization both within the diluted solution of aluminosilicate sol–gel and glycerol. This is likely because the octahedrally coordinated Al atoms interact with the silanol groups (these are the weak links in SBA-15 framework and are known to occur at defect sites), which not only further reduces the concentration of silanol
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C. Han et al. / Advanced Powder Technology 22 (2011) 20–25 Table 1 Textural properties of the samples before and after hydrothermal stability test.
a b c d e f g h i j k
Sample
Si/Ala
a0b (Å)
SBET (m2/g)
MIVc (cm3/g)
MEVd (cm3/g)
PSe (Å)
WTf (Å)
wSV1g
SBA-15 SBA-15h MSAMS-2A MSAMS-2A h MSAMS-2A j MSAMS-2G MSAMS-2G h MSAMS-2G j MSAMS-2G k MSAMS-2 MSAMS-2h MSAMS-2j MSAMS-2k Al-SBA-15 Al-SBA-15h
1 –i 100 –i –i 100 –i –i –i 100 –i –i –i 100 –i
116 –i 116 115 –i 117 116 115 –i 117 118 118 118 120 –i
616 295 357 344 215 361 355 333 197 353 375 370 362 914 341
0.22 0.11 0.11 0.11 0.09 0.12 0.12 0.11 0.10 0.12 0.12 0.12 0.12 0.27 0.13
0.77 0.67 0.62 0.61 0.55 0.63 0.64 0.62 0.57 0.62 0.64 0.65 0.65 0.99 0.74
80 –i 72 78 –i 72 77 88 –i 71 74 79 83 85 –i
36 –i 44 37 –i 45 39 27 –i 46 44 39 33 35 –i
6.4 –i 4.1 4.4 –i 4.1 4.3 4.7 –i 4.0 4.3 4.5 4.6 7.8 –i
Molar ratio, obtained by theoreticpcalculation. ffiffiffi Unit cell dimension a0 = d100 2/ 3:. MIV (micropore volume) is accumulated the pore volume by HK model from 0.9 nm to 2.0 nm in diameter. MEV (mesoporous volume) is BHJ cumulative pore volume of pores diameter between 2 and 50 nm. PS (Pore size) determined from adsorption branch of the N2 adsorption–desorption isotherms according to BJH method. WT (Wall thickness) = a0 – PS (pore size). Herein, w, S and V are the pore size, surface area, and mesopore volume, respectively. Treated in 100 °C water for 120 h. No data or the data is imponderable. Treated in 100 °C water for 360 h. Treated in 100 °C water for 434 h.
Fig. 5.
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Al MAS NMR spectra of (a) MSAMS-2A, (b) MSAMS-2G and (c) MSAMS-2.
Fig. 4. FT-IR spectra of (a) SBA-15, (b) MSAMS-2, (c) MSAMS-2A and (d) MSAMS-2G.
groups but acts to heal defect sites in the structure of SBA-15 [39], thus stabilizing the structure of MSAMS-2. The result of 27 Al MAS NMR provides a conceivable confirmation to prove that hydrothermal stability of MSAMS-2 is higher than that of MSAMS-2A and MSAMS-2G. Normally, surfactant-containing silica mesostructure, whether assembled from ionic or neutral surfactants, exhibits Q4/Q3 ratios that are less than 2.0, and their calcined derivatives typically have values near 3.0 [40–42]. For example, the Q4/Q3 ratio of SBA-15 containing surfactant P123 is 1.9 [26]. The 29Si MAS NMR spectra of MSAMS-2A, MSAMS-2G and MSAMS-2 are shown in Fig. 6. It is clearly that only a primary resonance at d = 110 ppm and a weak peak at d = 100 ppm correspond to Si(SiO)4 (fully cross-linked Q4 silica unit) and (HO)Si(SiO)3 (incompletely cross-linked Q3 silica unit), respectively, are detected for the three samples. As deduced from spectra, the ratio
of Q4/Q3 of MSAMS-2A, MSAMS-2G and MSAMS-2 is 5.9, 6.3 and 7.2, respectively, which may be due to multistage and high temperature re-crystallization [26,31,43]. It is well-documented that both the insertion of non-framework Al atoms into the framework which act to heal defect sites in the structure of mesoporous materials and the high degree of condensation silanol groups are responsible for significant improving their hydrothermal stability [26,37,41,43,44]. FE-SEM images of SBA-15, MSAMS-2A, MSAMS-2G and MSAMS-2 are displayed in Fig. 7. In general, the morphologies of MSAMS-2A, MSAMS-2G and MSAMS-2 are similar to those of parent SBA-15. However, some isolated sphere particles comprised of zeolite-like subunits with diameter from several tens to 200 nm arisen from the diluted solution of aluminosilicates sol–gel are still observed for MSAMS-2A and MSAMS-2G but not for MSAMS-2. As can be seen by comparing Fig. 7b and c, it is noticeable that the number of isolated sphere particles of MSAMS-2G is far less than that of MSAMS-2A. The most probable
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zeolite-like subunits and moreover glycerol can act as a stabilizing guest molecule.
4. Conclusions
Fig. 6.
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Si MAS NMR spectra of (a) MSAMS-2A, (b) MSAMS-2G and (c) MSAMS-2.
explanation for the difference between MSAMS-2A and MSAMS2G observed here is that glycerol is of benefit to the interaction between the isolated sphere particles and the silanol groups of SBA-15, which is well consistent with the previous reports that various silicate structures are formed during aging due to the interaction of glycerol with silicate species [45,46]. Moreover, the less sphere particles also indicates that the condensation degree of silanol groups for MSAMS-2G is higher than that of MSAMS-2A, as confirmed by 29Si MAS NMR. Therefore, we postulate that the difference of hydrothermal stability between MSAMS-2A and MSAMS-2G is likely because the high viscosity of glycerol will be in favor of the silanol groups interacting with
In summary, well-ordered hexagonal mesoporous aluminosilicates molecular sieves, designated as MSAMS-2A, MSAMS-2G and MSAMS-2, have been successfully synthesized via re-crystallization of SBA-15 within the diluted solution of aluminosilicates sol–gel, glycerol, or both of them. All the three sieves retain XRD order after treated with 100 °C water over 120 h, and the hydrothermal stability of them is in the order of MSAMS-2 > MSAMS2G MSAMS-2A > SBA-15. Compared with SBA-15, the formation five-membered ring subunits within MSAMS-2A taken with thicker pore wall might be responsible for the significant improvement of it hydrothermal stability. The hydrothermal stability difference between MSAMS-2G and MSAMS-2A is likely because the high viscosity of glycerol will be in favor of the silanol groups interacting with zeolite-like subunits and moreover glycerol can act as a stabilizing guest molecule. The hydrothermal stability of MSAMS-2 is higher than that of MSAMS-2G., which might be attributed to the synergistic effect of the higher condensation of silanol groups and insertion of all non-framework Al atoms into the framework of MSAMS-2. Acknowledgements This work was supported by National Natural Science Foundation of China (Grant Nos. 20867003, 90610035), Young Academic and Technical Leader Raising Foundation of Yunnan Province (No. 2008py010), and Natural Science Foundation of Kunming University of Science and Technology (Grant No. KKZ3200822027).
Fig. 7. FE-SEM images of (a) SBA-15, (b) MSAMS-2A, (c) MSAMS-2G, and (d) MSAMS-2.
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Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Characterization and investigation on the difference of hydrothermal stability for ordered mesoporous aluminosilicate sieves Caiyun Han a, Hua Wang b, Liuyi Zhang a, Rongtao Li a, Yanyan Zhang a, Yongming Luo a,b,*, Xiaoming Zheng c,* a b c
College of Environmental Science and Engineering, Kunming University of Science and Technology, Kunming 650093, PR China College of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, PR China Institute of Catalysis, Department of Chemistry, Zhejiang University (Xixi campus), Hangzhou 310028, PR China
a r t i c l e
i n f o
Article history: Received 11 December 2009 Received in revised form 28 February 2010 Accepted 1 March 2010 Available online 9 March 2010 Keywords: Ordered mesoporous sieves Synthesis Hydrothermal stability Characterization
a b s t r a c t Ordered hexagonal mesoporous aluminosilicates molecular sieves, designated as MSAMS-2A, MSAMS-2G and MSAMS-2, have been synthesized via re-crystallization of mesoporous SBA-15 within the diluted solution of aluminosilicate sol–gel, glycerol, and both of them, respectively. The three materials have been characterized by XRD, N2 adsorption–desorption, FT-IR, FE-SEM, 27Al MAS NMR and 29Si MAS NMR, and the corresponding hydrothermal stability of these three sieves is in the order of MSAMS2 > MSAMS-2G MSAMS-2A. The hydrothermal stability difference between MSAMS-2 and MSAMS2G might be attributed to the synergistic effect of the higher condensation of silanol groups and insertion of all non-framework Al atoms into the framework of MSAMS-2. The hydrothermal stability of MSAMS2G is higher than that of MSAMS-2A, which is likely because the high viscosity of glycerol will be in favor of the silanol groups interacting with zeolite-like subunits and moreover glycerol can act as a stabilizing guest molecule. Ó 2010 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Ordered mesoporous molecular sieves (OMMSs) with high surface area, well-defined and tunable pore diameters of 2–50 nm have attracted much attention for potential applications as versatile catalysts and catalysts supports, template, adsorption and separation materials [1–9]. However, compared with microporous aluminosilicate zeolites, the acidity and hydrothermal stability of OMMSs are relatively low, which severely limit them practical applications as catalytic support and/or catalysts in petroleum refining and fine chemical synthesis [1,2,6–9]. In general, both weak acidity and low hydrothermal stability of OMMSs could be attributed to the amorphous nature of their mesoporous pore wall. Therefore, in the past decade years, many efforts have been focused on improving both the hydrothermal stability and the acidity of OMMSs [9–19]. One might expect to improve both the stability and acidity of these materials if zeolite-like order could be introduced into the mesopore walls. It is well-documented that the amorphous walls of the mesostructure could indeed be converted
* Corresponding authors. Address: College of Environmental Science and Engineering, Kunming University of Science and Technology, Kunming 650093, PR China. Tel.: +86 871 5170905; fax: +86 871 5170906 (Y. Luo). E-mail addresses: [email protected], [email protected] (Y. Luo).
to a partially zeolitic product. For instance, van Bekkum et al. reported that the walls of Al-HMS and Al-MCM-41 could be transformed into partially zeolitic structures by ion-exchanged them with a microporous zeolites structure template such as TPA+[10]. Hydrothermally stable mesoporous aluminosilicate molecular sieves have been synthesized via assembly of zeolite seeds of ZSM-5, BEA, TS-1 and FAU by use of cetyltrimethyl ammonium bromide (CTAB) or HO(CH2CH2O)20(CH2CH(CH3)O)70(CH2CH2O)20H (P123) as a structure-directing agent [11–17]. A coated route was used to synthesis of ultrastable mesoporous aluminosilicate molecular sieves through ion-exchanged Al-SBA-15 and Al-MCF with ZSM-5 and FAU seeds, respectively [18,19]. More recently, SBA-16 and MCM-41 type mesoporous aluminosilicates with high hydrothermal stability have been synthesized by the use of the dissolution of zeolite Na-A [20,21] and the dissolved ZSM-5 [22] zeolite as a single-source of silicon and aluminum. In this paper, three well-ordered hexagonal mesoporous aluminosilicates molecular sieves, designated as MSAMS-2A, MSAMS-2G and MSAMS-2, have been successfully synthesized via re-crystallization of SBA-15 within the diluted solution of aluminosilicate sol–gel, glycerol, and both of them, respectively. XRD results clearly exhibit that all the three ordered mesoporous aluminosilicate molecular sieves exhibit higher hydrothermal stability than that of SBA-15, and the corresponding hydrothermal stability of them is in the order of MSAMS-2 > MSAMS-2G MSAMS-2A.
0921-8831/$ - see front matter Ó 2010 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. doi:10.1016/j.apt.2010.03.001
C. Han et al. / Advanced Powder Technology 22 (2011) 20–25
Furthermore, the difference of hydrothermal stability among the three ordered mesoporous Aluminosilicate molecular sieves was investigated and discussed in detail via the use of FT-IR, FE-SEM, 27 Al and 29Si MAS NMR characterization. 2. Experimental 2.1. Synthesis of materials 2.1.1. Synthesis of SBA-15 Hexagonal ordered mesoporous SBA-15 molecular sieve was synthesized by using triblock copolymer HO(CH2CH2O)20(CH2CH(CH3)O)70(CH2CH2O)20H (Pluronic P123, Aldrich) as a structuredirecting agent, and tetraethyl orthosilicate (TEOS) as a silica source. The chemical composition of the reaction mixture was 1 mol P123: 60 mol TEOS: 390 mol HCl: 8536 mol H2O. Hydrolysis and crystallization temperature is 38 °C and 96 °C, and the corresponding time is 20 and 24 h, respectively. Then, the reaction products were filtered, washed and dried at 45 °C for 48 h. Finally, the samples were calcined at 550 °C in air for 8 h. 2.1.2. Preparation of diluted solution of aluminosilicate sol–gel Tetrapropylammonium bromide (7.2 g) (27.0 mmol TPA+) was dissolved in 28.8 ml of deionized water to form a solution. Next to, 13.5 g of TEOS (64.8 mmol), 0.18–0.93 g of sodium aluminate (41.1wt% Al2O3 and 25.9 wt% Na2O) and 75 ml of H2O (4166.7 mmol) were added into the solution. Subsequently, the resulting mixture was stirred at 25 °C overnight. Finally, the reaction product was diluted with denionized water to yield 450 ml of aluminosilicate sol–gel solution. 2.1.3. Synthesis of MSAMS-2A Calcined SBA-15 (5 g) was added into the diluted solution of aluminosilicate sol–gel (150 ml) under vigorous stirring at room temperature (RT) for 2 h. Next to, the reaction product was transferred into Teflon-lined stainless autoclaves and crystallized at 110–130 °C for 12–36 h. After that, followed by filtered, washed and dried at 80 °C for 24 h. At last, the product was calcined at 550 °C in air for 8 h. 2.1.4. Synthesis of MSAMS-2G Calcined SBA-15 (5 g) was added into the diluted solution of aluminosilicate sol–gel (150 ml) under vigorous stirring at RT for 2 h. Subsequently, the reaction product was filtered, washed and dried at 80 °C for 24 h. After that, the product was suspended in 12.5 ml of glycerol and crystallized at 130–150 °C for 48 h. Finally, the product was filtered, washed, dried, and calcined at 550 °C in air for 8 h.
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300 mA. BET surface area and N2 adsorption–desorption isotherms were carried out on an Omnisorp - 100CX apparatus at 196 °C. All samples were degassed at 250 °C for 2 h prior to analysis. The BET specific surface area was calculated from adsorption data in the relative pressure range from 0.05 to 0.25. The pore size distribution (PSD) was calculated by employing an algorithm based on the concept of Barrett–Joyner–Halenda and using the adsorption branch data of the N2 adsorption–desorption isotherms. TEM images were obtained on a JEM-2010 HR transmission electron microscope, operating with an accelerate voltage of 200 kV. The 27Al and 29Si MAS NMR spectra were recorded on a Bruker DSX 300 spectrometer with a spinning frequency of 12 kHz and 4 kHz, respectively. FT-IR spectra of the samples in the form of KBr pellets were recorded by using a Nicolet 560 IR spectrometer. 2.3. Hydrothermal stability tests The hydrothermal stability of the synthesized samples was investigated by mixing ca. 0.5 g of the calcined sample with 50 g of deionised water and heating in a closed bottle at 100 °C under static conditions for different time periods. 3. Results and discussion Fig. 1 illustrates N2 adsorption–desorption isotherms and the corresponding XRD patterns of SBA-15, MSAMS-2A, MSAMS-2G and MSAMS-2. According to the IUPAC classification, the N2 adsorption–desorption isotherms of MSAMS-2A, MSAMS-2G and MSAMS-2 are founded to be of type IV, which are similar to that of SBA-15. Moreover, a clear type-H1 hysteresis loop is observed for each of them. These results indicate that MSAMS-2A, MSAMS2G and MSAMS-2 display the typical behavior of mesoporous materials (Fig. 1A). The relative pressure (P/P0) position of the inflection point is related to a diameter in the mesopore range, and the sharpness of these steps indicates the uniformity of the pore size [24,25]. As shown in Fig. 1A, the absorption branches of the isotherms for MSAMS-2A, MSAMS-2G and MSAMS-2 show the inflection point at lower relative pressure value than that of SBA-15, indicating that the mean pore size of them is smaller than that of SBA-15. A well resolved pattern with a prominent peak (1 0 0) and two weak peaks (1 1 0) and (2 0 0) are detected in the 2h region of 0.5–2° for all the four samples, which is an indication of good long-range hexagonal ordering. As can be seen by comparing with SBA-15 (Fig. 1B), scarcely any changes in the basal spacing (d1 0 0) and the diffraction intensity of (1 0 0) reflection for MSAMS-2A, MSAMS-2G and MSAMS-2 are observed, indicating that the unit cell dimensions of them are the same as that of SBA-15. On the basis of the characterization results from N2
2.1.5. Synthesis of MSAMS-2 Calcined SBA-15 (5 g) was added into the diluted solution of aluminosilicate sol–gel (150 ml) under vigorous stirring at RT for 2 h. After that, the reaction product was transferred into Teflonlined stainless autoclaves and crystallized at 110–130 °C for 12–36 h, followed by filtration, washed and dried at 80 °C for 24 h. Subsequently, the product was suspended in 12.5 ml of glycerol and crystallized at 130–150 °C for 48 h. Finally, the product was filtered, washed, dried, and calcined at 550 °C in air for 8 h. For comparison purposes, Al-SBA-15 was synthesized according to the procedure reported in Ref. [23]. 2.2. Characterization of materials Powder XRD patterns were performed on a Rigaku D/max 2550 PC diffractometer using Cu Ka radiation, operating at 40 kV and
Fig. 1. (A) N2 adsorption–desorption isotherms and (B) XRD patterns of (a) SBA-15, (b) MSAMS-2A, (c) MSAMS-2G and (d) MSAMS-2. The isotherms of (b), (c) and (d) are offset by 300, 500 and 750 cm3g1 for clarity, respectively.
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C. Han et al. / Advanced Powder Technology 22 (2011) 20–25
adsorption–desorption and XRD, the smaller pore size along with the unchanging unit cell dimension is an indication of thicker pore wall for MSAMS-2A, MSAMS-2G and MSAMS-2. The thicker pore wall is expected to provide significantly improved hydrothermal stability to them [8], which might be attributed to the five-membered ring subunits overgrow within the channels of SBA-15 with the aid of microporous zeolites structure template TPA+. Fig. 2 provides the XRD patterns of SBA-15, MSAMS-2A, MSAMS-2G and MSAMS-2 before and after hydrothermal stability tests. As shown in Fig. 2A, SBA-15 has maintained a well-ordered hexagonal symmetry mesostructure (p6mm) no more than 120 h, which is in excellent agreement with the previous reports [8,26]. MSAMS-2A retains XRD order after treated with 100 °C water over 120 h, but the mesostructure was almost destroyed after this treatment for 360 h (Fig. 2B). XRD results clearly exhibit that MSAMS2G retains ordered mesostructure after treated with 100 °C water for 360 h. However, with the treatment time increasing to 434 h, the mesostructure of MSAMS-2G was completely lost (Fig. 2C). As for MSAMS-2, an intense (1 0 0) diffraction peak and two well resolved higher order (1 1 0) and (2 0 0) peaks are still observed in spite of the treatment time increasing to 434 h (Fig. 2D). Furthermore, there were no significant changes in the basal spacing (d1 0 0), the linewidths and (1 0 0) reflection diffraction intensity of MSAMS-2 resulted from the hydrothermal stability treatment. However, it is noted that the ratio of the diffraction intensity of (200) to (110) decreases with the treatment time increasing, suggesting that the corresponding pore wall thickness becomes thinner [27–31]. The N2 adsorption–desorption isotherms and corresponding BJH pore size distribution curves for MSAMS-2 before and after hydrothermal stability tests are displayed in Fig. 3. All the samples after hydrothermal treatment still exhibit type IV isotherms with H1 hysteresis loops (Fig. 3A). Furthermore, a narrow pore size distribution is observed for each of them (Fig. 3B). Combining the results of XRD characterization which show that MSAMS-2 retains XRD order, it can be concluded that the mesoporous channels of MSAMS-2 with two-dimensional hexagonally ordered
Fig. 2. XRD patterns of (A) SBA-15, (B) MSAMS-2A, (C) MSAMS-2G and (D) MSAMS2 before and after treated with 100 °C water.
Fig. 3. (A) N2 adsorption–desorption isotherms and (B) BJH pore size distribution curves of MSAMS-2 before and after hydrothermal stability test. (a) Before, (b) after treated in 100 °C water for 120 h, and (c) after treated in 100 °C water for 434 h. The isotherms of (b) and (c) are offset by 200 and 400 cm3g1 for clarity, respectively.
arrays were perfectly maintained after these hydrothermal treatments. Table 1 gives the corresponding physicochemical properties of MSAMS-2 and compares them with those for SBA-15, MSAMS-2A and MSAMS-2G under equivalent hydrothermal treatment conditions. It is very interesting to note that the surface area and mesoporous volume of MSAMS-2 increase slightly even after treated with 100 °C water for 434 h, while the surface area and total pore volume of SBA-15, MSAMS-2A and MSAMS-2G reduce with the hydrothermal treatment time increasing. In light of these results, the hydrothermal stability of the four samples is in the order of MSAMS-2 > MSAMS-2G MSAMS-2A > SBA-15. In following of this paper, other characterization methods are used to investigate the difference of hydrothermal stability for SBA-15, MSAMS-2A, MSAMS-2G and MSAMS-2 in detail. The band centered in the 550–600 cm1 region is well-expressed in the spectra of MSAMS-2, MSAMS-2A and MSAMS-2G but not for the SBA-15 (Fig. 4), which was assigned to characteristic vibration of five-membered ring subunits [32–34]. The band presence indicates that zeolite-like subunits comprised of AlO4 and SiO4 tetrahedra were formed within the framework of these ordered mesoporous materials, which should play an important role in improving their hydrothermal stability [12]. A distinct IR vibration centered at 960 cm1 was detected for the four samples, which is the characteristic of non-condensed silanol groups [35,36]. However, the intensity of the band for MSAMS-2, MSAMS-2A and MSAMS-2G is much weaker than that of SBA-15, suggesting that the condensation degree of silanol groups of the three samples was far higher than that of SBA-15, as further confirmed by 29Si NMR characterization result in the following of the paper. Fig. 5 illustrates 27Al MAS NMR spectra of MSAMS-2A, MSAMS2G and MSAMS-2. It is clearly that only a single resonance centered near 53 ppm is detected for the 27Al MAS NMR spectrum of MSAMS-2, which has been attributed to tetrahedral aluminum center (framework Al) and indicated that all the Al atoms were incorporated into the mesoporous framework of MSAMS-2. An intense along with a weak resonance, centered near 53 and 0 ppm, are observed for the 27Al MAS NMR spectra of MSAMS-2A and MSAMS-2G, and the latter resonance is characteristic of octahedrally coordinated Al (non-framework Al, d = 0 ppm) [37,38]. The 27Al NMR results indicate that non-framework Al atoms can be inserted into the framework of MSAMS-2 by re-crystallization both within the diluted solution of aluminosilicate sol–gel and glycerol. This is likely because the octahedrally coordinated Al atoms interact with the silanol groups (these are the weak links in SBA-15 framework and are known to occur at defect sites), which not only further reduces the concentration of silanol
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C. Han et al. / Advanced Powder Technology 22 (2011) 20–25 Table 1 Textural properties of the samples before and after hydrothermal stability test.
a b c d e f g h i j k
Sample
Si/Ala
a0b (Å)
SBET (m2/g)
MIVc (cm3/g)
MEVd (cm3/g)
PSe (Å)
WTf (Å)
wSV1g
SBA-15 SBA-15h MSAMS-2A MSAMS-2A h MSAMS-2A j MSAMS-2G MSAMS-2G h MSAMS-2G j MSAMS-2G k MSAMS-2 MSAMS-2h MSAMS-2j MSAMS-2k Al-SBA-15 Al-SBA-15h
1 –i 100 –i –i 100 –i –i –i 100 –i –i –i 100 –i
116 –i 116 115 –i 117 116 115 –i 117 118 118 118 120 –i
616 295 357 344 215 361 355 333 197 353 375 370 362 914 341
0.22 0.11 0.11 0.11 0.09 0.12 0.12 0.11 0.10 0.12 0.12 0.12 0.12 0.27 0.13
0.77 0.67 0.62 0.61 0.55 0.63 0.64 0.62 0.57 0.62 0.64 0.65 0.65 0.99 0.74
80 –i 72 78 –i 72 77 88 –i 71 74 79 83 85 –i
36 –i 44 37 –i 45 39 27 –i 46 44 39 33 35 –i
6.4 –i 4.1 4.4 –i 4.1 4.3 4.7 –i 4.0 4.3 4.5 4.6 7.8 –i
Molar ratio, obtained by theoreticpcalculation. ffiffiffi Unit cell dimension a0 = d100 2/ 3:. MIV (micropore volume) is accumulated the pore volume by HK model from 0.9 nm to 2.0 nm in diameter. MEV (mesoporous volume) is BHJ cumulative pore volume of pores diameter between 2 and 50 nm. PS (Pore size) determined from adsorption branch of the N2 adsorption–desorption isotherms according to BJH method. WT (Wall thickness) = a0 – PS (pore size). Herein, w, S and V are the pore size, surface area, and mesopore volume, respectively. Treated in 100 °C water for 120 h. No data or the data is imponderable. Treated in 100 °C water for 360 h. Treated in 100 °C water for 434 h.
Fig. 5.
27
Al MAS NMR spectra of (a) MSAMS-2A, (b) MSAMS-2G and (c) MSAMS-2.
Fig. 4. FT-IR spectra of (a) SBA-15, (b) MSAMS-2, (c) MSAMS-2A and (d) MSAMS-2G.
groups but acts to heal defect sites in the structure of SBA-15 [39], thus stabilizing the structure of MSAMS-2. The result of 27 Al MAS NMR provides a conceivable confirmation to prove that hydrothermal stability of MSAMS-2 is higher than that of MSAMS-2A and MSAMS-2G. Normally, surfactant-containing silica mesostructure, whether assembled from ionic or neutral surfactants, exhibits Q4/Q3 ratios that are less than 2.0, and their calcined derivatives typically have values near 3.0 [40–42]. For example, the Q4/Q3 ratio of SBA-15 containing surfactant P123 is 1.9 [26]. The 29Si MAS NMR spectra of MSAMS-2A, MSAMS-2G and MSAMS-2 are shown in Fig. 6. It is clearly that only a primary resonance at d = 110 ppm and a weak peak at d = 100 ppm correspond to Si(SiO)4 (fully cross-linked Q4 silica unit) and (HO)Si(SiO)3 (incompletely cross-linked Q3 silica unit), respectively, are detected for the three samples. As deduced from spectra, the ratio
of Q4/Q3 of MSAMS-2A, MSAMS-2G and MSAMS-2 is 5.9, 6.3 and 7.2, respectively, which may be due to multistage and high temperature re-crystallization [26,31,43]. It is well-documented that both the insertion of non-framework Al atoms into the framework which act to heal defect sites in the structure of mesoporous materials and the high degree of condensation silanol groups are responsible for significant improving their hydrothermal stability [26,37,41,43,44]. FE-SEM images of SBA-15, MSAMS-2A, MSAMS-2G and MSAMS-2 are displayed in Fig. 7. In general, the morphologies of MSAMS-2A, MSAMS-2G and MSAMS-2 are similar to those of parent SBA-15. However, some isolated sphere particles comprised of zeolite-like subunits with diameter from several tens to 200 nm arisen from the diluted solution of aluminosilicates sol–gel are still observed for MSAMS-2A and MSAMS-2G but not for MSAMS-2. As can be seen by comparing Fig. 7b and c, it is noticeable that the number of isolated sphere particles of MSAMS-2G is far less than that of MSAMS-2A. The most probable
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C. Han et al. / Advanced Powder Technology 22 (2011) 20–25
zeolite-like subunits and moreover glycerol can act as a stabilizing guest molecule.
4. Conclusions
Fig. 6.
29
Si MAS NMR spectra of (a) MSAMS-2A, (b) MSAMS-2G and (c) MSAMS-2.
explanation for the difference between MSAMS-2A and MSAMS2G observed here is that glycerol is of benefit to the interaction between the isolated sphere particles and the silanol groups of SBA-15, which is well consistent with the previous reports that various silicate structures are formed during aging due to the interaction of glycerol with silicate species [45,46]. Moreover, the less sphere particles also indicates that the condensation degree of silanol groups for MSAMS-2G is higher than that of MSAMS-2A, as confirmed by 29Si MAS NMR. Therefore, we postulate that the difference of hydrothermal stability between MSAMS-2A and MSAMS-2G is likely because the high viscosity of glycerol will be in favor of the silanol groups interacting with
In summary, well-ordered hexagonal mesoporous aluminosilicates molecular sieves, designated as MSAMS-2A, MSAMS-2G and MSAMS-2, have been successfully synthesized via re-crystallization of SBA-15 within the diluted solution of aluminosilicates sol–gel, glycerol, or both of them. All the three sieves retain XRD order after treated with 100 °C water over 120 h, and the hydrothermal stability of them is in the order of MSAMS-2 > MSAMS2G MSAMS-2A > SBA-15. Compared with SBA-15, the formation five-membered ring subunits within MSAMS-2A taken with thicker pore wall might be responsible for the significant improvement of it hydrothermal stability. The hydrothermal stability difference between MSAMS-2G and MSAMS-2A is likely because the high viscosity of glycerol will be in favor of the silanol groups interacting with zeolite-like subunits and moreover glycerol can act as a stabilizing guest molecule. The hydrothermal stability of MSAMS-2 is higher than that of MSAMS-2G., which might be attributed to the synergistic effect of the higher condensation of silanol groups and insertion of all non-framework Al atoms into the framework of MSAMS-2. Acknowledgements This work was supported by National Natural Science Foundation of China (Grant Nos. 20867003, 90610035), Young Academic and Technical Leader Raising Foundation of Yunnan Province (No. 2008py010), and Natural Science Foundation of Kunming University of Science and Technology (Grant No. KKZ3200822027).
Fig. 7. FE-SEM images of (a) SBA-15, (b) MSAMS-2A, (c) MSAMS-2G, and (d) MSAMS-2.
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