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Synthesis and characterization of mesoporous aluminosilicate molecular sieve from K-feldspar
Microporous and Mesoporous Materials 83 (2005) 277–282 www.elsevier.com/locate/micromeso
Synthesis and characterization of mesoporous aluminosilicate molecular sieve from K-feldspar Shiding Miao a, Zhimin Liu a,*, Hongwen Ma b, Buxing Han a, Jimin Du a, Zhenyu Sun a, Zhenjiang Miao a a
CAS Key Laboratory of Colloid, Interfacial and Chemical Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China b National Laboratory of Mineral Materials, China University of Geosciences, Beijing 100083, China Received 7 April 2005; received in revised form 4 May 2005; accepted 5 May 2005 Available online 16 June 2005
Abstract Mesoporous aluminosilicate molecular sieves have been synthesized using K-feldspar, a natural ore rich in silicon and aluminum sources, as starting material. In this method, the mixture of K-feldspar and K2CO3 with mass ratio of 2:3 was first calcined at 880 °C for 1.2 h. The calcined materials were then dissolved in water together with cetyltrimethylammonium bromide as structure-directing agent, resulting in mesoporous molecular sieves after heating the solution at 130 °C for 60 h. Scanning electron microscopy and transmission electron microscopy observations indicated that the resulting materials were spherical particles with size of about 100 nm. The mesoporous structure of the as-synthesized materials was confirmed by low angle X-ray diffraction and nitrogen sorption analysis. The BET surface area of the as-prepared material after calcined at 550 °C was 507 m2 g1 and the pore volume was 0.854 cm3 g1. 27 Al MAS NMR analysis showed that nearly all of aluminium is incorporated into the framework of the mesoporous material with 4-coordinated state. The acidity of the product was analyzed by FTIR spectroscopy of pyridine-adsorbed product. The product was treated in water at 100 °C for 360 h, and its mesoporous structure was still intact, suggesting its high hydrothermal stability. Ó 2005 Elsevier Inc. All rights reserved. Keywords: Mesoporous material; Aluminosilicate; Molecular sieves; K-feldspar
1. Introduction Since the discovery of the mesoporous M41S materials in 1992 [1], the ordered mesoporous materials have attracted much attention. Among the mesoporous materials, hexagonally ordered silica MCM-41, characterized with pore size between 2 and 10 nm with a narrow pore size distribution, has been investigated extensively. However, due to the amorphous nature of their walls, silica MCM-41 materials have relatively low acidity and hydrothermal stability, which severely hinders their industrial applications in catalytic reactions [2]. The *
Corresponding author. Tel./fax: +86 1062562821. E-mail address: [email protected] (Z. Liu).
1387-1811/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.05.006
incorporation of Al (called Al-MCM-41) and transition metal elements within the silica framework has been implemented, which improved the hydrothermal stability and acidity to some extent [3]. Al-MCM-41 with tetrahedral coordinated Al into the silica framework can generate Bro¨nsted acid sites, which has been the focus of much recent research. A number of papers concerning the synthesis and characterization of Al-MCM-41 mesoporous materials have been published [4], and tetraethoxysilane, aluminum isopropoxide, sodium aluminate, and other aluminosilicates were generally used as Si, Al sources. Many natural ores are rich in silicon and aluminum, which are very cheap, and can be easily obtained. Obviously, using these ores as raw materials to synthesize Al substituted molecular sieves is of great importance.
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However, study on the synthesis of mesoporous molecular sieves containing Al and Si oxides using natural ores as aluminum and silicon sources is scarce. Recently, Kang et al. [5] synthesized Al-MCM-41 using metakaolin as aluminum source and water glass as silicon source, and Liu et al. [6] used NaY and kaolin as a starting material to synthesize kaolin/NaY/MCM-41 composites, which exhibits good hydrothermal stability. In this work, we report the synthesis of mesoporous molecular sieves using K-feldspar as silicon and aluminum sources, in which silicon and aluminum atoms are mixed uniformly in atomic scale. In this method, K-feldspar ore was first calcined with K2CO3 at high temperature to break down the framework of aluminosilicate in the K-feldspar sample, generating water-soluble silicon and aluminum sources. Mesoporous materials were then prepared under basic condition using cetyltrimethylammonium bromide (C16TMAB) as the template. The as-synthesized material was characterized by different techniques, such as X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), 27Al NMR, inferred spectroscopy (IR), and nitrogen sorption. The hydrothermal stability of resulting mesoporous molecular sieves was also studied via heating the synthesized sample in water at 100 °C for 360 h.
milled to such a grain size that more than 95% particles were 120 minus mesh, which was calcined in the temperature range of 860–880 °C for some time to form soluble potassium compounds. In a typical experiment to synthesize molecular sieves, 2.3 g of the calcined materials were dissolved in distilled water (20 mL), and 10 mL of C16TMABr aqueous solution containing 1.12 g C16TMABr was added into the solution. After stirring for 1 h, the pH of the solution was adjusted to 10.5 with HBr (20%), resulting in a gel under continuous stirring. The gel was then transferred into a 100 mL PTFE-coated stainless steel autoclave after stirred for another 1 h at room temperature. The autoclave was sealed and placed in an oven of 130 °C. After heating for 60 h, the autoclave was cooled to room temperature naturally, and the resulting solid product was filtered out and then redispersed into 1 M NH4NO3 solution to exchange K+/Na+ with NHþ 4 . The obtained slurry was filtered again, and washed with deionized water. Some of the as-prepared sample was dried at 105 °C for 6 h in a vacuum, which is named as Sample A, and the other was calcined at 550 °C for 6 h, termed as Sample B. Part of Sample B was heated in water in a sealed autoclave at 100 °C for 360 h, termed as Sample C after dried at 105 °C for 6 h in a vacuum. 2.3. Characterization Thermogravimetric-differential thermal analysis (TGDTA) of the mixture of K-feldspar and K2CO3 with mass ratio of 2:3 was carried out in a Rheometric scientific (NETZSCH4) thermobalance. The XRD patterns of the as-prepared samples were collected on an X-ray diffracometer (XÕPERT SW) operated at 40 kV and 10 mA with nickel filtered CuKa radiation (k = ˚ ). The morphologies of the samples were 1.54060 A observed by means of SEM (JEOL, JSM-6700F) and TEM (JEOL, JEM-2010) equipped with an energy dispersive X-ray spectrometer (EDS) at an accelerating voltage of 200 kV, respectively. The nitrogen sorption analysis was performed on an ASAP-2405N instrument at liquid nitrogen temperature. Prior to the adsorption, the sample was degassed at 300 °C for 12 h at 104 Torr. FT-IR spectra of samples were recorded with a TENSOR27 (BRUKER) spectrometer in KBr pellet. 27Al MAS NMR measurements were collected on a VARIAN UNITYINOVA 300M spectrometer with pulse width of 0.3 ls, recycle delay time of 1 s and spinning speed of 7 kHz, using 1.0 M Al(NO3)3 aqueous solution as the reference solutions.
2. Experimental 2.1. Materials K2CO3, C16TMABr and other chemicals were supplied by Beijing Chemical Reagent Center, which were used as received. K-feldspar ore used in this work was collected from Henan Qianhe Gold Mine in mid-China. The chemical composition of the K-feldspar sample is listed in Table 1. The main components of K-feldspar sample are silicon and aluminium, with less amount of other elements including Fe, Mn, Ca, Na, K, P etc., and the main minerals in the K-feldspar determined by XRD were microcline (KAlSi3O8), quartz (SiO2) and illite (K{Al2[(Si3Al)O10](OH)2}), whose contents were 91.10 wt%, 7.52 wt% and 2.38 wt%, respectively, calculated by LINPRO method [7]. 2.2. Procedures to synthesize mesoporous molecular sieves Before synthesis of molecular sieves, the mixture of K-feldspar and K2CO3 with mass ratio of 2:3 were Table 1 Chemical composition of K-feldspar (wt%) Sample
SiO2
TiO2
Al2O3
Fe2O3
FeO
MnO
MgO
CaO
Na2O
K2O
P2O5
H2O+
H2O
Total
HN-1
65.00
0.22
16.38
0.66
0.48
0.03
0.00
0.86
0.89
13.83
0.12
1.23
0.17
99.87
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3. Results and discussion 3.1. Calcination of K-feldspar ore In order to break down the aluminosilicate in the framework, the K-feldspar needs to be calcined at high temperatures. Before calcination, a TG-DTA analysis was performed, and the weight loss vs temperature plot is shown in Fig. 1. From the figure, a weight loss of 3.0 wt% appeared in the temperature range of 80– 110 °C, which corresponded to the desorption and removal of the water in the mixture, and a second weight loss occurred between 840 and 880 °C, which implied that the reactions between K-feldspar and K2CO3 took place. Based on the TG-DTA analysis results, the mixtures of K-feldspar and K2CO3 with K-feldspar/K2CO3 mass ratio of 2:3 was calcined at 800, 840, 880 °C, respectively, and the calcined materials were determined by XRD. From the XRD patterns (not shown), it was found a new phase K3Al3O6 (JCPDS: 27-1336) formed when the mixture was calcined at 840 °C, suggesting the reactions between K-feldspar and K2CO3 occurred, while when the mixture was calcined at 880 °C, KAlSi3O8 disappeared and most of the peaks in the XRD pattern are assigned to K3Al3O6, indicating the completion of the reactions. Due to the amorphous nature, no characteristic peaks for K2SiO3 could be observed on the XRD patterns. From the XRD results, we can deduce that the following representative reactions occurred during the calcination process. KAlSi3 O8 ðmicroclineÞ þ 3K2 CO3 ! 3K2 SiO3 þ 1=3 K3 Al3 O6 þ 3CO2 " KfAl2 ½ðSi3 AlÞO10 ðOHÞ2 g ðilliteÞ þ 4K2 CO3
ð1Þ
! 3K2 SiO3 þ K3 Al3 O6 þ CO2 " þH2 O " SiO2 ðquartzÞ þ K2 CO3 ! K2 SiO3 þ CO2 "
ð2Þ ð3Þ
10
95
8
TG
90
6
85
4 DSC
80
849.9
2
75 70
0
86.81 0
200
400
600
800
1000
1200
Temperature (°C) Fig. 1. TG/DSC curve of the K-feldspar and K2CO3 system.
DSC/(mW/mg)
Residue Mass/(wt %)
100
279
The K2SiO3 and K3Al3O6 formed during the calcination process may be used as the precursors of molecular sieves. To prepare the precursors, the mixture of K-feldspar and K2CO3 with the K2CO3/K-feldspar mass ratio of 1.51 was calcined at 880 °C for 1.2 h, and 99.4 wt% of the K-feldspar were turned into soluble inorganic salts, which were mainly composed of K2SiO3 and K3Al3O6 with the K2SiO3/K3Al3O6 mass ratio of about 1.8, estimated from their amount in the K-feldspar. Directed by C16TMAB as the structure-inducing reagent, mesoporous materials were produced using the above calcined materials as precursors in basic aqueous solution. The resulting material was characterized by the following techniques. 3.2. Nitrogen sorption analysis The nitrogen sorption analysis was performed for Sample B, and Fig. 2 shows its nitrogen adsorption– desorption isotherms and pore size distribution. As displayed in Fig. 2a, the sample exhibits two hysteresis loops, one of which is in the relatively low p/p0 range of 0.20–0.40, indicating the presence of framework-confined mesopores, and the other larger hysteresis loop is encountered in the higher p/p0 range from 0.40 to 1.0, which is believed to be attributive to the mesoporous structures. The pore size distribution of the sample, shown in Fig. 2b, illustrates the existence of a stepwise-distributed pore structure with an average pore diameter of 5.59 nm. The nitrogen analysis indicates that less ordered mesoporous molecular sieves were produced. From the t-plot, the BET surface area of the asprepared material is 507 m2 g1 and the pore volume is 0.854 cm3 g1. 3.3. XRD analysis The powder XRD patterns of the as-synthesized Samples A, B, C, are shown in Fig. 3. A peak in the 2h range of 1.80–2.20° appears on each XRD pattern, suggesting the mesoporous structure of the as-prepared materials. Compared to Sample A, the diffraction peak intensities of Samples B and C are weaker, and peak shapes become wider, demonstrating that the calcined and hydrothermally treated samples shrink to some extents, however, Samples B and C still remain the mesoporous structures. This indicates that the mesoporous materials prepared in this work have relatively good thermal and hydrothermal stability. For each sample three diffraction peaks appear at 36.64°, 38.28°, 44.56° on the XRD patterns, and the dvalues were similar to those reported by Inagaki et al. [8]. Therefore, the diffraction peaks may be attributed to the crystalline features of the walls of the resulting mesoporous materials, which was also verified by ED diffraction during the TEM observation for Sample B
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Fig. 2. (a) N2 sorption isotherms and (b) corresponding pore size distribution of Sample B.
improve their hydrothermal stability to some extent, Al-MCM-41 prepared via method reported by Kresge [1] has relatively poor hydrothermal stability. However, the mesoporous molecular sieves prepared in this work still keep their mesoporous structure after treated in boiled water for 360 h. This may result from its special microstructures, which will be discussed in the following sections.
Intensity (a.u.)
Intensity (a.u.)
c
b
c
a
b a
0
2
4
6
8
36
39
42
45
3.4. Morphology 48
Fig. 3. XRD patterns of Samples A (a), B (b) and C (c) in the low angle range (left) and wide angle range (right) regions.
(see the inset of Fig. 4b). The intensity and shape of the peaks for the three samples are similar, which indicates that heating at high temperature of 550 °C and hydrothermally treated in boiled water for long time cannot destroy the crystalline structure of the mesoporous material, further confirming good thermal and hydrothermal stability of the as-prepared materials. In general, although the incorporation of Al into the framework of the silica MCM-41 molecular sieves can
The morphologies of the resulting materials were observed by SEM and TEM, and some SEM and TEM images are shown in Fig. 4. From the SEM and TEM observation, the as-prepared samples are spherical particles with size of about 100 nm. Relatively regular arrays of the mesopores in the samples can also be observed clearly (Fig. 4b, c) with pore size of 5–6 nm, which is consistent with the XRD results calculated by BraggÕs law. The size and microstructure of Sample C are similar to those of Sample B, showing that hydrothermal treatment cannot destroy the microstructures of as-prepared sample. This provides further evidence about good hydrothermal stability of the as-prepared
Fig. 4. SEM image of Sample B (a) and TEM images of Sample B (b) and Sample C (c).
S. Miao et al. / Microporous and Mesoporous Materials 83 (2005) 277–282
mesoporous materials. The corresponding electron diffraction (inset of Fig. 4b) appears as a few spots, suggesting that the wall of the sample is partially crystalline. 3.5. IR analysis Fig. 5a shows IR spectra of Samples A and B. On each IR spectrum curve, the vibrational bands at 1237, 1084, 801, 460 cm1 are attributed to the characteristic of silica framework in MCM-41 [8]. Sample A exhibits absorption bands around 2921 and 2851 cm1 assigning to C–H vibrations of the surfactant molecule, while they disappear in Sample B because of the removal of surfactant. The broad bands around 3500 cm1 may be attributed to surface silanols and adsorbed water molecules, while deformational vibrations of the adsorbed water molecules cause the absorption bands at 1633 cm1, which was also observed by Selvaraj et al. [9]. These IR spectra demonstrate structural transformations with significant vibration bands at 1071, 968, 801 and 458 cm1 for Al-MCM-41. The vibration peak at 968 cm1 is assigned to the Al incorporation into the framework of the mesoporous silica materials. The samples prepared in this work also exhibit distinguishable bands at 610, 480, and 440 cm1, which are similar to those of zeolite L [10], assigned to characteristic of 5-ring and 6-ring T–O–T. This suggests that the as-made samples in this work possess features of zeolite structure, which can explain its crystalline characteristics and high hydrothermal stability. To evaluate the acidity of the as-prepared samples, Sample B was evacuated at 150 °C for 5 h, followed by exposure to pyridine at the same temperature for 30 min. The sample was then evacuated at this temperature under vacuum (105 Torr) for 1 h to remove physically adsorbed pyridine. Finally, IR spectrum of pyridine-adsorbed on Sample B was recorded at room temperature, which is shown in Fig. 4b in the range of
281
1400–1750 cm1. Compared with that of Sample B, IR spectrum of pyridine-adsorbed on Sample B exhibits four bands at 1447, 1490, 1550 and 1596 cm1, respectively. The bands at 1447 and 1596 cm1 are attributed to Lewis acid sites, and that at 1550 cm1 is assigned to Bro¨nsted acid centers, while the band at 1490 cm1 is ascribed to a combinational signal associated with both Lewis and Bro¨nsted acid sites [13]. The results indicates that the sample prepared in this work has good acidity, which may be used as acidic catalysts for some reactions. 3.6.
27
Al NMR analysis
Fig. 6 shows the solid 27Al NMR spectrum of Sample B. Obviously, only a strong and sharp signal appears at 61.03 ppm, assigning to tetrahedrally coordinated framework aluminum (Td–Al), and there is no other signal ascribed to other Al states (such as 6-coordination Al at 10 ppm, octahedrally coordinated non-framework aluminum at 0 ppm, non-framework Td–Al at 30–40 ppm, etc) in the product, implying that the aluminum was incorporated mainly
61.030
-200
-100
0
100
200
300
ppm Fig. 6.
27
Al MAS NMR spectrum of Sample B.
Fig. 5. (a) IR spectra of Samples A (Spectrum 1) and B (Spectrum 2); (b) IR spectra of Sample B (Spectrum 1) and pyridine adsorbed on Sample B (Spectrum 2).
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with tetrahedral coordination in the molecular sieves framework. The 27Al chemical shift of the as-made sample is similar to the values observed for most zeolites (59–65 ppm) [11], and much higher than that of most previously reported mesostructured aluminosilicates (51–56 ppm) [12]. Liu et al. [3] synthesized aluminosilicate mesoporous molecular sieves derived from zeolite type Y seeds, which exhibited a chemical shift at 60 ppm and high steam-stability. The mesoporous material prepared in this work probably has a similar microstructure to zeolite, which results in high hydrothermal stability.
4. Conclusion Mesoporous aluminosilicate molecular sieves with crystalline framework have been successfully prepared by hydrothermal synthesis using K-feldpar as silica and aluminium sources simultaneously, and C16TMABr as the template. SEM and TEM observations indicate that the resulting materials are spherical particles with size of about 100 nm. The BET surface area and the pore volume of the as-prepared materials are 507 m2 g1 and 0.854 cm3 g1, respectively. Nearly all of the aluminium is incorporated into the framework of the mesoporous material with 4-coordinated state, and the materials show acidity and high hydrothermal stability, which may be used as acidic catalysts for some reactions. The method to synthesize mesoporous aluminosilicates developed in this work is advantageous for large-scale industrial production because the raw material K-feldspar is abundant and very cheap.
Acknowledgement Financial support from the National Natural Science Foundation of China (No. 20374057, 50472096) is gratefully acknowledged.
References [1] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [2] S.S. Kim, W. Zhang, T.J. Pinnavaia, Science 282 (1998) 1032; A. Corma, A. Martinez, V. Martinez-Soria, J.B. Monton, J. Catal. 153 (1995) 25. [3] Y. Liu, W. Zhang, T.J. Pinnavaia, J. Am. Chem. Soc. 122 (2000) 8791; Y. Cesteros, G.L. Haller, Micropor. Mesopor. Mater. 43 (2001) 171. [4] Z.H. Luan, C.F. Cheng, W.Z. Zhou, J. Klinowski, J. Phys. Chem. 99 (1995) 1018; Y. Cesteros, G. Haller, Micropor. Mesopor. Mater. 39 (2000) 171. [5] F. Kang, Q.Y. Wang, S.H. Xiang, Mater. Lett., MLBLUE-05741, available online. [6] H. Liu, X. Bao, W. Wei, G. Shi, Micropor. Mesopor. Mater. 66 (2003) 117. [7] H.W. Ma, Software on Thermodynamics in Crystalline Petrology, Geological Publishing House, Beijing, 1999, p. 38 (in Chinese). [8] S. Inagaki, S. Guan, T. Ohsuna, O. Terasak, Nature 416 (2002) 304. [9] M. Selvaraj, A. Pandurangan, K.S. Seshadri, P.K. Sinha, K.B. Lal, Appl. Catal. A: General 242 (2003) 347. [10] H. Maas, A. Khatyr, G. Calzaferri, Micropor. Mesopor. Mater. 65 (2003) 233. [11] R.J. Davis, Heterogen. Chem. Rev. 1 (1994) 41. [12] E. Lippma, A. Samoson, M. Ma¨gi, J. Am. Chem. Soc. 108 (1986) 1730. [13] S.R. Zhai, J. Zheng, X. Shi, Y. Zhang, L. Dai, Y. Shan, Catal. Today 93–95 (2004) 675.
Synthesis and characterization of mesoporous aluminosilicate molecular sieve from K-feldspar Shiding Miao a, Zhimin Liu a,*, Hongwen Ma b, Buxing Han a, Jimin Du a, Zhenyu Sun a, Zhenjiang Miao a a
CAS Key Laboratory of Colloid, Interfacial and Chemical Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China b National Laboratory of Mineral Materials, China University of Geosciences, Beijing 100083, China Received 7 April 2005; received in revised form 4 May 2005; accepted 5 May 2005 Available online 16 June 2005
Abstract Mesoporous aluminosilicate molecular sieves have been synthesized using K-feldspar, a natural ore rich in silicon and aluminum sources, as starting material. In this method, the mixture of K-feldspar and K2CO3 with mass ratio of 2:3 was first calcined at 880 °C for 1.2 h. The calcined materials were then dissolved in water together with cetyltrimethylammonium bromide as structure-directing agent, resulting in mesoporous molecular sieves after heating the solution at 130 °C for 60 h. Scanning electron microscopy and transmission electron microscopy observations indicated that the resulting materials were spherical particles with size of about 100 nm. The mesoporous structure of the as-synthesized materials was confirmed by low angle X-ray diffraction and nitrogen sorption analysis. The BET surface area of the as-prepared material after calcined at 550 °C was 507 m2 g1 and the pore volume was 0.854 cm3 g1. 27 Al MAS NMR analysis showed that nearly all of aluminium is incorporated into the framework of the mesoporous material with 4-coordinated state. The acidity of the product was analyzed by FTIR spectroscopy of pyridine-adsorbed product. The product was treated in water at 100 °C for 360 h, and its mesoporous structure was still intact, suggesting its high hydrothermal stability. Ó 2005 Elsevier Inc. All rights reserved. Keywords: Mesoporous material; Aluminosilicate; Molecular sieves; K-feldspar
1. Introduction Since the discovery of the mesoporous M41S materials in 1992 [1], the ordered mesoporous materials have attracted much attention. Among the mesoporous materials, hexagonally ordered silica MCM-41, characterized with pore size between 2 and 10 nm with a narrow pore size distribution, has been investigated extensively. However, due to the amorphous nature of their walls, silica MCM-41 materials have relatively low acidity and hydrothermal stability, which severely hinders their industrial applications in catalytic reactions [2]. The *
Corresponding author. Tel./fax: +86 1062562821. E-mail address: [email protected] (Z. Liu).
1387-1811/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.05.006
incorporation of Al (called Al-MCM-41) and transition metal elements within the silica framework has been implemented, which improved the hydrothermal stability and acidity to some extent [3]. Al-MCM-41 with tetrahedral coordinated Al into the silica framework can generate Bro¨nsted acid sites, which has been the focus of much recent research. A number of papers concerning the synthesis and characterization of Al-MCM-41 mesoporous materials have been published [4], and tetraethoxysilane, aluminum isopropoxide, sodium aluminate, and other aluminosilicates were generally used as Si, Al sources. Many natural ores are rich in silicon and aluminum, which are very cheap, and can be easily obtained. Obviously, using these ores as raw materials to synthesize Al substituted molecular sieves is of great importance.
278
S. Miao et al. / Microporous and Mesoporous Materials 83 (2005) 277–282
However, study on the synthesis of mesoporous molecular sieves containing Al and Si oxides using natural ores as aluminum and silicon sources is scarce. Recently, Kang et al. [5] synthesized Al-MCM-41 using metakaolin as aluminum source and water glass as silicon source, and Liu et al. [6] used NaY and kaolin as a starting material to synthesize kaolin/NaY/MCM-41 composites, which exhibits good hydrothermal stability. In this work, we report the synthesis of mesoporous molecular sieves using K-feldspar as silicon and aluminum sources, in which silicon and aluminum atoms are mixed uniformly in atomic scale. In this method, K-feldspar ore was first calcined with K2CO3 at high temperature to break down the framework of aluminosilicate in the K-feldspar sample, generating water-soluble silicon and aluminum sources. Mesoporous materials were then prepared under basic condition using cetyltrimethylammonium bromide (C16TMAB) as the template. The as-synthesized material was characterized by different techniques, such as X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), 27Al NMR, inferred spectroscopy (IR), and nitrogen sorption. The hydrothermal stability of resulting mesoporous molecular sieves was also studied via heating the synthesized sample in water at 100 °C for 360 h.
milled to such a grain size that more than 95% particles were 120 minus mesh, which was calcined in the temperature range of 860–880 °C for some time to form soluble potassium compounds. In a typical experiment to synthesize molecular sieves, 2.3 g of the calcined materials were dissolved in distilled water (20 mL), and 10 mL of C16TMABr aqueous solution containing 1.12 g C16TMABr was added into the solution. After stirring for 1 h, the pH of the solution was adjusted to 10.5 with HBr (20%), resulting in a gel under continuous stirring. The gel was then transferred into a 100 mL PTFE-coated stainless steel autoclave after stirred for another 1 h at room temperature. The autoclave was sealed and placed in an oven of 130 °C. After heating for 60 h, the autoclave was cooled to room temperature naturally, and the resulting solid product was filtered out and then redispersed into 1 M NH4NO3 solution to exchange K+/Na+ with NHþ 4 . The obtained slurry was filtered again, and washed with deionized water. Some of the as-prepared sample was dried at 105 °C for 6 h in a vacuum, which is named as Sample A, and the other was calcined at 550 °C for 6 h, termed as Sample B. Part of Sample B was heated in water in a sealed autoclave at 100 °C for 360 h, termed as Sample C after dried at 105 °C for 6 h in a vacuum. 2.3. Characterization Thermogravimetric-differential thermal analysis (TGDTA) of the mixture of K-feldspar and K2CO3 with mass ratio of 2:3 was carried out in a Rheometric scientific (NETZSCH4) thermobalance. The XRD patterns of the as-prepared samples were collected on an X-ray diffracometer (XÕPERT SW) operated at 40 kV and 10 mA with nickel filtered CuKa radiation (k = ˚ ). The morphologies of the samples were 1.54060 A observed by means of SEM (JEOL, JSM-6700F) and TEM (JEOL, JEM-2010) equipped with an energy dispersive X-ray spectrometer (EDS) at an accelerating voltage of 200 kV, respectively. The nitrogen sorption analysis was performed on an ASAP-2405N instrument at liquid nitrogen temperature. Prior to the adsorption, the sample was degassed at 300 °C for 12 h at 104 Torr. FT-IR spectra of samples were recorded with a TENSOR27 (BRUKER) spectrometer in KBr pellet. 27Al MAS NMR measurements were collected on a VARIAN UNITYINOVA 300M spectrometer with pulse width of 0.3 ls, recycle delay time of 1 s and spinning speed of 7 kHz, using 1.0 M Al(NO3)3 aqueous solution as the reference solutions.
2. Experimental 2.1. Materials K2CO3, C16TMABr and other chemicals were supplied by Beijing Chemical Reagent Center, which were used as received. K-feldspar ore used in this work was collected from Henan Qianhe Gold Mine in mid-China. The chemical composition of the K-feldspar sample is listed in Table 1. The main components of K-feldspar sample are silicon and aluminium, with less amount of other elements including Fe, Mn, Ca, Na, K, P etc., and the main minerals in the K-feldspar determined by XRD were microcline (KAlSi3O8), quartz (SiO2) and illite (K{Al2[(Si3Al)O10](OH)2}), whose contents were 91.10 wt%, 7.52 wt% and 2.38 wt%, respectively, calculated by LINPRO method [7]. 2.2. Procedures to synthesize mesoporous molecular sieves Before synthesis of molecular sieves, the mixture of K-feldspar and K2CO3 with mass ratio of 2:3 were Table 1 Chemical composition of K-feldspar (wt%) Sample
SiO2
TiO2
Al2O3
Fe2O3
FeO
MnO
MgO
CaO
Na2O
K2O
P2O5
H2O+
H2O
Total
HN-1
65.00
0.22
16.38
0.66
0.48
0.03
0.00
0.86
0.89
13.83
0.12
1.23
0.17
99.87
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3. Results and discussion 3.1. Calcination of K-feldspar ore In order to break down the aluminosilicate in the framework, the K-feldspar needs to be calcined at high temperatures. Before calcination, a TG-DTA analysis was performed, and the weight loss vs temperature plot is shown in Fig. 1. From the figure, a weight loss of 3.0 wt% appeared in the temperature range of 80– 110 °C, which corresponded to the desorption and removal of the water in the mixture, and a second weight loss occurred between 840 and 880 °C, which implied that the reactions between K-feldspar and K2CO3 took place. Based on the TG-DTA analysis results, the mixtures of K-feldspar and K2CO3 with K-feldspar/K2CO3 mass ratio of 2:3 was calcined at 800, 840, 880 °C, respectively, and the calcined materials were determined by XRD. From the XRD patterns (not shown), it was found a new phase K3Al3O6 (JCPDS: 27-1336) formed when the mixture was calcined at 840 °C, suggesting the reactions between K-feldspar and K2CO3 occurred, while when the mixture was calcined at 880 °C, KAlSi3O8 disappeared and most of the peaks in the XRD pattern are assigned to K3Al3O6, indicating the completion of the reactions. Due to the amorphous nature, no characteristic peaks for K2SiO3 could be observed on the XRD patterns. From the XRD results, we can deduce that the following representative reactions occurred during the calcination process. KAlSi3 O8 ðmicroclineÞ þ 3K2 CO3 ! 3K2 SiO3 þ 1=3 K3 Al3 O6 þ 3CO2 " KfAl2 ½ðSi3 AlÞO10 ðOHÞ2 g ðilliteÞ þ 4K2 CO3
ð1Þ
! 3K2 SiO3 þ K3 Al3 O6 þ CO2 " þH2 O " SiO2 ðquartzÞ þ K2 CO3 ! K2 SiO3 þ CO2 "
ð2Þ ð3Þ
10
95
8
TG
90
6
85
4 DSC
80
849.9
2
75 70
0
86.81 0
200
400
600
800
1000
1200
Temperature (°C) Fig. 1. TG/DSC curve of the K-feldspar and K2CO3 system.
DSC/(mW/mg)
Residue Mass/(wt %)
100
279
The K2SiO3 and K3Al3O6 formed during the calcination process may be used as the precursors of molecular sieves. To prepare the precursors, the mixture of K-feldspar and K2CO3 with the K2CO3/K-feldspar mass ratio of 1.51 was calcined at 880 °C for 1.2 h, and 99.4 wt% of the K-feldspar were turned into soluble inorganic salts, which were mainly composed of K2SiO3 and K3Al3O6 with the K2SiO3/K3Al3O6 mass ratio of about 1.8, estimated from their amount in the K-feldspar. Directed by C16TMAB as the structure-inducing reagent, mesoporous materials were produced using the above calcined materials as precursors in basic aqueous solution. The resulting material was characterized by the following techniques. 3.2. Nitrogen sorption analysis The nitrogen sorption analysis was performed for Sample B, and Fig. 2 shows its nitrogen adsorption– desorption isotherms and pore size distribution. As displayed in Fig. 2a, the sample exhibits two hysteresis loops, one of which is in the relatively low p/p0 range of 0.20–0.40, indicating the presence of framework-confined mesopores, and the other larger hysteresis loop is encountered in the higher p/p0 range from 0.40 to 1.0, which is believed to be attributive to the mesoporous structures. The pore size distribution of the sample, shown in Fig. 2b, illustrates the existence of a stepwise-distributed pore structure with an average pore diameter of 5.59 nm. The nitrogen analysis indicates that less ordered mesoporous molecular sieves were produced. From the t-plot, the BET surface area of the asprepared material is 507 m2 g1 and the pore volume is 0.854 cm3 g1. 3.3. XRD analysis The powder XRD patterns of the as-synthesized Samples A, B, C, are shown in Fig. 3. A peak in the 2h range of 1.80–2.20° appears on each XRD pattern, suggesting the mesoporous structure of the as-prepared materials. Compared to Sample A, the diffraction peak intensities of Samples B and C are weaker, and peak shapes become wider, demonstrating that the calcined and hydrothermally treated samples shrink to some extents, however, Samples B and C still remain the mesoporous structures. This indicates that the mesoporous materials prepared in this work have relatively good thermal and hydrothermal stability. For each sample three diffraction peaks appear at 36.64°, 38.28°, 44.56° on the XRD patterns, and the dvalues were similar to those reported by Inagaki et al. [8]. Therefore, the diffraction peaks may be attributed to the crystalline features of the walls of the resulting mesoporous materials, which was also verified by ED diffraction during the TEM observation for Sample B
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Fig. 2. (a) N2 sorption isotherms and (b) corresponding pore size distribution of Sample B.
improve their hydrothermal stability to some extent, Al-MCM-41 prepared via method reported by Kresge [1] has relatively poor hydrothermal stability. However, the mesoporous molecular sieves prepared in this work still keep their mesoporous structure after treated in boiled water for 360 h. This may result from its special microstructures, which will be discussed in the following sections.
Intensity (a.u.)
Intensity (a.u.)
c
b
c
a
b a
0
2
4
6
8
36
39
42
45
3.4. Morphology 48
Fig. 3. XRD patterns of Samples A (a), B (b) and C (c) in the low angle range (left) and wide angle range (right) regions.
(see the inset of Fig. 4b). The intensity and shape of the peaks for the three samples are similar, which indicates that heating at high temperature of 550 °C and hydrothermally treated in boiled water for long time cannot destroy the crystalline structure of the mesoporous material, further confirming good thermal and hydrothermal stability of the as-prepared materials. In general, although the incorporation of Al into the framework of the silica MCM-41 molecular sieves can
The morphologies of the resulting materials were observed by SEM and TEM, and some SEM and TEM images are shown in Fig. 4. From the SEM and TEM observation, the as-prepared samples are spherical particles with size of about 100 nm. Relatively regular arrays of the mesopores in the samples can also be observed clearly (Fig. 4b, c) with pore size of 5–6 nm, which is consistent with the XRD results calculated by BraggÕs law. The size and microstructure of Sample C are similar to those of Sample B, showing that hydrothermal treatment cannot destroy the microstructures of as-prepared sample. This provides further evidence about good hydrothermal stability of the as-prepared
Fig. 4. SEM image of Sample B (a) and TEM images of Sample B (b) and Sample C (c).
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mesoporous materials. The corresponding electron diffraction (inset of Fig. 4b) appears as a few spots, suggesting that the wall of the sample is partially crystalline. 3.5. IR analysis Fig. 5a shows IR spectra of Samples A and B. On each IR spectrum curve, the vibrational bands at 1237, 1084, 801, 460 cm1 are attributed to the characteristic of silica framework in MCM-41 [8]. Sample A exhibits absorption bands around 2921 and 2851 cm1 assigning to C–H vibrations of the surfactant molecule, while they disappear in Sample B because of the removal of surfactant. The broad bands around 3500 cm1 may be attributed to surface silanols and adsorbed water molecules, while deformational vibrations of the adsorbed water molecules cause the absorption bands at 1633 cm1, which was also observed by Selvaraj et al. [9]. These IR spectra demonstrate structural transformations with significant vibration bands at 1071, 968, 801 and 458 cm1 for Al-MCM-41. The vibration peak at 968 cm1 is assigned to the Al incorporation into the framework of the mesoporous silica materials. The samples prepared in this work also exhibit distinguishable bands at 610, 480, and 440 cm1, which are similar to those of zeolite L [10], assigned to characteristic of 5-ring and 6-ring T–O–T. This suggests that the as-made samples in this work possess features of zeolite structure, which can explain its crystalline characteristics and high hydrothermal stability. To evaluate the acidity of the as-prepared samples, Sample B was evacuated at 150 °C for 5 h, followed by exposure to pyridine at the same temperature for 30 min. The sample was then evacuated at this temperature under vacuum (105 Torr) for 1 h to remove physically adsorbed pyridine. Finally, IR spectrum of pyridine-adsorbed on Sample B was recorded at room temperature, which is shown in Fig. 4b in the range of
281
1400–1750 cm1. Compared with that of Sample B, IR spectrum of pyridine-adsorbed on Sample B exhibits four bands at 1447, 1490, 1550 and 1596 cm1, respectively. The bands at 1447 and 1596 cm1 are attributed to Lewis acid sites, and that at 1550 cm1 is assigned to Bro¨nsted acid centers, while the band at 1490 cm1 is ascribed to a combinational signal associated with both Lewis and Bro¨nsted acid sites [13]. The results indicates that the sample prepared in this work has good acidity, which may be used as acidic catalysts for some reactions. 3.6.
27
Al NMR analysis
Fig. 6 shows the solid 27Al NMR spectrum of Sample B. Obviously, only a strong and sharp signal appears at 61.03 ppm, assigning to tetrahedrally coordinated framework aluminum (Td–Al), and there is no other signal ascribed to other Al states (such as 6-coordination Al at 10 ppm, octahedrally coordinated non-framework aluminum at 0 ppm, non-framework Td–Al at 30–40 ppm, etc) in the product, implying that the aluminum was incorporated mainly
61.030
-200
-100
0
100
200
300
ppm Fig. 6.
27
Al MAS NMR spectrum of Sample B.
Fig. 5. (a) IR spectra of Samples A (Spectrum 1) and B (Spectrum 2); (b) IR spectra of Sample B (Spectrum 1) and pyridine adsorbed on Sample B (Spectrum 2).
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with tetrahedral coordination in the molecular sieves framework. The 27Al chemical shift of the as-made sample is similar to the values observed for most zeolites (59–65 ppm) [11], and much higher than that of most previously reported mesostructured aluminosilicates (51–56 ppm) [12]. Liu et al. [3] synthesized aluminosilicate mesoporous molecular sieves derived from zeolite type Y seeds, which exhibited a chemical shift at 60 ppm and high steam-stability. The mesoporous material prepared in this work probably has a similar microstructure to zeolite, which results in high hydrothermal stability.
4. Conclusion Mesoporous aluminosilicate molecular sieves with crystalline framework have been successfully prepared by hydrothermal synthesis using K-feldpar as silica and aluminium sources simultaneously, and C16TMABr as the template. SEM and TEM observations indicate that the resulting materials are spherical particles with size of about 100 nm. The BET surface area and the pore volume of the as-prepared materials are 507 m2 g1 and 0.854 cm3 g1, respectively. Nearly all of the aluminium is incorporated into the framework of the mesoporous material with 4-coordinated state, and the materials show acidity and high hydrothermal stability, which may be used as acidic catalysts for some reactions. The method to synthesize mesoporous aluminosilicates developed in this work is advantageous for large-scale industrial production because the raw material K-feldspar is abundant and very cheap.
Acknowledgement Financial support from the National Natural Science Foundation of China (No. 20374057, 50472096) is gratefully acknowledged.
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