Ind. Eng. Chem. Res. 2008, 47, 8211–8217
8211
MATERIALS AND INTERFACES Study and Characterization of Al-MCM-41 Prepared with the Assistance of Supercritical CO2 Shimin Li,† Qun Xu,*,‡ Jiafu Chen,‡ and Yiqun Guo† Department of Chemistry, Zhengzhou UniVersity, Zhengzhou 450052, People’s Republic of China, and College of Materials Science and Engineering, Zhengzhou UniVersity, Zhengzhou 450052, People’s Republic of China
Supercritical carbon dioxide (SC CO2) was used as a carrier/solvent for the “dry” alumination of mesoporous silica to prepare aluminum-containing mesoporous MCM-41. The effect of alumination on the structure and crystallinity of the materials was studied. In addition, the hydrothermal and thermal stability of this material have been investigated and compared with Al-MCM-41 prepared via conventional impregnation and direct hydrothermal synthesis. Irrespective of the preparation method, the surface area, pore diameter, pore volume, and crystallinity of Al-MCM-41 all decrease after hydrothermal and thermal treatments. However, Al-MCM41 materials prepared with the assistance of SC CO2 possess better hydrothermal and thermal stability. This method allows for the incorporation of aluminum onto rather than into pores wall, without disintegration of the mesoporous structure, compared to Al-MCM-41 that has been prepared via impregnation and direct hydrothermal synthesis. Introduction MCM-41 is a member of a family of mesoporous molecular sieves that were discovered by researchers at Mobil Corporation.1,2 Their narrow pore size distribution (in the range of 15-100 Å) and extremely high surface area (∼1000 m2/g) make these materials promising as catalysts or catalyst supports. Purely siliceous MCM-41 has no Brønsted acidity, and the isomorphous substitution of Si by a trivalent cation (such as Al3+) can create moderately acidic sites and generate active sites for adsorption and ion exchange. Al-substituted Si-MCM-41 materials have already been proven to be effective catalysts for cracking, hydrocracking, hydrogenation, isomerization, and alkylation reactions.3-7 Nevertheless, mesoporous aluminosilicate materials differ from crystalline zeolites in regard to the nature of ordering in their pore walls; the pore walls of mesoporous aluminosilicates are amorphous, while those of zeolites are ordered at the local level. Consequently mesoporous aluminosilicate materials are structurally less stable than zeolites. Furthermore for a similar amount of aluminum, the ion-exchange capacity of mesoporous aluminosilicates such as Al-MCM-41 is much lower than that of zeolites, because a large proportion of the aluminum is buried within their relatively thick pore walls and, therefore, is not readily accessible for ion-exchange purposes.8 Conventionally, there are two ways of synthesizing Al-MCM41: (i) the one-step method, which involves the addition of aluminum during the synthesis of materials and (ii) postsynthesis alumination. For the one-step method, there is great disadvantage, as mentioned previously (a large proportion of the aluminum is buried within their relatively thick pore walls). Compared to the one-step method, post-synthesis alumination is advantageous, but there is still some difficulties, such as the fact that dispersion must be overcome. Therefore, good techniques to prepare Al-MCM-41 materials with better aluminum * To whom correspondence should be addressed. E-mail address:
[email protected]. † Department of Chemistry. ‡ College of Materials Science and Engineering.
dispersion and exposure are necessary. One potential avenue for realizing this purpose is by ensuring that the incorporated Al atoms occupies framework positions on or near the surface of the pore walls.9 The incorporation of Al atoms on the surface/ near-surface region of the pore walls should provide the following advantages: (1) The structural integrity can be retained, because no aluminum is present deep within the framework, thus minimizing framework disruption;10 (2) Its stability can be maintained, because the surface aluminosilicate layer will act as a protective coating for the mesoporous framework;11-13 and (3) The ion-exchange capacity can be enhanced, because of easy access to the tetrahedrally coordinated (framework) Al, where the ion-exchange sites are located. The key of realizing Al atoms occupying positions on or near the surface of the pore walls is to find a solvent that can effectively transport Al into the pores of MCM-41 without or scarcely any hydrolysis during this process. Fortunately, supercritical fluids (SCFs) currently possess this ability. SCFs have been widely used in many processes, such as supercritical extraction, chromatography, chemical reaction engineering, recrystallization of some specific substances, and synthesis of some polymers.14-17 CO2 was chosen among SCFs because it is nontoxic, nonflammable, and inexpensive, and its mild critical conditions (critical pressure, Pc ) 7.38 MPa; critical temperature, Tc ) 31.1 °C) allow CO2 to be used, under safe laboratory and commercial operation conditions.18 Other advantages include efeatures such as high diffusivity, low viscosity, and zero surface tension, which allow the complete wetting of substrates. In addition, CO2 can be easily and completely removed from the products and a porous structure can be obtained without any structural collapse. These unique properties make supercritical CO2 (SC CO2) an optimum solvent in coating or impregnating processes.18-20 Here, we investigate the use of SCFs as the carrier/solvent for the “dry” alumination of mesoporous silica. Aluminum
10.1021/ie800046h CCC: $40.75 2008 American Chemical Society Published on Web 10/09/2008
8212 Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008
chloride (AlCl3) was introduced into pure MCM-41 using SCF as the carrier, and then it reacted with MCM-41 at high temperature to produce Al-grafted materials. Better dispersion (and surface coverage) of the aluminum via alumination under supercritical conditions could result in improved hydrothermal stability.21-23 The research of Poliakoff and co-workers has presented results demonstrating that the use of SCFs (as compared to aqueous or organic solvents) leads to Al-grafted MCM-41 materials that exhibit exceptional hydrothermal (steam) stability, even for relatively highly aluminated materials.21 In this article, we use SC CO2 as a carrier to prepare Al2O3/MCM41 mesoporous molecular sieve materials, and we have investigated the effect of supercritical conditions (pressure, temperature) on mass uptake, Brunauer-Emmett-Teller (BET) surface area, and pore structures of materials that were subjected to a molecular sieve. Furthermore, the thermal stability and hydrothermal stability of the materials are studied and compared with those of Al-MCM-41 materials prepared via direct hydrothermal synthesis and impregnation synthesis. Experimental Section Chemicals. Soluble glass was provided by Zhengzhou Huada Chemical Company and used as the silica source. H2SO4 was purchased from Luoyang Haohua Chemical Reagent Company and used to adjust the pH. Cetyltrimethylammonium bromide (CTAB) was supplied by Nanjing Lingjiang Technological Company. Aluminum nitrate and tetrahydrofuran (THF), which were offered by Tianjin Kermel Chemical Reagent Company, were respectively used as the aluminum source in the direct and impregnation synthesis and co-solvent in the supercritical grafting process. Anhydrous aluminum chloride (AlCl3) was obtained from Shanghai Sanpu Chemical Company and used as the aluminum source in the supercritical grafting process. CO2 (99.9% purity) was provided by Zhengzhou Gas Company and used as received. Preparation of Al-Containing Mesoporous MCM-41. Two samples with different molar Si/Al ratios (10 and 80) were prepared using a direct hydrothermal synthesis method in an alkali-free medium, in a manner similar to the method given in the literature,1 using CTAB as the template. During synthesis, the SiO2:Na2O:CTAB:H2SO4:H2O molar ratio was 7:2.8:3.5:1: 530, and adequate amounts of aluminum nitrate (AlNO3) were added to achieve the desired Si:Al ratios. The mixtures were first stirred at 50 °C for 2 h, then were transferred into Teflonlined stainless steel autoclaves and maintained at 110 °C for 72 h under autogenous pressure. Thereafter, the solid material was filtered off, washed with deionized water until free of bromide, air-dried overnight at ambient temperature, and finally calcined in air at 550 °C for 6 h, using a heating rate of 3 °C/ min. Purely siliceous MCM-41 was prepared in the same way but without adding any aluminum source to the synthesis mixture. For post-synthesis modification, pure silica MCM-41 was reacted with different aluminum compounds. The impregnation grafting of aluminum to the siliceous MCM-41 with AlNO3, according to the literature,24 was performed as follows: 3 g of MCM-41 and the desired amount of AlNO3 were added into 150 mL of deionized water; the mixture was stirred at 60 °C (water bath) for 3 h, filtered off, and dried at 120 °C overnight. The material was finally calcined in air at 550 °C for 6 h at a heating rate of 1 °C/min. As for the supercritical grafting process, 333 mg of anhydrous AlCl3 and 5 mL of THF, which was used as the co-solvent, were placed in the bottom of a stainless steel autoclave (Hai’an
Scheme 1. Schematic Illustration of an Autoclave and Cage Used To Treat Samples in Supercritical Carbon Dioxide (SC CO2)
High Pressure Autoclave Factory, China) with a capacity of 50 mL. A quantity (0.15 g) of the material that was subjected to a molecular sieve was placed in a stainless steel cage that was fixed at the upper part of the autoclave, without touching with the solution at the bottom. The detailed structure of this apparatus is described in Scheme 1. The temperature of the autoclave was adjusted to the desired experimental temperature, and CO2 was then injected into the autoclave by a syringe pump (DB-80, Beijing Satellite Manufacturing Factory), until the desired pressure was obtained. Subsequently, the supercritical conditions were maintained for several hours and the pressure was released by venting slowly. After cooling to room temperature, the sample was removed and dried at 120 °C overnight under normal atmospheric conditions and then was calcined in static air at 550 °C for 6 h. Hydrothermal and Thermal Treatment of Al-MCM-41. To investigate the hydrothermal and thermal stability, every sample was treated as follows: (1) Al-MCM-41 (300 mg) was placed into 250 mL of deionized water (for a concentration of 1.2 g/L), refluxed at 100 °C for 16 h, filtered off, and dried at 120 °C overnight;25 (2) Al-MCM-41 (300 mg) was calcined at 850 °C for 4 h at a heating rate of 3 °C/min.26 Characterization. The pore structure of the samples was measured before and after hydrothermal and thermal treatment, to evaluate their stability. The BET surface area, pore volume, and pore size distribution were obtained from nitrogen adsorption-desorption isotherms measured at 77 K with a QuantachromeNOVA2000porosimeter,usingtheBarrett-Joyner-Halenda (BJH) method.27 All samples were dehydroxylated under a nitrogen flow at 150 °C for 1.5 h prior to nitrogen adsorption. A Rigaku D/MAX-3B powder X-ray diffractometer with Cu KR radiation (35 kV, 35 mA) was used to analyze the X-ray diffraction (XRD) patterns of the samples. The scanning range was 2θ ) 1.4°-10°, and the step size was 0.02°. Results and Discussion Characteristics of Pure Silica MCM-41. A nitrogen adsorption-desorption study is an effective method to investigate changes in the surface area and pore volume of porous materials. Figure 1 shows the N2 adsorption-desorption isotherms and BJH pore radius distribution of template-free MCM41. The material gives typical irreversible Type IV sorption isotherms with an H1 hysteresis loop, and the first adsorption step is explained by a monolayer adsorption of N2 on the pore walls, which is followed by a sudden increase that occurs at
Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008 8213
Figure 1. Nitrogen adsorption-desorption isotherms and BJH pore radius distribution of calcined pure silica MCM-41.
Figure 2. Powder X-ray diffraction (XRD) pattern of calcined pure silica MCM-41.
relative pressures of P/P0 > 0.3. The increase suggests a large pore volume (the textural properties are given in Table 1); the sharpness of these steps and the BJH pore radius distribution (most of the pores are distributed at ∼3 nm) reveal the uniformity of the mesoporous size. As shown in Figure 2, the powder XRD pattern of as-synthesized MCM-41 exhibits an intense (100) diffraction peak and some well-resolved higherorder (110), (200), and (210) peaks, which are an indication of good long-range hexagonal ordering. Effect of Supercritical Conditions on the Characteristics of Al-MCM-41. For the SC CO2-assisted impregnating process, as described in our previous papers,19,20 different experimental conditions apparently have some influence on the coating or impregnating ratio of precursor. Experiments were first conducted to study the effect of temperature on the impregnating amount of anhydrous AlCl3. Figure 3A shows that, as the temperature increases, the weight uptake first increases to a maximum value at 50 °C, then decreases continuously from 50 °C to 60 °C. This series of experiments was performed at 20 MPa in SC CO2. There is a maximum point for the coating ratio. It is suggested that this is due to the co-solvent effect of THF; the detailed mechanism requires further study.
Next, the effect of the SC CO2 pressure on the impregnating ratio was investigated. All experiments were conducted at 45 °C and 4 h. The relative study on the SC CO2 pressure and weight uptake of precursor on the molecular sieve is shown in Figure 3B. This figure shows that the weight uptake first increases as the pressure increases from 12 MPa to 20 MPa, then decreases from 20 MPa to 28 MPa; a maximum value is observed at 20 MPa. Usually, solvent power increases as the experimental pressure increases under supercritical conditions, and more precursors could be dissolved in SC CO2 and supply the possibility for the precursor to coat or impregnate onto MCM-41. However, when the solvent power is too high, the dissolved precursors would rather remain in the solvent, rather than coat the MCM-41, and the amount of impregnation will decrease. Our experimental results showed this phenomenon. The structural parameters of the various Al2O3/MCM-41 samples listed in Tables 2 and 3 show that the structural parameters, such as total pore volume and BJH average pore diameter, changed with the weight uptake in the opposite way. In other words, when the weight uptake increases to the maximum, the pore volume and pore diameter decrease to minimum (see Table 2). We can describe this situation vividly
Table 1. Textural Properties of the Calcined Pure Silica MCM-41 and Calcined Aluminosilicate Al-MCM-41 Samples sample
Si/Al ratio
d(100) (nm)
treatment
surface area (m2/g)
pore diameter (nm)
pore volume (cm3/g)
3.36
0.9921
2.89 2.21 1.61
0.7933 0.6222 0.2225
2.12 2.59 2.70
0.8679 0.4289 0.1828
3.26 3.39
0.9988 0.8303 0.1236
4.21 5.53 3.42
1.1450 1.0450 0.2695
Pure Silica MCM-41 1
∞
39.26
initial
1180.26
Al-MCM-41 Prepared with the Assistance of SC CO2 2 3 4
1 1 1
39.41 38.42 32.94
5 6 7
10 10 10
35.92
8 9 10
80 80 80
37.72 33.95
11 12 13
10 10 10
36.48 37.09
initial refluxing calcining
1094.54 1125.98 549.46
Al-MCM-41 Prepared via the Impregnation Method
29.04
initial refluxing calcining
1636.94 660.54 270.72
Al-MCM-41 Prepared via Direct Synthesis (Si:Al ) 80) initial refluxing calcining
1225.45 979.08 49.89
Al-MCM-41 Prepared via Direct Synthesis (Si:Al ) 10) initial refluxing calcining
1087.21 755.49 314.46
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Figure 3. Relationship between weight uptake and (A) temperature and (B) pressure. Table 2. Comparison of the Structural Parameters of Various Al2O3/MCM-41 Samples Treated at 20 MPa and Different Temperatures sample
temperature, T (°C)
weight uptake (%)
total pore volume (cm3/g)
pore diameter (nm)
surface area (m2/g)
MCM-41 1 2 3 4 5
40 45 50 55 60
34.0 43.3 44.7 41.3 33.3
0.9921 0.7104 0.6814 0.6444 0.7064 0.7487
3.36 2.69 2.66 2.44 2.58 2.68
1180.3 1054.8 1025.9 1054.1 1094.5 1117.2
Table 3. Comparison of the Structural Parameters of Various Al2O3/MCM-41 Samples Treated at 45 °C and Different Pressures sample
pressure, P (MPa)
weight uptake (%)
total pore volume (cm3/g)
pore diameter (nm)
surface area (m2/g)
MCM-41 1 2 3 4 5
12 16 20 24 28
28.7 29.3 43.3 29.3 21.3
0.9921 0.7329 0.7992 0.6814 0.8174 0.7933
3.36 2.69 2.86 2.66 2.98 2.90
1180.3 1090.0 1119.3 1025.9 1097.8 1094.5
Scheme 2. Illustration of Alumination in the Following Processes: (A) Direct Synthesis, (B) Impregnation, and (C) Supercritical Grafting
in Scheme 2C, where the aluminosilicate layer is the product of the reaction of AlCl3 and silanol groups after high temperature. Given the lower viscosity, higher diffusion, and absence of surface tension, the precursors that are created by the SC CO2 process can enter into the pores more deeply and form a larger aluminosilicate layer than that observed with aqueous solution (see Scheme 2B). Nitrogen adsorption/desorption study is an effective method to investigate changes in the surface area and pore volume of porous materials. Figures 4 and 5 show the nitrogen adsorption/ desorption isotherms of Al-MCM-41 materials prepared under different supercritical CO2 conditions. According to the experimental results in Figures 4 and 5, we can draw the conclusion that the adsorption-desorption isotherms of the Al-MCM-41 materials exhibit typical mesoporous characteristics, and variation of the experiments under SCF conditions has no influence on the mesoporous structure. In addition, compared to materials prepared via the hydrothermal and impregnation methods, the crystallinity of the sample that was prepared with the assistance of SC CO2 was much better (see spectra a, c e, and g in Figure 6). Therefore, we can effectively produce eligible Al-MCM-41 mesoporous materials with the assistance of SC CO2.
Hydrothermal Stability of Various Al-MCM-41. The hydrothermal stability of the differently prepared samples was assessed by comparing the BET surface area, pore diameter, pore volume, and the intensity of the XRD diffraction peaks before and after refluxing the sample at 100 °C for 16 h. Figures 6 and 7 respectively show the powder XRD patterns and nitrogen adsorption-desorption isotherms of various Al-MCM41 samples before and after refluxing. From the information provided by Figures 6 and 7, and from the structural parameters
Figure 4. Nitrogen adsorption-desorption isotherms of Al-MCM-41 samples prepared with the assistance of SC CO2 at 20 MPa and different temperatures: (a) 40 °C, (b) 45 °C, (c) 50 °C, (d) 55 °C, and (e) 60 °C.
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Figure 5. Nitrogen adsorption-desorption isotherms of Al-MCM-41 samples prepared with the assistance of SC CO2 at 45 °C and different pressures: (a) 12 MPa, (b) 16 MPa, (c) 20 MPa, (d) 24 MPa, and (e) 28 MPa.
Figure 6. Powder X-ray diffraction (XRD) patterns of different Al-MCM41 samples: Al-MCM-41 (Si:Al ) 1), prepared with the assistance of SC CO2 (initial sample (spectrum a) and after refluxing in water at 100 °C for 16 h (spectrum b); Al-MCM-41 (Si:Al ) 10), prepared via the impregnation method) (initial sample (spectrum c) and after refluxing in water at 100 °C for 16 h (spectrum d); Al-MCM-41 (Si:Al ) 80), prepared via the direct synthesis method (initial sample (spectrum e) and after refluxing in water at 100 °C for 16 h (spectrum f); and Al-MCM-41 (Si:Al ) 10), prepared via the direct synthesis method ((initial sample (spectrum g) and after refluxing in water at 100 °C for 16 h (spectrum h)).
in Table 1, we can draw the conclusion that samples synthesized from the impregnated alumination had the worst stability, samples synthesized with the assistance of SC CO2 were the best, and samples prepared using direct synthesis were moderate (although their nitrogen adsorption-desorption isotherms after refluxing appeared to be the same as those before refluxing, their absolute sorption capacity decreased sharply). After refluxing, the impregnated sample not only lost its H1 hysteresis loop (see Figure 7, spectrum e), 60% of its surface area, and 51% of its pore volume, but it also abandoned the (100) diffraction peak absolutely (see spectrum d in Figure 6), which suggests that it sheds its mesoporous characteristics completely. Meanwhile, the supercritical sample retained 80% of its pore volume and 78% of its pore diameter. Its surface became larger
Figure 7. Nitrogen adsorption-desorption isotherms of different samples: pure silica MCM-41 (isotherm a); Al-MCM-41 (Si:Al ) 1), prepared with the assistance of SC CO2 (initial sample (isotherm b) and (c) after refluxing in water at 100 °C for 16 h (isotherm c)); Al-MCM-41 (Si:Al ) 10), prepared via the impregnation method (initial sample (isotherm d) and after refluxing in water at 100 °C for 16 h (isotherm e)); Al-MCM-41 (Si:Al ) 80), prepared using the direct synthesis method (initial sample (isotherm f) and after refluxing in water at 100 °C for 16 h (isotherm g)); Al-MCM-41 (Si:Al ) 10) prepared using the direct synthesis method (initial sample (isotherm h) and after refluxing in water at 100 °C for 16 h (isotherm i)).
than before the treatment, which suggests that the reflux process helped to remove some blockages in the pore channels of the Al-MCM-41, so the pore structure became more perfect.28 It is well-known that pure silica MCM-41 is not stable in an aqueous environment,29-31 because its structure will degrade, because of the hydrolysis of Si-O-Si linkages.26 Researchers improved its hydrothermal stability by incorporating a certain amount of aluminum into the material, which can form Al-O-Si linkages and prevent Si-O-Si linkages from hydrolyzing. Therefore, the greater the number of Al-O-Si linkages, especially on or near the surface, the more stable the material. In the direct synthesis samples, most of the Al atoms were buried into their framework (see Scheme 2A), and few could be exposed to the surface to form Al-O-Si leakages, so their hydrothermal stabilities were not good. However, for lower Si:Al ratios, there would be more Al-O-Si linkages in the framework and, therefore, few aluminosilicate layers near the surface. This is why sample 11 (Si:Al ) 10) was more stable than sample 8 (Si:Al ) 80), which could be proved from their XRD diffraction peaks (see Figure 6, spectra e-h). As for the impregnation sample, because of the capillary condensation, the Al atoms could not be transferred deeply into the inner pores. Because of the hydrolysis in this aqueous impregnation process, some of the Al atoms were incorporated into the framework and others formed a small aluminosilicate layer near the orifice (see Scheme 2B). Therefore, the hydrothermal stability of the impregnation samples was poor (see isotherm e in Figure 7 and spectrum d in Figure 6). In regard to the material prepared with the assistance of SC CO2, Table 1 shows that, for samples 1 and 2, the surface area, pore diameter, and pore volume obviously decreased after the aluminum was grafted. This experimental result, combined with the fact that the grafted aluminum greatly improved the hydrothermal stability of MCM41,21 demonstrate that aluminum coated the inner surface of the pore with an aluminosilicate layer. Unlike the alumination in aqueous media, the silica framework of pore walls hydrolyzes
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Figure 8. Nitrogen adsorption-desorption isotherms of different samples: Al-MCM-41 (Si:Al ) 1), prepared with the assistance of SC CO2 (initial sample (isotherm a) and after thermal treatment at 850 °C for 4 h (isotherm b)); Al-MCM-41 (Si:Al ) 10), prepared via the impregnation method (initial sample (isotherm c) and after thermal treatment at 850 °C for 4 h (isotherm d)); Al-MCM-41 (Si:Al ) 80), prepared using the direct synthesis method (initial sample (isotherm e) and after thermal treatment at 850 °C for 4 h (isotherm f)); Al-MCM-41 (Si:Al ) 10), prepared using the direct synthesis method (initial sample (isotherm g) and after thermal treatment at 850 °C for 4 h (isotherm h)).
and the aluminum will penetrate into the pore walls. At the same time, it can be expected that the better dispersion of aluminum helps to reduce the number of free silanol groups. According to the literature,32 only aluminum is present on the inner surface of the pore; it can protect the Si-O-Si bonding from hydrolyzing. Therefore, the introduction of aluminum via the supercritical grafting method resolved the disadvantages of the conventional impregnation method.33 Thermal Stability of Various Al-MCM-41 Samples. The thermal stability of the differently prepared samples was also assessed by comparing the BET surface area, pore diameter, pore volume, and intensity of the XRD diffraction peaks before and after calcination at high temperatures (see Table 1). Figures 8 and 9 show the nitrogen adsorption-desorption isotherms and powder XRD patterns of various Al-MCM-41 samples before and after calcination. From Figures 8 and 9, and from the structural parameters given in Table 1, we can conclude that the direct synthesis samples had the worst stability and lost their XRD diffraction peaks completely (see Figure 9, spectra f and h), samples synthesized with the assistance of SC CO2 were the best, and samples synthesized from the impregnated alumination were moderate. After calcining at 850 °C, the supercritical sample still retained more than half of its surface area, while others had
