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Effects of post-synthesis treatments on the pore structure and stability of MCM-41 mesoporous silica
Materials Letters 61 (2007) 3119 – 3123 www.elsevier.com/locate/matlet
Effects of post-synthesis treatments on the pore structure and stability of MCM-41 mesoporous silica Li feng Yin, Fang fang Wang, Ji quan Fu ⁎ Center of Chemistry Engineering, Beijing Key Lab., Beijing Institute of Clothing Technology, Beijing, 100029, PR China Received 17 January 2006; accepted 2 November 2006 Available online 27 November 2006
Abstract The effects of post-synthesis treatments on the structure and the stability of MCM-41 mesoporous material were investigated. It was found that, by adding ammonium salts and adjusting pH during the post-synthesis to alkalescently synthesized MCM-41, the regularity of the pore was improved dramatically and the stability of the mesostructure was retained; conversely, they were diminished by adding sodium salts. The results were studied by analyzing the samples using X-ray diffraction (XRD), Transmission electron microscopy (TEM), Differential Thermal Analysis–Thermal Gravimetry (DTA–TG). The change of the electrostatic surrounding and the formation of hydrogen bonds caused by different ions were confirmed to be the main factors. © 2006 Elsevier B.V. All rights reserved. Keywords: Post-synthesis treatment; Pore structure; Stability; MCM-41
1. Introduction In the last ten years, mesoporous materials have been paid more attention for their potential use in the field of catalysis, separation, synthesis of nanocluster and object embedded vector, drug delivery, etc [1–5]. Also the synthesis methods and types were developed constantly. As one member of the first mesoporous materials family, MCM-41, due to its excellent characteristics of large specific surface area, uniform pore size and structural stability, is always the focus of the study. The internal structure of MCM-41 material is an array of hexagonal pore, of which pore size ranges from 1.6 to 10 nm, which depends on the surfactant agent, various additives (swelling agents) and the synthesis conditions. With those characteristics, MCM-41 is usually used as supports for large chemical species and templates for nanosize architecture. However, MCM-41 mesoporous materials synthesized in common routes are malformed and low ordered [6,7]. Although the thermal stability of MCM-41 mesostructure is measured up in nonaqueous surroundings [8], it is very poor with aqueous solutions, such as ion exchange and catalyst preparation [9]. ⁎ Corresponding author. 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.11.029
In many literatures, additives are proposed to improve the regularity and the stability of mesoporous material MCM-41. Khushalani et al. [10] showed that the regularity of the pore could be improved and d100 parameter ranges from 0.3 to 0.4 nm, when silica mesophase is treated in mother liquid at 423 K for 24 h; Pan et al. [11] showed that a suitable amount in the synthesis process could enhance the diffraction peak intensity of XRD and the stability of mesoporous silica notably; Ryoo and Jun [12] found that the hydrothermal stability of MCM-41 could be improved dramatically by adding sodium salts in the hydrothermal crystallization process, such as sodium chloride, potassium chloride, sodium acetate, and ethylenediaminetetraacetic acid tetrasodium salt. However, it is before or during the synthesis process that the improvers were usually added, and post-synthesis treatments, especially methods of adding salt to improve the regularity and stability, are rarely reported. In this thesis, we have studied and reported the effects of postsynthesis treatments on the porous structure and the stability of MCM-41. The structural regularity can be improved with ammonium salt and pH adjustment, and decreased with alkali salt which can also reduce the heat stability. As supplements to the characteristics of MCM-41, these results could help us improve the quality of mesoporous materials and play an active role to avoid defective products.
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2. Experimental section 2.1. Synthesis The synthesis process of MCM-41 was carried out in a basic condition, with soluble glass (SiO2:28.2%, Na2O:8.7%) as silica resource, and cetyltrimethyl ammonium bromide (CH3 (CH2)15N(CH3)3Br, CTAB) as structure-directing agent. In a typical synthesis, 2.64 g of CTAB and 18.46 g of soluble glass were dissolved in 35 ml of deionized water stirred continuously until the solution turned clear, then 6.0 M HCl (aq) was added drop by drop until the pH value of latex reaches up to 11.0. The final gel was composed of: 1CTAB:15.7SiO2:4.76Na2O:22.4HCl:2.56H2O. After hydrothermally crystallized at 393 K for 24 h, the gel as solid was taken out to be washed, filtrated and collected for airing marked as A-MS (as-synthesized MCM-41). 2.2. Post-synthesis treatment Several copies of equally weighed solid (A-MS) were steeped separately in different amounts of solution, which have equal volume but different kinds of salt, making the mass rate of
salt to A-MS as 0%, 1%, 2% and 5%, or steeped separately in different amounts of deionized water, which have the same volume, and then the pH values of the solution were adjusted separately to 6, 8 and 10 by adding 6.0 M HCl. After a slow drying process in 333 K for 2 h, the final pieces of compound were heated to 823 K with a rate of 2 K/min and then roasted for 6 h. The products were marked as S-MS. 2.3. Characterization Powder XRD patterns were obtained in a D/max-rb diffractometer using CuKα radiation at a scanning speed of 2° min− 1. SEM images were obtained in a JEM-2100F operated at 20 keV. The DTA–TG was carried out in air at a heating rate of 2 K/min from 373 K to 923 K using TG–DTA6300 instruments. 3. Results and discussions A-MS showed a typical pattern of hexagonal structure, including a strong peak (100) at about 2θ ≈ 2° and three weaker lines at 2θ ≈ 3.5°, 4.2° and 6°(Fig. 1A-a). These samples (A-MS and all S-MS) showed similar peak width and resolution, but quite different peak intensities, especially peak (100). The peak intensity was remarkably weakened while the mass of the alkali salt ranged from 0 to 5 wt.% (Fig. 1A and B),
Fig. 1. (A) XRD patterns of calcined MCM-41 treated with NaCl: a: 0.0 wt.%; b: 1.0 wt.%; c: 2.0 wt.%; d: 5.0 wt.%. (B) Calcined MCM-41 treated with NaBr: a: 1.0 wt.%; b:2.0 wt.%; c: 5.0 wt.%. (C) Calcined MCM-41 treated with NH4Cl: a: 1.0 wt.%; b: 2.0 wt.%; c: 5.0 wt.%. (D) Calcined MCM-41 treated with NH4Br: a: 1.0 wt.%; b: 2.0 wt. %; c: 5.0 wt.%. (E) Calcined MCM-41 adjusted in pH: a: pH= 10; b: pH= 8; c: pH= 6. 6. MCM-41 calcined at different temperatures (for 6 h): a: 5% NaBr treated, at 923 K; b: 5% NaBr treated, at 1023 K; c: without treatments, at 923 K; d: without treatments, at 1023 K; e: 5% NH4Br treated at 923 K; f: 5% NH4Br treated at 1023 K.
L. Yin et al. / Materials Letters 61 (2007) 3119–3123
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Fig. 2. TEM (JEM-2100F, 200 keV) images of calcined samples treated by 5% NH4Br (a) and 5% NaBr (b).
but strengthened for ammonium salt (Fig. 1C and D). Besides, low pH value is good for the peak intensity, but ammonium salt is better (Fig. 1E). TEM images showed the differences between the two treatments (Fig. 2). While the calcined samples treated with 5% ammonium chloride had many skew walls, distorted channels and blocked pores, the ones treated with 5% sodium chloride were more regular. The results indicated that the structure of the synthesized MCM-41 mesoporous silica was dramatically changed when treated by salt solutions. In some previous similar studies about adding salt to MCM-41 gel before crystallization [11], the salt effects for the improvement of hydrothermal stability of MCM-41 were due to the moderation of the electrostatic interaction between micelles and the surrounding silicate anions, among surrounding silicate anions with a transition of surfactant– silica mesophases from hexagonal MCM-41 to disordered KIT-1 [12]. But there is a small possibility of the transition because the mesophase has become relatively stable with the hydrothermal reaction. Furthermore there is not enough space for moderation of electrostatic interaction when
most of the silicates have been condensed. We inferred that these are the salt effects occurring on the interface between micelles and the surrounding silicate. It is reasonable to achieve the formation of surfactant–silicate mesostructures by electrostatic templating route which can be affected by the electrostatic interaction in the surrounding high salt concentration [12]. However, the electrostatic interaction still works in the interspace between the silicate and surfactant micelles after crystallization by different means which account for both enhancements and diminishments of the acting force between the hydrophilic radical of the surfactant agent and silicate ions. The able ammonium ions not only can transform into ammonia in the hydrolyzation to result in the enrichment of protons but also form hydrogen bonds with groups containing nitrogen and oxygen atoms. Due to the shortage of hydrogen on the hydrophilic groups of CTAB, the excess of alkyls hampers the access of protons to the nitrogen atoms and it's difficult to form hydrogen bonds. The addition of ammonium ions makes up the shortage of the ability for
Fig. 3. The self-assembly phases containing surfactant agents and silica micelles (A). The interaction between hydrophilic groups and silicate walls without the addition of salt (B-a); with addition of NH+4 which enhances the formation of hydrogen bonds (B-b); with addition of Cl− which weakens the electrostatic interactions (B-c).
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Table 1 The effects of salts comprising different ions on interaction between surfactant agents and silica micelles Ways to affect structure of calcined MCM-41 materials
NaCl KCl NH4Cl NH4Br pH adjustment
Forming new hydrogen bonds Enhancing the hydrogen bonds Diminishing electrostatic √ interaction Eutectic influence √
√
√ √ √
√ √ √
√
√
the hydrophilic group to form hydrogen bonds, strengthens the severely weakened hydrogen condition, and enhances the interactions between the cationic surfactant micelles and surrounding silicate anions which bring structural regularity to the array of hydrophilic groups. The above mentioned effects are essential to maintain the stability of hexagonal channels. For the inability of providing electronegative groups, hydrogen ion is good for improving the ionic environment, strengthening the hydronitrogen bonds and oxyhydrogen bonds, but weak in enhancing the regularity of the pore. On the contrary, haloid anions, such as chloridion and bromonium ion, weaken the interactions between cationic surfactant micelles and surrounding silicate anions by entering
into the adsorption layer and impair the ionic strength (like disjunction of janney couplers). The opposite effects make the walls of the mesopores flexible and damage the hexagonal array (Fig. 3). The different abilities of salts' effects are presented in Table 1. It should be noticed that Na2O was used as flux calcined agent to reduce the melting point of SiO2 in traditional glass industry. Does the Na2O in MCM-41 obtained in basic media affect the mesostructures? It is shown in the following TG–DTA curves (Fig. 4). The lost mass of the sample treated with 5% NH4Br is two times the weight of that of the sample treated with 5% NaBr. It is because of the decomposable ability of NH4Br. But obviously, compared with the sample treated with NH4Br, the sample treated with NaBr showed a very small oxidation peak. This can be explained properly that the irregular channels of the sample treated with NaBr lead to poor heat emitting, which brings to the accumulation of the heat and the growth of the coke. The flux calcined effect of Na2O (the same as other alkali metal salts) makes the walls collapse, and blocks the heat and the space. The above assumptions were confirmed by some further experiments of removing the surfactant agent from mesoporous channels at different temperatures (Fig. 1F). The sample treated with NaBr can not maintain its structure integrally at 923 K. It was destroyed at 1023 K. On the contrary, the patterns of the sample treated with NH4Br calcined at 923 K are very similar with that obtained at 823 K. Even calcined at 1023 K, the intensity of peak (100) descended only a little.
Fig. 4. DTA–TG curves of as-synthesized MCM-41 treated with 5% NaBr (A) and with 5% NH4Br (B).
L. Yin et al. / Materials Letters 61 (2007) 3119–3123
Whereas the MCM-41 mesoporous materials without post-synthesis treatments showed fairly good thermal stability, the effects of ammonium salts did a few works. However, the addition of NaBr (maybe including other alkali metal salts) diminished the stability of MCM-41 mesostructure at high temperature. Therefore it implied that the addition of sodium salts should be considered in order to improve the capabilities of MCM-41. At the same time, it can't be ignored that the presence of alkali metal salts will damage the stability of the mesostructure in basic synthesis media.
4. Conclusions As a general consideration, the mesoporous material synthesized in acid surrounding obtains variable morphology while that synthesized in basic surrounding obtains perfect stability. Thus the basic media was usually adopted when stable products were desired. Besides, the inorganic salts were added to the synthesis gel to improve the hydrothermal stability of the mesostructure. However, the presence of inorganic salts may not only improve hydrothermal stability but also damage the regularity of MCM-41 mesophase, especially when the alkali metal ions are enriched on the surface of mesoporous interchannel. On the other hand, the method of adding ammonium salt can improve
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the regularity and thermal stability of MCM-41 mesopores. We hope that this discovery will not only avoid producing defective mesoporous materials but also improve the quality of mesoporous materials. Details will be reported in the future. References [1] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [2] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, T.W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834. [3] J.Y. Ying, C.P. Mehnert, M.S. Wong, Angew. Chem., Int. Ed. Engl. 38 (1999) 56. [4] P.T. Tanev, T.J. Pinnavaia, Science 267 (1995) 865. [5] A. Corma, Chem. Rev. 97 (1997) 2373. [6] S. Schacht, M. Janicke, F. Schüth, Microporous Mesoporous Mater. 22 (1998) 485. [7] C.H. Ko, S.S. Han, S.H. Cho, J. Porous Mater. 12 (2005) 87. [8] C.Y. Chen, H.X. Li, M.E. Davis, Microporous Mater. 2 (1993) 17. [9] R. Ryoo, C.H. Ko, J.M. Kim, R. Howe, Catal. Lett. 37 (1996) 29. [10] D. Khushalani, A. Kuperman, G.A. Ozin, K. Tanaka, J. Garce, M.M. Olken, N. Coombs, Adv. Mater. 7 (1995) 842. [11] D.H. Pan, D.C. Li, J.H. Ma, R.F. Li, Acta Petrolei Sin. 21 (2005) 62. [12] R. Ryoo, S. Jun, J. Phys. Chem., B 101 (1997) 317.
Effects of post-synthesis treatments on the pore structure and stability of MCM-41 mesoporous silica Li feng Yin, Fang fang Wang, Ji quan Fu ⁎ Center of Chemistry Engineering, Beijing Key Lab., Beijing Institute of Clothing Technology, Beijing, 100029, PR China Received 17 January 2006; accepted 2 November 2006 Available online 27 November 2006
Abstract The effects of post-synthesis treatments on the structure and the stability of MCM-41 mesoporous material were investigated. It was found that, by adding ammonium salts and adjusting pH during the post-synthesis to alkalescently synthesized MCM-41, the regularity of the pore was improved dramatically and the stability of the mesostructure was retained; conversely, they were diminished by adding sodium salts. The results were studied by analyzing the samples using X-ray diffraction (XRD), Transmission electron microscopy (TEM), Differential Thermal Analysis–Thermal Gravimetry (DTA–TG). The change of the electrostatic surrounding and the formation of hydrogen bonds caused by different ions were confirmed to be the main factors. © 2006 Elsevier B.V. All rights reserved. Keywords: Post-synthesis treatment; Pore structure; Stability; MCM-41
1. Introduction In the last ten years, mesoporous materials have been paid more attention for their potential use in the field of catalysis, separation, synthesis of nanocluster and object embedded vector, drug delivery, etc [1–5]. Also the synthesis methods and types were developed constantly. As one member of the first mesoporous materials family, MCM-41, due to its excellent characteristics of large specific surface area, uniform pore size and structural stability, is always the focus of the study. The internal structure of MCM-41 material is an array of hexagonal pore, of which pore size ranges from 1.6 to 10 nm, which depends on the surfactant agent, various additives (swelling agents) and the synthesis conditions. With those characteristics, MCM-41 is usually used as supports for large chemical species and templates for nanosize architecture. However, MCM-41 mesoporous materials synthesized in common routes are malformed and low ordered [6,7]. Although the thermal stability of MCM-41 mesostructure is measured up in nonaqueous surroundings [8], it is very poor with aqueous solutions, such as ion exchange and catalyst preparation [9]. ⁎ Corresponding author. 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.11.029
In many literatures, additives are proposed to improve the regularity and the stability of mesoporous material MCM-41. Khushalani et al. [10] showed that the regularity of the pore could be improved and d100 parameter ranges from 0.3 to 0.4 nm, when silica mesophase is treated in mother liquid at 423 K for 24 h; Pan et al. [11] showed that a suitable amount in the synthesis process could enhance the diffraction peak intensity of XRD and the stability of mesoporous silica notably; Ryoo and Jun [12] found that the hydrothermal stability of MCM-41 could be improved dramatically by adding sodium salts in the hydrothermal crystallization process, such as sodium chloride, potassium chloride, sodium acetate, and ethylenediaminetetraacetic acid tetrasodium salt. However, it is before or during the synthesis process that the improvers were usually added, and post-synthesis treatments, especially methods of adding salt to improve the regularity and stability, are rarely reported. In this thesis, we have studied and reported the effects of postsynthesis treatments on the porous structure and the stability of MCM-41. The structural regularity can be improved with ammonium salt and pH adjustment, and decreased with alkali salt which can also reduce the heat stability. As supplements to the characteristics of MCM-41, these results could help us improve the quality of mesoporous materials and play an active role to avoid defective products.
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L. Yin et al. / Materials Letters 61 (2007) 3119–3123
2. Experimental section 2.1. Synthesis The synthesis process of MCM-41 was carried out in a basic condition, with soluble glass (SiO2:28.2%, Na2O:8.7%) as silica resource, and cetyltrimethyl ammonium bromide (CH3 (CH2)15N(CH3)3Br, CTAB) as structure-directing agent. In a typical synthesis, 2.64 g of CTAB and 18.46 g of soluble glass were dissolved in 35 ml of deionized water stirred continuously until the solution turned clear, then 6.0 M HCl (aq) was added drop by drop until the pH value of latex reaches up to 11.0. The final gel was composed of: 1CTAB:15.7SiO2:4.76Na2O:22.4HCl:2.56H2O. After hydrothermally crystallized at 393 K for 24 h, the gel as solid was taken out to be washed, filtrated and collected for airing marked as A-MS (as-synthesized MCM-41). 2.2. Post-synthesis treatment Several copies of equally weighed solid (A-MS) were steeped separately in different amounts of solution, which have equal volume but different kinds of salt, making the mass rate of
salt to A-MS as 0%, 1%, 2% and 5%, or steeped separately in different amounts of deionized water, which have the same volume, and then the pH values of the solution were adjusted separately to 6, 8 and 10 by adding 6.0 M HCl. After a slow drying process in 333 K for 2 h, the final pieces of compound were heated to 823 K with a rate of 2 K/min and then roasted for 6 h. The products were marked as S-MS. 2.3. Characterization Powder XRD patterns were obtained in a D/max-rb diffractometer using CuKα radiation at a scanning speed of 2° min− 1. SEM images were obtained in a JEM-2100F operated at 20 keV. The DTA–TG was carried out in air at a heating rate of 2 K/min from 373 K to 923 K using TG–DTA6300 instruments. 3. Results and discussions A-MS showed a typical pattern of hexagonal structure, including a strong peak (100) at about 2θ ≈ 2° and three weaker lines at 2θ ≈ 3.5°, 4.2° and 6°(Fig. 1A-a). These samples (A-MS and all S-MS) showed similar peak width and resolution, but quite different peak intensities, especially peak (100). The peak intensity was remarkably weakened while the mass of the alkali salt ranged from 0 to 5 wt.% (Fig. 1A and B),
Fig. 1. (A) XRD patterns of calcined MCM-41 treated with NaCl: a: 0.0 wt.%; b: 1.0 wt.%; c: 2.0 wt.%; d: 5.0 wt.%. (B) Calcined MCM-41 treated with NaBr: a: 1.0 wt.%; b:2.0 wt.%; c: 5.0 wt.%. (C) Calcined MCM-41 treated with NH4Cl: a: 1.0 wt.%; b: 2.0 wt.%; c: 5.0 wt.%. (D) Calcined MCM-41 treated with NH4Br: a: 1.0 wt.%; b: 2.0 wt. %; c: 5.0 wt.%. (E) Calcined MCM-41 adjusted in pH: a: pH= 10; b: pH= 8; c: pH= 6. 6. MCM-41 calcined at different temperatures (for 6 h): a: 5% NaBr treated, at 923 K; b: 5% NaBr treated, at 1023 K; c: without treatments, at 923 K; d: without treatments, at 1023 K; e: 5% NH4Br treated at 923 K; f: 5% NH4Br treated at 1023 K.
L. Yin et al. / Materials Letters 61 (2007) 3119–3123
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Fig. 2. TEM (JEM-2100F, 200 keV) images of calcined samples treated by 5% NH4Br (a) and 5% NaBr (b).
but strengthened for ammonium salt (Fig. 1C and D). Besides, low pH value is good for the peak intensity, but ammonium salt is better (Fig. 1E). TEM images showed the differences between the two treatments (Fig. 2). While the calcined samples treated with 5% ammonium chloride had many skew walls, distorted channels and blocked pores, the ones treated with 5% sodium chloride were more regular. The results indicated that the structure of the synthesized MCM-41 mesoporous silica was dramatically changed when treated by salt solutions. In some previous similar studies about adding salt to MCM-41 gel before crystallization [11], the salt effects for the improvement of hydrothermal stability of MCM-41 were due to the moderation of the electrostatic interaction between micelles and the surrounding silicate anions, among surrounding silicate anions with a transition of surfactant– silica mesophases from hexagonal MCM-41 to disordered KIT-1 [12]. But there is a small possibility of the transition because the mesophase has become relatively stable with the hydrothermal reaction. Furthermore there is not enough space for moderation of electrostatic interaction when
most of the silicates have been condensed. We inferred that these are the salt effects occurring on the interface between micelles and the surrounding silicate. It is reasonable to achieve the formation of surfactant–silicate mesostructures by electrostatic templating route which can be affected by the electrostatic interaction in the surrounding high salt concentration [12]. However, the electrostatic interaction still works in the interspace between the silicate and surfactant micelles after crystallization by different means which account for both enhancements and diminishments of the acting force between the hydrophilic radical of the surfactant agent and silicate ions. The able ammonium ions not only can transform into ammonia in the hydrolyzation to result in the enrichment of protons but also form hydrogen bonds with groups containing nitrogen and oxygen atoms. Due to the shortage of hydrogen on the hydrophilic groups of CTAB, the excess of alkyls hampers the access of protons to the nitrogen atoms and it's difficult to form hydrogen bonds. The addition of ammonium ions makes up the shortage of the ability for
Fig. 3. The self-assembly phases containing surfactant agents and silica micelles (A). The interaction between hydrophilic groups and silicate walls without the addition of salt (B-a); with addition of NH+4 which enhances the formation of hydrogen bonds (B-b); with addition of Cl− which weakens the electrostatic interactions (B-c).
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Table 1 The effects of salts comprising different ions on interaction between surfactant agents and silica micelles Ways to affect structure of calcined MCM-41 materials
NaCl KCl NH4Cl NH4Br pH adjustment
Forming new hydrogen bonds Enhancing the hydrogen bonds Diminishing electrostatic √ interaction Eutectic influence √
√
√ √ √
√ √ √
√
√
the hydrophilic group to form hydrogen bonds, strengthens the severely weakened hydrogen condition, and enhances the interactions between the cationic surfactant micelles and surrounding silicate anions which bring structural regularity to the array of hydrophilic groups. The above mentioned effects are essential to maintain the stability of hexagonal channels. For the inability of providing electronegative groups, hydrogen ion is good for improving the ionic environment, strengthening the hydronitrogen bonds and oxyhydrogen bonds, but weak in enhancing the regularity of the pore. On the contrary, haloid anions, such as chloridion and bromonium ion, weaken the interactions between cationic surfactant micelles and surrounding silicate anions by entering
into the adsorption layer and impair the ionic strength (like disjunction of janney couplers). The opposite effects make the walls of the mesopores flexible and damage the hexagonal array (Fig. 3). The different abilities of salts' effects are presented in Table 1. It should be noticed that Na2O was used as flux calcined agent to reduce the melting point of SiO2 in traditional glass industry. Does the Na2O in MCM-41 obtained in basic media affect the mesostructures? It is shown in the following TG–DTA curves (Fig. 4). The lost mass of the sample treated with 5% NH4Br is two times the weight of that of the sample treated with 5% NaBr. It is because of the decomposable ability of NH4Br. But obviously, compared with the sample treated with NH4Br, the sample treated with NaBr showed a very small oxidation peak. This can be explained properly that the irregular channels of the sample treated with NaBr lead to poor heat emitting, which brings to the accumulation of the heat and the growth of the coke. The flux calcined effect of Na2O (the same as other alkali metal salts) makes the walls collapse, and blocks the heat and the space. The above assumptions were confirmed by some further experiments of removing the surfactant agent from mesoporous channels at different temperatures (Fig. 1F). The sample treated with NaBr can not maintain its structure integrally at 923 K. It was destroyed at 1023 K. On the contrary, the patterns of the sample treated with NH4Br calcined at 923 K are very similar with that obtained at 823 K. Even calcined at 1023 K, the intensity of peak (100) descended only a little.
Fig. 4. DTA–TG curves of as-synthesized MCM-41 treated with 5% NaBr (A) and with 5% NH4Br (B).
L. Yin et al. / Materials Letters 61 (2007) 3119–3123
Whereas the MCM-41 mesoporous materials without post-synthesis treatments showed fairly good thermal stability, the effects of ammonium salts did a few works. However, the addition of NaBr (maybe including other alkali metal salts) diminished the stability of MCM-41 mesostructure at high temperature. Therefore it implied that the addition of sodium salts should be considered in order to improve the capabilities of MCM-41. At the same time, it can't be ignored that the presence of alkali metal salts will damage the stability of the mesostructure in basic synthesis media.
4. Conclusions As a general consideration, the mesoporous material synthesized in acid surrounding obtains variable morphology while that synthesized in basic surrounding obtains perfect stability. Thus the basic media was usually adopted when stable products were desired. Besides, the inorganic salts were added to the synthesis gel to improve the hydrothermal stability of the mesostructure. However, the presence of inorganic salts may not only improve hydrothermal stability but also damage the regularity of MCM-41 mesophase, especially when the alkali metal ions are enriched on the surface of mesoporous interchannel. On the other hand, the method of adding ammonium salt can improve
3123
the regularity and thermal stability of MCM-41 mesopores. We hope that this discovery will not only avoid producing defective mesoporous materials but also improve the quality of mesoporous materials. Details will be reported in the future. References [1] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [2] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, T.W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834. [3] J.Y. Ying, C.P. Mehnert, M.S. Wong, Angew. Chem., Int. Ed. Engl. 38 (1999) 56. [4] P.T. Tanev, T.J. Pinnavaia, Science 267 (1995) 865. [5] A. Corma, Chem. Rev. 97 (1997) 2373. [6] S. Schacht, M. Janicke, F. Schüth, Microporous Mesoporous Mater. 22 (1998) 485. [7] C.H. Ko, S.S. Han, S.H. Cho, J. Porous Mater. 12 (2005) 87. [8] C.Y. Chen, H.X. Li, M.E. Davis, Microporous Mater. 2 (1993) 17. [9] R. Ryoo, C.H. Ko, J.M. Kim, R. Howe, Catal. Lett. 37 (1996) 29. [10] D. Khushalani, A. Kuperman, G.A. Ozin, K. Tanaka, J. Garce, M.M. Olken, N. Coombs, Adv. Mater. 7 (1995) 842. [11] D.H. Pan, D.C. Li, J.H. Ma, R.F. Li, Acta Petrolei Sin. 21 (2005) 62. [12] R. Ryoo, S. Jun, J. Phys. Chem., B 101 (1997) 317.