Solid State Sciences 13 (2011) 271e275
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(2-Hydroxyethyl)-trimethylammonium hydroxide as an organic base for the synthesis of highly ordered MCM-41 Abdolraouf Samadi-Maybodi*, Amir Vahid Analytical division, Department of Chemistry, University of Mazandaran, Babolsar, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 May 2010 Received in revised form 18 October 2010 Accepted 19 November 2010 Available online 26 November 2010
MCM-41material was synthesized using various ratios of (2-Hydroxyethyl)-trimethylammonium hydroxide and sodium hydroxide. The calcined samples were characterized by powder XRD, scanning electron microscopy, nitrogen physisorption, 29Si MAS NMR spectroscopy and transmission electron microscopy. The XRD pattern of all calcined samples shows at least four distinct peaks that indicates long range order of them. The nitrogen adsorption/desorption isotherm of all samples exhibits type IV isotherm which is the typical characteristic of mesoporous materials. When (2-Hydroxyethyl)-trimethylammonium hydroxide was used alone as a base for the synthesis of MCM-41, the highest value of surface area was obtained. Results also reveal that the pore size distribution of this sample is so narrow. The SEM images show that wiry MCM-41 samples were obtained when (2-Hidroxyethyl)-trimethylammonium hydroxide was used in the synthesis process. 29Si MAS NMR spectroscopy proves that the presence of pure organic base in synthesis medium improves framework condensation of MCM-41. TEM images illustrate well-ordered hexagonal array of mesopores in all samples. Ó 2010 Elsevier Masson SAS. All rights reserved.
Keywords: (2-Hydroxyethyl)-trimethylammonium hydroxide Mesoporous silicate material Highly ordered hexagonal structure MCM-41synthesis Characterization
1. Introduction
2. Experimental
MCM-41 is a mesoporous molecular sieve which formed from the close packed silica-coated micelles of a surfactant template [1,2]. Removal of the template by calcination or extraction leaves parallel nanochannels in a hexagonal array [3,4]. MCM-41 material is a promising catalyst support in many reactions because of its unique structural and physical properties; notably large surface area, tunable and large pore size, narrow pore size distribution and high thermal stability [5,6]. Application of MCM-41 was investigated in various fields such as separation [7], catalysis [8], optic [9], nanostructure synthesis [10] and environmental purification [11]. However, structural and morphological properties of the MCM-41 strongly affect its performance in relevant application. There are numerous reports on the synthesis of MCM41 with different reactants that result in products with different textural and morphological properties [12e15]. Our previous works approved that (2-Hydroxyethyl)-trimethylammonium hydroxide (2HETMAOH) influences the structure and distribution of silicate spices in alkaline silicate solution [16e18] and consequently affects the properties of final product in zeolite synthesis [19]. In this work, 2HETMAOH was used as an organic base for the synthesis of highly ordered MCM-41. All samples were characterized by X-ray diffraction, nitrogen adsorption/desorption and scanning electron microscopy.
2.1. Materials and methods
* Corresponding author. Tel./fax: þ98 1125342350. E-mail address:
[email protected] (A. Samadi-Maybodi). 1293-2558/$ e see front matter Ó 2010 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2010.11.027
All chemical compounds were purchased from Merck. (2Hydroxyethyl)-trimethylammonium chloride was converted to hydroxide form by dissolving it in appropriate amount of water and passing it down a column of amberlite resin IR-120(OH). The exchange process was repeated by another fresh anion-exchange column to obtain hydroxide rich solution. In a typical synthesis, Cetyltrimethylammonium bromide (CTAB) was dissolved in appropriate amount of deionized water and then a mixture of NaOH and 2-HETMAOH was added to this solution followed by stirring (at 250 rpm) for 15 min to obtain a clear solution. Tetraethylorthosilicate (TEOS) was then added to this solution under stirring. The obtained suspension was stirred for further 4 h and then was transferred into the Teflon-lined stainless steel and autoclaved at 343 K for 48 h. The white precipitate was filtered and washed with sufficient amount of deionized water and dried at 373 K overnight. The synthesized samples calcined at 823 K under air for 6 h with a heating rate of 1 K/min from room temperature to 823 K. Four samples of MCM-41 were synthesized using different molar mixtures of two bases. In all samples, the concentrations of TEOS, CTAB and H2O were the same. The final molar compositions of reactants were 1.0 TEOS: 0.15 CTAB: 120H2O: R (R is molar mixture of NaOH and 2-HETMAOH, i.e.,
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0.35 þ 0, 0.28 þ 0.07, 0.14 þ 0.21, 0 þ 0.35 for samples number 1 to 4 respectively).
2.2. Characterization X-ray diffraction (XRD) patterns were recorded on a Seifert TT 3000 diffractometer using Cu Ka radiation of wavelength 0.15405 nm. Diffraction data was recorded in the region of 1e10 2q at an interval 0.01 2q. A scanning rate of 1.0 2q/min was used. Scanning electron micrographs were recorded using a Zeiss DSM 962 (Zeiss, Oberkochen, Germany). Samples were deposited on a sample holder with an adhesive carbon foil and sputtered with
Fig. 2. Nitrogen adsorption/desorption isotherms of four calcined MCM-41 samples. a (red curve), b (brown curve), c (green curve) and d (blue curve) represent the corresponding samples 1, 2, 3 and 4 respectively. The isotherms for b, c and d are offset vertically by 150, 300 and 450 cm3 (STP) g1, respectively (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
gold. Physisorption of Nitrogen was measured at 77 K using a BELSORP-mini porosimeter. Textural analyses of the samples were performed on a BELSORP-mini apparatus. Measurements were carried out at 77 K. Prior to analysis the samples were outgassed invacuo for 4 h at 423 K until a stable vacuum of 0.1 Pa was reached. Pore size distribution was calculated from the desorption branch using the Barrett-Joyner-Halenda (BJH) method [23,24]. 29Si MAS NMR experiments were measured on a Bruker MSL-400 spectrometer using a 4-mm MAS probe at a spinning rate of 10 kHz. Transmission electron microscopy was performed on a CM 120
Fig. 1. XRD patterns of samples 1e4 (1aed) that synthesized with different molar mixture of NaOH and 2-HETMAOH, i.e., 0.35 þ 0, 0.28 þ 0.07, 0.14 þ 0.21 and 0 þ 0.35, respectively. Intensity of four corresponding dashed lines are multipled to 5. The XRD patterns for b, c and d are offset vertically by 8000, 16000 and 24000 (a.u.), respectively.
Fig. 3. Pore size distribution of four calcined MCM-41 samples. a (red curve), b (brown curve), c (green curve) and d (blue curve) represent the corresponding samples 1, 2, 3 and 4, respectively (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
A. Samadi-Maybodi, A. Vahid / Solid State Sciences 13 (2011) 271e275 Table 1 Physical properties of four calcined MCM-41 samples synthesized with different molar mixtures of NaOH and 2-HETMAhydroxide. (I þ O)a
SBET (m2/g)b
Vt (cm3/g)c
ad (nm)
Wd (nm)e
Wt (nm)f
Q4/Q3
(0.35 þ 0.0) (0.28 þ 0.07) (0.14 þ 0.21) (0.0 þ 0.35)
855 684 633 1098
0.75 0.58 0.57 0.94
4.34 4.34 4.43 4.43
3.56 3.36 3.41 3.69
0.95 1.14 1.18 0.92
70/30 62/38 63/37 76/24
Sample 1 2 3 4
a Molar composition of Inorganic (I: NaOH) and Organic (O: 2-HETMAH) bases respectively. b BET specific surface area. c Total pore volume. d Unit cell parameter obtained from XRD diffractograms (2d100/O3). e pore diameter (nm) calculated by geometrical (pressure independent) method [22,23]. f Wall thickness(nm) obtained by following equation: Wt ¼ a (Wd/1.05).
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corresponding patterns. Diffractograms of all samples exhibit intense d100 reflections with several small peaks located at higher 2q angels, which means that the MCM-41 samples have hexagonal pore structure. The XRD pattern of all samples also reveals that the d100 reflection of the corresponding samples appears at a nearly equal position on X axis. It is well-known that the number of peaks and their relative intensity and sharpness represent the long-range order of the MCM-41’s structure [6]. The XRD pattern of the sample 1, which was synthesized only with sodium hydroxide- shows four well-resolved peaks. The XRD patterns of the samples 2 and 3 (Fig. 1b and c respectively), which were synthesized with different mixtures of sodium hydroxide and 2-HETMAOH, are almost similar to the XRD pattern of sample 1. Nonetheless, the XRD pattern of sample 4 (Fig. 1d), which was only synthesized by 2-HETMAOH, consists five well-defined Bragg peaks which states achievement of highly ordered MCM-41.
Philips with a tension voltage of 120 kV. Samples were dispersed in ethanol and sonicated for 45 min and deposited on a copper grid. 3.2. N2 physisorption 3. Results and discussion 3.1. XRD analysis XRD patterns of MCM-41 samples were presented in Fig.1. Vertical expansion of each XRD patterns was inserted at the top of
Nitrogen adsorption/desorption isotherms of the calcined samples are shown in Fig. 2. According to the IUPAC nomenclature all isotherms can be classified as type IV isotherm which is a typical character for mesoporous materials [20]. Sample 1 (Fig. 2a), which was synthesized with sodium hydroxide, shows a slow increase of nitrogen uptake at low relative pressures due
Fig. 4. SEM images of four MCM-41 samples. The inset letter at top of each micrograph i.e., a, b, c and d related to the sample 1, 2, 3 and 4 that were synthesized with different molar mixtures of NaOH and 2-HETMAOH as follow: 0.35 þ 0, 0.28 þ 0.07, 0.14 þ 0.21, 0 þ 0.35, respectively.
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Fig. 5. 29Si MAS NMR spectra of four calcined MCM-41 samples. a (red curve), b (brown curve), c (green curve) and d (blue curve) represent the corresponding samples 1, 2, 3 and 4, respectively (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
to the monolayer/multilayer adsorption of nitrogen both on its external surface and inside the mesopores, followed by a broad capillary condensation step that liquid nitrogen filled inside the mesopores. On the basis of IUPAC classification, it can be deduced that there is a H2 hysteresis loop for this sample (which is the most common hysteresis loop in mesoporous materials). Results also reveal that at higher pressures, small multilayer nitrogen adsorption has occurred on the external surface of this sample. Finally, further increasing in gas uptake arisen near the saturated pressure (i.e., P/P0 ¼ 1) can be associated to the filling of the gas into the other available pores and voids between MCM-41 particles. Fig. 2b and c represent adsorption/desorption isotherms of samples 2 and 3 respectively. Their specific surface area, calculated from the linear part of the Brunauer-Emmett-Teller (BET) equation (i.e., P/Po ¼ 0.05e0.25), are lower than sample 1. In both samples, the inflection of capillary condensation/evaporation step and the type of hysteresis loop are approximately similar to the sample 1. Sample 4, which was only synthesized with 2-HETMAOH, has the highest specific surface area (Fig. 2d). Its nitrogen adsorption/desorption isotherm clearly specifies that the capillary condensation step for sample 4 is sharper than the other samples. This implies that the pore size distribution of sample 4 is too narrow. Fig. 3 display Pore size distribution of corresponding samples which obtained. As can be seen, sample 1 (Fig. 3d) exhibits the sharpest peak or in the other word, the narrowest pore size distribution. Nitrogen adsorption/desorption isotherm of sample 4 also reveal that the desorption branch at the
Fig. 6. TEM images of four MCM-41 samples. The inset letter at top of each micrograph i.e., a, b, c and d related to the sample 1, 2, 3 and 4 that were synthesized with different molar mixtures of NaOH and 2-HETMAOH as follow: 0.35 þ 0, 0.28 þ 0.07, 0.14 þ 0.21, 0 þ 0.35, respectively.
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relative pressure range of 0.27e0.34 (i.e., capillary condensation/ evaporation region) is almost superimposed with the adsorption branch. This means that most pores within sample 4 are open pores, linked to each other and connected directly to the surface of the sample and are therefore accessible for the guest species such as N2 molecules [21]. The textural properties of all calcined samples were calculated and given in Table 1 [22,23]. 3.3. SEM analysis Scanning electron microscopy was used to show the morphology of the samples. The SEM images of all samples are illustrated in Fig. 4. The results obtained from SEM experiments specify that the morphology of MCM-41 samples are influenced by the presence of 2-HETMAOH. They became wiry when 2-HETMAOH was used in their synthesis process. It is pertinent to point out that pH of the samples during synthesis process (i.e., before adding TEOS, after 4 h steering and after aging for two days) were almost equal. 3.4.
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Si MAS NMR
Fig. 5 shows the 29Si MAS NMR spectra of all calcined samples. The resonances at ca. 110 ppm and ca. 100 ppm correspond to Q4 (fully cross-linked Si atom: Si(OSi)4) and Q3 (Si(OSi)3(OH)), respectively. Generally, the Q4 content in the spectrum represents the degree of framework condensation. Samples 2 and 3, which synthesized with different mixture of 2-HETMAOH and sodium hydroxide, show a lower Q4/Q3 ratio than samples 1 and 4 which synthesized with sodium hydroxide or 2-HETMAOH, respectively. This results suggesting that the employment of pure 2-HETMAOH is favorable for the formation of MCM-41 with a highly condensed framework. The ratios of Q4/Q3 for all samples were given in Table 1. 3.5. TEM analysis It is well known that transmission electron microscopy is a powerful technique for the direct elucidation of the structural order MCM-41 material. TEM images in Fig. 6 provide details of the open framework structure of all samples. As can be seen, TEM image of sample 4 (Fig. 6d), which synthesized only with 2-HETMAOH, shows higher degree of structural order than other samples. However, all of them exhibit hexagonal array of mesopores. These features are in agreement with those found from XRD and nitrogen physisorption measurement.
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4. Conclusion In the present work, we introduced 2-HETMAOH as a novel reactant for the synthesis of MCM-41 which results in an alkali-free product with better quality such as 28% higher specific surface area than other samples which synthesized by sodium hydroxide or mixtures of 2-HETMAOH/sodium hydroxide. Interestingly, this organic base can be used for the Hþ-form and alkali-free synthesis of other types of macroporous, mesoporous, and microporous materials. Acknowledgements We are grateful to the Materials & Energy Research Center of Karaj for performing Nitrogen physisorption measurment. References [1] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710e712. [2] T. Yanagisawa, T. Shimizu, K. Kuroda, C. Kato, Bull. Chem. Soc. Jpn. 63 (1990) 988e992. [3] A. Corma, Chem. Rev. 97 (1997) 2373e2420. [4] F. Schuth, W. Schmidt, Adv. Mate. 14 (2002) 629e638. [5] X. Liu, H. Sun, Y. Yang, J. Colloid Interface Sci. 319 (2008) 377e380. [6] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834e10843. [7] M. Grun, A.A. Kurganov, S. Schacht, F. Schuth, K.K. Unger, J. Chromatogr. A 740 (1996) 1e9. [8] A. Taguchi, F. Schuth, Microp. Mesop. Mater. 77 (2005) 1e45. [9] M.A. Zanjanchi, A. Ebrahimian, Z. Alimohammadi, Opt. Mater. 29 (2007) 794e800. [10] W. Ying, Z. Dongyuan, Chem. Rev. 107 (2007) 2821e2860. [11] L. Lv, K. Wang, X.S. Zhao, J. Colloid Interface Sci. 305 (2007) 218e225. [12] Q. Zhang, F. Lü, C. Li, Y. Wang, H. Wan, Chem. Lett. 35 (2006) 190e192. [13] R. Mokaya, Microp. Mesop. Mater. 44 (2001) 119e127. [14] Q. Cai, Z.S. Luo, W.Q. Pang, Y.W. Fan, X.H. Chen, F.Z. Cui, Chem. Mater. 13 (2001) 258e263. [15] H.P. Lin, C.Y. Mou, Acc. Chem. Res. 35 (2002) 927e935. [16] A. Samadi-Meybodi, N. Goudarzi, Spectrochim. Acta A 65 (2006) 753e758. [17] A. Samadi-Meybodi, N. Goudarzi, H. Naderi-Manesh, Bull. Chem. Soc. Jpn. 79 (2006) 276e281. [18] A. Samadi-Meybodi, N. Goudarzi, C.W. Kirby, Y. Huang, J. Surfact. Deterg. 11 (2008) 49e54. [19] A. Samadi-Meybodi, N. Goudarzi, Bull. Chem. Soc. Jpn. 80 (2007) 789e793. [20] INTERNATIONAL UNION OF PURE AND APPLIED CHEMISTRY, Reporting physisorption data for Gas/Solid Systems, Pure Appl. Chem. 87 (1957) 603e608. [21] S. Hamoudi, S. Royer, S. Kaliaguine, Microp. Mesop. Mater. 71 (2004) 17e25. [22] A. Galarneau, D. Desplantier, R. Dutartre, F. Di-Renzo, Microp. Mesop. Mater. 27 (1999) 297e308. [23] M. Kruk, M. Jaroniec, Y. Sakamoto, O. Terasaki, R. Ryoo, C.H. Ko, J. Phys. Chem. B. 104 (2000) 292e301. [24] M. Kruk, M. Jaroniec, A. Sayari, Langmuir 13 (1997) 6267e6273.
