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Synthesis of highly ordered MCM-41 silica with spherical particles
Microporous and Mesoporous Materials 104 (2007) 52–58 www.elsevier.com/locate/micromeso
Synthesis of highly ordered MCM-41 silica with spherical particles ˇ ejka Arnosˇt Zukal a, Matthias Thommes b, Jirˇ´ı C a
a,*
J. Heyrovsky´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejsˇkova 3, CZ-18223 Prague, Czech Republic b Quantachrome Instruments, 1900 Corporate Drive, Boynton Beach, FL 33426, USA Received 16 October 2006; received in revised form 22 December 2006; accepted 3 January 2007 Available online 14 January 2007
Abstract Highly ordered MCM-41 silica was synthesized by using of optimized blend of cetyltrimethylammonium bromide and 1-methyl-3octylimidazolium chloride as a structure-directing agent. The employment of homogeneous precipitation of the solid product from a water solution of sodium metasilicate and the surfactant blend resulted in the formation of very well ordered spherical particles. The material was characterized by scanning as well transmission electron microscopy, X-ray powder diffraction, and nitrogen and argon adsorption using a proper non-local density functional theory approach for calculations of the textural parameters. The results obtained show that spherical particles are composed of domains of perfectly ordered hexagonal porous structure. 2007 Elsevier Inc. All rights reserved. Keywords: Silica; Spherical particles; Homogenous precipitation; Nitrogen adsorption; Argon adsorption
1. Introduction Mesoporous silicas are very frequently used as a catalyst supports or adsorbents. Their physical properties such as surface area, pore distribution and void volume have significant effect on the performance in the respective application. High surface areas and monodisperse pore sizes are essential to obtain the necessary adsorption capacity and molecular selectivity. The materials synthesized by the condensation of silicate species in solution using proper surfactants as the structure-directing agents exhibit pore sizes in the nanometer range, narrow pore size distribution and the surface area as large as 1000 m2/g. On the macroscale level the performance of the mesoporous silica critically depends on the size and shape of their particles. As mesoporous silica spheres uniform in size are expected to be advantageously applicable in processes associated with gas separation or catalysis, a particular attention was paid to the preparation of such materials. The communication [1] describes the first synthesis of *
Corresponding author. Tel.: +420 26605 3795; fax: +420 28658 2307. ˇ ejka). E-mail address: [email protected] (J. C
1387-1811/$ - see front matter 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2007.01.004
spherical silica particles featuring an MCM-41 structure. The synthesis procedure itself was based on a modification of Sto¨ber’s well-known synthesis of monodisperse silica spheres [2], which involves the hydrolysis of tetraalkyl orthosilicate in a mixture of a low-boiling alcohol and aqueous ammonia. This procedure was modified by adding of a cationic surfactant to the reaction mixture, which was operative as a structure-directing agent. The structure and pore size distribution of spherical particles with size ranging from 0.2 to 1 lm have been studied using nitrogen adsorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray powder diffraction (XRD) [3]. SEM images gave a proof of a spherical morphology, which was also confirmed by TEM. Although XRD patterns can be explained in terms of an ill-ordered hexagonal structure, this structure was not confirmed by electron microscopy. TEM provided an evidence for a spherical distribution of the pores. The pores were hexagonally packed only on a local scale; the pore ordering on a larger scale was disturbed. This was also in agreement with the XRD patterns, which showed broader peaks for the higher orders as compared with the first order one.
A. Zukal et al. / Microporous and Mesoporous Materials 104 (2007) 52–58
By means of emulsion biphase chemistry the preparation of hard spheres of mesoporous silicas was achieved [4]. Silica spheres were uniform in size, which could be controlled from 0.1 to 2 mm. Mesoporous silica spheres were prepared in one step in a procedure based on the phase separation resulting from the hydrophobic nature of tetrabutyl orthosilicate, which was used to provide the desired spherical morphology. Two examples of the pore structure within the spheres were revealed by TEM. In some sections ordered arrays of pores similar to those found in MCM-41 were observed, while pores appearing to be monodispersed in size but with random orientation were found in other areas. Micrometer-sized hard spheres of mesoporous SBA-15 were also synthesized under acidic conditions by using TEOS as a silica source. The neutral triblock copolymer Pluronic P123 (EO20PO70EO20) as the structuredirecting agent and cetyltrimethylammonium bromide (CTMABr) as a cosurfactant were applied [5]. The formation of spheres of SBA-16 silica different in size was achieved under acidic conditions using quaternary mixture of triblock copolymer Pluronic F127 (EO106PO70EO106), water, HCl and butanol. The particle size ranged in this case from micrometers to milimeters [6] and it was controlled by the amount of TEOS added. Submicrometer sized silica spheres were synthesized under basic conditions from tetramethyl orthosilicate using different alcohols or polyols as co-solvents [7–9]. The materials prepared are characterized by relatively small pore diameter (2 nm) and low pore volume (0.4–0.5 cm3/g). The mesoporous micrometer-sized spherical MSU-X silica particles were obtained using procedure based on a sodium fluoride aided hydrolysis of tetraethyl orthosilicate (TEOS) dissolved in a dilute solution of non-ionic polyethylene oxide-based surfactants [10,11]. The main idea of this approach lies in the capability to separate the assembly step and the hydrolysis step. The X-ray diffraction pattern exhibited a single narrow peak assignable to the pore center to center correlation length, characteristic of a worm-hole structure of the porous framework of MSU materials. Centimeter sized macrospheres of silica-surfactant nanocomposites were synthesized from TEOS and Pluronic P123 as a template under acidic conditions (pH < 1) [12]. The organic–inorganic macrosphere is crack-free and possesses high elasticity. After calcination eliminating the surfactant, the silica sphere was intact in outer appearance. Porosity data for the calcined silica sphere evidenced the large BET surface area (732 m2/g) and narrow pore size distribution with mode diameter of 5.2 nm. The above described mesoporous silicas with spherical particles proved the influence of spherical symmetry on the pore architecture and ordering. Some materials show the spherical distribution of the pores [3] whereas other spherical silicas consist of different domains characterized by either well ordered or ill-ordered mesoporous structure [4]. Silica prepared by the two-step pathway features a worm-hole mesoporous structure [10,11]. The formation of spherical particles of SBA-16 was also explained by an
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agglomeration of colloidal particles and a consequent filling of the interparticle voids with amorphous silica [6]. In the present work we report on the employment of an ionic liquid surfactant for the preparation of the spherical silica. As the use of pure 1-methyl-3-alkylimidazolium salts does not provide well ordered mesoporous silicas with regular morphology [13], an optimized mixture of 1-methyl-3-octylimidazolium chloride (MC8IMCl) and cetyltrimethylammonium bromide (CTMABr) has been used as the structure-directing agent. The employment of this mixture of surfactants in the homogeneous precipitation of mesoporous silica opened the new pathway towards reproducible preparation of spherical siliceous particles characterized by perfectly ordered mesoporous structure. 2. Materials and methods 2.1. Synthesis The reaction mixture was prepared in an autoclavable Nalgene bottle at the temperature of 308 K. In the typical synthesis, m g of MC8IMCl and (4–m) g of CTMABr followed by 4 g of solid Na2SiO3 were dissolved in 900 ml of distilled water, resulting in the formation of a clear solution. Then, 5 ml of ethyl acetate was quickly added under stirring, the mixture was homogenized and the stirring was stopped. (All the chemicals were purchased by Aldrich.) After 10 min a precipitate began to form and the mixture was allowed to stand at 308 K for 5 h. After this period the separation of solid particles from the solution by sedimentation occurred; the suspension of the solid product in the mother liquor was kept at 368 K for 48 h in a heating box. During the ageing, organic vapor was allowed to escape through leaks in the cap of the bottle. The resulting solid phase was recovered by filtration, extensively washed out with distilled water and ethanol, and dried at ambient temperature. The templates were removed by calcination at 813 K for 8 h (temperature ramp of 1 K/min). The prepared samples are labeled as MCM-41-m/n, where m and n denote the amounts of MC8IMCl and CTMABr in the reaction mixture, respectively. 2.2. SEM, TEM, and XRD characterization X-ray powder diffraction data were recorded on a Bruker D8 X-ray powder diffractometer equipped with a graphite monochromator and position sensitive detector (Va˚ntec-1) using Cu Ka radiation (at 40 kV and 30 mA) in Bragg–Brentano geometry. The size and shape of synthesized particles were evaluated by scanning electron microscopy images using a JEOL JSM-5500LV instrument. High-resolution transmission electron microscopy (HRTEM) images were obtained with a JEOL JEM-3010 instrument operating at an accelerating voltage 300 kV using LaB6 cathode and resolution 0.17 nm. The samples
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A. Zukal et al. / Microporous and Mesoporous Materials 104 (2007) 52–58
Intensity (a.u.)
were ultrasonically dispersed in ethanol and then dropped onto the carbon-coated copper grids prior to the measurements. 2.3. Adsorption measurements
2.4. Non-local density functional theory (NLDFT) method for pore size characterization
(b) Intensity (a.u.)
High-resolution nitrogen (77.4 K) and argon (77.4 K, and 87.3 K) adsorption/desorption measurements on the highly ordered MCM-41 silica were performed with an Autosorb-I-MP instrument (Quantachrome Instruments, Boynton Beach, FL) in the relative pressure range p/p0 from 1 · 10 6 to 1. The analysis station of the volumetric adsorption apparatus was equipped, in addition to the standard pressure transducers in the dosing volume (manifold) of the apparatus, with high precision pressure transducers (Baratron MKS) dedicated to read the pressure in the sample cell itself. Hence, the sample cell was isolated during equilibration that ensured a very small effective dead volume and therefore a highly accurate determination of the adsorbed amount. To provide high accuracy and precision in the determination of p/p0, the saturation pressure p0 was measured throughout the entire analysis by means of a dedicated saturation pressure transducer, which allowed to monitor the vapor pressure for each data point. The samples were outgassed overnight at 423 K prior to the adsorption analysis.
4
6
8 2 theta ( °)
(e)
10
(d)
(c)
(b)
(a) 0
2
4
6
8
10
2 theta (°) Fig. 1. XRD patterns of samples MCM-41-4/0 (a), MCM-41-3/1 (b), MCM-41-2/2 (c), MCM-41-1/3 (d), and MCM-41-0/4 (e). The inset shows enlarged pattern of the sample MCM-41-3/1.
Pore size analysis was performed by applying the NLDFT method dedicated to the systems nitrogen (at 77.4 K) and argon (at 87.3 K) sorption in siliceous pores with cylindrical pore geometry. It has been previously shown that this method gives quantitatively correct pore size distributions in silica materials with cylindrical mesopores [14,15]. 3. Results and discussion X-ray powder diffraction patterns of mesoporous molecular sieves prepared with various amounts of MC8IMCl and CTMABr in the reaction mixture are presented in Fig. 1. It can be clearly seen that the sample MCM-41-3/ 1 features the best ordered mesoporous structure, i.e. the used blend of MC8IMCl with CTMABr represents an optimum supramolecular template. The enlarged X-ray pattern of the sample MCM-41-3/1 (see inset in Fig. 1) exhibits six narrow diffraction lines typical for the hexagonal ordering of the porous framework. The HRTEM images of the sample MCM-41-3/1 are presented in Figs. 2 and 3. The Fig. 2 evidences that the particles contain domains of perfectly ordered hexagonal structure; Fig. 3 shows that the mesopores are oriented from the external surface of the particle towards its center. This was confirmed on a number of different particles investigated and prepared in different synthesis batches,
Fig. 2. HRTEM image of the sample MCM-41-3/1 (a view across the pores).
which show not only very good reproducibility of this synthetic method but also very good ordering of all spherical particles. However, the HRTEM images show only
A. Zukal et al. / Microporous and Mesoporous Materials 104 (2007) 52–58
55
30
a (mmol/g)
20
10
Fig. 3. HRTEM image of the sample MCM-41-3/1 (a view along the pores).
microscopic part of the spherical particle. As shown in the foregoing paragraph, the overall view of the structure ordering of a macroscopic amount of the sample obtained by means of XRD indicates excellent structural uniformity of the material. Nitrogen isotherm measured at 77.4 K and argon isotherm recorded at 87.3 K on sample MCM-41-3/1 are given in Fig. 4. Both isotherms are characterized by the sudden increase in the amount adsorbed in the range of relative pressures p/p0 of 0.35–0.40, which is typical of the volume filling of MCM-41 mesopores. The argon adsorption/ desorption isotherm obtained at 87.3 K shows a small genuine type H1 hysteresis loop, whereas the nitrogen isotherm is as to be expected for this pore size completely reversible. In the region of p/p0 > 0.4 the amount adsorbed depends for both the argon and the nitrogen isotherm only slightly on the relative pressure and the isotherm is fully reversible. This fact clearly indicates that the sample MCM-41-3/1, synthesized with the optimum ratio and amount of MC8IMCl and CTMABr, does not contain any additional mesoporosity. The NLDFT pore size distribution (PSD, by applying the NLDFT equilibrium transition kernel [14]) calculated from the desorption branch of the hysteretic argon isotherm is in an excellent agreement with the NLDFT-PSD calculated from the reversible nitrogen isotherm. Both NLDFT-PSD curves are shown in Fig. 5; they are centered at the pore diameter of 4 nm. The presence of a single maximum reveals that the sample does not contain any microporosity. Note that this fact is also supported by the transformation of the nitrogen isotherm into the as plot (Fig. 6) using the standard reduced nitrogen adsorption isotherm data published in Ref. [16]. The low-pressure part of the as plot exhibits excellent linearity starting from the
0 0.0
0.2
0.4
0.6
0.8
1.0
p/p0 Fig. 4. Nitrogen isotherm on sample MCM-41-3/1 at 77.4 K (s) and argon isotherm on the same sample at 87.3 K (h). Solid symbols denote desorption.
lowest as values and passes through the origin. This behavior is typical for MCM-41 silica and convincingly proves the absence of detectable volume of micropores. Structural parameters of the sample MCM-41-3/1 are summarized in Table 1. In this table the cumulative pore area, SCUM(NLDFT), cumulative pore volume VCUM(NLDFT), and mesopore mode diameter DME(NLDFT) obtained by NLDFT analysis of nitrogen and argon isotherms are listed. Further, the total surface area STOT(as), external surface area SEXT(as), and mesopore volume VME calculated using the back-extrapolation of linear parts of the as plot (Fig. 5) are given as well. The pore wall thickness, b(XRD), was calculated under assumption of the hexagonal pore geometry. In such a case, b(XRD) is equal to the unit cell parameter a, (a = 2d/31/2, where d is the XRD (1 0 0) interplanar spacing), minus the distance between the midpoints of the sides of the hexagonal cross section (for details see Ref. [17]). The inspection of structural parameters of the sample MCM-41-3/1 clearly evidences that they correspond to those of the best quality MCM-41; the pore wall thickness is typical for this mesoporous molecular sieve [17]. Thus, it appears that the whole sample MCM41-3/1 is formed of the MCM-41 porous framework and that there is no amorphous component. Nitrogen adsorption isotherms on samples MCM-41-4/ 0, MCM-41-2/2, MCM-41-1/3, and MCM-41-0/4 are
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A. Zukal et al. / Microporous and Mesoporous Materials 104 (2007) 52–58
25
2.0
a (mmol/g)
1.5
3
dVME/dDME (cm /g/nm)
20
1.0
15
10
0.5 5
0
0.0 0
2
4
6
8
10
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
αS
DME (nm) Fig. 5. NLDFT pore size distribution curves of the sample MCM-41-3/1 calculated from nitrogen (s) and argon (h) isotherms.
Table 1 Structural parameters of sample MCM-41-3/1 Parameters
Values from nitrogen data
Values from argon data
SCUM(NLDFT) (m2/g) VCUM(NLDFT) (cm3/g) DME(NLDFT) (nm) STOT(as) (m2/g) SEXT(as) (m2/g) VME(as) (cm3/g) b(XRD) (nm)
822 0.837 4.1 801 73 0.819 0.90
828 0.801 4.0 – – – –
SCUM: cumulative surface area; VCUM(NLDFT): cumulative NLDFT pore volume; DME(NLDFT): mesopore diameter; STOT(as): as total surface area; SEXT(as): as external surface area; VME(as):as pore volume; b(XRD): pore wall thickness.
provided in Fig. 7. The isotherm on the sample MCM-414/0 that was prepared with pure MC8IMCl is typical for silicas templated with surfactants with a short hydrocarbon chain. The nitrogen isotherms on silicas templated with surfactant blend containing larger amount of CTMABr are characterized by pronounced hysteresis loops in the region of p/p0 > 0.4. The existence of this loop evidences the presence of secondary mesoporosity of a wide pore size distribution between 10 and 50 nm. The mode pore diameter of primary mesopores ranges between 3.8 and 4.1 nm.
Fig. 6. The as plot of the sample MCM-41-3/1.
The quality of these materials is less than optimal, and therefore only BET surface area SBET, volume of primary mesopores VME, volume of secondary mesopores VSEC, and total pore volume VSUM = VME + VSEC are given in Table 2. The volume VME was determined from the as plot, the volume VSEC was calculated using BJH method. SEM investigation of the prepared materials revealed that the sample MCM-41-3/1 is characterized by the regular particle morphology while the particle shape and size of other samples are rather irregular (Fig. 8). The SEM image in Fig. 9 evidences that the sample MCM-41-3/1 is composed of spherical particles with diameter centered between 1.8 and 2.2 lm. This spherical symmetry seems to be in contrast to 2D hexagonal ordering of mesopores. The possible explanation of this apparent contradiction can be proposed with respect to the synthesis protocol applied. The combination of MC8IMCl with planar aromatic head group and CTMABr with bulky three-dimensional head group and a long alkyl chain has enabled us to control the packing of micelles into the optimum cylindrical form. The decrease in pH, which causes the formation of solid particles, is achieved by the hydrolysis of ethyl acetate added to the reaction mixture as the final component. This process can be separated into two-steps. In the first one, the decrease in pH of the initially homogeneous silicate-surfactant solution, which is stable for weeks, induces the selfassembly of surfactant/silicate particles. Due to a low
A. Zukal et al. / Microporous and Mesoporous Materials 104 (2007) 52–58
57
50
a (mmol/g)
40
30
20
Fig. 8. SEM image of the sample MCM-41-0/4.
10
0 0.0
0.2
0.4
0.6
0.8
1.0
p/p0 Fig. 7. Nitrogen isotherms on samples MCM-41-4/0 (s), MCM-41-2/2 ($), MCM-41-1/3 (h) and MCM-41-0/4 (n) at 77.4 K. Except for that on sample MCM-41-4/0, isotherms are shifted by 5 mmol/g each. Solid symbols denote desorption.
Table 2 Structural parameters of samples MCM-41-4/0, MCM-41-2/2, MCM-411/3, and MCM-41-0/4 Sample
SBET (m2/g) VME (cm3/g) VSEC (cm3/g) VSUM (cm3/g)
MCM-41-4/0 714.0 MCM-41-3/1 951.1 MCM-41-2/2 1174.7 MCM-41-1/3 1124.6 MCM-41-0/4 1076.0
0.329 0.819 0.772 0.752 0.731
0 0 0.180 0.466 0.577
0.329 0.819 0.952 1.218 1.308
SBET: BET surface area; VME: mesopore volume; VSEC: volume of secondary mesopores; VSUM: total pore volume.
extent of silica condensation and optimum cylindrical form of the micelles, these ‘‘liquid-like’’ particles are spherical in shape and consist of domains of perfectly ordered hexagonal porous structure. In the second step, the inorganic polymerization of silica leads to the ‘‘frozen’’ particles. The porous structure of samples MCM-41-2/2, MCM41-1/3 and MCM-41-0/4 is characterized by the presence of secondary mesopores (Table 2). This means that these samples contain structural defect holes amid the primary mesopores [18]. The increase in the volume of secondary mesopores with increasing amount of CTMABr in the reaction mixture indicates that this surfactant promotes the formation of secondary mesoporous structure.
Fig. 9. SEM image of the sample MCM-41-3/1.
4. Conclusion A procedure for the synthesis of siliceous mesoporous molecular sieve MCM-41 based on the precipitation of the solid product from a homogeneous water solution of sodium metasilicate and the surfactant blend as a structure-directing agent was developed. The materials prepared by using of the optimized blend of cetyltrimethylammonium bromide and 1-methyl-3-octylimidazolium chloride features the regular spherical particles with diameter of 2 lm. The characterization performed by means of scanning and high-resolution electron microscopy, X-ray powder diffraction, and nitrogen and argon adsorption has shown that the particles of MCM-41 silica with spherical symmetry consist of domains of perfectly ordered hexagonal porous structure. Acknowledgments ˇ . and A.Z. thank the Grant Agency of the Academy J.C of Sciences of the Czech Republic (I4040411), the Grant
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Agency of the Czech Republic (203/05/0197), and European Community (DeSANNS, SES6-CT-2005-020133) for a financial support. The authors also thank Dr. L. Brabec (J. Heyrovsky´ Institute) for recording SEM images. References [1] M. Gru¨n, I. Lauer, K.K. Unger, Adv. Mater. 9 (1997) 254.. [2] W. Sto¨ber, A. Fink, E. Bohn, J. Colloid. Interface Sci. 26 (1968) 62. [3] B. Pauwels, G. Van Tendeloo, C. Thoelen, W. Van Rhijn, P.A. Jacobs, Adv. Mater. 13 (2001) 1317. [4] Q. Huo, J. Feng, F. Schu¨th, G.D. Stucky, Chem. Mater. 9 (1997) 14. [5] D. Zhao, J. Sun, Q. Li, G.D. Stucky, Chem. Mater. 12 (2000) 275. [6] W.J.J. Stevens, M. Mertens, S. Mullens, I. Thijs, G. Van Tendeloo, P. Cool, E.F. Vansant, 93 (2006) 119. [7] K. Yano, Y. Fukushima, J. Mater. Chem. 13 (2003) 2577. [8] K. Yano, Y. Fukushima, J. Mater. Chem. 14 (2004) 1579.
[9] Y. Yamada, K. Yano, Micropor. Mesopor. Mater. 93 (2006) 190. [10] C. Boissie`re, A. van der Lee, A. El Mansouri, A. Larbot, E. Prouzet, Chem. Commun. (1999) 2047. [11] C. Boissie`re, A. Larbot, A. van der Lee, P.J. Kooyman, E. Prouzet, Chem. Mater. 12 (2000) 2902. [12] C.-P. Kao, H.-P. Lin, C.-Y. Mou, J. Phys. Chem. Solids 62 (2001) 1555. [13] K. Zhu, F. Pozˇgan, L. D’Souza, R.M. Richards, Micropor. Mesopor. Mater. 91 (2006) 40. [14] A.V. Neimark, P.I. Ravikovitch, Micropor. Mesopor. Mater. 44 (2001) 697. [15] P.I. Ravikovitch, A.V. Neimark, Colloids Surf. A 187 (2001) 11. [16] M. Jaroniec, M. Kruk, J.P. Olivier, Langmuir 15 (1999) 5410. [17] M. Kruk, M. Jaroniec, Y. Sakamoto, O. Terasaki, R. Ryoo, C.H. Ko, J. Phys. Chem. B 104 (2000) 292. [18] H.-P. Lin, S.-T. Wong, C.-Y. Mou, C.-Y. Tang, J. Phys. Chem. B 104 (2000) 8967.
Synthesis of highly ordered MCM-41 silica with spherical particles ˇ ejka Arnosˇt Zukal a, Matthias Thommes b, Jirˇ´ı C a
a,*
J. Heyrovsky´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejsˇkova 3, CZ-18223 Prague, Czech Republic b Quantachrome Instruments, 1900 Corporate Drive, Boynton Beach, FL 33426, USA Received 16 October 2006; received in revised form 22 December 2006; accepted 3 January 2007 Available online 14 January 2007
Abstract Highly ordered MCM-41 silica was synthesized by using of optimized blend of cetyltrimethylammonium bromide and 1-methyl-3octylimidazolium chloride as a structure-directing agent. The employment of homogeneous precipitation of the solid product from a water solution of sodium metasilicate and the surfactant blend resulted in the formation of very well ordered spherical particles. The material was characterized by scanning as well transmission electron microscopy, X-ray powder diffraction, and nitrogen and argon adsorption using a proper non-local density functional theory approach for calculations of the textural parameters. The results obtained show that spherical particles are composed of domains of perfectly ordered hexagonal porous structure. 2007 Elsevier Inc. All rights reserved. Keywords: Silica; Spherical particles; Homogenous precipitation; Nitrogen adsorption; Argon adsorption
1. Introduction Mesoporous silicas are very frequently used as a catalyst supports or adsorbents. Their physical properties such as surface area, pore distribution and void volume have significant effect on the performance in the respective application. High surface areas and monodisperse pore sizes are essential to obtain the necessary adsorption capacity and molecular selectivity. The materials synthesized by the condensation of silicate species in solution using proper surfactants as the structure-directing agents exhibit pore sizes in the nanometer range, narrow pore size distribution and the surface area as large as 1000 m2/g. On the macroscale level the performance of the mesoporous silica critically depends on the size and shape of their particles. As mesoporous silica spheres uniform in size are expected to be advantageously applicable in processes associated with gas separation or catalysis, a particular attention was paid to the preparation of such materials. The communication [1] describes the first synthesis of *
Corresponding author. Tel.: +420 26605 3795; fax: +420 28658 2307. ˇ ejka). E-mail address: [email protected] (J. C
1387-1811/$ - see front matter 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2007.01.004
spherical silica particles featuring an MCM-41 structure. The synthesis procedure itself was based on a modification of Sto¨ber’s well-known synthesis of monodisperse silica spheres [2], which involves the hydrolysis of tetraalkyl orthosilicate in a mixture of a low-boiling alcohol and aqueous ammonia. This procedure was modified by adding of a cationic surfactant to the reaction mixture, which was operative as a structure-directing agent. The structure and pore size distribution of spherical particles with size ranging from 0.2 to 1 lm have been studied using nitrogen adsorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray powder diffraction (XRD) [3]. SEM images gave a proof of a spherical morphology, which was also confirmed by TEM. Although XRD patterns can be explained in terms of an ill-ordered hexagonal structure, this structure was not confirmed by electron microscopy. TEM provided an evidence for a spherical distribution of the pores. The pores were hexagonally packed only on a local scale; the pore ordering on a larger scale was disturbed. This was also in agreement with the XRD patterns, which showed broader peaks for the higher orders as compared with the first order one.
A. Zukal et al. / Microporous and Mesoporous Materials 104 (2007) 52–58
By means of emulsion biphase chemistry the preparation of hard spheres of mesoporous silicas was achieved [4]. Silica spheres were uniform in size, which could be controlled from 0.1 to 2 mm. Mesoporous silica spheres were prepared in one step in a procedure based on the phase separation resulting from the hydrophobic nature of tetrabutyl orthosilicate, which was used to provide the desired spherical morphology. Two examples of the pore structure within the spheres were revealed by TEM. In some sections ordered arrays of pores similar to those found in MCM-41 were observed, while pores appearing to be monodispersed in size but with random orientation were found in other areas. Micrometer-sized hard spheres of mesoporous SBA-15 were also synthesized under acidic conditions by using TEOS as a silica source. The neutral triblock copolymer Pluronic P123 (EO20PO70EO20) as the structuredirecting agent and cetyltrimethylammonium bromide (CTMABr) as a cosurfactant were applied [5]. The formation of spheres of SBA-16 silica different in size was achieved under acidic conditions using quaternary mixture of triblock copolymer Pluronic F127 (EO106PO70EO106), water, HCl and butanol. The particle size ranged in this case from micrometers to milimeters [6] and it was controlled by the amount of TEOS added. Submicrometer sized silica spheres were synthesized under basic conditions from tetramethyl orthosilicate using different alcohols or polyols as co-solvents [7–9]. The materials prepared are characterized by relatively small pore diameter (2 nm) and low pore volume (0.4–0.5 cm3/g). The mesoporous micrometer-sized spherical MSU-X silica particles were obtained using procedure based on a sodium fluoride aided hydrolysis of tetraethyl orthosilicate (TEOS) dissolved in a dilute solution of non-ionic polyethylene oxide-based surfactants [10,11]. The main idea of this approach lies in the capability to separate the assembly step and the hydrolysis step. The X-ray diffraction pattern exhibited a single narrow peak assignable to the pore center to center correlation length, characteristic of a worm-hole structure of the porous framework of MSU materials. Centimeter sized macrospheres of silica-surfactant nanocomposites were synthesized from TEOS and Pluronic P123 as a template under acidic conditions (pH < 1) [12]. The organic–inorganic macrosphere is crack-free and possesses high elasticity. After calcination eliminating the surfactant, the silica sphere was intact in outer appearance. Porosity data for the calcined silica sphere evidenced the large BET surface area (732 m2/g) and narrow pore size distribution with mode diameter of 5.2 nm. The above described mesoporous silicas with spherical particles proved the influence of spherical symmetry on the pore architecture and ordering. Some materials show the spherical distribution of the pores [3] whereas other spherical silicas consist of different domains characterized by either well ordered or ill-ordered mesoporous structure [4]. Silica prepared by the two-step pathway features a worm-hole mesoporous structure [10,11]. The formation of spherical particles of SBA-16 was also explained by an
53
agglomeration of colloidal particles and a consequent filling of the interparticle voids with amorphous silica [6]. In the present work we report on the employment of an ionic liquid surfactant for the preparation of the spherical silica. As the use of pure 1-methyl-3-alkylimidazolium salts does not provide well ordered mesoporous silicas with regular morphology [13], an optimized mixture of 1-methyl-3-octylimidazolium chloride (MC8IMCl) and cetyltrimethylammonium bromide (CTMABr) has been used as the structure-directing agent. The employment of this mixture of surfactants in the homogeneous precipitation of mesoporous silica opened the new pathway towards reproducible preparation of spherical siliceous particles characterized by perfectly ordered mesoporous structure. 2. Materials and methods 2.1. Synthesis The reaction mixture was prepared in an autoclavable Nalgene bottle at the temperature of 308 K. In the typical synthesis, m g of MC8IMCl and (4–m) g of CTMABr followed by 4 g of solid Na2SiO3 were dissolved in 900 ml of distilled water, resulting in the formation of a clear solution. Then, 5 ml of ethyl acetate was quickly added under stirring, the mixture was homogenized and the stirring was stopped. (All the chemicals were purchased by Aldrich.) After 10 min a precipitate began to form and the mixture was allowed to stand at 308 K for 5 h. After this period the separation of solid particles from the solution by sedimentation occurred; the suspension of the solid product in the mother liquor was kept at 368 K for 48 h in a heating box. During the ageing, organic vapor was allowed to escape through leaks in the cap of the bottle. The resulting solid phase was recovered by filtration, extensively washed out with distilled water and ethanol, and dried at ambient temperature. The templates were removed by calcination at 813 K for 8 h (temperature ramp of 1 K/min). The prepared samples are labeled as MCM-41-m/n, where m and n denote the amounts of MC8IMCl and CTMABr in the reaction mixture, respectively. 2.2. SEM, TEM, and XRD characterization X-ray powder diffraction data were recorded on a Bruker D8 X-ray powder diffractometer equipped with a graphite monochromator and position sensitive detector (Va˚ntec-1) using Cu Ka radiation (at 40 kV and 30 mA) in Bragg–Brentano geometry. The size and shape of synthesized particles were evaluated by scanning electron microscopy images using a JEOL JSM-5500LV instrument. High-resolution transmission electron microscopy (HRTEM) images were obtained with a JEOL JEM-3010 instrument operating at an accelerating voltage 300 kV using LaB6 cathode and resolution 0.17 nm. The samples
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Intensity (a.u.)
were ultrasonically dispersed in ethanol and then dropped onto the carbon-coated copper grids prior to the measurements. 2.3. Adsorption measurements
2.4. Non-local density functional theory (NLDFT) method for pore size characterization
(b) Intensity (a.u.)
High-resolution nitrogen (77.4 K) and argon (77.4 K, and 87.3 K) adsorption/desorption measurements on the highly ordered MCM-41 silica were performed with an Autosorb-I-MP instrument (Quantachrome Instruments, Boynton Beach, FL) in the relative pressure range p/p0 from 1 · 10 6 to 1. The analysis station of the volumetric adsorption apparatus was equipped, in addition to the standard pressure transducers in the dosing volume (manifold) of the apparatus, with high precision pressure transducers (Baratron MKS) dedicated to read the pressure in the sample cell itself. Hence, the sample cell was isolated during equilibration that ensured a very small effective dead volume and therefore a highly accurate determination of the adsorbed amount. To provide high accuracy and precision in the determination of p/p0, the saturation pressure p0 was measured throughout the entire analysis by means of a dedicated saturation pressure transducer, which allowed to monitor the vapor pressure for each data point. The samples were outgassed overnight at 423 K prior to the adsorption analysis.
4
6
8 2 theta ( °)
(e)
10
(d)
(c)
(b)
(a) 0
2
4
6
8
10
2 theta (°) Fig. 1. XRD patterns of samples MCM-41-4/0 (a), MCM-41-3/1 (b), MCM-41-2/2 (c), MCM-41-1/3 (d), and MCM-41-0/4 (e). The inset shows enlarged pattern of the sample MCM-41-3/1.
Pore size analysis was performed by applying the NLDFT method dedicated to the systems nitrogen (at 77.4 K) and argon (at 87.3 K) sorption in siliceous pores with cylindrical pore geometry. It has been previously shown that this method gives quantitatively correct pore size distributions in silica materials with cylindrical mesopores [14,15]. 3. Results and discussion X-ray powder diffraction patterns of mesoporous molecular sieves prepared with various amounts of MC8IMCl and CTMABr in the reaction mixture are presented in Fig. 1. It can be clearly seen that the sample MCM-41-3/ 1 features the best ordered mesoporous structure, i.e. the used blend of MC8IMCl with CTMABr represents an optimum supramolecular template. The enlarged X-ray pattern of the sample MCM-41-3/1 (see inset in Fig. 1) exhibits six narrow diffraction lines typical for the hexagonal ordering of the porous framework. The HRTEM images of the sample MCM-41-3/1 are presented in Figs. 2 and 3. The Fig. 2 evidences that the particles contain domains of perfectly ordered hexagonal structure; Fig. 3 shows that the mesopores are oriented from the external surface of the particle towards its center. This was confirmed on a number of different particles investigated and prepared in different synthesis batches,
Fig. 2. HRTEM image of the sample MCM-41-3/1 (a view across the pores).
which show not only very good reproducibility of this synthetic method but also very good ordering of all spherical particles. However, the HRTEM images show only
A. Zukal et al. / Microporous and Mesoporous Materials 104 (2007) 52–58
55
30
a (mmol/g)
20
10
Fig. 3. HRTEM image of the sample MCM-41-3/1 (a view along the pores).
microscopic part of the spherical particle. As shown in the foregoing paragraph, the overall view of the structure ordering of a macroscopic amount of the sample obtained by means of XRD indicates excellent structural uniformity of the material. Nitrogen isotherm measured at 77.4 K and argon isotherm recorded at 87.3 K on sample MCM-41-3/1 are given in Fig. 4. Both isotherms are characterized by the sudden increase in the amount adsorbed in the range of relative pressures p/p0 of 0.35–0.40, which is typical of the volume filling of MCM-41 mesopores. The argon adsorption/ desorption isotherm obtained at 87.3 K shows a small genuine type H1 hysteresis loop, whereas the nitrogen isotherm is as to be expected for this pore size completely reversible. In the region of p/p0 > 0.4 the amount adsorbed depends for both the argon and the nitrogen isotherm only slightly on the relative pressure and the isotherm is fully reversible. This fact clearly indicates that the sample MCM-41-3/1, synthesized with the optimum ratio and amount of MC8IMCl and CTMABr, does not contain any additional mesoporosity. The NLDFT pore size distribution (PSD, by applying the NLDFT equilibrium transition kernel [14]) calculated from the desorption branch of the hysteretic argon isotherm is in an excellent agreement with the NLDFT-PSD calculated from the reversible nitrogen isotherm. Both NLDFT-PSD curves are shown in Fig. 5; they are centered at the pore diameter of 4 nm. The presence of a single maximum reveals that the sample does not contain any microporosity. Note that this fact is also supported by the transformation of the nitrogen isotherm into the as plot (Fig. 6) using the standard reduced nitrogen adsorption isotherm data published in Ref. [16]. The low-pressure part of the as plot exhibits excellent linearity starting from the
0 0.0
0.2
0.4
0.6
0.8
1.0
p/p0 Fig. 4. Nitrogen isotherm on sample MCM-41-3/1 at 77.4 K (s) and argon isotherm on the same sample at 87.3 K (h). Solid symbols denote desorption.
lowest as values and passes through the origin. This behavior is typical for MCM-41 silica and convincingly proves the absence of detectable volume of micropores. Structural parameters of the sample MCM-41-3/1 are summarized in Table 1. In this table the cumulative pore area, SCUM(NLDFT), cumulative pore volume VCUM(NLDFT), and mesopore mode diameter DME(NLDFT) obtained by NLDFT analysis of nitrogen and argon isotherms are listed. Further, the total surface area STOT(as), external surface area SEXT(as), and mesopore volume VME calculated using the back-extrapolation of linear parts of the as plot (Fig. 5) are given as well. The pore wall thickness, b(XRD), was calculated under assumption of the hexagonal pore geometry. In such a case, b(XRD) is equal to the unit cell parameter a, (a = 2d/31/2, where d is the XRD (1 0 0) interplanar spacing), minus the distance between the midpoints of the sides of the hexagonal cross section (for details see Ref. [17]). The inspection of structural parameters of the sample MCM-41-3/1 clearly evidences that they correspond to those of the best quality MCM-41; the pore wall thickness is typical for this mesoporous molecular sieve [17]. Thus, it appears that the whole sample MCM41-3/1 is formed of the MCM-41 porous framework and that there is no amorphous component. Nitrogen adsorption isotherms on samples MCM-41-4/ 0, MCM-41-2/2, MCM-41-1/3, and MCM-41-0/4 are
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25
2.0
a (mmol/g)
1.5
3
dVME/dDME (cm /g/nm)
20
1.0
15
10
0.5 5
0
0.0 0
2
4
6
8
10
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
αS
DME (nm) Fig. 5. NLDFT pore size distribution curves of the sample MCM-41-3/1 calculated from nitrogen (s) and argon (h) isotherms.
Table 1 Structural parameters of sample MCM-41-3/1 Parameters
Values from nitrogen data
Values from argon data
SCUM(NLDFT) (m2/g) VCUM(NLDFT) (cm3/g) DME(NLDFT) (nm) STOT(as) (m2/g) SEXT(as) (m2/g) VME(as) (cm3/g) b(XRD) (nm)
822 0.837 4.1 801 73 0.819 0.90
828 0.801 4.0 – – – –
SCUM: cumulative surface area; VCUM(NLDFT): cumulative NLDFT pore volume; DME(NLDFT): mesopore diameter; STOT(as): as total surface area; SEXT(as): as external surface area; VME(as):as pore volume; b(XRD): pore wall thickness.
provided in Fig. 7. The isotherm on the sample MCM-414/0 that was prepared with pure MC8IMCl is typical for silicas templated with surfactants with a short hydrocarbon chain. The nitrogen isotherms on silicas templated with surfactant blend containing larger amount of CTMABr are characterized by pronounced hysteresis loops in the region of p/p0 > 0.4. The existence of this loop evidences the presence of secondary mesoporosity of a wide pore size distribution between 10 and 50 nm. The mode pore diameter of primary mesopores ranges between 3.8 and 4.1 nm.
Fig. 6. The as plot of the sample MCM-41-3/1.
The quality of these materials is less than optimal, and therefore only BET surface area SBET, volume of primary mesopores VME, volume of secondary mesopores VSEC, and total pore volume VSUM = VME + VSEC are given in Table 2. The volume VME was determined from the as plot, the volume VSEC was calculated using BJH method. SEM investigation of the prepared materials revealed that the sample MCM-41-3/1 is characterized by the regular particle morphology while the particle shape and size of other samples are rather irregular (Fig. 8). The SEM image in Fig. 9 evidences that the sample MCM-41-3/1 is composed of spherical particles with diameter centered between 1.8 and 2.2 lm. This spherical symmetry seems to be in contrast to 2D hexagonal ordering of mesopores. The possible explanation of this apparent contradiction can be proposed with respect to the synthesis protocol applied. The combination of MC8IMCl with planar aromatic head group and CTMABr with bulky three-dimensional head group and a long alkyl chain has enabled us to control the packing of micelles into the optimum cylindrical form. The decrease in pH, which causes the formation of solid particles, is achieved by the hydrolysis of ethyl acetate added to the reaction mixture as the final component. This process can be separated into two-steps. In the first one, the decrease in pH of the initially homogeneous silicate-surfactant solution, which is stable for weeks, induces the selfassembly of surfactant/silicate particles. Due to a low
A. Zukal et al. / Microporous and Mesoporous Materials 104 (2007) 52–58
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50
a (mmol/g)
40
30
20
Fig. 8. SEM image of the sample MCM-41-0/4.
10
0 0.0
0.2
0.4
0.6
0.8
1.0
p/p0 Fig. 7. Nitrogen isotherms on samples MCM-41-4/0 (s), MCM-41-2/2 ($), MCM-41-1/3 (h) and MCM-41-0/4 (n) at 77.4 K. Except for that on sample MCM-41-4/0, isotherms are shifted by 5 mmol/g each. Solid symbols denote desorption.
Table 2 Structural parameters of samples MCM-41-4/0, MCM-41-2/2, MCM-411/3, and MCM-41-0/4 Sample
SBET (m2/g) VME (cm3/g) VSEC (cm3/g) VSUM (cm3/g)
MCM-41-4/0 714.0 MCM-41-3/1 951.1 MCM-41-2/2 1174.7 MCM-41-1/3 1124.6 MCM-41-0/4 1076.0
0.329 0.819 0.772 0.752 0.731
0 0 0.180 0.466 0.577
0.329 0.819 0.952 1.218 1.308
SBET: BET surface area; VME: mesopore volume; VSEC: volume of secondary mesopores; VSUM: total pore volume.
extent of silica condensation and optimum cylindrical form of the micelles, these ‘‘liquid-like’’ particles are spherical in shape and consist of domains of perfectly ordered hexagonal porous structure. In the second step, the inorganic polymerization of silica leads to the ‘‘frozen’’ particles. The porous structure of samples MCM-41-2/2, MCM41-1/3 and MCM-41-0/4 is characterized by the presence of secondary mesopores (Table 2). This means that these samples contain structural defect holes amid the primary mesopores [18]. The increase in the volume of secondary mesopores with increasing amount of CTMABr in the reaction mixture indicates that this surfactant promotes the formation of secondary mesoporous structure.
Fig. 9. SEM image of the sample MCM-41-3/1.
4. Conclusion A procedure for the synthesis of siliceous mesoporous molecular sieve MCM-41 based on the precipitation of the solid product from a homogeneous water solution of sodium metasilicate and the surfactant blend as a structure-directing agent was developed. The materials prepared by using of the optimized blend of cetyltrimethylammonium bromide and 1-methyl-3-octylimidazolium chloride features the regular spherical particles with diameter of 2 lm. The characterization performed by means of scanning and high-resolution electron microscopy, X-ray powder diffraction, and nitrogen and argon adsorption has shown that the particles of MCM-41 silica with spherical symmetry consist of domains of perfectly ordered hexagonal porous structure. Acknowledgments ˇ . and A.Z. thank the Grant Agency of the Academy J.C of Sciences of the Czech Republic (I4040411), the Grant
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Agency of the Czech Republic (203/05/0197), and European Community (DeSANNS, SES6-CT-2005-020133) for a financial support. The authors also thank Dr. L. Brabec (J. Heyrovsky´ Institute) for recording SEM images. References [1] M. Gru¨n, I. Lauer, K.K. Unger, Adv. Mater. 9 (1997) 254.. [2] W. Sto¨ber, A. Fink, E. Bohn, J. Colloid. Interface Sci. 26 (1968) 62. [3] B. Pauwels, G. Van Tendeloo, C. Thoelen, W. Van Rhijn, P.A. Jacobs, Adv. Mater. 13 (2001) 1317. [4] Q. Huo, J. Feng, F. Schu¨th, G.D. Stucky, Chem. Mater. 9 (1997) 14. [5] D. Zhao, J. Sun, Q. Li, G.D. Stucky, Chem. Mater. 12 (2000) 275. [6] W.J.J. Stevens, M. Mertens, S. Mullens, I. Thijs, G. Van Tendeloo, P. Cool, E.F. Vansant, 93 (2006) 119. [7] K. Yano, Y. Fukushima, J. Mater. Chem. 13 (2003) 2577. [8] K. Yano, Y. Fukushima, J. Mater. Chem. 14 (2004) 1579.
[9] Y. Yamada, K. Yano, Micropor. Mesopor. Mater. 93 (2006) 190. [10] C. Boissie`re, A. van der Lee, A. El Mansouri, A. Larbot, E. Prouzet, Chem. Commun. (1999) 2047. [11] C. Boissie`re, A. Larbot, A. van der Lee, P.J. Kooyman, E. Prouzet, Chem. Mater. 12 (2000) 2902. [12] C.-P. Kao, H.-P. Lin, C.-Y. Mou, J. Phys. Chem. Solids 62 (2001) 1555. [13] K. Zhu, F. Pozˇgan, L. D’Souza, R.M. Richards, Micropor. Mesopor. Mater. 91 (2006) 40. [14] A.V. Neimark, P.I. Ravikovitch, Micropor. Mesopor. Mater. 44 (2001) 697. [15] P.I. Ravikovitch, A.V. Neimark, Colloids Surf. A 187 (2001) 11. [16] M. Jaroniec, M. Kruk, J.P. Olivier, Langmuir 15 (1999) 5410. [17] M. Kruk, M. Jaroniec, Y. Sakamoto, O. Terasaki, R. Ryoo, C.H. Ko, J. Phys. Chem. B 104 (2000) 292. [18] H.-P. Lin, S.-T. Wong, C.-Y. Mou, C.-Y. Tang, J. Phys. Chem. B 104 (2000) 8967.