Article pubs.acs.org/JPCC
Formation and Shrinkage of Necks in Microporous Silica Molecular Sieve with Ordered Mesoporous Structure Kunimitsu Morishige*,† and Yoshiyuki Kondou† †
Department of Chemistry, Okayama University of Science, 1-1 Ridai-cho, Kita-ku, Okayama 700-0005, Japan S Supporting Information *
ABSTRACT: We examined the relationship between the removal of the triblock copolymer template and the formation of necks available for molecular sieving in microporous silica molecular sieve that is composed of spherical large cavities arranged in a face-centered-cubic array and connected through narrow necks of molecular dimensions. For as-synthesized silica molecular sieve, water molecules can diffuse through the interface between the single PEO chains embedded inside the silica walls and the surrounding silica framework, indicating the presence of small gaps around the single PEO chains occluded. When calcination temperature is increased beyond 200 °C, the BET surface area of nitrogen adsorption at 77 K steeply increases, attains a maximum at 290 °C, and decreases rapidly with a further increase of calcination temperature. For a sample calcined at 290 °C, the dimension of the narrow necks connecting the large cavities is estimated to be ∼0.8 nm from adsorptions of probe molecules. A rapid shrinkage of the necks with a further increase of calcination temperature offers a convenient way to finely tune the size of the necks available for molecular sieving in the size range smaller than 0.8 nm.
I. INTRODUCTION A wide variety of ordered mesoporous materials have been synthesized by using nonionic surfactants, especially poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers, as structure-directing agents.1 Among them, a highly ordered large cage-type mesoporous silica FDU-12, where almost spherical cavities are arranged in a face-centered-cubic (fcc) lattice and connected through narrow necks, can be synthesized by using PEO-PPO-PEO triblock copolymer Pluronic F127 as a micellar template and 1,3,5-trimethylbenzene (TMB) as a micelle expander.2 In previous studies, we have shown that FDU-12 silicas prepared without and with the hydrothermal treatment at low temperatures reveal a molecular sieving effect for simple gases such as carbon dioxide and methane depending on the hydrothermal treatment conditions (amorphous silica molecular sieve).3,4 We speculated that the effect arises from the necks of molecular dimensions formed as a result of the templating of the spherical copolymer micelles bridged by the single PEO chains.3 The assembly of the ordered mesoporous silica organized by triblock copolymer species in acid media appears to take place through a (S°H+)(X−I+) pathway, where S° is a nonionic surfactant, X− is a halide anion, and I+ is a protonated Si−OH moiety.5 In the structure of a block copolymer/silica composite, the cores of almost spherical micelles are mainly constituted by hydrophobic blocks, whereas the micelle corona, which consists of hydrophilic PEO blocks, interacts with the silica framework.6−8 A certain fraction of the PEO chains are expected to be tightly embedded inside the silica walls as a consequence of the molecular imprinting of single PEO chains.6,7,9 Since the PEO chains of adjacent micelles may be highly entangled, the © 2012 American Chemical Society
penetration of the PEO chains within the pore walls of the block copolymer/silica composite may provide connectivity between the spherical micellar templates. The removal of the block copolymer by calcination leads to ordered silica with spherical cavities connected through narrow necks of molecular dimensions. The size of the necks is controlled by the interface structure around the single PEO chains occluded within the silica walls, although its structure is still not certain. A similar mechanism has always been thought for the occurrence of complementary pores inherent to mesoporous materials synthesized by templating of copolymers composed of PEO blocks.10−15 However, a formation process of the complementary pores during the removal of the copolymer template has not been examined so far. Conventionally, molecular sieves are made of inorganic zeolites. Because of the ordered arrangement of the atoms and the rigidity of the bonds in such materials, a zeolite molecular sieve is made with a fixed pore size. This is beneficial when the pore size precisely fits the separation needs. However, when the size difference of the two gases is very small, a molecular sieve with the precise pore size is not always readily available. In such cases, pore-adjustable molecular sieves that can always meet the separation needs are highly desirable.16,17 When cagelike silicas are synthesized without the hydrothermal treatment at high temperatures, highly ordered closed-pore silicas for nitrogen adsorption at 77 K are obtained by calcination at relatively low temperatures.18−20 This indicates a rapid shrinkage of necks by calcination. Although at present the interface structure around Received: December 5, 2011 Revised: January 18, 2012 Published: January 20, 2012 3702
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the single PEO chains is not known, the neck size of the silica molecular sieve just after the removal of the PEO chains is thought to be larger than the diameter of a single PEO chain itself. Therefore, the shrinkage of the necks with increasing calcination temperature may lead to the production of the poreadjustable molecular sieve. The present study aims at elucidating the relationship between the removal of the copolymer template and the formation of necks available for molecular sieving in the silica molecular sieve as well as providing a convenient way of finely tuning the neck size of the silica molecular sieve according to target molecules.
II. EXPERIMENTAL SECTION 1. Materials and Characterization. The silica molecular sieve was synthesized by using Pluronic F127 triblock copolymer as a structure-directing agent and TMB as a solubilizing agent.2−4 Typical synthesis procedure was as follows: 3.0 g of F127 and 7.5 g of KCl were dissolved in 180 mL of 2 M HCl, then 6.6 g of TMB was added, and the mixture was stirred at 15 °C for 24 h in a capped container, although the stirring time can be shortened to 1 h without appreciable reduction in quality of a product. Then 12.3 g of tetraethyl orthosilicate (TEOS) was added to the above solution, which was left to stir for another 24 h at 15 °C. The solid product was collected by filtration and dried at room temperature. As-synthesized materials were calcined at various temperatures (a heating rate of 1 K min−1) for 5 h in air to remove the copolymer template. Adsorption isotherms of nitrogen at 77 K were measured volumetrically on a BELSORP-mini II. Thermogravimetry (TG) and difference thermoanalysis (DTA) were carried out under flowing air using a Rigaku Thermo Plus 2 with a heating rate of 2 K min−1 up to 1000 °C. 2. Measurement of Adsorption Isotherms. Prior to the measurements of adsorption isotherms, except for the assynthesized material, the materials were outgassed under vacuum at 473 K for 2 h. Adsorption isotherms of water, ethanol, o-xylene, and TMB were measured volumetrically at 283 K on a homemade semiautomated instrument equipped with a Baratron capacitance manometer (Model 690A) with a full scale of 1000 Torr and an additional gas dosing volume of ∼2600 cm3.21 Adsorption isotherms of carbon dioxide, methane, n-butane, and i-butane were measured volumetrically at 303 K on another type of homemade semiautomated instrument. CO2 (99.995%), CH4 (>99.9%), n-C4H10 (>98%), and i-C4H10 (>98%) were obtained from Sumitomo Seika Chemicals, Inc. C2H5OH (dehydrated, >99.5%) was obtained from Kanto Chemical Co., Inc. o-C8H10 (>98.0%) and TMB (>97.0%) were obtained from Tokyo Chemical Industry, Inc. All these gases and liquids were used as received.
Figure 1. Weight change derivative for the as-synthesized silica molecular sieve.
losses can be attributed primarily to the decomposition and desorption of the polymeric template and, to a smaller extent, to the release of water formed from the condensation of silanols in the silica framework. The pure block copolymer F127 decomposed at temperatures between 160 and 210 °C. The decomposition of the block copolymer confined inside the cagelike pores of silica molecular sieve takes place over a much wider temperature range, suggesting strong interactions of a certain fraction of the polymer chains with the silica pore walls. Most of the copolymer confined inside the cagelike pores of the silica molecular sieve can be desorbed by calcination at 300 °C in air. Large-pore cagelike silicas with necks of molecular dimensions are obtained by the templating of spherical block copolymer micelles at a low temperature.3,4 The copolymer used as a template contains PEO as a hydrophilic block and PPO as a hydrophobic block. In the structure of the block copolymer/silica composite, the cores of spherical micelles are mainly constituted by PPO blocks, whereas the micelle corona, which consists of PEO blocks, interacts with the silica framework.6−8 A certain fraction of the PEO chains are expected to be tightly embedded inside the silica walls as a consequence of the molecular imprinting of single PEO chains.6,7,9 Since the PEO chains of adjacent micelles may be highly entangled, the penetration of the PEO chains within the pore walls of the as-synthesized silica molecular sieve may provide connectivity between the spherical micellar templates. Therefore, formation of necks available for gas adsorption requires the removal of the PEO chains embedded inside the silica walls between neighboring spherical micellar templates. Figure 2 shows the adsorption isotherms of water at 283 K on the as-synthesized sample and a sample calcined at 300 °C. Most of the template polymer can be removed from the pores of the silica molecular sieve by calcination at 300 °C, as described in the above. For the sample calcined at 300 °C, capillary condensation of water inside the large cavities of the silica molecular sieve occurred at a relative pressure of 0.77. Contrary to the expectation, however, the as-synthesized sample adsorbed a large amount of water, too. The amount of water adsorbed on the as-synthesized sample at high relative pressures was almost one-half that on the sample calcined at 300 °C. This indicates that water molecules can diffuse through the interface between single PEO chains embedded inside the silica walls and the surrounding silica framework and also that the copolymer confined inside the spherical cavities of the assynthesized material is spongy. Indeed, it has been reported from modeling of small angle neutron scattering data that the
III. RESULTS AND DISCUSSION A. Formation of Necks. Thermogravimetric (TGA) and differential thermal (DTA) analyses of the as-synthesized silica molecular sieve showed four weight loss steps in the TGA curve, with a total weight loss of 44 wt %, and one endothermic and four exothermic peaks in the DTA curve. Figure 1 shows a weight derivative curve of the as-synthesized sample. A weight loss at 45 °C was accompanied by an endothermic DTA peak due to desorption of water, while weight losses at temperatures between 130 and 400 °C (a total weight loss of 37 wt %) were accompanied by exothermic DTA peaks. The latter weight 3703
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Emmett−Teller method24 to nitrogen adsorption isotherms at 77 K (Figure 1S of the Supporting Information). When calcination temperature was increased beyond 200 °C, SBET steeply increased, attained a maximum around 290 °C, and then decreased rapidly with a further increase of calcination temperature. Such a rapid increase of SBET with an increase of calcination temperature can be certainly related to the removal of the template polymer because a large fraction of the polymer chains are removed from the cagelike pores by calcination at 290 °C. Figure 4 shows the adsorption isotherms of nitrogen at 77 K and o-xylene and TMB at 283 K on the sample calcined at
Figure 2. Adsorption isotherms of water on the as-synthesized and 300 °C-calcined samples at 283 K. Triangles and circles denote the data points for the as-synthesized and 300 °C-calcined samples, respectively.
volume fraction of water in the spherical cores of triblock copolymer micelles is quite high.22 Similarly, the as-synthesized sample adsorbed a large amount of ammonia but did not adsorb carbon dioxide. The kinetic diameters of H2O, NH3, and CO2 are 0.26, 0.29, and 0.33 nm, respectively.23 Therefore, it can be estimated that the space around the single PEO chains occluded within the silica walls is ∼0.3 nm in dimension. In the process of micelle templating, silica condensation takes place around hydrated PEO chains at the conditions of low temperature and low pH.6,7,9 The hydration water molecules may remain around the single PEO chains occluded within the silica walls of the as-synthesized sample, leading to creation of the narrow space around the single PEO chains occluded. Indeed, the weight loss (7 wt %) due to desorption of water in the TGA curve of the as-synthesized sample was nearly identical to the amount (∼4 mmol/g) of water adsorbed at a relative pressure of 0.8 on the same sample outgassed at room temperature. It is expected that the removal of the block copolymer by calcination may lead to ordered silica with spherical cavities connected through narrow necks of molecular dimensions, the size of which is determined by the sum of the dimension of the vacant space around the single PEO chains occluded and the diameter (∼0.5 nm) of the single PEO chains themselves. Figure 3 shows the relationship between the specific surface area (SBET) and calcination temperature of the silica molecular sieve, where SBET was obtained by applying the Brunauer−
Figure 4. Adsorption isotherms of nitrogen at 77 K and o-xylene and TMB at 283 K on the sample calcined at 290 °C. Circles, triangles, and squares denote the data points for nitrogen, o-xylene, and TMB, respectively.
290 °C. Here, the volume adsorbed is expressed in a volume of the corresponding liquid because the saturated volume adsorbed in capillary condensation, when expressed as a volume of liquid, should be the same for all adsorbates on a given porous solid (the Gurvitsch rule).25 The adsorption isotherm of o-xylene on the sample calcined at 290 °C showed a step due to capillary condensation of the vapor; in a volume of liquid, the amount of o-xylene adsorbed at high relative pressures is almost the same as the saturation amount of nitrogen adsorbed at 77 K. On the other hand, the use of TMB as a probe molecule led to an appreciable decrease in the adsorption capacity expressed as a volume of liquid, indicating that a certain fraction of necks are blocked for the access of the molecules to the pore network due to the molecular size exclusion and thus the effective size of the necks is close to the kinetic diameter of TMB. The kinetic diameter of o-C8H10 and TMB are 0.68 and 0.78 nm, respectively.23,26 Therefore, the dimension of the narrow necks just after the removal of the single PEO chain occluded within the silica walls by calcination is estimated to be ∼0.8 nm, being in good agreement with the sum of the dimension (∼0.3 nm) of the vacant space around the single PEO chains occluded and the diameter (∼0.5 nm) of the single PEO chains themselves. The decrease in the surface area was accompanied by the decrease in height of the hysteresis loop due to capillary condensation of nitrogen in the large cavities. Therefore, it is evident that the observed decrease in the surface area is mainly caused by size exclusion of nitrogen molecules due to shrinkage of the necks. B. Shrinkage of Necks. For nitrogen with kinetic diameter of 0.37 nm, the saturated amount of capillary condensate decreased rapidly with a further increase of calcination temperature beyond 300 °C and was negligibly small for the samples calcined at temperatures higher than 400 °C. This
Figure 3. Surface area of the silica molecular sieve as a function of calcination temperature. 3704
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clearly indicates a rapid shrinkage of necks with an increase of calcination temperature, being consistent with the previous study.18−20 Such a rapid shrinkage of the necks offers a convenient way to finely tune the size of the necks available for molecular sieving in the size range smaller than 0.8 nm. Poreadjustable molecular sieves that can always meet separation needs are highly desirable.16,17 The adsorption isotherms of CO2, CH4, n-C4H10, and iso-C4H10 were measured at 303 K on samples calcined at various temperatures higher than 300 °C, where most of the template polymer is removed from the cagelike pores of silica molecular sieve. All isotherms showed the type I curve of the BDDT definition27 of an adsorption isotherm (Figure 2S of the Supporting Information). Figure 5 shows plots of the adsorption capacities for these gases against
Figure 6. Saturated amounts of adsorption for water and ethanol at 283 K as a function of calcination temperature.
material calcined at 600 °C are open to water molecules with the kinetic diameter of 0.26 nm. In the silica molecular sieve, large cavities are arranged in a fcc lattice and connected through narrow necks of molecular dimensions. Unlike crystalline zeolites, the necks in the material are not uniform in size. The molecular sieving effect arises as a result of bond-controlled percolation of gas molecules in the pore network.3 Therefore, the above-mentioned neck size represents a window in the neck size distribution of each sample, where the window is almost determined by the percolation threshold of the pore network.
IV. CONCLUSIONS For as-synthesized silica molecular sieve, water and ammonia molecules can diffuse through the interface between the single PEO chains occluded within the silica walls and the surrounding silica framework, suggesting that hydration water molecules remain around the occluded single PEO chains. After the removal of the occluded single PEO chains by calcination in air at 290 °C, the dimension of the narrow necks connecting the large cavities is estimated to be ∼0.8 nm from adsorptions of probe molecules. The effective neck size of the silica molecular sieve shrinks rapidly with an increase of calcination temperature beyond 300 °C: ∼0.50 nm at 350 °C, ∼0.40 nm at 400 °C, ∼0.35 nm at 450 and 500 °C, and ∼0.30 nm at 550 °C. This offers a convenient way to finely tune the size of the necks available for molecular sieving in the size range smaller than 0.8 nm. Therefore, the effective neck size of the silica molecular sieve can be finely tuned both by changing the hydrothermal treatment conditions of the synthesis3,4 and the calcination temperature of the assynthesized material, leading to the production of poreadjustable molecular sieve that can always meets the separation needs.
Figure 5. Adsorption capacities for CO2, CH4, n-C4H10, and i-C4H10 at 303 K as a function of calcination temperature. In order to improve clarity, the adsorption capacities for CH4 are multiplied by a factor of 5.
the calcinations temperature. Here the adsorption capacity was determined at 760 Torr. The sample calcined at 350 °C exhibited a molecular sieving effect between n-butane and ibutane, while the samples calcined at 450 and 500 °C exhibited a molecular sieving effect between carbon dioxide and methane of smaller molecular sizes. Similarly, the adsorption isotherms of water and ethanol were measured at 283 K. All isotherms of water showed steps due to capillary condensation of the vapor inside the large cavities of the silica molecular sieves, indicating that the sizes of the necks in these samples are still larger than the kinetic diameter of water (Figure 3S of the Supporting Information). On the other hand, capillary condensation of ethanol was observed only for the samples calcined at temperatures lower than 400 °C (Figure 4S of the Supporting Information). Figure 6 shows plots of the saturated amount of water and ethanol adsorbed on the silica molecular sieve against the calcination temperature. The samples calcined at temperatures higher than 450 °C exhibited a molecular sieving effect between water and ethanol. The kinetic diameters of CH4, nC4H10, i-C4H10, and C2H5OH are 0.38, 0.47, 0.53, and 0.45 nm, respectively.23 Based on the kinetic diameters of these probe molecules, we can estimate the size of the necks responsible for the molecular sieving effect in the silica molecular sieves calcined at various temperatures: ∼0.50 nm for a sample calcined at 350 °C, ∼0.40 nm for a sample calcined at 400 °C, ∼0.35 nm for samples calcined at 450 and 500 °C, and ∼0.30 nm for a sample calcined at 550 °C . Water molecules can access the large cavities even after calcination at 600 °C. Therefore, it is evident that a certain fraction of necks in the
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ASSOCIATED CONTENT
S Supporting Information *
Adsorption−desorption isotherms of nitrogen at 77 K, adsorption−desorption isotherms of CH4, CO2, n-butane, and isobutane at 303 K, and adsorption isotherms of water and ethanol at 283 K. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. 3705
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by matching fund subsidy for private universities from MEXT (Ministry of Education, Culture, Sports, Science and Technology).
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REFERENCES
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