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Control of the structural properties of mesoporous polymers synthesized using porous silica materials as templates
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Microporous and Mesoporous Materials 112 (2008) 319–326 www.elsevier.com/locate/micromeso
Control of the structural properties of mesoporous polymers synthesized using porous silica materials as templates Antonio B. Fuertes *, Marta Sevilla, Sonia Alvarez, Teresa Valdes-Solı´s Instituto Nacional del Carbo´n (CSIC) Apartado 73, 33080 Oviedo, Spain Received 21 December 2006; received in revised form 3 October 2007; accepted 4 October 2007 Available online 12 October 2007
Abstract In the present work, the fabrication of high-surface area mesoporous polymers is investigated. A nanocasting technique based on mesoporous silica materials as sacrificial templates is employed to prepare these materials. In the first step, the porosity of the silica is filled with a monomer (divynilbenzene), which is polymerized in situ. Subsequently, the silica framework is selectively removed by an etching agent (NaOH), which allows a porous polymer to be retrieved. The structural characteristics (i.e. morphology and particle size) of the template are retained by the polymeric materials, which exhibit high BET surface areas and a large porosity made up of mesopores. What is more, by selecting the appropriate template, it is possible to fabricate porous polymers with a large variety of structural properties, i.e. particle shape, particle size and pore size. The technique used for polymerization also allows the textural properties to be controlled. Thus, when the polymerization is carried out under vacuum conditions, the synthesized porous polymers exhibit very high BET surface areas (up to 1010 m2 g1) and large pore volumes (up to 1.7 cm3 g1). Such polymeric materials are highly hydrophobic and they can be easily dispersed in organic media (hydrophobic) where they form stable dispersions. Ó 2007 Elsevier Inc. All rights reserved. Keywords: Polymer; Silica; Mesopore; Template; Divynilbenzene
1. Introduction The synthesis of mesoporous materials has recently generated great interest due to their importance for a wide range of applications such as the adsorption/immobilization of large molecules (i.e. enzymes, proteins, drugs, dyes, etc.), as catalytic supports or as materials for energy storage (i.e. supercapacitors, Li-batteries, H2 adsorption, etc.) [1]. Much attention has been paid to the fabrication of mesoporous inorganic materials (i.e. carbon and silica) [2]. On the other hand, only a few works have been focussed on the synthesis of mesoporous materials with an organic framework. These materials are of great importance taking into account the fact that the easy functionalization of their surfaces can provide a pathway for the preparation of adsor-
*
Corresponding author. E-mail address: [email protected] (A.B. Fuertes).
1387-1811/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2007.10.004
bents/supports with functional groups that are useful for specific applications. Moreover, nanoporous materials with entirely organic frameworks are important for use in lyophilic environments where they are more compatible with organic solvents than porous inorganic substances [3]. Indeed, porous polymers have great interest as supports of noble metals employed as catalysts for reactions in organic media [4–8]. These materials are also important as adsorbents in numerous applications such as the recovery of precious metals [9], the separation of biomolecules [10,11] and the adsorption of organic compounds [12]. There are a number of procedures for synthesizing micro-or macroporous polymeric materials. These include the co-polymerization of styrene and divinylbenzene resulting in the formation of a microporous material with pores < 3 nm, polymerization in the presence of porogens, giving rise to materials with a macroporous pore network (sizes > 50 nm) [13] or the use of high internal phase emulsion (HIPE) to create highly porous materials with
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micrometer-sized pores [14,15]. However, synthetic routes for producing porous polymeric materials with a porosity made up almost exclusively of mesopores are scarce. As an example, the nanocasting technique constitutes a good alternative for fabricating mesoporous materials. This technique has been extensively employed for the synthesis of a large variety of porous materials because it allows strict control over the structural properties of the obtained products. Mesoporous silica materials prepared with the aid of surfactants have been widely used as sacrificial templates to obtain a variety of porous materials with well-controlled textural properties such as carbons, metals, inorganic compounds, polymers, etc. [2,16–20]. Much attention has been paid to the nanocasting route as a mean to prepare porous carbons and other inorganic compounds. However, only a few works have been focussed on the synthesis of mesoporous polymers. In a pioneering work, Johnson et al. [21] reported the preparation of ordered mesoporous polymers by using colloidal silica crystals as templates. Kim et al. [22] synthesized highly ordered three-dimensionally interconnected mesoporous polymers by using mesostructured MCM-48 and SBA-15 silica materials as templates. Lee et al. [23] employed a mesocellular silica foam as template to fabricate a porous polymer containing a bimodal porosity made up of two pore systems of a size centred at 3 nm and 17 nm. The same group also reported the fabrication of polymer capsules with a hollow macroporous core and a mesoporous shell structure [24]. Recently, the Zhao’s group reported a novel synthetic strategy to fabricate highly ordered mesoporous polymers, which is based on the organic-organic self assembly of triblock copolymers with soluble phenolic resin precursors [25–27]. The main purpose of the present work is to investigate strategies to control the structural characteristics of porous polymers fabricated by using porous silica materials as sacrificial templates. Special emphasis is placed on the synthesis of porous polymeric materials with large specific surface areas, high pore volumes, a controlled morphology and particle size and a porosity made up almost exclusively of mesopores. Thus, we explore the use as templates of several types of silica materials having different structural properties (i.e. particle size, pore structure, pore size, morphology) and analyze their effect on the physical properties of the templated polymers. Moreover, we have investigated the effect that the polymerization conditions have on the textural properties of the obtained products. 2. Experimental section
ostructured SBA-15 silica was synthesized according to the procedure reported by Zhao et al. [28] whereas the spherical bimodal mesoporous silica was prepared according to the method described by Schulz-Ekloff et al. [29]. 2.2. Synthesis of porous polymers Polydivynilbenzene (PDVB) was deposited inside the porosity of the silica. This was carried out by in situ polymerization as reported by Kim et al. [22]. Briefly, a precursor solution of divinylbenzene (DVB) (Aldrich) with a free radical initiator, 1,1 0 -Azobis(cyclohexane-carbonitrile) (ACC) (Aldrich) (DVB/ACC mol ratio = 12) was added dropwise until the structural pore volume of the silica was completely filled (S-3 silica) or partially filled (up to 75% for S-1 silica and 40% for S-2 silica). Polymerization was performed by heating the impregnated sample under nitrogen or under vacuum (<0.01 mbar) to 80 °C for 24 h. The resulting silica/polymer nanocomposite composite was immersed in a 2 M NaOH solution (water/ethanol = 1/1) at room temperature for 15 h to remove the silica framework. The porous polymer obtained as an insoluble fraction was washed with distilled water and then dried in air at 60 °C. The synthesized polymers were denoted as PX-y where X = N or V depending on the conditions used to polymerize (N = under nitrogen, V = under vacuum), and y = 1, 2 or 3 as a function of the type silica used as template. 2.3. Characterization Low range angle X-ray diffraction (XRD) patterns were obtained on a Siemens D5000 instrument operating at 40 kV and 20 mA, using Cu Ka radiation (k = 0.15406 nm). The morphology of the powders was examined by scanning electron microscopy (SEM, Zeiss DSM 942). Nitrogen adsorption and desorption isotherms were performed at 196 °C in a Micromeritics ASAP 2010 volumetric adsorption system. The BET surface area was deduced from the isotherm analysis in the relative pressure range of 0.04–0.20. The total pore volume was calculated from the amount adsorbed at a relative pressure of 0.99. The PSD was calculated by means of the Kruk–Jaroniec– Sayari method [30]. 3. Results and discussion 3.1. Structural characteristics of the silica materials used as templates
2.1. Silica templates To prepare the porous polymers we used as templates three types of mesoporous silica materials: (i) an ordered mesostructured SBA-15 silica (denoted as S-1), (ii) a bimodal mesoporous silica with a spherical morphology (denoted as S-2) and (iii) a commercial mesoporous silica gel (Aldrich, Cat No. 28,851-9) (denoted as S-3). The mes-
In order to investigate the replication of structural properties from the template to polymer, we selected three representative silica materials having very different structural properties (i.e. morphology, porosity and structure of the pore system). Two of these silica materials were synthesized by using surfactants as structure directing agents, i.e. a well-ordered mesostructured SBA-15 silica (S-1) and a
bimodal mesoporous silica with spherical morphology (S-2). The other template, the S-3 silica, is a common and widely available commercial mesoporous silicagel. The textural characteristics of these materials are listed in Table 1. They exhibit high BET surface areas and large pore volumes. Fig. 1 shows the nitrogen sorption isotherms and the PSDs of these materials. The porosity of these samples consists almost exclusively of mesopores, the S-1 silica exhibiting a porosity made up of very uniform mesopores, which have a size centred at 9.5 nm. Moreover, the porosity of this material (SBA-15) is formed by hexagonally ordered cylindrical nanochannels as evidenced by TEM images (data not shown) and the low-angle range XRD pattern (Fig. 2). The latter exhibits three well-resolved peaks, which can be assigned to the (1 0 0), (1 1 0) and (2 0 0) diffractions of the 2-d hexagonal space group (p6mm). On the other hand, the S-2 silica contains two pore systems
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Intensity (a.u.)
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PV-1
PN-1
(100)
(110)
Table 1 Physical properties of the mesoporous polymers and the silica templates Material
Code
SBET (m2 g1) 690 760 340
Vp (cm3 g1)
Pore size (nm)
1.2 1.9 0.8
S-1 S-2 S-3
Mesoporous Polymers
Polymerization under vacuum (<0.01 mbar) PV-1 1010 1.0 3.3 PV-2 630 0.6 2–20 PV-3 930 1.7 6.5 Polymerization under nitrogen (atmospheric pressure) PN-1 660 0.6 3.4 PN-2 370 0.4 2–30 PN-3 720 1.2 7.7
9.5 2.8, 28 12
28 nm
6
dV/dlog(D), cm3/g
Adsorbed Volume, cm3 STP/g
1000
4
2.5
3.5
4.5
of around 2.8 nm and 28 nm, while the silicagel (S-3) has a poorly structured porosity and, in consequence, a relatively broad PSD centred at 12 nm. These materials hardly contain any micropores as can be deduced by the application of the as-plot method to the adsorption branch of N2 isotherms. Their morphology is illustrated by the SEM images shown in Fig. 3. Indeed, the S-1 silica is formed by platelet-like particles with a diameter of 1 lm (Fig. 3a); the S-2 sample consists of spherical particles with a diameter in the 2–4 lm range (Fig. 3c) and the silicagel (S-3) is made up of irregular-shaped particles (size 5– 10 lm) as shown in Fig. 3e. 3.2. Mesoporous polymers
2
0 1
10
100
Pore size (D), nm
400
S-1
200
S-2 S-3 0 0.0
1.5
12 nm 2.8 nm
800
600
0.5
Fig. 2. XRD patterns at low-range angles for the silica S-1 (SBA-15) and the corresponding polymeric replicas.
1400
1200
S-1
2 Theta (º)
Silica (template)
9.5 nm
(200)
0.2
0.4
0.6
0.8
1.0
Relative pressure (p/po)
Fig. 1. Nitrogen sorption isotherms and pore size distributions (Insets) of the silica materials used as templates.
The templated mesoporous polymeric replicas were obtained after the selective removal of the silica framework in the polymer/silica composites. In this way, a porous polymeric material was obtained. The successful synthesis of polymer (PDVB) was confirmed by means of infrared spectroscopy. Fig. 4 shows the FTIR spectra for the polymeric samples obtained through vacuum polymerization. Bands at around 1600–1650 cm1 and 3050–3090 cm1 are attributed to the vinyl groups, whereas the bands at 1450 cm1 and 2930 cm1 reflect the vibrations of CH2 groups [22,31]. The morphological characteristics of synthesized porous polymers are illustrated by the SEM microphotographs displayed in Fig. 3. A comparison of these images with
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Fig. 3. SEM images of the silica samples used as templates (a: S-1, c: S-2; e: S-3) and their corresponding polymeric replicas synthesized through vacuum polymerization (b: PV-1, d: PV-2, f: PV-3).
Transmitance (a. u.)
PV-1
PV-2
PV-3
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1)
Fig. 4. FTIR spectra of the mesoporous polymeric samples obtained by vacuum polymerization.
those of the silica templates (Fig. 3a, c and e) reveals that the morphology of the silica templates is retained in the porous polymeric materials. In fact, the polymer samples
obtained by using the S-1 silica as template (Fig. 3b) exhibits a platelet-like morphology very similar to that of the silica (Fig. 3a). Similarly, the porous polymers derived from the S-2 and S-3 templates consist of perfectly spherical particles (Fig. 3d) and irregular-shaped particles (Fig. 3f), respectively. We did not detect any significant modification of the size of the mesoporous polymeric particles derived from the S-1 and S-3 silica templates. In contrast, the spherical mesoporous polymeric particles (diameter 1.5– 2.5 lm) derived from the S-2 silica exhibit a certain shrinkage with respect to the corresponding parent silica (diameter 2–4 lm) as can be seen by comparing Fig. 3c and d. This is probably due to the fact that a portion of the large pores observed in the S-2 silica (27 nm, see Fig. 1, inset) partially collapse during the replication process. These results prove that the mesoporous polymeric materials retain the morphology of the silica templates. This suggests that with the appropriate template it might be possible to prepare polymeric materials with a preselected shape. The textural characteristics of the polymeric samples are exemplified by the nitrogen sorption isotherms and PSD shown in Fig. 5. The sorption isotherms contain capillary condensation steps, which reveal the presence of a meso-
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dV/dlog(D), cm3/g
3.3 nm
400 2
Adsorbed volume, (cm3 STP/g)
600
3
dV/dlog(D), cm3/g
Adsorbed volume, (cm3 STP/g)
800
1
0
1
10
100
Pore size (D), nm 400
200
300
0.8
323
8 nm
0.6 0.4 0.2
0.0
1
10
100
Pore size (D), nm
200
100
PV-1
PV-2
PN-1 0 0.0
PN-2 0
0.2
0.4
0.6
0.8
0.0
1.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (p/po)
Relative pressure (p/po) 1200
Adsorbed volume, (cm3 STP/g)
1000
800
600
dV/dlog(D), cm3/g
4
6.5 nm
3 2 1 0
1
10
100
Pore size (D), nm 400
200
PV-3 PN-3
0 0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (p/po)
Fig. 5. Nitrogen sorption isotherms and pore size distributions (insets) of the mesoporous polymeric materials obtained from (a) S-1, (b) S-2 and (c) S-3 silica templates.
porous network. This is confirmed by the PSDs (Fig. 5, insets), which clearly show that the polymer samples derived from the S-1 and S-3 silica templates are made up of uniform mesopores with sizes centred at 3.3 nm and 6.5 nm, respectively. In contrast, the PSD of the porous polymer obtained from the S-2 template has a porosity consisting of mesopores with sizes ranging from 2 nm to 20 nm (Fig. 5b, inset). This broad PSD reflects the presence of two pore systems. One system is formed by the mesopores generated from the dissolution of the silica walls (pores with sizes of around 2 nm), whereas the other pore system corresponds to mesopores (centred at 8–10 nm) derived from the non-filled large pores present in the bimo-
dal S-2 silica (see Fig. 1, inset). The presence of these large mesopores is a consequence of the fact that, during synthesis, in order to replicate the bimodal porosity of the S-2 silica in the porous polymer, we only used 60–70% of the pore volume of the S-2 to be loaded by the polymeric precursor. The mesoporous structure of the templated polymers was also analyzed by means of transmission electron microscopy (TEM). The TEM images displayed in Fig. 6 clearly illustrate that these samples contain a large framework-confined porosity composed of mesopores. Thus, Fig. 6a, b and d show that the porous polymers obtained from the S-1 and S-2 silica materials contain a porosity
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Fig. 6. TEM images showing the microstructure of the mesoporous polymers. (a) PN-1, (b) PV-1 (c) PV-2 and (d) PV-3. The inset in (a) corresponds to S-1 silica and the inset in (b) shows a detail of the porosity of the PV-1 sample.
made up of uniform mesopores. This is coherent with the PSDs shown in Fig. 5a (inset) and c (inset). In contrast, the TEM image shown in Fig. 6c reveals that the polymeric sample derived from the S-2 silica contains, in addition to the narrow mesopores (<3 nm), large pores (up to 20 nm, see arrows in Fig. 6c) randomly distributed throughout the polymeric matrix. This corroborates the conclusions drawn from the analysis of the porosity by nitrogen adsorption (see PSD in Fig. 5b, inset). The above results then show that the porosity of templated mesoporous polymers can be tuned by selecting the appropriate template or by controlling the degree of infiltration of the polymer precursor into the silica pores. An important property of these porous polymeric adsorbents is their lyophilic character, which allows, unlike that of inorganic adsorbents (i.e. active carbons, alumina, zeolites, silica, etc.) their easy dispersion in an organic medium (hydrophobic). This characteristic is illustrated by a simple experiment that consists in adding a certain amount of this polymeric material to a bottle containing a mixture of water and n-hexane. It was observed that the polymer is
entirely located in the organic phase where it is well-dispersed. The photograph shown in Fig. 7 illustrates this experiment. Moreover, we found that the dispersion of the polymer in n-hexane is very stable and no precipitate appearing until after several hours. 3.3. Vacuum polymerization versus polymerization under nitrogen We explored two different routes to polymerize the monomer (DVB) embedded within the porosity of the silica template, i.e. under nitrogen at an atmospheric pressure and under vacuum (<0.01 mbar). The results show that vacuum polymerization constitutes the best option for producing porous polymers with well-developed textural properties. The textural characteristics measured for these materials are listed in Table 1. These data reveal that the polymeric samples obtained under vacuum conditions exhibit larger BET surface areas and a larger pore volume than those synthesized in the presence of nitrogen (at atmospheric pressure). Fig. 5 shows the N2 sorption isotherms
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n-Hexane + PV-3 Water
Fig. 7. Photograph showing the dispersion of a small amount of porous polymeric sample (PV-3) in a mixture of n-hexane and water.
and PSDs for the porous polymers obtained following both polymerization procedures. The PSDs (Fig. 5, insets) indicate that, whichever method is used for polymerization, the size of the mesopores is hardly affected. However, the route used for polymerization does have an effect upon the structural characteristics of the polymeric samples synthesized from a highly ordered silica template S-1 (SBA-15). Thus, the TEM image obtained for the PN-1 sample (Fig. 6a) shows that the structural order of SBA-15 (S-1) (see Fig. 6a, inset) is preserved in the polymer synthesized under nitrogen, having a framework which is an inverse replica of that of the S-1 silica. This is corroborated by a comparison of the XRD spectra obtained at the low-range angles (0.5– 5°) for the S-1 (SBA-15) silica and the templated PN-1 polymer (Fig. 2), which displays a sharp XRD peak at 2h = 1.02°. This result is similar to that observed by Kim et al. [22] for templated polymers derived from SBA-15 silica. In contrast, when polymerization is carried out under vacuum, the resulting polymeric material (PV-1) shows a disordered porous network, exemplified by the TEM images shown in Fig. 6b. However, although the material does not replicate the structural order of SBA-15, it does maintain a certain pore-pore correlation, as suggested by the XRD pattern presented in Fig. 2, where it can be seen that the PV-1 sample contains a broad band (2h 1.1– 1.7°) typical of the materials with a wormhole pore structure [32]. The TEM image presented in Fig. 6b (inset) illustrates this kind of the porous network. 4. Conclusions In summary, we have illustrated a nanocasting route for successfully fabricating a large variety of mesoporous polymeric materials made up of polydivynilbenzene. Such materials exhibit large surface areas (up to 1000 m2 g1), high pore volumes (up to 1.7 cm3 g1) and a porosity made up of uniform mesopores. The structural characteristics
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(i.e. morphology and particle size) of the templates are retained by the polymeric materials. By means of this methodology it is possible to fabricate mesoporous polymeric materials with a perfect spherical morphology and diameters of a narrow range (1.5–2.5 lm). The size of the mesopores in these materials can be easily controlled by selecting the appropriate silica template. Thus, porous polymers containing uniform mesopores of 3.3 nm can be obtained from SBA-15 silica (S-1), whereas materials with uniform mesopores of 6.5 nm can be prepared from a commercial silicagel (S-3). Moreover, porous polymeric adsorbents with two pore systems can be obtained by controlling the amount of polymer precursor allowed to infiltrate into the silica porosity. The technique used to synthesize the polymer is important: vacuum polymerization leads to materials with better textural properties compared to those obtained from samples prepared at atmospheric conditions (under N2). We have also demonstrated that these porous polymers, unlike inorganic adsorbents (e.g. active carbons, alumina, zeolites, silica, etc.), can be easily dispersed in an organic medium (hydrophobic) forming a stable dispersion. Acknowledgment The financial support for this research work provided by the Spanish MCyT (MAT2005-00262) is gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
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Microporous and Mesoporous Materials 112 (2008) 319–326 www.elsevier.com/locate/micromeso
Control of the structural properties of mesoporous polymers synthesized using porous silica materials as templates Antonio B. Fuertes *, Marta Sevilla, Sonia Alvarez, Teresa Valdes-Solı´s Instituto Nacional del Carbo´n (CSIC) Apartado 73, 33080 Oviedo, Spain Received 21 December 2006; received in revised form 3 October 2007; accepted 4 October 2007 Available online 12 October 2007
Abstract In the present work, the fabrication of high-surface area mesoporous polymers is investigated. A nanocasting technique based on mesoporous silica materials as sacrificial templates is employed to prepare these materials. In the first step, the porosity of the silica is filled with a monomer (divynilbenzene), which is polymerized in situ. Subsequently, the silica framework is selectively removed by an etching agent (NaOH), which allows a porous polymer to be retrieved. The structural characteristics (i.e. morphology and particle size) of the template are retained by the polymeric materials, which exhibit high BET surface areas and a large porosity made up of mesopores. What is more, by selecting the appropriate template, it is possible to fabricate porous polymers with a large variety of structural properties, i.e. particle shape, particle size and pore size. The technique used for polymerization also allows the textural properties to be controlled. Thus, when the polymerization is carried out under vacuum conditions, the synthesized porous polymers exhibit very high BET surface areas (up to 1010 m2 g1) and large pore volumes (up to 1.7 cm3 g1). Such polymeric materials are highly hydrophobic and they can be easily dispersed in organic media (hydrophobic) where they form stable dispersions. Ó 2007 Elsevier Inc. All rights reserved. Keywords: Polymer; Silica; Mesopore; Template; Divynilbenzene
1. Introduction The synthesis of mesoporous materials has recently generated great interest due to their importance for a wide range of applications such as the adsorption/immobilization of large molecules (i.e. enzymes, proteins, drugs, dyes, etc.), as catalytic supports or as materials for energy storage (i.e. supercapacitors, Li-batteries, H2 adsorption, etc.) [1]. Much attention has been paid to the fabrication of mesoporous inorganic materials (i.e. carbon and silica) [2]. On the other hand, only a few works have been focussed on the synthesis of mesoporous materials with an organic framework. These materials are of great importance taking into account the fact that the easy functionalization of their surfaces can provide a pathway for the preparation of adsor-
*
Corresponding author. E-mail address: [email protected] (A.B. Fuertes).
1387-1811/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2007.10.004
bents/supports with functional groups that are useful for specific applications. Moreover, nanoporous materials with entirely organic frameworks are important for use in lyophilic environments where they are more compatible with organic solvents than porous inorganic substances [3]. Indeed, porous polymers have great interest as supports of noble metals employed as catalysts for reactions in organic media [4–8]. These materials are also important as adsorbents in numerous applications such as the recovery of precious metals [9], the separation of biomolecules [10,11] and the adsorption of organic compounds [12]. There are a number of procedures for synthesizing micro-or macroporous polymeric materials. These include the co-polymerization of styrene and divinylbenzene resulting in the formation of a microporous material with pores < 3 nm, polymerization in the presence of porogens, giving rise to materials with a macroporous pore network (sizes > 50 nm) [13] or the use of high internal phase emulsion (HIPE) to create highly porous materials with
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micrometer-sized pores [14,15]. However, synthetic routes for producing porous polymeric materials with a porosity made up almost exclusively of mesopores are scarce. As an example, the nanocasting technique constitutes a good alternative for fabricating mesoporous materials. This technique has been extensively employed for the synthesis of a large variety of porous materials because it allows strict control over the structural properties of the obtained products. Mesoporous silica materials prepared with the aid of surfactants have been widely used as sacrificial templates to obtain a variety of porous materials with well-controlled textural properties such as carbons, metals, inorganic compounds, polymers, etc. [2,16–20]. Much attention has been paid to the nanocasting route as a mean to prepare porous carbons and other inorganic compounds. However, only a few works have been focussed on the synthesis of mesoporous polymers. In a pioneering work, Johnson et al. [21] reported the preparation of ordered mesoporous polymers by using colloidal silica crystals as templates. Kim et al. [22] synthesized highly ordered three-dimensionally interconnected mesoporous polymers by using mesostructured MCM-48 and SBA-15 silica materials as templates. Lee et al. [23] employed a mesocellular silica foam as template to fabricate a porous polymer containing a bimodal porosity made up of two pore systems of a size centred at 3 nm and 17 nm. The same group also reported the fabrication of polymer capsules with a hollow macroporous core and a mesoporous shell structure [24]. Recently, the Zhao’s group reported a novel synthetic strategy to fabricate highly ordered mesoporous polymers, which is based on the organic-organic self assembly of triblock copolymers with soluble phenolic resin precursors [25–27]. The main purpose of the present work is to investigate strategies to control the structural characteristics of porous polymers fabricated by using porous silica materials as sacrificial templates. Special emphasis is placed on the synthesis of porous polymeric materials with large specific surface areas, high pore volumes, a controlled morphology and particle size and a porosity made up almost exclusively of mesopores. Thus, we explore the use as templates of several types of silica materials having different structural properties (i.e. particle size, pore structure, pore size, morphology) and analyze their effect on the physical properties of the templated polymers. Moreover, we have investigated the effect that the polymerization conditions have on the textural properties of the obtained products. 2. Experimental section
ostructured SBA-15 silica was synthesized according to the procedure reported by Zhao et al. [28] whereas the spherical bimodal mesoporous silica was prepared according to the method described by Schulz-Ekloff et al. [29]. 2.2. Synthesis of porous polymers Polydivynilbenzene (PDVB) was deposited inside the porosity of the silica. This was carried out by in situ polymerization as reported by Kim et al. [22]. Briefly, a precursor solution of divinylbenzene (DVB) (Aldrich) with a free radical initiator, 1,1 0 -Azobis(cyclohexane-carbonitrile) (ACC) (Aldrich) (DVB/ACC mol ratio = 12) was added dropwise until the structural pore volume of the silica was completely filled (S-3 silica) or partially filled (up to 75% for S-1 silica and 40% for S-2 silica). Polymerization was performed by heating the impregnated sample under nitrogen or under vacuum (<0.01 mbar) to 80 °C for 24 h. The resulting silica/polymer nanocomposite composite was immersed in a 2 M NaOH solution (water/ethanol = 1/1) at room temperature for 15 h to remove the silica framework. The porous polymer obtained as an insoluble fraction was washed with distilled water and then dried in air at 60 °C. The synthesized polymers were denoted as PX-y where X = N or V depending on the conditions used to polymerize (N = under nitrogen, V = under vacuum), and y = 1, 2 or 3 as a function of the type silica used as template. 2.3. Characterization Low range angle X-ray diffraction (XRD) patterns were obtained on a Siemens D5000 instrument operating at 40 kV and 20 mA, using Cu Ka radiation (k = 0.15406 nm). The morphology of the powders was examined by scanning electron microscopy (SEM, Zeiss DSM 942). Nitrogen adsorption and desorption isotherms were performed at 196 °C in a Micromeritics ASAP 2010 volumetric adsorption system. The BET surface area was deduced from the isotherm analysis in the relative pressure range of 0.04–0.20. The total pore volume was calculated from the amount adsorbed at a relative pressure of 0.99. The PSD was calculated by means of the Kruk–Jaroniec– Sayari method [30]. 3. Results and discussion 3.1. Structural characteristics of the silica materials used as templates
2.1. Silica templates To prepare the porous polymers we used as templates three types of mesoporous silica materials: (i) an ordered mesostructured SBA-15 silica (denoted as S-1), (ii) a bimodal mesoporous silica with a spherical morphology (denoted as S-2) and (iii) a commercial mesoporous silica gel (Aldrich, Cat No. 28,851-9) (denoted as S-3). The mes-
In order to investigate the replication of structural properties from the template to polymer, we selected three representative silica materials having very different structural properties (i.e. morphology, porosity and structure of the pore system). Two of these silica materials were synthesized by using surfactants as structure directing agents, i.e. a well-ordered mesostructured SBA-15 silica (S-1) and a
bimodal mesoporous silica with spherical morphology (S-2). The other template, the S-3 silica, is a common and widely available commercial mesoporous silicagel. The textural characteristics of these materials are listed in Table 1. They exhibit high BET surface areas and large pore volumes. Fig. 1 shows the nitrogen sorption isotherms and the PSDs of these materials. The porosity of these samples consists almost exclusively of mesopores, the S-1 silica exhibiting a porosity made up of very uniform mesopores, which have a size centred at 9.5 nm. Moreover, the porosity of this material (SBA-15) is formed by hexagonally ordered cylindrical nanochannels as evidenced by TEM images (data not shown) and the low-angle range XRD pattern (Fig. 2). The latter exhibits three well-resolved peaks, which can be assigned to the (1 0 0), (1 1 0) and (2 0 0) diffractions of the 2-d hexagonal space group (p6mm). On the other hand, the S-2 silica contains two pore systems
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Intensity (a.u.)
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PV-1
PN-1
(100)
(110)
Table 1 Physical properties of the mesoporous polymers and the silica templates Material
Code
SBET (m2 g1) 690 760 340
Vp (cm3 g1)
Pore size (nm)
1.2 1.9 0.8
S-1 S-2 S-3
Mesoporous Polymers
Polymerization under vacuum (<0.01 mbar) PV-1 1010 1.0 3.3 PV-2 630 0.6 2–20 PV-3 930 1.7 6.5 Polymerization under nitrogen (atmospheric pressure) PN-1 660 0.6 3.4 PN-2 370 0.4 2–30 PN-3 720 1.2 7.7
9.5 2.8, 28 12
28 nm
6
dV/dlog(D), cm3/g
Adsorbed Volume, cm3 STP/g
1000
4
2.5
3.5
4.5
of around 2.8 nm and 28 nm, while the silicagel (S-3) has a poorly structured porosity and, in consequence, a relatively broad PSD centred at 12 nm. These materials hardly contain any micropores as can be deduced by the application of the as-plot method to the adsorption branch of N2 isotherms. Their morphology is illustrated by the SEM images shown in Fig. 3. Indeed, the S-1 silica is formed by platelet-like particles with a diameter of 1 lm (Fig. 3a); the S-2 sample consists of spherical particles with a diameter in the 2–4 lm range (Fig. 3c) and the silicagel (S-3) is made up of irregular-shaped particles (size 5– 10 lm) as shown in Fig. 3e. 3.2. Mesoporous polymers
2
0 1
10
100
Pore size (D), nm
400
S-1
200
S-2 S-3 0 0.0
1.5
12 nm 2.8 nm
800
600
0.5
Fig. 2. XRD patterns at low-range angles for the silica S-1 (SBA-15) and the corresponding polymeric replicas.
1400
1200
S-1
2 Theta (º)
Silica (template)
9.5 nm
(200)
0.2
0.4
0.6
0.8
1.0
Relative pressure (p/po)
Fig. 1. Nitrogen sorption isotherms and pore size distributions (Insets) of the silica materials used as templates.
The templated mesoporous polymeric replicas were obtained after the selective removal of the silica framework in the polymer/silica composites. In this way, a porous polymeric material was obtained. The successful synthesis of polymer (PDVB) was confirmed by means of infrared spectroscopy. Fig. 4 shows the FTIR spectra for the polymeric samples obtained through vacuum polymerization. Bands at around 1600–1650 cm1 and 3050–3090 cm1 are attributed to the vinyl groups, whereas the bands at 1450 cm1 and 2930 cm1 reflect the vibrations of CH2 groups [22,31]. The morphological characteristics of synthesized porous polymers are illustrated by the SEM microphotographs displayed in Fig. 3. A comparison of these images with
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Fig. 3. SEM images of the silica samples used as templates (a: S-1, c: S-2; e: S-3) and their corresponding polymeric replicas synthesized through vacuum polymerization (b: PV-1, d: PV-2, f: PV-3).
Transmitance (a. u.)
PV-1
PV-2
PV-3
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1)
Fig. 4. FTIR spectra of the mesoporous polymeric samples obtained by vacuum polymerization.
those of the silica templates (Fig. 3a, c and e) reveals that the morphology of the silica templates is retained in the porous polymeric materials. In fact, the polymer samples
obtained by using the S-1 silica as template (Fig. 3b) exhibits a platelet-like morphology very similar to that of the silica (Fig. 3a). Similarly, the porous polymers derived from the S-2 and S-3 templates consist of perfectly spherical particles (Fig. 3d) and irregular-shaped particles (Fig. 3f), respectively. We did not detect any significant modification of the size of the mesoporous polymeric particles derived from the S-1 and S-3 silica templates. In contrast, the spherical mesoporous polymeric particles (diameter 1.5– 2.5 lm) derived from the S-2 silica exhibit a certain shrinkage with respect to the corresponding parent silica (diameter 2–4 lm) as can be seen by comparing Fig. 3c and d. This is probably due to the fact that a portion of the large pores observed in the S-2 silica (27 nm, see Fig. 1, inset) partially collapse during the replication process. These results prove that the mesoporous polymeric materials retain the morphology of the silica templates. This suggests that with the appropriate template it might be possible to prepare polymeric materials with a preselected shape. The textural characteristics of the polymeric samples are exemplified by the nitrogen sorption isotherms and PSD shown in Fig. 5. The sorption isotherms contain capillary condensation steps, which reveal the presence of a meso-
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dV/dlog(D), cm3/g
3.3 nm
400 2
Adsorbed volume, (cm3 STP/g)
600
3
dV/dlog(D), cm3/g
Adsorbed volume, (cm3 STP/g)
800
1
0
1
10
100
Pore size (D), nm 400
200
300
0.8
323
8 nm
0.6 0.4 0.2
0.0
1
10
100
Pore size (D), nm
200
100
PV-1
PV-2
PN-1 0 0.0
PN-2 0
0.2
0.4
0.6
0.8
0.0
1.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (p/po)
Relative pressure (p/po) 1200
Adsorbed volume, (cm3 STP/g)
1000
800
600
dV/dlog(D), cm3/g
4
6.5 nm
3 2 1 0
1
10
100
Pore size (D), nm 400
200
PV-3 PN-3
0 0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (p/po)
Fig. 5. Nitrogen sorption isotherms and pore size distributions (insets) of the mesoporous polymeric materials obtained from (a) S-1, (b) S-2 and (c) S-3 silica templates.
porous network. This is confirmed by the PSDs (Fig. 5, insets), which clearly show that the polymer samples derived from the S-1 and S-3 silica templates are made up of uniform mesopores with sizes centred at 3.3 nm and 6.5 nm, respectively. In contrast, the PSD of the porous polymer obtained from the S-2 template has a porosity consisting of mesopores with sizes ranging from 2 nm to 20 nm (Fig. 5b, inset). This broad PSD reflects the presence of two pore systems. One system is formed by the mesopores generated from the dissolution of the silica walls (pores with sizes of around 2 nm), whereas the other pore system corresponds to mesopores (centred at 8–10 nm) derived from the non-filled large pores present in the bimo-
dal S-2 silica (see Fig. 1, inset). The presence of these large mesopores is a consequence of the fact that, during synthesis, in order to replicate the bimodal porosity of the S-2 silica in the porous polymer, we only used 60–70% of the pore volume of the S-2 to be loaded by the polymeric precursor. The mesoporous structure of the templated polymers was also analyzed by means of transmission electron microscopy (TEM). The TEM images displayed in Fig. 6 clearly illustrate that these samples contain a large framework-confined porosity composed of mesopores. Thus, Fig. 6a, b and d show that the porous polymers obtained from the S-1 and S-2 silica materials contain a porosity
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Fig. 6. TEM images showing the microstructure of the mesoporous polymers. (a) PN-1, (b) PV-1 (c) PV-2 and (d) PV-3. The inset in (a) corresponds to S-1 silica and the inset in (b) shows a detail of the porosity of the PV-1 sample.
made up of uniform mesopores. This is coherent with the PSDs shown in Fig. 5a (inset) and c (inset). In contrast, the TEM image shown in Fig. 6c reveals that the polymeric sample derived from the S-2 silica contains, in addition to the narrow mesopores (<3 nm), large pores (up to 20 nm, see arrows in Fig. 6c) randomly distributed throughout the polymeric matrix. This corroborates the conclusions drawn from the analysis of the porosity by nitrogen adsorption (see PSD in Fig. 5b, inset). The above results then show that the porosity of templated mesoporous polymers can be tuned by selecting the appropriate template or by controlling the degree of infiltration of the polymer precursor into the silica pores. An important property of these porous polymeric adsorbents is their lyophilic character, which allows, unlike that of inorganic adsorbents (i.e. active carbons, alumina, zeolites, silica, etc.) their easy dispersion in an organic medium (hydrophobic). This characteristic is illustrated by a simple experiment that consists in adding a certain amount of this polymeric material to a bottle containing a mixture of water and n-hexane. It was observed that the polymer is
entirely located in the organic phase where it is well-dispersed. The photograph shown in Fig. 7 illustrates this experiment. Moreover, we found that the dispersion of the polymer in n-hexane is very stable and no precipitate appearing until after several hours. 3.3. Vacuum polymerization versus polymerization under nitrogen We explored two different routes to polymerize the monomer (DVB) embedded within the porosity of the silica template, i.e. under nitrogen at an atmospheric pressure and under vacuum (<0.01 mbar). The results show that vacuum polymerization constitutes the best option for producing porous polymers with well-developed textural properties. The textural characteristics measured for these materials are listed in Table 1. These data reveal that the polymeric samples obtained under vacuum conditions exhibit larger BET surface areas and a larger pore volume than those synthesized in the presence of nitrogen (at atmospheric pressure). Fig. 5 shows the N2 sorption isotherms
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n-Hexane + PV-3 Water
Fig. 7. Photograph showing the dispersion of a small amount of porous polymeric sample (PV-3) in a mixture of n-hexane and water.
and PSDs for the porous polymers obtained following both polymerization procedures. The PSDs (Fig. 5, insets) indicate that, whichever method is used for polymerization, the size of the mesopores is hardly affected. However, the route used for polymerization does have an effect upon the structural characteristics of the polymeric samples synthesized from a highly ordered silica template S-1 (SBA-15). Thus, the TEM image obtained for the PN-1 sample (Fig. 6a) shows that the structural order of SBA-15 (S-1) (see Fig. 6a, inset) is preserved in the polymer synthesized under nitrogen, having a framework which is an inverse replica of that of the S-1 silica. This is corroborated by a comparison of the XRD spectra obtained at the low-range angles (0.5– 5°) for the S-1 (SBA-15) silica and the templated PN-1 polymer (Fig. 2), which displays a sharp XRD peak at 2h = 1.02°. This result is similar to that observed by Kim et al. [22] for templated polymers derived from SBA-15 silica. In contrast, when polymerization is carried out under vacuum, the resulting polymeric material (PV-1) shows a disordered porous network, exemplified by the TEM images shown in Fig. 6b. However, although the material does not replicate the structural order of SBA-15, it does maintain a certain pore-pore correlation, as suggested by the XRD pattern presented in Fig. 2, where it can be seen that the PV-1 sample contains a broad band (2h 1.1– 1.7°) typical of the materials with a wormhole pore structure [32]. The TEM image presented in Fig. 6b (inset) illustrates this kind of the porous network. 4. Conclusions In summary, we have illustrated a nanocasting route for successfully fabricating a large variety of mesoporous polymeric materials made up of polydivynilbenzene. Such materials exhibit large surface areas (up to 1000 m2 g1), high pore volumes (up to 1.7 cm3 g1) and a porosity made up of uniform mesopores. The structural characteristics
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(i.e. morphology and particle size) of the templates are retained by the polymeric materials. By means of this methodology it is possible to fabricate mesoporous polymeric materials with a perfect spherical morphology and diameters of a narrow range (1.5–2.5 lm). The size of the mesopores in these materials can be easily controlled by selecting the appropriate silica template. Thus, porous polymers containing uniform mesopores of 3.3 nm can be obtained from SBA-15 silica (S-1), whereas materials with uniform mesopores of 6.5 nm can be prepared from a commercial silicagel (S-3). Moreover, porous polymeric adsorbents with two pore systems can be obtained by controlling the amount of polymer precursor allowed to infiltrate into the silica porosity. The technique used to synthesize the polymer is important: vacuum polymerization leads to materials with better textural properties compared to those obtained from samples prepared at atmospheric conditions (under N2). We have also demonstrated that these porous polymers, unlike inorganic adsorbents (e.g. active carbons, alumina, zeolites, silica, etc.), can be easily dispersed in an organic medium (hydrophobic) forming a stable dispersion. Acknowledgment The financial support for this research work provided by the Spanish MCyT (MAT2005-00262) is gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
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