Tailoring Adsorption and Framework Properties of Mesoporous

Mar 10, 2010 - Resorcinol (C6H4(OH)2; 98%), formaldehyde (HCHO), and ... In contrast to the previous series, ICS was added 20 min after formaldehyde...
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Tailoring Adsorption and Framework Properties of Mesoporous Polymeric Composites and Carbons by Addition of Organosilanes during Soft-Templating Synthesis Joanna Go´rka and Mietek Jaroniec* Department of Chemistry, Kent State UniVersity, Kent, Ohio 44242 ReceiVed: December 13, 2009; ReVised Manuscript ReceiVed: February 12, 2010

A series of soft-templated mesoporous polymeric composites and carbons was synthesized under acidic conditions by using resorcinol and formaldehyde in the presence of organosilanes such as tetraethyl orthosilicate and tris(3-trimethoxysilylpropyl)isocyanurate. Poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer, Pluronic F127, was employed as a soft template. While the self-assembly of resorcinol, formaldehyde, and block copolymer combined with polymerization and proper thermal treatment affords mesoporous polymers and carbons with high surface area, large pore volume, and well-developed mesoporosity, the addition of organosilanes into this process opens new possibilities for tailoring adsorption, surface, and framework properties of these materials and for expanding their porosity. It is shown that the use of heterocyclic organosilanes in the aforementioned soft-templating synthesis affords heteroatom-doped mesoporous carbons, the properties of which can be further modified by controlled dissolution of silica species that results in creating substantial microporosity in these materials. Introduction The synthesis of ordered mesoporous carbons1 would be difficult to achieve without the discovery of surfactant-templated ordered mesoporous silicas, which have been used as hard templates. The hard-templating method (also known as nanocasting)2–4 begins with the fabrication of a silica nanostructure, which is followed by filling its pores with carbon precursor, carbonization, and dissolution of the silica template. Although nanocasting is very popular and versatile, this method is laborious and expensive compared to the recently reported soft-templating approach,5–8 which is carried out in the presence of block copolymers. In this case a polymeric-type carbon precursor is formed in the ordered mesostructure of a block copolymer, giving a polymer-polymer mesostructured composite, which after controlled thermal treatment is transformed into ordered mesoporous carbon structure. It has been shown that soft templating is a cheap and effective method to introduce inorganic species into the carbon framework either by using commercially available metal or metal oxides nanoparticles9,10 or by generating those directly during carbon self-assembly process. Regardless of the origin of incorporated nanoparticles, the latter are well-dispersed in the carbon matrix.11–13 The combination of this feature with very good structural and adsorption properties of carbon host materials make them promising candidates for the preparation of nanostructured catalysts.14–16 The fact that the addition of foreign species into the mixture of block copolymer and carbon precursor does not disintegrate the self-assembly process provides a great opportunity for the one-pot synthesis of organic-inorganic composites. Previously, the methods such as surface functionalization,17,18 filling of the pores of mesoporous silicas with organic moieties,19,20 or direct synthesis of periodic organosilicas (PMOs)21,22 were used to achieve ordered mesoporous organosilicas. The risk of nonuniform distribution, pore blockage, and also the use of rather * Corresponding [email protected].

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expensive organosilanes somewhat limited the preparation of that type of materials. The soft-templating approach helps to overcome the aforementioned drawbacks. One of the first reports in this area employed tetraethyl orthosilicate (TEOS)23 in the self-assembly of phenol and formaldehyde under basic conditions. Not only ordered polymer-silica and carbon-silica composites were obtained but also mesoporous silicas and carbons due to the dual functionality of these composites. The pore sizes of the final products could be easily tuned by using different block copolymers (L64 or Brij76); however, in this case only disordered phases were obtained.24 Besides carbon-silica materials also titania-containing mesostrucures were synthesized with the use of prehydrolyzed TiCl4 as an inorganic precursor.25 In contrast to silica, titania easily nucleates, leading to TiO2 nanocrystals embedded in the pore walls of ordered mesoporous carbons. The uniform distribution of titania nanocrystals proved the formation of an interpenetrating hybrid framework during co-assembly process. Recently, the same group reported an interesting approach to the synthesis of ordered mesoporous poly(furfuryl alcohol) (PFA)-silica nanocomposites.26 The use of furfuryl alcohol as a carbon precursor requires usage of a special alkoxysilane as a linker between PFA/TEOS and a soft template to improve the assembly of PFA/TEOS in hydrophilic domains of block copolymers. The resulting composites possess high organic content even up to 60 wt %, although the structural and adsorption properties slowly deteriorate with an increasing amount of PFA in the samples. Here we report the organosilane-assisted soft-templating synthesis of mesoporous carbons prepared under acidic conditions. In the first part of this study TEOS was employed in the preparation of these materials to examine how acidic polymerization conditions affect the TEOS incorporation into the carbon framework. Moreover, our goal was to show that the TEOS addition can be used to improve and easily tune microporosity in the final carbons. However, the main goal of this study was to introduce much larger organosilane than TEOS to obtain mesoporous polymers with functional groups. The use

10.1021/jp9117858  2010 American Chemical Society Published on Web 03/10/2010

Mesoporous Polymeric Composites and Carbons of organosilane such as tris(3-trimethoxysilylpropyl)isocyanurate (ICS) afforded mesoporous phenolic resins with high loadings of the ICS groups, which after carbonization became carbon mesostructures doped with Si and N atoms. A broad variety of commercially available organosilanes gives a lot of freedom in the design of mesoporous phenolic resins with various organic groups, which after carbonization can be converted to heteroatom-doped carbon mesostructures with desired properties; e.g., their hydrophilicity can be easily modified by controlled removal of silica from the pore walls.27

J. Phys. Chem. C, Vol. 114, No. 14, 2010 6299 SCHEME 1: Soft-Templating Strategy for the Synthesis of Mesoporous Polymers and Carbons Involving Polymerization of Resorcinol and Formaldehyde in the Presence of Block Copolymer and Organosilane (TEOS or ICS) under Acidic Conditionsa

Experimental Section Chemicals. Poly(ethylene oxide)-poly(propylene oxide)-poly (ethylene oxide) triblock copolymer (EO106PO70EO106; Pluronic F127) was provided by BASF Corp. Resorcinol (C6H4(OH)2; 98%), formaldehyde (HCHO), and tetraethyl orthosilicate (TEOS, 98%) were purchased from Acros Organics. Tris(3trimethoxysilylpropyl)isocyanurate (ICS, 95%) was acquired from Gelest, Inc., HCl (35-38%) from Fischer, and ethanol (95%) from Pharmco. Materials. Mesoporous carbons were synthesized according to a modified recipe of Wang et al.28 Approximately 1.25 g of resorcinol and 1.25 g of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer (Pluronic F127) were dissolved in deionized water and ethanol. The weight ratio of water to ethanol was fixed to be 5.5:10. After the mixture was stirred for about 10-15 min, the reaction mixture was supplied with 1.1 mL of 37% HCl and kept under stirring for an additional 30 min. Subsequently, 1.25 mL of formaldehyde and various amounts of TEOS were added to the synthesis mixture. When the resulting solution turned milky and after an additional 30 min of stirring, the aqueous and solid phases were allowed to separate. The polymer-containing bottom layer obtained after separation was spread on the Petri dish and left under the fume hood overnight. Then the sample was aged at 100 °C for 24 h. Thermal treatment was performed under a nitrogen atmosphere in the tube furnace using a heating rate of 2 °C/min up to 180 °C, keeping the sample at this temperature for 5 h, resuming heating with 2 °C/min up to 400 °C. Further thermal treatment was performed as follows: in the synthesis of porous polymers the samples were kept at 400 °C for 2 h, whereas in the synthesis of carbons the samples were heated from 400 to 850 °C using a heating rate of 5 °C/min and kept at 850 °C for 2 h. A second series of the samples was prepared using ICS instead of TEOS, whereas the rest of the synthesis route remained the same. In contrast to the previous series, ICS was added 20 min after formaldehyde. The resulting materials were labeled M-Sxt, where M indicates the type of material (P ) polymer, C ) carbon, or S ) silica), S refers to silane used (T ) TEOS or I ) ICS), x denotes the weight percentage of silane in the sample, and t indicates the postsynthesis treatment (HF ) silica dissolution in 15 wt % HF or c ) calcination in air at 550 °C for 2 h). Measurements. Nitrogen adsorption isotherms were measured at -196 °C on ASAP 2010 and 2020 volumetric analyzers (Micromeritics, Inc., GA). Prior adsorption measurements all samples were outgassed at 200 °C for at least 2 h. Thermogravimertic analysis was made using a TA Instrument Hi-Res TGA 2950 thermogravimetric analyzer from 30 to 800 °C under air flow with a heating rate of 10 °C/min. Quantitative analysis of nitrogen was made using a LECO CHNS-932 elemental analyzer (St. Joseph, MI).

a Initially, the polymeric mesostructure is formed, which after thermal treatment in flowing nitrogen at 400 °C gives mesoporous phenolic resin with incorporated organosilane (mesoporosity is formed due to the decomposition of block copolymer template) and after subsequent carbonization this resin transforms to mesoporous carbons with doped inorganic species depending on the organosilane used. Additional treatment of this carbon with HF or NaOH solutions removes silica species, which results in the creation of substantial microporosity.

FT-IR spectra were collected using a Bruker Vector 22 FTIR spectrometer in the frequency range of 4000-500 cm-1. Calculations. The BET specific surface area was calculated from nitrogen adsorption isotherms in the relative pressure range of 0.05-0.2.29 The total pore volume30 was estimated from the amount adsorbed at a relative pressure of ∼0.99. The pore size distributions were calculated from nitrogen adsorption isotherms at -196 °C using the improved KJS method31 for cylindrical mesopores with diameters up to 10 nm. Results and Discussion Mesostructured Materials Prepared from Phenolic Resin and TEOS. Our goal was to obtain mesoporous carbon-silica composites and related materials using different carbon precursors and polymerization conditions than those proposed by Liu et al.23 The latter report showed that the self-assembly of phenol-formaldehyde and triblock copolymer in basic media affords carbons with relatively small mesopores (∼3 nm). The addition of the third component (TEOS) to the latter system caused a significant increase in the pore widths up to ∼6 nm. On the other hand, it has been reported that the polymerization of resorcinol and formaldehyde is greatly enhanced under highly acidic conditions,28 leading to the materials with larger mesopores compared to those obtained in basic media (6.3 nm vs 2.9 nm, respectively). Based on that, we had seen the need to investigate the co-assembly of the resorcinol-formaldehyde/ TEOS system under acidic conditions. An illustration of this organosilane-assisted synthesis is shown in Scheme 1. Nitrogen adsorption isotherms and the corresponding pore size distributions (PSDs) for two representative sets of polymer-silica, carbon-silica, and carbon materials prepared with TEOS content of 30 and 60 wt % are shown in Figure 1. All isotherms show type IV with H1-type hysteresis loops, which clearly indicate the uniform and well-developed mesoporous structures. In both cases, the capillary condensation steps for polymer-silica composites are located at 0.7-0.8 p/po,

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Figure 1. Nitrogen adsorption isotherms (A) and the corresponding pore size distributions (B) for mesoporous polymers and carbons prepared using phenolic resin and TEOS. For clarity, the nitrogen adsorption isotherms denoted with triangles were offset by 375 cm3 STP/g and the corresponding pore size distributions by 0.25 cm3/g · nm.

which corresponds to quite large mesopores around 8-10 nm (see Table 1). After carbonization at 850 °C, the total adsorption of the carbon-silica samples is slightly reduced and the capillary condensation steps shifted toward smaller pores due to materials shrinkage at high temperatures. However, the biggest changes are observed for the samples treated with HF to remove silica. While the mesoporous structure is not altered much, there is a significant increase in adsorption at a lower range of relative pressures compared to the carbon-silica composites. Especially for the series with higher TEOS loading, the microporosity evolution can be clearly seen on the PSD curves in the range up to 4 nm. The structural parameters listed in Table 1 suggest excellent adsorption properties of all discussed materials. The high BET surface areas (range from 500 to 700 m2/g) are characteristic for all polymer- and carbon-silica composites. The latter also showed quite high total pore volume (0.62-0.84 cm3/g) with the main contribution coming from the mesopores. Initially, small amounts of micropores in the carbon-silica composites can be easily increased by silica dissolution, which proves that TEOS species were evenly incorporated into a carbon framework during the self-assembly process. Depending on the amount of TEOS added to the synthesis mixture, not only microporosity but also other parameters such as the BET surface area, mesopore volume, and the total volume of pores can be increased. The development of mesoporosity in the resulting carbon samples is particularly interesting. It can be affected by a partial blockage of mesopores and by accumulation of TEOS at the block copolymer and phenolic resin interface; note that an increase of TEOS from 30 to 60 wt % caused an increase of the mesopore width from 6.8 to 9.6 nm (Table 1), whereas the BET surface area and pore volumes increased even more pronouncedly. Remarkably, the C-T60HF carbon exhibited high specific surface area of 1462 m2/g, total pore volume of 1.42 cm3/g, and more than 11 times larger volume of micropores in comparison to that for the corresponding carbon-silica composite. An additional set of samples was synthesized to investigate the effect of a gradually increasing amount of TEOS (from 10 to 70 wt %) on the structural properties of silica-carbon and carbon materials. Nitrogen adsorption isotherms of the carbonsilica composites (Supporting Information, Figure S1) show that the capillary condensation steps shift progressively toward higher relative pressures, indicating an enlargement of mesopores with increasing TEOS content. However, this tendency

Go´rka and Jaroniec

Figure 2. Nitrogen adsorption isotherms (A) and the corresponding pore size distributions (B) for mesoporous polymers prepared from phenolic resin with different ICS loadings.

seems to be suppressed when TEOS loading is higher than 60 wt %. After silica etching, the resulting carbons exhibit much higher adsorption at lower relative pressures, which is especially well pronounced for the C-T50HF, C-T60HF, and C-T70HF carbons (Figure S2, Supporting Information). As can be seen from Table S1, Supporting Information) all changes in physicochemical properties of the materials studied are consistent withtheamountoftheTEOSused.Ingeneral,theresorcinol-formaldehyde-TEOS co-assembly under acidic conditions results in the materials with much larger mesopores (up to 9.6 nm) compared to those of the carbons synthesized from phenol-formaldehyde in the presence of TEOS under basic conditions, where the maximum pore widths achieved were ∼6.7 nm. The small-angle XRD profiles for the entire series of carbon-silica composites (Figure S3, Supporting Information) show some ordering of mesopores when the silica loading do not exceed 40%. For illustration, the mesoporous silicas obtained by burning off the carbon component in the carbon-silica composites with the highest TEOS loadings are shown in Figure S4, Supporting Information. Mesostructured Materials Prepared Using Phenolic Resin and Isocyanurate Organosilane. In contrast to the materials prepared in the presence of TEOS, where it was possible to introduce high loadings of silica into the carbon matrix, the incorporation of tris(3-trimethoxysilyl-propyl)isocyanurate (ICS) is more challenging. This large molecule composed of isocyanurate ring linked through flexible propyl chains with three trimethoxysilyls was used as a bridging group in the synthesis of PMOs.32,33 In general, the introduction of large bridging groups require much more effort to preserve a good structural ordering at higher concentrations of these groups than in the case of using small aliphatic or aromatic bridging groups.34,35 Even up to 90% of ICS can be accommodated into a silica framework; however, the loading exceeding 30% resulted in disordered PMOs.32 On the other hand, the introduction of ICS into phenolic resins can enhance the physicochemical properties of the resulting carbons due to the presence of N and Si atoms in the ICS molecule.27,36–39 It is well-known that N-doped carbons exhibit an improved conductivity, while Si addition increases hydrophilicity of the carbon surface. Taking the above issues into account, the co-assembly of phenolic resins with organosilanes possessing different heteroatoms seems to be a promising strategy for the synthesis of new hybrid nanomaterials of desired framework and surface properties. Nitrogen adsorption isotherms and the corresponding pore size distributions for the polymer-ICS composites are presented in Figure 2. Taking into account the large size of the ICS molecule, we decided to introduce a maximum of 40 wt % ICS to the samples. As can be seen from Figure 2, the observed capillary condensation steps are steep for all ICS-containing

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TABLE 1: Adsorption Parameters for Mesoporous Polymer and Carbon Samples Obtained by Co-assembly of Resorcinol, Formaldehyde, and TEOSa sample

SBET (m2/g)

Vt (cm3/g)

Vmi (cm3/g)

Vme (cm3/g)

wKJS (nm)

SiO2 (%)

P-T30 C-T30 C-T30HF P-T60 C-T60 C-T60HF

651 690 854 607 497 1462

0.70 0.62 0.74 0.84 0.66 1.42

0.12 0.07 0.13 0.11 0.04 0.47

0.54 0.44 0.51 0.72 0.57 0.97

8.2 7.1 6.8 10.5 9.9 9.6

8.9 10.4 2.9 39.2 42.8 6.0

a Notation: SBET, BET specific surface area; Vt, single-point pore volume; Vmi, volume of fine pores (mainly micropores) obtained by integration of PSD up to 4 nm; Vme, volume of mesopores obtained by integration of PSD from 4 to 20 nm; wKJS, mesopore diameter at the maximum of the PSD curve obtained by the improved KJS method;31 SiO2 (%), residue obtained from the TG curve recorded in air at 800 °C.

TABLE 2: Adsorption Parameters for the Polymer and Carbon Samples Obtained in the Presence of ICSa sample

SBET (m2/g)

Vt (cm3/g)

Vmi (cm3/g)

Vme (cm3/g)

wKJS (nm)

N (%)

P-I10 P-I20 P-I30 P-I30HF* P-I40 C-I10 C-I20 C-I30 C-I30HF* C-I40

569 509 519 211 1018 701 553 550 1208 460

0.73 0.64 0.59 0.29 1.17 0.63 0.53 0.50 0.86 0.40

0.12 0.12 0.12 0.05 0.28 0.07 0.06 0.06 0.33 0.07

0.61 0.52 0.46 0.28 0.87 0.44 0.38 0.34 0.42 0.25

10.0 10.2 9.9 7.9 10.6 7.5 8.4 8.0 7.9 8.0

1.3 2.7 3.4 2.0 4.4 0.3 0.4 0.9 1.4 0.8

a Notation: SBET, BET specific surface area; Vt, single-point pore volume; Vmi, volume of fine pores (mainly micropores) obtained by integration of PSD up to 4 nm; Vme, volume of mesopores obtained by integration of PSD from 4 to 20 nm; wKJS, mesopore diameter at the maximum of the PSD curve obtained by the improved KJS method;31 N (%), nitrogen elemental analysis data. Thermogravimetric analysis in air of the samples denoted with asterisk revealed the presence of residual silica of ∼8% and 5% for P-I30HF and C-I30HF, respectively.

samples, reflecting high uniformity of mesopores. As the ICS content increases in these materials, the total amount adsorbed successively drops, reaching a minimum for the P-I30 sample and suddenly increases by doubling it for the P-I40 sample. Another interesting feature of the latter sample is much higher adsorption at low relative pressures compared to the remaining samples, which indicates that, in the case of such high organosilane content and its fast co-condensation with phenolic precursors, the formation of micropores is greatly enhanced. The additional plot (open circles) presented in Figure 2 corresponds to the polymer obtained after silica dissolution (PI30HF) and shows reduced adsorption in the whole range of p/po with a still quite steep capillary condensation step. This indicates the reduction in both micro- and mesopore volumes, which is also visible on the PSD curves, where the maximum for mesopores is shifted toward smaller pore sizes in contrast to the remaining samples. The physicochemical properties of the materials studied are listed in Table 2. The BET surface areas for polymers with ICS loading up to 30% were found to be ∼550 m2/g, as a consequence of having the same micropore volume. However, a very high specific surface area of 1018 m2/g was obtained for the P-I40 sample due to high microporosity, reaching a value of 0.28 cm3/g. The mesopore volumes show a tendency to decrease as the ICS content approaches 30%, while larger ICS loading results in an increased mesopore volume (0.87 cm3/g). As can be seen from the PSD curves (Figure 2), all ICScontaining polymers possess large mesopores, ∼10 nm. In the case of the P-I30HF sample, the silica dissolution lowered all adsorption parameters, implying homogeneous distribution of ICS in the polymer matrix. Shown in Figure 3 are nitrogen adsorption isotherms and the corresponding PSD curves for the carbon-ICS composites. In general, the gradual introduction of ISC groups reduced the amount of nitrogen adsorbed. Also, the observed capillary

condensation steps are shifted to lower p/po values, indicating some shrinkage of the framework due to carbonization. This translates to smaller mesopores, as can be seen in the PSD curves, which show maxima at about ∼8 nm that are roughly 2 nm smaller than the mesopores found in the corresponding polymer-organosilica composites. Besides that, as more ICS is introduced, the capillary condensation steps became less steep, which is also reflected by flattening of the mesopore peaks on the PSD curves. In contrast to the ICS-polymer composite (PI30-HF), where the silica removal resulted in structure deterioration mostly due to a not fully condensed polymer framework, the ICS-carbon composite treated with HF (C-I30-HF) exhibited enhanced adsorption properties, not only in terms of much higher microporosity but also in terms of enhanced mesoporosity. This carbon possessed a very high BET surface area of 1208 m2/g and total pore volume of 0.86 cm3/g with a significant contribution arising from micropores (0.33 cm3/g). For the C-I samples, not treated with HF solution, the micropore volume does not change with increasing amount of the ICS used in the synthesis, but the mesopore volume and the total pore volume are gradually reduced (Table 2). The mesopore widths estimated at the maxima of the PSD curves are in the range from 7.5 to 8.4 nm, showing some framework shrinkage (about 20-25%) in comparison to that of the corresponding polymer samples, which is larger than that for carbons prepared without ICS addition (∼15%). The ICS loading in the samples was monitored by recording thermogravimetric profiles for the polymer-ICS composites in flowing nitrogen. Shown in Figure 4 are the differential thermogravimetric (DTG) plots (the thermogravimetric curves are presented in Figure S5, Supporting Information), where a single decomposition event occurring at ∼460 °C is usually associated with the decomposition of the ICS bridging group. The continuously increasing intensity of the DTG peak proves the ICS incorporation can be precisely controlled. Also, the data

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Go´rka and Jaroniec -CH2 groups in the polymer.40,41 A broad band is observed at 1047 cm-1 with the tiny peak at 1224 cm-1 characteristic of Si-OCH3 and Si-CH2 groups after removal of silica splits, which is usually caused by branching of the carbon atoms close to oxygen.42 Among several relatively small peaks found in the range from 675 to 900 cm -1, the one at 764 cm-1 is present in both samples and can be assigned to C-H out-of-plane bending vibrations. Conclusions

Figure 3. Nitrogen adsorption isotherms (A) and the corresponding pore size distributions (B) for mesoporous carbons obtained in the presence of ICS.

Figure 4. Differential thermogravimetric plots for mesoporous polymer-ICS composites.

The soft-templated mesoporous polymers and carbons were successfully synthesized under acidic conditions in the presence of organosilicas. The main goal was to monitor the mesostructure formation as a function of increasing amount of the organosilica used. Moreover, two different organosilicas were studied to determine how the size of organosilica affects the self-assembly process and mesostructure formation. In the case of small organosilanes such as TEOS, we were able to introduce up to 70 wt % TEOS without significant structure deterioration. Noteworthy, TEOS is mostly embedded in the carbon framework, which was proved by a remarkable increase in microporosity after silica dissolution. An analogous incorporation mechanism took place in the case of the larger organosilanes used such as ICS. The TGA and IR data clearly show the presence of isocyanurate rings in the polymer and carbon frameworks. All samples exhibited high surface area and pore volumes. The well-developed mesopores with diameters ∼8-10 nm ensure a good mass transfer in the polymer and carbon frameworks. This work shows that the soft-templating synthesis of mesoporous polymers and carbons can accommodate high loadings of various organosilanes, such as large isocyanurate organosilane, which creates an effective way for tailoring surface and framework properties of these materials. Acknowledgment. This material is based upon work supported by the National Science Foundation under CHE-0848352. The authors thank BASF Co. for providing the triblock polymer. Supporting Information Available: One table with adsorption and TG data for the carbon-silica, carbon, and polymer samples prepared by co-assembly of resorcinol, formaldehyde, and TEOS, three figures with nitrogen adsorption isotherms and the corresponding PSDs for these samples, and one figure with TG profiles for polymer composites. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 5. FTIR spectra for mesoporous polymer containing 30% ICS before and after silica dissolution.

obtained from elemental analysis are in good agreement with the TG data and show a gradual increase in the amount of nitrogen in the polymer-ICS composites with increasing amount of the ICS used. Even after carbonization at 850 °C all samples still possess nitrogen in the carbon framework. Shown in Figure 5 is the comparison of the IR spectra for the sample with 30% ICS in the polymer matrix before and after HF treatment (P-I30 and P-I30-HF, respectively). Both spectra exhibit a characteristic band at 1694 cm-1, which can be easily assigned to CdO vibrations in isocyanurate rings.40 Taking into account relative intensities, one may say that the isocyanurate rings can be partially removed during the silica dissolution, which was also shown by elemental analysis. The band found at 1605-1610 cm-1 is attributed to -OH groups, while the one at 1454-1462 cm-1 can be assigned to aliphatic

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