Fabrication and Characterization of Mesostructured Silica, HUM-1

High temperature treatment of ordered mesoporous carbons prepared by using various carbon precursors and ordered mesoporous silica templates. Kamil P...
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Fabrication and Characterization of Mesostructured Silica, HUM-1, and Its Ordered Mesoporous Carbon Replica Suk Bon Yoon,† Jeong Yeon Kim,† Jong-Sung Yu,*,† Kamil P. Gierszal,‡ and Mietek Jaroniec*,‡ Department of Chemistry, Hannam University, Daejeon 306-791, Korea, and Department of Chemistry, Kent State University, Kent, Ohio 44242

The consecutive replications initiated with MCM-48 silica were carried out to synthesize ordered mesostructures of carbon and silica as evidenced by powder X-ray diffraction and transmission electron microscopy. However, ordered mesoporous carbon obtained via replication of the MCM48 silica exhibited different structural symmetry than that of MCM-48, and its replication afforded a new silica mesostructure, HUM-1. The subsequent nanocasting of HUM-1 led to a new second-generation carbon replica. The pore size analysis performed on the basis of nitrogen adsorption isotherms indicates that the HUM-1 silica possesses larger mesopores in comparison to those present in the initial MCM-48 silica, which is probably due to the structural change occurring during the first replication of MCM-48. The pore size distributions are similar for both mesoporous carbons and show bimodal shape. 1. Introduction Carbon possesses an excellent chemical, mechanical, and thermal stability and is a very interesting material for a variety of applications including catalysis, electrode applications, adsorption, and host-guest reactions.1-5 Recently, remarkable progress has been made in the synthesis of carbons with ordered mesoporous structures in the range of micropores (50 nm) using inorganic templates such as zeolites,6-8 mesoporous molecular sieves,9-24 and silica gels.25-34 In particular, a new class of ordered mesoporous carbons (OMCs) that has drawn great interest has been reported by replication using both cubic9-14 and hexagonal15-22 silica mesostructures as templates. In these works, a carbon precursor was incorporated into ordered mesopores of the silica framework to generate a mesoporous carbon after dissolution of the silica template. It is interesting to note that pore size changes were usually observed between the silica template and the carbon product. This is due to the morphological alterations that occurred during the template replication process, where the pores and walls of the host were transformed to the walls and pores, respectively, of the resulting carbon network. Thus, the host scaffold materials are required to have interconnected pore systems, which allow for structural integrity of the templated carbon product after removal of the host. This is the case for the OMC materials synthesized by using SBA-15 as the template.15-22 SBA-15 is a hexagonally ordered mesoporous silica (OMS) showing irregular interconnecting microporosity.35,36 However, the MCM-48 silica, which has cubic space-group symmetry of Ia3d, possesses two non-interconnecting enantiomeric channel systems, which are three-dimensionally intertwined.37,38 Interestingly, the template replication of the MCM-48 silica host resulted in morphological * To whom correspondence should be addressed. E-mail: [email protected] (M.J.) and [email protected] (J.-S.Y.). † Hannam University. ‡ Kent State University.

alterations between the silica host and the resulting carbon. Therefore, the resulting OMC was not an inverse replica of the MCM-48 silica host but rather underwent a phase transition to a new cubic phase.9-12 The subsequent replication (second generation), which involves the use of OMC obtained by replication of the initially selected OMS, provides an excellent opportunity for the synthesis of novel inorganic mesoporous materials, especially when the carbon has a new framework structure. The feasibility of this approach was already demonstrated by recovering an ordered mesostructure of SBA-15 via replication of the OMC (CMK3), which was synthesized from SBA-15 used as a template.39,40 Note that CMK-3 is an exact inverse replica of the SBA-15 structure15 because the latter consists of hexagonally ordered cylindrical mesopores interconnected by complimentary, randomly distributed, micropores.35,36 Thus, the regenerated silica was essentially the starting hexagonally ordered SBA-15 itself, indicating reversibility of the replication process in the case of three-dimensional interconnected mesostructures. Recently, a silica material called HUM-1 (Hannam University Mesostructure-1) was reported by templated replication of the OMC product, which was obtained from the MCM-48 silica used as a host.41 Interestingly, HUM-1 was a highly ordered mesoporous silica but was distinctly different from the MCM-48 parent silica.41 HUM-1 is particularly interesting because this new mesoporous silica was prepared by two consecutive replications and it may not be possible to make this material using the conventional surfactant assembly methods currently employed for the synthesis of OMS. Then, the new mesoporous HUM-1 silica also can provide another incentive for subsequent replication to obtain a new OMC templated by using HUM-1. Thus, this new carbon can be treated as the third replication product of MCM-48 because it was obtained via three subsequent replications. The first one gave the C-MCM48 carbon from MCM-48, the second one provided a new silica HUM-1 from C-MCM-48, and the 3rd one, studied in this work, is a new carbon denoted as C-HUM-1

10.1021/ie048946v CCC: $30.25 © 2005 American Chemical Society Published on Web 05/05/2005

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obtained via replication of HUM-1. The main objective of this work is to study the synthesis process of the HUM-1 OMS and the corresponding OMC, C-HUM-1. Also, this work presents a detailed characterization of these materials by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), and nitrogen adsorption. 2. Experimental Section 2.1. Materials. The materials colloidal silica Ludox HS40 (Aldrich), hexadecyltrimethylammonium bromide (HTMABr; Aldrich), poly(oxyethylene) (4) lauryl ether (Brij 30; Aldrich), sodium hydroxide (93%), acetic acid (98%, Aldrich), hydrochloric acid (35%), ethanol (EtOH; 99.9%), divinylbenzene (DVB; 80%, Aldrich), azobis(isobutyronitrile) (AIBN; Aldrich), and tetraethyl orthosilicate (TEOS; 98%, Aldrich) were received and used without further purification. 2.2. Synthesis of MCM-48 Silica Templates. Mesoporous silica MCM-48 was prepared using HTMABr (C16H33N(CH3)3Br) and Brij 30 (C12H25O(C2H4O)4H) as surfactants and colloidal silica Ludox HS40 as a silica source. The molar ratio of the reaction mixture was 5.0: 0.85:0.15:1.25:400 SiO2/HTMABr/C12H25O(C2H4O)4H/ Na2O/H2O. This gelatinous mixture of the silica source and the surfactant solution was stirred for 30 min at room temperature and heated under static conditions for 48 h at 373 K. The reaction mixture was cooled to room temperature and subsequently adjusted to pH ) 10 with 30 wt % acetic acid. The pH-adjusted mixture was reheated to 373 K and maintained under static conditions for 48 h at 373 K. The precipitated product was filtered, washed with deionized water, and dried at room temperature to have as-synthesized MCM-48. The as-synthesized MCM-48 silica sample was washed with a 1.0 M HCl/EtOH solution to extract out surfactant molecules in the main channel. Thermogravimetric analysis indicated that about 90% of the surfactant had been removed after such an extraction process. The surfactant-extracted template was denoted as MCM48e. The MCM-48e sample was then further calcined in air at 823 K for about 7 h to remove the remaining surfactant. The calcined templates were denoted as MCM-48c. 2.3. Synthesis of OMC via Replication of MCM48. Each of the silica templates MCM-48e or MCM-48c was transferred to a reaction flask in a drybox and dried under vacuum at 373 K overnight prior to introduction of the carbon precursor. A carbon precursor solution of 80% DVB and the free radical initiator AIBN at a DVB/ AIBN molar ratio of about 25:1 was incorporated into the mesopores of the silica template. Excess precursor and moisture were removed by several evacuations at room temperature. In situ polymerization was performed by heating at 343 K overnight. The resulting polymer was heavily cross-linked in the template pores. The template/polymer composites were then heated under nitrogen (or argon) flow at a heating rate of 2 K/min to 1173 K and held under these conditions for 7 h to carbonize the polymer. Finally, the resulting template-free carbons C-MCM-48e and C-MCM-48c were obtained by dissolution of the corresponding silica frameworks, MCM-48e and MCM-48c in a 48% aqueous HF solution, followed by filtering, washing with excess water, and then drying at 373 K overnight. 2.4. Synthesis of HUM-1 Silica and C-HUM-1 Carbon. After drying of C-MCM-48e and C-MCM-48c

overnight at 373 K under vacuum, the pores of the carbon templates were infiltraed with a silica precursor (as-received 98% TEOS) using 1.3-1.5 times larger volume of TEOS in relation to the pore volume of the carbon template; the pore volumes of C-MCM-48e and C-MCM-48c were about 0.9 and 1.1 cm3/g, respectively. Following the removal of the excess TEOS by multiple evacuations, the carbon/silica composite was exposed to HCl vapor by placing it in a sealed chamber containing 1.0 M aqueous HCl and maintaining the temperature at 373 K for 24 h in order to induce hydrolysis and condensation of the silicate precursor. Finally, the carbon/silica composites were heated at 823 K for 6 h under oxygen to remove the carbon frameworks. The resulting white powders were retrieved to yield HUM-1 silicas. To distinguish the HUM-1 products obtained from C-MCM-48e and C-MCM-48c hosts, the resulting OMSs were denoted as HUM-1e and HUM-1c. The second-generation carbon replicas, C-HUM-1 (which is the product obtained via third replication of MCM-48), were synthesized using the same replication method as the one for first-generation carbon replicas, C-MCM-48 (C-MCM-48e and C-MCM-48c), except employing the HUM-1 silicas as templates instead of the MCM-48 silicas. Two carbon samples were obtained by replication of HUM-1e and HUM-1c, and they are denoted as C-HUM-1e and C-HUM-1c, respectively. About 80% of the pore volume of the C-MCM-48 carbon template was filled with TEOS to give HUM-1 silica. The degree of filling of the HUM-1 mesopores with carbon precursor to obtain C-HUM-1 was even higher, about ∼90%. This also indicates a high efficiency of the replication process. 2.5. Adsorption, XRD, and TEM Measurements. Nitrogen adsorption and desorption isotherms were measured at 77 K using a Micromeritics ASAP 2010 system. Prior to adsorption measurements, each sample was degassed under vacuum at 473 K for 2 h. The specific surface area for the samples studied was determined from nitrogen adsorption isotherms in the relative pressure range from 0.05 to 0.2 using the Brunauer-Emmett-Teller (BET) equation. The total pore volume was estimated from the amount of nitrogen adsorbed at a relative pressure of about 0.99. The pore size distribution (PSD) for each sample was determined from the adsorption branch of the nitrogen isotherm using the Barrett-Joyner-Halenda (BJH) algorithm,42 which was substantially improved and calibrated on the basis of nitrogen adsorption isotherms measured for a series of well-defined MCM-41 materials.43,44 The main goal of this modification was (i) to determine experimentally the relationship between the capillary condensation pressure and the pore width for the MCM41 samples of different pore widths (the Kelvin-type relation), (ii) to establish an accurate statistical film thickness as a function of the relative pressure (t curve) for the silica surface on the basis of adsorption isotherms measured for selected large-pore MCM-41 samples, and (iii) to use both of these relationships in the original BJH algorithm. The aforementioned BJH modification was reported in 1997 by Kruk et al.,43 and it is known as the Kruk-Jaroniec-Sayari (KJS) method. This method was used for the pore size analysis of the OMS samples. The pore size analysis of the OMC samples was also done by the KJS method, but in this case, the t curve for the BP280 reference carbon black45 was used instead of that for the silica surface. A great advantage

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Figure 1. XRD patterns for the MCM-48e silica host, the corresponding mesoporous carbon, C-MCM-48e, the HUM-1e silica obtained via replication of C-MCM-48e, and the corresponding C-HUM-1e carbon.

of the BJH and KJS methods is the easiness of incorporation of the surface properties of a solid into the pore size analysis algorithm through the use of proper t curve. Note that the KJS method is applicable for the cylindrical geometry of mesopores. However, the samples studied were MCM-48 with a cubic structure and its analogues obtained via subsequent replications. Thus, their pore size analysis needs to be treated as an approximate one because of the assumption of cylindrical geometry to the samples of cubic structure. It is a common practice in the characterization of materials to use the method developed for cylindrical pores to analyze porous systems of geometries different from the cylindrical one. XRD measurements for OMSs and OMCs were obtained with a Rigaku diffractometer with Cu KR radiation at a scan rate of 4°/min. The X-ray source was operated at 40 kV and 30 mA. Powder samples were mounted on glass slides. The microscopic features of the samples studied were observed with a transmission electron microscope (TEM; EM 912 Omega) operated at 200 kV. 3. Results and Discussion Figure 1 shows the XRD patterns for the surfactantextracted silica host template, MCM-48e, and its replica products obtained by successive replications: the mesoporous carbon, C-MCM-48e, its silica replica, HUM1e, and the second-generation carbon, C-HUM-1e. An intense (211) signal at 2θ ) 2.21° is visible on the XRD pattern (Figure 1) for the MCM-48e silica, whereas two intense signals (having almost equal signal intensities) are present on the pattern at 2θ ) 1.50 and 2.53° for the C-MCM-48e carbon. The new (110) signal at 2θ ) 1.50°, which is visible on the XRD pattern in the case of the mesoporous carbon but is lacking on the pattern for the silica host, reflects the phase transition of the cubic MCM-48 with space group Ia3d to a new cubic phase with I4132.41 This phase transition has been attributed to a change in the relative position of the enantiomeric pair of the two-nonintersecting mesopore channels filled with carbon. This change is thought to

Figure 2. XRD patterns for the MCM-48c silica host, the corresponding mesoporous carbon, C-MCM-48c, the HUM-1c silica obtained via replication of C-MCM-48c, and the corresponding C-HUM-1e carbon.

take place to ensure energy minimization during removal of the cubic silica framework. The HUM-1e silica obtained by using the C-MCM48e carbon as a template gave the XRD pattern with two signals slightly shifted to 2θ ) 1.58 and 2.67° but with a second signal being much weaker compared to that of C-MCM-48e. The XRD pattern of HUM-1e indicates a long-range structural order characteristic for the OMS materials. The signal intensities were estimated by integrating the area under the signals at 2θ in the range of 1.2-7.0°. The areas for the HUM-1e silica were more than 85% of those for MCM-48e, indicating the continuity of template integrity despite the serial replication processes. Because the XRD pattern for HUM-1e is quite different from that of MCM48e, the HUMe-1 silica represents a new cubic phase of the symmetry different from that of the original MCM48e silica host. The generation of a new silica mesostructure may be triggered by the symmetry change observed in the parent mesoporous carbon, C-MCM-48e. The second-generation carbon, C-HUM-1e, shows the XRD pattern similar to that of the parent HUM-1e silica with two (110) and (211) signals slightly shifted to higher 2θ values of 1.67 and 2.80° but with a second signal being much weaker compared with that of HUM1e. The XRD pattern for C-HUM-1e also indicates a long-range structural order with periodically ordered mesopores. The second-generation OMC, C-HUM-1e, is different from the first-generation OMC, C-MCM-48e, in terms of the (211) signal intensity and thus may represent a new structure of the cubic OMC. Figure 2 shows the XRD patterns for analogous series of the silica and carbon samples as in Figure 1. The series of samples shown in Figure 2 was generated from the MCM-48c silica, which was obtained by further calcination of the surfactant-extracted MCM-48 silica, i.e., MCM-48e, to remove the remaining surfactant. Basically three successive replications resulted in identical structural changes such as those observed on the XRD spectra in Figure 1. Because MCM-48c was exposed to the harsh calcination process, the intense (211) XRD signal was observed at a higher value of 2θ ) 2.28° than the value of 2.21° for the corresponding MCM-48e sample due to the structural shrinkage.12 This effect was visible in the first replication and

Ind. Eng. Chem. Res., Vol. 44, No. 12, 2005 4319 Table 1. Adsorption and Structural Parameters for the MCM-48 and HUM-1 Silicas Studied and the Corresponding Carbons Synthesized by Using Those Samples as Templates

sample

unit cell value (nm)a

BET surface area (m2/g)b

pore volume (cm3/g)c

MCM-48e C-MCM-48e HUM-1e C-HUM-1e MCM-48c C-MCM-48c HUM-1c C-HUM-1c

9.80 8.33 7.91 7.48 9.48 8.12 7.71 7.35

1100 980 570 750 1210 1040 540 870

1.24 0.88 1.28 0.94 1.36 1.09 1.17 1.57

pore width 1 (nm)d 2.82 3.32 2.93 3.92 3.21 2.94

pore width 2 (nm)d 4.02 5.52 5.85 5.52 4.03 5.52 5.85 5.76

a The XRD unit cell parameter was estimated by using 61/2d(211) for MCM-48 and 21/2d(110) for C-MCM-48, HUM-1, and C-HUM1. b The BET surface area was calculated in the relative pressure range from 0.05 to 0.2 for silicas and from 0.01 to 0.1 for carbons. c The single-point pore volume was determined by using an amount adsorbed at the relative pressure of about 0.99. d Pore widths 1 and 2 denote the existing maxima on each PSD curve.

became minimal in the subsequent replications. The structural parameters for the OMSs and OMCs studied are summarized in Table 1. Figure 3 shows TEM images for the HUM-1e and C-HUM-1e samples and their calcined analogues, HUM1c and C-HUM-1c. Both HUM-1 silicas clearly exhibit highly ordered mesoporous networks with evenly spaced pores and walls, indicating a well-developed long-range order consistent with the XRD results. Also, the corresponding carbons, C-HUM-1, show highly ordered mesoporous networks with evenly spaced pores and walls such as in the HUM-1 silicas, indicating a welldeveloped long-range order. Thus, XRD patterns and TEM images indicate that the host HUM-1 has an interconnected pore system, which ensures the structural integrity of the templated carbon obtained after removal of the host. Shown in Figures 4 and 5 are nitrogen adsorption isotherms at 77 K for two series of samples synthesized by consecutive replications of the MCMN-48e and MCM48c silicas, respectively. Although both sets of adsorption isotherms are analogous, the isotherm curves for the series of samples obtained via replication of MCM48e have slightly better pronounced capillary condensation steps. A visual inspection of the isotherm curve sets shows that the curve for each MCM-48 sample differs substantially from the remaining curves. The curve for MCM-48 exhibits a very steep step for capillary condensation at the relative pressure of about 0.38, which is typical for OMS with a very narrow distribution of mesopores. Also, the isotherm curve for the HUM-1 silica shows one step reflecting capillary condensation, but this step is much less pronounced and located at a higher value of relative pressure (about 0.58). Adsorption isotherms for the C-MCM-48 and C-HUM-1 carbons show similar behavior. In the range of low pressures, these isotherms exhibit much higher adsorption in comparison to that on the corresponding silica samples. Also, in the relative pressure range between 0.1 and 0.2, there is a visible step that is analogous to that observed for the samples with fine pores (about 2.5 nm).46 At higher relative pressures, there is a moderate increase in adsorption, which indicates the presence of larger mesopores.

Figure 3. TEM images for HUM-1e (a), C-HUM-1e (b), HUM-1c (c), and C-HUM-1c (d) samples.

The PSDs for the samples studied were obtained by the KJS method by using adsorption branches of the nitrogen isotherms presented in Figures 4 and 5. The PSD curves are shown in Figures 6 and 7. A comparison of the PSD curves displayed in these figures shows several interesting similarities and differences between them: (i) the PSD curves for both MCM-48 silicas have one peak, and those for the HUM-1 silicas have one not so well-resolved peak at about 3.2 nm and one distinct peak at about 5.85 nm, and while the curves for the corresponding carbons exhibit a clearly bimodal distribution, (ii) the shapes of the PSD curves for both carbons are very similar, which indicates that second and third replications do not cause significant structural changes

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Figure 4. Nitrogen adsorption isotherms at 77 K for the MCM48e (extracted), C-MCM-48e, HUM-1e, and C-HUM-1e samples. The isotherm curves for the second, third, and fourth samples are offset vertically by 400, 800, and 1200 cm3 STP/g.

Figure 5. Nitrogen adsorption isotherms at 77 K for the MCM48c (calcined), C-MCM-48c, HUM-1c, and C-HUM-1c samples. The isotherm curves for the second, third, and fourth samples are offset vertically by 400, 800, and 1200 cm3 STP/g.

in the resulting samples, (iii) there is a substantial difference in the PSD curves for the MCM-48 and HUM-1 silicas; the former has a very narrow PSD located at about 4 nm, while the PSD for HUM-1 is more broad and its maximum is at about 5.8 nm, and (iv) for both carbons the most intensive peak appears at about 3 nm, which is related to the primary pore walls in MCM-48; the less intensive peak reflects larger mesopores formed probably because of the incomplete carbon filling of some mesopore channels and/or coalescence of some neighboring pore walls. Adsorption parameters such as the BET surface area, single-pore volume, and pore widths at which the PSD curves have maxima are summarized in Table 1. As can be seen, the BET surface areas for the MCM-48 silicas and the corresponding carbons, C-MCM-48, are similar and exceeded 1000 m2/g. The surface areas of the HUM-1 silicas are almost twice as small as those for the corresponding MCM-48 samples. However, the surface areas of carbons synthesized by using the aforementioned HUM-1 silicas as templates reached again high values, about 900 m2/g, which is due to the

Figure 6. PSDs for the MCM-48e (extracted), C-MCM-48e, HUM1e, and C-HUM-1e samples.

Figure 7. PSDs for the MCM-48c (extracted), C-MCM-48c, HUM1c, and C-HUM-1c samples.

presence of fine pores with the pore widths at about 3 nm. The percentage of these fine pores in both C-MCM48 and C-HUM-1 carbons is high, about 50% and 20%, respectively. The pore volume for all samples studied is high and in most cases exceeds 1.0 cm3/g. 4. Conclusions This study shows that the consecutive replications initiated with the MCM-48 silica gave ordered mesostructures of silica and carbon, as evidenced by XRD and TEM. Because the first replication of MCM-48 afforded a new OMC of different structural symmetry than that of MCM-48, the subsequent replication of this new carbon mesostructure allowed us to synthesize new mesostructures of silica and carbon of similar structural properties. The pore size analysis performed on the basis of nitrogen adsorption isotherms indicates that the HUM-1 silica shows larger mesopores in comparison to those present in the initial MCM-48 silica, which is probably due to the structural change occurring during the first replication of MCM-48. However, both carbon mesostructures exhibit bimodal PSDs with distinct

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Received for review October 31, 2004 Revised manuscript received March 10, 2005 Accepted April 7, 2005 IE048946V