Studies in Surface Science and Catalysis 156 M. Jaroniec and A. Sayari (Editors) 9 2005 ElsevierB.V. All rights reserved
Pitch-based carbons synthesized by using silica colloids and ordered mesoporous silica particles as templates Kamil P. Gierszal and Mietek Jaroniec
Department of Chemistry, Kent State University, Kent, Ohio 44242, USA
Pitch-based carbons with unimodal and bimodal distributions of mesopores were synthesized by using silica colloids and ordered mesoporous silica particles such as MCM48, SBA-15 and SBA-16 as templates and mesophase pitch as carbon precursor. Bimodal distribution of mesopores in these carbons was created by employing a mixed template, which consisted of uniform silica colloids and particles of ordered mesoporous silica.
1. INTRODUCTION In recent years mesoporous carbons attracted much attention because of their potential applications in many fields such as adsorption, catalysis, chromatography and electrochemistry. A significant breakthrough in the area of mesoporous carbons was the synthesis of ordered mesoporous carbon (OMC) in 1999 by using ordered mesoporous silica (OMS) as template [1 ]. Since then many OMCs were prepared using various OMSs templates such as SBA-15 and MCM-48 and various carbon precursors . Also, inverse carbon replicas of siliceous mesocellular foams, MSU silicas and siliceous colloidal crystals were reported . In addition to the templating synthesis of OMCs, the colloidal imprinting synthesis was proposed , in which silica colloids were used to imprint uniform spherical mesopores in mesophase pitch particles. The templating synthesis was also used to create mesoporous carbons with bimodal distribution of pores, so-called bimodal carbons [7-13]. Schtith et al.  introduced furfuryl alcohol into mesopores of SBA-15 to prepare carbons with unimodal and bimodal distributions of pores depending on the degree of filling of mesopores by carbon precursor. In this approach one system of pores resulted from incompletely filled mesopores of SBA15 and another one was created by dissolution of the SBA-15 pore walls . This group synthesized also bimodal carbons by polymerization of acrylonitrile in SBA-15 mesopores . In this case bimodal pore size distribution was created because some regions of the SBA-15 mesostructure were not filled by carbon precursor. The other carbons with bimodal distribution of pores possessed ordered-disordered structures as a result of an incomplete filling of mesopores of the template by carbon precursor, which was affected by chemical properties of this precursor and synthesis conditions [ 10-13]. This work is devoted to the pitch-based carbons with unimodal and bimodal distribution of pores synthesized by using single or binary templates of MCM-48, SBA-15, SBA-16 and colloidal silica. Mesoporous carbons with bimodal distribution contain both ordered
582 mesopores that are an inverse replica of SBA-15, SBA-16 and MCM-48 as well as uniform spherical mesopores created by dissolution of silica colloids introduced into mesophase pitch. Unimodal carbons were prepared by using a single OMS template. Mesophase pitch is a very attractive carbon precursor because it affords OMCs with relatively high degree of graphitization [ 14-17].
2. EXPERIMENTAL Synthesis of carbons with bimodal distribution of pores is illustrated in Scheme 1. Analogous procedure was used for the preparation of ordered carbons with unimodal distribution of mesopores; in this case one needs to discard the symbols referring to the silica colloids in Scheme 1. This procedure involved heating of the pitch and siliceous template mixture in flowing nitrogen at temperature higher than pitch softening point, followed by stabilization of the mixture in air, carbonization in flowing nitrogen and silica dissolution. The first heating of the synthesis mixture in flowing nitrogen was carried out at sufficiently high temperature to decrease significantly the pitch viscosity, which is crucial for interpenetration of pitch and template particles as well as for introduction of pitch into mesopores of the OMS template. Stabilization in air was used to "freeze" the nanostructure of silica-pitch composite , i.e., to stop the interpenetration of silica and pitch particles during heating to reach the carbonization temperature. The final step of the synthesis was to dissolve the silica template to create porosity in the resulting carbon. He at Ireatment m nitrogen foUowed by atr stabihzahon
\ Mixture c o n t a i r m ~ : \ Heat treatment
carbonization and silica dissolution
9 pitch particles \ m nitrogen 9 colloidal silica 0 ordered mesoporous silicaparticles Scheme 1. Schematic illustration of the synthesis procedure of pitch-based carbons with bimodal distribution of mesopores. Synthesis of carbons with bimodal distribution of pores was carried out as follows: the mixture of 0.2 g of mesophase pitch particles (synthetic pitch AR-24 from Mitsubishi: particle size below 45~fn; softening point about 237~ 0.2 g of the SBA-15 particles (surface a r e a - 780 m/g;, pore volume - 1.0 cm3/g; pore width- 9.1 nm) or MCM-48 particles (surface area - 1130 mE/g; pore volume - 1.46 cm3/g; pore width - 4.5 nm) and 0.2 g of commercial colloidal silica (Ludox AS-30 from Aldrich with the surface area o f - 2 3 0 m2/g and particle size about 13 nm) was dispersed in ethanol-water solution, which was allowed to evaporate at temperature about 60~ during several hours of stirring. The resulting solid mixture was heated in flowing nitrogen using the heating rate of 5 deg/min to
583 reach the desired temperature and time of thermal treatment. After this thermal treatment the sample was stabilized in flowing air at 220~ for 10 hours and carbonized in flowing nitrogen at 900~ for 2 hours. In the aforementioned processes a 5 deg/min heating rate was used to attain the temperature specified for a given process. Finally, the silica templates used were dissolved in 49% HF solution at room temperature (a special caution is recommended when dealing with HF). The dissolution conditions were analogous to those reported previously [ 13,15]. The resulting carbon samples were denoted C-S-T-t, where C, S, T and t refer to the carbon, silica template (single or mixed), temperature and time (in hours) of the thermal treatment, respectively. The resulting carbons contained below 2% of the silica residue, which was estimated by thermogravimetric analysis in air at 800~ Synthesis of the carbon samples with unimodal distribution of pores was done similarly as that of bimodal carbons, except using single-component template, SBA-15 or MCM-48, instead of mixed template consisting of colloidal silica and OMS particles. The SBA-15 template, which is a hexagonally ordered mesoporous silica (P6mm structure), was synthesized using poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer (EO20PO70EO20; Pluronic P 123, BASF) and tetraethylorthosilicate (TEOS) according to the recipe reported elsewhere . The synthesis of MCM-48 (Ia3d cubic structure) was reported in  and involved the use of eicosyltrimethylammonium bromide, C20TMABr, as surfactant template. The SBA-16 (Im3m cubic structure) was prepared from TEOS using a proper mixture of EO106PO70EO106 (Pluronic F127, BASF) and EO20POToEO20 (Pluronic P123, BASF) triblock copolymers to obtain the average molar composition of the template, EOsoPO70EOs0 . SBA-16 is a body-centered cubic structure having cage-like spherical mesopores. Each cage is connected with eight neighbors through short channels of smaller diameter than that of primary mesopores. The sample studied had the BET specific surface area of 810 m2/g, total pore volume of 0.53 cm3/g and the pore width (calculated using the KJS method for cylindrical pores [21 ]) equal to 6.7 nm.
3. RESULTS AND DISCUSSION 3.1 Unimodal pitch-based carbon synthesized using SBA-16 as template o.10o
~o 200 [.., 150
0.4 0.6 0.8 Relative Pressure
4 6 8 Pore Size [ nm ]
Figure 1. Nitrogen adsorption isotherm at- 196~ and the corresponding pore size distribution (PSD) for the pitch-based carbon with unimodal distribution of mesopores synthesized using SBA-16 as template.
584 The interpenetration of the SBA-16 and pitch particles was carried out in nitrogen at 450~ for 5 hours. This resulting carbon replica of SBA-16 had the BET specific surface area of 360 m2/g and the total pore volume of 0.3 cm3/g. Shown in Figure 1 are nitrogen adsorption isotherm (left panel) and the corresponding pore size distribution (fight panel) for this carbon replica. At the relative pressure range from -~0.1 and -0.6 the adsorption isotherm shown in Figure 1 exhibits almost linear increase, which leveled off at pressures higher than 0.6, indicating a small amount of secondary porosity. The external surface area of this sample was about 10 m2/g. Its micropore volume and volume of primary mesopores constituted -~90% of the total pore volume (see pore size distribution for this carbon). The pore size of primary mesopores, evaluated by using the KJS method calibrated for cylindrical pores , was equal to 4.4 nm, which is reasonable result for the SBA-16 sample treated hydrothermally at 100~ for 1 day .
3.2 Bimodai pitch-based carbons synthesized using SBA-15 and colloidal silica 300
"~ 100 r.~
0.4 0.6 0.8 Relative Pressure
8 12 16 Pore Size [ nm ]
Figure 2. Nitrogen adsorption isotherms at-196~ and the corresponding pore size distributions (PSD) for the pitch-based carbons with bimodal distribution of mesopores prepared by' using SBA-15 and colloidal silica (CS) as template. Table !. The BET surface area (S). total pore volume (Vt), pore diameter formed by using the SBA15 silica particles (w~) and silica colloids (w2) as templates. Sample
Temperature of thermal treatment [~
[cm3/g] wl [nm]~ w_~[nm]j'
"Calculated in the relative pressure range of 0.04-0.25" 1'calculated at the maximum of PSD. Shown in Figure 2 are nitrogen adsorption isotherms at -196~ for bimodal carbons prepared by using SBA-15 and colloidal silica as templates as well as the corresponding pore size distributions calculated from the adsorption branches of the isotherms by employing the KJS method  with the statistical film thickness for nitrogen on the BP280
585 reference carbon black . Adsorption parameters for these bimodal carbons are summarized in Table 1. As can be seen from Table 1 an increase in the temperature and time of thermal treatment affected the degree of interpenetration of pitch and template particles, which is manifested by an increase in the nitrogen uptake and consequently, an increase in the pore volume and surface area of the resulting carbons. Adsorption isotherms shown in Figure 2 reveal two distinct capillary condensation steps reflecting two types of mesopores: (i) mesopores created after dissolution of the pore walls of the SBA-15 template and (ii) spherical mesopores created by dissolution of silica colloids. These two types of pores are clearly visible on the PSD curves (see right panel in Figure 2). The position of the first peak on PSD, related to the pores created by dissolving the SBA-15 structure, shifts in the direction of smaller pore widths with increasing temperature of the first thermal treatment . For the sample synthesized at 340~ the maximum of this peak is located at 3.46 nm and it is shifted to 3.18 nm for the sample prepared at 400~ It appears that at higher temperatures the viscosity of pitch is lower, which facilitates penetration of the pitch components imo mesopores of SBA-15 and ensures better organization of disk-like molecules inside SBA-15 pores, i.e., perpendicular orientation to the channel axis . Another important factor that influences the degree of pore filling, besides temperature, is time, which is presumably required for arrangement of quite large polyaromatic hydrocarbons that constitute the mesophase pitch. There is some difference between the estimated pore width (-~3.4 nm) and the wall thickness of SBA-15 (~2.5 nm), which may be due to the following reasons: (i) error in the pore width estimation arising from using the KJS method (calibrated for cylindrical pores ) to analyze the porosity of interconnected carbon nanorods, (ii) shrinkage of the pitch during stabilization and carbonization processes, and (iii) incomplete filling of mesopores of the template. From the shape of adsorption hysteresis, relatively high BET surface area and pore volume one can infer that the imerconnectivity between mesopores of both types is very good. In contrast to the ordered mesopores the diameter of spherical mesopores (13.3 nm), calculated at the maximum of PSD is virtually independent on the temperature of heat treatment. Its value is in a good agreement with the diameter of silica colloids. Since there is a large difference in the widths of two types of mesopores, the PSD curve consists of two well-separated peaks. The left side of the first peak contains a small contribution arising from micropores, which is manifested by a small spike. The volume ratio of both types of mesopores can be tuned by adjusting the composition of the mixed siliceous template. The contribution of both types of mesopores to the total pore volume can be estimated by integration of PSD in proper range of the pore widths. For the samples studied it was --50%. Another advantage of the carbons studied was a very small microporosity because of using mesophase pitch, which does not show tendency to form micropores during carbonization process. The as-plot for the bimodal carbons studied is quite interesting because it shows three distinct steps corresponding to the micropores and two types of mesopores (see exemplau" as-plot for C-SBA15-CS-400-2). The first step reflects the presence of a small amount of micropores; the second step is related to the filling of ordered mesopores created by dissolution of SBA-15, whereas the third step corresponds to the mesopores created by dissolution of silica colloids. For the C-SBA15-CS-400-2 sample the (zs-plot analysis gives: the micropore volume of 0.015 cm3/g and the volumes of primary mesopores created after dissolution of SBA-15 and colloidal silica equal to 0.15 cm3/g and 0.17 cm3/g, respectively.
The sum of these pore volumes is smaller than the value evaluated from the amount adsorbed at the relative pressure of 0.99, which is due to the presence of textural mesopores. The external surface area of this sample was-~20 m2/g. 300 250
200 Z 150 ~ lOO ~ 5o 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Standard Adsorption as Figure 3. t~-plot for nitrogen adsorption on the C-SBA15-CS-400-2 sample obtained by using the BP280 carbon black as the reference adsorbent.
3.3 Bimodal pitch-based carbons synthesized using MCM-48 and colloidal silica 240
-o 120 0 "0
~ < ~ 0
8 12 16 Pore Size [nm ]
Figure 4. Nitrogen adsorption isotherm at- 196~ C and the corresponding pore size distribution (PSD) for the pitch-based carbon with bimodal distribution of mesopores prepared by using MCM-48 and spherical silica colloids as template. The pitch-based bimodal carbon prepared by using MCM-48 and colloidal silica is an example of another structure. Figure 4 shows nitrogen adsorption isotherm and the corresponding pore size distribution for the bimodal carbon synthesized at 400~ for 2 hours using MCM-48 and colloidal silica as template. Similarly as for the C-SBA15-SC samples, the C-MCM48-SC-400-2 carbon exhibits two distinct capillary condensation steps related respectively to the mesopores created by dissolution of MCM-48 and colloidal silica. The sample under study had a moderate BET surface area (-~210 m2/g) and total pore volume of 0.32 cm3/g. The mesopore widths estimated at the maxima of PSD were 3.4 and 13.5 nm. respectively. Since the wall thickness of the MCM-48 silica was around 1.3 nm. the resulting structure with mesopores of-~3.4 nm is not a faithful replica of MCM-48. This
587 behavior of the MCM-48-templated carbons is known in literature . The MCM-48 structure consists of two intertwined separated channels, which after filling with carbon followed by removal of silica, can shift leading to the pore enlargement. Note that the pore size analysis of C-MCM48-SC-400-2 was done by the KJS method, which was developed for cylindrical pores . Therefore, the estimated value of the pore size has some error resulting from assumption of the cylindrical pore geometry to the cubic structure of MCM48.
4. CONCLUSIONS This work shows that the pitch-based carbons of unimodal and bimodal distributions of mesopores can be prepared respectively by using single and mixed templates of ordered mesoporous silicas and silica colloids. In the case of bimodal carbons, one type of mesopores is created by dissolution of the OMS template, whereas another type of mesopores is obtained by dissolution of silica colloids. Different OMS and colloidal silica templates can be used to tune the widths of both types of mesopores as well as their contribution to the total pore volume.
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