Communication pubs.acs.org/cm
Porous Montmorillonite Heterostructures Directed by a Single Alkyl Ammonium Template for Controlling the Product Distribution of Fischer−Tropsch Synthesis over Cobalt Qing-Qing Hao,† Zhong-Wen Liu,*,† Bingsen Zhang,‡ Guang-Wei Wang,† Chunyan Ma,§ Wiebke Frandsen,‡ Jinjun Li,§ Zhao-Tie Liu,† Zhengping Hao,§ and Dang Sheng Su‡,* †
Key Laboratory of Applied Surface and Colloid Chemistry (MOE), School of Chemistry & Chemical Engineering, Shaanxi Normal University, Xi’an 710062, China ‡ Department of Inorganic Chemistry, Fritz Haber Institute of the Max Planck Society, Faradayweg 4-6, 14195 Berlin, Germany § Research Center for Eco-Environmental Science, Chinese Academy of Sciences, Beijing 100085, China S Supporting Information *
KEYWORDS: layered compounds, porous clay heterostructures, supercritical carbon dioxide, Fischer−Tropsch, heterogeneous catalysis
C
Herein, with a single template of alkyl ammonium, we demonstrate the synthesis of ordered PCHs using a lowercharge-density clay of MMT. Our strategy is accomplished by the confined hydrolysis and polymerization of the metal alkoxide in the MMT gallery, which is inferred from the synthesis of ordered MCM-41 templated by a quaternary ammonium.17,18 Moreover, highly ordered PCH is formed in the medium of scCO2, and the function of scCO2 is clearly clarified. Significantly, without using any alkyl amine as a cosurfactant, highly ordered PCH can be synthesized in a green manner. We use the obtained PCH to synthesize Co/PCH catalyst by the incipient impregnation method, and applied it for controlling the product distribution of Fischer−Tropsch (FT) synthesis. Due to the mild acidity for cracking reactions and the small mesopores in PCH, a very high selectivity of liquid hydrocarbons is achieved. The representative XRD patterns of the PCH, which is synthesized using CTAB ion-exchanged MMT (CTAB-MMT) and TEOS as a silica precursor under normal conditions (ncPCH) and scCO2 conditions (scPCH) (for the detailed procedure, please see section 1 of the Supporting Information (SI)), are shown in Figure 1A. As indicated by the clear (00l) diffractions, PCH with a basal spacing of about 3.10 nm was formed irrespective of synthetic conditions, and the gallery height of 2.14 nm was determined after deducting the MMT layer thickness (0.96 nm). Moreover, the (002) reflection of scPCH was much stronger than that of ncPCH, which is similar to those of the synthetic saponite and MMT derived PCHs directed by two surfactants.10,11,19,20 Thus, with a single template of CTAB, MMT derived PCH was successfully synthesized, and its regularity was significantly enhanced under scCO2. This is confirmed by the N2 adsorption−desorption results (Figure 1B). Both of the PCHs showed a parallel part of the hysteresis loop in the regions of p/p0 > 0.8, implying open
lays have been known for a long time as low-cost adsorbents and catalysts due mainly to their ion-exchange and acidic and porous properties.1 Thus, different strategies have been developed to enhance these functions via structural tailoring.1−3 Indeed, pillaring, including in the medium of supercritical CO2 (scCO2),4,5 is an effective method to increase surface area and pore volume of the clay.3 However, applications of the pillared clay are limited due to its microporous nature.6 In 1995, a new kind of clay derived mesoporous material, that is, the porous clay heterostructures (PCH) intercalated with silica, was designed through the template-directed assembly of silica precursor in the clay interlayer.7 Its unique mesoporous structure and the intrinsic acidity of the lamellar clay renders PCH new opportunities in catalysis.8−11 Thus, after this pioneer work, much attention has been paid to the synthesis and application of PCH.11−13 For the synthesis of PCHs intercalated with metal oxide, the key steps are the ion exchange of a cationic surfactant (quaternary ammonium in general) into the interlayer of the host clay and the subsequent intercalation of an alkyl amine as a cosurfactant along with or followed by metal alkoxide. According to the mechanism proposed by Galarnearu et al,7 PCH cannot be formed without alkyl amine. Besides a template-directing function, the neutral amine is believed to swell the silicate interlayer and to catalyze the intragallery hydrolysis of the metal alkoxide, leading to the ordered PCH heterostructures.7 Following this mechanism, a large excess of alkyl amine relative to clay must be exclusively used for the synthesis of PCH so far.6,14 Noteworthy, although an alkyl amine templated pillaring of montmorillonite (MMT)15 or a synthetic magadiite16 can produce some mesopores, delamination cannot be avoided due to the significant hydrolysis of TEOS out of the clay galleries. Thus, many challenges remain to be solved yet,6 such as, controlling the delamination of the clay layers, and the synthesis of highly ordered PCH without using the toxic alkyl amine. Moreover, even if PCH based materials are promising catalysts or catalyst supports, their catalytic application is still very scarce. © 2012 American Chemical Society
Received: December 28, 2011 Revised: February 2, 2012 Published: February 13, 2012 972
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Figure 3. XRD patterns (A) and FT-IR spectra (B) of CTAB-MMT (a), ncPCH precursor (b), and scPCH precursor (c).
Figure 1. XRD patterns (A) and N2 adsorption−desorption isotherms (B) of scPCH (a) and ncPCH (b) synthesized at a CTAB-MMT/ TEOS weight ratio of 1:20.
CTAB-MMT under either normal or scCO2 conditions, that is, the precursors of ncPCH and scPCH, the d001 spacing was increased to be 3.55 nm (Figure 3A), indicating that CTAB molecules are rearranged accompanying with the hydrolysis and polymerization of the intercalated TEOS. This is strongly supported by FTIR results (Figure 3B). In comparison with CTAB-MMT, the IR bands of the C−H bonds of methylene groups in PCH precursors were clearly shifted to higher wave numbers, indicative of the formation of a liquid-crystal CTAB micelle (for the detailed discussion, please see section 4 of the Supporting Information).1,22 Thus, the formation of rodlike CTAB micelles surrounded by hydrolyzed and polymerized TEOS is reasonably expected for the PCH precursor (III in Scheme S1, SI). This mechanism can be further validated by the similar basal spacing, peak pore size, and pore size distribution of scPCH synthesized at different CMAB-MMT/ TEOS weight ratios (Figure 1 and Figure S3 in the Supporting Information), for which the average pore size of the final product is strongly dependent on the organoclay to TEOS ratios according to the pillaring mechanism.7 The hydrolysis and polymerization of TEOS confined into the MMT gallery is crucial for the formation of ordered PCH. This is clearly revealed from the ncPCH synthesized from CTAB-MMT dried in different extents (details in section 7 of the Supporting Information). Following the proposed mechanism (Scheme S1, SI), the intercalation and dispersion of TEOS in the interlayer of CTAB-MMT can be greatly promoted by scCO2 when the adequate solubility of TEOS and the characteristics of scCO223−25 are considered. Moreover, under scCO2 conditions, carbonate acid is formed by reacting the interlayer water with CO2 (pH = 2.8−2.9526), which accelerates the hydrolysis of TEOS via an acid catalyzed process. In the aspect of surfactant, carbon-chain mobility of CTAB has been reported to be greatly enhanced with scCO2 via solute−solvent interactions.27 Considering the isoelectric point of silica (pH = 2−3),28,29 the ion-dipole interaction between cationic head groups of CTAB and the O atoms in Si−OH groups (S+ I0) is the reasonable driving force for the rodlike assembly. Thus, on the basis of these factors, more ordered rodlike assembly under scCO2 is reasonably expected, leading to the highly ordered scPCH. Moreover, the thus developed approach is verified for the successful synthesis of PCH by using different quaternary ammonium (section 8 of the Supporting Information). The PCH impregnated with 20 wt % Co was evaluated as a catalyst for FT synthesis, and the main results are given in Table 1. The comparable CO conversions over all catalysts except Co/Na-MMT were consistent with the crystal properties of cobalt (Table S1, SI). Significantly, compared with Co/ SiO2, PCH-based catalysts showed much higher selectivity of
21
and cylindrical pores. Moreover, a nearly linear uptake of nitrogen in the relative pressure ranges of 0.05−0.25 indicated the presence of framework pores spanning from supermicropores to small mesopores in the range 1.5−2.5 nm.9 A peak pore size of about 2.0 nm was confirmed from the pore size distribution calculated by the BJH method (the inset of Figure 1B), which is well matched with the gallery height of 2.14 nm determined by XRD technique. The highly ordered mesoporous structure of scPCH was clearly reflected from the sharp pore size distribution. To confirm the highly ordered structure of scPCH, microscopic observations were conducted. From the scanning electron microscopy (SEM) images (Figure S1, SI), the flakelike morphology of the raw MMT was well reserved for scPCH. Moreover, aggregates of several MMT layers with varied sizes and orientations were observable from the transmission electron microscopy (TEM) images (Figure S2, SI). Figure 2a shows high-resolution TEM (HRTEM) image of
Figure 2. TEM images of scPCH.
scPCH flake with curly edges. The (001) lattice fringes of scPCH exhibit an interplanar spacing of 3.13 nm that was calculated by combining fast Fourier transform (FFT) pattern (the inset of Figure 2a). Furthermore, to explore the fine structure of (001) plane, the cross-section TEM specimen was investigated, as shown in Figure 2b and Figure S2, SI. Paralleled MMT layers with an average spacing of 3.13 nm were clearly revealed from the HRTEM images, which agree well with XRD and N2 adsorption results. To reveal the formation mechanism of PCH, in our case, the intermediate products were characterized by XRD and FTIR. From the d001 spacing of CTAB-MMT (2.20 nm, Figure 3A), its interlayer distance of 1.24 nm was much smaller than the estimated chain length of CTAB (2.15 nm). Thus, the arrangement of CTAB molecules in the MMT interlayer is reasonably expected to be in a tilted (∼35°) interdigitated manner (II in Scheme S1, SI). After introducing TEOS into 973
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(3) Stefanis, A. D.; Tomlinson, A. A. G. Catal. Today 2006, 114, 126−141. (4) Yoda, S.; Sakurai, Y.; Endo, A.; Miyata, T.; Yanagishita, H.; Otake, K.; Tsuchiya, T. J. Mater. Chem. 2004, 14, 2763−2767. (5) Yoda, S.; Nagashima, Y.; Endo, A.; Miyata, T.; Yanagishita, H.; Otake, K.; Tsuchiya, T. Adv. Mater. 2005, 17, 367−369. (6) Cool, P.; Vansant, E. F. In Handbook of Layered Materials; Aurbach, S. M., Carrado, K. A., Dutta, P. K., Eds.; Marcel Dekker, Inc.: New York, 2004, pp 261−311. (7) Galarneau, A.; Barodawalla, A.; Pinnavaia, T. J. Nature 1995, 374, 529−531. (8) Galarneau, A.; Barodawalla, A.; Pinnavaia, T. J. Chem. Commun. 1997, 1661−1662. (9) Polverejan, M.; Pauly, T. R.; Pinnavaia, T. J. Chem. Mater. 2000, 12, 2698−2704. (10) Polverejan, M.; Liu, Y.; Pinnavaia, T. J. Chem. Mater. 2002, 14, 2283−2288. (11) Pichowicz, M.; Mokaya, R. Chem. Commun. 2001, 20, 2100− 2101. (12) Chmielarz, L.; Kuśtrowski, P.; Dziembaj, R.; Cool, P.; Vansant, E. F. Catal. Today 2007, 119, 181−186. (13) Chmielarz, L.; Piwowarska, Z.; Kuśtrowski, P.; Gil, B.; Adamski, A.; Dudek, B.; Michalik, M. Appl. Catal., B 2009, 91, 449−459. (14) Tchinda, A. J.; Ngameni, E.; Kenfack, I. T.; Walcarius, A. Chem. Mater. 2009, 21, 4111−4121. (15) Kwon, O.-Y.; Park, K.-W.; Jeong, S.-Y. Bull. Korean Chem. Soc. 2001, 22, 678−684. (16) Kwon, O.-Y.; Shin, H.-S.; Choi, S.-W. Chem. Mater. 2000, 12, 1273−1278. (17) Monnier, A.; Schüth, F.; Huo, Q.; Kumar, D.; Margolese, D.; Maxwell, R. S.; Stucky, G. D.; Krishnamurty, M.; Petroff, P.; Firouzi, A.; Janicke, M.; Chmelka, B. F. Science 1993, 261, 1299−1303. (18) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834−10843. (19) Benjelloun, M.; Cool, P.; Voort, P. V. D.; Vansant, E. F. Phys. Chem. Chem. Phys. 2002, 4, 2818−2823. (20) Wei, L.; Tang, T.; Huang, B. Microporous Mesoporous Mater. 2004, 67, 175−179. (21) de Jong, K. P.; Zečević, J.; Friedrich, H.; de Jongh, P. E.; Bulut, M.; van Donk, S.; Kenmogne, R.; Finiels, A.; Hulea, V.; Fajula, F. Angew. Chem., Int. Ed. 2010, 49, 10074−10078. (22) Xi, Y.; Frost, R. L.; He, H. J. Colloid Interface Sci. 2007, 305, 150−158. (23) Ishii, R.; Wada, H.; Ooi, K. J. Colloid Interface Sci. 2002, 254, 250−256. (24) Cooper, A. I. Adv. Mater. 2003, 15, 1049−1059. (25) O’Neil, A. S.; Mokaya, R.; Poliakoff, M. J. Am. Chem. Soc. 2002, 124, 10636−10637. (26) Toews, K. L.; Shroll, R. M.; Wai, C. M.; Smart, N. G. Anal. Chem. 1995, 67, 4040−4043. (27) Thompson, M. R.; Liu, J.; Krump, H.; Kostanski, L. K.; Fasulo, P. D.; Rodgers, W. R. J. Colloid Interface Sci. 2008, 324, 177−184. (28) Iler, R. K. The Chemistry of Silica; John Wiley & Sons: New York, 1979; Chapter 3, pp 185−189. (29) Li, F.; Stein, A. Chem. Mater. 2010, 22, 3790−3797. (30) Liu, Z.-W.; Li, X.; Asami, K.; Fujimoto, K. Appl. Catal., A 2006, 300, 162−169. (31) Kang, J.; Cheng, K.; Zhang, L.; Zhang, Q.; Ding, J.; Hua, W.; Lou, Y.; Zhai, Q.; Wang, Y. Angew. Chem., Int. Ed. 2011, 50, 5200− 5203. (32) Hao, Q.-Q.; Wang, G.-W.; Liu, Z.-T.; Lu, J.; Liu, Z.-W. Ind. Eng. Chem. Res. 2010, 49, 9004−9011.
Table 1. Main FT Results over Different Catalysts hydrocarbon selectivity [mol %] catalysta
CO conv. [%]
C1
C2−C4
C5−C12
C13−C20
C21+
Co/SiO2b Co/Na-MMT Co/ncPCH Co/scPCH
48.8 2.0 30.9 34.2
12.3 42.0 14.0 13.6
6.6 17.9 11.3 11.3
29.7 14.0 37.6 41.9
11.7 11.3 12.8 15.8
39.7 14.8 24.3 17.4
Reaction conditions: H2/CO = 2, W/F = 5.05 g·h·mol−1, P = 1.0 MPa, T = 235 °C, TOS = 10 h; bFujisilicia Q-15, surface area = 200 m2 g−1, average pore size = 15 nm, pore volume = 1.0 mL g−1. a
C5∼C12 and considerably lower selectivity of heavy hydrocarbons (C21+). This can be reasonably explained as the cracking of long-chain FT hydrocarbons over acidic sites.30−32 In comparison with Co/ncPCH, a higher selectivity of liquid hydrocarbons and a lower selectivity of C21+ (Table 1) were observed over Co/scPCH. Because of the much greater size of Co (∼15 nm, Table S1, SI) than the pore size of PCHs, the majority of Co should be at the outside of the PCH mesopores. Moreover, the acidity of scPCH and ncPCH was almost the same (Figure S6, SI). Thus, these differences in product selectivity can be reasonably ascribed to the more ordered pore structure of scPCH than ncPCH. In summary, using a single template of alkyl ammonium, PCH was successfully prepared from a lower-charge-density Na-MMT in a green manner, and a reasonable mechanism was proposed. scCO2 is favorable for the formation of a highly ordered PCH, and its function was clearly clarified. The mild acidity, the small mesopores, and a large optimizing room of PCH make Co/PCH an important FT catalyst for selectively synthesizing liquid fuels.
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ASSOCIATED CONTENT
S Supporting Information *
Experimental details and characterization results of SEM/TEM, XRD, TG, and NH3-TPD. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] and
[email protected]. de. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The group from Shaanxi Normal University is thankful for financial support by the National Natural Science Foundation of China (20876095), the Program for New Century Excellent Talents in University (NCET-08-0799), the Program for Changjiang Scholars and Innovative Research Team in University (IRT1070), and the Fundamental Research Funds for the Central Universities (2010ZYGX012).
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REFERENCES
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