3D-Hexagonal Mesoporous Silica Having Exceptional H2 Adsorption

Mar 27, 2009 - Apart from that, three different batches have been synthesized using 12, .... Supporting Information, S1) three types of Si environment...
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J. Phys. Chem. C 2009, 113, 6839–6844

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3D-Hexagonal Mesoporous Silica Having Exceptional H2 Adsorption Capacity Mahasweta Nandi, Mohona Sarkar,† Krishanu Sarkar, and Asim Bhaumik* Department of Materials Science, Indian Association for the CultiVation of Science, JadaVpur, Kolkatas700 032, India ReceiVed: June 6, 2008; ReVised Manuscript ReceiVed: February 23, 2009

Highly ordered 3D-hexagonal mesoporous silica HMS-3 has been synthesized under alkaline condition at 277 K using tetraethyl orthosilicate as the silica source and cetyltrimethylammonium bromide as the structuredirecting agent. Powder X-ray diffraction and HR TEM results revealed a 3D-hexagonal pore structure for this HMS-3 mesophase. Electron microscopy image analysis further suggested a uniform spherically packed nanoparticle morphology having a dimension of 200-300 nm for this mesoporous material. Nitrogen adsorption results showed a type IV isotherm with well-defined mesopores of ca. 2.2 nm dimension having very high surface area (1353 m2 g-1) and moderately good pore volume (0.63 cc g-1). A very high cross-linking framework has been revealed from the high concentration of Q4 species as seen from 29Si MAS NMR. This material showed considerably high H2 adsorption capacity at 77 K under 1 bar atmospheric pressure vis-a`-vis related pure silica MCM-41 (ca. 38% enhancement). At a pressure of 40 bar, the amount of H2 adsorbed was ca. 2.0 wt %, which is comparable to good H2 storage materials known in the literature. 3D-hexagonal pore openings with interconnected cagelike mesopores, very high surface area, and spherically packed nanoparticle morphology could be responsible for very high H2 adsorption capacity of HMS-3 over other related mesoporous silica based materials. 1. Introduction A family of ordered mesoporous materials, designated as M41S, was first introduced by Mobil researchers.1,2 These M41S materials were synthesized through a surfactant templating pathway, which utilizes the 2D or 3D supramolecular assembly formed by surfactant molecules as a core around which a solid inorganic matrix is formed as a shell. Subsequent removal of surfactant molecules from this composite can generate mesoporosity according to the size of the self-assembled surfactants in those solid materials. The most popular members of this family are MCM-41 and MCM-48. MCM-41 has a hexagonal array of unidirectional pores, and MCM-48 has a cubic interconnected pore system. In the past decade intensive research work has been done on the synthesis of mesoporous materials having wide chemical compositions.3-8 Several synthesis routes have been developed for these materials, and extensive sorption studies have been carried out to explore their potential application as adsorbents. Simultaneously, highly ordered mesoporous structures, like SBA-2/12,9 with 3D hexagonal mesostructure have been attracting increasing interest among researchers of various disciplines. SBA-12 was synthesized by using nonionic alkyl poly(ethylene oxide) (PEO) surfactant C18H37(OCH2CH2)10OH under acidic synthesis conditions at room temperature. Owing to its 3D cagelike structure, it shows high thermal stability up to 1023 K. Periodic mesoporous organosilica synthesized with ethylene bridged organosilane precursor10 is another example of this family of 3D-hexagonal mesoporous materials. The regular pore network of these 3D-hexagonal materials providing more favorable mass transfer kinetics than the unidirectional * To whom correspondence should be addressed. E-mail: [email protected]. † On leave from Department of Chemistry, Indian Institute of Technology, Kharagpurs721 302, India.

pore system of MCM-41 could be a promising candidate for applicationsinadsorption,catalysis,andseparationtechnologies.11,12 In this context an economical and safe hydrogen-storage material is highly in demand for the fruitful utilization of this fuel energy in fuel-cell vehicles and portable electronics due to its eco-friendly combustion and high-energy efficiency.13 Carbon based nanotubes,14 mesoporous carbon,15 MOFs,16-18 Prussian Blue analogues,19 metal sulfides,20 and phosphates,21 etc., are known as good hydrogen storage materials. Although the metal hydrides22 and intermetallic compounds23 have high H2 adsorption capacity at low pressure, they cannot release hydrogen completely unless they are heated to a moderately high temperature. This irreversible adsorption behavior has promoted physisorption to be the most versatile method for H2 storage. Mesoporous materials with cagelike pores could trap the adsorbed gas molecules24 and thus be expected to have higher hydrogen storage capacity than other related mesoporous materials having cylindrical pores. Herein, we report a new method for the synthesis of spherically packed hexagonal mesoporous silica through a modified synthesis procedure employing a surfactant templating pathway and characterize this material in detail. Hydrogen storage capacity for this sample was measured at 77 K up to a pressure of 50 bar. High surface area and H2 adsorption capacity suggested that this mesoporous silica has the potential to be used as a hydrogen storage material. 2. Experimental Section Synthesis of HMS-3. We have prepared HMS-3 by modifying the synthesis procedure reported by Schumacher et al.6 for the synthesis of ordered mesoporous materials. In a typical synthesis, to a solution containing 240 mL of water and 100 mL of ethanol, 24 g of aqueous ammonia (25% aqueous, E-Merck) was added, and the mixture was homogenized in an ice bath. This was followed by the addition of 5.2 g of

10.1021/jp8114034 CCC: $40.75  2009 American Chemical Society Published on Web 03/27/2009

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TABLE 1: Synthesis of Different Mesoporous Silicas sample no.

amount of NH3 (mL)

temp. (K)

time (h)

pH of gel

mesophase

1 2 3 4 5

24 12 18 24 30

298 277 277 277 277

24 18 20 16 16

12.4 11.7 12.1 12.4 13.2

cubic cubic cubic 3D-hexagonal 2D-hexagonal

cetyltrimethylammonium bromide (CTAB, Loba Chemie) while stirring under ice-cold conditions (277 K). After stirring for another 30 min, a clear solution was obtained and then 6.8 g of tetraethyl orthosilicate (TEOS, Aldrich) was added to the solution. A white precipitate was formed after another 20 min. The whole mixture was continuously stirred for another 3 h under ice-cold conditions. Then the slurry was allowed to refrigerate (277 K) for 16 h, when a solid was precipitated and settled at the bottom of the beaker. On the following day the product was filtered, the residue was repeatedly washed with water, and then it was dried in a freeze-dryer. The as-synthesized sample was calcined at 698 K for 6 h to remove the template molecules. The total weight of the sample has been reduced by ca. 44% during calcination. The sample synthesized by Schumacher et al.6 differed from our HMS-3 sample in the synthesis conditions. Whereas they synthesized the ordered mesoporous silica at room temperature, our synthesis was conducted at a lower temperature of 277 K throughout the gel preparation. In order to compare our material with the reported sample of Schumacher et al.,6 we have synthesized their sample as a reference. Apart from that, three different batches have been synthesized using 12, 18, and 30 mL of ammonia, respectively, employing a similar synthesis procedure at a low temperature (277 K). Details of the synthesis parameters including synthesis time, temperature, and pH are given in Table 1. Characterization Techniques. Carbon, hydrogen, and nitrogen contents of the as-synthesized HMS-3 sample were analyzed using a Perkin-Elmer 2400 Series II CHN analyzer. Powder X-ray diffraction patterns were recorded on a Bruker D-8 Advance diffractometer operated at 40 kV voltage and 40 mA current and calibrated with a standard silicon sample, using Ni-filtered Cu KR (λ ) 0.154 06 nm) radiation. Nitrogen and hydrogen adsorption/desorption isotherms under low pressures were obtained by using a Quantachrome Autosorb 1C at 77 K. For the high-pressure gas adsorption, a Bel Japan Inc. BelsorpHP was used. Prior to gas adsorption, all the samples were degassed for 4 h at 423 K. It is pertinent to mention here that in the high-pressure measurements the adsorption amount varies to some extent for repeated runs. Thus we have repeated the high-pressure adsorptions several times to obtain the most authentic result. Transmission electron microscopic images were recorded in a JEOL 2010 TEM operated at 200 kV. A JEOL JEM 6700F field emission scanning electron microscope was used for the determination of the morphology of the particles. A Bruker AM-300 NMR was used for MAS NMR measurement with a 7 mm zirconia rotor and 2.5-3.5 kHz speed for more than 5 h at around 5000 scans. The 29Si NMR was referenced with respect to external TMS at 0 ppm. FT IR spectra of both as-synthesized and calcined samples were recorded by using a Nicolet MAGNA-FT IR 750 Spectrometer Series II. Thermogravimetry (TG) and differential thermal analysis (DTA) of the samples were carried out on a TA Instruments Q600 DSC/TGA thermal analyzer.

Figure 1. Electron microscopic images of spherically packed HMS-3 nanospheres: TEM (a) and FE SEM (b).

3. Results and Discussion Nanostructure and Morphology. Electron microscopic images of HMS-3 are shown in Figure 1. From Figure 1a it is clear that this material is composed of spherical particles of dimension 200-300 nm in diameter. Well-defined external morphologies of the material are also observed from the respective FE SEM image (Figure 1b), where these nanospheres are seen to adhere with each other at their surfaces to form large aggregated particles. Mesoporous materials with spherically packed hexagonal micro/mesoporous structures are rarely seen, and this is particularly important for the applications like adsorption and catalysis. The 3D-hexagonal structure and the lattice parameters were determined by transmission electron microscope (TEM) imaging on the sample. Hexagonal arrangements of the pores having dark contrast vis-a`-vis the pore-walls were seen throughout the specimen grid. Lattice images recorded along different zone axes are shown in Figure 2a and 2b with their corresponding selected area Fourier transform diffractograms (FTD) in the insets, which also confirmed the hexagonal mesophase. Along the zonal z-axis (Figure 2a), sixfold symmetry can be clearly seen. When the grid was tilted along the x-axis, a slightly distorted hexagonal pore arrangement was observed. Thus we see that the pore system reflects the symmetry of the silica framework and shows close similarity to the common hexagonal close-packed structure. Moreover, FTD taken from the same particle with different directions of the incident electrons indicated hexagonal patterns, suggesting 3D-hexagonal structure for HMS-3. Average pore size as estimated from the magnified HR TEM image of the calcined sample (Figure 2c) was ca. 2.0 nm, which agree reasonably well with the other mesoporous materials synthesized by using supramolecular assembly of CTAB1-8 as the structure directing agent. Powder XRD. In Figure 3, the low-angle powder diffraction pattern of the as-synthesized HMS-3 sample is shown. The sample showed many high-intensity diffraction peaks, which

3D-Hexagonal Mesoporous Silica HMS-3

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Figure 4. XRD pattern of calcined HMS-3.

Figure 5. XRD patterns of as-synthesized samples synthesized under different conditions: (a) 24 mL ammonia, 298 K, (b) 12 mL ammonia, 277 K, (c) 18 mL ammonia, 277 K, and (d) 30 mL ammonia, 277 K, keeping other constituents of the synthesis gel identical.

Figure 2. HR TEM image of calcined HMS-3 (a) viewed along the z-axis, (b) viewed from a tilted x-axis, and (c) high-magnification view. The Fourier transform diffractogram (FTD) is shown in the insets (a and b).

Figure 3. XRD pattern of as-synthesized HMS-3. 2θ ranges 3-7 and 2.3-2.8 have been magnified in the insets for clarity.

have been indexed in the figure, and the observed and calculated d spacings were found to agree well with the 3D-hexagonal

mesostructure of SBA-2/12. In the inset of the figure, magnified portions of the XRD pattern have been given where the peaks can be easily resolved. In Figure 3 the different hkl planes have been indexed and shown in the insets. Calculated d spacings agree well with the observed d values, suggesting that HMS-3 has 3D-hexagonal mesophase. Unit cell parameters were a ) 4.20 nm and c ) 6.92 nm. The unit cell parameter ratio is c/a ) 1.648, which is very close to the theoretical c/a ) 1.633 for the hexagonal close-packed (hcp) phase. The XRD pattern of the calcined sample has been illustrated in Figure 4. As seen from the figure, upon calcination the intensity of some strong intensity peaks drastically gets reduced. Figure 4 further revealed that unit cell parameters are also reduced after calcination. Unit cell parameters of HMS-3 after calcination were a ) 3.5 nm and c ) 5.8 nm. The wall thickness estimated from this hexagonal lattice was quite slim, ca. 1.3 nm. The HR TEM image (Figure 2b) also revealed similar wall thickness (ca. 1.2 nm). This wall thickness value is quite slim considering other ordered mesoporous materials synthesized by cationic surfactant templating methods.1-12 The search for ordered mesoporous materials with slim pore walls has been one of the key areas of research for a long time because of their potential use in adsorbents and catalysis. The powder XRD pattern of the sample synthesized according to the process of Schumacher et al.6 have been given in Figure 5a. We see that, on merely changing the temperature during the synthesis, a completely different phase was generated. Whereas this XRD pattern shows a cubic arrangement of pores, the sample synthesized by us exhibits a 3D-hexagonal arrangement of pores. In order to study the effect of added ammonia in the synthesis batches, three different batches with 12, 18,

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Figure 6. N2 adsorption/desorption isotherms of calcined HMS-3. Adsorption points are marked by filled circles and desorption points by empty circles. NLDFT pore size distribution is shown in the inset.

and 30 mL (Table 1) of ammonia have been prepared and the XRD patterns are shown in Figure 5b, c, and d, respectively. We see that, apart from the reaction temperature, the amount of ammonia added during the synthesis plays a crucial role in the formation of this 3D-hexagonal mesophase. When a somewhat lesser amount of ammonia, viz., 12 and 18 mL (Figure 5b and c), are used during the synthesis, a cubic phase is generated, but as the amount of ammonia is increased, viz., 24 mL (Figure 3) and 30 mL (Figure 5d), the phases are transformed from cubic to a 3D-hexagonal and finally into a 2D-hexagonal pore system. Thus XRD pattern and HR TEM image analysis revealed that HMS-3 is a new 3D-hexagonal mesoporous silica material synthesized through the supramolecular assembly of CTAB as template. N2 Sorption. In Figure 6, adsorption/desorption isotherms for the calcined HMS-3 sample are shown. The isotherms followed typical type IV as observed for other mesoporous materials.1-12 As seen from this figure, the capillary condensation occurred at a relatively low partial pressure (P/P0) range, ca. 0.01-0.18 of N2. This capillary condensation step for highly ordered HMS-3 sample was quite broad, which may be related to a cagelike pore structure of the material similar to that observed for SBA-1.12 From the N2 sorption isotherm, the BET surface area of the sample was found to be 1353 m2 g-1, which is very high compared to 2D-hexagonal and cubic mesoporous materials. In the inset of Figure 6, the pore size distribution (PSD) of calcined HMS-3 employing the NLDFT method25 is shown. As seen from this plot, the peak pore diameter obtained from this PSD was 2.2 nm. The PSD pattern is quite narrow, suggesting the presence of uniform mesopores of similar dimensions throughout the sample surface. Peak pore dimension estimated from this adsorption data agree reasonably well with the TEM image analysis. The pore volume estimated from this adsorption isotherm for HMS-3 was 0.63 cc g-1, which agrees quite well with mesoporous organosilica material with cagelike 3D-hexagonal arrangement of the pores and synthesized in the presence of CTAB surfactant.10 It is pertinent to mention here that among the different ordered mesoporous materials, the mesopore pore volume/surface area [(total pore volume micropore volume)/(BET surface area - external surface area)] ratio was lowest for cagelike 3D-hexagonal mesophase,12 which could be taken as a signature to distinguish different mesophases. In our HMS-3 also the pore volume was relatively low and surface area was very high; thus the, mesopore pore volume to surface area ratio was quite low, 0.33 nm compared to that for 2D-hexagonal or cubic mesoporous materials. Thus this low value is an indirect evidence for 3D-hexagonal cagelike structure for HMS-3.

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Figure 7. H2 adsorption/desorption isotherms of HMS-3 under low pressure. Adsorption points are marked by filled circles and desorption points by empty circles. 29

Si MAS NMR and FT IR. The 29Si MAS NMR spectrum of calcined HMS-3 shows (see Supporting Information, S1) three types of Si environment present in this sample. The peaks at ca. -111.1, 103.2, and 91.6 could be attributed to the Q4 [Si(OSi)4], Q3 [Si(OH)(OSi)3], and Q2 [Si(OH)2(OSi)2] species, respectively. These chemical shifts agree well with that reported for other ordered mesoporous silica materials.26 It is clear from the figure that the ratio of Q4/Q3 was more than 1.0 in this sample. Concentration of more Q4 for HMS-3 synthesized by this method suggested more cross-linked and condensed structure with less defect Si-OH groups. Low density of surface silanol groups is primarily responsible for the hydrophobic nature of these mesoporous materials, and thus the 3D-hexagonal mesoporous silica synthesized under controlled pH condition has a hydrophobic surface, which may be very helpful for its practical application in adsorption and catalysis. FT IR spectra of the as-synthesized sample shows (see Supporting Information, S2) two C-H vibrations in the 2800-2950 cm-1 range, corresponding to different identical C-H bonds in the CTAB molecules. However, for the calcined sample, these peaks disappeared, suggesting the complete removal of the template CTAB molecules during calcinations. Other stretching frequencies agree quite well with the ordered mesoporous silica materials.27 Thermal Analysis. The quantitative determination of the CTAB content in the as-synthesized HMS-3 sample was done from the thermogravimetric (TG) and differential thermal analysis (DTA) under N2 flow. The TGA of the as-synthesized sample showed the first weight loss at up to 373 K due to desorption of physisorbed water (about 9.5 wt %; see Supporting Information, S3A). This was followed by a gradual decrease in the weight in the temperature range 403-693 K by two steps, which corresponds to the burning and removal of organic template molecules. Total weight loss in this region (43.2 wt %) corresponds to the template CTAB molecules. On the other hand, the template-free HMS-3 (see Supporting Information, S3B) did not show any peak in the 373-953 K region of the DTA plot apart from the low-temperature endotherm due to desorption of adsorbed water molecules in the mesopores. Absence of any further peak in the DTA plot of this calcined HMS-3 sample suggested very high thermal stability of this 3D hexagonal mesoporous silica framework. H2 Storage Application. In Figure 7, low-pressure H2 adsorption/desorption isotherms for the calcined HMS-3 sample at 77 K are shown. From the figure it is clear that with the increase in P/P0 value of H2 the uptake increases. As seen from the figure that desorption points exactly followed the adsorption ones, suggesting reversible physical adsorption of H2 at the silica

3D-Hexagonal Mesoporous Silica HMS-3

Figure 8. H2 adsorption isotherm for MCM-41 (a), SBA-16 (b), and HMS-3 (c) at 77 K under high pressure. For plots a and b, after saturation adsorption/desorption points decreased considerably. Thus for clarity points until the plateau are shown.

Figure 9. H2 adsorption at room temperature for MCM-41 (a) and HMS-3 (b) under high pressure.

surface. At 1 bar pressure of H2 the volume adsorbed is 90 cc g-1 at STP, which corresponds to 0.81 wt % of H2. This value is ca. 38% higher than the H2 adsorption capacity of pure silica MCM-41 under identical measurement conditions.28 In Figure 8, the high-pressure adsorption/desorption isotherms (77 K) for HMS-3 as well as MCM-41 samples are shown. Here at 40 bar pressure the amount of H2 adsorbed was ca. 2.0 wt % for HMS3, whereas those at 15 bar (saturation point) for MCM-41 and SBA-16 were 0.94 and 1.1 wt %, respectively. For both samples these adsorption amounts are the saturation limit, and at higher pressure the respective curves showed a decreasing trend.29 The amount of H2 adsorption over HMS-3 is quite high and comparable with good hydrogen storage materials known in the literature.14-19 In Figure 9, results for H2 adsorption capacities of MCM-41 and HMS-3 at room temperature (298 K) under high pressure are shown. As seen from this figure, there is a drastic enhancement of H2 uptake from MCM-41 to HMS-3, considering the fact that the latter has relatively lower pore volume (0.63 vis-a`-vis 0.93 cc g-1). For MCM-41 with 2Dhexagonal structure, the maximum adsorption capacity was ca. 0.4 wt % at 15 bar at 298 K, and there is no further increase in H2 uptake at higher applied pressure, as it reached the plateau. On the other hand, the HMS-3 sample with 3D-hexagonal cagelike structure could adsorb ca. 0.64 and 2.0 wt % H2 at 40 bar pressure at room temperature and 77 K, respectively. The amount adsorbed for the HMS-3 sample was much higher than that shown by the MCM-41 sample.30 It is pertinent to mention that the heat of adsorption in small micropores is larger than that on an open surface. Thus generally the hydrogen adsorption should be larger in a microporous adsorbent than on an open surface or in a mesoporous adsorbent

J. Phys. Chem. C, Vol. 113, No. 16, 2009 6843 of the same chemical composition. HMS-3 has peak pore diameter of ca. 2.2 nm, which is very close to the borderline between the microporous and mesoporous solids. Consequently, it has a relatively large H2 adsorption capacity as compared to other silica based mesoporous materials having large mesopores. To understand further the role of interconnected cagelike mesopores, we have prepared SBA-1531 and SBA-1632 materials using pluronic triblock polymers P123 and F127 as templates and measured the H2 adsorption capacities. At 1 bar H2 pressure, the amount of adsorption over MCM-41, SBA-16, and HMS-3 was 0.58, 0.46, 0.53, and 0.81 wt %, respectively (Figure 8). SBA-15 has 2D-hexagonal mesopores, whereas SBA-16 has interconnected cagelike mesopores of cubic framework structure. Relatively large H2 adsorption capacity of SBA-16 over SBA15 suggested that interconnected cagelike mesopores are more suitable for holding the adsorbed H2 through slow diffusion. Thus our experimental results suggested that high surface area, interconnected cagelike 3D-hexagonal structure, and spherically packed nanoparticle morphology might be responsible for higher H2 uptake in HMS-3. Mechanism of 3D-Hexagonal Mesophase Formation. It is interesting to note that although HMS-3 is closely related to the SBA-2 and SBA-12 families of structures (mesoporous silica) based on close-packed arrays of spherical micelles, the former has been synthesized by conventional CTAB as template. However, SBA-2 was typically prepared by synthesis under basic conditions using the dicationic gemini surfactant [CH3(CH2)15N(CH3)2-(CH2)3N(CH3)3]Br2 (C16-3-1),11 and SBA12 was prepared under acidic conditions using the nonionic Brij76 surfactant.9 Here the supramolecular assembly of the cationic surfactant (CTAB molecules) in the aqueous solution directs the formation of the inorganic mesostructure from the dissolved inorganic silica species under alkaline pH conditions. The framework may be considered as being made up of silicate cages (stabilized under alkaline pH and low temperature) that are formed by condensation around spherical micelles to form the cagelike hexagonal inorganic-organic composite. Here the inorganic components (which were negatively charged at the high pH values used) preferentially interacted with the positively charged ammonium head groups of the surfactants and condensed into a solid, continuous framework. The resulting 3Dhexagonal arrays of organic-inorganic mesophase upon calcination retain the mesophase and generate interconnected cagelike mesopores with a very high surface area having a narrow distribution of mesopores. 4. Conclusions In conclusion, we described herein synthesis of highly ordered 3D-hexagonal mesoporous silica under controlled pH conditions by using the supramolecular assembly of cationic surfactant CTAB. XRD and TEM data analysis suggested the 3Dhexagonal structures in these samples. N2 sorption isotherms indicated a very high surface area of 1353 m2 g-1 for this material with a narrow pore size distribution and slim pore wall. Broad capillary condensation observed could be attributed to the connected cagelike mesostructure. Electron microscopic image analysis further revealed uniform spherical nanoparticles of dimension 200-300 nm in diameter for this material. These ordered mesoporous silica samples showed a good amount of hydrogen adsorption at 77 K. At 40 bar pressure the amount of H2 uptake over this material at 77 K was 2.0 wt %, which is much larger than that for MCM-41 (0.94 wt %) with 2Dhexagonal mesopores synthesized in the presence of CTAB as template. We believe that high H2 uptake over 3D-hexagonal

6844 J. Phys. Chem. C, Vol. 113, No. 16, 2009 mesoporous silica with cagelike interconnected pores and spherical nanoparticle morphology under cryogenic conditions could motivate the researchers to find an application of the mesoporous materials in hydrogen storage. Acknowledgment. This work was partly funded by the Ramanna Fellowship and NanoScience and Technology Initiative grants of the Department of Science & Technology, New Delhi. M.N. and K.S. thank CSIR, New Delhi for their respective senior research fellowships. Supporting Information Available: 29Si MAS NMR spectra, FT IR spectra, and TGA results of HMS-3. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature (London) 1992, 359, 710–712. (2) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. T.; 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. (3) (a) Yanagisawa, T.; Shimizu, T.; Kuroda, K.; Kato, C. Bull. Chem. Soc. Jpn. 1990, 63, 988. (b) Morey, M.; Davidson, A.; Eckert, H.; Stucky, G. D. Chem. Mater. 1996, 8, 486–492. (c) Taguchi, A.; Schu¨th, F. Microporous Mesoporous Mater. 2005, 77, 1–45. (4) (a) Wang, Y.; Lang, N.; Tuel, A. Microporous Mesoporous Mater. 2006, 93, 46–54. (b) Zukal, A.; Thommes, M.; Cejka, J. Microporous Mesoporous Mater. 2007, 104, 52–58. (5) (a) Pauly, T. R.; Pinnavaia, T. J. Chem. Mater. 2001, 13, 987– 993. (b) Zhang, B.; Davis, S. A.; Mann, S. Chem. Mater. 2002, 14, 1369– 1375. (c) Das, D. D.; Harlick, P. J. E.; Sayari, A. Catal. Commun. 2007, 8, 829–833. (6) Schumacher, K.; Ravikovitch, P. I.; Du Chesne, A.; Neimark, A. V.; Unger, K. K. Langmuir 2000, 16, 4648–4654. (7) (a) Bhaumik, A.; Inagaki, S. J. Am. Chem. Soc. 2001, 123, 691– 696. (b) Chandra, D.; Yokoi, T.; Tatsumi, T.; Bhaumik, A. Chem. Mater. 2007, 19, 5347–5354. (c) Chandra, D.; Mukherjee, N.; Mondal, A.; Bhaumik, A. J. Phys. Chem. C 2008, 112, 8668–8674. (8) (a) Xu, A.-W. J. Phys. Chem. B 2002, 106, 13161–13164. (b) Corriu, R. J. P.; Mehdi, A.; Reye, C.; Thieuleux, C. Chem. Mater. 2004, 16, 159–166. (9) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024–6036. (10) Inagaki, S.; Guan, S.; Fukushima, Y.; Ohsuna, T.; Terasaki, O. J. Am. Chem. Soc. 1999, 121, 9611–9614. (11) (a) Zapilko, C.; Anwander, R. Chem. Mater. 2006, 18, 1479–1482. (b) Garcia-Bennett, A. E.; Williamson, S.; Wright, P. A.; Shannon, I. J. J. Mater. Chem. 2002, 12, 3533–3540. (c) Che, S.; Sakamoto, Y.; Terasaki, O.; Tatsumi, T. Chem. Mater. 2001, 13, 2237–2239. (d) Kim, M. J.; Ryoo, R. Chem. Mater. 1999, 11, 487–491. (12) Kruk, M.; Jaroniec, M.; Ryoo, R.; Kim, J. M. Chem. Mater. 1999, 11, 2568–2572. (13) (a) Liu, C.; Fan, Y. Y.; Liu, M.; Cong, H. T.; Cheng, H. M.; Dresselhaus, M. S. Science 1999, 286, 1127–1129. (b) Orimo, S.; Majer, G.; Fukunaga, T.; Zu¨ttel, A.; Schlapbach, L.; Fujii, H. Appl. Phys. Lett. 1999, 75, 3093–3095. (14) (a) Ijima, S. Nature (London) 1991, 354, 56. (b) Dillon, A. C.; Jones, K. M.; Bekkedahl, T. A.; Kiang, C. H.; Bethune, D. S.; Heben, M. J.

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