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J. Phys. Chem. B 2005, 109, 22710-22714
Methane Sorption in Ordered Mesoporous Silica SBA-15 in the Presence of Water Li Zhou,*,† Xiuwu Liu,† Yan Sun,† Jingwen Li,† and Yaping Zhou‡ High-Pressure Adsorption Laboratory, School of Chemical Engineering and Technology, and Department of Chemistry, School of Science, Tianjin UniVersity, Tianjin 300072, P. R. China ReceiVed: August 15, 2005; In Final Form: October 16, 2005
Methane or natural gas is a practical alternative fuel of vehicles. To develop more efficient technology of on-board storage, a possibility of using SBA-15 as the methane carrier is tested. An ordered mesoporous silica molecular sieve SBA-15 is synthesized aimed at collecting the data of methane sorption in the presence of water. The synthesized material is examined with SEM, TEM, and XRD analyses, and the hexagonal channel structure is confirmed. Its textual parameters are evaluated on the basis of the adsorption data for nitrogen at 77 K, and it gives the results of 802 m2/g for the specific surface area, 1.31 cm3/g for the pore volume, a narrow pore size distribution centered at 7.7 nm, and 4 nm for the width of the partition wall. Methane sorption isotherms on SBA-15 are collected with a volumetric method for samples of different contents of water, and the enhancement of the sorption capacity due to the presence of water is observed. A discussion on the mechanism of the sorption enhancement is presented.
1. Introduction Following the appearance of ordered mesoporous material MCM-41 in 1992,1 different mesoporous materials have been synthesized using the template technique. An SBA series of ordered mesoporous silica was synthesized in 1998 using a neutral triblock template agent.2 This group of materials, especially SBA-15, attracted researcher’s attention due to its prominent property. It has a relatively high specific surface area and uniform pore size. In addition, the partition wall is as thick as 3 nm or larger; therefore, it has high hydrothermal stability. There are also micropores on the partition walls, through which the mesopore spaces are interconnected. The pore size is adjustable for a wide range of 5-30 nm using organic poreexpanders. Research on its potential applications has actively been carried out.3-5 A new potential application of the material in the field of clean energy is studied presently. Utilization of alternative fuels becomes an urgent task facing the fast increasing oil price. Natural gas or other methane-containing fuels can fuel vehicles with fairly clean emissions. The natural resource of methane in the form of flammable ice is much more abundant than natural gas. Therefore, methane fuel is as important as hydrogen, especially because it does not involve any technical barriers in utilization. Storage of gaseous fuels on board of vehicles is a key technology of using gas fuels for the transportation system. Presently, compression is the only method applied in practice for natural gas (CNG); however, the storage pressure of 20 MPa is still too high to allow for a low fuel cost and guaranteed safety. In addition, the energy density of natural gas at pressure 20 MPa is only 29% of that of gasoline for the same fuel tank volume. Therefore, how to decrease the storage pressure and increase the energy density remains a challenge in the field of NGV’s (natural gas vehicles). Adsorption was applied for this purpose, and ANG (adsorptive natural gas) was then developed. * To whom correspondence should be addressed. Phone and fax: +86 22 8789 1466. E-mail:
[email protected]. † School of Chemical Engineering and Technology. ‡ School of Science.
Although a lot of theoretical and technical progress has been made in the research of ANG, it has not been practically applied for NGV until today due to its technical deficiency. The storage of natural gas in the form of a hydrate (NGH) was proposed later. NGH is a solid and has a theoretical volumetric capacity of 164 or 174 v/v assuming complete filling of the water cages within the clathrate structure. However, the formation condition of NGH is rigorous, and the formation and conversion rate is low. In addition, the stored gas cannot be released from the hydrate just by reducing pressure. Therefore, NGH has not become a commercial technique for natural gas storage. It is experimentally shown that the pore space of solids catalyzes the formation of methane hydrates, and much more methane is stored/unit mass of adsorbents in comparison with the capacity without presence of water.6,7 Therefore, a new method to store natural gas appeared recently. Instead of dry carbon, water-wetted carbon was used to absorb natural gas. As consequence, all the technical drawbacks of ANG have been overcome, and larger storage capacity and less storage pressure are achieved.8 However, microporous adsorbents with pore sizes less than 2 nm are not suitable for the wet storage method9 because it needs a definite space to allow for the formation of the water cages. It is, therefore, assumed that mesoporous material may be a better choice of adsorbents to store natural gas in the presence of water, and tests on SBA-15 were carried out presently. 2. Experimental Section 2.1. Synthesis and Characterization of SBA-15 Material. SBA-15 with 2-dimensional ordered channels was synthesized under acidic conditions. A nonionic oligomeric alky-ethylene oxide surfactant (Pluronic P123) was used as the structuredirecting agent, and TEOS (tetraethyl orthosilicate) of analysis grade was used as the silica source. The specific synthesis procedure is as follows: 6.0 g of P123 is dissolved in 240 mL of solution of 0.1 M HCl, and then 12.5 g of TEOS was added in the solution and agitated for 24 h at 313 K. The mixture was poured into a Teflon lined stainless steel autoclave and aged at
10.1021/jp0546002 CCC: $30.25 © 2005 American Chemical Society Published on Web 11/18/2005
Methane Sorption in Mesoporous Silica SBA-15
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Figure 1. SEM photo of the SBA-15 sample in two scales.
Figure 2. TEM photo of the SBA-15 sample taken along the channel direction and that perpendicular to it.
393 K for another 24 h in the closed space. After filtration and drying, the reaction product was calcinated at 823 K to remove the template, and the SBA-15 sample was obtained. The synthesized product was examined with small-angle X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and the adsorption of nitrogen at 77 K. Small-angle X-ray diffraction (XRD) was performed on a Rigaku D/MAX 2500/PV diffractometer with Cu-butt at the conditions of λ ) 0.154 056 nm, 40 kV, and 200 mA over a range 0.2° < 2θ < 4.0° with step size 0.002° and step time 1 s. The instrument used for a SEM photo was the Philips scanning electron microscope model XL30ESEM. The instrument used for a TEM photo was the Philips field emission gun transmission electron microscope model TECNAI G2F-20. Software provided by the instrument supplier was used to determine the distance between two parallel lines in the photos. Nitrogen adsorption data were collected at 77 K on a volumetric setup for vacuum adsorption. Prior to adsorption measurements, the sample was degassed in situ at 473 K for 12 h. Measurements covered the relative pressures from about 10-6 to 0.995. The BET surface area was calculated on the basis of the data for relative pressures from 0.05 to 0.3.10 The total pore volume was evaluated on the basis of the amount adsorbed at relative pressure 0.99. The pore size distribution was determined by applying the Barrett-Joyner-Halenda (BJH) equation11 to the adsorption branch. 2.2. Collection of Methane Sorption Data with/without the Presence of Water. Before the sorption measurement, the SBA15 sample was dried in a vacuum at 300 °C for 12 h. The sorption isotherms of methane on the synthesized material were collected on a typical volumetric apparatus adapted for highpressure studies. Working principle and details of the apparatus were previously presented.12 Pressure was measured with sensitivity of 0.05% up to 11 MPa. Temperature was kept constant within (0.1 °C. The methane gas used is of purity
above 99.99%. Initially, the adsorption isotherm of methane on the dry SBA-5 sample was collected at 275 K, and then the dry sample was replaced with wet samples containing different amounts of water. The experiment with wet samples follows the procedure described previously.6 The sorption measurement on wet samples was kept at 275 K first, and then the measurement was moved to 277, 279, and 281 K to evaluate the enthalpy change of methane sorption in wet samples. Dynamic behavior on sorption was also discussed on the basis of the variation of pressure readings during the sorption process. 3. Results and Discussion 3.1. Textural Property of the Synthesized Material. The SEM photo of the synthesized material is shown in Figure 1 for two different scales. A fringy appearance composed of ropelike bars is shown in the photo as is previously reported.13 Two-dimensional ordered channel structure is characteristic for SBA-15, which is clearly seen in the TEM photo shown in Figure 2. The XRD pattern shown in Figure 3 exhibits three characteristic peaks for the 2-D hexagonal structures of SBA15.2 The three peaks indicate the existence of the crystal planes (100), (110), and (200), characteristic for the 2-D hexagonal structure. The a0 parameter of the crystal cell is determined as 11.8 nm on the basis of d100 ) 10.2 nm and a0 ) 2d100/x3. The adsorption/desorption isotherms of nitrogen at 77 K are shown in Figure 4. They show the feature of type-IV isotherms with a H1 type hysteresis according to the IUPAC classification;14 therefore, the pore size is in the mesopore range and the pores are open at two ends.15 These isotherm features are characteristic for the two-dimensional hexagonal channel structure of, for example, MCM-41 or SBA-15. The BET specific surface area and pore volume determined are 802 m2/g and 1.31 cm3/g, respectively. The pore size distribution determined with the BJH method is shown in Figure 5. This figure shows a very
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Figure 3. XRD pattern of the SBA-15 sample. Figure 6. Sorption isotherms of CH4 in SBA-15 with different contents of water: 1, Rw ) 0; 2, Rw ) 1.31; 3, Rw ) 1.5; 4, Rw ) 2; 5, Rw ) 2.69; 6, Rw ) 2.92.
Figure 4. Nitrogen adsorption-desorption isotherms of sample SBA15-393K1d.
Figure 5. Pore size distributions of sample SBA-15-393K1d.
narrow pore size distribution centered at 7.7 nm. Referring to the cell unit parameter, one determines the width of the partition walls of 4 nm. A much small peak can be recognized at about 2 nm, which indicates the existence of micropores on the
partition walls. The mesopore space is interconnected through these micropores. 3.2. Methane Sorption in SBA-15 with Different Contents of Water. 3.2.1. Equilibrium Sorption Data. The sorption isotherms of methane on the SBA-15 sample with different content of water obtained at 275 K are shown in Figure 6. The sorbed amount of methane is expressed as the weight percentage of the dry material to facilitate the calculation and comparison between samples with different content of water as was usually done in the drying operation. The water content in a wet sample is quantified with Rw, the weight ratio of water over the SBA-15 material. Therefore, Rw ) 0 is the dry sample, on which the isotherm (curve 1) shows typical type-I feature. However, all isotherms on wet samples show a discontinuous form with a sharp increase at an inflection pressure. The sorbed amount is almost zero before the inflection pressure, but the isotherm jumped up to a higher level and kept at the level on further increasing pressure. This kind of isotherm has been observed for methane on other porous materials too if water is preadsorbed, and it was explained as the formation of methane hydrate in the porous space. The inflection pressure shifted from 3.9 MPa down to 3.23 MPa following the increase of water content. The latter is almost the same as the formation pressure of methane hydrate at 275 K in water media. The amount of methane stored in the wet SBA-15 sample increases with the increasing water content. It reaches 23.2 wt % at Rw ) 2.92, which is 3.75 times higher than the sorption capacity of dry sample. The isotherm after the inflection pressure shows a descending trend with increasing pressure. This is a normal phenomenon for the sorption measurement at supercritical temperatures. The surface excess amount of sorption is experimentally recorded on the basis of the density difference between the gas phase and the sorbed phase. The gas-phase density keeps increasing with the increasing pressure, but the density of the sorbed phase, whether compressed gas or hydrates, cannot keep increasing as the gas phase does. Thus, a descending part follows. 3.2.2. Desorption BehaVior. The desorption isotherm is shown in Figure 7 for the sample with Rw ) 2.69. Although the desorption isotherm is not totally coincided with the sorption one, methane can be discharged totally on releasing pressure.
Methane Sorption in Mesoporous Silica SBA-15
Figure 7. Methane sorption/desorption isotherms in the SBA-15 sample with Rw ) 2.69.
Figure 8. Dynamic curve of methane sorption in the SBA-15 sample with Rw ) 2.69.
3.2.3. Dynamic BehaVior of Methane Sorption. The dynamic behavior of methane sorption in SBA-15 in the presence of water is typically shown in Figure 8. The pressure reading does not change for the initial 30 min (before point A), and then it drops from 4.62 MPa down fast to 3.82 MPa at 180 min (point B), after which the pressure reading almost does not change. It seems an induction period for about 30 min is required for the formation of the cage structure of water molecules inside the porous space. It took about 150 min for the methane molecules to get into the cages (to form the hydrate), and the whole process ends if all the cage spaces have been occupied by methane molecules. The actual time of hydrate formation must be much shorter than 150 min considering the thermal effect of hydrate formation. The heat of hydrate formation must be released inside the adsorption cell and then be transferred to the surrounding water bath through the cell walls via heat conduction. A stable pressure reading is reached only when the equilibrium of both mass and heat transfer has been reached. As the charging/ discharging experiment done with active carbon shows, methane hydrate can form quite fast, but the heat conduction through the container wall controls the time of reaching equilibrium. The time needed to reach equilibrium at the inflection pressure depends also on the content of water in the sample. For example,
J. Phys. Chem. B, Vol. 109, No. 48, 2005 22713
Figure 9. Comparison of methane isotherms on SBA-15 and on largepore silica gel: 1, SBA-15 with Rw ) 2.92; 2, silica gel with Rw ) 1.17; 3, silica gel with Rw ) 0.8.
it takes 72 h to reach equilibrium at the inflection pressure with Rw ) 2.92, while it usually takes much shorter time for lower water content as shown in Figure 8. It seems there is a critical content of water. The sample is not sticky if the water content is lower than the critical value, but it becomes sticky and looks darker if the water content becomes higher than the critical value. The critical value of water content can be determined with the titration method,16 which was used to determine pore volume of porous solid previously. The critical water content as determined is about 2.7-2.8 cm3/g, which is much larger than the pore volume (1.31 cm3/g) determined on the basis of the adsorption isotherm of nitrogen at 77 K. The large difference in the equilibrium time between the sample with Rw ) 2.92 and the sample with Rw ) 2.69 might be attributed to the surplus water existing between the solid particles when the water ratio is larger than the critical water content. The transport resistance would be much larger if the particles become sticky. 3.2.4. Comparison with the ObserVation on Mesoporous Silica Gels. Mesoporous silica materials attracted research interest17-19 since a vast quantity of methane was discovered in the permafrost zone and the submarine sediment in the form of methane hydrate, which was reserved in natural porous material, most likely in porous silicates. Previous studies were focused mainly on the influences of pore diameter and temperature on hydrate formation and dissociation. Compared to the similar experiments carried out on a large pore silica gel,20 two differences were observed. Representative samples are shown in Figure 9, where curve 1 is the isotherm on SBA-15 with Rw ) 2.92 and curves 2 and 3 are the isotherms on silica gel with Rw ) 1.17 and 0.8, respectively. Curves 1 and 2 represent the largest sorption amount observed on the two materials. The sorption capacity of methane increased from ca. 5 to ca. 23 wt % due largely to the larger pore volume of SBA-15. The isotherm shape also looks different. While the isotherm goes down after saturation on SBA-15 as shown by curve 1, an increasing section is shown on curve 3, which is considered due to the enlargement of the pores in silica gel.20 Therefore, SBA-15 is more stable in the presence of water. 3.2.5. Discussion on the Mechanism of the Enhanced Sorption. An explanation of the methane sorption mechanism in SBA-15 in the presence of water can be found in the enthalpy
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Zhou et al. of the methane state in the enclosed system. The enthalpy change of the state transition can be determined on the basis of the Clausius-Clapeyron equation; therefore, isotherms at different temperatures are collected and shown in Figure 10. The inflection pressures are 3.42, 4.11, 4.81, and 6.08 MPa respectively for temperatures 275, 277, 279, and 281 K. The value of enthalpy change equals the product of the gas constant R times the slope of the plot ln f versus 1/T shown in Figure 11, where f is the fugacity of methane. It equals -55.4 kJ/mol, which is quite close to the formation heat of methane hydrate in pure water (-59.5 kJ/mol21). Therefore, the discontinuous isotherms of methane on SBA-15 in the presence of water are recorded due to the formation of methane hydrate in the porous space, which enhances the capacity for methane sorption. Acknowledgment. The financial support of the National Natural Science Foundation of China (Grant Nos. 50376047 and 20336020) is greatly appreciated. References and Notes
Figure 10. Methane sorption isotherms on the SBA-15 sample with Rw ) 2.69 at different temperatures: 1, 275 K; 2, 277 K; 3, 279 K; 4, 281 K.
Figure 11. Plot to evaluate the enthalpy change corresponding to the isotherm inflection pressure and the formation pressure of methane hydrate in pure water: 1, in water;21 2, in SBA-15.
change of methane during the sorption. The discontinuity of sorption isotherms at the inflection pressure indicates a transition
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