Enhanced CO2 Sorption on Ordered Mesoporous Carbon CMK-3 in

Feb 19, 2016 - Department of Chemistry, School of Science, Tianjin University, Tianjin ... School for Engineering of Matter, Transport and Energy, Ari...
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Enhanced CO2 Sorption on Ordered Mesoporous Carbon CMK‑3 in the Presence of Water Jia Zhou,† Wei Su,†,§ Yan Sun,‡ Shuguang Deng,§ and Xiaojing Wang*,† †

Tianjin Key Laboratory of Membrane and Desalination Technology, School of Chemical Engineering and Technology, Tianjin University, Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, P. R. China ‡ Department of Chemistry, School of Science, Tianjin University, Tianjin 300072, P. R. China § School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, Arizona 85287-6106, United States ABSTRACT: Mesoporous carbon CMK-3 was synthesized via a hard template method. The structures of the pores were characterized by N2 adsorption at 77K, XRD, and TEM examinations. The BET surface area and total pore volume were 1115 m2/g and 1.11 cm3/g, respectively. Adsorption isotherms of carbon dioxide on the CMK-3 in the presence of water were collected. At 275 K and 3.6 MPa, the CO2 sorption capacity was 42 mmol/g with a water to dry carbon weight ratio (Rw) of 2.35. This is 2.26 times as high as that for the dry CMK-3 sample and 23% higher than the highest CO2 sorption previously obtained on activated carbon with preadsorbed water. The enthalpy change obtained from the sorption isotherms was −55.6 kJ/mol, which is lower than the enthalpy for the formation of CO2 hydrate in pure water.

1. INTRODUCTION Carbon dioxide is a major greenhouse gas, and the largest source of this gas is the burning of fossil fuels.1 Although some promising clean energy sources such as solar,2 wind,3 and hydrogen4 have made significant progresses in the past decade, fossil fuels will continue to be the world’s main energy source in the coming decades. Therefore, the reduction of CO2 emissions continues to be a vital area. Carbon capture is an important approach for the reduction of CO2 emissions. Carbon dioxide can be captured by pre-, post-, or oxygen-fuel-combustion processes.5 Precombustion capture has a distinct advantage over the other two processes because the partial pressure of CO2 in these processes can reach as high as 8 bar, and this is favorable for CO2 capture and storage.6 Gas hydrate technology is a promising method to capture and store CO2 in precombustion process.7 CO2 hydrates can form in pure water, but the hydrate formation kinetics are very slow. However, adding a hydrate promoter can enhance the kinetics and considerably decrease the hydrate formation pressure.7 Many compounds such as tetrahydrofuran (THF),8 propane,9 cyclopentane,10 tetra-n-butylammonium bromide,11 tetra-n-butylammoniumfluoride,12 tetra-n-butylammonium nitrate,13 and sodium dodecyl sulfate14 have been used as hydrate promoters. THF especially has been extensively studied as a hydrate promoter in carbon dioxide capture and hydrogen storage.15,16 In order to enhance the contact between the gas and liquid, a stirred tank reactor has been employed for CO2 hydrate formation.17 However, the rate of crystallization decreased due to agglomeration of the hydrate crystals. Linga et al.18 reported that the hydrate formation efficiency could be significantly © XXXX American Chemical Society

improved by employing a gas inducing impeller; however, the power consumption for this was significant. Another way to ensure a large gas/water contact area is to use a porous medium.19 Gas hydrates can easily form in the pores of porous media, and because stirring is not needed, power requirements are minimal. Many porous materials such as silica gel,20,21 KIT-6,22 various types of coal,23 and activated carbon24,25 have been employed as contact medium to form gas hydrates for CO2 separation and storage. Babu et al.26 has shown that silica sand is more effective than silica gel for the capture of CO2 from flue gas. The pore size distribution is a very important factor for the formation of hydrates in porous materials. Zheng et al.21 tested three types of silica gel. The highest sorption capacity was 3.2 mmol/g, and they found that CO2 hydrate did not easily form in the sample with the largest pores. Zhang et al.22 investigated CO2 storage in ordered mesoprous silica KIT-6 and obtained a sorption capacity of 12.8 mmol/g at 275 K and 2.2 MPa. Storage capacities as high as 31 mmol/g have been obtained in activated carbon BY-1 at 275 K and 2.2 MPa.24 To the best of our knowledge, this is the largest storage capacity achieved for a porous material in the presence of water. The carbon materials have nonpolar surfaces, which is more favorable for CO2 storage in the presence of water than the silica materials. CMK-3 is an ordered mesoporous carbon with large pore volumes and is a promising material for CO2 storage.27 In this study, CO2 sorption equilibrium data on CMK-3 was measured Received: December 16, 2015 Accepted: February 9, 2016

A

DOI: 10.1021/acs.jced.5b01063 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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2.4. Sorption Equilibrium Measurement. Because the uptake of CO2 by CMK-3 in the presence of water may not be adsorption, by convention, the phase equilibrium of CO2 on the wet sample is called “sorption”.25 The sorption equilibrium of CO2 was performed in a volumetric apparatus. Working principle and details have been presented in previous similar study.30 The sorption equilibrium was measured at 275−281 K, and the pressure was varied from 0 to 4.2 MPa. The weight ratio of water to dry sample is defined as Rw and the dry sample is represented by Rw = 0. In this work, Rw values of 0 to 3.55 were used.

in the presence of water. In addition, the CO2 storage mechanism was investigated.

2. EXPERIMENTAL SECTION 2.1. Materials. CMK-3 was synthesized using SBA-15 as the template. Both CMK-3 and SBA-15 were synthesized in our laboratory. The sources for the chemicals and gases used in this work as well as their purities are presented in Table 1. Table 1. Chemicals and Gases Used name Plouronic P123 hydrochloric acid tetraethylorthosilicate (teos) sucrose sulfuric acid hydrofluoric acid ethanol deionized water helium carbon dioxide

source

purity

Sigma-Aldrich (Shanghai) Trading Co., Ltd. Tianjin Jiangtian Co., Ltd. Tianjin Jiangtian Co., Ltd.

Mw = 5800

Tianjin Jiangtian Co., Ltd. Tianjin Jiangtian Co., Ltd. Tianjin Jiangtian Co., Ltd. Tianjin Jiangtian Co., Ltd. Nankai University Water Center Tianjin Liufang Industrial Gases Co., Ltd. Tianjin Liufang Industrial Gases Co., Ltd.

⩾99.7% ⩾99.7% ⩾99.7% ⩾99.0%

3. RESULTS AND DISCUSSION 3.1. Characterization of CMK-3. The adsorption isotherm of N2 on CMK-3 at 77 K is presented in Figure 1. The isotherm

⩾99.7% ⩾99.7%

⩾99.9995% ⩾99.9995%

2.2. Synthesis SBA-15 and CMK-3. SBA-15 was synthesized following the procedure of Zhao et al.28 Briefly, 3.0 g of P123 was dissolved in 120 mL of 0.1 mol·L−1 hydrochloric acid solution. Then, 6.25 g of TEOS was added to the solution, and the mixture was stirred for 24 h at 313 K. The mixture was then heated in an autoclave at 393 K for 24 h. After filtration, washing with ethanol, and drying, the solid was heated at 823 K to remove any remaining P123. CMK-3 was prepared via a hard temple method.29 First, 1.25 g of sucrose and 0.14 g of H2SO4 were dissolved in 5 g of water. Then, 1 g of SBA-15 was impregnated with the above solution and the mixture was transferred to an oven and maintained at 373 K for 6 h and then at 433 K for another 6 h. The obtained brown power was then impregnated again with a solution obtained by dissolving 0.8 g of sucrose and 0.09 g of H2SO4 in 5 g of water. The mixture was then treated again at 373 and 433 K. Finally, the carbonization was completed at 1173 K in a nitrogen atmosphere rather than in vacuum. The silica template was removed with 5 wt % HF at room temperature. After filtration and washing with ethanol, the carbon product was dried at 393 K. 2.3. Characterization. The transmission electron microscopy (TEM) images were acquired using a JEM-2100F field emission transmission electron microscope, respectively. Small angle X-ray diffraction (XRD) analysis was performed on a Rigaku D/MAX-2500 X-ray diffractometer at a scan rate of 0.5°/min in the 2θ range of 0.5−5°. The N2 adsorption− desorption isotherms on CMK-3 were collected using an ASAP 2020 apparatus at 77 K. The Barret−Joyner−Halenda (BJH) method was used to evaluate the pore size distribution (PSD). The specific surface areas were determined using the Brunauer−Emmett−Teller (BET) equation. The total pore volume Vt was obtained from the adsorption capacity of N2 at p/p0 = 0.98. The microporoe volume Vmicro was estimated from the N2 sorption data at p/p0 ∼ 0.3 and the mesopore volume Vmeso = Vt − Vmicro.

Figure 1. N2 adsorption−desorption isotherm of CMK-3 at 77 K.

is type-IV with a H2 hysteresis loop, which indicates that the CMK-3 has mesoporosity. As shown in Figure 2, CMK-3 has a

Figure 2. Pore size distribution of CMK-3.

narrow pore size distribution with a median pore size of 3.25 nm. The specific BET surface area, Vt, Vmicro, and Vmeso were calculated to be 1115 m2/g, 1.11 cm3/g, 0.56 cm3/g, and 0.55 cm3/g, respectively. Both specific BET surface area and Vt are smaller than those previously reported for CMK-3.29 This may be because that the carbonization in this work was performed in a nitrogen atmosphere rather than in vacuum. The TEM image for the CMK-3 is shown in Figure 3. The TEM image shows that the CMK-3 has an ordered structure. In Figure 4, the XRD pattern of CMK-3 shows well-resolved XRD peaks. The peaks are due to (100), (110), and (200) diffractions of a two-dimensional hexagonal channel space.29 B

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hydrates in the pores.25 This is similar to previous studies of other materials.22,24,25 The inflection pressure at 2.6 MPa is the hydrate formation pressure and this value is much higher than the hydrate pressure in pure water (1.5 MPa).31 In porous materials, higher pressures are needed to form gas hydrates.22 When Rw was increased to 1.55−2.35 (curves 3−5), an additional sharp increase occurred at about 2.05 MPa. Sorption isotherms with two inflection points have never been reported for the sorption of CO2 on porous materials with preadsorbed water. This will be discussed in more detail later. The highest sorption capacity (42 mmol/g) was obtained for Rw = 2.35 at 3.6 MPa. This is 2.26 times as high as that for dry CMK-3 (18.6 mmol/g). Sorption capacities for other materials are shown in Table 2. The highest sorption capacity obtained in Table 2. Sorption Capacity of CO2 in Porous Materials Figure 3. TEM image of CMK-3.

adsorbent

sorption capacity (mmol/g)

silica gel (SG-A)

2.5

silica gel (SG-B)

3.2

bamboo carbon

12.1

coconut shell carbon (BY-1) silica (KIT-6)

31

mesoporous carbon (CMK-3)

42.0

12.8

Rw/pressure

reference

0.75/3.5 MPa 0.81/3.5 MPa 2.36/2.5 MPa 1.65/2.2 MPa 2.48/2.2 MPa 2.35/3.6 MPa

21 21 25 24 22 this work

this work is 23% higher than that obtained for activated carbon with preadsorbed water. When Rw was increased to 3.55, it had a negative effect on the sorption capacity (13.0 mmol/g). This is because too much water makes it difficult to form gas hydrates.22 3.3. Discussion of the Sorption Mechanism. Prior to the inflection pressure, the sorption capacities of CO2 on all the CMK-3 samples with preadsorbed water were much lower than that for dry CMK-3. Sorption capacity is affected by both the solubility of CO2 in water and by adsorption on the wet surface.24 For Rw = 0.55, the inflection point (i.e., the step rise in the sorption capacity at 2.60 MPa) is due to the formation of CO2 hydrate.22 For Rw = 1.55−2.35, there are two inflection points in the isotherms at 2.05 and 2.60 MPa. This has not previously been reported. The total pore volume of CMK-3 was calculated from the N2 adsorption at 77 K to be 1.11 cm3/g. The volume of water in the Rw = 1.55−2.35 samples is 1.55−2.35 cm3/g. Therefore, for these samples, the water must occupy both the pores and the exterior space in the samples. The exterior space has a large dimension that could not be measured via the N2 adsorption at 77 K. In addition samples with larger pore sizes require lower pressures to form gas hydrates.31 Therefore, the inflection points in curves 2, 3, and 4 at 2.05 and 2.60 MPa are due to the formation of CO2 hydrate in the exterior space and pores, respectively. For Rw = 0.55, the pore volume is larger than the volume of water and so most of the water occupies the pores and CO2 hydrate formation is only seen at 2.60 MPa. Additional sorption isotherms were collected at 277, 279, and 281 K for Rw = 1.55 and are shown in Figure 6. Two inflection points are seen at 275 and 277 K, but they are less pronounced at 277 K. At 279 and 281 K, only one inflection is observed. It is possible that at higher temperatures, the pressure required for

Figure 4. Small angle XRD pattern of CMK-3.

3.2. Sorption of CO2. The CO2 sorption isotherms of CMK-3 at 275 K with different amount of water are shown in Figure 5. The highest pressure was about 3.6 MPa, which is

Figure 5. CO2 sorption isotherms on the CMK-3 sample with different water content. 1, Rw = 0; 2, Rw = 0.55; 3, Rw = 1.55; 4, Rw = 1.75; 5, Rw = 2.35; 6, Rw = 3.55.

lower than the saturation pressure of CO2 at 275 K (3.67 MPa). The CO2 sorption capacity is expressed in mmol CO2 per gram of dry CMK-3 sample. The CO2 sorption isotherm of dry CMK-3 (curve 1) is type IV due to its mesoporosity and the highest sorption capacity was about 18.6 mmol/g at 3.56 MPa. The curve for Rw = 0.55 (curve 2) has a sharp increase in the sorption capacity at 2.6 MPa which is due to the formation of C

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Possibly, the inflection is contributed both gas hydrates and adsorption. 3.4. Molar Ratio of H2O/CO2. CO2 forms a type I hydrate structure in pure water with a water to carbon dioxide molar ratio of H2O/CO2= 5.75.32 The molar ratios of H2O/CO2 calculated from the CO2 sorption capacities at 275 K and at 3.2 MPa are shown in Figure 8, along with the ratios for some

Figure 6. Sorption isotherms of CO2 on CMK-3 with Rw = 1.55. Curves: 1, 275 K; 2, 277 K; 3, 279 K; 4, 281 K.

hydrate formation is increased more for the first inflection and the two formation pressures just overlap. The inflection pressures in Figure 6 were used to evaluate the enthalpy change by using the eq 1. For curve 1 and 2 in Figure 6, the first inflection pressures were used

Figure 8. Molar ratios of H2O to CO2: ■, silica (KIT-6);22 ○, bamboo carbon;25 ▲, coconut shell carbon (BY-1);24 ▽, mesoporous carbon (CMK-3).

ΔH ln f = C − (1) RT where f is the fugacity of carbon dioxide, ΔH is the enthalpy change at the inflection pressure, T is the temperature (K), R is the universal gas constant, and C is a constant. The value for f was calculated from f = pΦ and p z − 1 ln Φ = dp p 0 (2)

other materials. In Figure 8, H2O refers to that loaded in the adsorbent and CO2 refers to the sorption amount at 275 K and at 3.2 MPa. The molar ratios in CMK-3 are in the range of 2.65−3.26 for Rw = 1.55−2.35. These values are much lower than the theoretical value, 5.75. Similar results have been reported for other carbon materials.24,25 The low molar ratios are due to multiple mechanisms for the fixing of CO2 including gas hydrate formation, dissolution in water, and adsorption.22 In contrast, the molar ratio in silica based KIT-6 is much larger than 5.75, which is due to the strong H-bonding between the H2O molecules and the silica hydroxyl groups.22

∫

where p is the inflection pressure, Φ is the fugacity coefficient, and z is the compressibility factor of carbon dioxide. Plots of ln( f) versus 1/T are shown in Figure 7. The enthalpy change was determined from the slope in Figure 7 to

4. CONCLUSIONS The sorption capacity of CO2 in CMK-3 is remarkably enhanced by the preadsorption of water. The largest sorption capacity, 42 mmol/g, was achieved at 275 K and 3.6 M Pa for Rw = 2.35. This is 2.26 times as high as that for dry CMK-3. It is also 23% higher than the highest sorption amount previously reported for activated carbon with preadsorbed water. This significant enhancement in the sorption capacity is attributed to multiple fixing mechanisms, which includes gas hydrate formation, adsorption and dissolution in water.

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AUTHOR INFORMATION

Corresponding Author

Figure 7. Clausius−Clapeyron plots to evaluate the enthalpy change.

*E-mail: [email protected]. Tel.:+86-022-27406301. Fax: +86-022-27406301. Notes

The authors declare no competing financial interest.

be −55.6 kJ/mol. This is in good agreement with the enthalpies for silica gel (−57.4 kJ/mol)32 and bamboo carbon (−54.0 kJ/ mol).25 However, much higher enthalpy values have been reported for the formation of CO2 hydrate in coconut shell carbon (−81.9 kJ/mol)24 and in silica KIT-6 (−97.1 kJ/mol).22 Water content, pore size distribution and surface properties can all affect the enthalpy of hydrate formation.25 It is lower than the formation of hydrates in pure water (−79.2 kJ/mol)24 and higher than the isosteric heats of adsorption on dry carbons.

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ACKNOWLEDGMENTS The support from the National Natural Science Foundation of China (No 21206108; 21406004) and the Tianjin Municipal Science and Technology Commission (No 14JCYBJC21200) is greatly appreciated. W.S. acknowledges the China Scholar Council Scholarship for supporting his visit to Arizona State University. D

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DOI: 10.1021/acs.jced.5b01063 J. Chem. Eng. Data XXXX, XXX, XXX−XXX