CO2 Sorption on Mesoporous Solids: Effect of Temperature and Water

Article pubs.acs.org/jced

CO2 Sorption on Mesoporous Solids: Effect of Temperature and Water Content Nurulhuda Azmi,† Suzana Yusup,*,† and Khalik M. Sabil‡ †

Biomass Processing Lab, Universiti Teknologi Petronas, 32610 Seri Iskandar, Perak, Malaysia Institute of Petroleum Engineering, Heriot-Watt University Malaysia, 62200 Putrajaya, Malaysia

‡

ABSTRACT: CO2 captured by solid adsorbents is considered one of the promising technologies for carbon capture and sequestration. The sorption equilibria of CO2 on commercial CO2 adsorbents (silica gel and Norit SX2) and synthesized CaO were measured experimentally by using a volumetric method in a temperature range of 0−8 °C, at which CO2 hydrate is stable to form. The CO2 uptake increases with decreasing temperature in silica gel and Norit SX2 on a dry basis; differs in CaO as the adsorption rate of CO2 in calciumbased sorbent is high at higher temperatures. The effect of water content on the samples studied was measured at a temperature of 2 °C on a wet basis. The highest CO2 amount adsorbed was obtained at the lowest value of water ratio studied (Rw = 0.03), which is close to CO2 sorption in a dry basis. However, the adsorption capacities on wet synthesized CaO were water-content dependent, of which the highest sorption capacity was obtained at a particular isotherm with a water ratio of 0.67. It can be concluded that CaO has the ability to be further utilized for CO2 separation in the presence of a minute quantity of water.

1. INTRODUCTION CO2 is a very substantial contributor, contributing more than 60% to the greenhouse gases (GHG) effect.1 The growth of CO2 in the atmosphere is the predominant cause of global warming. The atmospheric concentration of CO2 has increased dramatically in the last decades and is expected to continually increase for the next few decades.2 The high CO2 content in the gas streams produced by industry, transportation, and agricultural, etc. needs to be separated and captured in order to meet environmental limits. Carbon capture and sequestration (CCS) is a technology that is being explored to deal with capture, possibly utilizing and if necessary storing the CO2 from large stationary sources. Capturing CO2 is the first step in CCS and prevents large quantities of CO2 from being released into the atmosphere. Various methods have been employed for capturing CO2 such as chemical solvents, absorption, physical adsorption, chemical adsorption, membrane technology, and cryogenic process.3 However, most of the techniques rely on proven small-scale, and scaling-up of these processes are normally accompanied by increments of operational issues and cost.4 The cost for capturing CO2 is generally estimated to represent 70−80% of the total cost of a full CCS system.5 It is crucial to have a process that increases the efficiency of the capture system while reducing overall cost for creating a feasible GHG control and implementation plan.6 A good separation technique should have high separation efficiency, low or zero chemical solvent used, low operational cost and be able to cater for bulk amount of the gas stream to ensure the efficient separation of CO2 from the gas streams. Significant R&D © XXXX American Chemical Society

efforts are focused on the new efficient separation technology with low operating cost and energy consumption.1 Separation by adsorption is based on the selective accumulation of one or more components of a gas mixture on a solid surface (adsorbent). When gaseous mixtures are exposed to an adsorbent for sufficient time, equilibrium is established between the adsorbate (adsorbed material) and the gas phase. The gas phase becomes richer in the less selectively adsorbed component. The attractive force responsible for the adsorption is a van der Waals type force. In the adsorption process, solid adsorbents such as zeolites and activated carbon allow the gas to accumulate on their surface forming a film of molecules or atoms due to molecular attraction to the solid surface. The process depends upon the intermolecular forces between CO2 and the adsorbent. However, the concerns over this technology are scale up and the need to develop CO2 specific adsorbent materials. Many porous solid adsorbents are limited by low selectivity, and their performance is impaired in the presence of water because the adsorbents have an affinity stronger to water than to CO2, and hence CO2 is displaced as a result.7,8 The hydrate-based CO2 captures technology has been widely explored as this technique might offer a couple of advantages. The main process solvent required for hydrate-based technology is water, which provides the process with abundant Received: August 27, 2016 Accepted: November 22, 2016

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potential biomass source for calcium carbonate (CaCO3), since the process of mining large quantities of raw materials such as limestone results in extensive deforestation and top soil loss.23 Li et al. have reported that CaO composition in seashell is higher than in limestone.24 As waste cockle shell (CS) is abundantly available in Malaysia, the utilization of this calcium based as CO2 adsorbent is expanded in this work. The CaO can be extracted via a decomposition process from cockle shell as it contains a large amount of CaCO3. The process is also known as calcination in which heat is used to decompose the material and cause volatile removal or phase transition which is known as an endothermic reaction. Calcination can be carried out in lime kiln, furnace, incinerator tube furnace, thermal gravimetric analyzer (TGA), fluidized bed reactor, and fixed bed reactor. The development of an adsorption-based process requires basic adsorption equilibria data across a wide range of pressures and temperatures. There are many studies on the adsorption of gases by adsorbents, but high pressure adsorption is rarely measured. As the present work focuses on the hydrate formation on wetted porous media, it is necessary to understand the fundamentals of adsorption equilibria of a single gases on studied adsorbents at relatively low temperatures and high pressures. The objective of this study is to measure high pressure adsorption equilibria of CO2 on mesoporous solids: silica gel, activated charcoal Norit SX2, and synthesized CaO with presence of water at temperature of 2 °C. The efficiency of the process is measure by comparing the CO2 sorption equilibria on wetted and dry samples.

(cheap) and green raw chemical. The pressure and temperature required for hydrate formation is relatively mild (15−30 bar and 273.36 K to 278.7 K) possibly contributing to a lower operational cost for the separation process. The preferential formation of CO2 hydrate over other fossil fuel conversion effluent gases could be used as a method to capture CO2. The CO2 hydrate could later be dissociated, producing a pure stream of CO2. However,the agglomeration of hydrate crystals creates a barrier to efficient gas/water contact in the crystallizers, and as a result the rate of crystallization decreases and the conversion of water and gas to hydrate is limited.9−11 Initially the gas hydrate that forms at the gas/water surface resists the further contact of gas and water which leads to a slow formation rate as further formation of hydrate has been hindered. Besides, large amounts of power are required for mechanical agitation for capturing CO2 by hydrate crystallization in bulk water.12 One feasible approach to overcome the contact limitation between gas and water is by allowing the gas phase with water dispersed in the porous materials for formation of hydrate within the pores.11,13−15 The porous structure can increase the rate of hydrate formation due to the increase of local CO2 supersaturation sites in the porous media and decrease in the induction time needed for hydrate formation, thus increasing the amount of CO2 separated from the gas stream. The addition of water on porous media creates plenty of voids among and inside the porous media particles, which provide an efficient contact between water and gas. The water-to-hydrate conversion of pore-dispersed water is almost 4-fold greater than of bulk water.11 Power consumption due to stirring in hydratebased technology can be reduced by allowing gas hydrate to form inside the porous materials8c. To have the successful combination effect of hydrate formation and adsorption of CO2, the porous materials used must be wetted with water. It is crucial to determine the best type of porous materials for maximum CO2 separation, where the selected porous material should have a tendency to be saturated with water and perform at operating conditions of gas hydrate formation. It is expected that the combined hydrate and adsorption process through the use of wetted porous materials is a way forward for the capture of CO2 in industrial gas streams especially for high CO2 concentration streams. The success of the suggested approach is dependent on the development of efficient adsorbent materials with high storage capacity and selectivity.16 Both natural and synthetic porous sorbents are used for the separation of CO2 from various gas mixtures. Activated carbon and silica gel are two porous media often used in the recent studies.17−20 The sorption behavior of CO2 and CH4 on activated carbon with a wide pore-size distribution at 275 K in different amounts of water content can be found in ref 19. An inflection pressure at about 1.5−2.0 MPa and at 3 MPa are observed on sorption isotherms of CO2 and CH4 to indicate hydrate formation in the system. The formation of CO2 hydrates was influenced by the quantity of presorbed water and pore size distribution of the silica gel.20 Zheng and his co-workers also stated that a lesser amount of water was used for hydrate formation in silica gel as compared with activated carbon due to the strong H-bonding between the water and the silicon hydroxyl group on the surface.20 Calciumbased sorbents which are rooted in biomass material have been suggested as a potential CO2 adsorbent as it is technically feasible, cost-effective, and advantageous to a certain extent in capturing CO2.21,22 Mehta found that seashell was a reliable

2. EXPERIMENTAL SECTION 2.1. Materials. The commercial silica gel and Norit SX2 were purchased from Sigma-Aldrich and have been used without further treatment, while waste cockle shells were obtained from a stall in the Seri Iskandar area. Once received, the cockle shells were washed with water to remove dirt and oven-dried at 110 °C for 24 h to remove excess moisture. After that, the shells were crushed using a pestle mortar and ground into powder by using an electrical grinder. Then the products were sieved for 10 min in the shaker sieve to ensure the powder was segregated well into various particle size fractions. The particle size fraction of less than 0.1 mm was used, as this size has less pore diffusion resistance, and mass transfer of CO2 through the sample core can be improved.25 In addition, smaller particle size has a wider surface area which influences the adsorption capacity of the gas.26 The calcination process was carried out in tube furnace where 10 g of the powder was placed in a ceramic sample boat and calcined at 850 °C for 40 min in nitrogen atmosphere.27 The adsorption/desorption isotherm of N2 was performed on a Micrometics ASAP 2020 Accelerated Surface Area and Porosimetry instrument by degassing 0.3 g of sample at 623 K for 12 h and analyzed in liquid nitrogen at 77 K. Table 1 summarizes the properties of the porous materials used in this work. 2.2. Sorption Equilibrium in Dry and Wet Materials. The CO2 sorption isotherms were measured by using a static volumetric method in which the measurements were performed by using a static high pressure volumetric analyzer (HPVA II) from Particulate System. In the present work, CO2 sorption on silica gel, Norit SX2, and synthesized CaO was measured at temperatures of 0−8 °C with 2 °C increments. Prior to CO2 adsorption, 300−400 mg samples were placed inside a 2 cm3 sample chamber and inserted into a furnace for the degassing process. A filter gasket of size 60 μm was placed on top of the B

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expressed as Rw; the weight ratio of water to sample. Since each isotherm starts at a pressure reading of zero, it is necessary to place the system in a vacuum.28 The wet sample was cooled to −20 °C to minimize the loss of water due to vaporization. The temperature of the chamber was adjusted to the adsorption temperature of interest before sorption measurement was started.18 In the present work, the effect of water contents on CO2 adsorption were measured at 2 °C.

Table 1. Summary of Physisorption Analysis for Samples with Particle Size Less than 0.1 mm sample silica gel Norit SX2 synthesized CaO

BET surface area (m2/g)

pore volume (cm3 /g)

average pore width (Å)

483.16 605.18 8.79

76 × 10−2 64.07 × 10−2 1.72 × 10−2

63.32 42.35 78.16

3. RESULTS AND DISCUSSION 3.1. CO2 Sorption Isotherms on Dry Samples. Figure 1 shows the effect of temperatures on CO2 sorption equilibria of (a) silica gel, (b) Norit SX2, and (c) synthesized CaO. The pressure limit was dependent on the temperature studied where hydrate is stable to form. It is important to ensure that the pressure dosed is lower than the saturation pressure to avoid CO2 from condensation at high pressure. The amount of gas adsorbed in a porous material increases with increasing relative pressure, and those observations are similar to Horikawa et al.29 It is because more CO2 molecules interact with a particular surface at a higher pressure, and the increment amount of collisions with the surface result in an increment in the adsorption process. The adsorbed amount of CO2 does not stop increasing until the saturation pressure due to the condensation of CO2 in the micro- or mesopores.19 For silica gel and Norit SX2, the magnitude of adsorption increases with a decrease of temperature as illustrated in Figure 1 panels a and b, respectively. The phenomenon is observed as CO2 molecules are attached to the carbon surface via weak van der Waals forces and contribute to physisorption that favorably takes place at lower temperature.30 The total amount adsorbed could be predicted with standard deviations of about ±0.8 for both samples The highest CO2 uptakes at a temperature of 0 °C are 15.14 and 8.41 mmol/g for silica gel and Norit SX2, respectively. The surface area is one of the factors that may influence the CO2 adsorption equilibrium as the isotherm

sample chamber to prevent the fine samples from entering the valves. The samples were evacuated at temperatures of 200 °C for synthesized CaO and 120 °C for silica gel and Norit SX2 under a vacuum condition for overnight to remove any impurities retained inside the samples. After the degassing process was completed, the sample chamber was moved to the analysis port for the adsorption measurement. The experiment for CO2 sorption began by purging the line with helium for three times. Helium was also used to measure the volume of free space in the sample chamber before the experiment was run. The adsorption was started by dosing CO2 to the system and the valve between the loading and sample cell was opened to allow the gas to contact with the samples. During adsorption, the holding time for each pressure interval was set at 45 min to ensure an equilibrium state was achieved. Final equilibrium pressure and quantity of gas adsorbed was recorded when the sample reached equilibrium with CO2. This process was repeated at given pressure intervals until the maximum preselected pressure was reached. After the experimental work was completed, the weight of dry samples was recalculated. Each of the resulting equilibrium points (volume adsorbed and equilibrium pressure) was plotted to provide an isotherm. The amount of CO2 adsorbed in the materials was expressed in mmol CO2 on the basis of per gram of dry sample. While for CO2 adsorption onto wet samples, the degassing process was skipped in order to avoid removal of water vapor from the wet sample. Water content in a wet sample was

Figure 1. CO2 sorption isotherms on (a) silica gel, (b) Norit SX and (c) synthesized CaO: ●, 0 °C; C

▲,

2 °C; ■, 4 °C; ◆, 6 °C; -, 8 °C. DOI: 10.1021/acs.jced.6b00760 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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follows the order of increasing capacity with increasing surface area.31 However, Norit SX2 demonstrates unexpectedly low CO2 capacity considering its high surface area (see Table 1). Yet at low P, it presents higher adsorption loadings compared to silica gel. With an increase in pressure, its capacity does not increase significantly and is much less than that of silica gel at about 23 bar due to low monolayer capacity.31 As illustrated in Figure 1c the opposite trend is observed in the CO2 isotherm on synthesized CaO as adsorption rate of CO2 in calcium based sorbent is high at higher temperature. This type of adsorption process can be classified as chemisorption in which there is a reaction for CaO to adsorb CO2 and yield CaCO3. It requires an activation energy in chemisorption, in which the adsorption capacity first increases with increasing of temperature. Basically, the synthesized CaO has lower adsorption capacity as compared to commercial sorbents at any studied temperatures. The phenomenon is observed due to the lower BET surface area and pore volume of the former adsorbent (see Table 1). It implies that there is limited area for adsorption process to take place. There are different types of isotherms that can be observed in the studied materials. According to the IUPAC classification, the isotherm shows the typical feature of type IV form for silica gel. The phenomenon is observed to indicate CO2 is condensable in the mesopores of silica gel when the relative pressure reaches the value given by the Kelvin equation.20 For Norit SX2, the type II isotherm is obtained to represent a monolayer−multilayer adsorption, and the completion of the monolayer is followed by successive unlimited multilayer condensation,19 while a type III isotherm is observed in synthesized CaO due to a weak and nonspecific interaction between the adsorbent and adsorbate, as compared to adsorbate−adsorbate interaction. Figure 1c shows a convex and the looking upward curve which implies a gradual increase in the volume of gas adsorbed with the relative pressure.32 3.2. CO2 Sorption Isotherms on Wet Samples at 2 °C. Figure 2 panels a and b show the difference of CO2 uptakes compared to dry silica gel and Norit SX2 for different amounts of water ratio at a temperature of 2 °C. As illustrated in both panels, only a water ratio of 0.03 obviously shows a positive curve to indicate the increment of CO2 uptake as compared to the dry samples. The addition of more water on the studied sorbents gives no significant impact on CO2 uptake as it decreases with increasing of Rw value. The pores of the sorbents are blocked when there is too much water loading and hinder the CO2 adsorption from happening. The silica gel with molecular formula SiO2 has strong interactions between the water molecules, and the silicon hydroxyl groups on its surface prevent any water molecules close to the surface from being used in the formation of the CO2 hydrates.20 Regardless of the structure, type, and composition, mesoporous materials usually have plenty of silanol groups because of their amorphous surface structure. A large amount of water can be adsorbed on the studied sorbents as capillary condensation apparently occurs on mesoporous solids.33 The formation of CO2 hydrates was influenced by the quantity of presorbed water and pore size distribution of the silica gel.20 Only the pores with a definite size are suitable for hydrate formation as the impetus of pore space for hydrate formation disappears if the pore size is too large.19,20 As silica gel used in the present work has a larger pore size (mesoporous), it could be the reason for no CO2 hydrate formation on the wet surface of the silica gel. The same

Figure 2. Difference of CO2 uptakes compared to (a) dry silica gel and (b) Norit SX2 for different amounts of water at 2 °C.

explanation can be further described for the CO2 sorption isotherm on Norit SX2 with the presence of water in Figure 2b. An inflection pressure is shown where the sorbed amount increases to a higher level to indicate the beginning of hydrate formation.20 The amount of CO2 uptake increases as the pressure increases until the hydrate formation is complete. This phenomenon is observed in porous material as water activity is depressed by partial ordering and bonding of water molecules with hydrophilic pore surfaces.34 The CO2 hydrate starts to form at a pressure of 15.7 bar at 2 °C in the bulk state, and the equilibrium line is shifted to a higher pressure in porous media.35 The inflection pressures are observed at around 20 bar with Rw = 0.03 for both samples to indicate the formation of CO2 hydrate. On the other hand, the inflection pressures are not observed at Rw more than 0.03 as an excess amount of water hinders the formation of gas hydrate. Figure 3 panels a and b show the difference of CO2 uptakes compared to that of dry synthesized CaO with water ratio ranges of Rw < 1 and Rw > 1 at temperatures of 2 °C. As illustrated in both figures, the adsorption capacities of the wet sample are water-content dependence. There is a large impact on CO2 uptake in the presence of water with the highest amount obtained being at Rw = 0.67. The improvement may be due to the remarkable changes in the sorbent’s texture during the addition of some amount of water.36 However, the amount of CO2 adsorbed decreases as water content increases beyond the optimum value (Rw = 0.67). The phenomenon is observed D

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sorption amount is due to the higher solubility of CO2 in water. As the pores are filled by the water, Ca(OH)2 is produced and only the external surface is exposed to the adsorbing gas, CO2. Wang et al. have reported that CaO particles transform into small Ca(OH)2 particles, and a part of the Ca(OH)2 dissolves with the pore structure becoming loose.38 The saturation is marked by a plateau on the isotherm to indicate all pores have been filled with hydrates of/or condensed CO2 occurring before maximum pressure is reached. The effect of water content on CO2 uptake at various operating pressures is illustrated in Figure 4. The highest CO2

Figure 4. Effect of Rw on CO2 sorption amounts at various operating pressures for synthesized CaO at 2 °C. Figure 3. Difference of CO2 uptakes compared to dry synthesized CaO for different amount of water (a) Rw < 1 and (b) Rw > 1 at 2 °C.

uptake (11.03 mmol/g) is reached for Rw = 0.67, which is 9.4 higher than that in dry synthesized CaO (Rw = 0) for pressure at 36 bar. At this point, about 15.38 mmol of water has been used per gram of synthesized CaO. The familiar trend is observed for a pressure of 20 bar. Although more water loading leads to a higher CO2 sorption amount, too much water may plug up the passage and lead to fewer hydrates formed as in the case for Rw > 0.73. During the process, inhibition of CaCO3 in small pores occurs and more CO2 molecules being adsorbed on the surface of the sample is avoided. At 5 bar, the trend is slightly different than with the others two pressures. The phenomenon is observed as, at low temperature, greater reduced pressure is required to form a sufficient number of clusters of a certain size in order to facilitate the adsorption of water clusters in the mesopores.39 It is clear that excessive water does not have a big impact on the amount adsorbed, but an appropriate quantity of water added into synthesized CaO greatly enhances the CO2 adsorption. Table 2 shows the comparison of adsorption capacities and Rw value from this work with those values previously published. The CO2 uptake is comparable with other porous materials, but the Rw values are too much different. As reported by other studies, the sorption capacity increases with an increase of Rw value. However, the opposite trend has been observed in this work due to physical type properties of the solid. The samples studied have been classified as mesoporous material as the average pore diameters fall in the range of 20−500 Å (see Table 1). As CO2 molecules have a smaller molecular diameter which is 3.34 Å, the large pore width gives plenty of space to accommodate the molecules into them. Thus, excessive water will act as a trap for the penetrating water molecules, which are retained within the pores.

as excess water may limit the reaction due to the blockage of the pores in the solid.37 Less amount of CO2 can be adsorbed, and its condensation may lead to formation of Ca(OH)2. The hydrate formation pressure at around 25 bar is much higher than that of CO2 hydrate in pure water (15.7 bar) and slightly different from commercial adsorbents as mentioned earlier. The inflections could be observed at Rw = 0.54, 0.67, and 0.73 to indicate formation of CO2 hydrate. However, the inflections are not observed for Rw less than 0.54 due to insufficient amount of preadsorbed water to trap CO 2 molecules to form CO2 hydrate. Zhou et al. have reported that the inner diffusion resistance in pores and the related percolation within the pore structure causes the difference between the inflection pressure and the genuine formation pressure of hydrate.28 The same behavior can be observed at Rw higher than 0.73 as an excess amount of water hinders the formation of gas hydrate. Thus, it is believed water vapor hardly influenced the CO2 capture performance. The amount of CO2 uptakes starts to decrease at a pressure around 25 bar as the amount of presorbed water becomes larger than the dry sample (Rw > 1) as shown in Figure 3b. The phenomenon is observed as both the interior and exterior pore spaces between the sorbent particles have totally been occupied by extra water and prevent CO2 from getting into the pore spaces to form hydrates. The amount sorbed before the inflection pressure is quite large as compared with that of the dry sample. The CO2 could condense in small pores at low pressure and react with water to form HCO3−, which leads to more CO2 fixed before the inflection pressure.19 It is also believed that the higher E

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Table 2. CO2 Sorption in Other Porous Materials at 2 °C CO2 uptake in dry (mmol/g)

CO2 uptake in wet (mmol/g)

Rw

20.0

36.0

1.65

18

26.0 7.5 14.0 6.88 15.19

39.0 2.5 3.2 12.80 17.73

1.80 0.75 0.81 2.48 0.03

Norit SX2

8.53

9.88

0.03

synthesized CaO

1.24

11.03

0.67

19 20 20 8 this work this work this work

adsorbent activated carbon BY-1 bamboo chips silica gel SG-A silica gel SG-B silica KIT-6 silica gel

Separation using Hydration Method. Renewable Sustainable Energy Rev. 2016, 53, 1273−1302. (4) Herzog, H. J. Scaling up carbon dioxide capture and storage: From megatons to gigatons. Energy Econ. 2011, 33, 597−604. (5) Leung, D. Y. C.; Caramanna, G.; Maroto-Valer, M. M. An Overview of Current Status of Carbon Dioxide Capture and Storage Technologies. Renewable Sustainable Energy Rev. 2014, 39, 426−443. (6) Figueroa, J. D.; Fout, T.; Plasynski, S.; McIlvried, H.; Srivastava, R. D. Advances in CO2 Capture TechnologyThe U.S. Department of Energy’s Carbon Sequestration Program. Int. J. Greenhouse Gas Control 2008, 2, 9−20. (7) Choi, S.; Drese, J. H.; Jones, C. W. Adsorbent Materials for Carbon Dioxide Capture from Large Anthropogenic Point Sources. ChemSusChem 2009, 2, 796−854. (8) Zhang, Z.; Wang, H.; Chen, X.; Xie, R.; Gao, P.; Wei, W.; Sun, Y. CO2 Sorption in Wet Ordered Mesoporous Silica Kit-6: Effects of Water Content and Mechanism on Enhanced Sorption Capacity. Adsorption 2014, 20, 883−888. (9) Lee, J. D.; Susilo, R.; Englezos, P. Kinetics of Structure H Gas Hydrate. Energy Fuels 2005, 19, 1008−1015. (10) Linga, P.; Kumar, R.; Englezos, P. Gas Hydrate Formation from Hydrogen/Carbon Dioxide and Nitrogen/Carbon Dioxide Gas Mixtures. Chem. Eng. Sci. 2007, 62, 4268−4276. (11) Adeyemo, A.; Kumar, R.; Linga, P.; Ripmeester, J.; Englezos, P. Capture of Carbon Dioxide from Flue or Fuel Gas Mixtures by Clathrate Crystallization in a Silica Gel Column. Int. J. Greenhouse Gas Control 2010, 4, 478−485. (12) Linga, P.; Kumar, R.; Lee, J. D.; Ripmeester, J.; Englezos, P. A New Apparatus to Enhance the Rate of Gas Hydrate Formation: Application to Capture of Carbon Dioxide. Int. J. Greenhouse Gas Control 2010, 4, 630−637. (13) Seo, Y.-T.; Moudrakovski, I. L.; Ripmeester, J. A.; Lee, J.-w.; Lee, H. Efficient Recovery of CO2 from Flue Gas by Clathrate Hydrate Formation in Porous Silica Gels. Environ. Sci. Technol. 2005, 39, 2315− 2319. (14) Park, J.; Seo, Y.-T.; Lee, J.-w.; Lee, H. Spectroscopic Analysis of Carbon Dioxide and Nitrogen Mixed Gas Hydrates in Silica Gel for CO2 Separation. Catal. Today 2006, 115, 279−282. (15) Kang, S.-P.; Lee, J.-W.; Ryu, H.-J. Phase Behavior of Methane and Carbon Dioxide Hydrates in Meso- and Macro-Sized Porous Media. Fluid Phase Equilib. 2008, 274, 68−72. (16) Lu, X.; Jin, D.; Wei, S.; Wang, Z.; An, C.; Guo, W. Strategies to Enhance CO2 Capture and Separation Based on Engineering Absorbent Materials. J. Mater. Chem. A 2015, 3, 12118−12132. (17) Anderson, R.; Llamedo, M.; Tohidi, B.; Burgass, R. W. Experimental Measurement of Methane and Carbon Dioxide Clathrate Hydrate Equilibria in Mesoporous Silica. J. Phys. Chem. B 2003, 107, 3507−3514. (18) Sun, Y.; Wang, Y.; Zhang, Y.; Zhou, Y.; Zhou, L. CO2 Sorption in Activated Carbon in the Presence of Water. Chem. Phys. Lett. 2007, 437, 14−16. (19) Wang, Y.; Zhou, Y.; Liu, C.; Zhou, L. Comparative Studies of CO2 and CH4 Sorption on Activated Carbon in Presence of Water. Colloids Surf., A 2008, 322, 14−18. (20) Zheng, J.; Zhou, Y.; Zhi, Y.; Su, W.; Sun, Y. Sorption Equilibria of CO2 on Silica-Gels in the Presence of Water. Adsorption 2012, 18, 121−126. (21) Nakatani, N.; Takamori, H.; Takeda, K.; Sakugawa, H. Transesterification of Soybean Oil using Combusted Oyster Shell Waste as Catalyst. Bioresour. Technol. 2009, 100, 1510−1513. (22) Wang, K.; Guo, X.; Zhao, P.; Zhang, L.; Zheng, C. CO2 Capture of Limestone Modified by Hydration−Dehydration Technology for Carbonation/Calcination looping. Chem. Eng. J. 2011, 173, 158−163. (23) Mehta, P. K. Reducing the Environmental Impact of Concrete. Concr. Int. 2001, 23, 6. (24) Li, Y. J.; Zhao, C. S.; Chen, H. C.; Duan, L. B.; Chen, X. P. CO2 Capture Behavior of Shell during Calcination/Carbonation Cycles. Chem. Eng. Technol. 2009, 32, 1176−1182.

ref

4. CONCLUSIONS The conversion of waste cockle shell to CaO could not only reduce the preparation cost of the adsorbent but also contribute to environmental protection. Considerable difference is observed in the CO2 sorption behavior on commerical adsorbents and synthesized CaO at a temperature of hydrate formation. The sorption isotherm of CO2 in wet silica gel and Norit SX2 shows no effect as compared to that in wet synthesized CaO. The surface characteristics and the pore size are thus the determining factors affecting adsorption equilibrium. The CO2 uptake rapidly increases by allowing gas hydrate to form in pore spaces of porous material, and the highest CO2 uptake is obtained at a water ratio of 0.67 for synthesized CaO. It was found that the physical properties of porous materials used have a significant effect on the amount of water used for hdyrate formation over the proposed process. A less amount of water, as compared with reported values in the literature, is required in mesoporous materials because ample spaces are accessible for trapping CO2 molecules within the pores. The overall experimental results are very helpful in choosing a good adsorbent for CO2 sorption in the presence of water.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +605-368 7642. Fax: +605-368 8205. ORCID

Suzana Yusup: 0000-0001-8396-4320 Funding

The financial support from the Exploratory Research Grant Scheme (ERGS) funded by Ministry of Higher Education (MOHE) is greatly appreciated. Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acs.jced.6b00760 J. Chem. Eng. Data XXXX, XXX, XXX−XXX