Environ. Sci. Technol. 2010, 44, 1820–1826
Adsorption of CO2, CH4, N2O, and N2 on MOF-5, MOF-177, and Zeolite 5A DIPENDU SAHA, ZONGBI BAO,† FENG JIA, AND SHUGUANG DENG* Chemical Engineering Department, New Mexico State University, Las Cruces, New Mexico 88003
Received October 25, 2009. Revised manuscript received January 6, 2010. Accepted January 26, 2010.
Adsorption equilibrium and kinetics of CO2, CH4, N2O, and N2 on two newly discovered adsorbents, metal-organic frameworks MOF-5 and MOF-177 and one traditional adsorbent, zeolite 5A were determined to assess their efficacy for CO2, CH4, and N2O removal from air and separation of CO2 from CH4 in pressure swing adsorption processes. Adsorption equilibrium and kinetics data for CO2, CH4, N2O, and N2 on all three adsorbents were measured volumetrically at 298K and gas pressures up to 800 Torr. Adsorption equilibrium capacities of CO2 and CH4 on all three adsorbents were determined gravimetrically at 298 K and elevated pressures (14 bar for CO2 and 100 bar for CH4). The Henry’s law and Langmuir adsorption equilibrium models were applied to correlate the adsorption isotherms, and a classical micropore diffusion model was used to analyze the adsorption kinetic data. The adsorption equilibrium selectivity was calculated from the ratio of Henry’s constants, and the adsorbent selection parameter for pressure swing adsorption processes were determined by combining the equilibrium selectivity and working capacity ratio. Based on the selectivity and adsorbent selection parameter results, zeolite 5A is a better adsorbent for removing CO2 and N2O from air and separation of CO2 from CH4, whereas MOF-177 is the adsorbent of choice for removing CH4 from air. However, both MOF adsorbents have larger adsorption capacities for CO2 and CH4 than zeolite 5A at elevated pressures, suggesting MOF-5 and MOF-177 are better adsorbents for CO2 and CH4 storage. The CH4 adsorption capacity of 22 wt.% on MOF-177 at 298K and 100 bar is probably the largest adsorption uptake of CH4 on any dry adsorbents. The average diffusivity of CO2, CH4 and N2O in MOF-5 and MOF177 is in the order of 10-9 m2/s, as compared to 10-11 m2/s for CO2, CH4 and N2O in zeolite 5A. The effects of gas pressure on diffusivity for different adsorabte-adsorbent systems were also investigated.
Introduction Removal of CO2, CH4, and N2O from air and separation of CO2 from CH4 are important separation processes in energy production and environmental protection. These separations can be achieved in pressure swing adsorption processes if suitable adsorbents with sufficiently large selectivity and adsorption capacity can be identified. * Corresponding author phone: 575-646-4346; fax: 575-646-7706; e-mail:
[email protected]. † Permanent address for Dr. Zongbi Bao: Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, P. R. China. 1820
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Evaluation of various adsorbents for their equilibrium and kinetic properties is an effective way to screen suitable adsorbents for these applications; it also contributes to better understanding of fundamentals of adsorption processes. With the incessant increase in world population, advancement of industrialization and progresses of technologies, use of fossil fuels has generated increasing amounts of greenhouse gases that possess a serious threat to the environment through the global warming effect. Carbon dioxide (CO2) is the main source of greenhouse gas that contributes to 60% of the global warming effects (1). It was estimated that 82.4% of total CO2 was released from thermal power plants (2) and the major portions of the remaining fraction was contributed by automobiles. In order to limit the CO2 level in the atmosphere, U.S. Department of Energy has developed a roadmap that requires all fossil fuel utilization facilities must remove 99% of CO2 at less than 10% increase in energy services by 2012. CO2 emissions to the atmosphere can be decreased in three ways: energy intensity reduction, carbon intensity reduction, and carbon capture and sequestration (3). Among these options, carbon capture and sequestration is probably the most feasible solution in the long run (3, 4). However, the high cost and low efficiency of the separation media (adsorbents, membranes and etc.) are the main challenges to implement this technology at present (5). The commonly used techniques for CO2 separation from flue gases include ammonium absorption process (6, 7), dual-alkali absorption (8), membrane separation process (9, 10), and adsorption on solid adsorbents (11-20). Adsorption and storage of CO2 in various nanoporous adsorbents have gained increasing interests recently and many of the several research papers were published to report improved CO2 adsorption capacity. Przepiorski et al. (11) used NH3-treated CWZ-35 activated carbon to adsorb CO2 and obtained a capacity of 76 mg/g. Kim et al. (12) demonstrated the CO2 adsorption capacity on an amine-treated mesoporous silica to be 1.79 mmol/g at room temperature. Drage et al. (13) reported 3.86 wt.% of CO2 uptake by chemically activated urea-formaldehyde and melamine-formaldehyde resins. Xu et al. and Song et al. (14, 15) impregnated MCM-41 silica adsorbent with polyethylamine (PEI) and obtained CO2 capacity of 246 mg/g. Fauth et al. (16), Essaki et al. (17) and Kato et al. (18) evaluated some lithium-based adsorbents including lithium zirconate and lithium silicate for CO2 separation with a temperature swing approach. Yaghi et al. (19) measured CO2 adsorption on various Zn-based metal-organic frameworks and found that MOF-177 can adsorb 35 mmol/g of CO2 at 45 bar and room temperature. Despite contributing less toward the global warming, methane (CH4) and nitrous oxide (N2O) possess stronger effect as greenhouse agents as per unit mass basis (3). Himeno et al. (20) performed CH4 adsorption on various kinds of commercially available activated carbon and reported a methane adsorption uptake of 10 mol/kg at 3000 kPa and 273K. Lee et al. (21) tested phenol-based activated carbons for CH4 adsorption and obtained an adsorption amount of 8.055 mmol/g at 35.64 bar and 193.15K. Zhou et al. (22) investigated the adsorption of CH4 on dry and water-loaded multiwalled carbon nanotube and reported a CH4 uptake of 8 wt % at 10 MPa and 275 K. A much higher methane adsorption capacity of 30 wt.% on an activated carbon preloaded with water was also obtained by the same research group at 10 MPa and 277 10.1021/es9032309
2010 American Chemical Society
Published on Web 02/09/2010
K (23). Cavenati et al. (24) reported the adsorption equilibrium capacity of CH4 on zeolite 13X of 5.719 mol/ kg at 4.725 MPa and 298K. The main applications of removing CH4 from air include air purification in coal mining and separation of CH4 in biogas generated in waste disposal and biomass fermentation sites. Nitrous oxide is reported to be a 150-times stronger greenhouse gas than CO2 (25, 26), it is also considered as ozone depleting substance. The main sources of N2O release to the atmosphere are from nitric acid and adiptic acid production facilities (27). The key technique for N2O abatement from the tail gases is to decompose or reduce it by suitable catalysts that include zirconia (28), platinum (29, 30), R-manganese sesquioxide (31), and Fe-ZSM-5 zeolite (32-38). Recently, it was shown that bimetallic FER catalysts containing iron and ruthenium increase the catalytic activity (39). Catalytic decomposition of N2O typically occurs at high temperatures and cannot recover N2O as a valuable intermediate for the production of other fine chemicals (40, 41). N2O adsorption on silicalite-1 was performed by Groen et al. who reported a N2O uptake of 2.5 mol/kg at 273 K (42). Adsorption of N2O was also measured in certain type of pseudomorphs by Lamb and West (43). Recently developed numerous kinds of zinc-based metal-organic frameworks (MOF) are considered to be ideal adsorbents owing to their very high specific surface area, tunable pore size and large accessible pore volume (44-48). We have studied hydrogen adsorption equilibrium and kinetics on MOF-5 and MOF-177 at various conditions, evaluated their structural stability, and demonstrated these two MOF adsorbents to be promising adsorbents for hydrogen storage (49-51), and would like to explore the feasibility of removing the greenhouse gases CO2, CH4 and N2O from various gas streams by adsorption on these two new adsorbents. The objective of this work is to determine the adsorption equilibrium and kinetics of CO2, CH4, N2O, and N2 on MOF-5, MOF-177, and zeolite 5A; analyze the adsorption data with appropriate adsorption equilibrium and kinetics models; and compare the efficacy of removing CO2, CH4, and N2O from air, and separation of CO2 from CH4 by adsorption on MOF-5, MOF-177 and zeolite 5A. This information will be valuable for selecting appropriate adsorbents for gas separation and purification in a pressure swing adsorption process.
Materials and Methods The zeolite 5A sample evaluated in this work was kindly provided by Mr. Li Shenan of Nanjing Refinery, SINOPEC, China. It was originally developed for separating n-paraffin (C10-C13) from kerosene in a simulated moving bed, and later optimized for oxygen separation/concentration from air in pressure swing adsorption processes. Synthesis of MOF-5 and MOF-177. The MOF-5 and MOF177 samples were synthesized in our laboratory following the synthesis procedures reported in our previous publications (49-52). A brief description of the synthesis procedures is given below. For MOF-5 synthesis, all the chemicals were purchased from Fisher Scientific, and they are of the highest available commercial purity (99+%, except zinc nitrate hexahydrate of 98% purity). 0.832 g of zinc nitrate hexahydrate and 0.176 g of benzene dicarboxylic acid were dissolved in 20 mL of N,N-diethylformamide (DEF) under constant agitation at ambient conditions. The resulting mixture was first degassed thrice using the freeze-pump-thaw method, and then 20 mL reaction vials were filled for crystallization. The capped vials were immediately put in an oven (at 85-90 °C) for crystallization for about 24 h. At the end of the crystal-
lization step, clear golden crystals of MOF-5 emerged from the wall and base of the vials. The MOF-5 crystals were separated from the reaction solution, washed with DEF to remove the unreacted zinc nitrate, and followed by purification in chloroform. The chloroform purification was performed by adding chloroform into 20 mL vials containing the raw MOF-5 crystals. The vials were capped tightly and put back in the oven at 70 °C for another 3 days. Solvent in the vials was replenished with fresh chloroform every day. After the chloroform treatment, the MOF-5 crystals changed from a golden color to transparent. Because MOF-5 crystals are very susceptible to moisture and air, they have to be stored in chloroform or under a vacuum in a Schlenk flask. The synthesis of MOF-177 can be divided into two steps: synthesis of the benzene tribenzoate (BTB) ligand and formation of MOF-177 crystals. The BTB ligand was synthesized in our laboratory following the procedures reported by Furukawa et al. (48). To produce MOF-177, 0.32 g of zinc nitrate hexahydrate and 0.07 g of BTB ligand were dissolved in 20 mL of dimethyl formamide. The mixture was degassed three times using the freeze-pumpthaw method and then stored in a 20 mL reaction vial that was fully filled with the mixture and capped tightly. The vial was then put in an oven at 67 °C for 7 days. At the end of this step, clear and transparent MOF-177 formed and became visible in the wall as well as on the base of the vial. The vial was then removed from the oven, decanted, and washed with dimethyl formamide to remove the unreacted zinc nitrate. The raw MOF-177 crystals were then purified by the chloroform treatment process employed for MOF-5 crystal purification and stored in chloroform or in a Schlenk flask under vacuum because MOF 177 is susceptible to humid air. Material Characterization. MOF-5, MOF-177, and zeolite 5A samples were examined for their phase structure by powder X-ray diffraction (XRD) using a Rigaku Miniflex-II X-ray diffractometer with CuKR emission, 30 kV/15 mA current and kβ-filter. To examine the crystal structure and size of the adsorbents, all three adsorbent samples were analyzed with a table-top scanning electron microscope (Hitachi TM-1000). The adsorbents samples were also characterized for their pore textural properties with a Micromeritics ASAP 2020 adsorption apparatus at 77K. The XRD patterns and SEM images of MOF-5, MOF-177 and zeolite 5A can be found in Supporting Information (SI) Figures S1 and S2. Adsorption Measurements. Adsorption equilibrium and kinetics of CO2, CH4, N2O, and N2 on the adsorbent samples were measured volumetrically in a Micromeritics ASAP 2020 adsorption apparatus at 298 K and gas pressures up to 800 mmHg. The adsorabte gas was introduced into the adsorption system at a given pressure, and the changes of gas pressure with time were recorded and converted into the transient adsorption amount as a function of time. The transient adsorption uptakes generated the adsorption kinetics, and the final adsorption amount at the terminal pressure determined the adsorption equilibrium amount at a given pressure. Adsorption of CO2 and CH4 at elevated pressures was performed gravimetrically in a Rubotherm magnetic suspension balance at 298 K and pressures up 14 bar for CO2 and 100 bar for CH4. The high pressure was achieved by employing a compressor that is capable of compressing the adsorbate gas from the cylinder pressure to an elevated pressure level. Like all other gravimetric devices, this balance was also pre-examined with the blank run of empty balance and volume run of sample loaded balance in order to measure the weight and volume of empty sample holder and sample itself before introducing the particular gas of interest. VOL. 44, NO. 5, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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The detailed operation procedures for the Rubotherm magnetic suspension balance were described in our previous publications (50, 51, 53). Adsorption Theories. Isotherm Models. In order to evaluate the adsorption equilibrium selectivity and predict adsorption of gas mixture from pure component isotherms, the Henry’s law linear isotherm equation and the Langmuir model were used to correlate the N2 adsorption on MOF-5 and MOF-177. The Henry’s isotherm equation is
The diffusion time constants (Dc/rc2, s-1) were calculated from the slope of a linear plot of ln(1 - (mt/m∞)) versus t (time) at a given pressure. Only data points with (mt/m∞) greater than 70% and less than 99% were employed for estimating the diffusion time constants. The intracrystalline diffusivity (Dc) of CO2, CH4, and N2O was calculated by multiplying the diffusion time constant with rc2 values.
q ) KP
Adsorption Isotherms at Pressure up to 800 Torr. Adsorption and desorption equilibrium isotherms of CO2, CH4, and N2O, and adsorption isotherms of N2 on all three adsorbents at 298 K are plotted in Figure 1(a-d), respectively. The adsorption and desorption isotherms shown in these plots basically follow the same path, suggesting that the adsorption process is reversible and the adsorbed molecules can be recovered during desorption process, if necessary. It can be observed from the isotherm plots that zeolite 5A has the strongest affinity to all gases studied. CO2 uptake on zeolite 5A at 298 K and 800 Torr is about 20.8 wt.%, which is significantly higher than the adsorption capacity of several amine-treated adsorbents (11-13). Only the polyethylamine (PEI) treated MCM-41 adsorbent was reported to have a higher CO2 adsorption capacity, but the reversibility of the desorption was not investigated (14, 15). Similar to CO2 adsorption, adsorption capacity of N2O on zeolite 5A is 17.8 wt.% at 298 K and 800 Torr. This adsorption uptake is twice the amount reported in previous works on adsorptive removal of nitrous oxide (42, 43). Unlike CO2 and N2O adsorption, adsorption of CH4 and N2 on zeolite 5A is on the lower side, 1.35 wt.% of CH4 and 1.58 wt.% of N2 at 298 K and 800 Torr although zeolite 5A has the highest CH4 or N2 adsorption capacity among the three adsorbents evaluated in this work. Therefore, it can be concluded that zeolite 5A can preferentially adsorb CO2 or N2O over CH4 and N2. Between the two MOF adsorbents, MOF-177 exhibits higher adsorption capacities than MOF-5 for CO2 and CH4 adsorption, but MOF-5 adsorbs more N2O than MOF-177 at similar conditions. N2 adsorption isotherms on all three adsorbents were also measured to assist us to evaluate the adsorption selectivity and adsorbent selection. This information will allow us to compare adsorbent efficacy for removing CO2, CH4, N2O from air and separating CO 2 from CH 4 and other applications. Henry’s law was used to correlate the N2 isotherms on MOF-5 and MOF-177 because these two isotherms are basically linear, and Langmuir equation was used to fit all isotherms except for the two linear isotherms of N2 on MOF-5 and MOF-177. The Henry’s constants and Langmuir equation parameters were summarized in Table 1. Adsorption Equilibrium Selectivity and Adsorbent Selection Parameter. Table 1 summarizes the Henry’s constants or the product of the Langmuir equation constants (am × b) that is the equivalent to the Henry’s constant, the equilibrium selectivity and the adsorbent selection parameter for different gases on all three adsorbents. The adsorbent selection parameters were calculated at adsorption pressure of 1 bar and desorption pressure of 0.1 bar in this work, which represents typical vacuum swing adsorption process conditions for gas separation and purification. As shown in Table 1, for removing/separating CO2 from N2 (air), zeolite 5A is the most suitable adsorbent among the three adsorbents evaluated in this work because the equilibrium selectivity and adsorbent selection parameter of CO2 over N2 are the highest for zeolite 5A. However, for CH4 removal from air, MOF-177 is better than both MOF-5 and zeolite 5A. Once again, zeolite 5A is a better adsorbent than both MOF-5
(1)
where q is the adsorbed amount per unit weight of adsorbent (wt.%), P is the adsorbate gas pressure at equilibrium (torr), and K is the Henry’s law constant (wt.%/torr). The Langmuir isotherm is formulated as q)
ambP 1 + bP
(2)
where am (wt.%) and b (torr-1) are the Langmuir isotherm equation parameters. They can be determined from the slope and intercept of a linear Langmuir plot of (1/q) versus (1/P). Adsorption Equilibrium Selectivity. In order to evaluate the efficacy of an adsorbent for gas separation and purification such as removal/separation of CO2, CH4, N2O from air by adsorption, it is necessary to know the adsorbent properties including adsorption capacity and selectivity. The adsorption equilibrium selectivity R12 between components 1 and 2 is defined as R12 )
X1 Y2 K1 am1b1 * ≈ ≈ X2 Y1 K2 am2b2
(3)
where component 1 is the stronger adsorbate and 2 is the weaker adsorabte. X1 and X2 are the molar fractions of components 1 and 2 on the adsorbent surface (or in the adsorbed phase), Y1 and Y2 are the molar fractions of components 1 and 2 in the gas phase. am1 and am2 and b1 and b2 are the Langmuir equation constants for components 1 and 2. K1 and K2 are the Henry’s constants for components 1 and 2. The equilibrium selectivity defined in the above equation is basically the ratio of the Henry’s constants of the two components, which is the intrinsic selectivity that is only valid at very low gas pressure and low adsorption loading on the adsorbent. For pressure swing adsorption process, the adsorbent selection parameter S defined in the following equation is more useful in adsorbent evaluation and selection because it includes the ratio of adsorption capacity difference of components 1 and 2 (54): S)
∆q1 R ∆q2 12
(4)
where ∆q1 and ∆q2 are the working capacity that is calculated as the adsorption equilibrium capacity difference at adsorption pressure and desorption pressure for components 1 and 2, respectively. Adsorption Kinetics. A classical micropore diffusion model was applied to extract the intracrystalline diffusivity for the three gases within these adsorbents. The fractional adsorption uptake (mt/m∞) can be correlated with the diffusion time constant (Dc/rc2) by the following equation if the fractional adsorption uptake lies within 70-99% (55). 1-
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( )
-π2Dct mt 6 ≈ 2 exp m∞ π rc2
(5)
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Results and Discussion
FIGURE 1. Adsorption isotherms of CO2 (a), CH4 (b), N2O (c), and N2 (d) on MOF-5, MOF-177, and zeolite 5A at 298 K and pressures up to 800 Torr.
TABLE 1. Summary of Henry’s Constants, Langmuir Equation Parameters, Equilibrium Selectivity and Adsorbent Selection Parameter Calculated from the Pure Component Adsorption Isotherms Henry’s Constants K or am × b (wt.% × torr-1) MOF-5 CO2 CH4 N 2O N2
MOF-177
13.98 × 0.0005 26.59 × 0.0004 0.45 × 0.001 2.4 × 0.001 8.17 × 0.001 0.67 × 0.001 0.0004 0.0006
zeolite 5A 21.14 × 0.033 2.72 × 0.001 18.21 × 0.03 2.90 × 0.001
Selectivity R12 CO2/N2 17.48 17.73 240.56 CH4/N2 1.13 4.00 0.94 N2O/N2 20.43 1.12 188.38 CO2/CH4 15.53 4.43 256.47 Adsorbent Selection Parameter S (PAds )1 bar, PDes ) 0.1 bar) CO2/N2 213.19 233.87 992.77 CH4/N2 0.67 8.45 0.81 N2O/N2 220.29 0.66 708.82 CO2/CH4 318.99 27.68 1232.66
and MOF-177 for removing N2O from air and separating CO2 from CH4 in a pressure swing adsorption process. Adsorption Equilibrium at Elevated Pressures. Table 2 shows the adsorption equilibrium capacities of CO2 and CH4 on all three adsorbents at 298 K and elevated pressures (14 bar for CO2 and 100 bar for CH4). It is observed that the two MOF adsorbents exhibit higher adsorption uptake than zeolite 5A at an elevated pressure. The Carbon dioxide
TABLE 2. Summary of Adsorption Equilibrium Capacity of CO2 and CH4 on MOF-5, MOF-177 and Zeolite 5A at 298K and Elevated Pressures adsorbents
CO2, 298 K, 14 bar
CH4, 298 K, 100 bar
MOF-5 MOF-177 zeolite 5A
47.98 wt.% 39.69 wt.% 22.27 wt.%
17.15 wt.% 22.03 wt.% 14.31 wt.%
captured by MOF-5 and MOF-177 at 298 K and 14 bar are 47.97 and 39.69 wt.%, respectively, as compared to 22 wt.% by zeolite 5A, which is consistent with the CO2 adsorption results obtained on MOF-5 and MOF-177 by Millward and Yaghi (19). CH4 adsorption uptakes by MOF-5 and MOF177 at 298 K and 100 bar are also higher than zeolite 5A. To the best of our knowledge, the methane adsorption amount (22 wt.%) on MOF-177 at 298K and 100 bar is probably the highest methane adsorption capacity ever reported on a dry adsorbent by physical adsorption. The adsorption capacities shown in Table 2 suggest that both MOF-5 and MOF-177 are better adsorbents than zeolite 5A for CO2 or CH4 storage at elevated pressures although the MOFs are not as good as zeolite 5A for CO2, CH4 and N2O adsorption at ambient pressures. Adsorption Kinetics. Adsorption kinetics data of CO2, CH4 and N2O on all three adsorbents were collected at the VOL. 44, NO. 5, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Adsorption kinetics of CO2 on MOF-5, MOF-177, and zeolite 5A at 298 K and pressures up to 800 Torr.
TABLE 3. Diffusivity of CO2, CH4 and N2O in MOF-5, MOF-177, and Zeolite 5A at 298K and Pressures up to 800 Torr gases
diffusivity in MOF-5 (m2/s)
diffusivity in MOF-177 (m2/s)
CO2 CH4 N2O
1.17 × 10-9 1.82 × 10-9 2.19 × 10-9
2.3 × 10-9 1.46 × 10-9 1.09 × 10-9
diffusivity in zeolite 5A (m2/s) 2.65 × 10-11 5.65 × 10-11 1.93 × 10-11
same time when the isotherms were measured at 298 K in the Micromeritics ASAP 2020 adsorption unit. Typical kinetic uptake curves of CO2 on all thee adsorbents are shown in Figure 2. Only the fractional uptake of CO2 at 800 Torr is plotted, all other kinetics plots are of similar shape. It is observed from these plots that MOF-5 and MOF-177 reached the adsorption saturation level in a shorter interval of time (within 5-10 s) as compared with zeolite 5A (within 30-60 s). The radius rc of 0.5 × 10-6 m for zeolite 5A crystallite, 9.31 × 10-5 m for MOF-5 and 1.2 × 10-4 m MOF- 177 were estimated from their SEM images shown in SI Figure S2. The average diffusivities of these gases on to MOF-5, MOF-177, and zeolite 5A were listed in Table 3. The diffusivity of these gases on to both MOFs is in the order of 10-9 m2/s, and for zeolite 5A is in the order of 10-11 m2/s. The lower magnitude of diffusivity on to zeolite 5A could be contributed to the lower pore opening of zeolite compared to the MOFs. The magnitude of diffusivity in zeolite 5A is consistent with the reported values of diffusivities for small gas molecules in zeolite 5A (56). The variation of diffusivity with the change in pressure is shown in Figure 3(a-c) for CO2, CH4, and N2O on MOF5, MOF-177 and zeolite 5A, respectively. From the plots, it is obvious that the magnitude of diffusivity confirms a decreasing trend with the increase in pressure; except for CO2 adsorption on to zeolite 5A, it depicts a reverse trend. The decreasing nature of diffusivity can be attributed to the fact that the adsorbent pores were partially saturated and blocked at higher adsorption loading in the higher pressure range giving rise to sluggish kinetics. In the case of CO2 adsorption on to zeolite 5A, the very high adsorption loading (up to 18 wt.%) of the adsorbed species on the adsorbent causes a greater surface concentration resulting in slippage of molecules on to the surfaces. This slippage causes the surface diffusion to set in at the higher adsorption loading at the higher pressure and this surface 1824
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FIGURE 3. Variation of diffusivity with adsorbate pressure in MOF-5, MOF-177, and zeolite 5A at 298K and pressures up to 800 Torr for CO2 (a), CH4 (b), and N2O (c). diffusion enhances the overall intracrystalline diffusivity with the increase in pressure.
Acknowledgments We greatly appreciate the financial support provided by the U.S. Army Research Office through a grant W911NF-06-10200. Dipendu Saha acknowledges the Bruce Wilson Fellowship awarded by the Chemical Engineering Department at New Mexico State University.
Supporting Information Available Figures S1 and S2 are the XRD patterns and SEM images of MOF-5, MOF-177, and zeolite 5A, respectively. This material is available free of charge via the Internet at http:// pubs.acs.org.
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