ARTICLE pubs.acs.org/Langmuir
Adsorption of Ethane, Ethylene, Propane, and Propylene on a Magnesium-Based MetalOrganic Framework Zongbi Bao,†,‡ Sufian Alnemrat,§ Liang Yu,‡ Igor Vasiliev,§ Qilong Ren,† Xiuyang Lu,† and Shuguang Deng*,‡ †
Chemical and Biological Engineering Department, Zhejiang University, Hangzhou 310027, China Chemical Engineering Department, New Mexico State University, Las Cruces, New Mexico 88003, United States § Physics Department, New Mexico State University, Las Cruces, New Mexico 88003, United States ‡
ABSTRACT: Separation of olefin/paraffin is an energyintensive and difficult separation process in petrochemical industry. Energy-efficient adsorption process is considered as a promising alternative to the traditional cryogenic distillation for separating olefin/paraffin mixtures. In this work, we explored the feasibility of adsorptive separation of olefin/paraffin mixtures using a magnesium-based metalorganic framework, Mg-MOF-74. Adsorption equilibria and kinetics of ethane, ethylene, propane, and propylene on a Mg-MOF-74 adsorbent were determined at 278, 298, and 318 K and pressures up to 100 kPa. A dual-site Sips model was used to correlate the adsorption equilibrium data, and a micropore diffusion model was applied to extract the diffusivities from the adsorption kinetics data. A grand canonical Monte Carlo simulation was conducted to calculate the adsorption isotherms and to elucidate the adsorption mechanisms. The simulation results showed that all four adsorbate molecules are preferentially adsorbed on the open metal sites where each metal site binds one adsorbate molecule. Propylene and propane have a stronger affinity to the Mg-MOF-74 adsorbent than ethane and ethylene because of their significant dipole moments. Adsorption equilibrium selectivity, combined equilibrium and kinetic selectivity, and adsorbent selection parameter for pressure swing adsorption processes were estimated. The relatively high values of adsorption selectivity suggest that it is feasible to separate ethylene/ethane, propylene/propane, and propylene/ethylene pairs in a vacuum swing adsorption process using Mg-MOF-74 as an adsorbent.
1. INTRODUCTION Separation of olefin and paraffin mixtures is one of the most energy-intensive and difficult chemical separations in petrochemical industry due to the very small relative volatilities and close molecular sizes between the olefin and paraffin molecules. Typically, these separations are achieved by cryogenic distillation, an energy-intensive and costly process. A large number of distillation stages and a high reflux ratio are required in order to achieve polymer-grade products, and the process must be operated at low temperatures and high pressures. For example, ethane/ethylene separation is typically performed at 25 °C and 23 bar in a distillation column with over 100 trays. Similarly, the process for propane/propylene separation is carried out at about 30 °C and 30 bar.1 These processes are energy-intensive and very expensive to operate. It was estimated that cryogenic distillation alone consumes 0.1 quads of energy annually, accounting for 6.2% of the total energy used in all distillation processes.2 An alternative olefin/paraffin separation process with low energy consumption would significantly reduce operating expenses. Among all the separation techniques examined (e.g., extractive distillation, membrane separation, absorption, hydrogenation, physical adsorption), the adsorptive separation seems to be the most promising and energy-efficient process for olefin/paraffin separation.35 r 2011 American Chemical Society
Selection of a proper adsorbent with adequate selectivity and capacity is an important step in designing an adsorption process. To date, a number of adsorbents have been evaluated for adsorptive separation of olefin/paraffin mixtures, including commercial zeolites (such as zeolites 4A, 5A, and 13X),614 activated carbon,1517 carbon molecular sieves,1821 and Ag(I)- or Cu(I)doped adsorbents.2126 Among many of the commercial adsorbents, 13X and 4A zeolites were widely studied and have been proven to be effective adsorbents for a vacuum swing adsorption (VSA) process for producing high-purity products.2729 Da Silva and Rodrigues6 investigated single-component adsorption isotherms and mass-transfer kinetics of propylene and propane on commercial 13X and 4A zeolites at temperatures between 273 and 473 K. It was found that 13X zeolite shows a higher adsorption capacity and lower mass-transfer resistance than 4A zeolite, while the 4A zeolite shows at least 1 order of magnitude higher selectivity for propylene over propane than 13X zeolite. Padin et al. reported that the commercial 4A zeolite partially modified with Li+ cations (NaLiA zeolite) has a faster uptake rate of propane than commercial 4A zeolite.14 In the same work, they Received: August 4, 2011 Revised: September 24, 2011 Published: September 27, 2011 13554
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in binding with the olefins. The open metal sites in the coordinately unsaturated metalorganic frameworks may have selective interactions with olefin by the π-complexation. In this work, we reported the feasibility of using a magnesiumbased metal organic framework for olefin/paraffin separation. Adsorption equilibria and kinetics of ethane, ethylene, propane, and propylene are determined at temperatures from 278 to 318 K. For further comparison, GCMC simulations were conducted to calculate the adsorption isotherms and other information to elucidate the adsorption mechanism.
2. EXPERIMENTAL SECTION Figure 1. Crystal structure of Mg-MOF-74 viewed along the [001] direction. The open metal ions bonded with five oxygen atoms, locates in the center of square-pyramid coordination environment (O, red; C, gray; H atoms are omitted for clarity).
also reported that an aluminophosphate (AlPO4-14) is capable of separating propane and propylene by a kinetic effect.14 Rege et al. also reported that a kinetically controlled olefin/paraffin separation can be accomplished on a zeolite 4A and a molecular-sieve carbon as well as π-complexation adsorbents.21 Recently, novel types of adsorbents called metalorganic frameworks (MOFs) have also been examined for olefin/paraffin separation. Li et al.30 reported that the kinetic separation of propane/propylene was achieved on zeolitic imidazolate frameworks with selectivity as high as 125. Wang and Nicholson et al.31,32 conducted a grand canonical Monte Carlo (GCMC) simulation to predict the adsorption behavior of ethane and ethylene on Cu3(BTC)2. Lamia et al.4 demonstrated the feasibility of separation of propane/propylene on a commercial Cu3(BTC)2 adsorbent using both experimental measurements and molecular simulations. Most recently, Yoon et al.33 also explored the adsorptive separation of propylene and propane on Cu3(BTC)2 that was synthesized using a microwave heating method. MOFs are emerging as a new class of porous materials that have many potential advantages over the traditional adsorbents. They are synthesized using organic ligands and metal clusters that self-assemble to form crystalline materials with well-defined structures, controlled pore size, high surface area, and desired chemical functionalities.3436 These attractive properties make MOFs promising materials for gas separation and storage.37,38 In this work we attempted to evaluate a coordinately unsaturated metal organic framework named Mg-MOF-74 (alternatively labeled as CPO-27-Mg or Mg/DOBDC) for olefin/paraffin separation. This material that was first reported by Dietzel39 has one-dimensional hexagonal channels with 5-coordinate Mg(II) ions decorating the edges of channel (Figure 1). The honeycomb-like structure has 1D pores with a diameter ∼11 Å. A characteristic feature of the crystal structure is the presence of a high concentration of coordinately unsaturated metal cations (∼4.5 sites/nm3), which are the primary interaction sites for guest molecules. Caskey et al.40 reported that Mg-MOF-74 has a high affinity to CO2. An exceptional adsorption capacity up to 23.6 wt % CO2 was obtained at 0.1 bar and 296 K. Our previous study shows that the thermodynamic selectivity for CO2 and CH4 adsorption is as high as 283 at 298 K, and it has a higher CO2 adsorption capacity than 13X zeolite.41 A study on selective adsorption of olefin on π-complexation adsorbents also proves that coordinately unsaturated metal ions play an important role
2.1. Synthesis of Mg-MOF-74. The Mg-MOF-74 material studied in this work was synthesized following a previously reported procedure.41 Mg(NO3)2 3 6H2O (0.712 g, 2.78 mmol) and DOT (0.167 g, 0.84 mmol) were dissolved under sonication in a 15:1:1 (v/v/v) mixture of DMF (67.5 mL), ethanol (4.5 mL), and water (4.5 mL). The homogeneous solution was then transferred to a 125 mL Teflon-lined stainless steel autoclave. The autoclave was capped tightly and heated up to 125 °C in an oven. After the reaction under the autogenous pressure for 26 h, the samples were then removed from the oven and allowed to cool down to the room temperature. The mother liquor was then carefully decanted from the product and replaced with methanol. Fresh methanol was used to exchange the DMF and replenished six times over 3 days. The yellow microcrystalline precipitate was isolated by filtration and washed thoroughly with methanol. The guest molecules incorporated in the crystals were removed under a dynamic vacuum at 250 °C for 15 h, yielding some dark-yellow crystals. The structure and porosity of this adsorbent were confirmed by the powder X-ray diffraction and nitrogen adsorption measurements. The BET surface area of the adsorbent was measured at 1174 m2/g by N2 adsorption at 77 K. The median pore size was calculated to be 10.2 Å using the HorvathKwazoe method. 2.2. Adsorption Measurements. Adsorption equilibrium and diffusivity data of four gases were measured volumetrically in a Micromeritics ASAP 2020 adsorption apparatus. The adsorption isotherms were obtained at three temperatures (278, 298, and 318 K) and gas pressures up to 100 kPa. The temperatures were achieved by using a Dewar with a circulating jacket connected to a thermostatic bath with a precision of (0.01 °C. About 60 mg of adsorbent sample was used for the gas adsorption studies. The initial outgassing process was carried out under a vacuum at 250 °C for 15 h. The free space of the system was determined by using the helium gas. The degas procedure was repeated on the same sample between measurements for 2 h. Ultrahigh purity grade helium (99.999%), ethane (99.95%), ethylene (99.95%), and propane (99.88%) were purchased from Matheson Co. Polymer-grade propylene with a minimum purity of 99.5% was obtained from Air Liquide. All hydrocarbons were used as received without any purification. The adsorption kinetic uptake curves (adsorption amount as a function of time) were obtained at the same time when the adsorption equilibrium data were collected. After the adsorbate gas was introduced into the adsorption system at a given dose, the changes in gas pressure and adsorption volume with time were recorded and then 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. In all experiments, the uptake curves were measured at a stepped pressure increment from 1 to 10 mmHg. 2.3. GCMC Simulations. In order to validate the experimental results we performed molecular simulations of adsorption of ethane, ethylene, propane, and propylene on the Mg-MOF-74 adsorbent. All simulations were carried out using an Accelrys Material Studio Simulation Package.42 The equilibrium adsorption isotherms of four gases were 13555
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Figure 2. Adsorption equilibrium isotherms of ethane (a), ethylene (b), propane (c), and propylene (d) on Mg-MOF-74 at 278, 298, and 318 K. Solid lines represent the dual-site Sips model fits. calculated using the GCMC technique and the Metropolis Monte Carlo algorithm (MMC) method with periodic boundary conditions implemented in the Accelrys Sorption module.42,43 We constructed the structure of Mg-MOF-74 based on the crystal structure data reported by Dietzel et al.39 The original structure contains oxygen atoms bonded to the magnesium, corresponding to water ligands. These atoms were removed in our simulation to represent the activated sample. A 2 2 2 unit cell of the Mg-MOF-74 was used in the simulations. Mulliken charges44 were assigned to the atoms in the Mg-MOF-74 by performing density functional theory (DFT)45,46 calculations using DMOL3 module in the Accelrys Package. The MOF Cartesian and fractional atomic positions were kept fixed in the simulations to restrict the adsorption simulation to a specific region of interest. The exchange-correlation energy was approximated by the local density approximation47,48 using the PWC functional49 and double numerical with diffusion basis set. The adsorbate molecules, ethane, ethylene, propane, and propylene were modeled using a single Lenard-Jones united-atom description with each site electronically neutral and each CHn group considered as a single interaction center with effective potential parameters.4 These atom sites interact with other adsorbate and framework sites by a Lennard-Jones potential.50,51 All Lennard-Jones cross-interactions between adsorbate molecules and adsorbent were determined using the universal force field (UFF) impeded in the package.52,53 The UFF is parametrized for the full periodic table to accurately predict geometries and energy of metal complexes and to calculate the force field parameters by combining atomic parameters.54 The van der Waals potential energy was calculated at which the interaction energies are set to zero for atombased with a cut of radius 13 Å and a cubic spline truncation of width 1 Å. The Ewald summation method was used to handle the electrostatic interactions between adsorbate and adsorbent atoms with accuracy of 0.001 kcal/mol. The potential energies between adsorbate and adsorbent
and the density field of the adsorbate molecules were sampled by 25 points between two evaluations of the field data on a three-dimensional grid of 0.25 Å grid spacing. The Metropolis Monte Carlo moves include 2 106 Monte Carlo steps to adjust fugacity and temperature and the same number for the production. The simulation for the first fugacity starts with an empty framework. Subsequent simulations start from the configuration reached at the end the previous simulation.
3. RESULTS AND DISCUSSION 3.1. Adsorption Isotherms. The single-component equilibrium adsorption isotherms of ethane, ethylene, propane, and propylene on Mg-MOF-74 at 278, 298, and 318 K are shown in Figure 2. All the adsorption isotherms are reversible, which was confirmed by measuring the desorption braches. Because of the steepness of equilibrium data, the Langmuir model could not describe the adsorption isotherms shown in Figure 2 very well, so a dual-site Sips model was used to correlate the adsorption isotherms. This model has been applied successfully to describe gases adsorption on Cu3(BTC)2 including propane and propylene as well as other hydrocarbons.4,55,56 The dual site Sips model can be given by the following form:
qi ¼ qm, i, A
ðbi, A pÞ1=ni, A 1=ni, A
1 þ ðbi, A pÞ
þ qm, i, B
ðbi, B pÞ1=ni, B 1 þ ðbi, B pÞ1=ni, B ð1Þ
and
bi ¼ bi, 0 13556
Qst T0 1 exp RT0 T
with T0 at 318 K
ð2Þ
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Table 1. Summary of Equation Parameters for the Dual-Site Sips Model qm,i,A (mmol/g))
bi,A,0 (kPa1)
Qst,i,A (kJ/mol)
ni,A
qm,i,B (mmol/g)
ethane
0.88
0.020
32.3
0.28
7.87
ethylene
6.38
0.10
42.4
0.96
4.32
propane
6.67
0.14
33.2
0.80
propylene
6.38
1.04
56.0
0.82
adsorbate
bi,B,0 (kPa1)
Qst,i,B (kJ/mol)
ni,B
ARE (%)
0.013
27.2
0.89
5.24
0.0022
21.6
0.98
1.07
1.80
0.00054
59.2
1.44
4.58
2.86
0.013
29.8
1.31
2.39
where qi is the adsorbed amount of the pure component i (mmol/g), p is the pressure of the bulk gas at equilibrium (kPa), qm,i,A and qm,i,B are the saturated capacities of sites A and B (mmol/g), bi,A and bi,B are the affinity parameters of the pure component i to sites A and B (1/kPa), Qst (kJ mol1) is the isosteric heat of adsorption at zero coverage, and ni represents the deviation from an ideal homogeneous surface and is assumed to be independent of temperature. To obtain the parameters precisely, we use the average relative error (ARE) as an objective function, which is defined as follows: 100 N qexp qcal ð3Þ ARE ¼ N k ¼ 1 qexp
∑
k
where N is the number of data points and qexp and qcal are the experimental and calculated data, respectively. The optimal fitting parameters were obtained by minimizing the objective function. The optimization calculation was achieved in the MATLAB software (Mathworks, Inc.). The solid lines in Figure 2 represent the dual-site Sips model using the regressed equation parameters summarized in Table 1. The estimated values of these model parameters are listed in Table 1, from which it shows that the model fits the equilibrium data well. Because of the S-shape type of ethane and propane isotherms, larger deviations are observed. Qst,i,A is the heat of adsorption when preferential adsorption sites are occupied by the adsorbate, typically representing the strength of affinity between adsorbate and adsorbent. It can be concluded that propylene has the strongest interaction with the adsorbent, followed by ethylene. The heats of adsorption of ethane and propane are similar, suggesting a similar affinity between the alkanes and the adsorbent. 3.2. Isosteric Heat of Adsorption and Henry’s Constants. The isosteric heat of adsorption represents the strength of the interactions between adsorbate molecules and the adsorbent lattice atoms and can provide useful information about the energetic heterogeneity of a solid surface. It is also an important aspect when adsorbents are evaluated for potential adsorption processes. The isosteric heat of adsorption (Qst) at a given amount can be calculated by the ClausiusClapeyron equation as ∂ ln P Qst ¼ RT 2 ð4Þ ∂T na where Qst (kJ mol1) is the isosteric heat of adsorption, P is the pressure (kPa), T is the temperature, R is the gas constant, and na is the adsorption amount (mmol g1). Integration of eq 4 gives ln P ¼
Qst þ C RT
ð5Þ
In this study, adsorption equilibrium data at 278, 298, and 318 K were used to make the heat of adsorption plots. The heat of adsorption at a given uptake was calculated from the slopes of the isosteres according to eq 5. The plots of the variation of Qst as a
Figure 3. Isosteric heat of adsorption as a function of loading for ethane (diamond), ethylene (circle), propane (triangle), and propylene (square).
Table 2. Isosteric Heats of Adsorption of Propane and Propylene at Zero Coverage ΔHpropane
ΔHpropylene
(kJ/mol)
(kJ/mol)
4A zeolite
15.6
28.2
27, 28
5A zeolite
39
47
7
13X zeolite LiNaX zeolite
36.9 39.5
42.4 63.8
58, 59 10
silica gel
33
36
60
activated carbon
12.8
14.3
61
carbon molecular sieve
93.9
32.1
19, 62
Cu3(BTC)2
35
49
33
Mg-MOF-74
33.9
60.5
this work
adsorbent
ref
Table 3. Henry’s Constants at Different Temperatures and Heats of Adsorption at Zero Coverage
adsorbate ethane ethylene propane propylene
H278 K
H298 K
H318 K
(mmol g1
(mmol g1
(mmol g1
Qst
kPa1)
kPa1)
kPa1)
(kJ mol1)
0.12 1.83
0.063 0.61
35.4 42.6
0.99
0.49
33.9
4.30
60.5
0.29 6.23 3.09 115.6
18.5
function of loading are shown in Figure 3. It can be seen that the isosteric heats of all gases do not remain constant with increasing loading, indicating the energetic heterogeneity of the adsorbent surface. The heats of adsorption of ethane and propane slightly increase with loading, while the trend for ethylene is the opposite. In the case of propylene, the isosteric heat first increases with loading up to 4 mmol/g and then significantly decreases. 13557
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Figure 4. Comparison between experimental adsorption with simulated results of ethane (a), ethylene (b), propane (c), and propylene (d) at 298 K. Solid lines represent the experimental isotherms, and simulated results are shown as points.
The observation in increasing heat as rising loading is probably attributed to the intermolecular interaction of gases. The initial isosteric heats for propylene, propane, ethane, and ethylene are found to be 52.6, 34.2, 26.9, and 42.5 kJ/mol, respectively. These values for propylene and propane are significantly higher than that reported on Cu3(BTC)2 but are comparable with that on 13X zeolite.4 In zeolite 13X, the heat of adsorption for propylene is generally between 46.1 and 52.7 kJ/mol6 and 32.936.9 kJ/mol for propane.6,5759 For a further comparison, the isosteric heats of adsorption for propane and propylene on Mg-MOF-74 and other adsorbents are summarized in Table 2. It can be seen that much larger adsorption heats are generated from the interactions between gases with adsorbents containing metals (e.g., zeolites and MOFs) than those from the dispersion interaction (e.g., silica- or carbon-based adsorbents). In addition to isosteric heat, the values of Henry constants are an alternative way to understand the interaction between adsorbate and adsorbent. An extrapolation method was used to determine the Henry constants, which is based on the isotherm model in Virail form. The adsorption isotherm can be expressed by P¼
q expðA1 q þ A2 q2 þ :::Þ H
ð6Þ
where A1 and A2 are Virial coefficients and H is the Henry constant. According to eq 6, the plot of ln(P/q) vs q is a linear function of when q or P approaches zero. The following equation was obtained for the linear region: ! P ln ð7Þ ¼ A1 q ln H q
The Henry constant can be extracted by extrapolating the intercept of eq 7. The temperature-dependent Henry constants were used to calculate the heat of adsorption at zero-coverage by the van’t Hoff equation. These values are summarized in Table 3. It can be seen that propylene has the largest Henry constant at all temperatures, suggesting the strongest interaction with MaMOF-74 framework among all the gases studied. The ratio of Henry constant represents the intrinsic thermodynamic selectivity, which was used to evaluate the separate selectivity in section 3.4. Moreover, it can be seen that the magnitude of adsorption heats is in good agreement with the value of Qst,i,A obtained from the dual-site Sips model and that the heats of adsorption for olefins are clearly higher than those for paraffins, revealing a stronger interaction of olefins with specific sites of the MOF framework caused by the π-complexation. For propylene and propane, the adsorption heat at zero coverage is 60.5 and 33.9 kJ/mol, respectively. They are comparable with the values reported on a Li-exchanged 13X zeolite adsorbent.10 3.3. Molecular Simulation of Adsorption Equilibrium. The GCMC calculation was performed for all gases in a pressure range of 0100 kPa and at a temperature of 298 K. The calculated isotherms are shown in Figure 4 along with the experimental results. It can be seen that the simulations are significantly higher than the experimental data except for the adsorption of ethane at pressures below 30 kPa, and the differences between the simulation and experimental at high pressures are less pronounced. The discrepancy between the simulated and experimental isotherms is not unexpected because the GCMC simulations have been performed for perfect crystals of Mg-MOF-74. This is because the type of charge and its magnitude assigned to 13558
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Figure 5. Equilibrium snapshots of ethane (a), ethylene (b), propane (c), and propylene (d) in Mg-MOF-74 at 100 kPa.
each molecule play a crucial role in the van der Waals and electrostatic interactions, and the direct effect of these charges has a screening effect directly reflecting the polarizibility of each molecule.31,32,50 The Mg-MOF-74 structures used in the simulation were assumed to be defect-free and were built according to the crystal structure data reported by Dietzel et al.39 Although we have confirmed that the crystal structures of Mg-MOF-74 used in this work are consistent with the reported data,41 the Mg-MOF74 crystals are probably not defect-free, and the small defects might have caused discrepancies between the simulated and experimental isotherms. Figure 5 shows equilibrium snapshots of adsorbed C2H4, C2H6, C3H6, and C3H8 in the Mg-MOF-74 at 100 kPa. From these density distribution maps, it is clear that all adsorbates were preferentially adsorbed by the open metal sites, and each metal can adsorb one molecule, similar to previous observations for other gas molecules.6466 In addition, propylene and propane have a stronger binding with the adsorbent than ethane and ethylene because the former have significant dipole moments. Interestingly, it was observed that one more molecule of propylene was adsorbed although propylene has a larger collision diameter of (4.6 Å) than that of propane (4.3 Å). This might also be attributed to the difference in their dipole moment. Propylene has a significantly larger dipole moment than propane, so that dipoledipole interaction between propylene molecules could lead to 1/6 higher adsorption uptake, approximately corresponding to the experimental uptake difference at 100 kPa. Though both ethane and ethylene are nonpolar and have zero dipole moment, ethylene has a slightly larger quadruple moment than ethane. Theoretically, ethylene should have a stronger interaction with the Mg-MOF adsorbent, which is consistent with the experimental results on the adsorption heats. However, simulation results revealed that ethane has a significantly stronger affinity to the metal sites according to their snapshots. This observation in the simulation is mainly attributed to more negative partial charges of a carbon atom (0.123) in ethane than that of ethylene (0.035), resulting in a stronger electrostatic interaction with Mg(II) ions. Those atomic chargers were calculated by the Mulliken population analysis built in the DMOL3 package. Such a discrepancy between experimental with simulation might be caused by the fact that specific interaction between the
Figure 6. Experimental adsorption uptakes of ethane (blue), ethylene (black), propane (green), and propylene (red) at 298 K and a pressure of 10 mmHg.
π-orbitals of the sp2 sites of olefin molecules and the unsaturated metal sites was not accounted for in the force field used in this study.4 3.4. Adsorption Kinetics. During measuring adsorption equilibrium in the ASAP 2020 unit, the decrease of pressure in the system could be recorded at a set interval. The change of pressure as a function of time can be inverted into uptake profiles. The adsorption uptakes of ethane, ethylene, propane, and propylene recorded at a low pressure (∼10 mmHg) and 298 K are shown in Figure 6. It can be seen from figure that it took ethane or ethylene less than 100 s to reach the adsorption equilibrium, while for propane or propylene the adsorbed amount could only reach up to 90% of the equilibrium capacity even after 200 s. This implies that faster adsorption kinetics for ethane and ethylene are expected as compared to propane and propylene. To extract the intracrystalline diffusivity, a simplified micropore diffusion model was used. Assuming mass transfer resistance in micropores is more important and the adsorbent crystals can be regarded as an approximately spherical object, the adsorption uptake profiles can be described by the following equation:67 0
mt 6 ∞ 1 n2 π2 Dc t q̅ q0 ¼ 1 2 exp 0 ¼ 2 π n¼1 n rc 2 m∞ q0 q0 13559
∑
! ð8Þ
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Table 4. Comparison of Diffusion Time Constants of Ethane, Ethylene, Propane, and Propylene in Mg-MOF-74 and Other Adsorbents at 298 K adsorbent
pore size (Å)
Mg-MOF-74
10.2
adsorbate ethylene
Dc/rc2 (s1) 3
7.12 10
2
Table 5. Estimated Separation Selectivities for Ethylene/ Ethane, Propylene/Propane, and Propylene/Ethylene Pairs on Mg-MOF-74 at 298 K
ref gas mixture
this work
αi,j =
βi,j = (Hi/Hj)
Si,j = (Hi/Hj)
Hi/Hj
(Dc,i/Dc,j)0.5
(Δqi/)(Δqj)
Mg-MOF-74
10.2
ethane
1.39 10
this work
ethylene/ethane
15.3
10.9
13.9
Mg-MOF-74
10.2
propylene
3.14 103
this work
propylene/propane
18.7
18.3
8.9
Mg-MOF-74
10.2
propane
3.26 103
this work
propylene/ethylene
10.1
6.7
4.7
zeolite 4A zeolite 4A
3.8 3.8
ethylene ethane
5.12 103 1.64 104
21 21
zeolite 4A
3.8
propylene
8.49 105
14, 21
300
ethylene
1.03 104
69
300
ethane
1.07 104
69
Ag+-resin +
Ag -resin AgNO3/SiO2
23
propylene
1.43 103
21
AgNO3/SiO2
23
propane
8.70 103
21 0
where q is the average adsorption amount in the particle, q0 is the initial adsorption amount in the particle, q0 is the equilibrium uptake in the particle, and mt/m∞ is the fractional adsorption uptake. At short times, eq 7 is approximated by67 rffiffiffiffiffiffiffiffiffi mt 6 Dc Dc ¼ pffiffiffi t3 2t ð9Þ m∞ rc π rc 2 This expression is accurate to within 1% for mt/m∞< 0.85 (or Dct/rc2 < 0.4). In this study, the diffusion time constant (Dc/rc2, s1) was used as fitting parameters to correlate eq 9 to the experimental data. The intracrystalline diffusion time constants of ethane, ethylene, propane, and propylene in Mg-MOF-74 at 298 K are summarized in Table 4 along with the constants obtained in 4A zeolite and two π-complexation adsorbents. The Dc/rc2 for ethane, ethylene, propane, and propylene in the Mg-MOF-74 studied in this work were 1.39 102, 7.12 103, 3.26 103, and 3.14 103 s1, respectively. A small difference between the diffusivities of ethane/ethylene and propane/propylene pairs indicates that a kinetic-based separation is difficult. The faster diffusion time constants of ethane and ethylene are attributed to their smaller kinetic diameter as compared to propane and propylene. As shown in Figure 6 and Table 4, the diffusion time constants for propane and propylene are almost identical. Since the molecular sizes of the adsorbates are far smaller than pore size of adsorbent studied in this work, the faster kinetics in MOF sample was expected as compared to zeolite 4A. However, slower kinetics were observed in π-complexation adsorbents even though they have a larger pore opening. As micropore diffusion, intracrystalline diffusivity consists of surface diffusion and Knudsen diffusion. This can be described by Bosanquit’s equation:68 1 1 1 ¼ þ Dc Ds DK
ð10Þ
Since the Knudsen diffusivity (DK) is proportional to the pore diameter, the slower kinetics in π-complexation adsorbents with a larger pore size is probably attributed to the slower surface diffusivity (Ds), which is the restricted step in mass transfer. Hence, it might be explained by the fact that the Ag+ in π-complexation adsorbents is fully unsaturated coordinated while the magnesium in MOF adsorbent has only one accessible coordination site. Consequently, a high-energy barrier for surface
diffusion is probable in the system where the interaction between π-complexation adsorbents with the adsorbate is much stronger. A similar explanation can be given for faster diffusivity of paraffin than that of olefin in Mg-MOF and 4A zeolite. 3.5. Estimation of Separation Selectivity. Because of the difficulty of measuring adsorption equilibrium and process data of gas mixtures, several methods were developed and applied for estimating the separation selectivity of gas mixtures from purecomponent adsorption and equilibrium and kinetic data.7072 The separation selectivity (αi,j) based on equilibrium alone can be calculated from the ratio of Henry’s constants. αi, j ¼ Hi =Hj
ð11Þ
If both adsorption equilibrium and kinetics are considered, a combined separation selectivity (βi,j) can be defined as73 βi, j ¼ ðHi =Hj ÞðDc, i =Dc, j Þ0:5
ð12Þ
For pressure swing adsorption process, the adsorbent selection parameter (Sij) defined in the following equation is more useful in adsorbent evaluation and selection because it includes the ratio of adsorption capacity difference of components i and j:70 Si, j ¼ ðHi =Hj ÞðΔqi =ÞðΔqj Þ
ð13Þ
where Δqi and Δqj are the working capacity that is calculated as the adsorption equilibrium capacity difference at adsorption pressure and desorption pressure for components i and j, respectively. The adsorption and desorption pressures are assumed to be 100 and 1 kPa, respectively, which is a typical operating condition for vacuum swing adsorption processes. Table 5 summarizes the adsorption selectivities (αi,j, βi,j, and Si,j) on the Mg-MOF-74 adsorbent for several olefin/paraffin gas pairs of interest. As shown in Table 5, the Mg-MOF-74 adsorbent has a decent equilibrium selectivity (>10) for separating ethylene/ ethane, propylene/propane, and propylene/ethylene pairs. The combined equilibrium and kinetic selectivity is slightly reduced for the propylene/propane pair, and more than 30% decreases for the other two pairs. The adsorbent selection parameter changed the order with the ethylene/ethane having the highest value of 13.9, propylene/propane having 8.9, and propylene/ethylene having 4.7. Although it is difficult to predict the actual adsorption separation performance for these pairs without mixture data, the relatively high selectivity values shown in Table 5 strongly suggest that it is feasible to separate ethylene/ethane, propylene/propane, and propylene/ethylene pairs in a vacuum swing adsorption process using Mg-MOF-74 as an adsorbent. Further studies on adsorption equilibrium, kinetics, and separation process of mixtures of these pairs are needed in order to develop the pressure swing adsorption processes for separating the olefin/paraffin mixtures. 13560
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4. CONCLUSION Adsorption equilibria, isosteric heats of adsorption, Henry’s constants, and intracrystalline diffusivities for ethane, ethylene, propane, and propylene on a magnesium-based metalorganic framework (Mg-MOF-74) were determined experimentally. A dual-site Sips model fits well the adsorption equilibrium data. Intracrystalline diffusivities were obtained by fitting uptake profiles to simplified micropore diffusion model. The isosteric heats of adsorption for propylene and propane are comparable with those on zeolite 13X. The high zero-coverage adsorption heats and Henry’s constants of olefin suggested strong interactions with the open sites of MOF framework by the π-complexation. GCMC simulations indicated all adsorbates were preferentially adsorbed by the open metal sites where each metal binds one molecule. Propylene and propane have a stronger affinity to the Mg-MOF-74 adsorbent than ethane or ethylene because of their significant dipole moment. Adsorption equilibrium selectivity, combined equilibrium and kinetic selectivity, and adsorbent selection parameter for pressure swing adsorption processes were estimated to predict the adsorption performance of Mg-MOF-74 for separating a few olefin/paraffin pairs. The relatively high values of adsorption selectivity suggest that it is feasible to separate ethylene/ethane, propylene/propane, and propylene/ethylene pairs in a vacuum swing adsorption process using Mg-MOF-74 as an adsorbent. ’ AUTHOR INFORMATION Corresponding Author
*Tel: +1-575-646-4346; Fax: +1-575-646-7706; e-mail: sdeng@ nmsu.edu.
’ ACKNOWLEDGMENT This work was partially supported by US Army Research Office (W911NF-06-1-0200), US Department of Energy (DEFC36-08GO88008), and Zhejiang University (for partially funding Dr. Zongbi Bao’s visit to New Mexico State University). ’ REFERENCES (1) Keller, G. E.; Marcinkowsky, A. E.; Verma, S. K.; Williamson, K. D. Abstr. Pap. Am. Chem. Soc. 1988, 195, 82. (2) Chen, J.; Eldridge, R. B.; Rosen, E. L.; Bielawski, C. W. AIChE J. 2010. (3) Eldridge, R. B. Ind. Eng. Chem. Res. 1993, 32, 2208. (4) Lamia, N.; Jorge, M.; Granato, M. A.; Paz, F. A. A.; Chevreau, H.; Rodrigues, A. E. Chem. Eng. Sci. 2009, 64, 3246. (5) Ghosh, T. K.; Lin, H. D.; Hines, A. L. Ind. Eng. Chem. Res. 1993, 32, 2390. (6) Da Silva, F. A.; Rodrigues, A. E. Ind. Eng. Chem. Res. 1999, 38, 2051. (7) Grande, C. A.; Gigola, C.; Rodrigues, A. E. Ind. Eng. Chem. Res. 2002, 41, 85. (8) Brandani, S.; Hufton, J.; Ruthven, D. Zeolites 1995, 15, 624. (9) Berlier, K.; Olivier, M. G.; Jadot, R. J. Chem. Eng. Data 1995, 40, 1206. (10) Grande, C. A.; Gascon, J.; Kapteijn, F.; Rodrigues, A. E. Chem. Eng. J. 2010, 160, 207. (11) Grande, C. A.; Cavenati, S.; Barcia, P.; Hammer, J.; Fritz, H. G.; Rodrigues, A. E. Chem. Eng. Sci. 2006, 61, 3053. (12) Da Silva, F. A.; Rodrigues, A. E. AIChE J. 2001, 47, 341. (13) Da Silva, F. A.; Rodrigues, A. E. Ind. Eng. Chem. Res. 2001, 40, 5758.
ARTICLE
(14) Padin, J.; Rege, S. U.; Yang, R. T.; Cheng, L. S. Chem. Eng. Sci. 2000, 55, 4525. (15) Choi, B. U.; Choi, D. K.; Lee, Y. W.; Lee, B. K.; Kim, S. H. J. Chem. Eng. Data 2003, 48, 603. (16) Costa, E.; Calleja, G.; Marron, C.; Jimenez, A.; Pau, J. J. Chem. Eng. Data 1989, 34, 156. (17) Olivier, M. G.; Bougard, J.; Jadot, R. Appl. Therm. Eng. 1996, 16, 383. (18) Nakahara, T.; Wakai, T. J. Chem. Eng. Data 1987, 32, 114. (19) Grande, C. A.; Rodrigues, A. E. Ind. Eng. Chem. Res. 2004, 43, 8057. (20) Grande, C. A.; Silva, V. M. T. M.; Gigola, C.; Rodrigues, A. E. Carbon 2003, 41, 2533. (21) Rege, S. U.; Padin, J.; Yang, R. T. AIChE J. 1998, 44, 799. (22) Blas, F. J.; Vega, L. F.; Gubbins, K. E. Fluid Phase Equilib. 1998, 151, 117. (23) Park, J. H.; Han, S. S.; Kim, J. N.; Cho, S. H. Korean J. Chem. Eng. 2004, 21, 236. (24) Iucolano, F.; Aprea, P.; Caputo, D.; Colella, C.; Eic, M.; Huang, Q. Adsorption 2008, 14, 241. (25) Basaldella, E. I.; Vazquez, P. G.; Firpo, N. F. Stud. Surf. Sci. Catal. 2005, 158, 1081. (26) Aguilar-Armenta, G.; Patino-Iglesias, M. E. Langmuir 2002, 18, 7456. (27) Grande, C. A.; Poplow, F.; Rodrigues, A. E. Sep. Sci. Technol. 2010, 45, 1252. (28) Grande, C. A.; Rodrigues, A. E. Ind. Eng. Chem. Res. 2005, 44, 8815. (29) Gomes, P. S.; Lamia, N.; Rodrigues, A. E. Chem. Eng. Sci. 2009, 64, 1336. (30) Li, K. H.; Olson, D. H.; Seidel, J.; Emge, T. J.; Gong, H. W.; Zeng, H. P.; Li, J. J. Am. Chem. Soc. 2009, 131, 10368. (31) Wang, S. Y.; Yang, Q. Y.; Zhong, C. L. Sep. Purif. Technol. 2008, 60, 30. (32) Nicholson, T. M.; Bhatia, S. K. Adsorpt. Sci. Technol. 2007, 25, 607. (33) Yoon, J. W.; Jang, I. T.; Lee, K. Y.; Hwang, Y. K.; Chang, J. S. B. Korean Chem. Soc. 2010, 31, 220. (34) Banerjee, R.; Furukawa, H.; Britt, D.; Knobler, C.; O’Keeffe, M.; Yaghi, O. M. J. Am. Chem. Soc. 2009, 131, 3875. (35) Ferey, G. Chem. Soc. Rev. 2008, 37, 191. (36) Snurr, R. Q.; Hupp, J. T.; Nguyen, S. T. AIChE J. 2004, 50, 1090. (37) Li, J. R.; Kuppler, R. J.; Zhou, H. C. Chem. Soc. Rev. 2009, 38, 1477. (38) Ma, S. Q.; Zhou, H. C. Chem. Commun. 2010, 46, 44. (39) Dietzel, P. D. C.; Blom, R.; Fjellvag, H. Eur. J. Inorg. Chem. 2008, 3624. (40) Caskey, S. R.; Wong-Foy, A. G.; Matzger, A. J. J. Am. Chem. Soc. 2008, 130, 10870. (41) Bao, Z.; Alnemrat, S.; Yu, L.; Vasiliev, I.; Ren, Q.; Lu, X.; Deng, S. J. Colloid Interface Sci. 2011, 357, 504. (42) Accelrys. MS Modeling Getting Started Accelrys Software ed., San Diego, CA, 2004. (43) Metropolis, N.; Rosenbluth, A. W.; Rosenbluth, M. N.; Teller, A. H.; Teller, E. J. Chem. Phys. 1953, 21, 1087. (44) Mulliken, R. S. J. Chem. Phys. 1955, 23, 1833. (45) Hohenberg, P.; Kohn, W. Phys. Rev. B 1964, 136, 864. (46) Levy, R. M.; Karplus, M.; Mccammon, J. A. Chem. Phys. Lett. 1979, 65, 4. (47) Ceperley, D. M.; Alder, B. J. Phys. Rev. Lett. 1980, 45, 566. (48) Hedin, L.; Lundqvis, Bi. J. Phys. C: Solid State Phys. 1971, 4, 2064. (49) Perdew, J. P.; Wang, Y. Phys. Rev. B 1992, 45, 13244. (50) Babarao, R.; Hu, Z. Q.; Jiang, J. W.; Chempath, S.; Sandler, S. I. Langmuir 2007, 23, 659. (51) Bae, Y. S.; Mulfort, K. L.; Frost, H.; Ryan, P.; Punnathanam, S.; Broadbelt, L. J.; Hupp, J. T.; Snurr, R. Q. Langmuir 2008, 24, 8592. 13561
dx.doi.org/10.1021/la2030473 |Langmuir 2011, 27, 13554–13562
Langmuir
ARTICLE
(52) Rappe, A. K.; Casewit, C. J.; Colwell, K. S.; Goddard, W. A.; Skiff, W. M. J. Am. Chem. Soc. 1992, 114, 10024. (53) Casewit, C. J.; Colwell, K. S.; Rappe, A. K. J. Am. Chem. Soc. 1992, 114, 10035. (54) Skouras, E. D.; Burganos, V. N.; Payatakes, A. C. J. Chem. Phys. 1999, 110, 9244. (55) Jorge, M.; Lamia, N.; Rodrigues, A. E. Colloids Surf., A 2010, 357, 27. (56) Chmelik, C.; Karger, J.; Wiebcke, M.; Caro, J.; van Baten, J. M.; Krishna, R. Microporous Mesoporous Mater. 2009, 117, 22. (57) Abdulrehman, H. B.; Hasanain, M. A.; Loughlin, K. F. Ind. Eng. Chem. Res. 1990, 29, 1525. (58) Lamia, N.; Wolff, L.; Leflaive, P.; Gomes, P. S.; Grande, C. A.; Rodrigues, A. E. Sep. Sci. Technol. 2007, 42, 2539. (59) Lamia, N.; Wolff, L.; Leflaive, P.; Leinekugel-Le-Cocq, D.; Gomes, P. S.; Grande, C. A.; Rodrigues, A. E. Sep. Sci. Technol. 2008, 43, 1124. (60) Grande, C. A.; Rodrigues, A. E. Ind. Eng. Chem. Res. 2001, 40, 1686. (61) Mofarahi, M.; Sadrameli, M.; Towfighi, J. J. Chem. Eng. Data 2003, 48, 1256. (62) Grande, C. A.; Cavenati, S.; Da Silva, F. A.; Rodrigues, A. E. Ind. Eng. Chem. Res. 2005, 44, 7218. (63) Granato, M. A.; Vlugt, T. J. H.; Rodrigues, A. E. Ind. Eng. Chem. Res. 2007, 46, 7239. (64) Wu, H.; Zhou, W.; Yildirim, T. J. Am. Chem. Soc. 2009, 131, 4995. (65) Dietzel, P. D. C.; Johnsen, R. E.; Fjellvag, H.; Bordiga, S.; Groppo, E.; Chavan, S.; Blom, R. Chem. Commun. 2008, 5125. (66) Wu, H.; Simmons, J. M.; Srinivas, G.; Zhou, W.; Yildirim, T. J. Phys. Chem. Lett. 2010, 1, 1946. (67) Ruthven, D. M.; Farooq, S.; Knaebel, K. S. Pressure Swing Adsorption; VCH Publishers: New York, 1994. (68) Suzuki, M. Adsorption Engineering; Elsevier: Amsterdam, 1990. (69) Wu, Z. B.; Han, S. S.; Cho, S. H.; Kim, J. N.; Chue, K. T.; Yang, R. T. Ind. Eng. Chem. Res. 1997, 36, 2749. (70) Yang, R. T. Adsorbents: Fundamentals and Applications; WileyInterscience: New York, 2003; p 43. (71) Saha, D.; Bao, Z. B.; Jia, F.; Deng, S. Environ. Sci. Technol. 2010, 44, 1820. (72) Bao, Z.; Alnemrat, S.; Yu, L.; Vasiliev, I.; Ren, Q.; Lu, X.; Deng, S. J. Colloid Interface Sci. 2011, 357, 504. (73) Ruthven, D. M.; Farooq, S.; Knaebel, K. S. Pressure Swing Adsorption; VCH Publishers: New York, 1994.
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