Article pubs.acs.org/IECR
Adsorption and Separation of Carbon Dioxide Using MIL-53(Al) Metal-Organic Framework Prashant Mishra, Hari Prasad Uppara, Bishnupada Mandal,* and Sasidhar Gumma Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati−781039, Assam, India S Supporting Information *
ABSTRACT: In this work, we report adsorption isotherms of various industrially important gases, viz. CO2, CO, CH4, and N2 on MIL-53(Al) metal organic framework (MOF). The isotherms were measured in the range of 0−25 bar over a wide temperature range (294−350 K). The structural transformation of the adsorbent and the resulting breathing phenomenon were observed only in the case of CO2 adsorption at 294 and 314 K. Adsorption of CO (another polar gas), N2 and CH4 did not induce any structural transformation in this adsorbent for the experimental conditions considered in this work. Since the CO2 isotherms at 294 and 314 K involve structural transformation and show a distinct step, a conventional isotherm model cannot be used to describe such behavior. In order to model these isotherms, a dual-site Langmuir-type equation (one site each for the two structural forms, i.e., large pore phase and narrow pore phase) that includes a normal distribution function to account for structural transformation is proposed. This model successfully mimics the Type-IV isotherm behavior of CO2 on MIL-53(Al). Henry’s constants and adsorption enthalpies of CO2 on the two structural forms were calculated using this model. The Ideal Adsorbed Solution Theory (IAST) was used to predict the selectivity of CO2 at 350 K over other gases studied in this work.
1. INTRODUCTION At present, there is a growing concern worldwide on the anthropogenic emission of carbon dioxide (CO2) into the atmosphere, and its impact on global warming.1,2 About 80% of CO2 emissions are due to burning of the fossil fuels, with a major contribution from coal-fired power plants.3 Rapid industrialization and consumption of fossil fuels has resulted in gradual enhancement of CO2 concentration in the atmosphere. The major constituents in the power plant flue gas include CO2 (∼15%) and N2 (∼75%); capture of CO2 is desired at about ambient pressure and an elevated temperature (∼350 K). The conventional method for CO2 capture involves absorption into an aqueous amine solution; however, this technique has severe drawbacks such as equipment corrosion, amine degradation, and high energy consumption in the solvent regeneration step.4 A considerable amount of research has been focused on the development of adsorbent-based processes for CO2 capture. Other adsorptive separations of CO2 include its separation from CH4 at pressures as high as 70 bar in natural gas purification and from CO2/CH4/CO mixture in steam reforming of natural gas. In general, adsorptive separation techniques are mainly driven by synthesis of suitable porous adsorbents. A variety of adsorbents such as zeolites, activated carbon, silica gel, activated alumina, etc. have been widely investigated for adsorption and separation of CO2. In addition, over the past decade, a new class of porous adsorbent commonly known as metal-organic frameworks (MOFs) has been widely synthesized and investigated by several researchers for CO2 capture and separation applications.5−24 Metal-organic frameworks are formed by coordinative bridging of metal ions and organic linkers. The choice of metal and organic linker has significant influence on the structure and properties of the framework. These materials have several better characteristics (like high CO2 uptakes/selectivity, © XXXX American Chemical Society
easier regeneration, tunable structure, etc.), compared to other conventional adsorbents.13−24 However, some of the MOFs are not stable and easily degenerate upon exposure to water vapor and other gases.18,23,24 One of the MOFs from the MIL family, MIL-53(Al), has good stability, comparable to that of other conventional adsorbents6,24 and it is one of the main reasons behind its choice for this work. This MOF is composed of corner-sharing interconnected Al clusters with benzene dicarboxylate (BDC) organic ligands.6,7 Another important characteristic of this material is its structural transformation from the large pore (lp) domain to the narrow pore (np) domain and vice versa (so-called breathing phenomena) upon exposure to certain guest molecules like CO2, H2O, etc.,6,8,10 by applying mechanical pressure,25−27 or by change in its temperature.7 The cell volume of MIL-53(Al) decreases by ∼35%, from 1412 Å3 to 947 Å3 during its transformation from the lp domain to the np domain.6 Most of the available studies on MIL-53(Al) attempt to explain the breathing phenomena exhibited by this material upon adsorption of CO2 or H2O.6−8,10 However, adsorption isotherms of other polar gases such as CO are rarely reported in the literature.12,28 Moreover, in order to evaluate the potential of MIL-53(Al), which has better stability, compared to most of the other MOFs, it is necessary to measure its pure component adsorption capacity over the range of temperature and pressure of interest. In this work, pure gas adsorption isotherms on MIL53(Al) are measured for industrially relevant gases such as CO2, Special Issue: Energy System Modeling and Optimization Conference 2013 Received: February 12, 2014 Revised: May 15, 2014 Accepted: June 26, 2014
A
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N2, CH4, and CO. These isotherms are suitably modeled and Ideal Adsorbed Solution Theory (IAST)29 is used to predict the selectivity of their binary gas mixtures.
2. SYNTHESIS The synthesis of MIL-53(Al) was carried out under hydrothermal conditions using aluminum nitrate nonahydrate (Al(NO3)2·9H2O, Aldrich), benzene 1,4-dicarboxylic acid (BDC, Merck), dimethylformamide (DMF, Merck), and deionized water as per the procedure suggested by Loiseau et al.6 The starting materials with molar composition of 1(Al(NO3)3·9H2O):0.5(BDC):80(H2O) were mixed and placed in a Teflon-lined steel autoclave at 493 K for 3 days. The product was filtered and washed with deionized water. For removal of the unreacted BDC in the resultant product, typically 1 g of as-synthesized MIL-53(Al) and 25 mL of DMF were kept in a Teflon-lined autoclave for 15 h at 423 K.11 The filtered product was calcined overnight at 553 K.
Figure 1. Thermogram of MIL-53(Al) at a heating rate of 5 K min−1 under a flow of 40 cm3 min−1 of N2.
3. EXPERIMENTAL SECTION Thermogravimetric analysis (TGA) of the synthesized MIL53(Al) sample was performed in a thermogravimetric analyzer (Mettler Toledo, Model No. TGA/SDTA 851e). The temperature was ramped from 298 K to 973 K at a heating rate of 5 K min−1, and the measurements were carried out under a nitrogen atmosphere. A Beckman Coulter surface area analyzer (Coulter SA model 3100) was used for performing surface area and pore volume measurements. The specific surface area was calculated from nitrogen physisorption at 77 K in the relative pressure (P/ P0) range of 0.05−0.2, using the BET (Brunauer−Emmett− Teller) model. The pore volume was calculated at a relative pressure of P/P0 = 0.98. Powder X-ray diffraction (XRD) patterns were measured on a Bruker A8 Avance instrument operating at 40 kV and 40 mA using Cu Kα (λ = 1.5406 Å) radiation. Adsorption equilibria of all gases were measured gravimetrically in a Rubotherm Magnetic Suspension balance. Prior to each isotherm measurement, MIL-53(Al) was activated by heating it at 493 K under a purge flow of 30 cm3 min−1 of helium. Excess amount adsorbed was calculated from the raw measurements using buoyancy corrections.30,31 The impenetrable solid volume for buoyancy correction was obtained from helium measurements at 294 K in the pressure range of 0−26 bar, using nonadsorbing helium assumption. In order to account for gas phase nonideality at higher pressures, fugacity was used instead of pressure.31
Figure 2. N2 physisorption at 77 K on MIL-53(Al); filled circles represent adsorption and open circles represent desorption.
the microporous nature of this material. The BET surface area and pore volume were ∼1284 m2 g−1 and 0.64 cm3 g−1, respectively. The detailed comparison of these results with the existing literature is provided in Table 1. Table 1. Surface Area and Pore Volume of MIL-53(Al) BET surface area (m2 g−1) 1140 1235 1300 1664a 1284 a
pore volume (cm3 g−1)
ref
0.42 0.60 0.64
6 12 33 34 present study
Langmuir surface area.
The XRD patterns (Figure 3) of MIL-53(Al) were obtained after activating the sample at 493 K. The patterns obtained in this work are very close to that of the lp domain of MIL-53(Al) reported by Loiseau et al.6
4. RESULTS AND DISCUSSION TGA of the calcined MIL-53(Al) is shown in Figure 1. The TGA profile obtained in this work for MIL-53(Al) is similar to that reported in the literature.6,12 The initial weight loss below 373 K is due to the removal of water from the sample. Thereafter, the material continues to show stable weight up to ∼723 K. The sharp increase in weight loss above 723 K is due to the collapse of MIL-53(Al) structure, i.e., removal of structural BDC linkers from the framework.6 Thermal stability of this MOF (723 K) is higher than most of the other MOFs such as Zn/DABCO,20 CuBTC,30 MIL-101,18 MOF-210,17 Mg/DOBDC,32 etc. Superior thermal stability is also one of the reasons for selecting MIL-53(Al) for this work. Prior to measuring N2 physisorption at 77 K, MIL-53(Al) sample was degassed at 493 K for 2 h. MIL-53(Al) exhibits a reversible, Type-I N2 isotherm at 77 K (Figure 2), indicating
Figure 3. X-ray diffractogram of MIL-53(Al) after high-temperature activation. B
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matches well with that reported in the literature.8,10,28 Similarly, N2 and CO adsorption uptakes at 294 K closely follow the literature data.12,28 To the best of our knowledge, none of the literature reports adsorption isotherms of these gases at higher temperatures. As expected, relatively nonpolar CH4 (Figure 5) and N2 (Figure 6) gases did not induce any structural change and
The above TGA, BET, and XRD characterization results match well with previous literature reports; we therefore conclude that MIL-53(Al) MOF has been successfully synthesized for the adsorption studies. CO2 isotherms of MIL-53(Al) at 294, 314, 332, and 350 K are shown in Figure 4. The isotherm at 294 K is in good
Figure 4. CO2 adsorption isotherms on MIL-53(Al) at (●) 294 K, (▲) 314 K, (■) 332 K, and (◆) 350 K. Symbols are experimental data; lines are fits obtained using model parameters (given in Tables S6 and S7 in the Supporting Information).
Figure 5. CH4 adsorption isotherms on MIL-53(Al) at (●) 294 K, (▲) 314 K, and (◆) 350 K. Symbols are experimental data; lines are fits obtained using model parameters (given in Table S6 in the Supporting Information).
agreement with the earlier reported data.8,10 In general, microporous adsorbents having rigid framework, exhibit Type-I isotherms. However, a Type-IV isotherm was obtained for CO2 adsorption onto microporous MIL-53(Al) at 294 and 314 K. According to the literature,8,28 during CO2 adsorption at 294 K, MIL-53(Al) material transforms from the large-pore (lp) phase to the narrow-pore (np) phase at ∼0.8 bar and remains in the np phase up to ∼4 bar. After this pressure, reverse transformation from the np to the lp phase occurs resulting in a sudden increase in the adsorption uptake. This structural transformation results in the Type-IV isotherm. It is reported in the literature that MIL-53(Al) transforms from the lp phase to the np phase, because of hydrogen bonding between the adsorbate molecules and the oxygen atoms of the carboxylate and the μ2-hydroxo group.8 The npto-lp phase transformation occurs at high pressure, because of the stress in the framework.10 Both the lp-to-np and subsequent np-to-lp transformations at 314 K occur at a higher pressure, compared to those at 294 K. As the temperature is further increased (to 332 and 350 K), CO2 isotherms show a Type-I behavior and adsorbent does not exhibit structural transformation. This observation matches closely with the predicted temperature-vapor pressure phase diagram for CO2 adsorption on MIL-53(Al) by Boutin et al.,10 indicating the absence of structural transformation at temperatures above 340 K. The CO2 loading on the MIL-53(Al) sample under ambient conditions is higher than that on many other adsorbents such as IRMOF-1,35 IRMOF-3,35 MOF-177,35 ZIF-8,35 MIL-47,8 AC (Norit R1),36 H-mordenite,37 silicalite,38 and ZnDABCO;20 however, it is lower than that on benchmark Zeolite 13X,39 and some open metal sites containing MOFs such as CuBTC,39 and MgDOBDC.32 Although the CO2 capacity for CuBTC and MgDOBDC is better, unlike MIL-53(Al), these MOFs are not stable in a humid environment.24 Thus, MIL-53(Al) may be a potential MOF-based adsorbent for CO2 separation applications. The isotherms of other gases on MIL-53(Al) at 294, 314, and 350 K are shown in Figures 5−7. The CH4 isotherm at 294 K
Figure 6. N2 adsorption isotherms on MIL-53(Al) at (●) 294 K, (▲) 314 K, and (◆) 350 K. Symbols are experimental data; lines are fits obtained using model parameters (given in Table S6 in the Supporting Information).
exhibit Type-I isotherms for all three temperatures. In fact, the isotherms for even CO (which has a dipole moment) show Type-I behavior at the three experimental temperatures (see Figure 7). The adsorption capacity of CO2 on MIL-53(Al) is higher than that of the other three gases. Since the kinetic diameter of CO2 is smallest, it also exhibits higher saturation uptake than other gases. Although CO has a permanent dipole, its adsorption uptake is lower than that of CH4 (CH4 has higher polarizability than CO). N2 has the lowest adsorption uptake, because of its low polarizability and quadrupole moment (physical properties of studied gases are given in Table S1 in the Supporting Information). Attempts were made to model the experimental results using a Langmuir isotherm. A simple Langmuir equation did not fit the isotherm well enough, possibly because of its inability to describe the heterogeneity that is likely to exist in MOFs (because of the presence of several groups in their structure). On the other hand, a virial isotherm (eq 1) is versatile and was C
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ψ {T , f } =
used to describe the adsorption behavior of CO, N2, and CH4 on this MOF with good statistical significance: N exp(bN + cN 2) β
(1)
ψ=
where N (mol kg−1) is the amount adsorbed, f (bar) is the fugacity, β (mol kg−1 bar−1) is Henry’s constant, and b (mol−1 kg) and c (mol−2 kg2) are the second and third virial coefficients, respectively. The usual temperature dependency was considered for these parameters:
(4)
⎛ f − m ⎞⎤ 1⎡ ⎟⎥ ⎢1 + erf⎜ ⎝ 2 s ⎠⎦ 2⎣
(5)
where m is the mean and s is the standard deviation for the underlying Gaussian. This model successfully describes the CO2 isotherms (at 294 and 314 K) involving structural transition (Figure 4). The parameters for the model are given in Table S7 in the Supporting Information. Figure 8 shows the variation of ψ with fugacity at the two temperatures and can be used to identify the fugacity range
⎛β ⎞ β = β0 exp⎜ 1 ⎟ ⎝T ⎠ b1 T c1 c = c0 + T
Nlp{T , f } − Nnp{T , f }
where N denotes the actual amount adsorbed on the material at a temperature T and fugacity f, Nlp and Nnp denote the amounts adsorbed at the same bulk gas conditions, if the material were to exist in pure lp and pure np forms, respectively. Thus, ψ is the uptake capacity as a fraction of the difference between the uptakes on the lp and np phases under the same conditions. Since the uptake capacity at any given condition is related to the “true” structure of the bulk adsorbent material (i.e., a pure np phase or a pure lp phase or somewhere between these two phases), ψ will also be related to the extent of np to lp structural transformation. It must be mentioned that it will not be possible to obtain the extent of structural transformation, when the isotherm is modeled using the low-pressure and highpressure experimental data separately. In this work, we use a normal distribution function to represent the variation of ψ with fugacity at a given temperature:
Figure 7. CO adsorption isotherms on MIL-53(Al) at (●) 294 K, (▲) 314 K, and (◆) 350 K. Symbols are experimental data; lines are fits obtained using model parameters (given in Table S6 in the Supporting Information).
f=
N {T , f } − Nnp{T , f }
b = b0 +
(2)
where T is the temperature (in Kelvin). This model was also used to fit Type-I isotherms of CO2 at higher temperatures (332 and 350 K). The modeling results are included in Figures 4−7, along with the experimental data; the parameter values are given in Table S6 in the Supporting Information. In the literature, attempts were made to model the Type-IV behavior of the CO2 isotherm on this material by assuming that adsorption on each one of the two phases (np and lp) follows a Langmuir model; the two sets of model parameters (one per phase) were obtained by using experimental data in the lowpressure region (0−5 bar) for the np phase, and in the highpressure region (9−30 bar) for the lp phase.40 However, in this work, we have used a dual-site Langmuir-type model (eq 3) to fit the isotherm over the entire pressure: ⎛ N maxb f ⎞ ⎛ N maxb f ⎞ N = ⎜ 1 1 ⎟(1 − ψ ) + ⎜ 2 2 ⎟ψ ⎝ 1 + b2 f ⎠ ⎝ 1 + b1f ⎠
Nmax 1
Nmax 2
Figure 8. Variation of ψ with fugacity during adsorption of CO2 on MIL-53(Al) at 294 K (solid line) and 314 K (dashed line).
corresponding to the structural transition. At 294 and 314 K, the transition occurs approximately between 2.4−7.2 bar and 5.7−10.8 bar, respectively. In addition, one can also identify the bulk gas condition where the transition is maximum (corresponding to the parameter m). Using the model parameters, the Henry’s constant of CO2 at 294 K for the np phase (5.98 mol kg−1 bar−1) is higher than that for the lp phase (4.43 mol kg−1 bar−1). These values are similar to earlier reports (9 and 2.6 mol kg−1 bar−1, respectively, for the np and lp phases) and indicate higher affinity of the np phase for CO2 adsorption.40 The enthalpy of adsorption is a measure of the strength of adsorbate−adsorbent interactions and also governs the ease of
(3)
−1
where and (mol kg ) represent the saturation capacities of the np phase and lp phase, respectively; b1 and b2 (bar−1) are affinity parameters for the np phase and lp phase, respectively. In addition to the conventional Langmuir-type terms for describing the adsorption uptake on the two sites (corresponding to the np and lp phases), an additional parameter ψ appears in this model. The significance of this parameter can be readily seen by rearranging eq 3 to yield D
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selective for CO2 over CH4 and is slightly selective for CH4 over CO and N2.
regeneration. The adsorption enthalpies at zero loading (−Δhads,0) shown in Figure 9 are calculated from the model
5. CONCLUSIONS In this work, adsorption characteristics of MIL-53(Al) MOF were studied for industrially relevant gases over a wide range of temperatures and pressures. CO, CH4, and N2 exhibit Type-I isotherm behavior. CO2 isotherms at the two higher experimental temperatures (332 and 350 K) also follow this trend, indicating the absence of structural transformation in MIL-53(Al). However, adsorption of CO2 at the two lower temperatures (294 and 314 K) induced the transformation in the adsorbent structure and as a result the isotherms are of Type-IV. A temperature-dependent virial equation was successful in modeling the Type-I isotherms for all of the gases. On the other hand, for modeling of Type-IV isotherms of CO2, an additional parameter was included in the conventional dual-site Langmuir formulation to account for the transformation from the np phase to the lp phase. Higher adsorption enthalpy at zero coverage for CO2 on the np phase (∼41 kJ mol−1) than that on the lp phase (∼26.3 kJ mol−1) indicates better affinity between CO2 molecules and the np phase of MIL-53(Al). While the enthalpy of adsorption for CH4 is close to that of CO2 on the lp phase, the enthalpies for CO and N2 were significantly lower (
