Effect of Adsorbent History on Adsorption Characteristics of MIL-53(Al

Article pubs.acs.org/Langmuir

Effect of Adsorbent History on Adsorption Characteristics of MIL53(Al) Metal Organic Framework Prashant Mishra, Satyannarayana Edubilli, 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: Structural transformation of MIL-53(Al) metal organic framework from large pore to narrow pore form (lp → np) or vice versa is known to occur by adsorption of certain guest molecules, by temperature change or by applying mechanical pressure. In this work, we perform a systematic investigation to demonstrate that adsorbent history also plays a decisive role in the structural transitions of this material (and hence on its adsorption characteristics). By changing the adsorbent history, parent MIL-53(Al) is tuned into its np domain at ambient temperature such that it not only exhibits a significant increase in CO2 capacity, but also shows negligible uptake for CH4, N2, CO, and O2 at subatmospheric pressure. In addition, for the high pressure region (1−8 bar), we propose a method to retain the lp form of the sample to enhance its CO2 uptake.

1. INTRODUCTION The last century has witnessed humanity actively exploiting organic fossil fuels like coal, petroleum, and natural gas as energy sources. The firing of these fuels produces significant quantities of CO2. Typically by burning of 1 ton of solid fuel, 2.76 tons of CO2 is released into the atmosphere.1 CO2 can sustain as such in the earth’s atmosphere for about 120 years,1 resulting in its gradual accumulation. Absorption into aqueous amine solutions is one of the widely used techniques to capture CO2; however, the scientific community is actively involved in search of other alternatives due to known drawbacks of the amine absorption process such as solvent degradation, corrosion, high regeneration cost, and so forth. The development of an adsorption based process as an alternate technique for CO2 capture is mostly driven by synthesis of novel adsorbent materials which can selectively adsorb reasonable amounts of CO2. Metal organic frameworks (MOFs) are being widely investigated for their potential to efficiently remove of CO2 from process streams.2−17 Some MOFs (DOBDC series, for example) have shown good CO2 adsorption uptake and selectivity over other gases like N2, CH4, and so forth at the desired process conditions.16,17 But poor stability toward moisture makes them unrealistic for any practical separation application.18,19 Like other members of the MIL series,20 MIL-53 MOFs are among the few which are readily stable toward moisture.19 Considerable amount of research work is published on these materials due to their © 2013 American Chemical Society

fascinating breathing behavior and good stability. They exhibit structural transformation from lp (large pore) domain to np (narrow pore) domain and vice versa (so-called breathing phenomena) upon adsorption of certain guest molecules (like CO2, H2O, etc.),2,4,21−25 by change in temperature3 or by applying mechanical pressure. 26−28 In their work on coadsorption measurements of CO2 and CH4 in MIL-53(Al), Ortiz et al. state that the total amount adsorbed depends on the history (i.e., whether the measurements were performed during so-called pressure or composition swipes) of the system.29 However, the parent form of MIL-53 material has low CO2 capacity and selectivity.19 Attempts were made to exploit MIL-53 for adsorptive separation of gases like CO2 and CH 4 by structural modification of the framework material. Hydrated30 and functionalized31 (using 2-amino terephthalate linkers) forms of MIL-53 have been used to enhance CO2 selectivity by transforming its structure into the np form30−32 (which exhibits negligible capacity for CH4 at low pressures). The resultant materials showed higher CO2 selectivity over CH4. However, compared to parent MIL-53 material, the CO2 adsorption capacity of the hydrated form decreased significantly.30 In case of the amine functionalized form,31 at subatmospheric pressures Received: July 22, 2013 Revised: September 3, 2013 Published: September 5, 2013 12162

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a marginal increase in CO2 uptake capacity was observed, while at higher pressures the gravimetric uptake was actually lower than that of the parent material due to the presence of additional functional groups. In this work, we present a technique to tune MIL-53(Al) structure into its np domain at subatmospheric pressures to considerably increase both CO2 capacity and selectivity. Enhanced CO2 uptakes at higher pressures (1−8 bar) were obtained by following a procedure which retains the lp form of the material. The tuning in both cases is achieved by simply changing the history of the sample. Since functional groups and other guest molecules are not used in this methodology, there will be no negative effect on gravimetric uptake capacity, or competitive adsorption of the guest molecules.

Scheme 1. Structural transformations in MIL-53(Al)

2. MATERIAL SYNTHESIS The synthesis of this material was carried out under hydrothermal conditions using aluminum nitrate nonahydrate (Al(NO3)2·9H2O, aldrich), 1,4-benzenedicarboxylic acid (BDC, Merck), dimethylformamide (DMF, Merck), and deionized water as per the procedure suggested by Loiseau et al.2 For removal of unreacted BDC, typically 1 g of MIL-53(Al) as-synthesized was dispersed in 25 mL of DMF and introduced in Teflon lined steel autoclave for 15 h at 423 K. The product was cooled, filtered, and calcined overnight at 553 K.33 Further synthesis and characterization details (TGA, BET and XRD) are given in the Supporting Information.

sample exists in the np phase. As CO2 pressure is further increased, reverse transformation starts at about 4.5 bar and a step in the isotherm is observed accordingly. Finally, by about 8 bar, the sample completely transforms into its lp form. During desorption, as the pressure is decreased, the sample will undergo a transformation back into the np structure. The two hysteresis loops observed (the low pressure hysteresis is prominent in Figure 1b) are attributed to the differences in structure of the sample during adsorption and desorption branches.6 CO2 uptake at subatmospheric pressures merits focus (Figure 1b). In this region, while the adsorption branch includes the lp→np transition, during desorption the sample is continues to be in its np form. What happens when the sample is desorbed to very low pressure (say about 0.01 mbar)? As expected, the initial weight of the sample is regained (matching well with that of the sample lp0 in vacuo). However, we observed that the XRD pattern (Figure S3c in Supporting Information) of the sample is closer to that of the np structure reported in literature.2 This is to be expected because a hysteresis loop is observed in the low pressure region. While the existence of hysteresis in low pressure region has been reported elsewhere in literature,6 to the best of our knowledge no attempt has been made to correlate hysteresis with the phase of the sample (in this case, majority of the sample will be in the np domain). As we will demonstrate, this important observation can be exploited to tune the adsorption characteristics of the material. We name this CO2 desorbed sample as sample np0. The necessary steps involved to tune MIL-53(Al) into the desired structural form are summarized in Scheme 1. Adsorption isotherm of CO2 on the sample np0 (filled triangles, Figure 1) is different from that on sample lp0. First, in the low pressure region, the uptake of CO2 on sample np0 increases as compared to that of sample lp0 and almost matches desorption branch of the isotherm on sample lp0 (Figure 1b). At about 0.17 bar, CO2 loading capacity on sample np0 (1.75 mmol g−1) is significantly higher than that on sample lp0 (0.42 mmol g−1). The comparison is highlighted for this pressure due to its industrial relevance (in flue gases, partial pressure of CO2 is typically between 0.12 and 0.20 bar). This increase in CO2 loading on sample np0 is to be expected, since np structure has a larger Henry constant.34 Some of the earlier works have also attempted to exploit this feature of the np structure to enhance CO2 loading at low pressure.30,31 However, such a high enhancement in CO2 uptake has not been reported (possibly due to the presence of guest molecules or functional groups in

3. EXPERIMENTAL SECTION Adsorption measurements were carried out gravimetrically in a Rubotherm magnetic balance. The usual procedure for equilibrium measurement has been followed, and excess amount adsorbed was calculated from raw measurements using buoyancy corrections. The impenetrable solid volume of adsorbent for buoyancy correction was obtained from helium measurements at 293 K in the pressure range of 0−26 bar, using nonadsorbing helium assumption. High temperature activation of the sample was done by heating it at 493 K under vacuum (and about 30 cm3 min−1 purge flow of helium). Regular Adsorption Measurement. After high temperature activation, sample was cooled to the experimental temperature. The first measurement point on the isotherm was obtained by charging CO2 to the desired pressure inside the adsorption cell; sufficient time was allowed to reach equilibrium. The rate of pressurization was approximately 8 mbar s−1 (slow pressurization). Subsequent measurements for the isotherm were obtained by increasing the pressure in incremental doses. Fast Pressurization. Step change in pressure inside adsorption cell was achieved by opening an on−off valve between the adsorption cell and a gas reservoir. The desired target pressure was obtained by adjusting the initial pressure of the gas inside the reservoir. In addition, prior to adsorption measurement of each target pressure, the sample was activated at high temperature (493 K) unless stated otherwise.

4. RESULTS AND DISCUSSION An initial high temperature (493 K) activation of MIL-53(Al) yields the lp structure (Scheme 1) that is retained even after the sample is cooled to the experimental temperature (293 K).3,6 We name this as sample lp0 (material in its lp structure in vacuo). The XRD pattern (Figure S3a in Supporting Information) and CO2 uptake (circles, Figure 1) for this material are in good agreement with earlier reported data2,4,6,23 and the predominance of lp phase is obvious. As the sample is exposed to CO2, the first lp→np transformation takes place at low pressures (0−0.8 bar),34 followed by a saturation in CO2 capacity for the np structure at about 1 bar. The XRD pattern for the sample at this condition (Figure S3b in Supporting Information) differs from that of sample lp0 and majority of the 12163

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Figure 1. (a) CO2 isotherms at 293 K on different structures of MIL-53(Al). (b) Enlarged portion of the isotherms in the low pressure region. Blue filled circle represents adsorption on sample lp0; blue open circle represents desorption on sample lp0; red filled triangle represents adsorption on sample np0; red open triangle represents desorption on sample np0. Lines are drawn as a guide to the eye.

Figure 2. Isotherms at 293 K on different structures of MIL-53(Al). Blue filled circle represents adsorption on sample lp0; blue open circle represents desorption on sample lp0; red filled triangle represents adsorption on sample np0; red open triangle represents desorption on sample np0. Lines are drawn as a guide to the eye.

transformation is complete and both the adsorption and desorption branches coincide with that on the lp structure. The predominance of lp phase for the sample equilibrated with N2 at high pressure is further confirmed from the XRD pattern (Figure S3d in Supporting Information). This behavior is also observed for other industrially important gases like CH4, CO and O2 (Figure 2b−d). It is interesting to note that for all the four gases considered, the transformation is complete when loading is in between 1.5 and 2 mmol g−1. Thus, the range of pressure at which the transformation occurs is related to the adsorption affinity of these gases on to the sample. For example, the transformation during CH4 adsorption occurs at a lower pressure compared to that occurring during adsorption of CO, O2, or N2. On the other hand, no transformation occurs even up to 25 bar when sample np0 is exposed to a nonadsorbing gas like He. To the best of our knowledge, structural transformation of MIL-53(Al) upon adsorption of gases like CH4, N2, CO, and O2 at ambient temperature has not been reported so far.8 Figure 3 highlights the usefulness of the np tuned form of MIL-53 (sample np0) for separations. Even though MIL-53(Al) has greater stability toward moisture, it has not gained much attention for CO2 separation due to its poor capacity and

their samples). In addition, since the sample np0 is already in the np domain prior to adsorption, the first lp→np transition no longer occurs and corresponding hysteresis below 1 bar disappears (solid and open triangles in Figure 1b overlap). It is also worth mentioning that the enhancement of CO2 capacity is achieved without any additional regeneration penalty, since the desorption branches on lp0 or np0 samples follow the same curve (open circles and triangles in Figure 1b). Adsorption and desorption isotherms of N2 at 293 K on the samples lp0 and np0 are shown in Figure 2a. As reported in literature,6,8 structural transformation does not occur in the case of sample lp0 and isotherm does not exhibit hysteresis. In contrast, isotherms on sample np0 show several interesting features. N2 adsorption was negligible up to a certain pressure (∼ca. 1 bar), as the np form of the sample excludes N2 molecules with larger kinetic diameter and low adsorption energies.32 This is the lowest uptake reported for N2 on nonhydrated and nonfunctionalized form of MIL-53(Al) at 293 K. At higher pressures, increase in adsorption may be readily attributed to the np→lp transformation of the sample. As a result of this structural transformation, hysteresis is also observed in the isotherms (which was absent earlier in case of sample lp0). After a certain pressure (ca. 12 bar), the 12164

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Figure 3. Adsorption capacities at 293 K on np structured MIL53(Al), sample np0. CO2 (blue filled circle), N2 (×), CH4 (▲), CO (red filled square), and O2 (green filled tilted square). Lines are drawn as a guide to the eye.

Figure 4. Effect of fast pressurization on CO2 uptake at 293 K for sample lp0. Blue filled circle represents regular adsorption branch; blue open circle represents desorption branch; green filled square represents adsorption obtained by fast pressurization. Lines are drawn as a guide to the eye.

selectivity on the lp structure.4,8,21 However, now, we have removed these limitations to a large extent, by tuning the pores into np domain; CO2 capacity at about 0.17 bar (similar to its partial pressure in flue gas) is significantly higher than those reported elsewhere on any MIL-53 material (including the hydrated or functionalized forms).30,31 At the same time, the np form shows near zero uptake for CH4, N2, CO, and O2 at pressures below 1 bar and thus almost infinite CO2 selectivity is obtained. Since np phase of sample np0 is retained up to 1 bar during adsorption of all gases at 293 K, similar trend in CO2 selectivity will be obtained at subatmospheric pressure even for multicomponent adsorption of these gases. No further experiments (with relevant gas mixtures) are needed to validate the selectivity trend shown in Figure 3. However, selectivity at higher pressures (where np to lp transition occurs) can be obtained only by experimentation with gas mixtures. In the previous section, we have demonstrated how the CO2 loading and selectivity of MIL-53(Al) sample can be enhanced at low pressures, by tuning its structure into the np form. In this section, we provide a tuning method for enhancing CO2 uptake in the second transition region (between 1 − 8 bar). The samples (both lp0 and np0) are typically saturated with CO2 by about 1 bar. They remain in the np domain until about 4.5 bar, and a negligible increase in CO2 uptake is observed. On the other hand, if MIL-53(Al) structure can be retained in its lp form, CO2 loading is expected to increase in this region. The protocol followed so far to obtain the isotherm (circles, Figure 1) involved charging CO2 on to the sample in incremental pressure doses, followed by sufficient equilibration time for each measurement point. We have slightly modified this protocol to minimize the structural transition of the sample into the np form. The sample lp0 under vacuum is exposed to CO2 at the desired target pressure by fast pressurization (as described in the experimental section). It is necessary to perform fast pressurization using the sample in its lp form (i.e., high temperature activation is necessary for each target pressure). The CO2 uptake in the 1−8 bar region, measured by fast pressurization method, is significantly higher than that measured using the regular method (Figure 4). During this method, the pores of the sample are instantaneously filled with the bulk gas at the target pressure, even before CO2 molecules adsorb. As pores in the lp structure are already filled, CO2 adsorption will no longer be able to effect a significant structural transformation, which also involves an appreciable change in pore volume.35 As a result, the fast pressurization method retains the lp form predominantly. It is also interesting

to note that uptakes obtained on the adsorption branch by this method are very close to that on the desorption branch. In the fast pressurization method, as the rate of pressurization decreases transformation is induced and uptake decreases correspondingly. This decrease will be a function of the pressurization rate as well as the final target pressure. For example, the CO2 uptake on the sample lp0 at about 4.5 bar using additional intermediate pressure steps is shown in Figure 5. When pressurization is done in a single step the uptake

Figure 5. Effect of number of incremental pressure steps (used to reach ca. 4.5 bar) on CO2 uptake at 293 K for sample lp0. Green filled square represents one step; dark green/light green filled square represents two steps, and light green filled square represents four steps. For the sake of comparison, regular adsorption (blue filled circle) and desorption (blue open circle) branches on the same sample are also shown. Lines are drawn as a guide to the eye.

corresponds to that on the predominant lp form. With an increase in the number of intermediate steps, the uptake decreases and eventually matches with that on the np structure (about four steps were needed for uptake at 4.5 bar to match the isotherm on the np form). Thus, an appropriate choice of multiple pressure steps can be used to tune the structural transformation (and hence CO2 uptake) on the sample.

5. CONCLUSIONS It is widely believed that the structure of MIL-53(Al) depends on its equilibrium environment, that is, temperature and partial pressure of the adsorbate guest molecules. In this work, we have demonstrated that the adsorption and desorption history of the sample after the high temperature activation, the rate of pressurization, and the number of incremental pressure steps 12165

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used to reach the target pressure also strongly influence its structural transformation (and hence its adsorption properties). By choosing an appropriate path, the sample under vacuum was tuned into the np form which exhibits more than 4-fold increase in CO2 capacity (at about 0.17 bar and 293 K) along with exceptional enhancement in its selectivity over N2, CH4, CO, and O2 at subatmospheric pressures. Similarly, the structure of MIL-53(Al) was tuned into predominantly lp form for achieving enhanced CO2 uptake in the 1−8 bar region. We believe these results demonstrate a methodology that can be exploited to tune flexible materials for enhancement of adsorption capacity/selectivity and to design separation processes based on such adsorbents.

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis procedure, detailed information on characterization (TGA, BET and XRD), and uptake kinetics for a typical fast pressurization measurement. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*Tel/Fax: +91 361 2582261. E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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REFERENCES

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