Breathing-induced new phase transition in an MIL-53(Al)–NH2 metal–organic framework under high methane pressures

Linius Bolinois , Tanay Kundu , Xuerui Wang , Yuxiang Wang , Zhigang Hu , Kenny Koh and Dan Zhao *
Department of Chemical & Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, 117585 Singapore. E-mail: chezhao@nus.edu.sg

Received 9th April 2017 , Accepted 14th June 2017

First published on 21st June 2017


The high pressure methane sorption tests on a flexible metal–organic framework [MIL-53(Al)–NH2] reveal a new phase transition to a large pore (lp) phase above 45 bar. The mixed-ligand strategy and pressure cycling tests are suggested to reduce the pressure requirement of such a phase transition for applications in natural gas storage.


Natural gas, mostly composed of methane, represents an eco-friendly and economic substitute for gasoline as a fuel. Storing natural gas in highly compressed gas tanks (200 bar) has been suggested for on-board natural gas storage.1 However, this method has the disadvantages of large energy penalty and safety concerns, inhibiting its widespread usage.2 Instead, it would be possible to take advantage of the affinity of methane with adsorbent materials in order to increase its storage capacity in relatively lower pressure ranges. Based on the working range of inexpensive compressors, such adsorbents should be able to adsorb a large quantity of methane at the storage pressures (35 or 65 bar) and release most of it at the delivery pressure (5.8 bar), the difference of which is defined as the deliverable capacity.1 Using computational calculations, Simon et al.3 and Snurr et al.4 demonstrated that the deliverable capacity of almost all conventional adsorbents would not be higher than 196–206 cmmethane3 cmadsorbent−3 (v/v, defined at 298 K and 1 bar), which is very low compared to the target of 350 v/v set by the US Department of Energy (DOE).5 One of the reasons is that most adsorbents exhibit type-I Langmuir isotherms, i.e. the methane adsorbed at high pressures (35–65 bar) is still partially retained at low pressure (5.8 bar).6 In this context, flexible adsorbent materials with structural dynamics that could possibly increase the deliverable capacity of methane are highly sought after.1 Some metal–organic frameworks (MOFs), crystalline solids assembled by metal ions and organic linkers, can exhibit structural flexibility, resulting in different isotherm shapes favorable for high deliverable capacities.7 For instance, Long and co-workers showed that Co(bdp) (bdp2− = benzene-1,4-dipyrazolate) exhibits a non-porous to porous phase transition upon methane adsorption that improves the deliverable capacity (197 v/v at 298 K between 65 bar and 5.8 bar) and helps to mitigate the thermal effect during adsorption–desorption cycles.8 In this context, the MIL-53 family [composed of M3+ (M = Al, Fe, Ga, In) and substituted benzene-1,4-dicarboxylates (BDC)] accounts for one of the most famous flexible MOFs with breathing properties.9 They are close to practical applications due to the advantages of low material cost, easy synthesis, and high stability under atmospheric conditions.10 The rhombic pores of MIL-53 can switch between a narrow pore phase (np) and a large pore phase (lp) depending on the pressure, temperature, and type of adsorbate (ESI, Fig. S1). For example, MIL-53(Al) can exhibit the breathing properties for methane adsorption, but only at low temperatures (<240 K) and relatively low pressures (<2 bar)9a,c due to the low stability of the np phase. Interestingly, among all the functional variants of MIL-53,11 MIL-53(Al)–NH2 [Al(OH)BDC–NH2; BDC–NH2 = 2-aminobenzene-1,4-dicarboxylate] can show breathing behaviour at room temperature triggered by a large variety of gases (e.g., CO2, CH4, C2H6).12 It has been suggested that the breathing properties of MIL-53(Al)–NH2 at room temperature are mostly due to the N–H⋯O hydrogen bonding interaction between the –NH2 group of the linkers and [AlO6]chain, which stabilizes a very narrow pore phase (vnp) and the np phase of the framework. However, up to now most of the reported studies have been focusing on CO2/CH4 separation,12a,b,13 and the studies of the methane sorption properties of MIL-53(Al)–NH2 have been limited to medium pressure ranges (40 bar) to adopt the np phase.12a Certainly, a larger pressure range has remained unexplored in terms of applied storage pressure (e.g. 65 bar) and framework behavior.

In this work, we report the observation of a unique 2nd phase transition to form a third phase [hereafter denoted as a large pore (lp) phase] by investigating the methane sorption behavior of MIL-53(Al)–NH2 between 0 and 80 bar at various temperatures. As can be seen from Fig. 1, each methane isotherm exhibits two jumps and three plateaux. Taking the isotherm measured at 298 K for example, the 1st plateau (0–18 bar) corresponds to the vnp phase, followed by a jump corresponding to the vnp to np phase transition. The 2nd plateau defines the np phase (20–45 bar) and finally leads to a np to lp phase transition above 50 bar. Interestingly, the transition pressures decrease with temperature, with a comparatively faster rate of the np to lp phase transition. It is also important to note that this transition pressure requires a large pressure interval, and that it is not complete at 80 bar for the isotherms above 248 K. This actually leaves the sample in a mixture of np and lp phases at 80 bar. The desorption curves of all isotherms present a steady hysteresis, indicating that the phase which is triggered remains stable initially.8,9c The desorption curves of the isotherms at 273 K and 248 K actually increase in the beginning, taking the shape of a typical excess curve for a single phase product at high pressures (ESI, Fig. S10). Indeed, for an excess curve of a single phase, the density increase of the bulk phase is faster at high pressure, in comparison with the density increase of the adsorbed layer.14 By lowering the pressure further, the desorption curves show a lp to np phase transition, followed by a np to vnp phase transition to return to the most stable vnp phase.


image file: c7cc02743e-f1.tif
Fig. 1 CH4 excess uptakes of MIL-53(Al)–NH2 at various temperatures (filled, adsorption; empty, desorption).

In order to evaluate the methane storage performance of the adsorbent, a conversion of the excess uptakes, which are obtained experimentally, into total uptakes is necessary. This conversion needs to consider the methane compressed in the pores, regardless of the host–adsorbate interactions (ESI, page 14). Such calculation in flexible MOFs turns out to be quite challenging since the pore volume changes during the phase transition. In order to tackle the issue, we performed the adsorption of MIL-53(Al)–NH2 for both nitrogen and methane under conditions of pressure and temperature leading to liquefaction to determine the pore volume in each phase (Fig. 2). The 77 K N2 adsorption isotherm of MIL-53(Al)–NH2 exhibits a similar trend to previous studies,11d,15 and is also identical to the 185 K CH4 adsorption isotherm. In particular, the 77 K N2 adsorption isotherm displays a middle plateau around 0.02–0.1 P P0−1, which we associated with the np phase, and another plateau at higher pressure (around 0.5 P P0−1), which can be associated with the lp phase. Contrary to the high pressure CH4 isotherms, the vnp phase cannot be well observed in 77 K N2 isotherms possibly because it occurs at a very low pressure range that is beyond the equipment limitation. The total pore volume, obtained from the 77 K N2 isotherm in each plateau, was used to estimate the total CH4 uptake of MIL-53(Al)–NH2 under high pressures (ESI, Fig. S11 and Tables S6 and S7).


image file: c7cc02743e-f2.tif
Fig. 2 N2 isotherm at 77 K (P0 = 1.00 bar) and CH4 isotherm at 185 K (P0 = 39.03 bar) of MIL-53(Al)–NH2 (filled, adsorption; empty, desorption).

From the total CH4 uptake that we estimated, we could define the deliverable capacity of the material and compare it with that of MIL-53(Al) (Fig. 3). As can be seen from Fig. 3, the deliverable capacity of MIL-53 remains low (133 cm3 g−1, or 130 v/v, for a process between 65 bar during adsorption and 5.8 bar during desorption) and does not depend on the sample history.9b,16 This is mostly due to the stability of the lp phase of MIL-53(Al) under these conditions.9c In contrast, the deliverable capacity of MIL-53(Al)–NH2 can be dramatically improved depending on the CH4 storage processes. In particular, if the methane is stored at 35 or 65 bar and released at 5.8 bar, the deliverable capacity varies from 74 cmSTP3 g−1 to 115 cmSTP3 g−1. However, if the adsorbent is first pressurized by CH4 at 120 bar (ensuring the complete phase transition to the lp phase) and brought back at 65 bar, the deliverable capacity (which can then be defined between 65 bar during desorption and 5.8 bar during desorption) reaches as high as 177 cm3 g−1. This corresponds to a minimum of 178 v/v (ESI, page 15). One can therefore see that although the total uptake of MIL-53(Al)–NH2 is lower than that of MIL-53(Al) in the entire pressure range explored, its deliverable capacity can be higher if it is defined between 65 and 5.8 bar. This is due to the triggering of the lp phase at high methane pressures, leading to a sudden increase of the uptake. The hysteresis phenomenon of desorption makes the uptakes for adsorption and desorption differ in the pressure range of 10–120 bar. This hysteresis is very well located since it allows maintaining the stability of the triggered lp phase during adsorption at 65 bar (whatever the pathway taken) while ensuring the complete return of the vnp phase (and therefore the almost complete restitution of the gas) at 5.8 bar.


image file: c7cc02743e-f3.tif
Fig. 3 CH4 total uptakes of MIL-53(Al)–NH2 and MIL-53 at 298 K (filled, adsorption; empty, desorption). The blue, purple and red arrows represent the deliverable capacities of MIL-53(Al)–NH2 between 35 bar during adsorption and 5.8 bar during desorption, between 65 bar during adsorption and 5.8 bar during desorption, and between 65 bar during desorption (after being pressurized at 120 bar) and 5.8 bar during desorption, respectively. The black arrow shows the deliverable capacity of MIL-53 between 65 bar during adsorption and 5.8 bar during desorption.

One can therefore see that the triggering of the lp phase is crucial for the good performance of MIL-53(Al)–NH2. However, the complete lp phase transition requires very high pressures (>120 bar). This can be avoided if the np to lp phase transition is triggered at lower pressures. In order to achieve this transition, the stability of the lp phase over the np phase should be increased. It has been proven that the stabilities of the vnp and np phases depend on the extent of hydrogen bonding between the amino group of the ligand and oxygen atoms of the metal cluster, regardless of the gas adsorbed.17 Thus, one way to reduce np to lp transition pressure would be to lower these interactions, simply by reducing the abundance of the amino group. This can be achieved using a mixed-ligand approach, i.e., by judicially replacing the amino functionality of the ligand (BDC–NH2) with non-functional one (BDC) in the framework. MIL-53(Al) with BDC/BDC–NH2 mixed ligands was synthesised following the previous study by T. Lescouet et al.,15,18 and the powder XRD, FT-IR, 1H-NMR and N2 sorption tests were used to characterize and quantify the relative ratios of the ligands (ESI, Fig. S2–S4, Tables S1 and S2). The CO2 sorption isotherms of MIL-53(Al) with BDC/BDC–NH2 mixed-ligands have been reported in the literature to show that the np–lp transition pressure can be shifted to lower values by increasing the BDC/BDC–NH2 ratio.15,18 Such a trend is also observed in CH4 isotherms (Fig. 4). However, it is noted that the vnp to np transition is more profoundly affected than the np to lp one. Besides, both uptakes at 65 bar and 5.8 bar have increased, resulting in a small improvement of the deliverable capacity (ESI, Table S8).


image file: c7cc02743e-f4.tif
Fig. 4 CH4 excess uptakes of MIL-53(Al)–NH2, MIL-53(Al), and MIL-53(Al) with the BDC/BDC–NH2 mixed ligand at 298 K (filled, adsorption; empty, desorption).

Another way to improve the deliverable capacity of MIL-53(Al)–NH2 would be to explore the heat of adsorption and the memory effect of the framework. As argued by Long et al.,8 adsorption is an exothermic process, which can provide the energy for phase change in flexible MOFs. The stability of the generated lp phase forces the desorption not to immediately cause the reverse transition. Thus, the endothermic desorption only results in cooling the system if it is paused at a sufficiently high pressure (above 45 bar). Starting from the last pressure point, if a second adsorption is then performed, the energy provided to the system should again trigger the np–lp transition, leading to dominance in the lp phase and consequently a higher uptake. Therefore, by making several CH4 adsorption/desorption cycles of MIL-53(Al)–NH2 between 45 and 65 bar, it could be possible to gradually increase the CH4 uptake of this material without going to higher pressures. The limited configuration of our high pressure gas sorption analyzer prevented us from conducting such cycle tests under high pressures. In order to prove this hypothesis, we performed this test using low pressure N2 sorption at 77 K due to the similarity in triggering the phase transition of MIL-53(Al)–NH2 between low pressure N2 at 77 K and high pressure CH4 at 298 K. Interestingly, cycling adsorption and desorption for multiple scans can be used to improve the final N2 uptake of the MOF. Similar trends can be observed in mesoporous materials due to several mechanisms that are not fully understood yet (gas condensation, percolation, cavitation, etc.).19 However, for MIL-53(Al)–NH2, the increase of the gas uptake cannot be linked with the mesoporous effect since MIL-53 series are well known for their microporosity with pore sizes ranging from 8 Å to 13 Å.20 In addition to MIL-53(Al)–NH2, we also tested MIL-53(Al) and HKUST-1 for comparison owing to their rigid structures under similar test conditions. Their isotherms do not show any noticeable increase (ESI, Fig. S13), confirming the unicity of this phenomenon for MIL-53(Al)–NH2. Fig. 5 shows that the highest value of the gas uptake of MIL-53(Al)–NH2 depends both on the number of cycles and on the interval of the pressure chosen. Interestingly, the pressure interval remains more important, since 25 cycles between 0.1 and 0.25 bar (second scan) give a lower final gas uptake value than 10 cycles between 0.1 and 0.3 bar. Extrapolation of this trend in CH4 adsorption curves could lead to the definition of a new process for natural gas storage using relatively low pressures to reach gas uptakes equivalent to that obtained under high pressures. Briefly, this process contains adsorption–desorption cycles between 45 and 65 bar followed by a final desorption, which will be the focus of our future studies.


image file: c7cc02743e-f5.tif
Fig. 5 Multi-cycling N2 sorption tests of MIL-53(Al)–NH2 at 77 K. The complete cycle was a cycle of adsorption–desorption between 0 and 1.0 bar. The first scan was composed of 5 cycles of adsorption–desorption between 0.1 and 0.25 bar followed by desorption to 0 bar. The second scan was composed of 25 cycles of adsorption–desorption between 0.1 and 0.25 bar followed by desorption to 0 bar. The third scan was composed of 10 cycles of adsorption–desorption between 0.1 and 0.3 bar followed by desorption to 0 bar.

In summary, we have identified the lp phase in MIL-53(Al)–NH2 under high methane pressures and we have suggested and tested several approaches to enhance the methane deliverable capacity of this flexible MOF, such as the mixed ligand strategy and continuous adsorption–desorption cycles at low pressures. These approaches may be applied to other flexible MOFs to improve their methane deliverable capacities, which is an ongoing study in our laboratory.

This work is supported by the National University of Singapore (CENGas R-261-508-001-646 and R-279-000-474-112).

Notes and references

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Footnote

Electronic supplementary information (ESI) available: Experimental details and relevant calculations. See DOI: 10.1039/c7cc02743e

This journal is © The Royal Society of Chemistry 2017