Using Pressure to Provoke the Structural Transition of Metal–Organic Frameworks

Abstract

Easy and efficient energy storage is one of the problems treated by numerous researchers today. Hydrophobic nanoporous materials can potentially be used as actuators, but also as molecular springs, dampers, or shock absorbers. [1] In this case, the reversible intrusion of a liquid in nonwetting pores at high pressure, a process subject to hysteresis, is used to store or produce mechanical work. Herein, we show the possibility to store mechanical energy using porous metal-organic framework materials (MOFs) in which their flexibility is used instead of nonwetting properties. Indeed, MOFs have found increasing interest over the past few years in potential applications such as gas separation and storage, [2][3][4] liquidphase separation, [5] or drug delivery.[6]One of the unique properties of some of these materials is their high degree of framework flexibility, which has reached 230 % in the case of the MIL-88 series (MIL stands for Materials from Institut Lavoisier). In most cases, the flexibility of these materials has been induced by adsorption of guest species, [7] which produce various types of flexibility. [8,9] One of the most interesting classes of flexible solids are those of the MIL-53 series. These are metal(III) terephthalates built up from chains of corner sharing metal(III) octahedral (M = Al, Cr, Fe, Ga, In, …) and terephthalate groups that delimit one-dimensional microporous pore system.[2] The Al and Cr forms are found in the large pore (LP) form after thermal removal of the guest species, whereas in the presence of various fluids, a narrow pore (NP) form is observed (MIL-53(Cr) NP; space group C2/c; V % 1020 3 ) before re-expansion to the LP form (MIL-53(Cr) LP; space group Imcm; V % 1490 3 ; see Figure 1). In the case of MIL-53(Al), this reversible flexible character has also been observed as being dependent on the temperature [10] with hysteresis between the cooling and heating processes. The transition from LP to NP occurs on cooling in the 125-150 K range whereas that from NP to LP occurs on heating in the range 325-375 K. The presence of these different crystalline states opens the possibility for phase diagrams to be established.Although previous gas adsorption studies have been carried out under various pressures, [4,5,9] to the best of our knowledge, the response of these materials solely under pressure, without any adsorption effect in pores, has not been reported to date. In the recent study of Moggach et al., ZIF-8 samples were submitted to a high hydrostatic pressure that provokes crystal phase transitions, but with the presence of liquid inside pores.[11] Logically, one would expect that a transition from the LP to the NP phase would occur provided a high enough pressure is applied to the MOF phase in question. One reason for this behavior is that the MIL-53 phases are usually synthesized in the form of micrometersized particles and it is not easy to impose a mechanical stress around the particle in a controlled manner. However, the use of mercury porosimetry permits an isostatic press...

The Direct Heat Measurement of Mechanical Energy Storage Metal–Organic Frameworks

“…Indeed, similar behavior was previously observed for the MIL-47(V) sample [5] although not discussed: the transition with mercury occurs at around 85 MPa whereas the transition with the same silicone oil is observed by XRD at 145 MPa. This difference in transition pressure, which depends on the liquid used for creating the isostatic pressure, may be the indication that the silicone oil partially penetrates at the entrance of the pores, as suggested above.…”

“…[1][2][3][4] Recently, another area has been proposed where the mechanical properties of flexible MOFs can be used in energy-related applications such as dampers (partial dissipation of mechanical energy), shock absorbers (total dissipation), or springs (no dissipation). [5][6][7] …”

“…[6,14] This compression step occurs at a pressure which is smaller than that observed either for MIL-53(Cr) (around 55 MPa [4] ) or MIL-47(V) (around 85 MPa). [5] The volume variation during this step (0.24 mL g À1 ) is comparable to that calculated from structure parameters obtained by XRD of approximately 0.3 mL g À1 (calculated from a volume variation of 29 % from LP to NP and a density of 0.98 g mL À1 for LP form). [22,24] Contrary to the two other samples, the compression of MIL-53(Al) is not reversible in the analyzed pressure range: after decreasing the pressure, the LP form is not fully recovered.…”

“…[5] With the latter, an isostatic pressure is imposed around the MOF with a non-wetting transmission fluid. The simplest means to carry out such experiments is to use mercury, which allows the system volume to be followed as a function of pressure (P).…”

“…At room temperature and pressure, it is found in a large pore (LP) form with a channel opening of 7-8 in width. As in the case of MIL-53(Cr), [5] the sample was first studied by mercury porosimetry. Figure 1 shows the volume-pressure curve obtained with the MIL-53(Al) where three domains are observed along the intrusion curve.…”