TECHNICAL FIELD

[0001] The present invention is related to a method for storing a gas. BACKGROUND ART

[0002] The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.

[0003] Selective admission of guest molecules into microporous solids is the

cornerstone of a range of key applications such as gas separation, storage, and recognition. However, in many important cases the size difference between relevant molecules can be of order 0.1 A, and materials with the appropriately sized micropores necessary for discrimination are often very difficult to find.

[0004] Current popular hydrogen storage technology by physisorption is implemented by adsorption of hydrogen at low temperature and high pressure in highly porous materials. In order to increase the hydrogen storage capacity, efforts have been devoted to increase the gas-adsorbent affinity via enhancing chemical interaction and tailoring confined pore space. These strategies, however, render the stored hydrogen difficult to release, which requires increased regeneration cost.

SUMMARY OF INVENTION

[0005] In accordance with the present invention, there is provided a method for gas storage comprising the steps of: loading a gas storage material with gas, wherein the gas storage material has an admission temperature specific to the combination of the gas storage material and the gas, and the step of loading the gas storage material with gas is conducted above the admission temperature; encapsulating the gas in the gas storage material; and reducing the temperature below the admission temperature, and storing the gas in the gas storage material.

[0008] Preferably, the step of loading the gas storage material with gas is conducted above atmospheric pressure. More preferably, the pressure is greater than 10 bar. More preferably, the pressure is greater than 50 bar. More preferably, the pressure is greater than 100 bar.

[0007] In accordance with the present invention, there is provided a method for gas storage comprising the steps of: loading a gas storage material with gas, wherein the gas storage material has an admission temperature specific to the combination of the gas storage material and the gas, and the step of loading a gas storage material with gas is conducted above the admission temperature and at a pressure greater than 100 bar; encapsulating the gas in the gas storage material; and reducing the temperature below the admission temperature, and storing the gas in the gas storage material.

[0008] Preferably, the method of present invention comprises the further step of: decreasing the pressure, after the step of: reducing the temperature below the admission temperature. [0009] In one form of the invention, the step of: decreasing the pressure, comprises decreasing the pressure to about atmospheric pressure.

[0010] In accordance with the present invention, there is provided a method for gas storage comprising the steps of: loading a gas storage material with gas, wherein the gas storage material has an admission temperature specific to the combination of the gas storage material and the gas, and the step of loading a gas storage material with gas is conducted above the admission temperature and at a pressure greater than atmospheric pressure; encapsulating the gas in the gas storage material; reducing the temperature below the admission temperature, thereby storing the gas in the gas storage material; and decreasing the pressure.

[001 1 ] The admission temperature is inherent to the particular combination of the guest molecule (i.e. the gas) and the gas storage material. Above the admission temperature, the guest molecule can enter and leave the pore space in the gas storage material, while below said temperature it is encapsulated within or blocked from entering.

[0012] It will be appreciated that the stored gas may be released by increasing the temperature above the admission temperature.

[0013] The method of the present invention offers substantial potential for gas storage because gas can be encapsulated and prevented from escaping by lowering the system temperature to below the admission temperature. In addition, it is not necessary to maintain high pressures for storage as required by gas storage methods of the prior art.

[0014] Gas storage materials, include, without limitation, zeolites (such as low silica chabazite, RHO, LTA), caesium chabazites, rubidium chabazites, potassium chabazites, caesium LTA, rubidium LTA, potassium LTA, RHO, and K-KF123, metal- organic frameworks (such as CuTEI where TEI stands for 5- ((triisopropyls!lyl)ethynyi)isophthalic acid, microporous manganese formate), and ca!ixarenes (such as p-t-butylcalix[4]arene).

[0015] The gas may be selected from the group comprising hydrogen, helium, argon, nitrogen and methane. The present invention provides the advantage of storing the gas by encapsulation rather than relying solely upon gas adsorption. Non-polar gases like hydrogen, helium and argon adsorbs very weakly to many microporous compounds. Where the gas is stored by encapsulation and not reliant on adsorption, it is possible to dramatically increase the amount of stored gas by increasing the pressure at which the gas is dosed. Additionally, as a result, provided the temperature is below the admission temperature, high pressures do not need to be maintained. Indeed, it can be possible to store gases at pressure below atmospheric.

[0018] Preferably, the gas is hydrogen.

[0017] In one form of the invention, the gas storage material has surface properties that inhibit adsorption of gases. Where the encapsulated gas is not adsorbed to any appreciable extent, increasing the pressure increases the storage capacity of the gas storage material.

[0018] Advantageously, increasing the pressure during the step of loading the gas storage material with gas increases the loading capacity of the gas storage material.

[0019] Without being limited by theory, it is believed that the storage capacity of gases is linearly proportional to the pressure at which the gas is dosed.

[0020] Preferably, the method of present invention comprises the further step of: reducing the pressure of the gas storage material below atmospheric pressure, prior to the step of: loading the gas storage material with gas. [0021] Preferably, the step of reducing the pressure of the gas storage material to below atmospheric pressure comprises the step of: applying a vacuum to the gas storage material.

[0022] Advantageously, the present invention permits the storage of hydrogen at atmospheric pressure. This is a significant improvement over, for example, prior hydrogen storage methods that require high pressure to maintain storage. In conventional hydrogen storage based on physisorption in porous materials, a high storage capacity necessitates applying high pressure to ensure high adsorption amount. Maintaining a high storage pressure poses potential hazard and increase the weight of the gas storage system.

[0023] Advantageously, it is possible to control and manipulate the admission temperature of the gas storage material by manipulating the pore keeping group. For example, altering the charge balancing cation in a zeolite will alter the admission temperature of a combination of the zeolite and a gas to be stored within the zeolite. Specifically, replacing K ⁺ with Cs ⁺ in a trapdoor type low silica chabazite whose 8- member-ring pore aperture are kept by the cations can increase the admission temperature. Alternatively, decreasing the Si/AI ratio can increase the cation density and can increase the admission temperature - for example a tested K-form trapdoor chabazite with Si/AI ratio of 2.2 has an admission temperature of 266 Kelvin for methane. This increased to 323 Kelvin after decreasing the Si/AI ratio to 1 .8.

[0024] Where the gas storage material is a calixarene, changing the group para to the phenol can change the admissibility of the gas molecule.

[0025] Without being limited by theory, it is believed that the admission of guest molecules such as hydrogen, nitrogen, argon, and methane in calixarenes is via the turnstile effect wherein the para substituents such as tert-butyl can rotate and permit hydrogen transfer. Changing the groups para to the phenol groups making gas admission easier or harder will change the admission temperature. For example, it is known tert-butyl-biphenyl group will make the admission of the guest molecule more difficult than tert-butyl at the para position. Additionally, changing the number of phenol groups in the calixarene (e.g. five phenol groups in a calix[5]arene) will alter the admission temperature.

[0028] It will be appreciated that the calixarene may be one based on resorcinol or pyrogallol as well as phenol.

[0027] In one form of the invention, the method comprises the step of loading the gas storage materia! with gas at a pressure between 100 bar and 5000 bar.

[0028] In one form of the invention, the method comprises the step of loading the gas storage material with gas at a pressure between 500 bar and 2500 bar.

[0029] In one form of the invention, the method comprises the step of loading the gas storage material with gas at a pressure between 1000 bar and 2000 bar.

[0030] In one form of the invention, the method comprises the step of loading the gas storage material with gas at a pressure between 1250 bar and 1500 bar.

[0031 ] It will be appreciated that the method of the present invention may be used to separate gas mixtures. Where a gas storage material has a different admission temperature for two different gases (T _lower and T _higher respectively), the two gases may be separated by increasing the temperature to a temperature above T _lower but less than T _higher - Under said conditions, only the gas with the lower admission temperature T _lower could be encapsulated by the gas storage material. The temperature can then be lowered to prevent the encapsulated gas being released.

[0032] In accordance with the present invention, there is provided a method for gas separation comprising the steps of: loading a gas storage material with a mixture of gases, wherein the gas storage material has a first admission temperature specific to the combination of the gas storage material and a first gas, and the gas storage material has a second admission temperature specific to the combination of the gas storage material and a second gas, and the step of loading a gas storage material with a mixture of gases is conducted at a temperature higher than the first admission temperature and lower than the second admission temperature and at a pressure greater than atmospheric pressure; encapsulating the first gas in the gas storage material; and reducing the temperature below the admission temperature, thereby storing the first gas in the gas storage material.

[0033] Advantageously, the method of the present invention provides, in addition to gas storage, the ability to control and regulate the release of gases. It will be appreciated that changing the temperature during gas release has an effect on the rate of gas release.

[0034] In accordance with the present invention, there is provided a method for hydrogen storage comprising the steps of: loading a hydrogen storage material with hydrogen, wherein the hydrogen storage material has an admission temperature specific to the combination of the hydrogen storage material and hydrogen, and the step of loading the hydrogen storage material with hydrogen is conducted above the admission temperature; encapsulating hydrogen in the hydrogen storage material; and reducing the temperature below the admission temperature, and storing the gas in the gas storage material.

[0035] Preferably, the step of loading a hydrogen storage material with hydrogen is conducted above atmospheric pressure. Preferably, the pressure is greater than 10 bar. More preferably, the pressure is greater than 50 bar. More preferably, the pressure is greater than 100 bar

[0036] In accordance with the present invention, there is provided a method for hydrogen storage comprising the steps of: loading a hydrogen storage material with hydrogen, wherein the hydrogen storage material has an admission temperature specific to the combination of the hydrogen storage material and the hydrogen, and the step of loading the hydrogen storage material with hydrogen is conducted above the admission temperature and at a pressure greater than 100 bar; encapsulating hydrogen in the hydrogen storage material; and reducing the temperature below the admission temperature, thereby storing the hydrogen in the hydrogen storage material.

[0037] Preferably, the method of present invention comprises the further step of: decreasing the pressure, after the step of: reducing the temperature below the admission temperature. [0038] In one form of the invention, the step of decreasing the pressure comprises decreasing the pressure to about atmospheric pressure.

[0039] The admission temperature is inherent to the particular combination of the hydrogen and the hydrogen storage material. Above the admission temperature, the hydrogen can enter and leave the pore space in the hydrogen storage material, while below said temperature it is encapsulated within or blocked from entering.

[0040] It will be appreciated that the hydrogen may be released by increasing the temperature above the admission temperature. [0041 ] The method of the present invention offers substantial potential for hydrogen storage because hydrogen can be encapsulated and prevented from escaping by lowering the system temperature to below the admission temperature,

[0042] Hydrogen storage materials, include, without limitation, zeolites (such as low silica chabazite, RHO, LTA), caesium chabazites, rubidium chabazites, potassium chabazites, caesium LTA, rubidium LTA, potassium LTA, RHO, and K-KFI23 metal- organic frameworks (such as CuTEI where TEI stands for 5- ((triisopropylsiiyl)ethynyi)isophthalic acid, microporous manganese formate), and calixarenes (such as p-t-butylcalix[4]arene).

[0043] Preferably, the method of present invention comprises the further step of: reducing the pressure of the hydrogen storage material below atmospheric pressure, prior to the step of: loading the hydrogen storage material with hydrogen.

[0044] Preferably, the step of reducing the pressure of the hydrogen storage material to below atmospheric pressure comprises the step of: applying a vacuum to the hydrogen storage material.

[0045] In one form of the invention, the method comprises the step of loading the hydrogen storage material with hydrogen at a pressure between 100 bar and 5000 bar.

[0048] In one form of the invention, the method comprises the step of loading the hydrogen storage material with hydrogen at a pressure between 500 bar and 2500 bar.

[0047] In one form of the invention, the method comprises the step of loading the hydrogen storage material with hydrogen at a pressure between 1000 bar and 2000 bar. [0048] In one form of the invention, the method comprises the step of loading the hydrogen storage material with hydrogen at a pressure between 1250 bar and 1500 bar.

[0049] In accordance with the present invention, there is provided a storage apparatus for storing a gas comprising a pressure vessel with means for receiving and discharging the gas, heating means for heating a gas storage material above an admission temperature; pressure means to reduce the pressure in the storage apparatus to less than atmospheric pressure and pressure means to increase the pressure in the storage apparatus to greater than 100 bar.

[0050] Preferably, the gas is hydrogen.

[0051 ] It will be appreciated that the means for receiving and discharging hydrogen may be the same or different means.

BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Further features of the present invention are more fully described in the following description of several non-limiting embodiments thereof. This description is included solely for the purposes of exemplifying the present invention. It should not be understood as a restriction on the broad summary, disclosure or description of the invention as set out above. The description will be made with reference to the accompanying drawings in which:

Figure 1 is a schematic representation of the setup of an encapsulation apparatus;

Figure 2 shows adsorption isotherms for hydrogen, argon, nitrogen, and methane, respectively, on the potassium chabazite showing restricted pore accessibility at low temperatures (crossed symbols) but no restriction at high temperatures (open symbols);

Figure 3 shows measured amount of hydrogen decapsuiated from r2KCHA as a function of system re-heating temperature after dosing with different initial pressures; Figure 4 presents a schematic diagram of an example route for synthesizing a core-shell zeolite particle composed of a high porosity core and a stimuli-responsive shell structure;

Figure 5 shows hydrogen storage capacity on the potassium chabazite by molecular encapsulation as a function of initial dosing pressure;

Figure 6 shows calculated hydrogen storage as a function of initial dosing pressure; and

Figure 7 depicts adsorption isotherms of CH ₄ on p-tert-butyi-calix[4]arene. DESCRIPTION OF EMBODIMENTS

[0053] Throughout this specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers,

[0054] Chabazite with Si/AI ratio of 2.2 was synthesized from zeolite Y in accordance with known procedures. Ion exchange with the potassium form with a typical procedure: 5 g of chabazite in 200 mL of 1 M KCI was refluxed at 343 K under stirring for 24 hr, followed by filtration and wash with deionized water 2-3 times. The ion exchange procedure was repeated twice to obtain fully exchanged potassium chabazite (labelled as r2KCHA).

[0055] p-feri-butylcalix[4]arene was synfhesised according to known procedures. After the synthesis, the sample was heated under vacuum at 140 °C for 24 hr to remove any possible entrapped solvent toluene. The sample was sublimed at 280 °C under vacuum to give the final product used in analysis.

[0058] The species of inorganic ions and Si/AI ratios of the chabazites were determined by inductively coupled plasma-mass spectroscopy (ICP-MS). For in situ synchrotron powder X-ray diffraction experiments, the zeolite powder was pre-activated on

IVlicromeritics ASAP 2010 (stepwise heated up to 623 K and held at this temperature for 12 hours under high vacuum < 1 Pa), prior to being loaded in a 0.7 mm quarts capillary and sealed in Ar glove bag. Then the capillary was decanted at one end and mounted to synchrotron powder X-ray diffraction machine with this open end being connected to vacuum/gas line. Prior to experiments, the sample was re-activated in situ, followed by cooling the sample down to 213 K. For the scans under vacuum, the sample was heated from 213 to 383 K at 5 K/min, during which X-ray diffraction data was collected at a series of temperature points; the sample was held at each set point for 60 s (to ensure a stable sample temperature) before scanning for 600 s. For the scans in nitrogen atmosphere, a similar procedure was adopted and the only difference is that nitrogen was dosed into the capillary slowly to 1 bar at 423 K before cooling to 213 K and the nitrogen pressure was kept at 1 bar. Nitrogen was introduced at a relatively high temperature to assure the gas was admitted and the behaviour of the zeolite sample we studied was truly in the presence of the gas. A Mythen-!i detector was used for ail data collection with a X-ray wavelength of 0.5894 A for all zeolites.

[0057] The purity of p-f-butylcalix[4]arene was verified by nuclear magnetic resonance (NMR) and high performance liquid chromatography, and no impurities were detected by either technique. The crystal structure of the p-f-butyicalix[4]arene was also identified by synchrotron PXRD with a X-ray wavelength of 0.77502 A.

[0058] Prior to isotherm measurements, the samples were thoroughly dehydrated and degassed by heating stepwise to 413 K for p-t-butyicalix[4]arene and 623 K for r2KCHA under high vacuum overnight on a Micromeritics ASAP2010/2020 accelerated surface area and porosity analyser. After degas the samples were cooled to room temperature and backfilled with helium. Adsorption isotherms on r2KCHA were measured in the temperature range of 77-343 K for nitrogen, 77-303 K for helium, 197-305 K for argon, and 195-353 K for methane and at pressures up to 120 kPa. Temperatures of 77 K and 195 K were achieved with liquid nitrogen and ethanoi-dry ice baths, respectively. Other unconventional cryogenic temperatures below 243 K were achieved using a solid-liquid thermostatic bath of water-ethanol in different ratios. CH4 isotherms on p~t- butylcalix[4]arene were conducted in the temperature range from 195 to 323 K.

[0059] Ab initio density functional theory (DFT) calculations were employed to determine the energy barriers associated with the guest admission process as well as the snapshots of the trajectories of the gas molecules and the door-keeping cations in r2KCHA, Vienna Ab initio Simulation Package (VASP) as used with the projector augmented waves (PAW) approach. The cut-off energy of the plane wave basis-set was 405 eV. A gamma point only k-point mesh was used for one unit ceil of chabazite (including three double six-ring prisms or one and a half supercavities). Such cut-off energy and /c-point mesh have been tested to ensure the total energy value

convergence within 1 meV/atom. The atomic positions were optimized with the conjugate gradient method until the forces acting on atoms were below 0.015 eV/A. The DFT-D3 functional (with !VDW=1 1 ) was adopted to account for the van der Waals interactions, and the nudgedelastic-band (NEB) method for energy barrier calculations.

[0060] The temperature for dosing and quenching 5.93 g of r2KCHA was 323 K and 77 K, respectively. The setup for conducting the reversible hydrogen encapsulation and decapsulation experiment is described below and shown in Figure 1 .

[0061 ] The sample cylinder 10 is constructed for the purpose of containing the r2KCHA whilst it undergoes activation, dosing, quenching and decapsulation procedures. The main cylinder is a Swagelok double-ended TPED-compliant sample cylinder rated to 124 bar with 1/4 inch female NPT ends and a volume of 50cm3. Valve 18 is a Swagelok stainless steel bonnet needle valve with 1 /8 inch Swagelok tube fittings. During dosing, valves 12 and 14 are dosed and valves 16 and 18 are open. Hydrogen gas 20 at 100 bar passes through a feed gas regulator 22 and enters the sample cylinder 10 containing the zeolite. . At the quenching step, the cylinder 10 is immersed in a liquid nitrogen bath (not shown). At the evacuation step, the feed gas regulator 22 is turned off and valve 12 opened for venting; after closing 12, valve 14 is opened to allow for evacuation of free space hydrogen from the dosed sample cylinder by the vacuum pump 24. The evacuation was maintained at the vacuum level of below 1 mbar for approximately 10 min. The hydrogen encapsulated inside the r2KCHA sample is then released by warming up the sample cylinder 10 in ambient air; and the amount of hydrogen evolved is quantified by measuring the pressure increase in the system.

[0062] Figure 2 shows how pore accessibility can be abruptly and reversibiy switched on/off across a narrow temperature range for four different guest molecules on r2KCHA (potassium chabazite with Si/AI ratio of 2.2). This material is a typical small-pore zeolite in which eight-membered oxygen rings (8MRs) govern the access of guest molecules to internal cavities. Geometrically, the eight-membered oxygen rings are large enough for simple gas molecules such as hydrogen and argon to pass through, but in trapdoor zeolites like r2KCHA, this access is blocked by large cations which occupy the energetically favourable site at the centre of the oxygen rings doorway.

[0063] Figure 2 shows adsorption isotherms for hydrogen (Figure 2a), argon (Figure 2b), nitrogen (Figure 2c), and methane (Figure 2d), respectively, on the potassium chabazite showing restricted pore accessibility at low temperatures (crossed symbols) but no restriction at high temperatures (open symbols).

[0064] A sharp transition in the pore accessibility with temperature is clearly apparent for r2KCHA in the uptake of hydrogen, argon, nitrogen and methane (Figure 2). The ultra- low adsorption capacities observed for these molecules at 149, 197, 223, and 233 K, respectively, transition to considerable capacities with an increase of only 30 - 70 K.

[0065] Many other zeolites also exhibit such a pore-blocking effect including caesium chabazites, rubidium chabazites, potassium chabazites, caesium LTA, rubidium LTA, potassium LTA, RHO, and K-KFI23.

[0066] The temperature-regulated guest admission mechanism identified here offers substantial potential for such applications because pre-dosed hydrogen can be encapsulated and prevented from escaping by lowering the system temperature well below the admission temperature. As illustrated in Figure 3, this concept was explored using the r2KCHA trapdoor zeolite and dosing it at ambient temperature with hydrogen at various pressures up to 10 MPa. After quenching to cryogenic temperatures, the system was evacuated to remove all the free gaseous hydrogen, leaving only the hydrogen trapped inside the chabazite's intracrystal cavities. No hydrogen was evolved over an extended period under vacuum as long as the temperature remained well below To = 170 K. The appreciable quantity of hydrogen stored at this low pressure and low temperature condition was then measured by returning the system to ambient temperature (Figure 3) thereby completely releasing the stored hydrogen. The

cumulative amount of hydrogen evolved varied with the initial dosing pressure, which for 10 MPa amounted to about 0.95 wt%. This result demonstrates the ability to store physically substantial quantities of hydrogen in materials with temperature-regulated pore accessibility without sustained external pressure. The encapsulation capacity is proportional to the intracrystal pore volume as opposed to the surface area; in this example the hydrogen encapsulated was 30 times greater than the maximum

adsorption capacity at the same storage temperature. These preliminary results suggest the possibility of storing 3-7 wt% hydrogen at ambient pressures by quenching a chabazite initially dosed at ambient temperature to 70-250 MPa). The encapsulation temperature could be elevated to near ambient by using heavier door-keeping cations, e.g. , Rb+, while the storage capacity could be increased by designing a composite material with a high porosity core and a trapdoor shell.

[0067] The encapsulation temperature could be elevated to near ambient by using heavier door-keeping cations, e.g. , Rb ⁺ , while the storage capacity could be increased by designing a composite material with a high porosity core and a trapdoor shell.

[0068] Figure 4 presents a schematic diagram of an example route for synthesizing a core-shell zeolite particle composed of a high porosity core 26 and a stimuli-responsive shell structure 28. A proton type chabazite (H-CHA) 30 that has higher pore volume due to smaller cations is used as the template, followed by chemical vapour deposition (CVD) 32 to fill up the zeolite pores with carbon 34. The outer layer of the infiltrated carbon is removed under controlled combustion 36 in a dilute 0 ₂ atmosphere, and the exposed layer of H-CHA 38 undergoes an ion exchange 40 to replace the proton by Cs ⁺ , to provide a Cs-CHA shell 42. Finally the zeolite particle undergoes a complete combustion to remove the residual carbon and recover the H-CHA core 30.

[0069] The calculation of the amount of hydrogen stored by encapsulation is presented as follows. Note that total amount of hydrogen stored (n _recorded ) is the summation of three contributing terms: amount of desorbed (r _desorbed ) (previously adsorbed on the internal surface of the zeolite), amount of decapsulated (n _decapsulated ) (molecules previously trapped in the zeolite intracrystal pore cavity), and residual amount (n _residual hydrogen) (molecules re-adsorbed after decapsulation). For the residual amount, it is a function of decapsulation pressure and temperature; if the release pressure is 1 bar and this term n _residual hydrogen is negligible.

[0070] More specifically; • n _desorbed is the difference between of the quantity of hydrogen adsorbed at criticai encapsulation temperature and dosing pressure and the quantity of hydrogen adsorbed at the released gas temperature and pressure;

• n _encapsulated is the difference between the quantity of hydrogen in the zeolite at the dosing pressure and the quantity of hydrogen in the zeolite at the released gas pressure and

• n _residual hydrogen is the quantity of the hydrogen remaining in the apparatus at the quenching temperature and atmospheric pressure.

[0071 ] Noting that the storage capacity above 60 bar becomes linearly proportional to the initial dosing pressure, suggesting the term of n _desorbed flats off due to saturation on internal surface sites which is consistent with the weak adsorption of hydrogen on zeolites. The linearly increasing part of n _recorded is predominantly the contribution of n _decapsulated , which is proportional to the volume of internal pores of the chabazite available from literature. Therefore, the amount of hydrogen stored with much higher initial dosing pressures can be reasonably predicted using the knowledge of pore volume and the density of hydrogen at various temperatures and pressures. Figure 5 reflects the accuracy of the model (— ) against experimentally observed hydrogen loading (■). Figure 6 presents theoretically determined loading at higher pressures.

[0072] Calixarenes are supramolecular host materials composed of macro-cyclic polyphenols. The cyclic tetramer calix[4]arene has a stable basket configuration. Methane adsorption isotherms were measured on p-t-butylcalix[4]arene between 195 - 303 K at pressures up to 120 kPa (Figure 7). Modelling provided an admission temperature of 230 K for CH ₄ admission. The results can be interpreted in terms of the cooperative rotation of the tert-butyl groups producing a 'turnstile' effect which regulates the admission of guests into the CX[4] cavity.

[0073] Adsorption data reported for a number of MOFs (including mesh-adjustable molecular sieve MOFs, zinc-dicarboxylate-bipyridine MOF, microporous manganese formate) also exhibit the bell-shaped isobars characteristic of the temperature- dependent gas admission effect. Interestingly, all these MOFs have pore-keeping groups responsive to temperature changes For example, adsorption capacity data have been presented for a porous coordination nanocage, CuTEI, the exterior of which is covered by turnstile-like triisopropylsiiyl groups that regulate access to the nanocage.