WO2009068912A1

WO2009068912A1 - Clathrates for gas storage

Info

Publication number: WO2009068912A1
Application number: PCT/GB2008/051115
Authority: WO
Inventors: Andrew Ian Cooper; Christopher Laurence Bray; Fabing Su; Benjamin Owen Carter; Weixing Wang; David John Adams
Original assignee: Ulive Enterprises Limited
Priority date: 2007-11-26
Filing date: 2008-11-26
Publication date: 2009-06-04
Also published as: GB2454931A; GB0723071D0

Abstract

A gas, for example hydrogen, methane or carbon dioxide, is enclathrated, and/or dissociated from a clathrate, for example clathrate hydrate, in the presence of an emulsion-templated porous support, for example polyHIPE. The emulsion-templated porous support enhances the practical applicability of gas storage by clathrates.

Description

Clathrates for gas storage

This invention relates to the use of clathrates as gas storage materials, for example in the storage of hydrogen, methane, carbon dioxide or other gases.

Hydrogen is a useful material, and technology based on the use of hydrogen, for example in fuel cells, is likely to be of greater value in the future. Cost-effective hydrogen-based applications require effective means of storing hydrogen. At room temperature and pressure hydrogen is a gas which is difficult to handle because of its large volume and reactivity. Liquefaction requires expensive and costly apparatus and liquid hydrogen can be difficult and dangerous to handle. Known methods for producing hydrogen from chemical compounds in situ are generally unsatisfactory for many industrial uses.

Clathrate hydrates have been studied as media for the storage of hydrogen (see for example Struzhkin, V. V.; Militzer, B.; Mao, W. L.; Mao, H. k.; Hemley, R. J. Chem. Rev. 2007, 107 No. 10, 4133). Clathrate hydrates comprise "host" cages Of H₂O assemblies, in which are bound "guest" materials. Clathrate hydrates have the potential to provide a safe and environmentally friendly solution to the need for hydrogen storage. However, the incorporation of hydrogen into clathrate hydrates can take an extremely long time (days or weeks) and furthermore the timescale of "freezing" the H₂-H₂O clathrate structures can be unpredictable. Enhancement of hydrogen enclathration kinetics together with good rechargeability is still a challenge for developing clathrate hydrates as a feasible hydrogen storage material.

In principle a wide range of gases - not just hydrogen - maybe stored within clathrates and the clathrates need not necessarily be clathrate hydrates but can comprise any suitable host - not just H₂O. Numerous other hosts are possible, such as for example those listed in US patent application publication US 2004/0230084 (Yagi).

There is no particular limitation of the type of gas that may be stored as guest in a clathrate beyond the requirement that the host and guest must be sufficiently compatible to form a stable clathrate structure, optionally in the presence of stabilizing agents. Known clathrates include those comprising (as guest) H₂, O₂, N₂, CH₄, CO₂, air, H₂S, Ar, Kr, Xe, He and Ne, amongst others (see for example international patent application publication no. WO 2006/131738 (Heriot-Watt University) and Lokshin, K. A. et al._s Physical Review Letters 5 2004, 193, No. 12, 125503). The stored gas may also comprise a mixture - for example, natural gas.

Whilst improvements in the storage of gases in general would be desirable, there are particular needs to enhance the technology for storing some particular gases from a0 technological and environmental viewpoint. The importance of hydrogen storage and the possibility of enclathration of hydrogen have been mentioned above. Gas remediation solutions are required to trap undesirable gases. Climate change and in particular global warming has prompted a search for better means of sequestration of certain gases, for example CO₂. Methane and natural gas also act as valuable fuels and therefore their storage5 is also important. These gases can all be stored in clathrates. In some cases the host may be a useful commodity as well as the guest; for example a clathrate of H₂ within CH₄ or octane can act as a dual fuel.

However, to date the feasibility of clathrates as gas storage means has been severely limited0 by the poor speed, reliability and cyclablity of enclathration and dissociation.

From a first aspect the present invention provides a method comprising the enclathration of a gas, in the presence of an emulsion-templated porous support. 5 Consistent with conventional meanings in the art, the term "clathrate" also includes "semi- clathrate". As known in the art, a semi-clathrate is an association comprising a host and a guest wherein the guest forms part of the clathrate framework; for example the guest, or one of the guests if there is more than one, may both be physically bonded to the host cage structure, and occupy some of the cavities. 0

Emulsion-templated porous supports are known in the art (see for example Cameron, N. R. Polymer 2005, 46, 1439) and are formed by the curing or solidification of a continuous (non-droplet) phase around a template of internal (droplet) phase, followed by the removal of the internal (droplet) phase.

As exemplified below, the use of an emulsion-templated porous support increases the speed of gas clathrate formation. Thus the present invention allows gas to be incorporated into clathrate cages more quickly than has previously been possible. This results in easier and less expensive gas charging, whether in a specialized gas charging plant, or in situ.

The choice of the type of support is significant because other supports, such as powdered activated carbon, have been found to have no significant effect on the enclathration kinetics, even when used in large amounts.

The present invention also allows the gas to be released from the clathrate structures more quickly. This allows ease of use at the point where gas is required, for example in fuel cells, and this advantage is particularly relevant where the storage is within a self-contained, remote or mobile unit, such as a vehicle.

Transitions between liquid and solid phases can be unpredictable; for example a laboratory chemist may find crystallization of a chemical compound to be difficult in practice, and may try various techniques to initiate the seeding of crystals. Similar unpredictability has hitherto been seen in the formation of clathrate structures and is a significant barrier to industrial applicability. One of the commercial advantages of the present invention is that it allows enclathration and dissociation to occur predictably and reliably.

The present invention allows multiple storage/release cycles. Cyclability of gas enclathration means that gas storage units can be used repeatedly, thereby reducing the down-time, costs and practical difficulties associated with maintenance, conditioning or replacement. Data presented below show the reproducibility of gas enclathration when the same emulsion-templated porous support is used on consecutive runs.

The present invention also allows gas uptake to occur rapidly and reproducibly in the absence of any agitation or mechanical mixing. This is an advantage since the incorporation of mixing devices adds complexity to the apparatus as well as introducing additional system weight and energy requirements.

Without wishing to be bound by theory, it is believed that the emulsion-templated porous support results in advantages because of its particular porous and interconnected nature. A thin film is believed to form on the surface of the support thereby greatly increasing the interfacial area for gas mass diffusion.

Emulsion-templated porous supports are relatively inexpensive and can be produced as moulded monoliths, for example for use as fuel tanks.

Preferably the clathrate is a clathrate hydrate, i.e. the host comprises H₂O. Thus, gas molecules, for example H₂ molecules are stored within H₂O cages. Clathrate hydrates are particularly advantageous because the host is merely water, is extremely easy and cost- effective to obtain, store, and dispose of, and is particularly environmentally friendly.

Regardless of the type of host, the present invention is of particular utility in the storage of hydrogen, hydrocarbon gas (e.g. methane) or carbon dioxide. It is particularly preferred to store these gases in clathrate hydrates, i.e. in H₂O hosts.

Hydrocarbon gas includes up to C₄ hydrocarbon compounds which may be saturated or unsaturated, for example methane, ethane and propane.

The present invention is also applicable to different gases, such as for example O₂, N₂, air, H₂S, Ar, Kr, Xe₅ He and Ne, or mixtures of such gases.

It is known that "stabilizer" (also known in the art as "promoter") materials may be used to aid the formation of clathrates, and such materials are therefore optionally present (along with the gas to be stored) in the clathrate structures according to the present invention. Examples of stabilizers include tetrahydrofuran (THF) and tetra-n-butylammonium bromide (TBAB) amongst others. These examples are particularly useful when the clathrate is a clathrate hydrate, for example when the gas to be stored is hydrogen. Other examples of stabilizers include cyclic ethers (for example ethyleneoxide (EO), 1,3- dioxolane, 1,3- and 1,4-dioxane, and trimethyleneoxide), per alkyl-onium salts, alkylamines (for example tert-butylamine), diamines, diols, crown ethers and their complexes, methylcyclohexane and methyl-tert-butyl ether (MTBE). These stabilizers may for example be used when the clathrate is a clathrate hydrate.

Further examples of suitable stabilizers include tetra-w-butylammonium fluoride (TBAF) and tetra-iso-amylammonium bromide (TiAAB), of which TiAAB is preferred. These stabilizers may for example be used when the clathrate is a clathrate hydrate, and particularly when methane or hydrogen, preferably methane, is enclathrated in H₂O.

Stabilizers are believed to enhance the stability of certain types of clathrate structure. For example H₂ and THF can both be contained as guests within the so-called sll structural framework of clathrate hydrate such that THF sits within some larger cavities thereof. The use of a suitable stabilizer enhances the gas storage capacity of certain clathrates and allows lower gas enclathration pressures.

Stabilizers act as guests in the sense that they may be contained within the host (in a clathrate) or may form a cage together with the host (in a semi-clathrate).

Semi-clathrates share many of the physical and structural properties of true clathrates. The principal difference between the two groups is that, in true clathrates, guest molecules are not physically bonded to the host lattice; rather, they are held within and lend stability to cavities through van der Waals interactions, hi contrast, in semi-clathrates, guest molecules both physically bond with the water structure and occupy cavities. For example, in the quaternary (or peralkyl) ammonium salt semi-clathrate hydrates, the quaternary ammonium salt (QAS) hydrophobic cation takes a cage filling role, while the negatively charged anion is hydrogen bonded with the water lattice-work.

The emulsion of the emulsion-templated porous support may be a high internal phase emulsion (HIPE) as defined in Cameron, N. R. Polymer 2005, 46, 1439. As stated in this document and generally accepted in the art, a HIPE has an internal (droplet) phase volume ratio of 0.74 or greater.

The support may be a polymer. For example the support may be a polyHEPE, which is an emulsion-templated polymer wherein the template is a high internal phase emulsion (HIPE). The support preferably has abundant interconnected macropores and low bulk density, and optionally has micropores or mesopores. Thus the support can enhance gas storage without being of prohibitive weight for practical use. The support allows gas uptake kinetics to be accelerated by factors of more than 100 with good recycl ability.

The emulsion used to make the emulsion-templated support usually comprises two liquid phases, with droplets of one phase within the other. Typically one phase is aqueous and the other organic, though any two immiscible phases can be used. For example, the emulsion can contain aqueous droplets within an organic continuous phase, the latter being polymerized into the base material of the support. Alternatively, the emulsion can contain organic droplets (e.g, a solvent) within an aqueous continuous phase which contains polymerizable monomers, the latter being polymerized into the base material of the support. Such water-in-oil (w/o) or oil-in-water (o/w) emulsions allow the preparation of a wide variety of porous polymers and other materials. For example, the polymer may comprise polystyrene, in which case the starting materials in the continuous phase comprise styrene monomer(s) and other optional materials such as a crosslinker (e.g. divinyl benzene [DVB]). Other examples of suitable polymers prepared in an organic phase include those prepared from acrylates or methacrylates. Polymers can be prepared by polymerization of components in an aqueous phase, such as for example acrylamide or JV- isopropylacrylamide. Supercritical CO₂-in-water (c/w) emulsions can also be used to prepare polyacrylamide and poly(2-hydroxyethyl acrylate) materials, amongst others. Alternatively, inorganic materials may be prepared, for example by the sol-gel polycondensation of tetraethylorthosilicate (TEOS) in the aqueous phase of an o/w emulsion.

The support is prepared by mixing the two phases, curing or otherwise solidifying the continuous phase and removing the droplet phase as known in the art. To the support is added the material which will form the host. For example, if the clathrate is a clathrate hydrate, then water, which may be in the form of a stock solution with optional other components (such as for example a stabilizer, in which case an appropriate amount of stabilizer is present based on known effective H₂θ:stabilizer ratios), is mixed with the host.

The clathrate may be formed in apparatus known in the art, such as for example a high pressure stainless steel cell.

Thus, it is believed that a clathrate thin film forms on the surface of the support.

Preferably the emulsion-templated porous support comprises relatively large voids, which may for example be approximately spherical. Preferably there are interconnecting windows between voids. Typically the windows are smaller than the voids. Additionally, in some cases smaller pores may be present within the walls of certain emulsion-templated porous supports; these smaller pores may be micropores or mesopores, i.e. these smaller pores may preferably be no greater than 50, no greater than 30, no greater than 20, no greater than 10 or no greater than 2 nm.

Preferred void diameters fall within the ranges 1-100 micro-m, 5-50 micro-m and most preferably 15-25 micro-m. Preferably the average void diameter falls within the ranges 1- 100 micro-m, 5-50 micro-m and most preferably 15-25 micro-m.

Preferred window diameters fall within the ranges 0.05-50 micro-m, 0.1-20 micro-m and most preferably 0.5-10 micro-m. Preferably the average window size falls within the ranges 0.05-50 micro-m, 0.1-20 micro-m and most preferably 0.5-10 micro-m.

Most preferably the support has voids of diameter 15-25 micro-m and windows of size 0.5- 10 micro-m.

Preferably no more than 20%, more preferably no more than 15%, more preferably no more than 10%, more preferably no more than 5%, most preferably no more than 1%, weight-for- weight, of support is used, relative to the amount of host (or relative to the combined amount of host and stabilizer, if a stabilizer is used).

Preferably the pore volume of the support is at least 74%, more preferably at least 80%, more preferably at least 90%, more preferably at least 95%, most preferably at least 99%. For the avoidance of doubt, the pore volume is the percentage volume of voids, windows, pores and any other non-solid parts.

Preferably the bulk density of the support is no greater than 1 g/ cm³, more preferably no greater than 0.1 g/ cm³, more preferably no greater than 0.05 g/ cm³, more preferably no greater than 0.01 g/ cm³. For the avoidance of doubt, the bulk density is the overall density of the material.

Preferably the absolute density of the support is no greater than 5 g/ cm³, more preferably no greater than 2 g/ cm³, more preferably no greater than 1 g/ cm³, more preferably no greater than 0.1 g/ cm³. For the avoidance of doubt, the absolute density is the density of the solid part of the material, i.e. the density not including the voids, windows, and pores.

From a further aspect the present invention provides the use of an emulsion-templated porous support in enhancing the enclathration of a gas, and/or the dissociation of a gas from a clathrate.

From a further aspect the present invention provides a composition in the form of a clathrate or suitable for forming a clathrate, comprising a clathrate-forming host and an emulsion- templated porous support, and optionally a gas.

From a further aspect the present invention provides a composition comprising a gas clathrate and an emulsion-templated porous support.

From a further aspect the present invention provides a composition comprising a clathrate and an emulsion-templated porous support, and optionally a gas. From a yet further aspect the present invention provides an apparatus comprising an emulsion-templated porous support, optionally a clathrate-forming host and optionally a gas, wherein said apparatus is selected from one of the following devices or a component thereof: a fuel cell, an energy storage device, a gas storage device for example a modified gas tank, a gas separation device for example an in-line gas separation cartridge, a gas sequestration device for example an in-line gas sequestration cartridge, a gas transportation device for example a modified gas tank, and a vehicle for example an automobile.

The preferred features specified above in relation to the method of the invention apply mutatis mutandis to the uses, compositions and apparatus of the invention.

Thus the present invention allows improvements in gas storage, gas transportation/distribution, use of fuels, gas sequestration, and waste gas trapping. The present invention also brings about improvements in the separation of one or more gases from a mixture: for example by the preferential enclathration of methane over hydrogen, ethane or propane; carbon dioxide over methane or nitrogen; or hydrofluorocarbons from gas mixtures.

The present invention will now be described further by way of non-limiting example with reference to the following drawings in which:-

Figure 1 is a schematic illustration of clathrate hydrate dispersed on an emulsion-templated porous support;

Figure 2 is an electron micrograph of an emulsion-templated porous support;

Figure 3 shows the pore size distribution of an emulsion-templated porous support;

Figure 4 is a schematic diagram of experimental apparatus used to prepare the supported clathrate hydrate; Figure 5 shows cooling/heating curves for a clathrate-forming solution containing THF stabilizer with and without a support;

Figure 6 shows kinetic plots of hydrogen enclathration in the presence of THF stabilizer with and without a support;

Figure 7 shows temperature vs. time plots of a THF-H₂O system in the presence and absence of H₂, and with and without a support.

Figure 8 shows a kinetic plot of hydrogen enclathration in bulk THF-H₂O clathrate hydrate.

Figure 9 shows kinetic plots of hydrogen enclathration in the presence of THF stabilizer at higher pressures than in Figure 6, without a support, with an activated carbon support, and with an emulsion templated support;

Figures 10 to 13 are analogous to Figures 5 to 8 respectively except that they relate to experiments wherein TBAB is used as stabilizer in place of THF;

Figure 14 shows a pressure vs. temperature plot of enclathration within semi-clathrate hydrate in the presence of TBAB and subsequent dissociation under heating; and

Figures 15 and 16 are analogous to Figures 5 and 6 respectively except that they relate to experiments wherein the guest is methane in place of hydrogen in the absence of any stabilizers;

Figure 17 shows a pressure vs. temperature plot of enclathration of methane within clathrate hydrate in the absence of any stabilizers and subsequent dissociation under heating;

Figure 18 shows heating/cooling curves for a clathrate-forming composition containing TiAAB stabilizer; Figure 19 shows kinetic plots for CH₄ encapsulation in TiAAB-H₂O semi-clathrate supported within polyHIPE; and

Figure 20 illustrates the stability Of CH₄-TiAAB-H₂O clathrate hydrate.

5

Example 1 - Preparation and characterization of polyHIPE polymer support

Typically the organic phase 10 mL was comprised of 5 ml Divinylbenzene (DVB, Aldrich

10 80 vol% m- and p-divinylbenzene, the remainder m- and /?-ethylstyrene, purified by passing through a column of basic alumina to remove the inhibitor) and 5 ml porogen(s) (chlorobenzene -2-chloroethylbenzene 1 : 1 ratio by volume, Aldrich) followed by adding 2 ml sorbitan monooleate (Aldrich, Span 80, HLB=4.3) (20 vol% to monomer porogen mixture). The aqueous phase (90 ml) contained 0.2 g potassium persulfate (Aldrich) and

15 1.0 g calcium chloride (Aldrich). The separate organic and aqueous phases were purged with nitrogen for 15 min, and then the aqueous phase was added dropwise to the organic under nitrogen with constant mechanical stirring. Full details of the procedure for preparing PolyHIPE materials can be found elsewhere (Hainey, P.; Huxham, I. M.; Rowatt, B.; Sherrington, D. C; Tetley, L. Macromolecules 1991, 24, 117; and Cameron, N. R.;

20 Barbetta, A. Journal of Materials Chemistry 2000, 10, 2466). The morphology of polyHIPE was observed using a Hitachi S-4800 cold Field Emission Scanning Electron Microscope (FE-SEM). The dry polymers samples were prepared on 15 mm Hitachi M4 aluminium stubs using either silver dag or an adhesive high purity carbon spectro tab. The samples were then coated with a 2 nm layer of gold using an Emitech K550X automated

25 sputter coater. The FE-SEM measurement scale bar was first calibrated using certified SIRA calibration standards. The Brunauer-Emmett-Teller (BET) surface area (P/Po = 0.05 - 0.20) was measured with nitrogen adsorption at 77.3 K using an ASAP2420 volumetric adsorption analyzer (Micromeritics). Samples were degassed at 90 ⁰C for 15 h under vacuum before analysis. The pore size distribution analysis was conducted using

30 Macropore intrusion volumes, bulk densities, and macropore size distributions were recorded by mercury intrusion porosimetry using a Autopore Mercury Porosimeter IV 9500 (Micromeritics) over a pressure range of 0.10 - 60000 psia. Intrusion volumes were calculated by subtracting the intrusion arising from mercury interpenetration between beads (pore size >150 μm) from the total intrusion.

Figure 2 shows the SEM image of polyHIPE polymer. It can be clearly seen that the macroporous polyHIPE polymer is built with open cells (voids) containing a large number of interconnected pore windows. The pore size distribution of the windows, derived using a mercury porosimetry method in Figure 3 exhibits a narrow peak centered at 9.1 μm and a shoulder peak at around 6.7 μm. Its bulk density is 0.056 g/cm³. Its BET surface area obtained from nitrogen adsorption is around 230 m /g, consistent with a previous report (Hainey, P.; Huxham, I. M.; Rowatt, B.; Sherrington, D. C; Tetley, L. Macromolecules 1991, 24, 117). The macroporous structure of polyHIPE can be efficiently tuned by control over synthesis conditions (Hainey, P.; Huxham, I. M.; Rowatt, B.; Sherrington, D. C; Tetley, L. Macromolecules 1991, 24, 117; and Cameron, N. R.; Barbetta, A. Journal of Materials Chemistry 2000, 10, 2466).

Example 2 - Formation of clathrate hydrates

A stock solution of tetrahydrofuran (THF, Aldrich), 5.56 mol% THF in deionized water (THF^»17H2O) was prepared gravimetrically for stoichiometric experiments (with all the large cages of sll occupied by THF) (Hester, K. C; Strobel, T. A.; Sloan, E. D.; Koh, C. A.; Huq, A.; Schultz, A. J. J. Phys. Chem. B 2006, 110, 14024, and Strobel, T. A.; Taylor, C. J.; Hester, K. C; Dec, S. F.; Koh, C. A.; Miller, K. T.; Sloan, E. D. J. Phys. Chem. B 2006, 110, 17121). Similarly, 2.56 mol% solution of tetra-n-butylammonium bromide (TBAB, Fisher) with a stoichiometric composition of TB AB* 38H₂O was prepared (Chapoy, A.; Anderson, R.; Tohidi, B. J. Am. Chem. Soc. 2007, 129, 746). A 20.0 g THF or TBAB stock solution together with a given amount of polyHIPE polymer support was loaded in a 60-cm³ high pressure stainless steel cell (New Ways of Analytics, Lόrrach, Germany) for the formation and dissociation of clathrate hydrates and for hydrogen enclathration under continuous cooling or heating. For methane clathrate experiment, the pure water with a mount of 20.0 g in the absence of any stabilizers was used. The temperature of coolant in the circulator was controlled by a programmable thermal circulator (HAAKE Phoenix II P2, Thermo Electron Corporation). The temperature of compositions in the cell was measured using a Type K Thermocouple (Cole-Parmer, -250 - 400 ⁰C). The pressure was monitored using a High-Accuracy Gauge Pressure Transmitter (Cole-Parmer, 0 - 3000 psia). Both thermocouple and transmitter are connected to a Digital Universal Input Panel Meter (Cole- Parmer), which communicate with a computer. Prior to experiments, the cell was slowly purged with hydrogen (UHP 99.999%, BOC Gases, Manchester, UK) or methane (UHP 99.999%, BOC Gases, Manchester, UK) three times, and then pressurized to the desired pressure at a designated temperature. The temperature (T, K) and pressure (P, psia) with time (t, min) of compositions within the cell were automatically interval-logged into a computer by MeterView 3.0 software (Cole-Parmer). The experimental apparatus is shown in Figure 4.

Assuming that the true density of polyHIPE and clathrate hydrates is around LO g/cm³, the free space volume of the cell can be obtained by subtracting the sum volume of clathrate and support. The hydrogen or methane enclathration capacity is approximately evaluated using Idea Gas Law with a pressure drop A? and temperature. In addition, GASPAK v3.41 software (Horizon Technologies, USA) was employed to calculate the hydrogen enclathration capacity for comparison.

Example 3 - HVTHFZH₂O systems

Figure 5

Figure 5 shows P-T plots of cooling and heating for the H₂-THF-H₂O ternary system under hydrogen pressure (temperature ramp: 2.5 KTh): (A) without polyHIPE (for clarity, the curve was vertically shifted by 60 psi); (B) with macroporous polyHIPE support (20.Og THF-H₂O solution mixed with 3.O g polyHIPE).

It can be seen that for curve A - THF-H₂O without support - there is a linear trend during continuous cooling and heating, indicating the pressure-temperature (P-T) relations of hydrogen in the system obey the idea gas law (P/T = constant). There is no evidence for the formation and dissociation of hydrogen clathrate. By contrast, for curve B with added polyHIPE support, there is clear evidence for the formation and dissociation of hydrogen clathrate with a dramatic pressure drop on cooling and rapid rise on heating. During cooling, the clathrate formation was accompanied by a significant pressure reduction at 277.1 - 275.2 K associated with an exothermal peak. The pressure drop recovered to a linear trend after 275.2 K, indicating the cessation of hydrogen clathrate formation from the liquid phase to solid clathrate. During the warming process, the rapid pressure rise suggests the release of enclathrated hydrogen and the dissociation of the clathrate starting approximately at 275.2 K and ending at around 282.3 K. The result may be attributed to the use of the polyHIPE support, whose large internal surface can homogeneously disperse the THF-H₂O clathrate and accelerate the formation of hydrogen clathrate hydrate. The gravimetric hydrogen enclathration capacity derived from this pressure drop, -dP (158 psi) at 270.0 K is evaluated as approximately 0.18 wt% (THF clathrate) using the ideal gas law and around 0.15 wt% using GasPak v3.41 software based on the initial point (289.1 K, 1843 psi) and final point (268.4 K, 1553 psi) considering the non-ideality of the hydrogen gas at a high pressure.

Figure 6

Figure 6 shows the kinetic plots of hydrogen encapsulation in THF-H₂O clathrate hydrate with and without polyHIPE at 270.0 K with different mass ratios (THF-H₂O solution: support): (a) 23-cm³ glass beads (baseline); (b) 20:0 THF-H₂0:polyHIPE (that is, without support); (c) 20:1 THF-H₂0:polyHIPE; (d) 20:3 (1^st run); (e) 20:3 (2^nd); (f) 20:3 (3^rd); (g) 20:3 (4^th); (h) 20:3 (5^th).

The small pressure drop originating from the temperature change of hydrogen gas from cylinder to the cell was calibrated with 23 cm³ glass beads as a baseline shown in curve (a). There is no distinct pressure drop observed at 270.0 K even after 1200 min, suggesting no gas leakage occurred in the setup used here. It can be seen that without polyHIPE, the curve (b) shows a small pressure drop after 1200 min, indicating an extremely slow hydrogen encapsulation in bulk THF-H₂O clathrate hydrate. After adding polyHIPE support, a large pressure drop can be observed in curves (c - h). In the cases of the ratio for 20:3, the difference in pressure drop for first three runs (d, e, and f) could be related to the wettability of polyHIPE support. Curve (f) shows a large pressure drop and after 200 min there is no further discernable pressure drop, indicating the completion of hydrogen enclathration. The similar profile for three runs in curves (f, g, and h) suggests the good cyclabϊlity of hydrogen encapsulation process using polyHIPE support. The maximum pressure drop 5 (-dPmax) derived from the hydrogen enclathration at 270.0 K is around 150 psi for last three runs (at 1200 min), consistent with the observation in Figure 5.

Figure 7

10 Figure 7 compares the T - 1 plots of cooling and heating (2.5 K/h) for the THF-H₂O binary system under different conditions: (a) Processing at atmospheric pressure without polyHIPE and hydrogen (For clarity, the curve was backwardly shifted by 50 min.); (b) Processing at atmospheric pressure without hydrogen but with polyHIPE support; (c) Processing at high hydrogen pressure with polyHIPE support (see Figure 5B) (For clarity, the curve was

15 horizontally forwarded by 100 min.)

It can be seen that in plot (a), there is no evidence of clathrate formation when the THF-H₂O system is cooled at atmospheric pressure in the absence of hydrogen within the timescale of the present experiment. By contrast, after adding support, plot (b) shows an exotherm at

20 274.2 K during cooling and a disturbance at around 276.8 K during heating, suggesting the formation and dissociation OfTHF-H₂O binary clathrate hydrate in the absence of hydrogen gas. In the presence of high pressure hydrogen (around 1660 psi) plot (c), a similar enclathration and dissociation profile was observed. The simultaneous enclathration of small hydrogen molecules and larger THF molecules within the clathrate cages possibly

25 contributes to the higher temperatures and greater exotherm for formation and dissociation OfTHF-H₂O clathrate hydrate.

Figure 8

30 Figure 8 shows the kinetic plot of hydrogen encapsulation in bulk THF-H₂O clathrate hydrate at 270.0 K (corresponding to experiment [b] in Figure 6) and the linear-fitted line in the period of 2000 - 4300 min. The linear equation is: P = -0.0074** + 1640. At t_g =60 min for curve (h) in Figure 6 using support, P_g is around 1526 psi (90% of hydrogen enclathration capacity), and thus for curve (b) in Figure 6 without support, t_a is calculated to be around 15405 min (11 days) using the above linear equation, showing an increase in gas uptake kinetics by a factor of 257. Similarly, at t_g=200 min, P_g is around 1517 psi (97% of hydrogen enclathration capacity), and thus t_a is around 16621 min (12 days), suggesting an increase of 83 times in kinetics. Furthermore, the enhancement in kinetics is also effective at a higher gas pressure (Figure 9).

Figure 9

Figure 9 shows the efficacy of the present invention at higher pressures and also a comparison between different kinds of support.

Figure 9 shows the kinetic plots of hydrogen encapsulation in THF-H₂O clathrate hydrate at high pressure and 270.0 K: (a) 23-cm³ glass beads (baseline); (b) THF-H₂O (20.0 g); (c) THF-H₂O (20.0 g) + activated carbon powder (10.Og); (d) THF-H₂O (20.0 g) + polyHIPE (3-O g).

Curve (a) shows the baseline obtained with 23-cm³ glass beads at higher pressure. Curves (b) and (d) show a similar trend as in Figure 6, indicating that kinetic enhancement can occur at a higher pressure. The maximum pressure drop (ΛP_max) derived from the hydrogen enclathration at 270.0 K is around 178 psi for curve (d), slightly higher than that at low pressure in Figure 5 and 6.

It can be seen that the use of powdered activated carbon in curve (c) has no obvious effect on the enclathration kinetics even though a large amount of carbon support (10.0 g) is used. Thus, using macroporous polyHIPE polymer as a support with a homogeneous, highly porous, open-cell structure and low apparent density properties is significant. Example 4 - H₇ZTBABZH₂O systems

We have also tested the applicability of using a polyHIPE support in the formation of a semi-clathrate hydrate with tetra-n-butylammonium bromide (TBAB) as a stabilizer. A similar accelerated formation of hydrogen clathrate hydrate was observed.

Figure 10

Figure 10 shows the P-T plots of cooling and heating for H₂-TBAB-H₂O ternary system (semi-clathrate hydrate) under hydrogen pressure (temperature ramp: 2.0 K/h): (A) TBAB- H₂O solution (20.0 g) without polyHIPE support; (B) TBAB-H₂O solution (20.0 g) mixed with polyHIPE support (3.4 g). It can be seen that the pressure drop {ΔPi= 23 psi) of semi- clathrate hydrate without using polyHIPE support (A) is much less than that using the support (B) [ΔPf= 34 psi), indicating the enhancement of hydrogen encapsulation in TBAB- H₂O semi-clathrate hydrate when using polyHIPE as a support.

Figure 11

Kinetic enhancement can be found at different hydrogen enclathration temperatures (273.2 and 278.2 K).

Figure 11 shows the kinetic plots of hydrogen enclathration in TBAB-H₂O semi-clathrate hydrate conducted under different conditions: (a) 23-cm³ glass beads (baseline) at 273.2 K; (b) TBAB-H₂O solution (23.0 g) without polyHIPE at 278.2 K; (c) TBAB-H₂O solution (23.0 g) without polyHIPE at 273.2 K; (d) TBAB-H₂O solution (20.0 g) with polyHIPE support (3.4 g) at 278.2 K; (e) TBAB-H₂O solution (20.0 g) with polyHIPE support (3.4 g) at 273.2 K (1^st run); (f) TBAB-H₂O solution (20.0 g) with polyHIPE support (3.4 g) at 273.2 K (2^nd run). The maximum pressure drop (ΛP_max) derived from the hydrogen enclathration at 273.2 K is around 78 psi for curve (e) or (f). Figure 12

Figure 12 shows the T- 1 plots of cooling and heating for TBAB-H₂O system (2.5 KJh): (a)

Processing at atmospheric pressure without polyHIPE and hydrogen (for clarity, the curve was backwardly shifted by 100 min.); (b) Processing at atmospheric pressure without hydrogen but with polyHIPE support; (c) Processing with polyHIPE support and high pressure hydrogen (see Figure 10B) (for clarity, the curve was forward shifted by 100 min).

It can be seen that there is a similar pattern of temperature disturbance observed for all curves. Plots (a) and (b) suggest the formation and dissociation of TBAB-H₂O semi- clathrate hydrate with or without support in the absence hydrogen gas. Plot (c) shows a similar pattern of clathrate formation in the presence of hydrogen under pressure.

Figure 13

We calculate that, to reach 90% of hydrogen enclathration capacity, 32222 min (22 days) would be needed in the absence of support, but only 300 min in the presence of support, indicating a large increase in gas uptake kinetics by a factor of 107.

Figure 13 shows the kinetic plot of hydrogen encapsulation in bulk TBAB-H₂O clathrate hydrate 273.2 K and the linear-fitted line in the period of 1000 - 4000 min. The linear equation is P = -0.0018*/ + 1473. At a time of 300 min for Figure 1 l(e) using support, the pressure is around 1415 psi near to equilibrium, and thus without support in Figure ll(b), the time needed to the same pressure is calculated to be 32222 min (22 days) using the above linear equation, showing an increase in kinetic behaviour by a factor of 107 when using polyHIPE as support.

Figure 14

Figure 14 shows the P-T plot of enclathration and subsequent dissociation for the H₂- TBAB-H₂O ternary system under hydrogen pressure with 20.0 g TBAB-H₂O solution and

3.4 g polyHIPE support: a→b, enclathration conducting at 273.2 K (bath temperature) (see

Figure l l(e)); b→c- >d→e, heating process with a temperature ramp of 4.0 K/h. The pressure drop during the enclathration process (a→b) is consistent with the pressure jump during the dissociation process (b— >c-→d-→e), implying hydrogen incorporation into the TBAB-H₂O semi-clathrate hydrate at around 273.2 K (a→b).

Example 5 - CHU/EbO systems

Figure 15

Figure 15 shows P-T plots for cooling and heating for a CH₄-H₂O system under CH₄ pressure (temperature ramp: 2.0 KTh): (A) H₂O (20.Og) without polyHIPE support; (B) H₂O (20.0 g) with polyHIPE support (3.0 g).

The CH₄-H₂O system is consistent with the above observations obtained for the THF-H₂O- H₂ system in Figure 5. There is no clear evidence for the formation and dissociation of the CH₄-H₂O clathrate in the absence of the polyHIPE support under these unmixed conditions (curve A). By contrast, in the presence of the support (curve B), there is evidence for the formation and dissociation of methane clathrate with a dramatic pressure drop upon cooling and a rapid rise upon heating, indicating the enhancement of methane encapsulation in the H₂O clathrate hydrate when using polyHIPE as a support.

Figure 16

Figure 16 shows the kinetic plots of CH₄ enclathration in H₂O clathrate hydrate at 271.0 K; (a) H₂O without support; (b) H₂O (20.0 g) with polyHIPE (3.0 g).

The very small pressure drop after 1200 min in curve (a) is consistent with very slow methane enclathration kinetics in the absence of mixing when no polyHIPE support is used. By contrast, a relatively large and rapid pressure reduction is observed in the presence of the polyHIPE support, curve (b). The methane enclathration capacity derived from the pressure drop in curve (b) at 271.0 K after 1200 min (AP = 290 psi) was estimated to be approximately 2.6 wt % CH₄ (37 v/v STP) based on the mass of water added using the ideal gas law and around 3.4 wt % as calculated using GasPak v3.41 software which takes account of the non-ideality of the gas. These calculations were based on the initial pressure (0 min, 271.0 K, 1330 psi) and the final pressure (1200 min, 271.0 K, 1037 psi) assuming that the free space volume in the cell is constant (37 cm³). The data demonstrate the greatly enhanced kinetics for methane enclathration in the presence of the support.

Figure 17

Figure 17 shows a P-T plot of enclathration and dissociation for the CH₄-H₂O system under CH₄ pressure with H₂O (20.0 g) and polyHIPE support (3.0 g): a→b, enclathration conducting at 271.0 K (see Figure 16(b)); b→c→d-→e, heating process with a temperature ramp of 2.0 K/h. Thus Figure 17 shows methane enclathration within H₂O clathrate hydrate at 271.0 K (bath temperature) and subsequent dissociation under continued heating (2.0 K/h) in the presence of the polyHIPE support. The pressure drop during the enclathration process (a→b) is consistent with the pressure jump during the dissociation process (b→c→d→e), implying that the methane was incorporated into the clathrate hydrate at around 271.0 K (a→b).

Example 6 - CHVTiAAB/H₂θ systems

We have also investigated the storage of methane within a tetra-/so-amylammonium bromide (TiAAB) semi-clathrate, and the high temperature stability of this material.

Nucleation of a dilute TiAAB solution in the presence of methane produced semi-clathrate crystals of Ti AAB.38H₂O of suitable quality for structural analysis by X-ray diffraction (X- ray diffraction data not shown). The resulting material is similar to that of TBAB semi- clathrates, except that there is an expansion of unit cell dimensions to accommodate the larger alkyl chains. As for TBAB, the semi-clathrate for TiAAB forms a structure where dodecahedral cavities are vacant and available for hosting gas molecules.

Three stable polyhydrates of TiAAB have a stoichiometry of 38, 32 and 26 water molecules per TiAAB. The melting points of all of these are approximately 303K, with the 1 :26 hydrate having the highest melting point of 303.3K. We focused on two different water compositions, that of a 3.7 mol% solution (nominally a composition of TAAB^«26H₂O) and that of a 2.6 mol% solution (TAAB.38H₂O).

To improve the kinetics of gas uptake, we investigated the storing of methane within a TiAAB semi-clathrate supported on the ultralow-density, emulsion template polymerized high internal phase emulsion (polyHIPE) material. The interconnected pore structure and very low bulk density means that it is possible to support 20 g of TiAAB solution on just 3.4 g of the polyHIPE.

Stock solutions of tetra-zsø-amylammonium bromide (TiAAB) with a stoichiometric composition of TiAAB^»26H₂O and TiAAB ^»38H₂O were prepared. To carry out the gas uptake kinetic experiments, 20.0 g of a stock solution of TiAAB was loaded into a 68.0 cm³ high pressure stainless steel cell (New Ways of Analytics, Lόrrach, Germany) together with the polyHIPE support. The temperature of the coolant in the circulator bath was controlled by a programmable thermal circulator (HAAKE Phoenix II P2, Thermo Electron Corporation). The temperature of the compositions in the high pressure cell was measured using a Type K Thermocouple (Cole-Parmer, -250-400 ⁰C). The gas pressure was monitored using a High-Accuracy Gauge Pressure Transmitter (Cole-Parmer, 0-3000 psia). Both thermocouple and transmitter were connected to a Digital Universal Input Panel Meter (Cole-Parmer), which communicates with a computer. Prior to experiments, the cell was slowly purged with methane (UHP 99.999%, BOC Gases, Manchester, UK) three times at atmospheric pressure to remove any air, and then pressurized to the desired pressure at the designated temperature. The temperature (T, K) and pressure (P, psia) and time (t, min) were automatically interval-logged using MeterView 3.0 software (Cole-Parmer). Using this set-up it was possible to obtain high resolution data (for example, 2 seconds between individual [T, P₅ t] points, 120,000 data points in a 2000 min experiment). Control experiments using glass beads showed that the system did not leak: that is, no pressure drop occurred over 2000 min. Figure 18

Figure 18 shows the pressure-temperature dependence of TiAAB^«26H₂O and TiAAB-38H₂O (3O3K) (20 g and 3.4 g polyHIPE), cooled and heated at 2 Khr^"1 under pressure of CH₄.

With no polyHIPE support, the P-T relationship for CH₄ in the system approximated to the ideal gas law during the heating/cooling cycle (data not shown). By contrast, there was strong evidence for clathrate formation and subsequent dissociation in the presence of the polyHIPE as shown by the dramatic pressure drop on cooling and rapid pressure rise on heating. Repeating this cycle resulted in broadly the same behaviour. However, the clathration onset temperature shifts to significantly higher temperature (31 OK as compared to 302K), with increased kinetics of clathrate formation as demonstrated by the quicker drop in pressure. Further repeats of the heating/cooling cycle then closely follow that for the 3rd cycle shown in Figure 18. We hypothesise that this effect results from the more uniform distribution of the TiAAB over the hydrophobic, high surface area support after the first cycle. However, it may also arise from a memory effect. Data for the 2.6 mol% solution closely follows that for the 3.7 mol% solution.

Figure 19

Figure 19 shows kinetic plots for CH₄ encapsulation in TiAAB-H₂O semi- clathrate supported within polyHIPE at 3O3K (20 g TiAAB.26H₂O and 3.4 g polyHIPE). The upper line shows results of a control experiment and the lower line shows an example in accordance with the present invention.

The kinetics of clathration are greatly enhanced in the presence of the polyHIPE. Again, this enhanced effect persists over several cycles. At an optimum temperature of 303 K, methane capacity of 36 v/v can be attained (80% of the maximum for this semi-clathrate system), with a t90 of ~1 OOmin. Figure 20

One issue with regard to the use of semi-clathrates for the storage of methane and other gases is the stability of the final material.

Figure 20a shows the stability OfCH₄-TiAAB-H₂O clathrate hydrate (2Og 3.7mol% TiAAB solution supported within 3.4 g polyHIPE). The photographs of figure 20b show CH₄- TiAAB-H₂O stability at 273 K / 1 bar (5 h), 293 K / 1 bar (4 h) and CH₄ dissociation and release at 313 K.

The stability of the CH₄-TiAAB-H₂O semi-clathrate hydrate was found to be higher than the si methane gas hydrate (MGH) (Figure 20a). We also compared the stability of the CH₄- TiAAB-H₂O system to the stability of the CH₄-TBAB-H₂O system used to store H₂. TiAAB methane semi-clathrate exhibits enhanced stability over the TBAB system. The iso- amyl chains exert a greater stabilizing effect than n-butyl chains on the H₂O cages. Indeed, this system is capable of retaining enclathrated methane after several hours at room temperature. Figure 20b demonstrates the absence of evolved methane after 5 hours at 273K, as shown by the deflated balloon. On warming to 293K, only a small amount of gas is evolved even after 4 hours. However, on warming to 303 K, a significant amount of gas is evolved, resulting in inflation of the balloon. Volumetric release experiments (not shown) demonstrate that the methane is released from the clathrate only at temperatures above room temperature.

These experiments show that supporting TiAAB on a polyHIPE significantly increases the kinetics of enclathration in the presence of methane. This system also high stability, with stability above room temperature being achieved. It should also be highlighted that this system is also extremely recyclable. Thus, utilizing a semi-clathrate allows an effective method for the trapping of methane. In summary, the present invention provides a method to improve gas enclathration kinetics and cyclability in clathrates using emulsion-templated supports. The work is of particular significance in promoting hydrogen clathrate hydrates, and methane clathrate hydrates, as practical means of gas storage.

Claims

1. A method comprising the enclathration of a gas, and/or the dissociation of a gas from a clathrate, in the presence of an emulsion-templated porous support.

2. The use of an emulsion-templated porous support in enhancing the enclathration of a gas, and/or the dissociation of a gas from a clathrate.

3. A composition in the form of a clathrate or suitable for forming a clathrate, comprising a clathrate-fomiing host and an emulsion-templated porous support, and optionally a gas.

4. A composition comprising a gas clathrate and an emulsion-templated porous support.

5. A composition comprising a clathrate and an emulsion-templated porous support, and optionally a gas.

6. An apparatus comprising an emulsion-templated porous support, optionally a clathrate- forming host and optionally a gas, wherein said apparatus is selected from one of the following devices or a component thereof: a fuel cell, an energy storage device, a gas storage device for example a modified gas tank, a gas separation device for example an inline gas separation cartridge, a gas sequestration device for example an in-line gas sequestration cartridge, a gas transportation device for example a modified gas tank, and a vehicle for example an automobile.

7. A method, use, composition or apparatus, as claimed in any preceding claim, wherein the clathrate comprises clathrate hydrate.

8. A method, use, composition or apparatus, as claimed in any preceding claim, wherein the gas comprises hydrogen.

9. A method, use, composition or apparatus, as claimed in any preceding claim, wherein the gas comprises methane.

10. A method, use, composition or apparatus, as claimed in any preceding claim, wherein the gas comprises carbon dioxide.

11. A method, use, composition or apparatus, as claimed in any preceding claim, wherein the gas comprises hydrocarbon gas.

12. A method, use, composition or apparatus, as claimed in any preceding claim, wherein the clathrate comprises a stabilizer.

13. A method, use, composition or apparatus, as claimed claim 12, wherein the stabilizer comprises THF or TBAB, preferably THF.

14. A method, use, composition or apparatus, as claimed claim 12, wherein the stabilizer comprises TBAF or TiAAB, preferably TiAAB.

15. A method, use, composition or apparatus, as claimed in any preceding claim, wherein the emulsion-templated porous support comprises voids of diameter 5-50 micro -m and smaller interconnecting windows of diameter 0.1-20 micro-m.

16. A method, use, composition or apparatus, as claimed in any preceding claim, wherein the emulsion comprises a HPE (high internal phase emulsion).

17. A method, use, composition or apparatus, as claimed in any preceding claim, wherein the support comprises a polymer.

18. A method, use, composition or apparatus, as claimed in claim 16, wherein the polymer comprises a vinyl polymer.

19. A method, use, composition or apparatus, as claimed in claim 17, wherein the polymer comprises polystyrene or a derivative thereof.

20. A method, use, composition or apparatus, as claimed in any preceding claim, wherein the emulsion-templated porous support comprises polyHIPE.