GB2457464A - Use of clathrates in gas storage - Google Patents

Abstract

A gas, for example hydrogen, methane, is enclathrated, and/or dissociated from a clathrate, for example clathrate hydrate, in the presence of an gellable material, for example poly acrylic acid sodium salt. The gellable material 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 matenals, for example in the storage of hydrogen, methane, carbon dioxide, or other gases.

1-lydrogen is a useful material, and technology based on the use of hydrogen, for example in ftiel 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 1 0 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 bc difficult and dangerous to handle. Known methods for producing hydrogen from chemical compounds in situ are generally unsatisfactory for many industrial uses.

1 5 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. Clieni. Rev. 2007, 107, 4133). CJathrate hydrates comprise "host" assemblies of H20 cages, in which are entrapped "guest" materials, such as gases. 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 elathrate hydrates can take a very long time (days or weeks, see for example Strobel, T. A.; Taylor, C. J.; I-lester, K. C.; Dee, S. F.; Koh, C. A.; Miller, K. T.; Sloan, E. D. I P/In. C'he,n. B 2006, 110, 17121) and furthermore the timeseale of "freezing" the H2-H20 elathrate structures can be unpredictable. Enhancement of hydrogen enelathration 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 -may be stored within clathrates and the clathratcs need not necessarily be clathrate hydrates, but can comprise any suitable host -not just 1-120. Numerous other hosts are possible, such as for example those listed in US patent application publication US 2004/0230084 (patent number US 7,220,294) of Yagi.

There is no particular limitation of the type of gas that may be stored as guest in a clathratc beyond the requirement that the host and guest must be sufficiently compatible (for example, in terms of the guest size) to form a stable clathrate structure, optionally in the presence of additional stabilizing agents. Known clathrates include those comprising (as S guest) H2, 02, N2, 01-14, 002, 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., Phys. Rev. Left 2004, 93, 125503). The stored gas may also comprise a mixture -for example, natural gas or air.

While improvements in the storage of gases in general would be desirable, there arc particular needs to enhance the technology for storing particular gases from a technological and envirom-riental 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 1 5 prompted a search for better means of sequestration of certain gases, for example 002.

Methane and natural gas also act as valuable fuels (e.g. for automotive applications) and therefore their storage is also important. These gases can all in principle be stored in elathrates. In some cases the host may be a useful commodity as well as the guest; for example a elathrate of H2 within OH4 or within octane could act as a dual fuel.

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

From a first aspect the present invention provides a method comprising the cnclathration of a gas, in the presence of a gellable material.

Thus the gellable material, together with the host, forms a gel prior to or during the enclathration of the gas into the host.

Preferably the clathrate formed is a clathrate hydrate: that is, preferably the host comprises 1-120 and the gel is a hydrogel. Thus, gas molecules, for example H2 molecules, are preferably stored within H20 cages formed within hydrogel. Clathrate hydrates are particularly advantageous because the major host component, water, is extremely easy and cost-effective to obtain, store, and dispose of, and is particularly environmentally friendly.

Consistent with conventional meanings iii the art, the term "clathrate" also includes here "semi-clathrate". As known in the art, a semi-clathratc is an association comprising a host and a guest wherein the guest forms part of the clathrate framework; fhr example the guest, or one of the guests if there is more than one. may both form part of the host cage structure, as well as occupying some of the cavities.

The terms "gellable material" and "gel" are known in the art. A gcllable material is a material which can be combined with a fluid or fluid mixture to form a gel, a gel being a substance intermediate between the liquid and the solid states. The gellable material acts like a mesh which is dissolved or suspended in the fluid or fluid mixture to font a jellylike, high viscosity material, namely a gel. The forces holding gels together include electrostatic, hydrogen bonding, and Van der Waals forces.

The fluid or fluid mixture, as well as forming a gel when combined with the gellable material, also functions as a "host" in the present invention. In other words, the gellable material is gellable in the sense that it is able to form a gel with a elathrate host.

In the event that the fluid or fluid mixture is predominantly water then the gel is referred to herein as a hydrogei and the gellable material as a hydrogellable material. Hydrogels and hydrogellable materials are specific sub-classes of gels and gellable materials, respectively.

Optionally the gellable or hydrogellable material may be a superabsorbent or water-swellable polymer network material. Superabsorbent materials are materials which have the ability to absorb large amounts of fluid (usually water or aqueous solution) and those which absorb and retain water and body fluids are commonly used in the absorbent products and personal care products industry (for example in diapers). Typically superabsorbent materials may swell so that they can produce materials which contain greater than 90% water weight-for-weight, in many cases more.

As exemplified below, the use of a gellable or hydrogellable material increases the spccd of gas clathratc 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 thu.

The use of a gellable or hydrogellahlc material is significant because bulk hosts (e.g. bulk ice) have been found to have extremely poor enelathration kinetics.

The present invention also allows the gas to be released from the elathrate structures rapidly. 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 or non stochastic; for 1 5 example a laboratory chemist may find crystalli4'ation 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. Moreover, the invention can be used over multiple gas charge/discharge cycles without significant deterioration of performance.

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.

One way of forming a gel is to imbibe a gellable material with a fluid or fluid mixture (for example, water, in the case of a hydrogellable material). One example of such a hydrogellable material is cross-linked poly(sodium acrylatc) but many more arc known in the art. Alternatively, a gellable material may be dissolved in a fluid under certain physical conditions to form a gel-precursor known in the art as a "sol" and then a gel formed by changing the conditions -for example, the temperature or pH of the sol. Another method of forming a gel is to carry out a chemical reaction in a fluid -for example, the polymerization of water-soluble monomers (which monomers do not gel the water) to form a polymeric material (which polymeric material causes the water to gel).

S

Gas incorporation by clathrate formation can for example be achieved by addition of the gas to a standard pressure reactor or other containment vessel, as known in the art, in the absence of any mechanical agitation. The gas uptake can be monitored for example by observing the pressure drop in the vessel as a thnction of time, using standard apparatus known in the art.

Without wishing to be bound by theory, it is believed that the gellable or hydrogcllable material results in advantages because it helps avoid the formation of bulk solid, e.g., bulk icc. For example, in hydrogel formation the water imbibcs into the hydrogellable material to form a swollen gel. Small gel domains enhance the effect of the present invention because they compartmentalize the water, in contrast to bulk water systems. Thus the path length for diffusion of the gas into or out of the water/ice during elathratc formation or dissociation is reduced.

The gel exhibits sufficiently high viscosity such that it is stable (and non-coalescing) in the form of small particles or domains over the operating temperature range of the gas storage system, thus presenting an enhanced surface-to-volume ratio in the gel host for accelerated gas clathrate formation. This differs, for example, from small icc particles which are known in the art to melt at the ice melting temperature and hence form a low viscosity fluid (water) which then coalesces to fonn bulk water.

The fluid or fluid mixture is predominantly responsible for acting as the clathrate host.

Without wishing to be bound by theory, the advantages of the present invention may in part be a consequence of the gellable material stabilizing and/or forming part of the clathrate framework.

Gellable and hydrogellable materials arc often inexpensive and commercially available, For example, hydrogellable polymers arc used as absorbents in diapers.

Regardless of the type of host, the present invention is of particular utility in the storage of S hydrogen, hydrocarbon gas (e.g., methane) or carbon dioxide. It is particularly preferred to store these gases in clathratc hydrates, for example, in the presence of hydrogellable materials, i.e. where the host comprises H20.

Hydrocarbon gas according to the present invention includes up to C4 hydrocarbon compounds which may be saturated or unsaturated, for exaniple methane, ethane. ethcne, propane, propene, and butane.

The present invention is also applicable to different gases, such as for example 02, N2, FI2S, Ar, Kr, Xc, He and Ne, or mixtures of such gases (e. g., air, natural gas).

It is known that a "stabilize?' (also known in the art as "promote?') may be used to aid the fonnation of clathrates and in particular to lower the gas clathration pressure, 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-butylarnmonium 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-butylaminc), diamines, diols, crown ethers and their complexes, methylcyclohexane and methyl-tert-butyl ether (MTBE). These stabilizers may for example be used when the elathrate is a clathrate hydrate. The gellablc material may also itself act as a stabilizer depending on its functionality.

Stabilizers are believed to enhance the stability of certain types of clathrate structure. For example, H2 and TI-I F can both be contained as guests within the so-called sli structural framework of clathrate hydrate such that TI-IF sits within some larger cavities thereof The usc of a suitable stabilizer eithances the gas storage capacity of certain clathrates at a given gas pressure and allows lower gas cnclathration pressures.

Stabilizers act as guests in the sense that they may he contained within the host (in a clathrate) or may form part of the cage structure together with the host (in a serni-clathrate).

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

To the hydrogellable material is added the material (often water) 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 H20: stabilizer ratios), is mixed with the host. The ratio of the fluid phase to the hydrogellable material may be controlled to achieve high fluid content (and hence high gravimetric gas storage capacity) while not sacrificing the gel viscosity and rigidity and thus losing the host compartmentalization effect referred to above.

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

The gellable or hydrogcllable material may be any known gellable or hydrogellable material. Some classes of suitable gellable or hydrogellable material may be formed by polymerization of viny] monomers, by ring-opening polymerization of ethylene oxide monomers or similar compounds, or by step-growth polymerization.

By way of non-limiting example, the gellable or hydrogellable material may bc a synthetic polymer: for example polyvinyl alcohol (PVA), poly(sodium acrylate) (PSA), poly(potassium acrylate), poly(acrylic acid) sodium salt-graJi-poly(ethylcne oxide), poly(aerylie acid co-acrylamide) potassium salt, poly(ethylcne oxide), or a poly(acrylamide); each of these may or may not be cross-linked.

In further alternatives the gellable or hydrogellable material may be a natural material: for example agarose, cellulose, ehitosan. gelatin, an alginate or earrageen.

Further options for the gellable or hydrogellable material include chemically-modified natural materials (e.g., methyl cellulose).

Preferably the gel or hydrogel comprises particles prior to or during the enelathration process. The use of particulate gels or hydrogels increases the surface to volume ratio of the material and decreases the diffusion path for the gas in the elathration process. Preferably the particles are less than about 2 mm in average size. The upper limit may alternatively be one of 1 mm, 100 miero-m, 10 micro-m, 1 miero-m, 100 nm or 50 nm. Even though the gel or hydrogel may optionally be particulate and the particles may optionally have a size of less than about 2 nmi (or one of the other upper limits mentioned above), such particles may be agglomerated to form agglomerated structures which are themselves larger than 2 mm in size (or larger than one of the other limits mentioned above). Such agglomerated sthietures may for example be up to 5 mm in size or larger In an agglomerate, discrete particles are still visible by microscopy though they may be conjoined. When the gel or hydrogel is in particulate form, for example of the sizes noted above, this further enhances the enclathration or dissociation by avoiding the formation of bulk solid (e.g., bulk ice). Each particle is itself a small unit, and the presence of small units of compartmentalized water (in the case of a hydrogel) greatly increases the interfaeial area for gas mass diffusion. Thus the path length (for diffusion of the gas into or out of the water/ice during clathrate formation or dissociation) is reduced. The gellable or hydrogellable material acts as a support for the host during clathrate formation.

The particulate hydrogellable material may he produced with an appropriate average particle size by a number of methods known in the art. For example, a bulk hydrogellahie material may he reduced to smaller particles by grinding or ball milling, and such materials arc commercially available. Also, thc hydrogellable material may hc synthesized in the form of spherical particles by heterogeneous polymerization techniques such as emulsion pol ymenzation, sus pension polymerization, inverse suspension polymerization, dispersion polymerization, oil-in-water-in-oil "sedimentation" polymerization (e.g., Zhang, El. and Cooper, A. I. C/win. Mater 2002, 14, 4017) or related methods. Again, a number of hydrogellable materials are commercially available in such forms. Alternatively, the hydrogellable material may be produced in an emulsion-teniplated format (e.g., Zhang, H. and Cooper, A. I. Soft Matter, 2005, 1, 107) whereby a high interfacial area and short diffusion path is facilitated by the macroporous structure of the material.

Optionally the absorption capacity of the gellable or hydrogellable material may be such that the gel or hydrogcl may contain at least 50%, 75%, 90%, 95%, or 99% liquid weight-for-weight.

Optionally the swellability of the gcllable or hydrogellable material may he such that the amount of liquid absorbed per g of material may be at least I ml, 3 ml, 9rn1, l9ml, or 99m1.

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 gellable or hydrogellable material 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 absolute density of the gellable or hydrogellable material (prior to the addition of liquid) is no greater than 5 g/ em3, more preferably no greater than 2 W em3, more preferably no greater than I g/ em3, more preferably no greater than 0.1 g/ em3.

When the gel or hydrogel is desired in particulate form, this may be prepared by addition of water (or other fluids) to suitably sized particles of gellablc or hydrogellable material.

Alternatively, or additionally, the particulate gel or hydrogel may be prepared by reducing in size bulk gel or larger particles, for example by grinding.

From a further aspect the present invention provides the use of a gellable material in enhancing the cnelathration of a gas, and/or the dissociation of a gas from a elathrate.

From a further aspect the present invention provides a composition in the form of a elathrate or suitable for forming a elathrate, comprising a elathrate-forming host and a gellable material, and optionally a gas.

From a further aspect the present invention provides a composition comprising a gas elathrate and a gellable materiaL From a further aspect the present invention provides a composition comprising a elatbrate and a gellable material, and optionally a gas.

From a yet further aspect the present invention provides an apparatus comprising a gellable material, optionally a elathrate-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 inodi lied 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 mutalis mutandis to the uses. compositions and apparatus of the invention. For example, the gellable material may by a hydrogellable material and the gel may be a hydrogel.

Thus the present invention allows improvements in gas storage, gas transportationldistribution, 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 dioxidc over methane or nitrogen: or hydrofluoi-ocarhons from gas mixtures.

The present invention will now he described ftrthcr by way of non-limiting example with reference to the following drawings in which:-Figure 1 is a schematic illustration of clathrate hydrate formed within a hydrogellable material; Figure 2 is a schematic diagram of experimental apparatus used to prepare and test the elathrate hydrate; Figure 3 shows photographs of particles of a hydrogellable material (PSA) hefhre and after swelling with water solution; Figure 4 shows kinetic plots of hydrogen enelathration in the presence of THF stabilizer with and without a hydrogellable material present; Figure 5 shows pressure versus temperature plots of hydrogen enelathration within elathrate hydrate in the presence of a hydrogel and subsequent dissociation under heating; Figure 6 shows kinetic plots of methane enelathration with and without a hydrogellable material; Figure 7 shows pressure vs. temperature plots of methane enelathration within elathrate hydrate in the presence of a hydrogel and subsequent dissociation under heating.

Figure 1 shows a schematic illustration of elathrate hydrate formed within a hydrogellable material (the same principle applies to elathrates in gellable materials generally). The hydrogellable material particles are first dispersed in TI-IF-H20 solution and then subsequently swo)len to form hydrogel at room temperature. Upon cooling, the THF-H20 solution within hydrogel particles further forms hydrogen-free clathrate hydrate (named eiathrate hydrate hydrogel), and, inversely, upon heating. clathrate hydrate hydrogel converts to hydrogel under appropriate conditions. Since the Iiydrogellable materials typically possess a super absorption property for aqueous solutions, they can hold the THF-H20 solution at above inciting or dissociation temperature of hydrogen clathrate hydrate even if using a very low mass ratio of hydrogellable material. Thus, the use of hydrogellable material as a scaffold overcomes the hurdle of recyclability during hydrogen enelathration and dissociation. By contrast, ground ice particles can not be recovered in situ after being melted tbr hydrogen release.

Example 1 -Formation of elathrate hydrates A stock solution of tetrahydrofuran (5.56 mol. % TFIF, Aldrich in deionized water; THF* 1 7H20) was prepared. To carry out the gas uptake kinetic experiments, an amount of the THF*i7H2O stock solution was homogeneously mixed with the hydrogellable material poly(aerylie acid) sodium salt (PSA) (Aldrich, lightly erosslinked, 436364-250, 99%<l000 gm particle size) at a given mass ratio and then loaded into a 60-em3 high pressure stainless steel cell (New Ways of Analytics. Lörraeh, Germany). For methane elathrate experiments, pure deionized water was used. The temperature of the coolant in the circulator bath was controlled by a programmable thermal circulator (HAAKE Phoenix 11 P2, Thenno Electron Corporation). The temperature of the compositions in the high pressure cell was measured using a Type K Thermocouple (Cole-Parmer, -250--400 °C).

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-Panner), which communicates with a computer. Prior to experiments, the cell was slowly purged with hydrogen (Ul-IP 99.999%, BOC Gases, Manchester, UK) or 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, iniri) 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 [7 F, t] points, 90,000 data points in a 1500 mm experiment).

Different control experiments were conducted. Assuming the true density of the PSA and elathrate hydrates to bc approximately 1.0 g/cm3, the free space volume of the cell (around 37 cmi) was obtained by subtracting the sum volume of clathrate and support. The inventors tried to keep this volume for each experiment. This was confirmed independently by measuring the free volume with helium gas at 270.0 K (hydrogen enclathration temperature). The hydrogen enclathration capacity was evaluated approximately using the ideal gas law, the pressure drop (AP), and the temperature. hi addition, GASPAK v3.4l software (Horizon Technologies, USA) was employed to calculate the hydrogen enclathration capacity, taking into account non-ideality factors. The experimental apparatus is shown in Figure 2.

Figure 3 Figure 3 shows (at varying scales): (a) a photograph of PSA particles before swelling (these particles having an average particle size of just under 1 mm) at approximately actual size; (b) fresh THF-H20 hydrogelled PSA particles [the photograph (b) being reduced in scale relative to the photograph in (a) by a linear factor of 2.5]; (e) THF-H20 hydrogel in a beaker of diameter 5 cm after several cycles of hydrogen enclathration and dissociation; (d) PSA gellable particles prior to the addition of water [this being a magnified image of the same particles photographed in (a)], viewed under an optical microscope; (e) PSA hydrogel particles after swelling with water, viewed under an optical microscope (at the same scale as (d)].

The white PSA gel particles (Figure 3a) were swollen after being dispersed in THF-H20 solution with a mass ratio of 3:20 (3 g PSA to 20 g I-120-THF) and became transparent hydrogel particles (Figure 3b), suggesting the PSA particles could completely "hold" the water solution under an appropriate mass ratio. Figure 3c shows the morphology of THF-H20 hydrogel after several cycles of hydrogen enclathration at 270.0 K and dissociation up to room temperature (292.2 K), clearly indicating that there is no water in the bottom of beaker and further demonstrating the hydrogel retained its initial status even after several cycles. Thus PSA particles arc an ideal support for holding water solution and dispersing clathrate hydrate crystal particles.

Example 2-H/THF/H,O systems Figure 4 Figure 4 shows kinetic plots for F!2 enelathration in preformed THF-H20 clathrate hydrate with and without PSA at 270.0 K: (a) 20.0 cm3 glass beads and 3.0 g PSA, no THF solution; (b) 20.0 g pure H20 and 3.0 g PSA (baseline); (c) 20.0 g Tl-JF-H20 solution plus 3.0 cm3 glass beads, no PSA; (d) 22.0 g THF-H20 solution and 1.0 g PSA; (c-h) 20.0 g THF-H20 solution and 3.0 g NA (1" -41h runs).

As can be seen in curve (a) there was no pressure drop at 270.0 K even after 1200 mm, suggesting there is no leakage in our experimental system used here and no adsorption of hydrogcn on PSA occurred. Curve (b) (baseline here) presents the kinetic plot obtained with 20.0 g H-,O and 3.0 g PSA. There is no pressure drop at 270.0 K after 30 mm, suggesting that no hydrogen encapsulation occurred because the elathrate hydrate can not be formed in the pure water without THF here and again demonstrating there is no leakage in our setup used here. Curve (e) reveals that the kinetic plot obtained with 20.0 g T1-!F-I-!20 and 3.0 cm3 glass beads. It can be seen that using glass beads, the curve (e) of preformed THF-H20 hydrate shows a small pressure drop with time, for an example, zIP] 30 psi at 1000 mm and 270.0 K, indicating an extremely slow hydrogen encapsulation in bulk THF-H20 hydrate.

Curve (d) exhibits the kinetic plot for H2 encapsulation in prefornied THF-H20 clathrate hydrate (22.0 g TI-IF-I-hO solution) supported with 1.0 g of PSA at 270.0 K. Further increasing the PSA mass to 3.0 g and reducing THF solution to 20.0 g, curves (c-h), shows the kinetic plots for H2 encapsulation in preformed THF-i120 clathrate hydrate hydrogel at 270.0 K for 4 cycles. It can be seen that the pressure drops of these curves are bigger than that of curve (d) after 100 mm. For curves (g) and (h) at 1000 ruin, the pressure drop AP2,) is around 374 psi, which is 12.5 times that of curve (c) using 3.0 cm3 glass beads (bulk solution). The shows hydrogen enclathration capacity in the case of curve (g) or (h) is up to around 0.40 -0.45 wt, % (based on the mass of THF-H20 used), indicating the nearly complete formation of clathrate hydrates after 4000 mm. The similar profile for 2 runs in curves (g and h) suggests the good cyclability of hydrogen encapsulation process using PSA support because it can "hold" the THF-H20 solution even above the melting temperature for hydrogen release.

Moreover, based on the fitted linear equation derived from curve (e), to reach 90 % (AP, = 360) psi of hydrogen enclathration capacity (assuming 4P,,,, = 400 psi), 39059 mm (27 days) is needed in the absence of PSA for curve (c), hut only 675 mm (less than half a day) if using PSA support (20:3) lbr curve (h), showing an considerable increase in gas uptake kinetics by a factor of 58 for hydrogen-free bulk Tl-IF-H20 hydrate.

A volumetric release experiment was carried out to further confirm the H2 capacity calculated from the observed pressure drop in the curve (g). For the experiment where the reactor was vented at 270.0 K (the temperature of the kinetic experiments), the excess H2 was rapidly vented (in situ) and the vent valve closed as soon as atmospheric pressure was reached. Subsequent warming released the additional hydrogen, which was collected and measured by volumetnc displacement. In this case, measurements were calibrated to take account of gas expansion in the free space during warming. The amount of H2 evolved in curve (g) was around 0.43 wt % based on the mass of clathrate hydrate, consistent with hydrogen amount calculated from pressure drop in Figure 4(g), released by dissociation in Figure 5, and the values reported previously (Strobel, T. A.; Taylor, C. J.; 1-lester, K. C.; Dee, S. F.; Koli, C. A.; Miller, K. T.; Sloan, F. D. J. Phvs. (hem. B 2006. 110, 17121) when crushed Tl-IF-H20 ice particles (45 or 250 uun) were employed.

Figure 5 Figure 5 shows the P-T plot of hydrogen enclathration and subsequent dissociation for the H2-THF-1-120 system under hydrogen pressure with and without PSA. Curve A was obtained with 20.0 g of THF-H20 solution and 3.0 em3 of glass beads. Curve B was obtained with 20.0 g of TH-J-120 solution and 3.0 g of PSA.

it can be seen that in the absence or presence of PSA -in the process of al-*a2 (see curve (c) in Figure 4) or b1-b2 (sec curve (Ii) in Figure 4) respectively -the pressure drop indicates the enclathration occurred at 270.0 K (bath temperature); when heating with a temperature ramp of 1.0 Kfh (a2-a3--a4----+a5 or b2-*b3----*b4->b5), the projected pressure jump in the period of a3-*a4 or b3-*h4 suggests the dissociation of clathrate and hydrogen release. The large difference in pressure drop and jump between both curves is believed to result from the use of PSA as a support. These indicate that the hydrogen was really enclathrated into the THF-H20 preformed clathrate hydrate at around 270.0 K and the use of PSA particles largely enhanced the enclathration capacity of hydrogen compared with dense glass beads. Moreover, the pressure drop derived from clathrate dissociation (b2-�h3--�b4--�bS) of curve B is around 392 psi, consistent with that resulting from hydrogen enclathration (400 psi, bl-*b2) at 270.0 K. Example 3 -CH4/H20 systems Figure 6 Figure 6 shows kinetic plots of methane enclathration in pure F120 clathrate hydrate; (a) 23.0 g of pure H20 without PSA at 271.0 K; (b) Glass beads (20.0 cm3) with PSA (3.0 g) at 271.0 K; (c) F120 (20.0 g) with PSA (3.0 g) at 276.0 K; (d) H20 (20.0 g) with PSA (3.0 g) at 271.0 K (I" run); (e) H20 (20.0 g) with PSA (3.0 g) at 271.0 K (2 run) It can be seen that thc very small pressure drop after 1200 mm iii curve (a) is consistent with very slow methane enelathration kinetics in the absence of mixing whcn no PSA is used.

Similarly, when loaded with glass beads (20 cm3) and PSA (3.0 g), the pressure drop is observed to he negligible after 5 mm, indicating that no pressure drop was caused by PSA or glass beads and no leakage in our system occurred. By contrast, a relatively large and rapid pressure reduction is observed in the presence of both water and PSA support, even at different temperatures of 276.0 K (curve e) and 271.0 K (curves d and e). The methane enclathration capacity derived from the pressure drop in curve (e) at 271.0 K after 1200 mm (AP = 56 psi) shown in Figure 6 was estimated to be approximately 0.52 wt % methane (7 v/v STP) based on the mass of water added using the ideal gas law. Also, around 0.77 wt % was obtained using GasPak v3.41 software which takes account of the non-ideality of the gas on the basis of the initial point (0 mm, 271.0K, 1326 psi) and the final point (1200 mm, 271.0 K, 1270 psi) since the free space volume in the cell is constant (37 em3). Particularly, it can be observed that the slope of curve (e) after 400 mm is much larger than curve (a), indicating a faster methane enelathration process when using PSA as a support. The data demonstrate the greatly enhanced kinetics for methane cnclathration in the presence of the hydro gel.

Figure 7 Figure 7 shows the P-T plot of enclathration and subsequent dissociation for the CH4-H20 system under methane pressure with H20 (20.0 g) and PSA (3.0 g): a->b, enclathration conducted at 271.0 K (bath temperature); b-*c--*d--*e, heating process with a temperature ramp of 2.0 Kfh. The pressure drop during the enclathration process (a-'b) is consistent with the pressure jump during the dissociation process (h-+c-*d--�e), implying that the methane was incorporated into the clathrate hydrate at around 271.0 K (a-b).

In summary, the present invention provides a method to improve gas enclathration kinetics and cyclability in clathrates using gellable materials. The work is of particular significance in promoting hydrogen elathrate hydrates or methane elathrate hydrates as practical means of gas storage.

Claims (19)

1. CLAiMS 1. A method comprising the enclathration of a gas, and/or the dissociation of a gas from a clathratc, in the presence of a gellable material.
2. 2. The use of a gellable material in enhancing the ericlathration of a gas, and/or the dissociation of a gas from a clathrate.
3. 3. A composition in the form of a clathrate or suitable for forming a clathrate, comprising a elathrate-fonning host and a gellable material, and optiorial]y a gas.
4. 4. A composition comprising a gas clathrate and a gellable material.
5. 5. A composition comprising a clathrate and a gellable material, and optionally a gas.
6. 6. An apparatus comprising a gellable material, 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.
7. 7. A method, use, composition or apparatus, as claimed in any preceding claim, wherein the clathrate comprises clathrate hydrate and the gellable material is a hydrogellable material.
8. 8. A method, use, composition or apparatus, as claimed in any preceding claim, wherein the gas comprises hydrogen.
9. 9. A method, use, composition or apparatus, as claimed in any preceding claim, wherein the gas comprises methane.
10. 10. A method, usc, composition or apparatus, as claimed in any prccedirig claim, wherein the gas comprises carbon dioxide.
11. 11. A method, use, composition or apparatus, as claimed in any preceding claim, wherein thc gas comprises hydrocarbon gas.
12. 12. A method, usc, composition or apparatus, as claimed in any preceding claim, wherein the gas comprises natural gas.
13. 13. A method, usc, composition or apparatus, as claimed in any preceding claim, wherein the clathratc comprises a stabilizer.
14. 14. A method, use, composition or apparatus, as claimed claim 13, wherein the stabilizer comprises THF or TBAB, preFerably TITIF.
15. 15. A method, use, composition or apparatus, as claimed in any preceding claim, wherein the gellable material is a superabsorbent material.
16. 16. A method, use, composition or apparatus, as claimed in any preceding claim, wherein the gel or hydrogel comprises particles which are less than 2 mm in size.
17. 17. A method, use, composition or apparatus, as claimed in any preceding claim, wherein the gcllablc material has an absorbtion capacity such that it may fonn a gel containing at least 90% liquid weight-for-weight.
18. 1 8. A method, use, composition or apparatus, as claimed in any preceding claim, wherein the gellable material may absorb at least 19 ml of liquid per g of gellable material.
19. 1 9. A method, use, composition or apparatus, as claimed in any preceding claim, wherein the gellahlc material is selected from: a synthetic polymer, for example polyvinyl alcohol (PVA), poly(sodium aciylate) (PSA), poly(potassium acrylate), poly(acrylic acid) sodium salt-grqfi-poly(cthylene oxide), poly(acrylic acid co-acrylamide) potassium salt, poly(ethylenc oxide), or a poly(acrylarnide), each of these optiona'ly being cross-linked; a natural material, for example agarose, eellulosc, chitosan, gelatin, an alginate or carrageen or a chemically-modified natural materials, for example methyl cellulose.