Patent Description:
Liquid phases for the dissolution of gases are known. Solutions of various amines in water, or other solvents, are known to dissolve CO<NUM> and are applied industrially in natural gas "sweetening". However, these methodologies comprise the use of toxic materials; are corrosive towards steel, which limits their uses industrially; and require large amounts of energy to regenerate. They are also non-specific and therefore cannot be used for specific or targeted gas separation. An glycol ether solvent type called Genosorb is utilised in industry for separating CO<NUM> from CH<NUM>. However, the CO<NUM> uptake of Genosorb is limited, as is its selectivity for CO<NUM> over CH<NUM>.

Porous solids such as zeolites are useful in molecular separation due to their permanent porosity. Porous solid adsorbents have significant advantages, for instance in terms of lower energy penalties in adsorption-desorption cycles when compared with their liquid counterparts, but they are difficult to incorporate into conventional flow processes. The use of solid zeolite Rho as an adsorbent for CO<NUM>/CH<NUM> separation is described in <NPL>. <CIT> describes adsorbent materials for performing gas separation.

Porous liquids (liquids with permanent porosity, or dispersions) for use in molecular separation have subsequently been developed. These porous liquids have been categorised into three different types (<NPL>) as follows:.

Thus, each type of porous liquid comprises a "host" having a cavity into which, for example, gas molecules could be absorbed.

Disadvantages of known porous liquids include that their preparation involves several steps and requires highly specialised expertise. The solvents used to prepare them are often volatile, which restricts their use in applications utilising reduced pressure to remove dissolved gases. In addition, the solubility of gases in these liquids is difficult to predict due to a lack of available data.

Liquids having improved properties, particularly high CO<NUM> uptake and improved selectivity for CO<NUM> over CH<NUM>, are sought. This can provide improved efficiency in industrial processes where these properties are useful, for example by reducing circulation rates.

This invention relates to a method of adsorbing a gas into a liquid, comprising at least the step of bringing the gas into contact with a dispersion comprising porous particles dispersed in a liquid phase, wherein the porous particles comprise a zeolite and the liquid phase is a size-excluded liquid, whereby the term size-excluded liquid is defined as a liquid which is excluded from the pores of the porous particles, either because it has a molecular size which is too large to enter the pores of the porous particles, or its entry into the pores of the porous particles is thermodynamically or kinetically unfavourable.

Generally, a dispersion is a system in which particles are dispersed in a continuous phase of a different composition. The term "dispersion" is used in relation to the invention to refer to a system in which particles of a porous solid are dispersed in a liquid phase or medium. The dispersion may optionally comprise additives, such as surfactants, in order to increase the stability of the dispersion. Such additives are known to those skilled in the art.

It is known that some dispersions are time dependent before the start of separation of the solid particles in the liquid phase or medium, possibly based on requiring some form of agitation to continue as a dispersion. The present invention is not limited to the stability or transience of the dispersion.

More particularly, the dispersion may be a type <NUM> porous liquid.

In particular, the porous particles may be microparticles and/or nanoparticles. More particularly, the porous particles may be microparticles. Microparticles are generally defined as particles having a mean diameter in the range <NUM>-<NUM> (ie <NUM>-<NUM>,<NUM>). More particularly, the porous particles may have a mean diameter in the range <NUM>-<NUM> (ie <NUM>-<NUM>). Nanoparticles are generally defined as particles having a mean diameter in the range <NUM>-<NUM>.

More particularly, the pores of the porous particles may comprise micropores (ie that they have a pore diameter of less than <NUM>), mesopores (ie that they have a pore diameter in the range <NUM>-<NUM>) or a mixture of micropores and mesopores.

In some embodiments, the zeolite may be selected from zeolite Rho, zeolite Na-Rho, ECR-<NUM>, ZSM-<NUM> and PST-<NUM>. More particularly, the zeolite may be zeolite Rho.

Zeolites may be generally defined as aluminosilicate materials which are crystalline and porous. Zeolite Rho is a type of zeolite which may be defined as having an Si/Al ratio in the range <NUM>-<NUM>, more particularly <NUM>-<NUM>. In particular, zeolite Rho may have a mean pore diameter of <NUM>-<NUM> Angstroms, more particularly <NUM>-<NUM> Angstroms, even more particularly about <NUM> Angstroms. In particular, it may have a pore volume of <NUM>-<NUM><NUM>g-<NUM>, more particularly <NUM>-<NUM><NUM>g-<NUM>, even more particularly about <NUM><NUM>g-<NUM>. More particularly, zeolite Rho may have a body-centred cubic crystal structure.

A size-excluded liquid is defined in the context of the invention as a liquid which is excluded from the pores (ie the cavities) within the porous particles. This can be because the size-excluded liquid has a molecular size which is too large to enter the pores of the porous particles. Alternatively, entry into the pores of the porous particles may be thermodynamically or kinetically unfavourable.

In particular, the size-excluded liquid may be selected from a glycol; <NUM>-crown-<NUM>; a tertiary amine having substituted or unsubstituted aryl or alkyl substituents, for example each substituent can individually be C<NUM>-C<NUM> substituted or unsubstituted aryl or alkyl; the tertiary amine may be a trialkylamine where each alkyl group is individually C<NUM>-C<NUM> alkyl, more particularly C<NUM>-C<NUM> alkyl, for example trioctylamine; <NUM>-(tert-butylamino)ethyl methacrylate; a trialkyl phosphate where each alkyl group is individually C<NUM>-C<NUM> alkyl, more particularly C<NUM>-C<NUM> alkyl, for example tributyl phosphate; a dialkyl phthalate where each alkyl group is individually C<NUM>-C<NUM> alkyl, more particularly C<NUM>-C<NUM> alkyl, for example dioctyl phthalate; and bis(<NUM>-ethylhexyl) sebacate. More particularly, the glycol may be a polyalkylene glycol. In particular, the polyalkylene glycol may be a polyethylene glycol or a polypropylene glycol. More particularly, the polyethylene glycol may be selected from a polyethylene glycol dialkyl ether and a polyethylene glycol carboxylate. In particular, the polyethylene glycol dialkyl ether may be selected from a polyethylene glycol dimethyl ether and a polyethylene glycol dibutyl ether.

The polyethylene glycol dimethyl ether may be in the form of a mixture with one or more other components. Example mixtures include:.

The polyethylene glycol dibutyl ether may be in the form of a mixture with one or more other components. An example mixtures is polyethylene glycol dibutyl ether with diaryl-p-penylenediamines (for example, Genosorb <NUM> - mixture of diaryl-p-penylenediamines >=<NUM> - < <NUM> %w/w)).

In particular, the zeolite may be zeolite Rho and the size-excluded liquid may be a polyethylene glycol dimethyl ether, a polyethylene glycol dibutyl ether, <NUM>-crown-<NUM> or bis(<NUM>-ethylhexyl) sebacate. More particularly, the zeolite may be zeolite Rho and the size-excluded liquid may be a polyethylene glycol dimethyl ether. Even more particularly, the zeolite may be zeolite Rho and the size-excluded liquid may be a polyethylene glycol dimethyl ether in the form of a mixture with one or more other components as defined above.

In particular, the dispersion may comprise <NUM>-<NUM> wt% of the porous particles, more particularly <NUM>-<NUM> wt%. Even more particularly, the dispersion may comprise <NUM>-<NUM> wt% of the porous particles, more particularly <NUM>-<NUM> wt%.

In the dispersion according to the present invention, the pores of the porous particles may be accessible to a gas. Optionally, the gas may be CO<NUM>, CH<NUM>, N<NUM>, C<NUM>H<NUM>, C<NUM>H<NUM>, Xe, SF<NUM>, C<NUM>H<NUM> or H<NUM>, or a mixture thereof. More particularly, the gas may be selected from CO<NUM> and CH<NUM>, even more particularly the gas may be CO<NUM>.

In an embodiment, the gas in the method is in a gas mixture and the gas is selectively adsorbed by the dispersion.

In particular, the gas in the method may be CO<NUM>, CH<NUM>, N<NUM>, C<NUM>H<NUM>, C<NUM>H<NUM>, Xe, SF<NUM>, C<NUM>H<NUM> or H<NUM>, or a mixture thereof. More particularly, the gas may be selected from CO<NUM> and CH<NUM>, even more particularly the gas may be CO<NUM>.

In particular, the method of adsorbing a gas into a liquid may additionally comprise, after the step of bringing the gas into contact with the dispersion, the step of regenerating the dispersion. More particularly, the regeneration step may comprise applying a vacuum to the dispersion. In particular, the regeneration step may comprise heating the dispersion to a temperature of at least <NUM>, more particularly at least <NUM>, even more particularly at least <NUM>. In particular, the steps of applying the vacuum and heating may be carried out at the same time. More particularly, the steps of applying the vacuum and heating may be carried out for at least <NUM> minutes, even more particularly at least <NUM> minutes, more particularly at least <NUM> minutes.

Also described, but not claimed, is a method of preparing a dispersion comprising at least the step of: mixing (i) porous particles comprising a zeolite, and (ii) a size-excluded liquid. The porous particles may be as defined above. The size-excluded liquid may be as defined above. More particularly, the dispersion formed by the method may be as defined above.

Optionally, the mixing includes agitating, stirring, sonication or grinding or a combination thereof. More particularly, the method may comprise stirring the mixture.

Also described, but not claimed, is an assemblage of a dispersion comprising porous particles dispersed in a liquid phase, wherein the porous particles comprise a zeolite and the liquid phase is a size-excluded liquid, wherein the zeolite comprising a cavity and a gas contained within the cavity. More particularly, the gas may be CO<NUM>, CH<NUM>, N<NUM>, C<NUM>H<NUM>, C<NUM>H<NUM>, Xe, SF<NUM>, C<NUM>H<NUM> or H<NUM>, or a mixture thereof. In particular, the gas may be selected from CO<NUM> and CH<NUM>, even more particularly the gas may be CO<NUM>.

This invention will be further described by reference to the following Figures which are not intended to limit the scope of the invention claimed, in which:.

There are two reported methods to synthesize zeolite Rho according to literature. For the current study of zeolite Rho, method <NUM> was used to synthesize high crystallinity material.

Method <NUM> (see <NPL>): <NUM>-crown-<NUM> ether (<NUM>, <NUM> mmol), cesium hydroxide (<NUM>, <NUM> mmol) and sodium hydroxide (<NUM>, <NUM> mmol) were dissolved in <NUM> of deionised water. Sodium aluminate (<NUM>, <NUM> mmol) was added to this solution and stirred until fully dissolved. Ludox AS-<NUM> colloidal silica (<NUM>, <NUM> mmol) was then added. The resulting mixture was stirred overnight at room temperature under atmospheric pressure. The obtained precursor mixture was then placed in a Teflon-lined stainless steel autoclave at <NUM> for <NUM> days for crystallization. The resulting zeolite Rho was then washed with deionised water by filtration until neutral and calcined at <NUM> for approximately <NUM> hours to remove the organic template (<NUM>-crown-<NUM>).

Method <NUM> (see<NPL>): Caesium hydroxide (<NUM>, <NUM> mmol) and sodium hydroxide (<NUM>, <NUM> mmol) were dissolved in <NUM> of deionised water. Sodium aluminate (<NUM>, <NUM> mmol) was added to this solution and stirred until fully dissolved. Ludox AS-<NUM> colloidal silica (<NUM>, <NUM> mmol) was then added. The resulting mixture was stirred overnight at room temperature under atmospheric pressure. The obtained precursor mixture was then placed in a Teflon-lined stainless steel autoclave at <NUM> in an oil bath for <NUM> days for crystallization. The resulting zeolite Rho was then washed with deionised water by filtration until neutral.

Method <NUM>: Caesium hydroxide (<NUM>, <NUM> mmol) and sodium hydroxide (<NUM>, <NUM> mmol) were dissolved in <NUM> of deionised water. Sodium aluminate (<NUM>, <NUM> mmol) was added to this solution and stirred until fully dissolved. Ludox AS-<NUM> colloidal silica (<NUM>, <NUM> mmol) was then added. <NUM> of crystalline zeolite Rho (seeding) was then added to the resulting mixture and stirred overnight at room temperature under atmospheric pressure. The obtained precursor mixture was then placed in a Teflon flask at <NUM> in oil bath for <NUM> days for crystallization. The resulting zeolite Rho was then washed with deionised water by filtration until neutral.

The zeolite Rho data below is a result of testing carried out on material made by Method <NUM>.

The dispersion (also sometimes referred to as a "porous liquid") was prepared by mixing Genosorb <NUM> and zeolite Rho by stirring the components in laboratory flask until formation of homogeneous dispersion, typically about <NUM> mins. Other missing techniques such as grinding, milling or sonicating can also be used.

The zeolite Rho - Genosorb <NUM> porous liquid was characterized by Powder X-Ray Diffractometer (PXRD), Thermo-gravimetric Analysis (TGA) and Infrared Spectroscopy (IR). The PXRD spectrum of zeolite Rho in Genosorb porous liquid (see <FIG>) shows an identical pattern to that of the original zeolite Rho (see both <FIG> and <FIG>). This confirms that the zeolite components remain intact and crystalline after mixing with Genosorb.

An SEM image of the original zeolite Rho (ie not as a dispersion) is shown in <FIG>. This demonstrates that the particle size is around <NUM>-<NUM>.

Low pressure measurement (c. <NUM> bar condition; <NUM>) - Gas solubility studies were carried out by using a volumetric technique based on an isochoric method (see <NPL>).

All the measurements were carried out at around <NUM> bar and <NUM>. The results show that the addition of zeolite Rho to commercial solvent Genosorb <NUM> increases the CO<NUM>/CH<NUM> selectivity significantly (see Table <NUM> below). The zeolite Rho does not lose its gas capacity and the gas uptake is predictable.

High Pressure measurement (<NUM>-<NUM> bar, <NUM>-<NUM>) - High pressure gas solubility studies were carried out by using Parr reactor based on a mass flow (see <NPL>for a similar experimental set-up).

All the measurements were carried out from <NUM> to <NUM> bar at <NUM>, <NUM> and <NUM>. The high pressure measurements also show predictable outcomes. Table <NUM> below, and <FIG>, show the CO<NUM> uptake of Genosorb <NUM>, a <NUM>. 5wt% dispersion of zeolite Rho in Genosorb <NUM> and a 25wt% dispersion of zeolite Rho in Genosorb <NUM>. Table <NUM> below shows the experimental values for pure zeolite Rho at <NUM>, plus the predicted values for the <NUM>. 5wt% zeolite Rho in Genosorb <NUM> and 25wt% zeolite Rho in Genosorb <NUM> dispersions.

The measured CO<NUM> solubility of the dispersions is comparable to its predicted value at low pressure but slightly less than the predicted value at high pressure. The high pressure gas uptake measurements show that the addition of zeolite Rho to Genosorb <NUM> solvent significantly enhances CO<NUM> uptake and the operational range for a temperature pressure swing adsorption/desorption system.

Ease of material regeneration is a useful property which can provide a reduction in regeneration cost. It is difficult to achieve by amine-based technology nowadays due to the high energy penalty. The dispersions of the invention are understood to be easily regenerated by applying mild heating or vacuum. As shown in <FIG>, the porous liquid (<NUM>. 5wt% zeolite RHO in Genosorb <NUM>) shows about a <NUM>% recovery in CO<NUM> uptake capacity when it has undergone a room temperature regeneration under vacuum for <NUM> minutes. However, CO<NUM> uptake capacity recovers to around <NUM>% when the same regeneration conditions are used, but the temperature is increased to <NUM>.

Further dispersions comprising combinations of porous particles with various liquids were prepared by mixing the porous particles with the liquid as described above. The dispersions produced, and their theoretical and actual CO<NUM> uptake values in mg/g, are shown in Tables 4a-c below.

CH<NUM> uptake of the dispersions was also investigated and the results are shown in Table <NUM> below. This was carried out using the isochoric method described above (ie<NPL>).

Selectivity is estimated by ratio (Ammol/g/Bmmol/g). Values for CO<NUM> selectivity over CH<NUM> (CO<NUM>/CH<NUM>) were calculated for two of the dispersions and the results are shown in Table <NUM> below.

Claim 1:
A method of adsorbing a gas into a liquid, comprising at least the step of bringing the gas into contact with a dispersion comprising porous particles dispersed in a liquid phase, wherein the porous particles comprise a zeolite and the liquid phase is a size-excluded liquid, whereby the term size-excluded liquid is defined as a liquid which is excluded from the pores of the porous particles, either because it has a molecular size which is too large to enter the pores of the porous particles, or its entry into the pores of the porous particles is thermodynamically or kinetically unfavourable.