USING NANO-FUNCTIONALIZED CLAY MINERALS FOR GAS SEPARATION

Abstract

A smectite or vermiculite clay mineral in the form of a powder and having a plurality of layers wherein each layer comprises one octahedral type sheet sandwiched between two tetrahedral type sheets; wherein at least every other layer of said clay mineral comprises a hydroxide species comprising a cation selected from the group consisting of Ni, Mg, Fe, Mn or Zn.

Claims

1. A smectite or vermiculite clay mineral in the form of a powder and having a plurality of layers wherein each layer comprises one octahedral type sheet sandwiched between two tetrahedral type sheets; wherein at least every other layer of said clay mineral comprises a hydroxide species comprising a cation selected from the group consisting of Ni, Mg, Fe, Mn or Zn.

2. A clay mineral as claimed in claim 1 which comprises a nickel hydroxide containing species.

3. A clay mineral as claimed in any preceding claim which has a pfu of 0.2 to 0.5.

4. A clay mineral as claimed in any preceding claim wherein the clay is montmorillonite.

5. A clay mineral as claimed in any preceding claim wherein the water content of the clay is 1 to 6 water molecules per cation.

6. A clay mineral as claimed in any preceding claim wherein at least every other layer of said clay comprises a species comprising Ni.sup.2+, (OH-) and (H.sub.2O).

7. A clay mineral as claimed in claim 6 wherein the alternate layers comprise Ni.sup.2+ ions.

8. A process for separating carbon dioxide from a gas mixture comprising carbon dioxide comprising: (i) contacting a smectite or vermiculite clay mineral such as synthetic fluorohectorite or natural bentonite with an aqueous solution of cations that can form hydroxides that are attractive for carbon dioxide, such as Ni, Mg, Fe, Mn and Zn ions, at a pH of 7 or more; (ii) drying the clay mineral prepared in step (i) to form a powder; (iii) contacting a bed of said powder from step (ii) with said gas mixture, said gas mixture preferably being supplied to said bed under pressure.

9. A process for separating carbon dioxide from a gas mixture comprising carbon dioxide comprising: (i) contacting a smectite or vermiculite clay mineral such as synthetic fluorohectorite or natural bentonite with an aqueous solution of cations that can form hydroxides that are attractive for carbon dioxide, such as Ni, Mg, Fe, Mn and Zn ions, at a pH of 7 or more; (ii) drying the clay mineral prepared in step (i) to form a powder (iii) contacting a bed of said powder from step (ii) with a gas stream comprising said gas mixture, said gas mixture preferably being supplied to said bed under pressure; (iv) removing said bed from the gas stream; (v) treating said bed to release carbon dioxide therefrom.

10. A process for separating carbon dioxide from a gas mixture comprising carbon dioxide comprising: (i) contacting a smectite or vermiculite clay mineral such as synthetic fluorohectorite or natural bentonite with an aqueous solution of Ni, ions, at a pH of 7 or more; (ii) drying the clay mineral prepared in step (i) to form a powder (iii) contacting a bed of said powder from step (ii) with a gas stream comprising said gas mixture, said gas mixture preferably being supplied to said bed under pressure; (iv) removing said bed from the gas stream; (v) treating said bed to release carbon dioxide therefrom.

11. A process for separating carbon dioxide from a gas mixture comprising carbon dioxide comprising: (i) mixing a smectite or vermiculite clay mineral such as synthetic fluorohectorite or natural bentonite with an aqueous solution of cations that can form hydroxides that are attractive for carbon dioxide, such as Ni, Mg, Fe, Mn and Zn ions, at a pH of 7 or more; (ii) drying the clay mineral prepared in step (i) to form a powder; contacting a first bed of said powder from step (ii) with a gas stream comprising said gas mixture, said gas mixture preferably being supplied to said first bed under pressure; (iii) removing said first bed from the gas stream; (iv) redirecting said gas stream to contact a second bed of powder from step (ii); (v) optionally heating said first bed to release carbon dioxide therefrom.

12. A process as claimed in claim 11 further comprising redirecting said gas stream to contact the regenerated first bed whilst said second bed is regenerated.

13. A process as claimed in claim 8 to 11 wherein carbon dioxide is desorbed from said powder by heating or under reduced pressure.

14. A process as claimed in claim 8 to 13 wherein the gas mixture comprises methane.

15. A process as claimed in claim 8 to 14 wherein the aqueous solution which is contacted with the clay mineral preferably has at least a 10-fold excess of cations in solution relative to the CEC of the clay mineral, such as a 10 to 30 fold excess.

Description

[0072] The invention is now described with reference to the following non limiting examples and figures.

[0073] FIG. 1: The smectite group including montmorillonite and hectorite and the vermiculite group are organized in a 2:1 (TOT = Tetrahedral-Octahedral-Tetrahedral) layered structure where each layer is made up of one O sheet sandwiched between two T sheets. Depending on the composition of the sheets (e.g. by isomorphic substitutions of metal cations), the composed layer can have a net negative charge, which needs to be balanced by an interlayer cation (blue balls in the sketch).

[0074] FIG. 2 is an illustration of the 2 WL state for Na-fluorohectorite, with the hydrogen bond bonding between Na(H.sub.2O).sub.6].sup.+ and tetrahedral sheets fixing the stacking order.

[0075] FIG. 3: Summary of maximum smectite interlayer TOT-TOT spacing d.sub.001 after charging samples with CO.sub.2. Hydration states (0 WL, 1 WL etc.) relate to interlayer water layers. These depend on the interlayer cation, on relative humidity, temperature and pressure.

[0076] FIG. 4 shows that nickel-fluorohectorite forms an ordered interstratification with a chlorite-like condensed species in one interlayer, and a smectite-like structure in the adjacent interlayer. The chlorite-like phase is responsible for CO.sub.2 adsorption, where the CO.sub.2 forms a reversible bond with the intercalated nickel hydroxide as depicted to the right.

[0077] FIG. 5 shows that when nickel fluorohectorite was prepared with three different layer charges, 0.3, 0.5, and 0.7 pfu respectively, it was found that the adsorption capacity of nickel-exchanged fluorohectorite increases for lower layer charge, and the onset of adsorption and swelling in response to CO.sub.2 is at lower pressure for lower layer charge. Upon reversible release of CO.sub.2, the higher layer charge clay retains CO.sub.2 to a larger degree.

EXAMPLE 1

Formation of an Ordered Interstratification Upon Ni-Exchange

[0078] A smectic clay mineral (synthetic sodium fluorohectorite) was subjected to ion exchange with a aqueous solution of Ni hydroxide at a pH of 7.

[0079] By simple ion exchange a corrensite-like structure was obtained with a structural formula of {[Ni(OH).sub.0.83(H.sub.2O).sub.1.17].sub.0.37.sup.1.17+}.sub.Int.1{[Ni(H.sub.2O).sub.6].sub.0.28.sup.2+}.sub.Int.2[Mg.sub.5Li] < Si.sub.8>O.sub.20F.sub.4.

[0080] This was investigated using a combination of powder X-ray diffraction, thermal gravimetric analysis, and various spectroscopic techniques and this showed the presence of an ordered interstratification of smectite-like [Ni(H.sub.2O).sub.6].sub.0.28 .sup.2+ and condensed, chlorite-like [Ni(OH).sub.2-y(H.sub.2O).sub.y].sub.x.sup.y+ interlayers, where x refers to the degree of condensation.

[0081] Improvement of the contrast between the two distinct d-spacings and between the electron densities of the interlayers was obtained by partial ion exchange with a long chain alkylammonium cation or thermal annealing. This increased the intensity of superstructure reflections, rendering the ordered interstratified structures more clearly visible.

EXAMPLE 2

CO.SUB.2 Capture by Nickel Hydroxide Interstratified in the Nanolayered Space of a Synthetic Clay Mineral

[0082] The clay obtained in example 1 was subject to drying to yield a “dry” and “hydrated” clay (a corrensite-like fluorohectorite clay with a structural formula of {[Ni(OH).sub.0.83(H.sub.2O).sub.1.17].sub.0.37.sup.1.17+}.sub.Int..sub.1{[Ni(H.sub.2O).sub.6].sub.0.28.sup.2+}.sub.Int.2[Mg.sub.5Li] < Si.sub.8>O.sub.20F.sub.4) The corrensite-like clay was packed into a suitable column to form a bed and carbon dioxide gas was applied to the column under pressure.

[0083] Using a combination of powder X-ray diffraction, Raman spectroscopy and Inelastic Neutron Scattering it was demonstrated that both dried and hydrated clays show crystalline swelling and spectroscopic changes in response to CO.sub.2 exposure. These changes can be attributed to interactions of carbon dioxide with chlorite like [Ni(OH).sub.0.83(H.sub.2O).sub.1.17].sup.1.17+.sub.0.37 -interlayer species within the clay. Swelling occurs solely in the interlayers where this condensed species is present. This example demonstrates a hitherto overlooked important mechanism, where hydrogenous species present in the nano-space of a clay mineral create sorption sites for CO.sub.2.

EXAMPLE 3

CO.SUB.2 Adsorption Enhanced by Tuning the Layer Charge in a Clay Mineral

[0084] Synthetic fluorohectorite clay minerals with pfu 0.5 and 0.7, respectively were prepared via melt synthesis according to published procedures, followed by long-term annealing to improve charge homogeneity and phase purity. In addition, synthetic fluorohectorite with pfu 0.3 was prepared by layer charge reduction by employing the Hofmann-Klemen-Effect, following well established published procedures.

[0085] Each of the three pfu clay minerals were subjected to ion exchange using Ni hydroxide as described in example 1 and dried to give batches of corrensite like clay minerals with the three different pfu values.

[0086] Each of these clay mineral batches were packed into suitable columns to form a bed and a carbon dioxide gas was applied to the column under pressure stepwise up to 35 bars, see FIG. 5.

[0087] The excess adsorption capacity of the clay mineral at 35 bar is measured to be 8.6 wt.%, 6.5 wt.% and 4.5 wt.%, for the lowest, intermediate and highest layer charge measured.

[0088] Carbon dioxide was desorbed from the clay mineral by stepwise pressure reduction, see FIG. 5.

[0089] Using a combination of X-ray diffraction, neutron diffraction and gravimetric adsorption measurements, our results show a clear dependency of the layer charge for CO.sub.2 adsorption.

[0090] The adsorption capacity of the clay mineral increases with decreasing layer charge, and the threshold for adsorption and swelling in response to CO.sub.2 occurs at lower pressures for decreasing layer charge. We associate the mechanism for CO.sub.2 adsorption with a higher cohesion due to attractive electrostatic forces between the layers with higher layer charge, resulting in a higher onset pressure required for swelling.

[0091] Upon release of CO.sub.2 the highest layer charge clay mineral retains CO.sub.2 to a larger degree, which we associate to the same cohesion mechanism, where CO.sub.2 is first released from the edges of the clay mineral particles thereby closing exit paths and trapping the CO.sub.2 molecules in the center of the clay mineral particles.