BACKGROUND OF THE INVENTION

Fiber membranes have superior gas separation properties, i.e., good selectivity and high permeation flux. As such, they have been used to separate and purify gases in various industrial applications, including fuel cell, power generation, and natural gas/synthetic gas treatment. See Baker, Ind. Eng. Chem. Res. 2002, 41 : 1393-1411.

There is a need to develop durable membranes that are more efficient in separating gases.

SUMMARY OF THE INVENTION

One aspect of this invention relates to a hollow fiber membrane, which includes (i) a selective layer including a first polymer, a plurality of sulfonate ions, and a plurality of transition metal ions (e.g., Ag ⁺ , Cu ⁺ , Zn ²⁺ , or a mixture thereof), the sulfonate ions being attached to the first polymer via covalent bonding and the transition metal ions being attached to the sulfonate ions via ionic bonding, and (ii) a supporting layer including a second polymer. In this membrane, the selective layer has a thickness of 20- 300 nm (e.g., 20-100 nm), the supporting layer has a thickness of 35-250 μπι and contains voids having sizes of 1-100,000 nm (the size of a void is the longest possible distance between two points on the void), and the selective layer has a higher density than the supporting layer. Typically, the hollow fiber membrane has an outer diameter of 150- 1000 μιη and an inner diameter of 75-500 μπι. In one embodiment, the selective layer constitutes the outermost portion of the hollow fiber membrane (i.e., the outer skin of the membrane).

Examples of polymers contained in the the selective layer and the supporting layer include, but are not limited to, polysulfone, polyethersulfone, polyimide, polyamide, poly(phenylene oxide), polyetherketone, and polyetheretherketone.

Preferably, these polymers may have a molecular weight of 10,000 to 3,000,000. If desired, the two layers can be made of the same polymer. In other words, the first polymer and the second polymer can be identical.

The voids in the supporting layer may be non-uniform in size across the thickness of the layer. For example, the sizes of the voids in the outer part of the supporting layer may be smaller than the sizes of the voids in the inner part of the supporting layer (e.g., 1-10 nm vs. 1-10 μπι).

The hollow fiber membrane may have an additional supporting layer that includes a third polymer and contains voids having sizes of 1-100,000 nm. The third polymer, e.g., polysulfone, polyethersulfone, polyimide, polyamide, poly(phenylene oxide), polyetherketone, or polyetheretherketone, may be the same as or different from the first polymer and second polymer in the membrane.

Another aspect of this invention relates to a process of preparing a hollow fiber membrane. The process includes the steps: (i) providing a solution of a polymer, e.g., polysulfone, polyethersulfone, polyimide, polyamide, poly(phenylene oxide),

polyetherketone, or polyetheretherketone, in a solvent, (ii) continuously extruding the solution through a spinneret while introducing a bore fluid into the center portion of the extruded solution, (iii) immersing the resulting extruded solution in a coagulant to form a hollow fiber, (iv) treating the hollow fiber with a solution containing aldehyde and then with a solution containing a sulfite salt to conduct the chloromethylation reaction and the sulfonation reaction, thereby forming a sulfonated hollow fiber, and (v) contacting the sulfonated hollow fiber with a solution containing transition metal ions for ion exchange, thereby forming a transition metal ionic modified hollow fiber membrane.

The above-mentioned extrusion step includes co-extruding two polymer solutions through a spinneret having dual annular concentric orifices; the two solutions contain the same polymer or different polymers, e.g., polysulfone, polyethersulfone, polyimide, polyamide, poly(phenylene oxide), polyetherketone, and polyetheretherketone.

Also within the scope of this invention is a method of altering the composition of a gaseous mixture. This method includes passing a gaseous mixture through the membrane described above. The gaseous mixture contains two or more gases of H ₂ , N ₂ , 0 ₂ , CH ₄ , C ₃ H ₈ , CO, C0 ₂ , H ₂ S, and H ₂ 0. Examples include a mixture of N ₂ and 0 ₂ , a mixture of H ₂ and N ₂ , a mixture of H ₂ and CH ₄ , a mixture of C0 ₂ and N ₂ , a mixture of C0 ₂ or CH4, and a mixture of CH ₄ and C ₃ Hg.

The details of one or more embodiments of the invention are set forth in the description below. Other features, objects, and advantages of the invention will be apparent from the detailed description of several embodiments and also from the appending claims.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to a hollow fiber membrane having an ultrathin transition metal modified selective layer and one or more voids-containing supporting layers. This membrane unexpectedly exhibits superior features, including high permeability, good selectivity, and excellent mechanical strength, for gas separation applications.

To fabricate the hollow fiber membrane of this invention, one first dissolves a polymer in a suitable solvent to form a polymer solution. A preferred polymer solution contains 25-40% by weight the polymer.

The polymer used to practice this invention can be polysulfone (PSf), polyethersulfone (PES), polyimide (PI), polyamide, poly(phenylene oxide),

polyetherketone (PE ), or polyetheretherketone. Structures of some of these polymers are shown below:

The suitable solvent used to dissolve the polymers can be methanol, ethanol, acetone, 1-propanol, isopropanol, butanol, isobutanol, N-methyl-2-pyrrolidone (NMP), N, N-dimethylacetamide (DMAc), ethylene glycol, dimethyl sulfoxide (DMSO), glycerol, propylene glycol, dimethyl formamide (DMF), diethylene glycol (DG), or a mixture thereof.

The polymer solution is filtered (e.g., mesh size of 15 μηι) and then extruded through a spinneret having an annular orifice. During the extrusion, a bore fluid solution is continuously introduced into the center of the spinneret, thereby occupying the center space of the extruded polymer solution. The bore fluid solution can be preferably miscible with the solvent of the polymer solution and a coagulant used in the next step described below. Examples of a bore fluid solution include, but are not limited to, water, methanol, ethanol, 1-propanol, isopropanol, butanol, isobutanol, ethylene glycol, diethylene glycol, glycerol, propylene glycol, N-methyl-2-pyrrolidone, dimethyl sulfoxide, dimethyl formamide, dimethyl acetamide, and a mixture thereof. The volume flow rates of the polymer solution and bore fluid solution through the spinneret are 0.05 to 10 ml/min.

The polymer solution can be extruded into a gaseous atmosphere (e.g., air at the ambient temperature) before contacting a coagulant bath. The air gap between the spinneret outlet and the top surface of the coagulation bath is preferably 0.5 to 50 cm, and, more preferably, 0.5 to 10 cm. Such positioning facilitates the stretching of the extruded polymer solution prior to coagulation.

Dry-jet wet spinning can be utilized to control the hollow fiber dimension. See Qin et al., Journal of Membrane Science, 2001, 182: 57-75. During the spinning, the temperature of the polymer solution is preferably kept at 5-100°C and, more preferably, at 20 to 50°C. The extruded polymer solution is then immersed into a coagulant for phase inversion. The coagulant is a solvent in which the bore fluid solution is soluble or miscible, but the solvent of the polymer sloution is insoluble or substantially insoluble. Examples of a suitable coagulant include, but are not limited to, water, methanol, ethanol, 1-propanol, isopropanol, butanol, isobutanol, ethylene glycol, diethylene glycol, glycerol, propylene glycol, and a mixture thereof. The temperature of the coagulant bath is preferably 0 to 90°C and, more preferably, 20 to 50 ^° C.

In the coagulation step, the outermost portion of the extruded polymer solution contacts the coagulant in which the solvent of the solution is insoluble or insubstantially soluble. This leads to an instantaneous phase inversion that facilitates formation of a selective layer having a high density, e.g., 1.0-1.6 g/cm ³ . By contrast, the inner portion of the extruded polymer solution does not contact the coagulant. Instead, it contacts the bore fluid solution in which the solvent of the solution is soluble. This delays the phase inversion. As a result, the inner portion forms a supporting layer containing voids. Due to the existence of voids, the supporting layer has a low density, e.g., 0.1 - 1.1 g cm ³ .

The thus-formed hollow fiber membrane can be washed with a liquid to remove residual solvents. The wash liquid is preferably water, methanol, ethanol, isopropanol, ethylene glycol, or a mixture thereof. As an example, the hollow fiber membrane is washed by flowing water or immersed in water for 3 to 5 days. This washing step is performed preferably at 5-50°C and, more preferably, at 20-30°C.

In the hollow fiber membrane, hollow space is formed after the bore fluid solution has been removed. The thus-obtained hollow fiber membrane preferably has an outer diameter of 200 to 750 μπι, and an inner diameter of 100 to 300 μηι. The particularly preferred hollow fiber membrane produced in the present process has an outer diameter of 300 to 600 μπι, and a wall thickness of 80 to 150 μιη.

The hollow fiber membrane of this invention may contain dual or multiple supporting layers. Such configurations require a smaller amount of polymer that contains a functional group, thereby reducing costs. They can be fabricated by co-extrusion techniques described in literature. See, e.g., Li et al., Journal of Membrane Science, 2008, 325: 23-27. As an example, a dual-supporting-layer membrane can be fabricated as follows:

Two polymer solutions are respectively extruded through dual-annular concentric orifices in a spinneret and then immersed in a coagulant for phase inversion. The dimensions of the spinneret are preferably such that the outer diameter is 0.5 to 2.5 mm (preferably 0.75 to 1.5 mm) and the inner diameter is 0.3 to 1.5 mm (more preferably 0.3 to 1.0 mm). The dual-supporting-layer hollow fiber spinning can be separately adjusted to control the volume flow rates of the outer-layer and inner-layer polymer solutions so as to decouple the effects of elongational and shear rates and thus produce fibers with higher selectivity. See Li et al., Journal of Membrane Science, 2008, 325: 23-27.

The hollow fiber membrane thus obtained is subjected to chemical modification, e.g., chloromethylation-sulfonation-ion exchange. The chloromethylation can be conducted by circulating a reaction solution containing 37 wt% HCl aqueous solution, 37 wt% formaldehyde aqueous solution, and ZnCl ₂ along the shell side of the fibers at 20- 100°C under 0.1-2 atm for 1 - 10 hours. The preferred chloromethylation condition is 40- 60°C under 0.5-1 atm for 3-6 hours. The hollow fiber membrane can then be sulfonated by treating it with a mixture of Na ₂ S0 ₃ , ethanol, and de-ionized (DI) water at 20-100°C under 0.1-2 atm for 1-10 hours. The preferred sulfonation condition is 40-60°C under 0.5-1 atm for 3-6 hours. The modified fiber membrane is washed with DI water for 12- 48 hours, preferably 20-30 hours, to remove the unreacted chemicals. The reaction equations of chloromethylation and sulfonation are shown below:

ZnCl ₂

Chloromethylation: -C ₆ H →HCHO ₊ HCl ^C^CH^ -^O _{( 1 )} Sulfonation: -c ₆ H,(CH i)→Na ₇ ∞;→-c ₆ H,(.CH ₂ sa;Na ⁺ ) ₊ N _a ⁺ ₊ cr

The hollow fiber membrane is then immersed in a salt solution for 1-5 days (preferably 2-3 days) with stirring at 20-50°C (preferably 20-30°C) for ion exchange between sodium ions and transition metal ions. The salt solution contains 0.01-1 mol/1, preferably 0.05-0.2mol/l, CuCl, AgN0 ₃ , or ZnCl ₂ . After the ion exchange, the hollow fiber membrane can be washed using de-ionized water to remove the unreacted ions for 1-12 hours, preferably 3-6 hours. Before being air-dried, the hollow fiber membrane can be further rinsed, e.g., by immersing it in methanol three times to remove water and then immersing it in hexane three times to remove methanol.

Chemically modified hollow fiber membranes obtained by the above-described method can be packed into modules and tested for their gas separation performance. See, e.g., Li et al., Journal of Membrane Science, 2002, 198: 21 1-223; Li et al., Journal of Membrane Science, 2004, 243: 155-175; and Li et al., Journal of Membrane Science, 2008, 325: 23-27.

Hollow fiber membranes have been used in industrial processes to separate and purify various gases. See, e.g., Baker, Ind. Eng. Chem. Res. 2002, 41 : 1393-141 1. One skilled in the art would know how to use the hollow fiber membranes of this invention to achieve the same purposes.

Without further elaboration, it is believed that one skilled in the art can, based on the disclosure herein, utilize the present invention to its fullest extent. The following specific embodiments are, therefore, to be construed as merely descriptive, and not limitative of the remainder of the disclosure in any way whatsoever. All publications cited herein are incorporated by reference.

EXAMPLE 1

A dual-supporting-layer hollow fiber membrane was made as follows:

Commercial Radel ^® A PES (Solvay Advanced Polymers L.L.C., Georgia, USA) was dried at 120°C overnight in vacuo before use. The composition of the outer-layer polymer solution was 35/50/15 (wt%) PES/NMP/ethanol, and the composition of the inner-layer polymer solution was 23/41/36 (wt%) PES/NMP/DG. These two solutions were vigorously stirred to become homogenous and then degassed for 1 day. The dual- supporting-layer hollow fiber membranes were fabricated by the co-extrusion technique as described above. The flow rates of the bore fluid and both polymer solutions were controlled by syringe pumps (ISCO Inc.). The detailed spinning conditions are listed in Table 1. Table 1 : Spinning conditions for dual-supporting-layer PES hollow fiber membranes

EXAMPLE 2

Three modules for each PES hollow fiber sample were made. Each module included 10-15 fibers with an effective length of around 16-20 cm per fiber. The chloromethylation followed by sulfonation described above were performed to make the membranes having sodium ions. Both the chloromethylation and sulfonation conditions were 40°C and 1 atm for 3 h. After sulfonation, the hollow fiber membranes were immersed in the 0.1 mol/1 silver nitrate aqueous solution for 3 days under stirring at room temperature.

EXAMPLE 3

The dual-supporting-layer PES hollow fiber membranes were tested for their performance and morphology in both pure gas and mixed gas systems. For pure gas test, 3-4 modules, each including 10-15 fibers with an effective length of 15 cm per fiber, were assessed at room temperature under 7 atm. For the mixed gas test, 2 modules, each including 10 fibers with an effective length of 18 cm per fiber, were assessed at room temperature under 7 atm. The 0 ₂ N ₂ and C0 ₂ /CH selectivity and dense-selective layer thickness (i.e. skin thickness) were measured. They are defined as follows:

The ideal selectivity: a _n ,„ = (3)

° ^{> I N>} (PIL) _Ni

0 ₂ (dense film )

The dense-selective layer thickness: L = (4)

(dual-layer hollow fiber membrane)

where P/L is the gas permeance of hollow fiber membranes in GPU (1 GPU = 1 x 10 ^"6 cm ³ (STP)/(cm ² s cmHg) = 7.5005 x 10 ^~12 m s ^~ ' Pa ^"1 ) and P is the gas permeability of dense films made from outer-layer materials in Barrer (1 Barrer = 1 x 10 ^"10 cm ³ (STP) cm/(cm ² s cmHg) =

7.5005 x 10- ¹⁸ mV Pa ^"1 ).

The gas separation performance of hollow fiber membranes is summarized Tables 2, 3 and 4 below.

Table 2: Pure gases 0 ₂ /N ₂ separation performance of dual-supporting-layer PES hollow fibers

Ag ⁺ form Permeance (0 ₂ ) (GPU) 3.33 4.03 4.52 5.60 (after coating) Permeance (N ₂ ) (GPU) 0.359 0.424 0.518 0.71

5

Ideal selectivity 9.28 9.50 8.73 7.83

Skin thickness (A) 1321 1092 973 786

Table 3: Pure gases C0 ₂ /CH ₄ separation performance of dual-supporting-layer PES hollow fibers

Table 4: Mixed gases CO /CH ₄ (50/50%) separation performance of dual-supporting- layer PES hollow fibers

OTHER EMBODIMENTS

All of the features disclosed in this specification may be combined in any combination. Each feature disclosed in this specification may be replaced by an alternative feature serving the same, equivalent, or siniilar purpose. Thus, unless expressly stated otherwise, each feature disclosed is only an example of a generic series of equivalent or similar features.

From the above description, one skilled in the art can easily ascertain the essential characteristics of the present invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. Thus, other embodiments are also within the claims.