WO2016118581A1

WO2016118581A1 - Polyphosphazene membranes for carbon dioxide separation

Info

Publication number: WO2016118581A1
Application number: PCT/US2016/014055
Authority: WO
Inventors: Harry R. Allcock; Zhicheng TIAN; Andrew Hess; Hunaid Nulwala
Original assignee: The Penn State Research Foundation
Priority date: 2015-01-20
Filing date: 2016-01-20
Publication date: 2016-07-28

Abstract

Polyphosphazenes that can be used to separate carbon dioxide from a mixture of components are disclosed such as polyphosphazenes having aliphatic ether side groups, e.g., an alkanoxy, cycloalkanoxy, or aromatic ether side groups. Advantages of the present disclosure include polyphosphazenes that can be used to separate carbon dioxide from a mixture of components. These and other advantages are satisfied, at least in part, by a method of separating carbon dioxide from a mixture of components including carbon dioxide. The method comprises exposing the mixture to a membrane composed of a polyphosphazene, e.g., a polyphosphazene having aliphatic ether side groups and/or aromatic ether side groups, to separate the carbon dioxide from the mixture.

Description

POLY PHO SPHAZENE MEMBRANES FOR CARBON

DIOXIDE SEPARATION

CROSS-REFERENCE TO RELATED APPLICATION

[0001 ] This application claims the benefit of U.S. Provisional Application No.

62/105,415 filed January 20, 2015, the entire disclosure of which is hereby incorporated by reference herein.

STATEMENT REGARDING FEDERALLY^' SPONSORED RESEARCH

[0002] This invention was made with government support under Grant No. DE-

FE0004000, awarded by the Department of Energy. The Government has certain rights in the invention.

TECHNICAL FIELD

[0003] The present disclosure relates to polyphosphazene polymers having an ether side group and the use of such polymers for separating carbon dioxide from a mixture.

BACKGROUND

[0004] Polyphosphazenes are hybrid polymers including an inorganic backbone and organic side groups. The skeleton backbone of alternating phosphorus and nitrogen atoms is inherently flexible, fire-resistant, radiation and oxidative stable, while the side groups also play a vital role in defining the end-properties and applications. For example, rigid and bulky side groups restrict the torsional freedom of the backbone and raise the overall glass transition temperature, and polar side groups or symmetric substitutions may favor crystallization or decrease of solubility in organic solvents. Several hundred polyphosphazenes have been synthesized so far, and they demonstrated a large variety of practical applications including high performance elastomers, fire retardant materials, electrolytes, optical polymers, and biomedical materials. SUMMARY OF THE DISCLOSURE

[0005] Advantages of the present disclosure include polyphosphazenes that can be used to separate carbon dioxide from a mixture of components.

[0006] These and other advantages are satisfied, at least in part, by a method of separating carbon dioxide from a mixture of components including carbon dioxide. The method comprises exposing the mixture to a membrane composed of a polyphosphazene, e.g., a polyphosphazene having aliphatic ether side groups and/or aromatic ether side groups, to separate the carbon dioxide from the mixture.

[0007] Embodiments of the present disclosure include one or more of the following features individually or combined. For example, the polyphosphazene can include cycioalkanoxy side groups, e.g., Cs-g cycioalkanoxy side groups. In some embodiments, the polyphosphazene includes a plurality of monomer units having one or more of a CM₂ alkanoxy side group, a C5..₈ cycioalkanoxy side group, or a mixture thereof. The ether side groups can be unsubstitited or substituted, e.g., substituted with one or more of additional ether groups, halogens, alkoxides, f!uorinated alkoxides, or combinations thereof.

[0008] Another aspect of the present disclosure includes polyphosphazenes having at least one cycioalkanoxy side group, in some embodiments, the polyphosphazene includes a plurality of monomer units having two cycioalkanoxy side groups, e.g., C5.₈ cycioalkanoxy side groups, or a mixture of an aliphatic ether side group and a cycioalkanoxy side group, e.g., a mixture of a C_1-12 alkanoxy group and a C_5-8 cycioalkanoxy side group.

[0009] Additional advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiment of the invention is shown and described, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Reference is made to the attached drawings, wherein elements having the same reference numeral designations represent similar elements throughout and wherein:

[0011] Fig. 1 shows example polyphosphazenes including an ether side chain in accordance with embodiments of the present disclosure.

DETAILED DESCRIPTION OF THE DISCLOSURE

[0012] Polyphosphazenes are advantageous as polymeric membranes for carbon dioxide

(C0₂) separations due to the synthetic tunabihty, backbone flexibility, stability, and property optimization. The co-substitution of different side groups can multiply the properties at same time, winch is especially important in this application since the membranes not only require a high CO? affinity, but can advantageously be mechanically strong and self-standing.

[0013] The homopolymer, poIy[bis(2-(2-(methoxyethoxy) ethoxy)phosphazene] or

MEEP, is an amorphous adhesive gum with a low glass transition at -80 °C. It has been widely studied as a candidate of polymeric electrolytes for lithium battery or for hydrogel applications. However, it is believed that this polymer has not been known for use in C0₂ separations.

[0014] Ether side groups, such as a 2,2-methoxyethoxy ethoxy (MEE) group, provide polymers opportunities for interactions with C0₂ due to the dipole-quadrupole interactions. However, co-substitutions are required to increase the mechanical properties of the membrane. Bulky and rigid side groups such as aromatic derivatives are believed to generate too much hindrance to the backbone mobility, and as a consequence, the membranes demonstrated a significant decrease in gas permeability. Thus, a bulky but flexible side group, such as a cyclohexanoxy (C6), was linked to the polymer backbone to enhance the membrane's mechanical properties. The flexibility of the C6 unit generated by the chair to boat conformational transformations could maintain the overall flexibility of the polymer network without significantly decreasing gas permeability.

[0015] The performance of a polymeric membrane toward C0₂ separation is usually determined by two factors, the ability of the polymer to form a strong and self-standing membrane, and the capability of the membrane to selectively permeate CO? gas. For example, amorphous low molecular weight poly(ethylene oxide) shows high C0₂ affinity due to the polar etheric groups along the chain. However, it is too soft and adhesive to handle unless copolymerization to introduce hard polymeric segments which improve the physical properties. Polyphosphazenes provide a more straightforward and simpler pathway to achieve such properties by introducing co-substitution of the side groups thereof.

[0016] In practicing certain aspects of the present disclosure, carbon dioxide can be separated from a mixture of components that includes carbon dioxide by exposing the mixture to a membrane composed of a poiyphosphazene membrane to separate the carbon dioxide from the mixture. Such mixtures include, for example, components as a result of combusting a material which can include a variety of gases and possible particulate matter, e.g., carbon dioxide and one or more of SO?, SO₃, particulate matter, etc. In practicing certain aspects of this method, the poiyphosphazene allows carbon dioxide to pass through the membrane but substantially inhibits other components from passing through the membrane to separate the carbon dioxide from the mixture.

[0017] In addition to a poiyphosphazene, the membrane can include additives. Such additives can be included with one or inore polyphosphazenes to form a mixed matrix membrane (MMM). The additives can include, for example, silica gel, graphene, particles of polymers, ceramics, zeolites, a metal organic framework (MOF), e.g., a zirconium 1 ,4-dicarboxybenzen (UiO-66) MOF, SI I SIX MOF, etc. The membrane can have a thickness of from about 100 nanometers to 100 microns, e.g., from about 1 to 100 microns.

[0018] Polyphosphazenes including ether side chain groups are useful for practicing the present disclosure. Such polyphosphazenes include, for example, a poiyphosphazene having a plurality of monomeric units including one or more aliphatic ether side groups such as one or more alkanoxy side groups, e.g., one or more C_M alkanoxy side groups, one or more cycloalkanoxy side groups, e.g., one or more C5-₈ cycloalkanoxy side groups, or mixtures thereof. Alternatively, or in addition, the poiyphosphazene can include a plurality of monomeric units having one or more aromatic ether side groups. The one or more ether side groups can be further substituted such as with one or more additional ether groups, e.g., a methoxyethoxy ethoxy side group, a tetrahydrofuranyloxy side group, a tetrahydropyranyloxy side group, etc., or with one or more halogens, e.g., fluorine, alkoxides, fluorinated alkoxides, e.g., -OCF₂CH₃, or combinations thereof. In another aspect of the present disclosure, the polyphosphazene can include a plurality of monomelic units having both side groups as ether side groups, e.g., the polyphosphazene includes a plurality of monomeric units having two Cj_-12 alkanoxy side groups, or two C5-₈ cycloaikanoxy side groups or a mixture of C_M?, alkanoxy and C_5-8 cycloaikanoxy side groups. In some embodiments, the polyphosphazene includes a plurality of monomer units having two cycloaikanoxy side groups, e.g., C5._S cycloaikanoxy side groups, or a mixture of an aliphatic ether side group and a cycloaikanoxy side group, e.g., a mixture of a Ci.]₂ alkanoxy group and a C5.₈ cycloaikanoxy side group.

[001 9] In one aspect of the present disclosure, the polyphosphazenes can be represented by the following formula (I);

°^{^}R^' (I)

wherein "n" represents the number of monomer units and can be from about 5 to 50,000, e.g., from about 10 to about 20,000; OR and OR' represent the same or different aliphatic ether groups or aromatic ether groups which can be substituted with one or more ether groups, halogens, e.g., fluorine, alkoxides, fluorinated alkoxides, e.g., -OCF₂CH₃, or combinations thereof. In one aspect of the present disclosure, OR and OR' represent the same or different, substituted or unsubstituted, aliphatic ether group, e.g., a C_1-12 alkanoxy group, or C_5-8 cycloaikanoxy group. In an embodiment of the present disclosure, at least one of OR and OR' represents a cycloaikanoxy group, e.g., a C₅-₈ cycloaikanoxy group.

[0020] Several polyphosphazenes including ether side groups, e.g., cycloaikanoxy side groups, as well as mixed-substituent polyphosphazenes including both ether side groups and other groups were prepared. Figure 1 shows some examples of polyphosphazenes containing ether side groups including cycloaikanoxy side groups.

[0021] A senes of polyphosphazenes with co-substitutions of C6 and MEE side groups

(C6/MEE PZ) have been synthesized and characterized. CO? selectivity and permeability were measured and calculated based on the pure polymeric membranes prepared by a standard solvent casting technique. The polymers in this series are advantageous for C0₂ separation membranes because: (1) the syntheses are simple and straightforward: (2) the materials are robust and the processing is facile; (3) there is a high effectiveness to cost ratios, among other reasons. The syntheses can be scaled up when necessary.

[0022] Maeromoleeular substitution reactions were carried out in a two-step process.

First, poly(dichlorophosphazene) was synthesized by the catalytic ring-opening polymerization of hexachlorocy ci otriphosphazene at 220 °C in a sealed Pyrex tube. Second, replacement of chlorine atoms was accomplished based on a sequential addition of the bulkier eyclohexanoxy group (C6) first, followed by an excess of the methoxyethoxy ethoxy (MEE) group (Scheme 1). The addition of the smaller side groups m the last step can facilitate the replacement of the remaining chlorine atoms due to their better accessibility to the polymer backbone. Polymers with C6 loadings higher than 65 mol% were difficult to obtain due to the significant increase of steric hindrance that prevented the further substitution of the bulky groups. The obtained polymers are soluble in THF and chloroform, but are insoluble in methanol, acetone, or hexanes.

Palyfdie torophosphazerse) C6/ EE PZ

CS/!V!EE PZ 1 x-1.08; ~0,92

CS/ft/SEE PZ 2 x-1 ,30; y~0.?0

Scheme 1, Synthesis of polyphosphazenes by sequential additions

Measured by GPC, all the polymers showed high molecular weights with the of repeating units in excess of three thousand. With an increasing percentage of C6 groups, the polymers showed an increase of glass transition temperatures due to the presence of the bulky C6 groups which hindered the phospliazene backbone torsional flexibility. The variations of the final calculated ratios (^lH NMR) to the original target ratios were explained by the backbone substitution exchange reactions.

Table 1. Summary of Characterization Data of CH/'MEE PZ

[0024] The polyphosbazene polymers having one or more ether side group can be optimized toward robust and amorphous self-standing membranes. For example, C6/MEE PZ 1 is a transparent membrane with good physical properties. However, C6/MEE PZ 2 was a less desirable membrane being soft and self-adhesive at ambient temperature due to insufficient amount of CH groups linked to the backbone. Therefore, C6/MEE PZ 1 with about 65 mol% of CH groups is the optimized structure for membrane applications,

[0025] Additional polvphosphazenes containing cvcloalkanoxy side groups as well as mixed-subs tituent polvphosphazenes containing both cycloalkanoxy groups and a classical side group, tfifluoroethoxy (TFE), were prepared. Scheme 2 below shows examples of such polvphosphazenes containing cvcloalkanoxy side groups and mixed-substituent polvphosphazenes.

Scheme 2. The formula of this scheme shows that R represents a TFE or -CH₂(CF )xCF₂H, or both and R' represents one or more of a C5, C6,€7, C8.

[0026] The Table 2 below provides characterization data for certain pol y (cy c! oa!kanoxyjphosphazes .

Table 2

C5-100 represents a poly[(dieycloper!toxy)phosphazene]

C6- 100 represents a poly[(dicyclohexanoxy)phosphazene]

C^'7-100 represents a poly[(dieycloheptoxy)phosphazene] [0027] The relatively high glass transition temperature of these polymers (Table 2) initially seems to be in conflict with previous reports of the very low T_gs of phosphazenes with linear saturated alkanoxy side-groups (as low as -105 °C for the butoxy side group). However, the low T_g of the poiyphosphazenes with linear alkanoxy groups is due to the low energy barrier to torsional backbone motion that the side groups provide. This low energy barrier is due to their low steric bulk near the point of attachment to the phosphazene backbone and their ability to undergo low-energy avoidance motions during backbone torsion. Conversely, the cycloalkanoxy groups are stencally bulky near the point of attachment to the phosphazene backbone and have fewer degrees of conformational freedom due to their cyclic nature, thus hampering their ability to undergo avoidance motions during backbone torsion. This results in higher energy barriers to torsional backbone motion and thus in a higher T_g.

[0028] Indeed, those poiyphosphazenes with bulkier cycloalkanoxy side-groups had also had higher T«s. The extreme difference in T_B between C5-100 and C6-100 is due to the C5 group being locked into an essentially planar conformation which lowers the amount of steric hindrance the group can provide. It is should be noted that when interpreting the thermal data, for C7-100 that it does not reflect the properties of the fully substituted polymer due to the remaining 7.5% of chlorine along the backbone. This remaining chlorine is responsible for the deviations from the trend m decomposition behavior among the single-substituent polymers.

EXAMPLES

[0029] The following examples are intended to further illustrate certain preferred embodiments of the invention and are not limiting in nature. Those skilled in the art will recognize, or be able to ascertain, using no more than routine experimentation, numerous equivalents to the specific substances and procedures described herein.

[0030] Reagents. Tetrahydrofuran (TI1F) was purchased from EMD and dried using solvent purification columns. Sodium hydride (Nail, 60% in mineral oil, Sigma- Aldrich) was stored in an inert atmosphere and was used as received. 2,2-Methoxyethoxy ethanol (MEE, Sigma-Aldrich) was distilled over sodium metal and stored over 4A molecular sieves (EMD) in an argon atmosphere. Cyclohexanol (Sigma-Aldrich) was sublimed before use. All synthesis reactions were earned out using standard Sehlenk line techniques and a dry argon atmosphere. The glassware was dried overnight in an oven at 120 °C before use.

[0031] Synthesis of Polyphosphazenes (C6/MEE PZ). The synthesis of C6/MEE PZ I is described in detail as an example. A THF suspension (300 mL) of sodium cyciohexanoxide (NaOCVHu) prepared by the treatment of cyclohexanol (6.84 g, 64.73 mmol) with aH (2.59 g, 64.73 mmol, 60% in mineral oil) was added to a THF solution (600 mL) of poly(dichlorophosphazene) (5.00 g, 43.15 mmol). The reaction mixture was stirred under reflux for 2 days. Then, a THF solution of 2,2-methoxyethoxy ethoxy sodium salt, NaOCH₂CH₂OCH₂CH₂0CH_3> prepared by a reaction of NaH (2.59 g, 64.73 mmol, 60% m mineral oil) and 2,2-methoxyethoxy ethanol (7.62 mL, 64.73 mmol) in a separate flask was added to the polymer mixture. The reaction was further stirred for 2 days under reflux to complete the chlorine replacement. The reaction medium was concentrated to 1/3 in volume, followed by precipitation of the mixture from THF into water (x 3). The product was isolated and redissolved in 100 mL THF. The polymer solution was further purified by dialysis versus dichloromethane/methanol hexanes (40/20/40) for 4 days (Spectra/Por dialysis membrane, MWCO; 12-14,000). Solvent was removed, and the final product was obtamed as a rubbery polymer (yield: 68%). C6/MEE PZ 2 was an adhesive gum with a yield of 71%. ⁱP NMR (CDC1₃) δ (ppm): -9.62 (s). Ί I NMR (CDC1₃) δ (ppm): 4.30 (s, 1H, C6), 4.06 (s, 2FI, C ). 3.64- 3.37 (m, 9H, C . 1.90- 1.27 (m, lOH, C6).

[0032] Structural Characterizations. ^fH and ^{3 l}P spectra were recorded on a Bruker WM-

360 NMR spectrometer operated at 360 or 145 MHz, respectively. Ή NMR spectra were referenced to solvent signals, while ^{, l}P NMR chemical shifts are relative to 85% phosphoric acid as an external reference, with positive shift values downfield from the reference. ¹ F NMR spectra were collected using a Bruker CDPX-300 spectrometer operated at 282 MHz with trifluoroacetic acid as an internal standard.

[0033] Molecular Weights and Distributions. Molecular weights were estimated using a

Hewlett-Packard HP 1090 gel permeation chromatograph (GPC) equipped with an HP- 1047 A refractive index detector, American Polymer Standards AM gel 10 mm and AM gel 10 mm 104 A columns, and calibrated versus polystyrene standards (Polysciences). The samples were eluted at 40 °C with a 0.1 wt% solution of tetra-n-butylammoiiium nitrate (Sigma- Aldrich) in THF.

[0034] Thermal Analysis. The thermal characteristics of samples were measured with a

TA Instruments Q10 differential scanning calorimeter and a Perkin-Elmer thermogravimetric analyzer. About 10 mg of dried sample was used for each test. A heating rate of 10 °C/min with a temperature range from -100 to 200 °C was used for DSC, while a heating rate of 20 °C/min from 25 to 800 °C was applied for TGA. Both instruments used dry nitrogen as the purge gas.

[0035] Membrane Preparation. All the membranes were prepared by a solvent casting technique on the surface of Teflon (PTFE), Briefly, C6/MEE FZ (5 g) was dissolved in 50 mL THE (10% w/v). Then the polymer solution was cast onto a 20 cmx:20 cm PTFE flat-bottom tray. The solvent was allowed to evaporate slowly over 4 days, and the membrane was further dried under reduced pressure for 3 days. The transparent membrane was then removed from the PTFE surface for CO₂ separation tests.

[0036] Gas separation performance measurements. Pure C0₂ and N₂ permeation tests were performed at room temperature using an isochoric (constant volume, variable pressure) permeation system. Upstream pressures were measured with a pressure transducer (maximum pressure 150 psia, Viatran Inc., Model-345) and accompanying readout (Dalec Electronics digital panel). Downstream pressures were measured using a Baratron 627D capacitance manometer with a maximum pressure output of 10 Torr (MKS, Wilmington, MA), The down- stream volume was calibrated by using a standard simple mole balance method with a known volume of stainless steel balls. Membranes coupons were formed using a hole punch resulting in a membrane of diameter 2.5 cm. The thicknesses of the membranes were measured using a micrometer (Marathon Electronic digital micrometer) several times and the average value was used for the calculation of permeability.

[0037] Testing was carried out as follows. The membrane was loaded into a Millipore high pressure 25 mm filter holder resulting in an exposed area for transport of 2.7 cm^7'. The entire permeation system was degassed using a vacuum pump (Edwards nXDS 101 scroll pump) for 18 hours and then a leak rate was measured by isolating the permeation system from the vacuum pump. The feed gas was then introduced to the upstream side of the membrane, and the pressure nse in the downstream volume was recorded as a function of time. Two film samples were tested to get average permeation results.

[0038] The permeation of a gas through the membrane can be described using the solution-diffusion model. The permeability of a gas, i, is given by:

p . — f} ,^

where Di and Si represent the diffusion and solubility coefficients of component i, respectively. In terms of this model, the productivity of a membrane is defined by the permeability of the gas through the membrane and the selectivity of the membrane is the ratio of the perme abilities of the individual gases. Permeability was calculated by differentiating the pressure rise as a function of time and using the following equation:

where, Vd downstream volume (cm3), 1 film thickness (cm), pa upstream absolute pressure (cmHg), A film area (cm"), T temperature ( ), R gas constant (cnr cmHg mol^"1 K^{' !}), (dpv'd/jss rate of downstream pressure rise during testing (cmHg s^"'), (dpi/di)leak rate of downstream pressure rise under vacuum (cmHg s-1). It should be noted that there is a difference between the ideal selectivity of a membrane defined as the ratio of individual permeabilities of the gases during separate pure gas testing and the real selectivity of a membrane defined as the ratio of the individual permeabilities during a test using a mixture of both gases. All testing performed here was pure gas testing, therefore all selectivities are ideal.

[0039] Table 3 below provides permeability results for a polyphosphazene membrane film sandwiched between two porous polysulphone supports. The polyphosphazene membrane film was rubbery and mechanically very strong. Table 3

[0040] While the claimed invention has been described m detail and with reference to specific embodiments thereof, it will be apparent to one of ordinary skill in the art that various changes and modifications can be made to the claimed invention without departing from the spirit and scope thereof. Thus, for example, those skilled in the art will recognize, or be able to ascertain, using no more than routine experimentation, numerous equivalents to the specific substances and procedures described herein. Such equivalents are considered to be within the scope of this in vention, and are covered by the following claims.

Claims

WHAT IS CLAIMED IS:

1. A method of separating carbon dioxide from a mixture of components including carbon dioxide, the method comprising exposing the mixture to a membrane composed of a polyphosphazene to separate the carbon dioxide from the mixture.

2. The method of claim 1, wherein the polyphosphazene includes aliphatic ether side groups.

3. The method of claim 1 , wherein the polyphosphazene includes cycloalkanoxv side groups.

4. The method of claim 1, wherein the polyphosphazene includes a plurality of monomer units having one or more of a Cj_..₁₂ alkanoxy side group, a C5-8 cycloalkanoxv side group, or a mi ture t ereof.

5. The method of claim 1 , wherein the polyphosphazene includes a plurality of monomer units including two C_1-12 alkanoxy side groups, two Cj-s cycloalkanoxy side groups or a mixture of Ci-12 alkanoxy and C_{5 -}8 cycloalkanoxy side groups.

6. The method of any one of claims 2 to 5, wherem the ether side groups are substituted with one or more of additional ether groups, halogens, alkoxides, fluormated alkoxides, or combinations thereof.

7. A polyphosphazene having at least one of a cycloalkanoxy side group.

8. The polyphosphazene of claim 7, wherein the polyphosphazene includes a plurality of monomer units having two cycloalkanoxy side groups.

9. The polyphosphazene of any of claims 7 or 8, wherein the cycloaikanoxy side groups are C_5-8 cycloalkanoxv side groups.

10. The polyphosphazene of claim 9, wherein the polyphospliazene further includes a C_1-12 alkanoxy side group.

11. The method of claim 9, wherein the cycloalkanoxv side groups are substituted with one or more of additional ether groups, halogens, alkoxides, fiuorinated alkoxides, or combinations thereof,

12. The polyphosphazene of claim 7, wherein the polyphosphazene includes a plurality of monomer units having two Cs-g cycloaikanoxy side groups or a mixture of CM2 alkanoxy and C₅. s cycloaikanoxy side groups.