High-performance SPEEK/amino acid salt membranes for CO₂ separation

Abstract

Three types of amino acid salts (sodium lysine, sodium arginine, sodium histidine) were introduced into SPEEK polymer matrix to fabricate a series of SPEEK/amino acid salt membranes for CO₂ separation. The membranes could efficiently separate CO₂ from a CO₂/CH₄ mixture due to the simultaneous improvement of CO₂ permeability and separation performance. Firstly, the amino acid salt contained a larger number of amino groups (–NH₂), which could reversibly react with CO₂, improving the CO₂ molecules transfer performance. Secondly, the ether oxygen structure (–O–) in the polymer chain and the carboxyl groups (–COO⁻) in the amino acid salt had an excellent affinity for CO₂, enhancing the solubility selectivity. Thirdly, the introduction of amino acid salt increased the water uptake, and water was favored over CO₂ dissolution; this was also good for the increase of solubility selectivity. Among the three types of amino acid salts, the membrane doped with 20 wt% sodium lysine (SPEEK/LLS-20) showed the highest CO₂ permeability of 295.6 barrer and CO₂/CH₄ selectivity of 71.8 at 2 bar and room temperature, surpassing the 2008 Robeson upper bound line.

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1. Introduction

Membrane separation technology has been used in large scale industries due to its advantages of small footprint, simple operation and high separation efficiency compared with other separation methods (e.g. adsorption, cryogenic separation and the biological method).^1–9 Polymer-based membranes have shown great potential in CO₂ separation, while the well-known “trade-off” between permeability and selectivity has restricted the separation performance of polymeric membranes.^10–15

Doping various salts into polymeric materials has been proved to be an efficient method to improve CO₂ separation performance in polymer-based membranes.¹⁶ Li et al.¹⁷ fabricated Pebax membranes containing alkaline-earth metal salts for CO₂ capture, these membranes showed high CO₂ separation performance due to the increase of water content in the membranes. Cao's group¹⁸ fabricated polymer membranes doped with fluoride-containing salts, the introduction of salt improved the CO₂ separation performance of membranes. EI-Azzami et al.¹⁹ found that the addition of arginine-salt increased the number of amino groups and facilitated CO₂ transport, thus, both CO₂ permeability and selectivity increased about three times in membranes. Quinn and Laciak²⁰ incorporated the fluoride-containing organic and inorganic salts into poly(vinylbenzyltrimethylammonium fluoride) for gas separation, and found that the CO₂ permeability of membrane containing salts was three times higher than that of pure membrane.

To further improve membrane performance, we attempt to fabricate high-performance membranes by combining facilitating CO₂ transport through reversible reaction and increasing CO₂ solubility selectivity through polar groups in one membrane. Due to CO₂ is an acid gas, the high pH could remarkably enhanced membrane separation performance and the basic –NH₂ groups are widely used as carriers for CO₂ transport.^21–23 The amino acid salts contains a large number of –NH₂ groups could reversibly react with CO₂,^23–25 which is good for facilitating CO₂ transport. And the –COO⁻ groups in amino acid salt have an excellent affinity with CO₂, which favors the increase of solubility selectivity. Furthermore, the presence of amino acid salt helps to increase water content of membranes, water itself constitute a special water channel for CO₂ transport.^17,26,27 Sodium lysine (LLS), sodium arginine (LAS) and sodium histidine (LHS) are three types of amino acid salts, they are basic amino acid salt and have a large number of –NH₂ groups and one carboxyl groups per mole. The advantage of environmental friendliness and low cost of amino acid salts make them become the potential alternative materials for CO₂ capture.

Sulfonated polyether ether ketone (SPEEK) is a potential material for CO₂ separation, the polymer chains of SPEEK contain sulfonic acids (–SO₃H) groups and ether oxygen structure (–O–).^28–30 The lots of hydrophilic –SO₃H could absorb a larger amount of water, water is favored to CO₂ dissolution. And the ether oxygen structure (–O–) has a good affinity with CO₂, which could increase CO₂ solubility selectivity of membranes.^30,31

In this study, sodium lysine (LLS), sodium arginine (LAS) and sodium histidine (LHS) were introduced into SPEEK matrix and fabricated a series of SPEEK/amino acid salt membranes for CO₂ separation. The membranes were characterized by Fourier transform infrared spectra (FT-IR), X-ray diffraction (XRD) and thermal gravimetric analysis (TGA), respectively. The effects of amino acid salt types and amino acid salt content on water state of membranes were investigated. In addition, the effects of feed gas pressure and operating temperature on gas separation performance were also studied.

2. Experimental

2.1. Materials

Poly(ether ether ketone) (PEEK) was purchased from Shanghai Huipu Chemical Co., Ltd. Concentrated sulfuric acid (H₂SO₄, 95–98%) was purchased from Kelong Chemical Reagent Factory (Chengdu, China). N,N-Dimethyl acetamide (DMAc) was of analytical grade and purchased from Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China). Sodium hydroxide was purchased from Tianjin Yongsheng Ltd (Tianjin, China). L-Lysine (LL), L-histidine (LH) and L-arginine (LA) were supplied by Aladdin reagent (Shanghai, China).

2.2. Synthesis of SPEEK

Sulfonated poly(ether ether ketone) (SPEEK) with a sulfonation degree of 65% was prepared by direct sulfonation of PEEK.³² To be specific, prior to sulfonation, the PEEK powder was dried in an oven at 80 °C for 24 h. First of all, 10 g of fully dried PEEK powder was added gradually into 250 ml sulfuric acid solution and stirred vigorously at 50 °C for 3 h in a three neck flask to dissolve PEEK. Then, the resultant solution was slowly dropwise and was added to the ice-cold water under magnetic stirred for about 1 h, and let it overnight. Finally, the mixture was filtered to obtain the cake and washed with deionized water until pH = 7, and then the cake was dried at ambient temperature for 24 h. The polymer was dried at 50 °C for another 24 h in a vacuum oven to remove the residual solvent. The obtained sample was SPEEK pellets.

2.3. Membrane preparation

Three types of amino acid salts (LLS, LAS, LHS) were employed in the experiments. The pure SPEEK and SPEEK/amino acid salt membranes were fabricated by a solution-casting method. The membranes containing LLS were taken as an example to describe the step of membrane preparation. SPEEK pellets were dissolved in DMAc under heated (60 °C) for 5 h to obtain a 5 wt% homogeneous solution. Simultaneously, a specified amount of L-lysine and sodium hydroxide were dissolved in water at room temperature under ultrasonic treatment for 30 min to get the sodium lysine solution. Afterwards, when the polymer solution was cooled to the room temperature, the sodium lysine solution was slowly added into the polymer solution and stirred another for 3 h. Then, the mixture solution was cast on an ultra-flat surface glass dish and dried at room temperature for 72 h and at 50 °C for another 24 h to remove the residual solvent. For comparison, the pure SPEEK membrane was fabricated in the same way without the addition of amino acid salt.

The obtained membrane were designated as SPEEK/X–Y, where X represents the type of amino acid salt, and Y (0, 5, 10, 15, 20) denotes the percentage of the amino acid salt content relative to the weight of SPEEK. The membrane thickness was about 60–90 μm.

2.4. Membrane characterization

2.4.1. Fourier transform infrared spectroscopy (FT-IR). FT-IR spectra of pure SPEEK and SPEEK/amino acid salt membranes were measured using Nicolet AVATAR 360 FT-IR instrument at the scan range of 400–4000 cm⁻¹ and the resolution of 4 cm⁻¹ for each sample.

2.4.2. Thermal gravimetric analysis (TGA). TGA was performed on a NETZSCH STA 449 F3. At least 10 mg of pure SPEEK or SPEEK/amino acid salt membranes was placed into the sample holder, and the samples were heated from room temperature to 800 °C at a heating rate of 10 °C min⁻¹ in nitrogen atmosphere with a flow rate of 30 ml min⁻¹.

2.4.3. X-ray diffraction (XRD). The crystalline structure of pure SPEEK and SPEEK/amino acid salt membranes was measured by XRD using a Bruker D8 X-ray diffractometer with Cu Kα irradiation (λ = 1.5406 Å) at 40 kV and 40 mA. The recorded 2θ was 10–90° and the scanning speed was 2° min⁻¹ for the samples.

2.5. Gas permeation experiments

The gas permeation experiments of membranes were tested using the conventional constant pressure/variable volume method. Mixed-gas (CO₂/CH₄ = 10/90 vol%) permeation test was measured by a set of gas permeation apparatus (Fig. 1). H₂ was selected as sweep gas for CO₂/CH₄ mixed-gas test, and the permeate side was kept at atmospheric pressure. In the experiment, prior to the mixed-gas entered the membrane cell to contact the flat-sheet membranes, the gas should passing through a humidifier which was saturated with water vapor at 40 °C and then passing through a dehumidifier at room temperature to remove the condensate water. Meanwhile, the sweep gas was also humidified by entering a humidifier with full of water at room temperature at the permeate side. The flow rate and composition of gas on permeate side were recorded every 12 min until they no longer changed with time.

Fig. 1 Schematic of gas permeation apparatus.

The gas permeability (P_i, barrer, 1 barrer = 10⁻¹⁰ cm³ (STP) cm cm⁻² s⁻¹ cmHg) was defined by the following equation:

where Q_i is the gas volumetric flow rate (cm³ s⁻¹) (STP), l is the thickness of the membrane (cm), Δp_i is the transmembrane pressure difference of gas ‘i’ (cmHg), A is the effective membrane area (cm²), and A is a constant of 15.9 cm².

Because the permeate side is maintained at atmospheric pressure, the binary gas (CO₂/CH₄) separation factor (α_ij) could be calculated by eqn (2-2):

2.6. Measurement of water uptake, free water and bound water

Water uptake and water state were measured by the method reported in the literature.¹⁹ The membrane was weighed (m₁, mg) after the gas permeation under the humidified condition, and then, the membrane was dried at 100 °C for 6 h in a vacuum oven to remove the free water in the membrane and weighed the quality and denoted (m₂, mg). Finally, the membrane was heated at 150 °C in a vacuum oven for another 6 h to remove the bound water and reweighed again (m₀, mg). The total water (W_t, %), free water (W_f, %) and bound water (W_b, %) of the membranes could be calculated by the following formulas:

W_t = (m₁ − m₀)/m₀ × 100% (2-3)

W_f = (m₁ − m₂)/m₀ × 100% (2-4)

W_b = (m₂ − m₀)/m₀ × 100% (2-5)

3. Result and discussion

3.1. Membrane characterization

FT-IR spectra of pure SPEEK membrane and SPEEK/amino acid salt membranes were presented in Fig. 2. The major absorption bands at 1023 cm⁻¹, 1077 cm⁻¹ and 1221 cm⁻¹ which corresponded to the asymmetric stretching vibration of O=S=O groups, stretching vibration of O=S=O groups and stretching vibration of S=O groups in the –SO₃H of the SPEEK, respectively.³³ Compared with the pure SPEEK membrane, the new characteristic peaks of SPEEK/amino acid salt membranes at 3435 cm⁻¹ was the stretching vibration of N–H. These observed bands indicated that the three types of amino acid salts (LHS, LAS and LLS) have been successfully introduced into the SPEEK polymer.

Fig. 2 FT-IR spectra of pure SPEEK membrane and SPEEK/amino acid salt membranes.

The TGA curves of pure SPEEK membrane and SPEEK/amino acid salt membranes were shown in Fig. 3. All membranes exhibited three-stage weight loss. The first stage was from 50 °C to 170 °C, it belonged to the evaporation of water (free water, bound water) and the residual solvent in the membranes. The second stage started from 170 °C to 450 °C, it attributed to the degrading of –SO₃H groups in the SPEEK. The last stage began from 450 °C, which was the degradation of polymer chains. As can be seen from Fig. 3, the membranes doped with amino acid salt have the better thermal stability than the pure SPEEK membrane. And the SPEEK/LHS-20 has the best thermal stability among the three types of SPEEK/amino acid salt membranes. In addition, the residual weight of membrane increased with increasing of LLS content in SPEEK/LLS membranes. These phenomena mean that the introduction of amino acid salts improves the thermal stability of membranes.

Fig. 3 TGA curves of SPEEK membrane and SPEEK/amino acid salt membranes: (a) SPEEK; (b) SPEEK/LLS-5; (c) SPEEK/LLS-10; (d) SPEEK/LLS-15; (e) SPEEK/LLS-20; (f) SPEEK/LAS-20; (g) SPEEK/LHS-20.

XRD analysis was used to analyze the crystalline properties of the pure membrane and SPEEK/amino acid salt membranes, the results were shown in Fig. 4. As can be seen from XRD patterns of all membranes, the membranes showed a weak and broad peak at 15–25°. There was no new characteristic peak for SPEEK/amino acid salt membrane, which indicated that amino acid salt was homogeneously dispersed in the polymer matrix. The introduction of amino acid salt increased the crystalline of membrane, while the intensity of the peak (15–25°) obviously decreased with the increase of LLS content in SPEEK/LLS membranes. The phenomenon can be ascribed to the amino acid salt increased the crystallization of the polymer chain, which was same as the introduction inorganic salts into polymer matrix.¹⁷ The interaction of water and amino acid salt would offset the negative effects caused by the increase of crystalline under humidified state, and improves gas separation performance of membrane.

Fig. 4 XRD patterns of pure SPEEK membrane and SPEEK/amino acid salt membranes.

3.2. Water content and water state in membranes

Table 1 presented the results of amino acid salt type and amino acid salt content on water uptake, free water and bound water in membranes. The water uptake of SPEEK/amino acid salt membranes was significantly higher than that of pure SPEEK membrane. Furthermore, the free water and bound water of SPEEK/LLS membranes increased from 22.02% to 35.79% and 1.31% to 1.66% respectively, when the LLS content increased from 5 to 20 wt%. The increase of water uptake could swell the polymer matrix and decrease the crystallization of SPEEK/LLS membrane, which could increase CO₂ permeability.¹⁷ In addition, the increased of bound water and free water improve CO₂ dissolution in membrane, which are favored of improving solubility selectivity of membrane. The increase of water uptake in SPEEK/amino acid salt membrane was attributed to the strong water absorption of amino acid salt. Among three types of SPEEK/amino acid salt membranes, SPEEK/LLS-20 membrane has the highest water uptake, which is good for gas permeability.

Table 1 Water uptake, free water and bound water in membranes (feed gas: CO₂/CH₄; temperature: 25 °C; pressure: 2 bar)

Membrane	Water uptake (%)	Free water (%)	Bound water (%)
SPEEK	7.7	6.68	1.02
SPEEK/LLS-5	23.33	22.02	1.31
SPEEK/LLS-10	24.02	22.44	1.58
SPEEK/LLS-15	36.73	35.1	1.63
SPEEK/LLS-20	37.45	35.79	1.66
SPEEK/LHS-20	35.68	33.93	1.55
SPEEK/LAS-20	33.65	31.19	1.53

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3.3. Gas separation performance

3.3.1. Effect of amino acid salt types on membrane transport properties. The effect of amino acid salt types on gas separation performance of pure SPEEK membrane and SPEEK/amino acid salt membranes was shown in Fig. 5. For SPEEK/amino acid salt membranes, both permeability and selectivity were significantly enhanced. This phenomenon attributed to the following reasons: first, –NH₂ groups in amino acid salt can react with CO₂, facilitating CO₂ transport.^23,24,34 Second, –COO⁻ groups in amino acid salt and ether oxygen structure (–O–) in SPEEK polymer chains increased the solubility selectivity due to their excellent affinity with CO₂.^35–37 Third, amino acid salt possessed a strong ability of absorbing and holding water, this was also good for the increase of CO₂ permeability and solubility selectivity.^17,23,24 The simultaneously enhance of CO₂ facilitate transport and solubility selectivity could significantly improve gas separation performance. Among three types of amino acid salts, the SPEEK/LLS membrane shown the best gas separation performance due to SPEEK/LLS membrane had the highest water uptake.

Fig. 5 Effect of amino acid salt types on gas separation performance of pure SPEEK membrane and SPEEK/amino acid salt membranes.

3.3.2. Effect of LLS content on membrane gas separation performance. The effect of LLS content on gas separation performance of SPEEK/LLS membranes at room temperature and 2 bar feed gas pressure under humidified state was shown in Fig. 6. Both CO₂ permeability and CO₂/CH₄ separation factor of SPEEK/LLS membranes were significantly higher than that of pure SPEEK membrane, and the CO₂ permeability and CO₂/CH₄ separation factor of SPEEK/LLS membranes increased with the increase of LLS content. SPEEK/LLS-20 membrane showed a CO₂ permeability of 295.6 barrer and CO₂/CH₄ selectivity of 71.8. The membrane doped with LLS has the highest water uptake than other amino acid salts-containing membranes. Since the mechanical properties of membrane can not meet the test requirement when LLS content more than 20 wt%, the effect of operating conditions on gas separation performance was based on SPEEK/LLS-20 membrane.

Fig. 6 Effect of LLS content on gas separation performance of SPEEK/LLS membranes.

3.3.3. Effect of feed gas pressure on gas separation performance. Fig. 7 showed the effect of feed gas pressure on gas separation performance of pure SPEEK membrane and SPEEK/amino acid salt membranes under humidified condition. The CO₂ permeability and the CO₂/CH₄ separation factor of all membranes decreased with the increase of feed pressure. And SPEEK/LLS-20 membrane showed the best separation performance. As the feed gas pressure increased from 2 to 6 bar, CO₂ permeability decreased about 48% for pure SPEEK membrane, and 78% for SPEEK/LLS-20 membrane. But, the membrane separation performance of SPEEK/LLS-20 membrane was better than pure SPEEK membrane at 6 bar. There exist a reversibly reaction between CO₂ and –NH₂ groups under humidified condition. As feed gas pressure increasing, the CO₂ adsorption capacity of –NH₂ groups on the feed side of the membrane became saturated, which resulted in the decrease of CO₂ permeability. The decreased separation factor can be explained that the permeability of gases (CO₂ and CH₄) gradually decreased with feed gas pressure, while the CO₂ permeability decreased faster than CH₄ permeability.

Fig. 7 Effect of feed gas pressure on gas separation performance of pure SPEEK membrane and SPEEK/amino acid salt membranes: (a) CO₂ permeability; (b) CH₄ permeability; (c) CO₂/CH₄ separation factor.

3.3.4. Effect of operating temperature on gas separation performance. Fig. 8 showed the effect of operating temperature on gas separation performance of pure SPEEK membrane and SPEEK/amino acid salt membranes. Both CO₂ permeability and CO₂/CH₄ selectivity of all membranes decreased continuously with increasing operating temperature. On one hand, the reaction between CO₂ and H₂O was exothermic, the direction of reaction equilibrium shifted to the reactant direction at high temperature. On the other hand, water content in the membrane was lower at high temperature than that at low temperature, the decreased water content led to the decrease of CO₂ solubility. The CH₄ transport followed the solution diffusion mechanism in membranes, and the diffusion rate was quicker in high temperature than that of in low temperature, thus the CH₄ permeability increased with increasing temperature.²⁴ Therefore, the gas separation performance of all membranes decreased. However, the membrane separation performance of SPEEK/amino acid salt membrane were better than that of pure SPEEK membrane, and SPEEK/LLS-20 membrane showed the best separation performance among three types of SPEEK/amino acid salt membranes.

Fig. 8 Effect of operating temperature on gas separation performance of pure SPEEK membrane and SPEEK/amino acid salt membranes: (a) CO₂ permeability; (b) CH₄ permeability; (c) CO₂/CH₄ separation factor.

3.4. Comparison of results to the Robeson's upper bound curve

The comparison of gas separation performance against the Robeson's upper bound was shown in Fig. 9. As can be seen from the curves, gas separation performance of pure SPEEK and SPEEK/LLS membranes under the humidified closes to or even surpass the 2008 Robeson upper bound line.^11,12 From another point of view, both permeability and selectivity were significantly improved for membranes doped with sodium lysine.

Fig. 9 Robeson plots for CO₂/CH₄.

4. Conclusions

A high-performance membrane was successfully fabricated by incorporating amino acid salt (LLS, LAS, LHS) into SPEEK matrix for gas separation. Both CO₂ permeability and CO₂/CH₄ selectivity of SPEEK/amino acid salt membranes exhibits a significant increase. Salt-doped membranes endowed a high water uptake, leading to high CO₂ permeability, and SPEEK/LLS membrane presented the best gas separation performance. The CO₂ separation performance of SPEEK/LLS membrane was remarkably improved with the increase of LLS content due to the simultaneous enhancement of CO₂ facilitate transport and solubility selectivity. The FT-IR spectra confirmed the presence of –NH₂ groups, and the –NH₂ groups could reversibly react with CO₂. Compared with pure SPEEK membrane, the CO₂ permeability and CO₂/CH₄ selectivity of SPEEK/LLS-20 membrane increase by 263% and 90%, respectively. The performance of SPEEK/LLS membrane surpassed or was close to the 2008 Robeson upper bound line.

Acknowledgements

The authors gratefully acknowledge the supported by the National High Technology Research and Development Program of China (2012AA03A611), and the Start-Up Foundation for Young Scientists of Shihezi University (RCZX201508).

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