New Pebax®/zeolite Y composite membranes for CO2 capture from flue gas
Introduction
Carbon dioxide separation and capture from flue gas streams has been widely believed to be the most important solution to greenhouse gas driven climate change. Compared to conventional CO2 separation methods including solvent absorption, solid adsorption and cryogenic distillation, membrane separation is a more promising method due to its system compactness, energy efficiency, operational simplicity and kinetic ability to overcome the thermodynamic solubility limitation [1], [2], [3].
Polymer membranes and inorganic membranes are two major categories of CO2 separation membranes. Gas transport through most of the polymer membranes follows the solution-diffusion mechanism (Fig. 1(a)), which is based on both the solubility of gases in the membrane and the diffusivity of gases through the membrane. Most of the recent work on the polymeric solution-diffusion membranes has focused on copolymers which generally have a hard (glassy) polymer segment such as polyamide (PA) or polyester and a soft (rubbery) polymer segment such as polyethylene oxide (PEO) [4]. The hard segment provides the mechanical strength to the membrane, while the soft segment interacts with CO2 molecules for enhanced transport. Solution-diffusion polymer membranes usually suffer from a trade-off between permeability and selectivity, which can be represented by the Robeson upper bound for the membrane performance [5]. Recent work has shown that solution-diffusion polymer membranes such as the PolarisTM membrane from Membrane Technology and Research, Inc. (USA) and the ultra-thin Polyactive® membrane from GKSS Research Centre Geesthacht GmbH (Germany) have the potential to achieve high CO2 permeances (1000–2000 GPU) [6], [7], [8], but the selectivity was limited by the Robeson upper bound, which was usually below 30 at 57 °C (flue gas temperature).
Pebax® 1657 copolymer contains 60 wt% PEO and 40 wt% polyamide 6 (PA6), the structure of which is shown in Fig. 2. In this copolymer structure, PEO acts as the soft segment and provides the CO2-philic property, while PA serves as the hard segment and provides the film-forming ability and mechanical strength to the membrane. Yave et al. reported that Pebax® 1657 had a CO2 permeability of around 175 Barrers and a CO2/N2 selectivity of 32 at 50 °C [9]. Although permeability is one of the most widely used parameters for membranes, it can only represent the property of a certain membrane material, while the permeance is the one used for carbon capture system design and cost analysis. For industrial applications, no matter how high the permeability is, a thick membrane can reduce the permeance significantly and increase the membrane area, and further increase the capture cost. Although researchers did a lot of work on Pebax® membranes, they usually reported CO2 permeabilities for thick membranes [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], while very few reported high CO2 permeances for thin membranes (<2 µm). Car et al. reported that a Pebax®/PEG blend composite membrane of less than 2 µm showed a CO2 permeability of 122 Barrers at 30 °C [20]. Liu et al. reported a CO2 permeance of around 350 GPU and a CO2/N2 selectivity of 35 for a thin Pebax® 2533 membrane with a thickness of 0.7 µm at 25 °C [21], [22].
Inorganic membranes for CO2/N2 separation are microporous and have a different transport mechanism compared to solution-diffusion membranes. The CO2/N2 separation from microporous inorganic membranes depends on both molecular sieving and surface diffusion (Fig. 1(b)). The similar size of CO2 (3.3 Å) and N2 (3.64 Å) molecules indicates that achieving a CO2 rich permeate by the simple molecular sieving is not very effective. Surface diffusion, particularly in combination with the molecular sieving, then becomes a main factor for the separation. In microporous inorganics such as zeolite Y (with a pore size of 7.4 Å and a Si/Al ratio of 1.5–3.8), CO2 favorably adsorbs and diffuses along the surface wall, providing a high CO2 permeance as well as a high CO2/N2 selectivity in combination with the molecular sieving. The performances of such membranes approach or cross the upper bound set by polymer membranes. Recent literatures have shown that zeolite membranes on an inorganic substrate, such as alumina, showed a CO2 permeance of around 2000–3500 GPU and a CO2/N2 selectivity of around 30–500 at room temperature [23], [24], [25], [26]. Although these inorganic membranes have shown higher separation performances compared to polymer membranes, it is very difficult to fabricate defect-free inorganic layers reproducibly. Besides, due to the fact that the inorganic substrates are thick, brittle, expensive and not amendable to continuous fabrication, the scale-up of inorganic membranes is complicated and costly.
This work for the first time designs a Pebax®/zeolite Y composite membrane with three layers for CO2/N2 separation (Fig. 3). Zeolite Y with a particle size of about 40 nm was synthesized and successfully deposited onto the PES substrate. A thin Pebax® layer was then coated on top of the zeolite Y layer. The inorganic zeolite Y layer not only improved the adhesion between the polymer layer and the substrate, but also had a smaller pore size than the PES substrate, which could improve the CO2 permeance by reducing the penetration of the polymer solution into the pores underneath. A PDMS cover layer was coated on top to cover any possible defects of the thin Pebax® layer. To our best knowledge, this work for the first time reports such a composite membrane with a Pebax® layer thickness of less than 500 nm, showing a high CO2 permeance (~940 GPU) while maintaining a good CO2/N2 selectivity (~30) at 57 °C.
Section snippets
Materials
Poly(ethylene glycol) with an average molecular weight of 200 (PEG200), poly(ethylene glycol) dimethyl ether with an average molecular weight of 500 (PEG-DME500), heptane (99%), isopropanol (IPA, 99.9%), Ludox HS-30 colloidal silica (SiO2, 30%), aluminum isopropoxide (Al(O-CH(CH3)2)3, 98%), and tetramethylammonium bromide ((CH3CH2CH2)4N(2Br), 98%) were purchased from Sigma-Aldrich. Tetramethylammonium hydroxide ((TMA)2O(2OH), 25% aqueous) was purchased from SACHEM Inc. (Austin, TX). Ethanol
Zeolite Y synthesis
As shown in Fig. 5, the XRD pattern of the synthesized zeolite sample matched the reference of zeolite Y very well, indicating that zeolite Y has been successfully synthesized. The particle size of synthesized zeolite Y was measured using DLS. As shown in Fig. 6, the average diameter of the synthesized zeolite Y was around 40 nm.
Membrane morphology via SEM
The SEM image in Fig. 7 shows the cross-section of the composite membrane synthesized in this work. As shown in this figure, the composite membrane with three layers was
Conclusions
A new Pebax® 1657/ zeolite Y composite membrane was successfully synthesized. With the help of a nano zeolite Y layer on top of the PES substrate, the adhesion between the Pebax® 1657 layer and the substrate was improved, which was confirmed by the fact that the contact angle of the substrate reduced from 30° (bare PES) to 3° (zeolite Y layer/PES). This better adhesion could allow for the preparation of much thinner membranes, which provided a higher CO2 permeance while maintaining a good CO2/N2
Acknowledgments
We would like to thank Dongzhu Wu for acquiring the SEM images of membrane samples and Jose D. Figueroa of National Energy Technology Laboratory for helpful discussion and input. We would also like to acknowledge Arkema Inc. in King of Prussia, PA for providing us free samples of Pebax® 1657 and Wacker Silicones Inc. in Adrian, MI for donating free samples of PDMS. We would like to gratefully acknowledge the Department of Energy/National Energy Technology Laboratory (FE0007632) and the Ohio
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