New Pebax^®/zeolite Y composite membranes for CO₂ capture from flue gas

Highlights

• Pebax^®/zeolite Y composite membranes were synthesized.

• Zeolite Y layer improved the transport performance of the composite membrane.

• PDMS cover layer was effective for defect abatement.

• The membrane showed 940 GPU and 30 CO₂/N₂ selectivity at 57 °C.

• Membrane preparation conditions and compositions were studied and optimized.

Abstract

A new Pebax^®/zeolite Y composite membrane with three layers on top of the polyethersulfone (PES) substrate was designed and successfully synthesized for CO₂ capture from flue gas. The three layers were the zeolite Y inorganic layer, selective Pebax^® layer, and polydimethylsiloxane (PDMS) cover layer, respectively, from bottom to top. Zeolite Y with a particle size of about 40 nm was synthesized and deposited onto the PES substrate. On top of the zeolite Y layer, a selective polymer layer was deposited with the blend solution of Pebax^® and ethylene oxide (EO)-containing small molecules. A PDMS cover layer was applied on top of the Pebax^® layer for defect abatement. The composite membrane showed a better CO₂/N₂ separation performance compared to the membrane without the zeolite Y layer, which was due to two reasons. First, the hydrophilic zeolite Y layer improved the adhesion between the Pebax^® polymer layer and the substrate, which allowed for the preparation of much thinner and defect-free membranes. Second, the zeolite Y layer had a smaller interparticle pore size than the PES substrate and then minimized the penetration of polymer solution into the pores, which reduced the mass transfer resistance of CO₂ molecules through the membrane. The effects of different Pebax^® blend compositions and various coating conditions on the membrane separation performance were also investigated. This work for the first time reports a thin Pebax^® membrane with a high CO₂ permeance and a relatively good CO₂/N₂ selectivity at 57 °C. The resulting Pebax^®/zeolite Y composite membrane exhibited a CO₂ permeance up to 940 GPU and a CO₂/N₂ mixed gas selectivity of 30 at 57 °C, indicating promising potential 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 Polaris^TM 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.