Water-Based Membrane Selectively Filters Carbon Dioxide

Researchers at ETH Zurich in Switzerland have developed a water-based membrane that separates carbon dioxide from other gases with higher selectivity and permeability than existing state-of-the-art materials. Reported by Chemistry World, the technology offers a potential alternative to industrial carbon capture methods that are typically energy-intensive and rely on hazardous chemicals.

The membrane functions by using a thin layer of water stabilized by hydrophilic nanopores. To pass through the membrane, gases must first dissolve in the water, diffuse through the layer, and then desorb on the other side. The researchers found that the solubility of individual gases primarily governs both the permeability and the selectivity of the membrane.

Industrial Context and Technical Trade-offs

Current industrial applications for separating carbon dioxide often depend on cryogenic separation or amine scrubbing. While these methods are effective, they are noted for their high energy requirements and the use of hazardous chemical agents.

Membranes constructed from polymeric or nanoporous materials have been proposed as simpler and more efficient options. However, these materials typically suffer from a trade-off between gas permeability—the rate at which gas passes through the membrane—and selectivity—the ability to separate one specific gas from others.

Some existing attempts to achieve high carbon dioxide selectivity involve infusing porous supports with gas-selective liquids, such as ionic liquids. Despite their selectivity, these systems often experience a decline in permeability during operation as chemical sorption sites become saturated, which can lead to blowouts.

The new water-based membrane developed at ETH Zurich is intended to provide better performance in these areas while maintaining a smaller environmental impact than traditional materials.

Development and Proof of Concept

The project was led by Kian P. Lopez and Anthony Straub. The research began when Straub playfully pitched it to the group as a project to make a beer that never goes flat.

The focus shifted toward gas separation after Lopez observed that nitrogen and carbon dioxide permeated the membrane at different rates. For the initial proof of concept, the team used anodic aluminum oxide membranes featuring hydrophilic pores, onto which water was pipetted.

The resulting water layers in the experimental setup varied in thickness, ranging from 190 nanometers to 100 micrometers.

Comparative Membrane Performance

Data on water membranes for carbon dioxide separation indicates specific selectivity levels relative to other common gases. In certain constructions, carbon dioxide selectivity has been measured at 86 with respect to nitrogen (N2), 66 with respect to methane (CH4), and 74 with respect to hydrogen (H2).

This approach to water-based separation aligns with previous biologically inspired research. In 2018, researchers from the University of New Mexico and Sandia National Laboratories developed a water-based membrane known as a memzyme.

The memzyme membrane, which was 18 nanometers thick, incorporated the natural enzyme carbonic anhydrase to mimic the way living cells remove carbon dioxide. That specific technology was designed to capture 90 percent of the carbon dioxide released from the smoke of coal-fired power plants, representing a significant improvement over the thick, solid polymer membranes available at that time.