Researchers achieve commercially attractive carbon capture with a MOF

CHEMICAL engineers at the École Polytechnique Fédérale de Lausanne (EFPL), Switzerland have, for the first time, achieved commercially-attractive carbon capture with a metal-organic framework (MOF).

MOFs are versatile compounds that contain nano-sized pores within their crystal structure. They offer advantages over other nanoporous membranes and are used in a range of applications, including petrochemical separation, DNA mimicking, and removal of particles such as heavy metals and fluoride anions from water.

Scientists in the lab of Kumar Varoon Agrawal, Assistant Professor at EFPL’s Institute of Chemical Sciences and Engineering, have now improved the gas separation ability of MOFs – specifically for carbon capture. Previously, MOFs had not exhibited good CO₂ separation performance, which is essential to enable energy efficient carbon capture.

The researchers improved separation performance using a novel “postsynthetic rapid heat treatment,” (RHT) in which a MOF was heated at 360°C for a few seconds, making it more rigid.  

According to Agrawal, the lattice structure of MOFs is flexible, which allows the frameworks to absorb molecules larger than the pore aperture. This makes it difficult to achieve efficient membrane-based separation. Making the MOF structure more rigid prevents the pores from expanding to accommodate larger molecules, which increases separation selectivity.

In the study, the researchers worked with the common MOF zeolitic imidazolate framework 8 (ZIF-8). ZIF-8 has a pore aperture of 3.4 Å, which Agrawal said is ideal for separation of CO₂ from N₂ (post-combustion carbon capture), and CO₂ from CH₄ (biogas purification). CO₂, N₂, and CH₄ have kinetic diameters of 3.3 Å, 3.6 Å, and 3.8 Å, respectively. Kinetic diameter gives an indication of the size of a molecule as a target.

CO₂/N₂ and CO₂/CH₄ separation are not achieved because of lattice flexibility. ZIF-8’s aperture can expand enough to easily allow absorption of C₃H₆, which has a diameter of 4.0 Å. Agrawal said this fact has been exploited to enable separation of C₃H₆ from C₃H₈, which has a diameter of 4.2 Å. ZIF-8 membranes are “extremely popular” for this separation, he said.

Using the RHT, the researchers were able to achieve “unprecedented” separation selectivities. Whilst prior to treatment, separation selectivities were restricted to below five, afterwards the selectivities for CO₂/CH₄, CO₂/N₂, and H₂/CH₄ increased to 30, 30, and 175, respectively. C₃H₆ was completely blocked. Separation selectivity represents the degree of separation achieved, where higher figures denote better separation.

The ability to stiffen the ZIF-8 structure makes it commercially attractive for separations such as post-combustion carbon capture, biogas purification, and “recovery of hydrogen from off-gas in refineries” (H₂/CH₄), said Agrawal. “Rapid heat treatment is an easy and versatile technique that can vastly improve the gas‐separation performance of the MOF membranes,” he said.

According to Agrawal, the researchers are working to develop scalable methods for ZIF-8 membranes, to allow low-cost synthesis whilst maintaining high-performance.

The researchers discovered that lattice stiffening was caused by the introduction of defects into the structure. The researchers are also working to understand this phenomenon in more detail, said Agrawal. “ For example, we want to understand the nature (chemical composition) and concentration of these defects.”

The research was funded with contributions from the Swiss National Science Foundation and the Assistant Professor Energy Grants scheme.

Advanced Materials: http://doi.org/c7g4