Polymer sieve divides gases by size

CARDIFF U. (UK) — A new polymer efficiently separates gas mixtures by the size of the molecules, report researchers.

Gas separation is crucial for many industrial processes, including obtaining nitrogen or oxygen from air and purifying natural gas or hydrogen.

Currently, the most energy efficient method for separating gases involves polymer membranes, however, most polymers either let gases pass through slowly (i.e. have low permeability) or are not selective towards one gas over another.

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Gas separation would be cheaper and use less energy if polymer membranes could be made both highly permeable and selective.

As reported in Science, the polymer’s molecular structure is very contorted so that it cannot fill space efficiently, therefore leaving gaps for small gas molecules to move through quickly. However, the transport of larger gas molecules is hindered by the polymer’s extreme rigidity so that it acts as an efficient molecular sieve.

The polymer is very easy to make. The monomer and reagents are mixed together and stirred at room temperature until the solution is viscous. It is then poured into water, washed, and can be cast from solvent into films suitable for membrane use.

Collaborators at the Institute on Membrane Technology, ITM-CNR, Italy, confirm that membranes prepared from the polymer are both highly permeable to gases and demonstrate remarkable selectivity for smaller gases such as hydrogen or oxygen over larger gases such as nitrogen or methane.

Professor Neil McKeown, a member of the Cardiff University team behind the research says: “The preparation of this highly rigid and contorted polymer required us to develop a new polymerisation reaction. In fact we used some very old chemistry—the formation of Tröger’s base, which is a compound that was first prepared 125 years ago.

“This simple chemistry allows us to prepare highly rigid ladder polymers of high molecular mass from readily available starting materials. In addition to making polymers for efficient gas separation membranes, we anticipate that this new process will be useful for preparing polymers for a variety of different applications.”

Cardiff University has applied for a patent covering this new polymerization process.

Source: Cardiff University