Polymers get a grip on CO2

New microporous organic polymers (MOPs) that can selectively take up and separate off CO₂/light gas mixtures have been synthesized [Du et al., Nature Mater (2011) doi: 10.1038/nmat.2989]. Researchers from Canada and Korea created the MOPs by [2+3] cycloaddition of an aromatic nitrile group on a simple polymer with an azide, to create TZIPM, a material that has super-permeable characteristics and good CO₂ separation.

 

MOPs have very large surface areas, they can be manufactured on a large scale and their properties can be tailored according to the function by adjusting the surface area and the size and shape of the cavities within the framework. Polymers of intrinsic porosity (PIMs) are one such group of MOPs, with a ladder-like rigid backbone and sites that can contort to create large free-volume elements within the framework, making them especially suitable for gas separation. One example, PIM-1, chosen because of its simplicity, ease of fabrication, high molecular weight, and good mechanical strength contains two nitrile groups per repeat unit and it is these nitrile groups which can undergo cycloaddition with azides.

 

“The cycloaddition reaction on PIM-1 creates a tetrazole ring, which contains 4 nitrogen atoms in a 5-membered aromatic-like ring, together with an N-H group,” explains Michael Guiver. “The nitrogen on the resulting tetrazole-PIM (TZPIM) has basic character, which interacts strongly with carbon dioxide. CO₂ sorbs strongly on the membrane. The N-H group also forms a hydrogen bonded network with the oxygen atoms of the polymer chain, and ‘tightens’ the PIM network. The combination of these effects is that gas selectivity (for CO₂/N₂) for TZPIM is increased relative to PIM-1.”

 

The CO₂ binds to the rings and this causes the cavities in the framework to contract, impeding further transport of smaller gaseous molecules, such as N₂ or CH₄.

 

“What is of particular interest is that when mixed gases (CO₂ and N₂) are measured, as would be encountered in a real separation, the gas selectivity for CO₂ is significantly higher than the pure gas measurement,” Guiver adds. “In most polymer membrane systems, the presence of CO₂ in a mixed gas system almost always gives lower selectivity compared with pure gas measurements, because the CO₂ is a ‘condensable’ gas, and tends to plasticize or ‘loosen’ the polymer chains. In the TZPIM system, CO₂ plasticization does not appear to occur, and the absorbed CO₂ in the tighter hydrogen-bonded network acts to block the nitrogen when permeating mixed CO₂/N₂ gases.”

 

While these TZIPM membranes exhibit good gas separation, they will have to be further refined before they can be used for selective removal of CO₂ from flue gases or biogas refining, but they could provide a viable energy-saving alternative to current methods.