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.
Katerina Busuttil