Systematic Investigation of the Effect of Polymerization Routes on the Gas Sorption Properties of Nanoporous Azobenzene Polymers

O. Buyukcakir, S. H. Je, J. Park, H. A. Patel, Y. Jung, C. T. Yavuz*, A. Coskun*
Chem. Eur. J., 21 (43), 15320–15327, (2015). DOI: 10.1002/chem.201501233.



Functional-group-oriented polymerization strategies have contributed significantly to the initial development of porous polymers and have led to the utilization of several well-known organic transformations in the synthesis of these polymers. Because there are multiple polymerization routes that can be used to introduce the same chemical functionality, it is very important to demonstrate the effect of different polymerization routes on the gas-sorption properties of these chemically similar polymers. Herein, we have studied the rich chemistry of azobenzenes and synthesized four chemically similar nanoporous azobenzene polymers (NABs) with surface areas of up to 1021 m2 g−1. The polymerization routes have a significant impact on the pore-size distributions of the NABs, which directly affects the temperature dependence of the CO2/N2 selectivity. A pore-width maximum of 6–8 Å, narrow pore-size distribution, and small particle size (20–30 nm) were very critical for high CO2/N2 selectivity and N2 phobicity, which is associated with azo linkages and realized at warm temperatures. Our findings collectively suggest that an investigation of different polymerization routes for the same chemical functionalization is critical to understand fully the combined effect of textural properties, local environment, and chemical functionalization on the gas-sorption properties of nanoporous polymers.
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Nanoporous covalent organic polymers incorporating Tröger’s base functionalities for enhanced CO2 capture

J. Byun, S. H. Je, H. A. Patel, A. Coskun,* C. T. Yavuz*
J. Mater. Chem. A, 2, 12507-12512, 2014. [DOI] [pdf] [WoS]


The CO2 uptake capacity and CO2/N2 selectivity of Tröger’s base–bridged nanoporous covalent organic polymers (TB-COPs) were investigated. TB-COPs were synthesized by reacting amine terminals of tetrahedral monomers - namely, tetraanilyladamantane and tetraanilylmethane - with dimethoxymethane in a one–pot reaction under relatively mild conditions. Interestingly, these two tetrahedral monomers formed nanoporous polymers with substantially different surface areas. While the trögerization of tetraanilyladamantane monomer (TB-COP-1) exhibit high surface area of 1340 m2 g-1, that of tetraanilylmethane monomer (TB-COP-2) is found to be only 0.094 m2 g-1. This unusual phenomenon can be explained by the proximity of amino moieties to each other within the monomeric unit. Shorter distance between the amino groups enables intramolecular cyclization along with the intermolecular one, thus resulting in much lower porosity. TB-COP-1 exhibits significant CO2 uptakes up to 5.19 and 3.16 mmol g-1 at 273 and 298K under ambient pressure, and CO2/N2 selectivities of 79.2 and 68.9 at 273 and 298K at 1 bar for the gas mixture of CO2:N2 in the ratio of 0.15:0.85. It is noteworthy that TB-COP-1 showed remarkable selectivity retention with rising temperature from 273 to 298 K.
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Directing the Structural Features of N2-Phobic Nanoporous Covalent Organic Polymers for CO2 Capture and Separation

H. A. Patel, S. H. Je, J. Park, Y. Jung, A. Coskun*, C. T. Yavuz*
Chem. Eur. J., 20, 772-780, (2014). [DOI] [pdf] [WOS]


A family of azo-bridged covalent organic polymers (azo-COPs) was synthesized through a catalyst-free direct coupling of aromatic nitro and amine compounds under basic conditions. The azo-COPs formed 3D nanoporous networks and exhibited surface areas up to 729.6 m2 g−1, with a CO2-uptake capacity as high as 2.55 mmol g−1 at 273 K and 1 bar. Azo-COPs showed remarkable CO2/N2 selectivities (95.6–165.2) at 298 K and 1 bar. Unlike any other porous material, CO2/N2 selectivities of azo-COPs increase with rising temperature. It was found that azo-COPs show less than expected affinity towards N2 gas, thus making the framework “N2-phobic”, in relative terms. Our theoretical simulations indicate that the origin of this unusual behavior is associated with the larger entropic loss of N2 gas molecules upon their interaction with azo-groups. The effect of fused aromatic rings on the CO2/N2 selectivity in azo-COPs is also demonstrated. Increasing the π-surface area resulted in an increase in the CO2-philic nature of the framework, thus allowing us to reach a CO2/N2 selectivity value of 307.7 at 323 K and 1 bar, which is the highest value reported to date. Hence, it is possible to combine the concepts of “CO2-philicity” and “N2-phobicity” for efficient CO2 capture and separation. Isosteric heats of CO2 adsorption for azo-COPs range from 24.8–32.1 kJ mol−1 at ambient pressure. Azo-COPs are stable up to 350 °C in air and boiling water for a week. A promising cis/trans isomerization of azo-COPs for switchable porosity is also demonstrated, making way for a gated CO2 uptake.
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Unprecedented high-temperature CO2 selectivity in N2-phobic nanoporous covalent organic polymers

H. A. Patel, S. H. Je, J. Park, D. P. Chen, Y. Jung, C. T. Yavuz*, A. Coskun*
Nature Commun., 4, 1357, (2013). [DOI] [pdf] [WOS]



Post-combustion CO2 capture and air separation are integral parts of the energy industry, although the available technologies remain inefficient, resulting in costly energy penalties. Here we report azo-bridged, nitrogen-rich, aromatic, water stable, nanoporous covalent organic polymers, which can be synthesized by catalyst-free direct coupling of aromatic nitro and amine moieties under basic conditions. Unlike other porous materials, azo-covalent organic polymers exhibit an unprecedented increase in CO2/N2 selectivity with increasing temperature, reaching the highest value (288 at 323 K) reported to date. Here we observe that azo groups reject N2, thus making the framework N2-phobic. Monte Carlo simulations suggest that the origin of the N2 phobicity of the azo-group is the entropic loss of N2 gas molecules upon binding, although the adsorption is enthalpically favourable. Any gas separations that require the efficient exclusion of N2 gas would do well to employ azo units in the sorbent chemistry.



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