Highly Thermally Stable and Gas Selective Hexaphenylbenzene Tröger’s Base Microporous Polymers

Abstract

This study shows the multistep synthesis of a series of Tröger’s base polymers of intrinsic microporosity (TB-PIMs) based on a hexaphenylbenzene (HPB) core, with a focus on evaluating their thermal stability, porosity, and CO2 capture performance. Both ladder and linear structures were prepared, designed to feature tunable nitrogen content and porosity. Our findings demonstrate that polymers with higher nitrogen content, such as tetra-TB-HPB, exhibit superior CO2 affinity and selectivity, attributed to enhanced interactions with CO2 and optimized micropore sizes. Linear TB-polymers 1 and 2 are also made for comparison and show competitive performance in carbon capture, suggesting that cost-effective, simpler-to-synthesize materials can achieve efficient gas separation. The study reveals that increased porosity significantly enhances CO2 capacity and selectivity, particularly in networked TB-HPB-PIMs with high surface areas and narrow micropores, achieving values up to 544 m2 g-1, CO2 uptake of 2.00 mmol g-1, and CO2/N2 selectivity of 45.6. The thermal properties of these materials, assessed via thermogravimetric analysis (TGA), show that TB-HPB-PIMs maintain robust thermal stability in nitrogen atmosphere, with tetra- and hexa-TB-HPBs leading the series. However, in oxidative environments, denser polymers such as TB-HPB and linear TB-polymer 1 demonstrate higher performance, likely due to restricted air diffusion. Overall, our findings highlight the critical need to balance porosity and thermal stability in TB-HPB-PIMs for applications in gas separation, carbon capture, and the potential for these polymers as flame retardant materials. Tetra-TB-HPB stands out as the most promising material for CO2 capture and thermal stability under inert conditions, while denser polymers like TB-HPB offer superior performance in oxidative environments.