A triazine–resorcinol based porous polymer with polar pores and exceptional surface hydrophobicity showing CO2 uptake under humid conditions

Abstract

Several applications including post-combustion carbon capture require capturing CO₂ under humid conditions. To obtain a material capable of interacting more strongly with CO₂ than water, surface hydrophobicity and polarizing pores have been incorporated simultaneously into an ultra-microporous Bakelite-type polymer comprising of triazine–triresorcinol building units. Being built from C–C bonds, it exhibits exceptional chemical stability (survives conc. HNO₃(g) + SO₃(g) without losing any porosity). Triazine–phenol lined channels enable adsorption of CO₂ (2.8 mmol g⁻¹ with a good selectivity of 120 : 1 (85% N₂ : 15% CO₂) at 303 K, 1 bar) and the inherent surface hydrophobicity amply minimizes the affinity for H₂O. When the adsorption was carried out using a humid CO₂ stream (∼50% RH) the material loses only about 5% of its capacity. In a steam-conditioning experiment, the sample was exposed to high humidity (∼75% RH) for a day, and without any further activation, was tested for CO₂ adsorption. It retains more than 85% of its CO₂ capacity. And this capacity was intact even after 48 h of steam conditioning. The role of phenol in contributing to the surface hydrophobicity is exemplified by the fact that a ∼17% lithiation of the phenolic sites nearly removes all of the surface hydrophobicity. The local structure of the polymer has been modeled using tight-binding DFT methods (Accelrys) and three low energy conformers were identified. Only the CO₂ isotherm simulated using the lowest energy conformer matches the experimental isotherm quite well. The triazine–phenol polymer presented here has good hydrophilic–hydrophobic balance, where the basic triazine units and the phenol groups seem to co-operatively assist the CO₂ capture under humid conditions. These properties along with its excellent acid stability make the material a suitable candidate for post-combustion CO₂ capture. Also, the study presents a new approach for simultaneously introducing polarizing character and surface hydrophobicity into a porous material.