Geometry variation in porous covalent triazine polymer (CTP) for CO2 adsorption

Abstract

Covalent triazine-based organic polymers (CTPs), a sub class of covalent organic polymers (COPs), are promising materials for CO₂ adsorption although the impact of their dimensionality on the trapping process is not well-understood. Two different molecular geometries of phenyl-based diamine linkers, planar p-phenylenediamine and angular 1,2-diphenylethylenediamine, were subsequently chosen to afford 2 CTPs (ACTP-1 and ACTP-2) via catalyst-free polycondensation to acquire deeper fundamental understanding of the effect of their dimensionality on the adsorption efficiency. Although ACTP-2, with non-planar building blocks, showed a significant improvement in the porosity of its network in comparison with the planar structure polymer ACTP-1, it also displayed pore size distribution that is twice as large. Furthermore, the presence of an alkyl chain linking the triazine units in ACTP-2 yielded a semi-crystalline polymer. At low pressure (up to 1 bar) and 298 K, ACTP-1 adsorbed significantly higher amount of CO₂ (0.65 mmol g⁻¹), implying that a planar building block is preferable to locate CO₂ moieties. On the contrary, steric hindrance due to the bulkiness of the aromatic ring restricted the interaction between CO₂ and amine functional group in ACTP-2. Hysteretic sorption of CO₂ was observed in ACTP-2, indicating the flexibility of the polymer due to the pressure change effect. Isosteric heats for ACTP-1 and ACTP-2 were 35 kJ mol⁻¹ and 22 kJ mol⁻¹, respectively. An increase in the C/N ratio of the triazine polymer resulted in higher affinity towards CO₂ that that towards N₂. For ACTP-1, the CO₂/N₂ selectivity of CO₂ : N₂ in the ratio of 15 : 85 was 43 at 298 K and 1 bar, and it was superior than those of most of the covalent triazine frameworks (CTFs) reported in recent literature at low pressure.