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

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.