| CO2 emissions into the atmosphere were considered as the main reason to lead into the global warming. It was a hot field to investigate CO2 capture and separation from industrial discharge gas in the recent years. A number of crystalline and amorphous materials have been developed for CO2 adsorption, such as Covalent Triazine-based Framework (CTF), Covalent Organic Polymer (COP), Metal-Organic Framework (MOF), Porous Polymer Network (PPN), Polymeric Organic Framework (POF), and Element-Organic Framework (EOF). Covalent organic frameworks (COFs) are a class of crystalline porous materials that allow the atomically precise integration of building blocks into ordered architecture. The pre-designable porous size and geometry make them attractive as platforms for CO2 capture and storage. Although COFs have a generally larger pore size, which is disadvantage for gas adsorption, the improvement of CO2 capture and CO2/N2 selectivity has been realized through channel-wall functionalization of two-dimensional (2D) COFs. On the one hand, the decreased pore size through the introduction of various groups can enhance the host-guest interaction. On the other hand, the functional groups that interact with CO2 can be introduced into the pores of materials. In many cases, both factors would contribute to the increased CO2 adsorption and separation capacity, but one may not know what extent each factor alters the material's performance.Herein, we have synthesized a series of Schiff-base [OH]X%-TAPH-COFs with a condensation of 5,10,15,20-tetrakis (p-tetraphenylamino) porphyrin (TAPH) and a mixture of 2,5-dihydroxyterephthalaldehyde (DHTA) and 1,4-phthalaldehyde (PA) at different ratios (X=[DHTA]/[DHTA+PA]×100=25,50,75,100) To enhance the affinity of the framework toward CO2, the 4-phenylazobenzoyl (PhAzo) group with a CO2-philic and N2-phobic feature has been introduced into the pore walls of [HO]x%-TAPH-COFs (X=25,50,75,100) through an acylation reaction. By contrast, an analogue of PhAzo,4-stilbenecarbonyl chloride (PhSti) that has a similar structure with PhAzo but lacks special CO2-philic and N2-phobic functionalities, has also been used to modify the pore surface properties. The functionalized materials are nominated as [N=N]X%-TAPH-COFs and [C=C]X%-TAPH-COFs, respectively.1. A series of tailored covalent organic frameworks (COFs), [N=N]X%-TAPH-COFs and [C=C]X%-TAPH-COFs, were synthesized by post-fabrication of [HO]X%-TAPH-COFs with 4-phenylazobenzoyl chloride (PhAzo) and 4-stilbenecarbonyl chloride (PhSti), With increasing the content of the PhAzo groups from 25% to 50%,75%, and 100%, the BET surface area of [N=N]X%-TAPH-COFs decreased from 702 to 560,320, and 250 m2g-1, whereas the pore size decreased from 1.7 to 1.4,1.3, and 1.2 nm. The pore volume was also found to be decreased from 0.72 to 0.64,0.59, and 0.54 cm3 g-1, respectively, A similar trend was also observed for [C=C]X%-TAPH-COFs. The BET surface area (680-310 m2 g-1), pore volume (0.70-0.51 cm3 g-1), and pore size (1.7-1.2 nm) decreased when the content of the PhSti groups was gradually increased.2. Powder X-ray diffraction (PXRD) measurements, FT-IR, and Elemental analysis et al were applied to clarify the functional groups integrated in the pore channels. [N=N]X%-TAPH-COFs and [C=C]x%-TAPH-COFs exhibited XRD patterns similar to those of [HO]X%-TAPH COFs, indicating that the crystal structures of [N=N]X%-TAPH-COFs and [OC]X%-TAPH-COFs were similar to those of the pristine [HO]X%-TAPH COFs. After functionalization, the C=N bonds were remained, suggesting that the frameworks were still stable, as for [N=N]X%-TAPH-COFs, several new peaks appeared at 1740,1253, and 690 cm-1, which was assigned to the C=O, C-O-C, and C-H (trans-azobenzene) stretching bands, respectively, indicating that the azobenzene groups with primarily trans conformation were grafted onto the channel walls of the COFs. Similarly, the [C=C]X%-TAPH-COFs also held the amine-linked frameworks and the new peaks at 1739 and 1253 cm-1 were assigned to C=O and C-O-C, respectively, indicative of the successful introduction of the PhSti groups. Elemental analysis revealed that the actual components of the materials were close to the calculated values.3. Compared with [C=C]X%-TAPH-COFs, the [N=N]X%-TAPH-COFs exhibited higher CO2 uptake capacities up to 207 mg/g (273 K and 1 bar), isosteric heats of adsorption for CO2 (30.7-43.4 kJ/mol), and CO2/N2 selectivities up to 78 (273 K) because of the dipole interactions between the azo group and CO2 as well as the N2-phobic behavior of the azo group. Furthermore, although the decreased pore size was advantageous for increasing CO2 adsorption, the decreased surface area of COFs would undoubtedly decrease CO2 adsorption if too many functional groups were introduced. |
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