Two-Dimensional Porphyrin-Based Covalent Organic Frameworks for Heterogeneous Photocatalysis: Importance of Pore Structure Engineering

Abstract

Covalent organic frameworks (COFs), with well-designed structure and molecular functionality materials, showed great promising as metal-free catalysts for heterogeneous photocatalysis. Nevertheless, the influence of the pore structure of COFs on photocatalytic performances is always neglected in ever reports. Herein, by rational designation of the topology diagrams, two-dimensional porphyrin-based COFs (ETBA-por COF and ETBC-por COF) with different pore sizes were synthesized for the first time. Both as-synthesized COFs exhibited outstanding crystallinity, and rich in micropores and high surface area. Moreover, the COFs also have similar photoelectrochemical performances for the efficaciously production of superoxide radical anions under visible irradiation. To test the effect of pore structure of COFs on photocatalytic performances, two typical characteristic reactions for organics transformation, including the selective aerobic oxidation of sulfides and oxidative amine coupling were evaluated. Interestingly, although both COFs exhibited good recyclability and stability, the ETBA-por COF with smaller pore size has higher sulfides conversion (94 %) and sulfoxides selectivity (>99 %), while things were reversed in imines production (99 % of substrate conversion and 96 % of imines yield by ETBC-por COF that with larger pore size). Mechanisms for the activity of COFs in different reactions were revealed by combing experiments and simulation, which infers that the suitable pore structure of COFs for molecules accesses is critical to ensure the good photocatalysis performances. This study confirmed the importance of rational designation the pore structure of COFs for efficient photocatalysis transformation of organics.

Keywords: Covalent organic frameworks, Heterogeneous photocatalysis, Pore size adjusting, Organics transformation