A Pd confined hierarchically conjugated covalent organic polymer for hydrogenation of nitroaromatics: catalysis, kinetics, thermodynamics and mechanism

Abstract

Herein, we propose a fast and scalable synthesis of a triazine based hierarchically conjugated covalent organic polymer under solvent and additive free conditions through a single step process. The synthesized material CCTP (Cyanuric Chloride-Thiourea-Polymer) was thoroughly characterized by various physicochemical techniques. The CCTP exhibited regular sponginess and excellent chemical as well as thermal stability. The solvent and additive free approach for CCTP synthesis provides a sustainable alternative for classical solvothermal methods. The CCTP was immobilized with Pd (0) and subsequently a heterogeneous material Pd@CCTP was obtained, which was used as an efficient catalyst for the hydrogenation of nitroarenes. The rate constant and E_a were measured to be 2.08 × 10⁻² s⁻¹ and 15.67 kJ mol⁻¹ respectively and thereafter other thermodynamic parameters like ΔH, ΔS and ΔG for the hydrogenation of p-nitrophenol were also calculated. The obtained results indicate that the catalytic hydrogenation of p-nitrophenol is a non-spontaneous and endothermic process. We have also investigated the effect of surfactants (NH₄OH, FA, and N₂) on the reaction performance, and consequently NH₄OH and FA both slow down the reaction while N₂ doesn't affect the reaction medium. Further, we calculated the rate constant for the hydrogenation of 2,4-dinitrophenol and 2,4,6-trinitrophenol. An array of nitroarenes were further reduced to extend the substrate scope at RT; high TOFs were observed. Besides, Pd@CCTP showed excellent reusability in hydrogenation reactions without evident performance falloff.