Hydrogen adsorption in pyridine bridged porphyrin-covalent organic framework

Abstract

Covalent organic frameworks (COFs), a class of carbon-based polymeric materials have the potential to be used as hydrogen adsorbent. Three dimensional (3D) COFs, due to their low density and high surface area, although have higher hydrogen adsorption, they have less stability than two dimensional (2D) COFs. Here we studied porphyrin group containing 2D COF, namely H2,P-COF for hydrogen storage using density functional theory (DFT) and grand canonical Monte Carlo (GCMC) simulations and the results were compared with the most common 2D COFs, COF-1 and COF-5. Cylindrical shaped 2D COFs where isolated unit blocks are stacked in multiple layers due to van der Waals interactions between individual layers, increase the effective surface area for hydrogen storage. A further modification has been done by bridging the inter-layer gap by pyridine molecules. Insertion of pyridine increases the separation distance of layers of 2D COFs as well as the free volume. Feasibility of the structure formation and stability of all the structures were checked using DFT study. To ensure the structural stability of bridged COFs after hydrogen loading, alternating layers of COF were bridged. Single, bi, tri and tetra -pyridine molecules were chemically bonded with the existing carbon ring present in between two C2O2B rings to form pyridine bridged H2,P-COFs. Our GCMC results show a significant increase in storage capacity which is mainly due to an increase in the free volume of the material. The highest capacity of ∼ 5.1 wt% and 20 g H2/L at 298 K and 100 bar, above the gravimetric DOE goal, has been found at room temperature for tetra-pyridine doped porphyrin COF structure.