Tuning the Hierarchical Pore Structure and the Metal Site in a Metal-Organic Framework Derivative to Unravel the Mechanism for the Adsorption of Different Volatile Organic Compounds.

Abstract

Volatile organic compounds (VOCs) are one of the main classes of air pollutants, and it is important to develop efficient adsorbents to remove them from the atmosphere. To do this most efficiently, we need to understand the mechanism of VOC adsorption. In this work, we described how the metal organic framework (MOF), ZIF-8, was used as a precursor to generate MOF derivatives (Zn-GC) through temperature-controlled calcination, which had adjustable metal sites and hierarchical pore structure. It was used as a model adsorbent to study the adsorption and desorption characteristics of different VOCs. Zn-GC-850 with developed pores exhibited higher adsorption performance for the benzene series, whereas Zn-GC-650 with more metal sites had a better adsorption capacity for oxygen-containing VOCs. By tuning the molecular structure of the VOCs, we revealed the adsorption mechanism of different VOCs at the molecular level. The more developed hierarchical pore structure obtained at the higher temperature facilitates the diffusion of the benzene series, and the noncovalent interaction between their methyl group(s) and the carbonized MOF derivatives improves the adsorption affinity; while the higher exposure of Zn sites obtained at lower temperature favors the adsorption of oxygen-containing VOCs by Zn-O bonds. The mass transfers of VOCs and the role of the adsorbent were simulated by multiple theoretical models. This study strengthens the basis for the design and optimization of the adsorbent and catalyst for VOCs treatment.