Molecular Mechanism Behind the Capture of Fluorinated Gases by Metal–Organic Frameworks

Abstract

Fluorinated gases (F-gases) play a vital role in the chemical industry and in the fields of air conditioning, refrigeration, health care, and organic synthesis. However, the direct emission of waste gases containing F-gases into the atmosphere contributes to greenhouse effects and generates toxic substances. Developing porous materials for the energy-efficient capture, separation, and recovery of F-gases is highly desired. Recently, as a highly designable porous adsorbents, metal–organic frameworks (MOFs) exhibit excellent selective sorption performance toward F-gases, especially for the recognition and separation of different F-gases with highly similar properties, showing their great potential in F-gases control and recovery. In this review, we discuss the capture and separation of F-gases and their azeotropic, near-azeotropic, and isomeric mixtures in various application scenarios by MOFs, specifically classify and analyze molecular interaction between F-gases and MOFs, and interpret the mechanisms underlying their high performance regarding both adsorption capacity and selectivity, providing a repertoire for future materials design. Challenges faced in the transformation research roadmap of MOFs adsorbent separation technologies toward F-gases are also discussed, and areas for future research endeavors are highlighted.

1. Introduction

Fluorinated gases (F-gases), including perfluorocarbons (PFCs), hydrofluorocarbons (HFCs), chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), inhaled volatile anesthetics (VAs), and other fluorine-containing gaseous compounds (Fig. 1), are widely used in air conditioning, refrigeration, medical, cable, semiconductor, and metal-processing industries [1–4]. The increasing use and emissions of F-gases have raised widespread environmental concerns. Most F-gases are typical greenhouse gases that exhibit much higher global warming potential (GWP) and longer atmospheric lifetime than carbon dioxide (CO2) and methane (CH4), and even lead to the depletion of the ozone layer [5–8]. In recent years, the greenhouse effect resulting from F-gases and related substances (such as carbon tetrachloride) emitted annually by various industries is equivalent to emitting 870 million tonnes of CO2 equivalent, which is comparable to that of over 200 coal-fired power plants [9, 10]. In addition, the emissions and hydrolysis of F-gases are often accompanied by toxic or corrosive concomitants, such as HF, CO, and NOx, which further adversely affect air quality and human health [11–16]. Therefore, achieving the effective capture, separation and recovery of F-gases to realize the climate goals, and the sustainable development of corresponding industries is top priority.

To date, cryogenic distillation is the main technology for separating F-gases for industrial applications [17–21]. However, some components of F-gases blends are similar in thermophysical properties, exhibiting azeotrope or near-azeotrope. For example, the boiling points of NF3 and CF4 differ by approximately 1 K (143.35 K for NF3 and 144.95 K for CF4) and their relative volatility less than 1.05 at 193.15 K. Thus, cryogenic distillation is difficult to perform [22–25]. In addition, the huge energy consumption and anticorrosion equipment design of cryogenic distillation render it an infeasible choice [17, 23]. Membrane-based separation technologies are hampered by their low intrinsic permeability to F-gases and have low separation efficiency [23, 26–].