Reviews on propylene/propane separation by using ionic liquids materials

<sec><p indent="0mm">Propylene is an important part of the petrochemical industry, and its global market size has exceeded $120 billion. However, the existing propylene production process is frequently observed with the presence of propane. The separation process of propylene and propane is one of the most energy consuming processes in industrial development because of the similar structure and physical and chemical properties between them. Traditional cryogenic rectification techniques are beset with limitations such as inadequate separation efficiency and high energy consumption. Given the carbon peaking and carbon neutrality goals, it is imperative to develop environmentally friendly separation materials for propylene/propane that not only conserve energy but also enhance separation efficiency. Ionic liquids (ILs), with their characteristics of adjustable physical and chemical properties of structure, lower volatility than other organic solvents and rich hydrogen bond network, have wide significant interest in gas separation applications. ILs exhibit a unique absorption ability, which can selectively separate between propylene and propane through strong hydrogen bonding interactions or via π complexation with transition metal salts. ILs have been recognized as a new generation of efficient separation materials for propylene/propane separation, offering significant advantages in propylene absorption capacity, selectivity and cycle stability, and more. </sec><sec> In this work, the current research on propylene/ propane separation processes by using ionic liquids was firstly summarized, focusing on the optimal design of ILs, composite methods involving transition metal salts, and the underlying adsorption mechanisms. Subsequently, the influences of the structure of ILs (including the groups on the cation and anion of ILs, the length of side chain alkyl chain of cation, and the combination with transition metal salts), the properties of ILs (viscosity and cycle stability) and absorption conditions (temperature, pressure, concentration of transition metal salts) on the separation performance were discussed in detail. The results showed that the use of unsaturated groups on cations and anions or the reduction of side chain length both strengthen selective absorption capacity for propylene. Simultaneously, better gas separation performance can be achieved by reducing viscosity, increasing absorption pressure and increasing transition metal content. </sec><sec> Secondly, this article also outlines two methods for researchers to explore ILs-based functional materials for the purpose of making full use of the gas separation performance of ionic liquids and overcoming the influence of viscosity on gas diffusion. The initial strategy is to be combined ILs with porous adsorbents (such as metal-organic frameworks (MOFs)) to enhance their dispersion. For the second approach, ILs is utilized as raw material for membrane preparation to achieve propylene/propane separation. These studies about membrane preparation by using ILs not only investigated the separation properties of different ionic liquid composites, but also revealed the potential mechanism of their application in gas separation processes. </sec><sec> Finally, propylene/propane separation and the feasibility study of industrial applications were reviewed, such as the design and synthesis of ILs, the design and preparation of ILs membrane materials, the development of MOFs material combination, the application research of screening simulation and machine learning technology and so on. Hoping to provide useful reference for propylene/propane separation by using ILs. This work illustrates that the application of ILs shows great potential in the field of propylene/propane separation. The feature research direction of propylene/propane separation materials was proposed. In our opinion, many new ILs materials will be made great breakthrough in propylene/propane separation and industrial application in the future. </sec>