Gas Diffusion In One-Dimensional Pore Channels And Its Separation Mechanism

Porous materials are important in established processes such as catalysis and molecular separation,as well as in emerging technologies for energy development and environmental management.Porous zeolites have made the greatest contribution to society to date and are rapidly developing in areas such as new energy development and environmental protection.Porous materials are ubiquitous in our daily lives,where they are used to capture,screen,and separate liquids,solids,and gases.Advances in the science of porous materials have facilitated the production of a variety of porous adsorbents with controlled pore size and surface area for molecular adsorption.Zeolites and microporous carbon are typical industrial adsorbents that have made significant contributions to traditional gas storage and separation technologies.In addition to these advantages,the gas diffusion processes in these porous materials are still difficult to design and control rationally.The narrow,constricted pores in these materials are not only a fundamental requirement for controlling gas diffusion,but also play an important role in the flexibility of local and global frameworks to regulate the flow in the channels as well as the recognition and separation of guest molecules.Porous materials have a wide range of applications,but no single class of porous materials is suitable for all applications.For example,crystallinity and long-range order may improve the selectivity of molecular separations while also reducing mechanical stability or processability compared to less ordered structures.To have an impact on practical applications,porous materials must be scalable and must meet multiple functional criteria such as long-term stability,selectivity,adsorption kinetics,and processability,all within a feasible cost range.This presents a broad design challenge that requires the ability to control the structure and understand multiple structureproperty relationships at a detailed level.Energy costs associated with the separation and purification of industrial commodities such as gases,fine chemicals and fresh water currently account for approximately 15% of global energy production,and demand for porous materials is expected to triple by 2050.Materials that enable high-capacity storage and efficient separation of gases to meet today’s demands for energy efficiency and security cannot be achieved by simply improving conventional technologies.In this context,porous materials with nanoscale voids will make a significant contribution to gas handling science and technology and energy consumption.We focus on the structure of AFI(Al PO4-5)zeolites and covalent organic framework COFs in porous materials.To understand in more detail the diffusion mechanism of guest molecules in the pores of porous materials,we use classical molecular dynamics(MD)simulations and density flooding theory(DFT)calculations to study the diffusion of small gas molecules(He,H2,N2,CO2,and CH4)in the Al PO4-5 molecular sieves by considering the diffusion behavior of gas molecules with different van der Waals diameters(dm)and different loading numbers into the channels of Al PO4-5,and using density flooding theory calculations to investigate gas-gas and gas-AFI interactions and the energy barriers of gas molecules in the longitudinal direction of the Al PO4-5 channels.Covalent organic frameworks(COFs)are proposed as alternative candidates for molecular sieve membranes due to their chemical stability.We thus investigated the separation mechanism of CO2/CH4 molecules in one-dimensional(1-D)channels within COFs,considering the diffusion and separation behavior of gas molecules at different pore sizes and different loads,and the interaction of gases with COFs/modified COFs to predict the separation efficiency and separation selectivity,and the results of this study will contribute to a better understanding of the role of COFs in gas separation and provide an important reference for the design of efficient COFs materials to develop alternative separation technologies with low energy consumption and reduce global energy demand,thus having a beneficial impact on the environment.