Adsorptive Separation Of C1/C2 Hydrocarbons Within Aluminum-based Metal-organic Frameworks:Application And Mechanism Insights

Currently,environmental damage and energy dissipation are two significant issues for the whole world.Natural gas,as an important energy source,mainly contains methane（CH₄）and nitrogen（N₂）.Therefore,the purification and recovery of methane from natural gas can not only alleviate the problem resulted from CH₄ emission,but also achieve the improvement of the natural gas quality and effective utilization of energy.At the same time,acetylene and ethylene are basic raw materials for a variety of essential chemicals.Therefore,it’s of great significance for the recovery of acetylene and ethylene from C₂H₂/CO₂ and C₂H₂/C₂H₄mixtures.The trade-off effect between excellent selectivity and outstanding capacity makes the design and development of adsorbents with high performance tougher and more difficult.Herein,we synthesized a series of aluminum-based metal-organic frameworks（Al-MOFs）,involving the investigation of CH₄/N₂,C₂H₂/C₂H₄,C₂H₂/CO₂ gas mixtures separation performance,elucidation of the mechanism for host-guest and guest-guest interaction;estimation the potential in practical industrial production.Our work also can furnish valuable guidance to design high-performance adsorbents for gas adsorption and separation.For CH₄/N₂ separation,we herein further study the effects of pore geometry and linker polarity on separation performance of Al-MOFs as adsorbent for CH₄/N₂ mixtures.Utilization of two bent and two linear ligands with different polarity afforded two one-dimensional square-shaped Al-MOFs i.e.,CAU-10-H,MIL-160,and two rhombic-shaped counterparts i.e.,Al-Fumarate（Al-Fum）and MIL-53（Al）,respectively.Afterward,static CH₄ and N₂ adsorption experiments were conducted at273-313 K for assessing the CH₄/N₂ separation performance.The isotherm results indicated that all Al-MOFs exhibited superior affinity toward CH₄over N₂,and the CH₄ uptake followed the sequence of Al-Fum>CAU-10-H>MIL-53（Al）>MIL-160.Exhilaratingly,Al-Fum exhibited unprecedented CH₄/N₂ selectivity（17.2）and high CH₄ uptake(1.74 mmol g^-1)at 273 K and 1.0 bar.The mechanism underlying the disparity of Al-MOFs affinity toward CH₄ was deciphered via theoretical simulation,suggesting that the synergetic effects of accessibility of strong affinity sites（μ₂-OH）on Al O₆ chains and polar pore surface induced by varying linkers highly promoted the CH₄ uptake.Furthermore,the results of cyclic adsorption-desorption experiments and binary breakthrough tests validated the feasibility of Al-Fum for practical application.For C2 hydrocarbons separation,we proposed a strategy involving synergic combination of spatial confinement and commensurate stacking for enhanced C₂H₂ storage via maximizing the host-guest and guest-guest interactions.Two ultramicroporous metal-organic frameworks,MIL-160and MOF-303 were elaborately constructed and exhibited ultrahigh C₂H₂uptake of 235 and 195 cm³ g^-1,respectively,which resulted by the confinement effect of the suitable pore size and periodically dispersed molecular recognition sites.Specially,C₂H₂ capacity of MIL-160 sets a new benchmark for C₂H₂ storage.The exceptional separation performance of two materials for C₂H₂ over both CO₂ and C₂H₄,which is rarely observed,outperformed most of the benchmark materials for C₂H₂ capture.We scrutinized the underlying reasons of ultrahigh C₂H₂ loading in the confined channels via theoretical calculations.The superior separation efficiency for C₂H₂/CO₂ and C₂H₂/C₂H₄ mixtures with unprecedented C₂H₂trapping capacity(>200 L kg^-1)was further demonstrated by dynamic breakthrough experiments and simulations.