On the Role of Flexibility for Adsorptive Separation

Abstract

Conspectus Chemical separations, mostly based on heat-driven techniques such as distillation, account for a large portion of the world’s energy consumption. In principle, differential adsorption is a more energy-efficient separation method, but conventional adsorbent materials are still not effective for many industry-relevant mixtures. Porous coordination polymers (PCPs), or metal–organic frameworks (MOFs), are attractive for their well-defined, designable, modifiable, and flexible structures connecting to various potential applications. While the importance of the structural flexibility of MOFs in adsorption-based functions has been demonstrated, the understanding of this special feature is still in its infancy and mostly stays at the periodic structural transformation at the equilibrium state and the special shapes of single-component adsorption isotherms. There are many confusions about the categorization and roles of various types of flexibility. This Account discusses the role of flexibility of MOFs for adsorptive separation, mainly from the thermodynamic and kinetic points of view. As the classic type of framework flexibility, guest-driven structural transformations and the corresponding adsorption isotherms can be thermodynamically described by the energies of the host–guest system. The highly guest-specific pore-opening action showing contrasting single-component adsorption isotherms is regarded as a strategy for achieving molecular sieving without the need for aperture size control, but its effect and role for mixture separation are still controversial. Quantitative mixture adsorption/separation experiments showed that the common periodic (cooperative) pore-opening action leads to coadsorption of molecules smaller than the opened aperture, while the aperiodic (noncooperative) one can achieve inversed molecular sieving under a thermodynamic mechanism. The energy barrier and structure in the nonequilibrium state are also important for flexibility and adsorption/separation. With suitable energy barriers between metastable structures, new types of framework flexibility such as aperture gating can be realized. While kinetically controlled gating flexibility is usually ignored because of the difficulty of characterization or considered as disadvantageous for separation because of the variable aperture size, it plays a critical role in most kinetic separation systems, including adsorbents conventionally regarded as rigid. With the concept of gating flexibility, the meanings of aperture and guest sizes for judging molecular sieving need to be reconsidered. Gating flexibility depends on not only the host itself but also the guest, the host–guest interaction, and the external environment such as temperature, which can be rationally tuned to achieve special adsorption/separation behaviors such as inversed temperature dependence, molecular sieving, and even inversed thermodynamic selectivity. The comprehensive understanding of the thermodynamic and kinetic bases of flexibility will give a new horizon for next-gener...

Engineering Pore Environments of Sulfate‐Pillared Metal‐Organic Framework for Efficient C2H2/CO2 Separation with Record Selectivity

“…[17][18][19] But, in fixed adsorption columns, the possible co-adsorption will generate low-purity C 2 H 2 product in the recovery process. [20] Engineering pore environments exhibit great potential in improving gas adsorption and separation performances but require specific means for acetylene/carbon dioxide (C 2 H 2 /CO 2 ) separation due to their identical dynamic diameters and similar properties. Herein, a novel sulfate-pillared MOF adsorbent (SOFOUR-TEPE-Zn) using 1,1,2,2-tetra(pyridin-4-yl) ethene (TEPE) ligand with dense electronegative pore surfaces is reported.…”

“…[ 17–19 ] But, in fixed adsorption columns, the possible co‐adsorption will generate low‐purity C 2 H 2 product in the recovery process. [ 20 ]…”

Optimal Binding Affinity for Sieving Separation of Propylene from Propane in an Oxyfluoride Anion-Based Metal–Organic Framework

“…30−32 The sorbents can be properly designed to achieve desired responsiveness. 33,34 But such cooperative pore opening could also lead to co-adsorption of undesired gas and, worse, leakage or "slipping-off" in mixture separation processes. 35,36 More research efforts and insights are still needed to understand this phenomenon.…”

“…By virtue of rational design and pore engineering, one can precisely functionalize the pore surface, which creates thermodynamic preference toward propylene, enabling propylene-selective molecular recognition over propane in the competitive adsorption process. − However, the co-adsorption of propane is typically unavoidable and significantly hinders the selectivity and purity of recovered propylene. Diffusivity-based kinetic separation is proven beneficial by enlarging the difference in diffusion rate and, thus, the adsorption selectivity, especially when synergistically coupled with equilibrium selectivity. − On the other hand, gating effect-driven separation is found to be effective to realize selective propylene adsorption, thanks to the structural flexibility allowing it to capture propylene through “gate-opening”. − The sorbents can be properly designed to achieve desired responsiveness. , But such cooperative pore opening could also lead to co-adsorption of undesired gas and, worse, leakage or “slipping-off” in mixture separation processes. , More research efforts and insights are still needed to understand this phenomenon. After all, in the ideal scenario, the ultimate goal is to realize molecular sieving, namely, a complete exclusion of the undesired molecule by tailor-made pore architecture featuring suitable size and dynamics.…”

Isoelectric Substitution Strategy to Construct a Partially Charged MOF for C2H2 Capture and Selective Dye Adsorption

“…Over the past two decades, metal–organic frameworks (MOFs), an emerging class of porous materials, have been widely investigated due to their high porosity, − facile tunability, − and extensive application range, − even the very challenging and difficult gas separation such as C 2 H 2 /CO 2 , − C 2 H 4 /C 2 H 6 , , C 3 H 6 /C 3 H 8 , , and removal of pollutants in water. − In this period, the different strategies have been utilized to design and synthesize novel functionalized MOF materials and promote them with practical applications, − which is the goal of framework materials. , Recently, our group utilized the steric hindrance effect to construct the partially charged MOF ( FJU-118 ), where the methyl group of ligands restrict the rotation of its C–N bonds, resulting in indium ions with partially positively charged residues . The residues’ charge on the indium site provides an electrostatic force to anchor the acetylene molecule for C 2 H 2 /CO 2 separation.…”