Hydrocarbon separation is one of the most critically important and complex industrial separation processes, offering versatile bulk chemicals and vital support to the national economy. Traditional separation technologies, such as cryogenic distillation and solvent extraction, are energy-intensive and cause serious environmental stress. Moreover, the growth of industries and technologies and the greater requirements for products (e.g., purity) lead to challenges that cannot be met using traditional separation methods. Adsorptive and membrane-based separations are recognized as energy-efficient alternatives by which to revolutionize the current energy-intensive conditions and satisfy the new demands. This critical review presents the recent progress in metal-organic frameworks (MOFs) and related membranes (e.g., continuous MOF membranes and mixed-matrix membranes) for hydrocarbon separation. The contributions of the underlying separation mechanisms (e.g., enthalpy-driven thermodynamic equilibrium, molecular sieving, kinetic separation based on molecular size, and combined mechanisms) and the adopted strategies (e.g., defect and microstructure control, membrane thickness and interfacial compatibility) to the breaking of trade-off (e.g., permeability/selectivity and capacity/selectivity) and the design of novel materials and processing technologies are discussed. Moreover, this review also summarizes the potential barriers that exist from the academic to the ultimate industrial implementations and the prospects of future development.

Energy-efficient separation alternatives: metal-organic frameworks and membranes for hydrocarbon separation.

Isoreticular chemistry within metal–organic frameworks for gas storage and separation

Abstract The separation of C 2 H 2 /CO 2 is not only industrially important for acetylene purification but also scientifically challenging owing to their high similarities in physical properties and molecular sizes. Ultramicroporous metal-organic frameworks (MOFs) can exhibit a pore confinement effect to differentiate gas molecules of similar size. Herein, we report the fine-tuning of pore sizes in sub-nanometer scale on a series of isoreticular MOFs that can realize highly efficient C 2 H 2 /CO 2 separation. The subtle structural differences lead to remarkable adsorption performances enhancement. Among four MOF analogs, by integrating appropriate pore size and specific binding sites, [Cu(dps) 2 (SiF 6 )] (SIFSIX-dps-Cu, SIFSIX = SiF 6 2- , dps = 4.4’-dipyridylsulfide, also termed as NCU-100) exhibits the highest C 2 H 2 uptake capacity and C 2 H 2 /CO 2 selectivity. At room temperature, the pore space of SIFSIX-dps-Cu significantly inhibits CO 2 molecules but takes up a large amount of C 2 H 2 (4.57 mmol g −1 ), resulting in a high IAST selectivity of 1787 for C 2 H 2 /CO 2 separation. The multiple host-guest interactions for C 2 H 2 in both inter- and intralayer cavities are further revealed by dispersion-corrected density functional theory and grand canonical Monte Carlo simulations. Dynamic breakthrough experiments show a clean C 2 H 2 /CO 2 separation with a high C 2 H 2 working capacity of 2.48 mmol g −1 . 

/pdf/fine-pore-engineering-in-a-series-of-isoreticular-metal-288i6sdt.pdf

Fine pore engineering in a series of isoreticular metal-organic frameworks for efficient C2H2/CO2 separation

In comparison with the fast development of binary mixture separations, ternary mixture separations are significantly more difficult and have rarely been realized by a single material. Herein, a new strategy of tuning the gate-opening pressure of flexible MOFs is developed to tackle such a challenge. As demonstrated by a flexible framework NTU-65, the gate-opening pressure of ethylene (C2 H4 ), acetylene (C2 H2 ), and carbon dioxide (CO2 ) can be regulated by temperature. Therefore, efficient sieving separation of this ternary mixture was realized. Under optimized temperature, NTU-65 adsorbed a large amount of C2 H2 and CO2 through gate-opening and only negligible amount of C2 H4 . Breakthrough experiments demonstrated that this material can simultaneously capture C2 H2 and CO2 , yielding polymer-grade (>99.99 %) C2 H4 from single breakthrough separation.

Tuning Gate-Opening of a Flexible Metal-Organic Framework for Ternary Gas Sieving Separation.

Ethylene production from C2 hydrocarbon mixtures through one separation step is desirable but challenging because of the similar size and physical properties of acetylene, ethylene, and ethane. Herein, we report three new isostructural porous coordination networks (NPU-1, NPU-2, NPU-3; NPU represents Northwestern Polytechnical University) that are sustained by 9-connected nodes based upon a hexanuclear metal cluster of composition [Mn6(μ3-O)2(CH3COO)3]6+. NPU-1/2/3 exhibit a dual cage structure that was systematically fine-tuned in terms of cage size to realize selective adsorption of C2H2 and C2H6 over C2H4. Dynamic breakthrough experiments demonstrated that NPU-1 produces ethylene in >99.9% purity from a three-component gas mixture (1:1:1 C2H2/C2H4/C2H6). Molecular modeling studies revealed that the dual adsorption preference for C2H2 and C2H6 over C2H4 originates from (a) strong hydrogen-bonding interactions between electronegative carboxylate O atoms and C2H2 molecules in one cage and (b) multiple non-covalent interactions between the organic linkers of the host network and C2H6 molecules in the second cage.

Pore engineering for one-step ethylene purification from a three-component hydrocarbon mixture

Three-dimensional metal−organic frameworks (MOFs) are cutting-edge materials in the adsorptive removal of trace gases due to the availability of abundant pores with specific chemistry. However, the development of ideal adsorbents combining high adsorption capacity with high selectivity and stability remains challenging. Here we demonstrate a strategy to design adsorbents that utilizes the tunability of interlayer and intralayer space of two-dimensional fluorinated MOFs for capturing acetylene from ethylene. Validated by X-ray diffraction and modeling, a systematic variation of linker atom oxidation state enables fine regulation of layer stacking pattern and linker conformation, which affords a strong interlayer trapping of molecules along with cooperative intralayer binding. The resultant robust materials (ZUL-100 and ZUL-200) exhibit benchmark capacity in the pressure range of 0.001–0.05 bar with high selectivity. Their efficiency in acetylene/ethylene separation is confirmed by breakthrough experiments, giving excellent ethylene productivities (121 mmol/g from 1/99 mixture, 99.9999%), even when cycled under moist conditions. Designing efficient adsorbents for trace gas removal remains a serious challenge. Here, the authors show promise in layered 2D metal−organic frameworks, often overlooked in favor of 3D frameworks, for separating trace acetylene from ethylene with enhanced performance and high stability.

/pdf/simultaneous-interlayer-and-intralayer-space-control-in-two-35zb4woiqt.pdf

Simultaneous interlayer and intralayer space control in two-dimensional metal-organic frameworks for acetylene/ethylene separation.

C2 -C3 alkyne/alkene separation is of great importance; however, designing materials for an efficient molecular sieving of alkenes from alkynes remains challenging. Now, two hydrolytically stable layered MOFs, [Cu(dps)2 (GeF6 )] (GeFSIX-dps-Cu, dps=4,4'-dipyridylsulfide) and [Zn(dps)2 (GeF6 )] (GeFSIX-dps-Zn), can achieve almost complete exclusion of both C3 H6 and C2 H4 from their alkyne analogues. GeFSIX-dps-Cu displays a notable advanced threshold pressure for alkynes adsorption and thus substantial uptakes at lower pressures, providing record C3 H4 /C3 H6 uptake ratios and capacity-enhanced C2 H2 /C2 H4 sieving for a wide composition range. Metal substitution (Zn to Cu) affords fine tuning of linker rotation and layer stacking, creating slightly expanded pore aperture and interlayer space coupled with multiple hydrogen-bonding sites, allowing easier entrance of alkyne while excluding alkene. Breakthrough experiments confirmed tunable sieving by these MOFs for C3 H4 /C3 H6 and C2 H2 /C2 H4 mixtures.

Molecular Sieving of C2 -C3 Alkene from Alkyne with Tuned Threshold Pressure in Robust Layered Metal-Organic Frameworks.

Incorporation of Fluorescent Carbon Quantum Dots into Metal–Organic Frameworks with Peroxidase-Mimicking Activity for High-Performance Ratiometric Fluorescent Biosensing

Shaped layered two-dimensional fluorinated metal-organic frameworks for highly efficient acetylene/ethylene separation

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Papers

Simultaneous interlayer and intralayer space control in two-dimensional metal-organic frameworks for acetylene/ethylene separation.

Molecular Sieving of C2 -C3 Alkene from Alkyne with Tuned Threshold Pressure in Robust Layered Metal-Organic Frameworks.

Incorporation of Fluorescent Carbon Quantum Dots into Metal–Organic Frameworks with Peroxidase-Mimicking Activity for High-Performance Ratiometric Fluorescent Biosensing

Shaped layered two-dimensional fluorinated metal-organic frameworks for highly efficient acetylene/ethylene separation