Microregulation of Pore Channels in Covalent-Organic Frameworks Used for the Selective and Efficient Separation of Ethane

Systemic regulation of binding sites in porous coordination polymers for ethylene purification from ternary C2 hydrocarbons

The global demand for poly-grade ethylene (C2H4) is increasing annually. However, the energy-saving purification of this gas remains a major challenge due to the similarity in molecular properties among the ternary C2 hydrocarbons. To address this challenge, we report an approach of systematic tuning of the pore environment with organic sites (from -COOH to -CF3, then to -CH3) in porous coordination polymers (PCPs), of which NTU-73-CH3 shows remarkable capability for the direct production of poly-grade C2H4 from ternary C2 hydrocarbons under ambient conditions. In comparison, the precursor structure of NTU-73-COOH is unable to purify C2H4, while NTU-73-CF3 shows minimal ability to harvest C2H4. This is because the changed binding sites in the NTU-73-series not only eliminate the channel obstruction caused by the formation of gas clusters, but also enhance the interaction with acetylene (C2H2) and ethane (C2H6), as validated by in situ crystallographic and Raman analysis. Our findings, in particular the systematic tuning of the pore environment and the efficient C2H4 purification by NTU-73-CH3, provide a blueprint for the creation of advanced porous families that can handle desired tasks.

Hyper-Cross-Linked Resin Modified by a Micropore Polymer for Gas Adsorption and Separation

Polymerization confined to the pore was first adapted for the nanoscale structure adjustment of adsorption resin. The self-cross-linked polymer (P-1) formed in the pore of hyper-cross-linked resin (HR) by the Friedel-Crafts reaction of p-dichloroxylene (p-DCX), occupying the macropore of the HR resin and bringing about an external micropore. Compared with the raw HR resin, the volume of the micropore of HR@P-1 in 0.4 < D < 1 nm increased but the volume of the macropore has obviously decreased. After the loading of P-1 in the nanopore of HR, HR@P-1 has better gas adsorption performance. At 298 and 100 KPa, the adsorption capacity of CO2 is almost 30% higher than that of HR, reaching 35.7 cm3/g, due to the increase in the smaller micropore volume. Moreover, HR@P-1 has also been found to be the first C2H6-selective adsorption resin. The uptake of C2H6 is up to 56 cm3/g, and the IAST selectivity of C2H6/CH4 reaches 15.3. HR@P-1 can also separate syngas efficiently at ambient temperature and be regenerated by simple vacuum operation.

Integrating Multiscale Simulation with Machine Learning to Screen and Design FIL@COFs for Ethane-Selective Separation

Efficient and economical separation of C2H6/C2H4 is an imperative and extremely challenging process in the petrochemical industry. The C2H6-selective adsorbents with high working capacity and high selectivity are highly desirable from a practical application standpoint. In this study, we constructed a database of fluorinated ionic liquid@covalent organic frameworks (FIL@COFs) and screened out the high-performing FIL@COFs for C2H6-selective separation. Utilizing the optimal machine learning (ML) algorithm (XGBoost) and hyperparameters, we further revealed the key factors influencing the separation performance. The multiscale simulation not only validated the prediction accuracy of ML but also demonstrated that adjusting the largest cavity diameter of COFs with FILs could yield FIL@COFs with high performance for C2H6-selective separation. Our work provides essential guidance for designing new FIL@COF adsorbents for value-added gas purification.

A Novel Homoconjugated Propellane Triimide: Synthesis, Structural Analyses, and Gas Separation

Rigid three-dimensional (3D) polycyclic propellanes have garnered interest due to their unique conformational spaces, which display great potential use in selectivity, separation and as models to study through-space electronic interactions. Herein we report the synthesis of a novel rigid propellane, trinaphtho[3.3.3]propellane triimide, which comprises three imide groups embedded on a trinaphtho[3.3.3]propellane. This propellane triimide exhibits large bathochromic shift, amplified molar absorptivity, enhanced fluorescence, and lower reduction potential when compared to the subunits. Computational and experimental studies reveal that the effective through-space π-orbitals interacting (homoconjugation) occurs between the subunits. Single-crystal XRD analysis reveals that the propellane triimide has a highly quasi-D3h symmetric skeleton and readily crystallizes into different superstructures by changing alkyl chains at the imide positions. In particular, the porous 3D superstructure with S-shaped channels is promising for taking up ethane (C2H6) with very good selectivity over ethylene (C2H4), which can purify C2H4 from C2H6/C2H4 in a single separation step. This work showcases a new class of rare 3D polycyclic propellane with intriguing electronic and supramolecular properties.

Fine-tuning the pore environment of ultramicroporous three-dimensional covalent organic frameworks for efficient one-step ethylene purification

The construction of functional three-dimensional covalent organic frameworks (3D COFs) for gas separation, specifically for the efficient removal of ethane (C2H6) from ethylene (C2H4), is significant but challenging due to their similar physicochemical properties. In this study, we demonstrate fine-tuning the pore environment of ultramicroporous 3D COFs to achieve efficient one-step C2H4 purification. By choosing our previously reported 3D-TPB-COF-H as a reference material, we rationally design and synthesize an isostructural 3D COF (3D-TPP-COF) containing pyridine units. Impressively, compared with 3D-TPB-COF-H, 3D-TPP-COF exhibits both high C2H6 adsorption capacity (110.4 cm3 g-1 at 293 K and 1 bar) and good C2H6/C2H4 selectivity (1.8), due to the formation of additional C-H···N interactions between pyridine groups and C2H6. To our knowledge, this performance surpasses all other reported COFs and is even comparable to some benchmark porous materials. In addition, dynamic breakthrough experiments reveal that 3D-TPP-COF can be used as a robust absorbent to produce high-purity C2H4 directly from a C2H6/C2H4 mixture. This study provides important guidance for the rational design of 3D COFs for efficient gas separation.

Aminal-Linked Covalent Organic Frameworks with hxl-a and Quasi-hcb Topologies for Efficient C2 H6 /C2 H4 Separation

In reticular chemistry, topology is a powerful concept for defining the structures of covalent organic frameworks (COFs). However, due to the lack of diversity in the symmetry and reaction stoichiometry of the monomers, only 5% of the two-dimensional topologies have been reported to be COFs. To overcome the limitations of COF connectivity and pursue novel topologies in COF structures, two aminal-linked COFs, KUF-2 and KUF-3, are prepared, with dumbbell-shaped secondary building units. Linear dialdehydes and piperazine are condensed at a ratio of 1:2 to construct an aminal linkage, leading to unreported hxl-a (KUF-2) and quasi-hcb (KUF-3) structures. Notably, KUF-3 displays top-tier C2 H6 /C2 H4 selectivity and C2 H6 uptake at 298 K, outperforming most porous organic materials. The intrinsic aromatic ring-rich and Lewis basic pore environments, and appropriate pore widths enable the selective adsorption of C2 H6 , as confirmed by Grand Canonical Monte Carlo simulations. Dynamic breakthrough curves revealed that C2 H6 can be selectively separated from a gas mixture of C2 H6 and C2 H4 . This study suggests that topology-based design of aminal-COFs is an effective strategy for expanding the field of reticular chemistry and provides the facile integration of strong Lewis basic sites for selective C2 H6 /C2 H4 separation.

Thiazole functionalized covalent triazine frameworks for C2H6/C2H4 separation with remarkable ethane uptake

A highly stable thiazole functionalized covalent triazine framework, namely CTF-BT-500, was developed for C2H6/C2H4 separation, which exhibits a record-high ethane uptake (99.7 cm3 g-1) among all reported COFs at 298 K and 1 bar. This work not only presents an excellent C2H6-selective adsorbent, but also provides guidance for the construction of robust adsorbents for value-added gas purification.

High-Performance Adsorbent for Ethane/Ethylene Separation Selected through the Computational Screening of Aluminum-Based Metal-Organic Frameworks

The development of a high-performance ethane (C₂H₆)-selective adsorbent for the separation of ethane/ethylene (C₂H₆/C₂H₄) gas mixtures has been investigated for high-efficiency adsorption-based gas separation. Herein, we investigated Al-based metal-organic frameworks (MOFs) to identify an efficient C₂H₆-selective adsorbent (CAU-11), supported by a computational simulation study. CAU-11 exhibited numerous advantageous properties (such as low material cost, structural robustness, high reaction yield, and high C₂H₆/C₂H₄ selectivity) compared to other Al-based MOFs, indicating immense potential as a C₂H₆-selective adsorbent. CAU-11 exhibited preferential C₂H₆ adsorption in single-component gas adsorption experiments, and its predicted ideal adsorption solution theory selectivity of C₂H₆/C₂H₄ was over 2.1, consistent with the simulation analysis. Dynamic breakthrough experiments using representative compositions of the C₂H₆/C₂H₄ gas mixture confirmed the excellent separation ability of CAU-11; it produced high-purity C₂H₄ (>99.95%) with productivity values of 0.79 and 2.02 mol L^-1 while repeating the cyclic experiment with 1:1 and 1:15 v/v C₂H₆/C₂H₄ gas mixtures, respectively, at 298 K and 1 bar. The high C₂H₆/C₂H₄ separation ability of CAU-11 could be attributed to its non-polar pore environment and optimum pore dimensions which strengthen the interaction of its pores (via C-H···π interactions) with C₂H₆ to a greater extent than with C₂H₄.

One-Step Ethylene Purification from Ternary Mixtures in a Metal-Organic Framework with Customized Pore Chemistry and Shape

Adsorptive separation is an energy-efficient technology for the separation of C2 hydrocarbons. However, it remains a critical problem to directly produce high-purity C₂ H₄ from ternary C₂ H₂ /C₂ H₄ /C₂ H₆ mixtures by simultaneously trapping C₂ H₂ and C₂ H₆ . Herein, we report the one-step C₂ H₄ purification from the ternary mixture by a metal-organic framework Zn(ad)(int) (ad=adeninate; int=isonicotinate). The material combines dense heterocyclic rings and accessible uncoordinated O atoms as strong binding sites for C₂ H₆ and C₂ H₂ . Its spindle-like cage exhibits an interesting shape matching with the targeted molecules, affording Zn(ad)(int) not only high separation selectivity for C₂ H₆ /C₂ H₄ and C₂ H₂ /C₂ H₄ , but also excellent gas capacity. Breakthrough experiments show that polymer-grade C₂ H₄ can be separated from the ternary mixtures with a record productivity of 1.43 mmol g^-1 . In situ powder X-ray diffraction and Fourier transform infrared spectrum analyses further provide deep insights into the separation mechanism.

Assembling covalent organic framework membranes via phase switching for ultrafast molecular transport

Fabrication of covalent organic framework (COF) membranes for molecular transport has excited highly pragmatic interest as a low energy and cost-effective route for molecular separations. However, currently, most COF membranes are assembled via a one-step procedure in liquid phase(s) by concurrent polymerization and crystallization, which are often accompanied by a loosely packed and less ordered structure. Herein, we propose a two-step procedure via a phase switching strategy, which decouples the polymerization process and the crystallization process to assemble compact and highly crystalline COF membranes. In the pre-assembly step, the mixed monomer solution is casted into a pristine membrane in the liquid phase, along with the completion of polymerization process. In the assembly step, the pristine membrane is transformed into a COF membrane in the vapour phase of solvent and catalyst, along with the completion of crystallization process. Owing to the compact and highly crystalline structure, the resultant COF membranes exhibit an unprecedented permeance (water ≈ 403 L m^-2 bar^-1 h^-1 and acetonitrile ≈ 519 L m^-2 bar^-1 h^-1). Our two-step procedure via phase switching strategy can open up a new avenue to the fabrication of advanced organic crystalline microporous membranes.

Bottom-Up Synthesis of 8-Connected Three-Dimensional Covalent Organic Frameworks for Highly Efficient Ethylene/Ethane Separation

Developing cost-/energy-efficient separation techniques for purifying ethylene from an ethylene/ethane mixture is highly important but very challenging in the industrial process. Herein, using a bottom-up [8 + 2] construction approach, we rationally designed and synthesized three three-dimensional covalent organic frameworks (COFs) with 8-connected bcu networks, which can selectively remove ethane from an ethylene/ethane mixture with high efficiency. These COF materials, which are fabricated by the condensation reaction of a customer-designed octatopic aldehyde monomer with linear diamino linkers, possess high crystallinity, good structural robustness, and high porosity. Attributed to the well-organized micro-sized pores with a nonpolar/inert pore environment, these COFs display high ethane adsorption capacity and good selectivity over ethylene, making them among the best ethane-selective adsorbents for ethylene purification. Their excellent ethylene/ethane separation performance is validated by dynamic breakthrough experiments with high-purity ethylene (>99.99%) produced through a single adsorption process. The separation performance surpasses all reported C₂H₆-selective COFs and even some benchmark metal-organic frameworks. This work provides important guidance for the design of new adsorbents for value-added gas purification.

Rational Construction and Performance Regulation of an In(III)-Tetraisophthalate Framework for One-Step Adsorption-Phase Purification of C2H4 from C2 Hydrocarbons

The development of porous materials for ethylene (C₂H₄) separation and purification, a very important separation process in the chemical industry, is urgently needed but quite challenging. In particular, the realization of selectivity-reversed adsorption (namely, C₂H₄ is not preferentially adsorbed) and the simultaneous capture of multinary coexisting impurities such as ethane (C₂H₆) and acetylene (C₂H₂) will significantly simplify process design and reduce energy and cost consumption, but such porous materials are quite difficult to design and have not yet been fully explored. In this work, by employing an aromatic-rich bithiophene-based tetraisophthalate ligand, we solvothermally fabricated an anionic In(III)-based framework termed ZJNU-115 featuring In(COO)₄ as an inorganic secondary building unit as well as one-dimensional channels. Due to the absence of unsaturated metallic sites, together with aromatic-rich channel surface decorated with abundant hydrogen-bonding acceptors of carboxylate oxygen and thiophene sulfur atoms, desolvated ZJNU-115 exhibited an unusual adsorption relationship with respect to C₂ hydrocarbons, namely, simultaneous and preferable capture of C₂H₆ and C₂H₂ over C₂H₄ at the temperatures investigated, thus representing a rare metal-organic framework (MOF) with the promising potential for one-step adsorption-phase purification of C₂H₄ from a trinary C₂ hydrocarbon mixture. Compared to a few of the MOFs reported for such an application, ZJNU-115 displayed simultaneously good adsorption selectivities of both C₂H₂ and C₂H₆ over C₂H₄. Furthermore, its separation potential can be postsynthetically tailored by substituting dimethylammonium (Me₂NH₂⁺) counterions with tetraalkyl ammonium ions (NR₄⁺; R = Me, Et, or n-Pr). More importantly, ZJNU-115 was stable in various organic solvents as well as aqueous solutions with pH values ranging from 5 to 9, thus laying a solid foundation for its practical applications. The design principle and the performance regulation strategy adopted in this work will offer valuable guidance for the contrapuntal construction of porous MOFs employed for direct multicomponent purification of C₂H₄ with improved performance.

Construction of a novel 2D–2D heterojunction by coupling a covalent organic framework and In2S3 for photocatalytic removal of organic pollutants with high efficiency

TpPa-1 COF was coupled with an inorganic narrow-band gap semiconductor In2S3 to form a novel 2D–2D heterojunction by a facile hydrothermal method.