Data-Driven Discovery of Gas-Selective Organic Linkers in Metal-Organic Frameworks for the Separation of Ethylene and Ethane.

Metal-organic frameworks (MOFs) are potential candidates for gas-selective adsorbents for the separation of an ethylene/ethane mixture. To accelerate material discovery, high-throughput computational screening is a viable solution. However, classical force fields, which were widely employed in recent studies of MOF adsorbents, have been criticized for their failure to cover complicated interactions such as those involving π electrons. Herein, we demonstrate that machine learning force fields (MLFFs) trained on quantum-chemical reference data can overcome this difficulty. We have constructed a MLFF to accurately predict the adsorption energies of ethylene and ethane on the organic linkers of MOFs and discovered that the π electrons from both the ethylene molecule and the aromatic rings in the linkers could substantially influence the selectivity for gas adsorption. Four kinds of MOF linkers are identified as having promise for the separation of ethylene and ethane, and our results could also offer a new perspective on the design of MOF building blocks for diverse applications.

Graph Transformer with Convolution Parallel Networks for Predicting Single and Binary Component Adsorption Performance of Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) are considered one of the most important materials for carbon capture and storage (CCS) due to the advantages of porosity, multifunction, diverse structure, and controllable chemical composition. With the continuous development of artificial intelligence (AI) technology, more and more machine learning models are used to identify MOFs with high performance within a massive search space. However, current works have yet to form a model that uses graph-structured data only, which can predict the adsorption properties of single and binary components. In this work, we proposed and developed a graph transformer, combined with convolution parallel networks, called GC-Trans. The model can accurately and efficiently predict the adsorption performance of MOFs under the single- and binary-component adsorption conditions using only the features of the crystal diagram as inputs. By extracting and fusing local and global feature information, the model has stronger expression and generalization abilities. Thus, we used it to screen the ARC-MOF database and analyze the MOF structures that meet the target requirements. Additionally, to demonstrate the transferability of the model, we applied transfer learning methods to predict the CO2/CH4 separations and CH4 uptake, both of which showed good predictive performance.

Exquisitely Constructing a Robust MOF with Dual Pore Sizes for Efficient CO2 Capture.

Developing metal-organic framework (MOF) adsorbents with excellent performance and robust stability is of critical importance to reduce CO2 emissions yet challenging. Herein, a robust ultra-microporous MOF, Cu(bpfb)(bdc), with mixed ligands of N, N'-(1,4-phenylene)diisonicotinamide (bpfb), and 1,4-dicarboxybenzene (bdc) was delicately constructed. Structurally, this material possesses double-interpenetrated frameworks formed by two staggered, independent frameworks, resulting in two types of narrow ultra-micropores of 3.4 × 5.0 and 4.2 × 12.8 Å2, respectively. The above structural properties make its highly selective separation at 273~298 K with a CO2 capacity of 71.0~86.2 mg/g. Its adsorption heat over CO2 and IAST selectivity were calculated to be 27 kJ/mol and 52.2, respectively. Remarkably, cyclic breakthrough experiments corroborate its impressive performance in CO2/N2 separation in not only dry but also 75% RH humid conditions. Molecular simulation reveals that C-H···OCO2 in the pores plays a pivotal role in the high selectivity of CO2 adsorption. These results point out the huge potential application of this material for CO2/N2 separation.

A Robust Metal-Organic Framework with Scalable Synthesis and Optimal Adsorption and Desorption for Energy-Efficient Ethylene Purification.

Adsorptive separation is an energy-efficient alternative, but its advancement has been hindered by the challenge of industrially potential adsorbents development. Herein, a novel ultra-microporous metal-organic framework ZU-901 is designed that satisfies the basic criteria raised by ethylene/ethane (C2 H4 /C2 H6 ) pressure swing adsorption (PSA). ZU-901 exhibits an "S" shaped C2 H4 curve with high sorbent selection parameter (65) and could be mildly regenerated. Through green aqueous-phase synthesis, ZU-901 is easily scalable with 99 % yield, and it is stable in water, acid, basic solutions and cycling breakthrough experiments. Polymer-grade C2 H4 (99.51 %) could be obtained via a simulating two-bed PSA process, and the corresponding energy consumption is only 1/10 of that of simulating cryogenic distillation. Our work has demonstrated the great potential of pore engineering in designing porous materials with desired adsorption and desorption behavior to implement an efficient PSA process.

Upgrading Octane Number of Naphtha by a Robust and Easily Attainable Metal-Organic Framework through Selective Molecular Sieving of Alkane Isomers.

The separation of alkanes, particularly monobranched and dibranched isomers, is of paramount importance in the petrochemical industry for optimizing the feedstock of ethylene production as well as for upgrading the octane number of gasoline. Here, we report the full separation of linear/monobranched alkanes from their dibranched isomers by a robust and easily scalable metal-organic framework material, Co3 (HCOO)6 . The compound completely excludes dibranched alkanes but adsorbs their linear and monobranched isomers, as evidenced by single-component and multicomponent adsorption measurements. More importantly, the material exhibits excellent performance in separating naphtha and is capable of providing high quality feedstock for the production of ethylene and gasoline components with high octane number, making it a promising candidate for naphtha separation in petrochemical industry.

Splitting Mono- and Dibranched Alkane Isomers by a Robust Aluminum-Based Metal-Organic Framework Material with Optimal Pore Dimensions.

The separation of alkanes with different degrees of branching, particularly mono- and dibranched isomers, represents a challenging yet important industrial process for the production of premium gasoline blending components with high octane number. We report here the separation of linear/monobranched and dibranched alkanes through complete molecular sieving by a robust aluminum-based MOF material, Al-bttotb (H3bttotb = 4,4',4″-(benzene-1,3,5-triyltris(oxy))tribenzoicacid). Single- and multicomponent adsorption experiments reveal that the material adsorbs linear and monobranched alkanes, but fully excludes their dibranched isomers. Adsorption sites of alkanes within the MOF channels have been identified by single-crystal X-ray diffraction studies, and the adsorption mechanism was explored through computational calculations and modeling. The highly selective adsorption of the MOF should be attributed to its optimal pore dimensions.

Location matters: cooperativity of catalytic partners in porous organic polymers for enhanced CO2 transformation.

Herein, we show that by following molecular engineering of the inter-site distance between the two functionalities in porous organic materials, it is possible to enable them to work in a concerted manner. Specifically, the activity can be amplified by the placement of the hydroxyl group in the meta position of the phosphonium salts in the representative cycloaddition of epoxides and CO2.

Chromophore-Based Luminescent Metal-Organic Frameworks as Lighting Phosphors.

Energy-efficient solid-state-lighting (SSL) technologies are rapidly developing, but the lack of stable, high-performance rare-earth free phosphors may impede the growth of the SSL market. One possible alternative is organic phosphor materials, but these can suffer from lower quantum yields and thermal instability compared to rare-earth phosphors. However, if luminescent organic chromophores can be built into a rigid metal-organic framework, their quantum yields and thermal stability can be greatly improved. This Forum Article discusses the design of a group of such chromophore-based luminescent metal-organic frameworks with exceptionally high performance and rational control of the important parameters that influence their emission properties, including electronic structures of chromophore, coligands, metal ions, and guest molecules.

Direct structural evidence of commensurate-to-incommensurate transition of hydrocarbon adsorption in a microporous metal organic framework.

The efficiency of physisorption-based separation of gas-mixtures depends on the selectivity of adsorbent which is directly linked to size, shape, polarizability and other physical properties of adsorbed molecules. Commensurate adsorption is an interesting and important adsorption phenomenon, where the adsorbed amount, location, and orientation of an adsorbate are commensurate with the crystal symmetry of the adsorbent. Understanding this phenomenon is important and beneficial as it can provide vital information about adsorbate-adsorbent interaction and adsorption-desorption mechanism. So far, only sporadic examples of commensurate adsorption have been reported in porous materials such as zeolites and metal organic frameworks (MOFs). In this work we show for the first time direct structural evidence of commensurate-to-incommensurate transition of linear hydrocarbon molecules (C2-C7) in a microporous MOF, by employing a number of analytical techniques including single crystal X-ray diffraction (SCXRD), in situ powder X-ray diffraction coupled with differential scanning calorimetry (PXRD-DSC), gas adsorption and molecular simulations.

The moisture-triggered controlled release of a natural food preservative from a microporous metal-organic framework.

In this work we demonstrate that allyl isothiocyanate (AITC), a common food flavoring agent and food preservative, can be effectively captured by and released in a controlled manner from a microporous metal-organic framework (MOF). The extent of AITC-MOF interactions is quantitatively measured by orbital overlap population calculations. Controlled release experiments show that loaded AITC can be released by applying higher relative humidity. Further analysis reveals that the underlying mechanism of the controlled release is associated with the transformation of the MOF from a porous to a nonporous structure at high humidity. This study represents the first example of making use of MOF porosity in food preservation.

Effective Detection of Mycotoxins by a Highly Luminescent Metal-Organic Framework.

We designed and synthesized a new luminescent metal-organic framework (LMOF). LMOF-241 is highly porous and emits strong blue light with high efficiency. We demonstrate for the first time that very fast and extremely sensitive optical detection can be achieved, making use of the fluorescence quenching of an LMOF material. The compound is responsive to Aflatoxin B1 at parts per billion level, which makes it the best performing luminescence-based chemical sensor to date. We studied the electronic properties of LMOF-241 and selected mycotoxins, as well as the extent of mycotoxin-LMOF interactions, employing theoretical methods. Possible electron and energy transfer mechanisms are discussed.

Influence of gas packing and orientation on FTIR activity for CO chemisorption to the Cu paddlewheel.

In situ Fourier-transform infrared (FTIR) spectroscopy is able to probe structural defects via site-specific adsorption of CO to the Cu-BTC (BTC = 1,3,5-benzenetricarboxylate) metal-organic framework (MOF). The temperature-programmed desorption (TPD) of CO chemisorbed to Cu-TDPAT (TDPAT = 2,4,6-tris(3,5-dicarboxylphenylamino)-1,3,5-triazine) is virtually identical to Cu-BTC, suggesting CO chemisorbs to the open metal site at the axial position of the copper paddlewheel that is the building unit of both MOFs. Yet, despite an increased gravimetric CO : Cu ratio, CO chemisorbed to Cu-TDPAT is FTIR inactive. We rule out the presence of residual solvent, thermal degradation, adsorption temperature, and ligand-induced electronic effects at the adsorption site. TPD at increased pressure suggests the multiple CO per Cu site rearrange in Cu-TDPAT as a dynamic function of temperature and pressure. Thus, the FTIR inactivity of CO chemisorbed to Cu-TDPAT is attributed to orientation and/or packing of the CO relative to the Cu binding site. The results suggest dynamic chemisorption complicate extension of a site-specific in situ FTIR probe of gas adsorption. For both Cu-BTC and Cu-TDPAT, the in situ FTIR probe is a less sensitive probe of defects than X-ray photoelectron spectroscopy and nitrogen adsorption.

Evidence of Amine-CO2 Interactions in Two Pillared-Layer MOFs Probed by X-ray Crystallography.

Two pillared-layer metal-organic frameworks (MOFs; PMOF-55 and NH2 -PMOF-55) based on 1,2,4-triazole and terephthalic acid (bdc)/NH2 -bdc ligands were assembled and display framework stabilities, to a certain degree, in both acid/alkaline solutions and toward water. They exhibit high CO2 uptakes and selective CO2 /N2 adsorption capacities, with CO2 /N2 selectivity in the range of 24-27, as calculated by the ideal adsorbed solution theory method. More remarkably, the site and interactions between the host network and the CO2 molecules were investigated by single-crystal X-ray diffraction, which showed that the main interaction between the CO2 molecules and PMOF-55 is due to multipoint supramolecular interactions of C-H⋅⋅⋅O, C⋅⋅⋅O, and O⋅⋅⋅O. Amino functional groups were shown to enhance the CO2 adsorption and identified as strong adsorption sites for CO2 by X-ray crystallography.

Solution processable MOF yellow phosphor with exceptionally high quantum efficiency.

An important aspect in the research and development of white light-emitting diodes (WLEDs) is the discovery of highly efficient phosphors free of rare-earth (RE) elements. Herein we report the design and synthesis of a new type of RE-free, blue-excitable yellow phosphor, obtained by combining a strongly emissive molecular fluorophore with a bandgap modulating co-ligand, in a three-dimensional metal organic framework. [Zn6(btc)4(tppe)2(DMA)2] (btc = benzene-1,3,5-tricarboxylate, tppe = 1,1,2,2-tetrakis(4-(pyridin-4-yl)phenyl)ethene, DMA = dimethylacetamide) crystallizes in a new structure type and emits bright yellow light when excited by a blue light source. It possesses the highest internal quantum yield among all RE-free, blue-excitable yellow phosphors reported to date, with a value as high as 90.7% (λex = 400 nm). In addition to its high internal quantum yield, the new yellow phosphor also demonstrates high external quantum yield, luminescent and moisture stability, solution processability, and color tunability, making it a promising material for use in phosphor conversion WLEDs.

From 1D chain to 3D network: a new family of inorganic-organic hybrid semiconductors MO3(L)(x) (M = Mo, W; L = organic linker) built on perovskite-like structure modules.

MO3 (M = Mo, W) or VI-VI binary compounds are important semiconducting oxides that show great promise for a variety of applications. In an effort to tune and enhance their properties in a systematic manner we have applied a designing strategy to deliberately introduce organic linker molecules in these perovskite-like crystal lattices. This approach has led to a wealth of new hybrid structures built on one-dimensional (1D) and two-dimensional (2D) VI-VI modules. The hybrid semiconductors exhibit a number of greatly improved properties and new functionality, including broad band gap tunability, negative thermal expansion, largely reduced thermal conductivity, and significantly enhanced dielectric constant compared to their MO3 parent phases.

Design and synthesis of new 1D and 2D R-isophthalic acid-based coordination polymers (R = hydrogen or bromine).

Three new R-isophthalic acid-based (R = H or Br) coordination polymers have been designed and synthesized. By changing the N-containing ligand in the system, we are able to tune the dimensionality of coordination polymers from one-dimension (1D) to two-dimensions (2D) with the same basic building unit. Also, different metal ions can be incorporated into the same structures. Compound 1 [Cu(bipa)(py)2]·0.5(H2O) (H2bipa = 5-bromoisophthalic acid; py = pyridine) and compound 2 [Co(bipa)(py)2] are 1D chain structures. Compound 3 [Cu8(ipa)8(bpe)8]·2(bpe)·4(H2O) (bpe=1,2-bis(4-pyridyl)ethane) is a 2D layered structure.

MOFs for CO2 capture and separation from flue gas mixtures: the effect of multifunctional sites on their adsorption capacity and selectivity.

Microporous metal-organic frameworks (MOFs) have attracted tremendous attention because of their versatile structures and tunable porosity that allow almost unlimited ways to improve their properties and optimize their functionality, making them very promising for a variety of important applications, especially in the adsorption and separation of small gases and hydrocarbons. Numerous studies have demonstrated that MOFs with multifunctional groups, such as open metal sites (OMSs) and Lewis basic sites (LBSs), interact strongly with carbon dioxide and are particularly effective in its capture and separation from binary mixtures of CO(2) and N(2). In this feature article, we briefly review the current state of MOF development in this area, with an emphasis on the effect of multifunctional groups on the selectivity and capacity of MOFs for the CO(2) capture from flue gas mixtures.

A multifunctional organic-inorganic hybrid structure based on Mn(III)-porphyrin and polyoxometalate as a highly effective dye scavenger and heterogenous catalyst.

A two-step synthesis strategy has led to a unique layered polyoxometalate-Mn(III)-metalloporphyrin-based hybrid material. The hybrid solid demonstrates remarkable capability for scavenging of dyes and for heterogeneous selective oxidation of alkylbenzenes with excellent product yields and 100% selectivity.

CO catalytic oxidation by a metal organic framework containing high density of reactive copper sites.

A new metal organic framework containing high density of active Cu sites demonstrates 100% oxidative conversion of CO to CO(2).