Dahuan Liu

Influence of framework metal ions on the dye capture behavior of MIL-100 (Fe, Cr) MOF type solids

MIL-100(Fe) shows large adsorption uptakes for both MO and MB, while MIL-100(Cr) can selectively adsorb MB from a MO–MB mixture. This work demonstrates that MOFs are promising adsorbents for dye capture and highlights that framework metal ion replacement is an efficient way to tailor MOFs for different applications in liquid separation.

Li-modified metal–organic frameworks for CO2/CH4 separation: a route to achieving high adsorption selectivity

In this work three Li-modified metal–organic frameworks (MOFs) were constructed from MOF-5, by substituting the H atoms with O–Li groups in the organic linkers. A multiscale approach combining grand canonical Monte Carlo (GCMC) simulation and density functional theory (DFT) calculation was adopted to investigate the separation of CO2/CH4 mixtures in these new Li-modified MOFs, as well as in a previously proposed Li-doped MOF-5 for hydrogen storage and the original MOF-5. The results show that the selectivity of CO2 from CH4/CO2 mixtures in Li-modified MOFs is greatly improved, due to the enhancement of electrostatic potential in the materials by the presence of the metals. One of the new Li-modified MOFs, chem-4Li, shows a higher CO2 selectivity than any other known MOFs. Therefore, this work provides a route to improve the separation performance of MOFs for gas mixtures with components that have large differences in dipole and/or quadrupole moments. In addition, the mechanisms for selectivity enhancement in the Li-modified MOFs were elucidated at the molecular level, and we found that the location of doped metals can change the adsorption sites for CO2, and in turn may change the active sites in MOFs when used as catalysts.

Revealing the structure-property relationships of metal-organic frameworks for CO2 capture from flue gas.

It is of great importance to establish a quantitative structure-property relationship model that can correlate the separation performance of MOFs to their physicochemical features. In complement to the existing studies that screened the separation performance of MOFs from the adsorption selectivity calculated at infinite dilution, this work aims to build a QSPR model that can account for the CO(2)/N(2) mixture (15:85) selectivity of an extended series of MOFs with a very large chemical and topological diversity under industrial pressure condition. It was highlighted that the selectivity for this mixture under such conditions is dominated by the interplay of the difference of the isosteric heats of adsorption between the two gases and the porosity of the MOF adsorbents. On the basis of the interplay map of both factors that impact the adsorption selectivity, strategies were proposed to efficiently enhance the separation selectivity of MOFs for CO(2) capture from flue gas. As a typical illustration, it thus leads us to tune a new MOF with outstanding separation performance that will orientate the synthesis effort to be deployed.

Understanding gas separation in metal–organic frameworks using computer modeling

Metal–organic frameworks (MOFs) are a new family of nanoporous materials that combine the advantages of both inorganic and organic materials with great variety in functionality, pore size and topology. Gas separation is one of the fields that the first practical application of MOFs may be applied to; however, the study of MOFs as adsorbents in gas separation is still in its early stage, and their separation characteristics are not quite clear. Here, we summarize the recent advances on gas separation in MOFs using computer modeling, and show how computer modeling can help to understand the separation characteristics of MOFs. In addition, several strategies are proposed to improve the separation efficiency of MOFs, which are expected to be useful for designing new MOFs with improved separation performance for targeted properties.

An in situ self-assembly template strategy for the preparation of hierarchical-pore metal-organic frameworks

Metal-organic frameworks (MOFs) have recently emerged as a new type of nanoporous materials with tailorable structures and functions. Usually, MOFs have uniform pores smaller than 2 nm in size, limiting their practical applications in some cases. Although a few approaches have been adopted to prepare MOFs with larger pores, it is still challenging to synthesize hierarchical-pore MOFs (H-MOFs) with high structural controllability and good stability. Here we demonstrate a facile and versatile method, an in situ self-assembly template strategy for fabricating stable H-MOFs, in which multi-scale soluble and/or acid-sensitive metal-organic assembly (MOA) fragments form during the reactions between metal ions and organic ligands (to construct MOFs), and act as removable dynamic chemical templates. This general strategy was successfully used to prepare various H-MOFs that show rich porous properties and potential applications, such as in large molecule adsorption. Notably, the mesopore sizes of the H-MOFs can be tuned by varying the amount of templates.

A Reversible Crystallinity-Preserving Phase Transition in Metal–Organic Frameworks: Discovery, Mechanistic Studies, and Potential Applications

A quenching-triggered reversible single-crystal-to-single-crystal (SC-SC) phase transition was discovered in a metal-organic framework (MOF) PCN-526. During the phase transition, the one-dimensional channel of PCN-526 distorts from square to rectangular in shape while maintaining single crystallinity. Although SC-SC transformations have been frequently observed in MOFs, most reports have focused on describing the resulting structural alterations without shedding light on the mechanism for the transformation. Interestingly, modifying the occupancy or species of metal ions in the extra-framework sites, which provides mechanistic insight into the causes for the transformation, can forbid this phase transition. Moreover, as a host scaffold, PCN-526 presents a platform for modulation of the photoluminescence properties by encapsulation of luminescent guest molecules. Through judicious choice of these guest molecules, responsive luminescence caused by SC-SC transformations can be detected, introducing a new strategy for the design of novel luminescent MOF materials.

Covalent Triazine-Based Frameworks with Ultramicropores and High Nitrogen Contents for Highly Selective CO2 Capture

Porous organic frameworks (POFs) are a class of porous materials composed of organic precursors linked by covalent bonds. The objective of this work is to develop POFs with both ultramicropores and high nitrogen contents for CO2 capture. Specifically, two covalent triazine-based frameworks (CTFs) with ultramicropores (pores of width <7 Å) based on short (fumaronitrile, FUM) and wide monomers (1,4-dicyanonaphthalene, DCN) were synthesized. The obtained CTF-FUM and CTF-DCN possess excellent chemical and thermal stability with ultramicropores of 5.2 and 5.4 Å, respectively. In addition, they exhibit excellent ability to selectively capture CO2 due to ultramicroporous nature. Especially, CTF-FUM-350 has the highest nitrogen content (27.64%) and thus the highest CO2 adsorption capacity (57.2 cc/g at 298 K) and selectivities for CO2 over N2 and CH4 (102.4 and 20.5 at 298 K, respectively) among all CTF-FUM and CTF-DCN. More impressively, as far as we know, the CO2/CH4 selectivity is larger than that of all reported CTFs and ranks in top 10 among all reported POFs. Dynamic breakthrough curves indicate that both CTFs could indeed separate gas mixtures of CO2/N2 and CO2/CH4 completely.

Fabrication of mixed-matrix membrane containing metal–organic framework composite with task-specific ionic liquid for efficient CO2 separation

Mixed-matrix membranes (MMMs) have exhibited advantages in membrane-based gas separation in recent years, however, there is still intensive demand for the development of a proper method to design effective fillers to further enhance the gas separation performance of MMMs. In this work, a nanoporous material to selectively facilitate CO2 transport was proposed through the loading of a task-specific ionic liquid (TSIL) into a metal–organic framework (MOF). [C3NH2bim][Tf2N] and NH2-MIL-101(Cr) were selected as a demonstrative TSIL and MOF, respectively. The amine-containing TSIL worked as a selective CO2 transport carrier, which can be beneficial for the improvement of CO2 permeability and CO2/N2 selectivity. Simultaneously, NH2-MIL-101(Cr) is an appropriate porous host material that can control the good dispersion of TSIL and can effectively expose more active adsorption sites of the TSIL. Meanwhile, the amine-containing porous MOF is helpful for rapid CO2 transport and further increases the CO2 permeability. We further incorporated the porous composite into PIM-1 to fabricate MMMs with different loadings. The prepared TSIL@NH2-MIL-101(Cr)/PIM-1 membrane exhibits largely improved gas permeability and selectivity for CO2/N2 separation, with CO2 permeation values of 2979 Barrer and a CO2/N2 separation selectivity of 37 at 5 wt% loading. Compared with NH2-MIL-101(Cr)/PIM-1 and PIM-1 membranes, the CO2/N2 separation selectivity was increased by 116% and 119%, respectively, at the same loading.

Direct Measurement of Adsorbed Gas Redistribution in Metal–Organic Frameworks

Knowledge about the interactions between gas molecules and adsorption sites is essential to customize metal-organic frameworks (MOFs) as adsorbents. The dynamic interactions occurring during adsorption/desorption working cycles with several states are especially complicated. Even so, the gas dynamics based upon experimental observations and the distribution of guest molecules under various conditions in MOFs have not been extensively studied yet. In this work, a direct time-resolved diffraction structure envelope (TRDSE) method using sequential measurements by in situ synchrotron powder X-ray diffraction has been developed to monitor several gas dynamic processes taking place in MOFs: infusion, desorption, and gas redistribution upon temperature change. The electron density maps indicate that gas molecules prefer to redistribute over heterogeneous types of sites rather than to exclusively occupy the primary binding sites. We found that the gas molecules are entropically driven from open metal sites to larger neighboring spaces during the gas infusion period, matching the localized-to-mobile mechanism. In addition, the partitioning ratio of molecules adsorbed at each site varies with different temperatures, as opposed to an invariant distribution mode. Equally important, the gas adsorption in MOFs is intensely influenced by the gas-gas interactions, which might induce more molecules to be accommodated in an orderly compact arrangement. This sequential TRDSE method is generally applicable to most crystalline adsorbents, yielding information on distribution ratios of adsorbates at each type of site.

A force field for dynamic Cu-BTC metal-organic framework

A new force field that can describe the flexibility of Cu-BTC metal-organic framework (MOF) was developed in this work. Part of the parameters were obtained using density functional theory calculations, and the others were taken from other force fields. The new force field could reproduce well the experimental crystal structure, negative thermal expansion, vibrational properties as well as adsorption behavior in Cu-BTC. In addition, the bulk modulus of Cu-BTC was predicted using the new force field. We believe the new force field is useful in understanding the structure-property relationships for MOFs, and the approach can be extended to other MOFs.