Chongli Zhong

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.

Experimental study on the direct liquefaction of Cunninghamia lanceolata in water

Direct liquefaction of Cunninghamia lanceolata in water was carried out in an autoclave in the reaction temperature and time ranges of 280–360° C and 10–30 min, respectively, and three feedstock ratios of biomass/water were tested. The experimental results show that the yield of heavy oil has a maximum between 320 and 340° C, depending on the reaction time and the feedstock ratio adopted. At the experimental conditions of this work, the maximum heavy oil yield is about 24%, obtained at the feedstock of 8 g biomass and 100 ml distilled water, with the reaction time of 10 min and the reaction temperature of 320° C.

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.

Large breathing of the MOF MIL-47(VIV) under mechanical pressure: a joint experimental–modelling exploration

A joint experimental–modelling study has demonstrated a large flexibility of the MIL-47(VIV) upon mechanical pressure which strongly deviates from its rigid behaviour in presence of guest molecules. A structural transition suspected by mercury intrusion and further confirmed by X-ray powder diffraction and molecular dynamics simulations, leads to a closed MIL-47(VIV) form never observed so far corresponding to a cell contraction of up to 43%. The microscopic key features that govern this transition are then elucidated from complementary Raman experiments.

Confinement of Ionic Liquids in Nanocages: Tailoring the Molecular Sieving Properties of ZIF-8 for Membrane-Based CO2 Capture

Fine-tuning of effective pore size of microporous materials is necessary to achieve precise molecular sieving properties. Herein, we demonstrate that room temperature ionic liquids can be used as cavity occupants for modification of the microenvironment of MOF nanocages. Targeting CO2 capture applications, we tailored the effective cage size of ZIF-8 to be between CO2 and N2 by confining an imidazolium-based ionic liquid [bmim][Tf2 N] into ZIF-8s SOD cages by in-situ ionothermal synthesis. Mixed matrix membranes derived from ionic liquid-modified ZIF-8 exhibited remarkable combinations of permeability and selectivity that transcend the upper bound of polymer membranes for CO2 /N2 and CO2 /CH4 separation. We observed an unusual response of the membranes to varying pressure, that is, an increase in the CO2 /CH4 separation factor with pressure, which is highly desirable for practical applications in natural gas upgrading.

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.