Interaction Of Guest Molecules Research Articles

Understanding the role of open metal sites in MOFs for the efficient separation of benzene/cyclohexane mixtures

Separating C6 cyclic hydrocarbons, specifically benzene and cyclohexane, presents a significant industrial challenge due to their similar physicochemical properties. We conducted Monte Carlo simulations in the Grand-Canonical ensemble to acquire adsorption properties and separation performance data for benzene and cyclohexane in three metal–organic frameworks featuring coordinatively unsaturated metal sites (Ni-MOF-74, Ni-ClBBTA, and Ni-ClBTDD). The separation performance of these MOFs was analyzed and compared with literature data for adsorbents of different natures, demonstrating superior performance. Additionally, we explored the molecular origins of this effective separation, examining the pore-filling mechanism, interaction of guest molecules with metal centers, and mutual interactions of each adsorbate. Our results highlight that the selected adsorbents, with remarkable loading capacity, can efficiently separate both compounds in a mixture with exceptional effectiveness.

A stable paddle-wheel Co-MOF (FNU-2) for the efficient separation of light hydrocarbons

The separation of methane from higher hydrocarbons (C2–C3) in natural gas processing is a crucial and energy-intensive operation. This work presents a comprehensive study on the separation of propane and ethane from methane using cobalt-based metal-organic frameworks (FNU-2), which exhibit exceptional stability and selectivity and preferential adsorption towards C2–C3 hydrocarbons. The selectivity values of C2H6/CH4 and C3H8/CH4 calculated by IAST are 43.9 and 638.9, respectively. According to theoretical calculations, the Van der Waals interactions of guest molecules play crucial roles in influencing the separation performance. Moreover, dynamic breakthrough experiments have revealed the promising potential of this adsorbent in industrial separations. These experiments indicate that the adsorbent is highly suitable for effectively separating and purifying natural gas. This study overall presents a novel and efficient adsorbent that can be employed to separate and purify natural gas in various industrial applications.

Research progress in AIE-based crystalline porous materials for biomedical applications

Crystalline porous materials (CPMs) not only present the precise integration of molecular building blocks into extensible structures with periodic frameworks and regular pores, but also provide limited molecular spaces for the interactions of guest molecules, electrons and photons. Incorporating aggregation-induced emission (AIE)-based units into crystalline porous frameworks can result in unique luminescent properties. AIE-based CPMs have widely tunable composition, high luminescent efficiency and good photo-stability, which make them useful for biomedical applications involving bio-sensing, bio-imaging and imaging-guided therapies. This review focused on structure design and luminescent property modulation of AIE-based CPMs with highlights on their applications in biomedical fields. The prospect and challenges in the development of AIE-based CPMs from chemistry, materials to biomedical applications were also discussed.

Evaluating the performance of Cr-Soc-MOF Super-Adsorbents for CO2 capture from flue gas under humid condition through molecular simulation

One of the major criticisms for gas separation by utilizing Metal Organic Framework (MOF) porous material for all classes of adsorbents is their very poor performance under humidified gas streams. Very few MOFs show better CO2 separation from N2 mixture in the presence of moisture. Conversely, plenty of MOFs demonstrated good CO2 capture from dry gas streams. In order to explore the potential characteristics of gas separation under humid conditions by coordinatively unsaturated metal organic framework (CUS-MOF) with square octahedral (Soc) topology were exercised single component (CO2, N2) adsorption and co-adsorption (CO2/N2) in the presence of moisture at 298 K with pressure ranging from 0 to 10 bar. Herein, we investigated five exceptional Cr-Soc-MOFs similar to the experimentally established iso-structural topology by differing the polynuclear aromatic ring size and N-heteroatom to their pore wall. The specific interaction of guest molecules to the coordinatively unsaturated metal site (CUS), i.e., rigid μ3-oxygen-centered tri-nuclear metal carboxylate, of Cr-Soc-MOF-n series were identified with Density Functional Theory (DFT) calculations, this energy profile was used to derive specific force field. Further, Grand canonical Monte Carlo (GCMC) simulation was employed with DFT-derived force field to identify suitable CO2 capture for Cr-Soc-MOFs in the presence of moisture. Interestingly, the Cr-Soc-MOFs with larger pore volume were retained not only CO2 uptake in the range of 23 – 35 wt% but also selectivity about 20 – 50 range even up to 70% of RH. Importantly, we introduced a mathematical tool to theoretically analyze the dynamic co-adsorption in the form of breakthrough curve measurement from CO2/N2 simulated co-adsorption by mimicking the real flue gas condition, also working capacity (ΔN), Regenerability (R%) and Adsorbent Performance Indicator (API) of Cr-Soc-MOF-n series were investigated. The CO2 separation performances of Cr-Soc-MOF-n series suggested that the polynuclear aromatic ring to their pore wall withstand CO2 uptake in the presence of humidity rather than N-heteroatom decorated Cr-Soc-MOF due to its high pore volume and the accommodation of extra aromatic ring in their pore channels.

Guest-Mediated Hierarchical Self-Assembly of Dissymmetric Organic Cages to Form Supramolecular Ferroelectrics

Guest-Mediated Hierarchical Self-Assembly of Dissymmetric Organic Cages to Form Supramolecular Ferroelectrics

Thermodynamically Consistent Methodology for Estimation of Diffusivities of Mixtures of Guest Molecules in Microporous Materials.

The Maxwell–Stefan (M–S) formulation, that is grounded in the theory of irreversible thermodynamics, is widely used for describing mixture diffusion in microporous crystalline materials such as zeolites and metal–organic frameworks (MOFs). Binary mixture diffusion is characterized by a set of three M–S diffusivities: Đ1, Đ2, and Đ12. The M–S diffusivities Đ1 and Đ2 characterize interactions of guest molecules with pore walls. The exchange coefficient Đ12 quantifies correlation effects that result in slowing-down of the more mobile species due to correlated molecular jumps with tardier partners. The primary objective of this article is to develop a methodology for estimating Đ1, Đ2, and Đ12 using input data for the constituent unary systems. The dependence of the unary diffusivities Đ1 and Đ2 on the pore occupancy, θ, is quantified using the quasi-chemical theory that accounts for repulsive, or attractive, forces experienced by a guest molecule with the nearest neighbors. For binary mixtures, the same occupancy dependence of Đ1 and Đ2 is assumed to hold; in this case, the occupancy, θ, is calculated using the ideal adsorbed solution theory. The exchange coefficient Đ12 is estimated from the data on unary self-diffusivities. The developed estimation methodology is validated using a large data set of M–S diffusivities determined from molecular dynamics simulations for a wide variety of binary mixtures (H2/CO2, Ne/CO2, CH4/CO2, CO2/N2, H2/CH4, H2/Ar, CH4/Ar, Ne/Ar, CH4/C2H6, CH4/C3H8, and C2H6/C3H8) in zeolites (MFI, BEA, ISV, FAU, NaY, NaX, LTA, CHA, and DDR) and MOFs (IRMOF-1, CuBTC, and MgMOF-74).

Synthetic Strategies and Structural Arrangements of Isoreticular Mixed-Component Metal-Organic Frameworks.

In recent years, the synthesis of mixed-metal and mixed-linker metal-organic frameworks with multiple metals and/or linker molecules combined in one framework has become a growing field of interest. These mixed-component or multivariate metal-organic framework materials provide the possibility to introduce multiple functionalities inside one framework. The interaction of guest molecules with different functionalities in the same material is a promising approach in the fields of gas storage, separation, catalysis and drug delivery. Furthermore, the combination of different components may lead to synergistic effects that cannot be achieved otherwise. These mixed-component approaches open up new pathways to an even larger range of possible customizations in the field of metal-organic frameworks.

Frontispiece: The Interaction of Guest Molecules with Co‐MOF‐74: A Vis/NIR and Raman Approach

Frontispiece: The Interaction of Guest Molecules with Co‐MOF‐74: A Vis/NIR and Raman Approach

The Interaction of Guest Molecules with Co-MOF-74: A Vis/NIR and Raman Approach.

Co-MOF-74 rod like crystals with a length of several hundred micrometers are synthesized by a solvothermal procedure and their interaction with different gases is evaluated for selective gas sensing. We show strongly anisotropic absorption behavior of the Co-MOF-74 crystals when illuminated with polarized light. The interactions of guests (CO2 , propane, propene, Ar, MeOH, H2 O) with Co-MOF-74, is studied by various spectroscopic techniques. Vis/NIR shows peak shifts of Co-MOF-74 depending on the interaction with the guest. In the visible and the NIR the maximum absorbance is shifted selectively corresponding to the intensity of the CoII -guest interaction. Even propene and propane could be distinguished at room temperature by their different interactions with Co-MOF-74. Raman spectroscopy was used to detect a modified vibrational behavior of Co-MOF-74 upon gas adsorption. We show that the adsorption of H2 O leads to a characteristic shift of the peak maxima in the Raman spectra.

Understanding the Reactive Adsorption of H2 S and CO2 in Sodium-Exchanged Zeolites.

Purifying sour natural gas streams containing hydrogen sulfide and carbon dioxide has been a long-standing environmental and economic challenge. In the presence of cation-exchanged zeolites, these two acid gases can react to form carbonyl sulfide and water (H2 S+CO2 ⇌H2 O+COS), but this reaction is rarely accounted for. In this work, we carry out reactive first-principles Monte Carlo (RxFPMC) simulations for mixtures of H2 S and CO2 in all-silica and Na-exchanged forms of zeolite beta to understand the governing principles driving the enhanced conversion. The RxFPMC simulations show that the presence of Na+ cations can change the equilibrium constant by several orders of magnitude compared to the gas phase or in all-silica beta. The shift in the reaction equilibrium is caused by very strong interactions of H2 O with Na+ that reduce the reaction enthalpy by about 20 kJ mol-1 . The simulations also demonstrate that the siting of Al atoms in the framework plays an important role. The RxFPMC method presented here is applicable to any chemical conversion in any confined environment, where strong interactions of guest molecules with the host framework and high activation energies limit the use of other computational approaches to study reaction equilibria.

Influence of Pore Dimension on the Host–Guest Interaction in Metal–Organic Frameworks

Atomistic simulations were performed in order to investigated the effect of pore dimension on the interaction of guest molecules with the inner surface of metal–organic frameworks (MOFs). In these systems, which only differ in the metric of their open structure, the chemical nature is conserved, and less impact on the host–guest interaction is expected compared to chemically funcionalized MOFs. However, by performing molecular dynamics simulations of benzene loaded MOF-5 derivatives (IRMOFs), which differ just in the length of the organic linker, it can be shown that impacts are present. The influence of the soft-modification can be explained only by a detailed analysis of the free energy topology and the diffusion mechanism. Note that the calculated self-diffusivity of benzene Dself shows no change with respect to the elongation of the linkers. The apparent contradiction between the macroscopic observable Dself and the microscopic free energy landscape could be resolved by introducing a hopping model for...

Chemistry of Metal-organic Frameworks Monitored by Advanced X-ray Diffraction and Scattering Techniques.

The research on metal-organic frameworks (MOFs) experienced rapid progress in recent years due to their structure diversity and wide range of application opportunities. Continuous progress of X-ray and neutron diffraction methods enables more and more detailed insight into MOF's structural features and significantly contributes to the understanding of their chemistry. Improved instrumentation and data processing in high-resolution X-ray diffraction methods enables the determination of new complex MOF crystal structures in powdered form. By the use of neutron diffraction techniques, a lot of knowledge about the interaction of guest molecules with crystalline framework has been gained in the past few years. Moreover, in-situ time-resolved studies by various diffraction and scattering techniques provided comprehensive information about crystallization kinetics, crystal growth mechanism and structural dynamics triggered by external physical or chemical stimuli. The review emphasizes most relevant advanced structural studies of MOFs based on powder X-ray and neutron scattering.

Competitive Coadsorption of CO2 with H2O, NH3, SO2, NO, NO2, N2, O2, and CH4 in M-MOF-74 (M = Mg, Co, Ni): The Role of Hydrogen Bonding

The importance of co-adsorption for applications of porous materials in gas separation has motivated fundamental studies, which have initially focused on the comparison of the binding energies of different gas molecules in the pores (i.e. energetics) and their overall transport. By examining the competitive co-adsorption of several small molecules in M-MOF-74 (M= Mg, Co, Ni) with in-situ infrared spectroscopy and ab initio simulations, we find that the binding energy at the most favorable (metal) site is not a sufficient indicator for prediction of molecular adsorption and stability in MOFs. Instead, the occupation of the open metal sites is governed by kinetics, whereby the interaction of the guest molecules with the MOF organic linkers controls the reaction barrier for molecular exchange. Specifically, the displacement of CO2 adsorbed at the metal center by other molecules such as H2O, NH3, SO2, NO, NO2, N2, O2, and CH4 is mainly observed for H2O and NH3, even though SO2, NO, and NO2, have higher binding energies (~70-90 kJ/mol) to metal sites than that of CO2 (38 to 48 kJ/mol) and slightly higher than water (~60-80 kJ/mol). DFT simulations evaluate the barriers for H2O->CO2 and SO2->CO2 exchange to be - 13 and 20 kJ/mol, respectively, explaining the slow exchange of CO2 by SO2, compared to water. Furthermore, the calculations reveal that the kinetic barrier for this exchange is determined by the specifics of the interaction of the second guest molecule (e.g., H2O or SO2) with the MOF ligands.

Evaluation of procedures for estimation of the isosteric heat of adsorption in microporous materials

The major objective of this communication is to evaluate procedures for estimation of the isosteric heat of adsorption, Qst, in microporous materials such as zeolites, metal organic frameworks (MOFs), and zeolitic imidazolate frameworks (ZIFs). For this purpose we have carefully analyzed published experimental data on adsorption isotherms at different temperatures, T, for a variety of guest molecules (water, methanol, ethanol, dimethylether, cyclohexane, benzene, toluene, xylene, ethylbenzene, ethyne, propene, propane, iso-butane, 1-butene, n-hexane) in several zeolites, MOFs, and ZIFs.Our analysis shows that when the binding energy with the structural framework is dominated by molecule–molecule interaction forces, Qst≈ΔHvap, the molar enthalpy of vaporization. In such cases, the molar loading qi at any temperature T and pressure pi, is a unique function of (pi/Pisat) where Pisat is the saturated vapor pressure at T.For most guest/host combinations, however, the Qst values are significantly higher than ΔHvap because of strong interactions of guest molecules with the material framework. For several guest/host combinations, a reasonable estimation of Qst values is obtained by addition of the Tóth potential to ΔHvap.

Microimaging of transient guest profiles to monitor mass transfer in nanoporous materials

The intense interactions of guest molecules with the pore walls of nanoporous materials is the subject of continued fundamental research. Stimulated by their thermal energy, the guest molecules in these materials are subject to a continuous, irregular motion, referred to as diffusion. Diffusion, which is omnipresent in nature, influences the efficacy of nanoporous materials in reaction and separation processes. The recently introduced techniques of microimaging by interference and infrared microscopy provide us with a wealth of information on diffusion, hitherto inaccessible from commonly used techniques. Examples include the determination of surface barriers and the sticking coefficient's analogue, namely the probability that, on colliding with the particle surface, a molecule may continue its diffusion path into the interior. Microimaging is further seen to open new vistas in multicomponent guest diffusion (including the detection of a reversal in the preferred diffusion pathways), in guest-induced phase transitions in nanoporous materials and in matching the results of diffusion studies under equilibrium and non-equilibrium conditions.

Theoretical investigation of structures and compositions of double neon-methane clathrate hydrates, depending on gas phase composition and pressure

The region of existence of neon clathrate hydrates is an actual problem of hydrate chemistry. The current work presents theoretical study of the equilibrium formation conditions of pure neon clathrate hydrates and double clathrate hydrates of neon-methane mixture. The structures and properties of double clathrate hydrates were described within the scope of the previously developed molecular clathrate hydrate model that takes into account the influence of guest molecules on the host lattice, interaction of guest molecules between themselves, and the possibility of multiple filling of host lattice cages by guest molecules. The model makes it possible to find an equilibrium state and thermodynamic properties of clathrate hydrates at given values of p and T. In the present work, we considered the properties of double clathrate hydrates in the range of pressures from 0 to 4 kbar at 250 K. The results of modeling have shown that the mass fraction of neon in double clathrate hydrate of Ne and CH4 mixture of cubic structure I (sI) can reach 26%, and 22.5% in double hydrate of cubic structure II (sII) even at a low methane concentration (1%) in gas phase, at high pressure. It is shown that in double clathrate hydrates of the Ne and CH4 mixture at high pressures, phase transition sII-sI can occur.

Recent developments in first-principles force fields for molecules in nanoporous materials

There is a great need to identify high-performance nanoporous materials for gas storage, separation and catalysis to solve a wide range of industrially relevant challenges. Molecular simulations have become an important complement to experiments in the screening and selection of suitable porous materials. Successful implementation of molecular simulations requires accurate force fields (FFs) to describe the interactions of guest molecules with porous frameworks. However, developing accurate and transferable FFs for porous materials can be extremely challenging. While generic FFs and experimentally-derived FFs work well for many simple systems, they often fail to describe interactions in more complex porous materials. Since first-principles quantum mechanical (QM) approaches are capable of accurately predicting intermolecular interactions, deriving FFs from QM data without experimental input is a promising solution. In this paper, we review recent developments in deriving first-principles FFs for molecules in a variety of nanoporous materials. We summarize and classify the methodologies used in these studies, and discuss the factors that are responsible for the accuracy and reliability of FFs. Finally, we conclude the review with a discussion of current challenges and future directions for this rapidly growing field.

Effect of monomeric sequence on transport properties of d-glucose and ascorbic acid in poly(VP-co-HEMA) hydrogels with various water contents: molecular dynamics simulation approach

We have used full-atomistic molecular dynamics (MD) simulations of both random and blocky sequence hydrogel networks of poly(N-vinyl-2-pyrrolidone-co-2-hydroxyethyl methacrylate) (P(VP-co-HEMA)) with a composition of VP/HEMA = 37:13 to investigate the effect of the monomeric sequence and the water content on the transport properties of ascorbic acid and d-glucose at 310.15 K. The degrees of randomness of the monomer sequence for the random and the blocky copolymers were 1.170 and 0.104, respectively, and the degree of polymerization was fixed at 50. By analyzing the pair correlation functions, it was found that for both monomeric sequences, the guest molecules (i.e., ascorbic acid and d-glucose) have greater accessibility to the VP units than to the HEMA units due to the higher hydrophilicity of VP compared to HEMA units. While the monomeric sequence effect on the P(VP-co-HEMA) hydrogel is clearly observed with 20 wt. % water content, the effect is significantly reduced with 40 wt. % water content and disappears completely with 80 wt. % water content. This is because the hydrophilic guest molecules are more likely to be associated with water molecules than with the polymer network at the high water content. By analyzing the diffusion of the guest molecules and the inner-surface area, it is also found that the guest molecules are confined in the system at 20 wt. % water content, resulting in highly anomalous sub-diffusion. Therefore, at low water content, the diffusion of the guest molecules in the hydrogel is directly affected by the monomeric sequence through the interaction of guest molecules with the monomeric units, whereas such monomeric sequence effects are significantly reduced with increasing water content.

Ultrafast Photodynamics of Drugs in Nanocavities: Cyclodextrins and Human Serum Albumin Protein

In this feature article, we discuss recent advances in studying ultrafast dynamic and structural aspects of host-guest interactions. Steady-state and time-resolved techniques exploring events from the femto- to nanosecond regime were used to examine the ultrafast photodynamics and subsequent events in selected nanostructures of the formed complexes. These consist of aromatic systems, biologically relevant molecules, and drugs trapped within cyclodextrins (CD) and human serum albumin (HSA) protein pockets. We examine the effects exerted by these chemical and biological cavitands on internal twisting motions, proton transfer and charge transfer, and cis-trans isomerization reactions that may occur in the confined molecular systems. In addition, the influence of a restricting environment on the interaction of guest molecules with biological water is considered. The dynamic details of the complexes (diffusion, early interactions, formation, stability, internal guest diffusion, and conformational changes) and the excited-state relaxation pathways, rate constants of the involved processes, and changes in the electronic distribution within encapsulated guests gave clues to elucidate their photobehavior and are relevant to the photostability and delivery of drugs when using nanocarriers.

Stretched Polyethylene Films Probed by Single Molecules

Stretched films of low-density polyethylene (LDPE) doped with 2.3,8.9-dibenzanthanthrene (DBATT) were studied using polarization-selective single-molecule spectroscopy at 1.8 K. By measuring the in-plane component of the electronic transition-dipole moments of individual chromophores, the alignment of dopant molecules is determined without averaging. The distributions of chromophore orientations reveal the presence of two fractions of dopant molecules: those oriented along the stretching direction and randomly oriented molecules. With increasing drawing ratio of the polyethylene films, the ratio of oriented to randomly oriented guest molecules increases, whereas the extent of chromophore orientation, that is, the width of the orientation distribution, remains the same. The results are consistent with the interpretation that oriented chromophores reside on the surfaces of polyethylene crystals, instead of in the amorphous polyethylene regions. Guest molecules in stretched polyethylene are oriented due to the alignment of the crystallites on which they are adsorbed. As such, the shape and width of the distributions of chromophore orientations are determined by the interaction of guest molecules with the crystal surfaces.