Crystalline Nanoporous Materials Research Articles

An approach for predicting intracrystalline diffusivities and adsorption entropies in nanoporous crystalline materials

AbstractConfinement of molecules in nanoporous crystalline materials results in the unique and diverse characteristics of intracrystalline diffusion and adsorption, which can significantly affect the efficiency of gas separation and/or catalysis. However, understanding the interplay between confinement and intracrystalline diffusion and adsorption remains elusive at the quantitative level. In this work, it is found that the intracrystalline diffusion could be related to the hopping rate, which might be further connected to the translational and rotational motion of molecules and quantified by corresponding partition functions. Based on this analysis, the correlations capable of predicting the intracrystalline diffusivity and the adsorption entropy are developed. It is shown that the correlations can well capture the experimental and simulation results of more than 20 frameworks, including zeolites and MOFs, for a wide range of guest molecules. This approach can potentially serve as rapid screening tool for nanoporous crystalline materials in gas separation and catalysis.

Applicability of Tail Corrections in the Molecular Simulations of Porous Materials.

Molecular simulations with periodic boundary conditions require the definition of a certain cutoff radius, rc, beyond which pairwise dispersion interactions are neglected. For the simulation of homogeneous phases the use of tail corrections is well-established, which can remedy this truncation of the potential. These corrections are built under the assumption that beyond rc the radial distribution function, g(r), is equal to one. In this work we shed some light on the discussion of whether tail corrections should be used in the modeling of heterogeneous systems. We show that for the adsorption of gases in a diverse set of nanoporous crystalline materials (zeolites, covalent organic frameworks, and metal–organic frameworks), tail corrections are a convenient choice to make the adsorption results less sensitive to the details of the truncation.

Direct quantification of surface barriers for mass transfer in nanoporous crystalline materials

Mass transfer of guest molecules in nanoporous crystalline materials has gained attention in catalysis, separation, electrochemistry, and other fields. Two mechanisms, surface barriers and intracrystalline diffusion, dominate the mass transport process. Lack of methods to separately quantify these two mechanisms restricts further understanding and thus rational design and efficient application of nanoporous materials. Here we derive an approximate expression of uptake rate relying solely on surface permeability, offering an approach to directly quantify surface barriers and intracrystalline diffusion. By use of this approach, we study the diffusion in zeolitic materials, and find that the intracrystalline diffusivity is intrinsic to the topological structure of host materials at low molecular loading for the given guest molecules, while the surface permeability is sensitive to the non-ideality of a crystalline surface owing to the physical and chemical properties of the crystalline surface, host–guest interaction at the surface, and change of the environment.

Predicting performance limits of methane gas storage in zeolites with an artificial neural network

Crystalline nanoporous materials (i.e. shapes) were generated in the energy space using an artificial neural network.

Intercalation of first row transition metals inside covalent-organic frameworks (COFs): a strategy to fine tune the electronic properties of porous crystalline materials.

Covalent-organic frameworks (COFs) have emerged as an important class of nano-porous crystalline materials with many potential applications. They are intriguing platforms for the design of porous skeletons with special functionality at the molecular level. However, despite their extraordinary structural tunability, it is difficult to control their electronic properties, thus hindering the potential implementation in electronic devices. A new family of nanoporous materials, COFs intercalated with first row transition metals, is proposed to address this fundamental drawback - the lack of electronic tunability. Using first-principles calculations, we designed 31 new COF materials in silico by intercalating all of the first row transition metals (TMs) with boroxine-linked and triazine-linked COFs: COF-TM-x (where TM = Sc-Zn and x = 3-5). We investigated their structure and electronic properties. Specifically, we predict that the band gap and density of states (DOS) of COFs can be controlled by intercalating first row transition metal atoms (TM: Sc-Zn) and fine tuned by the concentration of TMs. We also found that the d-subshell electron density of the TMs plays a main role in determining the electronic properties of the COFs. Thus intercalated-COFs provide a new strategy to control the electronic properties of materials within a porous network. This work opens up new avenues for the design of TM-intercalated materials with promising future applications in nanoporous electronic devices, where a high surface area coupled with fine-tuned electronic properties is desired.

All-Nanoporous Hybrid Membranes: Redefining Upper Limits on Molecular Separation Properties.

New membrane-based molecular separation processes are an essential part of the strategy for sustainable chemical production. A large literature on "hybrid" or "mixed-matrix" membranes exists, in which nanoparticles of a higher-performance porous material are dispersed in a polymeric matrix to boost performance. We demonstrate that the hybrid membrane concept can be redefined to achieve much higher performance if the membrane matrix and the dispersed phase are both nanoporous crystalline materials, with no polymeric phase. As the first example of such a system, we find that surface-treated nanoparticles of the zeolite MFI can be incorporated in situ during growth of a polycrystalline membrane of the MOF ZIF-8. The resulting all-nanoporous hybrid membrane shows propylene/propane separation characteristics that exceed known upper-bound performance limits defined for polymers, nanoporous materials, and polymer-based hybrid membranes. This serves as a starting point for a new generation of chemical separation membranes containing interconnected nanoporous crystalline phases.

All‐Nanoporous Hybrid Membranes: Redefining Upper Limits on Molecular Separation Properties

AbstractNew membrane‐based molecular separation processes are an essential part of the strategy for sustainable chemical production. A large literature on “hybrid” or “mixed‐matrix” membranes exists, in which nanoparticles of a higher‐performance porous material are dispersed in a polymeric matrix to boost performance. We demonstrate that the hybrid membrane concept can be redefined to achieve much higher performance if the membrane matrix and the dispersed phase are both nanoporous crystalline materials, with no polymeric phase. As the first example of such a system, we find that surface‐treated nanoparticles of the zeolite MFI can be incorporated in situ during growth of a polycrystalline membrane of the MOF ZIF‐8. The resulting all‐nanoporous hybrid membrane shows propylene/propane separation characteristics that exceed known upper‐bound performance limits defined for polymers, nanoporous materials, and polymer‐based hybrid membranes. This serves as a starting point for a new generation of chemical separation membranes containing interconnected nanoporous crystalline phases.

Ultrasmall Metal Nanoparticles Confined within Crystalline Nanoporous Materials: A Fascinating Class of Nanocatalysts.

Crystalline nanoporous materials with uniform porous structures, such as zeolites and metal-organic frameworks (MOFs), have proven to be ideal supports to encapsulate ultrasmall metal nanoparticles (MNPs) inside their void nanospaces to generate high-efficiency nanocatalysts. The nanopore-encaged metal catalysts exhibit superior catalytic performance as well as high stability and catalytic shape selectivity endowed by the nanoporous matrix. In addition, the synergistic effect of confined MNPs and nanoporous frameworks with active sites can further promote the catalytic activities of the composite catalysts. Herein, recent progress in nanopore-encaged metal nanocatalysts is reviewed, with a special focus on advances in synthetic strategies for ultrasmall MNPs (<5 nm), clusters, and even single atoms confined within zeolites and MOFs for various heterogeneous catalytic reactions. In addition, some advanced characterization methods to elucidate the atomic-scale structures of the nanocatalysts are presented, and the current limitations of and future opportunities for these fantastic nanocatalysts are also highlighted and discussed. The aim is to provide some guidance for the rational synthesis of nanopore-encaged metal catalysts and to inspire their further applications to meet the emerging demands in catalytic fields.

Mixed-Node Metal–Organic Frameworks as Efficient Electrocatalysts for Oxygen Evolution Reaction

Metal organic frameworks (MOFs), an emerging class of nanoporous crystalline materials, have become increasingly attractive for solar energy applications. In this work, we report a newly designed mixed-node MOF catalyst, CoxFe1–x-MOF-74 (0 < x ≤ 1), which acts as a highly efficient electrocatalyst for oxygen evolution reaction (OER) in alkaline solution with remarkably low overpotential (280 mV at a current density of 10 mA/cm2), small Tafel slope (56 mV/dec), and high faradic efficiency (91%) and can deliver a current density of 20 mA/cm2 at 1.58 V for overall water splitting. Moreover, using the combination of multiple spectroscopic methods, including X-ray absorption, electron spin resonance, and X-ray photoelectron spectroscopy, etc., we unraveled the mechanistic origin of the enhanced catalytic performance of CoxFe1–x-MOF-74 compared to its single-metal counterparts. We show the mixed-node MOF can provide more open metal sites and an enhanced electron-rich environment, which facilitates efficient cha...

Electrophoretic Crystallization of Ultrathin High-performance Metal-organic Framework Membranes.

We report the synthesis of thin, highly intergrown, polycrystalline metal-organic framework (MOF) membranes on a wide range of unmodified porous and non-porous supports (polymer, ceramic, metal, carbon, and graphene). We developed a novel crystallization technique, which is termed the ENACT approach: the electrophoretic nuclei assembly for the crystallization of highly intergrown thin films (ENACT). This approach allows for a high density of heterogeneous nucleation of MOFs on a chosen substrate via the electrophoretic deposition (EPD) directly from the precursor sol. The growth of well-packed MOF nuclei leads to a highly intergrown polycrystalline MOF film. We show that this simple approach can be used for the synthesis of thin, intergrown zeolite imidazole framework (ZIF)-7 and ZIF-8 films. The resulting 500 nm-thick ZIF-8 membranes show a considerably high H2 permeance (8.3 x 10-6 mol m-2 s-1 Pa-1) and ideal gas selectivities (7.3 for H2/CO2, 15.5 for H2/N2, 16.2 for H2/CH4, and 2655 for H2/C3H8). An attractive performance for C3H6/C3H8 separation is also achieved (a C3H6 permeance of 9.9 x 10-8 mol m-2 s-1 Pa-1 and a C3H6/C3H8 ideal selectivity of 31.6 at 25 °C). Overall, the ENACT process, owing to its simplicity, can be extended to synthesize intergrown thin films of a wide range of nanoporous crystalline materials.

First-Principles-Derived Force Fields for CH4 Adsorption and Diffusion in Siliceous Zeolites

Most previous studies on development of force fields for molecules in porous materials focus on prediction of adsorption properties. However, accurately reproducing adsorption data is not sufficient to guarantee the accuracy of other properties, such as diffusivities. We demonstrate an approach to develop force fields based on periodic first-principles calculations that can accurately predict both adsorption and diffusion properties in crystalline nanoporous materials using CH4 in siliceous zeolites as an example. First, multiple dispersion-corrected density functional theory (DFT) methods were tested for describing CH4 in siliceous chabazite, and the two configurations (CH4 interacting with a framework all and an eight-ring window) that are relevant to CH4 adsorption and diffusion were investigated. By comparing with the results from high-level random phase approximation calculations, DFT/CC (coupled cluster) was found to be the optimum method for force field development. DFT/CC-derived force fields that...

Supercritical fluid processing for metal–organic frameworks, porous coordination polymers, and covalent organic frameworks

The synthesis and processing of porous materials is a particularly fertile area for supercritical fluid (SCF) technology. Well-designed metal–organic hybrid porous materials such as metal–organic frameworks (MOFs) and porous coordination polymers (PCPs), which are constructed from metal-containing nodes and organic linkers, have recently been recognized as an intriguing class of crystalline nanoporous materials with a wide variety of potential applications in areas including gas storage, compound separation, biomedicine, chemical sensing, and catalysis. Furthermore, covalent organic frameworks (COFs) constructed from light elements have been developed. Recent research in the rapidly expanding field of MOFs, PCPs and COFs has demonstrated that SCF technologies, such as drying, impregnation, and synthesis, are effective approaches for processing and improving MOFs, PCPs and COFs. This short review describes the processing of MOFs, PCPs and COFs using supercritical CO2 and its mixtures. Selected examples are presented, and current achievements and challenges are summarized.

Synthesis of highly stable MOF-5@MWCNTs nanocomposite with improved hydrophobic properties

Metal-organic frameworks (MOFs) are a promising class of nanoporous crystalline materials with potential applications in gas storage, catalysis, biomedical and clean energy. However, moisture adsorption dampens their structure and therefore capacity and selectivity. Improving the hydrostability of MOF-5 without compromising on its characteristic features can be a potential way to make the best use of this material. In this work, we used a solvothermal method to precipitate MOF-5 in well-dispersed carboxy derived MWCNTs. By inspiring from the hydrophobic properties of carbon nanotubes (CNTs), we have successfully utilized a solvothermal approach to precipitate MOF-5 in well-dispersed carboxy functionalized multiwalled carbon nanotubes (FMWCNTs) to produce a MOF-5@FMWCNTs nanocomposite. The experimental results in this study reveal that the FMWCNTs form stable nanocomposite with MOF-5 particles and the resultant hybrid MOF-5@FMWCNTs nanocomposite indicated an increase in the moisture stability and even after prolonged exposure to ambient moist environment (55% humidity at 28°C) showed less degradation as confirmed by the BET surface area, and powder XRD studies. So, our findings demonstrate a simple synthetic route to make use of FMWCNTs to stabilize the MOF-5 and it can be a choice for many other MOFs without much compromise on the surface area.

Determining the specific surface area of Metal Organic Frameworks based on a computational approach

A novel simulation approach is proposed for the representation and calculation of the specific surface area of crystalline nanoporous materials (e.g. MOFs). In brief, the simulation box that contains the framework atoms is discretized with voxels, which are filtered based on an atomistic forcefield according to specific criteria (i.e. whether a voxel can be occupied by a gas molecule or not). This produces a “phase” function that represents the spatial distribution of matter in the biphasic (solid–void) medium. The specific surface area is then derived by the correlation function of the phase function, a method well established for the calculation of the specific surface area of mesoporous media. The proposed methodology has the advantage of allowing the implementation of any atomic interaction potential and/or filter of choice. Thus, no assumption on the shape of the pores or even the framework atoms is in principle required (for instance non-spherical potentials including electrostatic interactions can be used). Furthermore, since the solid–void interface has been determined, an attempt to simulate the monolayer coverage process was carried out and pertinent surface areas where also calculated by simply counting the number of molecules that form the monolayer.The results were validated against full GCMC adsorption isotherm simulations and were further compared with experimental data indicating the suitability and effectiveness of the proposed non-stochastic computational approach for the reliable, fast and accurate determination of the surface area of nanoporous solids.

Osteogenic activity and antibacterial effect of porous titanium modified with metal-organic framework films.

As a new class of crystalline nanoporous materials, metal-organic frameworks (MOFs) have recently been used for biomedical applications due to their large surface area, high porosity, and theoretically infinite structures. To improve the biological performance of titanium, MOF films were applied to surface modification of titanium. Zn-based MOF films composed of zeolitic imidazolate framework-8 (ZIF-8) crystals with nanoscale and microscale sizes (nanoZIF-8 and microZIF-8) were prepared on porous titanium surfaces by hydrothermal and solvothermal methods, respectively. The ZIF-8 films were characterized by powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). The nanoZIF-8 film exhibited good biocompatibility, whereas the microZIF-8 film showed obvious cytotoxicity to MG63 cells. Compared to pure titanium and alkali- and heat-treated porous titanium, the nanoZIF-8 film not only enhanced alkaline phosphatase (ALP) activity, extracellular matrix mineralization, and expression of osteogenic genes (ALP, Runx2) in MG63 cells but also inhibited the growth of Streptococcus mutans. These results indicate that MOF films or coatings may be promising candidates for bone tissue engineering. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 834-846, 2017.

Nanoporous Crystalline Polymer Materials for Environmental Applications

SummaryNanoporous crystalline phases can be obtained with syndiotactic polystyrene (s‐PS) and poly(2,6‐dimethyl‐1,4‐phenylene)oxide (PPO) materials characterized by a large variety of morphologies such as film, powder, or aerogel. These nanoporous crystalline materials are characterized by a high sorption capacity of volatile organic compounds (VOC) also when present in traces in air and water and thus are potentially interesting for environmental applications such as air/water purification. Different aspects relative to the structure and the sorption properties of these innovative polymeric materials are presented in this report.

ChemInform Abstract: Vapor‐Phase Deposition and Modification of Metal—Organic Frameworks: State‐of‐the‐Art and Future Directions

AbstractReview: 58 refs.

Vapor-Phase Deposition and Modification of Metal-Organic Frameworks: State-of-the-Art and Future Directions.

Materials processing, and thin-film deposition in particular, is decisive in the implementation of functional materials in industry and real-world applications. Vapor processing of materials plays a central role in manufacturing, especially in electronics. Metal-organic frameworks (MOFs) are a class of nanoporous crystalline materials on the brink of breakthrough in many application areas. Vapor deposition of MOF thin films will facilitate their implementation in micro- and nanofabrication research and industries. In addition, vapor-solid modification can be used for postsynthetic tailoring of MOF properties. In this context, we review the recent progress in vapor processing of MOFs, summarize the underpinning chemistry and principles, and highlight promising directions for future research.

A generalized method for constructing hypothetical nanoporous materials of any net topology from graph theory

Here we present a method for constructing hypothetical crystalline nanoporous materials, such as metal–organic frameworks (MOFs), using a graph theoretical approach. The method takes as input the discrete secondary (or structural) building units (SBUs) with defined connection points, and a desired 3-dimensional net topology in the form of a labelled quotient graph. The hypothetical materials are constructed based on the principle that using a labelled quotient graph obtained, for example from the reticular chemistry structure resource (RCSR), one can construct a net embedding in 3-D Euclidean space with an infinite number of different representations. Thus, crystalline structures can be realized by manipulating a net's embedding such that vertices of the net match the geometries of the desired SBUs. To demonstrate the methodology, 46 different network topologies (i.e.tbo, pcu), are used to build MOFs from the same pair of 4-coordinate and 3-coordinate SBUs. We further show that the method can be used to generate hypothetical MOFs where the most common realization of a net, called the barycentric representation, will not produce a viable structure. When combined with a robust force field based geometry optimizer, the method can be used to generate large and structurally diverse hypothetical databases for virtual screening purposes.

ChemInform Abstract: Guest‐Induced Emergent Properties in Metal‐Organic Frameworks

AbstractReview: 82 refs.