Microporous Environment Research Articles

Structural and Adsorption Properties of ZIF-8-7 Hybrid Materials Synthesized by Acid Gas-Assisted and De Novo Routes.

The tuning of micropore environments in zeolitic imidazolate frameworks (ZIFs) by mixed-linker synthesis has the potential for enabling new molecular separation properties. However, de novo synthesis of mixed-linker (hybrid) ZIFs is often challenging due to the disparate chemical properties of the different linkers. Here, we elucidate the structure and properties of an unconventional ZIF-8-7 hybrid material synthesized via a controlled-acid-gas-assisted degradation and reconstruction (solvent-assisted crystal redemption, SACRed) strategy. Selective insertion of benzimidazole (ZIF-7 linker) into ZIF-8 using SACRed is used as a facile method to generate a ZIF-8-7 hybrid material that is otherwise difficult to synthesize by de novo methods. Detailed crystal structure and textural characterizations clarify the significant differences in the microstructure of the SACRed-derived ZIF-8-7 hybrid material relative to a de novo synthesized hybrid of the same overall linker composition as well as the parent ZIF-8 material. Unary and binary adsorption measurements reveal the tunability of adsorption characteristics as well as the prevalence of nonideal cooperative mixture adsorption effects that lead to large deviations from predictions made with ideal adsorbed solution theory.

Fluorinated metal-organic framework for methane purification from a ternary CH4/C2H6/C3H8 mixture

Herein we present a fluorinated metal-organic framework of {(Me2NH2)[Ni3(μ3-OH)(CF3-BPDC-CF3)3(tpt)]}n (1) constructed from 2,2'-bis(trifluoromethyl)biphenyl-4,4'-dicarboxylic (CF3-BPDC-CF32‒) and 2,4,6-tri(4-pyridyl)-1,3,5-triazine (tpt) ligands, which is developed for separating propane (C3H8) and ethane (C2H6) from natural gas. Compound 1 preferentially adsorbs C3H8 and C2H6 over CH4 demonstrated by gas adsorption experiments. The presence of trifluoromethyl groups on the biphenyl-4,4'-dicarboxylic ligands facilitates the highly polarized micropore environments for compound 1, thus providing suitable micorpores for capturing the C3H8 and C2H6 molecules with larger polarizabilities and sizes compared to CH4 molecule. The dynamic mixture breakthrough experiments showed that compound 1 can separate C3H8 and C2H6 from the ternary CH4/C2H6/C3H8 mixtures efficiently, endowing compound 1 with excellent methane purification ability.

Partial hydrogenation of anisole to cyclohexanone in water medium catalyzed by atomically dispersed Pd anchored in the micropores of zeolite

Cyclohexanone is an important raw material for manufacturing nylon and other chemicals. The sustainable conversion of biomass derived molecules to cyclohexanone is desired but still challenging. Herein, a palladium single atom catalyst anchored in the micropores of zeolite (namely Pd1/ZSM-5) is designed and exhibits remarkable catalytic activity in transforming anisole to cyclohexanone in water medium (H2O), without using of organic solvent or halogen promoter. The selectivity to cyclohexanone is 91.2% and the turnover rate (TOR) is 400.8 h−1 at 453 K. The Pd sites, acid sites, and microporous confinement environment of zeolite cooperate to promote the partial hydrogenation to cyclohexanone, and inhibit further hydrogenation as the side reaction. H2O not only acts as the solvent, but also participates significantly in the partial hydrogenation. Moreover, as revealed by isotope labelled experiments (D2O and H218O), the water participated hydrogenolysis and the consequent partial hydrogenation mechanism is proposed.

Shaping Synthetic Multicellular and Complex Multimaterial Tissues via Embedded Extrusion-Volumetric Printing of Microgels.

In living tissues, cells express their functions following complex signals from their surrounding microenvironment. Capturing both hierarchical architectures at the micro- and macroscale, and anisotropic cell patterning remains a major challenge in bioprinting, and a bottleneck toward creating physiologically-relevant models. Addressing this limitation, a novel technique is introduced, termed Embedded Extrusion-Volumetric Printing (EmVP), converging extrusion-bioprinting and layer-less, ultra-fast volumetric bioprinting, allowing spatially pattern multiple inks/cell types. Light-responsive microgels are developed for the first time as bioresins (µResins) for light-based volumetric bioprinting, providing a microporous environment permissive for cell homing and self-organization. Tuning the mechanical and optical properties of gelatin-based microparticles enables their use as support bath for suspended extrusion printing, in which features containing high cell densities can be easily introduced. µResins can be sculpted within seconds with tomographic light projections into centimeter-scale, granular hydrogel-based, convoluted constructs. Interstitial microvoids enhanced differentiation of multiple stem/progenitor cells (vascular, mesenchymal, neural), otherwise not possible with conventional bulk hydrogels. As proof-of-concept, EmVP is applied to create complex synthetic biology-inspired intercellular communication models, where adipocyte differentiation is regulated by optogenetic-engineered pancreatic cells. Overall, EmVP offers new avenues for producing regenerative grafts with biological functionality, and for developing engineered living systems and (metabolic) disease models.

Templated Reactive Crystallization of Active Pharmaceutical Ingredient in Hydrogel Microparticles Enabling Robust Drug Product Processing

Commercialization of most promising active pharmaceutical ingredients (APIs) is impeded either by poor bioavailability or challenging physical properties leading to costly manufacture. Bioavailability of ionizable hydrophobic APIs can be enhanced by its conversion to salt form. While salt form of the API presents higher solution concentration than the non-ionized form, poor physical properties resulting from particle anisotropy or non-ideal morphology (needles) and particle size distribution not meeting dissolution rate targets can still inhibit its commercial translation. In this regard, API physical properties can be improved through addition of non-active components (excipients or carriers) during API manufacture. In this work, a facile method to perform reactive crystallization of an API salt in presence of the microporous environment of a hydrogel microparticle is presented. Specifically, the reaction between acidic antiretroviral API, raltegravir and base potassium hydroxide is performed in the presence of polyethylene glycol diacrylamide hydrogel microparticles. In this bottom-up approach, the spherical template hydrogel microparticles for the reaction lead to monodisperse composites loaded with inherently micronized raltegravir-potassium crystals, thus improving API physical properties without hampering bioavailability. Overall, this technique provides a novel approach to reactive crystallization while maintaining the API polymorph and crystallinity.

Synergistic Effects of Lewis Acid-Base Pair Sites─Hf-MOFs with Functional Groups as Distinguished Catalysts for the Cycloaddition of Epoxides with CO2.

The incorporation of Lewis acid-base sites in catalysts has been considered as a significant approach to fabricating bifunctional catalysts with efficient catalytic activity for CO2 fixation. In this paper, a series of Hafnium-based metal-organic frameworks (Hf-MOFs), NU-912(Hf) and NU-912-X(Hf)-X (X = -NH2, -Br, -CN, and -I) derivatives assembled by Lewis acidic Hf6(μ3-O)4(μ3-OH)4(H2O)4(OH)4 (Hf6) clusters and Lewis base-attached organic linkers, are successfully synthesized by a facile ligand functionalization method. These isostructural Hf-MOFs, which exhibit diamond channels of 1.3 nm diameter, great chemical stability, and CO2 adsorption capacity, have been evaluated as catalysts for the CO2 cycloaddition reaction with epoxides. Catalytic experiments reveal that the micropore environments of these MOFs have an outstanding impact on catalytic activity. Remarkably, NU-912(Hf)-I serves as an efficient heterogeneous catalyst for this catalytic reaction under mild conditions due to the high density of Lewis acid Hf6 cluster centers and strong Lewis base functional groups, surpassing most of the reported MOF-based catalysts.

Bubble-particle dynamics in multiphase flow of capillary foams in a porous micromodel.

Surfactant-free capillary foams (CFs) are known to be remarkably tolerant to oil, and possess unique stability and flow properties. These properties result from the presence of oil-and-particle-coated bubbles that are interconnected by a dense particle-oil capillary network. In this work, we present a study of the dynamics of capillary foams flowing through a porous micromodel. We determine that despite the presence of oil-particle networks, CFs can flow through a microporous environment and that above a threshold flowrate, >80% of foam pumped through the micromodel can be recovered. In addition, we highlight the absence of steady state in CF flow and identify the underlying phenomena including the increasing apparent viscosity, reconfigurable flow paths, and intermittent clogging of the micromodel from an oil-particle composite and bubbles trapped in pores. We also characterize bubble dynamics and show that CFs surprisingly exhibit the same bubble generation and destruction mechanisms as classical foams despite the absence of surfactants. Our observations suggest that the porous medium plays a key role in generating uniformly sized bubbles and that capillary foams in a microporous environment tend to reconfigure their flow paths in a manner that may provide opportunities for increased sweep efficiency in enhanced oil recovery.

Atomically Precise Integration of Multiple Functional Motifs in Catalytic Metal-Organic Frameworks for Highly Efficient Nitrate Electroreduction.

Ammonia production plays a central role in modern industry and agriculture with a continuous surge in its demand, yet the current industrial Haber-Bosch process suffers from low energy efficiency and accounts for high carbon emissions. Direct electrochemical conversion of nitrate to ammonia therefore emerges as an appealing approach with satisfactory sustainability while reducing the environmental impact from nitrate pollution. To this end, electrocatalysts for efficient conversion of eight-electron nitrate to ammonia require collective contributions at least from high-density reactive sites, selective reaction pathways, efficient multielectron transfer, and multiproton transport processes. Here, we report a catalytic metal-organic framework (two-dimensional (2D) In-MOF In8) catalyst integrated with multiple functional motifs with atomic precision, including uniformly dispersed, high-density, single-atom catalytic sites, high proton conductivity (efficient proton transport channel), high electron conductivity (promoted by the redox-active ligands), and confined microporous environments. These eventually lead to a direct and efficient electrochemical reduction of nitrate to ammonia and record high yield rate, FE, and selectivity for NH3 production. A novel "dynamic ligand dissociation" mechanism provides an unprecedented working principle that allows for the use of a high-quality MOF crystalline structure to function as highly ordered, high-density, single-atom catalyst (SAC)-like catalytic systems and ensures the maximum utilization of the metal centers within the MOF structure. Further, the atomically precise assembly of multiple functional motifs within a MOF catalyst offers an effective and facile strategy for the future development of framework-based enzyme-mimic systems.

Maximum Impact of Ionic Strength on Acid-Catalyzed Reaction Rates Induced by a Zeolite Microporous Environment.

The intracrystalline ionic environment in microporous zeolite can remarkably modify the excess chemical potential of adsorbed reactants and transition states, thereby influencing the catalytic turnover rates. However, a limit of the rate enhancement for aqueous-phase dehydration of alcohols appears to exist for zeolites with high ionic strength. The origin of such limitation has been hypothesized to be caused by the spatial constraints in the pores via, e.g., size exclusion effects. It is demonstrated here that the increase in turnover rate as well as the formation of a maximum and the rate drop are intrinsic consequences of the increasingly dense ionic environment in zeolite. The molecularly sized confines of zeolite create a unique ionic environment that monotonically favors the formation of alcohol-hydronium ion complexes in the micropores. The zeolite microporous environment determines the kinetics of catalytic steps and tailors the impact of ionic strength on catalytic rates.

Maximum Impact of Ionic Strength on Acid‐Catalyzed Reaction Rates Induced by a Zeolite Microporous Environment

AbstractThe intracrystalline ionic environment in microporous zeolite can remarkably modify the excess chemical potential of adsorbed reactants and transition states, thereby influencing the catalytic turnover rates. However, a limit of the rate enhancement for aqueous‐phase dehydration of alcohols appears to exist for zeolites with high ionic strength. The origin of such limitation has been hypothesized to be caused by the spatial constraints in the pores via, e.g., size exclusion effects. It is demonstrated here that the increase in turnover rate as well as the formation of a maximum and the rate drop are intrinsic consequences of the increasingly dense ionic environment in zeolite. The molecularly sized confines of zeolite create a unique ionic environment that monotonically favors the formation of alcohol‐hydronium ion complexes in the micropores. The zeolite microporous environment determines the kinetics of catalytic steps and tailors the impact of ionic strength on catalytic rates.

Two Structurally Similar Co5 Cluster-Based Metal-Organic Frameworks Containing Open Metal Sites for Efficient C2H2/CO2 Separation.

To reasonably design and synthesize metal-organic frameworks (MOFs) with high stability and excellent adsorption/separation performance, the pore configuration and functional sites are very important. Here, we report two structurally similar cluster-based MOFs using a pyridine-modified low-symmetry ligand [H4L = 2,6-bis(2',5'-dicarboxyphenyl)pyridine], [(NH2Me2)2][Co5(L)2(OCH3)2(μ3-OH)2·2DMF]·2DMF·2H2O (1) and [Co5(L)2(μ3-OH)2(H2O)2]·2H2O·4DMF (2). The structures of 1 and 2 are built from Co5 clusters, which have one-dimensional open channels, but their microporous environments are different due to the different ways in which ligands bind to the metals. Both MOFs have extremely high chemical stabilities over a wide pH range (2-12). The two MOFs have similar adsorption capacities of C2H2 (144.0 cm3 g-1 for 1 and 141.3 cm3 g-1 for 2), but 1 has a higher C2H2/CO2 selectivity of 3.5 under ambient conditions. The difference in gas adsorption and separation between the two MOFs has been compared by a breakthrough experiment and theoretical calculation, and the influence of the microporous environment on the gas adsorption and separation performance of MOFs has been further studied.

Fischer-Tropsch synthesis to olefins boosted by MFI zeolite nanosheets.

Catalytic reactions are severely restricted by the strong adsorption of product molecules on the catalyst surface, where promoting desorption of the product and hindering its re-adsorption benefit the formation of free sites on the catalyst surface for continuous substrate conversion1,2. A solution to this issue is constructing a robust nanochannel for the rapid escape of products. We demonstrate here that MFI zeolite crystals with a short b-axis of 90-110 nm and a finely controllable microporous environment can effectively boost the Fischer-Tropsch synthesis to olefins by shipping the olefin molecules. The ferric carbide catalyst (Na-FeCx) physically mixed with a zeolite promoter exhibited a CO conversion of 82.5% with an olefin selectivity of 72.0% at the low temperature of 260 °C. By contrast, Na-FeCx alone without the zeolite promoter is poorly active under equivalent conditions, and shows the significantly improved olefin productivity achieved through the zeolite promoter. These results show that the well-designed zeolite, as a promising promoter, significantly boosts Fischer-Tropsch synthesis to olefins by accelerating escape of the product from the catalyst surface.

Effect of MnO2 morphology on the thermal properties and combustion behavior of nano-Al/MnO2 thermite

Al/granular MnO2 and Al/rod MnO2 thermite samples were prepared to investigate the effects of different morphologies of MnO2 on the thermal properties and combustion behavior of nano-Al/MnO2 thermite. The morphology and thermal properties of the thermite were characterized by field emission scanning electron microscopy (FE-SEM), x-ray diffractometry (XRD), and differential scanning calorimetry (DSC). DSC results show that the Al/rod MnO2 releases 1274.39 J·mol−1 heat, which is 292.58 J·mol−1 more than the Al/granular MnO2. The initial reaction temperature of Al/rod MnO2 is 567.39 °C, which is delayed by 17.39 °C versus 550 °C of Al/granular MnO2. Non-isothermal thermodynamic analysis was used to measure the activation energy of Al/rod MnO2 to be 234.36 kJ·mol−1, which is 46.53 kJ·mol−1 higher than that of Al/granular MnO2. This corresponds to an increase in the ignition temperature in the DSC curve and indicating a higher safety profile. In the open burning experiment, the burning time of Al/granular MnO2 was longer and sparks sputtered around the flame. The Al/rod MnO2 has a large combustion flame and a fast combustion rate. The light intensity peaks of the two groups of samples are close in the light intensity test. The light intensity existence time of Al/rod MnO2 is 0.0146 s, which is 0.094 s shorter than Al/granular MnO2. This shows that the combustion rate of Al/rod MnO2 is much faster than that of Al/granular MnO2. The closed-tube combustion experiment shows that the combustion wave velocity of Al/rod MnO2 increases first and then decreases; the maximum wave velocity reaches 339.6 m·s−1, and Al/granular MnO2 cannot self-propagate combustion in the microporous environment. In the constant volume combustion experiment, the peak combustion pressure of Al/rod MnO2 is 0.938 Mpa, and the peak value of Al/granular MnO2 combustion pressure is 0.581 Mpa; the difference is obvious. This shows that the rod MnO2 gas production performance is better. According to the duration of the pressure peak, the burning speed of Al/rod MnO2 in the light intensity test is confirmed again to be much faster than that of Al/granular MnO2. The Al/rod MnO2 is better than Al/granular MnO2 in thermal and combustion performance and is also safer. This provides a basis for future performance and safety research on aluminothermic materials.

Mechanism of coalbed methane migration and production based on micro-pore overpressure environment

A complete understanding of the mechanism and characteristics of the migration and output of coalbed methane (CBM) is of great significance for CBM development. For the coal reservoir in a micropore overpressure environment, the “Reservoir inner surface desorption-micropore diffusion-natural fracture seepage” cannot completely describe the CBM production process. To this end, starting from the three stages of CBM production, based on the microporous overpressure environment of coal reservoirs to reveal CBM's migration and production mechanism. The results show that: (1) In the stage of saturated single-phase water flow, CBM was transported by “solution mass transfer”; (2) At the single-phase unsaturated water flow stage, CBM become “microporous drainage”; (3) At the two-phase gas-water flow stage, CBM is transported and produced traditionally.

De Novo Design of a Pt Nanocatalyst on a Conjugated Microporous Polymer-Coated Honeycomb Carrier for Oxidation of Hydrogen Isotopes.

A booming demand for energy highlights the importance of an emergency cleanup system in the nuclear industry or hydrogen-energy sector to reduce the risk of hydrogen explosion and decrease tritium emission. The properties of the catalyst determine the efficiency of hydrogen isotope enrichment and removal in the emergency cleanup system. However, the aggregation behavior of Pt, deactivation effect of water vapor, and isotope effect induce a continuous decrease in the catalytic activity of the Pt catalyst. Herein, a de novo design of a Pt nanocatalyst is proposed for catalytic oxidation of the hydrogen isotope via modification of a conjugated microporous polymer onto honeycomb cordierite as a Pt support. The conjugated microporous polymer creates a microporous and hydrophobic environment to attenuate the deactivation effect of water vapor and shape Pt nanoparticles with a diameter of around 2.4 nm. Thus, the as-prepared catalysts exhibit excellent catalytic performance in the range of 25-65 °C and high space velocity (≤30 000 h-1) and a stable and high catalytic activity during 487 h of continuous and intermittent operation. Importantly, the charge of the Pt nanoparticles is redistributed by the conjugated skeletons, leading to a decreased energy barrier in the rate-limiting step of hydrogen isotope oxidation and a reduced isotope effect.

Fluorous Metal–Organic Frameworks with Unique Cage-in-Cage Structures Featuring Fluorophilic Pore Surfaces for Efficient C 2 H 2 /CO 2 Separation

Fluorous Metal–Organic Frameworks with Unique Cage-in-Cage Structures Featuring Fluorophilic Pore Surfaces for Efficient C ₂ H ₂ /CO ₂ Separation

Impact of microporous structures of esterified cellulose filter papers on Co (II) rejection in cross-flow microfiltration

• Ion rejection by a low-pressure filtration is possible by Donnan exclusion effect. • A microporous structure is important for establishing Donnan exclusion effect. • This phenomenon could be explained by the Gouy-Chapman-Stern (GCS) model. • Highly overlapped EDLs are responsible for developing Donnan exclusion effect. The Donnan exclusion effect is a unique phenomenon in which ions are electrostatically repelled by a surface bearing an identical charge. In this study, cellulose filter papers were esterified by ten different organic acids to systematically evaluate how surface properties regulate the development of the Donnan exclusion effect and subsequent Co(II) rejection. Among the esterified cellulose filter papers, those exhibiting high Co(II) rejection (>80% in 1.0 mg/L Co(II) in 1.0 mM NaCl background electrolyte solution) were noted to simultaneously possess positive zeta potential when in contact with Co(II), low adsorption affinity, and a relatively high microporous environment. This behavior could be explained by the Gouy–Chapman–Stern (GCS) model, in which the ion retention in the overlapped electrical double layers (EDLs) regulates the zeta potential development and consequently the Co(II) rejection. In addition to introducing reactive carbonyl and carboxyl groups, we suggest that the creation of a relatively microporous environment is essential for high Co(II) rejection. Our results are valuable for developing low-energy filtration systems for environmentally friendly water and wastewater treatment.

Micropore environment regulation of zirconium MOFs for instantaneous hydrolysis of an organophosphorus chemical

Protection from nerve agents is an important societal challenge requiring effective strategies for rapid degradation of these toxic chemicals. Here, we report a series of zirconium-based metal-organic frameworks (MOFs)—NU-912 and its derivatives—with a 4,8-connected scu topology. We explore their activity toward degradation of a nerve agent simulant, dimethyl 4-nitrophenyl phosphate (DMNP), to illustrate the virtues of controlling functional group distribution and regulating the environment across the micropore in heterogeneous catalysis. The micropore environments of these MOFs tuned by functional groups (-Br, -CN, -I, and -NH2) have significant influence on catalytic activity. NU-912-I, NU-912-CN, and NU-912-NH2 have half-lives (t1/2) of less than 3 min using only 3 mol % catalyst loading, which places them among the best zirconium (Zr)-MOF-based heterogeneous catalysts for hydrolysis of DMNP.

Nanoscale Cu(II) MOFs Formed via Microemulsion: Vibrational Mode Characterization Performed using a Combined FTIR, Synchrotron Far-IR, and Periodic DFT Approach

Two Cu(II) metal–organic frameworks (MOFs) were prepared on the nanoscale at room temperature using a microemulsion method, namely, [Cu3(BTC)2(H2O)3] (BTC = benzene-1,3,5-tricarboxylate), known as HKUST-1 (1), and [Cu2(OH)(BTC)(H2O)]·2H2O (2). Thermochemical and gas sorption properties of the microporous topologies were characterized by mid- and far-infrared vibrational spectroscopy, supported by periodic density functional theory calculations. The mid-infrared profile of 1 appeared altered in response to gas sorption under variable temperature and pressure conditions. Vibrational mode analysis indicated the most sensitive infrared peaks were associated with the internal vibrations of organic linker moieties indirectly coupled to the Cu(II)–gas coordination site, activated by a lowered symmetry induced by guest interactions. Synchrotron far-infrared spectroscopy was shown to be a useful diagnostic for the microstructure of 1 and 2 where different temperature dependences were displayed in the low-frequency region. The loss of residual water during the activation of 2 at elevated temperature coincides with peaks indicative of free paddle-wheel moieties emerging in the far-IR spectra. As demonstrated for both materials 1 and 2, vibrational mode analysis was effective in screening MOF materials for their propensity toward gas uptake and, inversely, the diffusion of guest species such as adsorbed water from the microporous environments.

Cover Feature: Size‐Selective Photoelectrochemical Reactions in Microporous Environments: Clark Probe Investigation of Pt@g‐C3N4 Embedded into Intrinsically Microporous Polymer (PIM‐1) (ChemElectroChem 18/2021)

The Cover Feature illustrates a platinum-modified graphitic carbon nitride photocatalyst under illumination with blue light and in the presence of quencher producing hydrogen that is guided via polymer of intrinsic microporosity through a Teflon membrane into a Clark sensor. More information can be found in the Article by Yuanzhu Zhao et al.