Calcium-Based Metal-Organic Framework for Simultaneous Capture of Trace Propyne and Propadiene from Propylene

The simultaneous capture of trace propyne and propadiene from propylene is one of the important but energy demanding industrial processes because of their similar physicochemical properties as well as the ultralow concentration in the mixtures. Herein, a highly stable Ca-based MOF, constructed from an inexpensive precursor (CaCO₃) and rigid squaric acid, is capable of preferentially capturing trace propyne and propadiene with record-high uptake capacities of 2.44 and 2.64 mmol/g at pressures as low as 5 mbar, respectively. Direct multicomponent breakthrough experiments confirm that Ca-based MOF exhibits an excellent performance for simultaneous removal of trace propyne and propadiene from propylene. DFT simulation and in situ single-crystal X-ray diffraction of propadiene- and propyne-adsorbed Ca-based MOFs reveal that the strong affinity of the framework toward two species is ascribed to the multiple types of cooperative binding including π-π stacking and C-H···O interactions. The calcium squarate framework sets a new benchmark for adsorptive purification of propylene, showing great potential in the practical application.

Coordinated Water as New Binding Sites for the Separation of Light Hydrocarbons in Metal-Organic Frameworks with Open Metal Sites

Metal-organic frameworks with open metal sites are promising materials for gas separations. Particularly, the M₂(dobdc) (dobdc^4- = 2,5-dioxidobenzenedicarboxylate, M²⁺ = Co²⁺, Mn²⁺, Fe²⁺, ...) framework has been the Drosophila of this research field and has delivered groundbreaking results in terms of sorption selectivity. However, many studies focus on perfect two-component mixtures and use theoretical models, e.g., the ideal adsorbed solution theory, to calculate selectivities. Within this work, we shed light on the comparability of these selectivities with values obtained from propane/propene multicomponent measurements on the prototypical Co₂(dobdc) framework, and we study the impact of impurities like water on the selectivity. Despite the expected capacity loss, the presence of water does not necessarily lead to a decreased selectivity. Density functional theory calculations of the binding energies prove that the water molecules adsorbed to the metal centers introduce new binding sites for the adsorbates.

Sustainable Biomass Glucose-Derived Porous Carbon Spheres with High Nitrogen Doping: As a Promising Adsorbent for CO2/CH4/N2 Adsorptive Separation

Separation of CO2/CH4/N2 is significantly important from the view of environmental protection and energy utilization. In this work, we reported nitrogen (N)-doped porous carbon spheres prepared from sustainable biomass glucose via hydrothermal carbonization, CO2 activation, and urea treatment. The optimal carbon sample exhibited a high CO2 and CH4 capacity, as well as a low N2 uptake, under ambient conditions. The excellent selectivities toward CO2/N2, CO2/CH4, and CH4/N2 binary mixtures were predicted by ideal adsorbed solution theory (IAST) via correlating pure component adsorption isotherms with the Langmuir-Freundlich model. At 25 °C and 1 bar, the adsorption capacities for CO2 and CH4 were 3.03 and 1.3 mmol g-1, respectively, and the IAST predicated selectivities for CO2/N2 (15/85), CO2/CH4 (10/90), and CH4/N2 (30/70) reached 16.48, 7.49, and 3.76, respectively. These results should be attributed to the synergistic effect between suitable microporous structure and desirable N content. This report introduces a simple pathway to obtain N-doped porous carbon spheres to meet the flue gas and energy gas adsorptive separation requirements.

Polycatenated Molecular Cage-Based Propane Trap for Propylene Purification with Recorded Selectivity

The propane (C₃H₈)-selective adsorption technology is recognized as a promising energy-efficient way to directly afford high-purity propylene (C₃H₆). Here, a novel strategy via cage construction, combining with multiple interaction and shape selectivity, was raised to achieve preferential C₃H₈ adsorption. We revealed that the polycatenated molecular cage within a microporous framework of [Ni(bpe)₂(WO₄)] (bpe = 1,2-bis(4-pyridyl)ethylene) showed preferential C₃H₈ adsorption behavior with recorded C₃H₈/C₃H₆ selectivity (1.62-2.75), as well as the high adsorption enthalpy around 42 kJ mol^-1. The cage afforded dense electronegative binding sites, enabling the multiple C^δ--H^δ+. . .C^δ- interaction with C₃H₈ molecule and thus the higher affinity for C₃H₈ than C₃H₆. Additionally, the cage exhibited shape selectivity to oblate C₃H₈, and was unfavorable to C₃H₆ with relatively planar configuration as indicated by modeling studies. The high purity propylene (99.6%) was directly obtained without the extra adsorption-desorption cycles through the column breakthrough experiment.

Predicting adsorption selectivities from pure gas isotherms for gas mixtures in metal-organic frameworks

We perform Grand Canonical Monte Carlo simulations on a lattice of Mg²⁺ sites (GCMC) for adsorption of four binary A/B mixtures, CH₄/N₂, CO/N₂, CO₂/N₂, and CO₂/CH₄, in the metal–organic framework Mg₂(2,5-dioxidobenzedicarboxylate), also known as CPO-27–Mg or Mg–MOF-74. We present a mean field co-adsorption isotherm model and show that its predictions agree with the GCMC results if the same quantum chemical ab initio data are used for Gibbs free energies of adsorption at the individual sites and for lateral interaction energies between the same, A⋯A and B⋯B, and unlike, A⋯B, adsorbed molecules. We use both approaches to test the assumption underlying Ideal Adsorbed Solution Theory (IAST), namely approximating A⋯B interaction energies as the arithmetic mean of A⋯A and B⋯B interaction energies. While IAST works well for mixtures with weak lateral interactions, CH₄/N₂ and CO/N₂, the deviations are large for mixtures with stronger lateral interactions, CO₂/N₂ and CO₂/CH₄. Motivated by the theory of London dispersion forces, we propose use of the geometric mean instead of the arithmetic mean and achieve substantial improvements. For CO₂/CH₄, the lateral interactions become anisotropic. To include this in the geometric mean co-adsorption model, we introduce an anisotropy factor. We propose a protocol, named co-adsorption mean field theory (CAMT), for co-adsorption selectivity prediction from known (experiment or simulation) pure component isotherms which is similar to the IAST protocol but uses the geometric mean to approximate mixed pair interaction energies and yields improved results for non-ideal mixtures.

A new mixing rule (geometric mean) is proposed with substantial improvements compared to the widely used ideal adsorbed solution theory for adsorbates with strong lateral interactions.

Microporous Metal-Organic Frameworks with Hydrophilic and Hydrophobic Pores for Efficient Separation of CH4/N2 Mixture

Highly selective removal of N₂ from unconventional natural gas is considered as a viable way to increase the heat value of CH₄ and reduce the greenhouse effect caused by the direct emission of CH₄/N₂ mixture. In this work, a three-dimensional Cu-MOF with two different types of micropores was synthesized, exhibiting a high selectivity for CH₄/N₂ (10.00-12.67) and the highest sorbent selection parameter value (65.73) among the reported materials. The CH₄ molecule interacts with the framework to form multiple van der Waals interactions both in hydrophilic and hydrophobic pores, indicated by density functional theory calculations to gain a deep insight into the adsorption binding sites. In contrast, the weak polarity feature of the hydrophobic pore and the occupied open-metal sites in the hydrophilic pore result in a very low adsorption uptake of N₂. The excellent separation performance combining the good stability and regenerability guarantees this Cu-MOF to be a promising adsorbent for an efficient separation of the CH₄/N₂ mixture.

Effects of Intrinsic Flexibility on Adsorption Properties of Metal-Organic Frameworks at Dilute and Nondilute Loadings

Molecular simulation of adsorption in nanoporous materials has become a valuable complement to experimental studies of these materials. In almost all cases, these simulations treat the adsorbing material as rigid. We use molecular simulations to examine the validity of this approximation for the adsorption in metal-organic frameworks (MOFs) that have framework flexibility without change in their unit cells because of thermal vibrations. All nanoporous materials are subject to this kind of framework flexibility. We examine the adsorption of nine molecules (CO₂, CH₄, ethane, ethene, propane, propene, butane, Xe, and Kr) and four molecular mixtures (CO₂/CH₄, ethane/ethene, propane/propene/butane, and Xe/Kr) in 100 MOFs at dilute and nondilute adsorption conditions. Our results show that single-component adsorption uptakes at nondilute conditions are only weakly affected by framework flexibility, but adsorption selectivities at both dilute and nondilute conditions can be significantly affected by flexibility. The most dramatic impacts of framework flexibility occur for adsorption uptake in the limit of dilute adsorption. These results suggest that the importance of including framework flexibility when attempting to make quantitative predictions of adsorption selectivity in MOFs and similar materials may have been underestimated in the past.

Challenges and opportunities for adsorption-based CO2 capture from natural gas combined cycle emissions

In recent years, the power sector has shown a growing reliance on natural gas, a cleaner-burning fuel than coal that emits approximately half as much CO2 per kWh of energy produced. This rapid growth in the consumption of natural gas has led to increased CO2 emissions from gas-fired power plants. To limit the contribution of fossil fuel combustion to atmospheric CO2 levels, carbon capture and sequestration has been proposed as a potential emission mitigation strategy. However, despite extensive exploration of solid adsorbents for CO2 capture, few studies have examined the performance of adsorbents in post-combustion capture processes specific to natural gas flue emissions. In this perspective, we emphasize the importance of considering gas-fired power plants alongside coal-fired plants in future analyses of carbon capture materials. We address specific challenges and opportunities related to adsorptive carbon capture from the emissions of gas-fired plants and discuss several promising candidate materials. Finally, we suggest experiments to determine the viability of new CO2 capture materials for this separation. This broadening in the scope of current carbon capture research is urgently needed to accelerate the deployment of transformational carbon capture technologies.

3D porous metal-organic framework for selective adsorption of methane over dinitrogen under ambient pressure

We report a highly porous 3D metal-organic framework (MOF) that shows potential for coal mine methane (CMM) capture.

Unusual Moisture-Enhanced CO2 Capture within Microporous PCN-250 Frameworks

Developing metal-organic frameworks (MOFs) with moisture-resistant feature or moisture-enhanced adsorption is challenging for the practical CO₂ capture under humid conditions. In this work, under humid conditions, the CO₂ adsorption behaviors of two iron-based MOF materials, PCN-250(Fe₃) and PCN-250(Fe₂Co), were investigated. An interesting phenomenon is observed that the two materials demonstrate an unusual moisture-enhanced adsorption of CO₂. For PCN-250 frameworks, H₂O molecule induces a remarkable increase in the CO₂ uptake for the dynamic CO₂ capture from CO₂/N₂ (15:85) mixture. For PCN-250(Fe₃), its CO₂ adsorption capacity increases by 54.2% under the 50% RH humid condition, compared with that under dry conditions (from 1.18 to 1.82 mmol/g). Similarly, the CO₂ adsorption uptake of PCN-250(Fe₂Co) increases from 1.32 to 2.23 mmol/g, exhibiting a 68.9% increase. Even up to 90% RH, for PCN-250(Fe₃) and PCN-250(Fe₂Co), obvious increases of 43.7 and 70.2% in the CO₂ adsorption capacities are observed in comparison with those under dry conditions, respectively. Molecular simulations indicate that the hydroxo functional groups (μ₃-O) within the framework play a crucial role in improving CO₂ uptake in the presence of water vapor. Besides, partial substitution of Fe³⁺ by Co²⁺ ions in the PCN-250 framework gives rise to a great improvement in CO₂ adsorption capacity and selectivity. The excellent moisture stability (stable even after exposure to 90% RH humid air for 30 days), superior recyclability, as well as moisture-enhanced feature make PCN-250 as an excellent MOF adsorbent for CO₂ capture under humid conditions. This study provides a new paradigm that PCN-250 frameworks can not only be moisture resistant but can also subtly convert the common negative effect of moisture to a positive impact on improving CO₂ capture performance.

Molecular Modelling for Reactor Design

Chemical reactor modelling based on insights and data on a molecular level has become reality over the last few years. Multiscale models describing elementary reaction steps and full microkinetic schemes, pore structures, multicomponent adsorption and diffusion inside pores, and entire reactors have been presented. Quantum mechanical (QM) approaches, molecular simulations (Monte Carlo and molecular dynamics), and continuum equations have been employed for this purpose. Some recent developments in these approaches are presented, in particular time-dependent QM methods, calculation of van der Waals forces, new approaches for force field generation, automatic setup of reaction schemes, and pore modelling. Multiscale simulations are discussed. Applications of these approaches to heterogeneous catalysis are demonstrated for examples that have found growing interest over the last few years, such as metal-support interactions, influence of pore geometry on reactions, noncovalent bonding, reaction dynamics, dynamic changes in catalyst nanoparticle structure, electrocatalysis, solvent effects in catalysis, and multiscale modelling.

Efficiently Exploring Adsorption Space to Identify Privileged Adsorbents for Chemical Separations of a Diverse Set of Molecules

Although computational models have been used to predict adsorption of molecules in large libraries of porous adsorbents, previous work of this kind has focused on a small number of molecules as potential adsorbates. In this study, molecular simulations were used to consider the adsorption of a diverse range of molecules in a large collection of metal-organic framework (MOF) materials. Specifically, 11 304 isotherms were obtained from molecular simulations of 24 different adsorbates in 471 MOFs. This information provides insight into several interesting questions that could not be addressed with previously available data. Highly computationally efficient methods are introduced that can predict isotherms for a wide range of adsorbing molecules with far less computation than traditional molecular simulations. By characterizing the 276 binary mixtures defined by the molecules considered, "privileged" adsorbents are shown to exist, which are effective for separating many different molecular mixtures. Finally, correlations that were developed previously to predict molecular solubility in polymers are found to be surprisingly effective in predicting the average properties of molecules adsorbing in MOFs.