Adsorption equilibrium of nitrogen dioxide in porous materials

Surface Modification Strategy for Enhanced NO2 Capture in Metal-Organic Frameworks

The interaction strength of nitrogen dioxide (NO₂) with a set of 43 functionalized benzene molecules was investigated by performing density functional theory (DFT) calculations. The functional groups under study were strategically selected as potential modifications of the organic linker of existing metal–organic frameworks (MOFs) in order to enhance their uptake of NO₂ molecules. Among the functional groups considered, the highest interaction energy with NO₂ (5.4 kcal/mol) was found for phenyl hydrogen sulfate (-OSO₃H) at the RI-DSD-BLYP/def2-TZVPP level of theory—an interaction almost three times larger than the corresponding binding energy for non-functionalized benzene (2.0 kcal/mol). The groups with the strongest NO₂ interactions (-OSO₃H, -PO₃H₂, -OPO₃H₂) were selected for functionalizing the linker of IRMOF-8 and investigating the trend in their NO₂ uptake capacities with grand canonical Monte Carlo (GCMC) simulations at ambient temperature for a wide pressure range. The predicted isotherms show a profound enhancement of the NO₂ uptake with the introduction of the strongly-binding functional groups in the framework, rendering them promising modification candidates for improving the NO₂ uptake performance not only in MOFs but also in various other porous materials.

Separation of SO2 and NO2 with the Zeolite Membrane: Molecular Simulation Insights into the Advantageous NO2 Dimerization Effect

NO₂ and SO₂, as valuable chemical feedstock, are worth being recycled from flue gases. The separation of NO₂ and SO₂ is a key process step to enable practical deployment. This work proposes SO₂ separation from NO₂ using chabazite zeolite (SSZ-13) membranes and provides insights into the feasibility and advantages of this process using molecular simulation. Grand canonical ensemble Monte Carlo and equilibrium molecular dynamics methods were respectively adopted to simulate the adsorption equilibria and diffusion of SO₂, NO₂, and N₂O₄ on SSZ-13 at varying Si/Al (1, 5, 11, 71, +∞), temperatures (248-348 K), and pressures (0-100 kPa). The adsorption capacity and affinity (SO₂ > N₂O₄ > NO₂) demonstrated strong competitive adsorption of SO₂ based on dual-site interactions and significant reduction in NO₂ adsorption due to dimerization in the ternary gas mixture. The simulated order of diffusivity (NO₂ > SO₂ > N₂O₄) on SSZ-13 demonstrated rapid transport of NO₂, strong temperature dependence of SO₂ diffusion, and the impermeability of SSZ-13 to N₂O₄. The membrane permeability of each component was simulated, rendering a SO₂/NO₂ membrane separation factor of 26.34 which is much higher than adsorption equilibrium (6.9) and kinetic (2.2) counterparts. The key role of NO₂-N₂O₄ dimerization in molecular sieving of SO₂ from NO₂ was addressed, providing a facile membrane separation strategy at room temperature.

Adsorption and the Chemical Reaction N2O4 ↔ 2NO2 in the Presence of N2 in a Gas Phase Connected with a Carbon Nanotube

The paper shows, by molecular simulations, that a CNT (9,9) carbon nanotube allows very efficient separation of nitrogen oxides (NO _x ) from N₂, that has in good approximation properties of the complete air mixture. Gibbs ensemble Monte Carlo simulations are used to describe the adsorption. The permanent chemical reaction between N₂O₄ and NO₂, which occurs simultaneously to adsorption, is treated by the reactive Monte Carlo simulation. A very high selectivity has been found. For a low pressure and at T = 298 K, an adsorption/reaction selectivity between NO _x and N₂ can reach values up to 3 × 10³.

Controlled Synthesis of Large Single Crystals of Metal-Organic Framework CPO-27-Ni Prepared by a Modulation Approach: In situ Single-Crystal X-ray Diffraction Studies

The size of single crystals of the metal-organic framework CPO-27-Ni was incrementally increased through a series of modulated syntheses. A novel linker modulated synthesis using 2,5-dihydroxyterephthalic acid and the isomeric ligand 4,6-dihydroxyisophthalic acid yielded large single crystals of CPO-27-Ni (∼70 μm). All materials were shown to have high crystallinity and phase purity through powder X-ray diffraction, electron microscopy methods, thermogravimetry, and compositional analysis. For the first time single-crystal structure analyses were carried out on CPO-27-Ni. High BET surface areas and nitric oxide (NO) release efficiencies were recorded for all materials. Large single crystals of CPO-27-Ni showed a prolonged NO release and proved suitable for in situ single-crystal diffraction experiments to follow the NO adsorption. An efficient activation protocol was developed, leading to a dehydrated structure after just 4 h, which subsequently was NO-loaded, leading to a first NO loaded single-crystal structural model of CPO-27-Ni.

Brick-CFCMC: Open Source Software for Monte Carlo Simulations of Phase and Reaction Equilibria Using the Continuous Fractional Component Method

We present a new molecular simulation code, Brick-CFCMC, for performing Monte Carlo simulations using state-of-the-art simulation techniques. The Continuous Fractional Component (CFC) method is implemented for simulations in the NVT/NPT ensembles, the Gibbs Ensemble, the Grand-Canonical Ensemble, and the Reaction Ensemble. Molecule transfers are facilitated by the use of fractional molecules which significantly improve the efficiency of the simulations. With the CFC method, one can obtain phase equilibria and properties such as chemical potentials and partial molar enthalpies/volumes directly from a single simulation. It is possible to combine trial moves from different ensembles. This enables simulations of phase equilibria in a system where also a chemical reaction takes place. We demonstrate the applicability of our software by investigating the esterification of methanol with acetic acid in a two-phase system.

Combined Adsorption and Reaction in the Ternary Mixture N2, N2O4, NO2 on MIL-127 Examined by Computer Simulations

A high selectivity of NO _x over N₂ (simulating air) is found in silico when studying the adsorption of the ternary mixture N₂O₄/NO₂/N₂ on the metal-organic framework MIL-127(Fe) by molecular simulations under consideration of the recombination reaction N₂O₄ ↔ 2NO₂. The number of N atoms in nitrogen oxides NO _x and that in N₂ is used to define a selectivity of the combined adsorption and chemical recombination that can reach values of about 1000.

Capture of nitrogen dioxide and conversion to nitric acid in a porous metal-organic framework

Air pollution by nitrogen oxides, NO_x, is a major problem, and new capture and abatement technologies are urgently required. Here, we report a metal-organic framework (Manchester Framework Material 520 (MFM-520)) that can efficiently confine dimers of NO₂, which results in a high adsorption capacity of 4.2 mmol g^-1 (298 K, 0.01 bar) with full reversibility and no loss of capacity over 125 cycles. Treatment of NO₂@MFM-520 with water in air leads to a quantitative conversion of the captured NO₂ into HNO₃, an important feedstock for fertilizer production, and fully regenerates MFM-520. The confinement of N₂O₄ inside nanopores was established at a molecular level, and the dynamic breakthrough experiments using both dry and humid NO₂ gas streams verify the excellent stability and selectivity of MFM-520 and confirm its potential for precious-metal-free deNO_x technologies.

The effects of confinement in pores built of folded graphene sheets on the equilibrium of nitrogen monoxide dimerisation reaction

In the current work we have used reactive Monte Carlo simulations to systematically study the effects of graphene folding on equilibria of NO dimerisation occurring at isolated surfaces and in porous networks built of corrugated graphene sheets. It has been demonstrated that the folding of isolated graphene sheets significantly improves the yield of reactions occurring on their surface. Then, it has also been shown that in slit-like pores formed by the folded graphene sheets the reaction yield depends on the corrugation and arrangement of the pore walls. It has been found that the reaction yield increases when the walls' corrugation is high because of the appearance of narrow regions and/or wedge-like regions in the pores. The condensation of reacting fluid in such places, where the bulges at both walls are close one to another, leads to much higher reaction yield than on the surface of isolated sheets. Thus, we recommended the highly corrugated graphene to control the chemical reactions.

Carbon Nanohorns as Reaction Nanochambers - a Systematic Monte Carlo Study

Carbon nanohorns (CNHs, one of the newest carbon allotropes) have been subjected to intensive experimental and theoretical studies due to their potential applications. One of such applications can be their use as reaction nanochambers. However, experimental studies on the reaction equilibria under confinement are extremely challenging since accurate measurements of the concentrations of reacting species in pores are a very hard task. So, the main ways to examine such phenomena are theoretical methods (e.g. the reactive Monte Carlo, RxMC). We have presented the first systematic RxMC study on the influence of the CNH’s geometric parameters (the apex angle, the diameter, and the length) on reaction equilibria, taking the nitrogen monoxide dimerisation as an example. All the investigated parameters significantly affect the reaction yield at low and moderate coverages. Short and narrow CNHs have been found to be preferred. However, the key factor influencing the reaction equilibria is the presence of a conical part. Energetics of interactions between the reacting molecules in this fragment of a nanohorn maximises the effects of confinement. In consequence, CNHs have the advantage over their nanotube counterparts of the same diameter. The obtained results have confirmed that CNHs can be considered as potential reaction nanochambers.