Theoretical Hydrogen Cryostorage in Doped MIL-101(Cr) Metal–Organic Frameworks

The cryoadsorption (77 K) of H₂ in the MIL-101­(Cr) [MIL: Materials from the Institute Lavoisier] metal–organic framework (MOF) material and its Li⁺, Mg²⁺, Mn²⁺, and Co²⁺-doped analogues was explored by grand canonical Monte Carlo simulations (GCMC). The optimal hydrogen uptake in this highly porous material is still experimentally unknown considering the experimental difficulty to fully activate this sample. Indeed, a H₂ adsorption isotherm has only been measured for a mildly activated version (MIL-101b­(Cr)). Moreover, the recent adsorption of CO₂ in better activated form (MIL-101c­(Cr)) shows an increase up to 30% of the saturation capacity in comparison to MIL-101b­(Cr). From GCMC simulations, we provide the optimal uptake and delivery of H₂ at 77 K in the MIL-101(Cr) and its doped analogues at 77 K. For the Li-doped material we predict a hydrogen uptake of 10 wt % and a delivery of 6 wt %, which achieve the mass storage and delivery density target established by the U.S. Department of Energy for 2015

Multiscale Modeling of the HKUST-1/Poly(vinyl alcohol) Interface: From an Atomistic to a Coarse Graining Approach

We present a computational multiscale study of a metal–organic framework (MOF)/polymer composite combining micro- and mesoscopic resolution, by coupling atomistic and coarse grained (CG) force field-based molecular dynamics simulations. As a proof of concept, we describe the copper paddlewheel-based HKUST-1 MOF/poly­(vinyl alcohol) composite. Our newly developed CG model reproduces the salient features of the interface in excellent agreement with the atomistic model and allows the investigation of substantially larger systems. The polymer penetrates into the open pores of the MOF as a result of the interactions between its OH groups and the O and Cu atoms in the pores, suggesting an excellent MOF/polymer compatibility. Polymer structure is affected by the MOF surface up to a distance of ∼2.4 times its radius of gyration. This study paves the way toward understanding important interfacial phenomena such as aggregation and phase separation in these mixed matrix systems

Diffusion of CH₄, CO₂, and Their Mixtures in AlPO₄‑5 Investigated by QENS Experiments and MD Simulations

Quasi-elastic neutron scattering (QENS) measurements in combination with molecular dynamics (MD) simulations have been performed to characterize the dynamics of CH₄, CO₂, and binary mixtures of different compositions in the zeolite-type AlPO₄-5 material. The experimental and simulated self-diffusion coefficients (<i>D</i>_s) for CH₄ in the presence of CO₂ are in very good agreement in a whole range of CO₂ concentrations, showing a decreasing profile when the CO₂ loading increases. Similar to the diffusion of light gases in other nanoporous materials, the experimental and simulation approaches both evidence a fast mobility for CH₄ at low loading in this zeolite. Complementary to this, the MD simulations predict a slightly faster diffusivity for CH₄ in binary mixtures with CO₂ when compared to its behavior as a single component, which is concomitant with a speeding up of the CO₂ molecules. QENS further reveals a nonmonotonous evolution of the transport diffusivity for CO₂ as a function of the loading. This peculiar behavior is reproduced by MD simulations, with the minimum being shifted to a higher concentration. A deep analysis of the MD spatial densities indicates that both CO₂ and CH₄ experience a 1D-type normal diffusion along the AlPO₄-5 channels in a hollow cylinder with a hexagonal base. Finally, QENS and MD allow the exploration of the rotational dynamics of CH₄ as a pure component and in a binary mixture

Engineering of an Isoreticular Series of CALF-20 Metal–Organic Frameworks for CO₂ Capture

A series of linker-substituted ultramicroporous CALF-20 metal–organic frameworks (MOFs) were built in silico, and their CO2 capture performances over N2 in flue gas conditions were systematically computationally explored. Among the various linker substitutions explored, squarate-linker-incorporated CALF-20 (SquCALF-20) was demonstrated to show a larger CO2 uptake at 0.15 bar (3.6 mmol/g) and higher CO2/N2 selectivity (500) in dry conditions compared to pristine CALF-20. Interestingly, this MOF was shown to maintain a high level of CO2 capture performance even in the presence of humidity, although it starts to adsorb H2O at lower relative humidity compared to CALF-20. Because squaric acid is a semiconductor industry feedstock and the few-already published squarate-based MOFs are chemically robust, this engineered SquCALF-20 offers a promising avenue for cost-effective CO2 capture via physisorption, with potential applications in addressing environmental concerns associated with CO2 emissions

Modeling of Adsorption Thermodynamics of Linear and Branched Alkanes in the Aluminum Fumarate Metal Organic Framework

Aluminum fumarate is one of the most stable metal–organic frameworks (MOFs), showing good cycling performance in water adsorption and desorption. Because of its rather small pore size, this MOF shows shape selectivity in the adsorption of linear and branched alkanes. In this work, the interaction of a broad series of alkanes with this MOF was studied through molecular simulations. We expand the transferability of a periodic density functional theory (DFT)-derived force field previously reported by Kulkarni and Sholl to the case of alkane adsorption on this aluminum fumarate MOF. With this force field and using configurational bias Monte Carlo simulations (CBMC), low coverage adsorption enthalpies, adsorption entropies, and Henry’s adsorption constants were calculated. Experimental enthalpies of adsorption (−Δ<i>H</i>₀) of C5–C8 <i>n</i>- and iso-alkanes are accurately reproduced by our calculations, e.g., within 5% relative error for <i>n</i>-alkanes. Interestingly, a compensation effect between adsorption enthalpy and adsorption entropy is found in the simulations, with a calculated slope almost identical to the experimental value. This indicates that the force field is very well capable of predicting tendencies with respect to the energetic interactions between the confined molecules and the MOF pore walls. Our calculations also predict separation between linear and branched alkanes with very good accuracy

Computational Exploration of the Water Concentration Dependence of the Proton Transport in the Porous UiO–66(Zr)–(CO₂H)₂ Metal–Organic Framework

The UiO–66­(Zr)–(CO₂H)₂ metal–organic framework (MOF) has been recently revealed as a promising proton conducting material under humidification. Here, aMS-EVB3 molecular dynamics simulations are performed to reveal at the molecular level the structure, thermodynamics, and dynamics of the hydrated proton in three-dimensional (3D)-cages MOF as a function of the water loading. It is found that the most stable proton solvation structure corresponds to a H₇O₃⁺ cation and that a transition between this complex and a Zundel cation likely governs the proton transport in this MOF occurring via a Grotthuss-type mechanism. It is further shown that the formation of a H₂O hydrogen-bonded bridge that connects the cages occurs only at high water concentration and this creates a path allowing the excess proton to jump from one cage to another. This leads to a faster self-diffusivity of proton at high water concentration, thereby supporting the increase of the proton conductivity with the water loading as experimentally evidenced

Highly Selective CO₂ Capture by Small Pore Scandium-Based Metal–Organic Frameworks

The selective CO₂ adsorption performance of a series of functionalized small pore scandium terephthalate MOFs was explored by quantum and force-field-based molecular simulations. The NO₂ derivative was predicted to be highly selective for CO₂ over N₂ and CH₄, outperforming most of the MOFs as well as other classes of porous solids reported so far. The potential of this solid for physisorption based-applications was further confirmed by (i) an adsorbent performance indicator (API) which exceeds that previously evaluated for many MOFs, (ii) an easy regeneration under mild condition as revealed by high-throughput manometric adsorption experiments although a relatively high CO₂ adsorption enthalpy was confirmed by microcalorimetry, and (iii) a good stability under moisture

Tailoring Metal-Ion-Doped Carbon Nitrides for Photocatalytic Oxygen Evolution Reaction

Poly(heptazine imides) (PHIs) have emerged as prominent layered carbon nitride-based materials with potential oxygen evolution reaction (OER) catalytic activity owing to their strong VIS light absorption, long excited-state lifetimes, high surface-to-volume ratios, and the possibility of tuning their properties via hosting different metal ions in their pores. A series of metal-ion-doped PHI-M (M = K+, Rb+, Mg2+, Zn2+, Mn2+, and Co2+) were first systematically explored using density functional theory calculations. These simulations led an in-depth understanding of the microscopic OER mechanism in these systems and identified PHI-Co2+ as the best OER catalyst of this family of PHIs, whereas PHI-Mn2+ can be an alternative promising OER catalyst. This level of performance was attributed to a thermodynamically favorable formation of the reaction intermediates as well as its red-shifted absorption in the VIS region involving the population of long-lived states, as revealed by time-dependent density functional theory calculations. We further demonstrated that the electronic properties of the *OH intermediates (Bader population, crystal orbital Hamilton population analysis, and adsorption energies) are reliable descriptors to anticipate the OER activity of this family of PHIs. This rational analysis paved the way toward the prediction of the OER performance of another PHI-M derivative, i.e., PHI-Fe2+. The computationally explored PHI-Fe2+, PHI-Mn2+, and PHI-Co2+ systems were then synthesized alongside PHI-K+, and their photocatalytic OER activities were assessed. These experimental findings confirmed the best photocatalytic OER performance for PHI-Co2+ with an oxygen production of 31.2 μmol·h–1 that is 60 times higher than the pristine g-C3N4 (0.5 μmol·h–1), whereas PHI-Fe2+ and PHI-Mn2+ are seen as alternative OER catalysts with attractive oxygen production of 11.20 and 4.69 μmol·h–1, respectively. Decisively, this joint experimental–computational study reveals PHI-Co2+ to be among the best of the OER catalysts so far reported in the literature including some perovskites

Revealing the Structure–Property Relationships of Metal–Organic Frameworks for CO₂ Capture from Flue Gas

It is of great importance to establish a quantitative structure–property relationship model that can correlate the separation performance of MOFs to their physicochemical features. In complement to the existing studies that screened the separation performance of MOFs from the adsorption selectivity calculated at infinite dilution, this work aims to build a QSPR model that can account for the CO₂/N₂ mixture (15:85) selectivity of an extended series of MOFs with a very large chemical and topological diversity under industrial pressure condition. It was highlighted that the selectivity for this mixture under such conditions is dominated by the interplay of the difference of the isosteric heats of adsorption between the two gases and the porosity of the MOF adsorbents. On the basis of the interplay map of both factors that impact the adsorption selectivity, strategies were proposed to efficiently enhance the separation selectivity of MOFs for CO₂ capture from flue gas. As a typical illustration, it thus leads us to tune a new MOF with outstanding separation performance that will orientate the synthesis effort to be deployed

A Joint Experimental/Computational Exploration of the Dynamics of Confined Water/Zr-Based MOFs Systems

A joint modeling (molecular dynamics simulations)/​experimental (broadband dielectric spectroscopy) approach was conducted to investigate the water adsorption in the UiO-66­(Zr) MOF, and its functionalized versions bearing acidic polar groups (−COOH or 2-COOH per linker). It was first pointed out that the proton conduction measured at room temperature increases with (i) the water uptake and (ii) the concentration of the free acidic carboxylic functions. This trend was further analyzed in light of the preferential arrangements of water within the pores of each MOF as elucidated by molecular dynamics simulations. Indeed, it was revealed that the guest molecules preferentially (i) form interconnected clusters within the UiO-66­(Zr)­s cages and generate a H-bond network responsible for the proton propagation and (ii) strongly interact with the −COOH grafted functions, resulting in the creation of additional charge carriers in the case of the hydrated functionalized solids. Broadband dielectric spectroscopy shed light on how these water configurations impact the local dynamics of both the water molecules and the MOF frameworks. The dielectric relaxation investigation evidenced the existence of one or two relaxation processes, depending on the nature of the UiO-66­(Zr) framework and its hydration level. Compared to the dielectric behavior of water confined in a large variety of media, it was thus concluded that the fastest process corresponds to the dynamics of the water molecules forming clusters, while the slowest process is due to the concerted local motion of water/ligand entities