Structural and Electronic Properties of Carbon Nanotubes and Graphenes Functionalized with Cyclopentadienyl–Transition Metal Complexes: A DFT Study

In order to explore possible ways of using metallocene compounds in heterogeneous catalysis and in sensor applications, we present a theoretical characterization of cyclopentadienyl (Cp) + transition metal (TM) complexes adsorbed on boron-doped carbon nanotubes (B-CNTs) and boron-doped graphenes. Using spin-polarized density functional theory calculations, we present a systematic study of the geometries, energetics, and electronic properties of CpTM (where TM = Fe, Ni, Co, Cr, Cu) adsorbed on both pristine and boron-doped carbon supports. Our work reveals significant increases of the binding energies between CpTM and boron-doped CNTs and graphenes (versus pristine carbon supports), surpassing even the adsorption strength of the isolated metals atoms (by about 2 eV). According to our electronic structure analysis, both the delocalization of the TM-d state by the presence of the Cp ring and the interactions between the Cp ring and the carbon substrate are responsible for the enhancement of the binding energies. This stabilization may play an important role in immobilizing ferrocene-based catalysts. Moreover, tunable metallicities of CpTMs adsorbed on pristine and on B-CNTs are observed, indicating potential applications of CpTM/B-CNT complexes in nanoelectronics and as sensors. Using these complexes, we also probed the adsorption of O₂ molecules, as an initial indicator of catalytic performance. Both chemisorption (with an elongated O–O bond) and dissociative chemisorption were found on CpFe/B-CNT (8,0) complexes

Water-Induced Interactions between Boron-Doped Carbon Nanotubes

Molecular dynamics (MD) simulations are used to investigate the hydration, the water-induced interactions, and the dispersion behavior of boron-doped single-walled carbon nanotubes (B-CNTs) within aqueous solutions. Models of B-CNTs with various diameters and B-doping patterns are developed, with partial charges calculated from first-principles density functional theory (DFT). Using MD simulations, the potential of mean force (PMF) of one, two, and three solvated B-CNTs are evaluated, and these results are compared to pristine CNTs. The hydration behavior of the B-CNTs is also quantified by evaluating the water density profiles and hydrogen bonds during the solvation. Our MD simulations indicate the presence of water-induced interactions with B-CNTs over prolonged distances, as compared to pristine CNTs. In particular, the B-CNTs are more resistant to reaggregation than pristine CNTs. These simulation results thoroughly characterize the effects of substitutional doping of CNTs on their dispersion behavior in aqueous solution, and our atomistic simulations of B-CNTs are used to parametrize coarse-grained models of the nanotube–nanotube interactions

Linking Carbon and Boron-Nitride Nanotubes: Heterojunction Energetics and Band Gap Tuning

We investigate the energetics of forming heteronanotubes, which are combinations of pure carbon nanotube (CNT) segments and boron-nitride nanotube (BNNT) segments. Our density functional theory calculations predict that the adverse impacts of heterojunctions on the nanotube stability can be minimized if the CNT and/or the BNNT building block segments are sufficiently large along the axial direction (corresponding to circular junctions). As such, carbon−boron-nitride heteronanotubes can be thermodynamically competitive in stability, as compared to pure CNTs and BNNTs of similar geometry, and this is in good agreement with previous experimental observations. In addition, we find that the highest occupied crystal orbital/lowest unoccupied crystal orbital (HOCO−LUCO) gap of carbon−boron-nitride heteronanotubes can be significantly tuned by modifying the CNT and BNNT combinations, the tube chirality, or the junction geometry (i.e., circular or linear)

Predicting Gaseous Solute Diffusion in Viscous Multivalent Ionic Liquid Solvents

Calculating solute diffusion in dense, viscous solvents can be particularly challenging in molecular dynamics simulations due to the long time scales involved. Here, a new scaling approach is developed for predicting solute diffusion based on analyses of CO2 and SO2 diffusion in two different multivalent ionic liquid solvents. Various scaling approaches are initially evaluated, including single and separate thermostats for the solute and solvent, as well as the application of the Arrhenius relationship and the Speedy–Angell power law. A very strong logarithmic correlation is established between the solvent-accessible surface area and solute diffusion. This relationship, reflecting Danckwerts’ surface renewal theory and the Vrentas–Duda free volume model, presents a valuable method for estimating diffusion behavior from short simulation trajectories at elevated temperatures. The approach may be beneficial for enhancing predictive modeling in similar challenging systems and should be more broadly evaluated

Predicting Gaseous Solute Diffusion in Viscous Multivalent Ionic Liquid Solvents

Calculating solute diffusion in dense, viscous solvents can be particularly challenging in molecular dynamics simulations due to the long time scales involved. Here, a new scaling approach is developed for predicting solute diffusion based on analyses of CO2 and SO2 diffusion in two different multivalent ionic liquid solvents. Various scaling approaches are initially evaluated, including single and separate thermostats for the solute and solvent, as well as the application of the Arrhenius relationship and the Speedy–Angell power law. A very strong logarithmic correlation is established between the solvent-accessible surface area and solute diffusion. This relationship, reflecting Danckwerts’ surface renewal theory and the Vrentas–Duda free volume model, presents a valuable method for estimating diffusion behavior from short simulation trajectories at elevated temperatures. The approach may be beneficial for enhancing predictive modeling in similar challenging systems and should be more broadly evaluated

Tuning the Adsorption Interactions of Imidazole Derivatives with Specific Metal Cations

In this work, we report a computational study of the interactions between metal cations and imidazole derivatives in the gas phase. We first performed a systematic assessment of various density functionals and basis sets for predicting the binding energies between metal cations and the imidazoles. We find that the M11L functional in combination with the 6-311++G­(d,p) basis set provides the best compromise between accuracy and computational cost with our metal···imidazole complexes. We then evaluated the binding of a series of metal cations, including Li⁺, Na⁺, K⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Ba²⁺, Hg²⁺, and Pb²⁺, with several substituted imidazole derivatives. We find that electron-donating groups increase the metal-binding energy, whereas electron-withdrawing groups decrease the metal-binding energy. Furthermore, the binding energy trends can be rationalized by the hardness of the metal cations and imidazole derivatives, providing a quick way to estimate the metal···imidazole binding strength. This insight can enable efficient screening protocols for identifying effective imidazole-based solvents and membranes for metal adsorption and provide a framework for understanding metal···imidazole interactions in biological systems

Molecular Dynamics Simulation of Bismuth Telluride Exfoliation Mechanisms in Different Ionic Liquid Solvents

Bismuth telluride (Bi₂Te₃) is a well-known thermoelectric material with potential applications in several different emerging technologies. The bulk structure is composed of stacks of quintuple sheets (with weak interactions between neighboring sheets), and the performance of the material can be significantly enhanced if exfoliated into two-dimensional nanosheets. In this study, eight different imidazolium-based ionic liquids are evaluated as solvents for the exfoliation and dispersion of Bi₂Te₃ at temperatures ranging from 350 to 550 K. Three distinct exfoliation mechanisms are evaluated (pulling, shearing, and peeling) using steered molecular dynamics simulations, and we predict that the peeling mechanism is thermodynamically the most favorable route. Furthermore, the [Tf₂N^–]-based ionic liquids are particularly effective at enhancing the exfoliation, and this performance can be correlated to the unique molecular-level solvation structures developed at the Bi₂Te₃ surfaces. This information helps provide insight into the molecular origins of exfoliation and solvation involving Bi₂Te₃ (and possibly other layered chalcogenide materials) and ionic liquid solvents

Electrostatic Potential within the Free Volume Space of Imidazole-Based Solvents: Insights into Gas Absorption Selectivity

In this work, a variety of molecular simulation tools are used to help characterize the selective absorption of CO₂ and CH₄ in imidazole-based solvents. We focus our efforts on a series of 1-<i>n</i>-alkyl-2-methyl-imidazoles and ether-functionalized imidazoles, over a temperature range from 293 to 353 K, and we perform detailed analysis of the free volume. We find that the electrostatic potential within the solvent free volume cavities provides a useful indication of the selective absorption of CO₂ and CH₄. The electrostatic potential calculation is significantly faster than the direct calculation of the chemical potential, and tests with the 1-<i>n</i>-alkyl-2-methyl-imidazoles and the ether-functionalized imidazoles indicate that this may be a useful screening tool for other solvents

Molecular Simulation of Ionic Polyimides and Composites with Ionic Liquids as Gas-Separation Membranes

Polyimides are at the forefront of advanced membrane materials for CO₂ capture and gas-purification processes. Recently, ionic polyimides (i-PIs) have been reported as a new class of condensation polymers that combine structural components of both ionic liquids (ILs) and polyimides through covalent linkages. In this study, we report CO₂ and CH₄ adsorption and structural analyses of an i-PI and an i-PI + IL composite containing [C₄mim]­[Tf₂N]. The combination of molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) simulations is used to compute the gas solubility and the adsorption performance with respect to the density, fractional free volume (FFV), and surface area of the materials. Our results highlight the polymer relaxation process and its correlation to the gas solubility. In particular, the surface area can provide meaningful guidance with respect to the gas solubility, and it tends to be a more sensitive indicator of the adsorption behavior versus only considering the system density and FFV. For instance, as the polymer continues to relax, the density, FFV, and pore-size distribution remain constant while the surface area can continue to increase, enabling more adsorption. Structural analyses are also conducted to identify the nature of the gas adsorption once the ionic liquid is added to the polymer. The presence of the IL significantly displaces the CO₂ molecules from the ligand nitrogen sites in the neat i-PI to the imidazolium rings in the i-PI + IL composite. However, the CH₄ molecules move from the imidazolium ring sites in the neat i-PI to the ligand nitrogen atoms in the i-PI + IL composite. These molecular details can provide critical information for the experimental design of highly selective i-PI materials as well as provide additional guidance for the interpretation of the simulated adsorption systems

Effects of TiO₂ in Low Temperature Propylene Epoxidation Using Gold Catalysts

Propylene epoxidation with molecular oxygen has been proposed as a green and alternative process to produce propylene oxide (PO). In order to develop catalysts with high selectivity, high conversion, and long stability for the direct propylene epoxidation with molecular oxygen, understanding of catalyst structure and reactivity relationships is needed. Here, we combined atomic layer deposition and deposition precipitation to synthesize series of well-defined Au-based catalysts to study the catalyst structure and reactivity relationships for propylene epoxidation at 373 K. We showed that by decorating TiO₂ on gold surface the inverse TiO₂/Au/SiO₂ catalysts maintained ∼90% selectivity to PO regardless of the weight loading of the TiO₂. The inverse TiO₂/Au/SiO₂ catalysts exhibited improved regeneration compared to Au/TiO₂/SiO₂. The inverse TiO₂/Au/SiO₂ catalysts can be regenerated in 10% oxygen at 373 K, while the Au/TiO₂/SiO₂ catalysts failed to regenerate at as high as 473 K. Combined characterizations of the Au-based catalysts by X-ray absorption spectroscopy, scanning transmission electron microscopy, and UV–vis spectroscopy suggested that the unique selectivity and regeneration of TiO₂/Au/SiO₂ are derived from the site-isolated Ti sites on Au surface and Au–SiO₂ interfaces which are critical to achieve high PO selectivity and generate only coke-like species with high oxygen content. The high oxygen content coke-like species can therefore be easily removed. These results indicate that inverse TiO₂/Au/SiO₂ catalyst represents a system capable of realizing sustainable gas phase propylene epoxidation with molecular oxygen at low temperature