Carbon Dioxide Separation with a Two-Dimensional Polymer Membrane

Carbon dioxide gas separation is important for many environmental and energy applications. Molecular dynamics simulations are used to characterize a two-dimensional hydrocarbon polymer, PG-ES1, that uses a combination of surface adsorption and narrow pores to separate carbon dioxide from nitrogen, oxygen, and methane gases. The CO₂ permeance is 3 × 10⁵ gas permeation units (GPU). The CO₂/N₂ selectivity is 60, and the CO₂/CH₄ selectivity exceeds 500. The combination of high CO₂ permeance and selectivity surpasses all known materials, enabling low-cost postcombustion CO₂ capture, utilization of landfill gas, and horticulture applications

Helium Separation Using Porous Graphene Membranes

Graphene has been demonstrated to be impermeable to gases but can be made selectively permeable by introduction of pores. The permeability of a recently synthesized porous graphene structure to He, Ne, and CH₄ is studied using MP2/cc-pVTZ potential energy surfaces. The role of quantum and classical transmission effects as a function of temperature are investigated. At room temperature, there is a 20 and 16% increase in transmission due to quantum tunneling for ³He and ⁴He, respectively, over the purely classical result. The large differences in classical barrier heights for transmission through this membrane (0.523, 1.245, and 4.832 eV for He, Ne, and CH₄, respectively) allow for highly selective separation. This is proposed as an economical means of separating He from the other noble gases and alkanes present in natural gas

Carbon Dioxide Separation with a Two-Dimensional Polymer Membrane

Carbon dioxide gas separation is important for many environmental and energy applications. Molecular dynamics simulations are used to characterize a two-dimensional hydrocarbon polymer, PG-ES1, that uses a combination of surface adsorption and narrow pores to separate carbon dioxide from nitrogen, oxygen, and methane gases. The CO₂ permeance is 3 × 10⁵ gas permeation units (GPU). The CO₂/N₂ selectivity is 60, and the CO₂/CH₄ selectivity exceeds 500. The combination of high CO₂ permeance and selectivity surpasses all known materials, enabling low-cost postcombustion CO₂ capture, utilization of landfill gas, and horticulture applications

Helium Tunneling through Nitrogen-Functionalized Graphene Pores: Pressure- and Temperature-Driven Approaches to Isotope Separation

Recently, we showed that nitrogen-functionalized nanopores obtained by removing two rings from a perfect graphene sheet provide suitable barriers for a separation of fermionic helium-3 from its bosonic counterpart helium-4 [<i>J. Phys. Chem. Lett.</i> <b>2012</b>, <i>3</i>, 209–213]. In this follow-up Article, we provide potential curves for helium passing through several different types of pores, discuss the relation of the barrier height to the effective pore size, give estimations for bound states of helium attached to the pores, and analyze the effects of isomeric and stoichiometric variations of the pore-rim nitrogen-passivation on the gas separation performance. Slight deviations in the tunneling probability for the two helium isotopes can lead to a high selectivity at an industrially acceptable gas flux if the gas temperature is kept sufficiently low. We also recently showed that the mass-dependence of quantum tunneling and zero-point energy differences at the top of the potential energy barrier allow for a classically prohibited steady-state thermally driven isotope separation [<i>Chem. Phys. Lett.</i> <b>2012</b>, <i>521</i>, 118–124]. The nitrogen-passivated nanopores studied here give rise to larger steady-state isotopic enrichment than that in previous work and are dominated by zero-point energy differences at both high and low temperatures

Bio-Inspired Electroactive Organic Molecules for Aqueous Redox Flow Batteries. 1. Thiophenoquinones

Redox flow batteries (RFB) utilizing water-soluble organic redox couples are a new strategy for low-cost, eco-friendly, and durable stationary electrical energy storage. Previous studies have focused on benzoquinones, napthoquinones, and anthraquinones as the electroactive species. Here, we explore a new class of moleculesthiophenoquinonesspecifically focusing on the caldariellaquinone-, sulfolobusquinone-, and benzodithio­phenoquinone-like frameworks that are used for metabolic processes in thermophilic aerobic <i>Sulfolobus</i> archaebacteria. We demonstrated that B3LYP/6-311+G­(d,p) thermochemical calculations (using the SMD solvation model) reproduce experimental reduction potentials to within ±0.04 V. We then studied the effect of amine, hydroxyl, methyl, fluoro, phosphonic acid, sulfonic acid, carboxylic acid, and nitro functional group modifications on the reduction potential and Gibbs energy of solvation in water (using density functional theory) and aqueous solubility (using cheminformatics). Next we enumerated all of the 10 611 possible combinations of functional group substitutions on these frameworks and identified 1056 potential molecules with solubilities exceeding 2 mol/L; of these, 36 molecules have reduction potentials below 0.25 V and 15 molecules above 0.95 V (versus the standard hydrogen electrode (SHE)). The combination of high solubility and wide voltage range makes these molecules promising candidates for high performance aqueous RFB applications. Finally, using our data set of <i>ab initio</i> reduction potentials, we developed a cheminformatics model that predicts <i>ab initio</i> reduction potentials to within ±0.09 V based solely on molecular connectivity. We found that a model trained with as few as 200 examples generates rank-ordered predictions allowed us to identify the highest performance candidates with half the number of <i>ab initio</i> calculations. This offers a strategy for improving the tractability of future computational searches for high performance RFB molecules

Gas Separation through Bilayer Silica, the Thinnest Possible Silica Membrane

Membrane-based gas separation processes can address key challenges in energy and environment, but for many applications the permeance and selectivity of bulk membranes is insufficient for economical use. Theory and experiment indicate that permeance and selectivity can be increased by using two-dimensional materials with subnanometer pores as membranes. Motivated by experiments showing selective permeation of H₂/CO mixtures through amorphous silica bilayers, here we perform a theoretical study of gas separation through silica bilayers. Using density functional theory calculations, we obtain geometries of crystalline free-standing silica bilayers (comprised of six-membered rings), as well as the seven-, eight-, and nine-membered rings that are observed in glassy silica bilayers, which arise due to Stone–Wales defects and vacancies. We then compute the potential energy barriers for gas passage through these various pore types for He, Ne, Ar, Kr, H₂, N₂, CO, and CO₂ gases, and use the data to assess their capability for selective gas separation. Our calculations indicate that crystalline bilayer silica, which is less than a nanometer thick, can be a high-selectivity and high-permeance membrane material for ³He/⁴He, He/natural gas, and H₂/CO separations

EQeq+C: An Empirical Bond-Order-Corrected Extended Charge Equilibration Method

The extended charge equilibration (EQeq) scheme computes atomic partial charges using the experimentally measured ionization potentials and electron affinities of atoms. However, EQeq erroneously predicts constant (environment independent) charges for high-oxidation-state transition metals in amine-templated metal oxide (ATMO) compounds, contrary to the variation observed in iterative Hirshfeld (Hirshfeld-I) charges, bond-valence sum calculations, and formal oxidation state calculations. To fix this problem, we present a simple, noniterative empirical pairwise correction based on the Pauling bond-order/distance relationship, which we denote EQeq+C. We parametrized the corrections to reproduce the Hirshfeld-I charges of ATMO compounds and REPEAT charges of metal organic framework (MOF) compounds. The EQeq+C correction fixes the metal charge problem and significantly improves the partial atomic charges compared to EQeq. We demonstrate the transferability of the parametrization by applying it to a set of unrelated dipeptides. After an initial parametrization, the EQeq+C correction requires minimal computational overhead, making it suitable for treating large unit cell solids and performing large-scale computational materials screening

Role of Noncovalent Interactions in Vanadium Tellurite Chain Connectivities

Structural differences in [V₂Te₂O₁₀]_<i>n</i>^2<i>n</i>– chain metrics are directly ascribed to variations in noncovalent interactions in a series of organically templated vanadium tellurites, including [C₆H₁₇N₃]­[V₂Te₂O₁₀]·H₂O, [C₅H₁₆N₂]­[V₂Te₂O₁₀], and [C₄H₁₄N₂]­[V₂Te₂O₁₀]. The noncovalent interaction (NCI) method was used to locate, quantify, and visualize intermolecular interactions in [C₄H₁₄N₂]­[V₂Te₂O₁₀] and [C₅H₁₆N₂]­[V₂Te₂O₁₀]. Variations in the van der Waals attractions between [1,4-diaminobutaneH₂]²⁺ and [1,5-diaminopentaneH₂]²⁺ result in divergent packing motifs for these cations, which causes a reorganization of N–H···O hydrogen bonding and variances in the [V₂Te₂O₁₀]_<i>n</i>^2<i>n</i>– chain metrics. The application of the NCI method to this type of solid-state structure provides a direct method to elucidate the structural effects of weak noncovalent interactions

Role of Hydrogen-Bonding in the Formation of Polar Achiral and Nonpolar Chiral Vanadium Selenite Frameworks

A series of organically templated vanadium selenites have been prepared under mild hydrothermal conditions. Single crystals were grown from mixtures of VOSO₄, SeO₂, and either 1,4-dimethylpiperazine, 2,5-dimethylpiperazine, or 2-methylpiperazine in H₂O. Each compound contains one-dimensional [VO­(SeO₃)­(HSeO₃)]_n^n‑ secondary building units, which connect to form three-dimensional frameworks in the presence of 2,5-dimethylpiperazine or 2-methylpiperazine. Differences in composition and both <i>intra</i>-secondary building unit and organic–inorganic hydrogen-bonding between compounds dictate the dimensionality of the resulting inorganic structures. [1,4-dimethylpiperazineH₂]­[VO­(SeO₃)­(HSeO₃)]₂ contains one-dimensional [VO­(SeO₃)­(HSeO₃)]_n^n‑ chains, while [2,5-dimethylpiperazineH₂]­[VO­(SeO₃)­(HSeO₃)]₂·2H₂O contains a three-dimensional [VO­(SeO₃)­(HSeO₃)]_n^n‑ framework. The use of racemic 2-methylpiperazine also results in a compound containing a three-dimensional [VO­(SeO₃)­(HSeO₃)]_n^n‑ framework, crystallizing in the noncentrosymmetric polar, achiral space group <i>Pca</i>2₁ (no. 29), while analogous reactions containing either (<i>R</i>)-2-methylpiperazine or (<i>S</i>)-2-methylpiperazine result in noncentrosymmetric, nonpolar chiral frameworks that crystallize in <i>P</i>2₁2₁2 (no. 18). The formation of these noncentrosymmetric framework materials is dictated by the structure, symmetry, and hydrogen-bonding properties of the [2-methylpiperazineH₂]²⁺ cations

Steric-Induced Layer Flection in Templated Vanadium Tellurites

A series of organically templated vanadium tellurites has been prepared under mild hydrothermal conditions. Single crystals were grown from mixtures of NaVO₃, Na₂TeO₃, and either 1,4-diaminobutane, 1,6-diaminohexane, or <i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-tetramethylethylenediamine in H₂O. Each compound contains similar [V₂Te₂O₁₀]_<i>n</i>^2<i>n</i>– layers. The layer metrics of [1,4-diaminobutaneH₂]­[V₂Te₂O₁₀], [1,6-diaminohexaneH₂]­[V₂Te₂O₁₀], [<i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-tetramethylethylenediamineH₂]­[V₂Te₂O₁₀], and [piperazineH₂]­[V₂Te₂O₁₀] reflect the steric bulk of the respective amines. Topotactic conversions between compounds through amine exchange are possible in reactions in which an increase in the strength of the amine–[V₂Te₂O₁₀]_<i>n</i>^2<i>n</i>– hydrogen-bonding network is observed