Thomas A. Manz

In this study, mixed matrix membranes (MMMs) consisting of a continuous polymer phase and a dispersed metal-organic framework (MOF) phase were designed to purify hydrogen from the solar water splitting process. We first identified 20 MOFs with a small pore limiting diameter of less than or equal to 3 Å to restrict the passage of larger molecules such as N2 and O2, thereby allowing the smaller H2 molecule to pass through. These MOFs were assessed for water stability, thermal stability, solvent removal stability, and to identify open-metal sites. We developed atomistic polarizable flexible force fields for these 20 MOFs and used these in molecular dynamics and Monte Carlo simulations to calculate diffusion constants and adsorption isotherms. Using these results, we selected stable MOFs with the highest permeability and ideal selectivity for hydrogen separation. We used experimental data for polymer permeability and selectivity. Finally, we used the Maxwell model to predict the permeability and ideal selectivity of each MMM. We observed that selectivity is primarily governed by polymer properties, and permeability increases by adding MOFs as the dispersed phase. Our computed results showed that MMMs have approximately a twofold reduction in required membrane area compared with pure polymers. Triptycene-based polybenzoxazole (TPBO)-1.0/ARIBOS performed the best, requiring the smallest membrane area.

In this work, an extensive study was carried out to quantify the effects of various reaction conditions on the rate and selectivity of liquid-phase butyronitrile hydrogenation. In this reaction, butylamine is the desired product, and dibutylamine is produced as an unwanted by-product. Liquid-phase catalytic butyronitrile hydrogenation reactions were carried out in batch reactors at hydrogen pressures from 1 to 15 atm and at temperatures from 20 to 75 °C. Methanol was used as a solvent. The stirring speed was set sufficiently high to avoid external mass transfer limitations. Experiments were performed using several Raney nickel catalysts: an unpromoted catalyst, Fe-promoted catalysts, Cr-promoted catalysts, and a Fe and Cr doubly-promoted catalyst. Fe and/or Cr promotion increases the reaction rate while having little impact on the selectivity to primary amine. NaOH, NaCl, and butylamine were added separately to the reaction mixture, and the resulting changes in selectivity and reaction rate were measured to investigate the reaction mechanism. Water was also sometimes added to the reaction mixture (e.g., 50 mL methanol plus 10 mL water for the 1 atm batch reactions). An effective activation energy of ∼45 kJ/mol was computed for the unpromoted catalyst. Consistent with prior reports, NaOH addition improved the selectivity to butylamine by suppressing the condensation pathway. It was observed that adding NaCl to the reaction mixture improved the selectivity to primary amine. Adsorption experiments showed that butylamine adsorbed on two different site groups. One site group was blocked by added [OH]−, and the other site group was blocked by adsorbed carbon monoxide (CO). We propose and discuss a kinetic model to explain the observed reactivity and selectivity trends.

In this study, mixed matrix membranes (MMMs) consisting of a continuous polymer phase and a dispersed metal-organic framework (MOF) phase were designed to purify hydrogen from the solar water splitting process. We first identified 20 MOFs with a small pore limiting diameter of ≤3 Å to restrict the passage of larger molecules such as N ₂ and O ₂ thereby allowing the smaller H ₂ molecule to pass through. These MOFs were assessed for water stability, thermal stability, solvent removal stability, and to identify open-metal sites. We developed atomistic polarizable flexible forcefields for these 20 MOFs and used these in molecular dynamics and Monte Carlo simulations to calculate diffusion constants and adsorption isotherms. Using these results, we selected stable MOFs with the highest permeability and ideal selectivity for hydrogen separation. We utilized experimental data for polymer permeability and selectivity. Finally, we used the Maxwell model to predict the permeability and ideal selectivity of each MMM. We observed that selectivity is primarily governed by polymer properties, and permeability increases by adding MOFs as the dispersed phase. Our computed results showed that MMMs have approximately twofold reduction in required membrane area compared to pure polymers. TPBO-1.0/ARIBOS performed the best, requiring the smallest membrane area.

This study measured various effects of iron and chromium promotion on the microstructures of Raney(r) nickel alloys and catalysts. The following alloys and catalysts were studied: (i) unpromoted, (ii) multiple levels of Fe promotion, (iii) multiple levels of Cr promotion, and (iv) doubly-promoted with Fe and Cr. Microstructures and phase compositions of some initial alloys were characterized using backscattered scanning electron microscopy (SEM), electron microprobe, optical microscopy, and x-ray diffraction. Backscattered SEM and electron microprobe analysis showed the microstructure is composed of several phases: Al ₃ Ni ₂ , Al ₃ Ni, promoter-rich phase(s) (which are absent in the unpromoted alloy), and a eutectic phase. The Fe and Cr promoters concentrate in the promoter-rich phases in these alloys. Based on XRD pattern and the phase diagram, we have assigned ‘Fe promoter phase 1’, present in some Raney nickel alloys promoted with ≥ 2 wt.% Fe, as the decagonite (Al ₇₁ Ni ₂₄ Fe ₅ ) quasicrystal. This was a major finding, because decagonite was previously thought to be rare. Also, ‘Fe promoter phase 2’, present in all of the Fe-only promoted alloys, Al ₉ Ni _(2-y) Fe _y (where y was 0.37 to 0.77) crystalline phase has been proposed, based on the findings of this study. A phi phase of composition Al _77.5 Cr ₁₂ Ni _10.5 has been proposed for ‘Cr promoter phase’ (present in Cr-only promoted alloys) in this study. A ‘Cr, Fe promoter phase’ of approximate composition Al _75.5 Ni _13-14 Cr _6-7 Fe ₄ was present in the Cr, Fe doubly-promoted alloys. The alloys were crushed into a powder and leached in aqueous sodium hydroxide solution. After leaching, the catalysts were washed and subsequently characterized. Nitrogen adsorption at 77 K was performed to extract the total surface area and pore size distribution of each catalyst. Carbon monoxide and hydrogen gas adsorption were performed to measure each catalyst’s metallic surface area. For some catalysts, bulk elemental composition was quantified using atomic absorption spectroscopy, and the surface compositions and oxidation states were studied using x-ray photoelectron spectroscopy (XPS). XPS results showed that Cr is oxidized to Cr(+III) during the leaching procedure to form a compound (likely chromium hydroxide) that increases the total surface area (while having negligible effect on the metallic surface area) and also leads to smaller pore diameters.

In this work, an extensive study was carried out to quantify the effects of various reaction conditions on the rate and selectivity of liquid-phase butyronitrile hydrogenation. In this reaction, butylamine is the desired product, and dibutylamine is produced as an unwanted by-product. Liquid-phase catalytic butyronitrile hydrogenation reactions were carried out in batch reactors at hydrogen pressures from 1–15 atm and at temperatures from 20–75 °C. Methanol was used as a solvent. The stirring speed was set sufficiently high to avoid mass transfer limitations. Experiments were performed using several Raney(r) nickel catalysts: an unpromoted catalyst, Fe promoted catalysts, Cr promoted catalysts, and a Fe and Cr doubly-promoted catalyst. Fe and/or Cr promotion increases the reaction rate while having little impact on the selectivity to primary amine. The effects of adding NaOH, NaCl, and butylamine to the reaction mixture were measured separately. Water was also sometimes added to the reaction mixture (e.g., 50 mL methanol plus 10 mL water for the 1 atm batch reactions). An effective activation energy of 45 kJ/mol was computed for the unpromoted catalyst. Consistent with prior reports, NaOH addition improved the selectivity to butylamine by suppressing the condensation pathway. It was observed that adding NaCl to the reaction mixture improved the selectivity to primary amine. Adsorption experiments showed that butylamine adsorbed on two different site groups. One site group was blocked by added [OH] ^- , and the other site group was blocked by adsorbed carbon monoxide (CO). We propose and discuss some specific kinetic models as potential explanations for the observed reactivity and selectivity trends.

Several Mn–Ni alloys having more than 50 at. % Mn were prepared, and their phase microstructures were examined using optical microscopy, electron microprobe analysis, and X-ray diffraction. These alloys were leached in concentrated acetic acid aqueous solutions. The resulting nanoporous materials were characterized using optical microscopy, element composition analysis, and liquid nitrogen physisorption. Three alloy compositions were studied in detail. The first alloy containing ∼57 at. % Mn was only leachable near the surface. The second alloy containing ∼86 at. % Mn was readily leachable in concentrated acetic acid aqueous solution. Butyronitrile hydrogen reaction was performed over the resulting porous catalyst. The third alloy containing ∼73 at. % Mn was strong, nonbrittle, and did not deteriorate over more than 25+ years of observation. When this alloy was leached in concentrated acetic acid aqueous solution for up to 7 days, a nanoporous brick-like microstructure was produced that is extremely strong, nonbrittle, and lightweight with a low overall density. This material should find uses in applications that require strong, lightweight materials. When leached for 14 days, enough of the manganese atoms were removed that the microstructure had transitioned to a powdery macroporous material.

A popular way to measure the surface areas of porous solids involves fitting a small section of the experimentally measured liquid nitrogen, liquid argon, or moisture adsorption isotherm to the Brunauer-Emmett-Teller (BET) model. A small section is used, because the BET isotherm's shape differs dramatically from the experimental isotherm's shape. Several studies proposed consistency criteria to automate choosing the small isotherm portion fitted during BET analysis. In this article, we follow a different approach that constructs a general-purpose model adsorption isotherm with enough flexibility to fit the experimental adsorption isotherm across its entire pressure range. This enables the whole adsorption isotherm to be used to extract surface areas of porous materials. Our MD model isotherm is formulated to apply to fluids both below and above the critical temperature and critical pressure. Mathematical derivations revealed this isotherm is non-negative, monotonically increasing with x_A, and has correct limiting behaviors. We explain relationships between the MD model isotherm and many model isotherms described in prior literature. When using a single sitegroup, the MD model has five fitted parameters. We introduce a bootstrapping algorithm that computes 95% confidence intervals on the optimized model parameters and whole isotherm surface area. We present a detailed derivation of this new model isotherm and illustrate its utility with diverse examples: (a) liquid N₂ adsorption onto several porous nickel alloy sponges and metal-organic frameworks (MOFs), (b) diverse examples of gas adsorption in MOFs, (c) moisture adsorption onto various hydrophilic and hydrophobic materials, (d) adsorption of several proteins onto hydrophobic interaction chromatography resins, (e) CO₂ adsorption on Zeolite 13X, (f) butylamine adsorption from methanol-water solution onto sponge nickel catalysts, (g) aqueous red dye adsorption onto beechwood, (h) step-like layer-by-layer adsorption of methane onto a MgO surface at 87 K. These examples spanned: (i) each of the six IUPAC physisorption isotherm types, (ii) positive cooperative, negative cooperative, and noncooperative adsorption, (iii) single-layer and multilayer adsorption, (iv) adsorption with and without capillary condensation, and (v) adsorption modeled by a single sitegroup and by multiple sitegroups. Results showed the MD isotherm is a good general-purpose model for physical adsorption under diverse conditions on diverse materials.

This article uses many characterization techniques to achieve new insights into effects of Fe and Cr promotion on microstructures of RANEY® nickel alloys and catalysts, which have widespread importance as hydrogenation catalysts.

Correction for ‘An automated protocol to construct flexibility parameters for classical forcefields: applications to metal–organic frameworks’ by Reza Ghanavati et al., RSC Adv., 2024, 14, 22714–22762, https://doi.org/10.1039/D4RA01859A.

Atom-centered point-charge models are computationally efficient and commonly used electrostatic models to build forcefields used in classical simulations of materials. To assess their performance, we evaluated various atomic charge assignment methods across the following material types: (a) organic molecules, (b) inorganic molecules, (c) heterodiatomic molecules, (d) transition metal complexes, and (e) nanoporous crystals. We compared 12 atomic charge assignment methods for molecular systems and 6 for nanoporous crystals. In this article, we introduce a computationally efficient quadrupole-dipole-resorption (QDR) method that improves the accuracy of stockholder-partitioning models (e.g., DDEC6) for approximately reproducing the electrostatic potential and molecular dipole and quadrupole moments. For each electrostatic model, we computed the electrostatic potential's root-mean-squared error (RMSE) and relative RMSE (RRMSE) using the material's QM-computed electrostatic potential as a reference. The electrostatic RMSE and RRMSE were computed for a training dataset containing 21 geometric conformations per material and a validation dataset containing 20 new geometric conformations per material. Raincloud plots were prepared to visualize the resulting data distributions. For each charge assignment method in the nonperiodic materials, we also computed and compared the root-mean-squared charge transfer magnitude, correlations to other charge assignment methods, conformational sensitivity, etc. Among point-charge models, multiframe ESP methods provided the best overall accuracy for reproducing the electrostatic potential across different system conformations, but they require a training set containing many geometric conformations. The QDR-DDEC6 and CM5 methods provided good conformational transferability and electrostatic potential accuracy across various conformations even when trained only on a single optimized ground-state geometry. A Pareto plot was prepared illustrating the tradeoff between conformational sensitivity and accuracy for reproducing the electrostatic potential of individual conformations. Including atomic dipoles (e.g., QDR-DDEC6_ad, DDEC6_ad, and MBIS_ad) greatly improved the electrostatic model for individual conformations, and QDR-DDEC6_ad outperformed all atom-centered point-charge models. We recommend that more computationally efficient methods be developed to use electrostatic models containing atom-centered point charges plus atomic dipoles in flexible forcefields. Finally, some electron-density partitioning approaches have the key advantage of providing accurate results even when applied to dense solids under high pressures, and we demonstrated this using 11 sodium chloride crystals having various stoichiometries.