Selective Synthesis of Defect-Rich LaMnO3 by Low-Temperature Anion Cometathesis.

The synthesis of complex oxides at low temperatures brings forward aspects of chemistry not typically considered. This study focuses on perovskite LaMnO3, which is of interest for its correlated electronic behavior tied to the oxidation state and thus the spin configuration of manganese. Traditional equilibrium synthesis of these materials typically requires synthesis reaction temperatures in excess of 1000 °C, followed by subsequent annealing steps at lower temperatures and different p(O2) conditions to manipulate the oxygen content postsynthesis (e.g., LaMnO3+x). Double-ion exchange (metathesis) reactions have recently been shown to react at much lower temperatures (500-800 °C), highlighting a fundamental knowledge gap for how solids react at lower temperatures. Here, we revisit the metathesis reaction, LiMnO2 + LaOX, where X is a halide or mixture of halides, using in situ synchrotron X-ray diffraction. These experiments reveal low reaction onset temperatures (ca. 450-480 °C). The lowest reaction temperatures are achieved by a mixture of lanthanum oxyhalide precursors: 2 LiMnO2 + LaOCl + LaOBr. In all cases, the resulting products are the expected alkali halide salt and defective La1-ϵMn1-ϵO3, where ϵ = x/(3 + x). We observe a systematic variation in defect concentration, consistent with a rapid stoichiometric local equilibration of the precursors and the subsequent global thermodynamic equilibration with O2 (g), as revealed by computational thermodynamics. Together, these results reveal how the inclusion of additional elements (e.g., Li and a halide) leads to the local equilibrium, particularly at low reaction temperatures for solid-state chemistry.

Enhanced CO2 sorption properties in a polarizable [WO2F4]2--pillared physisorbent under direct air capture conditions.

We report the CO2 capture properties of an ultramicroporous physisorbent [Ni(WO2F4)(pyrazine)2]n, WO2F4-1-Ni, which crystallizes in I4/mcm (a = 9.91785(6) Å, c = 15.71516(9) Å) and its structure is solved using laboratory X-ray powder diffraction. The WO2F4 anion is acentric with polarizable WO bonds offering unique potential properties within a porous structure. Despite isostructural compounds being previously reported, the effect of this distorted anion on CO2 capture properties has not been studied. In this context, at a 400 ppm partial pressure of CO2 (applicable for direct air capture), this primitive cubic (pcu) network captures 0.934 mmolCO2 gsorbent-1 under dry conditions and 0.685 mmolCO2 gsorbent-1 at 75%RH, the highest capacity for a physisorbent reported to date.

Densified HKUST-1 Monoliths as a Route to High Volumetric and Gravimetric Hydrogen Storage Capacity.

We are currently witnessing the dawn of hydrogen (H2) economy, where H2 will soon become a primary fuel for heating, transportation, and long-distance and long-term energy storage. Among diverse possibilities, H2 can be stored as a pressurized gas, a cryogenic liquid, or a solid fuel via adsorption onto porous materials. Metal-organic frameworks (MOFs) have emerged as adsorbent materials with the highest theoretical H2 storage densities on both a volumetric and gravimetric basis. However, a critical bottleneck for the use of H2 as a transportation fuel has been the lack of densification methods capable of shaping MOFs into practical formulations while maintaining their adsorptive performance. Here, we report a high-throughput screening and deep analysis of a database of MOFs to find optimal materials, followed by the synthesis, characterization, and performance evaluation of an optimal monolithic MOF (monoMOF) for H2 storage. After densification, this monoMOF stores 46 g L-1 H2 at 50 bar and 77 K and delivers 41 and 42 g L-1 H2 at operating pressures of 25 and 50 bar, respectively, when deployed in a combined temperature-pressure (25-50 bar/77 K → 5 bar/160 K) swing gas delivery system. This performance represents up to an 80% reduction in the operating pressure requirements for delivering H2 gas when compared with benchmark materials and an 83% reduction compared to compressed H2 gas. Our findings represent a substantial step forward in the application of high-density materials for volumetric H2 storage applications.

A multimodal analytical toolkit to resolve correlated reaction pathways: the case of nanoparticle formation in zeolites.

Unraveling the complex, competing pathways that can govern reactions in multicomponent systems is an experimental and technical challenge. We outline and apply a novel analytical toolkit that fully leverages the synchronicity of multimodal experiments to deconvolute causal from correlative relationships and resolve structural and chemical changes in complex materials. Here, simultaneous multimodal measurements combined diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and angular dispersive X-ray scattering suitable for pair distribution function (PDF), X-ray diffraction (XRD) and small angle X-ray scattering (SAXS) analyses. The multimodal experimental data was interpreted via multi-level analysis; conventional analyses of each data series were integrated through meta-analysis involving non-negative matrix factorization (NMF) as a dimensional reduction algorithm and correlation analysis. We apply this toolkit to build a cohesive mechanistic picture of the pathways governing silver nanoparticle formation in zeolite A (LTA), which is key to designing catalytic and separations-based applications. For this Ag-LTA system, the mechanisms of zeolite dehydration, framework flexing, ion reduction, and cluster and nanoparticle formation and transport through the zeolite are elucidated. We note that the advanced analytical approach outline here can be applied generally to multimodal experiments, to take full advantage of the efficiencies and self-consistencies in understanding complex materials and go beyond what can be achieved by conventional approaches to data analysis.

Mixed-metal hybrid ultramicroporous material (HUM) precursor to graphene-supported tetrataenite as a highly active and durable NPG catalyst for the OER.

Current interest in investigating non-precious group (NPG) metals for catalyzing the oxygen evolution reaction (OER) has revealed that doping of Ni hydroxides with Fe results in the dramatic enhancement of catalytic activity. Herein, a facile pathway to construct tetrataenite, an NiFe alloy of extraterrestrial origin and to address the limited electrical conductivity of metal oxides/hydroxides by directly constructing them atop graphene sheets is described. In this approach, a one-pot, bottom-up assembly of hybrid ultramicroporous materials (HUMs) was carried out, in the presence of suspended graphene (G), to homogeneously deposit the HUMs on unmodified graphene sheets, affording HUMs@G. Single metal (SIFSIX-3-Ni@G) and mixed metal (SIFSIX-3-NiFe@G) HUMs can be readily synthesized from their respective metal salts to afford a well-designed catalyst for the OER. The pyrolysis of SIFSIX-3-NiFe@G resulted in the deposition of the nanoalloy tetrataenite on G, demonstrating an exceptionally low OER onset potential of 1.44 V vs. RHE and reduced overpotential at 10 mA cm-2 (η10 = 266 mV). The synergy between the composition of the active catalyst and the electronically conductive support was attained by designing a reaction system encoding the self-assembly of a crystalline pre-catalyst on G sheets.

Active Reaction Control of Cu Redox State Based on Real-Time Feedback from In Situ Synchrotron Measurements.

We achieve a target material state by using a recursive algorithm to control the material reaction based on real-time feedback on the system chemistry from in situ X-ray absorption spectroscopy. Without human intervention, the algorithm controlled O2:H2 gas partial pressures to approach a target average Cu oxidation state of 1+ for γ-Al2O3-supported Cu. This approach represents a new paradigm in autonomation for materials discovery and synthesis optimization; instead of iterating the parameters following the conclusion of each of a series of reactions, the iteration cycle has been scaled down to time points during an individual reaction. Application of the proof-of-concept illustrated here, using a feedback loop to couple in situ material characterization and the reaction conditions via a decision-making algorithm, can be readily envisaged in optimizing and understanding a broad range of systems including catalysis.

Crystal engineering of a rectangular sql coordination network to enable xylenes selectivity over ethylbenzene.

Separation of the C8 aromatic isomers, p-xylene (PX), m-xylene (MX), o-xylene (OX) and ethylbenzene (EB), is relevant thanks to their widespread application as chemical feedstocks but challenging because of their similar boiling points and close molecular dimensions. Physisorptive separation could offer an energy-efficient solution to this challenge but sorbents which exhibit strong selectivity for one of the isomers remain a largely unmet challenge despite recent reports of OX or PX selective sorbents with high uptake capacity. For example, the square lattice, sql, topology coordination network [Co(bipy)2(NCS)2] n (sql-1-Co-NCS) exhibits the rare combination of high OX selectivity and high uptake capacity. Herein we report that a crystal engineering approach enabled isolation of the mixed-linker sql coordination network [Co(bipy)(bptz)(NCS)2] n (sql-1,3-Co-NCS, bipy = 4,4'-bipyridine, bptz = 4,4'-bis(4-pyridyl)tetrazine) and study of its C8 vapour and liquid sorption properties. sql-1,3-Co-NCS was found to exhibit high adsorption capacity from liquid xylenes (∼37 wt%) and is to our knowledge the first sorbent to exhibit high selectivity for each of xylene isomer over EB (S OX/EB, S MX/EB, S PX/EB > 5). Insights into the performance of sql-1,3-Co-NCS are gained from structural studies which reveal stacking interactions between electron-deficient bptz linkers and the respective xylenes. sql-1,3-Co-NCS is the first N-donor mixed-linker sql coordination network studied for its gas/vapour sorption properties and represents a large and diverse class of understudied coordination networks.

Defect-Accommodating Intermediates Yield Selective Low-Temperature Synthesis of YMnO3 Polymorphs.

In the synthesis of complex oxides, solid-state metathesis provides low-temperature reactions where product selectivity can be achieved through simple changes in precursor composition. The influence of precursor structure, however, is less understood in solid-state synthesis. Here we present the ternary metathesis reaction (LiMnO2 + YOCl → YMnO3 + LiCl) to target two yttrium manganese oxide products, hexagonal and orthorhombic YMnO3, when starting from three different LiMnO2 precursors. Using temperature-dependent synchrotron X-ray and neutron diffraction, we identify the relevant intermediates and temperature regimes of reactions along the pathway to YMnO3. Manganese-containing intermediates undergo a charge disproportionation into a reduced Mn(II,III) tetragonal spinel and oxidized Mn(III,IV) cubic spinel, which lead to hexagonal and orthorhombic YMnO3, respectively. Density functional theory calculations confirm that the presence of Mn(IV) caused by a small concentration of cation vacancies (∼2.2%) in YMnO3 stabilizes the orthorhombic polymorph over the hexagonal. Reactions over the course of 2 weeks yield o-YMnO3 as the majority product at temperatures below 600 °C, which supports an equilibration of cation defects over time. Controlling the composition and structure of these defect-accommodating intermediates provides new strategies for selective synthesis of complex oxides at low temperatures.

A thermal-gradient approach to variable-temperature measurements resolved in space.

Temperature is a ubiquitous environmental variable used to explore materials structure, properties and reactivity. This article reports a new paradigm for variable-temperature measurements that varies the temperature continuously across a sample such that temperature is measured as a function of sample position and not time. The gradient approach offers advantages over conventional variable-temperature studies, in which temperature is scanned during a series measurement, in that it improves the efficiency with which a series of temperatures can be probed and it allows the sample evolution at multiple temperatures to be measured in parallel to resolve kinetic and thermodynamic effects. Applied to treat samples at a continuum of tem-peratures prior to measurements at ambient temperature, the gradient approach enables parametric studies of recovered systems, eliminating temperature-dependent structural and chemical variations to simplify interpretation of the data. The implementation of spatially resolved variable-temperature measurements presented here is based on a gradient-heater design that uses a 3D-printed ceramic template to guide the variable pitch of the wire in a resistively heated wire-wound heater element. The configuration of the gradient heater was refined on the basis of thermal modelling. Applications of the gradient heater to quantify thermal-expansion behaviour, to map metastable polymorphs recovered to ambient temperature, and to monitor the time- and temperature-dependent phase evolution in a complex solid-state reaction are demonstrated.

Metal-Organic Material Polymer Coatings for Enhanced Gas Sorption Performance and Hydrolytic Stability under Humid Conditions.

Physisorbent metal-organic materials (MOMs) have shown benchmark performance for highly selective CO2 capture from bulk and trace gas mixtures. However, gas stream moisture can be detrimental to both adsorbent performance and hydrolytic stability. One of the most effective methods to solve this issue is to transform the adsorbent surface from hydrophilic to hydrophobic. Herein, we present a facile approach for coating MOMs with organic polymers to afford improved hydrophobicity and hydrolytic stability under humid conditions. The impact of gas stream moisture on CO2 capture for the composite materials was found to be negligible under both bulk and trace CO2 capture conditions with significant improvements in regeneration times and energy requirements.

Molecularly Mixed Composite Membranes: Challenges and Opportunities.

The fabrication of porous molecules, such as metal-organic polyhedra (MOPs), porous organic cages (POCs) and others, has given rise to the potential for creating "solid solutions" of molecular fillers and polymers. Such solid solutions circumvent longstanding interface issues associated with mixed matrix membranes (MMMs), and are referred to as molecularly mixed composite membranes (MMCMs) to distinguish them from traditional two-phase MMMs. Early investigations of MMCMs highlight the advantages of solid solutions over MMMs, including dispersion of the filler, anti-plasticization of the polymer network, and removal of deleterious interfacial issues. However, the exact microscopic structure as well as the transport modality in this new class of membrane are not well understood. Moreover, there are clear engineering challenges that need to be addressed for MMCMs to transition into the field. In this Minireview, the authors outline several scientific and technological challenges associated with the aforementioned questions and their suggestions to tackle them.

Trace CO2 capture by an ultramicroporous physisorbent with low water affinity.

CO2 accumulation in confined spaces represents an increasing environmental and health problem. Trace CO2 capture remains an unmet challenge because human health risks can occur at 1000 parts per million (ppm), a level that challenges current generations of chemisorbents (high energy footprint and slow kinetics) and physisorbents (poor selectivity for CO2, especially versus water vapor, and/or poor hydrolytic stability). Here, dynamic breakthrough gas experiments conducted upon the ultramicroporous material SIFSIX-18-Ni-ß reveal trace (1000 to 10,000 ppm) CO2 removal from humid air. We attribute the performance of SIFSIX-18-Ni-ß to two factors that are usually mutually exclusive: a new type of strong CO2 binding site and hydrophobicity similar to ZIF-8. SIFSIX-18-Ni-ß also offers fast sorption kinetics to enable selective capture of CO2 over both N2 (S CN) and H2O (S CW), making it prototypal for a previously unknown class of physisorbents that exhibit effective trace CO2 capture under both dry and humid conditions.

Highly selective CO2 removal for one-step liquefied natural gas processing by physisorbents.

Industrial specifications require CO2 concentrations in natural gas below 50 ppm during liquefaction because of corrosion and CO2 freezing. Herein, we report a physisorbent (TIFSIX-3-Ni) that exhibits new benchmark CO2/CH4 selectivity and fast kinetics, thereby enabling one-step LNG processing to CO2 levels of 25 ppm.

Coordination Network That Reversibly Switches between Two Nonporous Polymorphs and a High Surface Area Porous Phase.

We report a 2-fold interpenetrated primitive cubic (pcu) network X-pcu-5-Zn, [Zn2(DMTDC)2(dpe)] (H2DMTDC = 3,4-dimethylthieno[2,3- b]thiophene-2,5-dicarboxylic acid, dpe = 1,2-di(4-pyridyl)ethylene), that exhibits reversible switching between an as-synthesized "open" phase, X-pcu-5-Zn-α, and two nonporous or "closed" polymorphs, X-pcu-5-Zn-ß and X-pcu-5-Zn-γ. There are two unusual features of X-pcu-5-Zn. The first relates to its sorption properties, which reveal that the α form exhibits high CO2 uptake (ca. 255 cm3/g at 195 K) via reversible closed-to-open switching (type F-IV isotherm) of the type desirable for gas and vapor storage; there are only three other reports of porous materials that combine these two features. Second, we could only isolate the ß form by activation of the CO2 loaded α form and it persists through multiple CO2 adsorption/desorption cycles. We are unaware of a new polymorph having been isolated in such a manner. That the observed phase changes of X-pcu-5-Zn-α occur in single-crystal-to-single-crystal fashion enabled structural characterization of the three forms; γ is a coordination isomer of α and ß, both of which are based upon "paddlewheel" clusters.

Recyclable switching between nonporous and porous phases of a square lattice (sql) topology coordination network.

A nonporous square lattice (sql) coordination network [Co(bipy)2(NCS)2]n (sql-1-Co-NCS) exhibits recyclable switching induced by CO2. The sorption isotherms are stepped with moderate hysteresis, temperature controlled and saturation uptake is fixed. Such switching, which has rarely been observed, offers the promise of exceptional working capacity for gas storage.

Impact of partial interpenetration in a hybrid ultramicroporous material on C2H2/C2H4 separation performance.

Phases of a 2-fold pcu hybrid ultramicroporous material (HUM), SIFSIX-14-Cu-i, exhibiting 99%, 93%, 89%, and 70% partial interpenetration have been obtained. 1 : 99 C2H2/C2H4 gas separation studies reveal that as the proportion of interpenetrated component decreases, so does the separation performance.

Controlling the Uptake and Regulating the Release of Nitric Oxide in Microporous Solids.

Representative compounds from three classes of microporous solids, namely, metal-organic frameworks (MOFs), hybrid ultra-microporous materials (HUMs), and porous-organic polymers (POPs), were investigated for their nitric oxide gas uptake and release behavior. Low-pressure sorption studies indicated strong chemisorption of NO on the free amine groups decorating the MOF UiO-66-NH2 when compared to its non-amine-functionalized parent. The HUMs demonstrated reversible physisorption within the low-pressure regime, but interestingly in one case there was evidence for chemisorption following pressurization with NO at 10 bar. Significant release of chemisorbed NO from the UiO-66-NH2 and one of the HUMs was triggered by addition of acid to the medium, a pH change from 7.4 to 5.4 being sufficient to trigger NO release. An imidazole-based POP exhibited chemisorption of NO at high pressure wherein the ring basicity facilitated both NO uptake and spontaneous release upon contact with the aqueous release medium.

An Ideal Molecular Sieve for Acetylene Removal from Ethylene with Record Selectivity and Productivity.

Realization of ideal molecular sieves, in which the larger gas molecules are completely blocked without sacrificing high adsorption capacities of the preferred smaller gas molecules, can significantly reduce energy costs for gas separation and purification and thus facilitate a possible technological transformation from the traditional energy-intensive cryogenic distillation to the energy-efficient, adsorbent-based separation and purification in the future. Although extensive research endeavors are pursued to target ideal molecular sieves among diverse porous materials, over the past several decades, ideal molecular sieves for the separation and purification of light hydrocarbons are rarely realized. Herein, an ideal porous material, SIFSIX-14-Cu-i (also termed as UTSA-200), is reported with ultrafine tuning of pore size (3.4 Å) to effectively block ethylene (C2 H4 ) molecules but to take up a record-high amount of acetylene (C2 H2 , 58 cm3 cm-3 under 0.01 bar and 298 K). The material therefore sets up new benchmarks for both the adsorption capacity and selectivity, and thus provides a record purification capacity for the removal of trace C2 H2 from C2 H4 with 1.18 mmol g-1 C2 H2 uptake capacity from a 1/99 C2 H2 /C2 H4 mixture to produce 99.9999% pure C2 H4 (much higher than the acceptable purity of 99.996% for polymer-grade C2 H4 ), as demonstrated by experimental breakthrough curves.

The effect of centred versus offset interpenetration on C2H2 sorption in hybrid ultramicroporous materials.

Fine-tuning of hybrid ultramicroporous materials (HUMs) can significantly impact their gas sorption performance. This study reveals that offset interpenetration can be antagonistic with respect to C2H2 separation from C2H2/C2H4 gas mixtures.

Water Vapor Sorption in Hybrid Pillared Square Grid Materials.

We report water vapor sorption studies on four primitive cubic, pcu, pillared square grid materials: SIFSIX-1-Cu, SIFSIX-2-Cu-i, SIFSIX-3-Ni, and SIFSIX-14-Cu-i. SIFSIX-1-Cu, SIFSIX-3-Ni, and SIFSIX-14-Cu-i were observed to exhibit negative water vapor adsorption at ca. 40-50% relative humidity (RH). The negative adsorption is attributed to a water-induced phase transformation from a porous pcu topology to nonporous sql and sql-c* topologies. Whereas the phase transformation of SIFSIX-1-Cu was found to be irreversible, SIFSIX-3-Ni could be regenerated by heating and can therefore be recycled. In contrast, SIFSIX-2-Cu-i, which is isostructural with SIFSIX-14-Cu-i, exhibited a type V isotherm and no phase change. SIFSIX-2-Cu-i was observed to retain both structure and gas sorption properties after prolonged exposure to heat and humidity. The hydrolytic stability of SIFSIX-2-Cu-i in comparison to its structural counterparts is attributed to structural features and therefore offers insight into the design of hydrolytically stable porous materials.