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Synthesis and Structure of Vacancy-Ordered Perovskite Ba6Ta2Na2X2O17 (X = P, V): Significance of Structural Model Selection on Discovered Compounds
No full textBa6Ru2Na2X2O17 (X = P, V) with vacancy-ordered 12H-type hexagonal perovskite with a (c’cchcc)2 stacking sequence of [BaO3]c, [BaO3]h and [BaO2]c’ layers, where c and h represent a cubic and hexagonal stacking sequence, were previously reported by Quarez et al. in 2003. They also synthesized Ba6Ta2Na2V2O17, but the structural refinement was absent. Very recently, Szymanski et al. reported 41 new compounds, including 12H-type Ba6Ta2Na2V2O17, using large-scale ab initio phase-stability data from the Materials Project and Google DeepMind. But their structural refinement was very poor. Here, we report synthesis and structure of Ba6Ta2Na2V2O17, which does not have 12H-type structure, but has a vacancy-ordered 6C-type hexagonal perovskite with a (c’ccccc) stacking sequence of [BaO3]c and [BaO2]c’ layers. We also report the phosphite analog Ba6Ta2Na2P2O17 as a new compound. We claim an importance of careful structural characterization on newly discovered compounds; otherwise, the database constructed will lose credibility
Cofactor-free biocatalytic hydrogenation of nitro compounds for synthesis of amines
No full textWe report a new paradigm for chemoselective hydrogenation of nitro compounds to amines, under mild, aqueous conditions. Hydrogenase enzyme releases electrons from H2 to a carbon black support which facilitates nitro-group reduction. For 30 nitroarenes we demonstrate full conversion (isolated yields 78 – 96%), with products including pharmaceuticals benzocaine, procainamide and mesalazine, and 4-aminophenol – precursor to acetaminophen (paracetamol). We also showcase gram-scale synthesis of procainamide with 90% isolated yield. We demonstrate potential for extension to aliphatic substrates. The catalyst is highly selective for reduction of the nitro group over other unsaturated bonds, tolerant to a wide range of functional groups, and exhibits excellent stability in reactions lasting up to 72 hours and full reusability over 5 cycles, indicating scope for direct translation to fine chemical manufacturing
Species Selection for Automatic Chemical Kinetic Mechanism Generation
No full textMany important chemical kinetic systems require detailed chemical kinetic models to resolve. These detailed kinetic models can involve thousands of species and hundreds of thousands of chemical reactions, making them difficult to construct by hand. Modern automatic mechanism generation algorithms can mostly be divided into two classes: rule and rate based. Rule-based generators choose species based on user defined constraints on species and reaction classes. Rate-based generators generate a much larger set of potentially important species and reactions and then choose which ones to add based on running simulations of species and reactions deemed important and calculating the flux to potentially important species. In principle, the latter is preferable, as it requires the user to make far fewer assumptions about what is important in the system. However, while the effectiveness of the rate-based approach has been demonstrated in a wide variety of systems, it has also been demonstrated to have difficulty picking up important low-flux chemistries. Here we present a discussion of the challenges associated with rate-based mechanism generation and new algorithms that are able to efficiently mitigate these challenges in a set of case studies
A High Efficiency Approach to Carbon Capture & Permanent Sequestration Through Mineralization
No full textWe report a highly efficient approach to carbon capture and sequestration via the water catalyzed conversion of basic oxygen steel (BOS) slag and CO2 into CaCO3 and volumetrically stable aggregate for road construction using the Zero-Air-Pollution-Carbon-Capture-Capsule (ZAP-C3) units. The overall transformation is accomplished in a 2-step process. In the first component pH 7 water is sprayed through CO2 containing vapor, forming an acidic solution (pH 10 and a volumetrically stabilized slag with chemically incorporated CO2. In the second stage the 55% decreased CO2 containing vapor out of the first reactor is passed through the leachate using a micro bubble system, causing CaCO3 to precipitate from pH 7 water, yielding a total carbon sequestration of 309 kg CO2 per ton of slag. Over 95% of the water in the process is recycled in a system that total CO2 sequestration of ca. 1,900 ton/year of CO2 in the present configuration and an estimated CO2 sequestration of ca. 2,600 ton/year with the proposed dual reactor set-up
Photocatalytic H2O2 Production Over Photocatalysts Prepared By Phosphine-protected Au101 Nanoparticles on WO3
No full textPhotocatalytic H2O2 synthesis is an appealing and feasible strategy to replace the energy- intensive, tedious, and waste-generating anthraquinone process. Often, pure metal oxides show low activity in photocatalytic H2O2 production and therefore metal co-catalysts are required to improve the photoactivity. This work investigated photocatalytic H2O2 production using monodisperse gold nanoclusters Au101(PPh3)21Cl5 supported on WO3. From HRTEM imaging, the Au101 size in the uncalcined samples is in the cluster regime (<2 nm) and after calcination at 200 °C the size increases to ca. 4.5 nm. The roles of Au101 have been identified to reduce the charge carrier recombination and provide the active sites for O2 reduction which significantly enhances the photoactivity. Both uncalcined and calcined Au101/WO3 photocatalysts produce over 75 mM g-1 h-1 of H2O2 under UV light irradiation while the pure WO3 is inactive. At early times (up to 30 min), the production rate of H2O2 from calcined Au101/WO3 reaches 173 mM g-1 h-1 and is almost double the rate of the uncalcined catalyst (93 mM g-1 h-1). The higher photoactivity of calcined versus uncalcined Au101/WO3 can be attributed to the aggregated Au101 and removal of phosphine ligands from the Au core as verified by HRTEM and XPS. The reaction rate decreases over time which is attributed to the reverse reaction. Using a simple kinetic model, the rate constant of the H2O2 formation (kf) for uncalcined and calcined Au101/WO3 are 2.07 and 6.31 mM h-1, while the rate constant of the H2O2 decomposition (kd) for uncalcined and calcined Au101/WO3 are 0.49 and 2.93 h-1, respectively. This work highlights a simple preparation of highly active photocatalysts to produce H2O2 derived from Au101 clusters and WO3
[68Ga]Ga-THP-Tetrazine for bioorthogonal click radiolabelling: Pretargeted PET imaging of liposomal nanomedicines
No full textPretargeted PET imaging using bioorthogonal chemistry is a leading strategy for the tracking of long-circulating agents such as antibodies and nanoparticle-drug delivery systems with short-lived isotopes. Here, we report the synthesis, characterisation and in vitro/vivo evaluation of a new 68Ga-based radiotracer [68Ga]Ga-THP-Tetrazine ([68Ga]Ga-THP-Tz) for bioorthogonal click radiochemistry and in vivo labelling of agents with slow pharmacokinetics. THP-tetrazine (THP-Tz) can be radiolabelled to give [68/67Ga]Ga-THP-Tz at room temperature in less than 15 minutes with excellent radiochemical stability in vitro and in vivo. [68Ga]Ga-THP-Tz was tested in vitro and in vivo for pretargeted imaging of stealth PEGylated liposomes, chosen as a leading clinically-approved platform of nanoparticle-based drug delivery, and for their known long-circulating properties. To achieve this, PEGylated liposomes were functionalised with a synthesised transcyclooctene (TCO) modified phospholipid. Radiolabelling of TCO-PEG-liposomes with [68/67Ga]Ga-THP-Tz was demonstrated in vitro in human serum, and in vivo using both healthy mice and in a syngeneic cancer murine model (WEHI-164 fibrosarcoma). Interestingly in vivo data revealed that [68Ga]Ga-THP-Tz was able to in vivo radiolabel liposomes present in the liver and spleen, and not those in the blood pool or in the tumour. Overall, these results demonstrate the potential of [68Ga]Ga-THP-Tz for pretargeted imaging/therapy but also some unexpected limitations of this system
The n,π* States of Heteroaromatics: When are They the Lowest Excited States and in What Way Can They Be Aromatic or Antiaromatic?
No full textHeteroaromatic molecules are found in areas ranging from biochemistry to photovoltaics. We analyze the n,π* excited states of 6π-electron heteroaromatics with in-plane lone-pairs (n(sigma), herein n), and use qualitative theory and quantum chemical computations, starting at Mandado’s 2n+1 rule for aromaticity of separate spins. After excitation of an electron from n to π*, a (4n+2)π-electron species has 2n+2 π(alpha)-electrons and 2n+1 π(beta)-electrons (or vice versa), and becomes π(alpha)-antiaromatic and π(beta)-aromatic. Yet, the antiaromatic π(alpha)- and aromatic π(beta)-components seldom cancel, leading to residuals with aromatic or antiaromatic character. We explore vertically excited triplet n,π* states (3n,π*), which are most readily analyzed, but also singlet n,π* states (1n,π*), and explain which compounds have n,π* states with aromatic residuals as their lowest excited states (e.g., pyrazine and the phenyl anion). If the π-electron population becomes more (less) uniformly distributed upon excitation, the system will have an (anti-)aromatic residual. Among isomers, the one which has the most aromatic residual in 3n,π* is often of lowest energy in this state. Five-membered ring heteroaromatics with one or two N, O and/or S atoms never have n,π* states as their first excited states (T1 and S1), while this is nearly always the case for six-membered ring heteroaromatics with electropositive heteroatoms and/or highly symmetric (D2h) diheteroaromatics. For the complete compound set, there is a modest correlation between the (anti)aromatic character of the n,pi* state and the energy gap between the lowest n,π* and π,π* states (R2 = 0.42), while it is stronger for monosubstituted pyrazines (R2 = 0.84)
Boundary Conditions for Promotion versus Poisoning in Copper-Gallium-based CO2–to–Methanol Hydrogenation Catalysts
No full textCu-Ga-based CO2-to-methanol hydrogenation catalysts are known to display a range of catalytic performance depending on their preparation. Here, using surface organometallic chemistry, we have prepared a series of silica-supported 3-6 nm Cu1-xGaxOy nanoparticles with a range of xGa to establish how the concentration of Ga and alloy formation affect the activity. Cu is always fully metallic in this series, while Ga is partially alloyed with Cu in the core and partially oxidized on the surface. These materials display a volcano-type activity behavior, where methanol formation is promoted when xGa < 0.13-0.18 and is suppressed at higher values, indicating a poisoning of the catalysts. In situ X-ray absorption spectroscopy shows that GaOx species over promoted Cu0.93Ga0.07-SiO2 catalyst are much more redox active than those over the poisoned Cu0.77Ga0.23-SiO2. In situ infrared spectroscopy detected methoxy intermediates over the promoted Cu0.93Ga0.07-SiO2 catalyst, while no formate or methoxy species could be observed over the poisoned Cu0.77Ga0.23-SiO2. The absence of reactive intermediates and irreversible oxidation of GaOx over poisoned catalyst suggests encapsulation of Cu by GaOx shell resulting in low activity
Improved protocols for the synthesis of Precursors of Thiazol-2-ylidene N-Heterocyclic Carbenes
No full textWe report improved protocols for the synthesis of thiazolium precatalysts from primary amines, carbon disulfide and alpha-halogenoketones. For N-alkyl substituted derivatives, yields for the corresponding thiazolethiones can be dramatically improved when isolating the intermediate dithiocarbamates. In most cases, meta-chloroperbenzoic acid can replace advantageously H202 in acetic acid for the oxidation of thiazolethiones into thiazoliums. This approach was applied to the synthesis of a thiazolium featuring a 2-adamantyl N-substituent, the corresponding persistent carbene and its dimer
Alternative weighting schemes for fine-tuned extended similarity index calculations
No full textExtended similarity indices (i.e. generalization of pairwise similarity) have recently gained importance because of their simplicity, fast computation and superiority in tasks like diversity picking. However, they operate with several meta parameters that should be optimized. Earlier, we extended the binary similarity indices to ‘discrete non-binary’ and ‘continuous’ data; now we continue with introducing and comparing multiple weighting functions. As a case study, the similarity of CYP enzyme inhibitors (4016 molecules after curation) was characterized by their extended similarities, based on 2D descriptors, MACCS and Morgan fingerprints. A statistical workflow based on sum of ranking differences (SRD) and analysis of variance (ANOVA) was used for finding the optimal weight function(s). Overall, the best weighting function is the fraction (“frac”), while optimal extended similarity indices were also found, and their differences are revealed across different data sets. We intend this work to be a guideline for users of extended similarity indices regarding the various weighting options available. Source code for the calculations is available at https://github.com/mqcomplab/MultipleComparisons