Reaction Conditions Research Articles

Interface Effects in the Stability of 2D Silica, Silicide, and Silicene on Pt(111) and Rh(111).

Ultrathin two-dimensional silica films have been suggested as highly defined conductive models for fundamental studies on silica-supported catalyst particles. Key requirements in this context are closed silica films that isolate the gas phase from the underlying metal substrate and stability under reaction conditions. Here, we present silica bilayer films grown on Pt(111) and Rh(111) and characterize them by scanning tunneling microscopy and X-ray photoelectron spectroscopy. We provide the first report of silica bilayer films on Rh(111) and have further successfully prepared fully closed films on Pt(111). Interestingly, surface and interface silicide phases play a decisive role in both cases: On platinum, closed films can be stabilized only when silicon is deposited in excess, which results in an interfacial silicide or silicate layer. We show that these silica films can also be grown directly from a surface silicide phase. In the case of rhodium, the silica phase is less stable and can be reduced to a silicide in reductive environments. Though similar in appearance to the "silicene" phases that have been controversially discussed on Ag(111), we conclude that an interpretation of the phase as a surface silicide is more consistent with our data. Finally, we show that the silica film on platinum is stable in 0.8 mbar CO but unstable at elevated temperatures. We thus conclude that these systems are only suitable as model catalyst supports to a limited extent.

Retrosynthesis Zero: Self-Improving Global Synthesis Planning Using Reinforcement Learning.

The field of computer-aided synthesis planning (CASP) has witnessed significant growth in recent years. Still, many CASP programs rely on large data sets to train neural networks, resulting in limitations due to the data quality and prior knowledge from chemists. In response, we propose Retrosynthesis Zero (ReSynZ), a reaction template-based method that combines Monte Carlo Tree Search with reinforcement learning inspired by AlphaGo Zero. Unlike other single-step reaction template-based CASP methods, ReSynZ takes complete synthesis paths for complex molecules, determined by reaction rules, as input for training the neural network. ReSynZ enables neural networks trained with relatively small reaction data sets (tens of thousands of data) to generate multiple synthesis pathways for a target molecule and suggest possible reaction conditions. On multiple data sets of molecular retrosynthesis, ReSynZ demonstrates excellent predictive performance compared to existing algorithms. The advantages, such as self-improving model features, flexible reward settings, the potential to surpass human limitations in chemical synthesis route planning, and others, make ReSynZ a valuable tool in chemical synthesis design.

Anionic Conjugate Addition Oligomerization of Carbon Dioxide/Butadiene Derived Lactones.

Nucleophilic and non-nucleophilic bases have been employed in anionic oligomerization of unsaturated δ-valerolactone (3-ethylidene-6-vinyltetrahydro-2H-pyran-2-one) (1). Compared to the seminal findings with 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), the unsaturated lactone reacts with guanidines, disilazanes, and phosphazenes both in bulk and in solution with higher productivities and activities, reaching full conversion with turnover frequencies up to 382 h-1. Additionally, reactions using phosphazenes and NaHMDS were active at 1 mol % catalyst loadings both in solvent and in bulk monomer at room temperature. Characterization of the reaction products by 1H, 13C, FTIR, MALDI-MS, tandem mass spectrometry (MS/MS), and ion mobility mass spectrometry (IM-MS) revealed microstructural differences dependent on the nucleophilicity of the organocatalytic base and reaction conditions. The products from phosphazene-catalyzed reactions are consistent with selective vinylogous 1,4-conjugate addition, whereas both conjugate addition and ring-opening mechanisms are observed in TBD. DSC reveals that these microstructures can be tuned to have a Tg range between -18 and 80 °C, while SEC and MALDI-MS reveal that only low molar mass oligomers are formed (748-5949 g/mol). From these results, an approach for selectively favoring the vinylogous 1,4-conjugate addition pathway is obtained over ring-opening reactivity.

Tuning Adsorbate-Mediated Strong Metal-Support Interaction by Oxygen Vacancy: A Case Study in Ru/TiO2.

The adsorbate-mediated strong metal-support interaction (A-SMSI) offers a reversible means of altering the selectivity of supported metal catalysts, thereby providing a powerful tool for facile modulation of catalytic performance. However, the fundamental understanding of A-SMSI remains inadequate and methods for tuning A-SMSI are still in their nascent stages, impeding its stabilization under reaction conditions. Here, we report that the initial concentration of oxygen vacancy in oxide supports plays a key role in tuning the A-SMSI between Ru nanoparticles and defected titania (TiO2-x). Based on this new understanding, we demonstrate the in-situ formation of A-SMSI under reaction conditions, obviating the typically required CO2-rich pretreatment. The as-formed A-SMSI layer exhibits remarkable stability at various temperatures, enabling excellent activity, selectivity and long-term stability in catalyzing the reverse water gas-shift reaction. This study deepens the understanding of the A-SMSI and the ability to stabilize A-SMSI under reaction conditions represents a key step for practical catalytic applications.

Addition of Imidazolium-Based Ionic Liquid to Improve Methanol Production in Polyamine-Assisted CO2 Capture and Conversion Systems Using Pincer Catalysts.

Ionic liquids have been studied as CO2 capture agents. However, they are rarely used in combined CO2 capture and conversion processes. Utilizing imidazolium-based ionic liquids, the conversion of CO2 to methanol was greatly improved in polyamine assisted systems catalyzed by homogeneous pincer catalysts with Ru and Mn metal centers. Among the ionic liquids tested, [BMIM]OAc was found to perform the best under the given reaction conditions. Among the polyamine tested, pentaethylenehexamine (PEHA) led to the highest conversion rates. Ru-Macho and Ru-Macho-BH were the most active catalysts. Direct air capture utilizing PEHA as the capture material was also demonstrated and produced an 86 % conversion of the captured CO2 to methanol in the presence of [BMIM]OAc.

One-pot two-step biocatalytic upgrading of 5-hydroxymethylfurfural to 2, 5-furandicarboxylic acid (FDCA) through a sequential oxidation process

2, 5-furandicarboxylic acid (FDCA) is a versatile bio-based building block that can be used for the synthesis of degradable polymers. Recently, developing green and sustainable catalytic processes for producing FDCA from renewable platform chemical molecules has attracted continuing attention. In this study, we proposed a green and sustainable one-pot, two-step biocatalytic process for the sequential oxidation of biomass-derived 5-hydroxymethylfurfural (HMF) to form FDCA with good yield and excellent selectivity. The initial step involves the oxidation of HMF to 2, 5-diformylfuran (DFF) and 5-formyl-2-furancarboxylic acid (FFCA) catalyzed by recombinant E. coli cells expressing a 5-hydroxymethylfurfural oxidase. The second step is to convert DFF and FFCA into FDCA final product via aldehyde oxidation reaction catalyzed by Deinococcus wulumuqiensis R12 whole cells. Combining the two different whole-cell biocatalysts, 180 mM of HMF could be easily converted into FDCA with a yield of 99% under optimized reaction conditions. This study offers an efficient and sustainable approach for the valorization of biomass-based HMF to form value-added FDCA.

Direct Observation of Gas-Phase Hydroxymethylene: Photoionization and Kinetics Resulting from Methanol Photodissociation.

Carbene species play an integral role in high-energy chemistry, transition-metal-carbene chemistry, catalysis, photolytic formation of carbohydrates, and possibly even the formation of interstellar sugars. In 1921, "reactive formaldehyde"─now known as hydroxymethylene (HCOH)─was first implicated as an intermediate in photocatalytic processes. However, due to its transient nature, direct observation of HCOH has predominantly been attained using cryogenic isolation methods. As a result, HCOH gas-phase reactivity measurements have been limited. We directly observed HCOH using photoionization spectroscopy following UV photodissociation of methanol. Our measurements show it reacts slowly with O2 at room temperature. This work provides evidence for the formation mechanism of HCOH from CH3OH and its subsequent reactivity under gas-phase reaction conditions.

CO2 photoreduction integrated with oxidative coupling of amines over WO3/Cu(BTC) nanocomposite

Here, we report an efficient integrated CO2 photoreduction to methanol accompanied with the simultaneous oxidation of benzyl amines to corresponding imines using quadrangular WO3 nanoplates grafted with Cu(BTC) as a photocatalyst under visible light, enabling to provide efficient use of the photogenerated electrons and holes. Under the optimized reaction conditions, the maximum yield of methanol and N-(4-methoxybenzyl)-1-(4-methoxyphenyl)methanimine was found to be 642 μmol-g−1 and 98 %, respectively. The developed integrated dual functional system comprising CO2 reduction coupled with the oxidative organic synthesis opens a new window for value chain up gradation at both ends using sustainable solar energy.

Selective Hydrogenation of Diethyl Malonate to 1,3-Propanediol Over Ga-Promoted Cu/SiO2 Catalysts With Enhanced Activity and Stability.

Cu catalysts with different compositions and different Cu and promoter contents were prepared by precipitation-gel method and studied for the selective hydrogenation of syngas or biomass-based diethyl malonate (DEM) to valuable 1,3-propanediol (1,3-PDO). The Ga-promoted 70Cu6Ga/SiO2 catalyst was found to exhibit the highest catalytic performance, achieving 100 % DEM conversion and 76.6 % 1,3-PDO selectivity under reaction conditions of 160 °C and 8 MPa H2. The 70Cu6Ga/SiO2 bimetallic catalyst also presented obviously better stability than that of the monometallic 70Cu/SiO2 catalyst in a continuous flow reactor over 180 h time-on stream. Characterization results showed that the incorporation of Ga increased the interaction between Cu and Ga species, hindered the full reduction of Cu2+ species, and thus increased the proportion of Cu+ and the number of Lewis acidic sites on the catalyst surface. The synergistic effect between Cu0 and Cu+ enhanced the adsorption and activation of ester carbonyl groups and their subsequent hydrogenation, eventually contributed to the outstanding performances of the CuGa/SiO2 bimetallic catalysts.

Electrocatalytic dehydrogenative and defluorinative coupling between aldehyde-derived N,N-dialkylhydrazones and fluoromalonates: synthesis of 2-pyrazolines.

An electrocatalytic synthesis of 2-pyrazolines via dehydrogenative and defluorinative cross-coupling reactions between (hetero)arylaldehyde-derived N,N-dialkylhydrazones and fluoromalonates is disclosed. Salient features of this work include (i) readily available starting materials, (ii) practical reaction conditions, and (ii) a formal oxidative (4 + 1)-cycloaddition via triple C-H bond functionalization. Cyclic voltammetry analyses support the electrocatalytic formation of an α-fluoromalonyl radical.

Catalysts for C-N coupling in urea electrosynthesis under ambient conditions from carbon dioxide and nitrogenous species.

Urea is an indispensable nitrogen-containing organic compound in modern human life. However, the current industrial synthesis of urea involves ammonia, which is produced through the Haber-Bosch process under harsh reaction conditions, causing huge energy consumption and heavy environmental pollution. Electrochemical reduction of carbon dioxide (CO2) and nitrogenous species (N2, NOx- and NO) have achieved significant progress, offering a promising approach for the electrochemical C-N coupling to produce urea under ambient conditions. Urea synthesis driven by renewable electricity represents a suitable alternative to the traditional process, contributing to the goal of carbon neutrality and nitrogen cycles. However, challenges such as low yield rate, poor selectivity and unveiled reaction mechanisms still need to be addressed. This review provides a summary of the latest catalysts utilized in urea electrosynthesis, aiming to provide guidance and prospects for the development of high-performance catalysts.

Copper-Nitroxyl-Catalyzed α-Oxygenation of Cyclic Secondary Amines Including Application to Late-Stage Functionalization.

Cyclic secondary amines are prominent subunits in pharmaceutical compounds. Methods for direct functionalization of N-unprotected/unsubstituted piperidines and related heterocycles have limited precedent despite their potential to impact medicinal chemistry and organic synthesis. Herein, we report a Cu/nitroxyl co-catalyzed method for direct conversion of cyclic secondary amines to the corresponding lactams via aerobic dehydrogenation and oxidative coupling with water. The mild reaction conditions tolerate diverse functional groups, enabling application to molecules that cover broad chemical space. The method is showcased in selective functionalization of building blocks and complex molecules, including late-stage functionalization of bromodomain inhibitors.

Online Compositional Analysis of Complex Oligomers in Biomass Degradation by High-Pressure Flow-Through Reactor Coupled with High-Resolution Mass Spectrometry.

Online analysis of the composition and evolution of complex oligomeric intermediates in biomass degradation is highly desirable to elucidate the mechanism of bond cleavage and study the effect of conditions on the selective conversion of feedstocks. However, harsh reaction conditions and complicated conversion systems pose tremendous challenges for conventional, state-of-the-art analytical techniques. Herein, we introduce a continuous and rapid compositional analysis strategy coupling a high-pressure flow-through reactor with online high-resolution mass spectrometry, which enables the molecular-level characterization of most biomass-related products throughout the conversion for over 2 h. Catalytic depolymerization of one model compound was studied, and temperature-dependent data of over 50 intermediates as well as recondensation dimers and oligomers were obtained, which have rarely been reported in the literature. Thousands of products during the flow-through conversion of birch wood with molecular weights up to 1000 Da were presented, and 8 typical lignin dimers and oligomers with various interunit linkages were identified at the molecular level, demonstrating the potential to analyze more complicated systems far beyond conventional methods, especially for complex oligomers. The continuous evolutions of different components and typical products were unveiled for the first time, providing valuable insights into the investigation of the structure, composition, and decomposition mechanism of lignocellulose as well as the influence of reaction conditions. This method leads to the previously unattained ability to probe and reveal complicated chemical compositions in high-pressure reactions and can be applied to all other high-pressure heterogeneous aqueous reactions.

Investigating the Mechanism of Aluminum Fluoride Chelation.

Aluminum fluoride (AlF) complexes have been used over the past decade to incorporate [18F]fluoride into large biomolecules in a highly selective fashion by using relatively facile conditions. However, despite their widespread usage, there are a large number of variations in the reaction conditions, without a definitive discussion provided on the mechanism to understand how these changes would alter the end result. Herein, we report a detailed mechanistic investigation of the reaction, using a mixture of theoretical studies, fluorine-19 and fluorine-18 chemistry, and the consequences it has on the efficient clinical translation of AlF-containing imaging agents.

Photoredox/Bismuth Relay Catalysis Enabling Reductive Alkylation of Nitroarenes with Aldehydes.

The effective transition metal-free photoredox/bismuth dual catalytic reductive dialkylation of nitroarenes with benzaldehydes has been reported. The nitroarene redution through visible light-driven photoredox catalysis was integrated with subsequent reductive dialkylation of anilines under bismuth catalysis to enable the cascade reductive alkylation of nitroarenes with carbonyls. Salient features of this relay catalysis system include mild reaction conditions, no requirement for transition metal catalysts, easy handling, step-economy, and high selectivity.

Ionic Liquids Boost Anthraquinone Hydrogenation over Pd‐based Bimetallic Pincer Catalysts

A series of bimetallic‐ionic liquid (IL) pincer catalysts Pd‐M@IL were synthesized using low Pd‐loading amount (0.3%) and adopting six kinds of ion liquids with the cation containing the imidazole ring, and then evaluated the catalytic activities toward 2‐ethylanthraquinone hydrogenation reaction with the purpose to produce H2O2. Under the reaction conditions of 60 ℃, 0.30 MPa and 15 min reaction time, over the catalysts Pd‐La@[BMIm]Ac/alu and Pd‐La@[VBIM]Br/alu achieved high hydrogenation efficiency(9.4 g/L and 8.5 g/L) but also high selectivity toward the active anthraquinone (98.9% and 99.6%). Through characterizations of STEM, TPD, in‐situ XPS, etc., it illustrates that the pincer catalysts Pd‐La@[BMIm]Ac/alu and Pd‐La@[VBIM]Br/alu have much better dispersity than the bimetallic Pd‐La/alu, these two ionic liquids of [BMIm]Ac and [VBIM]Br provide different local environment around Pd‐La sites, which in turn modulate the catalytic activity of the pincer‐catalysts toward anthraquinone hydrogenation. DFT calculations disclose the unique electron transfer between ILs ([BMIm]Ac and [VBIM]Br) and Pd‐La clusters and subsequent changes of energy barriers for anthraquinone hydrogenation. The work illuminates a facile strategy to design novel hydrogenation catalysts through combining the pincer microenvironment partially encapsulated by the cation and the anion of ionic liquids with the internal bimetallic sites.

Photoinduced Remote Selective C(sp3)‐H Alkylation Mediated by Cesium formate

Herein, we developed a photoinduced remote selective C(sp3)−H alkylation of protected amines in the presence of cesium formate. The protected amines were synthesized from commercially available inexpensive 2‐iodo benzoyl chloride. Under mild reaction conditions, the in situ generated aryl radical is converted to alpha‐amidoalkyl radical via [1,5]‐HAT process, which combines with different Michael acceptors and affords corresponding alkylation products in good yields. Preliminary mechanistic studies revealed that formate anion acts as a source of carbon dioxide radical anion (CO2•−) and hydrogen atom donor was directly confirmed by isotope‐labeling studies.

Auto‐Pressurized Multi‐Stage Tesla‐Valve Type Microreactors in Carbon Monoliths Obtained Through 3D Printing: Impact of Design on Fluid Dynamics and Catalytic Activity

AbstractThe present research exploits an innovative methodology for producing auto‐pressurized carbon microreactors with a precise and controlled structure analyzing the influence of their design on the fluid dynamics and their catalytic performance. Carbon monoliths with Tesla‐valve shape channels (Tesla, T, and modified Tesla, Tm) are synthesized through the combination of 3D printing and sol–gel process and further probed as Ni/CeO2 supports on CO2 methanation. The experimental results and mathematical modeling corroborated the improved performance obtained through the complex design compared to a conventional one. In addition to chaotic fluid flow induced by the deviation in flow direction, which improves the reagents‐active phase interaction, local pressure increases due to convergence of flows may enhance the Sabatier reaction according to Le Châtelier's principle. Conversely to straight channels, T and Tm are not affected by flow rate and presented chemical control. Tesla‐valve with curved angle (Tm) improved the mass transfer, achieving higher conversion and ≈30% reaction rate increase regarding right angle (T). Thus, this auto‐pressurized multi‐stage Tesla‐valve monolith opens the gate to design specific and advanced functional materials for multitude chemical reactions where not only the reactant‐active phase contact can be maximized but also the reaction conditions can be controlled to maximize the reaction kinetics.

Ab Initio Design of Ni‐Rich Cathode Material with Assistance of Machine Learning for High Energy Lithium‐Ion Batteries

With the widespread use of lithium‐ion batteries in electric vehicles, energy storage, and mobile terminals, there is an urgent need to develop cathode materials with specific properties. However, existing material control synthesis routes based on repetitive experiments are often costly and inefficient, which is unsuitable for the broader application of novel materials. The development of machine learning and its combination with materials design offers a potential pathway for optimizing materials. Here, we present a design synthesis paradigm for developing high energy Ni‐rich cathodes with thermal/kinetic simulation and propose a coupled image‐morphology machine learning model. The paradigm can accurately predict the reaction conditions required for synthesizing cathode precursors with specific morphologies, helping to shorten the experimental duration and costs. After the model‐guided design synthesis, cathode materials with different morphological characteristics can be obtained, and the best shows a high discharge capacity of 206 mAh g−1 at 0.1C and 83% capacity retention after 200 cycles. This work provides guidance for designing cathode materials for lithium‐ion batteries, which may point the way to a fast and cost‐effective direction for controlling the morphology of all types of particles.

PEG modification increases thermostability and inhibitor resistance of Bst DNA polymerase.

Polyethylene glycol modification (PEGylation) is a widely used strategy to improve the physicochemical properties of various macromolecules, especially protein drugs. However, its application in enhancing the performance of enzymes for molecular biology remains underexplored. This study explored the PEGylation of Bst DNA polymerase, determining optimal modification reaction conditions. In comparison to the unmodified wild-type counterpart, the modified Bst DNA polymerase exhibited significantly improved of activity, thermal stability and inhibitor tolerance during loop-mediated isothermal amplification (LAMP). When applied for the detection of Salmonella in crude samples, the modified enzyme demonstrated a notably accelerated reaction rate. Therefore, PEGylation emerges as a viable strategy for refining DNA polymerases, helping in the development of novel molecular diagnostic reagents.