Important Reactions Research Articles

Multidimensionally ordered mesoporous intermetallics: Frontier nanoarchitectonics for advanced catalysis.

Ordered intermetallics contribute to a unique class of catalyst materials due to their rich atomic features. Further engineering of ordered intermetallics at a mesoscopic scale is of great importance to expose more active sites and introduce new functions. Recently, multidimensionally ordered mesoporous intermetallic (MOMI) nanoarchitectonics, which subtly integrate atomically ordered intermetallics and mesoscopically ordered mesoporous structures, have held add-in synergies that not only enhance catalytic activity and stability but also optimize catalytic selectivity. In this tutorial review, we have summarized the latest progress in the rational design, targeted synthesis, and catalytic applications of MOMIs, with a special focus on the findings of our group. Three strategies, including concurrent template route, self-template route, and dealloying route, are discussed in detail. Furthermore, physicochemical properties and catalytic performances for several important reactions are also described to highlight the remarkable activity, high stability, and controllable selectivity of MOMI nanoarchitectonics. Finally, we conclude with a summary and explore future perspectives in the field to contribute to wider applications.

One-pot Hydrogenolysis of Polyethylene Terephthalate (PET) to p-xylene over CuZn/Al2O3Catalyst.

Chemical upcycling of plastic wastes into valuable chemicals is a promising strategy for resolving plastic pollution, but economically viable methods currently are still lacking. Here, we report one-pot hydrogenolysis of PET plastic into p-xylene with an excellent yield (99.8 %) over a robust non-precious Cu-based catalyst, CuZn/Al2O3, in the absence of alcohol solvents. The presence of Zn species promotes the dispersion of Cu0 and increases the ratio of Cu+/Cu0, whereas the synergistic effect of Cu0 and Cu+ leads to a superior performance in the conversion of PET. The combination of GC-MS, 13C CP MAS NMR, 2D 1H-13C CP HETCOR NMR spectroscopy and kinetic studies for the first time demonstrates 4-methyl benzyl alcohol as an important reaction intermediate in the hydrogenolysis of PET. Mechanistic studies indicate that the conversion of PET mainly follows a hydrogenolysis process, involving the cleavage of ester bonds to alcohols and the C-O bond cleavage of alcohols to alkanes. This work not only brings new insight for understanding the upgrading pathway of PET, but also provides a guidance for the design of high-performance non-precious catalysts for the chemical upcycling of plastic wastes.

Freestanding Penta-Twinned Pd-Ag Nanosheets.

2D metal nanosheets have attracted significant attention as efficient catalysts for various important chemical reactions. However, the development of metal nanosheets with controlled compositions and morphologies has been slow due to the challenges associated with synthesizing thermodynamically unfavorable 2D structures. Herein, we report a synthesis route of freestanding Pd-Ag penta-twinned nanosheets (Pd-Ag ptNSs) with distinct 5-fold twin boundaries. Through the coreduction of Ag and Pd precursors on presynthesized Pd ptNSs, Ag could be homogeneously alloyed with Pd, leading to the formation of well-defined Pd-Ag ptNSs. The promotional effects of the bimetallic composition, 2D structure, and twin boundaries on catalysis were studied by using Pd-Ag ptNS-catalyzed H2 production from formic acid decomposition as a model reaction. Notably, the catalytic activity of the Pd-Ag ptNSs drastically outperformed those of monometallic, bimetallic, and 3D counterparts, such as Pd ptNSs, Pd-Ag nanosheets without a TB, and Pd-Ag octahedral nanocrystals, demonstrating the promising potential of the integration of twin boundaries and multiple compositions in the development of high-performance 2D nanocatalysts.

Diatomic Active Sites Embedded Graphyne as Electrocatalysts for Ammonia Synthesis.

Ammonia (NH3) is a vital chemical compound in industry and agriculture, and the electrochemical nitrogen reduction reaction (eNRR) is considered a promising approach for NH3 synthesis. However, the development of eNRR faces the challenge of high overpotential and low Faradaic efficiency. In this work, graphyne (GY) is anchored by 3d, 4d, and 5d dual transition metal atoms to form diatomic catalysts (DACs) and is roundly investigated as an electrocatalyst for eNRR via density functional theory calculations. Due to the protrusion of anchored metal atoms, the active sites of GY are better exposed compared to other substrates, exhibiting higher activity. Through four-step hierarchical high-throughput screening (ΔG*N2 < 0 eV, ΔG*N2→*N2H < 0.4 eV, ΔG*NH2→*NH3 < 0.4 eV, and ΔG*N2 < ΔG*H), the number of selected catalysts in each step is 325, 240, 145, and 20, respectively. Considering a series of factors, including stability, initial potential, and selectivity, 13 kinds of eligible catalysts are identified. Optimal eNRR paths studies show that the best catalyst Mn2@GY features no onset potential. For the three catalysts (Mn2@GY, Ir2@GY, and RhOs@GY), the onset potentials of the most favorable eNRR pathways are -0.07, -0.12, and -0.17 V, respectively. The excellent catalytic activity can be credited to the effective charge transfer and orbital interaction between the active site and adsorbed N2. Our work demonstrates the significance of DACs for ammonia synthesis and provides a paradigm for the study of DACs even for other important reactions.

Microdroplet-Mediated Multiphase Cycling in a Cloud of Water Drives Chemoselective Electrolysis.

Electrification of water in clouds leads to fascinating redox reactions on Earth. However, little is known about cloud electrochemistry, except for lightning, a natural hazard that is nearly impossible to harness. We report a controllable electrochemistry that can be enabled in microclouds by fast phase switching of water between the microdroplet, vapor, and bulk phase. Due to the size-dependent charge transfer between droplets during atomization, this process generates an alternating voltage arising from the self-electrification and discharging of microdroplets, vapor, and bulk phase by electron and ion transfer. We show that the microclouds with alternating voltage cause 1,2-dichloroethane (ClH2C-CH2Cl) to be converted to vinyl chloride (H2C═CHCl) at ∼80% selectivity. These findings highlight the importance of controlled cloud electrochemistry in accelerating the removal of volatile organic compounds and treating contaminated water. We suggest that this work opens an avenue for harnessing cloud electrochemistry to solve challenging chemoselectivity problems in aqueous reactions of environmental and industrial importance.

Unconventional structural evolution of an oxide surface in water unveiled by in situ sum-frequency spectroscopy.

Oxide-water interfaces host a wide range of important reactions in nature and modern industrial applications; however, accurate knowledge about these interfaces is still lacking at the molecular level owing to difficulties in accessing buried oxide surfaces. Here we report an experimental scheme enabling in situ sum-frequency vibrational spectroscopy of oxide surfaces in liquid water. Application to the silica-water interface revealed the emergence of unexpected surface reaction pathways with water. With ab initio molecular dynamics and metadynamics simulations, we uncovered a surface reconstruction, triggered by deprotonation of surface hydroxylated groups, that led to unconventional five-coordinated silicon species. The results help demystify the multimodal chemistry of aqueous silica discovered decades ago, bringing in fresh information that modifies the current understanding. Our study will provide new opportunities for future in-depth physical and chemical characterizations of other oxide-water interfaces.

New Insights into the Understanding of High-Pressure Air Injection (HPAI): The Role of the Different Chemical Reactions

High-pressure air injection (HPAI) is an enhanced oil recovery process in which compressed air is injected into deep, light oil reservoirs, with the expectation that the oxygen in the injected air will react with a fraction of the reservoir oil at an elevated temperature to produce carbon dioxide. The different chemical reactions taking place can be grouped into oxygen addition, thermal cracking, oxygen-induced cracking, and bond scission reactions. The latter reactions involve the combustion of a flammable vapor as well as the combustion of solid fuel, commonly known as “coke”. Since stable peak temperatures observed during HPAI experiments are typically below 300 °C, it has been suggested that thermal cracking and combustion of solid fuel may not be important reaction mechanisms for the process. The objective of this work is to assess the validity of that hypothesis. Therefore, this study makes use of different oxidation and combustion HPAI experiments, which were performed on two different light oil reservoir samples. Modeling of those tests indicate that thermal cracking is not an important reaction mechanism during HPAI and can potentially be ignored. The work also suggests that the main fuel consumed by the process is a flammable vapor generated by the chemical reactions. This represents a shift from the original in situ combustion paradigm, which is based on the combustion of coke.

Selective methanol production via CO2 reduction on Cu2O revealed by micro-kinetic study combined with constant potential model

High efficiency electrocatalysis of greenhouse gas CO2 to liquid fuel methanol attracts great attentions that allows energy transformation from renewable electric energy to storable chemical energy. Catalytically converting CO2 to methanol using green hydrogen is a promising pathway to achieve carbon neutrality. In this work, the catalytic activity and product selectivity of the electrochemical CO2 reduction reaction on Cu2O were studied using a micro-kinetic model based on Marcus charge transfer theory. The constant potential model under the grand-canonical ensemble enables the accurate description of binding energies and ground-state charge transfer upon surface adsorption as a function of electrode potential. Firstly, the potential-dependent activation energies and rate constants of all possible elementary reactions involved in CO2 reduction are calculated. Then, the surface coverage and its derivative can be obtained in a steady-state approximation. Finally, the total reaction rate and partial reaction rate can be deduced, allowing for direct comparison with catalytic activity (current density) and product selectivity (Faradaic efficiency). It is found that CH3OH is the dominant product on the Cu2O(111) surface, while CO is the main product on the Cu2O(110) surface. Based on the derived total and partial reaction rates under the steady-state approximation, it is demonstrated that Cu2O(111) exhibits the highest electrocatalytic activity and methanol selectivity at U = −0.6 V vs. SHE. This research presents a methodology for predicting apparent electrocatalytic performance from microscopic reaction energy calculations, which can be applied to other important electrocatalytic reaction mechanisms and performance predictions.

Differential binding of curcumin with chair and basket type anti-parallel DNA G-quadruplexes

Guanine rich DNA sequences are widespread throughout the genome and they are prone to form G-quadruplex structures both in-vitro and in-vivo. Owing to their regulatory roles in important biological reactions, DNA G-quadruplexes are the prime target for the development of anti-cancer drug molecules. In the present study, we have investigated the interaction of curcumin with anti-parallel chair and basket type DNA G-quadruplexes formed by thrombin binding aptamer (TBA) and human telomeric repeat (HTG) sequences, respectively. The results obtained from spectroscopic techniques demonstrated that curcumin interacts with both the DNA G-quadruplexes through external binding mode and the estimated apparent binding constant value (Kb) was slightly higher for HTG (Kb = (4.38 ± 0.09) × 105 M−1) than that estimated for TBA (Kb = (0.62 ± 0.02) × 105 M−1) DNA G-quadruplex structures. It was also revealed that upon binding with curcumin, TBA and HTG DNA G-quadruplexes were slightly stabilized and destabilized, respectively. Circular dichroism study revealed that upon binding with curcumin the global structure of both the DNA G-quadruplexes was unaltered, whereas the local structure of HTG DNA G-quaruplex was slightly altered. Conjointly all the aforementioned spectroscopic methods emphasize that curcumin has differential interaction with anti-parallel DNA G-quadruplex structures depending on their loop orientation.

How to simulate dissociative chemisorption of methane on metal surfaces.

The dissociation of methane is not only an important reaction step in catalytic processes, but also of fundamental interest. Dynamical effects during the dissociative chemisorption of methane on metal surfaces cause significant differences in computed reaction rates, compared to what is predicted by typical transition state theory (TST) models. It is clear that for a good understanding of the catalytic activation of methane dynamical simulations are required. In this paper, a general blueprint is provided for performing dynamical simulations of the dissociative chemisorption of methane on metal surfaces, by employing either the quasi-classical trajectory or ring polymer molecular dynamics approach. If the computational setup is constructed with great care-since results can be affected considerably by the setup - chemically accurate predictions are achievable. Although this paper concerns methane dissociation, the provided blueprint is, so far, applicable to the dissociative chemisorption of most molecules.

A Detailed Reaction Mechanism for Thiosulfate Oxidation by Ozone in Aqueous Environments.

The ozone oxidation, or ozonation, of thiosulfate is an important reaction for wastewater processing, where it is used for remediation of mining effluents, and for studying aerosol chemistry, where its fast reaction rate makes it an excellent model reaction. Although thiosulfate ozonation has been studied since the 1950s, challenges remain in developing a realistic reaction mechanism that can satisfactorily account for all observed products with a sequence of elementary reaction steps. Here, we present novel measurements using trapped microdroplets to study the pH-dependent thiosulfate ozonation kinetics. We detect known products and intermediates, including SO32-, SO42-, S3O62-, and S4O62-, establishing agreement with the literature. However, we identify S2O42- as a new reaction intermediate and find that the currently accepted mechanism does not directly explain observed pH effects. Thus, we develop a new mechanism, which incorporates S2O42- as an intermediate and uses elementary steps to explain the pH dependence of thiosulfate ozonation. The proposed mechanism is tested using a kinetic model benchmarked to the experiments presented here, then compared to literature data. We demonstrate good agreement between the proposed thiosulfate ozonation mechanism and experiments, suggesting that the insights in this paper can be leveraged in wastewater treatment and in understanding potential climate impacts.

Hydrogenation Reactions with Heterobimetallic Complexes.

Hydrogenations are fundamentally and industrially important reactions that are atom economical paths to synthesize value-added products from feedstock chemicals. The cooperative effects of two or more metal centers in multimetallic active sites is a successful strategy to activate small molecules and facilitate catalytic reactions, and this strategy has been recently applied to catalytic hydrogenation reactions. Furthermore, heterobimetallic complexes have been well-documented to provide novel reaction pathways and improved selectivity, compared to their homo-bimetallic and monometallic analogues. This minireview provides a historical perspective on the development of heterobimetallic catalysts for the hydrogenation of unsaturated substrates and describes recent developments in this burgeoning research area.

Reaction Thermodynamic and Kinetics for Esterification of 1-Methoxy-2-Propanol and Acetic Acid over Ion-Exchange Resin.

The esterification of 1-methoxy-2-propanol (PM) and acetic acid (AA) is an important reaction for the production of 1-methoxy-2-propyl acetate (PMA). Herein, we used the macroporous ion-exchange resin Amberlyst-35 as a catalyst to explore the effects of reaction conditions on the reaction rate and equilibrium yield of PMA. Under the optimized conditions of a reaction temperature of 353 K, using the initial reactant PM/AA with a molar ratio of 1:3, and a catalyst loading of 10 wt%, the PMA equilibrium yield reached 78%, which is the highest equilibrium yield so far. The reaction equilibrium constants and activity coefficients were estimated to obtain reaction thermodynamic properties, indicating the exothermicity of the reaction. Furthermore, pseudo-homogeneous (PH), Eley-Rideal (ER), and Langmuir-Hinshelwood-Hougen-Watson (LHHW) kinetic models were fitted based on experimental reaction kinetic data. The results demonstrate that the LHHW model is the most consistent with experimental data, indicating a surface reaction-controlled process and exhibiting an apparent activation energy of 62.0 ± 0.2 kJ/mol. This work represents a valuable example of calculating reaction thermodynamics and kinetics, which are particularly essential for promising industrial reactor designs.

Effect of 4,4′-dialkyl-2,2′-bipyridine ligands on the hydrolysis of dichlorodioxomolybdenum(VI) catalyst precursors and the switch from homogeneous epoxidation to heterogeneous systems

Molybdenum catalysts have been industrially recognized for decades for liquid phase epoxidation, which is an important chemical reaction process, since epoxides are used for many industrial applications. In this work, molybdenum oxide hybrid catalysts, prepared by a reflux hydrolysis methodology, performed effectively as heterogeneous catalysts or reaction-induced self-separating catalysts under mild reaction conditions; in the two cases, the catalyst separation and reuse are facilitated. Specifically, catalysts with the general formula [MoO3(L)], possessing polymeric chain-like (L = 4,4′-dimethyl-2,2′-bipyridine (1)) or oligomeric (L = 4,4′-dinonyl-2,2′-bipyridine (2)) structures comprising corner-sharing {MoO4N2} units, were synthesized and characterized by various complementary techniques (ATR FT-IR, Raman, 13C{1H} CP MAS NMR spectroscopy, PXRD, SEM, TGA, elemental analysis, ICP-OES and N2 sorption isotherms). Small chemical differences in the organic synthesis precursor had important structure directing effects on the type of hybrid material formed. The hybrids promoted olefin epoxidation with H2O2 or tert-butylhydroperoxide (TBHP) as oxidant. For example, 1 catalyzed the conversion of biobased olefins (70 °C) and lignin-based isoeugenol (50 °C) with TBHP to useful bioproducts, in heterogeneous phase, leading to an epoxide yield of 100 % for DL-limonene (3:1 M molar ratio of 1,2-epoxy-p-menth-8-ene to 1,2:8,9-diepoxy-p-menthane), 81 % epoxide yield for fatty acid methyl esters, and 80 % Licarin A selectivity at 40 % isoeugenol conversion. For dienes (DL-limonene, methyl linoleate), kinetic modelling studies suggested that the formation of the monoepoxides was faster than that of diepoxides, accounting for enhanced monoepoxide selectivity.

Shattering the Water Window: Comprehensive Mapping of Faradaic Reactions on Bioelectronics Electrodes.

It is generally accepted that for safe use of neural interface electrodes, irreversible faradaic reactions should be avoided in favor of capacitive charge injection. However, in some cases, faradaic reactions can be desirable for controlling specific (electro)physiological outcomes or for biosensing purposes. This study aims to systematically map the basic faradaic reactions occurring at bioelectronic electrode interfaces. We analyze archetypical platinum-iridium (PtIr), the most commonly used electrode material in biomedical implants. By providing a detailed guide to these reactions and the factors that influence them, we offer a valuable resource for researchers seeking to suppress or exploit faradaic reactions in various electrode materials. We employed a combination of electrochemical techniques and direct quantification methods, including amperometric, potentiometric, and spectrophotometric assays, to measure O2, H2, pH, H2O2, Cl2/OCl-, and soluble platinum and iridium ions. We compared phosphate-buffered saline (PBS) with an unbuffered electrolyte and complex cell culture media containing proteins. Our results reveal that the "water window"─the potential range without significant water electrolysis─varies depending on the electrolyte used. In the culture medium that is rich with redox-active species, a window of potentials where no faradaic process occurs essentially does not exist. Under cathodic polarizations, significant pH increases (alkalization) were observed, while anodic water splitting competes with other processes in media, preventing prevalent acidification. We quantified the oxygen reduction reaction and accumulation of H2O2 as a byproduct. PtIr efficiently deoxygenates the electrolyte under low cathodic polarizations, generating local hypoxia. Under anodic polarizations, chloride oxidation competes with oxygen evolution, producing relatively high and cytotoxic concentrations of hypochlorite (OCl-) under certain conditions. These oxidative processes occur alongside PtIr dissolution through the formation of soluble salts. Our findings indicate that the conventional understanding of the water window is an oversimplification. Important faradaic reactions, such as oxygen reduction and chloride oxidation, occur within or near the edges of the water window. Furthermore, the definition of the water window significantly depends on the electrolyte composition, with PBS yielding different results compared with culture media.

A tailored cytochrome P450 monooxygenase from Gordonia rubripertincta CWB2 for selective aliphatic monooxygenation.

Cytochrome P450 monooxygenases are recognized as versatile biocatalysts due to their broad reaction capabilities. One important reaction is the hydroxylation of non-activated C-H bonds. The subfamily CYP153A is known for terminal hydroxylation reactions, giving access to functionalized aliphatics. Whilst fatty derivatives may be converted by numerous enzyme classes, midchain aliphatics are seldomly accepted, a prime property of CYP153As. We report here on a new CYP153A member from the genome ofthe mesophilic actinobacterium Gordonia rubripertincta CWB2 as an efficient biocatalyst. The gene was overexpressed inEscherichia coli and fused with a surrogate electron transport system from Acinetobacter sp. OC4. This chimeric self-sufficient whole-cell system could perform hydroxylation and epoxidation reactions: conversions of C6-C14 alkanes, alkenes, alcohols and of cyclic compounds were observed, yielding production rates of, e.g., 2.69 mM h-1 for 1-hexanol and 4.97 mM h-1 for 1,2-epoxyhexane. Optimizing the linker compositions between the protein units led to significantly altered activity. Balancing linker length and flexibility with glycine-rich and helix-forming linker units increased 1-hexanol production activity to 350 % compared to the initial linker setup with entirely helical linkers. The study shows that strategic coupling of efficient electron supply and a selective enzyme enables previously challenging monooxygenation reactions of midchain aliphatics.

Aqueous hydrotropes: An efficient and reusable catalyst for the synthesis of 3-carboxy-coumarin motifs at room temperature

The coumarin moiety plays an important role in the large number of natural products possessing different kinds of biological diversity. Coumarin carboxylic acids show a wide range of biological activities in the pharmaceutical and agricultural fields. Knoevenagel condensation is one of the important reaction pathways for synthesizing coumarin derivatives, and many methodologies have been developed to synthesize this class of compounds. A more environmentally friendly method of synthesizing 3-carboxy coumarins has been successfully carried out using 50% aqueous NaPTS hydrotropes at room temperature, along with various substituted 2-hydroxy benzaldehydes and Meldrum’s acid. This process involves Knoevenagel condensation followed by intramolecular cyclization, providing better product yields (78-95%).

Crystal structure of l-threonine-O-3-phosphate decarboxylase CobC from Sinorhizobium meliloti involved in vitamin B12 biosynthesis

Vitamin B12 is involved in many important biochemical reactions for humans, and its deficiency can lead to serious diseases. The industrial production of vitamin B12 is achieved through microbial fermentation. In this work, we determine the crystal structures of the l-threonine-O-3-phosphate (Thr-P) decarboxylase CobC from Sinorhizobium meliloti (SmCobC), an industrial vitamin B12-producing bacterium, in apo form and in complex with a reaction intermediate. Our structures supported the Thr-P decarboxylase activity of SmCobC and revealed that the positively charged substrate-binding pocket between the large and small domains determines its substrate selectivity for Thr-P. Moreover, our results provided evidence for the proposition that the AP-P linker is formed by direct incorporation of AP-P in the biosynthetic pathway of vitamin B12 in S.meliloti.

An Experimental and Kinetic Modeling Study of the Laminar Burning Velocities of Ammonia/n-Heptane Blends

Ammonia is carbon-free and is a very promising renewable fuel. The ammonia/diesel dual-fuel combustion strategy is an important combustion strategy for ammonia internal combustion engines. To achieve clean and efficient combustion with a high ammonia blending ratio in ammonia engines, it is important to thoroughly investigate the combustion characteristics and chemical reaction mechanisms of ammonia/diesel fuel blends. Based on the constant volume combustion vessel experiments, the laminar burning velocities (LBVs) of ammonia/n-heptane blends were measured at the conditions of an ammonia–energy ratio of 60–100%, at initial pressures of 0.1–0.5 MPa and initial temperatures of 338–408 K, and under an equivalence ratio regime of 0.8–1.3. The experimental results indicate that the laminar burning velocities of ammonia/n-heptane fuel blends increase with a decreasing ammonia–energy ratio. Specifically, with an ammonia–energy ratio of 60%, an initial temperature of 373 K, an initial pressure of 0.1 MPa, and an equivalence ratio of 1.1, the measured LBV is approximately 20 cm/s, which is about 61% faster than that of pure ammonia flames under the same conditions. A previously developed chemical kinetic mechanism is employed to simulate the new experimental data, and the model exhibits overall good performance. The sensitivity analyses have been conducted to highlight the important reaction pathways. The elementary reaction O2 + Ḣ&lt;=&gt;Ö + ȮH demonstrates the most significant promotional effect on the laminar burning velocities, while the interaction reaction pathways of via H-abstraction from n-heptane by ṄH2 radicals are not showing obvious effects on the simulation results under the studied conditions.

Ballistic and delayed photodissociation channels in the B̃ 2A11 state of water studied with resonant inelastic x-ray scattering

Photodissociation is one of the most important photoinduced chemical reactions. It occurs when the potential energy curve along a chemical bond is repulsive in an excited state. Typically, “ballistic” ultrafast dissociation leads to the broadening of absorption resonances and the smearing out of vibrational fine-structure. We report on the photodissociation of H2O in the B̃ 2A11 electronic state, characterized by a |3a1−14a11〉 configuration, which can be reached via resonant inelastic x-ray scattering or direct ultraviolet absorption. In both cases the spectra show narrow vibrational resonances, in spite of the dissociative character of the state. We find that “delayed” dissociation pathways, caused by reflection of the nuclear wave packet, are responsible for this effect. In spite of the analogous topology of the potential energy surfaces of the core- and valence-excited states, the reflection of the wave packet takes place only in the latter. The two-dimensional wave packet of the O-H stretching coordinates becomes trapped in a “cavity” near the Franck-Condon region, resulting from a mismatch between the OH vibrational frequency in the cavity and the one at the dissociation limit. Published by the American Physical Society 2024