Enhancement Of Reaction Rate Research Articles

Family 92 carbohydrate-binding modules specific for β-1,6-glucans increase the thermostability of a bacterial chitinase

In biomass-processing industries there is a need for enzymes that can withstand high temperatures. Extensive research efforts have been dedicated to finding new thermostable enzymes as well as developing new means of stabilising existing enzymes. The attachment of a stable non-catalytic domain to an enzyme can, in some instances, protect a biocatalyst from thermal denaturation. Carbohydrate-binding modules (CBMs) are non-catalytic domains typically found appended to biomass-degrading or modifying enzymes, such as glycoside hydrolases (GHs). Most often, CBMs interact with the same polysaccharide as their enzyme partners, leading to an enhanced reaction rate via the promotion of enzyme-substrate interactions. Contradictory to this general concept, we show an example of a chitin-degrading enzyme from GH family 18 that is appended to two CBM domains from family 92, both of which bind preferentially to the non-substrate polysaccharide β-1,6-glucan. During chitin hydrolysis, the CBMs do not contribute to enzyme-substrate interactions but instead confer a 10–15 °C increase in enzyme thermal stability. We propose that CBM92 domains may have a natural enzyme stabilisation role in some cases, which may be relevant to enzyme design for high-temperature applications in biorefinery.

Membrane Electrode Assembly for Electrocatalytic CO2 Reduction: Principle and Application

AbstractElectrocatalytic CO2 reduction reaction (CO2RR) in membrane electrode assembly (MEA) systems is a promising technology. Gaseous CO2 can be directly transported to the cathode catalyst layer, leading to enhanced reaction rate. Meanwhile, there is no liquid electrolyte between the cathode and the anode, which can help to improve the energy efficiency of the whole system. The remarkable progress achieved recently points out the way to realize industrially relevant performance. In this review, we focus on the principles in MEA for CO2RR, focusing on gas diffusion electrodes and ion exchange membranes. Furthermore, anode processes beyond the oxidation of water are considered. Besides, the voltage distribution is scrutinized to identify the specific losses related to the individual components. We also summarize the progress on the generation of different reduced products together with the corresponding catalysts. Finally, the challenges and opportunities are highlighted for future research.

Membrane Electrode Assembly for Electrocatalytic CO2Reduction: Principle and Application.

Electrocatalytic CO2 reduction reaction (CO2RR) in membrane electrode assembly (MEA) systems is a promising technology. Gaseous CO2 can be directly transported to the cathode catalyst layer, leading to enhanced reaction rate. Meanwhile, there is no liquid electrolyte between the cathode and the anode, which can help to improve the energy efficiency of the whole system. The remarkable progress achieved recently points out the way to realize industrially relevant performance. In this review, we focus on the principles in MEA for CO2RR, focusing on gas diffusion electrodes and ion exchange membranes. Furthermore, anode processes beyond the oxidation of water are considered. Besides, the voltage distribution is scrutinized to identify the specific losses related to the individual components. We also summarize the progress on the generation of different reduced products together with the corresponding catalysts. Finally, the challenges and opportunities are highlighted for future research.

Microwave-Assisted Synthesis of Highly Active Single-Atom Fe/N/C Catalysts for High-Performance Aqueous and Flexible All-Solid-State Zn-Air Batteries.

The development of low-cost single-atom electrocatalysts for oxygen reduction reaction (ORR) is highly desired but remains a grand challenge. Superior to the conventional techniques, a microwave-assisted strategy is reported for rapid production of high-quality Fe/N/C single-atom catalysts (SACs) with profoundly enhanced reaction rate and remarkably reduced energy consumption. The as-synthesized catalysts exhibit an excellent ORR performance with a positive half-wave potential up to 0.90V, a high turnover frequency of 0.76 s-1 , as well as a satisfied stability with a lost half-wave potential of just 27mV over 9000 cycles (much better than that of Pt/C with 107mV lost) and good methanol resistance. The open-circuit voltages of as-constructed aqueous and flexible all-solid-state Zn-air batteries (ZABs) are 1.56 and 1.52V, respectively, higher than those of 20% Pt/C-based ones (i.e., 1.43 and 1.38V, respectively). Impressively, they afford a peak power density of 235mW cm-2 , which exceeds that of Pt/C (i.e., 186mW cm-2 ), and is comparable to the best ones of Fe/N/C-based ZABs ever reported.

Various methods for enhancement in heterogeneous catalysed biodiesel production reaction rate

The biodiesel can be produced by transesterification and esterification reaction with edible and non-edible oil respectively. These reactions are catalysed by both homogeneous and heterogenous catalyst. The transesterification reactions for heterogeneous catalysts proceed at a relatively slow rate. The heterogeneous reaction mixture constitutes a three-phase system, oil/alcohol/catalyst therefore the mass transfer limitation controls the reaction rate. In the present study Tetra-hydrofuran, Hexane and Heptane as co-solvent have been tested for the transesterification and esterification reaction. Tetra-hydrofuran, assists in decreasing the mass transfer between the oil and methanol phases. Tetra-hydrofuran was found to be the best co-solvent. It also represents that the presence of Tetra-hydrofuran only enhance the solubility of phases not final equilibrium yield. Results are compared with homogeneous and heterogeneous catalysed reaction. Ultrasonic irradiation influenced the mixing of the reaction mixture and resulted in higher yields, for each co-solvent case.

Performance optimization of metal hydride hydrogen storage reactors based on PCM thermal management

Metal hydrides (MH) are considered promising hydrogen storage materials. In applications, the heat transfer characteristics of MH hydrogen storage reactors are closely related to their storage performance. Phase change materials (PCM) were proven to integrate with MH reactors for effective thermal management. Nevertheless, the PCM based MH reactors (MH-PCM reactors) still need to overcome the heat transfer resistance and the loss of volumetric hydrogen storage capacity caused by the PCM volume proportion. The present work established a mathematical model to describe the transfer and reaction process in the MH-PCM reactor. Various parameters were discussed in the current work. It was found that the increased PCM's amount and latent heat have a restricted effect on the enhancement of heat transfer and absorption reaction rate. The higher PCM's thermal conductivity and hydrogen supply pressure effectively enhanced the heat transfer and reaction rate. Furthermore, the increased absorption pressure can improve PCM's sensible heat storage capacity, which provides a new approach for optimizing PCM consumption. Finally, the quantitative relationship among the volumetric hydrogen storage capacity, the volume fraction and thermophysical properties of PCM, and the absorption pressure were established by systematic analysis, which provides an essential theoretical basis for the optimization of MH-PCM reactors.

Diffusion Coupling Kinetics in Multisite Catalysis: A Microkinetic Framework

Multicomponent (or multisite) catalysts are gaining increasing importance and popularity in heterogeneous catalysis and yet the cooperation mechanism among them remains obscure and intriguing. In this work, a vital and characteristic phenomenon in the two-component catalytic systems, the diffusion of surface intermediates, and the induced activity enhancement by coupling with the surface reactions is systematically investigated with both rigorous theoretical deduction and microkinetic simulations. By virtue of the proposed pseudo-fluctuation method, a general analytical framework is established with the two-step reaction model and the influence and origin of intermediate diffusion are elaborated. We quantify that the difference of the chemical potential of surface intermediates on different sites leads to the diffusion and determines the diffusion flowing from the strong adsorption site to the weak adsorption site. More importantly, we demonstrate that an enhancement in the overall reaction rate can be achieved by integrating the advantages of different components. Specifically, the optimum promotion effect would appear when the adsorption abilities of the two-component catalysts for the surface intermediate are around the chemical potential of the reactant and product, respectively. From the aspect of practical preparation methods, the coupling kinetics of the mechanically mixed catalysts and the alloy-typed catalysts are theoretically compared and a preliminary strategy for choosing the coupled components is proposed. We believe that this work provides a fundamental insight into the understanding and manipulation of the intermediate diffusion process.

Aerobic oxidative lactonization of diols at room temperature over defective titanium-based oxides in water

Lactones are applied in pharmaceutical or agriculture/forestry industry as anesthetics/sedatives, pesticide intermediates or as plant growth regulators. Selective conversion of diols into value-added lactones under mild reaction conditions is still challenging. Here, we report on an environmentally benign catalytic approach using oxygen vacancies-enriched titanium-based oxides and water as solvent at room temperature for the lactonization of various aromatic and aliphatic diols. Lactonization was found to proceed in two consecutive steps: 1) the dehydrogenation of diols to hydroxyaldehyde intermediates catalyzed by the Pt cocatalyst; 2) the subsequent oxidative lactonization of the intermediates to the lactone products over defective titanium-based oxides. Diffuse reflectance infrared Fourier transform spectroscopy analysis using pyridine as probe molecule combined with electron paramagnetic resonance results suggest that oxygen vacancies as strong Lewis acid sites are pivotal for the superior catalytic activities at room temperature, which are attributed to the enhanced reaction rate of step 2 due to stronger adsorption of the hydroxyaldehyde intermediates on the Lewis acid sites.

Amine Functionalization Within Hierarchically‐Porous Zeotype Framework for Plasmonic Catalysis over PdAu Nanoparticles

AbstractPlasmonic catalysis has revealed improved product yield and selectivity in various chemical transformation reactions over the past decade. In this report, the effect of tertiary amine (‐NR3) functionalization on the surface of hierarchically‐porous zeotype (HP‐AlPO‐5) materials to enhance the plasmon‐mediated catalysis, utilizing a combination of the plasmonic antenna (Au) and catalytic reactor (Pd) nanoparticles (NPs) was investigated. The catalysts have been characterized using enhanced techniques such as HAADF‐STEM, FT‐EXAFS, and probe‐based FT‐IR to reveal the proximity and interaction between bimetallic NPs, and thermal stability of amines in the presence of Au or PdAu NPs. Interestingly, a four‐fold enhancement in the Suzuki‐Miyaura coupling reaction product yield was obtained over PdAu/HP‐AlPO‐5‐NR3 when compared with the analogous plasmonic catalyst with no amine functionalization under visible light irradiation. A range of amines were functionalized and their influence in the nucleation, uniform growth and stabilization of catalytic active site (Pd) and formation of electron‐rich species under visible light irradiation has also been investigated. The presence of tertiary amine in the nanostructured catalyst enhanced the turnover number (TON) significantly under light irradiation conditions in comparison to dark conditions. This study provides an enriched understanding of plasmon‐driven chemistry, where the maximized reaction rate enhancement requires the existence of active metal species and the formation of electron‐enriched species under light irradiation conditions.

Enzymatic Reactions using Ionic Liquids for Green Sustainable Chemical Process; Stabilization and Activation of Lipases.

The enzymatic reaction is highly respected from an environmentally-friendly point-of-view. Optimization of the reaction media and supporting materials of enzymes must be investigated in parallel with the effort to develop new enzymes. Lipases are frequently used for organic syntheses as synthetic tools even industry because of their acceptance of having a broad range of substrates, stability, and availability. We have investigated the possibility of ILs as both a solvent and activating or stabilization agent of enzymes, in particular, lipase as a model enzyme. ILs allowed recyclable use of a lipase and significant acceleration of transesterification, and also enhanced the stability and reaction activity of a lipase by immobilization through a lyophilization process. We discuss how we enhanced the enzyme capability using the IL engineering focusing on lipase-catalyzed reactions, i. e., realization of the recyclable use of an enzyme, how ILs mediated the enhanced reaction rate, and improved the stability of the enzyme.

Towards the first plasma-electron screening experiment

The enhancement of fusion reaction rates in a thermonuclear plasma by electron screening of the Coulomb barrier is an important plasma-nuclear effect that is present in stellar models but has not been experimentally observed. Experiments using inertial confinement fusion (ICF) implosions may provide a unique opportunity to observe this important plasma-nuclear effect. Herein, we show that experiments at the National Ignition Facility (NIF) have reached the relevant physical regime, with respect to the density and temperature conditions, but the estimated impacts of plasma screening on nuclear reaction rates are currently too small and need to be increased to lower the expected measurement uncertainty. Detailed radiation hydrodynamics simulations show that practical target changes, like adding readily available high-Z gases, and significantly slowing the inflight implosion velocity, while maintaining inflight kinetic energy, might be able to push these conditions to those where plasma screening effects may be measurable. We also perform synthetic data exercises to help understand where the anticipated experimental uncertainties will become important. But challenges remain, such as the detectability of the reaction products, non-thermal plasma effects, species separation, and impacts of spatial and temporal gradients. This work lays the foundation for future efforts to develop an important platform capable of the first plasma electron screening observation.

The formation of a lithium-iridium complex hydride toward ammonia synthesis.

Noble metal elements are focal catalytic candidates for many chemical processes, but have received little attention in the field of nitrogen fixation except ruthenium and osmium. Iridium (Ir), as a representative, has been shown to be catalytically inactive for ammonia synthesis because of its weak nitrogen adsorption and severe competitive adsorption of H over N that strongly inhibits the activation of N2 molecules. Here we show that, upon compositing with lithium hydride (LiH), iridium can catalyze ammonia formation at much enhanced reaction rates. The catalytic performance of the LiH-Ir composite can be further improved by dispersion on a MgO support with a high specific surface area. At 400 °C and 10 bar, the MgO-supported LiH-Ir (LiH-Ir/MgO) catalyst shows a ca. 100-fold increase in activity compared to the bulk LiH-Ir composite and the MgO-supported Ir metal catalyst (Ir/MgO). The formation of a lithium-iridium complex hydride phase was identified and characterized, and this phase may be responsible for the activation and hydrogenation of N2 to NH3.

Enzymes and Liquid-Liquid Phase Separation: A New Era for the Regulation of Enzymatic Activity

Liquid-liquid phase separation (LLPS) is recognized as a mechanism for regulation of enzymatic activity. Biochemical mechanisms include concentrating reactants to enhance reaction rates or sequester enzymes and reactants from each other to reduce the reaction rate. On the other hand, LLPS might also regulate the diffusion of small molecules or important parameters for enzymatic activity (such as modulators, macromolecular crowding and changing the media physicochemical features) increasing or decreasing the reaction rate of the enzymes. Furthermore, the co-compartmentalization of specific enzymes can favour or speed up specific metabolic fluxes. Here, we discuss how LLPS contributed to generate a new era for enzyme regulation and the new possible subtle regulation mechanisms still unexplored.

Application of Piszkiewicz model on the electron transfer reaction of dithionite ion and bis-(2-pyridinealdoximato)dioxomolybdate(IV) complex

The need to better understand the binding mode of antioxidants (sulfur oxyanions) kinetically is a concern in medicine. Hence, a spectrophotometric method was used to study the application of the Piszkiewicz model on the electron transfer reaction of dithionite ion (S2O42−) and bis-(2-pyridinealdoximato)dioxomolybdate(IV) complex at 303 K and an absorption maxima of 560 nm. It follows an acid dependent reductive pathway that is medium sensitive. Charge distribution from the reaction species contributes to the redox efficiency of the system, resulting in a primary salt effect (NaCl) with an enhanced reaction rate. Alteration of the reaction medium with ethanol led to an elevation of reduction time as the charge catalysis was distorted by a drop in the system permittivity. Likewise, sodium dodecyl sulfate in the system decreased the reduction rate of the complex due to the low impact of hydrophobic and ion interaction between the micelle and substrates. First order reaction kinetics was observed in the concentration of the redox partners and a 2:1 (complex: S2O42−) stoichiometry was obtained with the involvement of hydrogenated sulfite radical which resulted in the formation of sulfur dioxide and a Mo2+ deactivated complex. The occurrence of counterion catalysis is pronounced in the reaction system owing to the participation of like-charged substrates in the rate-controlling phase. The standard enthalpy (69.12 pm 0.05 kJ mol−1) and Gibbs energy (80.10 pm 0.07) kJ mol−1 suggest that the process is endothermic dependent. The investigation of the anionic surfactant effect on the reaction medium was quantitatively ascertained from the Piszkiewicz model of the complex interaction sequence.

Reactivity and Enantioselectivity in NHC Organocatalysis Provide Evidence for the Complex Role of Modifications at the Secondary Sphere

Secondary-sphere interactions are often harnessed to control reactivity and selectivity in organometallic and enzymatic catalysis. Yet, such strategies have only recently been explicitly applied in the context of organocatalytic systems. Although increased stability, reproducibility, and selectivity were obtained in previous work using this approach, the precise mechanistic pathway promoted by secondary-sphere modification in organocatalysis remained unclear. Herein, we report a comprehensive mechanistic study on the origin of the unique reactivity patterns and stereocontrol observed with boronic acids (BAs) as secondary-sphere modifiers of N-heterocyclic carbene (NHC) organocatalysts. Kinetic experiments revealed partial order in catalyst upon the addition of BA and unusual preactivation behavior, indicating the presence of stable off-cycle catalyst aggregation and BA-base adducts. These hypotheses were supported both by computations and by a series of NMR and nonlinear effect experiments. Furthermore, computations indicated a rate-limiting, water-assisted hydrogen atom transfer mechanism. This finding led to a considerable enhancement in the experimental reaction rate while maintaining excellent enantioselectivity by adding catalytic amounts of water. Finally, computations and racemization experiments uncovered an uncommon Curtin-Hammett-controlled enantioselectivity in the presence of secondary-sphere modifiers.

The effect of hydrogen addition on methane-air flame in a stratified swirl burner

This numerical study has focused on assessing the influence of adding 20% and 40% hydrogen (on the volume of fuel basis) in different slots of a turbulent methane-air flame. The injection configurations are as follows: in the (i) inner slot, (ii) outer slot, (iii) and both slots. RANS numerical results has been obtained using OpenFOAM code. The impact of enrichment configuration and level on various flow fields have been investigated in the stratified and premixed cases. The results showed that adding hydrogen in the outer slot does not change the flow characteristics significantly, and adding H2 in both slots showed a slight difference compared to adding hydrogen in the inner slot. Hydrogen addition in the inner slot and both slots increased the combustion intensity near the burner exit and resulted in a robust flame attachment which is mainly the result of the enhancement of reaction rates of OH+H2=H+H2O and OH+CH4=CH3+H2O in the premixed case. In the stratified case, in addition to the pathways mentioned above, the reaction rate of H+O2=O+OH also enhanced, which resulted in better flame attachment. Regarding pollutant emissions, H2 addition, decreased CO and increased NOx emission in all configurations of the premixed and stratified cases.

Visible-light-driven nanoscale zero-valent iron loaded rGO/g-C3N4 for fluoroquinolone antibiotics degradation in water

Fluoroquinolone antibiotics attract increasing attention in the water treatment field because of the potential adverse effects on aquatic ecosystems and human health. The graphitic carbon nitride (g-C3N4) based photocatalysis has been demonstrated as an economically feasible and environmentally benign process to control these persistent contaminants. In this study, a new visible-light-driven of reduced graphene oxide (rGO) and nanoscale zero-valent iron (nZVI) co-modified g-C3N4-based photocatalyst was synthesized via ultrasonication-assisted chemisorption method. The optimized nZVI-loaded rGO/g-C3N4 (10% IGCN) showed a reaction rate enhancement of 2.12∼3.69-fold and 1.20∼1.68-fold for the degradation of ofloxacin (OFL), norfloxacin (NOR), and ciprofloxacin (CIP) compared to that of carbon-doped g-C3N4 (MCB0.07) and rGO-supported g-C3N4 (7.5% GCN) under the irradiation of simulated visible light, respectively. The enhanced photocatalytic activity can be ascribed to the synergistic effect of nZVI and rGO to improve the separation of charge carriers and boost the harvest of visible light. The degradation mechanisms were explored by scavenger tests and X-ray photoelectron spectroscopy (XPS), indicating that holes (h+) played a dominant role in the decomposition of OFL, NOR, and CIP. The piperazine ring and C–N between the piperazine ring and benzene were the primary attack sites of h+. In addition, the ring-opening oxidation of benzene (C=C bond) connected by the C–F bond may also be an essential step. This study shed light on the degradation mechanism of OFL, NOR, and CIP under visible light irradiation of the 10% IGCN and provided theoretical support for the practical application of photocatalysis in treating antibiotics-containing water.

Organizing Enzymes on Self‐Assembled Protein Cages for Cascade Reactions

AbstractCells use self‐assembled biomaterials such as lipid membranes or proteinaceous shells to coordinate thousands of reactions that simultaneously take place within crowded spaces. However, mimicking such spatial organization for synthetic applications in engineered systems remains a challenge, resulting in inferior catalytic efficiency. In this work, we show that protein cages as an ideal scaffold to organize enzymes to enhance cascade reactions both in vitro and in living cells. We demonstrate that not only enzyme‐enzyme distance but also the improved Km value contribute to the enhanced reaction rate of cascade reactions. Three sequential enzymes for lycopene biosynthesis have been co‐localized on the exterior of the engineered protein cages in Escherichia coli, leading to an 8.5‐fold increase of lycopene production by streamlining metabolic flux towards its biosynthesis. This versatile system offers a powerful tool to achieve enzyme spatial organization for broad applications in biocatalysis.

Organizing Enzymes on Self-Assembled Protein Cages for Cascade Reactions.

Cells use self-assembled biomaterials such as lipid membranes or proteinaceous shells to coordinate thousands of reactions that simultaneously take place within crowded spaces. However, mimicking such spatial organization for synthetic applications in engineered systems remains a challenge, resulting in inferior catalytic efficiency. In this work, we show that protein cages as an ideal scaffold to organize enzymes to enhance cascade reactions both in vitro and in living cells. We demonstrate that not only enzyme-enzyme distance but also the improved Km value contribute to the enhanced reaction rate of cascade reactions. Three sequential enzymes for lycopene biosynthesis have been co-localized on the exterior of the engineered protein cages in Escherichia coli, leading to an 8.5-fold increase of lycopene production by streamlining metabolic flux towards its biosynthesis. This versatile system offers a powerful tool to achieve enzyme spatial organization for broad applications in biocatalysis.

One Pot Synthesis of Benzopyranones and Benzoxazinones Catalyzed by MMO

One pot synthesis of benzopyranones and benzoxazinones is aimed to develop an environment friendly strategy in which the requisite moiety is constructed by using functionality of chalcones and mixed metal oxides (MMO). MMO, as catalyst are the topic of continuing interest due to their versatile applications in the field of organic synthesis. A novel approach to the synthesis of benzopyranones and benzoxazines with high pharmaceutical activity is sought to eliminate the use of volatile organic solvents. A new methodology is designed to carry out the synthesis in a single step with enhance reaction rate with high yield.