Forward Rate Constants Research Articles

Quantum chemistry and kinetics of hydrogen sulphide oxidation.

A fundamental understanding of the acid gas (H2S and CO2) chemistry is key to efficiently implement the desulphurisation process and even the production of clean fuels such as hydrogen or syngas. In this work, we developed a new kinetic model for the pyrolysis and oxidation of hydrogen sulphide by merging two previously reported models with the goal of covering a wider range of conditions and including the effect of carbon dioxide. The resulting model, which consists of 75 species and 514 reactions, was used to conduct rate of production and sensitivity analysis in plug flow reactor simulations, and the results were used to determine the most prominent reactions in which hydrogen sulphide, molecular hydrogen, and sulphur monoxide are involved. The resulting list of important reactions was screened and the kinetics of three of them, i.e., SO2 + S2 → S2O + SO, S2O + S2 → S3 + SO, and SO + SH → S2 + OH, was found to warrant further investigation. With the goal of improving the accurancy of our new kinetic model, we carried out a robust quantum chemistry and Rice-Ramsperger-Kassel-Marcus master equation study to obtain, for the first time, the forward and reverse rate constants for those three reactions at temperatures and pressures of interest for combustion and atmospheric chemistry. This work is the first step of a kinetic study that is aimed at improving the understanding of the chemistry of the pyrolysis and oxidation of H2S, highlighting the importance of sulphur-sulphur interactions and providing a fundamental basis for future kinetic models of H2S not only in the field of combustion, but also in atmospheric chemistry.

Rate Equations for Reversible Disproportionation Reactions and Fitting to Time-Course Data.

Integrated rate equations are straightforward to fit to experimental data to verify a proposed mechanism and to extract kinetic parameters. Such equations are derived for reversible disproportionation/comproportionation reactions with any set of initial concentrations. Extraction of forward and reverse rate constants from experimental data by fitting the rate law to the data is demonstrated for the disproportionation of 2,2,6,6-tetramethyl-1-piperidinyl-N-oxyl (TEMPO) under acidic conditions where the approach to equilibrium is observed.

TO ISSUE INCREASING THE EFFICIENCY OF THE MASS TRANSFER PROCESS IN AGITATORS

This study investigates the impact of impeller design on the rate of esterification reaction in a batch reactor. Different impeller designs were tested and compared, including Backswept impeller, C-shaped, Double Blade impeller, and Pitched impeller. The results showed that the choice of impeller design has a significant effect on the rate of esterification reaction, with the Backswept impeller proving to be the most effective. In this work, it is aimed to investigate the esterification of acetic acid and methanol in a batch stirred reactor for the synthesis of methyl acetate. The temperature range of 303.15 K is examined, with sulphuric acid used as the homogeneous catalyst in varying concentrations of 0.0633 mol·L1 to 0.3268 mol·L-1. The feed molar ratio of acetic acid to methanol also varied from 1:1 to 1:4 to observe its influence on the reaction rate. The research delves into the effects of temperature, catalyst concentration, and reactant concentration on the reaction rate. The experimental data is correlated using a second-order kinetic rate equation, and the forward and backward reaction rate constants and activation energies are determined from the Arrhenius plot. The developed kinetic model is compared with previous models in literature. In order to find the most effective impeller for esterification reaction, diameter of impellers was accepted same in the constant volume and dimension of batch reactor. The obtained kinetic equation proves useful for the simulation of a reactive distillation column for the synthesis of methyl acetate. Keywords: the batch reactor, impeller, esterification, effectivity, mass transfer, mixing, mechanical mixers, Logarithmic Shear Rate, Eddy Diffusivity.

Thermal decomposition of 1-ethyl-3-methylimidazolium acetate

To realize the potential applications of ionic liquids as working fluids and solvents one should have knowledge about their long-term stability, which depends on their thermally initiated decomposition. However, the possible decomposition pathways are not yet known accurately. 1-Ethyl-3-methylimidazolium acetate ([C2Mim][OAc]) is widely studied and shows potential in cellulose dissolution. In this study, we investigate possible [C2Mim][OAc] decomposition pathways by combining decomposition experiments and ab initio rate constant computations. Both approaches yield activation energies around 135 kJ·mol−1, which significantly deviate from earlier TGA measurements with open surface yielding 100–110 kJ·mol−1 and are probably affected by evaporation. Our study uses an improved TGA measurement protocol with a sealed pan with a tiny hole that fosters measurement of the actual chemical process leading to mass loss. Ab initio computations of this study comprise vapor-liquid equilibria of reactants and products as well as forward and reverse reaction rate constants in the gas and liquid phase. Calculations for the gas phase closely match a first-order analysis of the experiments, but calculations for the liquid phase strongly depend on the quality of solvation modeling. The computations indicate that the SN2 pathway with an activation energy of 142–144 kJ·mol−1 dominates thermal decomposition of [C2Mim][OAc].

Long-Range Kinetic Effects on the Alternating Ring Opening Metathesis of Bicyclo[4.2.0]oct-6-ene-7-carboxamides and Cyclohexene.

We report an investigation of rates of ruthenium-catalyzed alternating ring opening metathesis (AROM) of cyclohexene with two different Ru-cyclohexylidene carbenes derived from bicyclo[4.2.0]oct-6-ene-7-carboxamides (A monomer) that bear different side chains. These monomers are propylbicyclo[4.2.0]oct-6-ene-7-carboxamide and N-(2-(2-ethoxyethoxy)ethanylbicyclo[4.2.0]oct-6-ene-7-carboxamide. The amide substitution of these monomers directly affects both the rate of the bicyclo[4.2.0]oct-6-ene-7-carboxamide ring opening and the rate of reaction of the resulting carbene with cyclohexene (B monomer). The resulting Ru-cyclohexylidenes underwent reversible ring opening metathesis with cyclohexene. However, the thermodynamic equilibrium disfavored cyclohexene ring opening. Utilization of triphenylphosphine forms a more stable PPh3 ligated complex, which suppresses the reverse ring closing reaction and allowed direct measurements of the forward rate constants for formation of various A-B and A-B-A' complexes through carbene-catalyzed ring-opening metathesis and thus gradient polymer structure-determining steps. The relative rate of the propylbicyclo[4.2.0]oct-6-ene-7-carboxamide ring opening is 3-fold faster than that of the N-(2-(2-ethoxyethoxy)ethanylbicyclo[4.2.0]oct-6-ene-7-carboxamide. In addition, the rate of cyclohexene ring-opening catalyzed by the propyl bicyclooctene is 1.4 times faster than when catalyzed by the ethoxyethoxy bicyclooctene. Also, the subsequent rates of bicyclo[4.2.0]oct-6-ene-7-carboxamide ring opening by propyl-based Ru-hexylidene are 1.6-fold faster than ethoxyethoxy-based Ru-hexylidene. Incorporation of the rate constants into reactivity ratios of bicyclo[4.2.0]amide-cyclohexene provides prediction of copolymerization kinetics and gradient copolymer structures.

Synthesis, NMR kinetics and dynamic structure of a 17-mer heptaloop RNA hairpin carrying a 3-N-methyluridine nucleotide residue in the loop region

A 17-mer RNA hairpin (5'GGGAGUXAGCGGCUCCC3') carrying 3-N-methyluridine (m3U) at position X (m3U7-RNA), designed to represent the anticodon stem-loop (ACSL) region of tRNAs to study an open loop state (O-state), was synthesized, purified by HPLC, and characterized by MALDI-ToF_MS and NMR methods. 1H-NMR data revealed primary (P-state in 56.1%), secondary (S-state in 43.9%) and tertiary (∼5-6%) ACSL conformations. Exchange rate constant (kex) for interconversion between P and S states is 112 sec−1 (<Δω ∼454 rad/sec), confirming a slow exchange regime between the two states. Forward (k PS ) and backward (k SP ) rate constants are 49.166 sec−1 and 62.792 sec−1, respectively, leading to a longer life-time (20.339 msec) for the P-state and a shorter life-time (15.926 msec) for the S-state. In accordance with conformational populations determined by 1H-NMR, dynamics of the P/S/tertiary states of m3U7-RNA and its wild-type counterpart (wt-RNA) were studied by three independent MD production simulations. Cluster analysis revealed that wt-RNA reflects the structural characteristics of the ACSL region of tRNAs. The P-state of m3U7-RNA was found to be structurally similar to wt-RNA but lacks an intraloop H-bond between m3U7 and C10 (U33 and nt36 in tRNAs). In the S-state of m3U7-RNA, m3U7 flips out of the loop region. O-state loop conformations of m3U7-RNA were also clustered (4.8%), wherein the loop nucleotides m3U7.A8.G9.C10.G11 stack one after another. We propose that the O-state of m3U7-RNA is the most suitable conformation that makes the loop accessible for complementary nucleotides and for non-enzymatic primordial replication of small circular RNAs. Communicated by Ramaswamy H. Sarma

Hydrogen Peroxide Production with High Solar to Chemical Conversion (0.16%) and Efficient Water Purification Using Carbon Quantum Dots-Modified Indium Sulfide

Photocatalytic production of hydrogen peroxide (H2O2) is the need of the hour for green synthesis and sustainability. Herein, we designed hierarchical carbon quantum dots (CQDs)-supported β-indium sulfide (β-In2S3) nanoflakes as an efficient catalyst for H2O2 generation via a two-electron oxygen reduction reaction. The composite material exhibited a high H2O2 production rate of 634 μM h–1, a solar-to-chemical conversion (SCC) efficiency of 0.16%, a high forward rate constant of ∼19 μM min–1, and a low H2O2 dissociation constant of ∼0.004 min–1, all of which are significantly higher than those of pure β-In2S3. Moreover, this material also manifested good water remediation photocatalytic performance by way of removal of 93% chromium(VI), 58% nickel(II), and 94% rhodamine B in 2 h. The apparent rate constant for chromium(VI) reduction of the composite was 0.023 min–1, which is 10-fold higher than that of the pure material. β-In2S3-CQDs exhibited superior results, which can be attributed to their large electrochemical surface area, good light harnessing ability, upconverted photoluminescence, and the ability to act as an electron reservoir preventing recombination.

The calculation of the decomposition products of C5F10O–CO2 mixtures with a chemical kinetic model

This paper presents a chemical kinetic model to investigate the physicochemical process in C5F10O-CO2 mixtures. The model contains 12 ions, including e, C+, O+, C2 +, O2 +, CO+, CO2 +, O−, C−, C2 −, O2 −, O3 − and 38 neutral species, containing C, O, C2, O2, CO, CO2, O3, C5F10O, C4F7Oa, C4F7Ob, C4F7Oc, C4F7Od, C3F5O, C3F4Oa, C3F4Ob, C3F4Oc, C3F4Od, C3F4Oe, C3F3Oa, C3F3Ob, COCF3, C2F2Oa, C2F2Ob, COFC, COF, C3F7a, C3F7b, C3F6a, C3F6b, C2F5, C2F4a, C2F4b, C2F3a, C2F3b, C2F2, CF3, CF2, CF. The total number of 180 reactions are considered. For the reactions whose forward and reverse rate constants can be obtained, the reactions are split into two one-way reactions. In addition, the reactions which produce photons are also considered as one-way reactions. The other reactions are reversable. The reverse rate constants can be obtained by the ratio of the forward rate constants and the equilibrium constants. The molar fractions of the decomposition products in C5F10O-CO2 mixtures with the C5F10O content to be 5%, 7% and 13% are calculated in the temperature range of 500–3500 K. The ratio of the electron temperature to the temperature of the heavy species is calculated to obtain the deviation from local thermodynamic equilibrium in different temperatures. The characteristic decomposition products in different temperature ranges are determined. In addition, the main reaction pathways for the main species are determined by analyzing the contributions of the corresponding reactions to the generation and consumption of the species. In order to validate the chemical kinetic model, the molar fractions of the species in pure CO2 are compared with those by Gibbs free energy minimization in a wide temperature range. This model can help better understand the physicochemical process from the aspect of the reactions during the over-heat fault, and the characteristic decomposition products in different temperature range can help identify the potential fault to avoid great power accidents.

Investigations of Vacancy-Assisted Selective Detection of NO2 Molecules in Vertically Aligned SnS2.

Two important methods for enhancing gas sensing performance are vacancy/defect and interlayer engineering. Tin sulfide (SnS2) has recently attracted much attention for sensing of the NO2 gas due to its active surface sites and tunable electronic structure. Herein, SnS2 has been synthesized by the chemical vapor deposition (CVD) method followed by nitrogen plasma treatment with different exposure times for fast detection of NO2 molecules. Plasma treatment created a substantial number of surface vacancies on SnS2 flakes, which were controlled by the exposure period to modify the surface of flakes. After 12 min of nitrogen plasma treatment, SnS2 nanoflakes show considerable improvement in NO2 sensing characteristics, including a high sensing response of ∼264% toward 100 ppm NO2 at 120°C. The enhancement in the relative response of the sensor is due to the electronic interaction between NO2 molecules and the S vacancies on the surface of SnS2. Density functional theory (DFT) computations indicate that the S-vacancy defects on the surface dominate the effective NO2 detection and the NO2 adsorption mechanism transition from physisorption to chemisorption. Adsorption kinetics of the NO2 gas over SnS2 nanoflake-based chemiresistor sensors were studied using the Lee and Strano model [ Langmuir 2005, 21(11), 5192-5196]. The irreversible rate of the reaction for various NO2 concentrations exposed to the gas sensor is extracted using this model, which also appropriately describes the response curves. The forward rate constant of the irreversible gas sensor increased with the increase of the N2 plasma treatment time and reached the maximum in the 12 min plasma-treated sample. Through defect engineering, this research may open up new vistas for the design and synthesis of 2D materials with enhanced sensing properties.

Shock-Tube Measurements of Atomic Nitrogen Collisional Excitation in 8000-12000 K Partially Ionized Nitrogen-Argon Mixtures.

We report on shock-tube experiments measuring the collisional excitation of atomic nitrogen using tunable diode laser absorption spectroscopy (TDLAS). Conditions behind the reflected shocks ranged from 8000 to 12000 K and 0.1 to 1.1 atm in mixtures of 1 or 2% molecular nitrogen (N2) in argon (Ar). Absorption from the transition between atomic nitrogen quantum states 4P to 4D at 868 nm was used to monitor the formation of electronically excited nitrogen. Population measurements of the 4P state were made at a rate of 50 kHz. In connection with these measurements, a multitemperature kinetic mechanism is proposed for nitrogen excitation. Measurements suggest a multistage process. In early test times, a period of induction due to N2 dissociation is followed by a rise via heavy particle excitation. The dominant channel causing this excitation is believed to be N + N ↔ N(4P) + N with a measured forward rate constant of 3.65 × 10-18 exp(-119892/T) [m3/s]. As test time evolves, the excitation of 4P, referred to as N*, is subsequently interrupted and then resumes, as the kinetic environment becomes increasingly electron-dominated. The most impactful reactions of the mechanism are optimized to reduce the residual between simulations and the measurements. The measured N* populations indicate strong, though indirect, sensitivity to adjacent processes, including the excitation of metastable nitrogen, and ionization channels.

Shrinking gate fluorescence correlation spectroscopy yields equilibrium constants and separates photophysics from structural dynamics

Fluorescence correlation spectroscopy is a versatile tool for studying fast conformational changes of biomolecules especially when combined with Förster resonance energy transfer (FRET). Despite the many methods available for identifying structural dynamics in FRET experiments, the determination of the forward and backward transition rate constants and thereby also the equilibrium constant is difficult when two intensity levels are involved. Here, we combine intensity correlation analysis with fluorescence lifetime information by including only a subset of photons in the autocorrelation analysis based on their arrival time with respect to the excitation pulse (microtime). By fitting the correlation amplitude as a function of microtime gate, the transition rate constants from two fluorescence-intensity level systems and the corresponding equilibrium constants are obtained. This shrinking-gate fluorescence correlation spectroscopy (sg-FCS) approach is demonstrated using simulations and with a DNA origami-based model system in experiments on immobilized and freely diffusing molecules. We further show that sg-FCS can distinguish photophysics from dynamic intensity changes even if a dark quencher, in this case graphene, is involved. Finally, we unravel the mechanism of a FRET-based membrane charge sensor indicating the broad potential of the method. With sg-FCS, we present an algorithm that does not require prior knowledge and is therefore easily implemented when an autocorrelation analysis is carried out on time-correlated single-photon data.

Larger Intrinsic Rate Constants of Alpha-amylase is Possible if Intrinsic Forward Rate Constant is ≠ Diffusion limited Rate of Encounter

Background: Previous research has shown that the intrinsic reverse (backward) and forward rate constants are larger than the effective or apparent rate constants for the formation and dissociation of an enzyme-substrate complex (ES). It is speculated that such intrinsic rate constants could be larger if an appropriate mathematical equation was adopted for their computation. &#x0D; Methods: Theoretical, experimental (Bernfeld method), and computational methods.&#x0D; Objectives: 1) To rederive the equations for calculating the intrinsic rate constants for forward (k1) and reverse (i.e., backward) (k2) reactions; 2) to calculate the intrinsic rate constants; and 3) to show that the probability (1/g) (or req(r)) that an enzyme is at a distance from the substrate is a variable. &#x0D; Results and Discussion: The equations for the determination of k2 and k1 were rederived. Unlike previous findings, the intrinsic (reverse) first order rate, k2 and forward second order rate, k1 were larger than their apparent counterparts, but they were, however, very similar in magnitude. The intrinsic rate constants were much larger than previously reported values when the enzyme (E) total concentration [ET] was much less than substrate’s total concentration [ST]. The k1 and apparent forward second order rate (kf) values where [ET] is much less than [ST] were &gt; where [ET] is less than [ST]. Therefore, the magnitude of the second order rate constant is a function of [ET]. The values of k1 and k2 where [ET] is much less than [ST] and vice-versa were respectively, 7.41 exp. (+6) L/mol. min and 81.34 exp. (+4) /min, and 15.76 exp. (+6) L/mol. min and 58.08 exp.(+4) /min. It was discovered that the probability (1/g) (or req(r))) that an enzyme is at a distance from the substrate with the possibility of mutual attraction is not constant. &#x0D; Conclusion: If the intrinsic forward rate constant (k1) is not equal to diffusion limited rate (kD) of encounter, the k1 and k2 values could be larger than values where k1 is equal to kD. The probability (1/g) (or req(r)) that an enzyme is at a distance from the substrate with the possibility of mutual attraction has been discovered to be a variable constant dependent on the concentration of the reaction mixture components and the enzyme's affinity for the substrate, and vice versa. Future research may attempt to derive an equation for the determination of an intrinsic catalytic rate constant for the formation of a product.

Rate constants are determinable outside the original Michaelis–Menten mathematical formalism wherein the substrate concentration range is  1.6  4.8 times enzyme concentration: A pre-steady-state scenario and beyond

For some time now, there has been growing interest in pre-steady-state (PSS) kinetic parameters for whatever reasons, the measurement of which needs high-tech equipment capable of transient time-scale duration of assay. The proposition, however, is that all kinetic parameters, PSS and beyond, can be determined with appropriate PSS derivable equations and the usual Michaelis-Menten (MM) and Briggs-Haldane (BH) equations, respectively. The objectives of the research were: 1) To derive equations, for the determination of reverse rate constant when the substrate concentration, [S] « MM constant, KM, 2) determine by calculation, the reverse rate constant, forward rate constant, and consequently, show that it is possible to determine rate constant often seen to be masked within original MM cum BH mathematical formalism, and 3) validate corollaries from the derivation that justify procedural issue. Theoretical, experimental (Bernfeld method), and computational methods were explored. Pre-steady-state equations for the determination of kinetic parameters, the reverse rate constant, k-1, for the process ES ® E + S, the 2nd order rate constant, k1, and the rate, v1, for the formation of enzyme-substrate complex, ES, were derived. The derived originating equations with associated corollaries were validated and have been seen to be capable of reproducing experimental variables and kinetic parameters; rate constants that seemed masked in MM formalism were unmasked. Steady-state (SS) cum zero order kinetic parameters were » their PSS values. “Negative” catalytic efficiency (k-1/KM) was » “positive” catalytic efficiency, (kcat/KM), with lower [ET]. In conclusion, the equations for PSS kinetic parameters were derivable. Previously masked kinetic parameters in the MM/BB mathematical formalism can now be calculated using MM data; thus, all kinetic parameters can be determined regardless of the reaction pathway's state, PSS, and SS. PSS kinetic parameters were « SS/zero order values.

Feasibility Analysis Upon Optimal Pollutant Degradation via Compartmental Modeling

Due to a lack of plausible kinetic modeling for contaminant attenuation, this study first proposed a “bi-exponential disposition” model to demonstrate the feasibility of wide-ranging applications for the biotic and abiotic degradation of pollutant(s). As a consequence of under-determined systems of bisphenol A (BPA) degradation via advanced oxidation processes (AOPs), a previous study proposed asymptotic approximation singular perturbation for kinetic modeling. The present study extended this model to provide the key performance indicator (KPI); namely, the area under the time course (AUC) of pollutant concentrations from time zero to the endpoint (i.e., AUC0→tf), quantitatively revealing the most promising strategy for pollutant (bio)degradation. Compared to the typical KPI (percentage of pollutant removal), AUC better illustrated the overall efficiency. Compartmental modeling predicted maximal pollutant mitigation through optimal schemes of operation for global optimization. The feasibility of adopting AUC for system prediction was confirmed in cases of azo and anthraquinone dye biodecolorization and acetaminophen (APAP), glyphosate, and bisphenol A (BPA) degradation. Regarding azo dye biodecolorization, the AUC and SDRmax indicated the need to consider cell concentrations. Compartment kinetics could be used for the serial acclimation of anthraquinone dye removal. Moreover, compartmental assessment upon glyphosate and acetaminophen abiotic degradation was also feasible for further applications. To minimize the AUC for optimal degradation of A⇌k3k1B→k2C, the maximal forward rate constants k1 and k2 and minimal backward rate constant k3 should be satisfied simultaneously. Thus, this AUC approach might be broadened to demonstrate overall optimization via Pontryagin’s maximum principle.

Catalytic Partial Desalination of Saline Water

More than 1 billion ha of land is adversely affected by salinization, including about 54 million ha of irrigated cropland. This study trials a batch flow, bubble column, static bed, catalytic, pressure swing adsorption−desorption, zero valent iron, and diffusion reactor train, which is designed to partially desalinate water, for use as either livestock feed water or partially desalinated irrigation water. ZVI desalination produces a partially desalinated water product, without producing a waste brine product. The trial demonstrates sequential batch processing of 50 (0.86 m3) batches of saline water (43 m3 total), using a single ZVI charge, without loss of activity. The trialed feed water contained between 1 and 9 g NaCl L−1. The average desalination was 41.84% (standard deviation was 15.61%). The optimum batch processing time is determined as being between 12 and 24 h. For each batch, the first-order forward rate constants, for both Cl− and Na+ ion removal, decline with time. The study considers the irrigation application of the product water on crop yield for 70 crop varieties; the application of the technology to desalinate a saline aquifer; applications of the technology to improve the environmental efficiency of conventional desalination plants.

Interfacial Reinitiation of Free Radicals Enables the Regeneration of Broken Polymeric Hydrogel Actuators

Interfacial Reinitiation of Free Radicals Enables the Regeneration of Broken Polymeric Hydrogel Actuators

F1-ATPase Rotary Mechanism: Interpreting Results of Diverse Experimental Modes With an Elastic Coupling Theory.

In this chapter, we review single-molecule observations of rotary motors, focusing on the general theme that their mechanical motion proceeds in substeps with each substep described by an angle-dependent rate constant. In the molecular machine F1-ATPase, the stepping rotation is described for individual steps by forward and back reaction rate constants, some of which depend strongly on the rotation angle. The rotation of a central shaft is typically monitored by an optical probe. We review our recent work on the theory for the angle-dependent rate constants built to treat a variety of single-molecule and ensemble experiments on the F1-ATPase, and relating the free energy of activation of a step to the standard free energy of reaction for that step. This theory, an elastic molecular transfer theory, provides a framework for a multistate model and includes the probe used in single-molecule imaging and magnetic manipulation experiments. Several examples of its application are the following: (a) treatment of the angle-dependent rate constants in stalling experiments, (b) use of the model to enhance the time resolution of the single-molecule imaging apparatus and to detect short-lived states with a microsecond lifetime, states hidden by the fluctuations of the imaging probe, (c) treatment of out-of-equilibrium “controlled rotation” experiments, (d) use of the model to predict, without adjustable parameters, the angle-dependent rate constants of nucleotide binding and release, using data from other experiments, and (e) insights obtained from correlation of kinetic and cryo-EM structural data. It is also noted that in the case where the release of ADP would be a bottleneck process, the binding of ATP to another site acts to accelerate the release by 5–6 orders of magnitude. The relation of the present set of studies to previous and current theoretical work in the field is described. An overall goal is to gain mechanistic insight into the biological function in relation to structure.

Mechanism and Kinetics of n-Butane Dehydrogenation to 1,3-Butadiene Catalyzed by Isolated Pt Sites Grafted onto ≡SiOZn–OH Nests in Dealuminated Zeolite Beta

An interest in the on-purpose production of 1,3-butadiene (1,3-BD) has grown, as a consequence of the decline in naphtha cracking for the production of ethene and propene, products that can now be produced economically by thermal dehydrogenation of ethane and propane contained in natural gas. In this study, the mechanism and kinetics of n-butane dehydrogenation to 1,3-BD are explored over atomically distributed Pt sites grafted onto dealuminated zeolite BEA (DeAlBEA) in the form of (≡Si–O–Zn)4–6Pt complexes. Reaction of n-butane dehydrogenation carried out at 823 K with 2.53 kPa n-butane/He and a weight-hourly space velocity (WHSV) of 14.5 h–1 produced 1,3-BD with a turnover frequency of 0.45 mol 1,3-BD (mol Pt)−1 s–1. Space-time studies and identification of the reaction intermediates suggest that n-butane first undergoes dehydrogenation primarily to 1-butene, which then rapidly isomerizes to produce an equilibrated mixture of 1-butene and 2-butene. 1-Butene then undergoes secondary dehydrogenation to produce 1,3-BD. We report, here, a detailed study of the kinetics of n-butane dehydrogenation to butenes and 1-butene dehydrogenation to 1,3-BD over isolated Pt sites. Both reactions exhibit a Langmuir–Hinshelwood dependence on n-butane and 1-butene partial pressures, respectively. Comparison of effective forward rate constants of n-butane dehydrogenation to butenes (k1f) and butene dehydrogenation to 1,3-BD (k2f) shows that the isolated Pt sites grafted onto DeAlBEA exhibit a very high activity for sequential dehydrogenation of n-butane to 1,3-BD relative to other Pt-based catalysts previously reported.

Quadrupolar Ultrafast Charge Transfer in Diaminoazobenzene-Bridged Perylenediimide Triads.

Strong push-pull interactions between electron donor, diaminoazobenzene (azo), and an electron acceptor, perylenediimide (PDI), entities in the newly synthesized A-D-A type triads (A=electron acceptor and D=electron donor) and the corresponding A-D dyads are shown to reveal wide-band absorption covering the entire visible spectrum. Electrochemical studies revealed the facile reduction of PDI and relatively easier oxidation of diaminoazobenzene in the dyads and triads. Charge transfer reversal using fluorescence-spectroelectrochemistry wherein the PDI fluorescence recovery upon one-electron oxidation, deterring the charge-transfer interactions, was possible to accomplish. The charge transfer state density difference and the frontier orbitals from the DFT calculations established the electron-deficient PDI to be an electron acceptor and diaminoazobenzene to be an electron donor resulting in energetically closely positioned PDIδ- -Azoδ+ -PDIδ- quadrupolar charge-transfer states in the case of triads and Azoδ+ -PDIδ- dipolar charge-transfer states in the case of dyads. Subsequent femtosecond transient absorption spectral studies unequivocally proved the occurrence of excited-state charge transfer in these dyads and triads in benzonitrile wherein the calculated forward charge transfer rate constants, kf , were limited to instrument response factor, meaning >1012 s-1 revealing the occurrence of ultrafast photo-events. The charge recombination rate constant, kr , was found to depend on the type of donor-acceptor conjugates, that is, it was possible to establish faster kr in the case of triads (∼1011 s-1 ) compared to dyads (∼1010 s-1 ). Modulating both ground and excited-state properties of PDI with the help of strong quadrupolar and dipolar charge transfer and witnessing ultrafast charge transfer events in the studied triads and dyads is borne out from the present study.

Molecular dynamics simulations and quantitative calculations on photo-responsive behavior of wormlike micelles constructed by gemini surfactant 12–3-12·2Br− and cinnamates with different ortho-substituents

Photo-responsive wormlike micelles constructed by photo-sensitive molecules and surfactants have attracted widespread attention from researchers. The subtle structural changes in photo-sensitive molecules have a significant effect on the photo-responsive behavior of wormlike micelles, while the underlying mechanism of this effect has not been fully investigated. In this work, photo-responsive wormlike micelles were constructed by Gemini surfactant trimethylene-1, 3-bis (dodecyldimethylammonium bromide) and three cinnamates with different ortho-substituents. The effects of ortho-substituents in cinnamates on the photo-responsive behavior of prepared micelles were explored in details by combining the experimental methods with multi-scale modelling. The experimental results have shown that the wormlike micelles containing different cinnamates exhibit different photo-responsive behaviors and the one containing trans-OMCA shows the best photo-responsiveness (viscosity decrease from 1.63 Pa·s to 0.10 Pa·s with morphology transition from wormlike micelles to spherical micelles) due to the highest conversion fraction (0.88) of the trans-OMCA and forward rate constants (1.09) speed under UV irradiation, which have been reconfirmed by quantitative calculation. Furthermore, the effect of UV irradiation time on the morphology of micelles were studied and the conformational transformation behaviors of wormlike micelles under different UV irradiation time were successfully simulated by adjusting the ratio of cis/trans-isomer. The quantitative calculation illustrates that the existence of methoxy and hydroxyl expands the difference in steric hindrance, hydrophilicity and the electrostatic distribution before and after UV irradiation, and increases the reaction equilibrium constant (methoxy is more effective), thereby enhancing photo-responsive ability of wormlike micelles. The present work provides a comprehensive understanding of photo-responsive morphology transition mechanism of wormlike micelles and the effects of structure of photo-sensitive molecule on the photo-responsiveness of wormlike micelles and thus can find a concrete way to optimize the photo-responsive self-assembly system containing surfactants and photo-sensitive molecule, which is of great importance for the design of smart responsive systems and expansion of their applications.