Formation Of Main Products Research Articles

Oxidation decomposition mechanism of hexamethyldisiloxane

Siloxane working fluid has good environmental performance and thermodynamic performance, widely used in high temperature heat source ORC system. During the filling process of working fluid and operation in ORC system operation, air may leak into the system. The reaction of siloxane working fluid with oxygen in high temperature environment will produce abrasive silica, which will cause serious damage to the system. The oxidative decomposition mechanism of hexamethyldisiloxane (MM) was analyzed by density functional theory (DFT) and ReaxFF molecular dynamic simulation. It is found that the initial reactions of MM oxidation decomposition can be divided into two categories. The first type is the thermal decomposition of MM itself. The second one is the collision reaction between MM molecule and O2 molecule. Comparing the two kinds of chemical reaction barriers, MM is more likely to react with O2. Pathways 2–3 involving the collision between O2 molecule and Si atom are the optimal pathway of initial reaction kinetics, and its reaction energy barrier is 204.1 kJ/mol. It was observed from ReaxFF MD simulations that the main products of MM oxidative decomposition were CH4, CH2O, CO, CO2, SiO and amorphous silica. In the process of oxidative decomposition, small free radicals such as CH3, O, H, OH played a very important role in the oxidative decomposition of working fluids, which greatly promoted the decomposition of working fluids and the formation of main products.

Activation effect of ozone and DME on the partial oxidation of natural gas surrogates and validation of pressure-dependent ozone decomposition

To address current challenges related to climate change, novel concepts, such as engine-based polygeneration under fuel-rich conditions, have recently been developed. In this context, understanding and validation of the associated chemical kinetics are a key factor for prediction and optimizations. In this work, the combined effect of ozone and DME as additives on the partial oxidation of natural gas mixtures is investigated in a plug-flow reactor at a pressure of 4 bar and at temperatures ranging from 373 to 973 K. Double-imaging photoelectron photoion coincidence (i2PEPICO) spectroscopy measurements are presented to study the conversion of the reactants and the formation of main products, and to identify important intermediates responsible for the low temperature chemistry phenomena and ignition characteristics of the natural gas/dimethyl ether/ozone mixture. Among the activation effect of ozone, the temperature-variant ozone dissociation at elevated pressures up to 20 bar is studied for the first time, revealing the start of ozone decomposition at 423 K in the investigated pressure range. In addition, the ozone decomposition is shown to be slightly pressure-dependent in the pressure range between 1 and 20 bar with lower ozone conversions at higher pressures. The experimental results are compared with model predictions using literature reaction mechanisms for validation and further analysis. The ozone decomposition initiates fuel conversion and the formation of oxygenates and hydroperoxides at very low temperatures, i.e., 423 K, resulting in an overall three-stage oxidation process at ∼450 K (1. stage), ∼550 K (2. stage), and ∼750 K (3. stage). Also, the overall fuel conversion is enhanced in the intermediate temperature range (between 550 K and 750 K) by up to 40%-points. The hydroperoxides, among other species, are clearly identified by mass-selected threshold photoelectron spectra from the literature. Excellent agreement between experiments and simulations is found, while deviations are observed for some oxygenates at very low temperatures showing the need for further model improvements.

Ab Initio-Based Global Potential Energy Surface and Reaction Dynamics for H2S + C

Dynamics of the reaction of hydrogen sulfide, H2S(X1A1), with ground state atomic carbon, C(3Pj), was investigated over the interpolated ab initio based potential energy surface using the quasi-classical trajectory (QCT) simulation. The reaction probability and total reactive cross section were calculated at wide range of collision energies from 2.6 kJ mol-1 to 78.8 kJ mol-1. The total rate constant of the reaction was calculated using collision theory. The energy distribution for the formation of main products (HCS/HSC + H) were also investigated. At 44.6 kJ mol-1 collision energy approximately 39.5% and 12% of total available energy release to translational and rotational energy levels of the HCS + H products, respectively, while for HSC + H these values were found to be about 61.6% and 25.7% of total available energy. The rest of total energy deposited in the vibrational modes of the products.

Self-sustained Oscillations in Oxidation of Propane Over Nickel: Experimental Study and Mathematical Modelling

The regular reaction rate oscillations were observed by mass-spectrometry in the oxidation of propane over the Ni foil. CO, CO2, H2, and H2O were detected as products. The oscillations of products and reactants in the gas phase were accompanied by the oscillations of the catalyst temperature. It was shown by using isotopic labeling 18O2 that CO and CO2 formed simultaneously by the propane oxidation when the catalyst was in the highly active state. The non-isothermal model of a continuous stirred-tank reactor based on a microkinetic scheme including 50 reaction steps was developed. The model takes into account periodic oxidation and reduction of nickel and formation of main products. Using the method of quasi-steady-state approximations, it was shown that the observed self-sustained oscillations are associated with periodic changes in the coverage of the Ni surface by adsorbed oxygen and carbon as well as nickel oxide species. It was shown that the microkinetic scheme coupled with a continuous stirred-tank reactor model qualitatively describes the experimental data if the model also considers the periodic processes of oxidation and reduction of the Ni bulk.

A ReaxFF-based molecular dynamics study of the oxidation decomposition mechanism of HFO-1336mzz(Z)

The oxidation decomposition mechanism of HFO-1336mzz(Z) were studied by ReaxFF simulations. The initial oxidation dissociation reactions of HFO-1336mzz(Z) can be divided into two types: thermal decomposition of HFO-1336mzz(Z) and the collision of O2 with HFO-1336mzz(Z). Most of the latter reactions are more likely to occur than the former ones. By comparing the energy barriers of initiation pathways, the pathway 2 and pathway 3 which involve the collision of CC bonds with O2 molecules are main reaction pathways and the energy barriers of these reactions are 153.3 and 135.3 kJ mol−1, respectively. HF, COF2 and CO2 are main products and the formation pathways of these products are observed from ReaxFF MD simulations. The main small radicals CF3, F, O, COF and COF3 play an important role in the oxidation decomposition of HFO-1336mzz(Z) because they can promote the decomposition of molecules and radicals and the formation of main products.

Thermal decomposition mechanisms of poly(vinyl chloride): A computational study

The studies on the pyrolysis mechanisms of waste PVC contribute to development and application of pyrolysis technology for mixed waste plastics. In the article, the thermal decomposition mechanisms of model compound of poly(vinyl chloride) (PVC) have been investigated by employing density functional theory methods at M06-2X/6–31++G(d,p) level in order to illuminate the elimination of HCl and the formation of hydrocarbons. Various possible pyrolysis paths for the formation of main products were proposed, and the thermodynamic and kinetic parameters in every path were calculated. The calculation results show that the HCl elimination can occur through the concerted reaction and the energy barrier of HCl elimination changes from 167.4 to 243.3 kJ/mol; allyl group can obviously reduce the activation energy of HCl elimination, and the branched-chain can lower the energy barrier of HCl elimination at the carbon sites near the branch chain; a free radical is more easily converted into aromatic compound through a series of isomerizations, cyclization and dehydrogenation; the conjugated polyene could be decomposed in parallel reaction channels: one is the evolution of aromatics, another is the formation of small molecule products. The above analysis is consistent with previous experimental results and analysis.

A Theoretical Study on the Dynamics of O(3P) + H2S(1A1) Reaction on an Interpolated Potential Energy Surface

Quasi-classical trajectory calculations at the DFT level and CVT calculations at the CCSD(T) level are performed to study the dynamics of O(3P) + H2S(1A1) reaction on the lowest triplet potential energy surface. In the dynamics part the potential energy surface has been constructed by interpolation technique following the method introduced by Collins and his coworkers. Total and individual classical reactive cross sections are calculated at collision energies from 13.1 to 126.0 kJ mol−1. The rate constants from QCT calculation are compared with those calculated from canonical variational transition state theory at the G3(MP2)B3 and CCSD(T)/Aug-cc-pVTZ levels. The energy partitioning in reactive collisions for the formation of main products (OH + SH and H + HSO) and in non-reactive collisions for the reactants is investigated. At 52.5 kJ mol−1 initial collision energy about 42% and 49% of the total available energy goes into the translational energy and internal motions of H + HSO products, respectively, w...

Aqueous Phase Conversion of Hexoses into 5-Hydroxymethylfurfural and Levulinic Acid in the Presence of Hydrochloric Acid: Mechanism and Kinetics

The kinetics of the acid-catalyzed dehydration of two common monosaccharides (glucose and fructose) to 5-hydroxymethyl-furfural (HMF) and levulinic acid is studied in this work. Reaction studies were performed in a stirred batch reactor using aqueous sugar solutions (150 g/L). The influence of reaction temperature (363–403 K) and catalyst concentration (0.05–0.25 mol/L) was also studied. The formation of the main products, as well as other minor products such as humins (in both cases) or glucose dimers and anhydroglucose (in glucose processing), is modeled considering a serial-parallel scheme reaction. The proposed model allows predicting the formation of both main and side products. Obtained results suggest that it is possible to work at total conversions of both monosaccharides, and to tune the selectivity to HMF-LA by changing the operation temperature and/or catalyst concentration. As a general trend, higher selectivities are obtained (to HMF or to LA, depending on reaction conditions) when fructose i...

Thermal cracking of 1-n-butyldecalin at high pressure (100 bar)

The pyrolysis of 1-n-butyldecalin at high pressure (100bar) was studied in gold sealed tube reactors at 400°C. It leads to 4 main chemical families of products in order of predominance: (a) normal alkanes (methane, ethane, propane and n-butane), (b) bicyclics (decalin and 1-methyldecalin), (c) hydroaromatics (tetralin and 5-methyltetralin) and (d) diaramatics (naphthalene and 1-methylnaphthalene). This distribution of products suggests that breaking of side alkyl chain and aromatization reactions should prevail at high pressure and low temperature. Reaction pathways are effectively proposed to account for the formation of main products. Both normal alkanes (except CH4 and C2H6) and bicyclics should mainly come from CC bond breaking reactions on the side alkyl chain of 1-n-butyldecalin. Methane and ethane should come from further decomposition of fuel radicals, including ring opening through beta-scissions given the absence of any C12 and C13 compounds in similar quantities. Both hydroaromatics and diaromatics are secondary products, which result from aromatization reactions of decalin and 1-methyldecalin. In our conditions, 1-n-butyldecalin and n-butylcyclohexane have a similar thermal reactivity. A higher yield in n-butane was observed in the case of 1-n-butyldecalin. In comparison with the pyrolysis of decalin at low pressure (≤1atm) and high temperature (>500°C), the production of naphthalene and the absence of any BTX (Benzene, Toluene and Xylene) in detectable quantities is here worth mentioning.

Theoretical study of the pyrolysis of vanillin as a model of secondary lignin pyrolysis

The unimolecular and bimolecular decomposition reactions in processes of vanillin pyrolysis were theoretically investigated by employing density functional theory (DFT) method at M06-2X/6-31 G+(d,p) level. The result shows that the homolytic cleavage of O–CH3 bond could be the dominant initial step in the pyrolysis of vanillin. The hydrogen abstractions from functional groups of vanillin by the formed radicals play important roles in the formation of main products. Both formyl, hydroxyl and methoxyl group contribute to the formation of CO. Benzene is formed from the hydrogen addition reaction between hydrogen radical and phenol at high temperature.

Theoretic studies on decomposition mechanism of o-methoxy phenethyl phenyl ether: Primary and secondary reactions

In order to understand the pyrolysis mechanism of lignin and identify the chemical pathways for the formation of main products during pyrolysis, the primary and secondary reactions of o-methoxy phenethyl phenyl (o-CH3O-PPE) pyrolysis are investigated by DFT method at (M06-2X)/6-31G+ (d,p) level. It is found that the o-methoxy group reduces the bond dissociation energies (BDEs) of CβO and C(aromatic)O bonds of the β-O-4 linkage. Kinetic analysis results support the concerted reaction pathway 12 as the major primary reaction channel which leads to the formation of guaiacol and styrene. Salicylaldehyde is arisen from the secondary pyrolysis of guaiacol via bimolecular reaction at higher temperature.

Fusion Growth of Gold Nanoparticles Induced by the Conformational Change of a Thermoresponsive Polymer Studied by Distance Distribution Functions

The growth kinetics of gold nanoparticles induced by the conformational collapse resulting from the coil–globule transition of thermoresponsive polymer was studied using small-angle X-ray scattering (SAXS) method. The growth process of aggregates and nanoparticles formed during the reaction was determined on the basis of distance distribution functions calculated from the SAXS profiles without having to approximate or simplify the actual structures. The size of aggregates and nanoparticles reached a maximum not in the final stage of the reaction but in the middle stage, indicating that shrinkage of the particles occurred in the growth process. The surface roughness increased up to the middle stage of the reaction and then significantly decreased in the final stage. Furthermore, the dependence of polymer concentration was also investigated. It was found that small particles were generated prior to the formation of main products, and size of the small particles was estimated to be ca. 7 nm in diameter. The ...

ChemInform Abstract: Synthesis of a Wide Range of Thioesters by Indium Triiodide Catalyzed Direct Coupling Between Alkyl Acetates and Thiosilanes.

AbstractThe method is highly selective for the acetate moiety as is shown by formation of main product (XVII).

Ni-based sol–gel catalysts as promising systems for crude bio-oil upgrading: Guaiacol hydrodeoxygenation study

Catalytic hydrotreatment or hydrodeoxygenation (HDO) has been researched extensively with the crude bio-oil and its model compounds over conventional sulfided Ni(Mo), Co(Mo) catalysts and supported noble metal catalysts. These types of catalysts showed themselves unsuitable for the target HDO process, which resulted in an urgent need to search for a new catalytic system meeting such requirements as low cost, stability against coke formation and leaching of active components due to adverse effect of the acidic medium (bio-oil). In the present work a series of Ni-based catalysts with different stabilizing components has been tested in the hydrodeoxygenation (HDO) of guaiacol (2-methoxyphenol), bio-oil model compound. The process has been carried out in an autoclave at 320°C and 17MPa H2. The main products were cyclohexane, 1-methylcyclohexane-1,2-diol, and cyclohexanone. The reaction scheme of guaiacol conversion explaining the formation of main products has been suggested. The catalyst activity was found to rise with an increase in the active component loading and depend on the catalyst preparation method. The most active catalysts in HDO of guaiacol were Ni-based catalysts prepared by a sol–gel method and stabilized with SiO2 and ZrO2. According to TPR, XRD, XPS, and HRTEM, the high activity of these catalysts correlates with the high nickel loading and the high specific area of active component provided by the formation of nickel oxide–silicate species. The effect of temperature on the product distribution and catalyst activity in the target process (HDO) has been investigated as well. The catalysts were shown to be very promising systems for the production of hydrocarbon fuels by the catalytic upgrading of bio-oil.

Theoretical study on the gas-phase reaction mechanism between palladium monoxide and methane

The gas-phase reaction mechanism between palladium monoxide and methane has been theoretically investigated on the singlet and triplet state potential energy surfaces (PESs) at the CCSD(T)/AVTZ//B3LYP/6-311+G(2d, 2p), SDD level. The major reaction channel leads to the products PdCH(2) + H(2)O, whereas the minor channel results in the products Pd + CH(3)OH, CH(2)OPd + H(2), and PdOH + CH(3). The minimum energy reaction pathway for the formation of main products (PdCH(2) + H(2)O), involving one spin inversion, prefers to start at the triplet state PES and afterward proceed along the singlet state PES, where both CH(3)PdOH and CH(3)Pd(O)H are the critical intermediates. Furthermore, the rate-determining step is RS-CH(3) PdOH → RS-2-TS1cb → RS-CH(2)Pd(H)OH with the rate constant of k = 1.48 × 10(12) exp(-93,930/RT). For the first C-H bond cleavage, both the activation strain ΔE(≠)(strain) and the stabilizing interaction ΔE(≠)(int) affect the activation energy ΔE(≠), with ΔE(≠)(int) in favor of the direct oxidative insertion. On the other hand, in the PdCH(2) + H(2) O reaction, the main products are Pd + CH(3)OH, and CH(3)PdOH is the energetically preferred intermediate. In the CH(2)OPd + H(2) reaction, the main products are Pd + CH(3)OH with the energetically preferred intermediate H(2)PdOCH(2). In the Pd + CH(3)OH reaction, the main products are CH(2)OPd + H(2), and H(2)PdOCH(2) is the energetically predominant intermediate. The intermediates, PdCH(2), H(2) PdCO, and t-HPdCHO are energetically preferred in the PdC + H(2), PdCO + H(2), and H(2)Pd + CO reactions, respectively. Besides, PdO toward methane activation exhibits higher reaction efficiency than the atom Pd and its first-row congener NiO.

Theoretical considerations of secondary organic aerosol formation from H-abstraction of p-xylene

Xylenes are important constituents of many liquid fuels, as well as precursors of secondary organic aerosols (SOAs). To examine the mechanisms for formation of SOAs in the atmosphere, the abstraction reaction of p-xylene with OH and the secondary degradation channels of its intermediates were first and extensively investigated with density functional theory at the B3LYP/6-31+G (d, p) level. The result indicates that H-abstraction from methyl groups is a barrier-less path while that from phenyl groups require a free energy barrier of approximately 2.8 kcal mol −1. Upon formation of p-xylyl, further addition by O 2 readily occurs to form peroxy radical. Subsequently, possible degradation channels for the formation of main products ( p-tolualdehyde and p-quinone methide) have been determined in presence of NO. The free energy profile constructed shows that the entire reaction process is exothermic. In addition, the dipole moment of p-tolualdehyde is higher than that of p-xylene, consistent with their relative hygroscopic values. This indicates that the degradation products of p-xylene can readily immerse into the SOA phase, while p-xylene may be subject to further atmospheric degradation to form non-volatile compounds.

Anaerobic biohydrogen production from wheat stalk by mixed microflora: Kinetic model and particle size influence

This study investigated the influence of particle size on anaerobic biohydrogen production from wheat stalk by mixed microflora. In addition, the kinetic model for the formation of main products was also mentioned. The results demonstrated that all the cumulative productions of hydrogen, acetate and butyrate decreased as the particle size increasing from 1 to 10mm at a constant TS value of 2%, 5% and 8%, respectively. However, this difference for aqueous products was not very obvious compared with hydrogen. A modified Gompertz equation was able to adequately describe the cumulative production of hydrogen, acetate and butyrate (R2 higher than 0.989). The results also indicated that the formation of the main products were all associated with the degradation of cellulose and hemicellulose (R2 higher than 0.855).

Changing the substrate specificity of P450cam towards diphenylmethane by semi-rational enzyme engineering

A focused library comprising nine residues of the active site of P450cam monooxygenase resulting in ∼ 300,000 protein variants was screened for activity on diphenylmethane (DPM). The assay was based on the depletion of NADH by an in vitro reconstituted P450cam system in a 96-well scale. The throughput was increased by the parallel cultivation, purification and analysis of 20 variants per well (cluster screening). Thus ∼ 20,000 protein variants could be screened in summary of which five were found to transform DPM with a specific activity of up to 75% of the wild-type activity on d-camphor and a coupling rate of 7-18%. One variant converting DPM to 4-hydroxydiphenylmethane (4HDPM) was subjected to site-directed mutagenesis and saturation mutagenesis, which revealed the particular importance of positions F87, Y96 and L244 for substrate selectivity and the possibility for further improvements of this variant. Moreover, a reduction in size of the amino acid at position 396 decreased specific activity dramatically but increased coupling and switched the main product formation from 4HDPM towards diphenylmethanol.

Computational study on the reactions of H2O2 on TiO2 anatase (101) and rutile (110) surfaces

This study investigates the adsorption and reactions of H(2)O(2) on TiO(2) anatase (101) and rutile (110) surfaces by first-principles calculations based on the density functional theory in conjunction with the projected augmented wave approach, using PW91, PBE, and revPBE functionals. Adsorption mechanisms of H(2)O(2) and its fragments on both surfaces are analyzed. It is found that H(2)O(2) , H(2)O, and HO preferentially adsorb at the Ti(5c) site, meanwhile HOO, O, and H preferentially adsorb at the (O(2c))(Ti(5c)), (Ti(5c))(2), and O(2c) sites, respectively. Potential energy profiles of the adsorption processes on both surfaces have been constructed using the nudged elastic band method. The two restructured surfaces, the 1/3 ML oxygen covered TiO(2) and the hydroxylated TiO(2), are produced with the H(2)O(2) dehydration and deoxidation, respectively. The formation of main products, H(2)O(g) and the 1/3 ML oxygen covered TiO(2) surface, is exothermic by 2.8 and 5.0 kcal/mol, requiring energy barriers of 0.8 and 1.1 kcal/mol on the rutile (110) and anatase (101) surface, respectively. The rate constants for the H(2)O(2) dehydration processes have been predicted to be 6.65 × 10(-27) T(4.38) exp(-0.14 kcal mol(-1)/RT) and 3.18 × 10(-23) T(5.60) exp(-2.92 kcal mol(-1)/RT) respectively, in units of cm(3) molecule(-1) s(-1).

Skeletal isomerization of 1-butene: A thorough kinetic study over ZSM-22

Zeolites and mesoporous materials are important catalysts not only for oil-refining processes, but nowadays even for the production of fine and specialty chemicals. Few thorough studies on the important factors in chemical reaction engineering: kinetics, deactivation, mass and heat transfer and flow modeling have been carried out. In the present work, a comprehensive analysis of 1-butene skeletal isomerization over ZSM-22 was conducted. Kinetics of the main product formation, i.e. isobutene, as well as side-products, i.e. propene and pentene, was modeled using Langmuir–Hinshelwood approach. The axial-dispersion model was used for describing the reactor. The behavior of the main products and byproducts was successfully explained. Uniform coking was applied to describe the deactivation behavior of the reaction and the formation of byproducts.