Hydrocarbon Free Radicals Research Articles

Study on the Effect of Oxygen on Free Radical Generation in Coal

ABSTRACT According to the theory of coal–oxygen combination, oxygen has an important effect on the spontaneous combustion of coal. In this study, Hailar lignite is taken as the research object to prepare and analyze the coal sample. The variation trends of the free radical concentration, g factor value, line width and line shape are measured by electron spin resonance (ESR) technology, and the influence of different oxidation times and oxygen concentrations on the free radical parameters in coal are studied. The results show that the relationship between the free radical reaction and the oxidation time is not linear. The critical point of the free radical reaction is the oxidation time of 5 h. With an increase in the oxygen concentration, the free radical concentration, g factor value and line width also increase. In a nitrogen environment, the free radical reaction still exists, but the new free radicals are hydrocarbon free radicals or alkyne free radicals, which will not lead to spontaneous combustion of coal. Oxygen molecules, meanwhile, make the free radical reaction more active. Oxygen atoms combine with carbon chains to form oxygen-containing free radicals and peroxide free radicals, which further decompose and oxidize to produce CO and CO2, eventually triggering the spontaneous combustion of coal.

High temperature surface graphitization of CVD diamond films and analysis of the kinetics mechanism

A plasma arc generator was used to heat treat the free-standing CVD diamond films at 1450–1900 °C. An Arrhenius curve was fit according to trends in the measured Raman peak intensity ratio of diamond to graphite as a function of heat treatment temperature, and the curve was extrapolated linearly. It was found that the extrapolation lines intersected at the Debye temperature, ΘD = 2021 K. By comparing the activation energies of diamond graphitization in literature and from different measurement modes in our study, it was found that the graphitization mechanisms were different on both sides of Debye temperature due to differences in the excited lattice vibration modes. When the heat treatment temperature was just below the Debye temperature, the graphitization process was promoted by generating vacancies or the breaking of single CC bonds, and the apparent activation energy was 366 ± 60 kJ·mol−1. When the heat treatment temperature was above the Debye temperature, the graphitization process was promoted by the simultaneous breaking of multiple CC bonds, and the apparent activation energy was 887 ± 141 kJ·mol−1. In combination with the morphology changes on the surface of the CVD diamond film after high temperature heat treatment, the mechanism of the micro-cyclic phase transition on the surface of the diamond film at temperatures lower than Debye temperature was summarized as follows. First, CC bonds on the diamond surface were activated and broken at high temperatures, and some diamond carbon atoms dissociated from solid phase sites to the gas phase by forming hydrocarbon free radicals. Then, the carbon atoms from these hydrocarbons entered the graphite lattice step by step to complete the spontaneous deposition of sp2 clusters.

Reaction mechanisms involving peroxy radical in the low-temperature oxidation of coal

The mechanisms whereby peroxy radicals (ROO·) participate in the low temperature oxidation of coal have been probed. Through charge distribution analysis, structural parameters, and frontier orbital analysis, the existence of active sites has been proven. Model optimization, transition-state calculations, and IRC verification was carried out on common reaction processes involving ROO· in coal. Quantum chemical calculations were applied to evaluate and analyze thermodynamic parameters. The results of these calculations showed that chemical adsorption of O2 on aliphatic hydrocarbon free radicals produced ROO·. This chemical adsorption process was associated with a zero-level energy barrier at room temperature, and its enthalpy change was −132.64 kJ/mol. The abstraction of H by ROO· was divided into H transfer between adjacent groups and within a group. Our calculations showed that H transfer within a group had to overcome an energy barrier of 162.14 kJ/mol, the heat release was 135.23 kJ/mol, and the reaction rate constant was 2.34 × 10−14 s−1m−1. H transfer between adjacent groups overcame an energy barrier of 116.39 kJ/mol, the heat absorbed was 35.96 kJ/mol, and the reaction rate constant was 2.45 × 10−6 s−1m−1. A compound antioxidant enzyme inhibitor containing superoxide dismutase (SOD) was used to eliminate ROO·. SOD could undergo a disproportionation reaction to eliminate ROO· with an energy barrier of 31.3 kJ/mol. This implied that SOD should automatically eliminate ROO· at room temperature. Our research results helped to further understand the mechanism of ROO· participation in the reaction and provide theoretical support for the coal spontaneous combustion prediction.

Investigation of the interaction between vitrinite and inertinite of Xinjiang Wucaiwan coal in pyrolysis by ReaxFF molecular dynamics simulation

The interaction mechanism between vitrinite and inertinite of Xinjiang Wucaiwan coal during coal pyrolysis was investigated via ReaxFF molecular dynamics simulation. The results indicated that the interaction reactions between vitrinite and inertinite of Wucaiwan coal could be divided into three categories: (1) two active hydrogen atoms respectively heated out of the two macromolecular structures and interacted with each other, producing H2; (2) the aliphatic hydrocarbon free radicals, oxygen-containing radicals and other small free radical fragments consisting of cyclic structure that are generated by the cracking of vitrinite macromolecule and interacted with radicals belonging to the inertinite macromolecule and formed new interaction products; (3) residual fragments of the vitrinite and inertinite macromolecules combined with each other and formed a large molecular structure interaction product with more than 40 carbon atoms.

Experimental study on viscosity reduction of heavy oil by hydrogen donors using a cavitating jet.

Cavitating jet technology is used to change the quality and to reduce the viscosity of heavy oil because bubble collapse can cause extreme conditions such as high pressure and temperature. The experimental study on the viscosity reduction of heavy oil using a cavitating jet was performed through a self-designed experimental system for heavy oil viscosity reduction by a cavitating jet. Tetrahydronaphthalene was used as the hydrogen donor, and after adding different amount of tetrahydronaphthalene, the viscosity change of the heavy oil was studied before and after treatment by the cavitation jet. The experimental results show that cavitating jet technology can be used to improve the quality and reduce the viscosity of heavy oil, and the addition of tetrahydronaphthalene can effectively reduce the viscosity of heavy oil. With the increase in the amount of tetrahydronaphthalene added, the viscosity reduction effect on the heavy oil improved, and the viscosity reduction rate increased. When a certain amount of tetrahydronaphthalene was added, the viscosity of the heavy oil decreased gradually and plateaued with the increase in the cycles of cavitation jet treatment. The addition of tetrahydronaphthalene can dilute the heavy oil and effectively reduce the viscosity and improve the quality of heavy oil by providing active hydrogen to close the macromolecular hydrocarbon free radicals generated by the cavitation effect.

Quantum chemistry calculation of reaction pathways of carboxyl groups during coal self-heating

During coal self-heating, reactions of carboxyl groups feature in the evolution of the spontaneous combustion of coal. However, their elementary reaction pathways during this process still have not been revealed. This paper selected the Ar–CH2–COOH as a typical carboxyl group containing structure for the analysis of the reaction pathways and enhancement effect on the coal self-heating process by quantum chemistry calculations. The results indicate that the hydrogen atoms in carboxyl groups are the active sites, which undergo the oxidation process and self-reaction process during coal self-heating. They both have two elementary reactions, namely (i) the hydrogen abstraction of –COOH by oxygen and the decarboxylation of the –COO· free radical and (ii) the hydrogen abstraction of –COOH and its pyrolysis. The total enthalpy change and activation energy of the oxidation process are 76.93 kJ/mol and 127.85 kJ/mol, respectively, which indicate that this process is endothermic and will occur at medium temperatures. For the hydrogen abstraction of –COOH by hydrocarbon free radicals, the thermal parameters are 53.53 kJ/mol and 56.13 kJ/mol, respectively, which has the same thermodynamic properties as the oxidation process. However, for the pyrolysis, the thermal parameters are –42.53 kJ/mol and 493.68 kJ/mol, respectively, and is thus exothermic and would not occur until the coal reaches high temperatures. They affect heat accumulation greatly, generate carbon dioxide, and provide new active centers for enhancing the coal self-heating process. The results would be helpful for further understanding of the coal self-heating mechanism.

Thermal and spectroscopic analysis of stabilization effect of copper complexes in EPDM

The thermal stabilization effect of copper complexes on ethylene-propylene-diene terpolymer (EPDM) is studied during an accelerated degradation of polymer samples by their exposure to γ(137Cs)-radiation. These complexes of Cu(II) with two Schiff base ligands provided an efficient improvement in the stabilization of basic polymer. For the evaluation of improvement action of inorganic additive on thermal behavior of EPDM, the calculation of carbonyl and hydroxyl indexes and the radiochemical yields of corresponding radiolysis products prove the long-term stability of EPDM. Isothermal and nonisothermal chemiluminescence investigations were also accomplished, and the protection proficiencies of studied additives are compared by means of kinetic features of degradation. The activation energies required by the oxidation of polymer substrate were calculated, and their values are significant higher in comparison with the similar value found for pristine EPDM. The stabilized polymer is assisted by the interaction between mobile protons of ligands and hydrocarbon free radicals.

The reinvestigation of the kinetics of the metathesis reactions t-C4H9• + HBr (HI) → i-C4H10 + Br• (I•) and of the t-C4H9• free radical thermochemistry.

A reinvestigation of the absolute rate constant of the metathesis reactions t-C4H9• + HBr → i-C4H10 + Br• (1) and t-C4H9• + HI → i-C4H10 + I• (2) was performed thanks to a recently developed apparatus consisting of a Knudsen reactor coupled to detection based on single-photon (VUV) photoionization mass spectrometry (SPIMS). It enables the generation of thermalized hydrocarbon free radicals owing to a source upstream of and external to the Knudsen reactor. The following Arrhenius expressions were obtained: k1 = 5.6(±1.4) × 10(–12) exp(−6.76(±0.94)/(RT)) and k2 = 2.0(±0.6) × 10(–11) exp(−8.48(±0.94)/(RT)) with R = 8.314 J mol(–1) K(–1) over the range 293 to 623 K. The mass balance of the reaction system based on closed shell product detection (CSPD) was checked in order to ensure the accuracy of the used reaction mechanism and as an independent check of k1 and k2. The wall-loss rate constants of the t-butyl free radical, kw(C4H9), were measured and found to be low compared with the corresponding escape rate constant, ke(C4H9), for effusion of t-C4H9• out of the Knudsen reactor. On the basis of the present results, the free radical standard heat of formation ΔfH298°(t-C4H9•) = 44.3 ± 1.7 kJ mol(–1) was obtained when combined with the kinetics of the inverse halogenation reaction taken from the literature and using S298°(t-C4H9•) = 322.2 J K(–1) mol(–1) following a “Third Law” evaluation method. The standard enthalpy for t-butyl free radical is consistent for both the bromination and iodination reactions within the stated uncertainties.

Reinvestigation of the Elementary Chemical Kinetics of the Reaction C2H5• + HBr (HI) → C2H6 + Br• (I•) in the Range 293–623 K and Its Implication on the Thermochemical Parameters of C2H5• Free Radical

A reinvestigation of the absolute rate constants of the metathesis reactions C2H5• + HBr → C2H6 + Br• (R1) and C2H5• + HI → C2H6 + I• (R2) has been performed and led to the following Arrhenius expressions: k1 = 3.69(±0.95) × 10–11 exp(−10.62(±0.66)/RT), k2 = 1.20(±0.38) × 10–11 exp(−7.12(±1.059)/RT) in the temperature range 293–623 K (A/cm3 molecule–1 s–1, Ea/kJ mol–1). The study has been performed using a Knudsen reactor coupled to single-photon (VUV) photoionization mass spectrometer (SPIMS). Hydrocarbon free radicals have been generated externally before admission into the Knudsen reactor according to two different chemical schemes, enabling the generation of thermalized C2H5• free radicals. A minor correction to k1 and k2 for the wall loss of C2H5• (kw) has been applied throughout the temperature range. The obtained results are consistent regarding both the disappearance of C2H5• and the formation of closed shell products (n-C4H10, C2H4, C2H6), indicating that the chemical mechanism is largely understood and complete. Thermochemical parameters for C2H5• free radical resulting from the present kinetic measurements are discussed and point toward a slightly lower value for the standard heat of formation ΔfH298°(C2H5•) compared to some presently recommended values. On the basis of the present results and suitable data on the reverse reaction taken from the literature, we recommend ΔfH298°(C2H5•) = 117.3 ± 3.1 kJ/mol resulting from an average of “third law” evaluations using S298°(C2H5•) = 242.9 ± 4.6 J/K mol. The present work yields a standard heat of formation in satisfactory agreement with the results obtained by W. Tsang (ΔfH298°(C2H5•) = 119 ± 2 kJ/mol) despite using two very different experimental techniques.

Effect of aromatic amine antioxidants on NOx emissions from a soybean biodiesel powered DI diesel engine

It is an overwhelming argument that the use of biodiesel instead of petrodiesel causes a reduction in harmful exhaust emissions from engines. A number of studies, however, indicate substantial increases in engine out NOx emissions with biodiesel fuel. Some studies have pointed out that the increased formation of prompt NOx is responsible for biodiesel NOx effect. Treatment of biodiesel with antioxidants is a promising approach because it reduces the formation of hydrocarbon free radicals, which are responsible for prompt NOx production in combustion process. Aromatic amine antioxidants are known as to be efficient inhibitors of free radicals. This study examines the use of p-phenylenediamine derived aromatic amine antioxidants for NOx reduction in a soybean biodiesel fuelled DI diesel engine. The antioxidant additives, N,N′-diphenyl-1,4-phenylenediamine (DPPD) and N-phenyl-1,4-phenylenediamine (NPPD) were tested on a computerised Kirloskar-make 4 stroke water cooled single cylinder diesel engine of 4.4kW rated power. Results show that significant reduction of NOx could be achieved by the addition of antioxidants but smoke, CO and HC emissions were found to have increased.

Triboelectricity: Macroscopic Charge Patterns Formed by Self-Arraying Ions on Polymer Surfaces

Tribocharged polymers display macroscopically patterned positive and negative domains, verifying the fractal geometry of electrostatic mosaics previously detected by electric probe microscopy. Excess charge on contacting polyethylene (PE) and polytetrafluoroethylene (PTFE) follows the triboelectric series but with one caveat: net charge is the arithmetic sum of patterned positive and negative charges, as opposed to the usual assumption of uniform but opposite signal charging on each surface. Extraction with n-hexane preferentially removes positive charges from PTFE, while 1,1-difluoroethane and ethanol largely remove both positive and negative charges. Using suitable analytical techniques (electron energy-loss spectral imaging, infrared microspectrophotometry and carbonization/colorimetry) and theoretical calculations, the positive species were identified as hydrocarbocations and the negative species were identified as fluorocarbanions. A comprehensive model is presented for PTFE tribocharging with PE: mechanochemical chain homolytic rupture is followed by electron transfer from hydrocarbon free radicals to the more electronegative fluorocarbon radicals. Polymer ions self-assemble according to Flory-Huggins theory, thus forming the experimentally observed macroscopic patterns. These results show that tribocharging can only be understood by considering the complex chemical events triggered by mechanical action, coupled to well-established physicochemical concepts. Patterned polymers can be cut and mounted to make macroscopic electrets and multipoles.

Free radicals produced from thermally-irradiated polyethylene polymers: An ion attachment mass spectrometric study

Many gas-phase hydrocarbon free radical species produced during thermal irradiation of polyethylene polymers have been studied in detail by means of Li+ ion attachment mass spectrometry (Li+IAMS). Spectra of the thermal decomposition products, detected by means of the Li+ ion attachment technique, clearly show that radicals, such as CnH2n+1 (n=5–12) and CnH2n−1 (n=5–11), were the predominant species produced. Many of these species have been identified for the first time by mass spectrometry. Our results present direct evidence that free radicals are formed in the pyrolysis environment and are detectable with mass spectrometric techniques.

Simulation of the initiated addition of hydrocarbon free radicals and hydrogen atoms to oxygen via a nonbranched chain mechanism

A reaction scheme is suggested for the nonbranched-chain free-radical oxidation of hydrocarbons. The scheme includes the formation of a low-reactivity radical R (e.g., o-CH3C6H4CH2O 4 · ) not participating in further chain propagation. This reaction can effectively compete with reactions of chain propagation via the reactive hydrocarbon radical R· and, as the oxygen concentration in the reaction mixture is increased, begins to inhibit the chain process. The kinetic equation derived from the reaction scheme using the quasi-steady-state treatment provide a description for the nonmonotonic (peaking) dependence of the rate of the chain formation of molecular oxidation products on the oxygen concentration. The energetics of key radical-molecular reactions is considered, and the reaction scheme suggested is applied to the nonbranched-chain hydrogen oxidation involving the low-reactivity radical HO 4 ·

Simulation of the initiated addition of hydrocarbon free radicals and hydrogen atoms to oxygen via a nonbranched chain mechanism

Simulation of the initiated addition of hydrocarbon free radicals and hydrogen atoms to oxygen via a nonbranched chain mechanism

The investigation of hydrocarbon cracking reaction energetics with composite energy methods

Hydrocarbo cracking reactions are one of the most commonly encountered reactions in the petroleum industry, and the energetics of the reactions are crucial in understanding the reaction mechanisms and predicting reaction rates. In this work, a modified composite energy method (CBS-RAD(MP2)) is created as a version of the CBS-RAD method which gives accurate energetics for hydrocarbon free radical reactions. It replaces the time consuming QCISD(fc)/6-31g* method in the geometry optimization and frequency calculation steps with MP2(full)/6-31g* level calculations. The accuracy of the new CBS-RAD(MP2) method is compared with the widely used G2, G3 and CBS-QB3 composite methods for predicting heats of reaction and activation barriers of 14 hydrocarbon cracking reactions. We find that the new CBS-RAD(MP2) method has the second least RMS error of 1.22 kcal/mol for heats of reaction calculations. For activation energy calculations, the new CBS-RAD(MP2) method has the least RMS error of 1.37 kcal/mol. Moreover, the CBS-RAD(MP2) method was found to require only 81% of the computational time required compared to the CBS-QB3 method, 32% of G3 and 15% of the G2 method, making it an attractive alternative for predicting hydrocarbon cracking reaction energetics.

Estimating Heats of Formation of Hydrocarbon Radicals by a Combination of Semiempirical Calculation and Family Correlation with Experimental Values

A semiempirical method, the PM3-family-correlation (PM3-FC) method, has been developed to estimate the heats of formation ({Delta}H{sup {degree}}{sub f}) of hydrocarbon free radicals. The PM3-FC method combines semiempirical PM3 calculations and family correlation with experimental data. The {Delta}H{degree}{sub f} values for a test set of 67 hydrocarbon free radicals, including alkyl, cycloalkyl, alken-{alpha}-ly, cycloalken-{alpha}-yl, arylalk-{alpha}-ly, alkyn-{alpha}-yl, and vinyl radicals, were calculated by using the Pm3 method and then correlated systematically with the experimental values from the literature. According to the structural analogy of the radicals and the observed correlation, the 67 radicals in the test set can be separated into five groups. The MP3-calculated and experimental values of the radicals in each group exhibit a very good linear correlation. Using the obtained regression parameters to scale the PM3-calculated {Delta}H{degree}{sub f} values leads to a very significant increase in estimation accuracy. For the test set of radicals, excluding the three cyclic hydrocarbon radicals with three carbon atoms in the ring and the three methyl radicals coordinated with three unsaturated groups, the average absolute deviation and the standard deviation between the PM3-FC estimated and experimental enthalpy values are 0.8 and 1.1 kcal/mol, respectively. The 95% confidence limit of the deviation between themore » PM3-FC-estimated and experimental values is in the interval of {+-}2.1 kcal/mol. The error analysis of the PM3-FC method was performed on the basis of statistical analysis by comparison with other estimation methods. Hydrocarbon free radicals play an important role in many thermal chemistry processes, including combustion, petroleum and coal coking, coal liquefaction and pyrolysis, oil shale retorting, thermal stability of fuels, and free radical polymerization.« less

Radiation processing of ethylene-propylene rubber

The effects of ionizing radiation on ethylene-propylene copolymer were evaluated over the range of total g doses up to 500 kGy. The influence of the irradiation dose was investigated by oxygen uptake and thermal analysis. Four testing tempera- tures (170, 180, 190, and 200°C) and two heating rates between 2.9 and 5.9 K/min were selected for oxygen uptake measurements and thermal analysis, respectively. The competition between crosslinking and scission was examined on the basis of kinetic parameters of postirradiation oxidation. The influence of the momentary concentration of hydrocarbon free radicals is discussed in regard to the contribution of the antago- nistic processes of crosslinking and oxidative degradation. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 298 -303, 2000

Kinetics of the reactions of unsaturated hydrocarbon free radicals (vinyl, propargyl, and allyl) with molecular bromine

The kinetics of the reactions of three unsaturated free radicals (vinyl, propargyl, and allyl) with molecular bromine have been studied by using a tubular reactor coupled to a photoionization mass spectrometer. The radicals were homogeneously generated by the pulsed photolysis of precursor molecules at 193 nm. The subsequent decays of the radical concentrations were monitored in time-resolved experiments as a function of Br 2 concentration to obtain the rate constants of these Br-atom metathesis reactions. Rate constants were measured as a function of temperature to obtain Arrhenius parameters

Theoretical study of lithium cation interactions with hydrocarbon radicals

The nature of the interaction between lithium cations and hydrocarbon free radicals has been examined by electronic structure calculations at the HF/6–31 G ** and MP2/6–31 G ** levels. Bonding in the radical cation systems CH 3Li +, C 2H 5Li +, 2-C 3H 7Li +, C 2H 3Li +, and C 3H 5Li + closely resembles that between lithium cations and closed-shell molecules.

Kinetics of the reactions of unsaturated free radicals (methylvinyl and i-C4H3) with molecular oxygen

The kinetics and mechanisms of the reactions of two unsaturated hydrocarbon free radicals, methylvinyl (CH3CH=CH) and i-C4H3 (CH2=C=C=CH) with molecular oxygen were investigated using a tubular reactor coupled to a photoionization mass spectrometer. Rate constants were measured as a function of temperature and density in time-resolved experiments. Searches were conducted for possible products to identify reactive routes. The methylvinyl+O2 reaction was studied betweeen 296 and 600 K. The rate constant is essentially constant throughout this range, 7.2(±1.4)×10−12 cm3 molecule−1 s−1. The only reactive route observed is that which yields CH3CHO and HCO, a route analogous to that of the C2H3+O2 reaction. The i-C4H3+O2 reaction rate constant was measured from 296 to 900 K and was found to decrease sharply with increasing temperature, from 4.5(±0.9)×10−12 cm3 molecule−1 s−1 at 296 to 6.4(±1.3)×10−13)×10−13 cm3 molecule−1 s−1 at 900 K corresponding to a T−1.8 power dependence in this temperature range. The reaction rate constants of both reactions studied did not depend on density. No products of the i-C4H3+O2 reaction could bedetected. Possible mechanisms of this reaction are discussed. The role of elementary reactions between unsaturated free radicals and molecular oxygen in combustion processes is briefly reviewed.