Ab initio study of carbon-chlorine bond cleavage in carbon tetrachloride

Effect of the degradation performance on carbon tetrachloride by anaerobic co-metabolism under different external energy sources

In this research, a comprehensive study was carried out on the removal of carbon tetrachloride (CT) in the anaerobic co-metabolism (ACM) reactor. The experiments showed that when the hydraulic retention time (HRT) was 36 h, pH was 7, and influent CT was 2.5mg/L, the average removal efficiency reached 82.45 ± 2.56% in the glucose co-metabolism substrate reactor, exhibiting a dramatic excellent difference in reaction performance from the other two reactors (p < 0.05) and a favorable tolerance on the CT shock loading. The content of extracellular polymeric substances (EPS) and volatile fatty acids (VFA) demonstrated that glucose could supply more energy to protect the microorganisms, which was the appropriate external energy source. Moreover, microbial community structure and biostatistics analysis demonstrated that Pseudomonas was the most important dechlorination bacteria in ACM reactors, which might via dehalogenation process mediate the transformation of CT. The succession of methanogenic bacteria further demonstrated that CT degradation using co-digestion require to destroy hydrogenotrophic methane generation pathway and the external energy substances could make up the lack of hydrogen in the treatment of CT. The change of intermediate products hinted that anaerobic dechlorination process of CT in an ACM reactor was a sequential dechlorination process, and major transformation products measured were CF. Overall, this study has improved our understanding of the roles of CT degradation process in ACM reactors.

Modeling of catalytically active metal complex species and intermediates in reactions of organic halides electroreduction

The results of quantum chemical modeling of organic and metal-containing intermediates that occur in electrocatalytic dehalogenation reactions of organic chlorides are presented. Modeling of processes that take place in successive steps of the electrochemical reduction of representative C1 and C2 chlorides - CHCl3 and Freon R113 (1,1,2-trifluoro-1,2,2-trichloroethane) - was carried out by density functional theory (DFT) and second-order Møller-Plesset perturbation theory (MP2). It was found that taking solvation into account using an implicit solvent model (conductor-like screening model, COSMO) or considering explicit solvent molecules gave similar results. In addition to modeling of simple non-catalytic dehalogenation, processes with a number of complexes and their reduced forms, some of which were catalytically active, were investigated by DFT. Complexes M(L1)2 (M = Fe, Co, Ni, Cu, Zn, L1H = Schiff base from 2-pyridinecarbaldehyde and the hydrazide of 4-pyridinecarboxylic acid), Ni(L2) (H2L2 is the Schiff base from salicylaldehyde and 1,2-ethylenediamine, known as salen) and Co(L3)2Cl2, representing a fragment of a redox-active coordination polymer [Co(L3)Cl2]n (L3 is the dithioamide of 1,3-benzenedicarboxylic acid), were considered. Gradual changes in electronic structure in a series of compounds M(L1)2 were observed, and correlations between [M(L1)2](0) spin-up and spin-down LUMO energies and the relative energies of the corresponding high-spin and low-spin reduced forms, as well as the shape of the orbitals, were proposed. These results can be helpful for determination of the nature of redox-processes in similar systems by DFT. No specific covalent interactions between [M(L1)2](-) and the R113 molecule (M = Fe, Co, Ni, Zn) were found, which indicates that M(L1)2 electrocatalysts act rather like electron transfer mediators via outer-shell electron transfer. A relaxed surface scan of the adducts {M(L1)2·R113}(-) (M = Ni or Co) versus the distance between the chlorine atom leaving during reduction and the corresponding carbon atom showed an energy barrier to electron transfer (the first stage of R113 catalytic reduction), while DFT optimization of the {Ni(L2)·R113}(-) adduct showed barrier-free decomposition. The difference between the stabilities of the {Ni(L1)2·R113}(-) and {Ni(L2)·R113}(-) adducts correlates with the difference between the catalytic activities of Ni(L1)2 and Ni(L2) in the electrochemical reduction of R113.

Degradation of chlorofluorocarbons using granular iron and bimetallic irons

Degradation of trichlorofluoromethane (CFC11) and 1,1,2-trichloro-1,2,2-trifluoroethane (CFC113) by granular iron and bimetallic (nickel- or palladium-enhanced) irons was studied in flow-through column tests. Both compounds were rapidly degraded, following pseudo-first-order kinetics with respect to the parent compounds. The average pseudo-first-order rate constants for CFC11 were similar among different materials, except for palladium-enhanced iron (PdFe), in which the rate of degradation was about two times faster than for the other materials. In the case of CFC113, the rate constants for bimetallic irons were about two to three times greater than for the regular iron material. The smaller than expected differences in degradation rate constants of chlorofluorocarbons (CFCs) between regular iron and bimetallic irons suggested little, if any, catalytic effect of the bimetallic materials in the initial degradation step. Subsequent degradation steps involved catalytic hydrogenation, however, playing a significant role in further degradation of reaction intermediates. The degradation intermediates and final products of CFC11 and CFC113 suggested that degradation proceeded through hydrogenolysis and α/β-elimination in the presence of regular iron (Fe) and nickel-enhanced iron (NiFe). Even though there is only minor benefit in the use of bimetallic iron in terms of degradation kinetics of the parent CFCs, enhanced degradation rates of intermediates such as chlorotriflouroethene (CTFE) in subsequent reaction steps could be beneficial.

CCl(4) dissociation on the ice I(h) surface: an excess electron mediated process

Dissociation of chlorofluorocarbons in the atmosphere is a heterogeneous process that takes place mainly on the surface of ice particles. Recently an enhancement of the dissociation rate due to excess electrons has been shown theoretically and correspondingly measured experimentally. Our density functional theory calculations show that CCl(4) dissociates due to an excess electron with an energy gain of 0.8 eV on the ice surface as opposed to in the gas phase. Through the use of ab initio molecular dynamics, an atomistic pathway for this dissociation has been elucidated, this pathway shows the capture of Cl(-) by the ice surface through a partial solvation mechanism, in agreement with recent experimental findings.

Density functional theory studies on the relative reactivity of chloroethenes on zerovalent iron

The gas-phase dissociation of perchloroethene (PCE), trichloroethene (TCE), and cis-dichloroethene (cis-DCE) on zerovalent iron Fe(110) was investigated using periodic density functional theory (DFT) with the generalized gradient approximation (GGA) and climbing image nudged elastic band method (CI-NEB). Activation energies and dechlorination rate constants for reductive beta-elimination of the chloroethene compounds were calculated using an Arrhenius equation with theoretically calculated vibrational frequencies for the compounds. Activation energies were found to decrease as the chlorination number increases. The reaction rate-limiting step for PCE dissociation occurs at the second chlorine cleavage, while the rate-limiting steps for TCE and cis-DCE occur at the first chlorine cleavage. The activation energies of PCE, TCE, and cis-DCE at their rate-limiting steps are 9.9, 16.6, and 23.8 kJ/mol, respectively. Energy profiles along the reaction coordinate for the dechlorination paths are presented. The relative gas-phase reactivity order among chlorothenes on Fe(110) was found to be PCE > TCE > cis-DCE. At room temperature (300 K), the PCE dechlorination rate is 14 and 338 times faster, respectively, than that of TCE and cis-DCE. Details regarding the electronic properties of the transition states of the dechlorinated compounds are reported.

Understanding reductive dechlorination of trichloroethene on boron-doped diamond film electrodes

This research investigated reduction of trichloroethylene (TCE) at boron-doped diamond (BDD) film cathodes using a rotating disk electrode reactor. Rates of TCE reduction were determined as functions of the electrode potential and TCE concentration over a temperature range between 2 and 32 degrees C. Reduction of TCE resulted in production of acetate and chloride ions with no detectable intermediate products. At a current density of 15 mA/cm2 and concentrations below 0.75 mM, reaction rates were first order with respect to TCE concentration, with surface area normalized rate constants 2 orders of magnitude greater than those for iron electrodes. Density functional theory (DFT) simulations were used to evaluate activation barriers for reduction by direct electron transfer, and for reaction with four functional groups commonly found on BDD surfaces. The DFT calculated activation barrier for direct electron transfer was more than 4 times greater than the experimentally measured value of 22 kJ/mol. In contrast, the DFT activation barrier for reaction at a deprotonated hydroxyl site on a tertiary carbon atom (triple bond C-O(-)) of 24 kJ/mol was in close agreement with the experimental value. Both experiments and quantum mechanical simulations support a TCE reduction mechanism that involves chemically adsorbed intermediates.

Understanding trichloroethylene chemisorption to iron surfaces using density functional theory

This research investigated the thermodynamic favorability and resulting structures for chemical adsorption of trichloroethylene (TCE) to metallic iron using periodic density functional theory (DFT). Three initial TCE positions having the plane defined by HCC atoms parallel to the iron surface resulted in formation of three different chemisorption complexes between carbon atoms in TCE and the iron surface. The Cl-bridge initial configuration with the HCC plane of TCE perpendicular to the iron surface did not result in C-Fe bond formation. The most energetically favorable complex formed at the C-bridge site where the initial configuration had the C=C bond in TCE at a bridge site between adjacent iron atoms. In the C-bridge complex, one C atom formed two a bonds to different Fe atoms, while the second C atom formed a sigma bond with a second Fe atom. Surface complexation atthe C-bridge site resulted in scission of all three C-Cl bonds and also resulted in a shortening of the C==C bond to a distance intermediate between a double and a triple bond. Initial configurations with the C==C bond adsorbed at top or hollow sites on the iron surface resulted in formation of C-Fe a bonds between a single C and two adjacent Fe atoms, and the scission of only two C==Cl bonds. Bond angles and bond lengths indicated that there were no changes in bond order of the C==C bond for top and hollow adsorption. Chemisorption at the C-bridge site had an activation energy of 49 kJ/mol and an early transition state where all three C-CI bonds were activated. The early transition state and the loss of all three Cl atoms upon chemisorption are consistent with most experimental observations that TCE undergoes complete dechlorination in one interaction with the iron surface. The absence of chemisorption and scission of only two C--Cl bonds at the Cl-bridge site is consistent with experimental observations that trace amounts of chloroacetylene may also be produced from reactions of TCE with iron.