OH Attack Research Articles

Enhanced adsorption and photocatalytic removal of PFOA from water by F-functionalized MOF with in-situ-growth TiO2: Regulation of electron density and bandgap

PFOA has caused enormous environmental risks with recalcitrance, toxicity, and bioaccumulation, while the degradation of PFOA is still a challenging topic related to the high energy of CF bonds. However, the conventional methods such as biological oxidation, electrochemical oxidation and membrane separation have their own drawbacks. Actually, adsorption-photocatalysis is considered a potential treatment due to effectiveness and thoroughness. Instead of preparing the composite material by adding TiO2, the method to in-situ grow TiO2 in the MOF (MIL-125(Ti)) was carried out, which achieved narrowed band gap and improved the photoresponse ability, simultaneously. Furthermore, the surface F-functionalized MOF provided more available special adsorption sites for PFOA enrichment. Linked fluorine groups not only improved the adsorption capacity (185.151 μmol/g) but also enhanced the photocatalytic rate (1.221 E−4/s) by altering the electronic density of VB and CB. The constructed F-TiO2@MIL-125 stepwise degraded PFOA through a dominant ·OH attack pathway and maintained the oxidize ability of h+ to PFOA. As a bifunctional material for adsorption and photocatalysis, F-TiO2@MIL-125 was able to be applied in various water environments repeatedly, which has unlimited environmental application potential.

Free radical chemistry of atenolol and propranolol investigated by pulse and gamma radiolysis

Using pulse- and γ-radiolysis the free radical chemistry of atenolol and propranolol was investigated in aqueous solutions evaluating also the consequences of radical reactions on the elimination of organic content and toxicity. In aerated solutions, the hydroxyl radicals (•OH) induce the degradation; at low dissolved O2 concentration, the hydrated electrons (eaq−) may also contribute to the reactions. Propranolol showed higher reactivity towards both •OH and eaq− than atenolol. About 80–90% of •OH attack occurred on the aromatic rings (benzene or naphthalene), the reactions on the oxypropanolamine side chains had lower rate. The reactions with the aromatic rings gave hydroxycyclohexadienyl radicals, by radical titration experiments α-aminoalkyl and aminium radicals were shown forming by •OH attack on the side chain. Only the end-products of propranolol showed toxicity at low doses which was attributed to the formation of naphthalene derivatives, e.g., substituted naphthols.

ROS-mediated photoaging pathways of nano- and micro-plastic particles under UV irradiation

Reactive oxygen species (ROS) generation is considered as an important photoaging mechanism of microplastics (MPs) and nanoplastics (NPs). To elucidate the ROS-induced MP/NP aging processes in water under UV365 irradiation, we examined the effects of surface coatings, polymer types and grain sizes on ROS generation and photoaging intermediates. Bare polystyrene (PS) NPs generated hydroxyl radicals (•OH) and singlet oxygen (1O2), while coated PS NPs (carboxyl-modified PS (PS-COOH), amino-modified PS (PS-NH2)) and PS MPs generated fewer ROS due to coating scavenging or size effects. Polypropylene, polyethylene, polyvinyl chloride, polyethylene terephthalate and polycarbonate MPs only generated •OH. For aromatic polymers, •OH addition preferentially occurred at benzene rings to form monohydroxy polymers. Excess •OH resulted in H abstraction, CC scission and phenyl ring opening to generate aliphatic ketones, esters, aldehydes, and aromatic ketones. For coated PS NPs, •OH preferentially attacked the surface coatings to result in decarboxylation and deamination reactions. For aliphatic polymers, •OH attack resulted in the formation of carbonyl groups from peracid, aldehyde or ketone via H abstraction and CC scission. Moreover, 1O2 might participate in phenyl ring opening for PS NPs and coating degradation for coated PS NPs. This study facilitates understanding the ROS-induced weathering process of NPs/MPs in water under UV irradiation.

Molecular Mechanism of the Mononuclear Copper Complex-Catalyzed Water Oxidation from Cluster-Continuum Model Calculations.

Cluster-continuum model calculations were conducted to decipher the mechanism of water oxidation catalyzed by a mononuclear copper complex. Among various O-O bond formation mechanisms investigated in this study, the most favorable pathway involved the nucleophilic attack of OH- onto the .+ L-CuII -OH- intermediate. During such process, the initial binding of OH- to the proximity of .+ L-CuII -OH- would result in the spontaneous oxidation of OH- , leading to OH⋅ radical and CuII -OH- species. The further O-O coupling between OH⋅ radical and CuII -OH- was associated with a barrier of 14.8 kcal mol-1 , leading to the formation of H2 O2 intermediate. Notably, the formation of "CuIII -O.- " species, a widely proposed active species for O-O bond formation, was found to be thermodynamically unfavorable and could be bypassed during the catalytic reactions. On the basis the present calculations, a catalytic cycle of the mononuclear copper complex-catalyzed water oxidation was proposed.

Electrocatalytic degradation of 2,4-dichlorophenol by a 3DG-PbO2 powdered anode: Experimental and theoretical insights

• 3DG and powdered PbO 2 composite anode was fabricated for electrochemical oxidation. • Experimental and theoretical methods were combined to study OH attack to 2,4-DCP. • DFT calculations were performed for HO -addition on benzene of 2,4-DCP. • Degradation pathway of 2,4-DCP was elucidated on the molecular level. A novel nano powdered PbO 2 (NP-PbO 2 ) electrode was fabricated in our previous study as a highly efficient anode material for electrocatalytic oxidation, but it is still challenged by its high charge transfer resistance. Herein, three-dimensional graphene (3DG) was composited with NP-PbO 2 to solve this problem. Due to the multidimensional electrical transmission channels provided by 3DG, the charge transfer resistance of NP-PbO 2 was greatly reduced from 3.08 × 10 5 to 7.74 × 10 3 Ω/cm 2 , thereby significantly boosting the hydroxyl radical generation capacity of NP-PbO 2 electrode. In electrocatalytic oxidation process of 2,4-dichlorophenol (2,4-DCP), the optimal 3DG-PbO 2 composite anode exhibited excellent eletrocatalytic activity. After 60 min of electrolysis, the mineralization current efficiency at 3DG-PbO 2 electrode was 1.68 and 3.38 times higher than those at NP-PbO 2 and electrodeposited PbO 2 (ED-PbO 2 ) electrodes, respectively. The influence of several important operation parameters on the 2,4-DCP removal efficiency was examined. The results show that the 2,4-DCP removal reached 97.67% after 80 min of electrolysis under the conditions of current density 4 mA/cm 2 , initial 2,4-DCP concentration 50 mg/L, pH 3 and Na 2 SO 4 concentration 0.05 M. In addition, the degradation mechanism of 2,4-DCP was explored via theoretical computation (frontier molecular orbitals, molecular electrostatic potential, total charges, Fukui functions and transient states) and experimental identification of intermediates. Accordingly, we proposed the mineralization pathway of 2,4-DCP at the 3DG-PbO 2 anode.

Oxidation reactions of carbaryl in aqueous solutions

Hydroxyl radical induced oxidation of carbaryl has been studied using steady state photolysis followed by high-resolution mass spectrometry (LC-Q-TOF-MS), pulse radiolysis, and theoretical (DFT) calculations. The reaction of ●OH with carbaryl resulted in a number of hydroxylated adduct radicals (λmax - 330 nm and 390 nm; k2 - 1.2 × 1010 dm3 mol-1 s-1). The DFT calculations and results obtained from LC-Q-TOF-MS analysis shows the possible addition of ●OH at C1 (energetically most stable) and C7 positions of carbaryl leading to the generation of resonance stabilized hydroxycyclohexadienyl-type radicals as the immediate intermediates, which eventually converted into naphthol and a hydroxylated naphthols. LC-Q-TOF-MS results also revealed the formation of other hydroxylated derivatives and naphthoquinones that are most likely originated from the consecutive ●OH attack on the initially formed products. Naphthoquinones are found to undergo ring opening and the corresponding products are identified. The reaction of SO4●- with carbaryl, on the other hand, results the radical cation of parent molecule (λmax - 320 nm and 390 nm) which exhibits reasonable stability in the pulse radiolysis timescale. Total organic carbon (TOC) analysis after H2O2/UV photolysis revealed that nearly 70% of the organic content is mineralized after 35 min of irradiation, which demonstrates the potential application of oxidative methods towards the degradation of carbaryl.

Electrocatalytic degradation of pesticide micropollutants in water by high energy pulse magnetron sputtered Pt/Ti anode

The increasing occurrence of pesticide micropollutants highlights the need for innovative water treatment technologies, particularly for small-community and household applications. Electro-oxidation is being widely studied in this area, unfortunately, safe, stable and efficient electrocatalytic anodes without released heavy metal ions are still highly required. In this study, we fabricated a Pt/Ti anode by high energy pulse magnetron sputtering (HiPIMS-PtTi) which was used to decompose dichlorvos (DDVP) and azoxystrobin (AZX) in water. The results show that the reaction rate constant (kENR) on HIPIMS was 35.7 min–1 (DDVP) and 41.3 min–1 (AZX), respectively, superior to electroplating Pt/Ti anode (EP-PtTi). The identification of radicals (•OH, 1O2, •O2−) and micro-area analyses evidenced that Pt atoms were embedded into the TiO2 lattice on the surface of Ti plate by high-energy ions, which resulted in more adsorbed hydroxyls, and higher production of •OH under polarization conditions. Besides, the electro-oxidation intermediates of DDVP and AZX were identified and the degradation pathways were speculated: (1) indirect oxidation dominated by •OH attack, and (2) direct electron transfer reaction of pesticides on the anode surface. The cooperated reactions achieve the complete degradation and highly efficient mineralization of DDVP and AZX.

Understanding the factors governing the water oxidation reaction pathway of mononuclear and binuclear cobalt phthalocyanine catalysts.

The rational design of efficient catalysts for electrochemical water oxidation highly depends on the understanding of reaction pathways, which still remains a challenge. Herein, mononuclear and binuclear cobalt phthalocyanine (mono-CoPc and bi-CoPc) with a well-defined molecular structure are selected as model electrocatalysts to study the water oxidation mechanism. We found that bi-CoPc on a carbon support (bi-CoPc/carbon) shows an overpotential of 357 mV at 10 mA cm−2, much lower than that of mono-CoPc/carbon (>450 mV). Kinetic analysis reveals that the rate-determining step (RDS) of the oxygen evolution reaction (OER) over both electrocatalysts is a nucleophilic attack process involving a hydroxy anion (OH−). However, the substrate nucleophilically attacked by OH− for bi-CoPc is the phthalocyanine cation-radical species (CoII–Pc–Pc˙+–CoII–OH) that is formed from the oxidation of the phthalocyanine ring, while cobalt oxidized species (Pc–CoIII–OH) is involved in mono-CoPc as evidenced by the operando UV-vis spectroelectrochemistry technique. DFT calculations show that the reaction barrier for the nucleophilic attack of OH− on CoII–Pc–Pc˙+–CoII–OH is 1.67 eV, lower than that of mono-CoPc with Pc–CoIII–OH nucleophilically attacked by OH− (1.78 eV). The good agreement between the experimental and theoretical results suggests that bi-CoPc can effectively stabilize the accumulated oxidative charges in the phthalocyanine ring, and is thus bestowed with a higher OER performance.

Chemically stable piperidinium cations for anion exchange membranes.

The chemical stability of the anion exchange membranes (AEMs) is determinative towards the engineering applications of anion exchange membrane fuel cells (AEMFCs) and other AEM-based electrochemical devices, yet remains a challenge due to deficiencies in the structural design of cations. In this work, an effective design strategy for ultra-stable piperidinium cations is presented based on the systematic investigation of the chemical stability of piperidinium in harsh alkaline media. Firstly, benzyl-substituted piperidinium was degraded by about 23% in a 7 M KOH solution at 100 °C after 1436 h, which was much more stable than pyrrolidinium due to its lower ring strain. The introduction of substituent effects at the α-C position was proved to be an effective strategy for enhancing the chemical stability of the piperidinium functional group. As a result, the butyl-substituted piperidinium cation showed no obvious structural changes after being treated in the 7 M KOH solution at 100 °C for 1050 h. Afterwards, GC-MS and NMR analysis indicated that the α-C atoms in the substituents of piperidinium are fragile to the nucleophilic attack of OH−. Based on the above results, the electronic and steric effects of different alkyl substitutions were analyzed. This work provides critical insights into the structural design of chemically stable piperidinium functional groups for the AEM and boosts its application in electrochemical devices, such as fuel cells and alkaline water electrolysis.

Influence of polystyrene microplastics on the volatilization, photodegradation and photoinduced toxicity of anthracene and pyrene in freshwater and artificial seawater

In this study, the influences of polystyrene microplastics (PS MPs) on the volatilization, photodegradation and photoinduced toxicities of anthracene and pyrene were determined in freshwater and artificial seawater. The PS MPs reduced the volatilization of anthracene and pyrene, and the volatilization reduction was highly dependent on the PS MPs sizes and concentrations. The PS MPs increased the photodegradation kinetics (kp) of anthracene by promoting 1O2 generation and altered the photodegradation pathways through OH attack of the photodegradation byproducts. However, the kp of pyrene was decreased by PS MPs suppressing the transfer of electrons from excited pyrene to oxygen. The PS MPs modified the pathways of pyrene photodegradation via OH attack of the photodegradation byproducts. Due to light shielding by DOM and/or PS MPs aggregates in seawater, the modification of the photodegradation pathways of anthracene and pyrene by PS MPs was hardly happened in seawater compared with in freshwater. By changing the concentrations of anthracene or pyrene and their photodegradation byproducts, the PS MPs greatly affected the photoinduced toxicities of anthracene and pyrene to Selenastrum capricornutum and Phaeodactylum tricornutum. The influences of PS MPs on the volatilization, photodegradation and photoinduced toxicity of anthracene and pyrene are important and should be carefully considered during environmental risk assessments of anthracene and pyrene.

Kinetic analysis on premixed oxy-fuel combustion of coal pyrolysis gas at ultra-rich conditions: Selective combustion and super-adiabatic flame temperatures

Oxy-fuel combustion of coal pyrolysis gas has recently been proposed to serve as internal heat source of a vertical low-temperature pyrolysis furnace, in order to make the output pyrolysis gas nearly free of nitrogen and widely useful. To keep the pyrolysis temperature and the heat carrier gas volume unchanged from air combustion to oxy-fuel combustion, the equivalence ratio has to be increased up to 8. To explore the flame temperature and species variation at this ultra-rich condition, freely propagating premixed oxy-fuel flames of a typical coal pyrolysis gas at equivalence ratios of 0.5–10 are numerically studied with detailed chemistry. It is found that super-adiabatic flame temperatures (SAFT) occur at equivalence ratios larger than 3 for the considered pyrolysis gas and the SAFT magnitude is 294 K at equivalence ratio of 8. Due to the high H2 mole fraction (46%) in the pyrolysis gas, preferential diffusion plays a negligible role in the SAFT feature. Global net production of CO and H2 by the rich combustion only occurs at moderate equivalence ratio ranges, which are 1.5–8 and 3–5.5 respectively for the two species. At equivalence ratio of 8, the three fuel components are all net consumed following the mole ratio of CH4:CO:H2 = 1:0.07:0.84. Kinetic analysis reveals three factors responsible for the reaction mechanism change with the increase in equivalence ratio. Firstly, the lack of H-radical and the decrease in temperature result in the disappearance of the H2 production peak in the initial stage. Secondly, HO2 attack to CO prevails and hence contribution of CO oxidation in the initial stage increases. Thirdly, the long lasting OH attack to CO and H2 leads to the weakened CO and H2 production rate in the final stage.

Ultraviolet photolysis of monochloro-p-benzoquinone (MCBQ) in aqueous solution: Theoretical investigation into the dechlorination

In this work, the UV-induced transformation of monochloro-p-benzoquinone (MCBQ) in aqueous solution has been systematically investigated through quantum chemical calculations. During the UV irradiation at 253.7 nm, the first triplet state of MCBQ (3MCBQ*) was from the intersystem crossing of its first excited singlet state (1MCBQ*). In aqueous solution, the nucleophilic attack of OH− on carbon atoms in 3MCBQ* was the central reaction. The addition of OH− to olefinic carbon atoms was much more kinetically feasible than that to carbonyl carbon atoms, even though the carbonyl carbon atoms were more positively charged. Moreover, OH− preferred to add to the ortho-position of C–Cl bond, where the unchlorinated atom was more negatively charged than the chlorinated one. The UV photolysis of the primary intermediate (HO-CBQ) was not the same as that of MCBQ. The attack of OH− on the para-position of C–Cl bond was the most efficient pathway. The addition of OH− to the chlorinated atom of 3HO-CBQ* was much more efficient than that in the case of 3MCBQ*, which reveals that more UV irradiation may promote the dechlorination. The findings in the present study may be helpful to enrich the understanding of the halobenzoquinones transformation in aqueous solution.

Strong hydrophobic affinity and enhanced •OH generation boost energy-efficient electrochemical destruction of perfluorooctanoic acid on robust ceramic/PbO2-PTFE anode

The progress on perfluorooctanoic acid (PFOA) removal from wastewater is of great importance. Electrochemical advanced oxidation processes (EAOPs) are effective, but always trigger concerns involving energy consumption and stability of electrode materials. In this work, a tubular polytetrafluoroethylene (PTFE) doped PbO2 film anode supported over ceramic (ceramic/PbO2-PTFE) was fabricated and used for energy-efficient destruction of ppm-level PFOA. Given initial PFOA concentrations of 20 mg L−1, this anode outcompetes conventional Ti/SnO2-Sb/PbO2 anode with 15-fold in apparent rate constant (kobs), 0.36-fold in electric energy per order of magnitude of PFOA removed (EEO), and 640-fold in released F− by energy consumption per unit ([F−] / E). Experimental results under different potentials and density functional theory (DFT) calculations confirm that PFOA degradation is initiated by direct electron transfer (DET) and enabled by •OH attack. Ceramic/PbO2-PTFE features stronger hydrophobic affinity with PFOA, and more •OH generation than Ti/SnO2-Sb/PbO2, which boosts its good performance for PFOA degradation/defluorination. This anode exhibits 2.7-fold in service life than conventional Ti/SnO2-Sb/PbO2, and high tolerance for free fluorides, which expands its application for concentrated PFOA in waste stream.

Photodegradation, toxicity and density functional theory study of pharmaceutical metoclopramide and its photoproducts

Pharmaceuticals as ubiquitous organic pollutants in the aquatic environment represent substances whose knowledge of environmental fate is still limited. One such compound is metoclopramide, whose direct and indirect photolysis and toxicological assessment have been studied for the first time in this study. Experiments were performed under solar radiation, showing metoclopramide as a compound that can easily degrade in different water matrices. The effect of pH-values showed the faster degradation at pH = 7, while the highly alkaline conditions at pH = 11 slowed photolysis. The highest value of quantum yield of metoclopramide photodegradation (ϕ = 43.55·10−4) was obtained at pH = 7. Various organic and inorganic substances (NO3−, Fe(III), HA, Cl−, Br−, HCO3−, SO42−), commonly present in natural water, inhibited the degradation by absorbing light. In all experiments, kinetics followed pseudo-first-order reaction with r2 greater than 0.98. The structures of the photolytic degradation products were tentatively identified, and degradation photoproducts were proposed. The hydroxylation of the aromatic ring and the amino group's dealkylation were two major photoproduct formation mechanisms. Calculated thermochemical quantities are in agreement with the experimentally observed stability of different photoproducts. Reactive sites in metoclopramide were studied with conceptual density functional theory and regions most susceptible to •OH attack were characterized. Metoclopramide and its degradation products were neither genotoxic for bacteria Salmonella typhimurium in the SOS/umuC assay nor acutely toxic for bacteria Vibrio fischeri.

Humidity induces the formation of radicals and enhances photodegradation of chlorinated-PAHs on Fe(III)-montmorillonite

Chlorinated-PAHs (ClPAHs) are widely detected in the soil surface and atmospheric particles. However, the underlying mechanisms of their photodegradation are not well understood. In the present study, the formation of radicals on ClPAHs-contaminated clay minerals was quantitatively monitored via electron paramagnetic resonance (EPR) spectroscopy, and the impact of relative humidity (RH) was systematically explored. ClPAHs removal (> 75%) was attributed to electron transfer and •OH attack. The degradation easiness of ClPAHs follows: 2-ClNAP >2-ClANT >9-ClPHE >1-ClPYR. Light irradiation significantly improved the generation of reactive oxygen species (ROS, such as •OH and •O2−), and further generate a series of hydroxylated products of ClPAHs. Persistent free radicals (PFRs) were only detected on clay minerals contaminated with 2-ClANT and 1-ClPYR. RH 10–80%, the concentration of •OH and •O2− increased by 1.07 and 62.79 times respectively, which facilitated transformation of PFRs and ClPAHs degradation. The results of quantum chemical calculations indicate that the initial reaction of ClPAHs photodegradation is mediated by the substitution of •OH for chlorine groups. The present work implies that higher humidity may decrease the generation of PFRs on clay minerals and help mitigate the threats of PFRs and ClPAHs to human health.

Radical-Driven Decomposition of Graphitic Carbon Nitride Nanosheets: Light Exposure Matters.

Understanding the transformation of graphitic carbon nitride (g-C3N4) is essential to assess nanomaterial robustness and environmental risks. Using an integrated experimental and simulation approach, our work has demonstrated that the photoinduced hole (h+) on g-C3N4 nanosheets significantly enhances nanomaterial decomposition under •OH attack. Two g-C3N4 nanosheet samples D and M2 were synthesized, among which M2 had more pores, defects, and edges, and they were subjected to treatments with •OH alone and both •OH and h+. Both D and M2 were oxidized and released nitrate and soluble organic fragments, and M2 was more susceptible to oxidation. Particularly, h+ increased the nitrate release rate by 3.37-6.33 times even though the steady-state concentration of •OH was similar. Molecular simulations highlighted that •OH only attacked a limited number of edge-site heptazines on g-C3N4 nanosheets and resulted in peripheral etching and slow degradation, whereas h+ decreased the activation energy barrier of C-N bond breaking between heptazines, shifted the degradation pathway to bulk fragmentation, and thus led to much faster degradation. This discovery not only sheds light on the unique environmental transformation of emerging photoreactive nanomaterials but also provides guidelines for designing robust nanomaterials for engineering applications.

Selective photocatalytic conversion of guaiacol using g-C3N4 metal free nanosheets photocatalyst to add-value products

Valorization of lignin into high valuable chemical is a critical challenge. Its availability is a key factor for the development of viable lignocellulosic processes to replace fossil derived compounds. In this work, new insights on the high photocatalytic conversion of guaiacol (82%) as a lignin model compound was achieved, also, high selectivity to p-benzoquinone (59%), catechol (27%), and pyrogallol (6%) was obtained using metal-free pyrolyzed g-C3N4 under visible light irradiation. To highlight the new insights, experimental parameters were modified to control the reaction mechanism to increase selectivity and photo-conversion. g-C3N4 photocatalyst was synthesized through urea calcination at 550 °C and the photocatalytic performance was assessed in terms of pyrolysis time, where higher time resulted in better photocatalytic activity. This effect was attributed to smaller structures and therefore better quantum confinement of the charges. The oxidation was promoted by OH radicals, which were detected through EPR operando mode and the addition of radical scavengers. A reaction pathway was proposed, in which the ·OH attacks guaiacol through a methoxy group. The photocatalytic reaction can be tuned using external oxidant agents such as O2 and/or H2O2 to promote certain radical formation, enhancing conversion rates and promoting selectivity for a specific product, where yield shifting from p-benzoquinone to pyrogallol was experimentally observed.

Unravelling the effects of complexation of transition metal ions on the hydroxylation of catechol over the whole pH region

Catechol pollutants (CATPs) serving as chelating agents could coordinate with many metal ions to form various CATPs-metal complexes. Little information is available on the effects of complexation of metal ions on CATPs degradation. This work presents a systematical study of •OH-mediated degradation of catechol and catechol-metal complexes over the whole pH range in advanced oxidation processes (AOPs). Results show that the pH-dependent complexation of metal ions (Zn2+, Cu2+, Ti4+ and Fe3+) promotes the deprotonation of catechol under neutral and even acidic conditions. The radical adduct formation (RAF) reactions are both thermodynamically and kinetically favorable for all dissociation and complexation species, and OH/O− group-containing C positions are more vulnerable to •OH attack. The kinetic results show that the complexation of the four metal ions offers a wide pH range of effectiveness for catechol degradation. At pH 7, the apparent rate constant (kapp) values for different systems follow the order of catechol+Ti4+ ≈ catechol+Zn2+ > catechol+Cu2+ > catechol+Fe3+ > catechol. The mechanistic and kinetic results would greatly improve our understanding of the degradation of CATPs-metal and other organics-metal complexes in AOPs. The toxicity assessment indicates that the •OH-based AOPs have the ability for decreasing the toxicity and increasing the biodegradability during the processes of catechol degradation.

A comprehensive study on low-temperature oxidation chemistry of cyclohexane. II. Experimental and kinetic modeling investigation

Low-temperature oxidation of cyclohexane is investigated in two jet-stirred reactors (JSRs) at 1.04 bar and the equivalence ratio of 0.25. Reactive hydroperoxides and highly oxygenated molecules are detected using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). The isomers of C6H10O (5-hexenal, cyclic ethers and cyclohexanone) are separated using gas chromatography combined with mass spectrometry (GC–MS). Detection of characteristic hydroperoxides verifies that the conventional two-stage oxygen addition channels and recently reported third oxygen addition channels both contribute to the low-temperature oxidation of cyclohexane. Conformation-dependent channels theoretically investigated in Part I of this work are found correlated with the experimental observations of ketohydroperoxide (KHP) and alkenyl-hydroperoxide (AnHP) intermediates. A new detailed kinetic model of cyclohexane oxidation is constructed with consideration of the investigated conformation-dependent pathways in Part I and the experimental revisit of OH attack reactions over 889–1301 K and 1.22–1.84 bar. The model is validated against the newly measured oxidation data in this work and previous experimental data over a variety of pressure, temperature and equivalence ratio conditions. Modeling analysis reveals that the KHP channel and AnHP channel dominate the chain-branching process under the investigated conditions. The third oxygen addition channels and bimolecular reaction channels are found to play less important roles under the investigated conditions, while these reactions can provide more significant contributions to OH formation under high-pressure and lean conditions.

Conformational Change of H64 and Substrate Transportation: Insight Into a Full Picture of Enzymatic Hydration of CO2 by Carbonic Anhydrase.

The enzymatic hydration of CO2 into HCO3 − by carbonic anhydrase (CA) is highly efficient and environment-friendly measure for CO2 sequestration. Here extensive MM MD and QM/MM MD simulations were used to explore the whole enzymatic process, and a full picture of the enzymatic hydration of CO2 by CA was achieved. Prior to CO2 hydration, the proton transfer from the water molecule (WT1) to H64 is the rate-limiting step with the free energy barrier of 10.4 kcal/mol, which leads to the ready state with the Zn-bound OH−. The nucleophilic attack of OH− on CO2 produces HCO3 − with the free energy barrier of 4.4 kcal/mol and the free energy release of about 8.0 kcal/mol. Q92 as the key residue manipulates both CO2 transportation to the active site and release of HCO3 −. The unprotonated H64 in CA prefers in an inward orientation, while the outward conformation is favorable energetically for its protonated counterpart. The conformational transition of H64 between inward and outward correlates with its protonation state, which is mediated by the proton transfer and the product release. The whole enzymatic cycle has the free energy span of 10.4 kcal/mol for the initial proton transfer step and the free energy change of −6.5 kcal/mol. The mechanistic details provide a comprehensive understanding of the entire reversible conversion of CO2 into bicarbonate and roles of key residues in chemical and nonchemical steps for the enzymatic hydration of CO2.