X-ray and Optical Circular Dichroism as Local and Global Ultrafast Chiral Probes of [12]Helicene Racemization

Chirality is a fundamental molecular property that plays a crucial role in biophysics and drug design. Optical circular dichroism (OCD) is a well-established chiral spectroscopic probe in the UV-visible regime. Chirality is most commonly associated with a localized chiral center. However, some compounds such as helicenes (Figure 1) are chiral due to their screwlike global structure. In these highly conjugated systems, some electric and magnetic allowed transitions are distributed across the entire molecule, and OCD thus probes the global molecular chirality. Recent advances in X-ray sources, in particular the control of their polarization and spatial profiles, have enabled X-ray circular dichroism (XCD), which, in contrast to OCD, can exploit the localized and element-specific nature of X-ray electronic transitions. XCD therefore is more sensitive to local structures, and the chirality probed with it can be referred to as local. During the racemization of helicene, between opposite helical structures, the screw handedness can flip locally, making the molecule globally achiral while retaining a local handedness. Here, we use the racemization mechanism of [12]helicene as a model to demonstrate the capabilities of OCD and XCD as time-dependent probes for global and local chiralities, respectively. Our simulations demonstrate that XCD provides an excellent spectroscopic probe for the time-dependent local chirality of molecules.

Understanding the intermediates and carbon dioxide adsorption of potassium chloride-incorporated graphitic carbon nitride with tailoring melamine and urea as precursors

In this study, we demonstrated the synthesis of potassium chloride (KCl)-incorporated graphitic carbon nitride, (g-C₃N₄, CN) with varying amounts of N-vacancies and pyridinic-N as well as enhanced Lewis basicity, via a single-step thermal polymerization by tailoring the precursors of melamine and urea for carbon oxide (CO₂) capture. Melamine, as a precursor, undergoes a phase transformation into melam and triazine-rich g-C₃N₄, whereas the addition of urea polymerizes the mixture to form melem and heptazine-rich g-C₃N₄ (CN11). Owing to the abundance of pyridinic-N and the high surface area, CN11 adsorbed higher amounts of CO₂ (44.52 μmol m^-2 at 25 °C and 1 bar of CO₂) than those reported for other template-free carbon materials. Spectroscopic analysis revealed that the enhanced CO₂ adsorption is due to the presence of pyridinic-N and Lewis basic sites on the surface. The intermediates of CO₂adsorption, including carbonate and bicarbonate species, attached to the CN samples were identified using in-situ Fourier-transform infrared spectroscopy (FTIR). This work provides insights into the mechanism of CO₂ adsorption by comparing the structural features of the synthesized KCl-incorporated g-C₃N₄ samples_. CN11, with an excellent CO₂ uptake capacity, is viewed as a promising candidate for CO₂ capture and storage.

Formation pathways of polychlorinated dibenzo-p-dioxins and dibenzofurans from burning simulated PVC-coated cable wires

Open burning of PVC-coated cables is a major source of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F). In the present study, the formation characteristics of PCDD/F from burning of PVC-based samples with and without metallic copper were evaluated over the dioxin formation temperature window (200-500 °C). This temperature range also inevitably occurs under open burning conditions. The PCDD/F yield from PVC added with Cu increased by factors of 1390 (300 °C), 65 (400 °C) and 17 (500 °C) compared with that from PVC alone, confirming the stimulatory effect of metallic Cu on PCDD/F production. For the first time, a relatively complete isomer-specific analysis is established for PVC acting as source of PCDD/F. Formation pathways of PCDD/F and the reaction mechanisms were investigated using a combined analysis of PCDD/F isomer signatures, thermogravimetric results and Cl K-edge X-ray absorption spectra. De novo synthesis is the major pathway leading to massive production of PCDD/F. Copper extends the temperature range for the concurrence of de(hydro)chlorination of PVC with cross-linking and aromatisation of polyenes and then stimulates cracking of the chlorine-embedded carbon network. Together, these processes contribute to the strongly enhanced formation of PCDD/F via de novo synthesis.

Incineration of carbon nanomaterials with sodium chloride as a potential source of PCDD/Fs and PCBs

The incineration of waste carbon nanomaterials will become an inevitable waste management strategy following the disposal of products containing carbon nanomaterials. We investigated the formation of polychlorinated dibenzo-p-dioxins and furans (PCDD/Fs) and polychlorinated biphenyls (PCBs) during the incineration of selected carbon nanomaterials [fullerene (C₆₀), single-walled carbon nanotubes (SWNTs), and graphene], with sodium chloride and trace copper at 850 °C in air using a laboratory-scale electric furnace. Most PCDD/Fs and PCBs were concentrated in particulate fly ash post-incineration, and in low-temperature zones in the furnace (54-670 °C). Notably, C₆₀ had a specific thermal behavior leading to the formation of high concentrations of high chlorinated PCDD/Fs and toxic 2,3,7,8-tetra-CDD/F (2,3,7,8-T4CDD/F). SWNTs had a lower potential to generate such compounds than C₆₀, but had a higher potential than graphene and graphite. Temperature, solid/gas phases, chlorine sources, and the thermal stability of carbon nanomaterials were the key controlling factors. There is a need to consider the generation of PCDD/Fs and PCBs during the incineration of waste streams containing carbon nanomaterials.

Quantitative speciation of insoluble chlorine in E-waste open burning soil: Implications of the presence of unidentified aromatic-Cl and insoluble chlorides

Open burning of electronic waste (E-waste) produces numerous organochlorine compounds (OCs). Although the presence of unidentified OCs has been suggested, the mass balance of identified and unidentified OCs in E-waste open burning soils (EOBSs) still remains unknown. In this study, the concentrations of Cl bonded with aromatic carbon (aromatic-Cl) and aliphatic carbon (aliphatic-Cl), and inorganic Cl in EOBSs were determined by focusing on chlorine (Cl) in water-insoluble fractions (insoluble Cl) and applying Cl K-edge X-ray absorption spectroscopy in conjunction with combustion ion chromatography. The concentrations of identified Cl (Cl in five individual OCs: polychlorinated biphenyls, polychlorinated dibenzo-p-dioxins, polychlorinated dibenzofurans, chlorinated polycyclic aromatic hydrocarbons and chlorinated benzenes) were calculated from the concentrations previously reported for the same samples. The proportions of identified Cl were less than 1% to aromatic-Cl, indicating the abundance of unidentified OCs. The concentrations of both aromatic-Cl and identified Cl were highest in the sample collected from the site in Vietnam (VN), where wires and cables were mainly burned, suggesting that unidentified aromatic-Cl were produced through pathways similar to those of identified OCs, and the pathway could be related to burning of wires and cables. Further, insoluble Cu (II) compound, Cu₂(OH)₃Cl were assumed to be present in EOBSs and the concentration was highest in VN, implying that insoluble inorganic chlorides could be related to the formation of aromatic-Cl and identified Cl.

Removal of gaseous elemental mercury by hydrogen chloride non-thermal plasma modified biochar

Hydrogen chloride (HCl) non-thermal plasma was applied to introduce Cl active sites on biochar prepared from sorghum straw in this study. Surface modified biochar was then placed in flue gas with typical components to investigate its elemental mercury (Hg⁰) capture ability. To elucidate the adsorption mechanism & binding properties, samples were characterized by N₂ adsorption, scanning electron microscopy with energy dispersive spectrometer (SEM-EDS) and X-ray absorption near edge structure (XANES) analysis of Hg L_III-edge, Cl K-edge and S K-edge. Experimental results showed that HCl plasma modification successfully increased Cl active sites on biochar and greatly increased its mercury removal efficiency. Both HCl treatments (w/without plasma involvement) altered biochar's surface structure and layered structure generated. XANES spectra revealed that adsorbed-Hg on HCl-treated biochars mainly in the form of Hg⁺. Gaseous Hg⁰ was believed to heterogeneously react with chlorinated sites through electron-transfer and formed Hg₂Cl₂ compounds. With the presence of NO or SO₂ in the system, adsorbed mercury existed on biochar mainly as Hg⁺. SO₂ competed and inhibited the adsorption of Hg⁰; while NO promoted Hg⁰ removal capacity by increasing the active sites and enhancing the adsorption kinetics of adjacent Cl-containing sites.

Iron chloride catalysed PCDD/F-formation: Experiments and PCDD/F-signatures

Iron chloride is often cited as catalyst of PCDD/F-formation, together with copper chloride. Conversely, iron chloride catalysis has been less studied during de novo tests. This paper presents such de novo test data, derived from model fly ash incorporating iron (III) chloride and established over a vast range of temperature and oxygen concentration in the gas phase. Both PCDD/F-output and its signature are extensively characterised, including homologue and congener profiles. For the first time, a complete isomer-specific analysis is systematically established, for all samples. Special attention is paid to the chlorophenols route PCDD/F, to the 2,3,7,8-substituted congeners, and to their relationship and antagonism.

Synergetic inhibition of thermochemical formation of chlorinated aromatics by sulfur and nitrogen derived from thiourea: Multielement characterizations

Nitrogen and sulfur (N/S)-containing compounds inhibit the formation of polychlorinated dibenzo-p-dioxins (PCDDs) and furans (PCDFs) in thermal processes. However, few studies have examined the inhibition mechanisms of N/S-containing compounds. In the present study, we focused on thiourea [(NH2)2CS] as such a compound and investigated its inhibition effects and mechanisms. The production of PCDD/Fs, polychlorinated biphenyls (PCBs), and chlorobenzenes (CBzs) were inhibited by >99% in the model fly ash in the presence of 1.0% thiourea after heating at 300 °C. Experimental results using real fly ash series were indicative of the thermal destruction of these chlorinated aromatics by thiourea. Multielement characterization using K-edge X-ray absorption fine structures of copper, chlorine, sulfur, nitrogen, and carbon revealed three possible inhibition paths, namely, (a) sulfidization of the copper catalyst to CuS, Cu2S, and CuSO4; (b) blocking the chlorination of carbon via the reaction of chlorine with N-containing compounds to generate ammonium chloride and other minor compounds; and (c) changing the carbon frame involved in attacking the carbon matrix by sulfur and nitrogen. Thus, thiourea plays a role as a sulfur and nitrogen donor to achieve multiple and synergistic inhibition of chlorinated aromatics. Our results suggest that other N/S-containing inhibitors function based on similar mechanisms.

Contrasting effects of sulfur dioxide on cupric oxide and chloride during thermochemical formation of chlorinated aromatics

Sulfur dioxide (SO2) gas has been reported to be an inhibitor of polychlorinated dibenzo-p-dioxins and furans (PCDD/Fs) formation in fly ash. However, other research has suggested little or no inhibitory effect of SO2 gas. Although these studies focused on reactions between SO2 gas and gas-phase chlorine (Cl) species, no attention was paid to thermochemical gas-solid reactions. In this study, we found contrasting effects of SO2 gas depending on the chemical form of copper (CuO vs CuCl2) with a solid-phase inorganic Cl source (KCl). Chlorinated aromatics (PCDD/Fs, polychlorinated biphenyls, and chlorobenzenes) increased and decreased in model fly ash containing CuO + KCl and CuCl2 + KCl, respectively, with increased SO2 injection. According to in situ Cu K-edge and S K-edge X-ray absorption spectroscopy, Cl gas and CuCl2 were generated and then promoted the formation of highly chlorinated aromatics after thermochemical reactions of SO2 gas with the solid-phase CuO + KCl system. In contrast, the decrease in aromatic-Cls in a CuCl2 + KCl system with SO2 gas was caused mainly by the partial sulfation of the Cu. The chemical form of Cu (especially the oxide/chloride ratio) may be a critical factor in controlling the formation of chlorinated aromatics using SO2 gas.

Coexistence of Cu, Fe, Pb, and Zn oxides and chlorides as a determinant of chlorinated aromatics generation in municipal solid waste incinerator fly ash

We investigated chemical determinants of the generation of chlorinated aromatic compounds (aromatic-Cls), such as polychlorinated biphenyls (PCBs) and chlorobenzenes (CBzs), in fly ash from municipal solid waste incineration. The influences of the following on aromatic-Cls formation in model fly ash (MFA) were systematically examined quantitatively and statistically: (i) inorganic chlorides (KCl, NaCl, CaCl2), (ii) base materials (SiO2, Al2O3, CaCO3), (iii) metal oxides (CuO, Fe2O3, PbO, ZnO), (iv) metal chlorides (CuCl2, FeCl3, PbCl2, ZnCl2), and (v) "coexisting multi-models." On the basis of aromatic-Cls concentrations, the ∑CBzs/∑PCBs ratio, and the similarity between distribution patterns, MFAs were categorized into six groups. The results and analysis indicated that the formation of aromatic-Cls depended strongly on the "coexistence condition", namely multimodels composed of not only metal chlorides, but also of metal oxides. The precise replication of metal chloride to oxide ratios, such as the precise ratios of Cu-, Fe-, Pb-, and Zn-chlorides and oxides, may be an essential factor in changing the thermochemical formation patterns of aromatic-Cls. Although CuCl2 acted as a promoter of aromatic-Cls generation, statistical analyses implied that FeCl3 also largely influenced the generation of aromatic-Cls under mixture conditions. Various additional components of fly ash were also comprehensively analyzed.

Thermochemical behavior of lead adjusting formation of chlorinated aromatics in MSW fly ash

In this study, we examined the thermochemical role of Pb in the formation of chlorinated aromatics (aromatic-Cls) in MSW fly ash at 300-400 °C, a key temperature window for maximum yield. In the presence of lead oxide alone, aromatic-Cls formation was suppressed. One of the mechanisms of suppression was partial chlorination of PbO by an inorganic chlorine source in the solid phase, based on in situ Pb L3-edge X-ray absorption near-edge structure (XANES) data. In contrast, quantitative GC/MS measurements revealed that PbCl2 promoted aromatic-Cls formation to an extent that depends on the Pb concentration, the heating temperature, and the presence of other metal catalysts. We identified two mechanisms of aromatic-Cls formation triggered by PbCl2 in MSW fly ash. First, promotion can occur by the thermochemical partial oxidation of PbCl2. More specifically, real complex solid phase increases the thermochemical oxidation reactivity of PbCl2, based on in situ Pb L3-edge XANES data. Second, Cl K-edge X-ray absorption spectroscopy revealed a coexistent effect of PbCl2 with other metal catalysts such as CuCl2 and FeCl3. The presence of PbCl2 influences the balance of the bonding state of chlorine with Cu and Fe atoms at various temperatures. Thus, Pb in real MSW fly ash functions as an "adjuster" in the generation of aromatic-Cls, the nature of which depends on the lead oxide/chloride ratio and the presence of other metal catalysts.

Thermochemical chlorination of carbon indirectly driven by an unexpected sulfide of copper with inorganic chloride

Unintentional anthropogenic thermal chlorination of carbon is known to be a contributor to global environmental pollution of organochlorine compounds. We found unexpected, serious chlorination of carbon promoted by a "sulfide" of copper, which has been generally thought of and studied as an inactive metal catalyst. Our quantitative and X-ray spectroscopic results show that a fraction of cupric sulfide indirectly promoted thermochemical solid-phase formation of a large quantity of organochlorine compounds such as polychlorinated dibenzo-p-dioxins, dibenzofurans, biphenyls, and benzenes that used inactive inorganic chloride as chlorine storage, which partly caused environmental pollution by organochlorine compounds.

Role of zinc in MSW fly ash during formation of chlorinated aromatics

In this study, we determined the thermochemical role of zinc in municipal solid waste (MSW) fly ash. Zinc's role depended on its chemical form and the presence of other metal catalysts. When only zinc was present or it dominated other metal elements, chlorinated aromatic compound (aromatic-Cl) formation was promoted by zinc chloride but blocked by zinc oxide. When only zinc was present, such as in zinc metallurgical plants, some aromatic-Cls were generated and contaminated the environment. When zinc coexisted with other metal promoters in a thermal postcombustion solid phase, such as MSW incineration, Fourier-transform Zn K-edge extended X-ray absorption fine structure (EXAFS) analysis showed that the chemical forms of zinc were primarily chloride and/or oxide, and zinc chloride (ZnCl(2)) was thermally stable in the solid phase. Thus, we used ZnCl(2) in coexistence experiments as a promoter to generate aromatic-Cls. Zinc chloride acted as a coexistent inhibitor of metal catalysis and precursor dimerization to generate aromatic-Cls. There were two coexistent inhibition mechanisms. First, a low-temperature transition of chlorine to the gas phase (low-Cl(g)) occurred with metal catalysts such as CuCl(2) and FeCl(3), confirmed by Cl K-edge near-edge X-ray absorption fine structure (NEXAFS) analysis. Second, X-ray photoelectron spectroscopy (XPS) analysis of the surface or near-surface concentration of ZnCl(2) indicated weak reactivity between the catalysts and the carbon matrix.