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- Slanina, Tomáš, et al.
(författare)
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Impact of Excited-State Antiaromaticity Relief in a Fundamental Benzene Photoreaction Leading to Substituted Bicyclo[3.1.0]hexenes
- 2020
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society. - 0002-7863 .- 1520-5126. ; 142:25, s. 10942-10954
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Tidskriftsartikel (refereegranskat)abstract
- Benzene exhibits a rich photochemistry which can provide access to complex molecular scaffolds that are difficult to access with reactions in the electronic ground state. While benzene is aromatic in its ground state, it is antiaromatic in its lowest ππ∗ excited states. Herein, we clarify to what extent relief of excited-state antiaromaticity (ESAA) triggers a fundamental benzene photoreaction: the photoinitiated nucleophilic addition of solvent to benzene in acidic media leading to substituted bicyclo[3.1.0]hex-2-enes. The reaction scope was probed experimentally, and it was found that silyl-substituted benzenes provide the most rapid access to bicyclo[3.1.0]hexene derivatives, formed as single isomers with three stereogenic centers in yields up to 75% in one step. Two major mechanism hypotheses, both involving ESAA relief, were explored through quantum chemical calculations and experiments. The first mechanism involves protonation of excited-state benzene and subsequent rearrangement to bicyclo[3.1.0]hexenium cation, trapped by a nucleophile, while the second involves photorearrangement of benzene to benzvalene followed by protonation and nucleophilic addition. Our studies reveal that the second mechanism is operative. We also clarify that similar ESAA relief leads to puckering of S1-state silabenzene and pyridinium ion, where the photorearrangement of the latter is of established synthetic utility. Finally, we identified causes for the limitations of the reaction, information that should be valuable in explorations of similar photoreactions. Taken together, we reveal how the ESAA in benzene and 6π-electron heterocycles trigger photochemical distortions that provide access to complex three-dimensional molecular scaffolds from simple reactants.
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- Wiinikka, Henrik, et al.
(författare)
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Emissions of heavy metals during fixed-bed combustion of six biomass fuels
- 2013
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Ingår i: Energy & Fuels. - : American Chemical Society (ACS). - 0887-0624 .- 1520-5029. ; 27:2, s. 1073-1080
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Tidskriftsartikel (refereegranskat)abstract
- Few studies examine heavy metal emissions during the small-scale combustion of various solid biofuels. This issue may become more important, as one can expect new regulations governing such emissions from biomass combustion similar to those governing waste incineration. This paper investigates the emissions of particulate-associated heavy metals (i.e., Sb, As, Cd, Co, Cr, Cu, Pb, Mn, Ni, Tl, V, Hg, and Zn) during the fixed-bed combustion of six solid biofuels (i.e., stemwood from birch and pine/spruce, bark from birch and pine, salix, and oat grains) and of peat and bituminous coal for comparison. The results indicate that the flue gas concentration (normalized to 11% O2) of the sum of all measured metals (Zn excluded) during the biomass combustion tests ranged from 57 μg Nm-3 for birch stemwood to 198 μg Nm-3 for birch bark. The concentration of Zn in the flue gas was generally considerably higher than those of the other metals, ranging from 646 μg Nm-3 for spruce/pine stemwood to 7948 μg Nm-3 for birch bark. Compared with coal and peat, the biomass fuels produced higher Zn emissions, but lower or similar emissions of the sum of the other metals. The volatile behavior and concentration of the metal in the flue gases as a function of the heavy metal in the fuel are also presented for selected heavy metals.
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- Wiinikka, Henrik, et al.
(författare)
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Influence of TiO2 additive on vaporisation of potassium during straw combustion
- 2009
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Ingår i: Energy & Fuels. - : American Chemical Society (ACS). - 0887-0624 .- 1520-5029. ; 23:11, s. 5367-5374
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Tidskriftsartikel (refereegranskat)abstract
- In this work, the influence of TiO2 on the vaporization of K during combustion of straw under fixed bed condition was investigated experimentally. Controlled combustion experiments with a varied amount of TiO2 in straw pellets were performed in an 8 kW pellet burner together with sampling of particles (impactor and absolute filter), analysis of the flue gas composition (Fourier transform infrared, FTIR), and chemical analyses of the collected particles and bottom ash (inductively coupled plasma-atomic emission spectroscopy (ICP-AES), scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS), and X-ray diffraction (XRD)). The experimental results showed that the vaporization of K from the fuel bed was significantly reduced when TiO2 was used as an additive. The vaporization of K was reduced by ~40−50% for an optimal amount of TiO2 additive. The optimal added TiO2 for the straw used in this work corresponds to a Ti/K (wt) ratio between ~0.6−1.0. If more TiO2 was added to the fuel, the release of K to the gas phase was not further reduced and unreacted TiO2 was found in the bottom ash.
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