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Methane Conversion on Single-Atom Catalysts
Published in Jianli Hu, Dushyant Shekhawat, Direct Natural Gas Conversion to Value-Added Chemicals, 2020
Xiaoyan Liu, Hua Liu, Aiqin Wang
It has been reported that methane can be activated by halogen radicals through cleaving C–H bands under relatively mild conditions to produce alcohols, ethers, or olefins (Ogura and Takamagari 1986; Lorkovic et al. 2004; Weissman and Benson 1984; Olah et al. 1985; Olah 1987; Arvidsson et al. 2017). However, it makes the potential industrial application quite challenging due to the toxicity and high corrosiveness of the halogenation or oxy-halogenation reactions. Based on the above discussion, it is an ideal approach for utilization of CH4 by directly converting CH4 to oxygenated products such as HCHO, CH3OH, and CH3COOH under mild conditions (Osadchii et al. 2018; Tang et al. 2018; Shan et al. 2017; Kwon et al. 2017; Tomkins et al. 2019; Sushkevich et al. 2017). For example, the direct partial oxidation of methane to methanol (MTM) is thermodynamically feasible even at a low temperature; nevertheless, there is no industrial process for the MTM reaction even though it has been the subject of active research for almost a century. In addition to the high barriers in activating the methane, the C–H bond of the methanol is much active than that of the methane (Ravi et al. 2017).
Chemicals from Olefin Hydrocarbons
Published in James G. Speight, Handbook of Petrochemical Processes, 2019
Halogenation is a chemical reaction that involves the addition of one or more halogens (fluorine, chlorine, bromine, or iodine) to a compound. The reaction pathway and the stoichiometry of the reaction depend on the structural features and functional groups of the organic substrate, as well as on the specific halogen. Inorganic compounds such as metals also undergo halogenation.
Halogenases with Potential Applications for the Synthesis of Halogenated Pharmaceuticals
Published in Peter Grunwald, Pharmaceutical Biocatalysis, 2019
Georgette Rebollar-Pérez, Cynthia Romero-Guido, Antonino Baez, Eduardo Torres
Halogenated compounds represent a large family of valuable materials widely used in all sectors of the chemical industry as synthetic intermediates, solvents, flame retardants, antifouling, pesticides and as active ingredients in health care from blood extenders to anticancer drugs. Halogenated compounds can come from natural or synthetic sources, and the number of halogenated natural products accounts for more than 4700 metabolites at present (Gribble, 2010), most of them being derived from marine ecosystems. Recent estimates indicate that around 20% of synthetic small molecule pharmaceuticals are halogenated molecules (Herrera-Rodriguez et al., 2011). The chemical structure of halogenated natural products can vary from simple halogenated compounds (indoles, terpenes, and phenols) to complex oligopeptides and polyketides. The reasons of the relevance of halogenated moieties in pharmaceutical products is that the introduction of a halogen atom can have an improved effect on the physicochemical properties of the molecules such as bioactivity, bioavailability, membrane permeability, thermal and oxidative stability and binding capacity, which are relevant properties for an efficient therapeutic action and decreased side effects. In addition, the incorporation of halogen atoms allows further selective modification through cross-coupling chemistry (Mahoney et al., 2014) and halogen bonding. This chemical diversity, and their wide biological activity, makes halogenated compounds important precursors for new drugs (Brown and O’Connor, 2015; Gribble, 2015). Although chemical halogenation is a well-established technology, its procedures are typically characterized by hazardous or even highly toxic halogenating reactants, low product regioselectivity and harmful byproducts. In this sense, biocatalysis represents an alternative sustainable technology because enzyme-catalyzed reactions are operated under soft environmental conditions (ambient temperature, atmospheric pressure, neutral pH), in aqueous media and the protection/deprotection steps are usually unnecessary because of the high enantio- and regioselectivity showed. These remarkable characteristics come with a significant reduction in the overall economic cost and in the environmental impact of the process (Woodley, 2008). Enzymatic halogenation is a relevant step for bioactivities of many natural products, and in principle, the number of halogenated metaboilites supposed the equal number of halogenating enzymes; however, only a few halogenases have been described and characterized to date. Haloperoxidases were the first halogenases reported, but over the last two decades, new halogenases have been discovered. Halogenases have been classified according to the chemical mechanism they followed: electrophilic, nucleophilic, or radical halogenation. This chapter reviews the different halogenases described to date.
UHMWPE Modified by Halogenating Reagents: Study on the Improvement of Hydrophilicity and Tribological Properties
Published in Tribology Transactions, 2022
Yunkai Li, Youqiang Wang, Ping Zhang, Guangxiao Jian, Heng Luo, Xiao Yu
Figure 3 shows the contact angle results for UHMWPE before and after modification. Regardless of the reagent concentrations of the halogenation modification and the modification durations, the contact angles of the modified materials decreased compared with that of the UHMWPE without modification (Fig. 3a); that is, the surface hydrophilicity was improved. However, a higher reagent concentration and longer modification duration did not necessarily correspond to a smaller contact angle of the modified material. The contact angle of the unmodified UHMWPE was 95.5° (Fig. 3b), and reagent concentrations and modification durations of the three groups with the best modification effect and the most significant hydrophilicity improvements are shown. The contact angle after immersion in a 15% NH4Cl solution for 60 h was 71.5° (Fig. 3c), the contact angle after immersion in a 10% NH4Cl solution for 60 h was 68.5° (Fig. 3d), and the contact angle after immersion in a 15% KBr solution for 30 h was only 60.5° (Fig. 3e).
Iodination of vanillin and subsequent Suzuki-Miyaura coupling: two-step synthetic sequence teaching green chemistry principles
Published in Green Chemistry Letters and Reviews, 2019
James J. Palesch, Beau C. Gilles, Jared Chycota, Moriana K. Haj, Grant W. Fahnhorst, Jane E. Wissinger
Our aim was to design a guided-inquiry experiment exemplifying similar learning outcomes to the nitration experiment through a greener, safer transformation. The ideal substrate would have multiple possible substitution positions so that EAS selectivity could be studied and would afford a crystalline product with instructive 1H NMR spectral features. Recently, we developed an oxidation of borneol to camphor using Oxone® and catalytic sodium chloride (3). This experiment has been a highly successful green addition to our organic chemistry laboratory curriculum. Oxone® is a stable triple salt consisting of 2KHSO5•KHSO4•K2SO4 which has found wide spread application in synthetic chemistry (4). One such application is the halogenation of aromatic rings using a combination of Oxone® and a halide salt in various solvents (5–7). This reaction works most efficiently with aromatic substrates containing one or more electron-donating substituents (8). The active oxidizing agent in Oxone® is potassium peroxymonosulfate (KHSO5) which is thought to react with halide salts (M + X-) to produce a source of the electrophilic X+ in the form of a hypohalous acid, HOX (4). After workup, the by-products of the reaction are environmentally-benign potassium sulfate salts.
Mononuclear oxidodiperoxido vanadium(V) complex: synthesis, structure, VHPO mimicking oxidative bromination, and potential detection of hydrogen peroxide
Published in Journal of Coordination Chemistry, 2018
Haimanti Adhikari, Kalyan K. Mukherjea
Vanadium haloperoxidase (VHPO) is the first isolated vanadium-dependent enzyme from the marine algae Ascophyllum nodosum. In 1984 and later, it was found in some lichens and other marine algae as well [1, 2]. Vanadium-containing complexes have obtained attention due to their wide biological and catalytic properties such as haloperoxidation, nitrogen fixation, metalloprotein function, insulin-mimicking activities [3, 4]. The incorporation of halogen atoms in many organic compounds by nature leads to the formation of halogenated antibiotics, drugs, or signaling molecules in biological systems [5]. Halogenated compounds, usually found in marine organisms, especially marine macroalgae, seem to be important factors of halogen transfers in the coastal marine environment [6–8]. Among halogenating enzymes, haloperoxidases utilize hydrogen peroxide for electrophilic halogenation via the oxidation of halides [5, 9–12].