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Bimodal Reaction Sequences in Oxidation of Hydrogen and Organic Compounds with Dioxygen
Published in Robert Bakhtchadjian, Bimodal Oxidation: Coupling of Heterogeneous and Homogeneous Reactions, 2019
One of the best catalysts of the oxidative coupling of methane is alkali metal doped metal oxides.129in Ch. 1 The oxidative coupling of methane over Li/MgO (Li-doped) occurs through the formation of CH3 radicals, transferred to the gas phase. In the presence of oxygen, part of methyl radicals transform to CH3O2 radicals. These radicals were detected by the method of matrix-isolation EPR and also their presence was confirmed by beam mass-spectrometric data, reported by Lunsford et al.129in Ch. 1;202 Other evidence about the role of CH3 radicals was obtained in experiments with isotopically labeled methane CD4 in the presence of the same catalysts. These experiments confirmed the formation of C2H6, CD3CH3, C2D6 as coupling products. However, direct experimental proof of the existence of methyl radicals in the gas phase in reaction conditions of the oxidative coupling of methane, generated on the catalyst Li/MgO, was not first obtained until 2013, using synchrotron VUV(vacuum-UV) photoionization mass spectrometry.203 The reaction, yielding mainly C2 hydrocarbons, was carried out at 750°C, using a mixture of 8% CH4 and 4% O2 in argon.
Methane Conversions
Published in Saeed Sahebdelfar, Maryam Takht Ravanchi, Ashok Kumar Nadda, 1 Chemistry, 2022
Saeed Sahebdelfar, Maryam Takht Ravanchi, Ashok Kumar Nadda
The oxidative coupling of methane consists of sequential partial oxidation of CH4 into C2H6 and finally C2H4 (Eqs. 4.46–4.49). At first, oxygen and methane reacts and water and ethane are produced. By in-situ conversion steps, the produced C2H6 is changed to C2H4 . Of course, higher hydrocarbons are produced as well in trace amounts. In the OCM reaction, ethylene is the favorable product; but according to most mechanistic studies, ethane is the first product. The drawback of this process is that any slight increase in O2 concentration may shift the reaction to the production of carbon oxides. In the large-scale application of this process, the type of oxygen feed (i.e., air or pure oxygen) used is important. If the main process is based on pure oxygen, an air separation unit is needed that increase the process cost. If air is used in the process, its nitrogen can handle the exothermic temperature rise. If a selective membrane reactor is used in the process, without the need to an ASU (air separation unit), almost pure oxygen can be obtained. By this approach, technological difficulties and cost implications of the direct air usage can be overcome (Lunsford, 1995; Galadima and Muraza, 2016).
Injectable enzyme-catalyzed crosslinking hydrogels as BMSCs-laden tunable scaffold for osteogenic differentiation
Published in Journal of Biomaterials Science, Polymer Edition, 2023
Hongwei Pan, Wanxin Li, Yue Qu, Simei Li, Ayixiemu Yusufu, Jia Wang, Lihua Yin
The hydrogels were formed by oxidative coupling reaction via horseradish catalase(HRP) and hydrogen peroxide(H2O2), which is an efficient method for the in-situ formation of hydrogels. Figure 2a and b showed that when HA-TA and ALG-TA were mixed in PBS, the mixture showed good fluidity in the absence of HRP and H2O2. Then, with the addition of HRP and H2O2, HA/ALG hydrogel was gradually formed. This is because HRP was oxidized by H2O2 to form an intermediate, which then oxidizes phenol and leads to crosslinking of TA-modified HA and ALG. The crosslinking time is also essential for the application of hydrogels. If it is too fast, it may have been solidified before reaching the damage site and cannot fill the whole defect site; at the same time, the precursor material is difficult to mix evenly, resulting in uncontrollable composition and structure of the hydrogel. If it is too slow, it will cause serious leakage of precursor materials, resulting in hydrogel cannot match the defect site well, as well as a severe loss of cells or growth factors. EP tube inversion test determined the sol-gel transition of HA/ALG hydrogel. The hydrogels can be formed within 15 s, similar to the previous study [43], and have good injection molding ability (Figure 2c).
Vegetable tannin composition and its association with the leather tanning effect
Published in Chemical Engineering Communications, 2020
Priscila Auad, Franciela Spier, Mariliz Gutterres
The minimum size and reactivity required to have a tanning capacity can be achieved in a number of ways, and the most common way in nature seems to be the binding of esters or gallic acids to the central part of a carbohydrate. These molecules are easily hydrolyzed with acids, bases, or enzymes and are therefore called hydrolyzable tannins (Grasel et al., 2016a). Despite their non-polymer origin, this family of tannins can form complex structures (Belgacem and Gandini, 2008). The hydrolyzable tannins are chemically divided into gallotannins and ellagic tannins. Galotannins are glucose and polyesters of gallic acid commonly found in nature, and they release gallic acid when hydrolyzed. Meanwhile, ellagic tannins are characterized by a glucose center esterified with at least one unit of hexahydroxydiphenyl acid, which is formed by the oxidative coupling of two units of gallic acid (Belgacem and Gandini, 2008). On the other hand, other polyphenolic mixtures present in plants do not undergo hydrolysis, with these being called condensed tannins or proanthocyanidins. They are oligomers or polymers of the basic structure of flavan-3-ol (Koleckar et al., 2008).
Catalytic wet air oxidation of phenol: Review of the reaction mechanism, kinetics, and CFD modeling
Published in Critical Reviews in Environmental Science and Technology, 2021
Tladi J. Makatsa, Jeffrey Baloyi, Thabang Ntho, Cornelius M. Masuku
The formation of p-hydroxybenzoic acid follows two routes: it can be formed when phenol interacts with oxygen groups found on the surface of activated carbon or from the oxidation of chemically adsorbed species on the surface of AC previously formed by oxidative coupling reaction of phenol and aromatics. During oxidation of intermediates, 90% of hydroquinone was converted to p-benzoquinone. When p-benzoquinone was oxidized 100% conversion was reached with maleic, malonic, acetic and formic acid identified as intermediates. However, a small difference was observed between TOC and p-benzoquinone conversion values in the reactor exit. Thus, suggesting that most of p-benzoquinone was converted to CO2 and H2O via oxalic acid. Oxidation of p-hydroxybenzoic acid produced maleic, acetic and formic acid. Unidentified species were neglected due to close values of measured and calculated TOC. Maleic acid was the main product of oxidation. At high reaction time, 100% of oxalic acid was converted to CO2 and H2O. In addition, oxidation of maleic acid produced fumaric, acetic and formic acid whereas formic acid concentrations were higher than acetic acid at TOC values ranging between 20 and 40%. The measured and calculated TOC values were in agreement indicating complete oxidation of maleic acid. Formic acid was completely mineralized when it was oxidized at 127 °C and 8 bar whereas no conversion was observed when acetic acid was oxidized. Furthermore, malonic acid was oxidized to acetic acid and CO2. However, traces of formic acid were detected but measured and calculated TOC values indicated that all intermediates were identified.