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Refinery Reactors
Published in James G. Speight, Refinery Feedstocks, 2020
In discussions of the chemistry of refinery processes (Chapter 7), in studies of the process (or reaction) kinetics, the terms mechanism or model receive frequent use and are used to indicate a plausible, but assumed sequence of steps for a given reaction. However, the various levels of detail in investigating reaction mechanisms, sequences, and steps are so different that the terms mechanism and model (with the associated descriptors) are often associated with much speculation. An example is the ongoing attempts to assign molecular parameters to the higher molecular weight species in viscous feedstocks (such as heavy crude oil, extra-heavy crude oil, and tar sand bitumen as well as atmospheric residua and vacuum residua) (Speight, 2014) and thence basing process designs and reactors design on these assumptions. As worthy is such efforts may seem, the assumptions employed can lead to erroneous design and development of refinery processes. As any chemically reacting system proceeds from reactants to products, a number of species (reactive intermediates) are produced, reach a reaction-specific concentration, and then move to produce the products.
Tribochemistry
Published in Czesław Kajdas, Ken'ichi Hiratsuka, Tribocatalysis, Tribochemistry, and Tribocorrosion, 2018
Czeslaw Kajdas, Ken’ichi Hiratsuka, Gustavo Molina, Akira Sasaki, Roberto C. Dante
Because the binding energy of the extra electron to M is usually less than the ionization energy of A, the first reaction is typically endothermic and can only occur if A and M contain sufficient energy, referred to as translational/internal energy [86]. Further details along with references until year 1992 are described and discussed in [1]. Electron attachment mass spectroscopy (EA-MS) allows determination of the types of reactive intermediates formed from organic compounds [87]. Under conditions of EA-MS, negative ion spectra can be obtained.
Alkyl Halides and Substitution Reactions
Published in Michael B. Smith, A Q&A Approach to Organic Chemistry, 2020
A reactive intermediate is the product of a reaction that is high in energy, and reacts further to give a stable, isolable product. The most common reactive intermediates that will be seen in this book involve carbon atoms including carbocations (also known as carbenium ions, such as those seen in the reactions of alkenes and alkynes with mineral acids in Section 7.3). Other intermediates include carbanions and carbon radicals.
Understanding oxidative addition in organometallics: a closer look
Published in Journal of Coordination Chemistry, 2022
Nabakrushna Behera, Sipun Sethi
Some examples of oxidative addition as well as ortho-metalation which occur intramolecularly are depicted in Scheme 17. Compounds 41a and 41 b undergo ortho-metalation rearrangement intramolecularly with elimination of MeH to produce 41a′ and 41b′, respectively [37]. This shows that the aromatic C–H is more reactive than the C–F bond towards PtII center. In contrast, compounds 41c-e give 41c′-e′ following oxidative addition [37]. These aryl-halogen oxidative additions occur with cis stereochemistry at platinum. Metals having higher oxidation state in the complexes, as in the case of RhIII and IrIII, are capable of exhibiting ortho-metalation intramolecularly, provided it has to be initially induced to expel some ligands to acquire coordinative unsaturation status [30]. In Scheme 18a, the elimination of benzene from 42 upon thermolysis at 58 °C in cyclohexane-d12results in a reactive d8-complex which subsequently undergoes ortho-metalation to 43 [30, 38]. Similarly, a molecule of hydrogen is lost from 44, triggered by photolysis, to give a reactive intermediate Cp*Ir(PPh3), after which it rearranges to 45 with ortho-metalation (Scheme 18b) [30, 39].
Green synthesis of mesoporous MoS2 nanoflowers for efficient photocatalytic degradation of Congo red dye
Published in Journal of Coordination Chemistry, 2021
Haseeb Ullah, Zaibunisa Khan, Jamal Abdul Nasir, Timuçin Balkan, Ian S. Butler, Sarp Kaya, Zia ur Rehman
A possible mechanism for the photocatalytic CR degradation under visible light irradiation results from the small band gap (1.75 eV), i.e. a large portion of the light can be absorbed by the MoS2 NFs, is shown in Figure 6. Absorption of photons can occur if the energy of the trapped photons is greater than the band gap of the photocatalyst. The photogenerated electrons are readily excited from the valance band (VB) to the conduction band (CB). The excited electrons can produce reactive intermediate products, such as superoxide radical (•O2−) [59] and hydroxyl radical (•OH) [60], as well as hydrogen by direct reduction reactions. Photoinduced holes (h+) can react with water and OH− in the dye solution thus generating •OH and some of these radicals can directly conduct oxidative degradation due to their strong oxidizing ability [61]. The intermediates and radicals generated can react with the CR molecules and degrade them into smaller molecules such as H2O and CO2. The proposed stepwise photodegradation pathway is illustrated in Equations (7)–(12):
Can (H2O) n (n = 1–2) as effective catalysts in the CH2OO + H2S reaction under tropospheric conditions?
Published in Molecular Physics, 2020
Rui Wang, Mingjie Wen, Xu Chen, Yongqi Zhang, Ximei Geng, Yingshi Su, Meng Liang, Xianzhao Shao, Wei Wang
As a highly reactive intermediate, the simplest Criegee intermediate, CH2OO, is known to play an important role in atmospheric oxidation chemistry and possibly in aerosol formation [1,2]. This intermediate can react quickly with SO2 with a rate coefficient of 4 × 10−11 cm3·molecule−1·s−1. Also, it can react with H2O, however, the rate of CH2OO + H2O reaction is significantly lower than that of CH2OO + SO2 reaction in the atmosphere [3–6]. H2S is not only a significant part of the atmospheric sulfur cycle [7,8], but also a factor in the formation of H2SO4, which contributes to aerosol nucleation and acid rain [9]. Since H2S only differs from H2O by the period of the central atom, the addition reaction of CH2OO + H2S has been widely studied in experimental [10] and theoretical [10,11] investigation.