Deactivated carbons

A multiply bonded nitrogen atom deactivates carbon atoms a or y to it toward electrophilic attack thus initial substitution in 1,2- and 1,3-dihetero compounds should be as shown in structures (110) and (111). Pyrazoles (110 Z = NH), isoxazoles (110 Z = 0), isothiazoles (110 Z = S), imidazoles (111 Z = NH, tautomerism can make the 4- and 5-positions equivalent) and thiazoles (111 Z = S) do indeed undergo electrophilic substitution as expected. Little is known of the electrophilic substitution reactions of oxazoles (111 Z = O) and compounds containing three or more heteroatoms in one ring. Deactivation of the 4-position in 1,3-dihetero compounds (111) is less effective because of considerable double bond fixation (cf. Sections 4.01.3.2.1 and 4.02.3.1.7), and if the 5-position of imidazoles or thiazoles is blocked, substitution can occur in the 4-position (112). 

A multiply bonded nitrogen atom deactivates carbon atoms a or t to it toward electrophilic attack thus initial substitution in 1,2- and 1,3-dihetero compounds should be as shown in structures (136) and (137). Pyrazoles (136 Z=NH), isoxazoles (136 Z = 0), isothiazoles (136 Z=S), imidazoles (137 Z=NH, tautomerism can make the 4- and 5-positions equivalent) and thiazoles (137 Z=S) do indeed... 

One of the most critical issues in developing catalytic reformers, especially for the reforming of hydrocarbon fuels, is the risk of carbon deposition on the catalyst surface and consequent catalyst deactivation. Carbon formation can occur in several regions of the steam reformer where hot fuel gas is present. Natural gas for example will decompose when heated in the absence of air or steam at temperatures above 650 °C via pyrolysis reaction as shown in Equation 2.4. 

According to these results, the activity decrease in catalysts regenerated by H2 may be attributed to the permanency of deactivating carbon forms and not to the Ni sintering, while the regeneration in O2 removes completely the carbonous forms but it allows metallic sintering. 

The mechanism of synergism in pairs of carbon black and sulfur-containing compounds is quite unknown. It is assumed only that the structure of the carbon black plays an important role here. Carbon blacks subjected to pyrolysis entirely lose their protective action and their ability to manifest synergism with antioxidants. It has been shown [76] that polyacenes (model of deactivated carbon black), for example, tetracene, pentacene, and perylene possess high effectiveness in conjunction with phenol sulfides and thiols. Thus, the addition of 0.1% perylene and 0.1% iS-naphthyl mercaptan greatly decelerates the oxidation of polyethylene at 140°C. In this case autocatalysis is not observed for more than 2000 hr. 

Michaels C A, Mullin A S, Park J, Chou J Z and Flynn G W 1998 The collisional deactivation of highly vibrationally excited pyrazine by a bath of carbon dioxide excitation of the infrared inactive (10°0), (02°0), and (02 0) bath vibrational modes J. Chem. Phys. 108 2744-55... 

Rosser W A Jr, Sharma R D and Gerry E T 1971 Deactivation of vibrationally excited carbon dioxide (001) by collisions with carbon monoxide J. Chem. Phys. 54 1196-205... 

Both objectives have been met by designing special hydrogenation catalysts The most frequently used one is the Lindlar catalyst, a palladium on calcium carbonate combi nation to which lead acetate and quinoline have been added Lead acetate and quinoline partially deactivate ( poison ) the catalyst making it a poor catalyst for alkene hydro genation while retaining its ability to catalyze the addition of H2 to the triple bond... 

The earhest frothing process developed was the Dunlop process, which made use of chemical gelling agents, eg, sodium fluorosiUcate, to coagulate the mbber particles and deactivate the soaps. The Talalay process, developed later, employs freeze-coagulation of the mbber followed by deactivation of the soaps with carbon dioxide. The basic processes and a multitude of improvements are discussed extensively in Reference 3. A discussion more oriented to current use of these processes is given in Reference 115. 

Hydrogenation of the oxides of carbon to methane according to the above reactions is sometimes referred to as the Sabatier reactions. Because of the high exothermicity of the methanization reactions, adequate and precise cooling is necessary in order to avoid catalyst deactivation, sintering, and carbon deposition by thermal cracking. 

The carboxyl group of acids appears to deactivate the hydrogens on the alpha carbon atom toward attack by the free-radical flux in oxidation reactions. Acetic acid, therefore, is particularly inert toward further oxidation (hydrogens are both primary and deactivated) (48). For this reason, it is feasible to produce acetic acid by the oxidation of butane (in the Hquid phase), even under rather severe oxidation conditions under which most other products are further oxidized to a significant extent (22). 

Shift Conversion. Carbon oxides deactivate the ammonia synthesis catalyst and must be removed prior to the synthesis loop. The exothermic water-gas shift reaction (eq. 23) provides a convenient mechanism to maximize hydrogen production while converting CO to the more easily removable CO2. A two-stage adiabatic reactor sequence is normally employed to maximize this conversion. The bulk of the CO is shifted to CO2 in a high... 

The heat released from the CO—H2 reaction must be removed from the system to prevent excessive temperatures, catalyst deactivation by sintering, and carbon deposition. Several reactor configurations have been developed to achieve this (47). 

The reaction is carried out over a supported metallic silver catalyst at 250—300°C and 1—2 MPa (10—20 bar). A few parts per million (ppm) of 1,2-dichloroethane are added to the ethylene to inhibit further oxidation to carbon dioxide and water. This results ia chlorine generation, which deactivates the surface of the catalyst. Chem Systems of the United States has developed a process that produces ethylene glycol monoacetate as an iatermediate, which on thermal decomposition yields ethylene oxide [75-21-8]. 

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