Carbon forming reactions

As Lewis acid, titanium tetrachloride, boron trifluoride or ethylaluminum dichloride is often used. The stereochemical outcome of the reaction strongly depends on the Lewis acid used. The Sakurai reaction is a relatively new carbon-carbon forming reaction, that has been developed into a useful tool for organic synthesis. ... 

For a clearer understanding of the behavior of syngases in a shift converter, we established another set of carbon isotherms when considering the shift reaction only (without methanation) in addition to the carbon-forming reactions. Figure 6 shows isotherms at a partial pressure of 270 psia for all components of a gas mixture, but excluding methane. 

Table 2 shows the thermodynamics of the three major carbon-forming reactions (6)-(8), at 600°C, 1 atm, using n-Cig and i-Cg as surrogates of diesel and gasoline, respectively. 

These results show that each of the carbon-forming reactions is thermodynamically favorable at typical reforming temperatures. The reactions of n-Ci6 and i-Cg are irreversible at these conditions. 

Table 2 Thermodynamics of Elemental Carbon-Forming Reactions at 600°C, 1 atm (Calculations using HSC Chemistry 4.0 )...

Another effect of aromatics is increased carbon formation, which has long been recognized as the primary means of catalyst deactivation in the ATR of hydrocarbons. Using carbon-forming reactions (6)-(8), an equilibrium line for carbon formation as a function of O2/C and S/C ratios can be calculated. Figure 9 shows the results of this calculation for n-Ci4, along with the experimental results for two Ni-based commercial catalysts. 

Alkali oxides such as K2O are used to minimize carbon formation on the Ni catalyst. The alkali may evaporate at elevated reaction temperatures however, its loss can be controlled by adding acidic components, such as silica.Oxides of alkali earth metals, such as magnesia or calcia are also added to the support to neutralize highly acidic sites, which are mainly responsible for the carbon forming reactions. Compositions of various commercial Ni-based SR catalysts are listed in Table 4. 

It seems clear from this brief overview that, since the seminal work of Mosbach in 1987 [10], imprinted polymers with hydrolytic activity have been going through important developments, which have led to some interesting results. The discussion will therefore move to examine the use of the same format in the application of the imprinting technology to the carbon-carbon forming reactions. 

The simplest sulphoxide, dimethyl sulphoxide, is an important aprotic solvent (Section 4.1.55, p. 412). Its use as a reagent in carbon-carbon forming reactions and as a reagent for the oxidation of alcohols to carbonyl compounds (p. 608) (the Pfitzner-Moffatt and Swern oxidations) has been extensively reviewed.244 An illustrative example of carbon-carbon bond formation using dimethyl sulphoxide is noted in Expt 7.3. 

Computational results are plotted in Figures 2, 3 and 4. The effectiveness factors are very small at elevated temperatures in line with the observations of Van Hook (16). Note that this simulation has been kept as simple as possible for illustrative purposes. At steam-carbon ratios below about 1.4,carbon forming reactions should be considered. The water-gas shift reaction might also be a factor, but the experimental evidence suggests that both CO and CO2 are primary reaction products (16) in agreement with the assumed kinetics model. 

A. Thermodynamics.—The deposition of carbon on catalysts can be a complication both in laboratory investigations and in full-scale processes. Carbon can be formed in many different ways during reforming depending upon the catalyst and reaction conditions, e.g. by catalytic and thermal decompositioii of the hydrocarbon itself, reactions (7) and (8), or of the reaction products, (9) and (10), or by polymerization and dehydrogenation of unsaturated intermediate products. The thermodynamics of the simple carbon forming reactions (8)—(10) were discussed in the earlier review. ... 

Carbon-Carbon Forming Reactions (Single Bonds)... 

The formation of carbon is detrimental to reformer operation. Solid carbon will deposit on the catalyst surface, inhibiting its performance, eventually plugging the tubes, and creating an excessive pressure drop. It is essential to establish process conditions that avoid formation of carbon formation during the reforming reaction. Possible carbon forming reactions are the following ... 

Nickel on an acidic support, such as that used for methane reforming, will promote the desired naphtha decomposition reaction, but it also promotes the cracking and polymerization reactions that are the basis for carbon formation. ICI has solved this problem by incorporating an alkali metal into their catalyst [7]. The alkali accelerates the reaction of carbon with steam (the primary carbon removal reaction) and at the same time neutralizes acidity in the support inhibiting the cracking and polymerization reactions (other carbon-forming reactions). The most effective alkali is K2OH (potash). Most naphtha reformers use the alkalized catalyst developed by ICI [7]. 

Suzuki coupling is a prototypical metal-catalyzed carbon-carbon forming reaction normally conducted in an organic solvent under anaerobic conditions. In this... 

Table 1.5 Basic reaction properties for carbon-forming reactions.

The carbon limits are a function of the atomic ratios 0/C, H/C and inert/C and of total pressure. As the gas is at equilibrium, it is necessary to consider only one of the carbon-forming Reactions R6-R8 in Table 5.2, provided that the whisker stmcture (p ) is independent of the carbonforming reaction. 

The formation of coke by pyrolysis increases strongly with temperatures as indicated in Figure 5.36 [405] which shows results from TGA measurements on cracking of ethylene. With no catalyst, the activation energy was estimated to 458 kJ/mol. In the presence of an alkali-promoted support, (Zr02, 0.5% K) (refer to Section 4.3.3), the coking was retarded, probably by the promotion of alkali of the reverse carbon-forming reaction R8 in Table 5.2 as shown in Reaction (5.13) below ... 

Table A2.3. Carbon-forming reactions. Coefficients in equilibrium function ...

Iron salts are easily accessible, inexpensive and abundant and the metal itself is non-toxic. Their use should therefore become attractive from an economic and environmental point of view in a wide variety of carbohydrate transformations, in either stoichiometric applications or as a catalyst. As stated in the introduction, this review concentrates on a few transformations promoted by ferric salts used as Lewis acids in our laboratories and does not present exhaustive work done in carbohydrate chemistry with these salts. Many more other applications have been reported. However, their uses could be far more developed for fast and selective transformations of carbohydrates to useful new molecular constructs. Besides the acidic properties of iron(iii) presented here, iron chemistry is rich and could be particularly fruitful with carbohydrates in generating new types of complexes for regioselective transformations or in carbon-carbon forming reactions based on iron-catalyzed cross-coupling reactions. The glycochemistry community should certainly expect many more useful accomplishments in the near future. 

Since the discovery of Sc(OTf)3 as a water-compatible Lewis acid, several immobilized scandium catalysts that work efficiently in water have been developed. Polymer-supported scandium-based Lewis acid (7) worked well in several carbon-carbon forming reactions in water (Schemes 12.67-12.69) [168]. It was suggested that the spacer could help to form hydrophobic reaction environments in water. As expected, (7) was easily recovered and reused. 

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