Oxidation at low temperature

Friedel-Crafts. 2-Phenylpropanol results from the catalytic (AlCl, FeCl, or TiCl reaction of ben2ene and propylene oxide at low temperature and under anhydrous conditions (see Friedel-CRAFTS reactions). Epoxide reaction with toluene gives a mixture of 0-, m- and -isomers (75,76). 

The promotor effect of SO2 increases with the amount added to the reaction medium (Fig.3). An effect of the addition of sulfur dioxide has also been observed on the oxidation of decane with an increase of the activation energy expected for such a poisoning. This addition leads to a noticeable decrease of the rate of oxidation at low temperature, where Cu sulfate is stable, but the effect becomes negligible at about 600 K. At this temperature, the conversion of decane estimated by the evolution of the peak e/m = 57, characteristic of the hydrocarbon, is close to 100% with CufTi02 catalysts in presence or not of SO2 (Figure 4). With Cu/Zr02 SO2 inhibits decane oxidation below 640 K. At 640 K a conversion of about 60% is observed in both the presence or absence of additive and an acceleration of oxidation is noticed at higher temperatures. 

Together with the fast oxidation (at low temperatures) of NO to N02, the plasma causes the partial HC oxidation (using propylene, the formation of CO, C02, acetaldehyde and formaldehyde was observed). Both the effects cause a large promotion in activity of the downstream catalyst [86]. For example, a "/-alumina catalyst which is essentially inactive in the SCR of NO with propene at temperatures 200°C allows the conversion of NO of about 80% (in the presence of NTP). Formation of aldehydes follows the trend of NO concentration suggesting their role in the reaction mechanism. Metal oxides such as alumina, zirconia or metal-containing zeolites (Ba/Y, for example) have been used [84-87], but a systematic screening of the catalysts to be used together with NTP was not carried out. Therefore, considerable improvements may still be expected. 

The conventional selective reduction of NOx for car passengers still competes but the efficient SCR with ammonia on V205/Ti02 for stationary sources is not available for mobile sources due to the toxicity of vanadium and its lower intrinsic activity than that of noble metals, which may imply higher amount of active phase for compensation. As illustrated in Figure 10.9, such a solution does not seem relevant because a subsequent increase in vanadium enhances the formation of undesirable nitrous oxide at low temperature. Presently, various attempts for the replacement of vanadium did not succeed regarding the complete conversion of NO into N2 at low temperature. Suarez et al. [87] who investigated the reduction of NO with NH3 on CuO-supported monolithic catalysts... 

Listed in the Table 6.1 are some of the more common sources of oxygen employed for oxidations of organic compounds. Dioxygen is not listed because it requires a catalyst for oxidation at low temperatures. Likewise, hydrogen peroxide and ozone exhibit different activities when used with the proper heterogeneous catalyst. 

Kisailus, D., Choi, J.H., Weaver, J.C., Yang, W. and Morse, D.E. (2005) Enzymatic synthesis and nanostructural control of gallium oxide at low temperature. Advanced-Materials, 17, 314—318. 

The constitution of the new compound was proved22 by the isolation of an acid XX after oxidation at low temperature with potassium permanganate in alkaline solution. This acid was converted to its diethyl ester (XXI), the melting point and other properties of which coincide with those of a compound synthesized by Fischer and Hussong.23... 

The thermal conductivity of bulk silicon (148 W K m ) is dominated by phonons electronic contributions are negligible. Due to restrictions of the mean free path of phonons in the porous network the thermal conductivity of micro PS is reduced by two or three orders of magnitude at RT, compared to the bulk value. Because of the larger dimensions of its network, meso PS shows a thermal conductivity several times larger than that of micro PS, for the same value of porosity. Thermal oxidation at low temperatures (300°C) is found to decrease the thermal conductivity of meso PS by a factor of about 0.5 [Pe9]. In contrast to bulk Si the thermal conductivity of PS is found to decrease with decreasing temperature [Be21, La4, Ge9, Lyl]. 

Thus, it has been shown that, in SOMC-catalyzed hydrogenolysis on an oxide at low temperature, side phenomena due to adsorption or chain walking could occur. These results with linear alkanes and polymers bring a better understanding of the catalytic activity of the ZrH catalyst. 

Using this scheme, we can track the original alkyl radical through the most likely mechanisms for oxidation at low temperatures that lead to chain-branching. Once formed from the parent molecule (R—H), an alkyl radical (R ) can react with molecular oxygen to form an alkene and hydroperoxyl (HO2 ) radical [Equation (2)], via... 

Nitrogen pentoxide is obtained by dehydration of pure nitric acid by phos-phorus(V) oxide at low temperatures around -10°C ... 

Bromo- and iododibenzofurans readily form Grignard reagents that undergo the usual reactions. They have been oxidized at low temperature by oxygen to give phenols, usually in low yield.2-Methoxy-dibenzofuran-l-ylmagnesium bromide, however, affords 2-methoxy-l-di-benzofuranol in 71% yield under these conditions. ... 

The catalytic homogeneous oxidation at low temperatures is therefore economically interesting, but also very difficult to achieve due to the high stability of CH-bonds. Partial oxidation is particularly hard in alkanes as classical oxidation procedures tend to over oxidize them. In the case of methane this would result in the formation of CH2O, CO and CO2. Low valent transition metals, however, are capable of activating the CH bond and rendering that problem less important as the difference in reactivity between the CH bond in methane and methanol is not that big. 

Traces of water can enhance the rate of CO oxidation at low temperature but if the water concentration is too high, much higher temperatures are required in order to avoid the reduction of cationic gold to metallic gold [264]. Haruta et al. [265] and Idakiev et al. [266] studied Ti02 and mesoporous 2 and both demonstrated their... 

Aromatic and aliphatic aldehydes can be oxidized after careful and individual optimization of the reaction conditions to carboxylic acids (Eq. (7), Table 12). With aromatic aldehydes yields are excellent, with aliphatic aldehydes good to satisfactory. The electrolyte has to be less alkaline than normal to suppress the aldol condensation. 2-Phenylpropanol is best oxidized at low temperatures to render the cleavage to benzoic acid more difficult, at 70 °C benzoic acid becomes main product (47 %). Double bonds in y,8- or even a,P-position are not touched in the oxidation. 

Hexafluoropropene (29) can be oxidized in a similar way as tetrafluoroethene, but it can also be oxidized by different methods. Gas-phase oxidations generally lead to mixtures of products. A mixture of products is obtained in the photoinitiated oxidation of 29 with oxygen in both the liquid and gaseous state in addition to low conversions into perfluoro(2-methyloxirane) (30) and other volatile compounds, a clear viscous polymer is obtained as the major product.46 The oxidation at low temperatures gives the epoxide as the dominant product,47 but the conversion is only 24%. Epoxide 30 is obtained in moderate yield, among other products, when a mixture of hexafluoropropene and oxygen difluoride is irradiated with UV light.48 Without irradiation, the mixture does not react at room temperature.48... 

In air, the mechanical properties are influenced by the oxidation processes [543], In materials with a fine overall porosity the oxidation at > 1100 °C closes the pores with the help of an Si02 surface layer. This layer protects the material from further oxidation and heals surface defects. This and the formation of compressive stresses due to the different thermal expansion coefficients between Si02 and RBSN are the reasons for strength increase after oxidation. Materials with a high amount of macropores (>1 pm) oxidise not only at the surface but also inside the volume due to longer closing times of the surface pores. In consequence these oxidation mechanisms result in more intensive oxidation at low temperatures < 1100 °C, due to the slow rate of pore closure and higher internal oxidation. 

The initiation efficiency and the thermal stability in step-by-step reactions are essentially linked to the dissociation energy, Ed, of the weakest bond. In the frame of oxidation processes, it is clear that the weakest bond is the 0-0 bond of peroxides (140 kJ mol-1, against typically 350 kJ mol-1 for a -C-C- aliphatic bond, and 380 kJ mol-1 for a C-H bond in an aliphatic methylene). This explains the quasi-exclusive role played by peroxides in the initiation of chain oxidation at low temperatures. 

Ito, T., Watanabe, T., Tashiro, T., Toi, K. (1989) Reaction of preadsorbed Methane with Oxygen over Magnesium Oxide at low Temperatures, J. Chem. Soc., Farad. Trans. 185, 2381-2395. 

Automobile and Hydrocarbon Emissions. The oxidation of carbon monoxide and hydrocarbons is catalyzed by platinum/palladium/rhodium on alumina. If catalyst poisons such as lead and phosphorus are not present, the major problems become initiation of oxidation at low temperature, thermal stability at high temperature, resistance to thermal schock, and a high external surface area catalyst configuration. 

---