Hydrocarbons, selective oxidation

Substantial improvements in the performance of several processes of hydrocarbon selective oxidation can be achieved solely by developing new reactor configurations. An important step in this direction is exemplified by the circulating fluidized bed reactor, which over the years has been proposed for use in several selective oxidation reactions and has, finally, found application in w-butane selective oxidation to maleic anhydride. Although production at the plant (built in Spain) was later stopped, because it was uneconomic, it remains an interesting example that may find application in other reactions. 

As for the participation of intermediate O and O species in selective run of the hydrocarbon oxidation, there are some doUbt on such mechanism. The reason for this conclusion can be the observed increase of the processes selectivity with the decrease of O and O concentration [16] and reaching the maximum selectivity at surface oxygen radicals content of about zero. So, it should be assumed mechanism for the hydrocarbons selective oxidation involving oxygen from surfece groiqs V=0. The same conclusion has been drawn for the bulk V2OS-P2OS catalysts in butene-1 [7] and benzene oxidation [17]. 

They catalyse the hydrocarbon selective oxidation at sufficiently low temperatures isobutene at 300-320 K, propene at 310-350 K, n-butane at 330-350 K. 

Liu J, liu R, Li H, Kong W, Huang H, Liu Y, Kang Z. Au nanoparticles in carbon nanotubes with high photocatalytic activity for hydrocarbon selective oxidation. Dalton Trans 2014 43 12982-8. 

Thompson, D. and Hodnett, B. (2008). Hydrocarbon Selective Oxidation on Vanadium Phosphorus Oxide Catalysts Insights from Electronic Structure Calculations, Top. Catal, 50, pp. 116-123. 

D. J. HucknaU, Selective Oxidation of Hydrocarbons, Academic Press, Inc., London, 1974. 

A derivative of the Claus process is the Recycle Selectox process, developed by Parsons and Unocal and Hcensed through UOP. Once-Thm Selectox is suitable for very lean acid gas streams (1—5 mol % hydrogen sulfide), which cannot be effectively processed in a Claus unit. As shown in Figure 9, the process is similar to a standard Claus plant, except that the thermal combustor and waste heat boiler have been replaced with a catalytic reactor. The Selectox catalyst promotes the selective oxidation of hydrogen sulfide to sulfur dioxide, ie, hydrocarbons in the feed are not oxidized. These plants typically employ two Claus catalytic stages downstream of the Selectox reactor, to achieve an overall sulfur recovery of 90—95%. 

In a similar manner, Sahner et al. [76, 77] utilized a Pt-ZSM-5 layer to reduce the cross-sensitivity of a hydrocarbon (propane) sensor toward CO, propene, H2, and NO at 673 K. The zeolite layer was put on the sensor as a paste. The improved cross-sensitivity is attributed to selective oxidation of aU considered components except propane. Trimboli et al. [78] demonstrated the same concept by using a Pt-Y zeohte for the CO oxidation, maintaining the sensitivity for propane. 

Spectroscopy of the PES for reactions of transition metal (M ) and metal oxide cations (MO ) is particularly interesting due to their rich and complex chemistry. Transition metal M+ can activate C—H bonds in hydrocarbons, including methane, and activate C—C bonds in alkanes [18-20] MO are excellent (and often selective) oxidants, capable of converting methane to methanol [21] and benzene to phenol [22-24]. Transition metal cations tend to be more reactive than the neutrals for two general reasons. First, most neutral transition metal atoms have a ground electronic state, and this... 

Beside their use in equilibrium-restricted reactions, CMRs have been also proposed for very different applications [6], like selective oxidation and oxidative dehydrogenation of hydrocarbons they may also act as active contactor in gas or gas-liquid reactions. 

This method is especially valid for the preparation of gold NPs mixed with activated carbon, which are active and stable for the selective oxidation of hydrocarbons and alcohols in water. Over activated carbon gold could not be directly deposited as NPs by using the techniques described above, such as DP and even by GG. Gold colloids with mean diameters from 2.5 to lOnm stabilized by poly vinyl alcohol or poly vinyl p5rrolidone are used. 

Attempts to achieve selective oxidations of hydrocarbons or other compounds when the desired site of attack is remote from an activating functional group are faced with several difficulties. With powerful transition-metal oxidants, the initial oxidation products are almost always more susceptible to oxidation than the starting material. When a hydrocarbon is oxidized, it is likely to be oxidized to a carboxylic acid, with chain cleavage by successive oxidation of alcohol and carbonyl intermediates. There are a few circumstances under which oxidations of hydrocarbons can be synthetically useful processes. One group involves catalytic industrial processes. Much effort has been expended on the development of selective catalytic oxidation processes and several have economic importance. We focus on several reactions that are used on a laboratory scale. 

Selective oxidations are possible for certain bicyclic hydrocarbons.285 Here, the bridgehead position is the preferred site of initial attack because of the order of reactivity of C—H bonds, which is 3° > 2° > 1°. The tertiary alcohols that are the initial oxidation products are not easily further oxidized. The geometry of the bicyclic rings (Bredt s rule) prevents both dehydration of the tertiary bridgehead alcohols and further oxidation to ketones. Therefore, oxidation that begins at a bridgehead position... 

Other successful selective oxidations of hydrocarbons by Cr(VI) have been reported— for example, the oxidation of c/s-decalin to the corresponding alcohol—but careful attention to reaction conditions is required. 

Microporous catalysts such as MAPO-36 (43,44), which are excellent for selective oxidation of hydrocarbons (45), are highly beam-sensitive. Yet HRTEM... 

Scheme 9.1 shows a generalized sequence of reactions for the oxidation of an alkane, via alcohol, ketone and carboxylic acid, to the completely oxidized products, water and carbon dioxide. The latter are often referred to as combustion products as they are the same as those formed by burning hydrocarbons. These are not normally desirable chemical products unless it is necessary to destroy a toxic, hazardous or otherwise unwanted waste material. Oxidation itself is not difficult to achieve, and is a highly exothermic or even explosive process. Selective oxidation, however, is a much greater challenge, as it is important to stop the sequence at the desired product without proceeding further down the oxidation pathway. 

The activity of elemental carbon as a metal-free catalyst is well established for a couple of reactions, however, most literature still deals with the support properties of this material. The discovery of nanostructured carbons in most cases led to an increased performance for the abovementioned reasons, thus these systems attracted remarkable research interest within the last years. The most prominent reaction is the oxidative dehydrogenation (ODH) of ethylbenzene and other hydrocarbons in the gas phase, which will be introduced in a separate chapter. The conversion of alcohols as well as the catalytic properties of graphene oxide for liquid phase selective oxidations will also be discussed in more detail. The third section reviews individually reported catalytic effects of nanocarbons in organic reactions, as well as selected inorganic reactions. 

Together with glutathione conjugation, hydration is a major pathway in the inactivation and detoxification of arene oxides. Exceptions to this rule will be treated when discussing polycyclic aromatic hydrocarbons. Arene oxides are good substrates for microsomal EH, as evidenced in Table 10.1, where hydration of selected arene oxides, alkene oxides, and cy-cloalkene oxides by purified rat liver epoxide hydrolase is compared. The hy- ... 

An interesting variant of selective oxidation catalysis is the catal)hic reduction of nitrogen oxides by ammonia or hydrocarbons. In this case the reducing molecule should specifically react with NO or NO2, that are present in very low concentrations, rather than become oxidized to CO2 + H2O by reacting with O2 that is present in much higher concentration. The distinction between oxo-ions and oxide particle is most convenient if these entities are present on a zeolite support. Techniques have been developed to study the interconversion by IR spectroscopy ... 

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