Activation of paraffins

On the other hand, it was proposed that acid catalyzed reactions such as skeletal isomerization of paraffin [2], hydrocracking of hydrocarbons [3] or methanol conversion to hydrocarbon [4] over metal supported acid catalysts were promoted by spillover hydrogen (proton) on the acid catalysts. Hydrogen spillover phenomenon from noble metal to other component at room temperature has been reported in many cases [5]. Recently Masai et al. [6] and Steinberg et al. [7] showed that the physical mixtures of protonated zeolite and R/AI2O3 showed high hydrocracking activities of paraffins and skeletal isomerization to some extent. 

Much interest is related to the use of HPC s as catalysts for the oxidative functionalization of light paraffins, since the multifunctional properties of these systems may be of utility for the activation of saturated organic substrates. On the other hand, the high temperatures which are usually necessary to activate the paraffins may be deleterious for the structural stability of HPC s, since the destruction of the primary structure leads to a loss of the unique properties of the compound and to a decrease in catalytic performance. Therefore, it is necessary to use stable salts, which, on the other hand, are less reactive. This seems to leave little room for the design of a catalyst suitable for the activation of paraffins in the gas phase. However, different possibilities exist to resolve these problems ... 

The rational design of catalysts has been a desired aim of catalyst researchers for a long time. Our current attempt at this goal centers on the partial oxidation of paraffins, and entails the incorporation of key catalytic elements into a structural freimework which by its very nature would favor structural isolation of such catalytic functionalities. It is well known by now, that vanadium is one of the key elements for the oxidative activation of paraffins [1-5]. It is also well known, that structural isolation of catalytic moieties is desirable to achieve selectivity to useful oxidized products, thereby preventing overoxidation to waste products, CO and CO2 [6-8]. 

Since naphthenes are saturated hydrocarbons, the chemical activity of the naphthenic compounds is similar to the chemical activity of paraffins. During thermal treatment of the naphthenes, it takes part in reactions involving C-C bond cleavage, dehydration and, to a lesser extent, aromatization reactions. 

The mechanism originally proposed by the writer for the cracking reaction invoked both carbonium ions and carbanions, depending on whether the hydrocarbon had unsaturated or saturated carbon-carbon bonds. With olefins or aromatics protonation to form a carbonium ion is relatively easy. However, at that time it was not obvious how a paraffin can be converted to a carbonium ion. So it was postulated that water might extract a proton from the paraffin to form in this case a carbanion. However, this concept was very soon abandoned because it was inconsistent with the observed rearrangements (isomerization) in reactants and products of cracking. An all-cationic mechanism was then proposed ( ) in which the activation of paraffins occurs by hydrogen transfer to form a carbonium ion intermediate. 

Convincing proof of the carbonium ion mechanism of activation of paraffins was obtained from a detailed study of hydrogen exchange between catalyst and isobutane (10). If the exchange occurs through a carbonium ion mechanism, the theory requires that only primary hydrogens will exchange. The... 

In either VPO or MoVTeNbO catalysts, V-sites are directly involved in the selective oxidative activation of paraffins, while the presence of a second element is generally required in order to facilitate the formation of a defined crystalline phase (i.e. vanadyl pyrophosphate or orthorhombic metal oxide bronze, respectively) (Figs. 24.2a and 24.2c), but also for facilitating the multifuctionality of the catalysts. However, although both catalytic systems are active for the oxidative activation in alkanes, different selectivities are achieved depending on the alkane feed. Thus, a... 

The many examples of recent CH-activation give hope that activation of paraffins by homogeneousatalysts may be achievable in the near future. 

The field began in the 1920s when Cario and Franck discovered how these so-called collisions of the second kind could be put to practical use, e.g., the Hg -sensitized activation of paraffins. This is due to initiation of chain reactions by radical formation,... 

Mobil MTG and MTO Process. Methanol from any source can be converted to gasoline range hydrocarbons using the Mobil MTG process. This process takes advantage of the shape selective activity of ZSM-5 zeoHte catalyst to limit the size of hydrocarbons in the product. The pore size and cavity dimensions favor the production of C-5—C-10 hydrocarbons. The first step in the conversion is the acid-catalyzed dehydration of methanol to form dimethyl ether. The ether subsequendy is converted to light olefins, then heavier olefins, paraffins, and aromatics. In practice the ether formation and hydrocarbon formation reactions may be performed in separate stages to faciHtate heat removal. 

Sulfochlorination of Paraffins. The sulfonation of paraffins using a mixture of sulfur dioxide and chlorine in the presence of light has been around since the 1930s and is known as the Reed reaction (123). This process is made possible by the use of free-radical chemistry and has had limited use in the United States. Other countries have had active research into process optimization (124,125). 

Fig. 10. A model of PVC lubrication mechanism showing (a) PVC adhesion to metal without lubricant (b) surface activity of calcium stearate (c) nonmetal releasing character of paraffin only and (d) synergy between calcium stearate and paraffin (62).

Only trace amounts of side-chain chlorinated products are formed with suitably active catalysts. It is usually desirable to remove reactive chlorides prior to fractionation in order to niinimi2e the risk of equipment corrosion. The separation of o- and -chlorotoluenes by fractionation requires a high efficiency, isomer-separation column. The small amount of y -chlorotoluene formed in the chlorination cannot be separated by fractionation and remains in the -isomer fraction. The toluene feed should be essentially free of paraffinic impurities that may produce high boiling residues that foul heat-transfer surfaces. Trace water contamination has no effect on product composition. Steel can be used as constmction material for catalyst systems containing iron. However, glass-lined equipment is usually preferred and must be used with other catalyst systems. 

JS/oble Metals. Noble or precious metals, ie, Pt, Pd, Ag, and Au, are ftequendy alloyed with the closely related metals, Ru, Rh, Os, and Ir (see Platinum-GROUP metals). These are usually supported on a metal oxide such as a-alumina, a-Al202, or siUca, Si02. The most frequently used precious metal components are platinum [7440-06-4J, Pt, palladium [7440-05-3] Pd, and rhodium [7440-16-6] Rh. The precious metals are more commonly used because of the abiUty to operate at lower temperatures. As a general rule, platinum is more active for the oxidation of paraffinic hydrocarbons palladium is more active for the oxidation of unsaturated hydrocarbons and CO (19). 

The alkanesulfonate melt is treated with 4 bar of water to give a paste containing 60% active matter (12) or pelletized by means of a cooling conveyor with a special feeding device (not shown in Fig. 2). The solid product is composed of 93 wt % of alkanesulfonate, 6 of sodium sulfate, and 1 of paraffin. 

The kinetics of hydrocracking reactions has been studied with real feedstocks and apparent kinetic equations have been proposed. First-order kinetics with activation energy close to 50 kcal/gmol was derived for VGO. The reactions declines as metal removal > olefin saturation > sulfur removal > nitrogen removal > saturation of rings > cracking of naphthenes > cracking of paraffins [102],... 

The increasing volume of chemical production, insufficient capacity and high price of olefins stimulate the rising trend in the innovation of current processes. High attention has been devoted to the direct ammoxidation of propane to acrylonitrile. A number of mixed oxide catalysts were investigated in propane ammoxidation [1]. However, up to now no catalytic system achieved reaction parameters suitable for commercial application. Nowadays the attention in the field of activation and conversion of paraffins is turned to catalytic systems where atomically dispersed metal ions are responsible for the activity of the catalysts. Ones of appropriate candidates are Fe-zeolites. Very recently, an activity of Fe-silicalite in the ammoxidation of propane was reported [2, 3]. This catalytic system exhibited relatively low yield (maximally 10% for propane to acrylonitrile). Despite the low performance, Fe-silicalites are one of the few zeolitic systems, which reveal some catalytic activity in propane ammoxidation, and therefore, we believe that it has a potential to be improved. Up to this day, investigation of Fe-silicalite and Fe-MFI catalysts in the propane ammoxidation were only reported in the literature. In this study, we compare the catalytic activity of Fe-silicalite and Fe-MTW zeolites in direct ammoxidation of propane to acrylonitrile. 

The initiating action of ozone on hydrocarbon oxidation was demonstrated in the case of oxidation of paraffin wax [110] and isodecane [111]. The results of these experiments were described in a monograph [109]. The detailed kinetic study of cyclohexane and cumene oxidation by a mixture of dioxygen and ozone was performed by Komissarov [112]. Ozone is known to be a very active oxidizing agent [113 116]. Ozone reacts with C—H bonds of hydrocarbons and other organic compounds with free radical formation, which was proved by different experimental methods. 

Dichlorine shortens the induction period of autoxidation of paraffin wax [187] and accelerates the oxidation of hydrocarbons [109]. Difluorine is known as very active initiator of gas-phase chain reactions, for example, chlorination [188,189]. 

A quantitative model requires knowledge of the diffusivity under reaction conditions and of the intrinsic activities for toluene disproportionation and xylene isomerization. While these are not easily obtained, the methodology has been worked out for the case of paraffin and olefin cracking (5). So far, we have obtained an approximate value for the diffusivity, D, of o-xylene at operation conditions from the rate of sorptive o-xylene uptake at lower temperature and extrapolation to 482°C (Table V). 

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