Alkanes conversion selectivity

We have summarized below recent results concerning spectroscopic / flow reactor investigations of hydrocarbons partial and total oxidation on different transition metal oxide catalysts. The aim of this study is to have more information on the mechanisms of the catalytic activity of transition metal oxides, to better establish selective and total oxidation ways at the catalyst surface, and to search for partial oxidation products from light alkane conversion. 

The reaction network for isobutane selective oxidation catalyzed by POMs consists of parallel reactions for the formation of methacrolein, methacrylic acid, carbon monoxide, and carbon dioxide. Consecutive reactions occur on methacrolein, which is transformed to acetic acid, methacrylic acid, and carbon oxides. ° Methacrylic acid undergoes consecutive reactions of combustion to carbon oxides and acetic acid, but only under conditions of high isobutane conversion. Isobutene is believed to be an intermediate of isobutane transformation to methacrylic acid, but it can be isolated as a reaction product only for very low alkane conversion. ... 

C-H transformation of alkanes by SET is still a developing area of preparative organic chemistry. Generation of cr-radical cations from alkanes in solution requires strong oxidants, and is achieved by photochemical and electrochemical oxidation. Under these conditions even unstrained strained alkanes may be functionalized readily. The C-H substitution is selective if the hydrocarbon forms a radical cation with a definite structure and/or deprotonation from a certain C-H position of the radical cation dominates. Overoxidations are the most typical side reactions that lead to disubstituted alkanes. This can usually be avoided by running the reactions at low alkane conversions. 

An important goal is, therefore, to develop effective methods for catalytic oxidations with dioxygen, under mild conditions in the liquid phase. Two substrates which are often chosen as models for alkane oxidations are cyclohexane and adamantane. Cyclohexane is of immense industrial importance as its oxidation products - cyclohexanone and adipic acid - are the raw materials for the manufacture of nylon-6 and nylon-6,6. Adamantane is an interesting substrate as the ratio of oxidation at the secondary versus the tertiary C-H bonds is used as a measure of radical versus nonradical oxidation pathways. Industrial processes for the oxidation of cyclohexane, to a mixture of cyclohexanol and cyclohexanone, generally involve low conversions (under 10%). Even at such low conversions, selectivities are modest (70-80%) and substantial amounts of overoxidation products, mostly dicarboxylic acids, are formed. 

The occurrence of consecutive reactions, leading to combustion, which lower the selectivity to MA when the alkane conversion, is increased. At n-butane conversions, up to 60-70%, the extent of the consecutive reaction to give combustion products is not substantial, but the decrease in selectivity becomes dramatic when the conversion exceeds 70-80%. This observation has been attributed to the development of local catalyst overheating associated with the highly exothermic oxidation reactions and to the poor heat-transfer properties of the catalytic material. This problem is obviously more important in fixed-bed rather than mixed (fluidized) reactors, in which the heat transfer is faster. 

Entry Alkane Catalyst TfC) Oxidant/others Conversion (%) Selectivity (%) Products Ref. 

Acetonitrile can be produced by catalytic ammoxidation of ethane and propane over Nb-Sb mixed oxides supported on alumina, with selechvities to acetonitrile of about 50-55% at alkane conversions of around 30% [133]. In both cases, CO forms in approximately a 1 1 molar ratio with acetonitrile, owing to a parallel reaction from a common intermediate. When feeding n-butane, the selectivity to acetonitrile halves. Bondareva and coworkers [134] also studied ethane ammoxidation over similar types of catalyst (V/Mo/Nb/O). 

The work of Guisnet, reviewed in the previous section, shows that the selectivity for alkane conversion strongly depends on the npt/ A ratio. It appears that a balance between the concentration of metal sites and that of the acid sites has to be struck for optimum activity and selectivity of metal/acid... 

Figure 5. Selectivity to the parent olefins(%) as a function of the alkane feeded and of the catalyst. Experimental conditions alkane conversion of 20% reaction temperature of 600 (ethane) or 550 °C (propane and n butane).

Pt-Re content % mass Reaction T C n-hexane conversion % Products yield vol % benzene toluene C,-Cs alkanes Benzene selectivity%... 

Introduction.—The oxidative dehydrogenation of alcohols to aldehydes and ketones over various catalysts, including copper and particularly silver, is a well-established industrial process. The conversion of methanol to formaldehyde over silver catalysts is the most common process, with reaction at 750—900 K under conditions of excess methanol and at high oxygen conversion selectivities are in the region 80—95%. Isopropanol and isobutanol are also oxidized commercially in a similar manner. By-products from these reactions include carbon dioxide, carbon monoxide, hydrogen, carboxylic acids, alkenes, and alkanes. 

Figure 3. Conversion, selectivity, and yield of ethylene formation by ethane oxidation on Pt coated alumina ceramic foam monoliths. Up to 70% C2H4 selectivity is obtained at -70% C2H6 conversion for a single pass yield of -55%. Addition of Sn to Pt increases the selectivity and alkane conversion significantly.

Figure 3 also shows conversion and selectivity to C2H4 with Sn added to the Pt catalyst[15]. Both the alkane conversion and the selectivity to olefins increase significantly with added Sn. X-ray diffraction and XPS of the Pt-Sn catalyst indicate intermetallic compound formation rather than fee metal, and this surface evidently increases the alkane conversion and reduces the decomposition of olefins. 

We have also examined olefin formation from higher alkanes. Propane and butane also produce up to 70% selectivity to olefins on Pt monolith ceramic foam monoliths at nearly 100% O2 conversion with alkane conversions of typically 80% at comparable flow rates and catalyst temperatures to those used for C2H6. However, olefins from these higher alkanes exhibit considerable cracking, with C2H4 the dominant product except at low temperatures and excess fuel. However, isobutane produces primarily isobutylene and C3H6 with litde C2H4. 

The chemical inertness of alkanes can be overcome if the transformations are carried out at high temperatures. However, the low selectivity of such processes motivates chemists into searching principally for new routes of alkane conversion which could transform them into very valuable products (hydroperoxides, alcohols, aldehydes, ketones, carboxylic acids, olefins, aromatic compounds etc.) under mild conditions and selectively. This is also connected with the necessity for the development of intensive technologies and for solving... 

Among catalytic alkane conversions, the most important is the Shilov system and its descendents [108]. Discovered around 1970, these involve Pt(II) salts in aqueous solvents. Initially, the reaction studied was H/D exchange with D2O, where polydeuteration of alkanes was seen. The selectivity for attack at the terminal methyl groups of long chain alkanes made it clear that one was not dealing with classical electrophilic chemistry. The intervention of colloidal Pt was also excluded. 

Another process which is commercially used in several petrochemical processes but not in ammoxidation is recycle (110). A recycle process is characterized by the partial conversion of the hydrocarbon feed while achieving high selectivity to the desired product. This type of process is particulaiy appropriate for an alkane conversion process where selectivity to products is generally inversely proportional to conversion of the alkane. The unreacted alkane is separated from the product and sent back to the reactor wdth fresh alkane feedstock added to replace the portion converted in the cycle. The selectivity to product then is essentially the overall yield of the process. In the case of ammoxidation, recycle not only of the alkane but also of ammonia is feasible and economically propitious (111). 

In spite of these advances in alkane chemistry, the development of a series of robust and selective catalysts for different alkane conversion reactions remains a continuing challenge in organometallic chemistry today. Another related and very challenging problem is C—F activation in perfluorocarbons. 

However, although these types of catalysts present relatively high selectivity to olefins at low alkane conversions in ODH reactions, their main drawback is related to their high catal5Tic activity for deep oxidation of the corresponding olefins. Thus, in most cases, the olefin oxidation is 5 to 10 times faster than the alkane oxydehydrogenation, limiting the yield to C3 or C4 olefins below ca. 40% " (Fig. 24.4). 

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