Light alkanes selective oxidation

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. 

For the non-oxidative activation of light alkanes, the direct alkylation of toluene with ethane was chosen as an industrially relevant model reaction. The catalytic performance of ZSM-5 zeolites, which are good catalysts for this model reaction, was compared to the one of zeolite MCM-22, which is used in industry for the alkylation of aromatics with alkenes in the liquid phase. The catalytic experiments were carried out in a fixed-bed reactor and in a batch reactor. The results show that the shape-selective properties of zeolite ZSM-5 are more appropriate to favor the dehydroalkylation reaction, whereas on zeolite MCM-22 with its large cavities in the pore system and half-cavities on the external surface the thermodynamically favored side reaction with its large transition state, the disproportionation of toluene, prevails. 

This paper summarized our current understanding of the factors that determine selectivity for dehydrogenation versus formation of oxygen-containing products in the oxidation of light alkanes. From the patterns of product distribution in the oxidation of C2 to C6 alkanes obtained with supported vanadium oxide, orthovanadates of cations of different reduction potentials, and vanadates of different bonding units of VO in the active sites, it was shown that the selectivities can be explained by the probability of the surface alkyl species (or the... 

The low cost of light alkanes and the fact that they are generally environmentally acceptable because of their low chemical reactivity have provided incentives to use them as feedstock for chemical production. A notable example of the successful use of alkane is the production of maleic anhydride by the selective oxidation of butane instead of benzene (7). However, except for this example, no other successful processes have been reported in recent years. A potential area for alkane utilization is the conversion to unsaturated hydrocarbons. Since the current chemical industry depends heavily on the use of unsaturated hydrocarbons as starting material, if alkanes can be dehydrogenated with high yields, they could become alternate feedstock. 

After this paper was accepted for publication in November, 1992, a number of reports have appeared that deal with the subject of oxidative dehydrogenation of light alkanes. The effect of the structure of vanadia on a support has been investigated for the oxidation of butane [87J and propane [88-90], The evidence supports the concepts that the bridging oxygen in V — O — V plays an important role in the oxidation reaction [87, 90], The data also show that vanadia species of different structures on these supports have different catalytic properties, and that isolated V04 units are the most selective [91]. 

The selectivity to each product is defined on the basis of CO consumed. The oxide precursor W03 gives mainly linear alkanes (68%) but also methanol and ethanol (20% alcohols). WC leads to higher alkanes (up to C13) with a selectivity of about 80% to hydrocarbons and the formation of light alkanes is lower than on W03. The selectivity to hydrocarbons and alcohols of WC resembles more that of W03 than W2C. The catalytic behavior of W2C is very different. It did not produce any alcohol and its selectivity to alkanes is larger than for WC (87%). 

The abundance and low cost of light alkanes have generated in recent years considerable interest in their oxidative catalytic conversion to olefins, oxygenates and nitriles in the petroleum and petrochemical industries [1-4]. Rough estimates place the annual worth of products that have undergone a catalytic oxidation step at 20-40 billion worldwide [4]. Among these, the 14-electron selective oxidation of -butane to maleic anhydride (2,5-furandione) on vanadium-phosphorus-oxide (VPO) catalysts is one of the most fascinating and unique catalytic processes [4,5] ... 

This paper is an attempt to summarize the situation with respect to the selective catalytic oxidation of light alkanes using heterogeneous catalysts. Methane oxidation reactions and the oxidation of butane to maleic anhydride will only be alluded to occasionally, because they have been reviewed in detail in a large number of papers. 

It is the aim of this contribution to highlight a few promising directions for research in the area of selective reactions of light alkanes with oxygen (oxidation and oxidative dehydrogenation). We shall emphasize three aspects ... 

We believe that supports could play a more important role in the oxidation of light alkanes than it did in allylic oxidation. But this role will be complex, and include better dispersion of the active phase, stabilisation of the selective phase, control of oxido-reduction, and/or facilitation of oxygen spillover. 

These results question the validity of many previous results on catalytic oxidation of light alkanes. One should reassess the data concerning the relative reactivity of the various alkanes [105] and selectivity. 

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