Dehydrogenation in light alkane oxidation

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]. 

Finally, we mention supported molten metal catalysis (SMMC), in which molten metal catalysts are dispersed as nanodroplets or as thin film on the surface of porous supports. Supported salt melts provide a well-defined volume, accessible to few reactant components, with a surface that is dynamically restructuring to give access to metal cations. The supported molten salt forms a thin layer on the top of the support that is stable up to high temperatures (600 °C). Usually, the whole surface is covered, but micro- and small meso-pores are preferentially filled. Such catalysts possess very interesting properties for the oxidative dehydrogenation of light alkanes [138]. 

Combinatorial chemistry can perhaps help discover new catalyst formulations for reactions presently of particular interest, such as oxidations or ammoxidation, and generally all reactions of alkanes. Reactions traditionally made in different kinds of processes are frequently shown to be also activated by heterogeneous catalysts (e.g., epoxidations). Reactors of unexpected design allow surprisingly selective reactions (e.g., monoliths for the oxidative dehydrogenation of light alkanes). However, the distance often remains long between these discoveries and the manufacture of active and selective catalysts adequately structured for particular use in an industrial reactor inserted in an industrial plant. 

One oxidation reaction that is of large industrial relevance is the oxidative dehydrogenation of light alkanes to the corresponding alkene (Scheme 3.20). This reaction has been reported to be promoted by r-GO as catalyst [29]. The importance of this reaction type is particularly high for the industrial preparation of propene from propane and butenes from butanes. Both reactions are carried out industrially in very large scale, because propene is the monomer of polypropene and also the starting material of propylene oxide, acrylonitrile, and other base chemicals. Butenes are mainly used for the preparation of 1,3-butadiene that is one of the major components of rubbers and elastomers. 

Scheme 3.22 Types of active centers presumed in SWCNTs (carboxylic, carbonyl, and hydroxyl groups) and their selective protection to check the activity of the modified SWCNTs for the oxidative dehydrogenation of light alkanes.

Nakagawa K, Kajita C, Ikenaga NO, Gamo MN, Ando T, Suzuki T (2003) Dehydrogenation of light alkanes over oxidized diamond-supported catalysts in the presence of carbon dioxide. Catal Today 84 149-157... 

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. 

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 ... 

Since the introduction of the Cyclar process by BP(1), much work has been carried out on the Influence of the procedure for incorporation of dehydrogenating components (Ga or Zn) into the ZSM-5 zeolite, as well as on the mechanism of the conversion of light alkanes to aromatics (2-7). The differences observed among the different catalyst preparations are related to the dispersion and the stoichiometry of the metal oxide. Indeed changes in selectivity to aromatics with time on stream (8) or with a pretreatment of the catalyst with have been associated with the formation of lower valences of Ga as the active dehydrogenating species (6). 

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