Rhenium oxide metathesis catalyst alumina-supported

In catalyst preparation, one can use this knowledge to determine the relative contributions of various hydroxyl groups before and after application of the active phase onto the support. In this way Sibeijn etal. [31] established that rhenium oxide attached to acidic sites of the alumina support exhibits higher activity for the metathesis of olefins than rhenium oxide on neutral or basic sites. As, however, rhenium species preferentially exchange with basic hydroxyls, one needs to increase the loading above a certain value (6 wt% for an alumina of 200 m2/gram) before the catalyst exhibits appreciable activity [31]. 

Solid catalysts for the metathesis reaction are mainly transition metal oxides, carbonyls, or sulfides deposited on high surface area supports (oxides and phosphates). After activation, a wide variety of solid catalysts is effective, for the metathesis of alkenes. Table I (1, 34 38) gives a survey of the more efficient catalysts which have been reported to convert propene into ethene and linear butenes. The most active ones contain rhenium, molybdenum, or tungsten. An outstanding catalyst is rhenium oxide on alumina, which is active under very mild conditions, viz. room temperature and atmospheric pressure, yielding exclusively the primary metathesis products. 

In 1998, Ookoshi and Onaka reported remarkable increase in activity of M0O3 when this was supported on hexagonal mesoporous silica instead of conventional one. With this catalyst (7 wt % Mo) they achieved high conversion of 1-octene into 7-tetradecene at 50°C. Similarly in 2002, Onaka and Oikawa found rhenium oxide dispersed on mesoporous alumina with uniform pore size (7 wt % Re) to be more active in 1-octene metathesis than rhenium oxide on conventional y-alumina. Although both works lacked detailed characterization of supports and prepared catalysts, they clearly showed the positive effect of organized mesoporous siqrport on catalyst activity in alkene metathesis. 

Olefin metathesis is the transition-metal-catalyzed inter- or intramolecular exchange of alkylidene units of alkenes. The metathesis of propene is the most simple example in the presence of a suitable catalyst, an equilibrium mixture of ethene, 2-butene, and unreacted propene is obtained (Eq. 1). This example illustrates one of the most important features of olefin metathesis its reversibility. The metathesis of propene was the first technical process exploiting the olefin metathesis reaction. It is known as the Phillips triolefin process and was run from 1966 till 1972 for the production of 2-butene (feedstock propene) and from 1985 for the production of propene (feedstock ethene and 2-butene, which is nowadays obtained by dimerization of ethene). Typical catalysts are oxides of tungsten, molybdenum or rhenium supported on silica or alumina [ 1 ]. 

In the J. Heyrovsk Institute we performed a detailed study of (a) M0O3 supported on siliceous mesoporous sieves of several types and (b) rhenium (VII) oxide supported on organized mesoporous aluminas in order to (i) verify positive effect of these supports on catalyst activity, (ii) to find out the most convenient way for catalyst preparation, (iii) to receive detailed information about their catalytic activity and selectivity, and (iv) to show the applicability of these catalysts in different types of metathesis reaction. 

Metathesis disproportionation of propene has been reported on a wide range of heterogeneous catalytic systems based mainly on earbonyl eom-poxmds, as well as oxides or sulfides of molbyde-num, tungsten, or rhenium supported on alumina, silica, or mixtures of oxides or phosphates. Among these catalytic systems, only a restricted number present good activity and sufficient selectivity to allow reasonable yields of ethene and 2-butene. Examples of the most efficient heterogeneous catalysts appear in Table 2 [1, 3, 5-11]. 

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