Catalyst alumina-supported rhenium oxide

In contrast to the X-ray diffraction pattern of alumina-supported rhenium oxide, the pattern for the silica-supported samples gives diffraction lines characteristic of metallic rhenium. The metal particle size is about 75 A. Initial kinetic studies with propylene indicated that the silica-supported samples were inactive for the disproportionation reactions up to 180°C. X.p.s. studies of rhenium-supported catalysts show that the state of the initial and reduced rhenium on silica surface is quite different from that on 7-alumina and is dependent on the rhenium compound used to prepare the catalysts. Because of a stronger interaction of Re with the alumina surface, the reducibility of rhenium on alumina is much less than on silica. 

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 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]. 

CO oxidation, 28 108 iron catalyst, 30 168 kinetics, 28 250-257 complicated, 28 257-263 latest developments in, 5 1 over amorphous metal alloys, 36 372-374 over iron, 36 24-25 on alumina support, 36 47 antipathetic behavior, 36 150, 152 particle size and, 36 131-132 promotion by potassium, 36 36-37 over rhenium. 36 24-25 promotion by potassium, 36 37 photocatalysis over perovskites, 36 304 Anunoxidation, 30 136-137 allyl alcohol, 30 157-158... 

The physical and chemical nature of the rhenium in platinum-rhenium catalysts has been considered by a number of investigators. Johnson and Leroy (63) concluded that the rhenium is present as a highly dispersed oxide at typical reforming conditions. They studied a series of alumina-supported platinum-rhenium catalysts with platinum contents ranging from 0.31 to 0.66 wt% and rhenium contents ranging from 0.20 to 1.18 wt%. Their conclusions were based on measurements of hydrogen consumption during reduction of the catalysts at 482°C and on X-ray diffraction studies of the metal component of the catalyst after the alumina had been leached from the catalyst by treatment with a solution of fluoboric acid. 

Later experiments by McNicol (66) and by a group of French workers (67-72), in which the water formed during reduction was removed by a trap at 78°K, were consistent with the results of Webb in showing a change in oxidation state of rhenium from +7 to 0. These workers indicated also that the properties of alumina-supported platinum-rhenium catalysts depend on the method of preparation. 

Based on TPR results it can be concluded that intimate contact between rhenium and platinum is provided in bimetallic alloy particles on the surface of alumina supported catalysts. Therefore, one can expect that the oxidation of the catalyst followed by reduction at moderate temperature result in the formation of platinum and/or Re-Pt metallic nanoclusters and rhenium ions in atomic closeness. 

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. 

Molybdenum and rhenium oxide eatalysts based on siliceous mesoporous sieves and OMA, respectively, proved enhanced catalytic activity in comparison witii corresponding catal) using conventional supports. The origin of tiiis enhanced activity is not completely clear and is still a subject of discussions and continuous research. The better accessibility of catalysts site located in mesopores undoubtedly represents tire essential contribution to the increased catalytic activity. Rhenium (VII) oxide on organized mesoporous alumina preserves known tolerance of Re catalysts to the polar-substituted olefins. The presence of cocatalysts like Mc4Sn is essential similarly as for conventional systems. However, tire catalysts with higher pore size were found to deliver better results. 

BP has developed an even more active rhenium oxide catalyst supported on alumina that operates at 20°-50°C, with an equilibrium conversion higher than 60%. This follows from an observation by Philhps that alumina is a more suitable support than silica. 

Purely parallel reactions are e.g. competitive reactions which are frequently carried out purposefully, with the aim of estimating relative reactivities of reactants these will be discussed elsewhere (Section IV.E). Several kinetic studies have been made of noncompetitive parallel reactions. The examples may be parallel formation of benzene and methylcyclo-pentane by simultaneous dehydrogenation and isomerization of cyclohexane on rhenium-paladium or on platinum catalysts on suitable supports (88, 89), parallel formation of mesityl oxide, acetone, and phorone from diacetone alcohol on an acidic ion exchanger (41), disproportionation of amines on alumina, accompanied by olefin-forming elimination (20), dehydrogenation of butane coupled with hydrogenation of ethylene or propylene on a chromia-alumina catalyst (24), or parallel formation of ethyl-, methylethyl-, and vinylethylbenzene from diethylbenzene on faujasite (89a). 

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 ]. 

These heterogeneous catalysts consist of muitimetallic clusters, containing metals, such as platinum, iridium, or rhenium, supported on porous acidic oxide supports, such as alumina. The catalysts are said to be bifunctional because both the metal and the oxide play a part in the reactions. The metal is believed to carry out reversible dehydrogenation of paraffins to olefins, while the oxide is believed to carry out isomerization. 

Some of the earliest catalysts for this transformation were heterogeneous metal oxides (typically of tungsten, molybdenum or rhenium) on a support such as silica or alumina. These are still the catalysts employed for all current large scale industrial processes. Ill-defined homogeneous systems comprising alcohol solutions of a metal halide in conjunction with a promoter were also employed extensively in the early years, and are currently still used for the... 

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