Molybdenum hexacarbonyl alumina

Infrared spectral studies on molybdenum hexacarbonyl-alumina were reported by Davie, Whan, and Kemball 78). Without any activation procedure they obtained a sharp carbonyl frequency corresponding to unchanged hexacarbon-yl on the support. This material was not active for disproportionation. After treatment for one hour under vacuum at 373 °K the catalyst had lost the sharp carbonyl band but showed two wider and broader bands and was active for dis-proportionating propylene. The authors stated that the active catalyst clearly had a lower symmetry than the hexacarbonyl and must have lost one or more of the carbonyl groups. After exposure of the activated catalyst to air, the catalyst was inactive and showed no absorption in the carbonyl region. 

Figure 9.4 Gas phase impregnation and decomposition of molybdenum hexacarbonyl on a fully hydroxylated alumina surface.

Fig. 3 Surface-reactivity of the molybdenum hexacarbonyl complex as a function of the area density of - OH groups on an alumina surface...

An unanticipated catalytic reaction of olefinic hydrocarbons was described in 1964 by Banks and Bailey.1 2 They discovered that C3-C8 alkenes disproportionate to homologs of higher and lower molecular weight in the presence of alumina-supported molybdenum oxide [Eq. (12.1)], cobalt oxide-molybdenum oxide, molybdenum hexacarbonyl, or tungsten hexacarbonyl at 100-200°C, under about 30 atm pressure ... 

The first catalysis of an olefin metathesis reaction was reported by Banks and Bailey in 1964 (56). They reported that activated molybdenum hexacarbonyl on alumina converted propylene, for example, into ethylene and 2-butene at 150°C and 30 atm. Oxides of rhenium are also powerful heterogeneous catalysts. 

Formally, pentavalent neutral metallocorroles have been prepared by Murakami and coworkers.The first of these was the oxomolybdenum(V) corrole derivative 2.179. ° This complex was prepared by heating free-base corrole 2.82 with molybdenum pentachloride in oxygen-free decalin (Scheme 2.1.56). Alternatively, molybdenum hexacarbonyl (Mo(CO)e) could be used as the metal source. In both cases, oxidation to the oxomolybdenum complex 2.179 was believed to occur during workup (involving chromatography on neutral alumina followed by recrystallization). In this way, complex 2.179 was isolated in c. 40% yield. Similar yields of the oxochromium(V) complex 2.180 could be achieved via the reaction of 2.82 with anhydrous chromium(II) chloride in DMF. Here too, spontaneous oxidation during workup was used to afford the formally pentavalent oxo-complex 2.180. 

Mo(CO)3 ads> which is 100% molecularly dispersed. Above 200° C remaining CO groups of Mo(CO)3 ads ar liberated, but detectable amounts of Mo(CO)2,ads Slid Mo(CO)ads do not accumulate. The likely dominant species after activation at 270° C is (a-0")2Mo . At higher temperatures, e.g 500° C, the zero-valent decarbonylated molybdenum is oxidized by the surface hydroxyl groups of alumina to an average oxidation number of about 5.6. Molybdenum hexacarbonyl on highly dehydroxylated alumina, le., pretreated at 950° C, behaves differently Mo(CO)3 a[Pg.114]

Olefin metathesis—the catalytically induced redistribution of alkyli-dene groups between olefins— is a rather recently discovered process. The first example was the conversion of propylene into ethylene and 2-butene in the presence of molybdenum hexacarbonyl on alumina (82). 

Banks and Bailey [2] discovered the first heterogeneous catalyst in 1964 molybdenum hexacarbonyl deposited on alumina. Since this time a plethora of other systems have been discovered. 

Mixtures of butene, pentene, hexene, and higher olefins are commonly obtained by a-olefin disproportionation over supported heterogeneous catalysts [1]. Examples of disproportionation of propene, 1-butene, 1-pentene, and 1-hexene on alumina-supported molybdenum hexacarbonyl are included in Table 3. Adequate separation units may provide fractions of C4, C5, Cfi, and higher olefins for petrochemical uses. 

As the scope of the reaction increased, the name Olefin Metathesis was introduced by Goodyear, who also pioneered the use of homogeneous catalysts. The reaction had first been recognized during experiments on the development of a heterogeneous catalyst to replace mineral acids in alkylation reactions. Molybdenum hexacarbonyl catalyst supported on alumina produced 2-pentene (40%) from mixed n-butenes together with propylene (51%) and hexene (9%). Tungsten hexacarbonyl was less active than the molybdenum catalyst, and in total contrast, it was found that chromium hexacarbonyl acted as a polymerization catalyst. 

Finally, highly dehydroxylated alumina were prepared by heating at 800 °C under vacuum and their interaction with [Mo(CO)e] followed by infrared [19]. At room temperature, the hexacarbonyl molybdenum complex is physisorbed and the interaction of one oxygen atom of a CO ligand with defective aluminium sites (tetrahedral and octahedral sites were identified)... 

In 1964, Bank and Bailey [2] reported the first catalytic reaction on the same substrate, using heterogeneous catalysts derived from molybdenum or tungsten hexacarbonyles supported on alumina. 

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