Uranium-based catalysts

A common feature of catalysts based on 4 and 5f block elements is that of being able to polymerize both butadiene and isoprene to highly cistactic polymers, independently of the ligands involved. Butadiene, in particular, can reach a cistacticity as high as 99% with uranium based catalysts (3) and cistacticity of > 98% with neodymium based catalysts (4). This high tacticity does not change with the ligand nature (Fig. 1) in contrast to conventional catalysts based on 3-d block elements. A second feature of f-block catalysts is that the cis content of polymer is scarcely... 

The catalyst activity is so high that uranium concentration lower than 0.1 millimoles per liter allows a complete conversion of butadiene to be obtained in a few hours, at 20°C, The transfer reaction of uranium based catalyst is similar to that of conventional 3d-block elements (titanium, cobalt, nickel) so that the molecular weight of the polymer is affected by polymerization temperature, polymerization time and monomer concentration in the customary way. This is in contrast, as we shall see later on, to some catalysts based on 4 f-block elements. Uranium based catalysts are able to polymerize isoprene and other dienes to high cis polymers the cis content of polyisoprene is 94%, somewhat inferior to titanium based catalysts. In contrast, with 3d-block elements an "all cis", random butadiene-isoprene... 

It has to be mentioned that shortly before attention returned to T.n-catalysts actinides came into the focus of industrial research when the potential of uranium-based catalysts was recognized by Eni and later by... 

Bayer [41-49]. Uranium-based catalysts yield BR and IR with a significantly higher cis- 1,4-content than the established Co- and Ti-catalysts. Because of radioactive residues present in the respective polymers, however, the efforts aiming at the large-scale application of uranium catalysts were abandoned soon after by both companies. 

More-detailed studies of uranium-based catalysts for NO reduction and CO oxidation have been published and concentrate on catalyst characterizahon [67]... 

Catalysts based on uranium oxide are also particularly active for the destruction of the chlorinated VOCs chlorobenzene and chlorobutane [77]. Both were destroyed by U3O8 at 350°C and 70,000 h space velocity, showing 99.7% and >99.5% conversions respectively. Time-on-line studies for the destruction of 0.12% chlorobenzene at 450°C showed that the catalyst was not deactivated as 99.9% conversion was maintained during 400 hours continuous operation. These catalysts were also active for the oxidative abatement of other VOCs and it has been demonstrated that toluene, butylacetate and cyclohexanone can also be destroyed at relatively low temperatures. Considering the high space velocities employed in these studies, uranium based catalysts are amongst some of the most active oxide catalysts investigated for VOC destruction. 

Taylor, H. and O Leary, R. A study of uranium oxide based catalysts for the oxidative destruction of short chain alkanes, Appl. Catal, B Environmental, 2000, Volume 25, Issues 2-3, 137-149. 

Predominantly cis-1,4-polybutadiene is produced by coordination polymerization with mixed catalysts.187,487,488 Three catalyst systems based on titanium, cobalt, or nickel are used in industrial practice. Iodine is an inevitable component in titanium-alkylaluminum sytems to get high cis content. Numerous different technologies are used 490,491 A unique process was developed by Snamprogetti employing a (Tr-allyl)uranium halide catalyst with a Lewis acid cocatalyst.492-494 This catalyst system produces poly butadiene with 1,4-ris content up to 99%. 

Historical Uses of Uranium Oxides as Catalysts 545 Table 13.2 Catalytic data for toluene oxidation using uranium oxide based catalysts [6]. 

More commonly, uranium has been used as a catalyst component for mixed-metal oxide catalysts for selective oxidation. Probably the most well known of these mixed oxide catalysts are those based on uranium and antimony. The uranium-antimony catalysts are exceptionally active and selective and they have been applied industrially. An interpretation of the catalyst structure and reaction mechanism has been reported by GrasselU and coworkers [42, 43] who discovered the catalyst The USb30io mixed oxide has been extensively used for the oxidation/ammoxida-tion reaction of propylene to acrolein and acrylonitrile. The selective ammoxida-tion of propylene was investigated by GrasseUi and coworkers [44], and it has been demonstrated that at 460 °G a 62.0% selectivity to acrolein with a conversion of 65.2% can be achieved. Furthermore, Delobel and coworkers [45] studied the selective oxidation of propylene over USb30io, which at 340 °C gave a selectivity to acrolein of 96.7%. 

The selective oxidation of toluene has been studied over a number of catalysts based on metal oxides, with the U/Mo oxide system being one of the most achve and selective[50, 51]. The main products in the oxidation of toluene, excluding the non-oxidative coupling products, were benzaldehyde, benzoic acid, maleic anhydride, benzene, benzoquinone, CO and CO2. Under the same reachon condihons toluene may also yield coupling products such as phthalic anhydride, methyldi-phenylmethane, benzophenone, diphenylethanone and anthraquinone, as shown by Zhu and coworkers [51]. A range of different uranium-based oxides were tested [51] and the results obtained are shown in Table 13.4. 

Derivation (1) From propylene oxygen and ammonia with either bismuth phosphomolybdate or a uranium-based compound as catalysts (2) addition of hydrogen cyanide to acetylene with cuprous chloride catalyst (3) dehydration of ethylene cyanohydrin. 

The hydrogen peroxide method has been used for determining titanium in uranium alloys [83], ilmenite ore [84], and silicon-based catalysts [85]. Titanium was determined in steel by derivative spectrophotometry [86]. 

The Haber-Bosch catalytic process for production of ammonia is perhaps an invention that had the most dramatic impact on the human race (Ritter 2008). The inexpensive iron-based catalyst for ammonia synthesis, which replaced the original, more expensive osmium and uranium catalysts, made it possible to produce ammonia in a substantially effective manner. The objective here was not improvement in selectivity but higher reaction rates for rapid approach to the equilibrium conversion at the specified temperatme and pressme. Higher rates meant lower catalyst volume and smaller high-pressme reactors. The iron catalyst was improved by addition of several promoters such as alkali metals. In contrast to this simple single reaction case of ammonia synthesis, most organic reactions are complex with multiple pathways. 

Hutchings, G., Heneghan, C. and Taylor, S. (1996). Uranium-oxide-based Catalysts for the Destmction of Volatile Chloro-organic Compounds, Nature, 384, pp. 341-343. 

Taylor, S. and O Leary, S. (2000). A Study of Uranium Oxide Based Catalysts for the Oxidative Destmction of Short Chain Alkanes, App/. Catal. B Environ., 25, pp. 137-149. 

Hutchings, G.J., Heneghan, C.S., Hudson, I.D., and Taylor, S.H. Uranium-oxide-based catalysts for the destruction of volatile chloro-organic compounds. Nature 1996, 384, 341-343. 

While for the early demonstration units, osmium and uranium had been used, it was the promoted iron (magnetite) catalyst developed by Mittasch that opened the door to commercialization of the Haber-Bosch process. Osmium had to be ruled out for cost and availability reasons, uranium is impracticable due to its sensitivity for permanent oxygen compound poisoning. Emphasizing the outstanding work of Haber, Bosch, and Mittasch, magnetite-based catalysts are still state of the art today. 

The preparation and structure determination of ferrocene marked the beginning of metallocene chemistry Metallocenes are organometallic compounds that bear cyclo pentadiemde ligands A large number are known even some m which uranium is the metal Metallocenes are not only stucturally interesting but many of them have useful applications as catalysts for industrial processes Zirconium based metallocenes for example are the most widely used catalysts for Ziegler-Natta polymerization of alkenes We 11 have more to say about them m Section 14 15... 

Although acrylonitrile manufacture from propylene and ammonia was first patented in 1949 (30), it was not until 1959, when Sohio developed a catalyst capable of producing acrylonitrile with high selectivity, that commercial manufacture from propylene became economically viable (1). Production improvements over the past 30 years have stemmed largely from development of several generations of increasingly more efficient catalysts. These catalysts are multicomponent mixed metal oxides mostly based on bismuth—molybdenum oxide. Other types of catalysts that have been used commercially are based on iron—antimony oxide, uranium—antimony oxide, and tellurium-molybdenum oxide. 

Homopolymerization of Butadiene. It appeared to us that catalysts based on f-transition metals were the ones most likely to enable us to prepare polybutadiene with an extremely high cis content. We began by investigating catalysts based on uranium compounds. Two such systems were known at the beginning of our work. 

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