Uranium antimony catalyst acrylonitrile

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%. 

GrasseUi, R.K. and Suresh, D.D. Aspects of structure and activity in uranium-antimony oxide acrylonitrile catalysts.,/ Catal 1972, 25, 273-291. 

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. 

We have found that the partial substitution of certain tetravalent metals for pentavalent antimony greatly increases catalytic activity. For example, catalysts with the empirical formula USb2MO2 2 Q, where M=Ti, Zr, or Sn, were respectively 6, 11, and 13 times as active as the original uranium-antimony oxide catalyst, while exhibiting as good or better acrylonitrile... 

Substituting titanium for antimony in the USb Oj g phase dramatically Increased catalytic activity. The relative activity for the USbj. Ti Oy series peaked at x=1.5. The best acrylonitrile selectivity was obtained at x=0.6 and x=1.0. Reduced activity and selectivity at higher titanium levels corresponded to USbO and UTiO formation. The USb2TiOy catalyst seemed to offer the best combination of activity and selectivity. Under optimum conditions (Table IV) it yielded 83-84 mol% acrylonitrile per pass compared to 78% for the old uranium-antimony oxide catalyst (1,2,4) which required six times the contact time to obtain comparable conversions. 

The Sohio technology is based on a catalyst of bismuth an4 molybdenum oxides. Subsequent catalyst improvements came from the use of bismuth phosphomolybdate on a silica gel, and more recently, antimony-uranium oxides. Each change in catalyst was motivated Jby a higher conversion rate per pass to acrylonitrile. 

Acrylonitrile is manufactured by passing propylene, ammonia, and air over a mixed-oxide catalyst at 400-500 C. The process is also a major source of acetonitrile and hydrogen cyanide which are obtained as the result of side reactions. Catalysts used in this process are generally mixed oxides of bismuth or antimony with other multivalent metals such as molybdenum, iron, uranium, and tin. At one time, the preferred catalyst for propylene... 

Another propylene ammoxidation catalyst that was used commercially was U-Sb-0. This catalyst system was discovered and patented by SOHIO in the mid-1960s (26,27). Optimum yield of acrylonitrile from propylene required sufficient antimony in the formulation in order to ensure the presence of the USbaOio phase rather than the alternative uranium antimonate compound USbOs (28-30). The need for high antimony content was understood to stem from the necessity to isolate the uranium cations on the surface, which were presumed to be the sites for partial oxidation of propylene. Isolation by the relatively inactive antimony cation prevented complete oxidation of propylene to CO2. Later publications and patents showed that the activity of the U-Sb-0 catalyst is increased by more than an order of magnitude by the substitution of a tetravalent cation, tin, titanium, and zirconium (31). Titanium was found to be especially effective. The promoting effect results in the formation of a solid solution by isomorphous substitution of the tetravalent cation for Sb + within the catalytically active USbaOio- phase. This substitution produces o gen vacancies in the lattice and thus increases the facility for diffusion of lattice o gen in the solid structure. As is discussed below, the enhanced diffusion of o gen is directly linked to increased activity of selective (amm)oxidation catalysts based on mixed metal oxides. 

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