Uranium-antimony oxide catalysts activity

Structure and Activity of Promoted Uranium-Antimony Oxide Catalysts... 

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. 

Replacing antimony with zirconium increased catalytic activity 11-fold. Figures 3 and 4 show that activity peaked at x-1.0. The USb2ZrOy catalyst was less selective than the corresponding titanium-substituted catalyst but compared favorably to the old uranium-antimony oxide catalyst. 

Table V shows the effect of Ti, Zr, and Sn addition when excess antimony was present. Although each Increased catalyst activity, the effect was much smaller than for the USb. M Oy compositions. Titanium addition about doubled the relative activity compared to the standard uranium-antimony oxide catalyst, while Zr and Sn addition had a smaller effect. The poor selectivity of the Distillers-type catalyst. No. 9, is attributed to the presence of USbO. ...

The catalytic activity of the uranium-antimony oxide catalyst for propylene ammoxidation has been increased an order of magnitude by modifying the catalytically active phase rather than by adding various promoters to the optimum uranium-antimony oxide composition. This modification was accomplished by substituting titanium, zirconium, or tin for antimony in compositions with the empirical formula USb3 M Oy. Titanium and zirconium replaced... 

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

In the 1960s, a number of binary oxides, including molybdenum, tellurium, and antimony, were found to be active for the reactions and some of them were actually used in commercial reactors. Typical commercial catalysts are Fe-Sb-O by Nitto Chemical Ind. Co. (62 -64) and U-Sb-O by SOHIO (65-67), and the former is still industrially used for the ammoxidation of propylene after repeated improvements. Several investigations were reported for the iron-antimony (68-72) and antimony-uranium oxide catalysts (73-75), but more investigations were directed at the bismuth molybdate catalysts. The accumulated investigations for these simple binary oxide catalysts are summarized in the preceding reviews (5-8). 

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%. 

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. 

Under the reaction conditions used, a U3O8 catalyst demonstrated appreciable selective oxidation activity. The best results, in terms of both activity and selectivity to benzaldehyde, were obtained with the mixed oxides with U Mo atomic ratios in the range 8 2 to 9 1. The maximum yield of benzaldehyde was 40 mol%. On the other hand, antimony-based uranium oxides were not found to be effective as catalyst for this reaction. U—Mo and Bi—Mo mixtures also exhibited promising activity and selectivity to benzaldehyde. Bi—Mo and Bi—Mo—P—Si catalysts were also tested. Qualitahvely there was little difference between the product distributions from the two catalysts. The major products formed were benzaldehyde, benzene and carbon oxides, as well as traces of anthraquinone and benzoic acid. 

While titanium substituted for antimony and this had a dramatic effect on catalytic activity as expected, there is a question as to how much of the uranium was converted from the +5 to the +6 oxidation state. The shifts in the infrared bands indicate a shortening of the bond distance and a lengthening of the Sb-0 bond distance which is consistent with an increase in hexavalent character, but the magnetic measurements show that a substantial portion of the uranium remained in +5 state. If the valence of uranium is not changed, then the replacement of Sb" by Ti must generate oxygen vacancies in the USb Oj Q lattice. It is these sites that may be responsible for the high activity of the promoted catalysts. 

Tn review completely the numerous examples of promoted catalysts, most of which are mentioned in the patent literature, would be entirely out of place here. It is, however, interesting to note that the term was used and the effect noticed early in the industrialization of the water gas reaction.- Additions of the oxide of chromium, thorium, uranium, beryllium, and antimony to the nickel, iron, or cobalt catalysts was found to increase greatly the activity of these materials toward this reaction.24... 

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