Molybdenum-uranium oxide catalyst

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). 

A study has been undertaken to compare the effectiveness of molybdenum and uranium oxide and iron sodalite catalysts with the homogeneous gas-phase oxida-hon of methane [54]. Catalyst performance was evaluated in a high-pressure annular reactor and data were compared to the reactivity of the empty reactor. It was concluded that none of the catalysts gave any advantage over the homogeneous reaction. Indeed, using a catalyst only reduced the selectivity to the desired partial oxidation products. Similar conclusions have been reached for many catalysts used for the partial oxidation of methane, and therefore it is perhaps not surprising that uranium oxide catalysts are no different... 

Nozaki F, Ohki K (1972) A study of catalysis by uranium oxide and its mixed catalysis, 3. Comparison of uranium oxide catalysts with vanadium oxide, molybdenum oxide and tungsten oxide catalysts for catalytic oxidation of carbon monoxide. Bull Chem Soc Jap 45 9473... 

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. 

Molybdenum In its pure form, without additions, it is the most efficient catalyst of all the easily obtainable and reducible substances, and it is less easily poisoned than iron. It catalyzes in another way than iron, insofar as it forms analytically easily detectable amounts of metal nitrides (about 9% nitrogen content) during its catalytic action, whereas iron does not form, under synthesis conditions, analytically detectable quantities of a nitride. In this respect, molybdenum resembles tungsten, manganese and uranium which all form nitrides during their operation, as ammonia catalysts. Molybdenum is clearly promoted by nickel, cobalt and iron, but not by oxides such as alumina. Alkali metals can act favorably on molybdenum, but oxides of the alkali metals are harmful. Efficiency, as pure molybdenum, 1.5%, promoted up to 4% ammonia. 

Shortly after the introduction of the bismuth molybdate catalysts, SOHIO developed and commercialized an even more selective catalyst, the uranium antimonate system (4). At about the same time, Distillers Company, Ltd. developed an oxidation catalyst which was a combination of tin and antimony oxides (5). These earlier catalyst systems have essentially been replaced on a commercial scale by multicomponent catalysts which were introduced in 1970 by SOHIO. As their name implies, these catalysts contain a number of elements, the most commonly reported being nickel, cobalt, iron, bismuth, molybdenum, potassium, manganese, and silica (6-8). 

It is claimed that a catalyst mixture of 93% uranium oxide and 7% molybdenum oxide gives relatively high yields. The oxidation is sometimes carried out in the liquid phase by using manganese dioxide/sulfuric acid at 40°C. 

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... 

Like the original uranium-antimony oxide catalyst, the titanium substituted catalysts were able to operate only a short time without regeneration. Otherewise, the catalyst became overreduced, the USb3O2 Q type phase decomposed, and selectivity suffered. The addition of small amounts of molybdenum or vanadium prevented over-reduction enabling the catalyst to operate without regeneration. 

Replacing 0.10 atom of uranium with molybdenum stabilized the catalyst with only a small effect on activity and acrylonitrile selectivity (Table VI). Further replacement of uranium by molybdenum markedly reduced catalyst activity, so that contact time had to be increased to maintain a high conversion. The addition of molybdenum resulted in a greater production of by-product HCN and correspondingly less carbon oxides (Table VII). A similar effect was obtained with vanadium. However, the vanadium seemed to increase activity as well as stabilize the catalyst. 

In the cooling-tube system, one catalytic screen only was used, hence, all the air that was to be introduced had to be given to the reaction at one time. With more than one catalytic screen, the air is introduced in portions before entering each screen, being distributed among the screens usually in equal amounts. The triple-screen system also makes possible the use of different catalysts. It has been found that uranium oxide catalyzes the oxidation to the aldehyde stage, and at present uranium oxide is used for screen 1 and the molybdenum oxide mixture for screens 2 and 3. An example of the experimental result is shown -Ba... 

Hydroxybenzonitrile can be synthesized directly from p-cresol over the bismuth-molybdenum oxide, iron-antimony oxide or uranium-antimony oxide catalysts [81] normally used for the ammoxidation of propylene, although the catalysts are rapidly deactivated by coke-like deposits [81]. 

Recent work has concentrated on the adjustment of conditions to give a single product, primarily by using aqueous hydrogen peroxide at low temperatures (5° to 15°c) in the presence of catalytic concentrations of soluble tungsten, molybdenum and uranium oxides . Oxidations with tungsten catalysts have been reviewed In this way, ketoximes can be obtained in high yields fix)m secondary-alkyl primary amines (equation 74) , but the rate is slow with bulky... 

The preliminary catalyst evaluation was carried out with slurries of thorium oxide fired at 900°C. Subsequent experience with molybdenum oxide has indicated that it is inactive with low-fired oxides, and its activity at least at low concentrations is decreased by the presence of uranium oxide (see the following discussion). Hence in slurry systems using low-fired thorium oxide or thorium-uranium oxides, silver, palladium, and platinum, which are active in these slurries, may prove to be useful [154]. 

Weiss and Downs described the use of vanadium pentoxide for the catalytic oxidation of toluene and naphthalene. Subsequently, Graver suggested a mixed oxide catalyst derived from uranium oxide and molybdenum trioxide in molar ratios ranging from 3-13 1. Copper oxide was also suggested as a possible promoter. When using the vanadium pentoxide catalyst, benzoic acid was the main product at temperatures below 400°C, with some benzaldehyde formed at higher temperatures. Selectivity was only about 50% at 5% conversion. 

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. 

Today, the air oxidation of toluene is the source of most of the world s synthetic benzaldehyde. Both vapor- and Hquid-phase air oxidation processes have been used. In the vapor-phase process, a mixture of air and toluene vapor is passed over a catalyst consisting of the oxides of uranium, molybdenum, or related metals. High temperatures and short contact times are essential to maximize yields. Small amounts of copper oxide maybe added to the catalyst mixture to reduce formation of by-product maleic anhydride. 

The manner in which this equilibrium is influenced by changes in temperature and pressure should be reviewed. In practice, ammonia is produced by the Haber process at temperatures ranging from 400 to 600°C and at pressures between 200 and 1000 atm. Catalysts that are suitable for use in this process include a mixture of the oxides of iron, potassium, and aluminum iron oxide alone mixtures of iron and molybdenum the metals platinum, osmium, uranium and a number of others as well. 

The early catalyst for AN production was a multicomponent metal oxide, mainly consisting of bismuth and molybdenum oxides. Its composition has evolved over the past 40 years, constantly improved by continuous development work for increasingly better performances. Other catalytic materials that have been used in commercial processes include in their compositions, iron-antimony oxides, uranium-antimony oxides and tellurium-molybdenum oxides. 

Derivation (1) Air oxidation of toluene with uranium or molybdenum oxides as catalysts (2) reaction of benzyl dichloride with lime (3) extraction from oil of bitter almond. 

Unless otherwise noted, catalysts were prepared by coprecipitating the hydrous oxides of uranium, antimony, and a tetravalent metal from a hydrocholoric acid solution of their salts by the addition of ammonium hydroxide. The precipitates were washed, oven dried, then calcined at 910 C overnight or at 930 C for two hours to form crystalline phases. Attrition resistant catalysts, containing 50% by weight silica binder, were prepared by slurrying the washed precipitate with silica-sol prior to drying. In some cases, small amounts of molybdenum or vanadium were added by impregnating the oven dried material with ammonium paramolybdate or ammonium metavanadate solution. The details of these preparations may be found elsewhere (5-8). 

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