Bismuth phosphomolybdate

Invented and developed independently in the late 1950s by D.G. Stewart in the Distillers Company, and R. Grasselli in Standard Oil of Ohio. The former used a tin/antimony oxide catalyst the latter bismuth phosphomolybdate on silica. Today, a proprietary catalyst containing depleted uranium is used. See also Erdolchemie, OSW, Sohio. 

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. 

Oxidation in the original Sohio process941,942 was carried out over a bismuth molybdate catalyst, which was later superseded by bismuth phosphomolybdate with various amounts of additional metal ions (Ce, Co, Ni), and multicomponent metal oxides based on Mo, Fe, and Bi supported on silica. 

The ammoxidation of propene to acrylonitrile is of great industrial importance and accordingly the literature is abundant. The reaction is very similar to the oxidation of propene to acrylonitrile and carried out at the same conditions and over the same kind of catalysts. The famous bismuth phosphomolybdate catalyst developed by Sohio was the first of a series of highly effective mixed-oxide catalysts. The optimum yields are generally obtained at temperatures of 400—500°C. Initial selectivities over 95% and yields up to 80% are feasible. The superior selectivity of the ammoxida-... 

Some of tlie salts of bismuth are used in medicines for the relief of digestive disorders because of the smooth, protective coating the compounds impart to imtated mucous membranes. Like barium, bismuth also is used as an aid in x-ray diagnostic procedures because of its opacity to x-rays. At one time, certain bismuth compounds were used in the treatment of syphilis. Bismuth oxychloride, which is pcarlcsccnt, has found use in cosmetics, imparting a frosty appearance to nail polish, eye shadow, and lipstick, but may be subject to increasing controls. Bismuth phosphomolybdate has been used as a catalyst in the production of acrylonitrile for use in synthetic fibers and paints. Bismuth oxide and subcarbonate are used as fire retardants for plastics. 

Improved catalytic performance, selectivity and resistance to fusion, over bismuth molybdate catalysts was reported by McClellan (90) for catalysts obtained by chemically combining bismuth, molybdenum, phosphorus, and silica. After calcination at 450°C, the bismuth phosphomolybdate-on-silica catalyst showed an X-ray pattern of mainly crystalline Bi2(Mo04)3 which subsequently was converted to a new, substantially amorphous, phase after calcination at 800°C. Substantially morphous meant that the X-ray diffraction lines were broad diffuse bands of low intensity. The pattern of lines for this novel phase indicated a scheelite structure. A special interaction of silica with bismuth molybdate was also suggested by Callahan et al. (91). 

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 modern beginning of the heterogeneous catalytic oxidation of olefins to aldehydes may be taken as the discovery of the oxidation of propylene to acrolein over cuprous oxide by Hearne and Adams (5 ). This reaction has been carried to commercial operation by Shell Chemical Company. More recently, the use of bismuth phosphomolybdate has been demonstrated for the oxidation of propylene to acrolein by Veatch and co-workers (88), and, in the presence of ammonia, to acrylonitrile by Idol (89). It was also shown, by Heame and Furman (90), that diolefins could be made from C4 and higher olefins by oxidative dehydrogenation over a bismuth molybdate catalyst. From these beginnings, information on olefin oxidation has increased very rapidly, both in journal and patent literature. We shall make no attempt to review the large number of patents that have issued, but shall limit ourselves mainly to journal literature. 

A catalyst containing bismuth and molybdenum oxides was used many years ago by Tanner (130) for the oxidation of acetylene, but only recently have similar catalysts been employed for the oxidation of mono-olefins. Often bismuth phosphomolybdates are referred to, although the phosphorus does not appear to be necessary. For simplicity we refer to catalysts with characteristic properties of the bismuth-molybdenum oxide group as bismuth molybdate, and this phrase is not intended to imply a definite crystalline compound. 

Veatch, Callahan, Milberger, and Forman (88) reported that a bismuth phosphomolybdate catalyst was quite selective for oxidation of propylene to acrolein at 450°. The catalyst was compounded with SiOg, and could be used at low CgHg/Og ratio, such as 0.5. At 92 % conversion of propylene a 60% selectivity to acrolein was reported. An outstanding development with catalysts of this type is the conversion of propylene to acrylonitrile by reaction with ammonia and oxygen, as... 

Sachtler (135) and Sachtler and de Boer (136) also found evidence for a symmetric intermediate from the oxidation of propylenes containing radioactive carbon over bismuth molybdate. When the 0 was at either end of the propylene molecule, half of it was found in the carbonyl group of the product acrolein. When it was the middle carbon, no was found in the carbonyl group. McCain, Gough, and Godin (137) found the same results using a bismuth phosphomolybdate supported on silica. 

Carbon dioxide, acetaldehyde, and acrylic acid are formed as side products. A technical breakthrough was achieved by Standard Oil of Ohio (SOHIO) with the discovery of the bimetallic bismuth molybdate and bismuth phosphomolybdate catalysts. Propene is oxidized with air on a Bi203/Mo03 catalyst at 300-400 °C and 1-2 bar in a fixed-bed tubular reactor, which allows effective removal of heat from the exothermic reaction [15]. 

Today the most cost-effective processes are those based on propylene as the starting material. There are three major variations of propylene processes, the Distillers process [21-23], the Sohio process [24], and the DuPont process [25,26]. All three processes are based on the ammonoxidation of propylene. The Distillers process is carried out in two stages. In the first, propylene is oxidized in air to form acrolein and water. These intermediate products are allowed to react in the second stage with ammonia in the presence of molybdenum oxide and air to form crude acrylonitrile. The pure monomer is recovered by a series of azeotropic distillations. The Sohio process is carried out in just one stage. Ammonoxidation of propylene takes place in air at 2-3 atmospheric pressure and 425-510°C. With catalysts, such as concentrated bismuth phosphomolybdate or other oxides of molybdenum and cobalt, the reaction takes place with over 50% yield in a reaction time of only about 15 s. In the DuPont version of this process, the ammonoxidation is brought about with nitric oxide at 500°C using silver on silica catalyst. The chemistry of acrylonitrile monomer has been reviewed by a number of authors [27-30]. 

In this method, large amounts of acetonitrile, CH3CN, occur as a by-product. Catalysis is by vanadium oxide on AI2O3, or borophosphate, titanium phosphate, or bismuth phosphomolybdate on silica acid. The mechanism is still unestablished. On the one hand, acrolein does not appear to be an intermediate product, since it is not formed in the absence of ammonia. On the other hand, acrylonitrile can also be obtained from acrolein with ammonia and oxygen, and molybdenum oxide as catalyst. In another variation, the conversion is not made with ammonia and air, but with nitric oxide ... 

Bismuth phosphomolybdate is an important catalyst which is used for the industrial ammonoxi-dation of propylene to make acrylonitrile. Dawson-type anions will crystallise with organic cations to give microporous cavity structures, for example, [ H3N(CH2)e NH3 jj PjWjgOgj-31120. These structures may have application as shape-selective catalysts. 

Molybdate-Based Catalysts. The first catalyst commercialized by SOHIO for the propylene ammoxidation process was bismuth phosphomolybdate, Bi9PMoi2052, supported on silica (9). The catalytically active and selective component of the catalyst is bismuth molybdate. In commercial fluid-bed operation, the bismuth molybdate catalyst is supported on silica to provide hardness and attrition resistance in the fluidizing environment. Bismuth molybdate catalysts can be prepared by a coprecipitation procedure using aqueous solutions of bismuth nitrate and ammonium molybdate (10). The catal3ret is produced by drying the precipitate and heat treating the dried particles to crystallize the bismuth molybdate phase. Heat treatment temperature for bismuth molybdate catalysts is generally arovmd 500°C. 

With the supply of large amounts of propane in the 1950s the search began to find a system for its direct oxidation with molecular oxygen to yield acrolein. Attempts with cuprous oxide marked the beginning of the technical development of alkene oxidation in the gas phase by metal oxide catalysts [22]. But this system showed weak points in the conversion (20%) [23,24] and in the selectivity, with the consequence that most of the propane added had to be recycled and many side products had to be removed. The development and introduction of the bismuth molybdate/bismuth phosphomolybdate system (Sohio, 1957) as a catalyst [25-27] and the following application for propane... 

Various catalyst systems have been developed to increase the elRciency of the process and these use bismuth phosphomolybdate (as used in the Sohio process) and the mixture of oxides of cobalt, molybdenum, antimony and tin (as described in the Distiller s process). 

SOHIO [Standard OHIO] The Standard Oil Company of Ohio (later BP Chemicals America) has developed many processes, but its Ammoxidation process, for converting propylene to acrylonitrile, is the one mostly associated with its name. The original catalyst was bismuth phosphomolybdate, but the composition has evolved over the years and now may contain one or more of the following Fe, Ni, Co. First operated in the... 

Gupta PK and Ramchandran R (1991) Indirect atomic absorption spectrometric determination of phosphorus in high purity electronic grade silicon using bismuth phosphomolybdate complex. Microchemical Journal 44 34 38. 

It was soon realized that commercial units would be based on the use of fluidized beds, which were then being introduced in refineries to produce gasoline. The most successful fluid bed process was introduced by Idol of Soldo in 1960 and used a bismuth phosphomolybdate catalyst supported on silica. Knapsack described a bismuth phosphomolybdate catalyst containing iron for aciylonitrile production in 1962. This rather more complex mixed oxide formulation, Fe7Bi2Moi2052, also supported on silica, foreshadowed improvements in the 1970s and the introduction of multicomponent catalysts. Since then several generations of improved catalysts have been introduced, e.g., by Sohio, as the reaction mechanism has been better understood. 

A more active bismuth phosphomolybdate (Table 4.13) was prepared simply by adding an appropriate volume of phosphoric acid to the initial solution. A typical catalyst composition was claimed to be Bi9PMoi2052-55 2Si02. The same catalysts could be used to produce both acrolein and acrylonitrile. 

It is conunon practice to use two reactors. The first reactor contains a conventional bismuth phosphomolybdate catalyst and is used to convert propylene to acrolein. The second reactor contains a selective vanadium molybdate catalyst promoted with tungsten, nickel, manganese or copper, " to convert acrolein to acrylic acid. Fixed bed tubular reactors ate used in both stages. Typical operating conditions ate shown in Table 4.15. 

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