Tellurium-based oxidant

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. 

Around the same time, but slightly later, Edison (later Montedison) was also developing a similar process, but based on different catalysts (tellurium-cerium oxides) [7]. Although at that time the performance of these catalysts were equal or slightly superior to bismuth molybdenum oxides, the process was never commercialized. 

Comprehensive reviews covering the electrochemical data for selenium- and tellurium-based tetrachalcogenafulvalenes have appeared, as mentioned in the introduction <87SR155,93SR245). Table 2 gives the half-wave oxidation potentials for selected representative examples of the selenium- and tellurium-based donors. 

The most effective molybdenum-based oxide catalyst for propane ammoxidation is the Mo-V-Nb-Te-0 catalyst system discovered and patented by Mitsubishi Chemical Corp., Japan, U.S.A. (140). Under single-pass process conditions, acrylonitrile yields of up to 59% are reported, whereas under recycle process feed conditions, the acrylonitrile selectivity is 62% at 25% propane conversion (141). Although the latter results show that the catalyst operates effectively under recycle feed conditions, the catalyst system was originally disclosed for propane ammoxidation under single-pass process conditions. The catalyst was derived from the Mo-V-Nb-0 catalyst developed by Union Carbide Corp. for the selective oxidation of ethane to ethylene and acetic acid (142). The early work by Mitsubishi Chemical Corp. used tellurium as an additive to the Union Carbide catalyst. The yields of acrylonitrile from propane using this catalyst were around 25% with a selectivity to acrylonitrile of 44% (143). The catalyst was also tested for use in a regenerative process mode much like that developed earlier by Monsanto (144) (see above and Fig. 8). Operation under cyclic reduction/reoxidation conditions revealed that the performance of the catalyst improved when it was partially reduced in the reduction cycle of the process. Selectivity to acrylonitrile reached 67%, albeit with propane conversions of less than 10%, since activity in... 

Early catalysts for acrolein synthesis were based on cuprous oxide and other heavy metal oxides deposited on inert siHca or alumina supports (39). Later, catalysts more selective for the oxidation of propylene to acrolein and acrolein to acryHc acid were prepared from bismuth, cobalt, kon, nickel, tin salts, and molybdic, molybdic phosphoric, and molybdic siHcic acids. Preferred second-stage catalysts generally are complex oxides containing molybdenum and vanadium. Other components, such as tungsten, copper, tellurium, and arsenic oxides, have been incorporated to increase low temperature activity and productivity (39,45,46). 

Soda. Ash Roasting. Some of the first processes to recover selenium on a commercial basis were based on roasting of copper slimes with soda ash to convert both selenium and tellurium to the +6 oxidation state. Eigure 1 shows flow sheets for two such processes. Slimes are intensively mixed with sodium carbonate, a binder such as bentonite, and water to form a stiff paste. The paste is extmded or peUetized and allowed to dry. Care in the preparation of the extmdates or pellets is required to ensure that they have sufficient porosity to allow adequate access to the air required for oxidation. 

The heavier chalcogens are more prone towards secondary interactions than sulfur. In particular, the chemistry of tellurium has numerous examples of intramolecular coordination in derivatives such as diazenes, Schiff bases, pyridines, amines, and carbonylic compounds. The oxidation state of the chalcogen is also influential sulfur(IV) centres engender stronger interactions than sulfur(II). For example, the thiazocine derivative 15.9 displays a S N distance that is markedly longer than that in the corresponding sulfoxide 15.10 (2.97 A V5. 2.75-2.83 A, respectively). ... 

N,N -Chelation is also exhibited by the dianionic P(III)/P(V) ligands (25) in the MejSn complex (31) [39] and in the magnesium complex (32) [40], which is prepared by oxidation of [Mg(thf)2[ BuNP(p-N Bu)2PN Bu] by elemental tellurium [40]. One of the endocychc N Bu groups in (32) is also weakly coordinated to magnesium, thus providing an intramolecular base-stabihzation similar to that observed for complexes of type (8). 

In the processes described in which the tellurium is precipitated in the elementary form, it is generally assumed (see p. 365) that the error due to oxidation of the precipitate is practically negligible under the conditions of the experiment. Browning and Flint,6 however, maintain that the results are liable to be inaccurate owing to this oxidation. Tellurium dioxide, on the other hand, is unaffected by the air, is anhydrous, non-hygroscopic and easily obtained in the pure condition, and Browning and Flint base a method for the estimation of tellurium on precipitation as dioxide. The tellurium compound is precipitated from a faintly acid solution by means of ammonia, the acidity being restored by the cautious addition of acetic acid. The mixture is heated for some time to render the precipitate crystalline. The method is applicable to the separation of tellurium from selenium.7... 

Test of the Uptake Model Based on the Assumption That Diffusion within the Particle is Rate Controlling. As discussed earlier, the plots of molybdenum and tellurium oxide vapor uptake data vs. diameters and diameters squared of the clay loam particles gave inconclusive evidence as to whether the rate-controlling step was a slow rate of reaction at the surfaces of the particles or a slow rate of diffusion of the condensed vapor into the particles. 

Test of Uptake Model Based on a Slow Surface Reaction Combined with Diffusion within the Particle. Since the simple diffusion model is inadequate to describe the uptake behavior of the molybdenum and tellurium oxide vapors by the clay loam particles, a more complex model is required, in which the effects of a slow surface reaction and of diffusion of the condensed vapor into the particle are combined. Consider the condensation of a vapor at the surface of a substrate (of any geometry) and the passage by diffusion of the condensed vapor through a thin surface layer into the body of the substrate. The change in concentration of solute per unit volume in the surface layer caused by vapor condensa-... 

---