Ammoxidation catalyst systems

An in-depth analysis of the solid-state chemistry of the Mo-V-Te-Nb-0 propane ammoxidation catalyst system reveals the details of the two primary phases designated as Ml and M2 (150,151). Correlations of catalytic activity and phase composition for this catalyst system establish the specific functions of the two catalytically active phases (152,153). Specifically, the Ml phase is the phase primarily responsible for propane activation and conversion to acrylonitrile via intermediate, adsorbed propylene. The M2 phase is essentially inactive for propane activation but is capable for conversion desorbed propylene intermediate to acrylonitrile. 

Oxidation Catalysis. The multiple oxidation states available in molybdenum oxide species make these exceUent catalysts in oxidation reactions. The oxidation of methanol (qv) to formaldehyde (qv) is generally carried out commercially on mixed ferric molybdate—molybdenum trioxide catalysts. The oxidation of propylene (qv) to acrolein (77) and the ammoxidation of propylene to acrylonitrile (qv) (78) are each carried out over bismuth—molybdenum oxide catalyst systems. The latter (Sohio) process produces in excess of 3.6 x 10 t/yr of acrylonitrile, which finds use in the production of fibers (qv), elastomers (qv), and water-soluble polymers. 

The reducibility of the catalyst systems was further examined using temperature programmed reduction with a 3X hydrogen/argon gas mixture. The TPR curves shown in Figure 7 illustrate the NnP oxide catalyst is not readily reduced at reaction temperatures. In contrast, the FeNo oxide catalyst begins to reduce at 250 0, and the rate of reduction is fast at temperatures of methanol ammoxidation activity (425 -475°C). The poor lability of lattice oxygen for the HnP oxide catalyst provides additional evidence for a non-redox process. 

Investigations into the scheelite-type catalyst gave much valuable information on the reaction mechanisms of the allylic oxidations of olefin and catalyst design. However, in spite of their high specific activity and selectivity, catalyst systems with scheelite structure have disappeared from the commercial plants for the oxidation and ammoxidation of propylene. This may be attributable to their moderate catalytic activity owing to lower specific surface area compared to the multicomponent bismuth molybdate catalyst having multiphase structure. 

Some progress has been made in explaining the splendid catalytic performance of multicomponent bismuth molybdates that are used widely for the industrial oxidations and ammoxidations of lower olefin. We have seen that the catalytic activity and selectivity are greatly enhanced by the multifunctionalization of the catalyst systems. Many functions newly introduced are... 

More than a decade after the publication of the MoVNb catalyst system, scientists at Mitsubishi Chemical reported that modifying this family of mixed metal oxides with Te produced a catalyst for the amoxidation of propane to acrylonitrile [4] and the oxidation of propane to acrylic acid [5], Modification of the Union Carbide catalyst system with Te was probably not a random choice as it is a known propylene activator [5 b] and the molybdate phase TeMoO oxidizes propylene into acrolein and ammoxidizes propylene to acrylonitrile [6], a key intermediate in the commercial production of acrylic acid using Mo-based oxides. Significant efforts to optimize this and related mixed metal oxides continues for the production of both acrylic acid and acrylonitrile, with the main participants being Asahi, Rohm Hass, BASF, and BP. 

The patenting activity in the field of rare earth catalysts during 1970 and 1985 is illustrated in Fig. 12.3a. The vertical scale is arbitrary and is based on a total of 580 publications in 1985. As compared to 1970, the total published papers increased by four times and the patents by three times. Further, the patent activity showed a shift in emphasis from petroleum refining to pollution control activity. Other commercial catalyst systems are ammoxidation and dehydrogenation in which rare earths play a crucial role. 

The desired selective oxidation reactions are, of course, thermodynamically favorable. The discovery of appropriate catalyst systems is necessary to overcome kinetic limitations. This is a challenging task in that nonselective, complete, or deep oxidations (e.g., formation of CO2, H2O, and HCN) are thermodynamically more favorable than the selective oxidations or ammoxidations (Table I) (4). Therefore, it is necessary to intercept the desired products kinetically. 

Our recent work on the bismuth-cerium molybdate catalyst system has shown that it can serve as a tractable model for the study of the solid state mechanism of selective olefin oxidation by multicomponent molybdate catalysts. Although compositionally and structurally quite simple compared to other multiphase molybdate catalyst systems, bismuth-cerium molybdate catalysts are extremely effective for the selective ammoxidation of propylene to acrylonitrile (16). In particular, we have found that the addition of cerium to bismuth molybdate significantly enhances its catalytic activity for the selective ammoxidation of propylene to acrylonitrile. Maximum catalytic activity was observed for specific compositions in the single phase and two phase regions of the phase diagram (17). These characteristics of this catalyst system afford the opportunity to understand the physical basis for synergies in multiphase catalysts. In addition to this previously published work, we also include some of our most recent results on the bismuth-cerium molybdate system. As such, the present account represents a summary of our interpretations of the data on this system. 

The complex solid state relations of the cerium-molybdenum- tellurium oxide system were studied to determine the boundaries of single phase regions and phase distributions of a typical multicomponent ammoxidation catalyst. Between 400 and 600 C in air the (Ce,Mo,Te)0 system contains the following phases ... 

This work shows the acquired experience in the preparation at pilot-scale of a novel propane ammoxidation catalyst based on a partially nitrided V-Al mixed oxide obtained hy co-precipitation. A systematic investigation of the different parameters controlling the preparation of the catalyst via a co-precipitation route at different scales was carried out. At lab-scale (50 to 100 g), the preparation parameters optimized were precipitation pH, V/Al atomic ratio, V concentration in solution and nitridation conditions, while at pilot-scale (1 kg), the optimized parameters were precipitation and ageing time, solution/solid ratio during the washing step, drying and calcination conditions, and extrusion parameters. Our results show that the optimum preparation conditions for the VAION system are pH = 5.5, V/Al atomic ratio = 0.25, concentration of V species in solution = 30.10 M. This catalyst shows the highest selectivity and yield in acrylonitrile. The samples prepared at different scales show the same activity profile in the propane ammoxidation reaction. 

This catalytic ammoxidation process was truly revolutionary. Since the introduction of this technology, INEOS has developed and commercialized several improved catalyst formulations. These catalyst advancements have improved yields and efficiencies vs. each prior generation to continually lower the cost to manufacture acrylonitrile. INEOS continues to improve upon and benefit from this long and successful history of catalyst research and development. In fact, many of INEOS s licensees have been able to achieve increased plant capacity through a simple catalyst changeout, without the need for reactor or other hardware modifications. INEOS s catalyst system does not require changeout overtime, unless the licensee chooses to introduce one of INEOS s newer, more economically attractive catalyst systems. 

The most industrially significant and well-studied allylic oxidation reaction is the ammoxidation of propylene ( eq. 8 ) which accounts for virtually all of the 8 billion pounds of acrylonitrile produced annually world-wide. The related oxidation reaction produces acrolein ( eq. 9 ), another important monomer. Although ammoxidation requires high temperatures, the catalysts are, in general the same fof both processes and include bismuth molybdates, uranium antimonates (USb30j Q), iron antimonates, and bismuth molybdate based multicomponent systems. The latter category includes many of todays highly selective and active commercial catalyst systems. 

The only molybdate catalyst other than bismuth molybdate-based catalysts used commercially for propylene ammoxidation was that based on Te-Ce-Mo-0. The catalyst system was discovered, patented, and used commercially by Monte-catini Edison SpA in 1960s (18,19). Commercial use of this catalyst system was eventually terminated, in part because of the loss of the volatile tellurium component of the catalyst and subsequent diminution in catalyst performance during use. 

Antimonate-Based Catalysts. In addition to the bismuth-molybdenum oxide catalyst system, several other mixed metal oxides have been identified as effective catalysts for propylene ammoxidation to acrylonitrile. Several were used commercially at various times. In particular, the iron-antimony oxide catalyst is currently used commercially by Nitto Chemical (now Dia-Nitrix Co. Ltd., Japan) and its licensees around the world, although the catalyst was originally discovered and patented by SOHIO (20,21) and by UCB (22). Nitto Chemical improved the basic iron-antimony oxide catalyst with the addition of several elements that promote activity and selectivity to acrylonitrile. Key among these additives are tellurium, copper, molybdenum, vanadium, and tvmgsten (23-25). 

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. 

Another antimony-based oxide catalyst that was used commercially for propylene ammoxidation was Sn-Sb-0. This catalyst system was patented and used commercially in a fixed-bed process by Distillers Co. Ltd. (32). The Distillers catalyst and process remained in use in the United Kingdom at a plant owned and operated by British Petroleum imtil the late 1980s. Although studied extensively... 

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... 

The Nb-Sb-0 ethane ammoxidation catalyst, when supported on alumina, gives about 50% to 55% selectivity to acetonitrile at around 30% conversion of ethane (172,173). Coproducts are ethylene (less than 10% selectivity), CO, and CO2. Selectivity to acetonitrile is close to that obtained with a Co-zeolite catalyst. However, the Nb-Sb-0 catalyst gives more coproducts CO and CO2 and less ethylene than Co-zeolite catalysts. Thus, with recycle of the coproduct ethylene along with the unreacted ethane, the Co-zeolite catalysts is expected to provide higher recycle jdelds of acetonitrile than the AI2O3-supported Nh-Sb-0 catalyst system. 

Typical propane ammoxidation catalysts are essentially constituted by a combination of metallic mixed oxides. To date, there are two catalytic systems i) vanadium-antimonates with a rutile-type structure, represented by the VSbxMyOz formula, where M are elements used as the promoter such as W, Al, Te, Nb, Sn, Bi, Cu, or andii) vanadium-molybdates with a bronze structure, rep-... 

---