Chapter 5: Ammoxidation of Propylene and Propane to Acrylonitrile

An account is presented describing the scientific basis that led to the discovery and subsequent commercialization of the 1st ammoxidation catalyst, which led to the development of the now world-famous SOHIO acrylonitrile process. Currently, about 100 plants world wide use this process and produce in excess of 6.5 billion kilograms of acrylonitrile yearly, which is about 1 kg/year for each inhabitant of planet earth. Each world-scale plant employs about 175 000 kg of catalyst, which amounts to a total world wide use of 17.5 million kg of catalyst. Luckily, the newer generations of catalysts have a life expectancy in excess of 10 years. The demand for this versatile product is so high because of its many uses and favorable pricing of the product because of the great efficiency of the newest catalysts and process innovations. Acrylonitrile finds its major uses in the fiber, polymer, rubber and specialty products industries. The scientific basis on which the SOHIO catalysts are based is derived from our, by now, well-recorded “7 principles of ammoxidation catalysis”. The 1st and probably most important of these principles that goes back to the early 1950s is the idea that the lattice oxygen of metal oxides, if properly selected, will serve as a better and more effective oxidant than gaseous oxygen, or surface adsorbed oxygen on metals. In this manner, the lattice oxygen of the metal oxide was to be directly involved in the ammoxidation process. Our 2nd and also very important postulate was the idea of site isolation. Herewith, it is postulated that active sites must be isolated from each other in order to limit the number of active lattice oxygens at the reaction site to achieve the desired selectivity. The 7th and last postulate is that of phase cooperation which states that two catalytic phases having different properties can cooperate with each other in a synergistic way to achieve higher desired yields. This latter postulate is particularly applicable in the newer, more sophisticated catalysts that are multicomponent and multiphase. The 1st-generation commercial catalyst that we discovered after about 2 years of exploratory research in the late 1950, was a BiPMoO catalyst that produced, for that early time a respectable 55% acrylonitrile from propylene. The lifetime was limited to about 2 years. Nonetheless, it was good enough for SOHIO to launch its acrylonitrile process. Interestingly, the 7th and latest generation of catalysts is still based on BiMoO, but is much more complex than the 1st generation and is a multicomponent, multiphase catalytic system, e.g., (K,Cs)(Ni,Co,Mg,Mn)(Fe,Cr)BiMoO. These later-generation catalysts produce about 83% acrylonitile and the systems have a lifetime of about 10 years. All of the commercial catalysts are supported on SiO₂ to impart physical strength and favorable surface area to the catalytic systems. All of these systems are crystalline in nature since it is important that the active phases are well crystallized as microcrystalline solids, because poorly crystalline or amorphous active phase compositions are much less effective catalysts than their crystalline cousins. Although the current catalyst productivity is in the 83% acrylonitrile yield range, there is still room for improvement since the thermodynamic limit lies at 100% acrylonitrile. With the propylene becoming ever more expensive, because of its many uses in the polymer industry, it is becoming ever more attractive to develop a paraffin- (propane-) based acrylonitrile process. Therefore, much research has been expanded along these lines to use propane as feed, which is much less expensive than propylene, being derived from natural gas that is plentiful and cheap. Thus far, the Mitsubishi/Asahi catalyst based on MoV(Nb,Ta)(Te,Sb)O leads the pack with about 65% acrylonitrile yield. The catalytic system is based on two phases, M1 the paraffin-activating phase and M2 the propylene mop-up phase in synergy or symbiosis with each other. We have determined the crystalline structures of these two phases and assigned the active sites, comprised of 7 key catalytic elements, to be located on the ab planes of these microcrystalline phases. All of the elements have their unique catalytic functions and the active sites are isolated from each other by Nb- or Ta-containing pentagonal bipyramids, again in turn obeying our long-ago postulated site isolation principle, to achieve the desired selectivity. The olefin- and paraffin-based ammoxidation area, while relatively mature, still presents challenges to the physical, inorganic, and synthetic chemist, to further explore the nature of the fascinating catalytic materials capable of the complex molecular transformations required of the ammoxidation process, and to discover new and still more effective compositions for this world wide useful product, acrylonitrile. There is still further to go to achieve the 100% thermodynamic limit.