Multifunctional catalysts used in the thermo-catalytic cracking of hydrocarbon feedstocks for the production of light olefins

摘要

Light olefins such as ethylene and propylene, are considered the backbone of the petrochemical industry. They are the precursors of numerous plastic materials, synthetic fibers and rubbers. Commercially proven light olefin production technologies such as Steam Cracking (SC), Fluid Catalytic Cracking (FCC), and Deep Catalytic Cracking (DCC) are believed to have reached their full potential and cannot accommodate current demands of the petrochemical industry. The market demand for ethylene and propylene is projected to be about 140 and 90 million tons by year 2010, respectively. These current technologies cannot respond sufficiently to the rapidly growing demand for propylene, since propylene is only produced as a co-product of ethylene production. In addition, the high-energy consumption and the high GHG emissions are major setbacks for SC, which is regarded as the main light olefin technology. Thus, it is imperative that a new alternative should be developed in order to improve the production of light olefins. Thermo-Catalytic Cracking (TCC) has been recognized as a promising alternative route for light olefins production. Although, this process is still in the development stage, preliminary results show that the TCC offers several major advantages when compared to conventional SC: higher combined yields of light olefins, and significant energy savings. In this dissertation, the TCC activities, kinetic study, and structural-textural-surface properties of different catalyst formulations, which have been investigated thoroughly for their potential use in the TCC process, will be discussed. We report on our efforts to date to develop a suitable and an efficient catalyst that is characterized by high activity, high selectivity to light olefins, and high stability. A particular formulation studied was the hybrid catalyst configuration in which two components, microporous (zeolite) and mesoporous co-catalyst (supported metal oxide (i.e. MoO 3 -CeO), were firmly bound to each other within a clay binder, such that a "pore continuum" effect was developed. Another version was the mesoporous supported bi-oxide catalyst, which is based on MoO-CeO 2 supported on high surface area-metal oxide. Explicitly, it was found that supported bi-oxide catalysts are quite active, stable and selective to light olefins in the Thermo-Catalytic Cracking of n-hexane, which was used as a model molecule for petroleum light naphtha. Furthermore, it was observed that the physicochemical properties and subsequently the catalytic performance of these catalysts were influenced by many factors. Yttria stabilized alumina aerogel, which was prepared via sol-gel synthesis using super critical drying techniques, was considerably more effective as a catalyst support. Our results showed unambiguously that yttria stabilized alumina aerogel did not only possess a high surface area, but also was thermally and hydrothermally stable. In addition, it demonstrated a high ability of inducing homogenous distributions of impregnated metal oxides at high calcination temperature. The latter has resulted in significant improvements in the dispersion degree of Mo, Ce and MoCe species, and the retardation of sintering and sublimation of Mo species. More significantly, it was found that the on-stream-long term stability and the selectivity to light olefins over aromatics were increased upon the addition of CeO 2 into the supported mono-oxide MoO 3 catalyst