(139b) Efficient Propylene Production by Catalytic Cracking of Light Naphtha with MFI-Zeolite/Metal-Oxide Composites

     Propylene is quite important as a raw material building block in the petrochemical industry and the global demand for propylene has been increased steadily. A majority of the propylene (ca. 60%) has been produced as a byproduct by steam-cracking of hydrocarbon feedstock such as naphtha, but this technology requires a huge amount of thermal energy as well as its low propylene/ethylene weight ratio (ca. 0.5). The shift to ethane-cracker in ethylene production also might affect the propylene supply, because this technology produces little propylene. Therefore, any new method for producing propylene efficiently from light-naphtha, being widely available feedstock, should be developed to meet future propylene demand and utilize various light paraffinic hydrocarbons (e.g., condensates recovered from shale gas, GTL-naphtha and light liquid hydrocarbons obtained from refinery processes of unconventional crude oil) by converting them into valuable light olefins.

     Catalytic cracking of light-naphtha over solid-acid catalysts to produce propylene have been actively investigated as an alternative technology. In particular, catalytic cracking using fixed-bed reactor has the following advantages compared with steam-cracking: (1) high propylene/ethylene weight ratio; (2) energy-saving. Furthermore, the cracking process in fixed-bed mode is superior to fluid catalytic cracking (FCC) processes in terms of operational simplicity in reaction unit. However, catalytic cracking process using fixed-bed reactor has not been commercialized, because cracking catalyst with long lifetime, being applicable to fixed-bed reactor, has been still undeveloped.

     From the above-mentioned viewpoints, we have focused on the development of highly active and stable catalyst for light-naphtha cracking in the fixed-bed mode to produce propylene efficiently under mild conditions. The proprietary zeolite-based catalysts for cracking, consisting of modified Al-MFI zeolites and metal oxides (e.g., aluminum oxide), were developed in the present research. Both characteristics of these catalysts and their excellent performance are summarized below.

< Characteristics of catalysts >

(1) Al-MFI-type zeolites (ZSM-5), containing two kinds of heteroatom(A, B) at optimized ratio, were adopted as matrix component. Heteroatom(A) in the zeolites accelerated dehydrogenation activities for paraffinic hydrocarbons and heteroatom(B) remarkably reduced acid strength of zeolite species, so that the unique catalysts (A-B-ZSM-5) gave high propylene yields with low aromatics yields due to a drastic change of acidity by combination of heteroatom A and B.

(2) Composite catalysts with cylindrical shape, consisting of the above modified ZSM-5 and metal-oxide as a binder (e.g., aluminum oxide), were employed for industrial application (e.g., enhancement of mechanical strength). The composite catalysts were highly resistant to coke formation by coverage of strong acid sites on external surfaces by metal-oxide binder.

< Excellent productivity of propylene >

(1) At low temperature of 550^oC, the proprietary catalysts exhibited both high propylene/ethylene weight ratio (above 1.8) and high propylene yields (20-31 wt%) in cracking of n-hexane (model compound of light-naphtha) diluted with steam or nitrogen. These yields were comparable with steam-cracking or other technologies (e.g., FCC processes). In the reaction test attaining propylene yield of 31 wt%, catalytic activity was kept constant during 24 h. Moreover, it is noteworthy that this reaction temperature (550^oC) was much lower than those (850-900^oC) in the steam-cracking.

(2) In the catalytic cracking of n-hexane without dilution at 565^oC, high propylene yields of 17-18 wt% were maintained for ca. 80 h in spite of severe conditions to cause coke formation easily. This long lifetime was achieved due to high resistance to coke formation. Furthermore, thermal energy required for producing propylene per unit weight was estimated to be reduced by ca. 70% compared with the steam-cracking, because no apparent heat of diluent such as steam was needed. The supply of hydrocarbon feedstock without dilution was thus confirmed to be advantageous in terms of energy consumption.

     As described above, the proprietary zeolite-based catalysts made it possible to produce propylene quite efficiently from light-naphtha under mild conditions and gave long lifetime that is applicable to cracking process using simplified fixed-bed reactor. It was suggested on the experimental basis that the energy-saving process for producing propylene efficiently from light-naphtha could be realized by using the present catalysts as an alternative on-purpose technology to steam-cracking.