(676f) On-Purpose Propylene Production from Light Hydrocarbons with MFI-Zeolite/Metal-Oxide Composite Catalysts

Propylene has become increasingly significant as a raw material in the petrochemical industry because of its increasing demand. The global consumption of propylene has continued to expand at growth rate of 4-5 wt% annually and conventional thermal-cracking of naphtha feedstock can no longer meet the demand due to its low propylene selectivity (Weight ratio of propylene to ethylene: ca. 0.5). In addition, thermal steam-cracking processes are unfavorable in terms of energy consumption. From these technological backgrounds, it is necessary to establish any alternative technology for producing propylene efficiently from widely available feedstocks such as light naphtha. Though catalytic cracking of light hydrocarbons over zeolites (e.g., ZSM-5) in fixed-bed operation have been actively investigated as a promising choice for on-purpose propylene production, this method has not been commercialized because stable catalysts, being applicable to fixed-bed reactor, are still undeveloped. In the present study, unique composite catalysts [1,2], consisting of Fe-Ga-Al-MFI zeolites and metal oxides, have been developed to demonstrate efficient propylene production from light hydrocarbons at moderate temperatures (< 600^oC) in fixed-bed mode. Both characteristics of these catalysts and their excellent performance are summarized below.

1. MFI-type zeolites containing iron, gallium and aluminum at optimized ratios (Fe-Ga-Al-MFI), giving high propylene selectivity, were synthesized as main component. The Fe-Ga-Al-MFI zeolites combined with metal-oxide binder (e.g., SiO₂, Al₂O₃) were employed as composite catalysts for practical use.

2. The composite catalysts exhibited the following excellent performance in n-hexane cracking: (A) High propylene productivity at lower temperatures than 600^oC (Overall propylene yield: 20-30 wt%, Weight ratio of propylene to ethylene: 1.5-2.0); (B) High stability in fixed-bed operation (Lifetime of Fe-Ga-Al-MFI/SiO₂ catalyst: ca. 1,000 h).

3. Energy consumption in reaction unit in n-hexane cracking without dilution over the Fe-Ga-Al-MFI/SiO₂ catalyst at 565^oC was estimated to be reduced by ca. 60% compared to conventional thermal-cracking, because no thermal energy for heating diluent such as steam, i.e., apparent heat, was needed as well as its low temperature.
   

On these experimental basis, it was suggested that catalytic cracking of light hydrocarbons in fixed-bed mode is feasible as an efficient method for on-purpose propylene production by using the present zeolite-based composites.

References

[1] S. Hodoshima, A. Motomiya, S. Wakamatsu, R. Kanai, F. Yagi, Res. Chem. Intermed. 2015, 41(12), 9615-9626.

[2] S. Hodoshima, A. Motomiya, S. Wakamatsu, R. Kanai, F. Yagi, Micropor. Mesopor. Mater., in press.