(131b) On-Purpose Propylene Production from Light Hydrocarbons with MFI-Zeolite/Metal-Oxide Composites

< Backgrounds & Objective >

     Propylene as a raw material in the petrochemical industry is becoming significant, because the global demand for propylene has been increased at growth rate of 4-5%. A majority of the propylene (ca. 60%) has been produced as a byproduct in thermal 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 alternative method for on-purpose propylene production from light hydrocarbons (e.g., light naphtha), being widely available feedstock, is intensively desired to meet future demand for propylene and reduce energy-consumption in olefin production. In addition, the conversion of various light hydrocarbons (e.g., condensates recovered from shale gas, GTL-naphtha and light liquid hydrocarbons obtained from refinery processes of unconventional crude oil) into valuable light olefins is also significant task in terms of the efficient use of resources.

     Catalytic cracking of light hydrocarbons over solid-acid catalysts (e.g., ZSM-5) 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 these viewpoints, we have focused on the development of highly active and stable catalyst for cracking of light hydrocarbons 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 study. Both characteristics of these catalysts and their excellent performance are summarized below.

< Characteristics of proprietary catalysts >

(1)   Al-MFI zeolites containing gallium and iron atoms at optimized ratio (Fe-Ga-Al-MFI) were adopted as matrix component. Gallium accelerated dehydrogenation activities for alkanes and iron reduced acid strength remarkably, so that the Fe-Ga-Al-MFI zeolites gave high propylene selectivity by suppressing formation of aromatics due to a drastic change of acidity by combination of Ga and Fe.

(2)   Composite catalysts with cylindrical shape, consisting of the Fe-Ga-Al-MFI zeolites and metal-oxides as a binder (e.g., aluminum oxide), were employed in terms of properties required for industrial use (enhancement of mechanical strength and reduction of pressure drop during reaction). The composite catalysts were highly resistant to coke formation by coverage of strong acid sites on external surfaces by metal-oxide binder.

< Excellent performance of the present catalysts >

(1)   The Fe-Ga-Al-MFI/metal-oxide composites exhibited both high propylene yields (20-30 wt%) and high propylene/ethylene weight ratio (ca. 2.0) in cracking of n-hexane as model compound of light-naphtha diluted with nitrogen or steam at 550^oC. It is noteworthy that the reaction temperature (550^oC) was much lower than those (850-900^oC) in the steam-cracking.

(2)   These composites were employed for cracking of C₆ hydrocarbons containing 1-hexene (above 50 wt%) at 550^oC and gave higher one-pass yields of propylene than 35 wt%. These results indicate that the proprietary catalysts are suitable for cracking olefinic hydrocarbons (e.g., olefinic light-naphtha obtained from FT-oil).

(3)   In catalytic cracking of n-hexane without dilution at 565^oC over the present catalysts, one-pass propylene yields of ca. 13 wt% were maintained for longer than 1,000 h, in spite of severe conditions to cause coke formation easily. The long lifetime, being applicable to fixed-bed operation, was achieved due to the high resistance to carbon deposit. Furthermore, overall propylene yields were estimated to be higher than 20 wt%, being comparable with steam-cracking or other technologies (e.g., FCC processes).

(4)   Total energy-consumption in reaction unit for catalytic cracking of n-hexane without dilution at around 550^oC was evaluated to be reduced by 50-70% compared with the steam-cracking, because no thermal energy for heating diluent such as steam, i.e., apparent heat, was needed as well as its low temperature. The supply of hydrocarbon feedstock without dilution was thus confirmed to be advantageous in terms of energy consumption.

< Summary >

     The Fe-Ga-Al-MFI/metal-oxide catalysts for cracking were developed on the basis of original concept. These proprietary catalysts made it possible to produce propylene quite efficiently from light hydrocarbons under mild conditions and gave long lifetime that is applicable to cracking process using simplified fixed-bed reactor. It was suggested from experimental results that the energy-saving cracking of light hydrocarbons as an on-purpose propylene-production technology is feasible by using the present catalysts.