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Mechanism of methane formation in thermal and catalytic cracking of n-octane

LI Fu-chao;ZHANG Jiu-shun;YUAN Qi-min;Research Institute of Petroleum Processing,SINOPEC;  
The thermal and catalytic cracking reactions of n-octane were carried out in a temperature range of550 ~ 650 ℃ with low conversions( x 15%) in a pulse micro-reactor over quartz and ZRP zeolite. Reaction mechanism of methane formation was analyzed. The results showed that ethylene,propylene and n-butylene were primary products and four paths contributed to methane formation in thermal cracking of n-octane. At 600 ℃,dehydrogenation of terminal C-H bond in the chain attacked by methyl radical led to methane production. Due to higher activation energy of cleavage of terminal C-C bond in octyl radical formed via dehydrogenation of central C-C bond,only methane can form at higher temperature. Protolytic cracking was predominant with relatively remarkable yield of normal paraffin in catalytic cracking of n-octane over ZRP zeolite. Methane was produced by protolytic cracking route as well. By comparison of methane formation between thermal and protolytic cracking,it revealed that methane formed through protolytic cracking below 600 ℃ while thermal cracking dominated the selectivity of methane at higher reaction temperatures.
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