2,5-二甲基己烷热裂化和催化裂化生成甲烷的机理研究

采用脉冲微反装置，在反应温度为550～650 ℃、低转化率（小于 15%）条件下，研究了2,5-二甲基己烷在石英砂和ZRP分子筛上的热裂化和催化裂化反应，分析了甲烷的生成机理。结果表明：2,5-二甲基己烷热裂化反应的主要产物是甲烷、丙烯和异丁烯，在链传递阶段，甲基自由基夺氢可由3条反应路径生成甲烷，叔C-H键对甲烷选择性的贡献大于90%；ZRP分子筛的择形催化作用影响2,5-二甲基己烷催化裂化的转化率和产物分布，甲烷由质子化裂化反应生成；分析热裂化反应与质子化裂化反应对甲烷生成的影响可知，甲烷主要由热裂化反应生成，且随反应温度升高，热裂化反应对甲烷生成的贡献逐渐增大。

STUDY ON METHANE FORMATION IN THERMAL AND CATALYTIC CRACKING OF 2,5-DIMETHYLHEXANE

The thermal and catalytic cracking reactions of 2,5-dimethylhexane were carried out at 550-650 ℃ at low conversion (x < 15%) in a pulse micro-reactor over quartz and ZRP zeolite，respectively. Formation mechanism of methane was analyzed. The results show that methane, propylene and i-butylene are the primary products in thermal cracking. There are three reaction pathways that contribute to the methane formation during chain propagation through the hydrogen extraction from C-H bond by methyl radical. The C-H bond attached to the tertiary carbon shows higher reactivity and accounts for more than 90% of methane selectivity during thermal cracking. The conversion and product distribution are clearly influenced by shape-selective catalysis and the methane is produced by protolytic cracking reaction during the catalytic cracking of 2,5-dimethylhexane over ZRP zeolite. The comparison of the contribution to methane between thermal and protolytic cracking pathways reveals that methane is mainly formed by thermal cracking, which gradually enhances along with the increase of reaction temperature.