正己烷在分子筛上的裂化反应机理研究Ⅰ.正己烷的裂化反应链长

STUDY OF THE MECHANISM OF THE CRACKING OF n-HEXANE OVER ZEOLITESⅠ. The Cracking Chain Length of n-Hexane over Zeolites

研究了正己烷在不同分子筛上的裂化反应 ,用裂化反应链机理的观点统一了正己烷裂化反应中的单、双分子两种反应机理 ,进一步明确了某些基元反应步骤的发生过程 ,较好地解释了正己烷裂化反应的产物分布以及产物选择性和分子筛孔径的关系。基于裂化反应链机理 ,提出了“裂化反应链长 (CCL )”的概念 ,并将裂化反应链长与分子筛的孔结构进行了关联。结果表明 ,在孔径较大的 Y,β等分子筛上 ,正己烷裂化反应有着较大的 CCL值 ,这意味着双分子的链传递反应在裂化链反应中占主要地位 ,有利于仅在链传递段中生成的异构烷烃的选择性 ;而在 ZSM-5 ,Ferrierite等小孔径的分子筛上 ,正己烷裂化反应的 CCL值较小 ,并接近 CCL 的理论极限值 2 ,这说明裂化反应主要由单分子的链引发和链终止反应来完成 ,其结果导致产物中烯烃选择性较高。与其它表征分子筛孔径或氢转移反应活性的参数相比 ,裂化反应链长既可用于表示氢转移、烷基转移等双分子反应在整个裂化反应中的比例 ,又和分子筛的孔结构有着良好的关联 ,因而 ,裂化反应链长 (CCL)是研究烷烃裂化反应机理以及关联裂化产物和分子筛孔结构性质的重要参数。

The effects of zeolite structure on the mechanism of n hexane cracking have been studied The results can be explained in terms of the chain mechanism of cracking reaction through the integration of mono and bimolecular reaction mechanisms Based on this mechanism,a special parameter called "cracking chain length (CCL)" has been proposed A high value of CCL means that bimolecular reaction of chain propagation predominates,such as the n hexane cracking over large pore Y and β zeolites Whereas,a low value of the paramete indicates that the monomolecular route is the main cracking pathway,for instance, n hexane cracking over small pore ferrierite zeolite Moreover,the minimum value of CCL has been obtained by theoretical derivation Comparing with other parameters which characterize zeolite pore structure or relative contribution of reaction pathways,CCL not only can be employed to express the share of mono and bimolecular reactions,but also tightly correlates to zeolite pore structure Hence,CCL is an important parameter for researchers to study paraffin cracking mechanism and relating product distribution of cracking to zeolite structure