C5烃催化裂解过程中氢转移反应的研究

Study on hydrogen transfer reaction in C5 hydrocarbons catalytic pyrolysis

对C5烃(正戊烷、1-戊烯)的裂解反应产物进行分析,按照理想正碳离子和自由基反应机理,正戊烷和1-戊烯裂解生成低碳烯烃(C2H4+C3H6+C4H8)的摩尔选择性分别达到50%和100%。但是使用MFI-30分子筛,在650℃反应条件下,正戊烷和1-戊烯催化裂解生成低碳烯烃的摩尔选择性分别为23.41%和56.79%,说明分别有26.59%和43.21%的低碳烯烃发生了氢转移反应。进一步考察了不同类型分子筛和关键反应温度对C5烃催化裂解过程中氢转移反应的影响,研究发现,小孔结构、低酸密度的分子筛和较高反应温度,可以不同程度地抑制氢转移反应,提高低碳烯烃的选择性。在650℃条件下,当分子筛由大孔结构、高酸量的FAU更换为小孔结构、低酸量的MFI-120时,正戊烷和1-戊烯催化裂解的氢转移系数HTC分别减小96.86%和50.58%,焦炭选择性分别由11.91%和20.77%减小到0.75%和0.89%,低碳烯烃(C2H4+C3H6+C4H8)的选择性分别由14.25%和25.14%增加到46.28%和62.58%。

The cracking reaction mechanism of C5 hydrocarbons(n-pentane,1-pentene)was analyzed.It is found that according to the ideal carbanion reaction mechanism and free radicals reaction mechanism,the molar selectivity of the cracking of n-pentane and 1-pentene to lower olefins(C2H4+C3H6+C4H8)is 50% and 100%,respectively.However,using MFI-30 zeolite,the molar selectivity of catalytic cracking of n-pentane and 1-pentene to light olefins at 650℃ is 23.41% and 56.79%,respectively,suggesting that 26.59% and 43.21% of light olefins have undergone hydrogen transfer reactions.The effects of different zeolites and key reaction temperature on the hydrogen transfer reaction during the catalytic pyrolysis of C5 hydrocarbons were further investigated.The results show that the zeolite with small pore structure and low acid density and higher reaction temperature can inhibit the hydrogen transfer reaction to varying degrees,thereby increasing the selectivity of light olefins.At 650℃,as the zeolite changes from the FAU with a large pore structure and high acid content to the MFI-120 with a small pore structure and low acid content,the hydrogen transfer coefficient HTC of the catalytic pyrolysis of n-pentane and 1-pentene is reduced by 96.86% and 50.58%,respectively,and the coke selectivity is reduced from 11.91% and 20.77% to 0.75% and 0.89%,respectively.However,the selectivity of the lower olefins increases from 14.25% and 25.14%to 46.28% and 62.58%,respectively.