环己烯/HY体系中环烯碳正离子的形成及其转化

IR and UV-Vis Investigations on the Generation and Transformation of Cyclic-Alkenyl Carbocation from Cyclohexene on Faujasite

采用紫外漫反射光谱和红外光谱研究丁环已烯/HY体系中环烯碳正离子的形成及其进一步转化,两种方法结果相当吻合且互相充实。环己烯在室温吸附后,在UV-VIS、IR谱图中分别于350 nm、1530 cm^(-1)处产生与环烯碳正离子相关的吸收谱带。沸石酸性是出现这一结果的决定因素。在低温下(≤150℃)环烯碳正离子可转化成苯,从UV-VIS谱图尚看到链状多烯碳正离子存在,在温度高于150℃时,可观察到明显的催化剂结炭,所形成焦炭具有多支链多环芳烃结构性质。

In the present paper, IR and UV-Vis spectroscopies have been utilized to investigate cyclohexene reaction over HY. IR evidence for the formation and transformation of cyclic-alkenyl carbocation on zeolite was provided. Attention is called to the IR band at 1530 cm-1, which was observed after cyclohexene adsorption and reaction at room temperature and became stronger upon longer contact time <60℃). It can be inferred from the appearance of the 1530 cm-1 band that cyclic- hexenyl carbocation has been formed from cyclohexene through H-shift reaction. To help elucidate the nature of the IR band at 1530 cm-1, UV-Vis spectroscopic investigation was conducted under the same experiment condition as IR. The spectra obtained were similar to that H. Forster had reported on the cyclohexene/HZSM-5 system. Two strong UV-Vis bands at around 320, 350 nm can be detected immediately upon adsorption of cyclohexene onto HY at room temperature. The band at 320 nm can only be assigned to an allyl type carbocation, while 350 nm band originates from the cyclic alkenyl carbocation, in correspondence with the IR results. Subsequent heating of the absorbate/catalyst system to 100℃, the 1530 cm-1 band became weak rapidly. In the meantime, another IR band at 1478 cm-1 attributed to low-molecular weight aromatics, such as benzene, emerged. This shows that the dehydrogenation of cyclic- hexene carbocation into aromatics may takes place easily. Coke formed upon heat treatment at temperatures higher than 150℃. IR and UV-Vis results both verify that the coke formed on HY appeared to be polyaromatics with some aliphatic side-chains. With the adsorption of 1-hexene, a linear olefine, on HY, the characteristic bands of cyclic-alkenyl carbocation could not be found at 1530 cm-1 in IR and 350 nm in UV- Vis spectra respectively. An argument that different intermediates are formed in the reaction of linear or cyclic olefines over HY is put forward.