摘要
随着Si/Al比的增加,1-已烯逐渐与小笼羟基发生作用,至Si/Al为8.87时,可完全作用,这与吡啶吸附实验结果一致。低Si/Al比时,1-已烯可与部分剩余超笼羟基作用形成“氢键”,在约3300cm^(-1)产生吸收谱带;在高Si/Al比时,1-已烯亦可与硅终端羟基作用,使其伸缩振动谱带发生位移,在3700cm^(-1)处产生红外吸收。结炭过程中,1-已烯首先脱氢、环化形成低环芳烃,并进一步发生加成、环化反应;到结炭反应后期,环缩合、氢转移和支链化反应加强,使焦炭发生“老化”,具有了多支链、贫氢、多环芳烃的性质。随着Si/Al比的增加,结炭反应变缓,饱和结炭量减少,焦炭的“老化”程度降低。
Coke formation is a side reaction which occurs almost ubiquitously with the conversion of organic substrates on acidic catalysts. In the majority of cases, the activity and/or the selectivity of a catalyst deteriorates with the formation of coke, thus making the coke formation undesirable. A detailed infrared spectroscopic study of the effect of the Si/Al ratio on the adsorption and coking of 1 -hexene over faujasites is presented in this paper.
The dealuminated faujasites with different Si/Al ratios are prepared by SiCl4 vapor substitution method. The samples and its skeleton Si/Al ratio are: NH4Y 2.57, DNH4Y-1 3.63, DNH4Y-2 6.38, DNH4 Y-3 8.87, DNH4Y-4 23.58, and DNH4Y-5 60.82. The samples are calcined at 450℃, 10-5 Torr first to eliminate the H2O, NH3 and converted into the hydrogen form, then used in the study of adsorption and coke formation of 1-hexene. Experimental results were recorded by a 983 G infrared spectrophotometer from Perkin Elmer.
With the increase of the Si/Al ratio of DHY, the OH stretching bands in supercages and cubooctahedras decrease in intensity and the external OH stretching band increases. At the same time 1 -hexene interacts gradually with the OH groups in the cubooctahedras. This is consistent with the results of pyridine adsorption experiments.
When adsorption temperature is lower than 100 ℃, part of the adsorbed 1-hexene cyclized into simple aromatics, which is indicated by the appearence of 1478cm-1 band, but no coke band at about 1600 cm-1 has been observed. With low Si/Al ratio samples ( NH4 Y, DNH4Y-1) a broad band around 3300 cm-1 appears, which is perhaps a 'H-band' formed by the interaction of π electrons of the weakly adsorbed 1 -hexene with the acidic OH groups in the supercages. with high Si/Al ratio samples (DNH4Y-2-5), the external OH stretching band shifts from 3740cm-1 to 3695cm-1 which is attributed to the weak weak interaction of π electrons of 1-hexene with the external OH groups.
When reaction temperature is higher 150 ℃, coke is formed, which is indicated by the appearence of an IR band at 1600 cm-1. The band for the simple aromatics (1478 cm-1 ) which is formed at lower temperature disappears, and a new band at about 1500 cm-1 emerges. This indicated the formation of polycyclic aromatics. Nevertheless, the intensity ratio of the bands at 2965, 2929cm-1, attributed respectively to the -CH3, -CH2 asymmetry stretching vibration, rises with the increase of reaction temperature, and a new band at 1340 cm-1 appeared gradually, which is assigned to the tertiary C-H vibration of the brached hydrocarbon.
According to the phenomena mentioned above, it may be concluded that, in coking process, simple aromatics are formed first and successively addition, cyclization, H-shift, isomerization and branching reactions take place. The coke finally formed is polycyclic aromatics in structure with more branches and low H/C ratio.
出处
《分子催化》
EI
CAS
CSCD
1991年第1期35-41,共7页
Journal of Molecular Catalysis(China)
基金
国家自然科学基金
