摘要
催化反应中生成的焦炭常会阻塞沸石孔道或覆盖活性中心,从而使反应活性和选择性大大下降。针对沸石结炭的研究报导已有不少,但至今人们对结炭过程的认识还很模糊,酸强度及酸种类对结炭反应的影响也有待进一步研究。 沸石表面酸性位既是烃吸附位又是重要的结炭活性中心,B酸和L酸与探针分子—吡啶作用后可分别在1540、1450cm<sup>-1</sup>处定量产生红外吸收谱带。本文在前文的基础上研究了SiCl<sub>4</sub>同晶取代Y沸石上酸种类及酸强度在结炭反应中的作用。
The catalytic cracking of long-chain hydrocarbons is invariably accompanied by the formation of coke on the catalyst surface. Early work suggested that Bronsted acid hydroxyl groups alone accounted for the catalytic activity and coking rate of zeolites in a number of cracking reactions. In this paper, the effect of the nature of acid sites on coke formation in the reaction of 1-hexene on dealuminated fauja-sites DH Y-2 has been studied by IR method.
DNH4Y sample pellets were calcined at 10-5 Torr, 450℃ first to get active hydrogen-form catalysts, then exposed to the vapor of 1 -hexene or pyridine.The latter was used as a probe molecule to get the information on acid sites. All spectra were recorded by 983G infrared spectrophotometer from Perkin-Elmer. The coke concentration was measured by the absorbance of coke peak at 1600 cm-1, concentrations of Bronsted and Lewis acid sites were measured respectively by absorbances of bands at 1540, 1450 cm-1 which emerge upon pyridine adsorption on the catalyst.
Bronsted and Lewis acid sites coexist on the fresh DH Y-2 sample. Lewis acid sites are mainly composed of strong acid sites, while Bronsted acid sites are distributed over intermediate, strong and weak acid sites, with the last in the least amount.
When reaction temperature is higher than 150 @, coking reaction takes place. Coke concentration grows with the increase of reaction temperature and coke structure becomes more complex. Further experiments on the pyridine poisoning of acid sites on the catalyst reveals that the intermediate and strong Bronsted acid sites play an important role in the coke formation, while the Lewis acid sites change little in the reaction. Bronsted acid sites are gradually consumed by the blockage of and/or poisoning by the coke.
出处
《分子催化》
EI
CAS
CSCD
1991年第2期173-177,共5页
Journal of Molecular Catalysis(China)
基金
国家自然科学基金
