Structure And Performance Of Hierarchical Molecular Sieve For Catalytic Cracking Of Light Hydrocarbons

Light alkene (ethylene, propylene, etc.) as the important chemical raw materials are produced mainly by thermal cracking of naphtha in the current chemical industry. Nowadays, the traditional methods for propylene production cannot satisfy the growing demand of high quality petrochemical materials. Therefore, some new technologies have been studied and developed, in which catalytic cracking of hydrocarbons to produce ethylene and propylene has been considered as one of the most effective technologies to produce light olefins since it can not only reduce reaction temperature but also adjust the product distributions compare with conventional thermal cracking process. In addition, more propylene can be obtained in catalytic cracking reaction, which can satisfy the increasing demand of propylene driven primarily by the high growth rate of polypropylene. In numerous factors, the development of catalysts with good performances is the key of studies on catalytic cracking.Molecular sieve has been considered as one of the most effective catatyst for catalytic cracking of light hydrocarbons. Therefore, in the present work, firstly, the catalytic performance of the zeolites with different pore structures and acid properties were investigated in cracking of n-heptane. The relationship between the catalytic performance of the zeolites and pore structures was disccussed as well as the acidic properties. It was found that the suitable acid properties, such as the strength and amount of acid sites and the appropriate ratio of microporosity/mesoporosity of catalysts were favorable for enthancing the conversion of n-heptane and selectivity of ethylene and propylene. In addition, the effect of SiO2/Al2O3ratio and rotating speed of homogeneous reactor in the process of ZSM-5synthesis was tested. The results indicated that ZSM-5with higher SSiO2/Al2O3ratio were helpful for the selectivity of propylene, because their lower strength and amount of acid sites suppressed the secondarty reaction of the alkenes. High silica ZSM-5with smaller particles had stronger acidic properties and showed better catalytic performance. Secondly, the physicochemical properties and catalytic performances of Y2O3-ZSM-5and Y2O3-ZRP-1were investigated, the effect of different Y2O3loading amounts、calcination temperature and SiO2/Al2O3ratio of ZSM-5on the Y2O3-ZSM-5were discussed. The results indicated that the physicochemical properties and catalytic performances of Y2O3-ZSM-5and Y2O3-ZRP-1samples were quite different from those of ZSM-5and ZRP-1, respectively. Their acid properties changed apparently compared with the physical properties in condition of lower Y2O3loading amounts, in this case, the acid properties were considered as the principal factor for their catalytic performances. On the other hand, both of the acid properties and pore structures simultaneously changed in the presence of relatively high Y2O3loading amounts. Accordingly the conversion of n-heptane and distributions of products were also changed. The reason should due to the integrated zeolite structures and suitable acid properties. It was also found that the calcination temperature and SiO2/Al2O3ratio had effect on the Y2O3-ZSM-5performance.Besides, a series of HZSM-5-ZRP-1hierarchical molecular sieve was synthesized by adding different contents of ZRP-1to the procedure of hydrothermal synthesis of ZSM-5. Analogously, HZSM-5-Hβ、HZSM-5-HY and HZSM-5-Mordenite were also obtained. It was indicated that the physicochemical properties and catalytic performances of ZSM-5-ZRP-1samples are quite different from those of HZSM-5or ZRP-1. ZSM-5-ZRP-1samples have a lot of mesopores and suitable amount of B+L acid sites compared with HZSM-5or ZRP-1as well as higher L/B ratio. The properties of HZSM-5-Hβ、HZSM-5-HY and HZSM-5-Mordenite had a significant change, not only changed the physical properties, but also the acid properties, such as the strength and amount of acid sites. Moreover, new acid site was formed. The results indicated that the higher conversion of n-heptane and selectivitiy of olefins (ethylene plus propylene) were obtained over ZSM-5-ZRP-1samples as well as HZSM-5-Hβ、HZSM-5-HY and HZSM-5-Mordenite hierarchical molecular sieve.