Modified Zeolites For The Catalytic Cracking Of Diesel Oil For Enhancing The Yield Of Propylene

Propylene is an important chemical raw material. In recent years, the growth rateof propylene demand even has exceeded that of ethylene. Considering the domesticgasoline and diesel oil production and the demand of propylene, it is an economic andfeasible way to choose a high selective catalyst for increasing production of propylenethrough catalytic cracking device. Adding ZSM-5to the FCC catalyst is an effectiveway to increase the production of light olefins. And the modulation of the zeolites’physico-chemical properties such as the surface acidity and pore structure through themodification of ZSM-5is the main implementation methods to increase the propyleneselectivity and the catalytic performance.The main purpose of this thesis is to produce high-performance and low-costcatalysts. The ZSM-5was modified by Au, Ce, Au-Ce, Au-Mo and Au-Mn, the betazeolite and Shenneng ZSM-5were also modified to expand research. In the reaction, thestraight-run light diesel oil was used as raw oil (boiling range of235℃~337℃) in themicroreaction-activity evaluation unit. The performance of various modified catalystsuch as the propylene selectivity and the microreaction-activity index was investigated.The structure and composition of the catalysts were characterized by XRD, NH3-TPD,BET, TEM-EDS and other technology. The following results were obtained:(1) In order to distinguish the influence of Au in the catalytical performance, thecomparisive reactions were done on the Au/ZSM-5and the ZSM-5catalysts treated atcertain pH values at which Au was deposited on ZSM-5in the process of preparation.The ZSM-5catalysts treated by the aqueous solution of HCl-KOH with pH of9~10canprevent the decrease of the acidity from the steam treatment at800oC. The possible K,Cl atoms introduced in the preparation process does not significantly influence thecatalytic properties of the catalysts in the tested range. The modification of ZSM-5byAu can significantly increase the acidity,both the strength and the density. The average pore size and the specific surface area of the catalysts was also increased slightly. Theresults showed that0.1%Au/ZSM-5has the better catalytic performance.(2) In the Au-Ce/ZSM-5catalysts,1%Au-5%Ce/ZSM-5shows relatively goodcomprehensive performance. The modification by Au-Ce maintained the integrity of theZSM-5structure type and the acidity of catalysts. Cerium species are located on theZSM-5substrate in the form of cerium oxide. Small amount of large gold particles werefound in the catalysts. The introduction of Au-Ce to the ZSM-5increased the porevolume and improved the propylene selectivity in the reaction.(3) The surface area and pore volume of ZSM-5were significantly reduced by themodification of Au-Mo. The acidity is strongly modified too. When the theoreticloading of gold in the catalyst was given, the surface acid strength of the catalystsdecreased with an increase of the theoretic loading of molybdenum; when the theoreticloading of molybdenum was given, the acid sites and acid strength increased with anincrease of gold content. When the content of Au is1.0wt%, theoretical content of Mois5wt%, the catalyst presented good catalytic performance.(4) In the case of manganese modified ZSM-5catalysts, with an increase of Mncontent, the surface acidity and hydrothermal stability decline gradually. Au-Mn/ZSM-5is more acidic than Mn/ZSM-5, and their total acidity and acid strength firstly increasedand then decreased with an increase of the content of Mn, when the content of gold was1wt%. The diffraction peak of ZSM-5was maintained in Au-Mn/ZSM-5catalyst evenafter the steam treatment at800oC.1%Au-3%Mn/ZSM-5presented better catalyticperformance than Mn/ZSM-5.(5) For beta zeolite, the modification of Au-Ce decreased the pore volume andspecific surface area to some extent. Smaller pores reflect the effect of shape-selectivethat benefit the passage of propylene but not other larger molecules. The yield ofpropylene was therefore enhanced. When the content of cerium was given, thepropylene selectivity increased with an increase of gold content,1.5%Au-10%Ce/βpresented better catalytic performance.