The Study Of Coke Species On ZSM-5Zeolite Catalysts

Propylene and p-xylene are very important basic petrochemical materials. At present, propylene and p-xylene mainly come from the cracking of petroleum hydrocarbons. Methanol to the propylene (MTP) and shape-selective methylation of toluene to p-xylene processes have many advantages, such as a wide range of raw materials, high yield, and low operating costs. These are especially suited to China’s resource situation, including a lack of oil, little natural gas and abundant coal.ZSM-5is a typical zeolite catalyst that has acid-catalyzed and shape-selective properties derived from Br(?)nsted acid sites and its characteristic pore structure. ZSM-5not only has high reactivity, but also has a high propensity for coke formation. Determining the properties of the coke species formed on ZSM-5zeolite catalysts contributes to a better understanding of the catalytic reaction mechanism, and will provide information about the catalyst’s regeneration forwards improving the performance of catalysts.We used HF to destroy the skeleton of spent ZSM-5zeolite to release coke in the channels, then we extracted the coke using methylene chloride. We used x-ray diffraction spectroscopy(XRD), Raman spectroscopy, thermal gravimetric analysis(TG), gas chromate-graphy(GC), N2-physical adsorption, NH3-TPD and gas chromatography and mass spectro-metry (GC-MS) to characterize the coke species.In MTP reaction process, through the analysis of coke amounts, we observe that the carbon deposition rate speeds up with the increase of catalyst acid strength, methanol content, weight hourly space velocity of methanol, and reaction temperature. Through studying the large zeolite B-ZSM-5over different reaction times, we examine the dynamic process of coke formation. As coke amount increase, the catalyst’s specific surface area and total pore volume decrease, with the inner specific surface and micropore volume decreasing first. The amount and strength of acid sites also decrease with the increase of coke formed. The strong acid sites are covered first. During the process of methanol conversion, aliphatic hydrocarbons are the initial coke species. Aromatic hydrocarbons are then formed as coke precursors through dehydrogenation. The coke precursors diffuse to the external surface of the catalysts and form polycyclic aromatic hydrocarbons, without the restrictions of the pore size. When the external surface can not accommodate more coke, the coke blocks the pore of the catalyst. The insoluble carbon species formed through graphitization. Coke species during the methylation process were similar with the coke species during MTP process.