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Nano Zsm-5 Zeolite Characterization And Catalytic Performance Study

Posted on:2008-10-16Degree:MasterType:Thesis
Country:ChinaCandidate:P WangFull Text:PDF
GTID:2191360212493271Subject:Organic Chemistry
Abstract/Summary:PDF Full Text Request
The analysis on the composition of a typical FCC gasoline in China show that C4~hydrocarbons are the dominating components. The olefins, which contribut greatly to Gasoline Octane Number (RON), are distributed between C4~C8, especially C5 and C6. Thermodynamics analysis, performed on reactions that can decrease olefin content while increasing RON, indicates that the increase of iso-paraffins and aromatics contents could be achieved by enhancing the skeleton isomerization, cyclization and hydrogen transfer reactions of olefins. Therefore, RON could be retained while the olefin content in gasoline reduced greatly.Nano-scale ZSM-5 zeolite, due to its specialties of higher activity, specific selectivity, stronger ability to resist coking, has been received extensive attention in catalytic field. It has been widely applied in hydrocarbons transformation processes due to its excellent ability for aromatization, isomerization and alkylation. It is possible that ZSM-5 can also be used in the transformation of olefins in gasline into iso-paraffins and aromatics and thus reduces the olefins content in gasoline and keep the RON unchanged.In this paper, we have investigated the effect of crystal size of ZSM-5 zeolite on the performance of reducing olefins content and found that the zeolite with the nano scale size (20-50 nm) shows not only the best catalytic ability but also very good stability. The olefin reducing process catalyzed by nano scale HZSM-5 (20-50 nm) was systematically investigated on several aspects as below.In order to make a thorough understanding of the reaction mechanism, special concern was given to the monitoring of the effects and the physicochemical properties of catalysts, namely adsorptivity and diffusivity, total acidity distribution and acid type distribution. The uptake rates of normal hexane (n-h) and cyclohexane (c-h), the relative diffusivity of n-h and c-h, viz, Dn/Dc were used to characterize its diffusivity property. In terms of acidity characterization, both NH3-TPD and FT-IR techniques were employed to describe the total acid content and acid type distribution.Concerning both fundmental and applying aspects of the research, the lighter fraction (≤70°C) in FCC with higher content of alkene and lower content of arene was selected as model compound, and reaction features and product distribution of single and mixed hydrocarbons over nano-HZSM-5 catalysts were investigated. The studies of model reactants offered theoretical evidence for hydrocarbon transformation in gasoline over both the parent and the modified nano-HZSM-5 catalysts, proved that keeping RON invariable while greatly reducing olefins content was feasible. Our research can provide theoretical guidance to the design of catalyst.Based on all of the above investigations, we got the main conclusions as listed below:The catalytic activity and stability of HZSM-5 are greatly influenced by crystal sizes. Nano-HZSM-5 presented higher catalytic activity and stability because of its small size, more acidic centers at porthole and extra-surface, and small diameter of micropore, which increased diffusion rate of 'fat molecules', such as alkyl-aromatics and iso-paraffins, and decreased influence of coke on the reactions of aromatization, alkylation and isomerization. As a catalyst of reducing the olefins content in gasoline, HZSM-5 should be modified to adjust the composition of product oil, especially to decrease the aromatics content in gasoline. And nano-HZSM-5 was ideal matrix of the modified catalyst.The research work is practically significant to promote gasoline cleanness and environmental protection.
Keywords/Search Tags:nano-HZSM-5 zeolite, physiochemical property, reduction olefins, isomerization, aromatization
PDF Full Text Request
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