Research On The Heptane Isomerization Reaction On Pt/WO₃-ZrO₂ Catalyst

Motor fuel in China is produced mainly from catalytic cracking process.The aromatics and olefins content of gasoline are strictly limited by the environmental protection laws,so isoparaffin is an ideal gasoline blending component the standard of environmental protection regulations.In this paper,the Pt/WO₃-ZrO₂ catalyst was prepared,kinetic model of n-heptane isomerization reaction on Pt/WO₃-ZrO₂ catalyst was founded and estimated by experiment data,and the reaction mechanism was analyzed via the product distribution.First,the crystal phases and pore structure of Pt/WO₃-ZrO₂ catalysts were studied,showing that the content of WO₃,the calcination temperature,pH value and P123 amount have great effect on the catalyst structure.The optimal preparation condition of Pt/WO₃-ZrO₂catalysts is that the calcination temperature is 700°C,the pH value is 9.6,the WO₃ content is15%and the molar ratio of P123/Zr is 0.03.Catalysts prepared under the optimal condition have excellent performance,with specific surface area of 114.1 m²,the domain ZrO₂ phase is tetragonal and WO₃ is evenly dispersed.Second,the reaction temperature,hydrogen pressure,residence time and liquid hourly space velocity have effect on the catalyst activity and selectivity.The optimized reaction conditions are 290°C,LHSV=1.2 h^-1,P_H2=1.0¹.5 MPa and the residence time about 1.0 s.Among mono-branched isomers,2-methyl hexane and 3-methyl hexane yield is almost equal,and 3-methyl hexane is the main product at various conversions.The ratio of 2-methyl hexane and 3-methyl hexane maintains constant;Among di-branched isomers of 2,2-dimethyl pentane,2,3-dimethyl pentane and 2,4-dimethyl pentane are the main products for the relative stability of carbon ion intermediate.Through the analysis of the change of content of components with the conversion,it is concluded that the control step of n-heptane isomerization reaction is the skeletal rearrangement of n-heptane carbonium ions to mono-branched carbenium ions and mono-branched carbonium ions to di-branched carbonium ions.Two n-heptane isomerization reaction models were set up,the first is applicable for reactions without cracking,and the second is applicable on a wider scale.Activation energy of n-heptane to mono-branched isomers,mono-branched isomers to di-branched isomers and cracking activation energy are calculated with 117022 J/mol,106560J/mol,and 134600 J/mol respectively.