Synthesis and Modification of ZSM-23 Zeolite as Catalyst Support for Pt/ZSM-23 Targeting at Hydroisomerization
Author:Chen Yu Jing
Supervisor:liang chang hai
ZSM-23 is a high-silica zeolite with one-dimensional 10 membered ring channals.It can improve the selectivity of monobranched isomers by inhibiting the cracking of multibranched alkanes on account of its optimized pore size and special structure.As a suitable support for the hydroisomerization catalysts,it can also reduce the coke formation and extend service life of the catalysts.Herein,ZSM-23 was chosen as support and bifunctional catalyst Pt/ZSM-23 was prepared and applied in the n-hexadecane hydroisom erization.The structure and the properties of ZSM-23 zeolite was tuned by modifying the synthesis process and post-treatment.The relation between the zeolite properties and its catalytic performance was also investigated to help improve the hydroisomerization performance of the Pt/ZSM-23 catalysts.The main contents and results were as follows.ZSM-23 zeolite was synthesized with dual-template pyrrolidine and isopropylamine.Compared with ZSM-23 samples synthesized with a single template,the SiO2/Al2O3 molar ratio range in the initial gel was extended to 80-200,and the products possessed larger Snicro and Vmicro in the dual-SDA system.More weak and medium acidic sites were found in the products which facilitates the hydroisomerization reaction.When the reaction temperature was lower than 290℃,Pt/ZSM-23 prepared using dual-template with a molar ratio of 100 showed high selectivity to isohexadecane,and it reached to 92 wt%when the temperature was 270 ℃.While the catalyst with a molar ratio of 200 was a better choice when the temperature was higher than 300 °C and the isohexadecane selectivity was ca.60 wt%when the temperature was 320 ℃.Seed-assisted synthesis of ZSM-23 free of alkali metal ions(denoted as SNNa)was conducted with 72 h hydrothermal treatment.The product was highly crystallized and can be used as support directly after calcination.After compared with the samples synthesized with Na+or K+,it can be found that the acidity distribution was tuned when the SDA was used to balance the framework negative charge.The catalysts possessed more weak Br(?)nsted acidic sites which can suppress the cracking effectively.Besides,ZSM-23 with smaller crystal size and more mesopores was obtained which favoured the diffusion.Therefore,Pt/H-SNNa catalysts showed better performance in the hydroisomerization of hexadecane,and its selectivity to isohexadecane was-60 wt%when the conversion of n-hexadecane was 75 wt%,Fe-substituted ZSM-23 were prepared to adjust the acidity and ZSM-23 samples with different framework Fe contents were synthesized.The results showed that the Fe incorporati on promoted the formation of mesopores,thus increased the external surface area and the pore volume of the samples.Meantime,the crystals size decreased with the Fe content increasing.The acidity strength in the Fe-substituted was decreased,and higher weak Br(?)nsted acidity content was obtained.It can promote the isohexadecane selectivity effciently.It is notable that more isohexadecane with methyl branching at more central positions was detected over Pt/Fe-ZSM-23 catalysts potentially because of more occurance of the key-lock adsorption of hexadecane.As extraframework Fe species are easily to be introduced during the synthesis of Fe-substitued zeolite,the effect of the extraframework Fe on hydroisomerization performance of ZSM-23 catalysts were investigated and the extraframe Fe modified samples were prepared via ion exchange method.No obvious positive influence happened for the ZSM-23 sample synthesized with NaOH but no seeds adding.In the ZSM-23 samples synthesized through the seed-assisted system without alkali metal cation,the extraframework Fe species could further decrease the acidity strength,and some interaction existed between Fe species and the Pt particles which promoted the Pt reduction.With the Fe content increasing,the component of Fe species changed and the Br(?)nsted acidic sites initially increased and decreased finally,and the synchronous change happened to the Pt dispertion.Consequently,the isohexadecane selectivity firstly increased and then come down.When the content of the extraframework Fe was 1.98 wt%,the catalyst Pt/1.98Fe/ZSM-23 showed the optimal performance,over which the n-hexadecane conversion and selectivity to isohexadecane were both-80 wt%with the contact time of 1.8 min when the temperature was 280 ℃.