Synthesis,Characterization And Hydroisomerization Properties Of ZSM-22 Zeolite

The main component of F-T synthetic oil is long linear paraffin,which is suitable as a high-grade lubricating base oil.However,long-chain paraffins have disadvantages such as high freezing point and poor low-temperature fluidity.Therefore,in order to obtain a lubricating base oil with good low-temperature performance,the F-T synthetic oil must be refined and upgraded to a higher valueadded lubricating base oil.At present,it mainly relies on hydroisomerization technology to convert long linear paraffins into branched isomers,reduce product freezing point and improve product low temperature fluidity.The hydroisomerization technology generally uses a bifunctional catalyst consisting of a metal center and acid center according to the bifunctional catalytic mechanism.The bifunctional shape-selective catalyst with one-dimensional 10-membered ring channels is the technical core of the alkane isomerization.ZSM-22 zeolite has become a research hotspot for isomerization owing to its shapeselective pore structure and suitable acidity.Studies have shown that the morphology of molecular sieves has a great influence on the adsorption,diffusion and separation of probe molecules in the catalytic process.Therefore,it is of great theoretical and practical significance to study the effects of different morphological structures of ZSM-22 on n-alkanes isomerization performance.In this work,the type of template,crystallization conditions and the concentration of NaOH are studied to investigate the influence to physicochemical property on the synthesis of ZSM-22.And the hydroisomerization performance of n-dodecane was investigated based on this.The results show that 1,6-hexanediamine,diethylamine and 1-ethylbromide pyridine can be used as template for ZSM-22 molecular sieve,and different morphological structures such as bundle,thick rod,ball and needle were synthesized by changing the template agent and crystallization conditions.The /samples synthesized by dynamic crystallization conditions have smaller crystal size,large specific surface area,larger pore volume stronger acidity and richer acid distribution,and exhibits better catalytic activity and selectivity in the isomerization reaction.The KOH was replaced gradually by NaOH in the synthesis system by using diethylamine as an organic template,and the substitution ration was adjusted in the range of 0% to 65%.The introduction of Na changed the physicochemical properties of the products,especially for the morphology structure.With the increase of sodium,the crystal showed different morphological feature such as irregular mass,ellipsoid,cross petal,acicular and needle-like grains self-polymerizing into hollow spheres.The results of hydroisomerization of n-dodecane showed that Pt/Z22-DEA-gel which was prepared by dynamic conditions with diethylamine as template was more excellent in catalytic activity and selectivity,and the dynamic synthesis conditions could improve the hydroisomerization performance of n-dodecane obviously.With 1-ethylbromide as template,the catalytic activity of Pt/Z22-EtPBr prepared under static conditions is significantly weaker than other catalysts,and higher reaction temperature is required to achieve the same conversion.Analysis of the distribution of each catalytic product revealed that changing the synthetic environment can modulate the ratio of mono-branched and multi-branched products in the isomerized products,but it is impossible to achieve the modulation of the position of the heterogeneous rearrangement in the methyl branched isomers.The 2-methy isomers dominated in the mono-branched isomers and the position of the methyl group moves toward to the middle of carbon chain gradually.The multi-branched products are mainly 2,x-dimethyl isomers and the position of the second methyl tend to be symmetric with the first methyl.