Study On The Isomerization Of 2,6-xylenol On Molecular Catalyst

ZSM-5 zeolite is a very important catalysis in petrochemical industry for it's uniform pores,adjustable acidity and good hydrothermal stability.However,the microporous in ZSM-5 zeolite will limit the diffusion of large molecules during some reactions,thereby hierarchical ZSM-5 can effectively solve the problem of diffusion limit,and then improve it's catalytic performance.In this paper,hierarchical ZSM-5 had been synthesized by hydrothermal crystallization method after reflux pre crystallization with TPAOH as template agent,APTMS as mesoporous structure directing agent.A series of characterization methods were used to study the relationship between the structure and properties of hierarchical ZSM-5 and the condition of synthesis.The hierarchical ZSM-5 zeolite were prepared with the following molar composition:SiO2/Al2O3=60,TPAOH/SiO2=0.2,APTMS/TEOS=0.05,without NaOH.Crystallization temperature is 170?,the crystallization time is 5 days.Under these conditions,the specific surface area of synthesized ZSM-5 zeolite was 470.65 m ·g-1,the pore volume was 0.58 cm ·g-1,the crystallinity of 87.70%,the mesopore volume accounts for 81.03%of the total pore volume.In order to increase mesoporous further,hierarchical ZSM-5 synthesized was aftertreated.The effects of post-treatment conditions on physical and chemical properties of ZSM-5 zeolite were studied.The optimum treatment conditions were as follows:the ratio of solid to liquid was 20,the temperature was about 80?,The alkali treatment time was 3 h,C(NaOH)=0.3 mol·L-1,C(S)=0.03 mo1·L-1.Under these conditions,the specific surface area of synthesized ZSM-5 zeolite was 693.66 m ·g-1,the pore volume was 0.92 cm ·g-1,the crystallinity of 86.88%,The mesopore volume accounts for 90.21%of the total pore volume.The catalytic performances of hierarchical ZSM-5 in the isomerization reaction of 2,6-xylenol were also investigated.The optimal temperature was 430?,the optimal space velocity is 2.1 h-1.Under this condition,the conversion was 85.91%,the selectivity of 3,5-xylenol was about 8.55%,and the selectivity of 2,5-xylenol was about 33.25%.The activity of hierarchical ZSM-5 zeolites is 5.49 times higher than microporous ZSM-5 zeolite.Compared with ZSM-5 zeolite without treatment,the conversion of the ZSM-5 zeolite after alkali treatment was improved by 2.7%,the selectivity of 3,5-xylenol was improved by 12.16%,the selective of 2,5-xylenol was improved by 0.7%.The ratio of mesoporous volume to total pore volume is the key factor affecting the selectivity of target products.