Synthesis Of SSZ-39 Molecular Sieve With N,N-diethyl-2,6-dimethylpiperidinium Cation As Structural Directing Agent

The eight-membered ring molecular sieves have attracted extensive research attention because of its unique structures and good hydrothermal stability.SSZ-39 with the three dimensional channel possessing eight-membered ring,excellent hydrothermal stability,tunable acidity and good catalytic performance in a variety of reactions,is a very attractive type of catalyst.In this paper,a rapid method for the synthesis of structure directing reagent-N,N-diethyl-2,6-dimethylpiperidinium hydroxide?DEDMP⁺OH^-?was developed,and SSZ-39 molecular sieve was synthesised in the presence of structure directing reagent.And the crystallization mechanism of SSZ-39 zeolite was investigated.The main research contents and conclusions are as follows:DEDMP⁺OH^-was prepared from 2,6-dimethylpiperidine as the starting material by alkylation,quaternization,followed by alkaline treatment.The synthesis process of intermediate product N,N-diethyl-2,6-dimethylpiperidinium iodide?DEDMP⁺I^-?was improved.DEDMP⁺I^-was synthesized with 2,6-dimethylpiperidine and iodoethane as raw materials,ethanol as a solvent,potassium bicarbonate as acid-binding agent in the high-pressure reacter.Compared with the traditional method,the reaction time was shortened from 64 h to 22 h,and the yield was increased from 69%to 93.3%.The optimal synthesis conditions for DEDMP⁺I^-were obtained by single factor experiments,which were n(C₇H₁₅N):n?C₂H₅I?:n?C₂H₅OH?=1:2.4:17,reaction temperature 120?,reaction time 22h.The yield of DEDMP⁺I^-can be up to 93.3%.The structure of the intermediate compound synthesized with raw materials of DEDMP⁺I^-through alkali displacement method.The optimal synthesis conditions were:n?DEDMP⁺I^-?:n?Na OH?=1:1.3,reaction temperature40?,and reaction time 30 h according to single factor experiments.Under these conditions,the yield of DEDMP⁺OH^-reached 53.82%.SSZ-39 zeolite was prepared by hydrothermal process with low temperature aging and high temperature crystallization period using the silica sol as adding silicon source,Y zeolite aluminum source,and designed synthesis N,N-diethyl-2,6-dimethylpiperidinium hydroxide?DEDMP⁺OH^-?as structure directing agent.The effects of synthesis factors such as the ratio of silicon to aluminum,aging temperature,crystallization temperature,addition of silicon source and Y molecular sieve on molecular sieve were investigated.The results showed that aging temperature was 35?,crystallization temperature was 180?,crystallization time was 48 h,the crystallinity of prepared SSZ-39 zeolite was higher.The obtained SSZ-39zeolites were characterized by X-ray powder diffraction?XRD?,N₂ physisorption techniques,scanning electron micrograph?SEM?,flourier transformation infrared spectroscopy?FT-IR?,and solid-state nuclear magnetic resonance(²⁹Si MAS NMR,²⁷Al MAS NMR).The crystallization mechanism of SSZ-39 zeolite was studied.The results showed that the crystallization process of SSZ-39 molecular sieve was amorphous silica-alumina gel firstly formed with the supplemented silicon source and Y molecular sieve,then was transformed into ANA zeolite,with the extension of crystallization time,the intermediate phase was dissolved and recombined to form SSZ-39 crystals in the presence of N,N-diethyl-2,6-dimethylpiperidinium hydroxide?DEDMP⁺OH^-?,and finally the formed SSZ-39 crystals induced ANA zeolite to transform into the pure phase SSZ-39 zeolite.