Synthesis And Characterization Of Hierarchical-Structured Mordenite

Hierarchical-structured zeolite has advantages of both zoelites(high acidity,high hydrothermal stability) and mesoporous materials(favorable mesopore structure),and it is an effective strategy to combine shape selectivity with efficient mass transfer.These features make it high desirable in many very important fields,especially in cracking of heavy oil and catalytic reactions bulk molecules involved.In this dissertation,mordenite as the main object was investigated.Firstly,the optimal synthetic conditions of normal mordenite were confirmed.Furthermore,the de-alumination disciplinarian for mordenite with different Si/Al under the acidic circumstance was quested for.Ultimately,hard template method and method by using zeolites as original silica-alumina source were adopted to prepare the hierarchical-structured mordenite with the traditional hydrothermal method,respectively.The main results obtained were as follows:(1) A better crystallinity of mordenite was obtained with crystallization time prolonging and crystallization temperature upgrading.As a result,the optimal crystallization time was 24 h and crystallization temperature was 175℃.The aging time of zeolite gel had little effect on the crystallinity and morphology of mordenite crystals.Si/Al in the initiative zeolite gel had great influence on the morphology of mordenite crystals.Mordenite nanocrystal aggregates were produced with Si/Al=5;yet mordenite crystals were formed as plates with Si/Al=15. And lower Si/Al was favored for the forming of mordenite crystals.H₂O/SiO₂ also played great role on the morphology of mordenite crystals,well-shaped hexagonal column was manufactured with H₂O/SiO₂=16.7.(2) Higher Si/Al in the initiative zeolite gel didn't always produce higher framework Si/Al.Framework of mordenite may still remain after beening treated in hydrochloric acid. And the framework Si could protect Al from being taken off from mordenite crystals under acid surroundings,just like the protection of the framework Al against Si in MFI zeolite under alkaline environment.The treatment in hydrochloric acid had much effect on acidity of mordenite.(3) Mesoporous mordenite crystals could be successfully synthesized by using commercial multi-wall carbon nanometer tubes as template.And while the self-prepared porous carbon was employed as template,two statuses occurred.In the case of firstly mixing silica source and carbon source then adding alumina source,mordenite nanocrystal aggregates with extracrystalline mesopores were obtained,and the size of single mordenite nanocrystal was about 200 nm.Yet when,silica source and alumina source were mixed earlier before adding the carbon source,hollow-structured mordenite single crystals were manufactured. The size of the hollow core was about 8μm,and its wall thickness was around 1μm with many mesoporosities whose size focuses on 9.0 nm existing in the wall.BET of hollow-structured mordenite was 358.1 m²/g.Hollow-structured mordenite really coorperated the high acidity of mordenite and favorable mesopore structure.(4) Framework of mordenite couldn't be destroyed within 30 min under the action of 2.0 mol/L NaOH solution at room temperature.Higher concentration of NaOH solution aggravated the damage of framework of mordenite,and in the meaning while the intensity of the diffraction peak for mesoporous molecular sieve increased.The composite molecular sieve was a new kind of three-hierarchical-structured material with micropores,mesopores and macropores in simultaneity.The composite molecular sieve MOR/MCM-41(0.5) obtained by dissolving in 0.5 mol/L NaOH solution largely maintained the regular morphology of normal mordenite,and its BET was 355.6 m²/g.Furthermore,it had high hydrothermal stability and acidity,and its mesopores still remained after being treated in the boiled water for 100 h.