TPAOH/NaOH混合碱体系对ZSM-5沸石的改性及其催化性能研究

采用四丙基氢氧化铵和氢氧化钠有机无机混合碱TPAOH/NaOH体系对ZSM-5分子筛进行改性处理,得到了具有介孔和微孔的ZSM-5分子筛。用XRD、N₂吸附脱附、NH₃-TPD、SEM等手段对改性后的产物进行表征。结果表明,碱处理ZSM-5分子筛会使分子筛同时脱硅和脱铝。当混合碱体系中总OH^-浓度一定时,随TPA⁺/OH^-比值的降低和处理时间的延长,均会使分子筛的微孔结构数量减少,介孔结构数量增加。同时,还会改变分子筛的酸性,TPA⁺/OH^-比值适合的混合碱体系可在生成介孔的同时最大限度保留微孔结构,并减少分子筛的强酸数量,在甲醇制烯烃反应(MTO)中表现出较高的丙烯选择性。与无机碱NaOH相比,TPAOH对分子筛的改性速率较慢而且温和,具有可控性,可以起到孔道生长调节剂的作用。

Modification and catalyst performance of ZSM-5 zeolite by treatment with TPAOH/NaOH mixed alkali

The composites with mesoporous and microporous structure were obtained by treatment of ZSM-5 zeolite in mixed alkali aqueous solution of tetrapropylammonium hydroxide and inorganic base NaOH.The products were characterized by XRD,N2-adsorption/desorption,NH3-TPD,SEM and so on.The results showed that the alkaline solution would cause the zeolite desilication and dealumination and either decreasing the TPA+/OH-ratio or prolonging the alkali treatment time will cause reduction of micropore structure and increasement of mesopore amount increasingly while keeping the concentration of OH-at a fixed value.And meanwhile,the treatment also changed the acidity of zeolite.Mixed alkali treatment with proper TPA+/OH-ratio could preserve micropore structure to the maximum extent while formed the mesopore,reduce zeolite’s strong acid quantity and increase the selectivity of propylene when performanced on methanol-to-olefins(MTO) reaction.Compared with the inorganic base NaOH,the modification rate of zeolite by TPAOH was slower,milder and more controllable that could act as pore-growth moderator.