含铁骨架Fe-Al-EU-1分子筛的设计合成和晶化

以溴化六甲双铵(HMBr2)为模板剂,在HMBr2-Na2O-Al2O3-SiO2-Fe2O3-H2O体系中,采用水热法合成了高结晶度Fe-Al-EU-1分子筛,通过X射线粉末衍射(XRD)、扫描电子显微镜(SEM)、热重-微分热重(TG-DTG)分析、N2吸附-脱附、固体核磁共振(NMR)、紫外-可见(UV-Vis)吸收光谱和X射线吸收谱的精细结构(XAFS)等测试手段对合成样品的物理化学性质、铁在分子筛骨架的成键状态进行表征.结果表明:随着原始溶胶中铁的质量分数增加,Fe-Al-EU-1分子筛的晶胞体积增大;铁掺杂使Al-EU-1分子筛的表面积(BET)从272m2·g-1增加到365m2·g-1;模板剂脱除和分解温度降低;铁离子加入溶胶,导致zeta电位降低,胶体粒子容易聚集,分子筛外形增大.UV-Vis结果显示:由于220-250nm处的分子筛骨架中氧原子的成键2p电子向骨架四配位铁原子的空d轨道产生p-d跃迁,导致其特征峰的吸收峰强度急剧增加,在373nm左右骨架位的四配位铁原子与相邻的硅氧基团配位键合,产生能级分裂的d-d电荷跃迁峰且峰强度增加.XAFS结果表明:随着晶化过程的进行,边前1s→3d吸收峰和1s→4p主吸收峰发生了明显变化,原始溶胶样品在形成四配位铁物种结构单元后,逐步转化形成四面体配位的铁-硅-氧骨架铁物种,铁-硅-氧离子键结构单元也随之转变为四面体配位共价键结构的骨架铁物种.

Designed Synthesis and Crystallization of Fe-Al-EU-1 Zeolites Containing Framework-Iron

Highly crystalline Fe-Al-EU-1 zeolites were hydrothermally synthesized in a HMBr 2 -Na 2 O- Al 2 O 3 -SiO 2 -Fe 2 O 3 -H 2 O system by using hexamethonium bromide(HMBr 2 )as a template.The physical and chemical properties,and the bonding state of Fe in the zeolite framework for the prepared Fe-Al-EU-1 samples were characterized by a series of techniques including X-ray diffraction(XRD),scanning electron microscopy(SEM),thermogravimetric-derivative thermogravimetric(TG-DTG)analysis,N 2 adsorption-desorption,solid-state nuclear magnetic resonance(NMR),UV-visible diffuse reflectance spectroscopy (UV-Vis DRS),and X-ray absorption fine structure(XAFS).The results show that with increasing the mass fraction of Fe in the original sol-gel,the unit cell volume of Fe-Al-EU-1 zeolite inflates;the zeolite doped with Fe causes an increase in its surface area(BET)from 272 to 365 m 2 ·g -1 and a reduced amount of template removal,as well as a decrease in decomposition temperature.Adding Fe ions into sol-gel also leads to lowered zeta potential,easily gathered colloidal particles,and increased shape of zeolite.The UV-Vis spectrum shows that there is a characteristic peak at about 220-250 nm with a sharply increased intensity due to the p-d transition of the bonding electrons from the 2p-orbital of O atom to the d-orbital of the four-coordinated Fe atom in the framework.Also at around 373 nm,the coordination bonding of four- coordinated Fe atoms and adjacent Si-O groups leads to a d-d charge transition peak with an energy level splitting and increased peak intensity.The XAFS results show that with the crystallization process going on,the pre-edge absorption peak for 1s→3d and main absorption peak for 1s→4p change significantly.Four-coordinated structural units of iron species formed from the original sol-gel samples are gradually transformed into a tetrahedral coordinated iron-silicon-oxygen skeleton,in which iron-silicon- oxygen ionic structural unit is also transformed into skeleton iron species with a tetrahedral covalent bonding structure.