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金属有机骨架(MOF)材料是由过渡金属离子与有机配体通过配位键连接构成的高度有序的超分子化合物.这类材料比表面积大,孔隙率高,热稳定性好,而且具有规整可调控的孔结构、易于功能化的骨架金属离子和有机配体,在多相催化领域具有潜在应用前景.将纳米尺寸的MOF材料等多孔材料作为催化剂,可以提高反应传质效率,从而提高催化反应活性,但纳米MOF催化剂的分离和回收困难.将磁性纳米粒子和MOF材料组装成核壳结构的磁性MOF材料,不仅可简化催化剂的分离回收,而且通过控制壳层材料的厚度可以实现催化剂的高活性和高选择性.我们曾将磁核Fe_3O_4纳米粒子交替放入含有一种MOF材料前体的DMF溶液中,采用层层组装法制备了磁性Fe_3O_4@UiO-66-NH_2纳米复合材料.经过十步组装后的材料的透射电镜(TEM)结果证实为核壳结构.但未出现明显的UiO-66-NH_2的X射线衍射(XRD)特征峰,说明壳层材料UiO-66-NH_2的结晶度较低;同时由于其孔结构的破坏或堵塞,在反应中出现明显失活.本文进一步改进自组装方法制备了核壳结构的磁性Fe_3O_4@UiO-66-NH_2纳米复合材料,用XRD、傅里叶变换红外光谱(FT-IR)、TEM、扫描电镜(SEM)和氮气吸附等方法对材料的组成和结构进行了表征,并考察了其在Knoevenagel缩合反应中的催化性能.结果表明,所制材料是以Fe_3O_4为核,以UiO-66-NH_2为壳的核-壳结构材料.经三次组装后出现了一系列UiO-66-NH_2的XRD特征峰,说明采用新方法制备的复合材料中壳层材料UiO-66-NH_2结晶度高,晶体结构规整.N_2吸附-脱附结果表明,材料具有较高的比表面积和孔容.该复合材料在Knoevenagel缩合反应中表现出与纳米UiO-66-NH_2相当或更好的催化活性和选择性,而且因壳层材料的孔道限阈效应而对底物表现出尺寸选择性.由于材料结晶度和晶体结构规整度的提高,催化剂稳定性更好,通过简单磁性分离即可分离和回收催化剂,循环使用4次而未出现明显失活.相对于本课题组之前报道的Fe_3O_4@CuBTC-NH_2,Fe_3O_4@IRMOF-3和Fe_3O_4@UiO-66-NH_2材料,本文所制的Fe_3O_4@UiO-66-NH_2是一类结构更加稳定的高效固体碱催化剂.
Metal-organic framework (MOF) is a highly ordered supramolecular compound composed of transition metal ions and organic ligands linked through coordination bonds. Such materials have large specific surface area, high porosity, good thermal stability, Adjustable pore structure, easily functionalized framework metal ions and organic ligands have potential applications in the field of heterogeneous catalysis.Using porous materials such as nano-sized MOF as catalysts can improve the efficiency of mass transfer reaction and improve the catalytic But it is difficult to separate and recover the nano-MOF catalyst.Magnetic MOF with magnetic nano-particles and MOF assembled into core-shell structure can not only simplify the separation and recovery of catalyst, but also can control the thickness of the shell material High activity and high selectivity.We have prepared magnetic Fe_3O_4 @ UiO-66-NH_2 nanocomposites by layered assembly method by alternately placing magnetic Fe_3O_4 nanoparticles into a DMF solution containing a MOF precursor. Transmission electron microscopy (TEM) results of the 10-step assembled materials were confirmed to be core-shell structures, but no apparent X-ray diffraction (XRD) characteristics of UiO-66-NH 2 were observed , Indicating that the crystallinity of the shell material UiO-66-NH_2 is low, and at the same time it is obviously inactivated in the reaction due to the destruction or blockage of its pore structure.In this paper, the magnetic Fe_3O_4 @ UiO with core-shell structure was prepared by self-assembly method -66-NH_2 nanocomposites were synthesized and characterized by XRD, Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and nitrogen adsorption. Knoevenagel condensation reaction.The results show that the prepared material is a core-shell structure material with Fe_3O_4 as core and UiO-66-NH_2 as shell, and a series of UiO-66-NH_2 The results indicate that the UiO-66-NH 2 composite has high crystallinity and regular crystal structure, and the results of N 2 adsorption-desorption show that the composite has high specific surface area and pore volume. Exhibits the same or better catalytic activity and selectivity as the nano-UiO-66-NH 2 in the Knoevenagel condensation reaction, but also exhibits size selectivity to the substrate due to the pore-crossing threshold effect of the shell material.Because of the crystallinity of the material and Improve the regularity of the crystal structure, The catalyst has better stability and can be separated and recovered by simple magnetic separation and recycled for 4 times without obvious inactivation.Compared with Fe 3 O 4 @ CuBTC-NH 2, Fe 3 O 4 @ IRMOF-3 and Fe 3 O 4 @ UiO-66-NH_2 material, Fe_3O_4 @ UiO-66-NH_2 prepared in this paper is a more stable and efficient structure of solid base catalyst.