【摘要】 BaX分子筛是用于从C8芳烃中分离高纯度对二甲苯的吸附剂.应用多晶XRD技术,通过电子密度函数法和Rietveld法,对吸附速率和分离度不同的两个BaX分子筛样品进行了结构的测定,并通过结构分析探讨了BaX分子筛的吸附机理.结果表明,不同工艺制备的BaX分子筛中Ba2+离子的定位略有不同.BaX分子筛中Ba2+离子主要分布在SⅠ′和SⅡ位置上,定位于SⅠ′位的Ba2+离子稳定了分子筛的骨架;而定位于超笼的SⅡ位置的Ba2+离子,当其接近占满并且每个Ba2+极化两个配位水时,可在超笼中产生一个对称性为D2群的吸附力场,这是BaX分子筛吸附分离对二甲苯的必要条件.若Ba2+离子在SⅡ位置的占有率下降或在超笼中存在着SⅡ位置以外的低占有率的阳离子,则破坏了上述吸附力场的对称性,削弱了分子筛的选择吸附能力.更多还原

【Abstract】 BaX zeolite is a good adsorbent for the adsorption and separation of paraxylene in high purity from C₈ aromatic isomers in industry. In an effort to understand the effect of Ba 2+ distribution in X zeolite on its adsorption and separation ability, the structure of two BaX zeolite samples, BaX-Ⅰand BaX-Ⅱ, with different adsorption speed and resolution was determined by polycrystalline X-ray diffraction with the electronic density function and Rietveld method. The results showed that the sites of Ba 2+ cations were slight different between the two samples. Ba 2+ cations mainly located on the S_Ⅰ′ sites and S_Ⅱ sites in BaX zeolite. The Ba 2+ （1） cations which situated at the S_Ⅰ′ sites in the beta cage made the framework of X zeolite more stable. In addition, when the occupancy of Ba 2+ （2） cations at S_Ⅱ sites was 1.0, and each Ba 2+ （2） cation polarized two coordinated water molecules, an adsorption force field with D₂ symmetry would be generated in the supercage. Paraxylene molecules belong to the D 2h symmetry group, which includes a D₂ subgroup. Therefore, the BaX zeolite with this adsorption force field could selectively adsorb and separate paraxylene. The occupancy of Ba 2+ cations at S_Ⅱ sites in BaX-Ⅱ was lower than that of BaX-Ⅰ. Meanwhile, there were 2.86 Ba 2+ （3） cations outside the S_Ⅱ sites in the supercage with a low occupancy in BaX-Ⅱ. Consequently the number of the supercage with adsorption force field of D₂ symmetry decreased, and the selective adsorption performance of BaX-Ⅱ was weakened. So the selective adsorption activity of BaX-Ⅰ was better than that of BaX-Ⅱ.更多还原