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.
BaX zeolite is a good adsorbent for the adsorption and separation of paraxylene in high purity from C4 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- Ⅰ and 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 D2h 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 SⅡ 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 .
出处
《催化学报》
SCIE
CAS
CSCD
北大核心
2005年第8期655-659,共5页
基金
国家自然科学基金资助项目(20373002).
