Optimized Synthesis and Structural Characterization of the Borosilicate MCM-70

A structure analysis of the borosilicate zeolite MCM-70, whose synthesis had been patented in 2003, was reported in 2005. Unfortunately, that structure analysis was somewhat ambiguous. Anisotropic line broadening made it difficult to model the peak shape, some peaks in the electron density map could not be interpreted satisfactorily, the framework geometry was distorted, and MAS NMR results were partially contradictory. In an attempt to resolve some of these points, an optimization of the synthesis was undertaken, and the structure was reinvestigated. The structure was solved from synchrotron powder diffraction data collected on an as-synthesized sample (Pmn21, a = 13.3167(1) Å, b = 4.6604(1) Å, c = 8.7000(1) Å) using a powder charge-flipping algorithm. The framework topology, with a 1-dimensional, 10-ring channel system, is identical to the one previously reported. However, the B in this new sample was found to be ordered in the framework, fully occupying one of the four tetrahedral sites. Two extra-framework K+ ion positions, each coordinated to five framework O atoms and one water molecule, were also found. The solid state 29Si, 11B and 1H NMR results are fully consistent with this ordered structure.

2005年，研究人员首次报道了合成方法于2003年获得专利的硼硅酸盐沸石(borosilicate zeolite)MCM-70的结构解析结果，但该结构分析存在一定模糊性：各向异性线宽展宽导致峰形拟合难度较大，电子密度图中的部分峰无法得到合理解释，骨架几何结构发生畸变，且魔角旋转核磁共振(Magic Angle Spinning Nuclear Magnetic Resonance, MAS NMR)结果存在部分矛盾。为解决上述部分问题，研究人员对该沸石的合成工艺进行了优化，并重新开展了结构解析工作。本研究以合成态样品的同步辐射粉末衍射(synchrotron powder diffraction)数据为基础，采用粉末电荷翻转算法(powder charge-flipping algorithm)完成结构解析，确定其空间群为Pmn2₁，晶胞参数为a = 13.3167(1) Å，b = 4.6604(1) Å，c = 8.7000(1) Å。该沸石的骨架拓扑结构与此前报道的一致，均拥有一维十元环孔道体系。但本研究发现，新合成样品中的硼(B)原子在骨架中呈有序排布，完全占据了四个四面体位点中的一个。此外，本研究还识别出两个骨架外钾离子(K⁺)位点，每个位点均与五个骨架氧原子及一个水分子配位。固态核磁共振(solid state NMR)中²⁹Si、¹¹B及¹H的谱图结果与该有序结构完全吻合。