再探MCM-41合成中碱介质的影响

A FURTHER INVESTIGATION OF EFFECT OF ALKALINE MEDIUM ON THE SYNTHESIS OF MCM-41

以十六烷基三甲基溴化铵为模板剂 ,正硅酸乙酯为硅源 ,考察了不同碱介质 (氢氧化钠、乙二胺和哌啶 )中纯硅介孔分子筛MCM 4 1的合成 ,探讨了碱的种类、用量对产品质量的影响以及这种影响的本质。结果表明 ,有机弱碱体系中得到的样品 ,其质量明显优于氢氧化钠体系中的样品 ,这不仅是因为合成过程中硅物种的聚合度、电荷密度等较为均匀 ,而且是因为有机弱碱的共轭酸离子能增加胶束表面的电荷密度 ,有利于无机物种与模板剂作用的缘故。此外 ,对于碱性条件下的MCM 4 1合成 ,不同碱介质出现不同的 pH范围。这一范围的大小 (宽窄 )受制于碱的强弱、共轭酸阳离子的体积以及共轭酸阳离子是否能增加胶束表面的电荷等几个方面。碱越弱、共轭酸阳离子的体积越小、共轭酸越易渗入胶束表面 ,合成所需的 pH范围就越宽。使用有机弱碱 ,尤其是乙二胺作碱介质 ,不仅可获得规整有序、结晶度高、稳定性好的样品 ,而且得到高质量MCM 4 1所需的 pH范围较宽 ,使得样品的合成更加容易。

Pure siliceous mesoporous MCM-41 samples were synthesized in different alkaline media (sodium hydroxide, ethylnediamine and hexahydropyridine) by using cetyltrimethylammonium bromide as the template and tetraethyl orthosilicate as the silica source. MCM-41 sythesized in a weak organic base is of a higher quality than that synthesized in NaOH medium. This was not only due to the relatively uniform polymerization degree and charge density of the siliceous species,but also because the conjugated acid ion of the weak organic base can increase the surface charge density of the micelles to facilitate the interaction between the inorganic species and the template. Different pH ranges occurred in synthesis of MCM-41 when using different alkaline media. The variation of this pH range was dependent on the strength of the base, the volume of the conjugated acid cation, and whether this cation can increase the surface charge of the micelles or not, etc. The weaker the base was, the smaller the volume of the conjugated acid cation was produced which can easily penetrate onto the surface of the micelles, thus a broader pH range can be exploited for synthesis. A weak organic base, like ethylenediamine, yields the product samples of higher structural order, higher crystallinity and better stability, and provides the possibility of using a broader pH range to make the synthesized MCM-41 of higher quality.