MCM- 21的低活性是由于介孔材料的弱酸性导致，常规Beta沸石的低活性及选择性是由于大分子反应物难以进入沸石体相内，反应发生在外表面。 3.5.2 氧化及羟基化反应
氧化反应是介孔沸石分子筛作为催化剂最早应用的催化反应之一。Jacobsen 等人[18]采用碳模板剂制备出介孔TS- 1 沸石评价 1-辛烯及环己烯与过氧化氢的氧化反应。结果表明，由于 1- 辛烯未受扩散作用影响，两者产品产率相近；而对于环己烯的氧化反应，介孔TS- 1 催化剂表现出良好的催化性能。 3.5.3 加氢反应
介孔沸石负载上贵金属最近被用于大分子芳香族化合物的加氢反应，例如，4,6- 二甲基苯并噻吩的加氢脱硫反应。采用介孔Pd/Beta 催化剂以及Pd/Al- MCM-41催化剂对比评价，结果表明，介孔Pd/Beta沸石催化剂比Pd/Al-MCM-41显著降低硫含量[66]。在523K 反应 3h，采用介孔 Pd/Beta沸石催化剂的硫含量为165 ppm，而 Pd/Al- MCM- 41催化剂中的硫含量为226 ppm，这是由于介孔Beta的酸性比Al-MCM-41的酸性强的缘故。
4.总结
介孔沸石的合成与催化应用的研究自从2006 年使用软模板合成介孔沸石的报道之后进入一个新的发展阶段。综合前期研究的特点, 以下几点可能是下一个阶段的研究发展趋势:
模板的实用性。目前研究比较多的模板是碳模板和高分子聚合物，这主要是由于这两种模板基本上都是已经商业化的产品, 成本比较低. 尽管一些新型的有机硅烷或者模板剂可以合成出更有特色的样品, 但是在催化活性没有明显提高的前提下,合成成本仍然是模板选择的重要参考条件之一。
沸石种类的拓展。 目前被合成最多的介孔沸石是 ZSM-5 沸石, 其他沸石如β, ZSM-11, ZSM-12,SOD, X, Y 和 A 沸石等也有少量报道。 但是相对于数百种沸石而言, 介孔沸石的种类还是很少的. 而且从骨架构成上看, 硅铝沸石或者全硅沸石(Silica-lite-1, MFI)是主体。 其他构成如钛硅沸石, 磷酸铝沸石报道都很少。因此寻找一个通用性的方法来合成更多种类的介孔沸石仍将是介孔沸石合成研究的重点之一。
应用的广泛性。目前大多数介孔沸石仍然局限于作为催化剂或者载体应用在传统炼化中的一些典型反应, 如裂化、烷基化等, 可以被认为是具有介孔的沸石催化剂。 但是从另一方面, 介孔沸石也是具有沸石功能(强酸性、高稳定性等)的介孔材料, 应该在介孔材料的应用领域有更广泛的应用, 例如作为一个复合功能的载体而应用在精细化工中的很多典型反应,
如缩合、酯化、Heck、偶合反应等。这一方面已经有了一些研究, 但是涉及到的反应类型还不够广阔。作为一类重要的多孔材料, 介孔沸石应该有更宽广的应用领域。
介孔结构的多样性。 目前大多数介孔沸石的介孔仍然是无序的, 只有纳米层状 ZSM-5 的结构可以看作是有序介孔, 其他有序结构(二维六方或者立方)介孔沸石的研究仍在深入中。
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