The influences of Al species and Ti species on the catalytic epoxidation over Si/Ti-pillared MCM-36 synthesized from MCM-22
The influences of Al species and Ti species on the catalytic epoxidation over Si/Ti-pillared MCM-36 synthesized from MCM-22
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DOI：
10.1039/c5cy00145e
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发表时间：
2015-01-01
影响因子：
5
通讯作者：
Huang, Yu-Wei
中科院分区：
文献类型：
--
作者：
Jin, Fang;Huang, Shingjong;Huang, Yu-Wei
以Si/Ti混合氧化物为支柱制备了一种新型介孔硅铝材料Si/Ti-MCM-36衍生物。研究了Al物种的酸性和Ti物种的配位状态对叔丁基过氧化氢(TBHP)作为氧化剂催化环己烯环氧化反应的影响。通过 XRD 和 N-2 吸附/解吸实验证实了介孔结构。通过NH3-TPD、Al-27 2D MQ MAS NMR和Si-29 MAS NMR表征酸度和表面极性。用紫外-可见光谱鉴定Ti配位态。通过使用 Al-27 2D MQ MAS NMR 技术，鉴定了来自 MWW 沸石晶体学上不同 8 个 T (Si, Al) 位的 Al 物种，这些 Al 物种与桥连羟基（例如布朗斯台德酸位点）有关。结果发现，Si/Ti-MCM-36 样品，无论是四面体（T-d）配位态还是八面体（O-h）配位态 Ti 物种，均对环己烯环氧化反应不活泼。对Si/Ti-MCM-36样品进行进一步的酸处理可以极大地从MWW结构中去除Al物质，从而大大降低酸度并将叔丁基过氧化氢的酸催化分解转移到Ti催化的环己烯环氧化。这表明，不仅Al物种的数量及其骨架晶体位置对环氧化活性有很大影响。此外，在酸处理的Si/Ti-MCM-36上接枝四异丙醇可以增加其疏水性和环氧化活性。此外，单元中的T-d TiO4已被证实是环氧化反应最活跃的位点。
A novel mesopore silica-aluminium material, i.e., Si/Ti-MCM-36 derivatives were prepared with Si/Ti mixed oxide as a pillar. The influences of acidity from Al species and the coordination state of Ti species on the catalytic cylcohexene epoxidation with t-butylhydroperoxide (TBHP) as the oxidant were investigated. The mesoporous structures were confirmed by XRD and N-2 ads/desorption experiment. The acidity and surface polarity were characterized by NH3-TPD, Al-27 2D MQ MAS NMR and Si-29 MAS NMR. The Ti coordination state was identified with UV-Vis spectra. By using Al-27 2D MQ MAS NMR technique, the Al species from the different eight crystallographically T (Si, Al) positions of MWW zeolite were identified, which are associated with bridging hydroxyl groups such as the Bronsted acid site. It was found that the Si/Ti-MCM-36 samples, whether with tetrahedrally (T-d) or octahedrally (O-h) coordinated state Ti species, are all inactive for the cyclohexene epoxidation. A further acid treatment of the Si/Ti-MCM-36 samples can greatly expel Al species from the MWW structure, which largely decrease the acidity and transfer the acid-catalyzed decomposition of t-butylhydroperoxide to Ti catalyzed cyclohexene epoxidation. It shows that not only the amount of Al species but also their framework crystallographic positions have a great influence on the epoxidation activity. Furthermore, the grafting of the tetra-isopropoxide on the acid treated Si/Ti-MCM-36 can increase its hydrophobicity and epoxidation activity. Moreover, the T-d TiO4 in the units has been confirmed to be the most active site for the epoxidation reaction.