Tompkins Jordan T, Mokaya Robert
University of Nottingham , University Park, Nottingham NG7 2RD, United Kingdom.
ACS Appl Mater Interfaces. 2014 Feb 12;6(3):1902-8. doi: 10.1021/am404911x. Epub 2014 Jan 14.
Evaluation of low and ultralow Al content (Si/Al between 50 and 412) aluminosilicate Al-MCM-41 materials synthesized via three contrasting alumination routes, namely, direct mixed-gel synthesis, post-synthesis wet grafting, and post-synthesis dry grafting, indicates that trace amounts of Al introduced via dry grafting can stabilize mesoporous silica MCM-41 to steaming at 900 °C for 4 h. It was found that trace amounts of Al (Si/Al > 400) introduced via so-called dry grafting of Al stabilize the virtually purely siliceous MCM-41 to steaming, whereas Al incorporated via other methods that involve aqueous media such as direct mixed gel synthesis or wet grafting of Al offer only limited protection at low Al content. It is particularly remarkable that a post-synthesis dry grafted Al-MCM-41 material possessing trace amounts of Al (i.e., Si/Al ratio of 412) and surface area and pore volume of 1112 m(2)/g and 1.20 cm(3)/g, respectively, retains 90% (998 m(2)/g) of the surface area and 85% (1.03 cm(3)/g) of the pore volume after exposure to steaming at 900 °C for 4 h. Under similar steam treatment conditions, the mesostructure of pure silica Si-MCM-41 is virtually destroyed and undergoes a 93% reduction in surface area (958 m(2)/g to 69 m(2)/g) and 88% decrease in pore volume (0.97 cm(3)/g to 0.12 cm(3)/g). The steam stable ultralow (i.e., trace) Al containing MCM-41 materials is found to be virtually similar to mesoporous pure silica Si-MCM-41 with hardly any detectable acidity. The improvement in steam stability arises from not only the presence of trace amounts of Al, but also from an apparent increase in the level of silica condensation that is specific to dry grafted alluminosilicate MCM-41 materials. The more highly condensed framework has fewer silanol groups and therefore is more resistant to hydrolysis under steaming conditions.
通过三种不同的铝化途径合成的低铝和超低铝含量（硅铝比在50至412之间）的铝硅酸盐Al-MCM-41材料，即直接混合凝胶合成、合成后湿法接枝和合成后干法接枝，评估结果表明，通过干法接枝引入的痕量铝可使介孔二氧化硅MCM-41在900℃下蒸汽处理4小时后仍保持稳定。研究发现，通过所谓的铝干法接枝引入的痕量铝（硅铝比>400）可使几乎纯硅质的MCM-41在蒸汽处理下保持稳定，而通过其他涉及水相介质的方法（如直接混合凝胶合成或铝湿法接枝）引入的铝在低铝含量时仅提供有限的保护。特别值得注意的是，一种合成后干法接枝的Al-MCM-41材料，其铝含量痕量（即硅铝比为412），表面积和孔体积分别为1112 m²/g和1.20 cm³/g，在900℃下蒸汽处理4小时后，仍保留90%（998 m²/g）的表面积和85%（1.03 cm³/g）的孔体积。在类似的蒸汽处理条件下，纯硅质Si-MCM-41的介孔结构几乎被破坏，表面积减少93%（从958 m²/g降至69 m²/g），孔体积减少88%（从0.97 cm³/g降至0.12 cm³/g）。发现蒸汽稳定的超低（即痕量）铝含量的MCM-41材料实际上与介孔纯硅质Si-MCM-41相似，几乎没有可检测到的酸度。蒸汽稳定性的提高不仅源于痕量铝的存在，还源于干法接枝的铝硅酸盐MCM-41材料特有的二氧化硅缩合水平的明显提高。缩合程度更高的骨架具有更少的硅醇基团，因此在蒸汽处理条件下更耐水解。
