Synthesis of Hierarchical Sn-MFI as Lewis Acid Catalysts for Isomerization of Cellulosic Sugars
Synthesis of Hierarchical Sn-MFI as Lewis Acid Catalysts for Isomerization of Cellulosic Sugars
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DOI：
10.1021/cs500295u
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发表时间：
2014-06-01
期刊：
影响因子：
12.9
通讯作者：
Fan, Wei
中科院分区：
文献类型：
--
作者：
Cho, Hong Je;Dornath, Paul;Fan, Wei
采用种子生长法在三维有序介孔碳（3DOm）的有限空间内成功合成了具有有序介孔和MFI拓扑结构的分级结构锡硅酸盐分子筛（3DOm-i）Sn-MFI。所获得的由形成乳白色结构的30 nm球形元件组成的3DOm-i Sn-MFI含有范围为4至11nm的高度有序的中孔。与传统的Sn-MFI相比，3DOm-i Sn-MFI对纤维素糖异构化反应具有上级催化性能。没有观察到扩散限制的异构化的丙糖，二羟基丙酮（DHA），成乳酸甲酯（ML）。3DOm-i Sn-MFI催化剂中弱布朗斯台德酸的存在通过催化中间体异丁醛（PA）的形成而促进反应。3DOm-i Sn-MFI通过极大地增强分子运输而为C-5和C-6糖（例如木糖和葡萄糖）的异构化提供显著的改进。木糖在3DOm-i Sn-MFI上的反应速率比在常规大体积尺寸的Sn-MFI上的反应速率高至少20倍。葡萄糖的反应速率也通过使用3DOm-i Sn-MFI而增强，但与木糖的反应相比在较小程度上增强，这可能是因为葡萄糖不能扩散到MFI的10元环孔中，并且反应仅在Sn-MFI催化剂的外表面上催化。此外，晶种生长与受限合成的组合允许我们使用市售的碳材料如炭黑和活性炭合成分级Sn-MFI，表明合成策略是用于定制分级沸石结构的通用且可靠的方法。
Hierarchical stannosilicate molecular sieves with ordered mesoporosity and MFI topology (three dimensionally ordered mesoporous imprinted (3DOm-i) Sn-MFI) were successfully synthesized within the confined space of three dimensionally ordered mesoporous (3DOm) carbon by a seeded growth method. The obtained 3DOm-i Sn-MFI consisting of 30 nm spherical elements forming an opaline structure contains highly ordered mesopores ranging from 4 to 11 nm. Compared with conventional Sn-MFI, 3DOm-i Sn-MFI exhibits superior catalytic performance for the isomerization of cellulosic sugars. No diffusion limitation was observed for the isomerization of a triose sugar, dihydroxyacetone (DHA), into methyl lactate (ML). The presence of weak Bronsted acid in the 3DOm-i Sn-MFI catalyst facilitates the reaction by catalyzing the formation of an intermediate, pyruvaldehyde (PA). 3DOm-i Sn-MFI offers significant improvements for the isomerizations of C-5 and C-6 sugars, such as xylose and glucose, by greatly enhancing molecular transport. The reaction rate of xylose on 3DOm-i Sn-MFI is at least 20 times higher than that on conventional bulky sized Sn-MFI. The reaction rate for glucose is also enhanced by using 3DOm-i Sn-MFI, but to a lesser extent as compared with the reaction of xylose, possibly because glucose cannot diffuse into the 10-membered-ring pore of MFI, and the reaction is catalyzed only on the external surface of the Sn-MFI catalysts. Moreover, the combination of seeded growth with confined synthesis allows us to synthesize hierarchical Sn-MFI using commercially available carbon materials, such as carbon black and activated carbon, indicating that the synthesis strategy is a versatile and reliable method for tailoring the structure of hierarchical zeolites.