(236g) Synthesis of Hierarchical Sn-MFI As Lewis Acid Catalysts for Isomerization of Cellulosic Sugars
Zeolites are crystalline microporous materials with distinct pore dimensions, connectivity and framework compositions, which are widely used in petrochemical and biomass processing. However, the catalytic performance of zeolites is frequently hindered by the slow molecular diffusion in the microporous networks (0.5 nm to 2 nm), in particular when bulky molecules are reacted. A proven strategy to overcome the diffusion limitation is to create secondary mesopores into zeolites, leading to the formation of hierarchical porous structures. The secondary mesopores could enhance the diffusion of molecules to the active sites and desorption of the formed products.
In this work, hierarchical stannosilicate molecular sieves with ordered mesoporosity and MFI topology (three dimensionally ordered mesoporous imprinted (3DOm-i) Sn-MFI) have been successfully synthesized within the confined space of three dimensionally ordered mesoporous (3DOm) carbon via a seeded growth approach. The obtained 3DOm-i Sn-MFI consists of uniform 30 nm spherical opaline structures having ordered mesopores with a pore size from 4 nm to 11 nm. The 3DOm-i Sn-MFI exhibits higher catalytic activity for the isomerization of cellulosic sugars than the conventional 300 nm Sn-MFI and 60 μm Sn-MFI catalysts. It is revealed that there is no diffusion limitation for the isomerization of a triose sugar, conversion of dihydroxyacetone (DHA) into methyl lactate (ML). 3DOm-i Sn-MFI offers significant improvements for the isomerizations of C5 and C6 sugars, such as xylose and glucose, by greatly promoting 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 over 3DOm-i Sn-MFI, but to a lesser extent as compared to the reaction of xylose. This is possibly because it is difficult for glucose to diffuse into the 10 membered-ring pore of MFI, and the reaction might only occur on the external surface of the Sn-MFI catalysts. Furthermore, the combination of seeded growth with confined synthesis provides us with synthesizing hierarchical Sn-MFI using commercially available carbon materials such as carbon black and activated carbon, indicating that the synthesis methodology is versatile and reliable for tailoring the structure of hierarchical zeolites.