Department of Chemical Engineering, University of Massachusetts Amherst , 686 North Pleasant Street, Amherst, Massachusetts 01003, United States.
Langmuir. 2013 Nov 12;29(45):13943-50. doi: 10.1021/la403706r. Epub 2013 Oct 30.
Fundamental understanding of the mass transport of petrochemical and biomass derived molecules in microporous and mesoporous solid catalysts is important for developing the next generation of heterogeneous catalysts for traditional hydrocarbon processing including biomass pyrolysis and upgrading. Hierarchical zeolites with both micropores and mesopores exhibit enhanced mass transport and unique catalytic performance in reactions involving large molecules. However, quantitative description of mass transport in such materials remains elusive, owing to the complicated structure of hierarchical pores and difficulty in the synthesis of the materials with controllable structures. In this work, zero length column chromatography (ZLC) was used to study temperature-dependent diffusion of cyclohexane in silicalite-1, self-pillared pentasil (SPP) zeolite, and three-dimensionally ordered mesoporous imprinted (3DOm-i) silicalite-1. The samples were synthesized with controllable characteristic diffusion lengths from micrometer scale (ca. 20 μm) to nanometer scale (ca. 2 nm), allowing systematic study of the effect of mesoporosity on the mass transport behavior of hierarchical zeolites. The results show that the introduction of mesoporosity can indeed significantly facilitate the mass transport of cyclohexane in hierarchical silicalite-1 by reducing diffusional time constants, indicating rapid overall adsorption and desorption. However, when the length scale of the material approaches several nanometers, the contribution from the surface resistance, or "surface barrier", to overall mass transfer becomes dominant.
对石化和生物质衍生分子在微孔和介孔固体催化剂中传质的基本理解，对于开发下一代用于传统烃类加工的多相催化剂，包括生物质热解和升级，是非常重要的。具有微孔和介孔的分级沸石在涉及大分子的反应中表现出增强的传质和独特的催化性能。然而，由于分级孔的复杂结构以及具有可控结构的材料的合成困难，这种材料的传质定量描述仍然难以捉摸。在这项工作中，零长度柱色谱（ZLC）用于研究环己烷在 silicalite-1、自支撑五硅沸石（SPP）沸石和三维有序介孔印迹（3DOm-i）silicalite-1 中的温度依赖性扩散。这些样品的合成具有从微米级（约 20 μm）到纳米级（约 2 nm）的可控特征扩散长度，允许系统地研究介孔对分级沸石传质行为的影响。结果表明，介孔的引入确实可以通过减少扩散时间常数来显著促进环己烷在分级 silicalite-1 中的传质，表明整体吸附和解吸迅速。然而，当材料的尺度接近几纳米时，表面阻力或“表面势垒”对整体传质的贡献变得占主导地位。
