Dominance of Surface Barriers in Molecular Transport through Silicalite-1

Development of microporous materials with hierarchical structures of both micro/mesopores leads to molecular transport at nanometer length scales. For novel microporous materials including three-dimensionally ordered mesoporous imprinted (3DOm-i) zeolites and zeolite nanosheets, particle dimensions are below 35 nm resulting in surface-dominated structures. At the same time, the existence of surface-controlled mass transport including undefined “surface barriers” has been observed to reduce apparent diffusivity of hydrocarbons by orders of magnitude. This paper systematically characterizes cyclohexane transport in silicalite-1 by zero length chromatography (ZLC) to determine apparent diffusivity varying over 3 orders of magnitude in particles ranging from 35 nm to 3 μm. Three proposed mechanisms for surface barriers including surface pore narrowing, surface pore blockage, or surface desorption are evaluated by comparison with particle-size/diffusivity data. It is concluded that transport control in small p...