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 particles was likely due to either pore narrowing at the surface or an extension of the diffusional length scale near the surface due to total pore blockages