Effect of surface resistance on cyclohexane uptake curves in Silicalite-1 crystals

Adsorption kinetics of cyclohexane in a variety of Silicalite-1 samples were measured by gravimetric uptake experiments. The kinetic appears as dependent upon the history of the crystals. Samples stored for several months before calcination (with their micropores full of template) exhibit a kinetic drop by almost an order of magnitude. Surprisingly, physico-chemicals analysis do not show any differences between these samples (no modification of their crystallinity or morphology, and no residual carbon is detected in the pore network). The kinetic drop is therefore attributed to a modification of the crystals surface, induced by a long-time contact with the template or cyclohexane molecules. Aged and as-synthesized (non-calcined) samples were etched by an HF solution, so as to "purify" the surface of the crystals. After a few minutes of treatment, the aged samples recover their initial adsorption kinetic, confirming that the kinetic drop is induced by the partial blocking of the entrance of the pores. Moreover, some of the as-synthesized crystals also show a rise of their adsorption kinetic, showing that surface resistance can be present ab initio, depending on the synthesis conditions. In an attempt to produce an accelerated aging effect, hydrothermal treatments were performed on non-calcined crystals. However, the treatments conditions are too severe, and induce variations in the crystal structure. (C) 2011 Elsevier Inc. All rights reserved.