A comparative study of template removal from silicalite-1 crystals in prolytic and oxidizing regimes

The removal of tetrapropylammonium cations (TPA(+)) from silicalite-1 crystals with a morphology of 90degrees-inter-growth and different crystal sizes was investigated. The effects of the nature of the gas (air or nitrogen) and of the hydrodynamics were examined using a stream flowing in parallel or across the crystal layer. The only process variable was the plateau temperature (T-max). Crystal domain boundaries became permeable to template degradation products when T-max exceeded 300 degreesC. The template removal was monitored by (i) light microscopy, (ii) degree of removal of the total organics alpha, of nitrogen species alpha(N) and of carbon species alpha(C) and (iii) accessibility of the channel system for N-2 molecules. The sorption isotherms for N-2 exhibited two steps. The upper step started to be perceptible for a degree of organic removal higher than 55%. Its height increased with alpha, and its position moved to lower pressure values. The plots of the gas accessibility versus the degree of removal were modelled. For template removal in air, all plots were linear. In a non-oxidizing atmosphere, the accessibility lagged behind alpha(N). Light microscopy showed that template degradation started along domain boundaries. Sorption kinetics of iodine into silicalite-1 crystals treated in a non-oxidizing atmosphere was considerably slower than that observed into crystals calcined in air. The tendency to crack formation increased with crystal size. The template removal efficiency was higher when using cross-flow than parallel flow calcination. (C) 2002 Elsevier Science Inc. All rights reserved.