CALCINATION OF LARGE MFI-TYPE SINGLE-CRYSTALS .1. EVIDENCE FOR THE OCCURRENCE OF CONSECUTIVE GROWTH FORMS AND POSSIBLE DIFFUSION-BARRIERS ARISING THEREOF

The removal of tetrapropylammonium (TPA) from large, prismatic-shaped single crystals of the MFI-type (silicalite-1) has been studied by light microscopy and microscope FTi.r. spectroscopy. The first stage in the calcination process, the Hofmann elimination of TPA to tripropylamine and propene, proceeds homogeneously below 300-degrees-C. The subsequent beta-elimination reactions to lower alkylamines takes place in two crystal sections at a somewhat lower calcination temperature as compared to other crystal parts and leads to an 'hourglass pattern'' within the crystal during calcination. This may be attributed to the hindered diffusion of water from these crystal sections, presumably as a result of interfaces between crystal parts, developed during the stepwise crystal growth process. The microscope FTi.r. technique provides for a direct observation of chemical and/or physical processes within one selected zeolite single crystal and allows for in situ scanning over the crystal for detailed observations.