The effect of crystallite size on the activity and selectivity of the reaction of ethanol and 2-propanol over SAPO-34

SAPO-34 was prepared according to a standardised recipe and separated into different crystal size fractions by sedimentation in water. Three fractions with mean crystal diameters of about 0.25, 0.5 and 2.5 mu m were obtained, and are referred to as the fine, medium and coarse fractions, respectively. The fractions were analysed for their chemical composition (microprobe), crystallinity and phase purity (XRD), crystal size and shape (SEM and TEM), micropore volume and total surface area (BET N-2 adsorption), Si distribution and content (Si-29 NMR), acidity (NH3 TPD), and adsorption properties for propane. Ethanol and 2-propanol were reacted over the different size fractions at 400 degrees C. Both alcohols very rapidly dehydrated to ethene and propene, respectively, and the consecutive reactions of these olefins under MTO-like conditions were investigated. Propene reacted primarily to ethene and linear butanes, These reactions were found to be diffusion-limited at particle sizes >2 mu m and to have an effectiveness factor close to 1 for particle sizes <0.2 mu m. The deactivation of the catalyst proceeded in a manner consistent with further diffusional barriers being created rather than just the loss of active sites. Ethene was virtually non-reactive over virgin SAPO-34. After activation of the catalyst with a propene pulse, however, ethene conversion was achieved. This indicates that ethene alone is not able to form stable and reactive carbocation intermediates, but that it reacts with such ions once they have been formed. The reactions of ethene were found not to be diffusion-limited at particle sizes <25 mu m. (C) 1999 Elsevier Science B.V. All rights reserved.