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The transformation of n-butane over Ga/SAPO-11 - The role of extra-framework gallium species

Abstract:

A series of Ga-supported SAPO-11 catalysts with Ga-loading between 0.25 and 2.2wt.% were studied during the direct transformation of n-butane to iso-butene at 773 K and atmospheric pressure. With the exception of the richest Ga sample H2-TPR profiles showed only one signal, centred at around 973 K and designated as β-peak, associated to [Ga-O]+ species. A model was advanced to illustrate a probable reduction mechanism to Ga+ species, which have been thought to be the active catalytic centres. The model found support on the invariability of the turn-over frequency (TOF) with respect to Ga-charge. The deviation from this behaviour observed for the catalyst with the highest Ga content was explained in terms of the presence of free Ga2O3 detected also by H2-TPR. that would lead to the formation of reduced Ga2O clusters in the neighborhood of the active Ga+ sites, strongly inhibiting their catalytic activity. The catalyst with 0.5wt.% of Ga showed an excellent activity and selectivity for the direct transformation of n-butane to iso-butene. In fact, dehydrogenation yield at the thermodynamic limit of about 15% with a skeletal isomerization efficiency of around 70% was achieved. © 2002 Elsevier Science B.V. All rights reserved.