A series of bi-functional catalysts was prepared by using Al-MCM-41 and Ga-MCM-41 with Si/Me ratios of 15 and 50 impregnated with 0,5 Wt% of Pt or Ga. The n-butane dehydroisomerization was studied at 773 K. Catalysts based on Pt/Al-MCM-41 were less selective (more hydrogenolyzing) than those based on Ga-MCM-41. For the latter, Ga species segregated to extra-framework positions might exercise a kind of geometric effect on the Pt clusters inhibiting hydrogenolysis. The catalyst Ga/Al-MCM-41 showed the closest approach to the ideal dehydroisomerization catalyst.

The catalytic transformations of n-pentane were performed over a platinum-promoted silicoaluminophosphate molecular sieve (Pt/SAPO-11). The catalytic results indicate high yield for the production of pentenes with a good approach to the equilibrium distribution between the linear pentenes and iso-pentenes. The space velocity (WHSV) variation ranging between 1.6 and 4.7h−1, and the catalyst regeneration did not affect the catalytic behavior of the Pt/SAPO-11. On the contrary, Pt reduction at a temperature below 500°C, the H2/n-pentane molar ratio increase and Pt content below 0.5wt.%, resulted in lower yield for the production of pentenes with an increase in the production of hydrocarbons with less than five carbon atoms (undesirable side reaction).

A series of Ga-supported SAPO-11 catalysts with Ga-loading between 0.25 and 2.2 wt.% 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 0.5 wt.% 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.

Ga-71 MAS n.m.r. analysis and the catalytic behavior during the transformation of l-butene and n-butane strongly suggest the incorporation of Ga into the AIPO(4)-11 (AEL) framework during the synthesis of GaAPSO-11. A unique clear signal at around +120 ppm is proposed to be associated with the presence of tetrahedral Ga in the AEL framework of the unmodified GaAPSO-11. Migration of structural Ga atoms to the silicon domain of the AEL framework seems to occur as a result of a mild hydrothermal treatment, giving rise to an additional signal at +156 ppm, previously associated with tetrahedral gallium in gallosilicates with the MFI topology. The fact that GaAPO-11 and GaAPSO-11 behaved in a way similar to their counterparts AIPO(4)-11 and SAPO-11 during the skeletal isomerization of n-butenes reinforces the idea of an isomorphous substitution of Al(III) by Ga(III) in the AEL framework. The transformation of n-butane was shown to be a valuable test for detecting the presence of small amounts of hydro-dehydrogenating extraframework gallium species (EFGS) in Ga-supported SAPO-11 (Ga/SAPO-11). The fact that the sample of GaAPSO-11 was completely inactive for Eh is transformation leads us to believe that the incorporation of gallium into the tetrahedral positions of the AEL framework was almost complete. (C) Elsevier Science Inc. 1997.