The skeletal isomerization of 1-butene over Zn-silicoaluminophosphate molecular sieves

The catalytic transformations of 1-butene were performed over a zinc-substituted silicoaluminophosphate molecular sieve (ZnAPSO-11), over a Zn-supported SAPO-11 molecular sieve (Zn/SAPO-11) and over the unpromoted SAPO-11 solid. The ZnAPSO-11 catalyst showed the highest selectivity towards skeletal isomerization as well as the highest skeletal isomerization efficiency (SIE). In addition, the largest number of acid sites, particularly strong acid sites, were observed with temperature-programmed desorption of ammonia (NH₃-TPD) over the ZnAPSO-11 catalyst, The acid strength over the latter was also the highest. The agreement between the catalytic and the acidity results obtained in the present work can be explained in terms of the model of Gielgens et al. In this model, the substitution of Al(III ) ions by Zn( II) ions leads to partially unsaturated Zn(II) ions in the vicinity of structural P-OH groups. The Brønsted acidity of the latter may be enhanced by Brønsted-Lewis interaction, thus rendering catalysts with stronger acidity necessary for the skeletal isomerization. The lower molar fraction of Al(III) cations found in the ZnAPSO-11 solid, compared to the SAPO-11 sample, strongly supports the above. The results for the Zn-supported system were extremely different. Significant decreases in the strong acidity as well as in the skeletal isomerization selectivity, compared to the ZnAPSO-11 catalyst, were observed. These differences reinforce the idea of incorporation of Zn(II) into the silicoaluminate framework for the ZnAPSO-11 solid.