Alkane conversion over Pd/SAPO molecular sieves: influence of acidity, metal concentration and structure

The activity and selectivity for the isomerization of heptanes over Pd-load ed SAPO molecular sieves with AEL and AFI structure were investigated. By s ynthesis of the AEL molecular sieves from Al-isopropoxide, the acid site co ncentration could be controlled by the amount of silicon in the synthesis g el. The resulting concentration of SiAl-OH groups ranged from 0.07 to 0.3 m mol/g, exhibiting the same acid strength, independent from the acid site co ncentration. Catalysts were impregnated with 0.1-2 wt.% Pd. The activity in creased with increasing Pd content up to a Pd to acid site ratio of 0.02-0. 03. Above this value, the metal loading did not influence the activity. A f urther enhancement of the activity was possible by increasing the number of SiAl-OH acid sites. The selectivity to isomerization was constant for the catalysts with an acid site concentration >0.19 mmol/g. The catalysts synth esized from Al-isopropoxide showed a lower activity compared to the catalys ts synthesized from aluminum oxide, probably due to diffusional restriction s caused by the formation of polycrystalline agglomerates. (C) 2001 Elsevie r Science B,V. All rights reserved.