Aluminum-promoted tungstated zirconia catalyst in n-butane isomerization reaction

The promotion effect of Al on tungstated zirconia (WZ) catalyst was studied in a butane isomerization reaction and compared with Fe-promoted catalyst (FWZ). XRD and EXAFS studies revealed an interaction between Al and WO_x, whereas XANES analysis indicates that the octahedral symmetry around W sites remained unchanged. Two Al³⁺ sites are present on co-precipitated Al-promoted WZ (AWZ_C) catalyst; one is formed by the substitution of Al species in the WO_x structure and the other is the terminal Al species anchoring on the WO_x surface. Only the terminal Al³⁺ sites are evident in impregnated AWZ catalyst. Our NH ₃-TPD and DRIFT results suggest that these Al³⁺ sites are Lewis acid in nature with at least moderate acid strength. They are stronger than those Brönsted and Lewis acid sites originally present in WO _x. Exposure of the catalyst to atmospheric water results in the poisoning of these Al³⁺ sites by chemisorbed water, forming octahedral species. In an n-butane isomerization reaction, Al addition onto WZ increases both the activity and the stability of the catalysts. The catalytic activity of fresh catalysts decreases in the order FWZ_I>AWZ _C≥WZ_C, whereas regenerated catalysts follow the order AWZ_C≥FWZ_I>WZ_C. The improved activity of AWZ_C over WZ_C catalyst is related to the increased acidity created by the Al promoter. Most importantly, a great enhancement in catalytic activity was observed for regenerated catalyst relative to the fresh catalyst. The regenerated AWZ_C catalyst is as active as the FWZ _I catalyst.