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 WOx, whereas XANES analysis indicates that the octahedral symmetry around W sites remained unchanged. Two Al3+ sites are present on co-precipitated Al-promoted WZ (AWZC) catalyst; one is formed by the substitution of Al species in the WOx structure and the other is the terminal Al species anchoring on the WOx surface. Only the terminal Al3+ sites are evident in impregnated AWZ catalyst. Our NH3-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 WOx. 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 FWZI>AWZC⩾WZC, whereas regenerated catalysts follow the order AWZC⩾FWZI>WZC. The improved activity of AWZC over WZC 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 AWZC catalyst is as active as the FWZI catalyst.