NMR study of tungstated zirconia catalyst: acidic properties of tungstated zirconia and influence of tungsten loading Original Research Article

The concentration of Brönsted acid sites on tungstated zirconia (WZ) increased as tungsten increased (for tungsten loadings of 2.8, 5.3, and 9.6 wt.%), while that of Lewis acid sites decreased, indicating that Brönsted acid sites were formed by the tungsten species and that the Lewis acid sites detected in 31P MAS NMR spectra of trimethylphosphine (TMP) were mostly due to Lewis acid sites on zirconia. WZ of 9.6 wt.% tungsten content showed the same spectra as WZ of 16.6 wt.% tungsten content, indicating that they have the same surface acid properties. Although the concentration of Brönsted acid sites on WZ increased with reduction treatment, Brönsted acid strength of WZ was lower than that of sulfated zirconia, even after reduction treatment. WZ of 16.6 wt.% tungsten contained 0.07 mmol g−1 of Brönsted acid sites. The number of Brönsted and Lewis acid sites corresponded to 3.3 and 10.0% of all the tungsten on the tungstated zirconia, and 6.1 and 18.3% of the estimated surface tungsten, respectively.