Temperature (°C)

Figure 10.12. Differential thermal analysis curve of a hydrous zirconia catalyst. The heat emission peak at 450°C arises from the exothermic phase transition shown in Fig. 10.11. [From D. R. Milburn et al., in Moser (1996), Chapter 6A, p. 137.)

donors, or Lewis acids, which are electron pair acceptors. Figure 10.13 sketches the structures of a Lewis acid site on the left and a Brensted acid site on the right located on the surface of a zirconia sulfate catalyst, perhaps containing platinum (Pt/ Z1O2-SO4). The figure shows a surface sulfate group SO4 bonded to adjacent Zr atoms in a way that creates the acid sites. In mixed-oxide catalysts the surface acidity depends on the mixing ratio. For example, the addition of up to 20 wt% Zr02 to gamma-alumina (y-Al203) does not significantly alter the activity of the Brensted acid sites, but it has a pronounced effect in reducing the strength of the Lewis acid sites from very strong for pure y-alumina to a rather moderate value after the addition of the 20wt% Zr02. The infrared stretching frequency of die —CN vibration of adsorbed CD3CN is a measure of the Lewis acid site strength, and Fig. 10.14 shows the progressive decrease in this frequency with increasing incorporation of Zr02 in the y-Al203.

0 0

Post a comment