Abstract
A series of ZrO2-supported WOx catalysts were prepared using atomic layer deposition (ALD) with W(CO)6, and were then compared to a WOx/ZrO2 catalyst prepared via conventional impregnation. The types of sites present in these samples were characterized using temperature-programmed desorption/thermogravimetric analysis (TPD-TGA) measurements with 2-propanol and 2-propanamine. Weight changes showed that the WOx catalysts grew at a rate of 8.8 × 1017 W atoms/m2 per cycle. Scanning transmission electron microscopy/energy-dispersive spectroscopy (STEM-EDS) indicated that WOx was deposited uniformly, as did the 2-propanol TPD-TGA results, which showed that ZrO2 was completely covered after five ALD cycles. Furthermore, 2-propanamine TPD-TGA demonstrated the presence of three types of catalytic sites, the concentrations of which changed with the number of ALD cycles: dehydrogenation sites associated with ZrO2, Brønsted-acid sites associated with monolayer WOx clusters, and oxidation sites associated with higher WOx coverages. The Brønsted sites were not formed via ALD of WOx on SiO2. The reaction rates for 2-propanol dehydration were correlated with the concentration of Brønsted sites. While TPD-TGA of 2-propanamine did not differentiate the strength of Brønsted-acid sites, H–D exchange between D2O and either toluene or chlorobenzene indicated that the Brønsted sites in tungstated zirconia were much weaker than those in H-ZSM-5 zeolites.
Highlights
In the 1980s, it was reported that tungstated zirconia (WOx /ZrO2 ) could be used as a solid acid catalyst; there are still many questions and apparent contradictions regarding the nature and strength of the acid sites in this material, as demonstrated in the recent review by Zhou et al [1].A major difficulty with tungstated-zirconia catalysts is that their properties depend on how they are made
In agreement with previous reports [1,17], our data here indicate that tungstated zirconia is a complex material that can exhibit Brønsted acidity, Lewis acidity, and oxidation activity, along with complex material that can exhibit Brønsted acidity, Lewis acidity, and oxidation activity, along with dehydrogenation activity on ZrO2 [22]
The concentrations of the various types of sites depend on how the material is made, as shown by the differences we observed in materials synthesized via how the material is made, as shown by the differences we observed in materials synthesized via impregnation and using atomic layer deposition (ALD)
Summary
In the 1980s, it was reported that tungstated zirconia (WOx /ZrO2 ) could be used as a solid acid catalyst; there are still many questions and apparent contradictions regarding the nature and strength of the acid sites in this material, as demonstrated in the recent review by Zhou et al [1]. Catalysts 2018, 8, 292 for tungstated zirconia to exhibit alkane-isomerization activity [5,6] would suggest that the sites are less strong, in addition to tungstated zirconia being selective to ether formation in the dehydration of alcohols [7], without forming olefins, unlike with protonic zeolites. While the WOx species may spread over the support surface during calcination, the implications of there being so many tungsten atoms together in the initial state implies that there will likely be large clusters on the support in all cases This is important because mono-tungstate species are not believed to be the active component [1]. In agreement with expectations based on proton affinities, higher temperatures are required for H–D exchange with chlorobenzene compared to that with toluene; the Brønsted sites in tungstated zirconia appear to be significantly weaker than those in H-ZSM-5
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