Nikolaos Soultanidis

Relating n-Pentane Isomerization Activity to the Tungsten Surface Density of WOx/ZrO2

Zirconia-supported tungsten oxide (WO(x)/ZrO(2)) is considered an important supported metal oxide model acid catalyst, for which structure-property relationships have been studied for numerous acid-catalyzed reactions. The catalytic activity for xylene isomerization, alcohol dehydration, and aromatic acylation follows a volcano-shape dependence on tungsten surface density. However, WO(x)/ZrO(2) has not been studied for more acid-demanding reactions, like n-pentane isomerization, with regard to surface density dependence. In this work, WO(x)/ZrO(2) was synthesized using commercially available amorphous ZrO(x)(OH)(4-2x) and model crystalline ZrO(2) as support precursors. They were analyzed for n-pentane isomerization activity and selectivity as a function of tungsten surface density, catalyst support type, and calcination temperature. Amorphous ZrO(x)(OH)(4-2x) led to WO(x)/ZrO(2) (WZrOH) that exhibited maximum isomerization activity at ∼5.2 W·nm(-2), and the crystalline ZrO(2) led to a material (WZrO(2)) nearly inactive at all surface densities. Increasing the calcination temperature from 773 to 973 K increased the formation of 0.8-1 nm Zr-WO(x) clusters detected through direct imaging on an aberration-corrected high-resolution scanning transmission electron microscope (STEM). Calcination temperature further increased catalytic activity by at least two times. Brønsted acidity was not affected but Lewis acidity decreased in number, as quantified via pyridine adsorption infrared spectroscopy. WO(x)/ZrO(2) exhibited isomerization activity that peaked within the first 2 h time-on-stream, which may be due to Zr-WO(x) clusters undergoing an activation process.

Solvothermal synthesis of ultrasmall tungsten oxide nanoparticles

The synthesis of catalytically useful, ultrasmall oxide nanoparticles (NPs) of group 5 and 6 metals is not readily achievable through reported methods. In this work, we introduce a one-pot, two-precursor synthesis route to <2 nm MO(x) NPs in which a polyoxometalate salt is decomposed thermally in a high-boiling organic solvent oleylamine. The use of ammonium metatungstate resulted in oleylamine-coated, crystalline WO(x) NPs at consistently high yields of 92 ± 5%. The semicrystalline NPs contained 20-36 WO(x) structural units per particle, as determined from aberration-corrected high-resolution scanning transmission electron microscopy, and an organic coating of 16-20 oleylamine molecules, as determined by thermogravimetric analysis. The NPs had a mean size of 1.6 ± 0.3 nm, as estimated from atomic force microscopy and small-angle X-ray scattering measurements. Carrying out the synthesis in the presence of organic oxidant trimethylamine N-oxide led to smaller WO(x) NPs (1.0 ± 0.4 nm), whereas the reductant 1,12-dodecanediol led to WO(x) nanorods (4 ± 1 nm × 20 ± 5 nm). These findings provide a new method to control the size and shape of transition metal oxide NPs, which will be especially useful in catalysis.