Solvothermal Synthesis and Supported Catalysis of Polyanion-derived Metal Oxide Nanoparticles

Supported metal oxides (SMOs) are important catalytic materials that find numerous applications in important industrial processes. Improving the structural properties of SMOs is a challenging objective due to material synthesis and characterization limitations. Recent developments in the characterization of SMOs, specifically tungstated zirconia (WO x /ZrO 2 ), have revealed structural information that renewed scientific interest in developing more sophisticated synthetic protocols for SMOs. The current work aims to provide a robust characterization of WO x /ZrO 2 by using different characterization techniques and probe reactions. Conventional and non-conventional synthetic methods are investigated to cover the whole spectrum of published methods in order to understand the properties and limitations of these techniques. In the second part of this work, a new synthetic approach is presented that successfully produces ultrasmall (smaller than 2 nm) tungsten oxide nanoparticles (WO x NPs). By using conventional tungsten precursors and oleylamine, WO x NPs are synthesized, characterized, and finally supported to test their propene metathesis activity. Conventional WO x /ZrO 2 catalysts were prepared and extensively studied by probing their n -pentane isomerization activity and methanol dehydration activity. WO x /ZrO 2 prepared via incipient wetness impregnation shows maximum n -pentane isomerization turnover rates ( TOR ) at intermediate surface densities ( � surf ). This method delivers the most active n -pentane isomerization WO x /ZrO 2 catalysts since it maximizes the number density of the active sub-nm slightly distorted Zr-WO x sites at � surf between 5.2-6.2 W/nm 2 . By comparing the n -pentane isomerization activity with the methanol dehydration activity of WO x /ZrO 2 , n -pentane isomerization is shown to be an excellent probe reaction for qualitatively identifying the relative (to the other species) population density of Zr-WO x clusters. Bimolecular n -pentane isomerization is the prevailing mechanism and requires a higher population density of Zr-WO x clusters than methanol dehydration. In the second part of this work, a new solvothermal synthesis route for the preparation of ultrasmall tungsten oxide nanoparticles (WO x NPs) is introduced. By using ammonium polyanionic salts and oleylamine, high yields (92±5%) of oleylaminecoated WO x NPs were consistently synthesized. The co-addition of an organic oxidant during the synthesis led to smaller WO x NPs thereby providing insight into the NP synthesis mechanism. Deposition and activation of the NPs on SiO 2 support by removal of oleylamine allows better control over the WO x domain size than conventional methods. Oleylamine suppresses WO x NP sintering during calcination and prevents the formation of larger polytungstates present in conventional catalysts. The supported WO x NPs were found to be up to 3 times more selective for metathesis products than conventionally prepared tungstated silica likely due to their controlled structure

Nature of catalytically active sites in the supported WO3/ZrO2 solid acid system: a current perspective

Tungstated zirconia (WO3/ZrO2) is one of the most well-studied solid acid catalyst systems and continues to attract the attention of both academia and industry. Understanding and controlling the properties of WO3/ZrO2 catalysts has been a topic of considerable interest over almost the past three decades, with a particular focus on discovering the relationship between catalytic activity and the molecular structure of the surface acid site. Amorphous tungsten oxide (WOx) species on ZrO2 surfaces were previously proposed to be very active for different acidic reactions such as alcohol dehydration and alkane isomerization. Recent developments in electron optical characterization and in situ spectroscopy techniques have allowed researchers to isolate the size, structure, and composition of the most active catalytic species, which are shown to be three-dimensional distorted Zr-WOx clusters (0.8–1.0 nm). Complementary theoretical calculations of the Brønsted acidity of these Zr-WOx clusters have confirmed that they possess the lowest deprotonation energy values. This new insight provides a foundation for the future characterization and theory of acidic supported metal oxide catalytic materials that will, hopefully, lead to the design of more active and selective catalysts. This perspective presents an up-to-date, comprehensive summary of the leading models of WO3/ZrO2 solid acid catalysts

Synthesis of ultrasmall metal oxide nanoparticles

The invention generally relates to the ultrasmall MOx nanoparticles that are made in a solvothermal method using water soluble inorganic ammonium salt precursors of the MOx and organic amines, and slow heating to generate uniform ultrasmall MOx nanoparticles of 5 nm or less, as well as methods to make and use same

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_<i>x</i> 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_<i>x</i> NPs at consistently high yields of 92 ± 5%. The semicrystalline NPs contained 20–36 WO_<i>x</i> 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 <i>N</i>-oxide led to smaller WO_<i>x</i> NPs (1.0 ± 0.4 nm), whereas the reductant 1,12-dodecanediol led to WO_<i>x</i> 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

Nature of Catalytically Active Sites in the Supported WO₃/ZrO₂ Solid Acid System: A Current Perspective

Tungstated zirconia (WO₃/ZrO₂) is one of the most well-studied solid acid catalyst systems and continues to attract the attention of both academia and industry. Understanding and controlling the properties of WO₃/ZrO₂ catalysts has been a topic of considerable interest over almost the past three decades, with a particular focus on discovering the relationship between catalytic activity and the molecular structure of the surface acid site. Amorphous tungsten oxide (WO_<i>x</i>) species on ZrO₂ surfaces were previously proposed to be very active for different acidic reactions such as alcohol dehydration and alkane isomerization. Recent developments in electron optical characterization and in situ spectroscopy techniques have allowed researchers to isolate the size, structure, and composition of the most active catalytic species, which are shown to be three-dimensional distorted Zr-WO_<i>x</i> clusters (0.8–1.0 nm). Complementary theoretical calculations of the Brønsted acidity of these Zr-WO_<i>x</i> clusters have confirmed that they possess the lowest deprotonation energy values. This new insight provides a foundation for the future characterization and theory of acidic supported metal oxide catalytic materials that will, hopefully, lead to the design of more active and selective catalysts. This perspective presents an up-to-date, comprehensive summary of the leading models of WO₃/ZrO₂ solid acid catalysts