Tungsten and niobium oxide nanostructures as solid acid and redox catalysts

Tungsten and niobium oxide nanostructures are active as solid acid and oxidation catalysts because of their unique redox properties. WO_x and NbO_x domains supported on ZrO₂ form dispersed structures that uniformly increase in size with increasing surface density. NbO x species on γ-Al₂O₃ form poorly dispersed structures. Amorphous three-dimensional NbO_x species deposit within γ-Al ₂O₃ voids, leading to an increase in the surface area and a decrease in the pore size with increasing Nb content. The surface density of supported WO_x and NbO_x domains strongly influences their structure, domain size, electronic properties, and catalytic rates.; Maximum rates for o-xylene isomerization at 523 K and for 2-butanol dehydration at 373 K on WO_x-ZrO₂ catalysts are observed at intermediate WO_x surface densities (8–12 W/nm2 ) on polytungstate domains. o-Xylene isomerization requires Bronsted acid sites; rates at 523 K increase when H₂ is added, because H ₂ partially reduces WO_x domains to form Hδ+ Bronsted acid sites. H₂ does not increase 2-butanol dehydration rates at 373 K, because WO_x domains are reduced by 2-butanol. 2-Butanol dehydration on WO_x-ZrO₂ also requires Bronsted acid sites, since rates decrease with the addition of small amounts of 2,6-di-tert-butyl-pyridine during reaction with 2-butanol. The density of Bronsted acid sites formed during dehydration and isomerization reactions is extremely low, as determined from total 2,6-di-tert-pyridine uptakes at 373 K during 2-butanol reactions and from H₂ chemisorption uptakes at 523 K. Bronsted acid site densities are also highly dependent on WO_x surface density.; A strong correlation among the number of reduced centers, the concentration of Bronsted acid sites, and the rate of 2-butanol dehydration or o-xylene isomerization as a function of WO_x surface density confirmed the in-situ formation of active H+δ(WO₃) _n−δ Bronsted acid sites via redox processes using 2-butanol at 373 K or H₂ at 523 K as the stoichiometric source of H-atoms. Polytungstate domains are required for the partial and temporary reduction of WO_x domains, because of their optimum reducibility and accessibility. Turnover rates (per Bronsted acid site) on WO_x-ZrO ₂ catalysts were determined using acid site density measurements; these rates were compared with other Bronsted (H-ZSM5, SiO₂-supported tungstophosphoric acid) and Lewis acids (γ-Al₂O₃).