Synergistic effects of TiO2 and palladium-based cocatalysts on the selective oxidation of ethene to acetic acid on Mo-V-Nb oxide domains.

Methanol carbonylation on Rh and Ir organometallic complexes with iodide co-catalysts is currently used to produce acetic acid. Catalyst cost and recovery, as well as the toxic and corrosive nature of iodide compounds, has led a search for alternate routes, such as zeolite acid catalysts for methanol or dimethyl ether carbonylation and dispersed metal oxide domains for the oxidation of ethane, ethene, and ethanol. A commercial process using Pd-promoted (1.5% wt. Pd) heteropolyacid (HPA) catalysts for ethene oxidation was reported to give approximately 80% acetic acid selectivities at low temperatures (ca. 420 K). These polyoxometalate clusters, promoted with high concentrations of noble metals, are expensive and do not maintain their structure during the oxidative processes required for the regeneration of deactivated catalysts. Mo–V oxides catalyze the (ammo)oxidation of alkanes and the oxidation of unsaturated molecules to carboxylic acids. Multicomponent oxides promoted by noble metals (Mo1V0.396Nb0.128Pd3.84810 4) gave high acetic acid selectivities (ca. 80%) using ethene as a reactant. Their high stability allowed their use as catalysts at relatively high temperatures (560 K), which led, in turn, to higher acetic acid productivities than on polyoxometalate-based catalysts. We have found that the presence of TiO2 as a colloidal suspension during precipitation of Mo–V–Nb oxides leads to markedly higher rates for oxidation of ethane, ethene, and ethanol to acetic acid. We report herein these effects for ethene oxidation and provide evidence for the marked influence of Pd sites, present as trace components (0.0025– 0.005% wt.) within physical mixtures, on the rate of conversion of ethene into acetaldehyde, a rate-determining step in acetic acid synthesis. The catalytic materials reported herein catalyze ethene oxidation to acetic acid with much higher rates than on other catalysts, while using only trace amounts of costly Pd components. The ability of Pd promoters to act effectively within physical mixtures allows independent optimization of the Pd and oxide functions and much more efficient use of costly noble metals. Figure 1 shows ethene conversion rates and acetic acid selectivities on Mo0.61V0.31Nb0.08Ox and Mo0.61V0.31Nb0.08Ox/ TiO2 catalysts. Both materials showed high initial acetic acid selectivities (70%), which decreased with increasing ethene conversion, as a result of secondary oxidation of acetic acid to CO and CO2 (COx). Catalysts prepared by precipitation of active oxides in the presence of a colloidal suspension of preformed TiO2 (24% Mo0.61V0.31Nb0.08Ox/TiO2) gave ethene oxidation rates approximately 10-times higher (per active component) and around 2.5-times higher (per mass) than powders with similar V–Mo–Nb composition (Figure 1a). The surface area of Mo0.61V0.31Nb0.08Ox/TiO2 is 34 m g 1 (from N2 physisorption at its normal boiling point), and approx-