Niobium-doped heteropolyacid included in silica--titania support as catalyst for selective sulfoxidation

In the field of catalysis, the use of heteropolyacids is widespread, since they operate under mild conditions, favoring selectivity in reactions and reducing environmental pollution by not producing large volumes of waste. Heterogeneous catalysis is preferred because allows for easy recovery and re-use. This work shows the preparation of mixed silica--titania materials to immobilize a Keggin-type heteropolyacid doped with niobium and to use as a heterogeneous catalysts in sulfoxidation reactions under eco-friendly conditions.

Niobium-doped heteropolyacid (PMoNb) was synthesized by way of a hydrothermal synthesis method from orthophosphoric acid, molybdenum trioxide, and niobium pentoxide. Subsequently, PMoNb was included in different silica and titania supports using the sol--gel method from the precursors, i.e., tetraethyl orthosilicate and titanium isopropoxide. The prepared catalysts were characterized by XRD, FT-IR, Raman, and potentiometric titration, and tested in the selective oxidation of diphenyl sulfide to diphenyl sulfoxide.

The characteristic bands of both supports were found in the FT-IR and Raman spectra, with no appreciable differences due to the presence of PMoNb. Similar results were observed from XRD patterns. All the solids obtained present high acidity.

Regarding catalytic performance, bulk PMo and PMoNb catalysts were tested in the reaction, and it was observed that the incorporation of Nb improved the catalytic behaviour: 94% conversion and 94% selectivity (PMoNb) vs. 36% conversion and 100% selectivity (PMo) after 7 h. Moreover, the activity improved when the heteropolyacids were included in the support. However, the presence of Nb did not enhance the activity of the heterogeneous catalysts.

On the other hand, the high activity of TiO₂, undesired since it reduces selectivity, could be controlled when silica is incorporated into the structure. In this way, the best result was obtained using PMo, which was included in a support of silica--titania 1:1, with a conversion of 99% and a selectivity towards diphenyl sulfoxide of 88% after one hour of reaction.