Catalyst preparation

Tetrabutylammonium (TBA) salt of Keggin-type HPAs (denoted as TBA-HPAs), such as (TBA)_n[PW_12-xV_xO₄₀] (x = 0, 1, 2; n = 3, 4, 5), were prepared according to the published procedure with some modifications^36,37,38,39. They were analyzed using IR spectroscopy (see Supplementary Information). As a reference catalyst, WO₃ (Kanto Chemical Co. Inc.) was used as supplied. All other chemicals were reagent-grade; they were used as supplied.

Keggin-type TBA-HPAs supported on CeO₂ (JRC-CEO-1) catalysts were prepared by impregnation with acetone as the impregnation solvent. The loading amount of TBA-HPAs was 40 wt%. First, acetone (30 mL) and CeO₂ (0.6 g) were added to a 300 mL eggplant flask and were stirred for 2 h using a rotary evaporator. Subsequently, TBA-HPAs (0.4 g) dissolved into acetone (10 mL) were added to the flask and were stirred for 2 h again. The resulting suspension was dried. Then the resulting solid was dried overnight at 393 K.

WO₃/CeO₂ catalyst containing 11.9 wt% W was prepared using impregnation with water as the impregnation solvent, as described in a previous report³². An ammonium metatungstate hydrate ((NH₄)₆H₂W₁₂O₄₀·H₂O) was used as a precursor. After impregnation, the resulting suspension was dried. Then the resulting solid was dried at 393 K overnight, followed by calcination for 3 h in air at 773 K under a ramping rate of 0.5 K min⁻¹.

Ce₂(WO₄)₃/CeO₂ catalyst containing 11.9 wt% W was prepared by impregnating CeO₂ with an aqueous solution of ammonium metatungstate hydrate using a similar method to that for WO₃/CeO₂, except that the calcination temperature was 1173 K³². As a reference, unsupported Ce₂(WO₄)₃ was prepared using a complex method combining ethylenediamine tetraacetic acid and citrate ions, as described in previous reports^40,41.