Preparation of WO3/Tio2 Catalyst via Ammonium Paratungstate

Tungsten oxide (WO3) has been used as a photocatalyst, gas sensor, and acid catalyst over the past several decades. WO3-based catalysts having acidic properties can be classified into several groups. One group includes WO3 crystals and those doped with foreign elements in a nanowire structure, which are used as catalysts and gas sensors.

The Titanium dioxide supported tungsten trioxide catalyst (WO3/TiO2) was prepared by the conventional impregnation method. The catalyst had been used in Friedel-Crafts (F-C) acylation of anisole with octanoic acid and yield of 90%

The experimental procedures are as below:

A solution of ammonium paratungstate, (APT, NH4)10H2W12O42·4H2O) dissolved in 60 ml of water was immersed in the oxide support. TiO2 (TIO-4, -16the anatase-type (TIO-16; Table 1, Table 2, entry 4) TiO2 was used as the support for TiO2. The ammonium paratungstate solution was evaporated on a water bath with continuous stirring in the presence of the oxide supports. The resulting powder was heat treated in a furnace in air at 773 K for 2 h. The color of the WO3/TiO2 samples, calcined in air, changed from white to yellow as the amount of tungsten oxide increased. The amount of WO3 charged was adjusted to be 4.5 nm−2 at the time of preparation. This corresponds to the stipulated amount to ensure a monolayer of WO3 according to the reported cross section of W on Al2O3 (0.22–0.245 nm2/W). In addition, samples in which the loading amount of tungsten oxide fixed be to 18% by weight were also prepared. In the case where TiO2 was used as a support, the loading of WO3 was varied in the range from 7 to 100 wt%.

In summary, the catalytic activity of WO3 in the F-C acylation with octanoic acid and anisole varies depending on the support used. The highest yield (90%) was obtained at 20 wt % loading of WO3. The highest activity was obtained when TiO2 was used as a support. The catalyst was also applicable for similar reactions involving carboxylic acids other than octanoic acid. TOF was found to be dependent on the electronegativity of the cation of the support, implying that the electron withdrawing or donating effect of the support determines the Brønsted acid strength and catalytic activity of the supported WO3.

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