Mechanically induced disordered VOHPO4 x 0.5 H2O structure and its behaviour in the phase transformation in the presence of ammonia

The mechanical treatment of VOHPO4 x 0.5 H2O (VHP) in different mills (vibration ball mill, planetary mill and disk vibration mill) has induced a significant decrease in primary crystallite size (from 1700 Angstrom to 250 Angstrom) and homogenisation of them. The habit of the crystallites is changed from plate-shaped to cube-shaped. The surface area is increased. The induced disturbances in the structure of VHP have influenced its reactivity. In presence of air at room temperature strong mechanically treated VHP is transformed into VOHPO4 x 4 H2O. In an ammonia-air-water vapour flow at a temperature of about 670 K VHP is transformed into mainly (NH4)(2)[(VO)(3)(P2O7)(2)] (AVP). Several crystalline and X-ray amorphous intermediate phases are formed during the transformation process from VHP into AVP in dependence on the mechanically generated disturbances in the starting material. The temperature of the transformation start of VHP is decreased (from 700 K to 390 K) and the temporal onset of the AVP formation is earlier (from 450 min to 190 min) with the reduction of the primary crystallite size of the starting material. The catalytic activity of the generated AVP was tested in the ammoxidation reaction of toluene to benzonitrile at 710 K. The mechanical treatment of the precursor VHP leads to an increase in the catalytic activity and to an optimum in the nitrile selectivity. The reason for the increased catalytic activity is due to the lowered particle size. Size and habit of the precursor primary crystallite play a role for an optimal nitrile selectivity. The formation process of AVP was studied by means of a specially adapted in situ-powder-XRD-catalytic reactor in the temperature range from room temperature to 710 K and in a time interval up to 480 min.