Bulk Molybdenum and Tungsten Phosphides for Selective Phenol Production from Guaiacol

Bulk MoP and WP were investigated and compared in guaiacol hydrodeoxygenation to phenol. The catalysts obtained were studied by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and temperature-programmed desorption of NH₃ (NH₃-TPD) analyses. MoP was shown to be more active than WP. However, WP was more selective in phenol production. Guaiacol conversion using MoP was 90-98%. The highest selectivity for phenol was 66% (340 °C). By increasing the temperature to 380 °C, phenol selectivity decreased to 31%, while selectivity for cyclohexane increased to 29%. Thus, MoP was active not only in hydrodeoxygenation but also in hydrogenation. Guaiacol conversion over WP was 53-90%. The highest selectivity for phenol was 84% (380 °C). Hydrogenation products were also detected but with low selectivity. Thus, WP was active in the partial hydrodeoxygenation of guaiacol and was more suitable for the selective production of phenol than MoP. It was shown that after a 30 h recycling test, the activity of MoP did not decrease (1st and 5th cycle conversion value was 91%), while the activity of WP reduced (1st and 5th cycle conversion values were 81 and 64%, respectively). However, the activity of both catalysts at average conversion values decreased. Selectivity for phenol remained unaltered over both catalysts. It was supposed that catalyst activity decreased due to partial destruction of the crystalline phosphide phase and the surface phosphide oxidation to phosphate.