Enhancement of the catalytic activity of VPO ammoxidation catalysts by use of vanadyl(iv) orthophosphate precursor compounds

Abstract

(VO)₃(PO₄)₂·7H₂O and (VO)₃(PO₄)₂·9H₂O vanadyl(IV) orthophosphate hydrates (V/P = 1.5) used as precursor compounds were transformed into highly active ammoxidation catalysts during different pretreatment procedures. These structural transformations have been investigated in the presence of ammonia-containing gases or under nitrogen, leading into materials that contain crystalline (NH₄)₂(VO)₃(P₂O₇)₂ (V/P = 0.75) and (VO)₂P₂O₇ (V/P = 1) specimens, respectively, as well as an additional X-ray-amorphous phase of partially crystalline vanadium oxides. These vanadium oxides represent the molar vanadium excess of the original precursor material in comparison to the defined vanadium amount of the crystalline proportion of the transformation product. Both components of these solids are tightly grown together to form microdomains. The solids generated this way were characterized by XRD, EDX, FTIR spectroscopy, ³¹P MAS NMR spectroscopy, XPS and chemical analyses and used as catalysts in the ammoxidation of toluene to benzonitrile. The results of the heterogeneous catalytic ammoxidation on the orthophosphate derived catalysts were compared with those runs obtained by the application of pure, as-synthesized (NH₄)₂(VO)₃(P₂O₇)₂, similar transformation products derived from VOHPO₄·0.5H₂O precursor (V/P = 1) and pure (VO)₂P₂O₇. Owing to the existence of mixed-valent vanadium oxides in increased portions of the orthophosphate derived catalysts, these solids reveal a significant enhancement of the toluene conversion rate at almost equal high nitrile selectivities in comparison to usual VPO catalysts.