Abstract:

Vanadium phosphorus oxide (VPO) catalysts were synthesized by the dihydrate method which involved the two steps for the preparation of the dihydrate (VOPO₄×2H₂O) and the precursor hemihydrate (VOHPO₄×0.5H₂O). Bi and Ni salt were added into the mixture of VOPO₄×2H₂O and isobutanol, and the obtained precursors were calcined in a flow of a n-butane/air mixture to produce the promoted VPO catalysts. The catalysts were characterized by X-ray diffraction (XRD), N₂ adsorption-desorption, inductively coupled plasma-atomic emission spectroscopy, scanning electron microscopy (SEM), and H₂ temperature-programmed reduction (H₂-TPR). Their catalytic properties were tested using a fixed-bed microreactor. All the catalysts gave main XRD peaks at 2θ = 22.9°, 28.5°, and 30.0°, attributing to the (020), (204), and (221) planes of the pyrophosphate phase (VO)₂P₂O₇, respectively. The promoted catalysts have smaller crystallite size and higher specific surface areas. SEM micrographs revealed the formation of more prominent plate-like crystallites that were arranged as rosette clusters. H₂-TPR results showed that the promoted catalysts had lower reduction peak temperatures and possessed higher amounts of V⁵⁺-O^2- and V⁴⁺-O^- pairs, which gave higher selectivity and activity in the selective oxidation of n-butane to maleic anhydride.

Key words: Vanadium phosphorus oxide catalyst, Promoter, n-Butane oxidation, Maleic anhydride selectivity