Redox interaction of ammonia with (VO)₂P₂O₇

Abstract

The interaction of ammonia with (VO), P, O, prepared by calcination of the precursor compound VOHPO, 0.5H20 under nitrogen has been studied using temperature-programmed desorption of ammonia (TPDA), temperature-programmed reaction spectroscopy (TPRS), and IR and EPR spectroscopy. Massspectrometric detection was applied to observe possible ammonia decomposition or oxidation products. The investigation revealed that ammonia is not only adsorbed on but also reacts with (VO), P, O, in a redox process generating nitrogen, water and an amorphous V"'-containing compound, the concentration of which could be directly determined by potentiometric titration. The high amount of V"' found pointed towards reduction of V'" not only on the surface but also in deeper layers of the bulk. This was also confirmed by EPR spectroscopy.Furthermore, this reaction results in a change of the Brsnsted and Lewis acidity observed by IR spectroscopy.The concentration of the Brsnsted-acid OH groups was strongly enhanced by hydrolysis of P-0-P and/or V-0-P links by water formed during the redox reaction. The increased concentration of Lewis sites was caused by the removal of oxygen from surface vanadyl groups, probably creating additional coordinatively unsaturated sites. The influence of the observed redox reaction on the characterization of the acidity and the formation of VPO catalysts in the ammoxidation reaction are discussed.

Amorphous Vanadium Phosphate Catalysts Prepared Using Precipitation with Supercritical CO2 as an Antisolvent

“…Indeed, recently Coulston et al (12) have shown that the presence of V 5+ appears to be essential for the initial activation of n-butane, an observation that is essentially in agreement with earlier TAP pulse reactor experiments (13). To some extent, the debate concerning the involvement of specific VPO phases is complicated by the observation that many vanadium phosphate catalysts contain significant amounts of disordered material (5,6,(9)(10)(11)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29). Some studies suggest the amorphous material is an active component (5,6,15,18,19,(26)(27)(28)(29), whereas others consider this not to be the case (8)(9)(10)(11)22).…”

“…Many studies have suggested that an amorphous overlayer on the crystalline VPO subsurface may be the active surface for this reaction (5,6,15,18,19,(26)(27)(28)(29). This has also been observed on related vanadium carbide catalysts for butane oxidation (21).…”

“…To some extent, the debate concerning the involvement of specific VPO phases is complicated by the observation that many vanadium phosphate catalysts contain significant amounts of disordered material (5,6,(9)(10)(11)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29). Some studies suggest the amorphous material is an active component (5,6,15,18,19,(26)(27)(28)(29), whereas others consider this not to be the case (8)(9)(10)(11)22). Amorphous VPO compounds have been used as one way to secure in situ regeneration in industrial reactors (30) by using a high temperature treatment with pure n-butane.…”

Partial oxidation of n-pentane over vanadium phosphorus oxide supported on hydroxyapatites

“…The selective and controlled oxidation of hydrocarbons using heterogeneous catalysts to produce these compounds, occupy an important place in the chemical industry 2,3 . Vanadium phosphorus oxide (VPO) was found to be efficient in converting n-pentane to maleic anhydride and phthalic anhydride 4,5 . This catalytic reaction is promoted by the existence of both the vanadyl pyrophosphate (VPP) as well as of VOPO 4 phases in VPO 5,6 .…”

“…Vanadium phosphorus oxide (VPO) was found to be efficient in converting n-pentane to maleic anhydride and phthalic anhydride 4,5 . This catalytic reaction is promoted by the existence of both the vanadyl pyrophosphate (VPP) as well as of VOPO 4 phases in VPO 5,6 . Supported VPO catalysts showed an enhancement in the selectivity towards maleic anhydride and phthalic anhydride.…”