An investigation on surface reactivity of Nb-doped vanadyl pyrophosphate catalysts by reactivity experiments and in situ Raman spectroscopy

Abstract

The effect of Nb when used as a promoter for vanadyl pyrophosphate, the catalyst for the oxidation of n-butane to maleic anhydride, was investigated. By means of both reactivity experiments and in situ Raman spectroscopy, it was found that the promoting effect shown by Nb occurred already for a very low amount of it, i.e., for a V/Nb atomic ratio as high as 150. The main role of Nb was to accelerate the formation of a limited amount of δ-VOPO 4 on the surface of vanadyl pyrophosphate, by prompting the oxidation of V 4+ into V 5+ under reaction conditions. On the other hand, the presence of Nb concentrations higher than the optimal amount led to the generation of an undesired excess quantity of VOPO 4 and the formation of α II -VOPO 4 , as well as having a detrimental effect on the catalytic behavior. ExperimentalCatalysts were prepared by means of the organic route described in previous references. 29,30 The Nb-doped V/P/O were prepared by adding the desired amount of Nb 5+ ammonium oxalate hydrate (Sigma-Aldrich) to the slurry containing V 2 O 5 , H 3 PO 4 and isobutanol, which are the raw materials for the synthesis of the catalyst precursor. V/P/O samples with

Structure–Reactivity Correlations in Vanadium‐Containing Catalysts for One‐Pot Glycerol Oxidehydration to Acrylic Acid

“…Different states of Vi ons can be evidenced; both V 5 + (binding energy (BE): 517.1 eV)a nd V 4 + (BE: 516.1 eV) ions can be detected on the WV sample, whereas ah igherV 5 + /V 4 + surfacer atio is shown for the extra-framework Vs pecies in the ion-exchanged sample, VO-WO x .C onsidering that the ion exchange was performed by using vanadyl ions (VO 2 + ), the XPS results point out ap artial oxidation process during catalystp reparation, consistently more severe in the case of the extra-framework Vs pecies. Moreover,t he VPP catalystp resentsV 4 + as the predominant species, but the presence of V 5 + is also significant, despite the fact that the XRD pattern only shows the presence of (VO) 2 P 2 O 7 ;t his observation agrees with previousr eports by other authors, [35][36][37] who attribute the presence of V 5 + speciest ot he coexistence of VOPO 4 domains in additiont o( VO) 2 P 2 O 7 on the surface of the catalyst. Finally,t he VCoAPO sample shows both V 5 + /V 4 + ions, whereas V 5 + is mainly observed on the VO x /VCoAPOs ample.…”

Aerobic Oxidations of Light Alkanes over Solid Metal Oxide Catalysts

“…30,35 On the other hand, surface-sensitive characterization via HRTEM and in situ XPS suggest that an amorphous layer on the catalyst surface is responsible for catalytic activity via formation of isolated VO x sites segregated from the bulk VPO material. 31,34,36,37 The VPO catalyst is synthesized via reaction of V 2 O 5 and H 3 PO 4 in either aqueous or organic media. This reaction leads to the formation of VOHPO 4 •0.5H 2 O, which is then heated to eliminate water, either in situ in the butane oxidation reactor or in a dedicated synthesis reactor.…”

“…31,40 Other studies found that dopants such as Nb led to weaker binding of maleic anhydride on the surface, decreasing the rate of overoxidation. 9,37 More detailed discussion on the VPO catalyst will be presented in section 5.4.3 of this review.…”

Nb-Doped Vanadium Phosphorus Oxide Catalyst for the Aldol Condensation of Acetic Acid with Formaldehyde to Acrylic Acid

“…There are many patents and papers describing catalysts that convert acetic acid and formaldehyde into acrylic acid. − The main catalyst system known to affect this transformation, a vanadium phosphorus oxide (VPO) catalyst, is also known for its ability to oxidize butane to maleic anhydride − and for its use in some condensation reactions. ,− Eastman Chemical Company has described a V, Ti, P mixed oxide catalyst and then modified the catalyst with alkali metal . BASF SE has invented a catalyst that includes VPO as an active ingredient, carrier and accelerator.…”

“…These doped VPO catalysts proved beneficial due to changes in their redox properties, particle size and catalytic performances. Caldarelli et al discussed the effect of Nb when used as a promoter for VPO catalysts in the oxidation of n -butane to maleic anhydride. Through the ex situ and operando / in situ experiments, it was shown that Nb plays two important roles.…”