Niobium Insertion into α_II-VOPO₄: Tuning the Catalytic Properties for Selective Oxidation

Abstract

A holistic understanding of the key catalytic features of vanadyl­(IV) pyrophosphate enabling high maleic anhydride (MAN) yields in n-butane oxidation has fostered a debate which has continued since the finding of the catalyst. Under reaction conditions, vanadium­(V) orthophosphate structure fragments were detected on the surface of the catalyst. However, single-phase αII- and β-VVOPO4 reveal a much lower catalytic performance. This study shows that introducing Nb into αII-VOPO4 forming a solid solution (V1-x Nb x )OPO4 yields a bulk material with tunable catalytic properties. Selectivities of S MAN = 48% at a conversion of X n‑butane = 30% on (V0.1Nb0.9)­OPO4 are shown to be related to the isolation of surface V-sites, which surpass known VOPO4 catalysts by far. A boost in the overall n-butane consumption and MAN selectivity under alkane-rich feed conditions is shown to be another characteristic of (V1-x Nb x )­OPO4, leading to a highly increased MAN productivity. XPS studies reveal that a progressive replacement of V by Nb induces a reduction of the averaged oxidation state of near-surface V from +4.7 to +4.3, a finding that correlates linearly with an elevated MAN selectivity. This study experimentally confirms site isolation and electronic environment of the near-surface V-species as the key catalytic properties, from which catalyst design rules are derived to optimize partial oxidation reactions.

The Catalytic Function of Phosphorus Enriched on the Surface of Vanadium‐based Catalysts in Selective Oxidations

“…In contrast, V is present in two different oxidation states V 4+ and V 5+ in all samples. A change in composition (V to Nb ratio) within the solid solution does lead to a significant switch in the V‐oxidation state at the near‐surface, which was identified as a precise descriptor for product selectivity in a previous study [43] . The modification of the surface with PO x , shown in this paper, does not lead to any significant changes in the oxidation state of V, based on the measurements of the spent samples, shown in Figure SI13.…”

“…The VPP sample, synthesized according to patent literature, revealed the presence of a minor phase‐impurity with vanadyl pyrophosphate as main phase. All synthesis protocols are adapted from literature and are briefly described in the Experimental Section [15,43,44] . Depending on the synthesis route, the samples differ in their specific surface area, varying between 6 and 50 m 2 g −1 (see Figure SI2, and Table SI1 for details on the N 2 ‐physisorption experiments).…”

“…The fact that the normalized catalytic activity is very similar to the phosphate catalysts, as shown in Figure 1, leads to hypothesis, that the presence of P does not predominantly contribute to C−H activation on V‐based oxidation catalysts. Next, the solid solution V 0.3 Nb 0.7 OPO 4 , which was recently introduced as novel material with a tunable performance for this reaction, [43,44] converts the n ‐butane with a higher selectivity towards MAN. Nevertheless, a significant consecutive combustion can be observed throughout a broad conversion‐range varying the contact time.…”

“…With this jump in performance, a promising novel V‐phosphate based catalyst, PO x ‐V 0.3 Nb 0.7 OPO 4 , has been identified. Especially the capability to reach these high product selectivities at alkane conversions above 70 % could not be achieved by other bulk catalysts other than VPP [43] . Speaking of the benchmark system for this reaction, the effect of the surface enrichment with PO x has been studied as well.…”

“…Although the product selectivities are still superior on the benchmark catalyst, it must be kept in mind that VPP was optimized throughout decades of research to reach the current performance standards. Recently, the solid solution V 1‐x Nb x OPO 4 was introduced as mixed‐metal phosphate catalyst with tunable catalytic properties in the partial oxidation of n ‐butane [43] . While the system could be used to experimentally confirm the site isolation and the vanadium oxidation state as key properties for a selective partial oxidation, the material additionally showed unique characteristics under alkane‐rich feed conditions.…”