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

Authors: Frederik Rüther, Rhea Machado, Esteban Gioria, Sylvia L. Kunz, Knut Wittich, Patricia Löser, Michael Geske, Stephan A. Schunk (hte GmbH), Robert Glaum, Frank Rosowski

ACS Catal. 
20 February 2023, 13, 5, 3295–3307  

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 α_II- and β-V^VOPO₄ reveal a much lower catalytic performance. This study shows that introducing Nb into α_II-VOPO₄ forming a solid solution (V_1-xNb_x)OPO₄ yields a bulk material with tunable catalytic properties. Selectivities of S_MAN = 48% at a conversion of X_n-butane = 30% on (V_0.1Nb_0.9)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 (V_1-xNb_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.