Niobium Insertion into α<sub>II</sub>-VOPO<sub>4</sub>: Tuning the Catalytic Properties for Selective Oxidation
Frederik Rüther, Rhea Machado, Esteban Gioria, Sylvia Lorraine Kunz, Knut Wittich, Patricia Löser, Michael Geske, Stephan A. Schunk, Robert Glaum, Frank Rosowski
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 α II - and β-V V OPO 4 reveal a much lower catalytic performance. This study shows that introducing Nb into α II -VOPO 4 forming a solid solution (V 1 -x Nb x )OPO 4 yields a bulk material with tunable catalytic properties. Selectivities of S MAN = 48% at a conversion of X n -butane = 30% on (V 0.1 Nb 0.9 )OPO 4 are shown to be related to the isolation of surface V-sites, which surpass known VOPO 4 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- x Nb x )OPO 4, 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.