Mesostructured VPO Phases

Recently reported meso- and macroscale self-assembly approaches conducted, respectively, in the presence of surfactant mesophases [134-136] and colloidal sphere arrays [137] are highly promising for the molecular engineering of novel catalytic mixed metal oxides. These novel methods offer the possibility to control surface and bulk chemistry (e.g. the V oxidation state and P/V ratios), wall nature (i.e. amorphous or nanocrystalline), morphology, pore structures and surface areas of mixed metal oxides. Furthermore, these novel catalysts represent well-defined model systems that are expected to lead to new insights into the nature of the active and selective surface sites and the mechanism of n-butane oxidation. In this section, we describe several promising synthesis approaches to VPO catalysts, such as the self-assembly of mesostructured VPO phases, the synthesis of macroporous VPO phases, intercalation and pillaring of layered VPO phases and other methods. 

Surfactant Micellar Hexagonal micelle rod surfactant array 

Hybrid array Ordered mesoporous mixed metal oxide 

Recently, Carreon and Guliants reported novel hexagonal, cubic and lamellar VPO phases, which displayed improved thermal stability, desirable chemistries (i.e. the P/V ratios and vanadium oxidation states), and pore structures for the partial oxidation of n-butane [143-145]. These novel VPO phases displayed the selectivities to maleic anhydride up to 40 mol. % at 673K at 10 % n-butane conversion [146]. A conventional organic VPO catalyst containing well-crystallized vanadyl(IV) pyrophosphate, the proposed active and selective phase for n-butane oxidation to maleic anhydride, displayed the selectivities to maleic anhydride 50 mol. % under the same reaction conditions. The low yields observed for mesoporous VPO catalysts confirmed the critical role of the vanadyl pyrophosphate phase (VO)2P207 in catalyzing the oxidation of -butane to maleic anhydride. Therefore, the amorphous nature of the mesoporous VPO 

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FIGURE 8 The transformation of the lamellar to the hexagonal phase of VPO precursors [Reprinted from Carreon, M.A., Guliants, V.V., Guerrero-Perez, M.O., and Banares, M.A., Microporous Mesoporous. Mater. 71, 57 (2004) Phase transformation in mesostructured VPO/surfactant composites, copyright (2004) with permission from... 

S. Dasgupta, M.Agarwal, A.Datta, 2004,Surfactant assisted organization of an exfoliated VOHPO4..5H2O to a mesostructured lamellar VPO phase, Micropor. Mesopor. Mater. 67, 229. 

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