M1 to M2 Phase Transformation and Phase Cooperation in Bulk Mixed Metal Mo–V–M–O (M=Te, Nb) Catalysts for Selective Ammoxidation of Propane

Abstract

In the present study we systematically explored hydrothermal synthesis of bulk mixed metal Mo-V-(TeNb)-O catalysts and investigated the bulk characteristics of the resulting M1 and M2 phases as well as their roles in the selective ammoxidation of propane. It was found that unlike Mo-V-Te-Nb-O M1 phases, the Mo-V-Te-O M1 phases may be quantitatively transformed into M2 phases of the same chemical composition indicating that Nb stabilizes the M1 structure. The stabilizing role of Nb and Te was further observed in high resolution TEM studies of Mo-V-(Te-Nb)-O catalysts which indicated that structural order and the M1 phase domain size progressively decreased in this order: Mo-V-Te-Nb-O [ Mo-V-Te-O [ Mo-V-O. The cooperation between the M1 and M2 phases in propane ammoxidation to acrylonitrile was observed only at low propane conversions suggesting that the M1 phase is the only crystalline phase required for the activity and selectivity of the Mo-V-Te-Nb-O catalysts in propane ammoxidation to acrylonitrile at practical propane conversions.

Role of MoVTeNb oxide catalyst constituent phases in propane oxidation to acrylic acid.

“…and M2 phases cooperation in providing a high yield of acrylic acid [16][17][18][19]. The M1 phase of the optimal chemical composition The M1 phase is sufficient for efficient proceeding of the 6 and VO 6 octahedrons [7].…”

On the nature of active sites in MoVTeO and MoVTeNbO catalysts: The influence of catalyst activation temperature

“…procedure, composition, etc [3,4,8,9,[11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29]. However, the catalytic behavior of these catalysts is strongly related to the presence of M1 phase [18,22,23], although the presence of M2, in some case, could have a synergic effect [17][18][19].…”

“…Accordingly, changes in the catalyst surface could occur during the catalyst activation, especially in Nb-containing catalysts. Thus, it has been proposed that the incorporation of Nb 5+ cations decreases the number of acid sites [16], stabilizes the crystalline structure [25], and could facilitate a less degradation of acrylic acid to carbon oxides [26].…”

Characterization of MoVTeNbOx Catalysts during Oxidation Reactions Using In Situ/Operando Techniques: A Review

“…Although the MoVTeO sample already has the M1 phase, the presence of Nb +5 ions in the MoVTeNbO samples decreases the number of acid sites, stabilizes the crystalline structures, and reduces further degradation of acrylic acid to carbon oxides (CO, CO2, etc. ), eventually resulting in much-improved acrylic acid yields [13,53,79]. Although the XRD patterns show the same M1 phase for MoVTeO and MoVTeNbO catalysts, Raman spectroscopy was able to detect the presence of NbOx species whose presence decrease the strength of acidity as In a study by Naraschewski et al [12], the activity of the M1 phase is claimed to be independent of the total Te concentration although Te +4 is suggested to be active for α-H abstraction [50].…”