Propane Ammoxidation over Mo–V–Te–Nb–O M1 Phase Investigated by DFT: Elementary Steps of Ammonia Adsorption, Activation and NH Insertion into π-Allyl Intermediate

Abstract

The selective ammoxidation of propane into acrylonitrile catalyzed by the bulk Mo-V-Te-Nb-O system has received significant attention because it is more environmentally benign than the current process of propene ammoxidation and relies on more abundant propane feedstock. The reaction mechanism is proposed to consist of a series of elementary steps including propane oxidative dehydrogenation, ammonia and O 2 activation, and NH x insertion into C 3 intermediates. In this study density functional theory calculations have been performed to investigate the energetics of ammonia adsorption and activation in the proposed active center in the ab plane of the M1 phase. The formation of NH x (x = 0, 1, 2, 3) species is found to be highly favored on reduced, oxodepleted metal sites. The reduced Mo site is determined to be the most favorable site for ammonia activation by comparing the reaction energy profiles for the sequential dehydrogenation of ammonia on the various metal sites. The activation barrier for the initial H abstraction from ammonia was found to depend strongly on the surface sites that stabilize H and NH 2 , and is as low as 0.28 eV when NH 2 is stabilized by the reduced Mo site and H is abstracted by the telluryl oxo group. The subsequent step of surface NH insertion into a p-allyl gas intermediate was also found to have a low activation energy barrier of 0.03 eV on the reduced Mo site.

Structure and electronic properties of MoVO type mixed-metal oxides – a combined view by experiment and theory

“…50,52 Although these results agree with the experimental finding that the MMOs of the type MoVTe(Nb)O with Te are catalytically more active than the simple MoVO system, 17 the barriers calculated at the GGA level agree only rather qualitatively with the experimentally determined values of 131 kJ mol −1 for MoVTeNbO, and 105 kJ mol −1 for MoVTeO. 53 In anticipation of section 3.1.2, we point out here that the nature of the functional is not the only critical issue when evaluating reaction barriers. 53 Analyzing the reactions of NH x species and C-N bond formation steps, it was shown, in accordance with experiment, that these steps are not crucial for the overall kinetics of the process.…”

“…The reaction at the surface VvO group is reported to be associated with a barrier in the range 164-206 kJ mol −1 , depending on the structure adjacent to the V center, while the barrier of the dehydrogenation at the TevO group was calculated in the range 53-97 kJ mol −1 . 53 Analyzing the reactions of NH x species and C-N bond formation steps, it was shown, in accordance with experiment, that these steps are not crucial for the overall kinetics of the process. 17 More recently, efforts towards a characterization of the full catalytic cycle of the ammoxidation process have been made with the same computational methods.…”

“…[49][50][51][52][53] These investigations have addressed, among others, the adsorption of molecular species, 49,53 reaction steps of propane ammoxidation on the MoVTeNbO system 50,52,53 as well as the clarification of the position of the Ta and Nb centers in MoVTeNbO and MoVTe-TaO systems. [49][50][51][52][53] These investigations have addressed, among others, the adsorption of molecular species, 49,53 reaction steps of propane ammoxidation on the MoVTeNbO system 50,52,53 as well as the clarification of the position of the Ta and Nb centers in MoVTeNbO and MoVTe-TaO systems.…”

“…17 Cluster models including a S9 site (Fig. 52 The models for MMOs of the MoVO-type mentioned here [48][49][50][51][52][53] were mainly constructed for studying reactions that occur at the basal surface, as proposed by experiments. 52 The models for MMOs of the MoVO-type mentioned here [48][49][50][51][52][53] were mainly constructed for studying reactions that occur at the basal surface, as proposed by experiments.…”

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

“…The different catalytic behaviors of these sites in Mo-V-O and Mo-V-Te-Nb-O systems can be attributed to the different locations of V +5 sites [55]. DFT studies, however, indicate that that the reduced Mo is the most favorable site for ammonia activation [56], whereas Te is more active than V for the H abstraction from propane [57].…”