State-of-the-Art Review of Oxidative Dehydrogenation of Ethane to Ethylene over MoVNbTeOx Catalysts

Ethylene is mainly produced by steam cracking of naphtha or light alkanes in the current petrochemical industry. However, the high-temperature operation results in high energy demands, high cost of gas separation, and huge CO2 emissions. With the growth of the verified shale gas reserves, oxidative dehydrogenation of ethane (ODHE) becomes a promising process to convert ethane from underutilized shale gas reserves to ethylene at a moderate reaction temperature. Among the catalysts for ODHE, MoVNbTeOx mixed oxide has exhibited superior catalytic performance in terms of ethane conversion, ethylene selectivity, and/or yield. Accordingly, the process design is compact, and the economic evaluation is more favorable in comparison to the mature steam cracking processes. This paper aims to provide a state-of-the-art review on the application of MoVNbTeOx catalysts in the ODHE process, involving the origin of MoVNbTeOx, (post-) treatment of the catalyst, material characterization, reaction mechanism, and evaluation as well as the reactor design, providing a comprehensive overview of M1 MoVNbTeOx catalysts for the oxidative dehydrogenation of ethane, thus contributing to the understanding and development of the ODHE process based on MoVNbTeOx catalysts.

Phase-pure M1 MoVNbTeOx/TiO2 nanocomposite catalysts: high catalytic performance for oxidative dehydrogenation of ethane

The introduction of TiO2 can improve the catalytic performance of phase-pure M1 MoVNbTeOx in the ODHE process, in which the STY enhancement of M1/40TiO2 at 400 °C and W/F = 7.55 gcat h molC2H6−1 reached ∼76%.

Discerning the Metal Doping Effect on Surface Redox and Acidic Properties in a MoVTeNbO x for Propa(e)ne Oxidation

Adding a small quantity of K or Bi to a MoVTeNbO x via impregnation with inorganic solutions modifies its surface acid and redox properties and its catalytic performance in propa(e)ne partial oxidation to acrylic acid (AA) without detriment to its pristine crystalline structure. Bi-doping encourages propane oxydehydrogenation to propene, thus enlarging the net production rate of AA up to 35% more. The easier propane activation/higher AA production over the Bi-doped catalyst is ascribed to its higher content of surface V leading to a larger amount of total V5+ species, the isolation site effect of NbO x species on V, and its higher Lewis acidity. K-doping does not affect propane oxydehydrogenation to propene but mainly acts over propene once formed, also increasing AA to a similar extent as Bi-doping. Although K-doping lowers propene conversion, it is converted more selectively to acrylic acid owing to its reduced Brønsted acidity and the presence of more Mo6+ species, thereby favoring propene transformation via the π-allylic species route producing acrylic acid over that forming acetic acid and CO x via acetone oxidation and that yielding directly CO x .

Effect of Ce doping on the structure–activity relationship of MoVOx composite metal oxides

Ce-doped MoVOx with disperse rod-shaped exhibits excellent catalytic performance in selective oxidation of benzyl alcohol.

High yield lactic acid selective oxidation into acetic acid over a Mo-V-Nb mixed oxide catalyst

In this paper, we report for the first time a one-pot reaction enabling total transformation of lactic acid to acetic acid over a Mo-V-Nb mixed oxide catalyst having an optimal atomic ratio 19:5:1. The mechanism of the reaction consists in two parallel ways leading to acetic acid: (i) oxi-dehydrogenation of lactic acid to pyruvic acid followed by decarboxylation and (ii) decarbonylation of lactic acid to acetaldehyde followed by oxidation. In the operating conditions we used, the catalyst is very active (total conversion of lactic acid) and selective towards acetic acid (100% selectivity). A 100% yield into acetic acid is hence obtained.

Atomic-level imaging of Mo-V-O complex oxide phase intergrowth, grain boundaries, and defects using HAADF-STEM

In this work, we structurally characterize defects, grain boundaries, and intergrowth phases observed in various Mo-V-O materials using aberration-corrected high-angle annular dark-field (HAADF) imaging within a scanning transmission electron microscope (STEM). Atomic-level imaging of these preparations clearly shows domains of the orthorhombic M1-type phase intergrown with the trigonal phase. Idealized models based on HAADF imaging indicate that atomic-scale registry at the domain boundaries can be seamless with several possible trigonal and M1-type unit cell orientation relationships. The alignment of two trigonal domains separated by an M1-type domain or vice versa can be predicted by identifying the number of rows/columns of parallel symmetry operators. Intergrowths of the M1 catalyst with the M2 phase or with the Mo(5)O(14)-type phase have not been observed. The resolution enhancements provided by aberration-correction have provided new insights to the understanding of phase equilibria of complex Mo-V-O materials. This study exemplifies the utility of STEM for the characterization of local structure at crystalline phase boundaries.

Mechanism of the Oxidation−Reduction of the MoVSbNbO Catalyst: In Operando X-ray Absorption Spectroscopy and Electrical Conductivity Measurements

The mechanism of the oxidation-reduction of the MoVSbNbO catalyst has been studied in dynamic conditions using X-ray absorption spectroscopy (XAS) and electrical conductivity measurements. XAS at Sb L1- and V/Mo K-edges permitted a better understanding of the chemical processes taking place in the M1 phase of the MoVSbNbO catalyst at different temperatures and atmosphere compositions. The reduction of antimony was already observed during the annealing of the M1 phase in He at 100 degrees C, which might be explained by the presence of hydrogen in the bronze-like structure of the M1 phase. Under operando conditions at 380 degrees C, we have found that Sb and V change their oxidation states depending on the C3H8/O2 ratio in the atmosphere. These changes occur simultaneously and with the same kinetics. Under the same conditions, variations in the oxidation state of Mo were not observed. These results prove that different types of oxygen (from the hexagonal channels and from the MO6 octahedral network) must be involved in the catalytic process although their relative contributions are different. It was found that the electrical conductance of the M1 phase correlates with the oxidation states of Sb and V and the concentration of oxygen vacancies.

Probe Molecule Chemisorption−Low Energy Ion Scattering Study of Surface Active Sites Present in the Orthorhombic Mo−V−(Te−Nb)−O Catalysts for Propane (Amm)oxidation

Methanol and allyl alcohol chemisorption and surface reaction in combination with low energy ion scattering (LEIS) were employed to determine the outermost surface compositions and chemical nature of active surface sites present on the orthorhombic (M1) Mo-V-O and Mo-V-Te-Nb-O phases. These orthorhombic phases exhibited vastly different behavior in propane (amm)oxidation reactions and, therefore, represented highly promising model systems for the study of the surface active sites. The LEIS data for the Mo-V-Te-Nb-O catalyst indicated surface depletion for V (-23%) and Mo (-27%), and enrichments for Nb (+55%) and Te (+165%) with respect to its bulk composition. Only minor changes in the topmost surface composition were observed for this catalyst under the conditions of the LEIS experiments at 400 degrees C, which is a typical temperature employed in these propane transformation reactions. These findings strongly suggested that the bulk orthorhombic Mo-V-Te-Nb-O structure may function as a support for the unique active and selective surface monolayer in propane (amm)oxidation. Moreover, direct evidence was obtained that the topmost surface VO(x) sites in the orthorhombic Mo-V-Te-Nb-O catalyst were preferentially covered by chemisorbed allyloxy species, whereas methanol was a significantly less discriminating probe molecule. The surface TeO(x) and NbO(x) sites on the Mo-V-Te-Nb-O catalyst were unable to chemisorb these probe molecules to the same extent as the VO(x) and MoO(x) sites. Our findings suggested that different surface locations for V(5+) ions in the orthorhombic Mo-V-O and Mo-V-Te-Nb-O catalysts may be primarily responsible for vastly different catalytic behavior exhibited by the Mo-V-O and Mo-V-Te-Nb-O phases. Although the proposed isolated V(5+) pentagonal bipyramidal sites in the orthorhombic Mo-V-O phase may be capable of converting propane to propylene with modest selectivity, the selective 8-electron transformation of propane to acrylic acid and acrylonitrile may require the presence of several surface VO(x) redox sites lining the entrances to the hexagonal and heptagonal channels of the orthorhombic Mo-V-Te-Nb-O phase. The study of allyl alcohol oxidation over the Mo-V-O and Mo-V-Te-Nb-O catalysts further suggested that water plays a critical role during the oxidation of acrolein intermediate to acrylic acid over the orthorhombic (M1 phase) Mo-V-Te-Nb-O catalysts. Finally, the present study strongly indicated that chemical probe chemisorption combined with low energy ion scattering (LEIS) is a novel and highly promising surface characterization technique for the investigation of the active surface sites present in the bulk mixed metal oxides.

Roles of Surface Te, Nb, and Sb Oxides in Propane Oxidation to Acrylic Acid over Bulk Orthorhombic Mo−V−O Phase

The outermost surfaces and subsurface layers of the orthorhombic (M1) Mo-V-O catalysts promoted with Te, Nb, and Sb oxide species at submonolayer surface coverage were examined by low-energy ion scattering (LEIS). This study indicated that the Nb oxide species was preferentially located at the topmost surface, while the subsurface Te and Sb concentrations declined gradually into the bulk. Although the original Mo-V-O catalyst was essentially unselective in propane oxidation to acrylic acid, significant improvement in the selectivity to acrylic acid was observed when Te, Nb, and Sb oxides were present as the surface species at submonolayer coverage. These findings further suggested that the formation of the surface V-O-M bonds (M = Nb, Te, or Sb) was highly beneficial for both the activity and selectivity of the orthorhombic Mo-V-O catalysts in propane oxidation to acrylic acid. The highest selectivity was observed when both Nb and Te (or Sb) oxide species were present at the surface. The selectivity trends established for the surface-promoted Mo-V-O catalyst parallel those found previously for the corresponding bulk Mo-V-M-O catalysts. These results further indicated that the introduction of surface metal oxide species is a highly promising method to prepare well-defined model catalysts for studies of the structure-activity/selectivity relationships as well as optimize the catalytic performance of the bulk mixed Mo-V-M-O catalysts for selective (amm)oxidation of propane.

A Study of the Surface Region of the Mo−V−Te−O Catalysts for Propane Oxidation to Acrylic Acid

The bulk mixed Mo-V-Te oxides possess high activity and selectivity in propane oxidation to acrylic acid and represent well-defined model catalysts for studies of the surface molecular structure-activity/selectivity relationships in this selective oxidation reaction. The elemental compositions, metal oxidation states, and catalytic functions of V, Mo, and Te in the surface region of the model Mo-V-Te-O system were examined employing low energy ion scattering (LEIS) and X-ray photoelectron spectroscopy (XPS). This study indicated that the surfaces of these catalysts are terminated with a monolayer, which possesses a different elemental composition from that of the bulk. The rates of propane consumption and formation of propylene and acrylic acid depended on the topmost surface V concentration, whereas no dependence of these reaction rates on either the surface Mo or Te concentrations was observed. These findings suggested that the bulk Mo-V-Te-O structure may function as a support for the unique active and selective surface monolayer in propane oxidation to acrylic acid. The results of this study have important practical consequences for the development of improved selective oxidation catalysts by introducing surface metal oxide components to form new surface active V-O-M sites for propane oxidation to acrylic acid.