Phase-pure M1 Research Articles

Microporous exposure on catalytic performance of MoVNbTeOx mixed metal oxides in the oxidative dehydrogenation of ethane

The catalytic performance of the non-purified MoVNbTeOx catalyst was optimized by changing the composition of the synthesis gel method in the hydrothermal synthesis process. However, the enhancement in activity was more pronounced for the post-treatment process of the non-purified catalyst. The catalytic performance of non-purified MoVNbTeOx catalysts and phase-pure M1 catalysts in the oxidative dehydrogenation of ethane (ODHE) were enhanced by water agitation post-treatment. The results showed that the phase-pure M1 catalyst obtained by treating the MoVNbTeOx catalyst with nitric acid had an increased activity per unit mass, but with a mass loss of catalyst materials during the purification process. After several days of post-treatment with water agitation, the catalytic activity of the non-purified MoVNbTeOx and the M1 samples gradually increased and stabilized, where both catalysts had a comparable catalytic performance. All the samples were systematically characterized, showing that the catalytic performance of the samples was mainly correlated with the enhanced specific surface area/pore volume. The results show that for the hydrothermally synthesized MoVNbTeOx catalysts, the reaction takes place in the micropores and the key to improve the performance of MoVNbTeOx catalysts is to effectively expose the active sites. Physical post-treatment process that does not require nitric acid or hydrogen peroxide for purification is also effective in enhancing catalyst activity by boosting the active site.

Determination of highly active and selective surface for the oxidative dehydrogenation of ethane over phase-pure M1 MoVNbTeOx catalyst

The original phase-pure M1 MoVNbTeOx catalyst was mechanically treated with stirring water for several days under ambient conditions to obtain particles with different sizes. The catalytic performance was significantly improved and stabilized after about 7 days of treatment. Among all the catalysts, the sample treated in water for 7 days can obtain 57.6 % ethane conversion and 0.79 kgC2H4/kgcat/h ethylene productivity while the original phase-pure M1 catalyst was only 19.2 % and 0.27 kgC2H4/kgcat/h at reaction temperature of 400 °C and contact time of 18.5 gcat·h/molC2H6. Meanwhile, the catalyst was systematically characterized and the results showed that there was essentially no difference in microstructure between the original and treated catalysts. TEM and SEM morphological characterizations presented that the particle size and aspect ratio of the catalysts changed significantly, in which the particle size of the catalyst become smaller and the proportion of basal surface was enhanced. The activity and selectivity of the catalysts exhibited a correlation with the amount and proportion of basal surface. It can be deducted that the basal surface is the most active and selective surface during oxidative dehydrogenation of ethane (ODHE) process, and the intrinsic activity based on the basal (001) plane of the catalyst remains stable after the mechanical treatment. Samples treated in water for 7 days showed excellent long-term stability in a 200 h stability test at reaction temperature of 440 °C and contact time of 9.3 gcat·h/molC2H6. The present work demonstrated that the differences in the catalytic properties of different M1 samples depend mainly on the amount and proportion of exposed basal surface, and the possibility of enhancing the activity of orthorhombic M1 phase by fragmentation method.

Identification of the Intrinsic Active Site in Phase-Pure M1 Catalysts for Oxidation Dehydrogenation of Ethane by Density Functional Theory Calculations

Heterogeneous catalysts for alkane conversion reactions are required to possess both high activity for C–H bond cleavage and selectivity to target products. This work employed an atomic substitution strategy to investigate the active site of the phase-pure M1 MoVNbTeOx catalyst for the oxidation dehydrogenation of ethane (ODHE) reaction. Density of states and crystal orbital Hamilton population (COHP) based on density functional theory calculations indicated that the transition metal–O (M–O) bonds were weakened after H adsorption. Both integrated COHP and Bader charge were useful descriptors to correlate the electronic structure with catalytic performance. The results showed that the content of V in phase-pure M1 catalysts had a linear relationship with ethane conversion. Synergetic interactions between Te–O and V–O sites were accordingly considered as the intrinsic active sites for the ODHE reaction.

Nanocomposite catalysts of non-purified MoVNbTeOx with CeO2 or TiO2 for oxidative dehydrogenation of ethane

The phase-pure M1 of MoVNbTeOx has been demonstrated as a highly active catalyst for oxidative dehydrogenation of ethane (ODHE) reaction. However, the MoVNbTeOx without further purification shows poor catalytic performance so that the M2 phase has to be removed from the bulk material to approach the high purity of M1 phase. This work presents new discovery that the nanocomposite catalyst by adding CeO2 to the MoVNbTeOx using sol–gel method can effectively promote its catalytic performance. As a comparison, the sample combining TiO2 and MoVNbTeOx is also prepared which presents limited influence. The characterization results show that the enhanced catalyst performance can be attributed to the increased active species at the catalyst surface due to the addition of CeO2. This work would provide a new solution to greatly simplify the preparation procedure of MoVNbTeOx catalysts for the ODHE process and avoid the mass loss during the purification procedure.

MoVNbTeOx M1@CeO2@Cordierite structured catalysts for ODHE process

• M1@Cordierithe structured catalyst for the ODHE process is successfully prepared. • A CeO 2 inter-layer is introduced to improve the M1 catalytic performance without decrease of ethylene selectivity. • M1@CeO 2 @Cordierithe structured catalyst shows an excellent ethylene productivity and long-term stability in ODHE process. The phase-pure M1 MoVNbTeO x catalyst was successfully coated on cordierite monolith by PVP-sol binder solution. In order to improve the activity of structured catalyst, an M1@CeO 2 @Monolith layered structure was further prepared through a two-step procedure: (i) Coating CeO 2 layer on monolith by sol–gel method, then (ii) coating the M1 on CeO 2 @Monolith. The M1@CeO 2 @Monolith structured catalyst exhibited superior catalytic performance and stability for ODHE reaction. At 400 °C, ethane conversion ( X C2H6 ) ∼ 60% and ethylene selectivity ( S C2H4 ) ∼ 85% with a space time yield of ethylene (STY) ∼ 0.9 kg C2H4 /kg cat /h (normalized to M1 ∼ 1.05 kg C2H4 /kg cat /h) have been achieved for the M1@CeO 2 @Cordierite structured catalyst, while the M1@Monolith has only about X C2H6 ∼ 40%, S C2H4 ∼ 92%, and STY ∼ 0.64 kg C2H4 /kg cat /h. In summary, these results inspired the catalyst design for ODHE process, presenting great potential for industry practice.

Mixed Metal Oxides of M1 MoVNbTeOx and TiO2 as Composite Catalyst for Oxidative Dehydrogenation of Ethane

Composite catalysts of mixed metal oxides were prepared by mixing a phase-pure M1 MoVNbTeOx with anatase-phase TiO2. Two methods were used to prepare the composite catalysts (the simple physically mixed or sol-gel method) for the improvement of the catalytic performance in the oxidative dehydrogenation of ethane (ODHE) process. The results showed that TiO2 particles with a smaller particle size were well dispersed on the M1 surface for the sol-gel method, which presented an excellent activity for ODHE. At the same operating condition (i.e., the contact time of 7.55 gcat·h/molC2H6 and the reaction temperature of 400 °C), the M1-TiO2-SM and M1-TiO2-PM achieved the space time yields of 0.67 and 0.52 kgC2H4/kgcat/h, respectively, which were about ~76% and ~35% more than that of M1 catalyst (0.38 kgC2H4/kgcat/h), respectively. The BET, ICP, XRD, TEM, SEM, H2-TPR, C2H6-TPSR, and XPS techniques were applied to characterize the catalysts. It was noted that the introduction of TiO2 raised the V5+ abundance on the catalyst surface as well as the reactivity of active oxygen species, which made contribution to the promotion of the catalytic performance. The surface morphology and crystal structure of used catalysts of either M1-TiO2-SM or M1-TiO2-PM remained stable as each fresh catalyst after 24 h time-on-stream tests.

Plasma treated M1 MoVNbTeOx–CeO2 composite catalyst for improved performance of oxidative dehydrogenation of ethane

High activity and productivity of MoVNbTeOx catalyst are challenging tasks in oxidative dehydrogenation of ethane (ODHE) for industrial application. In this work, phase-pure M1 with 30 wt% CeO2 composite catalyst was treated by oxygen plasma to further enhance catalyst performance. The results show that the oxygen vacancies generated by the solid-state redox reaction between M1 and CeO2 capture active oxygen species in gas and transform V4+ to V5+ without damage to M1 structure. The space-time yield of ethylene of the plasma-treated catalyst was significantly increased, in which the catalyst shows an enhancement near ∼100% than that of phase-pure M1 at 400 °C for ODHE process. Plasma treatment for catalysts demonstrates an effective way to convert electrical energy into chemical energy in catalyst materials. Energy conversion is achieved by using the catalyst as a medium.

Phase-pure M1 MoVNbTeOx/TiO2nanocomposite catalysts: high catalytic performance for oxidative dehydrogenation of ethane

The introduction of TiO2can improve the catalytic performance of phase-pure M1 MoVNbTeOxin the ODHE process, in which the STY enhancement of M1/40TiO2at 400 °C andW/F= 7.55 gcath molC2H6−1reached ∼76%.

Determination of Highly Active and Selective Surface for the Oxidative Dehydrogenation of Ethane Over Phase-Pure M1 Movnbteox Catalyst

Determination of Highly Active and Selective Surface for the Oxidative Dehydrogenation of Ethane Over Phase-Pure M1 Movnbteox Catalyst

MnO promoted phase-pure M1 MoVNbTe oxide for ethane oxidative dehydrogenation

Abstract M1 phase of MoVNbTe oxide catalyst is a promising catalyst for oxidative dehydrogenation of ethane (ODHE) process. Raising V5+ concentration in the catalyst is an effective way to improve the catalytic performance while thermal oxidation in air has limited effect on the increase of vanadium valence state. In this work, manganese is introduced into the catalyst system, acting as oxygen facilitator, making M1 oxidized by gas phase oxygen at lower temperature during the preparation process. The influence of manganese addition amount and combination method on catalyst activity is studied. BET, XPS, SEM, TEM, ICP, TPR, and TPD characterizations are performed to study the correlation between catalyst properties and catalytic performance. Corresponding to the valence state increment, the catalyst promoted by MnOx gives an improved catalytic performance, e.g., over 20% improvement in ODHE process at 400 °C based on the ethane conversion.

Catalytic performance of phase-pure M1 MoVNbTeOx/CeO2 composite for oxidative dehydrogenation of ethane

The introduction of cerium oxide to phase-pure M1 MoVNbTeOx with CeO2 loading from 0 up to 90 wt.% results in highly active catalysts for oxidative dehydrogenation of ethane (ODHE). These catalysts are characterized with BET, ICP, XRD, TEM, SEM and XPS techniques and tested for ODHE process. The results show that the increase of active site, V5+, is one of the dominant factors to improve the ethane conversion. In addition, the easier re-oxidation of V4+ to V5+ due to the presence of Ce4+ makes an extra contribution to the increase of turn-over frequency. Among all the catalysts, M1 catalyst with 30 wt.% CeO2 loading obtains the best productivity of 0.69 kgC2H4/kgcat·h while the corresponding performance of phase-pure M1 catalyst is 0.34 kgC2H4/kgcat·h at 400 °C with the contact time of 18.52 gcat·h/molC2H6. It is further demonstrated that M1/CeO2 catalyst (e.g., with 50 wt.% CeO2 as a representative) loading remains highly active after 3 cycles of refreshment in situ, and its crystal structure and surface morphology keep stable compared to the fresh catalyst.

Phase-pure M1 MoVNbTeOx catalysts with tunable particle size for oxidative dehydrogenation of ethane

This work presented a comprehensive study on structure sensitivity of phase-pure M1 catalyst in oxidative dehydrogenation of ethane (ODHE). We proposed an effective method to modulate the morphology of phase-pure M1 catalysts by oxalic acid treatment. The proposed approach significantly promoted the catalyst surface area by restraining the anisotropic growth of the basal (001) plane of the M1-phase in calcination and decreased the diameter of M1 needles. Experimental results showed that phase-pure M1 catalyst treated with 1.0mmol/L oxalic acid solution exhibited remarkably promoted catalyst performance of 73% ethane conversion, 85% ethylene selectivity and 0.77kC2H4/kgcat/h catalyst productivity at 400°C. The serial catalyst tests also demonstrated the structure sensitivity of M1 phase in the ODHE reaction. Active sites were located on the lateral faces as well as the basal plane of the M1 needles, but different effects on the catalyst selectivity were observed. The phase-pure M1 catalyst with a lower aspect ratio was suggested to provide better ethylene selectivity in ODHE process.

Performance of phase-pure M1 MoVNbTeOx catalysts by hydrothermal synthesis with different post-treatments for the oxidative dehydrogenation of ethane

Phase-pure M1 catalysts with different post-treatments have been prepared from the same precursor slurry by hydrothermal synthesis and used to study the key factors influencing their performance in the oxidative dehydrogenation of ethane (ODHE) process. Different purification processes (i.e. steam treatment and hydrogen peroxide treatment) result in different tellurium (Te) contents and V5+ concentration in the catalysts. Catalytic tests reveal that there is a direct correlation between the amount of V5+ present in the catalysts and the catalytic activity. A hydrogen peroxide treatment increases the V5+concentration and decreases the Te content which can improve the catalytic activity and stability in comparison with the steam treatment. A post-treatment with oxalic acid improves the catalyst surface area (54m2/g) but causes some vanadium leaching. The phase-pure M1 catalyst calcined at 650̊C and purified by H2O2 shows the best catalyst productivity of 0.77 kgC2H4/kgcat/h at 73% ethane conversion and 85% ethylene selectivity in ODHE process. The formation of reduced Te(0) aggregates blocking the active sites is identified as a main reason for catalyst deactivation. A low Te content favors a stable catalyst with less risk of Te aggregation. However, at harsh operating conditions (i.e. high oxygen concentration and high reactor temperature) the performance of phase-pure M1 catalysts with a low Te content can also reduce obviously due to the formation of a new (V, Nb)-substituted θ-Mo5O14 and MoO2 phases.

Oxidative dehydrogenation of ethane to ethylene over phase-pure M1 MoVNbTeOx catalysts in a micro-channel reactor

Due to its excellent heat transfer ability, the micro-channel reactor with coated phase-pure M1 catalysts can achieve reactor productivity nearly 5 times higher than that of a traditional fixed-bed reactor under the same reaction conditions in oxidative dehydrogenation of ethane (ODHE).

Valence variation of phase-pure M1 MoVNbTe oxide by plasma treatment for improved catalytic performance in oxidative dehydrogenation of ethane

This work presents a new method of catalyst surface modification by using oxygen plasma to change the oxidation state of active sites in metal oxide catalysts.

The reaction network in propane oxidation over phase-pure MoVTeNb M1 oxide catalysts

MoVTeNb oxide catalysts exclusively composed of the M1 phase (ICSD No. 55097) have been studied in the direct oxidation of propane to acrylic acid applying a broad range of reaction conditions with respect to temperature (623–633–643–653–663K), O2 concentration in the feed (4.5–6.0–9.0–12.0%), steam concentration in the feed (0–10–20–40%), and contact time (0.06–0.12–0.18–0.24–0.36–0.48–0.72–1.44sgcatNml−1). The molar fraction of propane was kept at 3.0%. Model experiments were performed to study the reactivity of possible intermediates propene, acrolein, and CO. The impact of auxiliary steam on the chemical nature of the catalyst surface was analyzed by in situ photoelectron spectroscopy, while in situ X-ray diffraction has been carried out to explore the structural stability of the M1 phase under stoichiometric, oxidizing, and reducing reaction conditions. Phase purity apparently accomplishes absolute stability in terms of the crystalline bulk structure and the catalytic performance over month even under extreme reaction conditions. In contrast, the catalyst surface changes dynamically and reversibly when the feed composition is varied, but only in the outermost surface layer in a depth of around one nanometer. The addition of steam causes enrichment in V and Te on the surface at the expense of Mo. Surface vanadium becomes more oxidized in presence of steam. These changes correlate with the abundance of acrylic acid detected in the in situ photoelectron spectroscopy experiment. Analysis of the three-dimensional experimental parameter field measured in fixed bed reactors revealed that the complexity of the reaction network in propane oxidation over MoVTeNb oxide is reduced compared to less-defined catalysts due to phase purity and homogeneity. The oxidative dehydrogenation of propane to propene followed by allylic oxidation of propene comprises the main route to acrylic acid. The oxygen partial pressure was identified as an important process parameter that controls the activity in propane oxidation over phase-pure M1 without negative implications on the selectivity. High O2 concentration in the feed keeps the catalyst in a high oxidation state, which provides an increased number of active sites for propane activation. Auxiliary steam increases activity and selectivity of M1 by changing the chemical nature of the active sites and by facilitating acrylic acid desorption.

Surface chemistry of phase-pure M1 MoVTeNb oxide during operation in selective oxidation of propane to acrylic acid

The surface of a highly crystalline MoVTeNb oxide catalyst for selective oxidation of propane to acrylic acid composed of the M1 phase has been studied by infrared spectroscopy, microcalorimetry, and in situ photoelectron spectroscopy. The acid–base properties of the catalyst have been probed by NH3 adsorption showing mainly Brønsted acidity that is weak with respect to concentration and strength of sites. Adsorption of propane on the activated catalyst reveals the presence of a high number of energetically homogeneous propane adsorption sites, which is evidenced by constant differential heat of propane adsorption qdiff,initial=57kJmol−1 until the monolayer coverage is reached that corresponds to a surface density of approximately 3 propane molecules per nm2 at 313K. The decrease of the heat to qdiff,initial=40kJmol−1 after catalysis implies that the surface is restructured under reaction conditions. The changes have been analyzed with high-pressure in situ XPS while the catalyst was working applying reaction temperatures between 323 and 693K, different feed compositions containing 0mol.% and 40mol.% steam and prolonged reaction times. The catalytic performance during the XPS experiments measured by mass spectrometry is in good agreement with studies in fixed-bed reactors at atmospheric pressure demonstrating that the XPS results taken under operation show the relevant active surface state. The experiments confirm that the surface composition of the M1 phase differs significantly from the bulk implying that the catalytically active sites are no part of the M1 crystal structure and occur on all terminating planes. Acrylic acid formation correlates with surface depletion in Mo6+ and enrichment in V5+ sites. In the presence of steam in the feed, the active ensemble for acrylic acid formation appears to consist of V5+ oxo-species in close vicinity to Te4+ sites in a Te/V ratio of 1.4. The active sites are formed under propane oxidation conditions and are embedded in a thin layer enriched in V, Te, and Nb on the surface of the structural stable self-supporting M1 phase.

Dynamics of the MoVTeNb Oxide M1 Phase in Propane Oxidation

Effects of framework and near surface composition of quinternary, phase-pure M1 MoVTeNb oxide catalysts on their catalytic performance in selective oxidation of propane to acrylic acid have been studied. The catalysts were prepared by hydrothermal synthesis, spray-drying, and superheated water vapor treatment. Electron microscopy, chemical analysis, nitrogen physisorption, and in situ photoelectron spectroscopy have been used to characterize the materials. The yield of acrylic acid normalized to the specific surface area of the catalyst increases with decreasing percentage of Mo and increasing molar ratio of Te/V at the surface. The metal stoichiometry at the surface differs from the stoichiometry in the crystalline bulk and changes in response to the composition of the gas phase. In situ valence band spectroscopy at 623 K in the presence of all reactants revealed a substantial covalent character of the metal−oxygen bonds in M1. The surface restructuring under formation of V- and Te-containing clusters an...

How important is the (001) plane of M1 for selective oxidation of propane to acrylic acid?

The role of the (001) crystallographic plane of the M1 phase of MoVTeNb mixed-oxide catalysts in selective oxidation of propane to acrylic acid was addressed by investigating a phase-pure M1 material preferentially exposing this surface. A model catalyst was prepared by complete silylation of M1, followed by breakage of the SiO 2-covered needles. Using this approach, the reactivity of the M1 (001) surface was investigated by combining a microreactor study of propane oxidation with high-sensitivity low-energy ion scattering (HS-LEIS). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to study the shape and microstructure of the model system and verify the surface exposure of the model catalyst. The specific rate of formation of acrylic acid on the model catalyst was found to be similar to that on the phase-pure M1 reference material, indicating that the (001) plane of the M1 crystal structure did not have better catalytic properties compared with the lateral surface of M1 needles in propane oxidation.

Preparation of Phase-Pure M1 MoVTeNb Oxide Catalysts by Hydrothermal Synthesis—Influence of Reaction Parameters on Structure and Morphology

This work presents a detailed investigation of the preparation of MoVTeNbO x catalysts by hydrothermal synthesis. Phase-pure synthesis of M1 has been achieved applying the metals in a molar ratio Mo/V/Te/Nb = 1/0.25/0.23/0.12. Raman, UV/Vis spectroscopy, and SEM/EDX analysis show that the elements are inhomogeneously distributed in the initial suspension that is formed after mixing the metal salts in an aqueous medium. Iso- and heteropoly anions of molybdenum, free telluric acid as well as supra-molecular polyoxometalate clusters are observed in the solution, whereas all metals have been found in the precipitate. Complete rearrangement of molecular building blocks under hydrothermal conditions is essential for formation of phase-pure materials. Optimized synthesis conditions with respect to temperature and time result in the formation of a precursor consisting of nano-structured M1 characterized by an extended periodic organization in the [001] direction and a fairly homogeneous distribution of the elements. Residual ammonium containing supra-molecular species in the precursor result in the formation of phase mixtures during the subsequent crystallization by heat treatment in inert gas. Phase-pure M1 exhibits a distinct degree of flexibility with respect to the chemical composition that becomes obvious by incorporating Nb not exclusively into pentagonal bi-pyramidal units, but also into octahedral coordinated positions as shown by EXAFS. Anisotropic growth of the needle-like M1 crystals has been observed during the final heat treatment performed at 873–923 K in inert atmosphere disclosing a potential method to control the catalytic properties of MoVTeNbO x catalysts.