Structure and Properties of Oxidative Dehydrogenation Catalysts Based on MoO3/Al2O3

Chen, Kaidong; Xie, Sujuan; Bell, Alexis T.; Iglesia, Enrique

doi:10.1006/jcat.2000.3125

Cited by 276 publications

(197 citation statements)

References 47 publications

Supporting

Mentioning

177

Contrasting

Unclassified

Order By: Relevance

“…Figure 1 shows incipient reduction rates for RuO 2 domains using H 2 as a stoichiometric reductant as the sample temperature increased from 298 to 793 K. Incipient reduction rates obtained by a kinetic analysis of the low-temperature part of the first reduction peak in Figure 1 are shown for each sample in Initial reduction rates were extracted by kinetic analysis of the incipient reduction region of the reduction profiles 15,16 in Figure 1. These incipient reduction processes are most relevant to those occurring in nearly stoichiometric RuO 2 during the redox cycles required for methanol oxidation turnovers, evidence for which is presented below from the observed kinetic effects of CH 3 OH and O 2 on catalytic oxidation rates.…”

Section: Resultsmentioning

confidence: 99%

“…Reduction rates were measured from H 2 consumption rates using previously reported protocols. 15,16 Methanol reactions were carried out in a packed-bed quartz microreactor. Catalyst powders (0.1-0.3 g) were diluted with quartz powder (∼0.5 g) to prevent temperature gradients and treated in 20% O 2 Reactants and products were analyzed by on-line gas chromatography (Hewlett-Packard 6890GC) using a methyl-silicone capillary column (HP-1; 50 m × 0.25 mm, 0.25 µm film thickness) and a Porapak Q packed column (80-100 mesh, 1.82 m × 3.18 mm) connected to flame ionization and thermal conductivity detectors, respectively.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Selective Oxidation of Methanol and Ethanol on Supported Ruthenium Oxide Clusters at Low Temperatures

2004

Self Cite

View full text Add to dashboard Cite

catalyze the oxidative conversion of methanol to formaldehyde, methylformate, and dimethoxymethane with unprecedented rates and high combined selectivity (>99%) and yield at low temperatures (300-400 K). Supports influence turnover rates and the ability of RuO 2 domains to undergo redox cycles required for oxidation turnovers. Oxidative dehydrogenation turnover rates and rates of stoichiometric reduction of RuO 2 in H 2 increased in parallel when RuO 2 domains were dispersed on more reducible supports. These support effects, the kinetic effects of CH 3 OH and O 2 on reaction rates, and the observed kinetic isotope effects with CH 3 OD and CD 3 OD reactants are consistent with a sequence of elementary steps involving kinetically relevant H-abstraction from adsorbed methoxide species using lattice oxygen atoms and with methoxide formation in quasi-equilibrated CH 3 OH dissociation on nearly stoichiometric RuO 2 surfaces. Anaerobic transient experiments confirmed that CH 3 OH oxidation to HCHO requires lattice oxygen atoms and that selectivities are not influenced by the presence of O 2 . Residence time effects on selectivity indicate that secondary HCHO-CH 3 OH acetalization reactions lead to hemiacetal or methoxymethanol intermediates that convert to dimethoxymethane in reactions with CH 3 OH on support acid sites or dehydrogenate to form methylformate on RuO 2 and support redox sites. These conclusions are consistent with the tendency of Al 2 O 3 and SiO 2 supports to favor dimethoxymethane formation, while SnO 2 , ZrO 2 , and TiO 2 preferentially form methylformate. These support effects on secondary reactions were confirmed by measured CH 3 OH oxidation rates and selectivities on physical mixtures of supported RuO 2 catalysts and pure supports. Ethanol also reacts on supported RuO 2 domains to form predominately acetaldehyde and diethoxyethane at 300-400 K. The bifunctional nature of these reaction pathways and the remarkable ability of RuO 2 -based catalysts to oxidize CH 3 OH to HCHO at unprecedented low temperatures introduce significant opportunities for new routes to complex oxygenates, including some containing C-C bonds, using methanol or ethanol as intermediates derived from natural gas or biomass.

show abstract

Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Selective Oxidation of Methanol and Ethanol on Supported Ruthenium Oxide Clusters at Low Temperatures

2004

Self Cite

View full text Add to dashboard Cite

show abstract

“…12MoAl showed bands at 1013 and 842 cm -1 , attributed to ModO and Mo-O-Mo stretches, respectively. 36,[59][60][61] Raman bands for crystalline MoO 3 were not detected on samples prepared using molybdenyl acetylacetonate as the precursor and a surface density of 5.0 Mo/nm 2 , corresponding to approximately one monolayer. 62 In contrast, MoAl samples prepared with ammonium heptamolybdate showed crystalline MoO 3 bands at surface densities above 4.5 Mo/nm 2 .…”

Section: Methodsmentioning

confidence: 92%

Oxidative Dehydrogenation of Propane over V₂O₅/MoO₃/Al₂O₃ and V₂O₅/Cr₂O₃/Al₂O₃: Structural Characterization and Catalytic Function

2005

Self Cite

View full text Add to dashboard Cite

The structure and catalytic properties of binary dispersed oxide structures prepared by sequential deposition of VO x and MoO x or VO x and CrO x on Al 2 O 3 were examined using Raman and UV-visible spectroscopies, the dynamics of stoichiometric reduction in H 2 , and the oxidative dehydrogenation of propane. VO x domains on Al 2 O 3 modified by an equivalent MoO x monolayer led to dispersed binary structures at all surface densities. MoO x layers led to higher reactivity for VO x domains present at low VO x surface densities by replacing V-O-Al structures with more reactive V-O-Mo species. At higher surface densities, V-O-V structures in prevalent polyvanadates were replaced with less reactive V-O-Mo, leading to lower reducibility and oxidative dehydrogenation rates. Raman, reduction, and UV-visible data indicate that polyvanadates predominant on Al 2 O 3 convert to dispersed binary oxide structures when MoO x is deposited before or after VO x deposition; these structures are less reducible and show higher UV-visible absorption energies than polyvanadate structures on Al 2 O 3 . The deposition sequence in binary Mo-V catalysts did not lead to significant differences in structure or catalytic rates, suggesting that the two active oxide components become intimately mixed. The deposition of CrO x on Al 2 O 3 led to more reactive VO x domains than those deposited on pure Al 2 O 3 at similar VO x surface densities. At all surface densities, the replacement of V-O-Al or V-O-V structures with V-O-Cr increased the reducibility and catalytic reactivity of VO x domains; it also led to higher propene selectivities via the selective inhibition of secondary C 3 H 6 combustion pathways, prevalent in VO x -Al 2 O 3 , and of C 3 H 8 combustion routes that lead to low alkene selectivities on CrO x -Al 2 O 3 . VO x and CrO x mix significantly during synthesis or thermal treatment to form CrVO 4 domains. The deposition sequence, however, influences catalytic selectivities and reduction rates, suggesting the retention of some of the component deposited last as unmixed domains exposed at catalyst surfaces. These findings suggest that the reduction and catalytic properties of active VO x domains can be modified significantly by the formation of binary dispersed structures. VO x -CrO x structures, in particular, lead to higher oxidative dehydrogenation rates and selectivities than do VO x domains present at similar surface densities on pure Al 2 O 3 supports.

show abstract

“…8,9 We have studied 13 wt% Mo/Al 2 O 3 and Mo/SiO 2 catalysts during successive propane dehydrogenation cycles at 550 uC. The three techniques are sensitive to changes in the oxidation/coordination states of Mo allowing us to obtain complementary information on the catalysts behaviour during dehydrogenation and regeneration.…”

mentioning

confidence: 99%

Adding a third dimension to operando spectroscopy: a combined UV-Vis, Raman and XAFS setup to study heterogeneous catalysts under working conditions

et al. 2005

View full text Add to dashboard Cite

The potential of combined operando UV-Vis/Raman/XAFS has been explored by studying the active site and deactivation mechanism of silica-and alumina-supported molybdenum oxide catalysts under propane dehydrogenation conditions.Ideally scientists would like to take real-time spectra inside a catalytic reactor when a catalytic process is operating, giving them detailed insight into the working principles of the catalytic material. 1 On this basis, it would then be possible to improve upon existing catalyst formulations or design completely new ones, which are more active and/or selective. Such rational catalyst design is still a dream since the experimental tools available to study the active catalyst do not yet provide sufficient insight. In this respect, it is advantageous to look on catalytic systems from different perspectives by making use of multiple characterisation techniques. In recent years, many attempts have been made to combine multiple spectroscopic techniques into one experimental set-up. The following combinations of two spectroscopic techniques have been recently reported for studying heterogeneous catalysts in action: EPR/UV-Vis, NMR/UV-Vis, XAFS/IR, UV-Vis/Raman and IR/UV-Vis. [2][3][4][5][6][7] Here, we describe a newly developed and powerful operando set-up to measure combined energy-dispersive (ED)-XAFS, UVVis and Raman to study a working catalytic solid. To our best knowledge, this is the first device which couples three spectroscopic techniques in one reactor, focuses on the same spot of a metal oxide catalyst under true reaction conditions and is capable of delivering sub second time resolution. A scheme of the set-up is given in Fig. 1. Further details are given in the ESI.{The operando device developed is widely applicable in the field of heterogeneous catalysis and its potential has been explored for the dehydrogenation of propane (5% in He) over supported Mo catalysts which have shown potential for alkane activation. 8,9 We have studied 13 wt% Mo/Al 2 O 3 and Mo/SiO 2 catalysts during successive propane dehydrogenation cycles at 550 uC. The three techniques are sensitive to changes in the oxidation/coordination states of Mo allowing us to obtain complementary information on the catalysts behaviour during dehydrogenation and regeneration. The set-up allows us to discriminate between the dynamics of both catalysts under reaction conditions and to identify the possible active site and deactivation pathways. Also the complementary aspects of this setup are demonstrated by showing how the catalyst undergoes changes which cannot be followed using one of the techniques alone and how it is possible to obtain quantitative Raman information without the use of an internal standard. Fig. 2 shows data collected using the three techniques during the first propane dehydrogenation cycle (PC1) for Mo/SiO 2 . The initial features observed in the spectra included a distinct 1s-4d pre-edge feature at 20002 eV in the ED-XANES, a strong LMCT band at ca. 350 nm in the UV-Vis and Raman bands at 992 (nMoLO), 82...

show abstract

Structure and Properties of Oxidative Dehydrogenation Catalysts Based on MoO3/Al2O3

Cited by 276 publications

References 47 publications

Selective Oxidation of Methanol and Ethanol on Supported Ruthenium Oxide Clusters at Low Temperatures

Selective Oxidation of Methanol and Ethanol on Supported Ruthenium Oxide Clusters at Low Temperatures

Oxidative Dehydrogenation of Propane over V₂O₅/MoO₃/Al₂O₃ and V₂O₅/Cr₂O₃/Al₂O₃: Structural Characterization and Catalytic Function

Adding a third dimension to operando spectroscopy: a combined UV-Vis, Raman and XAFS setup to study heterogeneous catalysts under working conditions

Contact Info

Product

Resources

About

Structure and Properties of Oxidative Dehydrogenation Catalysts Based on MoO3/Al2O3

Cited by 276 publications

References 47 publications

Selective Oxidation of Methanol and Ethanol on Supported Ruthenium Oxide Clusters at Low Temperatures

Selective Oxidation of Methanol and Ethanol on Supported Ruthenium Oxide Clusters at Low Temperatures

Oxidative Dehydrogenation of Propane over V2O5/MoO3/Al2O3 and V2O5/Cr2O3/Al2O3: Structural Characterization and Catalytic Function

Adding a third dimension to operando spectroscopy: a combined UV-Vis, Raman and XAFS setup to study heterogeneous catalysts under working conditions

Contact Info

Product

Resources

About

Oxidative Dehydrogenation of Propane over V₂O₅/MoO₃/Al₂O₃ and V₂O₅/Cr₂O₃/Al₂O₃: Structural Characterization and Catalytic Function