In Situ XRD Investigations of Heteropolyacid Catalysts in the Methacrolein to Methacrylic Acid Oxidation Reaction: Structural Changes during the Activation/Deactivation Process (original) (raw)

Methacrylic Acid: Not a Mere Product But Activator in the Catalytic Oxidation of Methacrolein?

Catalysis Letters, 2009

Pretreatment of Cs 2 H 2 PVMo 11 O 40 catalyst with methacrylic acid (MAA) induces better performance in the oxidation of methacrolein. Pulsed and continuous experiments revealed higher conversion of reagents and increased selectivity to MAA for the pretreated catalyst. XRD showed swelling of the crystal lattice, while FTIR confirmed removal of the MO x (M = V or Mo) entities from the Keggin units at 80-100°C for the MAA-pretreated samples.

Mechanistic Details and Reactivity Descriptors in Oxidation and Acid Catalysis of Methanol

ACS Catalysis

Acid and redox reaction rates of CH3OH-O2 mixtures on polyoxometalate (POM) clusters, together with isotopic, spectroscopic, and theoretical assessments of catalyst properties and reaction pathways, were used to define rigorous descriptors of reactivity and to probe the compositional effects for oxidative dehydrogenation (ODH) and dehydration reactions. 31P-MAS NMR, transmission electron microscopy and titrations of protons with di-tert-butylpyridine during catalysis showed that POM clusters retained their Keggin structure upon dispersion on SiO2 and after use in CH3OH reactions. The effects of CH3OH and O2 pressures and of D-substitution on ODH rates show that C–H activation in molecularly adsorbed CH3OH is the sole kinetically relevant step and leads to reduced centers as intermediates present at low coverages; their concentrations, measured from UV–vis spectra obtained during catalysis, are consistent with the effects of CH3OH/O2 ratios predicted from the elementary steps proposed. First-order ODH rate constants depend strongly on the addenda atoms (Mo vs W) but weakly on the central atom (P vs Si) in POM clusters, because C–H activation steps inject electrons into the lowest unoccupied molecular orbitals (LUMO) of the clusters, which are the d-orbitals at Mo6+ and W6+ centers. H-atom addition energies (HAE) at O-atoms in POM clusters represent the relevant theoretical probe of the LUMO energies and of ODH reactivity. The calculated energies of ODH transition states at each O-atom depend linearly on their HAE values with slopes near unity, as predicted for late transition states in which electron transfer and C–H cleavage are essentially complete. HAE values averaged over all accessible O-atoms in POM clusters provide the appropriate reactivity descriptor for oxides whose known structures allow accurate HAE calculations. CH3OH dehydration proceeds via parallel pathways mediated by late carbenium-ion transition states; effects of composition on dehydration reactivity reflect changes in charge reorganizations and electrostatic forces that stabilize protons at Brønsted acid sites.

Kinetic Analysis of Methacrolein and Lactone Formation Over Lanthanide Phosphomolybdate Catalysts

Eight lanthanum (La 0.25 H 2.25 [PMo 12 O 40 ], La 0.5 H 1.5 [PMo 12 O 40 ], La 0.75 H 0.75 [PMo 12 O 40 ], La[PMo 12 O 40 ]) and cerium (Ce 0.25 H 2.25 [PMo 12 O 40 ], Ce 0.5 H 1.5 [PMo 12 O 40 ], Ce 0.75 H 0.75 [PMo 12 O 40 ], Ce[PMo 12 O 40 ]) containing phosphomolybdate catalysts have been synthesized and analysed using a low-pressure steady-state technique. The products from isobutane oxidation using the catalysts were water, methacrolein, carbon dioxide and lactone. The methacrolein and lactone data were simulated using two different theoretical models in order to determine kinetic parameters. The activation barriers for methacrolein formation vary substantially throughout each lanthanum and cerium series, however the most active catalyst is determined to be Ce[PMo 12 O 40 ]. Only three of the eight catalysts produced significant quantities of lactone (La 0.75 H 0.75 [PMo 12 O 40 ], La[PMo 12 O 40 ] and Ce[PMo 12 O 40 ]) and its formation is correlated with methacrolein. The correlation between lactone and methacrolein formation is likely due to the presence of a distinct structural phase.

Proposal of a new kinetic model based on the remote control mechanism to fit experimental data during the selective oxidation of isobutene to methacrolein on biphasic catalysts

Catalysis Today, 1996

A new kinetic model based on the classical Mars-van Krevelen mechanism is proposed. The new model incorporates both the mechanism of site creation (via a remote control) and the catalytic reaction at the thus created (or controlled) active site. The model has been tested in the selective oxidation of isobutene to methacrolein on catalysts made of mechanical mixtures of a-Sb,O, + MOO,. The good fitting of the model with the experimental results shows that the remote control mechanism can explain the difficulties to represent the kinetics of allylic oxidation reactions by the traditional Mars-van Krevelen mechanism. The fraction of the surface of MOO, which selectively realises the oxidation via the oxidoreduction process depends on the degree of irrigation of MOO, by spillover oxygen produced on o-Sb,O, and cannot be considered as constant. The possible use of the model for further progress in the kinetic description of selective oxidation reactions is briefly outlined.

Evaluation and design of heteropolycompound catalysts for the selective oxidation of isobutane into methacrylic acid

Applied Catalysis A-general, 2004

Chemical modifications of the 12-phosphomolybdic acid by substitution of a Mo atom by a V in the so-called Keggin primary structure and/or by introduction of counter-cations (NH 4 + , Cs + ) in the secondary structure were studied. The effects of the proportion of ammonium salt in cesium-ammonium salts mixtures and of different preparation methods were also investigated. Concurrently to characterization analyses, the kinetic parameters of the Mars and Van Krevelen (MVK) model, representing well the rate of consumption of isobutane over the solids tested were determined by varying partial pressures of isobutane and oxygen. The results were used for catalysts evaluation and as a tool for the development of a new, more active and stable formulation.