EPR and diffuse reflectance studies of the physico-chemical phenomena occurring during the preparation of Mo/SiO[sub 2] catalysts by the grafting method

Journal Article · Fri May 01 04:00:00 EDT 1992 · Journal of Catalysis; (United States)

DOI:https://doi.org/10.1016/0021-9517(92)90277-O· OSTI ID:6975831

Louis, C; Che, M ^[1]

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1. Univ. Pierre et Marie Curie, Paris (France)

Mo/SiO[sub 2] catalysts were prepared by the grafting method, i.e., by air- and water-free reaction between MoCl[sub 5] and hydroxyl groups of silica support. This reaction may be performed either in a liquid medium (cyclohexane) or in the vapor phase. It is shown by EPR and diffuse reflectance spectroscopies that two types of molybdenum were deposited onto silica during the grafting reaction: (i) grafted Mo as [triple bond]SiOMoCl[sub 4] (g = 1.9652 and g = 1.968), and (ii) physically adsorbed Mo as the diamagnetic Mo[sub 2] Cl[sub 10] dimer; this compound turns blue in air because of both its partial oxidation and hydrolysis to form the so-called molybdenum blues. After the grafting reaction itself, the second step involves washing by water or ammonia solution. The physically adsorbed Mo is eliminated and the grafted Mo remaining on the silica is hydrolyzed and partially oxidized in air, so as to lead to the following species: [triple bond]SiOmO[sup v](OH)[sub 4] or [triple bond]SiOMo[sup vi](OH)[sub 5]. In consequence, at the very end of the preparation, only the grafted Mo species remains on the silica and the sample color is brown. The catalytic behavior of the unwashed and washed catalysts in the reaction of methanol oxidation emphasizes the differences in the Mo species. The unwashed sample containing molybdenum blues behaves like a polycrystalline MoO[sub 3] catalyst, producing mainly methylal at low reaction temperature (230 C) and formaldehyde at higher temperature (280 C), whereas the washed one, containing only grafted Mo, produces mainly methylformate and, to a lower extent, formaldehyde.

• OSTI ID: 6975831
• Journal Information: Journal of Catalysis; (United States), Journal Name: Journal of Catalysis; (United States) Vol. 135:1; ISSN 0021-9517; ISSN JCTLA5
• Country of Publication: United States
• Language: English

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ALCOHOLS
ALDEHYDES
CARBOXYLIC ACID SALTS
CATALYST SUPPORTS
CATALYSTS
CATALYTIC EFFECTS
CHALCOGENIDES
CHEMICAL PREPARATION
CHEMICAL REACTIONS
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CHLORINE COMPOUNDS
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COMPARATIVE EVALUATIONS
DECOMPOSITION
ELECTRON SPIN RESONANCE
ELEMENTS
EVALUATION
FORMALDEHYDE
FORMATES
HALIDES
HALOGEN COMPOUNDS
HYDROGEN COMPOUNDS
HYDROLYSIS
HYDROXIDES
HYDROXY COMPOUNDS
LYSIS
MAGNETIC RESONANCE
METALS
METHANOL
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MOLYBDENUM CHLORIDES
MOLYBDENUM COMPOUNDS
MOLYBDENUM HYDROXIDES
MOLYBDENUM OXIDES
OPTICAL PROPERTIES
ORGANIC COMPOUNDS
OXIDATION
OXIDES
OXYGEN COMPOUNDS
PHYSICAL PROPERTIES
REFLECTIVITY
REFRACTORY METAL COMPOUNDS
RESONANCE
SILICON COMPOUNDS
SILICON OXIDES
SOLVOLYSIS
SURFACE PROPERTIES
SYNTHESIS
TRANSITION ELEMENT COMPOUNDS
TRANSITION ELEMENTS
WASHING