Infrared study of water and pyridine adsorption on the surface of anhydrous vanadyl pyrophosphate

Puttock, Simon J.; Rochester, Colin H.

doi:10.1039/f19868202773

Cited by 31 publications

(9 citation statements)

References 2 publications

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“…This means that hydroxyls are very probably present, and that spillover of hydrogen is easy on this surface. To summarize this part, the surface of (100) should be covered mainly by ° and OH, which is in accordance with experiments made by infrared spectroscopy [42).…”

Section: Elementary Steps Involving Hydrogenmentioning

confidence: 82%

Surface Reactivity of Oxides

Bordes

1993

Elementary Reaction Steps in Heterogeneous Catalysis

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ABSTRACT. Considerations on the factors governing selectivity in different kinds of oxidation reactions of hydrocarbons and small oxygenated compounds are proposed. Among them the surface crystal field displayed by a catalyst, depending on its structure and on crystal morphology, is shown to be the primary factor. With the help of knowledge on methane coupling and butane oxidation, sites needed to activate and oxygenate molecules are proposed for alkanes. In this view the main assumptions obtained by a model (CMAS) describing the elementary steps of butane oxidation on vanadyl pyrophosphate are recalled. IntroductionIn heterogeneous catalysis, reactions performed on metals and oxides share the same complexity, particularly when solids are deposited on a support. The difficulty is to evaluate the real mechanisms which are involved at the surface. In the case of metals, the crystals take various forms as aggregates in which the coordination is different than in bulk metals, which give them a particular quality and favors the selectivity. Not only the support itself influences the reaction by means of acidic properties (for example), but also metal-support interactions can playa great role as for SMSI. However several parameters such as work function, atomic adsorption energies, etc, and even quantum chemistry calculations (SCF, Morse potential, ... ) allow to estimate the state of surface atoms as compared to the bulk and to draw models which are closer and closer from the reality. Oxides of transition metals M are generally used in the selective oxidation catalysis of hydrocarbons and of small oxygenated molecules like methanol. In metallic solids, only one kind of bond, the metallic bond, is to be considered, and the spacial distribution of bonds is mainly isotropic at least in the bulk. On the contrary, inside an oxide like V20S, V-O bonds range from covalent to ionic and their distribution around V is strongly anisotropic. Consequently the structural and electronic properties of MOx oxides are also anisotropic even in the bulk, and the diversity of the M-O bonds is precisely related to their catalytic specificity. The methods used to characterize the surface give generally an average value of a given property because the results account for the whole grains. The contribution of the morphology of the crystallites should be taken into account because of the variety of bonds distributed among the various facets, some of them being responsible for selectivity. Happily the most wellknown catalysts V20S and Mo03 display a layered structure, so that their catalytic properties are known since a long time because the contribution of their cleavage plane is preponderant. This is less true for W03. In fact, it has been shown recently that, if Mo03 is selective but non

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Section: Elementary Steps Involving Hydrogenmentioning

confidence: 82%

Surface Reactivity of Oxides

Bordes

1993

Elementary Reaction Steps in Heterogeneous Catalysis

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“…Pyridine interacts with both Lewis-and Bramsted-acidic sites on the surface of (V0),P,0,. 4 Lewis sites are converted to Brarnsted sites by the adsorption of water. 4 The poisoning effect of adsorbed pyridine and the promoting effect of adsorbed water on the rate of but-1 -ene isomerisation therefore suggest that Brarnsted-acidic surface hydroxy groups are involved in the catalytic reaction.…”

Section: Discussionmentioning

confidence: 99%

“…4 Lewis sites are converted to Brarnsted sites by the adsorption of water. 4 The poisoning effect of adsorbed pyridine and the promoting effect of adsorbed water on the rate of but-1 -ene isomerisation therefore suggest that Brarnsted-acidic surface hydroxy groups are involved in the catalytic reaction. An adsorption process which gradually converts hydroxy groups to alkoxy groups would hence account for the progressive poisoning of the isomerisation reaction during contact between butenes and (VO),P,O,.…”

Section: Discussionmentioning

confidence: 99%

Infrared study of the adsorption of but-1-ene, buta-1,3-diene, furan and maleic anhydride on the surface of anhydrous vanadyl pyrophosphate

Puttock¹,

Rochester²

1986

J. Chem. Soc., Faraday Trans. 1

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Brrnsted-acidic hydroxy groups on the surface of vanadyl pyrophosphate act as catalytically active sites for the isomerisation of but-l-ene to but-2-enes. Progressive poisoning of the reaction is attributed to the formation of alkyl species, possibly alkoxy groups, on the (VO),P,O, surface. Adsorbed alkyl groups also result from buta-1,3-diene adsorption. Furan adsorbs on (VO),P,O, through coordinative interactions with Lewis-acidic surface sites. The presence of oxygen promotes the oxidation of furan to maleic anhydride.The importance has been established of vanadyl pyrophosphate as a component of vanadium-phosphorus mixed-oxide catalysts for the oxidation of butane or but-1 -ene to maleic anh~dride.l-~ The surface character of (VO),P207 has been partly established by infrared studies of the adsorption of water, pyridine, CO, CO,, acetic acid and acetic a n h ~d r i d e . ~? ~The present paper reports an infrared study of the adsorption on (VO),P207 of the species (butane, but-1 -ene, buta-1,3-diene, furan and maleic anhydride) involved as reactants, intermediates or product in the oxidation reactions. ExperimentalVanadyl pyrophosphate (surface area 11 m2 g-l) was prepared by reacting orthophosphoric acid and vanadium pentoxide in strongly acidic solution and decomposing the reaction product by thermal activation in vacuum. The purity of the final product was confirmed by infrared spectroscopy and X-ray powder diffracti0n.l Compressed (3.3 ton) self-supporting discs (25 mm diameter) of (VO),P207 (ca. 55 mg) were mounted in an evacuable infrared cell with fluorite windows and an external furnace and were heated at 673 K (17 h) in vacuum before the admission of adsorbate gases. Spectra of discs at ca. 300 K were recorded using a Perkin-Elmer 580A spectrophotometer. High-purity commercial samples of butane, but-1 -ene, buta-1,3-diene and furan were freed from permanent gases by a series of freeze-thaw cycles. Maleic anhydride was purified by sublimation under reduced pressure to remove maleic acid. ResultsContact between butane vapour (1.3 kN m-2) and (VO),P,O, at temperatures in the range 293-723 K gave no detectable change in the infrared spectrum of the gas phase and no infrared bands due to adsorbed species. However, at elevated temperatures the disc turned black, implying that carbon deposition had occurred.Exposure of (VO),P,O, at the ambient temperature in the sample beam of the spectrophotometer (ca. 300 K) to but-1-ene (2.5 kN m-,) gave spectra [fig. 1 (b)] in which the observed bands could primarily be attributed to but-1-ene in the gas phase. The spectra did not vary with time but removal of the gas phase by evacuation left residual 100 3033 FAR 1

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“…2-Picoline afforded pyridine-2-carbaldehyde with a yield of 50.2% at a conversion of 69.2% and no picolinic acid was formed at 573 K. 4-Picoline afforded pyridine-4-carbaldehyde (yield, 8.9%), isonicolinic acid (yield, 84.3%) at a conversion of 98.7% at 598 K. NH 3 -TPD showed an increase in the number of acid sites by replacing V with small amount of P in CrVO 4 -I. The favorable effect of water addition and the results of DRIFTS measurement of pyridine adsorbed on the catalyst 16 in the presence of water suggest that Brønsted acid sites are the active species. Moreover, all the V species are present as VO 4 tetrahedra in the CrVO 4 -I structure 14 and considered to act as active sites via their redox properties.…”

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confidence: 83%