Bfm Ben Kuster

Engineering aspects of the aqueous noble metal catalysed alcohol oxidation.

Abstract The aqueous noble metal catalysed alcohol oxidation is a reaction which can profitably be applied in fine-chemistry and for carbohydrate conversion. In this paper engineering aspects of this reaction are treated, i.e. the reaction kinetics, oxygen mass transfer restrictions, catalyst deactivation and reactivation, and implications for reactor design and operation. First a reaction mechanism is proposed, which is very helpful for understanding the observed phenomena. Also a short summary is given on catalyst deactivation mechanisms. Two different reaction regimes can clearly be distinguished: the oxygen mass transfer limited regime and the intrinsic kinetic regime, which are treated separately. Oxidations using noble metal catalysts promoted with less noble metals, like Pb, Bi, generally fall in the first regime, those using unpromoted noble metals in the second. Reaction rate data are evaluated for the Pd/Bi catalysed oxidation of glucose and the Pt catalysed oxidation of methyl-glucoside, respectively, illustrating the typical kinetic behaviour in both regimes. From oxidation kinetics in the mass transfer limited regime, it is concluded that adherence of catalyst particles to the gas–liquid interface, is a major factor determining reaction kinetics. Oxygen transfer, direct from the gas to the catalyst particle, is likely. For the Pt catalysed oxidation, a kinetic model is presented for catalyst deactivation by over-oxidation and for catalyst reactivation. Finally specific reactor options and suggestions for future engineering research are given: slurry catalyst versus fixed bed catalyst operation, avoidance of explosion risks, redox cycle reactors, electrochemical reactors, and multi-functional reactors.

Deactivation of platinum catalysts by oxygen. 2. Nature of the catalyst deactivation

Abstract A study has been made of the kinetics of deactivation of a commercial pt/C catalyst being used in an aqueous slurry for the oxidation of d -gluconate to d -glucarate at 50 °C. It appears that the deactivation of the catalyst is an independent process, governed by the coverage of the platinum surface by oxygen atoms. Under steady-state conditions an exponential decay is observed. A mathematical model is presented, based on the processes occurring at the platinum surface, which describes the experimental results very well.

Lead modified platinum on carbon catalysts for the selective oxidation of (2-) hydroxycarbonic acids, and especially polyhydroxycarbonic acids to their 2-keto derivatives

D-Gluconic acid has been oxidised with oxygen and air at 55 C in water, using Pt/C catalysts. The primary hydroxyl group is preferentially oxidised using an unmodified catalyst. Addition of a lead(II)salt changes the preference dramatically towards oxidation at the position α to the car☐yl group. Study of the kinetics of the reactions is complicated by catalyst deactivation caused by as well oxygen as by (side)product adsorption. For the oxidation of D-gluconic acid to 2-keto-D-gluconic acid a dehydrogenation mechanism is proposed in which the α-H is activated by PbII-complexation of the car☐yl- and α-OH functions at the Pt-site.

“Hairy Foam”: carbon nanofibers grown on solid carbon foam. A fully accessible, high surface area, graphitic catalyst support

This paper describes the synthesis of carbon nanofibers (CNFs) on solid carbon foam (“Hairy Foam”) by catalytic decomposition of ethylene. The effect of nickel loading on fiber diameter and morphology, CNF coverage, and fiber layer thickness is studied using SEM and N2/Kr-physisorption. The surface area increased from 0.12 m2support g−1support for reticulated vitreous carbon (RVC) to 146 m2support g−1support for “Hairy Foam”. A nickel concentration of 0.5 gNig−1RVC results in fibers with a diameter of 30 to 90 nm. Increasing the nickel concentration results in fiber diameters of 30 to 1100 nm. Complete CNF coverage is obtained for a nickel deposition time ≥240 min and a nickel concentration ≥2.5 gNig−1RVC.

Platinum deactivation: in situ EXAFS during aqueous alcohol oxidation reaction

With a new set‐up for in situ EXAFS spectroscopy the state of a carbon‐supported platinum catalyst during aqueous alcohol oxidation has been observed. The catalyst deactivation during platinum‐catalysed cyclohexanol oxidation is caused by platinum surface oxide formation. The detected Pt–O co‐ordination at 2.10 Å during exposure to nitrogen‐saturated cyclohexanol solution is different from what is observed for the pure oxidised platinum surface (2.06 Å).

The selective oxidation of aldoses and aldonic acids to 2-ketoaldonic acids with lead-modified platinum-on-carbon catalysts

Abstract Aldoses and aldonic acids have been oxidised with oxygen and air at 55° in water, using Pt/C catalysts. After oxidation of the reducing group, if available, the primary hydroxyl group is preferentially oxidised using an unmodified catalyst. Addition of a lead(II) salt changes the preference dramatically toward oxidation at the position α to the carboxyl group. Provided that oxygen transfer to the liquid phase is carefully controlled in order to prevent deactivation of the catalyst, 2-ketoaldonic acids can be prepared in high yields.

Kinetic modeling for wet air oxidation of formic acid on a carbon supported platinum catalyst

A study of the intrinsic kinetics of the oxidation of formic acid over a 1% Pt/C catalyst has been performed, at oxygen concentrations in water varying from 0.6 to 2.8 mol m−3 and at formic acid concentrations varying from 30 to 400 mol m−3. Experiments were performed in a continuous-flow stirred slurry reactor. To avoid mass transfer limitation of oxygen, a temperature interval from 282 to 293 K and a total pressure in the reactor of 0.6 MPa has been used. The pH interval of the experiments ranged from 1.1 to 13.0. Single response non-linear regression has been used to fit the parameters for a kinetic model. The proposed model can predict the steady-state disappearance rate of formic acid, as a function of the oxygen concentration, the formic acid concentration, the temperature and of the pH. Electrochemical experiments have been carried out for a better understanding of the pH effect on the formic acid oxidation.

Selective oxidation of methyl alpha-D-glucoside on carbon supported platinum III. Catalyst deactivation

Abstract Platinum supported on activated carbon and platinum supported on graphite were characterized before, during and after oxidation of methyl α- d -glucoside in water. Ex situ information was obtained by X-ray photoelectron spectroscopy, cyclic voltammetry, scanning transmission electron microscopy and carbon monoxide chemisorption. In situ information was obtained by cyclic voltammetry. Furthermore, the open circuit potential of a platinized platinum foil was monitored during reaction. A distinction could be made between a reversible deactivation, occurring on a time scale of 10 ks and an irreversible deactivation, occurring on a time scale of 100 ks. The reversible deactivation is attributed to a slowly increasing oxygen surface coverage rather than to the formation of platinum oxide. Chemisorption takes place beyond the steady-state degree of oxygen coverage expected from the intrinsic initial reaction kinetics and causes a reversible decrease of the reaction rate. The irreversible deactivation is caused by the dissolution and subsequent redeposition of platinum. The preferential dissolution of small platinum particles and redeposition on larger ones leads to a decrease of the fraction of exposed platinum atoms from 0.7 to 0.5. The effect of the irreversible decrease of the platinum surface area on the specific reaction rate is strongly attenuated by an antipathetic structure sensitivity. An increase of the average platinum particle diameter results in an increase of the turnover frequency.

Platinum catalysed aqueous methyl α-d-glucopyranoside oxidation in a multiphase redox-cycle reactor

Abstract Platinum catalyst deactivation during aqueous alcohol oxidation is discussed, using the selective oxidation of methyl α- d -glucopyranoside as an example. The most important causes of platinum deactivation are catalyst over-oxidation and catalyst poisoning. Deactivation by over-oxidation can be reversed by applying a redox-cycle, i.e. cyclic exposure to oxidative and reductive circumstances. A kinetic model for methyl α- d -glucopyranoside oxidation, platinum deactivation, and reactivation, based on electrochemistry is presented and implemented into a three-phase stirred slurry reactor model, showing the advantages of applying redox cycles.

On-line characterization by EXAFS of tin promoted platinum graphite catalysts in the aqueous phase

The structure of tin promoted graphite supported platinum catalysts has been studied with extended X-ray absorption fine structure spectroscopy (EXAFS). A newly developed EXAFS cell allows on-line characterization avoiding contact to ambient or drying. Hereto catalyst samples are transferred from a slurry reactor to the EXAFS cell forming a ‘‘bed’’ of catalyst particles in the EXAFS cell. The cell design was based on considerations concerning possible mass transport limitations while performing reactions in the liquid phase. The structures of the tin promoted platinum catalysts were investigated directly after preparation, drying, treatments with hydrogen (363 K) and oxygen (RT) in aqueous phase and a hydrogen gas treatment at 573 K at both the Pt LIII and the Sn K-edge. After preparation, under aqueous hydrogen, reduced platinum can be detected with three coordinations: Pt‐Pt, Pt‐C and Pt‐Sn. Tin appears to be partly oxidic showing a Sn‐O and a Sn‐Pt coordination. A treatment with aqueous oxygen or exposure to ambient leads to oxidized platinum and tin. At the Pt LIII-edge only a Pt‐Pt and Pt‐O coordination for platinum are detected. At the Sn K-edge tin has only a Sn‐O coordination. An aqueous treatment with hydrogen at 363 K reduces platinum showing, however, different coordination numbers for the Pt‐Pt and Pt‐Sn coordination. Tin only shows a Sn‐O coordination. A treatment with hydrogen at 573 K reduces both the platinum and the tin. Platinum shows a Pt‐Pt, Pt‐C and Pt‐Sn coordination. Tin shows a Sn‐Pt and Sn‐O coordination indicating tin deposition on the platinum, tin being bonded via oxygen to the graphite support. Reductive treatments in the aqueous phase appear to reduce platinum and only the tin deposited on the platinum. The effects of drying and consecutive reductive treatments could only be studied since the developed EXAFS cell allowed catalyst preparation and treatments avoiding contact to ambient. # 1998 Elsevier Science B.V.