Platinum catalysed alcohol oxidation : kinetics, reaction engineering, and process design Citation for published version (APA): Gangwal, V. R. (2005). Platinum catalysed alcohol oxidation : kinetics, reaction engineering, and process design. Technische Universiteit Eindhoven. https://doi.org/10.6100/IR587459 DOI: 10.6100/IR587459 Document status and date: Published: 01/01/2005 Document Version: Publisher’s PDF, also known as Version of Record (includes final page, issue and volume numbers) Please check the document version of this publication: • A submitted manuscript is the version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. 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If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license above, please follow below link for the End User Agreement: www.tue.nl/taverne Take down policy If you believe that this document breaches copyright please contact us at: [email protected] providing details and we will investigate your claim. Download date: 29. Sep. 2021 Platinum catalysed alcohol oxidation: Kinetics, reaction engineering, and process design PROEFSCHRIFT ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven, op gezag van de Rector Magnificus, prof.dr.ir. C.J. van Duijn, voor een commissie aangewezen door het College voor Promoties in het openbaar te verdedigen op donderdag 21 april 2005 om 16.00 uur door Vikrant Ramesh Gangwal geboren te Chapaner-Aurangabad, India Dit proefschrift is goedgekeurd door de promotor: prof.dr.ir. J.C. Schouten Copromotor: dr.ir. B.F.M. Kuster CIP-DATA LIBRARY TECHNISCHE UNIVERSITEIT EINDHOVEN Gangwal, Vikrant R. Platinum catalysed alcohol oxidation : kinetics, reaction engineering, and process design / by Vikrant R. Gangwal. - Eindhoven : Technische Universiteit Eindhoven, 2005. Proefschrift. - ISBN 90-386-3006-9 NUR 913 Trefwoorden: chemische technologie ; reaktiekinetiek / heterogene katalyse ; platina / katalytische oxidatie ; alcoholen / fysisch-chemische simulatie en modellering ; stofoverdracht Subject headings: chemical engineering ; reaction kinetics / heterogeneous catalysis ; platinum / catalytic oxidation ; alcohols / physicochemical simulation and modelling ; mass transfer / catalyst deactivation Printed in the Netherlands by Ridderprint Offsetdrukkerij bv Summary Traditionally, due to fast production demands and one pot operation, synthesis routes of fine chemical and pharmaceutical compounds have been based, amongst others, on alcohol oxida- tion with stoichiometric quantities of inorganic oxidants. This oxidation process suffers from low atom efficiency and high toxic waste production. Alcohol oxidation by enzymes is practiced as an environmentally friendlier alternative, however product separation and waste production remain a problem. Homogeneous catalysis is an efficient alternative, however catalyst recovery, reactant recycling, and reactor corrosion limits its industrial scale utilization. Heterogeneous catalytic alcohol oxidation has been known for a long time. However, in the past 20 years there has been a growing demand for clean and efficient solid catalysts, for the production of fine and speciality chemicals. Noble metal catalyzed alcohol oxidation with cheap oxidants such as air or molecular oxygen, is a clean and elegant alternative with mild reaction conditions and high selectivities that can be obtained. In this PhD research work, the Pt catalysed selective oxidation of methyl α-D-glucopyranoside and D-glucose in a continuous or semi-batch reaction mode, with oxygen as an oxidant, was investigated. It is generally accepted that the reaction takes place via a dehydrogenation mech- anism followed by the oxidation of the adsorbed hydrogen atoms with dissociatively adsorbed oxygen. The nature of catalyst deactivation is largely influenced by the amount of oxygen present at the catalytic surface. Several types of catalyst deactivation mechanisms have been identified, viz. over-oxidation, the formation of carbonaceous deposits, metal particle-growth, metal leaching, and chemical poisoning. These deactivation mechanisms lead to a decrease of the active metal surface area and hence to a decrease of the reaction rate. It has been found that over-oxidation was the major cause of catalyst deactivation in this study and care has been taken to avoid other types of deactivations. A dynamic transport model is derived to describe the platinum catalysed aqueous alcohol oxi- dation, considering a single spherical catalyst particle surrounded by a stagnant liquid film. The transport model is based on a heterogeneous kinetic model with mass transfer and intra parti- cle diffusion resistances. The developed model uses formerly developed kinetic model and is validated with the formerly obtained experimental kinetic data of methyl α-D-glucopyranoside 3 4 Summary (MGP) oxidation. The model is used to investigate the effect of process conditions, catalyst and particle properties, and transport parameters on the performance of the catalyst for alcohol oxidation. It is found that the electron conductivity of the catalyst support affects the rate of MGP oxidation especially at low bulk liquid oxygen concentrations. At high bulk liquid oxy- gen concentrations, the catalyst support conductivity does not play a role. A major cause of catalyst deactivation is over-oxidation under oxygen rich conditions. This can be reversed by applying redox-cycle operation, an alternating exposure of the catalyst to ox- idative and reductive environments. The advantages of redox-cycle operation are demonstrated using the developed model. For reactions of negative order, such as MGP oxidation, at high > . / 3 bulk oxygen concentrations (Co2,L 0 3mol m ), concentrating the active catalytic material in a layer buried some distance from the surface (core) gives considerable better performance than the conventional ”egg shell” design of shallow deposition near the surface or uniform distri- ≤ . / 3 bution. However, at low bulk oxygen concentrations (Co2,L 0 3mol m ), the performance of the uniform or egg shell catalyst distribution is superior to the core catalyst, which suffers with strong diffusion limitations. The model predicts very high oxidation rates, at low bulk liquid oxygen concentrations or oxy- gen mass transport limited conditions. However, experiments at mass transport limited condi- tions did not verify these high rates. It is diagnosed that for negative order kinetics, such as the reaction under investigation, kinetic parameters were erroneously estimated, which resulted in inadequate model description for the observed reaction rates at mass transport limited con- ditions. Kinetic parameters of a reaction rate model are usually estimated using reaction rate measurements obtained at intrinsic kinetic conditions, viz., without limitation of the reaction rate by mass transport, irrespective of the order of the reaction kinetics. For positive order ki- netics, this is good practice and the kinetic rate parameters can be safely applied both at intrinsic and mass transport limited conditions. However, for negative order kinetics, erroneous results are obtained because the kinetic rate parameters estimated at intrinsic kinetic conditions fail to predict the reaction rate when mass transport limitation also plays a role. This is demon- strated for the simple case of Langmuir-Hinshelwood negative-order kinetics and for the, more complex, kinetics of the oxidation of methyl α-D-glucopyranoside on noble metal catalysts. It is concluded that for a negative-order reaction rate model, which is valid for a wide range of reaction conditions, the kinetic rate parameters need to be estimated both at intrinsic and mass transport limited conditions. The electrochemical kinetic model with proper kinetic parameters is then used to design the catalyst activity profile for alcohol oxidation. This was done by comparing experimental data obtained in a semi-batch reactor with reactor modelling results, for catalysts of different ac- Summary 5 tivity distributions viz., egg shell and uniform. Experimental results show that for methyl α- D-glucopyranoside oxidation at oxygen partial pressures below 40 kPa, the egg shell catalytic activity distribution gives a higher rate of oxidation than a uniform distribution. It was also ob- served that with increase in the oxygen partial pressure from 10 to 40 kPa, the rate of deactiva- tion due to catalyst