Development of microreactor technology for partial oxidation of ethanol on gold catalyst

G5 Doctoral dissertation (article)


Internal Authors/Editors


Publication Details

List of Authors: Erfan Behravesh
Publisher: Åbo Akademi University
Place: Åbo
Publication year: 2019
ISBN: 978-952-12-3896-3
eISBN: 978-952-12-3897-0


Abstract

Oxidation of alcohols from biological origin is of significant
industrial relevance for the production of fine and specialty chemicals.
Aldehydes from polyols are important intermediates in pharmaceutical
and alimentary industries. Gold nanoparticles dispersed on porous
materials are effective catalysts in the oxidation of hydroxyl (–OH) to
carbonyl groups (C=O) in the presence of environmentally friendly
oxidizers such as molecular oxygen. Microreactors are suitable tools for
especially oxidation reactions because of having excellent heat and
mass transfer properties due to a high surface-to-volume ratio. The
importance of microreactors lies in their small size causing a
remarkable drop in capital and energy costs along with positive
environmental impacts.

In this work, a range of gold catalysts supported on zeolites and
oxides were synthesized via a depositionprecipitation method. The
effects of the surface charge as well as the pH of the solution on the
gold particle size and loading were investigated. Moreover, the effect
of gold deposition on the support acidity was revealed.

Both a fixed bed and a microreactor were used for the partial
oxidation of ethanol at atmospheric pressure and temperature range of
100–250°C. Catalyst screening was conducted using neat and
gold-supported catalysts in the fixed bed reactor. The activities and
the selectivities of the catalysts were discussed taken into account the
effect of gold particle size and the support acidity.

Two catalytic coating methods of microreactor elements were
developed using an Au/Al2O3 catalyst selected from the screening step as
one of the most promising catalysts in terms of the activity and
selectivity to the desired products. The first coating method was based
on the use of a catalyst slurry. The advantage of this coating method
lies in using a pre-prepared Au/Al2O3 catalyst without incorporation of
any binders, addressing the importance of the interplay between the
catalyst particle size and the slurry viscosity in the optimization of
the uniformity, stability and thickness of the coating layer. The second
method was inkjet printing. In this method, the alumina suspension was
first printed into the microchannels followed by gold deposition via a
deposition-precipitating step. This method has the advantage of a higher
precision but a more complicated chemistry since additives are needed.

A model was generated to explain the experimentally observed fixed
bed reactor behavior in the ethanol oxidation on gold nanoparticles. The
model for this heterogeneously catalyzed gas-phase multireaction system
consisted of dynamic mass and energy balances as well as
Langmuir–Hinshelwood–Hougen–Watson (LHHW), Mars van Krevelen and power
law expressions for the reaction kinetics. The gas fluid flow was
described with convection and dispersion terms. The axial and radial
concentration and temperature profiles inside the reactor and the
concentration profiles within the catalyst particles were predicted by
the model. The estimated parameters were the rate constants and the
activation energies as well as the adsorption parameters for LHHW and
Mars van Krevelen kinetics and reaction order of oxygen for power law
kinetics. Numerical simulations were performed to illustrate the
influence of the feed temperature and the catalyst loading as well as
the effect of Péclet number on the obtained results.

A detailed modelling work was performed for the microreactor which
resulted in a kinetic model for the reaction system. Dynamic mass
balance-based generic modelling was applied to estimate the rate
parameters from the experiments obtained with the microreactor. The aim
of the study was to find out the operation conditions that favor the
generation of the desired oxidation product.

Finally, molecular modelling computations at the DFT level were
carried out to elucidate the reaction mechanism of the oxidative
dehydrogenation of ethanol. Using the mechanism inspired by the
computational studies, an improved kinetic model was developed by
revisiting the previous data obtained with the microreactor to explain
the measured rates of acetaldehyde formation. Moreover, the
concentration profiles in the catalyst particles and layers were
calculated numerically to evaluate the role of internal diffusion in the
catalyst pores. The simulation results indicated the absence of
internal mass transfer limitations for catalyst layer thicknesses less
than 200 μm.


Last updated on 2020-03-06 at 03:45