selective aerobic oxidation of hydrocarbons over supported gold catalysts

144 Pages·2011·18.58 MB·English

S A o
elective erobic xidAtion of
H S G
ydrocArbonS over upported old
c
AtAlyStS
Selectieve Aërobe oxidAtie vAn KoolwAterStoffen
over GedrAGen GoudKAtAlySAtoren
(met een samenvatting in het Nederlands)
p
roefScHrift
ter verkrijging van de graad van doctor aan de Universiteit Utrecht
op gezag van de rector magnificus, prof. dr. G. J. van der Zwaan, ingevolge het
besluit van het college voor promoties in het openbaar te verdedigen
op maandag 14 november 2011 des middags te 2.30 uur
door
b p c H
Art eter HriStiAAn ereijGerS
geboren op 6 maart 1983 te Soest
Promotor: Prof. dr. ir. B. M. Weckhuysen
Het in dit proefschrift beschreven onderzoek werd mede mogelijk gemaakt met
financiële steun van ACTS/ASPECT onder projectnummer 053.62.015.
-Wisdom is the domain of the Wis-
Frank Zappa
ISBN: 978-90-8891-324-2
Printed by: Proefschriftmaken.nl || Printyourthesis.com
Published by Uitgeverij BOXPress, Oisterwijk
Contents
Chapter 1 General Introduction 7
Part I: Partial Oxidation of Methanol over Gold-based Catalysts
Chapter 2 Selective Oxidation of Methanol to Hydrogen over Gold 27
Catalysts Promoted by Alkaline-Earth Metal and Lanthanum
Oxides
Chapter 3 Understanding the Promotion Effect of Lanthanum Oxide on 41
Gold-based Catalysts in the Partial Oxidation of Methanol by
in-situ XAFS and DSC Studies
Part II: Selective Alkane Oxidation over Gold-based Catalysts
Chapter 4 Aerobic Oxidation of Cyclohexane over Gold-based Catalysts: 59
New Mechanistic Insight by Thorough Product Analysis
Chapter 5 Selective Oxidation of Methane to Methanol over Supported 81
Gold-based Catalysts
Part III: Cyclohexyl Hydroperoxide for Olefin Epoxidation
Chapter 6 Cyclohexene Epoxidation with Cyclohexyl Hydroperoxide: 91
A Catalytic Route to Largely Increase Oxygenate Yield from
Cyclohexane Oxidation
Chapter 7 Mechanistic Insight in the Olefin Epoxidation with Cyclohexyl 111
Hydroperoxide
Chapter 8a Summary and Concluding Remarks 127
Chapter 8b Nederlandse Samenvatting 131
Appendix Safety Considerations 136
List of Publications and Presentations 138
Dankwoord 141
Curriculum Vitae 144

Chapter 1
General Introduction

1.1 Catalysis: Principle and Applications
Catalysis is one of the most important and prominent concepts of life. It is often claimed
that all products we use in daily life have met, or will meet, a catalyst at some point on their
trip from cradle to grave. Besides that, also life itself essentially depends on (bio)catalytic
processes. The growing of crops, digestion of food and cell reproduction are just a few examples
of the numerous processes in nature in which enzymes (bio-catalysts) do their dedicated tasks
with high efficiency. The first catalytic process deliberately performed by humans probably
is the production of wine by fermentation of sugars, although the understanding of the actual
chemistry involved at that time most likely was a minor concern.
A popular definition of a catalyst still is: “a substance that accelerates a chemical
reaction without itself being consumed”. Although this definition covers the main purpose of
catalysis, it is essentially incorrect. In 1895, a modern definition was formulated by Ostwald
stating that; “a catalyst accelerates a chemical reaction without affecting the position of
the equilibrium”. This describes that a catalyst indeed can affect the kinetics of a reaction
(accelerates), but does not in any way, alter the thermodynamics of the process (position of
the equilibrium).
As always, things are more complicated in practise and as the definition holds for all
catalysts, there is no general definition of how a catalyst actually does the job. However,
catalysis is a cyclic process which contains a few basic steps, which are the key to all catalytic
processes. In Figure 1.1, an elementary catalytic cycle of the reaction A + B  2 AB is
2 2
schematically illustrated. In the first step, the different reactants approach and adsorb on the
catalyst surface (I). Then, under influence of the catalyst bonds get broken and rearranged, i.e.
a chemical reaction takes place (II) and finally, the freshly formed product desorbs from the
catalyst surface and the catalyst remains behind unchanged (III). Obviously this is a simplified
picture and many variations on the scheme exist. However, all catalytic cycles involve these
three basic steps.
The field of catalysis is often divided into three subfields: homogeneous (reactants
and catalyst in the same phase), heterogeneous (reactants and catalyst in different phases)
7
Chapter 1
Figure 1.1. Schematic representation of the elemental steps in a catalytic cycle. Bonding of reactants A
2
and B (I), chemical reaction (II) and desorption of the product AB (III).
2
and bio-catalysis (using catalysts derived from nature, mainly enzymes). Because of the
robustness, ease of separation and recycling of the catalyst and consequently relatively
low costs, heterogeneous catalysts are the most popular in large scale industrial processes.
Homogeneous and enzymatic catalytic processes mainly find application in fine-chemistry,
for example in the pharmaceutical and food industry. Due to the constantly increasing prices
of raw materials and energy and the need to reduce waste streams, the demand for new and
more efficient catalytic production routes has largely increased over the past decade and will
continue like that.
1.2 Oxidation Catalysis: A Short Literature Survey
Worldwide, about 25 % of the most important industrial intermediates are produced
by selective oxidation catalysts, comprising roughly a quarter of the value of all catalytic
processes.[1, 2] This makes the controlled partial oxidation of hydrocarbons the single most
important technology for the conversion of natural gas and oil based feedstocks.[3-5] For the
large scale processes, molecular oxygen is the preferred oxidant due to its low price and low
waste production. However, applying O as selective oxidant is not straightforward.
2
All oxidation reactions have in common that they are mechanistically tremendously
complex and therefore difficult to control. This results from the fact that molecular oxygen
can react with most organic compounds even without a catalyst present.
In the ground state, the molecular oxygen diradical is in a triplet spin state. Because
interaction with a closed shell singlet state, in which most molecules generally are, requires
a quantum mechanically forbidden transition, this prevents reaction of oxygen with these
molecules at ambient temperature. However, when the temperature is sufficiently high to
overcome the energy barrier for the transition into a singlet state, the reactions proceeds
readily. This is characterized by the existence of an autoignition temperature for most organic
substances at which they spontaneously combust, without the need of an external flame
or spark. Singlet oxygen is thought to abstract a hydrogen atom from a hydrocarbon, thus
8
General Introduction
creating two radical species. Triplet oxygen reacts readily with the formed radicals, which
are in a doublet state, allowing it to subsequently react with a singlet state fuel molecule.
This causes a constant supply of radical species and keeps the reaction running. The large
amount of energy released during combustion and the consequent temperature rise speeds
up the reaction in an uncontrollable way, but also (locally) alters the conditions so that other
reactions than the desired reaction might become thermodynamically or kinetically favoured.
Such an uncontrolled reaction can yield all kind of products, inevitably decreasing the
selectivity dramatically.
Probably, still the most important form of an oxidation reaction is simply the burning of
organic material, yielding heat, light, CO and water. But even this seemingly straightforward
2
reaction deals with selectivity issues. Incomplete combustion might form toxic intermediates
(like carbon monoxide) and causes a serious loss of potential energy from the fuel. When not
the energy, but partially oxidized intermediates are the desired products, care has to be taken
not to over-oxidize the feedstock into useless CO and water. In both cases the use of a catalyst
2
might offer a solution to the selectivity problem, by facilitating certain reaction paths at lower
temperature. Consequently, the field of oxidation catalysis research is broad and encloses both
studies into selective partial oxidation[4-9] as well as efficient total oxidation.[10]
Several tricks can be played to increase the selectivity of oxidation reactions. For instance
stoichiometric oxidants like dichromates or permanganates can be used, which can oxidize
hydrocarbons in a controlled way at lower temperature. However, such methods yield large
streams of waste inorganic salts, some of which are a serious threat to the environment, e.g.
Cr6+-compoundsand therefore are becoming prohibitive.[11] Other processes employ strong

acids like HNO and / or HSO as oxidant. However the use of these mineral acids can
3 2 4
cause severe corrosion of equipment and yields large acidic waste streams that have to be
neutralized and/ or purified.
The application of redox molecular sieves can offer a solution for both waste and
corrosion problems. Reactive metal-ions are trapped in the zeolites where they are oxidized
by molecular oxygen. The oxidized metal ions can subsequently oxidize the reactants that are
present in the micropores. The products diffuse out of the pores leaving the reduced metals
behind for another cycle, thus reducing the waste. Alternatively highly reducible metal oxides,
such as CeO, can act as oxygen reservoir via a Mars-van Krevelen type mechanism. Also in
2
these methods molecular oxygen has to be present to reoxidize the catalyst and complete the
cycle.
Another method that gained a lot of attention in the past decades is the use of supported
noble metal catalysts for the activation of molecular oxygen in gas-phase oxidation and
oxidative dehydrogenation reactions at high temperatures.[12-16]
Alternative oxidants like hydroperoxides,[17] ozone[18] or NO[19] contain activated oxygen
2
and therefore can be employed at lower temperatures thus allowing to perform the reaction in
a more controllable way. In some cases these oxidants can be formed in-situ, as for instance in
Shell’s SM-PO[20-22] or Sumitomo Chemical’s cumene hydroperoxide[23] process. However, the
production and use of these oxidants is expensive and often desires complicated technology.
In the following sections, the processes relevant for this PhD thesis will be discussed.
9
Chapter 1
1.3 Catalysis by Gold
The noble metal gold has always had a large appeal on humans because of the beautiful
shine and lack of corrosion. Therefore, gold has been used throughout human history mainly
for decoration purposes and has been the most common basis for monetary policies. Even
today, 50 % of the gold production is used in jewellery, another 40 % as investments. Because
of its inertness, gold has never been of any interest for application in heterogeneous catalysis,
until recently. In 1973, Bond and co-workers reported on the application of a supported gold
catalyst for the hydrogenation of mono-olefins but-1,2-ene and but-2-yne.[24] However, only
until Haruta and co-workers reported in 1989 that supported gold nanoparticles on an oxidic
support yielded a highly active catalyst for low temperature CO oxidation, a true gold rush
in heterogeneous catalysis started.[25] To compare the impact of both publications, Haruta’s
paper has been cited 1260 times to 6 citations of Bond’s publication (Scopus search on July
15th, 2011).
Supported gold nanoparticles were found to exhibit remarkable properties in, most
importantly selective oxidation reactions and gold-based catalyst materials have been
extensively researched and reviewed the past years.[26-32] But also for various other reactions[33]
including water gas shift reaction,[34] selective hydrogenations,[24, 35] NO reduction,[36, 37] total
combustion of hydrocarbons,[38] and the hydrochlorination of acetylene[39] gold catalysts have
successfully been applied. However, the oxidation of CO remains the most important topic in
gold catalysis.
For all applications, there is a strong size/ activity relationship for the supported gold
particles and the key to activity is to obtain gold particles with a diameter < 5 nm.[28] In
Figure 1.2, a Transmission Electron Microscopy (TEM) image of a typical Au/TiO with
2
particles in the range of 2–6 nm is shown. The true origin of the catalytic activity is not
fully understood and there are many explanations given that relate the catalytic activity to
the electronic properties of the gold clusters. These properties can be influenced by the gold-
Figure 1.2. Transmission Electron Microscopy image of home-synthesized gold nanoparticles on a
porous TiO support. The gold particles (2-6 nm) are clearly visible as the dark round dots on the light
2
grey support. The larger central dark area represents a thicker TiO area.
2
10