Title: Spectroscopic study of copper and iron zeolites and their reactive oxygen complexes
Other Titles: Spectroscopische studie van koper en ijzer zeolieten en hun reactieve zuurstof complexen
Authors: Vanelderen, Pieter
Issue Date: 15-Oct-2014
Abstract: The boom in shale gas exploitation in the US raises a strong renewed interest in the selective oxidation of light alkanes with simple oxidants like N2O and O2. In the past, numerous groups have attempted these reactions with bio- and chemocatalysis, but the search for robust heterogeneous catalysts for the partial methane oxidation has been elusive. The discovery of the oxidation capacities of Fe-ZSM-5 (with N2O) and Cu-ZSM-5 (with N2O and O2), is promising and is, giving the similarities with Fe- and Cu-containing methane oxidizing enzymes, scientifically very attractive. There is a quest for the elucidation of the reaction mechanism and the identification of the reactive intermediates that are involved in these inorganic Cu- and Fe-ZSM-5 biomimics. Such molecular understanding is important with respect to upgrading abundantly available methane, but also to comprehend the working mechanism of genuine Cu- and Fe- containing oxidation enzymes.A breakthrough in the chemistry of Cu-ZSM-5 was recently obtained by the detailed spectroscopic description and identification of the reactive dicopper core for methane oxidation in CuĀ–ZSM-5. This active site is a bent mono(&#956;-oxo)-dicopper, formed upon contacting an autoreduced Cu zeolite with N2O or O2. This is, so far, the only comprehensively characterized copper oxygen site capable of methane oxidation. Its discovery in ZSM-5 moved the Cu-O-Cu moiety into the spotlight. In this work the formation pathway of the Cu-O-Co site in Cu-ZSM-5 from both N2O and O2 was studied in detail. O2 activation proceeds through the formation of a &#956;-&#951;2:&#951;2-peroxodicopper(II) site. Remarkably, almost at the same time, it was reported that the pMMO enzyme might also form such a peroxo dicopper site. Studying the activation mechanism of N2O is important in view of N2O decomposition and reduction of greenhouse gases. The N2O activation on Cu-ZSM-5 proceeds via a bridged &#956;-1,1-O binding fashion if two Cu+ atoms are closely separated (<4.2 &Aring;). This pathway is in line with that recently discovered for the N2O reductase enzyme. The discovery of the bent [Cu-O-Cu]2+ core in Cu-ZSM-5 automatically led to a search for other zeolite structures able to stabilize this core, in order to study the influence of the zeolite lattice on the identity of the Cu-O-Cu site. Mordenite (MOR) with the same pentasil building block as ZSM-5, was the obvious first choice. Similar to the work on ZSM-5, the Cu-O-Cu formation in Cu-MOR was investigated using a combination of (operando) spectroscopic tools and kinetic studies of the reactivity. The thesis presents new data showing firstly the overall picture of the copper distributions and secondly the Cu redox chemistry in the Cu-MOR zeolites. Experimental spectroscopic proof of square planar mononuclear copper sites was found. Next to these monomers, a large quantity of reactive Cu-O-Cu sites, viz. 60% for CuMOR versus 5% for Cu-ZSM-5, were determined and identified.Fe-ZSM-5 is also active in the selective oxidation of hydrocarbons via the so-called alfa-O sites. The structure of these sites is elusive due to the lack of selective spectroscopic probes. An Fe-ZSM-5 catalyst with a very high active site density was synthesized, allowing spectroscopic and theoretical characterization of these sites and the intermediates in the selective oxidation reaction. In conclusion, this work provides insights in many aspects of the reactive oxygen species on zeolites. Hopefully these understandings will be inspirational for future catalyst design, and helpful in the development towards an exploitable selective oxidation catalyst for the valorization of methane.
Table of Contents: Dankwoord I
Table of Contents V
List of Symbols and Abbreviations IX
Abstract XIII
Samenvatting XVII
Setting the scene XXI
Part 1: Copper chemistry inside zeolites 1
Chapter 1: Literature review of the coordination chemistry and reactivity of copper in zeolites 3
1.1 Introduction 3
1.2 Ion exchange and coordination to lattice oxygen 3
1.3 Complexation of copper in cavities and channels of zeolites 11
1.4 Oxo complexes of Cu2+ in zeolites 15
1.5 Catalysis with Cu-zeolites 15
1.6 Conclusion 16
1.7 References 17
Chapter 2: Cu-ZSM-5: A biomimetic inorganic model for methane oxidation 25
2.1 Introduction 25
2.2 Spectroscopic and computational characterization of the methane-oxidizing active site 28
2.3 Formation of the active site: the O2 activation pathway 32
2.4 Evaluating the methane to methanol reaction mechanism 37
2.5 Outlook and significance 39
2.6 Supporting information 41
2.7 References 43
Chapter 3: N2O activation mechanism in Cu-ZSM-5 51
3.1 Direct N2O decomposition on Cu-zeolites 51
3.2 Results and Analysis 53
3.2.1 Nature of Dinuclear CuI Active Sites 53
3.2.2 Mechanism of [Cu2O]2+ Core Formation from N2O 57
3.2.3 Geometric Requirements for [Cu2O]2+ Active Site Formation 60
3.2.4 Experimental observations of N2O interaction with other Cu-zeolites. 62
3.2.5 Comparison of Cu-ZSM-5 with the N2O activation in the N2OR Cu-enzyme 65
3.3 Conclusion 66
3.4 Supporting information 68
3.4.1 Computational details 70
3.5 References 72
Chapter 4: Spectroscopy and redox chemistry of copper in mordenite 77
4.1 Introduction 77
4.2 Results 80
4.2.1 UV-vis-NIR and EPR spectroscopy 80
4.2.2 Redox behavior 82
4.3 Discussion 86
4.4 Conclusion 91
4.5 Supporting information 93
4.6 References 100
Chapter 5: Mono-(µ-oxo) dicopper motifs in Cu-Mordenite: Structure-reactivity relationship 105
5.1 Introduction 105
5.2 Results and discussion 106
5.2.1 Detailed analysis of the 22200 cm-1 absorption band 106
5.2.2 Resonance Raman (rRaman) study of the active site(s) 111
5.2.3 X-ray diffraction analysis of Cu-MOR 120
5.2.4 Structure-related reactivity study 122
5.3 Conclusion 124
5.4 Supporting information 126
5.4.1 Crystallographic information files 129
5.5 References 139
Part 2 Fascinating oxidation chemistry of iron with nitrous oxide on a ZSM-5 zeolite 141
Chapter 6: Fe-ZSM-5: A literature review 143
6.1 Fe-ZSM-5 preparation methods 143
6.2 High temperature treatment: autoreduction 144
6.3 Interaction with N2O 145
6.4 Direct catalytic decomposition of N2O 148
6.4.1 Comparison of Cu-Zeolites and Fe-Zeolites 148
6.4.2 Fe-zeolites 149
6.5 Identification of α-O 152
6.6 Selective oxidation with Fe-ZSM-5 155
6.7 Conclusion 156
6.8 References 157
Chapter 7: Spectroscopic study of Fe-ZSM-5: Fe2+, α-O and reaction intermediates in the benzene hydroxylation 165
7.1 Introduction 165
7.2 Preparation of Fe-ZSM-5 166
7.3 Spectroscopy of Fe2+ in ZSM-5 166
7.3.1 Results and analysis 166
7.3.2 Discussion of the 15200 cm-1 electronic transition in ferrous Fe-ZSM-5 170
7.4 Spectroscopic probes for α-O 172
7.4.1 UV-vis-NIR study of the interaction of ferrous Fe-ZSM-5 with N2O 172
7.4.2 EPR study of the N2O interaction 177
7.4.3 Discussion of the obtained spectroscopic probes for α-O 179
7.5 Reaction intermediates in the benzene hydroxylation 182
7.5.1 Results 182
7.5.2 Discussion 184
7.6 Future prospects on Fe-ZSM-5 186
7.7 Conclusion 188
7.8 Supporting information 189
7.9 References 190
General conclusions and perspective 197
Experimental details 203
E.1 Sample preparation 203
E.1.1 Cu-zeolites 203
E.1.2 Fe-ZSM-5 203
E.2 Sample treatments 204
E.2.1 Cu-zeolites 204
E.2.2 Fe-ZSM-5 204
E.3 Spectroscopic measurements: UV-vis-NIR, EPR and rRaman 205
E.3.1 UV-vis-NIR 205
E.3.2 Electron paramagnetic resonance (EPR) 205
E.3.3 Resonance Raman 206
E.4 X-ray powder diffraction measurements 206
E.5 Operando UV-vis spectroscopy and kinetic studies 206
E.5.1 Activation energy of N2O and O2 activation with Cu-ZSM-5 207
E.5.2 Activation energy of CH4 oxidation with activated Cu-MOR 207
E.5.3 Temperature programmed desorption 207
E.6 Liquid extraction 207
E.7 H2 TPR and H2 pulse experiments 208
List of Publications 209
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Centre for Surface Chemistry and Catalysis

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