Polyoxometalate-based Catalysts for Oxidation Reactions: Redox-buffering Catalysts and Alkane Dehydrogenating Nanoparticles Restricted; Files Only
Lu, Xinlin (Spring 2023)
Abstract
Polyoxometalate (POM)-catalyzed oxidation reactions have been studied for over a century. Systems include thermal, photo or photoelectron-activation and are conducted both homogeneously and heterogeneously. However, with the rapid growth of POM synthetic chemistry, a significant number of new POM molecules and POM-based materials have been designed and prepared in the last few decades. While a great number of different oxidation reactions catalyzed by POMs and POM-based materials have been reported, fundamental studies focusing on understanding the functioning of these catalysts by thermodynamic and kinetic tools are limited. Therefore, this dissertation focuses on the design of new POM-based catalytic systems and materials while simultaneously studying their mechanisms in detail. Chapter 2, describes a two-component catalytic system based on the tetrabutylammonium salt of [PMo6V6O40]9-, TBA4H5[PMo6V6O40] (PV6Mo6), and Cu(ClO4)2, I discovered that exhibits a strong synergistic effect between PV6Mo6 and Cu(II) ion with respect to the activity. Detailed kinetic and electrochemical studies have been performed and reveal a new POM-based self-redox buffering oxidation mechanism. Chapter 3 extends the study of the redox buffering effect associated with TBA salts of PVxMo12-xO40(3+x)- (PVMo) and focuses on the impact of vanadium atom number (x = 0-4, and 6). Detailed electrochemistry studies give the basic electrochemical performances of PVMo in acetonitrile and facilitate the calculation of key redox buffering parameters, such as the number of buffer steps and the number of electrons transferred at each buffer step. Further kinetic studies show that vanadium atoms have much faster electron-transfer rates than molybdenum atoms in the PVMo complexes. This implies that the redox buffering effect of PVMo is more pronounced when a larger number of vanadium atoms reside in the POM. The third project involves the formulation of the homogeneous PVMo/Cu(II) system as a more utilitarian heterogeneous one by inserting the POMs into the metal-organic framework (MOF), HKUST-1, making POM@HKUST. Studies show that the reactivity synergism still exists between Cu(II) in the nodes of HKUST-1 and PVMo(x = 1-3) but not for the corresponding transition-metal-substituted polytungstates XPW11 (X = V, Co, Zn, and Co). In addition, synergism in activity also leads to synergism in stability by fast electron transfer between the POM and the MOF framework. For the XPW11@HKUST, the Cu(II) nodes decompose to Cu(I) structures discovered by X-ray photoelectron spectroscopy (XPS). Finally, the fourth project describes the synthesis and photochemical activity of Na4W10O32-stabilized Pt(0) nanoparticles (NPs). These NPs show efficient activity for photodehydrogenation of alkanes to alkanes with quantum yields around 0.10, turnover numbers around 100, and high selectivity (ca. 80%) to the non-thermodynamic 1-alkenes. The NPs are stable and reusable for several runs of reactions.
Table of Contents
Chapter 1: Introduction 1
Overview of Polyoxometalate-based materials in thermal- and photo(electro)-activated Oxidation 2
1.1 Overview of Polyoxometalates2
1.1.1 Polyoxometalate Structures 2
1.1.2 Polyoxometalate Redox Properties6
1.1.3 Reduced Polyoxometalates, Mixed-Valence Species9
1.2 Polyoxometalate Catalytic Reactions 10
1.2.1 Polyoxometalate-catalyzed Oxidation Reactions11
1.2.2 Polyoxometalate Photoactivities18
1.3 Polyoxometalate-based materials21
1.3.1 Polyoxometalates Captured in Metal-Organic Framework (MOF) pores 22
1.3.2 Polyoxometalate-stabilized Nanoparticles 26
1.4 Scope of Current Work 28
1.5 References 29
Chapter 2: Catalytic System for Aerobic Oxidation That Simultaneously Functions as Its Own Redox Buffer 43
2.1 Introduction 44
2.2 Experimental 46
2.2.1 General Materials and Methods 46
2.2.2 Electrochemistry 47
2.2.3 Synthesis of TBA salts of PMo12-nVnO40(3+n)- (n = 1-6)48
2.2.4 Conditions for Catalytic Reactions 48
2.3 Results and Discussion50
2.3.1 Characterization of TBA salts of PMo12-nVnO40(3+n)- (n = 1-6) 50
2.3.2 Reaction Stoichiometry and Product Detection54
2.3.3 Catalytic Activity and Synergistic Effect55
2.3.4 Evidence of Non-dissociation of Free Vanadium, VO2+ 57
2.3.5 Features of the Catalytic Aerobic Oxidation of 2-mercaptoethanol 60
2.3.6 Electrochemistry 64
2.3.7 Measurements of PV6Mo6 Reduction State under Turnover Conditions 67
2.3.8 Redox Buffering by PV6Mo6 71
2.3.9 Thermodynamics, Speciation, and Catalytic Cycle of Copper Complexes 74
2.3.10 Cu(II) Catalysis of PV6Mo6 Reduction by RSH under Ar 76
2.3.11 Reaction of Reduced PV6Mo6 with O2 Catalyzed by Cu(II) 78
2.3.12 Overall Mechanism under Turnover Conditions 81
2.3.13 Difference in Activity and Redox Buffering Ability of PV6Mo6 versus PV6W6 82
2.4 Conclusions 87
2.5 References 87
Chapter 3: Faster polyoxometalate/Cu(II)-catalyzed aerobic oxidations by polyvanadomolybdate-enabled redox buffering. Role of multiple vanadium centers on redox buffering and catalytic turnover rates 94
3.1 Introduction 96
3.2 Experimental 97
3.2.1 General Materials and Methods 97
3.2.2 Electrochemistry 98
3.2.3 31P NMR of TBA salts of PVxMo12-xO40(3+x)- (x = 0-4 and 6) 99
3.2.4 RSH Oxidation and Measurement of the Varying PVMo Reduction States 100
3.2.5 Stopped-Flow Measurements 101
3.3 Results and Discussion 102
3.3.1 Catalytic Activity 102
3.3.2 Electrochemistry of PVxMo12-xO40(3+x)- (PVMo) 103
3.3.3 Reduction State Measurement and Buffer Range Determination 117
3.3.4 Difference of Electron Transfer Rates Between V and Mo to Substrates 124
3.4 Conclusions 130
3.5 References 131
Chapter 4: Reactivity and stability synergism directed by the electron transfer between polyoxometalates and metal−organic frameworks 137
4.1 Introduction 138
4.2 Experimental 140
4.2.1 General Materials and Methods 140
4.2.2 Synthesis of POM-HKUST 141
4.2.3 Thiol (RSH) oxidation 142
4.2.4 Electrochemistry 143
4.3 Results and Discussion 143
4.3.1 POM-MOF materials and characterization143
4.3.2 POM leaching and solvent selection 152
4.3.3 Activity and stability synergism 154
4.4 Conclusion 172
4.5 References 172
Chapter 5: Decatungstate [W10O32]4--Stabilized Pt(0) Nanoparticles for Photochemical Dehydrogenation of Alkanes 179
5.1 Introduction 180
5.2 Experimental 182
5.2.1 General Materials and Methods 182
5.2.2 Synthesis of Na4W10O32-stabilized Pt Nanoparticles 182
5.2.3 Photocatalytic Alkane Dehydrogenation 183
5.3 Results and Discussion 184
5.3.1 Characterization of Na4W10O32-stabilized Pt Nanoparticles 184
5.3.2 Reactivity and Stability of W10-PtNP 186
5.4 Conclusion 190
5.5 References 190
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File download under embargo until 22 May 2025 | 2023-03-18 16:57:06 -0400 | File download under embargo until 22 May 2025 |
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