The Use of Visible Light Absorbing Bismuth-Containing Semiconductors As Heterogeneous Photocatalysts for Selective Chemical Transformations
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The Use of Visible Light Absorbing Bismuth-Containing Semiconductors as Heterogeneous Photocatalysts for Selective Chemical Transformations Christopher Adam Unsworth Doctor of Philosophy University of York Chemistry August 2017 Abstract Bismuth-containing semiconducting materials were used as visible light absorbing heterogeneous photocatalysts for selective chemical transformations. The work demonstrates the importance of kinetic control in achieving selectivity; either through photocatalyst inhibition or through the presence of reagents capable of fast reactions with known intermediates. Bismuth oxide (β-Bi2O3), bismuth ferrite (BiFeO3), bismuth tungstate microflowers (Bi2WO6) and bismuth vanadate nanoparticles (nan-BiVO4) were synthesised and characterised by PXRD, SEM, DRUVS and BET. The bismuth-containing oxides were compared as photocatalysts for the aerobic oxidation of benzyl alcohol. The highest benzyl alcohol conversion (88%) and benzaldehyde selectivity (95%) was achieved with the use of nan-BiVO4. Further modifications to nan-BiVO4 resulted in materials that were less active for selective benzyl alcohol oxidation than unmodified nan-BiVO4. Further study of nan-BiVO4 as a heterogeneous photocatalysts for the selective oxidation of para-substituted benzyl alcohols was carried out. It was found that alcohol conversions and aldehyde selectivities were affected by by-product inhibition. The addition of 1 mol% 4-methoxybenzoic acid significantly reduced 4-methoxybenzyl alcohol conversion (to 49%). Isotopically labelled benzyl alcohols were used to show that α C-H bond cleavage was not rate limiting. However, changes in charge carrier lifetimes observed using TRPLS suggested that the charge carriers associated to the lifetimes observed were relevant to benzyl alcohol oxidation. Bismuth-containing semiconductors were also investigated as trifluoromethylation heterogeneous photocatalysts. Nan-BiVO4 was capable of oxytrifluoromethylation of styrene via the reduction of Umemoto’s reagents. The highly selective reaction produced the corresponding trifluoromethylated ketone in an 88% yield. Platinised bismuth tungstate (0.15-Pt-Bi2WO6) was found to give high conversions and product selectivities for the formation of Photo-Giese products (phenylacetic acid conversion = 99%, Photo-Giese product selectivity = 94%). Several coupling products were synthesised from different carboxylic acids and electron deficient alkenes. Competitive adsorption from by-products inhibition had an impact on acid conversions and Photo-Giese product selectivities. 2 Table of Contents Abstract ............................................................................................................................. 2 Table of Contents .............................................................................................................. 3 Table of Figures, Schemes and Tables .............................................................................. 8 Accompanying materials ................................................................................................. 20 Acknowledgements ......................................................................................................... 21 Declaration ...................................................................................................................... 22 1 Introduction ............................................................................................................. 23 1.1 Photocatalysis ................................................................................................... 23 1.2 Light absorbing semiconducting materials ....................................................... 28 1.3 Application of heterogeneous semiconducting photocatalysts ........................ 35 1.4 Understanding heterogeneous photocatalytic selectivity ................................. 37 1.4.1 Substrate attraction .................................................................................... 38 1.4.2 Substrate adsorption .................................................................................. 39 1.4.2.1 Size exclusion .................................................................................... 39 1.4.2.2 Surface adsorbed transition states that aid selectivity ....................... 39 1.4.3 Intermediate and product desorption ......................................................... 46 1.4.4 Reactions in solution ................................................................................. 47 1.5 Summary of selectivity and opportunities for investigation ............................ 50 1.6 Project aims ...................................................................................................... 50 2 The Direct Comparison of Bismuth-Containing Semiconductors for the Heterogeneous Photocatalytic Oxidation of Benzyl Alcohol Selectivity into Benzaldehyde .................................................................................................................. 52 2.1 Introduction ...................................................................................................... 52 2.1.1 Bismuth containing semiconductors ......................................................... 52 2.1.2 Photocatalytic application of bismuth containing semiconductors ........... 54 2.1.3 Comparing photocatalysts ......................................................................... 56 2.1.4 Objectives .................................................................................................. 57 3 2.2 Materials synthesis and characterisation .......................................................... 57 2.2.1 Bismuth tungstate (Bi2WO6) ..................................................................... 58 2.2.2 Bismuth ferrite .......................................................................................... 62 2.2.3 Bismuth oxide ........................................................................................... 65 2.2.4 Bismuth vanadate ...................................................................................... 68 2.2.5 Photocatalyst comparison.......................................................................... 71 2.3 Modifying bismuth vanadate ............................................................................ 78 2.3.1 Heterojunction synthesis by milling ......................................................... 79 2.3.2 Doping ....................................................................................................... 81 2.3.3 Modified bismuth vanadate catalyst comparison ...................................... 83 2.4 Conclusions ...................................................................................................... 84 3 Selective Benzyl Alcohol Oxidation and Product Inhibition using Bismuth Vanadate Nanoparticles .................................................................................................. 86 3.1 Introduction ...................................................................................................... 86 3.1.1 Heterogeneous photocatalytic benzyl alcohol oxidation........................... 86 3.1.2 Objectives .................................................................................................. 87 3.2 Selective benzyl alcohol oxidation ................................................................... 87 3.2.1 Kinetics and comparison to bulk BiVO4 and Degussa P25 TiO2 .............. 87 3.2.2 Steric and electronic effects ...................................................................... 92 3.2.3 Inhibition ................................................................................................... 95 3.2.4 Ketone and carboxylic acid inhibition ...................................................... 97 3.2.5 Catalyst recycling and stability ............................................................... 102 3.2.6 Mechanism .............................................................................................. 103 3.2.6.1 Use of isotopically labelled benzyl alcohols.................................... 103 3.2.6.2 Time resolved photoluminescence................................................... 107 3.2.6.3 Proposed mechanism ....................................................................... 111 3.3 Conclusions .................................................................................................... 112 4 4 Heterogeneous Photocatalytic Trifluoromethylation ............................................ 114 4.1 Introduction .................................................................................................... 114 4.1.1 Trifluoromethylation reagents ................................................................. 114 4.1.2 Thermal trifluoromethylation with copper .............................................. 115 4.1.3 Photolytic trifluoromethylation ............................................................... 116 4.1.4 Objectives ................................................................................................ 119 4.2 Trifluoromethylation ...................................................................................... 119 4.2.1 Reduction of Umemoto 1 for the trifluoromethylation of styrene .......... 119 4.2.2 Reduction of Umemoto 1 for the trifluoromethylation of aromatics ...... 123 4.2.3 Attempted reduction of pyridyl trifluoroacetyl ester for the trifluoromethylation of aromatics ........................................................................