CONVERSION OF LEVULINIC ACID TO GAMMA VALEROLACTONE USING p CYMENE RU(II) n HETEROCYCLIC CARBENE COMPLEXES

CONVERSION OF LEVULINIC ACID TO GAMMAVALEROLACTONE USING p-CYMENE RUTHENIUM(II) NHETEROCYCLIC CARBENE COMPLEXES TAY BOON YING (B.Sc. (Hons.), NUS) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF SCIENCE DEPARTMENT OF CHEMISTRY NATIONAL UNIVERSITY OF SINGAPORE 2013 DECLARATION I hereby declare that this thesis is my original work and it has been written by me in its entirety, under the supervision of Associate Professor Huynh Han Vinh, Chemistry Department, National University of Singapore and Dr. Ludger Paul Stubbs, Polymer Engineering and Catalysis, Institute of Chemical and Engineering Sciences, between 8 August 2011 and 16 September 2013. I have duly acknowledged all the sources of information which have been used in the thesis. This thesis has also not been submitted for any degree in any university previously. TAY BOON YING Name 16 September 2013 Signature Date ACKNOWLEDGEMENT I would like to take this opportunity to express my gratitude and heartfelt appreciation to the following people for their help, encouragement and support throughout this research journey: Associate Professor Huynh Han Vinh (my supervisor at the National University of Singapore, NUS) for his insightful guidance, support, encouragement and friendship. I would also like to thank him for his patience and understanding as well as the time spent in the evenings after group meeting for discussions. Associate Professor Leong Weng Kee (Honours Year Supervisor) for encouraging me to continue with my graduate studies. Dr. Ludger Paul Stubbs (my supervisor at the Institute of Chemical and Engineering Sciences, ICES) for giving me the autonomy in this project. I would also like to thank him for his care, understanding, friendship and support. Dr. Phua Pim Huat (my mentor at the ICES) for sharing with me his knowledge on nanoparticles, and for assistance with HPLC analysis. Dr. Wang Cun (my mentor at the ICES) for sharing and guiding me in various aspects in synthesis and crystallographic analysis. I would also like to thank him for his assistance in solving the crystal structures obtained in this project and the discussions we had in the lab. Dr. Martin van Meurs for the discussion on reaction kinetics and catalysis. I Members of the Central Analytical Laboratory, ICES for their assistance and support: Ms. Seo Pei Nee for her guidance and assistance on the operation of TEM, Ms. Chia Sze Chen for X-ray crystallographic analysis, Mr. Ng Fu Song for elemental analysis, Mr. Jeffrey Ng and Ms. Doris Tan for the MS analysis, Ms. Ong Lili for NMR analysis, Mr. Lee Koon Yong and Mr. Lee Ah Teck for the gas line support. My company, ICES, for the encouragement and financial support for this project. My examiners for taking their precious time in reviewing my thesis. Last but not least, I would like to express my heartfelt appreciation to my family, close friends and colleagues of the Polymer Engineering and Catalysis group, Vinh’s group members for their constant understanding, encouragement, patience, support and advice. II TABLE OF CONTENTS ACKNOWLEDGEMENT ................................................................................................... I TABLE OF CONTENTS ................................................................................................. III SUMMARY .................................................................................................................... VI COMPOUNDS NUMBERING SCHEME ...................................................................... VIII LISTS OF TABLES ........................................................................................................ X LIST OF FIGURES ....................................................................................................... XII LISTS OF SCHEMES .................................................................................................. XVI LIST OF CHART ....................................................................................................... XVIII LIST OF ABBREVATIONS .......................................................................................... XIX 1. INTRODUCTION ........................................................................................................ 1 1.1. Global energy demands and environmental issues ........................................... 1 1.2. Biomass as an alternative energy source .......................................................... 2 1.3. Levulinic Acid and γ-valerolactone .................................................................... 3 1.4. Conversion of LA to GVL................................................................................... 5 1.4.1. Catalytic homogeneous hydrogenation using molecular H2 ........................ 7 1.4.2. Catalytic heterogeneous hydrogenation using molecular H2 ....................... 7 1.4.3. Using formic acid as the alternative hydrogen source ................................ 8 1.4.4. Water as the solvating media ................................................................... 10 1.4.5. Metal nanoparticles as catalysts............................................................... 10 III 1.5. N-heterocyclic carbenes .................................................................................. 11 1.5.1. Definition and properties .......................................................................... 11 1.5.2. Preparation of NHC complexes and characterisation ............................... 14 1.6. Objective of the study ...................................................................................... 17 2. RESULTS AND DISCUSSIONS ............................................................................... 19 2.1. Synthesis and characterization of p-cymene Ru(II) NHC complexes ............... 19 2.1.1. p-cymene Ru(II) NHC complexes bearing monodentate NHC ligands ...... 19 2.1.2. p-cymene Ru(II) NHC complexes bearing bidentate NHC ligands ............ 22 2.2. Catalytic hydrogenation of LA to GVL using molecular H2 as the hydrogen source (Route 1) ............................................................................................. 29 2.2.1. Optimisation of reaction conditions ........................................................... 29 2.2.2. Catalyst screening.................................................................................... 32 2.2.3. Kinetics .................................................................................................... 35 2.3. Catalytic hydrogenation of LA to GVL using FA as the hydrogen source (Route 2) .................................................................................................................... 42 2.3.1. Optimisation of reaction conditions ........................................................... 42 2.3.2. Catalyst screening.................................................................................... 45 2.4. Appearance of the reaction mixture for the two different routes ....................... 47 2.5. Methods to probe the catalytic nature.............................................................. 48 2.5.1. TEM Studies ............................................................................................ 48 2.5.2. Mercury poisoning test ............................................................................. 53 2.5.3. UV Measurement ..................................................................................... 55 IV 2.6. Effect of additive on Route 1 ........................................................................... 57 2.7. Catalyst recycling experiments ........................................................................ 60 2.8. Effects of water and FA on nanoparticle formation .......................................... 62 2.8.1. Hydrogenation of cyclohexanone to cyclohexanol .................................... 65 3. CONCLUDING REMARKS ....................................................................................... 67 4. FUTURE WORK ....................................................................................................... 69 5. EXPERIMENTAL SECTION ..................................................................................... 72 5.1. Methods and materials .................................................................................... 72 5.2. Analysis .......................................................................................................... 72 5.3. Catalysis ......................................................................................................... 73 5.3.1. General procedure of catalytic hydrogenation of LA to GVL using molecular H2 (Route 1)............................................................................................. 73 5.3.2. General procedure of catalytic hydrogenation of LA to GVL using FA as the H2 source (Route 2) ................................................................................. 74 5.4. Synthesis of Ligand Salt Precursors ................................................................ 74 5.5. Synthesis of p-cymene Ru NHC Complexes ................................................... 76 6. REFERENCES ......................................................................................................... 82 7. APPENDIX ............................................................................................................... 88 V SUMMARY Fossil fuel has always been the main source of energy since the eighteenth century. However due to its depletion and related environmental concerns, there has been a shift towards renewable sources of energy. Biomass is a suitable alternative as it is the only renewable source of organic carbon that is essential for the production of liquid hydrocarbon fuels to chemicals. One of the top twelve useful platform chemicals that can be derived from biomass is levulinic acid (LA) that can be converted to gammavalerolactone (GVL) via hydrogenation. This thesis describes the synthesis and characterisation of three p-cymene Ru(II) complexes bearing monodentate N-heterocyclic carbene (NHC) ligand and three pcymene Ru(II) complexes bearing bidentate NHC-NHC/NHC-py ligand and their catalytic activity on the catalytic hydrogenation of LA to GVL. [RuCl2(η6-p-cymene)(iPr2-bimy)] (2), [RuCl2(η6-p-cymene)(Bn2-bimy)] (3) and [RuCl(η6-p-cymene)(κ2C,C-diNHCme)][PF6] (4) were characterised by X-ray diffraction analyses. Two hydrogen sources, molecular hydrogen (Route 1) and formic acid (FA, Route 2) were used in the catalytic hydrogenation of LA to GVL. In Route 1, we found that the catalysis proceeded in a heterogeneous fashion where the ruthenium complexes act as a precursor to catalytically active ruthenium nanoparticles (RuNPs) rather than a soluble metal catalyst. Under reducing H2 atmosphere, the catalyst precursor will rapidly be converted to RuNPs. At the end of the reaction, metallic plating was found on the reaction liner and magnetic stir bar. RuNPs were seen during transmission electron microscopy (TEM) analysis. Mercury poisoning experiments also showed positive results where there was a significant drop in the GVL yield. The VI disappearance of absorption maxima with time in the UV spectroscopic study indicated that the catalyst precursor was being converted to RuNPs. In the solvent selection study, we discovered that water is the best solvent for our optimised reaction conditions. Water is also the best solvent in the catalytic hydrogenation of cyclohexanone to cyclohexanol using our p-cymene Ru(II) NHC complexes as it aids in RuNPs dispersion. In the effect of additive study on Route 1, we discovered that pH has an effect on the stability of the RuNPs. RuNPs tend to aggregate at low pH. p-cymene Ru(II) complexes bearing monodentate NHC ligands generally performed better as compared to the complexes bearing bidentate ligands and catalysis via Route 1 with p-cymene Ru(II) complexes bearing monodentate NHC ligands proceeded via zero-order kinetics with respect to LA and first-order kinetics with respect to H2. The best performing catalyst was 2 where almost quantitative yield was achieved in 120 min. In Route 2, the catalysis was found to proceed in a homogeneous fashion as FA present might aid in the formation of a formate-bridged ruthenium dimers that will first catalyse the decomposition of FA to H2 and CO2 followed by the formation of GVL. RuNPs were absent for most of the catalysts screened and mercury poisoning experiments showed no decrease in GVL yield. The absorption spectrum of for the Ru(II) species did not change throughout the course of the catalysis. With respect to the catalytic performance, the trend was similar to the catalyst performance in Route 1. p-cymene Ru(II) complexes bearing monodentate NHC ligands generally performed better as compared to the complexes bearing bidentate ligands. The best performing catalyst was 2 where 90.7% GVL was obtained. VII COMPOUNDS NUMBERING SCHEME Ligand Precursor Salts 1,3-Diisopropylimidazolium chloride 1,3-Diisopropylbenzimidazolium i ( Pr2-imy·HCl) Salt A bromide (iPr2-bimy·HBr) Salt B 1,3-Dibenzylbenzimidazolium 1,1'-Dimethyl-3,3'-methylene- bromide (Bn2-bimy·HBr) diimidazolium dibromide me (diimy ·2HBr) Salt C Salt D 1,1'-Dimethyl-3,3'-ethylene- 3-Methyl-1-(2-picolyl)imidazolium diimidazolium dibromide chloride et (py-imy·HCl) (diimy ·2HBr) Salt E Salt F VIII p-cymene Ruthenium(II) NHC complexes 6 i 6 [RuCl2(η -p-cymene)( Pr2-imy)] 1 i [RuCl2(η -p-cymene)( Pr2-bimy)] 2 6 6 [RuCl2(η -p-cymene)(Bn2-bimy)] 3 2 me [RuCl(η -p-cymene)(κ C,C-diNHC )][PF6] 4 6 2 et 6 [RuCl(η -p-cymene)(κ C,C- diNHC )][PF6] 2 [RuCl(η -p-cymene)(κ C,N-Me,pyimy)][PF6] 5 6 IX LISTS OF TABLES Table 1. Summary of some physical properties of LA.[11] ................................................. 4 Table 2. Summary of some physical properties of GVL.[9] ............................................... 5 Table 3. Summary of some thermodynamic properties of FA decomposition pathways.[18c] ................................................................................................................... 9 Table 4. Selected interatomic distances (Å) and angles (). .......................................... 22 Table 5. Ccarbene-Ru and Ru-Cl distances [Å] for 4, 5 and 6. .......................................... 28 Table 6. Catalyst loading optimisation using the reported optimised condition.[a] ........... 30 Table 7. Solvent screening using 0.1 mol% 1................................................................ 30 Table 8. Pressure screening using 0.1 mol% 1 and water as solvent. ........................... 31 Table 9. Pressure screening using 0.1 mol% 1, water as solvent and 12 bar H2. .......... 31 Table 10. Summary of the outcome for catalyst screening[a] ......................................... 33 Table 11. Summary of the rate constant and linearity of [RuCl2(p-cymene)]2 and complexes bearing monodentate NHC ligands 1 to 3. ................................................... 35 Table 12. Summary of the rate constant and linearity of [RuCl2(p-cymene)]2 and complexes bearing bidentate NHC ligands 4 to 6. ......................................................... 38 X Table 13. Summary of the rate of reaction under 1, 6 and 12 bars using 0.1 mol% 1 as the catalyst at 130 C under 1, 6 and 12 bar H2 for 160 min. ......................................... 40 Table 14. Temperature screening with/without water as solvent. .................................. 43 Table 15. Temperature screening without water as solvent........................................... 44 Table 16. Summary of the outcome for catalyst screening.[a] ........................................ 46 Table 17. Summary of mercury poisoning test for the conversion of LA to GVL via Route 1 and Route 2................................................................................................................ 54 Table 18. Summary of mercury poisoning test for the conversion of LA to GVL via Route 2.[a] ................................................................................................................................ 54 Table 19. Summary of the results obtained from the study of the effect of additives on Route 1. ........................................................................................................................ 57 Table 20. Summary of the pH of reaction mixtures taken before hydrogenation via Route 1[a]. ................................................................................................................................ 59 Table 21. Summary of the results obtained from various variations in reaction condition. ...................................................................................................................................... 63 Table 22. Summary of the results obtained from hydrogenation of cyclohexanone[a] using water and THF. .................................................................................................... 66 XI LIST OF FIGURES Figure 1. World energy consumption from 1990 to 2035.[1] ............................................. 1 Figure 2. Global CO2 emission from fossil fuels from 1900-2009.[3]................................. 2 Figure 3. Organic components of lignocelluloic biomass and examples of useful chemical/intermediates that can be obtained from acid catalysed hydrolysis. .................. 3 Figure 4. Derivatives of levulinic acid.[12] ......................................................................... 4 Figure 5. FA produced from acidic hydrolysis of lignocelluosic biomass can be directly used as the hydrogen source with a suitable catalyst to be decomposed to H2 and CO2 followed by the conversion of LA to GVL. ........................................................................ 8 Figure 6. Examples of Fischer carbene, Schrock carbene and NHC. ........................... 12 Figure 7. Grubbs second generation catalyst................................................................ 12 Figure 8. Common azole rings used to tune the electronics of NHC. ............................ 14 Figure 9. p-cymene Ru(II) NHC complexes with monodentate NHC ligands (1 to 3) and bidentate NHC ligands (4 to 6) used in the study. .......................................................... 18 Figure 10. Molecular structures of 2 (a) and 3 (b) with thermal ellipsoids drawn at 30% probability level. Hydrogen atoms are omitted for clarity. ............................................... 21 Figure 11. Molecular structure of 4 with thermal ellipsoids drawn at 30% probability level. Hydrogen atoms and hexafluorophosphate anion are omitted for clarity. Selected bond lengths [Å] and angles []: Ru(1)-C(4) 2.041(4), Ru(1)-C(6) 2.047(4), Ru(1)-Cl(1) XII 2.413(1), Ru(1)-Ct1* 1.739, C(4)-Ru(1)-C(6) 83.3(2), N(2)-C(1)-N(3) 109.9(4), C(4)Ru(1)-Cl(1) 86.3(1), C(6)-Ru(1)-Cl(1) 86.3(1). *Ct1 denotes centroid formed by C(10)C(15). ............................................................................................................................ 28 Figure 12. Reaction profile of catalytic hydrogenation of LA to GVL using 1 from 0 to 160 min. ............................................................................................................................... 32 Figure 13. Plot of LA depletion vs. time for [RuCl2(p-cymene)]2. ................................... 36 Figure 14. Plot of LA depletion vs. time for 1. ............................................................... 36 Figure 15. Plot LA depletion vs. time for 2. ................................................................... 37 Figure 16. Plot of LA depletion vs. time for 3. ............................................................... 37 Figure 17. Plot of LA depletion vs. time for 4. ............................................................... 38 Figure 18. Plot of LA depletion vs. time for 5. ............................................................... 39 Figure 19. Plot of LA depletion vs. time for 6. ............................................................... 39 Figure 20. Plot of reaction rate vs. hydrogen pressure.................................................. 41 Figure 21. Effect of base loading on GVL yield (yield reflected is an average of two runs). ............................................................................................................................. 44 Figure 22. Plot of GVL formation and LA depletion with time. ....................................... 45 Figure 23. Appearance of reaction mixture after reaction. A. Using molecular H2 (Route 1), colourless solution was obtained with metallic coating on liner wall and magnetic stir XIII bar. Reaction condition: 4.31 mmol LA, 0.1 mol% 1, 10 mL water, 12 bar H2, 130 C, 160 min. B. Using FA as the H2 source (Route 2), yellow solution was obtained with clean liner wall and magnetic stir bar. Reaction condition: 80 mmol LA, 80 mmol FA, 0.075 mol% 1, 3 mol% NaOH, 130 C, 12 h. ........................................................................... 47 Figure 24. TEM images of [RuCl2(p-cymene)]2 using molecular H2 at 30 min (left), 90 min (middle) and 160 min (right). ................................................................................... 49 Figure 25. TEM images of 1 using molecular H2 at 30 min (left), 90 min (middle) and 160 min (right). ..................................................................................................................... 50 Figure 26. TEM images of 2 using molecular H2 at 30 min (left), 90 min (middle) and 160 min (right). ..................................................................................................................... 50 Figure 27. TEM images of 3 using molecular H2 at 30 min (left), 90 min (middle) and 160 min (right). ..................................................................................................................... 50 Figure 28. TEM images of 4 using molecular H2 at 30 min (left), 90 min (middle) and 160 min (right). ..................................................................................................................... 51 Figure 29. TEM images of 5 using molecular H2 at 30 min (left), 90 min (middle) and 160 min (right). ..................................................................................................................... 51 Figure 30. TEM images of 6 using molecular H2 at 30 min (left), 90 min (middle) and 160 min (right). ..................................................................................................................... 51 Figure 31. TEM images of catalytic hydrogenation of LA to GVL using molecular H2 as the hydrogen source. A. Ru/C 4. B. [RuCl2(PPh3)]. ....................................................... 52 XIV Figure 32. TEM images of catalytic hydrogenation of LA to GVL using FA as the hydrogen source. A. Ru/C. B. Complex 4. C. Complex 6. ............................................. 53 Figure 33. UV spectra of 1 at 0.0004M for catalytic hydrogenation of LA to GVL using A. Route 1 from 0 – 160 min. B. Route 2 from 0 – 24 h. .................................................... 55 Figure 34. Colour changes of 0.004M of reaction mixtures throughout the course of reaction via A. Route 1; B. Route 2. .............................................................................. 56 Figure 35. TEM images of 1 when different additives were added. ............................... 58 Figure 36. TEM image of 1 after second catalytic run. .................................................. 61 Figure 37. TEM images of RuNPs formed in A. Scenerio 3 and B. Scenerio 4. ............ 63 Figure 38. Structure of formate-bridged ruthenium dimers.[47] ....................................... 64 Figure 39. TEM images obtained for 1 in the conversion of LA to GVL using different solvents. ........................................................................................................................ 64 XV LISTS OF SCHEMES Scheme 1. Reaction pathways of LA to GVL.[17n] ............................................................ 6 Scheme 2. Conversion of LA to GVL using molecular H2 or FA as the hydrogen source.6 Scheme 3. Synthesis scheme of the first stable NHC. .................................................. 13 Scheme 4. Electronic configuration and resonance structures of NHC.[29]..................... 13 Scheme 5. Common synthetic routes towards NHC complexes.................................... 16 Scheme 6. Synthesis of the p-cymene Ru(II) NHC complexes 1 to 3. ........................... 19 Scheme 7. Synthesis of 4. ............................................................................................ 23 Scheme 8. Synthesis of 5. ............................................................................................ 25 Scheme 9. Literature reported procedure for the synthesis of salt F.[34]......................... 25 Scheme 10. Modified procedure for the synthesis of salt F. .......................................... 26 Scheme 11. Synthesis of 6. .......................................................................................... 26 Scheme 12. Reaction scheme of catalytic hydrogenation of LA to GVL via Route 1 with optimised conditions by Yan et. al..[17m].......................................................................... 29 Scheme 13. Reaction scheme of catalytic hydrogenation of LA to GVL via Route 2 with optimised conditions by Deng et. al..[11q] ........................................................................ 42 Scheme 14. Hydrogenation of cyclohexanone to cyclohexanol. .................................... 65 XVI Scheme 15. Proposed ways to convert biomass-derived LA to GVL using our optimised routes. ........................................................................................................................... 69 Scheme 16. Hydrogenation of acetanilide to N-cyclohexylacetamide using 1. .............. 70 XVII LIST OF CHART Chart 1. Methods of synthesizing MNPs and methods of stabilizing MNPs. .................. 11 XVIII LIST OF ABBREVATIONS AcOH Acetic Acid API Atmospheric Pressure Ionisation Ar aryl bimy benzimidazolin-2-ylidene Bn benzyl bpy bipyridine Calcd. calculated CCD Charged Coupled Device CO2 carbon dioxide Ct Centroid d doublet DCM Dichloromethane dd doublet of doublet DMSO dimethyl sulfoxide EA Elemental Analysis eq equivalent ESI-MS Electrospray Ionization Mass Spectrometry et ethyl bridge et. al. and others (Latin et alii) e.g. for example (Latin exempli gratia) FA Formic Acid GVL γ-valerolactone XIX h hour HPLC High Performance Liquid Chromatography Hz hertz I Inductive effect imid imidazole imy imidazolin-2-ylidene i Pr isopropyl J coupling constant LA Levulinic Acid LC liquid chromatography M Mesomeric effect m multiplet m/z mass to charge ratio Me methyl me methyl bridge MeOH Methanol min minutes mL milliliter MNPs Metal Nanoparticles MS Mass Spectrometry NHC N-Heterocyclic Carbene nm nanometers NMR Nuclear Magnetic Resonance NREL National Renewable Energy Laboratory XX Ph phenyl py pyridine RuNPs Ruthenium Nanoparticles s singlet sept septet TEM Transmission Electron Microscopy THF Tetrahydrofuran UV-Vis Ultraviolet-Visible vs. versus δ NMR chemical shift in ppm λmax absorption maxima μL microliter XXI Conversion of LA to GVL using p-cymene Ru(II) NHC Complexes 1. INTRODUCTION 1.1. Global energy demands and environmental issues Since the eighteenth century, fossil fuel has been the main energy source for human and economical developments. With the increasing human population and technological advances, the energy needed to meet the economic and social demand increased (Figure 1) in order to sustain human well-being and raise the standard of living. World Energy comsumption from 1990-2035 History 2013 Quadrillion Btu 800 Projection 600 400 200 0 1990 2000 2010 2020 2030 Year Liquids Natural gas Coal World Figure 1. World energy consumption from 1990 to 2035. [1] Fossil fuels have been gradually declining due to increasing usage. Environmental problems like high carbon dioxide (CO2) emission (Figure 2) led to increasing drastic climatic changes. CO2 emission has led to global warming and increase in average global temperature of 0.8 C since 1880.[2] Hence, there is a pressing need in search of alternative energy source that is sustainable and environmentally benign. Page 1 Conversion of LA to GVL using p-cymene Ru(II) NHC Complexes Global CO2 emission from fossil fuels from 19002009 35000 CO2 emission (tetragrams CO2) 30000 25000 20000 15000 10000 5000 0 1900 1920 1940 1960 1980 2000 Year [3] Figure 2. Global CO2 emission from fossil fuels from 1900-2009. 1.2. Biomass as an alternative energy source Biomass is biological material that is derived from living, or recently living organisms. In the context of biomass for energy this is often used to mean plant based material, but biomass can equally apply to both animal and vegetable derived material.[4] Biomass is a suitable alternative as it is the only renewable source of organic carbon, which is essential for the production of liquid hydrocarbon fuels to chemicals. [5] In the Kyoto protocol, together with the vision to reduce crude oil dependence, researchers’ attention has been directed to use biomass as a source of energy and, more specifically, for transportation fuels.[5-8] The organic components present in lignocellulosic biomass consist of cellulose, hemicellulose and lignin.[9] Upon acid catalysed hydrolysis, it can be broken down into useful chemicals and intermediates (Figure 3). Page 2 Conversion of LA to GVL using p-cymene Ru(II) NHC Complexes Figure 3. Organic components of lignocelluloic biomass and examples of useful chemical/intermediates that can be obtained from acid catalysed hydrolysis. 1.3. Levulinic Acid and γ-valerolactone The National Renewable Energy Laboratory (NREL) has identified levulinic acid (LA) as one of the top 12 building block chemicals that can be produced from sugars via biological transformation or chemical hydrolysis of plant biomass.[10] LA, also known as 4-oxopentanoic acid, 4-oxovaleric acid and 3-acetylpropionic acid, is a C5 ketoacid and is a product formed from the hydrolysis of cellulosic biomass with acid. Formic acid (FA) and water are formed as the by-products of hydrolysis. Table 1 summarises some physical properties of LA. Page 3 Conversion of LA to GVL using p-cymene Ru(II) NHC Complexes Table 1. Summary of some physical properties of LA. Molecular weight 116.12 Melting point 33–35 C Boiling point 245–246 C [a] 3 Density 1.134 g/cm at 25 C pKa 4.5 Refractive Index [a] [11] [a] 1.4796 values given at 20 C Given the presence of the reactive functionalities (carbonyl group and carboxyl group), various derivatives can be obtained from LA (Figure 4). [12] Figure 4. Derivatives of levulinic acid. Page 4 Conversion of LA to GVL using p-cymene Ru(II) NHC Complexes Of the wide range of derivatives that can be obtained from LA, γ-valerolactone (GVL) has been identified by Horváth as a sustainable liquid for energy and carbon-based chemicals.[4] GVL is a natural occurring compound that is present in fruits and is also used in the flavour and fragrance industry. It is a colourless liquid with a low melting point of -31 C, high boiling point of 207 C and has an acceptable odour as well as low toxicity.[11-15] Table 2 shows a summary of some physical properties of GVL. Table 2. Summary of some physical properties of GVL. Molecular weight 100.12 Melting point -31 C Boiling point 207-208 C Density 1.05 g / cm Refractive Index [a] [a] [9] 3 1.432 values given at 20 C 1.4. Conversion of LA to GVL The conversion of LA to GVL has been of interest since the 1930s. It involves hydrogenation followed by intramolecular cyclisation and the loss of water. When alcohols are used as the reaction solvent, LA and GVL can undergo esterification (Scheme 1). Hydrogenation can be achieved using molecular H2 (route 1, Scheme 2) or formic acid (FA) (route 2, Scheme 2) as the hydrogen source with the use of either homogeneous[16] or heterogeneous catalysts.[17] Page 5 Conversion of LA to GVL using p-cymene Ru(II) NHC Complexes [17n] Scheme 1. Reaction pathways of LA to GVL. Scheme 2. Conversion of LA to GVL using molecular H2 or FA as the hydrogen source. Page 6 Conversion of LA to GVL using p-cymene Ru(II) NHC Complexes 1.4.1. Catalytic homogeneous hydrogenation using molecular H2 Homogeneous systems for the conversion of LA to GVL like [RuCl2(PPh3)3][16a], [Ru(acac)3]/phosphine system[16b, d, g] , RuCl3·3H2O/PPh3 with various bases[16c] and Ir pincer complexes[16f] have been reported. Geilen et. al. developed a homogeneous system based on [Ru(acac)3]/phosphine system with the use of different additives to selectively hydrogenate LA to a range of promising derivatives, including GVL.[16d] Despite its highly selective nature, homogeneous systems face separation problems especially for GVL targeted production as GVL is a high boiling liquid (208 C), which makes separation by means of distillation uneconomical.[13] 1.4.2. Catalytic heterogeneous hydrogenation using molecular H2 Schuette and Thomas employed a platinum oxide catalyst in an organic solvent to hydrogenate LA at 3 bar H2 in 44 h to yield 87% GVL.[17a] Christian et al. improved the GVL yield to 94% using Raney nickel and copper-chromium catalysts.[17c] The disadvantage of using Raney nickel and copper-chromium catalysts is the need for high temperature and H2 pressure. Since then, numerous other catalyst systems have been studied. Broadbent et. al reported the use of rhenium catalysts (Re black, Re(IV) oxide hydrate) for the hydrogenation of LA to GVL.[17d] Yan et. al reported the hydrogenation of LA to GVL using Ru/C and achieved a 92% conversion of LA with a 99% GVL selectivity at 130 C and 12 bar H2 pressure in methanol (MeOH).[17m] Page 7 Conversion of LA to GVL using p-cymene Ru(II) NHC Complexes Among the carbon supported noble metal catalysts (Ru, Pt, Pd)[17s], 5 wt% Ru/C showed the highest catalytic activity and product selectivity. The better performance of Ru/C is attributed to its high dispersion of metallic Ru on carbon in nano-sizes compared to the Pt and Pd catalysts. 1.4.3. Using formic acid as the alternative hydrogen source Formic acid (FA), containing 4.4 wt% hydrogen, has been identified as the most promising material for hydrogen storage.[18] It is commonly adopted as an effective hydrogen source to produce GVL as it is readily available and of low cost. Moreover, from the hydrolysis of biomass (Figure 5), FA is produced. By using FA as the hydrogen source, the separation of the mixture of LA and FA can be eliminated; hence reducing the high separation cost and also improves the atom economy of the process. Figure 5. FA produced from acidic hydrolysis of lignocelluosic biomass can be directly used as the hydrogen source with a suitable catalyst to be decomposed to H 2 and CO2 followed by the conversion of LA to GVL. Page 8 Conversion of LA to GVL using p-cymene Ru(II) NHC Complexes The decomposition of FA can occur via two different pathways, namely (A) dehydrogenation/decarboxylation and (B) dehydration/decarbonylation. Their thermodynamic properties are summarised in Table 3. Table 3. Summary of some thermodynamic properties of FA decomposition pathways. ΔG (kJ / mol) ΔH (kJ / mol) ΔS (J / mol K) HCOOH  CO2 + H2 -32.9 31.2 215 HCOOH  CO + H2O -12.4 28.7 138 Pathway A) Dehydrogenation/ decarboxylation B) Dehydration/ decarbonylation [18c] Pathway A is generally catalysed by transition metal catalysts[18a-g] while pathway B can be catalysed by acid catalyst.[18i] Pathway A is favourable as H2 is produced. FA decomposition generating H2 is desirable as it serves as a hydrogen source for portable application without the need to use a pressurised H2 source. Pathway B is undesirable as the CO produced may poison the catalysts. Hováth et. al. used [(η6-C6Me6)Ru(bpy)(H2O)][SO4] in water for the transfer hydrogenation of LA with FA as the hydrogen donor and obtained 25% GVL and 75% 1,4-pentandiol.[16b] Deng et. al. reported using RuCl3/PPh3/pyridine catalyst system to convert a 1:1 aqueous mixture of LA and FA selectively into GVL with high yield of about 80-90%.[16c] Page 9 Conversion of LA to GVL using p-cymene Ru(II) NHC Complexes 1.4.4. Water as the solvating media In a recent review on the methods of accessing environmental impact of chemical processes, Sheldon stated that “The best solvent is no solvent and if a solvent is needed then water is preferred”.[19] Water is an ideal solvent as it is non-toxic, environmentally benign, non-flammable and readily available. Recent studies have shown that water can be advantageous as solvent for the catalytic hydrogenation of LA to GVL. Al-Shaal et. al. hydrogenated LA to GVL in water with 5 wt% Ru/C and achieved a 99.5% LA conversion and 86.6% GVL selectivity.[17n] Delhomme et. al used [Ru(acac)3] with various water-soluble phosphine and was able to reduce LA to GVL at 140 C and 5 MPa H2, with a LA conversion of up to 99% and GVL selectivity of as high as 97% within 5 h.[17v] Most importantly, water can replace alcoholic solvents (such as methanol) which might form levulinate esters. 1.4.5. Metal nanoparticles as catalysts Using metal nanoparticles (MNPs) as catalysts is a growing area in homogeneous and heterogeneous catalysis as nanoparticle catalysts are efficient, selective and can be recycled, hence meeting the requirements of green catalysis.[20] Chart 1 is a simplified chart illustrating the synthesis of MNPs by chemical reduction/decomposition. MNPs tend to aggregate but can be stabilized by using polymers, alcohols, ionic liquids and ligands.[21] Page 10 Conversion of LA to GVL using p-cymene Ru(II) NHC Complexes Alcohol Chemical Methods of synthesizing of MNPs Decomposition / reduction of metal salt/organometallic precursor Stabilisation methods Ionic Liquids Polymers Physical e.g. laser ablation Ligands Chart 1. Methods of synthesizing MNPs and methods of stabilizing MNPs. Lately, the use of [Ru3CO12] as the precursor for the preparation of ruthenium nanoparticles (RuNPs) for the conversion of LA to GVL by molecular H2 or FA as the H2 source has been reported.[17w] This is an example of nanoparticle formation by reductive decomposition of an organometallic cluster. Ligands like N-heterocyclic carbenes (NHCs) have been reported to stabilise or modify MNPs.[22] 1.5. N-heterocyclic carbenes 1.5.1. Definition and properties Carbenes are defined as neutral carbon species containing a divalent carbon atom with six valence electrons. There are two different types of carbene, namely Fischer[23] and Schrock carbenes[24]. Wanzlick-Arduengo carbenes,[25] which are also known as NHC, are a type of Fischer carbene (Figure 6). Fischer carbene complexes have an electrophilic carbene carbon atom while Schrock carbene complexes have a nucleophilic carbene carbon. Because of the electrophilic nature of Fischer carbene carbon atom, it tends to have higher affinity to electron-rich metal center like those in the late-transition Page 11 Conversion of LA to GVL using p-cymene Ru(II) NHC Complexes metal series. On another hand, the nucleophilic carbene carbon atom of the Schrock carbene will tend to prefer metal center that is electron deficient like those in the early transition metal series. Figure 6. Examples of Fischer carbene, Schrock carbene and NHC. NHC are a class of ligands with excellent steric and electronic versatility, where the carbene carbon is incorporated into a heterocyclic ring. Since the first report of an NHCMercury complex in 1968[26], the use of NHCs as ligands in transition metal chemistry remained relatively dormant until Arduengo et. al. isolated the first stable crystalline carbene (Scheme 3) in 1991.[27] Subsequently, many NHC-metal complexes have been prepared and used in catalysis. One such example is the Grubbs second generation catalyst (Figure 7), which is highly active in olefin metathesis reactions[28]. Figure 7. Grubbs second generation catalyst. Page 12 Conversion of LA to GVL using p-cymene Ru(II) NHC Complexes Scheme 3. Synthesis scheme of the first stable NHC. The stability of singlet vs. triplet NHC can be explained by a combination of mesomeric (M) and inductive (I) effects, also collectively known as “push-pull” effect. The +M effect “pushes” the lone pair of electrons from the neighbouring nitrogen atoms into the empty pπ-orbital, hence increasing the electron density of the carbene center. On the other hand, the –I effect of the σ-electron withdrawing N atoms “pulls” electrons from the carbene center, hence stabilizing the σ-orbital. The “push-pull” effect increases the energy gap between the σ and pπ orbitals, thus stabilizing the singlet carbene (Scheme 4). Scheme 4. Electronic configuration and resonance structures of NHC. [29] Page 13 Conversion of LA to GVL using p-cymene Ru(II) NHC Complexes The attractive features of using NHC as ligands for transition metal catalysis are the ability of tuning their electronic and steric properties. The electronic properties of an NHC can be tuned by changing the azole ring. The three common azole rings commonly used include benzimidazole, imidazole and imidazoline are shown in Figure 8. Their electron donating power increases in the order of benzimidazole[...]... N- Heterocyclic Carbene nm nanometers NMR Nuclear Magnetic Resonance NREL National Renewable Energy Laboratory XX Ph phenyl py pyridine RuNPs Ruthenium Nanoparticles s singlet sept septet TEM Transmission Electron Microscopy THF Tetrahydrofuran UV-Vis Ultraviolet-Visible vs versus δ NMR chemical shift in ppm λmax absorption maxima μL microliter XXI Conversion of LA to GVL using p- cymene Ru(II) NHC Complexes. .. Schrock carbene complexes have a nucleophilic carbene carbon Because of the electrophilic nature of Fischer carbene carbon atom, it tends to have higher affinity to electron-rich metal center like those in the late-transition Page 11 Conversion of LA to GVL using p- cymene Ru(II) NHC Complexes metal series On another hand, the nucleophilic carbene carbon atom of the Schrock carbene will tend to prefer... hemicellulose and lignin.[9] Upon acid catalysed hydrolysis, it can be broken down into useful chemicals and intermediates (Figure 3) Page 2 Conversion of LA to GVL using p- cymene Ru(II) NHC Complexes Figure 3 Organic components of lignocelluloic biomass and examples of useful chemical/intermediates that can be obtained from acid catalysed hydrolysis 1.3 Levulinic Acid and γ -valerolactone The National Renewable... carbenes 1.5.1 Definition and properties Carbenes are defined as neutral carbon species containing a divalent carbon atom with six valence electrons There are two different types of carbene, namely Fischer[23] and Schrock carbenes[24] Wanzlick-Arduengo carbenes,[25] which are also known as NHC, are a type of Fischer carbene (Figure 6) Fischer carbene complexes have an electrophilic carbene carbon atom... hydrogen donor and obtained 25% GVL and 75% 1,4-pentandiol.[16b] Deng et al reported using RuCl3/PPh3/pyridine catalyst system to convert a 1:1 aqueous mixture of LA and FA selectively into GVL with high yield of about 80-90%.[16c] Page 9 Conversion of LA to GVL using p- cymene Ru(II) NHC Complexes 1.4.4 Water as the solvating media In a recent review on the methods of accessing environmental impact of. .. stabilized by using polymers, alcohols, ionic liquids and ligands.[21] Page 10 Conversion of LA to GVL using p- cymene Ru(II) NHC Complexes Alcohol Chemical Methods of synthesizing of MNPs Decomposition / reduction of metal salt/organometallic precursor Stabilisation methods Ionic Liquids Polymers Physical e.g laser ablation Ligands Chart 1 Methods of synthesizing MNPs and methods of stabilizing MNPs Lately,... using p- cymene Ru(II) NHC Complexes Scheme 3 Synthesis scheme of the first stable NHC The stability of singlet vs triplet NHC can be explained by a combination of mesomeric (M) and inductive (I) effects, also collectively known as “push-pull” effect The +M effect “pushes” the lone pair of electrons from the neighbouring nitrogen atoms into the empty p -orbital, hence increasing the electron density of. .. Complexes 1 INTRODUCTION 1.1 Global energy demands and environmental issues Since the eighteenth century, fossil fuel has been the main energy source for human and economical developments With the increasing human population and technological advances, the energy needed to meet the economic and social demand increased (Figure 1) in order to sustain human well-being and raise the standard of living... Broadbent et al reported the use of rhenium catalysts (Re black, Re(IV) oxide hydrate) for the hydrogenation of LA to GVL.[17d] Yan et al reported the hydrogenation of LA to GVL using Ru/C and achieved a 92% conversion of LA with a 99% GVL selectivity at 130 C and 12 bar H2 pressure in methanol (MeOH).[17m] Page 7 Conversion of LA to GVL using p- cymene Ru(II) NHC Complexes Among the carbon supported noble... use of [Ru3CO12] as the precursor for the preparation of ruthenium nanoparticles (RuNPs) for the conversion of LA to GVL by molecular H2 or FA as the H2 source has been reported.[17w] This is an example of nanoparticle formation by reductive decomposition of an organometallic cluster Ligands like N- heterocyclic carbenes (NHCs) have been reported to stabilise or modify MNPs.[22] 1.5 N- heterocyclic carbenes ... DISCUSSIONS 2.1 Synthesis and characterization of p- cymene Ru(II) NHC complexes 2.1.1 p- cymene Ru(II) NHC complexes bearing monodentate NHC ligands The synthesis of the p- cymene Ru(II) NHC complexes. .. late-transition Page 11 Conversion of LA to GVL using p- cymene Ru(II) NHC Complexes metal series On another hand, the nucleophilic carbene carbon atom of the Schrock carbene will tend to prefer... donor-functionalised NHCs), tripodal NHCs and Page 14 Conversion of LA to GVL using p- cymene Ru(II) NHC Complexes pincer-type NHCs Three common approaches to prepare NHC complexes are highlighted