Heterogeneous Iron and Ruthenium Catalysts for Carbohydrate Based Biomass Transformations

Abstract

Heterogeneous catalyst applications have increased attention in green chemistry. Polymer, metal oxide, and carbon are used as universal support materials to fabricate heterogeneous catalysts. Because the heterogeneous catalyst is generally insoluble in solvent, it can be separated easily from the reaction media by filtration and recycled numerous times. Moreover, it is possible to design the property of catalysts through chemical processes. Despite these advantages, the use of heterogeneous catalysts has not been increased in biomass conversion to produce value-added chemicals because of lower activities than homogeneous catalysts. Thus, the development of highly efficient heterogeneous catalyst still remains a challenge in biomass transformation. In this thesis, two types of heterogeneous transition metal catalysts were utilized for efficient carbohydrate transformations: iron catalyst for fructose dehydration and ruthenium catalyst for 5-hydroxymethyl furfural (HMF) hydrogenation/oxidation. In chapter 1, fructose dehydration into HMF by heterogeneous NHC-Fe catalyst is described. Developing HMF production process has been the most important task in C6 biomass transformation. So far, numerous HMF production methods from carbohydrates have been reported using various catalysts such as Bronsted and Lewis acid, and ionic liquids. N-heterocyclic carboene (NHC)-Fe complex grafted heterogeneous catalysts were prepared from polystyrene and graphene oxide support. NHC ligand provided a stable and strong metal-ligand bonding environment to form an efficient active site. Polystyrene supported NHC-FeIII catalyst showed excellent fructose dehydration activity comparing to other supported NHC-metal catalysts. Graphene oxide grafted with NHC-FeIII catalyst showed better catalytic performance than polystyrene supported one. Futhermore, the NHC-FeIII grafted catalysts could be reused for 5 cycles without significant loss of activity. In chapter 2, synthesis of HMF derivatives by heterogeneous ruthenium catalyst is described. HMF derivatives such as 2,5-furandicarboxylic acid (FDCA) and 2,5-bis(hydroxymethyl)furan (BHMF) have received a lot of attention because of their applicability. Zirconia was chosen as a suitable solid support with unique surface features, which can improve the catalytic ability of the metal active sites. Ruthenium active sites were immobilized on the surface of zirconia without forming Ru(0) metal or RuO2 nanoparticles (NPs). The well deposited Ru active site resulted in enhanced HMF hydrogenation and oxidation activity. The zirconia supported ruthenium catalyst showed excellent catalytic ability for oxidation of HMF into FDCA with molecular oxygen as a green oxidant. HMF was selectively reduced to BHMF with excellent yield under pressured hydrogen gas condition. Moreover, the zirconia supported ruthenium catalyst could be recycled over 10 times without significant loss of activity.