UC Santa Barbara

The global production of plant biomass waste is in the order of 140 Gt per year which offers a promising alternative to petroleum and a sustainable resource to produce fuels and chemicals. Inspired by innovative advancement in biomass valorization, two catalytic processes have been developed to produce chlorine dioxide and polar aprotic solvents from non-edible biomass residues. The application of chlorine dioxide (ClO2) in water treatment is growing because of its superior antimicrobial properties and lower tendency to generate harmful chlorinated organic by-products. Most of the previously investigated catalysts for the one electron oxidation of chlorite to ClO2 are based on manganese or iron porphyrin complexes which suffer from expensive ligand and catalyst syntheses as well as the catalyst instability in oxidative environment. Chlorine dioxide chemistry and its catalytic production are explained in depth in chapter 1. Second chapter describes a novel catalyst design based on molecules that can be derived from lignin for catalytic production of ClO2 in water. A lignin-derived ligand bis(2-hydroxy-3-methoxy-5-propylbenzyl) glycine, (DHEG) was synthesized from 2-methoxy-4-propylphenol (dihydroeugenol (DHE)) and the amino acid glycine. Two mononuclear iron and manganese complexes of DHEG were prepared, characterized, and employed for the oxidation of chlorite to chlorine dioxide in aqueous solution. Peroxyacetic acid (PAA) was used as a ‘green’ oxidant in the redox reactions for the catalyst activation generating high valent Fe and Mn(IV)-OH intermediates. EPR studies verified the formation of a high valent MnIV species. Both Fe and Mn activated complexes catalyzed chlorite oxidation with bimolecular rate constants of 32 and 144 M-1 s-1, respectively, at pH 4.0 and 25 °C. The Mn complex was found to be more efficient for chlorite oxidation with a turnover frequency of 17 h-1 and remained active during subsequent additions of PAA. The rate of ClO2 formation with PAA/Mn-DHEG was first-order in PAA and showed acidic pH dependence. A mechanism that accounts for all observations is presented. Chapter 3 highlights the need of more environmentally benign polar aprotic solvents (PAS) from sustainable resources. Of particular interest for this work is the catalytic conversion of cellulose to short chain polyols and the coupling of these polyols with N,N-dimethylurea (DMU) to produce cyclic PAS. Detailed chemistry background for this transformation are presented within this chapter. In the final chapter, a green and catalytic process is described for the synthesis of N,N'-dimethylimidazolidinone (DMI) and 1,3,4-trimethylimidazolidin-2-one (TMI) from cellulose, the most abundant and non-edible component of biomass. The physical and chemical properties of DMI and TMI including high boiling point, remarkable chemical stability, and being more eco-friendly than DMF make them appealing for use in the pharmaceutical industry. Cellulose depolymerization and reaction of intermediate products with N,N-dimethylurea (DMU) to produce PAS have been investigated in a one-pot, two-step process at elevated temperature. Ru/C is an effective multifunctional catalyst for both C-C bond cleavage of cellulose and subsequent hydrogenation of the unsaturated products in the second step; the catalyst also promotes the condensation hydroxy ketone intermediates with DMU to create cyclic PAS concurrently. Such tandem reactions are challenging to achieve particularly when incompatible conditions are required for each step. Solvent selection is also challenging with the low solubility of cellulose in most common organic solvents. Herein, the overall 85% selectivity for PAS was achieved from the reactions of cellulose or sugar with DMU over Ru/C in DMI (the product) as a solvent. The optimized conditions for coupling of 1,2-propylene glycol with DMU was used in a mechanistic study for the production of PAS with both homogeneous and heterogenous Ru catalysts. Catalytic oxidation of 1,2-PG to hydroxy acetone is the key step to produce TMI with a higher yield obtained using electron-donating phosphine ligands on Ru.