Valorization of Exoskeletons of Crustaceans in Seafood Wastes into Chemicals and Fuels

轉化在海鮮廚餘內的甲殼動物外骨骼為有用化學品及燃料

Student thesis: Doctoral Thesis

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Award date27 Nov 2019

Abstract

Gamma-valerolactone (GVL) is a valuable chemical which can be derived from various types of biomass ranging from food wastes to agricultural residues. The valorization of the exoskeletons of crustaceans in seafood wastes into useful chemicals was studied. The reaction steps of the acid-catalyzed conversion of chitin to GVL include (1) the hydrolysis of chitin to N-acetyl-glucosamine (NAG), (2) the deacetylation of NAG to give acetic acid (AA) and glucosamine (GluN), (3) the deamination and dehydration of GluN to form 5-hydroxymethylfurfural (HMF), (4) the hydration of HMF to yield levulinic acid (LA) and formic acid (FA), and (5) the transfer hydrogenation of LA with FA in the presence of the Shvo' s catalyst resulting in the formation of 4-hydroxyvaleric acid (4-HVA), which readily undergoes ring-closure to yield GVL. Chitin was used as a model compound to optimize the conditions for the conversion of the exoskeletons of crustaceans in seafood wastes. By using the optimized reaction conditions, chitin (0.41 g, equivalent to 2 mmol of NAG) was heated in a mixture of AA (10 mL) and 5 M H2SO4 (1.5 mL) at 150 ℃ for 4 hours to yield 23 w% (or 39 mol%) LA. Same conditions were applied to convert various pretreated seafood wastes such as the exoskeletons of crabs and lobsters, obtained from local restaurants. The yields of LA were between 6.3 – 10.3 w% due to the lower chitin content of the exoskeletons, which usually contain 17.0 – 72.1 w% chitin, 30 – 50 w% calcium carbonate, and 10 – 20 w% protein. GVL was also used as solvent for the production of LA and FA to simplify the product purification process. The reaction mixture of chitin (0.41 g, 2 mmol of NAG) in 10 mL GVL and 1.5 mL 5 M H2SO4 was heated at 150 ℃ for 4 hours followed by the neutralization with ammonium hydroxide to result in two phases, which were readily separated due to the salting out effect of (NH4)2SO4. The Shvo' s catalyst was then added to the organic phase for transfer hydrogenation of LA with FA as the hydrogen donor to give GVL. Uniformly labelled N-acetyl-[13C6]glucosamine (UL-13C6-NAG) was used to confirm the formation of 13C5-GVL in 12C5-GVL via 13C5-LA and 13C-FA.

GVL has several properties which make it an excellent choice as a sustainable alternative to conventional organic solvents. GVL and H2O are in equilibrium with 4-HVA in the presence of acids, as expected. Since GVL has become popular as an environmentally friendly solvent, it is important to investigate the rate and the extent of reversible ring-opening of GVL in neutral and acidic conditions. The equilibrium between GVL, H2O, and 4-HVA in acidic environment were studied at room temperature by in situ 1H-NMR. The experimental data were analyzed by ZiTa, a comprehensive program package for determining the kinetic parameters of a given model. The developed model indicated that there were several equilibria in the system competing for water. This was reflected by the fact that the equilibrium position between GVL, H2O, and 4-HVA was affected by the acid concentrations. The equilibrium and kinetics of pseudo-first order of reversible GVL ring-opening were investigated by NMR methods. Using one-dimensional exchange spectroscopy (1D EXSY), the exchange rates were measured at different temperatures. From the kinetic data, the activation energies of the hydration of GVL and the dehydration of 4-HVA were 60.82 and 63.49 kJ mol-1, respectively. enthalpy and entropy of the equilibrium were -3.49 kJ mol-1 and -65.86 J mol-1K-1, respectively.

In order to have molecular insights to the conversion of NAG to GVL for further reaction optimizations, catalyst design, and screening, mechanistic studies on the conversion of NAG to HMF have been carried out. In the acid-catalyzed conversion of NAG, the deacetylation of NAG was faster than the deamination of GluN. During the conversion of GluN to HMF, the formation of significant amount of humins, a mostly black and insoluble solid wastes, were observed. By using UL-13C6-NAG, two possible new species, the protonated salt of 1,6-anhydro-2-deoxy-2-ammonio-glucopyranose (AGluNPH+) and 1,6-anhydro-2-deoxy-2-ammonio-glucofuranose (AGluNFH+) were identified. It was established by using singly 13C-labelled NAG at the C-1 position, that the dehydration results in 5-(hydroxymethyl)furan-2-13C-carbaldehyde, which undergoes selective hydration to yield 13C-FA and LA. In the case of 13C-labelled NAG at the C-2 position, the dehydration resulted in 5-(hydroxymethyl)-2-13C-furan-2-carbaldehyde followed by the hydration to 1-13C-LA and FA.