Valorization of lignin through selective￾depolymerization to polyols for synthesis of non￾isocyanate polyurethane foam

Abstract

Plastic products are an integral part of our everyday lives. They are utilised in the form of packaging, building materials, insulation, and for providing comfort. The exclusive dependence on non-renewable fossil fuel resources as feedstock poses a negative impact on the environment. Its high demand by consumers is directly linked to high waste generation, which negatively affects river streams, underground water resources, human and animal health. Polyurethane is one of the fast-growing plastics that is utilized in many forms due to its versatility. The production, however, is fully dependent on toxic isocyanates and petroleum-derived polyols that are non-biodegradable. The transition from using fossil fuels into renewable feedstock such as lignin will reduce the environmental concern, while at the same time adding market value to lignin. The novel non-isocyanate polyurethanes (NIPU) whose production can be derived from the reaction between polycarbonates and polyamines presents an opportunity for this realization. This work presents three green pathways for modifying lignin into a green polycarbonate that can be used to synthesize NIPUs. The first phase involved the conversion of lignin into a hydroxyl and phenolic-rich bio-oil through hydrothermal liquefaction using alkali catalysis. Selective depolymerization was conducted by mixing Lignex (a Sappi waste sodium lignosulfonate that is used as a fuel source in the recovery boilers) and water at a mass ratio of 1:10 and 5% NaOH with respect to the mass of lignin. The liquefaction process was conducted in an autoclave semi-batch reactor at 300°C for a period of 20 minutes before the reaction was quenched with cooling systems. The second phase involved grafting of an epoxy group into the bio-oil through glycidylation. 5 g of bio-oil was dissolved in 13.3 g of epichlorohydrin at molar ratio of 1:15 and mixed with 12 ml of water. 0.2% TBAB with respect to mass of bio-oil was added as a catalyst. The contents were heated to 70°C before 12 ml of 10% NaOH solution was added. The reaction was allowed to proceed for 3 hours at a constant temperature while stirring. A high yield of epoxidized oil was attained after washing. The last phase of the modification method involved the carbonation of epoxidized oil into polycarbonates. The process was conducted in an industrial microwave fitted with gas insertion equipment. Bio-oil was mixed with TBAB at a molar ratio of 5% prior to being pressurized with carbon dioxide to 20 bars. The reaction was conducted at 120°C for a period of 1, 6, and 16 hours, where after each cycle the sample was collected for analysis. The chemical structure of Lignex and the reaction products were characterized using FTIR, H-NMR, GC-MS, and GPC. The presence of lignin was confirmed through FTIR characterization where the skeletal vibration of phenolic hydroxyl groups, the presence of G-unit phenols and the presence of the thiol-sulfur functional groups were identified. The presence of methoxyl, methyl, and methylene groups was confirmed using H-NMR scanning. The vibration of sulfonate and primary hydroxyl groups could not be confirmed due to peak overlapping. The molecular weight and polydispersity index of the Lignex structure was determined using GPC scanning. The selective depolymerisation of Lignex by attacking the β-O-4 and the C-C bonds was confirmed through GC-MS characterisation whereby high phenolic products and hydrocarbons in the bio-oil were obtained. The addition of NaOH proved to supress char formation and favour the production of methyl-branched phenols through weakening of the interunit linkages. FTIR and H-NMR characterisation confirmed the occurrence of demethoxylation, demethylation and hydrodeoxygenation. However, H-NMR characterisation of the bio-oil could not detect the presence of phenolic hydroxyls. Instead, a high intensity peak representing phenolic acetates was confirmed. The use of hydrogen donor solvents such as ethanol and ethyl acetate favoured the production of large molecular weight esters. The second phase of grafting epoxy rings into the structure of bio-oil was first confirmed by the presence of epoxy ring vibration through FTIR scanning. A reduction in the intensity of the phenolic hydroxyl confirmed the occurrence of the glycidylation reaction between the phenolic hydroxyls and epichlorohydrin. H-NMR characterisation revealed the resurface of phenolic hydroxyl, which could be a result of a ring-opening reactions. The obtained spectra also indicated the presence of residual epichlorohydrin in the sample although at different vibrations. Finally, the successful synthesis of cyclic carbonate was confirmed by the appearance of a carbonyl functionality characteristic of a cyclic carbonate. A direct proportionality relationship was also observed between the diminishing epoxy group and the formation of cyclic carbonate. The results were consistent with the H-NMR characterisation where the epoxy peak disappeared with an appearance of peaks characteristic of the cyclic carbonate rings. The detection of high-intensity aliphatic acetates confirmed the occurrence of the ring-opening reaction. In conclusion, the possibility of using high-quality bio-oil from lignin to synthesise a polycarbonate for the synthesis of NIPU was possible, yielding 82% of polycarbonate from 85% conversion of the epoxy group. The study on the effect of epichlorohydrin/bio ratio during epoxidation would prove to be significant in improving the epoxy value, hence offering the possibility of synthesizing high-quality polycarbonate.