Recycling food waste into value-added chemicals over carbon-based catalysts

Abstract

As the global shortage of resources and energy has aroused increasing public attention over the years, it is imperative to develop practical technologies for waste recycling to produce alternative materials and energy. This thesis focuses on biomass valorisation by investigating thermochemical conversion of food waste into a wide diversity of value-added chemicals, as well as fabricating different cost-effective carbon-based catalysts to promote the processes. Cellulose and starch in food waste are polymers comprising glucose units linked by glycosidic bonds, which can be hydrolysed to release glucose in the presence of Brønsted acids. Glucose can undergo isomerisation to form fructose in the presence of Lewis acid catalysts, and fructose can be dehydrated to hydroxymethylfurfural, which can be transformed to levulinic acid via rehydration. Tailoring green catalysts is of great significance to improve the yield and selectivity of these value-added chemicals. The findings in this thesis evidenced that wood derived biochars modified by H₂SO₄ sulfonation presented strong/weak Brønsted acidity (­SO₃H, -COOH, and -OH groups) to accelerate biomass conversions, and the catalytic activity was associated with the concentration of acid during biochar modification. As for Al-impregnated carbon-based catalysts, detailed characterisation suggested the Lewis acid sites in forms of β-Al(OH)₃, γ­Al(OH)₃, Al-O-C, AlO(OH), and γ-AlO(OH) in the amorphous phase. In particular, the biochar­supported Al catalyst showed higher carbon balance in glucose isomerisation compared to catalysts derived from graphitic oxide and graphene oxide, probably due to the higher pH buffering capacity of biochar. To close the carbon loop of biomass waste valorisation, the unavoidable biorefinery by-product, humins, were evaluated as a low-cost precursor of carbon supports. The humins derived biochars carrying Al-O species on the surface were proved active toward the isomerisation of glucose. These research efforts elucidate that carbon-based catalysts, especially engineered biochars synthesised in a low-cost and sustainable manner, show great prospect in serving as renewable catalysts for promoting bio-circular economy and sustainable development.