Sustainable process for the complete upgrading of orange peel waste into value-added chemicals and bio-carbon materials

This doctoral work is focused on the complete upgrading of orange peel waste (OPW) into value-added chemicals and bio-carbon materials through the development of sustainable strategies for the valorization of citrus waste. Chapter 1 contains a general overview of the sustainability concepts and the use of biomass for the production of various value-added products. Furthermore, an extensive overview of the chemical composition and the utilization of the OPW, along with the recent advances in the recovery and catalytic upgrading of value-added chemicals from OPW is provided. In particular, as described in Chapter 2, a simple and green protocol was first investigated to obtain hydrochar and high-added value products, mainly 5-hydroxymethylfurfural (5-HMF), furfural (FU), levulinic acid (LA), and alkyl levulinates, by using the Hydrothermal Carbonization (HTC) process of the OPW. Process variables, such as reaction temperature (180-300 °C), reaction time (60-300 min), biomass: water ratio, and initial pH were investigated in order to find the optimum conditions that maximize the yields of 5-HMF and levulinates in the light bio-oil. Data obtained evidence that the highest yield of hydrochar is obtained at a 210 °C reaction temperature, 180 min residence time, 6/1 w/w orange peel waste to water ratio, and a 3.6 initial pH. The bio-products distribution strongly depends on the applied reaction conditions. Overall, 180 °C was found to be the best reaction temperature that maximizes the production of furfural and 5-HMF in the presence of pure water as a reaction medium. Chapter 3 reports the complete characterization of the hydrochar samples, carried out by various complementary techniques such as TGA, XRD, SEM-EDX, FT-IR, and BET surface area highlight their different morphological and microstructural characteristics. These modifications are also accomplished by variations of the electrical and electrochemical properties, which are here exploited, for the first time, for the development of high performances chemical sensors based on HC as a sensing element. Hydrochar derived from OPW treatment at 300 °C HC300 shows the best characteristics, and thereby is used for the fabrication of conductometric NO2 and electrochemical dopamine sensors. The conductometric HC300-based sensor was demonstrated to be sensitive to up to 50 ppb of NO2 in the air at 100 °C. Dopamine at nanomolar concentration was detected with good performances (large linear detection range from 0 to 1000 μM, and low limit of detection (LOD) of 180 nM) on the HC300-based electrochemical sensor. Chapter 4 is dedicated to the transformation of limonene into p-cymene over a Pd/C catalyst under both batch and continuous gas-flow conditions. A complete transformation of limonene into p-cymene is shown to be achieved at 300 °C in a nitrogen atmosphere under continuous gas-flow conditions. Based on the selectivity values obtained for p-cymene and by-products, a mechanistic scheme for the transformation of limonene into p-cymene over Pd/C catalyst is proposed. In addition, the catalytic conversion of methyl and ethyl levulinates into γ-valerolactone (GVL) by using alcohols (methanol, ethanol, and 2-propanol) as the H-donor/solvent is presented. These reactions are promoted by the ZrO2 catalyst under both batch and continuous gas-flow conditions. Under batch conditions, ethyl levulinate provides the highest yield to GVL, with 2-propanol which are found to be the best H-donor molecule. The reactions occurring under continuous gas-flow conditions were found to be much more efficient, providing excellent yields in GVL with ethanol as the H-donor/solvent. These experiments clearly show that the high reducing activity of alcohols is the main factor driving the CTH process, while the tendency to attack the esteric group is the key step in the formation of transesterification products. In the last part, presented in Chapter 5, final conclusions of this Ph.D. thesis are described.