Process investigations on second generation lignin as a source of high-value molecules

There is an increasing demand of renewable biomass-derived fuels, generally known as biofuels, in order to overcome the need of fossil ones, and ultimately, deal with the environmental regulations imposed by the recent international agreements, which will hopefully lead to a limitation of global warming below 2 °C. The human being has used biomasses to solve its energetic and non-energetic needs for millennia before the Industrial Era, but in the 21st century there is the need to go a little bit further, for this reason, it is born the concept of biorefinery, or biofactory, in contrast with the oil-based refinery. In a fully integrated biorefinery, biomasses with variable origin are transformed through chemical and biochemical processes in a plethora of chemicals, fuels like ethanol and FAMEs, but also polymer monomers, drug precursors and food additives. The biorefinery will provide the scale dimensions and production flexibility to compete with the oil industry and, ultimately, will cause a global change in the geography of the energy and chemical industry. However, the development of efficient models of biorefinery based on non-edible second generation biomasses is still economically tricky, and there the need to couple fuels production with high-value chemical building blocks. Particular attention should be addressed to lignin fraction which is the most underutilized component of lignocellulosic biomass. Despite lignin is an incredible source of aromatic compounds, it is commonly intended to landfill or, in the best case, used as solid combustible. In this sense, this work has dealt with the development of processes for the transformation of industrial lignin streams into chemicals of interest for the industry, as alternative to pyrolysis or mere combustion. On one hand, we investigated the reductive route which involves the use of H2 firstly, to carry out the hydrogenolysis of lignin to give a phenols-rich bio-oil, and secondly, to transform this phenols mixture to other products through a hydrodeoxygenation reaction. On the other hand, we explored the oxidative route which involves the use of an oxidant to break down lignin matrix to functionalized phenols and carboxylic acids. Both pathways offer the opportunity to produce significant quantities of renewable building blocks, which will ultimately support the economic sustainability of the integrated biorefinery. The following paragraphs resume the content of the main chapters of this dissertation. The hydrodeoxygenation (HDO) of guaiacol has been chosen as a model process for the upgrading of lignin-derived bio-oils. Tests were carried out in a batch reactor at 350 °C, 40 bar of H2, in the presence of several Mo-based catalysts prepared by impregnation of ammonium molybdate on SiO2, Al2O3, NaY zeolite, MgO, activated carbon and graphite. These materials have been characterized by means of: N2 physisorption, XRD, FESEM/EDS, XPS, TPR-H2, TPD-NH3, with the aim of correlating the physical and chemical properties of the prepared samples with the resulting features in the HDO reaction. Mo on activated carbon showed the best performances towards guaiacol demethoxylation, exhibiting complete conversion, 72 % of selectivity to phenol and 19 % to p- and o-cresol. The high surface area and low acidity of activated carbon allow good dispersion of MoOx which exhibits characteristic fragments with lamellar shape, capable to provide large active surface with localized acidity. A phenolic bio-oil, derived from an industrial hydrogenolysis facility treating 2nd generation lignin, was used as starting material for the recovery of phenolic monomers by means of an ad hoc vacuum distillation apparatus. The initial 20 %wt content of monomers was efficiently separated with the 90 % of yield, showing guaiacol, phenol and ethylphenol as main components. Afterwards, the distillate was upgraded through an HDO reaction in order to narrow the compounds distribution and to improve the stability of the mix. The process was carried out under similar conditions as the ones adopted in Chapter 3, allowing achieving the complete demethoxylation of the mix with high selectivity. Notably, the presence of the Mo-base catalyst over activated carbon significantly enhanced the performances of the reaction. Wet air oxidation (WAO) of lignocellulosic biomasses is a promising route for the production of renewable and valuable compounds, involving air as primary oxidant and mild reaction temperatures. In this work, an industrial residue of bioethanol production, steam exploded lignin derived from wheat straw, undergoes a WAO process with the aim to achieve more insights on the process performances in terms of potential yields of aromatic compounds and carboxylic acids (CAs). The experiments were carried out in a pressurized 50 ml batch reactor loaded with water or other aqueous solutions as solvent, the standard conditions were 150 °C of temperature, 20 bar of initial air pressure and 2 h. Afterwards, several solvothermal pretreatments were applied in order to depolymerize and solubilize lignin under inert atmosphere; the residues-free solutions obtained in this way were used as substrate for the WAO reaction. The choice of the pretreatment temperature, solvent alkalinity and presence of perovskite catalysts were evaluated with regard to the mass yields of resulting aromatic compounds and CAs, their carbon content and the products distribution. Best performance exhibits a lignin dissolution ratio of 53 % with 1.3 % of yield towards aromatic compounds where vanillin is the principal product (59.1 %) but also the 32 % of yield in CAs with glycolic acid as major product (40.9 %). Acetovanillone (AV) was selected as lignin model molecule in order to investigate its behavior under the WAO reaction conditions. The experiments were carried out in a pressurized 50 ml batch reactor loaded with NaOH 2 M as solvent, the reaction takes 1 h with temperatures ranging from 130 to 190 °C and air pressures between 5 and 30 bar. A heterogeneous catalyst, the perovskite-type mixed oxide LaFeO3 was synthetized and used as catalyst in order to improve the activation of molecular oxygen. Vanillin yield resulted to benefit from high reaction temperature showing a maximum carbon yield of 22 %, instead the formation of carboxylic acids from the oxidative degradation of AV largely benefits from high pressure of air, exhibiting an overall carbon yield of 35 %. The produced compounds include oxalic, glycolic, lactic, malonic and levulinic acid.