Glycerol Oxidation with Zeolite and Hierarchical Zeolite as Catalysts and Electrocatalysts

Glycerol overproduction as a byproduct in biodiesel synthesis is envisioned to limit further growth of biodiesel industry. To overcome this, glycerol conversion through chemical and electrochemical oxidation toward more valuable products is a promising path. In the present PhD thesis different materials and processes were developed for the thermal catalytic oxidation and the electrochemical oxidation reaction of glycerol (GEOR). For the study of the thermal catalytic oxidation of glycerol, diluted hydrogen peroxide was selected as a green and promising alternative to O2. Four series of different zeolites (X, Y, ZSM-5, and BEA), either in sodic (Na+) and Protonic (H+) forms, were modified with the introduction of ions of transition metals (Cu, Fe, Ni, Zn) via ion-exchange method. The obtained materials were characterized (XRD, N2 physisorption, ICP-OES, NH2 adsorption microcalorimetry) and tested in different conditions to evaluate the effect on catalytic performance of the different zeolite structures, the nature and amount of the metal ions, and the GLY/H2O2 molar ratio. The main mechanism involved in the catalytic activity was reconducted to the Fenton reaction. Better glycerol conversion was achieved using zeolite exchanged with Cu and Fe ions in high amounts. HY and HZSM-5 zeolites exchanged with both Cu and Fe granted the best glycerol conversions. However, regardless of the specific zeolite used, a mixture of both partially- and over-oxidated products was obtained, with the exact composition mostly related to reaction time. With the use of higher H2O2 concentration it is possible to reduce reaction time, preserving the desired products from being further oxidized, while maintaining acceptable glycerol conversion values. For the study of GEOR, the main objective was the development of modified electrodes for the electrooxidation of glycerol into fine chemicals. The main drawback of zeolites as catalysts and electrocatalysts is the limited diffusivity inside the microporous system. An important part of the study was the development of different hierarchical zeolites to be used for the preparation of the modified electrodes. Thus, two series of hierarchical zeolites, based on Y and ZSM5, were prepared using the surfactant-mediated method with CTAB and NaOH. For the Y zeolite, the effect of the treatment time was studied in the range 2 48 h; for the ZSM-5 5the effect of the strength of the two step pretreatment was investigated. In the case of the Y zeolites, the efficient formation of an intraparticle mesoporosity with a narrow pore size distribution depends on the duration of the hydrothermal treatment. For ZSM-5 zeolites only disordered interparticle mesoporosity with a heterogeneous pore size distribution was obtained in the two-step pretreatment, with no evident modification after the hydrothermal treatment. With the obtained hierarchical zeolites and the starting microporous zeolites, three series of metal-exchanged zeolite, containing Ni, Cu, Fe and Mn, were used to prepare the modified electrodes for the GEOR, tested both in acid and in alkaline conditions. Alkaline conditions proved to be more interesting for the GEOR and different mechanisms were proposed for the process. The hierarchical samples showed better performance compared to the conventional zeolites. By comparing the hierarchical zeolite-electrocatalysts at equivalent glycerol conversion values, NiCuY_h6 shows the best yield toward GLAl, FeMnY_h12 grants interesting formation of DHA and dicarboxylic acids (TAC, OXAc), and MnCuY_h6 grants the same glycerol conversion value in less time than the other materials.