Título Tesis: “CORE@SHELL CATALYSTS BASED ON COBALT, COPPER, AND NICKEL FOR THE CONVERSION OF BIOMASS DERIVATIVES – STUDIES IN THE CONVERSION OF GUAIACOL AS MODEL COMPOUND OF BIO-OIL"
Profesor (a) Guía: Néstor Escalona / Dorothée Laurenti (Université Claude Barnard Lyon 1)
Defensa de Tesis Doctoral: 1 de julio del 2022
Resumen
The impact of the consumption of fossil fuels on the concentration of greenhouse effect gases, which increases them leading to global warming, is a matter of concern for the future of the humankind. Thus, it is necessary to change our commodities sources from oil-based to renewable sources such as agricultural residues. There are technologies that allow the transformation of the biomass residues into either gas or liquid form; the liquid, called bio-oil, is a complex mixture of oxygenated compounds including phenols, carboxylic acids, and carbonyl compounds. The high oxygen content of the mixture restricts its use and further processing, requiring an upgrading by hydrotreatment to enhance its stability and calorific power.
The main goal of the present research was to synthetize a series of Core@Shell catalysts based on cheap and available transition metals like cobalt, copper and nickel from the conversion of guaiacol as a model compound of lignin-derived bio-oil in batch-reactor conditions. During this research, modifications of the composition of the core and treatment with a surface modifier like salicylic acid or catechol coupled were analyzed, studying changes in activity and selectivity of the catalysts in the conversion of guaiacol. The presence of modifiers leads to changes in the interactions between the metal phase and the silica matrix and variations in the surface of the active phase, leading to a raised selectivity towards value-added products like cyclohexanol and benzene, with an increased activity of some of the samples.
The effect of the metal employed was analyzed, and the use of copper as an active phase was disregarded due to its low activity; for the Co-based systems, they exhibited a high selectivity towards phenol and cyclohexanol. Finally, the Ni-based materials showed a high activity, with 2-methoxycyxlohexanol as the main product of the conversion of guaiacol. Furthermore, the use of bimetallic formulations based on these metals was examined, and the most active formulation consisted in a 1 to 1 Co to Ni material, due to synergistic effects related to the enhanced formation of nickel phyllosilicate, as well as a raised formation of spill-over species of hydrogen, that lead to an increased stability of the formulation, as determined by continuous flow reactor experiments. The use of Cu as a metal improves the reducibility of the catalysts, improving their hydrogenating activity and raising the yields of saturated products. However, this improved reducibility has little effect on the catalytic activity in a Ni-Cu system; whilst in the Co-Cu system, this leads to a raise in the activity of the catalyst.
From our results, the use of salicylic acid proved to be beneficial in both cobalt and nickel-based catalysts, enhancing the activity in a direct relation with the loaded salicylic acid. The main factors involved in this promoting effect are the tuning of the formation of silicate phases with a dispersing effect of the metallic phase throughout the catalyst. However, this effect reaches an optimum value around 15% molar based on the metal for nickel-based systems, when the crystallite size becomes so small that the conversion of guaiacol is disfavored. The modification with catechol has different effects in the activity of catalytic systems, depending on the metal employed. Herein, the REDOX properties of the modifier lead to the formation of more metallic phase during the synthesis, that reduces the production of phyllosilicate species of nickel that are active for the conversion of guaiacol, whilst for the cobalt catalysts, this treatment enhances the activity compared to the treatment with salicylic acid using a 6% molar. Nevertheless, higher loads of catechol lead to a sharp decrease in activity in both systems, due to the polymerization of the catechol that obstructs the formation of small size particles in the case of cobalt-based catalyst, whilst for the nickel based systems, an increased formation of an inactive 1:1 Ni:Si phyllosilicate phase is responsible to the observed decrease in activity.
In conclusion, the use of organic molecules as modifiers of the catalytic phases, using a sol-gel based route of preparation allows for the obtention of catalysts with enhanced activity, and possible modification of the selectivity of the materials. This, in combination with bimetallic formulations opens a new venue for the development of catalytic materials using a facile methodology for tuning the catalysts based on cheaper metals to generate selected products, exemplified here with the obtention of cyclohexanol and benzene, among others, from guaiacol.