Catalysts synthesis with structured pores derivatives of hydrotalcite by combined use of sol-gel chemistry, surfactants and emulsion

Author(s)

Davi Domingos Petrolini

• Document type: Doctoral Thesis
• Press: Araraquara. 2019-03-21.
• Institution: Universidade Estadual Paulista (Unesp). Instituto de Química. Araraquara
• Defense date: 2019-03-08
• Advisor: Leandro Martins

Abstract

This thesis presents the development and the study of a new method that allows the synthesis of porous mixed oxides derived from hydrotalcite. Based on theunique structural and catalytic properties of the hydrotalcites, the thesis shows that modifications in the synthesis of these materials allow to explore their potential. The method is based on the combined use of supramolecular chemistry of surfactants and emulsions for the production of hydrotalcites with hierarchically structured pores in different size scales. What is interesting in the development of porous catalysts is that the creation and design of the pores not only interfere with the facilitated diffusion of reagents and products, but also some unique properties of surface activity arise. The presence of amphipathic molecules and apolar molecules altered the strength of the sites in the catalyst, due to the interactions of the templates with the surface of the metallic oxides. In this way, the efficiency of the material was amplified by modifying its structural and chemical properties, where the composition and organization of the surface atoms in differentiated microdomains. In addition to the porous properties, another important emerging factor that has been studied was the use of mixed oxides of aluminum and magnesium, such as oxides derived from hydrotalcite, which are widely diffused in catalysis. In this context, the thesis explored the synthesis of catalysts and support of catalysts derived from hydrotalcite with well-defined properties. Each stage of preparation (precursors of the reaction mixture, emulsified colloidal system and sol-gel transition) has a number of factors that influence the final property of the solid, and whose control is necessary for reproducibility in the preparations. The preparation required very different knowledge, such as the phenomena that govern gel formation, the colloid chemistry and drying operations, which wereaccompanied by technique of characterization of structure, texture and surface to establish the conditions of operation of synthesis and activity catalytic optimization. The catalytic activity of the mixed oxides was evaluated in the dehydration and dehydrogenation reaction of secondary alcohols, such as 2-propanol, wherein the conversion of 2-propanol under controlled mass and diffusion heat conditions in the mixed oxides of Al-Mg texturally improved evidenced the preparation of a catalyst more efficient than the reference having low porosity. The prepared material was also evaluated as catalyst support for impregnation of metals, from which copper was impregnated and the catalytic activity was evaluated in the dehydrogenation of ethanol and in the coupling reaction of alcohols, or also known as the Guerbet reaction. For the dehydrogenation reaction of ethanol, the porous catalyst was about 3 times more active than the reference, due to formation Cu0along with the more active Cu+species on the support surface. In fact, the simultaneous presence of Cu+and Cu0was advantageous for the catalytic performance, since the TOF(“turnover frequency”) were 122 and 166 h-1for the non-porous and porous reference catalysts, respectively. And finally, the Guebert'sreaction in which the Cu catalysts show improved yields and selectivities at elevated H2pressures. Without Cu, AlMgO catalysts were more selective to diethyl ether (45-55 C%) than higher alcohols and aldehydes (30-40 C%), while AlCaO catalysts had no activity for ethanol conversion. The addition of Cu inhibited production of diethyl ether while promoting dehydrogenation to acetaldehyde, the initial step in Guerbet coupling of ethanol. Copper addition increased the activity and the selectivity to higher alcohols and aldehydes (up to 55 C% selectivity) for all catalysts. Olefins and paraffins comprised the majority of the side products formed with Cu-doped. (AU)

• FAPESP's process: 15/05321-5 - Catalysts synthesis with structured pores derivatives of hydrotalcite by combined use of sol-gel chemistry, surfactants and emulsion
• Grantee: Davi Domingos Petrolini
• Support Opportunities: Scholarships in Brazil - Doctorate