Last 1 of February of 2016, D. Manuel Checa-Gómez, defended his PhD entitled "Glycerol transformation on several heterogeneous acidbase and supported metal catalysts under different redox conditions", marked with 'SOBRESALIENTE CUM LAUDE'. SUPERVISORS Dr. Francisco J. Urbano Navarro Dr. Alberto Marinas Aramendia ¡¡ Congratulations Manolo !!! | |
Summary of the doctoral thesis
Content of research
This dissertation can be divided into three blocks, concerning i) the identification of strong metal-support strong interaction (SMSI) in our catalytic systems and its influence on glycerol hydrogenolysis process, ii) the study of the deactivation process of the catalysts during the reaction and, finally, iii) the influence of the incorporation of dopants (modifiers) in the catalysts activity and selectivity of glycerol hydrogenolysis process.
The first block began with the study of the influence of the catalytic support on the reaction, synthetizing a set of supported Pt catalyst over different partially reducible metal oxides such as TiO2, ZrO2, SnO2 and ZnO. The parameters studied were the reaction temperature, catalyst reduction temperature, pH and the initial pressure of either hydrogen or an inert gas.
The second part was focused on the study of the surface acid-base properties of the supports and the catalyst deactivation process during the reaction. To this end, a series of supported metal catalysts was synthesized over supports with well-defined acid-base properties. Activated Al2O3 (acidic and basic), CeO2, La2O3 and ZnO were used as support while Pt was elected as active metal since it was found to provide with the best results in former experiments. The catalyst reduction temperature, initial hydrogen pressure, stability of reaction intermediates and catalyst deactivation were the variables under investigation.
Finally, the third block focused on the incorporation of modifiers, via impregnation, to a commercial ZrO2 that was used as support for Pt in order to prepare new catalytic systems. This new set of Pt catalysts supported on modified ZrO2 were characterized in terms of surface acidity. The modifiers selected were: borax (B2O3), sulphuric acid (H2SO4), phosphomolybdic acid (H3PMo12O40) and silicotungstic acid (H3SiW12O40). The catalysts were characterized for each stage of the synthesis, and were tested in the glycerol hydrogenolysis reaction, paying special attention to the temperature dependence on the distribution of the reaction products
Conclusions
The appearance of strong metal-support interactions (SMSI) was observed in the catalysts based on SnO2 and ZnO as Sn:Pt and Zn: Pt alloys, respectively. As far as the SMSI is concerned, higher glycerol conversions were obtained when catalysts with easily reducible support were present, especially those based on ZnO, which exhibited the best results in terms of 1,2-PDO production. After the obtained results, ZnO was selected as support for a further study concerning the effect of the noble metal incorporated to the catalyst (Pt, Au, Rh or Pd). Moreover, a study of the reaction conditions was carried out, thus gathering additional information on several aspects such as the behavior of the catalyst under inert or reductive atmosphere, the influence of the pH of the reaction mixture on the reaction mechanism and the dependence of the reaction products with the reaction time. In this way, there were established two important relationships dealing with the catalytic supports and the noble metal incorporated to the catalyst and its influence on glycerol hydrogenolysis:
• Activity order for supports: ZnO> SnO2> ZrO2> TiO2.
• Activity order for supported metals: Pt> Rh> >Pd >> Au.
For Pt supported over Al2O3 (acid and basic), CeO2, ZnO and La2O3, it was found that an increase in conversion does not necessarily lead to an increase in 1,2-propanediol yield due to oligomerization processes that lead to by-products that disturbed the mass balance. Studies using acetol and 1,2-propanediol as reagents showed that the acetol was able to generate some of these polymers, especially with bare supports, while the 1,2-propanediol was stable in presence of the catalyst. Characterization of the spent catalysts indicated that the deactivation process could be explained by the graphitization of formed by-products on the acid centres of the catalyst, along with some leaching of the catalytic supports during the reaction.
Finally, it was confirmed that doped catalyst can be a useful tool not only for yields improvement, but also to modify the products selectivity in favour of an interesting product or group of products. In this line, is noteworthy to highlight the role of W polyoxomethalates as the only dopant able to produce 1,3-propanediol (1,3-PDO), a hydrogenolysis product that has not been observed in any of the previous studies of this work. Furthermore, it was observed a large amount of monoalcohols, particularly n-propanol, obtained in the aqueous phase and a gas phase composed mainly of the propene / propane mixture, both products of industrial interest. In any case, for the same metal, products distribution would depend on the dopant employed, the reaction temperature and the reaction time.
Content of research
This dissertation can be divided into three blocks, concerning i) the identification of strong metal-support strong interaction (SMSI) in our catalytic systems and its influence on glycerol hydrogenolysis process, ii) the study of the deactivation process of the catalysts during the reaction and, finally, iii) the influence of the incorporation of dopants (modifiers) in the catalysts activity and selectivity of glycerol hydrogenolysis process.
The first block began with the study of the influence of the catalytic support on the reaction, synthetizing a set of supported Pt catalyst over different partially reducible metal oxides such as TiO2, ZrO2, SnO2 and ZnO. The parameters studied were the reaction temperature, catalyst reduction temperature, pH and the initial pressure of either hydrogen or an inert gas.
The second part was focused on the study of the surface acid-base properties of the supports and the catalyst deactivation process during the reaction. To this end, a series of supported metal catalysts was synthesized over supports with well-defined acid-base properties. Activated Al2O3 (acidic and basic), CeO2, La2O3 and ZnO were used as support while Pt was elected as active metal since it was found to provide with the best results in former experiments. The catalyst reduction temperature, initial hydrogen pressure, stability of reaction intermediates and catalyst deactivation were the variables under investigation.
Finally, the third block focused on the incorporation of modifiers, via impregnation, to a commercial ZrO2 that was used as support for Pt in order to prepare new catalytic systems. This new set of Pt catalysts supported on modified ZrO2 were characterized in terms of surface acidity. The modifiers selected were: borax (B2O3), sulphuric acid (H2SO4), phosphomolybdic acid (H3PMo12O40) and silicotungstic acid (H3SiW12O40). The catalysts were characterized for each stage of the synthesis, and were tested in the glycerol hydrogenolysis reaction, paying special attention to the temperature dependence on the distribution of the reaction products
Conclusions
The appearance of strong metal-support interactions (SMSI) was observed in the catalysts based on SnO2 and ZnO as Sn:Pt and Zn: Pt alloys, respectively. As far as the SMSI is concerned, higher glycerol conversions were obtained when catalysts with easily reducible support were present, especially those based on ZnO, which exhibited the best results in terms of 1,2-PDO production. After the obtained results, ZnO was selected as support for a further study concerning the effect of the noble metal incorporated to the catalyst (Pt, Au, Rh or Pd). Moreover, a study of the reaction conditions was carried out, thus gathering additional information on several aspects such as the behavior of the catalyst under inert or reductive atmosphere, the influence of the pH of the reaction mixture on the reaction mechanism and the dependence of the reaction products with the reaction time. In this way, there were established two important relationships dealing with the catalytic supports and the noble metal incorporated to the catalyst and its influence on glycerol hydrogenolysis:
• Activity order for supports: ZnO> SnO2> ZrO2> TiO2.
• Activity order for supported metals: Pt> Rh> >Pd >> Au.
For Pt supported over Al2O3 (acid and basic), CeO2, ZnO and La2O3, it was found that an increase in conversion does not necessarily lead to an increase in 1,2-propanediol yield due to oligomerization processes that lead to by-products that disturbed the mass balance. Studies using acetol and 1,2-propanediol as reagents showed that the acetol was able to generate some of these polymers, especially with bare supports, while the 1,2-propanediol was stable in presence of the catalyst. Characterization of the spent catalysts indicated that the deactivation process could be explained by the graphitization of formed by-products on the acid centres of the catalyst, along with some leaching of the catalytic supports during the reaction.
Finally, it was confirmed that doped catalyst can be a useful tool not only for yields improvement, but also to modify the products selectivity in favour of an interesting product or group of products. In this line, is noteworthy to highlight the role of W polyoxomethalates as the only dopant able to produce 1,3-propanediol (1,3-PDO), a hydrogenolysis product that has not been observed in any of the previous studies of this work. Furthermore, it was observed a large amount of monoalcohols, particularly n-propanol, obtained in the aqueous phase and a gas phase composed mainly of the propene / propane mixture, both products of industrial interest. In any case, for the same metal, products distribution would depend on the dopant employed, the reaction temperature and the reaction time.