Last 15 of February of 2015, D. Vicente Montes-Jiménez, defended his PhD entitled "Design of several supported metal systems for diverse industrially-relevant hydrogenation processes", marked with 'SOBRESALIENTE CUM LAUDE'. SUPERVISORS Dr. Francisco J. Urbano Navarro Dr. Alberto Marinas Aramendia | |
ABSTRACT
A global vision of catalysis is adopted following the sequence synthesis-characterization-catalytic activity with a view to stablish some structure-performance relationships for the catalysts. Synthesized systems consist in several supported metal nanoparticles. Special attention is paid to the control of diverse synthetic variables such as the method (microemulsion, deposition-precipitation or impregnation), or the metal precursor of choice (with or without chlorine). Some of the solids used as the supports include several partially-reducible oxides (such as ZnO or TiO2), with a view to study the effect of metal-support interaction on catalytic performance. Several porous ordered materials (USY zeolites and MCM-41) are also tested in order to favor adsorption (due to their high surface area) and evidence potential diffusional problems (when microporous instead of mesoporous systems are used).
Regarding glycerol hydrogenolysis on several ZnO-supported metal (Pt, Pd, Rh) systems, similar metal particle sizes (in the 2-4 nm range) were obtained through microemulsion technique. Even though synthesized metal particles were already in the zero oxidation state, their highest activity was obtained on thermal treatment at 200-300ºC. The presence of some remaining surfactant could account for that, thermal treatment favoring their loss and thus the access of substrate to the metal particle. Moreover, the presence of surfactant favors selectivity to acetol since it somehow limits hydrogenation activity of the metal. Further studies on Pt/ZnO systems evidenced the role of the metal not only on the hydrogenation of acetol to 1,2-PDO but also on the previous dehydration of glycerol into acetol. Moreover, the reaction requires a moderate acidity whereas strong acid sites favors some secondary reactions such as catalytic cracking, to the detriment of selectivity to 1,2-PDO.
As regards Fischer-Tropsch synthesis, microemulsion technique allowed us to obtain several Pt-modified TiO-supported cobalt systems with cobalt particle sizes in the 3.8-4.7 nm range. Simultaneous reduction of Co and Pt led to higher conversions than consecutive reduction. This was explained as a result of the higher Pt-Co and Co-support interaction in the former case, as evidenced by TPR, Raman and XPS.
As far as enantioselective reduction of ethyl pyruvate to ethyl lactate is concerned, a study on Pt/USY and Pt/MCM-41 solids showed that the optimum platinum particle size is in the 3-5 nm range. A screening of several commercial chiral molecules as additives resulted in the increase in e.e. values (R or S) through the addition to the reaction medium of OHDF in the presence of low concentration of CD or CN, respectively. UV-Vis studies evidenced the existence of some substrate-modifier-additive interaction in the liquid phase. A multivariable study on one of the catalysts (USY780-H2PtCl6-10ºC/min) varying the hydrogen pressure, amount of CD and OHDF, allowed us to obtain 79% e.e. at total conversion and confirmed the above-mentioned positive effect of OHDF on enantioselectivity.
Finally, the study of gas-phase chemoselective hydrogenation of crotonaldehyde to crotyl alcohol showed the existence of some diffusion problems when microporous (USY zeolite) materials were used as the support. The presence of some residual chlorine coming from the metal precursor used favors selectivity to the unsaturated alcohol. Furthermore, those solids using ZnO as the support exhibited the highest selectivity values to crotyl alcohol upon reduction at high temperatures which seems to suggest that strong Pt-Zn interaction is positive to the reaction.
A global vision of catalysis is adopted following the sequence synthesis-characterization-catalytic activity with a view to stablish some structure-performance relationships for the catalysts. Synthesized systems consist in several supported metal nanoparticles. Special attention is paid to the control of diverse synthetic variables such as the method (microemulsion, deposition-precipitation or impregnation), or the metal precursor of choice (with or without chlorine). Some of the solids used as the supports include several partially-reducible oxides (such as ZnO or TiO2), with a view to study the effect of metal-support interaction on catalytic performance. Several porous ordered materials (USY zeolites and MCM-41) are also tested in order to favor adsorption (due to their high surface area) and evidence potential diffusional problems (when microporous instead of mesoporous systems are used).
Regarding glycerol hydrogenolysis on several ZnO-supported metal (Pt, Pd, Rh) systems, similar metal particle sizes (in the 2-4 nm range) were obtained through microemulsion technique. Even though synthesized metal particles were already in the zero oxidation state, their highest activity was obtained on thermal treatment at 200-300ºC. The presence of some remaining surfactant could account for that, thermal treatment favoring their loss and thus the access of substrate to the metal particle. Moreover, the presence of surfactant favors selectivity to acetol since it somehow limits hydrogenation activity of the metal. Further studies on Pt/ZnO systems evidenced the role of the metal not only on the hydrogenation of acetol to 1,2-PDO but also on the previous dehydration of glycerol into acetol. Moreover, the reaction requires a moderate acidity whereas strong acid sites favors some secondary reactions such as catalytic cracking, to the detriment of selectivity to 1,2-PDO.
As regards Fischer-Tropsch synthesis, microemulsion technique allowed us to obtain several Pt-modified TiO-supported cobalt systems with cobalt particle sizes in the 3.8-4.7 nm range. Simultaneous reduction of Co and Pt led to higher conversions than consecutive reduction. This was explained as a result of the higher Pt-Co and Co-support interaction in the former case, as evidenced by TPR, Raman and XPS.
As far as enantioselective reduction of ethyl pyruvate to ethyl lactate is concerned, a study on Pt/USY and Pt/MCM-41 solids showed that the optimum platinum particle size is in the 3-5 nm range. A screening of several commercial chiral molecules as additives resulted in the increase in e.e. values (R or S) through the addition to the reaction medium of OHDF in the presence of low concentration of CD or CN, respectively. UV-Vis studies evidenced the existence of some substrate-modifier-additive interaction in the liquid phase. A multivariable study on one of the catalysts (USY780-H2PtCl6-10ºC/min) varying the hydrogen pressure, amount of CD and OHDF, allowed us to obtain 79% e.e. at total conversion and confirmed the above-mentioned positive effect of OHDF on enantioselectivity.
Finally, the study of gas-phase chemoselective hydrogenation of crotonaldehyde to crotyl alcohol showed the existence of some diffusion problems when microporous (USY zeolite) materials were used as the support. The presence of some residual chlorine coming from the metal precursor used favors selectivity to the unsaturated alcohol. Furthermore, those solids using ZnO as the support exhibited the highest selectivity values to crotyl alcohol upon reduction at high temperatures which seems to suggest that strong Pt-Zn interaction is positive to the reaction.