Last 17 of june of 2016, D. Francisco Javier López Tenllado, defended her PhD entitled "Selective oxidation reactions in Organic Chemistry promoted by light", marked with 'SOBRESALIENTE CUM LAUDE'.
SUPERVISORS
Dr. Francisco J. Urbano Navarro
Dr. Alberto Marinas Aramendia
¡Congratulations Fran!
Summary of the doctoral thesis
In this Doctoral Thesis, selective photo-oxidation of crotyl alcohol to crotonaldehyde using acetonitrile as the solvent and selective photo-oxidation of isopropanol to acetone in the gas phase have been carried out. In addition, the possible use of these processes as model reactions to evaluate the catalytic activity of photocatalysts have been studied.
Initially, the experimental conditions of these reactions were optimized. Afterwards the catalytic behavior of several catalysts based on TiO2, unmodified and modified with transition metals, were studied. Subsequently, in order to evaluate the influence of the semiconductor of choice on the photo-catalyzed reactions, the study was extended to other semiconductors such as CeO2 and Bi2WO6.
Moreover, the photocatalyzed hydrogen production from glycerol and propan-2-ol using TiO2 modified with gold, platinum or palladium as the photocatalyst has been studied. The textural, structural and chemo-surficial properties of the catalysts were studied trying to stablish some relationships between the activity in hydrogen photoproduction, the substrate used and the characteristics of the metal deposited on the TiO2.
If we look at results obtained for the photo-oxidation of crotyl alcohol and isopropanol on TiO2-based catalysts, we observe some similar trends. Thus, unmodified TiO2 catalysts obtained from titanium tetraisopropoxide were more active than those synthesized from titanium tetrachloride. When solids containing metals are considered, incorporation of iron, zinc or palladium proved to be detrimental to the activity, while zirconium, silver and especially gold exhibited greater activity than unmodified TiO2.
Although titania catalysts exhibited different activities, when the selectivities to crotonaldehyde at equivalent conversion values for all catalysts are compared, we observe that catalysts show similar selectivity values. This suggests that the selectivity of the process is determined by the nature of TiO2.
For propan-2-ol transformation, similar selectivities to acetone were obtained on all TiO2 catalysts, in the 60% range, regardless of the conversion obtained. The only exception were the TiO2 catalysts modified with platinum, which showed a significantly higher selectivity to acetone, about 80%, and a conversion of 22-28%. This suggests that this test reaction is more sensitive to the presence of metals and ultimately both test reactions can be used for a screening of the photocatalytic activity.
Subsequently, CeO2 and Bi2WO6 catalysts were tested in gas phase selective photo-oxidation of propan-2-ol to acetone and selective photo-oxidation of crotonaldehyde to crotyl alcohol in the liquid phase. Bi2WO6 catalysts were intrinsically more selective to the carbonyl compound. Several experiments showed that high selectivities to crotonaldehyde on Bi2WO6 could be explained by the lower adsorption of crotonaldehyde on this catalyst. Moreover, when a small amount of TiO2 (5% molar) is incorporated to Bi2WO6, the reaction rate is increased significantly, while selectivity to crotyl alcohol remains high. This opens the possibility of synthesizing new hybrid photocatalytic materials, Bi2WO6-TiO2, with high activity and selectivity to carbonyl compounds.
As regards photocatalytic hydrogen production, the behavior shown by glycerol and propan-2-ol was completely different. While for glycerol total photo-reforming leading to CO2 and H2 was observed, propan-2-ol underwent selective transformation to acetone and hydrogen. It was found that adsorption process of both substrates was consistent with a Langmuir-Hinshelwood dissociative mechanism. However, adsorption is stronger for glycerol than propan-2-ol, which could explain the observed differences.
As far as the influence of the metal content is concerned, under our experimental conditions, catalysts with lower metal loading (0.5% nominal) were those showing the best results. This confirms that a lower metal content favors the separation of the electron-hole pair, while a higher metal content favors recombination. As for the metal of choice, for similar metal loadings, hydrogen production from propan-2-ol followed the order Pt> Pd> Au.
Papers related:
- A comparative study of hydrogen photocatalytic production from glycerol and propan-2-ol on M/TiO2 systems (M=Au, Pt, Pd). Catalysis Today, in press
- A comparative study of Bi2WO6, CeO2, and TiO2 as catalysts for selective photo-oxidation of alcohols to carbonyl compounds. Applied Catalysis A: General 505 (2015) 375–381
- Selective photooxidation of alcohols as test reaction for photocatalytic activity. Applied Catalysis B: Environmental 128 (2012) 150–158