RICERCA ITALIANA     

Research program

INNOVATIVE CATALYTIC PROCESSES FOR THE SELECTIVE OXIDATION AND REDUCTION OF GLYCEROL IN WATER: STUDIES OF REACTION MECHANISMS AND KINETICS FOR THE PROCESS OPTIMISATION

University Co-ordinator

Università degli Studi di TORINO - CHIMICA ANALITICA - ()

Research Unit Leader

Claudio Minero

Description

A literature survey revealed that there are no studies on the photocatalytic transformation of glycerol over irradiated semiconductor oxides in water or organic media. Based on the previous experience of this Unit several processes, relevant for the synthesis of value-added chemicals from glycerol, can be realized by semiconductor photocatalysis under the appropriate conditions. The Scheme 1 reports the various chemical obtainable from glycerol through halogenation, dehydration, reduction and oxidation, grouped according to the mean oxidation state of the carbon atoms.


Scheme 1. Redoc processes involving glycerol. The left column reports the average oxidation number of carbon

Besides oxidation and reduction, also hydrolysis [16], halogenation [15] and condensation[17) processes can be carried out by photocatalysis. Scheme 2 summarizes the various concurrent processes involved. A more detailed discussion will be reported later. A careful modulation of operating conditions and photocatalyst properties can favour some processes over the other.


Schema 2. Photoinduced processes that take place at the irradiated semiconductor oxide / electrolyte interface


The Unit has also relevant expertise in the characterization of surface structure and morphology of anatase based photocatalyst, as well as the surface properties at molecular level[19, 20, 21, 22].
The research program of the Unit UNITO is focused on the evaluation of the photo-assisted transformations of glycerol on TiO2 catalysts through the investigation of the molecular events (adsorption/reaction/desorption) and concurrent reactions involving glycerol and other reactive species, according to Scheme 2, at the surface of TiO2, and of their dependence on the:
1 - intrinsic and extrinsic surface features of the semiconductor. The structure-properties are useful to guide the selection of the photocatalyst and/or its preparation method. The experiments will be carried out on different commercial anatase samples deeply studied by the Unit, as well as on transparent TiO2 films prepared according to a sol-gel procedure patented by the Unit[23].
2 - the operating parameters, and mainly the amount of O2, halogens and glycerol, that according to Scheme 2 influence the kinetic and selectivity of the process and permit to pursue maximum yields of the desired products.

The detailed research program will be broken down in the following 3 tasks that address the above two topics:
1) Development of the suitable analytical tools for identification and quantification of glycerol and derivatives. The quali-quantitative determination of these highly hydrophilic compounds is a complex analytical problem. The Unit has good expertise in the development of analytical procedures for such compounds [24, 25]. Fast quantitative HPLC or HPLC-MS protocols, quickly transferable to other Units, will be developed, along with more complex analytical strategies by using GC-MS coupled with suitable derivatization procedures, as well as LC-MS in order to obtain a more thorough identification of reaction intermediates. (months 1-8).
2) Modulation of selectivity and optimization of the yield of the process through variation of the extrinsic surface properties of selected catalysts and operating conditions, both for carbonic products and the production of H2 and H2O2, conjectured under O2 deficiency, also through the use of cocatalysts (polyoxometallates and/or Pt/Au). (months 2-22).
3) Study of the surface processes at molecular level with spectroscopyc and microscopic techniques (month 4-18)

Task 1: Development of the suitable analytical tools (months 1-8)

The glycerol and its derivatives (Scheme 1) are highly hydrophilic compounds, without good chomophores, fluorophores, and majority of them without electroactive functional groups. Their determination at trace level in water systems is a difficult analytical challenge. Current analytical methods involve the use of LC-MS/MS techniques on target analytes[26], or GC-MS techniques after a time consuming step of clean-up, water elimination and derivatization with water incompatible silylating reagents[27]. This task activity will involve the development of both fast screening analytical methods capable to identify the broadest spectrum of transformation intermediates and fast and simple analytical methods for the quantification of target analytes. Methods of the first category will be used in Task 2 to identify principal and trace transformation intermediates and assess the transformation mechanism, whereas methods of the second category will be transferred to other Units to assist their activities.
The screening analytical methods will be developed by using GC-MS also coupled with a fast derivatization technique for highly hydrophilic compounds in aqueous matrices previously developed by this Unit [24,25]. The derivatization technique is based on the use of alkyl chloroformates with very low hydrolysis rates in the presence of suitable catalysts. The preconcentration, if needed, will be carried out with automated solid phase extraction techniques (SPDE, Solid Phase Dynamic Extraction) which require minimal sample preparation. An alternative technique that will use fluorescent derivatives of chloroformate will be also investigated. The technique would allow the HPLC separation followed by fluorescent detection, a tool more common in the labs of other Units.
The task activities will involve:
1) Development of a fast screening analytical method for product identification: optimization of alkyl chloroformate derivatization conditions for glycerol and selected derivatives; optimization of SPDE parameters for derivatives extraction; optimization of GC-MS analytical method.
2) Application of the fast screening analytical method to the relevant activities of Task 2.
3) Development of simple analytical methods by HPLC and HPLC-MS for target molecules identified in Task 2 as key intermediates and end products. This activity would also imply the transfer of the analytical protocols to the other Unit of the research project.

Task 2: Modulation of the selectivity and yield of the process (months 2-22)

As mentioned in Scheme 2, many concurrent processes are occurring on the surface of irradiated TiO2, depending on the presence of different electron/hole scavenger (including halides and other ions oxidizable by VB holes), the nature of the surface (presence of surface free hydroxyls, of surface complexing agents like fluorides, reagents or intermediates), the acidity/basicity of the reaction media (that changes the surface speciation, the surface charge and solution acid/base equilibria and rates), and on the substrate concentration [28] (that could favor back reactions, see in Scheme 2 gly->R1->P1 or gly->R2->P2), as well as on the kinetic of mass transfer from/to the surface, which can be relevant in concentrated systems. This gives to the photocatalytic process a high degree of versatility, because a change in the operating conditions can drive the process to different end products. For these reasons a careful study of the effect of the working conditions should be done with the aim to direct the process toward more added-value products.
The expertise gained by this Unit in non classical (e.g. purely redox) photocatalytic conversions (halogenation/dehalogenation, photoassisted hydrolysis, condensation) [15,16,17], suggests that, by combining in the right way the concurrent processes in scheme 2, there is the possibility to tune the reaction conditions, driving the photocatalytic conversion of glycerol to value-added compounds like acrolein or epychloridrin. The proper surface properties of the catalyst also play a relevant role (Task 3).
Accordingly, in this task, also on the basis of results produced by Task 3, a large interval of operating conditions will be explored, to get insight in the transformation mechanism of glycerine and yield of products, and to address the selectivity of the process by changing the relative role of the concurrent processes in Scheme 2. It will be explored not only photocatalyzed oxidative transformations (see Scheme 2), but also reductive processes (e.g. formation of propylene glycol) and the non classical route previously described (e.g. dehydration and halogenation).
During the course of the photocatalytic processes in the presence of a sacrifical hole scavenger, H2 and H2O2 can be produced from water and O2 reduction respectively (see scheme 2). The production of these two value-added compounds can represent another viable use of the glycerol feedstock. Formation of H2 by using glycerine as sacrificial electron donor was recently demonstrated over lanthanide doped titania[29] and over polyoxometallates[5]. Conditions for sustained H2O2 production in photocatalytic systems were recently reported[18,32]. The surface complexation of anions/cations on active TiO2 sites is fundamental to avoid the adsorption of produced H2O2. The presence of a sacrificial electron donor to reduce O2 is also important, as glycerol is a good candidate. A relevant role is played also by the surface characteristic of the TiO2 (nature and density of surface OH groups) (relation with Task 3).
Two model catalysts (TiO2 Degussa P25 and Merck), which were[19] and will be deeply studied by this Unit regarding surface properties in Task 3, will be employed. Other catalysts in form of supported films will also be considered, also in order to assist the reactor implementation by other Units.
The set of operating conditions in experiments of irradiation and temporal quantification of the product evolution that will be explored are:

1. Substrate concentration, considering also concentrated glycerol aqueous solution (0.1-20% w/w), like those produced during biodiesel manufacture. From these experiments one can get insight in the formation of condensation products and in the problems of mass transfer that could be present in viscous liquids.
2. Acidity/basicity of the aqueous system. These parameters influence the surface speciation of the catalyst, depending also on its nature, the reactivity of the substrate, as well as introduce specific acid/base catalysis. Highly acid media could favour dehydration processes, whereas in alkaline condition the speciation of glycerol change (pKa = 14.15), potentially leading to different intermediates (hole transfer centred on an oxygen and not on a carbon)
3. Tuning of the surface properties of the catalyst through surface complexation, also on the basis of the results produced by Task 3. Past[19,30,31,32] and ongoing studies on the two model catalysts (TiO2 P25 and Merck) showed that the surface modification through complexation can have profound effect on the mechanism of interfacial charge transfer and on the adsorption of substrates/intermediates. The variation in surface charge is also relevant in order to change the surface activity of non specifically adsorbed species. The additional control of the process introduced by these modifications can be of relevance for the selectivity of glycerol photoconversion.
4. Presence, type and concentration of electron and hole scavengers (e.g. H2, O2, peroxides). The tuning of these operating conditions is needed to drive the process toward reduced/oxidized glycerol derivatives (see Scheme 1), and in particular toward volatile products, the stripping of which is a duty of the Bologna Unit.
5. The evaluation of time evolution of and selectivity toward H2 and H2O2, and the influence of the presence of cocatalysts (noble metals, polyoxometallates) on their production rate.

Task 3: Study of the surface processes at molecular level with spectroscopyc and microscopic techniques (months 4-18)

This task is focused on the evaluation of the photo-assisted transformations of glycerol on TiO2 catalysts through the investigation of the molecular events (sites of adsorption/reaction and mechanisms) involving glycerol at the surface of TiO2, and on their dependence on the surface features of the semiconductor in order to define structure-properties relationship useful to guide the selection of the photocatalyst and/or its preparation method, which are of interest for other Units in the project. This aspect involves the study of the molecular phenomena on the semiconductor surface, that is, adsorption, possible modifications in the dark and photo-induced transformations, by means of Infrared (IR) and Raman spectroscopy.
The operative conditions influences surface properties (see also task 2).Due to the chemical characteristics of glycerol solutions, the choice of appropriate instrumental set up for surface characterization is directly linked to the necessity to operate in a liquid-solid regime. In the case of IR spectroscopic measurement carried out in this regime, absorption signals due to free water molecules dominate the spectra. Under these conditions it is not generally possible to distinguish the contribution of surface species like hydroxyl groups and coordinated water molecules or of adsorbates on the catalyst surface.
In the case of TiO2 in the form of powder samples, which constitute a model for the present study, this limit can be overcome by developing specific IR methods of analysis by means of appropriate techniques, that are Attenuated Total Reflection Infrared Spectroscopy (ATR-IR), with the potential to detect adsorbed specie [33], Cylindrical Internal Reflection (CIR), that addresses the adsorption in situ of suspension [34], and Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) [35, 36], able to easily distinguish surface hydroxyls from noise due to the presence on the catalyst of water adsorbed from vapor [37].
Because Raman signals from water molecule is relatively weak, and does not interfere with the signals due to adsorbates on the TiO2 surface, Raman spectroscopic studies will be carried out on TiO2 powdered samples. Raman spectroscopy will also be a useful tool in the case of TiO2 samples in the form of thin films. The investigation of the molecular events involving glycerol at the surface of TiO2 films will also be carried out by Infrared Reflection Absorption Spectroscopy (IRRAS). IRRAS is an established analytical technique for the characterization of adsorbed matter and thin layers deposited on surfaces, which does not suffer from the disturbing atmospheric absorptions caused by water vapour.
Different TiO2 photocatalysts will be used in this task:
1. Two commercial photocatalysts will be generally examined, namely TiO2 P25 from Degussa (80% anatase, 20% rutile, BET specific surface area 50 m2g-1) and TiO2 from Merck (100% anatase, BET specific surface area 10 m2g-1). These two commercial samples have been already extensively characterized by this research sub-Unit. The surface morphology of the particles of these TiO2 photocatalysts was already evaluated by TEM investigation, while FT-IR characterization of the surface active sites was carried out on both semiconductors to predict and to explain the differences in the chemical and photocatalytic behaviour. These catalysts should then represent the starting point for the study of the molecular phenomena on TiO2 surface due to adsorption and phototransformation (photo-oxidation and/or photo-reduction) of glycerol.
2. samples of TiO2 or TiO2-based (with added chromophores) photocatalysts prepared by our research Unit [23] or by Palermo research Unit. Different synthesis methodologies will be used to modulate the morphological features of the semiconductor surface and their selectivity towards the phototransformation of glycerol.
3. samples of TiO2 films on silicon.[23]
4. Au modified TiO2, which is already known to be an efficient catalyst for the selective oxidation of glycerol and which is known to have a good photocatalytic activity in aqueous solution[38, 39].