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 PALERMO - INGEGNERIA CHIMICA, DEI PROCESSI E DEI MATERIALI - ()

Research Unit Leader

Vincenzo Augugliaro

Description

The general research program here proposed deals with the development of an advanced selective oxidation or reduction process of glycerol in water under mild reaction conditions. The oxidation of glycerol is of great interest since some of the partial oxidation products, e.g. dihydroxyacetone, glyceric acid and glyceraldehyde, are potentially useful as chemical intermediates. The reduction process of glycerol is also of great industrial importance as it allows the production of 1,3-propanediol. The case of 1,3-propanediol compound is very interesting; in fact the recent development of a new polyester called polypropylene terephtalate, with unique properties for the fiber industry, has determined the need of a drastic increase in the production of 1,3-propanediol.
The specific research activity planned to be carried out at the “Dipartimento di Ingegneria Chimica dei Processi e dei Materiali” (DICPM) of the University of Palermo will be focused on the use of the heterogeneous photocatalytic method for carrying out the glycerol partial oxidation and reduction in water.
In particular the investigation program will concern:
(i) the preparation of TiO2-based photocatalysts able to perform selective oxidation and reduction of glycerol in water under near-UV and visible light radiation;
(ii) the optimisation of catalyst preparation;
(iii) the characterisation of the photocatalysts by XRD, SEM observations, specific surface area and porosity measurements;
(iv) the testing of the photocatalytic activity of the different powders by using a batch photoreactor under different operative conditions;
(v) the immobilization of the optimal photocatalyst on suitable supports
(vi) the testing under different operative conditions of the photocatalytic activity of immobilized catalyst by using a continuous fixed bed photoreactor;
(vii) the reduction of glycerol to 1,3-propanediol will be checked by working under anaerobic conditions.
Among the various semiconductor materials tested as photocatalysts anatase TiO2, due to its low cost and high stability under irradiation, is considered the most reliable material. TiO2 catalyst however shows two major disadvantages:
(a) it shows low quantum efficiency, i.e. the rate of transformation of molecules is two orders of magnitude less than the rate of photon absorption; and
(b) it is effective only under ultraviolet irradiation (? < 380 nm) so that the achievement of high reaction rate can be performed by using lamps emitting light in the near-UV region.
The direct consequence of previous drawbacks is that the use of solar energy, whose aliquot in the near-UV region is very low, is allowed only when the economics of the photocatalytic process is dominated by the irradiation cost and therefore slow reaction rates may be accepted.
The improvement and optimisation of TiO2 photocatalytic activity will be therefore the first task to be accomplished by the DICPM research group.
In order to improve the activity of the photocatalyst, two strategies will be applied.
On one side, the procedures of TiO2 synthesis will be varied and optimised in order to produce new and highly active catalysts so that the quantum yield under artificial irradiation increases of at least one order of magnitude. The preparation methods influence significantly morphological, structural, surface physico-chemical and electronic properties that are related to the extent of photoactivity. Nanostructured semiconductors exhibit special photochemical characteristics: by decreasing particle size, the band gap of the semiconductor becomes larger as indicated by an absorption shift to shorter wavelengths. The shift of the conduction band to more negative potentials and of the valence band to more positive potentials may favour redox processes that cannot occur in bulk materials.
The sol-gel method will be employed for the preparation of nanoparticles, due to the inexpensive equipment required and the low temperatures involved. This method will be also used by the Operative Unit of Torino to prepare different photocatalysts. The metodologies of photocatalysts preparation and characterisation will be compared in order to optimize the procedure. The properties of the sol-gel derived samples are strongly dependent not only on the composition, but also on the preparation conditions including the starting materials and solvents, the solution preparation sequence and various other processing conditions as [precursor]/[solvent] ratio, temperature, stirring and aging time. It is important to outline that the TiO2 particles obtained by this method are amorphous in nature and require a calcination treatment at high temperatures to perform the transition from amorphous to anatase, the most photoactive phase of TiO2. Unfortunately, the high calcination temperatures give rise to an increase of the size of the nanoparticles and to a decrease of their specific surface areas. In order to overcome this drawback, it will be tried to prepare nanostructured TiO2 catalysts by hydrolysis of titanium isopropoxide or titanium tetrachloride; this preparation method offers the advantage that no calcination will be needed to obtain a highly efficient anatase phase.
On the other side visible light-responsive TiO2 will be developed for increasing the photon flow able to be absorbed under solar irradiation. The aim is that of producing photocatalysts whose absorption extends to wavelengths less than 500 nm. It is known that TiO2 nanoparticles, doped with substances such as nitrogen or metallophthalocyanine dyes (MPc) show a high reactivity under visible light irradiation. MPc are potent TiO2-sensitizers enhancing the activity of titania by injecting electrons in its conduction band. The electronic transfer occurs through the change of the oxidation state of cations, such as Cu and Fe, that can shift alternatively from +1 to +2, and from +2 to +3, respectively.
On this basis different TiO2 samples, consisting of nanostructured particles, will be prepared by changing the starting precursors, Ti(iso-OC3H7)4 or TiCl4, and/or modifying some operative parameters like molar ratio of the reagents, heating and calcination temperatures and times. Moreover, in order to use the visible-light irradiation, N-doped and MPc-doped TiO2 nanoparticles will be prepared.
Bulk and surface characterisations of all the prepared catalysts will be carried out (not only by DICPM but also by the other operative units) by means of X-ray diffractometry (XRD), infrared spectroscopy (FTIR), determination of the BET specific surface areas, porosity measurements, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and diffuse reflectance spectroscopy (DRS) techniques. An electron microprobe used in an energy dispersive mode (EDX) will be also employed. Others electrochemical characterization techniques will be performed by the Operative Unit of Cagliari.
The photoactivity of all the prepared catalysts will be tested for the glycerol partial oxidation or reduction in aqueous phase. A batch photoreactor with immersed lamp will be used in order to determine the activity of the photocatalyst aqueous suspension. The influence of the following operative parameters:
i) glycerol concentration in water;
ii) catalyst amount in the suspension;
iii) concentration of dissolved oxygen;
iv) intensity of near-UV radiation (obtained by medium pressure Hg lamps); and
v) intensity of visible radiation (obtained by lamps simulating the solar spectrum)
on the photoprocess performance will be studied.
In the course of this part of investigation photoreactivity runs will be carried out with all the prepared catalysts for performing the reduction process of glycerol for the production of 1,3-propanediol. These experiments will be carried out in anaerobic conditions and by adding to the reacting solution a suitable oxidable species (hole scavengers); solvents different from water will be also tested.
Besides to testing the performance of catalysts prepared by the Palermo’s operative unit, the nanometric powders of Ti, Zn and Sn semiconductor oxides prepared by Cagliari’s OU in different ways will also be tested. In the first part of the research program, the cooperation with the Cagliari’s OU will be very intense as that unit will also produce catalysts by coupling semiconductors with different band gap values with the aim of increasing the photoactivity. Moreover, the OU of Torino will furnish information on the analytical techniques to be followed in order to completely characterize the products obtained from the glycerol partial oxidation and reduction.
The main results obtained in this first part of the investigation will be the determination of the catalysts which show the highest quantum yields for near-UV radiation absorption and the highest reaction rates in the case of visible irradiation. In the course of accomplishment of the research program, all the obtained results of photoreactivity will be communicated to the other operative units in order to have information on the photoreaction mechanism and kinetics (OU of Torino) and on the quantum efficiency of the photoprocess (OU of Bologna).
In the second part of this investigation program the best catalysts will be immobilised with different techniques on suitable supports for carrying out the glycerol oxidation or reduction in an annular continuous fixed bed photoreactor.
Thin films of TiO2 catalysts will be produced on the supports by following the same procedure used to prepare the pure catalysts. The procedure will be repeated in order to immobilize different layers on the supports (1 to 5 layers).
Composite films of titania with organically modified silicates (ORMOSIL) will be also produced. ORMOSIL thin films are commonly used as homogeneous films of organosilica with thickness up to 1-10 µm; they can be prepared by the sol-gel method with a single dip-coated layer. The presence of TiO2 enables fine tuning of the refractive index by varying the preparative conditions, which is of primary relevance in application of the films in continuous photoreactors; increasing the Ti/Si ratio results in enhanced hardness whereas the organosilica in the matrix improves the film’s flexibility enabling the preparation of crack-free films with thickness in the range of microns.
It has been recently reported that the entrapping of MPc dyes by sol-gel in micro and mesoporous silica determines monomeric dispersion of the molecules in the sol-gel cages. Such sol-gel molecular encapsulation ensures protection of the dopant molecules against both thermal- and photo-degradation, preserving the accessibility of the entrapped dopant to external reagents.
On this basis mixed titania/ORMOSIL films doped with phthalocyanines may result in versatile photocatalysts under visible light irradation. Organosilica-titania films doped with Cu(II), Fe(II) and Fe(III) phthalocyanines synthesized by mild sol-gel hydrolytic polycondensation of titanium and silicon alkoxides will be prepared and tested in the continuous photoreactor.
The performance of continuous photoreactor will be tested by changing the following operative parameters:
i) liquid flow rate;
ii) glycerol concentration in water;
iii) oxygen concentration in water;
iv) number of photocatalyst layers deposited on the different supports;
v) intensity of near-UV radiation; and
vi) intensity of visible radiation.
The results obtained with the continuous photoreactor will be spread to the other operative units in order to determine the optimal operative conditions of the whole photoprocess, also taking into account the possibility of coupling the photocatalytic reaction with the pervaporation operation, as investigated by the Bologna’s operative unit, that also on the base of the previous results will perform an economical evaluation of the process.