RICERCA ITALIANA     

Research program

New redox catalysts for new reactor technologies.

University Co-ordinator

Università degli Studi di NAPOLI "Federico II" - CHIMICA - NAPOLI(NA)

Research Unit Leader

Elio SANTACESARIA

Description

This research program will be developed following three different goals that are: (i) to individuate the best preparation method for obtaining by grafting new catalysts active and selective in the oxidative dehydrogenation (ODH) of alkanes of low molecular weight, such as for example, propane and butane to respectively propene and butene; (ii) to find the best catalysts prepared by grafting active and selective in the oxidation at low temperature of methanol to formaldheyde, (iii) to use the grafting technique for preparing the active part in new catalytic reactors such as membrane reactors, redox-decoupling reactors and honeycomb reactors. Our group will also made an activity of reactor modeling on the different mentioned reactors for comparing their performances with respect to the ones of the conventional tubular reactors. Therefore, this research pursues both scientific and technologic objectives. One technological objective, for example, is related to the development of a simple and effective process, of industrial interest, for obtaining isobutene from butane through the reactions butane-butene-isobutene as the second step has already been developed successfully by our group. Another technological objective is the realization by grafting of membrane reactor or alternative redox reactor in collaboration with other research groups. The scientific objective, on the contrary, is related to the investigation of new preparation methods to obtain catalysts that are able to promote the before mentioned reactions. It is obvious that catalysts particularly active and selective for the mentioned reactions could be used for many other processes of industrial interest, in particular catalysts, having redox properties, can usefully be used for other ODH reactions. This research will be devoted essentially to a unique preparation method based on the grafting of metal alkoxides on the surface of oxides. This technique will be used not only for preparing catalysts for the mentioned reactions but also for supports having a surface with chemical properties more favourable to the catalytic action. The research on the butane oxidative dehydrogenation will be initiated with the preparation and study of vanadium based catalysts obtained by grafting vanadyl-tri-isopropoxide dissolved in a solvent, that could be polar or apolar, and put in contact with the surface of different oxides, commonly used as supports (silica, alumina, zirconia, titania, magnesia etc.). It will be possible, in this way, to evaluate the effect of both the solvents and the chemical environment surrounding the active site, on the catalysts performances. The opportunity of the choice of vanadium as a starting base for the catalysts is justified by both the examination of the literature and the previous activity developed by the proponent on the same field. The scientific literature has shown that vanadium pentoxide supported by impregnation, is a promising catalyst for the ODH of low molecular weight alkanes to give the corresponding alkenes, while, the proponent research team has demonstrated that activities and selectivities of vanadium catalysts, prepared by grafting vanadyl-tri-isopropoxide on the surface of an oxide, can be strongly affected by following different grafting procedures. The research will be oriented, therefore, to interpret and to deep some of the observed aspects and new features that will be found during the research activity, with the aim of improving both activity and selectivity of the catalysts in the ODH of propane and butane. From the literature examination, it is possible to derive important information, such as, for example the fact that silica is not a good support for vanadium based catalysts because the interaction between vanadium oxide and silica surface is weak and vanadium oxide agglomerates easily and this favours the formation of low dispersed catalysts containing crystalline vanadium oxide, that has low activity. The interaction of vanadium oxide with titania surfaces is much stronger and catalysts obtained, in this case, are more dispersed, and normally, vanadium oxide covers TiO2 surface with a monolayer. Only in the presence of a great amount of charged vanadium multilayered structures can be formed and crystalline V2O5 appears. TiO2 environment resulted particularly favourable to the catalytic action of vanadium in different reactions, as for example, the Selective Catalytic Reduction (SCR) of NOx with NH3. For this reason it has been proposed by different authors in the literature to prepare supports of silica coated with TiO2 by grafting titanium alkoxides on silica, creating, in this way, a support that has a chemical surface very favourable to the successive grafting of vanadium tri-isopropoxide and to the preparation of catalysts of high performances, because, retain the elevated specific surface of the original silica support. Our research started, therefore, with the preparation and the study of catalysts prepared by grafting vanadium tri-isopropoxide on a TiO2/SiO2 support, where SiO2 is coated with a multilayer of TiO2, obtained by grafting titanium alkoxide on silica in three different successive steps. Any step of grafting is, obviously, followed by both steaming, for eliminating residual alkoxide groups by hydrolysis, and calcination. In successive experiments we noted that in the case of incomplete TiO2 coating, that is, by realizing a coating corresponding to a sub-monolayer, the catalysts obtained by grafting vanadyl tri-isopropoxide on this kind of support are less active but more selective in the ODH of propane and isobutane than the ones using the TiO2/SiO2 support characterized by a multilayer of TiO2 coating. It is clear that vanadyl tri-isopropoxide on the described supports preferentially interacts with the TiO2 islands formed on the surface and has, therefore, better redox and acid-base properties. The same advantageous situation can be realized also by preparing a vanadium-titanium mixed alkoxide compound by hydrolyzing with a stoichiometric amount of water one of the two alkoxides, for example titanium alkoxide and by reacting, then, the obtained compound with vanadyl tri-isopropoxide, at last, by grafting directly on silica, the product of the reaction. The same result can be obtained simply by mixing the two alkoxides and reacting them with a stoichiometric amount of water that will be enough for inducing agglomeration but without producing precitation. In both cases, an excess of TiO2 has been used in the preparation, constituting the favourable chemical environment for the catalytic action of the vanadium pentoxide. The concept that we desire to develop is to prepare directly the precursor for both the active site and the favourable chemical environment to be grafted on a suitable support. Moreover, the described methods of preparation are simpler, cheaper and more effective than that in which a TiO2/SiO2 support must, preliminarily, be prepared with a multi-step grafting procedure. Some of the preliminary results achieved on the ODH of propane and isobutane are very promising. We desire, therefore, to improve the experimented techniques based on the partial hydrolysis of vanadyl tri-isopropoxide, in the presence of alkoxides of other elements modifying the redox and acid-base properties of vanadium before grafting this element on the surface of different oxides. Many problems must be solved in order to do this as, for example, to increase the catalytic activity by increasing the amount of vanadium charged; another problem is to do the hydrolysis of the alkoxides in a controlled way to avoid vanadium precipitation from the solution, expecially in the presence of aprotic and apolar solvents.
The research program will be developed by following, first of all, an investigation on the effect of the surface of the support (silica, alumina, zirconia, magnesia, etc.) on both the behaviour in the grafting of vanadyl-tri-isopropoxide and the performances of the corresponding catalysts in the mentioned reactions. In the meantime, it will be studied the hydrolysis of both vanadyl-tri-isopropoxide and titanium alkoxide alone or in mixtures, in different solvents respectively polar or apolar, in order to evaluate the different rates of hydrolysis and the characteristics of the induced aggregation. The alkoxide of other elements will, then, be introduced, as for example, the magnesium alkoxide with the scope of decreasing the acidity of the vanadium oxide clusters grafted on the surface. It is known, at this purpose, that basic properties have a positive role in the ODH of alkanes of low molecular weight, because formed alkenes are not retained by the surface catalyst as it occurs in the case of more acid catalysts. Consequently, the successive undesired reactions to CO and CO2 are limited and selectivity increases. Alkoxides of other elements increasing or decreasing the redox properties of vanadium will be individuated and added. The final scope is to become able to change in a tailored way both acid-base and redox properties of the catalytic precursor. This experience could, then, extended to other components different from vanadium. The research of the more suitable alkoxide precursor, containing two or more elements, will be made by modifying, when necessary, each alkoxide in order to obtain it in a form soluble in the solvent used for the grafting. The choice of the solvent is very important for the preparation considering that commercial alkoxides are often available dissolved in the parent alcohol. These solutions sometime are not suitable for the grafting reaction, because, the excess of the solvent alcohol has a negative influence on the reaction as a consequence of the following equilibrium:
Support –OH + Me(OR)n = Support –O-Me (OR)n-1 + ROH
In order to have higher grafting yields and consequently higher amounts of the active part anchored on the surface of the support it is necessary to transfer the alkoxide in a different solvent, preferably aprotic, polar or apolar. A method for increasing the solubility of an alkoxide in an apolar solvent, for example, is to exchange the alkoxide group, bounded to the metal, with another having a longer alkylic chain. Alkoxides obtained in this way have tenside properties and can, therefore, show singular behaviour during the grafting reaction with consequences more or less important on the catalytic performances. This unknown aspect will also be studied. The information acquired in the preparation and testing of vanadium based catalysts will, then, be used for preparing with the same methods catalysts containing also other elements such as molybdenum, wolframium and in particular chromium that recently has been succesfully used in the ODH of isobutane.
Prepared catalysts, in the different described ways, will be tested in the ODH of propane, butane and methanol. Catalysts giving the best performances in the model reactions will be submitted to an extended characterization with the help of all the other R.U. of the project.
Our research team will also be involved in a modeling activity concerning membrane reactors, honeycomb reactors and redox-decoupling reactors with the aim of a comparison with the traditional tubular plug flow reactor for testing the most convenient performances. This activity will be made in collaboration with other research teams working on the specific topic.