ZOL ŻEL, SOL GEL, AEROŻEL, AEROGELS, SILICA AEROGELS, AEROŻELE,SOL-GEL, CERAMICS


PREKURSORY DO PROCESW ZOL EL, SILANY, SILSESKWIOKSANY





PREKURSORY DO PROCESW ZOL EL, SILANY, SILSESKWIOKSANY














Skrcony opis projektu badawczego Brief description of the research project

 

The project aims to explore catalytic activity of immobilised transition metal complexes and to find its correlation with the nature of existing centres on the oxide carrier (support) surface binding them. Modified SiO2 aerogel systems will serve as a carrier model for surface metal complexes. Their modification will take place in the structure of a gel and on its surface. The support complexes will be obtained in the form of aerogels, using solvent extraction techniques with supercritical CO2. Support modification will be held in two phases - by modifying of its structure during preparation and by chemical modification of the surface reactive centres with the precursors of metal oxides, such as chlorides or alkoxides, to which metal complex will be anchored then. Modification of the oxides: TiO2, Al2O3, SnO2, B2O3, Fe2O3 (modifier, the second oxide component of the SiO2) in the phase of silica gel formation material allows for a well-defined porous structure, redox, acidic or basic centres and extended surface area.

Implementation of the project is based on the support obtaining by using the sol-gel method combined with supercritical drying technique. Using aerogel support with well-defined composition and structure, developed specific surface area and ensuring durable immobilization of metal complexes is to give a catalyst with the desired properties: selectivity, resistance to poisoning and leaching of the complex, characterized by high activity. In a further step thus obtained catalysts will be tested in a series of model reactions - hydrosilylation, coupling and silylating hydroformylation of vinylsilanes. Then it is planned to seek a correlation between the way a modification of catalytic activity obtained complexes (determined by the parameter TOF), with particular emphasis on the acid-base nature of the support modified through the introduction of the second oxide component to the structure of aerogels.

Using the knowledge and experience of project participants, the proposal call the method of preparation of oxide materials by sol-gel technique and preparation of highly active supported catalysts containing immobilised transition metal complexes. The resulting catalysts are to be characterized by high selectivity and coupling reactions of silylating hydroslilyltion, good wettability by solvents used as reaction medium and stability in a series of reaction cycles.

The ability to design more efficient and environmentally friendly chemical technology is an important aspect of catalytic processes with low environmental impact. To achieve this goal catalysts should yield easy recovery without contamination or leaching to the final product. For this reason, heterogeneous catalysis plays a dominant role in industrial products due to the simplicity of the catalyst recovered by filtration or decanting.

These assumptions can be met by introducing various improvements to existing processes of the chemical compounds synthesis. In the case of the proposed project objective is to be achieved by using two complementary strategies. First, the formal change of catalytic process, as achieved by the immobilization of metal complex on the support surface, a transfer process from an area of ​​homogeneous catalysis to heterogeneous catalysis. Is should be done maintaining high catalytic activity, characteristic for homogeneous catalysis. The second modification is to improve the methods of preparing the supports of immobilized complexes, using the specialized methods for porous materials. To support this, support synthesis route based on sol-gel processes was chosen.

Preparation of well-defined heterogenised catalysts comprehensive links with both the knowledge of how his bond with the surface of the support, as well as with obtaining support on a previously established properties, such as specific porous structure, the presence of some active centers and appropriate development of specific surface area.

Combining this knowledge in the manner described in the project is to help understand the influences of the catalyst with active sites, which are metal complexes immobilized on its surface. The importance of this knowledge may provide a continuously growing interest in well-defined catalysts based on transition metal complexes. In previous studies, most of these complexes obtained on oxide materials, mainly silica, were used as catalysts in anhydrous solvents, which were not subjected to the processes of surface modification before or after the immobilization of the complex. Synthesis design based on sol-gel technology and the ability to control the process parameters provide material with specific and desired by the subsequent use properties, such as high selectivity, resistance to poisoning, permanent immobilization, high activity described by the parameter TOF (Turn Over Frequency).

In the case of a positive outcome of the planned research one can assume its great practical importance. It is expected that the proposed catalytic systems will be important to streamline many processes of organic or organometallic compounds synthesis, and in particular in reducing the unit cost of production by being able to recycle the expensive noble metal-containing catalyst for further reaction cycles.

 

1. Cel naukowy projektu Scientific goal of the project

 

The project aims to explore catalytic activity of immobilised transition metal complexes and to find its correlation with the nature of existing centres on the oxide carrier (support) surface binding them. Modified SiO2 aerogel systems will serve as a carrier model for surface metal complexes. Their modification will take place in the structure of a gel and on its surface. The support complexes will be obtained in the form of aerogels, using solvent extraction techniques with supercritical CO2. Support modification will be held in two phases - by modifying of its structure during preparation and by chemical modification of the surface reactive centers with the precursors of metal oxides, such as chlorides or alkoxides, to which metal complex will be anchored then. Modification of the oxides: TiO2, Al2O3, SnO2, B2O3, Fe2O3 (modifier, the second oxide component of the SiO2) in the phase of silica gel formation material allows for a well-defined porous structure, redox, acidic or basic centres and extended surface area.

 

2. Znaczenie projektu The importance of the project

 

The ability to design more efficient and environmentally friendly chemical technology is an important aspect of catalytic processes with low environmental impact. To achieve this goal catalysts should yield easy recovery without contamination or leaching to the final product. For this reason, heterogeneous catalysis plays a dominant role in industrial products due to the simplicity of the catalyst recovered by filtration or decanting.

These assumptions can be met by introducing various improvements to existing processes of the chemical compounds synthesis. In the case of the proposed project objective is to be achieved by using two complementary strategies. First, the formal change of catalytic process, as achieved by the immobilization of metal complex on the support surface, a transfer process from an area of ​​homogeneous catalysis to heterogeneous catalysis. Is should be done maintaining high catalytic activity, characteristic for homogeneous catalysis. The second modification is to improve the methods of preparing the supports of immobilized complexes, using the specialized methods for porous materials. To support this, support synthesis route based on sol-gel processes was chosen.

Preparation of well-defined heterogenised catalysts comprehensive links with both the knowledge of how his bond with the surface of the support, as well as with obtaining support on a previously established properties, such as specific porous structure, the presence of some active centres and appropriate development of specific surface area.

Combining this knowledge in the manner described in the project is to help understand the influences of the catalyst with active sites, which are metal complexes immobilized on its surface. The importance of this knowledge may provide a continuously growing interest in well-defined catalysts based on transition metal complexes. In previous studies, most of these complexes obtained on oxide materials, mainly silica, were used as catalysts in anhydrous solvents, which were not subjected to the processes of surface modification before or after the immobilization of the complex. Synthesis design based on sol-gel technology and the ability to control the process parameters provide material with specific and desired by the subsequent use properties, such as high selectivity, resistance to poisoning, permanent immobilization, high activity described by the parameter TOF (Turn Over Frequency).

In the case of a positive outcome of the planned research one can assume its great practical importance. It is expected that the proposed catalytic systems will be important to streamline many processes of organic or organometallic compounds synthesis, and in particular in reducing the unit cost of production by being able to recycle the expensive noble metal-containing catalyst for further reaction cycles.

 

3. Istniejcy stan wiedzy w zakresie tematu bada Existing state of knowledge in the research topic

 

In the context of a well-defined surface complexes, a description of the phenomena that occur between the surface of the support associated coordination compound becomes relevant. Current research focuses mainly on the structure of surface complexes and there are few data on the effects of the interaction between a complex and a support. Description of the nature of these interactions is essential not only during surface species formation, but probably becomes more important when talking about the catalytic activity and selectivity of the resulting catalyst. To get an answer about the nature of complex interactions-carrier, looking into a model systems with well-known properties is useful. Preparation of model support is associated with the fulfilment of several requirements for that product. The carrier should have a well defined composition, porous structure and be well wetted with solvent used as reaction medium. To meet these conditions one should therefore use the method of synthesis of the support that ensures fulfilment of all these requirements.

Sol-gel technique is a widely used tool in obtaining the materials under carefully controlled, mild conditions of synthesis. The resulting gel has to undergo a solvent removal process from its mass and the bulk or other words, drying. Removing solvent from the pores of gel material using supercritical drying technique allows, by compensating for the pressure, to prevent collapse of the structure of the porous material [1]. Carbon, Al2O3 and Cr2O3 aerogels were studied in a wide range of processes in which they were used directly as a catalyst or a carrier of active phase in processes such as hydrogenation, dehydrogenation, isomerisation, reforming, three-way catalyst converters, selective oxidations, NOx abatement, CO or CO2 hydrogenation, VOC degradation, enzyme aerogel-encapsulated [2,3]

 

The project manager has pursued a number of tasks in research projects in which the major role played the sol-gel processes using alkoxylate precursors. As part of the work previously succeeded in developing several innovative methods of synthesis of homogeneous supports and binary oxide catalysts. The tested materials were based on Al2O3 and SiO2 matrix modified with magnesia [4], chromia [5], boria [6], tin [7,8] and germania [9].

Previous studies on the nature of supports obtained by sol-gel methods, with particular emphasis on metal-support interaction, allowed a convenient method of preparation and control of the composition and homogeneity. Carriers obtained in this manner were used for the preparation of catalysts with highly dispersed metallic phase. [10] Positive results of this work, in particular, obtaining valuable results describing the stabilization of highly dispersed metallic phase (platinum), justify the statement that the sol-gel method allows for model support, not only for the metallic phase, but also for the compounds of the complex nature.

Oxide materials prepared by sol-gel method are characterized by different physicochemical properties compared to manufactured using standard methods such as co-precipitation and thermal decomposition of precursor. In addition, the sol-gel method allows for a strict monitoring conditions for the introduction of the second component, contributing to a homogenic dispersion in a gel network or between. In the case of standard methods of synthesis, for example, fused silica, there is great difficulty in introducing and even distribution of the second component, and thus, it is not easy to obtain material of planned and desirable physicochemical properties from the catalytic point of view.

Use a carrier material in the form of aerogels is dictated by a number of its physical properties (porous structure, density, specific surface), which predisposes him as an excellent material in catalysis. Aerogels have widely documented use both as catalysts and supports for the catalytically active phase, and in particular works on this issue of Pajonk [11], Baikera [12] and Moreno-Castilla [13] should be mentioned here.

To wide the range of applications, silica aerogels are made hydrophobic by replacing the surface hydroxyl groups by alkyl or aryl groups using hydrophobic reagents like methyl trimethoxysilane (MTMS), trimethylchlorosilane (TMCS) and trimethyl ethoxysilane (TMES) [14]. Despite the strong interest of aerogel materials, there are not many reports of their use as carriers for complex compounds heterogenisation. Therefore, the proposed study, which was planned not only the use of silica aerogels, but also its comprehensive modification, may be an interesting and valuable step in that direction.

Currently, a number heterogenised silica transition groups metal complexes and, germanium and tin is known. All of them are obtained by the reaction of ligand substitution with silanol groups located on the surface of silica. As a continuation of professor Bogdan Marciniec group experience with soluble rhodium and iridium siloxide complexes, our recent studies are focused on immobilization of organometallic rhodium derivatives on silica surface and their application to catalysis [15].

Precursors of surface complexes must be able to react with support surface groups, mostly with the superficial hydroxyl groups. In the case of research on the immobilisation of the metal complexes, the most commonly used support include silica materials with well-defined physical and chemical parameters (surface area, acidity), presenting numerous and reactive hydroxyl groups on the surface.

 

 

Figure 1. The creation of complex surface to form a single core complex of rhodium phosphine (1a) and the dual complex of rhodium (1b).

 

As a result of research carried out by applicants of the project on the use of commercial AEROSIL silica support, results obtained suggest that heterogenised siloxide complexes (Scheme 1) may exhibit activity comparable to commercial homogeneous catalysts. Siloxide rhodium complexes, particularly binuclear ones (Scheme 1b) with siloxide bridged ligand, appeared to be much more effective than the respective chlorocomplex (as well as the commercially available Pt-Karstedt catalyst) in hydrosilylation of a variety of olefins such as 1-hexene [16], vinylsilanes [17] and allyl alkyl ethers [18-20] as well as in the silylative coupling [21] and hydroformylation of vinylsilanes [22]. Siloxide complexes including terminal and/or bridging siloxy ligands are regarded as a good molecular model of metal complexes immobilized on silica surface [23-25].

 

 

 

Scheme 2. Mechanism of heterogeneous catalysis of hydrosilylation by surface rhodium (diene) siloxide complex.

Literature

 

1. Catalyst preparation: Science and Engineering / edited by John Regalbuto, CRC, (2006).

2. G.M. Pajonk, Catal. Tod. 52, 3, (1999).

3. G.M. Pajonk, Recent Res. Devel. Catalysis 2, 1, (2003).

4. P.Kirszensztejn, R. Przekop, A. Szymkowiak, E. Mackowiak, J. Gaca Micro and Mesoporous Materials 89, 150-157 (2006).

5. Work accepted for publication, Annales UMCS (2011).

6. A.Martya, B.Olejnik, P.Kirszensztejn, R.Przekop, International Journal of Hydrogen Energy, 36, 14, 8358-8364, (2011).

7. P. Kirszensztejn, A. Szymkowiak, P. Marciniak, A. Martya, R. Przekop, Applied Catalysis A: General 245,159-166, (2003).

8. P. Kirszensztejn, A. Kawako, A. Toliska, R. Przekop, Journal of Porous Materials 18, 2, 241-249 (2011).

9. P. Kirszensztejn, P. Marciniak, A. Szymkowiak, R. Przekop, K. Jurek, Polish Journal of Chemical Technology, 8, 2, 115-117, (2006).

10. P. Kirszensztejn, A. Szymkowiak, R. Przekop, E. Mackowska, Polish Journal of Environmetal Studies, 15, 6A, 74-80, (2006).

11. G. M. Pajonk, Catal. Today, 35, 319337, (1997).

12. M. Schneider, A. Baiker, Catal. Today, 35, 339365, (1997).

13. C. Moreno-Castilla, F. J. Maldonado, Carbon, 43, 455465, (2005).

14. L. W. Hrubesh, Journal of Non-Crystalline Solids 225, 335 (1998).

15. B. Marciniec, K. Szubert, R. Fiedorow, I. Kownacki, M.J. Potrzebowski, M. Dutkiewicz, A. Franczyk,Journal of Molecular Catalysis A: Chemical, 310 (1-2), 9-16, (2009).

16. Besson B., Moroweck B., Smith A. K., Basset J. M., Psaro R., Fusi A., Ugo R., J. Chem. Soc., Chem. Commun., 569, (1980).

17. B. Marciniec, P. Baejewska-Chadyniak, I. Kownacki, K. Szubert, Pol. Pat. Apl. P-368 485 (2004).

18. B. Marciniec, E.Walczuk, P. Baejewska-Chadyniak, D. Chadyniak, M. Kujawa-Welten, S. Krompiec, Organosilicon Chemistry V from Molecules to Materials,Verlag Chemie, (2003).

19. B. Marciniec, P. Baejewska-Chadyniak, E. Walczuk-Guciora, M. Kujawa-Welten, Pol. Pat. 194667 (2001).

20. B. Marciniec, in: A. M. Trzeciak (Ed.), Perspectives of Coordination Chemistry, Pozna-Wrocaw, 195214, (2005).

21. B. Marciniec, E. Walczuk-Guciora, P. Baejewska-Chadyniak, J. Mol. Catal. A:Chem. 160, 165171, (2000).

22. E. Mieczyska, A. M. Trzeciak, J. J. Zikowski, I. Kownacki, B. Marciniec, J. Mol. Catal. A: Chem. 237, 246253, (2005).

23. B. Marciniec, H. Maciejewski, Coord. Chem. Rev. 223,301335, (2001).

24. F. J. Feher, T. A. Budzichowski, Polyhedron 14, 32393253, (1995).

25. P. T. Wolczanski, Polyhedron 14, 33353362, (1995).

 

4. Koncepcja i plan bada The concept and plan of research

 

Implementation of the project is based on the support obtaining by using the sol-gel method combined with supercritical drying technique. Using aerogel support with well-defined composition and structure, developed specific surface area and ensuring durable immobilization of metal complexes is to give a catalyst with the desired properties: selectivity, resistance to poisoning and leaching of the complex, characterized by high activity. In a further step thus obtained catalysts will be tested in a series of model reactions - hydrosilylation, coupling and silylating hydroformylation of vinylsilanes. Then it is planned to seek a correlation between the way a modification of catalytic activity obtained complexes (determined by the parameter TOF), with particular emphasis on the acid-base nature of the support modified through the introduction of the second oxide component to the structure of aerogels.

Using the knowledge and experience of project participants, the proposal call the method of preparation of oxide materials by sol-gel technique and preparation of highly active supported catalysts containing immobilised transition metal complexes. The resulting catalysts are to be characterized by high selectivity and coupling reactions of silylating hydroslilyltion, good wettability by solvents used as reaction medium and stability in a series of reaction cycles.

To realize these objectives, a series of research activities, that form a sequence which consists of a consecutive preparation stages of the support and its functionalisation will be taken:

 

1.      Obtaining of a structurally modified aerogel silica supports. Obtaining material with a highly developed surface area, large pore volume and a surface susceptible to modification.

2.      Modification of SiO2 gel. Introduction of other oxide components to the gel structure, such as TiO2, Al2O3, SnO2, B2O3, Fe2O3. By introducing the second oxide component the effect of nature change of the surface centres and consequently a change of the surface interactions with a rhodium complex be achieved.

3.      Modification of silica carrier using grafting technique of surface hydroxyl groups with alkoxylates or metal chlorides (Fig. 2), that produces changes in the electron density distribution, acid-base or redox centres, and consequently change the geometry of the resulting complex.

4.      Support surface hydrophobisation after the creation of a catalytic complex will gain increasing contact angle of the carrier by the non-polar solvents such as hexane, ethylbenzene, toluene, which is advantageous from the point of view of the target application of these catalytic systems.

5.      Activity and selectivity tests of the resulting materials.

 

Figure 2. Modification of surface hydroxyl groups by grafting of reactive metal chlorides and metal alkoxylates.

 

Figure. 3. Summary of unit operation for obtaining immobilized metal complexes.

 

1. Aqueous dispersion of TEOS (tetraethoxyorthosilicone) by decreasing the pH.

2. Rise of the pH to increase the degree of gel condensation.

3. Addition of the metal alkoxide to silica gel and co-gelation introduction of a structure modifier.

4. Obtaining of the unmodified gel.

5. Solvent replacement - water removal from the gel.

6. Removal of organic solvent from the gel structure by a supercritical CO2 extraction resulting in formation of aerogel.

7. Thermal processing of aerogels, removal of residual organic functional groups and formation of the final support structure.

8. Grafting alkoxylates or chlorides to the surface hydroxyl groups.

9. Formation of a superficial rhodium complex.

10. Modification of the support surface (hydrophobisation) with functionalised silanes.

 

The use of modified aerogel support allows to control structural change, through variation of the size and shape of pores in order to match the surface sites geometry between the carrier and the metal complex. In addition, during the gel synthesis of SiO2, with appropriate precursors, titania, alumina, tin and iron oxides will be introduced as a structural matrix modifiers.

Immobilization of the complexes is carried out either by creating a lasting bond between the metal complex and the OH groups present on the carrier surface, and with the help of the weak interaction, electrostatic interaction and the use of unsaturated bonds of complexes (donor-acceptor bonding) to the modified surface of the support groups. For the latter type of immobilization in these complexes it is important to find the strength of such complex - support interaction to ensure a proper immobilization in the environment of catalytic reaction. Following the adoption of a strategy for both the permanent immobilization of the complex with bonds to the surface of the support and the weak interactions of electrostatic type, further research task will be the behaviour and stability of rhodium complexes in the systems investigated, with particular emphasis on sustainability in the reaction. It is important for ensuring a strong bonding between the obtained supports and complexes used. The strength of the complex - carrier interactions will depend on the complex precursor type, or the presence of certain functions in the complex, such as amino, carboxyl or epoxy groups.

Simultaneously, the support interaction will be determined by the type admixtures introduced with second component to the aerogel oxide matrix. Within the project the authors intend to investigate several metal oxides commonly used as carriers of heterogeneous catalysts: TiO2, Al2O3, SnO2, Fe2O3 and B2O3.

Catalytic system heterogenisation is obtained by binding the active complex to the surface of the carrier, so there is a possibility of separating the insoluble catalyst from the mixture solution of substrates and products after reaction completion and its return back to the catalytic reaction cycle. So, ultimately obtained catalyst systems will be tested in a number of model reactions (Scheme 3a-d).

 

 

a)       hydrosilylation of allyl glycidyl ether with triethoxysilane

 

 

b)       hydrosilylation of 1-hexadecene with 1,1,1,3,5,5,5-heptamethyltrisiloxane

 

 

 

c)       1-octene hydrosilylation with poly(methylhydrogen) dimethylsiloxane

 

 

d) hydrosilylation of 4-allyl-1,2-dimethoxybenzene with 1,1,1,3,5,5,5-heptamethyltrisiloxane

 

 

 

e) Silylating phenylodimethylvinylosilane coupling with styrene

 

Scheme 3. The proposed test reactions.

 

 












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