Supported catalyst

The presented slides gives brief description about supported catalysis process, its mechanism, advantages with examples. The given presentation also helpful for those who are seeking for details on preparation of supported catalysts with different types of catalysis process. This presentation also covers some applications of supported catalysts use in various chemical reactions.

1. Supported catalysis Presented By: Ekta P. Tembhare Department of Pharmaceutical Chemistry

2. Introduction In chemistry, a catalyst support is the material, usually a solid with a high surface area, to which a catalyst is affixed Usually supported catalysts in view of better control of properties of catalyst Surface area and loading of the support with the active component as well as the distribution of the active component over the surface of the support determining • Extent of catalytically active surface area per unit volume • Thermo stability together with the interaction of the active component with the surface of the support The activity of heterogeneous catalysts and nanomaterial-based catalysts occurs at the surface atoms

3. Consequently , great effort is made to maximize the surface area of a catalyst by distributing it over the support The support may be inert or participate in the catalytic reactions Typical supports include various kinds of carbon, alumina, and silica

4. Examples: Process Reactants, product(s) Catalyst Support Ammonia synthesis (Haber–Bosch process) N2 + H2, NH3 iron oxides alumina Hydrogen production by steam reforming CH4 + H2O, H2 + CO nickel K2O Ethylene oxide synthesis C2H4 + O2, C2H4O silver with many promotors alumina Ziegler–Natta polymerization of ethylene propylene, polypropylene ; ethylene, polyethylene TiCl3 MgCl2 Desulfurization of petroleum (hydro-desulfurization) H2 + organosulfur compounds, RH + H2S Mo-Co alumina

5. Importance: stabilization of high/ optimal dispersion of active component(s) (e.g. of noble metal particles) against sintering reduction of costs utilization of important mechanical and morphological properties of the support General procedure : deposition of the precursor onto the support surface transformation of the precursors into the required active compound (oxide, sulfide, metal)

6. Dispersion = n(s) / N n(s ) = Number of surface atoms N = total number of atoms in the cluster/particle D = f (particle shape)

7. Preparation of supported catalysts Supported catalysts are prepared by deposition of the active metal on the support materials. The main purpose of using a support is to achieve an optimal dispersion of the catalytically active component and to stabilize it against sintering Methods Deposition Impregnation Grafting Transformation Ion exchange Dry impregnation Wet impregnation

8. 1. Impregnation A. Dry Impregnation In this method, a previously dried support is contacted with volume of solution equal to its pore volume The solution contains the required amount of the precursors of the active phase As soon as the support is placed in contact with the solution, the solution is drawn into the pores by capillary suction No excess solution remains outside the pore space

9. Part of the air present in the pores is imprisoned and compressed under the effect of capillary forces The pressure developed inside the imprisoned gas bubbles depends on the radius, r, of the curve of the liquid -gas meniscus and may reach several MPa when r < 100 nm as a result of Young - Laplace law, ∆ P = P - P = , where γ is the liquid- gas interfacial tension  

10. Mechanism In addition to diffusion and adsorption processes, another phenomenon occurs, which is the pressure driven capillary flow of the solution inside the empty pores This can be represented by Darcy’s law . An important parameter from introduction of Darcy law is the solution viscosity ‘μ ’ In case of aqueous solution and in the common range of concentration used for impregnation, viscosity increases almost proportionally with concentration

11. B. Wet/ diffusional impregnation In this method, the pore space of the support is first filled with the same solvent as used in the impregnating precursor solution The wetted support is then treated with the impregnating precursor solution Here the actual impregnation takes place in diffusional condition when solvent filled support is dipped in the precursor solution

12. The first phase of saturation of the support by solvent involves the characteristics of dry impregnation But in the second phase, when solvent saturated support is added to the impregnating solution, high pressure is not developed within the pores Wet impregnation should be avoided when the interaction between the precursors and the support is too weak to guarantee the deposition of the former

13. Mechanism In wet impregnation, the distribution of the solute inside the pores is assumed to be governed by two phenomena's, Diffusion of the solutes within the pores. It is described by Fick’s law Adsorption of the solute onto the support. This depends on the adsorption capacity of the surface and on the adsorption equilibrium constant Drying : Impregnation is followed by elimination of the solvent. The impregnated sample is heated in an oven in a flow of gas

14. 2. Ion exchange Ion exchange consists of replacing an ionic species by another ionic species in electrostatic interaction of the precursors with the surface of a support - The support containing the ion A is dipped into an excess volume (compared to the pore volume) of a solution containing ion B that is to be introduced - Ion B gradually penetrates into the pore space of the support and takes the place of ion A, which passes into the solution - This continues until equilibrium is established corresponding to a given distribution of the two ions between the solid and solution

15. Mechanis m Single ion exchange takes place when in the solid-solution system only two ions interact If ion A+ on the solid Z is to be replaced by ion B+ present in the solution then in the simple case of two monovalent cations , the exchange equilibrium can be written as, Az ⁺ + Bs ⁺ Bz ⁺ + As⁺ The subscripts ‘s’ and ‘z’ represent solution and solid, respectively .

16. In case of ideal exchange, C AZ = concentration of ion A⁺ in the solid C AS = concentration of ion A⁺ in the solution C BZ = concentration of ion B⁺ in the solid C BS = concentration of ion B⁺ in the solution C AZ + C BZ = C z - total conc. Of cations in solid C AS + C BS = C s - total conc. of cations in solution Exa. Na+ ions in NaY zeolites can be replaced by NH4⁺ ions.  

17. 3. Grafting Formation of strong (covalent) bonds between support (polymer or inorganic solid) and metal complexes (in the very first step of immobilization ) Exa . Chemical reaction between functional groups ( e.g.OH ) of the surface and the precursor compound   Typical precursors- 1. Metal halides and oxyhalides 2. Metal alkoxides and amides 3. Metalorganic compounds (e.g. alkyl-, allyl complexes) or metal carbonyls

18. Advantages : • highly dispersed metal ions at a (previously) dehydrated and partially dehydroxylated (oxide) surface • hindering of migration, agglomeration and sintering during subsequent thermal Treatment • control of (metal) loading (by degree of dehydroxylation , multiple grafting)

19. Applications of supported catalysis 1. Enantioselective additions to aldehyde and imines 2. Diels-Alder reactions 3. Michael addition reactions- Aluminium containing catalyst use in the reactions of thiols , amines and nitromethane 4. Hydroformylation - Silica supported rhodium catalyst used in the supercritical carbon dioxides in a continuous hydroformylation process 5. Miscellaneous C-C bond forming reactions 6. Olefin metathesis- Ruthenium carbene complex attach to polymeric support

20. Thank you.