Volume 1 of JACS: 1879 Volume 1 of Chem. Mater.: 1989

The new motto (80s) has been: Better Ceramics Through Chemistry

Publications in the sol-gel materials field

25 YEARS OF SOL-GEL RESEARCH: CONTRIBUTIONS TO CHEMISTRY David Avnir Institute of Chemistry The Hebrew University of Jerusalem 75 th Meeting of the Israel Chemical Society Tel-Aviv,

Better Ceramics Through Chemistry

The motto of this lecture: # What has chemistry gained from sol-gel research # How do solid matrices affect a chemical reaction? # What can be done in heterogeneous chemistry that homogeneous reactions cannot achieve?

My main partners over the years Jochanan Blum Sergei Braun Ovadia Lev Daniel Mandler Sharon Marx Michael Ottolenghi Renata Reisfeld

1.The Heterogeneous environment: Silica

Synthesis of silica by the sol-gel polycondensation Si(OCH 3 ) 4 + H 2 O (SiO m H n ) p + CH 3 OH Variations on this theme: –the metals, semi-metals and their combinations –the hydrolizable substituent –the use of non-polymerizable substituents –organic co-polymerizations (Ormosils) –non-hydrolytic polymerizations H + or OH -

Controlled nanoporosity and cage geometry Surface area and pore volume of silica as a function of pH and water/silane ratio Y. Polevaya, M. Ottolenghi, D. Avnir, J. Sol-Gel Sci. Tech., 5, 65-70, (1995)

SolGel Xerogel SolGel Xerogel sol - particle Entrapped species monomer oligomer - Organic heterogenization Physical entrapment of molecules within sol-gel matrices * Small molecules * Polymers * Proteins * Nanoparticles Monomers, oligomers The concept is general and of very wide sc ope

Important property: Reactivity is possible with the entrapped species Physical entrapment vs covalent entrapment

Matrix parameters which can affect chemical reactivity: # The fixation by entrapment within the matrix # The confined environment of cages and narrow pores # The porosity # The chemical modification of the matrix # The co-entrapment of a surfactant

2. Affecting reactivity by the entrapment * Hydrophobic catalysts in water * The back-reaction problem in energy storage * One-pot reactions with mutually destructive reagents

Using hydrophobic catalysts in water With F. Gelman. J. Blum, D. Avnir J. Molec. Catal., A: Chem., 146, 123 (1999) ee = 78% (BPPM) The advantages of sol-gel entrapment # Reactivity in incompatible solvents # Covalent bonding chemistry is not needed # Recyclability and separation

Electron transfer Py Light Py* - the donor Py* + MV 2+ - the acceptor MV.+ + Py + 2MV.+ + 2H 3 O + 2MV 2+ + H 2 + 2H 2 O The classical problem: MV.+ + Py + MV 2+ + Py The problem of back-reaction in energy storage Energy storing pair Useful reaction back-reaction

+ TV 2+ Four hours, 5% yield of separated pair The solution: I. Separate spatially the donor and the acceptor in a matrix II. Allow them to communicate with a shuttler A. Slama- Schwok, M. Ottolenghi and D. Avnir, Nature, 355, 240 (1992) TV + + Py MV + TV + TV +2 + TV + + Py + TV 2+ TV 2+ Py MV 2+

One-pot reactivity from opposing reagents The concept: Entrapped reagents are not accessible to each other, but are accessible to diffusing substrates One pot, one step Acid, Base, C The acid and base are entrapped, separately A D

32% One-pot acid/base reactions Base: TBD Acid: Nafion Faina Gelamn, J. Blum, D. Avnir, Angew. Chem. Int. Ed., 40, 3647 (2001)

F. Gelman, H. Schumann, J. Blum, D. Avnir J. Sol-Gel Sci. Tech., 26, 43 (2003); J. Am. Chem. Soc., 122, (2000) Opposing catalyst and acids

RhCl[P(Ph) 3 ] F. Gelamn, J. Blum, D. Avnir, New J. Chem., 27, 205 (2003) Simultaneous oxidation-reduction reactivity

One-pot lipase / catalyst pair Biocatalysis and organometallic catalysis in one pot F. Gelman, J. Blum, D. Avnir J. Am. Chem. Soc., 124, (2002)

All possible combinations:

3. Cage-confinement effects on reactivity # Radical photo-rearrangement # Synergism in catalysis # Unusual enzyme reactivity

The photo-Fries rearrangement

D. Avnir, P. de Mayo, J. Chem. Soc. Chem. Comm., 1109 (1978) Radical cage effect of silica Only 5% in pentane, but 37% in silica (at 40%, conversion)

Cl 3 OCH 2 CO 2 H Cl 3 24 h (75%) Cl (99%) C CCl 3 Two components in a cage: Catalytic synergism Hydrogenation of chlorinated environmental pollutants Cl C H CH 3 (90%) OH H Cl 6 h H 2 O ClCH 2 CH 2 Cl (44%)+ (26%) = OH O O O hexane O O 24 h ClCH 2 CH 2 Cl Cl (93%) 24 h A. Ghatas, R. Abu-Reziq, J. Blum, D. Avnir Green Chem., 5, 40 (2003) The combined catalyst: Pd nanoparticles + [Rh(cod)Cl] 2 Chlorophenols 2,4,5-T PCBs DDT Cl-dioxins

C. Bianchini, R. Psaro et al, J. Am. Chem. Soc., 128, 7065 (2006) Pd(0) is able to reduce benzene to cyclohexane with a mechanism that involves disproportionation of the cyclohexa-1,3-diene product and fast cyclohexene hydrogenation; Rh(I) is faster than Pd(0) in reducing cyclohexa-1,3-diene, yet slower in the conversion of cyclohexene to cyclohexane Mechanism suggested by Bianchini, Psaro et al:

C. Bianchini, R. Psaro et al, J. Am. Chem. Soc., 128, 7065 (2006) The confinement of the two catalysts within a cage

Un-orthodox reactivity and unusual stability of sol-gel entrapped enzymes Alkaline-phosphatase is active at pH 1! H. Frenkel-Mullerad, D. Avnir J. Am. Chem. Soc. 127, 8077 (2005) Blue: silica Green: with AOT Red: with CTAB

Enzymatic activity under very extreme conditions A basic enzyme is active at pH 1! Why is the narrow cage so efficient in protecting the enzyme?

The collapse of thermodynamics Two protonated water molecules out of 100 is ~ pH=0!

Acid phosphatase is active under extreme alkaline conditions Right: silica; left: silica/CTAB In solution: Zero activity above pH 10

4. Porosity effects on reactivity # The inherent porosity of silica # Imprinted porosity

Size-discrimination in disproportionation reactivity A. Rosenfeld, J. Blum, D. Avnir J. Catal. 164, 363 (1996) The catalyst: [RR 3 N] + [RhCl 4 ] R: (C 8 H 17 ), R: Me

[RR 3 N] + [RhCl 4 ] SG-1: R: (C 8 H 17 ), R: Me SG-2: RR 3 N: [Me 3 N(CH 2 ) 3 Si(OMe) 3 ] SG-1 () and SG-2 (– – –) Pore-accessibility effects on the disproportionation reactivity of 1,2-dihydronaphthalene naphthalene and tetralin dihydronaphthalene Si(OEt) 4 and RSi(OEt) 3

Directing reactivity through imprinting of the matrix Forcing a cis-product in the Pd-acetate catalyzed Heck reaction D. Tsvelikhovsky, D. Pessing, D. Avnir, J. Blum, Adv. Synth. & Catal., 350, 2856 (2008) 9:1 1:1

5. Affecting reactivity by covalent modifying the material C o-polycondensation of Si(OEt) 4 and RSi(OEt) 3 # All-hydrophobic catalytic reactions in water

The emulsion contains the substrate The emulsion spills its content into the porous catalyst material All-hydrophobic catalytic reactions in water Hydrophobic chains The catalyst is entrapped in a partially hydrophobic silica sol-gel matrix R. Abu-Reziq, J. Blum, D. Avnir Angew. Chem. Int. Ed., 41, 4132 (2002)

The catalytic process takes place A micelle is reassembled and leaves with products inside

Three-phase catalysis: The EST process A novel three-phase microemulsion/solid heterogenization and transport method for catalysis All-hydrophobic catalytic reactions in water R. Abu-Reziq, J. Blum, D. Avnir Angew. Chem. Int. Ed., 41, 4132 (2002)

Octyl derivatized matrix Catalyst: [CH 3 (C 8 H 17 ) 3 N] + [RhCl 4 ] - Surfactant: Cetyl(trimethylammonium)(p-toluenesulfonate) Conditions: 200 psi of H 2 and heated at 80 ° C for 20 h Ethyl derivatized matrix EST: Matrix induced selectivity

6. Affecting reactivity by co-entrapment of a surfactant Getting a library of acids from a single molecule Claudio Rottman et al, J. Am. Chem. Soc., 121, 8533 (1999); 123, 5730 (2001) E T (30)

Getting a library of acids from a single molecule: E T (30)

Surfactant- dye interactions greatly enhanced in the cage Hartleys rule: The most significant surfactant- induced changes in properties of charged dyes are observed when the charge of the dye is opposite to that of the surfactant G. S. Hartley, Trans. Faraday Soc., 30, 444 (1934)

Getting a library of acids from a single molecule: E T (30)

Acid fuchsin – CTAB interactions Solution pKi = 13

Huge pK i shift for AF: 8 orders of magnitude

Take-home message Materials chemists: Dont ask what chemistry can do for you, but what you can do for chemistry!