Nanoporosity – where is it useful in chemistry? David Avnir Institute of Chemistry The Hebrew University of Jerusalem Nano Center Meeting, Ashkelon, March 29-30, 2015

1. The material at focus - silica

Silica

Controlled nanoporosity Surface area and pore volume of silica as a function of pH and water/silane ratio in the sol- gel process

Functionality within a sol-gel matrix Monoliths Powders Particles This-films

2. Chemical sponges – diffusion considerations

Sol-Gel Sponges Hagit Frenkel-Mullerad

The reaction kinetics can be followed in two ways: 1.Following the visible absorption of bromine 2. Following the decrease in pH

Kinetics of the reaction through follow-up of Br 2 consumption Vinylated silica

Kinetics of the reaction as detected by HBr release pH Time (min) Kinetics of reactivity in nanopores depends on the analytical probe! X 10 slower

Time (s) VTS ATS BTS OTS A/Ao Time (s) Kinetics depends also on the fine details of the hybrid material, even if the functionality is the same: Vinyl, allyl, butyl, octyl. The shorter chains are much more reactive than the longer ones - why? Initial rates

A schematic view of the possible micellar nano-phase zones Reactivity depends on the specific nano structure of the hybrid material

3. Photochemistry

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 Example 1: Solar energy storage - solving the problem of back-reaction Energy storing pair Useful reaction back-reaction

+ TV 2+ Four hours, 5% yield of separated pair The nanoporosity approach: I. Separate spatially the donor and the acceptor by entrapment in a sol-gel matrix II. Allow them to communicate through the nanopores with a shuttler A. Slama-Schwok, M. Ottolenghi TV + + Py MV + TV + TV +2 + MV TV + + Py + + TV 2+ TV 2+ Py MV 2+ The redox potential of the MV pair is smaller than that of the TV pair

D. Levy Example 2: Affecting the direction of photochromism by tailoring the surface of the nanopores Isomerization of spiropyrans

Colorless Colored Controlling the directionality of photochromism Reversed photochromism in silica sol-gel matrices

…but normal photochromism in ethylated silica Colorless Colored

3. Sensors: Extraction of a library of reactivities from a single molecule

Getting a library of acid/base sensors from a single molecule nanopore effects Anionic AF Zwitterionic ET(30) + Claudio Rottman

Affecting the immediate environment by co-entrapment of surfactants within the nanocages

E T (30), an acid or a base – your choice: The interpretation

Continuous range of acids/bases by using a surfactant mixture at varying proportions E T (30)

Huge pK i shift for AF: 8 orders of magnitude

4. Catalysis

Cl 3 OCH 2 CO 2 H Cl 3 24 h (75%) Cl (99%) C CCl 3 1st example: Superior synergistic catalyst for green chemistry Two components in a nano-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 R. Abu-Reziq, J. Blum 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. Mechanism suggested by Bianchini, Psaro et al: The confinement of the two catalysts within a cage

2nd example: One-pot multistep catalytic processes with opposing reagents F. Gelman, J. Blum Cutting the need for separation steps

F. Gelamn, J. Blum Three steps oxidation/reductions in one pot RhCl[P(C 6 H 5 ) 3 ] 3 91%

5. Imprinted nanoporosity

4th example: Tailored nanoporosity by imprinting Directing the seterochemistry of a reaction Forcing a cis-product in the Pd-acetate catalyzed Heck reaction D. Tsvelikhovsky, J. Blum 9:1 1:1

Current /  A Electrochemical recognition of the imprinting molecule: Dopa Current (  A) L-Dopa D-Dopa Silica sol-gel thin films, 70 nm D. Mandler, S. Fireman

6. Enzymatic reactions - enhanced stability

Protection from heat Very large shifts in the denaturing temperatures New, very mild entrapment method in alumina: Al(C 3 H 7 O) 3, pH 7.3, ultrasound V. Vinogradov, 2014/5

Not only thermal stability, but increase in activity up to 60 o C … and stability to repeated cycles of heating to 60 o C and cooling Acid The activity at 75 0 C, is higher than at room temperature by about two orders of magnitude. # (AcP, 1) : Treatment of enzyme deficiency diseases

Arrhenius analysis The pre-factor of the entrapped enzyme A = sec -1 six orders of magnitude higher (!) than that of the free enzyme sec o C

7. Merging all of the above

Protection of an enzyme from strong oxidative conditions: Alkaline phosphatase protected from bromine H. Frenkel-Mullerad, R. Ben-Knaz, 2014

One-pot enzyme/catalyst pair 0.6 mmol acid, 2.5 mmol alcohol 0.01 mmol catalyst, 11U lipase F. Gelman, J. Blum

Conclusion Better materials based on chemistry Better chemistry based on materials