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Chiral Nanoporosity in silicates Institute of Chemistry The Hebrew University of Jerusalem Nanocenter meeting, Ashkelon, 21.6.10 David Avnir.

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Presentation on theme: "Chiral Nanoporosity in silicates Institute of Chemistry The Hebrew University of Jerusalem Nanocenter meeting, Ashkelon, 21.6.10 David Avnir."— Presentation transcript:

1 Chiral Nanoporosity in silicates Institute of Chemistry The Hebrew University of Jerusalem Nanocenter meeting, Ashkelon, David Avnir

2 1. The Holy Grail

3 Zeolites: Highly porous, highly symmetric crystalline materials ZSM-5, a silicate zeolite: Na n Al n Si 96-n O H 2 O Major industrial applications: * Catalysis * Heterogeneous chemistry * Separation * Adsorbents

4 Chiral zeolites Prime importance because of their enantioselective potential applications in: * Enantioselective catalysis * Enantiomers separation Known: Zeolite-like, open-pore crystals, MOF’s, etc. Out of over 700 zeolite structures only 5 are recognized as chiral Desired: Chiral aluminosilicate zeolites Only one was reported

5 We found 21(!) chiral silicate zeolites which have been under the nose all the time! a. Goosecreekite. b. Bikitaite. c. The two enantiomeric forms of Nabesite Ch. Dryzun et al, J. Mater. Chem., 19, 2062 (2009) Editor’s Choice, Science, 323, 1266 (2009)

6 2. The route to that finding: I.Chiral nanoporosity of amorphous materials II. The chiral crystal of quartz

7 Silica

8 Here is how one can induce chiral porosity in silicates: * Adsorb on the surface a chiral molecule * Silylate the surface with a chiral silylating agent * Polymerize a chiral trialkoxysilane * Entrap a chiral molecule using the sol-gel polycondenstion * Prepare a hybrid of silica with a chiral polymer * Imprint chirally the silica

9 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 -

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

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

13 chiral imprinting

14 Silica imprinted with aggregates of DMB was capable of separating the enantiomer-pairs of: BINAP Propranolol Naproxen Pirkle’s alcohol With S. Fireman, S. Marx, J. Am. Chem. Soc. 127, 2650 (2005) (1R,2S)-(-)-N-dodecyl-N- methylephedrinium bromide (DMB)

15 S R R General enantioselectivity by chirally imprinted silica With S. Fierman, S. Marx, Adv. Mater., 19, 2145 (2007)

16 S R R S R R Comparison of two methods: Doping and imprinting Before extraction: Chiral dopant (DMB) After extraction: Chiral holes The recognition handedness changes!

17 If an amorphous SiO 2 material is made chiral by a foreign molecule which either remains there or not, then: #How are the building blocks of the material affected? #Is it possible that an SiO 4 tetrahedron which is neighboring to the chiral event, becomes chiral itself? Conceptual questions:

18 Nature has already provided an answer –Yes, it is possible a chiral SiO 4 tetrahedron! Quartz

19 The building blocks of quartz: All are chiral! SiO 4 SiSi 4 -O(SiO 3 ) 7 - Si(OSi) 4 D. Yogev-Einot, Chem. Mater. 15, 464 (2003)

20 Induced circular dichroism of Congo-Red within silica The chiral inducer: DMB The achiral probe: CR With S. Fireman, S. Marx, J. Mater. Chem., 17, (2007) We shall compare: * Co-doping * Adsorption of CR on silica doped with DMB

21 (red line) and (blue line) The ICD spectra of co-entrapped CR-DMB in hydrophilic and hydrophobic silica sols S. Fireman CR-DMB in solution (blue line) and CR solution (red line) Does CR “feel” the chirality of only DMB?

22 The ICD signal of CR adsorbed on The only possibility is chiral skeletal porosity induced by the doped DMB Co-doping: CR adsorbed on Reversal of the ICD signal indicates that the chirality- inducer is different in the two cases.

23 Silica is a racemic mixture of distorted tetrahedra and of chiral pores

24 2. The route to that finding: II. The measurement of chirality

25 The building blocks of quartz: All are chiral! SiO 4 SiSi 4 -O(SiO 3 ) 7 - Si(OSi) 4 D. Yogev-Einot, Chem. Mater. 15, 464 (2003) SiSi 4 is much more chiral than SiO 4

26 A useful tool: Quantitative measure of chirality

27 Various degrees of chirality:

28 Calculating the degree of chirality G: The nearest achiral symmetry point group Achiral molecule: S(G) = 0 The more chiral the molecule is, the higher is S(G) H. Zabrodski Hel-Or, J. Am. Chem. Soc., 117, 462(1995); 120, 6152 (1998); 126, 1755 (2004). A. Zayit et al, Chirality, in press (2010)

29 Le Chatelier, H. Com. Rend de I'Acad Sciences 1889, 109, 264. The optical rotation of quartz: 120 years ago Le Chatelier and his contemporaries

30 Temperature (°K)  Le Chatelier   t  Chirality, SiSi 4 Chirality  t   More than 120 years later: An exact match with quantitative chirality changes D. Yogev, Tetrahedron: Asymmetry 18, 2295 (2007) SiSi 4

31 3. Back to the zeolites

32 The symmetry space groups of quartz -O(SiO 3 ) 7 - D. Yogev-Einot, Chem. Mater. 15, 464 (2003) A helical chiral space group: P or P3 2 21

33 # The relevant space group symmetries which are indicative of a chiral crystal, are the 65 space groups which lack any improper symmetry element (reflection, inversion, glide or roto-inversion), collectively known as Sohncke space groups # Surprisingly, not all are chiral # 22 of the 65 are chiral (helical) Systematic search for Sohncke space groups in zeolites

34 # 43 of the 65 are non-helical # They are achiral space groups, despite the fact that they do not contain improper symmetries! The non-helical Sohncke space groups # They provide chiral crystals if the asymmetric unit is chiral All of the chiral zeolites we found belong to that category

35 Example in focus Goosecreekite (GOO) Si, Al, O

36 The main finding: Out of 120 classical silicate zeolites, we found 21 that must be chiral, but were not recognized as such Ch. Dryzun et al, J. Mater. Chem., 19, 2062 (2009) Editor’s Choice, Science, 323, 1266 (2009)

37 The chiral TT’ 4 building blocks

38 Goosecreekite (GOO) Chiral zincophosphate I (CZP) α-Quartz TT’ SBU A.U Unit cell The chirality values are comparable or larger than the chirality values of the known chiral zeotypes and of quartz

39 Adsorption of D-histidine (the lower curve) or L-histidine (the higher curve) on Goosecreekite (GOO): The heat flow per injection The isothermal titration calorimetry (ITC) experiment L-histidine With Y. Mastai and A. Shvalb, Bar-Ilan

40 Why have they been overlooked?

41 Conclusion Always look under the lamp – it might be there!

42 The building blocks of quartz and of chiral zeolites SiO 4 SiSi 4 So, what is a left-handed SiO 4 tetrahedron? CIP rules distinguish between the enantiomers of A(bcde) molecules: Si(OSi) 4

43 The steps: 1.Find the triangle with the maximal perimeter. 2. Check the direction from the longest edge to the shortest one, facing the triangle. 3. Clockwise rotation (shown) is a right handed tetrahedron. (The CIP logic of hierarchy) R* 1: : : D. Yogev et al Tetrahedron: Asymmetry 18, 2295 (2007) A method to assign handedness to AB 4 species The Triangle-Method

44 The analyzed Goosecreekite (GOO) is a left-handed (S o ), as determined from the handedness of its most chiral TT` 4 unit, Al(1)Si 4


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