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Chiral Nanoporosity in silicates

Chiral Nanoporosity in silicates. David Avnir. Institute of Chemistry The Hebrew University of Jerusalem. Nanocenter meeting, Ashkelon, 21.6.10. 1. The Holy Grail. Zeolites: Highly porous, highly symmetric crystalline materials. Major industrial applications: * Catalysis

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Chiral Nanoporosity in silicates

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

  2. 1. The Holy Grail

  3. Zeolites: Highly porous, highly symmetric crystalline materials Major industrial applications: * Catalysis * Heterogeneous chemistry * Separation * Adsorbents ZSM-5, a silicate zeolite: NanAlnSi96-nO192•16H2O

  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: • 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(OCH3)4 + H2O (SiOmHn)p + CH3OH • 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. Physical entrapment of molecules within sol-gel matrices Monomers, oligomers Sol Sol Gel Gel Xerogel Xerogel • * Small molecules • * Polymers • * Proteins • * Nanoparticles monomer oligomer sol - - particle Entrapped species The concept is general and of very wide scope

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

  12. chiral imprinting

  13. Silica imprinted with aggregates of DMB (1R,2S)-(-)-N-dodecyl-N-methylephedrinium bromide (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)

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

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

  16. Conceptual questions: If an amorphous SiO2 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 SiO4 tetrahedron which is neighboring to the chiral event, becomes chiral itself?

  17. Nature has already provided an answer –Yes, it is possible a chiral SiO4 tetrahedron! Quartz

  18. The building blocks of quartz: All are chiral! SiO4 SiSi4 -O(SiO3)7- Si(OSi)4 D. Yogev-Einot, Chem. Mater. 15, 464 (2003)

  19. Induced circular dichroism of Congo-Red within silica The chiral inducer: DMB The achiral probe: CR We shall compare: * Co-doping * Adsorption of CR on silica doped with DMB With S. Fireman, S. Marx, J. Mater. Chem., 17, 536 - 544 (2007)

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

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

  22. Silica is a racemic mixture of distorted tetrahedra and of chiral pores

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

  24. The building blocks of quartz: All are chiral! SiO4 SiSi4 -O(SiO3)7- Si(OSi)4 SiSi4 is much more chiral thanSiO4 D. Yogev-Einot, Chem. Mater. 15, 464 (2003)

  25. A useful tool: Quantitative measure of chirality

  26. Various degrees of chirality:

  27. 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)

  28. The optical rotation of quartz: 120 years ago Le Chatelier and his contemporaries Le Chatelier, H. Com. Rend de I'Acad Sciences1889, 109, 264.

  29. Chirality, SiSi4 Le Chatelier a t/a Chirality a t/a 0 Temperature (°K) More than 120 years later: An exact match with quantitative chirality changes SiSi4 D. Yogev, Tetrahedron: Asymmetry 18, 2295 (2007)

  30. 3. Back to the zeolites

  31. The symmetry space groups of quartz -O(SiO3)7- A helical chiral space group: P3121 or P3221 D. Yogev-Einot, Chem. Mater. 15, 464 (2003)

  32. Systematic search for Sohncke space groups in zeolites # 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)

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

  34. Example in focus Goosecreekite (GOO) Si, Al, O

  35. 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)

  36. The chiral TT’4 building blocks

  37. The chirality values are comparable or larger than the chirality values of the known chiral zeotypes and of quartz

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

  39. Why have they been overlooked?

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

  41. The building blocks of quartz and of chiral zeolites SiO4 SiSi4 Si(OSi)4 So, what is a left-handed SiO4 tetrahedron? CIP rules distinguish between the enantiomers of A(bcde) molecules:

  42. 1 3 2 R* A method to assign handedness to AB4 species The Triangle-Method The steps: 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) 1: 5.774 2: 4.913 3: 4.369 D. Yogev et al Tetrahedron: Asymmetry18, 2295 (2007)

  43. The analyzed Goosecreekite (GOO) is a left-handed (So), as determined from the handedness of its most chiral TT`4 unit, Al(1)Si4

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