1 Pore diameter, d [nm] Material Example d > 50 Macroporous Aerogels 2 < d < 50 Mesoporous Xerogels d < 2 Microporous Zeolites Amorphous, disordered - silica xerogels Ordered, amorphous walls Mesoporous Materials 2 Pore size distribution 3 Mesostructure Assembly 4 MMS mesoporous molecular sieves MCM-n Mobil Composition of Matter M41S A - lamellar MCM-50 B - hexagonal MCM-41 C - cubic MCM-48 Inverse hexagonal Discovered 1992 Mesoporous Materials 5 Mesoporous Materials 6 Surfactants - amphiphilic molecules, polar (head group)and nonpolar (chain, tail) part lyophilic, lyophobic Ionic surfactants, cationic, anionic, zwitterionic Nonionic amines, polyethyleneoxides A - normal surfactant molecule B - gemini C - swallow tail Supramolecular Templating 7 Surfactants 8 Anionic • sulfates: CnH2n+1OSO3 - Na+ • sulfonates: CnH2n+1SO3H • phosphates: CnH2n+1OPO3H2 • carboxylates: CnH2n+1COOH Cationic • alkylammonium salts: CnH2n+1(CH3)3NX X = OH, Cl, Br, HSO4 • dialkylammonium salts: (C16H33)2(CH3)2N+ Br- Noionic • primary amines: CnH2n+1NH2 • polyethyleneoxides: HO(CH2CH2O)nH Surfactants 9 Supramolecular templating Phase diagram of C16TMABr CMC = critical micelle conc. 10 Micelles - Supramolecular Templates 11 Micellar shapes A -spherical, B - rod-like, C - lamellar Micelles in media A - normal, in polar solvent, H2O B - inverse, in nonpolar solvent, organics 12 Micellar shapes Micellar structures A ) sphere, B ) cylinder, C ) planar bilayer, D ) reverse micelles, E ) bicontinuous phase, F ) liposomes). 13 Critical packing parameter – CPP CPP = VH / a0 lc VH volume of the hydrophobic part, a0 surface area of the hydrophilic part, lc critical chain length: lc ≤ 1.5 + 1.265 n [Å] n number of carbon atoms. lc depends on the chain shape. 14 CPP surfactant micelle shape < 0.33 linear chain, large head spherical 0.33 - 0.5 linear chain, small head cylindrical 0.5 - 1.0 two chains, large head bilayers 15 Surfactant Molecules Conical (icecream cone, A) Inverse conical (champagne cork, B) 16 Surfactant Molecules 17 L1= micellar solution; Nc = nematic phase; H1 = normal hexagonal phase (MCM-41; SBA-15); V1 = normal bicontinuous cubic phase (MCM-48); Lα = lamellar phase (MCM-50) path A, the micellar solution route path B, the lamellar phase route path C, the nematic phase route 18 Mechanism of the mesoporous material formation (hexagonal, MCM-41) LCT Liquid Crystal Templating 19 General Liquid Crystal Templating (LCT) Mechanism 20 SLC Silicatropic Liquid Crystals 21 22 Lamellar to Hexagonal Transformation 23 Silicate Rod Assembly 24 • Electrostatic interactions a) S+ II = silicate S = trimethylammonium I S b) S- I+ I = Fe2+ , Fe3+ , Co2+ , Ni2+ , Mg2+ , Mn2+ , Pb2+ , Al3+ S = sulfonane SI SI c) S+ X- I+ I = silicate – polyelectrolyte positive charge X = Cl S = trimethylammonium SI X d) S- M+ II = aluminate M = Na S = phophate I SM 25 • Hydrogen Bond a) S0 I0 I = silicate S = ammine SI 00 b) N0 I0 I = silicate N = polyethylenoxide 0 0 I N • Covalent Bond a) S-I I = niobate, tantalate S = ammine I S 26 27 28 MCM-41 29 TEM micrograph of hexagonal molecular sieve 30 TEM image of the Pd-grafted mesoporous silicate material 31 Silicate Layer Puckering 32 Charge Density Matching 33 Folding Sheets 34 XRD of hexagonal MCM-41 35 XRD of lamellar MCM-50 36 37 3 2 100 0 d a = 38 Gas Adsorption Isotherms Pore filling 39 Template Removal 40 Mesoporous Platinum Metal H2[PtCl6] or (NH4)2[PtCl6] C16(EO)8 Assembly of liquid crystalline phase Reductants: Fe, Zn, Hg, NH2NH2 Washed with acetone, water, HCl SEM (upper) and TEM (lower) images of mesoporous Pt metal show particles 90-500 nm in diameter and a pore diameter of 30 A and a pore wall thickness of 30 A. 41 Surface Silanols in MCM-41 Pores 42 Chemistry inside the Pores 43