1 TEM image of the Pd-grafted mesoporous silicate material 2 Pore diameter, d [nm] Material Example d  50 Macroporous Aerogels 2  d  50 Mesoporous Xerogels d  2 Microporous Zeolites • Amorphous, disordered - silica xerogels • Ordered pores, amorphous walls Mesoporous Materials 3 Mesoporous Materials Pore diameter, d [nm] Material Example d  50 Macroporous Aerogels, foams 2  d  50 Mesoporous Xerogels, MCM-41, SBA-15 d  2 Microporous Zeolites, MOF, COF 4 MMS mesoporous molecular sieves MCM-n Mobil Composition of Matter M41S A - lamellar, 2D layers, MCM-50 B - hexagonal order, 1D channels, MCM-41 C – cubic, 3D channel structure (bicontinuous), MCM-48 Inverse hexagonal Discovered 1992 Mesoporous Materials 5 Pore size distribution Micelles - Supramolecular Templates 6 In zeolitic materials the template is a single molecule or ion Self assembled aggregates of molecules or ions can also serve as templates Surfactants aggregate into a variety of structures depending on conditions 7 Mesostructure Assembly 8 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 9 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 Supramolecular Templating 10 Phase diagram of C16TMABr CMC = critical micelle conc. Micellar Shapes 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 Surfactant Molecules 12 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. Conical (icecream cone, A) Inverse conical (champagne cork, B) 13 Micellar Shapes Micellar structures A ) sphere, B ) cylinder, C ) planar bilayer, D ) reverse micelles, E ) bicontinuous phase, F ) liposomes). 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 14 Surfactant Molecules 15 Mechanism of the Mesoporous Material Formation L1= micellar solution; Nc = nematic phase; H1 = normal hexagonal phase (MCM-41; SBA-15); V1 = normal bicontinuous cubic phase (MCM-48); L = lamellar phase (MCM-50) path A, the micellar solution route path B, the lamellar phase route path C, the nematic phase route General Liquid Crystal Templating (LCT) Mechanism 16 Mechanism of the Mesoporous Material Formation 17 Hexagonal, MCM-41 LCT Liquid Crystal Templating SLC Silicatropic Liquid Crystals Mechanism 18 19 Lamellar to Hexagonal Transformation Charge Density Matching 20 As condensation proceeds the charge on the silicate layer decreases SiO  SiOSi 21 22  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 23  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 24 Control of Pore Size MCM-41 25 Control of Pore Size Surfactant chain length - increasing the chain length = bigger pores Swelling agents – an organic additive, such as trimethylbenzene, enters the surfactant assembly (micelle) = bigger pores Post synthetic modification - after a material has been made the pore size can be reduced by modifying the interior surface = smaller pores 26 Control of Pore Size 27 Control of Pore Size 28 Control of Pore Size Silylation of hydroxyl groups in MCM-41 by Me3SiCl reduces the effective pore size EISA = Evaporation-induced self-assembly 29 EISA 30 31 TEM micrograph of hexagonal molecular sieve 32 XRD of Lamellar MCM-50 33 XRD of Hexagonal MCM-41 wt = wall thickness d(100) = interplanar distance in the (100) plane a0 = mesoporous parameter 3 2 100 0 d a  34 Gas Adsorption Isotherms Mesopore filling Micropore filling Pores filled with LN2 Pore volume BET Surface area Template Removal 35 36 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. 37 Surface Silanols in MCM-41 Pores 38 Chemistry inside the Pores 39 Hard Tempalting 40 A = microwave digestion - template removal B = introduction of metal salt solution C = calcination D = dissolution of SiO2 in HF or NaOH Cr2O3 crystalline nanowires (bar = 25 nm for A, 10 nm for A1) 41 Pore Size Regimes and Transport Mechanisms 42 Macropores = larger than 50 nm larger than typical mean free path length of typical fluid. Bulk diffusion and viscous flow. Mesopores = between 2 and 50 nm same order or smaller than the mean free path length. Knudsen diffusion and surface diffusion. Multilayer adsorption and capillary condensation may contribute. Micropores = smaller than 2 nm pore size comparable to the size of molecules. Activated transport dominates. Spinodal Decomposition Sol-Gel Methods 43 (a) Free energy of a binary system as a function of composition and the miscibility region showing the origin of the binodal and spinodal lines (b) Evolution of a blend microstructure phase separating by spinodal decomposition Spinodal Decomposition Sol-Gel Methods 44 A two component system with a composition, c, that is unstable to small fluctuations in concentration, where (G = the free energy), will spontaneously phase separate with the fluctuations increasing and coarsening over time. 02 2    c G Spinodal Decomposition 45 coarsening Sol–Gel with Phase Separation i = the volume fraction Pi (i = 1, 2) = the degree of polymerization of each component, 12 the interaction parameter The former two terms in the bracket express the entropic contribution, and the last term the enthalpic contribution 46 47 Hierarchically Porous Monoliths 48 Macroporous – good mass transport Mesoporous – large surface area available for active sites Microporous – catalytic selectivity 49 TMOS-Formamide-1M nitric acid (b) calculated composition. Reaction temperature 40 oC; circles with cross and shaded areas denote the composition where the interconnceted structure has been obtained. : nanoporous gel, : interconnected strucuture, : particle aggregates, : macroscopic two-phase. TiO2 SEM images of dried TiO2 gels prepared with varied water/TiO2 molar ratios in the overall starting 1:0.5:0.5:f Ti(OnC3H7)4:HCl:formamide:water composition: (a) f ) 20.50, (b) f ) 20.75, (c) f ) 21.00, (d) f ) 21.25, and (e) f ) 21.50. (f) Photo image of monolithic TiO2 gels prepared in Teflon tubes and a coin. 50 Hierarchically Porous Monoliths 51 Hierarchically Porous Monoliths 52 Hierarchically Porous Monoliths 53 Time evolution of a spinodally decomposing isotropic symmetrical system 54 55 56 57