Mesoporous Materials 1 TEM image of the Pd-grafted mesoporous silicate material IUPAC Classification of Porous Materials 2 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 • Disordered, amorphous - silica xerogels • Ordered pores, amorphous walls (MCM, SBA) 4 M41S - Group name of mesoporous MCM materials by Exxon Mobil (MCM = Mobil Crystalline Materials or Mobil Composition of Matter) MMS - Mesoporous Molecular Sieves OMS - Ordered Mesoporous Silicas A - hexagonal 1D channels, MCM-41, SBA-15 B - cubic 3D channel structure (bicontinuous), MCM-48, KIT-6 C - FCC 3D channels, FDU-12 D - BCC 3D channels, SBA-16 E - Lamellar, 2D layers, MCM-50 • US pat. 1971 - forgotten • Kuroda, Japan - 1990 • Independently rediscovered, pattented and published 1992 by Exxon Mobil Nomenclature of Mesoporous Materials 5 Nomenclature of Mesoporous Materials MCM - Mobil Crystalline Materials SBA - Santa Barbara Amorphous KIT - Korea Advanced Institute of Science and Technology COK - Centre for Surface Chemistry and Catalysis, Leuven FDU - Fundan University MSU - Michigan State University 6 Pore Size Distribution Micelles - Supramolecular Templates 7 In zeolitic materials, the template is a single molecule or ion – pores too small (micropores) Self assembled aggregates of molecules or ions can also serve as templates – larger pores (mesopores) Surfactants aggregate into a variety of structures depending on conditions (concentration, temperature, solvent,…) 8 Surfactants = amphiphilic molecules, polar (head group) and nonpolar (chain, tail) part Lyophilic and lyophobic Ionic surfactants, cationic, anionic, zwitterionic Nonionic amines, polyethyleneoxides A - normal surfactant molecule B - gemini C - swallow tail Supramolecular Templating 9 Types Surfactant Molecules Cationic alkylammonium salts CnH2n+1(CH3)3NX, X = OH, Cl, Br, HSO4 dialkylammonium salts (C16H33)2(CH3)2N+ Br- Anionic sulfates CnH2n+1OSO3 - Na+ sulfonates CnH2n+1SO3H phosphates CnH2n+1OPO3H2 carboxylates CnH2n+1COOH Noionic primary amines CnH2n+1NH2 polyethyleneoxides HO(CH2CH2O)nH triblock copolymers Pluronic Surfactant Molecules 10 Conical (icecream cone, A) Inverse conical (champagne cork, B) 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 11 Micellar Shapes A ) sphere, B ) cylinder/rod, C ) planar bilayer/ lamellar, 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 Micelles in media A - normal, in polar solvent, H2O B - inverse, in nonpolar solvent, organics Supramolecular Templating 12 Phase diagram of [(C16H33)N(CH3)3]Br cetyltrimethylammonium bromide (CTAB) CMC1 = critical spherical micelle conc. CMC2 = critical rod-like micelle conc. L1 = micellar solution Nc = nematic phase H1 = hexagonal liquid crystal phase (MCM-41; SBA-15) V1 = bicontinuous cubic liquid crystal phase (MCM-48) L = lamellar phase (MCM-50) L1 V1 L Nc Concentration, % Mechanism of the Mesoporous Material Formation 13 Hexagonal MCM-41 • Liquid Crystal Templating (LCT) • Silicate Rod Assembly • Silicatropic Liquid Crystals (SLC) • Lamellar to Hexagonal Transformation CTAB TEOS CMC2 MCM-41 Liquid Crystal Templating (LCT) Mechanism 14 • Formation of rod-like micelles (above CMC2) • Assembly of rod-like micelles into hexagonal liquid crystal phase H1 • TEOS deposition in between rod-like micelles • Hydrolysis and condensation to form solid amorphous walls of silica • Template removal to form MCM-41 H1 CMC2 TEOS MCM-41 Silicate Rod Assembly Mechanism 15 • Formation of rod-like micelles (above CMC2) • Silicate deposition on the surface of micelles • Silica/surfactant rods assemble to hexagonal arrays • Silica condensation to solid amorphous walls • Template removal to MCM-41 CMC2 TEOS MCM-41 Silicatropic Liquid Crystals (SLC) Mechanism 16 • Interaction/ion-exchange of surfactant and inorganic precursors • Cooperative self-assembly of silicate/surfactant micelles below CMC2 !! • Assembly to hexagonal liquid crystal phase • Silica condensation to solid amorphous walls SLC LCT 17 Lamellar to Hexagonal Transformation Charge Density Matching As condensation proceeds the charge on the silicate layer decreases SiO  SiOSi • Ion Exchange of Na+ for surfactant • Folding of silicate layers Precursor-Micelle Interactions 18 Electrostatic Interactions a) S+ I- I = silicate (Si-O-) S = trimethylammonium b) S- I+ I = Fe2+, Fe3+, Co2+, Ni2+, Mg2+, Mn2+, Pb2+, Al3+ S = sulfonate c) S+ X- I+ I = silicate – positive charge X = ClS = trimethylammonium d) S- M+ I- I = aluminate M = Na+ S = phosphate Precursor-Micelle Interactions 19 Hydrogen Bond a) S0 I0 I = silicate at IEP S = ammine b) N0 I0 I = silicate N = polyethylenoxide Covalent Bond a) S-I I = niobate, tantalate S = ammine EISA = Evaporation-Induced Self-Assembly 20 Critical parameters • Molar ratio of Surfactant/Inorganic precursor • Amount of water • Volatile cosolvent content (EtOH, THF,…) • Temperature • Relative humidity 21 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 MCM-41 22 Control of Pore Size Surfactant chain length - increasing the chain length = bigger pores XRD 23 Control of Pore Size Swelling agents – an organic additive, such as trimethylbenzene, enters the surfactant assembly (micelle) = bigger pores 24 Control of Pore Size Silylation of hydroxyl groups in MCM-41 by Me3SiCl reduces the effective pore size Post synthetic modification - after a material has been made the pore size can be reduced by modifying the interior surface = smaller pores Template Removal 25 Characterization of Mesoporous Materials 26 TEM micrograph of hexagonal molecular sieve 27 Characterization of Mesoporous Materials XRD of Lamellar MCM-50 Only (00l) diffractions observed Bragg’s Law n = 2d sin  28 Characterization of Mesoporous Materials wt = wall thickness d(100) = interplanar distance in the (100) plane a0 = mesoporous parameter 3 2 100 0 d a  XRD of Hexagonal MCM-41 Only (hk0) diffractions observed 29 Characterization of Mesoporous Materials Mesopore filling Micropore filling Pores filled with liquid N2 Pore volumeBET Surface area Gas Adsorption Isotherms (N2 at 77 K) 30 Characterization of Mesoporous Materials MCM-41 Pore size 3.4 nm, no Kelvin capillary condensation below 3.6–3.8 nm - no hysteresis, sharp inc-decr at B ads.-des. N2 at 77 K SBA-15 ( = ads.,  = des.) 6.5 nm Kelvin capillary cond. - hysteresis SBA-16 ( = ads.,  = des.) spherical pores 6 nm with windows 4 nm hexagonal hexagonal cubic Mesoporous Platinum Metal 31 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 Å and a pore wall thickness of 30 Å 32 Chemistry inside the Pores Surface silanols in MCM-41 pores (3-Aminopropyl)triethoxysilane Hard Tempalting 33 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) Hard Tempalting 34 Mesoporous carbon synthesis