Introduction to Porous Materials Introductory lecture objectives: • Definitions of micro/meso/macropore, pore volume, porosity • Surface area (how it relates to porosity, particle shape/size) • Experimental techniques used to determine particle size Source: https://en.wikipedia.org/wiki/Porous_medium Source: https://www.sigmalabs.com/fibrous-and-porous-materials 1 What is a Porous Material? Porous (Cambridge English Dictionary): something that is porous has many small holes, so liquid or air can pass through, especially slowly Interstices or interparticle voids MOF-74 Channels Zeolite A Cavities or cages 2 Classification of Pores Pores can come in all different shapes and sizes Micropore: pore with width not exceeding 2.0 nm (20 Å) Mesopore: pore of intermediate size, width between 2.0 – 50.0 nm (20 - 500 Å) Macropore: pore with width exceeding 50.0 nm (500 Å) <2.0 nm 2.0 – 50.0 nm >50.0 nm Micropore Mesopore All of these fall under the definition of nanopore (<100 nm) Macropore 3 Porosity and Pore Volume Pore Volume: the total internal void volume per unit mass of adsorbent cm3/g Porosity: (ΙΈ) void volume (Vv) = total volume (VT) ** usually given as a percentage Total volume = 10 m3, Void volume = 2 m3 Total volume = 10 m3, Void volume = 7.5 m3 4 Natural Porous Materials (Sedimentary Rocks) Sandstone – up to 40% Shale – up to 30% Limestone – up to 30% 5 Ordered vs. Nonordered Porous Solids Ordered Porous Solids: pores are arranged with high regularity or periodicity. Typically this means the solid has long-range order, meaning the materials are crystalline, eg. most zeolites and metal–organic frameworks Nonordered Porous Solids: pores are arranged randomly. This means the material does not possess long-range order and is therefore amorphous, eg. activated carbon Ordered porous MOF Nonordered porous polymer 6 Porosity – Why is it useful? 7 Porosity - Adsorption H O H = H2O = water Pores can be used to soak chemicals (adsorbates) up, and in some cases store them to be used later 8 Porosity - Separations Pores can be used to separate different chemicals from each other based on their size or chemical functionality 9 Porosity - Catalysis X+Y Z X+Y Z Z X+Y X+Y Z Pores can be used to perform chemical reactions 10 Surface Area An important property of porous materials that is related to porosity is surface area Surface area: the total area of the surface of a material, in the case of porous materials this includes all internal and external surfaces Square (2D) 1 surface Solid Cube (3D) 6 surfaces Porous Cube (3D) 6 square surfaces + 4 cylindrical internal surfaces 11 Surface Area 12 Surface Area The surface area of a powdered material is affected by the particle size and shape In the case of a porous material, the surface area is also affected by porosity – including the size and shape of the pores Particle size & Surface area Cut into smaller cubes with edge length 1 µm (1 x 10-6 m) - 1m 1m External surface area = 6 m2 Gives 1 x 1018 particles - Each with surface area of 6 x 10-12 m2 - Total surface area of 6 x 106 m2 *1 million times more surface area! 13 Surface Area Particle shape & Surface area Assume two particles with the same composition and equal mass (assume equal density since density is independent of particle shape). Cube with length ππ and sphere with radius r r ππ ππππππππππ = πππ π π π π π π π π π π 4 3 3 ππ = ππππ 3 ππππππππππππ οΏ½ ππ 6 = πππππ π π π π π π π π π π οΏ½ ππ ππππππππππππ πππππ π π π π π π π π π π = ππππππππππππ = 6ππ 2 πππππ π π π π π π π π π π = 4ππππ 2 3 2ππ ππ 14 Surface Area - Units Surface area unit = m2 Specific surface area unit = m2/g ο This is most commonly used in materials chemistry and is often just referred to as “surface area” ο This is the gravimetric unit for surface area (based on mass) ο Surface are can also be reported in volumetric units: m2/cm3 How do you convert gravimetric surface area to volumetric surface area? Multiply by the density of the material 15 Particle Size Analysis How does a chemist in the lab determine the particle size of a material? 1) Sieving: sizes particles based on their smallest dimension; no information about particle shape Sieving 16 Particle Size Analysis How does a chemist in the lab determine the particle size of a material? 2) Electron Microscopy: used to estimate particle size but typically only a few particles are viewed and may not be representative of the bulk Scanning Electron Microscope Electron Microscopy 17 Particle Size Analysis How does a chemist in the lab determine the particle size of a material? 3) Permeametric Methods: gives information about average particle size by passing fluid through a packed bed of material. Pressure drop and flow rate through the packed bed are measured Permeametric Method 18 Particle Size Analysis How does a chemist in the lab determine the particle size of a material? 4) Optical Measurements: particle attenuation of a light beam or measurement of scattering angles and intensity Laser Diffraction Dynamic Light Scattering Optical Measurements 19 Particle Size Analysis How does a chemist in the lab determine the particle size of a material? 5) Sedimentation Analysis: based on different particle sizes settling in a fluid with different velocity. Uses Stokes’ Law: π·π·π π π π = 18ππππ πππ π − ππππ ππ 1/2 π·π·π π π π = πππππππππππ π ′ ππππππππππππππππ ππ = ππππππππππ π£π£π£π£π£π£π£π£π£π£π£π£π£π£π£π£π£π£ πππ π = π π π π π π π π π π ππππππππππππππ ππππ = ππππππππππππ ππππππππππππππ π£π£ = π π π π π π π π π π π π π π π π π£π£π£π£π£π£π£π£π£π£π£π£π£π£π£π£ ππ = ππππππππππππππππππππππππ ππππππ π‘π‘π‘π‘ ππππππππππππππ Sedimentation Analysis 20 Classes of Porous Materials Zeolites Lecture objectives: • Definition, structure, formula • Zeolite database/classifications • History • Synthesis • Processing • Applications 21 Zeolites Definition (Collins English Dictionary): 1) Any of a large group of glass secondary minerals consisting of hydrated aluminum silicates of calcium, sodium, or potassium: formed in cavities in lava flows and plutonic rocks 2) Any of a class of similar synthetic materials used in ion exchange and as selective adsorbents Natrolite Chabazite Heulandite Synthetic 22 Zeolites Many different structures with different pore architectures and sizes 23 Zeolite Structure ο ο ο ο ο ο 3D Microporous Crystalline Ordered pore structure Made of aluminum, silicon and oxygen Pores contain cations for charge balancing as well as water 24 Zeolite Formula Mx/n[(AlO2)x(SiO2)y]βmH2O M = cation which can be H+, Ca2+, Na+, K+ and the valence of that cation is n AlO2− and SiO2 are the fundamental units of a zeolite structure. Each Al3+ and Si4+ is in tetrahedral geometry while each oxygen is bridging O O Si O O O Al O Zeolites can also be comprised of P5+, B3+, Ga3+, Be2+, Ge4+ O 25 Zeolite Formula Mx/n[(AlO2)x(SiO2)y]βmH2O M = cation which can be H+, Ca2+, Na+, K+ and the valence of that cation is n AlO2− and SiO2 are the fundamental units of a zeolite structure. Each Al3+ and Si4+ is in tetrahedral geometry while each oxygen is bridging O O Si O O O Al O O Important Notes: 1) The formula of a zeolite must always be charge balanced 2) The structure of a zeolite should* obey Löwenstein’s rule, meaning that no Al—O—Al linkages can be present **much like most rules in chemistry, it can be broken 26 Zeolite Structure The Si4+ and Al3+ tetrahedral building blocks are referred to a primary building units (PBUs), whereas the larger structures (shapes) that these PBUs form are referred to as secondary building units (SBUs) SBUs are units (shapes) that repeat throughout a structure O O Si O O O Al O O PBUs Zeolite SBUs can be 2D or 3D. The vertices of an SBU represent the center of the tetrahedral building block (ie., the Si or Al atoms). Symbols represent number of tetrahedra forming each face or chain SBUs 27 Zeolite Structure Taking SBUs one step further, zeolite structures can also be described by their composite building units (CBUs), sometimes referred to as cage building units or polyhedral building units 28 Zeolite Structure Most common composite building units (CBUs) 29 Zeolite Database All the zeolite structure types can be found in the International Zeolite Association’s Database of Zeolite Structures. Currently there are 241 unique structure types and each unique type is defined by a 3 letter code. http://www.iza-structure.org/databases/ and click on “All Codes” 30 Zeolite Database – Example 1 The 3 letter code is related to the name of the zeolite. FAU = Faujasite, named after Barthélémy Faujas de Saint Fond, a French geologist known for studying volcanos 31 Zeolite Database – Example 2 The 3 letter code is related to the name of the zeolite. LTA = Linde Type A, invented by chemists at Linde Air Products, a division of 32 Union Carbide History of Zeolites The first natural zeolites were discovered in 1756 In the late 1800s, zeolites were also found in sedimentary rocks Zeolite formation most commonly occurs by: 1) Crystals resulting from hydrothermal/hot spring activity – reactions between solution and basaltic lava flow 2) Deposits formed from volcanic sediments in closed alkaline and saline lake systems 3) Deposits from open freshwater lake or groundwater systems acting on volcanic sediments 4) Deposits formed from volcanic materials in alkaline soils 5) Deposits from hydrothermal or low temperature alteration of marine sediments 6) Formations resulting from low grade burial metamorphism 33 Zeolite Synthesis Researchers try to mimic some of the natural conditions in which zeolites are formed 1) Hydrothermal (or solvothermal) synthesis: reactants are mixed in water (or solvent) and the mixture is heated, in some cases at high pressure Zeolites can be classified by Reactants: their Si:Al ratio silica, and alumina cation source water (sometimes organic solvent) Low silica: Si:Al = 1.0-1.5 silica: base (reactions are normally carriedMedium out at pH >10Si:Al = 2.0-5.0 High silica: Si:Al = 10.0-100.0 Conditions: **also some rare examples of silica:alumina (and concentration) pure silica zeolites charge/size of cation reaction temperature reaction time pH of the reaction mixture ** sometimes called sol-gel synthesis 34 Zeolite Synthesis Researchers try to mimic some of the natural conditions in which zeolites are formed 2) Templating Methods: using “large” organic cations to direct the structure of zeolites, i.e., to create larger pores 35 Zeolite Synthesis Researchers try to mimic some of the natural conditions in which zeolites are formed 3) Microwave Assisted Synthesis: using microwave irradiation for heating purposes Microwaves heat by: a) Dipolar polarization: polar molecules align in the oscillating microwave field b) Ionic conduction: dissolved charged particles oscillate in the microwave field Both processes cause rotations/collisions which heat up the sample. Heating from the inside out – faster and more energy efficient 36 Zeolite Processing (or Activation) After synthesis, the guest molecules inside the pores of a zeolite must be removed (or replaced) to gain access to the empty space Guest molecules to be removed or replaced: ο water or solvent ο organic templating agents ο inorganic templating agents (cations, metal complexes) These guests normally interact with the zeolite framework via H-bonds, van der Waals forces, ionic or covalent bonding, so they must be removed carefully so as not to collapse the structure/pores 1) High temperature calcination: drives off water, solvent, and even decomposes organic molecules (if done in air or O2 environment) ο can be very harsh; two-step (i.e., slow) calcination can help 37 Zeolite Processing (or Activation) 2) Chemical detemplating: using NH3, H2O2, O3 or O2 containing plasmas to oxidize guest organic templating agents 3) Cation exchange: replacing “larger” inorganic cations with smaller ones to open up the pores in a structure, i.e., LTA-Na has a pore opening of 4 Å, if that is exchanged with Cs+, K+, or Ca2+ then the pore opening becomes 2 Å, 3 Å, or 5 Å, respectively. 38 Zeolite Applications 1) Ion-Exchange: ο ο Water softening: hard water contains a high concentration of Ca2+ and Mg2+ (and sometimes other multivalent cations). Using a Na-zeolite, the Ca2+ and Mg2+ in hard water can be replaced by Na+ Also used in detergents/soaps for softening purposes (in 2014, Proctor and Gamble phased out phosphates for softening and replaced them with zeolites) Why soften water? ο scale buildup on dishes/appliances ο dry/itchy skin ο stains in sinks/bathtubs ο plumbing damage ο clothing grey/faded 39 Zeolite Applications 1) Ion-Exchange: 40 Zeolite Applications 2) Adsorption: Removal of water; desiccants or molecular sieves ο ο Drying gas streams in the petroleum industry: to prevent the formation of hydrates which can freeze/block pipes, valves and other equipment In research or industrial laboratories: sieves are used to remove water from various organic solvents 41 Zeolite Applications 2) Adsorption: Air purification ο Breathing apparatus’ used by scuba divers and firefighters, zeolites help to concentrate the oxygen being supplied by removing N2 and other impurities Chemical Separations ο Separating xylene isomers in the petroleum industry based on shape o-xylene m-xylene p-xylene 42 Zeolite Applications 3) Catalysis: Hydrocarbon cracking: ο ο ο Taking large, straight-chain hydrocarbons and breaking them down into smaller more useful hydrocarbons; ideally branched or cyclic molecules (i.e., high octane molecules) + C15H32 + This is performed after the initial crude oil refining process where the oil is separated into different fractions based on boiling point (longer chain hydrocarbons have higher boiling points) The cracking occurs when Brønsted acid sites on the zeolite transfer a proton to an alkane to give an alkane and a carbocation which then becomes an alkene O H O O O O Al Si O O + + 43