Porosity

Definitions:

  • True, apparent and bulk density
  • Pore volume, V\(_p\)
  • Pore size, (width, diameter)
  • Porosity = V\(_p\)/V, V = apparent volume
  • Surface area: Accessible area of solid surface per unit mass.

Measuring:

  • Depends on method/material
  • Coastline paradox
  • Surface probing w/molecules
  • Bulk probing (Spectroscopy, diffraction, scattering)

Size regimes:

  • Microporoes: <2nm ~ molecules: activated transport
  • Mesopores: 2<r<50nm (or < Mean Free Path)
    • Knudsen or surface diffusion, capillary, condensation or multilayer adsorbtion.
  • Macropores: >50nm (or >MFP), Bulk diffusion + Viscous flow.

Types: Porosity#Metallic Foams Porosity#Aerogels Porosity#Zeolites Porosity#MOFs Porosity#MCMs

Metallic Foams

  • Composite of gaseous and solid phases.
  • Porous metals: High bulk density, independently distributed voids
  • Metallic Foams: Low bulk density, connected voids
    • Porosity: 30-90 vol%
    • Uses:
      • Impact absorbtion
      • Air + water permeability (Filtering, membrane)
      • Acoustic properties (sound absorbers)
      • Low T conductivity (Insulation)
      • Surface area (Porous electrodes, heat exchanger)
      • electromagnetic shielding

Synthesis methods

  • Casting:
    • Foaming: decomposition to gas while mixing and cooling->high porosity
    • Lost-Foam: Open porous polymer foam -> filled with inorganic(eg gypsum) -> pyrolysis of polymer foam -> fill with molten metal -> Remove mold (eg solvation) -> product w/open porosity, roughly same as start polymer
    • Infiltration: pour molten metal in cast, egt NaCl beads which is then dissolved.
  • Gas-eutectic: insert often H in metal, cool down from one side through eutectic composition-> gas is formed as metal solidifies. Often gives gas-rods in metal.
  • Powder metallurgy: Pack mold with powder(or fibres) -> sinter together.
  • Deposition: CVD, electrochemical, PVD: deposit on prous organics.

Aerogels

Supercritical drying of gel.

  • Mesoporous (2<r<50nm)
  • bulk densities: 0.004-0.5g/cm\(^3\)

Ambient pressure drying

  • Collapse caused by capillary forces, fixed by:
    • Strengthen network
    • Modify contact angle (solid-liquid)
    • Exchange water with waterfree solvent
    • Silylate Si-OH-group -> springback

Zeolites

  • Porous, hydrated aluminosilicates
  • Natural or synthetic
  • \(M^n_{x/n} Si_{1-x} Al_x O_2 \cdot yH_2O\)
  • Counter ions compensate charge from Al substitution
    • Usually mobile and in pores
  • Zeolitic water to measure porosity (removed by heat)
  • Loewensteins rule: Only 0.5 of Si can be exchanged for Al (both extremes exist)(Al-O-Al bonds not allowed)

Zeolite Usages:

  • Molecular sieves: discrete pore sizes allowing special molecules.
  • Ion exchange: Counter cations can move and may be exchanged. Used in detergents, waste water purification, pigs food.
  • Catalysis: Heterogeneous cat for petrochem. Zeolite largest use: Cracking catalyst (faujasite). Production of synthetic gasoline from methanol. Tuned by Si/Al ratio, chemistry and counter ions.

Zeolite Synthesis

  • Deville: Lab scale: K\(_2\)Si\(_2\)O\(_5\) + NaAlO\(_2\) in glass ampoule
  • Usually made from sol or gel in mild hydrothermal conditions (<350\(^\circ\)C)
  • Ingredients: H\(_2\)O, Si-source, Al-source, pH-regulators, templates.
  • Templating: Cations working as counter-ions also work as templates: modifying cell and filling void

Zeotype Structure

  • BBU: Basic building unit: the tetrahedron: They all share corners and have periodic structure
  • CBU: Composite building units: polymeric structures with rings and prisms
  • Tertiari building units: larger cases

Microporous, zeolite-like structures

  • Aluminophosphates: AlPO4
  • Si-doped SAPO
  • Metal doped MePO

MOFs

Metal organic frameworks

MOF Structure

Built by BBU’s consisting of metallic connectors and organic linkers. However, unlinke Porosity#Zeotype Structure, connector-linker bonds are coordinative or ionic, not covalent.

  • Among the largest pores of crystalline structures.
  • >1000 m\(^2\)/g surface area

Classification

  • 1st gen: Collapse upon guest / template removal
  • 2nd gen: Stable + robust, porous after guest removed.
  • 3rd gen: Flexible + dynamic, responds to external stimuli.
  • The last two can be used for gas storage or catalysts.

MOF Synthesis

  • Standard coordination chemistry methods are used
    • Metal ions are reacted with organic ligand
    • Low T, solvothermal synthesis
    • Products determined by thermodynamics, not kinetics
    • Linker flexibility important for properties (Usually rigid)
    • Flexible linker may allow several structures leading to poor crystallinity.
    • Self assembly of BBUs
  • Initially, pores are filled with guest template molecules, which are hard to remove due to MOF’s low thermal stability.

MCMs

Mobil Crystalline Material Mobil Composition of Matter

MCM Structure

  • Mesoporous (2<r<50nm) material from supramolecular species(eg micelles) (Not ions or organics) In reality 2-10nm
  • Amphiphilic surfactant molecules
  • Amorphous pore walls
  • Narrow pore size distribution

MCM Synthesis

Solvothermal#Hydrothermal synthesis: Water, amphophilic molecule(eg CTAB), soluble inorganic + catalyst

  1. Form supramolecular arrangement of molecules (Hydrophilic head, hydrophobic tail)
  2. Templating (They form tubes, 3d or lamellar with increasing consentration of CTAB). Either infused with Silicon, or silicon already on amphophilic molecule.
  3. Remove template ( By solvent extraction, calcination, O\(_2\) Plasma, Supercritical drying) Only hexagonal or 3d structures can survive this.

MCM Usage

MCM-41 used as Catalyst, catalyst support, adsorbent, host for nanomaterials

Opals

Produced by packing of solid material template Template requirements:

  • Must be removable
  • Must be compatible with process conditions
  • Precursor solution must wet the template
  • Must have narrow size distribution.

Two synthesis routes:

  • Stöber process: 50nm-2mm
  • Organic polymer spheres produced from emulsion polymerization.
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