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: (ɸ)
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 𝑙𝑙 and sphere with radius r
r
𝑙𝑙
𝑉𝑉𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 = 𝑉𝑉𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠
4 3
3
𝑙𝑙 = 𝜋𝜋𝑟𝑟
3
𝑆𝑆𝑆𝑆𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 � 𝑙𝑙
6
=
𝑆𝑆𝑆𝑆𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 � 𝑟𝑟
𝑆𝑆𝑆𝑆𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐
𝑆𝑆𝑆𝑆𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠
=
𝑆𝑆𝑆𝑆𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 = 6𝑙𝑙 2
𝑆𝑆𝑆𝑆𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 = 4𝜋𝜋𝑟𝑟 2
3
2𝑟𝑟
𝑙𝑙
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Surface Area - Units
Surface area unit = m2
Specific surface area unit = m2/g
 This is most commonly used in materials chemistry and is often just referred to
as “surface area”
 This is the gravimetric unit for surface area (based on mass)
 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
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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:
𝐷𝐷𝑠𝑠𝑠𝑠 =
18𝜂𝜂𝜂𝜂
𝜌𝜌𝑠𝑠 − 𝜌𝜌𝑙𝑙 𝑔𝑔
1/2
𝐷𝐷𝑠𝑠𝑠𝑠 = 𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑠𝑠 ′ 𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑
𝜂𝜂 = 𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓 𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣
𝜌𝜌𝑠𝑠 = 𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑
𝜌𝜌𝑙𝑙 = 𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑
𝑣𝑣 = 𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣
𝑔𝑔 = 𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎 𝑑𝑑𝑑𝑑𝑑𝑑 𝑡𝑡𝑡𝑡 𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔
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






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]∙mH2O
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]∙mH2O
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:
 water or solvent
 organic templating agents
 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)
 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:


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?
 scale buildup on dishes/appliances
 dry/itchy skin
 stains in sinks/bathtubs
 plumbing damage
 clothing grey/faded
39
Zeolite Applications
1) Ion-Exchange:
40
Zeolite Applications
2) Adsorption:
Removal of water; desiccants or molecular sieves


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

Breathing apparatus’ used by scuba divers and
firefighters, zeolites help to concentrate the
oxygen being supplied by removing N2 and
other impurities
Chemical Separations

Separating xylene isomers in the petroleum industry based on shape
o-xylene
m-xylene
p-xylene
42
Zeolite Applications
3) Catalysis:
Hydrocarbon cracking:



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