characterization of adsorbents
TRANSCRIPT
Characterization
of Adsorbents
Ali Ahmadpour
Chemical Eng. Dept.
Ferdowsi University of Mashhad
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Contents
Characterization
Different types of pore
Pores evaluation methods
Methods for PSD & PVD calculations
Adsorption Parameters of Activated Carbon
Activated Carbon Properties
Bulk density and porosity
Conclusions
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Characterization
The most important characteristic of an adsorbent is itshigh porosity.
Physical adsorption measurements are widely used forcharacterization of porous materials.
The physical characteristics of porous materials arisefrom their texture and morphology:
Pore size
Pore shape
Pore size distribution
Pore volume
Specific surface area
Density
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Different types of pore
Pore shape is mainly unknown, but it could be approximated by
the model. Three basic pore models exist:
Closed pores
Blind pores (open at one end)
Through pores (open at two ends)
Blind and through pores can be:
1) Ink-bottle pores having a narrow neck and wide body
2) Cylindrical pores, circular in cross section
3) Funnel shaped or slit shaped pores with parallel plates
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Pore models
Blind pore that results in over
estimation of surface area.
Variation of pore diameter
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Pore Shapes
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Pore size, shape and distribution
One of the most important adsorbent parameters is the
pore size and pore size distribution.
Pore size defines an ability of the analyte molecules to
penetrate inside the particle and interact with its inner
surface.
Pore size distribution is the secondary parameter. This
could be measured by mercury porosimetry, or low-
temperature gas adsorption-desorption (BET method).
8
Cont.
The surface of the micropores are usually accounted for in
the adsorbent surface area measured by BET method, but
most of the analysis molecules could not penetrate in this
small pores.
Mesopores are partially accessible for molecules but
molecular diffusion into the pore space are restricted by
steric hindrance effect, which significantly slows mass
transfer.
9
Cont..
Specific pore volume, Vp, is the sum of volumes of all
pores in one gram of adsorbent.
Pore volume (Vp), specific surface area (S), and mean pore
diameter (D) are correlated to each other.
Specific surface area is said to be inversely proportional to
D.
There is no exact relationship between these parameters.
The correlation strongly depends on the adsorbent pore
type and shape.
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Pore diameters and measurement
techniques
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Pores evaluation methods
Experimental techniques:
Gas adsorption
Pre-adsorption
Retention & Adsorption from solution
Mercury intrusion
Physical methods (SAXS)
Empirical methods:
Comparative methods (t and s plots)
BET, Langmuir, DR, DA and DS equations.
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Gas adsorption
He adsorption at 4.2K
N2 adsorption at 77K
CO2 adsorption at 273 and/or 298K
Hydrocarbons at room temp. (benzene, n-butane, iso-
butane, propane, iso-octane, cyclohexane, etc)
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Adsorption Measurement
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Different types of
adsorption isotherms
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Langmuir adsorption isotherm
(Type I)
Assumptions:
Homogeneous surface (all adsorption sites energetically
identical)
Monolayer adsorption (so no multilayer adsorption)
No interaction between adsorbed molecules
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Type II and IV isotherms
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Type III and V isotherms
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Surface area & monolayer
capacity
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Properties of adsorbates for
physisorption measurements
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He adsorption
Advantages of He:
He atom is the smallest spherical monoatomic molecule
and interact weakly with any solid surface. Therefore, He
adsorption at 4.2K is a promising method for the accurate
assessment of the microporosity of microporous solids.
The dispersion potential of He is very small.
Disadvantage of He:
At high pressure region micropore volume obtained from
He adsorption has more error than N2.
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N2 adsorption
Advantage of N2:
Adsorption of N2 in multilayer part on a solid is insensitive to
any change in the chemical nature of the surface (unique).
Disadvantages of N2:
The main disadvantage of N2 is that it is somewhat atypical in
its molecular size and shape and hence in its micropore filling
behaviour (gives higher micropore volume).
Another reason for higher micropore volume is that the packing
density of N2 molecules in narrow pores does not confirm to that
in the liquid state.
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CO2
adsorption
Advantages of CO2:
CO2 is readily available gas and the temperature of 273-298K
are easily maintained.
Because of low relative pressure of CO2 at room temp. only
narrow microporosity is measured.
At these temps. the activated diffusion problem at low temp.
is avoided.
The isotherm can be obtained within 24 hours.
Disadvantage of CO2:
Uncertainty about the state of CO2 in the micropores makes
confusion the selection of molecular diameter of CO2 and the
density of the adsorbed phase.
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Benzene adsorption
Advantage of Benzene:
In adsorption of benzene the whole range of relative
pressure can be covered at room temperature.
Because of its flat molecules it can penetrate into the slit-
shaped structure of carbons.
Disadvantage of Benzene:
The use of benzene in carbons with a large no. of surface
groups is questionable because specific interactions are
possible between the benzene molecule and the carbon
surface groups. These interactions are eliminated if light
alkanes are used.
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Main characteristics for good
adsorbate
Chemical inertness
Relatively large saturation pressure (wide range of P/P0
can be covered)
A convenient adsorption temp. attainable with simple
cryogenic systems (liquid N2, solid CO2, ice, etc)
A molecular shape as close as possible to a sphere (to
avoid uncertainties in the calculation of surface area due to
the different orientations on the solid surface)
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Classification of ACs based on N2
(77K) and CO2 (273K) adsorption
(expressed as surface area or micropore volume)
N2<CO2: carbonized materials, CMS, AC with very low
burn-off (<5%)
(very narrow microporosity)
N2CO2: ACs with low-to-medium (<35%) burn-off and
some CMS
(narrow & homogeneous microporosity)
N2>CO2: ACs with medium-to-high burn-off
(wide & heterogeneous microporosity)
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Pre-adsorption method
(used for determination of micropore volume or surface area)
The method involves filling the micropores with large
molecules (e.g. n-nonane) which are not removed by
outgassing the adsorbent at low temperature. (The carbon
impregnate with n-nonane at 298K and evacuate at 10-3
torr to constant weight and the n-nonane volume retained is
converted to surface area)
Disadvantage:
The interpretation is not easy for adsorbent with a wide
distribution of micropores.
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Retention of EG
In this method the carbon is wetted with EG and then
outgassed at 35C to constant weight. The amount of
adsorbate retained is converted to a surface area value.
Disadvantage:
The retention of EG is affected by:
Micropore size distribution
Existence of oxygen surface complexes
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Adsorption from solution
One of the solutes more commonly used is
paranitrophenol (PNP), considered by Giles, 1970. The
concentration of PNP is spectrophotometrically
measured.
Adsorption of iodine from solution determined by
titration is very much used, mainly in industrial tests.
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Comparative methods
t-plot: (Lippens & deBoer, 1964)
Simple and direct method of comparing the shapes of the
isotherm (for a given adsorptive) of the sample with that
of a standard nonporous reference. Amount adsorbed (n)
plot vs. multilayer thickness (t=n/nm) of the standard.
s-plot: (Sing, 1970)
To avoid the dependency, t is replaced by s=n/ns (ns is
the amount adsorbed at selected relative pressure).
In practice s=0.4 for N2 adsorption at 77K
s method is independent of BET theory.
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t- plot method
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Shape of t-plots
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t-curves
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The statistical layer thickness t(nm) versus reduced pressure for nitrogen
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Empirical equations
In practice major problems arise between values of surface
area using empirical equations obtained from:
Isotherms of different adsorbates at the same adsorption
temperatures.
Isotherms of the same adsorbates at different adsorption
temperatures.
Isotherms of different adsorbates at different or close
adsorption temperatures.
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Cont.
The differences come from:
Activated diffusion (at low temp.)
Molecular sieve effect
Cooperative effects (adsorbate-adsorbate interaction). It is
a function of pore diameter, pore shape, structure of
adsorbent surface and adsorbate molecule.
(multilayer adsorption in supermicropores at low relative
pressure of 0.1-0.2 which makes the value of monolayer
coverage unrealistically high)
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Methods for PSD & PVD
calculations
Molecular probe method:
semi-quantitative estimate of the micropore size
distribution.
TVFM:
DR, DA and DS equations appear superior to Langmuir or
BET equations in terms of describing the microporosities
of carbons. In general the basic is that characteristic energy
(E0) decrease as micropore are widened. (E0=k/x0)
N2 adsorption at 77K:
Suitable for mesopore size distribution.
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DR equation
P
PlnRTA :here w
E
AexpWW 0
2
0
0
E
RTD :re whe
P
PlnDWlnWln
2
0
020
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DS equation
22
0
22
220
22
00
Am212
Xerf1
Am21
AX mexp
Am212
WW
2
Ex=k and
k
1m :Where 00
2
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TVFM method for PSD
Advantage:
TVFM method is simple and applicable over most of the
range of micropore sizes.
Disadvantages:
At low pressures or low filling this method is
questionable.
The selection of micropore volume distribution
(constrain).
Uncertainties in the correlation of E0 with the slit width
and the local isotherm.
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PSD of -Alumina
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Mercury intrusion method
Pore size and pore size distribution can be determined.
Since the volume of mercury can be determined veryaccurately, pore size distribution can be determined quiteprecisely.
Mercury is forced into a dry pores with the volume of
mercury being determined at each pressure.
The relationship between pressure and pore size is given by
the Laplace equation. Because mercury does not wet the
pores, Laplace equation is modified to:
cos2
Prp
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Mercury Intrusion Porosimetry
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Cont.
44
N2 adsorption isotherms & pore
volume distributions
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N2 adsorption isotherms & PVD
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N2 adsorption isotherms & PVD
47
Adsorption Parameters of
Activated Carbon
Capacity vs. Kinetics (Rate):
Capacity parameters determine loading characteristics of activated
carbon. Maximum adsorption capacity of activated carbon is only
achieved at equilibrium.
Kinetic parameters only determine the rate of adsorption and have
negligible effect on adsorption capacity.
Surface Area: Adsorption capacity is proportional to surface area
(determined by degree of activation).
Pore Size: Correct pore size distribution is necessary to facilitate the
adsorption process by providing adsorption sites and the appropriate
channels to transport the adsorbate.
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Cont.
Particle Size: Smaller particles provide quicker rates of adsorption.
Note: Total surface area is determined by degree of activation and pore
structure and not particle size.
Temperature: Lower temperatures increase adsorption capacity
except in the case of viscous liquids.
Concentration of Adsorbate: Adsorption capacity is
proportional to concentration of adsorbate.
pH: Adsorption capacity increases under pH conditions, which
decrease the solubility of the adsorbate (normally lower pH).
Contact Time: Sufficient contact time is required to reach
adsorption equilibrium and to maximize adsorption efficiency.
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Activated Carbon Properties
Iodine Number
most fundamental parameter used to characterize activated carbon performance
measure of activity level (higher number indicates higher degree of activation)
measure of micropore (0 – 20 Å) content
equivalent to surface area of activated carbon between 900 – 1100 m2/g
standard measure for liquid phase applications
Methylene Blue
measure of mesopore structure (20 – 500 Å)
50
Cont.
Molasses No. (Caramel dp )
measure of macropore structure (>500 Å)
important for decolorizing performance
Surface Area
measure of adsorption capacity (Note: pore sizedistribution/pore volume is also important to determine ultimate performance)
Apparent Density
higher density provides greater volume activity and normally indicates better quality activated carbon
51
Cont.
Particle Size
smaller size provides quicker rate of adsorption which
reduces the amount of contact time required
smaller size results in greater pressure drop
Hardness/Abrasion Number
Hardness/Abrasion is a measure of activated carbon`s
resistance to attrition
important indicator of activated carbon to maintain its
physical integrity and withstand frictional forces
imposed by backwashing etc.
52
Cont.
Ash Content
reduces overall activity of activated carbon
reduces efficiency of reactivation
metals (Fe2O3) can leach out of activated carbon resulting in discoloration
acid/water soluble ash content is more significant than total ash content
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Bulk density and porosity
The bulk density (b) is an important property, especially for
storage and transportation, rather than separation processes.
b = (mass / total volume occupied by the material). Total
volume includes air trapped between the particles.
The volume fraction trapped between the particles is known as
the porosity ().
s
b1
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Cont.
True (Skeletal) density: measured with helium (mass / volume of the solid).
Apparent density: measured by liquid
displacement (mass / voids volume + solid volume).
Bulk densities:
Loose density: (mass / total volume occupied by the material).
Compact (tap) density: (mass / total volume occupied by the
material after mechanical compression).
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Conclusions
Microporous adsorbent such as ACs are extremely
difficult materials to characterize in terms of structure
and porosity.
There is no reliable procedure available for the
computation of the MPSD from a single isotherm.
Gas adsorption measurements are widely used for
determination of surface area and PSD of different solid
materials.
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Cont.
In adsorption studies each adsorbate provides unique but
partial information. Therefore, several appropriate
adsorbates should be used over a wide range of
adsorption temp. and pressure.
N2 adsorption method can be used for routine adsorption
studies of porous carbons but the limitation of the method
should be kept in mind.