Characterization of Adsorbents

Characterization

of Adsorbents

Ali Ahmadpour

Chemical Eng. Dept.

Ferdowsi University of Mashhad

2

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

3

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

4

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

5

Pore models

Blind pore that results in over

estimation of surface area.

Variation of pore diameter

6

Pore Shapes

7

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.

10

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

14

Different types of

adsorption isotherms

15

Langmuir adsorption isotherm

(Type I)

Assumptions:

Homogeneous surface (all adsorption sites energetically

identical)

Monolayer adsorption (so no multilayer adsorption)

No interaction between adsorbed molecules

16

Type II and IV isotherms

17

Type III and V isotherms

18

Surface area & monolayer

capacity

19

Properties of adsorbates for

physisorption measurements

20

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.

21

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.

22

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.

23

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)

25

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)

26

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

34

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.

35

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

42

Mercury Intrusion Porosimetry

43

Cont.

44

N2 adsorption isotherms & pore

volume distributions

45

N2 adsorption isotherms & PVD

46

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.

48

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

54

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.

56

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.