Co

5.02 Adsorption PropertiesY Hattori and K Kaneko, Shinshu University, Nagano, JapanT Ohba, Chiba University, Chiba, Japan

ã 2013 Elsevier Ltd. All rights reserved.

5.02.1 Introduction 255.02.1.1 Nanoporous System 255.02.1.2 Vapor and Supercritical Gas 265.02.1.3 Adsorption, Absorption, and Sorption 275.02.2 Gas–Solid Interaction 285.02.2.1 Dispersion Interaction 285.02.2.2 Molecule–Pore Interaction 285.02.2.3 Contribution of Electrostatic Interaction 315.02.3 Vapor Adsorption 315.02.3.1 Adsorption Isotherm and Adsorption Mechanism 315.02.3.1.1 Type I adsorption isotherm 315.02.3.1.2 Type II and III adsorption isotherms 325.02.3.1.3 Type IV and V adsorption isotherms 335.02.3.2 Role of Modeling in Adsorption in Nanopores 345.02.4 Supercritical Gas Adsorption 355.02.5 New Developments in Adsorption 375.02.5.1 Adsorption of Water Vapor in Hydrophobic Pores 375.02.5.2 Quantum Effect in Physical Adsorption 395.02.5.3 Gate Adsorption 415.02.6 Conclusion 42Acknowledgments 42References 43

5.02.1 Introduction

5.02.1.1 Nanoporous System

IUPAC classified pores into micropores, mesopores, and macro-

pores with the pore width w1 (see Table 1). Here, w is the slit

width of the slit pore and the diameter of the cylindrical pore, as

shown in Figure 1. The w is not defined by the internuclear

distance and it can be evaluated by the vapor adsorption tech-

nique. Although IUPAC does not define the term nanopore, the

nanopore should be defined as a pore whose width is less than

100 nm, embracing almost efficient pores according to the future

IUPAC recommendation. Representative microporous solids are

zeolites and activated carbons. Zeolites have micropores origi-

nating from the crystal structure. We can modify the preparation

method to add mesopores to zeolites because zeolites having

mesopores are better catalysts without intrapore diffusion sup-

pression, gathering a great affection.2 Activated carbon has an ill-

crystalline structure and so many activated carbons have a wide

pore width distribution. However, activated carbon fiber (ACF)3

and carbide-derived carbon4 have only uniform micropores.

Silica gel is a representative mesoporous solid, used widely as a

desiccant agent. These porous solids have a long history and have

been extensively applied to various technologies. As nanoporous

solids hold promise in the development of sustainable technol-

ogies, new nanoporous solids have been developed since

1990.5,6

Carbon nanotubes were found by transmission electron

microscopy and they were prepared by the arc discharge

mprehensive Inorganic Chemistry II http://dx.doi.org/10.1016/B978-0-08-09777

method, laser vaporization, and chemical vapor deposition.7,8

Recently, highly pure single-wall carbon nanotubes (SWCNTs)9

and double-wall carbon nanotubes (DWCNTs)10 were prepared

and their unique adsorption properties were reported.11,12

As carbon nanotubes have unique physical properties, such as

high electronic conductivity, high thermal conductivity, and

high mechanical strength, their adsorption properties have

been actively studied for application to energy storage and gas

sensors. Mesoporous silicas that show sharp x-ray diffraction

patterns due to the ordered mesopore structure were developed

in Japan and the United States using the template of supra-

molecular assemblies of surfactant micelles.6,13,14 The novel

family of mesoporous silica has been extending year by

year.15,16 New nanoporous materials, the so-called porous coor-

dination polymers (PCPs) or metal–organic frameworks

(MOFs), appeared on the scientific stage shortly after the carbon

nanotube and mesoporous silica.17–21 PCPs have a completely

different framework structure compared to other porous solids.

Zeolites, activated carbons, carbon nanotubes, and mesoporous

silicas have porewalls, whereas PCPs have pillars, not porewalls.

Consequently, PCPs offer the advantage of largepore (lp) vol-

ume and surface area. As PCPs have coordination bondings for

the linkage ofmolecular pillars, they have excellent designability

for the structure, giving rise to explosive research activity. Their

adsorption properties have attracted a great deal of attention.

The physical and chemical properties of zeolites, nano-

porous carbons including nanotubes, PCPs, and silicas includ-

ing mesoporous silicas are summarized in Table 2. As active

4-4.00502-7 25

26 Adsorption Properties

studies on PCPs have produced new types such as thermally

stable covalent organic frameworks,21 the characteristics of

PCPs can be changed. Table 2 indicates the uniqueness of

porous carbons. They exhibit high electronic conductivity

and so they are applied to electrochemical devices such as

supercapacitors.22,23 Although zeolites have no super high

surface area, they have been widely applied because of their

unique properties. Mesopore-range zeolites are new porous

solids, exhibiting better adsorptivity, ion exchangeability, and

catalysis.24 Thus, both traditional and newly developed nano-

porous solids have been evolving, stimulating research on their

gas adsorption properties.

5.02.1.2 Vapor and Supercritical Gas

We must distinguish vapor and supercritical gas based on the

description of their gas adsorption behaviors. Adsorption

behaviors of vapor and supercritical gas are completely differ-

ent from each other, even though the nature of the gas–solid

interaction is identical. Figure 2 shows a phase diagram of

substances. The line of coexistence of liquid and gas is

expressed by the curve OC in Figure 2. Both gas and liquid

are stable below the critical temperature Tc; the gas below Tc is

Table 1 Classification of pores

Micropore w<2 nmMesopore 2 nm<w<50 nmMacropore w>50 nm

‘Nanopore’ is not recommended by IUPAC, but it is often used for pores whose width is

less than 100 nm. Ultramicropore is often used for a pore whose width is less than

0.7 nm, although it is not a term officially recommended by IUPAC.

w

(b)

w

(a)

Figure 1 Schematic diagram of representative pores: (a) slit pore and(b) cylindrical pore.

Table 2 Characteristics of nanoporous solids

Zeolite C

Electrical conductivity � ○Thermal conductivity � ○Thermal stability ○ ○Antioxidation property ○ �Hydrophobicity ○ ○Ion exchangeability ○ �Pore size Micropore MUniform porosity ○ DTunability of pore size ○ DHigh surface area (>1000 m2 g�1) � ○

named vapor, which has an intrinsic saturated vapor pressure

P0 that depends on the temperature. Here the temperature

dependence of P0 is expressed by the OC curve. Once the

temperature goes over Tc (T>Tc), there is no coexisting region

between gas and liquid. The state at T>Tc and pressure

P>critical pressure Pc is called supercritical fluid state. Mole-

cules in supercritical fluid form clusters with short half-lives

and the density of the supercritical fluid fluctuates remarkably

in space and time. Supercritical fluid has different characteris-

tics compared to both liquid and gas. Saturated vapor pressure

cannot be defined above Tc and the state above Tc is known as

supercritical gas, including supercritical fluid. As the sorption

phenomenon is the formation of a liquid-like molecular

assembly on a solid surface, as described later, vapor coexisting

with the liquid gives a predominant sorption phenomenon.

On the contrary, supercritical gas cannot induce a marked

sorption on solids.

There are many important supercritical gases whose Tc is

lower than ambient temperature. O2, N2, NO, H2, and CH4 are

representatives of supercritical gas. The Tc of CO2 is 304.2 K

and then CO2 at an ambient temperature (<Tc) is called a

subcritical gas being close to supercritical gas rather than

vapor. Table 3 summarizes the physical properties of impor-

tant gases.25,26 Here Tb is the boiling temperature. The units of

the dipole and quadrupole moments are (C m) and (C m2),

respectively. Although NO and CO have both the dipole

and quadrupole moments, their quadrupole moments of

the higher order are not shown because of their negligible

contribution to the gas–solid interaction. These dipole and

arbon PCP Silica

� �� �D ○� ○� �� �

icro- and mesopore Micropore Mesopore○ ○○ ○○ ○

SolidLiquid

Gas

O

C

Supercritical gas

Tc T

P

Figure 2 Phase diagram of substance.

Table 3 Properties of important gases

Molecule Tb (K) Tc (K) Pc (MPa) sff (nm) eff/KB (K) Multipole moment Magnetism

H2 20.3 33.0 1.29 0.292 38.0 Quadrupoleþ2.1�10�40

Diamag

O2 90.2 154.6 5.04 0.338 126.3 Quadrupole�1.33�10�40

Paramag

N2 77.3 126.2 3.39 0.363 104.2 Quadrupole�4.90�10�40

Diamag

NO 121.4 180 6.48 0.347 119 Dipole0.158�10�30

Paramag

CO 81.6 132.9 3.50 0.359 110 Dipole0.112�10�30

Diamag

CO2 194.7 304.2 7.48 0.376 245.3 Quadrupole�14.9�10�40

Diamag

CH4 111.6 190.5 4.60 0.372 161.3 Octapole Diamag

Here Tb, Tc, and Pc are the boiling temperature, critical temperature, and critical pressure. The units of dipole and quadrupole moments are C m and C m2, respectively.

Although NO and CO have the quadrupole moment in addition to the dipole moment, their quadrupole moments are omitted. ‘Diamag’ and ‘Paramag’ denote diamagnetism

and paramagnetism, respectively.

Adsorption Properties 27

quadrupole moments induce an electrostatic interaction with a

solid, which can contribute to about 10% of the whole inter-

action at a maximum. The electrostatic interaction can play an

important role in intermolecular orientation in the adsorbed

state. (diamag) and (paramag) denote diamagnetism and para-

magnetism, respectively. Almost all gases exhibit diamagne-

tism and therefore O2 and NO are quite unique in this point.

The paramagnetic property of molecules may affect their

adsorption on a solid remarkably. NO at an ambient temper-

ature is a supercritical gas. Consequently, NO molecules are

predominantly not physically adsorbed even on nanoporous

solids at room temperature. With a spin–spin interaction, NO

molecules form the dimer, which is the vapor at room temper-

ature. Addition of iron oxide nanoparticles at the entrance of

carbon micropores can enhance the NO dimerization, leading

to the predominant physical adsorption of NO on the nano-

porous carbon; the adsorption amount of NO is over 30% of

the weight of the adsorbent.27,28 The dimer formation of O2

molecules in the pores of PCP at low temperatures is evidenced

by x-ray diffraction analysis.29 The dimer formation of O2 in

the pores of zeolite at low temperatures is also reported from

the magnetic susceptibility measurement.30 Thus, the magnetic

property of gas must be taken into account in adsorption on

nanoporous solids.31

5.02.1.3 Adsorption, Absorption, and Sorption

Brunauer describes the terminology of adsorption, absorption,

and sorption in his book32 thus: “The molecules that disappear

from the gas phase either enter the inside of the solid, or

remain on the outside, attached to its surface. The former

phenomenon is called absorption, the latter adsorption.

Often the two occur simultaneously; the total uptake of the

gas is then designated by the term sorption. (The term sorption

was introduced by J. W. McBain in 1909).” This definition of

adsorption, absorption, and sorption is still valid according to the

book by Rouquerol, Rouquerol, and Sing,33 although adsorp-

tion is more rigorously defined as ‘enrichment of one or more

of the components in the region between two bulk phases.’

Thus, the definition of adsorption, absorption, and sorption is too

phenomenological and unclear. One of the authors has pro-

posed the related terminologies of physical adsorption, chem-

isorption, absorption, and occlusion based on the structural

change in molecules and/or solids.34,35 These four interactions

are distinguished explicitly by the changes in the atomic struc-

ture of a molecule and/or a solid. No structural change occurs

in the molecule and the solid on physical adsorption. This is

because the main attractive interaction in physical adsorption

is the one of dispersion. On the other hand, chemisorption

induces an intensive interaction between the molecule and

the solid, changing the molecular structure because of the

chemical bond formation between the molecule and the

solid surface. The bulk solid does not change its structure on

chemisorption, although the solid surface does. The major

attractive interaction between a molecule and a solid is the

one of dispersion as in the case of absorption. Absorption of

molecules changes the solid structure but there is no molecular

structure change. Clay minerals often swell on contacting with

water vapor.36 This is a typical example of absorption. Gate

adsorption37 recently found in the PCPs is also a good example

of absorption, as described later. Also, absorption of molecules

in PCPs can be regarded as clathrate formation between the

molecules and PCP crystals.38 Occlusion follows structural

changes in the molecule and the solid. The well-known exam-

ple is the interaction between H2 and Pd solid; the H2molecule

dissociates into hydrogen atoms, forming a new lattice consist-

ing of hydrogen and Pd atoms. Thus, occlusion changes the

structure of both molecules and solid. This classification can

avoid confusion in understanding an interaction of molecules

with a nanoporous solid. Here, sorption can be used for the

process of physical adsorption and absorption in which mol-

ecules do not change their structure. However, the term ‘adsorp-

tion’ has often been used in the literature to mean sorption. In

this chapter too, adsorption is often used with the same mean-

ing as sorption. Of course, there can be a medium or a hybrid

interaction between two types of the interaction. In particular,

hydrogen bonding still presents an ambiguity in the classifica-

tion, because the hydrogen-bonding energy covers from several

kJ mol�1 to 160 kJ mol�1, being comparable to the dispersion

interaction to weak chemical bonding.39

z/ssf

Fsf/k

BK

1 2 3 4 5

-600

-400

-200

0

Figure 3 The relative change of the attractive interaction between amolecule and solid surface with the mutual distance.

28 Adsorption Properties

5.02.2 Gas–Solid Interaction

5.02.2.1 Dispersion Interaction

The predominant attractive interaction of sorption is the disper-

sion interaction. The origin of the dispersion interaction can be

understood using a quantum mechanical theory. We consider

two neutral molecules, (a) and (b), each of which has one

electron, numbered 1 and 2. The coordinates of these electrons

1 and 2 are assumed to be (x1, y1, z1) and (x2, y2, z2). When

electrons 1 and 2 interact with each other on approaching

the opposite molecule, the perturbation term H0 is expressed by

H∧ 0� e2

r3x1x2 þ y1y2 � 2z1z2ð Þ [1]

where r is the distance between the twomolecules. The primary

perturbation energy becomes zero and the secondary pertur-

bation is suggestive. When the wave functions of the two

molecules having electrons 1 and 2 are given by C1(1) and

C2(2), respectively, the wave function C of the system is

C ¼ 1ffiffiffi2

p C1 1ð Þ þC2 2ð Þf g [2]

Thus, the second order perturbation energy E000 can be

obtained as

E000 ¼ � coefficientð Þ e

4

r6r21r

22 [3]

Here

r2i ¼ðC*

i ið Þr2i Ci ið Þdti i ¼ 1, 2 [4]

If m2i ¼ e2r2i , mi is the mean value of a dipole moment when

the electron is continuously moving in the neutral molecule.

Then, the dispersion interaction Udisp is given by

Udisp ¼ � coefficientð Þm21m22r6

¼ �C6

r6[5]

where C6 is the constant. This dispersion interaction is in

inverse proportion to the sixth power of the intermolecular

distance. As the mi stems from an instantaneous deviation

of an electron, a larger molecule has a stronger dispersion

interaction.

We need to take into account the repulsive interaction. The

whole intermolecular interaction can be approximated by the

pair potential Uff(r).

Uff rð Þ ¼ 4effsffr

� �12 � sffr

� �6� �

[6]

which is known as the Lennard-Jones (LJ) potential for a single

component gas. (f) indicates a fluid molecule. Here sff is thesize parameter at Uff¼0 and eff is the potential depth. The LJ

potential is applicable to the interaction between neutral mol-

ecules and neutral solid surfaces. When the neutral molecule

(f) interacts with the ith surface atom having the mutual dis-

tance r, the molecule–solid atom interactionCsf (ri) is given by

Csf rið Þ ¼ 4esfssfr

� �12� ssf

r

� �6� �

[7]

Here, ssf and esf have the physical meaning corresponding

to eqn [6]. ssf and esf are simply approximated by Lorentz–

Berthelot rules as given by eqn [8].

esf ¼ ffiffiffiffiffiffiffiffiffiffieff ess

p,

ssf ¼ sff þ sssð Þ2

[8]

These LJ parameters are available in the literature.25,26 The

gas–solid interaction is obtained by addition of the pair-

interaction of eqn [7]. The gas–solid interaction Fsf (z) is

expressed by the vertical distance z between a molecule and

the solid surface, as given by eqn [9].

Fsf zð Þ ¼ Asfssfz

� �9� Bsf

ssfz

� �3[9]

The attractive interaction between a molecule and a solid is

still a short-range interaction. Figure 3 shows the relative

change in the attractive term in eqn [9] with the distance. The

abscissa is normalized by ssf. The effective range of the disper-sion interaction between a molecule and solid is four times the

distance of ssf; the effective distance is 1.5 nm at best for a gas

molecule. Then, the nano-range structure in a solid is essen-

tially important for gas adsorption.

5.02.2.2 Molecule–Pore Interaction

PCPs have a pillar-frame structure, different from the wall-

frame structure of zeolite and nanoporous carbon. Here, gas–

solid interactions are compared for both frame structures. We

introduce a neutral pillar consisting of 20 carbon atoms. A

nitrogen molecule is placed on the center of the pillar at the

vertical distance z. The dispersive interaction of the molecule

with the pillar is obtained by the LJ potential, providing the

potential minimum value. When the pillar number increases,

the potential becomes deeper. Figure 4 shows the potential

minimum change in the interaction with the carbon pillar

number.40 The potential becomes rapidly deeper up to the

Carbon pillars

Molecule

0 2 4 6 8 10 12

-1500

-1000

-500

Pillar number

Pot

entia

l min

imum

(K)

Figure 4 Relationship between the pillar number and interaction potential.

Pot

entia

l dep

th (k

BK

)

-0.2 -0.1 0 0.1 0.2-800

-600

-400

-200

0

Distance (nm)

2

3

1 4

Figure 5 Interaction potential profiles of N2 with two mutually parallel-oriented carbon pillars as a function of the interpillar distance of 2.0 (1), 2.2 (2),2.6 (3), and 3.0 (4) in terms of ssf. Configuration of a molecule and two pillars is shown.

Adsorption Properties 29

five pillar number; the molecule–five pillar interaction energy

is five times larger than the molecule–single pillar interaction

energy. The potential minimum is �1707 KkB for the 12 pillar

number. Consequently, a pillar belt of an optimum pillar

number is necessary for production of PCP having enough

interaction strength and high surface area. If the molecule is

inserted by two pillar belts, the interaction is significantly

enhanced.

Here, we discuss the interaction of an N2 molecule with two

carbon pillars. Figure 5 shows the interaction potential profiles

for N2 with two carbon pillars as a function of the interpillar

distance of 2.0–3.0 in terms of ssf. Here, the effective distance

between two carbon pillars for the interpillar distance¼2.0 is

only ssf. Therefore, the repulsion interaction between N2 and

two pillars is significant, giving the shallow minima. The inter-

pillar distance of two ssf provides the minimum value of

�733 KkB. Thus, putting a molecule between two pillars lowers

the interaction potential minimum. The overlapping effect

of the interaction potential from both pillars is remarkable.

When the carbon pillars form a regular triangular prism struc-

ture (the triangle length¼10sc and prism height¼7sc), the

deepest potential in the prism is �1020 KkB, which is slightly

shallower than that of the graphite slit of w¼0.7 (�1200 KkB).

When pillars build a three-dimensional structure, the structure

leads to a considerably deep interaction potential minimum.

On the contrary, the complex pillar structure sacrifices the

lp volume and surface area. Consequently, the pillar-frame

structure is suitable for adsorbing vapor rather than supercrit-

ical gas.

How can we describe the molecule–slit pore interaction?

The slit pore is the extreme case of the two belt-model. How-

ever, an ordinary slit pore model consists of a slit space

between two bulk slabs whose thickness and breadth are infi-

nite. If the slit width between two surface atoms is H, the

molecule–slit pore interaction ’p(z) is given by the summation

of both solid–molecule interactions:

’p zð Þ ¼ ’sf zð Þ þ ’sf H � zð Þ [10]

Here, the molecule is situated at the distance z from one

solid surface. It must be kept in mind that H (the so-called

physical width) is not w (experimentally measured pore

30 Adsorption Properties

width). The contribution of the effective thickness of the elec-

tron cloud of the solid surface must be subtracted. Kaneko et al.

showed the following relation of wide applicability for the

slit pore41:

w ¼ H � 2z0 � sffð Þ, z0 ¼ 0:856 ssf [11]

Here z0 is the distance of the closest approach. In the case of

N2 adsorption on graphite slit pores,

w¼H � 0:24 nmð Þ, effkB

¼101:5 K, sff ¼ 0:3615 nm

� �[12a]

the simple relation given by eqn [12a] is obtained.

We can use the Steele 10-4-3 potential for graphite slit

pores.42 Figure 6 shows the potential profile of an interaction

between a graphite slit pore and a one-center nitrogen mole-

cule or a one-center hydrogenmolecule. The interaction poten-

tial becomes deeper with a decrease in w. The potential

z (nm)

f/k B

K

N20.5 nm

0.7 nm

1.0 nmw = 1.6 nm

-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8-2500

-2000

-1500

-1000

-500

0

Figure 6 Molecular potential profiles of the slit-shaped pore of carbon mate

A B

InterstitialGrooveA

F/k

BK

0.0

0

-400

-800

-1200

Figure 7 The bundle structure of SWCNT and the interaction potential profi

minimum for the slit pore of w¼0.5 nm is much deeper than

that for the pore of w¼1.6 nm, which is slightly deeper than

that of the flat surface. The potential minimum for w¼0.5 nm

and N2 is �2020 KkB, whereas that for H2 is only �690 KkB,

twice that of thermal energy at room temperature. This is the

reason why H2 is not abundantly adsorbed in carbon micro-

pores at room temperature. The interaction profile of a mole-

cule confined with cylindrical pores such as SWCNTs gives a

deeper potential minimum than the slit pore.

Also, SWCNT has the interaction potential energy with a

molecule depending on the sign of the nanoscale curvature,

that is, the internal and external wall surfaces. The internal

surface–molecule interaction provides a deeper potential min-

imum than the external surface–molecule interaction. The dif-

ference between both depths is about 100 KkB for H2 and

SWCNT of the tube diameter of 3 nm; the potential depth in

the inner position is �420 KkB. SWCNTs form the bundle

structure of a hexagonal symmetry as shown in Figure 7.43

f/k B

K

H2

0.5 nm0.7 nm

1.0 nmw = 1.6 nm

-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8-2500

-2000

-1500

-1000

-500

0

z (nm)

rials as a function of pore width w for N2 and H2.

B

r (nm)0.5 1.0 1.5 2.0

Quantum effective (77 K)Classical

G

IC

IP

2.5 3.0

le of H2 with the bundle (the diameter of SWCNT¼1.356 nm).

Adsorption Properties 31

Figure 7 shows the interaction potential profile between H2

and the SWCNT bundle. The interstitial channel is surrounded

by three tube surfaces, giving the deepest potential minimum

even though all three tubes have the positive sign of the curva-

ture. The interstitial channel has the potential for adsorption of

supercritical H2. Nevertheless, the absolute capacity is too

small. As the interaction potential for carbon nanotubes can

be evaluated using analytical functions,44 carbon nanotubes

have explicit merit as a model system.

5.02.2.3 Contribution of Electrostatic Interaction

Electrostatic interaction must be taken into account in the

intermolecular interaction of a polar molecule. A water mole-

cule is a typical polar molecule. The electrostatic contribution

is added to the LJ potential, as expressed by eqn [12b]

Fff rð Þ ¼ 4effsffr

� �12� sff

r

� �6� �

þX4i

X4j 6¼ið Þ

1

4pe0

didjrij

[12b]

Here eff/kB¼80.5 K and sff¼0.312 nm for water. There are

many potential models for a water molecule. In the case of

TIP5P potential of a five-site model,45 the negatively charged

interaction sites are located symmetrically along the lone-pair

directions at an angle of 109.47�. A charge ofþ0.241e is placed

on each hydrogen site and charges of �0.241e are placed on

the lone-pair interaction sites. There is no charge on oxygen for

the TIP5P model. Thus, the electrostatic interaction can be

added to the dispersion interaction in the intermolecular inter-

action. The contribution of the charges of an adsorbedmolecule

to the molecule–solid interaction can be evaluated with the

image potential approximation for a conducting solid.46 When

we treat dipolar and/or quadrupolar molecules such as SO2,

CO, N2, and CO2, other than water, the electrostatic interaction

is the key to determine both the intermolecular orientation

structure and the adsorbed structure on the wall surface.

5.02.3 Vapor Adsorption

Adsorption properties are measured quantitatively using static

and dynamic methods. The static method gives the so-called

adsorption isotherm, which provides vapor–solid interaction

information in an equilibrium state. Accordingly, the adsorp-

tion isotherm measurement is indispensable to the discussion

of the adsorption properties of a solid. The adsorption iso-

therm of vapor can be measured with manometric and gravi-

metric methods. Details of the adsorption measurement are

not described here.33 Adsorption of vapor can now be mea-

sured from the relative pressure P/P0 of 10�6 by using a high

quality vacuum system and precision pressure gauges. The low-

pressure vapor adsorption isotherm from P/P0¼10�6 with

multi-measuring points is often called a high-resolution

adsorption isotherm. This is because the adsorption measure-

ments below P/P0¼10�3 provide information on the fine

structure of subnanometer pores and adsorption

mechanisms.47 One of the authors showed the effectiveness

of the adsorption measurement from P/P0¼10�9 for evalua-

tion of subnanometer pores.48

The amount of adsorption depends on the equilibrium

pressure of vapor, the adsorption temperature, vapor, and the

amount of solid adsorbent in the system. The adsorption is

measured at a constant temperature, and the relationship

between the adsorption amount and the equilibrium pressure

P is obtained. The equilibrium pressure and the adsorption

amount are expressed by P/P0 and the amount per unit

weight of the adsorbent, respectively. The adsorption

amount is expressed by the volume at STP (273.15 K

and 101.32 kPa) or weight, depending on whether the

manometric or gravimetric method is used. The adsorption

mechanism can be determined from a glance at the adsorption

isotherm. Accordingly, the shape of the adsorption isotherm

must be memorized before analyzing it in a routine way.

5.02.3.1 Adsorption Isotherm and Adsorption Mechanism

Adsorption of vapors on flat surfaces, mesoporous solids, and

micropores proceeds through inherent mechanisms of multi-

layer (layer-by-layer) adsorption, capillary condensation com-

bined with multilayer adsorption, and micropore filling,

respectively. The different adsorption mechanisms give the

inherent adsorption isotherms. IUPAC recommended six

types of vapor adsorption isotherms, as shown in Figure 8;

however, new recommendations will be made in the near

future. The type I, II, and IV isotherms stem from micropore

filling, multilayer adsorption, and capillary condensation with

multilayer adsorption, respectively. Isotherms of types I–V are

discussed in the following sections. A special type VI isotherm

is not discussed in this chapter.

5.02.3.1.1 Type I adsorption isothermMicropores have a deep interaction potential well for vapor

molecules and therefore a predominant adsorption begins

from a very low P/P0 range below 10�3, depending on the

interaction potential depth. Adsorption is almost complete

below P/P0¼10�1, leading to the sharp adsorption uptake

accompanying the plateau due to adsorption saturation. The

adsorption amount in the micropores is significantly larger

than that on the external surfaces at the higher P/P0 region.

Therefore, we can observe a type I isotherm for micropore

filling. In micropores, layer-by-layer adsorption on the pore

walls, which is explained later, does not proceed and almost all

pores are filled below the P/P0 corresponding to the monolayer

formation on the flat surface. This remarkably enhanced

adsorption, which starts even below P/P0¼10�3, is called

micropore filling. When analyzing the stepwise mechanism

of micropore filling, an adsorption isotherm whose abscissa

is expressed by the logarithm of P/P0 is preferred. Figure 9

shows N2 adsorption isotherms of ZMS-5 at 77 K.49 In the

linear P/P0 expression, N2 adsorption increases vertically

at P/P0�0 and then the adsorption curve bends sharply,

becoming parallel to the horizontal axis. This adsorption

isotherm is representative of the type I isotherm. When we

express the adsorption isotherm in the logP/P0, a pre-

dominant adsorption starts below P/P0¼10�3. The type I iso-

therm can be described by the Dubinin–Radushkevich (DR)

equation.50,51

(a) (b)

00

50

100

150

200

250

0.2 0.4 0.6

P/P0 P/P0

Am

ount

ad

sorb

ed (1

0−3m

3S

TPkg

−1 )

Am

ount

ad

sorb

ed (1

0−3

m3

STP

kg−

1 )

0

50

100

150

200

250

0.8 10−5 0.0001 0.001 0.01 0.1 11

Figure 9 The nitrogen adsorption and desorption isotherms of ZMS-5 at 77 K, (a) linear and (b) logarithmic scale.

Relative pressure P/P0

Am

ount

ad

sorb

ed

II III

V

I

VIIV

Figure 8 The six types of gas adsorption isotherms according to the IUPAC classification.

32 Adsorption Properties

W

W0¼ exp � A

bE0

� �2( )

, A ¼ RT lnP0P

[13]

Here W, W0, A, b, and E0 are the amount of adsorption at

the equilibrium pressure P, the micropore volume, the adsorp-

tion potential, an affinity coefficient, and the characteristic

adsorption energy, respectively. bE0 gives the isosteric heat of

adsorption at fractional filling f of e�1 using the enthalpy of

vaporization DHV as follows52:

bE0 þ DHV ¼ qst,’¼1=e [14]

The theoretical validity of the DR equation has been ques-

tionable owing to the non-transformation to Henry’s law at

P!0; the contradiction was resolved by statistical mechanical

study.53 Also, the Langmuir adsorption equation of eqn [15] is

effective for description of the type I isotherm, which has a

considerably steep uptake near P/P0¼0,

W

WL¼ aLP

1þ aLP[15]

aL and WL are the constant and the saturated adsorption

amount corresponding toW0 in the DR equation, respectively.

Nevertheless, the Langmuir adsorption equation was devel-

oped to describe chemisorption. In the case of micropore

filling, micropores have sufficiently strong interaction energy

to induce a Langmuir isotherm. Then,WL and aL have different

meanings from the chemisorption phenomenon. The evalua-

tion of the specific surface area of microporous solids must be

carried out carefully. An accurate determination of monolayers

in micropores is difficult because of the enhanced adsorption.

Such micropore effects must be subtracted to obtain a true

value of the specific surface area.54–56 The Brunauer, Emmett,

and Teller (BET) surface area described later gives a highly

overestimated value for small micropores.

5.02.3.1.2 Type II and III adsorption isothermsThe type II adsorption isotherm is the most familiar of exper-

imental isotherms. The multilayer adsorption of BET theory

was originally developed to interpret the type II isotherm.57

Hence, this adsorption isotherm is indicative of the multilayer

Adsorption Properties 33

adsorption mechanism, suggesting the presence of nonporous

flat surfaces or macropores. The so-called BET plot is given

by eqn [16]

P=P0V 1� P=P0ð Þ ¼

1

Vmcþ c� 1

Vmc

P

P0[16]

V and Vm are the adsorption amount at P/P0 and the mono-

layer capacity, respectively. c is the constant, which is approx-

imated by eqn [17].

c ¼ e E1�ELð Þ=RT [17]

where E1 and EL are heat of adsorption of the first layer (mono-

layer adsorption) and heat of liquefaction, respectively. The

plot of P/V(P0�P) against P/P0 gives a straight line for the

multilayer adsorption. The addition of the intercept and

slope provides Vm. The available P/P0 region is 0.05–0.35.

The BET surface area is derived from the above plot using the

occupied N2 molecular area of 0.162 nm2 in the case of N2

adsorption measurement at 77 K. The BET surface area can be

used for a common measure of the specific surface area of

nanoporous materials. Nevertheless, the BET surface area of

microporous materials whose width is more than twice the

probe molecular size (about 0.7–1.2 nm) is considerably over-

estimated; the surface area is overestimated by about 30% in

the slit micropores. Consequently, the subtracting pore effect

(SPE) method54,55 using the comparison plot58 is recom-

mended to evaluate the true specific surface area of the

wall-frame porous materials. As for the pillar-frame porous

materials, the applicability of the SPE method needs to be

examined. Type III adsorption isotherms are observed for

weakly interacting systems whose c is very small (c�1).

When E1 is almost equal to EL (c�1), the adsorption isotherm

is representative of type III. As the BET equation cannot

describe the adsorption isotherm above P/P0¼0.4, the

Frenkel–Halsey–Hill equation is used to describe the adsorp-

tion isotherm in the multilayer adsorption region.59

5.02.3.1.3 Type IV and V adsorption isothermsThe type IV adsorption isotherm comes from capillary conden-

sation after multilayer adsorption in mesopores. Vapor mole-

cules are adsorbed on the walls of mesoporous materials and

then the condensed adsorbed film is formed in the mesopores

due to the depression effect of the saturated vapor pressure of

confined vapor. The saturated vapor pressure-depression is

described by the Kelvin equation:

Relative pre

Am

ount

ad

sorb

ed

H1 H2

Figure 10 The IUPAC classification of hysteresis loops.

lnP

P0¼ � 2gVm cos y

rmRT[18]

Here, the mean radius is the rm of curvature of the meniscus

of the condensate; g and Vm are the surface tension and molar

volume of the condensate, respectively. y is the contact angle ofthe condensate against the pore wall, which is often approxi-

mated to zero (perfect wetting).33 The multilayer of the thick-

ness t is already formed on the pore wall before the capillary

condensation. A more rigorous discussion is carried out using

the modified Kelvin equation with (rm� t) instead of rm in eqn

[18]. The modified Kelvin equation has been used in evalua-

tion of the mesopore size distribution. The type IV and V

adsorption isotherms have an adsorption hysteresis consisting

of adsorption and desorption branches. Both adsorption iso-

therms are associated with capillary condensation. Capillary

condensation, however, does not necessarily give rise to

adsorption hysteresis. When the condensation pressure is

larger than the evaporation pressure, adsorption hysteresis

occurs. A test tube-type mesopore, that is, a cylindrical meso-

pore closed at one end that has a hemispherical shape does not

show adsorption hysteresis. This is because the meniscus is

hemispherical for both capillary condensation and capillary

evaporation and both the condensation and evaporation pres-

sures are identical. On the contrary, a cylindrical mesopore

open at both ends exhibits an explicit adsorption hysteresis.

The meniscus is cylindrical on adsorption, while it is hemi-

spherical on desorption. The rm of the cylindrical meniscus is

twice that of the spherical one, so that the condensation pres-

sure is larger than the evaporation pressure. Thus, the adsorp-

tion hysteresis depends on the geometrical structure of the

mesopores. This classical view has been confirmed for the

simple cylindrical pore in recent molecular simulation studies

by Sarkisov and Monson.60 However, these data are applicable

only to mesopores whose width is larger than ca. 10 nm.

de Boer distinguished five types of adsorption hysteresis

loops denoted by A–E. The types A, B, and E are commonly

observed. Later, IUPAC recommended four representative

loops denoted by H1, H2, H3, and H4. The A, E, and B loops

correspond to H1, H2, and H4.1 Figure 10 shows H1-, H2-,

and H3-types. The H1(A)-type hysteresis loop indicates the

presence of tubular mesopores open at both ends, tubular

mesopores with slightly widened parts, and ink bottles. The

H2(E) type loop is indicative of the corpuscular systems having

ill-defined pore sizes and shapes. When the pores giving H1-

type hysteresis have mutual networks, evaporation depends on

ssure P/P0

H3

34 Adsorption Properties

the neighbor pore, showing the-H2 type hysteresis. The meso-

pores having ink bottles open at both ends and sheet-like

shapes give H3(B)-type loops.61

Mesoporous silicas having regular pore structures have

stimulated fundamental studies on adsorption in mesopores.

The adsorption hysteresis for cylindrical silica mesopores open

at both ends depends on the pore width and adsorption

temperature.15,62–64 The hysteresis loop of the N2 adsorption

isotherm at 77 K disappears for pores of 3.2–4 nm. Also,

adsorption hysteresis is observed in N2 adsorption isotherms

on MCM41 (w¼4.4 nm)65 below 72.6 K. In the case of small

mesopore systems, the vapor–mesopore wall interaction must

be taken into account in addition to the surface tension term,

which is predominant in capillary condensation. Saam and

Cole66 expressed the chemical potential Dms of molecules

in an open cylindrical mesopore on condensation by using

eqn [19].

Dms ¼ � ascRp � t 3 � gVm

a[19]

Here, RP, a, and asc are the radius of mesopores, the effective

radius of multilayer-coated mesopores (a¼Rp� t), and the

constant, respectively. The first term corresponds to an average

molecule–solid interaction, which is evaluated by analysis

using the Frenkel–Halsey–Hill equation. The second term

expresses the intermolecular interaction. As the chemical

potential of the desorption course is given by a slightly differ-

ent equation whose second term is �2gVm/a, the plausible

adsorption process can be predicted from the chemical poten-

tial with the progress of adsorption or desorption. The pres-

ence of critical pore width of disappearance of adsorption

hysteresis is associated with pore wall corrugation.67

The adsorption hysteresis on well-defined mesopores has

been actively studied experimentally and theoretically.65

Briefly speaking, adsorption and desorption in uniform,

straight mesopores without networks like the mesopores in

MCM41 can be basically understood by the classical interpre-

tation described above, although independent cylinder-

mesopores of small width that are open at both ends do not

give the adsorption hysteresis as discussed earlier. However,

strictly speaking, the multilayer adsorbed film on the meso-

pore walls is stabilized in the metastable state; delayed

capillary condensation occurs above the equilibrium conden-

sation pressure. The delayed capillary condensation occurs

spontaneously near the vapor-like spinodal stemming from

classical van der Waals theory.68 This case gives the H1-type

adsorption hysteresis.

However, adsorption hysteresis of cage-like pores that are

mutually connected through small necks cannot be sufficiently

explained using the classical capillary condensation theory.

According to the classical theory, the wide cage space having

necks (ink-bottle pores) is filled at the pressure corresponding

to the delayed condensation pressure and it remains filled

during desorption until the narrow neck empties first at a

lower pressure; that is, the evaporation of the condensates in

the cage is suppressed by the adsorbed layer in the narrow

neck, which is called ‘pore blocking.’ When the pore blocking

mechanism is applicable, the desorption branch provides

information on the neck size. The concept of pore blocking

was suggested by McBain in 1935.69 Recent studies have

pointed out the importance of cavitation in evaporation from

the condensate in the cage space.70–72 Figure 11 shows sche-

matically the adsorption hysteresis loops for cavitation and

pore blocking mechanisms. The neck diameter determines

the evaporation mechanism of condensates in the cage. The

evaporation occurs at the pressure corresponding to the equi-

librium meniscus in the pore neck for the pore blocking mech-

anism. On the other hand, the evaporation (cavitation)

pressure is not associated with the neck size, and provides no

information on neck size. When the neck size is smaller than

the critical width of wc, spontaneous local density fluctuation

in the narrow neck gives rise to the cavitation in the cage of

large volume. Figure 12 shows the molecular-scale picture on

cavitation. The cage becomes vacant before evaporation of

molecules in the neck pores. The wc is not clearly determined

yet; Ravikovitch and Neimark68 suggested ca. 4 nm for N2

adsorption on mesoporous silicas, which may be associated

with the liquid-like spinodal. Thus, capillary condensation is

not necessarily fully understood, although theoretical

approaches have provided new insights. In particular, adsorp-

tion in mesopores of connectability is an essential issue to be

elucidated.

5.02.3.2 Role of Modeling in Adsorption in Nanopores

Macroscopic adsorption isotherm measurements cannot

directly show the adsorption mechanism and the structure of

molecules in nanopores and, moreover, surface science tools

are not effective for elucidation of molecular assembly struc-

tures in nanopores due to shielding of electrons and low-

energy lights by the pore walls. The authors applied x-ray to

show the molecular assembly structure confined in carbon

nanopores for water, alcohol, SO2, and organic solution.73–76

In the case of single-wall carbons, even infra-red light can

transmit and IR spectroscopy can be applied to unveil the

molecular motion in the nanopores.77 These experimental

studies on the structure of molecular assemblies in nanopores

using synchrotron x-ray have become popular recently. Thus,

structural understanding of adsorption of molecules in nano-

pores is becoming possible. Molecular simulation, being a set

of computer-based techniques derived from statistical mechan-

ics, is indispensable to the understanding of adsorption in

nanopores. This is because molecular simulation can predict

the properties of adsorbed molecules and molecular assembly

structures given a model of intra- and intermolecular inter-

action potentials. Equilibrium and nonequilibrium properties

can be predicted by averaging over many millions of molecular

configurations. The properties can be studied as a function of

external variables such as temperature and pressure. Conse-

quently, the adsorption isotherm is calculated for the model

pore system and the average snapshots can be compared with

the structural data obtained by, for example, x-ray diffraction.

Molecular simulation is able to provide a molecular level

understanding of adsorption in nanopores. Also, molecular

simulation can be applied to adsorption that cannot be exper-

imentally measured because of various restrictions. Thus,

modeling with molecular simulation is essentially important

in adsorption on nanoporous materials. Moreover, molecular

simulation is effective for supercritical gas adsorption as well as

vapor adsorption. However, this chapter cannot cover the

detailed explanation of molecular simulation.78–80 The

P/P0

Am

ount

ad

sorb

ed

00

0.2 0.4 0.6 0.8

Adsorption

Desorption1

0.2

0.4

0.6

0.8

1

Figure 12 The calculated isotherms and the molecular-scale pictures on cavitation in the neck pores. Figures are courtesy of Professor P. Monson.

w > wc

w < wc

P/P0

Am

ount

ad

sorb

ed

0 0.2 0.4 0.6 0.8 1

P/P00 0.2 0.4 0.6 0.8 1

(b)

(a)

Figure 11 Schematic illustration of cavitation (a) and pore blocking (b) phenomena. wc is the neck diameter inducing cavitation (spontaneousnucleation of a bubble in the condensate in the cage).

Adsorption Properties 35

methodology of density functional theory (DFT) is also not

given in this chapter although DFT studies are useful to predict

adsorption isotherms with shorter calculation time than ordi-

narily used grand canonical Monte Carlo simulation.

5.02.4 Supercritical Gas Adsorption

Little is understood about the fundamentals of supercritical gas

adsorption in spite of its importance; in particular, adsorption

storage of CH4 and H2, which is indispensable to the construc-

tion of a better sustainable society, is directly associated with

supercritical gas adsorption. This is because CH4 and H2 at

ambient temperature are supercritical gases (see Table 3).

One of the reasons for the poor understanding of supercritical

gas adsorption is the difficulty in determining the absolute

adsorption amount from the surface excess mass adsorption.

In the case of supercritical gas adsorption, the interaction

potential well depth is not enough to form the stable adsorbed

layer on the solid surface. Then, the application of high

Distance from surface

Ad

sorb

edd

ensi

tyA

dso

rbed

pot

entia

l

Figure 13 The molecular density and interaction potential profiles ofadsorbed layers of supercritical gas.

Adsorbed layer BulkSolid

rbulk

L0

Y Z

X

Distance from surface

Den

sity

Figure 14 Relationship between the surface excess mass and absoluteadsorbed amount. Region (X) expresses the surface excess mass. Thesum of regions (X) and (Y) corresponds to the absolute adsorption. L isthickness of adsorbed layer.

36 Adsorption Properties

pressure is imperative to increase the surface excess adsorption

amount for supercritical gas. Figure 13 shows the adsorbed

layer of supercritical gas schematically. The molecular density

profile has no sharp peaks due to the weakly bound state.

Molecules in the gas phase are also populated near the surface,

since the bulk gas phase concentration is high due to the

application of high pressure. Therefore, we must take into

account the contribution by the bulk gas phase in adsorption.

Figure 14 shows the molecular density profile near the

surface. Region (X) denotes the surface excess mass and the

sum of regions (X) and (Y) corresponds to the absolute adsorp-

tion. Here rbulk and L are the bulk gas phase density and the

thickness of the adsorbed layer, respectively. In the case of

vapor adsorption, the surface excess mass is much larger than

the amount corresponding to the region (Y) even below ambi-

ent pressure. Therefore, the absolute adsorption can be approx-

imated by the surface excess mass adsorption. As both

gravimetric and manometric adsorption methods give only

the surface excess mass adsorption, we need to add the bulk

gas phase contribution within an adsorbed layer (the region

(Y)) to obtain the true adsorption amount (absolute

adsorption). The serious issue with respect to the evaluation

of the absolute adsorption is the difficulty in determining the

thickness of the adsorbed layer (or adsorbed layer volume

Vad).81–85 There is no established method to determine L or

Vad, although Murata and Kaneko determined it, by the

analysis of the supercritical gas adsorption isotherm, to be

1–2 nm of L.82–84

The relationship between the surface excess mass, nex, and

the absolute adsorption amount, nab, is given by

nab ¼ðL0

rad rð Þdr ¼ðVad

rad rð Þdr [20]

nab can be evaluated from eqn [21].

nex ¼ nab � rbulkVad [21]

where rad is the density of the adsorbed layer. Here we can

directly measure nex and rbulk by using the adsorption measure-

ment. On the contrary, nab, Vad, L, and rad cannot be measured

directly. However, accurate information on the adsorption

mechanism needs an absolute adsorption isotherm. For exam-

ple, the isosteric heat of adsorption from the surface excess mass

adsorption isotherm gives the negative value.86 The adsorption

isotherm for supercritical gas is different from the vapor adsorp-

tion isotherm shown in Figure 8.87 Murata and Kaneko pro-

posed a general equation of supercritical gas adsorption

isotherms on the basis of classical DFT.82 This general equation

provides important parameters of the average fluid–pore wall

and fluid–fluid interaction energies from the analysis of the

adsorption isotherm. Also, it gives a simple classification of

supercritical gas adsorption isotherms depending on the relative

strength of the fluid–fluid and fluid–surface interactions.83 We

introduce the compression factor zad of the adsorbed layer,

which can be defined by the average adsorbed layer density

hriad, as given by eqn [22],

zad ¼ P

rh iadRT[22]

where P is the pressure of the bulk gas phase.

If we transform the adsorption isotherms into the zad�hriad relations, three types are obtained, as shown in Figure 15.

Figure 15 shows the horizontal line, the linear increase, and the

S-shaped increasing curve, which correspond to the Henry, virial,

and cooperative transition types, respectively. These three types

of zad versus hriad correspond to the three surface excess mass

adsorption isotherms shown in Figure 16. The virial and co-

operative transition types in the zad versus hriad are obtained

from Langmuir (L) andmaximum (M) types of the surface excess

mass adsorption isotherms. The H-type guarantees the validity of

Henry’s law, indicating the absence of fluid–fluid interactions;

the adsorbed gas can be regarded as an ideal gas. When the

fluid–solid interaction is too weak, the H-type is observed.

The L (or virial) type needs the second virial coefficient to

describe the hriad, indicating the presence of a medium range

of the fluid–fluid and fluid–solid interactions. When fluid mol-

ecules form a considerably thick adsorbed layer due to the strong

fluid–solid interaction, the repulsive interaction cannot be

neglected. This case gives the M-type or sharp uptake in the low

pressure. In the case of the surface excess mass adsorption iso-

therm of the M-type, the absolute adsorption amount can be

P

Sur

face

exc

ess

L

H

M

Figure 16 Three types of adsorption isotherms of supercritical gas.

0 2 4 6 80

100

200

300

400

Fugacity (MPa)

Ab

sorb

ed a

mou

nt o

f O2 (m

gg−1

)

Figure 17 Adsorption isotherms of O2 at 196 K in terms of surfaceexcess (♦) and absolute amount (•). The solid and open symbols denoteadsorption and desorption isotherms, respectively.

z a

r (mol l−1)

0 5 10 15 20 25

0.2

0.4

0.6

0.8

1

Figure 15 Three types of the compression factor of adsorbed layer, za,versus adsorbed density.

Adsorption Properties 37

estimated using the analysis of the excess adsorption isotherm.

The detailed procedure is given in the literature.84,88 Figure 17

shows the surface excess mass adsorption and absolute adsorp-

tion isotherms of supercritical O2 on Cu-based PCP(ELM-11) at

196 K. This ELM-11 shows a unique gate adsorption behavior.

A marked decrease in adsorption is observed after gate adsorp-

tion in the surface excess mass adsorption isotherm (Figure 17

(diamond symbols)). However, a reasonable absolute adsorp-

tion isotherm in terms of the absolute adsorption is obtained by

the analysis, as shown in Figure 17 (circle symbols). Thus,

analysis of the surface excess mass adsorption isotherm needs a

basis different from the vapor adsorption isotherm.89

Although a general understanding of supercritical gas

adsorption on nanoporous materials is still not easy, better

adsorbents for storage of CH4 and H2 need to be developed.

Adsorbent design using molecular simulation, which provides

adsorption isotherms for a target molecule-adsorbent system,

is quite promising. Above all, the theoretical PCP structure

designing of better adsorbents for supercritical gases is really

preferable, because PCPs have splendid structural diversity and

designability. A recent review by Getman et al.,80 for example,

has offered a valuable guideline for the synthesis of better

adsorbents for storage of CH4 and H2 based on molecular

simulation studies.

5.02.5 New Developments in Adsorption

Scientific understanding of adsorption on nanoporous mate-

rials has progressed as a result of continuous efforts including

introduction of in situ structural analysis, computer simulation,

and well-defined nanoporous materials. In particular, a funda-

mental understanding of adsorption processes in micropores

and small mesopores has been obtained. Here three novel

developments in adsorption are shown.

5.02.5.1 Adsorption of Water Vapor in Hydrophobic Pores

When we drop a water droplet on a hydrophobic surface, the

spherical droplet is formed; the more hydrophobic the surface,

the larger the contact angle. This contact behavior is reflected in

adsorption of water vapor on solid surfaces. Hydrophobic

surfaces and pores give the type III and type V adsorption

isotherms, respectively. Then water vapor adsorption isotherms

give a molecular measure of the hydrophobicity of a solid.

Unique water vapor adsorption has been observed on hydro-

phobic zeolites and hydrophobic microporous carbons.90–92

Microporous carbons having just a small amount of surface

oxygen groups exhibit a hydrophobic nature for water vapor;

less oxidized surface states can be evidenced by x-ray photo-

electron spectroscopy. Water forms hemispherical droplets on

microporous carbon surfaces of such hydrophobicity. Then,

such carbon surfaces can be regarded as hydrophobic surfaces.

Figure 18 shows water vapor adsorption isotherms of hydro-

phobic microporous carbons whose widths are 0.6 and

1.5 nm. The water vapor adsorption isotherms of nonporous

carbon black and microporous carbon having many surface

oxygen groups (hydrophilic carbon) are shown in Figure 18

0 0.2 0.4 0.6 0.8 1P/P0

Ad

sorb

ed a

mou

nt o

f H2O

Hydrophilic carbon

w = 1.1 nm

w = 0.6 nm

Carbon black

Figure 18 Adsorption isotherms of water vapor on hydrophilic carbon,nanoporous carbon (pore width (w)¼0.6 nm and w¼1.1 nm), andcarbon black.

38 Adsorption Properties

for comparison. Here, water vapor adsorption isotherms

of microporous carbons whose widths are in the range of

0.8–1.5 nm are almost similar to each other, although the

larger the pore width, the larger is the uptake P/P0. Although

nonporous carbon black has less than 10% of the surface area

of other microporous carbons, the adsorption amount of

nonporous carbon black is much smaller than 10% of the

adsorption amount of microporous carbons. The presence

of micropores is essentially important in the adsorption of

water vapor. We notice an unusual behavior in microporous

carbons whose pore width is less than�0.6 nm; a considerable

amount of adsorption begins even below P/P0¼0.2, leading to

a gradual adsorption increase up to �P/P0¼0.5 and then a

plateau. A more striking behavior is observed in microporous

carbons of 1.2 nm; the adsorption amount is nil below

P/P0¼0.4 and a marked adsorption begins above P/P0¼0.8.

Furthermore, a pronounced adsorption hysteresis is observed,

while almost negligible hysteresis is observed in the pores

of �0.6 nm. Similar water vapor adsorption behavior is

observed on SWCNTs and DWCNTs.93–96 Tao et al.95 found

the marked higher pressure shift of uptake pressure with van-

ishing subnanometer pores on high-temperature treatment

of DWCNTs.

How can we understand water vapor adsorption in hydro-

phobic carbon micropores? McBain et al. indicated already the

unique nature of water vapor adsorption on hydrophobic

microporous carbon in 1933.97 The water vapor adsorption

isotherm of type V cannot be interpreted by the capillary

condensation mechanism based on the Kelvin equation. Also,

water vapor is not adsorbed on the typical mesoporous carbon

of hydrophobicity below P/P0¼0.9.98 The water vapor adsorp-

tion in hydrophobic micropores cannot be understood by the

capillary condensation mechanism. As the water adsorption

does not start from an extremely low P/P0, the water vapor

adsorption in the hydrophobic micropores cannot be inter-

preted by the micropore filling mechanism either.

Water adsorbed in hydrophobic carbon micropores at

ambient temperature is not ordinary liquid. Iiyama et al.73

and Bellissent-Funel et al.99 showed with the aid of x-ray

scattering that the structure of water adsorbed in hydrophobic

carbon micropores is rather close to that of the solid. Very

recently, Futamura et al. reported negative thermal expansion

of water and cubic ice formation in hydrophobic carbon

micropores by x-ray diffraction and small angle x-ray scattering

experiments over a wide temperature range of 20–277 K.100

A similar conclusion is obtained for water adsorption on

SWCNT.96 Thus, the adsorbed state of water molecules in

hydrophobic micropores is very unique, similar to other mol-

ecules adsorbed in micropores with the micropore filling

mechanism, as mentioned earlier. New structural mechanisms

on water vapor adsorption in hydrophobic micropores must

be introduced. The following study on adsorption hysteresis of

water is helpful to understand the adsorption mechanism.

Equilibration time for measuring adsorption points has a

great effect on the hysteresis loop of water vapor adsorption

isotherms.101 Varying the equilibration time from 5 min to

16 h for each measuring point shifts the adsorption branch,

not the desorption branch; the longer the equilibration time,

the narrower is the breadth of the hysteresis loop. The relation-

ship between the experimental equilibration time and the

breadth of the hysteresis loop suggests a necessary equilibra-

tion time of more than a thousand years for each measuring

point; the metastable state in the adsorption branch is really

stable compared with the thermodynamically stable state in

the desorption branch. This indicates that the metastable state

is quite close to the equilibrium state. Ohba and Kaneko

showed that the formation of pentamers or hexamers of

water molecules of about 0.5–0.6 nm in size induces enough

stabilization of water molecules in the hydrophobic carbon

micropores.102 As the energy difference between associated

clusters and solid-like equilibrium structures is estimated to

be less than a few kJ mol�1, the metastable state on adsorption

is highly stable, as observed. The adsorption on the adsorption

branch can be associated with the growth of cluster size and the

increase of the cluster number.103 Strictly speaking, there are

three elementary structures of monolayers, associated clusters,

and uniformly hydrogen-bonded networks, which are formed

in the pores with the increase in P/P0. The route of the associ-

ated clusters to the uniformly hydrogen-bonded network is

metastable, giving the adsorption hysteresis loop.104 Figure 19

shows the changes in the stabilization energy for each model

adsorbed structure with the fractional filling for the carbon

slit pore of w¼0.9–1.1 nm. In the course of adsorption,

adsorption begins on the route of the cluster formation and

transfers not to the monolayer route of the greatest stability,

but to the uniform layer route, because the transfer from the

cluster model to the monolayer one is kinetically forbidden

according to the molecular dynamics study; the adsorption

process is metastable in the fractional range of 0.4–0.8. On

the contrary, desorption takes the uniform layer model route

and transforms into the monolayer model at the fractional

filling of 0.8. When the filling decreases to 0.4, the monolayer

model must transform into the cluster one. However, this is

also kinetically forbidden. However, desorption behavior

below the fractional filling of 0.4 does not greatly affect the

adsorption isotherm. The structural difference between the

monolayer structure and the uniformly hydrogen-bonded net-

work in the small micropores of less than 0.7 nm width is only

slight. The transformation between the monolayer model and

FH H2FH D2Classical LJ

0.3 0.4 0.5-40

-20

0

Distance (nm)

Pot

entia

l (k B

K)

Figure 20 The interaction potential profiles for quantum H2/quantumD2 and classical H2 at 77 K.

0 0.2 0.4 0.6 0.8 1

10

20

30

40

50

Fractional filling

Sta

bili

zatio

n en

ergy

(kJ

mol

−1 )

Cluster Monolayer Uniform layer

Figure 19 The change in the stabilization energy of each adsorbedmodel structure with the fractional filling for the carbon slit pore ofw¼1.1 nm. Clusters (triangle), monolayers (rectangle), and uniform(circle) distribution structure.

Adsorption Properties 39

cluster model is not kinetically forbidden. Then no hysteresis is

observed.

The above-mentioned adsorptionmechanismmay be noted

in the cluster-mediated fillingmechanism. Thismechanism can

be applied to water adsorption on hydrophobic silicalites90,105

and AlPO4-5.106–108 In the case of AlPO4-5, perpendicular

uptake is observed around P/P0¼0.2. Accordingly, the

hydrogen-bonding network between smaller clusters in the

cylindrical pores of hydrophobicity should be different from

that in the slit-shaped pores of microporous carbons. Still,

water adsorption on hydrophobic micropores has intriguing

issues,109 which many researchers have found challenging to

elucidate.

5.02.5.2 Quantum Effect in Physical Adsorption

Ordinary physical adsorption can be described in terms of

classical physics. However, a light molecule such as H2, D2,

and He has ameaningful fluctuation extent at low temperature.

Kaneko et al. pointed out that He in carbon micropores should

be larger than the classical size at 4.2 K.110,111 Moreh et al.

reported an anomaly of the kinetic energy of He in the carbon

micropores at 4.2 K.112 Tanaka et al. showed that micropore

filling of Ne at 27 K could not be interpreted using a classical

particle model.113 Beenakker et al. proposed a new concept of

quantum molecular sieving for hydrogen isotopes on a cylin-

drical pore using a simple model.114 The theoretical and sim-

ulation studies using the Feynman–Hibbs (FH) effective

potential or the rigorous path integral method have been

actively applied to adsorption of quantum fluids.115–118 The

applicability of the FH potential is evidenced for adsorption

above �40 K.

VFH ¼ VLJ þ ħ2

12mkBTV

00LJ þ

2

VV

0LJ

� �[23]

The quadratic FH effective potential is given by eqn [23].

Here VLJ is the LJ potential function. The FH effective potential

can distinguish the intermolecular interaction of hydrogen iso-

topes, whereas the LJ potential cannot. Figure 20 shows the

interaction potential profiles for quantum H2, quantum D2,

and classical H2 at 77 K. Here the potential of quantum H2 and

D2 is expressed by the effective FH potential, while the interac-

tion potential of classical H2 (or D2) is expressed by the LJ

potential. The LJ potential gives the deepest potential mini-

mum and the equilibrium distance at the potential minimum

is the smallest. The depth and equilibrium distance obtained

from the FH potential depend on the mass of H2 and D2 and

therefore the lighter H2 provides the shallowest potential min-

imum and the largest equilibrium distance at the potential

minimum. If the temperature is lowered, the difference in the

potential depth and the equilibrium distance becomes larger;

that is, the quantum effect is more evident at a lower temper-

ature. The above interaction potential profiles indicate that

lighter isotope molecules have a larger molecular size and

they interact more weakly than heavier ones. This is the reason

for quantum molecular sieving. We can evaluate qualitatively

the fluctuation extent of a quantum molecule with de Broglie

wave length lt¼h/(2pmkT )1/2. Here m is the molecular mass.

The de Broglie wave lengths of H2 and D2 at 20 and 77 K are

0.27; 0.17 and 0.14; 0.11 nm, respectively. The difference in

the effective molecular sizes of D2 and H2 is only 0.03 nm at

77 K. The absolute difference is very small; nevertheless, it is

quite meaningful. The classical molecular size difference

between N2 and O2 is only 0.02 nm, although the quadrupole

moments of both molecules are different from each other. This

difference is applied to the separation of N2 and O2 with

molecular sieve carbon and zeolite for the industrial produc-

tion of pure N2 and O2 from air, for the benefit of human

society. The effective molecular size difference of hydrogen

isotopes is essentially important for adsorption in the subna-

noscale pores. Figure 21 shows snapshots of quantum H2 and

classical H2 at 40 K in the straight pores of AlPO4-5 whose

diameter is 0.73 nm. The adsorbed molecular numbers of

Pressure (MPa)

Ad

sorp

tion

(mol

kg−

1 )

0 0.02 0.04 0.06 0.08 0.1

0

5

10

Figure 22 H2 and D2 adsorption isotherms of bundled and oxidizedSWCNTs at 77 K. H2, diamond; D2, circle; open symbol, nonoxidized;solid symbol, oxidized.

40 Adsorption Properties

classical H2 and quantum H2 are remarkably different regard-

less of the tiny molecular size difference. The adsorption dif-

ference between D2 and H2 has been experimentally shown for

SWCNH,119 SWCNT,120 ACF,121 AlPO4-5,122 and PCPs,123,124

using the adsorption isotherm measurement.

Quantum molecular sieving behavior on SWCNT is shown

here. Highly pure SWCNTs of 3.0 nm average diameter, pre-

pared by the CVD method and mutually isolated, were soni-

cated in toluene and dried after filtration. This drying

treatment after sonication in toluene gives fine bundles of

SWCNTs due to the capillary force on drying. This SWCNT

sample is denoted bund-SWCNT. On the other hand, the

caps were removed by oxidation treatment; thus-treated

SWCNT is denoted ox-SWCNT. The pore structural parameters

determined by N2 adsorption at 77 K are as follows.125,126

Surface area in m2 g�1: SWCNT: 1040, ox-SWCNT: 1900, and

bund-SWCNT: 600; pore volume in ml g�1: ox-SWCNT: 0.80,

bund-SWCNT: 0.27. As the geometrical surface area of an

empty SWCNT without caps is 2630 m2 g�1, ox-SWCNT sam-

ples are considerably open (30%). The surface area of SWCNT

is close to half that of ox-SWCNT, indicative of the mutually

isolated state. TEM observation evidences the fine bundle for-

mation whose interstitial pore width is 0.52 nm on average.

The small surface area of the bund-SWCNT supports the

fine bundle structure consisting of three to four SWCNTs.

Figures 22 and 23 show H2 and D2 adsorption isotherms of

pore-controlled SWCNTs at 77 and 20 K, respectively. The

adsorption amounts of H2 and D2 on SWCNT at 77 K are

almost comparable to those on bund-SWCNT, although the

surface area of SWCNT is nearly twice that of bund-SWCNT.

This is because H2 and D2 are supercritical at 77 K and adsorp-

tion of H2 and D2 needs strong sites such as interstitial pores,

which the isolated SWCNT does not have. All samples have a

greater adsorption amount of D2 than H2. The interesting

point is that the difference between D2 and H2 for bund-

SWCNT is close to that for ox-SWCNT irrespective of the

great difference in their surface area. This indicates strongly

that the interstitial pores of 0.52 nm in bund-SWCNT are

effective for quantum molecular sieving. H2 is vapor at 20 K.

Therefore, adsorption of hydrogen isotopes on SWCNT

Quantum H2 Classical H2

Figure 21 The snapshots of quantum H2 and classical H2 at 40 K in thestraight pore of AlPO4-5.

samples is very different from that at higher temperature. The

adsorption amount of D2 is much larger than that of H2 for all

three samples. The adsorption amount of each sample corre-

sponds to the surface area. Hydrogen isotope vapor can be

adsorbed even on nonporous surfaces at 20 K. The intriguing

point is that even SWCNT without pores can show an explicit

quantum molecular sieving effect; the adsorption amount of

D2 is much larger than that of H2. This quantum molecular

sieving effect stems from the interaction potential wells at the

external tube walls; that is, the potential well, which is not

necessarily surrounded by physical matters, can induce the

remarkable quantum molecular sieving effect. The advantage

of the quantummolecular sieving effect by the potential well of

the surface has no diffusion restriction. Kumar and Bhatia127

first indicated the importance of the kinetic quantum molecu-

lar sieving effect theoretically. Very recently, the quantum

molecular sieving separation of D2 from the D2–H2 mixture

was experimentally evidenced for nanoporous carbon and

zeolite,128 which should accelerate the practical application

Pressure (MPa)

Ad

sorp

tion

(mol

kg−

1 )

0 0.02 0.04 0.06 0.08

0

20

40

60

Figure 23 H2 and D2 adsorption isotherms of bundled and oxidizedSWCNTs at 20 K. H2, diamond; D2, circle; open symbol, nonoxidized;solid symbol, oxidized.

g−1 )

200

Adsorption Properties 41

of the quantum molecular sieving effect. The kinetic issue in

pore filling of quantum fluid is also developing a new adsorp-

tion science.129

Pressure (kPa)

Am

ount

of a

dso

rbed

CO

2 (m

g

0 20 40 60 80 100

0

50

100

150

Figure 24 CO2 adsorption isotherms of ELM-11 at differenttemperatures. Solid and open symbols indicate adsorption anddesorption, respectively, 248 K, □; 273 K, ◊; 298 K, ○.

5.02.5.3 Gate Adsorption

PCPs or porous MOFs display an extremely large range of

crystal structures. The combination of tunable porosity and

chemistry of the internal spaces has attracted great attention.

Above all, flexible PCPs exhibiting gate opening (transition

from nonporous phase to porous phase) and breathing (two

successive structural transitions) on gas adsorption have stim-

ulated research from fundamental and innovative perspectives.

Here, we focus gate adsorption in Cu-based PCP. Li and

Kaneko found a new type of adsorption on [Cu(bpy)

(H2O)2(BF4)2](bpy) crystals called gate adsorption in 2001.37

As the Cu-based PCP crystals have no effective pores from the

crystal structure and they change the structure for accepting gas

molecules above the threshold pressure, this crystal is named

‘latent porous crystal (LPC).’ The LPC has a layer-stacking

structure whose interlayer spacing can be controlled by the

anion, ligand, and central metal atom. Later, similar adsorp-

tion behaviors were observed for several PCPs having similar

layer-stacking structures.130–135 Therefore, a more general

name of ‘elastic layer-structured metal–organic framework

(ELM)’ is proposed for the family of PCPs; LPC is named

ELM-11 after the new nomenclature.135 The specification of

the ELM family is as follows. As Co2þ(2) and Ni2þ(3) in

addition to Cu2þ(1) and BF4�(1), CF3SO3

�(2), CF3BF3�(OTf)

(3) form similar structures, the sort of metal ion (X) and

anion (Y) are noted by numbers corresponding to metal ion

and anion, which are shown in parenthesis, after ELM as ELM-

XY. Therefore, ELM-23 expresses [Co(bpy)2(OTf)2]. The

adsorption of gases on ELM-11 begins at the threshold pressure

vertically, reaching the saturation; on decreasing the gas pres-

sure, the adsorption amount does not change up to a lower

threshold pressure than that on adsorption, dropping to nil at

the pressure. Therefore, this adsorption is named ‘gate adsorp-

tion/gate desorption,’ because the effective pores are produced

at the threshold pressure (adsorption gate pressure Pg,a), on

adsorption, while the effective pores are shut at the desorption

threshold pressure (desorption gate pressure Pg,d) on desorp-

tion. Figure 24 shows CO2 adsorption isotherms on ELM-11 at

different temperatures. As mentioned earlier, the adsorption

amount varies almost vertically at adsorption and desorption

gate pressures, giving rise to a rectangular adsorption hystere-

sis. When the temperature decreases, the rectangular hysteresis

loop shifts to a low-pressure side; Pg,a at 273 K is 37.5 kPa,

shifting to 11.8 kPa at 248 K. Similar gate behavior is observed

for other gases of CH4, N2, O2, and Ar on ELM-11.87,135–137

The gate adsorption can be expressed by the clathrate forma-

tion equilibrium between ELM-11 and gas molecules, as

shown in eqn [24]; the adsorption gate pressure is an equilib-

rium pressure for the chemical equilibrium of the clathrate

formation.

ELM�11 sð Þ þM gð Þ ¼ M:ELM0½ � sð Þ [24]

where M and [M:ELM0] denote a guest molecule and clathrate

compound.38,137 van der Waals and Platteeuw showed that the

Clapeyron–Clausius equation can be applied to binary

systems.138 The Pg is regarded as the vapor pressure of the

clathrate [M:ELM0](s). The volume change is approximated

by the volume of gas (¼zRT/P, z is the compression factor.

z¼1 for an ideal gas). The Clapeyron–Clausius equation is

given by eqn [25].

d ln P

d 1=Tð Þ ¼�DHf

R[25]

Here DHf is the enthalpy charge of the clathrate formation.

The ln Pg,a versus 1/T relation gives the linear plot. The DHf is

27 kJ mol�1 for CO2. The gate adsorption of CH4 is not critical

compared with CO2. The linear Clapeyron–Clausius relation

leads to DHf¼13.01.2 kJ mol�1. These values are reason-

able, because the interlayers are bound with the hydrogen

bonding, which has a stabilization of �10 kJ mol�1. In the

case of CO2, no adsorption occurs even after 14 h at the CO2

pressure slightly lower than Pg,a. This fact supports the above

clathrate formation equilibrium.

Gate adsorption and desorption are considerably rapid

processes. Figure 25 shows the changes in the CO2 adsorption

amount after increasing the CO2 pressure from 20 to 98 kPa on

going over each gate pressure at 273 and 298 K. The adsorption

at 273 and 298 K almost finishes within 8 min. When the first-

order adsorption kinetics is assumed, the reverse values of the

first-order adsorption kinetic constants are 1.0 min at 273 K

and 0.400.3 min at 298 K. Thus, gate adsorption is a rapid

process.

The chemical potential difference between the adsorption

and desorption gate effects is given by kBTln(Pg,a/Pg,d). When

we assume that the lattice gate is bound by the hydrogen-

bonded spring, the energy difference of the both springs

for adsorption and desorption is expressed by [(1/2)ka,hddi2�(1/2)kdhdai2]. Here hddi and hdai are the average displacements

on desorption and adsorption, respectively. hddi should be

close to hdai. Then, the average potential energy difference of

the lattice spring may be approximated by (1/2)(kd�kd)hdi2.When the lattice gate is exposed to the gas, the potential energy

H2O

Figure 26 The local structure change in ELM-11 around Cu ion sites onhydration.

Time (min)

Am

ount

of a

dso

rbed

CO

2 (m

gg−

1 )

0 10 20 30 40 50

20

40

60

80

100

120

140

160

Figure 25 The change in the CO2 adsorption with time after increasingCO2 pressure from 20 to 98 kPa at different temperatures. ○, 273 K; ●,298 K.

42 Adsorption Properties

of the lattice gate is balanced with the chemical potential of the

surrounding gas, leading to eqn [26].

1

2ka � kdð Þ dh i2 ¼ kBT ln

Pg, aPg, d

� �[26]

The assumption of hdi2¼0.005–0.01 nm provides Dk(¼ka�kd) of 1.4�10–6�10 N m�1 for CO2. The force con-

stants of O–H–O stretching and O–H–O bending of ice are

(1.6–1.9�10) and 9 N m�1, respectively.139 Accordingly, the

above estimation of the lattice spring for gate behavior is

plausible, suggesting the expansion and shrinkage of the lat-

tices on gate adsorption and desorption, respectively. This

layered Cu-based PCP was studied with synchrotron x-ray

diffraction, infra-red spectroscopy, and EXAFS and systematic

adsorption experiments, showing that gate adsorption was

observed after dehydration. The original chemical formula

of [Cu(bpy)(H2O)2(BF4)2]bpy, whose crystal structure has

been published,140 changes into [Cu(bpy)(BF4)2]bpy (ELM-

11) after dehydration. [Cu(bpy)(H2O)2(BF4)2]bpy has a

hydrogen-based 3D structure, while [Cu(bpy)(BF4)2]bpy has

a complete 2D layer structure. Figure 26 shows the local struc-

ture change in ELM-11 around Cu ion sites on hydration. The

layers in ELM-11 are stacked against each other without effec-

tive pore spaces.141 The dehydration and hydration processes

are almost reversible, being very unique in PCP materials.142

Surprisingly, these layers open at the gate pressure. The inter-

layer distance increases from 0.458 to 0.578 nm by 0.0120 nm

(26%) on CO2 adsorption, accompanied with sliding of the

staggered stacking layers.141 This unusual expansion of the

crystal lattice recovers on desorption. Accordingly, ELM-11

can aspire CO2 without collapsing the lattices. The collective

motion of the lattices in order to adapt to the surrounding

atmosphere is really interesting. Kondo et al. evidenced an

essential role of surface adsorption inducing the collective

lattice change.143 There are few ELM family compounds that

exhibit gate adsorption phenomena, although more than 50

related crystals were synthesized and analyzed.135,137,144–146

PCPs exhibiting breathing phenomenon such as MIL-53147

and MIL-88148are also important to develop efficient adsorp-

tion storage and separation processes as well as the ELM family

of the gate effect. We must elucidate theoretically the mecha-

nism of the gate and breathing effects. Coudert et al.134,149,150

introduced the osmotic statistical ensemble that accounts for

the presence of a flexible adsorbent with variable unit cells,

leading to the osmotic potential involving the free energy of

the adsorbent phase, the adsorption isotherm, and molar vol-

ume of the pure gas as a function of pressure. They discussed

the adsorption-induced structural transition of the flexible PCP

with the osmotic potential using the Henry constant and the

saturated adsorption amount given by the experimental

adsorption isotherm. This theoretical analysis can provide the

phase diagram, which indicates the thermodynamic stability

regions for the narrow-pore (np) form and the large-pore (lp)

one for the breathing effect. This osmotic potential analysis is

also effective in describing the gate effect. Watanabe et al.151,152

interpreted the gate adsorption phenomenon with grand free

energy calculation and grand canonical Monte Carlo simula-

tion using the model of LJ molecules. Their approach can

describe well the gate-opening and gate-closing pressures and

the adsorption isotherm.

These theoretical approaches can be applied to understand

the adsorption-induced structural transition for a target mole-

cule–PCP system. As PCPs showing the adsorption-induced

structure transition are promising to innovative and highly

efficient separation and storage system of valuable gases,135

the progress in the theoretical understanding should accelerate

the practical application of the PCPs to sustainable

technologies.

5.02.6 Conclusion

Adsorption on nanoporous materials has a great demand in

innovative eco-technologies. A fruitful bridging between mate-

rials chemistry and adsorption science and technology is

highly needed to accelerate the growth of eco-technologies.

Finally, the necessity of fundamental studies on gas adsorption

on nanoporous materials should be emphasized to promote

the construction of eco-societies. For a related chapter in this

Comprehensive, we refer to Chapter 9.34.

Acknowledgments

KK has been supported by Exotic Nanocarbons, Japan Regional

Innovation Strategy Program by the Excellence, JST. This work

is partially supported by Grant-in-Aid for Scientific Research

(A) (24241038) by JSPS. The authors wish to thank

Dr H. Tanaka for sending some figures of his work and for

the valuable discussions.

Adsorption Properties 43

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