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Sorption of CO, CH4, and N2 on Zeolite ZSM-5

with Different SiO2/Al2O3 Ratio

Chapter 5

Chapter-5 Sorption of CO, CH4 and N2 on Zeolite ZSM-5 with different SiO2/Al2O3 ratio

143

5.1. Introduction

Gas separation using porous adsorbents like zeolites, activated carbon, and carbon

molecular sieve potentially offers a cost-effective and environmentally friendly

option.1-14

Adsorption based separation technique has been widely used in different

industries for the purification, separation and recovery of different gases from their

gas mixtures.15-19

NaZSM-5 has been used for gas separation applications, while

proton type HZSM-5 has been extensively used in acid catalyzed reactions.20-26

Adsorption properties of ZSM-5 based adsorbents can be influenced by their

SiO2/Al2O3 ratio and chemical composition. ZSM-5 can be synthesized with a broad

range of SiO2/Al2O3 ratio ranging from 10 to ∞.

Zeolite ZSM-5 was first synthesized by Mobile Company in 1972, and showed two

different crystal faces depending on the temperature of crystallization; and each phase

presents different secondary building units (SBUs). Figure 5.1 shows the construction

of ZSM-5. At low temp ZSM-5 crystallizes in a monoclinic system with SBUs

formed by 24 tetrahedra, while at high temperature it presents an orthorhombic lattice

with a SBUs of 12 tetrahedra. The phase transition takes place at 340 K. The

arrangement of these pentasil subunits (SBUs) in one dimension generates sinusoidal

chains. Repeating chains in one plane produces a 10 member ring and finally the

superposition of planes generates the three dimensional structure, which is

characterized by two channel systems one straight and another one sinusoidal. 21,24,27

Figure 5.1. Schematic presentation for the construction of ZSM-5 framework.

Separation of CH4, CO2, and N2 by adsorption on carbon molecular sieve and zeolite

13X has been reported by Cavenati et al.10

The layered pressure swing adsorption

consist of a zeolite 13X to selectively removes CO2, and followed by a layer of

carbon molecular sieve 3K to separate CH4 from N2. Adsorption equilibrium of CO2,


144

CH4 and N2 onto Na and H-mordenite at high pressures has been reported by Delgado

et al.28

, Harlick et al.29

have studied pure and binary mixture adsorption of CO2, CH4,

and N2 on HZSM-5 having SiO2/Al2O3 ratio of 30 and found that CH4+N2 binary

isotherms exhibit behaviour similar to the pure component isotherms, with CH4 as the

dominant adsorbate. The separation factor steadily declines as the mole fraction of

CH4 in the gas phase increased. Yun et al.30

carried out experimental evaluation of

methods for the prediction of adsorption equilibrium of CH4, ethane, and their binary

mixtures in MCM-41. High pressure CH4 adsorption in NaX, NaY and clinoptilolite

zeolites with different SiO2/Al2O3 was studied.31-33

Two different coordination modes

of CO in Na-ZSM-5 zeolites, the formation of Na+ → CO and Na

+ → OC adducts

between adsorbed CO and extra-framework Na+ ions have also been reported.

34 CO,

CH4 and N2 adsorption on zeolite ZSM-5 has been studied by few researchers but a

systematic study on effect of SiO2/Al2O3 alumina ratio on adsorption properties of

CO, CH4 and N2 in zeolite ZSM-5 has not been reported yet. 27, 35-36

In present chapter, we have studied the CO, CH4, and N2 adsorption on ZSM-5 having

different SiO2/Al2O3 ratio, by both volumetric equilibrium and dynamic adsorption.

Volumetric adsorption measurements of CO, CH4 and N2 up to 1 atm pressure and

288 K and 303 K were carried out while dynamic adsorption studies are carried out

for CO, CH4 and N2 binary and ternary gas mixtures with different composition and at

fixed flow rate of 100 ml/min.

5.2. Experimental Section

5.2.1. Materials

The sodium form of ZSM-5 with SiO2/Al2O3 ratio 25, 40, 100, 400 and 900 in

powder form were procured from Zeochem, Switzerland and is used as received.

ZSM-5 (25), ZSM-5 (40), ZSM-5 (100), ZSM-5 (400) and ZSM-5 (900) represent

ZSM-5 with SiO2/Al2O3 ratio of 25, 40, 100, 400 and 900, respectively. For preparing

zeolite extrudes, 20 wt% Bentonite clay was used as binder. The average length and

diameter of extrudes is 5 mm and 3 mm, respectively. N2 (99.999%), CH4 (99.9%),

CO (99.99%) and He (99.999%) from Inox Air Products, India were used for the

adsorption isotherm measurements.


145

5.2.2. Characterization

X-ray powder diffraction, Surface Area, SEM, EDX, and ICP analysis of zeolite

ZSM-5 with different SiO2/Al2O3 ratios were carried out by the same procedure as

described in chapter 2 section 2.2.3, 2.2.5 and 2.2.6.

5.2.3. Equilibrium and Dynamic Adsorption Measurements

Equilibrium and dynamic adsorption measurements of the zeolite ZSM-5 with

different SiO2/Al2O3 ratios were carried out for obtaining equilibrium and dynamic

adsorption capacity and selectivity of CO, CH4, and N2 from their mixtures. The

detail procedure is same as described in section 2.2.7 and 2.2.8.

5.3. Results and Discussion

5.3.1. Structure of ZSM-5

The structure, unit cell, and pore opening of ZSM-5 are shown in figure 5.2. ZSM-5

has a three-dimensional network structure with straight channels of dimension 5.3

5.6 Å viewed along [010] plane and sinusoidal channels of dimension 5.1 5.5 Å

viewed along [100] plane. The sinusoidal channels connect subsequent layers of

straight channels. The unit cell of ZSM-5 is orthorhombic with dimension 20.07

19.92 13.42 Å. 36

Zeolite ZSM-5 with SiO2/Al2O3 ratio of 25, 40, 100, 400 and 900

possess 7.11, 4.57, 1.88, 0.48 and 0.22 sodium cations/unit cell, respectively. The

pore opening and channel structure of ZSM-5 is large enough to neglect any steric

effects of the studied adsorbate molecules within the adsorbent channels. However,

the cationic number and nature of the adsorbent surface predominantly determines the

difference in adsorption capacity of CO, CH4 and N2.

Figure 5.2 (a-d). Schematic presentation of (a) ZSM-5 Structure, (b) unit cell of ZSM-5 (c),

10-ring viewed along [100] and (d) 10-ring viewed along [010] plane.


146

5.3.2. X-ray Powder Diffraction and Surface Area Measurements

X-ray diffraction pattern of ZSM-5 samples having different SiO2/Al2O3 ratio is

shown in Figure 5.3. These are indicative of highly crystalline material typically

observed for ZSM-5 type zeolite. The chemical composition and surface area of

ZSM-5 samples with different SiO2/Al2O3 ratio are given in Table 5.1.

Figure 5.3. XRD pattern of ZSM-5 having different SiO2/Al2O3 ratio.

Table 5.1. Physicochemical properties of ZSM-5 having different SiO2/Al2O3 ratio.

Adsorbent Composition

(Anhydrous wt %)

Surface area,

(m2/g)

Average

particle

size, (μm) Na2O Al2O3 SiO2 Micropore External

ZSM-5 (25) 3.75 6.12 90.16 243 124 3.8

ZSM-5 (40) 2.41 3.97 93.61 251 103 4.5

ZSM-5(100) 1.0 1.65 97.35 217 145 2.4

ZSM-5(400) 0.26 0.42 99.32 219 94 4.7

ZSM-5(900) 0.12 0.19 99.69 182 122 4.0

The external surface in the ZSM-5 could be due to the size of the crystals/particles

among the samples studied. The particle size distribution for ZSM-5 with different

SiO2/Al2O3 ratio was analyzed by particle size analyzer Malvern Mastersizer 2000,

operated at air pressure 1 bar and feed rate 50. ZSM-5 with SiO2/Al2O3 ratio of 25,

40, 100, 400 and 900 describe as dry powder have average particle size of 3.8, 4.5,

0 10 20 30 40 50 60

0

ZSM5-900

ZSM5-400

ZSM5-100

ZSM5-40

ZSM5-25

Inte

nsi

ty (

a.u

.)

2 Theta


147

2.4, 4.7 and 4.0 μm, respectively. The observed differences in external surface area

for the samples correlate with the size variation as seen from Table 5.1. The samples

with small particle size show larger surface area.

5.3.3. Adsorption Isotherms, Capacity and Selectivity for ZSM-5 with Different

SiO2/Al2O3 Ratio

The adsorption isotherms for CO, CH4, and N2 in ZSM-5 having different SiO2/Al2O3

ratio, at 288 K and 303 K are shown in Figures 5.4 and 5.5 respectively. The

adsorption isotherms of CO, CH4 and N2 on ZSM-5 samples follow the Type I

isotherm which become linear with increase in SiO2/Al2O3 ratio. The adsorption

isotherm shows CH4 selectivity over N2 at all partial pressure and over CO at higher

partial pressure. However, the low silica ZSM-5 (25) and ZSM-5(40) shows CO

selectivity over CH4 and N2 in low pressure region. The pure component adsorption

selectivity data of CO over N2 and CH4 over CO & N2 for zeolite ZSM-5 with

different SiO2/Al2O3 ratio at 288 and 303 K are given in Table 5.2 and 5.3,

respectively. The selectivity of CH4 over CO and N2 increases with increasing

SiO2/Al2O3 ratio because of weak interaction of CO and N2 with framework oxygen

atoms in comparison to that of CH4. The selectivity of CO over N2 decreases with

increase in SiO2/Al2O3 ratio.

The equilibrium adsorption capacities for the adsorption of CO, CH4, and N2 on

ZSM-5 having different SiO2/Al2O3 ratio are determined from the adsorption

isotherms and the number of molecules of CO, CH4, and N2 adsorbed per unit cell is

calculated (Table 5.2 and 5.3). The number of molecules adsorbed per unit cell is

calculated by multiplying volume of gas adsorbed and conversion factor given in

Table 5.2 and 5.3. It is observed that the adsorption capacities of CO, CH4, and N2

increases with decrease in SiO2/Al2O3 ratio in ZSM-5. The CH4 adsorption capacity

in ZSM-5 with SiO2/Al2O3 ratios ranging from 25 to 900 is higher compared to that of

CO and N2. CO adsorption capacity is higher compared to that of N2. The adsorption

capacities of ZSM-5 with SiO2/Al2O3 ratio of 400 and 900 are nearly the same for all

the gases studied. As the number of cations per unit cell increases, the adsorption

capacity increases for all gases studied.

Adsorbate molecules can interact with the zeolite surface through lattice oxygen

atoms and accessible extra framework cations, Al and Si atoms. The Al and Si atoms


148

present at the centre of tetrahedra are not directly exposed to the sorbate molecules.

Consequently, their interactions with the sorbate molecules are negligible. Therefore,

the principal interactions of the sorbate molecules with the zeolite surface are with

lattice oxygen atoms and extra framework cations.

Figure 5.4 (a-e). Adsorption isotherms for ZSM-5 with different SiO2/Al2O3 ratios at 288 K

0 200 400 600 8000

2

4

6

8

Mo

lec

ule

s a

ds

orb

ed

/un

it c

ell

Pressure (mmHg)

CO

CH4

N2

(a)

ZSM-5 (25)

0 200 400 600 8000

2

4

6

8

Mo

lec

ule

s a

ds

orb

ed

/un

it c

ell

Pressure (mmHg)

CO

CH4

N2

(b)

ZSM-5 (40)

0 200 400 600 8000

1

2

3

4

5

6

Mo

lec

ule

s a

ds

orb

ed

/un

it c

ell

Pressure (mmHg)

CO

CH4

N2

(c)

ZSM-5 (100)

0 200 400 600 8000

1

2

3

4

5

6

Mo

lec

ule

s a

ds

orb

ed

/un

it c

ell

Pressure (mmHg)

CO

CH4

N2

(d)ZSM-5 (400)

0 200 400 600 8000

2

4

6

Mo

lec

ule

s a

ds

orb

ed

/un

it c

ell

Pressure (mmHg)

CO

CH4

N2

(e)

ZSM-5 (900)


149

Figure 5.5 (a-e). Adsorption isotherms for ZSM-5 with different SiO2/Al2O3 ratios at 303 K

The electrostatic interactions between the sorbate molecules and the extra framework

cations of the zeolite depend on the quadrupole moments, dipole moment and

polarizability of the sorbate molecules.

0 200 400 600 8000

2

4

6

Mo

lec

ule

s a

ds

orb

ed

/un

it c

ell

Pressure (mmHg)

CO

CH4

N2

(a) ZSM-5 (25)

0 200 400 600 8000

1

2

3

4

5

6

ZSM-5 (40)

Mo

lec

ule

s a

ds

orb

ed

/un

it c

ell

Pressure (mmHg)

CO

CH4

N2

(b)

0 200 400 600 8000

1

2

3

4ZSM-5 (100)

Mo

lec

ule

s a

ds

orb

ed

/un

it c

ell

Pressure (mmHg)

CO

CH4

N2

(c)

0 200 400 600 8000

1

2

3

4

ZSM-5 (400)

Mo

lec

ule

s a

ds

orb

ed

/un

it c

ell

Pressure (mmHg)

CO

CH4

N2

(d)

0 200 400 600 8000

1

2

3

4

ZSM-5 (900)

Mo

lec

ule

s a

ds

orb

ed

/un

it c

ell

Pressure (mmHg)

CO

CH4

N2

(e)


150

The electrostatic interaction between gas molecules and extra framework sodium ions

is the main driving force for the adsorption of CO, CH4 and N2 in ZSM-5. The

quadrupole moment for the used gases is in the order of CO > N2 > CH4, while their

order of polarizability is CH4 > CO > N2. CO is polar in nature while CH4 and N2 are

non polar. With the decrease in number of sodium cations per unit cell, the

electrostatic interaction of CO, CH4 and N2 molecule decreases which results into the

decreases in the adsorption capacity of CO, CH4 and N2 in ZSM-5 on increasing

SiO2/Al2O3 ratio. Zeolite ZSM-5 with SiO2/Al2O3 ratio of 25 shows maximum

adsorption capacity whereas ZSM-5 with SiO2/Al2O3 ratio of 900 shows minimum

adsorption capacity for CO, CH4 and N2 at 760 mmHg and 288 and 303 K.

Table 5.2. Adsorption capacity and selectivity for CO, CH4 and N2 in ZSM-5 with

different SiO2/Al2O3 ratios at 288 K and 760 mmHg.

Adsorbents Conversio

n factor

Adsorption capacity

(molecules/u. c.)

Selectivity

CO CH4 N2 CH4/N2 CH4/CO CO/N2

ZSM-5(25) 0.2651 6.5 7.0 4.0 1.8 1.1 1.7

ZSM-5 (40) 0.2622 5.0 6.7 3.1 2.1 1.4 1.6

ZSM-5(100) 0.2592 3.8 4.8 2.3 2.1 1.3 1.7

ZSM-5(400) 0.2578 2.3 4.7 1.7 2.8 2.1 1.3

ZSM-5(900) 0.2574 2.0 4.6 1.6 2.9 2.2 1.3

Table 5.3. Adsorption capacity and selectivity for CO, CH4 and N2 in ZSM-5 with

different SiO2/Al2O3 at 303 K and 760 mmHg.

Adsorbents Conversio

n factor

Adsorption capacity

(molecules/u. c.)

Selectivity

CO CH4 N2 CH4/N2 CH4/CO CO/N2

ZSM-5(25) 0.2504 5.5 5.7 3 1.9 1.0 1.9

ZSM-5 (40) 0.2480 3.9 5.2 2.3 2.2 1.4 1.7

ZSM-5(100) 0.2456 2.7 3.6 1.7 2.1 1.3 1.5

ZSM-5(400) 0.2442 1.7 3.4 1.3 2.6 2.1 1.3

ZSM-5(900) 0.2439 1.5 3.1 1.2 2.6 2.0 1.4


151

CH4 has no quadrupole moment but has high polarizability (Table 1.1). Therefore,

when CH4 molecule is in close proximity to the cation, an instantaneous shift in the

time averaged neutral electrostatic field occurs in the structure of CH4. This induced

polarity results into the high adsorption capacity. However, with decrease in number

of cations per unit cell, the induced polarity of CH4 also reduces and hence the

adsorption capacity decreases. Therefore with the increase in SiO2/Al2O3 ratio the

total number of sodium cations decreases and hence the CH4 adsorption capacity

decreases. CH4 shows dispersion interaction potential with framework oxygen atoms

and hence CH4 molecules are homogenously distributed in ZSM-5 channels, while

CO and N2 molecules are located in proximately to the cations. However, in the case

of CO and N2, the adsorption is mainly governed by the electrostatic interaction

between N2 molecules and cations.

5.3.4. Adsorption Model Fitting

The adsorption data obtained at 288 and 303 K were fitted into Langmuir equation

and the values of the slope and the Langmuir constants for the adsorption of CO, CH4,

and N2 on different SiO2/Al2O3 ratio of ZSM-5 at 303 K are given in Table 5.4. The

decrease in slope and Langmuir constant b for CO, CH4 and N2 with an increase in

SiO2/Al2O3 ratio is in agreement with the decrease in adsorption capacity.

Table 5.4. Langmuir constants and slope for adsorption of CO, CH4 and N2 at 303 K

on ZSM-5 having different SiO2/Al2O3 ratio.

Adsorbent Slope Langmuir constant (b)

CO CH4 N2 CO CH4 N2

ZSM-5(25) 0.089 0.069 0.024 3.041 1.970 0.854

ZSM-5(40) 0.065 0.054 0.019 2.453 1.386 0.917

ZSM-5(100) 0.032 0.044 0.012 1.902 1.859 0.534

ZSM-5(400) 0.012 0.024 0.008 0.604 0.546 0.277

ZSM-5(900) 0.010 0.022 0.007 0.247 0.492 0.257

The CO, CH4, and N2 adsorption data obtained at 303 K were also fitted in Virial

equation and the values for the Virial coefficient A and Henry’s constant K at 303 K

are given in Table 5.5. The Henry’s constant values for the adsorption of CO, CH4,


152

and N2 decrease in ZSM-5 with increase in the SiO2/Al2O3 ratio. The value of Henry

constant for CO adsorption was higher than CH4 for low silica zeolite i.e. ZSM-5

(25), ZSM-5 (40), and ZSM-5 (100), while it is less for ZSM-5 (400), and ZSM-5

(900). The magnitudes of the values of Henry constants are higher for CO and CH4

compared to N2. The higher values of Henry constants confirm the strong interaction

of CO molecule with the sodium cations of the ZSM-5, while CH4 interacts with both

sodium ions and framework oxygen atoms. The ZSM-5 with SiO2/Al2O3 ratio of 25

has the highest values for the Henry constants for the studied gases.

Table 5.5. Virial coefficient and Henry’s constant for adsorption of CO, CH4, and N2

at 303 K on ZSM-5 Having different SiO2/Al2O3 Ratio.

Adsorbent A Henry constant(K) (10-5

cc g-1

Pa-1

)

CO CH4 N2 CO CH4 N2

ZSM-5(25) 1.9 2.5 3.7 105 60.4 22.0

ZSM-5(40) 2.2 2.6 3.4 82 54.7 21.0

ZSM-5(100) 2.5 3.0 4.4 60 36.0 9.6

ZSM-5(400) 4.3 3.7 4.9 11 19.3 5.8

ZSM-5(900) 5.0 3.8 5.1 7 16.8 5.3


153

5.3.5. Isosteric Heat of adsorption

The isosteric heats of adsorption for CO, CH4 and N2 are given in Table 5.6. The heat

of adsorption decreases with an increase in the SiO2/Al2O3 ratio due to decrease in

numbers of sodium cations. The heat of adsorption was higher for CO than CH4 and

N2. The high heat of adsorption for CO in ZSM-5 is due to strong interaction of CO

molecules with sodium ions.

Table 5.6. Isosteric heat of adsorption for CO, CH4, and N2 on ZSM-5 with different

SiO2/Al2O3 ratio.

Adsorbent Isosteric heat of adsorption (kJ mol-1

)

CO CH4 N2

ZSM-5(25) 33 25.2 26.8

ZSM-5 (40) 30 22.8 25.3

ZSM-5(100) 28 22.5 23.2

ZSM-5(400) 25 19.8 19.0

ZSM-5(900) 23 20.2 19.5

5.3.6. Dynamic Adsorption Studies of CO and N2 from Binary Gas Mixtures

The breakthrough measurements for CO and N2 from binary gas mixture have been

carried out on ZSM-5 (25) at 303 K, 1 atm. pressure and feed flow of 100 ml/min.

The binary breakthrough data are given in Table 5.7. The CO + N2 binary adsorption

and desorption breakthrough curves are shown in Figure 5.6. The binary adsorption

study shows the CO selectivity over N2 in accordance with equilibrium selectivity. In

ZSM-5 (25) the breakthrough time of CO decreases while the dynamic capacity

increased with increasing CO percentage. This decrease in dynamic adsorption

capacity is due to decrease in partial pressure of CO. The adsorbed CO is easily

desorbed by counter current purging of N2 at 100 ml/min.


154

Figure 5.6 (a-d). Breakthrough curves for binary gas mixtures on ZSM-5 (25), (a) CO

(20%) + N2 (80%) (b) CO (12%) + N2 (88%) (c) CO (6%) + N2 (96%) (d) CO (1.6%) + N2

(98.4%).

Table 5.7. Breakthrough data for binary gas mixtures on ZSM-5 (25).

Adsorbent Feed gas

composition by

volume (± 1%)

Weight of

adsorbent

(g)

Breakthrough

time (min)

Dynamic

capacity

(cm3/g)

ZSM-5 (25) CO = 20

N2 = 80

45.3 9 4

ZSM-5 (25) CO = 12

N2 = 88

45.3 11 2.9

ZSM-5 (25) CO = 6

N2 = 94

45.3 13 1.7

ZSM-5 (25) CO = 1.6

N2 = 98.4

45.3 16 0.6

0 5 10 15 20 25 30 35 40

0.0

0.4

0.8

1.2

1.6

Vo

lum

e (

%)

Time (min)

CO Adsorption

CO Desorption

(d)

0 5 10 15 20 25 30 35

0

5

10

15

20

Vo

lum

e (

%)

Time (min)

CO Adsorption

CO Desorption

(a)

0 5 10 15 20 25 30

0

3

6

9

12

Vo

lum

e (

%)

Time (min)

CO Adsorption

CO Desorption

(b)

0 5 10 15 20 25 30 35

0

2

4

6

Vo

lum

e (

%)

Time (min)

CO Adsorption

CO Desorption

(c)


155

5.3.7. Dynamic Adsorption Studies of CO, CH4, N2 from Ternary Gas Mixtures

The breakthrough measurements for CO + CH4 + N2 ternary gas mixture has been

carried out on ZSM-5 (400) at 303 K and 1 atm pressure and feed flow of 100ml/min.

The ternary breakthrough data are given in Table 5.8. The ternary adsorption and

desorption/breakthrough curves for CO (15%) + CH4 (70%) + N2 (15%) gas mixture

on ZSM-5 (400) are shown in Figure 5.7.

Figure 5.7 (a-b). Breakthrough adsorption/desorption curves for CO (15%) + CH4 (70%) +

N2 (15%) ternary gas mixture on ZSM-5 (400).

The dynamic adsorption studies showed that ZSM-5 (400) has CH4 selectivity over

CO & N2 and CO selectivity over N2. The increased concentration of CO in outlet

during adsorption was due to replacement of initially adsorbed CO with high affinity

CH4 molecules. The adsorbed CO is easily desorbed by counter current purging of N2

at 100 ml/min. We studied gas mixture with different composition (CH4 = 30-85%,

CO = 5-30% and N2 = 10-70%) and found that the breakthrough time is not affected

by change in concentration, while the dynamic adsorption capacity decreases with

decrease in CH4 % composition.

0 5 10 15 20 25

0

20

40

60

80

100

120

Vo

lum

e (

%)

Time (min)

CO

CH4

N2

(a)

Adsorption

0 5 10 15 20 25 30

0

20

40

60

80

100

120

Vo

lum

e (

%)

Time (min)

CO

CH4

N2

(b)

Desorption


156

Table 5.8. Breakthrough data for ternary gas mixtures on ZSM-5 (400).

Adsorbent Feed gas

composition

by volume (±

1%)

Weight of

adsorbent

(g)

CH4

Breakthrough

time (min)

CO

Breakthrough

time (min)

CH4

Dynamic

capacity

(cm3/g)

ZSM-5

(400)

CH4 = 85

CO = 05

N2 = 10

84 9 8 9.1

ZSM-5

(400)

CH4 = 70

CO = 15

N2 = 15

84 9 7 7.5

ZSM-5

(400)

CH4 = 50

CO = 30

N2 = 20

84 9 6 5.4

ZSM-5

(400)

CH4 = 70

CO = 10

N2 = 20

84 9 6 7.5

ZSM-5

(400)

CH4 = 50

CO = 10

N2 = 40

84 9 6 5.4

ZSM-5

(400)

CH4 = 30

CO = 10

N2 = 70

84 10 6 3.6

5.4. Conclusions

The adsorption capacity for CO, CH4 and N2 decreases with increase in the

SiO2/Al2O3 of ZSM-5 at 288 and 303 K. The percentage decrease in CO and N2

adsorption capacity is higher than that of CH4. The selectivity of CH4 over CO and N2

increased while selectivity of CO over N2 decreased with increase in SiO2/Al2O3.

ZSM-5 with SiO2/Al2O3 ratio of 900 showed higher selectivity for CH4 over CO and

N2 whereas with SiO2/Al2O3 ratio of 25 it showed higher CO selectivity over N2. The

heat of adsorption for CO, CH4 and N2 decreased with increase in SiO2/Al2O3. The

high CH4 and CO adsorption capacity, selectivity and heat of adsorption for ZSM-5

with low SiO2/Al2O3 show stronger electrostatic interaction of CO and CH4 molecules

with sodium cations compared to those for high SiO2/Al2O3. The value of Henry’s

constant also confirms the result. With increase in the SiO2/Al2O3 the dispersion

interaction potential dominates and hence ZSM-5 with SiO2/Al2O3 ratio of 400 and

900 shows higher value of Henry’s constant for CH4 than CO.

The adsorption capacities and selectivity for CO over N2 are very promising for ZSM-


157

5 (25) as an adsorbent for the CO separation from its mixture with N2. The adsorption

of binary and ternary gas mixtures of CH4, CO and N2 with different composition

showed selective adsorption of CH4 over CO and N2 and CO over N2 in adsorbent

column. In binary gas mixture the breakthrough time decreases and dynamic capacity

increases with increasing CO percentage. In the ternary gas mixture the CH4

breakthrough time was not decreases while dynamic adsorption capacity increases

with increase in CH4 percentage composition. The adsorbed CH4 and CO desorbed

completely under counter current N2 flow. Zeolite ZSM-5 (400) can be used as an

adsorbent for CH4 separation from its mixture with CO and N2, while ZSM-5 (25) can

be used for separating the CO and N2 gas mixture.


158

References:

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