Journal of Engineering Science and Technology
Special Issue on SOMCHE 2014 & RSCE 2014 Conference, January (2015) 87 - 95
© School of Engineering, Taylor’s University
DEVELOPMENT OF NATURAL ZEOLITES ADSORBENT:
CHEMICAL ANALYSIS AND PRELIMINARY
TPD ADSORPTION STUDY
1,
1
1
S. K. WIRAWAN *, H. SUDIBYO , MUHAMMAD F. SETIAJI ,
2
3
I. W. WARMADA , ENDANG T. WAHYUNI
1
Chemical Engineering Department, Gadjah Mada University Jalan Grafika No.2 Sleman,
Yogyakarta, Indonesia
2
Geological Engineering Department, Gadjah Mada University Jalan Grafika No.2 Sleman,
Yogyakarta, Indonesia
3
Department of Chemistry, Gadjah Mada University Jalan Grafika No.2 Sleman,
Yogyakarta, Indonesia
*Corresponding Author: [email protected]
Abstract
Klaten Indonesian Natural Zeolites had been modified to become a potential
adsorbent for gas or liquid separation. To enhance the adsorption capacity and
stability, a pre-treatment by acid (HCl dan H2SO4) and Ba-cation activation and
modification were applied to the raw natural zeolite samples. The pre-treatment
could increase the Si/Al ratio which correspond to the low-hydrophilic characteristic
of the zeolites and increase the Bronsted acid sites. This enhancement will improve
the capability of the natural zeolites as adsorbents or catalysts. After the pretreatment, the adsorbent was characterized by AAS, XRD analysis and TPD
adsorption measurement. The result shows that higher Si/Al ratio was achieved
when natural zeolites was activated by strong acid of HCl and H2SO4 (0.1 M and 0.3
M). Si/Al ratio of the HCl-activated zeolite is 5.33 while Si/Al ratio of the H2SO4
activated zeolite is 6.65. For the untreated zeolite, the Si/Al ratio is 5.14. The result
of XRD analysis supports the AAS analysis result in which the composition of
mordenite and clinoptilolite increased after acid activation. The increase of
mordenite and clinoptilolite composition related to the increase of Si/Al ratio. The
TPD adsorption measurement of the Ba(NO3)2 activated zeolite gave results of CO2
peak partial pressure of 4.4×10-7 torr, greater than the raw zeolite’s which is only
9.9×10-8 torr. By observing the adsorption time, the Ba(NO3)2-activated zeolite
requires 137 minutes to reach it’s CO2 peak partial pressure compared to the raw
natural zeolites which requires 48 minutes. This indicated that the Ba(NO3)2
modification can improve the adsorption capacity of natural zeolites qualitatively.
Keywords: Si/Al ratio, Acid activation, Zeolite, Bronsted acid site,
CO2 TPD adsorption.
87
88
S. K. Wirawan et al.
Nomenclatures
Edes
N
PCO2
R
t
Tpeak
v
activation energy of desorption
specific coverage area
partial pressure of CO2
ideal gas constant
time of desorption
temperature of CO2 peak partial pressure
pre-exponential coefficient
Greek Symbols
β
heating rate
Abbreviations
AAS
IPA
TPD
XRD
Atomic Absorption Spectroscopy
Iso-Propyl Alcohol
Temperature – Programmed Desorption
X – Ray Diffraction
1. Introduction
Zeolite have unique characteristic by having large surface area/gram of zeolite
because of its porous characteristic. The active surface area of zeolite is up to 200
m2/g zeolite [1]. Zeolite applications in industrial sector are as adsorbents,
membranes, catalysts, etc. Adsorption is a surface-based process that occurs when
a gas or liquid solute accumulates on the surface of a solid (adsorbent) forming a
molecular or atomic film (adsorbate) [2]. The challenge for adsorption system in
industrial separation and purification process is reaching the comparable product
purity with common distillation system. The energy consumption is also a
relevant issue to be considered in comparing the both separation systems.
In the fragrance industry, the purification of the spent solvent is necessary for
reusing the solvent [3]. The common solvent used is isopropyl alcohol (IPA). To
be able to purify the solvent, selectivity of the selected adsorbent must be higher
toward IPA than water (hydrophilic adsorbent). To improve the hydrophilic
characteristic, it is important to modify their chemical structure and composition
by applying a suitable pretreatment.
The objectives of this paper are determining the Si/Al ratio of the zeolite after
acid pretreatment and carrying the preliminary study for natural zeolite activated
by Barium Nitrate (Ba-(NO3)2).
2. Experimental
Solutions of HCl (Merck) and H2SO4 (Merck) as activating agent were prepared
by diluting each strong acid with aquadest to form 0.1 M and 0.3 M
concentration for each acid. Ba(NO3)2 0.05 mM was also used as an activating
agent. An experimental procedure was designed to determine the adsorption
characteristics of Klaten Indonesian Natural Zeolite by measuring the Si/Al
ratio and the adsorption capacity. The natural zeolite was sieved with 80 mesh
Sieve Shaker JSGUS Type ELM. Then, the activation was completed by mixing
the activated zeolite and activating agent with the ratio of 1 g zeolite: 3 mL
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activating agent. Washing and drying step were carried out prior to
characterization of natural zeolite chemical properties.
Atomic absorption spectroscopy (AAS) was utilized for revealing the
composition of cation oxide chemical compound in the natural zeolite [4]. The
result of AAS analysis were used to calculate the Si/Al ratio inside the natural
zeolite. X-Ray Diffraction (XRD) powder diffraction was used to confirm the
composition analysis result. Both untreated natural zeolite and modified natural
zeolite with the acid, will be examined by AAS and XRD analysis method. The
Temperature Programmed Desorption analysis method was used to compare the
adsorption capacity of the zeolite activated by Ba(NO3)2 0.05 mM and the raw
natural zeolite. The adsorption capacity of zeolite was observed by measuring the
partial pressure of CO2 desorbed from the zeolite.
3. Results and Discussion
3.1. Atomic absorption spectroscopy (AAS) analysis
The characterization results of the samples with different activating agent and
concentration show a significant change after the activation. The AAS analysis results
have been used to calculate the Si/Al ratio (mole ratio) as reported in Table 1.
Table 1. AAS analysis result for raw zeolite and acid-treated zeolite.
Measurement, % wt
Average of
Sample
Component
the
I
II
III
measurement
SiO2
69.15
70.45
69.15
69.58
Raw Zeolite
Al2O3
13.44
13.25
13.63
13.44
Si/Al Ratio
4.40
SiO2
66.49
67.79
67.79
67.35
HCl 0.3 M
Al2O3
12.48
12.29
12.29
12.35
Si/Al Ratio
4.63
SiO2
71.71
73.01
71.71
72.14
H2SO4 0.3 M
Al2O3
10.78
11.35
10.97
11.04
Si/Al Ratio
5.56
The Si/Al ratio increase was also followed by the increase of adsorptivity
because the releasing of Al from the active site Si-O-Al caused the transformation
of the active site into Si-O-Si in which the Si was coming from the outter structure.
But, kept in mind that the zeolite tend to be a hydrophobic adsorbent when the Si/Al
ratio increased. The formation of localized electrostatic pole between cation and
anion resulted in hydrophilic characteristic to adsorb more water than IPA. The
releasing of Al (dealumination) would reduce the number of cation inside the
zeolite structure and reduced the hydrophylic characteristic of the zeolite.
3.2. Discourse of zeolite to be catalyst
Actually, the acid activation not only caused the dealumination but also provides an
ion exchange or counter ion thorugh H+ or alkali metal insertion. Naturally, the
alumina tetrahedral had the negative charge because of its form AlO4-. Silica
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S. K. Wirawan et al.
tetrahedral had the neutral charge (SiO2). Discourse to apply zeolite as catalyst
would be realized based on the Si/Al ratio result after the strong acid activation. The
Bronsted – Lowry theory of acid and base said that acid will be functioned as proton
donor and base as proton acceptor [5]. The parameters shifted to get deeper into
Bronsted acid site formed because acid activation tends to “acidify” the zeolite..
Bronsted acid site was the distorted tetrahedral structure of zeolite in which the
Al - O(H) – Si bond have longer chain than three Al – O – Si. Bronsted acid site
was explained as hydroxyl group bridge between Al and Si. Bronsted acid site was
strengthened by reducing the Al amount inside the zeolite and inserting the low
charge cation. This condition would push the zeolite to have a weak bond so that the
zeolite could experience an easy cation exchange [6]. Therefore, the zeolite would
be functioned as the catalyst. Further research was necessary to explore the
Bronsted acid site formed by strong acid activation.
3.3. X-ray diffraction (XRD) analysis
XRD analysis results in Fig.1 showed that the zeolite composed of crystalline
structures. The powder contains a mixture of zeolite mineral groups and the inert
minerals. Mordenit [Na3KCa2(Al8Si40O96)·28H2O], was one of zeolite forming
minerals found in zeolitic stone spread in D.I. Yogyakarta and Central Java region.
From difractogram resulted from XRD analysis, this mineral had peak value of d
13.671, 9.074Å, 6.582Å, 3.466Å and 3.216Å. Mordenite had such physical property
as orthorhombic structure with 28% void volume. Mordenite always exist together
with clinoptilolite [7]. XRD analysis result agreed with the term saying that
mordenite always existed together with clinoptilolite. In difractogram, clinoptilolite
could be seen from its d 3.983Å dan 2.890Å peak value. The difference between
clinoptilolite and mordenite is that clinoptilolite has monoclinic system with void
volume of 39%. Both mordneite and clinoptilolite had high thermal stability [7].
Other zeolite forming minerals existed in difractogram were mesolite, feldspar,
gibbsite, hematite, and clinochlore. Mesolite[(Na2Ca2)(Al6Si9O30)·8H2O] existed in
nature but usually it was only an accessory in zeolite structure [8]. Feldspar
[(K,Na)AlSi3O8] was a primary composer in zeolite so that it existed in
difractogram with peak value of d 3.755Å, 3.378Å and 3.343Å. Gibbsite [(Al(OH)3]
was an tetrahydrate alumina found on laterite ground and resulted from feldspar
weathering [9]. Gibbsite and hematite [Fe2O3] usually exist together [9]. Therefore
the difractogram analysis agree with the theory. Meanwhile, clinochlore
[(Mg10Al2)(Si6Al2)O20(OH)16] was just a variant of chlorite (Mg-rich chlorite)
resulted from weathering of pyroxene mineral or iron substitution by magnesium
mineral along the zeolite forming process. Clinochlore had peak value of d 4.258Å
in difractogram.
Based on the XRD analysis, mordenite zeolite type was the major component in
the zeolite, followed by clinoptilolite and mesolite. Mordenite zeolite type is a
common zeolite found in silica-rich rocks [10].
The acid pretreatment by HCl and H2SO4 sharpened the structure of each
mineral type as shown in Fig. 1. Application of higher acid concentration for zeolite
activation cause the angle of the peak become sharper (the angle got smaller). Acid
activation caused the peak height of mordenite and clinoptilolite higher than the raw
zeolite’s. Therefore, the increasing of mordenite and clinoptilolite concentration
Journal of Engineering Science and Technology
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Development of Natural Zeolites Adsorbent: Chemical Analysis and . . . . 91
showed that after acid pretreatment, there is Si/Al ratio increase. But, acid activation
did not change the structure of the zeolite itself.
Fig. 1. XRD analysis result of the raw zeolite and acid-treated zeolite.
3.4. Temperature – programmed desorption (TPD) analysis
For the TPD analysis, as shown in Fig. 2 the raw zeolite peak partial pressure of
CO2 were reached after the desorption process ran about 48 minutes. Its peak
partial pressure was 9.9×10-8 torr. For, zeolite treated by barium nitrate (ZeoliteBa(NO3)2 ), as shown in Fig. 3, reached its peak partial pressure of CO2 after the
desorption process ran about 137 minutes. Its peak partial pressure was
4.45×10-7 torr.
The height of CO2 peak partial pressure described the adsorption capacity of
CO2. The higher CO2 peak partial pressure of zeolite-Ba(NO3)2 proved that
Ba(NO3)2 activation increased the adsorption capacity of zeolite. As the
consequence, zeolite-Ba(NO3)2 needed longer time to desorb the CO2 adsorbed. The
time necessary to reach the peak partial pressure corresponded to the temperature of
the zeolite powder. Longer time meant higher temperature because in TPD analysis,
the desorption process was supported by the constant heating rate (50 oC/min). It’s
well-known that the desorption process were endothermic.
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S. K. Wirawan et al.
Fig. 2. TPD analysis result of raw zeolite.
50
PC O2, (x 10-8) torr
40
30
20
10
0
0
5000
10000
15000
20000
-10
t, seconds
Fig. 3. TPD analysis result of zeolite – Ba(NO3)2.
Based on the Poliwanyi-Wigner theorem, the rate of desorption of adsorbates
from an adsorbent could be stated in the first order or second order kinetic
equation. The procedure in the TPD analysis was analyzing the partial pressure of
Ar, N2, and CO2. Therefore, the desorption occurred in this case was categorized
as the recombinative desorption [11]. The appropriate equation to formulate the
rate of desorption for recombinative desorption was the second order kinetic
equation. The second order kinetic of desorption means that the Tpeak depend on
the specific coverage, N, heating rate, β, and with constant Edes as the result of
Tpeak measurement.
dN
 E 
= −v. exp − des .N 2
dt
 R.T 
(1)
Longer time necessary to reach the peak partial pressure of CO2 described that
the temperature of the zeolite powder is greater too. It could be concluded that the
activation energy of desorption process would be greater if the time was longer.
Journal of Engineering Science and Technology
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Development of Natural Zeolites Adsorbent: Chemical Analysis and . . . . 93
Back to the top of this paragraph that longer time equal to greater temperature,
therefore in this case, the desorption process of CO2 from zeolite-Ba(NO3)2 had
greater energy of activation than the raw zeolite did. This phenomenon explained
why 125 minutes was necessary for zeolit-Ba(NO3)2 to reach its peak partial
pressure of CO2. Meanwhile the raw zeolite needed 33 minutes.
Other possible interpretations was that the pre – exponential coefficient (v) or
frequency factor value was lower on greater desorption temperature to reach peak
partial pressure of CO2. Therefore, zeolite-Ba(NO3)2 had lower frequency factor (v)
than the raw zeolite did.
From the TPD analysis in the research, it’s impossible to determine the value of
frequency factor (v) or energy of activation for desorption process (Edes). Those
values were possibly determined if the desorption process was done at least with
two values of heating rate (β) such things as at 50 oC/min or 25 oC/min). The
explanation why two values of heating rates were necessary explained below. The
second order kinetic of desorption was used as basic because of the recombinative
desorption [11].
dN
 E 
= −v. exp − des .N 2
dt
 R.T 
(2)
dT
=β
dt
(3)
dN dt
v
 E 
= − . exp − des .N 2
.
β
dt dT
 R.T 
(4)
dN
v
 E 
= − . exp − des .N 2
β
dT
 R.T 
(5)
The equation above showed that the rate of desorption was temperaturedependent. The temperature when the peak partial pressure of CO2 was reached was
formulated below:

 v
 E 
d  − . exp − des .N 2 
 R.T 

 β
dT
=0
(6)
T =Tpeak
Because N was also a temperature – dependent variable the derivative of the
equation above was:
N 2 .v
β
N 2 .v
β
.
 E
E des
. exp − des
2
 R.T
R.T peak
peak

 v

 + . exp − E des
 R.T peak
 β



.2 N . dN = 0

dT

.
 E
E des
. exp − des
2
 R.T
R.T peak
peak

 v

 − . exp − E des
 R.T
 β
peak




.2 N . exp − E des

 R.T
peak


Journal of Engineering Science and Technology
(7)
 2
.N = 0


(8)
Special Issue 4 1/2015
94
S. K. Wirawan et al.

E des
E
v
= 2 N . . exp − des
2

β
R.T peak
 R.T peak
2
T peak
β
=
2
 T peak
ln
 β

 E
E des
. exp des
 R.T
2.N .v.R
peak









 E des 1
 E des 
=
+ ln
.


R
T
 2.N .v.R 
peak

(9)
(10)
(11)
By varying the heating rate value, there would be more than one Tpeak from the
heating rate (β) variations. Two value of β meant two value of Tpeak. Each data
would support the determination of specific coverage (N) which equal to the area
below the curve of partial pressure CO2 vs desorption time. The area below the
curve would propose the determination value of Edes. Edes and N would end with
calculation to get v. Because only one heating rate used in this research, there would
be only qualitative interpretation (without quantitative interpretation) for each
variable in the formula of second order kinetic of desorption.
4. Conclusions
The adsorption potential and acidity behavior of the Klaten natural zeolites can be
studied by an acid activation (HCl dan H2SO4) and continued by Ba-cation
exchanged. The acid treatment could increase the Si/Al ratio which correspond to
the low-hydrophilic characteristic of the zeolites and increase the Bronsted acid
sites. The adsorption behavior of zeolites could be studied qualitatively by a step
change TPD adsorption measurement. Higher Si/Al ratio could be achieved when
natural zeolites was activated by strong acid. However, H2SO4 was better than HCl
for increasing the Si/Al ratio of the natural zeolite samples. Mordenite concentration
in the zeolite powder increased after acid activation indicated the Si/Al ratio
increased. The TPD adsorption measurement indicated that the Ba(NO3)2
modification can improve the adsorption capacity of natural zeolites qualitatively.
Acknowledgment
The authors are grateful for the financial support of ‘Hibah Penelitian
Kerjasama Luar Negeri dan Publikasi Internasional DIKTI 2013-2014’.
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