n-PENTANE ORIGINATED PROTONIC ACID SITES
OVER ZINC PROMOTED HZSM-5
MALIK MUSTHOFA
A thesis submitted in fulfilment of the
requirements for the award of the degree of
Master of Science (Chemistry)
Faculty of Science
Universiti Teknologi Malaysia
JULY 2009
iii
Teristimewa untuk:
ibunda Siti Mulyani, Bp. Zaed Nurul Hadi (alm) dan semua kakakku
belahan jiwa tersayang, Tri Margiyati Sucini,
penyejuk mataku, Innaa Munaadiyah,
buah hatiku, Muhammad Mas’uud Elfarisi,
semoga semua ini diridhoi dan diberkahi Alloh SWT.
iv
ACKNOWLEDGEMENT
Thanks to Allah who has given me the opportunity and will to perform this
Master project. I wish to express my sincere appreciation to my supervisor, Assoc.
Prof. Dr. Sugeng Triwahyono, for professional advices, encouragements, guidance,
critics and friendship. Besides, I would like my truthful to my co-supervisor, Assoc.
Prof. Dr. Aishah Abdul Jalil, for knowledge, guidance, motivation, and for
supporting me throughout the undertaking of this research.
Grateful acknowledge to the Ministry of Science, Technology and Innovation
(MOSTI) Malaysia for financial support through EScience Fund Research Grant, No.
03-01-06-SF0020. I am very thankful to my research friend, especially to Aini
Mohamed Rozali, Nur Hanis Hayati Hairom, Ainul Hakimah Karim, and Imam
Sumpomo for giving me knowledge, assistances, information, opinions, and for their
contribution in my research. My appreciation also goes to all of the members of
Department of Chemistry and all staff of Ibnu Sina Institute for Fundamental Science
Studies, especially to Lim Kheng Wei, Mohd Nazri Nawi, and Wan Aklim
Nursalafiany Wan Ahmad, for the supports, and good relationship.
I am also indebted to Muhammadiyah University of Surakarta (UMS) for
agreement, supports and attentions. My sincere appreciation also extends to my
parents, family, and those who provide assistance in this research either intentionally
or unintentionally at various situations and occurrences during the progress of this
research.
v
ABSTRACT
The formation of protonic acid sites from hydrogen and n-pentane over
Zn/HZSM-5 was evidenced by pyridine adsorption IR spectroscopy. The results
showed that protonic acid sites were formed by heat treatment of Zn/HZSM-5 in the
presence of molecular hydrogen which started at room temperature with a
concomitant elimination of Lewis acid sites.
Removal of molecular hydrogen
decreased the protonic acid sites and restored the Lewis acid sites to its original
intensity at 573 K. These phenomena are consistent with the concept of ‘molecular
hydrogen-originated protonic acid sites’ in which the process is initiated by
dissociation of hydrogen molecules to hydrogen atoms, spilling over onto the
HZSM-5 and followed by surface diffusion. Then hydrogen atom converts into a
proton by releasing electron to Lewis acid site. The formation of protonic acid sites
on Zn/HZSM-5 induced by molecular hydrogen is a reversible process. The protonic
acid sites were also formed from n-pentane on Zn/HZSM-5. Lewis acid sites were
converted to protonic acid sites when the sample was heated in n-pentane, and the
formed protonic acid sites were subsequently eliminated by removal of n-pentane
with a restoration of Lewis acid sites. These are essentially the same as a formation
of protonic acid sites from molecular hydrogen. It is plausible that the formation of
protonic acid site was initiated by dissociation of n-pentane to form hydrogen atom
and certain molecule.
Then, the hydrogen atom spills over onto the HZSM-5
followed by surface diffusion to form protonic acid sites by releasing electron to the
Lewis acid sites. The formed protonic acid sites which act as active sites played
significant role in the reactions of n-pentane.
vi
ABSTRAK
Pembentukan tapak asid protonik daripada hidrogen dan n-pentana atas
Zn/HZSM-5 telah dibuktikan melalui penjerapan piridin menggunakan spektroskopi
inframerah.
Hasil kajian menunjukkan bahawa tapak asid protonik terbentuk
manakala tapak asid Lewis tersingkir apabila Zn/HZSM-5 dipanaskan bermula pada
suhu bilik dalam kehadiran molekul hidrogen. Penyingkiran molekul hidrogen ini
mengurangkan tapak asid protonik dan mengembalikan tapak asid Lewis kepada
kepekatan asalnya pada suhu 573 K.
Fenomena ini didapati menepati konsep
‘molecular hydrogen-originated protonic acid sites’ di mana ia dimulakan dengan
pemisahan molekul hidrogen kepada atom hidrogen yang melimpah ke atas HZSM-5
dan diikuti dengan penjerapan di permukaannya. Atom hidrogen ini seterusnya
bertukar menjadi proton dengan memindahkan elektron kepada tapak asid Lewis.
Pembentukan tapak asid protonik pada Zn/HZSM-5 yang dijana oleh molekul
hidrogen ini merupakan proses berbalik. Tapak asid protonik juga didapati terbentuk
daripada n-pentana pada Zn/HZSM-5.
Pada mulanya, tapak asid Lewis akan
ditukarkan kepada tapak asid protonik apabila sampel dipanaskan dalam n-pentana,
seterusnya tapak asid protonik yang terbentuk itu dihapuskan dengan menyingkirkan
n-pentana untuk menjanakan semula tapak asid Lewis.
Ini bertepatan dengan
pembentukan tapak asid protonik daripada molekul hidrogen.
Dengan ini
pembentukan tapak asid protonik berkemungkinan dimulakan dengan penguraian npentana bagi membentuk atom hidrogen dan molekul tertentu. Kemudian, atom
hidrogen akan melimpah ke atas HZSM-5 diikuti dengan pembauran permukaan
untuk membentuk tapak asid protonik dengan membebaskan elektron kepada tapak
asid Lewis. Tapak asid protonik yang terbentuk bertindak sebagai tapak aktif yang
berperanan penting dalam tindak balas n-pentana.
vii
TABLE OF CONTENTS
CHAPTER
TITLE
PAGE
TITLE
1
2
DECLARATION
ii
DEDICATION
iii
ACKNOWLEDGEMENTS
iv
ABSTRACT
v
ABSTRAK
vi
TABLE OF CONTENTS
vii
LIST OF FIGURES
x
LISTS OF SYMBOLS
xiii
LIST OF ABBREVIATIONS
xiv
LIST OF APPENDICES
xv
INTRODUCTION
1.1
Research Background
1
1.2
Objective of Research
3
1.3
Scope of Research
3
LITERATURE REVIEW
2.1
Isomerization of Saturated Straight Alkane
4
2.2
Solid Acid Catalysts for Alkanes Isomerization
5
2.2.1
Friedel-crafts Catalysts
6
2.2.2
Chlorinated Alumina
6
2.2.3
Zirconia-based Catalyst
6
2.2.4
Zeolite Material-based Catalyst
7
viii
2.2.4.1
3
HZSM-5
9
2.3
n-Pentane Isomerization
10
2.4
Reaction Mechanism of n-Pentane Isomerization
12
2.4.1
Reaction Pathway of Monofunctional Mechanism
12
2.4.2
Reaction Pathway of Bifunctional Mechanism.
13
2.5
Role of Hydrogen on the Isomerization of Alkanes
14
2.6
Catalyst Preparation
16
EXPERIMENTAL
3.1
3.2
Preparation of Catalysts
18
3.1.1
Preparation of HZSM-5
18
3.1.2
Synthesis of Zn/HZSM-5
18
3.1.3
Preparation of Zr(OH)4
19
3.1.4
Synthesis of Pt/SO42--ZrO2
19
Characterization
19
3.2.1
X-Ray Diffraction (XRD) Analysis
19
3.2.2
BET Surface Area Analysis
20
3.2.3
Fourier Transform Infra Red ( FTIR) Spectroscopy
20
3.2.4
Qualitative Analysis of Zn and Pt by Energy Dispersive
21
X-ray (EDX)
4
3.2.5
Infra Red (IR) of Pyridine Adsorption
21
3.2.6
Ammonia Temperature Programmed Desorption (TPD)
22
3.3
Catalytic Testing
23
3.4
Formation of Protonic Acid Sites
23
RESULTS AND DISCUSSION
4.1
Characterization of Catalysts
26
4.1.1
X-Ray Diffraction (XRD) Analysis
26
4.1.2
Fourier Transform Infra Red ( FTIR) Spectroscopy
28
4.1.3
BET Surface Analysis
30
4.1.4
FESEM and EDX Analysis
30
4.1.5
Distribution of Acid Sites
32
4.1.6
Nature of Acidity
34
ix
5
4.2
n-Pentane Isomerization on Zn/HZSM-5
38
4.3
Formation of Protonic Acid Sites
39
4.3.1
Hydrogen Molecule Originated Protonic Acid Sites
39
4.3.2
n-Pentane Originated Protonic Acid Sites
43
4.3.3
Hydrogenation of Chemisorbed Pyridine
49
CONCLUSION AND FUTURE WORK
5.1
Conclusion
52
5.2
Future Work
53
REFFERENCES
54
APPENDICES
60
x
LIST OF FIGURES
FIGURE NO.
TITLE
PAGE
2.1
The isomerization of n-pentane.
4
2.2
Acid sites on the surface of zeolite.
8
2.3
Structure of ZSM-5.
9
2.4
The possibilities of structure of Zn species in
10
Zn/HZSM-5.
2.5
Monofunctional
mechanism
of
n-pentane
13
isomerization.
2.6
Bifunctional mechanism of n-pentane isomerization
14
2.7
Model for the generation of protonic acid sites of
16
solid acid catalyst.
3.1
Formation of protonic acid sites apparatus.
25
4.1
XRD patterns of HZSM-5 and Zn/HZSM-5.
27
4.2
XRD pattern of Pt/SO42--ZrO2.
27
4.3
FTIR spectra of HZSM-5 and Zn/HZSM-5.
29
4.4
FTIR spectra of Pt/SO42--ZrO2.
29
4.5
FESEM image of Zn/HZSM-5
30
4.6
EDX analysis for Zn/HZSM-5
31
4.7
FESEM image of PSZ
31
4.8
EDX analysis for PSZ
32
4.9
Ammonia TPD plots of HZSM-5, Zn/HZSM-5 and
34
Pt/SO42--ZrO2.
xi
4.10
IR spectra of (a) HZSM-5 after treated at 623 K, (b)
35
Pyridine adsorbed on HZSM-5, (c) Pyridine adsorbed
on Zn/HZSM-5at 423 K followed by removal of
physisorbed of pyridine at 598 K.
4.11
A) IR spectra of pyridine adsorbed on Zn/HZSM-5
36
pretreated at 598 K followed by removal of
physisorbed of pyridine at a) 423 K, b) 473 K, c) 523
K and d) 598 K. B) The fraction of acid sites after
heating in the presence of hydrogen at different
temperature.
4.12
The IR spectra of (a) Pt/SO42--ZrO2 treated at 623 K
37
and pyridine adsorbed on Pt/SO42--ZrO2.
4.13
Activity and selectivity of Zn/HZSM-5 for n-pentane
39
isomerization in the presence and absence of
hydrogen.
4.14
IR spectra of pyridine adsorbed on Zn/HZSM-5. (A)
41
Spectral changes when pyridine-preadsorbed sample
was heated in hydrogen at b) 298 K, c) 323 K, d) 348
K, e) 373 K and f) 398 K. a) Before exposure to the
hydrogen.
(B) The change of spectrum (f) when
hydrogen was removed at g) 323 K, h) 373 K, i) 423
K, j) 473 K, k) 523 K and l) 573 K.
4.15
The fraction of acid sites (A) after heating in the
42
presence of hydrogen and B) removal of hydrogen at
different temperature. White square and circle are
Lewis and protonic acid sites before exposure to the
hydrogen, respectively.
4.16
Proposed mechanism for formation of protonic acid
43
sites from molecular hydrogen over solid acid
catalyst.
4.17
IR spectra of pyridine adsorbed on Zn/HZSM-5. (A)
Spectral changes when pyridine-preadsorbed sample
was heated in dried n-pentane a) room temperature, b)
44
xii
323 K, c) 373 K, d) 473 K and e) 578 K. (B) Spectral
changes when the sample of the spectrum (e) was
heated in a removal of dried n-pentane at f) 323 K, g)
373 K, h) 473 K, i) 573 K and j) 623 K.
4.18
The fraction of acid sites (A) after heating in the
45
presence of n-pentane and B) removal of dried npentane at different temperature.
4.19
IR spectra of pyridine adsorbed on Pt/SO42--ZrO2.
47
(A) Spectral changes when pyridine-preadsorbed
sample was heated in dried n-pentane a) 373 K, b)
393 K, c) 423 K, d) 473 K, e) 528 K and f) 573 K. (B)
Spectral changes when the sample of the spectrum (f)
was heated in a removal of dried n-pentane at g) 373
K, h) 423 K, i) 473 K, j) 523 K and k) 573 K.
4.20
The fraction of acid sites (A) after heating in the
48
presence of n-pentane and B) removal of dried npentane at different temperature.
4.21
Speculated mechanism for formation of protonic acid
48
sites from n-pentane over solid acid catalyst.
4.22
Spectral changes for pyridine adsorbed on Zn/HZSM5 caused by heating in hydrogen at a) room
temperature, b) 373 K, c) 523 K and d) 573 K.
51
xiii
LIST OF SYMBOLS
θ
-
Angle
ε
-
Extinction coefficient
K
-
Kelvin
e-
-
Electron
xiv
LIST OF ABBREVIATIONS
RON
-
Research Octane Number
ZSM-5
-
Zeolite Socony Mobil-Five
XRD
-
X-Ray Diffraction
FTIR
-
Fourier Transform Infra Red
TPD
-
Temperature-Programmed Desorption
CFR
-
Continuous Flow Reactor
L
-
Lewis
B
-
BrØnsted
Py
-
Pyridine
BET
-
Brunauer, Emmet and Teller
n
-
Normal
i
-
Iso
xv
LIST OF APPENDICES
APPENDICES
A
TITLE
List of publication
PAGE
60
CHAPTER 1
INTRODUCTION
1.1
Research Background
The considerable need for high quality fuel in an era of pressing
environmental concern demands the resurgence in chemical processes and catalysts
science. It has given challenges and opportunities in these research areas to obtain a
sustainable process and catalysts with high activity and selectivity as well as
acceptable poison resistance.
A lot of research projects on the chemical processes and catalysis science
have been done in order to enhance the quality of gasoline.
The skeletal
isomerization of hydrocarbon was recommended as the important processes to
produce high quality gasoline by improving its research octane number (RON) [1-3].
The effective catalysts for this process such as solid acid catalysts based on metal
oxide and zeolitic materials therefore were extensively observed as well. These solid
acid catalysts were more favourable as compared to the liquid catalysts which cause
significant corrosion and environment problems [3-5]. Moreover these types of solid
acid catalyst have been widely applied in the petroleum industries.
Solid acid catalysts such as ZSM5 and/or Beta zeolite-based catalysts and
chloride alumina–based catalysts are conventionally available for the skeletal
isomerization of alkanes.
Although zeolite-based catalysts have an outstanding
tolerance of feedstock poisons, high temperature is still required. The chlorinated
2
alumina–based catalysts suffer from extreme sensitivity to all kinds of feed
contaminants [2, 3]. Recently, zirconia-based catalysts such as Pt/SO42--ZrO2 and
Pt/WO3-ZrO2 were reported as efficient catalysts for the skeletal isomerization of
alkanes. Moreover, Pt/SO42--ZrO2 has been used as catalyst for industrial processes
and Pt/WO3-ZrO2 has been proposed as alternative catalyst for isomerization of
alkanes [1, 4-5]. However, Pt/SO42--ZrO2 and Pt/WO3-ZrO2 were less selective for
production of clean gasoline by the skeletal isomerization of n-heptane. Okuhara [3]
reported that metal loaded-zeolite based catalysts such as Pt/HZSM-5, Pd-Hβ, and
Pt/H-β have great potential for production of clean gasoline by the skeletal
isomerization of n-heptane. These solid acid catalysts were more active and selective
for isomerization of n-heptane due to the effect of pore structure.
Pt/SO42--ZrO2 and Pt/WO3-ZrO2 possess a high and stable activity for the
isomerization of alkanes when the reaction is conducted in the presence of hydrogen.
Hattori and co-workers [6-7] suggested that these catalytic performances were due to
the generation of protonic acid sites from hydrogen molecule and became active sites
for isomerization of alkanes.
Moreover, hydrogen stream is required in the
isomerization of heptane by Pt/HZSM-5, Pd-Hβ, and Pt/H-β to enhance and stabilize
the activity of catalysts [3].
However, recently Iglesia et al. [8] reported that the isomerization of n-alkane
to produce iso-alkene occurs in the absence of hydrogen on Fe/HZSM-5. They
proposed that reaction via dehydrogenation of n-alkane to form hydrogen molecule
and n-alkene and then molecular hydrogen reacts with n-alkene to produce isoalkene.
Based on that report, then we postulated that the active sites for
isomerization can be formed from the reactant i.e. alkanes. However, the direct
evidences and detail mechanism of the formation of active sites from reactant in the
acid catalyzed reaction are not observed yet.
Therefore, we intended to study the formation of protonic acid sites from
reactant i.e. alkanes over solid acid catalyst.
3
1.1
Objective of Research
The objective of this research is to elucidate the generation of protonic acid
sites from n-pentane over Zn/HZSM-5 solid acid catalyst.
1.3
Scope of Research
This research enclosed the synthesis and characterization of catalysts as well
FTIR study of pyridine adsorption for the generation of protonic acid sites.
Zn/HZSM-5 solid acid catalyst was prepared by ion exchange method. The structure
of catalysts was determined by X-ray Diffraction (XRD) technique, while the
specific surface area of catalyst was measured with BET surface area analyzer. The
functional group of catalysts was considered by Fourier Transform Infra Red (FTIR)
spectroscopy. Ammonia Temperature-Programmed Desorption (TPD) was used to
determine the distribution of acid sites on the catalyst. The activity of catalysts was
tested on a Continuous Flow Reactor (CFR) in the presence and absence of
hydrogen. The generation of protonic acid sites over solid acid catalysts was studied
by pyridine pre-adsorbed FTIR spectroscopy.
.
CHAPTER 2
LITERATURE REVIEW
2.1
Isomerization of Saturated Straight Alkanes
Isomerization can be defined as a rearrangement of the structure of compound
without gain or loss of its component [9]. Isomerization of saturated straight alkanes
was first used by the petroleum industry in the 1930’s, and the reaction received
theoretical examination at about the same time [10].
Nowadays, the skeletal
isomerization of saturated straight alkanes becomes an important process in modern
refining for a gasoline of high quality.
The main purpose for the production of branched isomers of saturated straight
alkane is to enhance the research octane number (RON) of gasoline pool. The
isomerization process converts alkanes which has low RON into iso-alkane which
relatively has higher RON. For example, RON of n-pentane and n-hexane are 61.7
and 24.8, respectively, while those of 2-methyl butane (iso-pentane) and 2,3dimethylbutane are 92.3 and 103.6, respectively [2,11]. The isomerisation of npentane into iso-pentane is described in Figure 2.1.
n-pentane (RON=61.7)
Figure 2.1
iso-pentane (RON=92.3)
The isomerization of n-pentane
5
Although a lot of works on the isomerization of alkane have been carried out,
the reaction mechanism and role of catalysts and hydrogen are still in controversial
[3,11-12].
The recent reports on the catalysts and catalytic-isomerization of n-
alkanes are reviewed shortly in this chapter.
2.1
Solid Acid Catalysts for Alkanes Isomerization
Solid acid catalyst is a solid material which has a tendency to denote a proton
or to accept an electron pair from a molecule according to definition of BrØnsted and
Lewis. Considering the behaviours of the acid sites reaction, the acid sites on solid
acid are classified into two groups, BrØnsted and Lewis acid sites. BrØnsted acid site
is known as protonic acid site usually exists in the form of acidic hydroxyl group and
Lewis acid site usually appears in a metal which has coordinate unsaturated structure
[9,13].
In the recent years, solid acid catalysts have been developed for the
alkane isomerization. These catalysts substitute the liquid acid catalysts which
cause significant containment, corrosion and environment problems.
The
outstanding properties of solid acid catalysts are stable, regenerable and low
operation-temperature [4].
There are two types of catalysts effective for the isomerization of alkane;
solid acid catalysts (monofunctional) and solid acids modified with a transition
metal (bifunctional catalysts). The activity and selectivity of solid acid catalysts are
influenced greatly by type, strength and amount of acid sites [3,11]. The following
materials are several solid acid catalysts which were studied and implemented for
the isomerization of alkane.
6
2.2.1 Friedel-crafts Catalysts
Friedel-crafts catalysts, such as AlCl3 with additive SbCl3 and HCl, were
used for the earlier industrial isomerization process. These catalysts are strongly
acidic and very active even at low temperature. The low-temperature activity is
very favourable since the equilibrium shifts to branched isomers at lower
temperature. However, use of these catalysts causes the problems of corrosion of
the reactor and the disposal of the used catalysts character by a chlorine compound
[11].
2.2.2 Chlorinated Alumina
Chlorinated alumina is an active catalyst for alkane isomerization, although
higher temperature is required compared to the zeolitic material–based solid acid
catalysts.
Chlorinated alumina can be prepared by chlorination of Al2O3 by
chlorine-containing organic compound to improve the acidity of Al2O3. The activity
will also increase greatly by introduction of transition metal on the chlorinated
alumina [9,11]. In the industrial process, the reactants are butane, pentane, hexane
and their mixture (light naphtha). The reaction conditions depend on the reactant
and the nature of catalysts [11].
2.2.3 Zirconia-based Catalyst
Sulphated zirconia (SZ), SO42--ZrO2, possesses strong acid sites and able to
convert n-alkane into iso-alkane at low temperature.
In the presence of metal
catalyst such as Pt, Co or Ni and hydrogen, the activity and stability of SO42--ZrO2
catalyst increase markedly [2,14]. Operating at a high temperature will deactivate
catalyst rapidly due to the decomposition of sulphate ion [4]. Pt/SO42--ZrO2 (PSZ)
was developed by Cosmo Oil Co., Ltd. and Mitsubishi Heavy Industries, Ltd. for the
7
isomerization of light naphtha and this process was commercialized by UOP LLC
[11].
Tungstated zirconia (WZ), WO3-ZrO2, is similar to SO42--ZrO2 which possess
strong acid sites but WO3-ZrO2 is able to be operated at a higher temperature
compared to SO42--ZrO2. Although there are a lot of similarity between WO3-ZrO2
and SO42--ZrO2, in general, WO3-ZrO2 gives a higher conversion and less selectivity
compare to that of SO42--ZrO2 [4,14].
2.2.4 Zeolite Material-based Catalyst
Zeolites are crystalline aluminosilicates with precisely defined microporous
structures. Zeolites have some properties that make them unique. They are highly
crystalline with well-defined structure.
Zeolites, also called molecular sieves,
possess the ability to be shape and size selective in catalytic molecular
rearrangement. Ions within the cavities are easily exchanged with a large number of
altervalent ions.
Electron distribution shifts in acidic hydroxyl group may be
tailored to give higher acidities [15,16].
Zeolites are built from Aluminium and Silicon atoms are tetrahedrally
coordinated to four bridging oxygen atoms. Zeolite can be classified according to
their pore size which depends on the number of oxygen atoms in the aperture ring
which can contain 8-member (small pore zeolite), 10-member (medium pore zeolite)
and 12-member oxygen rings (large pore zeolite) [9,15].
When Si4+ is replaced by Al3+ in the zeolites frame work, the negative AlO4building block has to be compensated by counter ion. BrØnsted acidity can be
introduced by protons as compensation ion, thus making the material a solid acid.
The concentration of BrØnsted sites is therefore directly related to the number of
8
frame work Al atoms per unit cell. The acid sites on surface of zeolite are shown in
Figure 2.2.
BrØnsted acid site
Lewis acid site
O
O
Si
O
Al
OO
Figure 2.2
H+
OO
O
Al
Si
Si
OO
O
O
O
OO
Acid sites on the surface of zeolite.
The strength of the acid sites depends on the polarization of the OH band and
therefore is influenced by the angle between the two bridging T-O bonds and the
distance between the T-atoms connected to the bridging oxygen. Therefore, the acid
strength depends on the structure of the tree-dimensional network and the local
atomic environment [15,16].
Since aluminium carries a lower charge than the silicon atom, the
electronegativity of material is strongly dependent on the ratio between the silicon
and aluminium atoms in the framework. A higher amount of silicon atoms in the
framework causes a strengthening of the BrØnsted acidic OH bond and with it lower
deprotonation energy (higher acid strength). Consequently the number of BrØnsted
acid sites decreases.
Zeolites are not thermodynamically stable material.
High temperature,
concentrated mineral acids and alkalines or steam can destroy the structure of
zeolite causing the aluminium atoms to leave the frame work.
aluminium sites feature Lewis acidic character [16].
These extra
9
2.2.4.1 ZSM-5
Many types of zeolites catalysts have been developed and used for
hydrocarbon conversion processes in the modern refining industry. A significant
stage in the development of zeolite catalysts was the discovery of synthesis of ZSM5 (Zeolite Socony Mobil-5) by Mobil oil. ZSM-5 is an aluminosilicate zeolite with a
high silica and low aluminium content. The structure of ZSM-5 is built by 5-1
secondary building units, which are linked together to form chains. ZSM-5 is a
highly porous material and throughout its structure it has an intersecting twodimensional pore structure [9,17-18]. Figure 2.3 gives the structure of ZSM-5.
Figure 2.3
Structure of ZSM-5
The enhancement of catalytic performance of ZSM-5 because of metal
loading was reported in several papers [19-21]. The addition of Zn, Cu or Ge
increases the ratio of Lewis/BrØnsted acid sites. Moreover, it was recently reported
that addition ZnO improves the product selectivity for the alkane isomerization [22].
However, the structure and location of metal, in particular Zn, are not clear yet. The
proposed models suggest that zinc is introduced in ZSM-5 as either Zn2+ ion or as
ZnOH+ ions or that Zn2+ replaces two protons in the neighbouring aluminiumoxygen tetrahedra resulting in formation binuclear (Al-O-Zn2+-O-Zn2+-O-Al)
bridging fragments [20,23]. Thus, the structure of zinc in ZSM-5 depends on the
sample preparation method, silica alumina ratio and Zn/Al ratio. Structures of Zn in
HZSM-5 proposed are described in Figure 2.4.
10
O
Zn2+
Zn2+
O
Si
Figure 2.4
2.2
Zn2+
O
O
Al Si
Al
Si
O
Al Si
Al
The possibilities of structure of Zn species in Zn/HZSM-5
n-Pentane Isomerization
Isomerization of naphtha is composed mainly of a conversion of n-pentane
and n-hexane into iso-pentane, mono-methyl and dimethyl-pentane.
Therefore
isomerization of n-pentane is among the most important reaction of alkane
isomerization [24]. Several works have been conducted to obtain effective catalysts
for n-pentane isomerization.
The most widely applied catalysts for n-pentane isomerization are platinum
chlorinated alumina and zeolite based catalysts. Platinum chlorinated alumina gave
conversion of 77 % and selectivity of 90 % at about 428 K [1,25]. However this
catalyst suffers from chlorine loss during the isomerization process. Therefore such
catalyst requires the continuous addition of chlorine-containing compounds that are
moisture-sensitive.
They are also highly corrosive and hence, environmentally
hazardous [25].
Several observations on n-pentane isomerization have been also performed
on HZSM-5 and H-Mordenite based catalysts. Loading Pt or Pd on these catalysts
improved the catalytic performance of catalysts. Although these catalysts require
high temperature, elevated activity and selectivity can be obtained. Conversion and
selectivity gained were 68 % and 93% respectively [1,3, 26].
11
Sulphated zirconia (SZ) catalysts have been proposed as alternative catalysts,
which was active for the isomerization of n-pentane at low temperatures (30–150oC).
The promotion of SZ catalysts by the addition of noble metals and transition metal
oxides improved their stability, activity and selectivity even further [1,3,27].
Catalytic activity of Platinum Sulphated zirconia (PSZ) was higher than that of the
zeolitic catalysts but lower than that of chlorinated alumina.
Conversion and
selectivity obtained were 72 % and 89 % respectively [1,3]. However this catalyst
has disadvantage of deactivation and sulphur loss during reaction.
Hino and Arata proposed tungstated zirconia (WZ) for the first time. WZ has
superior stability under both reducing and oxidizing conditions and appear to be
more suitable for industrial applications. The catalytic activity of WZ is greatly
improved by promotion with Pt or Pd and with transition metal oxides when
hydrogen is present in the reaction feed.
Platinum tungstated zirconia (PWZ)
catalysts appear to have found commercial application already [3,28]. Knozinger et
al. investigated the catalytic performance of WZ, PWZ, platinum iron oxide
(PtFeWZ) and platinum chromium oxide (PtCrWZ) in the isomerization of npentane. The most promising catalyst giving 98% selectivity and 70% conversion
was PtCrWZ at reaction temperature of 523 K [28].
It was reported that Al- or Ga-promoted WZ catalysts showed activity and
stability for n-pentane isomerization. Compared to the major commercial catalysts,
Al- or Ga-promoted WZ catalysts are composed of mixed oxides without noble metal
and are halogen-free catalysts. So the cost of Al- or Ga-promoted WZ catalysts is
low and isomerization of n-pentane over this catalyst is a green process. Conversion
and selectivity achieved are 70% and 90% [25].
Recently, platinum
tungstophophoric acid supported MCM-41 was reported as an active catalyst for npentane isomerization with 78% conversion and 83% selectivity [29]
Okuhara et al. reported that Pd-H4SiW12O40/SiO2 was evaluated with respect
to the skeletal isomerization of n-pentane.
The effects of the loading of
H4SiW12O40/SiO2 and Pd on the catalytic activity and selectivity were investigated.
The results demonstrated that 1 wt% Pd-20 wt% H4SiW12O40/SiO2 offered excellent
12
catalytic performance for the hydroisomerization of n-pentane with 43 % conversion
and 99 % selectivity [30]. Heteropolyacids compounds such as H3PW12O40, Cs2.5
H0.5PW12O40, and Pt-Cs2.5 H0.5PW12O40 were also studied for the isomerization of npentane. Davis and co-workers observed the effects of cesium substitution on the
hydrocarbon reactivity. Pt-Cs2.5 H0.5PW12O40 exhibited conversion of 50 % and
selectivity of 96 % [31-33].
2.3
Reaction Mechanism of n-Pentane Isomerization
The isomerization of alkane over solid acid catalyst is known to proceed by
both acid-catalyzed monofunctional and metal-acid bifunctional mechanism. In both
cases, carbenium ions are believed to be responsible for the skeletal rearrangement.
Therefore, the formation of carbenium ions and its rearrangement on the catalyst
surface are the most important steps in the isomerization [11].
2.3.1
Reaction Pathway of Monofunctional Mechanism
Isomerization of alkane by acids proceeds through elementary steps:
H
H
+ H+
+ H2
… (1)
H
+ L
+ LH
… (1’)
13
… (2)
+
+
+
Figure 2.5
+
H2
H+
… (3)
… (4)
Monofunctional mechanism of n-pentane isomerization
The formation of carbenium ions can be induced through the protonation of alkane
on BrØnsted acid sites (reaction (1)) or through the hydride abstraction from alkane
on Lewis acid sites (reaction (1’)). These processes generally require high acid
strength. The carbenium ions thus formed undergo skeletal rearrangement (reaction
(2)). The newly formed carbenium ions turn into isomerized alkane by a hydridetransfer reaction with reactant alkane (reaction (3)). The chain reactions composed of
reactions (2) and (3) make the isomerization catalytic. In isomerization of n-alkane
to branched alkane, reaction (3) usually converts a tertiary carbenium ion into a
secondary one. Under high pressure hydrogen, the reaction of carbenium ions with
dihydrogen reaction (4) becomes more important than reaction (3). Hydrogen in this
case is considered to be a chain transfer agent [3,11].
2.3.2
Reaction Pathway of Bifunctional Mechanism.
In the acid-catalyzed monofunctional mechanism, only acid sites are involved
in the reaction. In the metal-acid bifunctional mechanism, metal sites and acid sites
act as active sites in the reaction.
Thus, it is supposed that alkane is
dehydrogenated on metallic sites to the corresponding alkene, which is isomerized
by acid sites into a branched alkene. The branched alkene is then hydrogenated into
the branched alkane again on the metallic sites.
14
+ H2
+ H+
+ H+
+ H2
Figure 2.6
(on metallic sites)
… (5)
(on solid acid sites)
… (6)
(on solid acid sites)
… (7)
(on solid acid sites)
… (8)
(on metallic sites)
… (9)
Bifunctional mechanism of n-pentane isomerization
According to this mechanism, acidic sites do not participate in the difficult step of
alkane activation, which requires high acid strength.
When a loaded
amount of a transition metal exceeds a certain level, the hydrogenation
dehydrogenation steps, reactions (5) and (9) reach the equilibrium under hydrogen.
The rate-determining step of the isomerization is the rearrangement of carbenium
ions reaction (7) [11].
2.4
Role of Hydrogen on The Isomerization of Alkanes
The role of hydrogen on the isomerization of alkane is still in controversy. It
was suggested that hydrogen acts as a coke removal by hydrogenation. Coke is
carbonaceous material form on the surface of acidic catalysts upon high temperature
contact with hydrocarbons [17,34]. Coke is formed on the surface of catalysts due to
the thermodynamic instability of hydrocarbons at high temperature with revere to the
elements, that is, hydrogen and carbon. Formation of carbonaceous deposits can
15
destroy the acid sites and cover the catalyst surface. This phenomenon results in
catalysts deactivation.
Most of acid-catalyzed processes for hydrocarbon conversion are influenced
by the occurrence of catalyst coking and the need for request catalyst regeneration.
Reactivation is mostly obtained by burning, with air or pure oxygen, the coke
particles. Obviously, regeneration by burning is exothermic and causes temperature
peaks at the surface of catalyst. The catalyst must be consequently stabilized to stand
this treatment, in particular, if repeated frequently, without relevant damage and
surface area loss. Other regeneration techniques are hydrogenation which performed
if noble metal particle are present and catalyzed this reactivation reaction [17].
According to Tomishige et al. [12], the effect of hydrogen was dependence
on reaction condition (temperature, hydrogen partial pressure). At high reaction
temperature and low hydrogen pressure, the reaction order with respect to hydrogen
was close to zero, but at low reaction temperature and high hydrogen pressure, the
negative effect of hydrogen was observed. However, they suggested that hydrogen
spillover phenomena occur, and hydride plays an important role in the reaction
mechanism of isomerization.
Another role of hydrogen is reported by Hattori and Shishido [6]. They
proposed that hydrogen atoms convert into protonic acid sites and act as active sites
in the acid-catalytic reaction. The mechanism of generation of protonic acid sites
from hydrogen molecules is described in Figure 2.7.
The mechanism of generation of protonic acid sites from hydrogen molecules
is begun by dissociation of hydrogen molecules on centers such as platinum species
to form hydrogen atoms. The hydrogen atoms spill over onto the support and
migrate to Lewis acid sites, where the hydrogen atom releases an electron to become
a proton. The proton may be stabilized on the O atom near the Lewis acid site. The
electron trapped on the Lewis acid site may react with a second hydrogen atom to
form a hydride which is stabilized on the Lewis acid site. As a whole, the hydrogen
16
molecule converts into a protonic acid site and a hydride, and the Lewis acid site
loses its function. The protonic acid sites thus formed act as catalytically active sites
for acid-catalyzed reactions [6].
H2
Dissociation
H H
Active Site
Acidic Support
Figure 2.7
Spillover
Diffusion
H
+e-
Protonic Acid
H+
-e-
Lewis Acid Sites
Model for the generation of protonic acid sites of solid acid catalyst
The elucidation of hydrogen generating protonic acid sites was also reported
by Triwahyono et al. [7]. Using IR pyridine adsorption technique, they reported the
interconversion between protonic and Lewis acid sites in respect to the presence and
removal of hydrogen over Pt/WO3-ZrO2 and WO3-ZrO2.
2.5
Catalyst Preparation
Most solid catalysts have tree types of easily distinguishable components:
active components, support or carrier and promoters.
Active components are
responsible for the principal chemical reaction. Support or carrier perform many
functions, but most important is maintenance of high surface area for the active
component. A promoter is some agent which when added, often in small amounts,
results in desirable activity, selective or stability effects. Promoters are designed to
assist either the support or the active component. They are added to supports in
order to inhibit undesirable activity. Promotion of the active component may be
either structural or electronic [9].
17
Dispersion of oxides on high-area supports is carried out by one of methods:
precipitation, adsorption, ion exchange and impregnation.
Each technique has
advantages and disadvantages. Precipitation is the preferred deposition route for
loadings higher than 10-20%. Below this value, other techniques are usually
practiced [9,18].
In adsorption, a support material exposed to metal salt solution adsorbs
equilibrium quantities of salt ion and obeys adsorption isothermal. Adsorption is an
excellent method for depositing a small amount of support material. Powders or
particle are dehydrated and soaked in the appropriate solution for suitable periods.
Deposition is uniform, providing all pores are penetrated during the soaking time.
Adsorption from solutions may be either cationic or anionic depending on the
properties of the surface [9].
Ion exchange in catalyst preparation is very similar to ionic adsorption but
involves ion exchange of ions other than protons. Lower valency ions exchange with
ions having higher charge. Ion exchange is useful in removing harmful agent and
adding promoters. Because of the larger number of ion change possibilities, this
method promises to be important for the modification of catalytic materials [9].
Impregnation, also known as incipient wetness, is the simplest and most the direct
method of deposition. The object is to fill the pores with a solution of metal salt of
sufficient concentration to give the correct loading [9,18].
Recently, electrochemical method was developed as an effective method for
the preparation of catalysts. Aishah et al. (2002) reported that highly reactive zinc
was produced successfully by using electrolysis in which platinum acted as cathode;
zinc was an anode and naphthalene was used as a mediator [35].
CHAPTER 3
EXPERIMENTAL
3.1
Preparation of Catalysts
3.1.1
Preparation of HZSM-5
A commercial ammonium form ZSM-5 (Zeolyst) with Si/Al atomic ratio of
80 was used as a support material. The protonated-ZSM-5 (HZSM-5) was obtained
by treatment of ammonium form ZSM-5 at 823 K for 3 h.
3.1.2
Synthesis of Zn/HZSM-5
Zn/HZSM-5 sample was prepared by treating HZSM-5 with Zn2+/N,Ndimethylformamide solution followed by filtration, drying at 393 K and calcination
at 873 K in air [36]. Zn2+/N,N-dimethylformamide solution was prepared following
the procedure of Aishah et al. [35].
19
3.1.1
Preparation of Zr(OH)4
Zirconium hydroxide was prepared by hydrolysis of ZrOCl2.8H2O (Wako
Pure Chemical) with 2.5 wt% NH4OH (Wako Pure Chemical) aqueous solution [33].
The precipitate was filtered and washed with deionised water. The obtained gel was
dried at 383 K to form Zr(OH)4.
3.1.2
Synthesis of Pt/SO42--ZrO2
The Pt/SO42--ZrO2 was prepared as follows.
Zr(OH)4, which is denoted as
SO42--Zr(OH)4,
The sulphated ion–treated
was prepared by impregnation of the
Zr(OH)4 with 1N H2SO4 aqueous solution followed by filtration and drying at 383 K.
The sulphated zirconia (SO42--ZrO2) was obtained by calcinations of SO42--Zr(OH)4
at 873 K in air. The Pt/SO42--ZrO2 was prepared by impregnation of SO42--ZrO2 with
H2PtCl6.6H2O (Wako Pure Chemical) aqueous solution followed by drying and
calcinations at 873 K in air. The content of Pt was adjusted to 0.5 wt %.
3.2
Characterization
3.2.1
X-Ray Diffraction (XRD) Analysis
XRD technique offers the information about the phase structure of the
samples whether they are amorphous or crystalline, the relative crystallinity, the
average crystalline size, and the lattice crystal. A crystal lattice is a regular three
dimensional (cubic, rhombic, etc) of atoms in space [37,38].
About 1 g of dried sample was ground to fine powder, and then lightly
pressed onto a XRD sample holder to get a thin layer of sample.
The X-ray
diffractogram was then collected at room temperature on a Bruker AXS D8
20
Automatic Powder Diffractometer with a Cu Kα (λ= 1.5418 Å at 40 kV and 40 mA)
radiation sources.
3.2.2
BET Surface Area Analysis
Total surface area is the important molecule parameter specified without
regard to the type of surface. Measurement of surface area using BET (Brunauer,
Emmet and Teller) surface area analyzer involves the principles of physical
adsorption of gas molecules on solid surface. The concept of the theory is an
extension of the Langmuir theory, which is a theory for monolayer adsorption, to
multilayer adsorption with the hypothesis: gas molecules physically adsorb on a solid
in layers infinitely; there is no interaction between each adsorption layer; and the
Langmuir theory can be applied to each layer [9,39].
The specific surface area of the solid catalysts was measured by Quanta
chrome Autosorb-1. Approximately 0.05 g of sample was put into a sample tube
holder followed by evacuation at 573 K for 3 h. The adsorption of nitrogen was done
at 77 K.
3.2.3
Fourier Transform Infra Red ( FTIR) Spectroscopy
FTIR is an analytical tool for examining the chemical bonding of samples.
The analysis by infrared spectroscopy is based on the fact that molecules have
specific frequencies of internal vibration. These frequencies occur in the infrared
region of electromagnetic spectrum: 4000 to 200 cm-1. Infrared spectroscopy reveals
information about molecular vibrations that cause a change in the moment dipole of
molecules [40].
21
KBr pellets technique was used to determine the functional groups of
catalyst. The experimental was done as follow. Sample was ground thoroughly with
KBr at approximately 1 to 3% by weight and pressed. The spectra were recorded on
Perkin-Elmer Spectrum GX FT-IR Spectrometer at room temperature with a spectral
resolution of 4 cm-1 and with 5 scans in open beam air background.
3.2.4
Qualitative Analysis of Zn and Pt by Energy Dispersive X-ray (EDX)
Energy dispersive X-ray analysis (EDX) provides information about which
element the sample contain quantitatively and qualitatively. Thus, Zn on Zn/HZSM5 and Pt on Pt/SO42--ZrO2 were analyzed qualitatively by this technique. Field
Emission Scanning Electron Microscope (FESEM) was used to visualize very small
topographic details on the surface or entire or fractioned objects. The high-resolution
reached by FESEM allows the study of morphology analysis (particles shape and
size), structure uniformity determination and quantitative and qualitative elemental
analysis [9].
Field Emission Scanning Electron Microscopy (FESEM) images were
obtained by JSM-6701F microscope. The accelerating voltage was 15 kV with a
beam current 20 µA and working distance of 8 mm.
3.2.5
Infra Red (IR) of Pyridine Adsorption
The IR spectroscopic of pyridine adsorption has been accepted as a general
practice to identify the type of acidic sites present at the surface of catalysts. This
technique is widely used to distinguish between BrØnsted and Lewis acid sites [41].
Generally, two bands are observed which attributed to pyridine-acid sites interaction:
a band at 1540 cm-1 obviously assigned to BrØnsted acid sites and the band around
1445 cm-1 arises from pyridine adsorbed on Lewis acid sites [42]. By measuring the
22
intensity of those bands and from the value of extinction coefficient, it is possible to
calculate the number of BrØnsted and Lewis acid sites.
The experimental was done as follow. A self-supported wafer placed in an insitu stainless steel FTIR cell with CaF2 windows followed by heating in driednitrogen at 623 K for 3 h. Then, the sample was exposed to the pyridine at 423 K for
1 h followed by removal of physisorption of pyridine at 598 K for 30 min. All
spectra were recorded at room temperature with a spectral resolution of 4 cm-1 and
with 5 scans.
3.2.6
Ammonia Temperature Programmed Desorption (TPD)
NH3-TPD was used to determine the distribution of acid sites. When basic
molecule (NH3, pyridine, etc) are adsorbed on acid sites, a base adsorbed on a strong
acid site is more stable than one adsorbed on weak acid site, and is more difficult to
be desorbed. Thus, the acid strength is considered to be parallel to the desorption
temperature. The amount of a gaseous base which a solid acid can adsorb chemically
from the gaseous phase is a measure of the amount of acid sites on its surface.
Therefore, the concentration of acid sites is reflected in the peak area of TPD profile,
and the strength of the acid sites in the temperature at which base molecule is
desorbed [43].
The ammonia TPD was performed on ThermoQuest TPD/R/O 1100. Prior to
the adsorption of ammonia, the sample was pre-treated with nitrogen flow at 673 K
for 2 h. Then, the sample was exposed to the dehydrated ammonia at 373 K for 30
min followed by purging with He flow to remove the excess amount of ammonia on
the surface of catalyst. TPD was run from room temperature up to 1073 K with a
heating rate of 10 K/min.
23
3.3
Catalytic Testing
The activity of catalyst was tested on the isomerization of n-pentane in the
presence and absence of hydrogen at 523 K. Prior to the reaction, the sample was
treated by oxygen and hydrogen flow at 673 K for 5 h with flow rate of 10 ml/min.
Isomerization was carried out in an online Continuous Flow Reactor (CFR) coupled
with 6090N Agilent Gas Chromatography. The conversion, selectivity and yield of
reaction were determined by following equations.
Conversion =
C1-C4 + iC4 + iC5
[C5] feed
... (3.1)
Selectivity =
iC5
C1-C4 + iC4 + iC5
... (3.2)
Yield = selectivity x conversion
... (3.3)
Where C1-C4: concentration of methane, ethane, propane and butane in the product.
iC4: concentration of iso-butane in the product.
iC5: concentration of iso-pentane in the product.
[C5] feed: concentration n-pentane in the feed.
3.4
Formation of Protonic Acid Sites
Observation of generation of protonic acid sites was performed by IR of
pyridine adsorption study. The experimental was done as follow. A self-supported
wafer placed in an in-situ stainless steel cell IR cell with CaF2 windows was heated
in dried-nitrogen at 623 K for 3 h. Then, the sample was exposed to the pyridine at
423 K for 1 h followed by removal of physisorbed of pyridine at 598 K for 30 min.
For formation of protonic acid sites from hydrogen molecules, the pyridine-covered
sample was then exposed to the dried-hydrogen at room temperature (RT). The
sample was heated stepwise from RT to certain temperature in 25 K increment. In
the removal of hydrogen, the sample was flashed by dried-nitrogen stepwise from RT
24
in 50 K increment. At each heating and removal of hydrogen were continued for 15
min. These steps were repeated for n-pentane exposure. All gases were dried by
molecular sieve and double liquid-nitrogen traps. All spectra were recorded on
Perkin-Elmer Spectrum GX FT-IR spectrometer at room temperature. Figure 3.1
shows the schematic diagram of formation of protonic acid sites apparatus.
25
Flow
Controller
Valve
Pellet
Valve
Pyridine and
n-pentane port
Valve
Trapping System
Cell Reactor and Heating
System
H2
Cylinder
N2
Cylinder
Figure 3.1
Formation of protonic acid sites apparatus
CaF2
Window
CHAPTER 4
RESULTS AND DISCUSSION
4.1
Characterization of Catalysts
4.1.1
X-Ray Diffraction (XRD) Analysis
The diffractograms of HZSM-5 and Zn/HZSM-5 are shown in Figure 4.1.
All diffractograms showed peaks at range of 2θ = 7-10o and 22-25o, which
correspond to specific peaks of ZSM-5 [44]. By introduction of Zn on HZSM-5,
peak positions of specific peaks of HZSM-5 did not shift that indicating the HZSM-5
structure did not change. However, the crystallinity of HZSM-5 decreased to be 89
%. This decrease of crystallinity indicated that crystal of HZSM-5 collapsed that
then reducing surface area of HZSM-5 as suggested by BET analysis result. Degree
of crystallinity can be determined according to method published by Nicolaides et al.
and Silva et al. [45-46]. The intense peaks of zinc at 2θ = 31o and 2θ = 36o
corresponding to Zn (100) and Zn (101) were not observed.
Therefore, semi
qualitative analysis of the presence of zinc on HZSM-5 was performed by EDX
analysis.
Intensity / a.u
27
Zn/HZSM-5
HZSM-5
0
10
20
30
2θ /
Figure 4.1
40
50
O
XRD patterns of HZSM-5 and Zn/HZSM-5
Intensity / a.u
T
M
20
Figure 4.2
M
30
M
2θ / O
40
50
XRD pattern of Pt/SO42--ZrO2; T: tetragonal phase, M:
monoclinic phase.
28
Figure 4.2 shows the diffractogram of Pt/SO42--ZrO2. The sample showed
two well-established polymorphs, monoclinic and tetragonal phases. The peak at
about 2θ = 30o is assigned to tetragonal phase of zirconia, and the peaks at about 2θ =
28o, 32o and 35o are assigned to monoclinic phase of zirconia. The intense peaks of
metallic platinum at 2θ = 40o and 45o corresponding to Pt (111) and Pt (200) were
not observed which may show that the amount of Pt was too small [47]. Therefore,
semi qualitative analysis of the presence of platinum on PSZ was performed by EDX
analysis.
4.1.1
Fourier Transform Infra Red ( FTIR) Spectroscopy
Figure 4.3 shows FTIR spectra of HZSM-5 and Zn/HZSM-5. The bands at
1100 and 1225 cm-1 are corresponding to T-O-T asymmetric stretching vibration of
HZSM-5 structure. A correlation between the positions of the lattice vibration was
observed at 545 (ZSM-5 lattice deformation) and 795 cm-1, which assigned to T-O-T
symmetric stretching vibration. The next strong band also observed at 450 cm-1 is
assigned to T-O bending mode. The existence of bands at 450, 545, 795, 1100 and
1225 cm-1 are indication of the complete crystalline structure of ZSM-5 zeolites [48].
FTIR spectra of Pt/SO42--ZrO2 are shown in Figure 4.4. The sample showed
a strong peak at 1395 cm-1 which is assigned to the asymmetric S=O stretching mode
of sulphated groups bound by bridging oxygen atoms to the surface. Two other
bands at 1025 and 1040 cm-1 could be assigned to an inorganic chelating bidentate
complex, the asymmetric stretching frequencies of the S-O bonds [49].
29
Absorbance / a.u
Zn/HZSM-5
HZSM-5
1600
1300
1000
700
400
Wavenumber / cm-1
FTIR spectra of HZSM-5 and Zn/HZSM-5.
Absorbance / a.u
Figure 4.3
1600
1450
1300
1150
1000
Wavenumber / cm-1
Figure 4.4
FTIR spectra of Pt/SO42--ZrO2.
850
30
4.1.2
BET Surface Analysis
The surface area of HZSM-5 and Zn/HZSM-5 are 511 and 495 m2/g
respectively. By loading Zn, surface are of HZSM-5 decreases slightly which may
be due to the decrease of crystallinity of HZSM-5. The surface area obtained was
119 m2/g for Pt/SO42--ZrO2.
4.1.3
FESEM and EDX Analysis
Morphology of Zn/HZSM-5 and Pt/SO42--ZrO2 are given in Figure 4.5 and
4.7 while analysis of EDX for Zn/HZSM-5 and Pt/SO42--ZrO2 are given in Figure 4.6
and 4.8. Even though the images of catalysts are not clear enough, the EDX analysis
clearly provides the presence of Zn on the HZSM-5 and Pt on PSZ. Percentages of
Zn on the HZSM-5 and Pt on PSZ are 6.27 and 0.59 wt% respectively.
2.0
2.0 µm
µm
Figure 4.5
FESEM image of Zn/HZSM-5.
31
Counts
11000
10000
Si
9000
8000
7000
6000
5000
4000
O
3000
Zn Pt Pt
2000 C Zn Al Pt
1000
0
0.00 1.00 2.00 3.00
Pt Zn
Pt
Zn
4.00 5.00 6.00 7.00 8.00 9.00 10.00
keV
Figure 4.6
EDX analysis for Zn/HZSM-5.
3.0
3.0 µm
µm
Figure 4.7
FESEM image of PSZ.
32
2000
Pt
Zr
1800
Counts
1600
1400
1200
1000
800
600
400
C
O
Pt
Zr Pt
SS
Pt
Pt
200
0
0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00
keV
Figure 4.8
4.1.4
EDX analysis for PSZ.
Distribution of Acid Sites
Figure 4.9 shows ammonia TPD plots for HZSM-5, Zn/HZSM-5 and Pt/SO42-ZrO2.
The TPD plots for all samples consisted of three desorption peaks,
corresponding to desorption of ammonia on weak, medium and strong acid sites
respectively. HZSM-5 revealed peaks of weak, medium and strong acid sites at
about 433, 503, and 753 K respectively.
The introduction of Zn on HZSM-5
strengthened and increased the amount of weak and medium acid sites. These were
supported by shift of weak acid sites peak from 433 K to 453 K and medium acid
sites peak from 503 K to 533 K and increase of intensity of these acid sites peaks.
Zn addition on HZSM-5 also raised the amount of strong acid sites but weaken their
strength. These were supported by increase of intensity of such peak and shift the
peak from 753 K to713 K.
Increase of the amount of all acid sites and strengthen of weak and medium
acid sites related to the existence of extra framework Zn cation in HZSM-5 as
illustrated in Figure 2.4. Introduction of Zn on HZSM-5 led to further loss of
33
BrØnsted acid sites associated with the hydroxyl group to form extra framework Zn
cation which remarked as Lewis acid sites. Indication of substitution of Zn cation for
protons of hydroxyl groups was confirmed by IR of pyridine adsorption result [5051]. In this study, extra framework Zn cation formed appeared lower strength of
strong acid sites when interacted with ammonia.
It was supported that extra
framework Zn cation were not able to retain ammonia against at 753 K. Although, it
is not clear yet why the existence of framework Zn cation resulted lower the strength
of strong acid sites, we suggested it may cause by multi layer structure of Zn on
Zn/HZSM-5.
The plots for Pt/SO42--ZrO2 also consisted of three desorption peaks; peak
appearing at about 453 K corresponding to weak acid sites, peak appearing at about
623 K corresponding to medium acid sites and peak appearing at about 813 K
corresponding to strong acid sites. Although the distribution of acid sites of Pt/SO42-ZrO2 is essentially same with that of Zn/HZSM-5 which possesses weak, medium
and strong acid sites, the acidic sites of Pt/SO42--ZrO2 is slightly stronger than that of
Zn/HZSM-5. This was supported by position of peaks for Pt/SO42--ZrO2 at higher
temperature compared to that of Zn/HZSM-5. Overall, the both of Zn/HZSM-5 and
Pt/SO42--ZrO2 possess weak, medium and strong acid sites in which the strong acid
sites are required for n-alkane isomerisation.
Peak intensity / a.u
34
Zn/HZSM-5
PSZ
HZSM-5
273
Figure 4.9
4.1.5
473
673
873
Temperature / K
1073
Ammonia TPD plot of HZSM-5, Zn/HZSM-5 and Pt/SO42--ZrO2
Nature of Acidity
Figure 4.10 shows pyridine adsorbed IR spectra for HZSM-5 and Zn/HZSM5. Figure 4.10(a) shows the spectrum of HZSM-5 after treated at 623 K for 3 h under
dried-nitrogen.
Figures 4.10(b) and 4.10(c) show the spectra of HZSM-5 and
Zn/HZSM-5 after pyridine treatment. Since the samples were treated at 598 K after
exposure to pyridine vapour, the peaks in the spectra should be due to the pyridine
adsorbed only on the strong acid sites. The band at 1454 cm-1 is due to pyridine
adsorbed on Lewis acid site, the band at 1490 cm-1 to pyridine adsorbed on Lewis
acid site and on protonic acid site, and the band at 1545 cm-1 to pyridine adsorbed on
protonic acid sites [41]. The introduction of Zn on HZSM-5 decreased protonic acid
sites at 1545 and 1490 cm-1 and generated Lewis acid sites at 1490 and 1454 cm-1
markedly.
These changes indicated that Zn cation substituted for protons of
hydroxyl groups, which resulted in a decrease of the number of BrØnsted acid sites
and increased the number of Lewis acid sites caused by the existence of extra
framework Zn bonded with hydroxyl group on the surface of HZSM-5.
35
Py:L
Py:B+Py:L
c
Absorbance / a.u
Py:B
b
a
1600
Figure 4.10
1550
1500
1450
-1
Wavenumber / cm
1400
IR spectra of (a) HZSM-5 after treated at 623 K, (b) Pyridine
adsorbed on HZSM-5, (c) Pyridine adsorbed on Zn/HZSM-5 at 423 K followed by
removal of physisorbed of pyridine at 598 K.
The extinction coefficient of ratio, ε1454/ε1545 was 1.49, where ε1454 and ε1545
refer the extinction coefficients due to the pyridinium ion of 1545 cm-1 and
coordinated pyridine of 1454 cm-1, respectively.
The ratio of the extinction
coefficients at the bands 1454 and 1545 cm−1 was measured by converting the band
at 1454 cm−1 to the band at 1545 cm−1 by introduction of a small amount of water
[7]. The obtained ratio is different with the value 1.08 which Hughes and White
reported for the ratio of the apparent integrated absorption intensity of the band at
1450 cm−1 to that of 1540 cm−1 for alumina and Y-zeolite [41] and Triwahyono et al.
reported for Pt/WO3-ZrO2 which the ε1454/ε1545 was 1.82 [7]. These differences may
be resulted by the differences between the integrated absorbance and the absorbance
at the peak position. In this report, the quantitative ratio of pyridine adsorbed on
36
Lewis acid sites and protonic acid sites can be estimated by multiplication of the
ratio of the absorbance of the band at 1454 cm−1 to that of 1545 cm−1 by 0.67.
A
B
1
0.8
Fraction of Acid Sites
Absorbance / a.u
d
c
b
Lewis Acid Sites
0.6
0.4
0.2
Protonic Acid Sites
a
0
1600
1550
1500
1450
Wavenumber / cm-1
Figure 4.11
1400
400 450 500 550 600 650
Removal Temp. / K
A) IR spectra of pyridine adsorbed on Zn/HZSM-5 pretreated at 598
K followed by removal of physisorbed of pyridine at a) 423 K, b) 473 K, c) 523 K
and d) 598 K. B) The fraction of acid sites after heating in the presence of hydrogen
at different temperature.
Figure 4.11 shows the variations of the absorbencies at 1545 and 1454 cm-1
as a function of removal temperature after pyridine was adsorbed on Zn/HZSM-5
followed by removal of physisorbed of pyridine at 598 K.
If the removal
temperature is raised, pyridine molecules adsorbed on weak acid sites should be
desorbed at lower temperatures, and those adsorbed on strong acid sites should be
desorbed at higher temperatures. Figure 4.11(A) shows the absorbance at 1545 and
1454 cm-1 did not change much with the removal temperature of adsorbed pyridine
37
up to 598 K. These results indicated that most of protonic and Lewis acid sites are
strong enough to retain pyridine against removal at 598 K.
The strength of both protonic and Lewis acid sites are clearly seen in Figure
4.11(B) which the absorbance of the bands at 1454 and 1545 cm−1 are plotted against
the removal temperature. Figure 4.12 shows pyridine adsorbed IR spectrum for
Pt/SO42--ZrO2. The Pt/SO42--ZrO2 sample showed all three bands, indicating both
Lewis acid sites and protonic acid sites were presence.
Absorbance / a.u
Py:B+Py:L
Py:B
Py:L
b
a
1600
1550
1500
1450
1400
Wavenumber / cm-1
Figure 4.12
The IR spectra of (a) Pt/SO42--ZrO2 treated at 623 K and pyridine
adsorbed on Pt/SO42--ZrO2.
38
4.2
n-Pentane Isomerization on Zn/HZSM-5
Figure 4.13 shows the activity of Zn/HZSM-5 on n-pentane isomerisation in
the presence and absence of hydrogen. In the presence of hydrogen, the selectivity
and yields of iso-pentane were 17 and 40 %. In the absence of hydrogen, the
selectivity and yields of iso-pentane were 90 and 45 %, respectively. Although
Zn/HZSM-5 gave better selectivity and yields of iso-pentane in the absence of
hydrogen, the conversion of n-pentane is lower than the reaction with hydrogen. The
differences on the activity of catalysts with and without hydrogen may relate to the
ability in the formation of active sites on the surface of Zn/HZSM-5. The molecular
hydrogen acts as sources of active sites on the surface of Zn/HZSM-5 in the presence
of hydrogen. New report on the isomerization of alkane to iso-alkene in the absence
of hydrogen on Fe/HZSM-5 was published by Iglesia et al. [8]. They proposed that
reaction via dehydrogenation of n-alkane to form hydrogen molecule and n-alkene
and products were formed by the reaction between n-alkene and molecular hydrogen.
Therefore we proposed that active sites on the surface of Zn/HZSM-5 for the
isomerization of n-pentane in the absence of hydrogen can be generated from npentane.
39
100
With Hydrogen
Without Hydrogen
Selectivity ConversionYields
Selectivity ConversionYields
17
25
43
90
10
45
80
%
60
40
20
0
Selectivity Conversion
Yields
With Hydrogen
Figure 4.13
Selectivity Conversion
Yields
Without Hydrogen
Isomerization of n-pentane on Zn/HZSM-5 in the presence and
absence of hydrogen.
4.3
Formation of Protonic Acid Sites
4.3.1
Hydrogen Molecule Originated Protonic Acid Sites
Figure 4.14 shows the changes of IR spectra in the range 1400-1600 cm-1
when pyridine was adsorbed on Zn/HZSM-5 followed by heating in the presence of
hydrogen up to 398 K and removal of hydrogen up to 573 K. Since pyridine
treatment has been done at 598 K, the acid sites under consideration are those which
retained pyridine against treatment at 598 K, these included strong acid sites only.
Heating in the presence of hydrogen increased the intensity of the bands at 1545 and
1490 cm-1, while the intensity of the band at 1454 cm-1 decreased, as shown in Figure
4.14(A). Figure 4.14(B) shows the spectra when the hydrogen was removed after the
measurement of the spectrum (f). As the temperature was raised, the intensity of the
40
band at 1545 cm−1 decreased, and the intensity of the band at 1454 cm−1 increased
almost to its original intensity at 573 K. These results indicated that Lewis acid sites
were converted into protonic acid sites when Zn/HZSM-5 was heated in the presence
of hydrogen, and that the protonic acid sites formed were eliminated by heating in
the removal of hydrogen with restoration of Lewis acid sites.
The absorbance of the bands at 1454 and 1545 cm−1 are plotted against the
temperature of heating in the presence of hydrogen (A), and the temperature of
heating in a removal of hydrogen (B) in Figure 4.15. The conversion of Lewis acid
sites into protonic acid sites upon heating in the presence of hydrogen began to occur
at near room temperature. Restoration of Lewis acid sites by heating in a removal of
hydrogen became appreciable above 373 K.
The present IR study obviously
demonstrated that protonic acid sites are formed from hydrogen molecules
reversibly; protonic acid sites are formed in the presence of hydrogen and eliminated
by removal of hydrogen from gas phase.
Although there exists some differences, the phenomena observed on
Zn/HSZM-5 are essentially same as those observed for SO42--ZrO2 and WO3–ZrO2
type catalysts which follow the concept of ‘molecular hydrogen-originated protonic
acid sites [6-7,52-53]. Metal is prerequisite for the formation of protonic acid sites
from hydrogen molecule for HSZM-5 and SO42--ZrO2 type but it is not required for
WO3–ZrO2 types. The other difference is the ease of conversion of Lewis acid sites
to protonic acid sites. The conversion was appreciable at 473 K for Pt/SO42--ZrO2
and was not observed for Pt-free SO42--ZrO2. The conversion was observed at near
room temperature and 373 K for WO3–ZrO2 with and without Pt respectively. The
conversion of Lewis acid sites to protonic acid sites began to occur at near room
temperature for Zn/HSZM-5. And the interchangeable of Lewis to protonic acid
sites did not observed for Zn-free HSZM-5. Although, it is not certain in the present
what causes the difference in formation temperature of protonic acid sites on the
samples, we suggested that it may cause by the difference on the nature of acidity of
catalysts.
41
B
A
f
l
Absorbance / a.u
Absorbance / a.u
e
d
c
k
j
i
h
b
g
a
1600
1550
1500
1450
-1
Wavenumber / cm
Figure 4.14
1400
1600
1550
1500
1450
1400
-1
Wavenumber / cm
IR spectra of pyridine adsorbed on Zn/HZSM-5. (A) Spectral changes
when pyridine-preadsorbed sample was heated in hydrogen at b) 298 K, c) 323 K, d)
348 K, e) 373 K and f) 398 K. a) Before exposure to the hydrogen. (B) The change
of spectrum (f) when hydrogen was removed at g) 323 K, h) 373 K, i) 423 K, j) 473
K, k) 523 K and l) 573 K.
42
A
1
1
Fraction of Acid Sites
Protonic Acid Sites
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
Lewis Acid Sites
RT
0
0
275 325 375 425
H2 Exposure Temp. / K
Figure 4.15
275 325 375 425 475 525 575 625
H2 Removal Temp. / K
The fraction of acid sites (A) after heating in the presence of hydrogen
and B) removal of hydrogen at different temperature. White square and circle are
Lewis and protonic acid sites before exposure to the hydrogen, respectively.
This result also obviously indicated that the metal sites such as Pt and Zn act
as active sites for the dissociation of molecular hydrogen into hydrogen atom. For
metal loaded SO42--ZrO2, WO3-ZrO2 and HSZM-5, hydrogen molecules dissociated
to hydrogen atoms on metal sites which undergo spillover onto the surface of
catalyst, followed by surface diffusion. Then hydrogen atom converts into a proton
by releasing electron to Lewis acid site. The dissociation of hydrogen molecule is
likely to occur on acidic sites for metal-free WO3–ZrO2 to form hydrogen atoms
which undergo spillover and diffusion of hydrogen atom as well.
Figure 4.16
illustrates the proposal mechanism for formation of protonic acid sites from
molecular hydrogen over solid acid catalysts.
43
H2
Dissociation
Spillover
Protonic Acid
Diffusion
H
H+
Specific Site
+e- -eH H
Zn/HZSM-5
Figure 4.16
Lewis Acid Sites
Proposed mechanism for formation of protonic acid sites from
molecular hydrogen over Zn/HZSM-5.
4.3.2
n-Pentane Originated Protonic Acid Sites
Figure 4.17 shows the changes of IR spectrum in the region 1600 - 1400 cm-1
when the pyridine-preadsorbed Zn/HZSM-5 was heated in dried n-pentane up to 578
K, followed by removal of dried n-pentane in the sample with heating. Figure
4.17(A) shows the spectra when pyridine-preadsorbed Zn/HZSM-5 was heated from
room temperature to 578 K in dried n-pentane. On raising the temperature, the
intensity of the peak at 1454 cm-1 decreased with concomitant increase in the
intensity of the peak at 1545 cm-1. Figure 4.17(B) shows the spectra when the dried
n-pentane was removed from the cell with increasing the temperature after the
measurement of spectrum (e). On raising the temperature, the intensity of the peak at
1545 cm-1 decreased, and the intensity of the peak at 1454 cm-1 increased almost to
its original intensity. These results indicated that Lewis acid sites were converted
into protonic acid sites when the sample was heated in dried n-pentane, and the
formed protonic acid sites were eliminated by removal of dried n-pentane with
increasing the temperature with restoration of Lewis acid sites.
44
B
A
e
j
1600
i
Absorbance / a.u
Absorbance / a.u
d
c
1550
1500
1450
b
g
a
f
1400
Wavenumber / cm-1
Figure 4.17
h
1600
1550
1500
1450
1400
Wavenumber / cm-1
IR spectra of pyridine adsorbed on Zn/HZSM-5. (A) Spectral changes
when pyridine-preadsorbed sample was heated in dried n-pentane a) room
temperature, b) 323 K, c) 373 K, d) 473 K and e) 578 K. (B) Spectral changes when
the sample of the spectrum (e) was heated in a removal of dried n-pentane at f) 323
K, g) 373 K, h) 473 K, i) 573 K and j) 623 K.
45
These spectral changes would be more clearly shown if the fraction acid site
of the peaks at 1454 and 1545 cm-1 are plotted against dried n-pentane exposure
temperature and the temperature of heating in a removal of dried n-pentane. These
plots are shown in Figure 4.18(A) and 4.18(B). The conversion of Lewis acid sites
into protonic acid sites on heating in the presence of dried n-pentane and the reversal
process appear to occur at 323 K and above.
Fraction of Acid Sites
A
1
B 1
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0
0
200 300 400 500 600
n -Pentane Exposure Temp. / K
Figure 4.18
250
350
450
550
650
n -Pentane Removal Temp. / K
The fraction of acid sites (A) after heating in the presence of n-
pentane and B) removal of dried n-pentane at different temperature.
The formation of protonic acid sites from dried n-pentane is also observed on
Pt/SO42--ZrO2. Figure 4.19 shows the interconversion of protonic acid sites and
Lewis acid sites in response to the presence and absence of dried n-pentane. The
spectra changed when pyridine-preadsorbed Pt/SO42--ZrO2 was heated from room
46
temperature to 573 K in dried n-pentane. On raising the temperature, the intensity of
the peak at 1454 cm-1 decreased with concomitant increase in the intensity of the
peak at 1545 cm-1 as shown in Figure 4.19(A). The peaks at 1454 cm-1 restored to its
original intensity by removal of dried n-pentane with heating after the measurement
of spectrum (f), as shown in Figure 4.19(B). These results indicated that Lewis acid
sites converted into protonic acid sites when the sample was heated in dried npentane, and the formed protonic acid sites were eliminated by removal of dried npentane with increasing the temperature with restoration of Lewis acid sites.
These spectral changes would be more clearly shown if the fraction acid site
of the peaks at 1454 and 1545 cm-1 are plotted against dried n-pentane exposure
temperature and the temperature of heating in a removal of dried n-pentane. These
plots are shown in Figure 4.20(A) and 4.20(B). The conversion of Lewis acid sites
into protonic acid sites on heating in the presence of dried n-pentane and the reversal
process appear to occur at 393 K and above.
The phenomena observed on Pt/SO42--ZrO2 and Zn/HZSM-5 is essentially the
same, thought there exist some differences between both catalysts. The conversion
of Lewis acid sites to protonic acid sites in the presence of molecular hydrogen
occurs at near room temperature and at 423 K for Zn/HZSM-5 and Pt/SO42--ZrO2
respectively. The other difference is the ease of the conversion of Lewis acid sites to
protonic acid sites. While all the Lewis acid sites were converted to protonic acid
sites at 573 K for Pt/SO42--ZrO2, considerable amount of Lewis acid sites still exist
for Zn/HZSM-5.
47
A
B
f
k
j
Absorbance / a.u
Absorbance / a.u
e
d
i
c
h
b
a
1600
1550
1500
1450
Wavenumber / cm
1400
-1
g
1600
1550
1500
1450
Wavenumber / cm
1400
-1
Figure .4.19 IR spectra of pyridine adsorbed on Pt/SO42--ZrO2. (A) Spectral
changes when pyridine-preadsorbed sample was heated in dried n-pentane a) 373 K,
b) 393 K, c) 423 K, d) 473 K, e) 528 K and f) 573 K. (B) Spectral changes when the
sample of the spectrum (f) was heated in a removal of dried n-pentane at g) 373 K, h)
423 K, i) 473 K, j) 523 K and k) 573 K.
48
A
B
1
Fraction of Acid Sites
0.8
0.6
0.4
1
0.8
Protonic Acid Sites
Lewis Acid Sites
0.6
0.4
0.2
0.2
0
0
300
400
500
600
n -Pentane Exposure Temp. / K
300
400
500
600
n -Pentane Removal Temp. / K
Figure .4.20 The fraction of acid sites (A) after heating in the presence of npentane and B) removal of dried n-pentane at different temperature.
C5H12
Dissociation
Spillover
Protonic Acid
Diffusion
H
H+
Specific Site
+e- -eH
Acidic Support
Figure 4.21
Lewis Acid Sites
Speculated mechanism for formation of protonic acid sites from n-
pentane over solid acid catalyst;
: certain molecule.
49
The formation of protonic acid site from n-alkane is essentially same as that
observed for formation of protonic acid site from molecular hydrogen, though the
mechanism for formation of protonic acid site from n-alkane on Zn/HZSM-5 and
Pt/SO42--ZrO2 is not clear yet. It is plausible that the formation of protonic acid site
was initiated by dissociation of n-alkane to form hydrogen atom and certain molecule
on centre of specific sites such as acidic sites of catalyst or metal catalysts. Then,
hydrogen atom spillover onto the surface of catalyst followed by surface diffusion to
form protonic acid site by releasing electron into adjacent Lewis acid site as
illustrated in Figure 4.21.
Then, the formed protonic acid sites which act as
catalytically active sites should be taken into account in the reactions of alkanes. The
formation of protonic acid sites from n-pentane is a widely applicable concept of
active sites formation for solid acid catalyst.
Certain molecule, molecule formed simultaneously with hydrogen atom when
n-pentane dissociated on the acidic or metal sites of catalyst, could not yet be
described in this study. Even though Rumyantsev and Eganosova [54-55] proposed
that n-pentyl radical was generated when n-pentane was heated in the absence of
oxygen, it required high temperature [56]. They suggested that in the flow of argon,
n-pentyl radical decomposed to methane, ethane, ethylene, propylene, butane,
pentene through several steps of mechanism proposed. Further research will be
important to study of comprehensive description of this certain molecule.
4.3.3
Hydrogenation of Chemisorbed Pyridine
Figure 4.22 shows IR spectral changes for hydrogenation of pyridine
adsorbed on Zn/HZSM-5 at difference temperature. Hydrogenation of adsorbed
pyridine was begun at 523 K, the band ascribed to adsorbed piperidine at 1470 cm-1
was appeared with concomitant decrease in the intensity of bands at 1454 and 1545
cm-1.
The intensity of the band at 1470 cm-1 was intensified by increase the
temperature. Hydrogenation of chemisorbed pyridine on Pt/HZSM-5 was reported
by Fujimoto et al. [57]. Although they have reported that all chemisorbed pyridine
50
was converted into piperidine within 1 h of exposure of the catalyst to the hydrogen
at 473 K, it could not be observed the formation of piperidine on Zn/HZSM-5 at 473
K. This difference on the hydrogenation temperature may cause by the differences in
the strength of acidic sites and the ability to form hydrogen atom from molecular
hydrogen. Triwahyono et al. have also reported the hydrogenation of chemisorbed
pyridine on WO3–ZrO2 type catalyst [7]. Hydrogenation occurred at 373 K for Pt
loaded WO3–ZrO2, but not for Pt-free WO3–ZrO2. The difference is considered to be
due to the easiness of formation of protonic acid sites in which formation of protonic
acid sites starts to occur at room temperature and at 373 K for WO3–ZrO2 with and
without Pt, respectively.
51
Absorbance / a.u
d
c
b
a
1600
1550
1500
1450
1400
-1
Wavenumber / cm
Figure 4.22
Spectral changes for pyridine adsorbed on Zn/HZSM-5 caused by
heating in hydrogen at a) room temperature, b) 373 K, c) 523 K and d) 573 K.
CHAPTER 5
CONCLUSION AND FUTURE WORK
4.3
Conclusion
Generation of protonic acid sites from hydrogen and n-pentane over
Zn/HZSM-5 was evidenced by pyridine-adsorbed IR study.
Zn/HZSM-5 was
prepared by treating HZSM-5 with Zn2+/N,N-dimethylformamide solution.
Introduction of Zn to the HZSM-5 decreased slightly the crystallinity of HZSM-5,
strengthened weak and medium acidic sites but weakened strong acidic sites and also
increased the number of all acidic sites. Pyridine-preadsorbed FTIR showed that the
introduction of Zn to the HZSM-5 increased the Lewis acid sites and decreased
BrØnsted acid sites markedly. All active sites were strong after treating at 598 K.
Protonic acid sites were formed on Zn/HZSM-5 by heating in the presence of
molecular hydrogen which was started at room temperature and above with a
concomitant eliminating of Lewis acid sites. The protonic acid sites were eliminated
by heating in the removal of hydrogen, with restoration of Lewis acid sites. The
mechanism of generation of protonic acid sites from hydrogen molecule on
Zn/HZSM-5 follows the concept of ‘molecular hydrogen-originated protonic acid
sites’ which the formation was initiated by dissociation of hydrogen molecules to
hydrogen atoms and then hydrogen atoms spillover onto the surface of catalyst
followed by surface diffusion.
Then hydrogen atom converts into a proton by
releasing electron to Lewis acid sites.
53
Protonic acid sites were also generated on Zn/HZSM-5 and Pt/SO42--ZrO2
from n-pentane. Lewis acid sites were converted into protonic acid sites when the
sample was heated in n-pentane and the protonic acid sites formed were eliminated
by removal of n-pentane with heating with the restoration of Lewis acid sites. The
mechanism proposed for generation of protonic acid sites from n-pentane on
Zn/HZSM-5 and Pt/SO42--ZrO2 as follows. n-Pentane molecule dissociates to form
hydrogen atom and certain molecule on Zn or Pt sites. Then, the hydrogen atoms
spillover onto the surface of catalyst, followed by surface diffusion to form protonic
acid sites by releasing electron into adjacent Lewis acid sites. The formation of
protonic acid sites on solid acid catalysts induced by n-pentane is a reversible
process. Then, the formed protonic acid sites which act as catalytically active sites
should be taken into account in the reactions of alkanes. The concept of hydrocarbon
originated protonic acid sites is a widely applicable concept of active sites formation
for solid acid catalyst.
5.2
Future Work
Generation of protonic acid sites from saturated straight alkane on Zn/HZSM5 and Pt/SO42--ZrO2 was observed in this research. However, other hydrocarbons
such as cyclic alkane and alkene are not observed yet. This phenomenon on other
solid catalysts such as Pt/WO3-ZrO2 and Pt/MoO3-ZrO2 need also to be studied.
Detailed mechanism of generation of protonic acid sites from hydrocarbon will be
interesting topic for further research.
REFERENCES
1.
Kimura, T. Development of Pt/SO 24− /ZrO2 Catalyst for Isomerization of Light
Naphtha. Catalysis Today. 2003. 81: 57-63.
2.
Weyda, H., and Kohler, E. Modern Refining Concept-An Update NaphthaIsomerization to Modern Gasoline Manufacture. Catalysis Today. 2003. 81:
51-55.
3.
Okuhara, T. Skeletal Isomerization of n-Heptane to Clean Gasoline. Journal
of Japan Petroleum Institute. 2004. 47(1): 1-10.
4.
Barton, D. G., Soled, S. L., Meitzner, G. D., Fuentes, G. A., and Iglesia, E.
Structural and Catalytic Characterization of Solid Acid Based on Zirconia
Modified by Tungsten Oxide. Journal of Catalysis. 1999. 181: 57-72.
5.
Zhang, S., Zhang, Y., Tierney, J. W., and Wender, I. Hydroisomerization of
Normal Hexadecane with Platinum-Promoted Tungstate-Modified Zirconia
Catalysts. Applied Catalysis A: General. 2000. 193: 155-171.
6.
Hattori, H., and Shishido, T. Molecular Hydrogen-Originated Protonic Acid
Site as Active Site on solid Acid Catalyst. Catalysis Surveys from Japan 1997
1: 205 – 213.
7.
Triwahyono, S., Yamada, T., and Hattori, H. IR Study of Acid Sites on WO3ZrO2 and Pt/WO3-ZrO2. Applied Catalysis A: General. 2003. 242: 101-109.
8.
Li, X., and Iglesia, E. J. Catalytic Dehydroisomerization of N-Alkanes to
Isoalkenes. Journal of Catalysis. 2008. 252: 134-137.
55
9.
Richarrdson, J. T. Principles of Catalyst Development. New York: Plenum
Press, 1989, 240-241.
10.
Pines, H. The Chemistry of Catalytic Hydrocarbon Conversions. New York:
Academic Press, 1981, 12-13.
11.
Ono, Y. A Survey of The Mechanism in Catalytic Isomerization of Alkane.
Catalysis Today. 2003. 81: 3-16.
12.
Tomishige, K., Okabe, A., and Fujimoto, K. Effect of Hydrogen on N-butane
Isomerization over Pt/SO42--ZrO2, and Pt/SiO2 + SO42--ZrO2. Applied
Catalysis A: General. 2000. 194-195: 383-393.
13.
Tanabe, K., Misono, M., Ono, Y., and Hattori, H. New Solid Acids and Bases
Their Catalytic Properties. Studies in surface and Catalysis. 1989. 51: 1-4.
14.
Arata, K., Matsuhashi, H., Hino, M., and Nakamura, H. Synthesis of Solid
Super Acids and Their Activities for Reactions of Alkanes. Catalysis Today.
2003. 81: 17-30.
15.
Woltz, C. Kinetic Studies on Alkane Hydroisomerization over Bifunctional
Catalysts. Dissertation Report. Munchen University. German; 2005.
16.
Jennings, J., R. Selected Developments in Catalysis. Critical Reports on
Applied Chemistry. London. Blackwell Scientific Publications. 1985. Volume
12.
17.
Busca, G. Acid Catalysts in Industrial Hydrocarbon Chemistry. Chemical
Review. 2007. 107: 5366-5410.
18.
Satterfield, C. N, Heterogeneous Catalysis in Practice, 1980. New York:
McGraw-Hill.
19.
Iglesia, E., and Biscardi, J., A. Structure and Function of Metal Cations in
Light Alkane Reactions Catalyzed by Modified HZSM-5. Catalysis Today.
1996. 31: 207-231.
20.
Iglesia, E., Meitzner, G., and Biscardi, J., A. Structure and Density of Active
Zn Species Zn/HZSM-5 Propane Aromatization Catalysts. Journal of
Catalysis. 1998. 179: 192-202.
56
21.
Aguirre, J., Dorado, F., and Canizares, P. N-butane Hydroisomerization over
Pd/HZSM-5. Microporous and Microporous Material 2001. 42: 245-254.
22.
Kooli, F., Lim, C., E., and Liu, Y. Effect of ZnO Additives and Acid
Treatment
on
Catalytic
Performance
of
Pt/WO3/ZrO2
for
n-C7
Hydroisomerization. Journal of Catalysis. 2006. 244: 17-23.
23.
Sachtel, W., Van Santen, R, and El-malki, E., M. Introduction of Zn, Ga, and
Fe into HZSM-5 Cavities by Sublimation. Journal of Physical Chemistry.
1999. 103: 4511-4622.
24.
A. Chica and A. Corma. Hydroisomerization of Pentane, Hexane and
Heptane for Improving the Octane Number of Gasoline. Journal of Catalysis.
1999. 187, 167-176.
25.
Chen, X., R., Du, Y., Chen, C., L., Xu, N., P., and Mou, C., Y. Highly Active
and Stable n-Pentane Isomerization Catalysts without Noble Metal
Containing: Al- or Ga-Promoted Tungstated Zirconia. Catalysis Letters.
2006. Vol. 111, Nos. 3-4.
26.
A. Brito, F.J. Garcia, M.C. Alvarez-Galvan, M.E. Borges, C. Diaz, and V.A.
de la Phena OShea. Catalytic Behavior of Pt or Pd Metal Nano ParticlesZeolite Bifunctional Catalysts for n-Pentane Hydroisomerization. Catalysis
Communication. 2007.8: 2081-2086.
27.
Watanabe, K., Kawakami, T., Baba, K., Oshio, N., and Kimura, T. Effect of
Metals on Catalytic Activity of Sulfated Zirconia for Ligh Naphtha
Isomerization. Catalysis Surveys from Asia. 2005. Vol. 9, No 1.
28.
P. Lukinskas, S. Kuba, R. K. Grasselli, and H. Knozinger. Chromiun
Promotion of Tungstated Zirconia Catalysts for the Isomerization of nPentane. Topics in Catalysis. 2007. Vol. 46, Nos. 1-2.
29.
Xu, Y., Qi, Y., and Lu, G. Skeletal Isomerization of n-Pentane over PlatinumPromoted Tungstophosphoric Acid Supported on MCM-41. Catalysis Letters.
2008. 125:83-89.
57
30.
Miyaji, A., Echizen, T., Nagata, K., Yoshinaga, Y., and Okuhara, T. Selective
Hydroisomerization of n-Pentane to Isopentane over Highly Dispersed PdH4SiW12O40/SiO2. Joulnal of Molecular Catalysis A: Chemical 2003. 201
145-153.
31.
Y. Sun, Y. H. Yue, and Z. Gao, H3PW12O40 and its Casium Salts as Pentane
Isomerization Catalysts at Ambient Temperature. Reaction Kinetic Catalysis
Letter. 1998. Vol. 63. No 2, 349-354.
32.
Bardin, B., and Davis, R. J. A Comparison of Cesium-Containing
Heteropolyacids and Sulfated Zirconia Catalysts for Isomerization of Light
Alkanes. Topics and Catalysis. 1998. 6 77-86.
33.
Miyaji, A., Echizen, T., Li, L., Suzuki, T., Yoshinaga, Y., and Okuhara, T.
Selectivity and Mechanism for Skeletal Isomerization of Alkanes over
Typical Solid Acids and Their Pt-Promoted Catalysts. Catalysis Today. 002.
291-297.
34.
Bayraktar, O., and Kugler, E. L. Coke Content of Spent Commercial Fluid
Catalytic Cracking (FCC) Catalysts. Journal of Thermal Analysis and
Calorimetry. 2003. 71: 867-874.
35.
Aishah, A., J., Kurono, N., and Tokuda, M. Facile Synthesis of Ethyl 2arylpropenoates by Cross-coupling Reaction Using Electrogenerated Highly
Reactive Zinc. Tetrahedron. 2002. 58: 7477-7484.
36.
Triwahyono, S., Yamada, T., and Hattori, H. Effect of Na Addition, Pyridine
Preadsorption, and Water Preadsorption on the Hydrogen Adsorption
Property of Pt/SO42--ZrO2. Catalysis Letter. 2003. 85: 109-115.
37.
Jentys, A., and Lercher, J., A., Technique of Zeolite Characterization. In: Van
Bekkum, H., Flanigen, E. M., Jacobs, P. A., and Jansen, J. C. Introduction to
Zeolite Science and Practice. 2001. 2nd ed. Amsterdam: Elsevier: 347.
38.
Millini, R., Massara, E. P., Perego, G., and Bellussi, G. Framework
Composition of Titanium Silicate-1. Journal of Catalysis. 1992. 137:497-503.
39.
Magee, J., and Dolbear, G. Petroleum Catalysis in Nontechnical Language.
United State of America. Penn Well Publishing Company. 1998. p.: 25-28.
58
40.
Flanigen, E., M. Zeolite Chemistry and Catalysis. In: Rabo, J., A. ACS
Monograph Series. Washington DC; ACS. 1976. 171: 93.
41.
Hughes, T. R., and White, H. M. A Study of The Surface Structure of
Decationized Y Zeolite, Journal of Physical Chemistry. 1967. 71(7): 21922201.
42.
Stevens, R. W., Chuang, Jr. S. S. C., and Davis, B. H., Insitu Infrared Study
of Pyridine Adsorption/Desorption Dynamic over Sulfated Zirconia and Ptpromoted Sulfated Zirconia. Applied Catalysis A: General. 2003. 252: 57-74.
43.
Triwahyono, S. Molecular Hydrogen-Originated Protonic Acid Sites on
Zirconia-Based Catalysts. Dissertation Report. Hokkaido University. 2003.
44.
Treacy, M. M. J., and Higgins, J. B. Collection of Simulated XRD Power
Patterns for Zeolites. 4th ed. Amsterdam: Elsevier. 2001. 238.
45.
Antonio, S. A, Jose, A. M., and Anne, M. G. P. Crystallization of ZSM-12
Zeolite with Different Si/Al Ratio. Adsorption. 2005. 11: 159-165.
46.
C. P. Nicolaides. A noval Family of Solid Acid Catalysts: Substantially
Amorphous or Partially Crystalline Zeolitic Materials. Apllied Catalysis A:
General. 1999. 185: 211-2117.
47.
Triwahyono, S., Aishah, A. J., and Halimaton, H. Isomerization of
Cyclohexane to Methylcyclopentane over Pt/SO42--ZrO2 Catalyst. JournalThe Institution of Engineers, Malaysia. 2006. 67 (1): 30-35.
48.
Y. Cheng, L.J. Wang, J.S. Li, Y.C. Yang, X.Y. Sun, Preparation and
Characterization of Nanosized ZSM-5 zeolites in the Absence of Organic
Templete. Material Letters. 2005. 59: 3427-3430.
49.
Minesso, A., Genna, F., Finotto, T., Baldan, A., and Benedetti, A. Synthesis
and Characterization of Sulfated Zirconia Sol-Gel Systems. Journal of SolGel Science and Technology. 2002. 24: 197–206.
50.
Matsuura, H., Katada, N., and Niwa, M. Additional Acid Site on HZSM-5
Treated with Basic and Acidic Solution as Detected by Temperature
Programmed-Desorption of Ammonia. Microporous and Mesoporous
Materials. 2003. 66: 283-296.
59
51.
Li, Y. G., and Xie, W. H., and Yong, S. The Acidity and Catalytic Behavior
of Mg/HZSM-5 Prepared Via Solid-State Reaction. Applied Catalysis A:
General 1997. 150: 231-242.
52.
Aihua, Z., Nakamura, I., and Fujimoto, K. A New Probe Reaction for
Studying the Hydrogen Spillover Phenomenon. Journal of Catalysis. 1997.
168: 328-333.
53.
Ebitani, K., Tsuji, J., Hattori, H., and Kita, H. Dynamic Modification of
Surface Acid Properties with Hydrogen Molecule for Zirconium Oxide
Promoted by Platinum and Sulfate Ions. Journal of Catalysis. 1992. 135: 609617.
54.
Rumyantsev, A. N., and Oganesova, E. Y. Mechanism of Pyrolysis of nalkanes. 2. n-Pentane. Chemistry and Technology of Fuels and Oil. 1987,
Vol. 23, No. 2: 85-88.
55.
Rumyantsev, A. N., and Oganesova, E. Y. Mechanism of Pyrolysis of nalkanes. 4. Selectivity of Interaction of Hydrogen Atoms with C-H Bonds at
Primary and Secondary Carbon Atoms. Chemistry and Technology of Fuels
and Oil. 1987. Vol. 23, No. 7, 342-345.
56.
Poutsma, M. L. Fundamental Reaction of Free Radicals Relevant to Pyrolysis
Reaction. Journal of Analytical and Applied Pyrolysis. 2000. 54: 5-35.
57.
Ueda, R., Kusakari, T., Tomishige, K., and Fujimoto, K. Nature of Split-over
Hydrogen on Acid Sites in Zeolites: Observation of the Behavior of Adsorbed
Pyridine on Zeolite Catalysts by Means of FTIR. Journal of Catalysis. 2000.
194: 14-22.
60
Appendix A
Result of my work carried out in the Department of Chemistry; Ibnu Sina
Institute for Fundamental Science Studies; University Teknologi Malaysia from
November 2006 to November 2008 was under supervision of Assoc. Prof. Dr.
Sugeng Triwahyono and Assoc. Prof. Dr. Aishah Abdul Jalil. Part of my work
described in this thesis has been reported in the following publications or
presentation:
1.
Musthofa, M., Aini, N. M. R., Hayati, N. H. H., Aishah, A. J., and
Triwahyono, Dynamic Modification of H+ Active Site on HZSM-5 Catalyst.
Regional Annual Fundamental Science Seminar (RAFSS) 2008. Universiti
Teknologi Malaysia. 2008.
2.
Musthofa, M., Aini, N. M. R., Hayati, N. H. H., Aishah, A. J., and
Triwahyono, S. Generation of Protonic Acid Sites from Molecular Hydrogen
on HZSM-5 Based Catalyst. 2nd Penang International Postgraduate
Convenstion 2008. Universiti Sains Malaysia. 2008.
3.
Musthofa, M., Aini, N. M. R., Hayati, N. H. H., Aishah, A. J., and
Triwahyono, S. Simple Analysis Technique to Determine Lewis and Bronsted
Acid Sites of Solid Catalyst. International Conference on Environment 2008.
Penang. 2008.
4.
Musthofa, M., Aini, N. M. R., Hayati, N. H. H., Aishah, A. J., and
Triwahyono, S. The Mechanism of Generation of Protonic Acid Sites from
Hydrocarbon over Pt/SO42--ZrO2. International Graduate Conference on
Engineering and Science. Universiti Teknologi Malaysia. 2008.