preparation of zinc(ii) porphyrin with a mesoporous structure and its

PREPARATION OF ZINC(II) PORPHYRIN WITH A MESOPOROUS

STRUCTURE AND ITS PHOTOCTALYTIC PROPERTIES FOR THE OXIDATION

OF AROMATICS

MAHSA KHOSH KHOOY YAZDI

UNIVERSITI TEKNOLOGI MALAYSIA

PREPARATION OF ZINC(II) PORPHYRIN WITH A MESOPOROUS

STRUCTURE AND ITS PHOTOCTALYTIC PROPERTIES FOR THE OXIDATION

OF AROMATICS

MAHSA KHOSH KHOOY YAZDI

A project report submitted in partial fulfilment of the

requirements for the award of the degree of

Master of Science (Chemistry)

Faculty of Science

Universiti Teknologi Malaysia

MAY 2011

iii

Dedicated to my mother, the benevolent source of love and support.

iv

ACKNOWLEDGEMENT

This research would not have been possible without the guidance and the

help of several individuals who in one way or another contributed and extended their

valuable assistance in the preparation and completion of this study.

First and foremost, my utmost gratitude to Prof. Dr. Salasiah Endud, my

supervisor whose sincerity and encouragement I cherish and will never forget. Prof.

Salasiah has been my inspiration as I hurdle all the obstacles in the completion of this

research work. My appreciation also goes to all the lecturers and laboratory officers

at both the Department of Chemistry and Ibnu Sina Institute of Fundamental Science

Study, University Teknologi Malaysia.

Great gratitude to Ms. Nadirah Zawani bt Mohd Nesfu for knowledge sharing

and invaluable assistance, and to Mr. Ali Mohammad Qasim for all his support,

encouragement and help during the compilation of this work.

I would like to thank all of my friends especially Pooneh Raisdana for her

support and motivational activities that kept me sane along with all the pressure that

accompanied this work.

Last but not the least, I want to express my gratitude to my family. First, my

mother, the benevolent source of love and support whose endless giving have been

truly inspirational at every moment of my life. Her extensive phone calls and worry

were indeed a daily positive stimulus till this project was completed. Then, my lovely

Guinea-Pig pet Fandogh, who had always been standing on my shoulders while doing

my work, filling me with delight and kindness, witnessing every written word in this

piece of work , being one of the biggest parts of my life and a reason why I had

pleasurable and amusing working moments..

v

ABSTRACT

Mesoporous molecular sieve MCM-41 was used as support for the

immobilization of bulky zinc(II)-5, 10, 15, 20-tetraphenylporphyrin (ZnTPP). Zinc

porphyrin was prepared via the treatment of ZnCl2 with 5, 10, 15, 20-tetraphenyl-

21H, 23H-porphyrin in dimethylformamide (DMF). The product was crystallized and

characterized by Ultraviolet Visible spectroscopy (UV-Vis) and Fourier Transform

Infrared spectroscopy (FTIR spectroscopy). ZnTPP encapsulated inside the

mesoporouse of ordered structure of MCM-41 with three different concentrations were

prepared by treatment of zinc porphyrin with MCM-41 in DMF in room temperature.

The materials obtained were characterized by X-ray Diffraction (XRD), and Ultraviolet

Visible Diffuse Reflectance (UV-Vis DR) spectroscopy. The powder XRD data

confirmed that the ordered structure of mesoporouse MCM-41 remained intact after

encapsulation process. Characterization of ZnTPP composite with MCM-41 using

UV-Vis DR confirmed that the structure of ZnTPP in the support is similar with bare

ZnTPP. Encapsulated ZnTPP as catalyst in a photoexcited system was found to be

active and selective in the D-limonene biotransformation under mild condition with

hydrogen peroxide as the oxidant. The influence of different concentrations of catalyst

was studied for photocatalytic oxidation of D-limonene to carvone. The products

were identified by using Gas Chromatography (GC) and Gas Chromatography-Mass

Spectrometry (GC-MS).

vi

ABSTRAK

Bahan mesoliang penapis molekul MCM-41 telah digunakan sebagai

penyokong bagi memengunkan zink(II)-5, 10, 15, 20-tetrafenilporfirin (ZnTPP).

Zink porfirin telah disediakan melalui tindak balas ZnCl2 dengan 5, 10, 15, 20-

tetrafenil-21H, 23H-porfirin dalam dimetilformamid (DMF). Produk telah dihablurkan

dan dicirikan menggunakan spektroskopi Ultra lembayung Nampak (UV-Vis) dan

spektroskopi Inframerah Transformasi Fourier (FTIR). ZnTPP dipegunkan dalam

struktur mesoliang MCM-41 menggunakan tiga kepekatan yang berbeza telah

disediakan secara tindalak balas zink(II) porfirin dengan MCM-41 dalam DMF pada

suhu bilik. Bahan yang diperolehi dicirikan menggunakan pembelauan sinar-X

(XRD) dan spektroskopi difusi pemantulan Ultra lembayung (UV-VisDR). Data XRD

mengesahkan keseragaman struktur mesoliang MCM-41 masi utuh selepas proses

pemegunan. Pencirian komposit ZnTPP dengan MCM-41 menggunakan spektroskopi

UV-VisDR mengesahkan struktur ZnTPP berpenyokong adalah serupa dengan ZnTPP

asal. Mangkin ZnTPP terenkapsulasi dalam dibawah sistem fotopemangunan didapati

aktif dan selektif dalam biotransformasi D-limonin keadaan sederhana dengan

kehadiran hidrogen peroksida sebagai pengoksida. Pengaruh perbezaan kepekatan

mangkin telah dikaji bagi fotopemangkinan pengoksidaan D-limoni kepada karvon.

Hasil tindak balas telah dikenalpasti menggunakan kromatografi gas (GC) dan

kromatografi gas-spektrometri jisim (GC-MS).

vii

TABLE OF CONTENTS

CHAPTER TITLE PAGE

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENT iv

ABSTRACT v

ABSTRAK vi

TABLE OF CONTENTS vii

LIST OF TABLES ix

LIST OF FIGURES x

LIST OF ABBREVIATIONS xii

LIST OF SYMBOLS xv

LIST OF APPENDICES xvii

1 INTRODUCTION 1

1.1 Background of Study 1

1.2 Problem Statement 7

1.3 Research Objectives 7

1.4 Scope of Study 8

1.5 Project Outline 9

2 LITERATURE REVIEW 10

2.1 Introduction to Metalloporphyrins 10

2.2 Synthesis of Metalloporphyrins 10

2.3 Heterogenization of Metalloporphyrins 18

2.4 Immobilization of Metalloporphyrins into MCM-41 19

2.4.1 Introduction on MCM-41 19

2.4.2 Immobilization Strategies 21

2.5 Catalysis Reaction 22

2.5.1 Introduction on D-limonene and Carvone 22

2.5.2 Conversion of Limonene to Carvone 23

viii

3 THE RESEARCH METHODOLOGY 26

3.1 Preparation of materials 26

3.1.1 Preparation of Zinc(II) porphyrin 26

3.1.2 Synthesis of Purely Siliceous Si-MCM-41 27

3.1.3 Encapsulation of Zinc(II) porphyrin into

MCM-41 27

3.2 Characterization Methods 28

3.2.1 Ultraviolet-Visible spectroscopy (UV-Vis) 28

3.2.2 Fourier Transform Infrared Spectroscopy

(FTIR) 29

3.2.3 UV-Vis Diffuse Reflectance Spectroscopy

(UV-Vis DR) 30

3.2.4 X-ray Powder Diffraction (XRD) 31

3.3 Photocatalytic activity of D-limonene 34

3.3.1 Catalytic testing 34

3.3.2 Analysis of reaction products 34

3.3.2.1 Gas chromatography- Flame Ion-

ization detector (GC-FID) 34

3.3.2.2 Gas Chromatography-Mass Spec-

trometry (GC-MS) 35

4 RESULT AND DISCUSSION 37

4.1 Physical properties of porphyrin and zinc(II) porphyrin 37

4.1.1 Physical properties of Zinc(II) tetraphenyl-

porphyrin encapsulated in MCM-41 39

4.2 Photocatalytic activity of D-limonene 41

5 CONCLUSION AND RECOMMENDATIONS 46

5.1 Conclusion 46

5.2 Recommendations 48

REFERENCES 49

Appendices A – B 55 – 60

ix

LIST OF TABLES

TABLE NO. TITLE PAGE

4.1 Main FTIR bands of porphyrin and zinc(II) porphyrin. 38

4.2 Catalytic activity of H2TPP, ZnTPP and different concentrations

of ZnTPP in ZnTPP-MCM-41 after UV-Vis light. 44

x

LIST OF FIGURES

FIGURE NO. TITLE PAGE

1.1 Generic potential energy diagram showing the effect of a catalyst. 2

1.2 Porphyrin and Pyrrole rings. 3

1.3 The structure of heme. 4

1.4 Outline of main research activities. 9

2.1 Rhenium porphyrins. 13

2.2 Possible intermediates of metalloporphyrins. 13

2.3 Direct synthesis of Palladium porphyrin. 17

2.4 MCM-41 structure. 20

2.5 Synthesis of MCM-41 using templating method. 20

2.6 The structure of D-limonene. 23

2.7 The structure of R-(-)-carvone and S-(+)-carvone. 24

2.8 Limonene to carvone transformation. 25

3.1 Synthesis of ZnTPP. 26

4.1 The structure of Zinc(II) tetraphenylporphyrin. 37

4.2 FTIR bands of free porphyrin and zinc(II) porphyrin. 38

4.3 UV-Vis spectra of free porphyrin and zinc(II) porphyrin. 39

4.4 UV-Vis DR spectra of free porphyrin and zinc(II) porphyrin. 40

4.5 XRD patterns of zinc(II) porphyrins encapsulated in MCM-41. 41

4.6 Products of limonene photo transformation in the presence of

ZnTPP catalyst. 42

4.7 A possible mechanism for limonene photo-oxidation by ZnTPP. 43

A1 GC/FID plot for ZnTPP-MCM-41 with 25μM concentration of

ZnTPP. 55


ZnTPP. 56


ZnTPP. 57

xi

A4 GC/FID plot for ZnTPP. 58

A5 GC/FID plot for uncatalyzed reaction. 59

B1 GC-MS plot for the sample with no catalyst. 60

B2 GC-MS plot for the sample with ZnTPP catalyst. 61

B3 GC-MS plot for the sample with 100 μM ZnTPP encapsulated

in MCM-41 catalyst. 62

xii

LIST OF ABBREVIATIONS

A – Absorbance

acac – Acetylacetonate

AlCl3 – Aluminium Trichloride

AlH3 – Aluminium Hydride

ATR – Attenuated Total Reflectance

C6H5OH – Hydroxybenzene - Phenol

C6H5CN – Benzonitrile

cm – Centimeter

CO – Carbon Monoxide

Cr(CO)6 – Hexacarbonylchromium

CrCl2 – Chromium(II) Chloride

CTABr – Cetyltrimethylammonium Bromide

DMF – Dimethylformamide

DNA – Deoxyriboneucleic Acid

Ea – Activation Energy

EtOH – Ethyl Alcohol (Ethanol)

Fe(C2H3O2)2 – Iron(II) Acetate

FTIR – Fourier Transform Infrared

GC-FID – Gas Chromatography - Flame Ionization Detector

GC-MS – Gas Chromatography - Mass Spectrometry

H2 – Hydrogen molecule

H2O – Water

H2O2 – Hydrogen Peroxide

xiii

H2TPP – Free base tetraphenylporphyrin

HC2H3O2 – Acetic acid

HCl – Hydrochloric Acid

Hpor-M – Metalloporphyrin

IUPAC – International Union of Pure And Applied Chemistry

JCPPS – Joint Committee on Powder Diffractions Standards

KBr – Potassium Bromide

KOH – Potassium Hydroxide

LCT – Liquid Crystal Templating

M – Metal

MCM-41 – Mobile Composition of Matter No. 41

MoCl2 – Molybdeum Chloride

N – Nitrogen

NMR – Nuclear Magnetic Resonance

O2 – Oxygen molecule

Pd – Palladium

PorH2 – Porphyrin

R• – Hydrocarbon Radical

RHA – Rice Husk Ash

SBU – Secondary Building Unit

TBHP – tert-Butyl hydroperoxide

THF – Tetrahydrofuran

UV-Vis DR – Ultraviolet Visible Diffuse Reflectance

ZnCl2 – Zinc Chloride

ZnTPP – Zinc(II) Tetraphenylporphyrin

ZnTPPM25 – Zinc(II) Tetraphenylporphyrin encapsulated in MCM-41 with

25 μM concentration of zinc



xiv



ZnTPP• – Zinc(II) Tetraphenylporphyrin Radical

xv

LIST OF SYMBOLS

A – Angstrom 10−10 meters

θ – Theta

o – Degree

μ – Mue

λ – Wavelength

ε – Wavelength-dependent molar absorptivity

σ – Head to head overlap of two orbitals

• – Radical

h – Plank’s constant

c – Speed of light

Eo – Ground State energy

E1 – First excitation state

n – Diffraction order from n=1,2,3, ...

ao – Unit cell parameter

V – Volt

A – Amper

d – Interplanar spacing

I – Intensity of light

Io – Intensity of incident light

k – Kilo

K – Kelvin

h – Hour

g – Gram

xvi

m – Milli

min – Minute

mL – Millilitre

M – Mole

μM – Micromole

nm – Nanometer

R – Reflectance

xvii

LIST OF APPENDICES

APPENDIX TITLE PAGE

A GC-FID RESULTS 55

B GS-MS RESULTS 60

CHAPTER 1

INTRODUCTION

1.1 Background of Study

A catalyst is a substance that can change the rate of a reaction. A catalytic

reaction is a kind of reaction that has a different activation energy with none catalytic

reaction. There are different kinds of catalysts that can change the rate of a reaction,

whether increasing or decreasing it. There are two types of catalysts, homogeneous and

heterogeneous ones. A homogeneous catalyst is in the same phase with the reactants

while the heterogeneous catalyst is in a different phase with the reactants. Catalysts

are crucial entities in many chemical reactions. Different types of catalysts can be used

in different types of reactions depending on the need. The effects of the presence of

a catalyst on reactions can be well noticed in the reactions with very high activation

energies. In the absence of a catalyst, these kinds of reactions can impossibly take

place (Shriver et al., 1999).

The knowledge of the catalytic reactions mechanism has improved

dramatically in recent years because of the availability of isotopically labeled

molecules, improved methods for determining reaction rates and improved

spectroscopic and diffraction techniques (Shriver et al., 1999). Due to the findings,

the catalyst itself is not consumed by the reaction though it may participate in various

chemical transformations. A catalyst introduces a new pathway to the reaction, an

alternative one with a different transition state. The important fact is that it does not

have any effect on the chemical equilibrium because it changes the rate of both forward

and reverse reactions, as shown in Figure 1.1. Catalysts affect the rate of a reaction by

making a change in the transition state. When they work as accelerators, the reaction

will possess a different transition state with lower activation energy. On the other hand,

when they slow up a reaction, the transition state will possess higher activation energy.

They usually participate in the rate determination step of a reaction. They generally

2

react with one or more reactants to form the intermediates that lead to the products

with regenerate catalysts included (Robertson, 1970; Shriver et al., 1999)

Figure 1.1: Generic potential energy diagram showing the effect of a catalyst.

The catalytic reaction must be thermodynamically favorable and it should run

at a reasonable rate, also the catalyst must have a proper selectivity toward the desired

product and it must have a long life, enough to be considered economical. A selective

catalyst produces a high proportion of the desired product with a minimum amount of

side products. In industry, there is a substantial economic stimulus to develop selective

catalysts (Shriver et al., 1999).

Catalysts are important in chemical laboratories, chemical and pharmaceutical

industry as well as in biology. The important role of catalysis is underlined by the fact

that approximately ninety percent of all nowadays products in these areas are being

produced by processes that require at least one catalytic step. At the same time, using

the hydrocarbons as the main stock for goods and fine chemicals needs at least one

oxidation step that is usually carried out with transition metals. Therefore oxidation

catalysis is a main research field in academic and industrial chemistry. In most of these

processes, species with metal-carbon bonds are formed as important intermediates.

Using organic ligands, and therefore organometallic complex metal fragments, is the

best way to achieve efficient oxidation catalysis (Meyer and Limberg, 2007).

Porphyrins are abundant organic compounds. They are ring shaped, and can

be used as organic ligands to generate organometallic complexes. A porphyrin has

four pyrrole rings. A pyrrole has four carbon atoms and a nitrogen atom shaped

like a pentagon ring (C4H5N) as shown in Figure 1.2. Porphyrin can act as a bi, tri

or hexadentate ligand as well as the normal tetradentate ligand (Ostfeld and Tsutsui,

3

1974). Porphyrins as macrocycle molecules have highly conjugated systems. The

porphyrin ring has a total of twenty six pi electrons. The macrocycles have intense

absorption bands in the visible region that contribute to its strong color (Fleischer,

1970).

Different transition metals can be bound inside the porphyrin ring, which

consequently forms various organometallic complexes. The four nitrogens in the

middle of the porphyrin ring can make bonds with transition metals like iron (Fe),

copper (Cu), cobalt (Co) and Zinc (Zn). Each of these organometallic complexes can

be used as a source of catalysts for different reactions especially the redox reactions

(Kadish et al., 2000).

(a) (b)

Figure 1.2: (a) Pyrrole ring, (b) Porphyrin ring.

There are different types of porphyrins that can be used as a ligand. They

can be substituted by both organic and inorganic groups. These substituents can give

various characteristics to the porphyrin ring. Therefore, a large group of ligands can

be produced for specific needs (Chou et al., 2003).

One of the most famous porphyrins is ferroporphyrin or heme that has iron (Fe)

bonded with its nitrogens inside. The structure is shown in Figure 1.3. The hemoglobin

in red blood cells that transfers the oxygen all over the body has four subunits and each

of these subunits has a heme group. Amino acids are attached to these heme groups

to make hemoproteins. A Heme is the prosthetic group of these metalloproteins. In

a heme, Iron (II) is responsible for the oxygen transportation in the body. The sixth

position around the iron in the heme will be occupied by the oxygen (O2) and the fifth

one is coordinated with the histidin residue. While the oxygen is attached to the iron,

the porphyrin ring has a planar shape but when the oxygen is released, the porphyrin

will adopt the domain configuration (Lever and Gray, 1989).

4

Figure 1.3: The structure of heme.

Chlorophyll that is responsible for capturing the sun′s energy in plants is

another example. The central metal of the chlorophylls porphyrin is magnesium (Mg).

Photosynthesis is the most important process in green plants in which lights energy is

converted to chemical energy, stored in carbohydrates. Magnesium in the porphyrin

center absorbs a photon and one of its 3s electrons get excited to a higher energy state.

Consequently, the electron will be ejected out of the magnesium when more photons

are absorbed. The magnesium is oxidized in this situation and it gets its electron back

from the photolysis of water. The magnesiums electron enters a chain called electron

transport chain, after which the energy storage process starts (Gray, 1981).

Vitamin B12 is also a good example to show the important roles of porphyrin

rings. This compound has cobalt (Co) as a central metal. Vitamin B12 is a water

soluble vitamin that is normally involved in the metabolism of every cell of human

body. Also, it affects DNA synthesis and regulation, fatty acid synthesis and energy

production (Banerjee and Ragsdale, 2003; Eschenmoser and Wintner, 1977). Beside

of all the work that have been done on metalloporphyrins in the biological systems,

metalloporphyrins have been studied for other reasons as well, for example, the search

for superconductors (Adler, 2007), anti-cancer drugs (Macquet and Theophanides,

1972), catalysis (Hansen and Drenth, 1993) and chemical shift reagents (Gianferrara

et al., 2007).

5

Much work has been done on the catalytic activity of metalloporphyrins.

Different metalloporphyrins have been selected for catalysis of different reactions.

For example, researches show that Manganese porphyrins have uncommonly high

reactivity toward olefin epoxidation and alkane hydroxylation (Campestrini and

Meunier, 1992). Also, ruthenium porphyrins have been found to catalyze the aerobic

epoxidation of olefin under mild conditions (Groves and Quinn, 1985). In substituted

metalloporphyrins, for example, researches have been done in catalytic activity of

halogenated iron porphyrins in alkene and alkane oxidation by iodosylbenzene and

hydrogen peroxide (Guedes et al., 2005).

In addition, researches have been done on metalloporphyrins extracted from

natural sources like heme fragment in cytochrome c or magnesium porphyrin in

chlorophyll. For these non-synthetic metalloporphyrins, the mechanism of their

catalytic activity in the body or in plants has been found. In other cases, these

metalloporphyrins have been used as a catalyst in other reactions. For example, copper

porphyrin extracted from the feather of certain birds was used to study the mechanism

of oxidation of ascorbic acid in human body. In this research, they found that ascorbic

acid in the body is oxidized in the presence of copper porphyrin and iron porphyrin

together since copper porphyrin alone is insufficient for the oxidation to take place

(Keilin, 1951).

Metalloporphyrins as catalysts for oxidation reactions are highly selective as

well as being able to operate in mild conditions (Haber et al., 2004). Besides all the

advantages of using metalloporphyrins in the oxidation of hydrocarbons there are some

disadvantages in using them as homogeneous catalysts. The difficulty of separating

the product from the catalyst increases the cost of using homogeneous catalysis in

the commercial process. Due to this fact, heterogenization of metalloporphyrins by

using porous solid supports can be a good solution to keep their properties besides

solving the separation issues. There are three classes of materials that are used as

heterogeneous catalysts and adsorption media. One of them is microporous inorganic

solids with pore diameter of 20 A and less and the other is mesoporous inorganic solids

with pore diameter of 20 to 500 A. The third one is macroporous materials which its

pore diameter is greater than 500 A (Rouquerol et al., 1994).

Zeolites as a member of microporous materials with regular arrays of internal

channels have been extensively studied as an inorganic support. They are hydrated,

crystalline tectoaluminosilicates that are constructed from TO4 tetrahedra (T =

tetrahedral atom, e.g., Si, Al); each apical oxygen atom is shared between two adjacent

6

tetrahedral giving a framework ratio of O/T = 2. They have specific sites available

for active metal substitution which gives them the ability to be prepared for selective

processes (Davis and Lobo, 1992). Different Zeolites have been used for different

purposes. For example, zeolite Y was used for encapsulation of metalloporphyrins

such as cis-Mn(bypy)3 to be active catalysts toward cyclohexane oxidation (Skrobot,

2003). Zeolites have been used as supports for metalloporphyrins that provide a

physical separation of active site, therefore catalyst self-destruction and dimerization

of unhindered are minimized (Li, 2002). However, due to the small pore size of

the Zeolites, their application in reactions involved with large molecules is limited

(Thomas, 1998).

MCM-41 is a mesoporous material from the M41S family which has unique

pores and large well-defined internal surface area (Beck et al., 1992). Due to its

pore size, high molecular mass organic compounds can be fitted into the pores.

Recently, much effort was focused on immobilization of metalloporphyrins into

the silica MCM-41 surface. Fe, Mn and Co tetraphenylporphyrin complexes were

immobilized in MCM-41 and used as catalysts for cyclohexene oxidation by peroxide.

The study showed that the interaction between the support and metalloporphyrins

causes modifications in MCM-41 chemical environment and these materials appear to

be good catalysts for reported reaction (Costa et al., 2008). Ruthenium meso-tetrakis-

(2, 6-dichlorophenyl)porphyrin complex were immobilized into MCM-41 to catalyze

selective alkene epoxidations in order to easy catalysts recycling. The research showed

that Ru supported catalyst is effective toward norbornene and cyclooctene oxidations,

where zeolite-based TS-1-catalyzed condition is unsuccessful in transformationin (Liu

et al., 1998).

In this research the same interest in using modified MCM-41 with

metalloporphyrins as catalysts for selective oxidations is followed. The oxidation of

D-limonene to carvone is studied by using zinc tetraphenylporphyrin immobilized on

MCM-41 as a catalyst.

Porphyrin-based photocatalytic system for biotransformation of D-limonene

was first reported by Trytek et al. in 2005. In their work, some porphyrin complexes

such as zinc tetraphenylporphyrin and cobalt tetraphenylporphyrin were used as

catalysts in the presence of dioxygen as an oxidant. The aim of this work was to

study the photochemical excitation as a clean and an appropriate strategy of porphyrin-

based biomimetic catalysis. Their result showed that in the absence of light limonene,

oxidation by using porphyrin complexes as catalysts does not take place and in the

7

absence of catalysts using the light, the result does not give observable oxidation.

Cobalt tetraphenylporphyrin did not show any result of oxidation where zinc porphyrin

catalyzed the reaction (Trytek et al., 2005). The same method is used to catalyze the

oxidation of D-limonene to carvone in this research.

1.2 Problem Statement

Photocatalytic reaction based on zinc(II) tetraphenylporphyrin for oxidation

of D-limonene to carvone was done by (Trytek et al., 2005). Zinc(II)

tetraphenylporphyrin was used as a homogeneous catalyst, but the separation of the

catalyst from the product in this method is rather difficult. Moreover, homogeneous

catalysts are not recyclable and not reusable. Therefore, in this research, zinc(II)

tetraphenylporphyrin immobilized into MCM-41 is used as heterogeneous catalysts

to provide an easier method to separate and recycle the catalyst.

The photocatalytic reaction which was reported before carried out at room

temperature (Trytek et al., 2005). Hence, the same way is adopted in this study since

providing the appropriate conditions for the reaction is easy, and it does not need extra

energy to be used. As D-limonene can be obtained from citrus peel oil with an amount

of up to 50 kg as a byproduct annually it can be used as cheap and ready to use starting

material to produce more valuable fragrance compounds such as carvone (Trytek et al.,

2005).

1.3 Research Objectives

The main objectives of this research are:

i) To synthesize zinc(II) tetraphenylporphyrin and to characterize it by

Ultraviolet Visible spectroscopy (UV-Vis) and Fourier Transform Infrared

spectroscopy (FTIR spectroscopy).

ii) To immobilize zinc(II) tetraphenylporphyrin and tetraphenylporphyrin into

MCM-41 and characterize the products by X-ray Diffraction (XRD), and

Ultraviolet Visible Diffuse Reflectance (UV-Vis DR).

8

iii) To study and optimize the catalytic activity of encapsulated zinc porphyrin

and porphyrin in the photocatalysis of limonene to carvone using Gas

Chromatography (GC) and Gas Chromatography-Mass Spectroscopy (GC-

MS).

1.4 Scope of Study

This study involves the synthesis of zinc(II) tetraphenyporphyrin via

the treatment of ZnCl2 with 5, 10, 15, 20-tetraphenyl-21H, 23H-porphyrin in

dimethylformamide. The synthesized product will be encapsulated in MCM-41.

MCM-41 is not synthesized in this research and it is ordered from a third party.

The reactivity of the prepared catalysts will be tested in the photocatalysis of

D-limonene to carvone. In this reaction, hydrogen peroxide is used as an oxidant and

UV light is used as an excitation agent.

The synthesized zinc porphyrin will be characterized by Ultraviolet

Visible spectroscopy (UV-Vis) and Fourier Transform Infrared spectroscopy (FTIR

spectroscopy). The encapsulated products in MCM-41 will be characterized by X-

ray Diffraction (XRD), and Ultraviolet Visible Diffuse Reflectance (UV-Vis DR).

The catalytic properties and the reusability of the prepared catalysts will be tested

in the photocatalysis of D-limonene. Optimization of the reaction parameters involves

using different concentrations of catalysts. The products will be analyzed using Gas

Chromatography (GC) and Gas Chromatography-Mass Spectroscopy (GC-MS).

9

1.5 Project Outline

Figure 1.4: Outline of main research activities.

CHAPTER 2

LITERATURE REVIEW

2.1 Introduction to Metalloporphyrins

Metalloporphyrins are important compounds found in nature. Metallopor-

phyrins in protein systems show an important class of biochemical functions in nature.

Hemoglobin, myoglobin, chlorophyll, cytochromes, catalase, and peroxidase are all

well known examples of porphyrin in nature. Their chemistry relates principally to

redox reactions and the transport, storage, and activation of molecular oxygen. Over

the years great biochemical efforts have been done on these natural metalloporphyrins.

The understanding of structure and function of these compounds and the relationship

between them has been gained through various researches. A large number of models

for naturally occurring metalloporphyrins were synthesized in their effort to determine

the factors that rule the biological function of natural porphyrins. The simplest and

most frequently encountered class of synthetic porphyrins is the monometallic single-

porphyrin type where, in many cases, a central metal takes up an additional one or two

donor ligands to complete its coordination sphere. Various synthetic metalloporphyrins

and their dioxygen adducts are extensively studied examples of this type, which

frequently incorporate an ingeniously modified porphyrin ring (Berezin, 1981; Tatsumi

and Hoffmann, 1980).

2.2 Synthesis of Metalloporphyrins

The work on porphyrin chemistry can be considered to have begun with

Hoppe-Seylers isolation of hematoporphyrin from hemin and phylloporphyrin from

chlorophyll in 1880. This study shows that there are some similarities between these

two macrocycles (Ostfeld and Tsutsui, 1974).

11

The first novel metalloporphyrins, mesoporphyrin IX complexes of copper (II)

and zinc (II) were prepared in 1902. From this works it was found that mesoporphyrin

of zinc and copper, metals missing a common state of 3+, without chlorine. Therefore

they found that the chlorine in porphyrin iron (III) chloride should have been bound to

the iron and not the porphyrin itself. Two years later, the first successful insertion of

iron into a demetalated porphyrin was done by Laidlaw (Ostfeld and Tsutsui, 1974).

The work on synthetic porphyrin was continued whereas in 1964, Falk could list

porphyrin complexes for 28 metallic elements (Falk, 1964).

Porphyrins, in common with other macrocyclic ligands, have a central hole

of mainly fixed size. Hoard in 1971 showed that in some particular complexes metal

cannot be fixed into this hole and comes out of the porphyrin plane (Tsutsui and Hrung,

1973). For example, in hemoglobin, the iron atom lies approximately in plane of the

oxygenated form, since it lies out of the plane in deoxygenated form. It is thought that

the transition between these two states can be responsible for the cooperative nature of

oxygen binding in hemoglobin (Ostfeld and Tsutsui, 1974).

The characteristic of porphyrins complexes and the other macrocycles is the

essential of breaking all metal- ligand bonds at the same time to remove the ligand.

Busch and coworkers tested several macrocycle ligands in 1971 and showed that

breaking the bonds between a metal and a macrocycle in a complex is about 106 slower

than similar noncyclic ligand (Ostfeld and Tsutsui, 1974).

Porphyrins and particular unsaturated ligands (e.g. , bipyridyl) are well-known

because of their delocalized π systems. Overlap between π orbitals of the ligand

and metal orbitals in a suitable direction and symmetry produces a slightly high field

strength. Also, the back-accepting π electron density from complexed metals by the

ligands, facilitate the reduction of the complexed metal to a low oxidation state (Ostfeld

and Tsutsui, 1974). Therefore, it should be said that all unusual porphyrin oxidation

states cannot be explained easily. For example, the most stable silver porphyrin has an

oxidation state 2+, not 1+ (Dorough et al., 1951).

The insertion of a metal from a carbonyl complex to a porphyrin ring was the

first new synthetic method that was discovered in 1966. There is no simple ligand

exchange in this reaction instead, the porphyrin is oxidized by the protons of the

porphyrin pyrrole. For example:

12

Cr(CO)6+ PorH2185o−−−→

decline

[Por - CrII

]+ 6 CO ↑ + H2 ↑

where Por represents porphyrin in the reaction (Tsutsui et al., 1966). The

mechanism of the reaction is assumed that the hydrogen evolution and carbon

monoxide carries the reaction toward the metal insertion. It can be inferred that the

metal is oxidized in a series of one electron steps which each of them is gone along

with the reduction of a pyrrole proton (Ostfeld and Tsutsui, 1974).

M(0) + PorH2 −→[HPor - MI

]+ H2 ↑

[HPor - MI

] −→ [Por - MII

]+ H2 ↑

Some previously unknown complexes of chromium (Tsutsui et al., 1966),

molybdenum (Ostfeld and Tsutsui, 1974), ruthenium (Tsutsui et al., 1971), rhodium

(Ostfeld and Tsutsui, 1974), iridium (Ostfeld and Tsutsui, 1974), rhenium (Tsutsui

et al., 1971) and technetium (Tsutsui et al., 1971) have been synthesized by carbonyl

methods during 1968 to 1972 years by different scientists. The metalloporphyrins

which have been synthesized by carbonyl method are unique though some of them

that contain chromium, molybdenum, rhodium and rhenium have been synthesized

by other methods. In some of these cases like ruthenium(II), iridium(I), rhenium(I)

and technetium(I) the carbonyl ligand remain with the metal. Also, from the metal

carbonyls generally low oxidation states are gained. Some novel metal(I) porphyrin

complexes are unique to the carbonyl methods (Ostfeld and Tsutsui, 1974).

By studying two rhenium porphyrins additional geometry which metallopor-

phyrins can assume are illustrated. These two complexes are μ-(Por)[Re(CO)3]2 and

(PorH)Re(CO)3. At first, the way that metal atoms were bonded to the porphyrin ring

was not clear and the information about the structure of these compounds was almost

from the IR spectra. A work on single- crystal X-ray diffraction of mono- and di-

rhenium complexes has given the structures below in Figure 2.1 (Cullen et al., 1972).

In these reported structures each rhenium atom is bonded to three nitrogen

atoms of a distorted porphyrin ring. Also the metal-metal distance that was reported

13

in this study is long, though it is short enough to have some possible reasonable

interactions (Cullen et al., 1972).

(a) (b)

Figure 2.1: Rhenium porphyrins. a) μ-(Por)[ Re(CO)3]2, b) (PorH)Re(CO)3 (Ostfeld

and Tsutsui, 1974).

These complexes can be used as models for two proposed metal-insertion

intermediates. Fleischers proposed sitting atop complex in 1964 is similar to mono-

rhenium complex which was shown in Figure 2.1(a) and the other possible intermediate

that was proposed by Hambright in 1972 resembles the di-rhenium complex in Figure

2.1(b). The possible intermediates are shown in Figure 2.2. The latter comparison may

no longer apply, because the X-ray structure of di-rhenium depicts both metal atoms

lie outside the S2 axis normal to porphyrin plane (Ostfeld and Tsutsui, 1974).

Figure 2.2: Possible intermediates of metalloporphyrins. Left: monometal

intermediate complex, Right: dimetal intermediate complex (Ostfeld and Tsutsui,

1974).

After carbonyl method, a similar method which is involved with the oxidation

of hydride ions from a metal hydride complex was presented by Inhoffen and Buchler

in 1968. They worked with AlH3 (Ostfeld and Tsutsui, 1974):

14

AlH2+ PorH265o−−→THF

+H2O−−−→ [Por-AlOH]+ 3H2 ↑

This method is somehow limited because the number of metals which their

hydrides are available is not much (Ostfeld and Tsutsui, 1974).

These two methods, metal carbonyls and hydrides are not enhanced only by the

oxidation and reduction. A more probable explanation for their success is that: (1) the

evolution of gas impels the reaction forward to completion; (2) the organic solvents

which porphyrins are dissolved in, are proper for dissolving the metal carbonyls and

hydrides as well (Ostfeld and Tsutsui, 1974).

Previously the metal salt and the porphyrin had been dissolved by using an

acidic or basic medium like acetic acid (Thomas and Martell, 1959) or pyridine

(Ostfeld and Tsutsui, 1974).

Fe(C2H3O2)2+ PorH2118o−−−−−→

HC2H3O2

+O2,Cl-−−−−→ [Por - FeIIICl

]+ 2HC2H3O2

AlCl3+ PorH280o−−−−→

pyridine−−−−−→HC2H3O2

[Por-Al(C2H3O2)]+ 3HCl ↑

But as Alder pointed out, these reactions have some serious flaws. In

an acidic medium porphyrin mainly exists in the unreactive protonated form; in

basic media the reactivity of the metal is decreased by complexation with hydride,

pyridine, etc. (Ostfeld and Tsutsui, 1974). Tsutsui solved the solubility problem

by using organometallic compounds as metal source. He and his coworkers found

that diphenyltitanium reacts with mesoporphyrin IX dimethyl ester and gives titanyl

porphyrin (Ostfeld and Tsutsui, 1974).

(C6H5)2Ti + PorH2240o−−−−−→

mentylene

O2−→ [Por - Ti=O] + 2C6H5

15

The organometallic compound which is used here is soluble in nonpolar

organic solvents, and the moiety that is formed in the reaction helps drive the reaction

to desired products. It is assumed that air oxidizes the titanium to the oxidation state of

4+. The other organometallic compounds can be used in the similar manner. The use

of organometallic compounds is thought to be one of the most effective methods for

metal insertion into a porphyrin ring so far. Therefore, the problem existed in choosing

and preparing the organometallic compounds besides the availability of other methods

have averted greater use of organometallic compounds as starting materials (Ostfeld

and Tsutsui, 1974).

In 1971 Buchler and coworkers studied on metal acetylacetonates and they

found that these compounds are easily available and reasonably soluble in organic

solvents. They worked on a reaction of octaethylporphyrin with different metal

acetylacetonates, and melts of quinolone, phenol and imidazole as solvents (Ostfeld

and Tsutsui, 1974).

M(acac)n+ PorH2 −→ [Por - M(acac)n - 2]+ 2Hacac

Alder and coworkers in 1970 used N,N-dimethylformamide (DMF) as a solvent

to solve a solubility problem instead of using metal complexes which can be easily

solved in organic solvents. High dielectric properties of DMF can dissolve both a metal

salt and a porphyrin. Alder got enhanced yields of many known metalloporphyrins

by using DMF. This method is also, probably a useful synthetic method for new

compounds. Because of the simplicity of this method and the availability of starting

material chromium porphyrin was synthesized by Hanson and coworkers in 1973. Thus

it was once synthesized before by other methods and it did not have a reasonable yield

(Ostfeld and Tsutsui, 1974).

4PorH2+ 4CrCl2+ O2153o−−→DMF

4[Por - CrIIICl]+ 4HCl ↑ + 2H2O

Using of the polar reaction media had not been unknown before but as it seems

lower dielectric properties of other solvents like acetone, ethanol or dioxane were not

16

successful as DMF (Ostfeld and Tsutsui, 1974).

The synthesis of metalloporphyrins in a field of high dielectric constant was

extended by Buchler later on. He synthesized complexes of scandium, tantalum,

tungsten, osmium, and rhenium by using a phenol melt as a solvent and metal halides

as metal sources. Metal halides and porphyrins reacted in benzonitrile to produce

an equally novel metalloporphyrins such as chromium, molybdenum, tungsten and

niobium. For example:

PorH2+ H2WO4220o−−−−→

C6H5OH[Por - W = O(OC6H5)]

PoH2+ MoCl2195o−−−−→

C6H5CN[Por - Mo=O]2O

A tungsten porphyrin is especially remarkable, since the efforts for

synthesizing it by using tungsten hexacarbonyl were not unsuccessful (Ostfeld and

Tsutsui, 1974).

The first lanthanide complexes of meso-tetraphenylporphyrin and some of

its derivatives were synthesized by (Wong et al., 1974). Their synthetic procedure

was a modified method of the one Buchler and Treibs worked on. They used

hydrated tris(2, 4-pentanedionato)europium(III) and H2TPP and refluxed them at

214oC 1, 2, 4-trichlorobenzene for 3 to 4 hours. The product 2,4-pentanedionato-

meso-tetraphenylporphyrineuropium(III) was characterized by UV-visible spectrum

and NMR. From the spectroscopic evidences they showed that the reaction is a general

one which proceeds for the entire lanthanides. They repeated the reaction with different

substituted TPP derivativs and a variety of β-diketonate complexes of the lanthanides.

These complexes are stable to the air and they are soluble enough in the organic

solvents (Wong et al., 1974).

By using these new methods that had been proposed by researches the number

of metalloporphyrins that synthesized by them increased. Beside the using of different

metals to insert into the porphyrin ring and creating new compounds, scientists

have done many researches to apply substituents to porphyrin ring itself. These

substituents give different characteristics to the porphyrin and metalloporphyrins

consequently scientists can synthesize a wide range of compounds for different needs.

17

For example a group of scientists in 1979 used octaalkyl metalloporphyrin π cation

radicals to react with nucleophiles (pyridines, nitrite, chloride, imidazole, cyanide,

triphenylphosphine, acetate, thiocyanate and azide) and produced meso- (methane)

substituted metalloporphyrins. By demetalation the appropriate meso-substituted

porphyrins were gained. During their investigations they found that corresponding

π dication of the metalloporphyrins or the radical is not involved in the combination

step of the reaction (Smith et al., 1979).

In 2005, a different method of synthesizing palladium and copper porphyrins

was introduced. They described a direct synthesis method starting from 1-(p- toluolyl)-

5-phenyldipyrromethane (2a) treated with metal reagent in KOH and ethanol at reflux

for one hour for palladium and 18 hours for copper. The process is shown in Figure

2.3. In this method metal will be bonded inside the porphyrin during the formation of

porphyrin itself (Sharada et al., 2005).

Figure 2.3: Direct synthesis of Palladium porphyrin (Sharada et al., 2005).

18

2.3 Heterogenization of Metalloporphyrins

Metalloporphyrins are distinguished for their great selectivity and working at

mild conditions which they can be selective catalysts for oxidation reactions. However,

they are expensive for using in industrial environment and dealing with them is

rather difficult (Haber et al., 2004). Traditional heterogeneous catalysts are quite

strong and they can work under more severe conditions. Also they are generally

stable and they are produced at low costs. But their selectivity in most cases is

much lower in comparison with homogenous catalysts. Hence, the immobilizing

the active site of metalloporphyrins on the surface of heterogenous supports may be

effective strategy to improve catalysts with advantages of using metalloporphyrins and

heterogenous catalysts at the same time. This strategy seems to be a good way to satisfy

environmental needs and to get catalysts with keeping the properties of homogeneous

systems, which are more stable and can be easily recovered (Hamid, 2005).

Metalloporphyrins can be absorbed physically and make bond with active

groups at surface of supporters as an active site (Sheldon, 1994). The advantages of

heterogenized metalloporphyrins on high-surface oxides or porous materials over their

homogenous alternatives which have been mentioned in Helda Hamids research are:

a) They can be easily separated from the products mixture.

b) It is possible to operate them continuously.

c) There is no limitation for solubility of porphyrins.

d) Interaction between complex and supporter is possible.

e) Formation of porphyrin cluster, μ-oxo-dimers is decreased and;

f) The ability of auto-oxidation is decreased.

There are two wide classes of supported complex catalyst that have been

developed. The first one is the metal complex which is linked to the supporter by

attachment to one of the supporters ligands. The second class is the metal complex

which reacts with support and results in displacement of ligands bond to the metal and

their substitution by groups that form an essential part of the support. The two types

of support that are used in these two classes are organic polymers and inorganic oxides

(Hartley, 1985).

The support provides the local environment of the catalyst, and as a result

19

of the structures support, supporting metalloporphyrins can be beneficial in general

applications in the following ways (Sheldon, 1994):

a) They are oxidatively stable.

b) They are tough and have high resistance toward physical abrasion.

c) They can be used repeatedly.

d) Metalloporphyrins cannot be leached or removed easily.

e) Wide range of solvents and conditions can be used in their presence.

f) They are proper for batch or continuous flow systems, and

g) They are capable to be customized for selective oxidations

2.4 Immobilization of Metalloporphyrins into MCM-41

2.4.1 Introduction on MCM-41

There are three classes of inorganic solids that are widely used as heterogenuos

catalysts and adsorption media. The first class is microporous materials whose pore

diameter is approximately 20 A or less and the second class is mesoporous materials

with a pore diameter ranging from 20 to 500 A Macroporous materials are the third

class of the inorganic solids and they have pore diameterof greater than 500 A (Beck

et al., 1992; Rouquerol et al., 1994). The microstructure of these inorganic solids

allows molecules access to large internal surfaces and cavities that increases catalytic

activity and adsorption capacity. This property shows the important role of using these

materials in the catalytic reactions (Beck et al., 1992).

MCM-41 (Mobile Composition of Matter number 41), one of the members of

large family of mesoporous sieves, is a silicate produced by a templating mechanism.

The structure is shown in Figure 2.4. It has a hexagonal array of uniform mesopores.

The uniform and controllable channels of MCM-41 have sizes between approximately

15 A to greater than 100 A (Beck et al., 1992). MCM-41 is high thermal compound

which has a highly specific surface of up to 1500 m2g−1 with a pore volume of up to

1.3 mL g−1 (Grun, 1999).

For the first time, MCM-41 was synthesized (Beck et al., 1992). They used

20

Figure 2.4: MCM-41 structure.

a liquid crystal templating mechanism (LCT) for their synthesis in which surfactant

liquid structures serve as templates. In this method continuous solvent (water) region is

used to create inorganic walls among the surfactant liquid structure. The encapsulation

might happen because of the entrance of anionic inorganic species in the solvent

region to balance the cationic hydrophilic surface of the micelles. They proposed that

silicate condensation is not the controllable factor in the formation of the structure.

Kresge suggested that formation of the structure depends on the organization of the

surfactant molecules into the micellar liquid crystals. These miceller liquid crystals

serve as a template for the formation of the MCM-41. The process is shown in Figure

2.5. Therefore, the types of the materials which may be formed are not limited to

silicates. Also, these materials must not necessarily have the regularly repeating SBUs

(Secondary Building Units) in the pore walls in order to make the regularly repeating

porous networks observed (Beck et al., 1992).

Figure 2.5: Synthesis of MCM-41 using templating method. 1) liquid crystal face

initiated, 2) silicate anion initiated (Beck et al., 1992).

After Kresges work, much work has been done to understand the mechanism

of MCM-41 formation and improve the synthetic procedure of MCM-41 materials to

obtain high thermal stability. The variety of MCM-41s calcined products morphologies

depends on the synthetic method (Lin et al., 1998).

21

In 1996 Schacht and coworkers used oil/solution microemulsion as templates.

They showed that ordered mesopores silica can form hollow spherical, thin sheet or

fibrous morphology by careful control of the oil/water interface in the macroemulsion.

Ozin and coworkers in the same year reported an oriented mesoporous silica films

produced on air-water or mica-water interfaces. They acquired a high variety of

morphology of mesoporous silica, discoids, ropes and wheels among the others. In

1997 a group of scientists used a synthetic method in which multilamella vesicles used

as template to produce hollow mesoporous silica. All these sophisticated morphologies

are produced by complicated modifications of curvature changes in the meso structures

of the gels and silica condensations (Lin et al., 1998).

2.4.2 Immobilization Strategies

MCM-41 has arisen as a new material for heterogeneous catalysis. Properties

such as uniform and controllable pore size, large surface area and high adsorptive

characteristic of MCM-41 materials provide highly diverse applications especially

where chemical and mechanical stabilities are needed. The large pore size of MCM-

41 besides its high silanol content increase the capacity of MCM-41 in terms of

immobilization macromolecules such as metalloporphyrins (Costa et al., 2008).

In recent years, many researches have reported different applications of

supported macromolecules on MCM-41. In 2005 Kulkarni et al. studied the iron-

porphyrin immobilized in MCM-41s pores using different synthetic methods. Nur

and coworkers in 2002 showed that iron-porphyrins encapsulated in MCM-41 with

the ordered structure may have a high selectivity in benzene to phenol oxidation.

Other researches showed the potential of these materials as active catalysts in oxidation

reactions and established their high efficiency (Costa et al., 2008).

22

To encapsulate porphyrins into the MCM-41 (Sung-Suh et al., 1997),

chloroform is used to dissolve the H2TPP and then MCM-41 was immersed into this

mixture. The slurry of MCM-41 in porphyrin solution was stirred and allowed to

equilibrate for one day, and the solvent was then removed under vacuum.

Methylene chloride solution of ruthenium porphyrin with added MCM-41 was

used to immobilize the metalloporphyrins in MCM-41. The mixture was stirred in the

room temperature for 1 hour and then the resulting solid product after washing dried

in vacuum for one day (Liu et al., 1998).

Water soluble metalloporphyrins in a mixture of water and MCM-41 was stirred

for 12 h at room temperature. Then the product was filtered and washed with deionized

water (Holland et al., 1998). In the other work manganese porphyrins dissolved in

methylene chloride with proper amount of MCM-41. Then the slurry of MCM-41 in

Mn(II) porphyrin was stirred and allowed to equilibrate for 24 h. The solvent was then

removed under vacuum (Kim et al., 2001).

Complexes of iron, cobalt and manganese porphyrins were used to immobilize

within the MCM-41. Porphyrins were dissolved in dichloromethane and then MCM-

41 was added to the mixture. This mixture was kept under the magnetic stirring

for 48 hours, and then under reflux conditions for 1 hour. The solid product was

then washed in Soxhlet apparatus for 48 hours with dichloromethane in order to

remove the weakly absorbed metalloporphyrins on MCM-41 (Costa et al., 2008).

All these immobilizations strategies are based on ion exchanges between surfactant

and porphyrin complexes. In this project the same concept is used to insert

metalloporphyrin into MCM-41.

2.5 Catalysis Reaction

2.5.1 Introduction on D-limonene and Carvone

Limonene is a hydrocarbon with IUPAC name of 1-methyl-4-(1-

methylethenyl)-cyclohexane and molecular formula of C10H16. The structure is

shown in Figure 2.6. It is a colorless liquid with a strong smell of orange which is

classified as a cyclic terpene. Limonenes name derived from lemon and it is the major

compound of the oil extracted from citrus rind. As limonene is a chiral molecule,

23

it has R and S isomers and its racemic limonene is known as dipentene. Biological

sources only produce D-limonene ((R-(+)-enantiomer). D-limonene can be obtained

from citrus peel oil with an amount of up to 50 kg as a byproduct annually. This makes

D-limonene a cheap and readily available starting material which can be used for

biotransformation into more valuable fragrance compounds such as carvone (Pakdel,

2001; Trytek et al., 2007).

Figure 2.6: The structure of D-limonene.

The major use of limonene is as a precursor to carvone. It is commonly used in

cosmetic products and because of its odor it is used in food manufacturing. As a flavor

it is used in some medicines to cover the bitter taste of alkaloids and as a fragrant in

perfumery. Also it is used as botanical insecticide, as solvent for cleaning purposes

and for some model airplane glues and as a paint stripper when applied to the painted

wood (Fahlbusch et al., 2003; Trytek et al., 2007).

Carvone can be found naturally in many essential oils but it is most abundant

in caraway and dills seeds. It is a member of terpenoides family with IUPAC name

of 2-methyl-5-(1-methylethanyl)-2-cyclohexenone. Carvone has S and R enantiomers

which R-(-)-carvone (D-carvone) smells like spearmint while S-(+)-carvone (L-

carvone) smells like caraway. The structures are shown in Figure 2.7.Both carvones

are used in the flavor and food industry. S-(+)-carvone is used to protect potatoes

during storage from premature sprouting. R-(-)-carvone has been suggested to use as

a mosquito repellent (Leitereg et al., 1971).

2.5.2 Conversion of Limonene to Carvone

Transformation of limonene to other monoterpenoid compounds has been

reported by using a number of microorganisms, yet the amounts gained were

24

Figure 2.7: The structure of R-(-)-carvone and S-(+)-carvone.

insufficient for industrial purposes. The lack of effective biotransformation systems for

producing natural flavor and fragrance compounds in addition to the volatility of the

substrate, insolubility of terpenes in water and their toxicity toward the microorganisms

make terpene transformations not useful enough (Trytek et al., 2007).

Using metalloporphyrins for limonene biotransformation has been reported in

few numbers. In 2003, Skrobot and coworkers investigated the biomimetic oxidation

of limonene into limonene epoxide using Mn(III) porphyrin complexes. Also, Mn(III)

porphyrin complexes were used for oxidation of other terpenes by Maraval et al.

In 2007, Trytek and coworkers used porphyrin-based photoexcited system as an

efficient catalyst for D-limonene biotransformation under mild conditons and oxygen

or/and H2O2 as oxidants (Trytek et al., 2007). The conversion of D-limonene to

carvone is shown in Figure 2.8. However, due to solubility of porphyrins in organic

solvent, separation of porphyrins from products is rather difficult. Using heterogenized

metalloporphyrins seems to be an effective alternative for D-limonene to carvone

transformation.

25

Figure 2.8: Limonene to carvone transformation.

CHAPTER 3

THE RESEARCH METHODOLOGY

3.1 Preparation of materials

3.1.1 Preparation of Zinc(II) porphyrin

The insertion of zinc into the free porphyrin was started by adding N,N-

dimethylformamide (20 mL) to H2TPP (Aldrich, 99.0%) with an amount of 0.1 g.

Then ZnCl2 (0.11 g) was added to the mixture and the solution was refluxed using an

oil bath at 150oC for 30 minutes. The mixture then was cooled in room temperature in

an ice bath (5-10 min) and distilled water (15 mL) was added and was returned back to

the ice bath for an additional 5-10 min. The mixture was filtered, washed with distilled

water and was allowed to be crystallized in the air for one week. The synthesis process

is shown in Figure 3.1.

Figure 3.1: Synthesis of ZnTPP.

27

3.1.2 Synthesis of Purely Siliceous Si-MCM-41

Purely siliceous mesoporous MCM-41 was synthesized according to the

method used by (Hasim, 2009) and as shown below:

6SiO2 : 1.5 Na2O : CTABr : 1.5 (NH4)2O : 250 H2O

The sodium silicate solution (Na2SiO3, part A) was prepared by mixing 7.75 g

rice husk ash (RHA), 2.42 g of NaOH pellet, and 45 g double distilled water together.

Then the mixture was stirred for 2 hours at 353 K. while for the surfactant solution

(part B), 7.79 g of cetyltrimethylammonium bromide (CTABr) was dissolved in 45

g double distilled water and 1 mL of NH4OH 25 wt%. The mixture of part B also

stirred at 353 K for one hour. After both mixture solution were cooled to the room

temperature, part A was added to part B. The gel mixture was simultaneously stirred

vigorously for 15 min was then aged for 24 hours. The resulting gel was kept in an air

oven for crystallization at 373 K (Hasim, 2009).

The pH of the gel was then adjusted close to 10.2 by adding acetic acid 25 wt%

after cooled to room temperature. The subsequent 24 hours heating and pH adjustment

was repeated twice. The solid product was filtered, washed, and neutralized using

double distilled water. The solid sample dried in an oven at 373 K and was calcined

at 823 K in air for 10 hours with a heating rate of 1oC min−1 to remove the trapped

organic template (Hasim, 2009).

3.1.3 Encapsulation of Zinc(II) porphyrin into MCM-41

Three different concentration of zinc porphyrin was encapsulated into MCM-

41. ZnTPP (0.017 g, 0.034 g and 0.068 g) was dissolved in DMF (10 mL) and was

stirred for half an hour. Then MCM-41 (0.5 g) was added to the mixture and stirred for

24 hours in a desiccator using magnet stirrer. Then the solution was put in the 100oC

oven for an hour for solvent evaporation.

28

3.2 Characterization Methods

3.2.1 Ultraviolet-Visible spectroscopy (UV-Vis)

UV-Vis spectroscopy is the measurement of the wavelength and intensity of

absorption of near-ultraviolet and visible light by a sample. Ultraviolet and visible

light are energetic enough to promote outer electrons to higher energy levels. UV-

Vis spectroscopy is usually applied to molecules and inorganic ions or complexes in

solution. The UV-Vis spectra have broad features that are of limited use for sample

identification but are very useful for quantitative measurements. The concentration

of an analyte in solution can be determined by measuring the absorbance at some

wavelength and applying the Beer-Lambert Law (Univeristy of Adelaide, 1999). The

Beer-Lambert law (or Beers law) is the linear relationship between absorbance and

concentration of an absorbing species. The general Beer-Lambert law is usually

written as Equation 3.1:

A = a(λ)× b× c (3.1)

Where A is the measured absorbance, a(λ) is a wavelength-dependent

absorptivity coefficient, b is the path length, and c is the analyte concentration. When

working in concentration units of molarity, the Beer-Lambert law is written as Equation

3.2:

A = ε× b× c (3.2)

Where ε is the wavelength-dependent molar absorptivity coefficient with units

of M−1 cm−1 (Univeristy of Adelaide, 1999).

Method:

Ultraviolet-Visible (UV-Vis) spectra were recorded using Perkin-Elmer

Lambda 25 with UV Winlab software. The cuvette was filled with the diluted solution

of synthesized ZnTPP sample in DMF and DMF was used as the blank. UV-Vis was

run in the range of 400-800 nm.

29

3.2.2 Fourier Transform Infrared Spectroscopy (FTIR)

FTIR is most useful for identifying chemicals that are either organic or

inorganic. It can be utilized to quantify some components of an unknown mixture.

It can be applied to the analysis of solids, liquids, and gasses. The term Fourier

Transform Infrared Spectroscopy (FTIR) refers to a fairly recent development in the

manner in which the data is collected and converted from an interference pattern to

a spectrum. Today′s FTIR instruments are computerized which makes them faster

and more sensitive than the older dispersive instruments. By interpreting the infrared

absorption spectrum, the chemical bonds in a molecule can be determined. FTIR

spectra of pure compounds are generally so unique that they are like a molecular

”fingerprint”. While organic compounds have very rich, detailed spectra, inorganic

compounds are usually much simpler. For most common materials, the spectrum of an

unknown can be identified by comparison to a library of known compounds (WCAS,

1984).

Molecular bonds vibrate at various frequencies depending on the elements and

the type of bonds. For any given bond, there are several specific frequencies at which

it can vibrate. According to quantum mechanics, these frequencies correspond to the

ground state (lowest frequency) and several excited states (higher frequencies). One

way to cause the frequency of a molecular vibration to increase is to excite the bond

by having it absorb light energy. For any given transition between two states the light

energy (determined by the wavelength) must exactly equal the difference in the energy

between the two states [usually ground state (Eo) and the first excited state (E1)]. It

is shown in Equation 3.3. The energy corresponding to these transitions between

molecular vibrational states is generally 1-10 kilocalories/mole which corresponds to

the infrared portion of the electromagnetic spectrum (WCAS, 1984).

Differences in energy states = energy light absorbed

E1 − Eo = h c/I (3.3)

Where h = Planks constant, c = speed of light and I = the wave length of

light.

Samples for the FTIR can be prepared in a number of ways. For liquid samples,

the easiest is to place one drop of sample between two plates of sodium chloride (salt).

30

Salt is transparent to infrared light. The drop forms a thin film between the plates.

Solid samples can be milled with potassium bromide (KBr) to form a very fine powder.

This powder is then compressed into a thin pellet which can be analyzed. KBr is also

transparent in the IR. Alternatively, solid samples can be dissolved in a solvent such

as methylene chloride, and the solution placed onto a single salt plate. The solvent is

then evaporated off, leaving a thin film of the original material on the plate. This is

called a cast film, and is frequently used for polymer identification. Solutions can also

be analyzed in a liquid cell. This is a small container made from NaCl (or other IR-

transparent material) which can be filled with liquid. This creates a longer path length

for the sample, which leads to increased sensitivity. Sampling methods include making

a mull of a powder with hydrocarbon oil (Nujol) or pyrolyzing insoluble polymers and

using the distilled pyrolyzate to cast a film. Matrials can be placed in an Attenuated

Total Reflectance (ATR) cell and gases in gas cells (WCAS, 1984).

Method:

Fourier Transform Infrared (FTIR) spectra were recorded using a Perkin Elmer

Spectrum One FTIR spectrometer with Nujol (mineral oil) mulls between KBr plates.

The sample of ZnTPP grounded in a mortar with a small drop of Nujol and then placed

onto the sample holder. The second window was placed on top and was rubbed with

a gentle circular and back-and-forth motion of two windows to distribute the mixture

between the plates. Then the sandwiched plates were placed in the spectrometer to

obtain a spectrum in the mid-infrared region of 4000-400 cm−1.

3.2.3 UV-Vis Diffuse Reflectance Spectroscopy (UV-Vis DR)

UV-Vis Diffuse Reflectance spectroscopy (UV-Vis DR) is a powerful technique

for qualitative and quantitative determination of the absorption spectra of solid samples

or molecules embedded on the solid surfaces. The UV-Vis DR can reveal the chemical

valence of incorporated metal ion (Frei and MacNeil, 1973).

UV-Vis DR spectroscopy measures the amount of light reflected from the

sample surface with an integrating sphere. The data are reported as percent of

reflectance (%R) read on the transmittance scale of the instrument and correspond

to R= I/Io where Io is the intensity of the incident light and I is the intensity of the

light reflected from the sample. The percentage reflectance unit was then converted to

31

Kubelka Munk unit which is a more convenient way to display the reflectance spectra

(Frei and MacNeil, 1973).

Method:

Instrument used for this analysis is Perkin Elmer Lambda 900 UV-VIS-NIR

spectrometer. About 0.04 g of ZnTPP encapsulated in MCM-41 samples were placed

in the quartz cell holder and smoothed to even the powder. The sample holder was

then located and locked properly in the analyzer compartment. The sample was

scanned in the range of wavelength 800-200 nm. To detect the ZnTPP in the samples

the Soret peak and the Q band was searched. The Soret peak is an intense peak in

the blue wavelength region of the visible spectrum of porphyrin containing moieties

with the absorption of 400 to 450 nm. The Q band has the absorption in the region

of 500 to 650 nm in free base tetraphenylporphyrins which is red shifted in metal

tetraphenylporphyrins.

3.2.4 X-ray Powder Diffraction (XRD)

X-ray Powder Diffraction (XRD) is a rapid analytical technique primarily used

for phase identification of a crystalline material and can provide information on unit

cell dimensions. The analyzed material is finely ground, homogenized, and average

bulk composition is determined (Dutrow and Clark, 2011).

Max von Laue, in 1912, discovered that crystalline substances act as three-

dimensional diffraction gratings for X-ray wavelengths similar to the spacing of planes

in a crystal lattice. X-ray diffraction is now a common technique for the study of crystal

structures and atomic spacing (Dutrow and Clark, 2011).

X-ray diffraction is based on constructive interference of monochromatic X-

rays and a crystalline sample. These X-rays are generated by a cathode ray tube,

filtered to produce monochromatic radiation, collimated to concentrate, and directed

toward the sample. The interaction of the incident rays with the sample produces

constructive interference (and a diffracted ray) when conditions satisfy Bragg′s Law

(nλ=2d sinθ). This law relates the wavelength of electromagnetic radiation to the

diffraction angle and the lattice spacing in a crystalline sample. These diffracted X-

rays are then detected, processed and counted. By scanning the sample through a

32

range of 2θ angles, all possible diffraction directions of the lattice should be attained

due to the random orientation of the powdered material. Conversion of the diffraction

peaks to d-spacings allows identification of the mineral because each mineral has a set

of unique d-spacings. Typically, this is achieved by comparison of d-spacings with

standard reference patterns (Dutrow and Clark, 2011).

All diffraction methods are based on generation of X-rays in an X-ray tube.

These X-rays are directed at the sample, and the diffracted rays are collected. A

key component of all diffraction is the angle between the incident and diffracted rays.

Powder and single crystal diffraction vary in instrumentation beyond this (Dutrow and

Clark, 2011).

X-ray powder diffraction is most widely used for the identification of unknown

crystalline materials (e.g. minerals, inorganic compounds). Determination of unknown

solids is critical to studies in geology, environmental science, material science,

engineering and biology. Other applications include:

i. Characterization of crystalline materials.

ii. Identification of fine-grained minerals such as clays and mixed layer clays that

are difficult to determine optically.

iii. Determination of unit cell dimensions.

iv. Measurement of sample purity.

With specialized techniques, XRD can be used to:

i. Determine crystal structures using Rietveld refinement

ii. Determine of modal amounts of minerals (quantitative analysis)

iii. Characterize thin films samples

iv. Make textural measurements, such as the orientation of grains, in a

polycrystalline sample (Dutrow and Clark, 2011)

XRD peaks are intense and sharp only if the sample has sufficient long-range

order and become broader for crystallite size below about 100 nm, while amorphous

phases will produce no diffraction peak at all. The degree of crystallinity can also be

determined by referring to peak intensity. The unit cell parameter of MCM-41 can be

33

calculated from interplanar spacing by applying Equation 3.4.

ao = 2d100/√

3 (3.4)

where ao is the unit cell parameter in Aand d is the interplanar spacing.

However XRD technique does present some limitations when applied to

catalysis (Richards, 2006):

i. It can only detect crystalline phases, and fails to provide useful information on

the amorphous or highly dispersed solid phases so common in catalysts.

ii. Due to its low sensitivity, the concentration of crystalline phase in the sample

needs to be reasonable high in order to be detected.

iii. Probes bulk phases, and are not able to selectivity identify the surface

structures where catalytic reactions take place.

iv. Not useful for the detection of reaction intermediates on catalytic surfaces.

All crystalline solids have unique pattern in terms of the positions and

intensities of the observed reflections including MCM-41. Identification of impure

materials data can be cross-matched by online search or by comparing the three most

intense reflections at the database known as the JCPDS (joint Committee on Powder

Diffractions Standards).

The intensity of the diffraction peaks can be used to determine the crystallinity

of the sample. For the purpose, the intensity of one particular peak is compared with

the intensity of the same peak of standard sample.

Method:

ZnTPP encapsulated in MCM-41 samples were characterized by means of X-

ray Powder Diffraction (XRD) using a Bruker Advance D8 using 5000 diffractometer

with Cu-Ka radiation (λ = 1.5418 A, kV = 40, mA = 40). The powder samples were

put on a sample holder and then scanned in the range from 2θ = 1.5o to 10.0o with

0.05o step size.

34

3.3 Photocatalytic activity of D-limonene

3.3.1 Catalytic testing

The catalytic performance of the prepared catalysts was tested in the

photocatalysis of limonene to carvone. The reactions were carried out at room

temperature by using aqueous hydrogen peroxide 30% as an oxidant in a 4 mL

quartz cuvette. The reaction mixture was contained chloroform as solvent (2 mL),

D-limonene (0.12 mL), hydrogen peroxide 30% (0.15 mL) and the catalyst (0.2 g).

Before exciting the reaction with UV light, small aliquots were withdrawn from

the reaction mixture. This sample was covered with aluminum foil for further gas

chromatography (GC) analysis to determine the amount of product. Then the reaction

mixture was excited with UV light for 5 minutes. The sample was taken from the

reaction after excitation for gas chromatography (GC) and gas chromatography-mass

(GCMS) to identify the products. Catalysts which were used in the reaction were

ZnTPP encapsulated in MCM-41 with three concentrations (25μM, 50 μM and 100

μM), ZnTPP and H2TPP.

According to the previous studies, photocatalysis of limonene to carvone has

been found typically carried out in chloroform. Thus, chloroform is an ideal organic

solvent for this catalytic testing study. From both economic and environmental point of

view, hydrogen peroxide is the best choice among the terminal peroxides because of its

high oxygen content and also is a cheap mild oxidizing agent with only water is being

formed as waste product. Also, zinc(II) tetraphenylporphyrin was used as the catalyst

due to its selectivity throw the oxidation reaction so based on its easy synthesizing

method and its selectivity it is found to be a proper catalyst for photocatalysis of

limonene to carvone.

3.3.2 Analysis of reaction products

3.3.2.1 Gas chromatography- Flame Ionization detector (GC-FID)

The flame ionization detector (FID) is a non-selective detector used in

conjunction with gas chromatography. Because it is non-selective, there is a potential

for many non-target compounds present in samples to interfere with this analysis and

35

for poor resolution especially in complex samples. In GC/FID, the FID or flame

ionization detector detects analytes by measuring an electrical current generated by

electrons from burning carbon particles in the sample.

The FID works by directing the gas phase output from the column into a

hydrogen flame. A voltage of 100-200V is applied between the flame and an electrode

located away from the flame. The increased current due to electrons emitted by burning

carbon particles is then measured. Although the signal current is very small (the

ionization efficiency is only 0.0015%) the noise level is also very small (<10-13 amp)

and with a well-optimized system, sensitivities of 5 x 10−12 g/mL for n-heptane at a

signal/noise ratio of 2 can be easily realized. Except for a very few organic compounds

(e.g. carbon monoxide, etc.) the FID detects all carbon containing compounds. The

detector also has an extremely wide linear dynamic range that extends over, at least

five orders of magnitude with a response index between 0.98-1.02 (Library 4 science,

2009).

Method:

The chromatography analysis of the catalytic reaction was carried out by using

GC instrument of Agilent Technologies series 6890N equipped with Thermo Finnigan,

HP-5 30 m x 0.32 mm 0.25 μM column. Pure helium gas was used as carrier gas.

Sample (1 μL) was injected to GC and mixture was separated using temperature

programming: 70-170oC (14oC / min), 170-260oC (22oC / min), 260-285oC (8 oC /

min) and 285oC (hold for 2 minutes).

3.3.2.2 Gas Chromatography-Mass Spectrometry (GC-MS)

Gas chromatography mass spectrometry (GC-MS) is an instrumental

technique, comprising a gas chromatograph (GC) coupled to a mass spectrometer

(MS), by which complex mixtures of chemicals may be separated, identified and

quantified. This makes it ideal for the analysis of the hundreds of relatively low

molecular weight compounds found in environmental materials. In order for a

compound to be analyzed by GC-MS it must be sufficiently volatile and thermally

stable. In addition, functionalized compounds may require chemical modification

(derivatization), prior to analysis, to eliminate undesirable adsorption effects that

would otherwise affect the quality of the data obtained. Samples are usually analyzed

36

as organic solutions. The sample solution is injected into the GC inlet where it

is vaporized and swept onto a chromatographic column by the carrier gas (usually

helium). The sample flows through the column and the compounds comprising the

mixture of interest are separated by virtue of their relative interaction with the coating

of the column (stationary phase) and the carrier gas (mobile phase). The latter part of

the column passes through a heated transfer line and ends at the entrance to ion source

where compounds eluting from the column are converted to ions (Gates, 2008).

Method:

GC-MS instrument of Agilent 5973/6890N model with 30 m x 0.25 mm x 0.2

μM column has been used to analyze the product of the reaction. Analysis temperature

was increased from 60oC to 300oC with the rate of 15oC per minute. The temperature

was hold for 2 minutes at 200oC and 1 minute at 300oC. Sample volume for each

injection is 0.4 μL. Based on the highest peak, computer program will suggest the best

match structure that may contain in the sample.

CHAPTER 4

RESULT AND DISCUSSION

4.1 Physical properties of porphyrin and zinc(II) porphyrin

The molecular structure of ZnTPP has a square-planar configuration with metal

ion in its center. The sturcture is shown in Figure 4.1. It has an overall symmetry of

D4h as calculations also confirmed the idealized D4h point group symmetry for this

molecule (Bowman et al., 2011). The purple powder of tetraphenylporphyrin and red-

violet powder of zinc(II) tetraphenylporphyrin was characterized by FTIR (Figure 4.2).

Figure 4.1: The structure of Zinc(II) tetraphenylporphyrin.

The characteristic FTIR bands of free porphyrin and ZnTPP complex are shown

in Table 4.1. In the IR spectrum of H2TPP, most of the bands which are appear in the

38

region of 456 to 979 cm−1 , are due to the in-plane bending, out of plane bending, ring

rotation, and ring torsion modes of porphyrin skeletal. Weak absorption bands in the

region from 456 to 473 cm−1 assigned to the in-plane pyrrol ring rotation vibration

can be seen in the spectrum of H2TPP. The stretching of the C-C bands between C=C

and C=N bands are observed at 576 and 658 cm−1. In the region from 999 to 1463

cm−1 the porphyrin skeletal related vibrations are mainly symmetric and asymmetric

C=C and C-C bands and pyrrol symmetric and asymmetric half-ring stretching modes.

Bands at 1156 and 1173 cm−1 in the spectrum of H2TPP are assigned to C-H bands

asymmetric deformation modes in the pyrrol rings.

Table 4.1: Main FTIR bands of porphyrin and zinc(II) porphyrin.

Characteristic bands Wavenumber / cm−1

H2TPP ZnTPPC-N bonds 1377 1378

C=C conjugated of aromatics 1455 1456

C-H sp22 bonds 658 675

Figure 4.2: FTIR bands of free porphyrin and zinc(II) porphyrin.

After the insertion of zinc into the porphyrin, a slight blue shift of peaks 1377

cm−1 and 658 cm−1 to lower frequency is observed. This indicates that zinc exists in

the sample.

39

Zinc(II) tetraphenylporphyrin and tetraphenylporphyrin were characterized by

UV-Vis spectra. The UV-Vis spectra of both porphyrin and zinc(II) porphyrin are

shown in Figure 4.3. For the metal free porphyrin four Q bands are observed from at

514, 548, 590 and 649 nm. These Q bands are corresponding to the a2u (π) - eg∗ (π)

transition (Zheng et al., 2008). ZnTPP has two Q bands at 558 nm and 600 nm, but

in the graph below, due to the presence of impurities such as the excess of H2TPP in

the sample, the peaks of ZnTPP may overlap with H2TPP′s peaks, giving three peaks

instead of two. The Soret band is red shifted which is not shown here, which shows

that ZnTPP exists in the sample.

Figure 4.3: UV-Vis spectra of free porphyrin and zinc(II) porphyrin

4.1.1 Physical properties of Zinc(II) tetraphenylporphyrin encapsulated inMCM-41

Zinc(II) tetraphenyl porphyrin was encapsulated in MCM-41 in three different

concentration (25 μM, 50 μM and 100 μM). All these samples were characterized

by UV-Vis DR and the existence of Soret band and Q bands were studied to find out

whether zinc(II) porphyrin exists in samples or not. Figure 4.3 shows the UV-Vis DR

spectra of ZnTPP encapsulated in MCM-41.

40

The Soret band has an absorbance between 400 to 450 nm in the porphyrin

containing moieties and the Q bands are observed between 500 to 650 nm. The Soret

band is arising from the transition of a1u (π) - eg∗(π) (Zheng et al., 2008). In UV-

Vis DR of zinc(II) porphyrins encapsulated in MCM-41 the Soret bands for the three

samples are observed around 420 nm and Q bands are observed in the region from 550

to 550 nm. The pure ZnTPP in MCM-41 should have two Q bands. But due to the

possible existence of H2TPP in the samples, there are more than two observed bands.

Figure 4.4: UV-Vis DR spectra of free porphyrin and zinc(II) porphyrin.

All the three samples also were characterized by XRD to study the structure of

MCM-41. The MCM-41s structure might have been damaged during the encapsulation

process so using the XRD to study the MCM-41 structure is necessary. The XRD

pattern of pure MCM-41 exhibited an intense peak at 2θ = 1.8 - 2.1 due to (100) plane

and additional weak signals between 3.4- 6.0o due to (110), which shows the hexagonal

structure of the materials. In XRD patterns of encapsulated samples these signals are

observed with a slightly shift to higher 2θ. These observations show that the structure

of MCM-41 during the encapsulation process remained intact. The XRD patterns of

as-encapsulated MCM-41 samples are shown in Figure 4.5.

41

Figure 4.5: XRD patterns of zinc(II) porphyrins encapsulated in MCM-41.

4.2 Photocatalytic activity of D-limonene

The photocatalysis of D-limonene by using encapsulated zinc(II) porphyrin

into MCM-41 has been investigated. Three different concentration of ZnTPP

encapsulated into MCM-41 were used to study the effect of zinc concentration in the

biotransformation of D-limonene to carvone. Also H2TPP was used as the catalyst for

this reaction to find out the catalytic activity of the ZnTPP is zinc based or porphyrin

based. Zinc (II) tetraphenylporphyrin itself was also used for the photocatalysis of

D-limonene to compare its selectivity with the selectivity of ZnTPP encapsulated in

MCM-41.

Biotransformation of limonene to carvone based on porphyrins as

photocatalysts which was done before produced carvone and unknown product with

a verbenone-like mass spectrum (Trytek et al., 2005). So the expected result for this

reaction is a mixture of carvone and other compounds. The reaction is shown in Figure

4.6.

To investigate the effect of UV-Vis light on the biotransformation of D-

limonene to carvone small aliquots from all the samples before going throw the UV-

42

Figure 4.6: Products of limonene photo transformation in the presence of ZnTTP

catalyst.

Vis light were withdrawn. After running the UV-Vis light for the samples again small

aliquots from all the samples were withdrawn. These two series of samples (before and

after UV-Vis light) were analyzed by Gas Chromatography to investigate the expected

products. The GC results for all the samples before UV light show no productivity.

This shows that the presence of UV-Vis light for exciting the reaction is inevitable.

In the photochemical oxidation of limonene with ZnTPP, carvone and the

other products could be formed either through a dioxygen-ZnTPP complex-mediated

limonene intermediate or through a free radical chain pathway. In the first case,

43

photoexcitation would form a highly reactive complex between dioxygen and ZnTPP,

which would play a distinct role in the reaction, perhaps via a complex-mediate

limonene intermediate (Figure 4.7). In the second case, photoexcitation would liberate

high reactivity hydroxyl species, which would go on to react with the limonene radical.

The second possibility is more probable. As abundant evidence shows, hydroxylation

of hydrocarbons catalyzed by cytochrome P-450 occurs via a mechanism involving

hydrogen atom abstraction from the substrate (R-H) followed by a rapid transfer of

the metal-bound hydroxyl radical to an intermediate alkyl radical (R•) (Trytek et al.,

2005).

Figure 4.7: A possible mechanism for limonene photo-oxidation by ZnTPP.

Samples containing ZnTPP encapsulated in MCM-41 with 25μM and 50μM

concentration of ZnTPP show insignificant results after UV-Vis light where 100μM

concentration of ZnTPP encapsulated into MCM-41 shows new peaks in GC spectrum.

To identify these peaks the sample was analyzed by GC-MS. This shows that the zinc

presence in quiet high amount in this catalytic reaction is necessary.

Sample which had H2TPP as its catalyst after UV-Vis light show no

productivity in its GC results. This proves that the catalytic activity of ZnTPP in this

reaction depends on the zinc and without zinc inside the porphyrin there is no catalytic

activity.

Zinc(II) porphyrin can easily form (ZnTPP•) in presence of UV light and carry

on the reaction. Zinc as an electron rich transition metal can easily donate and accept

electron and due to its nature it is a good metal choice to cooperate with the porphyrin

as a catalyst (Harriman et al., 1988).

44

Zinc(II) tetraphenylporphyrin as the catalyst in this reaction after UV-Vis light

shows quiet various new peaks in GC spectrum which are related to the products. This

proves that ZnTPP is an active catalyst for this reaction.

The importance of catalysts presence in this reaction was studied by preparing

samples with no catalyst before and after UV-Vis light. The sample before UV-Vis

light shows no productivity in its GC spectrum where the sample after UV-Vis shows

quiet new weak peaks in its GC spectrum. In comparison with those peaks of ZnTPP

and ZnTPP-MCM-41 the peaks of uncatalyzed reaction are so weak and this shows

that the presence of the catalysts to have a better yield is necessary.

Samples that had productivities after being exposed to UV-Vis light were

analyzed by GC-MS to be identified. All the expected products are shown in Figure

4.6. In this work the selectivity of the catalyst toward carvone, limonene epoxides (cis

and trans) and trans-carveol have been investigated.

Based on GC and GC-MS analysis, the percentage (%) of conversion of

limonene and the percentage (%) of selectivity towards cis- and trans-limonene oxide,

carveol and carvone were calculated as follow and they were listed in Table 4.2

Conversion (%) =Amount of limonene reacted

Amount of limonene input(4.1)

Selectivity (%) =Amount of limonene resulting

Amount of product input(4.2)

Table 4.2: Catalytic activity of H2TPP, ZnTPP and different concentrations of ZnTPP

in ZnTPP-MCM-41 after UV-Vis light.

Sample ZnTPP Conversion Selectivity Selectivity Selectivity Selectivity

(mmol) toward toward toward toward towardlimonene (%) trans-limonene cis-limonene trans-carveole carvone (%)oxide (%) oxide (%) oxide (%)

No catalyst - 43.13 - - 0.40 -

H2TPP - - - - - -

ZnTPP 0.294 60.96 0.20 0.10 - -

ZnTPP0.025 - - - - --MCM-41

ZnTPP0.050 - - - - --MCM-41

ZnTPP0.100 68.47 0.44 - - --MCM-41

45

Based on the GC-MS results the sample with no catalyst gave 43.13%

conversion of limonene to products. Only carveol as the expected product was obtained

and the rest was byproducts like cis-verbenol, butylated hydroxyl toluene and alpha-

pinene. The low conversion of limonene to the products shows that without catalyst

the yield of the reaction is significantly low. This is because of less formation of O•2 in

the absence of the catalyst. Metal ions with low redox potential promote an effective

σ-overlap with the oxygen atom, improving the O-O cleavage and resulting in the

formation of active intermediates and cause higher yields.

The sample with ZnTPP as a homogeneous catalyst gave 60.96% conversion of

limonene to products. Cis- and trans-limonene oxides were only obtained as expected

products besides the other byproducts. The selectivity of the catalyst toward the main

products is shown in Table 4.2. The presence of zinc(II) as an active site of the catalyst

leads to form the intermediate such as [(ZnTPP)O2]• which improves the O-O cleavage

and promotes the oxidation. It can be inferred that due to excess of ZnTPP in this

reaction oxidation was continued and resulted more byproducts than the main expected

products.

Zinc(II) tetraphenylporphyrin encapsulated in MCM-41 with concentration of

100μM as heterogeneous catalyst gave higher conversion of limonene to products

than the other two reactions (ZnTPP as homogeneous catalyst and no catalyst). Cis-

limonene oxide was obtained and the calculated catalyst selectivity toward this product

was 0.44%. The byproducts were mostly cyclic compounds and bicyclic compounds.

It is assumed that the formation of the various byproducts in the photocatalysis

transformation of D-limonene was due to the radicalic reaction under the UV-Vis. The

Photochemical reactions are involved in the formation of different radical species.

These highly reactive molecules or atoms in these reactions can form different

byproducts beside the main products due to their highly random collisions.

In the uncatalyzed reaction and homogeneous catalyzed reaction the variety of

byproducts were higher. It can be concluded that in the sample with ZnTPP-MCM-41

as heterogeneous catalyst most of the bulky aromatic byproducts were trapped in the

pores of MCM-41.

CHAPTER 5

CONCLUSION AND RECOMMENDATIONS

5.1 Conclusion

In this study, Zinc(II) tetraphenylporphyrin was successfully synthesized and

was successfully immobilized into MCM-41 with three different concentrations (25

μM, 50 μM and 100 μM).

The materials obtained were characterized by Fourier Transform Infrared

spectroscopy (FTIR), Ultraviolet Visible spectroscopy (UV-Vis), Diffuse Reflectance

Ultraviolet-Visible spectroscopy and X-ray Diffraction (XRD). The results confirmed

that Zinc was inserted inside the free porphyrin and zinc(II) tetraphenylporphyrin

was immobilized into MCM-41 according the methodology. However, there are

some problems that cannot be eliminated during the synthesis such as occurrence of

impurities due to the incomplete removal of excess reactants like H2TPP.

The catalytic activity of the prepared catalysts was tested in the photocatalysis

of D-limonene with hydrogen peroxide as oxidant and dimethylformamide as solvent.

The effect of different concentration of ZnTPP encapsulated in MCM-41 and UV-Vis

light were studied in this reaction. The analysis of the samples by Gas Chromatography

(GC) showed that only ZnTPP-MCM-41 with the highest concentration of ZnTPP (100

μM) catalyzed the reaction among the encapsulated catalysts. The GC analysis for all

the samples also showed that without exposing the samples to the UV-Vis light no

limonene oxidation would occur.

47

Zinc(II) tetraphenylporphyrin as homogeneous catalyst was used in the same

reaction. The results showed that ZnTPP is active toward the photocatalytic oxidation

of D-limonene. The reaction was also carried out under the same condition but with

no catalyst. GC analysis showed that without catalyst the conversion of the limonene

to products after exposing it to UV-Vis light is rather low.

The importance of the zinc′s presence in the catalysts was verified by testing

the catalytic activity of H2TPP in the photocatalytic transformation of D-limonene. No

productivity was found for H2TPP in GCs results. However, in the previous study of

using H2TPP as the catalyst in photocatalytic biotransformation of D-limonene in the

presence of hydrogen peroxide as an oxidant by (Trytek et al., 2005) the high yield of

carvone was reported.

Gas Chromatography-Mass spectroscopy (GC-MS) analysis for supported

catalysts showed that cis-limonene epoxide and other byproducts were obtained and

the carvone did not exist. For ZnTPP as homogeneous catalyst GC-MS results showed

that cis- and trans-limonene oxide, bulky aromatics as byproducts but no carvone were

obtained. In the reaction with no catalyst trans-carveol was identified by GC-MS.

In this study the sample of ZnTPP-MCM-41 as heterogeneous catalyst had

the highest conversion among the other samples. This is because of the controlling

the local environment of oxidation by immobilizing the catalyst on the solid support

(Olsen et al., 2005). The following order was found for the conversion of limonene

to product in the samples: [ZnTPP-MCM-41] > [ZnTPP] >> no catalyst. For all

these three reactions a high amount of byproducts was obtained. Since the reaction is

a photoexcited reaction it is involved with free radicals and random collision of these

radicals results different compounds as products.

ZnTPP as the homogeneous catalyst in comparison with supported ZnTPP as

heterogeneous catalyst is less active and it cannot be recycled. The advantages of using

heterogeneous catalyst due to its high activity and its reusability are more than those

of homogeneous alternatives.

48

5.2 Recommendations

In this research UV-Vis light was used to carry out the photocatalytic in the

room temperature with hydrogen peroxide as an oxidant but the parameters such as

temperature and the time of exposing the UV-Vis light to the reaction mixture were not

optimized. In the future study these parameter should be optimized. Also changing

the oxidant to an organic oxidant that is more stable such as tert-Butyl hydroperoxide

(TBHP) should be studied due to the fast decomposition of H2O2 to water and oxygen.

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APPENDIX A

GC-FID RESULTS

Figure A1: GC/FID plot for ZnTPP-MCM-41 with 25μM concentration of ZnTPP.

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57


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Figure A4: GC/FID plot for ZnTPP.

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Figure A5: GC/FID plot for uncatalyzed reaction.

APPENDIX B

GS-MS RESULTS

Figure B1: GC-MS plot for the sample with no catalyst.

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Figure B2: GC-MS plot for the sample with ZnTPP catalyst.

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Figure B3: GC-MS plot for the sample with 100 μM ZnTPP encapsulated in MCM-41

catalyst.

preparation of zinc(ii) porphyrin with a mesoporous structure and its

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