INTERMOLECULAR PHOTOREACTIONS AND SELECTIVITY
STUDIES IN CONFINED SPACE OF Y ZEOLITES
YEOH KAR KHENG
UNIVERSITI TEKNOLOGI MALAYSIA
PSZ 19:16 (Pind. 1/97)
UNIVERSITI TEKNOLOGI MALAYSIA
BORANG PENGESAHAN STATUS TESIS♦
JUDUL :
INTERMOLECULAR PHOTOREACTIONS AND SELECTIVITY
STUDIES IN CONFINED SPACE OF Y ZEOLITES
SESI PENGAJIAN: 2004/2005
Saya :
YEOH KAR KHENG
(HURUF BESAR)
mengaku membenarkan tesis ( PSM / Sarjana / Doktor Falsafah )* ini disimpan di
Perpustakaan Universiti Teknologi Malaysia dengan syarat-syarat kegunaan seperti
berikut:
1. Tesis adalah hakmilik Universiti Teknologi Malaysia.
2. Perpustakaan Universiti Teknologi Malaysia dibenarkan membuat salinan untuk
tujuan pengajian sahaja.
3. Perpustakaan dibenarkan membuat salinan tesis ini sebagai bahan pertukaran antara
institusi pengajian tinggi.
4. **Sila tandakan ( √ )
√
SULIT
(Mengandungi maklumat yang berdarjah keselamatan atau
kepentingan Malaysia seperti yang termaktub dalam AKTA
RAHSIA RASMI 1972)
TERHAD
(Mengandungi maklumat TERHAD yang telah ditentukan
oleh organisasi/badan di mana penyelidikan dijalankan)
TIDAK TERHAD
Disahkan oleh
______________________________
(TANDATANGAN PENULIS)
________________________________
(TANDATANGAN PENYELIA)
Alamat Tetap:
429, LORONG 9,
TAMAN KAYA,
34000 TAIPING, PERAK.
ASSOC. PROF. DR. ABDUL
RAHIM YACOB
Tarikh:___27 MAY 2005
Tarikh:
Nama Penyelia
27 MAY 2005
CATATAN: * Potong yang tidak berkenaan.
** Jika tesis ini SULIT atau TERHAD, sila lampirkan surat daripada pihak
berkuasa/organisasi berkenaan dengan menyatakan sekali sebab dan tempoh
tesis ini perlu dikelaskan sebagai SULIT atau TERHAD.
♦ Tesis dimaksudkan sebagai tesis bagi Ijazah Doktor Falsafah dan Sarjana secara
penyelidikan, atau disertasi bagi pengajian secara kerja kursus dan penyelidikan,
atau Laporan Projek Sarjana Muda (PSM).
“We hereby declare that we have read this thesis and in our opinion this thesis is
sufficient in terms of scope and quality for the award of the degree of
Master of Science (Chemistry).”
Signature
:
………………………………………………
Name of Supervisor I :
Assoc. Prof. Dr. Abdul Rahim Yacob
Date
:
27 May 2005
Signature
:
………………………………………………
Name of Supersivor II :
Assoc. Prof. Dr. Farediah Ahmad
Date
27 May 2005
:
BAHAGIAN A ⎯ Pengesahan Kerjasama*
Adalah disahkan bahawa projek penyelidikan tesis ini telah dilaksanakan melalui
kerjasama antara _______________________ dengan ________________________
Disahkan oleh:
Tandatangan
: _________________________________ Tarikh: ______________
Nama
: _________________________________
Jawatan : _________________________________
(Cop rasmi)
* Jika penyediaan tesis/projek melibatkan kerjasama.
BAHAGIAN B ⎯ Untuk Kegunaan Pejabat Sekolah Pengajian Siswazah
Tesis ini telah diperiksa dan diakui oleh:
Nama dan Alamat Pemeriksa Luar
: Prof. Dr. Zakaria bin Mohd Amin
School of Chemical Science,
Universiti Sains Malaysia
11800, Minden, Pulau Pinang.
Nama dan Alamat Pemeriksa Dalam : Assoc. Prof. Dr. Zainab Ramli
Dept. of Chemistry, Faculty of Science,
Universiti Teknologi Malaysia,
81310 Skudai, Johor.
Nama Penyelia Lain (jika ada)
: _____________________________________
_____________________________________
_____________________________________
_____________________________________
Disahkan oleh Penolong Pendaftar di SPS:
Tandatangan : _________________________________ Tarikh: ______________
Nama
: Ganesan A/L Andimuthu
INTERMOLECULAR PHOTOREACTIONS AND SELECTIVITY STUDIES
IN CONFINED SPACE OF CATION-EXCHANGED Y ZEOLITES
YEOH KAR KHENG
A thesis submitted in fulfilment of the
requirements for the award of the degree of
Master of Science (Chemistry)
Faculty of Science
Universiti Teknologi Malaysia
MAY 2005
ii
I declare that this thesis entitled “Intermolecular Photoreactions and Selectivity
Studies in Confined Space of Cation-Exchanged Y Zeolites” is the result of my own
research except as cited in the references. The thesis has not been accepted for any
degree and is not concurrently submitted in candidature of any other degree.
Signature
: ____________________
Name
: YEOH KAR KHENG
Date
: 27 May 2005
iii
For my family, teachers and friends.
iv
ACKNOWLEDGEMENT
This thesis could not be completed without the help of many. I am most
grateful to my supervisors Assoc. Prof. Dr. Abdul Rahim Yacob and Assoc. Prof. Dr.
Farediah Ahmad who always gave me guidance and support.
I must also thank all the lectures in Institute Ibnu Sina including Prof. Dr.
Halimaton Hamdan, Assoc. Prof. Dr. Zainab Ramli, Assoc. Prof. Dr. Salasiah Endud,
Dr. Hadi and others who always provided me good suggestions and solutions
whenever I faced with difficulties in this research.
My grateful acknowledgement is also due to En. Kadir, En. Fuad, En. Khairul
and other staffs in Department of Chemistry who provided me a lot of helps in this
research.
I am also indebted to my parents, friends and fellow researches. It was from
their encouragements, motivations, and supports enable me to complete this work.
Lastly, I must record my profound thanks to the National Science Fellowship
(NSF) and IRPA vote 74505 for financially supports in this research.
v
ABSTRACT
Photochemistry in organized assemblies has attracted considerable attention
because of their potential use in controlling photophysical and photochemical
behaviour of organic molecules in a confined space. Conversion of a starting material
to product in a photoreaction involves selectivity by the reaction cavity to the
specified product. For solid and rigid media like zeolite, the size of the reaction
cavity plays an important role in products selectivity. The surface of NaY zeolite was
first studied with paramagnetic probe using Electron Spin Resonance spectroscopy
(ESR). Two favourable active sites were identified. The study of a confine space
reaction was first studied in the photosensitization of triethylamine by acetophenone
in NaY zeolite. ESR result showed that radical cation of amine dimer was formed
inside zeolite resulted from the confinement effect of the zeolite Y supercage. Ultraviolet (UV) irradiation of acetophenone in toluene solution results in photochemical
hydrogen abstraction and yielded a mixture of both symmetric (1,2-diphenylethane
and 1,2-diphenylethyl alcohol) and asymmetric (1,2-diphenylpinacol) coupling
products. These were identified and characterized by gas chromatography-mass
spectrometry (GC-MS) and nuclear magnetic resonance (NMR). With the
introduction of NaY zeolite, high yield of asymmetric product, 1,2-diphenylpinacol
was observed. It further proved the confinement effect played by the zeolite
produced a drastic change in product selectivity compared to homogenous reaction.
Photodimerization of 2-cyclohexenone in various cation-exchanged Y zeolites were
also studied in solid state and zeolite-solvent slurries. Both the reactions showed a
great reversal of head-to-tail (HT) cyclohexenone dimer, to head-to-head (HH)
cyclohexenone dimer with increasing pattern from LiY to CsY zeolite. The study of
regioselectivity in the photocycloaddition of 2-cyclohexenone to vinyl acetate was
also carried out in zeolite slurries, in which the result showed a drastically change of
product yield compared to the homogeneous reaction. However, the cationexchanged zeolites failed to control the selectivity. This is explained by the passive
cavity effect of zeolite.
vi
ABSTRAK
Fotokimia di dalam media teraturapi telah banyak menarik perhatian kerana
potensinya dalam mengawal sifat fotofizik dan fotokimia molekul organik dalam
ruang terhad. Pengubahan bahan pemula kepada produk dalam tindak balas
fotokimia melibatkan kepilihan kaviti tindak balas terhadap produk tertentu. Untuk
pepejal tegar seperti zeolit, saiz kaviti tindak balasnya memainkan peranan dalam
kepilihan produk. Permukaan zeolite NaY telah dikaji dengan prob paramagnet
menggunakan spektroskopi Resonans Spin Elektron (RSE). Dua tapak aktif telah
dikenalpasti. Tindak balas dalam ruang terhad pada mulanya telah dikaji dalam
pemfotopekaan trietilamina oleh asetofenon dalam zeolit NaY. Keputusan RSE
menunjukkan radikal kation dimer amina terbentuk dalam zeolit disebabkan oleh
kesan ruang terhad supersangkar zeolit. Penyinaran ultra-lembayung (UL) ke atas
asetofenon dalam pelarut toluena pula menyebabkan pengabstrakan hidrogen dan
menghasilkan campuran kedua-dua hasil gandingan simetri (1,2-difeniletana dan 1,2difeniletil alkohol) dan tidak simetri (1,2-difenilpinakol). Pengenalpastian dan
pencirian hasil ini seterusnya dilakukan menggunakan kromatografi gas-spektrometri
jisim (KG-SJ) dan resonans magnet nukleus (RMN). Penggunaan zeolit NaY pula
menghasilkan hasil utama tidak simetri, 1,2-difenilpinakol. Ini membuktikan bahawa
ruang terhad pada zeolit telah mengubah kepilihan hasil tersebut berbanding dengan
tindak balas homogen. Pemfotodimeran 2-sikloheksenon dalam pelbagai zeolit Y
tertukar kation juga dikaji dalam fasa pepejal dan buburan zeolit-pelarut. Kedua-dua
tindak balas menunjukkan keterbalikan daripada dimer sikloheksenon kepala-ekor
kepada dimer sikloheksenon kepala-kepala dengan penambahan corak daripada
zeolit LiY kepada CsY. Seterusnya, keregiopilihan dalam pemfototambahan 2sikloheksenon kepada vinil asetat telah dijalankan dalam buburan zeolit. Keputusan
menunjukkan perubahan besar dalam kepilihan produk berbanding dengan tindak
balas homogen. Kegagalan zeolit tertukar kation dalam mengawal kepilihan produk
adalah disebabkan oleh kesan kaviti pasif zeolit.
vii
TABLE OF CONTENTS
CHAPTER
1
2
TITLE
PAGE
INTRODUCTION
1
1.1
Objectives of the Research
2
1.2
Scope of the Studies
2
LITERATURE REVIEWS
3
2.1
Supramolecular Photochemistry Versus Zeolite
3
2.2
The Origins of Supramolecular Chemistry
5
2.3
Zeolite
6
2.4
2.5
2.3.1
Faujasite (FAU) Zeolite
8
2.3.2
Ion Exchange Behavior
9
2.3.3
Electrostatic Field
10
2.3.4
Adsorption
11
2.3.5 Diffusion
13
2.3.6
14
Photochemistry
15
2.4.1
Basic Laws of Photochemistry
15
2.4.2
Electronic Transitions
17
2.4.3
Pathways of Excited States
19
2.4.4
Frontier Orbital Approach in Photochemical Reactions
21
Photocycloaddition Reactions
2.5.1
2.6
Confinement Effect
22
Regiochemistry and Stereochemistry of
Photocycloaddition in Enones
23
Electron Spin Resonance (ESR) Spectroscopy
27
2.6.1
28
The ESR Spectrometer
viii
2.7
2.8
3
2.6.2
Basic Principle of ESR
29
2.6.3
Hyperfine Structure
30
X-Ray Diffraction (XRD)
32
2.7.1 Theory of XRD
33
Flame Emission Spectroscopy (FES)
35
2.8.1
Basic theory and Flame Photometer
35
2.8.2
Quantitative Analysis
36
EXPERIMENTAL
38
3.1
Instrumentations
38
3.2
Chemicals
39
3.3
UV Irradiation of H2 in NaY Zeolite
39
3.4
ESR Study of the Photosensitization of Triethylamine by
3.5
Acetophenone in NaY Zeolite
41
Preparation of Alkali Metal Cation-Exchanged Y Zeolites
42
3.5.1
Quantitative Analysis of the Cation-Exchanged
Y Zeolites
3.6
Photochemical Hydrogen Abstraction by Acetophenone
in Toluene Solution and NaY Zeolites Slurry
43
3.6.1
Homogeneous Reaction
43
3.6.2
Isolation of Photoproducts
44
3.6.2.1 Thin Layer Chromatography (TLC)
45
3.6.2.2 Gravity Column Chromatography (CC)
45
Photoreaction in NaY Zeolite Slurry
45
3.6.3
3.7
Photodimerizations of 2-Cyclohexenone
46
3.7.1
Homogeneous Reactions
46
3.7.2
Solid State Photoreactions in Cation-Exchanged
Y Zeolites
3.7.3
47
Photoreactions in Cation-Exchanged
Y Zeolite-Slurries
3.8
42
49
Photocycloaddition of 2-Cyclohexenone to Vinyl Acetate
50
3.8.1
Homogenous Photoreaction
50
3.8.1.1 Acid Test
51
3.8.2
Photoreactions in Cation-Exchanged
ix
Y Zeolite Slurries
4
RESULTS AND DISCUSSION
4.1 ESR Study of the UV Irradiation of H2 in NaY Zeolite
4.2
51
53
53
An ESR Investigation of Amine Dimers Radical
Cation in the Photosensitization of Triethylamine by
Acetophenone in NaY Zeolite Supercages
58
4.3
Alkali Metals Cation-Exchanged Y Zeolites
62
4.4
Photochemical Hydrogen Abstraction by Acetophenone in
4.5
Toluene Solution and NaY Zeolites Slurry
65
4.4.1
Homogenous Photoreaction
65
4.4.2
Photoreaction in NaY Zeolite Slurry
68
Regioselective Photodimerizations of 2-Cyclohexenone in
Alkali Metal Cation-Exchanged Y Zeolites
4.5.1
Photodimerizations of 2-Cyclohexenone
in Homogenous Solution
4.5.2
83
Photocycloaddition of 2-Cyclohexenone to Vinyl Acetate
(VA) in Alkali Metal Cation-Exchagned Y Zeolite-Slurries
4.6.1
Homogenous Solution
4.6.2
Photocycloadditions in Alkali Metal
Cation-Exchanged Y Zeolite Slurries
5
76
Photodimerizations of 2-Cyclohexenone in
Alkali Metal Cation-Exchanged Zeolite Slurries
4.6
72
Solid State Photodimerizations of 2-Cyclohexenone
in Alkali Metal Cation-Exchanged Y Zeolites
4.5.3
72
CONCLUSIONS
87
87
89
93
REFERENCES
95
APPENDIXES 1-13
113
x
LIST OF TABLES
TABLE NO.
TITLE
PAGE
2.1
Cation dependence of supercage free volume in FAU zeolites
14
3.1
GC-MS analysis of the supernatants in the photochemical
hydrogen abstractions in NaY zeolite slurries
46
3.2
GC-MS analysis of the tetrahydrofuran extracts in the photochemical
hydrogen abstractions in NaY zeolite slurries
3.3
GC peak ratios of the photoproducts in the photodimerizations
of 2-cyclohexenone in homogenous reactions
3.4
46
47
GC peak ratios of the photoproducts in the solid state
photodimerizations of 2-cyclohexenone carried on
different cation-exchanged Y zeolites
3.5
49
GC peak ratios of the photoproducts obtained in the
photodimerizations of 2-cyclohexenone carried in
cation-exchanged Y zeolite-slurries
50
GC peak ratios of the photoproducts in the photocycloadditions
of 2-cyclohexenone to vinyl Acetate in cation-exchanged
Y zeolite-slurries
52
4.1
Ion-Exchanged levels of alkali metal cations-exchanged Y zeolites
65
4.2
Product ratios calculated by GC in the photochemical
hydrogen abstraction by acetophenone in toluene solution
68
Product ratios in the tetrahydrofuran extract of the
photolysed NaY zeolite
70
3.6
4.3
xi
4.4
4.5
4.6
4.7
4.8
Product ratios of the photodimerization of 2-cyclohexenone
in n-hexane
74
Product ratios of the solid state photodimerizations of 2-cyclohexenone
in alkali metal cation-exchanged Y zeolites with
tetrahydrofuran extractions
77
Product ratios obtained by solid state photodimerizations of
2-cyclohexenone in alkali metal cation-exchanged Y zeolites
with HCl treatment and ethyl acetate extractions
79
Product ratios of the photodimerizations of 2-cyclohexenone in
alkali metal cation exchanged Y zeolite-hexane- slurries
86
Product ratios obtained in photocycloadditions of 2-cyclohexenone
to vinyl acetate in different mediums.
92
xii
LIST OF FIGURES
FIGURE NO.
TITLE
PAGE
2.1
Oxygen is shared between two tetrahedra
8
2.2
External surface and supercage of FAU zeolite
9
2.3
Adsorption and desorption isotherm curves of N2
in zeolite NaY at 77 K
12
Pictorial representation of the diffusion of molecules
in a zeolite particle
13
2.5
Orbital energy level description of absorption and emission
18
2.6
Jablonski Diagram
20
2.7
Frontier orbital interactions between a photochemically
excited molecule and a ground state molecule
of 1,3,5-hexatriene
22
2.8
[4 + 2] cycloaddition (a Diels-Alder reaction)
23
2.9
Alkene [2 + 2] photocycloaddition
23
2.10
Head-to head and head-to-tail regioisomers found in
photocyloaddition of cyclohexenone to unsymmetrical alkene
23
2.11
Photocycloaddition of cyclohexenone to methoxyethylene
24
2.12
Photocycloaddition of cyclohexenone to electron-rich alkenes
25
2.13
Photocycloaddition of methyl substitution cyclohexenone to alkene
25
2.14
Stereochemical disposition around the cyclobutane ring in
the cis-fused photoaddition products
25
2.15
Photocyclodimerization reaction of acenaphtylene
26
2.16
Regioselectivity on photocycloadditon reactions
2.4
xiii
of substituted cyclohexenone with cycloalkenylesters
27
2.17
The schematic diagram of an ESR spectrometer
28
2.18
The absorption and first derivative of ESR spectra
29
2.19
Zeeman energy levels of an electron in an
applied magnetic field
30
The interaction of an electron with a single nucleus
I = ½ and the resulting ESR spectrum
31
2.21
Simplified X-ray diffractometer
33
2.22
Pictorial view of Bragg’s Law
34
2.23
Schematic diagram of a flame photometer
36
2.24
Plot of emission intensity versus concentration
37
3.1
Sample cell for activation and UV irradiation
40
3.2
Vacuum line used for sample activation
and sample degassing
41
Experiment set up for UV irradiations in homogenous
solutions and zeolite- solvent slurries
44
3.4
Experiment set up of solid state photoreactions
48
4.1
ESR spectrum of H2 in NaY before UV irradiation
54
4.2
ESR spectrum of UV irradiation (after 45 minutes) of H2
in NaY zeolite supercages
55
Stucture of the FAU zeolite with cation position
type II and type III in the supercages
56
ESR spectrum of UV photolysis (after1 hour)
of Acetophenone in NaY zeolite
57
2.20
3.3
4.3
4.4
4.5
(a) Peak 1 intensity and (b) Peak 2 intensity
against UV irradiation time
4.6
ESR spectrum of UV photolysis (after1 hour) of
triethylamine in NaY zeolite
4.7
4.8
ESR spectrum of UV photolysis (after1 hour) of acetophenone
and triethlyamine in the NaY zeolite supercages
X-ray diffractograms of the alkali metal cation-exchanged
59
60
60
xiv
Y zeolites compared to parent NaY zeolite
63
4.9
Crystalinity versus cation-exchanged Y zeolites
64
4.10
Emission intensity versus concentration of Na analysis in
flame emission photometry
64
GC chromatograms (a) before and (b) after the homogenous
photoreaction of acetophenone in toluene solution
66
GC chromatograms of the supernatant and the
resulting tetrahydrofuran extract
69
The difference of molecule distributions in homogenous
solution and zeolite slurry (spectator approach)
70
GC chromatograms of the homogeneous photoreactions of
2-cyclohexenone compared to solid state photoreactions
73
4.15
Corey’s model
75
4.16
GC chromatograms (b) and (d) show the remained products
which trapped in the zeolites after tetrahydrofuran extractions.
78
GC chromatograms of the solid state photodimerizations
of 2-cyclohenone in alkali metal cation-exchangedY zeolites (a)-(e)
80
Ratio HT(16)/HH(17) obtained in this research
compared to ratio obtained by Lem et al.
82
GC chromatograms of the photodimerizations of 2-cyclohexenone
in alkali metal cation-exchanged Y zeolite-hexane slurries (a)-(e)
85
GC analysis on the reaction mixture in the photocycloaddition
of vinyl acetate to 2-cyclohexenone in hexane
88
4.21
Photocycloaddition of vinyl acetate to 2-cyclohexenone
88
4.22
GC chromatograms of the photoproducts in
photocycloadditons of 2-cyclohexenoen to vinyl acetate
in alkali metal cation-exchanged Y zeolite-slurries
91
4.11
4.12
4.13
4.14
4.17
4.18
4.19
4.20
xv
LIST OF SCHEMES
SCHEME NO.
4.1
TITLE
PAGE
The proposed mechanism of amine photosensitization
by acetophenone inside NaY zeolite supecages
61
The mechanism of photochemical hydrogen abstraction
by acetophenone in toluene solution and zeolite NaY slurry
71
4.3
Photodimerization of 2-cyclohexenone (1)
74
4.4
Various intermediates which can lead to cyclohexenone dimers
76
4.5
Photocycloadditon of 2-cyclohexenone to ethoxyethene
89
4.2
xvi
LIST OF SYMBOLS/ABBREVIATIONS
A
-
Ampere
Å
-
Meter-10
AcP
-
Acetophenone
cm
-
Centimeter
CH
-
2-Cyclohexenone
Cps
-
Count per second
Eq.
-
Equation
EtOAc
-
Ethyl acetate
g
-
Gram
HH
-
Head-to-head
HT
-
Head-to-tail
Hz
-
Hertz (Second-1)
1
-
Proton Nuclear magnetic Resonance
FAU
-
Faujasite zeolite
K
-
Kelvin
k
-
Kilo
L
-
Litre
Μ
-
Mol/Litre
M+
-
Molecular ion
H NMR
xvii
MY
-
Alkali metals Y zeolite
m
-
multiplet
min
-
Minute
mg
-
Milligram
mL
-
Millimeter
mT
-
Millitesla
m/z
-
mass per charge
N
-
Normality
Rf
-
Retention factor
Rt
-
Retention time
s
-
singlet
sec
-
Second
TEA
-
Triethylamine
THF
-
Tetrahydrofuran
V
-
Volt
VA
-
Vinyl acetate
W
-
Watt
xviii
LIST OF APPENDICES
APPENDIX
TITLE
PAGE
1.
MS spectrum of 1,2-diphenylethane (DPE) (10)
113
2.
MS spectrum of 2,3-diphenylpropan-2-ol (DPP) (11)
114
3.
MS spectrum of 2,3-diphenylbutan-2,3-diols (DPB) (12)
115
4.
1
116
5.
MS spectrum of CH dimer, HT (16)
117
6.
MS spectrum of CH dimer, HH (17)
118
7.
MS spectrum of CH dimer (18) or (19) (Peak 1 in Figure 4.14)
119
8.
MS spectrum of CH dimer (18) or (19) (Peak 3 in Figure 4.14)
120
9.
MS spectrum of cyclohexene-cyclobutene adduct (P 1)
121
10.
MS spectrum of cyclohexene-cyclobutene adduct (P2)
122
11.
MS spectrum of cyclohexene-cyclobutene adduct (P3)
123
12.
MS spectrum of cyclohexene-cyclobutene adduct (P4)
124
13.
MS spectrum of cyclohexene-cyclobutene adduct (P5)
125
H NMR spectrum of 2,3-diphenylbutan-2,3-diols (DPB) (12)
CHAPTER 1
INTRODUCTION
Zeolites have been an object of scientific research and a material beneficial to
mankind for more than two centuries since its discovery in 1756. However, it was
not until 10-15 years ago that zeolites attracted the keen interest of photochemists
who wanted to use them in their research. Photochemists are most interested in
controlling chemical reactions with the aid of supramolecular assemblies, aimed at
constructing artificial photosynthetic systems, controlling chirality and inventing
nanoscale advanced materials. Zeolites are found to be particularly useful for such
purpose since they can host various organic molecules in their cavities and channels;
such inclusions have often been shown to modify the photophysicals and
photochemistry of a given species. Besides, photochemical reactions pursued in
zeolites also provide product distributions considerably different from those in
solution [1].
Zeolite nanospace could be considered as “hard” because of the frameworks
of zeolite are rigid, and “active” because of the non-bonding interaction between the
walls of the supercage and the included molecules. On top of these, the most
desirable property of zeolite is that it is transparent to light in the near-UV and
visible regions. Thus it eliminates the possibility of competitive absorption between
the medium and the guest molecule presents [2].
The synthetic utility of intermolecular photodimerization of cyclic enones and
the cycloadditon to unsymmetrical alkenes can be limited by the formation of the
mixtures of the head-to-head (HH) and head-to-tail (HT) regioisomers [3, 4]. HT
2
regioisomers are always formed in much larger amount compared to HH isomer in
solution reaction [5, 6]. In this research, cation-exchanged Y zeolites were applied to
control the regioselectivity of photoproducts in photoreactions of 2-cyclohexenone.
1.1
Objectives of the Research
The objective of this research is to evaluate the feasibility of using zeolite as
reaction medium to carry out intermolecular organic photoreactions, i. e.
photocycloaddition and photodimerization. This could be further divided to two:
(i)
To compare the products selectivity of between the
conventional homomogenous photoreactions with solid state
and/or slurry photoreactions in zeolite supercage
(ii)
To utilize the cation-exchanged property of zeolite to control
the regioselectivity of desired photoproducts.
Faujasite-Y zeolite was used as host because it possesses large supercages volume
which enable us to study a variety of photochemical reactions.
1.2
Scope of the Studies
At the first part of this research, locations of the paramagnetic probe in
different adsorption sites of NaY zeolite were studied using Electron Spin Resonance
Spectroscopy (ESR). Most of the research in the supramolecular photochemistry
within zeolites deal with the intramolecular reaction. In order to study the different
approaches used in the intermolecular photoreaction, we have studied the triplet
sensitization technique and “spectator” method. The triplet sensitization technique
had been applied in the dimerization of triethylamine (TEA) within Y zeolite, while
3
the “spectator” method was used in gaining selective asymmetric coupling products
in the hydrogen abstraction of toluene by acetophenone (AcP).
After gaining experiences from the first part, we turned to the next part, the
utilization of the size constriction effect and cation-guest interactions of the cationexchanged zeolites to modify the selectivity of the regioisomers in photodimerization
of 2-cyclohexenone (CH) and photocycloaddition of CH to vinyl acetate (VA).
CHAPTER 2
LITERATURE REVIEWS
2.1
Supramolecular Photochemistry Versus Zeolite
The last two centuries have witnessed the growth of organic photochemistry
from relatively unknown to a more developed discipline. During this period,
photochemists have discovered new reactions, established mechanism of
photoreactions, laid out the ground rules for the behavior of molecules in excited
state and surfaces, and found applications of photochemistry in everyday life. In spite
of these achievements, photochemistry is yet to become a sought after tool in
industrial synthetic processes. Nowadays, organic photochemistry seemed to have
developed into three stages: (a) discovery of reaction; (b) mechanistic pursuit, and (c)
gaining control on the outcome of a reaction [7].
Supramolecular or guest@host chemistry (symbol @ represents noncovalent
binding of the guest and host) is the chemistry that is dominated by forces resulting
from molecular non-bonded, non-covalent electrostatic forces (due to static and
oscillating fixed charge interactions) and dispersion force (due to induced transient
charge interactions) to control the selectivity of reactions of geminate radical pairs
whose molecular chemistry involves random radical-radical reaction [8]. The guests
serve as molecular probes of the host structure. The host controls the initial sitting of
the guests, the sieving probabilities, the size shape selective diffusional dynamic, and
the topology of diffusion pathways available to the adsorbed guest and to the reactive
intermediates produces by the absorbed guest organic molecules [9].
5
Conversion of starting material to product in photoreaction (and any chemical
reaction) involves change in the shape of reaction cavity from reactant-like to
product like. A number of organized assemblies such as micelles, vesicles, mono and
bilayer, liquid crystal, cyclodextrins, silica clay and zeolite surfaces have been
examined as media to control the excited state of organic molecules. Each of them is
unique in their ability to modify in photoreaction. In the case of photochemical
reactions occurring in liquid or liquid-like media like micelles, this change is not
obvious since the surrounding media rearranges itself to accommodate this change.
However, this becomes different in the case of rigid structures like crystal.
Among these, zeolites are the most versatile host system to control the
reactions of a large variety of molecules [2, 7]. Zeolites are made-up of silica and
alumina. They are not that different from the glassware used in laboratory reaction,
except that the size of the “glassware” now is at molecular dimensions [2]. One of
the recent trends in photochemical research on zeolite cavities focuses on the way in
which the restricted spaces influence the geminatoselectivity [7, 9] regioselectivity
[10-12] and stereoselectivity [13, 14] of products and enhanced molecular interaction
of guest molecules [15-17]. Recently, attentions have been given to the subjects of
photochemical asymmetric synthesis [18-25] with the use of chiral auxiliaries, and
the study of photosensitization within zeolites [26-29]. Carrying out the
photoreactions in nanospace of zeolite cavity gives us a unique opportunity to
understand the role played by factors like confinement and electrostatic interactions
with the cation sites on the reaction pathway.
2.2
The Origin of Supramolecular Chemistry
The starting point in the history of supramolecular effects on the chemistry
may be traced back to 1934 [30], which J. Franck and Rabinowitch coined the term
‘cage effect’ to explain observations comparing the photochemistry of diatomic
molecules (e.g. I2) in the gas phase to their photochemistry in the liquid phase [8].
A molecule in solution may happen to dissociate after light absorption, and
the radicals or atoms formed in this way separate with a certain amount of kinetic
6
energy will be at once lost in collisions with the solvent. In addition to the ‘normal’
probability of recombination governed by the law of mass action, there will be an
additional probability of primary recombination of two particles, which have been
parts of the same molecule before dissociation. The recombination effect will
probably show wavelength dependence, decreasing with the increasing energy of the
absorbed quantum. A greater excess energy will permit the dissociation products to
find their way through the surrounding ‘walls’ of the solvent and to put more
molecular layers between them before coming to rest [8].
The brilliant insight and imagery of the importance of a radical pair in a
‘solvent cage’ may be considered as setting the stage for supramolecular chemistry,
which concerned with how non-covalent, intermolecular interactions can influence
the chemistry of ‘bimolecular’ systems. The solvent cage is a primitive but
fundamental supramolecular ‘host’ that exerts an influence on the chemistry and
reactivity of an incarcerated ‘guest’ molecule or pair of ‘guest’ molecule [8].
2.3
Zeolite
Zeolites are inorganic material with a three-dimensional structure resembling
a honeycomb. It is made up of interconnecting channels and cages that extend three
dimensionally throughout the structure. Organic guest molecules could be adsorbed
and held inside the cavities because of non-bonded interactions and electrostatic
forces inside these cavities [31-34].
The history of zeolite began from the discovery of the stilbite by a Swedish
mineralogist, Axel Cronstedt, in 1756 [35]. He found that the natural mineral stilbite
visibly lost water when heated. Accordingly, he named this class of mineral, zeolite
from the classical Greek words “zeo” (to boil) and “lithos” (stone). Zeolites can be
simply divided into two categories, natural and synthetic. Natural zeolites are usually
found in basaltic areas and volcanic regions as well as in the sedimentary deposits in
many part of the world [36]. The pioneering work in synthetic zeolite was carried
out by Barrer [37, 38] and Milton [39]. There are approximately 40 naturally
7
occurring and over 100 synthetic forms of zeolites. Some of the examples of the
natural zeolites are stilbite, analcime, chabazite, ferrierite, mesolite and clinoptilolite.
Predominant types of synthetic zeolites are type A, type X, type Y and ZSM-5 [32,
40].
The natural zeolites have not gained the commercial importance of the
synthetic zeolites due to limitation in availability, large variations in the mineral
composition, crystal size, porosity, and pore diameter. Application areas of the
natural zeolite include building materials, agriculture, water treatment, radioactive
waste treatment, and pet litter and odor control. On the other hand the synthetic
zeolites have large market volume in detergent builder, petroleum refining and
petrochemical
processing
catalysts,
and
a
variety
of
uses
such
as
adsorbents/desiccants (molecular sieves). Zeolite A for example was developed
specifically as an eco-friendly (environmentally preferable) detergent builder as an
alternative to phosphate builder which can cause eutrophication. The current global
value market for zeolite is estimated to be $2.15 billion per annum, the components
of natural and synthetic zeolites being $450 million and $1.7 billion, respectively
[40].
The primary building blocks of zeolite are [SiO4]4- and [AlO4]5- tetrahedra.
These tetrahedra are linked through oxygen atoms to form channels and cages of
discrete size with no two aluminum atoms sharing the same oxygen atom (Figure
2.1). As a result, the total framework charge is negative, and it must be balanced by
cations, typically of an alkali or alkaline earth metal cations. These cation could be
exchanged by conventional ion-exchange method. The position, size and the number
of cations can significantly alter the properties of the zeolite.
Zeolites can be broadly divided into two types based on the pore structure.
Zeolites with interconnecting cages (e.g. Faujasite, Zeolite A) and zeolites made up
of channels that might be or might not be interconnected (e.g. ZSM-5, Zeolite-Beta).
In short, zeolite can be considered as a polymeric porous crystalline hydrated
aluminosilicate based on an infinite three-dimensional structure which has a general
formula of [31-33]:
8
Mx/n[(SiO4)y(AlO4)x].zH2O
where
M=
exchangeable cations of valency n
[]=
zeolite framework
n =
valency of cation
x =
the number of AlO4 tetrahedra
y =
the number of SiO4 tetrahedra
z
= the number of moles of zeolitic water
Figure 2.1: Oxygen is shared between two tetrahedra [8].
2.3.1
Faujasite (FAU) Zeolite
The structure of faujasite (FAU) zeolites (Zeolite X and Y) is cubic and built
from sodalite cage (0.66 nm in diameter, with an entry aperture of 0.21 nm)
connected via the double 6-membered ring. The entry aperture of the sodalite cage is
too small for oxygen molecule to enter; however, water molecules are known to go
into it. Zeolite X and Y have different framework of Si/Al ratios: 1.0 < Si/Al <1.5 for
zeolite X and 1.5< Si/Al < 3 for zeolite Y. No faujasite with Si/Al ratio less than 1.0
has been prepared to date due to the unstable framework structure [1].
These zeolites form three-dimensional network of nearly spherical supercages
of about 1.3 nm in diameter connected tetrahedrally to four other supercages through
0.74 nm windows. The charge-compensating cations are mobile and distributed
among several types of sites. The supercage concentration in zeolite Y with Na+ ions
as charge-compensating cation (NaY) is estimated to be approximately 6 x 10-4
9
mol/g on the basis of the crystal structure. Figure 2.2 shows the external surface and
the internal supercage of a FAU zeolite [1,8].
Each supercage in FAU zeolites could accommodate up to five molecules of
benzene, two molecules of naphthalene, or two molecules of pyrene [41]. The
relatively large dimensions supercages can be used as a host for various
photoreactions, such as photodimerization [10, 42], photocycloaddition [43] and
recently, it was applied in the photochemical asymmetric synthesis [18-25], and the
study of photosensitization reactions [26-29].
FAU External Surface
Sodalite
cage
Window
opening
0.74 nm
FAU Framework
Diameter
1.3 nm
Internal
supercage
Figure 2.2: External surface (left) and supercage (right) of FAU zeolite [8].
2.3.2
Ion Exchange Behavior
The cations and water molecules are distributed within the zeolite
intracrystalline pore system. Unlike water, the cations are not free to leave the
crystals unless they are replaced by their electrochemical equivalent of other cations
[31]. The ion exchange behavior (selectivity and degree of exchange) mainly
depends on the size and charges of the hydrated cation, the temperature, the
concentration, and to some degree the anion species. Cation exchange may produce
10
considerable change in thermal stability, adsorption behavior, and catalytic activity.
The ion exchange process is presented by the following equation:
ZABZB (z)
+
ZBAZA (s)
ZABZB (s)
+
ZBAZA (z)
where ZA and ZB are the ionic charge of cations A and B, and (z) and (s) represent
the zeolite and solution.
The ion exchange property of zeolites has been used in commercial
applications such as detergent builders, radioisotope separation, and removal of
ammonium ions from wastewater streams. Besides these, zeolites are used to replace
phosphates as water-softening agents [40].
2.3.3
Electrostatic Field
Zeolite can be regarded as a solvent that dissolves or disperses molecules into
pores and channels similar to solvent cage. Despite the similarity of the zeolite pores
to solvent shells, zeolite pores are rigid and distinctly shaped in contrast to the soft
and featureless solvent shells [1, 44].
The solvation-like interaction can be expected for zeolite host-guest molecule
pairs. The negatively charged framework and the mobile cations combine to produce
an electrostatic field akin to solvent polarity inside the cavities where the molecules
reside. The electrostatic field strength in zeolites has been reported to be extremely
high [45, 46]. It is considered due to the fact that the cations exposed at the center of
the supercage being only partially shielded. The strength of the electric field is
dependent on both the cation size of the charge-compensating cation and Si/Al ratio
of the zeolite framework. For example, the smallest alkali metal cation, Li+ ion
induces a stronger field in its proximity than the largest Cs+ ion. Also, cations in
zeolite Y (which has higher Si/Al ratio) exhibit higher fields than those in zeolite X.
11
The effects of the fields have been explored with a number of fluroscence probes
incorporated into zeolite. [47, 48].
2.3.4
Adsorption
Adsorption is one of the fundamental issues in zeolite science. Figure 2.3
represents an adsorption isotherm of N2 in NaY zeolite at 77 K [1]. As shown by the
figure, both the adsorption and desorption curves are superimposed indicating the
adsorption and desorption processes are completely reversible. The adsorption
process occurs in two stages with increasing equilibrium pressure. Initially, the steep
rise part indicates that N2 is being absorbed into the internal supercages, while the
flat part indicates the process takes place very slow with the increase in pressure.
This accounts for N2 molecules covering the external surface of the zeolite at a
monoleyer level. In this case, more than 90% of the N2 molecules were adsorbed in
the supercages. It can be simply explain by the dominating volume of supercages in
the zeolite, and the adsorption takes place first in the cages mainly because of a big
gain in entropy [1].
Size exclusion is a basic guideline for adsorption of organic molecules in
zeolites. Results showed the maximum quantity of anthracene that could be adsorbed
in NaX zeolite from hexane solution at 296 K is 5.3 x 10-4 mol/g, nearly one
molecule per supercage. However, an abrupt decrease in adsorption (5 x 10-6 mol/g)
was observed for 9,10-dimethylanthracene that apparently has a size larger than the
aperture of the supercage [49]. The adsorbed organic molecules are mainly residing
inside the zeolites if they can meet the requirement of size limitation imposed by the
entry apertures to the cages [1].
A variety of techniques have demonstrated the cation-guest interaction plays
an important role during the adsorption of organic molecules in cation-exchanged Y
zeolites [50-53]. Cation-π interaction (also known as cation-quadrupolar interaction)
[54-56] has been recognized as the main force of binding between aromatic guest
molecules and the zeolite supercage. However, when a benzene ring contains a polar
12
group such as nitro (dinitrobenzene), the primary interaction is between the cation
and the oxygen of the nitro group (not the π cloud of the phenyl ring) [57]. Such a
type of dipolar interaction between the cation and the guest predominate even in
nonaromatic such as hyroflurocarbons within NaY [58-60].
It has been shown the emission spectra for larger aromatic species such as
naphthalene [61], and pyrene [62] in NaY or NaX zeolite are loading dependent. The
contribution of excimer emission increases at the expense of monomer as the loading
level increases. This indicates a heterogeneous distribution within the zeolites,
because the excimer emission is considered to arise from more than one molecule
occupying the same cage. An average occupancy of unity in zeolites does not mean
that most of the supercage contain one guest species on the average but rather that
most cages are empty, with some being multiply-occupied [63].
Figure 2.3: Adsorption and desorption isotherm curves of N2 in zeolite NaY at 77 K:
(o) adsorption curve; (●) desorption curve [1].
13
2.3.5
Diffusion
The diffuse of molecules within the intracrystalline cage network of zeolite is
important in influencing the chemical reactions of the guest species and for the
design and application of shape-selective catalysts [64,65]. The molecules introduced
externally into zeolites are not fixed at particular sites, but rather migrate by
executing a hopping motion from one adsorption site to another within the same cage
and occasionally to other cages through one of the connecting windows. Typically,
intercage hopping assumes a relatively high activation barrier and is a slow process,
allowing the guest molecules to stroll around the adsorption sites within a given cage
before they jump out. Figure 2.4 shows the pictorial representation of the diffusion
of molecules in a zeolite particle. The diffusional motion is classified as two types
according to the nature of the activation energy; (1) intercage jump and (2) intracage
jump [66, 67].
Figure 2.4: Pictorial representation of the diffusion of molecules in a zeolite particle.
The arrows in this picture represent the motion of molecules [1].
The cage-to-cage diffusivity of the guest species is highly possible dependent
on the molecular size because of the constraints on the diffusive motion imposed by
the windows and walls of the zeolite. It has been pointed out that the intercage
hopping dynamics of an organic molecule is largely affected by the adsorption
interaction with the host zeolite [68].
14
2.3.6
Confinement Effect
The reactions which take place within a zeolite actually occur in the
supercage, which can be considered as “reaction cavity”. This reaction cavity is of
molecular scale dimensions hence would influence the reactivity of the substrate and
the course of the reaction. In this confined space, the mobility and conformational
flexibility will be restricted. The “free volume” indicates the space in which the
reactants transform themselves to products. The volume available for an organic
molecule within a supercage depends on the number and the nature of the cation [69,
70]. Table 2.1 shows the available volume for a guest decreases as the cation size
increases from Li to Cs [70].
Table 2.1: Cation dependence of supercage free volume in FAU zeolites [70].
Cation (M+)
Radius of the
cation (Å)
Y- zeolite
X-zeolite
Vacant space within the supercage (Å3)
Li
0.60
834
873
Na
0.95
827
852
K
1.33
807
800
Rb
1.48
796
770
Cs
1.69
781
732
The spatial confinement in the cavities of zeolite is expected to provide
molecules with geometric restrictions. The cavities of zeolite may prevent or restrict
the approach of the guest molecules, in particular, reactive intermediate which may
due to unusual photophysic and/or photochemistry of the guest species. This idea
was tried with a molecule whose size is similar to the dimension of the entry
aperture, and thus the host/guest complex was expected to fix in very tightly [1].
Recently, the idea of confinement effect has led to the understanding of
electronic confinement. Marquez et al. [71-73] proposed the influence of the cavity
dimensions on the electronic structure of some guest molecules incorporated within
the zeolites framework can be related to the quantum confinement concept. This
explanation was similar to “electronic confinement effect” in which the electron
15
density of the guest is constrained and localized within the zeolite cavity as a result
of strong range repulsion with the electrons of the zeolite walls [74].
2.4
Photochemistry
Photochemistry is the study of chemical changes by visible or ultraviolet
light. It plays an important role in everyday processes that occur in nature such as
photosynthesis in plants and photodissociation of ozone in the atmosphere that
prevents harmful ultraviolet radiation of sun reaching the earth’s surface. The
process of vision itself involves the photochemical isomerization of the protein,
rhodopsin in the retina of eyes. [75, 2]
Quantum Chemistry predicts that molecules exist in a variety of electronic
states that differ in both electronic energies and wave functions. Classical chemistry
involves reactions of molecules in their electronic ground state (R0) (lowest
electronic energy state) [76].
The study of reactions that occur through high
electronic energy (R*) (electronic excited states) of organic molecules is called
organic photochemistry. Reactions that are thermodynamically unfavorable when the
reactants are in the ground states may occur from an excited state in photoreaction. In
addition, the other advantage of photochemical reactions is specific bonds could be
activated depending on the frequency of the radiation used for excitation [2, 77].
2.4.1
Basic Laws on Photochemistry
The First Law of Photochemistry or Grottus-Draper Law states, “Only
radiation absorbed in a system can produce a chemical change” [83]. The amount of
light absorbed is related to the concentration of the absorbing molecule in the path of
the irradiation; this relation is known as Beer-Lambert Law [77, 78] and is
represented by Eq.1:
A = log I0/I = εcl
(Eq. 1)
16
A: Beer-Lambert law absorbance
I0: incident radiant flux
I: transmitted radiant flux
ε: molar absorption coefficient (l mol-1 cm-1)
c: concentration of absorbing molecules (mol l-1)
l: absorbing path length (cm)
The Beer-Lambert law is valid except when very high intensities of radiation are
employed, such as laser light [79].
“A molecule which undergoes a photochemical change does so by an
absorption of a single quantum of light energy”. This statement is referred to the
Stark-Einstein Law of Photochemical Equivalence. In other word, it means that the
number of activated molecules equal to the number of quanta of radiation absorbed
(Eq. 2). Exceptions of
the law have been observed in two-photon absorption
processes [77, 80].
1hv + 1R0 = 1R*
(Eq. 2)
h : Planck’s constant
v : frequency of absorbed radiation
R0: reactant in ground state
R*: reactant in excited state
The energy required for electronic excitation is excitation energy (Eexc) [77], which is
defined as Eq. 3:
Eexc = E* - E0 = hv = hc/λ
E : electronic excitation energy
E*: energy of the excited state
E0: energy of the ground state
h : Planck’s constant
v : frequency of absorbed radiation (sec-1)
(Eq. 3)
17
c : velocity of light in vacuo (3 x 1010 cm sec-1)
λ: wavelength of absorbed light (cm)
Generally, electronic excitations that produce photochemical reactions are induced
by the absorption of ultraviolet (UV) or visible (VIS) electromagnet by a molecule.
The radiation energy corresponds to the excitation energies of organic molecules is
140 – 30 kcal mol-1 (λ = 200 –700 nm) [77]. The efficiency of a photochemical
process is variable and is expressed in term of quantum yield Φ which is showed in
Eq. 4.
Φ = Number of molecules reacting or formed
Number of quanta absorbed
2.4.2
(Eq. 4)
Electronic Transitions
Absorption of ultraviolet or visible light by an organic molecule results in the
excitation of an electron from an initial occupied, low energy orbital to a high
energy, previously unoccupied orbital [81]. Organic molecules that have the
capability of doing so contain chromophores, also defined as functional groups
which absorb near ultraviolet or visible radiation. Examples include C=O, Ph, NO2
and -N=N-. Chromophores absorb in the far ultraviolet (λ < 200 nm) are C=C and
C≡C [82, 83]. The absorption process can be simplified as in Figure 2.5 [81].
18
hv
+
S1
S0
hv
+
(Spin allowed
absorption)
electron jump
S1
S0
(Spin forbidden
absorption)
electron jump
and spin flip
S1
S0
electron jump
S1
S0
+
hv (fluorescence)
+
hv (phosphorescence)
electron jump
and spin flip
Figure 2.5: Orbital energy level description of absorption and emission. The arrows
intersected by the levels represent electrons. The direction of the arrow
represents the orientation of the electron spin [81].
Two excited electronic states derive from the electronic orbital configuration
produced by light absorption. In one state, the electron spins are paired (antiparallel)
and the other state the electron spins are unpaired (parallel). The state with paired
spins has no resultant spin magnetic moment, but the state with unpaired spins
possesses a net spin magnetic moment. A state with paired spins interacts remain a
single state in the presence of magnetic field, and is termed as a singlet state. A state
with unpaired spins interacts with the magnetic field and splits into three quantized
states, and is termed a triplet state. These are three states, which are most crucial to
an understanding of organic photoreactions [81]:
1. S0 = ground, singlet state
2. S1 = lowest energy excited, singlet state
3. T1 = lowest triplet state
The triplet state is slightly lower in energy than the corresponding singlet state
because two paired electrons have a greater electronic repulsion Hund’s rule [84].
19
2.4.3
Pathways of Excited States
Excited states are short-lived and lose excess energy by returning to the
ground state as rapidly as possible. There are several pathways it can occur; there are
[85]:
(i)
Radiative processes,
(ii)
Radiationless processes,
(iii)
Energy transfer,
(iv)
Chemical reactions.
The commonly encountered photophysical radiative and radiationless processes are
best shown by Jablonski Diagram (Figure 2.6) [81].
The Jablonski Diagram can be simply summarized as:
(A) Radiative processes (shown by arrow in Figure 2.6):
(a) “Allowed” or singlet-singlet absorption (S0 + hv → S1)
(b) “Forbidden” or singlet-triplet absorption (S0 + hv → T1)
(c) “Allowed” or singlet-singlet emission, called fluorescence (S1→ S0 + hv)
(d) “Forbidden” or singlet-triplet emission, called phosphorescence (T1→ S0
+ hv)
(B) Radiationless processes (shown by dotted arrow in Figure 2.6):
(e) “Allowed” transitions between states of the same spin, called internal
conversion (e.g., S1→ S0 + heat)
(f) “Forbidden” transitions between excited states of different spin, called
intersystem crossing (e.g., S1→ T1 + heat)
(g) “Forbidden” transitions between triplet states and the ground state – also
called intersystem crossing (e.g., T1→ S0+ heat)
20
Radiative process
Radiationless process
S3
T3
S2
S-S absorption
T2
T-T absorption
(f)
S1
T1
(e)
(a)
(c)
(b)
S0
(d)
(g)
S0
Figure 2.6: Jablonski Diagram [81].
The intersystem crossing occurs within 10-8 to 10-10 sec and is slower than internal
conversion (10-9 to 10-14 sec). The triplet state T1 is therefore longer-lived than
singlet state, S1. Chemical reactions are much more common for excited triplet
species because of the longer lifetimes of these states [81].
21
2.4.4
Frontier Orbital Approach in Photochemical Reactions
Knowledge of all the molecular orbitals in a compound is necessary to fully
understand its chemistry. However, a great deal can be learned by looking at only
two of the orbitals, that are the highest occupied molecular orbital (HOMO) and the
lowest unoccupied molecular orbital (LUMO). These two molecular orbitals are
known as the frontier orbitals [86].
The first step of bimolecular photoreactions is the excitation of a component
with the chromophore which most efficiently absorbs light. If a conjugated system
present in the component, promotion of a frontier electron from HOMO to LUMO
occurs on the excitation. The excited state produce is ππ* whilst nπ* when the
excitation results from the
non-bonding orbital, such as lone pair electron of
carbonyl group in ketone. The second step of the reaction is the interaction between
the excited molecule and the ground state molecule. The two effective frontier orbital
interactions are [87]:
(1) Interaction between the singly occupied π* of the excited molecule with
LUMO of the ground state molecule (shown at top part of Figure 2.7).
(2) Interaction of the singly occupied n or π orbital of the excited molecule
with HOMO of the ground state molecule (shown at bottom part of
Figure 2.7).
Therefore, the important frontier orbitals in a photochemical reaction are HOMO/
‘HOMO’ and LUMO/ ‘LUMO’ of the ground state and the excited state molecules
respectively. There are strong bonding since the orbitals are closest in energy [86,
87].
22
Energy
LUMO
‘LUMO’
‘HOMO’
Excited molecule
HOMO
Molecular orbital
Ground state
molecule
Figure 2.7: Frontier orbital interactions between a photochemically excited molecule
and a ground state molecule of 1,3,5-hexatriene [87].
2.5
Photocycloaddition Reactions
Two different π-bond-containing molecules react to form a cyclic compound
in a cycloaddition reaction. Each of the reactants loses a π bond, and the resulting
cyclic product has two new σ bonds. These are classified according to the number of
π electrons that interact in the reaction. The best-known example of cycloaddition
reaction is Diels-Alder [4 + 2] addition and the [2+2] photocycloaddition. The
examples of Diels-Alder [4 + 2] addition and [2 + 2] photocycloaddition are shown
in Figure 2.8 and Figure 2.9 respectively [86].
23
O
+
O
O
O
O
O
Figure 2.8: [4 + 2] cycloaddition (a Diels-Alder reaction) [86].
hv
+
Figure 2.9: Alkene [2 + 2] photocycloaddition [86].
2.5.1
Regiochemistry and Stereochemistry of Photocycloaddition of Enones
Head-to-head (HH) and head-to-tail (HT) regioisomers are two possible
orientations commonly found in the addition of an enone to an unsymmetrical
alkene. This example is given in Figure 2.10 [3-5].
R
hv
+
+
R
O
R
O
head-to-head (HH)
O
head-to-tail (HT)
Figure 2.10: Head-to-head and head-to-tail regioisomers found in photocyloaddition
of cyclohexenone to unsymmetrical alkene [3].
24
The orientation of the addition is control by the geometry of the intermediate
π-complex formed between the enone and the alkene in the photocycloaddition
process. This then proceeds via a 1,4-diradical to the cyclobutane photoproduct [5].
The exciplex (π-complex) formed results from dipolar interaction between the
excited enone (acceptor) and the ground state alkene (donor). Calculations of charge
distribution in the n→ π*excited state of planar α, β-unsaturated ketones show the Cβ
is quite negative relative to Cα [87-89]. Thus addition of excited 2-cyclohexenone
(CH) (1) to ground state methoxyethylene (2) leads largely to the HT photoadduct (3)
(Figure 2.11) [5].
δ−
δ+
O
(1)
+
OCH3
δ+
δ−
(2)
OCH3
OCH3
hv
O
O
(3)
Figure 2.11: Photocycloaddition of cyclohexenone (1) to methoxyethylene (2) [5].
The steric effect was also found to influence the regiochemistry of the
addition. It can be done by increasing the bulk of the enone β-substituent or alkene
substituent [90-92]. The presence of the nitro group on the β-position of
cyclohexenone (1) was found to increase the proportion of the HH regioisomers in
the adduct mixture [93].
Attempts also have been made to control the regiochemistry by placing
removable directing groups on the enone. For example, a trimethylsilyl group at the
α-position of the 2-cyclopentenone has been found to increase the proportion of the
HT isomer in the photocycloaddition reaction with 2-acetoxyproprene; the silyl
group can be than removed from the adducts by treatment with fluoride ion [94].
The stereospecificity of [2 + 2] photocycloaddition reaction is discussed base
on the relative stereochemistry of the newly created chiral centers of the cyclobutene
ring. Addition of acyclic alkenes to cyclic enones usually will form the cis-fused
photoproducts whereas with the electron-rich alkenes, the addition will give transfused adducts as the major products (Figure 2.12) [91, 92].
25
R
R
+
O
H R
hv
R
R = CH3, OCH3
H
O
Figure 2.12: Photocycloaddition of CH to electron-rich alkenes [91].
It was found that alkyl substitution at Cβ in cyclohexenone (1) [91, 92] also
tends to increase the proportion of cis-fused products (Figure 2.13).
R
R
+
O
R
hv
R
R = CH3, OCH3
O
H
Figure 2.13: Photocycloaddition of methyl substitution cyclohexenone to
alkene [91].
[2 +2] Photocycloaddition of the enones with cyclic alkenes give either cis or
trans-fused adducts, the preferred arrangement usually is determined by the ring size.
A further point of interest is the stereochemical disposition around the cyclobutane
ring in the cis-fused photoaddition products. The possible stereoisomers are cis-anticis and the cis-syn-cis isomers (Figure 2.14) [81].
O
O
cis-anti-cis
(a)
cis-syn-cis
(b)
Figure 2.14: Stereochemical disposition around the cyclobutane ring in the cis-fused
photocycloaddition products [81].
26
One
of
the
examples
of
this
isomeric
form is
found
in
the
photocyclodimerization reaction of acenaphtylene (4) (Figure 2.15) [95].
hv
syn
(4)
hv
triplet sens
anti
Figure 2.15: Photocyclodimerization reaction of acenaphtylene (4) [95].
Recently, Omar et al. [96] reported the experimental results of the triplet [2 +
2] photocycloaddition reactions of substituted CH with cycloalkenylesters,
cyclobutenylester (5), cyclopentenylester (6), and cyclohexenylester (7) gave
remarkable change in the regioselectivity of the products (Figure 2.16). The HT/HH
product ratio increases with the increment of the cycle-size. The changes in the
HT/HH ratio with the enlargement of the alkene ring size may be due to the
increment of the repulsion energy between the enone carbonyl and esters in the
alkenylesters, and large changes in the deformation energy of the reactants.
27
R h
t
R H
(5)
hv
O
H R
> 95%
R
(6)
t R
R H
hv
h
O
O
(4)
H R
R R
O
H H
40%
60%
R
R = CO2Me
hv
R R
R H
(7)
O
H R
O
< 5%
head-to-head (HH)
H H
> 95%
head-to-tail (HT)
Figure 2.16: Regioselectivity on photocycloadditon reactions of substituted
CH with cycloalkenylesters (5), (6), and (7) [96].
2.6
Electron Spin Resonance (ESR) Spectroscopy
Electron Spin Resonance (ESR) or Electron Paramagnetic Resonance (EPR)
spectroscopy is a physical method of observing resonance absorption of microwave
power by unpaired electron spins in magnetic field. It has developed into a most
direct, sensitive and powerful non-destructive method for the characterization and
measurement of species with unpaired electron [97].
ESR is a specific technique for systems with net electron spin angular
momentum [97]. These systems include: (i) free radicals in the solid, liquid, or
gaseous states; (ii) some point defects (localized crystal imperfections) in solids; (iii)
28
biradicals; (iv) systems in the triplet state; (v) systems with three or more electrons
and (iv) most transition-metal ions and rare-earth ions.
2.6.1
The ESR Spectrometer
The schematic diagram of the more common continuous-wave (CW) ESR
spectrometer is shown in Figure 2.17. It consists of a microwave source (klystron
oscillator), cavity in which the sample is inserted in a quartz container, a microwave
detector, and electromagnet with a field that can varied in the region of 0.3 T. The
ESR spectrum is obtained by monitoring the microwave absorption as the magnetic
field is varied. To minimize the noise from the diode in steady state measurements, a
magnetic field modulation scheme with phase sensitive detection is usually
employed. As a result, the detected signal appears as a first derivative of the
absorption intensity [98]. Figure 2.18 shows the absorption and first derivative of
ESR spectra.
Figure 2.17: The schematic diagram of an ESR spectrometer [98].
29
Figure 2.18: The absorption and first derivative of ESR spectra [98].
2.6.2
Basic Principle of ESR
In the presence of magnetic field, H, an interaction between the magnetic
moment of an unpaired electron and the applied field will occur and these energy
which yields different spin states known as “Zeeman Energy”. The Zeeman energy is
given by:
Ez = gβMsH
(Eq.4)
where Ez is the Zeeman energy, Ms represent the magnetic quantum number, β is the
electronic Bohr magneton with a value 9.2733 x 10-28 J/Gauss and g is the
spectroscopic splitting factor which has a value of 2.0023 for a free electron.
The possible values of Ms are Ms = + ½ and Ms = – ½ for an electron. Hence,
the two possible values of the energy levels (Zeeman levels) are Ez = + ½ gβH (α
state) and Ez = - ½ gβH (β state) which is represented in Figure 2.19.
30
∆E = gβH
Ez = - ½ gβH
H=0
H
Figure 2.19: Zeeman energy levels of an electron in an applied magnetic field [97].
The direction of the spin is changed by the absorption of microwaves when
the energy different (∆E = gβH) is equal to the quantum energy of an
electromagnetic wave, hv, where h is the Planck’s constant and v the frequency of an
electromagnetic radiation. This absorption of the electromagnetic wave (microwave)
by the unpaired electron is called “Electron Spin Resonance”. The resonance
condition is represented by
∆E = gβHr = hv
(Eq. 5)
where Hr is the resonance magnetic field.
This is the fundamental equation of ESR spectroscopy and to obtain an
absorption by paramagnetic species, we either fix the magnetic field and vary the
frequency, or fix the frequency and vary the magnetic field however, the later is
chosen with the frequency being in the microwave magnetic region (λ = 3 cm and v ≈
9 GHz) and the magnetic field being centered around 3000 gauss (300 mT) [97-100].
2.6.3
Hyperfine Structure
If the only effect observed in ESR were the interaction of an unpaired
electron with an external field, then the spectrum would consist of only one line.
However, one of the most important features of ESR spectra are their hyperfine
31
structure, the splitting of the individual electron resonance lines into components
with respect to nucleus with a spin. In spectroscopy, the term “hyperfine structure”
means the structure of the spectrum that can be traced to interactions of the electrons
with other nuclei as a result of the latter’s point electric charge [98]. The “hyperfine
coupling” is the term used to describe the magnetic coupling that occur between the
spin of the unpaired electron and those of the nearest magnetic nuclei in the molecule
[101]. Figure 2.20 shows the interaction of an unpaired electron with a single
nucleus with nuclear quantum number, I = ½ (upper part in figure) and the resulting
ESR spectrum (bottom part in figure); “A” represents the hyperfine coupling
constant (Hz) while gN and βN represent the spectroscopic splitting factor and Bohr
magneton of the nucleus. hA measures the interaction between the electron and
nucleus. Dashed line in the figure correspond to the allowed transition according to
selection rule ∆Ms = ± 1 and ∆MI = 0. The total number of lines is given by, N = 2nI
+ 1 where “n” is the number of equivalent nuclei which interact with the electron.
MI = - 1/2
- ¼ hA
Ms = + 1/2
H=0
geβH
E2
- gNβNH
MI = + 1/2
+ ¼ hA
MI = - 1/2
- ¼ hA
E1
E3
- gNβNH
Ms = - 1/2
MI = + 1/2
+ ¼ hA
Degenerate
Levels
Electronic
Zeeman
Splitting
Nuclear
Zeeman
Splitting
E4
Hyperfine
Interaction
A
Figure 2.20: The interaction of an electron with a single nucleus I = ½ (upper)
and the resulting ESR spectrum (bottom) [101].
32
2.7
X-Ray Powder Diffraction (XRD)
X-Rays were discovered by Wilhelm Röntgen in 1895. They are
electromagnetic radiation with wavelengths of the order of 10-10 m and are typically
generated by bombarding a metal with high-energy electrons. While the phenomena
of diffraction is the interference caused by an object in the path of waves. It occurs
when the dimensions of the diffracting objects are comparable to the wavelength of
the radiation. The pattern of varying intensity that results from the phenomena is
called the diffraction pattern [98].
XRD is an instrumental technique that is used to identified minerals, as well
as other crystalline materials. XRD provides the researcher with a fast and reliable
tool for routine mineral identification. Other information obtained can include the
degree of cystallinity, the structural state, possible deviations of the minerals from
their ideal compositions, and degree of hydration for minerals that contain water in
their structure.
In X-ray powder diffractometry, X rays are generated within a sealed tube
that is under vacuum. A current is applied that heats a filament within the tube. A
high voltage typically 15-60 kilovolts is applied within the tube. This high voltage
accelerates the electrons which then hit a target, commonly made of copper and Xrays are produced. These X-rays are collimated and directed onto the sample, which
has been ground to a fine powder. A detector detects the X-ray signal, which then
processed either by a microprocessor or electronically, converting the signal to a
count rate. Changing the angle between the X-ray source, the sample, and the
detector at a controlled rate between preset limits is an X-ray scan.
Figure 2.21 shows a simplified X-ray diffractometer which contain the X-ray
source (X-ray tube), X-ray detector, and the sample during and X-ray scan. In this
configuration, the X-ray tube and the detector both move through the angle (θ), and
the sample remains stationary [102].
33
Figure 2.21: Simplified X-ray diffractometer [102].
2.7.1 Theory of XRD
When X-ray radiation passes through matter, it interacts with the electrons in
the atoms resulting in scattering of the radiation. If the atoms are organized in planes
(i.e. the matter is crystalline) and the distances between the atoms are of the same
magnitude as the wavelength of the X-rays, constructive and destructive interference
will occur. This result in diffraction where X-rays are emitted at characteristic angles
based on the spaces between the atoms organized in crystalline structures called
planes.
There are many different sets of planes in crystal. Each set of planes has a
specific interplanar distance that will give rise to a characteristic angle of diffracted
X-rays [98]. The relationship between wavelength (λ), atomic spacing (d) and angle
was solved as the Bragg’s Law in Eq. 6. Figure 2.22 shows the pictorial
representative of the equation [102].
34
n λ = 2 d sin θ
(Eq. 6)
Where,
n = the order of the diffracted beam
λ=
wavelength of the incident X-ray beam
d=
the distance between adjacent planes of atoms (d spacing)
θ=
angle of the incidence X ray beam.
X-ray
Plane of
atoms
Figure 2.22: Pictorial view of Bragg’s Law [102].
Since λ is known and θ can be measured, then d-spacing can be calculated.
The characteristic set of d-spacings generated in a typical X-ray scan provides a
unique “fingerprint” of the material. When properly interpreted by comparison with
the standard reference patterns and measurements, this “fingerprint” allows for
identification of the material [102].
35
2.8
Flame Emission Spectroscopy (FES)
2.8.1
Basic theory and Flame Photometer
The early use of a flame as an excitation source for analytical emission dates
back to Herschel [103] and Talbolt [104], who identified alkali metals by flame
excitation. Flame emission spectroscopy (FES) is so named because of the use of a
flame to provide the energy of excitation to atoms introduced into the flame [105].
Flame spectrophotometry has been widely used in clinical application such as
analysis of cations in biological fluids and tissues, and also diagnosis and treatment
of many diseases. Besides, it also been used in the analysis of soils, plant materials,
plant nutrients, cement and glass [106].
The high stability of the flame source was recognized as the key to the
construction of simple instruments for the determination of easily excited elements
such as alkali metals, sodium and potassium [105]. This relies on the principle that
an alkali metal salt drawn into a non-luminous flame will ionize, absorb energy from
the flame and then emit light of a characteristic wavelength as the excited atoms
decay to the unexcited ground state. The intensity of emission is proportional to the
concentration of the element in the solution. A photocell detects the emitted light and
converts it to a voltage, which can be recorded. Since Na+ and K+ emit light of
different wavelengths, by using appropriate colored filters the emission due to Na+
and K+ (and hence their concentrations) can be specifically measured in the same
sample. Besides alkali metals, FES also can be used to analyze other elements such
as calcium, magnesium, iron, nickel and platinum [105,106].
Figure 2.23 shows the instrumental set up of a flame photometer. The
sequence processes occur in a flame photometer can be simply summarized as below
[105]:
1. Sample solution sprayed or aspirated as fine mist into flame. Conversion of
sample solution into an aerosol by atomizer.
2. Heat of the flame vaporizes sample constituents.
36
3. By heat of the flame and action of the reducing gas (fuel), molecules and ions
of the sample species are decomposed and reduced to give atoms.
eg. Na+ + e- --> Na
4. Heat of the flame causes excitation of some atoms into higher electronic
states.
5. Excited atoms revert to ground state by emission of light energy, hν, of
characteristic wavelength; measured by detector.
Figure 2.23: Schematic diagram of a flame photometer [105].
2.8.2
Quantitative Analysis
Plot of emission intensity versus concentration of ionic species in the solution
being measured is linear over wide range but with deviation at both low and high
concentrations [105]. Figure 2.24 shows the plot of emission intensity versus
concentration.
37
Figure 2.24: Plot of emission intensity versus concentration [105].
The plot in Figure 2.24 shows the following occurrence:
1. At very low concentration, emission falls below expected due to inoization,
some atoms converted back to ions (eg. K --> K+ + e-).
2. Linear region.
3. Negative deviation at high concentration due to self-absorption. Photons
emitted by excited atoms partly absorbed by ground state atoms in flame.
CHAPTER 3
EXPERIMENTAL
3.1
Instrumentations
ESR spectra in this work were recorded on a JEOL JES-FA 100 spectrometer,
operating at X-band frequencies and 100 kHz, interfaced to a computer with JEOL
system software. The ESR sample tube was made of quartz with diameter of 2.0 mm.
The peak intensity and g value were automatically calculated by the data analysis
progam. In-situ photolysis was carried out in the cavity of ESR spectrometer by
using JEOL Ultraviolet radiation, ES-USH 500 Hg lamp, 500 W.
The gas chromatography (GC) was performed by Hewlett-Packard
chromatometer Model 6890. Tetrahyrofuran (THF), or dichrolometane (CH2Cl2), or
ethyl acetate (EtOAc) was used as solvent with injection amount of 2 µL. Helium gas
was used as the mobile phase while column Ultra 1 (100% polymethylsiloksane)
with 0.11 µm thickness, 25.0 m length and internal diameter of 0.20 mm was used as
the stationary phase. The column was operated from the temperature of of 50oC
(maintained for 5 minutes) up to 250oC with the rate 8oC/min.
The MS spectra were taken using coupled GC-MS Agilent Technologies
spectrometer Model G 1540 N (GC) and G 2579 A (MS) with identical operation
condition as in the GC analysis. Nucleus Magnetic Resonance spectrum for proton
(1H NMR 400 MHz) was recorded with a Bruker spectrometer with deuterated
chloroform (CDCl3) as solvent. Plate Merck pre-coated silica gel F254 with 2.0 mm
thickness was used in the Thin Layer Chromatography (TLC). Silica gel Kieselgel
39
Merck with particle size 70-230 mesh was used as packing material for Gravity
Column Chromatography (CC).
X-ray diffractograms were obtained using D 500 Siemens Kristalloflex
automated powder X-ray diffractometer with CuKα1 as the radiation source with λ =
1.548 Å at 40 kV and 30 mA. All zeolite samples were measured in the range of 2θ
of 2 to 60 degree at room temperature with step intervals of 0.02 degree and scan
speed of 4 deg. min-1. The reflection position, d value and peak intensity were
calculated automatically by the data analysis program. Elemental analysis of Na in
zeolites were done by using Jenway Flame Photometer (Model PFP 7).
3.2
Chemicals
NaY zeolite (SiO2/Al2O3 = 5.1, unit cell size = 24.65 Ǻ, surface area = 900
2
m /g) was purchased from Zeolyst International. CH2Cl2 was purchased from
MERCK and used after purification. Triethylamine (TEA), acetophenone (AcP),
hexane, LiNO3 and KNO3 were obtained from MERCK. 2-Cyclohexenone (CH),
vinyl acetate (VA), 1,2-diphenyletane (DPE), RbNO3 and CsNO3 were purchased
from Fluka. Purified THF was obtained from Ajax Chemicals. HF (49 %) was
obtained from Clean Room® Electronic Chemical. Ethyl acetate (EtOAc) and HCl
(37 %) were obtained from Ashland and dehydrated MgSO4 was purchase from GCE
Loboratory Chemicals.
3.3
UV Irradiation of H2 in NaY Zeolite
A sample of NaY zeolite (50 mg) was activated at 300oC for 3 hours in a
specially designed pyrex cell attached with ESR sample tube (Figure 3.1) under
vacuum (10-4 Torr). Figure 3.2 shows the vacuum line used for all sample
activations and sample degas in this research. In this experiment, specially designed
pyrex cell replaced the sample activation bulk and the purified hydrogen was
40
introduced through the joint 2. Purified hydrogen was used straight from the tank and
added to the activated sample at room temperature. Typical pressures of ~ 100 Torr
were used. The pyrex cell was then taken off from the vacuum line and the activated
zeolite was transferred to the ESR quartz tube by tilting and gently tapping the cell.
This tube was then inserted into the ESR cavity and UV irradiated. The ESR spectra
were recorded every 3 minutes until 15 spectra were obtained at 298 K. The ESR
spectra showed 2 singlet peaks at 321.56 and 322.59 mT which correspond to g value
of 2.0073 and 2.0008 .
Ball joint
ESR tube
Valve
B 14 joint
Pyrex
activation
bulb
Figure 3.1: Sample cell for activation and UV irradiation.
41
Teflon
valve
Mercury
monometer
Figure 3.2: Vacuum line used for sample activation and sample degassing.
3.4
ESR Study of the Photosensitization of Triethylamine by Acetophenone in
NaY Zeolite
NaY zeolite (200 mg) was activated under vacuum at 400oC for 3 hours. The
zeolite was then added to a solution of acetophenone (AcP) (50 mg) in CH2Cl2 (2.5
mL) inside the glove box and stirred overnight. After filtration, the zeolite was then
added to a solution of triethylamine (TEA) (50 mg) in dichloromethane (2.5 mL) and
stirred overnight. The sample was then filtered and dried in air. The dried sample of
NaY zeolite (40 mg) which contained AcP and TEA was degassed under vacuum
(10-4 Torr) with a specially designed pyrex cell (Figure 3.1). The cell was then taken
off from the vacuum line and transferred to an ESR quartz tube by tilting and gently
tapping the cell. The sample tube was then inserted into the ESR cavity and UV
irradiated. The ESR spectra of zeolite-AcP, zeolite-TEA, and zeolite-AcP-TEA were
recorded after 1 hour of photolysis at room temperature.
The ESR spectrum of zeolite-AcP showed a singlet peak at 322.83 mT while
no ESR peak was observed for the zeolite-TEA sample. Zeolite-AcP-TEA sample
42
showed multiplet peaks at 319.21, 321.60, 322.13, 323.32, and 325.02 mT (A = 1.8
mT, g = 2.0072) and a singlet peak at 322.128 mT (g = 2.0023) in its ESR spectrum.
3.5
Preparation of Alkali Metal Cation-Exchanged Y Zeolites (MY)
Nitrate solutions (0.5 M) of different cations (Li, K, Rb, and Cs) were
prepared by dissolving the nitrates (LiNO3, KNO3, RbNO3, and CsNO3) with
deionized water into volumetric flasks (200 mL). The nitrate solution (~65 –70 mL)
was then added to NaY zeolite (5 g) in a Teflon bottle (200 mL) and put into the oil
bath (90oC) with continued stirring for one hour. Then the zeolite was separated from
the nitrate solution with centrifuge, and washed with deionized water. The exchange
process was repeated three times. After filtration, the exchanged zeolites were
washed thoroughly with distilled water and dried at 120oC for overnight. The dried
samples were then weight and characterized using XRD.
All the samples were saturated over concentrated NH4NO3 solution prior to
XRD measurement in order to ensure complete hydration. The sample was ground to
fine powder using pestle and mortar before mounting it on the sample holder. The
XRD profile of the zeolites was compared with the simulated XRD pattern of
standard. The crystallinity of the cation-exchanged zeolites were calculated using the
intensity of the 3 reflection peaks, namely {533}, {642} and {840}.
3.5.1
Quantitative Analysis of the Cation-Exchanged Y Zeolites
The samples of M+Y zeolite (M = Na, Li, K, Rb, and Cs) (~ 1 g) were dried
at 120oC for 24 hours. The dried samples (0.1 g) were dissolved using HF 49% (5
mL) in a plastic beaker (5 mL). The diluted solution were then added with deionized
water into a plastic volumetric flask (100 mL). Solution of HF was prepared as
blank. The cation solutions were then sent for Na analysis using Flame Photometer.
43
The exchanged level (%) of cations was calculated depending on the replacement of
Na cation in various MY zeolites.
3.6
Photochemical Hydrogen Abstraction by Acetophenone (AcP) in Toluene
Solution and NaY Zeolites Slurry
3.6.1
Homogeneous Reaction
Toluene solution of AcP (0.1 M, 5 mL) was added into a pyrex tube. The
sample was flushed with purified argon gas for 1 hour prior to UV-irradiation in
positive pressure condition. Figure 3.3 shows the experimental set up for the
irradiation of sample in homogeneous solution and zeolite-solvent slurry. The sample
was then UV-irradiated for 5 hours with continuous stirring in an inert condition. The
conversion rate was determined using GC with m-xylene as an external standard. The
sample was then concentrated under pressure using rotary evaporator and to give
transparent liquid (40 mg).The GC chromatogram of the concentrated sample
showed 5 peaks with retention time (Rt ) values of 18.62 , 19.60, 20.85 , 23.19, and
23.29 minutes.
Peak 1 (Rt 18.62 minutes, 31.0 %); MS: m/z 182 [M+, C14H14], 165, 152, 91,
77, 65, 51, 41.
Peak 2 (Rt 19.60 minutes, 2.0 %); MS: m/z 194, 179, 165, 116, 103, 91, 77,
65, 51.
Peak 3 (Rt 20.85 minutes, 37.0 %); MS: m/z 194 [M+, C15H16O - H2O], 179
[194- CH3], 165, 152, 139,121, 105, 91, 77, 65, 51, 43.
Peak 4 (Rt 23.19, 13.0 %); MS: m/z 210 [M+, C16H18O2 – H2O – CH3], 195
[210 – CH3], 181, 165, 121, 105, 91, 77, 65, 51, 43.
44
Peak 5 (Rt 23.29 minutes, 17.0 %); MS: m/z 210 [M+, C16H18O2 – H2O –
CH3], 195 [210 – CH3], 181, 165, 121, 105, 91, 77, 65, 51, 43.
Silicon oil
Positive
pressure
Dry gas
Dried
silica gel
Pryrex
tube
UV source
A
L
U
M
I
N
I
U
M
Solvent S
H
Zeolite E
E
T
Stirrer hot plate
Con.
H2SO4
Argon
Figure 3.3: Experiment set up for UV irradiations in homogeneous solutions and
zeolite-solvent slurries.
3.6.2
Isolation of Photoproducts
The reaction mixture was purified to further confirm the chemical structures
of the photoproducts.
45
3.6.2.1 Thin Layer Chromatography (TLC)
The sample was analyzed with TLC using CH2Cl2 (100%) as eluent. The
TLC showed 3 major components with Rf 0.76, 0.50, and 0.31.
3.6.2.2 Gravity Column Chromatography (CC)
The mixture (40 mg) was purified using CC. the gravity column (internal
diameter 1.5 cm, height 10.0 cm) was packed with silica gel (7 g) in CH2Cl2 as eluent
and 200 fractions were collected. Every fraction was analyzed by TLC. Fractions
with the same TLC profile were combined and concentrated.
However, only a pure compound was able to isolate. The combined fractions
120-160 after evaporation gave PH (13.8 mg, 34.5 %) with Rf 0.30 in CH2Cl2 (100
%). 1H NMR δ (CDCl3): 1.53 (3H, s, -CH3), 1.61 (3H, s, -CH3), 2.30 (1H, s, OH),
2.60 (1H, s, OH), 7.20-7.29 (10H, m, Aryl-H).
3.6.3
Photoreaction in NaY Zeolite Slurry
Stock solution of acetophenone (AcP) in toluene (4 mg/mL) was prepared in
a volumetric flask (25 mL). NaY zeolite (300 mg) was activated under vacuum at
300oC for 3 hours. Solution of AcP (3 mL = 12 mg AcP) was then added into the
activated zeolite in a pyrex tube. After 3 hours stirring, the zeolite was washed twice
with toluene (10 mL) and the combined solution was concentrated. The concentrated
solution was analysed by GC to detect the absence of AcP. The sample was then
degassed by three freeze-pump-thaw cycles. The resulting sample was irradiated for
5 hours under continuous stirring. The irradiated sample was centrifuged to separate
the solvent and the zeolite. The zeolite obtained was then extracted with
tetrahydrofuran (THF) for overnight. The concentrated solvent layer (supernatant)
and the resulting extract was then analysed using GC.
46
The GC-MS results of the supernatant and the resulting THF extract were
summarized in Table 3.1 and Table 3.2 respectively. The product ratios were
calculated using peak area of product over peak area of total products.
Table 3.1: GC-MS analysis of the supernatants in the photochemical hydrogen
abstractions in NaY zeolite slurries.
. Peak
Retention time
% of Area
Rt (minutes)
Molecular
Molecular
formula
weight
1
18.08
12.78
-
182
2
18.62
76.95
C14H14
182
3
19.60
3.56
-
194
4
20.85
6.53
C15H16O
194
Table 3.2: GC-MS analysis of the tetrahydrofuran extracts in the photochemical
hydrogen abstractions in NaY zeolite slurries.
.
Peak
Retention time
% of Area
Rt (minutes)
Molecular
Molecular
formula
weight
1
18.62
13.00
C14H14
182
2
19.60
11.00
-
194
3
20.85
72.00
C15H16O
194
4
23.29
4.00
C16H18O2
242
3.7
Photodimerizations of 2-Cyclohexenone
3.7.1
Homogeneous Reactions
Solution of 2-cyclohexenone (CH) (1) in hexane (10 mg/mL) was prepared in
a volumetric flask (50 mL). The solution (5 mL) was transferred into a pyrex tube.
The sample was flushed with purified argon gas for 30 minutes prior to UVirradiation in back pressure condition. The sample was UV-irradiated for 5 hours
47
with continuous stirring. Another 5 mL solution of CH (1) in hexane was UV
irradiated for 5 hours in an open-air condition as the control experiment. The
reaction mixture was then concentrated under reduced pressure using rotary
evaporator and analysed with GC and GC-MS. The conversion rates for both the
experiments were determined using GC with AcP as an external standard.
The GC analysis of the photoproducts showed the presence of four peaks,
Peak 1-4, each with Rt values of 23.40, 23.60, 23.89, and 24.12 minutes. All these
four peaks gave the same molecular ion peaks with similar ion fragmentation
patterns in the MS analysis; MS: m/z 192 [M+, C12H16O2], 175, 164, 149, 136, 121,
108, 96, 79, 68, 55, 41. Table 3.3 shows the obtained GC peak ratio of the
photoproducts in this experiment.
Table 3.3: GC peak ratios of the photoproducts in the photodimerizations of 2cyclohexenone in homogeneous reactions.
Condition
Peak 1
Peak 2
Peak 3
Peak 4
Inert
0.12
0.61
0.10
0.16
Open air
0.11
0.65
0.07
0.17
3.7.2
Solid State Photoreactions in Cation-Exchanged Y Zeolites
The MY zeolite (M = Na, Li, K, Rb, and Cs) (300 mg) was activated under
vacuum at 300oC for 3 hours. Hexane solution of CH (1.5 mL = 15 mg CH) prepared
in Section 3.5 was added into volumetric flask (5 mL). An additional hexane was
added until the solution reached a volume of 5 mL. The activated MY zeolite and the
diluted solution of CH were then added into centrifuge tubes (50 mL) and stirred for
3 hours. The zeolite was washed twice with hexane (10 mL) and the combined
washings were concentrated. The concentrated solution was analysed with GC to
detect the absent of CH. The MY zeolite containing CH (CH-MY) were dried under
vacuum (10-4 Torr) for 2 hours in a pyrex tube. The magnetically stirred dry powder
sample was irradiated for 5 hours in the pyrex tube (magnetically stirred) in vacuum
48
condition. Figure 3.4 shows the experiment set up of solid state photoreaction. The
irradiated zeolite was treated with HCl (1 N) and extracted with EtOAc. The
resulting extract was then analysed with GC and GC-MS.
The GC chromatograms showed 7 peaks with Rt 23.40, 23.60, 23.89, 24.12,
24.54, 25.66 and 25.87 minutes. Peak 1-6 gave the same molecular ion peaks with
similar fragmentation patterns in the MS analysis; MS: m/z 192 [M+, C12H16O2], 175,
164, 149, 136, 121, 108, 96, 79, 68, 55, 41. With the similar fragmentation patterns
with Peak 1-6, Peak 7 gave molecular ion peak at m/z 207 with molecular formula
C12O12H16. Table 3.4 summarizes the GC peak ratios of the photoproducts obtained
in photoreactions carried out in different MY zeolites.
To vacuum pump
Teflon valve
Rubber stopper
A
L
U
M
I
N
I
U
M
UV source
Zeolite
Stirrer hot plate
S
H
E
E
T
Figure 3.4: Experiment set up for the solid state photoreactions.
49
Table 3.4: GC peak ratios of the photoproducts in the solid state photodimerizations
of 2-cyclohexenone carried out in different cation-exchanged Y zeolites.
MY
Peak 1
Peak 2
Peak 3
Peak 4
Peak 5
Peak 6
Peak 7
LiY
0.18
0.10
0.06
0.50
0.05
0.10
0.02
NaY
0.17
0.15
0.09
0.31
0.10
0.15
0.04
KY
0.05
0.11
0.01
0.71
0.04
0.07
0.01
RbY
0.02
0.05
-
0.81
0.02
0.06
0.03
CsY
0.03
0.05
0.02
0.80
0.02
0.06
0.04
zeolite
3.7.3
Photoreactions in Cation-Exchanged Y Zeolite-Slurries
The MY zeolite (M = Na, Li, K, Rb, and Cs) (300 mg) was activated under
vacuum at 300oC for 3 hours. Hexane solution of CH (1.5 mL = 15 mg CH) prepared
in Section 3.5 was added into a volumetric flask (5 mL). An additional hexane was
added until the solution reached a volume of 5 mL. The activated MY zeolite and the
diluted solution of CH were then transferred into centrifuge tubes (50 mL) and stirred
for 3 hours. The zeolite was washed twice with hexane (10 mL) and the combined
washings were concentrated. The concentrated solution was analysed with GC to
detect the absent of CH. Hexane (5 mL) was then added to the washed sample in a
pyrex tube. After purging with argon for 30 minutes, the magnetically stirred
translucent MY-hexane slurry was irradiated for 2 hours. After the irradiation, the
hexane layer was separated and concentrated for GC analysis. The irradiated zeolite
was then dissolved with HCl (1N) followed by EtOAc extraction. The extract was
analyzed using GC and GC-MS.
The GC chromatograms showed 8 peaks with Rt 23.40, 23.60, 23.89, 24.12,
24.54, 25.66, 25.87 and 26.93 minutes. Peak 1-6 gave the same molecular ion peaks
with similar fragmentation patterns in the MS analysis; MS: m/z 192 [M+, C12H16O2],
175, 164, 149, 136, 121, 108, 96, 79, 68, 55, 41. Peak 7 gave molecular ion peak at
m/z 207 with molecular formula of C12O12H16. and Peak 8; gave the molecular ion
50
peak at m/z 277 with other fragment ions at m/z 222, 204, 160, 149, 135, 121, 104,
93, 76, 65, 57, 50. Table 3.5 summarizes the GC peak ratios obtained in these
photoreactions.
Table 3.5: GC peak ratios of the reaction mixture obtained in the photodimerizations
of 2-cyclohexenone carried in cation-exchanged Y zeolite-slurries.
MY
Peak 1
Peak 2
Peak 3
Peak 4
Peak 5
Peak 6
Peak 7
Peak 8
zeolite
LiY
0.14
0.05
0.06
0.32
0.14
0.10
0.08
0.12
NaY
0.28
0.06
0.07
0.16
0.19
0.16
0.06
0.02
KY
0.03
0.06
0.02
0.74
0.04
0.01
0.06
0.04
RbY
0.05
0.06
0.03
0.71
0.03
0.07
0.04
0.01
CsY
0.05
0.05
0.03
0.69
0.04
0.07
0.04
0.02
3.8
3.8.1
Photocycloaddition of 2-Cyclohexenone to Vinyl Acetate
Homogeneous Photoreaction
A hexane solution containing CH (1 g, 0.01 mol) and vinyl acetate (VA)
(12.8 g, 0.15 mol) was prepared in a volumetric flask (50 mL). The solution (10 mL)
was added into a pyrex tube and flushed with purified argon gas for 30 minutes prior
to 5 hours of irradiations in the inert condition. The sample was stirred magnetically
during the photolysis. The irradiated sample was then concentrated under pressure
using rotary evaporator. The resulted solution was then analyzed using GC and GCMS.
The GC chromatograms of the concentrated sample showed the presence of 5
peaks with Rt values of 19.58 (30.0 %), 19.71 (18.0 %), 19.83 (8.0 %), 19.94 (15.0
%), and 20.10 (29.0 %) minutes. These five peaks gave the same molecular ion peaks
and similar fragmentation patterns; MS: m/z 158, 139 [M+, C10H14O3 –COCH3], 122
[M+- HOCOCH3], 111, 97, 84, 79, 55, 43.
51
3.8.1.1 Acid Test
A small portion of the mixture (2 mL) was taken for the acid test.
Evaporation of the solvent gave a mixture of photoproducts which were redissolved
in HCl (1N) (5 mL) and hexane (1 mL), stirred for 15 minutes. The organic layer
was separated, dried over anhydrous MgSO4 and analyzed by GC. The GC
chromatogram showed no different in the peak profile or peak ratio as before the acid
test.
3.8.2
Photoreactions in Cation-Exchanged Y Zeolite-Slurries
The MY zeolite (500 mg) were activated under vacuum at 300oC for 3 hours.
Hexane solution (0.5 mL) containing CH (10 mg/mL) and VA (128 mg/mL) were
added into volumetric flask (10 mL). An additional hexane was added until the
solutions reached a volume of 10 mL. The activated MY zeolite and the diluted
solution were then transferred into a pyrex tube and stirred for 3 hours. The zeolite
was washed twice with hexane (10 mL) to get rid of excess VA in solution. The
washings were combined, concentrated and analyzed with GC. After purging with
argon for 30 minutes, the magnetically stirred sample (CHVA-MY-hexane slurry)
was irradiated for 5 hours under inert gas condition. After the irradiation, the hexane
layer was separated and concentrated for GC analysis. The irradiated zeolite was then
dissolved with concentrated HCl and isolated using EtOAc. The extract was then
analyzed using GC.
The GC chromatograms showed 5 peaks with Rt 19.58, 19.71, 19.83, 19.94,
and 20.10 minutes. These 5 peaks gave the same molecular ion peaks and similar
fragmentation patterns; MS: m/z 158, 139 [M+, C10H14O3 –COCH3], 122 [M+HOCOCH3], 111, 97, 84, 79, 55, 43. Table 3.6 summarizes the individual peak ratios
obtained in these photoreactions.
52
Table 3.6: GC peak ratios of the photoproducts in the photocycloadditions of 2cyclohexenone to vinyl acetate in cation-exchanged Y zeolite-slurries.
MY zeolite
Peak 1
Peak 2
Peak 3
Peak 4
Peak 5
LiY
NaY
KY
RbY
CsY
0.21
0.25
0.21
0.34
0.21
0.05
0.06
0.05
0.05
0.06
0.28
0.24
0.29
0.21
0.25
0.08
0.09
0.08
0.05
0.06
0.38
0.36
0.37
0.35
0.42
CHAPTER 4
RESULTS AND DISCUSSION
4.1
ESR Study of the UV Irradiation of H2 in NaY Zeolite
Paramagnetic probes may localize in sites characterized by different
environmental mobility and polarity at the zeolite surface. With electron spin
resonance (ESR) technique the precise structural and dynamical information about
the probe and their environments can be studied by means of an accurate analysis of
the spectral line shape [107]. Under conditions where the exchange rate among
different sites is slow, the ESR signals at each site will contribute to superimposed
adsorptions of overall spectra [108].
Due to their ability to interact with the surface sites, both paramagnetic metal
ions [109, 110] and nitroxide radicals [108, 111-114] can be used as good spin
probes in zeolite X and Y. The viscosity and the rotational motion of the adsorbed
radicals depend on the dehydration degree of the zeolite, the nature of the support
surfaces and the characteristic of the spin probe molecules [111].
In this research, locations of paramagnetic probe (H radical) in different
adsorption sites of NaY zeolite was studied using Electron Spin Resonance
spectroscopy in this section.
Non-irradiated sample (Figure 4.1) of H2 in NaY zeolite gives only one
singlet peak with the g value of 2.0001 is observed. This peak may due to the free
electron from the defect sites on the zeolite framework. The other six peaks with the
54
A value 8.5 mT are distributed by the Manganese marker (I of Mn = 5/2) which was
inserted as standard during recording of the spectrum.
g = 2.0001
A
mT
Figure 4.1: ESR spectrum of H2 in NaY before UV irradiation.
The resulting ESR spectrum for UV irradiation of H2 in NaY zeolite
supercages after 45 minutes is shown in Figure 4.2. It gives 2 singlet peaks, with
Peak 1 and Peak 2 corresponding to g values of 2.0008 and 2.0073. During UV
irradiation, the H2 molecule is homolytically cleaved and the resulting H• radicals,
being highly unstable on the zeolite surface, are easily ionised to produce H+ and a
free electron. This process is given by the following equations:
H2
2H•
hv
(Eq. 7)
2 H•
2H+ +
2e-
(Eq.8)
The free electron on proton is not seen as isotropic doublet lines (which
indicates the hyperfine interaction between the free electron and a proton with I = ½)
in the spectrum. This is further supported that the H radicals are ionised according to
equation (8).
55
g = 2.0073, Peak 2
Peak 1, g= 2.0008
mT
Figure 4.2: ESR spectrum of UV irradiation (after 45 minutes) of H2 in NaY zeolite
supercages.
Broussard et al. [115] have proved that both hydrated and dehydrated zeolites
are known to present a locally homogeneous distribution of cations and surface
groups (both O- and OH-), and it was proved that the FAU cavities bear different
adsorption sites [116].
From literatures [2, 117], the charge compensating cations are known to be
occupied in three main positions in FAU zeolites (zeolite X and zeolite Y). Type I
cations with 16 per unit cell are located on the hexagonal prism faces between the
sodalite cages. Type II, with 32 per unit cell is located in the open hexagonal faces.
While the type III cations, with 38 per unit cell in zeolite X and only 8 per unit cell in
the case of zeolite Y, is located on the walls of the large cavity. Cations in the zeolite
exhibit a high mobility and are not rigidly located at their cations position, thus
cations also can be found in sites I’ and site II’ in the sodalite units which are
slightly displaced from the ideal positions. These extraframework cation sites are
indicated in Figure 4.3. Only cations of site II and III in supercages are readily
accessible to organic molecules. It is because the site I cations enjoy an octahedral
coordination of six oxygens are well shielded from the guest molecules. The
locations of the cations also vary depending on the presence of water and other
molecules [2, 118, 119]. For the case of NaY, diffraction measurements have
56
determined that mainly two sites are occupied by Na+ cations in the dehydrated
zeolite Y: site I and site II [120].
Site III
Site I
Site II
Figure 4.3: Stucture of the FAU zeolite with cation position site II and site III in the
supercages, site I’ and site II’ in sodalite units, and site I in the centers of
hexagonal prisms [118].
From the 23Na synchronized double-rotation NMR spectra the cations on site
I are more shielded than site II [117]. Therefore, we can assign Peak 1 represented
the free electrons adsorb at sites II and Peak 2 represented the free electrons stayed at
sites I. Since, we do not observed any extra peaks of isotropic lines, the interaction of
unpaired electron and Na+ cation (I = 3/2) can be ignored. This is further proved by
Gutjahr et al. [114] who claimed the conventional continuous wave (CW) ESR is not
sufficient to resolve the hyperfine interaction of the unpaired electron with alkali
metal ions in Y zeolite.
Figure 4.4 shows Peak 1 and Peak 2 (refer to Figure 4.2) intensity plot
against UV irradiation time. From the graph, it shows that the electrons diffused from
site II to site I when the UV irradiation time was increased after 19 minutes.
However, the intensity of Peak 1 (1200-1350) is higher than Peak 2 (0-300). This
indicates that the electrons preferred to stay at sites II than sites I. The electric field is
stronger at sites II than at sites I and the cations at sites II are poorly shielded.
57
Therefore, the radicals should preferentially interact at site II where the free electrons
Peak 1 Intensity
experience a rather strong spin polarization.
1340
1320
1300
1280
1260
1240
1220
1200
0
10
20
30
40
50
UV Irradiation Time (min)
Peak 2 Intensity
(a)
300
250
200
150
100
50
0
0
10
20
30
40
50
UV Irradiation Time (min)
(b)
Figure 4.4: (a) Peak 1 intensity and (b) Peak 2 intensity against UV irradiation time.
58
4.2
An ESR Investigation of Amine Dimers Radical Cation in the
Photosensitization of Triethylamine by Acetophenone in NaY Zeolite
Supercages
One of the intermolecular photoreactions have been studied within zeolite
was the use of triplet sensitizer. In order to study this approach, the reaction
intermediate in the photosensitization of triethylamine (TEA) by AcP in NaY zeolite
was further investigated by using ESR.
Detailed mechanism were devised to explain the range of products and the
overall kinetic course of the reaction. ESR spectroscopy has extended mechanistic
studies since radical intermediates can now directly be detected and identified, and
the intermediate rates of decay and the interconversion of the radical can be
measured. Photolysis is probably the most common and versatile method to generate
radicals. Using a microwave cavity field with grating designed to let light shine onto
the cell, but not to perturb the microwaves, in-situ photolysis becomes quite simple.
The technique is very effective because high stationary concentration of the radicals
could be accumulated even when they are lost by diffusion controlled radical-radical
reactions [97, 101].
The photosensitizations of aliphatic amines by acetophenone inside NaY
zeolite have been studied by Scaino et al. [26] using laser flash photolysis (308 nm).
The results showed a distinctive pattern including detection of amine dimer radical
cations which were not observed for the same reaction in polar solvents. Analysis of
the products have revealed the formation of hydrazines. In this study ESR
spectroscopy is used instead of laser flash photolysis to study the free radical
mechanisms of the photosensitization of amines occurred inside zeolite NaY.
The disadvantage of laser flash photolysis is that the optical spectra are often
very broad, and hence may lack the detail needed to give firm identification. This
means that arguments based on kinetic studies, product analysis and simply chemical
expectation have often been used to identify a given intermediate [101]. Hopefully
with ESR, the mechanism could be further established.
59
The sample NaY zeolite containing AcP and TEA was prepared by sequential
adsorptions. In- situ UV irradiations were carried out on the dried sample in ESR
spectrometer.
The ESR spectrum of AcP-zeolite (Figure 4.5) showed only a singlet peak
with g = 1.9996 indicated the AcP triplet state, AcP*. AcP was reported to have its
two triplets so close in energy that they may be inverted simply by adjusting the
polarity of the solvent. In non-polar solvents the lowest triplet state of AcP is known
to be nπ* not ππ* which is slightly higher in energy than nπ* [121]. In hydrocarbon
glass, the emission from AcP is characteristic of the nπ* triplet, whereas in polar
hydrogen-bonding media such as silica gel, a long lived (~ 300 ms) emission
characteristic of the ππ* triplet is observed [122,123]. Shailaja et al. [124] have
reported the nature of the emitting triplet in AcP switches from the nπ* (nonpolar
media) to ππ* within zeolites. In LiY, NaY, and KY zeolites, the lowest triplet state
is identified as ππ*, whereas in RbY and CsY, two emissions characteristic of nπ* and
ππ* were observed. In this case, the cation-carbonyl interaction, not polarity was
reported responsible for the state switch. Beside, the nature and reactivity of the AcP
triplet also was found strongly depends on the phenyl ring substitution [125]. When
further experiment was performed on TEA-zeolite, no ESR peak was observed
(Figure 4.6). This may imply that TEA could not be easily excited.
O
g = 1.9996
CH3
AcP*
mT
Figure 4.5: ESR spectrum of UV photolysis (after1 hour) of AcP in NaY zeolite.
60
mT
Figure 4.6: ESR spectrum of UV photolysis (after1 hour) of TEA in NaY zeolite
When the zeolite sample containing AcP and TEA was UV irradiated, the
resulting ESR spectrum (Figure 4.7) shows several prominent peaks. Albeit
complex, the spectrum could be characterized with detail analysis. The five peaks at
318.04, 319.81, 321.60, 323.32, and 325.02 mT (labeled as “x” in the spectrum) with
hyperfine splitting constant, A = 1.8 mT are assigned to amine dimer cation radical (I
(spin) for nitrogen atom = 1). The total of lines observed for the amine dimer cation
radical are in agreement with the formula N = 2nI + 1, where ‘N’ is the number of
line and ‘n’ is the number of atoms with spin I. Even though the intensity of the
peaks does not exactly give the ratio 1: 2: 3: 2: 1 as suggested by Pascal’s Triangle
Theory, it could still be accepted considering the presence of anisotropic effect in a
solid state.
mT
Figure 4.7: ESR spectrum of UV photolysis (after1 hour) of AcP + TEA
in the NaY zeolite supercages.
61
The appearance of the peak with g value of 2.0023 (value for free electron)
suggests that upon light absorption, a single electron transfer (SET) from the amine
to AcP triplet excited state was taken place inside NaY zeolite supercages. In this
case, the TEA acts as electron donor while the AcP as the acceptor. The process of
SET of TEA was photoinduced by AcP (photosensitizer) for it to achieve the excited
triplet state. No peaks corresponding to TEA radical cation was detected in the ESR
spectrum. This could be due to its short lifetime.
The obtained results are comparable to the same reaction studied by Scaino et
al. [26] using laser flash photolysis. Scheme 4.1 shows the proposed mechanism
[26]. They reported that the most remarkable observation of the AcP-amine system
inside zeolite is the detection of radical cation of amine dimer. These species have
not been observed in solution, except in the case of rigid diamine holding the two
nitrogen atoms in close proximity [26, 126].
This reflected the fact that the
photolysis was taking place in a confined space. Thus, for those amine radical cation
sharing the zeolite cage with another molecule of neutral amine, the interaction
[N….N]+• has to be favored as the result of mobility and conformation restriction
imposed by the rigid framework. This so-called internal-pressure effect was
previously observed for charge-transfer complexes [127, 128]. The distinctive
behavior of NaY zeolite arises from the combined contribution of a polar
environment stabilizing positively charged intermediates and confined reaction
cavity favoring aggregation of amine radical cation.
O
CH3
hv
H
AcP
*
OH
AcP
CH3
AcP
Et N Et
Et
Et
N Et
Et
Et
Et N
Et
Et
N Et
Et
Scheme 4.1: The proposed mechanism of amine photosensitization by AcP inside
NaY zeolite supecages [26].
62
4.3
Alkali Metals Cation-Exchanged Y Zeolites
In order to study the size constriction effect and cation-guest interactions
played by different cations in the photoreaction, NaY zeolite was further exchanged
with alkali metal cations (Li, K, Rb, and Cs) from the respective nitrate solutions.
Figure 4.8 shows the X-ray diffractograms of the alkali metal cationexchanged Y zeolite, MY (M= Li, K, Rb, and Cs) compared to NaY zeolite. The
XRD profile of the parent NaY sample and all of its modified forms essentially
showed that the characteristic peaks closely match those of the reported data [129].
The relative intensities of the XRD peaks of cation-exchanged zeolites were found to
be affected to different extents depending on the nature and the concentration of
nonframework cationic size without any significant shift in the positions of
reflections [118].
The influences of the extraframework cationic size on the change in the
relative intensities of the characteristic peaks were also examined assuming identical
site occupancies of the different cations. The plane’s values of the Y zeolite were
obtained from literature [129]. The relative intensity of the peak on the plane {642},
2θ = 27.04o was found to be least affected by the size of the cation. However, the
decrease in the relative intensity due to plane {111}, 2θ = 6.27o and an increase in
the relative intensity due to plane {733}, 2θ = 29.63o with increased cationic size
were observed. The variation in peak intensity may be attributed to the higher/lower
scattering power of X-rays because of the variation in the charge-to-size ratio of
cationic species and the framework distortion to some extent [118, 130].
The diffractograms of the samples also showed an increased background
from LiY to CsY (compared to parent NaY), indicating an increased amorphous
fraction in the material. It may due to the increasing of basicity with the decreasing
electronegativity of the alkali cation.
63
3000
2900
2800
2700
2600
X
Relative Intensities (Cps)
2500
2400
2300
X
2200
2100
2000
X
1900
1800
NaY
1700
1600
1500
1400
1300
LiY
1200
1100
1000
900
KY
800
700
600
500
RbY
400
300
200
CsY
100
2
10
20
30
40
50
60
2-Theta - Scale
Figure 4.8: X-ray diffractograms of the alkali metal cation-exchanged Y zeolites
compared to parent NaY zeolite.
64
Graph of crystallinity versus various MY zeolites (Figure 4.9) was plotted
using average value (Cps) of total the intensities on plane {533}, {642} and {840}
correspondence to 2θ 23.65o, 27.04o, and 32.46o of the parent NaY. These peaks
were marked by a “X” in Figure 4.8.
Crystallinity
600
500
400
300
200
100
0
NaY
LiY
KY
RbY
CsY
Cation-exchanged Y zeolites
Figure 4.9: Crystallinity versus cation-exchanged Y zeolites.
The degree of ion exchange was determined by flame emission photometry
(FEP). The exchange levels were calculated based on the replacement of Na cation in
the zeolites (assuming that NaY was 100% ion exchanged). Figure 4.10 is the
calibration line of Na FEP analysis. Table 4.1 shows the concentration of Na+ in the
Emission Intensity
exchanged zeolites and its corresponding exchange levels.
1
y = 0.0069x + 0.0911
R2 = 0.9984
0.8
0.6
0.4
0.2
0
0
50
100
150
Concentration (mg/L)
Figure 4.10: Emission intensity versus concentration of Na analysis in flame
emission photometry .
65
Table 4.1: Exchanged levels of alkali metal cations exchanged Y zeolites.
MY zeolites
Emission Intensity
NaY
LiY
KY
RbY
CsY
0.584
0.208
0.031
0.211
0.263
Na+ concentration
(mg/L)
71.434
12.590
Negligible
17.377
20.565
Exchange level
100.0%
82.4%
~ 99%
75.7%
71.3%
The exchanged levels and distributions of cation in Y zeolite are very much
depends on the size and the nature of the cations. The exchange levels of RbY
(75.7%) and CsY (71.3%) are relatively low compared to LiY (82.4%) and KY
(almost all the Na+ have been replaced) which has been explained by their inability
to replace the site I cation in the hexagonal prism due to the increase in atom radius
of Rb+ and Cs+[119,131]. However, the exchange levels of 80 % have been achieved
in conventional exchange of NaY with Rb+ and Cs+ [132,133]. It was reported that
the Li+ ions are not preferred in exchanged the cations in site I and site I’ due to its
hydrated ions which has bigger size compared to K+ ions and weak coulombic
interactions between the hydrated counterions and anionic sites [131]. It gives the
reason of the low exchange level of Li+ compared to K+.
4.4
Photochemical Hydrogen Abstraction by AcP in Toluene Solution and
NaY Zeolites Slurry
In this part, we continued our study to another important approach in the
intermolecular photoreaction within zeolite, the “spectator” approach. This approach
was studied in the photochemical hydrogen abstraction by AcP in zeolite-toluene
slurry.
4.4.1
Homogeneous Photoreaction
Photochemical hydrogen abstraction by AcP (8) in toluene (9) homogeneous
solution and NaY zeolite slurry have been studied in this research.
The
66
homogeneous reaction gave 95 % conversion after 5 hours of irradiation under inert
condition. Figure 4.11 shows the GC chromatograms before and after the hydrogen
abstraction reaction. The GC analysis of the photoproducts showed the presence of 4
significant peaks, each with Rt value of 18.62, 20.85, 23.19, and 23.29 minutes.
Toluene
AcP
(a) Before
photoreaction
Peak 1
m-Xylene
Peak 2
Toluene
Peak 3 & 4
Unknown
product (14)
(b) After
photoreaction
AcP
5
10
15
20
25
30
min
Figure 4.11: GC chromatograms (a) before and (b) after the homogeneous
photoreactions of acetophenone in toluene solution.
Peak 1 at Rt 18.62 minutes, gave a molecular ion peak at m/z 182 (Appendix
1) in GCMS analysis which was in agreement with a molecular formula of C14H14.
The fragmentation pattern of this compound was matched with 1,2-diphenylethane
(DPE) (10) cited by the Wiley database of the GCMS system.
67
Peak 2 (Rt 20.85 minutes) which was the major peak in the mixture (37 %)
showed an ion peak at m/z 194 (Appendix 2). It’s fragmentation pattern was 90 %
matched with 2,3-diphenylpropan-2-ol (DPP) (11) which has a molecular formula
C15H16O (M+ 212). The ion peak at m/z 194 confirmed the loss of a molecule of H2O
from the molecular ion
Peak 3 (Rt 23.19 minutes) and 4 (Rt 23.29 minutes) gave the same
fragmentation patterns and the ion peak at m/z 208 in their GCMS spectrum
(Appendix 3). The fragmentation of both peaks were matched with 2,3diphenylbutan-2,3-diols (DPB) (12), with molecular formula C16H18O2 (M+ 242).
The ion peak at m/z 208 was suggested due to the loss of a H2O and CH3 from the
parent ion. The 1H NMR spectrum (Appendix 4) of the isolated compound (PH)
exhibited two singlet signals at δ 2.30 and 2.60 attributed to two hydroxyl protons of
the compound. Another two singlets at δ 1.53 and 1.61 were assigned to two methyl
groups. A multiplet signal which resonated at δ 7.20 – 7.29 was due to ten aryl
protons.
A small peak with Rt19.60 was also observed in the GC chromatogram. It
gave an ion peak at m/z 194 in the MS spectrum. The compound was unable to
identify and was named as unknown product (14). Interestingly, there was no 1,2phenylethanol (13) being observed as proposed in the mechanism (Scheme 4.2).
The products ratios in this photoreaction were calculated using peak areas in
the GC chromatogram, with the assumption of all the photoproducts produced the
same response to the FID detector. Table 4.2 shows the calculated ratios.
OH
OH
OH
(10)
(11)
(12)
68
Table 4.2: Product ratios calculated by GC in the photochemical hydrogen
abstraction by acetophenone in toluene solution.
Condition
Conversion
DPE (10)a
DPP (11)
DPB (12)
Homogeneous
95%
0.31
0.37
0.30
a
Unknown
product (14)
0.02
Numbers reported are the average of at least two measurements. Error limit of the analysis is ± 3%
From Table 4.2 photoproducts (10), (11), and (12) gave the ratio of 1:1:1
different from the reported ratio 1:2:1 by Lei and Turro [134]. The data cannot be
compared because the products ratio in this research was calculated based on the GC
peak area while the reported ratio was based on isolated yield.
4.4.2
Photoreaction in NaY Zeolite Slurry
The zeolite sample containing AcP was irradiated under degassed condition.
Extraction of the photoproducts from the zeolite was done by using THF. Figure
4.12 shows the resulting GC chromatograms of the photoreactions in NaY zeolite
slurry. The analysis of the supernatant (toluene solution after photoreaction) showed
the present of DPE (10) and an unidentified compound (15). Small amounts of AcP
and DPP (11) were also detected. The present of AcP shows that the starting material
(AcP) might “escaped” from the supercage of zeolite during process of stirring in the
photoreaction. DPE (10) and DPP (11) may formed by the hydrogen abstraction
occurred in toluene solution. The high yield of DPE (10) could also caused by the
benzyl radicals formed from zeolite supercage because these radicals are small in
size and less polar compared to AcP-OH radical.
The extraction of the photolysed zeolite by THF gave high yield of
asymmetric coupling product DPP (11) due to geminate cage combination. The
69
yields of free radical products DPE (10) and DPB (12) were drastically reduced
compared to the homogeneous reaction. The unknown compound (14) was also
detected. Table 4.3 shows the product ratios of the extract in the photolysed zeolite
calculated using product peaks area in the GC chromatogram. Although the
conversion rate was not calculated in this experiment, the ratio of total
products/starting material (AcP); 1.65, predicted that the conversion rate is much
more lower in the zeolite slurry as compared to the homogeneous reaction (the ratio
was 45.25) which occurred in the same reaction period.
DPE
(a) Supernatant
after photolysis
Unknown
product (15)
AcP
DPP
AcP
(b) THF extract of
the photolysed
zeolite
DPP
Unknown
product
(14)
DPE
DPB
12
14
16
18
20
22
24
Figure 4.12: GC chromatograms of the supernatant and the resulting tetrahydrofuran
extract.
70
There was no AcP detected after 2 hours of stirring in supernatant. It is due to
the polar carbonyl group of AcP which is strongly bound to the zeolite internal
surface. The loading level of 4 mg/100 mg (AcP/NaY) was reported to enable each
zeolite cavity contained an AcP molecule and a molecule of toluene [134]. The
excess toluene molecules will fill the void space of the NaY zeolite supercage. These
hydrocarbon molecules are expected to serve as blockers that inhibit the diffusion of
geminate radical pairs and therefore inhibit the free radical formation. Figure 4.13
shows the different molecules distribution in homogeneous solution and zeolite
slurry.
We can conclude that the different in chemoselectivity obtained in zeolite
slurry is the result from a combination of strong preferential adsorption of the AcP to
the internal surface of NaY zeliote and the inhibition of the diffusional motion of the
geminate radical pairs produce by the toluene radicals. Scheme 4.2 summaries the
reaction mechanism and also how the high yield of asymmetric coupling product can
be obtained by using the “spectator” approach in the zeolite slurry in this study [134].
Table 4.3: Product ratios in the tetrahydrofuran extract of the photolysed NaY
zeolite.
Condition
a
Zeolite
slurry
DPE (10)a
0.13
DPP (11)
DPB (12)
0.72
0.04
Unknown
product (14)
Total
products/AcP
0.11
1.65
Numbers reported are the average of at least two measurements. Error limit of the analysis is ± 2%
In homogeneous solution
In zeolite slurry
Figure 4.13: Molecule distributions in homogeneous solution and zeolite slurry
(spectator approach) [147].
71
O
Ph C CH3
O
Ph C CH3
hv
1
O
Ph C CH3
ISC
3
AcP (8)
O
Ph C CH3
OH
Ph C CH3
hv
3
+
Ph CH3
zeolite
-slurry
+
PhCH2
Geminate cage
combination
geminate radical pair
Toluene (9)
OH
Ph C CH2Ph
CH3
DPP (11)
major product
Free Radicals
combination
homogenous
PhCH2CH2Ph
+
DPE (10)
symmetric
statistical distributions:
OH
+
Ph C CH3
Ph CH3
(9)
1
OH OH
Ph C C Ph
CH3 CH3
OH
Ph C CH2Ph +
CH3
DPP (11)
asymmetric
:
2
DPB (12)
symmetric
:
1
OH
Ph C CH3
H
2-phenylethanol (13)
expected minor product
Scheme 4.2: The mechanism of photochemical hydrogen abstraction by
acetophenone in toluene solution and zeolite NaY slurry [134].
72
4.5
Regioselective Photodimerizations of 2-Cyclohexenone (CH) in Alkali
Metal Cation-Exchanged Y Zeolites
We continued our research on the confiment effect of zeolite by further study
the effect of alkali metal cation-exchanged Y zeolites in the photodimerization of
cyclohexenone.
4.5.1
Photodimerizations of 2-Cyclohexenone in Homogeneous Solution
Homogenoues photodimerization of 2-cyclohexenone (CH) was first carried
out in n-hexane. Reaction in open air was used as control experiments. The GC
analysis of the photoproducts (Figure 4.14) showed the presence of four peaks, each
with Rt values of 23.40, 23.60, 23.89, and 24.12 minutes.
The MS analysis showed that peaks 1-4 gave the same molecule ion peaks at
m/z 192 (Appendix 5-8) which was in agreement with a molecular formula of
C12H16O2.
The fragmentation pattern of this compound was matched with the
fragmentation pattern of CH dimer. Peak 2 (Rt 23.60 minutes) and peak 4 (Rt 24.12
minutes) were further confirmed to be dimers of head-to-tail (HT) (16) and head-tohead (HH) (17) with the comparison of literatures [6, 10,135,136]. Peak 1 (Rt 23.40
minutes) and peak 3 (Rt 23.89 minutes) were assigned to either dimer (18) or (19)
and classified under other products. Scheme 4.3 summarizes the products
distribution in the photodimerization of CH [10].
Products ratios were calculated using the peak area in GC chromatograms.
Table 4.4 shows the (16)/(17) dimer ratios in the homogeneous photodimerizations
of CH in n-hexane in different conditions. Figure 4.14 also shows the significant
different of product selectivity of the photoreactions conducted in homogeneous
condition compared to solid-state reaction.
73
HT (16)
2
3
1
(a)
Homogeneous
photoreaction in
inert Condition
(b)
Homogeneous
photoreaction in
free air condition
4 HH (17)
HT (16)
HH (17)
HH (17)
(c)
HT
(16)
20
22
24
26
Solid state
photoreaction
in CsY
28
30
Figure 4.14: GC chromatograms of the homogeneous photoreactions of 2-
cyclohexenone compared to solid state photoreactions (a)-(c).
74
O
O
O
O
O
O
hv
+
+
(16) O
anti-cis HT
(1)
(17)
anti-cis HH
O
(18)
trans-fused HH
O
O
or
+
(19a) O
alternative
trans-fused HT
(19b)
alternative
trans-fused HH
Scheme 4.3: Photodimerization of 2-cyclohexenone (1) [10].
Table 4.4: Product ratios of the photodimerizations of 2-cyclohexenone in n-hexane.
Condition
Conversion
HT (16)a
HH (17)
(16)/(17)
Inert
Open air
67%
10%
0.61
0.65
0.16
0.17
3.81
3.82
a
Other
products
0.23
0.28
Numbers reported are the average of at least two measurements. Error limit of the analysis is ± 3%
Table 4.4 shows that the ratio of (16)/(17) is independent from the % of
conversion. The existence of oxygen molecules in the open air experiment will not
inhibit the photoreaction but will slow the rate of the reaction. Oxygen usually will
form superoxide radical which can terminate the radical reaction. It explains the low
conversion compared to the same experiment carried in inert condition.
It is reported that the ratio of anti-dimers HT (16) and HH (17) is in the
function of both the solvent and the concentration of the starting material, CH. In
non-polar solvent, the photodimerization of CH gives predominantly the HT
photodimer (in n-hexane, ratio HT/HH 5.21:1), but the regioselectivity switched in
favor of the HH photodimer in polar solvent (in acetonitrile, ratio HT/HH 0.5:1).
Irradiation of 0.5 M and 3.0 M CH in benzene give the HT/HH ratio of 2.50:1 and
1.52:1 respectively and photodimerization of the neat ketone gave the dimer ratio
1.06:1 [6, 10,135, 136].
75
Lam et al. [135] and Wagner and Bucheck [6] have suggested that the
photodimerization of CH occurs via it lowest triplet states, ππ*. It is believed there
are two distinct structural possibilities for the metastable intermediate: an excited
state charge-transfer complex and a 1,4-biradical. The charge-transfer complex itself
cannot provide a completely satisfying explanation of the stereochemistry of the
photodimerizations [6]. Corey’s model (Figure 4.15) would predict the HH dimer is
predominant in this reaction.
O δ−
O δ−
O
δ+
δ+
δ+
better than
δ+
O
ππ*
(b) HT alignment
3
ππ*
(a) HH alignment
3
Figure 4.15: Corey’s model [5].
Another factor influences the course of cycloaddition is the dipole moment of
the collision complex that precedes chemical reaction. A HT alignment of the excited
enone and ground state enone should be greatly favored over a HH approach in non
polar solvents, but less so in polar solvents. The Scheme 4.4 incorporates the various
intermediates which can lead to dimers. A and B are probably π complex
(exciplexes) but may be simply collision complexes intermediates. In either case, the
specific charge-transfer interactions suggested by Corey would favor B while dipole
effects would favor A. In non-polar solvents, biradicals b and c would probably
rotate to conformations with lower dipole moments but which would have their
radical sites too far apart for effective coupling. Polar solvent however would help to
maintain the dipole moments in b and c [6].
However, evidences have been suggested that exciplexes may not involved in
cycloadditon reaction of alkenes with cyclic enones and the regiochemistry can be
explained in terms of the properties of triplet 1,4-biradiacl intermediates [4, 137139].
76
O*
O*
O
HT
O
+
A
O*
O
O
O
a
O
O
O
O
or
B
b
HH
c
Scheme 4.4: Various intermediates which can lead to 2-cyclohexenone dimers.
4.5.2
Solid State Photodimerizations of 2-Cyclohexenone in Alkali Metal
Cation-Exchanged Y Zeolites
The [2+2] photocycloaddition of enone to alkene is one of the most widely
used photochemical reactions. Several factors in modifying the regioselectivity and
stereoselectivity such as chain length [140, 141], substituents of the system [142,143]
and incorporation of the conjugated double bond into a ring [143], have been
reported. In addition, reaction medium also seems to be a main factor in controlling
the regiochemistry and stereochemistry. In this research, alkali metal cation
exchanged Y zeolites were used to obtain the selectivity of the photoproducts in
[2+2] photodimerization of CH. The relative efficiency of dimerization was
calculated by irradiating all five zeolite complexes under identical condition, thus the
results of this experiment are comparable
The zeolite samples containing CH were prepared by stirring the zeolite in
hexane. The samples were then degassed and UV irradiated as dry powder. The
photoproducts were firstly obtained by stirring the photolysed samples overnight in
THF. Table 4.5 shows the product ratios obtained from of the solid state
photodimerization of CH in cation-exchanged Y zeolites using THF extraction.
77
Table 4.5: Product ratios of the solid state photodimerizations of 2-cyclohexenone in
alkali metal cation-exchanged Y zeolites with tetrahydrofuran extractions.
a
M+Y
LiY
NaY
KY
RbY
CsY
HT (16)a
0.06
0.13
0.03
0.06
0.02
HH (17)
0.70
0.21
0.88
0.78
0.86
(16)/(17)
0.09
0.62
0.03
0.08
0.02
Other products
0.24
0.66
0.09
0.16
012
Numbers reported are the average of at least two measurements. Error limit of the analysis is ± 2%
However, when the remained zeolites were further dissolved in 1N HCl and
extracted using ethyl acetate (EtOAc), the resulting GC chromatograms showed there
were still some products (residue) trapped in the zeolites. Thus we concluded that the
THF extraction was not a suitable method to extract the product. GC chromatograms
of LiYCH-THF-HCl (Figure 4.16 (b)) and CsYCH-THF-HCl (Figure 4.16 (d))
clearly show the remained products which trapped in the zeolites after THF
extractions. The photoreactions were repeated by using other extraction method.
78
HH
(a) LiYCH-THF
HT
1
4
HH
2
HT
3
5
(b) LiYCH-THF-HCl
6
7
HH
(c) CsYCH-THF
HH
HT
20
22
(d) CsYCH-THF-HCl
24
26
28
Figure 4.16: GC chromatograms (b) and (d) show the remained products which
trapped in the zeolites after tetrahydrofuran extractions.
30
79
The photolysed zeolites were dissolved in acid 1 N HCl followed by ethyl
acetate (EtOAc) extraction. The dimers have been proved to be stable under acid
condition [10]. The resulting chromatograms (Figure 4.17) gave 7 peaks (can be
observed clearly in LiY and NaY) instead of 4 peaks in homogeneous reaction
(Figure 4.14).
The extra peaks 5, 6, and 7 (Figure 4.17) (show with arrows in
chromatogram) were observed at Rt 24.54, 25.66 and 25.87 minutes. Peak 5 and 6
gave ion peaks at m/z 192 in their MS spectra. Their fragmentation patterns were
matched with the fragmentation patterns of CH dimer. While peak 7 gave the ion
peak at m/z 207. Their MS pattern is similar to CH dimers. It is believed that these
unknown products are CH dimer with a methyl substitution. The product ratios were
calculated based on the peak areas of these 7 peaks in the GC chromatogram and was
shown in Table 4.6. The dimer ratios did not change even the dimers were allowed
to remain within the zeolite for up to 24 h after irradiation.
Table 4.6: Products ratio obtained by solid state photodimerization of 2-
cyclohexenone in alkali metal cation-exchanged Y zeolites with HCl
treatment and ethyl acetate extraction.
a
M+Y
HT (16)a
HH (17)
(16)/(17)
Other
products
LiY
NaY
KY
RbY
CsY
0.10
0.15
0.11
0.05
0.05
0.50
0.31
0.71
0.81
0.80
0.20
0.48
0.15
0.06
0.06
0.40
0.54
0.14
0.15
0.15
Total
products/
(CH+ Total
products)
0.37
0.81
0.96
0.63
0.98
Numbers reported are the average of at least two measurements. Error limit of the analysis is ± 2%
80
4
HH
1 HT
2
(a) CH-LiY-HCl
HH
HT
1
6
5
3
7
(b) CH-NaY-HCl
HH
HT
(c)CH-KY-HCl
HH
4
1
5
HT
6
(d) CH-RbYHCl
HH
(e) CH-CsY-HCl
HT
20
22
24
26
28
30
Figure 4.17: GC chromatograms of the solid state photodimerizations of 2-
cyclohexenone in alkali metal cation-exchanged Y zeolites (a)-(e).
81
It is observed that the solid state photoreactions give a distinctive difference
in products selectivity compared to homogeneous reactions (shown in Figure 4.14).
The major photoproduct in homogenous reaction is HT (16) (Table 4.4) while HH
(17) become dominant in all the solid state photoreactions (Table 4.6). In most
zeolites, the minor (other) products are formed in comparable amounts (Table 4.6).
In NaY and LiY they had became prominent products. In NaY, these products
formed (54 % of total products) even greater than HT (16) and HH (17) dimers. It
further showed that the solid state photodimerization of CH can result in totally
different product selectivity compared to solution reaction (Table 4.4).
It is suggested that the complexing effect of the charge compensating cation
and the size constriction factor in the zeolite supercage is the main factor which
contributes to the different products selectivites compared to homogenous reaction
[10, 144]. There is a great decrease in the HT (16)/ HH (17) ratio from NaY (0.48)
to CsY (0.06) in solid state photoreactions (Table 4.6). The greatest reversal in the
regiochemisrty from HT to HH dimer in RbY and CsY could be due to the smallest
supercage volume and weakest electrostatic interaction of ion in the zeolites [10,
145].
In RbY and CsY, the constriction factor most probably is the dominant factor.
It was reported even at very low loading levels there are supercages with double
occupancies [146]. It has been established through solid state NMR and diffusion
measurement studies, the translation and rotation motions of aromatic as well as
aliphatic molecules are reduced within zeolites [147]. Thus, the formation of high
HH in RbY and CsY are caused mainly by the confined space.
While in LiY, the relatively large effect of Li+ (strong van der Waals) are
expected to provide strong interaction/binding between the carbonyl group of CH
with the zeolite surface [28,144]. LiY has been reported to give the greatest
enhancement of Norrish Type I products in the photolysis of macrocyclic ketone
within zeolites [144]. Lithium ions have been suggested to exert a large effect due to
their small size, give rise to high charge density [148,149]. The factor of strong
electrostatic field in LiY may suggest why LiY give the higher HH product
compared to NaY with smaller supercage volume.
82
The results obtained are compared to experiments done by Lem et al. [10].
Most of the ratio HT(16)/HH(17) are quite similar (for the case NaY, KY, and CsY).
While the ratio gives a significant different in LiY (this research 0.20, Lem et al.
0.46). It probably due to the higher exchanged level of Li obtained in this
experiment. RbY was not tested by Lem et al.
Figure 4.18 shows the ratio
HT(16)/HH(17) obtained in this research compared to Lem et al. [10].
Conversion range of 5-40% was reported by Lem et al. [10] correspondent to
1.25 hours of irradiation time. Although conversion was not calculated in this
experiment, the wide range differences between the ratio of total products/(starting
material (CH) + total products) (Table 4.6) indicated the big different of product
conversion rates in various cation-exchanged zeolites. It was shown that products
ratios also do not change accordingly as a function of irradiation times (5 hours in
this experiment compared to 1.25 hours in Lem et al.’s experiment) (Figure 4.18).
Ratio (16)/(17)
0.6
0.5
0.4
This Research
Lem et al.
0.3
0.2
0.1
0
LiY
NaY
KY
RbY
CsY
Zeolites
Figure 4.18: Ratio HT(16)/HH(17) obtained in this research compared to ratio
obtained by Lem et al. [10].
83
4.5.3
Photodimerizations of 2-Cyclohexenone in Alkali Metal CationExchanged Y Zeolite Slurries
Different handling in the procedures for sample preparing as well as the
different method of sample preparation sometimes will induce contradicting
observations even for similar system [1]. Majority of the studies of photochemistry in
zeolite are concerned with photolyses of organic molecules in dry powder zeolite
(solid state) in the absent of solvent. It has been shown that the product distribution
obtained upon UV irradiation of organic molecules included in zeolite-solvent
slurries is distinctly different from conventional dry powder photolysis.
The difference in the product distribution obtained between zeolite-solvent
slurry and a homogeneous solution is often higher than that between the dry powder
zeolites and homogeneous solution. Solvent present within the supercages of zeolite
X and Y was reported to provide constraint on the mobility of the included guest
molecules, thus one might able to modify the photoreactivity of the guest molecules.
Photolysis of acenapthylene [12] in the RbY-hexane slurry gave a high yield of cis
dimer compared to solid state reaction and it was believed that the migration of
acenaphtlylene between cages was blocked or inhibited by the solvent hexane.
In order to study the effect of solvent on the HT(16) /HH (17) ratio, the
reactions were carried out in zeolite-hexane slurry. Since the adsorption of CH is
achieved by the same process (stirring in hexane) both for dry (hexane evaporated
off) and slurry (hexane left within zeolite) irradiations, we may assume that the
distribution pattern remains the same for both the dry and slurry samples. Although
the dimerization of CH is more complicated compared to dimerization of
acenaphtlylene (which gives only two products), we tried to make a relation between
the formations of HH (17) to cis dimer of acenaphtlylene within zeolites. If the high
yield of HH (17) in the solid state reaction are mainly due to the size constriction
effects within the supercage, this would be expected to get a higher yield of HH (17)
in zeolite-hexane slurry system. Hexane was always reported to be the best solvent
for zeolite-solvent slurry preparations since total adsorption of the reactant molecules
to zeolites occurred in hexane slurries [12,150].
84
Figure 4.19 shows the resulted GC chromatograms of the photoproducts
obtained from the photodimerizations of CH in alkali metal cation-exchanged Y
zeolite-hexane slurries. Most of the chromatograms show similar pattern with those
obtained in dry powder (solid state) reaction (Figure 4.17), except in the NaY and
LiY zeolite- slurry systems. These systems showed a significant increase of
background peaks at the Rt 20-23 minutes (shown with circles in the chromatogram),
which were not observed in the solid state systems. These products could not be
identified. Solid state reaction seen to be provided a “cleaner” reaction compared to
slurry system.
An extra peak (marked with arrow in Figure 4.19, Peak 8) with the Rt 26.93
minutes could be clearly observed in LiY, NaY and KY-slurry systems compared to
dry powder systems. This peak gave a molecular ion peak at m/z 277 in the MS
analysis. Its fragmentation patter did not matched with any of the compounds cited in
the Wiley database of the GC-MS system. However, this unknown compound (Peak
8) formed an important portion (12 %) of total products in LiY-slurry system. The
product distributions are calculated based on the peak areas of these 8 peaks in the
GC chromatogram. Table 4.7 shows the calculated results.
85
HH
8
(a) CH-LiY-S
HT
HH
1
4
5
HT
6
2
3
7
8
(b) CH-NaY-S
HH
(c) CH-KY-S
HT
4
1 HT
HH
5
6
(d) CH-RbY-S
HH
HT
20
22
(e) CH-CsY-S
24
26
28
30
Figure 4.19: GC chromatograms of the photodimerizations of 2-cyclohexenone in
alkali metal cation-exchanged Y zeolite-hexane slurries (a)-(e).
86
Table 4.7: Product ratios of the photodimerizations of 2-cyclohexenone in alkali
metal cation-exchanged Y zeolite-hexane-slurries.
MY zeolites
HT (16)a
HH (17)
(16)/(17)
Other
products
LiY
NaY
KY
RbY
CsY
0.05
0.06
0.06
0.06
0.05
0.32
0.16
0.74
0.71
0.69
0.16
0.38
0.08
0.08
0.07
0.63
0.78
0.20
0.23
0.26
a
Total
products/
(CH+ Total
products)
1.00
0.94
0.92
0.91
0.97
Numbers reported are the average of at least two measurements. Error limit of the analysis is ± 2%
In LiY, NaY and KY-slurry systems, the formations of HT (16) are lesser,
thus provided lower HT/HH value compared to solid state systems (Table 4.6). The
formations of HT (16) in hexane solution are more than 60% (Table 4.4). However,
the formations of HT (16) and HH (17) decreased in NaY-slurry system, the “other
products” become the main portion (78%) and Peak 1 (trans-fused photodimers (18)
or (19) is the prominant compound (28 %). Except for the KY-system, the formations
of all HH dimers (17) are lesser compared to solid-state reactions. The zeolite-slurry
system gave rise to more “other products” compared to HT (16) and HH (17).
Introducing of the solvent (hexane) molecules to zeolite supercages in this
expeiment did not give higher yield of HH (17) as expected. Hence, it is further
proved that the size constriction effect is not the single factor that bringing higher
formation of HH (17) within the zeolite supercage.
Generally, conversion of the starting material to products is higher in zeoliteslurry compared to dry powder. It can be observed on the value of total products/(CH
+ total products). All the zeolite-slurry systems gave a value greater than 0.9 even in
a shorter irradiation time (3 hours shorter compared to the solid state reaction).
Thus, we concluded that photodimerization of CH in alkali metal cationexchanged Y zeolites in solid state and slurry system has successfully gaining
favorable result in controlling the regio and steroeselectivity of the photodimer. The
results obtained are comparable to the previous report [10]. The selectivity of the
87
photoproducts is most probably due to the combine factors of size constriction
effects and the complexing ability of the charge-compensating cation of the zeolites.
In overall, the modified (cation-exchanged) zeolites gave better product selectivity
compared to the parent NaY zeolite. The presence of solvent molecules in the system
did not give big different in the ratio of HT (16)/HH (17).
4.6
Photocycloadditions of 2-Cyclohexenone to Vinyl Acetate (VA) in Alkali
Metal Cation-Exchanged Y Zeolite-Slurries
After the photodimerization of 2-cyclohexenone (CH) within zeolites has
successfully gaining high regioselective of HH dimer, we tried to apply the same
approach in a more complicated reaction. Photodimerization only involved a single
reactant. We further our study to another similar cycloaddition reaction, by
introducing an alkene (vinyl acetate), VA to the enone within the zeolite.
4.6.1
Homogeneous Solution
Photocycloadditon of CH (1) to VA (21) (1: 15 mol) was carried out in
hexane solution, in inert condition for 5 hours. The reaction mixture was analyzed
using GC and GC-MS. The GC analysis of the mixture (Figure 4.20) showed the
presence of five significant peaks, each with Rt values of 19.58, 19.71, 19.83, 19.94,
and 20.10 minutes. All these five peaks gave similar fragmentation patterns and the
ion peaks at m/z 158, 139, 122, 111, 97, and 43 in their GCMS analysis (Appendix 9
-13). Their fragmentation patterns were matched with the cyclohexane-cyclobutene
adduct (22) or (23) (Figure 4.21) which has a molecular formula of C10H14O3 (M+,
182). The ion peak at m/z 122 confirmed the loss of HOCOCH3 from the molecular
ion. Small amounts of CH dimer were also detected in this reaction.
88
1
2
45
3
16
18
20
22
24
Figure 4.20: GC analysis on the reaction mixture in the photocycloaddition of vinyl
acetate to 2-cyclohexenoen in hexane.
O
O
OAc
O
OAc
hv
+
+
OAc
(1)
(21)
HT (22)
HH (23)
Figure 4.21: Photocycloaddition of vinyl acetate to 2-cyclohexenone.
In early 1960, Corey et al. [5] have reported the photocycloadditon of CH to
VA, they have only able to characterize three HT streoisomeric acetoxy ketones as
major products. However, we believe the cycloadducts distribution of the addition of
CH to VA (CH2=CHOAc) are similar to the addition of CH to ethoxyethene. Seven
racemic cycloadducts (25)-(31) in the ratio of 7: 2: 10: 23: 28: 7: 23 were reported by
Maradyn et al. [139] in the photocycloaddition of CH (1) to ethoxyethene
(CH2=CHOEt) (24) (Scheme 4.5).
Although we are not able to determine the stereochemistry of these
cycloadducts, we hope to compare the different of product distribution between
solution reaction and reaction in confinement space of zeolite. The cyclobutene
89
adducts were then named as (P1)-(P5) and the product ratios were calculated based
on the peak areas obtained in the GC analysis.
O
H
OEt
O
OEt
+
(1)
H
hv
HH (25)-(27)
(24)
O
H
H
OEt
HT (28), (29)
O
H
H
OEt
HT (30), (31)
Scheme 4.5: Photocycloadditon of 2-cyclohexenone to ethoxyethene.
4.6.2
Photocycloadditions in Alkali Metal Cation Exchanged Y Zeolite Slurries
At first, we faced with difficulties in finding a suitable sample preparation
method. We tried to include both the CH and VA in zeolite and irradiated it in solid
state. However, the products analysis only revealed the CH dimer. It is believed that
the VA had all evaporated off in the process of drying and degassing the sample
because VA is a highly volatile compound. Thus, the “solid state” method is not
suitable for this reaction. Then, we applied the “spectator” zeolite-slurry method (as
what we have done in the hydrogen abstraction experiment) by absorbing the CH in
VA solution. After the irradiation, the sample turned to become a gummy transparent
liquid, stuck with the zeolite. The gummy liquid was believed to be the polymerized
form of VA, polyvinyl acetate. Again, this method failed. The triplet sensitization
technique was not in our consideration, because it will only complicate the problem.
Finally, the reactions were carried out by absorbing both the CH and VA in
MY zeolites and photolysed it as hexane slurry. The products were extracted by
dissolving the irradiated zeolites with concentrated HCl and isolated with EtOAc. No
new products or a difference in the product ratio were observed relative to the
90
mixture before acid treatment (refer to the acid test in Section 3.8.1.1). Figure 4.22
shows the resulted GC chromatograms of the extracted photoadducts and the resulted
product ratios in different mediums are reported in Table 4.8.
There is no new compound obtained in the photocycloaddition within zeoliteslurries system. However, distinctly difference of product selectivity was observed
compared to the homogeneous reaction. There are drastically decrease of product
portion of (P2) and increase of the ratio of (P3). Surprisingly, all the MY zeolites
provided a similar pattern of product distributions. There is no significant change of
product ratios from LiY to CsY. LiY and KY systems almost provided the same
product distribution as solution reaction. RbY system gave the highest yield of (P1)
while CsY system provided (P5) as the largest portion in the products mixture. We
also tried to increase the loading level of CH (4 mg CH/100 mg zeolite) to increase
the confinement effect. However the obtained results provided a high yield of CH
dimer, indicated that photodimerization reaction was more prominant compared to
photocycloaddition of VA.
Thus, we tried to reason the failure of cation to vary the selectivity in this
reaction is caused by the “passive” cavity. A zeolite reaction cavity has been
characterized to be “active” when the interaction between a guest molecule and the
cavity is attractive or repulsive. While there is no significant interaction, it is
considered to be “passive”. The interactions may vary from weak van der Waals
forces, to hydrogen bonds to strong electrostatic forces between charged centers [69].
VA (CH2=CHOCOCH3) molecule has two polar groups, C-O and C=O which can
compete with the C=O group in the CH molecule to interact electrostatically with the
surface cations. Because the VA molecules are present in a large amount in the
supercage, it may also shielded and weaker the electric field created by the cations. It
explains why the alkali metal cation-exchanged Y zeolites are not able to control the
products selectivity in this system.
1
91
3
5
(a) CHVA-LiY
(b) CHVA-NaY
(c) CHVA-KY
1
3
5
(d) CHVA-RbY
(e) CHVA-CsY
16
18
20
22
24
Figure 4.22: GC chromatograms photproducts in photocycloadditons of 2-
cyclohexenone to vinyl acetate in alkali metal cation-exchanged Y
zeolite-slurries.
92
Table 4.8: Product ratios obtained in photocycloadditions of 2-cyclohexenone to
vinyl acetate in different mediums.
Medium
(P1)(a)
(P2)
(P3)
(P4)
(P5)
Hexane
LiY
NaY
KY
RbY
CsY
0.30
0.21
0.25
0.21
0.34
0.21
0.18
0.05
0.06
0.05
0.05
0.06
0.08
0.28
0.24
0.29
0.21
0.25
0.15
0.08
0.09
0.08
0.05
0.06
0.29
0.38
0.36
0.37
0.35
0.42
a
Total
products/
(CH+
Total
products)
1.00
0.97
1.00
0.83
0.70
1.00
Numbers reported are the average of at least two measurements. Error limit of the analysis is ± 2%
CHAPTER 5
CONCLUSIONS
In summary, this research have achieved all the stated objectives. The ESR
study of paramagnetic probe (H radical) in NaY zeolite showed the radicals stayed in
two different adsorption sites. The radicals are more preferable to stay in site II most
probably because it is less shielded. ESR results also showed that the radical cation
of amine dimer was formed resulted from the constriction effect of the zeolite Y
supercage in the photosensitization of triethylamine by acetophenone.
Photoreactions in the confined space of zeolite Y supercage produced
remarkable differences in product distribution comp red to conventional solution
reactions. In the photochemical hydrogen abstraction by acetophenone in toluene, the
NaY zeolite-toluene slurry provided high yield of asymmetric product (0.72)
compared to homogeneous reaction (0.37).
Cation-exchanged Y zeolites has successfully gaining favourable result in
controlling the regio and setereoselectivity of photodimer in the solid state
photodimerization of 2-cyclohexenone. The reactions showed a great reversal of
head to tail (HT) cyclohexenone dimer, to head to head (HH) cyclohexenone dimer
increasing from LiY to CsY zeolite. The complexing effect of the charge
compensating cation and the size constriction factor is the main factor which
contributes to the different product selectivities.
The appearance of solvent
molecules in zeolite slurry reactions, however did not give any significant change in
product distributions compared to solid state reactions.
94
The study of regioselectivity in the photocycloaddition of 2-cyclohexenone to
vinyl acetate in zeolite slurry also showed a drastically change of product yield
compared to homogeneous reaction. However, the cation-exchanged Y zeolites did
not play an important role in controlling the product selectivity. The failure of the
alkali metal cations to vary the selectivity was due to the passive cavity effect.
The use of zeolites in controlling product selectivity produced favourable
results. However, attentions must be given in choosing the suitable zeolites
(regarding their pore size, framework structure, etc) and reactants (molecular size,
polarity, volatility, etc). The possibility of using other type of zeolites (ZSM-5 or
zeolite-beta) in controlling the regioselectiviy of the photocycloaddition of 2cyclohexenone to vinyl acetate should be further investigated. Further experiments
can be carried out using different organic molecules from other functional group
such as alkene.
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M+, C14H14
m/z 182
113
Appendix 1: MS spectrum of 1,2-diphenylethane (DPE) (10).
M+, C15H16O- H2O
m/z 194
114
Appendix 2: MS spectrum of 2,3-diphenylpropan-2-ol (DPP) (11).
M+, C16H18O2-H2O-2CH3
m/z 195
115
Appendix 3: MS spectrum of 2,3-diphenylbutan-2,3-diols (DPB) (12).
116
Appendix 4: 1H NMR spectrum of 2,3-diphenylbutan-2,3-diols (DPB) (12).
M+, C12H16O2
m/z 192
Appendix 5: MS spectrum of CH dimer, HT (16).
117
M+, C12H16O2
m/z 192
118
Appendix 6: MS spectrum of CH dimer, HH (17).
M+, C12H16O2
m/z 192
119
Appendix 7: MS spectrum of CH dimer (18) or (19) (Peak 1 in Figure 4.14).
M+, C12H16O2
m/z 192
120
Appendix 8: MS spectrum of CH dimer (18) or (19) (Peak 3 in Figure 4.14).
M+, C10H14O3 – COCH3
m/z 139
121
Appendix 9: MS spectrum of cyclohexene-cyclobutene adduct (P1) (Peak 1 in Figure 4.20).
M+, C10H14O3 – COCH3
m/z 139
122
Appendix 10: MS spectrum of cyclohexene-cyclobutene adduct (P2) (Peak 2 in Figure 4.20).
Appendix 11: MS spectrum of cyclohexene-cyclobutene adduct (P3) (Peak 3 in Figure 4.20).
123
M+, C10H14O3
m/z 182
Appendix 12: MS spectrum of cyclohexene-cyclobutene adduct (P4) (Peak 4 in Figure 4.20).
124
M+, C10H14O3
m/z 182
Appendix 13: MS spectrum of cyclohexene-cyclobutene adduct (P5) (Peak 5 in Figure 4.20).
125