preparation and immobilization of palladium(ii) and nickel …

PREPARATION AND IMMOBILIZATION OF

PALLADIUM(II) AND NICKEL(II) SALEN COMPLEXES

ONTO MCM-41 FROM RICE HUSK FOR

SUZUKI-MIYAURA AND OXIDATION REACTIONS

SALIH HAMZA ABBAS

UNIVERSITI SAINS MALAYSIA

2015

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i

PREPARATION AND IMMOBILIZATION OF

PALLADIUM(II) AND NICKEL(II) SALEN COMPLEXES

ONTO MCM-41 FROM RICE HUSK FOR

SUZUKI-MIYAURA AND OXIDATION REACTIONS

by

SALIH HAMZA ABBAS

Thesis submitted in fulfillment of the requirement for the degree of

Doctor of Philosophy

August 2015

ii

DEDICATIONS

This thesis is dedicated to

My parents. May Allah forgive them

and

My family

iii

ACKNOWLEDGEMENT

In the Name of Allah, Most Gracious, Most Merciful

Praise is due to Allah Whose worth cannot be described by speakers.

I am pleased to place on record my special gratitude and thanks to the

government of Iraq, the government of Malaysia, University of Basrah and

University Science Malaysia for the financial and technical supports.

My sincere thanks go to my supervisor, Professor Farook Adam for his

guidance, insightful comments and constant support during the experimental part and

in the writing of the thesis.

I would like to thank the staff members of Institute of Postgraduate Studies

(IPS), School of Chemical Sciencess, School of Physics and School of Biological

Sciences, USM for their help in the use of the necessary equipments and analysis.

I would like to give special thanks to my past and present labmates and my

friends for their help, the knowledge sharing and instilling confidence.

Last but not least, my heartfelt gratitude towards my family: particularly my

sisters, my brother, my loving wife and my kids for their continuous love, prayer,

support and patience throughout my PhD study. Thank you all so much.

iv

TABLE OF CONTENTS

Page

DEDICATIONS ............................................................................................... ii

ACKNOWLEDGEMENT .............................................................................. iii

TABLE OF CONTENTS ................................................................................ iv

LIST OF TABLES ........................................................................................... x

LIST OF SCHEMES ...................................................................................... xx

LIST OF SYMBOLS .................................................................................... xxi

LIST OF ABBREVIATIONS ..................................................................... xxiv

ABSTRAK ................................................................................................. xxvii

ABSTRACT ................................................................................................ xxix

CHAPTER 1

INTRODUCTION .......................................................................................... 1

1.1 General entrance....................................................................................... 1

1.2 Heterogenization of homogeneous catalysts ............................................ 2

1.3 Silica as support material ......................................................................... 4

1.3.1 Silica abundance............................................................................... 4

1.3.2 Surface nature of silica ..................................................................... 4

1.3.3 Rice husk ash ................................................................................... 5

1.4 Mesoporous hybrid materials ................................................................... 7

1.4.1 Amorphous silica ............................................................................. 8

1.4.2 Ordered mesoporous silica ............................................................... 8

1.4.3 Synthesis of ordered mesoporous silica ......................................... 10

1.5 Organically functionalized mesoporous materials ................................. 12

1.6 Metal incorporation in mesoporous materials ........................................ 14

v

1.6.1 Incorporation of metals without organic ligands ........................... 15

1.6.1.1 Metal salts as metal precursors .................................................. 15

1.6.1.2 Metal complexes as metal precursors ........................................ 15

1.6.2 Incorporation of metal ions within organic ligands ....................... 16

1.6.2.1 Grafting incorporation (post-synthesis method)........................ 16

1.6.2.1.1 Indirect grafting incorporation ............................................ 17

1.6.2.1.2 Direct grafting method ........................................................ 19

1.6.2.2 One-pot incorporation ............................................................... 21

1.7 Salen Schiff base ligand and metal-salen complex ................................ 22

1.7.1 Incorporation of metal-salen complexes onto mesoporous

materials ......................................................................................... 23

1.7.2 Application of metal-salen in catalysis .......................................... 27

1.7.2.1 Suzuki-Miyaura reaction ........................................................... 31

1.7.2.2 Oxidation of benzyl alcohol ...................................................... 32

1.8 Objectives of the study ........................................................................... 36

1.9 Outline of thesis ..................................................................................... 37

CHAPTER 2

EXPERIMENTAL METHODS .................................................................. 40

2.1 Raw material and chemicals................................................................... 40

2.2 Extraction of silica ................................................................................. 40

2.2.1 Washing of rice husk ...................................................................... 40

2.2.2 Treatment of rice husk with acid .................................................... 41

2.2.3 Rice husk calcination ..................................................................... 41

2.3 Preparation of silica MCM-41 ............................................................... 41

2.4 Functionalization of silica MCM-41 with CPTES ................................. 42

2.5 Synthesis of MCMSalenPd catalyst ....................................................... 43

vi

2.5.1 Synthesis of N,Nʹ-bis(4-hydroxysalicylidene)ethylenediimine

palladium(II) complex ................................................................... 43

2.5.2 Immobilization N,Nʹ-bis(4-hydroxysalicylidene)ethylenediimine

palladium(II) complex on functionalized silica Cl-MCM-41 ........ 44

2.6 Synthesis of MCMSalenNi catalyst ....................................................... 45


Schiff base ..................................................................................... 45


nickel(II) complex ......................................................................... 45

2.6.3 Immobilization N,Nʹ-bis(4-hydroxysalicylidene)ethylenediimine

nickel(II) complex on functionalized silica Cl-MCM-41 .............. 46

2.7 Physco-chemical characterization .......................................................... 46

2.7.1 Fourier Transform Infrared Spectroscopy (FT-IR) ........................ 46

2.7.2 Powder X-ray Diffraction (XRD) .................................................. 47

2.7.3 1H,

13C Liquid State NMR Spectroscopy ....................................... 48

2.7.4 13

C, 29

Si CP/MAS NMR Spectroscopy .......................................... 49

2.7.5 Scanning Electron Microscopy-Energy Dispersive (SEM/EDX) .. 49

2.7.6 Transmission Electron Microscopy (TEM) ................................... 49

2.7.7 Nitrogen Sorption Analysis (BET/BJH) ........................................ 50

2.7.8 Atomic Absorption Spectrometry (AAS)....................................... 51

2.8 Catalysts reactions .................................................................................. 52

2.8.1 MCMSalenPd catalyst .................................................................... 52

2.8.1.1 Suzuki-Miyaura reaction ........................................................... 52

2.8.1.2 Gas chromatography and mass spectroscopy analysis (GC &

GC-MS) ..................................................................................... 53

2.8.1.3 Leaching and reusability of MCMSalenPd ............................... 54

2.8.1.4 Oxidation reaction ..................................................................... 55

2.8.1.5 Gas chromatography and mass spectroscopy analysis (GC &

GC-MS) ..................................................................................... 55

vii

2.8.1.6 Leaching and reusability of MCMSalenNi ............................... 56

CHAPTER 3

CHARACTERIZATION OF SALEN LIGAND & METAL-SALEN

COMPLEXES ............................................................................................... 58

3.1 Introduction ............................................................................................ 58

3.2 H2((OH)2-salen) Schiff base ligand ........................................................ 58


3.2.2 Liquid State 1H NMR spectroscopy ............................................... 59

3.3 N,Nʹ-bis(4-hydroxysalicylidene)ethylenediimine palladium(II)

complex ................................................................................................. 60


3.3.2 Liquid state 1H NMR Spectroscopy ............................................... 61

3.3.3 Liquid State 13

C NMR Spectroscopy ............................................. 62

3.4 N,Nʹ-bis(4-hydroxysalicylidene)ethylenediimine nickel(II) complex ... 63


3.4.2 Liquid State 1H NMR Spectroscopy .............................................. 64

3.4.3 Liquid state 13

C NMR spectroscopy .............................................. 65

CHAPTER 4

RESULT AND DISCUSSION ..................................................................... 67

4.1 Characterization of the ordered silica MCM-41 .................................... 67


4.1.2 X-ray Diffraction (XRD)................................................................ 68

4.1.3 Nitrogen sorption studies ............................................................... 69

4.1.4 Electron Microscope (SEM and TEM) .......................................... 70

4.2 Characterization of Cl-MCM-41 ............................................................ 74



viii



4.3 Characterization of MCMSalenPd catalyst ............................................ 80





4.3.5 13

C solid state NMR ....................................................................... 88

4.3.6 29

Si solid state NMR ...................................................................... 88

4.3.7 AAS and EDX ................................................................................ 89

4.4 Characterization of MCMSalenNi catalyst ............................................ 90



4.4.3 Nitrogen Sorption studies ............................................................... 93


4.4.5 13

C solid state NMR ....................................................................... 99

4.4.6 29

Si solid state NMR ...................................................................... 99

4.4.7 AAS and EDX .............................................................................. 100

CHAPTER 5

CATALYTIC ACTIVITY OF THE CATALYSTS ................................ 102

5.1 Catalytic activity of MCMSalenPd ...................................................... 102

5.1.1 Effect of time and temperature ..................................................... 102

5.1.2 Effect of mass of catalyst ............................................................. 105

5.1.3 Effect of mole ratio of reactants ................................................... 106

5.1.4 Effects of solvents and bases for the Suzuki-Miyaura reaction ... 107

5.1.5 Leaching test of MCMSalenPd catalyst ....................................... 109

ix

5.1.6 Reusability of MCMSalenPd catalyst .......................................... 110

5.1.7 Reaction kinetics .......................................................................... 111

5.1.8 Summary ...................................................................................... 114

5.2 Catalytic activity of MCMSalenNi ...................................................... 114

5.2.1 Effect of time and temperature ..................................................... 115

5.2.2 Effect of mass of catalyst ............................................................. 118

5.2.3 Effect of mole ratio of reactants ................................................... 119

5.2.4 Effects of oxidants for the oxidation of benzyl alcohol ............... 119

5.2.5 Leaching test of MCMSalenNi catalyst ....................................... 120

5.2.6 Reusability of MCMSalenNi catalyst .......................................... 121

5.2.7 Reaction kinetics .......................................................................... 122

5.2.8 The proposed mechanism of oxidation of benzyl alcohol ........... 125

5.2.9 Summary ...................................................................................... 126

CHAPTER 6

CONCLUSION AND RECOMMENDATIONS ..................................... 127

6.1 Conclusion ........................................................................................... 127

6.2 Probable future works .......................................................................... 128

REFERENCES ............................................................................................. 129

APPENDICES .............................................................................................. 143

x

LIST OF TABLES

Page

Table ‎1.1: Comparison of homogeneous and heterogeneous catalysts. ....................... 3

Table ‎1.2: Amorphous dispersed systems. ................................................................... 9

Table ‎1.3: Some Jacobsen catalysts and their applications with different metal ions

and bearing different substituents. .......................................................... 29

Table ‎1.4: Heterogeneous catalysts for the oxidation of benzyl alcohol. .................. 34

Table ‎2.1: GC and GC-MS program for identification and confirmation of the

product of Suzuki-Miyaura reaction. ...................................................... 54

Table ‎2.2: GC and GC-MS program for identification and confirmation of the

product of oxidation of benzyl alcohol. .................................................. 57

Table ‎4.1: Physicochemical textural parameters of MCM-41. .................................. 73

Table ‎4.2: Physicochemical textural parameters of Cl-MCM-41. ............................. 79

Table ‎4.3: Physicochemical textural parameters of MCM-41, Cl-MCM-41 and

MCMSalenPd. ......................................................................................... 87

Table ‎4.4: Physicochemical textural parameters of MCM-41, Cl-MCM-41 and

MCMSalenNi. ......................................................................................... 98

Table ‎5.1: The effect of catalyst’s mass, temperature and time for the Suzuki-

Miyaura reaction using MCMSalenPda. ................................................ 104

xi

Table ‎5.2: Effects of different solvents and bases for the Suzuki-Miyaura cross-

coupling reaction using MCMSalenPd as the catalysta. ........................ 108

Table ‎5.3: The kinetic parameters for the conversion of bromobenzene using

MCMSalenPd catalyst. .......................................................................... 113

Table ‎5.4: Effect of the temperature, weight of catalyst, and mmols of oxidant for

the oxidation reaction of benzyl alcohola. ............................................. 117

Table ‎5.5: The kinetic parameters for the conversion of benzyl alcohol using

MCMSalenNi catalyst. .......................................................................... 124

xii

LIST OF FIGURES

Page

Figure ‎1.1: The various types of siloxanes and silanols in silica matrix. .................... 5

Figure ‎1.2: The conversion of RH (a) to RHA (b, c) and their physical appearances.

RHA (b) is a result of incomplete burning resulting black coloured char

(Turmanova et al., 2011). .......................................................................... 7

Figure ‎1.3: Structures of M41S materials: a) MCM-41 (2D hexagonal, p6mm), b)

MCM-48 (cubic, Ia3d) and c) MCM-50 (lamellar, p2) (Kresge et al.,

1992)........................................................................................................ 10

Figure ‎1.4: Probable pathways for the templated synthesis of ordered MCM-41 by

structure-directing agents: a) true liquid-crystal template, b) cooperative

liquid-crystal template mechanism (Monnier et al., 1993). .................... 12

Figure ‎1.5: Three synthesis approaches for mesoporous hybrid materials (Hoffmann

et al., 2006). ............................................................................................. 14

Figure ‎1.6: Indirect grafting of palladium(II) complex onto silica gel (Sarmah et al.,

2012)........................................................................................................ 18

Figure ‎1.7: Indirect grafting of nickel(II) complex onto silica MCM-41 (a)

functionalization, b) condensation, c) metal complexation) (Bhunia and

Koner, 2011)............................................................................................ 19

Figure ‎1.8: Synthesis of copper(II) complex covalently anchored on silica (s = silica)

(Mishra et al., 2012). ............................................................................... 20

xiii

Figure ‎1.9: Immobilization of palladium complex onto silica gel through Schiff-

base condensation (Sarmah et al., 2013). ................................................ 20

Figure ‎1.10: Preparation of immobilized catalysts via sol-gel process. ..................... 21

Figure ‎1.11: Synthesis of salen Schiff base ligand and salen complex (Jacobsen

and Larrow, 1998). .................................................................................. 23

Figure ‎1.12: Sol-gel immobilization of Co-salen and Cu-salen complexes on silica

gel (Baiker et al.2001). ............................................................................ 24

Figure ‎1.13 Immobilization of Mn (salen) catalysts via the linkage groups axially

connected to Mn atoms (Zhang et al., 2008). .......................................... 25

Figure ‎1.14: Different tethered M-Salen-SBA catalysts (M = metal) (Yang et al.,

2011)........................................................................................................ 26

Figure ‎1.15: Salen Schiff base ligand of Jacobsen catalyst. ...................................... 28

Figure ‎2.1: Hexagonal pore structure of ordered silica MCM-41. ............................. 48

Figure ‎2.2: Types of gas sorption isotherms. ............................................................. 51

Figure ‎3.1: The FT-IR spectrum of H2((OH)2-salen) Schiff base ligand. .................. 59

Figure ‎3.2: 1H NMR spectrum of H2((OH)2-salen) ligand in DMSO-d6 (300 MHz). 60

Figure ‎3.3: The FT-IR spectrum of N,Nʹ-bis(4-hydroxysalicylidene)

ethylenediimine palladium(II) complex. ................................................. 61

xiv

Figure ‎3.4: 1H NMR spectrum N,Nʹ-bis(4-hydroxysalicylidene)ethylenediimine

palladium(II) complex in DMSO-d6 (500 MHz)..................................... 62

Figure ‎3.5: 13

C NMR spectrum of N,Nʹ-bis(4-hydroxysalicylidene)

ethylenediimine palladium(II) complex in DMSO-d6 (125 MHz). ......... 63

Figure ‎3.6: The FT-IR spectrum of N,Nʹ-bis(4-hydroxysalicylidene)

ethylenediimine nickel(II) complex complex. ........................................ 64

Figure ‎3.7: 1H NMR spectrum of N,Nʹ-bis(4-hydroxysalicylidene)

ethylenediimine nickel(II) complex in DMSO-d6 (300 MHz). ............... 65

Figure ‎3.8: 13

C NMR spectrum of N,Nʹ-bis(4-hydroxysalicylidene)

ethylenediimine nickel(II) complex in DMSO-d6 (125 MHz). ............... 66

Figure ‎4.1: FT-IR spectrum of silica MCM-41. ......................................................... 68

Figure ‎4.2: The X-ray diffraction pattern of the prepared MCM-41. ........................ 69

Figure ‎4.3: The nitrogen adsorption-desorption isotherm of MCM-41. .................... 70

Figure ‎4.4: The SEM image of MCM-41 at 30,000 magnification. .......................... 71

Figure ‎4.5: The TEM images of MCM-41 at 60,000 magnification. ......................... 72

Figure ‎4.6: The X-ray diffraction patterns of Cl-MCM-41. ...................................... 75

Figure ‎4.7: The nitrogen adsorption-desorption isotherm of Cl-MCM-41. ............... 76

Figure ‎4.8: The SEM image of Cl-MCM-41 at 30,000 magnification. ..................... 77

Figure ‎4.9: The TEM images of Cl-MCM-41 at 60,000 magnification..................... 78

xv

Figure ‎4.10: FT-IR spectrum of Cl-MCM-41, Pd(4-OH)2salen and MCMSalenPd. . 80

Figure ‎4.11: The X-ray diffraction patterns of MCMSalenPd catalyst. ..................... 81

Figure ‎4.12: Low angle XRD diffraction patterns of Cl-MCM-41 and

MCMSalenPd catalyst compared to MCM-41. ....................................... 82

Figure ‎4.13: The nitrogen adsorption-desorption isotherms of MCM-41,

Cl-MCM-41 and MCMSalenPd catalyst. ................................................ 83

Figure ‎4.14: BJH adsorption pore size distributions for the MCM-41, Cl-MCM-41

and MCMSalenPd catalyst. ..................................................................... 84

Figure ‎4.15: The SEM image of MCMSalenPd catalyst at 10,000 magnification. ... 85

Figure ‎4.16: The TEM images of MCMSalenPd catalyst at 60,000 magnification. .. 86

Figure ‎4.17: The 13

C solid state NMR spectrum of MCMSalenPd catalyst

(100 MHz). .............................................................................................. 88


Si solid state NMR spectrum of MCMSalenPd catalyst (79.4

MHz). ...................................................................................................... 89

Figure ‎4.19: Certain area, table of element continent and EDX pattern of

MCMSalenPd catalyst. ............................................................................ 90

Figure ‎4.20: FT-IR spectrum of Cl-MCM-41, Ni(4-OH)2salen and MCMSalenNi. . 91

Figure ‎4.21: The X-ray diffraction patterns of MCMSalenNi catalyst. ..................... 92

xvi

Figure ‎4.22: Low angle XRD diffraction patterns of Cl-MCM-41 and

MCMSalenNi catalyst compared to MCM-41. ....................................... 93

Figure ‎4.23: The nitrogen adsorption-desorption isotherm of MCM-41,

Cl-MCM-41 and MCMSalenNi catalyst. ................................................ 94

Figure ‎4.24: BJH adsorption pore size distributions for the MCM-41, Cl-MCM-41

and MCMSalenNi catalyst. ..................................................................... 95

Figure ‎4.25: The SEM image of MCMSalenNi catalyst at 10,000 magnification. .... 96

Figure ‎4.26: The TEM images of MCMSalenNi catalyst at 60,000 magnification. .. 97


C solid state NMR spectrum of MCMSalenNi catalyst (100

MHz). ...................................................................................................... 99


Si solid NMR spectrum of MCMSalenNi catalyst (79.4 MHz). 100

Figure ‎4.29: One spot, table of element continent and EDX pattern of

MCMSalenNi catalyst. .......................................................................... 101

Figure ‎5.1: The effect of the time and temperature on Suzuki-Miyaura reaction

using MCMSalenPd, Reaction conditions: bromobenzene (1 mmol),

phenylboronic acid (1 mmol), base K2CO3 (2 mmol), catalyst (0.05 g,

0.184 mol% pd), DMF:H2O (8:1 mL) solvent. ..................................... 103

Figure ‎5.2: The variation of conversion with catalyst weight loaded for the

Suzuki-Miyaura reaction with MCMSalenPd as the catalyst................ 105

xvii

Figure ‎5.3: Effect of mole ratio of reactants for the Suzuki-Miyaura reaction with

MCMSalenPd, Reaction conditions: base K2CO3 (2 mmol), catalyst

(0.05 g, 0.184 mol% pd), DMF:H2O (8:1 mL) solvent, 2 h at 130 oC. . 106

Figure ‎5.4: Leaching test of MCMSalenPd catalyst. The catalyst was removed

after 0.5 h, subsequent bar charts show the conversion without

catalyst in the reaction. .......................................................................... 110

Figure ‎5.5: Reusability of MCMSalenPd catalyst, Reaction conditions:

bromobenzene (1 mmol), phenylboronic acid (1 mmol), base K2CO3

(2 mmol), catalyst (0.05 g, 0.184 mol% pd), DMF:H2O (8:1 mL), 2 h

at 130 oC. ............................................................................................... 111

Figure ‎5.6: The first order rate plots for the conversion of bromobenzene using

MCMSalenPd catalyst at different temperature, R2 is the correlation

coefficient. ............................................................................................. 112

Figure ‎5.7: Arrhenius plot for the conversion of bromobenzene using

MCMSalenPd catalyst, R2 is the correlation coefficient. ...................... 113

Figure ‎5.8: The effect of the time and temperature on the oxidation reaction of

benzyl alcohol, Reaction conditions: benzyl alcohol (1.0 mmol),

periodic acid (1.1 mmol), catalyst (0.05 g, 0.132 mol% Ni), acetonitrile

(12 mL). ................................................................................................. 116

Figure ‎5.9: The variation of conversion with catalyst mass in oxidation of benzyl

alcohol, Reaction conditions: benzyl alcohol (1.0 mmol), periodic acid

(1.1 mmol), acetonitrile (12 mL), 90 min at 90 oC................................ 118

xviii

Figure ‎5.10: Effect of mole ratio of benzyl alcohol and oxidant, Reaction

conditions: catalyst (0.05 g, 0.132 mol% Ni), acetonitrile (12 mL),

90 min at 90

oC. ..................................................................................... 120

Figure ‎5.11: Leaching test of MCMSalenNi catalyst. The catalyst was removed

after 15 min, subsequent bar charts show the conversion without the

catalyst in the reaction. .......................................................................... 121

Figure ‎5.12: Reusability of MCMSalenNi catalyst, Reaction conditions: benzyl

alcohol (1.0 mmol), periodic acid (1.1 mmol), catalyst (0.05 g, 0.132

mol% Ni), acetonitrile (12 mL), 90 min at 90 oC. ................................. 122

Figure ‎5.13: The pseudo first order rate plots to determine the ka at different

temperature for the conversion of benzyl alcohol using MCMSalenNi

catalyst, R2 is the correlation coefficient. .............................................. 123

Figure ‎5.14: Arrhenius plot for the conversion of benzyl alcohol using

MCMSalenNi catalyst, R2 is the correlation coefficient. ...................... 124

Figure ‎5.15: The proposed mechanism of oxidation of benzyl alcohol. .................. 126

Figure A.1: FT-IR spectrum of the obtained biphenyl. ............................................ 143

Figure A.2: 1

H NMR spectrum of obtained biphenyl in CD2Cl2-d2 (500 MHz). ..... 144

Figure A.3: 13

C NMR of obtained biphenyl in CD2Cl2-d2 (125 MHz). ................... 144

Figure A.4: GC chromatograms of Suzuki-Miyaura reaction at 0 h and after 2 h

respectively............................................................................................ 145

xix

Figure A.5: GC-MS chromatogram of Suzuki-Miyaura reaction. ........................... 146

Figure A.6: GC-MS data for the produced biphenyl of Suzuki-Miyaura reaction. . 146

Figure A.7: GC chromatograms of oxidation of benzyl alcohol reaction at 0 min

and after 90 min respectively. ............................................................... 147

Figure A.8: GC-MS chromatogram of oxidation of benzyl alcohol reaction. ......... 148

Figure A.9: GC-MS data of oxidation of benzyl alcohol, (a g)............................. 155

xx

LIST OF SCHEMES

Page

Scheme ‎1.1: General mechanism for the Suzuki-Miyaura cross-coupling reaction. . 32

Scheme ‎2.1: The preparation steps of MCM-41. ....................................................... 42

Scheme ‎2.2: Functionalization of MCM-41 with CPTES linking agent. .................. 43

Scheme ‎2.3: Immobilization of the metal salen complex on functionalized silica

Cl-MCM-41. ............................................................................................ 44

Scheme ‎5.1: The general reaction scheme for the Suzuki-Miyaura using the

MCMSalenPd catalyst. .......................................................................... 102

Scheme ‎5.2: The general reaction scheme for the oxidation of benzyl alcohol

using MCMSalenNi catalyst. ................................................................ 115

xxi

LIST OF SYMBOLS

A Frequency factor

ao Hexagonal unit cell parameter

Co Initial concentration

Ct Concentration at time

cm-1

Wavenumber

d100 Diffraction plane at 100

Ea Activation energy

g Gram

h Hour

Hz Hertz

K Kelvin

k Kilo, (1000)

ka Apparent rate constant

kcal Kilocalorie

kHz Kilo Hertz

kv Kilo volt

L Litter

ln Natural logarithm

log Logarithm

M Molarity

mA Milli amper

min minute

mL Milliliter

xxii

mm Millimeter

mm Hg millimeter of mercury

mmol Millimole

mol Mole

nm Nanometer

o Degree

oC Degree Celsius (degree centigrade)

P/Pο Relative Pressure

Q2

Silicon atom bearing two hydroxyl groups and bonded to two

silicon atoms via oxygen bond

Q3

Silicon atom bearing one hydroxyl groups and bonded to three

silicon atoms via oxygen bond

Q4

Silicon atom not bonded to any hydroxyl group and bonded to

four other silicon atoms via oxygen bond

R Gas constant

T Absolute temperature

T Tesla (unit for measuring magnetic induction)

t Time

Vp Pore volume

Wd Pore diameter

X Halogen

x Unreacted concentration

μL Microliter

λ Wavelenght

δ Chemical shift

xxiii

θm magic angle-spinning

% Percentage

%T Percentage Transmitance

√ The square root

xxiv

LIST OF ABBREVIATIONS

a Primary concentration

APTES 3-aminopropyltriethoxysilane

BET Brunauer, Elmmet and Teller method

BJH Barrett, Joyner and Halenda method

ca. Calculated

cm3/g Cubic meter per gram

13C NMR Carbon-13 Nuclear Magnetic Resonance

CPTES 3-(chloropropyl)triethoxysilane

CTAB Cetyltrimethylammonium bromide

CuKα X-ray spectral line for copper bombarded with energy

sufficient to give intense X-ray emission

d Doublet

d6 Deuterated solvent

DMF Dimethylformamide

DCM Dichloromethane

DMSO Dimethyl sulfoxide

EDX Energy Dispersive X–ray

Et3N Triethylamine

etc. et cetera

FAO Food and Agriculture Organization of the United Nations

FID Flame ionization detector

FT-IR Fourier Transmitance Infrared Spectroscopy

xxv

GC Gas Chromatography

GC-MS Gas Chromatography Mass Spectroscopy

H4 Hysteresis loop type 4

1H NMR Hydrogen-1 Nuclear Magnetic Resonance

IUPAC The International Union of Pure and Applied Chemistry

M41S Mesoporous silicas

m2/g Square meter per gram

MAS NMR Magic angle–spinning Nuclear Magnetic Resonance

MCM-41 Mobil Crystalline Material Number 41

PAG Polyethylene glycol

pg page

pH The Hydrogen Function

PMOs Periodic measoporous organoslicas

ppm Part per Million

R Alkyl group

RH Rice husk

RHA Rice husk ash

RMM Rice Market Monitor

Rt Retention time

r.t. Room temperature

s singlet

salen Salicyladimine

SBA-15 Santa Barbara Amorphous Material Number 15

SDAs Structure-directing agents

SEM Scanning Electron Microscopy

xxvi

29Si NMR Sillicon-29 Nuclear Magnetic Resonance

t Triplet

TEM TEM Transmission Electron Microscopy

TEOS Tetraethyl orthosilicate

TMOS Tetramethyl orthosilicate

t-BuOOH tert-butyl hydroperoxide

v/v Volume/ volume

XRD Powder X–ray Diffraction

xxvii

PENYEDIAAN DAN SEKATGERAK KOMPLEKS SALEN PALADIUM(II)

DAN NIKEL(II) KE ATAS MCM-41 DARIPADA SEKAM PADI UNTUK

TINDAK BALAS SUZUKI-MIYAURA DAN TINDAK BALAS

PENGOKSIDAAN

ABSTRAK

MCM-41 daripada abu sekam padi (RHA) telah berjaya disintesis dangan

menggunakan cetiltrimetilammonium bromida sebagai agen pegasal melalui kaedah

sol-gel pada sahu 100 oC. MCM-41 telah difungsikan dengan 3-

kloropropiltrietoksisilana (CPTES) sebagai agen pencantuman dan pepejal yang

diperoleh dilabel sebagai Cl-MCM-41. Ini diikuti dengan dua kali immobilisasi

kompleks N,N'-bis (4-hidroksisalisilidina)etilenadiimina-paladium(II) dan N,N'-bis

(4-hidroksisalisilidina)etilenadiimina-nikel(II) melalui dua kumpulan hidroksil pada

Cl-MCM-41. Dua mangkin heterogen ini telah diperoleh dan dilabel sebagai

MCMSalenPd dan MCMSalenNi. Pencirian kedua-dua mangkin dan

immobilisasinya telah buktikan dengan menggunakan spektrometer infra merah

(FT-IR), Pembelauan Sinar-X (XRD), penyerapan N2, mikroskop transmisi elektron

(TEM), mikroskop elektron imbasan (SEM), resonan nuklear magnet C-13 (13

C

MAS NMR), resonan nuklear magnet Si-29 (29

Si MAS NMR), tenaga serakan sinar-

X (EDX) dan spektroskop serapan atom (AAS). Mangkin MCMSalenPd telah

digunapakai untuk tindak balas Suzuki-Miyaura antara bromobenzena dengan asid

fenilboronik. Mangkin ini sangat stabil dan memberikan aktiviti yang berkesan

dalam masa 2 jam pada 130 oC. Peratusan penukaran adalah 98.7% dengan 100%

kepilihan. Kajian kinetik menunjukkan tindak balas mematuhi hukun kadar pseudo-

pertama. Tenaga pengaktifan adalah 0.393 kcal mol-1

. Mangkin ini telah diguna

semula lebih daripada lima kali tanpa menjejaskan aktiviti pemangkin. Mangkin

MCMSalenNi telah digunakan untuk tindak balas pengoksidaan benzil alkohol

xxviii

dengan asid periodik sebagai agen pengoksidaan. Mangkin ini sangat stabil dan

menunjukkan aktiviti pemangkinan yang berkesan. Peratusan penukaran adalah

98.4% dengan 48.8% kepilihan apabila tindak balas dilakukan untuk 1.5 jam pada

suhu 90 oC. Kajian kinetik menunjukkan hukum kadar pseudo-pertama dan tenaga

pengaktifan adalah 0.0538 kcal mol-1

. Mangkin ini boleh digunakan lebih daripada

enam kali.

xxix

PREPARATION AND IMMOBILIZATION OF PALLADIUM(II) AND

NICKEL(II) SALEN COMPLEXES ONTO MCM-41 FROM RICE HUSK

FOR SUZUKI-MIYAURA AND OXIDATION REACTIONS

ABSTRACT

MCM-41

was successfully prepared from rice husk ash (RHA) using

cetyltrimethylammonium bromide as a directing agent via sol-gel method at 100 oC.

MCM-41 was functionalized with 3-chloropropyltriethoxysilane (CPTES) as a

functioning agent and the solid obtained was labeled as Cl-MCM-41. This was

followed by double immobilization of N,Nʹ-bis(4-

hydroxysalicylidene)ethylenediimine palladium(II) and N,Nʹ-bis(4-

hydroxysalicylidene)ethylenediimine nickel(II) complexes via their two hydroxyl

groups onto Cl-MCM-41. Two new heterogeneous catalysts were obtained and

designated as MCMSalenPd and MCMSalenNi. The two prepared catalysts were

fully characterized and the double immobilization of the two complexes was

confirmed using fourier transform infrared (FT-IR), X-ray diffraction (XRD), N2

sorption measurements, transmission electron microscopy (TEM), scanning electron

microscopy (SEM), solid state C-13 nuclear magnetic resonance (13

C MAS NMR),

solid state Si-29 nuclear magnetic resonance (29

Si MAS NMR), energy dispersive

X-ray elemental analysis (EDX) and atomic absorption spectroscopy (AAS). The

MCMSalenPd catalyst was examined for Suzuki-Miyaura C-C coupling reaction of

bromobenzene with phenylboronic acid. The catalyst had good stability and excellent

activity within 2 hour at 130 oC. The conversion percentage was 98.7% with 100%

selectivity. The kinetic studies showed the reaction followed a pseudo first order rate

law. The activation energy was determined to be about 0.393 kcal mol-1

. The catalyst

was reused for more than five catalytic cycles without significant loss of activity.

The MCMSalenNi catalyst was examined for the oxidation reaction of benzyl

xxx

alcohol using periodic acid as oxidant. The catalyst was found to have good stability

and excellent activity. The conversion percentage was 98.4% with 48.8% selectivity

when the reaction was carried out for 1.5 hour at 90 oC. The kinetic study showed a

pseudo first order rate law and the activation energy was determined to be about

0.0538 kcal mol-1

. The catalyst was reused for more than six catalytic cycles with

good reusability.

1

1 CHAPTER 1

INTRODUCTION

1.1 General entrance

Catalysis is the increase in the rate of a chemical reaction due to the

partnership of a substance called a catalyst. Catalysts have become an even more

important area of chemical research since metals started to be used in 1796 by Von

Marum for dehydrogenation of alcohols. The term ―catalyst‖ was introduced as early

as 1836 by John Jacob Berzelius when he supposed that catalysts had special

abilities that could cause the convergence of chemical substances. Catalysis can be

divided into two types: homogeneous catalysis, in which either a gas (or a liquid)

phase was engaged in both as the catalyst and the reactant and heterogeneous

catalysis, in which the catalysts and the reactant are in different phases (Satterfield,

1991); solid catalysts are used generally with liquids or gaseous reactants.

Broadly speaking, there are three main functions for a substance which acts

as a catalyst; activity, selectivity and stability. The preference between them depends

on the specific demands in a certain chemical process (Hagen, 2006).

The term ―catalyst activity‖ refers to the rate of one or more reactions

continuing to chemical equilibrium in the presence of the catalyst. It depends on

many variable parameters, like pressure, temperature, the mole ratio of reactants and

products and the catalyst quantity. The term ―catalyst selectivity‖ means the extent

to which the catalyst expedites the reaction to shape one or more of the coveted

products. The term ―catalyst stability‖ means its active time in the industrial process;

it depends on several factors, like decomposition, cocking and poisoning. The

2

catalyst damping can be tracked by detecting either its activity or selectivity as a

function of time.

Furthermore, the important considerations which are most concerned about

in the designing of catalysts are that they be economically useful and friendly

towards the environment in their industrial applications. Many of these qualities can

be found in heterogeneous catalyst systems mainly due to their inherent properties;

the heterogeneous catalyst can be recovered easily by a simple filtration process so

the coveted product will be un-contaminated by transition metals or complexes

though some leaching which may occur. In addition, the catalyst usually can be

reused in subsequent reactions. In industrial use, both types of catalysts have

advantages and disadvantages, which are briefly compared in Table 1.1 (Hagen,

2006).

1.2 Heterogenization of homogeneous catalysts

To implement the separation and reusability of the homogeneous catalysts,

lots of approaches have been evolved to immobilize the homogeneous catalysts to

achieve heterogenization. The immobilization of the homogeneous catalyst can be

accomplished by supporting the homogeneous catalyst molecules by different ways

involving suitable inorganic, organic and so called hybrid materials. For example,

this can take place on inorganic support like silica, clay, zeolites and heteropoly

acids (Kuźniarska-Biernacka et al., 2011; Uruş et al., 2013; Yu et al., 2013) and for

organic support like carbon, dendrimers, polymeric ligands and and polyelectrolytes

(Chang et al., 2008; Deraedt et al., 2014; Wang et al., 2012). The theme of

immobilization has been a subject of diverse publications and several prior reviews

3

(Selvam et al., 2012; Wight and Davis, 2002). In this way many homogeneous

catalysts have been immobilized on different supports in order to get a better

understanding of this strategy in recent years (Steiner et al., 2004).

Table ‎1.1: Comparison of homogeneous and heterogeneous catalysts (Hagen, 2006).

Homogeneous Heterogeneous

Effectivity

Active centers All metal atoms Only surface atoms

Concentration Low High

Selectivity High Lower

Diffusion problems Practically absent Present

Reaction conditions Mild (50–200 oC) Severe (often >250

oC)

Applicability Limited Wide

Activity loss Cluster formation;

poisoning

Crystallites; poisoning

Catalyst properties

Structure/stoichiometry Defined Undefined

Modification possibilities High Low

Thermal stability Low High

Catalyst separation Often laborious: chemical

decomp., distillation,

extraction

Fixed-bed: unnecessary

Suspension: filtration

Catalyst recycling Possible Fixed-bed: unnecessary

Suspension: easy

Cost of catalyst losses High Low

4

1.3 Silica as support material

1.3.1 Silica abundance

A lot of homogeneous catalytic systems have been successfully immobilized

in relation to diverse silica supports. The silica and oxygen are the most abundant

components in the earth’s crust, with about 55% w/w of the earth's surface and its

existence either as silicate SiO3= or as silica with anhydrous formula SiO2 or with

hydrated formula SiO2.xH2O (Scott, 1993). The silica also exists in mineral ores like

quartz and granite and in cereal plants like rice and barley (Unger, 1979).

1.3.2 Surface nature of silica

Studies on silica started in the 1930s and led to the understanding of the

nature of the silica surface (Zhuravlev, 2000). Due to the considerable spectral and

chemical data, it has become clear that silica has two different functional groups, i.e.

the siloxane (≡Si–O–Si≡) in the bulk and numerous forms of silanol groups (≡Si–

OH) on the surface.

According to Nuclear Magnetic Resonance (NMR) studies, there are three

types of siloxane groups (≡Si–O–Si≡) which can be represented by the following

representation Qn = Si(OSi)n(OH)4-n, where n = 2–4 which refers to the number of

bridging bonds (–O–Si) bonded to the central Si atom, i.e. Q2, two siloxane bonds to

the central silicon atom, Q3, three siloxane bonds to the central silicon atom and Q

4,

four siloxane bonds to the central silicon atom as shown in Figure 1.1. Moreover,

experimentally there are two types of silanol groups found on the surface of silica:

the isolated group (a single hydroxyl group bonds to the silicon atom; ≡SiOH),

which is called the vicinal silanol group when it is bonded by an intramolecular

hydrogen bond with other silanol group; and the geminal group (two hydroxyl

5

groups bonded to one silicon atom =Si(OH)2) (Yang et al., 2006; Zhuravlev, 2000).

The surface of silica can be activated using modification reactions with an organic

compound to get new bonds, i.e. (≡Si–O–C≡, ≡Si–C≡, ≡Si–N꞊) through the reaction

of the silanol functional group.

Si

Si

Si O

Si

OSi

O Si

Si

SiOO

OO

O

OO

OHOH

OH

O

OO

O

O

HH

O

O

HH

O

O

H

H

Hydrated surface

Vicinal silanolGeminal silanol

Isolated silanol

Q2 Q3Q4

O OO

Siloxane

Figure 1.1: The various types of siloxanes and silanols in silica matrix.

1.3.3 Rice husk ash

Rice husk (RH), also called rice hull, is the hard outermost layer of rice seeds

which protect the seed during the growing season. It forms on average 22% of the

rice which is separated out during the husking process in the rice mills.

The world’s paddy production in 2014 was 744.4 million metric tonnes of

rice according to the rice market monitor, a report made by the Food and Agriculture

Organization of the United Nations at October 2014 and, consequently, more than

6

150 million metric tonnes of rice husk which are mostly burnt outdoors or thrown on

fallow land. Such cause disposal systems environmental problems. In many cases

the rice husk are ignited resulting in poisons fumes. Since RH is highly resistant to

moisture and fungus, it works as a good insulator (Mohanta et al., 2012).

Rice husk consists of 20% ash, 38% cellulose, 22% lignin, 18% pentose and

2% other organic trace compounds (Adam and Chua, 2004; Adam et al., 2006).

Because of its large silica content (SiO2), it rots slowly. When it is brought back to

the fields, it causes poor food returns and, when burnt, the rice husk gives out 3,410

kcal/kg. Therefore, it represents a cheap source of renewable energy, but when used

for energy generation, it gives about 17-26% ash, higher than the common fuels like

wood 0.2-2% and coal 12.2% (Flörke, 2008).

The rice husk ash (RHA) contains 87-97% of amorphous silica (Della et al.,

2002) and it has a variety of applications in material science, like being used in the

production of Portland cement, supplying a very compact concrete, as an absorbent

for oils and chemicals, a source of silicon, in the ceramics industry and as an

insulation material (Flörke 2008).

A more specialized application is as a natural, perfect source of amorphous

reactive silica, which is employed as an inorganic support for heterogeneous

catalysts (Chumee et al., 2009). Figure 1.2 shows the conversion of yellow rice husk

into white amorphous silica RHA.

7

Figure ‎1.2: The conversion of RH (a) to RHA (b, c) and their physical appearances.

RHA (b) is a result of incomplete burning resulting black coloured char

(Turmanova et al., 2011).

1.4 Mesoporous hybrid materials

Mesoporous organic-inorganic hybrid materials, a novel type of substance

with large surface areas and a tunable pore size (Fujita et al., 2009), have been

synthesised through the pairing of organic and inorganic constituents by using the

template system. There are three ways to incorporate functionalities; sequent

grafting of organic components onto the matrix of silica, one-pot condensation

reaction of silica as an inorganic component with silylated compound as organic

component and using bissilylated organic components to give periodic mesoporous

organosilicas (PMOs) (Hoffmann et al., 2006).

8

1.4.1 Amorphous silica

In nature, silica is inert and it exists in amorphous and crystalline forms. The

crystalline form comprises of a major portion of soil, sand and other various

minerals like quartz, cristobalite and tridymite (Iler, 1979). Amorphous silica is an

irregular form which is more highly hydrated than its crystalline form and is found

as opal and infusible earth (Gregg and Sing, 1982). Silica exists in various

amorphous forms called the dispersed system as described in Table 1.2.

1.4.2 Ordered mesoporous silica

The improvement of porous materials with high specific surface areas is

actually an attractive research area, especially with respect to prospective applied

fields such as gas stockpiling, sensor technology, adsorption, chromatography and

catalysts. Since 1990, a big breakthrough in material synthesis took place when a

Japanese group of researchers synthesized the first mesoporous nanoparticals of

silica (Kato et al., 1990). Later, in 1992, the researchers of Mobil oil company

developed periodic mesoporous silicate materials called Mobil Crystalline Materials

(M41S) (Beck et al., 1992). These M41S materials subrogated zeolite molecular

sieves which were held on a pore size about of 15Å; however, the M41S materials

posses pore diameters of about 2 10 nm and manifest amorphous pore walls.

According to International Union of Pure and Applied Chemistry (IUPAC)

classification, based on their pore size, microporous materials are classified as

materials whose pore diameters are below 2 nm, while mesoporous materials are

classified as materials whose pore diameters are in the range 2-50 nm and

9

macroporous materials are classified as materials whose pore diameters are above 50

nm.

Table ‎1.2: Amorphous dispersed systems.

Silica sol (colloidal) It is an important type of amorphous silica, which can be

prepared by sol-gel route (Brinker and Scherer, 1990) by

polycondensation and polymerization reactions of a soluble

silicate by adjusting the pH using an acid or base.

Silica hydrogel It is rigid, coherent and includes three dimensional

network, which can be prepared by proceeding of

condensing reaction until the gelation point.

Silica xerogel It is a partial collapsed gel, which can be made during the

dehydration process of hydrogel and losing its pore filling

liquid (mostly water).

Silica aerogel It is a porous ultralight material with high pore volume,

which can be prepared through supercritical drying process

of a gel and replacing the liquid component with a gas.

Precipitated silica It is dry silica with short distance characteristics structure

(Parfitt and Sing, 1976). It occurs when the ultimate silica

particles are coagulated as loose aggregates in the aqueous

medium.

Pyrogenic silica

(fumed)

It is anhydrous silica which is extremely low in bulk

density and has a high surface area which can be

synthesized by heating the silica at elevated temperatures

(Otto et al., 2008).

http://en.wikipedia.org/wiki/Ultralight_material

http://en.wikipedia.org/wiki/Supercritical_drying

http://en.wikipedia.org/wiki/Liquid

10

The structures of M41S materials can be divided into: MCM-41 with a

hexagonal arrangement of the mesopores; MCM-48 with a cubic arrangement of the

mesopores; and MCM-50 with a lamellar structure (Figure 1.3) (Kresge et al., 1992).

Figure ‎1.3: Structures of M41S materials: a) MCM-41 (2D hexagonal, p6mm), b)

MCM-48 (cubic, Ia3d) and c) MCM-50 (lamellar, p2) (Kresge et al.,

1992).

Eight years after the discovery of M41S materials, researchers from the

University of California in Santa Barbara prepared silica nanoparticles which were

called Santa Barbara Amorphous materials (SBA-15). These also involve a

hexagonal mesoporous arrangement with large surface areas and pore volumes and

act as excellent supports for several applications in chemistry (Zhao et al., 1998a).

1.4.3 Synthesis of ordered mesoporous silica

The synthesis of ordered mesoporous materials has been mentioned vastly in

the literature. A suggestion of the steps for the synthesis of MCM-41 is shown in

Figure 1.4. There are two routes: route (a) (true liquid-crystal templating procedure),

which was proposed initialy by Mobil’s researchers (Beck et al., 1992). The

11

synthesis of hexagonal silicate materials depends on the hydrophilic and

hydrophobic properties of the molecules of the structure-directing agents, mostly

long-chain alkyltrimethylammonium halides like cetyltrimethylammonium bromide

(CTAB). When it is dissolved in water, its molecules will arrange themselves to

form a micelle; the hydrophilic heads form the outside connect with the polar

solvent (i.e. water) and the hydrophobic tails are directed inwards of the micelle

because they are insoluble in polar solvents. These micelles will arrange themselves

together to form a micellar rod. Sequentially over time, these new micellar rods will

arrange to form a hexagonal array of micellar rods in solution. At the time of the

preform process, when adding the silicate source like the tetramethylorthosilicate to

the bulk solution, it will form a coating on the outside of the formed micelle rods.

Route (b) follows another procedure suggested by Monnier et al. (1993), which is

called cooperative liquid-crystal template mechanism. It implies that the silica and

the structure-directing agents are first mixed, and then the surfaces of micelles in the

silica environment are developed from sphere to rod and cluster-driven by non-

covalent weak interactions like hydrogen bonding, van der Waals forces and

electrostatic attraction (Zhao et al., 2012). After the process is completed, the

obtained white product is filtered, washed, dried and the template molecules are

removed by either calcination at 500 to 600 oC (Beck et al., 1992) or solvent

extraction, i.e., by using ethanol, acid ethanolic solution or using the ion-exchange

procedure (usually using NaCl or NH4Cl) (Inagaki et al., 1999) to get the

mesoporous material MCM-41.

The mesoporous material SBA-15 are synthesized by the same pathway, but

the difference is that the SBA-15 is synthesized using non-polar surfactants like

12

polyethylene glycol as template agent in acidic conditions while MCM-41 is

synthesized using CTAB in basic conditions (Zhao et al., 1998a, 1998b).

Figure ‎1.4: Probable pathways for the templated synthesis of ordered MCM-41 by

structure-directing agents: a) true liquid-crystal template, b) cooperative

liquid-crystal template mechanism (Monnier et al., 1993).

1.5 Organically functionalized mesoporous materials

The amalgamation of the properties of organic and inorganic construction

units within a single material is a most interesting notion for scientists’s estimation

of materials. This would merge an enormous collection of organic functionality with

the advantages of the inorganic substrate giving it robustness and thermal stability.

The participation of organic and inorganic components can produce a united

material whose properties vary comparatively from those of the solo, isolated

components. This is particularly applicable to the heterogenization of the

homogeneous catalysts when the inorganic materials can be modified with organic

functionalities, like alcohols, thiols, amines, carboxylic and sulfonic acids, so that a

real reaction can occur on a stable, hard inorganic matrix (Hoffmann et al., 2006).

13

Recently, more extensive studies have been developed to use hybrid organic-

inorganic materials for different applications, such as optics, sensing,

microelectronics, separation and catalysis.

In terms of the large pore size and high abundance of silanol groups on the

surface of the mesoporous silica, this makes it a serious nominee for supporting

different organic functions which can be located in the channels or in the pore walls.

In this respect, there are three main approaches for the synthesis of porous hybrid

materials depending on the organoalkoxysilane units (Figure 1.5);

1) Grafting approach; which is the subsequent amendments of the inner

walls of the inorganic mesoporous silica.

2) Co-condensation approach (one-pot synthesis); which is a potential direct

synthesis of mesoporous silica phase by the co-condensation of tetraalkoxysilanes

[(RO)4Si; i.e. Tetraethyl orthosilicate (TEOS) or Tetramethyl orthosilicate (TMOS)]

with terminal trialkoxyorganosilanes of the type (RO)3SiR using a structure-

directing agent (SDA).

3) Periodic mesoporous organosilica production approach; this was first

used in 1999 (Inagaki et al., 1999; Melde et al., 1999) by hydrolysis and subsequent

condensation reactions of the bridged organosilica precursors of the type (RO)3Si–

R–Si(OR )3, which had long been known from sol-gel chemistry (Loy and Shea,

1995; Shea and Loy, 2001).

14

Figure ‎1.5: Three synthesis approaches for mesoporous hybrid materials (Hoffmann

et al., 2006).

1.6 Metal incorporation in mesoporous materials

Metals incorporated in mesoporous materials have been widely reported for

various numerous approaches inflation to catalytic applications. Many different

metals have been inserted in the mesoporous materials, such as B, Al, Ga, In, Ti

(Adam and Ahmed, 2008), and different transition metals, such as Mn, Fe, Co, Ni,

Cu (Adam et al., 2013b; Sayari and Liu, 1997). The following sections will discuss

two types of metal incorporation onto silica:

a) Incorporation of metal ions onto silica without organic ligands.

15

b) Incorporation of metal ions within organic ligands onto silica.

1.6.1 Incorporation of metals without organic ligands

1.6.1.1 Metal salts as metal precursors

The metals incorporated within mesoporous materials can be done by one-

pot method or by post-treatment. In the one-pot method, metal salt is added from the

beginning in the initial gel while the post-treatment can be carried out either by

impregration or by ion-exchange; in both procedures the interaction occurs between

the metal salt and the initially formed mesoporous silica. The resulting mesoporous

materials resulting from one route is not necessarily similar to that of the other. The

post-treatment also can be carried via grafting method. Tilley et al. (2002)

synthesized single-site grafted catalysts using a technique called as precursor route.

(iPrO)Ti[OSi(

tBu)3]3 and Ti[OSi(

tBu)3]4 were used as precursor grafted onto MCM-

41 and SBA-15, respectively (Fujdala, 2003; Nozaki et al., 2002). By utilizing this

approach various mesoporous materials have been obtained with different single-site

metal centers, such as Cr, Fe and V.

1.6.1.2 Metal complexes as metal precursors

In some cases, the right selection of the metal precursor in the post-treatment

can produce a mesoporous material with high metal dispersion. (Bonneviot et al.,

1990) have noticed that when Ni(NH3)x(H2O)(6-2x) complexes are adsorbed on silica,

they loose the aqua ligands through the replacement by ≡SiO- from the silica

surface. Moreover, the water molecules can easily replace the monodentate NH3

ligands through drying and calcination process, forming nickel phyllosilicates and

nickel hydroxide on the mesoporous material (Che et al., 1995). Using a bidentate

16

ligand such as ethylenediamine instead of NH3 can produce more isolated metal

species after calcination with high dispersion on the surface of silica (Sun et al.,

2006).

1.6.2 Incorporation of metal ions within organic ligands

There are four identified methods for anchoring metal complexes onto

mesoporous materials; i) encapsulation, ii) exchanging, iii) post-synthesis (grafting),

and iv) one-pot synthesis. The first two methods are insignificant, with weak

electrostatic interactions between the metal complex and the mesoporous support.

Subsequently, the anchored complexes of these two methods are normally easily

leached through the reaction process. The significant immobilization of complexes

on solid support are usually accomplished by post-synthesis and one-pot synthesis

methods by virtue of the formation of stable covalent bonds between a functional

group (R) and the solid support ≡Si-O-Si-R.

1.6.2.1 Grafting incorporation (post-synthesis method)

There are numerous strategies for grafting metal complexes into the channel

of mesoporous materials to create the active centers in the solid support. The

activation of the mesoporous materials can be achieved efficiently due to their

confinement effect and changeable of the hydrophilic-hydrophobic chemical

environment compared to that of homogeneous complexes, which suffer

deactivation due to their oligomerization probability. Herein, we are tallying the

various grafting strategies to two kinds: direct grafting and indirect grafting.

17

1.6.2.1.1 Indirect grafting incorporation

The grafting of metal complexes can be established indirectly by grafting

organic functional groups onto mesoporous materials such as numerous functioned

propyltrialkoxysilanes to tethered organic moieties. A further modification can be

done by a suitable nucleophilic substitution on the functionalized ligand leading to a

metal complex through complexation with acertain transition metal. However, it is

not possible to be certain whether all of the added metal salt reacts and is

coordinated with the ligand or whether it is just physically adsorbed on the surface

or pores of the support, so it can not be proven that the catalytic activity is facilitated

due to the metal complex or the adsorbed free metal salt.

Herein, are selected examples from literature to illustrate this approach. In

2012, Sarmah et al. reported the indirect grafting of palladium(II) Schiff base

complex onto silica (see Figure 1.6), where the Schiff base has been formed by the

condensation between 3-aminopropyltriethoxysilane (APTES) functionalized silica

gel and acetamide and then developed into a complex with palladium acetate

(Sarmah et al., 2012). The materials obtained have been utilized for the Suzuki-

Miyaura cross-coupling reactions of aryl halides. For the Suzuki-Miyaura reaction,

the general catalyst used is Pd(OAc)2. However this is a homogeneous catalyst

which has all the disadvantages discussed earlier.

18

Figure ‎1.6: Indirect grafting of palladium(II) complex onto silica gel (Sarmah et al.,

2012).

In 2011, Bhunia and Koner reported the preparation of nickel(II) Schiff base

complex via post-synthesis method. The Schiff base obtained by the condensation of

salicylaldehyde and 3-aminopropyltriethoxysilane (APTES), was anchored on

MCM-41. The activity of the prepared catalyst was examined for epoxidation of

olefins by using tert-butyl hydroperoxide as an oxidant (Bhunia and Koner, 2011)

(Figure 1.7).

19

Figure ‎1.7: Indirect grafting of nickel(II) complex onto silica MCM-41 (a)

functionalization, b) condensation, c) metal complexation) (Bhunia and

Koner, 2011).

1.6.2.1.2 Direct grafting method

In the direct grafting, the metal complex is first produced at the molecular

level, then it is immobilized as a whole onto the surface of mesoporous materials

through a suitable organic silanes, whether it is inside the molecular construction of

the metal complex or functioning on the surface of the mesoporous materials, as

shown in the examples in Figure 1.8 and Figure 1.9 respectively.

20

Figure 1.8: Synthesis of copper(II) complex covalently anchored on silica (s = silica)

(Mishra et al., 2012).

Figure 1.9: Immobilization of palladium complex onto silica gel through Schiff-base

condensation (Sarmah et al., 2013).

21

1.6.2.2 One-pot incorporation

Successful mesoporous materials containing metal complex in their pore wall

can be synthesized by co-condensation of a silica source such as common

tetraethoxysilane (TEOS) or tetramethoxysilane (TMOS) and terminal trialkoxy

organosilanes precursors with structure-directing agents (SDAs) by sol-gel

chemistry. This is shown in Figure 1.10.

Figure 1.10: Preparation of immobilized catalysts via sol-gel process (Lijuan et al.,

2009).

22

1.7 Salen Schiff base ligand and metal-salen complex

In 1864, the German scientist Hugo Schiff described the condensation of an

aldehyde with an amine leading to the production of imine with general chemical

formula R1R2C=N-R3, known as the so-called Schiff base. A wide range of

numerous types of Schiff base ligands can be coordinated with different metals

through the imine nitrogen and another heteroatom, which can be oxygen, nitrogen

or phosphine usually linked to the original aldehyde or ketone molecule to construct

metal complexes (Cozzi, 2004).

The preparation of new metal complexes of Schiff bases is still interesting in

catalyst chemistry. This is due to the ligands being easily prepared, supports

substrate chirality, tuning the located metal, enhancing the solubility and stability.

As a result, they were widely used in homogeneous and heterogeneous catalysts

(Yoon and Jacobsen, 2003).

Salicyladimines (salens) are an example of Schiff base complexes which

have been known since 1933 (Pfeiffer et al., 1933) and which can be formed by the

condensation of salicyladehyde bearing different substituents with a primary 1,2-

diamine (Figure 1.11). The salen complexes have been comprehensively studied,

especially when Jacobsen (Jacobsen et al., 1990) and Katsuki (Katsuki et al., 1990)

detected the enantioselective epoxidation of unfunctionalized alkenes by using

catalysts of chiral Mn-salen complexes and went on to investigate various catalytic

reactions by these types of complexes such as oxidation of hydrocarbons (Lee et al.,

1998), aziridination of alkenes (Omura et al., 2004), Diels-Alder reaction (McGilvra

and Rawal, 2004), hydrolytic kinetic resolution of epoxides (Shin et al., 2004),

alkylation of aldehydes (Maeda et al., 2004) and oxidation of sulfides to sulfoxides

23

(Kim and Rajagopalba, 2003). In this work, the salen ligand was synthesized by

condensation of 4-hdroxysalicyladehyde with 1,2-ethylenediamine and complexed

with palladium and nickel ions.

O

OH

salicylaldehyde

H2N NH2

ethylendiamine

O

N N

O

M

OH

N N

HO

Salen ligand

Salen complex

Alcohol solvent

reflux

Metal salt

reflux

Figure ‎1.11: Synthesis of salen Schiff base ligand and salen complex (Jacobsen and

Larrow, 1998).

1.7.1 Incorporation of metal-salen complexes onto mesoporous materials

The incorporation of metal-salen complexes onto mesoporous silica can be

accomplished by grafting or by one-pot synthesis methods as referred to the above.

In 2002, Baiker et al. was the first to immobilize the cobalt-salen and copper-salen

complexes on xerogel and aerogel silica by sol-gel approach via co-condensation of

N,N-ethylenebis(salicylidenaminato) (salen) metal complexes with tetraethoxysilane

under acid condition, with the complex possessing two imine bonds with metal

24

without any bridge connection for two nitrogen atoms. The material produced was

used as catalyst for aerobic allylic oxidation of isophorone (Baiker et al., 2001;

Murphy and Baiker, 2002) (Figure 1.12).

Figure 1.12: Sol-gel immobilization of Co-salen and Cu-salen complexes on silica

gel (Baiker et al.2001).

Below are some reported examples for the incorporation of salen complexes

formed by different linkages of the grafting or one-pot synthesis method.

In 2008, Zhang et al. synthesized the mesoporous silica-supported chiral Mn

(salen) catalysts via the linking groups which are axially connected to Mn atoms via

a linkage connected to the salen ligand. The prepared homogeneous and

heterogeneous Jacobsen catalysts were evaluated in the asymmetric epoxidation of

β-methylstyrene with NaClO as an oxidant (Zhang et al., 2008) (Figure 1.13).

preparation and immobilization of palladium(ii) and nickel …

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