Design and Applications of Charge-Separated Metal-Organic Frameworks

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Sheela Thapa University of New Mexico - Main Campus

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Sheela Thapa Candidate

Chemistry and Chemical Biology Department

This dissertation is approved, and it is acceptable in quality and form for publication:

Approved by the Dissertation Committee:

Prof. Yang Qin, Chairperson

Prof. Brian Gold

Prof. Christine Mai Le

Prof. Gayan Rubasinghege

Design and Applications of Charge-Separated Metal-Organic Frameworks

Sheela Thapa

M.Sc. Chemistry, Tribhuvan University, 2009

DISSERTATION

Submitted in Partial Fulfillment of the Requirements for the Degree of

Doctor of Philosophy Chemistry

The University of New Mexico Albuquerque, New Mexico

December 2019

DEDICATION

To my beloved parents and my loving husband!

iii

ACKNOWLEDGEMENTS

First of all, I would like to express my sincere gratitude to my advisor, Professor

Yang Qin for providing me an opportunity to work under his supervision. I would like to thank him for his encouragement, guidance and research training during my PhD research.

The research knowledge and skills I have learned from him are the most valuable achievements of my PhD degree.

My cordial thanks to my dissertation committee members, Professor Brian Gold,

Professor Christine Mai Le and Professor Gayan Rubasinghege for their time and suggestions about my work. I would also like to thank my research proposal committee members Professor Ramesh Giri and Professor Wei Wang for their feedback and advice on my research work. Special thanks to Professor Jeff Rack for all the insightful comments and advice on my research work. Thanks to Dr. Diane Dickie from University of Virginia for all single crystal XRD analysis. Thanks to Professor Gayan Rubasinghege and Eshani

Hettiarachchi from New Mexico Tech for the research collaboration.

Additionally, I would like to thank all the past and present group members of the

Qin research group for their help and assistance in the lab and my research work. I will always remember and cherish every happy and funny moments I spent with my lab mate and a very good friend Lingyao. Many thanks to my friends Rajani, Surendra, Jillian,

Griffin, Sangita, Umesh, Prakash, Tefera and Ranjana for their friendship and making my time at UNM memorable. I am also thankful to all the professors and staffs of the UNM

Chemistry Department for their assistance throughout my PhD study.

Finally, I would like to thank my family, my father Narayan B. Thapa, my late mother Krishna K. Thapa, my sisters and brothers-in-law for their unconditional love and

iv support. I would like to thank my sweet and loving husband Dr. Shekhar KC for being my best friend. He has always inspired me to pursue my dream and supported me during my good and bad times. My PhD research would not have been possible without his love, help, guidance and encouragement.

Design and Applications of Charge-Separated Metal-Organic Frameworks

Sheela Thapa

M.Sc. Chemistry, Tribhuvan University, Nepal, 2009

Ph.D. Chemistry, University of New Mexico, USA, 2019

Abstract

Ionic tetrapodal ligands with colinear coordinating arms are very effective for designing hybrid porous materials with unusual structure and properties. The novelty of this research work lies in the utilization of a unique borate ligand that leads to charge- separated MOF structure with tailor designed properties. Borate ligands being tetrahedral afford 3D materials and the negative charge of borate anion can compensate the positive charge of metal ion in the framework. The borate ligands designed in this research consists of quaternary ammonium cation and anionic borate with four pyridine arms. These ligands upon coordination with Cu(I), Cu(II), Ag(I) and Co(II) metal cations formed six different charge-separated MOFs (UNM-1 to UNM-6).

Chapter 1 covers a brief review on the design, synthesis, classification and applications of MOFs. Additionally, MOF interpenetration, control and applications of interpenetration are discussed with examples.

Chapter 2 is about the synthesis, structural characterization and application of a charge-separated diamondoid UNM-1 MOF assembled from tetrakis(4-

vi pyridyltetrafluorophenylethynyl)borate (T1) and Cu(I) metal cation. UNM-1 MOF structure displays 4-fold interpenetration, resulting in high environmental stability, and at

2 the same time possesses relatively large surface area (SABET = 621 m /g) due to the absence of free ions. Gas adsorption measurements revealed temperature-dependent CO2 adsorption/desorption hysteresis and large CO2/N2 ideal selectivities up to ca. 99 at 313 K and 1 bar, suggesting potential applications of this type of charge-separated MOFs in flue gas treatment and CO2 sequestration.

In Chapter 3, synthesis and single-crystal structural characterization of four new charge- separated MOFs (UNM-2, UNM-3, UNM-4 and UNM-5) based on two tetrapodal borate ligands: (T1) and tetrakis(4-pyridyltetrafluorophenyl)borate (T2) having coordinating arms of different lengths and pyridine groups at the end of each arm are discussed. Coordination of these tetrapod with Cu(I)/Cu(II), and Ag(I) ions under specific conditions led to a series of new charge-separated MOFs in single crystalline forms. UNM-2/UNM-3 in monoclinic

C 2/c space group, are derived from Cu(NO3)2 upon coordination with T1. On the other hand, coordination of T2 with Cu(CH3CN)4BF4 and AgBF4 respectively formed UNM-4 and UNM-5 in monoclinic I 2/a space group. These MOFs possesses several degrees of interpenetration that are correlated with the arm lengths of ligands. All these MOFs, except

UNM-3 are 3D, 2-fold interpenetrated crystalline frameworks. UNM-3 is 1D framework containing coordinated solvent molecules in the crystal structure.

In Chapter 4, synthesis of UNM-6, characterization, post-synthetic ionic exchange, applications in chemical fixation of CO2 and CO2/N2 adsorption are discussed.

UNM-6 is synthesized via solvothermal synthesis from the assembly of T2 with Co(NO3)2.

UNM-6 is crystallized in cubic crystal system with P -4 3 n space group and is 4-fold

vii interpenetrated, 3D framework containing coordinated nitrate ion in the crystal structure.

The nitrate ion in UNM-6 is replaced with different anions like halides, cyanide and azide by post-synthetic ionic exchange reaction. Furthermore, UNM-6 and bromide exchanged

Br-UNM-6 MOFs are used as heterogenous catalysts in the cycloaddition reaction of CO2 to epichlorohydrin without co-catalyst. Both catalysts can be easily separated from the product, very efficient and stable, with ability to be reused multiple times. Additionally,

UNM-6 showed very high CO2/N2 separation selectivity of 1022 at 313 K under 1 atm pressure.

In summary, this dissertation highlights the versatility of tetrapodal borate ligands in engineering charge-separated MOFs with diverse structures and controlled functionality.

viii

Table of Contents

DEDICATION ...... iii

ACKNOWLEDGEMENTS ...... iv

Abstract ...... vi

List of Figures ...... xii

List of Schemes ...... xviii

List of Tables ...... xix

List of Abbreviations ...... xxii

Chapter 1. A Review on Metal-Organic Frameworks ...... 1

1.1 Introduction ...... 1

1.2 Metal Organic Frameworks (MOFs) ...... 2

1.2.1 Design and Synthesis of Metal-Organic Frameworks...... 4

1.2.2 Classification of Metal-Organic Frameworks ...... 8

1.2.3 Applications of MOFs ...... 9

1.2.4 Interpenetration in MOFs ...... 12

Chapter 2. Synthesis, Characterization and Application of Charge-Separated

Diamondoid UNM-1 MOF ...... 18

2.1 Introduction ...... 18

2.2 Results and Discussion ...... 21

2.2.1 Synthesis and Characterization of Ligand T1 and UNM-1 ...... 21

2.2.2 Stability Analysis of UNM-1 ...... 25

2.2.3 Gas Adsorption Analysis of UNM-1 ...... 28

2.3 Conclusion ...... 43

2.4 Experimental Procedure ...... 43

Chapter 3. Synthesis and Characterization of Modified Ligand T2 and Charge-

Separated MOFs (UNM-2, UNM-3, UNM-4 and UNM-5) ...... 51

3.1 Introduction ...... 51

3.2 Results and Discussion ...... 52

3.2.1 Synthesis and Characterization of Ligand T2 and MOFs (UNM-2 to UNM-5)

...... 52

3.3 Conclusion ...... 64

3.4 Experimental Procedure ...... 66

Chapter 4. Synthesis, Characterization and Applications of a Cubic MOF (UNM-6)

...... 70

4.1 Introduction ...... 70

4.2 Results and Discussion ...... 71

4.2.1 Synthesis and Structural Analysis of UNM-6...... 71

4.2.2 Anion Exchange in UNM-6 ...... 74

4.4.3 Chemical Fixation of CO2 by UNM-6 and UNM-6-Br...... 77

4.4.4 Gas Adsorption Analysis of UNM-6 ...... 84

4.4 Conclusion ...... 95

4.5 Experimental Procedure ...... 96

References ...... 101

List of Figures

Chapter 1

Figure 1.1. Schematic representation of assembly of inorganic units and organic linkers resulting metal-organic frameworks and examples of most studied MOFs ...... 3

Figure 1.2. Some selected examples of rigid ligands ...... 5

Figure 1.3. Some selected examples of flexible ligands ...... 6

Figure 1.4. Schematic representation of interpenetration ...... 12

Figure 1.5. Representation of (A) 5-fold interpenetration and (B) 10-fold in 3D, diamondoid frameworks ...... 13

Figure 1.6. Schematic representation of common methods used to control interpenetration

...... 14

Chapter 2

Figure 2.1. Examples of some common ligands used for the synthesis of charge-separated

MOFs ...... 20

1 12 19 11 Figure 2.2. H, C, F and B NMR spectra of T1 in CDCl3 ...... 22

Figure 2.3. Single crystal X-ray structure of UNM-1: (A) partial view of crystal structure showing coordination environment around copper; (B) partial view of crystal structure showing coordination environment around boron; (C) extended structure; (D) space-filling model of a 2×2×2 unit cell viewed from the b-axis; and (E) space-filling model of a 2×2×2 unit cell viewed from the c-axis showing 4-fold interpenetration (Each colors; red, pink, grey and green represents 1-fold interpenetration). Carbon atoms appear in grey, nitrogen in blue, boron in yellow, fluorine in green, and copper in cyan. Hydrogen atoms are omitted for clarity ...... 25

xii

Figure 2.4. PXRD patterns of UNM-1, from bottom to top: simulated from single crystal

XRD data (black), as synthesized (red), sample after soaking for 48 h in HCl solutions of pH 1 (maroon) and pH 4 (pink), neutral water of pH 7 (blue), NaOH solutions of pH 10

(navy blue) and pH 13 (purple) and, after all adsorption experiments (green) ...... 27

Figure 2.5. Thermogravimetric analysis (TGA) histogram of UNM-1 under N2 environment ...... 28

Figure 2.6. (A) N2 adsorption/desorption isotherm at 77 K; (B) BET plot; (C) Langmuir plot and (D) NLDFT pore size distribution of UNM-1 ...... 29

Figure 2.7. Fitting of CO2 and N2 adsorption isotherm at 273 K. Dots represents experimental data, lines are fitting curves ...... 31

Figure 2.8. Fitting of CO2 and N2 adsorption isotherm at 298 K. Dots represents experimental data, lines are fitting curves ...... 33

Figure 2.9. Fitting of CO2 and N2 adsorption isotherm at 303 K. Dots represents experimental data, lines are fitting curves ...... 35

Figure 2.10. Fitting of CO2 and N2 adsorption isotherm at 313 K. Dots represents experimental data, lines are fitting and interpolating curves. N2 (313 K) adsorption data was linearly interpolated because none of the other available model of pyIAST produced the good fit ...... 37

Figure 2.11. Fitting of CO2 and N2 adsorption isotherm at 323 K. Dots represents experimental data, lines are interpolating curves. Both N2 (323 K) and CO2 (323 K) adsorption data were linearly interpolated because good fit was not obtained by any other model available in pyIAST ...... 39

xiii

Figure 2.12. (A) Adsorption/desorption isotherms of CO2 and N2 on UNM-1 at various temperatures; insert: hysteresis percentages of CO2 adsorption/desorption isotherms; (B) ideal CO2/N2 selectivities at different temperatures and pressures ...... 41

Figure 2.13. The Van’t Hoff isochores for CO2 adsorption on UNM-1 at different loadings

...... 42

Figure 2.14. Isosteric heat of CO2 adsorption for UNM-1 at different loading calculated by Clausius-Clapeyron method (Left) and virial fitting of CO2 adsorption isotherm for

UNM-1 at zero coverage (Right). Experimental data are the dots, lines are fitting and the parameters are given in the table within the plot for virial method ...... 43

Chapter 3

Figure 3.1. Structure of tetrapodal ligand T1 and T2 ...... 52

1 13 11 19 Figure 3.2. H, C, B and F NMR spectra of T2 in CDCl3 ...... 54

Figure 3.3. Single crystal X-ray structure of UNM-2; (A) Partial view of crystal structure showing coordination environment around copper; (B) coordination environment around

Boron; (C) extended network; (D) space-filling model of a 2×2×2 unit cell viewed from the a-axis; (E) space-filling model of a 2×2×2 unit cell viewed from the b-axis and (F) space-filling model of a 2×2×2 unit cell viewed from the c-axis showing 2-fold interpenetration (red and grey colors each indicates 1-fold interpenetration). Atoms of carbon appears in grey, nitrogen in blue, boron in yellow, fluorine in green, oxygen in red and copper in cyan. Hydrogen atoms are omitted for clarity ...... 56

Figure 3.4. Single crystal X-ray structure of UNM-3; (A) framework structure showing chain like structure; (B) a crystalline unit showing coordination environment around boron

xiv molecules; (D) space-filling model of a 2×2×2 unit cell viewed from the a-axis; (E) space- filling model of a 2×2×2 unit cell viewed from the b-axis; and (F) space-filling model of a

2×2×2 unit cell viewed from the c-axis. Atoms of carbon appears in grey, nitrogen in blue, boron in yellow, fluorine in green, oxygen in red and copper in cyan. Hydrogen atoms are omitted for clarity ...... 58

Figure 3.5. Single crystal X-ray structure of UNM-4; (A) Partial view of crystal structure showing coordination environment around copper; (B) coordination environment around

Boron; (C) extended network; (D) space-filling model of a 2×2×2 unit cell viewed from the a-axis; (E) space-filling model of a 2×2×2 unit cell viewed from the b-axis and (F) space-filling model of a 2×2×2 unit cell viewed from the c-axis showing 2-fold interpenetration (red and grey colors each indicates 1-fold interpenetration). Atoms of carbon appears in grey, boron in yellow, fluorine in green and copper in cyan. Hydrogen atoms are omitted for clarity ...... 61

Figure 3.6. Single crystal X-ray structure of UNM-5; (A) Partial view of crystal structure showing coordination environment around copper; (B) coordination environment around

Boron; (C) extended network; (D) space-filling model of a 2×2×2 unit cell viewed from the a-axis; (E) space-filling model of a 2×2×2 unit cell viewed from the b-axis; and (F) space-filling model of a 2×2×2 unit cell viewed from the c-axis showing 2-fold interpenetration (red and grey colors each indicates 1-fold interpenetration). Atoms of carbon appears in grey, boron in yellow, fluorine in green and silver in light grey. Hydrogen atoms are omitted for clarity ...... 63

Chapter 4

Figure 4.1. Single crystal X-ray structure of UNM-6; (A) Partial view showing coordination environment around cobalt; (B) Partial view showing coordination environment around boron; (C) extended network; (D) single pore environment (E) space- filling model of a 2×2×2 unit cell viewed from the a-axis showing 4-fold interpenetration

(each color represents 1-fold interpenetration). Color Scheme: C (grey); N (blue); B

(yellow); F (green); O (red) and Co (cyan). Hydrogen atoms are omitted for clarity ...... 73

Figure 4.2. Pictures of UNM-6 before and after exchange with halides, cyanide and azide

...... 75

Figure 4.3. FTIR of UNM-6 before and after PSIE...... 76

Figure 4.4. PXRD of UNM-6 before and after PSIE...... 77

Figure 4.5. PXRD of UNM-6 catalyst before and after catalytic runs ...... 83

Figure 4.6. PXRD of UNM-6-Br catalyst before and after catalytic runs ...... 83

Figure 4.7. (A) N2 adsorption/desorption isotherm at 77 K; (B) 7-point BET plot; (C) 7- point Langmuir plot and (D) NLDFT pore size distribution of UNM-6 ...... 85

Figure 4.8. CO2 and N2 adsorption/ desorption isotherm of UNM-6 at 273 K, 298 K, 303

K, 313 K and 323 K ...... 86

Figure 4.9. Fitting of CO2 and N2 adsorption isotherm at 273 K. Dots represents experimental data, lines are fitting curves ...... 87

Figure 4.10. Fitting of CO2 and N2 adsorption isotherm at 298 K. Dots represents experimental data, lines are fitting curves ...... 89

Figure 4.11. Fitting of CO2 and N2 adsorption isotherm at 313 K. Dots represents experimental data, lines are fitting curves ...... 91

xvi

Figure 4.12. Ideal CO2/N2 selectivities of UNM-6 at different temperatures and pressures

...... 93

Figure 4.13. The Van’t Hoff isochores for CO2 adsorption on UNM-6 (first seven plots) and summary of Qst at different loadings calculated by Clausius-Clapeyron method (last plot) ...... 94-95

xvii

List of Schemes

Chapter 2

Scheme 2.1 Synthesis of ligands and Schematic representation of UNM-1 ...... 22

Chapter 3

Scheme 3.1. Synthesis of ligand T2 ...... 54

Chapter 4

Scheme 4.1. Application of UNM-6 and UNM-6-Br as heterogeneous catalysts ...... 79

Scheme 4.2. A reaction mechanism for the CO2 insertion into epoxide catalyzed by an acid in the presence of a tetraalkylammonium halide co-catalyst ...... 79

xviii

List of Tables

Chapter 2

Table 2.1. Crystallographic data of UNM-1...... 24

Table 2.2. N2 (273 K) - Quadratic Parameters ...... 31

Table 2.3. CO2 (273K) - Langmuir Parameters ...... 31

Table 2.4. IAST adsorption selectivity calculated for mixed gas (15% CO2: 85% N2) at

273 K ...... 32

Table 2.5. N2 (298 K) - Quadratic Parameter ...... 33

Table 2.6 CO2 (298K) - Langmuir Parameters ...... 33

Table 2.7. IAST adsorption selectivity calculated for mixed gas (15% CO2: 85% N2) at

298 K ...... 34

Table 2.8. N2 (303 K) - Quadratic Parameter ...... 35

Table 2.9. CO2 (303K) - Langmuir Parameters ...... 35

Table 2.10. IAST adsorption selectivity calculated for mixed gas (15% CO2: 85% N2) at

303K ...... 36

Table 2.11. CO2 (313 K) - Dual-Site Langmuir Parameters ...... 37

Table 2.12. IAST adsorption selectivity calculated for mixed gas (15% CO2: 85% N2) at

313K ...... 38

Table 2.13. IAST adsorption selectivity calculated for mixed gas (15% CO2: 85% N2) at

323K ...... 40

Chapter 3

Table 3.1. Summary of crystallographic data and some important properties of UNM-2 and UNM-3 synthesized from the assembly of T1 and Cu(II) salt ...... 59

xix

Table 3.2. Summary of crystallographic data and some important properties of UNM-4 and UNM-5 synthesized from the assembly of T2 with Cu(I) and Ag(I) salts ...... 64

Chapter 4

Table 4.1. Crystallographic data for UNM-6 ...... 72

Table 4.2. Catalytic performance evaluation of different catalyst in the CO2 cycloaddition reaction of epichlorohydrin ...... 80

Table 4.3. Comparison of relative conversion of chloropropene carbonate obtained from the CO2 cycloaddition to epichlorohydrin at different temperature ...... 80

Table 4.4. Comparison of conversion of chloropropene carbonate at 10 mg catalyst loading ...... 81

Table 4.5. Comparison of conversion of chloropropene carbonate at 30 mg catalyst loading ...... 81

Table 4.6. Comparison of conversion of chloropropene carbonate at 50 mg catalyst loading ...... 81

Table 4.7. Investigation of recyclability of catalysts in the cycloaddition of CO2 to epichlorohydrin under optimized conditions ...... 82

Table 4.8. N2 (273 K) - Langmuir Parameters ...... 87

Table 4.9. CO2 (273 K) - BET Parameters ...... 87

Table 4.10. IAST adsorption selectivity calculated for mixed gas (15% CO2: 85% N2) at

273 K ...... 88

Table 4.11. N2 (298 K) - Quadratic Parameter ...... 89

Table 4.12. CO2 (298 K) - BET Parameters ...... 89

Table 4.13. IAST adsorption selectivity calculated for mixed gas (15% CO2: 85% N2) at

298 K ...... 90

Table 4.14. N2 (313 K) - Quadratic Parameter ...... 91

Table 4.15. CO2 (313 K) - Quadratic Parameter ...... 91

Table 4.16. IAST adsorption selectivity calculated for mixed gas (15% CO2: 85% N2) at

313 K ...... 92

xxi

List of Abbreviations

% Percentage

[Ni(timpt)2] (ClO4) Nickel(2,4,6-tri[4-(imidazol-1-ylmethyl)phenyl]-1,3,5-triazine)

perchlorate

°C Degree Celsius

11B Boron nuclear magnetic resonance

13C Carbon nuclear magnetic resonance

1D One-dimensional

19F Fluorine nuclear magnetic resonance

1H Proton nuclear magnetic resonance

1st First

2D Two-dimensional

3D 3-dimensional

3rd Third

Å Angstrom

Ag Silver

AgBF4 Silver tetrafluoroborate

AgoTf Silver trifluoromethanesulfonate

B Boron bdc 1,4-benzene dicarboxylic acid

BET Brunauer, Emmett and Teller

BF3.Et2O Boron trifluoride diethyl etherate bpdc 4,4′-biphenyldicarboxylate

xxii

Br¯ Bromide anion

Bu4NBr Tetrabutylammonium bromide

Bu4NF Tetrabutylammonium fluoride

C Carbon

C6F6 Hexafluorobenzene

C6H4 Phenylene

CaH2 Calcium hydride

CDCl3 Deuterated chloroform

CH2Cl2 Dichloromethane

CH3CN Acetonitrile

CH3OH Methanol

ClO¯ Perchlorate anion

Cm-1 Reciprocal centimeter (Wavenumber)

Co Cobalt

Co(NO3)2 . 6H2O Cobalt nitrate hexahydrate

CO2 Carbon dioxide

Cu Copper

Cu Kα X-ray energy frequency (λ = 1.5406 Å)

Cu(CH3CN)4BF4 Tetrakis(acetonitrile)copper(I) tetrafluoroborate

Cu(NO3)2 . xH2O Copper(II) nitrate hydrate

DMF N, N-Dimethylformamide

F Fluorine

FTIR Fourier-transform infrared spectroscopy

xxiii g/cm3 Gram per centimeter cube h Hour

H2 Hydrogen

HCl Hydrochloric acid

HKAUST-1 Hongkong University of Science and Technology MOF-1

Htb S-heptazine tribenzoate

IAST Ideal Adsorbed Solution Theory

IR MOFs Isoreticular Metal-Organic Frameworks

KAUST-7 King Abdullah University of Science and Technology MOF-7

K2CO3 Potassium carbonate

KI Potassium iodide

KJ/mol Kilojoule per mole m2/g Meter squared per gram mg Milligram

MHz Megahertz

MIL Material Institute Lavoisier mmol/g Millimoles per gram

Mo Kα X-ray energy frequency (λ = 0.71075 Å)

MOFs Metal-Organic Frameworks

N(CH2CH3)3 Triethylamine

N Nitrogen

N2 Nitrogen gas

Na Sodium

xxiv

NaCl Sodium chloride

NaCN Sodium Cyanide

NaN3 Sodium Azide

NaNO3 Sodium nitrate

NaOH Sodium hydroxide

NLDFT Non-local density functional theory

NMR Nuclear magnetic resonance

¯ NO3 Nitrate anion

O Oxygen

PCN Porous Coordination Network

Pd(PPh3)4 Tetrakis(triphenylphosphine)palladium(0)

¯ PF6 Hexafluorophosphate anion ppm Parts per million

PSIE Post-synthetic ionic exchange

PSM Post-synthetic modification

PXRD Powder X-ray diffration

QSDFT Quenched solid density functional theory

Qst Isosteric heat of adsorption

SABET Brunauer, Emmett and Teller

SALangmuir Langmuir surface area

SBUs Secondary building units

2¯ SiF6 Hexafluorosilicate anion

SO3H Sulfonic acid

xxv

T1 Tetrapod 1

T2 Tetrapod 2

TGA Thermogravimetric analysis

THF Tetrahydrofuran

UHP Ultra-high purity

UNM University of New Mexico

2+ UO2 Uranyl cation

XRD X-ray diffraction

ZIFs Zeolitic imidazolate frameworks

Zn Zinc

δ Chemical shift

λ Wavelength

xxvi

Chapter 1. A Review on Metal-Organic Frameworks

1.1 Introduction

Porous materials that can be inorganic, organic, hybrid composite have attracted long lasting attention of scientific community because of their widespread application in gas adsorption, shape/size selective catalysis, drug storage/delivery and purification.1 Based on the pore size, porous materials are classified into three different types. Materials with pore size not exceeding 20 Å are microporous. Mesoporous materials have pore size in the range of 20-500 Å and those with pore size above 500 Å are macroporous.2 Activated carbons which can be easily obtained from any high carbon containing materials is the most common commercially available cheap microporous material. They possess high surface area with density 2 g/cm3 and have applications such as gas storage, water purification and as a metal support in catalysis. Despite the multiple application in diverse field and low cost, activated carbon lacks ordered structure which makes their structural characterization very difficult.3

Zeolites also known as molecular sieves are the ordered inorganic porous materials composed of most earth abundant minerals such as silica and alumina with loosely held cations. They may be both naturally occurring or synthetic and have been synthesized in industries simply by heating aqueous solution of sodium hydroxide with alumina and silica.4-5 They have very regular pore structure with high surface area and density 4 g/cm3.

Because of their high thermal and mechanical stability, they have been used commercially in adsorption, water purification, heterogeneous catalysis and ion exchange for smaller molecules that fall within the range of their accessible pore dimension.6 As the composition of zeolites is only limited to alumina, silica and chalcogens, structural diversity is lacking

1 in these materials and also the modification of structure, property and topology is almost impossible because the covalently bonded framework is likely to collapse during modification.7

As mentioned above both organic and inorganic microporous materials are limited by the lack of order, stability and structural diversity. Metal organic frameworks (MOFs); also, known as porous coordination polymers often referred as an organic analogue of inorganic zeolites are the new class of organic inorganic hybrid, ordered crystalline materials which could be the potential candidate to address these limitations. Compared to other porous materials, structure and properties of MOFs can be controlled to a far greater extent by introducing different functionalities. This chapter covers a brief review on the design, synthesis of metal-organic frameworks (MOFs), classification, selected applications and

MOF interpenetration.8

1.2 Metal Organic Frameworks (MOFs)

Hybrid porous coordination polymers formed by the assembly of metal cations with a variety of organic bridging ligand or struts via coordination bond are known as metal organic frameworks (MOFs). These materials have wide range of physical as well as chemical properties and known for their structural diversity, chemical tunability, porosity, high surface area, high pore volume, low density 0.5 g/cm3, stability and rigidity. Because of these unique characteristics of MOFs over traditional porous materials, they are expected to hold strong impact on the application of other porous materials in future.7

Figure 1.1 Schematic representation of assembly of inorganic units and organic linkers resulting metal-organic frameworks9 and examples of most studied MOFs.10-11

In 1965, Tomic in his first paper mentioned the simple synthesis of coordination polymers containing metals like Zn, Ni, Fe and Al coordinated by aromatic carboxylic acids.12 Later

Robson introduced the node and spacer model for coordination polymers which set the basis for the discovery of all the existing hybrid porous materials up to now. In the structure of Robson’s polymer, ditopic ligands are the spacer and metal ion or cluster are the nodes.

These units when held together by a coordination bond formed porous coordination polymeric material.13-14 Later in 1995 Yaghi et al. used the term MOF to describe a layered porous coordination material of Co-trimesate that displayed reversible sorption behavior.15-

16 Kitagawa et al. in 1997 reported a 3-dimensional MOF that showed room temperature gas adsorption properties.17 The chemistry of MOF was coming into light in 1999, after the

3 discovery of MOF-5 and HKUST-1 having exceptionally high surface area of that time.18-

19 These MOFs are among the most studied MOFs of all the time. In 2000, Ferey et al. reported several flexible and non-flexible MOFs such as MIL-47, MIL-53 and MIL-88 respectively.20-22 The discovery of reticular chemistry concept in 2002 for the synthesis of

MOF for a series of Zn terephthalate MOFs was a breakthrough in the field of material chemistry. The use of mixed, extended as well as shorter ligands to expand or contract pore resulted MOF material with exceptionally high surface area and allowed to tune the pore size.23-25 Furthermore, by bridging the rigid multidentate building units such as square, octahedra to organic linkers rather than a simple metal node and organic spacer construction of conventional porous coordination polymers, Yaghi et al. took MOF chemistry to another level which attracted the attention of many researchers of that time.

In the same year, the family of MOF expanded by the addition of new type of MOFs based on imidazole known as zeolitic imidazole frameworks (ZIFs).26-27 Hoping to break the record and to set the new trends in MOF research, several new exciting robust solid materials have been developed to date and it is one of the fastest growing research field in chemistry.

1.2.1 Design and Synthesis of Metal-Organic Frameworks

Organic linkers and metal ions are the two crucial components for the design of MOFs.

Potential application of MOFs in different fields such as separation, purification, storage, catalysis, energy and others depend mainly on MOF composition i.e. linker functionality and type of metal node.28 First and second row transition metals are commonly used as nodes or secondary building units (SBUs). These SBUs are either a single metal ion or cluster which exist with different geometries depending upon the coordination number of

4 metal ion.29 Some of the most usual geometries are linear, trigonal-bipyramidal, tetrahedral, octahedral, T or Y-shape, trigonal prismatic and pentagonal bipyramidal.30

Occasionally, inner transition metals with high coordination numbers such as lanthanides are also used as inorganic building unit in MOFs construction.31-32 Organic ligands with desired functionality acts as a spacer to link the metal ions within the framework. Both rigid and flexible organic ligands are used in MOF construction. But usually, conformationally rigid ligands are preferred as they result robust MOFs with targeted topology and properties. These frameworks possess high thermal and mechanical stability which help to preserve framework porosity after solvent removal. The most common rigid organic ligands used for MOF synthesis are multidentate benzene carboxylates, azolates, pyridines and their derivatives to obtain a variety of MOFs with preferred features. 32-37

Figure 1.2 Some selected examples of rigid ligands. 32-38

Figure 1.3 Some selected examples of flexible ligands.39

The use of flexible ligands on an account of their ability to acquire different conformations forms flexible MOFs with different symmetries often hard to predict. Flexible MOFs are very delicate and may lose framework porosities upon solvent removal as the ligand flexibility cannot preserve the crystalline framework.39 However, the use of flexible ligands has some unique advantages over rigid ligands when used to build structurally distinct

MOFs. Because of their mobility, conformational flexibility and variable coordination modes, multifunctional flexible organic ligands have been used to synthesize a variety of polynuclear MOFs, homochiral MOFs.40-44 Some of the common example of flexible organic ligands include amino acids, peptides, chiral organic molecules and their derivatives.45

MOFs are synthesized in relatively mild synthetic condition by using aqueous/nonaqueous medium via different synthetic techniques such as hydrothermal, solvothermal,

6 electrochemical, mechanochemical, micro-wave, ultrasonic, layer by layer deposition and high throughput synthesis.46-48 Careful selection of constituents and reaction medium such as solvent, temperature, nucleation time is very important to yield highly stable MOFs with high crystallinity and porosity. The versatility of MOF synthesis lies on the different post- synthetic modification (PSM) methods and category-wise synthetic route used to introduce multiple functionalities into the crystalline framework without altering the underlying topology of MOFs. These methods are proven for integrating pore complexity in a controlled fashion to obtain stable MOF structure with tailor designed pores and novel properties. By post synthetic modification one can do the chemical modification of already assembled MOFs without affecting MOF lattice at the same time. 49 Based on the robustness and framework porosity, many diverse chemical routes are available for PSM.

However, it is very important that the chosen framework applied to post-synthetic transformation should retain its integrity after the chemical treatment. Sometimes PSM can be achieved very easily via protonation and chemical doping. 50-54 The type of chemical interactions used in these processes are mainly covalent, non-covalent and coordinative interactions.13, 55-59 The concept of PSM for the first time was introduced by Robson and

Hoskin who mentioned in their paper about the possibility of incorporating chemical functionalization to polymeric framework by free movement of chemical species throughout the lattice after the framework construction.13 Although the concept of PSM is not new regarding MOFs, this unique technique appeared to gain attention only after more than a decade when MOF scientists successfully modified porous framework by means of covalent interaction.60 Since then this research field has been extensively grown to expand

7 the number of MOF materials with broader applications and to upgrade the knowledge of

PSM concept for controlling as well as manipulating MOF properties.49

1.2.2 Classification of Metal-Organic Frameworks

Based on the porosity, MOFs are classified into three different categories and they are as follows:

❑ First generation MOFs: These MOFs are the earliest examples of porous coordination

polymers prepared by linking single metal ion to monodentate N-donor polytopic

pyridine-based ligands.61-65 Additionally, nonlinear ditopic N-donor ligands such as

pyrimidine, azolates with specific angles were applied to construct frameworks with

specific structures, such as zeolite-like nets.66 In these types of porous materials, the

structure and topology of obtained framework is controlled by the ligand geometry.64,

67 These MOFs lacked permanent porosity and rigidity. The material often collapses

upon the removal of solvent molecules from the framework and it is very hard to predict

their structure due to the lack of directionality.68-71 This behavior makes these MOFs

similar to charged MOFs containing counter anions in the pores.72

❑ Second generation MOFs: Compared to first generation, 2nd generation MOFs are rigid

and can retain their porosity upon guest removal. They are similar to zeolites and

neutral MOFs and built from the rigid ligands containing carboxylate groups. 72 The

incorporation of rigid carboxylate groups into MOF structure helped to lock the metal

ions in place and resulted frameworks with higher crystallinity, rigidity and stability.73

❑ Third generation MOFs: These frameworks are highly flexible and respond well to

guest exchange or external stimuli. These MOFs are also known as breathing MOFs.

In contrast to 2nd generation MOFs, which becomes stationary after the incorporation

of metal ions, flexible MOFs show guest responsive behavior which is very important

for selective adsorption of guest molecules.74

❑ Fourth generation MOFs: MOFs materials modified post synthesis falls under this

category. These are the recently developed MOFs which retains their underlying

topology and integrity after various chemical modifications post synthesis. These

MOFs contain various complex functionalities within the pore environment for more

sophisticated applications.75

1.2.3 Applications of MOFs

MOFs have widespread applications in many fields because of their high surface area, porosity, stability and structural diversity. By introducing a variety of building units and applying different synthetic techniques during framework construction, it is possible to modify some of their properties like pore volume (up to 50% of total volume), surface area as high as 10,000 m2/g, pore size up to 98 Å and density as low as 0.126 g/cm3. It means the properties of MOFs are dictated by the type and structure of the building blocks used during the synthesis which in turn determine their applications as well.76-77

One of the most important application of MOF is gas storage as they can physically adsorb gas molecules without forming chemical bonds. Practically, the structure of a MOF can be considered as a sponge which can adsorb a huge amount of gas because of its porous structure. Storage of fuel gas like H2 require high pressure and low temperature which limits its practical usage as a fuel gas.78 Though porous MOF having high surface area can adsorb large amount of hydrogen, it is not always an essential condition.79 Indeed, better interaction between MOFs and hydrogen lead to higher uptake of hydrogen. Increased interaction can be achieved via adjusting the pore size either by creating an interpenetrated

9 framework or by synthesizing MOF with pore size around 6 Å.80-81 Also in some MOFs, it is possible to increase hydrogen-metal interaction by creating an unsaturated metal center through the removal of any loosely coordinated solvent molecule.82 MOF-5 is one of the earliest MOF known to adsorb hydrogen.83 One of the major breakthrough in this research field is the use of MOF hydrogen fuel tank in Mercedes-Benz F125.84

Natural gas like methane can serve as an alternative fuel source to hydrogen and gasoline.

So far MOFs like MOF-177, MOF-200 and MOF-210 are known to have highest methane storage capacity of 345, 446 and 476 mg/g respectively and this technology has been practiced by BASF researcher to develop natural gas tanks.85-86 In 2015, a new flexible

MOF was reported to exhibit phase transition behavior for an elevated adsorption and desorption resulting greater methane storage capacity.87 MOFs can also be applied for the selective uptake of carbon dioxide from flue gas and atmosphere. Generally compared to hydrogen and methane, carbon dioxide interacts strongly with MOFs because of its quadrupole moment.76 Ultra-high microporous MOFs like MOF-200 and MOF-177 can be filled with larger amount of carbon dioxide and are the best candidate for the selective carbon dioxide separation from nitrogen even at low pressure.85, 88 Nowadays selective separation of hydrocarbons by MOFs is a new hot research topic because separation process in chemical industry is highly energy intensive and it is very hard to maintain the pore size, functionality in conventional adsorbent like zeolites, carbons for the selective separation of two different gases having almost similar kinetic diameter. For an example, the gate opening phenomenon in MOF; KAUST-7 has been applied for the selective exclusion of propane from propylene.89

Another potential field where MOF finds its application is catalysis. The presence of

10 continuous, consistent and penetrable channels offers an active space and the reactants/products can move in and out of the inner pore with great facility depending on shape, size and chiral selectivity.90 One of the interesting examples of chiral selectivity has been explored in chiral MOF abbreviated as MOF-520. The chirality of this MOF was used for the allocation of absolute configuration of molecules of different functionality crystallized within its framework.91 Several heterogeneous catalysis reactions like coupling reaction, alcohol oxidation, cycloaddition of CO2 to epoxides, cyclopropanation of olefins, three components coupling reactions of amines, alkynes, azides, etc. have been carried out in MOFs containing different transition metal ions. To expand this field further, homochiral

MOFs containing coordinatively unsaturated metal center have been successfully applied in asymmetric catalysis like Mukaiyama Aldol Condensation. Besides this, redox and

Lewis acid catalytic chemistry of MOF have also been demonstrated in many literatures.72

Applications of MOF in other fields like drug storage/delivery and chemical sensor has also been explored. Mostly biodegradable MOFs are used as a storage receptacle for controlled drug delivery. Inorganic unit of MOF helps in control release of drug whereas organic unit acts as a biocompatible counterpart by providing different functionality and pore size in drug storage and release process. MCM-41 functionalized with aminopropyl group has been used for the controlled delivery of ibuprofen from the siliceous matrix.92

Similarly, tunability and structural diversity of MOF materials makes them an ideal nominee for the selective and quick recognition of both vapor and gas phase of a material under investigation. MOF as a chemical sensor find application in many critical industrial processes such as food quality control, chemical threat detection, medical diagnosis, occupational safety and environmental monitoring.93 One of the most common early MOF

HKUST-1 is capable of adsorbing about 40% of water which makes them very useful in sensing humidity.94 Furthermore, application of MOF in other fields like molecular separation, purification, semiconductor, mixed matrix membrane, nanofabrication has also been demonstrated in past years.95

1.2.4 Interpenetration in MOFs

Highly porous MOFs are considered best in the applications like storage and separation.96

These MOFs are constructed by longer organic linkers which often results interpenetrated

MOFs structure. The existence of interpenetration is very common in both coordination polymers and MOFs which is defined as a phenomenon of physical entanglement of two or more independent networks without any chemical bonding within a structure. The formation of interpenetrated network is highly accidental as it is never expected while designing a highly porous material.

Figure 1.4 Schematic representation of interpenetration.97

Interpenetration is very common in MOFs which are built by using long ligands of certain

topology such as primitive cubic.98 The use of these ligands in MOF construction results

large voids enough to accommodate another fold of structurally alike framework. Instead

of not being chemically bonded, these interpenetrated networks are almost impossible to

separate without breaking the bonds. This phenomenon greatly reduces the pore size of

MOFs and is considered as an actual obstacle in the field of MOF chemistry in the

beginning.99 Different degree of interpenetration from 1-fold to 10-fold with highest

being 54-fold has been reported to date.100 As interpenetration seems quite bad for the

framework porosity, it is beneficial for framework stability and sometimes to improve

the applications of MOFs.101-102 Compared to non-interpenetrated MOFs, interpenetrated

MOFs are more flexible because the forces between individual nets are supramolecular

interactions such as Vander Waals, hydrogen bonding and π-π stacking instead of

chemical bond.103 In last decade large number of 1D, 2D and 3D interpenetrated networks

with exceptional structural flexibility and diverse structure have been reported in the

literature.

Figure 1.5 Representation of interpenetration: (A) 5-fold and (B) 10-fold in 3D, diamondoid frameworks.104

1.2.4.1 Interpenetration Control and Applications

Compared to non-interpenetrated and partially interpenetrated frameworks, highly interpenetrated frameworks have smaller pores, lower surface area and high density which limits the potential applications of these materials.105 Hence, to obtain highly porous

13 materials, several strategies have been developed by the researchers to minimize the interpenetration. Some of the most effective methods are listed below.

Figure 1.6 Schematic representation of common methods used to control interpenetration.

❑ Use of infinite SBUs for MOF construction: Yaghi et al. first conducted the use of

infinite SBUs in the construction of series of non-interpenetrated MOFs. In a typical

example of 4,4′-biphenyldicarboxylate (bpdc) based Zn-MOFs containing Zn-O-C

units, the self-assembly of Zn-O links bearing SBUs with C6H4 links in the structure

has successfully prevented the 2-fold interpenetration.106

❑ Control of reaction concentration and temperature: It is found that the reaction

parameters such as concentration and temperature play key role in determining the

interpenetration. Various studies on effects of reaction medium on MOF

interpenetration has shown that the lower concentration and temperature are the best

reaction condition to afford non-interpenetrated MOFs.107 The use of dilute reaction

medium can effectively suppress the degree of interpenetration in the synthesis of

isoreticular MOFs (IR MOFs) most of which are constructed from longer organic

ligands.23 In a work of Xu et al., they demonstrated that nonporous MOF can be

synthesized as microporous MOF by decreasing the temperature which can be further

converted into mesoporous just by reducing the reaction concentration and reaction

time.108

❑ Template-directed control: The use of template to grow MOF is a very useful method

to afford highly porous MOF materials. The growth of MOFs on the template surface

avoid interpenetration of framework nets. Zhou et al. introduced the concept of

template-directed interpenetration control in MOFs. In their experiment, they have

confirmed that the addition of oxalic acid and large ligand htb (s-heptazine tribenzoate)

as a template can greatly reduce interpenetration of PCN-6 MOF.109-111

❑ Ligand modification: The use of sterically hindered groups on organic ligand and

shorter rigid ligand are among other approaches for the effective prevention of

framework interpenetration. The degree of structural interpenetration is usually

determined by the mode of coordination and the arm length of organic ligands.112 In a

synthesis of a doubly interpenetrated (6, 3) net by connecting dicarboxylate ligand bdc

2+ with uranyl cation (UO2 ), the existing 2-fold interpenetration in this type of

framework is controlled by the introduction of sterically bulky group on the bdc ligand

prior to the synthesis resulted a framework with same topology.113

❑ Layer by layer assembly: Interpenetration control by layer by layer synthesis technique

is a method of MOF synthesis where a properly synthesized organic medium is soaked

in two separate solutions of metal salt and organic ligands with intermittent rinsing. By

applying this method, Yaghi et al. successfully made non-interpenetrated MOF-508

which was otherwise a 2-fold interpenetrated framework when solvothermal method of

synthesis was applied.114

❑ Control by adding or removing coordinated/uncoordinated solvent: Solvent molecules

triggers interpenetration not only during MOF crystallization but also after synthesis.

Sometimes addition and removal of uncoordinated solvent molecules from the MOF

pores leads reversible transformation between interpenetrated and non-interpenetrated

frameworks. In a silver-based MOF designed by Chen et al., water desorption

rearranged the 5-fold interpenetrated MOF into 6-fold interpenetrated framework

which on exposure to water vapor converted back into the original framework.115

Similarly, coordinated solvents removal/addition also results the transformation from

interpenetrated to non-interpenetrated MOFs. An example of this phenomenon was

observed in MOF-123 which was transformed into doubly interpenetrated MOF-246

upon the removal of DMF solvent molecules coordinated to Zn centers.116

Most of the flexible interpenetrated MOFs seemed to have higher sorption capacity for gases on an account of increased interaction between pore surface and guest molecules.

Therefore, interpenetrated MOFs are proven to be an ideal candidate for the adsorption, selective catalysis, storage and separation of gases like CO2 and H2. In contrast to non- interpenetrated MOFs with large pore size and volume, interpenetrated MOFs have higher gas adsorption capacity at lower pressure because of its small pore volume.81, 117-118 A typical example of MOFs which showed this sort of behavior includes a doubly interpenetrated PCN-6 and PCN-6’. In the meantime, the smaller pores of interpenetrated

MOFs can find special application in selective catalysis.119 These MOFs are ideal to

16 perform the catalysis reactions in smaller substrates. Furthermore, by varying the degree of interpenetration to create stable interpenetrated materials, the applications of these materials can be extended to other fields such as magnetism and sensing as well.

Chapter 2. Synthesis, Characterization and Application of Charge-Separated

Diamondoid UNM-1 MOF

2.1 Introduction

A vast number of MOF materials have been developed from limitless combinations of metal centers and organic ligands, leading to microporous materials with tunable surface areas, pore volumes, pore sizes and shapes, and surface functionalities.121-124 Majority of these MOFs are either charge neutral or ionic where non-charged or charged organic ligands are used to bridge the metal cluster or a metal node in the framework. Charge- neutral MOFs are very promising with tailor design properties and have been used for different useful applications such as catalysis,125-126 sensors,127-128 electronics,129-130 medicine,131-132 gas storage,133-134 and gas/liquid separation.135-136 But these frameworks possess very weak electrostatic attraction because of their limited charged surface. The other class of MOFs are ionic, which are built by bridging charged organic ligands to the metal cluster or a metal node in the framework.137-138 These MOFs contain exposed charge species which result local electric field within the pore. Hence, they can exert stronger electrostatic interaction and coordinating effects on the polarizable guest molecules. But to make these ionic MOFs stable, a cationic or anionic species must be present in the pore which will unnecessarily reduce the pore size and makes them unavailable to the guest molecules. In this regard, it is better to incorporate both positive and negative charges at fixed distances and precisely controlled locations into one framework structure, forming charge-separated, or zwitterionic MOFs that can possess both the favorable interaction properties of ionic MOFs and free pore spaces as in non-ionic MOFs. Charge-separated

MOFs have infinite electrostatic binding sites. Therefore, sorption properties of these materials are based on multi-point interactions which is better than those of conventional

MOFs based on single point interactions localized in isolated region of pore environment.

The charged surfaces within the pore environment of these MOFs cause polarization of gas molecules resulting increased electrostatic interaction and increased adsorption enthalpy.

This effect is more pronounced on the quadrupole gases such as CO2 with high polarizability. Until now, very limited number of charge-separated MOF materials are designed mainly by utilizing zwitterionic ligands containing both cationic (pyridinium,139-

148 imidazolium,149-150 metalloporphyrin151 or, ammonium152) and anionic moieties

(carboxylate, sulphonate) to bind metals in different oxidation states.

Anionic organic tetrahedral borate ligands have also been used by Ziegler and coworkers for the purposeful design of charge separated MOFs for enclathration and anion exchange applications.153-156 Borate ligands being tetrahedral afford three dimensional materials and the negative charge of borate anion can compensate the positive charge of metal ions in the framework.157-158 When the metal ion have an oxidation state of +1 and coordination number of 4, three dimensional charge-separated MOFs are formed with an overall charge neutrality without any charge balancing free ions in the pores.159 Although in Ziegler’s anionic borate ligand, the four B-N bonds are arranged in tetrahedral geometry at the boron center, due to the off-set angle between the B-N and N-metal bonds at ca. 145°, rotation around these single bonds can lead to different conformations causing the construction of ordered three dimensional structures tough and very hard to predict.160 In this regard, borate ligands that possess collinear boron-organic-metal arms are preferred.

In our research, we have demonstrated the design of a rigid tetrahedral ionic borate ligand named as Tetrapod 1 (T1) containing colinear tetraphenylethynyl pyridine arms which upon coordination with Cu(I) forms charge-separated diamondoid MOF referred as UNM-

In this chapter, synthesis, structural characterization and application of UNM-1 MOF is presented.

Figure 2.1. Examples of some common ligands used for the synthesis of charge-separated

MOFs. 141, 150, 153-156, 161

2.2 Results and Discussion

2.2.1 Synthesis and Characterization of Ligand T1 and UNM-1

2.2.1.1 Tetrabutyl ammonium tetrakis(4-pyridyltetrafluorophenylethynyl)borate

(T1). Ionic ligand T1 was synthesized from the Sonogashira coupling of previously reported tetrabutylammonium tetrakis(tetrafluorophenylethynyl)borate162 and 4- bromopyridine hydrochloride in presence of triethylamine base and is summarized in scheme 1. T1 is characterized by multi-nuclear NMR spectroscopy and the respective NMR spectra with peak assignments are presented in Figure 2.2. A single sharp signal at −16.3 ppm in 11B NMR and two 19F signals at −129.9 and −139.7 ppm were observed, which is in agreement with the values reported for compound 3.163 However, the 13C NMR for compound 3 were not available, so a similar compound, lithium tetrakis(4- bromotetrafluorophenyl)borate is used as a reference compound for 13C signal

164 assignments. the F4-phenyl carbon atoms ortho- and meta- to the boron center are

1 represented by two sets of doublets at 148.2 and 145.7 ppm having JCF coupling constants of 241 and 255 Hz respectively. A broad signal ranging from 132 to 134 ppm caused by the splitting effects from 1J boron and 2J fluorine atoms is assigned as an ipso carbon. The

2 triplet at 98.9 ppm with a JCF coupling constant of 14 Hz is from the F4-phenyl carbon para- to boron center. The two triple bond carbon signals appear at 96.0 and 80.5 ppm, while the enhanced signals located at 149.9, 130.5 and 125.5 ppm are from pyridine rings, respectively. The presence of one tetrabutylammonium cation per borate anion is confirmed by the integration of proton NMR signals.

Scheme 2.1 Synthesis of ligands and Schematic representation of UNM-1.

Proton Carbon

Fluorine Boron -16.32

-129.94 -139.69

1 12 19 11 Figure 2.2. H, C, F and B NMR spectra of T1 in CDCl3.

2.2.1.2 Synthesis and structural analysis of UNM-1. Slow diffusion of T1 solution in dichloromethane and Cu(CH3CN)4BF4 solution in acetonitrile for 48 h at room temperature afforded yellow needle shaped crystals of UNM-1 of approximate composition

C52H16BCuF16N4. UNM-1 is a 3-dimensional neutral framework with diamondoid architecture containing positively charged Cu(I) cation bridged by negatively charged borate anion at a fixed distance without any charge balancing counter ion inside the pores.

Crystallographic data and crystal structures are presented in Table 2.1 and Figure 2.3. The crystal system of UNM-1 is tetragonal with space group of I-4. The dihedral angles around boron centers ranges from ca. 101° to 116° and those around copper centers from ca. 100° to 121° indicating that the coordination environment around both Cu(I) and boron is tetrahedral as appeared in Figure 2.3A. The distance between the core boron atoms of two ligands sharing a common copper atom is ca. 20.72 Å and boron-copper distance is ca. 13.3

Å resulting 4-fold interpenetration and reduced pore size. The interpenetrated networks are shown in Figure 2.3D and 2.3E with each color representing 1-fold interpenetration. Cu-N bond length in UNM-1 ca. 2.07, 2.09 and 2.0 Å and the distance between two boron atoms sharing a common Cu atom is ca. 20.73 Å. Instead of the interpenetration, the framework is porous with two different pore sizes and shape as revealed in a 2×2×2 unit cell space filling model in Figure 2.3D and 2.3E. The larger pores along b-axis are circular ca. 7.4 Å in diameter. The smaller channels along c-axis have alternating square and octahedral shapes, both of which possess pore size of ca. 2.7 Å.

Table 2.1. Crystallographic data of UNM-1.

Chemical formula C52H16BCuF16N4 Formula weight 1075.04 g/mol Temperature 100(2) K Wavelength 0.71073 Å Crystal size 0.380 x 0.428 x 0.488 mm Crystal system tetragonal Space group I -4 Unit cell dimensions a = 23.5586(7) Å, α = β = γ= 90° b = 23.5586(7) Å, c = 24.6516(9) Å Volume 13681.8(10) Å3 Z 8 Density (calculated) 1.044 g/cm3 Absorption coefficient 0.392 mm-1 F (000) 4272 Theta range for data collection 2.10 to 25.71° Index ranges -24<=h<=28, -28<=k<=24, - 30<=l<=24 Reflections collected 31438 Independent reflections 12964 [R(int) = 0.0408] Coverage of independent 99.8% reflections Absorption correction Multi-Scan Max. and min. transmission 0.8650 and 0.8320 2 2 2 Function minimized Σ w(Fo - Fc ) Data / restraints/ parameters 12964 / 0 / 668 Goodness-of-fit on F2 0.937 R1 (I>2σ(I), wR2 0.0314, 0.0657 R1, wR2 (all data) 0.0459, 0.0697

2.2.2 Stability Analysis of UNM-1

Power x-ray diffraction analysis and thermogravimetric analysis is carried out to determine the environmental and thermal stability of UNM-1.

2.2.2.1 Powder XRD analysis

Powder X-ray Diffraction (PXRD) analysis of UNM-1 showed that it is stable under ambient conditions. The PXRD pattern of UNM-1 closely matches that of the simulated pattern from single crystal X-ray data after drying under high vacuum at room temperature for 24 h. To determine the water stability of UNM-1, few MOF crystals were soaked in water for 48 h and dried under high vacuum for 24 h. PXRD measurements of this sample displayed no significant changes in diffraction pattern. However, the XRD patterns of

UNM-1 changed slightly after soaking the crystals in pH 4 and pH 10 aqueous solutions for 48 h, but the major scattering peaks remained, which means UNM-1 can tolerate mildly acidic and basic conditions. Under highly acidic conditions of pH 1 and pH 13, UNM-1 is unstable as indicated by the complete disappearance of scattering XRD signals. Even after repeated gas adsorption trials with CO2 or N2 at various temperatures, the main PXRD pattern remained, except becoming broader indicating retention of the basic crystal structure but loss of long-range order.

After adsorption

pH 13

pH 10

pH 7 Intensity pH 4

pH 1

As synthesized Simulated

5 10 15 20 25 30 35 40 2 (Degree)

Figure 2.4. PXRD patterns of UNM-1, from bottom to top: simulated from single crystal

XRD data (black), as synthesized (red), sample after soaking for 48 h in HCl solutions of pH 1 (maroon) and pH 4 (pink), neutral water of pH 7 (blue), NaOH solutions of pH 10

(navy blue) and pH 13 (purple) and, after all adsorption experiments (green).

2.2.2.2 Thermogravimetric analysis

In the thermogravimetric (TGA) analysis of UNM-1 (Figure 2.5) under N2, we observed ca. 2-3% weight loss up to 150 °C, which is likely due to the loss of trapped solvent and water molecules. There is no significant decomposition up to ca. 300 °C. At 600 °C a total weight loss 50% is noticed which indicates the beginning of UNM-1 MOF crystals disintegration. The observed stability of UNM-1 at such a high temperature is because of its 4-fold interpenetration geometry that interlocks each layer and prevents dislocation.

110 Weight (%)

100

70 Weight (%) Weight

40 100 200 300 400 500 600 700 T (C)

Figure 2.5. Thermogravimetric analysis (TGA) histogram of UNM-1 under N2 environment.

2.2.3 Gas Adsorption Analysis of UNM-1

2.2.3.1 Surface area and Pore size determination

The surface area of UNM-1 is determined by multi-point Brunauer-Emmett-Teller (BET) and Langmuir method through N2 adsorption measurements at 77 K. For the pore size distribution, the N2 adsorption isotherm at 77K is fitted by non-local density functional theory (NLDFT). Type-I adsorption behavior is obtained at 77 K which confirms the microporous nature of UNM-1. The average BET surface area (SABET) of UNM-1 is obtained as ca. 621 m2/g from the linear fit of isotherm at 77 K between pressures (P/P0)

0.05 and 0.30. As Type-I shape of N2 adsorption isotherm is the characteristic of microporous solids, we assumed the absence of meso- and macro-pores in UNM-1 MOF structure. Furthermore, the fitting of isotherm with Langmuir method (Figure 2.6) obtained 28

2 the average SALangmuir of ca. 915 m /g. Hence, the surface area of UNM-1 is among the highest in MOFs with 4-fold interpenetrated structures,165-166 which is likely resulted from the rigid borate arms and absence of charge compensating free ions. It means that, even though the pore size and surface area of UNM-1 is greatly reduced due to interpenetration, the large porous area is retained in its final crystal structure. From NLDFT fitting, the pore size of UNM-1 is found to be ca. 6.14 Å, which is consistent with the microporous structure as revealed by single crystal x-ray structure.

1.1 S = 621 m2/g BET 1/[W(Po/P)-1)] 2 180 1.0 R = 0.99809 Linear fit

(77 K-N2) Des 0.9 (77 K-N2) Ads 170 0.8

0.7

160 0.6

1/[W((Po/P)-1) 0.5 V (cc/g) V 150 0.4

0.3

140 0.2

100 200 300 400 500 600 700 800 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 P (torr) P/Po

2 1.0 S = 915.47 m /g Lang (P/P0)/W 0.10 R2= 0.99985 Linear fit of (P/Po)/W 0.9 dV(r) 77K N 0.08 2 0.8 Half pore width= 3.07 Å

0.7 0.06

0.6

(P/P0)/W 0.04

0.5 dV(r) (cc/Å/g) dV(r) 0.4 0.02

0.3

0.00 0.2

0 20 40 60 80 100 0.05 0.10 0.15 0.20 0.25 Pore Width (Å) P/P0

Figure 2.6. (A) N2 adsorption/desorption isotherm at 77 K; (B) BET plot; (C) Langmuir plot and (D) NLDFT pore size distribution of UNM-1.

2.2.3.2 CO2/N2 adsorption selectivity calculation (IAST Method)

Ideal Adsorbed Solution Theory (IAST) is a thermodynamic model developed by Myer and Prausnitz which predicts the multicomponent adsorption isotherm from pure component adsorption isotherm at same temperature.167 This method is widely used for the adsorption selectivity calculation of metal organic framework and covalent organic framework.

The N2 and CO2 adsorption/desorption isotherms of UNM-1 is measured at 273 K, 298 K,

303 K, 313 K and 323 K which are shown in Figure 2.12A. These temperatures are relevant

168-169 in real-world applications including CO2 capture from industrial flue gases. From the plot, it can be clearly observed that the adsorption of CO2 by UNM-1 is much elevated compared to that of N2 at all temperatures applied, up to ca. 27 cc/g CO2 at 273 K and 1 bar. It is noticed that, the adsorption/desorption isotherms hysteresis reaches a maximum of 52% at 313 K (Figure 2.12A, insert) even though the amount of CO2 adsorbed decreases

170 with increasing temperature. The adsorption selectivity of UNM-1 between CO2 and N2 is calculated based on the ideal adsorbed solution theory (IAST)171-172 by using the pyIAST

173 code developed by Simon et. al., assuming a flue gas like mixture containing 15% CO2

174-176 and 85% N2. Detailed isotherm fitting parameters and ideal selectivity at various pressures and temperatures are given below, and summarized in Figure 2.12B. For the temperatures, 273 K, 298 K, 303 K and 323 K under the pressure range of 0-1, the ideal

CO2/N2 selectivities are relatively constant and the value is above 10. However, at 313 K the selectivities are increased with increasing pressure and reaches a value of ca. 99 at 1 bar suggesting the potential application of UNM-1 for effective CO2 capture at flue gas conditions.

A. 273 K

1.2 T=273 K Langmuit fit of CO 2 Quadratic fit of N 1.0 2

0.8

0.6

n (mmol/g) 0.4

0.2

0.0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 P (bar)

Figure 2.7. Fitting of CO2 and N2 adsorption isotherm at 273 K. Dots represents experimental data, lines are fitting curves.

Table 2.2. N2 (273 K) - Quadratic Parameters.

-1 -2 2 M (mmol/g) KA (bar ) KB (bar ) R 0.139144 0.545797 0.624341 0.99973

(퐾 + 2퐾 푃)푃 ( ) 퐴 퐵 푛 푃 = 푀 2 (퐸푞. 2.1) 1 + 퐾퐴푃 + 퐾퐵푃

Where, n is the amount adsorbed (mmol/g), P is the pressure (bar), M is the saturation

1 -2 loading and KA (bar ), KB (bar ) are constants.

Table 2.3. CO2 (273K) - Langmuir Parameters.

M (mmol/g) K (bar-1) R2 2.703687 0.765012 0.99718

퐾푃 푛(푃) = 푀 (퐸푞. 2.2) 1 + 퐾푃

Where n is the amount adsorbed (mmol/g), P is the pressure (bar-1) M is the saturation loading and K is a constant.

Table 2.4. IAST adsorption selectivity calculated for mixed gas (15% CO2: 85% N2) at

273 K.

IAST CO molar IAST N molar Total Pressure 2 2 Selectivity fraction fraction 0.06 0.80103 0.19896 22.8144853 0.11 0.78786 0.21214 21.0452531 0.16 0.77904 0.22096 19.9790007 0.21 0.77308 0.22692 19.3054234 0.26 0.76916 0.23084 18.8813608 0.31 0.76679 0.23321 18.6318911 0.36 0.76559 0.2344 18.5082907 0.41 0.76533 0.23467 18.4807176 0.46 0.76581 0.23419 18.5302105 0.51 0.7669 0.2331 18.6433576 0.56 0.76847 0.23153 18.8082034 0.61 0.77044 0.22956 19.0182378 0.67 0.77323 0.22676 19.3227936 0.72 0.77584 0.22416 19.6128956 0.77 0.77866 0.22134 19.9349718 0.82 0.78165 0.21835 20.2855507 0.87 0.78477 0.21523 20.6617572 0.92 0.78799 0.21201 21.0616323 0.97 0.79131 0.20869 21.4868465 1 0.79332 0.20667 21.7519717

B. 298 K

1.0

T=298 K Langmuit fit of CO 2 0.8 Quadratic fit of N 2

0.6

0.4 n (mmol/g)

0.2

0.0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 P (bar)

Figure 2.8 Fitting of CO2 and N2 adsorption isotherm at 298 K. Dots represents experimental data, lines are fitting curves.

Table 2.5. N2 (298 K) - Quadratic Parameter.

-1 -2 2 M KA (bar ) KB (bar ) R (mmol/g) 0.110403 0.363959 0.421614 0.99967

Table 2.6 CO2 (298K) - Langmuir Parameters.

M (mmol/g) K (bar-1) R2 8.600484 0.113949 0.99963

Table 2.7. IAST adsorption selectivity calculated for mixed gas (15% CO2: 85% N2) at

298 K.

IAST CO molar IAST N molar Total Pressure 2 2 Selectivity fraction fraction

0.06 0.78101 0.21899 20.2097052 0.11 0.76445 0.23555 18.3905045 0.16 0.75219 0.24781 17.2003148 0.21 0.74283 0.25717 16.3680445 0.26 0.73559 0.26441 15.7646962 0.31 0.72998 0.27002 15.3194331 0.36 0.72566 0.27424 14.9944331 0.41 0.7224 0.27776 14.7379032 0.46 0.72001 0.27999 14.5721514 0.51 0.71835 0.28165 14.452867 0.56 0.71732 0.28268 14.3795576 0.61 0.71683 0.28327 14.3398054 0.67 0.71683 0.28317 14.3448694 0.72 0.71726 0.28274 14.3753036 0.77 0.71803 0.28197 14.4300339 0.82 0.71908 0.28092 14.5051497 0.87 0.7204 0.2796 14.6003815 0.92 0.72193 0.27807 14.7118951 0.97 0.72366 0.27634 14.8394731 1 0.72479 0.27521 14.9236704

C. 303 K

0.7 T=303 K Langmuit fit of CO 0.6 2 Quadratic fit of N 2 0.5

0.4

0.3 n (mmol/g) 0.2

0.1

0.0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 P (bar)

Figure 2.9 Fitting of CO2 and N2 adsorption isotherm at 303 K. Dots represents experimental data, lines are fitting curves.

Table 2.8. N2 (303 K) - Quadratic Parameter.

-1 -2 2 M (mmol/g) KA (bar ) KB (bar ) R 0.066380 0.224045 0.580506 0.99948

Table 2.9. CO2 (303K) - Langmuir Parameters.

M (mmol/g) K (bar-1) R2 4.738390 0.151683 0.99992

Table 2.10. IAST adsorption selectivity calculated for mixed gas (15% CO2: 85% N2) at

303K.

IAST CO molar IAST N molar Total Pressure 2 2 Selectivity fraction fraction

0.06 0.83417 0.16583 28.5048745 0.11 0.80811 0.19189 23.8641409 0.16 0.78997 0.21003 21.313606 0.21 0.77655 0.22345 19.69322 0.26 0.76636 0.23364 18.5871711 0.31 0.75858 0.24142 17.8055671 0.36 0.75264 0.24736 17.2419146 0.41 0.74818 0.25182 16.8361793 0.46 0.74491 0.25509 16.5477152 0.51 0.74264 0.25736 16.3518431 0.56 0.7412 0.2588 16.2292633 0.61 0.74046 0.25953 16.1674566 0.67 0.74035 0.25965 16.1575839 0.72 0.7408 0.25201 16.65754 0.77 0.74167 0.25833 16.2692262 0.82 0.74291 0.25709 16.3749011 0.87 0.74445 0.25555 16.5077284 0.92 0.74626 0.25374 16.6659047 0.97 0.74831 0.2517 16.8471328 1 0.74963 0.25037 16.9665029

D. 313 K

0.6 T=313 K Dual-site Langmuit fit of CO 0.5 2 Interpolation of N 2

0.4

0.3

0.2 n (mmol/g)

0.1

0.0

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 P (bar)

Table 2.11. CO2 (313 K) - Dual-Site Langmuir Parameters.

-1 -1 2 M1 (mmol/g) M2 (mmol/g) K1 (bar ) K2 (bar ) R -2.802783 6.146601 -0.047517 0.063223 0.9992

퐾1푃 퐾2푃 푛(푃) = 푀1 + 푀2 (퐸푞. 2.3) 1 + 퐾1푃 1 + 퐾2푃

Where M1 and M2 (mmol/g) are the number of adsorption sites of type 1 and type 2. K1 and K2 are the constants.

Table 2.12. IAST adsorption selectivity calculated for mixed gas (15% CO2: 85% N2) at

313K.

IAST CO molar IAST N molar Total Pressure 2 2 Selectivity fraction fraction

0.06 0.85075 0.14925 32.3009492 0.11 0.8265 0.1735 26.9942363 0.16 0.81487 0.18513 24.9424549 0.21 0.80942 0.19058 24.0671284 0.26 0.81033 0.18967 24.2097854 0.31 0.81633 0.18367 25.185768 0.36 0.82528 0.17472 26.7661783 0.41 0.83576 0.16424 28.8356876 0.46 0.84694 0.15306 31.3558517 0.51 0.85829 0.14171 34.3211018 0.56 0.86945 0.13051 37.7510025 0.61 0.88035 0.11965 41.6936899 0.67 0.89273 0.10727 47.1595351 0.72 0.90244 0.09756 52.4172475 0.77 0.91156 0.08844 58.4069049 0.82 0.92006 0.07994 65.2198315 0.87 0.92795 0.07205 72.9824196 0.92 0.93524 0.06476 81.8359069 0.97 0.94195 0.05805 91.9503302 1 0.9457 0.0543 98.6918355

E. 323 K

0.5 T=323 K Interpolation of CO 2 Interpolation of N 0.4 2

0.3

0.2 n (mmol/g)

0.1

0.0

0.15 0.30 0.45 0.60 0.75 0.90 1.05 P (bar)

Table 2.13. IAST adsorption selectivity calculated for mixed gas (15% CO2: 85% N2) at

323K.

IAST CO molar IAST N molar Total Pressure 2 2 Selectivity fraction fraction

0.06 0.86208 0.13792 35.4199536 0.11 0.8248 0.1752 26.6773212 0.16 0.80018 0.19982 22.6921896 0.21 0.78123 0.21877 20.235727 0.26 0.76688 0.23311 18.6420717 0.31 0.75576 0.24424 17.5345562 0.36 0.74975 0.25025 16.977356 0.41 0.74797 0.25203 16.8174291 0.46 0.7492 0.2508 16.927698 0.51 0.75259 0.24741 17.2372849 0.56 0.75749 0.24251 17.7000674 0.61 0.76345 0.23655 18.2888043 0.67 0.77154 0.22846 19.1370918 0.72 0.7788 0.22121 19.9502735 0.77 0.78634 0.21366 20.8552217 0.82 0.79404 0.20596 21.8467664 0.87 0.80182 0.19818 22.9268678 0.92 0.80959 0.19041 24.0936751 0.97 0.81729 0.18271 25.3478737 1 0.82186 0.17813 26.1449877

2.2.3.4 Calculation of isosteric heat of CO2 adsorption

Isosteric heat of CO2 adsorption of UNM-1 is calculated by applying Clausius-Clapeyron equation and virial method. The isosteric heat of CO2 adsorption (QSt) on UNM-1 is found to be 27±2 kJ/mol at 0.15 mmol/g CO2 adsorption and 21±4 kJ/mol at 0.5 mmol/g CO2

adsorption using Clausius-Clapeyron equation. All the fitted plots and summary of QSt at different loadings are presented in Figure 2.13 and 2.14. From virial method, QSt of ca. 16

177-178 kJ/mol at 0 mmol/g CO2 adsorption is estimated. The low QSt values for CO2 adsorption at zero coverage are relatively small. This suggests the pure physical interaction of UNM-1 with CO2. The fitted isotherm and fitting parameters for CO2 used in virial method are given in Figure 2.14.

6.0 n=0.15 mmol/g lnP 6.0 Q =26.73 2 KJ/mol n=0.2 mmol/g lnP st Linear fit of ln P Q =25.88 2.38 KJ/mol st Linear fit of lnP 5.5 5.5

5.0 5.0

4.5 4.5 y = a+b*x

ln P (torr) Equation ln P (torr) Weight No Weighting Residual Sum 0.00927 Equation y = a+b*x of Squares Weight No Weighting Pearson's r -0.9958 Residual Sum 0.00662 Adj. R-Square 0.98322 of Squares Value Standard Error 4.0 Pearson's r -0.99718 4.0 lnP Intercept 15.56304 0.97632 Adj. R-square 0.98873 Slope -3112.76665 286.30486 Value Standard Error lnP lnP Intercept 15.5997 0.82501 lnP Slope -3214.40642 241.9329

3.5 3.5 0.0032 0.0033 0.0034 0.0035 0.0036 0.0037 0.0038 0.0032 0.0033 0.0034 0.0035 0.0036 0.0037 0.0038 -1 1/T (K-1) 1/T (K )

6.5 6.5 n= 0.25 mmol/g lnP n= 0.3 mmol/g lnP Q = 25.13 2.67 KJ/mol Q = 24.15 2.71 KJ/mol st Linear fit of lnP st Linear fit of lnP

6.0 6.0

5.5 5.5

5.0 5.0 ln P (torr) ln P (torr) Equation y = a+b*x Weight No Weighting Equation y = a+b*x Residual Sum 0.01205 Weight No Weighting of Squares Residual Sum 0.01164 Pearson's r -0.99374 of Squares 4.5 Adj. R-Square 0.97506 4.5 Pearson's r -0.99442 Value Standard Error Adj. R-Square 0.97773 lnP Intercept 15.3023 1.11297 Value Standard Error lnP Slope -2904.21368 326.37898 lnP Intercept 15.50201 1.09371 lnP Slope -3022.20752 320.72838 4.0 4.0 0.0032 0.0033 0.0034 0.0035 0.0036 0.0037 0.0038 0.0032 0.0033 0.0034 0.0035 0.0036 0.0037 0.0038 -1 1/T (K-1) 1/T (K )

6.5 n= 0.35 mmol/g lnP n= 0.4 mmol/g lnP Q = 22.59 3.26 KJ/mol Q = 23.21 2.9 KJ/mol Linear fit of lnP st Linear fit of lnP st 6.5

6.0

5.5

5.5 ln P (torr) ln P (torr) ln P Equation y = a+b*x Equation y = a+b*x Weight No Weighting Weight No Weighting Residual Sum 0.01324 Residual Sum 0.01739 of Squares of Squares 5.0 Pearson's r -0.99258 5.0 Pearson's r -0.98975 Adj. R-Square 0.97041 Adj. R-Square 0.95922 Value Standard Error Value Standard Error lnP Intercept 15.09385 1.16661 lnP Intercept 14.99883 1.33702 lnP Slope -2791.93775 342.10826 lnP Slope -2717.66385 392.08001

4.5 4.5 0.0032 0.0033 0.0034 0.0035 0.0036 0.0037 0.0038 0.0032 0.0033 0.0034 0.0035 0.0036 0.0037 -1 1/T (K-1) 1/T (K )

7.0 7.0 n= 0.45 mmol/g lnP n= 0.5 mmol/g lnP Q = 21.94 3.6 KJ/mol Q = 20.95 3.5 KJ/mol Linear fit of lnP st Linear fit of lnP st

6.5 6.5

6.0

5.5

ln P (torr) P ln ln(torr) P Equation y = a+b*x No Weighting Weight Equation y = a+b*x 0.02112 Residual Sum Weight No Weighting of Squares Residual Sum 0.02031 -0.98687 5.5 Pearson's r of Squares Adj. R-Square 0.94781 5.0 Pearson's r -0.98615 Value Standard Error Adj. R-Square 0.94499 Intercept 14.87139 1.47336 lnP Value Standard Error lnP Slope -2639.51275 432.06172 lnP Intercept 14.57841 1.44487 lnP Slope -2519.49322 423.70763

4.5 5.0 0.0032 0.0033 0.0034 0.0035 0.0036 0.0037 0.0038 0.0032 0.0033 0.0034 0.0035 0.0036 0.0037 0.0038 -1 1/T (K ) 1/T (K-1)

Figure 2.13. The Van’t Hoff isochores for CO2 adsorption on UNM-1 at different loadings.

n = 0 mmol/g 273 K 28 7.0 Q = 15.85 KJ/mol Q 298 K st st 313 K 6.5

26 6.0

5.5 Model Virial chi^2 8E-4 24 R^2 0.999

5.0 a0 1906.4871

(KJ/mol) lnP (torr)

st a1 742.73244 Q a2 -863.75639 4.5 a3 588.34402 22 T1 273 K 4.0 T2 298 K T3 313 K

20 3.5 0.2 0.3 0.4 0.5 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 n (mmol/g) n (mmol/g)

Figure 2.14. Isosteric heat of CO2 adsorption for UNM-1 at different loading calculated by Clausius-Clapeyron method (Left) and virial fitting of CO2 adsorption isotherm for

UNM-1 at zero coverage (Right). Experimental data are the dots, lines are fitting and the parameters are given in the table within the plot for virial method.

2.3 Conclusion

We have designed and synthesized a new charge separated MOF from an ionic tetrapodal borate ligand T1 and Cu(I) metal ion. The obtained UNM-1 MOF is free of counter-ions, possessing high environmental stability and relatively large surface area. It has diamondoid topology and 4-fold interpenetrated structure which contributed to its exceptional environmental stability. Furthermore, UNM-1 showed promising characteristics for

CO2/N2 separation. UNM-1 exhibited very unusual sorption behavior for CO2 at 313 K with increasing hysteresis, increased selectivity and low isosteric heat of adsorption.

2.4 Experimental Procedure

Materials and Methods

Solvents and chemicals were purchased from Sigma-Aldrich or VWR unless otherwise noted. Anhydrous solvents such as ether, acetonitrile (CH3CN), triethylamine

(N(CH2CH3)3) and dichloromethane (CH2Cl2) were prepared by distillation over calcium hydride (CaH2) followed by degassing through several freeze-pump-thaw cycles before use. NMR experiments, such as 300.13 MHz proton, 96.25 boron and 282.23 MHz fluorine spectra, were done on a Bruker Advance III 300 MHz solution spectrometer. The Bruker

Advance 500 MHz solution spectrometer was used for 125.76 MHz carbon NMR. For proton and carbon NMR spectra, internal solvent signal was used as a reference. External reference such as BF3. Et2O (δ = 0 ppm) and C6F6 (δ = −164.9 ppm) were used for boron and fluorine NMR respectively. Single crystal X-ray data were collected on a Bruker

Kappa APEX II CCD system equipped with a graphite monochromator and a Mo K-α fine- focus tube (λ = 0.71073 Å). Acidic and basic solutions of different pH is prepared by diluting the concentrated aqueous solutions of HCl and NaOH. pH meter is used to measure the pH of acidic and basic solutions. Thermogravimetric analysis was carried out using a

TA SDT Q600 TGA/DSC instrument at a heating rate of 2 °C/minute from room temperature to 600°C under nitrogen. All gas adsorption isotherms were measured on a

Quantachrome Autosorb AS1 instrument. UHP grade gases were used for the gas adsorption analysis. Before the analysis, about 100 mg of sample was outgassed for 25 h at 60 °C. Liquid nitrogen was used as a cooling bath to maintain the temperature of 77 K.

Other temperatures were maintained by using water bath. To confirm the accuracy of the results, all gas adsorption experiments were repeated 3 times. For selectivity calculations,

PyIAST code is used. All the data were fitted by using Origin Pro.

Synthesis of T1, Tetrabutylammonium tetrakis(pyridine-4-ethynyl-2,3,5,6- tetrafluorophenyl)borate [TBA+ (B(PhF-in-py)4)–]

Compound 3 (400 mg, 0.42 mmol), 4-bromopyridine hydrochloride (351 mg, 1.806 mmol), Pd(PPh3)4 (24 mg, 0.021 mmol), and CuI (7 mg, 0.042 mmol) were dissolved in a mixture of triethylamine (1.5 mL) and CH3CN (6 mL) into a pressure vessel equipped with a magnetic stir bar under argon. The vessel was sealed and immersed into an oil bath preset at 90 °C with stirring for 24 h. The reaction mixture was then cooled to room temperature, diluted with a large excess of 1:5 dichloromethane: hexane mixture, and stirred for 6 h. After filtration, the residue obtained was recrystallized from methanol and water to afford borate ligand T1 as a light-yellow compound (380

1 3 mg, 73% yield). H NMR: (300.13 MHZ, CDCl3, 298 K): δ (ppm) = 8.62 (d, JHH = 6

3 Hz, 8H), 7.39 (d, JHH = 6 Hz, 8H), 3.01(m, 8H), 1.58 (m, 8 H), 1.36 (m, 8H), 0.95 (t,

3 13 JHH = 7.5 Hz, 12 H). C NMR: (125.76 MHz, CDCl3, 298 K): δ (ppm) = 13.3, 19.5,

2 1 23.6, 58.7, 80.5, 96.0, 98.9 (t, JCF = 14 Hz), 125.5, 130.5, 132-134 (br), 145.7 (d, JCF

1 11 = 255 Hz), 148.2 (d, JCF = 241 Hz), 149.9. B NMR: (96.25 MHz, CDCl3, 298 K): δ

19 (ppm) = −16.32. F NMR (282.40 MHz, CDCl3, 298 K): δ (ppm) = −129.94 (s, 8F),

−139.69(s, 8F).

Synthesis of UNM-1. 20 mg (0.015 mmol) of T1 was dissolved in 1 mL of CH2Cl2 in a long 20 mL glass vial and 7 mL of CH3CN was layered on the top. Then a solution of 15 mg (0.045 mmol) of (CH3CN)4CuBF4 in 1 mL of CH3CN was added on the top of the two layers. The contents were mixed slowly under argon for 48 h to obtain yellow colored needle shaped crystals. The crystals were washed several times with

45 acetonitrile, dichloromethane and air dried to obtain 12 mg (74.4%) of UNM-1 based on T1.

Single crystal X-ray Diffraction.

For the X-ray crystallographic analysis, a single X-ray quality crystal of each MOFs was coated with Paratone oil and mounted on a MiTeGen Micro Loop that had already been attached to a metallic pin using epoxy. The frames were integrated with the Bruker SAINT software package using a narrow-frame algorithm. Data were corrected for absorption effects using the Multi-Scan method (SADABS). The structure was solved and refined using the Bruker SHELXTL Software Package. Non-hydrogen atoms were refined anisotropically. SQUEEZE was used to account solvent accessible voids.

Preparation of samples for powder X-Ray diffraction measurements

Prior to the analysis about 400 mg of UNM-1 MOF crystals were separated from the mother liquor and washed several times with anhydrous CH3CN followed by drying under vacuum for 24 h. For each analysis about 30-40 mg of sample was used. The samples soaked in aqueous solution of different pH was washed several times with water and dried under vacuum for 48 h before subjecting to the PXRD analysis. The sample of UNM-1 after 12 adsorption/desorption cycles was used for PXRD measurement without any solvent or heat treatments.

Surface area determination: As UNM-1 displayed type I adsorption isotherm for N2 gas at 77 K, following methods are used for the surface area calculation.

Brunauer-Emmett-Teller (BET) method: The BET theory is developed by Stephen

Brunauer, Paul Hugh Emmett, and Edward Teller in 1938. It is a method for the determination of surface area of solid surface by adsorbing gas molecules. The gas used

46 for the BET analysis should not react chemically with the adsorbing surface. Usually N2 gas at its boiling temperature (77 K) is used as an adsorbate gas. The analysis is carried out in the P/P0 range of 0.05-0.35. The standard BET equation is given as:

1 1 퐶 − 1 푃 = + ( ) (퐸푞. 2.4) 푊 (푃0⁄푃) − 1 푊푚퐶 푊푚퐶 푃0

Where, W = weight of gas adsorbed, P/P0 = relative pressure, Wm = weight of adsorbate forming a monolayer and C is the BET constant, which is related to the magnitude of the adsorbent/adsorbate interactions.

The weight of a monolayer of adsorbate Wm can then be obtained from the slope s and intercept i of the BET plot as follows:

퐶 − 1 푠 = (퐸푞. 2.5) 푊푚퐶

1 푖 = (퐸푞. 2.6) 푊푚퐶

Combining equation 2.5 and 2.6, Wm can be calculated as:

1 푊 = (퐸푞. 2.7) 푚 푠 + 푖

The total surface area of the sample St can be obtained by using the following equation:

푊 푁퐴 푆 = 푚 푐푠 (퐸푞. 2.8) 푡 푀 where N is Avogadro’s number (6.023x1023 molecules/mol), M is the molecular weight

2 of the adsorbate and Acs is the cross-sectional area of an adsorbate (16.2 Å for N2).

The BET surface area SBET of the solid can be calculated from the total surface area St and the sample weight w by using the equation given below:

푆 푆 = 푡 (퐸푞. 2.9) 퐵퐸푇 푤

Langmuir method: When a sample is microporous and does not contain any meso or macropores, it will exhibit a Type I or Langmuir isotherm. The Langmuir equation is applied for the adsorption of a single molecular layer of adsorbate. Langmuir equation used for the surface area calculation is given below:

푊 퐶(푃⁄푃 ) = 0 (퐸푞. 2.10) 푊푚 1 + 퐶(푃⁄푃0)

Where W is the weight of adsorbate, Wm= weight in a monolayer at P/P0 respectively. C is the constant associated with the energy of adsorption. Equation (2.10), in the form of a straight line can be written as:

푃⁄푃 1 푃⁄푃 0 = + 0 (퐸푞. 2.11) 푊 퐶푊푚 푊푚

A linear plot of (P/P0)/W as a function of P/P0 produce the slope (1/Wm) and the intercept

(1/CWm). Wm can be determined from the slope and intercept according to equation 2.11.

Finally, the Langmuir surface area can be calculated by applying equation 2.9 as in BET surface area measurement.

Pore size distribution

Pore size of UNM-1 was estimated by fitting N2 adsorption isotherm at 77 K using non- local density functional theory (NLDFT). Here, N2 adsorption at 77K on carbon for slit pore was used as the calculation model with QSDFT (Quenched Solid Density Functional

Theory) equilibrium model as the kernel in the Quantachrome AS1Win software. QSDFT has the advantage on the contrary to regular NLDFT, as it considers surface roughness and heterogeneity. This model is applicable to micro and meso porous materials with 0.35 nm to 40 nm of pore size.

PyIAST method for selectivity calculation

In order to determine the selectivity CO2/N2 selectivity of UNM-1 by pyIAST model, all experimental pure component isotherms data for CO2 and N2 at 273 K, 298 K, 303 K, 313K and 323 K were fitted by the analytical models like Langmuir or quadratic. For some of the data linear interpolation is also used. From fitted data, spreading pressure is calculated.

Then the following equation was applied to calculate the selectivity of UNM-1 for CO2/N2 separation:

푥푎⁄푦푎 푆푎/푏 = ( 퐸푞. 2.12) 푥푏⁄푦푏

Where, xa and ya are the mole fraction of component ‘CO2’ in adsorbed and gas phase.

Similarly, xb and yb are the mole fraction of component ‘N2’ in adsorbed and gas phase.

Heat of adsorption

Isosteric heat of CO2 adsorption of UNM-1 is calculated by the following two methods.

The adsorption isotherms measured at 273 K, 298 K and 313 K in the pressure range from

0 to 760 torr are used for the calculation.

A. Direct method. Clausius-Clapeyron equation for the determination of isosteric heat of adsorption (Qst) given as:

푑푙푛푃 푄 = − [ ] 푛 (퐸푞. 2.13) 푠푡 푑(1⁄푇)

Integration of equation 2.13 gives:

(푄 ⁄푅) ln (푃) = − 푠푡 + 퐶 (퐸푞. 2.14) 푛 (1⁄푇)

Where P is the Pressure in torr at temperature T in K, n is the amount of gas adsorbed in mmol/g, R is universal gas constant and C is a constant. The corresponding value of ‘P’ at each given ‘n’ is determined from the fitted isotherm at 273 K, 298 K and 313 K. The slope

49 obtained from the linear regression of ln P versus 1/T at a given ‘n’ is used to calculate Qst according to equation 2.14.179-180

B. Virial method

The coverage dependent Qst is determined by fitting the isotherms data to the following

‘virial equation’:

푚 1 ln 푃 = ln 푛 + ∑ 푎 푛푖 (퐸푞. 2.15) 푇 푖 푖=0

Where, n is the amount adsorbed in mmol/g at pressure P (torr) and temperature T (K). ai is the virial coefficient and m represent the number of coefficients required to adequately describe the isotherms (m is increased until the best fit is obtained, and the average value of the squared deviations is minimized). The values of these virial coefficients are then used to calculate the isosteric heat of adsorption according to the following expression:

푚 푖 푄푠푡 = −푅 ∑ 푎푖푛 (퐸푞. 2.16) 푖=0

’ ‘Qst at zero coverage is calculated according to equation 2.17.

푚

푄푠푡 = −푅 ∑ 푎0 (퐸푞. 2.17) 푖=0

Chapter 3. Synthesis and Characterization of Modified Ligand T2 and Charge-

Separated MOFs (UNM-2, UNM-3, UNM-4 and UNM-5)

3.1 Introduction

Researchers have put significant efforts for the development of MOFs with extraordinary properties. So far, large number of MOFs with interesting structures and properties have been synthesized by using classical synthetic methodologies such as reticular chemistry, topology-guided design, ligand exchange/insertion/modification and post-synthetic routes.181 All of these synthetic strategies are based on the approach which involves the use of variety of ligands with different functionality, lengths and binding sites. Ligand modification allows for the design of MOFs with tunable pore size, surface area and functionality. At the same time by reducing the length of the coordinating arm, interpenetration in MOF structure can be minimized.182 Based on this approach, we have synthesized a new tetrahedral ionic borate ligand referred as Tetrapod 2 (T2) containing coordinating arms without ethynyl group which are relatively shorter compared to ligand

T1. This newly developed borate ligand has tetrabutyl ammonium cation and borate anion.

As UNM-1 MOF synthesized by using ligand T1 is interpenetrated, we used new ligand

T2 to synthesize a variety of charge-separated MOFs named UNM-4 and UNM-5 to analyze if reduction in ligand arm length will reduce interpenetration. During MOF synthesis copper (I) and silver (I) metal salts are used to coordinate to ionic borate resulting

UNM-4 and UNM-5 respectively with overall charge neutrality. Because of the extended ligand arm length, all the resulting MOF crystal structures possessed 2-fold interpenetration which greatly reduced porosity and void size. Also, under the similar synthetic condition the change of metal cation from Cu(I) in UNM-4 to Ag(I) in UNM-5

51 has negligible effect on overall framework’s crystal structure. Additionally, UNM-2 and

UNM-3 MOFs are synthesized from T1 and Cu(II) metal ions. Although these MOFs are synthesized from same metal precursor and ligand, they have different structures depending upon the solvent used for crystallization. In these MOFs, the overall charge neutrality is balanced by the nitrate ion present within the pores. UNM-2 is 2-fold interpenetrated structure which is because of the inclusion of nitrate ion inside the pores.

Compared to UNM-1 mentioned in Chapter 2, all of these MOFs are unstable after solvent removal because of reduced degree of interpenetration. In this chapter synthesis and characterization of ligand T2 and charge-separated MOFs (UNM-2, UNM-3, UNM-4 and

UNM-5) are discussed.

Figure 3.1. Structure of tetrapodal ligand T1 and T2.

3.2 Results and Discussion

3.2.1 Synthesis and Characterization of Ligand T2 and MOFs (UNM-2 to UNM-5)

3.2.1.1 Tetrabutylammonium tetrakis(4-pyridyltetrafluorophenyl)borate (T2). In the synthesis of T2, ionic borate compound 5 was synthesized according to the literature.183

Simple ionic exchange reaction of compound 5 with tetrabutylammonium bromide produced compound 6 with tetrabutylammonium cation and tetrakis(4- bromotetrafluorophenyl)borate anion which is fully characterized by proton, carbon, boron and fluorine NMR spectroscopy (chemical shift values are provided in experimental section). Compound 6 was converted into the final borate ligand T2 with an overall yield of 80% by Suzuki coupling with 4-pyridineboronic acid pinacol ester in presence of excess

K2CO3. The formation and purity of T2 is confirmed by proton, carbon, boron and fluorine

NMR spectroscopy. The integration of 1H NMR protons confirmed 1:1 ratio of tetrabutylammonium cation and borate anion. 11B NMR showed a sharp signal at −16.3 ppm and 19F showed two sharp signals at −129.9 and −146.8 ppm. In 13C NMR spectrum the F4-phenyl carbon atoms ortho- and meta- to the boron center appears as two sets of

1 doublets at 148.8 and 142.4 ppm having JCF coupling constants of 252 and 251 Hz, respectively. The broad signal ranging from 132 to 134 ppm caused by the splitting effects from 1J boron and 2J fluorine atoms is assigned as an ipso-carbon atom. The triplet at 113.1

2 ppm is a F4-phenyl carbon para- to boron center with JCF coupling constant of 15 Hz. The sharp enhanced signals at 149.9, 137.4 and 124.8 ppm are from the pyridine rings, while all other carbons from tetrabylammonium group are observed in between 13 and 59 ppm.

Scheme 3.1. Synthesis of ligand T2.

1 13 11 19 Figure 3.2. H, C, B and F NMR spectra of T2 in CDCl3.

3.2.1.2 Synthesis and structural analysis of UNM-2. Two different crystal system with different crystal structures were obtained by combining Tetrapod 1 and Cu(NO3)2 . xH2O depending upon the solvents used for the crystallization. Slow mixing of T1 solution in dichloromethane and methanolic solution of Cu(NO3)2 . xH2O resulted in the formation of

X-ray quality blue rod-shaped crystals named as UNM-2. The obtained 3-dimensional crystal has an approximate composition of C52H18BCuF16N4O. Crystallographic data and crystal structures of UNM-2 are presented in Table 3.1 and Figure 3.3. UNM-2 crystallizes in monoclinic crystal system with a space group of C 2/c. As shown in Figure 3.3A, copper atom in UNM-2 is coordinated to four pyridyl units from different borate ligand and a water molecule resulting a square pyramidal geometry. Since the copper atoms in UNM-2 are in

+2 oxidation state and the equally numbered borate units possess a −1 charge, there must be one −1 charged counterion, likely NO3¯ ion, per Cu(II) atom within the framework in order to ensure the overall charge neutrality. These nitrate ions are likely disordered and mobile since they could not be resolved in the single crystal X-ray analyses. The N-Cu-N bond angles average at ca. 116° and the N-Cu-O bond angles are at ca. 96°, while the bond angles around boron atoms range from 100° to 116°, similar to those found in UNM-1. The

Cu−OH2 bond length in UNM-2 is found at ca. 2.52 Å and the boron-boron and boron- copper distances are measured at about 26.39 Å and 13.25 Å, respectively. Cu-N bond lengths averages at ca. 2.06 Å. Despite the same ligand arm lengths as in UNM-1, UNM-

2 is found to be 2-fold interpenetrated as shown in Figure 3.3D, 3.3E and 3.3F. The reduced degree of interpenetration in UNM-2 is likely caused by the inclusion of the nitrate ions in the framework, which also possibly led to a relatively small BET surface area of ca. 40.3

55 m2/g despite the pore sizes of 10.7 Å (viewed from a-axis), 10.27 (viewed from b-axis) and

14.06 Å (viewed from c-axis) (Figure 3.3D, 3.3E and 3.3F).

Figure 3.3. Single crystal X-ray structure of UNM-2; (A) Partial view of crystal structure showing coordination environment around copper; (B) coordination environment around boron; (C) extended network; (D) space-filling model of a 2×2×2 unit cell viewed from the a-axis; (E) space-filling model of a 2×2×2 unit cell viewed from the b-axis and (F) space- filling model of a 2×2×2 unit cell viewed from the c-axis showing 2-fold interpenetration

(red and grey colors each indicates 1-fold interpenetration). Atoms of carbon appears in grey, nitrogen in blue, boron in yellow, fluorine in green, oxygen in red and copper in cyan.

Hydrogen atoms are omitted for clarity.

3.2.1.3 Synthesis and structural analysis of UNM-3. When a DMF solution of T1 and a water solution of Cu(NO3)2 . xH2O were slowly mixed at room temperature and let stand still for ca. 15 days, light blue rod-shaped crystals of UNM-3 were obtained.

Crystallographic data and crystal structures are presented in Table 3.1 and Figure 3.4. The approximate crystal composition of UNM-3 is C110H54B2CuF32N10O6, indicating the presence of water and DMF molecules within the crystal structures. The crystal system of

UNM-3 is monoclinic with a space group of C 2/c. The Cu2+ center is coordinated to four pyridyl groups from different ligands and to two water molecules in trans positions, resulting in an octahedral geometry as shown in Figure 3.4A and 3.4C. The C-B-C angle around the boron atom ranges from ca. 100° to 114°, consistent with the tetrahedral coordination environment of boron atoms. The N-Cu-N angle and N-Cu-O angle around the copper center ranges from ca. 89°−173° and ca. 88°−94° respectively. All the Cu-N bond lengths are normal as found in other copper pyridyl complexes (2.017 and 2.027

104, 184-185 Å). However, the diaxial Cu−OH2 bond length in UNM-2 is ca. 2.45 Å and lengthened demonstrating the possible Jahn-Teller distortion because of the d9 configuration of Cu(II) in UNM-3. Although the ligand T1 is tetrahedral in shape and expected to form 3-dimensional MOF structures upon coordination with metal centers,

UNM-3 is found to be a one-dimensional coordination polymer. In UNM-3, only two out of the four pyridine arms of each T1 ligand are coordinated to the copper atoms, leaving the other two dangling, which leads to a chain-like structures. Therefore, the framework couldn’t form 3D structure which results a wavy sheet that are very loosely held together by the hydrogen bond between water and DMF solvent molecules. As shown in the Figure

3.4C, two of the four pyridine independent N atoms (N1 and N4) coordinated to copper

57 sits on a 2-fold rotation axis. A 3rd pyridine N (N2) is hydrogen bonded to the water molecule (O1) that is also coordinated to copper. That water also hydrogen bonds to an uncoordinated water (O3), and the uncoordinated water makes a hydrogen bond to DMF

(O2). The final pyridine N3 and one hydrogen on the O3 water do not appear to form hydrogen bond or coordinate to anything. Attempts to determine the porosity of UNM-3 resulted an amorphous structure as it involved the removal of coordinated solvent molecule, making it difficult for the BET surface area analysis. From single crystal structure, the framework appears non porous along a- and b-axis while along c-axis the pore size is ca. 9.26 Å.

Figure 3.4. Single crystal X-ray structure of UNM-3; (A) framework structure showing chain like structure; (B) a crystalline unit showing coordination environment around boron

(C) coordination environment around copper and hydrogen bonding between solvent molecules; (D) space-filling model of a 2×2×2 unit cell viewed from the a-axis; (E) space- 58 filling model of a 2×2×2 unit cell viewed from the b-axis; and (F) space-filling model of a

Table 3.1. Summary of crystallographic data and some important properties of UNM-2 and UNM-3 synthesized from the assembly of T1 and Cu(II) salt.

Parameters UNM-2 UNM-3 Solvent of crystallization CH2Cl2/CH3OH H2O/DMF Crystal yield (%) 74 27 Complex with Cu(NO3)2 . xH2O Cu(NO3)2 . xH2O Color blue blue Shape rod rod Chemical formula C52H18BCuF16N4O C110H54B2CuF32N10O6 Formula weight 1093.05 g/mol 2304.79 g/mol Temperature (K) 100(2) 100(2) Crystal system monoclinic monoclinic Space group C 2/c C 2/c a (Å) a = 29.313(3) a = 36.955(18) b (Å) b = 20.394(2) b = 21.618(10) c (Å) c = 38.674(4) c = 16.249(8) α (°) 90 90 β (°) 90.874(5) 103.122(14) γ (°) 90 90 V (Å3) 23117.(4) 12642.(18) Z 8 4 3 Dcalc (g/cm ) 0.628 1.212 Total Reflections 51445 47295 Unique 9047 [R(int) = 0.1971] 9091 [R(int) = 0.0846] 2 2 2 2 2 2 Function minimized Σ w(Fo - Fc ) Σ w(Fo - Fc ) Data / restraints/ 9047 / 603 / 679 9091 / 0 / 732 parameters Goodness-of-fit on F2 1.214 1.007 R1 (I>2σ(I), wR2 0.1014, 0.2471 0.0603, 0.1552 R1, wR2 (all data) 0.2338, 0.3117 0.1183, 0.1862 Surface area (m2/g) 40 N/A

3.2.1.4 Synthesis and crystal structure analysis of UNM-4. Slow mixing of a T2 solution in dichloromethane and a Cu(CH3CN)4BF4 solution in acetonitrile followed by slow self- mixing for 48 h at room temperature afforded needle shaped yellow crystals of UNM-4 which were big enough to be characterized by single crystal X-ray diffraction. The 3- dimensional framework formed by bridging of copper(I) to ionic borate formed a charge neutral framework with an approximate composition of C44H16BCuF16N4. Crystallographic data and crystal structures are presented in Table 3.2 and Figure 3.5. UNM-4 is a monoclinic crystal system with a space group of I 2/a. The use of T2 without a triple bond in UNM-4 synthesis was expected to form a network without interpenetration. However, the framework is still 2-fold interpenetrated because the separation distance between boron-boron of two ligands sharing a common copper atom is ca.16.07 Å and boron-copper distance is ca. 10.8 Å which is large enough to cause 2-fold interpenetration. A space fill model of 2×2×2 unit cell displaying interpenetration is shown in Figure 3.5D, 3.5E and

3.5F. In UNM-4, both boron and copper have tetrahedral coordination environment. Each copper atom is coordinated to four pyridyl units from different borate ligand and vice- versa. The Cu-N bond lengths are 2.03 and 2.01 Å. The bond angles around boron centers ranges from ca. 102° to 116° and those around copper centers from ca. 101° to 119°. The

BET surface area of UNM-4 is ca. 9 m2/g indicating low framework porosity. Based on the single crystal structure, the framework is non porous when viewed from b-axis but when viewed from a- and c-axis, the pore sizes were found as ca. 13.05 Å and ca. 11.09 Å respectively.

Figure 3.5. Single crystal X-ray structure of UNM-4; (A) Partial view of crystal structure showing coordination environment around copper; (B) coordination environment around boron; (C) extended network; (D) space-filling model of a 2×2×2 unit cell viewed from the a-axis; (E) space-filling model of a 2×2×2 unit cell viewed from the b-axis and (F) space- filling model of a 2×2×2 unit cell viewed from the c-axis showing 2-fold interpenetration

(red and grey colors each indicates 1-fold interpenetration). Atoms of carbon appears in grey, boron in yellow, fluorine in green and copper in cyan. Hydrogen atoms are omitted for clarity.

3.2.1.5 Synthesis and crystal structure analysis of UNM-5. To see if replacing a copper ion from the framework with a bigger metal cation can bring any changes in the crystal structure, a new MOF, UNM-5 is synthesized from T2 and AgBF4. UNM-5 is a colorless

61 rod-shaped crystal formed by the slow diffusion of T2 solution in dichloromethane and

AgBF4 solution in acetonitrile for 48 h at room temperature. The approximate composition of UNM-5 is C44H16BAgF16N4 and it crystallizes in monoclinic crystal system with a space group of I 2/a. Crystal structures and crystallographic data are presented in Figure 3.6 and

Table 3.2. Similar to UNM-4, it is formed as a charge neutral 3-dimensional framework as

+1 charge of silver cation is compensated by -1 charge of boron anion. In UNM-5, silver is coordinated tetrahedrally to four pyridyl group from different borate ligand and the bond angle around the silver atom ranges from ca. 95° to 128°. The coordination environment around boron is also tetrahedral with C-B-C bond angles of ca. 101° to 115°. All Cu-N bond lengths are 2.30 Å. The separation distance between the boron-boron of two ligands bonded to same silver atom is ca. 19.66 Å and boron-copper is ca. 11.09 Å which indicates that the ligand arms are little bit extended compared to UNM-4. However, the use of bigger cation like silver didn’t get rid of the interpenetration completely. UNM-5 is 2-fold interpenetrated as shown in Figure 3.6D, 3.6E and 3.6F. The BET surface area of UNM-5 is very low as ca. 3 m2/g indicating that the crystals are less porous after solvent removal.

Single crystal structure of the framework appears non porous from b-axis but from a- and c-axis, the pore sizes were found as ca. 13.73 Å and ca. 10.56 Å respectively.

Figure 3.6. Single crystal X-ray structure of UNM-5; (A) Partial view of crystal structure showing coordination environment around silver; (B) coordination environment around boron; (C) extended network; (D) space-filling model of a 2×2×2 unit cell viewed from the a-axis; (E) space-filling model of a 2×2×2 unit cell viewed from the b-axis; and (F) space- filling model of a 2×2×2 unit cell viewed from the c-axis showing 2-fold interpenetration

(red and grey colors each indicates 1-fold interpenetration). Atoms of carbon appears in grey, boron in yellow, fluorine in green and silver in light grey. Hydrogen atoms are omitted for clarity.

Table 3.2. Summary of crystallographic data and some important properties of UNM-4 and UNM-5 synthesized from the assembly of T2 with Cu(I) and Ag(I) salts.

Parameters UNM-4 UNM-5

Solvent of crystallization CH2Cl2/CH3CN CH2Cl2/CH3CN Crystal yield (%) 84 77 Complex with Cu(CH3CN)4BF4 AgBF4 Color yellow colorless Shape needle needle Chemical formula C44H16BCuF16N4 C44H16AgBF16N4 Formula weight 978.96 g/mol 1023.29 g/mol Temperature (K) 100(2) 100(2) Crystal system monoclinic monoclinic Space group I 2/a I 2/a a (Å) a = 15.6357(11) a = 15.197(3) Å b (Å) b = 29.2695(12) b = 30.954(4) Å c (Å) c = 16.0766(7) c = 16.340(2) α (°) 90 90 β (°) 98.025(2) 100.440(2) γ (°) 90 90 V (Å3) 7285.4(7) 7559.(2) Z 4 4 3 Dcalc (g/cm ) 0.893 0.899 Total Reflections 28037 54294 Unique 6707 [R(int) = 0.1530] 7227 [R(int) = 0.1635] 2 2 2 2 2 2 Function minimized Σ w(Fo - Fc ) Σ w(Fo - Fc ) Data / restraints/ 6707 / 276 / 299 7227 / 0 / 299 parameters Goodness-of-fit on F2 0.914 1.003 R1 (I>2σ(I), wR2 0.0726, 0.1892 0.0646, 0.1698 R1, wR2 (all data) 0.1725, 0.2319 0.1865, 0.2323 Surface area (m2/g) 9 3

3.3 Conclusion

In conclusion borate centered tetrapodal ionic ligands are very effective for the construction of charge-separated MOFs. By employing solvent diffusion crystallization technique, four different MOFs; UNM-2, UNM-3, UNM -4 and UNM-5 are synthesized from ionic organic borate ligand T1 and T2 of different arm lengths. All these MOFs

64 crystallized in different space group depending upon the oxidation state of coordinated metal ions. In order to reduce the degree of interpenetration caused by longer arm length of T1 ligand, Cu ion in +2 oxidation is used as an inorganic unit in UNM-2 and UNM-3.

This approach successfully reduced the interpenetration but not productive enough to increase the stability and porosity of resulting materials. UNM-2 containing Cu(II) is 3D,

2-fold interpenetrated framework with small surface area of 40.3 m2/g due to the presence of charge balancing nitrate anions inside the pores. Although both UNM-2 and UNM-3 synthesized from same Cu(II) salt and ionic T1 crystallized with similar space group, these have different ligand arrangement around metal center depending upon the solvent system used for crystallization. UNM-3 is 1D MOF containing hydrogen bonded solvent molecules in the final crystal structure. This framework is very unstable after solvent removal due to the loss of coordinated solvents and included guests from the larger channels.

Furthermore, we have successfully synthesized and characterized a new tetrahedral ionic borate ligand T2 with relatively shorter coordinating arms without ethynyl group compared to ligand T1. Through solvent diffusion crystallization technique, UNM-4 and UNM-5 are synthesized by coordinating T2 with Cu(I) and Ag(I) respectively. Both of these MOFs are

2-fold interpenetrated and crystallized in monoclinic crystal system with similar space group of I 2/a. It is fascinating to see that even after replacing Cu(I) with bigger metal cation of Ag(I), similar type of crystal system is obtained. Instead of charge neutrality both frameworks have very low surface area and are unstable after solvent removal caused by reduced degree of interpenetration. Together these results demonstrate the potential of tetrapodal ionic borate ligands in construction of charge-separated MOFs with unique

65 architectures and properties. Shorter ligands expectedly lead to lower degrees of interpenetration but not necessarily higher stability or surface areas. The properties of these materials are closely related to the crystal structural details that are sensitive to the changes in experimental condition including metal oxidation states, crystallization methodologies, and solvent additives.

3.4 Experimental Procedure

Materials and Methods

Solvents and chemicals were purchased from Sigma-Aldrich or VWR unless otherwise noted. Anhydrous ether is prepared by distillation over calcium hydride (CaH2) followed by degassing through several freeze-pump-thaw cycles before use. THF was distilled from

Na/benzophenone prior to use. NMR experiments, such as 300.13 MHz proton, 96.25 boron and 282.23 MHz fluorine spectra, were done on a Bruker Advance III 300 MHz solution spectrometer. The Bruker Advance 500 MHz solution spectrometer was used for

125.76 MHz carbon NMR. For proton and carbon NMR spectra, internal solvent signal was used as a reference. External reference such as BF3. Et2O (δ = 0 ppm) and C6F6 (δ =

−164.9 ppm) were used for boron and fluorine NMR respectively. Single crystal X-ray intensity data for UNM-2 and UNM-4 were measured on a Bruker Kappa APEX II CCD system equipped with a graphite monochromator and a Mo Kα fine-focus tube (λ = 0.71073

Å). For UNM-3, Bruker X8 APEX II system equipped with a Double Bounce Multilayer

Mirrors monochromator and a CuKα Micro-focus rotating anode (λ = 1.54178 Å) was used.

Single crystal X-ray intensity data for UNM-5 was measured on a Bruker Kappa APEX II

Duo system equipped with a fine-focus sealed tube (Mo Kα, λ = 0.71073 Å) and a graphite monochromator. BET surface area was measured on a Quantachrome Autosorb AS1

66 instrument using UHP grade nitrogen gas. About 100 mg of sample was outgassed for 25 h at 60 °C before the surface area measurement.

Synthesis of compound 6, [Tetrabutylammonium tetrakis(4- bromotetrafluorophenyl) borate]

3.24 mg (3.48 mmol) of Compound 5, 6.74 g (20.91 mmol) of tetrabutylammonium bromide were dissolved in 20 mL of acetone and stirred for 24 h at room temperature. After solvent evaporation, the residue was dissolved in large excess of dichloromethane and filtered through silica. Solvent was evaporated to obtain white residue which was dissolved in little amount of dichloromethane and precipitated in hexane. After filtration white

1 colored compound was obtained in 95% (3.58 g) yield. H NMR: (300.13 MHZ, CDCl3,

3 13 298 K): δ (ppm) = 2.9 (m, 8H), 1.5 (m, 8 H), 1.3 (m, 8H), 0.9 (t, JHH = 7.2 Hz, 12 H). C

3 NMR: (125.76 MHz, CDCl3, 298 K): δ (ppm) = 13.1, 19.3, 23.4, 58.6, 94.3 (t, JCF = 22

1 1 11 Hz), 129-131 (br), 143.7 (d, JCF = 245 Hz), 148.5 (d, JCF = 244 Hz). B NMR: (96.25

19 MHz, CDCl3, 298 K): δ (ppm) = −16.3. F NMR (282.40 MHz, CDCl3, 298 K): δ (ppm)

= −128.8 (s, 8F), −137.1(s, 8F).

Synthesis of T2 [Tetrabutylammonium tetrakis(4-pyridinetetrafluorophenyl)borate].

400 mg (0.34 mmol) of Compound 6, 365.6 mg (1.7 mmol) of 4-pyridineboronic acid pinacol ester, 11.7 mg (0.01 mmol) of Pd(PPh3)4, and 282 mg (2.04 mmol) of potassium carbonate were dissolved in 6 mL of anhydrous tetrahydrofuran into a pressure vessel equipped with a magnetic stir bar under argon. The vessel was sealed tightly and heated at

80 °C with stirring for 24 h. After cooling to room temperature, the reaction mixture was diluted with large excess of 1:5 dichloromethane: hexane and stirred for 12 h. After filtration, 315 mg of T1 was obtained as a white powder in 80% yield. 1H NMR: (300.13

3 3 MHZ, CDCl3, 298 K): δ (ppm) = 8.7 (d, JHH = 4.2 Hz, 8H), 7.4 (d, JHH = 4.2 Hz, 8H), 2.9

3 13 (m, 8H), 1.5 (m, 8 H), 1.29 (m, 8H), 0.9 (t, JHH = 7.2 Hz, 12 H). C NMR: (125.76 MHz,

2 CDCl3, 298 K): δ (ppm) = 13.3, 19.4, 23.5, 58.7, 113.1 (t, JCF = 15 Hz), 124.9, 132-134

1 1 11 (br), 137.4, 142.4 (d, JCF = 251 Hz), 147.8 (d, JCF = 252 Hz), 149.9. B NMR: (96.25

19 MHz, CDCl3, 298 K): δ (ppm) = −16.3. F NMR (282.40 MHz, CDCl3, 298 K): δ (ppm)

= −129.9 (s, 8F), −146.8(s, 8F).

Synthesis of UNM-2, UNM-3, UNM-4 and UNM-5

UNM-2. 20 mg (0.015 mmol) of T1 was dissolved in 2 mL of CH2Cl2 in a long 20 mL glass vial and 3 mL of CH3OH was layered on the top. Then a solution of 6 mg (0.032 mmol) of Cu(NO)3 . xH2O in 2 mL of CH3OH was added on the top of the two layers. The contents were mixed slowly for 48 h to obtain 13 mg of blue colored rod like crystals in

74% yield based on T1.

UNM-3. A solution of 6 mg (0.032 mmol) of Cu(NO)3 . xH2O in 2 mL of H2O was prepared in a long 20 mL glass vial and 3 mL of DMF was layered as an intermediate layer. Then

20 mg (0.015 mmol) of T1 solution in 2 mL of DMF was added on the top of two layers.

The contents were mixed slowly for fifteen days to obtain 10 mg of light blue colored rod like crystals in 27% yield based on T1.

UNM-4. 20 mg (0.017 mmol) of T2 was dissolved in 1 mL of CH2Cl2 in a long 20 mL glass vial and 5 mL of CH3CN was layered on the top. Then a solution of 10.6 mg (0.034 mmol) of (CH3CN)4CuBF4 in 1 mL of CH3CN was added on the top of the two layers. The contents were mixed slowly for 48 h at room temperature to obtain yellow colored needle shaped crystals. The crystals were formed in 114 mg (84%) yield based on T2.

UNM-5. 20 mg (0.017 mmol) of T2 was dissolved in 1 mL of CH2Cl2 in a long 20 mL glass vial and 6 mL of CH3CN was layered on the top. Then a solution of 6.6 mg (0.017 mmol) of AgBF4 in 1 mL of CH3CN was added on the top of the two layers. The contents were mixed slowly under argon for 48 h to obtain colorless rod-shaped crystals in 13.5 mg

(77%) based on T2.

Single crystal X-ray Diffraction

A single crystal of each MOF was coated with Paratone oil and mounted on a MiTeGen

MicroLoop. The frames were integrated with the Bruker SAINT software package using a narrow-frame algorithm. Data were corrected for absorption effects using the Multi-Scan method (SADABS).186 Each structure was solved and refined using the Bruker SHELXTL

Software Package within APEX3186 and/or OLEX2.187-188 Non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed in geometrically calculated positions with Uiso = 1.2Uequiv of the parent atom. During the structure solution, electron density difference maps revealed that there was diffuse solvent dispersed throughout the pores that could not be successfully modeled with or without restraints. Thus, the structure factors were modified using the PLATON SQUEEZE technique, in order to produce a “solvate- free” structure factor set. 189

Chapter 4. Synthesis, Characterization and Applications of a Cubic MOF (UNM-6)

4.1 Introduction

As discussed and demonstrated in Chapter 2 and 3, binding of ionic borate ligands to metal cations in different oxidation states result 3-dimensional charge-separated MOF materials.

The obtained frameworks displayed very interesting properties because of their interpenetrated crystal structure. Though interpenetration is known very bad considering the pore size and surface area, it offers higher stability to the framework. Besides interpenetration, the counter ion present within the MOF structure also play an important role in its stability and coordination interactions.190 While the removal of counter ions from the pores make the framework unstable, they can be exchanged with other ions by means of post-synthetic ionic exchange (PSIE) to construct stable frameworks with same topology and improved properties. PSIE can also be used to exchange ions which are weakly coordinated to the metal center in MOFs. Cohen et al. adopted this concept to exchange

¯ triflate anion by PF6 anion in MOF-Co/AgOTf-1. The MOF obtained after anion exchange

¯ retained its morphology and stability. The newly incorporated PF6 anion occupied the same position in the framework as the replaced triflate anion.191 Similarly Wan et al. reported anion exchange in a novel polymeric coordination complex [Ni(timpt)2](ClO4)2.

Their MOF was a self-penetrating and synthesized from flexible three-connecting ligand.

Anion exchange was performed by soaking the powder of this MOF in an aqueous solution

¯ ¯ of NaNO3. However, during the process ClO4 anions were partially exchanged with NO3 anions. MOFs which can undergo anion exchange are found to have many useful applications such as ion separation, adsorption and most importantly catalysis. There are some reports demonstrating the application of anion exchanged MOFs in heterogeneous

70 catalysis.192 For example, S. Wang and coworkers reported a copper (II) based MOF, and

2- 2- its catalytic properties. The MOF contained SiF6 within its 1D channel. These SiF6 can

¯ be readily exchanged with NO3 simply by suspending the compound in a solution of

NH4NO3. Both MOF and exchanged MOF were used as catalysts for the selective oxidation of benzylic compounds to the corresponding carbonyl functionality.193 Furthermore, they found that under the similar reaction condition compared to original MOF sample, the anion-exchanged sample is more efficient catalyst for the oxidation of diphenylmethane to

194 benzophenone. Recently, MOFs are being used as a heterogeneous catalyst in CO2 cycloaddition reactions using epoxide precursors to produce cyclic carbonates. Ma et al. recently reported a MOF heterogeneous catalyst represented as IL@MIL-101−SO3H by incorporating ionic liquid within MIL-101−SO3H. The catalyst was found to be very

195 efficient for the cycloaddition reaction of CO2 with epichlorohydrin.

There are many reports on the application of MOFs supported ionic liquids for the chemical

196-199 fixation of CO2. However, there are limited number of reports on the application of anion exchanged MOF as a heterogeneous catalyst. In our research, we have synthesized a new MOF denoted as UNM-6 which undergo anion exchange and can be used as a heterogeneous catalyst for the chemical fixation of CO2. In this Chapter, synthesis, characterization, post-synthetic ionic exchange, catalytic and gas adsorption application of highly symmetrical, cubic, 4-fold interpenetrated UNM-6 MOF is presented.

4.2 Results and Discussion

4.2.1 Synthesis and Structural Analysis of UNM-6. UNM-6 is violet colored block shaped-crystal formed by solvothermal synthesis. In a typical synthesis, solution of T2 in dichloromethane and methanolic solution of Co(NO3)2 . 6H2O were mixed and heated at

100°C for 48 h. The approximate composition of UNM-6 is C132H48B3Co4F48N16O12.

UNM-6 is crystallized in cubic crystal system with a space group of P -4 3n.

Crystallographic data and crystal structures are presented in Table 4.1 and Figure 4.1.

Table 4.1. Crystallographic data for UNM-6.

Chemical formula C132H48B3Co4F48N16O12 Formula weight 3230.01 g/mol Temperature 100(2) K Wavelength 1.54178 Å Crystal size 0.079 x 0.094 x 0.110 mm Crystal system cubic Space group P -4 3 n Unit cell dimensions a = 18.5448(8) Å Å, α = 90° b = 18.5448(8) Å, β = 90° c = 18.5448(8) Å, γ = 90° Volume 6377.7(8) Å3 Z 2 Density (calculated) 1.682 g/cm3 Absorption coefficient 5.267 mm-1 F(000) 3206 Theta range for data collection 3.37 to 69.84° Index ranges -21<=h<=17, -18<=k<=22, - 21<=l<=21 Reflections collected 18566 Independent reflections 2026 [R(int) = 0.1034] Coverage of independent 100.0% reflections Absorption correction Multi-Scan Max. and min. transmission 0.6810 and 0.5950 2 2 2 Function minimized Σ w(Fo - Fc ) Data / restraints/ parameters 2026 / 3 / 169 Goodness-of-fit on F2 1.047 R1 (I>2σ(I), wR2 (1599 data) 0.0590, 0.1538 R1, wR2 (all data) 0.0748, 0.1697

The crystal structure of UNM-6 consist of a cobalt cation coordinated to three pyridyl group from different borate ligand and a nitrate ion. The dihedral angle around the cobalt atom ranges from ca. 26° to 145°. The coordination environment around boron is

72 tetrahedral with C-B-C angles of ca. 100° to 114°. The separation distance between the core boron atoms of two ligands bonded to same cobalt atom is ca. 17.34 Å and boron- cobalt is ca. 10.75 Å which indicates that the ligand arms are little bit extended. All three

Co-N bond lengths are ca. 2.004 Å and Co-NO3 length averages at ca. 1.91 Å. UNM-6 is

4-fold interpenetrated as shown in Figure 4.1E. UNM-6 is porous 3D framework with pore opening of 3.64 Å indicating its microporous structure.

(each color represents 1-fold interpenetration). Color Scheme: C (grey); N (blue); B

(yellow); F (green); O (red) and Co (cyan). Hydrogen atoms are omitted for clarity.

4.2.2 Anion Exchange in UNM-6

UNM-6 contains coordinated nitrate ion in its crystal structure. It is weakly bounded to the

Co (II) cation and can be replaced by the anions of similar sizes. During the anion exchange reaction, crystalline sample of UNM-6 MOF is suspended in different aqueous solutions containing excess of NaCl, Bu4NF, Bu4NBr, KI, NaCN and NaN3. The ion exchange occurs with change in color from violet to blue or purple for different anions exchanged (shown in Figure 4.2). Exchange reaction with NaCl, KI and NaN3 produced dark blue colored powder denoted as UNM-6-Cl, UNM-6-I and UNM-6- N3 which appears less crystalline than the original MOF. Similarly exchange with Bu4NF, Bu4NFBr also produced dark blue colored MOFs powder (UNM-6-F and UNM-6-Br) but appears crystalline as UNM-6.

With NaCN, crystalline purple colored powder (UNM-6-CN) is obtained. Furthermore, the anion exchange is confirmed by solid FTIR analysis. All the exchanged samples are stable as demonstrated by the PXRD analysis.

Figure 4.2. Pictures of UNM-6 before and after exchange with halides, cyanide and azide.

4.2.2.1 FTIR analysis

Prior to FTIR analysis, all the exchanged samples are filtered, washed with water several times and dried under high vacuum for 24 h. The IR spectrum of all the anion-exchanged

¯ -1 MOF samples lacked characteristic band of NO3 at 1384 cm . In case of N3UNM-6, a band in between 2000-2100 cm-1 is observed which is a characteristic band for azide. All

¯ these results indicate that the trigonal planar NO3 anion in UNM-6 can be replaced by

¯ ¯ spherical halide anions and linear anions such as CN and N3 .

N-O stretch disappeared N3 UNM-6-N3

UNM-6-CN

UNM-6-I

UNM-6-Br

% T UNM-6-Cl

UNM-6-F

UNM-6

500 750 1000 1250 1500 1750 2000 2250 -1 Wavenumber (Cm )

Figure 4.3. FTIR of UNM-6 before and after PSIE.

4.4.2.2 PXRD analysis

The samples were dried under high vacuum for 24 h at room temperature before every analysis. Powder X-ray Diffraction (PXRD) analysis of UNM-6 and exchanged MOF samples showed that they are stable before and after exchange. The PXRD pattern of as synthesized UNM-6 and all the exchanged samples closely matches that of the simulated pattern from single crystal X-ray data. However, PXRD spectrum of UNM-6-I lacked some of the peaks between 2θ value of 5 and 13 but majority of the peaks are retained. The exceptional stability of UNM-6 in different ionic solution is acquired from its 4-fold interpenetrated structure which holds each layer together and prevents any disintegration during the exchange reaction.

UNM-6-N 3

UNM-6-CN

UNM-6-I

UNM-6-Br

UNM-6-Cl Intensity UNM-6-F

UNM-6

Simulated

5 10 15 20 25 30 35 40 45 50 2 theta (degree)

Figure 4.4. PXRD of UNM-6 before and after PSIE.

4.4.3 Chemical Fixation of CO2 by UNM-6 and UNM-6-Br

As CO2 is one of the major greenhouse gases, it’s captures and utilization is always a major

200-201 concern of scientific community. CO2 is most abundant, non-expensive and non-toxic gas which is a major renewable source of one carbon (C1).202 It can be converted into valuable organic compounds such as five membered cyclic carbonates via CO2 cycloaddition reaction to epoxides. This reaction is very promising environmental and

100% atom economic reaction because it is carried out in a solvent less condition and occurs without any byproduct formation.203-205 In a typical reaction mechanism, an acid catalyst which may be either metal ion or proton gets attached to the epoxide followed by the nucleophilic attack by the co-catalyst (tetraalkylammonium halide) resulting a halo- 77 alkoxide intermediate. This intermediate then reacts with CO2 in a cycloaddition reaction to form a cyclic carbonate thereby releasing co-catalyst.206-207

Both UNM-6 and UNM-6-Br contains acidic active catalytic metal site and nucleophiles such as NO3¯ and Br¯, they offer potential heterogeneous catalysts for the chemical fixation of CO2. Based on the mechanism proposed in reported literature, we assume that the reaction starts with a ring activation of epoxide by Lewis acidic metal Co(II) center via coordination with the oxygen atom of epoxide. Then the anion either NO3¯ or Br¯ attacks the less hindered carbon atom of the ring forming an intermediate which reacts with CO2.

The latter step involves the cyclization reaction resulting cyclic carbonate and catalyst release. 195

To test the effectiveness of UNM-6 and UNM-6-Br as heterogeneous catalysts, cycloaddition of CO2 with epichlorohydrin in carried out. In a typical reaction, few milligrams of MOF crystals were suspended in epichlorohydrin in a Schlenk flask and CO2 gas under 1 atm is passed into the vessel. The reaction was stirred at designated temperature for several hours to get the high conversion percentage. After that, the reaction mixture is filtered, and the residue obtained was washed with methanol several times. The filtrate was subjected to NMR analysis and product yield was calculated by comparing the ratio of carbonate to epoxide. From the preliminary result, we observed significant conversion of about 86 % and 92% into the product with UNM-6 and UNM-6-Br respectively. This sort of catalytic activity of UNM-6 and UNM-6-Br is conferred to its ionic framework structure which enhanced CO2 adsorption during the catalysis reaction and facilitated the cycloaddition of CO2. Furthermore, both catalysts were found to be stable after multiple catalytic cycle as confirmed by PXRD analysis.

Scheme 4.1. Application of UNM-6 and UNM-6-Br as heterogeneous catalysts.

Scheme 4.2. A reaction mechanism for the CO2 insertion into epoxide catalyzed by an acid in the presence of a tetraalkylammonium halide co-catalyst. 207-208

4.4.3.1 Evaluation of catalytic performance of UNM-6 and UNM-6-Br

Epichlorohydrin was used to perform the cycloaddition of CO2 under 1 atm pressure. Four different experiments were performed without any catalyst (control), with Co(NO3)2 .

6H2O, UNM-6, UNM-6-Br under CO2 gas and one additional control experiment was done without catalyst under N2 environment to compare the yield of chloropropene carbonate.

Control experiment with epichlorohydrin and CO2 gas under 1 atm pressure under optimized reaction condition produced trace amount of product. With Co(NO3)2 . 6H2O as a catalyst, reaction did not happen. Control experiment under N2 environment without catalyst did not form any product. However, when reactions were performed in presence of UNM-6 or UNM-6-Br, significant amount of product was obtained. In each experiment, the progress of reaction was monitored by 1H NMR. The relative conversion of product was calculated by comparing the ratio of epichlorohydrin to chloropropene carbonate.

Furthermore, the conditions for the chemical fixation of CO2 were optimized by varying the catalyst amount and reaction time under 1 atm CO2 pressure with epichlorohydrin as a reactant.

Table 4.2. Catalytic performance evaluation of different catalyst in the CO2 cycloaddition reaction of epichlorohydrin.

Entry Catalyst Relative Conversion (%) 1 None 8.25 2 None No reaction 3 Co(NO3)2 . 6H2O No reaction 4 UNM-6 86.6 5 UNM-6-Br 92.5 Reaction Condition: Epichlorohydrin (6.38 mmol), catalyst (30 mg), reaction time (72 h), reaction temperature (115°C) and CO2 pressure (1 atm). Entry 2 (N2, 1 atm).

Table 4.3. Comparison of relative conversion of chloropropene carbonate obtained from the CO2 cycloaddition to epichlorohydrin at different temperature.

Entry Temperature(°C) Catalyst Relative Conversion (%) 1 65 UNM-6 34 UNM-6-Br 40 2 95 UNM-6 40.5 UNM-6-Br 65 3 115 UNM-6 86.6 UNM-6-Br 92.5

Reaction Condition for Table 4.3: Epichlorohydrin (6.38 mmol), catalyst (30 mg), reaction time (72 h) and CO2 pressure (1 atm).

Table 4.4. Comparison of conversion of chloropropene carbonate at 10 mg catalyst loading.

Entry Time (h) Catalyst Relative Conversion (%) 1 24 UNM-6 1.5 UNM-6-Br 2 2 48 UNM-6 9.4 UNM-6-Br 10 3 72 UNM-6 13 UNM-6-Br 14.5

Table 4.5. Comparison of conversion of chloropropene carbonate at 30 mg catalyst loading.

Entry Time (h) Catalyst Relative Conversion (%) 1 24 UNM-6 6.5 UNM-6-Br 19 2 48 UNM-6 31.5 UNM-6-Br 89.5 3 72 UNM-6 86.6 UNM-6-Br 92.5

Table 4.6. Comparison of conversion of chloropropene carbonate at 50 mg catalyst loading.

Entry Time (h) Catalyst Relative Conversion (%) 1 24 UNM-6 2.5 UNM-6-Br 22.6 2 48 UNM-6 29 UNM-6-Br 82 3 72 UNM-6 59 UNM-6-Br 94.7 Reaction Condition for Tables 4.4-4.6: Epichlorohydrin (6.38 mmol), reaction temperature (115°C) and CO2 pressure (1 atm).

4.4.3.2 Evaluation of recyclability of UNM-6 and UNM-6-Br

Recyclability of recovered UNM-6 and UNM-6-Br after the 1st catalytic run was tested for at least two times. PXRD of both UNM-6 and UNM-6-Br after 1st catalytic cycle showed no significant difference indicating the stability of both catalysts. However, after 3rd catalytic run, some of the PXRD peaks disappeared but the samples are still crystalline as most of the major peaks were retained. These results indicate the stability and recyclability of UNM-6 and UNM-6-Br as heterogeneous catalysts.

Table 4.7. Investigation of recyclability of catalysts in the cycloaddition of CO2 to epichlorohydrin under optimized conditions.

Entry Catalytic cycle Catalyst Relative Conversion (%) 1 1st UNM-6 86.6 UNM-6-Br 92.5 2 2nd UNM-6 60 UNM-6-Br 77 3 3rd UNM-6 39 UNM-6-Br 50 Reaction Condition: Epichlorohydrin (6.38 mmol), catalyst (30 mg), reaction time (72 h), reaction temperature (115°C) and CO2 pressure (1 atm).

UNM-6 (after 3rd catalytic run)

UNM-6 (after 1st catalytic run)

Intensity UNM-6 (As synthesized)

UNM-6 (Simulated)

5 10 15 20 25 30 35 40 45 50 2 theta (degree)

Figure 4.5. PXRD of UNM-6 catalyst before and after catalytic runs.

UNM-6-Br (after 3rd catalytic run)

UNM-6-Br (after 1st catalytic run) Intensity

UNM-6-Br (As synthesized)

5 10 15 20 25 30 35 40 45 50 2 theta (degree)

Figure 4.6. PXRD of UNM-6-Br catalyst before and after catalytic runs.

4.4.4 Gas Adsorption Analysis of UNM-6

4.4.4.1 Surface area and pore size determination of UNM-6

The surface area of UNM-6 is determined by multi-point Brunauer-Emmett-Teller (BET) and Langmuir method through N2 adsorption measurements at 77 K. For pore size distribution, N2 adsorption isotherm at 77 K is fitted by non-local density functional theory

(NLDFT). Type-I adsorption behavior is obtained at 77 K which confirms the microporous

2 nature of UNM-6. The average BET surface area (SABET) of UNM-6 is ca. 178 m /g which is calculated from the linear fit of isotherm at 77 K between partial pressures (P/P0) of 0.05 and 0.30. As Type-I shape of N2 adsorption isotherm is the characteristic of microporous materials, we assumed the absence of meso- and macro-pores in UNM-6 MOF structure.

Furthermore, the fitting of isotherm with Langmuir method obtained the average SALangmuir of ca. 267 m2/g. From NLDFT fitting, the pore size of UNM-1 is found to be ca. 9.6 Å, which indicates that UNM-6 MOF is microporous.

8 (N -77 K) Ads 2 2 S = 178 m /g 70 BET 1 / [ W((P0/P) - 1) ] (N -77 K) Des 2 7 2 Linear fit R = 0.994

65 6

5 60 4

55 3 Volume (cc/g)

1 / [ W((P0/P)[ 1)-] / 1 2 50 1

45 0 0.0 0.2 0.4 0.6 0.8 1.0 0.05 0.10 0.15 0.20 0.25 0.30 0.35 Pressure (bar) Relative Pressure (P/P0)

5.0 0.024 2 S = 267 m /g dV(r) Lang P/P0/W 77 K N 4.5 2 Linear fit 0.021 2 R = 0.999 Half pore width: 4.8 Å 4.0 0.018

3.5 0.015

3.0 0.012

2.5 0.009 P/P0/W

2.0 dV(r) (cc/ Å/ g) 0.006 1.5 0.003 1.0 0.000 0.5 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0 20 40 60 80 100 120 Relative Pressure (P/P0) Pore Width (Å)

Figure 4.7. (A) N2 adsorption/desorption isotherm at 77 K; (B) 7-point BET plot; (C) 7- point Langmuir plot and (D) NLDFT pore size distribution of UNM-6.

4.4.4.2 CO2/N2 adsorption selectivity calculation (IAST method)

IAST method is used for the adsorption selectivity calculation of UNM-6. The N2 and CO2 adsorption/desorption isotherms of UNM-6 is measured at 273 K, 298 K, 303 K, 313 K and 323 K which are shown in Figure 4.11. Usually, these temperatures are used in real- world applications including CO2 capture from industrial flue gases. From the plot, it can be clearly observed that the adsorption of CO2 by UNM-6 is higher compared to that of N2 at all temperatures applied, up to ca. 29.6 cc/g CO2 at 273 K and 1 bar.

The adsorption selectivity of UNM-6 between CO2 and N2 is calculated based on the ideal adsorbed solution theory (IAST)171-172 by using the pyIAST code assuming a flue gas like mixture containing 15% CO2 and 85% N2 at 273 K, 298 K and 313 K. Detailed isotherm fitting parameters and ideal selectivity at various pressures and temperatures are given below. For the temperatures, 273 K and 298 K under the pressure range of 0-1, the ideal

CO2/N2 selectivities are 37 and 111. However, at 313 K the selectivities increased with increasing pressure and reaches a value of ca. 1022 at 1 bar suggesting the potential application of UNM-6 for effective CO2 capture at flue gas conditions in near future.

(273 K-CO2) Ads (273 K-CO2) Des (298 K-CO2) Ads 1.4 (298 K-CO2) Des (303 K-CO2) Ads 1.2 (303 K-CO2) Des (313 K-CO2) Ads (313 K-CO2) Des 1.0 (323 K-CO2) Ads (313 K-CO2) Des 0.8 (273 K-N2) Ads (273 K-N2) Des 0.6 (298 K-N2) Ads

(298 K-N2) Des n (mmol/g) n (303 K-N ) Ads 0.4 2 (303 K-N2) Des (313 K-N2) Ads 0.2 (313 K-N2) Des (323 K-N2) Ads 0.0 (323 K-N2) Des

0.0 0.2 0.4 0.6 0.8 1.0 1.2 Pressure (bar)

Figure 4.8. CO2 and N2 adsorption/ desorption isotherm of UNM-6 at 273 K, 298 K, 303

K, 313 K and 323 K.

273 K

1.4 T=273 K BET fit of CO 2 1.2 Langmuir fit of N2

1.0

0.8

0.6 n(mmol/g) 0.4

0.2

0.0

0.0 0.2 0.4 0.6 0.8 1.0 Pressure (bar)

Figure 4.9. Fitting of CO2 and N2 adsorption isotherm at 273 K. Dots represents experimental data, lines are fitting curves.

Table 4.8. N2 (273 K) - Langmuir Parameters.

M (mmol/g) K (bar-1) R2 0.358994 0.379200 0.999

퐾푃 푛(푃) = 푀 (퐸푞푛. 4.1) 1 + 퐾푃

Where n is the amount adsorbed (mmol/g), P is the pressure (bar-1) M is the saturation loading and K is a constant.

Table 4.9. CO2 (273 K) - BET Parameters.

-1 -2 2 M (mmol/g) KA (bar ) KB (bar ) R 1.066835 5.946062 0.280896 0.998

퐾 푃 푛(푃) = 푀 퐴 (퐸푞푛. 4.2) (1 − 퐾퐵푃)(1 − 퐾퐵푃 + 퐾퐴푃) where n is the amount adsorbed (mmol/g), P is the pressure (bar), M is the saturation loading, KA is the Langmuir constant for the first layer of adsorbate molecules, and KB is the constant for the second and higher layers of adsorbate molecules.

Table 4.10. IAST adsorption selectivity calculated for mixed gas (15% CO2: 85% N2) at

273 K.

IAST CO molar IAST N molar Total Pressure 2 2 Selectivity fraction fraction 0.05 0.89642 0.10358 49.0414494 0.1 0.900971 0.099029 51.5556285 0.15 0.905255 0.094745 54.1429979 0.2 0.909291 0.090709 56.804165 0.25 0.913099 0.086901 59.5416355 0.3 0.916695 0.083305 62.3564612 0.35 0.920094 0.079906 65.2499937 0.4 0.92331 0.07669 68.2238884 0.45 0.926356 0.073644 71.2800862 0.5 0.929244 0.070756 74.42077 0.55 0.931984 0.068016 77.6470634 0.6 0.934585 0.065415 80.9597442 0.65 0.937057 0.062943 84.3618777 0.7 0.939407 0.060593 87.8534869 0.75 0.941644 0.058356 91.4384582 0.8 0.943774 0.056226 95.1170751 0.85 0.945804 0.054196 98.8920954 0.9 0.947739 0.052261 102.763456 0.95 0.949586 0.050414 106.735973 1 0.951348 0.048652 110.806791

298 K

T=298 K BET fit of CO 0.9 2 Quadratic fit of N 2 0.8

0.7

0.6

0.5

0.4

n (mmol/g) 0.3

0.2

0.1

0.0

0.0 0.2 0.4 0.6 0.8 1.0 Pressure (bar)

Figure 4.10. Fitting of CO2 and N2 adsorption isotherm at 298 K. Dots represents experimental data, lines are fitting curves.

Table 4.11. N2 (298 K) - Quadratic Parameter.

-1 -2 2 M (mmol/g) KA (bar ) KB (bar ) R 0.150930 0.478034 0.240235 0.999

(퐾 + 2퐾 푃)푃 ( ) 퐴 퐵 ( ) 푛 푃 = 푀 2 퐸푞. 4.3 1 + 퐾퐴푃 + 퐾퐵푃

Where, n is the amount adsorbed (mmol/g), P is the pressure (bar) M is the saturation

1 -2 loading and KA (bar ) and KB (bar ) are constants.

Table 4.12. CO2 (298 K) - BET Parameters.

-1 -2 2 M (mmol/g) KA (bar ) KB (bar ) R 0.714323 4.070726 0.311615 0.999

Table 4.13. IAST adsorption selectivity calculated for mixed gas (15% CO2: 85% N2) at

298 K.

IAST CO molar IAST N molar Total Pressure 2 2 Selectivity fraction fraction 0.05 0.867686 0.132314 37.160749 0.1 0.861825 0.138175 35.344129 0.15 0.857898 0.142102 34.210792 0.2 0.855286 0.144714 33.491028 0.25 0.853626 0.146374 33.046948 0.3 0.8526809 0.1473191 32.798588 0.35 0.852287 0.147713 32.696014 0.4 0.852327 0.147673 32.706405 0.45 0.8527102 0.1472898 32.80624 0.5 0.85337 0.14663 32.979356 0.55 0.854253 0.145747 33.213493 0.6 0.855316 0.144684 33.499148 0.65 0.856525 0.143475 33.82918 0.7 0.857852 0.142148 34.197888 0.75 0.859275 0.140725 34.600995 0.8 0.860775 0.139225 35.034836 0.85 0.862335 0.137665 36.3896059 0.9 0.8639481 0.136056 35.983021 0.95 0.865589 0.134411 36.492581 1 0.867262 0.132738 37.023947

313 K

0.7 T=313 K Quadratic fit of CO 2 0.6 Quadratic fit of N2

0.5

0.4

0.3 n (mmol/g) 0.2

0.1

0.0

0.0 0.2 0.4 0.6 0.8 1.0 Pressure (bar)

Figure 4.11. Fitting of CO2 and N2 adsorption isotherm at 313 K. Dots represents experimental data, lines are fitting curves.

Table 4.14. N2 (313 K) - Quadratic Parameter.

-1 -2 2 M (mmol/g) KA (bar ) KB (bar ) R 0.029628 0.36718 0.026054 0.998

Table 4.15. CO2 (313 K) - Quadratic Parameter.

-1 -2 2 M (mmol/g) KA (bar ) KB (bar ) R 0.761215 1.826306 0.0412119 0.997

Table 4.16. IAST adsorption selectivity calculated for mixed gas (15% CO2: 85% N2) at

313 K.

IAST CO molar IAST N molar Total Pressure 2 2 Selectivity fraction fraction 0.05 0.987491 0.012509 447.3405 0.1 0.984869 0.015131 368.84042 0.15 0.98367 0.01632 341.55208 0.2 0.983276 0.016724 333.16774 0.25 0.983366 0.016634 335.00104 0.3 0.983782 0.016218 343.73934 0.35 0.984414 0.015586 357.90748 0.4 0.985184 0.014816 376.80274 0.45 0.986037 0.013963 400.1678 0.5 0.986931 0.013069 427.92937 0.55 0.987837 0.012163 460.22716 0.6 0.988733 0.011267 497.27703 0.65 0.989605 0.010395 539.46721 0.7 0.990441 0.009559 587.1429 0.75 0.991235 0.008765 640.84408 0.8 0.991983 0.00817 688.0339 0.85 0.992681 0.007319 768.57389 0.9 0.993331 0.006669 844.03594 0.95 0.993933 0.006067 928.34795 1 0.994487 0.005513 1022.2068

1200 273 K 298 K 313 K, 1022 1000 313 K

800

600

Selectivity 400

200 273 K, 111 298 K, 37 0 0.0 0.2 0.4 0.6 0.8 1.0 Pressure (bar)

Figure 4.12. Ideal CO2/N2 selectivities of UNM-6 at different temperatures and pressures.

4.4.4.3 Calculation of isosteric heat of gas adsorption

Isosteric heat of CO2 adsorption of UNM-6 is calculated by applying Clausius-Clapeyron equation. The isosteric heat of CO2 adsorption (QSt) on UNM-6 is found to be 30.80 ± 1.36 kJ/mol at 0.3 mmol/g and 27.79 ± 1.13 kJ/mol at 0.6 mmol/g using Clausius-Clapeyron

equation. Qst values for different amount CO2 adsorption are given below. All these values indicate the pure physical adsorption of CO2 by UNM-6.

6.0 6.0 n = 0.35 mmol/g n = 0.3 mmol/g ln P ln P 5.8 5.8 Q = 30.18 1.28 KJ/mol Linear fit of ln P Qst= 30.80 1.36 KJ/mol Linear fit of ln P st 5.6 5.6 5.4 5.4 5.2 5.2 5.0 5.0 4.8 4.8

4.6 4.6 ln P (torr) P ln 4.4 (torr) P ln 4.4 Equation y = a + b*x Weight No Weighting Equation y = a + b*x Residual Sum 0.00268 4.2 Weight No Weighting 4.2 of Squares Residual Sum 0.00303 Pearson's r -0.9991 of Squares 4.0 4.0 Adj. R-Square 0.99641 -0.99903 Pearson's r Value Standard Error Adj. R-Square 0.99611 ln P Intercept 17.23626 0.52518 3.8 Value Standard Error 3.8 ln P Slope -3630.3481 154.00826 ln P Intercept 17.23486 0.55771 3.6 ln P Slope -3704.94075 163.54745 3.6

0.0032 0.0033 0.0034 0.0035 0.0036 0.0037 0.0032 0.0033 0.0034 0.0035 0.0036 0.0037 -1 1/T (K ) 1/T (K-1)

6.2 n = 0.4 mmol/g ln P 6.4 n = 0.45 mmol/g ln P 6.0 Qst= 29.64 1.18 KJ/mol Linear fit of ln P 6.2 Qst= 29.14 1.13 KJ/mol Linear fit of ln P 5.8 6.0 5.6 5.8 5.4 5.6 5.2 5.4 5.0 5.2 ln P (torr) P ln 4.8 Equation y = a + b*x (torr) P ln 5.0 Equation y = a + b*x Weight No Weighting Weight No Weighting 4.6 Residual Sum 0.00229 0.00211 of Squares 4.8 Residual Sum of Squares Pearson's r -0.9992 4.4 Pearson's r -0.99924 Adj. R-Square 0.99681 4.6 Adj. R-Square 0.99698 Value Standard Error Value Standard Error ln P Intercept 17.23645 0.48562 4.2 ln P Intercept 17.23739 0.46523 ln P Slope -3564.67709 142.40685 4.4 ln P Slope -3505.92469 136.42956 4.0 4.2 0.0032 0.0033 0.0034 0.0035 0.0036 0.0037 0.0032 0.0033 0.0034 0.0035 0.0036 0.0037 -1 1/T (K ) 1/T (K-1)

6.6 6.8 n = 0.5 mmol/g ln P n = 0.55 mmol/g ln P 6.4 6.6 Qst= 28.66 1.09 KJ/mol Linear fit of ln P Qst= 28.09 1.0 KJ/mol Linear fit of ln P 6.2 6.4 6.0 6.2

5.8 6.0

5.6 5.8

5.4 5.6 ln P (torr) P ln

5.2 ln(torr) P y = a + b*x 5.4 Equation Equation y = a + b*x 5.0 Weight No Weighting Weight No Weighting Residual Sum 0.00196 5.2 Residual Sum 0.00162 of Squares of Squares 4.8 Pearson's r -0.99927 Pearson's r -0.99937 Adj. R-Square 0.99708 5.0 Adj. R-Square 0.99749 Value Standard Error Value Standard Error 4.6 ln P Intercept 17.21924 0.44923 4.8 ln P Intercept 17.1504 0.40812 ln P Slope -3447.02391 131.73545 ln P Slope -3378.20954 119.68148 4.4 4.6 0.0032 0.0033 0.0034 0.0035 0.0036 0.0037 0.0032 0.0033 0.0034 0.0035 0.0036 0.0037 -1 -1 1/T (K ) 1/T (K )

7.0 32 n = 0.6 mmol/g ln P Q 6.8 st Qst= 27.79 1.13 KJ/mol Linear fit of ln P 31 6.6 6.4 30

6.2 29 6.0

5.8 (KJ/mol) 28

st ln P (torr) P ln

5.6 Q Equation y = a + b*x 27 5.4 Weight No Weighting Residual Sum 0.0021 of Squares 5.2 Pearson's r -0.99917 Adj. R-Square 0.99668 26 Value Standard Error 5.0 ln P Intercept 17.18875 0.4651 ln P Slope -3342.88516 136.39043 4.8 25 0.0032 0.0033 0.0034 0.0035 0.0036 0.0037 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 1/T (K-1) n (mmol/g)

Figure 4.13. The Van’t Hoff isochores for CO2 adsorption on UNM-6 (first seven plots) and summary of Qst at different loadings calculated by Clausius-Clapeyron method (last plot).

4.4 Conclusion

A new charge-separated MOF (UNM-6) is synthesized by coordinating Co(II) to ionic borate ligand T2 through solvothermal crystallization method. The obtained MOF; UNM-

6 is 4-fold interpenetrated, cubic crystal system containing weekly coordinated nitrate ion in its crystal structure. At room temperature, this MOF can undergo post-synthetic ionic exchange with halides, cyanide and azide to replace nitrate ion as confirmed by color change and FTIR analysis. PXRD analysis of all the exchanged samples revealed that they exhibit same XRD pattern and same structure as UNM-6. Furthermore, the catalytic performance of the UNM-6 and bromide exchanged sample UNM-6-Br for the cycloaddition of CO2 with epichlorohydrin was investigated. Compared to UNM-6, a significant increase in the yield of chloropropene carbonate was obtained with UNM-6-Br as a heterogeneous catalyst. Both catalysts were easily separated from the product, very efficient and stable, with ability to be reused multiple times. Additionally, UNM-6 showed

95 promising CO2/N2 separation capacity at 313 K with increased selectivity and reasonable value of isosteric heat of adsorption.

4.5 Experimental Procedure

Materials and Methods

Solvents and chemicals were purchased from Sigma-Aldrich or VWR unless otherwise noted. Single crystal X-ray intensity data were measured on a Bruker Kappa APEXII Duo system equipped with Incoatec Microfocus IμS (Cu Kα, λ = 1.54178 Å) and a multi-layer mirror monochromator. Rigaku Smart Lab Diffractometer was used to record powder XRD in Bragg-Brentano mode using Cu K-α radiation (λ = 1.54 Å) at room temperature. Solid

FTIR spectra were recorded using a Shimadzu Fourier Transform Infrared

Spectrophotometer in transmission mode at room temperature. The dry KBr pellets were used to prepare samples for IR measurements. Adsorption isotherms were measured on a

Quantachrome Autosorb AS1 instrument using UHP grade nitrogen and carbon-dioxide gases. About 100 mg of sample was outgassed for 25 h at 60 °C before each adsorption/ desorption analysis. For 77 K, liquid nitrogen was used as a cooling bath. Other temperatures were maintained by using water bath. To confirm the accuracy of results, all gas adsorption experiments were repeated three times and averaged to get the final data.

Synthesis of UNM-6

UNM-6. 40 mg (0.035 mmol) of T2 was dissolved in 1 mL of CH2Cl2 in a long 20 mL glass vial and 9 mL of CH3OH was layered on the top. Then a solution of 20 mg (0.07 mmol) of Co(NO3)2 . 6H2O in 1 mL of CH3OH was added on the top of the two layers. The contents were heated at 100°C for 48 h to obtain violet colored block-shaped crystals in 40 mg (36%) based on T2.

Single crystal X-ray Diffraction.

A single crystal of UNM-6 was coated with Paratone oil and mounted on a MiTeGen

Procedure for Post-Synthetic Ionic Exchange (PSIE)

PSIE between UNM-6 and tetrabutyl ammonium fluoride. UNM-6 (50 mg, 0.015 mmol) and tetrabutyl ammonium fluoride (23.53 mg, 0.09 mmol, 6 equiv. of UNM-6) were mixed in a vial containing 2 mL of water. The mixture was left at room temperature for 2 days. After that, the mixture was filtered followed by washing the residue with 5×10 mL of water. The dark blue colored solid was then collected in a vial and dried under vacuum for at least 24 h at room temperature before PXRD and FTIR analysis.

PSIE between UNM-6 and sodium chloride. UNM-6 (50 mg, 0.015 mmol) and sodium chloride (7.89 mg, 0.135 mmol, 9 equiv. of UNM-6) were mixed in a vial containing 2 mL of water. The mixture was left at room temperature for 3 days. After

97 that, the mixture was filtered followed by washing the residue with 5×10 mL of water.

The shiny crystalline blue colored solid was then collected and dried under vacuum for at least 24 h at room temperature before PXRD and FTIR analysis.

PSIE between UNM-6 and tetrabutyl ammonium bromide. UNM-6 (50 mg, 0.015 mmol) and tetrabutyl ammonium bromide (29 mg, 0.09 mmol, 6 equiv. of UNM-6) were mixed in a vial containing 2 mL of water. The mixture was left at room temperature for 3 days. After that, the mixture was filtered, and the residue obtained was washed with 5×10 mL of water. The dark blue colored solid was then collected in a vial and dried under vacuum for at least 24 h at room temperature before PXRD and

FTIR analysis.

PSIE between UNM-6 and potassium iodide. UNM-6 (50 mg, 0.015 mmol) and potassium iodide (22.41 mg, 0.135 mmol, 9 equiv. of UNM-6) were mixed in a vial containing 2 mL of water. The mixture was left at room temperature for 3 days. After filtering the mixture, the solid obtained was washed with 5×10 mL of water. The navy- blue colored powder was then collected in a vial and dried under vacuum for at least

24 h at room temperature before PXRD and FTIR analysis.

PSIE between UNM-6 and sodium cyanide. UNM-6 (50 mg, 0.015 mmol) and sodium cyanide (2.21mg, 0.135 mmol, 3 equiv. of UNM-6) were mixed in a vial containing 2 mL of water. The mixture was left at room temperature for 3 days. After filtration of the mixture, the solid obtained was washed with 7×10 mL of water. The purple colored powder was then collected in a vial and dried under vacuum for at least

24 h at room temperature before PXRD and FTIR analysis.

PSIE between UNM-6 and sodium azide. UNM-6 (50 mg, 0.015 mmol) and sodium azide (5.85 mg, 0.09 mmol, 6 equiv. of UNM-6) were mixed in a vial containing 2 mL of water. The mixture was left at room temperature for 3 days. After filtration of the mixture, the solid obtained was washed with 5×10 mL of water. The purple colored powder was then collected in a vial and dried under vacuum for at least 24 h at room temperature before PXRD and FTIR analysis.

Catalysis Reaction. Control experiment was done in a 25 mL Schlenk flask with 590.6 mg of epichlorohydrin (6.38 mmol) and CO2 gas under 1 atm pressure at 115°C for 72

nd h. Similar reaction condition was used for 2 control reaction except gas used was N2

(1 atm). In a typical experiment, in presence of catalyst, a 25 mL Schlenk flask was charged with 590.6 mg of epichlorohydrin (6.38 mmol), 30 mg of catalyst (either

Co(NO3)2, UNM-6 or UNM-6-Br) and a stir bar. CO2 gas under 1 atm pressure was passed into the flask for five minutes after which the flask was sealed very tightly and heated at designated temperature for several hours. The progress of reaction was monitored by 1H NMR for which a small amount of sample was withdrawn and diluted with deuterated chloroform and filtered. The percentage relative conversion of product was calculated by comparing the ratio of epichlorohydrin to chloropropene carbonate.

For the reaction condition optimization catalytic reactions were performed with UNM-

6 and UNM-6-Br at different temperatures (65, 95 and 115 °C), catalyst loadings (10,

30 and 50 mg) and time (24 h, 48 h and 72 h) following the similar procedure.

Recyclability Analysis. After the completion of cycloaddition reaction, both catalyst

UNM-6 and UNM-6-Br were filtered and washed with 5×5 mL methanol and dried at 60°C under vacuum for 24 h. After that, about 30 mg of UNM-6 and UNM-6-Br were placed in

99 two different 25 mL Schlenk flasks loaded with 590.6 mg of epichlorohydrin (6.38 mmol) and stir bar. CO2 gas under 1 atm pressure was passed into the flask for five minutes and the flask was sealed very tightly and heated for 72 h at 115°C. The progress of reactions was monitored in every 24 h by 1H NMR. The relative conversion% of product was calculated by comparing the ratio of epichlorohydrin to chloropropene carbonate. Similar procedure was followed for 3rd catalytic run.

Surface Area and Pore Size Determination. Surface area of UNM-6 was determined by

BET and Langmuir method. Pore size was determined by non-local density functional theory (NLDFT). For all these calculation N2 adsorption isotherm at 77 K was used. The details of BET, Langmuir and NLDFT methods are summarized in the experimental section of chapter 2.

PyIAST Method for Selectivity Calculation. The selectivity of UNM-6 for CO2/N2 separation was calculated by pyIAST model at 273 K, 298 K, and 313 K. Adsorption isotherms were fitted by the analytical models like Langmuir, quadratic and BET. The detail procedure is given in chapter 2.

Isosteric Heat of Adsorption. Isosteric heat of adsorption of UNM-6 for CO2 adsorption at 273 K, 298 K and 313 K was calculated by Clausius- Clapeyron equation. The method is explained in detail in chapter 2.

100

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