Synthesis of zeolitic imidazolate frameworks and their application in selective co₂ capture

Abstract of the Dissertation The Synthesis of Zeolitic Imidazolate Frameworks and their Application in Selective CO2 Capture Crystalline structures of a series of ZIFs were determined

VIETNAM NATIONAL UNIVERSITY, HCM CITY

UNIVERSITY OF SCIENCE

SYNTHESIS OF ZEOLITIC IMIDAZOLATE FRAMEWORKS AND THEIR APPLICATION IN

SELECTIVE CO2 CAPTURE

Major: THEORETICAL AND PHYSICAL CHEMISTRY

Specialization: MOLECULAR AND NANO ARCHITECTURE Code: 62440119

Reviewer 1: Prof Phan Thanh Son Nam

Reviewer 2: Dr Pham Cao Thanh Tung

Reviewer 3: Assoc Prof Tran Dai Lam

Blind Reviewer 1: Dr Pham Cao Thanh Tung

Blind Reviewer 2: Dr Truong Vu Thanh

ASSURANCE

I swear that this dissertation is my research under the scientific guidance of Assoc Prof Nguyen Thi Phuong Thoa and Dr Hiroyasu Furukawa The results in this work are honest and reliable If any fraud is detected, I am responsible for the contents of my dissertation

TPHCM, October 26, 2016

Nguyen Thi Tuyet Nhung

TABLE OF CONTENTS

LIST OF TABLES vi

LIST OF FIGURES vii

GLOSSARY OF TERMS AND ABBREVIATIONS xii

ACKNOWLEDGEMENTS xv

Abstract of the Dissertation xvi

Chapter One 1

Introduction: Literature review 1

1.1 History of Porous Materials 1

1.2 Zeolitic Imidazolate Frameworks (ZIFs) 7

1.3 Carbon-dioxide Capture 19

1.4 Scope of this dissertation 24

Chapter Two 26

Experimental Section 26

2.1 Analytical techniques 26

2.1.1 Powder X-ray diffraction 26

2.1.2 Single crystal X-ray diffraction 26

2.1.3 Thermal gravimetric analysis 27

2.1.4 Proton nuclear magnetic resonance 28

2.1.5 Gas adsorption analysis 28

2.1.6 Breakthrough experiments 29

2.1.7 Scanning electron microscope images 30

2.1.8 Elemental analysis 30

2.1.9 Inductively coupled plasma optical emission spectrometry (ICP-OES) 30

2.1.10 Fourier transform infrared spectroscopy 30

2.2 Chemicals used in this work 31

2.3 Synthesis procedures 32

2.3.1 Synthesis of ZIF-300, -301, and -302 based on mixed linkers 32

2.3.2 Synthesis of ZIF-202, -203, and -204 based on copper imidazole complex 33

2.3.3 Synthesis of multivariate ZIFs 35

Chapter Three 39

Design, Synthesis, Characterization, and Gas Adsorption of ZIF-300, -301, and -302 Based on Mixed Linkers 39

Chapter Four 55

Design, Synthesis, Characterization, and Gas Adsorption of ZIF-202, -203, and -204 Based on Copper Imidazole Complex 55

Chapter Five 71

Design, Synthesis, Characterizations and Gas Adsorption of Chabazite Multivariate ZIFs 71

Chapter Six 83

Dynamic Gas Separation Studies of ZIF-300, -301, and -302, and ZIF-204 83

Concluding Remarks and Outlook 93

PUBLICATIONS 96

REFERENCES AND NOTES 97

APPENDIX A 113

APPENDIX B 141

APPENDIX C 162 APPENDIX D 171

LIST OF TABLES

Table 1.1 Adsorption properties of representative solid porous materials 22 Table 3.1 Composition, surface area, crystal density, CO2 uptake, and selectivity for ZIF-300, -301, and -302 51

Table 3.2 ZIF surface area and CO2/N2 selectivity for selected ZIFs 53

Table 4.1 Composition, surface area, CO2 uptake, and selectivity for ZIF-204 70

Table 5.1 Cell parameters of MTV-ZIF-ADE, -ADF, -ADG, -ACDE, and –ADEG

in comparison with those of parent CHA-ZIFs including ZIF-AB (ZIF-300), -AC

(-301), and -AD (-302) All materials have the trigonal crystal system with space group R-3 73

Table 5.2 Ratio of linkers determined for MTV-ZIFs In each case the ratios were

normalized to a value of one for linker A 78

Table 5.3 Chemical compositions, CO2, N2 uptake at 298 K and 100 Torr, and rough estimation CO2/N2 selectivity at 298 K of the selected MTV-ZIFs in comparison with

parent CHA-ZIFs 82

Table 6.1 Average breakthrough time (n = 3) and CO2 uptake capacity under dry and wet conditions of ZIF-300, -301, and -302, and ZIF-204 87

LIST OF FIGURES

Figure 1.1 The formation of MOF-5 from the combination of octahedral inorganic

and linear organic SBUs (a) Examples of inorganic (a) and organic (b) SBUs from carboxylate MOFs 2

Figure 1.2 Illustration for pore structures of activated carbon 4 Figure 1.3 The bridging angle in ZIFs (left) and in zeolites (right) 7 Figure 1.4 Crystal structures of representative ZIFs were grouped according to their

topology (three-letter symbol) 9

Figure 1.5 Schematic illustration of ZIFs; a) from left to right, 4, 6 and 8

membered-rings are mainly constructed into cages in ZIFs; b) sod cage [46.68] (β cage) in ZIF-8

(left), lta cage [412.68.86] (α cage) in ZIF-11 (middle) and in ZIF-20 (right) are typical

example of cages; c) simplified frameworks in ZIF-8 (SOD, left), ZIF-11 (RHO,

middle), and ZIF-20 (LTA, right) which were resulted from fusing sod cages,

cross-linking of a lta and a double 8-member ring cage in a ratio 1 : 3, and cross-cross-linking of

a lta, a sod and a double 4-member ring cage in a ratio 1 : 1 : 3, respectively 10

Figure 1.6 Single crystal structure of crb-ZIF-1 (a), SOD-ZIF-8 (b), and

RHO-ZIF-11 (c) 13

Figure 1.7 Illustration of SiO2 in zeolite (a); Zn2(2-methylimidazole), mIm, in

ZIF-8 with SOD topology (b); Zn2(benzimidazole), bIm, in ZIF-11 with RHO topology

(c), and Zn2(5-chlobenzimidazole), cbIm, in ZIF-95 and ZIF-100 with previously unknown topologies (d) 13

Figure 1.8 The positions of the nitrogen atoms in the imidazolate-type linkers are

significant in determining which ZIF topology (SOD, RHO, DIA and LTA) is

produced The numbering of all linkers is the same as that indicated for benzimidazolate 14

Figure 1.9 X-ray single-crystal structure of ZIF-20 with LTA topology as zeolite A

including a α-cage (a), a β-cage (b) and a cube (c) The formation of linkers between cubes is important in the reticulation of the structure (d) 15

Figure 1.10 Structures of imidazolate links and their abbreviations Reaction of nIm

results in the SOD topology Reaction of nIm plus any other imidazole linker shown results in the GME topology, whose tiling is shown at the above 17

Figure 1.11 The presynthesized four- and three-connected boron-centered

precursors which were used to synthesize BIF and MOP materials 18

Figure 1.12 Global monthly mean CO2 globally averaged over marine surface sites 19

Figure 1.13 Reaction of CO2 with monoethanolamine (MEA) to give a carbamate product (upper), and the corresponding reaction with triethanolamine (TEA) resulting

in a bicarbonate species (lower) 21

Figure 2.1 Schematic representation of breakthrough experimental 30

Figure 3.1 Imidazolate linkers used in sysnthesis of CHA-ZIFs 40 Figure 3.2 Reaction of 2-methylimidazole (2-mImH) with Zn(NO3)2 and 5(6)-bromobenzimidazole (bbImH), 5(6)-chlorobenzimidazole (cbImH), or 5(6)-methylbenzimidazole (mbImH) resulting in ZIF-300, -301, and -302, respectively The yellow sphere represents the space in the framework 41

Figure 3.3 The asymmetric unit of ZIF-300 showing zinc surrounded by four

imidazolate links with imposed disorder 43

Figure 3.4 The asymmetric unit of ZIF-301 showing zinc surrounded by four

imidazolate links with imposed disorder 44

Figure 3.5 The asymmetric unit of ZIF-302 showing zinc surrounded by four

imidazolate links with imposed disorder 45

Figure 3.6 a) Ball-and-stick model depicting the CHA net The yellow and orange

sphere represents the space in the framework b-c) The crystal structure of CHA

ZIF-302 is comprised of two types of cages, cha (b) and hpr (c) d) These two types of cages, cha (large) and hpr (small) are cross-linked together Light red and grey

imidazolate rings indicate the specific position of 2-mIm and the corresponding bIm, respectively The green imidazolate rings indicate the nonspecific positions, in which

2-mIm or bIm can occupy e) CHA tiling, with both the cha (yellow polyhedra) and

hpr (orange polyhedra) cages, demonstrating which links occupy each of the unique

edges (same color scheme as represented in d) 46

Figure 3.7a Comparison of the simulated (blue) PXRD pattern with the experimental

as-synthesized (red) and activated [guest-free, (green)] PXRD patterns of ZIF-300 47

Figure 3.7b Comparison of the simulated (blue) PXRD pattern with the

experimental as-synthesized (red) and activated [guest-free, (green)] PXRD patterns

of ZIF-301 48

Figure 3.7c Comparison of the simulated (blue) PXRD pattern with the experimental

as-synthesized (red) and activated [guest-free, (green)] PXRD patterns of ZIF-302 48

Figure 3.8 Ar isotherms of ZIF-300 (red), ZIF-301 (green), and ZIF-302 (blue) at

87 K 50

Figure 3.9 CO2 (circles) and N2 (triangles) isotherms of ZIF-300 (red), -301 (blue), and -302 (green) at 298 K 53

Figure 4.1 Illustration of pth, pts, and ast nets resulting from the combination of

square planar and tetrahedral SBUs 56

Figure 4.2 The asymmetric unit of [Cu(HIm)4](NO3)2 including copper surrounded

by four imidazolate linkers and two NO3- ions for charge compensation 57

Figure 4.3 PXRD pattern of experimental [Cu(HIm)4](NO3)2 (red) Calculated PXRD pattern of simulation from single crystal data was also overlaid (blue) 58

Figure 4.4 TGA curve of [Cu(HIm)4](NO3)2 59

Figure 4.5 Reaction of [Cu(HIm)4](NO3)2 with Zn(CH3COO)2 in DMF/MeOH/H2O

at pH = 5.4, MeCN at pH = 8.5, and DMF/MeCN/H2O at pH = 8.5 resulted in

ZIF-202, -203, and -204, respectively The yellow sphere represents the space in the framework 60

Figure 4.6 Asymmetry unit (a), coordination environment of two metal sites in the

structure (b), single crystal structure (c) of ZIF-202 61

Figure 4.7 Asymmetry units of ZIF-203 (a) and ZIF-204 (b) 62

Figure 4.8 Single crystal structures of ZIF-203 (a) and ZIF-204 (b) The yellow

spheres represent the space in the frameworks 63

Figure 4.9 The connectivity among metal nodes in ZIF-203 (a) and ZIF-204 (b) All imidazole linkers are omitted for clarity 63

Figure 4.10 SEM image of ZIF-204 64

Figure 4.11 PXRD patterns of as-synthesized (read) and activated (green) ZIF-204 Calculated PXRD pattern (blue) from the single crystal structure is also overlaid 65

Figure 4.12 Illustration of color change from purple for as-synthesized (left) to red for activated (middle) ZIF-204 The red color of the activated sample is reversed to purple when re-immersing in DMF (right) 66

Figure 4.13 N2 isotherms of ZIF-204 at 77 K 67

Figure 4.14 CO2 (red circles) and CH4 (blue triangles) isotherms of ZIF-204 at 298 K 69

Figure 5.1 Imidazolate linkers and corresponding MTV-ZIFs formed from mixing of various linkers 72

Figure 5.2 PXRD patterns of MTVZIFADE (red), ADF (blue), ADG (green), -ACDE (purple), and -ADEG (brown) Calculated PXRD pattern of simulated CHA-ZIF (black) is also overlaid 74

Figure 5.3a 1H NMR spectrum of digested MTV-ZIF-ADE 76

Figure 5.3b 1H NMR spectrum of digested MTV-ZIF-ADF 76

Figure 5.3c 1H NMR spectrum of digested MTV-ZIF-ADG 77

Figure 5.3d 1H NMR spectrum of digested MTV-ZIF-ACDE 77

Figure 5.3e 1H NMR spectrum of digested MTV-ZIF-ADEG 78

Figure 5.4 N2 isotherms of MTV-ZIF-ADE (black), -ADF (blue), -ADG (red) at 77 K 79

Figure 5.5 CO2 isotherms of MTV-ZIF-ADE (red), -ADG (green), -ACDE (purple) in comparison with the parent CHA-ZIFs, ZIF-AB (ZIF-300, black), -AC (-301, brown red), and -AD (-302, blue) 80

Figure 5.6 N2 isotherms of MTV-ZIF-ADE (red), -ADG (green), -ACDE (purple)

in comparison with the parent CHA-ZIFs, ZIF-AB (ZIF-300, black), -AC (-301,

brown red), and -AD (-302, blue) 81

Figure 6.1 PXRD patterns of ZIF-300, -301, and -302 collected after suspension in

water at 100 C over the course of the 7 days The PXRD patterns of the

corresponding as-synthesized CHA-ZIFs is provided for reference 84

Figure 6.2 Water isotherms of ZIF-300 (blue), -301 (red), and -302 (green) at 298

K 85

Figure 6.3 A binary mixture of CO2 and dry N2 (a) or wet (80% relative humidity)

N2 (b) is flown through a fixed bed of all three CHA-ZIFs The breakthrough time is

indicated by the dashed line 86

Figure 6.4 PXRD patterns of ZIF-204 collected after suspension in water at RT over

the course of the 7 days The PXRD pattern of as-synthesized ZIF-204 (black) is also provided for reference 88

Figure 6.5 N2 isotherms at 77 K of ZIF-204 before (blue) and after suspension in water (red) at RT for 7 days 89

Figure 6.6 Water isotherms of ZIF-204 at 298 K 90 Figure 6.7 A binary mixture of CO2 and dry CH4 (a) or wet (60% relative humidity)

CH4 (b) is flown through a fixed bed of ZIF-204 The breakthrough time is indicated

by the dashed line 91

GLOSSARY OF TERMS AND ABBREVIATIONS

ESI-MS Electrospray ionization mass spectrometry

Isoreticular Sharing the same underlying network topology ICP-OES Inductively coupled plasma atomic emission

Permanent porosity Refers to the reversible inclusion and removal of

guest molecules (usually gaseous) in the pores of

a structure in the low pressure region

P0 Saturation pressure

Topology A property of figures or surfaces which is

independent of size and shape which remains unchanged by any continuous deformation, or more simply the way in which constituent parts are interrelated, arranged, or connected

I would like to thank Kyle E Cordova for his useful discussions in every work presented in this dissertation and for his support and insightful contributions

I would like to thank Dr Felipe Gándara for his critical inputs towards the crystallographic work

I also would like to thank every member of MANAR In particular, I wish to thank Lo Nu Hoang Tien and Le Bao Toan for working together on the ZIF project, Nguyen Thi Diem Huong for useful inputs in CO2 separation applications and for her kindness and friendship, and Pham Quang Hung for his computational calculations help and for his smile I thank MANAR group members in the present and the past, Assoc Prof Hoang Dung, Nguyen Thanh Binh, Nguyen Thi Kieu Phuong, Tu Ngoc Thach, Doan Le Hoang Tan, Nguyen Lac Ha, Mai Toan, Assoc Prof Nguyen Thai Hoang, Dr Le Viet Hai, Dr Nguyen Thi Le Anh, and Dr Anh Phan for their discussion, support, and help

Sincere appreciation is also extended to Prof Omar M Yaghi for his global mentoring idea, from which MANAR was born

Abstract of the Dissertation

The Synthesis of Zeolitic Imidazolate Frameworks and their

Application in Selective CO2 Capture

Crystalline structures of a series of ZIFs were determined by X-ray diffraction studies, some of which show high BET surface area (420, 680, 240 and 715 m2 g-1for ZIF-300, -301, -302, and ZIF-204, respectively) and water and chemical stability Gas adsorption measurements revealed that these ZIF materials are able to adsorb larger amount of CO2 compared to other common gases, such as N2 and CH4 For good performers [ZIF-300, Zn(2-mIm)0.86(bbIm)1.14; ZIF-301, Zn(2-mIm)0.94(cbIm)1.06; ZIF-302, Zn(2-mIm)0.67(mbIm)1.33; ZIF-204, Cu3Zn2(Im)10; where 2-mIm = 2-methylimidazolate, bbIm = 5(6)-bromobenzimidazolate, cbIm = 5(6)-chlorobenzimidazolate, mbIm = 5(6)-methylbenzimidazolate, and Im = idazolate], dynamic CO2 separation measurements were performed using artificial flue gas and biogas streams containing water The results indicate that ZIF-300 to 302

(CHA-ZIFs) are equally effective at dynamic separation of CO2 over N2 in the presence of water (80% relative humidity) as dry condition ZIF-204 also successfully plucks out CO2 from CO2/CH4 mixture in the presence of water (60% relative humidity) and the breakthrough time is nearly identical to that under the dry condition The breakthrough experiments of these ZIFs were repeated over three consecutive cycles No change in performance indicates all of these materials are able

to be regenerated by using a N2 flow at room temperature

In summary, three complementary synthetic strategies were tested to demonstrate how desired ZIF materials are prepared without losing porosity and chemical stability By introducing functionalities into the porous framework, ZIF materials are able to adsorb CO2 even under the wet condition, while adsorbed CO2

is liberated under mild regeneration condition that is beneficial to the reduction of separation costs Therefore, it is believed that the developed strategies including new ZIF materials would pave the way for the realization of inexpensive carbon capture processes

Publications list

1) Nguyen, N.T.T., Furukawa, H., Gándara, F., Nguyen, H.T., Cordova, K.E.,

Yaghi, O.M (2014), Selective Capture of Carbon Dioxide under Humid Conditions

by Hydrophobic Chabazite-Type Zeolitic Imidazolate Frameworks, Angew Chem

Int Ed., 53(40), pp 10645-10648

2) Nguyen, N.T.T., Furukawa, H., Gándara, F., Trickett, C.A., Jeong, H.M.,

Cordova, K.E., Yaghi, O.M (2015), Three-Dimensional Metal-Catecholate

Frameworks and their Ultrahigh Proton Conductivity, J Am Chem Soc., 137(49),

pp 15394-15397

3) Nguyen, N.T.T., Furukawa, H., Gándara, F., Trickett, C.A., Jeong, H.M.,

Cordova, K.E., Yaghi, O.M., 9th Vietnam National Conference of Solid Physics and

Materials Science (SPMS-2015), 08-10/11/2015, Vietnam National University - Ho Chi Minh City (VNU-HCM)

4) Nguyen, N.T.T., Furukawa, H., Gándara, F., Nguyen, H.T., Cordova, K.E.,

Yaghi, O.M., Zeolitic Imidazolate Frameworks: Hydrophobic Nature and Carbon Dioxide Capture in the Presence of Water, 4th International Conference on Metal Organic Frameworks & Open Framework, 28/09/2014-01/10/2014, Kobe, Japan

Tóm tắt Luận án

Tổng hợp vật liệu ZIF (Zeolitic Imidazolate Framework) mới

định hướng ứng dụng trong phân tách khí CO2

Nhung T T Nguyen

Bắt giữ CO2 có đóng góp đáng kể vào việc giảm thiểu phát thải CO2 trong vài thập kỉ tới; tuy nhiên kỹ thuật bắt giữ CO2 chiếm 70-80% tổng chi phí Vì vậy việc giảm thiểu chi phí này là vấn đề được quan tâm trên thế giới Mục tiêu của luận văn này là thiết kế và tổng hợp vật liệu khung kết tinh có độ xốp cao (ZIF), định hướng bắt giữ chọn lọc CO2 trong điều kiện thực tế Cụ thể là, ba phương pháp tổng hợp đã được sử dụng tạo thành mười một vật liệu ZIF mới sau đó được tiến hành đo hấp phụ

CO2: (i) sử dụng hỗn hợp hai linker, (ii) tổng hợp phức vuông phẳng đồng-imidazole như tiền chất trong phản ứng tổng hợp vật liệu ZIF và (iii) kết hợp nhiều nhóm chức khác nhau trong cùng một cấu trúc (phương pháp MTV)

Cấu trúc tinh thể của các vật liệu ZIF được xác định bằng phương pháp nhiễu

xạ tia X, trong đó có vật liệu có diện tích bề mặt riêng BET lớn (420, 680, 240 và 715

m2 g-1 đối với ZIF-300, -301, -302 và ZIF-204), có độ bền trong nước và độ bền hóa học cao Kết quả đo đường hấp phụ khí đẳng nhiệt cho thấy các vật liệu ZIF này có khả năng hấp phụ nhiều CO2 hơn so với các khí khác như N2 và CH4 Đối với các vật liệu tiềm năng như [ZIF-300, Zn(2-mIm)0.86(bbIm)1.14; ZIF-301, Zn(2-mIm)0.94(cbIm)1.06; ZIF-302, Zn(2-mIm)0.67(mbIm)1.33; ZIF-204, Cu3Zn2(Im)10; trong

đó 2-mIm = 2-methylimidazolate, bbIm = bromobenzimidazolate, cbIm = chlorobenzimidazolate, mbIm = 5(6)-methylbenzimidazolate và Im = idazolate], được tiến hành nghiên cứu khả năng tách chọn lọc khí CO2 từ dòng hỗn hợp khí mô phỏng đúng thành phần thể tích các khí từ dòng khí ống khói và khí sinh học có sự

5(6)-hiện diện của nước Kết quả cho thấy ZIF-300, -301 và -302 (CHA-ZIF) có khả năng

tách chọn lọc CO2/N2 trong điều kiện ẩm (80% độ ẩm tương đối) như trong điều kiện khô ZIF-204 cũng thể hiện thành công khả năng tách CO2/CH4 khi có sự hiện diện của nước (60% độ ẩm tương đối) và dung lượng lưu trữ khí CO2 trong điềù kiện ẩm cao hơn cả trong điều kiện khô Thí nghiệm tách chọn lọc khí của các vật liệu ZIF này được lặp lại ba lần liên tiếp Đặc biệt là các vật liệu được tái tạo dễ dàng bằng cách thổi dòng khí N2 qua vật liệu ở nhiệt độ phòng

Tóm lại, ba phương pháp tổng hợp được sử dụng đã góp phần phát triển phương pháp tổng hợp vật liệu ZIF mới với diện tích bề mặt lớn và độ bền hóa học cao Bằng cách đưa các nhóm chức vào khung sườn của vật liệu xốp, các vật liệu ZIF có khả năng hấp phụ CO2 trong điều kiện ẩm và các phân tử CO2 bị hấp phụ có thể được giải hấp dễ dàng, góp phần làm giảm chi phí tách chọn lọc CO2 Do đó, phát triển các phương pháp tổng hợp vật liệu ZIF mới có khả năng mở ra con đường khám phá quy trình bắt giữ CO2 rẻ tiền

Các công trình công bố

1) Nguyen, N.T.T., Furukawa, H., Gándara, F., Nguyen, H.T., Cordova, K.E.,

Yaghi, O.M (2014), Selective Capture of Carbon Dioxide under Humid Conditions

by Hydrophobic Chabazite-Type Zeolitic Imidazolate Frameworks, Angew Chem

Int Ed., 53(40), pp 10645-10648

2) Nguyen, N.T.T., Furukawa, H., Gándara, F., Trickett, C.A., Jeong, H.M.,

Cordova, K.E., Yaghi, O.M (2015), Three-Dimensional Metal-Catecholate

Frameworks and their Ultrahigh Proton Conductivity, J Am Chem Soc., 137(49),

pp 15394-15397

3) Nguyen, N.T.T., Furukawa, H., Gándara, F., Trickett, C.A., Jeong, H.M.,

Cordova, K.E., Yaghi, O.M., 9th Vietnam National Conference of Solid Physics and Materials Science (SPMS-2015), 08-10/11/2015, Vietnam National University - Ho Chi Minh City (VNU-HCM)

4) Nguyen, N.T.T., Furukawa, H., Gándara, F., Nguyen, H.T., Cordova, K.E.,

Yaghi, O.M., Zeolitic Imidazolate Frameworks: Hydrophobic Nature and Carbon Dioxide Capture in the Presence of Water, 4th International Conference on Metal Organic Frameworks & Open Framework, 28/09/2014-01/10/2014, Kobe, Japan

Chapter One

Literature Review

1.1 History of Porous Materials

Metal-organic frameworks (MOFs) are extended porous materials which were constructed from organic linkers and metal-oxide secondary building units (SBUs) through strong bonds (Figure 1.1).[31,84] The diversity in library of organic compounds (mainly carboxylic acid compounds) and the variety of metal SBUs have resulted in over 20000 MOF compounds reported so far.[31] Most of their structures were well elucidated by single crystal X-ray diffraction analysis (SXRD) to provide an insight into the design of new MOF materials.[84] Moreover, these materials attract much attention due to their high permanent porosity resulted from the rigid architecture The history of MOFs is relatively short compared to classical porous compounds Therefore, prior to review the history of MOFs, the author will briefly introduce conventional porous materials in relation to the advantages and drawbacks of these materials, which can emphasize the difference between MOFs and other conventional porous solids

Figure 1.1 The formation of MOF-5 from the combination of octahedral inorganic

and linear organic SBUs (a) Examples of inorganic (b) and organic (c) SBUs from carboxylate MOFs The yellow sphere represents for the empty space inside the pore Atom colors: metal-oxygen polyhedra, blue; polyhedron defined by carboxylate carbon atoms, red; C, black; O, red; all H atoms are omitted for clarity

Activated carbon[46] was known over 2000 year ago for water treatment These are mainly composed of carbon atoms, but small amount of hetero-atoms like oxygen and hydrogen are also found in the structure The interlayer spacing of hexagonal layers ranges between 0.340.35 nm and the pores are channels created within the random arrangements of disordered layers stacked unevenly creating nooks, crannies, cracks, and crevices between carbon layers (Figure 1.2) Therefore, many activated carbon materials show high porosity (500-2000 m2 g-1), while crystallinity is generally poor compared to graphite

To improve the porosity, carbonization of procedures has been studied During the carbonization, non-carbon atoms like oxygen and hydrogen were removed, leaving internal pores The numbers and sizes of pores were furthered developed by activation process where walls of adjacent pores were burnt out, resulting in highly porous materials In addition to the surface area improvement, functionalization of carbon surface is also important for their practical use It is known that the surface of activated carbon can be functionalized by oxidation reactions; however, the nature of functional groups on the surface cannot be well understood so far The pore structure and functionalities in activated carbons are not well-characterized such that materials may not be suitable for systematic studies of CO2 capture in relation to the pore size and functionality effects

Figure 1.2 Illustration for pore structures of activated carbon.[45]

Zeolites[9,106] were discovered in 1756 and synthetic zeolites were produced at the end of 1940s These materials are constructed from tetrahedral Si-O-Si units connected together by oxygen bridges and part of Si atoms can be replaced by Al3+ The presence of Al makes zeolite frameworks negatively charged and so it needs to

be compensated by counter cations inside the pore The general formula of zeolite can be described as Mn+

x/n[(AlO2)x(SiO2)y]x- ·wH2O (M = e.g Na, Ca, K) Since these cations can be exchanged easily, the relationship between cationic spices and CO2uptake capacity was studied.[108] However, functionalizing zeolites frameworks with organic functionalities (e.g -NH2 and -NO2 group)is difficult to be done due to framework composition and small pore diameters

Mesoporous silicas are another class of porous materials The first mesoporous silica materials have been reported in 1990.[117] Alkyltirimethylammonium ions were used as template such that the pore diameter of these materials (2-4 nm) was altered

by changing the length of surfactant molecules, and one of the porous members showed 900 m2 g-1 of surface areas Shortly after, MCM-41[55] was reported by Mobil The pore structure and diameter can be easily controlled by varying the surfactants (structure directing agents, SDAs) such that various mesoporous silica materials have

been reported, including MCM-48, FSM-16, SBA-15, SBA-16, KSW-2, and

MSU-1 Due to the large pore diameter, these materials are not suitable to capture CO2 (i.e weak interaction with CO2) Although the surface of mesoporous silica has been postsynthetically modified with aminopropyltrimethxysilane (and related organosilane) to introduce amine functionalities,[40] it is still a challenge to control the pore environment to capture CO2 effectively

As is shown above, classical porous solids (carbon, zeolite, and mesoporous silica) are porous and these pore environment can be tuned by postsynthetic modification and/or ion exchange However, it is fair to say that these are not suitable

to tune gas adsorption properties in a precise manner It was sought to take advantage

of the use of hybrid-materials that are composed of organic and inorganic parts The abilities for combinations of metal ions and organic ligands to form polymeric structures were first reported in 1959 for co-ordination polymer [Cu(NO3)(adiponitrile)2]n by Kinoshita et al.[51] Another example for typical coordination polymer is Zn(L)2(ClO4)2 [where L = N,N-bis(4-pyridyl)urea].[56] In these structures, metal ions are linked together by multifunctional ligands such as cyanide or pyridine to form extended frameworks In addition, organic linkers are weakly bound to the metal center, leading to the cationic framework Since counter anions occupy the pore to compensate the charge balance, these materials do not show large open space Furthermore, due to the weak interaction between metal ion and organic linker, it was not possible to remove the occluded guest molecules from the pore without damaging the framework structure

To overcome this problem, organic linker with carboxylic acid was used to form

a rigid framework structure In 1998, Yaghi et al prepared a framework structure

from Zn salt and benzenedicarboxylic acid, Zn(BDC)[63] (MOF-2, BDC = bezendicarboxylate) During the synthesis, the carboxylic terminal is deprotonated to form a strong metal-oxygen bond and it was possible to make the neutral framework More importantly, the pore structure of MOF-2 was retained even after the evacuation, which was proven by 77 K N2 isotherm measurements Indeed, this was the first

example to show the presence of permanent microporosity in MOF materials.[90] A year later, MOF-5 [Zn4O(BDC)3][64] and HKUST-1 [Cu3(BTC)2, also known as MOF-199][18] were discovered and are among the most studied MOFs so far, attributed to their high surface areas Shortly after the first report of MOF-5, 15 MOF-

5 type structures (IRMOF-2 to -16) having functionalities (e.g -Br, -NH2, -OC3H7,

-OC5H11, -C2H4, -C4H4) and/or longer organic linkers (biphenyl, terphenyl, pyrene, and tetrahydropyrene) have been synthesized These are later called isoreticular MOFs (those have the same framework topology) It should be noted that these isoreticular MOF series clearly indicate that pore size and functionalities of MOFs are systematically designed and tuned.[26]

Numbers of functionality that can be incorporated into MOF structure are not always unity In 2008, the first example of MOF based on two different organic linkers, UMCM-1 [Zn4O(BDC)(BTB)4/3, BTB = 4,4,4-benzene-1,3,5-triyl-trisbenzoate], was successfully synthesized by Matzger et al.[53] Two year later, Yaghi

et al reported MOF-210 {Zn4O(BTE)4/3(BPDC), BTE = 4,4,4tris(ethyne-2,1-diyl)]tribenzoate, BPDC = biphenyl-4,4-dicarboxylate} as the second example for mixed two type linker based MOF.[32] Both of these MOFs have ultrahigh permanent porosity, and MOF-210 also exhibited an exceptional CO2uptake capacity

Furthermore, in 2010, the idea of incorporating multivariate functionalities (MTV) in one structure was demonstrated using prototype MOF, MOF-5 In this work, up to eight different functional groups were incorporated into MOF-5 framework to form MTV-MOF-5.[23, 25] The interesting discovery of the MTV approach is that MTV-MOF showed improved CO2 uptake capacity; CO2 uptake in MTV-MOF-EHI [E, -NO2; H, -(OC3H5)2; and I, -(OC7H7)2 indicating functionalities

on MOF-5 frameworks] was greater than MOFs having single type of functionality Consequently, MTV-MOF-5-EHI showed 400% higher CO2/CO selectivity compared to that of the single-linker counterparts.[25] These findings made an impress

on synthesizing new MOFs with improved properties from mixing organic linkers Due to the exceptionally porous properties, well-defined and designable structures, MOFs may have applications in many fields including gas storage, separations and catalysis.[19, 43, 94, 101, 116, 121]

1.2 Zeolitic Imidazolate Frameworks (ZIFs)

The development of MOF chemistry implies that stable framework can be made

if one can combine metal ions and deprotonated organic linkers As expected, it was shown that imidazolate (Im) linkers that can be easily deprotonated were able to form framework structures by connecting with transition metal ions such as Zn,[47, 61] Co,[100, 102, 103] Fe,[91] and Cu [73] The first example of framework structure with imidazole linker is reported in 1975 where two Co2+ ions were bridged by imidazole linker.[98]

In 2003, Zn and benzimidazole were used to form 3D framework whose topology is identical to the sodalite type zeolite.[85] The atomistic connectivity of these materials can be easily modified by employing different types of imidazolate linkers In 2006,

12 zeolitic imidazolate framework (ZIF) structures possessing zeolite framework topologies have been reported By careful examination of these structures, it was found that the angle of 145 made by Im-M-Im within these structures is very close

to the angle of Si-O-Si in zeolites (Figure 1.3), which can be the reason why like topology is often obtained Connectivity of metal ion and imidazolate linkers in ZIF structures will be described below

zeolite-Figure 1.3 The bridging angle in ZIFs (left) and in zeolites (right)

145o

145o

There are over 100 ZIF materials with around 40 distinct topologies reported so far and crystal structures of the representative ZIFs were shown in Figure 1.4 In most

of ZIF structures, the inorganic containing SBUs are composed of single metals tetrahedrally coordinated with N atoms of imidazolate linkers These imidazolate linkers act as bridges connecting metal centers through N atoms at the position 1 and

3 on the five member ring of imidazoles As mentioned, due to Im-M-Im angle of

145, most of ZIFs possess zeolite framework topology (Figure 1.4) As representatives, the detail illustration of structures of ZIF-8, ZIF-11, and ZIF-20 with

SOD, RHO, and LTA topologies, respectively are shown in Figure 1.5

Figure 1.4 Crystal structures of representative ZIFs were grouped according to their

topology (three-letter symbol).[2,83] The largest cage in each ZIF is shown with ZnN4

in blue and CoN4 in pink polyhedra, and the links in ball-and-stick presentation The yellow sphere represents space in the cage Atom colors: Zn, blue polyhedra; Co, pink polyhedral; C, black; N, green; O, red; Cl, light pink; all H atoms are omitted for clarity

Figure 1.5 Schematic illustration of ZIFs; a) from left to right, 4, 6 and 8

membered-rings are mainly constructed into cages in ZIFs; b) sod cage [46.68] (β cage) in ZIF-8

(left), lta cage [412.68.86] (α cage) in ZIF-11 (middle) and in ZIF-20 (right) are typical

example of cages; c) simplified frameworks in ZIF-8 (SOD, left), ZIF-11 (RHO,

middle), and ZIF-20 (LTA, right) which were resulted from fusing sod cages,

cross-linking of a lta and a double 8-member ring cage in a ratio 1 : 3, and cross-cross-linking of

a lta, a sod and a double 4-member ring cage in a ratio 1 : 1 : 3, respectively The

yellow spheres represent for empty spaces inside the pores Atom colors: Zn, blue polyhedra; N, green; C, black; all H atoms and terminal linkers are omitted for clarity

Notably, ZIF structures are diverse They are not always based on purely tetrahedral nodes ZIF-5, In2Zn3(Im)12, comprising octahedral InN6 and tetrahedral ZnN4 SBUs were the first example of ZIFs based on mixed coordination.[85] Moreover,

many reported ZIFs possessed new topologies such as cag, frl, zeb, zni, poz, and

moz which are unknown in zeolites However, a number of topologies in ZIFs are

smaller than in zeolites, with around 40 topology types for the former as compared to around 180 for the later.[88] With that being said, there are a lot of chances to discovery ZIFs with new topology Another notable thing in ZIF structures is that, ZIF cages are lined by imidazolate organic components with functionalities pointing into the pore (Figure 1.4 and 1.5) Therefore, pore functionalities and sizes can be systematically varied by designing imidazolate linkers incorporated within ZIF structures for studies of CO2 capture

Besides the structural diversity and tunability, the thermal and chemical stability, and porosity of ZIF materials are also attracting many interests as shown in the typical ZIF-8, Zn(2-methylimidazole)2 This materials exhibited high thermal stability, up to

550 C, and extremely stable in boiling alkaline water and organic solvents These properties were attributed to the strength of Zn-N bonds in ZIFs Due to its rigid framework, the guest molecules occluded inside the pores can be fully evacuated, leaving the highly porosity framework of 1800 m2 g-1 Hence, ZIF-8 with these salient features has been reported for many important application so far, especially in gas separation.[12, 48, 49, 57, 60, 77, 78, 81, 104, 118, 123, 127]

As discussed above, ZIF materials are diverse in structures Their pores can be functionalized for systematically studies the effects of functionalities and/or pore sizes in CO2 capture More importantly, they are thermally and chemically stable and highly porous Because these significant properties, these materials have been studying intensively on strategies for synthesis of new ZIFs In the following, the general procedures for ZIFs synthesis and methods for discovering new ZIFs were demonstrated

General synthetic procedure of ZIFs

The general synthetic protocol of ZIFs involves combining transition metal salts

and imidazolate linkers in an amide solvent such as N,N-dimethylformamide (DMF),

N,N-diethylformamide (DEF), or N,N-dimethylacetamide (DMA) The reaction

mixtures are then heated from 85-150 °C.[39, 85] Under these conditions, the solvent is thermally decomposed to amine which helps the deprotonation of imidazolate and normally, after three days, the crystalline ZIF compounds are yielded The powder X-ray diffraction patterns (PXRD) are then collected for the resulting materials, and

it is followed by SXRD analysis for samples exhibiting new phases evaluated from PXRD analysis It is noted that the small change of molar ratio of reactants, polarity

of solvents and temperatures strongly effect on not only the quality of crystals but also the topology of ZIFs.[4, 109] Below are three main methods for synthesis of ZIFs with new topologies

Link-link interactions

It was found in ZIF structures that the interaction between imidazolate linkers plays an important role in directing the structures of resulting ZIFs One of the main reasons is that the length of imidazole linkers is much shorter than any other organic linkers in MOFs As consequences, these imidazolate linkers are easy to be influenced

by each other, especially when new functionalities were inserted Indeed, ZIF-1 composed of zinc and imidazole has the structural symbol crb When imidazolate

linker with a side chain -CH3 as 2-methylimidazole was employed, ZIF-8 with SOD

structure type was obtained.[85] It was reasoned that the -CH3 functionality in

imidazole was not favor to accommodate into the pore space of crb framework

Instead, it produced new interactions between 2-methylimidazolate linkers for the

formation of sod cage of ZIF-8 (Figure 1.6) Similarly, the use of benzimidazole as a

bulkier aromatic ring functionalizing imidazolate linker showed sufficient steric

effects to prevent the formation of old topology, resulting in the RHO framework

type of ZIF-11 (Figure 1.6) Furthermore, if benzimidazole crowded at 5 position as

5-chlorobenzimidazole was used to incorporate with zinc salts, ZIF-95 and ZIF-100 with unprecedented topologies were obtained (Figure 1.7).[109]

Figure 1.6 Single crystal structure of crb-ZIF-1 (a), SOD-ZIF-8 (b), and

RHO-ZIF-11 (c) The yellow spheres represent the empty spaces inside the pores Atom colors:

Zn, blue polyhedra; C, black; N; green; all H atoms and terminal linkers are omitted for clarity

Figure 1.7 Illustration of SiO2 in zeolite (a); Zn2(2-methylimidazole), mIm, in

ZIF-8 with SOD topology (b); Zn2(benzimidazole), bIm, in ZIF-11 with RHO topology

(c), and Zn2(5-chlobenzimidazole), cbIm, in ZIF-95 and ZIF-100 with previously unknown topologies (d) Atom colors: Si, purple; O, red; N, yellow; H, blue; C, black;

Zn, grey; Cl, green.[109]

It was reported that with the use of benzimidazole linker, the obtained materials

have SOD[47] or RHO[85] topology which has one type of cage,  cage Otherwise,

LTA topology has two cage types, α and β cage The authors found out that the

replacing of nitrogen for carbon atom at the position 5 of benzimidazole linker has

the profound impact on achievement of LTA structure.[39] Accordingly, LTA ZIFs

were obtained when 5-azabenzimidazolate or purinate were used (Figures 1.8 and 1.9) It is likely the new interactions among imidazolate linkers were produced not only by the bulkiness of functionalities but also by the presence of N atoms on

aromatic rings Indeed, the position 5 of nitrogen on benzimidazolate backbone was

found to produce the suitable link-link interaction for the formation of a cube which

is considered as the requisite starting stage for directing the obtained structure to LTA

(Figure 1.9) This principle was confirmed by using 4-azabenzimidazolate as a linker

leading to the formation of DIA ZIF structure

Figure 1.8 The positions of the nitrogen atoms in the imidazolate-type linkers are

significant in determining which ZIF topology (SOD, RHO, DIA and LTA) is

produced The numbering of all linkers is the same as that indicated for benzimidazolate

Figure 1.9 X-ray single-crystal structure of ZIF-20 with LTA topology as zeolite A

including a α-cage (a), a β-cage (b) and a cube (c) The formation of linkers between cubes is important in the reticulation of the structure (d) The yellow spheres represent the empty spaces inside the pores Note the pairs of interacting C/N atoms highlighted

in red; Zn, blue polyhedra; carbon and nitrogen at Purinate position 7, black; all H atoms and terminal linkers are omitted for clarity.[39]

Mixing of two distinct linkers

Due to the ease to receive influences among imidazolate linkers from their sizes and functionalities, it was thought that it is not easy to design isoreticular ZIFs with systematically varying in pore sizes and/or functionalities as shown for MOF-5.[26]

However, this challenge was solved as a series of eight isoreticular GME-ZIFs was

reported That is, the use of mixed linkers can be an effective way to functionalize ZIF frameworks for systematically study the effect of pore sizes and/or functionalities It was observed that with the sole linker 2-nitroimidazole employed,

ZIF-65 with SOD topology was achieved.[4] However, when 2-nitroimidazole was simultaneously combined with another imidazolate linker as a mixture of two distinct

linkers, series of GME ZIFs were obtained (Figure 1.10) As discovered, Rahul

Banerjee et al successfully prepared a range of eight GME ZIFs which were known

as ZIF-68, -69, -70, -78, -79, -80, -81, and -82 (Figure 1.10).[3,4] The salient features

of using mixed linkers in these ZIFs are that the obtained ZIFs not only have the desired topology but also have interesting properties Specifically, the polarity of functionalized pores are in the order NO2 (ZIF-78) > CN, Br, Cl (ZIF-82, -81, -69) >

C6H6, Me (ZIF-68, -79) > H (ZIF-70) and the sequence of CO2 uptake capacity was

in agreement with this order This result was explained by the great interaction between polar functional group of ZIFs (i.e -NO2, and -CN) and high quadrupole moment CO2 molecules In addition, the pore diameters within these ZIFs were systematically tuned from 7.1 to 15.9 Å, although they have no correlation with CO2uptake

Figure 1.10 Structures of imidazolate links and their abbreviations Reaction of nIm

results in the SOD topology Reaction of nIm plus any other imidazole linker shown results in the GME topology, whose tiling is shown at the above The name of the

ZIF resulting from reaction with nIm is given under each linker At the bottom are

illustrated the hpr, gme, and kno cages, which make up the GME topology Red edges

indicate nIm linkers and gray edges indicate substituted Im linkers Atom colors: Zn, blue tetrahedral; C, black; N, green; O, red; all H atoms are omitted for clarity

GME cages:

SOD-ZIF-65

Using presynthesized metal-imidazole complex as a precursor

As are shown in the two methods above, conventionally mixing imidazolate linkers and metal salts resulted in ZIFs based on almost tetrahedral SBUs One method of using boron imidazolate complex as a precursor introduced three connected SBUs into imidazolate frameworks Indeed, three and four connected boron imidazolate complex can be presynthesized and then reacted with metal salt to form new porous materials with three connected or (3,4)-connected framework topologies as shown in MOP-100 and -101[71] (MOP = metalorganic polyhedron) and in boron imidazolate frameworks (BIFs) (Figure 1.11).[125] It is notable that three connected Zn(Im)3- units are unlikely to be prepared and so most of ZIF frameworks based on tetrahedral metal nodes Therefore this strategy helps to increase the diversity of topology in imidazolate frameworks through introducing new metal containing SBUs In fact, over twenties BIF materials[16, 114, 125, 126] had been made

and the topologies such as srs, ths, and ctn[125, 126] have not been seen previously in imidazolate frameworks

Figure 1.11 The presynthesized four- and three-connected boron-centered

precursors which were used to synthesize BIF and MOP materials

1.3 Carbon-dioxide Capture

Carbon dioxide (CO2) is the main component of greenhouse gas and therefore the emissions of CO2 are the most important cause of global warming CO2 emission rates rise in line with economic growth because the development of countries relies

on the carbon intensive industries The average growth of CO2 emission in last decade was around 3% per year.[80]The Global Monitoring Division of NOAA (National Oceanic and Atmospheric Administration)/Earth System Research Laboratory has measured concentration of CO2 in air for several decades and showed recent monthly mean CO2 globally averaged over marine surface sites as can be seen from the graph below (Figure 1.12)

Figure 1.12 Global monthly mean CO2 averaged over marine surface sites.[44]