Hydrogen storage in metal organic framword mil 88s a computational study = nghiên cứu lưu trữ khí hydro sử dụng vật liệu khung hữu cơ kim loại mil 88s bằng phương pháp mô phỏng

Volume optimization for Co-MIL-88A structure by Murnaghan fitting method: a Relative energy as a function of the lattice constant a for each c/a ratio, where the solid lines are the fi

VIETNAM NATIONAL UNIVERSITY HO CHI MINH CITY

HO CHI MINH CITY UNIVERSITY OF TECHNOLOGY

Nguyen Thi Xuan Huynh

HYDROGEN STORAGE IN METAL-ORGANIC FRAMEWORK

MIL-88S: A COMPUTATIONAL STUDY

A dissertation submitted for the degree of

Doctor of Philosophy

Ho Chi Minh City – 2019

VIETNAM NATIONAL UNIVERSITY HO CHI MINH CITY

HO CHI MINH CITY UNIVERSITY OF TECHNOLOGY

Nguyen Thi Xuan Huynh

HYDROGEN STORAGE IN METAL-ORGANIC FRAMEWORK

MIL-88S: A COMPUTATIONAL STUDY

Major: ENGINEERING PHYSICS

Major code: 62520401

Independent Reviewer 1: Assoc Prof Dr Pham Tran Nguyen Nguyen

Independent Reviewer 2: Assoc Prof Dr Nguyen Thanh Tien

Reviewer 1: Assoc Prof Dr Phan Bach Thang

Reviewer 2: Assoc Prof Dr Huynh Quang Linh

Reviewer 3: Dr Phan Hong Khiem

SCIENTIFIC SUPERVISORS:

1 Dr Do Ngoc Son

2 Dr Pham Ho My Phuong

DECLARATION

I declare that this doctoral dissertation was written by myself, that the work contained herein is my own except where explicitly stated otherwise in the text, and that this work has not been submitted for any other degrees or professional qualification except as specified

Parts of this dissertation were published in the following papers:

[1] T T T Huong, P N Thanh, N T X Huynh, and D N Son, “Metal-Organic

Frameworks: State-of-the-art Material for Gas Capture and Storage,” VNU Journal of

Science: Mathematics – Physics, vol 32, pp 67-84, 2016

[2] N T X Huynh, O M Na, C Viorel, and D.N Son, “A computational approach

towards understanding hydrogen gas adsorption in Co-MIL-88A,” RSC Advances, vol

17, pp 39583-39593, 2017

[3] N T X Huynh, C Viorel, and D.N Son, “Hydrogen storage in MIL-88 series,”

Journal of Materials Science, vol 54, pp 3994-4010, 2019

Author of dissertation

Nguyen Thi Xuan Huynh

ABSTRACT

Fossil fuel-based energy consumption causes serious environmental impacts such

as air pollution, greenhouse effect, and so on Therefore, searching clean and renewable energy sources is urgent to meet the demand for sustainable development of the global society and economy Hydrogen gas (H2) is a reproducible, clean, and pollution-free energy carrier for both transportation and stationary applications Hydrogen gas has a much higher energy density than other fuels; and thus, it becomes one of the most promising candidates to replace petroleum Therefore, in recent years, the interest in the research and development of hydrogen energy has grown constantly

A safe, efficient, and commercial solution for hydrogen storage is based on adsorption

in porous materials, which have the exceptionally large surface area and ultrahigh porosity such as metal-organic framework (MOF) materials In order to be selected as porous materials for gas storage, MOFs must be stable to avoid collapsed under humid conditions MIL-88 series (abbreviated as MIL-88s including MIL-88A, MIL-88B, MIL-88C and MIL-88D) is highly stable and flexible sorbents For these reasons, MIL-88s becomes a suitable candidate for the storage of hydrogen gas based on the physisorption Moreover, coordinatively unsaturated metal centers in MIL-88s are able

to enhance gas uptakes significantly at ambient temperatures and low pressures These materials have been investigated and highly evaluated for various applications such as gas storage/capture and separation of binary gas mixtures in recent years; however, they have not yet been evaluated for hydrogen storage These outstanding features have attracted my attention to consider the hydrogen storage capacity in MIL-88 series

In this dissertation, the van der Waals dispersion-corrected density functional theory (vdW-DF) calculations were used to examine the stable adsorption sites of the hydrogen molecule in MIL-88s and clarify the interaction between H2 and MIL-88s via electronic structure properties This observation showed an implicit role of electronic structures on the H2 adsorption capacity at the considered temperature and pressure conditions Besides, it was found that the H2@MIL-88s interaction is dominated by the bonding state () of the hydrogen molecule and the p orbitals of the

O and C atoms in MIL-88s For MIL-88A and B, the d orbitals of the metals also play

an important role in the interaction with H2

Moreover, grand canonical Monte Carlo (GCMC) simulations were used to compute hydrogen uptakes in MIL-88s at the temperatures of 77 K and 298 K and the pressures up to 100 bar For Fe based-MIL-88 series, we found that MIL-88D is very promising for the gravimetric hydrogen storage (absolute/excess uptakes

= 5.15/4.03 wt% at 77 K and 0.69/0.23 wt% at 298 K), but MIL-88A is the best alternative for the absolute/excess volumetric hydrogen storage with 50.69/44.32 g/L

at 77 K and 6.97/2.49 g/L at 298 K Via this research, scandium (Sc) was also found as the best transition metal element for the replacement of Fe in MIL-88A for the hydrogen storage, in which absolute/excess uptakes are 5.30/4.63 wt% at 77 K and 0.72/0.29 wt% at 298 K for gravimetric uptakes; 51.99/45.51 g/L at 77 K and 7.08/2.83 g/L at 298 K for volumetric uptakes The hydrogen storage capacity is the decrease in the order: Sc-, Ti-, V-, Cr-, Mn-, Fe-, and Co-MIL-88A The calculations showed that the results are comparable to the best MOFs for the hydrogen storage up

to date The results also elucidated that the gravimetric hydrogen uptakes depend on the specific surface area and pore volume of the MIL-88s These important structural features, if properly improved, lead to an increase in the capability of hydrogen storage

in MIL-88s

có thể tái tạo và không gây ô nhiễm môi trường cho cả các ứng dụng di chuyển và tại chỗ Hydro lại có mật độ năng lượng cao hơn nhiều so với các nhiên liệu khác nên nó được chọn như là ứng viên sáng giá cho việc thay thế xăng dầu Với những đặc tính như vậy nên sự quan tâm đến nghiên cứu và phát triển năng lượng hydro đã tăng lên không ngừng trong những năm gần đây Một giải pháp đảm bảo tính an toàn, hiệu quả

và kinh tế cho lưu trữ hydro đó là hấp phụ khí vào trong vật liệu xốp Những vật liệu xốp được đánh giá cao cho khả năng lưu trữ khí hydro hiện nay đó là các vật liệu xốp

có diện tích bề mặt rất lớn và tính xốp cực cao như họ vật liệu khung hữu cơ kim loại (MOF) Để được chọn làm vật liệu lưu trữ khí, các vật liệu MOF phải có tính ổn định

và bền để tránh hiện tượng phá vỡ cấu trúc trong môi trường ẩm Chuỗi MIL-88A, MIL-88B, MIL-88C và MIL-88D (viết tắt là MIL-88s) có thể đáp ứng được yêu cầu trên vì chuỗi vật liệu này có cấu trúc rất linh hoạt và rất bền trong môi trường ẩm Do

đó, chuỗi MIL-88s này được dự đoán là những ứng viên sáng giá cho lưu trữ hydro dựa trên tính chất hấp phụ Hơn nữa, MIL-88s còn chứa các tâm kim loại chưa bão hòa

mà đặc tính này được cho là một trong những giải pháp chiến lược có thể tăng cường đáng kể lượng khí hấp phụ vào trong MOF ở điều kiện nhiệt độ phòng và áp suất thấp Trong thời gian gần đây, chuỗi MIL-88s đã từng được nghiên cứu và được đánh giá cao cho nhiều ứng dụng như lưu trữ, bắt giữ và tách khí; tuy nhiên, chúng chưa được đánh giá cho khả năng lưu trữ hydro bằng cả phương pháp thực nghiệm và tính toán Với những tính năng nổi bật như trên, các phương pháp tính toán được sử dụng để xem xét khả năng lưu trữ hydro của MIL-88s và giải thích chi tiết tương tác giữa các trạng thái điện tử của phân tử H2 với các nguyên tử của MIL-88s

Trong luận án này, phương pháp lý thuyết phiếm hàm mật độ (DFT) có hiệu chỉnh van der Waals (vdW-DF) được sử dụng để tính năng lượng liên kết hay năng

lượng hấp phụ và từ đó tìm các vị trí hấp phụ bền cho H2 trong chuỗi MIL-88s Cụ thể hơn, dựa trên tính chất cấu trúc điện tử, tương tác giữa H2 và MIL-88s được làm sáng

tỏ Kết quả tính toán chỉ ra được các vị trí hấp phụ bền nhất của H2 trong các cấu trúc MIL-88s Kết quả cũng chỉ ra rằng tương tác giữa H2 và MIL-88s được đóng góp chính bởi trạng thái liên kết ( - trạng thái bonding) của phân tử H2 tương tác với các

quỹ đạo p của các nguyên tử O và C trong MIL-88s Đối với MIL-88A và B, tính toán vdW-DF cũng chỉ ra các quỹ đạo d của kim loại cũng đóng vai trò quan trọng trong

tương tác với H2

Bên cạnh đó, để đánh giá định lượng khả năng lưu trữ hydro trong MIL-88s ở nhiệt độ 77 K và 298 K với áp suất lên đến 100 bar, phương pháp mô phỏng Monte Carlo chính tắc lớn (GCMC) được sử dụng Phương pháp này cũng được dùng để đánh giá độ mạnh tương tác giữa H2 và MIL-88s qua nhiệt hấp phụ Qst

Đánh giá khả năng lưu trữ hydro của chuỗi MIL-88s (với thành phần kim loại là Fe), kết quả chỉ ra rằng MIL-88D có tiềm năng nhất cho khả năng lưu trữ hydro tính theo phần trăm trọng lượng (dung khối) với dung lượng hấp phụ toàn phần/bề mặt tương ứng là 5,15/4,03 wt% ở 77 K và 0,69/0,23 wt% ở 298 K, nhưng nếu đánh giá theo dung tích thì MIL-88A lại tốt hơn cho lưu trữ hydro (dung tích toàn phần/bề mặt tương ứng là 50,69/44,32 g/L ở 77 K, và 6,97/2,49 g/L ở 298 K) Kết quả của luận án cũng chỉ ra rằng với việc thay thế Fe trong MIL-88A lần lượt bởi một số kim loại chuyển tiếp có thể nâng cao khả năng lưu trữ H2 và kim loại tốt nhất trong chuỗi kim loại được khảo sát đó là scandium (Sc) Cụ thể, kết quả lưu trữ toàn phần/bề mặt đạt 5,30/4,63 wt% ở 77 K và 0,72/0,29 wt% ở 298 K tính theo dung khối; 51,99/45,51 g/L

ở 77 K và 7,08/2,83 g/L ở 298 K tính theo dung tích Khả năng lưu trữ hydro trong M-MIL-88A theo thứ tự giảm dần là Sc-, Ti-, V-, Cr-, Mn-, Fe- và Co-MIL-88A Kết quả nghiên cứu bước đầu cho thấy tiềm năng của chuỗi MIL-88s cho lưu trữ hydro và kết quả này có thể so sánh được với nhóm vật liệu MOF đã từng được đánh giá cao cho lưu trữ H2 đến nay Các kết quả cũng giải thích được khả năng hấp phụ hydro phụ thuộc mạnh vào các đặc tính cấu trúc như diện tích bề mặt riêng, thể tích lỗ rỗng của MIL-88s Những đặc tính quan trọng này nếu được cải thiện phù hợp sẽ tăng được khả năng hấp phụ hydro trong MIL-88s

ACKNOWLEDGEMENTS

This work couldn’t be completed without the help and support of many people to whom I would like to express my gratitude

First of all, I would like to thank my supervisors, Dr Do Ngoc Son and Dr Pham

Ho My Phuong at Ho Chi Minh City University of Technology in VNU-HCM (HCMUT), for their guidance and helpful comments throughout my doctorate course I

am grateful to express my deepest appreciation to Dr Do Ngoc Son when I perform

my research at HCMUT His enthusiasm, valuable suggestions and comments were very helpful during this research

I would like to thank Prof Viorel Chihaia (Institute of Physical Chemistry “Ilie Murgulescu” of the Romanian Academy, Splaiul Independentei 202, Sector 6, 060021 Bucharest, Romania) for helpful comments to my research papers I acknowledge the usage of the computer time and software granted by the Institute of Physical Chemistry of Romanian Academy, Bucharest (HPC infrastructure developed under the projects Capacities 84 Cp/I of 15.09.2007 and INFRANANOCHEM 19/01.03.2009)

I am very thankful to Prof Vo Van Hoang for helping me more advantageous in this work at the Computational Physics Laboratory I will also remember nice and fruitful conversations with other members of this Lab Additionally, I also would like

to thank all lecturers and secretaries of Faculty of Applied Science, HCMUT giving

me useful knowledge and helping during my research herein

I am very thankful to the lecturers and co-workers at Department of Physics, Quy Nhon University (QNU) who helped me to have a chance to participate in PhD project

at HCMUT and do well my work at QNU I would like to acknowledge the financial support from Quy Nhon University and the Vallet scholarship foundation

Last but not least, I would like to thank my beloved husband for taking care, understanding and sympathy He is the one who strongly believed in me, encouraged

me to start the PhD project and was always with me in good and bad times I had during that four years I am grateful to my sons for understanding, sympathy and they are always lovely babies giving me a great motivation to complete this study I am also

big thanks to other members in my family for understanding and helping me during the time for my PhD course

This research was funded by the Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 103.01-2017.04; the research project T2015.460.05 of Quy Nhon University; and the research project

TNCS-2015-KHUD-33 of Ho Chi Minh City University of Technology

TABLE OF CONTENTS

DECLARATION i

ABSTRACT ii

TÓM TẮT LUẬN ÁN iv

ACKNOWLEDGEMENTS vi

TABLE OF CONTENTS viii

LIST OF FIGURES xi

LIST OF TABLES xvii

LIST OF ABBREVIATIONS xix

INTRODUCTION 1

1 Motivation for study 1

2 Structure of PhD dissertation 3

LITERATURE REVIEW OF METAL-ORGANIC FRAMEWORKS 5

CHAPTER 1: General overview of metal-organic frameworks 5

1.1 1.1.1 Definition of metal-organic frameworks 5

1.1.2 Structural aspects of MOFs 6

1.1.3 History of MOFs 8

1.1.4 Nomenclature of MOFs 11

1.1.5 Current research of MOFs in Vietnam 12

Major applications of MOFs 13

1.2 1.2.1 Gas storage, capture, and separation 13

1.2.2 Biomedical applications 20

Overview of synthesis and research methods for MOFs 21

1.3 1.3.1 Synthesis methods for MOFs 21

1.3.2 Theoretical studies 23

MIL-88s for hydrogen storage 24

1.4 COMPUTATIONAL METHODS 26

CHAPTER 2: Density functional theory calculations 26 2.1

2.1.1 The Schrödinger equation 26

2.1.2 Born-Oppenheimer and adiabatic approximations 28

2.1.3 Thomas-Fermi theory 29

2.1.4 Hohenberg- Kohn theorems 29

2.1.5 Variational condition and Levy constrained search formulation 30

2.1.6 The Kohn-Sham equations 31

2.1.7 Exchange-correlation functional 34

2.1.8 The basis sets 37

2.1.9 Pseudopotentials 38

2.1.10 Self-consistent field methods 39

2.1.11 Van der Waals density functional (vdW-DF) calculations 40

2.1.12 Computational details 41

Grand canonical Monte Carlo simulations 42

2.2 2.2.1 Introduction 42

2.2.2 Computational details 47

HYDROGEN ADSORPTION IN Co-MIL-88A 49

CHAPTER 3: Optimization of Co-MIL-88A unit cell 49

3.1 Searching stable adsorption sites of hydrogen 50

3.2 Adsorption isotherms of hydrogen in Co-MIL-88A 56

3.3 Short summary 59

3.4 HYDROGEN STORAGE IN MIL-88 SERIES 60

CHAPTER 4: Geometry optimization of MIL-88 series 60

4.1 Isotherms and isosteric heats of hydrogen adsorption 61

4.2 The most favourable H2 adsorption configurations 67

4.3 Electronic structure properties of H2 – MIL-88s interaction 69

4.4 Short summary 72

4.5 EFFECTS OF TRANSITION METAL SUBSTITUTION IN MIL-88A CHAPTER 5: ON HYDROGEN ADSORPTION 74

Optimization of M-MIL-88A unit cell 74 5.1

Stable hydrogen adsorption sites 755.2.

Isotherms and isosteric heats of hydrogen adsorption 795.3

Short summary 825.4

CONCLUSIONS 83CHAPTER 6:

The main findings 836.1

Scientific contributions 846.2

Outlook 846.3

LIST OF PUBLICATIONS 85REFERENCES 88APPENDIX 107

LIST OF FIGURES

Figure 1.1 Simple topology of MOFs 5

Figure 1.2 Structure of MOF-5 [26] 6

Figure 1.3 Several common coordination geometries of metal ions used for MOF construction The numbers indicate the numbers of functional sites [30] 7

Figure 1.4 Several common organic ligands used for MOF construction [28] 7

Figure 1.5 Several common SBUs of MOFs [31, 32] 8

Figure 1.6 1D, 2D, and 3D MOF structures from 1970 to 2015 [35] 9

Figure 1.7 NU-110 structure with the highest BET surface area of MOFs reported until now [23] with CCDC data taken from [38] 10

Figure 1.8 BET surface areas (m2/g) and pore volumes (cm3/g) of representative MOFs, compared to conventional porous materials (zeolites, silicas, and activated carbons) From Ref [29] Reprinted with permission from AAAS 11

Figure 1.9 Application areas of porous reticular materials (MOFs, ZIFs and COFs) [35] 13

Figure 1.10 H2 storage in MOFs relatively compared to zeolite and empty tank Reproduced with permission of Royal Society of Chemistry from Ref [74] 15

Figure 1.11 CH4 storage in MOFs relatively compared to active carbon and empty tank Reproduced with permission of Royal Society of Chemistry from Ref [74] 18

Figure 1.12 Volumetric CO2 capacity of MOF- 177 relatively compared to zeolite 13X pellets, MAXSORB carbon powder, and pressurized CO2 Reproduced with permission from Ref [37] Copyright © 2005, American Chemical Society 19

Figure 1.13 CO2 uptakes of MOFs at 298 K Reprinted from Ref [36] Reprinted with permission from AAAS 19

Figure 1.14 Schematic diagram of the drug and biomedical gas delivery by MOFs [23] 20

Figure 1.15 (a) Synthesis conditions commonly used for MOF preparation; (b) indicative summary of the percentage of MOFs synthesized using the various preparation routes [75] 22

Figure 1.16 Schematic representation of PMS [110] 22

Figure 1.17 Multiscale methodology scheme, showing the different levels of theory and the corresponding size of systems under study Reproduced with permission of Royal Society of Chemistry from Ref [109] 24

Figure 2.1 Schematic illustration for all-electron (solid lines) and pseudo wave potentials (dashed lines) and their corresponding wave functions [117] 39

Figure 2.2 Flow chart of a self-consistent loop of the Kohn Sham equation 40 Figure 2.3 The IUPAC classification for adsorption isotherms [143] 45 Figure 2.4 The scheme for the multi-scale simulation method [151] 46 Figure 3.1 The unit cell of Co-MIL-88A: (a) side view, (b) top view of the unit cell,

(c) the μ3-O-centered trimer of Co metals, and (d) the fumarate ligand of MIL-88A

The blue, red, brown, and white coloured balls represent Co, O, C, and H atoms, respectively 50

Figure 3.2 Volume optimization for Co-MIL-88A structure by Murnaghan fitting

method: (a) Relative energy as a function of the lattice constant a for each c/a ratio,

where the solid lines are the fitting curves, while the points are the calculated values

by vdW-DF; The minimum relative energy points (fitted from (a)) are plotted versus

(b) the lattice constant a and (c) the c/a ratio 50

Figure 3.3 The favourable adsorption configurations of H2 in Co-MIL-88A The bond distance to the reference atoms is correspondingly shown for each configuration H1 and H2 are hydrogen atoms of hydrogen gas 51

Figure 3.4 CDD for the favourable adsorption configurations of H2 in Co-MIL-88A

The orbitals are drawn at an isosurface value of 0.0002 e/Bohr3 Yellow (positive) and cyan (negative) clouds indicate charge gain and loss The blue, red, brown, white balls denote Co, O, C, and H atoms of MIL-88A, respectively 53

Figure 3.5 The electronic density of state of the hydrogen molecule and the s and d

orbitals of the Co atoms of the Co-MIL-88A at the sites: (a) hollow, (b) ligand, (c) metal (side-on), and (d) metal (end-on) 54 Figure 3.6 The real part of the wave functions of the H atom of the H2 molecule and

the Co atom of the MIL-88A along the x-direction The dots indicate for the position

of the atoms 54

Figure 3.7 (a) A demonstration of the overlapping of DOS, the filled area (turquoise area); (b) the overlapping area of the H2 DOS with the total DOS (blue line), and the DOS area of the adsorbed H2 molecule (magenta line) versus the binding energy 55

Figure 3.8 Excess H2 uptake for Co-MOF-74 at 77 K: (a) GCMC simulation with the

generic force field for MOFs and the DDEC charge assignment, compared to the

experimental data extracted with permission from Ref [159] (b) GCMC simulation

with the DDEC charges of this work and the charges from the library of RASPA 57

Figure 3.9 Absolute (red solid line) and excess (green dash line) adsorption isotherms for the Co-MIL-88A at (a) 77 K and (b) 298 K 58 Figure 3.10 The density of the adsorbed hydrogen molecules in the Co-MIL-88A

The blue, red, brown, white balls represent the cobalt, oxygen, carbon, and hydrogen

atoms of MOF, respectively Each pair of green balls represents an adsorbed hydrogen molecule The cyan, red and blue bands refer to the hollow, ligand and metal sites, respectively 58

Figure 4.1 The unit cell of MIL-88s with the different ligands: (a) FMA, (b) BDC, (c) NDC, and (d) BPDC The copper, red, brown, and white colours represent the iron,

oxygen, carbon, and hydrogen atoms, respectively 60

Figure 4.2 Absolute (abs.) and excess (ex.) gravimetric hydrogen adsorption isotherms of MIL-88s (s = A, B, C, and D) at (a) 77 K and (b) 298 K Solid and dash

lines imply the absolute and excess adsorption isotherms 62

Figure 4.3 Absolute (abs.) and excess (ex.) volumetric H2 adsorption isotherms of

MIL-88s at (a) 77 K and (b) 298 K Solid and dash lines imply the absolute and excess

adsorption isotherms 64

Figure 4.4 Size view of the density of H2 adsorbed in MIL-88s at 77 K and 1 bar The copper, red, brown, white small balls represent the Fe, O, C, and H atoms of MIL-88s, respectively Each pair of the green balls represents a hydrogen molecule 65

Figure 4.5 Correlation between the maximum absolute and excess uptakes and the

specific surface area (SSA) of MIL-88s at (a) 77 K and (b) 298 K 66

Figure 4.6 Correlation between the maximum absolute and excess uptakes and the

pore volume (V p) of MIL-88s at (a) 77 K and (b) 298 K 66

Figure 4.7 The isosteric heats of adsorption versus the number of the H2 molecules adsorbed per the unit cell of MIL-88s 67

Figure 4.8 The average adsorption energy and the isosteric heat of the H2 adsorption

in MIL-88s 69

Figure 4.9 DOS of the adsorbed H2 and the C, O, Fe atoms of MIL-88s in the most stable adsorption configurations 70

Figure 4.10 The p orbitals of the C and O atoms of MIL-88s for the most favourable

adsorption configurations: (a) 88A, (b) 88B, (c) 88C and (d) 88D 71

MIL-Figure 4.11 The d orbitals of the Fe atoms for the most favourable adsorption

configurations: (a) MIL-88A, (b) MIL-88B, (c) MIL-88C, and (d) MIL-88D 71

Figure 4.12 The CDD of the most favourable adsorption site of each MOF: (a)

MIL-88A, (b) MIL-88B, (c) MIL-88C, and (d) MIL-88D Yellow and cyan clouds represent charge gain and loss, respectively 72

Figure 5.1 Relationship between the volume of the unit cell and the ionic radius of

metal of the M-MIL-88A structures Data of Fe-MIL-88A and Co-MIL-88A were taken from Refs [162] and [18], respectively 75

Figure 5.2 CDD of H2 and atoms of V-MIL-88A: (a) side-on (isosurface is 410-4

e/Bohr3); (b) end-on (isosurface = 310-5 e/Bohr3) Cyan/yellow clouds indicate charge loss/gain The slate-gray, red, brown, white balls denote V, O, C, and H atoms of MIL-88A, respectively 77

Figure 5.3 DOS of the adsorbed H2 and the orbitals of C, O and M atoms of 88A configurations at side-on site: (a) Sc-MIL-88A, (b) Ti-MIL-88A, (c) V-MIL-88A, (d) Cr-MIL-88A, (e) Mn-MIL-88A and (f) Fe-MIL-88A 78

M-MIL-Figure 5.4 Absolute (solid) and excess (dashed) gravimetric adsorption isotherms of

hydrogen in M-MIL-88A series at: (a) 77 K and (b) 298 K Data of Fe-MIL-88A and Co-MIL-88A were taken in Ref [162] and Ref [18], respectively 80

Figure 5.5 Absolute (solid) and excess (dashed) volumetric adsorption isotherms of

H2 in M-MIL-88A series at: (a) 77 K and (b) 298 K Data of Fe-MIL-88A was taken in Ref [162] 80

Figure 5.6 Isosteric heats of H2 adsorption in M-MIL-88A series Data of 88A was taken in Ref [162] 81

Fe-MIL-Figure A1 Electrostatic charges and LJ parameters for the H2 molecule according to the TraPPE force field Hydrogen molecule is modeled as a three-site rigid model at

the center of mass (dH-H = 0.74 Å) 111

Figure A2 DOS of the isolated hydrogen molecule Fermi level is set to zero 111 Figure A3 Volume optimization for Fe-MIL-88A structure by Murnaghan fitting

method: (a) Relative energy as a function of the lattice constant a for each c/a ratio,

where the solid lines are the fitting curves, while the points are the calculated values

by vdW-DF; The minimum relative energy points (fitted from (a)) are plotted versus

(b) the lattice constant a and (c) the c/a ratio 111

Figure A4 Volume optimization for MIL-88B structure by Murnaghan fitting

method: (a) Relative energy as a function of the lattice constant a for each c/a ratio,

where the solid lines are the fitting curves, while the points are the calculated values

by vdW-DF; The minimum relative energy points (fitted from (a)) are plotted versus

(b) the lattice constant a and (c) the c/a ratio 112

Figure A5 Volume optimization for MIL-88C structure by Murnaghan fitting

method: (a) Relative energy as a function of the lattice constant a for each c/a ratio,

where the solid lines are the fitting curves, while the points are the calculated values

by vdW-DF; The minimum relative energy points (fitted from (a)) are plotted versus

(b) the lattice constant a and (c) the c/a ratio 112

Figure A6 Volume optimization for MIL-88D structure by Murnaghan fitting

method: (a) Relative energy as a function of the lattice constant a for each c/a ratio,

where the solid lines are the fitting curves, while the points are the calculated values

by vdW-DF; The minimum relative energy points (fitted from (a)) are plotted versus

(b) the lattice constant a and (c) the c/a ratio 112

Figure A7 Most favourable adsorption sites of the H2 molecule in Fe-MIL-88A 113

Figure A8 Most favourable adsorption sites of the H2 molecule in MIL-88B 114

Figure A9 Most favourable adsorption sites of the H2 molecule in MIL-88C 115

Figure A10 Most favourable adsorption sites of the H2 molecule in MIL-88D 116

Figure A11 Volume optimization for Sc-MIL-88A structure by Murnaghan fitting

method: (a) Relative energy as a function of the lattice constant a for each c/a ratio,

where the solid lines are the fitting curves, while the points are the calculated values

by vdW-DF; The minimum energy points (fitted from (a)) are plotted versus (b) the

lattice constant a and (c) the c/a ratio 117

Figure A12 Volume optimization for Ti-MIL-88A structure by Murnaghan fitting

method: (a) Relative energy as a function of the lattice constant a for each c/a ratio,

where the solid lines are the fitting curves, while the points are the calculated values

by vdW-DF; The minimum energy points (fitted from (a)) are plotted versus (b) the

lattice constant a and (c) the c/a ratio 117

Figure A13 Volume optimization for V-MIL-88A structure by Murnaghan fitting

method: (a) Relative energy as a function of the lattice constant a for each c/a ratio,

where the solid lines are the fitting curves, while the points are the calculated values

by vdW-DF; The minimum energy points (fitted from (a)) are plotted versus (b) the

lattice constant a and (c) the c/a ratio 117

Figure A14 Volume optimization for Cr-MIL-88A structure by Murnaghan fitting

method: (a) Relative energy as a function of the lattice constant a for each c/a ratio,

where the solid lines are the fitting curves, while the points are the calculated values

by vdW-DF; The minimum energy points (fitted from (a)) are plotted versus (b) the

lattice constant a and (c) the c/a ratio 118

Figure A15 Volume optimization for Mn-MIL-88A structure by Murnaghan fitting

method: (a) Relative energy as a function of the lattice constant a for each c/a ratio,

where the solid lines are the fitting curves, while the points are the calculated values

by vdW-DF; The minimum energy points (fitted from (a)) are plotted versus (b) the

lattice constant a and (c) the c/a ratio 118

Figure A16 Adsorption configurations of the hydrogen molecule in the M-MIL-88A

structure: (a) side-on configuration and (b) end-on configuration 118

Figure A17 PDOS of the adsorbed H2 and the orbitals of metals of M-MIL-88A configurations at side-on site: (a) Sc, (b) Ti, (c) V, (d) Cr, (e) Mn, and (f) Fe 119

Figure A18 PDOS of the adsorbed H2 and the orbitals of carbon atoms of 88A configuration at side-on site with the M is: (a) Sc, (b) Ti, (c) V, (d) Cr, (e) Mn, and (f) Fe 120

M-MIL-Figure A19 PDOS of the adsorbed H2 and the orbitals of oxygen atoms of 88A configuration at side-on site with the M is: (a) Sc, (b) Ti, (c) V, (d) Cr, (e) Mn, and (f) Fe 121

M-MIL-LIST OF TABLES

Table 1.1 High hydrogen uptakes in MOFs at 298 K and pressures below 100 bar 16

Table 2.1 The GCMC simulation box of MIL-88s 47

Table 2.2 The LJ parameters for atom types and atomic partial charges used in GCMC simulation 48

Table 2.3 The atomic partial charges of MIL-88s (s = A, B, C and D) used in GCMC simulation 48

Table 3.1 The adsorption energy (Eads in kJ/mol) between H2 and Co-MIL-88A for the favourable adsorption sites The average distance between the H2 molecule and the reference atoms of the MOF is denoted by 2 H A d  and the Bader point charge of H2 is denoted by 2 H q 51

Table 3.2 Overlapping between the DOS of the adsorbed H2 molecule with the DOS of different components of Co-MIL-88A 55

Table 4.1 Cell parameters of the hexagonal MIL-88s compared to the data in Ref [115] 61

Table 4.2 The maximum absolute and excess gravimetric hydrogen uptakes in MIL-88s at 77 K and 298 K, the pressures below 100 bar The specific surface area (SSA) and the pore volume (Vp) are also listed 63

Table 4.3 Maximum absolute and excess volumetric H2 uptakes of MIL-88s at 77 K and 298 K, the pressures under 100 bar 64

Table 4.4 Adsorption energy of H2 in MIL-88s (Eads in kJ/mol), the average H2 – MOF distance ( 2 H -MOF d in Å), and the Bader charge of the adsorbed H2 ( 2 H q in e -) 68

Table 5.1 Optimal parameters of the M-MIL-88A unit cell and the parameters correlating to the investigated metals 75

Table 5.2 Adsorption energies, H2 – metal distance (in Å) and Bader charges of the adsorbed H2 in M-MIL88A compared to that of isolated H2 with (-) for the charge loss and (+) for the charge gain and other relative parameters 76

Table 5.3 Maximum absolute and excess and gravimetric volumetric uptakes of hydrogen in M-MIL-88A at 77 K and 298 K and the pressures under 100 bar 81

Table A1 The organic ligand/linker of MOFs 107

Table A2 Chemical formula of MOFs 108

Table A3 The name of metal-organic frameworks 109

Table A4 The maximum absolute and excess gravimetric H2 uptakes in MIL-88s at

77 K and 298 K and the pressures below 100 bar, shown in many other units (mmol/g and cm3/g) 110

Table A5 The maximum absolute and excess gravimetric H2 uptakes in M-MIL-88A

at 77 K and 298 K and the pressures below 100 bar, shown in many other units (mmol/g and cm3/g) 110

IUPAC International Union of Pure and Applied Chemistry

INTRODUCTION

1 Motivation for study

Hydrogen gas (H2) is an attractive source for potential clean energy because it is

most abundant in the universe as part of water, hydrocarbons, and biomass, etc

Moreover, using energy from the H2 gas does not emit the CO2 gas and not pollute the environment like the burning of fossil fuels In recent years, the material-based hydrogen storage is expected to provide the safe, efficient and commercial solution for hydrogen storage in both transportation and stationary applications However, in order

to use the hydrogen energy source, most commonly used in the fuel cell technology, it

is necessary to develop a comprehensive system of generating production, storage, delivery, and fuel cell technologies for hydrogen In which, the H2 gas storage has been challenging because of its low density Therefore, seeking advanced storage materials plays a vital role in the success of hydrogen energy technology [1–4] The

2025 targets for the H2 storage set by the U.S Department of Energy (DOE) are 1.8 kWh/kg (55 mg H2 per gram of the (MOF+H2) system, i.e 5.5 wt% H2) for gravimetric storage capacity and 1.3 kWh/L (40 g H2/L) for volumetric storage capacity under moderate temperatures and pressures [5] Various materials have been studied for hydrogen storage such as metal hydrides, carbon-based materials, zeolites, zeolitic imidazolate frameworks (ZIFs), covalent organic frameworks (COFs), and MOFs [4–7] Among them, MOFs having the ultrahigh surface area, high porosity and controllable structural characteristics are the most promising candidates for the commercial hydrogen storage [8–11] Although thousands of MOFs have been successfully synthesized, only a few of them have been tested for hydrogen storage MIL-88 series (hereafter denoted as MIL-88s, where s = A, B, C and D; MIL = Materials from Institut Lavoisier) has attracted my attention due to consisting of the coordinatively unsaturated metal sites (CUS) or open metal sites [12], one of the most effective strategic solutions for improving the gas storage capacity [13, 14]

Furthermore, MIL-88s structures have high flexibility and thermal stability [15–17]; and hence, they are expected to be good candidates for long-term hydrogen storage

[18].Although MIL-88s structures have been assessed for catalyst [19], photo-catalyst

[20], NO adsorption [21], and CO2 capture [22], they have not yet been explored for hydrogen storage

In this dissertation, vdW-DFT calculations are utilized to examine favourable adsorption sites of H2 in the MIL-88s via the adsorption energy The interaction of the

H2 molecule with MIL-88 series is also clarified through electronic structure properties such as the electronic density of states (DOS), charge density difference (CDD), Bader charge, overlapping DOS between the gas molecule and MOF, and the overlapping of the wave functions Besides, grand canonical Monte Carlo (GCMC) simulations are used to assess quantitatively the H2 storage capability via the H2 adsorption isotherms

of MIL-88s and the strength of the H2 – MOF interaction through the isosteric heat of adsorption Three main directions of research are performed as follows:

- First of all, we choose a MOF in MIL-88s family to perform our research Among MIL-88s, MIL-88A has the smallest number of atoms per a primitive unit cell This helps DFT calculations become faster for the [H2+MOF] system For Co-MIL-88A, the H2 molecule was found to adsorb most favourably at the hollow site of the metal trimers because of the maximum overlap between the bonding () state of the H2 molecule and the total density

of state of the Co-MIL-88A Additionally, the hydrogen adsorption isotherms were also assessed by grand canonical Monte Carlo simulations The results showed that Co-MIL-88A is comparable to efficient H2 storage materials

- By changing the ligands in MIL-88, including MIL-88A, MIL-88B, MIL-88C and MIL-88D, the absolute (total) and excess H2 uptakes are estimated quantitatively via GCMC simulations The results showed that MIL-88D has the highest absolute and excess gravimetric H2 storage with 5.15 wt% (100 bar), 4.03 wt% (25 bar) at 77 K, and 0.69 wt%, 0.23 wt% at 100 bar and 298

K This result is comparable with the best MOFs for hydrogen storage/adsorption While, MIL-88A has the highest total (and excess) volumetric H2 storage with 50.69 g/L (44.32 g/L, 15 bar) at 77 K and 6.97 g/L (2.49 g/L) at 298 K The ability for volumetric H2 storage of MIL-88s and particularly MIL-88A is worth noticing Moreover, by utilizing vdW-DF calculations to calculate the adsorption energy as well as the combination

with the isosteric heat of adsorption, we elucidated that the H2 – MIL-88C interaction is strongest despite its lowest storage capacity The interaction between H2 and MIL-88s is mainly dominated by the bonding state of H2 and

the p orbitals of O atoms, and the d orbitals of Fe atoms In MIL-88 series,

MIL-88D is the best choice for hydrogen storage based on the commercialization, stability and high storage capacity However, if the volumetric H2 storage is concerned, MIL-88A is noteworthy

- To enhance the storage/adsorption capacity of hydrogen in MIL-88s, changing the metal centers in MIL-88s should be evaluated MIL-88A having high volumetric H2 storage is selected to substitute the metal component of this MOF material with the trivalent transition metals that are Sc, Ti, V, Cr, and Mn, compared to Fe- and Co-based MIL-88A in the previous parts The calculations show the binding between H2 and V-MIL-88A is strongest with -17 kJ/mol at the side-on configuration on the metal Nevertheless, the H2

adsorption isotherm and heat of adsorption Qst of Sc-MIL-88A is strongest among the investigated M-MIL-88A structures The highest absolute and excess loadings of H2 in Sc-MIL-88A are 5.13 wt% (50 bar), 4.63 wt% (10 bar) at 77 K; and 0.72, 0.29 wt% at 298 K and 100 bar, respectively

- Chapter 1: Literature review of metal-organic frameworks

In this chapter, an overview of the metal-organic framework, the main applications of MOFs, the overview of experimental and computational research methods in the literature are introduced

- Chapter 2: Computational methods

In this part, I introduce the theory of the computational methods that are density functional theory (DFT) using revPBE functional and Grand canonical

Monte Carlo (GCMC) simulations We also provide computational details for the concerns of this dissertation

- Chapter 3: Hydrogen adsorption in Co-MIL-88A

In this chapter, the hydrogen adsorption of Co-MIL-88A is studied and the physical origin for the interaction between H2 and Co-MIL-88A is explained Firstly, searching for the most favourable adsorption sites of H2 is performed via computing the adsorption energy, and then the electronic properties are analyzed based on vdW-DFT calculations Finally, hydrogen adsorption isotherms of the Co-MIL-88A are computed by GCMC simulations

- Chapter 4: Hydrogen storage in MIL-88 series

MIL-88 series including MIL-88A, B, C, and D is considered in this chapter for hydrogen storage capacity GCMC simulations quantitatively assess the H2uptakes of the MIL-88s sorbents via the H2 adsorption isotherms at 77 K and

298 K with the pressures below 100 bar using the GCMC simulations The vdW-DF calculations elucidate the interaction between the H2 molecule and the MIL-88 series

- Chapter 5: Effects of transition metal substitution in MIL-88A on hydrogen adsorption

In this research, I perform to evaluate hydrogen storage capacity of MIL-88A and the effects of transition metal substitution on hydrogen storage capacity in M-MIL-88A where M is the trivalent transition metal (Sc, Ti, V, Cr, Mn, Fe, and Co) Moreover, the adsorption energies of H2 with M-MIL-88A at the side-on and end-on adsorption configurations closing to the metal centers are calculated by the vdW-DF approach to search the most stable configurations Besides, electronic properties are also clarified for the stable adsorption configurations Via the GCMC simulations, the hydrogen adsorption isotherms

at 77 K and 298 K and the isosteric heats of hydrogen adsorption in 88A series are also studied

M-MIL Chapter 6: Conclusions

This chapter highlights the main findings, scientific contributions, and an outlook in the near future

LITERATURE REVIEW OF METAL-ORGANIC CHAPTER 1:

FRAMEWORKS

In this chapter, we provide an overview of metal-organic frameworks and their common applications such as gas storage, gas capture, separation of mixture gas, biomedical application and so on in recent years Moreover, this dissertation shows the overview of synthesis methods and theoretical researches for MOF materials Furthermore, the prediction of MIL-88s for hydrogen storage is introduced

The results of this chapter are only partly reproduced from the paper Organic Frameworks: State-of-the-art Material for Gas Capture and Storage” by T T

“Metal-T Huong, P N Thanh, N “Metal-T X Huynh, and D N Son, VNU Journal of Science:

Mathematics – Physics, vol 32, pp 67-84, 2016 [23]

General overview of metal-organic frameworks

1.1.

1.1.1 Definition of metal-organic frameworks

Metal-organic framework (MOF) materials are the compounds constructed by two main components that are inorganic metal ions/clusters and organic ligands/linkers

[24] Figure 1.1 shows a simple topology of MOF [25] consisting of metal nodes

connected to organic linkers to form a three-dimensional (3D) framework For example, Figure 1.2c represents MOF-5 structure with 1,4-benzenedicarboxylate (H2BDC) or BDC2- linker (Figure 1.2a) and Zn4O(CO2)6 cluster (Figure 1.2b)

Figure 1.1 Simple topology of MOFs

+

Organic linker

Metal ion/

cluster

a) BDC2- linker b) Zn4O(CO2)6 cluster c) MOF-5

Figure 1.2 Structure of MOF-5 [26]

1.1.2 Structural aspects of MOFs

Primary building units

1.1.2.1.

The metal ions ( i.e connectors) connecting with the organic linkers (or ligands)

are basic primary units resulting in the porous 3D structure of MOFs [26–30] Therefore, metal ions and organic compounds are used as the primary building units (PBUs) of MOFs

a Metal ions (connectors)

Transition metal ions are often used as versatile connectors in the construction of MOFs Common transition metal ions in MOFs are Zn2+, Co2+, Ni2+, Cu2+, Fe2+, Fe3+,

Cr3+ that are the first-row transition metal ions Besides, other metal ions are also often used for constructing MOFs such as alkali metal ions (Mg2+), alkaline-earth metal ions (Al3+) and rear-earth metal ions (In3+, Ga3+) The important features of metal connectors are coordination numbers and coordination geometries Depending on the metals and their oxidation states, coordination numbers can commonly be 2 – 6 for transition metals, or 6 – 12 for lanthanides with various geometries (Figure 1.3) including linear, T- or Y-shaped, square-planar, tetrahedral, square-pyramidal, octahedral, or polyhedral coordination geometries which plays a vital role in the construction of the MOF structures [30]

Figure 1.3 Several common coordination geometries of metal ions used for MOF

construction The numbers indicate the numbers of functional sites [30]

b Organic ligands

The organic ligands/linkers generally contain coordinating functional groups such as carboxylate, phosphate, sulfonate, amine, and nitrile Common ligands of MOFs are benzene-dicarboxylate (BDC), benzene-tricarboxylate (BTC), polycarboxylate (BTB), imidazole, pyrazole, triazole, tetrazole, and mixed ligands (seen more detailed in Table A1) Figure 1.4 shows some examples of organic ligands

Figure 1.4 Several common organic ligands used for MOF construction [28]

Secondary building units

1.1.2.2.

Organic linkers of MOFs are connected via metal-oxygen-carbon clusters, instead of metal ions alone These metal-oxygen-carbon clusters are called secondary building units (SBUs) [31, 32] SBUs have intrinsic geometric properties, facilitating

the topology of MOFs Figure 1.5 shows several common SBUs for MOFs, where metal polyhedrons, O, and C atoms are in blue, red, and black, respectively The polygons or polyhedrons defined by carboxylate carbon atoms (extension points) are

in red The carboxylate carbons of these units are at the vertices of the trigonal prismatic geometries

Figure 1.5 Several common SBUs of MOFs [31, 32]

1.1.3 History of MOFs

Metal-organic crystals with open spaces were firstly identified by Alfred Werner, father of coordination chemistry, over 100 years ago The term coordination compound originally was referred to as the Co(NH3)6Cl3 compound Since the earliest days of solid-state chemistry, these crystal structures have an apparent capacity for the host-guest adsorption Nevertheless, these frameworks were usually too fragile to maintain permanent porosity, and the removal of guest molecule resulted in the collapse of these structures, leading them non-porous MOFs, initially known as porous coordination networks (PCNs), microporous coordination polymers (MCPs),

a) Three points of extension b) Four points of extension

c) Four points of extension d) Six points of extension

e) Six points of extension f) Eight points of extension

zeolite-like metal-organic frameworks or porous coordination polymers (PCPs), invented in the 1990s overcame these limitations [33] In 1990s, the research group of Prof Omar Yaghi (University of California Berkeley) successfully synthesized a series

of MOFs named isoreticular metal-organic frameworks (IRMOFs), including

IRMOF-1 or MOF-5 (i.e Zn4O(BDC)3) (Figure 1.2) [26] with the Brunauer-Emmett-Teller surface area (SBET) of 3800 m2/g, which is one of the most common MOFs nowadays and the first MOF investigated for hydrogen adsorption [34] Subsequently, many new MOFs have been designed and synthesized with much progress in both quality and quantity Figure 1.6 shows the number of 1D, 2D, and 3D MOFs designed and synthesized up to 2015, reported in the Cambridge Structural Database (CSD) [35]

During the last two decades, MOFs continuously set new records in terms of specific surface areas, pore volumes, and gas storage capacities MOF-177 and MOF-

210 are the two of MOFs which have been technically tested for hydrogen (H2) storage and carbon dioxide (CO2) capture with exceptionally high storage capacity at 77 K and relatively low pressure (under 100 bar) [36, 37] Reported to date, NU-109 and NU-

110 (Table A2) exhibited the highest experimental SBET with 7000 m2/g and 7140 m2/g (Figure 1.7), respectively [38] The theoretical researchers also estimated based on the

limit of the MOF surface areas, and the result showed that the hypothetical maximum

BET surface area of MOF could reach ca 14600 m2/g or even higher [38] Nowadays, thousands of different types of MOFs have been known, and they have been continuously developing further [39] In general, the specific surface area of MOFs is much larger than the average surface area of other traditional inorganic materials such

as zeolites and silicas (smaller than 1000 m2/g), and activated carbons (< 2000 m2/g), seen in Figure 1.8 Pore volume is also one of the most important characteristics affecting the adsorption capacity of the porous materials Figure 1.8 shows that the pore volume of MOFs is much larger than that of traditional porous materials such as zeolites, silicas and carbons leading to enhance gas storage/capture

Figure 1.7 NU-110 structure with the highest BET surface area of MOFs reported until now

[23] with CCDC data taken from [38]

Figure 1.8 BET surface areas (m2/g) and pore volumes (cm3/g) of representativeMOFs, compared to conventional porous materials (zeolites, silicas, and activated carbons) From

Ref [29] Reprinted with permission from AAAS

1.1.4 Nomenclature of MOFs

MOFs have been named either by a sequence of isoreticular (same topologies) synthesis, the sequential number of synthesis/chronological order of discovery or the initials of the Institution or Laboratory where they were first synthesized [33]

Naming by the sequence of isoreticular synthesis

1.1.4.1.

Some MOFs are named based on the presence of the same network topologies,

i.e they share a common cubic topology constructed from the same type of organic

ligands For example, the zinc-based isoreticular MOF (IRMOF) series was synthesized including from IRMOF-1 to IRMOF-16 [40]

Naming by the sequential number of synthesis

Naming by the initials of Institution or Laboratory of discovery

1.1.4.3.

Another method of naming MOFs is that the formation of an acronym from the name of the institution or laboratory where the MOF was originally synthesized For example, HKUST = HongKong University of Science and Technology, MIL= Material Institute Lavoisier, NENU = North East Normal University China, VNU = Vietnam National University, NU = Northwestern University (Table A3)

1.1.5 Current research of MOFs in Vietnam

In Vietnam, MOFs have been studied by several research groups, e.g., the experimental research group of Nam T S Phan (Faculty of Chemical Engineering, HCMC University of Technology) This group has synthesized many MOFs such as MOF-5 (IRMOF-1), IRMOF-3, IFMOF-8, MOF-199, Cu(BDC), Cu2(BDC)2,

Co2(BDC)2(DABCO), Cu2(BPDC)2(DABCO) and has studied their applications in heterogeneous catalysis [41–49] Some noticeable results have also been achieved by the other research groups such as the Center for Innovative Materials and Architectures (INOMAR), VNU-HCM [50, 51]; Institute of Materials Science and Institute of Chemistry, Vietnam Academy of Science and Technology (VAST), Ha Noi [52, 53], Institute of Chemical Technology, Vietnam Academy of Science and Technology [54]; University of Sciences, Hue University [55, 56], University of

Science, VNU-HCM, etc Remarkably, VNU MOF series (VNU = Vietnam National

University), which are VNU-10, VNU-15, VNU-17, VNU-18, VNU-20, VNU-23 and

so on, has been completely synthesized and evaluated for several potential applications such as heterogeneous catalysis, proton conduction [57–61] In addition to our computational research group, the groups of Dr Nguyen-Nguyen Pham-Tran (Faculty

of Chemistry, University of Science, VNU-HCM) [62–64] and Dr Hung M Le (INOMAR) [65–67] also have studied the MOFs by DFT and GCMC simulation methods

Major applications of MOFs

1.2.

Figure 1.9 Application areas of porous reticular materials (MOFs, ZIFs and COFs) [35]

Due to the flexible combination of organic and inorganic components, MOFs offer many outstanding structural characteristics such as exceptionally large surface areas, high pore volume, ultrahigh porosity, complete exposure of metal sites, and high mobility of guest species in the nanopores of frameworks MOFs can be widely used for many applications such as catalysis, gas capture and storage, gas

separation/purification, sensing, biological application, and semiconductors, etc

(Figure 1.9) [26, 35, 39, 68–75]

1.2.1 Gas storage, capture, and separation

Current techniques for gas storage are cryogenic vessels, high-pressure tanks or storing by chemisorption, physisorption methods, and so forth These methods have achieved the storage target at nearly practical levels; nevertheless, essential improvements and cost reduction are required to most of them For example, the

Applications

of Reticular Materials

Gas storage

Gas capture

Gas separation

Gas purification

Water capture

Proton conductors

Sensors Opto-

electronic materials

Luminescent materials

Drug carriers

Heterogeneous catalysts

catalysts

Photo- catalysts

Electro-Electrical energy storage

pressurized tank-based hydrogen storage suffers from safety and economic issues The chemisorption approach allows the formation of chemical bonds between the adsorbed gases and the storage materials, leading to a greater gas storage density, but the kinetics, reversibility and heat management are still challenging [39] MOFs, one class

of porous materials, have researched for gas storage or capture primarily based on physical adsorption The advanced characteristics of MOF-based storage technologies compared to other techniques that are fast kinetics and absolute reversibility; therefore, MOFs used for gas storage could reduce the cost due to easy desorption of the adsorbed gases and the reusability of MOFs

Hydrogen storage

1.2.1.1.

As introduced above, the use of energy from hydrogen gas is environmentally friendly and non-toxic under normal conditions of temperatures and pressures Thus, it can meet the global consumption requirement for the crisis of energy in the near future Owing to the volatile property of hydrogen under ambient conditions, hydrogen storage for onboard usage must be in extremely high-pressure conditions leading to expensive and unsafe, and extremely dangerous problems MOF materials having ultra-high surface areas, large pore volumes with the advantages of physisorption-based materials are of particular interest for gas storage, especially in hydrogen storage

So far, various MOF materials have proved high hydrogen storage capacity compared to other methods such as the empty tank and the zeolite X13 (Figure 1.10)

[7–9, 11, 74] In 2003, hydrogen storage was firstly investigated on MOF-5 with the gravimetric uptakes of 4.5 wt% (at 78 K, 0.8 bar) and 1 wt% (at 298 K, 20 bar) [34] This report has attracted much attention and opened a new research direction for

computational simulations In 2004, Hüber et al was the first group using MP2 (the

second-order Møller-Plesset perturbation theory) calculations to clarify the interaction

of hydrogen with benzene and naptalin via the adsorption energy of molecular hydrogen with the obtained values were 3.91 and 4.28 kJ/mol, respectively [76] Besides, quantitatively evaluate of hydrogen storage in MOF-5 was first calculated in

2004 using GCMC simulations with the universal force field (UFF) by Ganz group

[77] This research showed a high energy binding site at the corners of the MOF and

the uptake quickly saturated with 1.27 hydrogen molecules per Zn4O(BDC)3 formula

unit (i.e 4.5 wt %) at 78 K By GCMC simulations, many research groups also

investigated the adsorption isotherms of H2 gas for MOF structures with force fields including DREIDING force fields [78], the optimized potential for liquid simulations (OPLS) force fields [79] (see Ref [80] for a detailed overview of the use of these force fields)

Figure 1.10 H2 storage in MOFs relatively compared to zeolite and empty tank Reproduced

with permission of Royal Society of Chemistry from Ref [74]

The experimental record in the highest total (or absolute) hydrogen storage capacity reported so far was found in MOF-210 with 17.6 wt% (176 mg/g) at 77 K and

80 bar, while in this same condition the excess uptake obtained 8.6 wt% (86 mg/g)

[36] Meanwhile, the greatest excess H2 uptake is of NU-100 with the value of 9.95 wt% at 56 bar and 77 K (absolute uptake: 16.4 wt% at 70 bar) [81] Additionally, there are a huge number of potential MOFs that demonstrated a considerable capability for hydrogen storage such as MOF-200 with 7.4 excess wt% and 16.3 total wt% at 77K and 100 bar [81], MOF-205 with 7.0 excess wt% and 12.0 total wt% at 77 K and 80 bar [36] Due to the weak H2@MOF interaction and the low isosteric heat of H2adsorption (typically 4 – 13 kJ/mol), MOFs exhibited significant hydrogen uptakes only at cryogenic temperature and quite low hydrogen uptakes at room temperatures and the pressures under 100 bar (Table 1.1) At room temperatures, the highest uptakes

to date are ca 1 wt% for excess uptake and 2.3 wt% for absolute uptake Although

none of MOFs has reached the DOE targets under moderate temperature and pressure

conditions, they contain several key characteristics that are expected to improve and ultimately produce new MOFs with exceptional properties for hydrogen storage Various strategies for improving the storage capacity at ambient temperature have been suggested One of the most strategic solutions to enhance hydrogen adsorption is using the MOF containing open metal sites Isosteric heat of hydrogen adsorption in the range of 15 – 25 kJ/mol is also recommended to increase the ability to adsorb hydrogen in MOFs required for onboard applications [82] In recent years, the supports from computer simulations allow predicting and designing new MOFs that can significantly improve hydrogen uptakes [9, 83]

Table 1.1 High hydrogen uptakes in MOFs at 298 K and pressures below 100 bar

Volumetric (g/L)

Methane storage

1.2.1.2.

Methane (CH4) gas is one of the most important hydrocarbon-based fuels providing high energy density together with low carbon emission after the combustion process because of its great hydrogen-to-carbon ratio [101] CH4 storage in MOFs was first established from the research group of Kitagawa and co-workers in 1997 but with very limited methane uptake (2.3 mmol/g at 30 atm, 298 K) [102] They synthesized the coordination polymers with 3D frameworks and large cavities, which were used to adsorb a significant amount of CH4 by the diffusion of the gas into the cavities compared to the empty tank and the active carbon (Figure 1.11) [74] Since then, many MOFs have been studied for methane storage New MOFs have been synthesized with

a variety of important factors such as high surface areas, ligand functionalization, open

metal sites, etc., which have led to the significant improvements in CH4 adsorption capacity [26, 72] Additionally, computational simulations by first-principles methods have indicated that the creating of open metal sites within MOFs can increase the binding strength of methane with the metals by high affinity created at these metal areas [103] Recently, H Wu et al has examined on six promising MOFs for CH4storage including PCN-14, UTSA-20, HKUST-1, Ni-MOF-74 (Ni-CPO-27), NU-111 and NU-125 [104] The result showed that HKUST-1 reached the highest volumetric uptakes of methane that are 230 cc(STP)/cc at (298 K, 35 bar) and 270 cc(STP)/cc at (298 K, 65 bar), which holds the record of methane uptake to date and meets the new volumetric target recently set by the DOE that is 263 cc(STP)/cc at 298 K and 65 bar Meanwhile, other MOFs such as NU-111, Ni-MOF-74 and PCN-14 have reached up to 70% of the new DOE gravimetric and volumetric targets For the gravimetric target, MOFs only obtained under 0.5 g/g of DOE target [104]

Generally speaking, methane storage have been met the DOE′s targets but further development is required for more economical competence while the targets for hydrogen storage are currently unreachable However, many strategies to enhance the hydrogen storage capacity have been developed such as the creation of open metal sites, doping with metal ions, fabrication of metal nanoparticles to utilize the spillover effect, functionalization of the ligands, and catenation/interpenetration of the frameworks These strategies have shown a considerable improvement of the storage

capacity of the gases that makes MOFs becoming the leading material for both methane and hydrogen storage

Figure 1.11 CH4 storage in MOFs relatively compared to active carbon and empty tank Reproduced with permission of Royal Society of Chemistry from Ref [74]

CO 2 capture

1.2.1.3.

MOFs are reported to be useful for reducing the CO2 level in the atmosphere For the capture of CO2 in MOFs at high pressures, the first systematic study was carried out on nine MOF structures (in 2005) to find out the relationship between the surface area and CO2 uptake capacity [37] The results showed that the saturated gravimetric and volumetric CO2 uptakes are qualitatively correlated with the surface areas of the MOFs They found that MOF-177 has the highest Langmuir surface area of 5640 m2/g and the CO2 uptake of 33.5 mmol/g at 35 bar and room temperature, which surpass any reported porous materials including the benchmark of zeolites 13X and activated carbon (MAXSORB) [37], seen in Figure 1.12 In investigated MOF series, MOF-200 and MOF-210 showed the CO2 uptake approximately 2400 mg/g at 298 K and 50 bar and set a new record for the adsorption capacity of CO2 among all porous materials (Figure 1.13)

Figure 1.12 Volumetric CO2 capacity of MOF- 177 relatively compared to zeolite 13X pellets, MAXSORB carbon powder, and pressurized CO2 Reproduced with permission from

Ref [37] Copyright © 2005, American Chemical Society

Figure 1.13 CO2 uptakes of MOFs at 298 K Reprinted from Ref [36] Reprinted with

permission from AAAS

Gas separation

1.2.1.4.

The research efforts have been dedicated not only to the development of MOFs for gas storage/capture applications but also for gas separation Up to date, a large number of MOFs have shown the promising potential of the highly selective separation of gas mixtures by rationally and systematically tuning pore/window sizes

[68, 69, 73, 105] In the problem of gas separation or purification, the hydrogen separation from gas mixtures is an important application in the industry With regard

to hydrogen-containing mixtures, most attention has been paid to the separation of