Defect Engineering Of UiO-66 Metal- Organic Framework (MOF) For .

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Defect engineering of UiO-66 metalorganic framework (MOF) forimproved hydrogen storageapplicationsByMPHO LEDWABASubmitted in partial fulfilment of the requirements for the degree ofDoctor of Philosophyin the Department of Chemistry and Polymer ScienceFACULTY OF SCIENCESTELLENBOSCH UNIVERSITYSupervisor: Prof. Leonard BarbourCo-Supervisor: Prof. Jianwei RenMarch 2021

Stellenbosch University https://scholar.sun.ac.zaDECLARATION BY CANDIDATEI hereby declare that the work found in this dissertation submitted for a degree at Stellenbosc hUniversity is solely my own original work and has not previously been submitted to any otherhigher education institution. I further confirm that all sources referenced or quoted herein areconfirmed and acknowledged by virtue of a list of references.March 2021Copyright 2021 Stellenbosch UniversityAll rights reservedii

Stellenbosch University https://scholar.sun.ac.zaDEDICATION“The mind of the prudent acquires knowledge”“The ear of the wise seeks knowledge”“It is not good for a person to be without knowledge”“Listen to counsel, receive instruction and accept correction that you may be wise in the timeto come.”“Train up a child in the way she should go, teaching her to seek Gods wisdom and will for hisabilities and talents.”These are a few of many teachings from the book of ProverbsI dedicate this research to my mother, Priscilla Ledwaba who firmly instilled these teachingsin my life, encouraging me to pursue what my heart desires regardless of the circumsta ncesthat seemed like a barrier. In her financial struggles, she ensured that I achieved my goals andpushed me forward to where I am today. I write this with tears of joy in my eyes, grateful thatyou are alive to see the product of your strength.iii

Stellenbosch University https://scholar.sun.ac.zaACKNOWLEDGEMENTSI would like to first and foremost give thanks my heavenly Father, the Sovereign Master andCreator of all things. It has not been by power or wisdom but only by Your Spirit and Gracethat I arrived at the finish line. You have carried me through the ups and downs (and yes therewere plenty of downs) and when I wanted to give up, You lifted me up encouraged me withyour unfailing word and gave me new strength.Your word in Psalms says “weeping may last for a night, but JOY comes in the morning”Oh LORD my GOD I give Glory to YOU forever!!!This would not have been possible without the assistance of my supervisors Profs Len Barbourand Jianwei Ren. Thank you for transferring knowledge, for the guidance and for pushing meto achieve my goals. According to the Oxford Dictionary a supervisor is someone who directsand oversees the work of another, and I can say that you have fulfilled that duty exceptiona lly.The advice you gave me showed that not only were you doing what you had to, but you wantedto better me as a person and a researcher not only for this project but also for the future. I amtruly grateful.I acknowledge the Hydrogen South Africa (HySA) team for being such a great supportstructure. Thank you for every contribution you have made towards my work. Dr Musyoka, Ithank you for all your assistance. You are the backbone of this work. Your endless supply ofadvice and guidance didn’t go unnoticed. Dr H and Dr North, thank you for all the technica lwork you did backstage. Thank you for the regular check-up; I appreciate each of you.I thank my husband, Kgositsile Mogaswane, for always being there for me. The late nights atwork, listening to my complaints when things were not working in the lab, I would have brokendown if you were not by my side. Thank you for being my pillar of strength.iv

Stellenbosch University https://scholar.sun.ac.zaAdditionally, I would like to acknowledge CSIR for hosting me and financing my studies undera Doctoral Studentship. Through this studentship, you provided the training and facilitiesneeded for me to realise completion of this research. I would also like to thank Stellenbosc hUniversity for all the support especially Dr Marike Du Plessis for all the administrative sopportas well as the National Research Foundation (NRF) for the financial support.Finally, I would like to extend sincere gratitude to my fellow EC colleagues/friends for theirassistance and for creating a friendly research environment.v

Stellenbosch University https://scholar.sun.ac.zaABSTRACTMetal–organic frameworks (MOFs) as a relatively new class of porous crystalline materia lshave attracted much interest in many applications due to their high porosity, diverse structures,and controllable chemical structures. However, the specific geometrical morphologies, limitedfunctions and unsatisfactory performances of pure MOFs hinder their further applicatio ns .Several modifying strategies for engineering MOF crystals have been developed based on theirdesired features and applications. In nature there are no "ideal crystals" with an infinite periodicrepetition or ordering of the same groups of atoms in space. The structure of "real crystals"often deviates from the ideal arrangement and includes a significant density of structuralirregularitiesor ositio na linhomogeneities, and this term is often used interchangeably. By using defective technologystrategies during their synthesis, crystal imperfections may be introduced into the MOFframework, thereby enhancing their performance in the envisioned applications. Defectengineering is one of the most effective approaches that one can use to change their physicaland chemical features such as thermal stability, textural properties, mechanical properties andgas adsorption abilities. In order to achieve the desired changes, it is essential to control thedefects, otherwise these defects may have an adverse effect on the MOFs. Therefore, it is vitallyimportant to apply synthetic control over defects; the exact nature and concentration of thedefects may be controlledby modifying the syntheticconditions and post-syntheticmodifications. Structurally characterising inherent or engineered defects is very challengingand this challenge has not been addressed substantially.This thesis explores the experimental creation of structural defects via post-syntheticmodification, the role of structural defects and their relationship to gas adsorption, withvi

Stellenbosch University https://scholar.sun.ac.zaemphasis on hydrogen adsorption. Through a combination of techniques, including powder Xray diffraction (PXRD), thermogravimetric analysis (TGA), acid-base titration and Brunauer–Emmett–Teller (BET) surface area and pore size measurements, missing linkers and missingcluster defects have been identified and analysed. In Chapter 4, we seek to understand therelationship between some of the major synthetic parameters and the physicochemic a lcharacteristics of UiO-66 (Universitetet i Oslo) MOF and discover a "non-defective" samplingtechnique for this material. The technique produces non-defective UiO-66 MOFs at a specifictemperature (493 K), with the linker ratio being greater than that of the salt previously reportedby Shearer et al. As described in Chapter 5, by varying the concentrations of modulator and thelinker, we demonstrate that the linker vacancies can be systematically tuned, leading tosignificantly increased surface areas. The defects are caused by partial terephthalic acidreplacement with smaller formate groups from the formic acid modulator. The BET surfaceareas of the obtained samples range from 1200 to 1600 m2 .g-1 , and the best sample has a surfacearea that is about 30% higher than the theoretical value of the surface area of defect-free UiO66. Additionally, linker vacancies are proven to have profound effects on the gas adsorptionbehaviour of UiO-66 by improving the hydrogen uptake from 1.51 wt. % to 2.0 wt. % at 77 Kand 1 bar. Chapters 5 and 6 include detailed studies of two conventional methods for generatingdefects (i.e., de novo defect technology and post-synthetic modification (PSM)) discussed ineach chapter respectively. Still in chapter 5, experimental investigations are discussed thatshow the impact of modulator and linker concentration on H2 adsorption and thermal stability.Chapter 6 provides insight into the impact on thermal stability and adsorption propertiesbrought about by the post-synthetic modification methods. The resultant materials typicallyhave high surface areas, large pore volumes and structures with hierarchical pores, whichmakes them more practical for hydrogen storage applications.vii

Stellenbosch University se raamwerke (MOFs) as ′n relatief nuwe klas van poreuse, kristallyne materiale lokbaie belangstelling in vele toepassings as gevolg van hul hoë poreusheid, diverse strukture enbeheerbare chemiese strukture. Die spesifieke geometriese morfologie, beperkte funksies enonbevredigende werkverrigting van suiwer MOFs beperk egter hul verdere toepassing. Heelwatmodifiseringstrategieë vir die ontwerp van MOF kristalle, gebaseer op hul gewensde eienskappe, isreeds ontwikkel. In die natuur is daar nie "ideale kristalle" met ′n oneindige, periodiese herhaling ofgespasieerde orde van dieselfde groepe atome nie. Die struktuur van "regte kristalle" wyk dikwels afvan die ideale rangskikking en sluit ′n aansienlike digtheid van strukturele onreëlmatigheid of gebrekein. Kristalonreëlmatighede kan voorkom wanneer die samestelling nie homogeen is nie en hierdie termeword dikwels afwisselend gebruik. Deur gebruik te maak van defek-tegnologie strategieë gedurendesintese, kan kristalonvolmaaktheid in die MOF raamwerk ingebou word en so hul werkverrigting in diebeoogde toepassings versterk. Defek-konstruksie is een van die mees effektiewe benaderings watgebruik kan word om die fisiese en chemiese eienskappe, soos byvoorbeeld termiese stabiliteit,teksturele eienskappe, meganiese eienskappe en gassorpsievermoë, van MOFs te verander. Om dieverlangde veranderinge te bereik, is dit noodsaaklik om die defekte te beheer, anders kan die defekte ′nongunstige effek op die MOF hê. Daarom is dit uiters belangrik om beheer oor defekte tydens sintesetoe te pas; die presiese aard en konsentrasie van defekte kan beheer word deur die toestande tydenssintese te wysig, asook deur na-sintetiese modifikasie. Om inherente-, of ingeboude defekte struktureelte karakteriseer, is uitdagend en hierdie uitdaging is nog nie aansienlik aangespreek nie.Hierdie tesis ondersoek die eksperimentele inbou van strukturele defekte deur middel van de-novo enna-sintetiese modifikasie. Hierin ondersoek ons ook die rol van strukturele defekte en hoe hul verbandhou met gasadsorpsie, met ′n klem op waterstof adsorpsie. Deur gebruik te maak van ′n kombinasie vantegnieke, insluitend poeier X-straaldiffraksie (PXRD), termogravimetriese analise (TGA), suur-basistitrasie, Brunauer–Emmett–Teller (BET) oppervlak-area en meting van porie-grootte, kon vermisteskakels en bondeldefekte geïdentifiseer en geanaliseer word. In Hoofstuk 4 poog ons om die verhoudingtussen sommige van die hoof sintetiese parameters en die fisikochemiese eienskappe van UiO-66viii

Stellenbosch University https://scholar.sun.ac.za(Universitetet i Oslo) MOF te verstaan en ontdek ′n "nie-defektiewe" monsternemingstegniek virhierdie materiaal. Hierdie tegniek produseer nie-defektiewe UiO-66 MOFs by ′n spesifieke temperatuur(493 K), met ‘n skakelverhouding groter as dié van die sout wat voorheen deur Shearer et al.gerapporteer is. Soos beskryf in Hoofstuk 5, deur die konsentrasies van die modulator en die skakel tevarieer, demonstreer ons dat die skakelgapings sistematies gestel kan word, wat lei tot ′n merkwaardigetoename in oppervlak-areas. Die defekte word veroorsaak deur die gedeeltelike vervanging vantereftaalsuur met kleiner formaatgroepe, afkomstig van die formaatsuur modulator. Die BET oppervlakareas van die monsters wat verkry is, strek van 1200 to 1600 m2 .g-1 en die beste monster het ′noppervlak-area ongeveer 30% hoër as die teoretiese waarde vir die oppervlak-area van defek-vrye UiO66. Verder is dit bewys dat skakelgapings groot effekte het op die gasadsorpsie gedrag van UiO-66 deurdie waterstofopname van 1.51 wt. % tot 2.0 wt. % by 77 K en 1 bar te verbeter. Hoofstukke 5 en 6 sluitnoukeurige studies in van twee konvensionele metodes om defekte te genereer (naamlik de novo defektegnologie en na-sintetiese modifikasie (PSM)) soos bespreek in elke hoofstuk afsonderlik. Steeds inHoofstuk 5, word eksperimentele ondersoeke bespreek, wat die impak van modulator en skakelkonsentrasie op H2 -adsorpsie en termiese stabiliteit wys. Hoofstuk 6 verskaf insig ten opsigte van dieinwerking op termiese stabiliteit en adsorpsie eienskappe teweeggebring deur die na-sintetiesemodifikasie metodes. Die gevolglike materiale het tipies hoë oppervlak-areas, groot porievolumes enstrukture met hiërargiese porieë, wat dit meer prakties maak vir waterstof-storingstoepassings.ix

Stellenbosch University https://scholar.sun.ac.zaTABLE OF CONTENTSDECLARATION BY CANDIDATE . IIDEDICATION . IIIACKNOWLEDGEMENTS . IVABSTRACT. VIOPSOMMING . VIIILIST OF FIGURES . XVLIST OF TABLES . XXIIPUBLICATIONS.XXVSYMPOSIUMS/CONFERENCE PRESENTATIONS .XXVCHAPTER 1: INTRODUCTION . 11.1Background . 11.2Problem statement . 51.3Significance . 51.4Purpose of study . 71.5Aims of the project . 71.6Objectives of the project are to: . 71.7Thesis outline . 81.8References . 10CHAPTER 2: LITERATURE REVIEW . 132.1Design and Synthesis of MOFs . 132.2Characterisation of MOFs . 17x

Stellenbosch University https://scholar.sun.ac.za2.3MOFs for energy applications. 182.4Background on UiO-66 . 222.5Designation and classification of defects . 232.6Formation of structural defects in MOFs . 26Solvent systems . 28Nature of modulating systems. 29Quantity of modulators . 31The nature of the metal clusters . 32Inherent defects . 322.7Defect engineering . 33Defects formed during de novo synthesis. 34Defect formation by post-synthetic treatment. 362.8Detection of structural defects in MOFs . 37PXRD: The presence of prohibited symmetry reflections . 38HRNPD: Presence of missing-linker defects . 39DRIFTS spectra with vibrational modes . 40TGA: less weight loss than theoretically expected . 41SBET/Vpore measurements . 43FTIR: multiple OH stretching bands . 442.92.10Conclusion. 46References. 47CHAPTER 3: INSTRUMEN TATION . 66xi

Stellenbosch University https://scholar.sun.ac.za3.1Scanning Electron Microscopy (SEM) . 663.2Powder X-ray diffraction (PXRD) . 663.3Thermogravimetric analysis (TGA) . 673.4Differential scanning calorimetry (DSC) . 673.5BET surface area and pore size determination . 673.6Hydrogen adsorption measurements . 693.7Mechanical studies . 693.8Hydrogen adsorption at high pressure. 703.9Fourier-transform infrared spectroscopy (FT-IR) . 713.10Potentiometric acid-base titrations . 723.11References. 73CHAPTER 4: SYNTHESIS OF NON-DEFECTIVE UIO-66 MOF. 744.1Abstract . 744.2Synthesis. 75Reagents and che micals . 75Procedure . 754.3Results . 75SEM, TEM and EDS results . 76TGA and DSC results . 77PXRD patterns . 78IR spectra. 79Raman Spectroscopy . 80Pore size distribution . 83xii

Stellenbosch University https://scholar.sun.ac.zaN2 adsorption isotherm . 83Hydrogen uptake isotherm. 84High pressure hydrogen uptake . 854.4Adsorption theories . 86Adsorption equilibrium . 86Adsorption kinetics . 87Activation energy for diffusion and heat of adsorption . 884.5Conclusion. 944.6References . 96CHAPTER 5: DE-NOVO DEFECT ENGINEERING . 995.1Modulator variation . 99Abstract. 99Synthesis. 100Results and discussion . 1025.2Linker variation . 107Abstract. 107Synthesis. 107Results and discussion . 1085.3Conclusion. 1155.4References . 117CHAPTER 6: POST-SYNTHETIC MODIFICATION. 120xiii

Stellenbosch University https://scholar.sun.ac.za6.1Post-synthetic modification via acid treatment . 120Abstract. 120Preparation . 120Results and discussion . 1206.2Post-synthetic modification via exertion of mechanical force . 127Abstract. 127Preparation . 127Results and discussion . 1286.3Comparison of optimised conditions. 1336.4Conclusion. 1346.5References . 135CHAPTER 7: CONCLUSION AND RECOMMENDATIONS . 1377.1References . 140xiv

Stellenbosch University https://scholar.sun.ac.zaLIST OF FIGURESFigure 1.1 An evaluation of hydrogen and petrol in the energy supply chain currency . 3Table 1.1. Combustion characteristics of hydrogen compared to other fuels . 4Figure 2.1. Methods of synthesis, potential temperatures and finished products. . 14Figure 2.2. The building block, or ‘modular' approach behind the formation of coordinatio npolymers. 15Figure 2.3. A linear ligand (purple spacing) joining a range of octahedral building blocks(yellow clusters) such as cations (A), metal clusters (B), or octahedral metal organicmolecules (C) can be fashioned to form porous primitive cubic networks (D) (metal,green; oxygen, red; nitrogen, blue; carbon, grey; hydrogen, white). Polyhedral buildingcomponents such as C provide a great level of control over the resulting structure and canbe assembled step-by-step in a reversible way. . 20.Figure 2.4. Left panel: Crystal structures of four representative MOFs, (A) MOF-5, (B)HKUST-1, (C) Mg-MOF-74, (D) ZIF-8. Colours: C, grey; O, red; and N, blue. The yellowsphere signifies the porosities, and the yellow cylinders display the topology of ZIF-8.Reprinted with permission from Ref. [50]. Right panel: An illustration of the UiO-66structure. (A) The fcc UiO-66 structure composed of the metal node (blue) and ligand(gray) with an atomic representation of the node and 12-connected terepthalic acid linkers.(B) The node and ligand structure with the 12 Å UiO-66 octahdron cage. (C) The nodeand ligand structure with the 7.5 Å tetrahedron cage. (D) The color scheme for the atomicrepresentation. . 23Figure 2.5. Defining defects: missing and improperly located atoms cause vacancies andmaterial dislocation. . 24xv

Stellenbosch University https://scholar.sun.ac.zaFigure 2.6. Structural description of UiO-66(Hf) defective nano-region. (a) Polyhedralillustration of the (ordered) reo defect structure single unit cell. (b) Defect-rich nanoregions are dispersed across the defect-free fcu matrix. (c) Atomic depictions of defectnano-regions in UiO-66(Hf) may be used for defect concentration and domain sizeparameters which were experimentally identified. . 26Figure 2.7. Different strategies to introduce defects in MOF structures . 28Figure 2.8. Proposed PSE process for non-defective and defective UiO-66. MeOH enables theexchange of ligands by the formation and stabilization of defects. Differences in enthalpyare given in kJ mol-1 at 313 K. . 29Figure 2.9. (a) modulated stepwise growth of the UiO-type Zr-MOF network. (b) The proposedreaction scheme for defective Zr-MOF development by means of H2 bpdc as a ligand inexcess formic acid modulator (reaction I), and the replacement of anions, capping defects,with organic acid (here, acetic acid) (reaction II). . 30Figure 2.10. Schematic showing thermal removal of TFA from UiO ‐66 creating open sites. 33Figure 2.11. Representation of all key MOF defects procedures. . 34Figure 2.12: Left panel: An indication of the number of Zr6 formula deficiencies in UiO-66 inrelation to different modulators. Reproduced with permission from Ref. Right panel: RuHKUST-1 mixed linker approach resulting in node errors. . 35Figure 2.13. (a) UHM-3 ball-and-stick representation: oxygen (red), hydrogen (grey), carbon(dark grey), copper (magenta) and silicon (yellow). (b) PXRD patterns for UHM-3SURMOF samples which were annealed at various temperatures. . 37Figure 2.14. (a) Missing linkers in UiO-66 partial unit cells, (b) diffuse reflections (110), (c)diffuse reflections (100), (500), and (d) diffuse reflections (500). . 38xvi

Stellenbosch University https://scholar.sun.ac.zaFigure 2.15. PXRD patterns recorded at various temperatures after heating UiO-66 samples for12 h. (a) 373 K; (b) 433 K; (c) 493 K. The two reflections labelled 'FR' and marked outby vertical dotted lines are prohibited. 39Figure 2.16. Experimental neutron powder diffraction profiles for dehydroxylated deuteratedUiO-66 at 4 K (circles), calculated (lines) and difference (line underneath the observedand estimated patterns). The missing linker defects were identified in the structural modelof Wu et al. by a linker occupancy parameter, of which 0.917 was the refined value. Theestimated locations of the B

characteristics of UiO-66 (Universitetet i Oslo) MOF and discover a "non-defective" sampling technique for this material. The technique produces non-defective UiO-66 MOFs at a specific temperature (493 K), with the linker ratio being greater than that of the salt previously reported by Shearer et al.

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