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Examensarbete 30 hp Juni 2018

Novel Hybrid Nanomaterials

Combining Mesoporous Magnesium Carbonate and Metal-Organic Frameworks

Viktor Sanderyd

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Teknisk- naturvetenskaplig fakultet UTH-enheten

Besöksadress:

Ångströmlaboratoriet Lägerhyddsvägen 1 Hus 4, Plan 0

Postadress:

Box 536 751 21 Uppsala

Telefon:

018 – 471 30 03

Telefax:

018 – 471 30 00

Hemsida:

http://www.teknat.uu.se/student

Abstract

Novel Hybrid Nanomaterials - Combining Mesoporous Magnesium Carbonate with Metal-Organic

Frameworks

Viktor Sanderyd

Nanotechnology as a field has the potential to answer some of the major challenges that mankind faces in regards to environmental sustainability, energy generation and health care. Though, solutions to these concerns can not necessarily rely on our current knowhow. Instead, it is reasonable to expect that humanity must adapt and learn to develop new materials and methods to overcome the adversities that we are facing. This master thesis has involved developing novel materials, serving as a small step in the continuous march towards a bright future where this is possible. More specifically, this work sought to combine mesoporous magnesium carbonate with various metal-organic frameworks to utilize the beneficial aspects from each of these constituents. The ambition was that these could be joined to

render combined micro-/mesoporous core-shell structures, with high surface areas and many active sites whilst maintaining a good

permeability. Numerous different synthesis routes were developed and explored in the pursuit of viable routes to design novel materials with potential future applications within for instance drug delivery, water harvesting from air and gas adsorption. Core- shell structures of the hydrophilic mesoporous magnesium carbonate covered with the hydrophobic zeolitic imidazole framework ZIF-8 was successfully synthesized for the first time, and practical studies demonstrated a dramatically enhanced water stability, which is perceived to have an impact on further research on these materials. ZIF-67 was also combined with mesoporous magnesium carbonate in a similar manner. Further, Mg-MOF-74 was grown directly from mesoporous magnesium carbonate, where the latter acted as a partially self-sacrificing template, with the aim of rendering a porous hierarchical structure with contributions from the micro- and mesoporous ranges. The outcomes of all these syntheses were characterized using several analyzing methods such as scanning electron microscopy, X-ray diffraction, energy

dispersive spectroscopy and nitrogen sorption analysis.

Handledare: Chao Xu

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Populärvetenskaplig sammanfattning

Benämningen nanoteknologi syftar till att designa och tillverka strukturer som har en längd i spannet 1-100 nm (nanometer) i åtminstone en dimension. Ett materials sammansättning på nanonivå påverkar dess egenskaper, och nanote- knologi går därmed ut på att styra denna uppbyggnad för att ge en önskad effekt på vår makronivå. För att få lite perspektiv på vad nanoskala faktiskt innebär, kan det nämnas att tjockleken på ett vanligt A4-ark motsvarar 100 000 nm. En nm kan alltså tyckas närmast infinitesimalt litet, och vid så små stor- lekar så ändras spelreglerna som vi känner till från vår makrovärld drastiskt - en nanopartikel kommer exempelvis att vara helt opåverkad av gravitation. Det handlar således om en värld där våra sinnen såsom syn och känsel inte är till någon nytta, och där krafterna som verkar är kontraintuitiva. Svåra utmaningar brukar dock vara förenade med stora belöningar. Lyckas mänskligheten fort- sätta bemästra nanoteknologin och dess möjligheter på det sätt som inletts de senaste decennierna, finns potential att revolutionera cancerbehandling, datala- gring, energiomvandling, vattenrening samt en uppsjö andra tillämpningar, där det i vanlig ordning endast är fantasin som sätter gränser.

Hybridmaterial är även det ett aningen diffust samlingsbegrepp, som avser sam- manslagning eller kombination av egenskaper. Inom materialvetenskap kombin- eras ofta strukturer för att få ut det bästa av olika materials parametrar, eller för att minimera någon oönskad effekt.

Mesoporös magnesiumkarbonat (MMC) är ett mycket poröst material som up- ptäcktes 2013 vid avdelningen för Nanoteknologi och Funktionella Material vid Uppsala Universitet. Det är ett amorft material, vilket innebär att dess struktur saknar en ordnad uppbyggnad på atomär nivå. Trots detta, går det att reglera storleken på dess porer genom syntesvariabler. När upptäckten tillkännagjordes erhöll det stort internationellt genomslag, eftersom gängse uppfattning varit att ett sådant magnesiumbaserat material varit mycket svårt att framställa.

Metallorganiska nätverk (Metal Organic Frameworks, MOF:s) är strukturer som utgörs av noder av metalljoner som binds samman av kolvätekedjor, så kallade ligander. Dessa ligander arrangeras på ett specifikt och repititivt sätt i alla dimensioner, vilket innebär att MOFs till skillnad från MMC är kristallina ma- terial som har en ordnad uppbyggnad. Gravt förenklat går det att se på MOFs som ett Legobygge - ligander är långsmala bitar, noderna är stora klossar. Sät- ten bitarna kan monteras ihop på må vara begränsade, men strukturerna som kan konstrueras blir i det närmaste oändliga till följd av att det finns så många olika varianter av bitar. MOFs har ofta en extremt hög porositet, och väldigt små porstorlekar - så kallade mikroporer, som är en storleksordning mindre än mesoporerna i MMC. Bland annat porositet, kemisk- och termisk stabilitet kan ändras utifrån vilka noder och ligander som nyttjas i MOF-strukturen. Detta medför också att egenskaperna olika MOF:s kan uppvisa är många, intressanta och kan specialiseras för olika tillämpningar.

Detta examensarbete har inriktats på att kombinera dessa grupper av mate-

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rial, och att utveckla syntesmetoder där de gynnsamt förenas. Ifall det på ett framgångsrikt sätt går att kombinera mikroporösa MOFs med mesoporös mag- nesiumkarbonat öppnar detta upp ett helt nytt spelfält av möjligheter. För att åskådliggöra potentialen, har en ny syntesmetod utvecklats där den något fuk- tkänsliga MMC:n ges ett hölje av en zinkbaserad MOF för att förbättra dess vattentålighet, något som varit eftersträvansvärt. En magnesiumbaserad MOF har också framställts direkt från MMC som utgångsmaterial, i syfte att bilda ett hierarkist poröst material, där porer av olika storlekar är gynnsamt förenade.

Dessa kombinerade mikro- och mesoporösa material har sedan noga karaktäris- erats med en rad analysmetoder. Strukturerna som tagits fram och besläk- tade material utifrån dessa har en mängd tänkbara framtida applikationer inom exempelvis läkemedelstransport i kroppen, vattengenerering från luft samt för gasadsorption och som sensormaterial.

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Contents

1 Introduction 1

1.1 Aim and purposes . . . 3

2 Background 3 2.1 Mesoporous Magnesium Carbonate . . . 3

2.2 Zeolitic Imidazolate Frameworks . . . 4

2.3 MOF-74 . . . 5

3 Experimental 6 3.1 Synthesis of MMC . . . 6

3.2 Pure MOFs . . . 6

3.2.1 ZIF-8 . . . 6

3.2.2 MOF-74 . . . 7

3.3 ZIFs@MMC . . . 8

3.3.1 Physical mixing of MMC and ZIF-8 . . . 8

3.3.2 In situ-reaction . . . 8

3.3.3 Chemical modification of MMC . . . 9

3.4 MOF-74@MMC. . . 10

3.5 Materials Characterization. . . 11

3.5.1 Scanning Electron Microscopy with Energy Dispersive X- ray Spectroscopy (SEM-EDS) . . . 11

3.5.2 Nitrogen Sorption Measurements . . . 11

3.5.3 X-ray Diffraction (XRD). . . 12

3.5.4 Thermogravimetric Analysis (TGA) . . . 12

3.5.5 Fourier Transform Infrared Spectroscopy (FTIR) . . . 12

3.6 Water Adsorption Analysis of ZIF-8@MMC . . . 13

3.6.1 Liquid Water Stability . . . 13

3.6.2 Adsorption of water vapor from air . . . 13

4 Results and Discussion 14 4.1 Mesoporous Magnesium Carbonate . . . 14

4.2 ZIF-8 . . . 14

4.3 ZIFs@MMC - Physical mixing. . . 16

4.4 ZIFs@MMC - In situ-route . . . 18

4.5 ZIFs@MMC - Chemical modification route. . . 19

4.5.1 SEM-EDS . . . 19

4.5.2 Nitrogen sorption analysis . . . 24

4.5.3 X-ray diffraction . . . 26

4.5.4 Thermogravimetric analysis . . . 28

4.6 Water Stability Analysis of ZIF-8@MMC . . . 29

4.6.1 Liquid water stability . . . 29

4.6.2 Adsorption of water vapor from air . . . 31

4.7 MOF-74 . . . 32

4.7.1 SEM-analysis . . . 33

4.7.2 Nitrogen sorption analysis . . . 35

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4.7.3 X-ray Diffraction . . . 36

4.7.4 Fourier Transform Infrared Spectroscopy. . . 37

5 Conclusions 38 6 Acknowledgements 40 7 Appendix - EDS-spectra 45

List of Figures

1 Scheme of physical mixing to form ZIFs@MMC . . . 8

2 Scheme of in situ-route to ZIFs@MMC . . . 9

3 Scheme chemical modification ZIFs@MMC. . . 10

4 SEM-images Pure ZIF-8 . . . 15

5 XRD-pattern ZIF-8. . . 15

6 XRD-pattern physical mixing ZIF-8@MMC . . . 16

7 SEM-images of ZIFs@MMC from physical mixing . . . 17

8 SEM-EDS of ZIF-8-cluster from physical mixing . . . 17

9 SEM-images in situ ZIF@MMC . . . 18

10 XRD-pattern in situ ZIF-8@MMC . . . 19

11 SEM-EDS of MMC etched with Zn . . . 20

12 SEM-EDS of decanted solvents post etching . . . 21

13 SEM-images ZIF-8@MMC . . . 22

14 SEM-images ZIF-67@MMC . . . 23

15 EDS-mapping of ZIFs@MMC . . . 24

16 Nitrogen sorption isotherms ZIF-8@MMC . . . 25

17 Pore size distributions ZIF-8@MMC . . . 26

18 XRD-patterns for ZIF-8@MMC and ZIF-67@MMC. . . 27

19 XRD-patterns pure MMC, ZIF-8@MMC, Zn-etched MMC . . . . 28

20 TGA-curves for pure MMC, ZIF-8@MMC, Zn-etched MMC . . . 29

21 XRD-patterns after water stability . . . 30

22 Moisture uptake analysis. . . 32

23 SEM-images MOF-74@MMC . . . 33

24 SEM-images MOF-74 on MgO and MgCO3 . . . 34

25 Nitrogen sorption isotherms MOF-74@MMC . . . 35

26 Pore size distributions MOF-74@MMC . . . 36

27 XRD-pattern MOF-74@MMC . . . 37

28 FTIR MOF-74@MMC . . . 38

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List of Tables

1 Synthesis parameters ZIF-8 . . . 7

2 ZIF-8 yield and crystal sizes . . . 14

3 Nitrogen sorption analysis ZIFs@MMC . . . 25

4 Water stability - nitrogen sorption analysis . . . 29

5 Water vapor uptake ZIF-8@MMC. . . 31

List of Abbreviations

API Active Pharmaceutical Ingredient DIW Deionized water

DMF N,N-dimethylformamide

EDS Energy Dispersive X-ray Spectroscopy FDA US Food and Drug Administration FTIR Fourier Transform Infrared Spectroscopy GRAS Generally Recognized As Safe

H2BDC 1,4-benzodicarboxylic acid H4DOBDC 2,5-dihydroxyterephtalic acid MMC Mesoporous Magnesium Carbonate MOF Metal Organic Framework

POP Porous Organic Polymer RH Relative Humidity

SEM Scanning Electron Microscopy SSA Specific Surface Area

TGA Thermogravimetric Analysis XPS X-ray Photoelectron Spectroscopy XRD X-ray Diffraction

ZIF Zeolitic Imidazolate Framework

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1 Introduction

As Richard Feynman famously stated back in 1959, there is still plenty of room at the bottom.[1] Nanotechnology has taken large strides since then, yet it remains a valid statement as copious challenges still lies ahead before we can harvest all of its potential.

Hybrid nanomaterials can be generally classified as synthetic materials having inorganic and organic constituents that are linked together either through co- valent or non-covalent bonds at the submicron level.[2] Thus, it is no wonder that these materials have attracted considerable attention in recent years, owing to them displaying features and properties distinctly different from their pure components. Not only can beneficial properties of each constituent be favorably combined in a hybrid material - the method can also be exploited to mitigate or eliminate undesired properties from either constituent.[3] As the sheer num- ber of possible components and combinations are virtually unlimited, it is easily recognized that hybrid nanomaterials are considered for a broad range of ap- plications. One conceptual way of combining nanomaterials is in a core-shell structure. As the name implies, such structures consists of an inner core sur- rounded by an outer shell made of other constituents. Initially, research within the semiconductor industry found that when small metal particles were covered with a thin surface layer, they demonstrated drastically different chemical re- activity and magnetic, optical and electronic properties.[4, 5] Subsequently, the term core-shell structure was coined, and the fields of research adopting it has been much expanded, nowadays involving biomedicine and pharmaceutics.[6–8]

Creating core-shell structures typically serves the same purpose as hybrid ma- terials in general - combining the properties of two different materials within a single structure.[9] There is also the added benefit that the geometry and design of the core-shell structures themselves will influence the resulting properties.[10]

Adding a shell material can drastically enhance or decrease the reactivity of the core, and alter its thermal and hygroscopic stability, which can be exploited for improving its compatibility for various applications.[11]

Mesoporous magnesium carbonate (MMC) is a porous material that was dis- covered at the Ångström laboratory during 2013. It is an amorphous material consisting of aggregated magnesium oxide and magnesium carbonate.[12] Porous materials in general have interesting intrinsic properties such as high surface ar- eas, enabling a small amount of material to have a large interface, with many potential interaction sites. For materials with tunable pore sizes, topologies and structures such as zeolites, metallosilicates [13], metal organic frameworks (MOFs), aluminaphosphates [14] and porous organic polymers (POPs) [15] it is possible to adjust both the general and the qualitative throughput through size and shape selectivity. This enables porous materials to be employed for vari- ous applications including gas and energy storage, within metal ion adsorption for water purification, as sensor materials and as catalysts for a broad range of reactions. Furthermore, porous materials have been exploited as drug delivery vehicles, where an active pharmaceutical ingredient (API) is loaded into pores

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whose sizes have been precisely controlled. This method of effectively trapping the API in a cage may serve to protect it from a hostile environment, control the drug release rate, and maintain the API in an amorphous state to increase its solubility. As it is possible to adjust the pore sizes of MMC, despite it being an amorphous material, considerable research efforts within a broad range of these applications of porous materials has been conducted.[16–21]

MOFs are another particularly interesting class of porous hybrid nanomaterials, consisting of coordination polymers arranged in 3D-networks. Essentially, they consist of either metal ions or metal ion clusters, that are linked together in a coordinated manner using multidentate organic linkers.[22, 23] As indicated, MOFs displays tremendously high surface areas, structural and chemical diver- sity along with other admirable properties that has led to them being considered as prime candidates for industry usage within gas separation and storage, catal- ysis, water harvesting from air and drug delivery.[22, 24–26] However, there is also a drawback associated with these structures. Their narrow pore sizes are most often in the microporous regime, which means that the pores are smaller than 2 nm in accordance with IUPAC’s definition. This provides the large sur- face areas that MOFs are well-renounced for. Though, such small pores also limits diffusion, causing potential throughput to be decreased, and it prevents bulky molecules from being transported in the pores.[25, 27] Several mesoporous MOFs have been developed, though in order to form mesopores, the ligands must expand in length, making it difficult for those to maintain their rigidity, leading to reduced thermal stability.[28] Also, the synthesis conditions for mesoporous MOFs tend to be challenging, expensive and time-consuming.[29] Conclusively, any promising material feature in itself needs to be adapted in order for it to sustain its desired properties under various circumstances so that it is useful in practical applications. Much research efforts are therefore currently spent on forming combined micro-/mesoporous structures.[30]. This would serve to utilize both the good accessibility of the mesoporous structure, and the many active sites as well as increased thermal stability and hydrophobic properties provided by microporous networks.[31] Industrial applications of micro-/mesoporous ma- terials are also researched.[32]

With this in mind, it was envisioned that combining the mesoporous structure of MMC with the microporous structure of MOFs could render hybrid materials with near-endless potential, enabling utilization of the benefits of each respective structure. Such novel materials could enhance existing properties in a variety of applications, spanning from drug delivery vehicles to gas sensors. Develop- ing new multifunctional materials with tunable structures, functionalities and properties that can be tailored for certain circumstances is currently a main research field within material science. There might undoubtedly be much to gain by studying porous hybrid nanomaterials as perceived applications involve environmentally beneficial and humanitarian areas. Screening through some of the options will provide solid ground for a more extensive research project. This master thesis will serve to explore formation of novel hybrid nanomaterials, more specifically core-shell micro-/mesoporous structures based on MMC and various

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MOFs. In these core-shell structures, the inner mesoporous network of MMC would act as the backbone, and would remain accessible through the micropores of the surrounding MOF. Focus has been on investigating syntheses routes, to determine whether or not such structures can be formed. Developing new mate- rials is not straightforward – it is more like being placed in a maze of dark caverns without neither lantern nor a road map, where it is only gradually possible to learn of ones surroundings. Considerable efforts were therefore spent on explor- ing synthesis parameters. MOFs based on different transition metal ions and ligands were investigated in the attempts of forming core-shell structures using MMC as the starting material. The obtained materials were then meticulously characterized using a number of analysis methods.

1.1 Aim and purposes

This master thesis sought to determine whether or not it was possible to com- bine MMC with various different MOFs. In order to accomplish this, a literature study to gain insight into porous nanomaterials was required, and based on this a number of different potential synthesis routes were to be designed and evaluated.

To verify if the intended core-shell structures could form, the products were then to be thoroughly characterized, both in terms of their morphology, composition and properties. This work would thereby serve as the first step in exploring these kinds of materials, where the long-term ambition is to better utilize the mesoporous structure of MMC and enhance its water stability through combi- nations with microporous MOFs. Thus, the objective was to investigate how MMC would interact with MOFs to see if hybrid materials could be formed, and also to determine how these modifications affected the porosity and hygroscopic properties of MMC.

2 Background

2.1 Mesoporous Magnesium Carbonate

Mesoporous magnesium carbonate has the trademark name Upsalite®, and it is a material discovered by a research group at the Department of Nanotechnology and Functional Materials at Uppsala University in 2013.[12] It is a nearly an- hydrous material and consists of aggregations of magnesium oxide nanoparticles and amorphous magnesium carbonate. The free space between these aggrega- tions provides the material with its pore structure, which is tunable based on synthesis parameters.[17] MMC has recorded a specific surface area (SSA) of up to 800 m2g−1, which is unprecedented by any other alkali earth metal car- bonate.[16] It also has a narrow pore size distribution, typically around 5 nm, though as mentioned its pore size can be tailored. Ordinary magnesium car- bonate holds the US Food and Drug Administrations (FDA) classification as Generally Recognized As Safe (GRAS), meaning it is allowed to be used as a

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food additive as well as within pharmaceutical formulations.[33] Furthermore, cytotoxic studies specifically for MMC have been undertaken, demonstrating promising results.[34] Several studies involving MMC as a drug delivery vehicle have been published[19, 20, 35], and MMC has displayed antibacterial[18] and an- ticoagulant properties.[21] MMC is a hygroscopic material, capable of extremely high water adsorption uptake.[16, 36] Improving its water stability would further expand its potential applications as a porous material, for instance within drug delivery[36].

2.2 Zeolitic Imidazolate Frameworks

Zeolitic Imidazolate Frameworks (ZIFs) are a subcategory within MOFs, first dis- covered and termed during 2006.[37, 38] Structurally, they are composed of diva- lent transition metal cations (Zn, Co, Fe, etc) that are tetrahedrally coordinated with imidazolate acting as bridging ligands. As the coordination bond angles for the metal-imidazole-metal is only slightly perturbed from 145°, ZIFs are essen- tially topologically isomorphic with zeolites, in which the Si-O-Si bond angles are 145°.[37, 39] Though there are exceptions[40], most ZIFs are thereby said to be zeolite-like.[41] Due to this stable coordination bond and its favorable structure, there are ZIFs that have demonstrated remarkable thermal and chemical sta- bility versus organic solvents and alkaline solutions, which are properties where organic based materials normally falters.[37, 41, 42] Naturally though, the chemi- cal stability of ZIFs differs amongst different structures. ZIFs are formed through a self-assembling process. Although most of the structures can be formed un- der standard conditions using a facile and environmentally benign hydrothermal approach, other methods such as solvothermal or sonochemical routes can also be employed, were a broad array of polar solvents such as methanol and N,N- dimethylformamide (DMF) may be used instead of water.[41]

Some of the most promising members of the ZIF-family is ZIF-8 and ZIF-67.

They are isostructural, have this sodalite topology, and consist of zinc or cobalt cations (ZIF-8 and ZIF-67, respectively) that are tetrahedrally coordinated to four imidazole ligands through Zn/Co-N-bonds.[37, 41, 43] Since ZIF-8 is com- monly synthesized in water, it is no wonder that ZIF-8 has demonstrated ideal hydrophobic properties, both in simulations[44] and in numerous empirical ad- sorption studies.[45–47] Thus, it would be favorable to combine this ZIF with MMC to form a core-shell structure, where the outer layer of microporous ZIF-8 protects the mesoporous core of MMC against moisture degradation. One of the potential applications for this would be as a drug delivery vehicle, as both MMC[19, 20, 35] and ZIF-8 has been successfully examined on their own for such applications.[48, 49] Another would be to tune the water adsorption prop- erties of MMC, so that it is easier to release captured water, since this might be used to capture large amounts of water from air with moderate relative humid- ity (RH) which after a facile regeneration then releases liquid water as another MOF recently has had reported progress with.[26] For practical applications, ZIF-8 would be more desirable than ZIF-67, due to the major ethical, envi-

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ronmental and health issues associated with extraction and use of cobalt.[50]

Though, it may be perceived that applications were ZIF-67 are superior exists, for example within electrochemistry where cobalt’s various oxidation states can be exploited. The potential formation of ZIF-8@MMC as well as ZIF-67@MMC will be explored in this study, though application studies will be limited to a wa- ter stability assessment of ZIF-8@MMC, as further applications would require an even more ample timescale. Cytotoxicity studies of ZIF-8 have been incon- sistent, as initial reports suggested it was harmless[51], though recently it has been indicated that high concentrations may elicit cell response.[52, 53] Further research is needed before any definitive verdicts could be made.

2.3 MOF-74

Structurally, the metal centers of MOF-74 consists of either Mg, Co, Ni or Zn, and these are joined together by 2,5-dihydroxyterephtalic acid (H4DOBDC) serv- ing as linkers.[54] Each metal cluster can be coordinated to 6 atoms, of which 5 are oxygen atoms in the shape of either terephtalic- or hydroxy groups in the ligands. The last remaining one is an open site, that used to be occupied by a water- or other solvent molecule, which is removed during activation of the MOF.[24] Since it contains these unsaturated metal coordination sites that are prone to adsorb various small gas molecules such as N2, CO2, H2and CH4, MOF- 74 is a microporous material with high SSA that is intensely researched as a gas adsorption material.[55] There are large discrepancies in the adsorption proper- ties of MOF-74 with different metal centers, where Mg-MOF-74 is arguably the most interesting.[54, 56] In fact, Mg-MOF-74 has the highest recorded adsorption uptake of CO2of any solid material at sub-atmospheric pressure regimes.[56–58]

Further, the majority of the adsorbed CO2 can easily be regenerated at room temperature, explaining why it has attracted vast interest.[24] Mg-MOF-74 is structurally stable when exposed to humidity, yet its adsorption properties can be severely reduced, which has led to considerable research efforts to mitigate these effects.[54, 55] Recently, MOF-74 has been investigated as a gas adsorption membrane.[56] Since micropores have the drawback of being diffusion limited when it comes to kinetics, increasing the permeability by connecting microp- ores to mesopores in a hierarchical porous network has gained much research attention.[59, 60] If MOF-74 could be prepared in an ordered manner, such as in a core-shell structure with hierarchical porosity, which would be a promis- ing new concept in the global challenge of solving difficulties associated with CO2-adsorption.

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3 Experimental

3.1 Synthesis of MMC

To be used as a starting material, MMC was synthesized in accordance with a well-established route.[17] In a 350 ml pressure reaction vessel, 15 g MgO (Sigma Aldrich, USA) and 225 ml methanol (VWR, Sweden) were mixed. The reaction vessel was sealed, and connected to a CO2-line were the pressure was manually adjusted to around 4 bars. Reaction was performed at room temperature, and the solution was stirred at 500 rpm. After 22 hours, a white, hazy solution had been obtained, and the CO2-pressure was released. The contents of the vessel were transferred to five 50 ml Falcon tubes (VWR, Sweden) that were centrifuged at 3800 rpm for 20 minutes to remove unreacted MgO-particles. Solution was then poured into a 250 ml glass flask that was sealed with a rubber stopper and para film and kept in a refrigerator to prevent gelation. The unreacted MgO that had been discarded amounted to 63 mg, which rendered that 1 ml of the solution contained 1.647 mmol magnesium. It was presumed that this could be interchangeably used as 1.647 mmol MMC. MMC-solution refers to this reaction product.

To form MMC-powder, MMC-solution was placed in a glass beaker. This beaker had a N2-flow added to it, and it was in turn placed in a large petri dish filled with 55°C water. This caused the solution to commence gelling almost instanta- neously, and after 10 minutes, a gel had formed. This was heat-treated, first in a 70°C convection oven for 1 hour, followed by 16-22 hours in a 150°C convection oven to render the product MMC-powder. It has been proved that the energy input during MMC powder formation has a vital influence on the resulting pore size and thereby also its specific surface area.[17] Based on three separate yield calculations, 1 ml of MMC-solution corresponded to 126 mg MMC-powder. The powder was coarsely ground using a ceramic mortar to render a slightly narrower size distribution.

3.2 Pure MOFs 3.2.1 ZIF-8

With the intention of being used as a starting material, ZIF-8 was synthesized under various different conditions. These are summarized in Table1

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Table 1: Synthesis parameters for various versions of pure ZIF-8

Sample Zinc salt Molar ratios

Zn:Ligand:Solvent Reaction time ZA (Standard) Zn(NO3)2•6 H2O 1:8:740 1 h ZB (Long-stirred) Zn(NO3)2•6 H2O 1:8:740 24 h

ZC (Chloride) ZnCl2 1:8:740 2 h

ZD (Excess ligand) Zn(NO3)2•6 H2O 1:35:740 1 h ZE (Low ligand) Zn(NO3)2•6 H2O 1:4:740 1 h

All reactions were performed in room temperature, in accordance with estab- lished literature.[42, 61] Methanol was selected as solvent as it complies with MMC (for subsequent synthesis of ZIF-8@MMC), and it also provides the most narrow size distribution of ZIF-8-particles.[62] 5 mmol zinc salt (either zinc ni- trate hexahydrate (Sigma Aldrich, USA) or zinc chloride (Sigma Aldrich, USA)) and amounts of 2-methylimideazole (HMIM) (Alfa Aesar, USA) that are specified in Table 1 were each dissolved in 75 ml methanol (VWR, Sweden) in separate beakers. Once completely dissolved, the beaker containing the zinc salt was poured into the beaker with ligand under vigorous stirring for the time stipu- lated in Table 1. Once the reaction was complete, samples were centrifuged at 11 000 rpm for 20 minutes in order for the ZIF-8-crystals to precipitate, before solvent was decanted. Samples were then washed with methanol twice. Lastly, 25 ml methanol was added so that the final product was ZIF-8-crystals dissolved in MeOH. Yield calculations were made by drying 2 ml of each ZIF-8-solution in a 70°C convection oven for five hours, and then weighing the obtained ZIF-8- powder.

3.2.2 MOF-74

Pure MOF-74 was prepared in accordance with an established procedure.[24, 63] Briefly, duplicate samples were prepared through a solvothermal reaction in 20 ml vials. 140 mg Mg(NO3)2· 6 H2O (0.55 mmol) (Sigma Aldrich, USA) was dissolved in 5 ml of a solvent mixture of DMF (VWR, Sweden), deion- ized water (DIW) and ethanol (15:1:1 v/v/v), while 34 mg (0.17 mmol) 2,5- dihydroxyterephtalic acid (H4DOBDC) (Sigma Aldrich, USA) was seperately dissolved in 9 ml of the same solvent mixture through sonication. Then, these solutions were combined under stirring before they were sealed with Teflon tape and capped. A solvothermal reaction was then performed in a convection oven at 125°C for 25 hours. Samples were cleaned with DMF once, then soaked in methanol for a total of three days, where methanol was exchanged on a daily basis. Samples were then dried through evaporation at 70°C and combined.

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3.3 ZIFs@MMC

To form core-shell structures of ZIF-8/ZIF-67 around MMC, a number of dif- ferent potential syntheses routes were designed. Initially, several options were available, as the starting material MMC could be used either as a solid (MMC- powder) or while it was still dissolved in methanol (MMC-solution). In a similar manner, ZIF-8 and ZIF-67 could either be synthesized in accordance with the procedure mentioned above and then allowed to react with MMC, or it could be formed during the reaction with MMC in an in situ-route through addition of zinc salt and HMIM under controlled circumstances. With the assumption that the reaction mechanisms were similar for ZIF-8 and ZIF-67 as they are isostructural, initial screenings were done using only ZIF-8, whereas later exper- iments also included syntheses of ZIF-67@MMC. Through iterative processes, various different synthesis approaches and conditions such as stoichiometric ra- tios, amounts and reaction time were investigated and gradually improved. Spec- ified procedures and ratios mentioned below are those that these screenings have indicated are most viable.

3.3.1 Physical mixing of MMC and ZIF-8

This was initially perceived as the most straightforward approach - simply dis- persing MMC in methanol, and adding already synthesized ZIF-8-solution. Stir- ring these together would then render ZIF-8-crystals to attach onto MMC- particles, as illustrated in figure1. Several variations of the synthesis conditions were made; the aggregation state of the MMC (solution, powder) was altered, ZIF-8-solutions in different concentrations and crystal sizes were applied, and numerous different ratios between MMC, ZIF-8 and solvent were attempted.

Figure 1: Illustration of how physical mixing would render ZIF-8@MMC

3.3.2 In situ-reaction

Much like the synthesis of pure ZIF-8, this method aimed at forming a core- shell structure by allowing zinc nitrate to react with HMIM in the presence of MMC-particles. A similar method was developed by Sorribas et al where

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they successfully formed core-shell structures of silica spheres covered with ZIF- 8 through an in situ-reaction followed by a seeding procedure.[64] Essentially, the synthesis of ZIF-8 described above was repeated, though MMC had been dispersed in the respective reactant solutions before these were mixed, a scheme is presented in figure 2. Molar ratios of MMC, Zn, HMIM and methanol were altered in subsequent experiments, as was reaction time and at which step MMC- powder was added.

Figure 2: Scheme of in situ-route to form ZIFs@MMC

3.3.3 Chemical modification of MMC

During experiments with the in situ-reaction, a similar synthesis method was developed in parallel, which was attempted for both ZIF-8@MMC and ZIF- 67@MMC. This was based on subjecting MMC-powder to an etching process to substitute some of the magnesium in MMC with zinc/cobalt using an acidic metal salt solution, followed by removing of excess metal ions and introducing HMIM, in accordance with figure 3. In a typical synthesis of ZIF-8@MMC, triplicate samples were made where each sample had 126 mg (1.65 mmol) MMC- powder mixed with 49 mg Zn(NO3)2· 6 H2O (0.17 mmol) (Sigma Aldrich, USA) in 5 ml MeOH (VWR, Sweden). Samples were sonicated for ten minutes and then stirred at moderate speed (400 rpm) for a specified time before they were centrifuged at 3800 rpm for 20 minutes. Solvent containing excess metal ions along with nitrate ions was then decanted. Meanwhile, 79 mg (0.96 mmol) HMIM (Alfa Aesar, USA) had been dissolved in 5 ml methanol, and this solution was added to the MMC/Zn-powder, and samples were stirred at moderate speed (500 rpm) for 72 hours. Lastly, they were cleaned with 5 ml methanol three times and dried at 70°C in a convection oven. ZIF-67 was prepared in a similar manner, though zinc nitrate was replaced with 48 mg Co(NO3)2· 6 H2O (0.17 mmol), and the amount of methanol was increased to 10 ml. Variations of this synthesis included altering molar ratios of all constituents, varying the concentration of the Zn-/Co-metal solution and the etching time, exchanging the solvent to DMF and elevating the reaction temperature to 120°C, applying different reaction

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times and using different aggregation states of MMC (MMC-solution, MMC- powder, MMC-solution co-gelled with zinc/cobalt nitrate). To analyze some of the basic mechanisms behind the reactions, some batches were only subjected to the etching reaction, to see how this procedure influenced the MMC.

Figure 3: Illustration of concept of chemically modifying MMC with Zn/Co, to ultimately render ZIFs@MMC.

3.4 MOF-74@MMC

Synthesis was based on literature for generating pure MOF-74, where a mag- nesium salt such as magnesium nitrate typically acts as source of Mg-ions for the metal nodes in the framework.[24, 63, 65] Though, in this procedure, MMC along with commercial grades of MgO and MgCO3were employed as magnesium sources, to establish whether or not Mg-MOF-74 could be synthesized from these materials. Furthermore, modifications of the solvent used in the references [24, 63, 65] were made, and the final procedure also included a minor amount of diluted nitric acid, as initial experiments showed that it had a beneficial effect on forming Mg-ions from MMC.

Triplicate samples were made, each in a 20 mL glass vial. In each respective vial, 126 mg (1.6 mmol) ground MMC-powder and 33 mg (0.16 mmol) 2,5- dihydroxyterephtalic acid (H4DOBDC) was dispersed in 14 ml of a solvent mix- ture of DMF and ethanol (15:2 v/v). 500 µL 1 M HNO3 was added to each

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sample. To dissolve the solid constituents, vials were sonicated. Vials were then capped using Teflon tape and placed in a convection oven at 125°C for 24 hours.

Once removed from the oven, a fine yellow powder had been produced. Samples were transferred to Falcon tubes, centrifuged at 3800 rpm for 20 minutes, and solvent was decanted. Then, samples were soaked in methanol and cleaned with methanol on a daily basis for three consecutive days. Lastly, samples were dried in a convection oven at 70° for 24 hours.

3.5 Materials Characterization

To determine the outcome of the syntheses and characterize the obtained prod- ucts, several analyzing methods were conducted.

3.5.1 Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM-EDS)

SEM was the primarily used method since it provided a rather quick feedback as to whether or not an intended material had formed, as it visualizes the mor- phology of the materials, making it possible to deduce if MOFs were present, and if so if a core-shell structure had been obtained. Two different SEM instru- ments were used, as one has higher resolution and the other is equipped with an EDS-detector, prompting the use of both. For recording SEM-images, a Zeiss LEO 1530 scanning electron microscope (Oberkochen, Germany) with an in-lens detector was used with an acceleration voltage of 3 kV. Working distance varied between 2.5-3 mm. Since specimens were in powder shape, they were mounted on aluminum stubs using double-sided carbon tape. To avoid charging effects on these non-conducting samples, they were coated with roughly 5 nm Au/Pd using a Quorum Technologies Ltd Polaron SC7640 sputter coater (Ashford, UK) prior to analysis.

SEM-EDS images and spectra were obtained using a Zeiss LEO 1550 scanning electron microscope (Oberkochen, Germany), that was fitted with a X-maxN SDD-detector from Oxford Instruments (Abingdon, UK) which was used for element mapping. Electron images were taken using an in-lens detector. Accel- eration voltage was set to 10 kV with working distance 6.5 mm. The EDS signal was not calibrated, as there was a lack of suitable reference materials, causing the accuracy of these measurements to be sub-optimal.

3.5.2 Nitrogen Sorption Measurements

To obtain values for the specific surface areas and information about the poros- ity of the obtained materials, nitrogen sorption isotherms were collected using a Micromeritics ASAP 2020 surface area analyzer (Norcross, GA, USA). Measure- ments were conducted under cryogenic conditions at 77 K, using liquid nitrogen

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as coolant. The Bruner-Emmett-Teller (BET) method was applied to estimate the specific surface area, using data from the adsorption branches between rel- ative pressures 0.05 to 0.30, were calculations were performed with the ASAP 2020 V3.04 software from Micromeritics. In a similar manner the pore size dis- tribution was determined using density functional theory (DFT), executed by the DFT Plus software from Micromeritics based on the obtained isotherms.

All samples were degassed prior to analysis, through heating under dynamic vacuum (10−4Pa) using a Micromeritic SmartVacPrep sample preparation unit.

Temperature and time of pre-treatment were based on previously reported ac- tication temperatures. Degassing of MMC and ZIFs@MMC were performed at 150° for 12 hours[17], samples including MOF-74 were degassed at 250° for 6 hours[24], and those containing MOF-5 were treated at 120° for 6 hours[65].

3.5.3 X-ray Diffraction (XRD)

For the initial powder XRD analysis, a Bruker D5000 X-ray diffractometer (Bre- men, Germany) with CuKα radiation (λ = 0.154 nm) in the 2θ range spanning from 5° to 60° with a measuring time of 1 s per step, and a step size of 0.1 was used. Applied voltage and current were 40 kV and 40 mA, respectively.

To enhance the signal intensity and resolution, XRD patterns included in this report were obtained using a Bruker D8 Advanced XRD TwinTwin instrument (Bremen, Germany) with CuKα radiation (λ = 0.154 nm) in the 2θ range span- ning from 5°to 60°, with applied voltage 40 kV and current 40 mA. Step size was adjusted to 0.02, with a measuring time of 0.1 s per step.

All samples were ground with ethanol prior to analysis, and were placed in silicon zero background sample holders. Obtained XRD-patterns were treated to remove background and strip Kα2-peaks using HighScore software from Malvern Panalytical.

3.5.4 Thermogravimetric Analysis (TGA)

TGA measurements were conducted with a Mettler Toledo TGA instrument (Graifensee, Switzerland). A temperature span of 25°C to 800°C was used, with a heat ramping of 5°C per minute, under a nitrogen flow of 60 ml/minute.

Samples were placed in aluminum oxide crucibles.

3.5.5 Fourier Transform Infrared Spectroscopy (FTIR)

A Bruker Tensor27 spectrometer equipped with an attenuated total reflectance (ATR) sample holder was used to record FTIR analysis. A background scan was carried out prior to all measurements, and later subtracted from the samples spectra. 64 scans were signal-averaged for each spectrum, which were taken over

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a range of 4000 to 400 cm−1 with a resolution of 4 cm−1. Samples were ground before each measurement.

3.6 Water Adsorption Analysis of ZIF-8@MMC

As one of the driving forces behind the desire to form core-shell structures of ZIF-8@MMC is to improve the water stability of MMC, a series of experiments were designed to be able to evaluate the hygroscopic properties of the obtained material. As starting materials, ZIF-8@MMC was prepared using two different molar ratios of Zn to MMC (1:10, termed 10% Zn and 1:2 termed 50% Zn). As reference materials, pure MMC and pure ZIF-8 were also prepared.

3.6.1 Liquid Water Stability

To analyze the water stability of the obtained materials, nitrogen adsorption measurements were conducted prior and after they were dipped in deionized water (DIW). Each of the materials were degassed, and rapid nitrogen adsorption measurements were conducted at 77 K using the ASAP 2020 from Micromeritics.

Then, the powders were collected in 20 ml glass vials, and had 5 ml DIW added to them. After 120 seconds, the water was decanted, and vials were immediately placed in a convection oven at 150°C to dry for 10 hours. Then, samples were thoroughly degassed at 150°C under dynamic vacuum for an additional 12 hours, before new nitrogen sorption measurements were conducted to investigate how the porosity had been effected, compared to the reference samples of MMC and ZIF-8.

3.6.2 Adsorption of water vapor from air

Another method of examining whether or not a core-shell structure with in- creased water stability had formed, was to investigate the moisture uptake in air. Since pure MMC is hydrophilic and has a high moisture sorption capacity, whereas ZIF-8 is hydrophobic, a core-shell structure of ZIF-8@MMC ought to have a reduced water vapor adsorption capacity. A TGA-program was designed, with the intention of drying the sample in a nitrogen environment at 150°C, followed by a gradual temperature decrease and settling at room temperature, when the nitrogen atmosphere was replaced by a constant air flow of 60 ml/min.

Since air has a certain RH (in this environment it was measured to 29.8%), an air flow provides water molecules that can be adsorbed by the sample in the sample holder, resulting in a weight increase that is detected by the sensitive scale. Thus, moisture uptake causes a gradual weight increase up until the satu- ration point, giving a qualitative as well as quantitative water vapor adsorption capacity for the different materials. It was assumed that the sole source of mass increase would be water vapor, as amounts of other gases adsorbed ought to be negligible. Starting amounts of sample was roughly 35 mg.

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4 Results and Discussion

4.1 Mesoporous Magnesium Carbonate

Examination of the obtained material in SEM indicated that MMC had indeed formed. Subsequent XRD and FTIR measurements further strengthened this perception, and nitrogen gas sorption analysis proved that a highly porous ma- terial had formed, with a hysteresis resembling previously reported results for MMC.[17] The BET SSA was found to be 536 m2g−1 with an average pore size around 3.7 nm, i.e. in the lower mesoporous region.

4.2 ZIF-8

A number of different methods were used to fabricate ZIF-8, where the ambition was to form small crystals with a high yield. This would provide insights as to which basic recipe that ought to be used to later form ZIF-8@MMC. The outcome of these parameters is briefly summarized in table 2.

Table 2: Yield of ZIF-8 based on Zn for various methods, values obtained through mass calculations.

Also, rough estimations of particle sizes are included, that were approximated using SEM-images, though these should not be considered exact.

Sample Yield Average particle size ZA (Standard) 42.9% 40 nm

ZB (Long-stirred) 46.4% 50 nm ZC (Chloride) 3.2% 800 nm ZD (Excess ligand) 57.0% 35 nm ZE (Low ligand) 41.5% 40 nm

These results of the yield calculations indicated that using ZnCl2 was not suit- able, as such a low yield was highly unlikely to ever result in sufficient ZIF-8 to render complete coverage of MMC-particles. This yield was unexpectedly low, whereas yield of ZIF-8 from Zn(NO3)2· 6 H2O was in accordance with previ- ously reported values.[42, 66] Values for particle sizes are given only as rough estimations, as no actual size measurements were conducted other than manual estimations from SEM-images, such as those depicted in figure 4. The pre- dominant conclusion from these images was that the obtained materials have an expected morphology, and that size differed dramatically between ZnCl2

and Zn(NO3)2· 6 H2O. This was expected, though perhaps not that the dif- ference would be this substantial - ZnCl2 yielded slightly larger crystals, and Zn(NO3)2· 6 H2O yielded smaller crystals than anticipated.[61, 66]

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a)ZIF-8 from ZnCl2 b) ZIF-8 from Zn(NO3)2· 6 H2O, standard conditions

Figure 4: SEM-images of ZIF-8 based on different zinc sources. ZnCl2 (sample ZC) yielded large particles, whereas Zn(NO3)2· 6 H2O under standard conditions (sample ZA) resulted in particles that were significantly smaller. Notice that scale bars differ between images.

It could be concluded that ZIF-8 derived from Zn(NO3)2•6 H2O gave a much higher yield, as well as significantly smaller particles, prompting subsequent synthesis to be based on this zinc source. A slightly higher yield could be noticed with excess ligand, which was reasonable[67], though no definitive verdicts could be made from the reaction time, nor if there is any major difference between a low or standard ratio of Zn to HMIM. Thus, ratio of ligand and reaction times were parameters that were separately investigated in the synthesis of ZIF-8@MMC.

Figure 5: Obtained XRD-pattern for synthesized ZIF-8 (red) is a great fit when compared with a

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To confirm beyond any doubt that the synthesized material was in fact ZIF-8, XRD was also conducted on ZIF-8 synthesized under standard conditions. The obtained XRD-pattern was an excellent fit with a simulated pattern for ZIF-8, thereby proving that the desired material had formed. These XRD-patterns are presented in figure 5.

4.3 ZIFs@MMC - Physical mixing

Physical mixing of MMC and ready-made ZIF-8 was the most simplistic ap- proach, where the idea was that stirring these materials together would result in interactions and the formation of a core-shell structure.

XRD confirmed that ZIF-8 remained in sample, figure 6. An initial cause of concern had been that since ZIF-8-crystals are so small, they might remain dissolved during the centrifugation and thus be discarded. Though, these XRD- patterns proved otherwise. This pattern does however not display any signs of the amorphous MMC, that lacks distinct peaks but can produce signals as broad shoulders around 30° 2θ.[12, 16] This may have been due to the treatment of the pattern, as the slight indications of MMC might have been stripped when determining the background signal since its relative intensity compared to the crystalline ZIF-8 (which no doubt was present in a high degree in this case) is low.

Figure 6: XRD-pattern of ZIF-8@MMC obtained through physical mixing of MMC and ZIF-8 (orange).

This pattern corresponds nicely with that of pure ZIF-8 (black).

Samples were investigated using SEM, which gave an uneven impression, see figure 7. There were several particles with various sizes that seemed to be fully covered with a thick layer of ZIF-8, just as intended. Though, these were in- evitably also accompanied by particles that clearly only had partial coverage.

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a) ZIF-8-coverage between MMC-particles seemed un- even; the top left particle appears to be fully covered, whilst other particles lack ZIF-8-crystals.

b) Another particle with seemingly great ZIF-8- coverage

c)High magnification image of MMC-particle that did not seem to have any ZIF- 8-coverage

Figure 7: SEM-images of ZIF-8@MMC through physical mixing. It appears that ZIF-8-coverage varies significantly between particles.

Since it was strange that the coverage itself and the thickness of the ZIF-8- layers varied so much, SEM-EDS was conducted to gain information about which elements were present in the samples, figure 8. This also provided a reasonable explanation, as it became evident that while there were many MMC-particles with shifting amounts of ZIF-8-crystals, there were also particles in various sizes that entirely consisted of ZIF-8. Thus, ZIF-8-crystals had agglomerated to form large ZIF-8-clusters. EDS-mapping was conducted on a number of particles of different sizes that turned out to consist only of Zn, N, C and O, which are the elements of ZIF-8. No traces of Mg could be found in these clusters, meaning that they did not contain any MMC and had formed independently. Variations of the synthesis were made with the intention of triggering more interactions between ZIF-8 and MMC, though in all cases ZIF-8-crystals seemed more prone to form large clusters than to cover MMC-particles.

Figure 8: EDS-mapping of particle with seemingly good ZIF-8-coverage proves to be a cluster of only ZIF-8. Color spectra to the right of SEM-image represents elemental distribution in particle, and no Mg is found meaning no MMC is present.

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4.4 ZIFs@MMC - In situ-route

The concept of in situ-formation of ZIF-8 was also explored, with the ambition that such formation would result in MMC-particles being covered with ZIF-8.

Figure 9 shows SEM-images of the outcome of the synthesis. Though some ZIF-8-crystals can be seen, the general impression is that yield of ZIF-8 was low, and that the distribution of ZIF-8-crystals on MMC-surfaces were partial and uneven. This would indicate that the majority of the zinc ions and HMIM either did not react at all, or formed nanoparticles that did not interact strongly with MMC and might have been removed during centrifugation of the sample.

Based on the SEM-images, ZIF-8 only exists in small amounts, and where it does exist it is unevenly distributed. It was believed that one of the issues with forming core-shell structures in this case was that unlike the zeolite spheres used by Sorribas et al[64], MMC-particles lacked shape and size regularity. Therefore, various treatments of the MMC involving dispersion, sonication and incremental centrifugation was conducted prior to reaction. Though, this did not yield any better results, and during these attempts it could be seen that ZIF-8 did not appear to have any structural or size preferences of its support - similar results were obtained regardless of size and morphology of the MMC.

a)Particles are generally de- rived of ZIF-8-coverage

b) Even with high magni- fication, only single ZIF-8- crystals can be seen on sur- face

c) Some MMC-particles does have partial coverage of ZIF-8-crystals.

Figure 9: SEM-images of ZIFs@MMC derived from the in situ-route. Though there are exceptions, MMC-particles are generally not covered with ZIF-8, and the crystals that do exist appears to be physisorbed on surfaces of MMC-particles.

XRD-analysis was conducted to determine if the intended MOF had formed.

The obtained pattern is compared with pure ZIF-8 in figure 10. It can be seen that peaks align decently, indicating that ZIF-8 is present in sample. Though, the XRD-pattern is reletively blurry in general. A broad shoulder around 30°

2θ can also be seen, and an increased intensity in this region is consistent with previously reported XRD-patterns for MMC.[16, 17] It should be mentioned that the measured intensity of this particular XRD-pattern for in situ ZIF-8@MMC was several magnitudes lower than for other ZIF-8-samples. There are several reasons why signal intensities in XRD may vary between samples, though there is a general correlation between amount of crystalline sample and signal intensity.

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Figure 10: XRD-pattern of ZIF-8@MMC obtained through in situ-route (red), compared with pure ZIF-8 (black). Patterns align quite well, though peaks are less distinct in ZIF-8@MMC as a result of low signal intensity. Broad shoulder around 30° 2θ indicates presence of MMC.

All things considered, it seems reasonable that this method can produce mi- nor amounts of ZIF-8, though these are not evenly distributed over the MMC- particles. Therefore, the desired core-shell morphology could not be obtained.

This conclusion was reached despite numerous alterations of synthesis parame- ters.

4.5 ZIFs@MMC - Chemical modification route

From the outcome of the previous methods, the chemical modification route was developed. Since it proved to be the most viable route to form the intended core-shell structure, the most emphasis was put on this route. Therefore, this section is also more detailed.

4.5.1 SEM-EDS

It was assumed that the chemical modification of MMC through etching relied on the occurrence of transition metal ions (Zn or Co partly substituting Mg in the MMC-structure). Therefore, the etched materials as well as the solvent that was decanted post etching were analyzed using SEM-EDS to determine which elements they contained. These etched samples are portrayed in figure11, which is a representative image of the elemental distribution in the particles of this sample.

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Figure 11: EDS-mapping of MMC that has been subjected to etching with 20% Zn. Color spectra beside SEM-image represents elemental distribution in particle

From this mapping, it was evident that Zn was present in the sample. Pure MMC does not contain any Zn, meaning that this was a result of the etching procedure. For this particular image, the quantitative EDS-analysis found that Mg represented 14.64 wt%, and Zn stood for 14.84 wt%, with the remaining weight contributions originating from oxygen and carbon, as would be expected.

Since the molar mass of Mg is roughly one third of that of Zn (24.3 u compared to 65.4 u), the actual molar ratios are reasonable. It should be emphasized that these values for the wt% should not be considered exact - no element calibration measurement of the detector was made, which increases the already quite large standard deviations for EDS-measurements. It should also be noticed that the transition metal content varies between particles within the same batch, where values spanning from 2-23 wt% were obtained. Values in the same vicinity were obtained for MMC etched with cobalt nitrate. Though, it is a conceptually valid indication that when a transition metal salt is mixed with MMC in a solution, there is a chemical interaction were Zn-Co-ions are substituted into the amorphous MMC-structure, to render new materials. To further gauge the functionality of this concept, chemical modification of MMC with Cu was also attempted, with the intention of later forming Cu-based MOF HKUST-1. EDS- mapping of Cu-etched MMC revealed corresponding images, meaning that at least three transition metals can be substituted into MMC to render new hybrid structures.

During the etching procedure, the MMC and the transition metal salt were centrifuged, and the solvent that contained excess metal ions was decanted before adding HMIM, to avoid forming ZIF-structures anywhere but on the MMC- particles. This solvent was evaporated, leaving behind a minor amount of solid material, which was also subjected to SEM-EDS analysis, to further investigate the theory of ion substitution. Results can be seen in figure12.

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a)Decanted solvent after Zn-etching b) Decanted solvent after Co-etching

Figure 12: SEM-images and element maps constructed in SEM of the particles originating from the decanted solvent after etching of MMC. Magnesium (blue maps) are found in high amounts, indicating that magnesium ions have been substituted from the MMC structure, and been replaced by zinc and cobalt, respectively.

It is evident that the decanted solvent contains magnesium ions, meaning that a substitution reaction is likely to have occurred. The fact that zinc and cobalt is also present in each respective solvent (purple element map in figure12a, orange element map in figure 12b) indicates that the 1:2 ratio of transition metal to MMC was exaggerated, as it is apparent that far from all zinc/cobalt ions are substituted into the MMC-structure. Figure 12 also displays high content of nitrogen and oxygen, originating from the nitrate ions that the transition metal salts contained.

To illustrate the outcome of the ZIF@MMC syntheses through chemical mod- ification, a number of SEM-images of the obtained materials are portrayed in figures 13 (ZIF-8@MMC) and figure 14 (ZIF-67@MMC). Since zinc and cobalt appears to exist within the MMC-structure after transition metal etching, it is reasonable that when ZIF-8 and ZIF-67 forms, they will exist as integrated particles. This can be clearly seen in figure13d, where a layer of embedded ZIF- crystals in various sizes can be seen. Throughout the images in figure 13, there are also ZIF-crystals that appear to be only physically attached on the surfaces

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of the MMC-particles. These are smaller in size, and might have formed either from excess zinc ions that were not removed after centrifugation, or it might be that some of the ions were only adsorbed within the amorphous MMC, and that these were liberated and formed ZIF-8-crystals that later just happened to be physisorbed on the surface of the MMC-particles. All of these samples had been washed with methanol three times, so the physical interaction needs to be relatively strong, since these particles were still not separated from the surface.

a) ZIF-8@MMC, overview of repre- sentative particle

b) Integrated ZIF-8-crystals that are quite large, as well as smaller ph- ysisorbed ZIF-8-crystals are found on surface of MMC-particle

c)Larger ZIF-8@MMC-particle, inte- grated particles throughout, save for a minor part along the edge

d)Highly magnified image displaying integrated ZIF-8-crystals

Figure 13: Crystals of ZIF-8 are present on MMC-particles. Majority of ZIF-8-crystals are truly integrated in particle, smaller ZIF-8-crystals that appears to be physisorbed onto surfaces can also be seen

ZIF-67@MMC displays a similar morphology, which is logical based on the struc- tural similarities of the ZIFs and that both zinc and cobalt could be substituted into the MMC-structure. Though, there are some vital differences. First of all, the sheer amount of ZIF-67-crystals appears to be fewer, and when it comes to size they are generally larger than their Zn-counterparts. Also, ZIF-67-crystals are not as blatantly integrated in the MMC-surface. Overall, there are more ZIF-67@MMC-particles lacking proper coverage than what was seen for ZIF- 8@MMC. Nonetheless, as figure 14 clearly displays, there are lots of particles with the intended morphology, with evenly distributed ZIF-67-crystals on the surface.

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a) ZIF-67@MMC, particle with good coverage

b)Higher magnification image of ZIF- 67@MMC, particles does not appear to be integrated in the same manner as ZIF-8

Figure 14: SEM-images of ZIF-67@MMC. Relatively even distribution of ZIF-67-crystals on surfaces.

Particles are larger than those seen in ZIF-8@MMC, less obvious integration of particles throughout.

These presented SEM-images are all from representative particles, though it should be mentioned that there are discrepancies in the appearances of parti- cles within the same sample. In general, there is a high degree of coverage, though some particles only have a partial coverage, meaning that an ideal core- shell structure is not present throughout all of the particles within the samples, despite optimization attempts. Though, it is at least evident that there is a ther- modynamic driving force to form ZIF-8 and ZIF-67. For comparison, chemical modification of MMC was also conducted to form other MOFs; the Cu-based HKUST-1 and the Zn-based MOF-5, which proved to be entirely unsuccessful.

In those cases, it might of course have been simply due to unfortunate reaction kinetics - that the probability of the organic linker finding and connecting to a metal node was simply too low - and that it could be circumvented by tuning reaction parameters. Though, that is entirely speculative, whereas in the case of ZIFs@MMC, there are strong indications that this is a viable formation path, that just needs some tinkering.

EDS-mapping was also conducted for ZIFs@MMC and images are found in figure 15. Elemental mapping displays presence of Zn/Co as well as N, which is a strong indication that the desired ZIFs have formed since the source of the latter ought to be the organic linker HMIM. When compared to figure 11, there is an undeniable change in morphology, which along with the presence of nitrogen in the elemental analysis is a compelling argument that ZIF-formation has occurred.

In figure15a, there is a region along the edge of the MMC-particle where ZIF-8- coverage is missing which is clearly indicated in the elemental maps, as Mg and O are brighter and Zn, N and C are correspondingly darker.

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a) Elemental mapping in SEM-EDS for ZIF-8@MMC. Notice how area of exposed MMC is reflected in the ele- mental maps.

b) SEM-image of ZIF-67@MMC with corresponding elemental map- ping from EDS-detector

Figure 15: EDS-mapping of ZIFs@MMC, presence of transition metal along with nitrogen marks presence of ZIF-8 and ZIF-67, respectively.

Values obtained for the composition of these particles during the EDS-measurements were 2.0 wt% Mg, 18.4 wt% Zn and 20.6 wt% N for ZIF-8@MMC, and 10.7 wt%

Mg, 8.8 wt% Co and 7.0 wt% N for ZIF-67@MMC. The remaining contributions originated from C, O and Pd, the latter being a result of the thin metal layer that was sputtered on the sample to render them conducting. The amount of Mg reg- istered in the case of ZIF-8 was surprisingly low, even though it appears to have quite a thick layer of ZIF-8 covering it. Again though, it is worth emphasizing that these values were obtained without any calibration measurements, and they do not necessarily hold their ground upon careful scrutinizing. However, EDS is an esteemed qualitative measurement, and the elemental maps correspond well to the assumption that the SEM-images display MMC-particles covered with an outer layer of ZIF.

4.5.2 Nitrogen sorption analysis

Obtained values of SSA for different samples are briefly summarized in table 3

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Table 3: Summarized results of SSA from nitrogen sorption analysis of ZIFs@MMC, compared with pure MMC and MMC etched with Zn/Co.

Sample BET SSA [m2g−1]

Pure MMC 536

Zn-etched MMC (10% Zn) 381 Zn-etched MMC (50% Zn) 110 Co-etched MMC (50% Co) 357

ZIF-8@MMC 10% 634

ZIF-8@MMC 50% 927

ZIF-67@MMC (50% Co) 671

To illustrate the effect of the different synthesis steps, nitrogen sorption isotherms for pure MMC, MMC that has been etched with 10 and 50 molar% Zn, and isotherms for ZIF-8@MMC derived from MMC etched in these manners are all summarized in figure 16. It can be seen that the Zn-etching (as well as the Co- etching, though that is not displayed) has a downside as it lowers the uptake, resulting in a lower SSA. Thus, substituting transition metal ions into MMC affects the porosity in the structure. Though, when etched MMC is later used to form ZIFs@MMC, a dramatic increase of uptake in the microporous regime (be- low relative pressure 0.2 in figure16) can be seen, causing these materials to have a higher SSA than pure MMC after all. Most importantly, the obtained material is a combined micro-/mesoporous one, with potential hydrophobic properties. It is also worth noticing the similarity of the isotherm shapes for the samples that have the same content of Zn - Zn-etched MMC 10% Zn and ZIF-8@MMC 10%

Zn have distinct hysteresis present around relative pressure 0.5, something that is not present in samples with 50% Zn.

Figure 16: Nitrogen sorption isotherms comparing pure MMC with ZIF-8@MMC, as well as with Zn-etched MMC with the same concentrations of Zn that was used to synthesize the ZIF-8@MMC- samples. Isotherms indicates that etching procedure lowers uptake, whereas subsequent ZIF-8-formation drastically increases it. Notice the similarity in the isotherm shapes of Zn-etched MMC and ZIF- 8@MMC with the same concentrations of Zn.

(33)

Pore size distributions can be seen in figure17where incremental pore volume is plotted against pore width, based on DFT calculations. It can be seen that while the pore width of pure MMC is practically maintained both in the Zn-etched MMC 10% Zn and in the ZIF-8@MMC 10% Zn, it is lost when the amount of Zn is increased to 50%. This is likely due to an increased amount of Zn resulting in a solution with a lower pH, and once MMC is placed in a solvent that is too acidic, it is degraded. That 50% Zn results in a much lower SSA than 50% Co follows this reasoning, as the concentration between these samples differs - the Zn-sample had a much higher concentration, providing a lower pH and yielding a lower SSA. Thereby, it seems evident that there is a balance to obtain where as much ZIF-8 as possible is formed (meaning a high content of Zn), whilst ensuring that the acidity does not become too low. Unfortunately, simply diluting a sample with high Zn-content so that it has the same concentration as 10% Zn (namely, 1.7 mM) renders an inadequate sample with low yield of ZIF-8 where many MMC-particles lack coverage. Despite all the efforts spent so far, much work still lies ahead in forming ideal ZIF-8@MMC-particles.

Figure 17: Pore size distribution calculated from isotherms in figure16. Pore widths of pure MMC is consistent with those for Zn-etched MMC 10% Zn and ZIF-8@MMC 10% Zn.

4.5.3 X-ray diffraction

To confirm that the obtained materials contained the desired ZIFs, XRD-patterns were generated. These are displayed in figures 18 and 19. It can be seen in figure 18 that ZIF-8@MMC and ZIF-67@MMC display very similar patterns.

This is no surprise, since ZIF-8 and ZIF-67 are isostructural and unit cells with the same dimensions yields identical diffraction patterns. Simulated patterns for both these materials further reinforces the assumption that ZIF-8 and ZIF- 67 should have similar patterns[68], and they are also in accordance with the obtained pattern of pure ZIF-8 in figure5. Since a crystalline material is bound to have a higher intensity, the amorphous pattern of MMC generally drowns in the background, and it is hard to distinguish it in these patterns.

References

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