Luận án tiến sĩ Kỹ thuật hóa học: Synthesis of cobalt and iron-based metal-organic frameworks and their applications

ABSTRACT The synthesis, structural identification of four novel cobalt and iron-based metal-organic frameworks MOFs, named VNU-10 cobalt-based MOF, VNU-15 iron-based MOF, VNU = Vietnam N

THE CHEMISTRY & APPLICATIONS OF METAL ORGANIC

Definition of Metal Organic Framework

Metal organic frameworks (MOFs) is the compound which are consisted of metal clusters and linker, typically, polytopic organic carboxylates was employed, for example 1,4-benzenedicarboxylic acid (H2BDC), to construct two-, or three- dimensional structures which can be porous (Figure 1) 6

Fig 1 Structure of MOF-5 constructed from Zn4O(CO2)6 cluster and BDC 2- linker 6

Fig 2 Recent progress on synthesizing high surface area material 1

Recently, more than 20.000 different MOFs have been reported 1 Among these materials, the highest surface area is 7140 m 2 g -1 , which far exceeding those of traditional porous materials such as zeolites and activated carbons (Figure 2) 7,8

Applications of Metal-organic Frameworks

Due to high porosity and the modular nature of MOF design and synthesis, in which the backbone components [e.g inorganic and organic secondary building units (SBUs)], can be easily tailored, MOFs is promised for diverse applications such as gas storage and separation, 9 catalysts, 10 proton conduction, 3 sensor, 11 light harvest, 12 drug delivery, 13 batteries and supercapacitors, 14 and so on 1,15

1.2.1 Applications of Metal-organic Frameworks as Heterogeneous Catalysis

Catalysts, generally, were classified into homogenous and heterogeneous, in which, the homogenous catalysts were recognized for fast kinetic and high conversion in organic synthesis, albeit, several drawbacks have been accounted for, which including the difficulties to separate the catalysts for recycling investigations as well as desired products were usually contaminated by catalyst or decomposed products of catalyst On the other hand, heterogeneous catalyst was recognized as greener pathway for organic synthesis owning to its convenience for recycling, in which, the catalysts can be easily separated from the reaction mixture Despite significant advantage of heterogeneous catalysts, organic synthesis employed these catalysts, mostly resulted in low conversion, hence, one of interesting research direction in the catalytic field has been devoted to develop more efficiently heterogeneous catalysts

Traditional heterogeneous catalysts include metal oxides, polymer resin, silica gel and zeolites, for which, low surface area of metal oxides, polymer resin as well as the small pore aperture of zeolite, preventing the large organic substrates from reaching catalytic centers, thus limiting the use for the transformation of large organic substrates

Another platform, mesoporous silica gel, which possessed large pore and high surface

5 area, however, the structure and pore size of the materials are not uniform and the immobilization of active centers within its pore has maintained challenges

Oxidative transformation of large organic substrates, commonly required the formation of active radicals or high oxidation state of the metal centers, which is unstable with very short decay time, hence required fast diffusion of organic substrate onto the catalytic active sites Recently, MOFs have been employed as the platform for catalytic synthesis of diverse organic compounds 10 In fact, most of published MOFs possessed the small pore aperture with low surface area (less than 8 Å and 2000 m 2 g -

1), some of most noticeable MOFs have large internal surface areas and ultralow densities 7 Due to the large and uniform pore size and definitely coordinative environment of metal active centers, a few MOFs catalysts exhibited interesting properties for oxidative transformation of large organic substrates, however, the examples for these class of catalytic reactions are still very rare

1.2.1.1 Metal-organic Frameworks as Scaffold for Oxidative Transformation of Organic Substrates

1.2.1.1.1 Cobalt-based Metal-organic Frameworks for Oxidative Transformation of Small Organic Substrates

The pyrazolate-based materials, namely, [Co II 4O(bdpb)3]n were prepared by Volkmer in the reactions of H2bdpb and CoCl2ã6H2O The structure of [Co II 4O(bdpb)3]n was deduced to adopt pcu net which is similar to MOF-5 with encloses octahedral

{Co4O(dmpz)6} nodes instead of {Zn4O(CO2)6} The pore size of the material was found to be 18.1 Å [Co II 4O(bdpb)3]n has permanent porosity, which was confirmed by an argon gas sorption experiment The BET surface areas of [Co II 4O(bdpb)3]n were calculated from the adsorption data to give of 1525 m 2 g -1 Similar reaction scheme of H2bdpb with Co(NO3)2ã6H2O leading to the formation of [Co II (bdpb)]n which constructed from the cobalt rod SBU and bdpb 2- in order to form three dimension square grid framework [Co II (bdpb)]n possessed 1D channel with the diagonal length of 18.6 Å 16

Scheme 1 Formation of the observed products through the reaction of tert-butyl peroxy radicals with cyclohexene (b) Mechanism for the formation of tert-butylperoxy radicals catalysed by the cobalt(II) centres in [Co II 4O(bdpb)3] Their further reaction with cyclohexene, forming the main product 16,17

Liquid-phase oxidation of cyclohexene using TBHP as the oxidant of [Co II 4O(bdpb)3]n and [Co II (bdpb)]n catalysts were investigated The maximum cyclohexene conversion after 22 h for [Co II 4O(bdpb)3] is 27.5% and 16% for [Co II (bdpb)] The main reaction products obtained using both catalysts were tert-butyl- 2-cyclohexenyl-1-peroxide, followed by 2-cyclohexen-1-one and cyclohexene oxide (Scheme 1) 17

Recently, significant advances have been observed in the cyclohexene oxidation reaction using cobalt-based MOFs For example, a novel cobalt-based MOF, formulated as Co3(OH)2-(tpta)(H2O)4 (tpta = terphenyl-3, 2’’, 5’’, 3’-tetracarboxyate) has been synthesized Material characterization revealed that the material could be dehydrated by heating to transform into dehydrated Co3(OH)2(tpta) Heterogeneous catalytic experiments on allylic oxidation of cyclohexene show that Co3(OH)2(tpta) has 6 times enhanced catalytic activity than Co3(OH)2-(tpta)(H2O)4, hence coordinatively unsaturated Co II sites in Co3(OH)2(tpta) have played a significant role in oxidation of cyclohexene The maximum conversion for the system was observed around 73.6% 18 Subsequently, similar oxidative transformation of cyclohexene was carried on Ni-

MOF-74, Co-MOF-74 and the mixed of Co & Ni-MOF-74 by Zhaohui Li et all The results revealed that introduction of active Co into the Ni-MOF-74 framework enabled the inert Ni-MOF-74 to show activity for cyclohexene oxidation with the maximum conversion of 54.7% Furthermore, the superior catalytic performance, compared with pure Co-MOF-74, was observed 19

1.2.1.1.2 Metal-organic Frameworks for Oxidative Conversation of Large Organic Substrates

Although the oxidative transformation of small organic substrates could be proceeded by MOFs, it is rare examples for which the oxidative transformation large organic substrates, except for the couple cases, in which, the MOFs catalyst possessed large pore size with porphyrin active centers

Fig 3 a) Crystal structure of PCN-222; b) Peroxidase-like oxidation reaction of pyrogallol catalyzed by PCN-222(Fe) 20

Recently, Zhou et al published a porphyrin-based MOF, named PCN-222 and took advantage of large pore size of material with porphyrin active center to use for catalyst

The self-assembly of tetrakis(4-carboxyphenyl)porphyrin) and zirconium cluster leaded to csq framework, in which, the architecture contained hexagonal and triangular one dimension channels with diameter of 36 and 8 Å (Figure 3a) The iron analogue

PCN-222(Fe) employed to test the catalytic activity for the oxidative conversation of several substrates, included pyrogallol, 3,3,5,5-tetramethylbenzidine, and o- phenylenediamine Taken in note, very high kinetic parameters (k cat, 16.1 min -1 ) for oxidative conversation of pyrogallol by PCN-222(Fe) which was several times faster free homogeneous hemin catalyst (Figure 3b) 20

Fig 4 a) Crystal structure of PCN-600(Fe); b) Enzyme mimetic co-oxidation of phenol and 4- aminoantipyrine catalyzed by PCN-600(Fe) 21

Later on, Zhou group published the iron-based MOF, named PCN-600(Fe), which adopted stp-a topology and possessed a giant pore size (3.1 nm) (Figure 4a)

Subsequently, the iron analogue, PCN-600(Fe), was employed for enzyme mimetic co- oxidation of phenol and 4-aminoantipyrine (Figure 4b) Albeit lower reaction speed of the catalyst was observed in comparing with cytochrome (k cat: 0.66 min -1 for PCN-600(Fe) and 9.59 min -1 for cytochrome), the authors could claim for the higher efficiency of PCN-600(Fe) active centers despite the low speed, which caused by low diffusion of substrates 21

1.2.1.2 Strategy for Design the Catalytic Active Centers in MOFs

SYNTHESIS OF THE NOVEL METAL-ORGANIC FRAMEWORKS

Introduction

In MOFs chemistry, choice of metals as well as organic linker determined structure and properties of obtained materials The linkers for MOFs usually contain multifunctional chelating organic groups in which the most common are carboxylates, 80 pyridine, 81 imidazole, 82 phosphonates and sulfonates, 83 among others In addition, the linkers with different functional groups, length and bond angle significantly contributed to define framework structure and properties (Table 3) 84–87

Recently, more than 20.000 different MOFs have been reported Several of these were found to have the capabilities to solve challenge which encountered in modern ages Despite the significant progress in synthesis and applications of MOFs, there are maintained challenges sought to overcome by novel MOFs, which possess novel or enhanced properties, for example, the quest to synthesize better proton conducting membrane that can maintain high conductivity (>10 -2 S cm -1 ) at medium temperature (T ≥ 100 °C) or a demand for larger pore aperture porous material which can serve as host scaffold for various doping of active guest molecules as well as played the catalyst for the transformation of large organic substrates

Table 3 Famous MOFs that was synthesized by commercial linkers

In last two decades, a vast number of MOFs have been synthesized from the cheap and commercial linkers, taken notices, our survey in Cambridge structural database gave approximately 5092 structures, in which terephthalic acid (H2BDC) was found to be the key constructed component However, respecting to cheap cost for consequent vast production, in scope of exploration, we targeted to employ the cheap and commercial linkers as well as earth abundant metals such as iron and cobalt to synthesize the novel metal-organic frameworks

Subsequently, our newly discovered crystal structure were employed as standpoints for initially justifying the interesting properties of novel MOFs in order to be employed in relevant applications

During the last two decades, the huge number of MOFs have been synthesized by several common organic building blocks, for examples, 1,4-diazabicyclo[2.2.2]octane (DABCO); terephthalic acid (H2BDC); trimesic acid (H3BTC); aminoterephthalic acid (NH2-H2BDC) and 1,6-naphthalene dicarboxylic acid (H2NDC) In fact, there are maintained vast majority of unexplored synthetic conditions, in which the mixture of several organic building blocks, incorporating various metal sources, have been carried out yet to synthesize metal-organic frameworks

Hence, we employed single linker as well as the mixed linker strategy, which incorporated with cobalt and iron metal sources to approach diverse novel metal- organic frameworks which possess the enhanced or novel properties, in which the new material can be utilized for relevant applications.

Materials and Instrumentation

9,10-Anthraquinone was purchased from Merck Co Iron sulfate heptahydrate (FeSO4ã7H2O, 99% purity), copper chloride dihydrate (CuCl2ã2H2O, 99% purity),

25 cobalt nitrate hexahydrate (Co(NO3)2ã6H2O, 98%), 1,4-Diazabicyclo[2.2.2]octane (DABCO, 98%), trimesic acid (H3BTC, 95%), aminoterephthalic acid (NH2-H2BDC, 99%), benzene-1,4-dicarboxylic acid (H2BDC, 98% purity) and 2,6-naphthalene dicarboxylic acid (H2NDC, 98% purity) were purchased from Sigma-Aldrich

Anhydrous N,N-dimethylformamide (DMF, 99% extra dry grade) and dichloromethane (DCM, 99% extra dry grade), acetic acid (CH3COOH, 99.8%), hydrochloride acid (HCl, 35-37%) were obtained from Sigma-Aldrich, Acros and Merck for using without further purification

All reagents use for catalysis study were obtained commercially from Sigma- Aldrich and Merck, and were used as received without any further purification unless otherwise noted

2.2.2 Single Crystal X-ray Diffraction (SC-XRD) and Powder X-ray Diffraction (PXRD) Data Collection

The single X-ray diffraction data for VNU-10, VNU-15, Fe-NH2BDC, Fe-BTC were collected on a Bruker D8 Venture diffractometer equipped with a PHOTON-100 CMOS detector Monochromatic microfocus Cu Kα radiation (λ = 1.54178 Å) operated at 50 kV and 1.0 mA was used Prior to data collection, a single crystal of VNU-10 and VNU-15 were cooled down to 100 K by chilled nitrogen flow controlled by a Kryoflex II system Unit cell determination was performed in the Bruker SMART APEX II software suite The data sets were reduced and a multi-scan spherical absorption correction was implemented in the SCALE interface The structures were solved with direct methods and further refined by the full-matrix least-squares method in the SHELX-97 program package After location of all framework atoms in the difference Fourier maps, the SQUEEZE routine in PLATON was run to remove scattering from the highly disordered guest molecules inside the pores 88 Crystallographic data for VNU-10, VNU-15 has been deposited in the Cambridge Crystallographic Data Centre (deposition no 1063411 and 1443413), CIF files can be obtained free of charge via the Crystallographic Data Centre website Powder X-ray diffraction patterns (PXRD) were

26 collected using a Bruker D8 Advance equipped with a Ni filtered Cu Kα radiation (λ 1.54178 Å) source The diffractometer was also equipped with an anti-scattering shield that prevented incident diffuse radiation from hitting the detector

2.2.3 Instruments for Characterization of VNU-10, VNU-15, Fe-NH 2 BDC, Fe-BTC

Nitrogen physisorption measurements of VNU-10 were conducted using Autosorb IQ volumetric adsorption analyzer system Samples were pretreated by heating under vacuum at 100 °C for 12 h using MasterPrep system N2, CO2, CH4 physisorption measurements of VNU-15 were conducted with VNU-15 samples which were pretreated by heating under vacuum at 100 °C for 12 h in MasterPrep system using auto-sorp IQ volumetric gas adsorption analyzer The experiment temperature was controlled via water circulator Ultrahigh-purity-grade N2 and He (99.999% purity) were used throughout adsorption experiments High grade CO2 (99.95% purity) was used for the respective adsorption experiments Water uptake of VNU-15 was measured using a BELSORP-aqua 3 with the experiment temperature being controlled via water circulator Thermal gravimetric analysis (TGA) was measured by a TA Instruments Q- 500 thermal gravimetric analyzer under a gas mixture of O2 (20%) and N2 (80%) with temperature ramp of 5 °C min -1 Fourier transform infrared spectroscopy (FT-IR) was measured by a Bruker ALPHA FTIR spectrometer using potassium bromide pellets

The amount cobalt in VNU-10 was measured by atomic absorption spectroscopy (AAS) using an AA-6800 Shimadzu analyzer, the amount copper in VNU-15 was measured by atomic absorption spectroscopy (AAS) was performed at Vietnam Academy of Science And Technology using a Shimadzu AA 6200 analyzer Elemental analysis of VNU-10 and VNU-15 was performed in the Microanalytical Laboratory of the College of Chemistry at UC Berkeley using a Perkin Elmer 2400 Series II combustion analyzer

Crystal picture of VNU-10 and VNU-15 was taken using a Nikon SMZ1000 microscope

Material Synthesis, Single Crystal Structure Analysis and Characterization for VNU-10

Scheme 2 Synthetic scheme for crystallizing green, needle VNU-10

In a typical synthesis procedure, a mixture of 1,4-benzenedicarboxylic acid (H2BDC) (0.1 g, 0.60 mmol), 1,4-Diazabicyclo[2.2.2]octane (DABCO) (0.075 g, 0.67 mmol), and Co(NO3)2ã6H2O (0.1 g, 0.34 mmol) was dissolved in a solvent mixture of

N,N-dimethylformamide (DMF) (20 mL), CH3COOH (2 mL, 0.01 mmol), and HCl (20 μL, 0.24 μmol) The resulting solution was then dispensed equally to ten vials (10 mL)

The vials were heated at 120 °C in an isothermal oven for 12 h After cooling the vials to room temperature, the solid product was removed by decanting the mother liquor and then washed with DMF (3 × 10 mL) to remove any unreacted species The DMF solvent was exchanged with dichloromethane (DCM) (3 × 10 mL) at room temperature

The product was then dried at 120 °C for 4 h under vacuum, yielding green needle- shaped crystals of VNU-10 (76% based on Co(NO3)2ã6H2O) (Scheme 2) EA: Calcd for Co2C22H26O11N2 = [Co2(BDC)2(DABCO)]∙3H2O: C, 43.15; H, 4.28; N, 4.58%

Found: C, 43.19; H, 4.35; N, 4.50% AAS indicated cobalt amount of 20.0%, which matched with calculated value of 20.9%

Adding the mixture of CH3COOH and HCl to the solution of H2BDC, DABCO and Co(NO3)2ã6H2O in DMF leading to new cobalt MOF with kgm topology, which was termed as VNU-10 89

Fig 13 Structure of VNU-10, the paddle wheel cluster are connected with BDC 2- by two different way to form the DABCO connected kgm layers of VNU-10 and DABCO connected sql layer of Co2(BDC)2(DABCO) C, black; O, red; Co, light blue; N, blue; H was omitted for clarity

Fig 14 Crystal structure of VNU-10 represented in DABCO connected kgm layers; a) Vertexes and edges assignment for cobalt nodes and linkages of VNU-10; b) Structure of VNU-10 represented in DABCO connected kgm layers Black, BDC 2- ; Blue, DABCO; light blue, paddle wheel cobalt nodes; yellow, linkages between iron nodes

The structure of VNU-10 was solved by single crystal X-ray diffraction (Figure 15 and Table 4) The architecture of VNU-10 was further revealed to consist of cobalt paddlewheel units that are connected, in a slightly bent fashion, by four BDC 2- linkers

This cluster formation coupled with the bent nature of the linkage allowed the formation of two-dimensional kagome (kgm) sheets Additionally, DABCO ligands on the apical sites of the paddle wheel cobalt clusters to link each kgm sheet Consequently, a three- dimensional framework is formed with 4.5 Å triangular and 15 Å hexagonal channels running along the c axis and a ~3.5 Å rectangular channel along the a and b axes (Figure 13 and 14)

Fig 15 Thermal ellipsoid plot of the asymmetric unit of VNU-10 with 30% probability C, black; O, red; Co, light blue; N, blue; H, white

Table 4 Crystal data and structure refinement for VNU-10

Identification code VNU-10 VNU-10 with SQUEEZE

Empirical formula C11H10NO11Co C11 H10 NO4Co

Crystal size (mm 3 ) 0.2 × 0.04 × 0.04 θ range for data collection 2.37 ° to 65.10 °

R 1 , wR 2 (all data) R 1 = 0.1010, wR 2 = 0.2533 R 1 = 0.0644, wR 2 = 0.1551

Largest diff peak and hole (eãÅ -3 ) 0.831 and -0.321 0.365 and -1.171

Although DABCO connected kgm layers has been observed for Zn2(BDC)2(DABCO) kgm and Ni2(BDC)2(DABCO) kgm isomer and the cobalt MOF which were constructed from BDC 2- linker and cobalt paddle wheel SBU to form DABCO connected sql layers which composed square-grid layers and DABCO pillars were well-known, DABCO connected kgm layers structure which was constructed from BDC 2- and cobalt paddle wheel SBU was first reported of the Co-based version

2.3.3 Characterization of VNU-10 2.3.3.1 Microscope Image of VNU-10

Fig 16 Green needle crystal of VNU-10 at forty zooming times

Phase purity of the bulk sample was confirmed by PXRD analysis For this analysis, PXRD patterns were calculated and generated based on structural models of Co2(BDC)2(DABCO) for both the pillared kgm and sql topological isomers and compared with the measured experimental pattern for VNU-10 The results indicated that the structure of the as-synthesized VNU-10 was consistent with the single crystal data for the pillared kgm Co2(BDC)2(DABCO) isomer (Figure 17)

Fig 17 The calculated PXRD pattern of VNU-10 from single crystal data (red) compared with the experimental patterns from the as-synthesized VNU-10 (orange) and Co2(BDC)2DABCOsql

2.3.3.3 FT-IR Analysis of activated VNU-10

Fig 18 FT-IR of activated VNU-10; inset: zooming with wavelength from 1450 to 1690 cm -1

FT-IR of activated VNU-10 indicated the presence of DABCO ligand and bonded carboxylate organic linkers The partial overlapping peak at 1620 cm -1 is assigned as stretching of coordinated carboxylate, sharp peaks at 1043 cm -1 are assigned as the vibration of C-N bond (Figure 18)

Fig 19 Thermogravimetric analysis of VNU-10 in air stream under 20% O2 and 80% N2

The framework thermal stability and architectural robustness of VNU-10 was assessed by thermal gravimetric analysis (TGA) Accordingly, as-synthesized VNU- 10 exhibited significant weight lost (40% by weight) at low temperature (T < 150 °C) which corresponded to the removal of H2O and DMF solvent in the pore Activated VNU-10 demonstrated high thermal stability and architectural robustness (T ~ 350 °C) and Co3O4 residue at 500 °C matched well with amount of Co3O4 derived from structure formula (29.6 wt% versus 28.8 wt% for the experimental and theoretical Co3O4, respectively) (Figure 19)

Nitrogen isotherm measurements at 77 K confirmed the permanent porosity of VNU-10 with calculated Brunauer-Emmett-Teller and Langmuir surface areas of 2396 m 2 g -1 and 2604 m 2 g -1 , respectively (Figure 20)

Fig 20 N2 adsorption isotherm of VNU-10 at 77 K

Taking from high surface area of VNU-10, we sought to analyze the CO2, CH4 and N2 adsorption of VNU-10 Accordingly, VNU-10 exhibited high CO2 uptake at room temperature Moreover, CO2 / CH4 and CO2 / N2 selectivity were calculated using ration between initial slopes of CO2 isotherm over CH4 and N2 isotherm under Henry’s law pressure to be 3.5 and 11.8 times, respectively (Figure 21, 22 and Table 5)

Fig 21 CO2, CH4, N2 adsorption isotherm of VNU-10 at 273 K

Table 5 CO2, CH4, N2 uptake at 802 Torr and selectivity in adsorption of CO2 over CH4 and N2 of VNU-10

Material Synthesis, Single Crystal Structure Analysis and Characterization for the

A mixture of H2BDC (60 mg, 0.36 mmol), H2NDC (60 mg, 0.27 mmol), 9,10- anthraquinone (30 mg, 0.25 mmol), FeSO4ã7H2O (60 mg, 0.143 mmol), and CuCl2ã2H2O (60 mg, 0.345 mmol) were dissolved in 10 mL DMF The solution was

36 sonicated for 10 min and then divided between six borosilicate glass tubes (1.7 mL each tube) The glass tubes were subsequently flame sealed under ambient conditions and placed in an isothermal oven, preheated at 165 °C, for four days to yield reddish-yellow crystals of VNU-15

Scheme 3 Synthetic scheme for crystallizing reddish-yellow, octahedral VNU-15

These crystals were washed with 10 mL DMF (6 times) and immersed in DMF three days before exchanging the solvent with 10 mL DCM over two days (6 times for exchanging solvent) Thereafter, VNU-15 was activated at 100 °C to obtain 34 mg (0.051 mmol) of dried VNU-15 (71.3% yield based on iron) (Scheme 3) Note: MIL- 53 was formed in the absence of 9,10-anthraquinone to the reaction mixture 85 Furthermore, MIL-88 was formed without CuCl2ã2H2O added to the reaction mixture 90 EA of activated VNU-15: Calcd for Fe4C37.8H71.4N4.68O38.64S4 {[Fe4(NDC)(BDC)2DMA4.2(SO4)4]ã0.4DMF}ã10H2O: C, 29.38; H, 4.62; N, 4.25; S, 8.29% Found: C, 28.95; H, 4.64; N, 4.74; S, 8.13% Atomic absorption spectroscopy (AAS) of activated VNU-15: 0.036 wt% copper

The Assembly of H2BDC, H2NDC and anthraquinone, Fe(SO4)2ã7H2O and CuCl2ã3H2O in DMF leading to new iron based MOF with new fob topology, which was termed as VNU-15 89

Fig 23 Crystal structure of VNU-15 is constructed from BDC 2- and NDC 2- linkers that stitch together corrugated infinite rods of [Fe2(CO2)3(SO4)2(DMA)2]∞ (a) These corrugated infinite rods propagate along the a and b axes to form the three-dimensional architecture The structure is shown from the [110] and [001] plans (b, c, respectively) Atom colors: Fe, orange and blue polyhedra; C, black; O, red; S, yellow; N, blue; and DMA cations, light blue All other H atoms are omitted for clarity

Single crystal X-ray diffraction (SCXRD) analysis revealed that VNU-15 crystallized in the orthorhombic space group, Fddd (No 70), with unit cell parameters, a = 16.7581, b = 18.8268, and c = 38.9998 Å (Table 6 and Figure

25) The architecture of VNU-15 is based on two distinct linkers, namely BDC 2- and NDC 2- , that stitch together corrugated iron infinite rod SBUs These infinite rod SBUs, formulated as Fe2(CO2)3(SO4)2(DMA)2]∞, are composed of two independent octahedral iron atoms that alternate consecutively in order (Figure 23a) The coordination environment of each distinct iron atom is highlighted by two equatorial corner-sharing vertices derived from μ2-O atoms of the carboxylate functionality in NDC 2- It is noted that these μ2-O atoms, which are cis to one another, are what promote the infinite rod SBU to arrange in a

38 corrugated fashion The coordination sphere of each iron is then completed through bridging axial sulphate ligands and bridging carboxylate functionalities from BDC 2- (Figure 23a)

Fig 24 Representation of the fob topology that VNU-15 adopts a) Vertexes and edges assignment for iron nodes and linkages of VNU-15; b) Structure of VNU-15 represented in fob topology Atom colors: Fe, orange and blue polyhedra; C, black; O, red; S, yellow; N, blue; and DMA cations, light blue All other H atoms are omitted for clarity

Two BDC 2- and one NDC 2- linkers, relatively close together in space (aromatic π–π interaction distance, 3.4 Å), connect infinite rods together periodically in a perpendicular manner (83.4°) (Figure 23b and c) This propagates a three-dimensional architecture with the fob topology (Figure 24)

We deduce that the π–π interactions played an important role in forming the realized fob topological structure Finally, DMA counterions were found to line the infinite rod SBUs due to hydrogen bonding with the axial bridging sulphate ligands (N-

HãããO-S distances of 1.90 - 1.96 Å) (Figure 23) Taken together, the resulting pore size of VNU-15, as calculated by PLATON, is 2.52 Å

Crystal size (mm) 0.131 × 0.143 × 0.234 θ range (°) 3.6931 to 67.9142

Largest diff peak and hole (eãÅ -3 ) 0.651 and -0.401

Fig 25 Thermal ellipsoid plot of the asymmetric unit of VNU-15 with 50% probability C, black; O, red; Fe, orange; S, yellow; N, blue; H, white

Fig 26 Orange octahedral crystal of VNU-15 at forty zooming times

Phase purity of the bulk VNU-15 was confirmed by PXRD analysis For this analysis, PXRD patterns were calculated and generated based on structural models of VNU-15 and compared with the measured experimental pattern of VNU-15 The results indicated that the structure of the as-synthesized and activated VNU-15 was consistent with the single crystal data (Figure 27)

Fig 27 The calculated PXRD pattern of VNU-15 from single crystal data (black) compared with the experimental patterns from the as-synthesized sample (blue) and samples after activation at 100 °C (red)

Fourier transform infrared (FT-IR) spectroscopy analysis highlighted the presence of hydrogen-bonded DMA to sulphate ions that bridged two iron atoms in the activated sample of VNU-15 Specifically, a broad peak originating at 3400-3500 cm -1 , in conjugation with a sharp stretching peak at 2781 cm -1 , were assigned to N-H vibrations and C-H stretches of the DMA molecules, respectively Furthermore, the vibration modes of the bridging sulphate ligands were clearly identified (sharp peaks, centred at 983, 1037, 1110, 1143 cm -1 )

Finally, the partial overlapping peak at 1606 cm -1 was assigned as stretching of coordinated carboxylate (Figure 28)

Fig 28 FT-IR spectra of activated VNU-15

Fig 29 Thermogravimetric analysis of VNU-15 in air stream with 20% O2 and 80% N2 The framework thermal stability and architectural robustness of VNU-15 was assessed by thermal gravimetric analysis (TGA) The TGA curve of VNU-15

43 exhibited a small weight percentage loss ( 14 Å) as highly active heterogeneous catalyst for large organic substrates transformation while the smaller pore size MOFs could not proceed the reaction (Scheme 7)

Gas chromatographic (GC) analyses were performed using a Shimadzu GC 2010-

Plus equipped with a flame ionization detector (FID) and an SPB-5 column (length 30 m, inner diameter = 0.25 mm, and film thickness = 0.25 μm) The temperature program for GC analysis held samples at 120 °C for 1 min; heated them from 120 to 180 °C at 50 °C/min; held them at 180 °C for 1 min; heated them from 180 to 280 °C at 50 °C/min and held them at 280 °C for 3 min Inlet and detector temperatures were set constant at 280 °C Diphenyl ether was used as an internal standard to calculate reaction conversions GC-MS analyses were performed using a Hewlett Packard GC-MS 5972 with a RTX-5MS column (length = 30 m, inner diameter = 0.25 mm, and film thickness = 0.5 μm) The temperature program for GC-MS analysis heated samples from 60 to 280 °C at 10°C/min and held them at 280 °C for 2 min Inlet temperature was set constant at 280 °C MS spectra were compared with the spectra gathered in the NIST library 1 H and 13 C NMR spectra were recorded in CDCl3 using TMS as an

61 internal standard on a Bruker NMR spectrometer at 500 MHz and 125 MHz, respectively

From the data obtained from GC diagram, which is in form of peak position and peak’s area, the conversion of the reaction based on product was calculated by the following formula

 C(t) : The conversion of reaction based on product at the time t

 S product : The peak’s area of product

 S internal standard : The peak’s area of internal standard

 t : The time of each aliquot was withdrawn

 tc : The time when conversion achieved 100 %

The S(product) S(internal standard)(tc) ratio was figured out when the proceeding ratio of

A mixture of benzoxazole (0.101 mL, 1 mmol), acetic acid (0.114 mL, 2 mmol), and diphenyl ether (0.15 mL, 0.95 mmol), as an internal standard, in acetonitrile (5 mL) was added together into a 25 mL round bottom flask containing the Co2(BDC)2(DABCO) (VNU-10) catalyst (0.0143 g, 5 mol%) The catalyst amount was calculated with respect to the cobalt/benzoxazole molar ratio The reaction mixture was magnetically stirred for 3 min to disperse the catalyst entirely the catalyst throughout the liquid phase Piperidine and tert-butyl hydroperoxide (TBHP) were then added The resulting mixture was continuously stirred at room temperature for 1 h Reaction

62 conversion was monitored by withdrawing aliquots from the reaction mixture at different time intervals, followed by quenching with aqueous KOH solution (5% (w/w), 1 mL) The organic components were then extracted into ethyl acetate (2 mL), dried over anhydrous Na2SO4, and then analyzed by GC with reference to diphenyl ether The product identity was further confirmed by GC-MS, 1 H-NMR, and 13 C-NMR In recycling studies, the catalyst was separated from the reaction mixture by centrifugation, washed, and heated with copious amounts of DMF at 100 °C for 2 h

The recovered VNU-10 was then activated under vacuum at room temperature for 4 h, and reused for the next run under identical conditions For the leaching test, the catalytic reaction was stopped after 5 min, analyzed by GC, and centrifuged to remove the solid catalyst The reaction solution was then stirred for an additional further 55 min

Reaction progress, if any, was monitored by GC as previously described

Ni2(BDC)2(DABCO) sql , Cu2(BDC)2(DABCO) sql , Co2(BDC)2(DABCO) sql ,Co-ZIF- 67 were synthesized based on previous reported procedure 23,124–126

3.1.6 Investigations on VNU-10 Catalytic Performance for Direct Oxidative Amination of Benzoxazole with Piperidine

3.1.6.1 Conditions Screening for Direct Oxidative Amination of Benzoxazole with Piperidine Using Heterogeneous VNU-10

In order to find the optimized condition for amination of benzoxazole, systematic investigation with respecting the affection of reagent ratios (piperidine : benzoxazole, eq.), amounts of VNU-10 catalyst (mol% catalyst to benzoxazole, reaction solvents (acetonitrile, dichloromethane, toluene, dioxane, ethanol), types and amounts of proton donor (acetic acid, benzoic acid, formic acid, trifluoroacetic acid) were carried out

According to previous reports, several initial factors, for example, solvent (CH3CN), types of proton donors (provide by acetic acid), oxidant (tert-butyl hydroperoxide, TBHP) was chosen for the catalytic oxidative amination of benzoxazole with piperidine, in which VNU-10 as the catalyst (Scheme 8)

Scheme 8 Initial screening factors for direct oxidative amination of benzoxazole with piperidine

3.1.6.1.1 Effect of Reagent Ratio on GC Yield

VNU-10 was assessed for its catalytic activity in the direct amination via C-H functionalization of benzoxazole with piperidine to form 2-(piperidin-1-yl)benzoxazole as the principal product

Scheme 9 Initial factors to investigate the effect of reagent ratio on GC yield of 2-

Fig 45 Effect of benzoxazole/piperidine molar ratio on GC yield of 2-

HIGH PROTON CONDUCTIVITY AT LOW RELATIVE HUMIDITY IN AN

3.2.1 Introduction of Hydrogen Fuel Cell, Impedance and Nyquist Plot of Impedance

A Hydrogen fuel cell is a device that converts chemical potential energy (energy stored in molecular bonds) into electrical energy A PEM (Proton Exchange Membrane) cell uses hydrogen gas (H2) and oxygen gas (O2) as fuel The products of the reaction in the cell are water, electricity, and heat

Fig 63 Typical structure of Hydrogen fuel cell

Typically, the fuel cell were assembled by three parts which included anode, Proton conducting membrane and cathode (Figure 62)

The anode which consisted of thin film of high surface area carbon and a catalyst which can split H2 into hydrogen atoms, the hydrogen atoms was further oxidized to H + and release electron which travel along thin activated carbon layer to external circle to the cathode in order to generate the electric current

Proton conducting membrane was inserted between anode and cathode, which allowed H + travel from anode to cathode while serve as insulator for electron and force electron to travel along external circle to cathode For hydrogen fuel cell, the membrane, commonly was hydrated in order to function and remain high proton conductivity

The cathode was constructed from the catalyst coated on porous carbon, which allows the distribution of O2 and flush of electron from external circle in order to combine with H + to form H2O as the final product

The catalyst is a special material that facilitates the reaction of oxygen and hydrogen It is usually made of platinum nanoparticles very thinly coated onto porous carbon The catalyst is rough and porous so that the maximum surface area of the platinum can be exposed to the hydrogen or oxygen The platinum-coated side of the catalyst faces the PEM

By converting chemical potential energy directly into electrical energy, fuel cells avoid the "thermal bottleneck" (a consequence of the 2 nd law of thermodynamics) and are thus inherently more efficient than combustion engines, which must first convert chemical potential energy into heat, and then mechanical work

Direct emissions from a fuel cell vehicle are just water and a little heat This is a huge improvement over the internal combustion engine's litany of greenhouse gases

Fuel cells have no moving parts They are thus much more reliable than traditional engines

Hydrogen can be produced in an environmentally friendly manner, while oil extraction and refining is very damaging

3.3.1.2 Definition of Impedance and Nyquist Plot of Impedance

Electrical impedance (Z) is the opposition that a circuit presents to alternating current (AC) when AC voltage is applied Impedance is the voltage–current ratio for a single complex exponential at a particular frequency ω (Hz)

Briefly, impedance will be a complex number, with the same units as resistance, for which the SI unit is the ohm (Ω) Basically, impedance is defined as Z = Z’(ω) – jZ’’(ω) where Z’ is the real resistance (Ω) and Z’’ is the imagine resistance (Ω)

Fig 64 Typical Nyquist plot and an equivalent circuit used for fitting Schematic representations: R c /R m , resistor; W, Warburg diffusion element; C, capacitor

Nyquist plots is the plot of Z’ (Ox axis) and Z’’ (Ox axis) into Cartesian coordinate system at particular frequency ω (Hz)

Equivalent Circuit is the electrical circuit that was used to generated Nyquist plots which coincidently matched with experimental Nyquist plots Equivalent Circuit can be used to obtain the resistance in a proton conducting membranes (Figure 63)

3.2.2 The Quest of Proton Conducting Membrane that Maintain High Conductivity at High Temperature and Low Humidity

The development of novel electrolyte materials for proton exchange membrane fuel cells has received considerable attention owing to the need for alternative energy technologies 128,129 Traditional electrolyte materials, such as fully hydrated Nafion, are capable of reaching proton conductivities of 1 × 10 -1 S cm -1 at 80 °C However, to reach these levels, the material must remain in a relatively high humid environment (98% relative humidity, RH) 130,131 This poses significant challenges, including substantial costs associated with maintaining the appropriate level of humidity as well as the possibility of flooding the cathode leading to a loss in fuel cell performance 2 Furthermore, high operating temperatures, which lessen CO poisoning at Pt-based catalysts and increase efficiency, lead to decreased conductivities as a result of dehydration of the electrolyte material 132 Therefore, the development of novel electrolyte materials that maintain ultrahigh proton conductivity at elevated temperatures and under low relative humidity are highly sought after 133

Recently, metal-organic frameworks (MOFs) have been explored as potential candidates for use as electrolyte materials 40 This is primarily due to the modular nature of MOF design and synthesis, in which the backbone components [e.g inorganic and organic secondary building units (SBUs)] can be easily tailored to satisfy particular applications 1 Indeed, previous work on developing MOFs as proton conducting materials have focused on incorporating proton transfer agents within the pores, 41–43 functionalizing coordinatively unsaturated metal sites, 44,45 tuning the acidity of the pore channels through incorporating specific functional groups, 46–51 and controlling and modifying defect sites, 52,53 among others 3 These strategies have led to significant developmental progress, in which proton conductivities in MOFs have been achieved on the order of 10 -2 S cm -1 , but require high working relative humidity (≥ 90% RH) On the other hand, proton

85 conductivity under anhydrous conditions (T ≥ 100 °C) in MOFs has reached ultrahigh levels (10 -2 S cm -1 ), albeit in a limited number of reports.66,76,77,79

Achieving high proton conductivity at elevated temperature (T ≥ 95 ºC) under low humidity (RH ≤ 60%)

As the synthesis and full characterization of a novel iron-based MOFs (VNU- 15), formulated as Fe4(BDC)2(NDC)(SO4)4(Me2NH2)4 (BDC = 1,4- dicarboxylate; NDC = 2,6-napthalenedicarboxylate), have been done in previous investigation, the architecture of VNU-15 was known to adopts the three- dimensional fob topology with new iron rod-shaped SBUs, previously unseen in MOF chemistry

As densely occupation of coordinated sulfate ligands to the iron SBUs, ordered dimethylammonium (DMA) cations were found to occupy the pore channels of VNU-15 via hydrogen bonding leading to a plausible conduction pathway Accordingly, VNU-15 could exhibit ultrahigh conductivity under low relative humidity (RH ≤ 60%)

3.2.5 Method for Proton Conductivity Measurement

3.2.5.1 Preparation of Pelletized VNU-15 and Proton Conductivity Measurement

Impedance analysis of VNU-15 pelleted samples (13 mm diameter, pressing at 3 ton cm -2 ) was measured by a Gamry potentiostat (model Interface 1000™) using the two-probe method Humidity was controlled by an Espec humidity chamber (model SH-222) The measuring frequency ranged from 1 MHz to 10 Hz The applied voltage varied from 1 mV to 30 mV depending on open circle voltage The thickness of VNU-15 pallet was measured using a Nikon SMZ1000 microscope, with a typical pellet thickness ranging from 0.4 to 0.5 mm

Aiming to achieve ultrahigh proton conductivity at elevated temperature and under medium RH, we carried out the impedance analysis of VNU-15 at RH ≤ 60% while gradually changing temperature between 25 °C to 95 ºC

3.2.5.2 Data Proceeding to Obtain Proton Conductivity

Fig 65 An equivalent circuit used for fitting Schematic representations: R 1/R 2/R 3, resistor; W 1, Warburg diffusion element; Q 1/Q 2/Q 3, imperfect capacitor

Fig 66 Nyquist plot derived from equivalent circuit (black line) and experimental Nyquist plot

(blue circles) of pelletized VNU-15 under 60% RH at 25 °C Frequency ranged from 1 MHz to 10 Hz Inset: Zoom of Nyquist plot at high frequency

Fig 67 Nyquist plot derived from equivalent circuit (black line) and experimental Nyquist plot

(blue circles) of pelletized VNU-15 under 60% RH at 95 °C Frequency ranged from 1 MHz to 10 Hz Inset: Zoom of Nyquist plot at high frequency

Synthetic scheme for crystallizing green, needle VNU-10