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Title: Metal-organic polyhedral framework materials for hydrogen storage
Author: Yan, Yong
Awarding Body: University of Nottingham
Current Institution: University of Nottingham
Date of Award: 2011
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This thesis describes the design, synthesis and characterisation of a series of novel metal-organic frameworks constructed from Cu(II) paddlewheels and polydentate aromatic carboxylate ligands. Gas sorption applications of these porous materials have been studied, with an emphasis on hydrogen storage. The effects of cage wall functionalisation within the frameworks, internal BET surface areas and pore volumes, and open Cu(II) sites on H2 adsorption by these materials are investigated. Chapter 1 introduces the current status of H2 storage in metal-organic framework materials. Extended metal-organic structures exhibiting large surface areas and high total pore volumes are favourable for H2 storage at high pressures and cryogenic temperatures. The strategy of utilising metal-organic polyhedra as secondary building blocks in the assembly of highly porous frameworks with high structural stability is discussed. Chapter 2 describes the synthesis of a highly porous (3,24)-connected metal-organic polyhedral framework (denoted NOTT-112) constructed from a nanosized hexacarboxylate linker and {CU2(COO)4} paddlewheels. The framework of NOT- 112 is composed of three types of metal-organic polyhedral cages: cuboctahedra, truncated tetrahedra and truncated octahedra. Desolvated NOTT-112 shows a very large apparent surface area of 3800 m2 g-l (BET) and high H2 storage capacity of 7.07 wt% (excess) at 35 bar at 77 K (total H2 uptake of 10.0 wt% at 77 bar and 77 K), making it one of the best materials for H2 storage at cryogenic temperatures and high pressures. In Chapter 3, neutron powder diffraction (NPD) studies on D2 (deuterium)-loaded NOTT-1l2 give the insight into the mechanism of H2 adsorption in this polyhedral MOF with coordinatively unsaturated Cu(II) sites. NPD experiments reveal that the exposed Cu(II) sites within the smallest cuboctahedral cages in NOTT-112 are the first and strongest binding sites for D2 in this material giving an overall discrimination between the two types of exposed Cu(II) sites within one {CU2} paddlewheel unit. Thus, NPD studies provide, for the first time, direct structural evidence demonstrating that a specific geometrical arrangement of exposed Cu(II) sites, in this case within a [Cu24(isophthalate)24] cuboctahedral cage, strengthens the interactions between D2 molecules and open metal sites. In the second part of this chapter, partially deuterated MOF NOTT-112-d13 was synthesised for inelastic neutron scattering experiments. In Chapter 4, four isostructural metal-organic polyhedral cage based frameworks (denoted NOTT-l13, NOTT-114, NOTT-115 and NOTT-118) with (3,24)-connected topology have been synthesised by combining hexacarboxylate isophthalate linkers with {CU2(COO)4} paddlewheels. All four frameworks have the same cub octahedral cage structure constructed from 24 isophthalates from the ligands and 12 {CU2(COO)4} paddlewheel moieties. The frameworks differ only in the functionality of the central core of the hexacarboxylate ligands with phenyl, trimethylphenyl, phenyl amine and triphenylamine moieties in NOTT-118, NOTT-113, NOTT-114 and NOTT-115, respectively. The desolvated framework materials shows high BET surface areas of 3265, 2970, 3424 and 3394 m2 g-l for NOTT-118, NOTT-l13, NOTT-114 and NOTT-115, respectively. Desolvated NOTT-l13 and NOTT-114 show high total H2 adsorption capacities of 6.7 wt% and 6.8 wt% at 77 K and 60 bar, respectively. Desolvated NO TT -118 and NOTT -115 have significantly higher total H2 uptakes of 8.0 wt% and 7.5 wt% under the same conditions, respectively. Analysis of the heats of adsorption (QsD for H2 reveals that with a triphenylamine moiety in the cage wall, desolvated NOTT-115 shows the highest value of Qs/ for these four materials indicating that functionalisation of the cage walls with more aromatic rings can enhance the H2/framework interactions. In contrast, measurement of Qs/ reveals that the amine substituted tris-alkynylbenzene core used in NOTT-114 gives a notably lower H2/framework binding energy. NOTT-112 to NOTT-115 also show high CH4 adsorption capacities (104-124 cm\STP) cm -3) at 20 bar and room temperature. The amine-functionalised NOTT-114 exhibits strong CO2-framework interaction and high CO2 storage capacity of 22.9 mmol g-1 at 20 bar and 298 K. Chapter 5 describes the linker expansion strategy used in construction of highly porous MOFs. Ultrahigh porosity can be achieved by expansion of the C3-symmetric hexacarboxylate linkers in the polyhedral frameworks. Two isostructural noninterpenetrated (3,24)-connected frameworks NOTT-116 and NOTT-119 have been synthesised based on elongated poly-aromatic hexacarboxylate linkers. Both frameworks of NO TT -116 and NOTT -119 consist of mesoporous cages with diameters up to ~ 3 nm. These two mesoporous materials show good thermal stability and can be fully desolvated without framework collapse by traditional activation method (heating the samples under vacuum). Desolvated NOTT -116 exhibits a significantly high BET surface area (4664 m2 g-I) and a high H2 adsorption capacity (saturated excess H2 uptake of 6.4 wt% at 77 K; total uptake of 9.2 wt% at 77 K and 50 bar). NO TT -119 shows a lower specific surface area of 4118 m2 g -I and a lower saturated excess H2 uptake of 5.6 wt% at 77 K, but a larger pore volume of 2.32 cm3 g-l, leading to a high total H2 uptake capacity of 9.2 wt% at 77 K and 60 bar. Another Cu(II) based MOF NOTT-120 constructed from a hexacarboxylate linker as large as 3.0 nm shows ultrahigh porosity with ultrahigh total pore volume of 2.81 cm3 g-l. Chapter 6 summarizes the H2 storage properties of the (3,24)-connected frameworks and draws an overall conclusions from this study.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available