Gao, Shan
(2016)
Gas adsorption and selectivity studies for porous metal-organic frameworks.
PhD thesis, University of Nottingham.
Abstract
This thesis describes research on the preparation and characterisation of porous metal-organic framework (MOF) materials for gas storage and separation applications, with particular focus on light hydrocarbons. This work is of great importance in the structural flexibility studies of MOF materials upon the physisorption of gas molecules. It also involves comprehensive gas sorption and separation studies of a Zr-based MOF, MFM-601, with a pioneering investigation for NH3 uptakes for this species of MOFs.
Chapter 1 outlines the background and reasons for designing the targeted MOFs in this work. It reviews a series of porous materials on their general syntheses, structures, properties and potential applications. Among these porous materials, MOFs are highlighted, with reviews mainly focusing on design of flexible MOFs and isostructural MOFs due to their important applications. In addition, this chapter summarises the importance and interests of porous materials for light hydrocarbon sorption and separation, toxic gas storage and carbon capture.
Chapter 2 describes and discusses a comprehensive study of a flexible MOF, MFM-202a, in the applications of light hydrocarbon (CH4, C2H2, C2H4, C2H6, C3H6 and C3H8) sorption and separation. MFM-202a is known to be a flexible crystalline material that shows dramatic structural flexibility upon adsorption of CO2 and SO2 gases, which is indicated by the stepwise adsorption and hysteretic desorption. Interestingly, the light hydrocarbon sorption of this flexible MOF also exhibits stepwise adsorption and hysteretic desorption isotherms for C2H2, C2H4 and C3H6 at ~195 K. However, for the saturated hydrocarbons, such as CH4, C2H6 and C3H8, no correlated hysteresis is observed. To gain insights into the structural change for the C2 hydrocarbons, in situ synchrotron power diffraction (in situ PXRD) experiments have been carried out. The results suggest that MFM-202a again exhibits structural flexibility upon the sorption of C2 hydrocarbons. Le Bail refinements have been performed to give the specific unit cell parameters upon sorption of the three C2 hydrocarbons. In addition, MFM-202a shows very high uptakes of C2H2 (18.3 mmol g-1) and C2H4 (14.8 mmol g-1) and C3s (11.9 mmol g-1) at 195 K, compared with other MOFs.
The isotherms of these light hydrocarbons at 273-303 K have also been carried out and show remarkable differences in sorption capacities, suggesting potential hydrocarbon separation applications. In this chapter, two methods have been adopted in the calculations of selectivities, dual-site Langmuir Freundlich-Ideal adsorbed solution theory (DSLF-IAST) and numerical integration-IAST (NI-IAST). MFM-202a exhibits excellent selectivities for C2s/CH4 (~7-12), C3s/CH4 (~63-105) and an inverse selectivity for C2H4/C2H6 (~0.7).
Chapter 3 describes the uptake of NH3 for MFM-202a at 293 K over multiple cycles. In the first cycle, MFM-202a exhibits a high uptake capacity of 8.5 mmol g-1. However, in the second cycle, the uptake shows a dramatic decrease. As NH3 is a polar molecule, it may interact strongly with the framework and lead to a possible structural change. To investigate the structural response upon NH3 adsorption of MFM-202a, in situ PXRD and in situ X-ray pair distribution function (PDF) experiments have been carried out for MFM-202a during the adsorption and desorption of NH3. Interestingly, the time- dependent in situ PXRD patterns of the NH3-loaded MFM-202a shows a gradual loss of peaks, indicating the loss of crystallinity upon interaction with NH3. However, after a very short time period, new diffraction peaks appear which differ from the peaks of the activated material. This suggests that the framework undergoes a fast rearrangement of the structure after the collapse of the original structure upon adsorption of NH3. Upon desorption, the in situ PXRD patterns of the new phase is retained, indicating an irreversible phase transition upon the uptake of NH3.
Chapter 4 presents the synthesis and characterisation of four novel Zr- and Hf-based MOFs. The two Hf-based MOFs have been synthesised using the strategy of isostructural MOF design. Zr6(L1)2(μ3- O)4(μ3-OH)4(OH)4(H2O)4 and its isostructural Hf6(L1)2(μ3-O)4(μ3-OH)4(OH)4(H2O)4 are synthesised from the linker of biphenyl-3,3',5,5'-tetra-(phenyl-4-carboxylic acid) (H4L1) and both crystallise in space group Fmmm. In comparison, the Zr6(L2)2(μ3-O)4(μ3-OH)4(OH)4(H2O)4 (denoted MFM-601) is synthesised from the amine-functionalised linker of 4,4'-diamino-3,3',5,5'-tetra(4- carboxyphenyl)biphenyl (H4L2) and crystallises in Cmmm space group. The difference in structure owes to the different conformation of the two linkers. H4L1 adopts a twisted conformation for the two internal phenyl rings whereas H4L2 adopts a flat style. With similar porosity to MFM-202a, MFM-601 has also been investigated for a series of gas storage and separation applications. Light hydrocarbon adsorption measurements carried out at low temperature show dramatic stepwise adsorption, which could be due to the mesopore effect. Remarkably, MFM-601 shows even higher uptakes of C2H2, C2H4, C2H6, C3H6 and C3H8 than MFM-202a at ~195 K. Furthermore, the hydrocarbon uptakes at 293 K also exhibit significant difference in the capacities. DSLF-IAST has been adopted to calculate the selectivities. It shows high selectivity for C2s/CH4 and C3s/CH4. In addition to the light hydrocarbon sorption, CO2 adsorption for MFM-601 has also been carried out, which shows very high CO2 storage (31.3 mmol g- 1) at ~198 K, with stepwise adsorption and hysteretic desorption. This has been further studied by in situ PXRD experiments to observe structural changes during the adsorption of CO2. The CO2-loaded PXRD patterns show significant peak shifts compared with the bare material, confirming the structural change upon the adsorption of CO2. In addition, as a pioneering study of NH3 sorption on a Zr-based MOF, MFM-601 exhibits reasonable repeatability in multiple cycles.
Chapter 5 summarises the experimental results and conclusions of the thesis and gives the perspectives for future investigations of the work.
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