Synthesis and Characterizations of MgAl and LiAl Layered Double Hydroxides and Their Application for Hydrogen Storage and Transesterification
Date Issued
2014
Date
2014
Author(s)
Huang, Yu-Wei
Abstract
This thesis focused on the preparation and applications of layered double hydroxides (LDHs) intercalated with carbonate, acetate, terephthalate, and p-toluate by one-pot coprecipitation method. During the calcination, LDHs were transformed mixed metal oxides (MMOs). The MMOs were resolved in an aqueous solution, and the MMOs would be rearranged in the form of layer structure. On the other hand, when MMOs dissolved into the anionic solution, the structure of MMOs would recover that of LDHs. This special phenomenon is called “Memory Effect”. In order to study the memory effect, synchrotron radiation in-situ X-ray diffraction was utilized to monitor the phase transformation during the calcinations and rehydration process. When temperature rised to 350 oC, the MgAl layered structure started to collapse and the (003) diffraction of LDHs obviously shifted to the higher angle. Subsequently, its corresponding MMO was formed, i.e.: MgO and/or Al2O3, when temperature reached to 450 oC. In contrast to MgAl-LDH, the layer structure of LiAl-LDH would destroy at 200oC, and its derived MMO was appeared when the temperature achieved to 500 oC. The LDH calcined at 600 oC is more difficult to recover back to the layered structure than that calcined at 400 oC. After rehydration, the sample was heated again. The phase-transformation of LDHs to MMOs still observed at the same temperature. However, the recovery rate is slower than as-synthesized sample. Layered double hydroxides are used to prepare various mixed metal oxides with high surface area. In this thesis, layered double hydroxides intercalated with carbonate are calcined around 300-500 oC, and the hydroxides decompose to form mixed metal oxides. Both of LDHs and MMOs are used as the heterogeneous catalysts in the transesterification of tripalmitin and methanol at reflux temperature. Methanol to tripalmitin molar ratio is among 30-90:1; the catalyst amount is based on the amount of used tripalmitin in the range of 1 to 9 wt%. The reaction becomes faster with increasing the ratio of methanol to tripalmitin. When the ratio of methanol to tripalmitin fixed at 60:1 for 3 h, the content of the catalyst amount changed, it is found that the yield is increased with increasing the content until upon to 6 wt%. However, the yield is decreased, when the catalyst amount is above 6 wt%. The yield of LiAl-CO3-LDH- C400 can achieve 99% methyl palmitate when the ratio of methanol to tripalmitin of 60:1 and 6 wt% catalyst at reflux temperature for 3 h. For reused measurement, the LiAl-CO3-C400 dried at 200 oC for several hours after the reaction can be repeatedly used for three times, and the yield still can maintain 87%. In addition, the thesis contains a new approach to measure hydrogen adsorption for layered double hydroxides intercalated with various organic acids, e.g.: carbonate, acetate, terephthalate, p-toluate. In order to synthesize the materials which have high surface area and π-π interaction, the acetate and arylate are added in the precursor solution with the varied molar ratio of acetate and arylate. The surface area and pore volume of the LDHs increase when a portion of the arylate anions are replaced by acetate ions; as a result, their hydrogen uptakes are also increased. The hydrogen adsorption capacities of LDHs are proportional to the surface area. For MgAl-LDHs, the hydrogen adsorption uptake of MgAl-3pTA+4aa-LDH is 0.42 wt% at -196 oC and 1 atm; 0.08 wt% at room temperature and 10 atm. Compared with MgAl-LDHs, LiAl-LDHs have higher hydrogen storage capacities. The hydrogen capacity of LiAl-LDH intercalated with TA;aa in 1:4 ratio under nitrogen is 0.69 wt% at -196 oC and 1atm; 0.10 wt% at room temperature and 10 atm.
Subjects
MgAl-LDH
LiAl-LDH
Memory Effect
Hydrogen Adsorption
Transesterification
Type
thesis
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