鎂鋁和鋰鋁層狀雙氫氧化物的合成與鑑定以及應用於氫氣吸附和轉酯化催化反應

Abstract

本博士論文以共沉澱法製備出鎂鋁和鋰鋁層狀雙氫氧化物，其中崁入層間的陰離子為下列羧酸，例如：碳酸、醋酸、對苯二甲酸、對甲基苯甲酸。層狀雙氫氧化物燃燒後會形成混和金屬氧化物，將混和金屬氧化物分散於水溶液中，混和金屬氧化物會重新排列成層狀雙氫氧化物，為記憶效應。為了瞭解記憶效應的過程，針對崁入碳酸層狀雙氫氧化物，利用同步輻射原位X-ray繞射觀察結構變化，發現鎂鋁層狀雙氫氧化物，當溫度上升到350 oC，層狀結構就會坍塌，往大角度偏移，並且在450 oC生成氧化物；而鋰鋁層狀雙氫氧化物，則是在200 oC層狀結構發生偏移，直到500 oC才生成氧化物。另外，比較鍛燒到400 oC和600 oC混和金屬氧化物的水合現象，鍛燒溫度越高會越難恢復成層狀結構。將水合後的樣品再次升溫，並不會影響轉相的溫度，但是可以看到層狀坍塌的過程變慢。 層狀雙氫氧化物常被用來製備高表面積混和金屬氧化物，本論文以崁入碳酸層狀雙氫氧化物鍛燒到不同溫度的混和金屬氧化物為觸媒，探討應用於三棕櫚酸與甲醇生成棕櫚酸甲酯和甘油的轉酯化反應，並且調整醇油比以及觸媒含量，找到最佳化條件測試重覆使用。發現當醇油比越高，反應速率越快。當固定醇油比為60:1的條件下反應三小時，改變觸媒用含量(1~9 wt%)，發現產隨著觸媒含量增加而增加，直到觸媒含量為6 wt%時，產率達到99%，然後下降。在迴流的溫度下、醇油比為90:1、觸媒量為3 wt%，以崁入碳酸根鋰鋁層狀雙氫氧化物鍛燒400 oC的觸媒具有最佳活性，可以達到95%的產率，並且將使用過的觸媒，直接放入200 oC烘箱數小時，可以重複使用三次還維持87%產率。 此外，將層狀雙氫氧化物崁入各種有機酸(醋酸、對苯二甲酸、對甲基苯甲酸)，以製備出高表面積且含有苯環的結構，應用於氫氣吸附反應。發現當醋酸和含有苯環的有機酸同時崁入的層狀氫氧化物與只有崁入純有機酸的樣品比較會有較大的表面積，導致有較高的氫氣吸附量。其中，以鎂鋁層狀雙氫氧化物系列，對甲基苯甲酸和醋酸以三比四的比例崁入鎂鋁層狀雙氫氧化物，在-196 oC和一大氣壓下，可以吸附0.42 wt%；在室溫和約10 MPa氣壓下，可以吸附0.08 wt%。以鋰鋁層狀雙氫氧化物系列，則是對苯二甲酸和醋酸以一比四的比例崁入鋰鋁層狀雙氫氧化物，則在低溫常壓可以吸附0.67 wt%氫氣，並且在常溫高壓有0.10 wt%氫氣吸附量。

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.