以溶膠－凝膠法製備鈦酸鹽類及其在碳酸甘油酯合成之應用

Abstract

將生質柴油製程中過剩的副產物甘油轉換為高經濟價值的化學品，已成為異相觸媒領域中一項蓬勃發展的議題。本研究中，吾人使用溶膠凝膠法中的檸檬酸聚合錯和物法（polymeric citrate complex method，即Pechini method）製備鹼金與鹼土族鈦酸鹽類觸媒，以應用於甘油與碳酸二甲酯（dimethyl carbonate）轉酯化合成碳酸甘油酯（glycerol carbonate）。文中探討之鹼金與鹼土族包含鋰、鈉、鉀、鎂、鈣、鍶，製備所得觸媒材料使用X光粉末繞射儀（powder XRD）、拉曼光譜儀（Raman）、氮氣吸脫附儀 (BET method)、原子吸收光譜儀（AAS）、程溫脫附儀（CO2-TPD）、熱重分析儀（TGA）與場發式掃描電子顯微鏡（FE-SEM）等儀器鑑定，以觀察材料結構、組成與其表面形貌。 經批次的活性篩選測試結果顯示，使用鈦酸鈉與鈦酸鉀之碳酸甘油酯產率可高於50%，但其結構在反應後易衰退；鹼土族鈦酸鹽活性較差（產率普遍低於30%），但其反應後之結構相較前者穩定，而在鹼土族金屬鈦酸鹽類中摻入鹼金族金屬能保有其結構，並大幅度地改善甘油轉化率與碳酸甘油酯產率；以煅燒溫度攝氏700度、鈉：鈣莫爾比0.4：1.0之鈦酸鈣（命名為0.4Na-Ca-Ti-C700）作為觸媒，對甘油轉酯化反應之促進作用最大，推測其活性位置可能來自於煅燒過程中形成的鈉物質。吾人更進一步由催化反應參數－時間、溫度、反應物比例及觸媒添加量尋求最適化之反應條件。取用0.200克之0.4Na-Ca-Ti-C700投入至含有碳酸二甲酯/甘油莫爾比為 4.0之批次反應器，於攝氏75度下反應 2 小時，可獲得產率高達91% 之碳酸甘油酯與93%之甘油轉化率，對碳酸甘油酯之選擇率為98%。

The valorization of surplus glycerol produced from biodiesel industry has become a prosperous issue in the application of heterogeneous catalysts. Herein, we designed a series of novel titanate catalysts for the synthesis of glycerol carbonate from glycerol. The titanates catalysts were prepared via the sol-gel method using polymeric citrate complex route (i.e. Pechini method), and they were evaluated for the transesterification of glycerol with dimethyl carbonate in a batch system. These titanate catalysts were characterized by powder XRD, Raman, AAS, N2 physical adsorption (BET method), temperature-programmed desorption of CO2 (CO2-TPD), TGA, and FE-SEM. The findings in the screening test indicate that applying sodium titanate and potassium titanates obtained YGLC of more than 50%; however, the structures of them were relatively unstable. Moreover, it was revealed that the activity of alkaline metal (Mg, Ca, Sr) titanates with additions of alkali metal (Li, Na, K) were highly improved on the conversion of glycerol (XGLY) and the yield of GLC (YGLC). The sodium promoted calcium titanate with a molar ratio of Na/Ca = 0.4 designated 0.4Na-Ca-Ti-C700 demonstrates the best promotion effect. The active sites might be attributed to the sodium species formed during calcination. The influence of the reaction time, temperature, the molar ratio of reactants (DMC/GLY), and calcination temperature on the catalytic activity of 0.4Na-Ca-Ti-C700 were investigated. The optimized result shows that under the condition of time = 2h, temperature = 75 °C, the molar ratio of DMC/GLY = 4, and amount of catalyst = 0.200 g, the YGLC of ca. 91%, XGLY of 93%, and selectivity of 98% toward GLC were achieved over 0.4Na-Ca-Ti-C700.