中孔洞複合材料之合成、分析與對二氧化碳吸附之研究

Abstract

應用在中高溫吸附二氧化碳的材料上佔有日益重要的地位，本論文提供兩種金屬氧化物中孔洞複合材料之合成與二氧化碳吸附之研究。第一部分為利用含浸法和離子交換法合成含奈米氧化鈣之中孔洞氧化矽，接著以XRD、BET進行結構鑑定與結晶型態分析，並藉由TEM觀察微結構，再利用TGA測量二氧化碳之吸附量。經由含浸法合成的中孔洞複合材料S15-CaO在孔道中及外部皆有氧化鈣分佈，隨著含浸溶液濃度增加，氧化鈣的含量亦增加，相對的經由離子交換法合成的孔洞複合材料S15COOH2-CaO在孔道中含有較多的氧化鈣，也較不易有過多的氧化鈣團聚於孔道外。在二氧化碳吸附研究，可分為孔道外的氧化鈣所產生的快速化學吸附與孔洞內的氧化鈣所產生的緩慢擴散吸附。450~650℃下，溫度越高二氧化碳吸附量越大，吸附速率也越快。 第二部分為利用尿素經水熱法合成中孔洞鈣-鋁氧化物，亦以XRD、BET、TEM進行組成與結構分析，再利用TGA測量二氧化碳之吸附量及吸附/脫附迴圈。在150℃下，鈣離子和尿素產生的OH-沉澱形成氫氧化物於氧化鋁表面上，經過600℃鍛燒後，會導致鈣離子摻雜進入氧化鋁，形成一個中孔洞鈣-鋁氧化物。在二氧化碳吸附研究，由於中孔洞鈣-鋁氧化物表面積相當高，且鈣-鋁氧化物的反應層相當薄，導致氣體可迅速與鈣-鋁氧化物反應，產生快速的化學吸附，無須藉由擴散吸附行為。在300~700℃下，隨著鈣/鋁比例越高，二氧化碳最佳吸附量之溫度越高，吸附速率也越快。當鹼濃度較高時，較易形成中孔洞鈣-鋁氧化物，也就是說有較多的鈣離子摻雜進入氧化鋁，故具有較好的二氧化碳吸附/脫附迴圈。

This work focuses on developing a series of mesoporous CaO-based adsorbents for CO2 capture at high temperature. First, mesoporous CaO/SBA-15 composites have been successfully synthesized by wet impregnation using SBA-15 and ion-exchange method with the SBA-15-COOH as the support. The composites undergo calcination at 800℃. These mesoporous CaO/SBA-15 composites have been characterized by XRD, N2 adsorption-desorptgion isotherms, TEM and CO2 adsorption capacity by TGA at 450~650°C. The mesoporous CaO/SBA-15 by impregnation show CaO in the channels and aggregation on the external surface of SBA-15. The CaO/SBA-15 by ion-exchange method have more CaO in the channels. A CO2 adsorption capacity as high as 16.32% of adsorbent was obtained for S15-CaO-3 adsorbent at 650 °C, equal to 100.74% of CaO, with a CaO/SBA-15 ratio of 1/5. The CO2 adsorption mechanism was investigated by adsorption kinetics. Second, a series of mesoporous Ca-Al mixed oxide composites with Ca/Al ratios of 3 and 7 have been synthesized through homogenous decomposition of urea under hydrothermal conditions and subsequent calcination at 600℃. These mesoporous Ca-Al mixed oxides have been characterized by XRD, N2 adsorption-desorptgion isotherms, TEM and CO2 adsorption capacity and cycles by TGA at 300~700°C. At 150℃, Ca2+ and OH- form Ca(OH)2 and CaCO3 on theγ-Al2O3 surface. The calcined samples showed that Ca2+ was introduced into the mesoporous γ-Al2O3 support to form the mesoporous Ca-Al mixed oxides. The mesoporous Ca-Al oxides revealed a high CO2 adsorption capacity and a fast CO2 adsorption rate. The mesoporous Ca-Al oxides using high base concentration have a better reversibility of adsorption/desorption cycles. The Ca-Al oxides provides a stable framework inhibiting deactivation of Ca(OH)2.