製備氧化鈰觸媒應用於二氧化碳與二元醇直接合成聚碳酸酯

Abstract

近年來，二氧化碳排放隨著化石燃料的燃燒及石油化學品的開發而日漸增加。以二氧化碳及生物質衍生物二元醇直接合成生物可降解的聚碳酸酯，為兼具二氧化碳再利用及高環境效益的綠色反應。 本研究第一階段以水熱法製備具棒狀(Nanorod, NR)及立方體(Nanocube, NC)結晶形貌的奈米氧化鈰，並與市售的奈米級(Nanoparticle, NP)及半微米級(Submicroparticle, SMP)氧化鈰作比較，探討不同結晶形貌氧化鈰對反應活性的影響。 研究結果顯示，粒徑較大的NC及SMP因比表面積小，表面的活性位點較少而不易使CO2吸附於觸媒表面，不利於聚碳酸酯合成；NR及NP的比表面積較大，其中，NR又因表面具有更多的氧空缺位，可助CO2以Bidentate Carbonate (BC)及Hydrogen Carbonate (HC)結構吸附於觸媒表面，促進催化聚碳酸酯寡聚物的合成，有最佳的97% 1,4-丁二醇轉化率及1.11 g聚碳酸酯寡聚物產量。 本研究第二階段以與Ce4+：(a)不同離子半徑的Zr，(b)不同離子價態的La及Pr及(c)離子半徑、價態皆不同的Cu四種金屬摻雜於NR內，探討摻雜不同金屬於NR後對氧空缺濃度的提升及反應活性的影響。 研究結果顯示，四種金屬摻雜後皆能提升NR的氧空缺濃度。摻雜Zr後，CO2僅以BC結構吸附於觸媒表面，降低了CO2吸附時的立體障礙及競爭吸附而使寡聚物產量些微提升至1.18 g；摻雜La及Pr後CO2的吸附行為不變，但吸附強度增加而抑制聚合反應發生，寡聚物產量降至0.74 g (La)及0.64 g (Pr)；摻雜Cu後形成部分Cu及CuO散佈於表面，造成CO2吸附時產生立體障礙及競爭吸附，伴隨吸附強度增加而使寡聚物產量大幅下降至0.10 g。

In recent years, carbon dioxide emission is proportional to burning of fossil fuels and the development of petrochemicals. Direct synthesis of biodegradable polycarbonate from carbon dioxide and biomass derivative diols is a green process with high environmental benefits. In the first stage of this work, cerium oxide with nanorod (NR) and nanocube (NC) morphology were prepared by hydrothermal method and compared with commercial cerium oxide nanoparticle (NP) and submicroparticle (SMP). Investigating the effect of different morphology of cerium oxide on the activity. The results show that NC and SMP with large crystalline size have low specific surface area and less active sites on the surface, which are hard to activate CO2 and synthesize polycarbonate; NR and NP have relatively high specific area, among them, CO2 can adsorb as Bidentate Carbonate (BC) and Hydrogen Carbonate (HC) structure on NR surface due to the more oxygen vacancy in the NR structure, which can promote polycarbonate synthesis and has the best 97% 1,4-butanediol conversion and 1.11 g polycarbonate yield. In the second stage of this work, we doped Zr (different ion radius with Ce4+), La and Pr (different ion valence with Ce4+), and Cu (both different ion radius and valence with Ce4+) in NR structure. Investigating the promotion of oxygen vacancy concentration and the activity of polycarbonate synthesis after doping. The results show that NR doped with Zr, La, Pr or Cu can enhance oxygen vacancy concentration in the structure. After Zr doping, CO2 only adsorbed on the surface as BC structure which may reduce the steric hindrance and competitive adsorption. Slightly increasing the oligomer yield to 1.18 g; After La and Pr doping, the adsorption structure of CO2 on the surface was unchanged. The adsorption strength increased which inhibited polycarbonate synthesis. Decreasing the oligomer yield to 0.74 g (in La doping case) and 0.64 g (in Pr doping case); After Cu doping, Cu and CuO may disperse on the surface. Result in steric hindrance and competitive adsorption while CO2 adsorption. Nevertheless, the adsorption strength also increased after Cu doping. Dramatically decreasing the oligomer yield to 0.10 g.