二氧化碳的非還原性轉換：氧化鈰觸媒的機械化學合成和聚碳酸酯之二階段製程

Abstract

本研究中分為兩主題：(1)透過球磨、鍛燒方法提升半微米氧化鈰觸媒於甲醇與二氧化碳合成碳酸二甲酯之反應活性；(2)以二階段製程合成聚碳酸酯。 (1) 球磨處理氧化鈰對於碳酸二甲酯合成活性之影響 近年來由二氧化碳與甲醇以非還原反應合成碳酸二甲酯逐漸受到關注，其特點在於二氧化碳的再利用以及合成碳酸酯的過程中避免使用有毒的光氣。目前市售氧化鈰觸媒中，半微米等級往往價錢便宜但催化活性不佳。在本研究中，我們嘗試透過球磨處理提升市售半微米等級氧化鈰觸媒於碳酸二甲酯反應之合成活性。活性測試結果顯示觸媒經過球磨處理後可顯著提升其反應活性，從6 μmol g-cat-1 h-1提升至378 μmol g-cat-1 h-1，進一步將觸媒以鍛燒處理可提升至538 μmol g-cat-1 h-1。透過X光吸收光譜發現球磨能提升觸媒表面的氧空缺濃度，從原位擴散反射傅立葉轉換紅外線光譜(DRIFTS)結果得知，球磨所產生的氧空缺有利於二氧化碳活化，而鍛燒處理則可降低二氧化碳以及甲醇吸附物與觸媒表面間的作用力，進而有利於碳酸二甲酯的合成。 (2) 聚碳酸酯之二階段製程 以二氧化碳以及雙醇透過氧化鈰催化輔以2-氰基吡啶作為脫水劑為一綠色製程。然而此新興製程目前一大困難在於其聚合度受限，產物分子量低。本研究將透過二階段製程合成出高子量之聚碳酸酯：第一階段以二氧化碳與1,4-丁二醇透過非還原轉換合成碳酸酯寡聚物，並在第二階段以減壓濃縮使其自聚合成聚碳酸酯。為順利實行二階段製程，解決在第一階段中碳酸酯寡聚物受脫水劑封端的狀況為首要目標。我們發現透過改變製程條件(如時間、溫度)可有效抑制脫水劑封端產物的情況。在第二階段製程中，使用製程純化後之碳酸酯寡聚物可藉由減壓濃縮將分子量從1000 Da提升至20000 Da，並且透過調整減壓濃縮的溫度及時間可達到分子量可控之目的。

This thesis includes two parts. (1) treatment of ceria catalyst with ball-milling and calcination to enhance its activity for dimethyl carbonate synthesis; (2) development of two-step process for polycarbonate polyol synthesis. (1) Enhanced production of dimethyl carbonate from carbon dioxide and methanol over ball-milled ceria catalysts. Recently the direct synthesis of dimethyl carbonate (DMC) from CO2 and CH3OH over CeO2 catalysts has attracted massive attention for its usage of CO2 as reactant and absence of toxic regents in the process. Commercial CeO2 submircoparticle (CeO2) is generally inexpensive while much less catalytically active than commercial CeO2 nanoparticle. In this study, we have treated CeO2 by ball milling with an attempt to enhance its catalytic activity for direct synthesis of DMC from CO2 and CH3OH. Reaction testing shows that ball milling of CeO2 could significantly increase the DMC yield from 6 to 378 μmol g-cat-1 h-1, and further air calcination of ball-milled CeO2 leads to a higher DMC yield of 538 μmol g-cat-1 h-1. X-ray absorption spectroscopy (XAS) characterizations show ball milling increases the amount of oxygen vacancies at CeO2 surface. In-situ temperature-dependent diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) results suggest that mechanochemically-generated surface oxygen vacancies are beneficial for CO2 activation, and air calcination decreases the adsorption strengths of both CO2-derived (i.e., bidentate carbonate) and CH3OH-derived adspecies (i.e., on-top methoxy) to facilitate the production of DMC. (2) Polycarbonate polyol synthesized through two-step Process. Direct copolymerization of CO2 and diols over CeO2 catalysts with 2-cyanopyrinde (2-CP) as the dehydration reagent is a green process to produce polycarbonate polyol (PCPO) oligomer. Nevertheless, this emerging process is currently limited by the low molecular weight of PCPO product obtained. In this study, we have strived to synthesize high-molecular-weight poly(butylene carbonate) (or PBC) by a two-step process involving direct copolymerization of CO2 and 1,4-butanediol to yield PBC oligomer followed by melt polycondensation. In order to enable this two-step process, it is essential to prevent the end group of PCPO oligomer from capping with 2-CP derivative, a key requirement for subsequent melt polymerization. It is found that the capping with 2-CP derivative in PBC oligomer can be efficiently suppressed by engineering the reaction conditions (e.g., reaction time and reaction temperature) of direct copolymerization. Our results show that melt polymerization of PBC oligomer with suppressed capping 2-CP derivative can substantially increase the molecular weight of PBC from ca. 1000 to ca. 20000 Da. Furthermore, the molecular weight of obtained PBC can be controlled by tuning the temperature and period during melt polycondensation.