二氧化碳甲烷化反應之鎳金屬觸媒改質與二氧化碳非還原性轉換之除水系統探討

Abstract

二氧化碳還原與非還原轉換成燃料與高價值化學品之觸媒系統為近年二氧化碳相關研究趨勢。於二氧化碳還原部分，本研究探討添加鑭作為催化助劑(catalytic promoter)對於鎳/二氧化鈦觸媒低溫催化二氧化碳甲烷化反應之影響。我們結合表面性質鑑定、原位光譜鑑定與催化反應數據，探討表面活性位點與反應機制，並分析催化助劑在觸媒中扮演的角色。催化結果顯示添加鑭能有效增加觸媒於低溫環境的催化表現，250 °C下二氧化碳轉化率由20%提升至76%。透過氫氣程溫還原、X光繞射光譜與相關物化性質鑑定，我們確認添加鑭有助於提升鎳金屬分散度。我們亦由X光吸收光譜判斷鎳金屬與二氧化鈦擔體之間金屬擔體交互作用會隨著鑭擔載量增加而減弱。由二氧化碳程溫脫附實驗，我們發現隨著鑭擔載量增加，觸媒表面中強鹼性位點的數量也隨之增加。我們進行一系列紅外線原位光譜實驗以探討鑭對反應機制的影響，發現擔載鑭之後，二氧化碳甲烷化之反應途徑由一氧化碳路徑改變為甲酸根路徑，由於甲酸根路徑之反應活化能較低，使鎳鑭/二氧化鈦於低溫之下的催化活性比鎳/二氧化鈦高。最後我們將消除鎳金屬分散度影響後的反應速率與中強鹼性位點量做線性迴歸，發現兩者成完美的線性關係，顯示隨著鑭擔載量增加而提升的中強鹼性位點量與觸媒活性提升有直接關係。 本研究的第二部分著重於開發化學或物理除水方法，移除二氧化碳非還原性轉換生成有機碳酸酯反應之副產物水，以符合工業化生產條件之原則突破反應熱力學限制提升有機碳酸酯的產率。化學性除水方面，我們使用商用二氧化鈰觸媒分別搭配氰基甲烷(acetonitrile)及苯甲?(benzonitrile)作為化學除水劑，催化甲醇與二氧化碳反應生成碳酸二甲酯(dimethyl carbonate, DMC)，DMC產率由平衡產率(0.07%, 170 °C)分別提升至2.1%和5.2%。隨著反應時間增長，氰基甲烷水合產物乙醯胺的衍生性副產物增加限制了DMC產率；苯甲?水合產物苯甲醯胺則有毒化觸媒之情形。物理性除水方面，我們使用常壓流動CO2半批次反應系統進行常壓CO2和1,4-丁二醇(1,4-butanediol, 1,4-BDO)共聚反應，利用流動CO2以氣提分離法將水分子帶離反應系統，成功合成出脂肪族聚碳酸酯寡聚物(Polycarbonate oligomer, PCPO)(X1,4-BDO = 35%, Mn, GPC = 540 g mol-1)。產物混合物以真空系統減壓濃縮進行縮和聚合，使分子量由540 g mol-1上升至6143 g mol-1。我們以反應溫度、CO2流量和溶劑為變因進行反應系統測試，反應溫度提升可加快反應速率，但也因氣化增加1,4-丁二醇的損失；CO2流量增大有助於增加反應溶液的CO2濃度提升反應速率；使用疏水性溶劑則能有效提升除水效率，進而提升1,4-丁二醇轉化率與聚碳酸酯寡聚物(PCPO)聚合度。最佳化後之1,4-丁二醇轉化率約35% (1,4-丁二醇40 mmol, 二苯醚6 g, 180 °C, CO2 200 mL/min, 48 h)，所得聚碳酸酯寡聚物(PCPO)產量仍難以符合工業化條件，需研究更多操作變因以找出提升除水效率的關鍵。

Owing to increasing carbon dioxide (CO2) emissions and exhaustion of fossil fuels, the conversion of CO2 toward value-added chemicals and fuels has attracted much attention. Research has been focused on developing catalyst with enhanced low temperature activity in CO2 methanation to avoid stability loss and catalyst deactivation. In this work, titanium-supported nickel-lanthanum catalyst prepared by deposition precipitation method was studied for CO2 methanation. The CO2 conversion at 250 °C increase from 20% (10Ni/TiO2) to 76% (10Ni-24La2O3/TiO2) with the incorporation of lanthanum. Results suggest that the addition of lanthanum enhanced the dispersion of Ni particle (H2-TPR, XRD, TEM), while reducing the interaction between nickel and support (XAS). CO2-TPD and CO2 DRIFTS-TPD results show that medium basic sites on catalyst surface effectively increased with increased loading of lanthanum, promoting catalyst’s CO2 adsorption ability. The presence of medium basic centers favors formation of carbonate species, which are intermediates in formate pathway. In-situ IR surface reaction result indicated that different reaction pathway occurred on the surface of Ni/TiO2 (CO pathway) and Ni-La2O3/TiO2 (formate pathway). Formate pathway with lower activation is the main reason for enhanced catalytic performance at lower temperature. We also confirmed the linear relation between reaction rate and medium basic sites by normalizing the effect of Ni dispersion. In the second part of the thesis, we focused on developing a feasible process for direct synthesis of organic carbonate through carbon dioxide and alcohols with the assistance of chemical or physical dehydrating methods to overcome the thermodynamic limitation on product yields. With the promotion of acetonitrile and benzonitrile, the yield of dimethyl carbonate (DMC) raised from the equilibrium yield (0.07% at 170 °C) to 2.1% and 5.2%, respectively. As reaction time increase, the DMC yield of acetonitrile dehydrating system is limited by increase of by-product from acetamide. Benzamide was also observed (transmission IR) to strongly adsorb on CeO2 catalyst surface which results in catalyst poison and the decrease in DMC production rate. An atmospheric CO2 flow semi-batch reactor was used as an alternative dehydrating method for direct synthesis of polycarbonate oligomer from CO2 and 1,4-butanediol with CeO2 as catalyst. The conversion of 1,4-butanediol reaches 35% and the product reaches 540 g/mol without using any dehydrating agent and was further directly polymerized to 6000 g/mol by melt polycondensation. The effects of solvent, reaction temperature and CO2 flow rate were also studied. The reaction rate increases with increasing temperature, resulting in increase of oligomer molecular weight but more 1,4-butanediol loss due to higher vaporization rate. Higher CO2 flow rate results in higher CO2 concentration in the reaction media and hence higher reaction rate. Adding hydrophobic organic solvent accelerates the separation of water and reaction mixture, thus promoting the water removal efficiency.