醇類輔助低溫二氧化碳氫化生成甲醇:觸媒研發及反應機制探討

Abstract

本研究旨在探討乙醇輔助二氧化碳(CO2)氫化低溫合成甲醇之反應機制。所使用之觸媒為以共沉澱法製備的CuZnCeOx系列觸媒。研究表明，甲酸乙酯(EtFm)為關鍵中間體，係由CO2與H2生成的甲酸根與催化溶劑乙醇反應形成。該中間體經加氫裂解(hydrogenolysis)可快速轉化為甲醇，從而降低CO2氫化合成甲醇的反應溫度。鑒於過去研究多以銅鋅氧化物作為觸媒，本研究引入鈰(Ce)作為促進劑，探討其對與甲醇產率及選擇率之影響。 觸媒活性測試結果顯示，相較於CuZnO，部分以Ce取代Zn可提升EtFm及甲醇的生成量。透過調控Ce與Zn的比例，本研究進一步釐清兩者在反應中的角色。Ce有助於促進EtFm的生成，惟過量的Ce則會抑制其後續轉化為甲醇；相對的，Zn則可有效促進EtFm經由氫解反應生成甲醇。研究結果顯示，適量添加Ce不僅有助於中間體EtFm的生成，亦能兼顧其高效轉化，進一步提升甲醇之產率與選擇率。 本研究使用原位擴散反射紅外線傅立葉轉換光譜(in-situ DRIFTS)技術，探討Ce與Zn在反應機制中所扮演的角色。光譜結果顯示，乙酸乙酯(EtFm)在CuZnO上的訊號較弱且持續時間短，顯示其吸附較弱並易於轉化；相對地，在CuCeO₂上則觀察到較強且持續時間較長的EtFm吸附訊號，顯示其在表面具有較強的吸附力。根據這些觀察，推論EtFm在Ce含量較高的觸媒上傾向以單點氧(η¹-O)模式吸附於表面，造成氫解反應受阻，進而降低其轉化效率；而Zn含量較高的觸媒則促進EtFm以雙點碳氧(η²-(C,O))模式吸附，有助於羰基活化，進一步與銅表面解離之氫反應，有效轉化為甲醇。 綜上所述，本研究指出促進劑表面之吸附結構為影響觸媒性能的關鍵因素，並提出可藉由調控觸媒組成，達成提升甲醇生成效率、抑制副反應之設計準則，對推動高效、低溫CO2氫化生成甲醇製程的發展具有實質貢獻。

This study investigates ethanol (EtOH)-assisted CO2 hydrogenation for low-temperature methanol (MeOH) synthesis using a series of CuZnCeOx catalysts synthesized via the co-precipitation method. Ethyl formate (EtFm) is identified as a key intermediate, formed through the reaction between formate species (derived from CO2 and H2) and EtOH, which serves as a catalytic solvent. Subsequent hydrogenolysis of EtFm enables MeOH production at temperatures lower than those required for conventional CO2 hydrogenation. While previous studies have focused primarily on CuZnO system, this work introduces cerium (Ce) as a promoter aimed at improving MeOH yield and selectivity. Catalytic testing demonstrated that partial substitution of Zn with Ce markedly increases EtFm formation and concurrently boosts MeOH yield compared to CuZnO. Systematic variation of the Ce/(Ce+Zn) ratio revealed bifunctional behavior: Ce promotes EtFm generation but excessive Ce content shows inhibitory effect in EtFm conversion, whereas Zn effectively converts EtFm into MeOH via hydrogenolysis. An optimal Ce/Zn ratio was identified, achieving a balance between intermediate formation and conversion, resulting in the optimal MeOH yield and selectivity. Based on the results of in-situ DRIFT spectroscopy, this study proposes that the adsorption configuration of ethyl formate (EtFm) on the catalyst surface is a key factor governing the distinct catalytic behaviors. Spectral observations revealed that the EtFm signal on CuZnO was relatively weak and short-lived, suggesting weak adsorption and facile conversion. In contrast, CuCeO₂ exhibited a stronger and more persistent EtFm signal, indicating stronger surface binding. These differences imply that EtFm is preferentially stabilized in an η¹-O adsorption geometry on Ce-rich catalysts, which inhibits hydrogenolysis to methanol (MeOH). Conversely, Zn-rich catalysts promote an η²-(C,O) adsorption mode that facilitates C=O activation and enables effective conversion to MeOH via surface hydrogen species. These findings underscore the crucial role of adsorption geometry in determining reaction pathways and product selectivity. This work offers valuable design principles for optimizing Cu-based catalysts in alcohol-assisted CO2 hydrogenation, paving the way for efficient, low-temperature MeOH synthesis.