Cu-MFI催化剂上Cu^(δ+)催化乙醇脱氢制乙醛

Ethanol dehydrogenation to acetaldehyde over a Cu^(δ+)-based Cu-MFI catalyst

作为一种清洁的可再生能源产品,乙醇可用于制造乙醛、丙烯、正丁醇、1,3-丁二烯和芳香烃等高值化学品.对于大多数的乙醇催化转化过程,第一步是乙醇脱氢生成乙醛,因而该反应具有重要的研究意义和应用价值.铜基催化剂因在乙醇脱氢反应中具有高活性和高选择性,受到广泛关注与研究.然而,由于催化剂上铜配位结构较为复杂,价态多变,催化剂的关键活性位点目前仍难以确定.此外,铜的Tamman温度较低,在反应条件下因铜活性位容易烧结或团聚而导致催化剂失活,因此需要进一步提升铜催化剂的稳定性.构筑具有明确结构和高稳定性的铜基催化剂是获得高性能乙醇脱氢反应的关键,也是深入认识该反应中催化剂结构与性能构效关系的前提.本文首先采用蒸氨法制备了一系列的Cu-MFI催化剂,再通过酸处理将催化剂上不稳定的CuOx物种除去,留下与MFI载体强相互作用的Cu物种.通过优选Cu负载量和优化反应条件,发现在250°C和WHSV=0.64 h-1的反应条件,在最佳催化剂5%Cu-MFI-de Cu上实现了95%的乙醛选择性和约87%的乙醇转化率,且可稳定运行120 h.系统表征(氢气程序升温还原、高分辨扫描透射电镜、氘代乙腈红外光谱和X射线吸收光谱)结果表明:催化剂中稳定的Cu物种以(SiO)3Cu-OH的形式高度分散在载体上,在反应过程中被反应物或H2部分还原为[≡SiO-Cu]^(δ+)(1<δ<2).不同于之前广泛报道的Cu催化活性中心以Cu+和Cu0物种存在,本文中[≡SO-Cu]^(δ+)物种被证实是乙醇脱氢制乙醛反应的高活性位点.飞行时间二次离子质谱结果表明,[≡SiO-Cu]^(δ+)数量与乙醇转化率之间存在较好的线性关系.吡啶红外光谱结果进一步表明,路易斯酸(LAS)密度与乙醇转化率之间也存在相似的线性关系.据此可知,5%Cu-MFI-de Cu催化剂上的LAS位点来自于高度分散在载体上的Cu^(δ+)物种.动力学实验结果表明,与同时存在Cu^(2+),Cu+和Cu0的5%Cu-MFI催化剂相比,仅含有Cu^(δ+)的5%Cu-MFI-deCu催化剂在乙醇脱氢反应中表现出更低的表观活化能,因而具有更好的催化性能.综上,本文构建的具有明确结构的高活性、高稳定性Cu催化剂的策略,将为发展更多的新型高效乙醇转化催化剂提供有价值的参考.

Copper catalysts have been extensively investigated for the dehydrogenation of ethanol to acetaldehyde.However,identifying the essential active sites for this reaction is difficult because of the complex coordination structure and variable valences of the Cu species during the reaction.The stability of the Cu catalysts in the reaction also needs to be substantially improved.In this study,Cu-MFI,a well-defined Cu-based Lewis acid catalyst,was prepared using a post-acid treatment method for ethanol dehydrogenation.Different from the widely reported Cu^(+) and Cu^(0) species accounting for the activity of Cu catalysts,conditional experiments and in situ characterizations revealed that the highly dispersed Cu^(δ+)(1<δ<2)species on the MFI support are the essential active sites for ethanol dehydrogenation.Due to the strong interaction between Cu and silica via the Cu-O-Si linkage,the Cu^(δ+) species were very stable in the reaction and played the role of a Lewis acid catalyst in promoting ethanol activation and dehydrogenation.Over the optimal catalyst 5%Cu-MFI-deCu,95%selectivity of acetaldehyde and approximately 87%ethanol conversion were obtained at 250°C and a weight hourly space velocity of 0.64 h^(-1) in 120 h time on stream.