作者:Tobias C. Keller、Stéphane Isabettini、Danny Verboekend、Elodie G. Rodrigues、Javier Pérez-Ramírez
DOI:10.1039/c3sc51937f
日期:——
For more than four decades, the design of zeolite base catalysts has relied on the application of aluminium-rich frameworks exchanged with alkali metal cations (preferably Cs+). However, moderate activity associated with access and diffusion limitations, and high manufacturing costs associated with high caesium content (typically over 30%) have hampered their industrial implementation so far. Herein, we have discovered that high-silica USY zeolites outperform their Al-rich counterparts in a variety of base-catalysed reactions of relevance in the fine chemical industry, as well as in the upgrading of biofuels. The benefits of this class of materials are amplified upon the alleviation of diffusion constraints through the introduction of a network of intracrystalline mesopores by post-synthetic modification. For example, the resulting cation-free hierarchical USY provides an up to 30-fold Knoevenagel condensation activity compared to the benchmark Cs–X, and similar observations were made upon application in liquid-phase (nitro)aldol reactions. Moreover, in the gas-phase aldol condensation of propanal, high-silica zeolites provide superior activity, selectivity, and lifetime compared to caesium-containing zeolites and even a strong solid base such as MgO. We decouple the complex interplay between mesoporosity and intrinsic zeolitic properties such as crystallinity, and quantify the increase in catalyst effectiveness upon hierarchical structuring as a function of reactant size. The obtained results are a major step to resolve the drawbacks of zeolites catalysis and thereby revitalise their potential for industrial application.
在过去的四十多年中,沸石基催化剂的设计依赖于富铝框架与碱金属阳离子(优选Cs+)的交换。然而,受限于接入和扩散的中等活性,以及高铯含量(通常超过30%)所带来的高制造成本,迄今为止,这些催化剂的工业应用一直受到阻碍。在此,我们发现高硅USY沸石在精细化工行业相关的多种碱催化反应及生物燃料升级中,优于铝富集的同类催化剂。通过后合成改性引入晶内介孔网络,缓解扩散限制,这类材料的优势得以增强。例如,所得到的无阳离子层级USY在Knoevenagel缩合反应中的活性比基准的Cs–X高出多达30倍,而在液相(硝基)醛缩合反应中也获得了类似的结果。此外,在丙醛的气相醛缩合反应中,高硅沸石相比含铯沸石甚至强固体碱(如MgO)提供了更优越的活性、选择性和使用寿命。我们将介孔性和内在沸石特性(如晶体结构)之间复杂的相互作用进行解耦,并量化了依赖于反应物大小的层级结构对催化剂有效性的提升。获得的结果为解决沸石催化的缺陷迈出了重要一步,从而重新激发了其在工业应用中的潜力。