www.afm-journal.de FULL PAPER © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 www.MaterialsViews.com wileyonlinelibrary.com Danny Verboekend, Tobias C. Keller, Sharon Mitchell, and Javier Pérez-Ramírez* 1. Introduction Zeolites are a class of aluminosilicate catalysts of paramount importance to the chemical and petrochemical industries. Their unique properties, e.g., strong Brønsted acidity, high surface area, high (hydro)thermal stability, and shape selectivity, com- bined with an extensive tunability, render them the catalyst of choice in many catalyzed reactions. [1,2] Nevertheless, often only a fraction of their potential is exploited, due to diffusion and access limitations. The size of the micropores (0.3–1 nm) is similar to that of many molecules, enforcing an intracrystalline “single file” diffusion. The latter regime is associated with dif- fusivities orders of magnitude lower compared to outside the pores, implying an underutilization of the zeolite volume. [3] To alleviate diffusion limitations, wide-pore zeolites and hier- archical zeolites were conceived. [3–6] Whereas the former class aims at enhancing diffusion in the micropores, [7] hierarchical zeolites facilitate access to the active sites in the micropores using a secondary net- work of meso- or macropores. [8–13] The increased external surface associated with hierarchical zeolites renders the crystal interior more accessible to large mol- ecules, [14] which is particularly advanta- geous in the catalytic processing of bulky substrates. [15] Hierarchical zeolites can be obtained by reducing the zeolite crystal in one or more dimensions, resulting in nanometer-sized, [8] two-dimensional, [9,10] or lamellar zeolites, [11] or by introducing intracrystalline mesopores and thereby forming mesoporous crystals. [12,13] Of the plethora of bottom-up and top- down methods available to prepare hier- archical zeolites, [3–13] base leaching, or desilication, is widely applied owing to its experimental simplicity and to the efficiency of the resulting multiporous crystal. [13] Nevertheless, in many cases, desilication by aqueous NaOH should be considered only part of the post-synthetic modification strategy. [13] For example, in the case of Y zeo- lites, [16] the execution of a dealumination step prior to base leaching facilitates the desired formation of intracrystalline mesopores. Additionally, mild acid washing has proved of cru- cial importance to remove Al-rich species residual from alka- line treatment. In the case of steamed and acid-leached USY zeolites, [16,17] the inclusion of pore-directing agents (PDAs) in the alkaline solution permits the introduction of extensive mes- oporosity while preventing amorphization. The premeditated use of these tools has enabled the preparation of hierarchical zeolites of numerous framework types over a wide composi- tional range (i.e., from pure silica down to Si/Al ≈ 4). [13,16] Con- sequently, Al-rich zeolites, e.g., X or A (Si/Al ≈ 1), pose the last compositional challenge to be amended to hierarchical form by post-synthetic strategies. Hierarchical X [18] and A [19] zeolites have been obtained using bottom-up strategies, involving costly organic templates that after the synthesis need to be thermally removed. However, it should be emphasized that the commercial synthesis of zeolites with Si/Al < 5 does not involve the use of organic templates as structure-directing agents, which significantly reduces manu- facturing expenses (ca. 2–5 USD per kg). [20] The resulting eco- nomic constraints infer that most preferably affordable routes, e.g., post-synthetic modifications, should be used to derive their mesoporous analogues. Hierarchical FAU- and LTA-Type Zeolites by Post-Synthetic Design: A New Generation of Highly Efficient Base Catalysts Hierarchical FAU- and LTA-type catalysts are prepared by post-synthetic modifications and evaluated in the base-catalyzed Knoevenagel condensa- tion of benzaldehyde with malononitrile. A novel route to attain mesoporous Al-rich zeolites (A and X) is demonstrated, while mesoporous Y and USY zeolites are prepared using recently developed methods. Base functionality is introduced by alkali ion exchange (Cs, Na) or by high-temperature nitridation in ammonia. A thorough characterization of the zeolites’ structure, composi- tion, porosity, morphology, and basicity demonstrates that the presence of a secondary mesopore network enhances the ion-exchange efficiency and the structural incorporation of nitrogen. The modified USY zeolites display twice the conversion, while the hierarchical A, X, and Y are up to 10 times more active based on the enhanced accessibility. These results demonstrate that the Knoevenagel condensation takes place predominately at the external sur- face, highlighting secondary porosity as a key criterion in the design of basic zeolite catalysts. DOI: 10.1002/adfm.201202320 D. Verboekend, T. C. Keller, Dr. S. Mitchell, Prof. J. Pérez-Ramírez Institute for Chemical and Bioengineering Department of Chemistry and Applied Biosciences ETH Zurich, Wolfgang-Pauli-Strasse 10 CH 8093, Zurich, Switzerland E-mail: jpr@chem.ethz.ch Adv. Funct. Mater. 2012, DOI: 10.1002/adfm.201202320