DOI: 10.1002/chem.200903426 Hierarchical ZSM-5 Zeolites in Shape-Selective Xylene Isomerization: Role of Mesoporosity and Acid Site Speciation Christian Fernandez,* [a] Irina Stan, [a] Jean-Pierre Gilson, [a] Karine Thomas, [a] AurØlie Vicente, [a] Adriana Bonilla, [b] and Javier PØrez-Ramírez* [b, c] Introduction The history of zeolite catalysis is firmly rooted in the oil and petrochemical industry. [1] Over the years, a series of synthet- ic zeolites (X and Y, MOR, ZSM-5, beta, MCM-22, ZSM- 22, ZSM-23, etc.) have been discovered and subsequently applied with considerable success in important oil-refining and petrochemical processes. Parallel to this wave of innova- tion in industrial catalysis, the scientific foundations of their unique selectivity were laid down by industrial and academ- ic scientists. [2] It is remarkable that, although so many zeolite frameworks exist, [3] only a handful is responsible for their major industrial applications. The explanation lies, in part, in the extraordinary creativity of scientists and engineers to design post-synthesis treatments for a few zeolites to fine- tune their catalytic properties to many reactions and reactors. Modern catalyst design is a challenge of optimization, be- cause activity and selectivity often have contradictory re- quirements. For instance, when shape-selective zeolite catal- ysis is based on diffusion constraints on either reactants or products, selectivity is enhanced by working with large zeo- lite crystals. [4] The use of such crystals results, however, in lower activity due to suboptimal utilization of all the active sites present. Smaller crystals, on the other hand, could dis- play an increased activity but a lower selectivity. Hierarchi- cal porous zeolites, that is, zeolites containing their native micropores along with an auxiliary network of mesopores have attracted much attention due to their potential in catal- Abstract: The isomerization of o- xylene, a prototypical example of shape-selective catalysis by zeolites, was investigated on hierarchical porous ZSM-5. Extensive intracrystalline mes- oporosity in ZSM-5 was introduced by controlled silicon leaching with NaOH. In addition to the development of sec- ondary porosity, the treatment also in- duced substantial aluminum redistribu- tion, increasing the density of Lewis acid sites located at the external sur- face of the crystals. However, the strength of the remaining Brønsted sites was not changed. The mesoporous zeolite displayed a higher o-xylene con- version than its parent, owing to the re- duced diffusion limitations. However, the selectivity to p-xylene decreased, and fast deactivation due to coking oc- curred. This is mainly due to the dele- terious effect of acidity at the substan- tially increased external surface and near the pore mouths. A consecutive mild HCl washing of the hierarchical zeolite proved effective to increase the p-xylene selectivity and reduce the de- activation rate. The HCl-washed hier- archical ZSM-5 displayed an approxi- mately twofold increase in p-xylene yield compared to the purely micropo- rous zeolite. The reaction was followed by operando infrared spectroscopy to simultaneously monitor the catalytic performance and the buildup of carbo- naceous deposits on the surface. Our results show that the interplay between activity, selectivity, and stability in modified zeolites can be optimized by relatively simple post-synthesis treat- ments, such as base leaching (introduc- tion of mesoporosity) and acid washing (surface acidity modification). Keywords: acidity · catalyst design · deactivation · IR spectroscopy · zeolites [a] Prof. C. Fernandez, Dr. I. Stan, Prof. J.-P. Gilson, Dr.K. Thomas, Dr. A. Vicente Laboratoire Catalyse et Spectrochimie, ENSICAEN, UniversitØ de Caen, CNRS 6 Bd du MarØchal Juin, 14050 Caen (France) Fax: (+ 33) 231-45-2822 E-mail : christian.fernandez@ensicaen.fr [b] Dr. A. Bonilla, Prof. J. PØrez-Ramírez Institute of Chemical Research of Catalonia (ICIQ) Avinguda dels Països Catalans 16, 43007 Tarragona (Spain) [c] Prof. J. PØrez-Ramírez Institute for Chemical and Bioengineering Department of Chemistry and Applied Biosciences ETH Zurich, HCI E125, 8093 Zurich (Switzerland) E-mail : jpr@chem.ethz.ch Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.200903426. # 2010 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim Chem. Eur. J. 2010, 16, 6224 – 6233 6224