crystAlzeo · Crystallography-assisted synthesis of zeolites with tailored acid site distributions

Crystallography-assisted synthesis of zeolites with tailored acid site distributions

Zeolite catalysts occupy a strategic position among the catalysts employed for industrial processes, as they are widely used in petro-chemistry, oil-refining, and fine chemistry (production of high added value compounds). Zeolites are crystalline aluminosilicates whose frameworks are composed of corner-sharing tetrahedral [SiO4] and [AlO4] units, giving rise to a network of channels and cavities of molecular dimensions. The incorporation of Al in place of Si generates a negative charge in the structure, which is counter-balanced by inorganic cations or organic species employed as structure directing agents (SDAs). These species are replaced by protons in the post-synthesis treatment (ion-exchange and/or calcination) to transform zeolites into their catalytically active form (acid form). Although Al is the most common trivalent atom that partially replaces Si in the zeolite framework, other trivalent elements can also be employed, such as boron or gallium, leading also to the formation of Brönsted sites. In a similar way, Brönsted sites in aluminophosphates –analogous to zeolites but with composed of AlO4 and PO4 tetrahedra- are formed by the partial replacement of phosphorous by a trivalent element (usually silicon) or aluminum by a divalent element (Zn, Mg, Co or Fe among others). The structure of zeolites and the concentration and distribution of acid sites are known to determine their activity and selectivity in catalysis. However, less attention has been paid to the latter probably due to the difficulty in identifying the exact location of the active sites. The general objective of this project was to employ crystallography as a tool to study the factors affecting the location of the active sites in nanomaterials. The fellow was not an expert in the use of crystallographic techniques, and therefore her training was also an important objective of the project. Another objective was to apply the new findings to the synthesis of materials with controlled catalytic properties. This has been performed in the laboratory of the Molecular Sieves Group of the Institute of Catalysis and Petroleum Chemistry (Spanish Research Council, CSIC) in Madrid. Finally, as specified in the Call for proposals for Marie Curie IEF, the creation of European networks of scientists that collaborate in common projects was also an important task to develop during the two years of the fellowship. The fellow has established collaborations with Prof. Jeroen van Bokhoven from ETH Zurich, Prof. Russell Morris from the University of St. Andrews (Scotland), Prof. Valentin Valtchev from the CNRS in Caen (France), Prof. Brad Chmelka from the University of California Santa Bárbara (U.S.A.), Dr. Benoit Louis from the University of Strasbourg (France) and Dr. Santiago Gómez from the University Rey Juan Carlos (Spain). It has been found that the organic species employed as templates in the synthesis can establish strong interactions with the negatively charged oxygen atoms that are bonded to Al in the inorganic framework. This has allowed characterizing the acid sites distribution of zeolite samples by studying the location of the organic templates within their pores. A series of zeolite ferrierite samples synthesized following a strategy to tailor their acid sites distribution has been studied. We have been able to find the most likely location for Al atoms in different samples, and this explains the previous results on acid sites accessibility and catalytic activity. To our knowledge, this is the first time that a synthesis approach to control the acid sites distribution in zeolites is reported together with evidences on the preferential interaction of the organic templates with certain oxygens of the zeolite framework. This opens the possibilities to tailor the acid sites distribution of zeolites to be employed in particular catalytic reactions. Other heteroatoms, such as Ge or Zn, that partially replace Si have also been located in different samples, as well as other extraframework species like water, inorganic cations or fluoride anions. This has allowed, for example, understanding the relationship between the incorporation of Ge to the double-four rings of zeolite A and the presence of fluoride in the middle of those rings. All these structural details, only accessible through an in depth crystallographic study, provide with valuable information about zeolites that can help in making zeolite synthesis a more rational process than the usual trial and error approach.

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