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Infrared Spectroscopic Investigation of the Acidity and Availability of the Surface Hydroxyls of Three-Dimensional 12-Ring Zeotype H-ITQ-7

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posted on 23.06.2011, 00:00 by Ragnhild Skorpa, Stian Forselv, Karina Mathisen, Silvia Bordiga, Morten Bjørgen, Stian Svelle
Zeotype H-ITQ-7 is among the very few microporous materials comprising a three-dimensional large pore channel based topology other than the well studied zeolite H-beta. Thus, H-ITQ-7 is of substantial interest as an acid catalyst for petrochemical processing of bulky molecules where H-beta is employed today. A more exact fundamental understanding of the availability of the surface sites to bulky species and the acidic properties of this little studied zeotype is highly desirable. Quantitative knowledge of this nature constitutes a prerequisite for potential rational process design based on fundamental insights. To illuminate these aspects, we have employed infrared spectroscopy in combination with adsorption of nonpolymerizing, sterically demanding probe molecules (1,2,4,5-tetramethylbenzene and hexamethylbenzene) that undergo infrared visible symmetry changes upon protonation. This approach can be viewed as a general approach to obtain detailed information about proton transfer reactions and the acidic character of such porous solids, in particular when the experimentally based interpretations are supported by spectra obtained from density functional theory (DFT) calculations. On the basis of the degree of protonation and the persistence of the cations within H-ITQ-7, it can be established that the Brønsted acidic strength of H-ITQ-7 is slightly weaker than that reported for H-beta. Moreover, information regarding the microstructure of the H-ITQ-7 particles has been obtained, as it appears that a large fraction of the surface hydroxyl groups are located in internal crystal voids inaccessible to bulky molecules.