Infrared studies of the surface acidity of oxides and zeolites using adsorbed probe molecules

Abstract

The use of infrared spectroscopy to probe the surface acidity of oxides and molecular sieves is reviewed. The experimental requirements and the type and nature of probe molecules available are also discussed. Special emphasis is given to the criteria that have to be met to arrive at a characterization of the solid that is useful for its catalytic application.

The Influence of Si/Al Ratio on the Distribution of OH Groups in Zeolites with MWW Topology

“…The latter band is slightly blue-shifted after pyridine adsorption, which is sometimes observed during pyridine adsorption [33]. For all zeolites the OH groups in hexagonal prisms (band at 3,582 cm -1 ) are interacting with pyridine because even if pyridine is not able to enter hexagonal prisms, it is able to withdrawn protons from their original positions [5]. During thermodesorption this band is not restored after pyridine desorption at 520°C (spectra not shown).…”

“…As a result, more and more detailed knowledge about this complex phenomenon is being acquired, enabling not only better understanding of the properties of existing and functioning catalysts but also invention and tailoring the new ones. Real catalytic studies require not only the knowledge about the concentration of acid sites but also on their relative strength, availability and type and to achieve this goal multiple probe molecules are employed [4,5]. The acidity of the external surface of zeolites has gathered a lot of attention, mainly in the aspect of enhancing the shape selectivity of zeolites by passivating their external surface activity [6,7].…”

Silica Surface Features and Their Role in the Adsorption of Biomolecules: Computational Modeling and Experiments

“…In view of the importance of the problem for heterogeneous catalysis, 167 the most common use of probe molecules is to titrate Brönsted acidic sites (also known as Brönsted acidic centres, BACs) and in that case they must of course be Brönsted bases. Pure silica does not have strong Brönsted acidity.…”

Characterization of the acid sites in dealuminated nanosized HZSM-5 zeolite with the probe molecule trimethylphosphine

“…Most authors ascribed this to the existence of Lewis acid sites with the combination of Brönsted acid sites after dealumination of zeolite [6][7][8]. They employed a lot of methods to characterize the acidity in solid cata-lysts [9]. Among them, the infrared spectroscopy of adsorbed pyridine has been used extensively to provide evidence for both Brönsted and Lewis acid sites [9].…”

“…They employed a lot of methods to characterize the acidity in solid cata-lysts [9]. Among them, the infrared spectroscopy of adsorbed pyridine has been used extensively to provide evidence for both Brönsted and Lewis acid sites [9]. However, the limitation of this technique lies in the difficulty to determine the strength of the acidity and the number of acid sites in a quantitative way for the different extinction coefficients of each adsorbed species.…”