B. Staudte

Low-Temperature 1H MAS NMR Investigations on the Nature of Acid Sites Causing Enhanced Catalytic Activity in H-Zeolites

Summary The present low-temperature 1 H MAS NMR investigations yield new information concerning the nature of acid sites responsible for the enhanced catalytic activity of “mildly” hydrothermally treated H-ZSM-5 zeolites. The enhancement of catalytic activity is either caused by Br0nsted acid sites of enhanced strength of acidity with a relatively broad distribution of deprotonation energies or by a synergism between Brensted acid sites of “normal” strength of acidity and Lewis acid sites. Furthermore, the existence of a new signal due to bridging hydroxyl groups influenced by an additional interaction with the zeolitic framework could be shown in H-ZSM-5 zeolites.

NMR-studies concerning brönsted acidity of HY zeolites

Abstract In the present work nuclear magnetic resonance has been applied to the study of hydroxyl groups in HY zeolites. The proton mobility in activated zeolites and in zeolites loaded with pyridine, and the proton exchange in the system zeolite/benzene have been investigated. The ratio nA/τ is used as a relative measure for the acidity of OH groups if nA denotes the number of protons involved in the particular process and τ is the mean residence time of a hydroxyl proton at a lattice oxygen. The following conclusions could be drawn: In ammonium exchanged Y zeolites pretreated at temperatures between 300 and 500 °C the proton mobility is controlled by the residual amount of ammonium. Measurements of the pyridine induced mobility of hydroxyl protons show, for deep bed activated samples, an increasing acidity (by one order of magnitude) for an increasing degree of decationization from 36 to 91% or for a decreasing activation temperature from 500 to 300 °C. The quantity nA/τ measured from proton exchange in the system zeolite/benzene, increased in the same way with the degree of decationization.

Detection of Lewis acid sites using NO as probe molecule

Abstract EPR spectra of H-ZSM-5 activated at 773 and 1073 K for non-treated and hydrothermally treated samples were measured without loading and loaded with two molecules of NO per three aluminum atoms of the zeolite framework. In accordance with the literature, the strong EPR signal with g =1.98 was ascribed to NO adsorbed on “true” Lewis acid sites (LAS). Interestingly, we have found that, inspite of the loading with NO, this signal is absent for the hydrothermally treated sample activated at 773 K, while it appears for the corresponding untreated sample and the treated and non-treated samples activated at 1073 K. Taking into account the time dependence of the signal intensity, we must conclude that only in the latter two cases, stable Lewis acid sites (SLAS) are formed. This finding should provide an additional aspect for the explanation of the enhancement of the catalytic activity of H-ZSM-5 through a mild hydrothermal treatment.

Low-temperature 1H MAS NMR and FTIR investigations on the interaction of tetrachloroethylene with surface hydroxyl groups in H-zeolites

Abstract The interaction of surface hydroxyl groups in zeolites with adsorbed C 2 Cl 4 molecules was investigated by lowtemperature 1 H MAS NMR and FTIR spectroscopy. It could be shown that the induced 1 H NMR chemical shift Δδ of free surface hydroxyl groups caused by the adsorption of C 2 Cl 4 reflects the strength of acidity of the hydroxyl groups. A linear correlation between Δδ and the induced wavenumber shift Δν of the stretching vibration band of the surface hydroxyl groups has been found. Therefore, Δδ can also be used for the calculation of the deprotonation energy ΔE dp of surface hydroxyl groups by a previously developed method.

Variable-Temperature 1H MAS NMR Investigations on the Interaction Between Brønsted Acid Sites and Carbon Monoxide Adsorbed on H-ZSM-5 Zeolites

Summary An analysis of the influence of absorbed carbon monoxide upon the low-temperature 1 H MAS NMR signals of surface hydroxyl groups in zeolites allows the determination of the geometry of the formed adsorption complexes. Information concerning the strength of acidity and the accessibility of the different types of hydroxyl groups in H-ZSM-5 can be derived from the 1 H MAS NMR spectra.

NMR and IR studies on the adsorption of methane and trimethylgallium on zeolite HY

Abstract The adsorption of trimethylgallium on H-zeolites is accompanied by the conversion of methyl groups into methane. A detailed NMR and IR spectroscopic study of the adsorption of methane and trimethylgallium on zeolite HY is provided in the present contribution.

A new method for the NMR-spectroscopic measurement of the deprotonation energy of surface hydroxyl groups in zeolites

A new 1 H MAS NMR-spectroscopic method for the determination of the deprotonation energy Δ E DP of surface hydroxyl groups in zeolites is described. This method is based on the measurement of the induced 1 H NMR chemical shift Δδ caused by the interaction between the surface hydroxyl groups and weakly basic probe molecules such as C 2 Cl 4 or CO. The new 1 H MAS NMR-spectroscopic method is compared with the formerly established methods based on the measurement of the induced wavenumber shift Δν of the O−H stretching vibration caused by the adsorption of weakly basic probe molecules [1] or on the measurement of the 1 H NMR chemical shift δ H in activated samples [2,3].

Magic-Angle-Spinning Nuclear Magnetic Resonance and Infrared Studies on Modified Zeolites

Abstract 11B MAS NMR yields quantitative information about the incorporation of boron into zeolite frameworks. 1H MAS NMR and IR spectroscopy show that OH groups introduced into the framework by boron substitution are non-acidic. 2D proton spin diffusioin measurements of the zeolite SAPO-5 reveal that defect OH groups are adjacent to acidic bridging hydroxyl groups and do not exist in an amorphous phase. Strongly adsorbed water molecules in mildly steamed zeolites H-Y can be explained by Lewis sites.