Catalytically important metal ion intermediates on zeolites and silica surfaces

High field 27Al ENDOR reveals the coordination mode of Cu2+ in low Si/Al zeolites

The siting of Cu2+ in zeolites with low exchange levels has been a subject for debate due to the lack of experimental evidence that provide directly the interaction between the Cu2+ ion and the zeolite framework. High field 27Al ENDOR provided highly resolved orientation selective ENDOR spectra from which both the 27Al hyperfine and quadrupole principal components and orientations relative to the g tensor principal axis system were determined for a dehydrated Cu2+ exchanged zeolite X with Si/Al = 1. The results show that all three Cu-Al distances in the six-member ring are equivalent, in contrast to DFT predictions using cluster models.

High field ENDOR as a characterization tool for functional sites in microporous materials

The determination of the details of the spatial and electronic structure of functional sites (centers) in any system, be it in materials chemistry or in biology, is the first step towards understanding their function. When such sites happen to be paramagnetic in any point of their activity cycle, the tool box offered by a variety of high resolution electron paramagnetic resonance (EPR) spectroscopic techniques becomes very attractive for their characterization. This tool box has been considerably expanded by the developments in high field (HF) EPR in general, and HF electron nuclear double resonance (ENDOR), in particular. These have led to numerous new applications in the fields of biology, physics, chemistry and materials sciences. This overview focuses specifically on recent applications of pulsed HF ENDOR spectroscopy to microporous materials, such as zeotype materials, presenting the new opportunities it offers. First, a brief description of the theoretical basis required for the analysis of the HF ENDOR spectrum is given, followed by a description of the pulsed techniques used to record spectra and assign the signals, along with a brief presentation of the required instrumentation. Next, specific applications are given, including transition metal ions and complexes exchanged into zeolite cages, transition metal substitution into frameworks of zeolites, aluminophosphate molecular sieves, and silicious mesoporous materials, the interaction of NO with Lewis sites in zeolite cages and trapped S. We end with a discussion of the advantages and the shortcomings of the method and conclude with a future outlook.