Maria Castro

Application of NMR crystallography to the determination of the mechanism of charge-balancing in organocation-templated AlPO STA-2

The combination of solid-state MAS NMR spectroscopy and first-principles calculations is used to elucidate the structure of an as-prepared microporous AlPO (STA-2), in which the organocation template (bis-diazabicyclooctane-butane) is charge balanced by hydroxyl groups coordinated to framework aluminium species. 27Al MAS NMR spectra show Al exists in both tetrahedral and five-fold coordination, with the latter directly coordinated to hydroxyl O atoms as well as framework O atoms. The relative intensities of these resonances can be used to determine that the hydroxyls bridge between the two types of Al. Calculation of NMR parameters using a periodic density functional theory approach are able to suggest assignments for the resonances in both 27Al and 31P NMR spectra. 31P chemical shifts are shown to depend not only on the topologically-distinct sites in the SAT framework but also on whether or not the P atoms form part of a six-membered P(OAl)2OH ring, where OH is a bridging hydroxyl. By diffraction six possible sites for the charge-balancing hydroxyls have been identified, but all are refined with an average occupancy of 0.33. However, predicted peak positions in two-dimensional J-HETCOR NMR spectra indicate that only one hydroxyl is found in each cancrinite cage, and suggest that the most likely arrangements are those that avoid the close approach of bridging hydroxyl groups in adjacent cages. We show that the use of a combination of NMR spectroscopy and calculation to elucidate structure, often referred to as “NMR Crystallography”, offers a promising route for the future study of as-made and substituted microporous materials.

Nanometre resolution using high-resolution scanning electron microscopy corroborated by atomic force microscopy

The resolving power of high-resolution scanning electron microscopy was judged using topographical height data from atomic force microscopy in order to assess the technique as a tool for understanding nanoporous crystal growth.

High-Resolution scanning electron and atomic force microscopies: observation of nanometer features on zeolite Surfaces

It is now possible to observe nanometer features on the surfaces of zeolitic materials using high-resolution scanning electron microscopy. By taking ibidem measurements in combination with atomic force microscopy we are able to illustrate the strengths and weaknesses of both techniques and judge respective resolving power.

A computational study of the stretching frequencies of Brønsted acid sites in SAPO STA-7: preliminary comparison with infrared

Abstract The acidity of the Bronsted sites in SAPO STA-7 material is investigated by atomistic simulation techniques. The long-range interactions mainly govern the acidity where as the short-range contributions have only a limited degree of influence. The calculated population of the OH stretching frequencies for a STA-7 sample with 4.8 silicon atoms per unit cell, predicts a broad distribution ranging from 3560 cm –1 to 3585 cm –1 which is confirmed by Infrared Spectroscopy.