Zeolite neutron diffraction studies

Neutron Diffraction Studies of Zeolite-A and Synthetic Faujasite... 

The resurgence of interest in the field of zeolite chemistry, which has been stimulated by the appreciation of their enormous potential as catalysts, has led to the application of several sophisticated physical methods in the study of their structural properties. Important advances have already been made using high resolution, solid state NMR (1,2) and electron microscopy (3), and in this paper we discuss the scope and limitations of neutron diffraction studies with powder samples, with some specific applications to zeolite-A and synthetic faujasite. 

Neutron Diffraction Experiments The sample of Na-A zeolite used in the neutron diffraction studies was prepared by the standard technique and had a Si Al ratio slightly greater than unity (1.06), corresponding to the composition NaQ.97SiiQ3Alo.97 O4. The sample was dehydrated by slowly heating in air in a shallow dish up to 300°C, maintained at 300°C for 16 hours, rapidly transferred to a desiccator and allowed to cool. This procedure does not yield a completely anhydrous material. The sample was finally loaded into a cylindrical aluminum holder under an atmosphere of dry helium. 

The structure of the internal acid site in ZSM-5 can be inferred from the similarity of infrared and NMR data to those of the acidic protons in zeolite HY, where neutron diffraction studies (ref. 21) have shown that the proton is located on an oxide ion bridging between silicon and aluminium ... 

While these computational studies [34] were awaiting publication, a neutron diffraction study on H-SAPO-34 provided evidence for a protonated water molecule [38]. Whereas comments in the more popular press [39] stressed the apparent disagreement with previous calculations ( much of the confusion about how zeolites work stems from quantum calculations ) and used the entertaining title Quantum mechanics proved wrong , a comment to the original paper in the same issue... 

Neutron diffraction study of protons in four lanthanum exchanged X and LSX zeolites... 

Without any doubt, the zeolite framework porous characteristics (micropores sizes and topology) largely govern the zeolite properties and their industrial applications. Nevertheless for some zeolite uses, as for instance, host materials for confined phases, the zeolite inner surface characteristics should be precised to understand their influence on such low dimensionality sorbed systems. In that paper, we present illustrative examples of zeolite inner surface influence on confined methane phases. Our investigation extends from relatively complex zeolite inner surface types (as for MOR structural types) to the model inner surface ones (well illustrated by the AFI zeolite type). Sorption isotherm measurements associated with neutron diffraction experiments are used in the present study. 

The above results augur well for the efficacy of neutron diffraction as a tool for studying zeolites. The sensitivity to... 

A High Resolution Silicon-29 NMR and Neutron Powder Diffraction Study of Na-A Zeolite Loewenstein s Rule Vindicated... 

This rather unexpected result stimulated a number of other investigations, including the present one, with the conclusion that the 4 0 ordering scheme is correct.(13) In addition, the NMR and neutron diffraction data reported in referenced) have been re-interpreted in favor of 4 0 rather than 3 1 order,(14, 15) and it is concluded that the rhombohedral distrotion observed in the neutron pattern is strongly dependent upon the identity of the exchangeable cations. However, in an independent neutron study of Na-A zeolite(16), Adams and Haselden reported no evidence for such a distortion, and conclude that the symmetry must depend subtly upon the method of preparation. 

It has also been observed that zeolite A (10,11,12) and Losod (7) both with Si/Al = 1 have chemical shifts of -88.9 ppm, outside of the range proposed for Si-4A1. At first, this was attributed to S1-3A1 ordering in violation of Lowenstein s rule. However, recent neutron diffraction work on zeolite A (14) and NMR studies of its high silica analogue ZK-4, (8,13) have shown that the single resonance in zeolite A does correspond to S1-4A1. The unusual chemical shift for zeolite A may be a consequence of the strained double four rings in the structure and the presence of one nearly linear T-O-T angle. 

Up till today, relatively few systems have been studied using the most direct technique, the neutron diffraction of deuterated zeolite sam-... 

Numerous studies have been performed in order to elucidate the structure and the dynamics of confined water using a variety of experimental techniques — NMR, quasi-electron neutron diffraction, IR absorption spectroscopy, or molecular dynamics simulation. Most of these studies use model porous media such as clays, polymer membranes, different types of silica and zeolites. Some of these systems (cf Fig. 2) may be used to study water radiolysis in nanoporous media. 

Vitale G., Bull L. M., Morris FL E. and Cheetham A. K., Combined neutron and X-ray powder diffraction study of zeolite CaLSX and a H NMR study of its complex with benzene. J. PAys. Chem. 99 (1995) pp. 16087-16092... 

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