Zeolites neutron diffraction

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. 

We have observed large variations in the sorption capacities of zeolite samples characterized by (ID) channel systems, as for instance AFI (AIPO4-5 zeolite) and MTW (ZSM-12 zeolite) architectural framework types. Indeed, for such unconnected micropore networks, point defects or chemisorbed impurities can annihilate a huge number of sorption sites. Detailed analysis, by neutron diffraction of the structural properties of the sorbed phase / host zeolite system, has pointed out clear evidence of closed porosity existence. Percentage of such an enclosed porosity has been determined. 

Figure 5. Evolution of the main neutron diffraction peak intensity versus methane confined phase loading in AlP04-5 zeolite.

Feuerstein, M. and Lobo, R.F. (1998) Characterization of Li cations in zeolite LiX by solid-state NMR spectroscopy and neutron diffraction. Chem. Mater., 10, 2197-2204. 

The former way is not useful when X-ray diffraction is used, because the difference between the scattering curves of Si and A1 is too small. If neutron diffraction is used, the neutron diffraction curves differ by as much as 25 %, so that the Si/Al ratio can be satisfactorily refined. Unfortunately, large crystals are needed at present (nearly 1mm in volume), therefore this method can only be applied to a restricted number of zeolites. 

HRMAS Al and Si-NMR spectra frequently give useful information for the determination of Si-Al distribution in zeolites, but this technique should be considered as complementary to x-ray or neutron diffraction. Their combined use often provides a most complete and reliable description of the Si-Al distribution in zeolites. The limits of NMR spectra in zeolites with disordered Si-Al distributions and three or more symmetrically independent T sites must also be considered. 

The five years since last considering specifically recent developments in X-ray and neutron diffraction methods for zeolites [1] have witnessed substantial progress. Some techniques, such as high resolution powder X-ray diffraction using synchrotron X-rays, have blossomed from earliest demonstrations of feasibility to widespread and productive application. Others, such as neutron powder diffraction, have shown steady progress. For still others, notably microcrystal diffraction, a variety of circumstances have contributed to extended gestation periods. Additionally, opportunities scarcely considered earlier (such as single crystal Laue diffraction, and certain developments in computer simulations that complement diffraction work) now command broad attention and warrant the commitment of substantial further investment. 

The structures of the zeolite frameworks have been determined by X-ray and neutron diffraction techniques. Some of the naturally occurring minerals were characterized in the 1930s, and the synthetic zeolites have been investigated from 1956 onward. Unfortunately, it is extremely difficult for diffraction techniques to determine a structure unequivocally because A1 and Si are next to each other in the Periodic Table and thus have very similar atomic scattering factors (Chapter 2). It is possible to determine the overall shape of the framework with accurate atomic positions but not to locate the Si and A1 atoms precisely. 

Improved NMR, XRD, neutron-diffraction, IR, and adsorption techniques are helping to sort out the physical properties of these various A1 species in the dealuminated zeolite Y samples, and their relationship to catalytic activity. 

Measurements of Crystal Structures. - (a) Zeolites. Improved understanding of the catalytic behaviour of zeolites has tended to proceed in parallel with improvements in their structural characterization. The recent advent of magic-angle spinning n.m.r. (m.a.s.n.m.r.)for instance, has reawakened interest in the ordering of the Si and A1 atoms in zeolites and its catalytic consequences. This renewed interest has in turn lead to a re-examination of the neutron diffraction patterns of certain zeolites in order to obtain confirmatory evidence for the the predictions of the n.m.r. experiments. A useful introduction to the benefits of using neutrons rather than X-rays for powder diffraction can be found in ref. 55. 

In the conventional view of the structure of zeolite A each Si atom is surrounded by four A1 atoms and vice-versa, but recent m.a.s.n.m.r. data56 was interpreted to indicate that the structure was actually one in which each Si atom was surrounded by one Si and three A1 atoms and vice-versa. In support of this work a powder neutron diffraction experiment found a rhombohedral distortion in a sample of Na zeolite A.57... 

These revolutionary ideas lead to further neutron measurements of the structure of zeolite A, which have confirmed the correctness of the traditional 4 0 ordering scheme.58 59 Neutron diffraction traces for several samples of a dehydrated Na zeolite A with Si to A1 ratios of 1.03, 1.09 and 1.12 failed to show any rhombohedral distortions similar to those reported in ref. 57, and in each case the data was consistent with a cubic structure.58 Neutron diffraction experiments on a T1 exchanged sample of the same Na zeolite that had shown the rhombohedral distortion in ref. 57 showed that the crystals now had cubic symmetry59 60 and therefore the distortion that had been measured for the Na zeolite A must be very sensitive to the identity of the exchangeable cations. Profile refinement of this neutron data56 57 also showed a pronounced bimodal distribution of the bond lengths as would be predicted by the 4 0 model. In conclusion it appears that the chemical shifts observed in the n.m.r. experiments can be influenced by factors such as local strain, as well as by the local environment of each Si atom. 

The Na ZK-4 and Y zeolites have Si to A1 ratios of 1.65 and 2.61, respectively, but although the ZK-4 zeolite is a variant of zeolite A, powder neutron diffraction data61 showed that no superlattice reflection could be found at the angle where it occurs in the T1 zeolite A.59 60 The small unit cell implied that in this zeolite the Si and A1 atoms were no longer preferentially located on alternate tetrahedral sites. A similar result was found for the Y zeolite. 

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. 

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

In a series of recent papers the structure of zeolite A has been re-examined with particluar reference to the Si and A1 ordering, and a different arrangment has been proposed,(5, 6 ) based mainly upon results from three different techniques, electron diffraction, (5, 7) 29Si-NMR(5, ], j ) and neutron diffraction, C5, 9 ) and is at variance with Loewenstein s rule. (10)... 

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. 

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