Aluminum siting in silicon-rich zeolite frameworks: a combined high-resolution (27)Al NMR spectroscopy and quantum mechanics / molecular mechanics study of ZSM-5.

Zeolites are crystalline microporous aluminosilicates widely used as molecular sieves and catalysts in industrial chemical processes. Silicon-rich zeolites (Si/Al> 12) such as ZSM-5 (MFI framework) have found particular attention. The catalytically active species, that is, protons, metal cations, and metal-oxo cations, compensate the negative charge of the microporous aluminosilicate frameworks [Sin mAlmO2n] m made of corner-sharing TO4 tetrahedra (T= Si, Al ). A typical feature of many silicon-rich zeolites is a high number of crystallographically distinguishable T sites. Since the cationic species bind to the AlO4 tetrahedra, the crystallographic position of aluminum in zeolite frameworks governs the location of the active sites, which in turn affects the catalytic activity and selectivity. Thus, understanding the Al siting in zeolite structures is a priority, but it has remained a challenge. Diffraction methods are of limited use because of the similar scattering properties of Si and Al but also because of the lowAl content in the most active zeolite catalysts. Solid-state Si magic-angle spinning (MAS) NMR spectroscopy succeeded early in distinguishing between Si in different crystallographic positions of the MFI framework, but for the quadrupolar Al nucleus, the development of multiple quantum (MQ) NMR spectroscopy experiments opened such possibilities only in the last decade. It is also not clear if there are preferred T sites for Al substitution or whether the T sites are occupied statistically. An X-ray diffraction study found three Cs sites in extraframework positions of ZSM-5, thus indicating nonrandom Al siting, which is also supported by the effect of the Al concentration on the Al MQMASNMR spectra of ZSM-5 and zeolite b. As lattice energy minimizations with reliable force fields, for example, for MFI, yielded only small energy differences for Al in different positions, the Al distribution might be kinetically controlled. This assumption implies that different synthesis procedures and different templates and cations could lead to different Al substitution patterns, thus increasing the total number of resolved Al signals in the MQMAS NMR spectra of a variety of samples. This strategy is followed herein. For a set of 11 differently synthesized ZSM-5 samples, ten distinct resonances have been identified by Al MQ MAS NMR spectroscopy, extending over a shift range of Dd= 13.6 ppm. Quantum-chemical calculations for simulated structures with Al in 24 different T sites yield a shift range of Dd= 14.1 ppm and show that the observed resonances belong to Al in different crystallographic sites. We conclude that the Al siting in ZSM-5 is not random and can be substantially varied by the conditions of zeolite syntheses. A set of Na-ZSM-5 samples (A–K) with Si/Al framework ratios from 14 to 45 was prepared by using different silicon, aluminium, and sodium sources as well as different structuredirecting agents; for details, see Table S1 of the Supporting Information. Al 3Q MAS NMR spectroscopy experiments were carried out on the calcined samples, which were dried only at ambient temperature to retain their full hydration inside the pores. Figure 1 shows a typical 2D plot of the Al 3Q MAS NMR spectrum together with a simulation of the projections and selected spectra slices. Figure 2 shows the F1 and F2 projections of all spectra. Their shapes were simulated assuming three to four resonances per sample. The chemical shifts in the isotropic (dF1) and in the observed dimension (dF2) for all ten resonances together with the isotropic chemical shifts obtained according to diso= (17dF1+ 10dF2)/27 are listed in Table 1. Resonances I, VII, VIII, and X arise from small shoulders, and it may be argued that they are less safely determined than the other six resonances. However, each of resonances I–VII [*] Dr. S. Sklenak, Dr. J. Dědeček, Dr. C. Li, Dr. B. Wichterlov%, Dr. V. G%bov% J. Heyrovský Institute of Physical Chemistry Academy of Sciences of the Czech Republic v.v.i. , Dolejškova 3, 182 23 Prague (Czech Republic) Fax: (+420)28658-2307 E-mail: stepan.sklenak@jh-inst.cas.cz