Dedicated to Süd-Chemie on the occasion of its 150th anniversary 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 [Si nÀm Al m O 2n ] mÀ made of corner-sharing TO 4 tetrahedra (T = Si, Al À ). A typical feature of many silicon-rich zeolites is a high number of crystallographically distinguishable T sites. Since the cationic species bind to the AlO 4 À 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 low Al content in the most active zeolite catalysts. Solid-state 29 Si magic-angle spinning (MAS) NMR spectroscopy succeeded early in distinguishing between Si in different crystallographic positions of the MFI framework, [1] but for the quadrupolar 27 Al nucleus, the development of multiple quantum (MQ) NMR spectroscopy experiments [2] opened such possibilities only in the last decade. [3][4][5][6] 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, [7] which is also supported by the effect of the Al concentration on the 27 Al MQ MAS NMR spectra of ZSM-5 [4] and zeolite b.[5] As lattice energy minimizations with reliable force fields, for example, for MFI, [8,9] 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 27 Al signals in the MQ MAS 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 27 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 differe...

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The Al siting in the ZSM-5 zeolite was investigated by (27)Al 3Q MAS NMR spectroscopy and QM/MM calculations. It was found that the occupation of the framework T-sites by Al and the concentration of Al in these T-sites are neither random nor controlled by a simple rule. They both depend on the conditions of the zeolite synthesis. At least 12 out of the 24 distinguishable framework T-sites of ZSM-5 are occupied by Al in the set of the investigated zeolite samples. A partial identification of the Al sites is possible. The calculated (27)Al NMR shielding values were converted to (27)Al isotropic chemical shifts using the experimental isotropic chemical shift of 60.0 ppm referenced to the aqueous solution of Al(NO(3))(3) and the corresponding calculated NMR shielding of 490.0 ppm of a silicon rich (Si/Al 38) chabazite structure zeolite as a secondary internal standard. The observed (27)Al isotropic chemical shifts of 50.0 and 54.7 ppm correspond to Al atoms in the T20 and T6 sites, respectively. The pair of measured isotropic chemical shifts of 52.9 and 53.7 ppm can be assigned to the T4, T8 pair. At the low-shielding end, two assignments are plausible. The smallest deviations between the calculated and observed isotropic chemical shifts are reached for the assignment as follows: T24 (64.8 ppm) is not occupied in the samples and that the observed isotropic chemical shifts 63.6, 62.8, and 60.0 ppm belong to T1, T17, and T7, respectively. It follows then that T-sites T12 (60.8 ppm), T3 (61.7 ppm), and T18 (62.0 ppm) are most likely not occupied by Al in our ZSM-5 samples. If we assume that the calculated isotropic chemical shifts are systematically larger than the observed ones then we can assign the largest observed isotropic chemical shifts of 63.6 and 62.8 ppm to the least shielded T24 and T1 sites, respectively, and 60.0 ppm to T12. Then the sites T3 (61.7 ppm), T18 (62.0 ppm), and T17 (62.5 ppm) would be unoccupied by Al in our ZSM-5 samples. It was further shown that there is no simple linear relationship between the observed (27)Al isotropic chemical shifts and the average Al-O-Si angles.

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Sets of ZSM-5 zeolites with different distributions of Al atoms in the framework were synthesized from the mixtures of Si/Al gel 25−90, varying in the sequence of mixing and using Na-silicate or tetraethoxy orthosilicate as the Si source, AlCl 3 or Al(NO 3 ) 3 as the Al source, NaCl or Na-silicate as the Na source, and tetrapropylammonium hydroxide as the structure directing agent. The distribution of Al atoms in the ZSM-5 framework between single Al atoms and Al pairs of Al−O−(Si− O) 2 −Al sequences was monitored by the 29 Si MAS NMR, Co(II) ion exchange capacity and UV−vis spectroscopy of bare Co(II) ions in the dehydrated zeolites. The results showed that the distribution of Al atoms in the ZSM-5 framework can be controlled in a wide range of concentrations (6−66% of Al atoms in Al pairs for the given Si/Al) and Si/Al prod 15−37. From the analysis of both the initial gels and crystalline products by the 13 C, 27 Al, and 29 Si (SP and CP) MAS NMR and FTIR, it was concluded that the electrostatic and van der Waals interactions, governed mainly by the presence of hydrophobic (TPA + ) and charged (Na + ) cations and anions (Cl − , NO 3 − ) of different polarization ability and by the reactivity of alumosilicate species, control the incorporation of Al atoms into the framework of ZSM-5 in the Al pairs located in the rings of three cationic sites and distant not-cooperating single Al atoms in different rings. The high concentration of Al pairs in the zeolite can be reached by the increase of the positive charge of the central N atom of TPA + , which supports incorporation of the Al pair in its close vicinity or by the increase in relative concentration of entities containing Al pairs in the synthesis gel. The predominant incorporation of single Al atoms into the framework occurs in the presence of highly reactive q 3 Al species containing an OH group or under the addition of Na + ions to the synthesis mixture. Single AlO 4 − balanced by Na + ions can be located at each framework T site, not only in the vicinity of TPA + .