Synthesis of ZSM-5 Zeolites with Defined Distribution of Al Atoms in the Framework and Multinuclear MAS NMR Analysis of the Control of Al Distribution

Abstract

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 + .

Structure–performance descriptors and the role of Lewis acidity in the methanol-to-propylene process

“…al. 46 Operando UV-Vis diffuse reflectance spectra were obtained using an AvaSpec 2048L spectrometer connected to a high temperature UV-Vis optical fiber probe. The measurements were performed in the wavenumber range 11 000 -50 000 cm -1 .…”

Location of Framework Al Atoms in the Channels of ZSM‐5: Effect of the (Hydrothermal) Synthesis

“…It was shown that the addition of Na + ions to the structure‐directing agent (SDA) tetrapropylammonium cation (TPA + ) in the synthesis mixture can markedly affect the incorporation of single isolated Al atoms (hereafter single Al atoms) and Al−O−(Si−O) 2 −Al sequences in one ring (hereafter Al pairs) into the zeolite framework . Moreover, our 27 Al triple quantum magic angle spinning (3Q MAS) NMR spectroscopic measurements revealed that the addition of Na + also alters the Al siting in the framework T sites . These findings suggest that the presence of Na + ions during the synthesis can possibly affect the Al location in the ZSM‐5 channel system.…”

“…These findings suggest that the presence of Na + ions during the synthesis can possibly affect the Al location in the ZSM‐5 channel system. Since no syntheses of ZSM‐5 zeolites have been reported with Si/Al≲22 and with Na + absent from the synthesis gel, all such ZSM‐5 zeolites, which are highly relevant for catalysis, exhibit an organization of framework Al atoms that is affected by the presence of Na + in the synthesis gel. Thus, knowledge of the effect of Na + ions on the location of Al atoms in the zeolite channel system is of crucial importance.…”

Reaction path analysis for 1-butanol dehydration in H-ZSM-5 zeolite: Ab initio and microkinetic modeling

“…The MFI orthorhombic unit cell has twelve different tetrahedral sites and the replacement of a Si atom requires selection of an appropriate site. The location of the Al atom in the unit cell was reported to be non-random [26] and to depend on the zeolite synthesis conditions [27]. Nevertheless, few other studies specify the T12 site to be a preferred location for the Al atom associated with the Brønsted acid site [28,29].…”