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/Algel 25–90, varying in the sequence of mixing and using Na-silicate or tetraethoxy orthosilicate as the Si source, AlCl3 or Al(NO3)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 29Si 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/Alprod 15–37. From the analysis of both the initial gels and crystalline products by the 13C, 27Al, and 29Si (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–, NO3–) 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 q3 Al species containing an OH group or under the addition of Na+ ions to the synthesis mixture. Single AlO4– balanced by Na+ ions can be located at each framework T site, not only in the vicinity of TPA+.