Incorporation of Al at ZSM-5 hydrothermal synthesis. Tuning of Al pairs in the framework

• Concentration of single Al atoms and Al pairs in ZSM-5 framework can be tuned in a wide range.
• Presence of Na+ ions in the synthesis mixture supports stabilization of single Al atoms in the zeolite framework.
• Polarization ability of anions present in the synthesis mixture controls Al distribution in the zeolite framework.
• Application of different Si and Al sources for zeolite synthesis can affect Si and Al networking in the synthesis mixture.

The effect of Si- (TEOS, Tixosil and Na-silicate), Al- (AlCl3, Al(NO3)3, NaAlO2, Al(OH)3 and metallic Al) and Na-reactants (NaCl, NaOH, Na3PO4, NaAlO2) in the presence of TPAOH was investigated with regard to the networking of Si and Al atoms in gels and incorporation of Al atoms and their distribution in the hydrothermally synthesized ZSM-5 framework. At Si/Alprod from 12 to 60 the Al–Si–Si–Al sequences (Al pairs) in one 6MR and single Al atoms located in different rings were the most populated (>99% of total Al), reaching 4–70% of the Al atoms in Al pairs in 6MRs adjacent to TPA+. It has been shown that Na+ ions stabilize single SiAlO4− units in Si–Al gels network and lead to a high relative degree of Al incorporation and high population of single Al atoms charge-balanced by TPA+ and Na+ ions. The occurrence of Al pairs in 6MRs is suggested to be controlled by the positive charge on the N atom of TPA+ resulting from its polarization by OH− and other anions present. The decreasing polarization ability of OH− > Cl− ∼ PO43− > NO3− anions present next to OH− reduces the charge on N+ and results in a decreasing population of Al pairs and increasing population of single Al atoms located in the vicinity of TPA+ ions.

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