Al–Mn-silicate nanobubbles phase as an intermediate in zeolite formation

• An original Al–Mn silicate with nanobubbles morphology was prepared.
• This morphology is attributed to the presence of CO2 gas bubbles.
• The prepared Al–Mn silicate contains both Mn(IV) and Mn(II).
• It has a significant surface area and a very high cation exchange capacity.
• It is an intermediate in the Mn-analcime formation

Manganese (II) carbonate, silicic acid and aluminum nitrate were treated hydrothermally at different temperatures (120–210°C) and for different durations (6–72 h) in an aqueous basic medium. Different compositions of starting mixtures were used. The synthetized materials were characterized by powder X-ray diffraction, FTIR-spectroscopy, N2 adsorption analysis, Transmission Electron Microscopy (TEM), energy dispersive X-ray analysis (EDX), thermal analysis (TG-DTG), Temperature-programmed reduction (TPR), X-fluorescence, X-ray photoelectron spectroscopy (XPS), electronic paramagnetic resonance (EPR) and 29Si and 27Al MAS-NMR spectroscopy. The formation of kaolinite, smectite-like and Mn-containing lamellar phases was observed, but only in minor amounts contrary to previous studies. The majority phase was an original Al–Mn silicate with nanobubble-like morphology, a high surface area and mesoporosity, containing both Mn(IV) in lattice positions and Mn2 + as exchangeable cations. This amorphous Al–Mn-silicate nanobubbles phase seems to be an intermediate in a zeolite formation. Indeed, the increase of the reaction temperature, the reaction time or the reagent concentrations promoted the crystallization of a zeolite of the analcime type by transformation of the Al–Mn-silicate nanobubbles.

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