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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