Ionothermal Synthesis of MnAPO-SOD Molecular Sieve without the Aid of Organic Structure-Directing Agents

An
SOD-type metalloaluminophosphate molecular sieve (denoted as SOD-Mn)
was ionothermally synthesized by introducing manganese­(II) cations
into the reaction mixture via MnO–acid or MnO2–reductant
reactions. Composition and structure analyses results show that two
kinds of manganese­(II) cations exist in the SOD-Mn structure. Part
of the manganese­(II) cations isomorphously substitute the framework
aluminum­(III) with a substitution degree of ∼30%. The rest
of the manganese­(II) cations occupy a fraction of the sod cages in their hydrated forms. A comprehensive investigation of
the synthesis parameters, crystal sizes, and crystallization kinetics
indicates that the in situ released hydrated manganese­(II) cations
direct the formation of SOD-Mn. Such structure-directing effect may
be inhibited by both the fluorination of manganese­(II) cations and
the water accumulation during crystallization. In the fluoride anion-containing
reaction mixture with a low ionic liquid content, the crystallization
process is strongly suppressed, and large SOD-Mn single crystals of
over 200 μm in size are yielded. SOD-Mn is free from organics
and shows improved thermal stability compared with metalloaluminophosphates
synthesized by using organic structure-directing agents.