Crystal growth of RHO-type zeolitic imidazolate framework in aqueous phase

Here we report the synthesis of a zeolitic imidazolate framework with RHO topology (RHO-Zn(eim)₂; eim is the deprotonated anion of 2-ethylimidazole (Heim)) in the aqueous phase. Zn(eim)₂ crystals were prepared by the reaction between Heim and zinc acetate in deionized water. The products prepared at relatively high Heim/Zn molar ratios were Zn(eim)₂ whose structure assigned to RHO, qtz and ANA topologies. Zn(eim)₂ obtained under static condition had porous RHO structure, while under stirred condition, nonporous dense qtz and ANA structures were formed. This study revealed that the formation of RHO porous structure requires the template effect of excess Heim. The RHO-Zn(eim)₂ crystals possessed high surface area and micropore volume, whose morphology consisted of a rhombic dodecahedron. RHO-Zn(eim)₂ exhibited high adsorption capacity (4 mmol/g) for hexane and cyclohexane. Due to the hydrophobic nature of RHO-Zn(eim)₂, water vapor was hardly adsorbed. Although RHO-Zn(eim)₂ was stable in the presence of water vapor, it became nonporous upon hydrolysis in aqueous solution. In contrast, partial carbonization of topmost surface improved the structural stability against hydrolysis by water, while maintaining the adsorption capacity and increasing the adsorption rate.