Synthesis of ZIF-11 – Influence of the synthesis parameters on the phase purity

The synthesis of ZIF-11 in N,N-Diethylformamide (DEF) at varying reaction temperatures, times and different molar compositions of the reaction mixture was investigated in detail in order to identify ranges of the formation of phase-pure ZIF-11. Since it is known that water in the solvent DEF leads to the formation of the phase impurity ZIF-7-III during the ZIF-11 synthesis, all reactions were carried out in DEF exhibiting water contents below 0.1 wt%. The variations of the molar ratio of substrates Zn:bIm:DEF = 1:x:y as well as the synthesis temperature and time revealed a number of phase transitions within the ZIF-11 synthesis. Highly crystalline ZIF-11 was obtained for a molar ratio Zn:bIm:DEF of 1:15:1400 already after 3 h of reaction at 60 °C and 100 °C. Depending on the reaction vessel, the water content of the solvent as well as the temperature, ZIF-11 transformed to the dense phase ZIF-7-III within different reaction times. It was concluded that ZIF-11 represents a metastable reaction product. SEM images of the samples obtained after different synthesis times showed that ZIF-11 undergoes successive dissolution-recrystallization cycles in combination with heterogeneous nucleation. For a reduced amount of solvent, the formation of ZIF-11 proceeded faster, but ZIF-11 with lower maximum relative crystallinity was formed. For lower ratios of solvent to zinc, the time range of stable ZIF-11 formation was smaller as compared to higher solvent amounts. The variations of the ratio of zinc to the linker benzimidazole showed that ZIF-11 formation was slower for lower molar ratios, e.g. 1:2:1400 and 1:4:1400. ZIF-11 with the highest relative crystallinity and a pore volume of 0.43 cm3 g−1 was formed after 3–12 h of reaction at molar ratios Zn:bIm:DEF of 1:8:1400 and 1:12:1400. Another phase transition was observed for synthesis times of 6–96 h for a molar ratio of 1:2:1400. Under those conditions, an unidentified layered crystalline product was found and similarities to the layered ZIF-L were identified.