Crystal formation and size control of zeolitic imidazolate frameworks with mixed imidazolate linkers

Mikaela Gustafsson,Xiaodong Zou

doi:10.1007/s10934-012-9574-1

Abstract

The effect of synthesis parameters, such as batch composition and synthesis temperature, on the formation and crystal size of zeolitic imidazolate frameworks (ZIFs) using mixed linkers, imidazole (Im) and benzimidazole (bIm), were studied by X-ray powder diffraction (XRPD) and scanning electron microscopy (SEM). ZIF-7 and ZIF-62 were identified as the main phases and a phase diagram relating the batch composition and the final product was constructed based on XRPD and SEM. The range of batch composition giving rise to ZIF-7 and ZIF-62 was identified from the phase diagram. Changes of crystal size and morphology of ZIF-7 and ZIF-62 in terms of batch composition and synthesis temperature were studied by SEM and different trends were observed for the two ZIFs. Thermogravimetric analysis (TGA) and in situ XRPD showed that ZIF-62 possesses a similar high thermal stability as ZIF-7.

Highlights

Zeolitic imidazolate frameworks (ZIFs) are a sub-class of metal–organic frameworks (MOFs) that have higher surface areas than zeolites and higher thermal, hydrothermal and chemical stability than most of MOFs
We have studied how the batch composition and the synthesis temperature affect the formation of zeolitic imidazolate frameworks (ZIFs) in the system with hetero-linkers [Zn:Im:bIm] under solvothermal conditions
The effect of the batch composition on the formation of ZIF-7 and ZIF-62 at 110 °C was constructed into a phase diagram

Summary

Introduction

Zeolitic imidazolate frameworks (ZIFs) are a sub-class of metal–organic frameworks (MOFs) that have higher surface areas than zeolites and higher thermal, hydrothermal and chemical stability than most of MOFs. Since the M–Im–M angle is 145°, similar to the Si–O–Si angle in zeolites [2], extended 3D frameworks that possess identical tetrahedral topologies of zeolites are expected for ZIFs. The combination of fourcoordinated transition metal ions and imidazolate-based linkers opened up new opportunities to synthesize porous materials with large pores, high surface areas and tunable functionalities [1]. Even though many ZIFs with interesting properties have been synthesized, to the best of our knowledge, no systematic study has been reported yet on the crystal formation, size and morphology control by changing the synthesis parameters such as batch composition and temperature. We present a study of how different synthesis parameters, such as batch composition and temperature, influence on the crystal formation, size and morphology of zeolitic imidazolate frameworks (ZIFs) when mixed linkers, imidazole (Im) and benzimidazole (bIm), were used. The crystal size and morphology of the two main products, ZIF-7 and ZIF-62, were further studied by scanning electron microscopy (SEM)

Synthesis of ZIFs

Characterization

Effect of the batch compositions on the final products

Effects of synthesis parameters on crystal size and morphology

Thermal stability of ZIF-62

Conclusions