ZIF-21 Crystals: Its Morphology Control and Potential as an Adsorbent for Ammonia Capture

The selective adsorption of NH3 over N2 and H2 is of interest for ammonia separation and synthesis. Zeolitic imidazolate frameworks (ZIFs) having tunable pores, high surface area, and chemical affinity for ammonia are suitable candidates for this particular application. Herein, we explore the influence of synthesis parameters on the morphology of ZIF-21 formed by hydrothermal synthesis. Among the many variables examined, it was found that the addition of weak bases such us sodium formate (SF) and weak acids such as ethylenediaminetetraacetic acid (EDTA) acts as crystal morphology modifiers to improve the crystallinity and morphology of ZIF-21 crystals. The addition of EDTA led to the formation of crystals with significant improvements in ammonia adsorption performance without modifying the characteristic LTA topology of ZIF-21. The resultant ZIF-21 crystals led to octahedral uniform shapes displaying remarkably high NH3/N2 and NH3/H2 ideal selectivities. Specifically, ideal selectivities of NH3/N2 = 400 and NH3/H2 = 390 were observed, which correspond to an ∼7- and 8-fold improvement as compared to pristine ZIF-21 (without the addition of EDTA), respectively. Besides the improved and regular ZI-21 morphology, two main factors led to the high observed NH3/N2 and NH3/H2 ideal selectivities of the EDTA-assisted ZIF-21 crystals: ammonia pore size confinement and enhanced polar–polar interactions between NH3 and polar ZIF-21 walls. Extended exposure to ammonia partially degraded the ZIF-21 crystal structure by reaction with the purine linkers.