(77e) Additive-Induced Polymorphism in a Zn-Based Metal Organic Framework

While polymorphism has been long known in many crystalline systems, its presence in Metal Organic Frameworks (MOFs) has only recently been discovered. In the past decade, polymorphism has been induced in several MOF families via a multitude of control mechanisms. Pressure and temperature studies completed on ZIF-4 have elucidated several polymorphs, one of which exhibited resistance to both high temperatures and pressures. Alternative synthesis solvents have also been found to induce polymorphism in the UHM family, where the use of either DMA or a DMF/water/dioxane mixture produced polymorphic forms. In many cases, polymorphism has also been induced via modulating species, which were used to synthesize the first known polymorph of UiO-66 with enhanced porosity. These control mechanisms and others have produced MOF polymorphs with more desirable properties than their previously known counterparts.

In this work, we explore the polymorphic control of ZIF-8 and ZIF-L based on metal/linker ratio and examine the morphological characteristics of this transition. We show that a narrow window of MOF metal:linker ratio exists where the formation of ZIF-8 and ZIF-L can be tightly controlled. We show through the use of dilute additives during ZIF synthesis alters the phase tipping point between ZIF-8 and ZIF-L, indicating additives can be used as handles for polymorphic control. Finally, we propose a mechanism inspired by previously postulated templating effects and supported by DFT calculations to explain the observed polymorphic control.

This work suggests that modulators and additives may provide a new path of polymorphic control in MOFs and elaborates on the previously hypothesized templating effects of modulating species.