Researchers Stabilize the Collapsing Metal-organic Frameworks by Adding a Polymer

High internal surface area is a highly sought after asset in material design, bringing metal-organic frameworks (MOFs) at the forefront of materials research. In fact, the main focus in this field is to create innovative methods to maximize the surface area of the MOF. Nevertheless, macroporous MOFs, especially those with mesopores, still face the problem of pore collapse during activation. In a study published in the Journal of the American Chemical Society, the researchers have solved this problem by adding a small amount of polymer to the MOF pores, which prevents the pores from collapsing.

MOFs are a special kind of sponge-like materials with nano-scale pores which have many applications, such as carbon capture and water purification. Nanopores result in internal surface areas, up to 7,800 square meters per gram, making MOFs extremely versatile materials for a variety of purposes, such as separating petrochemical products and gases, simulating DNA, generating hydrogen and removing heavy metals, fluoride ions and even gold from water.

One of the key features of MOFs is the aperture. MOFs and other porous materials are classified according to the diameter of their pores: MOFs with a pore size of up to 2 nm are called “micropore”, and any pores higher than that are called “mesopores.” Most MOFs today are microporous, so they are not useful in applications that require them to trap large molecules or catalyze reactions between them-basically, these molecules are not suitable.

Therefore, mesoporous MOFs have recently played a role because they have shown broad prospects in macromolecular applications. However, they also have problems: when the pore size enters the mesoporous state, they tend to collapse. Understandably, this reduces the internal surface area of mesoporous MOFs and therefore reduces their overall use. Since the main focus in this field is to find innovative ways to maximize the surface area and pore size of the MOF, it is imperative to solve the collapse problem.

In this study, the researchers demonstrated an easy method to inhibit this problem via the introduction of small quantities of polymer. For several mesoporous, isostructural MOFs, known as M2(NDISA) (where M = Ni²⁺, Co²⁺, Mg²⁺, or Zn²⁺), the accessible surface areas are increased dramatically, from 5 to 50 times, as the polymer effectively pins the MOFs open. Postpolymerization, the high surface areas and crystallinity are now readily maintained after heating the materials to 150 °C under vacuum. These activation conditions, which could not previously be attained due to pore collapse, also provide accessibility to high densities of open metal coordination sites. Molecular simulations are used to provide insight into the origin of instability of the M2(NDISA) series and to propose a potential mechanism for how the polymers immobilize the linkers, improving framework stability. Last, the researchers demonstrated that the resulting MOF-polymer composites, referred to as M2(NDISA)-PDA, offer a perfect platform for the appendage/immobilization of small nanocrystals inside rendering high-performance catalysts. After decorating one of the composites with Pd (average size: 2 nm) nanocrystals, the material shows outstanding catalytic activity for Suzuki-Miyaura cross-coupling reactions.

Wendy Lee Queen, the study leader of the research groups, believed that this method for polymer-induced stabilization will allow us to prepare many new mesoporous MOFs that cannot be obtained due to collapse. Therefore, this work can open up new and exciting applications involving the separation, conversion, or delivery of large molecules.

Reference

1. Peng, Li, et al. Preserving Porosity of Mesoporous Metal–Organic Frameworks through the Introduction of Polymer Guests.Journal of the American Chemical Society. 2019, 31: 12397-12405.