Molecular rivets keep porous materials from collapsing in humid air

Covalent organic frameworks (COFs) have been hailed as next-generation materials for capturing water from air, powering dehumidifiers, and driving energy-efficient heat pumps. Built from lightweight and organic building blocks, these crystalline and highly porous materials are akin to molecular Lego sets: their geometry and chemistry can be tailored with precision. However, a significant issue remained: in humid air, COFs may collapse.

Besides chemical degradation, which breaks the covalent bonds that hold COFs together, the collapse issue can also be attributed to physical instability. When COFs adsorb and release water vapor, capillary forces exerted within their pores can shift layers, collapse channels, or even transform the frameworks into amorphous polymers. Such changes reduce water uptake and shorten device lifetimes.

In our recent work published in Advanced Functional Materials, we synthesized COFs with varying pore sizes, ranging from 1.4 to 3.3 nanometers, and tested them through repeated water vapor adsorption,desorption cycles. A trade-off was noticed immediately: Smaller pores were more stable but held less water, while larger pores stored more water but tended to collapse under capillary forces.

The breakthrough occurred when we introduced keto-enamine linkages, which create intralayer hydrogen bonds (see fig. 1). These molecular reinforcements acted like rivets, holding the COF layers more rigidly together, which reduces flexibility and prevents interlayer slipping.

Molecular dynamics simulations and density functional theory calculations confirmed the stabilizing role of these hydrogen bonds. Experimentally, the keto-enamine COFs maintained crystallinity and porosity even after 200 water-vapor cycling tests. In contrast, imine-linked COFs degraded quickly under similar conditions.

To showcase practical relevance, we scaled up the synthesis using a microwave-assisted aqueous method, producing grams of COFs within hours. We then coated a fin-tube heat exchanger with the stable one (Tp_OMe-BpyD COF) and demonstrated effective air dehumidification at regeneration temperatures as low as 60°C.

Compared with a benchmark metal,organic framework (aluminum fumarate), the COF-coated device showed nearly doubled the moisture removal capacity (MRC), while operating with low-grade heat that could be sourced from waste or solar energy (see fig. 2).

The study offers new design guidelines for engineering COFs that strike a balance between water capacity and resilience. By reinforcing COFs at the molecular scale, we have made them durable enough for real-world applications from air dehumidification to sustainable water harvesting.

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Dr. Wei Zhao earned his B.S. (2015) from Hunan University and M.S. (2018) from Sichuan University under Prof. Xikui Liu. He completed his Ph.D. at the University of Liverpool in 2022 with Prof. Andrew I. Cooper and is now a research fellow in Prof. Dan Zhao’s group at the National University of Singapore. His work centers on covalent organic frameworks for photocatalysis, batteries, and gas sorption.

Prof. Dan Zhao received his Ph.D. in Inorganic Chemistry from Texas A&M University in 2010 under Prof. Hong-Cai Joe Zhou, followed by postdoctoral research at Argonne National Laboratory. He joined NUS in 2012, where he became Professor in 2025. His research focuses on porous materials and hybrid membranes for clean energy and environmental sustainability.