Metal–organic frameworks (MOFs) are crystalline, porous materials composed of metal centers and organic linkers. They have applications, e.g., in catalysis or gas separation. However, the limited chemical, thermal, and mechanical stability of MOFs, as well as their poor electrical conductivity, can hamper some applications. Composite materials that combine MOFs with other materials can help to alleviate these problems.

Carbon nanotubes (CNTs), for example, can be useful in this context, and composites with an irregular distribution of MOF particles on a CNT network have been synthesized. However, a more regularly structured composite could be useful in retaining the desired properties of both components.

Carlos Martí-Gastaldo, Universitat de València, Spain, and colleagues have prepared MOF crystals with a carbon nanotube coating. The team’s synthetic strategy is based on the presence of tetrazine units in the MOF, which control the formation of an ultrathin single-wall nanotube coating on the surface of the MOF crystal. They prepared the MOF UiO-68-TZDC, which is composed of 4,4′-(1,2,4,5-tetrazine3,6-diyl)dibenzoic acid (H₂tzdc) linkers and Zr₆-based metal-oxo clusters. Then the team used the reactive tetrazine groups to graft CNTs onto the surface. To obtain the desired composite, the team combined a dispersion of single-wall carbon nanotubes (SWCNTs) in dioxane with freshly prepared crystals of UiO68-TZDC, followed by incubation at 65 ºC for 48 h.

Scanning electron microscopy (SEM) analysis of the surface of the crystals confirmed the formation of a clean, chainmail-like coating layer. According to the researchers, the functionalization is facilitated by an inverse-demand [4+2] Diels–Alder reaction between the nanotube sidewall and the tetrazine groups. The resulting composites combine desirable porosity, conductivity, and stability at low carbon weight contents. The work could, thus, offer a new approach to the design of MOF composites.

• Wrapping up Metal-Organic Framework Crystals with Carbon Nanotubes,
  Belén Lerma-Berlanga, Natalia M. Padial, Marta Galbiati, Isaac Brotons-Alcazar, Josep Albero, Hermenegildo García, Alicia Forment-Aliaga, Carolina R. Ganivet, Carlos Martí-Gastaldo,
  ChemRxiv 2023.
  https://doi.org/10.26434/chemrxiv-2023-fl2wp
  The research has been published as a preprint and has not yet been peer-reviewed.