HKUST-1 (Cu-BTC): structure, synthesis and properties

HKUST-1 is built from two parts. The node is a copper paddlewheel: two Cu²⁺ ions 2.63 Å apart, held together by four carboxylate groups that bridge them like the blades of a paddlewheel, Cu₂(CO₂)₄. Each copper also carries one water molecule, pointing out along the Cu···Cu axis (Chui et al. 1999).

The linker is trimesate (BTC³⁻), a benzene ring with three carboxylates at 120°. Each linker joins three paddlewheels, and each paddlewheel takes four linkers. Together they make the tbo net (Kong et al. 2013), a cubic framework of cages (Fm-3m, a ≈ 26.3 Å).

The result is a three-dimensional system of channels about 1 nm across, with about 40% of the solid accessible to guests (Chui et al. 1999). The dehydrated framework has three kinds of pore, about 12.6, 10.6 and 5.0 Å across (Wong-Ng et al. 2015).

1. Solvothermal synthesis. Copper(II) nitrate trihydrate and trimesic acid (H₃BTC) in water/ethanol are heated in a sealed, Teflon-lined vessel at 180 °C for 12 h. Turquoise octahedral crystals of [Cu₃(BTC)₂(H₂O)₃]ₙ form in 80% yield (Chui et al. 1999).
2. Washing. The crystals are separated and washed to remove unreacted reagents.
3. Activation. Heating under vacuum removes the water from the copper ions and empties the pores; the crystals turn darker and the framework survives (Prestipino et al. 2006). Water binds to these copper sites in the as-made crystals, so activated samples are kept dry when the open sites are needed.
4. Other routes. HKUST-1 has also been made by sonochemistry, where the sonication conditions set the particle size (Armstrong et al. 2017), by continuous extrusion with little solvent (Crawford et al. 2015), and as thin films by seeded growth (Brower et al. 2017).

When it was reported in 1999, HKUST-1 offered something zeolites could not: a highly porous, thermally stable crystal whose channel linings could be changed chemically. The water molecules on the copper ions can be replaced by pyridines, functionalising the pores without destroying the framework (Chui et al. 1999).

It became the classic example of a MOF with open metal sites. Once the water is removed, each Cu²⁺ is left with a free coordination site facing the pore. Small molecules bind there: carbonyl and dihydrogen adducts of the copper ions were observed by infrared spectroscopy at low temperature (Prestipino et al. 2006). The copper paddlewheel it is built on is one of the most widely used building units in MOF chemistry.

• Gas adsorption. Its open copper sites bind small molecules such as CO and H₂ (Prestipino et al. 2006), which makes HKUST-1 a reference material in gas storage and separation studies.
• Thin films and electronics. HKUST-1 grows as thin films (Brower et al. 2017); filling the pores of such films with TCNQ makes them electrically conducting, with a conductivity that can be tuned (Talin et al. 2014).
• Scale-up. It was among the MOFs made continuously by extrusion at large scale with little or no solvent (Crawford et al. 2015).
• Limitation. Water binds to the copper sites, so the activated material has to be handled dry when the open sites are needed.

For a paper, slide or poster, show the paddlewheel (two copper atoms and four carboxylates, with or without the axial water), one trimesate linker, and the cubic framework of cages. The figure above has all three, plus the synthesis and the open copper sites, and opens as an editable LabCanvas figure. The crystal structure SVG is drawn from the HKUST-1 crystal file by the LabCanvas figure editor, where you can switch between atoms and polyhedra and change the number of cells; the 3D model on this page opens in the LabCanvas 3D Studio, where you can turn it and export a high-resolution image.