MOF-74 (CPO-27): structure, synthesis and CO₂ capture

Most famous MOFs are built from separate clusters, such as the Zn₄O unit of MOF-5. MOF-74 is different: its node is an infinite rod, a chain of metal–oxygen polyhedra that winds as a helix along the c axis of the crystal (Rosi et al. 2005). The chains repeat every 6.9 Å along c (c = 6.935 Å in the Zn₂(dobdc) crystal file).

The linker is dobdc⁴⁻, the fully deprotonated form of 2,5-dihydroxyterephthalic acid. Both its carboxylate and its phenoxide oxygens bind to metals, so each linker ties two neighbouring rods together. Seen down the c axis, the rods sit at the corners of hexagons and the linkers form the walls, giving a honeycomb of one-dimensional hexagonal channels.

In the as-made solid each metal also holds one solvent molecule (water or DMF) pointing into the channel. Heating under vacuum removes it, and the metal is left with an empty coordination site, an open metal site. Every channel is lined with these bare cations along its whole length (Britt et al. 2009; Bloch et al. 2011), which is what makes MOF-74 unusual.

In the Zn₂(dobdc) crystal file (trigonal, R-3, a = 26.19 Å) the channel centres are 15.1 Å apart, and the channel is about 11–12 Å across once the size of the atoms lining it is taken into account (both calculated from the file).

1. Solvothermal synthesis. For Mg-MOF-74, magnesium nitrate hexahydrate and 2,5-dihydroxyterephthalic acid are dissolved in DMF/ethanol/water (15:1:1) and heated at 125 °C (Caskey et al. 2008). The Co, Ni and Zn members are made in a similar way from their metal salts and the same linker (Caskey et al. 2008).
2. Solvent exchange. The crystals are washed and the high-boiling DMF in the channels is exchanged for a more volatile solvent.
3. Activation. Heating under vacuum removes the solvent bound to the metals and opens the metal sites. The activated material must then be kept dry, because water binds strongly to the open sites and competes with CO₂.

MOF-74 made open metal sites a design tool. Its channels hold one exposed metal cation per metal atom, packed densely along every wall, and guests such as CO₂, O₂ and olefins bind directly to them (Dietzel et al. 2008; Bloch et al. 2011, 2012).

It is also the clearest example of tuning a framework by changing the metal alone. Caskey, Wong-Foy and Matzger made four isostructural versions (Mg, Co, Ni and Zn) and found that the CO₂ uptake at low pressure changed dramatically with the metal, with the magnesium version on top (Caskey et al. 2008). Britt et al. then showed in a packed bed that Mg-MOF-74 separates CO₂ from methane with 8.9 wt% dynamic capacity and gives the CO₂ back at only 80 °C (Britt et al. 2009). Rosi et al. used MOF-74 and its relatives to show that MOFs built from rod-shaped units can also be permanently porous (Rosi et al. 2005).

• CO₂ capture. Mg-MOF-74 is a benchmark adsorbent for CO₂ from natural gas and flue gas (Caskey et al. 2008; Britt et al. 2009). Infrared and diffraction studies of the nickel version show CO₂ binding end-on to the open Ni sites (Dietzel et al. 2008).
• Hydrocarbon separations. Fe₂(dobdc) separates ethylene from ethane and propylene from propane at 318 K; neutron diffraction shows the olefins bound side-on at the open Fe(II) sites (Bloch et al. 2012).
• Oxygen from air. Fe₂(dobdc) binds O₂ in preference to N₂: at 211 K reversibly, one O₂ per iron (Bloch et al. 2011).
• Toxic gas removal. MOF-74 was one of six MOFs benchmarked against eight harmful gases, including ammonia and sulfur dioxide, by breakthrough measurements (Britt et al. 2008).
• Limitation. Water competes for the open metal sites, so humid gas streams lower the CO₂ capacity, and powders have to be shaped into pellets for use in beds.

Draw MOF-74 looking straight down its channels: the honeycomb view shows the metal rods at the corners of the hexagons, the dobdc⁴⁻ linkers as the walls and the open metal sites pointing into each channel. Add a close-up of one open metal site with a bound CO₂ molecule to explain capture. The Mg-MOF-74 figure above has both, plus the synthesis and a breakthrough experiment, and opens as an editable LabCanvas figure. Below it, the crystal structure of Zn₂(dobdc) is drawn as a high-resolution SVG from its crystal file, and the same file opens as a live 3D model you can turn to look along the channels and export as an image.