Porous Materials Stores Acetylene

MATERIALS SCIENCE

A hybrid copper-organic microporous material, designed and synthesized by scientists in Japan, stores acetylene in preference to its close molecular cousin carbon dioxide at room temperature and pressure (Nature 2005, 436, 238).

Acetylene, a key starting material for the synthesis of many chemical products and electric materials in the petrochemical and electronics industries, is highly reactive and explodes even in the absence of oxygen when compressed at more than 2 atm at room temperature, note Kyoto University professor of inorganic chemistry Susumu Kitagawa, postdoc Ryotaro Matsuda, and coworkers.

"Our material permits acetylene to be stored safely at a density 200 times the safe compression limit of free acetylene at room temperature," Kitagawa says.

The material, Cu₂(pzdc)₂ (pyz)--where pzdc is pyrazine-2,3-dicarboxylate and pyz is pyrazine--has permanent one-dimensional channels with nanoscale cross sections. It exhibits high levels of C₂H₂ adsorption at room temperature and pressure and adsorbs CO₂ only at high pressures and low temperatures, the researchers find.

"The selective adsorption of C₂H₂ compared with CO₂ has never been achieved with conventional adsorbents, such as zeolites and activated carbons," Kitagawa says. "The precise control of the functionalization is one of the important advantages of metal-organic microporous materials. In addition, pores with cross sections of 1 nm or less allow highly efficient sorption and storage with minimum wasted space."

The Japanese scientists used a technique known as maximum entropy method, based on synchrotron X-ray powder diffraction data, to determine the mechanism of C₂H₂ adsorption in the hybrid material and to elucidate the overall crystal structure and electron density of the C₂H₂-containing complex.

Hydrogen bonding between the guest molecules and the microporous host facilitates selective adsorption, they showed. "The acetylene molecules are held at a periodic distance from one another by hydrogen bonding between two basic noncoordinated oxygen atoms in the nanoscale pore wall of the microporous material and the acidic hydrogen atoms of the acetylene molecule," Kitagawa explains.

The CO₂ molecule, on the other hand, although it is similar in structure and dimensions to the C₂H₂ molecule, has no acidic protons.

In a Nature commentary, chemistry professor Grard Frey of Lavoisier Institute at the University of Versailles Saint Quentin, in France, observes that one of the striking features of the work is its quality. He writes that the work combines creativity, accurate characterization, and energy calculations to demonstrate and explain the incarceration behavior of acetylene.

One other striking feature is the stoichiometric 1:1 ratio between the number of adsorbed C₂H₂ molecules and the number of available pores, according to Frey. The ratio is reached quickly at the lowest pressures.

"The possible extension of their methods to other strategically important gases makes their paper an outstanding contribution to this field of research," Frey remarks.

The Japanese group is now planning to design and synthesize metal-organic microporous materials that target other molecules. These include nitrogen oxide (NO_x) and sulfur oxide (SO_x) gas molecules, which pollute the environment, and the energetically important gas molecules methane and hydrogen.