15 July 2005: Explosive acetylene leads to blocked pores

Japanese chemists and physicists have discovered that a novel type of metal-organic material is selectively able to adsorb and store acetylene much more effectively than any other known material. The finding could lead to the development of similar materials able to absorb and store other industrially-important molecules.

Acetylene is the starting point in the manufacture of a wide range of chemical products, including acrylonitrile, vinyl chloride and polyvinyl alcohol. There are a number of methods for generating acetylene, including the partial oxidation of natural gas and the thermal cracking of oil, but they tend to produce a mixture of acetylene and other gases, such as carbon dioxide, which need to be removed. In addition, acetylene is highly explosive, especially at slightly elevated pressures.

The researchers from universities across Japan investigated whether metal-organic microporous materials (MOMs), which comprise a porous skeleton of organic and inorganic molecules that form large numbers of small pores, might prove adept at selectively storing acetylene. To do this, they tested how well a MOM with the chemical formula Cu₂(pyrazine-2,3-dicarboxylate)₂(pyrazine) adsorbed acetylene compared with how it adsorbed carbon dioxide. 

At room temperature and low pressures, the researchers discovered that their MOM adsorbed 26 times as much acetylene as carbon dioxide. Furthermore, the MOM stored acetylene at a density that was 200 times larger than the normal safe compression limit for the gas.

The researchers then investigated the adsorption process in more detail by analysing the overall crystal structure and electron density of the MOM and the adsorbed acetylene molecules. In most microporous materials, gases tend to be adsorbed into the pores through van der Waals-type forces, which are quite weak. In the MOM, however, each pore contains two oxygen atoms, which strongly bind with the two hydrogen atoms in an acetylene molecule. This leads to each pore preferentially adsorbing and holding on to one acetylene molecule.

The researchers are now developing different MOMs, with pores that have different sizes and interaction sites, to investigate their adsorption properties. ’We are going to design and synthesise various MOMs targeting other molecules such as the environmental pollutants NOx and SOx, and the energetically important gas molecules CH₄ and H₂,’ team leader Susumu Kitagawa of Kyoto University told Chemistry World. Jon Evans