Hydrogen generation from formic acid decomposition with a ruthenium catalyst promoted by functionalized ionic liquids.

Hydrogen has attracted increasing attention as an important alternative secondary energy resource particularly when combined with fuel-cell technology, which may play a very significant role in power generation in the future. Currently most H2 is produced industrially by methane or alcohol reforming and by the water–gas-shift reaction at high temperatures. However, special purification steps are needed to remove any remaining CO, which severely inhibits the efficiency of the fuel cell. On the other hand, the storage and transfer of hydrogen are also problematic because of its low volumetric energy density. At present most of the hydrogen storage materials, such as metal hydrides, carbon nanotubes, metal–organic frameworks, and ammonia, can only store low amounts; high temperatures are required to release the stored hydrogen; and metal-based compounds have disadvantages in terms of toxicity, price, and safety. Formic acid, which is nontoxic and a liquid at room temperature, with a density of 1.22 gcm , has been widely used as a hydrogen source for transfer hydrogenation. The decomposition of formic acid (HCOOH!CO2+H2), which is the reversible reaction of CO2 hydrogenation (CO2+H2!HCOOH), is considered a promising hydrogen-generation process. Recently, excellent catalytic activities were independently obtained by two groups. Beller and co-workers investigated the catalytic decomposition of formic acid/amine mixtures at 40 8C or room temperature, and excellent catalytic activity was obtained. Although the highest catalytic activity for formic acid decomposition reported to date was obtained, a high ratio of organic amine to formic acid was needed. The volatile amine should be removed before application in fuel cells, so the use of a large amount of volatile organic amine will result in an increased complexity and, accordingly, increase the cost of application. At the same time, Laurenczy and co-workers developed the decomposition of HCOOH/HCOONa (9:1) under aqueous conditions. HCOONa is nonvolatile, but was only effectively when heated to 80 8C. This severely inhibits its usage in ambient conditions. Nevertheless, it should be noted that the only products of formic acid decomposition by both of the methods mentioned above were gaseous H2 and CO2. Fuel-cellgrade hydrogen could be obtained by simple separation. Ionic liquids (ILs), defined as organic salts with melting points below 100 8C, have attracted considerable attention as versatile media and materials because of their peculiar physicochemical properties. Moreover, functionalized ILs, incorporating different functional groups into the alkyl chains of the imidazolium or pyridinium units, have been synthesized and used as solvents and catalysts in chemical processes. For example, H2N-functionalized ILs have been used to absorb CO2, [17]