A team at Delft University of Technology in The Netherlands has developed an efficient heterogeneous catalyst for the production of hydrogen from formic acid (HCOOH). In a paper in the journal ChemSusChem, they reported initial turnover frequencies (TOFs) up to 27 000 h−1 and turnover numbers (TONs) of more than one million in continuous operation. Hydrogen production rates of more than 60 mol L-1 h-1 were obtained at high catalyst loadings of 16 wt % Ir, making it attractive towards process intensification.
Formic acid is a liquid that contains 4.4 wt.% of hydrogen with a volumetric capacity of 53.4 g/l at standard temperature and pressure. The hydrogen in formic acid can be released by decomposing formic acid on a catalytic surface. For practical use, the catalysts need to decompose formic acid via dehydrogenation (HCOOH → H2 + CO2) rather than dehydration (HCOOH → H2O + CO); the latter process produces unwanted CO.
In a 2014 paper exploring selective hydrogen production from formic acid using Pd-Au catalysts, Yu et al. noted:
Recently, considerable advances have been made in the selective dehydrogenation of HCOOH at ambient and near-ambient temperatures using homogeneous catalysts. Nevertheless, the separation issues associated with homogeneous catalysts and requisite use of organic solvents and additives hamper their practical applications. Use of heterogeneous catalysts may circumvent these issues, but improved catalytic performance under ambient conditions is still required.
The Delft heterogeneous molecular catalyst is based on IrIIICp* (Cp*=pentamethylcyclopentadienyl) attached to a covalent triazine framework (CTF).
The CTF support, with a surface area of 1800 m2 g-1, was constructed from an optimal 2:1 ratio of biphenyl and pyridine carbonitrile building blocks. Biphenyl building blocks induce mesoporosity, facilitating the diffusion of reactants and products. Free pyridinic sites activate formic acid towards β-hydride elimination at the metal, rendering unprecedented rates in hydrogen production. The catalyst is air-stable, produces CO-free hydrogen, and is fully recyclable.
Bavykina, A. V., Goesten, M. G., Kapteijn, F., Makkee, M. and Gascon, J. (2015), “Efficient production of hydrogen from formic acid using a Covalent Triazine Framework supported molecular catalyst.” ChemSusChem. doi: 10.1002/cssc.201403173
Wen-Yueh Yu, Gregory M. Mullen, David W. Flaherty, and C. Buddie Mullins (2014) “Selective Hydrogen Production from Formic Acid Decomposition on Pd–Au Bimetallic Surfaces” Journal of the American Chemical Society 136 (31), 11070-11078 doi: 10.1021/ja505192v