Conversion of Formic Acid into Methanol Using a Bipyridine-Functionalized Molecular Heterogeneous Catalyst

Abstract

Although the conversion of carbon dioxide (and its derivatives) into methanol has attracted remarkable attention in the last two decades, performing this process over a heterogeneous catalyst under mild conditions is still a challenging task. We report bipyridine-functionalized iridium-based heterogeneous catalysts for the hydrogenation of formic acid to produce methanol at low temperature. The solid catalysts were obtained by postsynthetic metalation of bipyridine-functionalized organosilica nanotubes with a [Cp*Ir(H2O)3]SO4 (Cp* = η5-pentamethylcyclopentadienyl) complex. Detailed studies including N2 physisorption, TEM, XPS, and 13C CP MAS NMR confirmed the stable structures of nanotube supports and the molecular nature of the active species. The catalysts showed competitive methanol selectivities compared to their homogeneous counterpart under similar reaction conditions. Addition of strong acids (such as triflic acid) showed improved methanol selectivity, whereas the presence of free bipyridine groups was found to promote the dehydrogenation of formic acid, resulting in low methanol selectivity. The catalyst showed excellent reusability over four consecutive cycles without any significant loss in activity and maintained its heterogeneous nature in extremely high acidic environment.