Catalytic Disproportionation of Formic Acid to Methanol by an Iridium Complex Immobilized on Bipyridine‐Periodic Mesoporous Organosilica

Abstract

We report the first example of using heterogeneous Iridium (Ir) catalysts under atmospheric pressure conditions to achieve the catalytic disproportionation of formic acid to methanol. IrCp* complex (Cp* = pentamethylcyclopentadienyl (� 5 -C 5 Me 5 )) was immobilized on the pore surface of the periodic mesoporous organosilica synthesized from a precursor mixture of ethane (Et) and 2,2'-bipyridine (BPy)-bridged organosilanes (Ir-Et-BPy-PMO). Ir-Et-BPy-PMO showed unique catalysis with higher selectivity of methanol compared to homogeneous Ir-BPy complexes.Furthermore, Ir-Et-BPy-PMO exhibited high reusability without any noticeable decrease in activity for at least five times with no leaching of Ir species during the reaction. The increase in methanol selectivity of Ir-Et-BPy-PMO would be attributed to the unique confinement effect in the mesochannels with their high aspect ratio for the retention of generated H 2 /CO 2 , which suppresses the competing formic acid dehydrogenation and promotes the formic acid hydrogenation.[a] Dr. Scheme 1. Disproportionation of formic acid. Scheme 2. Dehydrogenation of formic acid.Scheme 3. Hydrogenation of formic acid.

Mechanism of Formic Acid Disproportionation Catalyzed by an Iridium Complex Immobilized on Bipyridine‐Periodic Mesoporous Organosilica: A Case Study Based on Kinetics Analysis

“…When the PMO‐based material 1 and the homogeneous catalyst 3 , having an electron withdrawing CF 3 group, were applied, the amount of FA slowly decreased over time while the yield of MeOH increased. The use of 1 generated 87 μmol MeOH after 96 h. As noted in a previous report, the increased MeOH yield obtained with 1 cannot be explained solely by the electronic effect of substituents on the BPy ligands. Thus, a kinetics study was performed based on the above time course measurements.…”

“…Here, A is the cross sectional area of the PMO pores [m 2 ], S BET is the specific surface area with a value of 802 m 2 g −1 , w is the mass of catalyst used with a value of 0.286 g, S sa is the cross‐sectional pore area calculated as π( d DFT /2)×2 [m 2 ] and S area is the surface area per pore, calculated as 2π( d DFT /2)×2 dx [m 2 ]. A standard pore length, L , of 3.2 μm was used on the basis of scanning electron microscopy (SEM) observations, and so the average diffusion distance of H 2 was found to be 1.6 μm. The experimental results were substituted into Eq.…”

“…We recently reported FA disproportionation using an Ir complex ([IrCp*(BPy)(H 2 O)](SO 4 )) immobilized on a periodic mesoporous organosilica (PMO) obtained from a precursor mixture composed of ethane (Et) and BPy‐bridged organosilanes (Ir−Et−BPy‐PMO ( 1 )) . This complex had a high stability and recyclability under the disproportionation of FA and showed unique catalytic properties with higher selectivity for MeOH compared to a series of homogeneous Ir‐BPy complexes having 5,5′‐substituents on the BPy ring.…”

Homogeneous Catalysis for Sustainable Energy: Hydrogen and Methanol Economies, Fuels from Biomass, and Related Topics

“…The analogous heterogeneous catalyst Ir17a was found to be more selective (up to 8.3%) for methanol production in comparison to the homogeneous Ir-bipyridine catalysts (1.4–4.3%). 401 Neary and Parkin reported that the molybdenum complex Cp R Mo(PMe 3 ) 3– x (CO) x H afforded 21% selectivity of MeOH and methyl formate formation in benzene at 100 °C. 402 In 2016, the Cantat group also reported a metal-free system for the formic acid disproportionation reaction using stoichiometric quantities of dialkylborane reagents ( B01 ), where boroformate and borohydride intermediates were formed via the decarboxylation of formate and followed by undergoing disproportionation of formates to formaldehyde and methanol.…”

Immobilization of an Iridium(I)-NHC-Phosphine Catalyst for Hydrogenation Reactions under Batch and Flow Conditions

“…In addition, chemical hydrogen storage is also an environmentally important research topic. Several types of iridium-catalyzed hydrogen storage reactions are known in the literature [28][29][30][31][32][33][34][35][36][37][38][39][40], however, only a few heterogeneously catalyzed, iridium-based hydrogen storage systems have been described recently [41][42][43].…”