Second Sphere Ligand Promoted Organoiridium Catalysts for Methanol Dehydrogenation under Mild Conditions

Abstract

Hydrogen produced from renewable resources is a potential source of clean energy. And Methanol is a liquid at room temperature and has a hydrogen content of up to 12.6 wt%, makes it a promising hydrogen source and can provide great benefits for sustainable hydrogen production. In this work, the modification of the picolinic acid ligand along with changes in the type and substituent position of the groups enhances the activity in iridium‐catalyzed methanol dehydrogenation catalysis. These second sphere ligand modifications are found to be effective for achieving highly active organoiridium catalysts for methanol dehydrogenation reaction under mild conditions, i. e. 1 atm, 78 °C, with a weak base and without using an organic solvent, exhibiting a TOF of 377 h−1. The electronic population analysis of each ligand is theoretically calculated, which shows that catalytic activity is dramatically affected by modifications to the metal's secondary coordination sphere environment, and the high activity originates from appropriate coordination by the picolinic acid ligand and carboxylic acid moiety in addition to the hydroxyl functionality. Moreover, kinetic experiments suggest that the C−H bond of methanol is linked to the iridium core and broken in the rate‐determining step of the reaction. This study will be valuable for catalyst development in the field of methanol dehydrogenation.

Recent progress in molecular transition metal catalysts for hydrogen production from methanol and formaldehyde

“…Furthermore, kinetic experiments confirmed the involvement of a hydrogen atom in methanol, with the cleavage of the C-H bond as the rate-determining step. 38 Milstein et al reported the base-free aqueous phase dehydrogenation of methanol (MeOH : H 2 O 4 : 1 v/v) using an acridine-based Ru-PNP catalyst (C-12), where the catalytic activity of C-12 could be enhanced by incorporating one equivalent of a thiol additive. The catalyst C-12 achieved an impressive TOF of 643 h −1 for hydrogen production.…”

“…Furthermore, kinetic experiments confirmed the involvement of a hydrogen atom in methanol, with the cleavage of the C–H bond as the rate-determining step. 38…”

“…Subsequently, the formic acid dehydrogenates to produce a third molecule of H 2 along with CO 2 . 11 Various homogeneous 11,21–53 and heterogeneous 12–20 catalysts have been explored for hydrogen production from methanol and formaldehyde, where most of the heterogeneous catalytic systems work well at harsh reaction temperatures, while homogeneous molecular catalysts work well at lower operational temperatures. 21 Moreover, molecular catalysts also aid in a better understanding of the reaction mechanism, which further helps in designing efficient catalysts.…”

CO-Free H2 Production by Aqueous-Phase Reforming of Methanol Utilizing Heterogenized Ru-MACHO at Low Temperature

“…In 2013, Nielsen et al published an aqueous-phase reforming of methanol (APRM) method that employed homogeneous Ru-MACHO under 100 °C, resulting in a maximum TOF of 2670 and 350,000 TON for 600 h, without generating CO . Since the publication, there has been investigation into APRM using a homogeneous catalyst. − ,− However, the homogeneous nature of the Ru-MACHO catalyst poses difficulties in separation, resulting in significant challenges for its industrial application. ,− Due to the advantage of a heterogeneous catalyst in separation, studies have been conducted using various transition metals in APRM with heterogeneous catalysts, but most of them require high temperatures (>200 °C) yet. ,− Therefore, demonstrating the activity of heterogeneous catalysts under mild conditions (<100 °C) like Ru-MACHO remains a significant challenge in the effective application of fuel cell systems.…”

Enabling alcohol as a hydrogen carrier using metal–organic framework-stabilized Ir–Sc bifunctional catalytic sites

“…16 Recently, Zhou group also demonstrated that the hydroxyl group next to the pyridyl N in pyridine-carboxylate-based iridium catalysts could promote its catalytic activity in methanol dehydrogenation. 17 However, the alkaline reagents such as NaOH or NaOMe used in these catalytic systems prevent acetal formation reaction and further dehydrogenation to form esters. To the contrary, these reactions between alcohol and aldehyde can be catalyzed by Lewis acid that is incompatible with the alkaline condition.…”