Pincer–Ruthenium-Catalyzed Reforming of Methanol─Selective High-Yield Production of Formic Acid and Hydrogen

Abstract

A series of NNN pincer−ruthenium complexes of the type ( R2 NNN)RuCl 2 (CH 3 CN) based on bis(imino)pyridine ligands were synthesized and characterized. These pincer ruthenium acetonitrile complexes, along with their phosphine and carbonyl counterparts, were tested for the reforming of methanol in water in the presence of a base. The catalyst ( Cy2 NNN)-RuCl 2 (PPh 3 ) was found to be the most efficient in comparison to other considered catalysts. Among the bases screened, KO t Bu (1.5 equiv with respect to water) was found to give the best results at temperatures as low as 100 °C. Under these conditions, while ( Cy2 NNN)RuCl 2 (PPh 3 ) (0.2 mol %) in a mixture of methanol and water in a 2:1 ratio gave a yield of up to 81% each of hydrogen and formic acid (FA) at 100% selectivity, the corresponding reaction with 2 mol % ( Cy2 NNN)RuCl 2 (PPh 3 ) gave up to 90% of hydrogen and 73% of FA at 80% selectivity. On the other hand, the ( Cy2 NNN)RuCl 2 (PPh 3 ) (0.8 mol %) catalyzed reforming of a 3:1 methanol/water mixture gave good yields (84%) of hydrogen with 81% FA at 95% selectivity. The yield of hydrogen was cross-verified by using it to reduce unsaturated compounds and determining the corresponding yield of the reduced product, which was found to be consistent. Isotope-labeling studies suggest the involvement of C−H activation as a part of the catalytic cycle and not as a part of the rate-determining step (RDS) with an average secondary KIE of 1.96. The reaction was observed to have a first-order dependence of rate on the concentration of both ( Cy2 NNN)RuCl 2 (PPh 3 ) and methanol. DFT studies are in agreement with this, and the σ-bond metathesis leading to the elimination of the first molecule of hydrogen is computed to be the RDS either for the cycle leading to FA and 2 moles of hydrogen or for the cycle that results in carbon dioxide and 3 moles of hydrogen. The Ru−H species ( Cy2 NNN)RuCl(H) plays a decisive role in the unprecedented selectivity toward FA. In its choice to undergo a σ-bond metathesis either with the O−H of methanol (that completes the FA cycle) or with the O−H of FA that leads to carbon dioxide, it chooses the former as it is kinetically more favored by 4.58 kcal/mol. The current catalytic system comprising of NNN pincer-ruthenium phosphines based on bis(imino)pyridine ligands that gives high yields of H 2 and FA at unprecedented selectivity at low operating temperature offers immense promise in the transformation of methanol to clean-burning hydrogen and high-value FA.

Reforming of ethanol to hydrogen and acetic acid catalyzed by pincer-ruthenium complexes

“…2 a , c Despite these advancements in hydrogen production, state-of-art hydrogen technologies are hampered by critical issues related to their low volumetric energy density, 5 safe handling, 6 and the need for expensive cryogenic 7 and high-pressure compression cylinders 6 a that pose serious limitations to the transportation and storage of H 2 . 6 b ,8 While hydrogen adsorption based technologies have failed to deliver the goods, 9 hydrogen storage in the chemical bonds of tiny organic molecules such as methanol, 10 ethanol 10 a – d and formic acid 10 c – e ,11 among many others fondly tagged as liquid organic hydrogen carriers (LOHCs) and their subsequent dehydrogenation to generate on-demand H 2 has been widely-accepted as an attractive strategy. 10 a ,12…”

Panoramic Mechanistic Insights into Hydrogen Production via Aqueous-Phase Reforming of Methanol Catalyzed by Ruthenium Complexes of Bis-N-Heterocyclic Carbene Pincer Ligands

“…To this end, efforts have been devoted to H 2 production by catalytically dehydrogenating certain liquid organic hydrogen carrier (LOHC) molecules that are otherwise perfectly stable under ambient conditions. − Being inexpensive and readily available, methanol is one of the most attractive LOHCs; three molecules of H 2 can be generated by aqueous-phase reforming of methanol (APRM, eq ). ,,− normalC normalH 3 O H + H 2 normalO → 3 H 2 + normalC normalO 2 …”

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.…”