Reduction of CO₂ to Methanol Catalyzed by a Biomimetic Organo-Hydride Produced from Pyridine

Abstract

ABSTRACT:We use quantum chemical calculations to elucidate a viable homogeneous mechanism for pyridine-catalyzed reduction of CO2 to methanol. In the first component of the catalytic cycle, pyridine (Py) undergoes a H + transfer (PT) to form pyridinium (PyH + ) followed by an e -transfer (ET) to produce pyridinium radical (PyH 0 ). Examples of systems to effect this ET to populate PyH + 's LUMO (E 0 calc ~ -1.3V vs. SCE) to form the solution phase PyH 0 via highly reducing electrons include the photo-electrochemical p-GaP system (ECBM ~ -1.5V vs. SCE at pH= 5) and the photochemical [Ru(phen)3] 2+ /ascorbate system. We predict that PyH 0 undergoes further PT-ET steps to form the key closed-shell, dearomatized 1,2-dihydropyridine (PyH2) species. Our proposed sequential PT-ET-PT-ET mechanism transforming Py into PyH2 is consistent with the mechanism described in the formation of related dihydropyridines. Because it is driven by its proclivity to regain aromaticity, PyH2 is a potent recyclable organo-hydride donor that mimics the role of NADPH in the formation of C-H bonds in the photosynthetic CO2 reduction process. In particular, in the second component of the catalytic cycle, we predict that the PyH2/Py redox couple is kinetically and thermodynamically competent in catalytically effecting hydride and proton transfers (the latter often mediated by a proton relay chain) to CO2 and its two succeeding intermediates, namely formic acid and formaldehyde, to ultimately form CH3OH. The hydride and proton transfers for the first reduction step, i.e. reduction of CO2, are sequential in nature; by contrast, they are coupled in each of the two subsequent hydride and proton transfers to reduce formic acid and formaldehyde.

Aluminum Nitride Hydrolysis Enabled by Hydroxyl-Mediated Surface Proton Hopping

“…[45][46][47][48][49][50][51][52] While surface Grotthuss-like mechanisms where OH * surface species mediate the PT and H2O * transiently forms are less common, proton conduction mechanisms mediated by adsorbed hydroxyl groups have also been previously reported. 43,[53][54] However, to the best of our knowledge, this is the first reported ab initio description of a Grotthuss-like proton transport mechanism across a solid surface mediated by immobile surface hydroxyl groups in the absence of solvation.…”

What Is the Role of Pyridinium in Pyridine-Catalyzed CO2Reduction on p-GaP Photocathodes?

“…On the basis of this experimental evidence, PyH + reduction to the pyridinyl radical (PyH•) has been proposed as a crucial mechanistic step in two studies: Bocarsly and coworkers conjectured that PyH• itself is the active catalyst for CO2 reduction, 4 while Musgrave and coworkers recently proposed that PyH• will further react to form dihydropyridine (DHP) as the active (homogeneous) catalyst. 5 However, computational studies [6][7][8][9] showed that PyH + reduction to PyH• is energetically unfeasible in solution when using metal electrodes and under the experimentally applied potential (~-0.6 V vs SCE 4 ). In contrast, Musgrave and coworkers argued that PyH + may be reduced to PyH• when using p-GaP photoelectrodes because the photoexcited electrons might have enough energy.…”

Recent progress on advanced design for photoelectrochemical reduction of CO2 to fuels

“…13b. For the moment, the formation of dihydropyridine intermediate seems more convictive than others based on the investigation evidences, which was kinetically and thermodynamically feasible in electron and proton transfers to CO 2 [150,153]. In this pathway, both HCOOH and HCHO could act as intermediate products and be further reduced to ultimately form CH 3 OH (Fig.…”

“…. dynamically unfavorable process with a high overpotential; 2) the pK a of the pyridinyl radical was calculated to be as high as 27, suggesting that the deprotonations can hardly take place so that the formation of pyridinyl radical-CO 2 complex was disfavored. On this account, some other hypotheses were put forward to unravel the pathway of pyridine-catalyzed CO 2 reduction, including the surface adsorption of pyridinyl radical [149], the generation of surface hydride [151], the formation of carbamate radical by utilizing the bridged water molecules [152], the formation of 4,4'-bipyridine by coupling two pyridinyl radicals [147], and the multielectron reduction to dihydropyridine intermediate [153] as depicted in Fig. 13b.…”