Catalytic Reduction of CO₂ by Renewable Organohydrides

Abstract

Dihydropyridines are renewable organohydride reducing agents for the catalytic reduction of CO2 to MeOH. Here we discuss various aspects of this important reduction. A centerpiece, which illustrates various general principles, is our theoretical catalytic mechanism for CO2 reduction by successive hydride transfers (HTs) and proton transfers (PTs) from the dihydropyridine PyH2 obtained by 1H(+)/1e(-)/1H(+)/1e(-) reductions of pyridine. The Py/PyH2 redox couple is analogous to NADP(+)/NADPH in that both are driven to effect HTs by rearomatization. High-energy radical intermediates and their associated high barriers/overpotentials are avoided because HT involves a 2e(-) reduction. A HT-PT sequence dictates that the reduced intermediates be protonated prior to further reduction for ultimate MeOH formation; these protonations are aided by biased cathodes that significantly lower the local pH. In contrast, cathodes that efficiently reduce H(+) such as Pt and Pd produce H2 and create a high interfacial pH, both obstructing dihydropyridine production and formate protonation and thus ultimately CO2 reduction by HTPTs. The role of water molecule proton relays is discussed. Finally, we suggest future CO2 reduction strategies by organic (photo)catalysts.

Recent Trends, Benchmarking, and Challenges of Electrochemical Reduction of CO2 by Molecular Catalysts

“…[121,125] In addition, Saveant and his co-workers also summarized the "methodological misadventures" in the catalytic mechanisms established for protonated pyridine. [127] In a later 2016, Yang et al achieved the similar FE using a Cu-Pd cage for PYD to produce alcohols. This is, not a topic of discussion in this review as Lim et al summarized the mechanistic hypotheses surrounding pyridines very well in their recent paper.…”

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.…”

In Pursuit of Sustainable Hydrogen Storage with Boron‐Nitride Fullerene as the Storage Medium

“…[21] The thermodynamic driving force of this reaction lies in the formation of the exceptionally stable nitrogen molecule. [24] We find the same technique can be applied to hydrogenate NiPr 2 = BH 2 and this turns out to be thermodynamically and kinetically feasible (see ing barrier (RDB) = 16.5 kcal mol À1 ]a nd hydrogenation of last B 2 N 2 ring (RDB = 9.8 kcal mol À1 ). Hydrazine itself is produced from ammonia feedstock and, in turn, ammonia in mass scale is produced through av ery energy intensive process by reactingN 2 with H 2 in aH aeber-Boschp rocess.…”

“…[21] However, the long term sustainabilityo fs uch efforts mayh ave some issues. [24] One may anticipatet hat B 24 N 24 H n species may form solidst hrough NÀH d + ··· dÀ HÀBd ihydrogeni nteractions. [22] Alternatively, if one looks at reduction of carbon dioxide, the thermodynamic cost is slightly higher( CO 2 + H 2 = HCOOH; + 7.9 kcal mol À1 ), which has gained prominence owing to the recent drive for developing ah ydrogen storage route based on the formic acid and carbon dioxide cycle.…”

“…[26] Alternatively, cis-LCo(H) 2 (H 2 )c an transform to trans-LCo(H) 2 (H 2 )t hrough LCo(H) 4 and self-dehydrogenation of trans-LCo(H) 2 (H 2 )r egeneratest he catalytic active species LCo(H) 2 and hydrogen gas (see Figure S11i nt Figure 2). [24] However,d eactivation of LCo(H) 2 through such an ucleophilic attack is not expected forB 24 N 24 owing to its steric bulk. We also identified the added advantage of the dehydrogenation of the B 24 N 24 H 32 pathway over that of amineborane.…”