Dynamically Reversible Interconversion of Molecular Catalysts for Efficient Electrooxidation of Propylene into Propylene Glycol

Abstract

For the electrooxidation of propylene into 1,2-propylene glycol (PG), the process involves two key steps of the generation of *OH and the transfer of *OH to the CC bond in propylene. The strong *OH binding energy (E B(*OH)) favors the dissociation of H2O into *OH, whereas the transfer of *OH to propylene will be impeded. The scaling relationship of the E B(*OH) plays a key role in affecting the catalytic performance toward propylene electrooxidation. Herein, we adopt an immobilized Ag pyrazole molecular catalyst (denoted as AgPz) as the electrocatalyst. The pyrrolic N–H in AgPz could undergo deprotonation to form pyrrolic N (denoted as AgPz-Hvac), which can be protonated reversibly. During propylene electrooxidation, the strong E B(*OH) on AgPz favors the dissociation of H2O into *OH. Subsequently, the AgPz transforms into AgPz-Hvac that possesses weak E B(*OH), benefiting to the further combination of *OH and propylene. The dynamically reversible interconversion between AgPz and AgPz-Hvac accompanied by changeable E B(*OH) breaks the scaling relationship, thus greatly lowering the reaction barrier. At 2.0 V versus Ag/AgCl electrode, AgPz achieves a remarkable yield rate of 288.9 mmolPG gcat –1 h–1, which is more than one order of magnitude higher than the highest value ever reported.

Bipyridine-Confined Silver Single-Atom Catalysts Facilitate In-Plane C–O Coupling for Propylene Electrooxidation

“…2,12−14 Some strategies such as facet engineering, doping modification, and dynamic ligand regulation have been applied to tune the energy barrier of the C−O coupling process during the propylene electro-oxidation. 2,12,15 Despite the efforts mentioned above, the catalytic performance of propylene electrooxidation is still unsatisfactory (yield of <23 g PG m −2 h −1 ). 1,2,13,15,16 To the best of our knowledge, the C−O coupling reactions in homogeneous synthesis typically rely on mononuclear metal complexes.…”

“…2,12,15 Despite the efforts mentioned above, the catalytic performance of propylene electrooxidation is still unsatisfactory (yield of <23 g PG m −2 h −1 ). 1,2,13,15,16 To the best of our knowledge, the C−O coupling reactions in homogeneous synthesis typically rely on mononuclear metal complexes. 17 Ag SAs/UiO-bpy was prepared via chelation between pyridinic N in UiO-bpy and silver nitrate (AgNO 3 ) (Figure S1).…”

“…1,2-Propylene glycol (PG) is a basic chemical that can be used for syntheses in the pharmaceutical, cosmetics, and food manufacturing industries. − The global market size of PG in 2023 has reached USD 4.5 billion, which is expected to increase at a compound annual rate of 5.8% . Conventionally, the manufacture of PG from propylene is classified into one-step and two-step pathways. − The one-step pathway using exogenous hydrogen peroxide (H 2 O 2 ) as the oxidant has been restricted by the insufficient supply of H 2 O 2 . , In the two-step process, propylene is first transformed into propylene oxide (PO) via either chlorohydrin or Halcon processes, followed by the hydrolysis from PO to PG.…”

“…Conventionally, the manufacture of PG from propylene is classified into one-step and two-step pathways. − The one-step pathway using exogenous hydrogen peroxide (H 2 O 2 ) as the oxidant has been restricted by the insufficient supply of H 2 O 2 . , In the two-step process, propylene is first transformed into propylene oxide (PO) via either chlorohydrin or Halcon processes, followed by the hydrolysis from PO to PG. This operation produces a large amount of wastewater and requires additional purification steps, resulting in uncontrolled environmental pollution and massive energy input. ,− With the ever-increasing deployment of renewable energy, the mild electrooxidation of propylene into PG with water as the oxygen source presents a promising approach for the one-step production of PG.…”

“…The C–O coupling step, namely the combination of adsorbed alkene and oxygenated species (*O or *OH), is normally considered to be vital in determining the catalytic performance of alkene electrooxidation. ,− Some strategies such as facet engineering, doping modification, and dynamic ligand regulation have been applied to tune the energy barrier of the C–O coupling process during the propylene electrooxidation. ,, Despite the efforts mentioned above, the catalytic performance of propylene electrooxidation is still unsatisfactory (yield of <23 g PG m –2 h –1 ). ,,,, To the best of our knowledge, the C–O coupling reactions in homogeneous synthesis typically rely on mononuclear metal complexes. − Inspired by this, heterogeneous single-atom catalysts (SACs) with individual metal sites possess the potential to promote the formation of C–O bonds. Due to the restricted spatial environment, conventional SACs with saturated planar coordination structures are unfavorable to the co-adsorption of the multiple intermediates, thus limiting the efficiency of the C–O coupling. − Therefore, constructing SACs with rational local structures to enable the simultaneous adsorption of intermediates holds promise for achieving the efficient electrooxidation of propylene into PG.…”