Electrifying Hydroformylation Catalysts Exposes Voltage-Driven C-C Bond Formation

Abstract

Electrochemical reactions can access a significant range of driving forces under operationally mild conditions and are thus envisioned to play a key role in decarbonizing chemical manufacturing. However, many reactions with well-established thermochemical precedents remain difficult to achieve electrochemically. For example, hydroformylation (thermo-HFN) is an industrially important reaction that couples olefins and carbon monoxide (CO) to make aldehydes. However, the electrochemical analogue of hydroformylation (electro-HFN), which uses protons and electrons instead of hydrogen gas, represents a complex C-C bond-forming reaction that is difficult to achieve at heterogeneous electrocatalysts. In this work, we import Rh-based thermo-HFN catalysts onto electrode surfaces to unlock electro-HFN reactivity. At mild conditions of room temperature and 5 bar CO, we achieve Faradaic efficiencies of up to 15% and turnover frequencies of up to 0.7 h^-1. This electro-HFN rate is an order of magnitude greater than the corresponding thermo-HFN rate at the same catalyst, temperature, and pressure. Reaction kinetics and operando X-ray absorption spectroscopy provide evidence for an electro-HFN mechanism that involves distinct elementary steps relative to thermo-HFN. This work demonstrates a step-by-step experimental strategy for electrifying a well-studied thermochemical reaction to unveil a new electrocatalyst for a complex and underexplored electrochemical reaction.

Figure 1

Electrifying a thermo-HFN catalyst to access electro-HFN reactivity. (a) Thermochemical and electrochemical hydroformylation (HFN) reactions. In this work, a thermo-HFN catalyst was sequentially electrified to achieve electro-HFN. Black ovals represent carbon black and blue lines represent Nafion binder. (b) Batch reaction setup within a pressurized Parr reactor used for thermo-HFN experiments. (c) Batch two-compartment, two-electrode electro-HFN cell configuration for elevated pressure electro-HFN experiments. (d) Thermo-HFN turnover frequency (TOF) as a function of partial H₂ pressure. (e) Electro-HFN TOF as a function of applied reductive current. (f) Electro-HFN product distribution, given as Faradaic efficiency, as a function of applied reductive current. All data were collected at 25 °C, 5 bar CO, 0.52 M styrene, 0.1 M TBAOTf, 25 mM HOTf in a 50% v/v IPA/H₂O mixture. Error bars represent standard deviation with n ≥ 3.

Figure 2

XAS. (a) Normalized XANES data zoomed in on rising edge, shown for ex situ and operando samples as well as known rhodium-containing standards. (b) Processed operando XANES data showing Rh K-edge energy as a function of applied potential (black stars), as well as control experiments without CO and/or styrene. (c) R-space EXAFS spectra of ex situ and operando samples, as well as known rhodium-containing standards. (d) Processed operando EXAFS data showing fitted Rh–O coordination numbers as a function of applied potential (red circles), as well as a control condition with neither CO nor styrene (black circles). Error bars in (d) represent fitting errors given by the Artemis fitting software. All data collected at ambient temperature and pressure in electrolyte composed of 0.1 M TBAOTf and 25 mM HOTf in IPA/H₂O. Voltages reported vs SCE.

Figure 3

Rate data for electro-HFN at ambient temperature and pressure. (a) Control tests with and without application of voltage in the explicit presence of H₂ gas. “V on”: constant application of −1.45 V vs SCE for the entire duration of the experiment, “V off”: no voltage applied, and “V init.”: −1.45 V vs SCE was applied for only the first 5 min of a 2 h experiment (b) Tafel dependence. (c) CO partial pressure dependence. (d) Styrene activity dependence. Data correspond to styrene concentrations ranging from 0.13 to 0.28 mol/L. (e) Acid concentration dependence. Panels (c–e) plotted on log–log scales, with data at higher driving forces (−1.45 V vs SCE) in black and data at lower driving forces (−1.05 V vs SCE) in red. (f) Kinetic isotope effect measurements, where the reaction was run with D₂O, isopropanol-d₈, and DOTf to replace possible proton sources with deuterium. Plotted is the rate with protons divided by the rate with deuterons. All data were collected at 25 °C, 1 bar CO, 0.52 M styrene, 25 mM HOTf, and 0.1 M TBAOTf in a 50% v/v IPA/H₂O mixture unless explicitly labeled otherwise. Error bars represent standard deviation [propagated through division in (f)] with n ≥ 3.

Figure 4

Extension of the reactor and substrate scope. (a) Electro-HFN TOF as a function of applied reductive current for cells containing either a sacrificial Al anode and AEM (gray) or a Pt anode and a cation exchange membrane (black). (b) Cell voltages corresponding to the reaction rate data shown in (a). (c) Electro-HFN TOF as a function of applied reductive current for styrene (black), 1-hexene (red), and 1-decene (blue). (d) Electro-HFN regioselectivity for the same three olefins, plotted as a percentage of the aldehyde products with linear selectivity. Unless labeled otherwise, all data were collected beyond the saturation limit of the olefin and at 25 °C, 5 bar CO, 0.1 M TBAOTf, 25 mM HOTf in a 50% v/v IPA/H₂O mixture, with a Pt anode and Nafion membrane. Error bars represent standard deviation with n ≥ 3.