This document describes an integrated solid electrolyte reactor system that couples electrochemical and chemical reactions to efficiently produce ethylene glycol from ethylene oxidation. Specifically, it leverages the high concentration of hydrogen peroxide generated at the electrode/electrolyte interface during oxygen reduction to boost the subsequent ethylene epoxidation reaction in the same reactor. Experimental results showed the integrated reactor approach improved ethylene glycol production rates 3-fold and hydrogen peroxide utilization 2-fold compared to a traditional tandem reactor system. The interfacial reaction coupling strategy allows full utilization of unique concentration gradients in electrocatalysis and could be applied to other concentration-sensitive electrochemical-chemical processes.
This document describes an integrated solid electrolyte reactor system that couples electrochemical and chemical reactions to efficiently produce ethylene glycol from ethylene oxidation. Specifically, it leverages the high concentration of hydrogen peroxide generated at the electrode/electrolyte interface during oxygen reduction to boost the subsequent ethylene epoxidation reaction in the same reactor. Experimental results showed the integrated reactor approach improved ethylene glycol production rates 3-fold and hydrogen peroxide utilization 2-fold compared to a traditional tandem reactor system. The interfacial reaction coupling strategy allows full utilization of unique concentration gradients in electrocatalysis and could be applied to other concentration-sensitive electrochemical-chemical processes.
This document describes an integrated solid electrolyte reactor system that couples electrochemical and chemical reactions to efficiently produce ethylene glycol from ethylene oxidation. Specifically, it leverages the high concentration of hydrogen peroxide generated at the electrode/electrolyte interface during oxygen reduction to boost the subsequent ethylene epoxidation reaction in the same reactor. Experimental results showed the integrated reactor approach improved ethylene glycol production rates 3-fold and hydrogen peroxide utilization 2-fold compared to a traditional tandem reactor system. The interfacial reaction coupling strategy allows full utilization of unique concentration gradients in electrocatalysis and could be applied to other concentration-sensitive electrochemical-chemical processes.
This document describes an integrated solid electrolyte reactor system that couples electrochemical and chemical reactions to efficiently produce ethylene glycol from ethylene oxidation. Specifically, it leverages the high concentration of hydrogen peroxide generated at the electrode/electrolyte interface during oxygen reduction to boost the subsequent ethylene epoxidation reaction in the same reactor. Experimental results showed the integrated reactor approach improved ethylene glycol production rates 3-fold and hydrogen peroxide utilization 2-fold compared to a traditional tandem reactor system. The interfacial reaction coupling strategy allows full utilization of unique concentration gradients in electrocatalysis and could be applied to other concentration-sensitive electrochemical-chemical processes.
Context & scale Electrochemical synthesis of chemicals and fuels plays an important role in decarbonizing the chemical manufacturing sector due to the merits of using renewable electricity, mild reaction conditions, and environmentally benign operations. Despite the significant achievements of individual electrochemical reactions, coupling electrolyzers with downstream reactors is increasingly attractive from more complicated chemical fabrications that are typically challenging for individual reactors. Current coupling reactions focus on the assembly strategies of multiple individual reactors by feeding the electrochemically synthesized species into the followed-by reactors, which have not yet leveraged the unique property of electrolysis: the high concentration of generated species at the electrode interface. Different from the traditional tandem system, we designed an integrated solid electrolyzer that fully leverages the interfacial high concentration of species to boost the consecutive chemical reactions. Summary Coupling electrochemical and chemical reactions has been demonstrated in traditional tandem reactor systems, but their practical applications are still distilled down to individual reactor optimizations. Here, we demonstrate a fully integrated system that presents significantly improved catalytic performance when compared with traditional tandem systems. Using electrosynthesis of hydrogen peroxide followed by olefin epoxidation reaction as a representative example, we demonstrated that, by confining the chemical reaction right at the electrode/electrolyte interface in our solid electrolyte reactor, we can fully leverage the interfacial high concentration of H2O2 product from electrocatalysis to boost the following ethylene epoxidation reaction, which represented a 3-fold improvement in electrolyte-free ethylene glycol generation when compared with a tandem reactor system. This integration strategy can be extended to other electrochemical-chemical coupling reactions, especially when the coupled reaction is sensitive to reactant concentrations, which could avoid energy-intensive separation or concentration steps typically needed between the electrochemical and chemical reactions. Graphical abstract Introduction Electrochemical synthesis of chemicals and fuels is becoming increasingly important to decarbonize the chemical manufacturing sector.1,2,3,4,5,6,7,8 It can utilize renewable electricity as the energy input, be operated under mild reaction conditions, and use atmospheric molecules as reactants.9,10,11,12,13 As the economics of electrochemical synthesis routes gradually improve with the decreased renewable electricity price and further technological development, the chemical industry will witness more and more electrolyzers playing significant roles along industrial supply chains.14,15,16 Although the technological improvements and device scaling-up of individual electrochemical reactions, such as water splitting and CO2 reduction,17,18,19,20 remain the focal points in the field, we have begun to see more recent reports on coupling electrochemical reactors with upstream or downstream reactors for more complicated products that are typically impossible from electrolyzers alone.21,22,23,24,25,26 One typical coupling strategy is the tandem system, where the electrochemically generated intermediate products are continuously fed into a downstream reactor for product upgradation (Figure 1A). Examples include coupling electrochemical devices with bioreactors for the conversion of CO2 into high-value chemicals (Figure 1A).27,28,29 Another type of coupling is the mediator coupling reaction, in which the anodically or cathodically generated reaction mediators, such as Cl− to ClO− or Cl radicals, as well as O2 to ⋅OH, can homogeneously react with a fed-in reactant for obtaining the target product and are converted back to their original status for the next round of reaction (Figure 1B).30,31,32,33,34 Although exciting progress has been made in the development of these coupling reactions, their design and engineering are still distilled down to individual reaction optimizations, which have not yet leveraged one unique property of electrocatalysis: the high concentration of generated products/intermediates at the catalyst/electrolyte interface.35,36 As is well documented in electrocatalysis, the surface-generated molecules are typically accumulated at the catalyst/electrolyte interface before they slowly diffuse away into the bulk electrolyte or the gas chamber. For example, in the formation of liquid products, due to the sharp catalyst/electrolyte interface (typically in micrometer scales) compared with bulk electrolyte (typically in millimeter scales), the product concentration at this interfacial region could be orders of magnitude higher than the average concentration when all the molecules are well dispersed into the electrolyte.37 However, in a traditional electrochemical-chemical tandem reaction system, this concentration gradient will disappear before the product stream flows into the downstream reactor, losing the opportunity to make full use of the high interfacial concentrations to accelerate the follow- up reaction. How to leverage this unique property in electrocatalysis and the impacts of this interfacial high concentration on the follow-up reaction activity are still unclear.38 Different from traditional coupling systems, here, we demonstrated an interfacial electrochemical-chemical reaction coupling design to fully leverage the interfacial high product concentrations in electrochemistry to boost the consecutive chemical reactions (Figure 1C). Using olefin epoxidation reaction as a model reaction, which is sensitive to the concentration of oxidants,39 we integrated oxygen reduction reaction (ORR) to hydrogen peroxide (H2O2) with a subsequent ethylene (C2H4) epoxidation reaction (to ethylene glycol [EG]) into one solid electrolyte (SE) reactor. This integration allowed us to achieve a 3-fold improvement in EG production rates and 2-fold in H2O2 utilization efficiencies when compared with a traditional tandem reactor system. Specifically, by directly feeding ethylene into the SE layer of our integrated reactor, under a 25 mA/cm2 ORR current density, the EG production rate can reach 583 μmol/h, delivering a H2O2 utilization efficiency of 96% and an overall Faradaic efficiency of 98.7%. This interfacial reaction integration strategy can be extended to other electrochemical-chemical coupling reactions, especially when the coupled chemical reaction is sensitive to reactant concentrations. Glycols, especially EG and propylene glycol (PG), are important feedstocks for industrial polymer synthesis and antifreezes, which are normally obtained from the direct oxidation of olefins40 or hydrolysis of olefin oxides.41,42 Recently, using H2O2 instead of chlorine for olefin epoxidation on titanium silicalite-1 (TS-1) catalyst, especially the well-known hydrogen peroxide to propylene oxidation (HPPO) process,43 has attracted extensive attention in both industry and academia due to its high product selectivity and environmentally benign process (producing H2O as the byproduct).44,45,46,47,48 In our group’s recent studies on the electrochemical synthesis of H2O2, we developed a unique SE reactor that enables direct and continuous synthesis of high purity H2O2 solutions via 2e−– ORR.49,50,51 By performing 2e−–ORR electrolysis on the cathode, H2O2 molecules could be continuously formed at the membrane/SE interface via ionic recombination between HO2− (transported from the cathode) and H+ (transported from the anode). When mixing TS-1 catalyst particles inside the middle SE layer, this integrated SE reactor can serve as a perfect platform to demonstrate the interfacial electrochemical-chemical reaction coupling strategy—we can leverage the high local concentration of H2O2 molecules to boost the following olefin epoxidation reaction before they gradually diffuse and are diluted into the bulk deionized (DI) water stream. In order to prove this concept, we designed a novel three-chamber SE reactor to realize an interfacial integration of electrochemical and chemical reactions (Figures 2 and S1). The main chamber (middle layer) of the SE cell was filled with a mixture of SE and TS-1 particles and was separated by an anion exchange membrane (AEM) and cation exchange membrane (CEM) from the cathode and anode, respectively. The cathodic ORR via 2e−– ORR catalyst (carbon black) could selectively generate HO2− species that were subsequently transported through AEM into the middle layer (Figure 2B, Equation 1). These anion species were recombined with the proton flux that was transported from the anode chamber via the CEM and SE particles (proton conductors) (Figure 2B, Equations 2 and 3). Therefore, the AEM/SE interface has the highest H2O2 concentration during electrolysis. By co-feeding ethylene and DI water into the middle chamber during ORR electrolysis, ethylene can be efficiently oxidized by the interfacial H2O2 to generate ethylene oxide (EO) (Figure 2B, Equation 4), followed by the hydrolysis on the surface of SE to form EG as the final product (Figures 2B, Equation 5 and S2; Notes S1 and S2). Combined with half-cell oxygen evolution reaction (OER) at the anode (Figure 2B, Equation 2) and the coupled chemical epoxidation reaction in the main chamber (Figure 2B, Equations 3–5), we can conclude the overall reaction using O2, ethylene, and DI water as reactants and EG as the only product with an excellent atom economy (Figure 2B, Equation 6). To have a rough understanding of how high the interfacial H2O2 concentration could be in our SE reactor when compared with the downstream output concentration (average bulk concentration), we constructed a two-dimensional (2D) COMSOL model to map out the H2O2 concentration distribution near the catalyst surface (Figures 2C–2E and S3). As expected, the H2O2 concentration is highest at the left-hand boundary, where the reaction occurs and H2O2 flux is introduced. The H2O2 concentration reaches above 2.06 wt % closer to the surface at 25 mA/cm2, whereas the average concentration in the channel is around 0.081 wt % (Figure 2C). This results in an interfacial concentration that is more than 25 times higher than the average concentration in the channel. This simulation result suggests that, if we can confine the following epoxidation reaction right at the catalyst/electrolyte interface, the overall efficiency of the coupling reaction could be dramatically boosted.