Toshihiro
Akashige
a,
Adlai B.
Katzenberg
a,
Daniel M.
Frey
a,
Debdyuti
Mukherjee
a,
César A.
Urbina Blanco
ab,
Brian
Chen
a,
Yoshiyuki
Okamoto‡
a and
Miguel A.
Modestino
*a
aDepartment of Chemical and Biomolecular Engineering, New York University, Brooklyn, NY 11201, USA. E-mail: modestino@nyu.edu
bLaboratory for Chemical Technology (LCT), Department of Materials, Textiles and Chemical Engineering, University of Ghent, Ghent, 9052, Belgium
First published on 22nd March 2024
Olefin–paraffin separations are large-volume energy-intensive processes for preparing purified monomers such as ethylene and propylene. Currently, these separations are performed using cryogenic distillations, accounting for approximately 250 trillion BTU per year of industrial energy consumption. This work demonstrates an alternative olefin–paraffin separation method based on an electrochemically modulated swing absorption system. Nickel maleonitriledithiolate, an electrochemically active organometallic complex, is dispersed in the ionogel binder of a membrane electrode assembly (MEA). When exposed to propylene–propane gas mixtures, propylene is selectively captured during the oxidation of the complex and is then released when the complex is reduced. Our results suggest that transport limitations of olefins to electrochemical active sites play an important role in determining separation efficacy. Experiments conducted under varying oxidative potentials (from 1 to 3 V) and a reductive potential of −2 V demonstrated the operational robustness of the MEA over multiple capture-and-release cycles. This proof-of-concept demonstration represents a new non-thermal route for purifying the large olefin commodities in the chemical industry.
Several alternative separation methods for olefin purifications have been demonstrated in the past century, but their scale-up and deployment have proven challenging. Membrane-based separations of gases are a promising non-thermal alternative with the potential to reduce the energy requirement by at least 85% relative to cryogenic distillation.9,10 Membrane separation often relies on a solution–diffusion mechanism.6,11 This separation method however often proves ineffective because of the similarity in molecular size and dipole moments of 2-carbon and 3-carbon olefins and paraffins.6,12 An improved version of this diffusion-based separation method involves molecular-sieving, in which carefully engineered metal–organic frameworks or zeolite structures, accurately designed to angstrom range, help improve selectivity between molecules of similar sizes.6,11,13 Alternatively, metallic species that selectively complex olefins can facilitate the separations. For example, silver(I) ions have been known to create weak pi-bond complexes with alkenes.14,15 Liquid solutions saturated with silver(I) ions are known to show increased sorption of olefin into the solution media when exposed to an olefin-rich environment.15 Similarly, polymer membranes doped or blended with silver(I) species exhibit an increase in selective sorption behavior towards olefin, enhancing the selectivity of olefin/paraffin separation.11,13,14,16 However, using silver ions has often been challenging because of possible poisoning by impurities found in gas mixtures (i.e., hydrogen sulfide or acetylene), reducing their lifespan.17,18 Recent review articles on membrane separations provide a comprehensive and in depth perspective on the strategies explored for olefin–paraffin mixtures.6,8
A promising yet underexplored nonthermal approach to olefin separations is electrochemical swing adsorption. In this process, olefins reversibly bind to redox-active organometallic complexes and their affinity can be electrochemically modulated. Similar electrochemical separation methods have been demonstrated for carbon dioxide (CO2) capture where quinone-based redox-active species selectively interact with CO2.19,20 The quinone species capture CO2 at its reduced state which can then be released when the species are oxidized. This selective capture-and-release mechanism can also be applied to olefin separation, as previously demonstrated using copper(I)-containing solutions which have high affinity to olefin complexation.21,22 However, because copper(I) ions are vulnerable to poisoning, a more robust olefin-complexing species (i.e., nonreactive to the impurities) is sought to facilitate this capture-and-release mechanism. To overcome this limitation, Wang and Steifel demonstrated that solutions containing Ni(mnt)2n complexes (mnt = [S2C2(CN)2]2−, maleonitriledithiolate; n = 0, −1, −2), where the nickel oxidation state (OS) can be IV, III, or II respectively, could be used to reversible bind olefins in its oxidized state, and release them in its reduced state.23 As shown in the reversible redox cycle depicted in Fig. 1, different oxidation states of Ni(mnt)2n species can be electrochemically accessed, in which one state has a high affinity to complex olefins (n = 0, OS = IV), while the other state has low affinity towards them (n = −2, OS = II). Each OS can be accessed via carefully tuning the electrode potential, enabling control over the olefin capture and release process. Since these early demonstrations, computational and experimental studies have sought to understand the mechanism of the reaction of Ni(mnt)2n species with olefins.24–29 This electrochemical swing separation approach is fundamentally different than that of membrane separations, as it relies on the differences in reactivity of olefins and paraffin towards redox-active species, rather than on their transport properties across a medium. Given that paraffins cannot react with Ni(mnt)2n complexes, this electrochemical separation has the potential to be highly selective towards the capture of olefins in mixtures. On the other hand, electrochemical separations present additional complexities for scale-up, as they require the integration of electrodes with large electrochemically-active surface areas and large interfaces with gas mixtures, such as those incorporated in membrane-electrode assemblies (MEAs).
Fig. 1 (A) Propylene can be separated from propane-containing gas mixture via selective capture and release of alkene molecules facilitated by a membrane electrode assembly (MEA) as depicted above in the top-right. (B) The MEA's reaction surface consists of a layer of porous gas-diffusing electrodes that is made primarily of carbon nanoparticles scaffolded by a hybrid ionogel mixture of [BMIM][PF6] ionic liquid and Nafion™ polymers. SEM images of the porous electrode/ionogel structures are shown in the top-left. (C) Ni(mnt)2n species are dispersed within the ionogel whose close vicinity to both the carbon electrode surface and the gas-phase interface containing propane/propylene mixture allows electrochemically modulated complexation and de-complexation with propylene via a reversible redox olefin separation process. The selective pi-bond complexation affinity between propylene and Ni(mnt)2n is dependent on the oxidation state of Ni(mnt)2n as depicted in the lower-left. At an oxidation state of 0, propylene can complex to the Ni(mnt)2 species, while at an oxidation state of −2, complexation is not favored. (D) The sample MEA is installed into an electrochemical flow cell device, as shown in the lower-right, which is connected to a potentiostat (for modulating potential in the MEA), a mass flow control meter feeding propane/propylene gas mixture or nitrogen into the MEA during appropriate phases of the experiment (refer to Fig. 2), and a GC to measure the concentration of the outlet gas stream. Because the flow cell device requires a counter electrode and a balanced ion transfer in the central Nafion-117 membrane, water oxidation and reduction were utilized on the side opposite to the gas-separating side of the flow cell. |
In this work, we demonstrate that Ni(mnt)2n species can be effectively utilized for propylene–propane gas separation when appropriately embedded into a membrane electrode assembly (MEA) as depicted in Fig. 1. The Ni(mnt)2n species can be distributed within a hybrid ionogel medium composed of an ionic-liquid-swollen ionomer. This ionogel acts as a binder for electrically conductive carbon particles deposited on porous gas diffusion electrodes (GDEs). The porous nature of the GDEs provides a large interface between the gas phase and the redox sites (Fig. 2). The incorporation of Ni(mnt)2n species into the ionogel composite electrode layer was partly inspired by works involving porous coordination polymers (PCP) with integrated metal bis(dithiolene) units that have been used in electrochemically modulated separation of olefins.30 PCP can be utilized as a scaffold to hold olefin-complexing metal dithiolene species in thin, porous layers that can be electrochemically modulated with oxidative and reductive charge to capture and release olefins. MEA-based devices are advantageous over solution-based electrochemical separation systems because the latter usually operates in a liquid organic media prone to evaporation requiring frequent replenishment, and imposes significant mass transport resistances to olefin gases with low solubility. Unlike solution-based systems, ionogel-containing MEAs are not prone to evaporative losses of media, and the presence of ionic liquids in the gels enhances the solubility of olefins.31–33
Ionogels have been extensively studied for electrochemical energy conversion, catalysis, and gas separations.34–38 Most importantly, ionogels are ionically conductive, allowing 3-dimensional transport of organometallic species and supporting ions. To demonstrate the concept, we prepared electrode layers with ionogels of Nafion and 1-n-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) ionic liquid (IL) along with the Ni(mnt)2n species. These ionic liquids are known to provide high ionic conductivity exceeding 10 mS cm−139,40 and have an inherently higher solubility for olefins than paraffins, further compounding the effective selectivity towards olefins in gas separations.32,33
The separation performance of the device was characterized by GC analysis of the outlet gas stream. Our experimental results (Fig. 2 and 3) show strong evidence of olefin separation facilitated by our Ni(mnt)2n-containing MEA. Moreover, the device separated a 50:50 propylene–propane gas mixture to a higher propylene ratio in its final output, reaching as high as 80:20 propylene–propane ratio, excluding the nitrogen flush gas (see Fig. S4D in ESI†). The separation performance was quantified as the number of moles of propylene captured and released per MEA projected area from a propylene–propane gas mixture of known composition (i.e., usually of 50:50 by volume mixture unless otherwise stated). Carbon electrode nanoparticles and Nafion ionomer loadings were kept constant throughout this study to help minimize the number of parameters affecting the MEA's separation performance. Control experiments without Ni(mnt)2n species did not exhibit any sign of propylene gas separation from the propane mixture (see Fig. S1A in ESI†).
As depicted in Fig. 3A1, at [N(Et)4]2[Ni(mnt)2] loadings between 2–4 mg cm−2, the propylene separation performance is a strong function of the complex loading, but eventually approaches a plateau as the [N(Et)4]2[Ni(mnt)2] loading approaches >4 mg cm−2. This asymptotic behavior suggests that at high complex loadings, the separation is hindered due to transport limitations that limit the access of [Ni(mnt)2]n to the electrode surface, or the ability for propylene to reach all reaction sites. One of the transport limitation sources can be attributed to possible phase separation of [N(Et)4]2[Ni(mnt)2] crystals out of the ionogel medium due to oversaturation (see Fig. S7 in ESI†), which could be related to the plateauing separation performance since the ionogel (of a specified IL vs. polymer ratio) can only dissolve a finite maximum amount of [N(Et)4]2[Ni(mnt)2]. In subsequent experiments, the [N(Et)4]2[Ni(mnt)2] loading was kept constant at 8 mg cm−2 since it exhibited a sufficiently high throughput of purified propylene (for further details regarding Fig. 3A1 see Section S1 in ESI†).
There is a local maximum in the relationship between IL loading and separation performance because of transport limitations induced by increased concentration of IL in the MEA. As shown in Fig. 3B, the separation performance decreased progressively after reaching an IL loading of 4 mg cm−2. It is possible that excessive IL in the ionogel can inadvertently lower the gas/ionogel interfacial area, consequentially restricting the amount of propylene that can be absorbed into the MEA. Furthermore, the ionogel can easily be swelled with an increased IL ratio, increasing the physical distance between the carbon electrode surface and the gas interface and mass transport resistance. Therefore, a carefully optimized IL loading should balance both the positive effect of increased mobility of [Ni(mnt)2]n species in the ionogel and the negative effect of both the ionogel swelling and the reduced interfacial area to achieve a high separation performance. Since an IL loading of 4 mg cm−2 demonstrated optimal functionality with 8 mg cm−2 of [N(Et)4]2[Ni(mnt)2] in terms of propylene separation performance, these parameters were kept constant in subsequent studies (for further details regarding Fig. 3B see Section S2 in ESI†).
Furthermore, the effect on separation performance from varying reductive potentials at a constant oxidative potential (of +1.5 V) was studied (Fig. 4B). A minimum thermodynamic potential of −1.80 V is required to reduce the [Ni(mnt)2·C3H6] species back to [Ni(mnt)2]2− and propylene, which agrees with the experimental data shown in Fig. 4B (see Section S14 in ESI†). However, it must be noted that excessive reductive potential can defeat the targeted goal of olefin purification due to parasitic propylene hydrogenation. In Fig. 4C, propylene separation is achieved at −2 V where propane's concentration profile exhibited no activity upon applying the reductive potential (polarity switch) to the MEA. However, applying −3 V reductive potential released propylene and a significant amount of propane, likely arising from hydrogenation side reactions (for further details regarding Fig. 4B and C see Section S5 in ESI†). Furthermore, hydrogen gas was commonly observed as a side product during reductive phases in almost all experimental runs. Approximately 10–20% of reductive charge contributed to hydrogen gas production (see Sections S1–S3 in ESI†). Therefore, carefully controlling the reductive potential is necessary to ensure the released propylene is not hydrogenated.
Realizing the proposed scalable concepts will rely in overcoming the scientific and technical barriers identified in our study, including faradaic efficiency, utilization fraction of [Ni(mnt)2]n species, mass transport limitations, and stability. Typically, a low faradaic efficiency (FE) was observed (≤1%), which is calculated as the fraction of the total transferred charge used for the electrochemical modulated capture and release of propylene from a 50:50 propylene–propane mixture. The utilization fraction (UF) of the embedded [Ni(mnt)2]n species in the MEA was typically <2% when separating propylene from a 50:50 mixture. The low FE and UF arise from parasitic reactions (e.g., HER or OER from water molecules that diffused from the counter electrode) and mass transport limitation in the ionogel (see Fig. 3B), impinging the accessibility of [Ni(mnt)2]n species to the interfacial area with the gas phase. These limitations can be addressed by controlling electrode side reactions and developing highly porous electrodes that can scaffold a high loading of [Ni(mnt)2]n species such as by using PCP30 or by using alternative ionogel materials and compositions that exhibit high solubility and ion mobility of embedded [Ni(mnt)2]n species with minimal negative effect on porosity and interfacial area. Lastly, long-term stability is an important factor in the scale-up of this separation process. Degradation of separation performance has been observed over consecutive cycles during our study (see Fig. S1C and D in ESI†). Even though [Ni(mnt)2]n can undergo reversible redox processes, excess voltage coupled with the presence of water, oxygen, and hydrogen (from the counter-side of the MEA) can lead to undesirable parasitic reactions that irreversibly degrade [Ni(mnt)2]n species. Previous computational studies have also suggested that a [Ni(mnt)2]− anion participates as a catalyst for the complexation of olefins, and that without the presence of this anion the complex can undergo degradation.26 Strategies such as the swing cycle shown in Fig. 5 as well as minimizing overpotentials in the cell can help mitigate the irreversible degradation of the Ni complex.
The MEA components of specified compositions were then carefully installed into an electrochemical flow cell device like the one depicted in Fig. 1 with torque wrenches (set to 6.00 N m) to ensure good contact between different components of the device. The composition of each deposited layer can be found in the figure captions in Sections S1–S6 in ESI.† Workable electric potentials to induce the selective olefin complexation mechanism were also identified for the MEA through cyclic voltammetry as shown in Section S11 in ESI.†
Each separation test cycle typically lasts for 4 hours and consists of four distinct phases to enable the electrochemical flow cell device to separate propylene–propane gas mixture in an experimentally observable manner. Temperature and pressure inside the flow cell are kept constant at approximately 25 degrees celsius and 1 atm, respectively. Fig. 2 depicts a typical separation test cycle. In phase (1), a mixture of propylene and propane gas is fed into the flow cell for 30 minutes at a combined total flow rate of 20 sccm. Simultaneously, an oxidative potential is applied to the gas-diffusing electrode layer (+1.5 V at the standard procedure) vs. counter iridium-coated electrode. The oxidative current converts [Ni(mnt)2]−2 to [Ni(mnt)2]0 near the surface of the carbon nanoparticles within the ionogel.
After enough propylene complexation takes place, phase (2) involves the removal of propylene and propane gas molecules in the environment of the MEA through a steady nitrogen flush at a constant flow rate of 5 sccm. Propylene and propane feed flow ceases to 0 sccm at start of phase (2). The main purpose of this flush is to lower the concentration of the propylene and propane molecules in the gas stream and increase the GC sensitivity to detect small changes in the concentration of these gases. Furthermore, sufficient time was required to flush the system of propylene and propane molecules that were absorbed into the MEA system via concentration-gradient-driven sorption and avoid non-electrochemically complexed propylene and propane to substantially interfere with our measurements. Nitrogen carrier flow rate will maintain at 5 sccm for the remainder of the cycle including phases (3) and (4).
Phase (3) can be initiated once the propylene–propane concentration is low enough. A reductive potential is applied to the gas-diffusing electrode layer (−2 V vs. counter-side at the standard procedure) so that the complexed species can decouple the propylene at the carbon electrode surface. Concentration-gradient-driven diffusion would allow more of the propylene-containing complex to travel to the electrode surface to get reduced and decoupled. The propylene concentration should theoretically increase initially near the carbon electrode surface and eventually diffuse towards the gas-phase interface where a bulk nitrogen stream can carry the propylene into the GC sampling port. By measuring the concentration of the propylene at periodic time intervals after applying this polarity switch in electric potential, the total moles of propylene gas that were captured and released by this MEA system can be quantified through area integration of the characteristic “peak” as shown in deconvoluted propylene concentration profiles in Sections S1–S6 in ESI.† Phase (4) was a maintenance-purpose flush to ensure the flow cell device restarts in a similar condition for the next experimental cycle. Data that support the findings of this study have been deposited in GitHub publicly accessible (requiring no access codes) via web link: https://github.com/ta1535/Nickel_Dithiolene_Research_Experimental_Data.git.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ya00508a |
‡ This project is dedicated to the late Dr Yoshiyuki Okamoto who devoted much of his life to polymer science research. Achieving a practical system for olefin–paraffin separation was his last dream. |
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