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Electrolytic membrane extraction enables fine chemical production from biorefinery sidestreams Stephen J Andersen, Tom Hennebel, Sylvia Gildemyn, Marta Coma, Joachim Desloover, Jan Berton, Junko Tsukamoto, Christian Victor Stevens, and Korneel Rabaey Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es500483w • Publication Date (Web): 21 May 2014 Downloaded from http://pubs.acs.org on May 29, 2014
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Electrolytic membrane extraction enables fine chemical
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production from biorefinery sidestreams
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Stephen J Andersen, a,# Tom Hennebel,a,b,# Sylvia Gildemyn,a Marta Coma,a Joachim
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Desloover,a Jan Berton,c Junko Tsukamoto,c Christian Stevens,c and Korneel Rabaeya*
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a Laboratory of Microbial Ecology and Technology, Ghent University, Coupure Links 653,
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B-9000 Ghent, Belgium., Tel:+3292645976, Fax: +329266248, E-mail:
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[email protected] 10
b Department of Civil and Environmental Engineering, 407 O’Brien Hall, University of
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California, Berkeley, CA 94720-1716, USA
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c SynBioC, Department of Sustainable Organic Chemistry and Technology, Ghent University,
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Coupure Links 653, B-9000 Ghent, Belgium
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# - These authors contributed equally
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TOC Art
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Abstract
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Short chain carboxylates such as acetate are easily produced through mixed culture
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fermentation of many biological waste streams, though routinely digested to biogas and
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combusted rather than harvested. We developed a pipeline to extract and upgrade short chain
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carboxylates to esters, via membrane electrolysis and biphasic esterification. Carboxylate rich
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broths are electrolyzed in a cathodic chamber from which anions flux across an anion
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exchange membrane into an anodic chamber, resulting in a clean acid concentrate with neither
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solids nor biomass. Aqueous carboxylic acid concentrate reacts with added alcohol in a water-
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excluding phase to generate volatile esters. In a batch extraction, 96 ± 1.6% of the total
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acetate was extracted in 48 h from bio-refinery thin stillage (5 g.L-1 acetate) at 379 g.m-2.d-1
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(36 % coulombic efficiency). With continuously regenerated thin-stillage, the anolyte was
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concentrated to 14 g/L acetic acid, and converted at 2.64 g (acetate).L-1.h-1 in the first hour to
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ethyl acetate by the addition of excess ethanol and heating to 70°C, with a final total
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conversion of 58 ± 3%. This processing pipeline enables direct production of fine chemicals
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following undefined mixed culture fermentation, embedding carbon in industrial chemicals
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rather than returning them to the atmosphere as carbon dioxide.
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INTRODUCTION Microbial fermentation and petrochemical production can both generate industrially
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valuable building block chemicals. The petrochemical industry dominates chemical
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production due to the relatively low cost of the oil and gas substrates and well-entrenched
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separation and recovery technology, such as liquid-liquid extraction, adsorption, distillation
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and more. So-called biorefineries can utilize microorganisms in fermentation processes to
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generate sustainable products from biological resources. Genetically modified organisms and
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single strain cultures can generate products with high specificity at relatively high production
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rates, but such processes are cost intensive and thereby limit the application to high value
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products only. Mixed culture fermentations can be operated at lower costs on more complex
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feedstocks, including various bio-industrial wastes and syngas, but generally without
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specificity at biologically constrained rates.1 - 5 Example products include acetate, butyrate,
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caproate and other short chain carboxylates, common intermediates in the anaerobic digestion
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of organic wastes to biogas. These are industrially valuable building block chemicals, though
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often not valuable enough to overcome the cost of recovery from low titer broths. The
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separation and recovery of fermentation products, even from highly specific pure cultures, can
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account for over 60% of the total plant costs, thus providing an incentive for low cost
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separation technologies.6 Alternatively, biogas can be generated from a broad range of
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organic waste and though it is significantly less valuable, it is easily extracted and utilized, a
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key factor in the success of anaerobic digestion.
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Separation and recovery processes tailored to specific compounds and low titer broths
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can improve the competitiveness of biorefineres, both for pure and mixed culture fermentation
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processes.7, 8 We present a bio-production alternative to digestion leading to non-fuel
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outcomes in a biorefinery context, a processing pipeline to recover and upgrade the ubiquitous
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short chain carboxylate fermentation products as high value esters. The processing pipeline
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extracts carboxylates directly from a broth and upgrades SCFAs to esters in a second step.
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Membrane Electrolysis (ME) generates a concentrate stream which is delivered to a reactive
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extraction step, Biphasic Esterification (BE) (Figure 1). The ME process is a two-chamber
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electrochemical water treatment step where the fermentation broth or carboxylate rich stream
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is electrolyzed by a cathode and the applied electrical potential draws carboxylate ions across
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a single polymeric anion exchange membrane (AEM) into a clean, highly saline and low pH
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anolyte, without solids nor biomass. Ion exchange membranes are designed for selective
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transport of ions, and an AEM is capable of excluding solids, micro-organisms and uncharged
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molecules larger than the effective pore size of the membrane. A number of carboxylate ions
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have been demonstrated to cross AEMs, for example synthetic lactate salts.9, 10 ME stands
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apart from conventional electrodialysis and bipolar electrodialysis in which the treated
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streams are not engaged in the electrochemical reactions. In addition, only a single membrane
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is required for ME while electrodialysis requires many.11 The ME process generates
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hydroxide ions and hydrogen gas in the broth (cathodic electrolysis) and protons and oxygen
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gas in the extractant (anodic electrolysis). The hydroxide ions can replace caustic soda
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management of a fermentation broth. In the anode chamber, the extracted ionic fermentation
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product is protonated to its acidic conjugate by electrolytically generated protons. This
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product is then esterified in the BE step, increasing the volatility of the product for separation.
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Conventional esterification is generally performed in the complete absence of water or in
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organic solvents, as water kinetically constrains the esterification reaction.12 The BE process
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consists of an unreactive water exclusion phase (eg solvent or ionic liquid) and an extractant
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(anolyte) phase. Esterification proceeds by excluding the water component following the
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addition of excess alcohol and heat. In a fully realized process the higher value, volatile esters
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are extracted from a low affinity, non-volatile water-exclusion layer, such as an ionic liquid,
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and the aqueous extractant is returned to the anode to accrue more fermentation product.
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The objective of this research was to present and demonstrate a viable processing
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pipeline for extracting and upgrading short chain carboxylates. As presented, ME and BE are
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two interdependent processes that enable ester production directly from fermenter broths.
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Acetate was used as the model carboxylate for the ME and BE pipeline since acetate is among
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the most common fermentation products. The ME process was demonstrated at low current
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density with a low cost membrane and then with a higher cost, proton excluding membrane at
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a greater range of current densities. The anode salinity was investigated as a means of
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maximizing carboxylate flux in broths where the relatively low concentration of product
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results in low coulombic efficiency extraction. The ME and BE pipeline was ultimately
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demonstrated with an acid concentrate extracted from biorefinery thin stillage, and with this
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concentrate BE was demonstrated with xylene and a xylene / ionic liquid mixture. The ME
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and BE pipeline successfully extracted acetate from biorefinery thin stillage and upgraded the
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product to methyl acetate and ethyl acetate.
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MATERIALS AND METHODS
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Electrochemical Cells. Electrochemical cells (two chambers separated by a membrane, each
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with internal dimensions: 80 x 80 x 20 mm) were used as previously described,13 with spacer
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material (ElectroCell A/S, Denmark) between the surface of the electrode and the AEM.
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Membranes (AM-7001 Anion Exchange Membranes, Membranes International Inc, USA, and
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fumasep FAB, FumaTech GmbH, Germany) were pre-treated in accordance with
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manufacturer specifications. The anode was an Ir MMO coated titanium electrode (IrO2/TaO2:
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0.65/0.35), 80 mm x 80 mm, with a centrally attached, perpendicular current collector
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(Magneto Special Anodes BV, The Netherlands). The cathode was a stainless steel wire mesh
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with a stainless steel sheet metal current collector. For high current, high concentration
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experiments only (Figure S2) a low volume two chamber electrochemical cell was used
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(internal dimensions 70 x 10 x 20mm with 50 x 10 mm effective membrane area) with a
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single pass of electrolytes at 1.54 L.d-1 per chamber. The anode was an Ir MMO coated
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titanium electrode (IrO2/TaO2: 0.65/0.35), 50 mm x 20 mm (Magneto Special Anodes BV,
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The Netherlands) and the cathode stainless steel, 60 mm x 40 mm.
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The anolyte consisted of sodium sulfate, corrected to pH 2 with sulfuric acid.
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Synthetic catholyte consisted of a sodium acetate, sodium butyrate or sodium caproate
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solution (Sigma Aldrich, Belgium), corrected to pH 5.5 with sulfuric acid prior to operation.
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Fermenter broth from bioethanol production was sourced from the thin stillage of Alco Bio
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Fuel NV (Gent, Belgium). To avoid clogging within the laboratory scale connectors and to
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allow consistency in the reactor design, this broth was centrifuged at 10 000 rpm for 10 min
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and suspended solids were removed. This broth was dosed with sodium acetate to bring the
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concentration to 5 g.L-1 acetate to simulate conditions after carboxylate fermentation. For a
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consistent concentration of acetate, active fermentation was avoided by centrifugation of the
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thin stillage, a low residence time in the cathode chamber and daily replacement of the feed.
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The initial concentration of acetate in the broth was only 0.7 g L-1, as the fermentation process
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is focused on bioethanol production. In all tests, sodium sulfate salt was used in the electrolyte
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rather than sodium chloride to prevent the formation of chlorine gas at the anode while
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increasing the conductivity.
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For continuous experiments, the electrochemical cells were fed at a rate of 0.8 L.d-1 to
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achieve a residence time of 5 hours. The anode and cathode batch experiments were
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performed with equal volumes of electrolyte at 1 L. Both compartments were continuously
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stirred at a recirculation rate of 6 L.h-1.
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All electrochemical experiments were controlled with a VSP multipotentiostat
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(Princeton Applied Research, France) and an Ag/AgCl reference electrode (+0.197 V vs.
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SHE, Princeton Applied Research, France) in the cathode compartment. The applied current is
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reported as current density, defined as the set current divided by the exposed surface area of
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the anion exchange membrane. The anode reaction was the oxidation of water (2 H2O → 4 H+
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+ O2 + 4e-) which replenishes the sulfuric acid/sulfate medium. The cathodic reaction was the
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reduction of water (2 H2O + 2e- → H2 + 2 OH-).
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Membrane Electrolysis. ME was tested with continuously fed synthetic solution for flux
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characterization, as further described below. Acetate, butyrate or caproate in synthetic broths
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were tested for flux through an AEM at various current densities. For two synthetic
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experiments with acetate, the anolyte was altered. In one experiment the concentration of
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sodium sulfate was increased in the anode to observe the trend of the flux of acetate relative to
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anode salinity. Similarly, the concentration of sodium acetate at pH 2 was increased in the
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anode to observe the trend of the acetate flux relative to acetate accumulation as acetic acid.
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For these experiments the fluxes were tested at 20 A.m-2 only.
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Next, the process was tested with thin stillage from an bioethanol plant (Alco Biofuels
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bv, Belgium) to demonstrate the extraction process with a real matrix and native ions. By
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using this broth, we were able to demonstrate the full pipeline from a consistent and practical
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source fluid. Both the cathode compartment (broth) and the anode compartment (extractant)
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were first tested in a batch extraction. The cathode was then continuously fed while the anode
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compartment was run in batch to better mimic realistic operating conditions (accumulation in
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the anode, excess broth relative to extractant). For these experiments, the current remained set
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at 20 A.m-2.
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In all measurements of flux, the cell was set at 1.6 mA.m-2 (a galvanostatic setting of -
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0.1 mA controlled at the cathode) for at least 4 h prior to the run to allow for electrode and
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membrane polarization, then was changed to the set current density of interest. Hydroxide
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ions generated by water reduction were mitigated by 1 M sulfuric acid by a pH controller
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(Consort, Belgium).
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Carboxylate concentrations were measured by gas chromatography (GC), as described in Supporting Information. All ME experiments were performed in triplicate, at a minimum.
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Biphasic Esterification. Esterification experiments were performed with both a synthetic
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anolyte and a real (ie generated in a previous experiment) anolyte. The synthetic anolyte
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consisted of 20 g.L-1 acetate (as acetic acid, pH 2) and 0.5 M Na2SO4. In all cases, 20 mL of
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anolyte was used with a xylene (Sigma Aldrich, Belgium) solvent layer of 20 mL in a 100 mL
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bottle, for approximately 60 mL of headspace. The layered liquids were vacuumed to a slight
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under-pressure and heated to 70 °C. At t = 0 h, 5 mL of either ethanol, methanol or distilled
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water (control) was injected into the biphasic solutions. The reactors were then stirred and
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maintained at 70 °C for 20 hours. The aqueous fraction was sampled at t = 0 and t = 20 h, and
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analyzed for carboxylate concentration by GC. The GC conditions are described in
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Supporting Information. These BE experiments were performed in quadruplicate.
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Biphasic esterification was also performed with an ionic liquid for water exclusion.
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The ionic liquid was trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl) amide
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(Sigma Aldrich, Belgium). Real anolyte (6 mL), xylene (2 mL) and ionic liquid (1 g at 1.07
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g/mL) were heated to 70°C and stirred in a high pressure flask (15 mL). Ethanol (room
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temperature, 1 mL) was added at t = 0, and the xylene was tested for esters after t = 5 min, 30
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min, 3 h, 18 h and 24 h by GC-MS, as described in Supporting Information.
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RESULTS
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Membrane electrolysis extraction. In the first phase, we demonstrated the basic ME concept
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on synthetic solutions with various concentrations of the model carboxylate, acetate (Figure
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2a & 2b) and other carboxylates of interest, butyrate and caproate (Figure S1) from 0 to 30
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A.m-2 projected membrane surface. For an initial catholyte concentration of 10 g L-1 the flux
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of acetate was 1.05 kg.m-2.d-1 at 20 A.m-2, corresponding to a coulombic efficiency of 99.4 ±
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0.1%. At 30 A.m-2, the maximum flux of acetate was 1.38 ± 0.03 kg.m-2.d-1, corresponding to
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a coulombic efficiency of 87.0 ± 1.8 %. The extraction efficiency was lower for 1 and 5 g.L-1
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acetate broth, corresponding to less availability of the target anion in the bulk (molar
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concentration), and similarly butyrate and caproate show a low flux rate and coulombic
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efficiency (Figure S1). Transport limitations that manifest as membrane resistance and ion
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transport number have been previously demonstrated for low concentration salt solutions.14 A
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target molecule at high concentration will flux efficiently at low current densities, as was seen
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with acetate in Figure 1, and at high current densities for both acetate and caproate as shown
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in Figure S2. High current density extraction, and therefore higher flux, is preferable in a fully
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realized system. Acetate and caproate were tested at equal molar concentrations (170 mM: 10
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g.L-1 acetate and 19.6 g.L-1 caproate) and display good extraction for current densities up to
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100 A/m2 (Figure S2), though caproate did not show efficiency equal to acetate at a current
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density of less than 30 A.m-2. This demonstrates the importance of the molecular availability
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for flux (molar concentration), as opposed to the mass concentration in the bulk.
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The coulombic efficiency in ME can be managed by adjusting the salinity of the
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anolyte. An applied current must be balanced by an equal transport of charge across the
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membrane. This is demonstrated experimentally in Figure 3 with two different membranes,
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the Membrane International AMI-7001(MI) and the proton-blocking FumaTech fumasep FAB
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(FB), and represented in the illustration (Figure 3; A, B and C). The targeted circuit for
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acetate extraction in ME is represented by (Fig 3A), with the applied current balanced by the
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flux of acetate anions. High anolyte salinity promotes diffusion across the membrane against
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the potential gradient as in (Fig 3B). Diffusion of anionic species from the anolyte to the
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catholyte will be balanced by the transport of an equal charge in the opposite direction, such
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as acetate from the catholyte to the anolyte, or an opposite charge in the same direction, such
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as proton flux from the anolyte to the catholyte (Fig 3C). Low coulombic efficiency is a result
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of acetate (Fig 3A) competing with other ions in the broth or proton diffusion from the
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anolyte to the catholyte (Fig 3C). With the FB membrane, acetate flux across the membrane
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increases as the salt gradient increases due to diffusion of sulfate from the anolyte to the
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catholyte to maintain the charge balance, as in (Fig 3B). Acetate flux does not increase for
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MI, as this membrane is more prone to proton flux and the charge imbalance caused by sulfate
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diffusion drives protons across the membrane, as in (Fig 3C), rather than the targeted acetate.
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This result highlights the importance of membrane selection and management of the anolyte
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(salinity and pH) for greater efficiency.
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Net acetate flux from the catholyte to the anolyte continues despite acetic acid
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accumulation in the anode (Figure S3). Accumulation of the organic product is possible by the
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low pH of the anolyte, driven by the local electrolytic production of protons. The acetate is
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protonated and the uncharged acidic product does not pass back across the anion exchange
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membrane, if the system is sufficiently resistant to diffusion. However, the acetate/acetic acid
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equilibrium will result in some portion of the product remaining in ionic form and will impact
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the net transfer of acetate. This can be seen in the inverse relationship between acetate flux
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and acetic acid concentration in Figure S3. In addition to concentration gradient, this will also
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be a function of membrane properties and applied current.
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Up to 1.07 kg.d-1 of sodium hydroxide equivalent can be generated electrolytically in
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the cathode per m2 membrane by 30 A.m-2 of applied current, assuming total coulombic
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efficiency. This decreases the chemicals requirement for pH control in a fermentation process.
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However, inefficiencies in extraction can lead to an excess of hydroxide per molecule of
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product extracted, thus disproportionally increasing the pH. In a fully realized system, the pH
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correction aspect must be carefully managed.
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Extraction and valorization from thin stillage. We tested the ME on thin stillage of a
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bioethanol refinery, spiked to 5 g.L-1 sodium acetate (0.7 g.L-1 of acetate native to the broth).
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With equal volumes of anode and cathode compartments, 96 ± 12% of the acetate was
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transferred to the anolyte after 48 hours (Figure 4a). The flux through the AEM was 0.379
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kg/m2.d at 20 A.m-2, corresponding to a coulombic efficiency of 35.9 %. Acetate was
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extracted from the catholyte and accumulated in the anolyte as acetic acid at a linear rate. This
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result exhibits a good transference from a real matrix to the clean anolyte. The relatively low
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coulombic efficiency here is due to the competitive flux of other native anions in the broth
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and proton backflux. To mimic a more practical operation of ME, the cathode was
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continuously fed and the acetic acid accumulated in the anode in batch mode. Figure 4b
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presents the combined ME and BE experiment. The ME extraction had accumulated 14 g.L-1
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of acetic acid in the anolyte by the conclusion of the six-day experiment, and was slowly
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accumulating other native anions. Over time, these anionic species would reach equilibrium
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across the membrane, enabling an optimum net flux of acetate from cathode to anode (as in
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Figure 3, A and B). As the acetic acid concentration approached 12 g.L-1 the net flux began to
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decrease as the acetic acid product accumulated, demonstrating the need to remove the
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product.
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Aqueous acetic acid can be consumed and recovered as an ester with the BE step.
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Water is a product of carboxylate esterification, as follows: R-CO2H + R’-OH ↔ R-CO2-R’
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+ H2O, in the presence of an acid catalyst. The high salinity and low pH of the anolyte can
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support a phase transition of the acetic acid to the water-excluding phase where it is esterified
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with added alcohol and heat. This was tested with both synthetic and real anolyte using xylene
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as model solvent and vector for ester measurement. Gas chromatography and mass
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spectrometry confirmed the presence of methyl acetate and ethyl acetate, and the absence of
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esters in the control test with distilled water instead of alcohol. For the synthetic anolyte,
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acetic acid concentration fell by 26 ± 5 % for methyl acetate production, 10 ± 3 % for ethyl
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acetate, and no change in acetic acid was observed (0 ± 1 %) where only water was added,
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after taking dilution and transference to the solvent layer into consideration. The same process
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was applied to the real anolyte collected at t = 144 of the continuous cathode / batch anode
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experiment, as shown in Figure 4b. The drop in acetic acid concentration was higher for the
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true anolyte, at 58 ± 3% for the production of ethyl acetate and 65 ± 2% for methyl acetate.
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This is likely due to improved catalysis by the lower pH in the real anolyte, measured at pH
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0.94 compared to the synthetic anolyte’s pH 2, as the batch was under electrolytic conditions
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for 144 h prior to the esterification. Acetic acid consumption proceeded rapidly in the initial
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period, at 2.82 g.L-1.h-1 over the first hour during BE with ethanol (Figure S3, A). This is
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equivalent to a maximum esterification rate calculated at 4.14 g.L-1.h-1 in the first hour of the
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reaction (per g of ethyl acetate per L anolyte, assuming complete stoichiometric conversion).
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The presences of ethyl acetate and methyl acetate in the respective reactions were confirmed
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by GC-MS but could not be quantified due to high decomposition in the xylene between
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sampling and GC-MS measurement. With respect to the reaction of other organic species,
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butyrate was also present in the broth, and butyric acid also accumulated in the anolyte up to
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0.56 g.L-1. This butyric acid showed 37 ± 1% conversion in the BE step after six hours
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(Figure S3, B). Although the target species demonstrated a high rate of esterification, the
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reaction of side-species may present a challenge in an applied system when aiming to produce
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high purity esters.
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Effective recovery of esters ideally requires a water excluding phase that has a low
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affinity for the product and low volatility to allow for a simple separation and regeneration of
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the water excluding phase. Xylene was used as the experimental model solvent as it is water
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excluding, relatively low cost and can be analyzed for esters in a GC-MS, however its
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physical properties would be non-ideal in a fully realized system due to its relatively high
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volatility. The biphasic esterification can be enabled by an ionic liquid water exclusion layer,
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and the resulting esters could be extracted from this layer by applying an under-pressure.
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Ionic liquids have been previously demonstrated to enable esterification15 and therefore an
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ionic liquid was introduced in a unique setup to demonstrate its capacity to enable BE with
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the real anolyte. The production of ethyl acetate was observed at up to 2.05 g (ethyl
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acetate).L-1.h-1 and a final concentration of 14 g.L-1 ethyl acetate (Figure S5). This
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demonstrates the capacity for the BE to be driven with an ionic liquid layer. The production
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rate is lower than the pure xylene esterification, as this experiment was performed in different
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equipment with an adjusted method for rigorous quantification of the ethyl acetate in the
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ester-trapping xylene, rather than rate optimization.
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DISCUSSION
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Energy and fuel production in a biorefinery context have been successful due to
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fermentation processes that generate energy-dense products such as ethanol, and biogas
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production by anaerobic digestion. Anaerobic digestion (AD) has been successful as the first
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microbial technology dealing with waste biomass and is applied worldwide in the treatment of
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organic rich streams. As a bio-production technology, anaerobic digestion is not an
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economically strong case in itself, as considering a typical power production of approximately
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1 kWh per kg biomass converted and a market price of 0.1 EUR, only 100 EUR is generated
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per ton biomass converted to biogas. The main benefit of AD in most cases is the reduction of
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waste such as sludge and manure. In the current economic environment, electrical power
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remains a low value product and further, one can argue that the final destination of carbon
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should preferentially be embedded in industrial bio-chemicals, rather than returned to the
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atmosphere as carbon dioxide. However, bio-production of chemicals is still in an
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overwhelmingly one-sided competition with petrochemical production and therefore often
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also dependent on subsidies. Thanks to next-generation biofuels and bioplastics, the industry
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is beginning to find its footing. Advances in mixed culture fermentation now enables
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successful production of organic acids such as poly-hydroxybutyrate and succinate.16 These
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products can be generated from renewable waste or low cost biomass and have a higher value
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than biogas and ethanol, and yet expensive processes for recovery of the final product still
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contributes to the rarity of economically sound biorefineries.1
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Here we have presented an alternate pipeline for a sustainable bio-production route
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that extracts and valorizes short chain carboxylates, though this valorization must be sufficient
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to overcome the cost of extraction. At 10 g.L-1 the power cost of extraction is calculated at 6.2
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MJ per kg acetate based on this study, which corresponds to 1.27 kWh per kg of acetic acid
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produced. Considering an energy cost of 0.1 EUR per kWh and an acetic acid value of 0.5
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EUR per kg, there is a margin of 0.373 EUR per kg to deal with other operation costs and
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write-off. At the highest current tested for 10 g.L-1 acetate, (100 A/m², 65% CE) the power
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input is 24 MJ per kg Acetate/m², achieving a flux three times higher for four times more
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power input. This demonstrates that the ME extraction would ideally fit a high production
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rate, high concentration fermentation, which will be important when optimizing operation
331
costs against unit size and investment cost. For ethyl acetate in the BE step, for 1 kg of acetic
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acid at 0.5 EUR per kg, the reaction requires 0.8 kg ethanol at 0.4 EUR per kg to produce 1.5
333
kg ethyl acetate at 0.9 EUR per kg, stoichiometrically. From an input value of 0.82 EUR (0.5
334
EUR, 1kg of acetic acid and 0.32 EUR, 0.8 kg ethanol) this translates to a product value of
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1.35 EUR. This is an attractive valorization for an effective and cost-optimized BE step.
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Though only a superficial economic assessment is practical at this stage, the presented case is
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sufficiently attractive to further explore the potential of this bio-production pipeline.
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Projecting forward to a fully realized system, the ME and BE pipeline is conceptually
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non-invasive and would require minimal chemical intervention. The electrolysis of the water
340
replaces caustic and acid dosing in the broth and concentrate streams, respectively. After the
341
ME and BE process, the organic fraction that was unsuitable for fermentation could, in
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principal, be digested and combusted to reduce the remaining solid fraction and help drive the
343
applied current, while ethanol for esterification could be sourced from separate fermentations
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within an integrated biorefinery. Ionic liquids, though currently expensive, tend to be stable
345
and can be regenerated, but this must be assessed within the specific bounds of this process.
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The anode (in this study, iridium oxide coated titanium) is also an expensive feature, but is
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active in a clean, saline, acid medium under relatively mild electrical duress. The cathode that
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interacts with the broth is inexpensive stainless steel. Other process engineering challenges of
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the ME and BE pipeline include power management, reactor design, heat management, bio-
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fouling and possible degradation of the AEM by the alcohol and/or water excluding phase,
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and not least of all, optimization of the fermentation process.
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The feed and the fermentation profile (eg target product, production rate,
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concentration, specificity, and more) is integral to the performance of the ME and BE
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pipeline. On feedstock such as sludge and municipal solid waste, the carboxylate
355
concentrations typically do not exceed ~17 g.L-1 COD,17 which is in excess of the
356
concentration range demonstrated in this study. However, waste, residue or other low value
357
side-streams from existing biorefineries such as sugar or bio-ethanol production still contain
358
high amounts of organics which could be further processed for chemicals production through
359
the carboxylate platform at potentially high concentrations and rates. An ideal situation would
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see such streams optimized for high rate production of valuable carboxylates for ME
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extraction. Mixed culture fermentations can produce higher value products such as caproate
362
(C6) and caprylate (C8) by microbial chain elongation. In this process, acetate is elongated to
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butyrate, and in a similar cycle butyrate is elongated to caproate.6, 18 Though caproate is a
364
higher value product, the commercial viability for biological caproate production remains
365
unproven due to the high cost of separation relative to product value. The extraction of such a
366
product is a challenging candidate for the ME and BE pipeline, as the substrates (acetate,
367
butyrate) also traverse the AEM, as demonstrated in this study. However, if the chain
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elongation can be steered to generate caproate at a greater concentration relative to the
369
substrates, then the ME step becomes particularly attractive as the concentration of the target
370
compound is linked to the extraction efficiency, and the relative concentration is linked to the
371
extraction selectivity. Alternatively, ME could generate an acetic and butyric acid concentrate
372
that could conceptually be used as substrate in a separate chain elongation, or other biological
373
reaction, as the AEM should exclude non-ionic inhibiting compounds such as furfural and
374
phenolic compounds.19, 20 Similarly, another opportunity is to use the technology to remove
375
toxic acetate from ethanol fermentation, simultaneously solving a toxicity issue while
376
recovering acetic acid. However such processes can only be judged with greater knowledge of
377
economic viability and product selectivity.
378
Targeted, selective extraction is a defining feature of a high performing separation
379
system. Harvesting a specific product from a low titer, mixed-species broth is a seminal
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challenge in biorefinery separation technology, even with high value products.21 For a review
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of separation technologies in biorefineries, see Huang et al., 2008.19 In its current incarnation,
382
the ME step has not been demonstrated to selectively extract specific molecules, although the
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flux profile and coulombic efficiency clearly varies for acetate, butyrate and caproate within
384
the range applied currents tested here (Figure S1 and S2). By extension, it is possible that
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selectivity can be manipulated through membrane development and/or applied current scheme
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relative to, for example, the carbon chain length. However, a size exclusion model is highly
387
unlikely to represent the electrochemical transport of the carboxylates, as many other
388
parameters come in to play including diffusion in the bulk electrolyte, the membrane surface
389
interface (eg the molecular arrangement of the double layer), interaction of the ion with bulk
390
membrane (both the polymer backbone and charged group), among others. Indeed, it is
391
known that the performance of electrodialysis is highly dependent on the intrinsic properties
392
of the AEM.20 A deeper understanding of the electrochemical transport properties of organic
393
molecules with respect to the electrolytic membrane extraction process will reveal the
394
potential for selective extraction of, for example, caproate rather than acetate or butyrate.
395
With respect to BE, opportunities for improved selectivity and yield exists in a deeper
396
knowledge of the reaction kinetics, operating conditions and the ionic liquid’s inherent
397
properties (solubility, stability, catalytic activity, etc). Ionic liquids have been demonstrated to
398
drive esterification and organic recovery even at room temperature,14, 21 and their participation
399
in the reaction also remains open to research and optimization. The ionic liquids used in this
400
study are ideal candidates for separating volatile organic compounds since ionic liquids have a
401
very low vapor pressure by definition and those used in this study are not water miscible. The
402
ionic liquids have a good thermal stability and can be heated. In a fully realized system,
403
applying a reduced pressure (vacuum) on the system lowers the boiling point of the esters so
404
that only the esters volatilize, and the esters are then passed through a condenser and captured
405
as a liquid product.
406
Other than upgrading waste streams, the ME process is also interesting in cutting edge
407
industrial biotechnology. For example, microbial electrosynthesis has been shown to generate
408
carboxylates from carbon dioxide and electricity.22, 23 The electromotive force generated by
409
delivering electricity to the microorganisms can theoretically be used to drive products out of
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the biocathode to the anode in the ME step. Continuous in situ extraction directly from the site
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of the reaction could potentially increase the production rate by preventing product
412
accumulation. Microbial electrosynthesis at present generates products in the more neutral pH
413
range at relatively low titers, as with mixed culture fermentation. For a product such as acetate
414
additional steps to recover the product would be required. Distillation, for example, would
415
require an acidification step to convert the acetate to acetic acid and a high energy input. The
416
ME approach may lower the input chemical and energy demands for product recovery and
417
improve production rates. Rigorous economic assessment, process development and selective extraction is
418 419
required to bring the ME and BE pipeline forward. Though biorefineries may be uniquely
420
poised to take advantage of this process due to chemical production infrastructure that
421
traditional wastewater treatment facilities may lack, this work has presented an attractive
422
alternative for biomass that would otherwise be destined for combustion; generating products
423
instead of combusting waste.
424 425
ASSOCIATED CONTENT
426
Supporting Information
427
Description of GC Technique, two tables and five figures. This material is available free of
428
charge via the Internet at http://pubs.acs.org
429 430
AUTHOR INFORMATION
431
Corresponding Author
432
* Phone: + 32 (0) 9/264 59 76. Fax: + (0) 9/264 62 48. Email:
[email protected] 433
Notes
434
The authors declare no competing financial interest.
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ACKNOWLEDGEMENTS
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SA and TH are supported by the European Union Framework Programme 7 project
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“ProEthanol 2G.” SA, TH, MC and KR are supported by Ghent University Multidisciplinary
439
Research Partnership (MRP)—Biotechnology for a sustainable economy (01 MRA 510W).
440
TH is also supported by a postdoctoral fellowship from the Research Foundation Flanders
441
(FWO Vlaanderen). SG is funded by the Special Research Fund (BOF) of the University of
442
Ghent (Belgium). JD is supported by the Institute for the Promotion and Innovation through
443
Science and Technology in Flanders (IWT-Vlaanderen, SB-091144). JT is supported by the
444
IWT-Vlaanderen. KR is supported by European Research Council Starter Grant
445
ELECTROTALK. We acknowledge Alco Bio Fuel nv for generously providing thin stillage.
446
We thank Bram Wuyts for preliminary esterification work and Michail Syrpas for technical
447
assistance with the GC-MS. Tim Lacoere is warmly thanked for his graphical contribution.
448
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REFERENCES
450 451
1.
Zhang, Y; Zhu, Y; Zhu, Y; Li, Y; The importance of engineering physiological
452
functionality into microbes, Trends Biotechnol. 2009 27 (12), 664-672
453 454
2.
Temudo, M.F; Muyzer, G; Kleerbezem, R; van Loosdrecht, M.C; Diversity of
455
microbial communities in open mixed culture fermentations: impact of the pH and carbon
456
source, Appl. Microbiol. Biotechnol. 2008 80 (6), 1121-1130
457 458
3.
Holzapple, M.T; Granda C.B; (2009) Carboxylate Platform: The MixAlco Process
459
Part 1: Comparison of Three Biomass Conversion Platforms. Appl Biochem Biotechnol, 2009
460
156 (1-3), 525-536
461 462
4.
Agler, M.T; Wrenn, B.A; Zinder, S.H; Angenent, L.T; Waste to bioproduct conversion
463
with undefined mixed cultures: the carboxylate platform, Trends Biotechnol. 2011, 29 (2),
464
70-78
465 466
5.
Latif, H; Zeidan, A.A; Nielsen, A.T; Zengler, K; Trash to treasure: production of
467
biofuels and commodity chemicals via syngas fermenting microorganisms. Current Opinion
468
in Biotechnology. 2014, 27 (0), 79-87
469 470
6.
Bechtold, I; Bretz, K; Kabasci, S; Kopitzky, R; Springer, A; Succinic Acid: A New
471
Platform Chemical for Biobased Polymers from Renewable Resources, Chem. Eng. Technol.
472
2008, 31 (5), 647-654
473
20 ACS Paragon Plus Environment
Page 20 of 27
Page 21 of 27
Environmental Science & Technology
474
7.
Lyko, H; Deerberg, G; Weidner, E; Coupled production in biorefineries - Combined
475
use of biomass as a source of energy, fuels and materials, J. Biotechnol,2009, 142 (1), 78-86
476 477
8.
Joyce, B.L; Stewart Jr, C.N; Designing the perfect plant feedstock for biofuel
478
production: Using the whole buffalo to diversify fuels and products, Biotechnol. Adv. 2012,
479
30 (5), 1011-1022
480 481
9.
Dohno, R; Azumi, T; Takashima, S; Permeability of mom-carboxylate ions across an
482
anion exchange membrane, Desal. 1975, 16 (1), 55-64
483 484
10.
Saxena, A; Gohil, G.S; Shahi, V.K; Electrochemical Membrane Reactor: Single-Step
485
Separation and Ion Substitution for the Recovery of Lactic Acid from Lactate Salts, Ind. Eng.
486
Chem. Res. 2007, 46, 1270-1276
487 488
11.
Baker, RW; Ion Exchange Membrane Processes – Electrodialysis, in Membrane
489
Technology and Applications, Third Edition, John Wiley & Sons, Ltd.: Chichester, U.K., 2012
490 491
12.
Liu, Y; Lotero, E; Goodwin Jr, J.G; Effect of carbon chain length on esterification of
492
carboxylic acids with methanol using acid catalysis, J. Mol. Catal. A. Chem, 2006 243 (2),
493
132-140
494 495
13.
Desloover, J; Woldeyohannis, A.A; Verstraete, W; Boon, N; Rabaey, K;
496
Electrochemical Resource Recovery from Digestate to Prevent Ammonia Toxicity during
497
Anaerobic Digestion, Environ. Sci. Technol, 2012 46 (21), 12209-12216
498
21 ACS Paragon Plus Environment
Environmental Science & Technology
499
14.
Dlugolecki, P; Anet, B; Metz, SJ; Nijmeijer, K; Wessling, M; Transport limitations in
500
ion exchange membranes at low salt concentrations, J Membr Sci 2010 346:163 – 171
501 502
15.
Izak, P; Mateus, N.M.M; Afonso, C.A.M; Crespo, J.G; Enhanced esterification
503
conversion in a room temperature ionic liquid by integrated water removal with
504
pervaporation, Sep. Pur. Tech., 2005 41 (2), 141 – 145
505 506
16.
Sauer, M; Porro, D; Mattanovich, D; Branduardi, P; Microbial production of organic
507
acids: expanding the markets, Trends Biotechnol, 2008 26 (2), 100-108
508 509
17.
Pratt, S; Liew. D; Batstone, D.J; Werker, A.G; Morgan-Sagastume, F; Lant, P.A;
510
Inhibition by fatty acids during fermentation of pre-treated waste activated sludge J.
511
Biotechnol, 2012 159 (1-2), 38-43
512 513
18.
Steinbusch KJJ, Hamelers H.V.M; Plugge, C.M; Buisman, C.J.N; Biological
514
formation of caproate and caprylate from acetate: fuel and chemical production fromlow
515
grade biomass, Energy Environ. Sci., 2011 4, 216 - 224
516 517
19.
Palmqvist, E; Grage, H; Meinander, NQ; Hahn-Hägerdal, B; Main and interaction
518
effects of acetic acid, furfual, and p-hydroxybenzoic acid on growth and ethanol productivity
519
of yeasts, Biotechnol Bioeng 1999 63(1): 46 – 55
520 521
20.
Palmqvist, E; Hahn-Hägerdal, B; Fermentation of lignocellulosic hydrolysates. I:
522
inhibition and detoxification, Biores Technol 2000 74: 17 – 24
523
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Page 23 of 27
Environmental Science & Technology
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21.
Kurzrock. T; Weuster-Botz. D; Recovery of succinic acid from fermentation broth,
525
Biotechnol. Lett. 2010 32 (3), 331-339
526 527
22.
Huang, H.J; Ramaswamy, S; Tschirner, U.W; Ramarao, B.V; A review of separation
528
technologies in current and future biorefineries, Sep Purif Technol,2008 62 (1), 1-21
529 530
23.
Strathmann H, Membranes and Membrane Separation Processes, Ullmann’s
531
Encylcopeadia of Industrial Chemistry, Wiley, Germany, 2005
532 533
24.
Poole, C.F; Poole, S.K; Extraction of organic compounds with room temperature ionic
534
liquids, J. Chromatog. A., 2010 1217 (16), 2268-2286
535 536
25.
Logan, B.E; Rabaey, K; Conversion of Wastes into Bioelectricity and Chemicals by
537
Using Microbial Electrochemical Technologies, Science, 2012 337 (6095), 686-690
538 539
26.
Lovley, D; Nevin, K.P; Electrobiocommodities: powering microbial production of
540
fuels and commodity chemicals from carbon dioxide with electricity Curr. Op. Biotech 2013.,
541
24 (3), 1-6
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Figures
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548 549
Figure 1. Schematic for carboxylate extraction from a fermenter by membrane electrolysis
550
and biphasic esterification. Fermentation generates short chain carboxylates which are driven
551
across the anion exchange membrane by an applied current and are protonated. Alcohol and
552
heat reacts with the carboxylic acid in the water exclusion phase to generate esters.
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555 556
Figure 2. Extraction profile for acetate across a Membrane International AMI-7001 anion
557
exchange membrane (MI) from a starting catholyte concentration of 1, 5 and 10 g/L acetate.
558
(A) Mass flux per area of membrane per day, (B) Coulombic efficiency for the same data
559
points. The error bars are occasionally obscured by the medallions.
560
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562 563
Figure 3. Coulombic efficiency (molar flux of acetate / moles of applied current) can be
564
improved with membrane selection and diffusion of anions against the applied current.
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Experimental changes in efficiency of two different anion exchange membranes versus
566
anolyte concentration of Na2SO4 (left). Illustration of the mechanism (right).
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569 570
Figure 4. Extraction of acetate from a biorefinery thin stillage at 5 g.L-1 acetate. (A) Batch
571
extraction with equal volumes of catholyte and anolyte. (B) Continuously fed waste stream
572
(catholyte) with batch operation of acid concentrate (anolyte), with biphasic esterification at
573
144 h. See Figure S4 for the acetic acid concentration in the first 6 h of the esterification.
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