Electrolytic Membrane Extraction Enables Production of Fine

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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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Environmental Science & Technology

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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]

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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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1 ACS Paragon Plus Environment


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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

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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

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kg ethyl acetate at 0.9 EUR per kg, stoichiometrically. From an input value of 0.82 EUR (0.5

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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.

336

Though only a superficial economic assessment is practical at this stage, the presented case is

337

sufficiently attractive to further explore the potential of this bio-production pipeline.

338

Projecting forward to a fully realized system, the ME and BE pipeline is conceptually

339

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

342

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

344

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.

346

The anode (in this study, iridium oxide coated titanium) is also an expensive feature, but is

347

active in a clean, saline, acid medium under relatively mild electrical duress. The cathode that

348

interacts with the broth is inexpensive stainless steel. Other process engineering challenges of

349

the ME and BE pipeline include power management, reactor design, heat management, bio-

350

fouling and possible degradation of the AEM by the alcohol and/or water excluding phase,

351

and not least of all, optimization of the fermentation process.

352

The feed and the fermentation profile (eg target product, production rate,

353

concentration, specificity, and more) is integral to the performance of the ME and BE

354

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



360

see such streams optimized for high rate production of valuable carboxylates for ME

361

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

363

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

368

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

380

challenge in biorefinery separation technology, even with high value products.21 For a review

381

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

383

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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385

selectivity can be manipulated through membrane development and/or applied current scheme

386

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



410

the biocathode to the anode in the ME step. Continuous in situ extraction directly from the site

411

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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435 436

ACKNOWLEDGEMENTS

437

SA and TH are supported by the European Union Framework Programme 7 project

438

“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



449

REFERENCES

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Holzapple, M.T; Granda C.B; (2009) Carboxylate Platform: The MixAlco Process

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Platform Chemical for Biobased Polymers from Renewable Resources, Chem. Eng. Technol.

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Chem. Res. 2007, 46, 1270-1276

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Baker, RW; Ion Exchange Membrane Processes – Electrodialysis, in Membrane

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Technology and Applications, Third Edition, John Wiley & Sons, Ltd.: Chichester, U.K., 2012

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Liu, Y; Lotero, E; Goodwin Jr, J.G; Effect of carbon chain length on esterification of

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carboxylic acids with methanol using acid catalysis, J. Mol. Catal. A. Chem, 2006 243 (2),

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Desloover, J; Woldeyohannis, A.A; Verstraete, W; Boon, N; Rabaey, K;

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Dlugolecki, P; Anet, B; Metz, SJ; Nijmeijer, K; Wessling, M; Transport limitations in

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ion exchange membranes at low salt concentrations, J Membr Sci 2010 346:163 – 171

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conversion in a room temperature ionic liquid by integrated water removal with

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pervaporation, Sep. Pur. Tech., 2005 41 (2), 141 – 145

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acids: expanding the markets, Trends Biotechnol, 2008 26 (2), 100-108

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Biotechnol, 2012 159 (1-2), 38-43

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formation of caproate and caprylate from acetate: fuel and chemical production fromlow

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Palmqvist, E; Hahn-Hägerdal, B; Fermentation of lignocellulosic hydrolysates. I:

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Strathmann H, Membranes and Membrane Separation Processes, Ullmann’s

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Logan, B.E; Rabaey, K; Conversion of Wastes into Bioelectricity and Chemicals by

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Using Microbial Electrochemical Technologies, Science, 2012 337 (6095), 686-690

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Lovley, D; Nevin, K.P; Electrobiocommodities: powering microbial production of

540

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24 (3), 1-6

542 543 544 545



546

Figures

547

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.

553


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554

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



561

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.

565

Experimental changes in efficiency of two different anion exchange membranes versus

566

anolyte concentration of Na2SO4 (left). Illustration of the mechanism (right).

567


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568

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.

574


Electrolytic Membrane Extraction Enables Production of Fine

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