Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
Vinyl Addition Copolymers of Norbornylnorbornene and Hydroxyhexafluoroisopropylnorbornene for Efficient Recovery of n‑Butanol from Dilute Aqueous Solution via Pervaporation Beom-Goo Kang, Dong-Gyun Kim, and Richard A. Register* Department of Chemical and Biological Engineering, Princeton University, Princeton, New Jersey 08544, United States S Supporting Information *
ABSTRACT: The high energy cost to recover heavier alcohols, such as n-butanol, from dilute aqueous solution is a significant practical barrier to their large-scale bioproduction. Membrane pervaporation offers an energy-efficient alternative, provided membrane materials can be developed which provide both good alcohol selectivity and high flux. Previous work has revealed that vinyl addition polynorbornenes bearing substituentsespecially hydroxyhexafluoroisopropylwith an affinity for n-butanol have potential in this application, as their high glass transition temperature allows the formation of thin but mechanically robust selective layers in thin-film composite (TFC) membranes. In the present work, we synthesize both microphase-separated gradient copolymers, and homogeneous random copolymers, of hydroxyhexafluoroisopropylnorbornene (HFANB) with norbornylnorbornene (NBANB) and evaluate their n-butanol/water pervaporation performance. Compared with analogous copolymers of HFANB and n-butylnorbornene (BuNB), the greater n-butanol permeability and selectivity of PNBANB vs PBuNB lead to a more-than-2-fold increase in membrane selectivity for n-butanol transport; the best HFANB−NBANB copolymers show n-butanol selectivities and fluxes which compare favorably with those of the best commercial TFC membranes, which contain cross-linked polydimethylsiloxane selective layers. Moreover, vinyl addition copolymers offer a straightforward route to further flux enhancement, simply by reducing the selective layer thickness.
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INTRODUCTION The energy-efficient recovery of alcohols from dilute aqueous solution is an essential step in the practical production of liquid biofuels. This is especially true for alcohols heavier than ethanol, such as n-butanol,1,2 that are considered as potential “advanced biofuels”; these heavier alcohols are more highly toxic than ethanol to the organisms which produce them,3 and consequently the fermentation broth must be maintained dilute in the alcohol product, typically ∼1 wt % (∼10 g/L) for nbutanol.4 The high energy input required to recover such a dilute component by distillation motivates the search for alternative separation processes, such as membrane pervaporation,5−7 where the liquid feed contacts one side of the membrane, and the permeate is withdrawn as a vapor and condensed downstream. The current state-of-the-art commercial membranes for recovery of n-butanol from dilute aqueous solutions are thinfilm composite membranes containing a thin dense “skin” layer of cross-linked polydimethylsiloxane (PDMS) coated onto a thicker porous membrane which provides mechanical support.8−10 PDMS is modestly selective for butanol over water (membrane selectivity α in the range of 1−25,8,10,11), but its low glass transition temperature (Tg) and consequent low modulus and strength mean that the selective layer must be relatively thick, adversely affecting transmembrane flux. Recently, vinyl addition polynorbornenes bearing various substituents, notably hexafluoroisopropanol (HFA), have been investigated as pervaporation membranes for n-butanol recovery.12,13 Vinyl addition polynorbornenes14,15 have a saturated backbone which © XXXX American Chemical Society
retains the bicyclic norbornene structure in every repeat unit, leading to exceptionally high values of Tg (385 °C for the unsubstituted polynorbornene16). Although Tg will be reduced somewhat by plasticization (principally by n-butanol) during pervaporation, the high Tg of the neat polymers ensures that they remain glassy during operation. Consequently, even very thin selective layers can be mechanically robust. The homopolymer (PHFANB) of the HFA-substituted norbornene (HFANB) monomer is soluble in n-butanol but insoluble in water,13 providing the basis for the requisite nbutanol selectivity: differences in solubility between n-butanol and water in PHFANB, in a separation based on the usual solution-diffusion process for dense membranes.17 (The much larger molecular size of butanol vs water virtually guarantees that the “diffusion” aspect of the process will favor water permeation over butanol, so a large solubility difference is essential; even with the solubility difference, PHFANB homopolymer shows a membrane selectivity α = 0.53, i.e., it selectively transports water over n-butanol.13) To tailor the physical properties of the membranes, our group recently synthesized and evaluated a series of block copolymers, and a series of random copolymers, of HFANB with n-butylnorbornene (BuNB).13 PBuNB homopolymer shows both poor flux and separation for n-butanol/water mixtures, but its insolubility in both water and n-butanol provided a convenient and effective Received: March 4, 2018 Revised: April 26, 2018
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DOI: 10.1021/acs.macromol.8b00470 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
vial containing a magnetic stirring bar, 0.15 mL of a 0.50 M solution of MePd(i-Pr t-Bu2P)Cl in TFT (26 mg, 0.075 mmol) and 0.15 mL of a 0.50 M solution of Li[FABA] in TFT (65 mg, 0.075 mmol) were reacted for 20 min to activate the Pd proinitiator. 0.20 mL of the MePd(i-Pr t-Bu2P)Cl/Li[FABA] solution (0.050 mmol each of MePd(i-Pr t-Bu2P)Cl and Li[FABA]) was rapidly injected into the flask containing the NBANB solution with vigorous stirring. The reaction was allowed to proceed for 48 h, after which the solution was poured into a large amount of methanol to precipitate the resulting polymer. The polymer was recovered by filtration and dried under vacuum at 60 °C. Synthesis of NBANB−HFANB Gradient Copolymers. Vinyl addition gradient copolymers were synthesized via a batch reaction of the mixed monomers at room temperature in the same glovebox. The gradient copolymers are designated hereafter as GCP#, where “GCP” indicates “gradient copolymer” and “#” indicates the wt % of HFANB in the final polymer. The initial monomer mixture concentration was 5 wt %. The following procedure was used to prepare GCP83: a mixture of NBANB (0.75 g, 4.0 mmol), HFANB (3.3 g, 12 mmol), and toluene (81 g) was placed into a 250 mL round-bottomed flask with a magnetic stirring bar. To a scintillation vial with a magnetic stirring bar, 0.12 mL of a 0.50 M solution of (η3-allyl)Pd(i-Pr3P)Cl in TFT (21 mg, 0.060 mmol) and 0.12 mL of a 0.50 M solution of Li[FABA] in TFT (52 mg, 0.060 mmol) were reacted for 20 min to activate the Pd proinitiator. 0.16 mL of (η3-allyl)Pd(i-Pr3P)Cl/Li[FABA] solution (0.040 mmol each of (η3-allyl)Pd(i-Pr3P)Cl and Li[FABA]) was rapidly injected into the flask containing the monomer mixture solution under vigorous stirring. The polymerization was allowed to proceed for 3 h, and the reaction solution was poured into a large amount of methanol/water (50/50 v/v) to precipitate the resulting copolymer. Synthesis of HFANB−NBANB Random Copolymers. Vinyl addition random copolymers were synthesized similarly to the GCPs, but with slow dropwise addition of the monomer mixture. The random copolymers are designated as RCP#, where “RCP” indicates “random copolymer” and “#” indicates the wt % of HFANB in the polymer. The following procedure was used to prepare RCP80: 0.16 mL of (η3-allyl)Pd(i-Pr3P)Cl/Li[FABA] solution (0.040 mmol each of (η3-allyl)Pd(i-Pr3P)Cl and Li[FABA]) was first injected into a 250 mL round-bottomed flask containing toluene (25 g). A mixture of NBANB (0.75 g, 4.0 mmol), HFANB (3.3 g, 12 mmol), and toluene (56 g) was placed into a dropping funnel, and the monomer mixture solution was added dropwise to the flask (15 s per drop) with vigorous stirring of the reaction medium. The addition of monomer solution proceeded over more than 10 h. Polymer Characterization Techniques. Monomer conversions were obtained by 1H NMR spectroscopy on reaction aliquots diluted in CDCl3, and copolymer compositions were determined by quantitative 13C NMR spectroscopy in THF-d8,24 both using a Bruker AVANCE III 500 MHz spectrometer (see Supporting Information Figure S1, for a representative copolymer 13C NMR spectrum). Weight-average molecular weights (Mw) and dispersities (Đ) of the polymers were determined by gel permeation chromatography (GPC) in tetrahydrofuran (THF), using a Waters 515 HPLC pump, two 30 cm Agilent PLgel Mixed-C columns operating at 35 °C, and Wyatt OptiLab T-rEX (differential refractive index, RI) and miniDAWN TREOS (three-angle light scattering, LS) detectors, both operating at 658 nm and 25 °C. Mw values were taken from the light scattering data via the Wyatt Astra software (averaging over the elution time distribution); the specific refractive index increment (dn/dc) for each GCP or RCP was calculated as the weight-fraction-weighted25 average of the dn/dc values of the corresponding homopolymers (0.1565 mL/ g for PNBANB and 0.0437 mL/g for PHFANB24), using the weight fractions determined from 13C NMR spectroscopy. The values of Đ were calculated from the GPC-RI elution time distribution, calibrated with narrow-distribution polystyrene standards. Prior to injection into the GPC, polymers were dissolved in THF, stirred over activated charcoal for 2 h, and filtered through an alumina plug to remove Pd residue.
means of controlling membrane swelling. In the block copolymers, one might imagine that the PBuNB domains simply act as impenetrable obstacles to transport, such that the flux is reduced but the selectivity is unaffected by BuNB incorporation;18 for the random copolymers, one might expect both the flux and selectivity to drop with BuNB incorporation. But surprisingly, for BuNB contents up to ∼20 wt %, selectivity actually increased with BuNB content, by up to ∼1.5× (α = 0.82).13 (Flux decreased monotonically with increasing BuNB content, as expected.13) This result reflects the reduced swelling in the presence of BuNB counits; the less-swollen copolymer membranes are more selective than the more-swollen PHFANB homopolymer membrane. The pervaporation performances of BuNB-HFANB random vs block copolymers were similar, though the block copolymers showed a systematically better flux−selectivity balance (moderately higher flux at the same selectivity, or vice versa). Since the BuNB units clearly impact the transport behavior, one might ask whether the chemical identity of these counits could affect the pervaporation performance of HFANB-based membranes. Specifically, in collaboration with our industry partner, Promerus LLC, we hypothesized that replacing the nbutyl substituent with a rigid aliphatic unita norbornyl groupcould provide extra free volume7,19−21 and thereby enhance the transport of n-butanol by increasing its permeability (solubility and/or diffusivity) through the membrane. In the present work, we synthesized block-like (gradient) and random copolymers of HFANB with norbornylnorbornene (NBANB) and found that NBANB provides a much enhanced selectivity over BuNB; the best NBANB− HFANB copolymers show values of α approaching 2, i.e., as good as or better than commercial PDMS membranes, with a high flux (∼2000 g m−2 h−1) enabled by the ability to utilize thin permselective layers formed from these glassy polymers.
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EXPERIMENTAL SECTION
Materials. All monomers and Pd proinitiators were provided by Promerus LLC. NBANB (2,2′-bis(bicyclo[2.2.1]heptan-5-ene), 83/17 endo/exo attachment to the norbornenyl ring, endo/exo ratio for the norbornane ring ∼80/20 based on ratio for the vinylnorbornane precursor) was synthesized via high-temperature Diels−Alder reaction of cyclopentadiene (CPD, charged as the dimer, dicyclopentadiene, DCPD) with ∼80/20 endo/exo-vinylnorbornane.21 HFANB (2(bicyclo[2.2.1]hept-5-en-2-ylmethyl)-1,1,1,3,3,3-hexafluoropropan-2ol, 82/18 endo/exo, >99.5%) was purchased from Central Glass Co., Ltd. (Japan). Both monomers were degassed by freeze−pump−thaw cycles and stored over 3 Å molecular sieves in a nitrogen-filled MBraun UNIlab glovebox (
