NMR Investigation of the Fractionation of WaterâEthanol Mixtures with

Article pubs.acs.org/EF

NMR Investigation of the Fractionation of Water−Ethanol Mixtures with Starch and Its Cross-Linked Forms Leila Dehabadi and Lee D. Wilson* Department of Chemistry, University of Saskatchewan, 110 Science Place, Saskatoon, Saskatchewan, Canada S7N 5C9 S Supporting Information *

ABSTRACT: The development of low-cost materials and efficient methods for the fractionation of water and ethanol in binary mixtures is of great interest for food processing and biofuels production. Herein, we report a systematic study of a series of linear and branched starch-based biopolymers along with their modified forms via cross-linking with epichlorohydrin (EPI) at variable EPI composition. The fractionation properties of these adsorbent materials were studied in binary ethanol−water solutions and compared against those of neat solvents. 1H NMR spectroscopy was used to assess the binary solvent composition for the study of isotherms for the respective solvent components. The monolayer adsorption capacity (Qm) for starch and its cross-linked forms varied from 0.01 to 2.70 g·g−1 for water and ethanol in binary mixtures according to the Sips isotherm model. The fractionation selectivity [Rselectivity; Qm(W)/Qm(E)] of starch-EPI adsorbents for water (W) and ethanol (E) in binary mixtures ranges from 3.8 to 80. At saturative conditions in binary W−E mixtures, the Rselectivity isotherm parameter reveals unique solventselective uptake that depends on the amylose versus amylopectin composition and the EPI content of these cross-linked materials. The unique water uptake properties of starch and its cross-linked forms illustrate the role of textural properties and relative hydrophobic character of the polymer network based on the selective adsorption properties. Starch and its modified forms represent a promising class of adsorbent materials and a sustainable technology for the adsorptive-based fractionation of W−E binary mixtures.

1. INTRODUCTION Bioethanol is often produced from agricultural waste through fermentation and is considered as a clean and sustainable biofuel as a blending additive with conventional fossil fuels.1 A key challenge associated with bioethanol production is achieving more effective and economical methods for water (W) removal. Several techniques such as azeotropic distillation, extractive distillation, reactive distillation, and adsorptive distillation can be employed for the fractionation and enrichment of ethanol in binary water−ethanol (W−E) mixtures.2 Among these techniques, distillation is one of the commonly used separation methods employed by the fine chemicals industry. However, there are some disadvantages with conventional distillation that employ the use of azeotropes like benzene to separate ethanol−water mixtures, e.g., high input energy consumption and costly operational requirements. Distillation relies on well-defined differences in the volatility and temperature stability of the components for adequate multicomponent separation. Membrane-based pervaporation is gaining attention for the separation of labile liquid mixtures because of its safe operation, reduced energy footprint, and good efficiency.3 However, there is a persistent search for effective, green and environmentally friendly methods for the separation of water and ethanol in binary mixtures. Adsorption represents a relatively low-cost, low energy input, and promising method for water removal in such W−E mixtures,4 especially in the condensed phase at ambient conditions. Heterogeneous adsorption processes at the liquid−solid interface involve specific surface interactions that may result in favorable fractionation of multicomponent mixtures due to specific adsorbent−adsorbate interactions.5 © 2016 American Chemical Society

Recent efforts have been focused on the development of materials and methods to enrich the separation of W−E binary mixtures.6 Diverse types of industrial adsorbents such as zeolites, clays, and biomass adsorbents have been employed for water fractionation in W−E mixtures.4,7,8 Diatomaceous molecular sieves and zeolites are important types of inorganic dessicants because of their unique textural properties and extensive pore network structure. The lack of uniformity in size and tortuosity of various synthetic molecular sieves is a drawback that offsets the separation efficacy.9,10 In contrast, biomaterials and modified forms have some advantages over inorganic dessicants for the separation of liquid mixtures such as their variable morphology, good recyclability, and tunable adsorption properties. Starch is an abundant and sustainable biopolymer that has attracted attention as an adsorbent for the separation of W−E mixtures.11−13 Starch biopolymers possess unique structural features such as abundant hydrophilic groups with variable accessibility due to chain branching and adjustable morphology. Previous reports indicate that starch-based materials show evidence of solvent fractionation in the case of alcohol−water mixtures.11 Chemical modification of polysaccharides via chemical cross-linking is a strategy for modifying the adsorption properties of adsorbents such as cellulose.14,15 In the food processing industry, cross-linked starch represents a valuable material with properties suitable for the fractionation of water in mixtures.16 Cross-linking of starch results in chain branching and three-dimensional (3-D) Received: February 23, 2016 Revised: June 2, 2016 Published: June 16, 2016 5684

DOI: 10.1021/acs.energyfuels.6b00437 Energy Fuels 2016, 30, 5684−5692

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Energy & Fuels

with shaking (Poly Science, Dual Action Shaker) for 24 h at 160 rpm at ambient conditions to achieve equilibrium. Thereafter, the samples were phase separated by centrifuging (Precision Micro-Semi Micro Centricone, Precision Scientific Co.) at 1800 rpm for 1 h. Thereafter, 1 g of supernatant solution was mixed with a 0.1 g mixture containing tetrahydrofuran (THF) with deuterated dimethyl sulfoxide (DMSOd6, 99.9%, from Sigma-Aldrich). THF serves as an internal standard, and DMSO-d6 serves as the field-lock solvent, where the weight content of THF in DMSO-d6 was held constant for all measurements. All 1H NMR spectra were recorded using a wide-bore (89 mm) 11.7 T Oxford superconducting magnet system (Bruker Bio Spin Corp; Billerica, MA, United States) equipped with a 5 mm PaTx1 probe. NMR acquisition parameters were controlled using a SSSC 500 console and workstation running X WIN-NMR 3.5. Standard pulse programs utilized were available from the TopSpin 1.3 software. In this study, a quantitative NMR method (qNMR) was used to analyze the relative solvent composition water−ethanol binary solutions by independent calibration curves derived from various standard binary W−E solutions prepared at variable W−E composition (w/w; %). Each solution contained an analytical internal standard (THF) and a locking solvent (DMSO-d6). The recycle delay times were determined based on a previous T1 relaxation study of analogous binary W−E mixtures and their neat solvents. The equilibrium time of the adsorption process was evaluated by varying the contact times of 20 mg samples of adsorbent with the binary W−E mixtures. Quantitative determination of the respective solvent adsorbed at each mixing time was evaluated using 1H NMR spectroscopy under fully relaxed and steady-state conditions. The relaxation time (T1) of the pure solvents and their binary W−E mixtures was measured by the inversion−recovery method.15 2.3. Models and Equations. Adsorption isotherms were obtained by using eq 1 where Qe,i is the equilibrium sorption capacity (mg/g) of the adsorbent materials with the ith solvent component (E or W) in a mixed binary (W+E) solvent system; Co,i (mg/L) is the the weight (g) of the initial solvent concentration before adsorption of the ith component, and Ce,i is the weight-based equilibrium concentration after adsorption of the ith component (W or E). The composition of the respective solvent components (W or E) were determined by their respective weight, where Wsolution is the total weight of solution (W+E) and m is the weight of polymer adsorbent (g). The equilibrium concentration (Ce,i) of ethanol and water in the supernatant solutions were estimated from the corresponding 1H NMR peak areas for E and W, before and after sorption.

network structures that display enhanced swelling and gel formation in aqueous solution.17 Thus, cross-linked starch may serve as an improved adsorbent for water fractionation in food and biofuels processes for condensed phases. However, there are sparse reports in the literature describing the application of such biopolymers for the separation of W−E mixtures (ethanol dehydration).18,19 As well, there are sparse reports that use corn meal, corn grits, and pristine starch as adsorbents.20−24 In a previous study,15 we reported the utility of cellulose and its cross-linked forms as adsorbents for the fractionation of W− E mixtures. Modified cellulose biomaterials display low molecular selectivity [Rselectivity; Qm(W)/Qm(E)] that varies from 1.10 to 2.03 at saturative conditions in binary W−E mixtures.15 Low Rselectivity values and the close structural similarity of cellulose and starch have motivated this study on the adsorption properties of starch materials in binary W−E systems. Farhad et al.25 recently reported that starch-coated surfaces have higher affinity toward water vapor over silica gel coatings, which provides support that starch has good potential utility as a sorbent for vapor-phase water. In this study, we report on the adsorption properties of starch-based adsorbent materials for the fractionation of solvent components in W−E binary mixtures using 1H NMR spectroscopy as a quantitative analytical method. Structural variants of starch derived from corn, maize, and soluble starch were evaluated because of the variable content of linear versus branched polysaccharides. The starch materials were cross-linked with epichlorohydrin at variable levels to evaluate the fractionation behavior of starch with cellulose and its cross-linked forms.15 This study contributes to the field of sustainable energy and biomass by demonstrating the tunable adsorption properties of starch materials in W−E systems in accordance with the structural modification via cross-linking of such adsorbent materials. A key objective of this study was to investigate the adsorption properties of a series of starch adsorbents at variable crosslinker content for the fractionation of water−ethanol binary mixtures. To this end, 1H NMR spectroscopy was used for the in situ quantitative analysis of W−E mixtures and to elucidate the role of cross-linking effects on the adsorptive fractionation of water and ethanol in binary mixtures. Herein, we report on the unique solvent-selective uptake behavior of starch-based components and their potential utility as adsorbents for the fractionation of W−E mixtures. In a broader context, this study contributes to the future development of adsorbent materials in the food and fuels industry for the chemical separation of water−alcohol mixtures.

Q e, i =

(Ce, i − Co, i) m

Wsolution

(1)

Finally, equilibrium adsorption results were fit by various isotherm models, such as the Langmuir, Freundlich, and Sips models,28,29 to evaluate the adsorption parameters in the W−E binary solvents. The Langmuir model (eq 2) describes monolayer sorption between adsorbate and adsorbent, where Qm represents the monolayer adsorption capacity and KL represents the Langmuir adsorption constant for the respective solvent component (W or E) in the binary W−E solvent system. The other terms (Ce,i and Qe,i) are defined by eq 1.30

2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. Starch from various sources (corn or maize) containing variable levels of amylose and amylopectin were chosen as the polysaccharide materials. Epichlorohydrin, sodium hydroxide, and ethanol (reagent grade, Commercial Alcohols Inc. Brampton, ON, Canada) were purchased from Sigma−Aldrich (Oakville, ON, Canada) and used as received. The cross-linked starch biopolymers containing different percentages of amylose/amylopectin (AM/AP) such as (SSE, 50% AM/AP; MSE, 98% AP; and AMCE, 98% AM) and various mole ratios of EPI were prepared by a modified method reported previously.26,27 2.2. Methods. To examine the sorption capacity of the starch materials with W−E in binary solvents, a batch adsorption method was used to obtain solid−solution isotherms for each adsorbent−solvent system. A 20 mg sample of starch (or modified starch) was added to 10 g of solvent with variable ethanol−water content in 4 dram glass vials. Samples of adsorbent and the solvent system were incubated

Q e, i = Q m

KLCe, i 1 + KLCe, i

(2)

The Freundlich model (eq 3) accounts for nonequivalent binding sites of heterogeneous surfaces, where the heterogeneity factor is denoted by the exponential term (nf).

Q e, i = KFCe1/ nf

(3)

The Sips isotherm is likened as a hybrid model with combined features of the Langmuir and Freundlich isotherms at certain limiting conditions.28 The Sips model provides an estimate of the monolayer surface coverage (Qm) of the adsorbate (E or W) that depends on the uptake properties of the adsorbent. The Sips model accounts for 5685


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Energy & Fuels adsorption processes that display surface heterogeneities according to the heterogeneity factor (ns) in eq 4. When ns = 1, Langmuir isotherm behavior (no heterogeneity) is predicted; whereas Freundlich behavior is predicted when (KsCe,i)ns ≪1 (ns ≠1). Q e, i

properties (surface area and pore structure), where the surface accessibility of the hydroxyl groups is reported to contribute to the solvent fractionation process in W−E binary solutions, as the cross-linker content varies.5,7,13−15,30 3.2. Quantitative Analysis of Water−Ethanol Components by qNMR. 1H NMR spectroscopy is a unique method for the structural characterization and identification of organic compounds because of the relative abundance of 1H nuclei and its overall sensitivity.32 The qNMR method used herein employs 1H NMR spectroscopy for the quantitative analysis of mixtures when the spectral signatures are well-resolved and instrumental conditions account for the relaxation dynamics of the accordant nuclei.15 The relaxation times (T1) for selected nuclei of water and ethanol binary solvents were reported previously.15 qNMR was used to estimate the solvent composition before and after the sorptive fractionation of the starch materials in binary W−E mixtures. Molecular selective adsorption for such systems indicates that the physical separation of one solvent component, such as water (W) over ethanol (E), occurs because of preferential uptake in a W− E solution. The relative solvent composition in the W−E mixtures was calculated using calibration curves over a defined W−E composition range via qNMR. Tetrahydrofuran (THF) was chosen as the internal calibration standard because its 1H spectral signatures do not overlap with those of water or ethanol (−CH2 and −CH3 groups) in the W−E solutions (cf. Figure 1 in ref 15). Among the various NMR solvents, DMSO-d6 is optimal as a field-locking solvent because of its miscibility in W−E mixtures and the spectral resolution it affords. Quantitative integration of the NMR spectra was achieved using the longest relaxation delay (D1) as determined by the inversion−recovery determination of the relaxation time (T1) values of the various nuclei for E and W for pure and binary W−E mixtures.33 The NMR spectra of W−E mixtures reveal resolved resonance lines for OH appearing at δ = 4.50 ppm (W) and δ = 5.20 ppm (E), as observed in Figure 1 of ref 15. Quantitative analysis of water and ethanol was achieved by measuring the integrated signal intensity of relevant signatures (−OH, −CH2, and −CH3) for E and W. Spectral resolution of the solvent signatures of fully relaxed spectra allow for the quantitative analysis by eq 5. The relative water content (w/w; %) in binary mixtures can be calculated according to the respective gravimetric weight (nW and nE) contributions by eq 5. Similarly, ethanol content was calculated according to the peak area for the −CH3 group.

Q m(K sCe, i)nS 1 + (K sCe, i)nS

(4)

3. RESULTS AND DISCUSSION 3.1. Selection of Polysaccharide Materials. The selection criteria for starch was based on commercially available types which vary from a linear form (amylose) to branched forms (amylopectin), according to the source of the polysaccharide. The various structural forms of starch possess variable water solubility, steric effects, and variable accessibility of primary versus secondary hydroxyl groups due to branching.31 Branching of amylose was reported15,26 to yield differences in the relative hydrophobicity of these materials; whereas, incremental cross-linking of starch yields materials with tunable adsorption properties where differences in hydrophobicity can be inferred. Table 1 lists the various types Table 1. Selected Physicochemical Properties of Starch and Its Cross-Linked Polymers at Variable Epichlorohydrin Content (L, M, H)a PS-EPI copolymer soluble starch-EPI (L) soluble starch-EPI (M) soluble starch-EPI (H) soluble starch maize starch-EPI (L) maize starch-EPI (M) maize starch-EPI (H) maize starch high amylose starchEPI (L) high amylose starchEPI (M) high amylose starchEPI (H) high amylose starch cellulose-EPI (L) cellulose-EPI (M) Cellulose-EPI (H) cellulose

polymer ID codea

yield (%)

SSE-L SSE-M SSE-H SSE MSE-L MSE-M MSE-H MSE AMCE-L

93 91 96 ND 86 87 85 NA 89

35 67 117 157 25 52 107 153 63

AMCE-M

93

188 ± 11

2±1

AMCE-H

90

307 ± 16

18 ± 4

AMCE CE-L CE-M CE-H CE

NA 94 88 91 NA

248 244 205 161 166

SW (%) ± ± ± ± ± ± ± ± ±

± ± ± ± ±

4 12 18 21 5 7 9 9 7

12 1 9 3 9

SE (%) 8 13 27 8 4 4 3 2 1

14 120 92 15 2

± ± ± ± ± ± ± ± ±

± ± ± ± ±

3 7 3 2 2 1 2 0.5 0.3

6 1 8 5 0.1

1 H (solvent) n Area(solvent) = 1 nuclei × solvent Area(THF) nsolvent H nuclei(THF)

a

L, M, and H denote the mole ratios of Starch-EPI, as follows: L = low (1:2 mol ratio); M = medium (1:3.6 mol ratio); and H = high (1:5.4 mol ratio). NA denotes not applicable.

(5)

The relative area for each solvent component refers to the integrated peak area for the 1H NMR signals; 1Hnuclei refers to the number of nuclei of the solvent (E or W); nsolvent is the number of moles of solvent (ethanol or water); and nTHF refers to the mole content of THF. The results of such determinations were obtained in triplicate with standard deviations below 5%. 3.3. Sorption Study. On the basis of the results in Table 1, it can be inferred that starch-based materials with greater amylose content display greater swelling in water over materials richer in amylopectin. The swelling results for starch-based materials in neat ethanol are notably smaller by a factor of 5- to 10-fold compared with those in water. The amylose-based materials reveal comparable swelling in ethanol except those

of starch-based materials with their sample ID codes, where corn starch contains amylose, soluble starch contains ca. 1:1 amylopectin/amylose content, and maize contains a relatively greater fraction of amylopectin. Also, a summary of the synthetic yield (%) and equilibrium swelling properties in water (SW) and in ethanol (SE) for starch and its cross-linked forms (starch-EPI) at variable composition is provided in Table 1. Accordingly, the starch materials with variable levels of amylose/amylopectin were cross-linked using epichlorohydrin with low (1:2; L), medium (1:3.6; M), and high (1:5.4; H) levels of cross-linker to afford starch-based adsorbents variable framework structure and surface-accessible hydroxyl groups. Cross-linking of starch affords materials with variable textural 5686


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Figure 1. Adsorption isotherms for the uptake of solvent components [water (W) and ethanol (E)] against initial solvent composition (fractional weight scale) in binary W−E solutions at 295 K: (A) MSE and its cross-linked polymers, (B) SSE and its cross-linked forms, and (C) AMCSE and its cross-linked forms. The best-fit line corresponds to the Sips models, and the error bars for the corresponding isotherm results denote the error contributions to eq 1 according concentration and mass.

materials containing amylopectin, where the least swelling was observed in ethanol. By contrast, the swelling of cellulose materials is relatively high in water and ethanol solvents, where the origin of this effect relates to the unique fibril structure of cellulose. Nitrogen adsorption results for cellulose show relatively high uptake when compared with starch-based materials, in accordance with the rigid nature and the micropore network structure of cellulose owing to its unique fibril morphology. Starch-based materials have lower surface area and reduced pore volume that relate to the greater conformational entropy of amylose and amylopectin. This

conformational motility of starch results in denser packing and collapse of the starch framework, in agreement with the nitrogen adsorption and water swelling results. In this study, the adsorption isotherms for three classes of starch (MSE, SSE, and AMCSE) with variable cross-linking are listed in Table 1. The value of Qe,i was calculated using eq 1 and analyzed by several isotherm models. Estimates of the error contributions to Qe,i (ΔQe,i) were obtained by differential error analysis of eq 1 that relate to uncertainties in the initial solvent concentration (Co,i and Ce,i) of either component (E or W) and the mass error of the sorbent (Δm). The adsorption isotherm 5687


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Table 2. Sips Isotherm Parameters for the Uptake of Ethanol at 295 K with Variants of Starch at Variable Cross-Linker Content starch ID code AMCSE AMCSE-L AMCSE-M AMCSE-H SSE SSE-L SSE-M SSE-H MSE MSE-L MSE-M MSE-H

Ks (g/g) 3.08 4.14 3.45 3.47 3.91 15.4 3.49 5.79 3.21 3.50 3.39 3.59

± ± ± ± ± ± ± ± ± ± ± ±

0.220 0.671 0.137 2.24 1.38 4.55 0.301 1.19 0.179 0.217 0.663 0.929

ns 3.08 3.11 5.51 1.14 0.895 0.668 5.28 2.11 1.68 3.36 2.13 2.43

± ± ± ± ± ± ± ± ± ± ± ±

1.43 1.39 2.37 0.544 0.156 0.119 2.65 0.693 0.102 0.602 0.875 0.638

Qm (g/g)

R2

± ± ± ± ± ± ± ± ± ± ± ±

0.803 0.983 0.949 0.884 0.993 0.988 0.862 0.943 0.999 0.984 0.869 0.966

0.0084 0.0142 0.145 0.152 0.0509 0.0134 0.0263 0.0643 0.0321 0.0532 0.022 0.0151

0.0025 0.0042 0.0098 0.0261 0.0079 0.0011 0.0030 0.0089 0.00125 0.00311 0.00374 0.0043

Table 3. Sips Isotherm Parameters for the Uptake of Water at 295 K with Variants of Starch at Variable Cross-Linker Content starch ID code AMCSE AMCSE-L AMCSE-M AMCSE-H SSE SSE-L SSE-M SSE-H MSE MSE-L MSE-M MSE-H

Ks (g/g) 3.91 3.76 6.25 3.84 2.81 2.40 3.64 3.79 2.78 8.14 3.26 3.17

± ± ± ± ± ± ± ± ± ± ± ±

0.923 0.578 0.459 0.224 1.05 0.870 0.585 0.418 0.422 5.20 0.481 0.322

ns 2.02 2.40 2.76 4.79 2.44 2.06 2.56 1.85 3.86 0.555 2.17 2.76

± ± ± ± ± ± ± ± ± ± ± ±

0.666 0.804 0.433 0.224 1.76 0.749 1.43 0.239 1.23 0.116 0.519 0.668

Qm (g/g)

R2

± ± ± ± ± ± ± ± ± ± ± ±

0.871 0.952 0.977 0.969 0.848 0.944 0.909 0.978 0.974 0.993 0.977 0.974

0.608 1.38 2.64 2.89 0.202 0.0504 0.885 1.52 0.206 0.540 1.16 1.24

0.111 0.205 0.0935 0.202 0.0752 0.0159 0.162 0.113 0.0308 0.0871 0.143 0.127

materials, cross-linking results in a pillaring ef fect of such biopolymers, as evidenced by a comparison of the textural properties of the pristine and modified materials. Evident variation of the surface area and pore structure properties of pristine and modified materials was observed by the variable dye uptake in such systems with increased cross-linking. Adsorption isotherms of W−E binary solvent systems can be used to determine the relative uptake levels of each respective solvent component (W and E) and to estimate the relative solvent selectivity of the adsorbent systems. According to Figure 1 and Table 2, the uptake capacity (Qm) of AMCE-H displays the highest Qm value for water and ethanol. Qm values for the various starch materials are listed in descending order for W and E as follows: AMCE-H > SSE-H > MSE-H. According to the value of the heterogeneity parameter (ns) in Tables 2 and 3, the surface of starch and its modified forms is heterogeneous in nature because ns deviates measurably from unity. There are two factors which contribute to the heterogeneity of the starch surface: cross-linking effects and the disordered pore network arrangement of the polysaccharide due to branching or folding of the biopolymer. By contrast, cellulose is a more rigid polysaccharide with a linear morphology where it is noted that the values of ns are closer to unity for water uptake (cf. Table 5 in ref 26). Thus, crosslinking of starch via the EPI cross-linker contributes to an increase in the surface heterogeneities35 for several reasons: (i) alteration of the morphology of the polysaccharide due to steric effects, (ii) the formation of micropore domains, and (iii) alteration of the relative hydrophobicity of the starch network due to a net reduction of the −OH groups. These heterogeneities affect the sorptive uptake and the solvent-

of a solid−solution system describes the thermodynamic relationship between the bound and unbound adsorbate (W or E) species at constant temperature and at equilibrium. The corresponding adsorption isotherms for the uptake of water and ethanol in W−E binary solutions are illustrated in Figure 1. The isotherm parameters for the starch-based systems are listed in Tables 2 and 3, according to the best-fit results for the Langmuir, Freundlich, and Sips models. The isotherm results show that starch-based materials display greater overall uptake of water over ethanol, in agreement with the swelling results in Table 1. The greater adsorptive affinity of water over ethanol with cross-linked starch parallels the greater Qm values for each solvent component. The Langmuir and Freundlich models show reduced R2 values, which further illustrate that these isotherms show poorer agreement with the experimental results (cf. Supporting Information, Figure S1). By contrast, the Sips model provides a better description of the isotherm results because the R2 values are closer to unity. A comparison of Qm values of AMCE, SSE, and MSE reveals that the cross-linked forms of starch display variable water adsorption capacity. The greater uptake of water over ethanol relates to differences in the physicochemical properties of each solvent. The greater polarity and smaller molecular size of water enables favorable interaction with the polar hydroxyl groups of starch. In the case of cross-linked starch materials, the accessibility of hydroxyl groups within the polysaccharide framework varies because of steric effects and the net loss of one −OH group per cross-linked formed via Williamson etherification. Cross-linking of starch results in the formation of a network structure between adjacent starch polymer units (cf. Scheme 2 in ref 26). In the case of cellulose14 and chitosan34 5688


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for canola meal (before and after protein extraction) and other sorbents provide additional support for the important role of surface accessibility of polar functional groups in the fractionation of W−E mixtures. This trend parallels the importance of variable textural properties and surface chemistry of polysaccharides that result from cross-linking effects.26 In the case of cellulose with variable cross-linking, changes in the surface accessible hydroxyl groups and pillaring effects of the fibril structure were shown by variable decolorization of phenolphthalein at alkaline conditions (cf. Figure 6 in ref 14). The equilibrium swelling properties of starch materials are a qualitative indicator of solvent uptake capacity of singlecomponent solvents at ambient conditions. Although the swelling parameters relate to the combined effects from adsorption and absorption (sorption), swelling enables the solvent to permeate into polymer networks and membrane microstructures.39 Biopolymer materials such as starch may undergo swelling through expansion of the pore network due to the inclusion of solvent via physical interactions. Swelling depends on various factors including the pH of solution, ionic strength, temperature and so forth.40−42 With reference to cellulose materials,26 cross-linked starch shows greater uptake of water over ethanol (cf. Table 1), along with the isotherm results in Figure 1 and Tables 2 and 3. Furthermore, amylosebased polymers such as AMCSE showed the highest adsorption capacity for water, while SSE and SSE-L materials show greater uptake of ethanol over water. Differences in the preferred uptake of water over ethanol for starch and its modified forms are illustrated in Figure 2. The observed trends in Rselectivity relate to the effects of cross-linking due to surface heterogeneity effects (see below) that favor uptake of solvent (W or E) according to the relative accessibility of polar functional groups and textural properties of the adsorbent.43 An increase in the cross-linker content results in greater Rselectivity values due to steric or pillaring ef fects, surface accessibility of polar functional groups, and textural properties (cf. Scheme 2 of ref 26). These factors contribute to the swelling and solvent selectivity of starch biopolymers, however, a detailed understanding of the role of cross-linking on the structure and function of these materials is the subject of ongoing and future work. The greater Rselectivity observed herein for starch and its modified forms finds additional support reported elsewhere.43,44 Physicochemical properties of the

selective uptake as compared with unmodified starch, according to the nature of each solvent (W or E) and the adsorptive interactions that govern the adsorption process. In Tables 2 and 3, notable differences in the Qm value for each solvent (W or E) reveal that the starch materials show preferential uptake of water over ethanol in W−E mixtures. Molecular selective adsorption relies on differences in the physicochemical properties of the adsorbate (e.g., polarity) in order to achieve adsorptive fractionation of multicomponent systems. The molecular selectivity of starch-based materials in W−E mixtures can be understood according to variation in the relative hydrophobicity of the adsorbent, according to the level of cross-linking and the type of starch (amylose versus amylopectin). The molecular selectivity (Rselectivity) value of water over ethanol was calculated using eq 6. R selectivity =

Q m(Water) Q m(Ethanol)

(6)

The Rselectivity values for starch materials were estimated using the Qm values for water and ethanol from Tables 2 and 3. The Rselectivity values varied with increased cross-linking and the nature of the starch biopolymer. In contrast with cellulose materials (Rselectivity ≈ 1−2), pristine and cross-linked starch materials show greater Rselectivity, especially for amylose-based materials where Rselectivity is greater (4−80). An exception was noted for SSE materials, where SSE-L displays a maximum Rselectivity value for ethanol. The Rselectivity values in Tables 2 and 3 for starch-based adsorbents are among the highest reported in the open literature, where a notably high Rselectivity value was observed for MSE-H. According to the composition of polar functional groups of biomass and molecular sieves, the results in Table 4 provided Table 4. Selected Studies of Rselectivity for Various Biomaterials adsorbent

selectivity

canola meal corn meal protein-extracted canola meal molecular sieves

8−20 0.6 1.9 78

C0 (w/w%) 4−100% 15−75% 80−95% 80−95%

(water) (ethanol) (ethanol) (ethanol)

ref 36 37 38 38

Figure 2. Equilibrium solvent selectivity ratios for cellulose and starch-based adsorbents in binary W−E systems at 295 K: (A) native cellulose (CE) and its cross-linked forms and (B) starch and its cross-linked forms. The sample ID code for cellulose is defined in Table 1 (0, native biopolymer; 1, low; 2, medium; 3, high cross-linker content). 5689


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Figure 3. Conceptualized molecular structure of cross-linked polysaccharide materials: (A) amylose and (B) amylopectin, where i denotes low crosslinking and ii denotes higher cross-linking. The red bars represent the cross-linkers and the black lines are segment of the polysaccharide backbones. The hydroxyl groups are not explicitly shown, and the diagrams are not drawn to scale.

the micropore network of adsorbent. The specific adsorption capacity is often controlled by the third step and strongly relates to the role of pore structure and surface functionalization.26 Crini13 has suggested that polysaccharides display rapid adsorption kinetics due to the favorable solvation of the polar groups, in agreement with the swelling results for water over ethanol summarized in Table 1. The variable branching of amylose versus amylopectin and its cross-linked forms (cf. Figure 3) influence the kinetics of adsorption in the above steps, in agreement with the observed solvent selectivity for such starch materials. Among the various polysaccharides investigated, AMCSE showed notably higher adsorption performance and selectivity relative to SSE and MSE. Amylose is a linear starch based on α1 → 4 glycosidic bonds with a molecular weight ca. 105−106 amu and a moderate degree of polymerization (n = 600).20 Amylopectin is a branched form of starch containing α-1 → 4 and α-1 → 6 glycosidic bonds with a relatively high molecular weight (ca. 107−109 amu). The linear and branched structural variants of starch influence the physical and biological properties of these materials that parallel the water and ethanol uptake properties of starch.45,49 Highly branched amylopectin versus linear starch variants influence the relative surface accessibility of the hydroxyl groups and their adsorptive interactions. The third step in the kinetic process of adsorption described above is influenced by cross-linking and the steric accessibility of polar functional groups. However, the structural details of such complex polysaccharides is currently lacking and require further study. The fractionation properties of cellulose were reported for alkyl carboxylate anions and their mixtures in aqueous solution by Udoetok et al.14 The fractionation behavior of cellulose paralleled the surface accessibility of polar functional groups because an alteration of the relative hydrophobicity and textural properties of the biopolymer was related to the level of cross-linking. Similarly, the high solvent selectivity of carboxymethyl cellulose and other polysaccharide polyelectrolyte membrane materials for dehydration of aqueous alcohol mixtures (under pervaporation conditions) is readily understood in terms of variable hydrophobicity.50 The findings presented by Reineke et al.50 parallel the observed trends in solvent selectivity in this study. This is due to the effects of solvent size and polarity and the relative hydrophobicity of

solvent such as the relative size and polarity of water and ethanol are key factors that influence the uptake of water over ethanol observed for starch-based materials. The offset in molar volume (Vm) of water (Vm = 18 cm3/mol) and ethanol (Vm = 58.0 cm3/mol) and the difference in relative polarity (H2O, 16 D; and C2H5OH, 8.8 D)15 contribute to the observed fractionation properties and provide insight into the kinetic and thermodynamic adsorption properties of starch materials. The foregoing is consistent with the effects of cross-linking on the textural properties and hydrophobic character of the adsorbent according to the Rselectivity values for starch- and cellulose-based adsorbents. In general, starch shows greater uptake selectivity toward water over ethanol according to the greater accessibility of polar (−OH) groups of starch. Cellulose materials have reduced accessibility of polar groups due to more extensive intra- and intermolecular hydrogen bonding, owing to its fibril morphology and lower water solubility. Thus, small-sized solvents like water with greater polarity afford more stable binding interactions when compared with ethanol over a range of surface accessible donor and acceptor groups at the polysaccharide surface. The variable Rselectivity values for starch materials at different cross-linker content account for the variation in hydrophobic character inferred for such materials.44 Cross-linkers alter the relative hydrophobicity of starch biopolymers by decreasing the net number of hydroxyl groups and presence of chain branching.45 The stabilizing effect of hydrogen bonding with water and ethanol differ according to the level of cooperative interactions due to the structured nature of each solvent.45 Thus, the role of molecular size, polarity, and adsorption site accessibility is highlighted in a study of the adsorption of apolar organics and water molecules using polysaccharides where the separation of smaller molecules such as urea with dimensions close to those of water present challenges in achieving molecular selectivity.46−48 The relevance of polysaccharide structure on the kinetic adsorption properties is evidenced by the uptake of the adsorbate via three successive steps: (i) transport of molecules from the bulk of the solution to the exterior surface of adsorbent particles through a boundary layer of two phases; (ii) diffusion processes that relate to transport of the adsorbate to the interior active sites of the adsorbent (intraparticle or internal diffusion); and (iii) adsorption at the active sites within 5690


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(4) Wang, Y.; Gong, Ch.; Sun, J.; Gao, H.; Zheng, S.; Xu, Sh. Separation of ethanol/water azeotrope using compound starch-based adsorbents. Bioresour. Technol. 2010, 101, 6170−6176. (5) Mohamed, M. H.; Wilson, L. D.; Headley, J. V.; Peru, K. M. Thermodynamic Properties of Inclusion Complexes Between βCyclodextrin and Naphthenic Acid Fraction Components. Energy Fuels 2015, 29 (6), 3591−3600. (6) Jiang, L. Y.; Wang, Y.; Chung, T.-S.; Qiao, X. Y.; Lai, J.-Y. Polyimides membranes for pervaporation and biofuels separation. Prog. Polym. Sci. 2009, 34, 1135−1160. (7) Wilson, L. D.; Mohamed, M. H.; Headley, J. V. Novel materials for environmental remediation of oil sands contaminants. Rev. Environ. Health 2014, 29, 5−8. (8) Ivanova, E.; Damgaliev, D.; Kostova, M. Adsorption separation of ethanol-water liquid mixtures by natural clinoptilolite. J. Chem. Technol. Metall. 2009, 44 (3), 267−274. (9) Al-Asheh, S.; Banat, F.; Al-Lagtah, N. Separation of ethanol-water mixtures using molecular sieves and biobased adsorbents. Chem. Eng. Res. Des. 2004, 82 (A7), 855−864. (10) Kang, Q.; Huybrechts, J.; Van der Bruggen, B.; Baeyens, J.; Tan, T.; Dewil, R. Hydrophilic membranes to replace molecular sieves in dewatering the bio-ethanol/water azeotropic mixture. Sep. Purif. Technol. 2014, 136, 144−149. (11) Wu, P.; Gao, H.; Sun, J.; Ma, T.; Liu, Y.; Wang, F. Biosorptive dehydration of tert-butyl alcohol using a starch-based adsorbent: Characterization and thermodynamics. Bioresour. Technol. 2012, 107, 437−443. (12) Hsu, S.-H.; Hsu, W.-Ch.; Chung, T.-W.; Liao, C.-Ch. Dynamic adsorption study for water removal from ethanol solution by using a novel immobilized starch sorbent. J. Taiwan Inst. Chem. Eng. 2013, 44, 952−956. (13) Crini, G. Recent developments in polysaccharide-based materials used as adsorbents in wastewater treatment. Prog. Polym. Sci. 2005, 30, 38−70. (14) Udoetok, I. A.; Dimmick, R. M.; Wilson, L. D.; Headley, J. V. Adsorption Properties of Cross-linked Cellulose-Epichlorohydrin Polymers in Aqueous Solution. Carbohydr. Polym. 2016, 136, 329− 340. (15) Dehabadi, L.; Wilson, L. D. An NMR investigation of the fractionation of water-ethanol mixtures with cellulose and its crosslinked biopolymers. Energy Fuels 2015, 29, 6512−6521. (16) Shukri, R.; Zhu, L.; Seib, P. A.; Maningat, C.; Shi, Y.-C. Direct in-vitro assay of resistant starch in phosphorylated cross-linked starch. Bioact. Carbohydr. Diet. Fibre 2015, 5, 1−9. (17) Mostafa, K. M.; Samarkandy, A. R.; El-Sanabary, A. A. Preparation of poly (DMAEM)-cross linked pregelled starch graft copolymer and its application in waste water treatments. Carbohydr. Polym. 2011, 86, 491−498. (18) Ernest, R. Roth.; Villanova, Pa. Production of low-ethanol beverages by membrane extraction. U.S. Patent 4,306,884, Dec. 22, 1981. (19) Gubicza, L.; Kabiri-Badr, A.; Keoves, E.; Belafi-Bako, K. Largescale enzymatic production of natural flavour esters in organic solvent with continuous water removal. J. Biotechnol. 2000, 84, 193−196. (20) Ladisch, M. R.; Voloch, M.; Hong, J.; Bienkowski, P.; Tsao, G. T. Cornmeal adsorber for dehydrating ethanol vapors. Ind. Eng. Chem. Process Des. Dev. 1984, 23 (3), 437−443. (21) Chang, H.; Yuan, X.-G.; Tian, H.; Zeng, A.-W. Experiment and prediction of breakthrough curves for packed bed adsorption of water vapor on cornmeal. Chem. Eng. Process. 2006, 45, 747−754. (22) Chang, H.; Yuan, X.-G.; Tian, H.; Zeng, A.-W. Experimental Study on the Adsorption of Water and Ethanol by Cornmeal for Ethanol Dehydration. Ind. Eng. Chem. Res. 2006, 45, 3916−3921. (23) Chang, H.; Yuan, X.; Tian, H.; Zeng, A. Experimental Investigation and Modeling of Adsorption of Water and Ethanol on Cornmeal in an Ethanol-Water Binary Vapor System. Chem. Eng. Technol. 2006, 29, 454−461.

starch biopolymers and their modified forms, which is in agreement with cross-linking effects illustrated in Figure 3.

4. CONCLUSION 1 H NMR spectroscopy is a versatile and rapid screening technique for the estimation of the composition in W−E binary solution and to provide an understanding of the fractionation properties of starch and its cross-linked forms. Starch-based materials show preferential uptake of water (2.68 g/g) over ethanol (0.152 g/g) in binary W−E solutions where variable solvent fractionation occurs at different levels of cross-linking using epichlorohydrin. A cross-linked form of high amylose (AMCSE-H) starch showed the highest uptake of ethanol and water, when compared with other starch materials (MSE and SSE). The uptake selectivity of water over ethanol (Rselectivity) was attributed to the unique structure and textural properties of starch and its modified forms (cf. Figure 3). Overall, starchbased materials display greater Rselectivity (ca. 80) for water over ethanol when compared with cellulose-based materials. This research will contribute to the development of improved adsorbents with tunable solvent fractionation properties according to the nature of the polysaccharide and modification via a facile cross-linking method. Further studies are underway to develop a more detailed understanding of the structure− activity relationship related to the adsorption properties of these starch-based biopolymers. We envisage that such adsorbent technology will contribute significantly to the use of sustainable biomaterials for the production of food and fuels.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b00437. Adsorption isotherms at equilibrium for starch and its cross-linked forms (Figure S1) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +1-306-966-2961. Fax: +1306-966-4730. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the University of Saskatchewan and the Government of Saskatchewan (Ministry of Agriculture) through the Agriculture Development Fund (Project 20110162) for supporting this research.

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REFERENCES

(1) Lu, L.; Shao, Q.; Huang, L.; Lu, X. Simulation of adsorption and separation of ethanol−water mixture with zeolite and carbon nanotube. Fluid Phase Equilib. 2007, 261, 191−198. (2) McCready, R. M.; Hassid, W. Z. The Separation and Quantitative Estimation of Amylose and Amylopectin in Potato Starch. J. Am. Chem. Soc. 1943, 65, 1154−1157. (3) Magalad, V. T.; Gokavi, G. S.; Nadagouda, M. N.; Aminabhavi, T. M. Pervaporation Separation of Water Ethanol Mixtures Using Organic Inorganic Nanocomposite Membranes. J. Phys. Chem. C 2011, 115, 14731−14744. 5691


Article

Energy & Fuels (24) Li, G.; Bai, P. New Operation Strategy for Separation of Ethanol−Water by Extractive Distillation. Ind. Eng. Chem. Res. 2012, 51, 2723−2729. (25) Fathieh, F.; Dehabadi, L.; Wilson, D. L.; Besant, R.; Evitts, R.; Simonson, C. Sorption Study of Biopolymer (High Amylose Starch) as an Alternative Desiccant for Rotary Desiccant Wheels. ACS Sustainable Chem. Eng. 2016, 4 (3), 1262−1273. (26) Dehabadi, L.; Wilson, L. D. Polysaccharide-based materials and their adsorption properties in aqueous solution. Carbohydr. Polym. 2014, 113, 471−479. (27) Ashish, P.; Khanderao, J.; Deelip, D. Cross linked high amylose starch biodegradable implants for sustained delivery of ciprofloxacin hydrochloride. Int. J. Pharm. Pharm. Sci. 2013, 5, 391−397. (28) Foo, K. Y.; Hameed, B. H. Insights into the modeling of adsorption isotherm systems. Chem. Eng. J. 2010, 156, 2−10. (29) Jiang, Y.; Huang, C.; Pang, H.; Liao, B. Progress in cellulosebased adsorbents. Huaxue Tongbao Sci. 2008, 71 (12), 891−899. (30) Morin-Crini, N.; Crini, G. Environmental applications of waterinsoluble beta-cyclodextrin-epichlorohydrin polymers. Prog. Polym. Sci. 2013, 38, 344−368. (31) Lee, B.; D’Alessio, M.; Vissing, H.; Ramirez, F.; Steinmann, B.; Superti-Furga, A. Characterization of a large deletion associated with a polymorphic block of repeated dinucleotides in the type III procollagen gene (COL3A1) of a patient with Ehlers-Danlos syndrome type IV. Am. J. Hum. Genet. 1991, 48 (3), 511−7. (32) Ratanapariyanuch, K.; Shen, J.; Jia, Y.; Tyler, R. T.; Shim, Y. Y.; Reaney, M. J. T. Rapid NMR method for the Quantification of organic compounds in Thin Stillage. J. Agric. Food Chem. 2011, 59, 10454− 10460. (33) Schmitt, P.; Griswold, M. A.; Jakob, P. M.; Kotas, M.; Gulani, V.; Flentje, M.; Haase, A. Inversion Recovery True FISP: Quantification of T1, T2, and Spin Density. Magn. Reson. Med. 2004, 51, 661−667. (34) Mohamed, M. H.; Udoetok, I. A.; Wilson, L. D.; Headley, J. V. Fractionation of Carboxylate Anions from Aqueous Solution Using Chitosan Cross-Linked Sorbent Materials. RSC Adv. 2015, 5, 82065− 82077. (35) Singh, J.; Kaur, L.; McCarthy, O. J. Factors influencing the physico-chemical, morphological, thermal and rheological properties of some chemically modified starches for food applicationsA review. Food Hydrocolloids 2007, 21, 1−22. (36) Ranjbar, Z.; Sun, N.; Niu, C. H.; Dalai, A. K. Adsorption of Water and Ethanol by Canola Meal in a Batch Liquid System. CSBE/ SCGAB 2013 Annual Conference, Saskatoon, Saskatchewan, July 7− 10, 2013; Paper No. CSBE13-108, pp 1−10. (37) Chang, H. X.; Yuan, H. T.; Tian, H.; Zeng, A.-W. Experimental Study on the Adsorption of Water and Ethanol by Cornmeal for Ethanol Dehydration. Ind. Eng. Chem. Res. 2006, 45, 3916−3921. (38) Ranjbar, Z.; Tajallipour, M.; Niu, C. H.; Dalai, A. K. Water Removal from Ethanol Vapor by Adsorption on Canola Meal after Protein Extraction. Ind. Eng. Chem. Res. 2013, 52 (40), 14429−14440. (39) Ying, W.; Kumar, R.; Herzberg, M.; Kasher, R. Diminished Swelling of Cross-Linked Aromatic Oligoamide Surfaces Revealing a New Fouling Mechanism of Reverse-Osmosis Membranes. Environ. Sci. Technol. 2015, 49, 6815−6822. (40) Mandala, I. G.; Bayas, E. Xanthan effect on swelling, solubility and viscosity of wheat starch dispersions. Food Hydrocolloids 2004, 18, 191−201. (41) Blazek, J.; Copeland, L. Pasting and swelling properties of wheat flour and starch in relation to amylose content. Carbohydr. Polym. 2008, 71, 380−387. (42) Démé, F.; Peuvrel-Disdier, E.; Vergnes, B. Loss of crystalline structure and swelling kinetics of maize starch and flour granules in glycerol excess: The role of the envelope structure. Ind. Crops Prod. 2015, 70, 149−157. (43) Liu, Yu-W.; Tang, T.; Chung, T.-W.; Huang, Ch.; Lin, Y.-S. Equilibrium Isotherms of Water and Ethanol Vapors on Immobilized Starch Sorbents. J. Chem. Eng. Data 2010, 55, 5807−5811.

(44) Wang, K.-S.; Liao, C.-C.; Chu, R. Q.; Chung, T.-W. Equilibrium Isotherms of Water and Ethanol Vapors on Starch Sorbents and Zeolite 3A. J. Chem. Eng. Data 2010, 55, 3334−3337. (45) Zou, W.; Yu, L.; Liu, X.; Chen, L.; Zhang, X.; Qiao, D.; Zhang, R. Effects of amylose/amylopectin ratio on starch-based superabsorbent polymers. Carbohydr. Polym. 2012, 87, 1583−1588. (46) Westgate, P. J.; Ladisch, M. R. Sorption of Organics and Water on Starch. Ind. Eng. Chem. Res. 1993, 32, 1676−1680. (47) Xue, C.; Wilson, L. D. Kinetic Study of Urea Uptake with Chitosan Based Sorbent Materials. Carbohydr. Polym. 2016, 135 (1), 180−186. (48) Ho, Y. S.; Porter, J. F.; Mckay, G. Equilibrium isotherm studies for the sorption of divalent metal ions onto peat: copper, nickel and lead single component systems. Water, Air, Soil Pollut. 2002, 141, 1− 33. (49) Foo, K. Y.; Hameed, B. H. Value-added utilization of oil palm ash: a superior recycling of the industrial agricultural waste. J. Hazard. Mater. 2009, 172 (2−3), 523−31. (50) Reineke, C. E.; Jagodzinski, J. A.; Denslow, K. R. Highly water selective cellulosic polyelectrolyte membranes for the pervaporation of alcohol-water mixtures. J. Membr. Sci. 1987, 32, 207−221.

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