Letter Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
pubs.acs.org/journal/ascecg
Valorization of Grass Lignins: Swift and Selective Recovery of Pendant Aromatic Groups with Ozone Andrew M. Danby,† Michael D. Lundin,† and Bala Subramaniam*,†,‡ †
Center for Environmentally Beneficial Catalysis, University of Kansas, 1501 Wakarusa Drive, Lawrence, Kansas 66047, United States Department of Chemical and Petroleum Engineering, University of Kansas, 1530 W. 15th Street, Lawrence, Kansas 66045, United States
‡
S Supporting Information *
ABSTRACT: The lack of a simple process for valorizing the lignin produced during cellulosic ethanol manufacture is a major hurdle preventing such biorefineries from becoming economically sustainable. Here, we demonstrate a facile ozonolysis process in a continuous stirred tank reactor (CSTR) to generate a value-added product stream from lignin dissolved in either acetic acid (as in acetosolv lignin) or formic acid. At ambient pressure, relatively mild temperature (70 °C), and short residence times (few minutes), ozone is bubbled through the solution to selectively cleave the CC bonds associated with specific pendant groups present in corn stover and wheat straw lignins. The resulting products, mainly vanillin and 4-hydroxybenzaldehyde, constitute approximately 7 wt % of the lignin and command high value with established applications in the flavoring, pharmaceutical, and electronics industries. These products are easily separated from the remaining lignin via membrane nanofiltration of the product mixture. The lignin resulting from ozone pretreatment and nanofiltration retains its polymeric structure and is available for further valorization. The lack of detectable secondary ozonide intermediates and the dominant formation of aromatic aldehyde monomers as the main products strongly suggest that the short chain carboxylic acid solvents may be acting as participating solvents, interrupting the classic Criegee mechanism. KEYWORDS: Ozonolysis, Agricultural waste, Vanillin, 4-Hydroxybenzaldehyde, Biorefinery
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INTRODUCTION After cellulose, lignin is the most abundant polymer produced by nature and is the only renewable feedstock containing aromatics.1 Grasses and woody plants form lignin by the oxidative polymerization of hydroxycinnamyl alcohol monomers which vary in the degree of ring methoxylation,2 leading to a variety of intermonomer bonds that make lignin hard to degrade,3 a trait that is good for the plant but one which makes it difficult to deconstruct lignin to recover valuable aromatics.4 Even if a depolymerization technique that is capable of breaking all types of interunit bonds becomes available, it will yield a complex mixture requiring tedious separation schemes to isolate value-added products.5,6 Clearly, simple lignin valorization techniques that selectively produce value-added fractions that are relatively clean and easy to isolate are preferred. As explained below, grass lignins are amenable to such a strategy. Many naturally occurring lignins incorporate preacylated monolignols in the Cγ position during the polymerization process.7 Several studies have shown that various types of acids may be added to the monolignols,8−10 depending upon the plant species. For example, acetates are observed in low abundance in hardwoods and at high levels in abaca, kenaf, palms, and sisal.7 In contrast, p-hydroxybenzoates are found in palms and Populus © XXXX American Chemical Society
species such as aspen, poplar, and willow, while p-coumarates are abundant in grasses with maize (Zea mays L.) having the highest abundance of all common species.11 The acylation has been shown to occur predominantly on syringyl units, with 7−12% of p-coumarates reported on guaiacyl units.12,13 Depending on the species, grass lignins may contain up to 20 wt % of p-coumaric and ferulic acids.11,14 These acids are bound primarily as pendant esters in the Cγ-position of the β-O-4 linkages of the polymer (Scheme 1). As depicted in Scheme 1, ozone should readily attack the olefinic bond between the α and β carbons in the pcoumaric acid of the acylated lignin to yield 4-hydroxybenzaldehyde and/or 4-hydroxybenzoic acid upon cleavage. Ozonolysis of lignin was first reported in 1913.15 Much of the early focus was on ozone’s penchant to attack aromatic rings, thereby effecting complete delignification in the paper and pulping industries. The application of ozone for determining lignin structure, particularly of the aliphatic side chains, has been widely studied.16,17 In this context, batch ozonolysis of lignin has been reported to yield oxygenated aromatics18 but only at short Received: August 26, 2017 Revised: November 6, 2017 Published: November 22, 2017 A
DOI: 10.1021/acssuschemeng.7b02978 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Letter
ACS Sustainable Chemistry & Engineering Scheme 1. β-O-4 Ether Linkages in Lignin Polymers: (A) Hydroxylated at Cγ, Typical of Woody Lignins. (B) pCoumarate Ester at Cγ with an Olefinic Bond between α and β Carbons in Coumarate, Typical of Grass Lignins
acids.24 Ozone is considered sustainable because it is generated from oxygen on site when required, is fully consumed in CC cleavage reactions when fed in stoichiometric amounts or less, forms nontoxic products, and has a relatively short half-life decomposing to oxygen without any environmental persistence issues.
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EXPERIMENTAL SECTION
Materials and Methods. Lignins from three sources were used in the studies. Corn (Zea mays L.) stover-derived lignin was provided by Archer Daniels Midland Company (Decatur, IL). Two lignins were extracted in our laboratories, one from wheat straw and the other from American white oak hardwood, using the techniques described in the Supporting Information (SI). All solvents and reagents were supplied by either Fisher Scientific (Pittsburgh, PA, USA) or Sigma-Aldrich (St. Louis, MO, USA) and were used as supplied. An ozone/oxygen gas mixture, containing ∼3.5 mol % O3, was generated from oxygen (extra dry grade, 99.6 wt %, supplied by Matheson) using a Praxair-Trailigaz Uniozone LO ozone generator. Ozonolysis. The continuous ozonolysis of lignin was performed in a stirred Parr reactor (Scheme S1). The lignin was dissolved in either acetic or formic acid containing up to 20 v/v% water at a concentration of ca. 1 wt %. The lignin solution was filtered through a 0.45 μm frit and pumped into the heated and stirred reactor (500 rpm) using either an HPLC pump or a Teledyne-ISCO 500D syringe pump, depending on the flow rate. Typical liquid flow rates ranged between 0.71 to 14.2 mL min−1. A gaseous feed stream, containing approximately 3.5 mol % ozone in oxygen, was simultaneously sparged through the liquid phase at high flow rates (70 std L h−1). The liquid and the gas streams exit the reactor in a multiphase flow via a dip tube, the depth of which determines the liquid holdup (14.2 mL) in the reactor. The lignin solution was exposed to the ozone-laden gas stream for relatively short “contact times”, defined as the ratio of the liquid holdup in the CSTR relative to the volumetric flow rate of the lignin solution. The contact time was typically on the order of a few minutes and was varied by changing the feed rate of the liquid feed stream. In a typical experiment, a steady reactor temperature was achieved after establishing the liquid and gas flow rates at desired values. Following this step, the ozone generator was switched on to introduce ozone in the gas stream. The reactor contents were collected as they were purged from the reactor by the gas stream with a collection interval of 0.5 times the contact time. Excess ozone was purged from the collected liquid samples by bubbling nitrogen gas through the sample for 10 s. Analysis revealed that a steady state was always achieved within three contact times. The liquid samples were analyzed using gel
reaction times beyond which total oxidation products dominate. Controlled ozonolysis that minimizes over oxidation of the cleaved pendant groups and the lignin backbone may be achieved in two ways: (i) using short residence times in a continuous stirred reactor and (ii) choosing a solvent that inhibits ozone’s propensity for oxidizing aromatic moieties. Short chain organic acids are often used as solvents for organosolv extraction of lignin from biomass. In the present study, we chose to use either acetic acid or formic acid as solvents for lignin ozonolysis. It has been reported that the use of formic acid instead of ethyl acetate as a solvent for ozonolysis reduces the degree of aromaticity loss and also prevents the oxidation of the aldehyde products to carboxyl groups.19 Recently, it has been shown that for woody lignins the use of either formic acid20 or formaldehyde21 significantly improves aromatic monomer yields via elegant depolymerization strategies based on the existence of Cγ- and Cα-hydroxy species in the β-O-4 linkage. However, such chemistries would be less feasible with grass lignins since they are partially acylated in the Cγ position (Scheme 1). Ozone is often considered a “green” alternative to less sustainable oxidants. It is used in pulp bleaching,22 water treatment facilities,23 and in the industrial ozonolysis of fatty
Table 1. Yields of Aromatic Monomers from Continuous Ozonolysis of Corn Stover Lignin in a Stirred Reactora Yield of monomers (wt % of original lignin) Solvent Glacial Acetic Acid Acetic Acid (12 v/v % water)
Acetic Acid (20 v/v % water)
Formic Acid (12 v/v %)
Contact time (min)
Vanillinb
4-Hydroxybenzaldehydeb
Vanillic acidc
4-Hydroxy-benzoic acidc
Total aromatic monomers
2.5 5.9 1.0 2.5 5.0 10.0 2.5 5.0 10.0 1.0 2.5 5.0 10.0
1.08 ± 0.07 0.94 ± 0.06 1.5 ± 0.2 1.39 ± 0.04 1.2 ± 0.2 0.60 ± 0.2 1.41 ± 0.03 1.4 ± 0.1 0.80 ± 0.02 0.58 ± 0.02 0.89 ± 0.02 0.94 ± 0.02 0.45 ± 0.02
2.85 ± 0.07 2.6 ± 0.1 5.2 ± 0.4 5.1 ± 0.3 4.8 ± 0.2 3.2 ± 0.5 4.2 ± 0.1 4.52 ± 0.07 3.3 ± 0.2 1.35 ± 0.08 4.4 ± 0.4 4.2 ± 0.4 4.15 ± 0.05
0.10 0.09 0.18 0.20 0.17
0.48 0.45 0.62 0.82 0.60
4.51 4.08 7.50 7.29 6.77
0.21 0.22 0.18 trace 0.19 0.15 0.10
0.84 0.87 0.61 0.22 0.59 0.71 0.62
6.66 7.01 4.89 2.15 6.07 6.00 5.32
Reaction conditions: P = 1 atm, T = 70 °C. bPerformed in at least triplicate, ± one standard deviation. cMeasured in duplicate, estimated uncertainty ca. 20%.
a
B
DOI: 10.1021/acssuschemeng.7b02978 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Letter
ACS Sustainable Chemistry & Engineering
Figure 1. 2D HSQC NMR spectra of corn stover lignin (A) before ozonolysis, (B) ozonized lignin (contact time of 1 min), and (C) ozonized lignin (contact time of 5 min). PCA, p-coumarate; FA, ferulate. Reaction conditions: acetic acid (12 v/v% H2O), P = 1 atm, T = 70 °C. permeation chromatography (GPC), gas chromatography (GC), high performance liquid chromatography (HPLC), and 1H and 13C nuclear magnetic resonance (NMR) spectroscopy. Detailed analytical procedures are described in the Supporting Information. Membrane Filtration. A stirred Millipore high pressure filter holder fitted with a 47 mm diameter SolSep BV NF030306 membrane sheet was used to isolate the ozonized products. Prior to use, the membrane disk was washed with and soaked in solvent for ca. 10 h. To improve the flux across the membrane, mixtures of acetic acid with either ethyl acetate or ethanol were tried as solvents. The solvent was removed from the membrane apparatus without allowing the membrane to dry. An ISCO pump was used to pump the ozonized lignin solution (glacial acetic acid, 2.5 min R.T) diluted with an equal volume of ethanol such that the products remain dissolved. A pressure of approximately 30 atm was used for the filtration. Samples of the filtrate were collected periodically and were analyzed by gas chromatography and by gel permeation chromatography (GPC).
structure of the processed lignin is largely preserved at shorter contact times. Although GPC data suggest that there is no extensive depolymerization when increasing the contact time from 1 to 5 min, 2D HSQC NMR data (Figures 1 and S4) indicate that there is increasing loss of aromaticity. While the intensity of the aromatic resonances in the ozonized lignin does not change significantly from the original lignin at a contact time of 1 min, the signals at δC/δH 104.0/6.70, 111.0/6.96, 115.5/ 6.30, and 119/6.78 decrease at the longer contact time of 5 min. Additionally, the 2D HSQC NMR spectra of the products collected at the shorter contact time (1 min) show that while the ozonized lignin shares many of the structural features in the untreated lignin some features are absent. The 13C resonances for p-coumarate functionality of the lignin ester (Table 2) are in good agreement with literature values.11 However, as revealed in Figure 1, some distinctive resonances of the p-coumaric acid esters are lost upon ozonolysis. Prominent signals corresponding to the p-coumaric acid (PCA)25,26 observed at δC/δH 145.0/ 7.51 (PCA α position), 130.5/7.48 (PCA 2,6), 115.5/6.70 (PCA 3,5), and 114.1/6.28 (PCA β) (green signals) in the unreacted
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RESULTS AND DISCUSSION Ozonolysis of acetosolv lignin isolated from maize (Zea mays L.) in a CSTR at 70 °C and ambient pressure yields 4hydroxybenzaldehyde and vanillin along with minor quantities of the corresponding acids, 4-hydroxybenzoic acid, and vanillic acid. At a contact time of 1 min, 4-hydroxybenzaldehyde and vanillin were produced almost exclusively (Figure S1) at steady yields of up to 5.2 and 1.5 wt %, respectively (based on the weight of dry lignin). Unexpectedly, the acid products (4-hydroxybenzoic acid and vanillic acid) were present in significantly lower yields of 0.62 and 0.18 wt %, respectively, rather than in stoichiometrically equivalent amounts relative to the aldehydic products. The total yield of these four products (ca. 7.5 wt % of the initial lignin) corresponds to substantial recovery of the aromatic portion of the pendant hydroxycinnamic acid groups which are commonly present in maize at ∼10 wt %.14 As shown in Table 1, longer contact times lower the monomer yields presumably due to their oxidation upon extended exposure to ozone. Gel permeation chromatography (GPC) and NMR analyses (Figures S2 and S3, respectively) confirm that the
Table 2. Measured 13C NMR Resonance Values for Lignin Acylated with p-Coumaric Acid in the γ-Position
C
DOI: 10.1021/acssuschemeng.7b02978 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Letter
ACS Sustainable Chemistry & Engineering lignin agree well with literature values (Figure 1A). Following ozone treatment at 70 °C with 1 min contact time, the signals corresponding to the bound PCA (Figure 1B) have much reduced intensity and are replaced by new resonances at δC/δH 134.0/7.74, 117.1/6.91, and 128.7/9.75, characteristic of 4hydroxybenzaldehyde (red signals) and at δC/δH 126.2/7.39, 114.6/6.91, and 110.7/7.35 for vanillin (blue signals). These comparative NMR data conclusively show that the pendant aromatic groups are being completely converted to the oxoaromatic products. As expected, the red and blue signals corresponding to the products decay at the longer contact time of 5 min (Figure 1C). Instead of the equimolar amounts of acids and aldehydes that would be expected from the decomposition of the secondary Criegee ozonide, we observe an excess of the aldehyde products (in the GC product analysis) and low concentrations of acid products [confirmed by NMR (Figure S5) and HPLC analysis]. According to the Criegee mechanism of olefin ozonolysis,27 the initial intermediate is a primary ozonide (1,2,3-trioxolane) formed by the 1,3-dipolar cycloaddition of the O3 molecule across the CC double bond of the alkene. This cleaves to form an aldehyde or ketone fragment and a carbonyl oxide, also known as the Criegee intermediate. A further cycloaddition of the carbonyl oxide with a dipolarophile, usually the aldehyde or ketone resulting from the scission of the primary ozonide, yields a secondary ozonide (1,2,4-trioxolane). Secondary ozonides are relatively stable28,29 and can be observed via their 13C NMR resonances at ca. 105 ppm.30 However, they were not observed in either the NMR spectra of the ozonolysis products (Figures S3 and S4) indicating that the olefin cleavage may not be proceeding via the conventional Criegee mechanism. Further, the yields of monomer products at identical contact times (1 min) were similar at both 30 and 70 °C, with the aromatic aldehyde monomers dominating the product spectrum. Clearly, this will not be the case if the observed products were formed by thermal decomposition of the secondary ozonide. While there exists ample evidence in the literature to validate the Criegee mechanism for alkene ozonolysis in nonparticipating solvents,27,31 it has also been reported that in participating nucleophilic solvents the Criegee mechanism can be interrupted. For example, acetic acid (the solvent in this case) can interrupt the recombination of the carbonyl oxide and aldehyde fragment by trapping the carbonyl oxide species, thereby preventing the formation of the secondary ozonide.32 Further, higher yields of the two aromatic aldehydes were observed when small quantities of water were added to the solvent (Figure 2). Direct production of aldehydes via ozonolysis by using an organic solvent containing low concentrations of dissolved water has been previously reported.33,34 For the ozonolysis of anethole to anisaldehyde in a solution of ethyl acetate with 10 wt % water,35 the mechanism was shown to involve the addition of water to the carbonyl oxide to yield a gem-hydroperoxy alcohol which decomposes to form hydrogen peroxide and the aldehyde in 99.5% purity. While the analytical data in this work clearly rule out product formation via the classic Criegee mechanism, the NMR data do not provide conclusive evidence for these alternate mechanisms. Clearly, detailed mechanistic studies are required to clarify the underlying mechanism. The ozonolysis pretreatment was applied to two additional lignins, one extracted from wheat straw (a grass lignin) and the other from American white oak (a woody lignin), using an ethyl acetate organosolv process. Wheat straw lignin is reported have similar acylation as maize lignin but with lower abundance.14 The
Figure 2. Effect of solvent composition on the yields of vanillin (diagonal lines) and 4-hydroxybenzaldehyde (cross-hatch) during continuous ozonolysis of corn stover lignin. Reaction conditons: P = 1 atm, T = 70 °C, contact time = 2.5 min.
yields of vanillin (1.29 wt %) and 4-hydroxybenzaldehyde (1.77 wt %) confirm the decreased presence of the hydroxycinnamic acids. Lignins derived from hardwoods are partially acetylated in the γ-position of the alkyl side chain and consequently have no peripheral olefin bound aromatic groups that would yield aromatic monomers after ozone treatment. Indeed, no 4hydroxybenzaldehyde was observed from ozonolysis of the white oak lignin, and the yield of vanillin was approximately a third of that observed with maize lignin. To determine if the aromatic monomer products were indeed derived from the hydroxycinnamic acids bound to the lignin polymer rather than from depolymerization of the lignin backbone, the ozone-treated lignin was separated and subjected again to ozonolysis. The products from an initial ozonolysis reaction, with a contact time of 2.5 min in acetic acid (12 v/v% water), were extracted with diethyl ether to remove the aromatic monomers. The residual polymeric material was dried and redissolved in a fresh solution of acetic acid. It was reozonized in the CSTR with a contact time of 2.5 min. Yields of vanillin and 4hydroxybenzaldehyde were significantly lower (
