Parinaz
Hafezisefat
a,
Jake K.
Lindstrom
a,
Robert C.
Brown
*a and
Long
Qi
*b
aDepartment of Mechanical Engineering, Iowa State University, Ames, Iowa 50011, USA. E-mail: rcbrown3@iastate.edu
bU.S. DOE Ames Laboratory, Iowa State University, Ames, Iowa 50011, USA. E-mail: lqi@iastate.edu
First published on 15th September 2020
We demonstrate for the first time non-catalytic, oxidative cracking with molecular oxygen (O2) to depolymerize native lignin into oxygenated phenolic monomers. Maximum monomer yield of 10.5 wt% was achieved at 250 °C after only 10 min of reaction and included vanillin, syringaldehyde, vanillic acid, and syringic acid. High rates of oxidation are attributed to the use of perfluorodecalin as solvent. Perfluorodecalin is a perfluorocarbon (PFC), characterized by their chemical stability and exceptionally high solubility for O2. Monomer yields were typically five-fold higher in perfluorodecalin compared to solvents more commonly employed in lignin conversion, such as methanol, butanol, acetonitrile, and ethyl acetate. Phenolic monomer production in perfluorodecalin favors high temperatures and short reaction times to prevent further oxidation of the produced monomers. Lignin oil obtained under oxidative conditions in perfluorodecalin showed lower molecular weight and smaller polydispersity compared to other solvents. Increasing the reaction time further decreased the molecular weight, while increasing reaction time in an inert atmosphere increased the molecular weight of the lignin oil. High concentrations of O2 in perfluorodecalin not only increased lignin depolymerization but suppressed undesirable condensation reactions. Depolymerization is likely initiated by thermally induced homolytic cleavage of ether linkages in lignin to form phenoxy and carbon-based radicals. These radicals bind with O2 as a radical scavenger and further react to form phenolic monomeric products rather than repolymerizing to large oligomers. The PFC process was scaled from 5 mL to 250 mL without any loss of yield. Because most organic compounds are not soluble in perfluorodecalin, recycling is easily achieved via liquid–liquid separation.
Reductive depolymerization of lignin employs redox catalysts to break β-O-4 and α-O-4 ether linkages2 with molecular hydrogen or hydrogen donor molecules as the reducing agent to stabilize deconstruction products. However, most of these reductive methods have been ineffective in breaking carbon–carbon bonds, and hydrogen is a relatively expensive input to the process. Alternatively, oxidative deconstruction of lignin using molecular oxygen (O2) or even air is potentially much cheaper than reductive depolymerization, although this possibility is just emerging.8 Interestingly, nature has harnessed oxidation as the main pathway to biologically degrade lignin, employing enzymes at ambient temperatures with molecular oxygen from air.9
Lignin oxidation can produce a range of functionalized chemicals of significant economic value such as aromatic acids, aromatic aldehydes, and aliphatic carboxylic acids.10,11 Several challenges stand in the way to its practical implementation. Strong oxidants, including hydrogen peroxide,12 ozone,13 and peroxy acids,14 and expensive and difficult to recycle catalysts15,16 are commonly required for effective oxidation of lignin. Recently, lignin oxidation by O2 has attracted increasing attention. However, O2 is a weak oxidizing agent, requiring either highly acidic17 or alkaline18 reaction conditions to catalyze the oxidation of lignin.10 For example, Beckham and co-workers systematically optimized the alkaline aerobic oxidation of lignin which is the heart of alkaline pulping process. In the presence of 2 M NaOH and 5 bar O2 in 30 min at 150 °C, maximal 14% and 23% yields of monomers were obtained from whole poplar biomass with or without additional copper sulfate respectively.19 The generation of highly alkaline wastewater when processing under basic conditions and the possibility of oxidizing organic solvents under acidic processing conditions are major challenges associated with oxidative depolymerization using O2. Moreover, many organic solvents used in deconstruction of lignin20 are highly flammable and explosive, making their use in the presence of oxygen problematic.
We hypothesize that radicals generated during high temperature homolytic cleavage of lignin ether bonds (phenoxy radicals and carbon-based radicals) can catalyze the oxidation of lignin without the need for acid or base catalysts. This hypothesis is motivated by the observation that thermal depolymerization of lignin under inert conditions shows clear evidence of homolytic cleavage to produce lignin radicals.21,22 Based on the bi-radical nature of oxygen, we further hypothesize that a balance between O2 and freshly generated carbon-based radicals from lignin suppresses undesirable repolymerization reactions to generate carbon–carbon bonds23 which is well established for radical polymerization chemistry.24
To investigate these questions, a solvent with high solubility toward both O2 and lignin is desirable; unfortunately, as described below, these are mutually exclusive criteria. Three major interactions influence the solubility of one substance in another. The most general of these is the nonpolar interaction, which is known as the dispersion interaction (δD). Since this interaction arises from atomic forces, it is a feature of all types of molecules. The second is dipolar interaction (δP) caused by permanent dipole forces between molecules. Intermolecular hydrogen bonding (δH) is the third major interaction.25 From these three so-called Hansen solubility parameters (HSPs), the total solubility parameter, δt, and solubility parameter distance, Ra, are calculated:
(Ra)2 = 4(δD2 − δD1)2 + (δP2 − δP1)2 + (δH2 − δH1)2 | (1) |
δt2 = δD2 + δP2 + δH2 | (2) |
To screen solvents for their ability to dissolve O2, Ra was calculated for several perspective solvents including water, acetonitrile, and butanol (Tables S1 and S2†). Solvents frequently used in lignin utilization have large Ra relative to O2 resulting in low O2 solubility. On the other hand, Ra for perfluorodecalin relative to O2 is very small (2.9 MPa0.5), making it an intriguing solvent for oxidative deconstruction of lignin. Perfluorodecalin is a type of perfluorocarbon (PFC) containing only carbon and fluorine atoms. Indeed, as shown in Fig. 1, PFCs dissolve 40–50 vol% oxygen at room temperature and pressure, which is several-fold higher than most other potential solvents.
Fig. 1 Solubility of oxygen (vol%) in PFCs at ambient temperature and atmospheric pressure (estimated by solubility parameter distance of oxygen and solvents). |
Perfluorodecalin is a non-toxic, non-flammable, thermally stable, non-bio-accumulating and non-ozone-depleting solvent which is used as artificial blood. Although perfluorocarbons in general are classified as greenhouse gases, perfluorodecalin has a high boiling point (142 °C) in contrast to the very low boiling points of PFCs (typically much below 0 °C) used in refrigeration and semi-conductor manufacturing. Perfluorodecalin's high boiling point mitigates its fugitive emissions. If employed in an industrial process, perfluorodecalin would be recovered in a sealed decantation process to protect against fugitive emissions. Industrial decanting equipment ranges from simple settling tanks, in which the heavy perfluorodecalin would be drained from the bottom, or decanter centrifuges, for continues decanting.
The high gas solubility of PFCs arises from their feeble van der Waals forces. Perfluorocarbons are more stable and chemically more inert than their hydrocarbon counterparts. In contrast to highly flammable hydrocarbons, PFCs are so inert that they can be used as fire extinguishants, and are resistant to thermal decomposition up to 400 °C.26 Their high O2 solubility and low toxicity27,28 make them attractive as artificial blood.29 As a reaction media, the low solubility of most organic compounds in PFCs facilitates product recovery, a fact that has been exploited in a biphasic catalytic system for methane oxidation.30,31
For polymers like lignin, only solvents within a certain range of HSPs can dissolve them. This range is often represented as Hansen sphere and only solvents within this space are likely to dissolve the polymer. In a three-dimensional HSP space, the center point of a solute's Hansen sphere corresponds to its three HSP values and RO (interaction radius) as the radius of the sphere that encompasses all good solvents of the polymer while excluding the poor solvents.25 Hansen solubility sphere of lignin is illustrated in Fig. S1.† The Ra/RO ratio is called the RED number. RED numbers less than 1.0 show the high affinity of solvent and solute and higher RED numbers show lower affinities.25Fig. 2 is a two-dimensional projection of the Hansen sphere for lignin (dashed circle) and several organic solvents (dots) in the δH−δP plane. Methanol and butanol are closer to the center of lignin circle, with Ra ranging between 8 to 16 MPa0.5, indicating that they are more likely to dissolve lignin than the other solvents. Acetonitrile and ethyl acetate, with Ra ranging between 16 to 24 MPa0.5, are better solvents than perfluorodecalin, benzene, and water, but not as good as methanol and ethanol. In general, solvents with high lignin solubility potential have low O2 solubility and vice versa.
Fig. 2 Hansen solubility parameters of lignin and solvents25 (the dashed circle is the maximum circumference of the Hansen solubility sphere in the δH–δP plane for lignin). |
To explore our hypotheses about non-catalytic oxidation of lignin and the role of O2 in suppressing undesirable repolymerization reactions, a solvent with high O2 solubility is desired. We have selected perfluorodecalin as our reaction media despite its poor solubilization of lignin. To compare its performance to more traditional solvents, we also performed experiments with two polar protic solvents (methanol and ethanol) and two polar aprotic solvents (acetonitrile and ethyl acetate). In a typical experiment, ball milled red oak powder and a solvent were sealed in a batch reactor, purged and pressurized at ambient temperature with O2 to the desired partial pressure (50–300 psi) and then further pressurized with molecular nitrogen (N2) to reach a total pressure of 300 psi. The reactor was heated to the desired temperature for the desired reaction time followed by cooling and recovery of liquid and solid products. The liquid was extracted with acetone and perfluorodecalin was recycled for other experiments. The acetone-extracted lignin oil was then analyzed by gas chromatography with flame ionization detection or mass spectrometry (GC-FID and GC-MS) and gel permeation chromatography (GPC) and HSQC NMR.
Fig. 3 Comparing aromatic monomer yields from lignin depolymerization for different solvents in nitrogen and oxygen environments (250 °C, 300 psi, 10 min reaction time). |
Fig. 4 Gas chromatograms of acetone-extracted lignin oils produced from oxidation of red oak in different solvents at 250 °C and 300 psi O2 pressure for 10 min reaction time. |
In contrast, the major monomeric products in the presence of O2 included benzaldehydes (1.7 wt% vanillin and 4.1 wt% for syringaldehyde) and benzoic acids (1.3 wt% for vanillic acid and 2.9 wt% for syringic acids). No cinnamaldehyde was detected, suggesting enhanced oxidation of intermediates after the lignin was thermally decomposed. Syringol was also detected, yielding about 0.5 wt%, possibly formed from the thermally induced decarbonylation of benzaldehydes or decarboxylation of benzoic acids.
For all other solvents, the major product for reaction in the absence of oxygen was 1,2,3-trihydroxybenzene. Monomer yields were either not improved or even decreased when O2 was admitted to these other solvent processes. Besides syringaldehyde and vanillin, cinnamaldehyde was found in all cases suggesting insufficient oxygen dissolved in these other solvents.
In contrast to other studies on oxidative depolymerization of lignin, we achieved significant yields of phenolic monomers in only 10 min without catalysts (acidic, basic, or transition metals) by operating at high temperatures, which induced thermal generation of radicals from lignin that further reacted with O2 dissolved in perfluorodecalin. Non-catalytic lignin depolymerization via hydrogenolysis in ethanol/isopropanol mixtures has been reported to produce monomers at high yield.33 However, to the best of our knowledge, effective non-catalytic, reagent-free oxidation of lignin has not been previously reported in the literature. From a safety standpoint, this was achieved without the use of flammable, peroxide-generating solvents. In comparison, Welton et al.34 and Prado et al.35 oxidized lignin in ionic liquids with O2 and hydrogen peroxide, but monomer yield after 5 h at 100 °C was less than 1 wt% and the bio-oil was contaminated with ionic liquid cations. Voitl et al.17 oxidized Kraft lignin in methanol/water (170 °C, 20 min, 5 bar pressure) with phosphomolybdic acid as catalyst to yield 5.0 wt% phenolic monomers. Deng et al.20 oxidized organosolv lignin in methanol (185 °C, 24 h, 1 bar pressure) with Pd/ceria as catalyst to produce 7.6 wt% phenolic monomers.
Thermal degradation of lignin without acid/base catalysts is initiated by lignin-derived radicals. We previously have shown that thermal degradation of lignin can produce radicals.21 The presence of cinnamaldehyde observed under N2 is also an indicator for the thermal decomposition of lignin model compounds and lignin via a radical-based mechanism.36,37 The hydroxyl and methoxy groups in lignin promote the formation of stable radicals. To demonstrate this, we used benzyl phenyl ether (BPE) as a model compound containing an α-O-4 lignin ether linkage. Only 4% conversion for BPE was observed for 10 min of reaction time at 250 °C and 300 psi O2. The absence of substantial oxidative cleavage arises from the failure to efficiently generate radicals from non-hydroxylated aryl ethers. Therefore, cleavage of lignin ether linkages generates radicals stabilized by hydroxylated phenyl rings,22 and phenoxy and carbon-based radicals drive non-catalytic oxidation of lignin.
In the presence of O2, increasing reaction time further decreased the intensity of high MW regions and increased monomer peaks, as shown in Fig. 5a and Table 1. In contrast, the progression with time under N2 shifts the MW of lignin oil toward larger molecules, suggesting a condensation process in the absence of O2 arising from the irreversible formation of C–C bonds.
Gas | Time (min) | M n | M w | PD |
---|---|---|---|---|
O2 | 5 | 323 | 511 | 1.58 |
10 | 272 | 314 | 1.16 | |
20 | 256 | 287 | 1.12 | |
N2 | 5 | 334 | 505 | 1.52 |
10 | 347 | 557 | 1.65 | |
20 | 383 | 661 | 1.73 |
GPC profiles of lignin oils produced in various solvents in the presence of O2 are depicted in Fig. 6. Lignin oils produced in alcohols, methanol, and butanol are primarily composed of high molecular-weight species centered at ∼3000 Da, reflecting the fact that short-chain alcohols are good solvents for lignin dissolution and possible participation in solvolysis. Ethyl acetate and acetonitrile are polar aprotic solvents, delivering bio-oils of similar distribution, centered at ∼400 Da. None of the solvents produced appreciable quantities of monomeric products, in agreement with the GC analysis in Fig. 4. Mn, Mw, and PD of these lignin oils are listed in Table S3.† These results indicate that O2 solubility plays a crucial and dominant role in oxidative depolymerization of lignin. The time-dependent GPC results suggest that perfluorodecalin is an outstanding carrier of O2 and boosts thermal oxidation of lignin via kinetic control. As a result of enhanced oxidation, condensation of the intermediate products of depolymerization was greatly reduced.
Monomer yield suffered as temperature increased to 300 °C, suggesting that the rate of deep oxidation of monomeric products was increasing faster than the rate of lignin depolymerization. Notably, PFC reaches the supercritical state above 293 °C.38 Under supercritical conditions, rates of diffusion and reaction could be significantly altered, which could be another reason monomer yield decreased at 300 °C.
To further study the role of O2 in mitigating condensation reactions, the effect of O2 pressure on the yield of monomeric products was investigated. Oxidation was performed under O2 partial pressures between 50 and 300 psi at 250 °C. Under N2, the monomeric yield was very low, and almost no oxidation products were produced (vide supra, Fig. 3). However, under the same conditions of temperature and time, increasing oxygen partial pressure increased monomer yield from 6.0 wt% at 50 psi to 10.5 wt% at 300 psi (Fig. 8). Under increasing O2 pressure, vanillin and syringaldehyde yields remained almost constant, but vanillic acid and syringic acid yields increased. Because vanillic acid and syringic acid are products of the oxidation of vanillin and syringaldehyde, respectively, their increases show that vanillin and syringaldehyde are being oxidized rather than condensed as O2 partial pressure is increased.
The positive effect of O2 on monomer yield can be explained by the specific interaction of carbon-based radicals with O2. Free radical polymerization (FRP) is the process by which radicals catalyze polymerization. FRP is initiated from radicals and propagates by the addition of monomers (such as olefins) to the radicals to form C–C bonds, thus growing the polymer chain. Termination of radical polymerization requires an inhibitor. Molecular oxygen, a bi-radical, is a common inhibitor in FRP.24 The active end of the growing polymer chain can react with O2, possibly producing a peroxy radical, which is much less susceptible to further reactions with monomers to form O–C bonds.24
Radicals generated from thermal depolymerization of lignin can directly react with O2 to form oxidation products instead of condensing as occurs in inert environments (Fig. 9a). However, under N2 (Fig. 9b), the carbon-based radicals will be more easily coupled via repolymerization reactions, which eventually reduces monomer yield and increases the MW of the lignin oil (vide ante). If O2 can be transferred from the gas phase to the liquid phase at sufficient rates, it can serve as both depolymerization promoter and capping agent and enhance the formation of phenolic monomers.
Fig. 9 Proposed mechanism of non-catalytic oxidation of lignin. (a) Monomers stabilized by O2 and (b) undesirable lignin condensation products in absence of O2. |
The lignin oil was further characterized by HSQC NMR (Fig. 10). As one of the few techniques to deliver structural insights for oligomers, HSQC NMR can be applied to directly identify chemical changes to the interunit linkages in lignin,39 while GPC only provides information on molecular weight. The lignin oil obtained under N2 mainly showed guiacyl units (G5/6) and syringyl (S2/6) units. Lignin oil obtained under O2 contained less guaicyl and syringyl units. Instead, oxidized guiacyl units (G′2) and oxidized syringyl units (S′2/6) increased significantly with the emergence of α-ketone structures.
Fig. 10 HSQC NMR spectra of lignin oils obtained under 300 psi of (a) N2 and (b) O2. Reaction conditions: 100 mg red oak, 5 ml perfluorodecalin, 250 °C, 10 min reaction time. |
Fig. 10 displays the aldehyde region for δH/δC in the ranges of 9–10.2 ppm and 188–195 ppm. Lignin oil generated under N2 showed only small areas of benzaldehyde and cinnamaldehyde end groups, while lignin oil obtained under oxygen showed a significant region for benzaldehyde due to syringaldehyde and vanillin production. Cinnamaldehyde radicals are known to initiate condensation reactions.40 As the NMR data illustrates, the oligomer end group with cinnamaldehyde moiety disappeared in the presence of O2. As shown in Fig. 10, carbohydrate-derived products including levoglucosan are also produced under oxidative conditions (δH/δC in the ranges of 3–5 ppm and 50–80 ppm). Some of them have been also identified by GC-MS (Fig. S2–S6†).
In a mass transfer limited system, illustrated in Fig. 11a, the gas–liquid interface is rich in O2, while the bulk of the liquid remains O2-deprived because of low O2 solubility and high consumption. In this circumstance highly reactive radicals from lignin are expected to readily repolymerize and produce condensed oligomeric lignin oil of high MW. However, in a kinetically controlled system, illustrated in Fig. 11b, enough O2 diffuses into the bulk liquid to react with lignin radicals and produce less reactive radicals. These oxygenated radicals further transform to produce oxidation products like benzaldehydes and benzoic acids and suppress repolymerization reactions.
Fig. 11 The influence of mass transfer on the selectivity of lignin oxidation by O2. (a) Mass transfer-controlled system and (b) kinetically controlled system. |
The scheme of the proposed oxidative depolymerization process is illustrated in Fig. 12. Perfluorodecalin is stable at reaction conditions (≤300 °C) in this study and PFCs are not miscible with most organic compounds, which allows simple, facile liquid–liquid separation for recovering and recycling PFCs without emulsion formation (Fig. S7†). Recovered perfluorodecalin was extremely pure: no contaminants or decomposition of perfluorodecalin were identified by GC or NMR in the recovered PFC (Fig. S8–S10†). Recycled perfluorodecalin was used for the second runs performed for the purpose of statistical analysis.
Fig. 12 Schematic diagram of the proposed process for oxidative depolymerization of lignin in perfluorodecalin with solvent recycle to produce valuable phenolic monomers at high selectivity. |
Furthermore, GC analysis of the lignin oil revealed no residual perfluorodecalin. In contrast, other solvents are likely to decompose or be oxidized if employed in high-temperature oxidation reactions. For instance, butanol is not inert under oxidative conditions (Fig. S5†), and side products, including 1,4-butanediole, 2-methyl butane, 2-methyl butanoic acid, and 1,3-butanediol, were identified by GC.
In these experiments, molecular oxygen was not only effective in promoting depolymerization but also blocked condensation to recalcitrant phenolic oligomers. Increasing reaction time in perfluorodecalin shifted the lignin oils to lower molecular weight and lower polydispersity. GC analysis revealed that oxygen increased yields of oxygenated phenolic monomers while HSQC NMR showed that phenolic oligomers were also oxygenated. Lignin depolymerization in perfluorodecalin under N2 only produced 1.3 wt% of monomers. GPC analysis of the lignin oil obtained under N2 suggests mostly oligomeric products, and average molecular weight increased with increasing reaction time, suggesting condensation predominated under non-oxidative conditions. This is consistent with the role of O2 as an inhibitor in free radical polymerization reactions. Under oxidative condition, phenoxy and carbon-based radicals, produced by homolytic cleavage of lignin ether linkages, reacted with O2 to produce less reactive species. However, under inert conditions, these radicals were easily repolymerized, causing an increase in the molecular weight of lignin oil. GPC characterization of lignin oils from other solvents indicated products were primarily phenolic oligomers even under O2 atmospheres, suggesting that mass transfer was rate limiting for these solvents.
Non-catalytic lignin oxidation in perfluorocarbons shows promise as a safe, efficient, and economical method to convert lignin to value-added products. The process is readily scalable, and perfluorocarbons are thermally stable. Since PFCs do not dissolve the products of lignin depolymerization, they can be readily recovered by simple liquid phase separation.
Red oak (100 mg) and perfluorodecalin (5 ml) were added to the reactor. In order to assure mass transfer of products to solvent was not rate limiting in the microreactors, we used a high solvent to substrate mass ratio, which was based on our previous research.42 The reactors were closed and purged five times with nitrogen gas, pressurized with oxygen to the desired oxygen partial pressure, and then nitrogen added to the desired total operating pressure of 300 psi. The reactors were then connected to a custom-built shaker and submerged into a preheated sand bath, which heated the reactor at a rate of 3.5 °C s−1. Reaction time was defined as the time interval between reaching the desired reactor temperature and the conclusion of the test when the reactor was removed from the sand bath and quenched in water.
For each experiment, the empty reactor was preheated to the desired temperature using a 1000 W electrical heater and fast-response temperature controller. The feeder autoclave was filled with 250 mL solvent, 4 g red oak, and stirred for 30 min. At this point, the feeder and reactor were purged and pressurized with nitrogen and nitrogen/oxygen mixtures, respectively, to desired partial and total pressures. The valve between feeder and reactor was then opened to rapidly transfer the mixture of reactants and solvent from the feeder to the reactor. This system made possible the heating of the reactants and solvent to desired reaction temperature in less than 3 min, much faster than a conventionally operated autoclave reactor. At the completion of an experiment, the valve between the reactor and bottoms condenser was opened and the pressurized contents were rapidly expelled into the unpressurized bottom condenser, cooling the products below 100 °C within a few seconds.
(3) |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0gc02505d |
This journal is © The Royal Society of Chemistry 2020 |