Integrated Biorefining: Coproduction of Renewable Resol Biopolymer

Research Article pubs.acs.org/journal/ascecg

Integrated Biorefining: Coproduction of Renewable Resol Biopolymer for Aqueous Stream Valorization A. Nolan Wilson,† Mariel J. Price,† Calvin Mukarakate,† Rui Katahira,† Michael B. Griffin,† John R. Dorgan,‡ Jessica Olstad,† Kimberly A. Magrini,† and Mark R. Nimlos*,† †

National Bioenergy Center, National Renewable Energy Laboratory, 15013 Denver W Parkway, Golden, Colorado 80401, United States ‡ Department of Chemical Engineering and Material Science, Michigan State University, 428 South Shaw Lane, Room 2100, East Lansing, Michigan 48824, United States S Supporting Information *

ABSTRACT: Phenol-formaldehyde resins are major material classes that are used in a range of applications including composites, adhesives, foams, electronics, and insulation. While efforts have been made to produce renewable resins, there has yet to be an approach that offers potential for economic viability and meets all critical quality metrics. This failure can be attributed largely to the use of phenol and cresol homologues and to high separation costs. In this work, the use of phenol, cresol, and alkyl phenols derived from the aqueous phase generated from catalytic fast pyrolysis of biomass to produce a high-quality biobased resin is demonstrated. Production, through catalytic fast pyrolysis (CFP), separation, through distillation and adsorption unit operations, and synthesis, through typical resol chemistry, produced a resin with properties, such as curing kinetics and molecular weight, competitive with petroleum-derived resin. This work explores a pathway to value-added coproducts from a CFP waste stream, which has the potential to improve the economic viability of biofuels production. KEYWORDS: Biorefining, Resin, Valorization, Biopolymer, Coproduct

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products with high oxygen content,6 which adds complexity and cost to material production due to additional separation processes and the generation of a substantial wastewater stream.7 Chemical diversity and oxygen content can be reduced through catalytic fast pyrolysis (CFP), a demonstrated thermochemical approach to biomass upgrading in which pyrolysis vapors are catalytically upgraded.8,9 CFP produces multiple product streams including fuel vapors, condensed biooil, and an aqueous stream that contains useable biogenic carbon and is currently treated as a waste stream. A proposed strategy for treatment of the aqueous phase is regenerative thermal oxidation to remove carbonaceous compounds and generate process heat; 10 however, valuable commodity chemicals are found in this waste stream including phenol and alkyl phenols (phenolics). Commonly used in resins, applications of phenolics include electronics,11 flame retardant materials,12 wood adhesives and laminates,13 foaming polymers,14 among others.15 These molecules have increased market value relative to nonoxygenated aromatics such as

INTRODUCTION To enable better sustainability, augmentation of petroleumderived materials with renewably derived material is necessary, and this effort has gained traction as a primary focus of research over past decades.1,2 Currently, approximately 15% of oil resources are used for the production of commodity chemicals.3 If fossil fuel use diminishes due to the success of the emerging but not yet realized sustainable economy,4 the appeal to produce renewably resourced chemicals and materials concomitantly with sustainable biobased energy production is selfevident. Such an approach can enable wider adoption of biofuels while also providing humanity with a source of carbonbased materials. Thermochemically derived materials are continually being explored as replacements for petroleum-based materials and as coproducts during biofuel production.5 However, challenging limitations must be overcome in upstream processing, separations, and material synthesis before bioderived materials can succeed. Advantages of thermochemical conversion include access to certain monomers (e.g., aromatics) that are difficult to produce in biocatalytic processes at the production scales typical of the petrochemical industry. 5 Drawbacks to thermochemical conversion include a wide diversity of mixed © 2017 American Chemical Society

Received: March 21, 2017 Revised: July 7, 2017 Published: July 13, 2017 6615

DOI: 10.1021/acssuschemeng.7b00864 ACS Sustainable Chem. Eng. 2017, 5, 6615−6625

Research Article

ACS Sustainable Chemistry & Engineering

however, resins from fully bioderived resources often exhibit reductions in thermal stability, increased curing times, odorous characteristics, and reduced mechanical strength.21,23 The reduced properties are attributed to fewer reactive sites on the bioderived phenolic compounds and to impurities within the starting material.24 Others have utilized condensation fractionation of phenolics from pyrolytic oils producing a phenolics phase for partial replacement in resins,25 but this approach neglects a substantial fraction of the phenolics, which partition into the aqueous phase during CFP. Additionally, the aqueous stream is chemically less diverse, resulting in fewer impurities in the resulting phenolics phase.26 To the best of our knowledge, there has yet to be a scalable, robust separation train, which produces a high-quality (i.e., low chemical diversity) biobased phenolics stream, that uses the stream as the only source of phenolics in a resin, and that the resulting resin demonstrates comparable properties with petroleumderived resins. The purpose of this work is to demonstrate a separation train capable of extracting a high-quality phenolics stream from a CFP aqueous stream, synthesize a resol resin from the extracted biophenolics, and show the bioresin is similar to petroleum-derived resins synthesized under similar conditions.

benzene and toluene, and phenol retains an annual market of about $1 billion.16 Table 1 provides pricing for the benzene, Table 1. Prices for Petrochemicals compound

pricea ($/kg)

benzene toluene mixed xylenes phenol

1.63 1.41 1.44 2.12

a

All prices scaled to 2014 dollars using the Producing Price IndexCommodities.27,28

toluene, mixed xylene, and phenol. Petroleum-derived phenolics are produced through an energy intensive, caustic multistep cumene process, while bioderived phenolics are byproducts of lignin pyrolysis. The production of phenolic compounds from CFP is expected as the phenolic moiety is present in the lignin structure and is an extremely stable aromatic structure. Additionally, these phenolics are unattractive fuel molecules because of their high melting points, high boiling points (for gasoline), low cetane number (for diesel), and poor solubility in hydrocarbons.17 In keeping with the concepts of sustainability, separation of these molecules for use in synthesizing materials can improve the economics of bioderived fuel through production of value-added coproducts.18 Phenolic monomers are used in an array of materials such as polycarbonates, phenol-formaldehyde epoxies, and resins.19,20 Specifically for resins, substantial investigation toward finding a biobased substitute has occurred in past decades.21 These attempts include partial or full replacement of phenol with lignin derivatives or phenolic fractions.22 For partial replacement (25−50% substitution of petroleum phenol), resins often have comparative or improved properties over the phenol resin;

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EXPERIMENTAL SECTION

The experimental work describes a process to generate a phenolic resin from woody biomass. First, generation of the aqueous phase through CFP is discussed. The separation process, designed for the benchtop scale (i.e., 100−250 mL), used to extract biogenic phenolics from the CFP aqueous phase, Figure 1, is then discussed. The formulation and composition of a petroleum-based phenolics phase, to serve as a control for interpreting biogenic phenolics properties, is defined. The synthesis process of the resol resins is outlined, and, finally, characterization techniques for the CFP aqueous phase, phenolic phases, and resol resins analysis are described.

Figure 1. Separation scheme, which separates phenolics from initial CFP aqueous waste stream using distillation, polyvinyl pyro PVP resin adsorption, and solvent recovery unit operations. 6616


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ACS Sustainable Chemistry & Engineering Materials. Phenol, o-cresol, p-cresol, 2,3-dimethylphenol, polyvinylpyrrolidone (PVP), NaOH, formaldehyde, and deuterated chloroform were purchased from Sigma-Aldrich (St. Louis, MO). Bioderived aqueous phase was generated from the Davidson Circulating Riser Reactor at NREL using mixed hardwood and an HZSM-5 type catalyst that was provided from Johnson Matthey (London, UK). CFP Aqueous Stream Production. Catalytic upgrading of fast pyrolysis vapors from mixed hardwood was conducted in a DCR (Davison Circulating Riser reactor system), comprised of three reaction vessels (regenerator, riser, and stripper). The DCR is operated adiabatically. Similar to industrial fluid catalytic cracking (FCC) units, the riser and gas feed rates are set during operation once and maintained through the run. Upgrading zeolite catalyst (1.8 kg) is charged into the regenerator and moved through the system via pressure differentials. Hot gas filtered fast pyrolysis vapors are fed into the DCR via a heated transfer line (400 °C) through an injection port located at the base of the riser. The catalyst circulation rate (the primary source of heat to the riser) varies to maintain the desired target temperatures. Air is introduced into the regenerator for in situ catalyst regeneration, and the produced flue gas is analyzed to determine coke deposition on the catalyst. The product stream (composed of nitrogen, steam, and hydrocarbons) is sent through a reflux-style condenser that uses a countercurrent down flow of cold product liquids to scrub the product gases swept out from the catalyst steam stripper. The whole condensed product is allowed to drain and separate into a hydrocarbon phase and an aqueous phase, which are analyzed separately. Phenols were recovered from the aqueous phase for subsequent resin synthesis. Residual product gases are analyzed by online gas chromatography. For the experiment that generated the mixed hardwood aqueous feedstock, 1−2 mm hardwood particles were fed to the pyrolyzer with a biomass/N2 ratio of 0.5. Resulting pyrolysis vapors were fed to the DCR at 1.0 kg vapor/h for catalytic upgrading. The regenerator, stripper, riser outlet, and the feed preheater temperatures were set to 700, 500, 521, and 150 °C, respectively. These operating temperatures were chosen because they are commonly used in industrial FCC processes. For further detail, the reader is directed to Black et al.26 Separation − Azeotropic Distillation. 100−250 mL of CFP aqueous phase was loaded into a round-bottom flask. Attached to the flask was a 20 cm, jacketed, vigreux distillation column. The top of the column had an attached 18 cm condenser, which was packed with 3 mm borosilicate beads to increase surface area for condensation. The condenser was chilled with 10 °C water from a recirculation bath. Distillation proceeded by constantly stirring and heating the bottom flask with a heating mantle, which utilized a PID control loop through a Watlow EZ-ZoneTM controller. The set point for distillation was 105 °C, which is 10 °C above the boiling point of the azeotrope 95 °C at 5700 ft above sea level. Distillate was continually collected from the condenser in a round-bottom flask until the boiler temperature went above the boiling point of the azeotrope, at which point the heat source was removed and fractions collected. Separation − PVP Resin. The separation of the phenolics from the distillate occurs through a three-step process: (1) resin loading, (2) water desorption of bound species from the resin, and (3) methanol desorption of bound species from the resin. Preparation of PVP resin was performed by first washing the resin in EtOH using 5× the estimated bed volume of the resin for a minimum of 20 min. Bed volume density was 0.625 g/mL. The EtOH was subsequently filtered off, and the resin was rinsed in ultra high-purity deionized water (UHP DI) with 10−15× the estimated bed volume. The resin was loaded into a buret with a metering stopcock using UHP DI to fluidize the bed. The UHP DI was eluted from the column leaving ∼1 cm of UHP DI above the top of the PVP bed, which ensured air pockets in the column did not form. For resin loading, CFP aqueous phase was then added to the column and eluted at ∼1 mL/min. Maintaining a low flow rate, 1 mL/ min, results in a transport-limited system; that is, binding occurs more quickly than bulk transport through the column. This maximizes the binding driving force, the amount of free sites relative to the

concentration of unbound species, and minimizes the mass of resin required to retain all species on the column. To determine the respective amounts of solvent required for each elution, a packed bed of PVP resin was loaded by passing 50 mL of distillate through the column. After loading, 50 mL of DI water was passed through the column and fractionated. Finally, after all DI water was passed through the column, 50 mL of methanol was passed through the column. Before elution started, the bed was fluidized using a stir rod to remove air pockets. All three effluents from resin loading, water desorption, and methanol desorption were collected and analyzed via GC-MS and GC-FID. The final methanol effluent was dried using NaSO4, which was filtered before the final methanol phase was rotovapped on a Buchi R300 rotovap at 220 mbar and bath set point of 50 °C. To aid in residual water removal, chloroform was added at the end of the solvent recovery process and subsequently distilled as chloroform azeotropes with water. Phenolics Formulation. Qualitative and quantitative analysis of the separation process streams was performed to determine the similarity of the bioderived phenolics (biophenolics) to petroleumderived phenol (petro-phenol) and phenolics (petro-phenolics). The petro-phenolics are a mixture of phenolics with a composition similar to that of the biophenolics. The petro-phenolics contained phenol (61 wt %), o-cresol (15 wt %), p-cresol (19 wt %), and 2,3-dimethyl phenol (5 wt %) in a 16:3:4:1 molar ratio, respectively. Resol Synthesis. The resol synthesis performed here preceded as typical resol synthesis, using excess formaldehyde relative to the phenolics in the presence of a base catalyst, NaOH, to drive the reaction. Five grams of petro-phenol, petro-phenolics, or biophenolics was added to a round-bottom flask. EtOH and NaOH were added to the flask in 1.6:1 molar (3.95 g) and 0.05:1 mass (0.25 g) ratios to the phenolics, respectively. The flask was constantly mixed and heated under reflux at 80 °C. After reaching temperature, 37% formaldehyde was added dropwise to the flask such that the final formaldehyde:phenolics molar ratio was 1.3:1 (5.6 g of formaldehyde solution). The reaction continued for 4 h under reflux at 80 °C and constant mixing. After completion, the EtOH was rotovapped from the resulting products. The final product was placed under vacuum for >24 h to remove residual solvent. Characterization − Aqueous Phase. The aqueous phase feedstock used in these experiments was comprehensively characterized with gas chromatography mass spectroscopy (GCMS), inductively coupled plasma spectroscopy (ICP), proximate analysis, and carboxylic acid and carbonyl analyses to understand relevant chemistry and contaminants that may impact downstream separations and resin production. The starting CPF aqueous phase and distillation products were analyzed on an Agilent Technologies 7890A, 5975c GC-MS, and GCFID with a DB-5 column for identification and quantification. All samples were prepared by a 90:10 MeOH:sample dilution in the GC vial. Quantification proceeded via standard curves created for phenol, o-cresol, p-cresol, catechol, and hydroquinone, and adjustments to the response factors were made per Faiola et al.29 for species for which standard curves were not created; see Table S2. Integration was performed using Agilent’s Enhanced Chemstation software using the Chemstation Integrator for area determination and NIST library v2.0A 2001 for identification. The resulting areas were quantified using Python 2.7.11. Analysis code is available on github: https://github. com/wilsoa6/FID-Analysis_Mac. Characterization − Phenolics. The final biophenolics phase, petro-phenol, and a petro-phenolics phase were analyzed on a 400 MHz Bruker NMR in deuterated chloroform, and 1H spectra were generated using Bruker Topsin 3.2. Characterization − Resols. Thermogravimetric analysis was performed for each resin to investigate the thermal stability and degradation profiles on the resin. Thermograms were generated using 10−25 mg samples with platinum pans on a TA Q500 TGA. Each resin was cured in the TGA prior to degradation isothermal incubation in the TGA at 150 °C until the rate of mass loss was less than 0.02%/ min, which required 12−18 min depending on the sample mass. Cured 6617


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ACS Sustainable Chemistry & Engineering resins cooled back to room temperature before being degraded in the TGA using the high-RES mode with a maximum heat rate of 20 °C/ min, resolution of 4, and sensitivity of 1. During degradation, resins were heated from room temperature to 1000 °C under the flow nitrogen as the carrier gas at 40 and 60 mL/min for the sample and balance gas, respectively. Dynamic scanning calorimetry was used to determine peak exothermic temperatures during curing. Aluminum T-zero pans were purchased from TA Instruments (New Castle, DE). 2−5 mg of the synthesized resin was loaded into aluminum Tzero pans. The pans were then heated in a TA Q1000 DSC from −40 to 220 °C at heating rates of 10, 15, 20, 25, and 30 °C/min with nitrogen at 50 °C/min as the carrier gas. Temperature at peak exotherm was determined using TA Universal Analysis 2000 v4.5A software with 3 °C smoothing to reduce signal noise. Gel permeation chromatography samples of the synthesized resins were prepared by dissolving them in tetrahydrofuran (THF, Baker HPLC grade) with 0.5 mg/mL concentration. The dissolved samples were filtered (0.45 μm PTFE syringe filters) before GPC analysis. GPC analysis was performed using an Agilent HPLC with 3 GPC columns (Polymer Laboratories, 300 × 7.5 mm) packed with polystyrene-divinylbenzene copolymer gel (10 μm beads) having nominal pore diameters of 104, 103, and 102 Å. The eluent was THF, and the flow rate was 1.0 mL/min. An injection volume of 25 μL was used. The HPLC was attached to a diode array detector measuring absorbance at 260 nm (bandwidth 80 nm). Retention time was converted into molecular weight (Mw) by applying a calibration curve established using polystyrene standards of known molecular weight (1 × 106 to 580 Da) plus toluene (92 Da). Fourier Transform Infrared Spectroscopy (FTIR). FTIR absorption spectra were collected for each resin using a Thermo Nicolet iS50 spectrometer equipped with an integrated diamond attenuated total reflectance (ATR) module. Spectra were collected at room temperature and represent the average of 64 accumulations at 4 cm−1 resolution.

pounds, such as acetic acid, and a bottom oil phase containing the residual species. Leveraging the water-phenolics azeotrope in an initial distillation process offers an advantage at the laboratory scale as compared to other approaches such as liquid−liquid extraction followed by distillation. Given the low carbon content of the aqueous stream, dewatering the organics first would result in high losses during the fractional distillation due to holdup in the apparatus. Additionally, the azeotrope enables distillation at a lower temperature, 99.5 versus 182 °C, the boiling point of phenol. While this approach is appropriate for lab scale applications, high energy costs would be incurred during scaled-up production due to the energy demands for water distillation. An industry scale separations process would likely proceed by dewatering the organics first with liquid− liquid extraction (LLE) or resin adsorption; this staging would substantially reduce the energy demands of distillation. Resin Adsorption of Distillate. The distillate from azeotropic distillation contains a high weight percent of water, the phenolics, some acetic acid, and any residual light organic compounds. A PVP resin is used to recover the phenolic compounds from this mixture. PVP resins adsorb Lewis acid compounds through hydrogen bonding with the tertiary amine.31 Resin loading is performed by passing the distillate over a packed bed of PVP resin. To determine the mass of resin required to retain all target species, a breakthrough curve was generated by passing distillate over the column and collecting the effluent in fractions, Figure 2A. From this curve, the breakthrough point, VB, which is the last fraction from which no

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RESULTS AND DISCUSSION CFP Aqueous Phase Production. The composition of the mixed hardwood derived aqueous phase is shown in Table S3. ICP determined primary elemental species comprising Na, Al, P, S, Ca, Mg, and Fe at concentrations of 124, 107, 92, 48, 25, 10, and 1.9 ppm, respectively. The carboxylic acid numbers (CAN) of both CFP oil and aqueous phases are similar at 0.114 mol/kg and indicate that the acids partition equally between the two phases. This result is generally observed for other oil phases as well. The carbonyl content was 0.23 mol/kg of liquid and showed that some likely lighter carbonyl compounds migrated to the aqueous phase as compared to the CFP oil. Proximate analysis provided CHN and O by difference content with a C content of 1.49 wt %. Note that other CFP-derived aqueous phases can contain up to 30 wt % C. Here, we used the DCR-derived aqueous phase as it is well characterized and significantly enriched in phenolic species.26 To synthesize the desired polymeric materials, the separation train must produce a phase consisting of phenol, o-cresol, p-cresol, and residual alkyl phenols (e.g., 2,3-dimethyl phenol). As shown in Figure 1, the optimized separation train, which achieves this, uses three unit operations: (1) azeotropic distillation, (2) resin adsorption, and (3) solvent recovery. Azeotropic Distillation of CFP Aqueous Phase. A wellknown azeotrope between phenol, cresols, and water exists at 99.5 °C.30 This azeotrope was utilized to separate the phenolic compounds from other heavier compounds such as catechol, alkyl-catechols, and indene. This separation results in a distillate aqueous phase containing phenolics and other light com-

Figure 2. (A) Breakthrough curve containing concentration of individual species (left y-axis) and amount of occupied binding sites (right y-axis) as a function of loading volume, and (B) elution curve showing mass of species in effluent as a function of effluent volume for water and methanol desorption. 6618


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ACS Sustainable Chemistry & Engineering target species is present, can be determined. Additionally, N*B and NT can be determined, which are the amount of occupied amines at breakthrough and the total capacity of the resin in mmol, respectively. Determination of N*, the amount of retained species at the ith fraction, equal to the number of occupied amines, can be found using eq 1. Equation 2 is then used to determine the fractional coverage of the resin at breakthrough, θB.

phenolics remain bound to the column during water desorption as the chemical potential of the bound phenolics is lower than that of the water solubilized phenolics. When the solvent is switched to one with increased solubility (i.e., methanol), the chemical potential of the solubilized phenolics is lowered relative to the bound phenolics and desorption occurs. Conversely, the hydrophilic acetic acid readily elutes into the water and is completely removed before the solvent is switched. This provides a viable and simultaneous dewatering and separation method that has potential to be commercially scaled. An additional benefit is the removal of the characteristic “smokey” odor of the phenolics as the odor producing species are removed during water desorption.32 The odor associated with lignin-derived compounds has proven to be a significant stumbling block in commercialization activities including resins produced from biomass-derived phenolics.21,23 Characterization of Separated Phenolics. For 1H NMR analysis, petro-phenol and petro-phenolics were used as controls to compare with bioderived phenolics phase. The results of the NMR spectra are shown in Figure 3A. For petrophenol, characteristic shifts were present between 6.82 and 7.26 ppm, associated with aromatic protons, and 4.97 ppm, associated with the hydroxyl proton. The petro-phenolics exhibited similar aromatic proton shifts between 6.60 and 7.21 ppm. Broadening and shifting of the hydroxyl proton shift from 4.97 to 5.92 ppm is observed, which can be attributed to chemical exchange of this proton in the multicomponent system. Additional proton shifts are observed at 2.22 ppm, which is associated with the methyl groups on the cresols. In comparison, the biophenolics exhibited similar proton shifts in comparison to the petro-phenolics. In addition to the aromatic proton shifts, the biophenolics demonstrated similar behavior of the hydroxyl proton shift with a low, broad shift to 6.35 ppm and similar proton shifts at 2.22 ppm. The biophenolics show some residual peaks between 1 and 2 ppm and 3−6 ppm, which are likely due to minor impurities in the phase; however, a qualitative comparison of the petro-phenolics and biophenolics exhibited no major chemical shifts associated with different chemical functionalities within the two phases. GC-MS and GC-FID were used to identify and quantify chemical species present in the initial CFP aqueous phase, distillate, and PVP eluent. Characteristic GC-FID chromatograms are shown in Figure 3B. The chromatograms demonstrate the effectiveness of the separation scheme by disappearance of the acetic acid, catechol, hydroquinone, and alkyl catechol peaks in the PVP eluent phase. Concentrations, quantified using GC-FID, of the species in each separation stream are shown in Figure 3C. The removal of catechol, alkyl catechols, hydroquinone, and acetic acid demonstrates progressive purification of the phenolics. The concentrations determined here are in agreement with literature values.26 The distillate from the azeotropic distillation is free of all species with a boiling point higher than catechol, 246 °C. The distillation process improves the mass purity of phenolics from 54% to 83% with a recovery of 92% by mass. The major remaining impurity, acetic acid, is subsequently removed using the PVP resin, which results in the phenolics in methanol at an aggregate concentration of 3.5 g/L. The PVP resin improves the phenolic purity to >99% and has an associated mass recovery of >99%. The overall separation process mass recovery is 91% from the initial amount of phenolics in the CFP aqueous phase.

F

N* =

∑ OHiin − OHiout

(1)

i

θB =

NB* NT

(2)

OHini

where F is the total number of fractions collected, is the total amount of Lewis acid groups passed through the column at the ith fraction, and OHout is the amount of Lewis acid i groups measured in the fraction effluent at the ith fraction. By determining [N], which is NT normalized to the mass of resin in the breakthrough experiment, θB, and using eq 3, the total amount of resin required to retain all target species for a given volume of distillate with a known concentration of binding species can be calculated.

M resin =

OH [N ]θB

(3)

where Mresin is the amount of resin required to bind all target species and OH is the amount of Lewis acid groups in the solution in mmol. Calculated values of θB and [N] are provided in Table 2. Table 2. Calculated PVP Resin Separation Parameters parameter θB [N] [VH2O] [VCH3OH]

value 32 1.0 5.1 4.4

% mmol g−1 mL g−1 mL g−1

After resin loading, the next two steps to recover purified phenolics from the distillate are water desorption and methanol desorption. For efficient desorption, it is necessary to know the minimum volume of the desorbant that will remove all species from the column. The concentrations of species, determined via GC-FID, in the collected fractions are shown in Figure 2B. The minimum volume of solvent for each elution process was measured as the total volume of eluting solvent passed through the column once the concentration of the species reached zero. To ensure enough eluting solvent was used for separation, the determined volume was increased by 20% as a safety factor. The required volume was then normalized to Mresin resulting in the normalized elution volumes for water and methanol, [VH2O] and [VCH3OH], respectively. This normalization allows for scaling of the elution volume as a function of column size, and the normalized elution volumes for each eluent are provided in Table 2. Figure 2B shows that acetic acid desorbs into the water eluent but the phenolics remain on the column. The phenolics only elute with methanol. In addition to acetic acid, this method has been used to separate other hydrophilic compounds, such as cyclopentenone, data not shown. The 6619


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Figure 3. (A) 1H NMR spectra of petro-phenol, petro-phenolics, and biophenolics. (B) GC-FID chromatogram of initial CFP aqueous phase and resulting methanol PVP eluent. (C) Quantified values of compounds from the separation unit operations.

Figure 4. (A) Molecular weight distribution of the three synthesized resins. (B) TGA thermograms of the three synthesized resins. (C) FTIR spectra of cured and uncured resins.

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ACS Sustainable Chemistry & Engineering Resol Synthesis and Characterization. A synthesis scheme33 for a phenol-formaldehyde resin is provided in Figure S1. The resulting product is a viscous, light to dark brown fluid comprised of phenolic oligomers, which can be further cross-linked upon heating to the set temperature. The gelation of the cross-linked network through aromatic substitutions on the ortho (o) and para (p) positions, originally described by Flory and Stockmayer for branching polymer systems, can be effected by the functional groups on the aromatic rings. To compare the petro-based and biobased phenolic feedstocks, three resins can be synthesized from the phenolic mixtures, petro-phenol, petro-phenolics, and biophenolics, which resulted in three resols, pPF, pPCF, and bPCF, respectively. Relevant is not only the molar ratios for the monomers, but also the relative number of reactive sites available to form the network. The pPF resin contains 14% more reactive sites relative to the pPCF resin (1 para and 2 orthos = 3 sites/phenol), which can directly affect reactivity and branching. The pPCF resin was formulated to closely match the phenol, o-cresol, p-cresol, and dimethyl phenol distribution of the bPCF as determined through GCMS; however, variations in substitution location, for example, 2,3- versus 2,4-dimethyl phenol, can alter the number of available reactive sites, ultimately changing the polymer structure. To better understand how the biophenolics resin compared to a petroleum resin, several analytical methods were employed.34,35 To compare processability, GPC is used to determine relative molecular weight, which is directly related to solution viscosity. To compare thermal stability, resin samples are analyzed via TGA. Finally, the relative reactivity, which informs curing time, can be assessed through DSC. It is well established that reaction conditions such as solvent, temperature, temperature ramp rate, time, [NaOH], and consecutive additions of NaOH or formaldehyde can be used to control substitutions, branching, and molecular weight, among other properties;36−38 for this reason, the synthesis conditions of pPF, pPCF, and bPCF resins were the same to ensure comparability within the presented work. Molecular weight distribution can affect processability and internal bond strength (tensile strength orthogonal to wood panels) through viscosity,39 curing temperature,40 and gross penetration into wood flakes.41 To assess the differences in molecular weight distributions, the synthesized resols were analyzed via GPC. Normalized distributions as a function of molecular weight and quantitative values from the analysis are provided in Figure 4A and Table 3, respectively. From these

molecular weights. With a slightly higher MW and higher Pd, it is expected the resin will perform sufficiently well in comparison to petro-based resins in adhesive and binding applications. The reason for the observed difference in MW between the pPCF and bPCF resin is not readily apparent; however, a potential explanation is the difference in composition of the biophenolics and petro-phenolics. The biophenolics contained more variation in the types of phenolics (e.g., 2,4-dimethyl phenol, 2,5-dimethyl phenol, etc.). The full composition and the molar ratio relative to phenol of the biophenolics are provided in Table S1. Additionally, Black et al.26 performed more extensive analysis using GC-MS and LC-MS on the CFP phase and identified o-, m-, and p-cresol in addition to other substituted phenolics, Table S3. m-Cresol was not readily apparent in the GC-MS analysis performed here; however, it is possible the chromatography did not provide sufficient separation of m- and p-cresol given their similar boiling points and polarity. The types of substitutions on the phenolic ring, primarily a function of the shape-selectivity of the CFP catalysts,43 affect the reactivity of the phenolic ring with formaldehyde.44 Higher reactivity occurs for compounds with an unsubstituted para position due to electronic stabilization through hydrogen bonding of pendent hydroxyl groups.45 mCresol has been shown to directly lead to higher MW resins with higher Pd relative to o- and p-cresol under the same reaction conditions.46 The presence of m-cresol will additionally affect the degree of branching as p-cresol will form linear structures, through available substitution at the ortho positions, while m-cresol has the capacity for branching, through available substitutions at the ortho and para positions. If the biophenolic phase contains a variety of alkyl phenol and/or m-cresol, the biophenolics could exhibit a higher reactivity relative to the petro-phenolics phase due to increased concentrations of unsubstituted para positions. The higher reactivity of the phenolic rings would coincide with the observed higher MW. Another way composition could effect MW is through isomers of dimethyl phenol by altering resin structure during network formation. Dimethyl phenols, formed as intermediates on the catalyst during vapor upgrading,8 could change the resin structure. In the case of 3,5-dimethyl phenol where positions 2, 4, and 6 are available for reaction, branching would increase, or in the case of 2,4-dimethyl phenol, where only the 6 position is available for reaction, the monomer would be terminal. pPCF used 2,3-dimethyl phenol as a model compound, which would reduce the degree of branching and explain the increase in apparent MW relative to pPF. The bPCF contained the terminating 2,4-dimethyl phenol isomer, which could reduce the MW of the resin; however, the bPCF resin exhibited a higher apparent MW relative to the pPCF resin, so increased reactivity through m-cresol is a more feasible explanation. TGA was used to compare the degradation properties of the three resins, and calculate the weight percent of material (TG) and derivative of the weight percent of material (DTG) as a function of temperature. The resulting profiles for these values are shown in Figure 4B, top and bottom, respectively. The degradation profiles of each resin demonstrated up to five distinct degradation regimes apparent from the peaks A−E marked on the DTG profiles. The peak temperature and weight loss at the peak are provided in Table 4. The first three peaks are associated with three well-described degradation phenomena: (A) removal of small terminal groups and additional polycondensation cross-linking reactions, (B) degradation of

Table 3. Molecular Weight Data for the Three Synthesized Resins resin

Mp

Mn

Mw

Pd

pPF pPCF bPCF

540 520 520

410 420 490

510 550 790

1.26 1.30 1.63

data, it is apparent the number-average molecular weight, Mn, weight-average molecule weight, Mw, and polydispersity index, Pd, of the bioderived resin are all higher than the petroleumderived resins. Because the MW resin is higher for the bPCF resin, the viscosity of this resin will be higher as predicted through the Mark−Houwink equation.42 It is noted that the degree of branching in the pPF, pPCF, and bPCF may bias the hydrodynamic volume, ultimately affecting the relative 6621


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Extensive work has been performed elsewhere to understand the reaction chemistry of curing resol resins.47,51 Briefly, during the curing process, methylol groups undergo condensation with unsubstituted ortho or para positions on phenolic rings. This process results in the formation of methylene bridges, the primary cross-linking moiety in the resin, and reduces the number of methyol groups in the resin. Here, FTIR spectroscopy was utilized to probe changes to methyol and methylene functional groups during curing. As illustrated in Figure 4C, a broad peak in the 912−1095 cm−1 spectral region is apparent for the uncured resins but is nondistinct for the cured resins. This peak is assigned to the C−O deformation mode associated with uncondensed methylol groups.47 Dimethylene ether may also exhibit molecular vibrations in this region, but contributions from this functional group are expected to be minor due to its low concentration as formation of ether linkages is not preferred under base-catalyzed conditions.52 The reduction in the methylol C−O deformation mode upon curing is concomitant with an increase in peaks observed in the 1400−1495 cm−1 spectral region, which are assigned to the C−H bending modes of methylene bridges.47 The ratio of the C−O deformation peak area (912−1095 cm−1) to the C−H bending peak area (1400−1495 cm−1) is given in Table 5 and provides a comparative assessment of methylol

Table 4. Thermal Degradation Peaks, Associated Temperatures, and Associated Weight Percent Remaining for pFP, pPCF, and bPCF Resins temperature (°C)

weight % remaining

peak

pPF

pPCF

bPCF

pPF

pPCF

bPCF

A B C D E end

210 393 525 603 791 1000

204 394 527 601

205 360 474 590

98% 92% 81% 76%

97% 87% 69% 59%

1000

1000

98% 92% 80% 74% 68% 64%

71%

55%

the methylene bridge between aromatic units resulting in phenol and cresol homologues, and (C) degradation of the resin structure and homologues and generation of carbonaceous material through dehydrogenation reactions.47 Peak (D) is relatively small and is present in all three resins. This peak, which does not have a well-identified literature mechanism, is likely due to similar processes as peak (C). The ability to identify this subtle peak is attributed to the use of the HIGHRES Dynamic Rate algorithm as opposed to a constant heating rate. This algorithm dynamically adjusts the heating rate in response to the samples decomposition to maximize the weight change resolution. Finally, peak (E) was only identified in the pPF resin. The mechanism and species associated with this peak are also not well described; however, it is likely degradation of coked material given its high temperature. For thermal comparison of the three resins, the first three major peaks A−C are considered. Peak A for all resins occurred within 6 °C of each other and have negligible weight losses, ≥3%. Peak B occurred at similar temperatures for both pPF and pPCF; however, the bPCF resin showed degradation of the methylene bridge at ∼30 °C below the two petroleum-based resins. While the temperature for the bioderived resin is lower, it demonstrated thermal stability up to 360 °C, a reasonable temperature for many applications (IEC 60085:2007, NEMA Insulation Class A). Additionally, mass loss for the bPCF resin was within 5% of the petroleum-based resins at peak B, which indicates the bulk of the material did not degrade prior to this peak. The reduced temperature of peak B in the bPCF resin could be contributed to relative abundance of the phenolic groups next to the methylene bridge,48 the extent of crosslinking,49 or increased branching.50 The bPCF resin, while similar in chemical composition to the pPCF resin, has a wider diversity of alkyl phenols, which could potentially effect the position of the methylene bridge relative to the phenolic group. Additionally, from the MW distribution, the bPCF resin has both small and large oligomers, which could lead to a more cross-linked network. Finally, increased branching has been shown to decrease the degradation temperature of phenolic resins, which would be consistent with the presence of m-cresol in the bPCF resin, but not present in the pPCF resin. Determination of the direct cause of the shift in methylene bridge degradation temperature will require further experimentation, which could include 13C NMR to identify o−o, o− p, and p−p bond distriubtions or further purification to improve degradation profiles. Peak C was also shifted to a lower temperature for the bPCF resin. This is likely due to the resin having already undergone some decomposition processes and subsequently having increased concentrations of the phenol and cresol homologues, which begin to degrade at peak C.

Table 5. Ratio of C−O Deformation and C−H Bending Areas Observed in FTIR Spectra for As-Prepared and Cured Resins resin uncured

cured

C−O:C−H area pPF pPCF bPCF pPF pPCF bPCF

2.59 2.12 2.59 0.25 0.29 0.74

versus methylene functional groups in the uncured and cured resins. All three resins undergo a significant reduction in the relative methylol:methylene peak area when cured, which is consistent with the condensation of methylol groups and formation of methylene bridges. These data indicate that the biobased resins are capable of cross-linking chemistries similar to those of their petroleum-derived analogues. Interestingly, the calculated ratio of the cured bPCF is higher than that of the petroleum-derived resins, which indicates more unreacted methylol groups. The cause of the residual methylol groups is unclear, and further mechanical testing will be required to determine the ultimate effect. To understand the curing kinetics of the biobased resin relative to petro-based resins, the resins were cured using conventional DSC. Originally described by Murray and White53 and further expanded by Kissinger,54,55 the Kissinger equation given by eq 4 can be linearized with respect to 1/Tp, giving eq 5. This linearization is used to determine activation energies and frequency factors from experimentally measured peak temperature as a function of ramp rate as shown in Figure 5A, top. Additional application of the Crane equation, eq 6, enables the determine of reaction order for cases where Ea/(nR) ≫ 2Tp.56

EaβRTp2 = A e−Ea / RTp 6622

(4) DOI: 10.1021/acssuschemeng.7b00864 ACS Sustainable Chem. Eng. 2017, 5, 6615−6625

Research Article


Table 6. Scan Rate, Peak Exotherm Temperature, Calculated Activation Energies, and Calculated Order of Reaction for the Three Synthesized Resins resin pPF

pPCF

bPCF

(5)

−d ln(β)/d(1/Tp) = Ea /(nR ) + 2Tp

(6)

Tp,cross (°C)

Ea (kJ/mol)

n (−)

10 15 20 25 30 10 15 20 25 30 10 15 20 25 30

150 155 159 161 164 151 157 190 164 167 143 151 155 159 160

113 ± 7

0.94

99 ± 6

0.93

89 ± 9

0.93

the pPF resin; however, the bPCF and pPCF did not demonstrate significantly different activation energies at a 95% confidence. Given the similar MW of the pPF and pPCF, the difference between the activation energies of PF and PCF resins cannot be attributed to MW as described in other studies.49 An alternative explanation is the difference in composition of the PF versus PCF starting material. This was previously discussed as an explanation for the differences in MW for the synthesized resols. A follow-up study by GrenierLoustalot et al., which looked at the condensation of methylols on phenols, also showed the location of substitution methylol on the ring affects its reactivity in the cross-linking reaction.61 This was attributed to steric hindrance or electronic interactions of the aromatic hydroxyl and methylol groups. With different starting compositions of PF and PCF resins, the synthesized resins could exhibit differences in reactivity, which would manifest in lower activation energies. Additionally, the branched nature of the PF versus the PCF resins could contribute to the difference in activation energies. Increased branching will increase Tg, decreasing initial mobility of the reactants.50 For a fixed cure temperature, Tcure, the higher Tg will also decrease the time, or conversion, required to induce gelled-glass phase separation where the Tg = Tcure (i.e., vitrification).62,63 As this transition occurs, the mobility of the reactants is reduced and the system transitions from reaction to diffusion-controlled.64 With reduced mobility of reactants, either from higher initial Tg or from vitrification, the activation energy will increase. As previously discussed, the PF resin has higher potential for branching, which will coincide with the observed higher activation energy. Finally, reaction order for all three resins was found to be approximately first order, which is in good agreement with predicted theoretical first-order kinetics51 and similar results found elsewhere.65 Resol resins, while simple to synthesis experimentally, are highly complex polymeric structures with properties dependent on starting material and synthesis conditions. The starting material of the bPCF resin was compared to pPF and pPCF, and differences in methyl substitutions on the aromatic ring likely affected properties. Compositional differences resulted in an observed increase in relative molecular weight for the bPCF, likely due to more reactive sites from m- versus p-substitutions in the bPCF versus pPCF resin, respectively. Differences in

Figure 5. (A) DSC temperature scans at three different scan rates for the bPCF resin (top) and scans of the three synthesized resins (bottom) showing heat flow as a function of temperature. (C) Kissinger (top) and Crane (bottom) plots showing relation between scan rate and peak exotherm temperature of curing.

−ln(βTp2) = (1/Tp)(Ea /R ) − ln(AR /Ea)

β (°C/min)

In the above, Ea is the activation energy, β is the heating rate, R is the gas constant, Tp is the peak temperature, A is the frequency factor, and n is the reaction order. DSC curves for the three resins are shown in Figure 5A, bottom. Mechanistically, the curing of resol resins can be considered a two-step process with two exothermic peaks that characteristically fall between 100 and 130 °C, associated with the addition of formaldehyde to the aromatic rings resulting in hydroxymethylphenols, and between 140 and 150 °C, associated with the cross-linking reaction.57 A third process, the conversion of dibenzyl ether linkages to methylene bridges, has also been implicated in resol curing;58 as previously stated, formation of dibenzyl ether linkages is not preferred under alkaline reaction conditions.52 Others have observed only one exothermic peak under different synthesis conditions.59 The variations in phenomena in the literature have been considered and attributed to wide ranges of MW of the uncured resins between reports.60 From the data set, only the higher temperature peak was reliably apparent; as such, the associated temperatures for the cross-linking reaction, Tp,cross, are provided in Table 6. To further elucidate curing properties of the three resins, Tp,cross was use in the Kissinger and Crane equations resulting in the linear model shown in Figure 5B, top and bottom, respectively. The calculated values of Ea and n are provided in Table 6. To verify there was no biasing in the data set due to thermal lag from high heating rates (i.e., β > 15 °C/min), the residuals from the Kissing and Crane equation were plotted against scan rate, Figure S2. From the experimental values, it is apparent Tp,cross of the bPCF is lower for all heating rates. This corresponds to the observed lower activation energy of the bPCF resin relative to 6623



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degradation profiles were observed, which have been shown elsewhere to be dependent on the degree of branching. Branching is also hypothesized to have affected observed differences in activation energies for curing through chain mobility and changes to time−temperature-transition behavior. While measurable differences for the bPCF resin were observed, the bioresin still offers reasonable comparison to the petroleum-based resin for the testing performed. Further testing to understand branching structure and performance within a composite material (e.g., MOE, MOR) will be required to determine the resin’s suitability for direct substitution.

CONCLUSIONS In this work, a significant new path toward a valuable coproduct derived from a waste stream in biorefining is demonstrated. The process for separating a phase containing phenol, cresols, and residual alkyl phenols from a waste CFP aqueous stream is shown. The overall separation recovery is 91%. The phenolics phase obtained is used to synthesize a resol resin and compared to petroleum-based resins synthesized under the same conditions. The bioresin demonstrated similar properties when interrogated through GPC, TGA, FTIR, and DSC analytical methods. The overall process shows the viability of separating and synthesizing phenolic-based materials from bioderived resources. Continuing the development of renewable resins will require rigorous techno-economic analysis and further composite development to fully demonstrate the economic viability and performance acceptability of these bioderived resins. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00864. Synthesis scheme for resol, residuals of fits to Kissinger and Crane equations, species and composition of separated biophenolics, GC-FID factors for concentration determination, and chemical characterization of CFP aqueous streams (PDF)

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REFERENCES

(1) Vardon, D. R.; Rorrer, N. A.; Salvachua, D.; Settle, A. E.; Johnson, C. W.; Menart, M. J.; Cleveland, N. S.; Ciesielski, P. N.; Steirer, K. X.; Dorgan, J.; Beckham, G. T. Cis,cis-Muconic Acid: Separation and Catalysis to Bio-Adipic Acid for Nylon-6,6 Polymerization. Green Chem. 2016, 18, 3397−3413. (2) Vardon, D. R.; Franden, M. A.; Johnson, C. W.; Karp, E. M.; Guarnieri, M. T.; Linger, J. G.; Salm, M. J.; Strathmann, T. J.; Beckham, G. T. Adipic Acid Production from Lignin. Energy Environ. Sci. 2015, 8 (2), 617−628. (3) Davis, S. Petrochemical Industry Overview. IHS Chemical 2007, 1−85. (4) ORNL. 2016 Billion-Ton Report, 2016; Vol. I. (5) Bridgwater, A. V. Review of Fast Pyrolysis of Biomass and Product Upgrading. Biomass Bioenergy 2011, 38, 68−94. (6) Elliott, D. C.; Wang, H.; Rover, M.; Whitmer, L.; Smith, R.; Brown, R. Hydrocarbon Liquid Production via Catalytic Hydroprocessing of Phenolic Oils Fractionated from Fast Pyrolysis of Red Oak and Corn Stover. ACS Sustainable Chem. Eng. 2015, 3 (5), 892− 902. (7) Dutta, A.; Sahir, A.; Tan, E.; Humbird, D.; Snowden-swan, L. J.; Meyer, P.; Ross, J.; Sexton, D.; Yap, R.; Lukas, J.; Dutta, A.; Sahir, A.; Tan, E.; Humbird, D.; Snowden-swan, L. J.; Meyer, P.; Ross, J.; Sexton, D.; Yap, R.; Lukas, J. Process Design and Economics for the Conversion of Lignocellulosic Biomass to Hydrocarbon Fuels, 2015. (8) Mukarakate, C.; Watson, M. J.; ten Dam, J.; Baucherel, X.; Budhi, S.; Yung, M. M.; Ben, H.; Iisa, K.; Baldwin, R. M.; Nimlos, M. R. Upgrading Biomass Pyrolysis Vapors over β-Zeolites: Role of Silica-toAlumina Ratio. Green Chem. 2014, 16 (12), 4891−4905. (9) Carlson, T. R.; Cheng, Y.; Jae, J.; Huber, G. W. Production of Green Aromatics and Olefins by Catalytic Fast Pyrolysis of Wood Sawdust. Energy Environ. Sci. 2011, No. 4, 145−161. (10) Dutta, A.; Schaidle, J. A.; Humbird, D.; Baddour, F. G. Conceptual Process Design and Techno-Economic Assessment of Ex Situ Catalytic Fast Pyrolysis of Biomass: A Fixed Bed Reactor Implementation Scenario for Future Feasibility. Top. Catal. 2016, 59 (1), 2−18. (11) Rimdusit, S.; Ishida, H. Development of New Class of Electronic Packaging Materials Based on Ternary Systems of Benzoxazine, Epoxy, and Phenolic Resins. Polymer 2000, 41, 7941−7949. (12) Weil, E. D.; Levchik, S. A Review of Current Flame Retardant Systems for Epoxy Resins. J. Fire Sci. 2004, 22, 25−40. (13) Kamke, F. A.; Lee, J. N. Adhesive Penetration in Wood: A Review. Wood Fiber Sci. 2007, 39, 205−220. (14) Guo, C.; Zhou, L.; Lv, J. Effects of Expandable Graphite and Modified Ammonium Polyphosphate on the Flame-Retardant and Mechanical Properties of Wood Flour-Polypropylene Composites. Polym. Polym. Compos. 2013, 21, 449−456. (15) Pilato, L. Phenolic Resins: 100 Years and Still Going Strong. React. Funct. Polym. 2013, 73 (2), 270−277. (16) Linak, E.; Buchholz, U.; Kishi, A.; Guan, M. Phenolic Resins. IHS 2014, 1−287. (17) Ruddy, D.; Schaidle, J.; Ferrel, J.; Wang, J.; Moens, L.; Hensley, J. Green Chem. 2014, 16 (2), 454−490. (18) Langeveld, J. W. A.; Dixon, J.; Jaworski, J. F. Development Perspectives Of The Biobased Economy: A Review. Crop Sci. 2010, 50 (April), 142−151. (19) Carvajal, J.; Byrne, D. Phenol. IHS Chemical 2014, 1−80. (20) Janshekar, H.; Greiner, E.; Kumamoto, T.; Zhang, E. Cresols, Xylenols, and Cresylic Acids. IHS Chemical 2004, 1−94. (21) Effendi, A.; Gerhauser, H.; Bridgwater, A. V. Production of Renewable Phenolic Resins by Thermochemical Conversion of Biomass: A Review. Renewable Sustainable Energy Rev. 2008, 12 (8), 2092−2116. (22) Sen, S.; Patil, S.; Argyropoulos, D. S. Thermal Properties of Lignin in Copolymers, Blends, and Composites: A Review. Green Chem. 2015, 17 (11), 4862−4887. (23) Regi Giroux, E.; Freel, B.; Graham, R. Natural Resin Formulations. U.S. Patent US6326461B1, 2001.

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Research Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

A. Nolan Wilson: 0000-0002-9002-3585 Calvin Mukarakate: 0000-0002-3919-7977 Mark R. Nimlos: 0000-0001-7117-775X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Laboratory Directed Research and Development Program at the National Renewable Energy Laboratory and in collaboration with the Chemical Catalysis for Bioenergy Consortium, an Energy Materials Network Consortium funded by the Bioenergy Technologies Office. We would also like to thank Brenna Black for support around the analysis of the CFP aqueous phase. 6624


Research Article

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