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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 Robert Dorgan, Jessica Olstad, Kimberly A. Magrini, and Mark R Nimlos ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.7b00864 • Publication Date (Web): 13 Jul 2017 Downloaded from http://pubs.acs.org on July 26, 2017

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ACS Sustainable Chemistry & Engineering

Integrated Biorefining: Coproduction of Renewable Resol Biopolymer for Aqueous Stream Valorization A. Nolan Wilsona, Mariel J. Pricea, Calvin Mukarakatea, Rui Katahiraa, Michael B. Griffina, John R. Dorganb, Jessica Olstada, Kimberly A. Magrinia and Mark R. Nimlosa* a

National Bioenergy Center, National Renewable Energy Laboratory, 15013 Denver W Pkwy, Golden, CO 80401, USA. b

Dept. Chemical Engineering and Material Science, Michigan State University, 428 S. Shaw Lane, Room 2100, East Lansing, MI 48824 *Corresponding Author: [email protected]

KEYWORDS: Biorefining, Resin, Valorization, Biopolymer, Coproduct

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 homologs and to high separation costs. In this work, the use of 1 ACS Paragon Plus Environment


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phenol, cresol and alkyl phenols derived from the aqueous phase generated from catalytic fast pyrolysis of biomass to produce a high quality bio-based 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 co-products from a CFP waste stream, which has the potential to improve the economic viability of biofuels production.

Introduction To enable better sustainability, augmentation of petroleum derived materials with renewablyderived resource materials 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 bio-based energy production is selfevident. Such an approach can enable wider adoption of biofuels while also providing humanity with a source of carbon based materials. Thermochemically derived materials are continually being explored as replacements for petroleum-based materials and as co-products 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 2 ACS Paragon Plus Environment

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conversion include a wide diversity of mixed 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 bio-oil 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 non-oxygenated aromatics such as benzene and toluene and phenol retains an annual market of about $1 billion 16. Table 1 provides pricing for the benzene, 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 co-products 18.

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Phenolic monomers are used in an array of materials such as polycarbonates, phenolformaldehyde epoxies and resins

19,20

. Specifically for resins, substantial investigation towards

finding a bio-based 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; however, resins from fully bio-derived 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 bio-derived 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) bio-based phenolics stream, which uses the stream as the only source of phenolics in a resin, and which the resulting resin demonstrates comparable properties with petroleum derived 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 bio-phenolics and show the bio-resin is similar to petroleum derived resins synthesized under similar conditions.


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Table 1. Prices for petro-chemicals Compound

Price* ($/kg)

Benzene

1.63

Toluene

1.41

Mixed Xylenes

1.44

Phenol

2.12

*All prices scaled to 2014 dollars using the Producing Price Index-Commodities 27,28

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. Then the separation process, designed for the bench top scale (i.e. 100-250mL), used to extract biogenic phenolics from the CFP aqueous phase, Figure 1, is 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. Materials. Phenol, o-cresol, p-cresol, 2,3-dimethylphenol, polyvinylpyrrolidone (PVP), NaOH, formaldehyde, deuterated chloroform were purchased from Sigma-Aldrich (St. Louis, MO). Bioderived aqueous phase was generated from the Davidson Circulating Riser Reactor at NREL



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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 ºC) through an injection port located at the base of the riser. The catalyst circulation rate (the primary source of heat to the riser) varies in order 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 on-line 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/hr for catalytic upgrading. The regenerator, stripper, riser outlet, and the feed pre-heater temperatures were set to 700 °C, 500 °C, 521 °C,


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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 18cm condenser, which was packed with 3mm 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

CH3OH

CFP Aqueous

PVP Resin

H 2O

MeOH Desorption Effluent

Solvent Removal

CH3OH

Distillate

Distillation

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H 2O

H2O Desorption Effluent H2 O

Bottoms

Adsorption Effluent

Phenolics for Resol Synthesis

Figure 1. Separation scheme, which separates phenolics from initial CFP aqueous waste stream using distillation, polyvinyl pyro PVPresin adsorption and solvent recovery unit operations. 7 ACS Paragon Plus Environment


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controller. The setpoint for distillation was 105 °C, which is 10 °C above the boiling point of the azeotrope 95 °C at 5700’ 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 5X the estimated bed volume of the resin for a minimum of 20min. 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 10-15X the estimated bed volume. The resin was loaded into a burette with a metering stopcock using UHP DI to fluidize the bed. The UHP DI was eluted from the column leaving ~1cm of UHP DI above the top of the PVP bed which ensured air pocks 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, 1mL/min, results in a transport-limited system, i.e. 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 8 ACS Paragon Plus Environment

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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 R-300 rotovap at 220 mbar and bath setpoint 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 bio-derived phenolics (bio-phenolics) to petroleum-derived phenol (petro-phenol) and phenolics (petro-phenolics). The petro-phenolics are a mixture of phenolics with a composition similar to the bio-phenolics. 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. 5 g of petro-phenol, petro-phenolics or bio-phenolics were added to a round bottom flask. EtOH and NaOH were added to the flask in a 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 drop-wise to the flask such that the final formaldehyde:phenolics molar ratio was a 1.3:1 (5.6 g of formaldehyde solution). The reaction continued for 4 hrs 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 hrs to removed residual solvent. 9 ACS Paragon Plus Environment


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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 GC-FID 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, Supporting Information Table S2: calculated response factors. 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 bio-phenolics phase, petro-phenol and a petrophenolics phase were analyzed on a 400MHz Bruker NMR in deuterated chloroform and 1H spectra 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 10 ACS Paragon Plus Environment

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of mass loss was less than 0.02 %/min which required 12-18 min depending on the sample mass. Cured resins cooled back to room temperature before being degraded in the TGA using the HighRESTM 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. Aluminium T-zero pans were purchased from TA Instruments (New Castle, DE). 2-5mg of the synthesized resin was loaded into aluminum Tzero pans. The pans were then heated in a TA Q1000 DSC from -40 °C 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 x 7.5 mm) packed with polystyrene-divinyl benzene copolymer gel (10 µm beads) having nominal pore diameters of 104, 103, and 102 Å. The eluent was THF and the flow rate 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 (band width 80 nm). Retention time was converted into molecular weight (Mw) by applying a calibration curve established using polystyrene standards of known molecular weight (1 x 106 to 580 Da) plus toluene (92 Da).



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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.

Results and Discussion CFP Aqueous Phase Production. The composition of the mixed hardwood derived aqueous phase is shown in Supporting Information Table S3: chemical characterization of ex situ CFP aqueous streams. 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 moles/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 mole/kg of liquid and showed that some likely lighter carbonyl compounds migrated to the aqueous phase compared with 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 well-known azeotrope between phenol, cresols and water exists at 99.5 °C

30

. This azeotrope was utilized to separate the phenolic


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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 compounds, such as acetic acid, and a bottoms oil phase containing the residual species. Leveraging the water-phenolics azeotrope in an initial distillation process offers an advantage at the laboratory scale 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 °C vs. 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.



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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 target species is present, can be determined. Additionally, N∗ 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 equation ( 1 ). Equation ( 2 ) is then used to Figure 2. (A) Breakthrough curve containing concentration of individual species (left y-axis)

determine the fractional coverage of the resin at breakthrough, θB.

and amount of occupied binding sites (right yaxis) as a function of loading volume, (B) Elution curve showing mass of species in effluent as a function of effluent volume for 14 water and methanol desorption. ACS Paragon Plus Environment

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(1)

∗

(2)

∗

Where F is the total number of fractions collected, OH""# is the total amount of Lewis acid groups passed through the column at the ith fraction, OH"$%& is the amount of Lewis acid 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 equation ( 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.

)*+,

[]

(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

Value

32

%

[]

1.0

mmol g-1

3456 7 8

5.1

mL g-1



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3495: 75 8

4.4

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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 which 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, 3456 7 8 and 3495: 75 8, 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 phenolics remain bound to the column during water desorption as the chemical potential of the bounds phenolics is lower than 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 16 ACS Paragon Plus Environment

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“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 phenolics21,23. Characterization of Separated Phenolics. For 1H NMR analysis, petro-phenol and petrophenolics were used as controls to compare with bioderived phenolics phase. The results of the NMR spectra are shown in Figure 3A. For petro-phenol, characteristic shifts were present between 6.82-7.26 ppm, associated with aromatic protons, and 4.97 ppm, associated with the hydroxyl proton. The petro-phenolics exhibited similar aromatic proton shifts between 6.60-7.21 ppm. Broadening and shifting of the hydroxyl proton shift from 4.97 ppm to 5.92 ppm is observed which can be attributed to chemical exchange of this proton in the multi-component system. Additional proton shifts are observed at 2.22 ppm, which is associated with the methyl groups on the cresols. In comparison, the bio-phenolics exhibited similar proton shifts in comparison to the petro-phenolics. In addition to the aromatic proton shifts, the bio-phenolics demonstrated similar behaviour of the hydroxyl proton shift with a low, broad shift to 6.35 ppm and similar proton shifts at 2.22 ppm. The bio-phenolics show some residual peaks between 1-2 ppm and 3-6 ppm, which are likely due to minor impurities in the phase; however, a qualitative comparison of the petro-phenolics and bio-phenolics 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 17 ACS Paragon Plus Environment


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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. Resol Synthesis and Characterization. A synthesis scheme 33 for a phenol-formaldehyde resin is provided in the Supporting Information Figure S1: resol synthesis scheme. 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 crosslinked network through aromatic substitutions on the ortho (o) and para (p) positions, original 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 bio-based phenolic feedstocks three resins can be synthesized from the phenolic mixtures, petro-phenol, petro-phenolics and bio-phenolics, 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 & 2 orthos = 3 sites / phenol), which can directly affect reactivity and branching. The pPCF resin was 18 ACS Paragon Plus Environment

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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, e.g. 2,3- vs. 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 employed34,35. 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, 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.



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Figure 3. (A) 1H NMR spectra of petro-phenol, petro-phenolics and bio-phenolics. (B) GC-FID chromatogram of initial CFP aqueous phase and resulting methanol PVP eluent. (C) Quantified values of compounds from the separation unit operations. Molecular weight distribution can affect processability and internal bond strength (tensile strength orthogonal to wood panels) through viscosity

39

, curing temperature


40

and gross

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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 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 petroleum derived resins. Since 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 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.



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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. 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 bio-phenolics and petro-phenolics. The bio-phenolics contained more variation in the types of phenolics (e.g. 22 ACS Paragon Plus Environment

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2,4 dimethyl phenol, 2,5 dimethyl phenol, etc). The full composition and the molar ratio relative to phenol of the bio-phenolics are provided in Supporting Information Table S1: species and compostion of separated bio-phenolics. 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, Supporting Information Table S3: chemical characterization of ex situ CFP aqueous streams . m-Cresol was not readily apparent in the GCMS 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 catalysts43, 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

. m-Cresol 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 bio-phenolics could exhibit a higher reactivity relative to the petro-phenolics phase due 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 upgrading8, could change the resin structure. In the case of 3,5 dimethyl phenol where positions 2, 4 and 6 available for reaction, branching would increase



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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.

Table 3. Molecular weight data for the three synthesize resins. Resin

Mp

Mn

Mw

Pd

pPF

540

410

510

1.26

pPCF

520

420

550

1.30

bPCF

520

490

790

1.63

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 is 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 the methylene bridge between aromatic units resulting in phenol and cresol homologs and (C) degradation of the resin structure and homologs and generation of carbonaceous material through dehydrogenation reactions 24 ACS Paragon Plus Environment

47

.

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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 HIGH-RESTM 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 is also not well described; however, it is likely degradation of coked material given its high temperature. 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

210

204

205

98%

98%

97%

B

393

394

360

92%

92%

87%

C

525

527

474

80%

81%

69%

D

603

601

590

74%

76%

59%

E

791

-

-

68%

-

-

End

1000

1000

1000

64%

71%

55%



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For thermal comparison 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 bio-derived 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

branching50. 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 C13 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 subsequently having increased concentrations of the phenol and cresol homologs, which begin to degrade at peak (C).


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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 crosslinking 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 non-distinct 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 catalysed 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 vs. 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 bio-based resins are capable of similar crosslinking chemistries as their petroleum derived analogues. Interestingly, the calculated ratio of the cured bPCF is higher than 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. 27 ACS Paragon Plus Environment


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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. Table 5. Ratio of C-O deformation and C-H bending areas observed in FTIR spectra for asprepared and cured resins. 28 ACS Paragon Plus Environment

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Resin

C-O:C-H Area

Uncured

Cured

pPF

2.59

pPCF

2.12

bPCF

2.59

pPF

0.25

pPCF

0.29

bPCF

0.74

To understand the curing kinetics of the bio-based resin relative to petro-based resins, the resins were cured using conventional DSC. Originally described by Murray and White expanded by Kissinger

54,55

53

and further

, the Kissinger equation given by Equation 4 can be linearized with

respect to 1⁄XY , giving Equation 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, Equation 6, enables the determine of reaction order for cases where [\ ⁄(]^) ≫ 2Tp 56.

[\ `^XYa bc def⁄gh

(4)

i]j`XYa k (1⁄XY )([\ ⁄^) i](b^ ⁄[\ )

(5)

li](`)⁄lj1⁄XY k [\ ⁄(]^) m 2XY

(6)



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Page 30 of 44

In the above, Ea is the activate 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 100-130 °C, associated with the addition of formaldehyde to the aromatic rings resulting in hydroxymethylphenols, and between 140-150 °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 has 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 value of Ea and n are provided in Table 6. To verify there was no biasing in the dataset 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, Supporting Information Figure S2: calculated residuals of fits to Kissinger and Crane equations, and showed no apparent trend. 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 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 30 ACS Paragon Plus Environment

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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 vs PCF starting material.

This was previously discussed as an explanation for the differences in MW for the synthesized resols. A follow on study by Grenier-Loustalot 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 crosslinking 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 vs. 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 decreased 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-controlled64. 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 kinetics 51 and similar results found elsewhere 65. Table 6. Scan rate, peak exotherm temperature, calculated activation energies and calculated order of reaction for the three synthesized resins.



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Resin

pPF

pPCF

Page 32 of 44

β

Tp,cross

Ea

n

(°C/min)

(°C)

(kJ/mol)

(-)

10

150

15

155

20

159

113±7

0.94

25

161

30

164

10

151

15

157

20

190

99±6

0.93

25

164

30

167

10

143

15

151 89±9

0.93

bPCF 20

155

25

159


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30

160

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- vs p- substitutions in the bPCF vs. pPCF resin, respectively. Differences in degradation profiles were observed which have been shown elsewhere to be dependent on 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 difference for the bPCF resin were observed, the bio-resin 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 towards a valuable co-product 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 bio-resin demonstrated similar properties when interrogated through GPC, TGA, FTIR and DSC analytical methods. 33 ACS Paragon Plus Environment


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The overall process shows the viability of separating and synthesizing phenolic-based materials from bio-derived 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 bio-derived resins.

Supporting Information Synthesis scheme for resol, residuals of fits to Kissinger and Crane equations, species and composition of separated bio-phenolics, GC-FID factors for concentration determination, chemical characterization of CFP aqueous streams.

Acknowledgements 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.

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For Table of Contents Use Only Biomass

CFP

fuels

bio-Phenolics

Synthesis

Separa ons Integrated BioRefinery

Materials bio-Phenolics

petro-Phenolics

SYNOPSIS: This work demonstrates synthesis of a PF resin using purely bioderived phenolics through catalytic fast pyrolysis, separations and synthesis operations.


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Integrated Biorefining: Coproduction of Renewable ... - ACS Publications

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