Reaction analysis of diaryl ether decomposition under hydrothermal

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Reaction analysis of diaryl ether decomposition under hydrothermal conditions David Alam, Matthew Yuk-Yu Lui, Alexander Kah Liem Yuen, Thomas Maschmeyer, Brian S. Haynes, and Alejandro Montoya Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04754 • Publication Date (Web): 22 Jan 2018 Downloaded from http://pubs.acs.org on January 24, 2018

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Industrial & Engineering Chemistry Research

Reaction analysis of diaryl ether decomposition under hydrothermal conditions David Alam1, Matthew Y. Lui2, Alexander Yuen2, Thomas Maschmeyer2, Brian S. Haynes1, Alejandro Montoya1,* 1

The University of Sydney, School of Chemical and Biomolecular Engineering, NSW 2006 Australia 2

The University of Sydney, School of Chemistry, NSW 2006 Australia

* Corresponding author address: [email protected]

KEYWORDS: Kinetic Analysis, Lignin, Model Compounds, Hydrothermal Decomposition ABSTRACT: The reactivity and decomposition pathway of models for α–O–4 and β–O–4 linkages, found within lignin, have been examined using methoxy-substituted (–OCH3) and – unsubstituted (–H) aryl groups under hydrothermal conditions. α–O–4 model compounds readily underwent conversion at comparatively mild temperatures (140–300 °C) and short reaction times (5–80 minutes), in contrast with the β–O–4 containing model compounds which required temperatures up to 340 °C and longer reaction times up to 240 minutes. Pseudo-first-order rate constants and apparent activation energies were calculated for hydrothermal conversion of the model compounds based on experimental data. The cleavage of these linkages proceeded via

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hydrolysis and direct elimination pathways, with the resulting products prone to undergoing further reactions including condensation, and dehydration. The presence of methoxyfunctionalities on the aromatic rings was found to destabilize both the α–O–4 and β–O–4 ether linkages, decreasing the temperature and reaction times required to decompose them under hydrothermal conditions. In addition, the methoxy-substituents were partially hydrolyzed under hydrothermal conditions at temperatures exceeding 280 °C, resulting in a number of substituted guaiacol products.


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INTRODUCTION Lignocellulosic biomass has garnered interest as a renewable, carbon-neutral feedstock which can be valorized via biochemical or thermochemical processes to yield a wide variety of industrially vital chemical commodities and fuels.1–4 Lignin is one of the principal components of lignocellulosic biomass, constituting between 15–35 % of the total dry weight: lignin is a complex polymer primarily of the monolignols: p-coumaryl, coniferyl and sinapyl alcohols.5–8 These monolignols contain several distinct sites of reaction which allow for the formation of a diverse network of interlinked ether (α–O–4, β–O–4, 4–O–5) and carbon (β–5, β–1, β–β, 5–5) linkages between monomer units during the radical enzyme initiated polymerization mechanism which forms lignin.9–11 The relative distribution of each linkage varies depending on the role and type of biomass; however, the β–O–4 ether linkage is typically the most abundant accounting for 45–60 % of the total bonds present in lignin.

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Combined, ether linkages represent over two-

thirds of the total bonds within lignin and are the primary bonds of interest as they are relatively reactive compared to carbon linkages and can be cleaved to liberate monomeric aromatic products.12–15 For the future production of renewable aromatic chemical commodities, lignin is the only significant aromatic source available capable of supplementing global chemical demand. Low molecular weight lignin fragment products are currently produced in vast quantities as a byproduct of biorefineries and paper pulping industries.7,16 The majority of these residues is combusted to recover heat energy as the highly stable, amorphous structure of lignin is resistant towards decomposition and degradation.17 Developing processes able to overcome these barriers could allow for the renewable production of valuable aromatic chemical commodities from lignin, including o-cresol, guaiacol, catechol and vanillin, shifting reliance away from fossil resources.18–20



Hydrothermal decomposition (HTD)21 is a thermochemical process that utilizes the unique properties of superheated water22 under subcritical temperatures (140–374 °C) and high pressures (5–25 MPa). Under these conditions, the autolysis of water increases so that the concentration of hydronium and hydroxide ions in solution is raised by as much as 3–4 orders of magnitude compared to room temperature in addition to a decrease in dielectric constant.23,24 This change in properties allows water to not only behave as a reagent for hydrolysis reactions and as a polar solvent able to solubilize typically hydrophobic compounds and stabilize charged intermediates, but also act as a source of catalytic acid and base. This multifunctionality is what makes water so effective for the depolymerization of biomass into low molecular weight products.25–27 HTD is particularly appealing compared to other biomass processing methods such as pyrolysis as it results in a bio-oil with significantly higher energy-density and relatively low oxygen content without requiring an energy intensive drying pre-treatment of the feedstock.28,29 The decomposition of lignin under HTD conditions proceeds by means of two processes occurring consecutively: An initial depolymerization phase forming water soluble lignin fragments followed by a slower condensation phase where reactive fragments recombine forming insoluble oligomers and char.30 The depolymerization of lignin involves the cleavage of the ether linkages, primarily the β–O–4 and α–O–4 linkages, forming monomeric and oligomeric aromatic products.31 The resulting products then recombine through a series of reactions forming larger oligomers which can eventually lead to the formation of insoluble, solid char material. Investigations by Pińkowska et al. and Yong & Matsumura noted the formation of these recombination products are enhanced at long reaction times under hydrothermal conditions and especially at supercritical temperatures (>374 °C).32,33


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Determining the fundamental chemistry of lignin during HTD is typically accomplished using model compounds representative of the structures and linkages forming lignin in order to reduce the number of primary and secondary reactions and simplify product identification. Unsubstituted diaryl compounds such as benzyl phenyl ether (BPE, CAS # 946-80-5)34–38 and phenylethyl phenyl ether (PPE, CAS # 40515-89-7)39–41 have been used to gain insight into the decomposition of the α–O–4 and β–O–4 ether linkages under HTD conditions. HTD studies using these model compounds indicate that the cleavage is slow and proceeds by hydrolysis, with the resulting monomeric products undergoing dehydration, alkylation and condensation. Recently it was shown that the decomposition under HTD conditions of diaryl compounds with β–O–4 linkages with hydroxyl groups in α-positions leads to the formation of aldehydes via the enol tautomer,42 similar to the decomposition mechanism under acidolysis conditions.43 The presence of hydroxyl groups in α- or β-position promotes the formation of monomeric aldehydes 42–44

while the absence of these groups promotes the formation of styrene in decomposition

products from β–O–4 linkages.40 This study examines the HTD of substituted and unsubstituted diaryl compounds containing α– O–4 and β–O–4 linkages, without the presence of neighboring hydroxyl groups. The effect of methoxy (–OCH3) substituents in the aromatic ring is analyzed to determine the effect on the rate of decomposition and product distributions. Identified products have been quantified and reaction pathways have been hypothesized.



EXPERIMENTAL SECTION Hydrothermal Decomposition: The decompositions of diaryl model compounds under HTD conditions were carried out in a custom built 316 stainless steel (316SS) batch reactor with a total internal volume of 10 mL. The reactor was constructed of a 150 mm length of ½′′ outer diameter 316SS section of tubing sealed at one end; whereas the other was subsequently fitted with a gasket and attached to a pressure-head, specifically engineered to handle pressures up to 250 bar at 350 °C. The reactor was based on a design used in our previous work, with detailed schematics of the position of valves, temperature and pressure indicators available elsewhere.45 The batch reactor used in this study has some modifications with the previous design; specifically a smaller total volume (10mL vs 20mL) and narrower reactor outer diameter (½′′ vs ¾′′). Both of these factors contribute to more rapid heating and shorter cooling profiles to reach experimental temperatures greatly improving the reactors applicability for this study. The reactor was loaded with the model compound and 5.0 mL of deionized water to yield a concentration of 2000 mg/L (0.2 wt.%) unless stated otherwise. The volume of liquid loaded into the batch reactor was limited to 5.0 mL to allow for adequate headspace for thermal expansion as the water was heated during the experiment. Atmospheric gases within the reactor were then purged several times with nitrogen before pressurization with the same gas at room temperature. The pressure at which the reactions took place was controlled by adjusting the initial pressure of nitrogen in the reactor. The pressurized reactor was then immersed in a pre-heated fluidized sand bath (Techne SBL-2D) set to the required reactor temperature. In all cases, the reactor pressure exceeded the saturation pressure of water by 10–20 bar at each of the temperatures investigated to maintain water in a liquid phase. The temperature in the reaction solution was measured with a tip-sensitive K-type thermocouple. The contents of the reactor could be rapidly heated upon submerging into the sand


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bath; e.g. for a desire temperature of 350 °C, the reactor reached temperatures of 250 and 350 °C after 0.5 and 5 minutes, respectively. After this heating period, the sand bath temperature controller would maintain its set point and keep the reactor within 0.5 °C for the duration of the experiment. Further information on the reactor heating profile can be found in Figure S3 in the Supporting Information. The reaction times were taken to commence after these initial heating periods since the heating times are relatively small compared with reaction times that extend up to 240 minutes. Upon completing the reaction, the reactor was quenched in an ice water bath which cooled the temperature inside the reactor down to room temperature within 60 seconds. After quenching, the reactor was vented before opening and collecting the liquid sample. The batch reactor was then washed using 2 × 10 mL aliquots of methanol to solubilize any remaining material which may have adhered to the reactor walls. The methanol washings were collected and combined with the liquid sample for analysis. Experiments were carried out at least in duplicates for each data point with the mean values and error bars reported. The carbon balance after each reaction was calculated based on the yields of the species. The yield was calculated based on the molar carbon content of the feed following equation (1). The product yields are defined in terms of elemental carbon content relative to the feedstock where Yci corresponds to the carbon yield of species i (in mol %); represents its molar concentration in the products stream, and is the number of carbon atoms per molecule. The nfeed and Cfeed |0 corresponds to the number of carbon atoms and molar concentration of the feed species.

Yci =

n i ·Ci

(1)

nfeed · Cfeed |0



Analytical methods: The products in the liquid phase were identified using a Shimadzu QP2010 GC-MS equipped a Restek Rtx-5MS column (Length 30 m, Internal Diameter, 0.25 mm; 0.25 µm thickness). Samples of 1 µL were injected with a split ratio of between 1:10 and 1:50, with a heating program of 50 °C for 3 minutes, followed by an increase of 15 °C/minute until 300 °C which was then held for 5 minutes. Peak identity was assigned by confirmation of retention time with known standards and similarity with electron impact mass spectrum data from the National Institute of Standards and Technology (NIST) spectral library. The products identified by GC-MS were then quantified where possible by HPLC (Varian 385LC). A 250 × 4.6 mm GRACESMART© column (5 µm diameter C18 packing, Grace Davidson) and a 150 × 2.1 mm ACE© column (3 µm diameter C18-PFP packing) were used to separate the products formed. Several HPLC methods were used depending on the decomposition products formed from each diaryl ether compound, utilizing both isocratic and gradient elution methods. The mobile solvents used for each method involved 0.05M acetic acid aqueous solution and CHROMASOLV® grade methanol (≥ 99.9 %). A Photo Diode Array (PDA) detector scanned over a wavelength range of 190–400 nm. Known standards were injected to identify elution times and develop calibration curves for quantification. A full description of the HPLC methods is provided in Section 3 of the Supporting Information.

Chemicals and Reagents: The chemical structure of the four diaryl ethers examined in this study are shown in Figure 1. BPE and 1,2‐dimethoxy-4-((2-methoxyphenoxy)methyl)benzene (LM–A, CAS # 10548-82-0) contain α–O–4 ether linkages, while PPE and 1,2‐dimethoxy-4‐(2‐ (2‐methoxyphenoxy)ethyl)benzene (LM–B, CAS # 90293-77-9) contain β–O–4 ether linkages. The BPE and PPE diaryl ethers contain no additional oxygenated groups other than the α–O–4


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and β–O–4 linkages; LM–A and LM–B contain –OCH3 functionalities attached to the aromatic rings, more representative of the functionalized nature of lignin. The chemical structure of LM– A and LM–B differ only in terms of the ether linkages (α–O–4 vs β–O–4), otherwise having similar positions of the –OCH3 groups in the aromatic rings. Studying them allows a direct comparison of reactivity of the substituted aromatic rings with α–O–4 and β–O–4 linkages.

Figure 1. Chemical structures of the α–O–4 and β–O–4 ether model compounds investigated under HTD conditions BPE and PPE were sourced from Sigma Aldrich (Australia) and Frinton Laboratories (USA), respectively, with purities exceeding 99 % and were used without any additional purification whereas LM–A and LM–B were synthesized in-house following Mikawa, and Sergeev and Hartwig.46,47 The synthesis method for both compounds commenced with the bromination of 3,4dimethoxybenzyl (veratryl) alcohol for LM–A and 3,4-dimethoxyphenethyl (homoveratryl) alcohol for LM–B. The resultant benzyl bromide was coupled to guaiacol under standard conditions to promote SN2 displacement of the bromo-substituent. The desired compound was then recrystallized, affording analytically pure (> 99 %), colorless flakes. The purities and structures of the synthesized products were confirmed using nuclear magnetic resonance (NMR) at 300 K and referenced internally to residual solvent signals at 7.26 and 77.16 ppm, respectively, for CDCl3 (Bruker Avance DPX300 spectrometer) and GC-MS. Further information on the synthesis methodologies used and the NMR data can be found in Section 1 of the Supporting Information. Deionized water was generated in-house by a Millipore Elix 5 system, and was further vacuum filtered through a 0.2 µm nylon filter before use. Calibration standards analyzed by HPLC were



prepared using unadulterated samples of benzyl alcohol (99.8 %), phenol (> 99 %), veratryl alcohol (96 %), guaiacol (98 %), phenethyl alcohol (99 %), styrene (> 99 %) and homoveratryl alcohol (> 98 %) sourced from Sigma Aldrich (Australia). CHROMASOLV® grade methanol for HPLC (≥ 99.9 %) and acetic acid (> 99 %) were also sourced from Sigma Aldrich (Australia) and used without further purification.

RESULTS & DISCUSSION Decomposition of α–O–4 linkages: Figure 2 shows the conversion profiles and the first-order kinetics of the decomposition of LM–A under hydrothermal conditions. The conversion of LM– A was significantly affected by changes in temperature between 140–200 °C below 120 minutes, reaching up to X = 90% conversion at 200 °C in 20 minutes. The decomposition can be well represented by a pseudo first-order reaction, indicated by the linear trends shown in Figure 2b. The rate constants (k ×10−3 min−1) deduced from the first-order kinetic rates including error margins at 140, 155, 170, 185 and 200 °C were determined to be k = 2.6 ± 0.3, k = 9.6 ± 1.5, k = 28.2 ± 0.8, k = 50.3 ± 2.7 and k = 101.0 ± 30.2. The larger deviation in k observed at 200 °C is attributed to the short reaction times (5–20 minutes) which were more sensitive to variations in heating and fluctuations during the reactor heating and cooling periods. The k values, were used to derive the pre-exponential factor ( ) and apparent activation energy (Ea) using the Arrhenius rate expression, k=A∙exp‐Ea/RT. Values of Ea = 97.8 ± 7.5 kJ·mol–1 and lnA(min–1) =22.7 ± 2.0 were deduced from the calculated the first-order rate constants.

Figure 2. Conversion profiles (left) and pseudo first-order kinetics (right) of the decomposition of LM–A under HTD conditions (140 − 200 °C) over a range of reaction times (5–120 minutes)


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In contrast to the reactivity of α–O–4 ether linkage in the methoxy-substituted LM-A, the unsubstituted analogue BPE was relatively unreactive. After 120 minutes of reaction time, there was less than 10% conversion at temperatures up to 200°C; at 260 °C, conversion reached ≈70%. Similar observations and levels of conversion were reported by Wu & Lü who investigated the decomposition of BPE under hydrothermal conditions at temperatures between 220–250 °C and reaction times up to 5 hours.35 The disparity in the overall reactivity between BPE and LM–A is attributed to the steric hindrance induced by the ortho- methoxy-functionality present in LM–A, which weakened the α–O–4 linkage and allowed for its cleavage to proceed at lower temperatures.7,48 A combination of HPLC and GC-MS was used to quantify the major products formed during HTD and to identify minor secondary products which contributed to unquantified carbon. Figure 3 shows the yields of the primary products quantified from the decomposition of LM–A. It is hypothesized that hydrolysis is the dominant initial cleavage mechanism, resulting in the formation of veratryl alcohol and guaiacol in equimolar quantities. The carbon balance taken as the sum of the yield of the species quantified typically exceeded 80% and unquantified carbon was typically greater for longer reaction times. The respective carbon yields of both veratryl alcohol and guaiacol were found to increase with increasing temperatures and reaction times. At a reaction time of 10 minutes, the yield of guaiacol increased with increasing temperatures, accounting for 2.3 %, 7.0 % and 17.7 % of the total carbon at 155 °C, 170 °C and 200 °C; corresponding yields of veratryl alcohol were 1.4 %, 4.8 % and, 12.1 % respectively.

Figure 3. Carbon yields of the species identified by HPLC from the decomposition of LM–A under HTD conditions at 140–200 °C over a range of reaction times between 5 and 120 minutes



However, the carbon yields of veratryl alcohol were generally lower compared with those for guaiacol, suggesting that the former alcohol is more reactive than guaiacol, undergoing secondary reactions and forming compounds not easily identified or quantified by HPLC. Solid, char-like products were not observed in any of the sample solutions post experiment indicating high molecular weight oligomeric products were not formed. To determine the possible identity of other products and secondary reactions occurring after the cleavage of the α–O–4 ether linkage, an aqueous solution with approximately equal concentrations of veratryl alcohol and guaiacol (1000 mg/L each) was reacted under HTD conditions at 200 °C for 10 minutes. A dimeric compound was identified with structural similarity to LM–A and diveratryl ether as shown in Figure 4. The presence of diveratryl ether indicates the self-condensation of veratryl alcohol had occurred under HTD conditions whereas the formation of a dimer structurally similar to LM–A suggests condensation or alkylation between veratryl alcohol and guaiacol. Dimers of guaiacol self-condensation were not detected using HPLC or GC-MS under the HTD conditions of this study indicating a lower reactivity towards these reactions. These results are in agreement with studies by Yong and Yukihiko which found the formation of oligomeric products from guaiacol in sub- and supercritical water (300–450 °C) occurred exclusively in the supercritical range.49 The lack of demethylated products in samples analyzed by GC-MS (see supplementary information and Figure 4) indicates the temperatures used for HTD were mild enough to efficiently cleave the α–O–4 ether linkage, without promoting the hydrolytic demethylation of the methoxy groups present in LM–A or in any of its decomposition products.

Figure 4. GC-MS chromatogram of guaiacol and veratryl alcohol decomposition under HTD conditions at 200 °C after 10 minutes. Products identified: guaiacol (1), veratryl alcohol (2),


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unidentified dimeric product (3) and diveratryl ether (4). Remaining unlabeled peaks represent base line noise or eluted packing materials. *The MS spectra for this peak was consistent with a dimeric product similar in structure to diveratryl ether (peak 4). Figure 5 shows the carbon yields of products after the decomposition of BPE between 260 °C and 300 °C. The presence of benzyl alcohol and phenol was observed at each reaction times indicates that hydrolysis is a primary reaction of the α–O–4 linkage under the HTD conditions investigated. The total carbon recovery was found to decrease at each of the temperatures as the reaction time increases. At 300 °C, the total carbon recovery decreased from ≈ 106 % after 10 minutes to ≈ 81 % at 40 minutes. The decrease in the carbon balance indicates that the formation of dimers and higher molecular weight oligomers proceeded at a slow rate and was therefore favored at longer reaction times and high temperatures. Overall the yield of phenol was consistently lower in comparison to benzyl alcohol. The lower yields potentially suggests a greater tendency for phenol to undergo secondary reactions forming dimers and oligomers. Lower yields of phenol in comparison to benzyl alcohol was similarly observed by He et.al in their study on the decomposition of BPE under hydrothermal conditions at 250°C, 40 bar N2 and reaction times up to 110 minutes.50 A study by Wu and Lü investigating BPE under comparable experimental conditions (220–250 °C,

Reaction analysis of diaryl ether decomposition under hydrothermal

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