Enhanced Levoglucosan Yields from the Copyrolysis of Cellulose and

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Enhanced Levoglucosan Yields from the Copyrolysis of Cellulose and High Density Polyethylene Melisa Nallar, and Hsi-Wu Wong ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00765 • Publication Date (Web): 29 Apr 2019 Downloaded from http://pubs.acs.org on May 2, 2019

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ACS Sustainable Chemistry & Engineering

Enhanced Levoglucosan Yields from the Co-pyrolysis of Cellulose and High Density Polyethylene Melisa Nallar and Hsi-Wu Wong* Department of Chemical Engineering, University of Massachusetts Lowell One University Avenue, Lowell, Massachusetts 01854, United States *Corresponding Author E-mail: [email protected]; Tel: 1-978-934-5290 (H.-W. Wong)

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ABSTRACT: The widely available biomass resources could present a sustainable and alternative feedstock to fossil fuels for the production of energy sources and platform chemicals. Fast pyrolysis is one of the simplest methods for biomass conversion, but producing value added chemicals via fast pyrolysis is still challenging due to diverse product distributions and low product selectivity. Thermoplastics that melt at high temperatures are potential candidates to trap escaping products from biomass pyrolysis, promoting secondary cellulose pyrolysis reactions for increased product selectivity. In this work, a strategy to promote the yields of levoglucosan (LG), a potential high-value chemical from cellulose pyrolysis, is presented using molten high density polyethylene (HDPE) for the inhibition of product escape. Cellulose and HDPE mixtures were pyrolyzed with controlled mixing patterns in a custom-made laboratory reactor to investigate the interplay between chemical kinetics and mass transfer. Our experimental results showed that the yields of LG and low molecular weight products (LMWPs) both increased due to the presence of HDPE. This is likely resulted from the inhibition of evaporation of anhydrosugar oligomers in cellulose pyrolysis, leading to increased secondary pyrolysis reactions in the molten HDPE phase. The morphology of the HDPE layers on top of cellulose is the determining factor affecting the interplay between chemical kinetics and mass transfer in cellulose/HDPE copyrolysis, thus altering product distributions. The highest LG yield observed in this work reached 74.2% at 400 ºC, an approximately 30% increase, when significant amount of HDPE was coated on top of cellulose. The HDPE pyrolysis was also accelerated by the presence of cellulose, likely due to the catalytic effect caused by LG.

KEYWORDS: Biomass, plastics, thermochemical conversion, fast pyrolysis, co-pyrolysis, cellulose, high density polyethylene, levoglucosan 2 ACS Paragon Plus Environment

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INTRODUCTION Our current dependence on fossil-derived resources, which are limited in supply and will one day be depleted, is unsustainable.1-3 With growing global population, the world’s demands for clean energy and sustainable chemicals will increase proportionately. Renewable resources such as biomass will play an increasingly vital role over the next century as alternative feedstocks for the production of energy sources and platform chemicals.4-7 According to the 2016 Billion-Ton Report from the U.S. Department of Energy,8 the potential supply of biomass (composed of agricultural, forestry, waste, and algal materials) in the United States is at least one billion dry tons per year, sufficient to replace 30% of the U.S. petroleum consumption for the production of drop-in fuels and commodity chemicals without adversely affecting the environment.8 Two types of biomass conversion methods, biochemical and thermochemical, are typically considered.9-11 Biochemical biomass conversion has the advantages of high selectivity and conversion efficiencies but they suffer from low throughput and feedstock specificity.10 Thermochemical biomass conversion has the advantages of more diverse liquid products, shorter reaction times, no need for sterilization, and lower overall cost.12 Fast pyrolysis is one of the thermochemical biomass conversion methods that attracts the most interest due to its scalability, low capital cost, high liquid product yields, and self-sustainability.13-17 Under fast pyrolysis conditions, biomass is decomposed at moderate temperatures (400-650 ºC) in the absence of oxygen with short contact times (1-5 seconds). The main drawback to biomass fast pyrolysis is its diverse product distributions, where many products with distinct physical and chemical properties are formed with low selectivity.18-20 Cellulose is the most abundant biomass constituent with a simple chemical structure and thus has gained the most attention in fast pyrolysis studies.21 Fast pyrolysis of cellulose can be 3 ACS Paragon Plus Environment


described as a competition between the formation of low molecular weight products (LMWPs, which are C1 to C6 oxygenated hydrocarbons) and levoglucosan (LG).22-24 Traditionally, LMWPs are desired since they can be easily upgraded into biofuel or value-added chemicals. Recently, LG has gained increased consideration as a potential platform chemical for polymer, pharmaceutical, and consumer product manufacturing.25-27 The LG yields from cellulose fast pyrolysis are dependent on experimental conditions. For instance, Paulsen et al.28 investigated cellulose pyrolysis using samples of different dimensions. It was found that thin-film samples resulted in a lower LG yield (~27%), as opposed to conventional powder samples (~49%), at a reaction temperature of 500 ºC. Zhou et al.22,29 investigated the effect of pyrolysis temperature on LG yields. They reported a 54.5% LG yield at 500 ºC, whereas the highest LG yield was found at 69.5% at a lower reaction temperature of 400 ºC. Westerhof et al.30 showed that the escape rate of pyrolysis products plays a key role on product distributions. They studied cellulose fast pyrolysis under different reaction pressures and observed that total yields of anhydrosugar molecules (LG, cellobiosan, cellotriosan, etc.) increased with decreasing pressure, although the highest LG yield was from reactions conducted near atmospheric pressure. Similar pressure dependence was also reported by Pecha et al.31 The scattered LG yields reported in the literature suggest that there is a complex interplay between chemical kinetics and mass transfer processes (e.g., product escape via evaporation or aerosol ejection31-33) in cellulose fast pyrolysis. Manipulation of this interplay may provide a means to alter the yields of key products, particularly LG. Thermoplastics that melt at high temperatures are potential candidates to trap the escaping products from cellulose pyrolysis, promoting secondary cellulose pyrolysis reactions. High density polyethylene (HDPE), for example, has a degradation timescale 75 to 500 times longer 4 ACS Paragon Plus Environment

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than that of cellulose between 400-600 ºC34 and can be used in cellulose pyrolysis to suppress product escape without decomposition. Since waste plastics, particularly those into the ocean, are also an increasing environmental concern,35 using molten thermoplastics to alter biomass pyrolysis chemistry will also help mitigate the plastic waste problem. Experimental results for biomass/plastics co-pyrolysis in the literature36-49, however, remain contradictory. Most studies observed higher quality bio-oils40,42,45-46 and lower bio-char yields.38,41 On the contrary, a few studies reported no interactions at all, particularly at short contact times with more thermally stable synthetic thermoplastics.47-49 The traditional consideration is to utilize the abundant hydrogen sources in synthetic plastics for bio-oil upgrading via reaction coupling. However, no coupling products between biomass and plastics are typically observed unless gas-phase reactions are allowed, suggesting that the synergy between the two components in the molten phase during co-pyrolysis, if any, is resulted from the change of reaction rates of the individual pyrolysis pathways (e.g., catalytic or inhibitory effects). Since biomass and polymers are immiscible under fast pyrolysis conditions, an inhibition of the escape of the biomass-derived volatile products30-33,50 may occur in the molten polymer phase, promoting secondary reactions. However, possible mechanisms of molecular trapping due to the presence of molten polymers have never been proposed or discussed in the literature. In this work, a strategy to promote LG yields via the inhibition of product escape using molten HDPE was examined. HDPE was selected as the thermoplastic of interest because (1) HDPE has no functional groups and is least likely to incur chemical effects; (2) HDPE has a simple linear chain structure, unlike low density polyethylene or other cross-linked polymers, so that the identification of HDPE-derived products is straightforward; and (3) HDPE has a degradation timescale much longer than that of cellulose. Cellulose and HDPE mixtures were 5 ACS Paragon Plus Environment


pyrolyzed with controlled mixing patterns to investigate the interplay between chemical kinetics and mass transfer. A depiction of possible inhibition pathways of product escape in cellulose pyrolysis resulted from the presence of HDPE and its impact on the LG yields are presented.

EXPERIMENTAL Materials. Microcrystalline cellulose (Alfa Aesar) and HDPE (Sigma-Aldrich, Mw ~35,000 g/mol, Mn ~7,700 g/mol, and density of 0.906 g/mL) were used for the pyrolysis experiments. Methanol (Alfa Aesar, environmental grade, 99.8+%) and dichloromethane (Alfa Aesar, environmental grade, 99.8+%) were used as solvents for product extraction. Standard chemicals used for gas chromatography (GC) calibration include: o-terphenyl (Sigma-Aldrich, 99%), acetic acid (Alfa Aesar, glacial, 99+%), 2-furaldehyde (Alfa Aesar, 98%), 5-hydroxymethyl-2furaldehyde (Alfa Aesar, 97%), 1,6-anhydro-beta-D-glucopyranose (Acros Organics, 99+%), 1decene (Sigma-Aldrich, 94%), 1-dodecene (Alfa Aesar, 96%), 1-tetradecene (Alfa Aesar, 94%), and n-eicosane (Alfa Aesar, 99%). Note that o-terphenyl was used as an external standard in the experiments and during the preparation of calibration standards. Preparation of Layered Thin-Film Samples. Cellulose thin film samples were prepared following the procedure by Paulsen et al.28 Briefly, microcrystalline cellulose powders were first suspended in distilled water. A small amount of the suspension was then transferred into a copper holder (1/8 tube cap, Elkhart Products Corporation). Water in the samples was subsequently removed by evaporation on a hot plate (Fisher Scientific) at 150 ºC for 5 minutes before the samples were being evacuated, resulting in a microscale cellulose film of 5 mg or 10 mg in the copper holder. To prepare HDPE thin-films, different amount of HDPE powders 6 ACS Paragon Plus Environment

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(approximately 5 mg, 7 mg, or 10 mg) was loaded into a copper holder. The holder was then positioned on the hot plate at 250 ºC with different duration (1, 3 and 5 minutes) for melting HDPE into a layer. Note that single component thin-film cellulose samples were also heated on the hot plate at 250 ºC for 5 min prior to pyrolysis experiments so that the comparison of the experimental results was on the same basis.

CE

PE

PECE

2PECE

CEPE

3PECE

Figure 1. Schematic illustrations of single component (CE and PE) and layered thinfilm samples (CEPE, PECE, 2PECE, and 3PECE) used in our pyrolysis experiments to investigate the effects of mixing pattern and PE to CE ratio.

Single component and layered thin-film samples with different mixing patterns were prepared, as illustrated in Figure 1. For single component samples, 10 mg of materials (cellulose or HDPE) were loaded in the copper holder. For samples containing both cellulose and HDPE, three different high density polyethylene (PE) to cellulose (CE) ratios in mass (1:1, 2:1, and 3:1) were prepared (Figure 1), while keeping the total combined mass the same at 20 mg. For samples with 1:1 PE to CE ratio, two orientations of samples were prepared: cellulose on top (denoted as 7 ACS Paragon Plus Environment


CEPE) or HDPE on top (denoted as PECE). For samples with 2:1 and 3:1 PE to CE ratios, only samples with HDPE on top were prepared to investigate the effect of product trapping (denoted as 2PECE and 3PECE, respectively). Pyrolysis Experiments. A custom-made batch pyrolysis reactor used in our previous work5152

was employed for the pyrolysis experiments (Figure 2). The reactor containing the sample

holder was first evacuated down to below 0.1 torr before it was closed and placed into a hot furnace, which was controlled at a desired reaction temperature with PID controllers (Omega Engineering, CN742). The pyrolysis reactions were carried out at 400 ºC, 450 ºC and 500 ºC with a reaction time of 15 minutes. Since our reactor system does not have a quenching mechanism to allow rapid cooling, this reaction time was chosen to be sufficiently long for cellulose pyrolysis to reach equilibrium compositions rather than acquiring time evolution of individual products. A reaction time of 15 minutes is also sufficiently long to allow observations of HDPE pyrolysis affected by the presence of cellulose. The copper sample holder was initially positioned in the cold zone above the furnace. To initiate the reaction, the sample holder was rapidly dropped into the hot zone kept at the desired temperature using a hammer. This ensures rapid heating of the samples. The condenser placed in the liquid nitrogen bath was used to collect pyrolytic products throughout the reaction. Between 2-5 mg of o-terphenyl were placed inside the condenser prior to the experiments as an external standard. This design takes advantage of the rapid thermal diffusion of the pyrolysis products between the hot reactor and the cold condenser driven by the large temperature gradient (> 600 ºC), mimicking semi-batch reactions where products are continuously removed. Once the reaction was completed, the reactor was then removed from the hot furnace and cooled in a water bath at ambient temperature for two minutes. The gas-phase products were first extracted by connecting the reactor assembly to a


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sampling chamber under vacuum. The condensed phase products collected in the condenser were then extracted by methanol and dichloromethane in equal volume. Both gas phase and condensed phase products were analyzed by gas chromatography (GC). The experiments were repeated three times to ensure reproducibility. The average values are reported in the figures, and the standard deviation of the three repeated runs are used as error bars.

Figure 2. The custom-made, drop-in, pyrolysis reactor used in this work.

Product Analysis. The identification and quantification of the reaction products was performed using a Shimadzu GC-2010 Plus GC system equipped with a mass spectrometer (MS) and a flame ionization detector (FID) and a Shimadzu GC-2014 system equipped with a thermal conductivity detector (TCD) and a FID. For gas phase products, 2 mL of the samples were injected into a Restek packed column (80486-810, 2 m) connected to a TCD and a Restek Rt-Q9 ACS Paragon Plus Environment


Bond column (30 m) connected to a FID. Peak identification and calibration were achieved from results obtained using standard gas mixtures containing CO, CO2, and C1 to C4 hydrocarbons (Scotty Specialty Gases). The GC was programmed with an injection temperature of 250 ºC and a split ratio of 10.0. The programmed temperature regime for the GC oven was: start at 35 ºC, ramp up to 50 ºC at 7.5 ºC min-1, hold for 1 min, ramp up to 100 ºC at 5 ºC min-1, hold for 3 min, ramp up to 200 ºC at 10 ºC min-1, hold for 7 min, ramp up to 250 ºC at 25 ºC min-1, and hold at 250 ºC for 10 min. The identification of extracted liquid phase products was achieved by injecting 3 µL of samples into the GC-MS system equipped with a Shimadzu SH-Rxi-5Sil MS column (30 m). The GC was programmed with an injection temperature of 280 ºC and a split ratio of 5.0. The programmed temperature regime for the GC oven was: start at 35 ºC, hold for 5 min, ramp up to 100 ºC at 5 ºC min-1, hold for 5 min, ramp up to 150 ºC at 5 ºC min-1, hold for 5 min, ramp up to 175 ºC at 5 ºC min-1, hold for 5 min, ramp up to 190 ºC at 5 ºC min-1, hold for 5 min, ramp up to 275 ºC at 10 ºC min-1, and hold at 275 ºC for 18 min. Quantification of the extracted liquid phase products was performed using the same GC system equipped with an FID. 3 µL of samples were injected into the GC-FID system equipped with a Shimadzu SH-Rxi-5ms column (15 m). The GC was programmed with the same aforementioned operating parameters, except that the oven temperature was not hold at 190 ºC for 5 min. Mixtures of o-terphenyl (external standard, C18H14), levoglucosan (1,6-anhydro-beta-Dglucopyranose, C6H10O5), 5-hydroxymethyl-2-furaldehyde (C6H6O3), 2-furaldehyde (C5H4O2), and acetic acid (CH3CO2H) in four different ratios, dissolved in 125 mL of methanol and 125 mL of dichloromethane, were used as calibration standards for the cellulose-derived products in GC analysis. Mixtures of o-terphenyl (C18H14), n-eicosene (C20H42), 1-tetradecene (C14H28), 110 ACS Paragon Plus Environment

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dodecene (C12H24), and 1-decene (C10H20) in four different ratios, dissolved in 1500 mL of methanol and 1500 mL of dichloromethane, were used as the calibration standards for the HDPE-derived products in GC analysis.

RESULTS AND DISCUSSION Effect of Sample Composition and Mixing Pattern. The yields of cellulose-derived products from neat cellulose (CE) pyrolysis and cellulose/HDPE co-pyrolysis of two different mixing patterns (layered samples with cellulose and HDPE on top, respectively) are shown in Figure 3 and tabulated in Table S1 of Supporting Information. Three types of reaction products are reported in the figure, including levoglucosan (LG), low molecular weight products (LMWPs, which are oxygenated C1 to C6 hydrocarbons), and residue, which likely consists of unidentified anhydrosugar oligomers and char (measured to be ~5% in neat cellulose pyrolysis). The LG yield for neat cellulose (CE) pyrolysis in our experiments reached 43.4%. The presence of HDPE in the samples enhanced LG yields, and sample mixing pattern was found to have a significant impact. When CE layer was positioned on top of a HDPE layer during co-pyrolysis (CEPE), the LG yield increased to 48.4%. Since the heating of the samples in our experimental apparatus relies purely on the conduction from the bottom of the copper holder, the heating of the CE layer of the CEPE samples could be slower due to the low thermal conductivity of the HDPE layer, causing the CE layer to experience lower reaction temperature. Since it is acknowledged that low pyrolysis temperature produces higher LG yields in cellulose pyrolysis22, this heat transfer effect could be the cause of increased LG yields in CEPE pyrolysis. When a HDPE layer was positioned on top of a CE layer (PECE), the LG yield further increased to



51.3%. The increase in LG yields was also accompanied by increased LMWP yields and decreased residue yields, which can be explained by the inhibition of the evaporation of anhydrosugar oligomers in cellulose pyrolysis due to an increase of their retention time through the molten HDPE phase. The breakdown of LMWPs also shows that the presence of HDPE increased the yields of small (C1 and C2) LMWPs (i.e., CO, CO2, acetic acid, and glycolaldehyde), which were likely produced from secondary reactions including retro-aldol fragmentation, dehydration, and decarbonylation of various forms of sugar molecules,22 suggesting that these secondary reactions were also promoted in the molten HDPE phase.

Figure 3. Yields of cellulose-derived products from single component and layered thin-film pyrolysis. Yields are shown for residue, low molecular weight products (LMWPs), and levoglucosan (LG). LMWPs include 1,6-anhydroglucofuranose (AGF), 5hydroxymethylfurfural (HMF), dianhydroglucopyranose (DAGP), levoglucosenone (LGO), furfural (FF), 2(5H)-furanone (FO), acetic acid (AA), glycolaldehyde (GA), carbon dioxide (CO2), and carbon monoxide (CO).

Effect of HDPE Layer Coverage and Thickness. To further test our hypothesis that molten HDPE inhibits anhydrosugar evaporation and promotes secondary pyrolysis reactions, PECE 12 ACS Paragon Plus Environment

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samples prepared with different HDPE melting times (1 min, 3 mins, and 5 mins, respectively) were reacted. It was found that increased HDPE melting time further promotes LG yields and reduces the yields of residue (Figure 4 and Table S2 of Supporting Information). The images of the samples prior to pyrolysis reactions taken by optical microscope (Olympus CX41) show that only a fraction of cellulose surface was covered with 1 min HDPE melting time, whereas a longer HDPE melting time of 5 mins produced almost a complete HDPE coverage (Figure 5). This suggests that increased HDPE surface coverage prohibits more anhydrosugar oligomers from evaporating, leading to further degradation into LG. On the other hand, a melting time of 3 mins resulted in the lowest LMWP yield. Since the same amount of HDPE mass (i.e., 10 mg) was used in all three experiments, samples with larger HDPE surface coverage (i.e., longer melting time) produced thinner HDPE layers. The thickness of the HDPE layers has a direct effect on the amount of time that a molecule stays within the molten HDPE phase. Consequently, thinner HDPE layers could allow LMWP precursors (e.g., glucose or LG) to diffuse through the molten HDPE phase more easily, leading to lower LMWP yields. Eventually, a complete HDPE surface coverage (i.e., melting time of 5 mins) could produce sufficient LMWP precursors from large anhydrosugar oligomers to compensate this effect. In summary, our experiments showed that the morphology of the HDPE layers on top of cellulose is the determining factor affecting the interplay between chemical kinetics and mass transfer in cellulose/HDPE co-pyrolysis, thus altering product distributions.



Figure 4. Yields of cellulose-derived products from layered thin-film pyrolysis with different PE melting times. Yields are shown for residue, low molecular weight products (LMWPs), and levoglucosan (LG). LMWPs include 1,6-anhydroglucofuranose (AGF), 5hydroxymethylfurfural (HMF), dianhydroglucopyranose (DAGP), levoglucosenone (LGO), furfural (FF), 2(5H)-furanone (FO), acetic acid (AA), glycolaldehyde (GA), carbon dioxide (CO2), and carbon monoxide (CO).

Figure 5. Images taken from optical microscope prior to pyrolysis reactions show that longer HDPE melting time during sample preparation provides increased HDPE surface coverage but thinner layers. 14 ACS Paragon Plus Environment

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Since fractional surface coverage is difficult to control quantitatively in our experiments, samples with a complete HDPE surface coverage (i.e., 5 mins of melting time) with different thickness (namely PECE, 2PECE, and 3PECE) were then prepared for the pyrolysis experiments. The yields of cellulose-derived products from these samples are shown in Figure 6 and tabulated in Table S3 of Supporting Information. We observed that increased HDPE thickness on top of cellulose elevated LG yields from 51.3% (PECE) to 70.5% (3PECE), a nearly 20% increase. The total yield of LMWPs also increased from 5.7% (CE) to 22.8% (3PECE), with increased yields of C1 and C2 products, particularly CO, CO2, and glycolaldehyde. These results further support our hypothesis that the evaporation of large anhydrosugar oligomers is inhibited, which leads to the formation of LG (via further degradation) or small LMWPs (via the secondary reactions).

Figure 6. Yields of cellulose-derived products from the co-pyrolysis of layered HDPE/CE samples with different thickness of HDPE layers on top. Yields are shown for residue, low molecular weight products (LMWPs), and levoglucosan (LG). LMWPs include 1,6anhydroglucofuranose (AGF), 5-hydroxymethylfurfural (HMF), dianhydroglucopyranose (DAGP), levoglucosenone (LGO), furfural (FF), 2(5H)-furanone (FO), acetic acid (AA), glycolaldehyde (GA), carbon dioxide (CO2), and carbon monoxide (CO).



Our experimental results suggest that molten HDPE can be used to selectively inhibit the mass transfer processes in cellulose pyrolysis by trapping the escaping molecules, promoting secondary pyrolysis reactions. A simple depiction of this process is shown in Figure 7. We hypothesize that anhydrosugar oligomers (e.g., cellobiosan and cellotriosan) and ejected aerosols do not have the mobility to maneuver through the molten polymer phase, as opposed to LG and LMWPs that can move through the molten HDPE layer and evaporate relatively rapidly. This physical inhibition leads to increased formation of (1) LG, the most desired anhydrosugar product, due to increased glycosidic bond cleavage of anhydrosugar oligomers trapped in the molten HDPE phase, and (2) LMWPs, due to increased secondary pyrolysis reactions of the LMWP precursors (e.g., glucose22 in the illustration). The extent of the inhibition can be controlled by the molten HDPE properties, specifically the thickness, viscosity, and surface coverage of the molten HDPE layer. Since diffusivity of a molecule through a liquid medium is inversely proportional to its molar volume (i.e., molecular weight divided by density) to the 0.6 power, according to the Wilke-Chang equation53, it is expected that the inhibition is more pronounced for larger molecules or ejected aerosols due to their relatively low diffusivity in the molten HDPE phase, resulting in increased mass transfer timescales.


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Figure 7. Hypothesized physical inhibition by molten HDPE: the presence of HDPE layer traps the escaping products in the molten HDPE layer, suppressing the mass transfer (evaporation/ejection) processes and promoting the secondary pyrolysis reactions to form LG and LMWPs.

Although this study only focuses on cellulose, the same physical inhibition effect is expected to exist during the co-pyrolysis of thermoplastics and other biomass constituents, such as hemi-cellulose and lignin. The extent of this effect, however, is a topic for future studies. It is likely that the reaction timescales of the secondary reactions relative to the diffusion timescales caused by the molten polymer layers play a critical role.

Effect of Reaction Temperature. It was observed in the experiments by Zhou et al.22,29 that lower reaction temperature favors the formation of LG. This is likely due to the higher reaction barrier of cellulosic chain dehydration, which produces water vapor for the formation of glucose, 17 ACS Paragon Plus Environment


the hypothesized LMWP precursor22. In our work, pyrolysis of layered HDPE/cellulose samples was conducted at two more reaction temperatures, 400 ºC and 450 ºC, in addition to 500 ºC discussed earlier. The LG yields from neat cellulose pyrolysis in our experiments slightly increased with decreasing reaction temperature, consistent with literature findings. The effect of temperature, however, is not as marked as reported by Zhou et al.22,29 (Figure 8) and is more inline with the powder samples studied by Paulsen et al.28 The presence of molten HDPE layers on top of CE consistently resulted in higher LG yields compared to neat CE runs, regardless of reaction temperatures. The highest LG yield in this work reached 74.2% at 400 ºC for the 3PECE samples, a nearly 30% increase.

Figure 8. Levoglucosan yields from neat CE pyrolysis and HDPE/CE co-pyrolysis (3PECE samples) at different reaction temperatures.


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Co-Pyrolysis of Levoglucosan and HDPE. To understand if levoglucosan (LG) produced from cellulose pyrolysis is reactive under our reaction conditions, single component and layered LG/HDPE samples were also prepared for the pyrolysis experiments. The yields of LG-derived products are shown in Figure 9 and tabulated in Table S4 of Supporting Information. Our results show that LG was polymerized under our reaction conditions, producing residue.54 This suggests that the potential increase in LG yields from cellulose/HDPE co-pyrolysis could be even higher if LG polymerization can be minimized, possibly with shorter pyrolysis times. Another observation is that yields of LMWPs increased with the thickness of the HDPE layers, suggesting that LG also undergoes degradation in the molten HDPE phase.

Figure 9. Yields of LG-derived products from neat LG pyrolysis and LG/HDPE copyrolysis. Yields are shown for residue, 1,6-anhydroglucofuranose (AGF), 5hydroxymethylfurfural (HMF), dianhydroglucopyranose (DAGP), levoglucosenone (LGO), α-methyl furan (MFUR), furfural (FF), furan (FUR), 2(5H)-furanone (FO), acetaldehyde (ACAL), acetic acid (AA), glycolaldehyde (GA), carbon dioxide (CO2), carbon monoxide (CO), and levoglucosan (LG). 19 ACS Paragon Plus Environment


High Density Polyethylene (HDPE)-derived Products. Yields of HDPE-derived products from the co-pyrolysis of cellulose (CE) and HDPE are shown in Figure S1 of Supporting Information for molecules up to C33 (which represents the largest size that our GCs could detect). The products can be grouped into three families, linear alkanes, linear alkenes, and linear dienes. The cumulative yields of the three product families are shown in Figure 10 and tabulated in Table S5 of Supporting Information. The yields of alkenes are higher than that of alkanes and dienes, and the ratio of alkenes, alkanes, and dienes is approximately 3:2:1, consistent with literature findings.55-56 Compared to neat HDPE pyrolysis, the yields of HDPE-derived products increased with the presence of cellulose. A possible explanation for this increase is the catalytic effect from the cellulose-derived pyrolysis products while they are moving through the molten HDPE phase.

Figure 10. Yields of HDPE-derived products from different sample mixing patterns during the co-pyrolysis of cellulose (CE) and high density polyethylene (PE) or the co-pyrolysis of levoglucosan (LG) and high density polyethylene (PE). Note that yields of hydrocarbon products larger than C33 could not be detected by the GCs and are not included in this figure. 20 ACS Paragon Plus Environment

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The yields of HDPE-derived products during the co-pyrolysis of levoglucosan (LG) and HDPE are also shown in Figure 10. As opposed to neat HDPE pyrolysis, the presence of LG enhanced the overall yields of HDPE-derived products. However, this enhancement decreased with increased thickness of the HDPE layers, unlike what was observed in CE/HDPE copyrolysis. Since LG yields decreased with the thickness of the HDPE layers (Figure 9), these results suggest that LG may catalyze the degradation of HDPE, although the exact mechanism of this catalytic effect needs to be further investigated in future studies.

CONCLUSIONS Co-pyrolysis of cellulose (CE) and high density polyethylene (HDPE) was conducted using a custom-made batch reactor. Samples of different mixing patterns of CE and HDPE were studied. Our experiments showed that the yields of levoglucosan (LG) and low molecular weight products (LMWPs) both increased due to the presence of HDPE. This is likely resulted from the inhibition of the evaporation of anhydrosugar oligomers in cellulose pyrolysis, leading to increased secondary pyrolysis reactions in the molten HDPE phase. The morphology of the HDPE layers on top of cellulose is the determining factor affecting the interplay between chemical kinetics and mass transfer in cellulose/HDPE co-pyrolysis, thus altering product distributions. The highest LG yield observed in this work reached 74.2% at 400 ºC, an approximately 30% increase, when significant amount of HDPE was coated on top of cellulose. The HDPE pyrolysis was also accelerated by the presence of CE, likely due to the catalytic effect caused by LG.



NOTES The authors declare no competing financial interest.

SUPPORTING INFORMATION The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. The file contains tabulated mass yields of cellulose-derived, levoglucosanderived, and HDPE-derived products from the pyrolysis experiments, and graphical representation of the mass yields of HDPE-derived products for each carbon number during the co-pyrolysis of cellulose and HDPE.

ACKNOWLEDGMENT The work was supported by University of Massachusetts Lowell Faculty Startup Funds.

ABBREVIATIONS LG, levoglucosan; HDPE, high density polyethylene; PE, high density polyethylene; CE, cellulose; CO, carbon monoxide; CO2, carbon dioxide; ACAL, acetaldehyde; FUR, furan; MFUR, α-methyl furan; GA, glycolaldehyde; AA, acetic acid; FO, 2(5H)-furanone; FF, furfural; LGO, levoglucosenone; DAGP, dianhydroglucopyranose; HMF, 5-hydroxymethylfurfural; AGF, 1,6-anhydroglucofuranose; GC, gas chromatography; MS, mass spectrometer; TCD, thermal conductivity detector; FID, flame ionization detector 22 ACS Paragon Plus Environment

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For Table of Contents Use Only Synopsis The presence of HDPE layer traps the escaping products in the molten HDPE phase, suppressing the mass transfer (evaporation and aerosol ejection) processes during cellulose pyrolysis and promoting the formation of levoglucosan (LG) and low molecular weight products (LMWPs).

Graphical Abstract


Enhanced Levoglucosan Yields from the Copyrolysis of Cellulose and

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