Formation of Anhydro-sugars in the Primary Volatiles and Solid

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Formation of Anhydro-Sugars in the Primary Volatiles and the Solid Residues from Cellulose Fast Pyrolysis in a Wire-Mesh Reactor Xun Gong, Yun Yu, Xiangpeng Gao, Yu Qiao, Minghou Xu, and Hongwei Wu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef501112q • Publication Date (Web): 03 Jul 2014 Downloaded from http://pubs.acs.org on July 14, 2014

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Formation of Anhydro-Sugars in the Primary Volatiles and the Solid Residues from Cellulose Fast Pyrolysis in a Wire-Mesh Reactor

Xun Gong a, Yun Yu b*, Xiangpeng Gao b, Yu Qiao a, Minghou Xu a *, Hongwei Wu b

RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to) a

State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan

430074, China b

School of Chemical and Petroleum Engineering, Curtin University, GPO Box U1987, Perth WA 6845,

Australia

* Contact authors. Dr Yun Yu, E-mail: [email protected]; Tel: +61-8-92669202; Fax: +61-8-92662681 Prof Minghou Xu, Email: [email protected]; Tel: +86-27-87546631; Fax: +86-27-87545526

A manuscript submitted to Energy and Fuels for consideration of publication

Short Running Title: Formation of Anhydro-Sugars from Cellulose

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ABSTRACT. The unique design of wire-mesh reactor (WMR) enables the collection of primary volatiles 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

with minimised secondary reactions from fast pyrolysis of solid fuels. This paper reports the formation of anhydro-sugars in both primary volatiles and solid residues from the fast pyrolysis of microcrystalline cellulose in a WMR. Under fast pyrolysis, cellulose is rapidly converted into an intermediate phase and the maximal yield of the water-soluble intermediates achieved in this study is ~21% on a carbon basis at 450 °C, much higher than those achieved in other pyrolysis reactor systems. The solid residue consists of sugar and anhydro-sugar oligomers with a wide range of degrees of polymerization (DPs). However, only anhydrosugars with DPs up to 3 can be identified in the primary volatiles, although the presence of these anhydrosugars is evident even at pyrolysis temperatures as low as 300 °C. Because of high boiling points, cellobiosan and cellotriosan are impossible to release into the vapour phase via evaporation under the conditions. These high-DP anhydro-sugars are also unlikely formed due to oligomerization of levoglucosan in the vapor phase because such secondary reactions are minimized in the WMR. Therefore, cellobiosan and cellotriosan are most likely released into the vapor phase as aerosols, driven by the ejection mechanism (i.e., carryover by the intensive release of volatiles). Among the anhydro-sugars in the primary volatiles, levoglucosan has yields of 27−44% (based on weight) depending on pyrolysis temperature, while cellobiosan and cellotriosan have yields of 3−9% and 1.0−2.3%, respectively. This leads to the highest selectivity of 34−60% (based on weight) in the condensed liquid product for levoglucosan. The yields of anhydro-sugars initially increase with pyrolysis temperature and achieve the maximal value between 400 and 450 °C. Further increases in the pyrolysis temperature lead to substantial reductions in the yields of anhydro-sugars in the primary volatiles (although the yield of liquid product remains unchanged), indicating the increased formation of water-insoluble compounds in the primary volatiles at increased temperatures.


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1. Introduction 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fast pyrolysis is a promising technology to convert biomass into high-energy-density biofuels,1-4 i.e., bio-oil and biochar, which are more convenient to be stored, transported, and refined into transport fuels in a central biorefinery plant. Part of biochar can be also used as a soil amendment to achieve nutrient recycling5,6 and carbon sequestration,7,8 enhancing the overall sustainability of the pyrolysis process. However, the poor fuel properties of bio-oil largely limit the further commercialisation of fast pyrolysis and refining technology for the production of transport fuels.9-11 A sound understanding of biomass pyrolysis mechanism is of critical importance to develop advanced pyrolysis and refining technologies to produce biofuels and biochemicals from biomass. Due to the complex structure of biomass, cellulose is generally used as a model compound for biomass pyrolysis studies. Previous studies12-16 and recent reviews17,18 have indicated that pyrolysis of cellulose and lignocellulosic materials occurs through a molten intermediate liquid, which subsequently evaporates into volatiles or thermally degrades to more volatile compounds. However, the identification of this intermediate phase is challenging because of its short life-time and the complexity of compounds. Recent studies19,20 have provided new knowledge on the composition of the water-soluble portion of reaction intermediates from cellulose pyrolysis. Both sugar and anhydro-sugar oligomers with a wide range of degrees of polymerisation (DPs) are present in the water-soluble portion of reaction intermediates from cellulose pyrolysis.19 The anhydro-sugar products are likely to be the primary pyrolysis products from cellulose pyrolysis via the breaking of glycosidic bonds.20 However, the anhydro-sugars have different fates within the liquid intermediate phase (so-called condensed-phase), depending on their volatilities. The volatile anhydro-sugars (mainly levoglucosan) diffuse from liquid bulk to the liquid-vapor interface, and then escape the liquid intermediate phase. The anhydro-sugars may experience significant degradation to form other small volatile compounds within the liquid intermediate phase, and the extent of degradation depends on the relative rates of secondary pyrolysis reactions and mass transport (i.e., diffusion) within the liquid intermediate phase.21 The non-volatile anhydro-sugars largely retain in the liquid intermediate phase till decompose into volatile compounds, which then evaporate into vapor phase. The complex chemistry of primary pyrolysis products in the liquid intermediate phase is obviously one of the important reasons leading to the complicated composition of bio-oil. Typical bio-oil from ACS Paragon Plus Environment

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cellulose pyrolysis mainly includes anhydro-sugars (e.g., levoglucosan), pyrans, furans and light oxygenates, 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

while their distribution was found to depend upon the chain length, with longer chains producing more anhydro-sugars.22,23 To improve the bio-oil quality, it is important to have a sound understanding in the complex chemistry of condensed-phase reactions within the liquid intermediate phase. However, it requires the detailed information of primary volatiles from cellulose pyrolysis with the minimization of their secondary reactions in the vapor phase. Unfortunately, such knowledge on the primary products from the fast pyrolysis of biomass and its model compounds is still lacking. Due to the limitations of the reactor systems, most of the previous studies were carried out under the conditions where the significant secondary reactions take place in both the solid/molten and gaseous phases in the same reactor. This study deploys a wire-mesh reactor (WMR) to facilitate the collection of primary volatiles from cellulose pyrolysis, via rapid separation and quench of these products. The WMR minimizes not only the interactions between evolving volatiles and pyrolysing cellulose but also the secondary reactions of the primary volatiles in the vapor phase. Therefore, the condensed liquid products from a WMR are close to the true primary volatiles from pyrolysing cellulose. As anhydro-sugar products (e.g., levoglucosan) are widely reported as the main product from cellulose pyrolysis,24 this study mainly focuses on the formation of anhydro-sugar products in the primary volatiles and solid residues from cellulose fast pyrolysis at various pyrolysis temperatures (300–700 °C).

2. Experimental Section 2.1 Materials Microcrystalline cellulose (Avicel PH-101) purchased from Sigma-Aldrich was sieved to a large size fraction of 106–250 µm, in order to make sure cellulose particles are not carried away by sweep gas during pyrolysis in the WMR. The cellulose was washed by double distilled water at room temperature to remove any water-soluble compounds in the cellulose sample. The sample was then dried in an oven at 105 ºC prior to use.

2.2 Pyrolysis Experiments


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Cellulose pyrolysis experiments were undertaken in a WMR at Huazhong University of Science and 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Technology, and a schematic diagram of the WMR is shown in Figure 1. The WMR was first invented by Loison and Chauvin25 in 1964, and later improved/used by Howard and co-workers26,27 and other researchers,28-31 just to name a few. It consists of a stainless steel wire-mesh sample holder stretched between two electrodes, where the mesh also severs as a resistance heater. A stream of helium was used as sweep gas to pass through the wire-mesh sample holder with a linear gas velocity of 0.1 m s-1 (equivalent to 4 L min-1 under ambient conditions), due to its high thermal conductivity. The rapid sweeping of the wiremesh sample holder via the sweep gas minimizes the intra-particle mass transfer limitations and also the interactions between the pyrolysing cellulose particles and the evolving volatiles. In each experiment, a thin layer of cellulose particles (~10 mg) were sandwiched between two layers of wire-mesh, which were heated directly with an electrical current to the desired pyrolysis temperature with further holding at the temperature for a specified period. The experimental program considered various pyrolysis temperatures (300−700 ºC) under fast heating conditions (with a heating rate of 100 ºC s-1). The primary volatiles were rapidly quenched in a liquid nitrogen-cooled trap for suppressing the secondary reactions in the vapor phase hence condensed as the liquid products (so-called tar) in the trap. The solid samples were also rapidly cooled to ambient temperature in the helium to facilitate the collection of reaction intermediates in the solid residue. After experiment, the trap was thoroughly washed by the mixture of chloroform and methanol (80:20 by volume) to remove all the condensed liquid in the trap. Then the collected liquid sample was evaporated at 35 ºC to remove the solvents for determining the yield of liquid products, and the produced water during cellulose pyrolysis also evaporated during this process. Therefore, the yield of liquid product in this study is defined as the compounds remained after evaporation normalized by the loaded cellulose. The weight loss of cellulose pyrolysis was determined by weighing the sample-laden wire-mesh sample holder before and after the experiment, considering the moisture in the samples. Each experiment was run in triplicate.

2.3. Characterisation of Anhydro-Sugars in the Solid Residues To quantify the anhydro-sugars in the reaction intermediates, the solid samples after cellulose pyrolysis were washed by double distilled water to extract the anhydro-sugars in the solid sample. In this study, the sampleladen wire-mesh sample holder was soaked in ~20 ml double distilled water at ambient temperature. After ACS Paragon Plus Environment

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thorough shaking, the mixed sample was filtered to obtain the liquid sample containing the water-soluble 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

compounds in the solid samples. This liquid samples are referred to hereafter as “water-soluble intermediates”. The total carbon contents of the water-soluble intermediates were immediately measured by a total organic carbon (TOC) analyzer. Then the yield of the water-soluble intermediates on a carbon basis can be calculated as total carbon in the water-soluble intermediates divided by total carbon in raw cellulose. All the liquid samples were further analysed by a high-performance anion exchange chromatography with pulsed amperometric detection (HPAEC-PAD) to characterise the sugar and anhydro-sugar oligomers with a wide range of DPs, using a technique detailed elsewhere.32 A Dionex ICS-3000 ion chromatography (IC) system equipped with a CarboPac PA200 analytical column (3 × 250 mm) and guard column (3 × 50 mm) was used for HPAEC-PAD analysis, following a gradient method eluting 20−225 mM NaOAc and 100 mM NaOH over 30 min with a flow rate of 0.5 mL min-1. The peaks detected by HPAEC-PAD were verified by various sugar and anhydro-sugar standards. In this paper, the sugar and anhydro-sugar oligomers are named according to DP values (e.g., glucose as C1, cellobiose as C2; levoglucosan as AC1, cellobiosan as AC2). The quantification of anhydro-sugars were also done by IC with a CarboPac PA20 analytical column (3 × 150 mm) and guard column (3 × 30 mm) using an isocratic method, which elutes 100 mM NaOH only with a flow rate of 0.5 mL min-1. The yields of anhydro-sugars (on a carbon basis) were calculated as the carbon in each anhydro-sugar divided by total carbon in raw cellulose, while their selectivities in the watersoluble intermediates (on a carbon basis) were determined as the carbon in each anhydro-sugar divided by total carbon in the water-soluble intermediates. In order to calculate the percentage of the water-soluble intermediates in the solid residues after cellulose pyrolysis, elemental analysis was conducted to measure the carbon contents of solid residues. It should be noted that under some conditions (e.g., high temperature or long holding times) it is not possible to obtain enough solid residues for elemental analysis.

2.4. Quantification of Anhydro-Sugars in the Primary Volatiles To quantify the anhydro-sugars in primary volatiles from cellulose pyrolysis, another set of experiments were conducted under identical conditions. Then the trap was thoroughly washed by double distilled water with fixed volume, to extract the anhydro-sugars in the condensed liquid products. The liquid samples are ACS Paragon Plus Environment

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referred to hereafter as “water-soluble liquid”. The water-soluble liquid samples were first subjected to TOC 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

analysis. The yield of the water-soluble liquid on a carbon basis can then be calculated as total water-soluble carbon normalized by the total carbon in raw cellulose. All the fresh liquid samples were analysed by the HPAEC-PAD to quantify the anhydro-sugar products, using the same isocratic method. The yield and selectivity of those anhydro-sugar products in the condensed liquid products were calculated based on weight. It is noteworthy that this analysis also reports the formation of some large-DP anhydro-sugars (e.g., cellobiosan and cellotriosan) in the volatiles, which were generally neglected in the previous works due to the limitations of GC-MS in detecting large-DP anhydro-sugars.

3. Results and Discussion 3.1. Weight Loss and Yields of Pyrolysis Products Weight Loss as a Function of Holding Time and Pyrolysis Temperature. As shown in Figure 2, cellulose pyrolysis proceeds slowly at 300 °C, with the weight loss is only ~38% after holding for 120 s. Further experiments showed that the weight loss is only increased to 65% at a long holding time of 10 mins, indicating that the reaction intermediates are difficult to evaporate as volatiles at 300 °C. However, when the pyrolysis temperature increases to 350 °C, the cellulose pyrolysis reaction rate increases significantly and cellulose appears to be completely pyrolysed within 120 s. The reaction time needed for complete pyrolysis reduces further increases with increasing pyrolysis temperature. The pyrolysis reactions at temperatures above 500 °C are so rapid that the reactions have virtually completed during the heating-up period.

Yields of Liquid and Solid Products from Cellulose Pyrolysis. To facilitate the collection of reaction intermediates from cellulose pyrolysis, the solid products at zero holding time were collected to maximize the yield of reaction intermediates during pyrolysis. The yields of solid and condensed liquids products at zero holding time and complete conversion for 300–700 °C are shown in Figure 3. It should be noted that as the reaction time required to achieve complete conversion at 300 °C is too long, the yield of liquid products at 300 °C with complete conversion is not presented in this figure. Also, the pyrolysis reactions are very fast at temperatures above 400 °C, the majority of the liquid products were produced during heating-up period. It is interesting to see that, although pyrolysis temperature has a substantial effect on the reaction rate of ACS Paragon Plus Environment

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cellulose pyrolysis, the yield of liquid products at complete conversion does not change significantly with 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

pyrolysis temperature. For example, the liquid products even have a yield of ~81% at 350 °C, compared to that of ~80% at 500 °C. A recent study21 reported a liquid product yield of 89% during the pyrolysis of cellulose at 500 °C and 60 °C s-1 using a WMR, but the study did not report the results at different temperatures. The lower liquid product yield at 500 °C in this study was likely due to that the majority of volatile compounds (including water) were removed during the evaporation process. However, the yield of solid product at complete conversion is low and decreases with increasing pyrolysis temperature, from ~4% at 350 °C to ~1.5% at 600 °C. During cellulose pyrolysis, the evaporation and polymerization of reaction intermediates are two competing reactions.33-35 The slightly higher yield of solid products at low temperatures indicates that the polymerization reactions are promoted at lower temperatures. Further increases in the temperature do not change the yields of liquid and solid products, as the pyrolysis reactions are complete at 600 °C. In a recent work,19 the char yield for cellulose pyrolysis at 350 °C and a heating rate of 10 °C min-1 is ~8%, which is higher than that achieved in this study at a fast heating rate of 100 °C s-1. The low char yield in this study indicates that the polymerization reactions from intermediates for char formation were largely suppressed during cellulose fast pyrolysis in the WMR. Some other reactions (e.g., cross-linking36) favored for char formation seem also be minimized.

3.2. Formation of Anhydro-Sugars in the Solid Residues Yield of the Water-Soluble Intermediates at Zero Holding Time. As shown in Figure 4, the yield of the water-soluble intermediates initially increases with increasing pyrolysis temperature. It achieves a maximal yield of ~21% at 450 ºC, which is much higher than those (i.e., ~3%) obtained in our previous studies using a quartz reactor under slow and fast pyrolysis conditions.19,37 Further increases in the pyrolysis temperature result in a significant decrease in the yield of the water-soluble intermediates. In consistence with the results under slow pyrolysis of cellulose,19 a comparison between the data in Figure 3 and Figure 4 suggests that the maximal yield of the water-soluble intermediates is always achieved when significant weight loss occurs. It is also evident that the fast heating and rapid quenching conditions indeed promote the formation of the water-soluble intermediates from cellulose pyrolysis.


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Furthermore, most of the solid residues at high temperatures are water-soluble. The contribution of 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the water-soluble intermediates in the solid residue increases with increasing pyrolysis temperature. For example, the content of the water-soluble intermediates for the solid residue increases from only ~1.6% at 300 ºC to ~68% at 450 ºC. Unfortunately, data for the solid residues at higher temperature (>500 ºC) are not available due to the little amount of samples available for collection from experiments. However, it is plausible that at temperatures > 450 ºC, the cellulose is converted into mostly water-soluble intermediates that rapidly evaporate or decompose into more volatile compounds, leading to close to zero char yields.

Characterisation of Anhydro-Sugars in the Water-Soluble Intermediates. Figure 5 shows the IC chromatograms of the water-soluble intermediates obtained from the fast pyrolysis of cellulose in a WMR. Similar to those under slow pyrolysis conditions,19 sugar and anhydro-sugar oligomers were also present in the water-soluble intermediates from cellulose fast pyrolysis. At low temperatures (i.e., 300 ºC), the majority of the oligomers exist as the form of sugar oligomers with a DP range of 1−8. At 350 ºC, the anhydro-sugar oligomers start to appear. For example, the sugar oligomers with DPs of 1−8 and anhydro-sugar oligomers with DPs of 1−11 are present in the water-soluble intermediates obtained at 350 ºC. Further increases in pyrolysis temperature lead to an increase in the production of anhydro-sugar oligomers but a decrease in the production of the sugar oligomers. However, it is interesting to see that at 450 ºC, only the anhydro-sugar oligomers with DPs of 1−5 can be found in the water-soluble intermediates. The higher-DP anhydro-sugar oligomers, which appear in the water-soluble intermediates at low temperatures, seem to decompose into lower-DP anhydro-sugar oligomers or other unidentified oligomers (i.e. the so-called partially decomposed sugar-ring-containing oligomers - PDSRCOs19). At 500 ºC, no sugar or anhydro-sugars can be detected, because almost all the cellulose has decomposed at such a high temperature even without holding. The above results clearly show that during fast pyrolysis cellulose is rapidly converted into reaction intermediates which are different from the original cellulose. This newly formed intermediate phase is at least partially water-soluble and consists of sugar and anhydro-sugar oligomers. The formation of non-sugar compounds is very likely at high temperatures or longer holding times due to the decomposition of those oligomer sugars.


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Figure 6 presents the concentrations (on a carbon basis) of typical small anhydro-sugars (e.g., 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

levoglucosan, cellobiosan and cellotriosan) found in the water-soluble intermediates. It can be found that the yields of these anhydro-sugars in the solid residues are quite low. The highest yield for levoglucosan in the water-soluble intermediates is only ~1.1% at 400 °C. For cellobiosan and cellotriosan, the maximal yields are all obtained at 450 °C, with the values of ~0.7% and ~0.5%, respectively. Further increases in the pyrolysis temperature will lead to the disappearance of those anhydro-sugars. Therefore, those anhydrosugars either evaporate into the vapor phase or decompose into other compounds, when the pyrolysis temperature increases to above 450 °C. Also shown in Figure 6 are the selectivities of anhydro-sugars in the water-soluble intermediates. Levoglucosan has a selectivity of ~10% at 300 °C, but its selectivity increases to ~19% at 350 °C. Further increases in the pyrolysis temperature lead to an increase in the formation of levoglucosan but a reduction in its selectivity. The selectivities of cellobiosan and cellotriosan follow the similar trends but with lower values. For example, the maximal selectivities of cellobiosan and cellotriosan are obtained at 350 °C, with the values of ~8% and ~3.4%, respectively. There are at least two possible reasons which are responsible for the increase in yields but reduction in selectivities of those anhydro-sugars with increasing pyrolysis temperature at 400 °C. One is the formation of more high-DP oligomers with increasing temperature, which could be the reason between 350 and 400 °C since higher peaks of oligomeric sugars can be found in Figure 5. However, those higher-DP oligomeric sugars even become less when further increasing the temperature to 450 °C. This demonstrates that the majority of the water-soluble intermediates at high

temperatures (e.g., >450 °C) are contributed by some PDSRCOs or non-sugar

products, as evidenced in some recent studies.19 Unfortunately, the detailed information on those compounds in the water-soluble intermediates is largely unknown due to the limitations in analysis.

Distribution of Anhydro-Sugars in the Primary Volatiles and the Solid Residues. Further analysis was made to understand the distribution of anhydro-sugars in the primary volatiles and the solid residues at zero holding time, and the data are shown in Figure 7. The yield of anhydro-sugars in the primary volatiles at zero holding time can be found in the section below. At 300 °C, the anhydro-sugars are only detected in the solid residues as reaction intermediates, indicating they have not evaporated into vapor phase at zero holding time. At 350 °C, the majority of anhydro-sugars retain in the reaction intermediates, and only ~19% of total ACS Paragon Plus Environment

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levoglucosan, ~8% of total cellobiosan, and ~6% of total cellotriosan are found in the primary volatiles. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Further increases in pyrolysis temperature significantly promote the evaporation of anhydro-sugars, thus increasing their fractions in the primary volatiles. At temperatures above 500 °C, the anhydro-sugars only can be found in the primary volatiles. It is worthy to mention that anhydro-sugars with lower DPs have a higher tendency to evaporate as volatiles at the same temperature, obviously due to the higher volatility of anhydro-sugars with lower-DP.

3.3. Formation of Anhydro-Sugars in the Primary Volatiles Anhydro-sugar products, i.e., levoglucosan, are known to be the main components in the liquid product of cellulose pyrolysis and the presence of inorganic catalysts (e.g., Na, K, Mg, Ca) can significantly reduce the levoglucosan yield,38,39 due to the promoted cross-linking reactions by inorganic catalysts.37,40 Some anhydro-sugar oligomers (i.e., cellobiosan) were also identified in the tar from cellulose pyrolysis.41-43 However, it is not clear if the anhydro-sugar oligomers were indeed present in the primary volatiles, as the secondary reactions of primary volatiles were not minimized in previous experiments. Furthermore, the yields of high-DP anhydro-sugars in the bio-oil were seldom reported, mainly due to the limitations of GCMS system, which was normally used for bio-oil analysis but cannot detect the high-DP anhydro-sugars in the bio-oil. This study deploys a newly-designed method to quantify the anhydro-sugars in the primary volatiles from cellulose pyrolysis. Anhydro-sugars have high solubilities and can be easily extracted by water. The trap was therefore thoroughly washed by double distilled water to obtain a liquid sample containing the water-soluble compounds in the liquid products from cellulose pyrolysis. As shown in Figure 8, the yield of the water-soluble liquid after complete conversion increases initially with the pyrolysis temperature, achieves the maximum at 450 °C then followed by a substantial reduction as the pyrolysis temperature further increases to 500 °C. As the sample almost achieves complete pyrolysis at 500 °C, further increase in the pyrolysis temperature above 500 °C leads to little change in the yield of the water-soluble liquid. It seems that, although the yield of total liquid products remains unchanged at 350–700 °C (see Figure 3), the compositions of liquid products have experienced substantial changes. As there is little increase in the gas yield based on the calculations of mass balance, the reduction in the yield of the water-soluble liquid appears ACS Paragon Plus Environment

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to result from some water-insoluble compounds produced from these reaction intermediates at increased 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

temperatures. The water-soluble liquid samples were further analysed by HAPEC-PAD in order to quantify the anhydro-sugar products. Despite the existence of sugar and anhydrosugar oligomers with a wide range of DPs in the water-soluble intermediates from cellulose fast pyrolysis (see Figure 5), only anhydro-sugars with DPs up 3 (i.e., levoglucosan, cellobiosan and cellotriosan) were detected as in the liquid products from cellulose pyrolysis. Some oxygenate compounds such as hydroxyacetaldehyde (HAA, also called glycolaldehyde) and small amount of glucose also exist in the water-soluble liquid samples. Figure 9 presents the yields of typical anhydro-sugar oligomers (e.g., levoglucosan, cellobiosan and cellotriosan) on a weight basis after complete conversion, together with those at zero holding time for comparisons. It is interesting to see that the yields of anhydro-sugars at complete conversion all follow the similar trend as that for the water-soluble liquid, i.e., increasing from 350 to 450 °C, suddenly decreasing at 500 °C and remaining unchanged at 500−700 °C. Figure 9 shows that the yield of levoglucosan (on a weight basis) increases from ~40% at 350 °C to ~44% at 400 and 450 °C, but reduces significantly to ~27% at 500 °C. The decreased yield of levoglucosan is more likely due to the reactions in the intermediate phase, since the secondary reactions of primary volatiles have been minimized in the WMR system used in this study. The use of HPAEC-PAD system in this study also provides us an opportunity to quantify all the anhydro-sugar products in the liquid products, and such high-DP anhydro-sugars were seldom reported in previous studies44 due to the limitations of GCMS system. As shown in Figure 9, cellobiosan and cellotriosan have the similar trends as that of levoglucosan. The yield of cellobiosan increases from ~4% at 350 °C to ~9% at 400 and 450 °C, but decreased largely to ~3% at 500 °C. For cellotriosan, its yield increases from ~1.2% at 350 °C to ~2.3% at 400 and 450 °C. A further increase in the temperature to 500 °C leads to a reduction of the yield to ~1.0%. The selectivities of those anhydro-sugars in the liquid products were further calculated, and the results are presented in Figures 9c and d. As a reference, the selectivities of anhydro-sugars in the liquid products at 300 °C and 50% weight loss are also presented in Figure 9. The selectivities of those anhydrosugars initially increase with temperature at low temperatures and achieve the maximum at 400 °C, i.e., ~60% for levoglucosan, ~11% for cellobiosan, and ~3% for cellotriosan. When the pyrolysis temperature ACS Paragon Plus Environment

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increases to 500 °C or above, their selectivities decreased significantly to ~34% for levoglucosan, ~7% for 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

cellobiosan, ~2% for cellotriosan. The above results clearly indicate that at high temperatures the reaction intermediates were converted into other water-insoluble compounds. Such compounds are volatile and still can be condensed in the tar trap, therefore it does not lead to the change in yield of total liquid products. Based on the above results, several important findings can be observed. First of all, the lowest temperature at which three anhydro-sugars appear in the liquid products is 300 °C, indicating that three anhydro-sugars start to escape the pyrolysing cellulose and enter into the vapor phase at ~300 °C. Their yields and selectivities increase with pyrolysis temperature until 400 °C. Our experimental results for levoglucosan seem to be in agreement with its true boiling point of ~300 °C.45 A recent study shows that levoglucosan can be completely recovered in the liquid products at a temperature of 250 ºC and a heating rate of 6000 K s-1 when pyrolysing in a WMR for 1 s holding.21 This indicates that levoglucosan can completely evaporate with little decomposition at high heating rates. Suuberg et al.46 also estimated a lower levoglucosan boiling point of ~260 °C for a fresh condensate of cellulose tar, but levoglucosan may escape together with other volatiles at a lower temperature than its true boiling point since the tar is enriched with low-boiling substances.45 As for cellobiosan and cellotriosan, Lédé et al47 estimated their boiling points to be ~581 and ~792 °C, respectively. Those boiling points are much higher than the temperature (300 °C) at which they were found in the liquid products in this work. It seems that the mechanism of high-DP anhydrosugars entering into vapor phase from pyrolysing cellulose cannot be explained by evaporation mechanism only. The formation of high-DP anhydro-sugars is also not due to the oligomerization of levoglucosan in the vapor phase (as suggested elsewhere48), as the secondary reactions of primary volatiles have been minimized in the WMR. Therefore, it seems that, some other mechanisms, e.g., physical entrainment,21 are likely to be responsible for the presence of high molecular weight anhydro-sugars in the liquid products at the pyrolysis temperature much lower than their true boiling points. Recently, Teixeira et al.49 reported their experiments on the ejection mechanisms of aerosols using high speed photography technique. Their results support the mechanism of interfacial gas bubble collapse forming a liquid jet which subsequently fragments to form ejected aerosols. The aerosols results from two possible pathways: evaporation of molten cellulose followed by nucleation/condensation, and direct ejection of liquids which is capable of transporting high molecular


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weight non-volatiles and inorganics into vapor phase.49 It seems that once the intermediate sugar oligomers 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

are melted, they can form aerosols which can be swept away by carrier gas from pyrolysing cellulose. Secondly, only anhydro-sugars with DP up to 3 have been identified in the liquid products under all the conditions in this study, although the sugar and anhydro-sugar oligomers with a wide range of DPs have been identified in the water-soluble intermediate as precursors of primary volatiles from cellulose pyrolysis. The ejection mechanism is capable of transporting high molecular weight non-volatiles into vapor phase, but the non-volatiles have to be melted first. The melting point is about 170−180 °C for levoglucosan,50,51 and about 285 °C for cellobiosan52. It is obvious that the melting point of anhydro-sugar increases with its DP. It seems like the anhydro-sugars with DP>3 either have high melting points or have decomposed before melting, so the high-DP anhydro-sugars cannot be carried away into volatiles and condensed as liquid products. The yields of anhydro-sugars in the liquid products decrease with their DPs, with majority of anhydro-sugars existed as levoglucosan (see Figure 9). Accordingly, the selectivity of levoglucosan is also much higher than those of cellobiosan and cellotriosan. Thirdly, fragmentation or other decomposition reactions may have occurred for those anhydro-sugars in the liquid intermediate phase, since we have detected the formation of HAA in the liquid products obtained from cellulose pyrolysis. The decreased yields of the water-soluble liquid and anhydro-sugars at high temperatures (>450 °C) indicate that reactions in the liquid intermediate phase become increasingly important at high temperatures, resulting in the formation of water-insoluble compounds. Besides, a small amount of glucose is always present in the liquid products at all temperatures. It should be mentioned that glucose is also detected in the reaction intermediate from solid residue, but the exact mechanism for glucose formation is not clear yet. It can be produced via the hydrolysis of glycosidic bonds by water produced from dehydration.53 But once glucose is formed in the intermediate phase, it can easily evaporate into vapor phase at low temperatures (e.g., ~250 °C21).

4. Conclusions This study reports the formation of anhydro-sugars in the primary volatiles and the solid residues from fast pyrolysis of cellulose, using a WMR which minimizes the interactions between evolving volatiles and pyrolysing cellulose as well as the secondary reactions in the vapor phase. It can be observed that cellulose ACS Paragon Plus Environment

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is rapidly converted into reaction intermediates under fast pyrolysis, with the maximal water-soluble 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

intermediate yield of ~21% obtained at 450 °C. The sugar and anhydro-sugar oligomers with a wide range of DPs are present in the water-soluble intermediates from cellulose pyrolysis. However, only anhydro-sugars with DPs up to 3 were identified in the primary volatiles. Because of their high boiling points, the presence of cellobiosan and cellotriosan in the primary volatiles are more likely due to aerosol ejection rather than evaporation. Anhydro-sugars contribute to up to ~74% of condensed liquid product from cellulose pyrolysis, depending on pyrolysis temperature, with levoglucosan as the major anhydro-sugar product. The yields of anhydro-sugars initially increase with temperature and achieve the maxima between 400 and 450 °C. Further increases in the pyrolysis temperature lead to substantial reductions in the yields of anhydro-sugars but not the yield of condensed liquid products, indicating that the reaction intermediates tend to form some waterinsoluble compounds in the primary volatiles at increased temperatures.

Acknowledgements The authors are grateful to the partial supports from the National Science Foundation of China (51306062 and U1261204) and the Australian Research Council.

References (1) Huber, G. W.; Iborra, S.; Corma, A. Chem. Rev. 2006, 106, 4044. (2) Bridgwater, A. V.; Meier, D.; Radlein, D. Org. Geochem. 1999, 30, 1479. (3) Mohan, D.; Pittman Jr., C. U.; Steele, P. H. Energy Fuels 2006, 20, 848. (4) Bridgwater, A. V. Therm. Sci. 2004, 8, 21. (5) Wu, H.; Yip, K.; Kong, Z.; Li, C.-Z.; Liu, D.; Yu, Y.; Gao, X. Ind. Eng. Chem. Res. 2011, 50, 12143. (6) Kong, Z.; Liaw, S. B.; Gao, X.; Yu, Y.; Wu, H. Fuel 2014, 128, 433. (7) Lehmann, J. Nature 2007, 447, 143. (8) Laird, D. A.; Brown, R. C.; Amonette, J. E.; Lehmann, J. Biofuels Bioprod. Bioref. 2009, 3, 547. (9) Anex, R. P.; Aden, A.; Kazi, F. K.; Fortman, J.; Swanson, R. M.; Wright, M. M.; Satrio, J. A.; Brown, R. C.; Daugaard, D. E.; Platon, A.; Kothandaraman, G.; Hsu, D. D.; Dutta, A. Fuel 2010, 89, S29. (10) Oasmaa, A.; Czernik, S. Energy Fuels 1999, 13, 914. (11) Diebold, J. P., A review of the chemical and physical mechanisms of the storage stability of fast pyrolysis bio-oils, National Renewable Energy Laboratory, NREL/SR-570-27613, Jan 2000. (12) Vladars-Uasa, A., Thermal decomposition of cellulose, 1993, University of Waterloo, Ontario, Canada. (13) Boutin, O.; Ferrer, M.; Lédé, J. J. Anal. Appl. Pyrolysis 1998, 47, 13. (14) Piskorz, J.; Majerski, P.; Radlein, D.; Vladars-Usas, A.; Scott, D. S. J. Anal. Appl. Pyrolysis 2000, 56, 145. (15) Lédé, J.; Blanchard, F.; Boutin, O. Fuel 2002, 81, 1269. (16) Wooten, J. B.; Seeman, J. I.; Hajaligol, M. R. Energy Fuels 2004, 18, 1. (17) Lédé, J. J. Anal. Appl. Pyrolysis 2012, 94, 17. (18) Mettler, M. S.; Vlachos, D. G.; Dauenhauer, P. J. Energy Envrion. Sci. 2012, 5, 7797. ACS Paragon Plus Environment

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(19) Yu, Y.; Liu, D.; Wu, H. Energy Fuels 2012, 26, 7331. (20) Liu, D.; Yu, Y.; Wu, H. Energy Fuels 2013, 27, 1371. (21) Hoekstra, E.; Van Swaaij, W. P. M.; Kersten, S. R. A.; Hogendoorn, K. J. A. Chem. Eng. J. 2012, 187, 172. (22) Patwardhan, R. R.; Satrio, J. A.; Brown, R. C.; Shanks, B. H. J. Anal. Appl. Pyrolysis 2009, 86, 323. (23) Mettler, M. S.; Paulsen, A. D.; Vlachos, D. G.; Dauenhauer, P. J. Green Chem. 2012, 14, 1284. (24) Antal Jr., M. J. Advances in Solar Energy 1983, 1, 61. (25) Loison, R.; Chauvin, R. Chim. Ind. 1964, 91, 269. (26) Anthony, D. B.; Howard, J. B.; Meissner, H. P.; Hottel, H. C. Rev. Sci. Instrum. 1974, 45, 992. (27) Anthony, D. B.; Howard, J. B.; Hottel, H. C.; Meissner, H. P. Fuel 1976, 55, 121. (28) Hamilton, L. H.; Ayling, A. B.; Shibaoka, M. Fuel 1979, 58, 873. (29) Gibbins, J. R.; King, R. A. V.; Wood, R. J.; Kandiyoti, R. Rev. Sci. Instrum. 1989, 60, 1129. (30) Gibbins-Matham, J.; Kandiyoti, R. Energy Fuels 1988, 2, 505. (31) Wu, L.; Qiao, Y.; Gui, B.; Wang, C.; Xu, J.; Yao, H.; Xu, M. Energy Fuels 2012, 26, 112. (32) Yu, Y.; Wu, H. Ind. Eng. Chem. Res. 2009, 48, 10682. (33) Kawamoto, H.; Murayama, M.; Saka, S. J. Wood Sci. 2003, 49, 469. (34) Ronsse, F.; Bai, X.; Prins, W.; Brown, R. C. Environmental Progress & Sustainable Energy 2012, 31, 256. (35) Liu, D.; Yu, Y.; Wu, H. Ind Eng Chem Res 2013, 52, 12785-12793. (36) Chaiwat, W.; Hasegawa, I.; Tani, T.; Sunagawa, K.; Mae, K. Energy Fuels 2009, 23, 5765. (37) Yu, Y.; Liu, D.; Wu, H. Energy Fuels 2014, 28, 245. (38) Patwardhan, P. R.; Satrio, J. A.; Brown, R. C.; Shanks, B. H. Bioresour. Technol. 2010, 101, 4646. (39) Hoekstra, E.; Westerhaf, R. J. M.; Brilman, W.; Van Swaaij, W. P. M.; Kersten, S. R. A.; Hogendoorn, K. J. A. AIChE J. 2012, 58, 2830. (40) Liu, D.; Yu, Y.; Long, Y.; Wu, H. Proc. Combust. Inst. 2014, DOI: 10.1016/j.proci.2014.05.026. (41) Tsuchiya, Y.; Sumi, K. J. Appl. Polym. Sci. 1970, 14, 2003. (42) Radlein, D.; Grinshpun, A.; Piskorz, J.; Scott, D. S. J. Anal. Appl. Pyrolysis 1987, 12, 39. (43) Arisz, P. W.; Lomax, J. A.; Boon, J. J. Anal. Chem. 1990, 62, 1519. (44) Paulsen, A. D.; Mettler, M. S.; Vlachos, D. G.; Dauenhauer, P. J. Energy Fuels 2013, 27, 2126. (45) Mamleev, V.; Bourbigot, S.; Bras, M. L.; Yvon, J. J. Anal. Appl. Pyrolysis 2009, 84, 1. (46) Suuberg, E. M.; Milosavljevic, I.; Oja, V. Symposium (International) on Combustion 1996, 26, 1515. (47) Lédé, J.; Diebold, J. P.; Peacoke, G. V. C.; Piskorz, J.; Bridgwater, A. V.; Czernik, S.; Meier, D.; Oasmma, A.; Radlein, D., Fast pyrolysis of biomass: a handbook. 1999, Newbury, UK: CPL Press. (48) Patwardhan, P. R.; Dalluge, D. L.; Shanks, B. H.; Brown, R. C. Bioresour. Technol. 2011, 102, 5265. (49) Teixeira, A. R.; Mooney, K. G.; Kruger, J., S.; William, C. L.; Suszynski, W.; Schmidt, L. D.; Schmidt, D. P.; Dauenhauer, P. J. Energy Environ. Sci. 2011, 4, 4306-4321. (50) Golova, O. P.; Andrievskaya, E. A.; Pakhomov, A. M.; Merlis, N. M. Russian Chemical Bulletin 1957, 6, 399. (51) Schwenker Jr., R. F.; Bech Jr., L. R. J. Polym. Sci. Part C Polym. Symp. 1963, 2, 331. (52) Bridgwater, A. V.; Boocock, D. G. B., Developments in Thermochemical Biomass Conversion, Volume 1. 1997: Springer. 525. (53) Shafizadeh, F.; Fu, Y. L. Carbohydr. Res. 1973, 29, 113.


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Figures 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. A schematic diagram of the wire mesh reactor used in this study.


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o

300 C o 350 C

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Figure 2. Weight loss as a function of pyrolysis temperature and holding time.


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Yield of Liquid Products (wt%)

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Figure 3. The yields of liquid and solid products as a function of pyrolysis temperature at zero holding time and complete conversion.


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Percentage in Solid Residue (C%)

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Figure 4. Yield of the water-soluble intermediates (based on carbon in cellulose) and its percentage in solid residue as a function of pyrolysis temperature at zero holding time.


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300 C, at zero holding time o 350 C, at zero holding time o 400 C, at zero holding time o 450 C, at zero holding time o 500 C, at zero holding time

AC1 & AC2 AC3

Detector Responce (nC per unit of cellulose)

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Figure 5. IC chromatograms of the water-soluble intermediates produced from cellulose pyrolysis at various temperatures. Note that AC1-AC11 are the anhydro-sugar oligomers with DPs of 1-11, and C1-C8 are the sugar oligomers with DPs of 1-8.


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1.5 Levoglucosan (AC1) Cellobiosan (AC2) Cellotriosan (AC3)

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Figure 6. Yields and selectivities of anhydro-sugars (on a carbon basis) in the water-soluble intermediates as a function of pyrolysis temperature.


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Levolgucosan (wt%)

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 7. Distribution of anhydro-sugars in the intermediates and condensed liquid products as a function of pyrolysis temperature at zero holding time. Empty: fraction in the intermediates; shade: fraction in the condensed liquid products.


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At Complete Conversion At Zero Holding Time

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Figure 8. Yield of the water-soluble liquid (based on carbon in cellulose) as a function of pyrolysis temperature.


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Yields of Anhydro-Sugars in the Liquid Products (wt%)

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Figure 9. Yield and selectivity of anhydro-sugars (on a weight basis) in the liquid products as a function of pyrolysis temperature. (a) Yield of levoglucosan (AC1); (b) yields of cellobiosan (AC2) and cellotriosan (AC3); (c) selectivity of levoglucosan (AC1); and (d) selectivity of cellobiosan (AC2) and cellotriosan (AC3). Solid: at complete conversion; empty: at zero holding time.


Formation of Anhydro-sugars in the Primary Volatiles and Solid

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