Effect of Pyrolysis Temperature and Sulfuric Acid During the Fast

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Effect of Pyrolysis Temperature and Sulfuric Acid During the Fast Pyrolysis of Cellulose and Douglas Fir in an Atmospheric Pressure Wire Mesh Reactor Zhouhong Wang,† Shuai Zhou,† Brennan Pecha,† Roel J. M. Westerhof,‡ and Manuel Garcia-Perez*,† †

Biological Systems Engineering, Washington State University, Pullman, Washington 99164, United States Thermo-Chemical Conversion of Biomass Group, Faculty of Science and Technology, University of Twente, Postbus 217, 7500AE Enschede, The Netherlands

‡

ABSTRACT: The goal of this study is to better understand important reactions responsible for the suppression of anhydrosugars during the pyrolysis of microcrystalline Avicel, ball-milled Avicel, levoglucosan, cellobiosan, and Douglas fir at varied pyrolysis conditions (heating rate 100 °C/s, temperature 300−500 °C, H2SO4 addition 0−0.6 wt %) in an atmospheric pressure wire mesh reactor. Pyrolysis of levoglucosan at 300 °C yielded 67 wt % of itself, indicating that this is a reactive molecule. Pyrolysis of cellobiosan at 300 °C resulted in the production of relatively large quantities of an unidentified compound (estimated in 22 wt %) and a solid residue (18 wt %) with small quantities of levoglucosan. This result suggests that cellobiosan is an important intermediate for char formation during cellulose pyrolysis. When sulfuric acid (0.04 wt %) was added in small amounts to the control and ball-milled cellulose, the yield of levoglucosan decreased and 1,6-anhydrogulcofuranose was formed through accelerated dehydration reactions. In the case of the Douglas fir, an increase in levoglucosan yield (from 30 to 40 wt %) may occur via mitigation of cellulose−lignin interactions in this material or through passivation of the remaining AAEMs (0.013 wt %). At higher acid concentrations the levoglucosan yield decreases, likely due to the acceleration of cellulose dehydration reactions.

1. INTRODUCTION Fast pyrolysis is a promising method to convert up to 75 wt % of lignocellulosic materials into a crude bio-oil that can be further refined for the production of fuels and chemicals.1,2 Although fast pyrolysis results in a high yield of oil, in most cases a significant fraction of cellulose is fragmented to produce C1−C4 oxygenated molecules. Under very specific reaction conditions, cellulose can be converted into levoglucosan, which is intensively studied as a hydrolyzable sugar for the production of fuels and chemicals.1,3−6 Recent molecular modeling studies7,8 based on density functional theory (DFT) have identified a “concerted” mechanism (glycosidic bond cleavage and levoglucosan formation) for levoglucosan production that is more favorable than previously proposed mechanisms based on the formation of radical or ionic intermediates. The yields of levoglucosan obtained from the pyrolysis of cellulose vary widely in the literature.9 In 1979 Shafizadeh et al. reported a hydrolyzable sugar yield of 58 wt % levoglucosan, which is equivalent to 77 wt % of D-glucose.10 The same author conducted vacuum pyrolysis studies of cellulose (Whatman CF 11 powder) at temperatures between 300 and 500 °C and reported yields of hydrolyzable sugars (as D-glucose) close to 60 wt %, while the yield of levoglucosan was between 34 and 40 wt %.11 The reason for the difference in the yield of hydrolyzable sugars between slow and fast heating rate conditions is not fully undertood but seems to be associated with secondary reactions of cellulose primary products in the solid, liquid intermediate phase, and vapor phase.12,13 Kawamoto et al. studied the polymerization of levoglucosan into polysaccharides as a key reaction for the formation of carbonized products.13 Bai et al. studied the role of © 2014 American Chemical Society

levoglucosan physicochemistry in cellulose pyrolysis and concluded that the polymerization of this compound is at least in part responsible for the formation of extra char.14,15 This causes lower levoglucosan yields. In a vacuum, a higher levoglucosan yield is typically obtained.16,17 Cellobiosan is one of the other sugar products that has been identified from cellulose pyrolysis.12,18,19 Though the formation of cellobiosan has been studied,20,21 the literature on the secondary thermochemical reactions of cellobiosan and larger oligo-anhydrosugars is very limited.22 The yield of levoglucosan obtained from the pyrolysis of lignocellulosic materials is affected by many factors.23−31 Interestingly, the yield of levoglucosan from lignocellulosic materials is generally much lower (1.89 wt %,32 2−3 wt %,33 1.8−3.9 wt %,34 4−10 wt %,35 17.3 wt %36) than the yields reported for cellulose. It is known that even small quantities of alkali metals such as sodium and potassium naturally found in biomass can decrease the yield of usable sugars.23−26 So far, two strategies have been proposed to mitigate the undesirable effect of AAEMs. The first one consists on the removal of these AAEMs by washing with aqueous solutions containing acid.28 The aqueous phase of pyrolysis oil, which is rich in acetic acid, has been proven to effectively remove AAEMs to increase levoglucosan production.28 The second strategy consists of the passivation of the catalytic effect of AAEM by adding small quantities of strong acid able to form stable salts.27,37 These strategies increase levoglucosan yields from lignocellulosic Received: May 2, 2014 Revised: July 10, 2014 Published: July 11, 2014 5167

dx.doi.org/10.1021/ef500999z | Energy Fuels 2014, 28, 5167−5177

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Marathon MT) was used to measure the temperature of the mesh. A PID controller was employed to control the heating rate (maximum 120 °C/s based on current setting) and hold temperature. The actual temperature was calibrated by sandwiching an insulated thermal couple (Omega XC series, type K) between two pieces of mesh. Samples (∼40 mg) were sandwiched between two pieces of mesh and tightly clamped on the reactor. Then they were heated to designed temperature (300, 375, 450, and 500 °C) with a heating rate of 100 °C/s. The temperature was held for 10 s (at 375, 450, and 500 °C) or until the reactions finish (30 s for 300 °C). The reactor chamber was flushed with nitrogen (0.4 L/min) continuously to help carry the vapors through the condenser. The condenser was filled with liquid nitrogen in the outer tube so the pyrolysis vapors could be collected on the mesh filter, the paper filter, and the inner tube. The products collected on the condenser wall and wire mesh filter were washed with HPLC grade methanol. The methanol with products was collected, and the weight was recorded. The paper filters with products were immersed in this methanol. The weight of the mesh was carefully measured before and after each experiment. The resulting methanol solution was analyzed by HPLC and GC/MS. Yield of levoglucosan, acetol, and glycolaldehyde was monitored and calculated. The calculation of product yields followed this equation:

materials, but the yields obtained are still lower than those obtained from pure cellulose. The reasons for this phenomenon are not well-known, but there is abundant information in the literature suggesting that levoglucosan formation is affected by the presence of other organic compounds in lignocellulosic materials.18,38−42 In 1982, Shafizadeh and Stevenson observed an important increase in the yield of levoglucosan when acid washed pine wood was pyrolyzed in the presence of sulfuric acid.27 The authors noted that this phenomenon only occurred in materials containing lignin. However, they did not propose any explanation for this phenomenon. Radlein et al. also studied the effect of sulfuric acid on the yield of sugars and observed a similar phenomenon.43 Cellulose conversions into levoglucosan as high as 53.9 wt % was obtained when 0.1 wt % of sulfuric acid was used. In a preliminary semiquantitative Py-GC/MS study, we also observed a significant increase in the yield of levoglucosan when sulfuric acid was added.44,45 The main cause for the phenomena is still unknown. Thus, the main purpose of this paper is to investigate the effect of temperature and sulfuric acid on cellulose thermochemical reactions at atmospheric pressure with the aid of a wire mesh reactor to advance our understanding of the nature of the intermediates formed during cellulose pyrolysis.

concentration of product in methanol yield =

(

g g of methanol

)

M (g of biomass)/M methanol (g of methanol)

(1) M: amount of biomass used in experiment. Mmethanol: amount of methanol used in washing the condensing tube and mesh filter. 2.2.2. Fast Speed Camera. A fast speed camera focused on the mesh reactor allows for recording the changes in cluster size during the fast heating of the samples. The camera was set at a 60° angle to the wire mesh surface. The top lid was removed from the reaction chamber, and the flushing gas was changed from nitrogen to argon, which is slightly denser than air. A piece of quartz glass was placed on the mesh to prevent small particle from falling through mesh openings, so the reaction took place on the quartz rather than on the wire mesh itself. The reaction temperature was measured directly from the quartz piece with a thermocouple before the experiment, which also verified the temperature calibration of the wire mesh experiment. A constant light source (USHIO So̅ larc LB50 Fiber-Optic Illuminator) was used for better visibility of the samples. The mesh and quartz glass were preheated for 10 s. The camera then started recording, and samples were sprinkled from above. The videos were recorded at 250 frames/s and analyzed in Midas 4.0 Express to determine the reaction times of the samples by counting frames in which each cluster was still visible. Cellulose cluster sizes were measured by counting the pixels for wire mesh openings (0.074 mm) and wire diameters (0.053 mm). 2.3. Analytical Methods. 2.3.1. Scanning Electronic Microscopy (SEM). SEM pictures of the solid residue, obtained after pyrolysis of cellobiosan, were obtained on a FEI 200F SEM system with a large field detector and a low vacuum environment (130 Pa). High voltage was set at 30 kV, and the magnification was set to 1000×. 2.3.2. GC/MS. The pyrolysis condensates dissolved in methanol were first analyzed by GC/MS (Agilent 7890A equipped with HP-5 MS column, 30 m × 0.250 mm, 0.25 μm and 5975C inert XL EI MS detector). Two microliters of sample were injected for every analysis. The split ratio was set at 5:1, and 1 mL/min He was used to flush the column. The inlet was heated to 200 °C. The column was kept at 40 °C for 1 min and then heated to 190 °C at a heating rate of 3 °C/min. Next, the temperature of the oven was ramped up to 280 °C with heating rate of 20 °C/min and then kept for 10 min to remove any product left over in the column. Standard curves were created for levoglucosan, acetol, and glycolaldehyde. 2.3.3. HPLC. Product condensates dissolved in methanol were weighed, vacuum-dried, and carefully weighed again. The dried samples were redissolved in a known amount of E-pure water. The redissolved samples were injected in a Varian Prostar 230 HPLC with Varian Prostar 350 RI (refractive index).23 The water flow rate was

2. MATERIALS AND METHODS 2.1. Materials. The cellulose samples (control and ball-milled) used in this work have been described elsewhere.30,31 Briefly, the microcrustalline cellulose (Avicel PH-101 ∼50 μm) was purchased from Sigma-Aldrich, and ball-milled cellulose was obtained by ballmilling control cellulose at 300 rpm for 24 h. Control cellulose had a crystallinity of 60.5%, and the ball-milled cellulose had a crystallinity of 6.5% based on X-ray diffraction (XRD) measurements.30 Levoglucosan and cellobiosan used in for the pyrolysis studies were purchased from Carbosynth. The levoglucosan used for standards in the GC/MS, IEC, and HPLC analysis was purchased from Sigma-Aldrich. Both anhydrosugars (levoglucosan and cellobiosan) are primary products of cellulose pyrolysis.46 The Douglas fir feedstock was harvested from the cascade mountain range in Washington State, USA, kindly provided by Herman Brothers Logging & Construction (Port Angeles, WA). The feedstock was ground by hammer mill (Model number 400 HD, serial 2404, Bliss Industries) in the Composite Material and Engineering Center (CMEC) at Washington State University (WSU) and sieved to 2 mm diameter or smaller. This woody material was then ball-milled at 300 rpm for 30 h (Across International PQ-N2, ceramic 100 mL jar and balls). Then, the ball-milled Douglas fir powder was immersed in 0.1% H2SO4 (w/w = 1:10) overnight to dissolve the ash. Then the liquid was disposed, and the sample was washed by E-pure water (w/w = ∼1:200−300) to remove acid and dissolve ash. The acid level was monitored by electric conductivity meter until the conductivity remained constant after three washes. This feedstock has ∼46% of cellulose, as reported elsewhere.45,47 The ash content of Douglas fir particles was 0.24 wt %. ICP-MS tests confirmed that the biomass after acid washing had 0.013 wt % of AAEMs remaining. Samples (acid washed Douglas fir, microcrystalline and ball-milled cellulose, and levoglucosan) were impregnated with sulfuric of different weights. Sulfuric acid solution (1 wt %) was prepared for this addition to prevent fast dehydration from sulfuric acid. Approximately 0.07, 0.13, 0.26, 0.40, and 0.60 wt % of acid was added into samples. Tests with cellobiosan were conducted at four levels: 0, 0.07, 0.13, and 0.32 wt %. Acid added samples were left under a vacuum to dry for a week before testing. 2.2. Reactor. 2.2.1. Wire Mesh Reactor. Wire meshes (TWP 200 Mesh T304 Stainless 0.0533 mm Wire Dia., cut into pieces 50.8 mm wide and 63.5 mm long) were used to hold and heat samples. A welder was used to provide a controlled current. An infrared sensor (Raytek 5168


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used at 0.4 mL/min on a Bio-Rad Aminex HPX-87P column. Cellobiosan (1, CL), 1,6-anhydroglucofuranose (2, AGF), 1,4;3,6dianhydro-glucopyranose (3, DGP), levoglucosenone (4, LVS), levoglucosan (5, LVG), and one unknown peak (presumably a product of dehydration reactions) were monitored. Yields of these compounds were calculated.

the pyrolysis of the microcrystalline control and ball-milled cellulose between 300 and 500 °C. Levoglucosan and an unknown peak triplet at residence time of ∼24.2 min can be seen. Although the unknown peak has the same residence time as 1,6-anhydro-glucofuranose, it cannot be assigned to this compound because we did not observe that compound (which is volatile) in the corresponding GC/MS chromatogram. Surprisingly, no cellobiosan was observed in the HPLC chromatograph. This result contrasts with a yield of cellobiosan close to 10−16 wt % reported in vacuum pyrolysis tests by Westerhof et al. (2013) and earlier results of 6−15 wt %.18,19 The GC/MS chromatogram of Avicel and ball-milled Avicel pyrolysis oil between 300 and 500 °C (not shown) had the same peaks. The cellobiosan and the 1,6-anhydro-glucofuranose peaks did not show up. The absence of the cellobiosan peak confirms the HPLC results. The absence of the unknown peak in the GC/MS indicates that it should be a relatively heavy compound that can be evaporated at the pyrolysis temperatures studied (300−500 °C) but not at the analytical injection conditions (around 200 °C) in the GC/MS. The yields of levoglucosan obtained by GC/MS and by HPLC, the unknown compound(s) by HPLC, and the solid residue are plotted in Figure 3 as a function of pyrolysis temperature. The maximum yield of 41 wt % levoglucosan was achieved by both the microcrystalline control and ball-milled cellulose at 300 °C. This levoglucosan yield is lower than the one reported using Py-GC/MS by Patwardhan et al. (2010)49 but is comparable with the yield reported by Paulsen et al. (2012).50 The difference in our yields can be due to the way some Py-GC/MS systems are calibrated49 and/or because of the effect of sample mass33 and the shape of the biomass processed.50 The unknown compound(s) (estimated with the calibration of levoglucosan) was found to slightly decrease with temperature increasing (from 19 to 13 wt %) in the microcrystalline control cellulose but slightly increased in ball-milled cellulose (from 15 to 17 wt %). The yields obtained from control cellulose and from ballmilled cellulose were very similar, suggesting that cellulose crystallinity has a very limited effect on the outcome of fast pyrolysis under atmospheric pressure and very fast heating rates. The mechanism by which this compound is formed is unknown, but likely spawns from the liquid intermediate. In both cases, the yield of levoglucosan decreased as the pyrolysis temperature increased. Minor peaks, using the calibration of levoglucosan, accounted for approximately 5 wt % (microcrystalline cellulose) and 7 wt % (ball-milled cellulose) of total yield.

3. RESULTS AND DISCUSSION 3.1. Effect of Pyrolysis Temperature. 3.1.1. Pyrolysis Time of Cellulose As a Function of the Cluster Size. The particle size of the microcrystalline cellulose was ∼0.05 mm and the ball-milled cellulose even smaller. Cellulose samples were sprinkled through the same mesh used for heating between 300 and 500 °C. The pyrolysis time and the formation of liquid intermediates at very high heating rate conditions of small particles in the wire mesh reactor were observed with a fast speed camera. The pyrolysis reaction times for the clusters were measured by counting frames. The calculation of the effective diameter followed the method described by Dauenhauer et al.48 The average total conversion time of particles with an effective diameter smaller than 0.2 mm is plotted in Figure 1 as a

Figure 1. Reaction time vs temperature of microcrystalline (control) and ball-milled cellulose.

function of the wire mesh reactor temperatures (300, 375, 450, and 500 °C). At low temperatures (below 450 °C), ball-milled cellulose pyrolyzed much faster than the control cellulose. This is mainly due to the formation of liquid intermediates from ballmilled cellulose in the whole range of temperatures. In the microcrystalline (control) cellulose, the liquid intermediate forms only at relatively high temperatures (over 400 °C). At a temperature over 400 °C when both samples formed liquid intermediates, the total reaction time was very similar. 3.1.2. Analysis of Cellulose Volatile Products. Figure 2 shows the HPLC chromatographs of condensates derived from

Figure 2. HPLC chromatograms of products from mesh reactor, in terms of heating temperature (results normalized, control, and ball-milled cellulose). From left to right: cellobiosan, unknown compounds, levoglucosan. 5169


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Figure 3. Yield of levoglucosan (by HPLC and GC/MS) and unknown compound(s) (by HPLC) from pyrolysis of microcrystalline (control) cellulose and ball-milled cellulose.

Figure 4. GC/MS total ion chromatogram of (A) levoglucosan and (B) cellobiosan pyrolysis products at 300 °C.

Figure 5. HPLC chromatogram of products from pyrolysis of levoglucosan and cellobiosan.

The yields of solid residues were between 2 (500 °C) to 6 (375 °C) wt % for microcrystalline cellulose and between 1 (500 °C) to 4 (375 °C) wt % for ball-milled cellulose. The yield of glycolaldehyde was lower than 1% and no acetol was found. 3.1.3. Pyrolytic Behavior of Cellobiosan and Levoglucosan. In order to understand the mechanism responsible for the formation of the unknown peak detected in the HPLC studies of cellulose pyrolytic condensate, we decided to study the pyrolysis of two of the main cellulose primary products (levoglucosan and cellobiosan). Levoglucosan has an estimated boiling point of ∼300 °C at atmospheric pressure;51−53 cellobiosan has a boiling point of ∼581 °C, and the cellotriosan has a boiling point of ∼792 °C.53,54 Although in most pyrolysis cases the temperature will not exceed the range of 500−600 °C, cellobiosan and larger oligomers are still observed in fast pyrolysis,12,46 possibly released from the liquid intermediate through thermal ejection (spitting).46 Figure 4 shows the GC chromatograms of cellobiosan and levoglucosan products. The GC/MS total ion chromatogram of levoglucosan shows the peak of levoglucosan only. In addition

to levoglucosan, glycolaldehyde, acetol, and cellobiosan were also observed in the pyrolysis of cellobiosan. A second unknown compound which was not found in either cellulose or levoglucosan was observed from cellobiosan pyrolysis. The fragmentation pattern of the unknown peak (not shown) looks similar to that of levoglucosenone and may be a partially fragmented oligomer. The HPLC chromatogram obtained for the pyrolytic products of levoglucosan and cellobiosan is shown in Figure 5. Levoglucosan is the well-known major product from cellulose pyrolysis and is detected in the chromatograph. However, levoglucosan pyrolysis produced no detectable cellobiosan (retention time: 20.8 min). Instead, the peak of the previously discussed unknown compound(s) can be observed in the range of 21−27 min. The pyrolysis of cellobiosan produced of very small quantities of levoglucosan, large quantities of the aforementioned unknown compound(s), a small quantity of cellobiosan, and small quantities of other compounds that are likely dehydration products. The HPLC result also shows the 5170


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Figure 6. Yields of levoglucosan (by HPLC and GC/MS), cellobiosan (by GC/MS), and unknown compound(s) (by HPLC) from the pyrolysis of levoglucosan (left) and cellobiosan (right).

Figure 7. SEM pictures of mesh exhibit a thin layer of carbonaceous residue after cellobiosan pyrolysis (contrast and brightness adjusted).

presence of a second unknown peak that may or may not be the compound identified by GC/MS. The yields of products obtained from levoglucosan and cellobiosan are shown in Figure 6. The yields reported were obtained by calibrating the HPLC with standards of levoglucosan (∼70 min) and cellobiosan (20.8 min). At 300 °C 67 wt % of the levoglucosan is recovered as levoglucosan, 4 wt % is recovered as the unknown sugar and 2 wt % is recovered as a solid residue. In the case of cellobiosan, 6 wt % is recovered as levoglucosan, 22 wt % is recovered the unknown compound(s) and 18 wt % is recovered as a solid residue. The yield of levoglucosan decreased as the temperature increased, while the yield of the unknown compound(s) remained nearly constant with temperature. Pyrolysis of levoglucosan produced a smaller amount of the first unknown compound(s) compared to pyrolysis of cellobiosan. Our

experiments did not reveal significant amounts or changes of products from fragmentation reactions (0.2 wt %) did not increase the yield of char significantly in the high temperature experiments (500 °C), indicating the reaction with sulfuric acid might be a catalyzed dehydration/crosslinking reaction which could also occur in a liquid intermediate.30 The concentration at which the yield of levoglucosan is highest has been correlated by Kuzhiyil et al. (2012) with the

cellobiosan from pyrolysis of cellobiosan was most likely thermally ejected.46,48 Figure 7 shows the SEM picture of the solid residue formed during the pyrolysis of cellobiosan. The pictures at low temperature show the formation of a liquid intermediates and bubbles. As the temperature increased, the liquid layer could still be found covering the wire. Considering the high boiling point (∼581 °C53,54), the following scenarios are likely: (1) Unboiled liquid has a high tendency to crosslink/dehydrate/polycondense, thus forming char and unknown compounds (liked products of dehydration reactions); (2) thermal ejection of oligo-sugars needs the involvement of low boiling point compounds to thermally eject the liquid intermediate (rich in heavy oligosugars). 3.1.4. Analysis of Douglas Fir Volatile Products. The effect of temperature on the yield of levoglucosan and the unknown peak is shown in Figure 8. The yields of levoglucosan, obtained

Figure 8. Effect of temperature on the yield of levoglucosan from Douglas fir (yields on cellulose basis).

by both HPLC and GC/MS, decreased as the temperature increased. The levoglucosan yield (on a cellulose base) was ∼9 wt % lower (18−32 wt %) in Douglas fir than for cellulose (27−41 wt %). The yield of the unknown compound from Douglas fir (33−34 wt % on a cellulose basis) was twice the yield estimated for microcrystalline cellulose (13−19 wt %) and ball-milled cellulose (15−17 wt %). The main cause for the lower levoglucosan yield and the higher yield of the unknown compound(s) in the lignocellulosic material is not known, but other researchers suggest it may be due to cellulose−lignin interactions.27,43 3.2. Effect of Sulfuric Acid. 3.2.1. Effect of Sulfuric Acid Concentration on the Yield Products from Douglas Fir. The effect of sulfuric acid on the yield of levoglucosan and other

Figure 9. Effect of sulfuric acid concentration on the yield of levoglucosan (on cellulose basis) and other products from Douglas fir pyrolysis at 500 and 300 °C by mesh reactor (detected by HPLC and GC/MS). 5172


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Figure 10. Effect of sulfuric acid concentration on the yield of dehydrated sugar products (by Douglas fir) and char yield from Douglas fir pyrolysis at 500 and 300 °C by mesh reactor (detected by GC/MS).

Figure 11. Product yields from 500 °C pyrolysis of cellulose (microcrystalline control and ball-milled) and levoglucosan under the effect of sulfuric acid impregnation.

AAEM content.37 Because our material was acid washed before adding sulfuric acid, the concentration at which the maximum levoglucosan is formed could be associated with the concentration needed to passivate the AAEM left in the biomass or due to the concentration at which the interactions between cellulose and the other biomass components are mitigated.60 To investigate the main causes for the maximum yield of levoglucosan at sulfuric acid concentrations of 0.04−0.08 wt %, in the next section we study whether this increase in yield can

be associated with the behavior of cellulose or its primary products (levoglucosan or cellobiosan). 3.2.2. Effect of Sulfuric Acid Concentration on the Yield and Composition of Cellulose and Levoglucosan Pyrolysis Products. Microcrystalline cellulose, ball-milled cellulose, and levoglucosan were pyrolyzed in mesh reactor, and their pyrolysis products were analyzed by GC/MS and HPLC. Since pyrolysis of Douglas fir had a much bigger increment in the yield of levoglucosan at 500 °C, the pyrolysis tests were also conducted at 500 °C. Figure 11 shows the yields of levoglucosan (HPLC and GC/MS), the unknown compound, 5173


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Figure 12. Effect of sulfuric acid concentration on the yield of residue and dehydrated products (by GC/MS) from microcrystalline control cellulose, ball-milled cellulose, and levoglucosan for pyrolysis at 500 °C on a wire mesh reactor.

(microcrystalline and ball-milled cellulose) was similar to the behavior observed for levoglucosan (the main product of cellulose degradation reactions); this suggests that the intramolecular dehydration reactions will also occur after the levoglucosan intermediate is formed. 3.2.3. Effect of Sulfuric Acid Concentration on Yield and Composition of Cellobiosan Pyrolysis Products. The thermal behavior of cellobiosan in the presence of different concentrations of sulfuric acid was also studied in the mesh reactor. Our previous results have shown that untreated cellobiosan does not produce much levoglucosan but gave one unknown compound shown in GC/MS and a large amount of other unidentified compounds that can be clearly seen by HPLC. With the mesh reactor, we pyrolyzed cellobiosan at 500 °C with three different sulfuric acid concentrations. Cellobiosan samples were dissolved in water during the sample preparation and vacuum-dried after applying acid. The potential hydrolysis of cellobiosan in the presence of sulfuric acid was studied by analyzing the unpyrolyzed cellobiosan by GC/MS and HPLC. The HPLC chromatograms (see Figure 13) clearly show that the simple addition of small quantities of sulfuric acid did not result in the production of levoglucosan. However, an unidentified heavier compound seems to be produced even at the very low concentration of sulfuric acid studied. This heavy compound could be cellobiose, but this hypothesis should be confirmed experimentally. When the cellobiosan with different concentrations of sulfuric acid

DGP, levoglucosenone (HPLC) as a function of sulfuric acid addition. Levoglucosan yields obtained by HPLC and GC/MS were very similar and in all cases decreased as sulfuric acid concentration increases even with less than 0.1 wt % of sulfuric acid added. Ball-milled cellulose had a more drastic change in yield of levoglucosan than control cellulose, indicating stronger action of sulfuric acid either on the primary reaction or on the secondary reactions in the liquid intermediate. The yield of the unknown compound also decreased as sulfuric acid concentration increased, which might indicated the production of these compounds are related to liquid intermediate or even yield of levoglucosan. The yield of DGP and levoglucosenone gradually increased as the sulfuric acid concentration increased. The yields of GC/MS detectable compounds are shown in Figure 12. Levoglucosenone, DGP, and one unknown GC/MS peak was found to increase with more sulfuric acid. The yield of 1,6-anhydroglucofuranose followed the same trend found in Douglas fir. These results confirm that the addition of sulfuric acid can enhance cellulose dehydration reactions responsible for the formation of anhydrosugars and extra-char. As discussed before, in slow pyrolysis the sulfate can bind onto cellulose and drive the reactions toward water, char, and small molecules.59 The yield of levoglucosan was much less affected by sulfuric acid in the microcrystalline cellulose than in ball-milled cellulose. This indicates that the existence of crystalline structure protects the cellulose from the penetration of sulfuric acid. On the other hand, the behavior of cellulose samples 5174


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4. CONCLUSIONS The primary products of cellulose pyrolysis (levoglucosan, cellobiosan, and heavier anhydrosugars) form a liquid intermediate, under high heating rates, that evaporates, is either thermally ejected from the solid/liquid phase, or further cross-links and polycondenses to form of bio-oil and a solid residue. Ball-milled cellulose pyrolyzed formed a liquid intermediate and evaporated much faster than the control cellulose at temperatures below 400 °C. At higher temperatures both samples form liquid intermediates. However, these phenomena do not significantly affect the yield of levoglucosan produced. Our experimental results highlight the importance of the thermal behavior of cellobiosan (and likely the heavier anhydrosugars derived from cellulose) under atmospheric pressure. At atmospheric pressure, cellobiosan cannot evaporate as it does under a vacuum. If not removed by thermal ejection, this anhydrosugar could be a major intermediate for char production. The formation of levoglucosan from cellobiosan (and perhaps from the other anhydrosugars) may explain why the yield of levoglucosan obtained under a vacuum is much lower than the yield obtained at atmospheric pressure. Most of the levoglucosan formed in the primary reactions can be evaporated at atmospheric pressure in the wire mesh reactor with a very small fraction converted into the unknown compound and solid residue. Higher yields of hydrolyzable (fermentable) sugars could be obtained if cellobiosan secondary reactions can be mitigated using a vacuum or by other methods. The yield of levoglucosan obtained from pyrolyzing acid washed Douglas fir at 300 and 500 °C in a wire mesh reactor is 32 and 18 wt % (on cellulose weight basis), respectively, lower than the yields obtained for control and ball-milled cellulose at the same temperature. The main reason for the relatively lower levoglucosan yield obtained with acid wash Douglas fir is unknown, but it can be due to undesirable cellulose−lignin interactions or the presence of very small quantities of AAEMs. There is an optimum concentration of the sulfuric acid infused in Douglas fir, at which the yield of levoglucosan reached 43 and 37 wt % (cellulose weight basis). At 500 °C, the yield of levoglucosan obtained was even higher than those obtained from pure cellulose. This increase in levoglucosan yield cannot be explained by the effect of sulfuric acid on the cellulose major pyrolysis product levoglucosan. However, the tests with cellobiosan showed that the levoglucosan yield could also increase because of the modification of this sugar dimer or even heavier sugars in the existence of a small amount sulfuric acid and high temperature. Analysis showed that cellobiosan was modified by sulfuric acid before pyrolysis and that hydrolysis reactions are possible even at the inlet temperature

Figure 13. HPLC of unpyrolyzed cellobiosan samples with sulfuric acid (in wt %).

was dissolved in methanol and then injected into the GC/MS, the chromatograph (not shown) indicated the formation of levoglucosan and an unknown peak. The apparent contradiction between the HPLC and GC/MS analysis suggests that the cellobiosan containing small quantities of sulfuric acid can be hydrolyzed at the GC/MS inlet (which is at 200 °C) to form levoglucosan and an unknown product previously identified in cellobiosan pyrolysis. Cellobiosan pyrolysate HPLC chromatographs can be seen in Figure 14. After acid was applied, the yield of the solid residue was 1 and 2 wt % for acid concentration at 0.07 and 0.13 wt %; at 0.32 wt % acid, the char yield increased to 6 wt % (originally 4 wt % at 500 °C). The yield of levoglucosan increased from 4 wt % (no acid) to 13 wt % with 0.32 wt % of sulfuric acid. The change might be due to the acceleration of acid hydrolysis reactions. The yield of the unidentified compound (2) was not affected with 0.07 wt % of sulfuric acid usage and decreased with higher sulfuric acid concentration. Yields of levoglucosenone and DGP were only detected when the acid concentration was at 0.32 wt %. Our experimental results reinforce the hypothesis that the sulfuric acid does not increase levoglucosan yields through direct interactions (hydrolysis) with cellulose; it is likely a suppression of cellulose−lignin or cellulose−AAEM interactions. Note that this contradicts results from similar experiments using phosphoric acid56,58 but can be attributed to the chemical properties of sulfuric acid. It is logical to conclude that sulfuric acid acts more like a strong dehydrator that catalyzes cross-linking and polycondensation reactions leading to the formation of char.61

Figure 14. HPLC of cellobiosan pyrolysates (left) and the evaluation of their yields (right). 5175


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to 200 °C. The increase in the yield observed when small quantities of sulfuric acid added to Douglas fir could be due to (1) the effect of this additive mitigating the poorly known cellulose−lignin interactions allowing one to achieve the maximum yield expected for cellulose under the same reaction conditions or (2) its passivation effect on the remaining AAEM.

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AUTHOR INFORMATION

Corresponding Author

*Address: Biological Systems Engineering, Washington State University, LJ Smith Hall, Room 205, Pullman, WA, 991646120 Phone: 509-335-7758, Fax: 509-335-2722 e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was financially supported by the U.S. National Science Foundation (CBET-0966419, CAREER CBET1150430), the Sun-Grant Initiative (Interagency Agreement: T0013G-A), and the Washington State Agricultural Research Center. This work was also partially funded by USDA/NIFA through Hatch Project No. WNP00701. The authors are very thankful for their support.

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