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Journal of Analytical and Applied Pyrolysis 99 (2013) 58–65

Contents lists available at SciVerse ScienceDirect

Journal of Analytical and Applied Pyrolysis

journa l h o me page: www.elsev ier .com/ locate / jaap

ole of levoglucosan physiochemistry in cellulose pyrolysis

ianglan Baia,b,∗, Patrick Johnstona, Sunitha Sadulaa, Robert C. Browna,c,∗

Center for Sustainable Environmental Technologies, Iowa State University, Ames, IA 50011, USADepartment of Aerospace Engineering, Iowa State University, Ames, IA 50011, USADepartment of Mechanical Engineering, Iowa State University, Ames, IA 50011, USA

r t i c l e i n f o

rticle history:eceived 22 August 2012ccepted 16 October 2012vailable online 15 November 2012

eywords:elluloseevoglucosan

a b s t r a c t

The fate of levoglucosan after it forms during cellulose pyrolysis was investigated experimentallyusing time-resolved thermogravimetric analysis/differential scanning calorimetric combined with massspectrometry measurements followed by high performance liquid chromatography and gel filtrationchromatography of the pyrolysis residue. This study indicates that levoglucosan formed during cellu-lose pyrolysis is initially a liquid that undergoes two simultaneous, competing processes of evaporationand polymerization. Levoglucosan that evaporates escapes the high temperature pyrolysis zone whilelevoglucosan that polymerizes is trapped in the pyrolysis zone and dehydrates to low molecular weight

yrolysisvaporationolymerizationligosaccharide

volatile products and char. The oligosaccharides that form during polymerization are subject to two simul-taneous reaction pathways: direct decomposition to low molecular weight products such as water, carbondioxide, 5-hydroxymethylfurfural, furfural, furan and acetic acid, and formation of polysaccharides thateventually dehydrate to char and low molecular weight volatiles. Based on the experimental observa-tions and quantitative measurements, a modified cellulose pyrolysis pathway involving levoglucosan as

prop

the major intermediate is

. Introduction

Biomass is a renewable and sustainable resource of fuels andhemicals [1,2]. When biomass is pyrolyzed in an oxygen free envi-onment, it converts to gases, condensable vapors, aerosols, andhar. The recovered liquid bio-oils can be further upgraded to bio-uels and valuable chemicals.

Lignocellulosic biomass consists of three major fractions: cel-ulose, hemicellulose and lignin. Cellulose is the largest fraction,ccounting for about 50% of most feedstocks [3]. Cellulose, aolysaccharide with the formula (C6H10O5)n, consists of a linearhain of thousands of d-glucose units connected by glycosidic link-ges. Anhydrosugars, mainly LG (C6H10O5), char, and LMWV suchs furans, acids, water and carbon dioxide are the final pyrolysisroducts of cellulose.

The pyrolysis behavior of cellulose has been studied by manyesearchers and several models have been developed [4–11].ccording to the Broidio–Shafizadeh model [4], cellulose ther-

ally decomposes to form “active cellulose,” a poorly characterized

ntermediate, which subsequently decomposes via two competi-ive first-order reactions to form volatiles such as LG or char and

∗ Corresponding authors at: Biorenewables Research Laboratory, Ames, IA 50011,SA. Tel.: +1 515 294 7669; fax: +1 515 294 3091.

E-mail addresses: bxl9801@iastate.edu (X. Bai), rcbrown@iastate.eduR.C. Brown).

165-2370/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.jaap.2012.10.028

osed.© 2012 Elsevier B.V. All rights reserved.

gases. Mok and Antal [5] proposed that cellulose forms active cel-lulose and LG as intermediates through competing processes thatsubsequently participate in secondary reactions to form volatilesand char. Shen and Gu [6] suggested that LG (and other anhy-drohexoses) and LMVW are formed directly from cellulose. Theyalso suggested that these anhydrosugars can further decompose toLMWV such as 5-HMF, furfural and acids via the formation of glu-copyranose as an intermediate. Patwardhan et al. [7] determinedthat LG and LMWV are formed through competitive pyrolysis reac-tions involving cellulose rather than sequential decompositionreactions; they based their conclusions on a comparison of thepyrolysis products of cellulose with those of several mono-, di- andpolysaccharides.

Furthermore, several researchers have reported that cellulosemelts in the early stage of cellulose pyrolysis prior to volatile evolu-tion. For example, Schroeter and Felix [12] observed cellulose meltsinto a colorless liquid during pyrolysis. Although they suspectedthat the liquid was chemically cellulose, they determined from IRspectral analysis that the melt contained reduced OH bonds. Byadopting high speed photography, Dauenhauer et al. [13] observedthat cellulose particles placed on a hot plate at 700 ◦C rapidlymelted and subsequently shrank in size as the melt vaporized.Lede et al. [14] found that when cellulose was fast pyrolyzed in

a radiant furnace, the fibrillar structure of cellulose disappearedin the early stage of pyrolysis and melted. Upon cooling the liquidbecame a solid that was soluble in water and, thus, was no longercellulose.

X. Bai et al. / Journal of Analytical and A

Nomenclature

LG levoglucosanLMWV low molecular weight volatileTGA thermogravimetric analysisDSC differential scanning calorimetricDTG differential thermogravimetricHPLC high performance liquid chromatographyGFC gel filtration chromatographyGC gas chromatographyMS mass spectrometryFID flame ionization detectorDa DaltonDP degree of polymerization

oco(spgpwP2sact

ipLlv7sp

LtopItpLtgdctLewttti

u

HMP hydroxymethylfurfural

This melt phase is thought to consist of LG and anhydro-ligosaccharides [14–17], but the exact composition is likely tohange as pyrolysis proceeds. Lede et al. [14] found the presencef LG and oligosaccharides with molecular weights up to 1134 Daseptomers) in the liquid intermediate products. They hypothe-ized that oligosaccharides preceded LG as the products of celluloseyrolysis. Pouwels et al. [15] also found anhydrosaccharides ran-ing from monomers to hexamers (162–972 Da) in the pyrolysisroducts of cellulose, and suggested a pyrolysis mechanism inhich oligosaccharides are the initial decomposition products.

iskorz et al. [16] reported yields of anhydro-oligosaccharides up to0% from cellulose fast pyrolysis and suggested that cellulose can beelectively depolymerized into oligosaccharides. Radlein et al. [17]nalyzed condensed pyrolysis residue of cellulose and identified aonsiderable amount of cellobiosan, along with LG, and suggestedhat cellobiosan is a primary pyrolysis product.

Although oligosaccharides formed during pyrolysis havensignificant vapor pressure, LG has sufficient vapor pressure atyrolysis temperatures to rapidly evaporate [18]. High yields ofG can be produced when pure cellulose is pyrolyzed under venti-ated conditions. Shafizadeh et al. [19] reported 58% yield of LG byacuum pyrolysis of cotton cellulose. Kwon et al. [20] obtained a0% yield of LG by pyrolyzing cellulose using a low pressure gram-cale pyrolysis reactor. Other studies [21,22] have shown that theyrolysis products of LG are identical to those of cellulose.

Several studies have been conducted to elucidate the role ofG in cellulose pyrolysis. Suuberg and co-workers [23] found thathe enthalpy of cellulose tar vaporization is very similar to thatf LG. They hypothesized that the tar intermediate from celluloseyrolysis could evaporate as LG vapor or dehydrate to char [24].

t is also well known that LG can polymerize at elevated tempera-ures [25–29]. Kawamoto et al. [30] proposed that LG is the primaryroduct of cellulose pyrolysis and can either directly decompose toMWV such as furans and acids or polymerize to polysaccharideshat eventually carbonize to char. More recently, Hosoya et al. sug-ested that non-condensable gases, such as CO2 and CO, are formeduring decomposition of vapor phase LG. They also suspected thathar and condensable LMWV products are related to polymeriza-ion of the liquid/solid phase LG. However, detailed mechanisms ofMWV and char in LG pyrolysis are not described [31]. Kawamotot al. [32] found that char formation does not occur when celluloseas pyrolyzed in sulfolane, a good solvent of LG. They reported that

he yield of LG in the liquid pyrolysis residue decreased as the reten-ion time was increased; thus, they suspected that the LG formed in

he early stages of pyrolysis is less stable and eventually convertsnto products other than char through an unknown intermediate.

Although previous studies clearly reveal the existence of a liq-id phase during fast pyrolysis of cellulose, most descriptions of

pplied Pyrolysis 99 (2013) 58–65 59

pyrolysis do not account for its effect on the physiochemistry ofpyrolysis. We hypothesize that the LG produced from the thermaldepolymerization of cellulose initially exists as liquid monomerthat either evaporates, which allows it to escape from the hightemperature pyrolysis zone, or polymerizes to oligosaccharides,which are not able to escape from the pyrolysis zone and eventuallydehydrates to LMWV and char. To test this hypothesis, the physio-chemistry of LG is studied using time-resolved TGA/MS, HPLC, andGPC analytical methods.

2. Materials and methods

2.1. Materials

Avicel PH200 cellulose was purchase from FMC Biopolymer,Inc. and LG (1,6-anhydro-�-d-glucopyranose) was purchased fromFisher Scientific. Cellobiosan was purchased from Santa CruzBiotechnology. Yellow dextrin (pyrodextrin) was purchased fromSpectrum. All chemicals had a purity of at least 99%.

2.2. Fast pyrolysis of LG

Levoglucosan samples were pyrolyzed in a CDS 5200 modelPyroprobe, which simulates the conditions of fast pyrolysis. Sam-ples in the range of 324–927 �g were placed inside an open-endedquartz tube reactor with loose quartz wool packing at both sides.The reactor was then heated resistively using an electric coilwrapped around the tube at a rate estimated to be over 100 ◦C/s. Thefinal pyrolysis temperature was set at 500 ◦C and the temperaturesof the valve oven and transfer line were kept at 325 ◦C to preventcondensation of pyrolysis vapor. Helium was used as sweep gasand the flow rate was 107 ml/min. The pyrolysis vapors were ana-lyzed with an online GC (model CP3800, Varian, USA) and a VarianFID. The injector temperature of the GC was 300 ◦C, and the splitratio was 1:100. A 60 m length of DB-1701 capillary column witha constant flow rate of 1.0 ml/min was used to separate the pyrol-ysis products. The GC oven was first held at 35 ◦C for 3 min, andthen heated to a final temperature of 300 ◦C with a heating rateof 5 ◦C/min. The final temperature was held for 4 min with a totalrun time of 60 min. The common pyrolysis products of cellulosereported in the literature [7,22] were calibrated in the GC/FID andused to identify pyrolysis products of LG.

2.3. Time-resolved TGA with online MS

TGA analysis was performed using a Mettler Toledo TGA/DSCsystem. In the case of cellulose pyrolysis, each cellulose sample(20 mg) was placed inside a 100 �l aluminum cup and heatedfrom room temperature to final temperatures between 310 ◦C and350 ◦C at a heating rate of 60 ◦C/min. In the case of LG pyrolysis,20 mg of LG sample was placed in a 100 �l aluminum cup andheated from room temperature to the desired final pyrolysis tem-perature (600 ◦C unless otherwise specified) using a heating rateof 5 ◦C/min. Nitrogen gas was used as sweep gas at a flow rateof 100 ml/min.

During LG pyrolysis, the TGA system was connected to a Pfeifferquadrupole MS. The MS system, which had a Faraday and sec-ondary electron multiplier, is capable of measuring compoundswith molecular weights up to 300 Da. Volatiles evolved duringpyrolysis in the TGA were introduced into the MS via a 1.5 m deacti-vated quartz capillary (0.23 mm outer diameter and 0.15 mm interdiameter) heated to 250 ◦C. Compounds were analyzed according

to the ions produced (m/z ratio). The recording of mass ions in theMS was triggered by a starting signal from the TGA.

In the initial TGA–MS experiments, the MS system was operatedin scan mode to identify the type of mass ions produced from the

6 and Applied Pyrolysis 99 (2013) 58–65

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Fig. 1. HPLC chromatograms of water-soluble residue obtained from the TGA pyrol-ysis of cellulose at 340 ◦C at heating rate of 60 ◦C/min clearly shows cellulose is

pyrolysis.

0 X. Bai et al. / Journal of Analytical

olatile products. For later tests, the MS system was operated inrend mode to monitor mass ions identified during the scan modehroughout the pyrolysis process. The dwell time for each ion waset to 0.1 ms at each cycle. TGA–MS tests were repeated three timesor each test case and the measurement results were found to beeproducible.

.4. Extraction of pyrolysis residue at different stages of heating

A methodology was developed to rapidly remove and quenchhe residue of TGA pyrolysis upon the sample reaching a prescribedemperature. Both cellulose and LG samples were prepared andyrolyzed as described in the previous section. LG samples wereeated from room temperature to final temperatures ranging from60 ◦C to 310 ◦C at 10 ◦C intervals with two additional tests at 200 ◦Cnd 285 ◦C for a total of eight tests. Upon reaching the desiredaximum pyrolysis temperatures, the automated pyrolysis pro-

ess was terminated by reset function and the sample cups wereuickly removed from the oven. The sample cup containing the liq-id/solid pyrolysis residue was then immediately dropped into alastic tube containing refrigerated (6 ◦C) deionized water (2.0 ml)o prevent further reactions. The pyrolysis residue separated into

dissolved water-soluble fraction and a water-insoluble fractionnd these two fractions were separated using a 0.45 �m syringelter prior to chemical analyses.

.5. HPLC analysis

The water-soluble fraction of the pyrolysis residue was directlynjected into a Dionex Ultimate 3000 series HPLC System to analyzets sugar composition. Separation of the peaks for quantification

as achieved using a resin based Bio-Rad Aminex HPX-87P columnt 40 ◦C with a water flow rate of 0.2–0.4 ml/min. A Varian 385-LCvaporative light scattering detector (ELSD) was used to detect theeaks. Standards of LG, cellobiosan and dextrin were calibrated forolecular identification.

.6. GFC analysis

The water-soluble fraction of the residues obtained in the LGyrolysis was also subjected to GFC analysis. GFC analysis wasarried out using the Dionex Ultimate 3000 series HPLC System.wo PL-aquagel-OH-20 5 �m columns were connected in seriest 25 ◦C using a deionized water mobile phase at a flow rate of

ml/min. A refractive index detector was used to detect the peak.elative molecular weight determination was based on a standardolyethylene glycol calibration curve. LG, cellobiosan and dextrintandards were also calibrated to obtain the relative moleculareight distribution of each compound.

. Results and discussion

.1. Liquid LG formation in cellulose pyrolysis

The pyrolysis residues of cellulose were yellowish to brown-sh in color and partly soluble in the water. Fig. 1 showshe HPLC spectrum of the water soluble fraction of residuebtained in cellulose pyrolysis. The solid residue contained bothnhydro-monosaccharide (LG) and anhydro-oligosaccharide andolysaccharide (cellobiosan and dextrin). Since the melting pointsf LG and the oligosaccharides were lower than the pyrolysis tem-erature, they likely existed as liquids during pyrolysis [5]. We

ypothesize that cellulose initially depolymerizes to either LG orligosaccharide during pyrolysis.

Evidence that the initial product of cellulose pyrolysis is LGather than oligosaccharides is found by measuring the peak-area

depolymerized to liquid LG.

ratios of cellobiosan (oligosaccharide) to LG and dextrin (polysac-charide) to cellobiosan in HPLC chromatogram. As shown in Fig. 2,the relative amount of cellobiosan compared to LG increased overthe increasing temperature, which would be expected if LG is con-verting to oligosaccharide instead of the reverse. Although this alsocould be explained by LG preferentially evaporating compared tonon-volatile oligosaccharides, a similar increase in dextrin com-pared to cellobiosan with time is also observed. Since both of thesecompounds are non-volatile, it suggests that larger molecules arebeing formed from smaller molecules instead of the reverse.

3.2. Secondary reaction of LG during fast pyrolysis

The results above in combination with observations of previ-ous studies [12–14] suggest that during pyrolysis, cellulose directlydepolymerizes to LG at a rate faster than it can evaporate, result-ing in a liquid melt of LG. Evidence that this also occurs underthe much higher heating rates characterizing fast pyrolysis wasfound by heating 324–927 �g of LG in a Pyroprobe instrument at(500 ◦C/s) to a temperature of 500 ◦C. The GC/FID chromatogram(see Fig. 3) obtained from this experiments shows peaks for LG aswell as for several smaller molecules typical of cellulose pyrolysisincluding 5-HMF, furan, 5-methylfurfural, acetic acid [7,22]. As sub-sequently demonstrated, these are also products in the dehydrationof oligosaccharides thought to form during high temperature poly-merization of LG. The dehydration of LG is visually evident in thequartz reactor tubes that held the LG samples during the Pyro-probe tests (Fig. 4). After pyrolysis the quartz tubes were stainedwith a brownish-to-blackish residue, which increased as the LGsample increased in size. Clearly, not all of the LG could evaporatefaster than it could polymerize and dehydrate to char despite thetubes being highly ventilated with an inert sweep gas during fast

Fig. 2. Ratios of cellobiosan to LG and dextrin to LG increased with increasingtemperature in the water-soluble residues produced during the TGA pyrolysis ofcellulose at heating rate of 60 ◦C/min.

X. Bai et al. / Journal of Analytical and Applied Pyrolysis 99 (2013) 58–65 61

Fp

3

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mrhrophrte

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ig. 3. Furans and acetic acid were found in GC/FID chromatogram obtained fromyrolysis of levoglucosan.

.3. Mass changes and heat flow during LG pyrolysis

The physiochemistry of LG at elevated temperatures wasxplored by pyrolyzing LG at TGA. As shown in the DSC curve ofig. 5a, two endothermic valleys were observed before any signif-cant reduction in sample mass occurred around 200 ◦C. The firstndothermic valley (labeled 1) at 118 ◦C was due to the solid phaseransition of LG into a plastic crystal state [5]. The second val-ey, near 186 ◦C (labeled 2), corresponded to LG melting. The thirdndothermic valley in the DSC curve (labeled 3) was due to thevaporation of LG from the liquid melt. Evaporation was followedy an exothermic reaction near 290 ◦C (labeled 4), an endothermiceaction (labeled 5), and an exothermic reaction (labeled 6) as theemperature increased. The TGA curve indicates that about 2 mg ofhe original 20 mg of LG remained in the sample cup at the end ofhe test.

The DTG curve of Fig. 5b shows two relatively broad peaks in theass loss rate. The first peak at 275 ◦C corresponds to the evapo-

ation of LG. This peak’s subsidence suggests that most of the LGad evaporated by 275 ◦C, subsequently reducing the mass lossate; examination of the TGA plot (Fig. 5a), however, shows thatnly half of the LG mass had been lost. Furthermore, a DTG secondeak (Fig. 5b) at 315 ◦C clearly indicates that something else must

ave occurred. We attribute the sudden reduction in the mass lossate between 275 ◦C and 290 ◦C to the polymerization of LG, whichhen would have almost no vapor pressure and could no longervaporate. The second peak in the DTG curve, therefore, can be

ig. 4. Increased LG to char was found in the quartz reactor tube of Pyroprobe duringest of larger sample.

Fig. 5. TGA curves obtained in levoglucosan pyrolysis clearly suggest thedevolatilization of LG follows two different mechnisms (heating rate = 5 ◦C/min,sample size = 20 mg, N2 flow rate = 100 ml/min).

attributed to the dehydration of the polymerized material to charand gas.

3.4. Polymerization of LG

To investigate the hypothesis that LG polymerizes underpyrolysis conditions, samples of pyrolyzed LG were immediatelyquenched in distilled water and the residues were subjected tochemical and physical analyses (see Experimental Section). Allpyrolysis residues were completely soluble in water except fora black solid residue (later determined to be char) produced at310 ◦C. The water-soluble residues exhibited a gradual darkeningin color with increasing pyrolysis temperature from colorless toyellow to light brown and finally dark brown (see Fig. 6). The col-orless solution suggests that the residue was LG while the othercolors are thought to indicate the presence of pyrodextrins, which

are products of the dry thermal processing of starch [33]. Thiscolor transition suggests that cellulose undergoes a similar seriesof depolymerization and repolymerization steps as starch duringpyrolysis.

Fig. 6. Melted LG gradually changes color and eventually dehydrates to char. (Forinterpretation of the references to color in this figure legend, the reader is referredto the web version of the article.)

62 X. Bai et al. / Journal of Analytical and Applied Pyrolysis 99 (2013) 58–65

F resenc( 8 = dex

adaoitoappc

ptt(cpbmuwpbiitotbd

yw

ig. 7. HPLC of LG pyrolysis residues produced at different temperatures show the pnot shown) were similar to that of 290 ◦C. (Peak 1 = LG; peak 6 = cellobiosan; peak

The water-soluble pyrolysis residues were subjected to HPLCnalysis for sugar identification (see Fig. 7). The residue pro-uced at 200 ◦C consisted of pure LG. The residues producedt higher temperatures contained LG and cellobiosan, a dimerf LG. Above 280 ◦C, dextrin, a polysaccharide of LG, was alsodentified. Some additional, unidentified peaks were observed inhe chromatograms of the residues that likely correspond to LGligosaccharides with various degrees of polymerization. It shouldlso be noted that the HPLC chromatograms of the residues of LGyrolysis shown here are very similar to the residue of celluloseyrolysis shown in Fig. 1. In both the cases, water-soluble residuesonsisted of LG and oligosaccharide.

MW distributions from the GFC analysis of the water-solubleyrolysis residues are shown in Fig. 8. The MW distribution ofhe residue at 200 ◦C has a single peak of LG (Fig. 8a), whereashree peaks can be identified in the residue produced at 260 ◦CFig. 8b). The first and second peaks were identified as LG andellobiosan, respectively, while the third higher-molecular weighteak is believed to be an LG trimer. (A linear relationship was notedetween the true MW and the corresponding relative MW of theonomer and dimer, and assumed trimer; this relationship was

sed to convert between relative MW and true MW.) The moleculareight distributions extend into higher MW regions as the tem-erature increased up to 300 ◦C (Fig. 8c–g) and are accompaniedy decreases in the LG peaks. This implies that LG was converted

nto polymers and both the molecular size and quantity of polymerncreased with increasing temperature. At 290 ◦C, the quantity ofhe polysaccharide (the right peak in Fig. 8f) increased, while that ofligosaccharide (i.e., the middle peak in Fig. 8f) decreased. The samerend was observed at 300 ◦C (Fig. 8g). At 310 ◦C, the quantities ofoth the oligosaccharide and the polysaccharide show significant

ecreases, which we consider evidence of their dehydration to char.

Changes in the weight average MW of the water-soluble pyrol-sis residues are shown in Fig. 8i. The DP of the residue increasedith increasing temperature (expect for 310 ◦C) due to the

e of oligosaccharides and polysaccharides. The chromatograms of 300 ◦C and 310 ◦Ctrin; peaks 2, 3, 4, 5, 7 = unknown.)

polymerization of LG. Residues produced at the temperaturesbelow 290 ◦C mainly consisted of oligosaccharide, whereas theresidues produced between 290 ◦C and 300 ◦C were dominatedby polysaccharide. This suggests that rapid formation of polysac-charide took place between 290 ◦C and 300 ◦C. The very smallamount of LG remaining in the residue at temperatures above290 ◦C suggests that the polysaccharide is the result of oligosac-charide combination reactions rather than direct polymerizationof LG to polysaccharide.

3.5. Dehydration of polymerized LG

Evidence of dehydration of the polymerized LG was found inthe mass spectra of vapors and gases released during the TGAexperiment. As illustrated in Fig. 9, no decomposition productswere released at temperatures below 290 ◦C, even within the tem-perature range of the first DTG (Fig. 5b). It should be noted thatalthough LG was likely evaporating in this temperature range, asection near the outlet of the TGA was cool enough to condense LGvapor, thus preventing its detection. In fact, upon disassemblingthe TGA after this test, condensed LG powder was found at the out-let of the TGA. Starting at 290 ◦C, which corresponds to the rapidmass loss associated with the second DTG peak (Fig. 5b), light gasesand vapors were detected by the MS including water (H2O), carbondioxide (CO2), 5-HMF, furfural, furan and acetic acid (see Fig. 9).These are well-known dehydration and decarboxylation productsfrom the pyrolysis of cellulose [7,22]. After dehydration, a porousblack residue was found in the sample cup. This residue, whichreleased no volatiles upon heating in an inert atmosphere to 900 ◦C,is thought to be char. These results suggest that char formed dur-ing cellulose pyrolysis is the result of the dehydration of LG or its

polymerization products.

To explore whether it was LG monomer or LG polymerizationproducts that dehydrated in the previous experiments, the moltenresidue remaining after pyrolysis at different temperatures was

X. Bai et al. / Journal of Analytical and Applied Pyrolysis 99 (2013) 58–65 63

Fig. 8. (a)–(h) MW distributions of LG pyrolysis residues produced at different tempeatures. (i) Weight average MW of pyrolysis residues with increasing temperature(numbers associated with the data points is DP for the sample).

64 X. Bai et al. / Journal of Analytical and A

Fig. 9. Above 290 ◦C, LMWVs were detected by MS during TGA pyrolysis of LG(shown in Fig. 1).

Ft

smLmropeAdot5LtAt

in the physiochemistry of the liquid phase.

ig. 10. LG and polymer contents in the pyrolysis residue changes with increasingemperature.

ubjected to HPLC analysis, which quantified the amount of LGonomer in the sample. Assuming that the sample consisted of

G and LG-derived polymers, the amount of polymer was deter-ined by difference. Fig. 10 shows that the mass of LG decreased

apidly with increasing temperature. Only about 1 mg (5% of theriginal weight) of LG remained in the residue at 290 ◦C, which isrior to the appearance of dehydration products in the gas streamxiting the TGA, indicating that dehydration of LG did not occur.bove 290 ◦C, further loss of LG was very slow indicating that anyehydration of LG would be minimal. On the other hand, the massf the polymer increased rapidly with increasing temperature upo 290 ◦C, at which point the amount of polymer was 11.2 mg, or6% of the original LG mass. (Due to the evaporation of some of theG, the total sample weight at 290 ◦C was only 12.2 mg, indicating

he remaining sample was composed of 8% LG and 92% polymer.)bove 290 ◦C, the mass of the polymer decreased, corresponding to

he appearance of dehydration products in exist gas stream. These

Fig. 11. Proposed pathway for pyrolysis of cel

pplied Pyrolysis 99 (2013) 58–65

results strongly suggest that char is formed from the dehydrationof polymerized LG rather than monomeric LG.

The TGA/MS measurements showed that LMWV were releasedduring LG pyrolysis at 290 ◦C, indicating that something wasdecomposing, however, char was not detected in the pyrolysisresidue until 310 ◦C. We propose the following explanation: pyrol-ysis residue below 290 ◦C is mainly composed of oligosaccharide,which dehydrates to LMWV such as water, CO2, HMF, furfural, furanand acetic acid but no char. Between 290 ◦C and 300 ◦C, pyrolysisresidue is mainly composed of polysaccharide (see Figs. 6 and 7).Above 310 ◦C, the disappearance of polysaccharide is accompaniedby the rapid dehydration to char and LMWV. Therefore, duringLG pyrolysis dehydration of oligosaccharide is thought to precededehydration of polysaccharide.

To further understand whether LG polymerization is a reversibleprocess, 10 mg of reagent grade dextrin, a polysaccharide found inthe reaction residue of LG (see Fig. 7), was pyrolyzed at 310 ◦C inthe TGA–MS system. While dextrin was observed to dehydrate toLMWV products and char, small quantities of LG, cellobiosan andother oligosaccharides were also identified in the water-solublefraction of the pyrolysis residue, indicating partial depolymeriza-tion of the dextrin. Depolymerization can occur in a closed systemwhen the temperature exceeds a ceiling temperature for a closedsystem at equilibrium [34]. Although the ceiling temperature fordextrin is unknown for an open system such as the TGA used inthe present experiments, depolymerization may occur even belowthe polymer’s predicted ceiling temperature. The results not onlyconfirm that dehydration of polysaccharide forms both LMWV andchar, but also suggests that LG polymerization is a partly reversibleprocess.

The thermal characteristics of LG polymerization and subse-quent dehydration of the polymer can be understood by the DSCcurve shown in Fig. 3. Polymerization of LG is an exothermic reac-tion (peak 4 in Fig. 3). Devolatilization of LMWV is an endothermicprocess (peak 5 in Fig. 3) and carbonization of polymer is anexothermic process (peak 6 in Fig. 3).

3.6. The role of the liquid phase in fast pyrolysis

Although the appearance of a liquid phase may seem improba-ble at the high temperatures of fast pyrolysis, previous experimentshave conclusively demonstrated its existence at reactor tem-peratures as high as 700 ◦C. Although few carbohydrate-derivedcompounds can survive these temperatures, the process of heatinga particle of biomass is a transient phenomena: the particle may noteven reach “pyrolysis temperature” before it depolymerizes, melts,and evaporates. The fact that levoglucosan is the primary product ofthe thermal depolymerization of cellulose and has a melting pointbelow pyrolysis temperatures suggests that it plays defining role

Most models of cellulose pyrolysis do not account for thephysiochemistry of this liquid phase. We propose a pyrolysis path-way for cellulose involving an LG intermidiate (see Fig. 11). The

lulose based on the results of this study.

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iquid LG produced from the depolymerization of cellulose under-oes two competing reactions: evaporation and polymerizationo oligosaccharides. The oligosaccharides then undergo furtherolymerization to form polysaccharides and dehydration to formMWV but not char. The polysaccharides dehydrate to the sameMWV as the oligosaccharides plus char, with the possibility of alsoartially undergoing depolymerization back to oligosaccharides.

While previous studies [31] indicate the formation of non-ondensable gas is due to LG decomposition in vapor phase, theresent study suggests it can be produced in the processes of dehy-ration and decarboxylation of oligosaccharide and polysaccharide.hile oligosaccharide was suggested as a primary product in cel-

ulose pyrolysis by some researchers [14–17], our finding suggestshat liquid oligosaccharide can be produced when liquid LG poly-

erizes during pyrolysis.

. Conclusions

The main findings derieved from the present experimental studyn the fate of LG after it is formed during cellulose pyrolysis can beummarized as follows:

. LG formed in cellulose decomposition is initially a liquid.

. In an open pyrolysis system, evaporation and polymerizationof LG are two competing simultaneous processes. EvaporatedLG vapor can escape from the high temperature pyrolysis zone,while the non-volatile polymer of LG is not able to escape fromthe pyrolysis zone and eventually dehydrates to LMWV and char.

. LG polymerization yields both oligosaccharides and polysaccha-rides, which have distinctive fates:

) Oligosaccharides can further polymerize to polysaccharides ordehydrate to form LMWV including water, carbon dioxide, HMF,furfural, furan,and acetic acid.

) Polysaccharides dehydrate to the same LMWV as from oligosac-charide dehydration and then form char.

) Depolymerization of polysaccharide can also occur during dehy-dration, although at a relatively slow rate.

. A modified model for cellulose pyrolysis is proposed involvingliquid LG as an intermediate to dehydration products.

cknowledgement

Authors greatly appreciate the financial support provided byational Advanced Biofuels Consortium under subcontract No.CE-0-40625-01.

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