Structural Characterization and Pyrolysis Behavior of Cellulose and

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Structural characterization and pyrolysis behavior of cellulose and hemicellulose isolated from softwood Pinus armandii Franch Shurong Wang, Haizhou Lin, Li Zhang, Gongxin Dai, Yuan Zhao, Xiaoliu Wang, and Bin Ru Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00650 • Publication Date (Web): 21 Jun 2016 Downloaded from http://pubs.acs.org on June 28, 2016

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Title page

Structural characterization and pyrolysis behavior of cellulose and hemicellulose isolated from softwood Pinus armandii Franch

Shurong Wang*, Haizhou Lin, Li Zhang, Gongxin Dai, Yuan Zhao, Xiaoliu Wang, Bin Ru

State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China

﹡Corresponding author (Shurong Wang) Postal address: State Key Laboratory of Clean Energy Utilization, Zhejiang University Zheda Road 38, Hangzhou 310027, China Tel: +86 571 87952801; Fax: +86 571 87951616 Email address: [email protected]

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Structural characterization and pyrolysis behavior of cellulose and hemicellulose isolated from softwood Pinus armandii Franch Shurong Wang*, Haizhou Lin, Li Zhang, Gongxin Dai, Yuan Zhao, Xiaoliu Wang, Bin Ru

State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou, 310027, China

Abstract: Cellulose and hemicellulose were isolated directly from softwood Pinus armandii Franch by alkali method for pyrolysis mechanism study. Structural characterization based on FTIR,

1

H-NMR and

13

C-NMR showed that the

hemicellulose was mainly composed of galactoglucomannan and arabinoxylan with abundant branches. While the cellulose was mainly composed of β-1,4-glucan with a few residual hemicellulose fragments. Pyrolysis of extracted cellulose showed high char yield due to the lower crystallinity. Hemicellulose pyrolysis showed a shoulder peak at low temperature and high char yield because of its abundant branches. A double-Gaussian distributed activation energy model (DG-DAEM) was introduced for pyrolysis kinetics analysis. The devolatilization processes for both cellulose and hemicellulose was dominated by parallel decomposition reaction pathway. The distributions of their pyrolysis products were comparatively analyzed by Py-GC/MS. It is suggested that the utilization of isolated sample as model compound could get a better understanding of biomass pyrolysis mechanism. 2


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Key word: Cellulose, Hemicellulose, Pyrolysis, Structural analysis, Kinetic analysis

1. Introduction

Pyrolysis is a promising thermochemical route for converting biomass into liquid bio-oil

1, 2

. However, the bio-oil from biomass pyrolysis has complicated

compositions, such as acids, ketones, aldehydes and phenols 3. These abundant oxygenic chemicals contribute to the poor quality of crude bio-oil, like low calorific value, viscid, high acidity and easy aging, which impedes the large scale industrial application of biomass pyrolysis technology

4-6

. The extremely complex structure of

biomass containing various oxygenic functional groups results in the difficulties to optimize pyrolysis conversion to improve the bio-oil quality. Therefore, it was significant to study the structural properties of biomass and the relevant pyrolysis behavior.

Biomass mainly consists of 40-50wt% cellulose, 20-40wt% hemicellulose and 15-30wt% lignin 7. Particularly, cellulose and hemicellulose, the primary components of biomass, are polysaccharide carbohydrates. They impressively affect the kinetic characteristic of biomass pyrolysis and the oxygenated products distribution (acids, aldehydes and ketones etc.) in bio-oil. Cellulose is a regular and ordered polymer of β-1,4-glucan, showing relatively simple structure 8. It is common to use commercial microcrystalline cellulose (MCC) as a model compound to study the pyrolysis behavior of cellulose. For example, Wang et al. 8 analyzed the product distributions of MCC pyrolysis by using Py-GC/MS, and they found that the main products could be 3


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classified into pyrans, furans and linear small molecule compounds. They also proposed the formation pathways of the major products by performing the density functional theory (DFT) simulation. However, the structural characteristics of cellulose, such as degree of polymerization and crystal form, all depend on the biomass sources. Based on above, it is necessary to investigate the cellulose isolated directly from some special biomass.

In contrast to cellulose, hemicellulose is much more complex. It is composed of short chain heteropolysaccharides that were polymerized by various monosaccharide unites, showing an amorphous and branched structure, which makes the isolation of hemicellulose very difficult 9. Therefore, many researchers used commercial xylan as model compound in their studies on hemicellulose pyrolysis

10, 11

. However, the

structure and composition of hemicellulose varied in different biomass, and xylan is just one of the hemicellulose compositions. So, more attentions should be paid to the investigation of extracted hemicellulose pyrolysis. Hemicellulose isolation methods mainly include alkali, organic solvent, mechanization and hydrotherm methods

12

.

Alkali method usually uses alkali (NaOH, KOH) solution to extract and separate hemicellulose from delignified biomass

12

. This method has been proved to be very

effective to isolate hemicellulose as it can obtain a high yield of hemicellulose and preserve the original structure of hemicellulose with high degree of polymerization. Currently, alkali method is widely accepted and applied

12, 13

. Lv et al

13

isolated

hemicellulose from corn stalk by alkali method and analyzed its pyrolysis characteristics by TG-MS and Py-GC/MS. The results showed that the hemicellulose 4


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mainly decomposed at the temperature range of 206-346 °C and released a large amount of acetic acid, furfural and ketones at the temperature around 300 °C. Patwardhan et al

14

pyrolyzed the hemicellulose isolated from switchgrass by alkali

method, and found that the primary products were mainly composed of CO, CO2, formic acid, acetol, furfural etc. They suggested that hemicellulose pyrolysis encountered competing reaction pathways.

At present, studies on pyrolysis of isolated softwood hemicellulose are limited. Hosoya et al

15

isolated glucomannan from softwood Japanese cedar and analyzed its

pyrolysis products at 800 °C. Moreover, few studies compared the difference of softwood hemicellulose and cellulose pyrolysis, in spite of their difference in the composition of monosaccharide units. Therefore, in order to get a deeper insight into the pyrolysis of softwood cellulose and hemciellulose, we isolated cellulose and hemicellulose from softwood Pinus armandii Franch by alkali method, and then carefully characterized their chemical structures, followed by in-depth investigation their pyrolysis behaviors. Meanwhile, the difference of pyrolysis behaviors between isolated samples and common samples (MCC and xylan) were compared.

2. Experimental

2.1 Isolation of cellulose and hemicellulose

Softwood Pinus armandii Franch was procured from a local timber mill in Zhejiang province, China. The ‘as-received’ Pinus armandii Franch was ground and sieved into 180–250 µm. The component composition of Pinus armandii Franch was determined 5


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by Van Soest method 16, 17. Cellulose and hemicellulose were isolated according to the method modified from previous studies

18

. The major modification was that

hemicellulose was extracted by KOH solution instead of KOH/H3BO3 solution 18. The dried raw material (10 g) was extracted with toluene/ethanol (2:1, v/v) in a Soxhlet for 8 h to remove the extractives. Subsequently, the dewaxed sample was delignified in sodium chlorite (7.0 wt%, 300 mL) at 75 °C for 3 h with stirring, and this process was repeated three times. After that the resulting holocellulose was extracted with KOH (10 wt%, 100 mL) at 50 °C for 5 h and the crude cellulose was filtered out and collected. The pH of filtrate was adjusted to 5.5-6.0 by acetic acid, and then the filtrate was diluted with excessive ethanol (1000 mL) and the precipitation was collected as crude hemicellulose. Finally, the crude cellulose and hemicellulose were purified by washing with ethanol and lyophilization (-0.1 MPa, -50 °C, Freezone, LABCONCO, USA) prior to further analyses. MCC was purchased from Aladdin Industrial Corporation.

2.2 Structural characterization

The elemental composition of the cellulose and hemicellulose samples was determined by a Vario MICRO Elemental Analyzer (Elementar Analysensysteme GmbH, Germany). The apparent molecular mass distribution of hemicellulose sample was measured by a PL GPC 50Plus (Varian Polymer Laboratories, UK) equipped with a PL aquagel-OH column and a refractive index detector. Hemicellulose sample was dissolved in 0.1 mol/L sodium nitrate solution at a concentration of 3 mg/mL. The

6


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eluent was 0.1 mol/L sodium nitrate solution and the flow rate was kept 0.8 mL/min. The column oven temperature was maintained at 30 °C. The obtained relative molecular weight was calibrated with polyethylene oxide standards.

The neutral sugar composition of the cellulose and hemicellulose was determined as follows. The sample was hydrolyzed in 72% H2SO4 at 20 °C for 3 h and then in 1 mol/L H2SO4 at 100 °C for 2 h. The obtained sugar solution was adjusted to neutral by NaOH solution. The neutral sugar composition of the cellulose and hemicellulose was analyzed on a Dionex HPLC system (Ultimate 3000) equipped with a Shodex SP0810 sugar column (Showa Denko Kabushiki-gaisha, Japan) and an RI 2000 refractive index detector. Pure water was used as the mobile phase. The flow rate and column temperature were kept at 0.6 mL/min and 85 °C , respectively. The concentrations of neutral sugars were determined by comparison against standard calibration curves. The total uronic acid content was determined on a Shimadzu UV-3150 spectrophotometer according to the method proposed by Blumenkrantz et al

19

. The

wavelength was 520 nm and the glucuronic acid was used as standard for quantification.

X-ray diffraction analysis of the cellulose sample was performed on a PANalytical X’Pert PRO Diffractometer with the scanning angle (2θ) of 5-50°. The crystalline index (CrI) of sample was determined from X-ray diffraction patterns via Eq. (1) 20. CrI =

( )

× 100% Eq. (1)

where I002 is the peak intensity corresponding to 002 lattice plane (at 2θ=22.5°), and 7


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Iam is the peak intensity corresponding to the amorphous phase (at 2θ=18.5°).

The functional groups in the cellulose and hemicellulose structure were identified by a Nicolet 5700 FTIR spectrometer (Thermo Fisher Scientific Corporation, USA). The spectra were recorded in the range of 400-4000 cm-1 with a resolution of 4 cm-1; each spectrum was composed of 36 scans. The structure of hemicellulose was further elucidated by 1H-NMR and

13

C-NMR on an Agilent 600 MHz DD2 spectrometer

(Agilent Technologies Corporation, USA). The hemicellulose sample was dissolved in D2O with tetramethylsilane (TMS) as internal reference. 1H-NMR spectrum was recorded at 25 °C after 100 scans. The pulse angle was 45° and the relaxation time was 1 s.

13

C-NMR spectrum was recorded at 25 °C with a pulse angle of 45° and a

relaxation time of 2 s; and the spectrum was composed of 10,000 scans.

2.3 Pyrolysis behavior analysis

The thermal mass loss process of the sample was analyzed by a Netzsch STA 409 thermobalance. The sample was heated from 50 °C to 800 °C at a rate of 40 °C/min with nitrogen (99.99% pure) as inert atmosphere. The product distribution of the sample pyrolysis was analyzed by a CDS5200 micro pyrolyzer coupled with a Thermo Scientific Trace DSQⅡ gas chromatograph–mass spectrometer (Py-GC/MS). The sample was heated to 600 °C for 10 s at a heating rate of 1000 °C/s with helium (99.995% pure) as the inert carrier gas. A DB-WAX chromatography column (30 m × 0.25 mm × 0.25 µm) was used for product separation. The GC oven temperature was programmed from 40 °C (held for 1 min) to 240 °C (held for 20 min) at a rate of 8


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5 °C/min. The MS detector was operated in electron ionization (EI) mode (70 eV) with a scan range of m/z 35–450. All the detected chemicals were identified according to the NIST (National Institute of Standards) MS library. The semi-quantitative results of detected chemicals were done by using the individual peak area of the corresponding chemicals divided by the sample weight.

The structural characterization and pyrolysis analytical methods above were repeated three times and the standard deviations were provided.

2.4 Pyrolysis kinetic analysis

A double-Gaussian distributed activation energy model (DG-DAEM) was used to analyze the kinetics of the cellulose and hemicellulose pyrolysis. DG-DAEM model is developed in recent years and has been proved to offer high accuracy in simulating devolatilization over the whole temperature range

21, 22

. The DAEM model is

described by Eq. (2), in which the double-Gaussian distribution function is presented as Eq. (3). The two Gaussian functions hypothetically correspond to two reaction types that lead to devolatilization during pyrolysis: the first is the decomposition and release of volatiles (reaction I); the second is the polymerization to form char and gas (reaction II). "

1 − () = − / ! #($)$ f(E) = w σ

(

)* √,π

exp

(0 0* ) ,σ)*

! + (1 − w) σ

(

) √,π

exp

Eq. (2) (0 0 ) ,σ)

!

Eq. (3)

In Eqs. (2) and (3), T is the temperature, α(T) is the conversion rate (mass loss rate); 9


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E is the apparent activation energy; k0 and R are the pre-exponential factor and universal gas constant, respectively, and β is the heating rate used for the TG experiments. E01 and E02 are the average activation energies for the first and second Gaussian distributions, and σE1 and σE2 are the corresponding standard deviations. The weight factor, w, scales the devolatilization contributions for the two reactions. A pattern search algorithm was adopted to perform the iterative operation and obtain kinetic parameters 21, 22.

3. Results and Discussion

3.1 Structural characterization

According to the Van Soest component analysis, the contents of cellulose, hemicellulose, lignin and extractives in Pinus armandii Franch were 48.4%, 17.8%, 24.1% and 9.5%, respectively. The isolated cellulose was white flocculent particle which was insoluble in water and ethanol, and the extraction yield was about 80% (eliminating residual hemicellulose and water). The isolated hemicellulose was off-white powder particle that was soluble in water but not in organic solvent like ethanol and dimethylsulfoxide (DMSO). The extraction yield of hemicellulose was about 90%, indicating the high isolation efficiency of alkali method. The elemental composition of the cellulose and hemicellulose are listed in Table 1. The structural formula of the cellulose was (C6H10.08O5.19)n, and the O/C ratio (0.86) was slightly higher than the (0.83). This might be due to the dispersive distribution of residual hemicellulose in cellulose as the O/C ratio of short hemicellulose fragment was higher 10


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than 0.83. The structural formula of the hemicellulose was (C6H10.05O6.53)n (hexosan basis) or (C5H8.37O5.44) (pentosan basis). It was also different from the theoretical formulae of hexosan ((C6H10O5)m) or pentosan ((C5H8O4)n), both of which were the main compositions of hemicellulose. In particular, as shown in Table 2, the O/C ratio (1.09) was higher than the theoretical value (0.83 or 0.80), resulting from the existence of abundant oxygenated side branches, such as uronic acid, in the hemicellulose structure.

The weight apparent average molecular mass

(Mw) and number apparent average

molecular mass (Mn) of the hemicellulose sample were 44,622Da and 23,747Da, respectively, which were obviously higher than that extracted by organic solvent. Wang et al. found the Mw and Mn of hemicellulose extracted from softwood by DMSO were around 30,000Da and 18,000Da, respectively

21

. Meanwhile, the This

result was in agreement with that larger molecular mass of hemicellulose was easier to be dissolved in alkali solution 23, thus alkali method could retain the high degree of polymerization of natural structure well. Polydispersity (PD), the ratio of Mw and Mn, is an index of the width of the molecular mass distribution. PD of the extracted hemicellulose in this study was 1.88, which was close to the results obtained by Wang et al

21

, indicating the narrow molecular distribution of hemicellulose as it was

composed of short chain heteropolysaccharides. According to the results of Huang et al 24, the Mw and Mn of cellulose isolated from softwood pine by alkaline method was about 1,800,000Da and 190,000Da, respectively, which were much greater than that of MCC (about 150,000Da and 39,000Da)25. Meanwhile, the PD of the isolated 11


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cellulose (9.33) was much higher than that of MCC (3.89) due to the degradation of cellulose matrix during the pretreatment to obtain microcrystalline cellulose 24, 25.

Fig. 1 shows the X-ray diffraction patterns of the extracted cellulose and MCC. Both celluloses displayed typical diffraction peaks at 2θ = 14.8° (101 plane), 16.5° _

(101 plane) and 22.5° (002 plane), indicating the crystal structure of cellulose I

26

.

The CrI for the extracted cellulose and MCC were 64.2% and 78.5%, respectively, suggesting the higher proportion of amorphous phase in the structure of the extracted cellulose.

The neutral sugar composition of cellulose and hemicellulose are presented in Table 2. It could be found that the cellulose was mainly composed of glucose, whose proportion was as high as about 90%. In addition, there were a small content of mannose, galactose, xylose, arabinose, etc., indicating the presence of few hemicellulose residual in the extracted cellulose. Hemicellulose contained over 50% hexose, and the content of mannose was about 30%. Moreover, the ratio of mannose, glucose and galactose was approximately 3:1:1, suggesting that galactoglucomannan was the primary polysaccharide in softwood hemicellulose

12

. It also belonged to

galactose-rich type, in contrast to galactose-poor type with a mannose, glucose and galactose ratio of 3:1:0.1

12

. Galactoglucomannan was mainly composed of a

backbone of β-1,4-linked mannose and glucose residues with α-1,6-galactosyl side chains

12, 21

. The hemicellulose also contained 19.31% xylose and 9.46% arabinose,

indicating arabinoxylan was another polysaccharide component in the hemicellulose,

12


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which had a backbone of β-1,4-linked xylose residues with α-1,3-arabinosyl side chains 18, 21. The content of uronic acid in hemicellulose was 16.53%. Usually, uronic acid was linked at the polysaccharide backbone as side branch. This indicates that the hemicellulose may present high branched structure and acidity 27.

The FTIR spectra of the cellulose and hemicellulose samples are presented in Fig. 2. The peaks centered at 3416 cm-1 and 1320 cm-1 were due to the axial and angular deformation of O–H in hydroxyl groups, respectively, indicating the widely distribution of hydroxyl group in the structures of cellulose and hemicellulose 21. The peak at 2924 cm-1 was ascribed to the axial deformation of C–H in methyl and methylene 22. The peak at 1610 cm-1 might be assigned to water of hydration 28. The peaks at 1457 and 1220 cm-1 corresponded to the bending and stretching vibrations of –CH2– in the sugar rings. It should be noted that the peaks at 1395 cm-1 and 1268 cm-1 in hemicellulose spectrum were the characteristic absorptions of methyl groups and carboxylic groups, respectively. This showed the existence of uronic acid in hemicellulose 29.

The spectra of cellulose and hemicellulose showed intensive and complex absorptions at 1200-1000 cm-1. This region corresponded to the sugar ring vibrations overlapped with stretching vibrations of side groups C–OH and glycosidic bonds C–O–C

30

. It was notable that the spectra of hemicellulose featured broad and sharp

peak around 1047 cm-1, which was due to the overlap of stretching vibrations of glucosidic bonds in glucomannan and xylan over a wide range of 1064–1026 cm-1

13


28,

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30, 31

. This also indicated the existence of backbone of β-1,4-glucomannan and

β-1,4-xylan in hemicellulose structure. The cellulose spectrum showed a broad and smooth peak around 1047 cm-1, which might result from the overlap of peaks at 1041cm-1, 1026cm-1 and 1106cm-1, among which peaks at 1041cm-1 and 1026cm-1 were assigned to β-1,4-glycosidic bonds, and the peak at 1106 cm-1 was assigned to asymmetric ring vibration in the crystal structure of cellulose

30, 32, 33

. Moreover, the

peak at 1162 cm-1 was ascribed to the asymmetric bridge vibration of C-O-C in β-glucosidic bond of cellulose

32, 33

. The peak at 893 cm-1 was assigned to C1 group

frequency or ring frequency in the β-glycosidic linkages 34. Particularly, There was a weak peak at 1504 cm-1 in the hemicellulose spectrum, which corresponded to the aromatic skeletal vibration, suggesting the residual of lignin fragment

35

. As for

cellulose, no obvious signal was observed in this region. This was due to that hemicellulose and lignin were connected by the covalent bond, making the complete removal of lignin difficult 36.

The composition and structure of the hemicellulose was much more complex than cellulose, so 1H-NMR and 13C-NMR analyses were further performed to elucidate its structural properties. The obtained

1

H-NMR and

13

C-NMR spectra of the

hemicellulose sample are presented in Fig. 3. The spectra showed the signals of glucose clearly. The 1H-NMR signals at 4.38ppm, 3.17ppm, 3.47ppm, 3.56ppm and 3.71ppm could be assigned to protons on C1–C5 of glucose respectively. And the 13

C-NMR signals at 103.90ppm, 73.84ppm, 78.97ppm, 79.83ppm, 75.29ppm and

63.89ppm could be assigned to C1–C6 of glucose respectively. Similarly, signals of 14


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mannose were observed in the spectra. The 1H-NMR signals at 4.34ppm, 3.98ppm, 3.67ppm, 3.68ppm and 3.45ppm could be assigned to protons on C1–C5 of mannose respectively. The

13

C-NMR signals at 100.11ppm, 71.77ppm, 72.58ppm, 76.24ppm,

75.88ppm and 62.45ppm could be assigned to C1–C6 of mannose respectively. The H-1 proton signals of glucose and mannose were in the region of 4.30-4.90 ppm, corresponding to the β-anomeric protons

37, 38

. This indicated that the glucose and

mannose units are linked by β-1,4-glycosidic bonds 37, 38. The weak 1H-NMR signal at 5.02ppm, corresponding to the H-1 proton of D-galactose, was in the α-anomeric proton region of 4.90-5.60ppm. This suggested the D-galactosyl side chains was linked to glucomanna by α-1,6-linkages 37, 39. The signals assigned to protons of xylose unit were obvious in 1H-NMR spectrum, which appeared at 4.48ppm (H-1), 3.29ppm (H-2), 3.50ppm (H-3) and 3.78ppm (H-4). Moreover, the

13

C-NMR signals assigned to C1–C5 were also strong, appearing at

102.71ppm, 74.78ppm, 75.29ppm, 75.88ppm and 65.45ppm, respectively. In particular, the H-1 of xylose was β-anomeric proton, indicating the β-1, 4-glycosidic bonds in xylan structure

23, 38

. The 1H-NMR signal at 5.26ppm (H-1) and 3.49ppm

(–OCH3) showed the existence of 4-O-methyl-D-glucuronic acid, which connected with the C2 of xylose unit by α-1,2-linkage 40. 4-O-methyl-D-glucuronic acid was also detected in the 13C-NMR spectrum, in which the signals at 176.40ppm and 58.33ppm corresponded to the characteristic chemical shifts of carboxyl and methoxyl group respectively. The notable signal at 5.28ppm in 1H-NMR spectrum was ascribed to the α-anomeric proton (H-1) of arabinose, indicating that the arabinose was linked to the 15


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xylan backbone as side chain by α-1,3-linkage 38. There was no obvious peak at the region of 2.1-2.2 ppm, implying that the acetyl groups in branched chain structure were damaged in alkali-extraction

39

. In addition, the weak signals in

13

C-NMR

spectrum at 110-160 ppm corresponded to aromatic carbons, indicating the residue lignin fragment in hemicellulose structure 41.

3.2 Pyrolysis kinetics

The thermal decomposition kinetics of the cellulose and hemicellulose samples were studied by thermogravimetric analysis. The resulting TG/DTG curves are presented in Fig. 4. Both the thermal mass loss process of cellulose and hemicellulose could be divided into three stages. The first stage corresponded to the removal of free water and crystal water at low temperature. The second one was the main decomposition stage that involved the release of a large amount of volatiles, showing big peak in the DTG curve. The last one was ascribed to the slow carbonization at elevating temperature. The temperature range of the main decomposition stage for hemicellulose was distinctly lower than that for cellulose. In addition, the temperature corresponding to maximum mass loss rate for hemicellulose was 298 °C, which was much lower than that for celllulose (382 °C), indicating the poor thermal stability of hemicellulose. This was related to the amorphous and branched structure of hemicellulose, in contrast to the ordered crystal structure of cellulose. In particular, the temperature corresponding to the maximum mass loss rate for the extracted cellulose was obviously higher than that for MCC (358 °C), which mainly resulted

16


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from the higher molecular mass of extracted cellulose 42. It should be pointed out that the a shoulder peak appeared in the DTG curves for the hemicellulose 250 °C and cellulose appeared at 312 °C, respectively. This was very different from the DTG curves of xylan and MCC that only one major peak appeared 10. The shoulder peak of hemicellulose might be due to the easy removal of uronic acid and arabinose branches from the main chain at low temperature 10. Moreover, the degradation of the smaller molecular mass component in the hemicellulose sample could also contribute to the shoulder peak, as Bian et al.

27

found that small molecular mass of hemicellulose

showed poorer thermal stability than the large molecular mass of hemicellulose. The shoulder peak of cellulose was due to the decomposition of residual hemicellulose.

The final residue yield of hemicellulose at 800°C was 40.1%, which was greatly higher than that of cellulose (26.8%). It was in line with that the pentosan was easier to form char compared to glucan, as pentose cation formed in hemicellulose pyrolysis could not be stabilized by the formation of uronicanhydride via intramolecular dehydroxy reaction, leading to the occurrence of polymerization to form char

43

.

Meanwhile, some metal cation such as Na+ and K+ might be combined in the hemicellulose sample, which would catalyze the formation of char in the pyrolysis process

14, 23, 41

. The char yield of hemicellulose in this study was higher than that of

softwood hemicellulose isolated by DMSO, which were about 33% 21. This might be because of the large molecular mass and the better structural diversity of the hemicelluloses isolated by alkali method, which could produce more kinds of macromolecular radicals through the cleavage of glucosidic bonds and lead to 17


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polymerizing to form char

10, 21

. The char yield of the hemicellulose was also higher

than that of xylan (24%). Similarly, Hosoya et al 15 found that the pyrolysis char yield of glucomannan isolated from softwood Japanese cedar (30.4%) was higher than that of xylan (20.1%) at 800 °C, suggesting that glucomannan was easier to form char compared to xylan. Moreover, the char yield of cellulose in this study was also much higher than that of MCC. Wang et al 44 obtained a char yield of about 5% from MCC pyrolysis. Gronli et al 42 organized a round-robin study of MCC Pyrolysis and the char yields were in range of 3.3-11.3% under comparable condition. It was mainly due to the lower crystallinity of the isolated cellulose as mentioned above. This could be confirmed by Zhang et al

26

and Kim et al

45

, who indicated that the cellulose char

yield increased with the decrease of crystallinity.

To further illuminate the thermal decomposition properties of the cellulose and hemicellulose, kinetic analysis was performed using a double-Gaussian distributed activation energy model (DG-DAEM). Fig. 5 shows the experimental and calculated relative mass loss rate curves of the cellulose and hemicellulose. It can be seen clearly that the model simulation results fitted experimental data well. The distributions of apparent activation energies and the corresponding kinetic parameters for the cellulose and hemicellulose pyrolysis are displayed in Fig. 6. For both cellulose and hemicellulose, reaction I showed sharp peak with low E01 and narrow σE1, while reaction II presented as shoulder peak with high E02 and wide σE2. The value of w for cellulose and hemicellulose were 0.78 and 0.75, respectively. Both were higher than 0.5, indicating that the thermal mass loss mainly resulted from the reaction I. The 18


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corresponding average activation energies were 211.8kJ/mol and 174.3kJ/mol, respectively, confirming that the hemicellulose with amorphous and branched structure was easier to decompose at low temperature. The average activation energy of the whole pyrolysis process (including the reaction I and II) for the cellulose and hemicellulose were 215.8 kJ/mol and 181.0 kJ/mol, respectively. They were higher than the results obtained by Cai et al 46, in which the corresponding average activation energies for the pyrolysis of MCC and xylan were 210kJ/mol and 178.3kJ/mol, respectively. This implied the different pyrolysis behavior of isolated samples from commercial samples due to the difference in their structural properties. Wang et al. 21 used a DG-DAEM model to analyze the pyrolysis kinetics of two softwood hemicelluloses isolated by DMSO, and found that their average activation energies were about 160kJ/mol, which were lower than that in this paper. This might be due to the removal of acetyl groups in hemicellulose during alkali extraction, leading to the improvement of its thermal stability.

3.3 Pyrolysis product distributions

Table 3 lists the typical products of the cellulose and hemicellulose pyrolysis determined by Py-GC/MS. They could be divided into five categories according to their structural characterization: acids, furans, cyclic ketones, linear ketones and aldehydes, sugars. The abundance of acetic acid from hemicellulose (11.16×108/mg) was much higher than that from cellulose (4.34×108/mg), which might result from the high content of uronic acid in hemicellulose, as the decomposition of uronic acid

19


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could lead to the production of acetic acid

Page 20 of 35

47

. The furans were mainly composed of

furfural, 2-methylfuran, hydroxymethylfurfural (HMF) and furfuryl alcohol etc. The kinds of furans from the cellulose pyrolysis were more than that from hemicellulose, suggesting the complex reaction pathways of glucose decomposition due to the existence of the hydroxymethyl group in the sugar ring

48, 49

. Furfural was a typical

product of pentose pyrolysis, and it was mainly generated from the multi-step dehydration of loop-opened xylose units after the depolymerization of the xylan backbone

50

. HMF was the representative dehydration product of hexose, and the

secondary reaction of HMF through dehydroxymethylation could also form furfural 49. However, this reaction was not favorable because of the high energy barrier, resulting in the low yield of furfural from cellulose pyrolysis

51

. Therefore, the abundance of

furfural from hemicellulose (5.21×108/mg) was obviously higher than that from cellulose (2.26×108/mg).

Both the pyrolysis products of cellulose and hemicellulose were rich in cyclic ketones, including cyclopentenone, furanone, cyclopentanone, pyranone, etc. Furan ketones and cyclopentene ketones were from the cyclization of cracked pentose ring, leading to the production of cylic ketones from hemicellulose higher than that from cellulose. In addition, the ring-opening of the pentose rings generated a considerable amount of linear ketones and aldehydes, such as hydroxyacetone, 2-butanone, methoxyacetaldehyde and so on. Sugars were composed of some anhydrosugars, and levoglucosan was the most representative one. Levoglucosan was formed from cellulose via the rupture of glycosidic bond and the subsequent dehydration 20


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condensation of hydroxyl on C6 and oxygen on C1

52

. The pentosyl cations formed

from the rupture of glycosidic bond in pentosan could not be stabilized to further generate anhydride due to the absence of C6-hydroxyl, leading to the low yield of sugar anhydride for hemicellulose 14, 43.

The categories of volatile compounds from the extracted cellulose and hemicellulose pyrolysis were consistent with the results from MCC and xylan pyrolysis, but there were differences between their contents due to the various chemical structures. For example, the formation of anhydrosugars (levoglucosan) from cellulose was influenced by the crystallinity and cellulose with higher crystallinity formed more anhydrosugars

53

. This could lead to lower yield of

anhydrosugars from the extracted cellulose compared to MCC. Therefore, from the results above, the extracted samples and the common model samples showed obviously different pyrolysis behaviors, including the mass loss process, reaction activation energies, char yield and volatiles composition. Hence, it was recommended to isolate their components for pyrolysis, which could be helpful to get a better insight into the pyrolysis mechanism of biomass. In particular, it was beneficial for modeling the pyrolysis behaviors of biomass and predicting the conversion of biomass precisely on the basis of the pyrolysis of isolated components in future studies. Nevertheless, the effect of isolation on the structure change of the isolated sample needed to be taken into account. For example, the removal of acetyl group in hemicellulose by alkaline extraction could reduce the formation of acetic acid and increase the activation energy for hemicellulose pyrolysis. 21


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4. Conclusions

In this study, cellulose and hemicellulose samples isolated from softwood Pinus armandii Franch were subjected to pyrolysis study. The structural characterization by elemental analysis, neutral sugar analysis, FTIR, 1H-NMR and

13

C-NMR revealed

that the isolated hemicellulose was mainly constituted by galactoglucomannan and arabinoxylan, while the isolated cellulose was composed of β-1,4-glucan. Thermogravimetric analysis showed that the thermal stability of hemicellulose was poorer because of its amorphous and branched structure. Further thermal kinetics analysis by DG-DAEM model found that the devolatilization process for both cellulose and hemicellulose was dominated by decomposition reaction. And the activation energy for hemicellulose pyrolysis was obviously lower than that for cellulose pyrolysis. The Py-GC/MS results showed the various product distributions of cellulose and hemicellulose. Hemicellulose formed more acids, cyclic ketones, linear ketones and aldehydes due to its high content of pentose and uronic acid, while cellulose generated more furans and anhydrosugars because of the prominence of glucose.

AUTHOR INFORMATION Corresponding Author * Tel.: +86-571-87952801. Fax: +86-571-87951616. E-mail: [email protected].

Notes The authors declare no competing financial interest. 22


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Energy & Fuels

Acknowledgements The authors are grateful for the financial support from the National Natural Science Foundation of China (51276166), the National Basic Research Program of China (2013CB228101), and the National Science and Technology Supporting Plan Through Contract (2015BAD15B06).

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Government Printing Office: Washington, DC, 1970. (17) Liu, Q.; Wang, S.; Zheng, Y.; Luo, Z.; Cen, K., J. Anal. Appl. Pyrolysis 2008, 82, (1), 170-177. (18) Xue, B. L.; Wen, J. L.; Xu, F.; Sun, R. C., Carbohydr. Res. 2012, 352, 159-165. (19) Blumenkrantz.N; Asboehansen.G, Anal. Biochem. 1973, 54, (2), 484-489. (20) Segal, L.; Creely, J.; Martin, A.; Conrad, C., Textile Research Journal 1959, 29, (10), 786-794. (21) Wang, S.; Ru, B.; Lin, H.; Sun, W., Fuel 2015, 150, 243-251. (22) Wang, S. R.; Lin, H. Z.; Ru, B.; Sun, W. X.; Wang, Y. R.; Luo, Z. Y., J. Anal. Appl. Pyrolysis 2014, 108, 78-85. (23) Bian, J.; Peng, F.; Xu, F.; Sun, R.-C.; Kennedy, J. F., Carbohydr. Polym. 2010, 80, (3), 753-760. (24) Huang, F.; Ragauskas, A. J., Ind. Biotechnol. 2012, 8, (1), 22-30. (25) Trache, D.; Donnot, A.; Khimeche, K.; Benelmir, R.; Brosse, N., Carbohydr. Polym. 2014, 104, 223-230. (26) Zhang, J. X.; Luo, J.; Tong, D. M.; Zhu, L. F.; Dong, L. L.; Hu, C. W., Carbohydr. Polym. 2010, 79, (1), 164-169. (27) Bian, J.; Peng, F.; Peng, P.; Xu, F.; Sun, R.-C., Carbohydr. Res. 2010, 345, (6), 802-809. (28) Kacurakova, M.; Belton, P. S.; Wilson, R. H.; Hirsch, J.; Ebringerova, A., J. Sci. Food Agric. 1998, 77, (1), 38-44. (29) Buslov, D. K.; Kaputski, F. N.; Sushko, N. I.; Torgashev, V. I.; Solov'eva, L. V.; Tsarenkov, V. M.; Zubets, O. V.; Larchenko, L. V., J. Appl. Spectrosc. 2009, 76, (6), 801-805. (30) Kacurakova, M.; Capek, P.; Sasinkova, V.; Wellner, N.; Ebringerova, A., Carbohydr. Polym. 2000, 43, (2), 195-203. (31) Sun, X. F.; Sun, R. C.; Fowler, P.; Baird, M. S., J. Agric. Food. Chem. 2005, 53, (4), 860-870. (32) Bian, J.; Peng, F.; Peng, X. P.; Xiao, X.; Peng, P.; Xu, F.; Sun, R. C., Carbohydr. Polym. 2014, 100, 211-217. (33) Siroky, J.; Blackburn, R. S.; Bechtold, T.; Taylor, J.; White, P., Cellulose 2010, 17, (1), 103-115. 24


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(34) Fang, J. M.; Sun, R. C.; Tomkinson, J., Cellulose 2000, 7, (1), 87-107. (35) Moore, A. K.; Owen, N. L., Appl. Spectrosc. Rev. 2001, 36, (1), 65-86. (36) Choi, J. W.; Choi, D. H.; Faix, O., J. Wood Sci. 2007, 53, (4), 309-313. (37) Tenkanen, M.; Makkonen, M.; Perttula, M.; Viikari, L.; Teleman, A., J. Biotechnol. 1997, 57, (1-3), 191-204. (38) Chiarini, L.; Cescutti, P.; Drigo, L.; Impallomeni, G.; Herasimenka, Y.; Bevivino, A.; Dalmastri, C.; Tabacchioni, S.; Manno, G.; Zanetti, F.; Rizzo, R., J. Cyst. Fibros. 2004, 3, (3), 165-172. (39) Lundqvist, J.; Teleman, A.; Junel, L.; Zacchi, G.; Dahlman, O.; Tjerneld, F.; Stalbrand, H., Carbohydr. Polym. 2002, 48, (1), 29-39. (40) Gabrielii, I.; Gatenholm, P.; Glasser, W. G.; Jain, R. K.; Kenne, L., Carbohydr. Polym. 2000, 43, (4), 367-374. (41) Wang, S.; Ru, B.; Lin, H.; Sun, W.; Luo, Z., Bioresour. Technol. 2015, 182, 120-127. (42) Gronli, M.; Antal, M. J.; Varhegyi, G., Industrial & Engineering Chemistry Research 1999, 38, (6), 2238-2244. (43) Ponder, G. R.; Richards, G. N., Carbohydr. Res. 1991, 218, (0), 143-155. (44) Wang, S. R.; Liu, Q.; Liao, Y. F.; Luo, Z. Y.; Cen, K. F., Korean J. Chem. Eng. 2007, 24, (2), 336-340. (45) Kim, U. J.; Eom, S. H.; Wada, M., Polym. Degrad. Stab. 2010, 95, (5), 778-781. (46) Cai, J. M.; Wu, W. X.; Liu, R. H.; Huber, G. W., Green Chem. 2013, 15, (5), 1331-1340. (47) Wang, S.; Ru, B.; Lin, H.; Luo, Z., Bioresour. Technol. 2013, 143, 378-383. (48) Mettler, M. S.; Mushrif, S. H.; Paulsen, A. D.; Javadekar, A. D.; Vlachos, D. G.; Dauenhauer, P. J., Energy Environ. Sci. 2012, 5, (1), 5414-5424. (49) Shen, D. K.; Gu, S., Bioresour. Technol. 2009, 100, (24), 6496-6504. (50) Shen, D. K.; Gu, S.; Bridgwater, A. V., Carbohydr. Polym. 2010, 82, (1), 39-45. (51) Shin, E. J.; Nimlos, M. R.; Evans, R. J., Fuel 2001, 80, (12), 1697-1709. (52) Ponder, G. R.; Richards, G. N.; Stevenson, T. T., J. Anal. Appl. Pyrolysis 1992, 22, (3), 217-229. (53) Wang, Z. H.; McDonald, A. G.; Westerhof, R. J. M.; Kersten, S. R. A.; Cuba-Torres, C. M.; Ha, S.; Pecha, B.; Garcia-Perez, M., J. Anal. Appl. Pyrolysis 2013, 100, 56-66. 25


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Figure captions

Fig. 1. X-ray diffraction patterns of the extracted cellulose and MCC

Fig. 2. FTIR spectra of the cellulose and hemicellulose. Fig. 3. 1H NMR and 13C NMR spectra of the hemicellulose.

Fig. 4. TG and DTG curves for the cellulose and hemicellulose.

Fig. 5. DG-DAEM simulation for pyrolysis of the cellulose and hemicellulose.

Fig.6. Distribution of activation energies for pyrolysis of the cellulose and hemicellulose.

26


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Table 1 Elemental composition of extracted cellulose and hemicellulose Cdaf

Hdaf

Odaf

(wt%)

(wt%)

(wt%)

cellulose

43.61 (±0.09)a

6.10 (±0.02)

hemicellulose

38.61 (±0.87)

5.39 (±0.06)

Sample

a

O/C

H/C

Formula

50.28 (±0.11)

0.86

1.68

(C6H10.08O5.19)n

56.01 (±0.81)

1.09

1.68

(C6H10.05O6.53)n(C5H8.37O5.44)m

Standard deviation

27


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Table 2 The contents of neutral sugars and uronic acid. Glucose

Mannose

Galactose

Xylose

Arabinose

Uronic acid

(wt%)

(wt%)

(wt%)

(wt%)

(wt%)

(wt%)

cellulose

88.36 (±0.91)a

4.87 (±0.16)

1.75 (±0.10)

2.651 (±0.17)

1.75 (±0.16)

-

hemicellulose

10.82 (±0.55)

31.61 (±0.66)

11.68 (±0.23)

19.31 (±0.64)

9.46 (±0.18)

16.53 (±0.58)

a

Standard deviation

28


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Table 3 The typical pyrolysis products of the cellulose and hemicellulose pyrolysis identified by GC-MS (peak area/sample weight)

acids

furans

cyclic ketones

Cellulose

Hemicellulose

8

×10 /mg

×108/mg

4.34 (±0.03)b

11.16 (±0.10)

RTa

Product

Formula

13.40

Acetic acid

C2H4O2

15.58

Propanoic acid

C3H6O2

1.36 (±0.01)

-

11.13

2-Methylfuran,

C5H6O

2.74 (±0.09)

5.16 (±0.06)

13.68

Furfural

C5H4O2

2.26 (±0.06)

5.21 (±0.07)

14.37

2-ethyl-5-methylfuran,

C7H10O

1.60 (±0.06)

1.19 (±0.03)

14.77

5-Ethylfurfural

C7H8O2

-

1.82 (±0.03)

16.40

5-Methylfurfural

C6H6O2

0.82 (±0.03)

-

18.26

Furfuryl alcohol

C5H6O2

-

1.96 (±0.03)

33.2

5-Hydroxymethylfurfural

C6H6O3

1.54 (±0.02)

-

7.07

Cyclopentanone

C5H8O

0.77 (±0.03)

1.87 (±0.06)

7.74

3-Methylcyclopentanone

C6H10O

-

1.30 (±0.03)

11.55

2-Cyclopenten-1-one, 2-methyl-

C6H8O

1.84 (±0.02)

5.36 (±0.09)

15.24

2-Cyclopenten-1-one, 3-methyl-

C6H8O

1.39 (±0.04)

3.23 (±0.01)

15.76

2-Cyclopenten-1-one, 2,3-dimethyl-

C7H10O

1.10 (±0.03)

3.91 (±0.03)

19.61

3-Methyl-2(5H)-Furanone,

C5H6O2

0.52 (±0.01)

-

20.36

2(5H)-Furanone

C4H4O2

1.29 (±0.03)

-

20.85

2-Cyclopenten-1-one, 2-hydroxy-

C5H6O2

4.97 (±0.11)

2.70 (±0.06)

22.19

1,2-Cyclopentanedione, 3-methyl-

C6H8O2

8.21 (±0.10)

8.71 (±0.08)

C8H12O2

0.46 (±0.01)

1.54 (±0.04)

C7H10O2

2.46 (±0.09)

2.70 (±0.08)

C8H12O2

0.56 (±0.03)

-

23.19

23.58

25.19

1,3-Cyclopentanedione, 2-ethyl-2-methyl2-Cyclopenten-1-one, 2-hydroxy-3-ethyl2-Cyclopenten-1-one, 2-hydroxy-3-propyl-

linear ketones and

2.80

2-Butanone

C4H8O

0.60 (±0.03)

2.94 (±0.08)

aldehydes

3.87

Methoxyacetaldehyde

C3H6O2

-

1.86 (±0.03)

4.66

2,3-Pentanedione

C5H8O2

0.42 (±0.01)

-

9.39

2-Butanone, 3-hydroxy-

C4H8O2

0.39 (±0.01)

1.34 (±0.03)

9.74

Hydroxyacetone

C3H6O2

9.21 (±0.07)

16.26 (±0.18)

12.59

Butanedial

C4H6O2

1.25 (±0.02)

-

30.31

2,3-Anhydro-D-mannosan

C6H8O4

1.79 (±0.02)

-

32.84

1,4:3,6-Dianhydro-D-glucopyranose

C6H8O4

1.74 (±0.04)

-

50.07

Levoglucosan

C6H10O5

2.04 (±0.05)

-

sugars

a

RT = retention time, min. b Standard deviation 29


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Fig. 1

30


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Fig. 2.

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Fig. 3.

32


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Fig. 4.

33


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Fig. 5.

34


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Fig. 6.

35


Structural Characterization and Pyrolysis Behavior of Cellulose and

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