Study of the Crystallization Parameters of Bamboo Fibers with Pretreatment

Chu Jie Zhang Junhua Ma Li Lu Haidong

(Northwest Agriculture and Forestry University Yangling 712100)

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Study of the Crystallization Parameters of Bamboo Fibers with Pretreatment

Chu Jie Zhang Junhua Ma Li Lu Haidong

(NorthwestAgricultureandForestryUniversityYangling712100)

【Objective】 Chemical pretreatment is a critical step for transforming a biomass to its value-added lignocellulosic biomass polymeric products, and the elucidation of pretreatment mechanism is important for improving the thermal treatment efficiency. 【Method】 Four-year-old bamboo was pretreated with acid, alkali and glycerine at 117 ℃ and 135 ℃,respectively. The structural features of the pretreated and the native samples were characterized and compared using a set of spectroscopy and wet chemistry methods including FTIR, XRD,respectively. 【Result】The results showed that the holocellulose and cellulose yields increased significantly, and the lignin removal rate was better for dilute alkali (NaOH) pretreatment than that for dilute acid (H2SO4) and glycerin pretreatments. Furthermore, for the same solutions, the compositional changes of samples were more remarkable at 135 ℃ than those at 117 ℃, and the same degradation of hemicelluloses was observed for different processing.All of the pretreatment samples exhibited relatively low average degree of polymerization (DP) values, indicating that the pretreatment processing could change the super molecular structure.The X-ray diffraction results showed that after different chemical heat treatments, the 002 peak position was clearly offset, the crystalline width and half peak width decreased, but the crystalline intensity increased significantly, and the relative crystallinity declined at 117 ℃ and gradually increased at 135 ℃. The results obtained by Fourier infrared spectrum were consistent with those of the X-ray diffraction.【Conclusion】 For both 117 ℃ and 135 ℃ pretreatment temperature in sand bath, alkali pretreatment effects were stronger than that of the dilute acid and glycerin. These results would provide foundation and basis for the research of energy transformation of lignocellulose raw material.

bamboo； pretreatment； Flourier infrared spectrum； average degree of polymerization； X-ray diffraction； thermogravimetric analysis

Chemical heat treatment is a critical step for transforming a biomass to its value-added lignocellulosic biomass polymeric products (Zhaoetal.， 2012; Xieetal.， 2015). Pretreatment can quickly remove hard dissolved lignin, leading to the physical separation of hemicellulose, increasing lignocellulose production. Currently, despite the large number of studies of the thermal chemical conversion method, general conclusions that are applicable to the specific treatment schemes have not been obtained. This is because most studies have focused on the yield of cellulose and sugar production during the process of chemical pretreatment. However, chemical pretreatment of biomasss is much more complicated, and therefore, the influence of composition changes and the effect of pretreatments on the obtained functional groups and crystallinity should be further investigated. In addition, the lignocelloses consists of complex structures, including cellulose, hemicelluloses, lignin, and a small amount of extract (Chundawatetal.， 2011; Zhouetal.， 2004; Merino-Pérezetal.， 2014). The pretreatment solution and treatment temperature have an important effect on the variation of the content of each component. Due to the importance and high cost of the pretreatment process and the chemical conversion of functional groups, the change of the crystallinity and cellulose molecular weight during pretreatment will be subject to increasing attention from researchers in the future.

Based on the current pretreatment processing technology, Lietal.(2014) studied the development of bamboo cellulose crystallinity changes using X-ray diffraction and Rezendeetal.(2011) indicated bamboo fiber crystallinity changes after radiation treatment.The results obtained by Li and Rezende agree with intensity changes of the X-ray diffraction of bamboo fiber. Abdulkhanietal.(2014) studied the trends in the bamboo crystal area changes after alkali treatment and found that these differed from the trends obtained in previous studies, with the fiber crystallinity changes of bamboo first increasing and then decreasing. Wangetal.(2008) suggested that the two important factors leading to the worse performance of bamboo are high crystallinity and the orientation structure; additionally, she found an effective method for using alkali treatment of bamboo to quickly reduce the bamboo crystallinity index. Lietal.(2010) provided pretreated bamboo to isolate cellulose for the further utilization of bioethanol.

Among the representative technologies for the pretreatment of wood biomass, Xinetal.(2015) researched the digestion absorption rate of the bamboo structure after different pretreatments with acid, alkali and sulfate; Zhangetal.(2013)and Karpetal.(2015) studied the chemical composition changes due to bamboo pretreatment; Lietal. (2013)showed that the application of dilute organic acids in the pretreatment of bamboo can be effective and clearly influenced the cell-wall structure and enzymatic digestibility of bamboo; He also provided that the pretreatment of bamboo with the same sodium hydroxide loading at higher pretreatment temperature led to greater hemicellulose and lignin removal. These investigations showed that acid and alkali pretreatment of bamboo is a more effective way to improve wood fiber content than other approaches used in the production of bioethanol and other fields (Lietal.， 2014). However, to the best of our knowledge, less attention has been focused on the chemistry and molecular structure changes, with fewer studies on the mechanism causing the changes of the chemical composition of lignocellulosic biomass materials after chemical treatment. Nevertheless, a considerable amount of research has been performed on bamboo cellulose fermentation and the saccharification process under the condition of alkali treatment in the past few years. To the best of our knowledge, there are few results on cellulose crystallinity size changes. In particular, the obtained results are not consistent with the scale change rule of bamboo fiber crystallinity under the different chemical treatment conditions, such as for the use of acid alkali, and glycerol (Himmeletal.， 2007).

This paper is based on four years of bamboo studies that used X-ray diffraction(XRD) with different temperatures and pretreatments with acid, alkali, and glycerol, focusing on the determination of the average degree of polymerization(DP) content of cellulose, hemicelluloses and lignin, extract and ash of pretreated bamboo; additionally, using Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction, we further investigated the influence of different heat treatment methods on the bamboo chemical composition. We used the changes of the functional groups and the mechanism of the chemical pretreatment of wooden biomass resources to reveal the rules governing the compositional changes found after the application of the different chemical treatment methods.

1 Materials and methods

1.1 Materials

The bamboo powder sample (Phyllostachysedulis) was acquired from USDA Forest Service, Southern Research Station in Pineville, USA, in the summer of 2015. Prior to chemical pretreatment, air-dried bamboo was milled using a hammer mill with a screen opening size of 1.0 mm. The average moisture content of the ground air-dried bamboo was 6.18% (wt). The bamboo was sealed in a zipper-lock bag and kept at 4 ℃ in a refrigerator until use.

H2SO4, NaOH, NaClO2, toluene, methanol, ethanol, and glacial acetic acid were used; these were purchased from Fisher Scientific (Pittsburgh, PA) and used as received. Pretreatment equipment mainly included the electronic sand bath pot and thermometer produced in JAMs experimental equipment factory (USA).

1.2 Pretreatment processing

Samples were obtained by different processing methods, with the processing parameters and processes shown in
Fig.1.

Fig.1 Scheme for the preparation of samples and cellulose, hemicelluloses, and lignin

Fig.1-5 illustrate the pretreatment processing of six bamboo samples. This includes a native sample, the sample treated by sulfuric acid at 135 ℃(SA135) and at 117 ℃(SA117), the sample treated by alkali at 135 ℃(SH135) and at 117 ℃(SH117), and the sample treated by glycerol at 117 ℃.

The moisture content of the bamboo samples was measured by drying in an oven for 24 h at (105±2) ℃. The holocellulose and cellulose contents of the samples were determined using a Soxhlet extractor. An electric mufe furnace (programmable ramping) was used to determine the ash in the samples of bamboo. The lignin content of unpretreated and pretreated bamboo was determined according to the National Renewable Energy Laboratory (NREL)analytical procedure: determination of the structural carbohydrates and lignin in biomass.15 each test was performed for three parallel samples at the same time, with the average used as the measured value.

1.3 Measurements

1.3.1 Chemical analysis The bamboo samples were evaluated in accordance with ASTM D1105-96 (ASTM 1996) and ASTM D-1102-84(ASTM 1990). Each evaluation was performed twice.

Fig.2 Dry sample under different processing conditionsa. The native bamboo is original; b. The SH117 sample is pretreated with sulfuric acid at 117 ℃; c.The SH135 sample is pretreated with sulfuric acid at 135 ℃; d. The SA 117 sample is pretreated with alkali at 117 ℃; e. The SA135 sample is pretreated with alkali at 135 ℃; f. The GL117 sample is pretreated with glycerin at 117 ℃.

Fig.3 Filtrate of different processing conditions

Fig.4 Cellulose samples of different processing conditions

Fig.5 Liquid and solid samples of lignin and ash

1.3.2 DP and molecular content The average degree of polymerization (DP) and molecular content were determined using the method specified by the British Standards restrictions, and the viscosity was determined according to the Part I.Cupriethylenediamine (CED) method (BS 6306: Part 1: 1982). The sample viscosity was determined based on its cupriethylenediamine viscosity [η], which was obtained using the following hydroxide (cuene) solution equation:

(1)

where DP is the average degrees of polymerization andηis the intrinsic viscosity in mL·g-1. The crystallinity of the samples was determined by X-ray diffraction using an XRD-6000 instrument (Shimidzu, Japan). A capillary viscometer was used for the viscosity measurement (Shimidzu, Japan),D=(0.80±0.05) mm, with calibration using a capillary viscometer,D=(0.57±0.05)mm.

1.3.3 Fourier transform-infrared spectroscopy (FTIR) Image acquisition of Fourier infrared spectrum (FTIR). We used a NicoletiS10 Fourier trans form infrared spectrometer produced by the Thermo Nicolet company (USA) with the following test conditions: the spectral range was 4 000-400 cm-1, the spectral resolution was 4 cm-1, and the scans were performed 64 times.

1.3.4 Analysis of crystallinity by X-ray diffraction (XRD) The crystallinity of the samples was determined by X-ray diffraction (XRD) using Rigaku Ultima3 x (Louisiana State University, USA) with the scanning parameters of Ni-filtered Cu K α radiation (λ=1.540 60).

2 Results and discussion

2.1 Changes in the chemical composition of pretreated bamboo

Fig.6 shows the main chemical components obtained for different pretreatment conditions. The holocellulose content decreases in the order of SH117> SH135 > SA117 > SA135 > GL117 > NA; the cellulose content change decreases in the order of SH135 > SH117 > SA117 > SA135 > GL117 > NA; the amount of lignin removal decreases in the order of SH117 > SH135 > SA135 > > SH117 > GL117 > NA; and the degree of hemicellulose degradation decreases in the order of SH135>GL117>SA117> SA135 > SH117.

2.2 Comparison of pretreatments with sodium hydroxide and sulfuric acid

The results obtained by pretreatment with sodium hydroxide are much better than those obtained after sulfuric acid pretreatment at both 117 ℃ and 135 ℃; in particular, the cellulose yield showed a clear increase after the sodium hydroxide pretreatment at 135 ℃, with a 29.6% higher average ratio than that of the native material. However, pretreatment with glycerin was found to be not ideal. A remarkable lignin removal effect was also observed for the sodium hydroxide pretreatment at 117 ℃, with the average removal rate reaching 14.69%.

Fig.6 Main chemical components obtained with the use of different pretreatment conditions

2.3 Degree of polymerization

To investigate the cellulose depolymerization behavior in the pretreatment, the viscosity of the sample was determined and the related parameters were calculated. The obtainedηvalues were in the 103.88-132.62 range, and the obtained DP values were in the 336-396 range (Tab.1).

Tab.1 Viscosity and DP values of samples with different pretreatments

Inspection of the data from
Tab.1 shows that for all of the pretreatments, relatively low DP values are obtained, the reason might that pretreatment processing could change the super molecular structure of plant fiber raw material, that lead to changes for cellulose crystalline region size, on the other hand, better pretreatment can be part of the degradation of lignin and hemicellulose, increasing accessibility, which is benefit to improve the efficiency of plant cellulose enzyme hydrolysis.The DP value of the native sample is mainly higher that maybe due to the low value deformation of the cellulose crystal lattice in the processing.

2.4 Analysis of bamboo samples by Infrared spectroscopy

The obtained FTIR spectra in the 4 000-400 cm-1range and the changes in the absorption peak of the functional group are shown in
Fig.7 and
Fig.8.

The absorbance band at 897 cm-1is the characteristic peak of ? glucose anhydride and the C-H bond bending vibration of cellulose (Li, 2003). While the absorbance band at 897 cm-1exhibits only one peak in the native spectrum, it can be divided into two peaks at 880 and 895 cm-1for the SH117, SH135, SA135, and SA117 samples. The absorption peak at 828 cm-1(
Fig.7) was enhanced, indicating the change in cellulose content after pretreatment. After acid treatment, the crystallization of water O-H weakened the cellulose peak at 828 cm-1(Shaoetal., 2008; Abdulkhanietal., 2014). Other characteristic absorption peaks of the cellulose are found at 2 900 cm- 1and 3 308 cm- 1;these peaks change slightly and exhibit no other obvious absorption peak near the band. The relative ranking for the change in the cellulose was obtained as SH135 > SH117 > SA117> SA135 > GL117, namely, the order of the 1, 2, 3, 4, and 5 (
Fig.7); this is consistent with the standard method for the determination of cellulose content increase presented above in
Fig.6.

The analysis of the spectra is more complicated for the functional groups of lignin. Usually, the functional groups of lignin are benzene and benzene with C—C bonds combined with functional groups; a stretching vibration C—C absorption peak is found at approximately 1 420 cm-1(Sunetal., 2010; Singhetal., 2014; Gabovetal., 2014).Quite strong absorbance bands are present at 1 230 cm-1, which are due to the diaryl ether bond of the native sample that become weaker for the pretreated sample. In addition，from 1 420 cm-1to 1 482 cm-1, the absorption peak also exhibits a steep fall (
Fig.7). These feature demonstrate the presence of a remarkable lignin removal effect after pretreatment.

Based on the changes of the characteristic band of lignin, it can be speculated that the strength of lignin removal is ranked as SH117 > SH135 > SA135?SH117 > GL117 > NA, which is in agreement with the previous test results.

For glycerol pretreatment, an obvious change of hemicelluloses with glycerol pretreatment was observed, with no significant removal of lignin and only incremental changes in the cellulose content; elucidation of the origin of this effect requires further study.

2.5 Analysis of the bamboo samples by XRD

According to the diffraction spectroscopy characteristics of bamboo fiber, the 002 peak and the 004 peak indicate the length and width of the cellulose crystal area, respectively. In most cases, the 040 peak is not obvious, and we therefore calculated the diffraction intensity and position of the 002 peak to discuss the changes in the cellulose crystallization area. We use the Scherer formula (Segaletal., 1959) to calculateD,dandCrIas follows.

whereDis the length or width of the crystalline region；Dis the crystal layer spacing (nm)；λis the incident wavelength(0.154 nm); Bhkl is the diffraction peak half width;θis the diffraction angle; andkis diffraction constant,0.89;CrIis the relative degree of crystallinity(%);I002is the great strength of the 002 diffraction angle; andIamis the background diffraction scattering intensity when 2θis equal to 180°.

Six different test samples were prepared based on the use of different alkali and glycerol media and heating temperatures; XRD scans showed obvious changes of the 002 peak position and intensity (
Fig.9).

Fig.7 Compositional changes in the infrared spectra of pretreated and native bambooa:SA117； b:SA135；c: SH117； d: SH135； e: NATIVE；f:GL117Note: cellulose changes;Lignin changes;Hemicellulose changes;1, 2, 3, 4, 5, and 6 are the chemical composition changes of the different processing conditions, respectively.

Fig.8 Infrared spectra of pretreatment and native bambooa:SA117； b:SA135； c: SH117； d: SH135； e: NATIVE； f:GL117.

Fig.9 002 crystal plane diffraction peak position changes

Examination of Fig.9 shows that the angle of diffraction (2θ), that is, the position of the 002 crystal plane diffraction peak, changed significantly, and the peak exhibits an obvious offset after different pretreatments. This phenomenon shows that the lattice parameters of the crystalline region and the interplanar spacing decrease (Foremanetal., 1993; Mansfieldetal., 1999; Liuetal., 2008).The effect of the chemical pretreatment is beneficial, as the decreased bamboo crystal spacing improves the dimensional stability of bamboo and provides a strong reference value for composite processing. These results consistent with previous research (Tamuraetal., 2003). The change rate of chemical pretreatment at 135 ℃ is significantly higher than that at 117 ℃.

2.6 Analysis of various crystallization parameters of the different pretreatments

Based on the 002 crystal crystallization index and Eqns. (2), (3), and (4)，the crystallization parameters can be obtained for different pretreatment conditions.

Fig.10 shows the change of the crystalline region widthD. The width of the crystalline region increases after different pretreatments, showing that the grain size of the crystalline region decreased. Examination of the data shows that the rate of change rate is significantly higher for pretreatment at 135 ℃ than at 117 ℃. However, no obvious differences were observed between the SA135, SH135, SA117 and SH117 samples. This indicated that the pretreatment temperature has a stronger influence on the grain size of the crystalline region. After glycerol pretreatment, a significantly smaller grain size was found.

Obtaining the fine grain structure is the most effective approach for improving the strength and toughness of bamboo,therefore, the improvement of the degree of crystalline parameters through chemical heat treatment has important implications for the enhanced use and performance of bamboo.

The change of the half peak width (f) of the crystalline region indicated the grain size of the crystalline region: a smaller peak width(f) implies that the grain size is smaller as well (Nishimiyaetal., 1998; Inarietal., 2007).
Fig.11 further illustrates the change of the crystalline region grain size. The half peak width (f) is lower for pretreatment at 117 ℃ than at 135 ℃; this is especially pronounced for treatment with alkali at 117 ℃, where the half peak width of the crystallization area is minimum, showing that alkali treatment will be the most effective approach for obtaining tiny crystalline bamboo particles.

Fig.10 Width variation for the 002 crystal plane

Fig.11 FWHM variation for the 002 crystal plane

Fig.12 Layer spacing variation for the 002 crystal plane diffraction peak

Fig.12 shows the changes of the crystalline region layer spacing (d), with the layer spacing dimension exhibited a trend of significant decrease after pretreatment and a more obvious concentration at 135 ℃ compared to that at 117 ℃. Investigation of the origin of this effect showed that adsorption on the cell walls of bamboo was gradually enhanced and that the oligosaccharide in the crystalline region of cellulose surface molecular chain was dissolved (Yangetal., 2008). That due to the interaction between hemicelluloses and a weak base, a peeling reaction may occur, leading to decreased layer spacing (Foremanetal.,1993).

Fig.13 crystal strength of crystal plane variation

Fig.14 The value of crystal plane variation

As shown in Fig.13,the 002 peak is high and sharp, with the degree of crystallinity increasing significantly after pretreatment.
Fig.14 showed the trend of a first reduced and then increased intensity, motivating the present research to achieve a consistent increase of crystallinity of pretreated bamboo. Bamboo crystallinity declined at 117 ℃; however, the diffraction peaks become sharp; when the temperature rose to 135 ℃, the diffraction peaks increased gradually. The results showed that after pretreatment with sulfuric acid at 135 ℃, the bamboo crystallinity fell by 5% and the relative crystallinity increased by 3%; by contrast, the bamboo crystallinity fell by 15% and the relative crystallinity increased by 9% after pretreatment with alkali at 135 ℃; after heat treatment, the crystallinity increased and the most obviously decreased pretreatment was with glycerol, where two irregular peaks appeared near the 040 peak; the elucidation of the reason for this effect requires further study.

Due to the stronger cell wall adsorption, following the alkali pretreatment for bamboo, the oligosaccharide in the crystalline region of the cellulose surface molecular chain is dissolved and the reactive amorphous zone reactive is exposed. Moreover, the interaction between hemicellulose and a weak base led to decreased crystallinity because of the swelling of the cellulose matrix and the increase in the amorphous area (Yangetal., 2008). Similarly, this led to a relative reduction in the shaped area of the cellulose, making an additional contribution to the further decrease of crystallinity. Hemicelluloses of bamboo hydrolysis are used to produce acetic acid involving the degradation of microfibrils in the amorphous region and in the setting area of cellulose (Yangetal., 2007) for the pretreatment temperature of 135 ℃, resulting in the decrease of the cellulose crystallinity.

Additionally, the density of the bamboo crystalline region increased after pretreatment that caused the enhancement of the diffraction peak.

3 Conclusions

Comparison of the hemicellulose degradation degree and rate of lignin removal obtained with FTIR shows that the yield of cellulose is increasing; for the lignin removal efficiency and hemicelluloses after different pretreatments, the effect of pretreatment with sodium hydroxide is relatively better than that with sulfuric acid under the same conditions, with an obvious enhancement of cellulose output with sodium hydroxide at 135 ℃, as shown by the 29.6% increase in the average ratio relative to native bamboo; for sodium hydroxide treatment at 117 ℃, the removal of lignin reaches to 14.69%. Investigations of the cellulose depolymerization behavior in the pretreatment shows that t all of the pretreatments, relatively low DP values are obtained, the reason might that pretreatment processing could change the super molecular structure of plant fiber raw material, that lead to changes for cellulose crystalline region size, on the other hand, better pretreatment can be part of the degradation of lignin and hemicellulose, increasing accessibility. The yield of holocellulose and cellulose increased significantly after pretreatment, with a stronger effect observed for dilute alkali (NaOH) than for dilute acid (H2SO4) and glycerin (glycerin). The XRD results exhibit a high and sharp peak, with an obvious enhancement of the diffraction peak intensity and large changes in the angle of the 002 peak after pretreatment.

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(責任編輯 石紅青)

預處理竹材的結晶度分析*

楚 杰 張軍華 馬 莉 路海東

(西北農林科技大學 楊凌 712100)

【目的】 化學預處理是生物質聚合物產品高值化利用的關鍵步驟，闡明預處理機制有助于提高熱處理效率。【方法】 以4年生毛竹為研究對象，采用稀酸、堿、甘油分別在117 ℃和135 ℃進行砂浴預處理，并采用光譜和濕化學方法對預處理前后樣品的結構進行表征和比較，包括傅里葉紅外光譜、聚合度、X射線衍射等?！窘Y果】 綜纖維素和纖維素的產量顯著增加，堿(NaOH)預處理比稀硫酸(H2SO4)和甘油(丙三醇)脫木素效果好； 在相同預處理條件下，135 ℃比117 ℃樣品的結構變化更明顯。平均聚合度結果顯示，所有預處理樣品均表現出較低的聚合度，說明纖維素結晶區尺寸發生變化，部分木質素和半纖維素降解，從而增加可及度。X射線衍射結果表明，002峰位置明顯偏移，晶體寬度和半峰寬下降，不同化學預處理后的結晶強度明顯增加，相對結晶度在117 ℃下降，在135 ℃逐漸恢復，傅里葉紅外光譜與X射線衍射研究結果一致?！窘Y論】 無論采用117 ℃還是135 ℃砂浴預處理，堿預處理纖維分離的效果均強于稀酸和甘油，可為后續木質纖維素原料水解和能源化轉換方面的相關研究提供基礎和依據。

竹材； 預處理； 傅里葉紅外光譜； 平均聚合度； X射線衍射； 熱重分析

S781.46

A

1001-7488(2017)02-0100-10

10.11707/j.1001-7488.20170212

Received date：2015-12-09; Revised date：2016-10-31.

Funding Project：Shaanxi Science and Technology Research Project (2014 k02-12-04); Basic Science and Technology Found of North West Agriculture and Forestry University (2452015165).

* Zhang Junhua is corresponding author.