Characterization of residue and bio-oil produced by liquefaction of loblolly pine at different reaction times.
The objective of this study was to analyze the effect of liquefaction time on the properties of residue and bio-oil produced by liquefaction of loblolly pine using ethylene glycol at 150[degrees]C. Liquefaction was carried out in a glass reactor for 30, 60, 90, 120, and 150 minutes. The lowest residue content of 42 percent was obtained from 90 minutes of liquefaction. Fourier transform infrared analysis of residue and bio-oil revealed that as liquefaction time was prolonged, more hemicellulose underwent degradation, resulting in the formation of compounds containing carbonyl groups. An inverse correlation between the intensity of peaks at 1,734 [cm.sup.-1] (carbonyl groups) and 3,293 [cm.sup.-1] (OH stretching) in the infrared spectra of bio-oil was observed. X-ray diffraction analysis of residue supported the degradation of hemicellulose in the early stage of liquefaction as well as cellulose degradation as the liquefaction was prolonged. Finally, the hydroxyl number of bio-oil was determined by the phthalic anhydride esterification method and was found to be in the range of 1,230 to 1,427 mg KOFI/g, depending on liquefaction time.
Forest biomass is receiving increased attention as an alternative feedstock to petroleum for the production of fuel and chemicals due to environmental concerns, the decrease in petroleum resources, and the instability in the petroleum market. Approximately 1,400 million dry tons of forest biomass per year is being produced in the United States (Perlack et al. 2005); therefore, there is a tremendous amount of research on the utilization of forest biomass as renewable and sustainable feedstock in the field of bioenergy and bioproduct. Moreover, the US Department of Energy and the US Department of Agriculture set a goal to produce 18 percent of the current US chemical commodities from biomass by 2020 and 25 percent by 2030 to reduce the country's dependency on petroleum (Perlack et al. 2005). This should also lower fossil fuel emissions, which have increased by 3.4 percent since 2010 (Bergman et al. 2014).
Organic solvent liquefaction, pyrolysis, and hydrothermal liquefaction are thermomechanical conversion techniques to convert forest biomass into a liquid product called "bio-oil." High hydroxyl (OH) functionality of bio-oil makes it a great candidate to be used as a bio-based polyol to synthesize a variety of polymers, including phenolic resins (Alma and Basturk 2006), polyurethane (Hu and Li 2014), epoxy resin (Kuo et al. 2014), polyester (Yu et al. 2006), and melamine formaldehyde and melamine urea-formaldehyde resin precursors, for the forest products industry (Kunaver et al. 2010). Replacement rates of phenol for various adhesives and polymers can often reach 50 percent, offering significant environmental supplementation to an otherwise petroleum-dependent process (Wei et al. 2014, Zhang et al. 2015).
In the organic solvent liquefaction technique, bio-oil is produced by liquefying biomass using an organic solvent in the presence of an acid or base catalyst at moderate temperatures (100[degrees]C to 300[degrees]C). The optimum liquefaction temperature was reported to be around 150[degrees]C to 200[degrees]C in terms of low residue content (RC) and high bio-oil yield when ethylene glycol (EG) was used as the liquefying solvent in the presence of acid catalyst (Celikbag et al. 2014). Different types of solvents have been previously studied. Phenol has been reported as the best liquefying solvent for biomass in terms of bio-oil yield (Zhang et al. 2006); however, owing to the toxicity of phenol, more environmental friendly solvents, such as EG, diethylene glycol (DEG), and glycerol, have gained more attention for liquefaction. Liquefaction of bagasse with EG (Zhang et al. 2007), spruce with DEG-glycerol (Jasiukaityte et al. 2010), pine with EG (Rezzoug and Capart 2002), switchgrass with DEG (Wei et al. 2014), and bamboo with EG (Ye et al. 2014) have been previously reported. These studies focused mainly on the effect of liquefaction time and temperature on bio-oil yield and bio-oil composition; however, information is sparse on residue formation and OH analysis of bio-oil, which are important considerations because they positively impact the financial feasibility for use as bioenergy or bioproducts (Kim et al. 2015).
Characterization of residue and bio-oil is necessary to improve our understanding behind organic solvent liquefaction of forest biomass, which enhances the utilization of bio-oil for the production of bio-based value-added products. Therefore, the objective of this study was to investigate the effect of liquefaction time on residue production and OH content of bio-oil.
Materials and Methods
Clean loblolly pine (Pinus spp.) wood chips were obtained from a local chipping plant in Opelika, Alabama. Physiochemical properties of loblolly pine were determined by traditional wet-chemistry analysis (Zhou et al. 2015). Extractives, cellulose, hemicellulose, and lignin content were also determined by the near-infrared (NIR) and Fourier transform infrared (FTIR) spectroscopy models generated by Jiang et al. (2014) and Zhou et al. (2015), respectively. Physiochemical properties determined by wet chemistry, NIR, and FTIR models are summarized in Table 1. All chemicals were purchased from VWR in reagent grade and used as received.
Loblolly pine was ground to 20-mesh particle size by a hammer mill (New Holland grinder model 358) for particle size reduction. The wood particles were kept in an oven at 105[degrees]C for 24 hours to remove moisture. Liquefaction was carried out in a three-neck atmospheric glass reactor equipped with a mechanical stirrer and condenser. The reactor was charged with 400 g of EG, 12 g of sulfuric acid (3%, wt/wt, based on EG), and 100 g of pine wood particles. The reactor was then immersed in a preheated silicon oil bath at 150[degrees]C. After the preset time (30, 60, 90, 120, and 150 min), the reaction was stopped, and the glass reactor was immersed in cold water to quench the reaction.
Determination of RC
Liquefied wood was diluted with 1,4-dioxane--water--acetone (4:1:2, vol/vol) mixture and filtered with Whatman no. 1 filter paper under vacuum. The insoluble solid portion was labeled as residue, which was dried in an oven at 105[degrees]C for 24 hours and then cooled to room temperature in a desiccator. RC was calculated as follows:
RC (%) = Weight of residue (g)/Weight of starting pine wood (g) x 100 (1)
The excess 1,4-dioxane--water--acetone was removed from the liquid portion by a rotary evaporator, and resulting liquid was labeled bio-oil. The liquefaction and separation methods are illustrated in Figure 1.
Degree of crystallinity index of residue
X-Ray diffractograms were collected by X-ray diffraction (Bruker D-8 Advanced, with a lynx eye detector) in the range of 0[degrees] to 40[degrees] at a scanning speed of 1[degrees]/s. The crystallinity index (CrI) was then calculated according to Equation 2 (Segal et al. 1959) as follows:
CrI (%) = [I.sub.002] - [I.sub.am]/[I.sub.002] (2)
where [I.sub.002] is the maximum intensity of the (002) plane at 2[theta] = 22.6[degrees] and [I.sub.am] is the intensity diffraction of the amorphous band at 2[theta] = 18[degrees].
Attenuated total reflection FTIR
Attenuated total reflection FTIR (ATR-FTIR) spectra of residue and bio-oil were acquired between 4,000 and 650 [cm.sup.-1] with an ATR-FTIR spectrometer (Model Spectrum400, Perkin Elmer Co.) to identify functional groups. A small amount of residue (1 to 2 g) was put on the diamond crystal, and spectra were collected at room temperature (22[degrees]C [+ or -] 1[degrees]C).
Determination of hydroxyl number of bio-oil
Total hydroxyl number (OHN) of bio-oil was determined using the phthalic esterification technique as described by Carey et al. (1984). In this technique, the phthalation reagent was prepared by dissolving phthalic anhydride (113.5 [+ or -] 2.5 g) and imidazole (17 [+ or -] 1 g) in 700 mL of pyridine. An appropriate amount of bio-oil was esterified with 25 mL of phthalation reagent in a pressure bottle at 100[degrees]C for 20 minutes. After cooling to room temperature, 50 mL of pyridine and 10 mL of distilled water were added to the pressure bottles, and OHN was then quantitatively determined by titrating the esterified bio-oil with 0.5 N NaOH. OHN was defined as milligrams of potassium hydroxide (KOH) per gram of sample and calculated as follows:
OHN (mg KOH/g) = ([V.sub.B] - [V.sub.S]) x 56.1 x 0.5/W (3)
where [V.sub.B] and [V.sub.S] are the volume of NaOH consumed by blank (phthalation reagent only) and sample (bio-oil), respectively; W is the weight of sample (g); 56.1 is the equivalent weight of KOH (mg/meq); and 0.5 is the normality of NaOH.
Results and Discussion
RC was calculated in the range of 42 to 48 percent, depending on liquefaction time. As can be seen in Figure 2, the RC decreased from 47 to 42 percent at 90 minutes of liquefaction and then increased again to 48 percent as the liquefaction was prolonged to 120 minutes. Other researchers have observed the RC decrease followed by an increase as well (Celikbag et al. 2014, Wei et al. 2014). Two reactions take place during liquefaction: alcoholysis and recondensation (Zhang et al. 2006, Zou et al. 2009). In alcoholysis reactions, wood polymers are degraded by solvent to low-molecular-weight components. As the liquefaction time is prolonged, highly reactive degraded wood components react with each other, and nonsoluble high-molecular-weight components are produced as the result of recondensation reactions. Therefore, a decrease and then an increase in RC are attributable to the alcoholysis reaction and the recondensation reaction, respectively.
Our laboratory has previously studied the liquefaction of loblolly pine using a sealed Parr reactor where the RC was found to be 25 percent when liquefaction was carried out at 150[degrees]C for 60 minutes under 70 to 80 lb/[in.sup.2] of pressure (Celikbag et al. 2014). However, in this study, an atmospheric glass reactor was used for liquefaction, and the RC was calculated to be 47 percent (Fig. 2) at 150[degrees]C for 60 minutes. Although liquefaction in both a sealed Parr and a glass reactor was carried out at the same biomass-tosolvent ratio (1:4, wt/wt) and acid catalyst concentration (3%, wt/wt, based on EG) as a similar study (Celikbag et al. 2014), the glass reactor was determined to be less efficient and lower in RC. Similar trends have also been reported by other researchers who studied wood liquefaction using different reactors (Pan et al. 2007). In their study, the RC was found to be 70 and 50 percent when an atmospheric glass reactor and a sealed Parr reactor were used for liquefaction, respectively. Less RC when using a Parr reactor could be attributable to the pressurized effect, which facilitates the penetration of liquefying solvent through biomass so that more biomass undergoes decomposition, resulting in less residue (Vasilakos and Austgen 1985, Toor et al. 2013).
FTIR analysis of residues
Because woody biomass is a highly complex composite material composed of cellulose, hemicellulose, and lignin, as shown in Table 1, it has various functional groups in its structure. Therefore, many peaks exist in the IR spectra of pine and residues, as shown in Figure 3, and their respective peak assignments are presented in Table 2.
There was a significant difference between pine and residues in the intensity of spectral peaks at 1,736 [cm.sup.-1]. owing to the carbonyl (C=0) stretching of acetyl or carboxylic acid. Acetyl ester groups in xylan, the major carbohydrate in hemicellulose, is the main source of C=0 stretching (Perez et al. 2002, Pan et al. 2007); therefore, the peak at 1,736 [cm.sup.-1] is considered the characteristic peak of hemicellulose (Pan et al. 2007, Rana et al. 2010, Huang et al. 2012, Casas et al. 2013). As the liquefaction time was prolonged, it was observed that the intensity of peak at 1,736 [cm.sup.-1] decreased. Intensity reductions were most pronounced after 30 minutes and then continued to decrease significantly after 90 minutes of exposure time. This finding suggests that the longer liquefaction time yields more hemicellulose degradation products. This agrees with the literature where hemicellulose was reported as the wood polymer most subject to degradation under acidic conditions (Xiao et al. 2013). Lee et al. (2010) also reported that hemicellulose degraded faster than lignin and cellulose, as observed in our study. Longer residence times yielded additional decomposition, but at much slower rates. Other studies that characterized residues from liquefaction of cypress biomass in hot-compressed water found a complete disappearance of the peak at 1,722 [cm.sup.1] as liquefaction time and temperature increased. Their study likewise witnessed the disappearance of hemicellulose before lignin and cellulose when the residues were analyzed with FTIR (Liu et al. 2013).
Another significant change in the IR spectra was observed at the peaks at 1,508 [cm.sup.-1], which was because of the G=C stretching in the aromatic ring. The peak at 1,508 [cm.sup.-1] was considered the characteristic peak of lignin (Colom et al. 2003, Rana et al. 2010, Huang et al. 2012, Casas et al. 2013, Salehian et al. 2013). Lignin is also responsible for the peak at 1,367 [cm.sup.-1, which was attributed to phenolic OH groups (Gonzalez Alriols et al. 2009, Zhao et al. 2013). It was observed that the intensity of these two characteristic peaks of lignin at 1,508 and 1,367 [cm.sup.-1] decreased as the reaction was prolonged, indicating that the degradation of lignin increased during liquefaction. But the degradation rates observed for lignin appeared less pronounced than hemicellulose based on the spectrum in this analysis.
The peaks at 1,155, 1,101, 1,052, and 897 [cm.sup.-1] are assigned to cellulose (Table 2). As opposed to hemicellulose and lignin, the intensity of the peaks associated with cellulose was increased with liquefaction time. Most of the hemicellulose and lignin were decomposed during liquefaction, while the relative content of cellulose within the residues increased. The increase in cellulose content was indicative of higher resistance to degradation than hemicellulose and lignin in EG solvolysis because of (1) its highly crystalline structure and (2) the structure of the cell wall, where lignin and hemicellulose shield cellulose. Therefore, the peaks attributed to cellulose became more visible (sharper) in the IR spectra. Zheng et al. (2012) also reported that cross-linking of cellulose caused an increase in intensity at 1,156 [cm.sup.-1].
Degree of crystallinity index of residue
Degree of crystallinity of residue, which is defined as the weight fraction of crystalline material (crystalline cellulose) in residue (Jiang et al. 2007), was calculated by X-ray diffraction (XRD) analysis. The XRD patterns and the crystallinity index of residues are shown in Figure 4. There are typically three diffraction peaks of cellulose at 2[theta] of 15[degrees], 16.5[degrees], and 23[degrees] due to 101, 10[bar.I], and 002 reflections, respectively (Popescu et al. 2008); however, owing to the complex structure of wood, peaks of 101 and 10[bar.I] diffractions overlapped and generated a broad peak at around 2[theta] of 14[degrees] to 16[degrees].
The crystallinity index of loblolly pine before liquefaction was calculated to be 23 percent, as shown in Figure 4. After liquefaction, the crystallinity indices of residues were found to be 41, 42, 46, 42, and 41 percent for 30, 60, 90, 120, and 150 minutes of liquefaction, respectively. This dramatic increase in crystallinity index from 23 to 41 percent for just 30 minutes of liquefaction time may be attributable to hemicellulose degradation, which is an amorphous polymer and very sensitive to temperature. Via et al. (2013) reported that hemicellulose was the first depolymerized wood component under thermal treatment.
Thus, hemicellulose degradation in the early stage of liquefaction resulted in an increase in relative crystallinity. As the liquefaction was carried out for 90 minutes, the highest crystallinity index of 45 percent was obtained. As discussed in the FTIR analysis of residues, an increase in the degradation of lignin was observed as liquefaction was prolonged, using the decrease in intensity of characteristic peaks of lignin (1,508 and 1,367 [cm.sup.-1]) as an indicator. An increase in crystallinity index as a result of decomposition of amorphous components in wood has been observed by other researchers as well (Pan et al. 2007, Liu et al. 2013). Therefore, lignin and hemicellulose degradation likely contributed to the increase in the crystallinity index at 90 minutes of liquefaction because more amorphous components of wood were decomposed. It was observed that the crystallinity index of residues obtained from 120 and 150 minutes of liquefaction started decreasing. By definition, the crystallinity index is the weight fraction of crystalline material in residue; thus, a decrease in the crystallinity index could be interpreted as the indication of decomposition of the crystalline portion of the wood, which is mostly cellulose.
The authors are, respectively, Graduate Research Assistant and Associate Professor, Forest Product Development Center, School of Forestry and Wildlife Sci., and Center for Bioenergy and Bioproducts, Auburn Univ., Auburn, Alabama (firstname.lastname@example.org, email@example.com [corresponding author]). This paper was received for publication in December 2014. Article no. 14-00110.
OHN of bio-oil
Bio-oil is used to produce a variety of polymers, including phenolic resin, polyurethane, epoxy, and polyester, and OH groups in bio-oil are utilized to synthesize these polymers (Pan 2011). The reaction behavior of bio-oil based polymers is assumed to depend on the interaction between OH groups from the bio-oil and the formaldehyde (C[H.sub.2]O) for phenol-formaldehyde resin, isocyanate (R-N=C=O) for the polyurethane, the epoxide ([C.sub.2][H.sub.4]O) group for epoxy resin, and ester (-COOH) groups for polyester production. Therefore, characterization of the OH groups plays a crucial role in process optimization of bio-oil-based polymer synthesis.
The total OHN of bio-oils was quantitatively calculated by the esterification method. The OHN of pure EG was also calculated by the esterification method in order to confirm the accuracy of the method. OHN of EG was calculated to be 1,830 [+ or -] 10 mg KOH/g, which was quite close to the theoretical OHN value of EG, 1,808 mg KOH/g.
The total OHN of bio-oil from the liquefaction time treatments ranged from 1,230 to 1,427 mg KOH/g, as shown in Figure 5. Analysis of variance (single factor, 95% confidence interval, and three replications) was performed, and it was found that liquefaction time had a significant effect on OHN (P < 0.0001). The high OHN across the entire range was consistent with other studies (Budija et al. 2009, Kunaver et al. 2010, Wei et al. 2014). However, relatively lower OHN, such as 212 to 450 mg KOH/g, has also been reported by other researchers (Zou et al. 2012, D'Souza and Yan 2013, Xiao et al. 2013). Considerable differences in OHN may be attributable to solvent type, solvent-to-biomass ratio, heating source (microwave, heat bath, and autoclave), and liquefaction time and temperature. Celikbag et al. (2014) reported that unreacted liquefying solvent retained in the bio-oil was the major source for OH groups and accounted for 70 to 94 percent of the OHN, depending on liquefaction conditions. Therefore, the high OHN in this study could be attributable to the unreacted EG retained in bio-oil after liquefaction.
The first decrease in OITN was observed at 90 minutes of liquefaction (Fig. 5) and could be attributable to the biomass decomposition by EG (alcoholysis reaction). It was expected that a decreasing trend in the OHN as the liquefaction time was prolonged would be seen because of the alcoholysis reaction; however, this was not the case. We observed that OHN increased again at 120 minutes of liquefaction. An increase in OHN at prolonged liquefaction times was also reported by other researchers who conjectured that condensation reactions during liquefaction resulted in increased OEIN (Sun et al. 2011, Wei et al. 2014).
FTIR analysis of bio-oil
The IR spectra of pure EG and bio-oils were obtained from different reaction times and are shown in Figure 6. The IR spectra of bio-oils and EG were quite similar, indicating that EG was the dominant factor driving the distribution of reaction products of bio-oil (Fig. 6). The unreacted residual EG in the bio-oil could also cause similar IR spectra (Celikbag et al. 2014) as observed for bio-oil. Bio-oils are labeled BO-30, BO-60, BO-90, BO-120, and BO-150 for 30, 60, 90, 120, and 150 minutes of liquefaction, respectively, for this discussion. All bio-oils exhibited a broad peak at 3,293 [cm.sup.-1], which is because of the OH groups, indicating that all liquefaction oils contain a significant amount of OH groups, as evidenced by phthalic anhydride esterification. The peak at 1.736 [cm.sup.-1] was owing to the carbonyl (C=0) stretching groups. Oxidation of EG is the main reason for the generation of carbonyl groups containing compounds such as aldehydes, ketones, and carboxylic acids (Xiao et al. 2011). Levulinic acid, a compound that contains carbonyl and carboxyl group in its structure, was reported as the main component that contributed to intensity of the peak at 1,736 [cm.sup.-1] (Budija et al. 2009).
Compounds containing carbonyl groups are formed by the reaction of six carbon sugars (degradation products of cellulose or hemicellulose) with OH groups of the EG. Formation of carbonyls, therefore, is related to the consumption of OH groups. This phenomenon explains the inverse correlation between the intensity of peaks at 1.736 and 3,293 [cm.sup.-1] (attributed to OH stretching) that was observed in the IR spectra (Fig. 6). For example, BO-90 exhibited the lowest peak intensity at 3,293 [cm.sup.-1] and the highest peak intensity at 1,736 [cm.sup.-1]. Because more OH groups underwent oxidation reactions, a lower intensity was observed at the peak at 3,293 [cm.sup.-1]. Conversely, more OH consumption yielded more carbonyls, resulting in higher peak intensity at 1,736 [cm.sup.-1] for BO-90. Peak intensity of bio-oils at 1,736 [cm.sup.-1] shows the following order (Fig. 6): BO-30 < BO-60 < BO150 [approximately equal to] BO-120 < BO-90. This sequence was perhaps the consequence of condensation reactions. After 90 minutes of liquefaction, the intensity at 1.736 [cm.sup.-1] decreased, suggesting the presence of condensation reactions. One study showed that insoluble residues could be generated from hydroxymethyl furfural derivatives in the presence of water (Yamada et al. 2007). In fact, we observed an increase in RC after 90 minutes of liquefaction.
Summary of Results
Loblolly pine was liquefied in a glass reactor using EG at 150[degrees]C in the presence of acid catalyst for five different reaction times: 30, 60, 90, 120, and 150 minutes. RC was found to be in the range of 42 to 48 percent, depending on liquefaction time. The lowest RC was obtained from 90 minutes of liquefaction. The intensity of the hemicellulose peak (1,736 [cm.sup.-1]) in the IR spectra of residues was observed to be decreasing as the liquefaction time was prolonged, indicating significant hemicellulose degradation. XRD analysis of residues also supported the hemicellulose degradation in the early stages of liquefaction. The crystallinity index of residues was found to be 41, 42, 46, 42, and 41 percent for 30, 60, 90, 120, and 150 minutes of liquefaction, respectively. All bio-oils were found to be rich in OH groups attributable to the excess EG, which generated broad peaks at 3,293 [cm.sup.-1] in the IR spectra. OHN of bio-oil were calculated to be 1,230 to 1,427 mg KOH/g, depending on the liquefaction time.
This study shows that loblolly pine could be successfully converted to a liquid product called bio-oil using EG at 150[degrees]C. Bio-oil has the potential to be used as a bio-based polyol because of the high OHN. With continuous research, this field can provide a profitable and renewable resource that will reduce the US dependence on petrochemical polyols. Continued research must explore the source and variation of OHN found within the bio-oil as well as the effect of liquefaction parameters, such as temperature, pressure, and the biomass-to-solvent ratio, on OHN.
The Auburn University Intramural Grants Program is recognized for their start-up funding that allowed part of these data to be obtained. Part of the stipend for the graduate student was obtained from the School of Forestry and Wildlife Sciences (Auburn University) as matching funds for a grant obtained from the NSF Auburn IGERT: Integrated Biorefining for Sustainable Production of Fuels and Chemicals (NSF award no. 1069004). This work was supported by the Agriculture and Food Research Initiative (AFRI) CAP "Southeast Partnership for Integrated Biomass Supply Systems," which is exploring bio-oil as a fuel source (project no. TEN02010-05061). Regions Bank provided support, and their goal is to develop value-added products from low-value trees. The Forest Products Development Center is acknowledged for supplementary funding of materials and supplies. The Center for Bioenergy and Bioproducts is acknowledged for use of their facilities.
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Table 1.--Content of extractives, lignin, cellulose, and hemicellulose determined by wet chemistry, near-infrared (NIR), and Fourier transform infrared (FTIR) models. Content (%, wt/wt) Wet chemistry NIR FTIR Extractives 4.5 4.6 2.5 Lignin 25.8 28.3 28.3 Cellulose 48.5 43.1 46.1 Hemicellulose 31.3 28.9 26.6 Table 2.--Characteristic Fourier transform infrared bands of pine and residues obtained from different liquefaction times. Wavenumber ([cm.sup.-1]) Functional group Assignment 3,341 -OH stretch Cellulose, hemicellulose, lignin (alcohols, phenols) 2,916 -C-H, -CH2- stretching in Cellulose, methyl and methylene extractives group, hydrocarbon chains 1,736 C=O stretching of acetyl Hemicellulose or carboxylic acid (frequently carbohydrate origin) 1,599, 1,508 C=C stretching in Lignin aromatic ring 1,452 C-H bending Lignin 1,419 C-[H.sub.2] symmetric Cellulose bending 1,367 -OH (phenolic) Lignin 1,264, 1,230, 1,216 Vibration of guaiacyl Lignin ring 1,155 C-O-C asymmetric Cellulose stretching 1,101, 1,052 C-0 stretching Cellulose, hemicellulose 1,026 C-O-C stretching Cellulose 897 Out-of-phase ring Cellulose stretching Wavenumber ([cm.sup.-1]) References 3,341 Pan et al. (2007), Huang et al. (2012) 2,916 Huang et al. (2012), Salehian et al. (2013) 1,736 Pan et al. (2007), Rana et al. (2010), Huang et al. (2012), Casas et al. (2013), Salehian et al. (2013) 1,599, 1,508 Colom et al. (2003), Rana et al. (2010), Huang et al. (2012), Casas et al. (2013), Salehian et al. (2013), Shibata et al. (2013) 1,452 Bakirtzis et al. (2012) 1,419 Colom et al. (2003), Salehian et al. (2013) 1,367 Gonzalez Alriols et al. (2009), Zhao et al. (2013) 1,264, 1,230, 1,216 Li et al. (2010), Casas et al. (2013) 1,155 Colom et al. (2003), Salehian et al. (2013) 1,101, 1,052 Chenet al. (2011) 1,026 Pholosi et al. (2013) 897 Colom et al. (2003), Salehian et al. (2013)
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|Author:||Celikbag, Yusuf; Via, Brian K.|
|Publication:||Forest Products Journal|
|Date:||Jan 1, 2016|
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