Effects of reaction conditions on the phenol liquefaction of peanut shells

Han, S., Cui, J., Gan, L., and Zhou, X. (2019). "Effects of reaction conditions on the phenol liquefaction of peanut shells," BioRes. 14(1), 1899-1914.

Abstract

Peanut shells were liquefied in phenol using sulfuric acid as the catalyst. The effects of the liquefaction conditions, such as the phenol/peanut shells ratio, catalyst loading, reaction temperature, and time, on the residue ratio, percent combined phenol, and formaldehyde reactivity of the liquefied products were investigated. The results showed that increasing the phenol/peanut shells ratio decreased the residue ratio and increased the combined phenol and formaldehyde reactivity. Increasing the catalyst loading decreased the residue ratio and increased the combined phenol, but a higher catalyst loading caused the formaldehyde reactivity to decrease. A higher reaction temperature and prolonged reaction time did not lead to a further decrease in the residue ratio and it further increased the combined phenol and formaldehyde reactivity. Fourier transform infrared spectroscopy and gas chromatography-mass spectroscopy showed that the liquefied products contained a large amount of polyphenol compounds and a small quantity of furan compounds and furfural. The SEM images showed small particles were the main portion of the liquefaction products and cellulose fibers could still be clearly seen.

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Effects of Reaction Conditions on the Phenol Liquefaction of Peanut Shells

Shuguang Han,* Juqing Cui, Lu Gan, and Xiaoyan Zhou

Keywords: Peanut shells; Phenol liquefaction; Residue ratio; Combined phenol; Formaldehyde reactivity

Contact information: College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, China; *Corresponding author: hanshg@njfu.edu.cn

INTRODUCTION

Phenolic resins, the first synthesized thermosetting resins, are still widely used in several applications, such as wood-based panel adhesives, coatings, and high-performance composite matrices. Phenolic resins are generally synthesized from phenol and formaldehyde. Phenol as a raw material for phenolic resins is produced from non-renewable fossil resources through the cumene method and crude phenol refining technology. The world reserves of economically exploitable fossil resources are finite, and the global expected consumption of fossil resources is gradually increasing. The phenol supply is challenged by the declining reserves of fossil resources. Under these circumstances, switching from fossil resources to renewable resources may provide an alternative way to deal with the sustainable growth requirements of organic raw materials (Karagöz et al. 2004; Ragauskas et al. 2006). Thus, the development of phenol alternatives from renewable resources has attracted great interest from both academic and industrial sectors.

Two approaches have been considered for obtaining renewable phenol alternatives. One approach is to extract chemical components, such as tannins, from wood, bark, or other plant resources that are rich in plant polyphenols (Pizzi 2006; Ping et al. 2011; Zhang et al. 2017a). The other method is to convert renewable biomass via various thermochemical processes (phenolysis, pyrolysis, etc.) into “bio-phenols” (Feng et al. 2015). Phenolysis is a liquefaction process conducted in phenol with an acidic catalyst at a moderate temperature or without a catalyst at elevated temperatures to produce phenolic compounds (Feng et al. 2013). Acid-catalyzed phenol liquefaction of biomass is a well-known method with immense potential for substituting liquefied products for phenol to synthesize more environmentally-friendly phenolic resins (Alma et al. 1998; Alma and Basturk 2006; Ahmadzadeh et al. 2009; Pan et al.2009; Lee et al. 2012; Li et al. 2012; Zhao et al. 2013; Yan et al. 2017). The products from white birch bark phenol liquefaction have been successfully applied in the synthesis of bark-based phenol formaldehyde resole, and the substitution rate of phenol was up to 50% (Feng et al. 2016). Phenol formaldehyde-type moldings, which are produced by liquefying wood into phenolic compounds with use of inorganic and organic acidic catalysts, have physical and mechanical properties that are comparable to those of commercial phenol-formaldehyde-type moldings (Alma et al. 1996a,b).

As the largest peanut producer in the world, China produces 5 million ton of peanut shells each year (Wu et al. 2015). Peanut shells are by-products of peanut production and are usually used as animal feed or fuel without any economic value-added applications (Xu et al. 2017). Furthermore, because the collection and disposal of this residue is generally becoming more difficult and expensive, large quantities of peanut shells are left without any treatment or simply burned in fields, which leads to various environmental problems (Biswas et al. 2017), such as air pollution and CO₂ emissions. The direct combustion of such residues in fields contributes 5% to 12% of the particulate matter concentration in the air, which has contributed to the recent severe hazy weather in China (Yao et al. 2010).

Peanut shells can be used as a precursor material to prepare biochar or activated carbon for the removal of heavy metals and some organic pollutants from contaminated water (Ahmad et al. 2012; Zhong et al. 2012; Zhang et al. 2014; Wu et al. 2015; Georgin et al. 2016; Chu et al. 2017; Xu et al. 2017), for generating high energy density liquid fuel by pyrolysis as a substitute for fossil fuels (Zhang et al. 2011; Gurevich Messina et al. 2015; Gurevich Messina et al. 2017), and they can be liquefied to polyol by polyethylene glycol 400 and glycerol for use in preparing rigid polyurethane foams (Zhang et al. 2016; Zhang et al. 2017b). Additionally, peanut shells can be converted into a phenol alternative via various thermochemical conversions, such as pyrolysis (Alma and Kelley 2002; Mamaeva et al. 2017). However, all of these methods still cannot consume all of the excess peanut shells. Many attempts at phenolysis have established that almost all lignocellulosic biomass can be successfully liquefied in phenols (Alma and Acemioglu 2004). Compared with wood and bark, peanut shells are more abundant in lignin and polyphenols (Gao et al. 2011; Zhang et al. 2013), which makes them suitable for phenolysis (Effendi et al. 2008; Feng et al. 2013). However, there have only been a few studies that focused on the liquefaction of peanut shells in phenol. Considering the large annual output of peanut shells worldwide, it would be highly beneficial to explore phenol liquefaction as a method to convert this waste to a useful feedstock for large-scale industrial applications, such as making wood adhesives. Yet, it is still unclear how the reaction conditions affect the liquefaction yield and liquefied products when peanut shells are used.

Therefore, in this study, peanut shells were liquefied in phenol with an acid catalyst. The effects of different liquefaction reaction conditions on the residue ratio, percent combined phenol, and formaldehyde reactivity of the liquefied peanut shell fraction were investigated to better understand the efficacy of phenol liquefaction as a conversion technique for peanut shells.

EXPERIMENTAL

Materials

Peanut shells were purchased from a farmer’s market in Nanjing, China. They were washed with deionized water to remove impurities from the surface, dried in an oven at 105 °C for 12 h, then crushed and sieved through a 100-mesh sieve. The portion with particle size greater than 100-mesh was used for liquefaction. The peanut shells were found to contain 61.8 wt.% holocellulose measured according to the Chinese national standard GB/T 2677.10 (1995), 34.4 wt.% acid insoluble lignin measured according to the Chinese national standard GB/T 2677.8 (1994), 4.46 wt.% ethanol-toluene extractives measured according to the Chinese national standard GB/T 2677.6 (1994), and 3.45 wt.% ash measured according to the Chinese national standard GB/T 2677.3 (1993). Phenol, concentrated sulfuric acid (96%), formaldehyde solution (37%), methanol, and other chemicals were used as analytical reagents without further purification (Aladdin Company, Shanghai, China).

Methods

Preparation of the phenol liquefied peanut shells

The phenol and peanut shells were added to a three-necked flask equipped with a reflux condenser and stirrer. The reaction was performed in a heating oil bath at the settled temperature (140, 160, 180 °C) under atmospheric pressure. After the liquefaction reaction, the system was cooled down to room temperature using cold water.

To investigate the effect of different liquefaction reaction conditions on the residue ratio, percent combined phenol, and formaldehyde reactivity of the liquefied peanut shells, 100 g of peanut shells were used for each experiment and the experiments were conducted following an L₉ (3⁴) orthogonal design. The liquid to solid ratio (phenol/peanut shells, w/w) (A), catalyst loading (loading rate of sulfuric acid as a catalyst, based on the weight of the peanut shells, %) (B), liquefaction reaction temperature (°C) (C), and liquefaction reaction time (min) (D) were selected as the four factors with three levels for the orthogonal experiment. The orthogonal experiment factors and levels are shown in Table 1. The experiment scheme is shown in Table 2, and to assure the accuracy of the experimental data, each experimental group had three replicates.

Table 1. Factors and Levels for the Orthogonal Experiment of Phenol Liquefaction of Peanut Shells

Measurement of the residue ratio

The liquefied products were diluted using excessive methanol, and then they were vacuum filtered using a vacuum pump and millipore filter paper to separate the solid residue (methanol-insoluble part) from the methanol-soluble part. Simultaneously, the filter residue was washed in methanol until the filtrate ran clear. The methanol-insoluble residue was dried in an oven at 105 °C to a constant weight. The residue ratio was calculated using Eq. 1,

R = W_r / W_o × 100% (1)

where R is the residue ratio of the peanut shells in the liquefaction reaction (%), W_r is the oven-dried weight of the methanol-insoluble residue after liquefaction (g), and W_o is the original oven-dried weight of the peanut shells before liquefaction (g).

After the methanol was removed from the filtrate by distillation under vacuum, the remaining liquefied peanut shells were used for further analysis.

Measurement of the percent combined phenol of the liquefied peanut shells

The amount of free phenol remaining in the methanol-soluble shells was first measured by using high performance liquid chromatography (HPLC) (1260 Infinity, Agilent, Santa Clara, USA) with a DAD ultraviolet-visible detector (254 nm) and ZORBAX Eclipse plus C18 column (4.6 mm × 100 mm, 5 μm) ( Agilent, Santa Clara, USA). Measurements were done at 40 °C and a flow rate of 1.0 mL/min using a methanol/water solution (1/2, v/v) as the mobile phase. The methanol-soluble liquefied products were dissolved in methanol at a concentration of 0.1% and 10 mL were injected into the HPLC apparatus. Meanwhile, solutions of pure phenol at different methanol concentrations (0.1%, 0.2%, 0.3%, 0.4%, 0.5%, and 0.6%) were used as standard solutions for calibration purposes. The percent combined phenol was calculated using Eq. 2,

C_p = (P_o – P_f) / (P_o + W_o – W_r) × 100% (2)

where C_p is the percent combined phenol (%), P_o is the initial amount of phenol (g), and P_f is the amount of phenol remaining after liquefaction (g).

Measurement of the formaldehyde reactivity of the liquefied peanut shells

The formaldehyde reactivity of the liquefied peanut shells was determined using the hydroxymethylation reaction method. The liquefied peanut shells were hydroxymethylated by the reaction for 5 h at 60 °C with excessive formaldehyde in the presence of sodium hydroxide solution (50%, w/w) and a ratio of liquefied peanut shells to excessive formaldehyde to sodium hydroxide solution of 10:30:10 (g, w/w). Briefly, 10 g of the liquefied peanut shells was added to a 500 mL conical flask. 30 g of formaldehyde solution (37%, w/w) and 10 g of sodium hydroxide solution (50%, w/w) were added to the conical flask. The mixture was reacted in a thermostatic water bath through condensate reflux device at 60 °C for 5 h. After reaction finished, cooling the entire reaction system for 30 min, then using 150 mL of distilled water to swash glass wall and dilute the reaction system. The filtrate was collected through a funnel. The free formaldehyde content in the filtrate was determined by the sodium sulfite method, according to the Chinese national standard GB/T 14074 (2006). The formaldehyde reactivity was calculated using Eq. 3,

F_r = (F_e – F_f) / (P_o + W_o – W_r) × 100% (3)

where F_r is the formaldehyde reactivity of the liquefied peanut shells (%), F_e is the weight of the excessive formaldehyde (g), and F_f is the weight of the free formaldehyde (g).

FTIR measurements

The FTIR measurements of the peanut shell powder, liquefied peanut shells, and residue were done using an FTIR TENSOR 27 spectrometer (Bruker, Karlsruhe, Germany) over a frequency range of 4000 cm^-1 to 400 cm^-1. The samples were dried under vacuum freezing and the KBr pellet method was used for sample preparation.

GC-MS analysis of the liquefied peanut shells

The chemical composition of the liquefied peanut shells was analyzed using GC-MS (Agilent 7890A/5975C, Santa Clara, USA) with a 30-m × 0.25-mm × 0.25-μm capillary column (DB-5ms, Agilent, Santa Clara, USA). Helium (99.999 %) was used as the carrier gas with a constant flow of 1.0 mL/min. For all analysis, the samples were dissolved in a mixture of dichloromethane and methanol (1:1 v/v, p.a. grade) to a concentration of approximately 0.1 mg/mL from where 1.0 μL of this solution was introduced to the GC-MS. The oven was programmed to increase at a rate of 5 °C/min until it reached a final temperature of 250 °C, which was held for 20 min. The compounds were identified by comparison with the NIST17 library (reference).

SEM observation of the liquefaction products

Scanning electronic microscopy (SEM) (JSM-7600F, Hitachi, Tokyo, Japan) was used to examine the morphology of the liquefaction products. The samples were dried under vacuum freezing, and sputter coated with gold prior to scanning.

RESULTS AND DISCUSSION

Analysis of the Orthogonal Experiment

From Table 2, it was suggested that the liquid to solid ratio had the most impact on the residue ratio and formaldehyde reactivity. The catalyst loading was found to markedly affect the percent combined phenol. Furthermore, it was concluded from the range analysis that the liquefaction conditions for the minimum residue ratio, maximum percent combined phenol, and maximum formaldehyde reactivity were A₃B₃C₂D₃, A₃B₃C₃D₃, and A₃B₂C₂D₂, respectively. From the point of view of substituting phenol for phenolic resin synthesis, A₃B₂C₂D₂seemed to be the most reasonable liquefaction conditions, whose levels were a liquid to solid ratio of 4, catalyst loading of 8%, liquefaction reaction temperature of 160 ℃, and liquefaction reaction time of 80 min.

Effect of the Reaction Conditions on the Residue Ratio, Percent Combined Phenol, and Formaldehyde Reactivity

Residue ratio

The effect of the phenol to peanut shells ratio on the residue ratio from liquefaction is shown in Fig. 1 (line A). The residue ratio consistently decreased as the phenol to peanut shells ratio increased from 2 to 4, and a remarkable reduction in the residue ratio was observed when the phenol to peanut shells ratio increased from 2 to 3. When the phenol to peanut shells ratio exceeded 3, the level of reduction was not notable, even though the residue ratio decreased further. The same trend also appeared during wood liquefaction in phenol (Lin et al. 1994; Pan et al. 2007; Zhao et al. 2013). The peanut shell components underwent decomposition, phenolation, and recondensation during liquefaction. When the liquid ratio was lower than a certain value and as the liquefaction reaction time increased, recondensation of the liquefied peanut shell components became the preferable reaction, rather than phenolysis. Meanwhile, decomposition and recondensation are competing reactions, and a higher amount of phenol used in the liquefaction retarded the recondensation reactions among the degraded components and prevented the formation of new larger molecular weight insoluble residues (Lin et al. 1997).

Table 2. L₉ (3⁴) Orthogonal Experiment Scheme and Results

A, B, C, and D denote liquid to the solid ratio, the catalyst loading, the liquefaction reaction temperature and the liquefaction reaction time, respectively; R, C_p, and F_r denote the residue ratio, the percent combined phenol and the formaldehyde reactivity, respectively; Values are the mean ± standard deviation of three replicates; ^a K1^R, K2^R, and K3^R denote the average residue ratio for levels 1, 2, and 3, respectively; R^R denotes the residue ratio range; ^b K1^P, K2^P, and K3^P denote the average percent combined phenol of levels 1, 2, and 3, respectively; R^Pdenotes the range of percent combined phenol; ^c K1^F, K2^F, and K3^F denote the average formaldehyde reactivity of levels 1, 2, and 3, respectively; R^F denotes the range of formaldehyde reactivity.

The effect of the catalyst loading on the residue ratio from liquefaction is shown in Fig. 1 (line B). When the catalyst loading increased, the residue ratio decreased gradually. A higher catalyst loading promoted the extent of liquefaction through hydrolysis and degradation of the cellulose and hemicellulose, but recondensation between the phenol and decomposed products occurred at a rather high catalyst dosage, which led to the residue ratio decreasing slowly.

Fig. 1. Effects of the reaction conditions on the residue ratio of the peanut shell liquefaction

Figure 1 (line C) shows the relationship between the residue ratio and liquefaction temperature. When the liquefaction temperature was increased from 140 °C to 160 °C, the residue ratio decreased noticeably and reached a minimum value. When the liquefaction temperature was increased from 160 °C to 180 °C, the residue ratio increased slightly. It was reported that the recondensation reaction among the decomposed components might have occurred at 180 °C, which could have affected the residue ratio (Pan et al. 2007).

Figure 1 (line D) shows that with an increasing liquefaction reaction time, the residue ratio decreased, especially from 60 min to 80 min. When the reaction time was increased from 80 min to 100 min, there was only a minor change in the residue ratio. The liquefaction efficiencies were not remarkably improved by increasing the liquefaction time when the liquefaction reaction time reached a reasonable value.

Percent combined phenol

Figure 2 (line A) shows that the percent combined phenol increased as the phenol to peanut shells ratio increased. Higher phenol/peanut shells ratios promoted liquefaction and retarded the secondary condensation reaction of the decomposed components, which resulted in more percent combined phenol after liquefaction. For the acid-catalyzed phenol liquefaction, the phenol acted not only as a liquefaction solvent, but also as a cross-linker for the degraded components (Zhang et al. 2006). Figure 2 (line B) shows that the catalyst loading was the most effective factor that affected the percent combined phenol, as it increased remarkably when the catalyst loading increased.

A higher catalyst loading promoted liquefaction of the lignin and hydrolysis of the hemicellulose and cellulose. The acid catalyst not only catalyzed the liquefaction reaction, but it also played a major role in accelerating recondensation reactions among the degraded components and phenol. Figure 2 (lines C and D) shows that the reaction temperature and reaction time had a similar trend effect on the percent combined phenol. With higher reaction temperatures and longer reaction times, the phenol reacted more with the peanut shell components during liquefaction. The percent combined phenol increased gradually with an increasing reaction temperature and time.

Fig. 2. Effects of the reaction conditions on the percent combined phenol from peanut shell liquefaction

Formaldehyde reactivity

The formaldehyde reactivity of the liquefied products was determined to formulate resins that can assist in the preparation of bio-based phenolic resins. Figure 3 (line A) shows that the formaldehyde reactivity of the liquefied peanut shells linearly increased as the phenol to peanut shells ratio increased. Because phenol was added as the liquefaction solvent, the amount of formaldehyde consumed by the reaction accounted for the phenol content, which had three reactive sites. The formaldehyde consumption levels generally agreed with the amount of phenol used during liquefaction (Hassan et al. 2009).

Figure 3 (line B) shows the relationship between the formaldehyde reactivity and catalyst loading. When the catalyst loading increased from 6% to 8%, the formaldehyde reactivity increased remarkably and reached a maximum value. When the catalyst loading increased from 8% to 10%, the formaldehyde reactivity decreased. Liquefaction broke down the lignin and carbohydrates, and the acid catalyst played a major role in accelerating recondensation reactions among the degraded components and phenol. There was the possibility of some phenol reacting with the liquefaction products and deriving a high molecular weight polymer that was not be an active site for formaldehyde reaction. For that reason, higher reaction temperatures and prolonged reaction times had similar trend effects on the formaldehyde reactivity.

Fig. 3. Effects of the reaction conditions on the formaldehyde reactivity of peanut shell liquefaction

FTIR analysis

Figure 4 shows three FTIR spectra of the raw peanut shells, liquefied peanut shells, and residue. The OH and CH₂ group stretching vibrations had peaks at 3442 cm^-1 and 2925 cm^-1, respectively. Stretching vibration peaks at 1635 cm^-1 and 1124 cm^-1 were attributed to C=O and C-O bonds present in the celluloses, respectively, and the sharp peak indicated some residual celluloses that were un-liquefied in the phenol might have been present in the liquefaction products and residue.

The absorption bands at 1511 cm^-1, 1452 cm^-1, and 831 cm^-1 representing benzene rings were seen in spectra a and b, but completely disappeared in spectrum c, which indicated that all of the lignin was sufficiently liquefied (Long et al. 2015). The band at 1382 cm^-1 was associated with crystalline cellulose, and the intensity of this band was more pronounced in spectra a and c compared with in spectrum b, which may have been because of the high cellulose content.

The absorbance peaks at 1228 cm^-1 and 1035 cm^-1 represented the C-O-C unit attributed to aryl-alkyl ether linkages in the lignin. The absorbance peak at 1170 cm^-1 was attributed to the skeletal pyranose ring with C–O–C stretching vibration (Yang et al. 2007). The peaks at 752 cm^-1 and 693 cm^-1 represented C-H in the benzene rings and were seen in spectrum b, which when combined with the absorption peak at 3442 cm^-1, indicated that the liquefaction reaction led to the degradation of lignin and that the liquefaction products consist of polyphenols.

Fig. 4. FTIR spectra of the raw peanut shells, liquefied peanut shells, and residue; liquefaction conditions: phenol/peanut shells ratio = 4, catalyst loading = 8%, reaction temperature = 160 °C, and reaction time = 80 min

GC-MS analysis of the liquefied peanut shells

The products from liquefaction of the peanut shells were determined using GC-MS. The results are presented in Table 3 and show that more than 20 kinds of aromatic products were detected, among which the phenolic monomer was dominant. Aromatic acid ethylester, ketone, furan compounds, furfural, and ester were also detected. The results listed in Table 3 indicated that all of these aromatic structural units originated from liquefaction of the lignin, the furan compounds came from liquefaction of the celluloses, and the presence of furfural was attributed to liquefaction of the hemicelluloses.

SEM observation of the liquefaction products

Figure 5 shows SEM images of the liquefaction products generated with a phenol/peanut shells ratio of 4, catalyst loading of 8%, reaction temperature of 160 °C, and reaction time of 80 min. Small particles were the main portion of the liquefaction products and the cellulose fibers could still be clearly seen. It was interpreted that the liquefaction conditions dissolved lignin preferentially, which was the most sensitive part of phenol liquefaction. Meanwhile, the cellulose, considered to be the hardest part of phenol liquefaction, became part of the residues.

Table 3. GC-MS Spectrum Analysis of the Liquefied Peanut Shells

Liquefaction conditions: phenol/peanut shells ratio = 4, catalyst loading = 8%, reaction temperature = 160 °C, and reaction time = 80 min

Fig. 5. SEM micrographs of the liquefaction products; liquefaction conditions: phenol/peanut shells ratio = 4, catalyst loading = 8%, reaction temperature = 160 °C, and reaction time = 80 min

CONCLUSIONS

In this study, peanut shells were effectively liquefied in phenol using sulfuric acid as a catalyst.

The results showed that the reaction conditions had different effects on the residue ratio, combined phenol, and formaldehyde reactivity.
During the liquefaction process, the lignin and hemicellulose were converted to methanol-soluble compounds, while less cellulose fiber was left in the residues.
The liquefied products contained a large amount of polyphenol compounds and a small quantity of furan compounds and furfural. This liquefaction process provides a potential approach to generate high-quality chemical feedstocks, which can be used directly to prepare bio-based phenolic resins.

ACKNOWLEDGMENTS

This project was supported by the National Natural Science Foundation of China (Grant Number 31470590).

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Article submitted: October 15, 2018; Peer review completed: January 1, 2019; Revised version received and accepted: January 15, 2019; Published: January 18, 2019.

DOI: 10.15376/biores.14.1.1899-1914