Comparative Study of Effective Pretreatments on the Structural Disruption and Hydrodepolymerization of Rice Straw

Abstract

Rice straw (RS) is the most potentially renewable agricultural waste resource widely distributed in nature. Considering the complex recalcitrant structure and components of RS, three pretreatment methods, including high-temperature hydrothermal, medium-temperature microwave, and low-temperature cryocrushing pretreatment were performed. The components and structure of RS residues were examined and analyzed after the pretreatments. Pretreatment with hydrothermal yielded the lowest rice straw recovery (59.0%); after being pretreated at 180 °C for 10 min, the hemicellulose recovery was only 14.1%, and the removal efficiency of lignin was the largest (41.3%), which was 32.2% and 18.8% higher than that achieved from cryocrushing and microwave pretreatment, respectively. Pretreatment with cryocrushing yielded the highest recovery rates of rice straw (92.9%), hemicellulose and cellulose (88.8% and 90.4%, respectively). Results of scanning electron microscopy, X-ray diffraction, Fourier-transform infrared spectroscopy, and the analysis of specific surface area and apertures demonstrated that all three pretreatments could effectively disrupt the structure of RS and reduce its cellulose crystallinity. The three pretreatments were found to enhance the hydrodepolymerization of RS residues. Furthermore, cryocrushing pretreatment yielded the highest cellulose conversion rate (56.8%), and the yields of glucose, xylose, and arabinose were 29.6%, 56.2%, and 17.8%, respectively. Apart from the use of acids and enzymes, hydrodepolymerization of RS was among the few methods that can effectively degrade cellulose, presenting an ideal solution for the degradation of biomass.

1. Introduction

With increasing economic development, the depletion of fossil resources has become a crucial problem; thus, the search for alternative renewable resources has gained tremendous focus [1,2]. With a global production of 370–520 million tons per year, rice straw (RS) is the most potential renewable agricultural waste resource [3,4]. RS is widespread with abundant natural polymers, which are primarily composed of cellulose (32–47%), hemicellulose (19–27%), lignin (5–24%), and small amounts of ash and protein [5,6]. Compared to wooden materials, RS has a low content of lignin and sufficient content of holocellulose (cellulose and hemicellulose) that can be converted into relevant sugars just as corn straw, wheat straw, rape stalk, bagasse and other agricultural wastes [7]. The three primary components of cellulose, hemicellulose, and lignin form a macromolecular network resulting in a three-dimensional recalcitrant structure. More specifically, the outer layer of straw contains lignin, which is intertwined with hemicellulose wrapped with cellulose inside to form a hard shell of lignocellulose [8,9,10]. Although the rigid shell protects plants from external factors, it is also an obstruction in the utilization of lignocellulose, hindering the entry of catalysts and violently limiting their effective performance [11,12].

Because the direct degradation and depolymerization of RS are difficult, pretreatment of RS before the degradation and transformation processes is indispensable [13,14]. Effective pretreatment can facilitate the degradation of the dense RS network structure, either by eliminating the lignin, reducing the crystallinity of cellulose, or both. This will prove conducive to the performance of a catalyst and to the depolymerization of hemicellulose and cellulose into oligosaccharides or monosaccharides, which can be directly used as valuable chemical and fuel resources such as bioethnol, biogas, biohydrogen, and so forth [15,16,17,18,19]. The current pretreatment and degradation methods include physical methods (ball milling [20], microwaving [21], and ultrasonication [22]), physical and chemical methods (steam explosion [23], supercritical fluid extraction [24], and hydrothermal methods [25]), chemical methods (acids [26], alkaline chemicals [27], ionic liquids [28], organic solvents [29], deep eutectic [30] and metal salts [31]), and biological methods (microorganisms [32] and enzymes [33]). However, some limitations for certain pretreatment methods have been reported: the high cost of reagents in pretreatment with chemical ionic liquids, high investments for the equipment required in ultrasonication, steam explosion, and supercritical pretreatments, consumption of high levels of mechanical energy in the ball milling pretreatment, secondary pollution caused by pretreatments with chemical acids, alkali reagents, organic solvents, and metal salts, or demanding process controls during pretreatments with microorganisms and enzymes [34]. Among these methods, hydrothermal pretreatment (HP) has been reported to be economical and ecofriendly; the water undergoes self-ionization to generate protons that cleave the acid-labile glycosidic bonds, resulting in a more acidic medium, contributing to the further depolymerization of hemicellulose and removal of lignin, increasing cellulose accessibility in the biomass [35]. Microwave pretreatment (MP) is a convenient and high-speed method. Microwave heat disrupts the waxy surface, degrades the lignin–hemicellulose complex, partially dissolving the lignin and hemicellulose in RS, which is beneficial for subsequent hydolyzation [36]. Cryocrushing pretreatment (CP) is a novel method of freezing materials and crushing them into powders using sub-zero temperatures. Dehydration using liquid nitrogen renders plant cell walls brittle and destroys the recalcitrant structures [37].

This article describes three pretreatment methods to degrade the original structure of RS, including hydrothermal, microwave, and cryocrushing methods. The effects of pretreatment methods on the physical structure and chemical composition of RS were compared using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and analysis of the specific surface area and apertures. After pretreatments, the recycled contents of RS residues and feed solution were analyzed. The subsequent depolymerization and degradation of RS residues was conducted under H₂ atmosphere using a novel hydrodepolymerization method.

2. Materials and Methods

2.1. Materials and Reagents

The raw RS used in this study was obtained from Nanjing, Jiangsu Province. RS stalks were cut into small segments of 1–5 cm, washed with tap water to remove the ash on the surface, placed in an oven to dry at 100 °C for 6 h, and then crushed using a pulverizer. Subsequently, various particle sizes of the crushed raw materials were selected and individually packed in sealed bags. The starch content in the straw has been reported to affect the degradation of sugars [38]; thus, the rice spikes containing starch were removed from the harvested straw for future research. All chemical reagents including potassium sodium tatrate, 3,5-dinitrosalicylic acid and sodium hydroxide were obtained from Sinopharm Chemical Reagent Co. Ltd. and were all analytical reagent; deionized water was purchased from Wahaha Group Co., Ltd., Hangzhou, China.

2.2. Pretreatment of Rice Straw

2.2.1. Hydrothermal Pretreatment

Deionized water and 20 g of RS were weighed and added to the reactor (0.25 L, Weihai Chemical Machinery, Weihai, China). The solid–liquid ratio of straw to water was 1:10, the temperature was set at 100 °C–200 °C, the heating rate was approximately 2 °C/min, the pretreatment time was set to be 5 min–30 min, and the stirring speed was 400 r/min. The feed solution was filtered after the reaction was bundled. The remaining residue was washed with deionized water until colorless, dried in an oven at 100 °C, and sealed in a bag until further use. The filtrate and filter residue were characterized and analyzed, and the changes in the physical structure and chemical composition of RS after hydrothermal pretreatment were analyzed. The strength coefficient logR₀ was used to describe the comprehensive influence of pretreatment temperature and time and was calculated as shown in Equation (1) [39].

$$R_0=t\times \exp \frac{T-100}{\omega }$$

(1)

t: pretreatment time, min; T: pretreatment temperature, °C; ω: 4.6, empirical constant.

2.2.2. Microwave Pretreatment

RS (20 g) was pretreated for 5–30 min using 65 W–650 W microwave power in a microwave reactor (WBFY201, Yuhua Instrument Co., Ltd., Zhengzhou, China). Subsequently, the reaction feed solution was filtrated and measured, the filter residue was washed with deionized water until colorless, and the filter cake was dried at 100 °C for 6 h. The pretreated samples obtained using the microwave method were characterized and analyzed.

2.2.3. Cryocrushing Pretreatment

For the freezing pretreatment, 20 g of RS was sealed using tin foil and immersed in liquid nitrogen for different time intervals. RS was then taken out and ground fully using a mortar. Liquid nitrogen was poured into the mortar while grinding to ensure that the straw in the mortar was continually immersed in liquid nitrogen. The straw was then washed with deionized water. Ash and other substances were filtered from the extract after liquid nitrogen treatment. The filter cake was then dried at 100 °C for 6 h and sealed for further use.

2.2.4. Hydrodepolymerization of Pretreated RS Residues

The hydrodepolymerization of pretreated RS residues was performed according to our previous study [40]. RS (10 g) pretreated using different methods was added to the reactor and subjected to hydrogen leaching for 3 h at 160 °C, wherein the solid-to-water ratio was 1:15, rotating speed was 600 r/min, and the ratio of catalyst to residue mass was 3:10.

2.2.5. Analysis and Characterization of RS Hydrolyzates

The primary components of RS, including cellulose, hemicellulose, lignin, ash, and ethanol extract, were analyzed using the National Renewable Energy Laboratory’s method [41] in triplicates. The recovery of RS, cellulose, hemicellulose, and the removal of lignin were calculated as shown in Equations (2)–(5).

$$RS\mathrm{recovery}=\frac{m_{{}_{RSresidue}}}{m_{{}_{RS}}}\times 100\%$$

(2)

$$C\mathrm{ellulose}\mathrm{recovery}=\frac{m_{{}_{R\mathrm{S}}{}_{\mathrm{recovery}}}\times C_{{}_{R\mathrm{S}}{}_{\mathrm{recovery}}}}{m_{cellulose}}\times 100\%$$

(3)

$$H\mathrm{emicellulose}\mathrm{recovery}=\frac{m_{{}_{R\mathrm{S}}{}_{\mathrm{recovery}}}\times H_{{{}_{R\mathrm{S}}}_{\mathrm{recovery}}}}{m_{Hemicellulose}}\times 100\%$$

(4)

$$L\mathrm{ignin}\mathrm{removal}=(1-\frac{m_{{{}_{R\mathrm{S}}}_{\mathrm{recovery}}}\times L_{{}_{R\mathrm{S}}{}_{\mathrm{recovery}}}}{m_{Lignin}})\times 100\%$$

(5)

C_{RS recovery}, H_{RS recovery}, and L_{RS recovery} denote the cellulose, hemicellulose, and lignin contents in the pretreated RS residue. After hydrodepolymerization, the RS residues were dried in an oven at 100 °C for 6 h. The conversion rate of RS and its cellulose was calculated as shown in Equation (6).

$$Conversion=\frac{m_c-m_a}{m_c}\times 100\%$$

(6)

where m_c (g) is the dry weight of RS, cellulose, or hemicellulose in RS before the reaction; m_a (g) is the dry weight of RS, cellulose, or hemicellulose in RS residues after the reaction.

High-performance liquid chromatography (HPLC) (LC-20AT, Shimadzu, Kyoto, Japan) was used to determine the concentration of the sugars in the reaction solution. An RID-10A differential refraction detector and HPX-87H (300 mm × 7.8 mm, Bio-Rad, Hercules, CA, USA) column were equipped with a column temperature of 50 °C. Deionized water (injection volume of 10 µL) was used as the mobile phase and had a flow rate of 0.6 mL/min. Each sample was injected three times in parallel, and the average value was recorded. Prior to HPLC injection, the reaction liquid samples were all filtered and passed through a 0.45 µm water filter. The corresponding peak area of sugar was determined using HPLC. According to the standard curve regression equation of various sugars, the concentration levels of glucose, xylose, and arabinose were calculated (oligosaccharides and unexpected substances were excluded from this study). The yield was calculated based on the location of the cellulose or hemicellulose. Sugar yield was calculated as shown in Equation (7).

$$\mathrm{Monosaccharide}yield=\frac{m_{\mathrm{m}}}{m_{\mathrm{r}}}\times 100\%$$

(7)

where m_r (g) is the mass of cellulose or hemicellulose in the RS before the hydroreaction; m_m (g) is the mass of monosaccharide obtained after hydroreaction.

Reduction sugar contents were analyzed using the 3,5-dinitrosalicylic acid method as reported previously [42], and reduction sugar yield was calculated according to Equation (8).

$$Reductionsugaryield=\frac{C_{Reductionsugar}\times V}{m_{Ricestraw}}$$

(8)

where C_{Reduction sugar} (g/L) is the concentration of the total reduction sugar in the RS; V is the volume of reaction; m_{Rice straw} (g) is the mass of RS.

The conductivity, total dissolved solids (TDS), and salinity of solution after HP and MP pretreatments were examined using a conductivity meter (DDSJ-308A, INESA Sci. Inst. CO., Ltd., Shanghai, China).

2.2.6. Characterization of the Straw Residue

SEM (SEM-EDS; JSM-5900, Electronics Co., Tokyo, Japan) was used to study the microphysical characteristics of RS and residue surfaces. The samples were first fixed to a needle-type sample holder with carbon-coated conductive tape and were coated with gold for approximately 20 min. Images of random points were captured under 15 kV voltage for element determination.

The crystallinity of RS samples was also analyzed using XRD (Scientific K-Alpha, Thermo, Waltham, MA, USA). Test conditions included: Cu λα radiation source (λ = 0.1542 nm), tube voltage 40 kV, tube current 100 mA, scanning range (2 θ) 10°–30°, and scanning speed 2°/min. The sample crystallinity (CrI) was calculated as follows:

$$CrI=\frac{I_{002}-I_{am}}{I_{002}}\times 100\%$$

(9)

I₀₀₂ and I_am of cellulose in straw and residues represented the crystallization strength of the sample (2 θ = 22.0°) and the amorphous part (2 θ = 15.3°) [43].

FTIR (Nicolet 380, Thermo, Waltham, MA, USA) was used to detect the changes in the functional groups of the straw and its residue samples obtained after hydrogenation. The RS and residues were pressed with KBr, and FTIR spectra were recorded in the range of 4000–400 cm⁻¹.

The specific surface area and aperture analyzer (JW-BK122W, Beijing Jingwei Gaobo Instrument Co., Ltd., Beijing, China) were used to measure the RS and residues. With the presence of He and N₂, the temperature was increased to 100 °C at a rate of 10 °C/min, and the samples was stored for 120 min to eliminate other impurities. Meanwhile, the samples were vacuumed for 2 h to below 0.008 kPa, and after cooling to room temperature, liquid nitrogen (approximately −196 °C) was added into the cold liquid nitrogen well for analysis of the specific surface and apertures.

3. Results and Discussion

3.1. Effects of Hydrothermal Pretreatment on RS

Effects of the strength coefficients corresponding to temperature and time on the high-temperature hydrothermal pretreatment of RS were investigated (

Table 1. Effects of temperature and time on the hydrothermal pretreatment of RS.

Pretreat Condition (°C, min)	logR0	RS Recovery (%)	Cellulose Recovery (%)	Hemicellulose Recovery (%)	Lignin Removal (%)	Cellulose (%)	Hemicellulose (%)	Lignin (%)	Reduction Sugar Yield (%)
RS	_	_	_	_	_	37.8 ± 0.3	20.5 ± 0.3	19.0 ± 0.2	_
100, 5	0.7	87.2	90.9	86.8	9.6	39.4 ± 0.3	20.4 ± 0.7	19.7 ± 0.4	0.6
100, 10	1.0	86.9	89.0	83.9	11.7	38.7 ± 0.2	19.8 ± 0.5	19.3 ± 0.3	0.8
100, 30	1.5	86.2	88.3	79.9	13.8	38.7 ± 0.2	19.0 ± 0.4	19.0 ± 0.3	0.9
120, 5	2.6	86.0	88.0	79.3	14.4	38.7 ± 0.3	18.9 ± 0.5	18.9 ± 0.1	0.9
120, 10	2.9	85.5	87.8	78.8	15.9	38.8 ± 0.5	18.9 ± 0.2	18.7 ± 0.3	1.0
120, 30	3.4	84.9	87.6	78.7	19.1	39.0 ± 0.6	19.0 ± 0.5	18.1 ± 0.3	1.0
140, 5	4.5	84.7	86.5	74.0	23.8	38.6 ± 0.9	17.9 ± 0.7	17.1 ± 0.1	1.8
140, 10	4.8	84.0	87.6	69.2	25.3	39.4 ± 1.0	16.9 ± 0.5	16.9 ± 0.8	2.0
140, 30	5.3	81.8	85.5	69.8	25.9	39.5 ± 0.2	17.5 ± 0.3	17.2 ± 0.4	1.8
160, 5	6.4	80.0	87.4	63.6	27.2	41.3 ± 1.0	16.3 ± 0.8	17.3 ± 0.5	5.1
160, 10	6.7	78.2	86.1	61.4	25.5	41.6 ± 0.8	16.1 ± 0.7	18.1 ± 0.2	5.6
160, 30	7.0	73.5	84.6	58.1	26.1	43.5 ± 0.5	16.2 ± 0.4	19.1 ± 0.8	5.2
180, 5	8.3	62.2	79.5	22.5	36.2	48.3 ± 0.4	7.4 ± 0.1	19.5 ± 1.0	8.4
180, 10	8.6	59.0	74.6	14.1	41.3	47.8 ± 0.8	4.9 ± 0.0	18.9 ± 0.7	14.9
180, 30	9.0	58.7	75.9	9.5	40.1	48.9 ± 1.0	3.3 ± 0.0	19.4 ± 0.5	8.8
200, 5	10.1	58.3	75.6	9.1	50.0	49.0 ± 0.2	3.2 ± 0.1	16.3 ± 0.3	9.0
200, 10	10.4	54.8	71.5	4.8	53.3	49.3 ± 0.5	1.8 ± 0.0	16.2 ± 0.4	14.6
200, 30	10.9	54.1	72.6	4.0	54.2 ±	50.7 ± 1.1	1.5 ± 0.0	16.1 ± 0.6	5.8

).

The recovery of RS varied greatly with ranging strength coefficients (0.7–10.9) that corresponded to different temperatures (100 °C–200 °C) and time intervals (5 min–30 min). Thus, strength coefficients corresponding to temperature and time had a great impact on the recovery of RS after hydrothermal pretreatment. With an increase in the strength coefficient after hydrothermal pretreatment, RS recovery gradually decreased. The recovery efficiency of RS was >80% when the strength coefficient was in the range of 0.7–6.4. When the strength coefficient ranged from 6.4 to 8.6, the recovery of RS decreased to 59.0% by 21.0% because of an increase in the percentage of substances that dissolved in the solvent, which was consistent with the reduction of RS recovery in solid residue [39]. The recovery of RS was similar to the results of Imman et al. [44], and they believed that increasing mass loss during pretreatment was attributed to the increase in the solubility of RS polysaccharides. However, the RS recovery was as low as 54.1% when the strength coefficient increased from 8.6 to 10.9. This may be due to the gasification of straw and the formation of gaseous compounds during pretreatment under harsh conditions [45].

At a pretreatment temperature of 180 °C, treatment time of 10 min, and strength coefficient of 8.6, the removal of lignin was 41.3%. Simultaneously, the recovery of hemicellulose and cellulose were 14.1% and 74.6%, respectively, and reduction sugar yield reached the highest value, 14.9%. This indicated that the pretreatment temperature and time had a greater impact on the removal of hemicellulose and lignin than on cellulose. The removal efficiency of hemicellulose reached 85.9%, which was similar to the results of 85.0% reported by Yu et al. [46] at the pretreatment temperature of 180 °C for 30 min. Santucci et al. [47] also demonstrated a hemicellulose removal efficiency of 94.5% under hydrothermal pretreatment at 190 °C for 67 min. Li et al. [48] studied the effect of hydrothermal pretreatment on corn straw and reported that the maximum hemicellulose recovery efficiency was 14.0% during pretreatment performed at 210 °C for 25 min. These results corresponded with those of Jiang et al. [49], who reported that when straw was pretreated with hot water, the overheated water ionized into H⁺, promoting the cleavage of the ester bond of the acetyl side chain in hemicellulose, generating acetic acid in situ. This acid, in its free state, catalyzed the hydrolysis of hemicellulose, altered the structure of lignocellulose, and increased catalyst accessibility to cellulose. Therefore, they also hypothesized that the pretreatment temperature had a larger impact on the hemicellulose than on the cellulose in the straw [49]. Under the pretreatment condition of 180 °C for 10 min, the content of cellulose in the solid residue increased from 37.8% to 47.8%, but the recovery efficiency of cellulose decreased to 74.6%, indicating that the increase in the content may be attributed to the sharp decrease in hemicellulose content from 20.5% to 4.9%. Zhang et al. [50] also studied the relationship between hemicellulose and cellulose under different conditions of temperature and time during the hydrothermal pretreatment of wheat straw; as hemicellulose was hydrolyzed, the cellulose content increased significantly.

In summary, the appropriate temperature and time for hydrothermal pretreatment was 180 °C for 10 min in order to achieve subsequent depolymerization and degradation.

3.2. Effects of Microwave Pretreatment on RS

3.2.1. Effects of Microwave Power on Microwave Pretreatment of RS

The effects of microwave power on microwave pretreatment were studied under the following conditions: the solid–liquid ratio of straw and water was 1:10, and the treatment time was 30 min (

Table 2. Effect of microwave power on microwave pretreatment of RS.

Power (W)	RS Recovery (%)	Cellulose Recovery (%)	Hemicellulose Recovery (%)	Lignin Removal (%)	Cellulose (%)	Hemicellulose (%)	Lignin (%)	Reduction Sugar Yield (%)
65	88.1	83.4	78.2	15.1	35.8 ± 0.8	18.2 ± 0.0	18.3 ± 0.2	1.0
195	87.9	83.0	73.8	19.1	35.7 ± 0.4	17.2 ± 0.3	17.6 ± 0.5	1.1
325	87.8	82.5	72.4	19.6	35.5 ± 0.5	16.9 ± 0.1	17.4 ± 0.2	1.2
455	87.7	82.8	71.4	20.6	35.7 ± 0.5	16.7 ± 0.1	17.2 ± 0.3	1.3
585	87.6	81.8	71.4	22.2	35.3 ± 0.5	16.7 ± 0.0	16.9 ± 0.4	1.4
650	87.6	81.1	70.5	22.5	35.0 ± 0.8	16.5 ± 0.1	16.8 ± 0.9	1.9

).

With an increase in power from 65 W to 650 W, the recovery rate of straw ranged from 88.1% to 87.6%. The recovery efficiency of cellulose was almost >80.0%, that of hemicellulose was >70.0%, and the removal rate of lignin was >15.0%. The recovery rate of the reducing sugars was only 1.9% when the power increased from 65 W to 650 W. During microwave pretreatment, the solvent was heated in a contactless manner through microwave radiations. This is a kind of energy heat transfer as compared to simple heat transfer; thus, it had a high heating rate. Norazlina et al. [51] reported the highest yield of xylose for RS produced at an optimum microwave power of 320 W; however, they used dilute sulfuric acid as an auxiliary catalyst.

Compared with the hydrothermal pretreatment conditions of 100 °C for 30 min (Table 1), the recovery efficiencies of RS cellulose and hemicellulose and the removal efficiency of lignin were higher when the feed liquid temperature was 100 °C. This might be because of the destruction of the ester bond between lignin and hemicellulose caused by microwave radiations, increasing the removal efficiency of lignin [52].

According to the results shown in Table 2, microwave radiation seemed to have little effect compared to thermal pretreatment. The maximum power of 650 W was selected as the optimal microwave power for lignin removal.

3.2.2. Effects of Time on Microwave Pretreatment of RS

The effects of microwave radiation time on pretreatment were studied under the following conditions: solid–liquid ratio of straw and water was 1:10, and the microwave power was 650 W. The results are listed in

Table 3. Effects of time on microwave pretreatment of RS.

Time (min)	RS Recovery (%)	Cellulose Recovery (%)	Hemicellulose Recovery (%)	Lignin Removal (%)	Cellulose (%)	Hemicellulose (%)	Lignin (%)	Reduction Sugar Yield (%)
5	88.9	84.2	80.2	16.2	35.8 ± 0.3	18.5 ± 0.1	17.9 ± 0.3	0.4
10	88.7	83.8	78.7	15.0	35.7 ± 0.5	18.2 ± 0.5	18.2 ± 0.6	0.6
15	88.3	82.9	77.1	17.7	35.5 ± 0.9	17.9 ± 0.4	17.7 ± 0.5	0.7
20	87.9	82.8	76.3	17.2	35.6 ± 0.4	17.8 ± 0.1	17.9 ± 0.9	1.5
25	87.8	82.2	75.0	22.4	35.4 ± 0.6	17.5 ± 0.1	16.8 ± 0.3	1.7
30	87.6	81.1	74.5	22.5	35.0 ± 1.0	17.4 ± 0.0	16.8 ± 0.3	1.9

.

The recovery of straw was found to change slightly (88.9–87.6%) during the 5–30 min of microwave pretreatment, as described in Table 3. The microwave pretreatment time had little effect on RS. Yan et al. [53] studied the effect of microwave pretreatment on corn straw. They reported that microwave pretreatment power of 640 W for 15 min led to the quality loss of straw of 17.5%, which was similar to the results of this study. After microwave pretreatment for 30 min, the recovery rates of cellulose and hemicellulose in the solid residue were 81.1% and 74.5%, respectively. The removal rate of lignin was 22.5%, and the yield of reducing sugars was the highest at 1.9%. Pang et al. [54] also believed that although microwave radiations have an important influence on the recrystallization of materials, the intensity of microwave power and radiation time were unimportant.

When 30 min of microwave pretreatment was compared with hydrothermal pretreatment at 100 °C for 30 min, the recovery rate of RS decreased, indicating that microwave radiations had a greater effect on the pretreatment of RS at the same temperature.

In summary, microwave radiation at 650 W for 30 min was deemed as the optimal condition for subsequent RS depolymerization and degradation.

3.3. Effects of Cryocrushing Pretreatment on RS

Low-temperature cryocrushing pretreatment of RS was performed to degrade plant tissue using the ultra-low temperature of liquid nitrogen. The effect of soaking time in low-temperature liquid nitrogen on straw pretreatment was studied. The composition of the pretreated straw was determined, and the results are demonstrated in

Table 4. Effects of soaking time in liquid nitrogen on the cryocrushing pretreatment of rice straw.

Soaking Time (h)	RS Recovery (%)	Cellulose Recovery (%)	Hemicellulose Recovery (%)	Lignin Removal (%)	Cellulose (%)	Hemicellulose (%)	Lignin (%)
0	92.9	90.4	88.8	9.1	36.8 ± 0.3	19.6 ± 0.3	18.6 ± 1.0
0.16	91.8	91.8	87.3	9.6	37.8 ± 0.8	19.5 ± 0.7	18.7 ± 0.9
0.5	90.3	92.5	87.2	12.0	38.7 ± 0.0	19.8 ± 0.2	18.3 ± 0.4
1	89.9	91.3	85.1	12.3	38.4 ± 0.6	19.4 ± 0.5	18.2 ± 0.8
5	89.6	89.8	84.4	12.9	37.9 ± 0.2	19.3 ± 0.8	18.2 ± 0.4
10	89.5	89.0	84.3	13.3	37.6 ± 0.9	19.3 ± 0.7	18.4 ± 0.3
20	89.4	89.2	83.7	14.8	37.7 ± 0.2	19.2 ± 0.4	18.1 ± 0.9

.

Soaking time in liquid nitrogen did not affect the recovery of RS. RS was pretreated with liquid nitrogen for 0–20 h, and the recovery efficiency of RS was 92.9–89.4%, indicating that low-temperature cryocrushing had little effect on RS recovery. RS residue that was directly ground in liquid nitrogen had a recovery rate of cellulose of 90.4%; the recovery rate of hemicellulose was 88.8%, and the removal rate of lignin was 9.1%. We hypothesize that the protective hard outer structure of the straw was damaged under ultra-low temperature conditions of liquid nitrogen (−196 °C). Furthermore, direct grinding of RS in liquid nitrogen can also lead to the destruction of the outer structure. When RS was pretreated using liquid nitrogen, the primary contents loss included the salt released during grinding and washing. Castoldi et al. [37] used liquid nitrogen to pretreat rice husk, and the contents of cellulose and hemicellulose in the straw decreased after pretreatment. Moreover, the rice husk reached a freezing state and turned brittle under ultra-low temperatures, and the outer structure was damaged after grinding.

When liquid nitrogen was added to RS for direct grinding, the structure of RS was damaged, as shown in Table 4. The large difference in temperature rapidly rendered the plant cells to turn brittle under low temperatures, and the physical structure of the RS was damaged after grinding. Therefore, the pouring of liquid nitrogen for direct grinding of RS was considered optimal for subsequent depolymerization and degradation.

3.4. Comparison of RS and Pretreated RS Residues and Solutions Parameters

We compared the composition, straw recovery efficiency, conductivity, and salinity of the pretreated and untreated RS reaction solutions under optimal conditions, as represented in

Table 5. (a) Comparison of RS residue pretreatment parameters. (b) Comparison of parameters of pretreatment reaction solutions.

a
Samples	RS Recovery (%)	Cellulose Recovery (%)	Hemicellulose Recovery (%)	Lignin Removal (%)	Cellulose (%)	Hemicellulose (%)	Lignin (%)
RS	-	-	-	-	37.8	20.5	19.0
HP	59.0	74.6	14.1	41.3	47.8	4.9	18.9
MP	87.6	81.1	74.5	22.5	35.0	16.5	16.8
CP	92.9	90.4	88.8	9.1	36.8	19.6	18.6
b
Liquid Samples		pH	Conductivity (µs/cm)	TDS (mg/L)	Salinity (%)	Reduction Sugar Yield (%)
HP		3.9	2250.0	1126.0	0.1	14.9
MP		6.4	613.0	305.0	0.0	1.9
CP		-	-	-	-	-

a,b.

Based on Table 5a, the recovery rate of RS was low compared to that of RS after three pretreatments. The recovery rate of RS was the lowest (59.0%) after hydrothermal pretreatment at 180 °C for 10 min, whereas it was the highest (92.9%) after cryocrushing pretreatment, followed by 87.6% after microwave pretreatment. In contrast, as shown in Table 5a, the recovery rate of hemicellulose was the minimum (14.1%) after hydrothermal pretreatment, while the maximum recovery of hemicellulose was 88.8% when pretreated with the cryocrushing pretreatment; that after microwave pretreatment was 74.5%. As the hemicellulose composition of RS was disrupted, dissolved, or stored in aqueous solutions in other forms during this process, RS was partially depolymerized. The recovery rate of cellulose was relatively high as compared to that of hemicellulose, indicating that cellulose crystallinity is difficult to destroy. The cellulose recovery after cryocrushing pretreatment was the highest at 90.4%. Pretreatment via cryocrushing not only preserves cellulose but also preserves the majority of the hemicellulose. Moreover, pretreatment of RS through cryocrushing using liquid nitrogen had the least effect on the removal of lignin, whereas the lignin removal efficiency of the hydrothermal pretreatment was the largest (41.3%), which was 32.2% and 18.8% higher than that achieved from cryocrushing and microwave pretreatment, respectively.

After hydrothermal pretreatment, the conductivity of the solution was 2250.0 µs/cm, TDS was 1126.0 mg/L, and salinity was 0.11, which were all higher than the values of the microwave pretreatment (Table 5b). These results can be attributed to the partial depolymerization or leaking out of the main components, such as cellulose, hemicellulose, lignin, and ash. These components were dissolved in an aqueous solution, indicating that the RS was largely degraded through the hydrothermal pretreatment. The pH was 3.9, and the solution was acidic after the hydrothermal pretreatment because of the acid generated in situ by the RS under high-temperature conditions (Table 5b) [55]. The pH after microwave pretreatment was 6.4, indicating that temperature largely influenced the in situ acid formation during RS pretreatment as microwave pretreatment was performed at 100 °C, whereas hydrothermal pretreatment was conducted at 180 °C. Similar research reported by Moirangthem et al. [36] indicated that when RS was pretreated using microwave radiation at 100 °C for 5 min, the pH value was 6.1, whereas at 180 °C for 5 min, the pH was 4.7. They reported a decrease in both the pH of the hydrolysate liquor and the recovered weight of the solid residue, along with subtle changes in the recovered RS residue. However, in this study, the recovered rate of RS after hydrothermal pretreatment was much lower (59.0%) than that after microwave pretreatment (80.4%); this disparity may be attributed to the shorter time of exposure to microwave radiations than hydrothermal pretreatment.

As shown in Table 5b, the total reduction sugar obtained after the hydrothermal pretreatment was 14.9%, which can be subsequently valorized to chemicals, making up for the loss of hemicellulose in most pretreatments. However, the RS loss was relatively higher, and the recovery rate was lower than the other two pretreatments. In general, cellulose and hemicellulose were lost during all three pretreatments, especially hemicellulose, indicating a drawback of the pretreatments. Almost all pretreatments have the same limitation.

3.5. Characterization and Comparison of RS and Pretreated RS Residues

3.5.1. SEM-EDS Results

The SEM characterization results of coarse straw after pretreatment using various methods are demonstrated in Figure 1A–D, and the corresponding EDS characterization results are shown in

Table 6. Main elements in RS before and after pretreatments.

Samples	C (%)	O (%)	Si (%)	Cl (%)	K (%)
RS	72.2	24.4	2.1	0.4	1.0
HP	59.7	38.5	1.6	0.0	0.3
MP	59.6	35.9	4.4	0.1	0.0
CP	58.7	39.6	1.6	0.1	0.0

.

According to the SEM images, the untreated RS displayed a smooth, well-organized, and compact structure covered with a thick layer of wax [56]. This structure was not conducive to the dissolution of RS in organic matter and the diffusion of the catalyst. Figure 1B,C reveal that hydrothermal and microwave pretreatments resulted in RS disintegration. The waxy surface of RS was destroyed, the dense network structure is shown to collapse, and the cell wall became loose with the appearance of pores and cracks. After pretreatment, the physical and structural alteration of RS facilitated the action of the catalyst on the carbohydrates in the inner layer. Figure 1D indicates that the RS tissue became hard and brittle, the waxy surface of the RS residue disintegrated completely, the rigid framework loosened, and obvious gullies, pores, and cracks appeared on the wall. The secondary cell wall was exposed outside, and parts of the fiber bundles were leaking out. Therefore, the function of the cell wall function to act as a barrier was lost, rendering it unable to protect cellulose and hemicellulose, which was conducive to the subsequent hydrolysis of RS. Zhang et al. [57] also reported that SEM results showed that the straw was visibly disassembled after pretreatment with liquid nitrogen (low-temperature cryocrushing). Patil et al. [58] also reported a similar phenomenon via ozonolysis pretreatment; the superficial surface area and reaction site exposure present in RS fibers increased enormously.

The main elements summarized in Table 6 suggest that the contents of K and Cl in the pretreated straw significantly reduced after hydrothermal, microwave, and cryocrushing pretreatments. This indicates that the salt dissolved in the solvent during pretreatment, which is consistent with the salt content of the feed solution shown in Table 5b. Si was also released during hydrothermal pretreatment. The C/O ratio decreased in all RS residues, probably because the lignin and cellulose contents reduced and increased simultaneously, respectively, during the pretreatments.

3.5.2. FTIR Results

Changes in the RS chemical bonds due to pretreatments were analyzed using FTIR characterization. The results of the samples are displayed in Figure 2.

The characteristic adsorption peak of the infrared spectrum reveals the structural changes of RS residues after pretreatments in the range of 4000–400 cm⁻¹ wave number. The band corresponding to O−H stretching at 3341 cm⁻¹ indicates that the weakened vibration of the hydroxyl groups in cellulose and lignin after catalysis. The absorption band at 1512 cm⁻¹ represents the characteristic peak of lignin that is related to aromatic ring vibration. After microwave and hydrothermal pretreatments, the peak value at 466 cm⁻¹ decreased, indicating that the proportion of lignin decreased [59]. The absorption peak of the aromatic ring of lignin was the lowest; however, the peak value at 466 cm⁻¹ did not change significantly after cryocrushing pretreatment. The two peaks at 1727 cm⁻¹ and 1367 cm⁻¹ represent the alkyl ester of acetyl in hemicellulose. After pretreatment, both peaks were weak in the spectrum, indicating that the bonds between hemicellulose and lignin were broken [60]. Comparable results have also been reported after pretreatment using NaOH/urea and electrohydrolysis by Cai et al. [10] and after thermal pretreatment by Rajput et al. [61]. The absorption peaks at 2923 cm⁻¹, 1419 cm⁻¹, and 1321 cm⁻¹ correspond to the C–H tensile vibration from the CH₂ group represented in the CH₂OH group of C6 in cellulose. The decrease in these bands indicated that the cellulose content had changed after pretreatments [62]. The characteristic band at 1034 cm^–1 decreased after pretreatment, representing C–O–C ether vibrations in cellulose and hemicellulose and C–O stretching in cellulose, hemicellulose, and lignin. The β-glycoside bond stretched at 897 cm^–1 indicates that the cellulose content in solid residues after hydrothermal, microwave, and cryocrushing pretreatments increases due to the removal of hemicellulose [60]. The changes in chemical bonds were consistent with the SEM images. Figure 1B–D shows the damaged surface structure of RS after hydrothermal, microwave, and cryocrushing pretreatments.

3.5.3. XRD Results

Cellulose fibers consist of both amorphous and crystalline regions, whereas hemicellulose is primarily amorphous in nature. As crystalline materials are relatively more resistant to degradation than amorphous materials, the crystallinity of a substance can be used to assess the effectiveness of pretreatment of lignocellulosic biomass [61].

The XRD characterization results of solid RS residue after pretreatment are shown in Figure 3. The maximum absorption of cellulose crystalline and amorphous zones is 2 θ = 22.0° and 2 θ = 15.3°, respectively. The crystallinity of coarse straw was 55.0%, which decreased to 52.3%, 52.1%, and 52.1% after hydrothermal, microwave, and cryocrushing pretreatments, respectively. These results were consistent with those reported by Sorn et al. [63], Shi et al. [64], and Patil et al. [58]. Sorn et al. [63] showed that the crystallinity of RS subjected to [Bmim]Cl-only pretreatment (46.0%) and microwave-[Bmim]Cl pretreatment (42.5%) were both lower than that of the untreated RS (52.3%). Patil et al. [58] discovered that ozonolysis pretreatment led to the highest reduction in crystallinity (29.0%) of RS fiber. Shi et al. [64] believed that the unique heating characteristics of microwave radiations might lead to the explosion of particles, destroying the stubborn structure of lignocellulose and possibly damage of the clean area of cellulose, leading to the decrease of crystallinity. The decrease in crystallinity indicates that the cellulose crystallizing zone was destroyed during RS pretreatments [65]. The crystallinity of lignocellulose in compounds such as RS is an important factor in resisting the action of enzymes and other catalysts. The signal response value of the amorphous area of XRD was higher, which may be because of the transformation of crystalline cellulose into an amorphous form. RS is a highly heterogeneous complex polymer, primarily composed of lignin, cellulose, and hemicellulose, and some polymers may be coupled with carboxylic acid that acts as a catalyst, which is released during the self-hydrolysis of hemicellulose during pretreatment [54]. Ling et al. [66] summarized effective ways to reduce the crystallinity of cellulose, including mechanical treatment, such as ball milling, and a dissolving regenerating method.

However, some studies have demonstrated that RS pretreatment could increase its crystallinity. Cai et al. [10] reported that the crystallinity of cellulose in RS increased after pretreatment with NaOH/urea and electrohydrolysis. Sahoo et al. [67] also reported that the crystallinity of RS pretreated with acid and alkali was much higher than that of untreated RS. These conflicting findings may be attributed to the different interactions of tow specific with biomass, in which the underlying mechanisms were dependent on cations or anions [10].

Overall, hydrothermal, microwave, and cryocrushing pretreatments of RS could effectively disrupt the chemical bonds between hemicelluloses and lignin, decreasing the crystallinity and the porosity of cellulose. This would prove beneficial for improving the hydrodepolymerization of RS cellulose.

3.5.4. N₂ Physical Adsorption–Desorption Results

The N₂ physical adsorption–desorption characterization results reflect the physical structure changes after three pretreatments through the comparison of specific surface area and pore diameter between RS and the pretreated RS residues as shown in

Table 7. Analysis of specific surface area and pore size of RS and residues.

Samples	Specific Surface Area (m2/g)	Aperture (nm)
RS	2.5	13.0
HP	9.3	15.8
MP	6.3	23.1
CP	3.0	28.8

.

Based on Table 7, compared with untreated RS, the specific surface area of RS residues increased after all three pretreatments. The increase in specific surface area after cryocrushing pretreatment was the lowest (increased by 0.5 m²/g), and that after hydrothermal treatment was the highest (increased by 6.8 m²/g). The specific surface area increased by 3.8 m²/g after microwave pretreatment. Meanwhile, the pore diameter of RS increased after hydrothermal, microwave, and cryocrushing pretreatments. However, the cryocrushing pretreatment had the largest impact on the pore diameter of RS, increasing it by 15.8 nm. Li et al. [68] also obtained the same phenomenon, whereas RS was pretreated with a surface active agent that was auxiliary with acid and alkali. Microwave radiation not only led to the dissolution of impurities such as inorganic salts in RS but also degraded the internal RS structure. During the cryocrushing pretreatment, the RS was in a cold and brittle state at low temperatures and was easier to be crushed. Hence, the structure easily degraded, and the pore diameter increased by a large extent. Hydrothermal pretreatment also destroyed the internal components of RS.

3.6. Effects on Hydrodepolymerization of RS Residues after Various Pretreatments

Due to the external composite structure of RS, the RS residues that were pretreated under optimal conditions were selected for degradation under the optimal conditions of hydrodepolymerization. The results are shown in Figure 4.

As shown in Figure 4, the degradation conversion rate of RS cellulose was 42.9% without pretreatment, and the conversion rate of cellulose increased after all pretreatments. After the cryocrushing pretreatment, the cellulose conversion rate was the highest (56.8%), and the yields of glucose, xylose, and arabinose were 29.6%, 56.2%, and 17.8%, respectively. Compared with the untreated RS, the conversion of cellulose increased by 13.9%, the yield of glucose increased by 14.1%, and that of xylose increased by 3.2%. Although the hemicellulose and cellulose in RS residue were both lost after cryocrushing pretreatment and the recovery rates were 88.8% and 90.4%, respectively, based on the results in Table 5a; compared to untreated RS, the yield of xylose in the RS residue was increased, and the yield of glucose was also elevated. It is reasonable to assume that the waxy components on the surface of the RS residue were completely destroyed after the cryocrushing pretreatment according to the characterization results in Figure 1D. As a result, the RS cell wall had lost the function of the protective barrier for cellulose, and the catalyst could establish better contact with the inner layer to induce degradation into sugars. During hydrodepolymerization, increased cellulose conversion exposed the heterogeneous catalyst that was employed to degrade RS residues by hydrocracking the C–O bonds in cellulose [40]. Castoldi et al. [37] also reported similar results, showing that the enzymatic hydrolysis of rice husk increased after pretreatment with liquid nitrogen. Huang et al. [6] performed one-pot pretreatment with lactic acidic–choline chloride, and the saccharification took place at 120 °C for 3 h. A biomass loading enzyme saccharification system of 15% achieved a total yield of 75.7%.

Hydrothermal pretreatment converted RS cellulose by 55.0%. The yields of glucose and xylose were 22.7% and 32.9%, respectively. Compared with RS, the yield of glucose increased, but that of xylose was decreased. This was related to the higher loss of hemicellulose during pretreatment. According to Table 1, the recovery rates of RS and hemicellulose were only 59.0% and 14.1%, respectively, after 10 min of hydrothermal pretreatment at 180 °C. Microwave pretreatment had the lowest effect on the production of sugars through hydrodepolymerization degradation. The conversion rate of cellulose was 54.9%, and the yields of glucose, xylose, and arabinose were 25.5%, 47.6%, and 14.4%, respectively. According to our previous study, the C–O bond in the hemicellulose was preferentially broken when the RS was hydrodepolymerized by copper; thus, the degradation and conversion of cellulose in the RS were difficult [40].

The observed increase in cellulose conversion and glucose yield was not large enough. Hydrodepolymerization of RS is among the few methods that can effectively degrade cellulose apart from the use of acids and enzymes owing to the virtues of the shorter reaction time, fewer reaction procedures, no follow-up operations, and environmentally friendly protocols, thus presenting an ideal solution for the utilization of this abundant resource [40].

4. Conclusions

Three different pretreatment methods were employed and compared to degrade the structure of RS, including low-temperature cryocrushing, medium-temperature microwave, high-temperature hydrothermal pretreatments. The physical structure and chemical composition of RS were altered after various pretreatments. The analysis and comparison of the straw recovery rate and residue composition after pretreatments revealed the loss of cellulose and hemicellulose, especially hemicellulose, during the structural degradation of RS. Pretreatment with hydrothermal yielded the lowest rice straw recovery (59.0%) after pretreated at 180 °C for 10 min, the hemicellulose recovery was only 14.1%, and the lignin removal efficiency was the largest (41.3%), which was 32.2% and 18.8% higher than that achieved from the cryocrushing and the microwave pretreatment, respectively. Pretreatment with cryocrushing yielded the highest recovery rates of rice straw (92.9%), hemicellulose and cellulose (88.8% and 90.4%, respectively). Pretreated rice straw residues were hydrodepolymerized into monosaccharides, whereas the cellulose conversion rate increased. The highest cellulose conversion rate was obtained via pretreatment with cryocrushing (56.8%), which was 13.9% higher than that obtained from untreated RS. Furthermore, the yields of glucose, xylose, and arabinose were 29.6%, 56.2%, and 17.8%, respectively, which indicated an increase of 14.1% in glucose yield.