- Email: [email protected]
Microwave Pretreatment
C H A P T E R
9 Microwave Pretreatment Jian Xu National Key Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, China
9.1 INTRODUCTION 9.1.1 Microwave Microwave (MW) is electromagnetic waves with wavelengths in the range of 0.01e1 m and a corresponding frequency ranging from 0.3 to 300 GHz. The MW region in the whole electromagnetic spectrum is between the infrared and radio frequencies. As a kind of electromagnetic wave, MW has both electric and magnetic fields. Based on the interaction between MW irradiation and materials, a material can be categorized into three types [1]: (1) absorbing materials such as water and glycerol, which can absorb MW, are also called MW dielectric; (2) conductors (mainly metals), which MW cannot penetrate, and most of which is reflected; and (3) insulators or MW-transparent materials, including quartz and Teflon, which allow MW to pass through without loss. MW dielectric is commonly used as the heating medium [2].
9.1.2 MW Heating Mechanism MW heating is also called dielectric heating, because of the dielectric employed to absorb MW irradiation. To avoid interference with telecommunications and cellular phone frequencies, 915 MHz (896 in the United Kingdom) and 2450 MHz are two frequencies usually used for MW heating for industrial, scientific, and medical applications [3]. A domestic MW oven normally operates at 2450 MHz. The way a material is heated by MW depends on its shape, size, and dielectric constant, and the nature of the MW equipment used. The main MW heating mechanisms consist of dipolar polarization, conduction, and interfacial polarization [4e6]: (1) Dipolar polarization is responsible for most MW heating in the solvent systems. Polar molecules characterized with an electrical dipole moment will align themselves in an electromagnetic field. In an electromagnetic wave with rapid oscillation, the polar molecules will rotate continuously, aligning with it. This is called dipolar polarization. As Pretreatment of Biomass http://dx.doi.org/10.1016/B978-0-12-800080-9.00009-8
157
Copyright Ó 2015 Elsevier B.V. All rights reserved.
158
9. MICROWAVE PRETREATMENT
TABLE 9-1
Comparison of conventional and MW heating
MW irradiation
Conventional heating
Conversion of energy
Transfer of energy
In corn volumetric and uniform heating
Superficial Heating via Convection/conduction
Rapid and efficient
Slow, inefficient
Selective
Non selective
Hot spots
No hot spot
Dependent of the properties of the material
Less dependent
Precise and controlled heating
Less controllable
the field alternates, the molecule reverses direction to align itself with it. This causes energy to be lost from the dipole by molecular friction and collisions, giving rise to dielectric heating. For this reason, MW heating is always called dielectric heating. (2) Conduction mechanism happens when the dissolved charged particles in a sample (electrons, ions, etc.) oscillate back and forth under the influence of the electric component of MW irradiation. They collide with the adjacent molecules or atoms, which cause agitation or motion, and heat is thus created. (3) Interfacial polarization is a phenomenon viewed as a combination of conduction and dipolar polarization. It is an important mechanism for systems composed of conducting and nonconducting materials.
9.1.3 MW Effects The action of MW irradiation results from materialewave interactions leading to thermal effects, specific MW effects and nonthermal effects [7e10]. A combination of these contributions is responsible for the observed effects. Heat from conventional heating is transferred from the surface toward the center of the material by conduction, convection, and radiation. It is relatively slow and inefficient, and depends on the thermal conductivity of the material and convection currents. MW heating is characterized by converting electromagnetic energy into thermal energy, which is a kind of energy conversion rather than heating. Compared with conventional heating, the heat produced by MW irradiation is throughout the volume of the materials rather than an external source. The differences between conventional and MW heating are presented in Table 9.1 [11].
9.2 MW APPLICATION Since the first MW oven built in 1947 by Raytheon Corporation and an early commercial model introduced in 1954, MW heating has gained popularity in food processing because of its ability to achieve high heating rates, significant reduction in cooking time, more uniform heating, safe handling, ease of operation, and low maintenance [12,13]. Over the decades, this
A. LIGNOCELLULOSIC BIOMASS
9.2 MW APPLICATION
159
technology has spread widely to such applications as analytical chemistry, heating and vulcanization of rubber, plasma processing, chemical synthesis and processing, and waste remediation. MW irradiation has acquired a great deal of attention in domestic, industrial, and medical applications. It has been used in many applications including mineral and metallurgic processes [14e16], chemical catalysis [17,18], organic/inorganic syntheses [19e24], pyrolysis [25e27], phase separation and extraction processes [28e34], remediation of soil and hazardous/radioactive waste [35e41], and sewage sludge treatment [42,43]. The rapid and effective heating properties of MW have led to its application in biomass pretreatment since 1984 [44]. As a promising alternative feedstock to replace crude oil, biomass has gained attention recently owing to its potential in producing energy, materials, and chemicals. However, the recalcitrant nature of biomass makes the pretreatment step necessary to improve its enzymolysis efficiency. A good biomass pretreatment produces high enzyme-digestible biomass, low-cost inhibitors, and ease of operation. MW has been a powerful tool in the pretreatment of various biomass including agricultural residues, woody biomass, grass, energy plants, and industrial residuals. This chapter addresses the most commonly preferred MW-assisted biomass pretreatments: (1) MW/water alone, (2) MW/alkali, (3) MW/acid, (4) MW/ionic liquid, (5) MW/ salt, and (6) other combined MW-assisted pretreatment. Although MW-assisted pyrolysis and torrefaction have been widely used in biofuel production [45e49], it is a direct conversion technology rather than a pretreatment, which is not included in this chapter.
9.2.1 Microwave/Water Alone Ooshima et al. (1984) were the first to report the use of MW heating for the pretreatment of rice straw and bagasse [44]. It was found that the enzymatic accessibilities increased by 1.6 and 3.2 times for rice straw and bagasse, respectively, compared with untreated samples. In the same year, sugar cane bagasse, rice straw, and rice hulls were pretreated by Azuma et al., who observed no substantial change in cellulose crystallinity detected by short heating (<8 min) below 230 C [50]. The enzymatic hydrolysis of all the biomass was markedly improved by MW pretreatment above 160 C and showed a maximum at 223e228 C, independent of the source. The increase in surface area of cellulose resulting from the degradation of lignin and hemicellulose is considered a rate-determining factor for cellulose enzymolysis because of the stability of cellulose crystallinity under MW irradiation. The effects of temperature and duration of MW pretreatment of wheat straw on its enzymatic saccharification to fermentable sugars were evaluated [51]. The yield of 544 mg/g on monomeric sugars was obtained from straw from MW (200 C, 10 min) pretreated (8.6%, w/v, in water) after enzymatic saccharification (45 C, pH 5.0, 120 h) using a cocktail of three commercial enzyme preparations. Switchgrass soaked in water was also pretreated by MW irradiation [52]. The total sugar (xylose þ glucose) yield of 34.5 g/100 g biomass was obtained, which was 53% higher than that from conventional heating. A comparison made of pretreatment between MW and ultrasound was carried out by Nikolic et al, who found that MW pretreatment resulted in higher yields of glucose and bioethanol compared with ultrasound pretreatment. This investigation also indicated that the time for simultaneous saccharification and fermentation was markedly decreased by MW pretreatment, and thus, the bioethanol production cost could be reduced [53]. Lu et al. [54]
A. LIGNOCELLULOSIC BIOMASS
160
9. MICROWAVE PRETREATMENT
also pointed out that MW generates higher power densities, enabling higher production rates and lower production costs compared with conventional heating. Ma et al. used a BoxeBehnken design (BBD) and response surface methodology optimizing MW pretreatment on rice straw [55]. MW intensity, irradiation time, and substrate concentration were identified as three main factors affecting enzymatic saccharification. It was also found that the silicified waxy surface and the ligninehemicellulose complex were broken down and silicon and lignin were partly removed under MW pretreatment. MW was also employed to pretreat milled barley, spring wheat, winter wheat, and oat straw for biogas [56]. However, the performance of anaerobic digestion for these four pretreated materials did not improve, which suggests that MW pretreatment might be not suitable for milled straw types in biogas plants.
9.2.2 Microwave/Alkali Lignin has been considered a barrier to enzymatic hydrolysis, and the removal of lignin has been confirmed to enhance biomass susceptibility for enzymes [57,58]. It was postulated that alkali treatment releases hemicelluloses as oligomers [59], causing less sugar degradation at a low alkali concentration [60,61]. Higher alkali used in pretreatment tends to form hemicellulose with a higher molecular weight, which is considered to expand its economic value [62]. However, a relatively long residence time is required to obtain high yields for alkaline pretreatment at a lower temperature [61]. It has been reported that 80% of hemicelluloses and 60% of lignin could be released from wheat straw by sodium hydroxide pretreatment at room temperature with a long time of 144 h [63]. As a rapid and effective heating source, MW irradiation has been widely employed in alkali pretreatment to obtain the higher temperature required in the delignification process. Some studies with MW-assisted alkali pretreatment on wheat straw and rice straw confirmed its effectiveness in enhancing the enzymatic hydrolysis and ethanol production [64e67]. Compared with pretreatment with alkali alone, biomass pretreated by MW and alkali had a higher hydrolysis rate and glucose content in hydrolysate. A higher yield of 90% maximum potential sugars was achieved with switchgrass pretreated by alkali (NaOH) at 0.1 g/g biomass with MW as the heat source [52]. It was also found from scanning electron microscope (SEM) images that MW was more effective in disrupting the recalcitrant structures compared with conventional heating. Another investigation with MW-assisted alkali pretreatment on wheat straw for saccharide- and lignin-releasing potential was carried out; it was confirmed effective by extracting more than 80% of hemicellulose and 90% of lignin at a very short pretreatment time and low degree of cellulose solubilization [68]. Rodrigues et al. [69] focused on the potential of MW-assisted alkali pretreatment to improve the rupture of the recalcitrant structures of the cashew apple bagasse (CAB). With an alkali concentration of 0.2 and 1.0 mol/L, solid percentage of 16% (w/v), and enzyme load of 30 FPU/g CAB, pretreatment time and MW power had no significant effect on glucose concentration. Zhao et al. [70] used an NaOH solution to pretreat rice hulls with MW irradiation; they mentioned that increased accessibility of the substrates by MW pretreatment was mainly achieved by rupturing the rigid structure of rice hulls, and the reducing sugar content was increased by 13% compared with that of rice hulls without
A. LIGNOCELLULOSIC BIOMASS
9.2 MW APPLICATION
161
pretreatment. Hu and Wen [52] described the pretreatment of switchgrass using MW. With MW-assisted alkali pretreatment, they reported that the total sugar yield was 53%, which was higher than that obtained by conventional heating. MW-assisted alkali pretreatment was also used to pretreat vinasse for L-lactic acid production. The maximum lactic acid concentration of 17.5 g/L was obtained with the alkali concentration of 8% and MW power of 700 W, higher than those of the independent method used. In addition, the fermentation time could be 1 day earlier than other methods [71]. Another follow-up study was done on L-lactic acid from vinasse, and an optimized condition for the MW-alkali coupling pretreatment process was obtained as follows: 0.06 g/g NaOH, 523 W for 8 min, and solid-to-liquid ratio at 1:2, which resulted in 86.0 mg/g L-lactic acid [72]. Compared with independent NaOH and MW pretreatment methods, MWeNaOH coupled pretreatment reduced hemicellulose and lignin in vinasse more markedly by 28.24% and 10.53%, respectively. The crystallinity of vinasse was also reduced significantly from 69.34% to 33.68%. Pretreated vinasse by MWeNaOH coupling exhibited more surface holes, resulting in a wider specific surface area. Orthogonal design and BBD have been used to optimize MW-assisted alkaline pretreatment [73,74]. The ethanol yield was 148.93 g/kg wheat straw at this condition, much higher than that from the untreated material, which was only 26.78 g/kg. Pretreatment on rice straw and hulls was optimized by Singh et al. using a BBD [73]. It was found that alkali (NaOH) concentration, irradiation time, and substrate concentration were main factors affecting the efficiency of saccharification. The optimal conditions for these three factors were 2.75%, 22.50 min, and 30 g/L, respectively. A study by Xu et al. used an orthogonal design to optimize MW pretreatment on wheat straw for ethanol production; pretreatment with a ratio of biomass to liquid of 80 g/kg, NaOH concentration of 10 kg/m3, and MW power of 1000 W for 15 min was confirmed to be optimal [74]. Energy consumption was compared between alkali-assisted MW pretreatment (AAMP) on cotton plant residue (CPR) and high-pressure reactor pretreatment [75]. The energy requirement to pretreat a unit quantity of CPR by AAMP was calculated to be 5 times less than by a high-pressure reactor treatment. Ammonia has also been widely used in pretreatment and has demonstrated great success in the delignification of sugarcane bagasse, sorghum bagasse, corn stover, barley hull, and municipal waste [76e79]. Combined pretreatment of MW and dilute ammonia proved effective in removing 48% of the original lignin from sorghum bagasse at comparatively low ammonia concentrations and lower temperature, and in a relatively short reaction time compared with other technologies. The best results in terms of glucose (4.2 g glucose/10 g dry biomass) and ethanol yields (2.1 g ethanol/10 g dry biomass) were observed with 1- to 2-mm biomass fibers pretreated at 130 C for 1 h. SEM images indicated that the biomass structure was deformed as a result of the swelling of cellulose and delignification, with both factors contributing to improved digestibility. In addition, glycerol, organic acid, and furfural concentrations were too insignificant to have caused toxicity to the enzymes and yeast during enzymatic hydrolysis and fermentation studies, respectively [80]. In addition to NaOH and ammonia, lime is another alkaline used in biomass pretreatment. MW-assisted lime pretreatment on sweet sorghum bagasse (SSB) was evaluated and the optimal pretreatment condition was identified as 0.1 g lime and 10 mL water/g SSB in 4 min. Under these conditions, a sugar yield of 32.2 g/100 g SSB (equivalent to 52.6% of maximal potential sugars) was achieved. With the same water content and exposure time,
A. LIGNOCELLULOSIC BIOMASS
162
9. MICROWAVE PRETREATMENT
but without lime, a sugar yield of 39.8 g/100 g SSB (equivalent to 65.1% of maximal total sugars) was observed. The higher sugar recovery without lime was mainly due to high sugar release during pretreatment. However, with lime, sugar degradation took place, which resulted in less sugar yield although lime made cellulose more accessible to enzymes, as evidenced by a higher percentage of increase of total reducing sugars during enzymatic hydrolysis. Results from this study were strongly supported by Fourier transformeinfrared spectroscopy (FTIR) and SEM images. Overall, in a short time and with a simple setup, MW irradiation showed great promise as a leading pretreatment technique for SSB [81]. Saha compared pretreatment on wheat straw between MW-alone and MW-assisted lime; the release of 604 mg total sugars/g straw after enzymatic hydrolysis was achieved from MW pretreatment with lime (0.1 g/g straw) at 160 C for 10 min, higher than that from pretreatment by MW alone (200 C, 10 min), which was 544 mg/g straw [51].
9.2.3 Microwave/Acid Dilute acid pretreatment is able to convert hemicellulose contained in biomass to soluble sugars and facilitates the subsequent enzymatic hydrolysis of cellulose [82e84]. However, the hydrolysis of polysaccharides also leads to the formation of sugar degradation products: namely, inhibitors such as furfural and 5-hydroxymethylfurfural (HMF) [85], which not only reduce the yield of sugar monomers but also act as fermentation inhibitors. It is therefore necessary to choose reaction conditions that keep the generation of inhibitors at a lower level. To achieve dilute acid pretreatment, the process is usually carried out in a high-temperature environment using conventional heating. Apart from the conventional heating, MW heating is another way to accomplish disruption of recalcitrant structures in biomass. This arises from the fact that water, cellulose, hemicellulose, and other low-molecular compounds (e.g., organic acid) belong to dielectrics [44]. Consequently, the dilute acid solution can be heated by MW through a process of dielectric heating [86e88]. MW-assisted dilute acid pretreatment had remarkable results in promoting the enzymatic saccharification of water hyacinth [89]. After pretreatment, hemicellulose was hydrolyzed into monosaccharide units, and a large amount of cellulose was released from the degraded lignocellulose matrix to react with cellulase on larger surface areas. As a result, glucose yield increased after enzymatic hydrolysis. When 20 g/L water hyacinth feedstock was pretreated with MW at 140 C for 15 min with 1% H2SO4, the highest reducing sugar yield obtained after enzymatic hydrolysis was 48.3 g/100 g hyacinth, which was 94.6% of the theoretical reducing sugar yield. In the study of Palmarola-Adrados et al. [90], starch-free wheat fibers were pretreated in dilute sulfuric acid solutions using MW heating; biomass pretreatment with MW heating was able to give a higher sugar yield compared with steam explosion. Li et al. [91] carried out pretreatment of swine manure in sulfuric acid solutions heated by MW; MW irradiation led to a higher yield of reducing sugar, shorter reaction time, and lower energy consumption, and was a suitable technique for the saccharification process of swine manure. Sugarcane bagasse was pretreated via MW-assisted heating in a dilute sulfuric acid solution (0.2 M); an increase in reaction temperature significantly intensified destruction of the biomass structure of bagasse and the pretreated bagasse particles were simultaneously characterized by fragmentation and swelling [92]. Another investigation was made by the same research group with acid concentrations in the range of 0e0.02 M at 180 C for 30 min [93].
A. LIGNOCELLULOSIC BIOMASS
9.2 MW APPLICATION
163
The derivative thermogravimetric analysis (DTG) curve evolved from a double-peak distribution to a single-peak one owing to hydrolysis of most of the hemicellulose. However, the characteristics of crystalline cellulose and lignin were obviously exhibited in X-ray diffraction and FTIR analyses, revealing that the two constituents were barely affected by the pretreatment. However, a higher acid concentration led to higher yields of furfural and HMF, stemming from the further degradation of xylose and glucose. From the operating point of view for sugarcane bagasse pretreatment, an optimal acid concentration of 0.005 M and a heating temperature of 180 C for 30 min were suggested by this study. A comparison of pretreatment with MW alone, MWelime and MWedilute acid revealed that the highest yield of total sugars (glucose, xylose, arabinose, and galactose) after enzymatic hydrolysis was 651 mg/g straw, obtained from wheat straw pretreated by MW dilute acid (0.5% H2SO4, w/v) at 160 C for 10 min, higher than that from MW pretreatment with lime (0.1 g/g straw) at 160 C for 10 min (604 mg total sugars/g straw) and MW alone at 200 C for 10 min (544 mg/g straw) [51].
9.2.4 Microwave/Ionic Liquid The main advantage of using ionic liquids for bioethanol production is represented by the possibility of complete solubilization of the biomass. Swatloski et al. suggested that solubilization results from breakdown of the H-bonds of the polysaccharides by the anion of the ionic liquids [94]. A variant of the pretreatment with ionic liquids is represented by the MW-assisted pretreatment of biomass in ionic liquids [95,96]. The method is characterized by a shorter reaction time (owing to MW irradiation) and better solubilization of the biomass. According to Zhu et al., the raw biomass is directly solubilized in the ionic liquid in the presence of MW and cellulose is precipitated by adding water. The other organic compounds (such as lignins) remain in solution. Experimental results [96] showed that the yields in ethanol are similar to the ones obtained through steam explosion or chemical pretreatment. Liu et al. used 1-n-butyl-methylimidazolium chloride ([Bmim]Cl) with MW to pretreat cellulose and found that degree of polymerization of cellulose decreased after being pretreated [97]. The saccharification rate reached a maximal value of 18.3 g/L/h at 300 W, 90 C after 1 h, increased by 160% compared with untreated cellulose. SEM and FTIR spectroscopic analysis showed that the cellulose was depolymerized into fragments and the crystallinity of the cellulose significantly decreased after pretreatment. Ha et al. compared different pretreatments with and without MW irradiation on cotton cellulose [98]. They observed that the rate of enzymatic hydrolysis of cotton cellulose increased by at least 12-fold after pretreatment at 110 C, and by 50-fold after ionic liquid dissolution with MW irradiation, showing that cellulose pretreatment with ionic liquid and MW irradiation could be a potential alternative pretreatment method. A novel alternative extraction method for the fast extraction and determination of patchouli alcohol from Pogostemon cablin with MW irradiation-assisted ionic liquid pretreatment (MRAILP) was developed [99]. Under optimized conditions, the extraction yield of patchouli alcohol by MRAILP was 1.9%, which increased to 166% compared with MW-assisted extraction. MW-assisted ionic liquid pretreatment (MILP) was also used to destroy the cell walls of medicinal plants (Cynanchum paniculatum) before solvent extraction, to increase the accessibility of active ingredients to the solvents. The paeonol extraction yield by MILP was higher
A. LIGNOCELLULOSIC BIOMASS
164
9. MICROWAVE PRETREATMENT
than that of ionic liquid aqueous solution and Soxhlet extraction. In addition, the results of bioassay test showed that the antibacterial effect of crude extract by MILP was almost the same with that of Soxhlet extraction [100]. Because of the strong dissolution abilities of ionic liquids for plant cell walls, integrated MW-assisted ionic liquids pretreatment followed by hydro-distillation (MILPeMHD) was carried out for the efficient extraction of essential oil from the rare Dryopteris fragrans [101]. The MILPeMHD was effective in obtaining much higher yield (0.9%) of essential oil within a shorter duration (14.2 min) than that from the reported solvent-free MW extraction (0.3% and 94 min). The antioxidant efficacy of the extracted essential oil in the stabilization of sunflower oil was comparable to the commonly employed atocopherol (VE), indicating that MILPeMHD might be a potential and highly efficient technique for the extraction of valuable essential oils in flavor food industries. Another investigation into MILT-MHD for isolating essential oil from Fructus forsythiae seed was done by Jiao et al. [102]. Compared with conventional hydro-distillation (HD) and emerging MW-assisted aqueous ionic liquid hydro-distillation (MAILHD), MILT-MHD obtained a yield of essential oil (9.6%) in 29.3 min, much higher than those from HD (4.1% and 100 min) and MAILHD (5.4% and 45 min).
9.2.5 Microwave/Salt Molecular dynamics study has been done on MW heating of water, ice, and saline solution [103]. Dilute salt water was significantly more heated than pure water because of the fieldinduced motion of salt ions, especially for large ions, by MW electric field and energy transfer to water molecules by collisions. Other research explored the effects of selected salts (NaCl, CaCl2, FeCl3, Na4P2O7, and Na3P3O10) on the MW heating profile of a starch solution. Results showed that samples containing salt heated faster than those that contained no salt for heating times in excess of 40 s [104]. The reason might be that the addition of salt could increase water’s loss factor, causing the water to heat faster. Because of this, if slat is added to MW-assisted pretreatment, it might lead to better results. In fact, Liu et al. [105] demonstrated that MW-assisted pretreatment with FeCl3 could easily and effectively solubilize hemicellulose into monomeric and oligomeric sugars and disrupt ether and ester linkages between lignin and carbohydrates, but did not affect delignification. Enzymatic hydrolysis of pretreated corn stover resulted in an optimum yield of 98.0%, which was significantly higher than that from untreated corn stover (22.8%). SEM and XRD analysis indicated that MW-assisted FeCl3 pretreatment apparently damaged the surface of corn stover. The conditions for MW-assisted FeCl3 pretreatment of rice straw were optimized using BBD [106]. Optimal conditions were as follows: 0.14 mol/L FeCl3, 160 C, 19 min with substrate concentration at 109 g/L. Reducing sugar of 6.6 g/L was produced from the degradation of pretreated rice straw with Trichoderma viride and Bacillus pumilus, 2.9 times higher than that of untreated rice straw. Li and Xu [107] used CaCl2-assisted MW pretreatment to pretreat corn stover using the central composite design of response surface methodology. Temperature and time were identified as the main factors affecting the enzymatic digestibility of corn stover. The enzymatic hydrolysis ratio of cellulose was 90.7% and glucose recovery was 65.5% under optimal conditions at 162.1 C for 12 min with a solid-to-liquid ratio of 10% (w/v). This process achieved a temperature of about 160 C, necessary for lignocellulose pretreatment under atmospheric pressure using cheap calcium chloride as the heating medium.
A. LIGNOCELLULOSIC BIOMASS
9.3 MW PRETREATMENT REACTORS
165
9.2.6 Other MW-Based Combined Pretreatments Pretreatment of beech wood with MW-assisted ammonium ions and peroxometal complexes was evaluated. A maximum sugar yield of 59.5% was obtained by MW-assisted ammonium molybdate and H2O2 at 140 C for 30 min, whereas external heating in an autoclave gave a sugar yield of 41.8% [108]. MW-assisted pretreatment of recalcitrant softwood in aqueous glycerol containing a series of organic and inorganic acids was carried out by Liu et al. [109]. The pulp obtained by organosolvolysis with 0.1% hydrochloric acid at 180 C for 6 min gave the highest sugar yield of 53.1%. The pretreatment efficiency correlated linearly with the pK(a) of the acids, with the exception of malonic and phosphoric acids. Organosolvolysis with 1.0% phosphoric acid (pK(a) 2.15) gave a saccharification yield 50.6% higher than that expected from its pK(a), whereas the catalytic effect of malonic acid (pK(a) 2.83) was negligible. Extensive exposure of crystalline and noncrystalline cellulose by glycerolysis with strong inorganic acids was demonstrated using fluorescent-labeled recombinant carbohydrate-binding modules. Because of the low concentration of the acid catalysts and the availability of glycerol as a byproduct from biodiesel and fatty acid production, organosolvolysis in glycerol is an appealing process for the pretreatment of recalcitrant softwood. MWealkali (1% NaOH) followed by acid pretreatment (1% H2SO4) was developed by Binod et al. (2012). The enzymatic hydrolysis gave a higher reducing sugar yield of 0.83 g/g dry sugarcane bagasse [110]. MW-assisted ammonium hydroxide (NH4OH) followed by phosphoric acid (H3PO4) treatments was used to release monomeric sugars from Miscanthus sinensis. Treatment with NH4OH at 120 C for 15 min liberated 2.9 g of monomeric sugars per 100 g of dried biomass, whereas the corresponding yield for a treatment with H3PO4 at 140 C for 30 min was 62.3 g/100 g. The two-stage pretreatment, treatment with NH4OH at 120 C for 15 min followed by treatment with H3PO4 at 140 C for 30 min, impressively provided the highest total monomeric sugar yield of 71.6 g/100 g dried biomass [111]. A novel combined pretreatment of ball milling (BM) and MW irradiation (MWI) was developed by Peng et al. [112]. To achieve the same or higher glucose yield of BM for 3 and 6 h, BM for 1 h with MWI for 20 min could save 54.8% and 77.4% energy consumption, respectively. Moreover, chemicals were not required in this process, which made it an environmentally friendly, economical, and effective pretreatment approach.
9.3 MW PRETREATMENT REACTORS Although MW irradiation has been widely used to pretreat many kinds of biomass for several decades, most pretreatments still take place in the laboratory in domestic MW ovens or are modified as listed in Table 9.2 In the first report on MW pretreatment, Ooshima et al. [44] pretreated rice straw and bagasse with water individually placed in sealed glass vessels in a domestic MW oven. Rose and Inglett used a special MW with well energyecontrolled system to heat wheat bran and induce the autohydrolysis of arabinoxylan in wheat bran when heated to 200e210 C [117]. A continuous flowetype MW irradiation system with a capacity of 30 kg/h at 5.0 kW (2.45 GHz) equipped with two magnetrons was successfully used to
A. LIGNOCELLULOSIC BIOMASS
166
9. MICROWAVE PRETREATMENT
TABLE 9.2 Summary of MW Reactors Used to Pretreat Different Biomass Feedstock
MW Device
References
Barley husk
MLS-1200 Mega MW workstation, Milestone, Sorisole, Italy
[113]
Barley straw Spring wheat straw Winter wheat straw Oat straw
CEM MW Max asphalt oven
[56]
Beech wood
Biotage Initiator 2.0 MW reactor (Biotage Co. Ltd., Uppsala, Sweden)
[108]
Corn stover Cotton cellulose
CEM DiscoverÔ unit (CEM company Matthews, USA)
[98,114]
Cotton plant residue Sugarcane bagasse
CE2877 N MW oven (Samsung, Korea)
[75,110]
Cynanchum paniculatum
MP23C-BF (KA) MW oven (Midea, China)
[100]
Dryopteris fragrans Fructus forsythiae
MAS-II (2450 MHz) MW system(Shanghai Sineo MW Products Company (Shanghai, China)
[101,102]
Microcrystalline cellulose Corn stover
MCR-3 MW reactor (Yuhua Equipment Co., Ltd., China)
[107,112]
Pogostemon cablin
MW-assisted extraction unit (Media, Shunde, China)
[99]
Rice straw
WP800T MW (Galanz)
[55]
Rice straw and hulls Wheat straw
M510 laboratory MW oven (Whirlpool)
[43,74]
Switchgrass
Customized Sharp/R-21 HT MW oven
[52]
Switchgrass
NN-S954 MW oven (Panasonic Corporation)
[115]
Water hyacinth
WX-4000 MW system (Shanghai Yiyao MW Chemistry Company)
[89]
Wheat straw
MLS ETHOS PLUS MW reactor (MLS GmbH, Leutkirch/Germany)
[68]
Wheat bran
NE-1756 MW oven (Panasonic Corporation)
[116]
pretreat biomass including softwoods, hardwoods, and monocotyledons [118]. Recently, a continuous MW biomass pretreatment reactor was invented by Xu et al. (Chinese patent application no. CN201210107472.8) [119], which consists of a feeding system, a continuous linear adjustable power MW reaction system, anti-MW leakage systems, control and detection systems, heat/humidity systems, material mixing systems, and discharge systems, as shown in Figure 9.1.
A. LIGNOCELLULOSIC BIOMASS
REFERENCES
167
FIGURE 9.1 Continuous MW irradiation reactor. ① Reversing gear control bracket; ② Feed hopper; ③ MW suppressor; ④ Power box; ⑤ Exhaust heat shield; ⑥ Humidity-discharging device; ⑦ Heating cabinet; ⑧ Safety gate; ⑨ Control panel; ⑩ Conveyor belt; ⑪ Discharge hopper; ⑫ End bracket; ⑬ Vibration motor.
9.4 SUMMARY AND PROSPECTS MW-assisted pretreatment has been successfully employed to pretreat various biomass, and has many advantages: (1) It improves pretreatment speed, and thus reduces the reaction time; (2) it increases pretreatment selectivity; (3) it decreases the pretreatment energy input; and (4) it enhances enzymatic hydrolysis efficiency. All of these are mainly caused by the thermal and nonthermal effects of MW irradiation. Because the heat triggered by MW irradiation occurs mainly by two main mechanismsddipolar polarization and ionic conductiondit is critical to choose an appropriate reaction medium. In general, a reaction medium with a high loss tangent is required for good absorption, and consequently, for efficient heating. Although water has been widely used as a good absorbing solvent, the addition of polar or ionic substances such as salt, acid, alkali, or ionic liquids can increase the absorbance level of the medium. In addition, the additives can act on the biomass directly and accelerate rupture of the biomass structure under MW heating. In addition, the combination of MW irradiation and other pretreatment technologies, such as MW with ball milling [118], has also proven effective. Despite the successful investigations of MW-assisted pretreatment of biomass, there are several aspects still need to be improved: (1) a reactor design for biomass pretreatment and scale-up for commercialized purposes; (2) modeling of MW-assisted pretreatment for different origins of biomass; (3) the establishment of a product chain based on MW-assisted pretreatment; and (4) exploration of an MW heating medium similar to ionic liquid, but at a low cost.
Acknowledgments The author gratefully acknowledges support from the National High Technology Research and Development Program of China (863 Program: 2012AA022301), the National Basic Research Program of China (973 Program: 2011CB707401), the National Natural Science Foundation of China (No. 21276259), and the 100 Talents Program of the Chinese Academy of Sciences.
References [1] Motasemi F, Muhammad TA. A review on the microwave-assisted pyrolysis technique. Renew Sustainable Energy Rev 2013;28:317e30. [2] Kappe CO, Stadler A, Dallinger D, Mannhold R, Kubinyi H, Folkers G. Microwaves in organic and medicinal chemistry. John Wiley & Sons; 2012.
A. LIGNOCELLULOSIC BIOMASS
168
9. MICROWAVE PRETREATMENT
[3] Sun Y, Wang Y, Ni X, Gao Y, Shao Q, Liu L, et al. Comparison of ablation zone between 915- and 2450-MHz cooled-shaft microwave antenna: results in in vivo porcine livers. Am J Roentgenol 2009;192:511e4. [4] Gabriel C, Gabriel S, Grant EH, Halstead BSJ, Mingos DMP. Dielectric parameters relevant to microwave dielectric heating. Chem Soc Rev 1998;27:213e23. [5] Bradshaw SM, van Wyk EJ, de Swardt JB. Microwave heating principles and the application to the regeneration of granular activated carbon. J South Afr Inst Min Metall 1998;98:201e12. [6] Mingos DMP, Baghurst DR. Applications of microwave dielectric heating effects to synthetic problems in chemistry. Chem Soc Rev 1991;20:1e47. [7] Collins JM, Leadbeater NE. Microwave energy: a versatile tool for the biosciences. Org Biomol Chem 2007;5:1141e50. [8] Kuhnert N. Microwave-assisted reactions in organic synthesisdare there any nonthermal microwave effects? Angew Chem Int Ed 2002;41:1863e6. [9] Chemat F, Esveld E. Microwave super-heated boiling of organic liquids: origin, effect and application. Chem Eng Technol 2001;24:735e44. [10] Shibata C, Kashima T, Ohuchi K. Nonthermal influence of microwave power on chemical reactions. Jpn J Appl Phys Part 1 1996;35:316e9. [11] Yin C. Microwave-assisted pyrolysis of biomass for liquid biofuels production. Bioresource Technol 2012;129:273e84. [12] Salazar-Gonzalez C, San Martin-Gonzalez MF, Lopez-Malo A, Sosa-Morales ME. Recent studies related to microwave processing of fluid foods. Food Bioprocess Technol 2012;5:31e46. [13] Zhang M, Tang J, Mujumdar AS, Wang S. Trends in microwave-related drying of fruits and vegetables. Trends Food Sci Technol 2006;17:524e34. [14] Ferrando CJ, Andres M, Membrado L. Coal desulphurization with hydroiodic acid and microwaves. Coal Sci Technol 1995;24:1729e32. [15] Kingman SW, Rowson NA. The effect of microwave radiation on the magnetic properties of minerals. J Microwave Power Electromagn Energy 2000;35:144e50. [16] Koleini SMJ, Barani K, Rezaei B. The effect of microwave treatment upon dry grinding kinetics of an iron ore. Miner Process Extr Metall Rev J 2012;33:159e69. [17] Bond GR, Moyes B, Whan DA. Recent applications of microwave heating in catalysis. Catal Today 1993;17:427e37. [18] Zhang HD, Hayward O, Mingos DMP. Effects of microwave dielectric heating on heterogeneous catalysis. Catal Lett 2003;88:33e8. [19] Adachi K, Iwamura T, Chujo Y. Microwave assisted synthesis of organic-inorganic polymer hybrids. Polym Bull 2005;55:309e15. [20] Bhattacharyya S. Advances in organic synthesis using polymer-supported reagents and scavengers under microwave irradiation. Mol Diversity 2005;9:253e7. [21] Ni Z, Masel R. Rapid production of metal-organic frameworks via microwave-assisted solvothermal synthesis. J Am Chem Soc 2006;128:12394e5. [22] Kappe CO. Controlled microwave heating in modern organic synthesis. Angew Chem Int Ed 2004; 43:6250e84. [23] Kappe CO. Microwave dielectric heating in synthetic organic chemistry. Chem Soc Rev 2008;37:1127e39. [24] Wu DS, Han CY, Wang SY, Wu NL, Rusakova IA. Microwave-assisted solution synthesis of SnO nanocrystallites. Mater Lett 2002;55:155e9. [25] Huang YF, Kuan WH, Lo SL, Lin CF. Total recovery of resources and energy from rice straw using microwaveinduced pyrolysis. Bioresour Technol 2008;99:8252e8. [26] Menendez JA, Dominguez A, Inguanzo M, Pis JJ. Microwave-induced drying, pyrolysis and gasification (MWDPG) of sewage sludge: vitrification of the solid residue. J Anal Appl Pyrolysis 2005;74:406e12. [27] Menendez JA, Inguanzo M, Pis JJ. Microwave-induced pyrolysis of sewage sludge. Water Res 2002;36:3261e4. [28] Ericsson M, Colmsjo A. Dynamic microwave-assisted extraction coupled on-line with solid-phase extraction and large-volume injection gas chromatography: determination of organophosphate esters in air samples. Anal Chem 2003;75:1713e9. [29] Haswell SJ, Howarth N. Perturbation of a solid phase separation process by a non-thermal microwave effect. Anal Chim Acta 1999;387:113e20.
A. LIGNOCELLULOSIC BIOMASS
REFERENCES
169
[30] Perez-Cid B, Albores AF, Gomez EF, Lopez EF. Use of microwave single extractions for metal fractionation in sewage sludge samples. Anal Chim Acta 2001;431:209e18. [31] Perez-Cid B, Lavilla I, Bendicho C. Application of microwave extraction for portioning of heavy metals in sewage sludge. Anal Chim Acta 1999;378:201e10. [32] Prevot AB, Gulmini M, Zelano V, Pramauro E. Microwave-assisted extraction of polycyclic aromatic hydrocarbons from marine sediments using nonionic surfactant solutions. Anal Chem 2001;73:3790e5. [33] Punt MM, Raghavan GSV, Belanger JMR, Pare JRJ. Microwave-assisted process for the extraction of contaminants from soil. J Soil Contam 1999;8:577e92. [34] Routray W, Orsat V. Microwave-assisted extraction of flavonoids: a review. Food Bioprocess Technol 2012;5:409e24. [35] Abramovitch RA, ChangQing L, Hicks E, Sinard J. In situ remediation of soils contaminated with toxic metal ions using microwave energy. Chemosphere 2003;53:1077e85. [36] Abramovitch RA, Huang B, Abramovitch DA, Song J. In situ decomposition of PAHs in soil and desorption of organic solvents using microwave energy. Chemosphere 1999;39:81e7. [37] Kawala Z, Atamanczuk T. Microwave-enhanced thermal decontamination of soil. Environ Sci Technol 1998;32:2602e7. [38] Jou CJ. An efficient technology to treat heavy metal-lead-contaminated soil by microwave radiation. J Environ Manage 2006;78:1e4. [39] Lin L, Yuan S, Chen J, Wang L, Wan J, Lu X. Treatment of chlorophenicol-contaminated soil by microwave radiation. Chemosphere 2010;78:66e71. [40] Komatsu F, Takusagawa A, Wada R, Asahina K. Application of microwave treatment technology for radioactive wastes. Waste Manage 1990;10:211e5. [41] Yuan S, Tian M, Lu X. Microwave remediation of soil contaminated with hexachlorobenzene. J Hazard Mater 2006;137:878e85. [42] Wojcichowska E. Application of microwaves for sewage sludge conditioning. Water Res 2005;39:4749e54. [43] Yin GQ, Liao PH, Lo KV. An ozone/hydrogen peroxide/microwave enhanced advanced oxidation process for sewage sludge treatment. J Environ Sci Health A 2007;42:1177e81. [44] Ooshima H, Aso K, Harano Y, Yamamoto T. Microwave treatment of cellulosic materials for their enzymatic hydrolysis. Biotechnol Lett 1984;6:289e94. [45] De Wild PJ, Reith H, Heeres HJ. Biomass pyrolysis for chemicals. Biofuels 2011;2:185e208. [46] Mohan D, Pittman Jr CU, Steele PH. Pyrolysis of wood/biomass for bio-oil: a critical review. Energy Fuels 2006;20:848e89. [47] Demirbas A. Effect of initial moisture content on the yields of oily products from pyrolysis of biomass. J Anal Appl Pyrolysis 2004;72:243e8. [48] Medic D, Darr M, Shah A, Potter B, Zimmerman J. Effects of torrefaction process parameters on biomass feedstock upgrading. Fuel 2012;91:147e54. [49] Chen WH, Kuo PC. Torrefaction and co-torrefaction characterization of hemicellulose, cellulose and lignin as well as torrefaction of some basic constituents in biomass. Energy 2011;36:803e11. [50] Azuma J, Tanaka F, Koshijima T. Enhancement of enzymatic susceptibility of lignocellulosic wastes [sugar cane bagasse, rice straw and rice hulls] by microwave irradiation. J Ferment Technol 1984; 624:377e84. [51] Saha BC, Biswas A, Cotta MA. Microwave pretreatment, enzymatic saccharification and fermentation of wheat straw to ethanol. J Biobased Mater Bioenergy 2008;2:210e7. [52] Hu Z, Wen Z. Enhancing enzymatic digestibility of switchgrass by microwave-assisted alkali pretreatment. Biochem Eng J 2008;38:369e78. [53] Nikolic S, Mojovic L, Rakin M, Pejin D, Pejin J. Utilization of microwave and ultrasound pretreatments in the production of bioethanol from corn. Clean Technol Environ Policy 2011;13:587e94. [54] Lu X, Xi B, Zhang Y, Angelidaki I. Microwave pretreatment of rape straw for bioethanol production: focus on energy efficiency. Bioresour Technol 2011;102:7937e40. [55] Ma H, Liu W, Chen X, Wu Y, Yu Z. Enhanced enzymatic saccharification of rice straw by microwave pretreatment. Bioresour Technol 2009;100:1279e84. [56] Sapci Z. The effect of microwave pretreatment on biogas production from agricultural straws. Boresour Technol 2013;128:487e94.
A. LIGNOCELLULOSIC BIOMASS
170
9. MICROWAVE PRETREATMENT
[57] Tanaka M, Taniguchi M, Morita T, Matsuno R, Kamikubo T. Effect of chemical treatment on solubilization of crystalline cellulose and cellulosic wastes with Pellicularia filamentosa cellulase. J Ferment Technol 1979;57:186e90. [58] Taniguchi M, Tanaka M, Matsuno R, Kamikubo T. Evaluation of chemical pretreatment for enzymatic solubilization of rice straw. Eur J Appl Microbiol Biotechnol 1985;14:35e9. [59] Chang VS, Holtzapple MT. Fundamental factors affecting biomass enzymatic reactivity. Appl Biochem Biotechnol 2000;84e86:5e37. [60] Binod P, Sindhu R, Singhania RR, Vikram S, Devi L, Nagalakshmi S, et al. Bioethanol production from rice straw: an overview. Bioresour Technol 2010;101:4767e74. [61] Kumar P, Barrett DM, Delwiche MJ, Stroeve P. Methods for pretreatment of lignocellulosic biomass for efficient hydrolysis and biofuel production. Ind Eng Chem Res 2009;48:3713e29. [62] Persson T, Ren JL, Joelsson E, Jonsson AS. Fractionation of wheat and barley straw to access high-molecular-mass hemicelluloses prior to ethanol production. Bioresour Technol 2009;100:3906e13. [63] Sun R, Lawther JM, Banks WB. Influence of alkaline pre-treatments on the cell wall components of wheat straw. Ind Crops Prod 1995;4:127e45. [64] Zhu S, Wu Y, Yu Z, Liao J, Zhang Y. Pretreatment by microwave/alkali of rice straw and its enzymic hydrolysis. Process Biochem 2005;40:3082e6. [65] Zhu S, Wu Y, Yu Z, Zhang X, Wang C, Yu F, et al. Simultaneous saccharification and fermentation of microwave/alkali pre-treated rice straw to ethanol. Biosyst Eng 2005;92:229e35. [66] Zhu S, Wu Y, Yu Z, Chen Q, Wu G, Yu F, et al. Microwave-assisted alkali pre-treatment of wheat straw and its enzymatic hydrolysis. Biosyst Eng 2006;94:437e42. [67] Zhu S, Wu Y, Yu Z, Zhang X, Wang C, Yu F, et al. Production of ethanol from microwave-assisted alkali pretreated wheat straw. Process Biochem 2006;41:869e73. [68] Janker-Obermeier I, Sieber V, Faulstich M, Schieder D. Solubilization of hemicellulose and lignin from wheat straw through microwave-assisted alkali treatment. Ind Crops Prod 2012;39:198e203. [69] Rodrigues THS, Rocha MVP, Macedo GRD, Goncalves LRB. Ethanol production from cashew apple bagasse: improvement of enzymatic hydrolysis by microwave-assisted alkali pretreatment. Appl Biochem Biotechnol 2011;164:929e43. [70] Zhao X, Zhou Y, Zheng G, Liu D. Microwave pretreatment of substrates for cellulase production by solid-state fermentation. Appl Biochem Biotechnol 2010;160:1557e71. [71] Liu JG, Wang QH, Ma HZ, Li J. Effect of pretreatment methods on L-lactic acid production from vinasse fermentation. Adv Mater Res 2010;2:113e6. [72] Liu J, Wang Q, Wang S, Zou D, Sonomoto K. Utilisation of microwave-NaOH pretreatment technology to improve performance and L-lactic acid yield from vinasse. Biosyst Eng 2012;112:6e13. [73] Singha A, Tuteja S, Singh N, Bishnoi NR. Enhanced saccharification of rice straw and hull by microwaveealkali pretreatment and lignocellulolytic enzyme production. Bioresour Technol 2011;102: 1773e82. [74] Xu J, Chen H, Ka´da´r Z, Thomsen AB, Schmidt JE, Peng H. Optimization of microwave pretreatment on wheat straw for ethanol production. Biomass Bioenergy 2011;35:3859e64. [75] Vani S, Binod P, Kuttiraja M, Sindhu R, Sandhya SV, Preeti VE, et al. Energy requirement for alkali assisted microwave and high pressure reactor pretreatments of cotton plant residue and its hydrolysis for fermentable sugar production for biofuel application. Bioresour Technol 2012;112:300e7. [76] Kim M, Aita G, Day DF. Compositional changes in sugarcane bagasse on low temperature, long-term diluted ammonia treatment. Appl Biochem Biotechnol 2010;161:34e40. [77] Aita GA, Salvi DA, Walker MS. Enzyme hydrolysis and ethanol fermentation of dilute ammonia pretreated energy cane. Bioresour Technol 2010;102:4444e8. [78] Salvi DA, Aita GM, Robert D, Bazan V. Ethanol production from sorghum by a dilute ammonia pretreatment. J Ind Microbiol Biotechnol 2009;37:27e34. [79] Salvi DA, Aita GM, Robert D, Bazan V. Dilute ammonia pretreatment of sorghum and its effectiveness on enzyme H hydrolysis and ethanol fermentation. Appl Biochem Biotechnol 2010;161:67e74. [80] Chen C, Boldor D, Aita G, Walker M. Ethanol production from sorghum by a microwave-assisted dilute ammonia pretreatment. Bioresour Technol 2012;110:190e7. [81] Choudhary R, Umagiliyage AL, Liang Y, Siddaramu T, Haddock J, Markevicius G. Microwave pretreatment for enzymatic saccharification of sweet sorghum bagasse. Biomass Bioenergy 2012;39:218e26.
A. LIGNOCELLULOSIC BIOMASS
REFERENCES
171
¨ z C. Progress in bioethanol processing. Prog Energy Combust Sci 2008;34:551e73. [82] Balat M, Balat H, O [83] Gomez LD, Steele-King C, McQueen-Mason SJ. Sustainable liquid biofuels from biomass: the writing’s on the walls. New Phytol 2008;178:473e85. [84] Keshwani DR, Cheng JJ. Switchgrass for bioethanol and other value-added applications: a review. Bioresour Technol 2009;100:137e45. [85] Neureiter M, Danner H, Thomasser C, Saidi B, Braun R. Dilute-acid hydrolysis of sugarcane bagasse at varying conditions. Appl Biochem Biotechnol 2002;98e100:49e58. [86] Jones DA, Lelyveld TP, Mavrofidis SD, Kingman SW, Miles NJ. Microwave heating applications in environmental engineering e a review. Resour Conserv Recycl 2002;34:75e90. [87] Chen WH, Jheng JG, Yu AB. Hydrogen generation from a catalytic water gas shift reaction under microwave irradiation. Int J Hydrogen Energy 2008;33:4789e97. [88] Chen WH, Lin BC. Effect of microwave double absorption on hydrogen generation from methanol steam reforming. Int J Hydrogen Energy 2010;35:1987e97. [89] Xia A, Cheng J, Song W, Yu C, Zhou J, Cen K. Enhancing enzymatic saccharification of water hyacinth through microwave heating with dilute acid pretreatment for biomass energy utilization. Energy 2013; 61:158e66. [90] Palmarola-Adrados B, Galbe M, Zacchi G. Combined steam pretreatment and enzymatic hydrolysis of starchfree wheat fibers. Appl Biochem Biotechnol 2004;115:989e1002. [91] Li J, Yang Y, Chen H, Jiang F, Ling J, Liu M, et al. Comparison of saccharification process by acid and microwave-assisted acid pretreated swine manure. Bioprocess Biosyst Eng 2009;32:649e54. [92] Chen WH, Tu YJ, Sheen HK. Disruption of sugarcane bagasse lignocellulosic structure by means of dilute sulfuric acid pretreatment with microwave-assisted heating. Appl Energy 2011;88:2726e34. [93] Chen WH, Ye SC, Sheen HK. Hydrolysis characteristics of sugarcane bagasse pretreated by dilute acid solution in a microwave irradiation environment. Appl Energy 2012;93:237e44. [94] Swatloski RP, Spear SK, Holbrey JD, Rogers RD. Dissolution of cellose with ionic liquids. J Am Chem Soc 2002;124:4974e5. [95] Zhang Z, Zhao ZK. Solid acid and microwave-assisted hydrolysis of cellulose in ionic liquid. Carbohydr Res 2009;344:2069e72. [96] Zhu S, Wu Y, Chen Q, Yu Z, Wang C, Jin S, et al. Dissolution of cellulose with ionic liquids and its application: a mini-review. Green Chem 2006;8:325e7. [97] Liu Z, Hao J, Dai H, Wang J. Cellulose saccharification after ionic liquid pretreatment with microwave. J Microwaves 2011;21:93e6. [98] Ha SH, Mai NL, An G, KOO YM. Microwave-assisted pretreatment of cellulose in ionic liquid for accelerated enzymatic hydrolysis. Bioresour Technol 2011;102:1214e9. [99] Fan L, Jin R, Liu Y, An M, Chen S. Enhanced extraction of patchouli alcohol from Pogostemon cablin by microwave radiation-accelerated ionic liquid pretreatment. J Chromatogr B 2011;879:3653e7. [100] Jin R, Fan L, An X. Microwave assisted ionic liquid pretreatment of medicinal plants for fast solvent extraction of active ingredients. Sep Purif Technol 2011;83:45e9. [101] Jiao J, Gai Q, Fu Y, Zhu Y, Luo M, Wang W, et al. Microwave-assisted ionic liquids pretreatment followed by hydro-distillation for the efficient extraction of essential oil from Dryopteris fragrans and evaluation of its antioxidant efficacy in sunflower oil storage. J Food Eng 2013;117:477e85. [102] Jiao J, Gai Q, Fu Y, Zhu Y, Luo M, Zhao C, et al. Microwave-assisted ionic liquids treatment followed by hydro-distillation for the efficient isolation of essential oil from Fructus forsythiae seed. Sep Purif Technol 2013;107:228e37. [103] Tanaka M, Sato M. Microwave heating of water, ice, and saline solution: molecular dynamics study. J Chem Phys 2007;126:034509. [104] Rozzi NL, Sing RK. The effect of selected salts on the microwave heating of starch solutions. J Food Process Preserv 2000;24:265e73. [105] Liu L, Sun JS, Li M, Wang SH, Pei HS, Zhang JS. Enhanced enzymatic hydrolysis and structural features of corn stover by FeCl3 pretreatment. Bioresour Technol 2009;100:5853e8. [106] Lu¨ J, Zhou P. Optimization of microwave-assisted FeCl3 pretreatment conditions of rice straw and utilization of Trichoderma viride and Bacillus pumilus for production of reducing sugars. Bioresour Technol 2011;102:6966e71.
A. LIGNOCELLULOSIC BIOMASS
172
9. MICROWAVE PRETREATMENT
[107] Li H, Xu J. Optimization of microwave-assisted calcium chloride pretreatment of corn stover. Bioresour Technol 2013;127:112e8. [108] Verma P, Watanabe T, Honda Y, Watanabe T. Microwave-assisted pretreatment of woody biomass with ammonium molybdate activated by H2O2. Bioresour Technol 2011;102:3941e5. [109] Liu J, Takada R, Karita S, Watanabe T, Honda Y, Watanabe T. Microwave-assisted pretreatment of recalcitrant softwood in aqueous glycerol. Bioresour Technol 2010;101:9355e60. [110] Binod P, Satyanagalakshmi K, Sindhu R, Janu KU, Sukumaran RK, Pandey A. Short duration microwave assisted pretreatment enhances the enzymatic saccharification and fermentable sugar yield from sugarcane bagasse. Renewable Energy 2012;37:109e16. [111] Boonmanumsin P, Treeboobpha S, Jeamjumnunja K, Luengnaruemitchai A, Chaisuwan T, Wongkasemjit S. Release of monomeric sugars from Miscanthus sinensis by microwave-assisted ammonia and phosphoric acid treatments. Bioresour Technol 2012;103:425e31. [112] Peng H, Li H, Luo H, Xu J. A novel combined pretreatment of ball milling and microwave irradiation for enhancing enzymatic hydrolysis of microcrystalline cellulose. Bioresour Technol 2013;130:81e7. [113] Palmarola-Adrados B, Galbe M, Zacchi G. Pretreatment of barley husk for bioethanol production. J Chem Technol Biotechnol 2005;80:85e91. [114] Shi J, Pu Y, Yang B, Ragauskas A, Wyman CE. Comparison of microwaves to fluidized sand baths for heating tubular reactors for hydrothermal and dilute acid batch pretreatment of corn stover. Bioresour Technol 2011;102:5952e61. 5953. [115] Zhu S, Wu Y, Yu Z, Zhang X, Li H, Gao M. The effect of microwave irradiation on enzymatic hydrolysis of rice straw. Bioresour Technol 2006;97:1964e8. [116] Wang TH, Lu S. Production of xylooligosaccharide from wheat bran by microwave assisted enzymatic hydrolysis. Food Chem 2013;138:1531e5. [117] Rose D, Inglett G. Two-stage hydrothermal treatment of wheat (Triticum aestivum) bran for the production of feruloylated arabinoxylooligosaccharides. J Agric Food Chem 2010;58:6427e32. [118] Azuma J, Tsubaki S, Sakamoto M, Yudianti R, Hermiati E. Refinery of biomass by utilization of specific effects of microwave irradiation. Procedia Chem 2012;4:17e25. [119] Xu J, Peng H, Li H. A microwave continuous reactor using for biomass pretreatment. Chinese Patent. Application No.: CN201210107472.8.
A. LIGNOCELLULOSIC BIOMASS
9 Microwave Pretreatment Jian Xu National Key Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, China
9.1 INTRODUCTION 9.1.1 Microwave Microwave (MW) is electromagnetic waves with wavelengths in the range of 0.01e1 m and a corresponding frequency ranging from 0.3 to 300 GHz. The MW region in the whole electromagnetic spectrum is between the infrared and radio frequencies. As a kind of electromagnetic wave, MW has both electric and magnetic fields. Based on the interaction between MW irradiation and materials, a material can be categorized into three types [1]: (1) absorbing materials such as water and glycerol, which can absorb MW, are also called MW dielectric; (2) conductors (mainly metals), which MW cannot penetrate, and most of which is reflected; and (3) insulators or MW-transparent materials, including quartz and Teflon, which allow MW to pass through without loss. MW dielectric is commonly used as the heating medium [2].
9.1.2 MW Heating Mechanism MW heating is also called dielectric heating, because of the dielectric employed to absorb MW irradiation. To avoid interference with telecommunications and cellular phone frequencies, 915 MHz (896 in the United Kingdom) and 2450 MHz are two frequencies usually used for MW heating for industrial, scientific, and medical applications [3]. A domestic MW oven normally operates at 2450 MHz. The way a material is heated by MW depends on its shape, size, and dielectric constant, and the nature of the MW equipment used. The main MW heating mechanisms consist of dipolar polarization, conduction, and interfacial polarization [4e6]: (1) Dipolar polarization is responsible for most MW heating in the solvent systems. Polar molecules characterized with an electrical dipole moment will align themselves in an electromagnetic field. In an electromagnetic wave with rapid oscillation, the polar molecules will rotate continuously, aligning with it. This is called dipolar polarization. As Pretreatment of Biomass http://dx.doi.org/10.1016/B978-0-12-800080-9.00009-8
157
Copyright Ó 2015 Elsevier B.V. All rights reserved.
158
9. MICROWAVE PRETREATMENT
TABLE 9-1
Comparison of conventional and MW heating
MW irradiation
Conventional heating
Conversion of energy
Transfer of energy
In corn volumetric and uniform heating
Superficial Heating via Convection/conduction
Rapid and efficient
Slow, inefficient
Selective
Non selective
Hot spots
No hot spot
Dependent of the properties of the material
Less dependent
Precise and controlled heating
Less controllable
the field alternates, the molecule reverses direction to align itself with it. This causes energy to be lost from the dipole by molecular friction and collisions, giving rise to dielectric heating. For this reason, MW heating is always called dielectric heating. (2) Conduction mechanism happens when the dissolved charged particles in a sample (electrons, ions, etc.) oscillate back and forth under the influence of the electric component of MW irradiation. They collide with the adjacent molecules or atoms, which cause agitation or motion, and heat is thus created. (3) Interfacial polarization is a phenomenon viewed as a combination of conduction and dipolar polarization. It is an important mechanism for systems composed of conducting and nonconducting materials.
9.1.3 MW Effects The action of MW irradiation results from materialewave interactions leading to thermal effects, specific MW effects and nonthermal effects [7e10]. A combination of these contributions is responsible for the observed effects. Heat from conventional heating is transferred from the surface toward the center of the material by conduction, convection, and radiation. It is relatively slow and inefficient, and depends on the thermal conductivity of the material and convection currents. MW heating is characterized by converting electromagnetic energy into thermal energy, which is a kind of energy conversion rather than heating. Compared with conventional heating, the heat produced by MW irradiation is throughout the volume of the materials rather than an external source. The differences between conventional and MW heating are presented in Table 9.1 [11].
9.2 MW APPLICATION Since the first MW oven built in 1947 by Raytheon Corporation and an early commercial model introduced in 1954, MW heating has gained popularity in food processing because of its ability to achieve high heating rates, significant reduction in cooking time, more uniform heating, safe handling, ease of operation, and low maintenance [12,13]. Over the decades, this
A. LIGNOCELLULOSIC BIOMASS
9.2 MW APPLICATION
159
technology has spread widely to such applications as analytical chemistry, heating and vulcanization of rubber, plasma processing, chemical synthesis and processing, and waste remediation. MW irradiation has acquired a great deal of attention in domestic, industrial, and medical applications. It has been used in many applications including mineral and metallurgic processes [14e16], chemical catalysis [17,18], organic/inorganic syntheses [19e24], pyrolysis [25e27], phase separation and extraction processes [28e34], remediation of soil and hazardous/radioactive waste [35e41], and sewage sludge treatment [42,43]. The rapid and effective heating properties of MW have led to its application in biomass pretreatment since 1984 [44]. As a promising alternative feedstock to replace crude oil, biomass has gained attention recently owing to its potential in producing energy, materials, and chemicals. However, the recalcitrant nature of biomass makes the pretreatment step necessary to improve its enzymolysis efficiency. A good biomass pretreatment produces high enzyme-digestible biomass, low-cost inhibitors, and ease of operation. MW has been a powerful tool in the pretreatment of various biomass including agricultural residues, woody biomass, grass, energy plants, and industrial residuals. This chapter addresses the most commonly preferred MW-assisted biomass pretreatments: (1) MW/water alone, (2) MW/alkali, (3) MW/acid, (4) MW/ionic liquid, (5) MW/ salt, and (6) other combined MW-assisted pretreatment. Although MW-assisted pyrolysis and torrefaction have been widely used in biofuel production [45e49], it is a direct conversion technology rather than a pretreatment, which is not included in this chapter.
9.2.1 Microwave/Water Alone Ooshima et al. (1984) were the first to report the use of MW heating for the pretreatment of rice straw and bagasse [44]. It was found that the enzymatic accessibilities increased by 1.6 and 3.2 times for rice straw and bagasse, respectively, compared with untreated samples. In the same year, sugar cane bagasse, rice straw, and rice hulls were pretreated by Azuma et al., who observed no substantial change in cellulose crystallinity detected by short heating (<8 min) below 230 C [50]. The enzymatic hydrolysis of all the biomass was markedly improved by MW pretreatment above 160 C and showed a maximum at 223e228 C, independent of the source. The increase in surface area of cellulose resulting from the degradation of lignin and hemicellulose is considered a rate-determining factor for cellulose enzymolysis because of the stability of cellulose crystallinity under MW irradiation. The effects of temperature and duration of MW pretreatment of wheat straw on its enzymatic saccharification to fermentable sugars were evaluated [51]. The yield of 544 mg/g on monomeric sugars was obtained from straw from MW (200 C, 10 min) pretreated (8.6%, w/v, in water) after enzymatic saccharification (45 C, pH 5.0, 120 h) using a cocktail of three commercial enzyme preparations. Switchgrass soaked in water was also pretreated by MW irradiation [52]. The total sugar (xylose þ glucose) yield of 34.5 g/100 g biomass was obtained, which was 53% higher than that from conventional heating. A comparison made of pretreatment between MW and ultrasound was carried out by Nikolic et al, who found that MW pretreatment resulted in higher yields of glucose and bioethanol compared with ultrasound pretreatment. This investigation also indicated that the time for simultaneous saccharification and fermentation was markedly decreased by MW pretreatment, and thus, the bioethanol production cost could be reduced [53]. Lu et al. [54]
A. LIGNOCELLULOSIC BIOMASS
160
9. MICROWAVE PRETREATMENT
also pointed out that MW generates higher power densities, enabling higher production rates and lower production costs compared with conventional heating. Ma et al. used a BoxeBehnken design (BBD) and response surface methodology optimizing MW pretreatment on rice straw [55]. MW intensity, irradiation time, and substrate concentration were identified as three main factors affecting enzymatic saccharification. It was also found that the silicified waxy surface and the ligninehemicellulose complex were broken down and silicon and lignin were partly removed under MW pretreatment. MW was also employed to pretreat milled barley, spring wheat, winter wheat, and oat straw for biogas [56]. However, the performance of anaerobic digestion for these four pretreated materials did not improve, which suggests that MW pretreatment might be not suitable for milled straw types in biogas plants.
9.2.2 Microwave/Alkali Lignin has been considered a barrier to enzymatic hydrolysis, and the removal of lignin has been confirmed to enhance biomass susceptibility for enzymes [57,58]. It was postulated that alkali treatment releases hemicelluloses as oligomers [59], causing less sugar degradation at a low alkali concentration [60,61]. Higher alkali used in pretreatment tends to form hemicellulose with a higher molecular weight, which is considered to expand its economic value [62]. However, a relatively long residence time is required to obtain high yields for alkaline pretreatment at a lower temperature [61]. It has been reported that 80% of hemicelluloses and 60% of lignin could be released from wheat straw by sodium hydroxide pretreatment at room temperature with a long time of 144 h [63]. As a rapid and effective heating source, MW irradiation has been widely employed in alkali pretreatment to obtain the higher temperature required in the delignification process. Some studies with MW-assisted alkali pretreatment on wheat straw and rice straw confirmed its effectiveness in enhancing the enzymatic hydrolysis and ethanol production [64e67]. Compared with pretreatment with alkali alone, biomass pretreated by MW and alkali had a higher hydrolysis rate and glucose content in hydrolysate. A higher yield of 90% maximum potential sugars was achieved with switchgrass pretreated by alkali (NaOH) at 0.1 g/g biomass with MW as the heat source [52]. It was also found from scanning electron microscope (SEM) images that MW was more effective in disrupting the recalcitrant structures compared with conventional heating. Another investigation with MW-assisted alkali pretreatment on wheat straw for saccharide- and lignin-releasing potential was carried out; it was confirmed effective by extracting more than 80% of hemicellulose and 90% of lignin at a very short pretreatment time and low degree of cellulose solubilization [68]. Rodrigues et al. [69] focused on the potential of MW-assisted alkali pretreatment to improve the rupture of the recalcitrant structures of the cashew apple bagasse (CAB). With an alkali concentration of 0.2 and 1.0 mol/L, solid percentage of 16% (w/v), and enzyme load of 30 FPU/g CAB, pretreatment time and MW power had no significant effect on glucose concentration. Zhao et al. [70] used an NaOH solution to pretreat rice hulls with MW irradiation; they mentioned that increased accessibility of the substrates by MW pretreatment was mainly achieved by rupturing the rigid structure of rice hulls, and the reducing sugar content was increased by 13% compared with that of rice hulls without
A. LIGNOCELLULOSIC BIOMASS
9.2 MW APPLICATION
161
pretreatment. Hu and Wen [52] described the pretreatment of switchgrass using MW. With MW-assisted alkali pretreatment, they reported that the total sugar yield was 53%, which was higher than that obtained by conventional heating. MW-assisted alkali pretreatment was also used to pretreat vinasse for L-lactic acid production. The maximum lactic acid concentration of 17.5 g/L was obtained with the alkali concentration of 8% and MW power of 700 W, higher than those of the independent method used. In addition, the fermentation time could be 1 day earlier than other methods [71]. Another follow-up study was done on L-lactic acid from vinasse, and an optimized condition for the MW-alkali coupling pretreatment process was obtained as follows: 0.06 g/g NaOH, 523 W for 8 min, and solid-to-liquid ratio at 1:2, which resulted in 86.0 mg/g L-lactic acid [72]. Compared with independent NaOH and MW pretreatment methods, MWeNaOH coupled pretreatment reduced hemicellulose and lignin in vinasse more markedly by 28.24% and 10.53%, respectively. The crystallinity of vinasse was also reduced significantly from 69.34% to 33.68%. Pretreated vinasse by MWeNaOH coupling exhibited more surface holes, resulting in a wider specific surface area. Orthogonal design and BBD have been used to optimize MW-assisted alkaline pretreatment [73,74]. The ethanol yield was 148.93 g/kg wheat straw at this condition, much higher than that from the untreated material, which was only 26.78 g/kg. Pretreatment on rice straw and hulls was optimized by Singh et al. using a BBD [73]. It was found that alkali (NaOH) concentration, irradiation time, and substrate concentration were main factors affecting the efficiency of saccharification. The optimal conditions for these three factors were 2.75%, 22.50 min, and 30 g/L, respectively. A study by Xu et al. used an orthogonal design to optimize MW pretreatment on wheat straw for ethanol production; pretreatment with a ratio of biomass to liquid of 80 g/kg, NaOH concentration of 10 kg/m3, and MW power of 1000 W for 15 min was confirmed to be optimal [74]. Energy consumption was compared between alkali-assisted MW pretreatment (AAMP) on cotton plant residue (CPR) and high-pressure reactor pretreatment [75]. The energy requirement to pretreat a unit quantity of CPR by AAMP was calculated to be 5 times less than by a high-pressure reactor treatment. Ammonia has also been widely used in pretreatment and has demonstrated great success in the delignification of sugarcane bagasse, sorghum bagasse, corn stover, barley hull, and municipal waste [76e79]. Combined pretreatment of MW and dilute ammonia proved effective in removing 48% of the original lignin from sorghum bagasse at comparatively low ammonia concentrations and lower temperature, and in a relatively short reaction time compared with other technologies. The best results in terms of glucose (4.2 g glucose/10 g dry biomass) and ethanol yields (2.1 g ethanol/10 g dry biomass) were observed with 1- to 2-mm biomass fibers pretreated at 130 C for 1 h. SEM images indicated that the biomass structure was deformed as a result of the swelling of cellulose and delignification, with both factors contributing to improved digestibility. In addition, glycerol, organic acid, and furfural concentrations were too insignificant to have caused toxicity to the enzymes and yeast during enzymatic hydrolysis and fermentation studies, respectively [80]. In addition to NaOH and ammonia, lime is another alkaline used in biomass pretreatment. MW-assisted lime pretreatment on sweet sorghum bagasse (SSB) was evaluated and the optimal pretreatment condition was identified as 0.1 g lime and 10 mL water/g SSB in 4 min. Under these conditions, a sugar yield of 32.2 g/100 g SSB (equivalent to 52.6% of maximal potential sugars) was achieved. With the same water content and exposure time,
A. LIGNOCELLULOSIC BIOMASS
162
9. MICROWAVE PRETREATMENT
but without lime, a sugar yield of 39.8 g/100 g SSB (equivalent to 65.1% of maximal total sugars) was observed. The higher sugar recovery without lime was mainly due to high sugar release during pretreatment. However, with lime, sugar degradation took place, which resulted in less sugar yield although lime made cellulose more accessible to enzymes, as evidenced by a higher percentage of increase of total reducing sugars during enzymatic hydrolysis. Results from this study were strongly supported by Fourier transformeinfrared spectroscopy (FTIR) and SEM images. Overall, in a short time and with a simple setup, MW irradiation showed great promise as a leading pretreatment technique for SSB [81]. Saha compared pretreatment on wheat straw between MW-alone and MW-assisted lime; the release of 604 mg total sugars/g straw after enzymatic hydrolysis was achieved from MW pretreatment with lime (0.1 g/g straw) at 160 C for 10 min, higher than that from pretreatment by MW alone (200 C, 10 min), which was 544 mg/g straw [51].
9.2.3 Microwave/Acid Dilute acid pretreatment is able to convert hemicellulose contained in biomass to soluble sugars and facilitates the subsequent enzymatic hydrolysis of cellulose [82e84]. However, the hydrolysis of polysaccharides also leads to the formation of sugar degradation products: namely, inhibitors such as furfural and 5-hydroxymethylfurfural (HMF) [85], which not only reduce the yield of sugar monomers but also act as fermentation inhibitors. It is therefore necessary to choose reaction conditions that keep the generation of inhibitors at a lower level. To achieve dilute acid pretreatment, the process is usually carried out in a high-temperature environment using conventional heating. Apart from the conventional heating, MW heating is another way to accomplish disruption of recalcitrant structures in biomass. This arises from the fact that water, cellulose, hemicellulose, and other low-molecular compounds (e.g., organic acid) belong to dielectrics [44]. Consequently, the dilute acid solution can be heated by MW through a process of dielectric heating [86e88]. MW-assisted dilute acid pretreatment had remarkable results in promoting the enzymatic saccharification of water hyacinth [89]. After pretreatment, hemicellulose was hydrolyzed into monosaccharide units, and a large amount of cellulose was released from the degraded lignocellulose matrix to react with cellulase on larger surface areas. As a result, glucose yield increased after enzymatic hydrolysis. When 20 g/L water hyacinth feedstock was pretreated with MW at 140 C for 15 min with 1% H2SO4, the highest reducing sugar yield obtained after enzymatic hydrolysis was 48.3 g/100 g hyacinth, which was 94.6% of the theoretical reducing sugar yield. In the study of Palmarola-Adrados et al. [90], starch-free wheat fibers were pretreated in dilute sulfuric acid solutions using MW heating; biomass pretreatment with MW heating was able to give a higher sugar yield compared with steam explosion. Li et al. [91] carried out pretreatment of swine manure in sulfuric acid solutions heated by MW; MW irradiation led to a higher yield of reducing sugar, shorter reaction time, and lower energy consumption, and was a suitable technique for the saccharification process of swine manure. Sugarcane bagasse was pretreated via MW-assisted heating in a dilute sulfuric acid solution (0.2 M); an increase in reaction temperature significantly intensified destruction of the biomass structure of bagasse and the pretreated bagasse particles were simultaneously characterized by fragmentation and swelling [92]. Another investigation was made by the same research group with acid concentrations in the range of 0e0.02 M at 180 C for 30 min [93].
A. LIGNOCELLULOSIC BIOMASS
9.2 MW APPLICATION
163
The derivative thermogravimetric analysis (DTG) curve evolved from a double-peak distribution to a single-peak one owing to hydrolysis of most of the hemicellulose. However, the characteristics of crystalline cellulose and lignin were obviously exhibited in X-ray diffraction and FTIR analyses, revealing that the two constituents were barely affected by the pretreatment. However, a higher acid concentration led to higher yields of furfural and HMF, stemming from the further degradation of xylose and glucose. From the operating point of view for sugarcane bagasse pretreatment, an optimal acid concentration of 0.005 M and a heating temperature of 180 C for 30 min were suggested by this study. A comparison of pretreatment with MW alone, MWelime and MWedilute acid revealed that the highest yield of total sugars (glucose, xylose, arabinose, and galactose) after enzymatic hydrolysis was 651 mg/g straw, obtained from wheat straw pretreated by MW dilute acid (0.5% H2SO4, w/v) at 160 C for 10 min, higher than that from MW pretreatment with lime (0.1 g/g straw) at 160 C for 10 min (604 mg total sugars/g straw) and MW alone at 200 C for 10 min (544 mg/g straw) [51].
9.2.4 Microwave/Ionic Liquid The main advantage of using ionic liquids for bioethanol production is represented by the possibility of complete solubilization of the biomass. Swatloski et al. suggested that solubilization results from breakdown of the H-bonds of the polysaccharides by the anion of the ionic liquids [94]. A variant of the pretreatment with ionic liquids is represented by the MW-assisted pretreatment of biomass in ionic liquids [95,96]. The method is characterized by a shorter reaction time (owing to MW irradiation) and better solubilization of the biomass. According to Zhu et al., the raw biomass is directly solubilized in the ionic liquid in the presence of MW and cellulose is precipitated by adding water. The other organic compounds (such as lignins) remain in solution. Experimental results [96] showed that the yields in ethanol are similar to the ones obtained through steam explosion or chemical pretreatment. Liu et al. used 1-n-butyl-methylimidazolium chloride ([Bmim]Cl) with MW to pretreat cellulose and found that degree of polymerization of cellulose decreased after being pretreated [97]. The saccharification rate reached a maximal value of 18.3 g/L/h at 300 W, 90 C after 1 h, increased by 160% compared with untreated cellulose. SEM and FTIR spectroscopic analysis showed that the cellulose was depolymerized into fragments and the crystallinity of the cellulose significantly decreased after pretreatment. Ha et al. compared different pretreatments with and without MW irradiation on cotton cellulose [98]. They observed that the rate of enzymatic hydrolysis of cotton cellulose increased by at least 12-fold after pretreatment at 110 C, and by 50-fold after ionic liquid dissolution with MW irradiation, showing that cellulose pretreatment with ionic liquid and MW irradiation could be a potential alternative pretreatment method. A novel alternative extraction method for the fast extraction and determination of patchouli alcohol from Pogostemon cablin with MW irradiation-assisted ionic liquid pretreatment (MRAILP) was developed [99]. Under optimized conditions, the extraction yield of patchouli alcohol by MRAILP was 1.9%, which increased to 166% compared with MW-assisted extraction. MW-assisted ionic liquid pretreatment (MILP) was also used to destroy the cell walls of medicinal plants (Cynanchum paniculatum) before solvent extraction, to increase the accessibility of active ingredients to the solvents. The paeonol extraction yield by MILP was higher
A. LIGNOCELLULOSIC BIOMASS
164
9. MICROWAVE PRETREATMENT
than that of ionic liquid aqueous solution and Soxhlet extraction. In addition, the results of bioassay test showed that the antibacterial effect of crude extract by MILP was almost the same with that of Soxhlet extraction [100]. Because of the strong dissolution abilities of ionic liquids for plant cell walls, integrated MW-assisted ionic liquids pretreatment followed by hydro-distillation (MILPeMHD) was carried out for the efficient extraction of essential oil from the rare Dryopteris fragrans [101]. The MILPeMHD was effective in obtaining much higher yield (0.9%) of essential oil within a shorter duration (14.2 min) than that from the reported solvent-free MW extraction (0.3% and 94 min). The antioxidant efficacy of the extracted essential oil in the stabilization of sunflower oil was comparable to the commonly employed atocopherol (VE), indicating that MILPeMHD might be a potential and highly efficient technique for the extraction of valuable essential oils in flavor food industries. Another investigation into MILT-MHD for isolating essential oil from Fructus forsythiae seed was done by Jiao et al. [102]. Compared with conventional hydro-distillation (HD) and emerging MW-assisted aqueous ionic liquid hydro-distillation (MAILHD), MILT-MHD obtained a yield of essential oil (9.6%) in 29.3 min, much higher than those from HD (4.1% and 100 min) and MAILHD (5.4% and 45 min).
9.2.5 Microwave/Salt Molecular dynamics study has been done on MW heating of water, ice, and saline solution [103]. Dilute salt water was significantly more heated than pure water because of the fieldinduced motion of salt ions, especially for large ions, by MW electric field and energy transfer to water molecules by collisions. Other research explored the effects of selected salts (NaCl, CaCl2, FeCl3, Na4P2O7, and Na3P3O10) on the MW heating profile of a starch solution. Results showed that samples containing salt heated faster than those that contained no salt for heating times in excess of 40 s [104]. The reason might be that the addition of salt could increase water’s loss factor, causing the water to heat faster. Because of this, if slat is added to MW-assisted pretreatment, it might lead to better results. In fact, Liu et al. [105] demonstrated that MW-assisted pretreatment with FeCl3 could easily and effectively solubilize hemicellulose into monomeric and oligomeric sugars and disrupt ether and ester linkages between lignin and carbohydrates, but did not affect delignification. Enzymatic hydrolysis of pretreated corn stover resulted in an optimum yield of 98.0%, which was significantly higher than that from untreated corn stover (22.8%). SEM and XRD analysis indicated that MW-assisted FeCl3 pretreatment apparently damaged the surface of corn stover. The conditions for MW-assisted FeCl3 pretreatment of rice straw were optimized using BBD [106]. Optimal conditions were as follows: 0.14 mol/L FeCl3, 160 C, 19 min with substrate concentration at 109 g/L. Reducing sugar of 6.6 g/L was produced from the degradation of pretreated rice straw with Trichoderma viride and Bacillus pumilus, 2.9 times higher than that of untreated rice straw. Li and Xu [107] used CaCl2-assisted MW pretreatment to pretreat corn stover using the central composite design of response surface methodology. Temperature and time were identified as the main factors affecting the enzymatic digestibility of corn stover. The enzymatic hydrolysis ratio of cellulose was 90.7% and glucose recovery was 65.5% under optimal conditions at 162.1 C for 12 min with a solid-to-liquid ratio of 10% (w/v). This process achieved a temperature of about 160 C, necessary for lignocellulose pretreatment under atmospheric pressure using cheap calcium chloride as the heating medium.
A. LIGNOCELLULOSIC BIOMASS
9.3 MW PRETREATMENT REACTORS
165
9.2.6 Other MW-Based Combined Pretreatments Pretreatment of beech wood with MW-assisted ammonium ions and peroxometal complexes was evaluated. A maximum sugar yield of 59.5% was obtained by MW-assisted ammonium molybdate and H2O2 at 140 C for 30 min, whereas external heating in an autoclave gave a sugar yield of 41.8% [108]. MW-assisted pretreatment of recalcitrant softwood in aqueous glycerol containing a series of organic and inorganic acids was carried out by Liu et al. [109]. The pulp obtained by organosolvolysis with 0.1% hydrochloric acid at 180 C for 6 min gave the highest sugar yield of 53.1%. The pretreatment efficiency correlated linearly with the pK(a) of the acids, with the exception of malonic and phosphoric acids. Organosolvolysis with 1.0% phosphoric acid (pK(a) 2.15) gave a saccharification yield 50.6% higher than that expected from its pK(a), whereas the catalytic effect of malonic acid (pK(a) 2.83) was negligible. Extensive exposure of crystalline and noncrystalline cellulose by glycerolysis with strong inorganic acids was demonstrated using fluorescent-labeled recombinant carbohydrate-binding modules. Because of the low concentration of the acid catalysts and the availability of glycerol as a byproduct from biodiesel and fatty acid production, organosolvolysis in glycerol is an appealing process for the pretreatment of recalcitrant softwood. MWealkali (1% NaOH) followed by acid pretreatment (1% H2SO4) was developed by Binod et al. (2012). The enzymatic hydrolysis gave a higher reducing sugar yield of 0.83 g/g dry sugarcane bagasse [110]. MW-assisted ammonium hydroxide (NH4OH) followed by phosphoric acid (H3PO4) treatments was used to release monomeric sugars from Miscanthus sinensis. Treatment with NH4OH at 120 C for 15 min liberated 2.9 g of monomeric sugars per 100 g of dried biomass, whereas the corresponding yield for a treatment with H3PO4 at 140 C for 30 min was 62.3 g/100 g. The two-stage pretreatment, treatment with NH4OH at 120 C for 15 min followed by treatment with H3PO4 at 140 C for 30 min, impressively provided the highest total monomeric sugar yield of 71.6 g/100 g dried biomass [111]. A novel combined pretreatment of ball milling (BM) and MW irradiation (MWI) was developed by Peng et al. [112]. To achieve the same or higher glucose yield of BM for 3 and 6 h, BM for 1 h with MWI for 20 min could save 54.8% and 77.4% energy consumption, respectively. Moreover, chemicals were not required in this process, which made it an environmentally friendly, economical, and effective pretreatment approach.
9.3 MW PRETREATMENT REACTORS Although MW irradiation has been widely used to pretreat many kinds of biomass for several decades, most pretreatments still take place in the laboratory in domestic MW ovens or are modified as listed in Table 9.2 In the first report on MW pretreatment, Ooshima et al. [44] pretreated rice straw and bagasse with water individually placed in sealed glass vessels in a domestic MW oven. Rose and Inglett used a special MW with well energyecontrolled system to heat wheat bran and induce the autohydrolysis of arabinoxylan in wheat bran when heated to 200e210 C [117]. A continuous flowetype MW irradiation system with a capacity of 30 kg/h at 5.0 kW (2.45 GHz) equipped with two magnetrons was successfully used to
A. LIGNOCELLULOSIC BIOMASS
166
9. MICROWAVE PRETREATMENT
TABLE 9.2 Summary of MW Reactors Used to Pretreat Different Biomass Feedstock
MW Device
References
Barley husk
MLS-1200 Mega MW workstation, Milestone, Sorisole, Italy
[113]
Barley straw Spring wheat straw Winter wheat straw Oat straw
CEM MW Max asphalt oven
[56]
Beech wood
Biotage Initiator 2.0 MW reactor (Biotage Co. Ltd., Uppsala, Sweden)
[108]
Corn stover Cotton cellulose
CEM DiscoverÔ unit (CEM company Matthews, USA)
[98,114]
Cotton plant residue Sugarcane bagasse
CE2877 N MW oven (Samsung, Korea)
[75,110]
Cynanchum paniculatum
MP23C-BF (KA) MW oven (Midea, China)
[100]
Dryopteris fragrans Fructus forsythiae
MAS-II (2450 MHz) MW system(Shanghai Sineo MW Products Company (Shanghai, China)
[101,102]
Microcrystalline cellulose Corn stover
MCR-3 MW reactor (Yuhua Equipment Co., Ltd., China)
[107,112]
Pogostemon cablin
MW-assisted extraction unit (Media, Shunde, China)
[99]
Rice straw
WP800T MW (Galanz)
[55]
Rice straw and hulls Wheat straw
M510 laboratory MW oven (Whirlpool)
[43,74]
Switchgrass
Customized Sharp/R-21 HT MW oven
[52]
Switchgrass
NN-S954 MW oven (Panasonic Corporation)
[115]
Water hyacinth
WX-4000 MW system (Shanghai Yiyao MW Chemistry Company)
[89]
Wheat straw
MLS ETHOS PLUS MW reactor (MLS GmbH, Leutkirch/Germany)
[68]
Wheat bran
NE-1756 MW oven (Panasonic Corporation)
[116]
pretreat biomass including softwoods, hardwoods, and monocotyledons [118]. Recently, a continuous MW biomass pretreatment reactor was invented by Xu et al. (Chinese patent application no. CN201210107472.8) [119], which consists of a feeding system, a continuous linear adjustable power MW reaction system, anti-MW leakage systems, control and detection systems, heat/humidity systems, material mixing systems, and discharge systems, as shown in Figure 9.1.
A. LIGNOCELLULOSIC BIOMASS
REFERENCES
167
FIGURE 9.1 Continuous MW irradiation reactor. ① Reversing gear control bracket; ② Feed hopper; ③ MW suppressor; ④ Power box; ⑤ Exhaust heat shield; ⑥ Humidity-discharging device; ⑦ Heating cabinet; ⑧ Safety gate; ⑨ Control panel; ⑩ Conveyor belt; ⑪ Discharge hopper; ⑫ End bracket; ⑬ Vibration motor.
9.4 SUMMARY AND PROSPECTS MW-assisted pretreatment has been successfully employed to pretreat various biomass, and has many advantages: (1) It improves pretreatment speed, and thus reduces the reaction time; (2) it increases pretreatment selectivity; (3) it decreases the pretreatment energy input; and (4) it enhances enzymatic hydrolysis efficiency. All of these are mainly caused by the thermal and nonthermal effects of MW irradiation. Because the heat triggered by MW irradiation occurs mainly by two main mechanismsddipolar polarization and ionic conductiondit is critical to choose an appropriate reaction medium. In general, a reaction medium with a high loss tangent is required for good absorption, and consequently, for efficient heating. Although water has been widely used as a good absorbing solvent, the addition of polar or ionic substances such as salt, acid, alkali, or ionic liquids can increase the absorbance level of the medium. In addition, the additives can act on the biomass directly and accelerate rupture of the biomass structure under MW heating. In addition, the combination of MW irradiation and other pretreatment technologies, such as MW with ball milling [118], has also proven effective. Despite the successful investigations of MW-assisted pretreatment of biomass, there are several aspects still need to be improved: (1) a reactor design for biomass pretreatment and scale-up for commercialized purposes; (2) modeling of MW-assisted pretreatment for different origins of biomass; (3) the establishment of a product chain based on MW-assisted pretreatment; and (4) exploration of an MW heating medium similar to ionic liquid, but at a low cost.
Acknowledgments The author gratefully acknowledges support from the National High Technology Research and Development Program of China (863 Program: 2012AA022301), the National Basic Research Program of China (973 Program: 2011CB707401), the National Natural Science Foundation of China (No. 21276259), and the 100 Talents Program of the Chinese Academy of Sciences.
References [1] Motasemi F, Muhammad TA. A review on the microwave-assisted pyrolysis technique. Renew Sustainable Energy Rev 2013;28:317e30. [2] Kappe CO, Stadler A, Dallinger D, Mannhold R, Kubinyi H, Folkers G. Microwaves in organic and medicinal chemistry. John Wiley & Sons; 2012.
A. LIGNOCELLULOSIC BIOMASS
168
9. MICROWAVE PRETREATMENT
[3] Sun Y, Wang Y, Ni X, Gao Y, Shao Q, Liu L, et al. Comparison of ablation zone between 915- and 2450-MHz cooled-shaft microwave antenna: results in in vivo porcine livers. Am J Roentgenol 2009;192:511e4. [4] Gabriel C, Gabriel S, Grant EH, Halstead BSJ, Mingos DMP. Dielectric parameters relevant to microwave dielectric heating. Chem Soc Rev 1998;27:213e23. [5] Bradshaw SM, van Wyk EJ, de Swardt JB. Microwave heating principles and the application to the regeneration of granular activated carbon. J South Afr Inst Min Metall 1998;98:201e12. [6] Mingos DMP, Baghurst DR. Applications of microwave dielectric heating effects to synthetic problems in chemistry. Chem Soc Rev 1991;20:1e47. [7] Collins JM, Leadbeater NE. Microwave energy: a versatile tool for the biosciences. Org Biomol Chem 2007;5:1141e50. [8] Kuhnert N. Microwave-assisted reactions in organic synthesisdare there any nonthermal microwave effects? Angew Chem Int Ed 2002;41:1863e6. [9] Chemat F, Esveld E. Microwave super-heated boiling of organic liquids: origin, effect and application. Chem Eng Technol 2001;24:735e44. [10] Shibata C, Kashima T, Ohuchi K. Nonthermal influence of microwave power on chemical reactions. Jpn J Appl Phys Part 1 1996;35:316e9. [11] Yin C. Microwave-assisted pyrolysis of biomass for liquid biofuels production. Bioresource Technol 2012;129:273e84. [12] Salazar-Gonzalez C, San Martin-Gonzalez MF, Lopez-Malo A, Sosa-Morales ME. Recent studies related to microwave processing of fluid foods. Food Bioprocess Technol 2012;5:31e46. [13] Zhang M, Tang J, Mujumdar AS, Wang S. Trends in microwave-related drying of fruits and vegetables. Trends Food Sci Technol 2006;17:524e34. [14] Ferrando CJ, Andres M, Membrado L. Coal desulphurization with hydroiodic acid and microwaves. Coal Sci Technol 1995;24:1729e32. [15] Kingman SW, Rowson NA. The effect of microwave radiation on the magnetic properties of minerals. J Microwave Power Electromagn Energy 2000;35:144e50. [16] Koleini SMJ, Barani K, Rezaei B. The effect of microwave treatment upon dry grinding kinetics of an iron ore. Miner Process Extr Metall Rev J 2012;33:159e69. [17] Bond GR, Moyes B, Whan DA. Recent applications of microwave heating in catalysis. Catal Today 1993;17:427e37. [18] Zhang HD, Hayward O, Mingos DMP. Effects of microwave dielectric heating on heterogeneous catalysis. Catal Lett 2003;88:33e8. [19] Adachi K, Iwamura T, Chujo Y. Microwave assisted synthesis of organic-inorganic polymer hybrids. Polym Bull 2005;55:309e15. [20] Bhattacharyya S. Advances in organic synthesis using polymer-supported reagents and scavengers under microwave irradiation. Mol Diversity 2005;9:253e7. [21] Ni Z, Masel R. Rapid production of metal-organic frameworks via microwave-assisted solvothermal synthesis. J Am Chem Soc 2006;128:12394e5. [22] Kappe CO. Controlled microwave heating in modern organic synthesis. Angew Chem Int Ed 2004; 43:6250e84. [23] Kappe CO. Microwave dielectric heating in synthetic organic chemistry. Chem Soc Rev 2008;37:1127e39. [24] Wu DS, Han CY, Wang SY, Wu NL, Rusakova IA. Microwave-assisted solution synthesis of SnO nanocrystallites. Mater Lett 2002;55:155e9. [25] Huang YF, Kuan WH, Lo SL, Lin CF. Total recovery of resources and energy from rice straw using microwaveinduced pyrolysis. Bioresour Technol 2008;99:8252e8. [26] Menendez JA, Dominguez A, Inguanzo M, Pis JJ. Microwave-induced drying, pyrolysis and gasification (MWDPG) of sewage sludge: vitrification of the solid residue. J Anal Appl Pyrolysis 2005;74:406e12. [27] Menendez JA, Inguanzo M, Pis JJ. Microwave-induced pyrolysis of sewage sludge. Water Res 2002;36:3261e4. [28] Ericsson M, Colmsjo A. Dynamic microwave-assisted extraction coupled on-line with solid-phase extraction and large-volume injection gas chromatography: determination of organophosphate esters in air samples. Anal Chem 2003;75:1713e9. [29] Haswell SJ, Howarth N. Perturbation of a solid phase separation process by a non-thermal microwave effect. Anal Chim Acta 1999;387:113e20.
A. LIGNOCELLULOSIC BIOMASS
REFERENCES
169
[30] Perez-Cid B, Albores AF, Gomez EF, Lopez EF. Use of microwave single extractions for metal fractionation in sewage sludge samples. Anal Chim Acta 2001;431:209e18. [31] Perez-Cid B, Lavilla I, Bendicho C. Application of microwave extraction for portioning of heavy metals in sewage sludge. Anal Chim Acta 1999;378:201e10. [32] Prevot AB, Gulmini M, Zelano V, Pramauro E. Microwave-assisted extraction of polycyclic aromatic hydrocarbons from marine sediments using nonionic surfactant solutions. Anal Chem 2001;73:3790e5. [33] Punt MM, Raghavan GSV, Belanger JMR, Pare JRJ. Microwave-assisted process for the extraction of contaminants from soil. J Soil Contam 1999;8:577e92. [34] Routray W, Orsat V. Microwave-assisted extraction of flavonoids: a review. Food Bioprocess Technol 2012;5:409e24. [35] Abramovitch RA, ChangQing L, Hicks E, Sinard J. In situ remediation of soils contaminated with toxic metal ions using microwave energy. Chemosphere 2003;53:1077e85. [36] Abramovitch RA, Huang B, Abramovitch DA, Song J. In situ decomposition of PAHs in soil and desorption of organic solvents using microwave energy. Chemosphere 1999;39:81e7. [37] Kawala Z, Atamanczuk T. Microwave-enhanced thermal decontamination of soil. Environ Sci Technol 1998;32:2602e7. [38] Jou CJ. An efficient technology to treat heavy metal-lead-contaminated soil by microwave radiation. J Environ Manage 2006;78:1e4. [39] Lin L, Yuan S, Chen J, Wang L, Wan J, Lu X. Treatment of chlorophenicol-contaminated soil by microwave radiation. Chemosphere 2010;78:66e71. [40] Komatsu F, Takusagawa A, Wada R, Asahina K. Application of microwave treatment technology for radioactive wastes. Waste Manage 1990;10:211e5. [41] Yuan S, Tian M, Lu X. Microwave remediation of soil contaminated with hexachlorobenzene. J Hazard Mater 2006;137:878e85. [42] Wojcichowska E. Application of microwaves for sewage sludge conditioning. Water Res 2005;39:4749e54. [43] Yin GQ, Liao PH, Lo KV. An ozone/hydrogen peroxide/microwave enhanced advanced oxidation process for sewage sludge treatment. J Environ Sci Health A 2007;42:1177e81. [44] Ooshima H, Aso K, Harano Y, Yamamoto T. Microwave treatment of cellulosic materials for their enzymatic hydrolysis. Biotechnol Lett 1984;6:289e94. [45] De Wild PJ, Reith H, Heeres HJ. Biomass pyrolysis for chemicals. Biofuels 2011;2:185e208. [46] Mohan D, Pittman Jr CU, Steele PH. Pyrolysis of wood/biomass for bio-oil: a critical review. Energy Fuels 2006;20:848e89. [47] Demirbas A. Effect of initial moisture content on the yields of oily products from pyrolysis of biomass. J Anal Appl Pyrolysis 2004;72:243e8. [48] Medic D, Darr M, Shah A, Potter B, Zimmerman J. Effects of torrefaction process parameters on biomass feedstock upgrading. Fuel 2012;91:147e54. [49] Chen WH, Kuo PC. Torrefaction and co-torrefaction characterization of hemicellulose, cellulose and lignin as well as torrefaction of some basic constituents in biomass. Energy 2011;36:803e11. [50] Azuma J, Tanaka F, Koshijima T. Enhancement of enzymatic susceptibility of lignocellulosic wastes [sugar cane bagasse, rice straw and rice hulls] by microwave irradiation. J Ferment Technol 1984; 624:377e84. [51] Saha BC, Biswas A, Cotta MA. Microwave pretreatment, enzymatic saccharification and fermentation of wheat straw to ethanol. J Biobased Mater Bioenergy 2008;2:210e7. [52] Hu Z, Wen Z. Enhancing enzymatic digestibility of switchgrass by microwave-assisted alkali pretreatment. Biochem Eng J 2008;38:369e78. [53] Nikolic S, Mojovic L, Rakin M, Pejin D, Pejin J. Utilization of microwave and ultrasound pretreatments in the production of bioethanol from corn. Clean Technol Environ Policy 2011;13:587e94. [54] Lu X, Xi B, Zhang Y, Angelidaki I. Microwave pretreatment of rape straw for bioethanol production: focus on energy efficiency. Bioresour Technol 2011;102:7937e40. [55] Ma H, Liu W, Chen X, Wu Y, Yu Z. Enhanced enzymatic saccharification of rice straw by microwave pretreatment. Bioresour Technol 2009;100:1279e84. [56] Sapci Z. The effect of microwave pretreatment on biogas production from agricultural straws. Boresour Technol 2013;128:487e94.
A. LIGNOCELLULOSIC BIOMASS
170
9. MICROWAVE PRETREATMENT
[57] Tanaka M, Taniguchi M, Morita T, Matsuno R, Kamikubo T. Effect of chemical treatment on solubilization of crystalline cellulose and cellulosic wastes with Pellicularia filamentosa cellulase. J Ferment Technol 1979;57:186e90. [58] Taniguchi M, Tanaka M, Matsuno R, Kamikubo T. Evaluation of chemical pretreatment for enzymatic solubilization of rice straw. Eur J Appl Microbiol Biotechnol 1985;14:35e9. [59] Chang VS, Holtzapple MT. Fundamental factors affecting biomass enzymatic reactivity. Appl Biochem Biotechnol 2000;84e86:5e37. [60] Binod P, Sindhu R, Singhania RR, Vikram S, Devi L, Nagalakshmi S, et al. Bioethanol production from rice straw: an overview. Bioresour Technol 2010;101:4767e74. [61] Kumar P, Barrett DM, Delwiche MJ, Stroeve P. Methods for pretreatment of lignocellulosic biomass for efficient hydrolysis and biofuel production. Ind Eng Chem Res 2009;48:3713e29. [62] Persson T, Ren JL, Joelsson E, Jonsson AS. Fractionation of wheat and barley straw to access high-molecular-mass hemicelluloses prior to ethanol production. Bioresour Technol 2009;100:3906e13. [63] Sun R, Lawther JM, Banks WB. Influence of alkaline pre-treatments on the cell wall components of wheat straw. Ind Crops Prod 1995;4:127e45. [64] Zhu S, Wu Y, Yu Z, Liao J, Zhang Y. Pretreatment by microwave/alkali of rice straw and its enzymic hydrolysis. Process Biochem 2005;40:3082e6. [65] Zhu S, Wu Y, Yu Z, Zhang X, Wang C, Yu F, et al. Simultaneous saccharification and fermentation of microwave/alkali pre-treated rice straw to ethanol. Biosyst Eng 2005;92:229e35. [66] Zhu S, Wu Y, Yu Z, Chen Q, Wu G, Yu F, et al. Microwave-assisted alkali pre-treatment of wheat straw and its enzymatic hydrolysis. Biosyst Eng 2006;94:437e42. [67] Zhu S, Wu Y, Yu Z, Zhang X, Wang C, Yu F, et al. Production of ethanol from microwave-assisted alkali pretreated wheat straw. Process Biochem 2006;41:869e73. [68] Janker-Obermeier I, Sieber V, Faulstich M, Schieder D. Solubilization of hemicellulose and lignin from wheat straw through microwave-assisted alkali treatment. Ind Crops Prod 2012;39:198e203. [69] Rodrigues THS, Rocha MVP, Macedo GRD, Goncalves LRB. Ethanol production from cashew apple bagasse: improvement of enzymatic hydrolysis by microwave-assisted alkali pretreatment. Appl Biochem Biotechnol 2011;164:929e43. [70] Zhao X, Zhou Y, Zheng G, Liu D. Microwave pretreatment of substrates for cellulase production by solid-state fermentation. Appl Biochem Biotechnol 2010;160:1557e71. [71] Liu JG, Wang QH, Ma HZ, Li J. Effect of pretreatment methods on L-lactic acid production from vinasse fermentation. Adv Mater Res 2010;2:113e6. [72] Liu J, Wang Q, Wang S, Zou D, Sonomoto K. Utilisation of microwave-NaOH pretreatment technology to improve performance and L-lactic acid yield from vinasse. Biosyst Eng 2012;112:6e13. [73] Singha A, Tuteja S, Singh N, Bishnoi NR. Enhanced saccharification of rice straw and hull by microwaveealkali pretreatment and lignocellulolytic enzyme production. Bioresour Technol 2011;102: 1773e82. [74] Xu J, Chen H, Ka´da´r Z, Thomsen AB, Schmidt JE, Peng H. Optimization of microwave pretreatment on wheat straw for ethanol production. Biomass Bioenergy 2011;35:3859e64. [75] Vani S, Binod P, Kuttiraja M, Sindhu R, Sandhya SV, Preeti VE, et al. Energy requirement for alkali assisted microwave and high pressure reactor pretreatments of cotton plant residue and its hydrolysis for fermentable sugar production for biofuel application. Bioresour Technol 2012;112:300e7. [76] Kim M, Aita G, Day DF. Compositional changes in sugarcane bagasse on low temperature, long-term diluted ammonia treatment. Appl Biochem Biotechnol 2010;161:34e40. [77] Aita GA, Salvi DA, Walker MS. Enzyme hydrolysis and ethanol fermentation of dilute ammonia pretreated energy cane. Bioresour Technol 2010;102:4444e8. [78] Salvi DA, Aita GM, Robert D, Bazan V. Ethanol production from sorghum by a dilute ammonia pretreatment. J Ind Microbiol Biotechnol 2009;37:27e34. [79] Salvi DA, Aita GM, Robert D, Bazan V. Dilute ammonia pretreatment of sorghum and its effectiveness on enzyme H hydrolysis and ethanol fermentation. Appl Biochem Biotechnol 2010;161:67e74. [80] Chen C, Boldor D, Aita G, Walker M. Ethanol production from sorghum by a microwave-assisted dilute ammonia pretreatment. Bioresour Technol 2012;110:190e7. [81] Choudhary R, Umagiliyage AL, Liang Y, Siddaramu T, Haddock J, Markevicius G. Microwave pretreatment for enzymatic saccharification of sweet sorghum bagasse. Biomass Bioenergy 2012;39:218e26.
A. LIGNOCELLULOSIC BIOMASS
REFERENCES
171
¨ z C. Progress in bioethanol processing. Prog Energy Combust Sci 2008;34:551e73. [82] Balat M, Balat H, O [83] Gomez LD, Steele-King C, McQueen-Mason SJ. Sustainable liquid biofuels from biomass: the writing’s on the walls. New Phytol 2008;178:473e85. [84] Keshwani DR, Cheng JJ. Switchgrass for bioethanol and other value-added applications: a review. Bioresour Technol 2009;100:137e45. [85] Neureiter M, Danner H, Thomasser C, Saidi B, Braun R. Dilute-acid hydrolysis of sugarcane bagasse at varying conditions. Appl Biochem Biotechnol 2002;98e100:49e58. [86] Jones DA, Lelyveld TP, Mavrofidis SD, Kingman SW, Miles NJ. Microwave heating applications in environmental engineering e a review. Resour Conserv Recycl 2002;34:75e90. [87] Chen WH, Jheng JG, Yu AB. Hydrogen generation from a catalytic water gas shift reaction under microwave irradiation. Int J Hydrogen Energy 2008;33:4789e97. [88] Chen WH, Lin BC. Effect of microwave double absorption on hydrogen generation from methanol steam reforming. Int J Hydrogen Energy 2010;35:1987e97. [89] Xia A, Cheng J, Song W, Yu C, Zhou J, Cen K. Enhancing enzymatic saccharification of water hyacinth through microwave heating with dilute acid pretreatment for biomass energy utilization. Energy 2013; 61:158e66. [90] Palmarola-Adrados B, Galbe M, Zacchi G. Combined steam pretreatment and enzymatic hydrolysis of starchfree wheat fibers. Appl Biochem Biotechnol 2004;115:989e1002. [91] Li J, Yang Y, Chen H, Jiang F, Ling J, Liu M, et al. Comparison of saccharification process by acid and microwave-assisted acid pretreated swine manure. Bioprocess Biosyst Eng 2009;32:649e54. [92] Chen WH, Tu YJ, Sheen HK. Disruption of sugarcane bagasse lignocellulosic structure by means of dilute sulfuric acid pretreatment with microwave-assisted heating. Appl Energy 2011;88:2726e34. [93] Chen WH, Ye SC, Sheen HK. Hydrolysis characteristics of sugarcane bagasse pretreated by dilute acid solution in a microwave irradiation environment. Appl Energy 2012;93:237e44. [94] Swatloski RP, Spear SK, Holbrey JD, Rogers RD. Dissolution of cellose with ionic liquids. J Am Chem Soc 2002;124:4974e5. [95] Zhang Z, Zhao ZK. Solid acid and microwave-assisted hydrolysis of cellulose in ionic liquid. Carbohydr Res 2009;344:2069e72. [96] Zhu S, Wu Y, Chen Q, Yu Z, Wang C, Jin S, et al. Dissolution of cellulose with ionic liquids and its application: a mini-review. Green Chem 2006;8:325e7. [97] Liu Z, Hao J, Dai H, Wang J. Cellulose saccharification after ionic liquid pretreatment with microwave. J Microwaves 2011;21:93e6. [98] Ha SH, Mai NL, An G, KOO YM. Microwave-assisted pretreatment of cellulose in ionic liquid for accelerated enzymatic hydrolysis. Bioresour Technol 2011;102:1214e9. [99] Fan L, Jin R, Liu Y, An M, Chen S. Enhanced extraction of patchouli alcohol from Pogostemon cablin by microwave radiation-accelerated ionic liquid pretreatment. J Chromatogr B 2011;879:3653e7. [100] Jin R, Fan L, An X. Microwave assisted ionic liquid pretreatment of medicinal plants for fast solvent extraction of active ingredients. Sep Purif Technol 2011;83:45e9. [101] Jiao J, Gai Q, Fu Y, Zhu Y, Luo M, Wang W, et al. Microwave-assisted ionic liquids pretreatment followed by hydro-distillation for the efficient extraction of essential oil from Dryopteris fragrans and evaluation of its antioxidant efficacy in sunflower oil storage. J Food Eng 2013;117:477e85. [102] Jiao J, Gai Q, Fu Y, Zhu Y, Luo M, Zhao C, et al. Microwave-assisted ionic liquids treatment followed by hydro-distillation for the efficient isolation of essential oil from Fructus forsythiae seed. Sep Purif Technol 2013;107:228e37. [103] Tanaka M, Sato M. Microwave heating of water, ice, and saline solution: molecular dynamics study. J Chem Phys 2007;126:034509. [104] Rozzi NL, Sing RK. The effect of selected salts on the microwave heating of starch solutions. J Food Process Preserv 2000;24:265e73. [105] Liu L, Sun JS, Li M, Wang SH, Pei HS, Zhang JS. Enhanced enzymatic hydrolysis and structural features of corn stover by FeCl3 pretreatment. Bioresour Technol 2009;100:5853e8. [106] Lu¨ J, Zhou P. Optimization of microwave-assisted FeCl3 pretreatment conditions of rice straw and utilization of Trichoderma viride and Bacillus pumilus for production of reducing sugars. Bioresour Technol 2011;102:6966e71.
A. LIGNOCELLULOSIC BIOMASS
172
9. MICROWAVE PRETREATMENT
[107] Li H, Xu J. Optimization of microwave-assisted calcium chloride pretreatment of corn stover. Bioresour Technol 2013;127:112e8. [108] Verma P, Watanabe T, Honda Y, Watanabe T. Microwave-assisted pretreatment of woody biomass with ammonium molybdate activated by H2O2. Bioresour Technol 2011;102:3941e5. [109] Liu J, Takada R, Karita S, Watanabe T, Honda Y, Watanabe T. Microwave-assisted pretreatment of recalcitrant softwood in aqueous glycerol. Bioresour Technol 2010;101:9355e60. [110] Binod P, Satyanagalakshmi K, Sindhu R, Janu KU, Sukumaran RK, Pandey A. Short duration microwave assisted pretreatment enhances the enzymatic saccharification and fermentable sugar yield from sugarcane bagasse. Renewable Energy 2012;37:109e16. [111] Boonmanumsin P, Treeboobpha S, Jeamjumnunja K, Luengnaruemitchai A, Chaisuwan T, Wongkasemjit S. Release of monomeric sugars from Miscanthus sinensis by microwave-assisted ammonia and phosphoric acid treatments. Bioresour Technol 2012;103:425e31. [112] Peng H, Li H, Luo H, Xu J. A novel combined pretreatment of ball milling and microwave irradiation for enhancing enzymatic hydrolysis of microcrystalline cellulose. Bioresour Technol 2013;130:81e7. [113] Palmarola-Adrados B, Galbe M, Zacchi G. Pretreatment of barley husk for bioethanol production. J Chem Technol Biotechnol 2005;80:85e91. [114] Shi J, Pu Y, Yang B, Ragauskas A, Wyman CE. Comparison of microwaves to fluidized sand baths for heating tubular reactors for hydrothermal and dilute acid batch pretreatment of corn stover. Bioresour Technol 2011;102:5952e61. 5953. [115] Zhu S, Wu Y, Yu Z, Zhang X, Li H, Gao M. The effect of microwave irradiation on enzymatic hydrolysis of rice straw. Bioresour Technol 2006;97:1964e8. [116] Wang TH, Lu S. Production of xylooligosaccharide from wheat bran by microwave assisted enzymatic hydrolysis. Food Chem 2013;138:1531e5. [117] Rose D, Inglett G. Two-stage hydrothermal treatment of wheat (Triticum aestivum) bran for the production of feruloylated arabinoxylooligosaccharides. J Agric Food Chem 2010;58:6427e32. [118] Azuma J, Tsubaki S, Sakamoto M, Yudianti R, Hermiati E. Refinery of biomass by utilization of specific effects of microwave irradiation. Procedia Chem 2012;4:17e25. [119] Xu J, Peng H, Li H. A microwave continuous reactor using for biomass pretreatment. Chinese Patent. Application No.: CN201210107472.8.
A. LIGNOCELLULOSIC BIOMASS