Microwave assisted chemical pretreatment of Miscanthus under different temperature regimes
© Zhu et al. 2015
Received: 16 May 2015
Accepted: 15 September 2015
Published: 6 October 2015
Miscanthus is a major bioenergy crop in Europe and a potential feedstock for second generation biofuels. The most efficient and realistic method to produce fermentable sugars from lignocellulosic biomass is by enzymatic hydrolysis, assisted by thermo-chemical pretreatment. Recently, microwave technology has drawn growing attention, because of its unique effects and performance on biomass.
In this work, microwave energy was applied to facilitate NaOH and H2SO4 pretreatment for Miscanthus under different temperatures (130–200 °C) for 20 min. The yields of reducing sugars from Miscanthus during the pretreatment process increased up to 180 °C and then declined with increasing temperature. Out results here showed a remarkable sugar yield from available carbohydrate (73 %) at the temperature of 180 °C by using 0.2 M H2SO4. In comparison with conventional heating pretreatment studied at same temperature with same biomass material, the reducing sugar release in this study was 17 times higher within half the time. It was highlighted that the major sugar component could be tuned by changing pretreatment temperature or pretreatment media. Optimally, the glucose and xylose yield from available carbohydrate are 47 and 22 % by using 0.2 M H2SO4 and NaOH respectively when temperature was 180 °C. The digestibility of pretreated Miscanthus was 10 times higher than that of untreated biomass. 68–86 % of the lignin content was removed from biomass by 0.2 M NaOH. Simultaneous saccharification fermentation (SSF) results showed an ethanol production of 143–152 mg/g biomass by using H2SO4/NaOH microwave assisted pretreatment, which is 7 times higher than that of untreated Miscanthus. Biomass morphology was studied by SEM, showing temperature has a strong influence on lignin removal process, as different lignin deposits were observed. At the temperature of 180 °C, NaOH pretreated biomass presented highly exposed fibres, which is a very important biomass characteristic for improved enzymatic hydrolysis.
Compared to conventional pretreatment, microwave assisted pretreatment is more energy efficient and faster, due to its unique heating mechanism leading to direct interaction between the polar part of biomass and electromagnetic field. The results of this work present promising potential for using microwave to assist biomass thermo-chemical pretreatment.
KeywordsMicrowave pretreatment Temperature dependence Miscanthus NaOH H2SO4 Crystalline cellulose percentage Hemicellulose Lignin Digestibility SSF
Nowadays, there is a global rise in energy demand and rising concerns about increasing greenhouse gas emissions, hence biofuels derived from lignocellulosic biomass based on the biorefinery philosophy is drawing growing attention [1, 2]. Second generation bioethanol is produced from lignocellulosic biomass following three main processing steps: pretreatment, hydrolysis, and fermentation . Pretreatment is crucial in the conversion of biomass into biofuel via biochemical hydrolysis, so that the recalcitrant structure of biomass can be accessed and sugars released for fermentation .
A number of pretreatments have been studied to improve the yields of fermentable sugars from cellulose and hemicellulose, such as mechanical [4, 5], steam explosion [6, 7], ammonia fibre explosion [5, 8], hot water , sub/supercritical fluid [10–12], ozone , biological , ultrasound , acid or alkaline pretreatments [14–16], ionic liquid [17, 18] and so forth. It is worth mentioning that novel pretreatment media, such as sub/supercritical fluids and ionic liquid are also drawing attention due to their unique solvent properties. For instance, sub/supercritical pretreatment offers several advantages, such as particle size reduction and low toxicity. Additionally, cellulose accessibility would be improved, because sharply reduced pressure leads to explosive decompression of feedstock , Nevertheless, this technology requires equipment capable of withstanding high temperature and pressure [10–12]. Alternatively, ionic liquids have high thermal stability and high solvent power, and can be easily recycled. While ionic liquid toxicity and biodegradability have been controversial issues, [17, 18, 20] recent advances in ionic liquid design have improved this situation. It is also suggested that some of ionic liquid could be as cheap as conventional organic solvent.  Therefore, aqueous acid and alkali are more extensively used as pretreatment media during biomass pretreatment [21–30]. In comparison with HCl, HNO3 and H3PO4, H2SO4 is cheaper, less corrosive, non-oxidative and stronger, hence it was used in this study. Compared with ionic liquid and sub/supercritical fluids pretreatments, microwave assisted pretreatment offers great advantages because of its unique heating mechanism. In microwave, energy transmission is contributed by dielectric losses, and the magnitude of heating depends on the dielectric properties of the subject . It is more direct, uniform and much faster, due to the direct interaction between the object to be heated and an applied electromagnetic field [31, 32]. When microwave is applied to lignocelluloses, it selectively heats the more polar parts throughout the material (as opposed to conventional heat sources which heat from the outside towards the inside), and creates a ‘hot spot’ within heterogeneous materials . Hence, it is hypothesized that an ‘explosion’ effect could occur in the particles, improving the disruption of the recalcitrant structures of lignocellulose. Additionally, it has been claimed that the electromagnetic field used in the microwave might create non-thermal effects that also accelerate the destruction of the crystal structures .
Microwaves have been used in the acid or alkali pretreatment of sugar cane bagasse , oilseed rape straw , switchgrass , crystalline cellulose , and wheat straw . Ma et al. reported that by using microwave pretreatment of rice straw, the maximal efficiencies of the cellulose, hemicellulose and total saccharification of pretreated biomass were increased by 30.6, 43.3 and 30.3 % respectively. Additionally, microwave pretreatment disrupted the silicified waxy surface on rice straw, broke down the lignin-hemicellulose complex and partially removed silica and lignin . Lu et al. reported that the glucose yield of pretreated rape straw from enzymatic hydrolysis was greatly enhanced (56.2 %) after microwave pretreatment, (11.5 % for untreated rape straw) . These works show that microwave thermo-chemical processes are effective and promising pretreatment methods. Their results focused on pretreated biomass solid fraction, while little data has been reported concerning sugar removal during the pretreatment process. In current work, we monitored the effects of microwave assisted pretreatment in the presence of acid and alkali on Miscanthus, at different temperatures (between 130 and 200 °C). In this work, the results were focused on the sugar removal efficiency during pretreatment, rather than on the residual biomass, Moreover, to our knowledge, potential ethanol production from microwave pretreated Miscanthus has not been reported yet.  In addition, we used the SSF (simultaneous saccharification fermentation) process to investigate the potential ethanol production from microwave pretreated Miscanthus solid fraction. Hence, the overall sugar yield in the pretreatment liquid fraction and potential ethanol production from biomass solid fraction are studied here. Biomass morphological characteristics were studied by scanning electron microscopy.
Results and discussion
Microwave pretreatment of lignocellulosic material enhances its hydrolysis, and temperature plays a significant role during the pretreatment . A higher temperature typically achieves higher biomass solubility, shortens the pretreatment time, and reduces the biomass recalcitrance more effectively . However, high temperature also leads to the formation of compounds that are harmful to subsequent hydrolysis and fermentation . Hence, different temperatures ranging from 130 to 200 °C were assayed in this work, in order to investigate temperature influence on biomass under microwave irradiation.
Monosaccharides analysis in the pretreatment media
A number of pretreatments have been studied on Miscanthus before. Yu et al. pretreated Miscanthus by using aqueous ammonia/hydrogen peroxide under lower temperature (90–150 °C) with longer holding times (1–4 h), and the results showed lower cellulose removal during the pretreatment (2.4–19.1 %) . Haverty et al. studied peroxide/formic acid assisted pre-treament for Miscanthus under autothermal conditions, and the results showed 0.3–4.37 % cellulose removal across conditions assayed . One of our co-author, Gomez et al. studied conventional thermo-chemical pretreatment for Miscanthus material (20–180 °C, holding time 40 min), and their results shows 6–12 mg reducing sugar release/mg biomass (yield from total carbohydrate: 1.88–3.76 %) when temperature is 180 °C . The reducing sugar yield in this work is 19 times higher within half the time than the result from Gomez et al. In comparison with other pretreatment methods, our microwave assisted pretreatment led to significant yield of reducing sugar release during pretreatment process.
For Miscanthus, microwave assisted pretreatment is therefore more efficient in releasing reducing sugars during pretreatment; the reason could be its unique heating mechanism (magnitude of heating depends on the dielectric properties of the subject) leading to more efficient biomass decomposition.
Effect of microwave assisted pretreatment on different biomass fractions
In general, increasing of pretreatment temperature removed increasing proportions of hemicellulose from biomass. Water and NaOH pretreatments have similar effects on hemicellulose degradation, in agreement with previous results on monosaccharides in the pretreatment media (Fig. 2). H2SO4 removes hemicellulose more efficiently into pretreatment media, and when temperature was 200 °C, the hemicellulose was completely removed from biomass.
Lignin is composed of phenolic units, although it has multiple potentials for its use as a product feedstock or as a fuel in its own right, it is also considered as a barrier for the efficient enzyme hydrolysis of biomass . Hence, the presence of lignin is considered one of the most important factors limiting the hydrolysis of lignocellulose . Alkaline and oxidative treatments, such as alkaline peroxide and lime and oxygen, have been utilized to remove lignin [27, 52, 53].
Pretreatment is an important step to make cellulose more accessible to cellulases, enhancing glucose production . The percentage of crystalline cellulose in raw Miscanthus is 36 % (Fig. 4b). Microwave assisted water pretreatment has little effect on crystalline cellulose when the temperature was 130 °C. When the temperature was increased to 160, 180, and 200 °C, it increased to 44, 45, and 53 % due to lignin and hemicellulose removal. Under H2SO4 pretreatment, crystalline cellulose percentages in solid fraction are similarly enhanced when temperatures were between 130 and 180 °C, but dropped remarkably to 9 % when pretreatment temperature was 200 °C, showing that under more severe acid condition (200 °C), crystalline cellulose was degraded and carbonized (see “SEM”). In the case of NaOH pretreatment, the crystalline cellulose percentage in solid fraction was enhanced to 67 % when the pretreatment temperature was 180 °C, in good agreement with the extensive removal of lignin and hemicellulose observed in these conditions. At 200 °C, the crystalline cellulose percentage was 39 %, showing a reduction of cellulose crystallinity at this temperature . Under microwave irradiation the heat is produced by direct interaction between polar part of biomass and oscillating electromagnetic field. The cellulose fibres could be described as being ionic conducting (crystalline) and non-conducting (amorphous) . A very ordered hydrogen bonded network is contained in the crystalline cellulose which could lead to a proton transport network under an electromagnetic field under right condition . Therefore, the crystalline cellulose is able to act as an active microwave absorber, promoting the biomass decomposition. Along with the process of lignin/hemicellulose removal, crystalline cellulose percentage goes up, enhancing the microwave absorbing effect and promoting biomass degradation.
Digestibility analysis of solid fraction of biomass
Digestibility of pretreated biomass solid fraction were measured and compared, in order to find out the effectiveness of pretreatment conditions. The pretreatment is to remove lignins to make the remaining biomass fraction more accessible to the enzymes in the enzyme hydrolysis. Miscanthus digestibility was increased after all microwave assisted pretreatments, albeit to widely differing extents (Fig. 4c). For untreated Miscanthus, the digestibility is 10.25 nmol/mg biomass h, meaning 10.25 nmol glucose is produced from 1 mg biomass during each hour of enzymatic hydrolysis (the total enzymatic hydrolysis is 4 h). Water treatment slightly increased digestibility at 130 °C. It was further enhanced to 40–50 nmol/mg biomass h when the pretreatment temperature was increased from 160 to 200 °C. In the case of H2SO4, the digestibility was marginally increased when the holding temperature was 130 °C, thereafter it declines with the temperature. At 200 °C, the digestibility was only 8.7 nmol/mg biomass h. Conversely, NaOH pretreatment remarkably improves Miscanthus digestibility. At 130–160 °C, the digestibility of NaOH pre-treated Miscanthus was 10 times higher than that of untreated biomass. Because of the delignification effect of NaOH, alkaline pretreated Miscanthus with low lignin percentage and higher cellulose percentage generates more sugar in the hydrolysis process. The difference in saccharification after acid or alkali pretreatments can be explained by the fact that the easily hydrolysed sugars are released into the pretreatment liquor, reducing the amount of sugar available for enzymatic digestion.
SEM analysis of microwave pre-treated Miscanthus
Simultaneous saccharification fermentation (SSF) of hydrothermal-microwave pre-treated samples
Miscanthus is one of the most promising energy crops in Europe and improving processing alternatives is a priority for second generation biofuel production. In this work, we tested microwave assisted pretreatments in the presence of water, H2SO4 and NaOH. Different temperature was assayed here, and the maximum sugar yield (73 %) is obtained by using 0.2 M H2SO4 at 180 °C, which is 17 times higher than conventional heating pretreatment within half time less. It was highlighted that xylose and glucose were selectively produced by tuning pretreatment temperature or media, and significant amount of glucose (yield: 47 %) was obtained from available carbohydrate when 0.2 M H2SO4 was used for pretreatment under 180 °C. The temperature has a strong influence on the lignin removal process, as different form of lignin deposits are observed from SEM images of biomass surface. Additionally, lignin removal process was improved with microwave assistance, especially in the case of H2O and H2SO4. Due to the effective removals of lignin and hemicellulose, NaOH pretreatment significantly enhances Miscanthus digestibility, which was up to 10 times higher than that of untreated Miscanthus. It is worth mentioning that the fermentability of pretreated Miscanthus is more than 7 times higher than that of untreated biomass, and it can be optimized by changing pretreatment media and pretreatment time. Morphological study showed more exposed biomass fibres characteristic after 0.2 M NaOH pretreatment at 180 °C, which is a very important feature for following enhanced biomass digestibility. Temperature plays a significant role in pretreatment process. Under microwave conditions, 180 °C is a crucial point in the biomass degradation process, as the polar groups could be involved in a localized rotation in the microwave radiation and promote biomass degradation . In our study, remarkable sugar yields and promising bioethanol production were achieved at 180 °C, which was identified as the optimal condition for our microwave assisted pretreatment. Overall, this work extensively studied the microwave assisted pretreatment for Miscanthus, and the results showed promising potential of using Microwave to assist thermo-chemical conversion of biomass to second generation biofuels.
Untreated biomass material and constituents
Miscanthus giganteus was grown under field conditions near York, UK, and harvested at maturity. The biomass was ground using a hammer mill to produce and average particle size of 100 μm × 57 μm. The biomass compositions of untreated Miscanthus are cellulose (34 % ± 2.5 %), hemicellulose (42 ± 2.8 %), lignin (30.4 ± 2 %) and ash (0.83 ± 0.03 %).
Microwave pretreatment methods
The pretreatment was conducted in a CEM Discover microwave machine (CEM Discover SP-D, US). The CEM microwave reactor vessel (30 ml) was charged with 0.4 g of Miscanthus and 16 ml H2SO4 or NaOH solution (0.2, 0.4 and 1). Pretreatment was carried out at various temperatures between 130 and 200 °C for a period of 20 min. After pretreatment, the liquid fraction was separated from biomass solid fraction by filtration. Liquid fraction was neutralized with 150 mM Ba(OH)2 or 1 M HCl. The solid fraction was rinsed with absolute ethanol (3 × 10 ml) and dried at 50 °C overnight.
The CEM MARS 6 (CEM, US) was used for the scale up microwave pretreatment in order to perform Simultaneous saccharification and fermentation (SSF). 3 g of Miscanthus and 80 ml H2SO4 (0.2 M) or NaOH solution (0.2 M) were added in a 100 ml reaction vessel. The pretreatment was carried out at 180 °C for various holding time (5–20 min). Same procedures (separation, washing and drying) as above were performed in order to conduct SSF.
Analysis of carbohydrates in liquid fraction
The liquid fraction resulting from alkaline and acid pretreatments was neutralized by 1 M HCl or 1 M NaOH solutions, respectively. Then monosaccharide in the liquid fraction was analysed by High Performance Ion Exchange Chromatography (Dionex, ICS-3000PC, Thermal scientific, USA) equipped with electrochemical detector to quantify the sugar content .
Hemicellulose was analysed by using the method developed by Foster et al. . 4 mg of biomass were hydrolysed using 0.5 ml 2 M TFA. After flushing the vial with dry Argon, the vials were heated at 100 °C for 4 h. TFA was removed completely by centrifugal evaporation with fume extraction overnight. Then the biomass was washed with 500 μl of Propan-2-ol twice. The samples were re-suspended in 200 μl of deionised water. After thorough mixing, the supernatant was put into a new tube for analysis using Dionex in order to measure monosaccharides in hemicellulose.
Analysis of crystalline cellulose
To determine the percentage of crystalline cellulose in biomass, 10 mg untreated or pre-treated biomass was hydrolysed using 500 µl 2 M TFA (trifluoroacetic acid) at 100 °C for 4 h. The solids were subsequently hydrolysed using Acetic acid:Nitric Acid:Water (8:1:2 v/v) at 100 °C for 30 min. Finally, the resulting residue was hydrolysed into glucose using 175 µl 72 % H2SO4 at room temperature for 45 min and then diluted to 3.2 % H2SO4 and heated at 120 °C for 2 h. Anthrone assay was used to quantify corresponding glucose .
Analysis of biomass digestibility
The digestibility of biomass was investigated by using a high throughput saccharification assay which is based on a robotic platform that can carry out the enzymatic digestion and quantification of the released sugars in a 96-well plate format. Enzymatic hydrolysis was carried out using an enzyme cocktail with a 4:1 (v/v) ratio of Celluclast and Novozyme 188 (both Novozymes, Bagsvaerd, Denmark). The enzymes were filtered using a Hi-Trap desalting column (GE Healthcare, Little Chalfont, Buckinghamshire, UK) before use. 0.1 mg biomass was hydrolysed for 8 h with 250 μl enzyme cocktail, in 250 ml of 25 mM sodium acetate buffer at pH 4.5, at 30 °C. Determination of sugars released after hydrolysis was performed using a modification of the method by Anton and Barrett using 3-methyl-2-benzothiazolinonehydrozone (MTBH) .
Morphological characteristics of the raw materials and pre-treated biomass residue were studied using a scanning electron microscope fitted with tungsten filament cathode (JEOL, JSM-6490LV, Japan). Samples were sputter-coated with 7 nm Au/Pd to facilitate viewing by SEM. Images were obtained under vacuum, using a 5 kV accelerating voltage and a secondary electron detector.
Simultaneous saccharification and fermentation (SSF)
The SSF experiments were performed in 100 ml conical flask with 1 g of untreated/pre-treated biomass, 10.75 ml sterile water, 0.250 ml NaOAc buffer, 1 ml enzyme solution (4:1 v/v ratio of Celluclast and Novozyme 188, Novozymes, Bagsvaerd, Denmark), 1.365 ml ATCC medium, and 200 μl yeast extract (the yeast was grown until optical density 5 and added). The flasks were incubated for 48 h in a shaking incubator under at 30 °C and 150 rpm. Samples for ethanol determination were collected after 1, 6, 24, 48 h in GC vials containing 500 µL of 1 M sodium chloride and 0.04 % 1-propanol. Ethanol concentrations at different time points were measured by using a standard curve of ethanol.
simultaneous saccharification and fermentation
DJM and LDG planned the pre-treatments. ZZ carried out the pretreatments and the determination of monosaccharides, chemical compositions, scanning electron microscopy study and SSF, as well as the analysis of the results. RH measured biomass digestibility. ZZ, DJM and LDG prepared the manuscripts. SMM coordinated the overall study. All authors suggested modifications to the draft. All authors read and approved the final manuscript.
The present work was funded by the European Community’s Seventh Framework Programme SUNLIBB (FP7/2007-2013) under the grant agreement no. 251132. The authors gratefully acknowledge Dr Andrew Hunt for advising and support.
Compliance with ethical guidelines
Competing interests The authors declare that they have no competing interests.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.
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