Optimization of furfural and 5-hydroxymethylfurfural production from wheat straw by a microwave-assisted process

Bioresource Technology 109 (2012) 215–223

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Optimization of furfural and 5-hydroxymethylfurfural production from wheat straw by a microwave-assisted process Oktay Yemisß a,b,⇑, Giuseppe Mazza a a b

Pacific Agri-Food Research Center, Agriculture and Agri-Food Canada, 4200 Highway 97, Summerland, BC, Canada V0H 1Z0 Department of Food Engineering, Faculty of Engineering, Pamukkale University, Kinikli, 20070 Denizli, Turkey

a r t i c l e

i n f o

Article history: Received 2 August 2011 Received in revised form 24 December 2011 Accepted 9 January 2012 Available online 16 January 2012 Keywords: Lignocellulosic biomass Acid-catalyzed conversion Furfural HMF Microwave effect

a b s t r a c t Optimization of acid-catalyzed conversion conditions of wheat straw into furfural, 5-hydroxymethylfurfural (HMF), glucose, and xylose was studied by response surface methodology (RSM). A central composite design (CCD) was used to determine the effects of independent variables, including reaction temperature (140–200 °C), residence time (1–41 min), pH (0.1–2.1), and liquid:solid ratio (15–195 mL/g) on furan and sugar production. The surface response analysis revealed that temperature, time and pH had a strong influence on the furfural, HMF, xylose and glucose yield, whereas liquid to solid ratio was found not to be significant. The initial pH of solution was the most important variable in acid-catalyzed conversion of wheat straw to furans. The maximum predicted furfural, HMF, xylose and glucose yields were 66%, 3.4%, 100%, and 65%, respectively. This study demonstrated that the microwave-assisted process was a very effective method for the xylose production from wheat straw by diluted acid catalysis. Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.

1. Introduction Agricultural and agro-industrial residues are potential carbohydrate sources for production of chemicals, but only a minor fraction of the huge natural sources is used by the chemical industry (Lichtenthaler and Peters, 2004). Straws (wheat, flax, triticale, etc.), sugarcane bagasse, sawdust, rice husk, corn stover are lignocellulosic materials that are biodegradable agricultural and industrial wastes. Lignocellulosic materials have a great industrial potential since they are available in large quantities worldwide, cheap, and renewable source. Wheat straw is one of the major agricultural residues in terms of low-cost and available quantity. Taking into consideration the annual production of wheat (681 million metric tons (Mt)) in the world and the yield of straw (1.5 straw/grain, w/w), approximately 600–900 million (Mt) of wheat straw is produced as a by-product per year (Lal, 2005). Furfural and HMF are the most promising members of the family of furan. Furfural, HMF, and their derivatives are known to have many applications as strategic chemicals (Chedda et al., 2007). Although the production rate and market price of furfural vary depending on demand, the annual world production is about 250,000 (Mt) and, the current market price of furfural is approximately $1000/Mt. The lack of an economically-viable process for di⇑ Corresponding author at: Department of Food Engineering, Faculty of Engineering, Pamukkale University, Kinikli, 20070 Denizli, Turkey. Tel.: +90 258 296 3120; fax: +90 258 296 3262. E-mail address: [email protected] (O. Yemisß).

rect catalytic conversion of carbohydrates in biomass to furans is an important issue for the production of furans. The currently used batch and continuous production processes for furfural and HMF are expensive, energy-intensive (Win, 2005) and non-environmentally friendly techniques that have cause acid wastes (Dias et al., 2010). Thus, the present studies regarding furans are focused on developing novel processes that will be both economically and environmentally acceptable. Heating rate, residence time, temperature, substrate concentration, catalyst concentration and types are key factors that can influence the thermochemical conversion processes (Zeitsch, 2000). Response surface methodology (RSM) is an effective statistical procedure frequently used for optimization of process when there are many factors and interactions among these factors (Myers et al., 2009). Wheat straw consists of three main polymers, mainly cellulose (30–35%), hemicellulose (15–25%) and lignin (10–25%) (Tamaki and Mazza, 2010). The hemicellulose fraction which is a heterogeneous polysaccharide composed of mainly pentoses and hexoses, is the main polymer responsible for the formation furfural (Buranov and Mazza, 2010). There are two main steps in the production of furfural and HMF from biomass; a hydrolysis step that breaks down cellulose and hemicellulose polymers into their six and five carbon sugar monomers, and a dehydration step at which the sugars are converted into furanic compounds. Industrial furfural processes are usually carried out with convective–conductive heating sources (Zeitsch, 2000). In considering the energy efficiency of conventional and microwave heating, the use of microwaves as energy source provides an up to 85-fold savings in energy

0960-8524/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2012.01.031

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(Dallinger and Kappe, 2007). In addition, carrying out extractions and conversion reactions in water is critical from a green chemistry point of view (Cacace and Mazza, 2007). In this context, a microwave-assisted process can reduce the cost of production of furan from biomass. To the best of our knowledge, there has been no study on the optimization of the acid-catalyzed conversion products from wheat straw by microwave-assisted process. Thus, the objective of this study was to determine the optimum process conditions for maximizing the acid-catalyzed conversion products of wheat straw by a microwave-assisted reaction. 2. Methods 2.1. Biomass Wheat (Triticum aestivum L.; cv. Andrew) straw grown near Lethbridge, AB, Canada in 2008 was used as a biomass feedstock. Samples were ground with a cutting mill (Retsch SM 2000, Retsch GmbH, Haan, Germany) equipped with a 2-mm discharge screen. Samples were screened to obtain a uniform size with a tapping sieve shaker (Retsch AS200, Retsch GmbH, Haan, Germany) equipped with a 20-mesh and 80-mesh. The material in the middle fraction (850–180 lm) was used for the microwave-assisted processing experiments. The chemical composition of wheat straw was determined according to the NREL Laboratory Analytical Procedures (Sluiter et al., 2008). 2.2. Acid-catalyzed conversion process of wheat straw by microwaveassisted reaction The acid-catalyzed conversion of wheat straw into furans and sugars by microwave-assisted reaction was carried out according to the method previous described by Yemisß and Mazza (2011) and hydrochloric acid (HCl) was used as catalyst in all treatments. For that purpose, a microwave extraction system (ETHOS EX, Milestone, Italy) equipped with a 16-closed-vessel position carousel was used. The microwave extraction system utilized an optical fiber temperature sensor for temperature control. A control program with the appropriate temperature, ramp time, residence time, and cooling time was defined using the system software (EasyControl terminal 640). Before conducting any experiments, the affect of ramp time on product yields was determined to ascertain the effect of heating rate. A series of experiments were run in the microwave extraction system with ramp times of 5, 10, 15, and 20 min. No significant differences in furfural yield were observed (data not shown), and it was concluded that the heating rate had not any influence on furfural yield. Thus, a ramp time of 10 min for reaching the target temperature in all treatments was used. Each acid-catalyzed conversion of wheat straw by microwave-assisted reaction was carried out in four 75-mL Teflon vessels under the identified conversion conditions including temperature, substrate concentration, acid concentration and time as outlined in Table 1. According to the L:S ratio, ground wheat straw samples were accurately weighed and transferred to the Teflon extraction vessels. The HCl solution at different pH were added to each sample in the proportions dictated by the experimental trial to a constant final volume of 20 mL for all treatments. The extraction vessels were closed and the mixture was homogenized with a magnetic stirrer for 3 min before the microwave process. A fiber optic sensor was inserted into the reference vessel to monitor the internal temperature. After processing in the microwave extraction system according to the experimental conditions (Table 1), the extraction vessels were allowed to cool down to room temperature before removal from the system. The extracts were filtered with Whatman

filter paper (no. 1) and the clear liquid extracts were used for analysis of furfural, HMF and sugars. 2.3. Analysis of furans (furfural and HMF) and sugars Decomposition products (sugars) and dehydration products (furans) were detected and quantified in reaction media after the conversion of biomass samples by microwave-assisted reaction. Before injection into the HPLC, extracts were neutralized to pH 6.5–7.0 with NaOH solution, and then made up to 25 mL with Milli-Q water. Furans and sugars were determined simultaneously using an Agilent 1100 series liquid chromatography system (Agilent Technologies Inc., Palo Alto, CA) equipped with DAD and RID detectors. Chromatographic separations were carried out on a Bio-Rad Aminex HPX-87P column (300  7.8 mm, Bio-Rad Laboratories, Hercules, CA) with a cation H Refill Cartridge guard column (30  4.6 mm, Bio-Rad Laboratories, Hercules, CA) operated at 75 °C with Milli-Q water as mobile phase. The flow rate was 0.5 mL/min and the injection volume of the extract was 50 lL. The yields of furans and sugars on a dry weight basis (DWB) were calculated using the following equation:

YieldDWB ¼

mass of product formed dry mass of substrate utilized

ð1Þ

where the yields of furfural and HMF were expressed as a ‘‘g furan/ 100 g pentose content (xylan + arabinose) or hexose content (glucan + galactose + mannose) of straw on a dry weight basis’’. The yields of sugars were expressed as a ‘‘g sugar/100 g xylan content or glucan content available in wheat straw on a dry weight basis’’. 2.4. Solid residue The mass of the solid residue in the reaction mixture was determined gravimetrically by filtration with Whatman no. 1 filter. The solid residue on the filter was precisely weighed after drying at 105 °C overnight. The solid residue (g/100 g) is expressed relative to the initial straw. 2.5. Response surface methodology (RSM) Response surface methodology (RSM) was used to determine the optimum conditions and main effects of the microwave-assisted process variables on the acid-catalyzed conversion of wheat straw. Temperature (X1), residence time (X2), pH (X3), and liquid:solid (L:S) ratio (X4) were selected as independent process variables by taking into consideration the industrial practice. The ranges of variables were chosen on the basis of information from a previous study (Yemisß and Mazza, 2011). Experiments for the optimization of process variables were set up according to the central composite design (CCD). The 28 treatments were run in a random order to minimize the effect of any unexplained variability in the observed responses due to extraneous factors. The responses measured were the yields of furfural, HMF, glucose, xylose and solid residue. Statistical analysis was performed using the statistical software package of Design Expert 8.0.5.2 (Stat-Ease., Inc., MN) to generate a regression model. Significance test on the regression models and individual model coefficients were performed by analysis of variance (ANOVA). Regression models were evaluated for each response variables and tested possible linear, quadratic, cubic and linear cross-product relationship. Backward elimination strategy was used to reach the appropriate model coefficient by reducing the insignificant terms. In this procedure, the term that was least significant in the model was removed at each step. The reduced cubic models containing the significant terms (p < 0.05)

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O. Yemisß, G. Mazza / Bioresource Technology 109 (2012) 215–223 Table 1 Four factor, five-level central composite design used for RSM and experimental results for the different combinations of microwave-assisted process conditions. Standard order

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 a b c

Factor 1 (X1)

Factor 2 (X2)

Factor 3 (X3)

Factor 4 (X4)

Furansa

Temperature (°C)

Time (min)

pH

Liquid:solid ratio (mL/g)

Furfural (%)

HMF (%)

Glucose (%)

Xylose (%)

155 155 155 155 155 155 155 155 185 185 185 185 185 185 185 185 140 200 170 170 170 170 170 170 170 170 170 170

11 (1) 11 (1) 11 (1) 11 (1) 31 (+1) 31 (+1) 31 (+1) 31 (+1) 11 (1) 11 (1) 11 (1) 11 (1) 31 (+1) 31 (+1) 31 (+1) 31 (+1) 21 (0) 21 (0) 1 (2) 41 (+2) 21 (0) 21 (0) 21 (0) 21 (0) 21 (0) 21 (0) 21 (0) 21 (0)

1.6 0.6 1.6 0.6 1.6 0.6 1.6 0.6 1.6 0.6 1.6 0.6 1.6 0.6 1.6 0.6 1.1 1.1 1.1 1.1 1.1 1.1 2.1 0.1 1.1 1.1 1.1 1.1

60 (1) 60 (1) 150 (+1) 150 (+1) 60 (1) 60 (1) 150 (+1) 150 (+1) 60 (1) 60 (1) 150 (+1) 150 (+1) 60 (1) 60 (1) 150 (+1) 150 (+1) 105 (0) 105 (0) 105 (0) 105 (0) 15 (2) 195 (+2) 105 (0) 105 (0) 105 (0) 105 (0) 105 (0) 105 (0)

1.4 32.9 1.8 34.7 4.8 46.9 6.3 51.3 21.4 33.9 31.1 40.7 39.9 23.8 38.3 22.1 3.3 37.9 4.8 40.8 21.9 32.9 2.4 15.0 33.4 35.2 41.9 38.6

0.40 0.64 0.37 0.61 0.55 1.27 0.56 1.38 1.12 0.54 1.39 0.35 2.29 0.00 2.09 0.00 0.48 0.45 0.65 1.61 0.89 1.10 0.38 0.00 1.11 1.22 1.43 1.23

5.6 23.1 6.3 24.0 6.6 43.7 7.4 40.2 19.0 8.8 23.9 2.5 38.3 0.0 34.3 0.0 6.1 1.1 6.9 36.9 17.8 20.4 3.9 0.0 28.1 30.1 33.1 30.7

95.3 48.7 77.9 52.1 90.2 10.6 99.1 5.9 70.8 0.0 56.4 0.4 28.2 4.1 23.4 10.6 93.9 5.0 97.1 18.9 71.6 26.2 78.6 5.0 36.4 38.4 30.0 37.8

(1) (1) (1) (1) (1) (1) (1) (1) (+1) (+1) (+1) (+1) (+1) (+1) (+1) (+1) (2) (+2) (0) (0) (0) (0) (0) (0) (0) (0) (0) (0)

(1) (+1) (1) (+1) (1) (+1) (1) (+1) (1) (+1) (1) (+1) (1) (+1) (1) (+1) (0) (0) (0) (0) (0) (0) (2) (+2) (0) (0) (0) (0)

Sugarsb

Solid residuec (%)

55.6 43.1 53.9 35.5 54.0 33.4 55.4 26.7 46.0 18.3 45.8 13.0 38.0 21.9 40.3 16.3 51.1 17.9 53.8 37.0 52.2 36.2 52.4 17.2 41.8 40.1 39.1 36.1

Furan yield values were expressed as ‘‘g furfural–HMF/100 g pentose or hexose content available in straw DWB’’. Sugar yield values were expressed as ‘‘g sugar/100 g pentose or hexose content available in straw DWB’’. Solid residue yield values were expressed as ‘‘g solid residue/100 g straw DWB’’.

were obtained. The criterion for rejection (a out) of a parameter in the model equation was 0.05. 3. Results and discussion 3.1. Chemical composition of wheat straw The main constituents of wheat straw used in this study were glucan (34.2% DWB), xylan (19.7% DWB), lignin (15.3% DWB), ash (1.7% DWB), galactose (0.8% DWB), arabinose (2.2% DWB), mannose (0.3% DWB), and extractives (18.5% DWB). The structural carbohydrates (glucan and xylan) which are the major substrates for furan production, accounted for approximately 57% of the wheat straw. The extractives including waxes, chlorophyll, nonstructural sugars, organic acids, inorganic material, and nitrogenous materials were a significant fraction that accounted for 18.5% of wheat straw. These results are in agreement with those reported by Tamaki and Mazza (2010). It is well known that pentoses and hexoses, respectively, are the precursors for furfural and HMF formed during the conversion of wheat straw (Yemisß and Mazza, 2011). Determination of the initial quantity of the main constituents available in wheat straw allowed for the calculation of the yields and theoretical conversion rates of furans and sugars from wheat straw.

1.43–51.30%, 0.00–2.29%, 0.00–43.60%, 0.00–99.1%, and 13.04– 55.55%, respectively. The ‘‘fit summary’’ report produced by Design-Expert recommends the quadratic model for furfural. The ‘lack of fit’ was 0.0501 for quadratic model of furfural and the significant parameters shown were temperature (X1), time (X2), pH (X3), the interaction of temperature  pH (X1 X3), and pH square (X 23 ) (data not shown). However, the lack of fit value of the models was also significant for the other response variables as well as furfural. Thus, the quadratic model suggested by Design-Expert was modified by addition of the interactive terms of X1X2X3 and X1X2X3X4, and then the reduced cubic model equations were reached with the elimination of insignificant terms. To obtain the best-fit model, the f -test, ‘‘lack-of-fit’’ test and the analysis of variance (ANOVA) technique were considered for determination of the adequacy of the models. The best regression equations represent the description of furfural, HMF, glucose, xylose, and solid residue yield were obtained below:

Y Furfural ¼ 105:0945 þ 0:74834X 1 þ 14:93577X 2 þ 20:51717X 3  0:08778X 1 X 2  0:000966X 1 X 3  10:53525X 2 X 3  19:33042X 23 þ 0:0643X 1 X 2 X 3 Y HMF ¼ 23:11 þ 0:25667X 1 þ 0:64508X 2 þ 1:98108X 3  0:003818X 1 X 2  0:000016X 1 X 3  0:51992X 2 X 3

3.2. Fitting models Values of the independent process variables studied and the responses obtained from 28 different combinations of microwave-assisted reaction conditions are shown in Table 1. The yields of furfural, HMF, glucose, xylose and solid residue from wheat straw during optimization trials were in the range of

 0:000719X 21  0:9225X 23 þ 0:031833X 1 X 2 X 3 Y Xylose ¼ 744:60  4:25978X 1  31:4644X 2  406:62X 3 þ 0:17722X 1 X 2 þ 2:64779X 1 X 3 þ 27:08596X 2 X 3  0:15854X 1 X 2 X 3

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Y Glucose ¼ 596:454 þ 7:31036X 1 þ 13:62604X 2  24:0083X 3

ables affecting the yields where smaller p values indicate higher significance of the corresponding coefficient (Oomah and Mazza, 2001). The analysis of variance (ANOVA) showed that temperature, time and pH had a significant effect on the furfural yield at the 0.01, 0.05 and 0.01 levels, respectively. Similar results were also obtained for HMF, but the order of significance of process variables affecting yield was different from that of furfural. The effect of temperature on the HMF yield was lower than that of time and pH. As shown in the ANOVA table, the L:S ratio in the studied range of 15–195 mL/g did not have a significant effect on furfural, HMF, glucose and xylose yields (p > 0.05). Sugars, mainly xylose and glucose, were other major products formed from the acid-catalyzed conversion of wheat straw. This study revealed the optimal pretreatment conditions for the maximizing sugar production from wheat straw by microwave-assisted process as well as furans at the same time. The surface response analysis for xylose yield indicated that temperature, time and pH were highly significant terms (p < 0.001), whereas only time had a significant effect on glucose yield (p < 0.001). Lack of fit was insignificant for the furfural, HMF, glucose and solid residue yields at the level of 0.05 and for the xylose yield at the level of 0.1.

 0:07781X 1 X 2 þ 0:41984X 1 X 3  10:6416X 2 X 3  0:02266X 21  22:0352X 23 þ 0:06307X 1 X 2 X 3 Y Solid residue ¼ 26:5608 þ 1:46535X 1  7:10776X 2  106:032X 3  0:05132X 4 þ 0:04051X 1 X 2 þ 0:84830X 1 X 3 þ 5:46925X 2 X 3  0:00886X 21  7:6405X 23  0:0323X 1 X 2 X 3 Regression coefficients and analysis of variance of the reduced polynomial cubic models for furfural, HMF, glucose, xylose and solid residue are presented in Table 2. ANOVA analysis for the reduced cubic model showed that although the interaction terms of X1  X2 and X2  X3 were not a significant parameter for furfural (p value > 0.05), these terms were included in the model to support the hierarchy model. Therefore, these terms were added as hierarchical term. Similarly, the interaction term of X1  X2 for HMF, X1  X2, X1  X3 and X2  X3 for xylose, X1, X3, X1  X2, X2  X3 for glucose, and X1  X2, X1  X3 and X2  X3 for solid residue was the hierarchical term added after backward elimination. The coefficient of determination (R2), which demonstrates the agreement between the observed and predicted results, was used to check the quality of fit of the equations. R2 has been suggested to be at least 0.80 for a good fit of a model (Joglekar and May, 1987). Higher R2 values were obtained for the reduced cubic model than for the quadratic model (data not shown). R2 values for the reduced polynomial cubic models for furfural, HMF, glucose, xylose and solid residue were found to be 0.8079, 0.9411, 0.8934, 0.9196 and 0.9496, respectively. The associated the probability (p) values of less 0.05 for the regression models and the independent variable coefficients show that both are statistically significant. The probability (p) values are an indication of the order of significance of process vari-

3.3. Optimization of process variables Three-dimensional response surface plots were generated by plotting the yield on the Z-axis against two independent variables. These 3D-plots that are the graphical representations of the regression equations were used to evaluate the interactions between variables and to predict the optimum values of each variable for maximum yield. 3.3.1. Furfural Furfural was the major product obtained from acid-catalyzed conversion of wheat straw by the microwave-assisted process.

Table 2 Regression coefficients and analysis of variance of the models for furfural, HMF, glucose, xylose and solid residue. Variablea

Furfural k

Intercept X1 X2 X3 X4 X1 X2 X1 X3 X1 X4 X2 X3 X2 X4 X3 X4 X1 X2 X3

HMF p Value

k

105.0945 0.74834 14.93577 20.51717

Xylose p Value

k

0.0023 0.0140 0.0005

23.11 0.25667 0.64508 1.98108

0.087780 0.000966

0.2232A <0.0001

10.53525

Glucose

Solid residue

p Value

k

0.0378 <0.0001 <0.0001

744.60 4.25978 31.4644 406.62

<0.0001 0.0002 <0.0001

596.454 7.31036 13.62604 24.0083

0.1153A 0.0001 0.7736A

0.003818 0.000016

0.2963A <0.0001

0.17722 2.64779

0.8970A 0.1288A

0.07781 0.41984

0.3421A

0.51992

0.0271

27.08596

0.8378A

0.06430

0.0283

0.0031833 0.000719

<0.0001 0.0002

0.15854

0.0015

19.33042

0.0057

0.92250

<0.0001

X 21

p Value

k

p Value

0.3210A <0.0001

26.5608 1.46535 7.10776 106.032 0.05132 0.04051 0.84830

<0.0001 0.0058 <0.0001 0.0084 0.4415A 0.1965A

10.6416

0.7471A

5.46925

0.9101A

0.06307 0.02266

0.0013 <0.0001

0.03230 0.00886

0.0205 0.0147

22.0352

<0.0001

7.64050

0.0187

X 22 X 23 X 24 Model R2 Lack of fit Main effects Temperature Time pH L:S ratio

<0.0001

<0.0001

<0.0001

<0.0001

<0.0001

0.8079 0.0922

0.9411 0.3276

0.8934 0.0283

0.9196 0.07

0.9496 0.2043

***

**

***

NS

***

**

***

***

***

***

***

***

***

***

NS

NS

NS

NS NS

Lack of fit is insignificant at p > 0.05, NS: not significant. a Polynomial models were adjusted by backward elimination (a out = 0.05). A Hierarchical term added after backward elimination regression. ** Significant at p < 0.05. *** Significant at p < 0.01.

***

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The maximum level of furfural yield was 51.3% of pentose content in wheat straw at 155 °C, pH of 0.6 for 31 min, L:S ratio of 150 mL/g (run 8). The order of significance of each coefficient was evaluated by probability values, which are presented in Table 2. The reaction temperature, reaction time and initial pH of solution had a positive effect on furfural yield. The initial pH of the solution among the independent variables had the greatest effect on the furfural yield while the L:S ratio was an insignificant factor for the conversion of wheat straw. A similar result was reported by Riansa-ngawong and Prasertsan (2011a), who found that the L:S ratio had no influence on the furfural production from hemicellulose extracted from delignified palm pressed fiber. They found that a maximum furfural yield of 8.67 g/L (17.34%) was achieved at a temperature of 135 °C, a concentration of 5.7% H2SO4, and in 90 min using a two-stage process where hydrolysis and dehydration reactions occur in two different reactors. However, another study by the same authors (Riansa-ngawong and Prasertsan, 2011b) found that the furfural yield from a one-stage process where hydrolysis and dehydration reaction occur simultaneously within the same reactor was only 0.86 g/L (3.44 wt.%). Singh et al. (1984) reported that a maximum furfural yield of 11.5 (% of bagasse) and 10.9 (% of rice husk) could be obtained by 0.4% H2SO4 at 180 °C. Similar results were also reported by Mansilla et al. (1998), who achieved a maximum furfural yield of 10.5% and 3.34% (dry rice hull basis) using twostage and one-stage technology, respectively. Amiri et al. (2010) studied the production of furans from rice straw by single and biphasic systems and found that the highest yield of furfural in a single-phase hydrolysis and biphasic system was 5.9% and 11.8% (g/100 g straw), respectively, at 150 °C and for 5 h. In our study, the maximum furfural yield of 51.3% (g/100 g pentose in straw DWB) (run 8) was equivalent to 14.87% (g/100 g straw DWB) of actual yield based on straw basis. These yield values obtained from the one-stage technology in our study were high compared to those reported in the literature. The higher yields obtained by one-stage microwave-assisted process may be related to the fact that microwaves interacted with the wheat straw at a molecular level and led to considerable physical disruptions in the internal composition of the cell. This microwave effect leads to a release of cell constituents and an increased the rate of mass transfer. It can be concluded that the microwave-assisted process was an effective method for the acid-catalyzed conversion of wheat straw to furfural. The yield of furfural from acid-catalyzed conversion increased linearly with increasing temperature and time (Fig. 1A). The slower rise of furfural yield at elevated temperatures and long residence times was likely due to thermal degradation of furfural. The surface response analysis for furfural yield revealed that the interaction effect between pH and temperature was highly significant (p < 0.0001) (Fig. 1C). The interactions between the other pair of independent variables (temperature–time and time–pH) were not significant (p > 0.05). The linear increase in furfural yield was observed with decreasing initial pH at low temperatures (<170 °C). However, furfural yield drastically decreased at temperatures above 170 °C in the range of low initial pHs (Fig. 1C). This is in full agreement with the result of O’Neill et al. (2009) who also reported the stability of furfural was low and that it degraded quickly at high temperatures. The linear increase in furfural yield with decreasing initial pH (up to 1.1) was observed at short residence time treatments, but a further decrease in initial pH resulted in complete disappearance of this effect (Fig. 1B). In agreement with this finding, Riansa-ngawong and Prasertsan (2011a) reported that the yield of furfural from H2SO4-catalyzed conversion increased linearly with increasing concentration of H2SO4 (up to 5.5%), however, this linear effect disappeared at higher H2SO4 concentrations. The strong effect of initial pH may be attributed to the change of H+ and Cl concentrations with change in HCl concentra-

219

Fig. 1. Response surface plot showing the effect of temperature, time, and initial pH on the acid-catalyzed conversion of wheat straw to furfural by microwave-assisted reaction. (A) Fixed at a pH of 1.1 and L:S ratio of 105 mL/g. (B) Fixed at a temperature of 170 °C and L:S ratio of 105 mL/g. (C) Fixed at a residence time of 21 min and L:S ratio of 105 mL/g.

tion. The nature of the solvent chosen and the matrix are known to be major factors influencing microwave absorption (Camel, 2000). The presence of excessive acid in reaction media may lead to rapid heating of the solution in microwave field; resulting in thermal stress depending on microwave absorption. In addition, the consecutive condensation reactions (cross- and self-polymerization) are known to cause loss of furfural during the acid-catalyzed

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Table 3 Possible solutions for the optimum conditions that maximize furfural yield. Solution

Temperature (°C)

Time (min)

pH

L:S ratio

Furfural yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

145.8 146.2 199.7 145.5 145.5 195.4 144.3 191.2 196.1 148.5 198.9 198.9 198.9 154.5 154.9

30.7 31.4 40.8 29.8 29.8 34.1 28.8 37.6 32.2 31.3 26.0 34.2 26.0 34.6 40.2

0.21 0.22 1.45 0.21 0.21 1.77 0.28 1.76 1.66 0.38 1.94 0.53 1.94 0.33 0.75

130.7 115.3 103.2 83.9 126.1 90.4 52.5 100.5 149 125.7 45.2 94 164.8 91.2 119.3

66 66 64 64 64 64 62 61 60 59 59 59 59 56 52

conversion process and is a most plausible reason for the dramatic decrease in furfural yield at low initial pH and high temperatures (Zeitsch, 2000). Determination of the optimum conditions for furfural yield by microwave-assisted heating was obtained by Design Expert V.8.0.5.2 using numerical optimization. The best possible solutions that maximized the furfural yield are presented in Table 3. There are two different possible regions in which furfural yield is maximized; one of low temperature–low pH and another of high temperature–high pH. From these results, the optimum points for maximizing furfural yield from acid-catalyzed conversion of wheat straw by microwave-assisted process from the two regions are: (1) a temperature of 146 °C, a residence time of 31 min, initial pH of 0.22, and L:S ratio of 84 mL/g in low temperature–low pH region; and (2) a temperature of 195 °C, a residence time of 34 min, initial pH of 1.77, and L:S ratio of 90 mL/g in high temperature–high pH region. These results show that the initial pH of solution and reaction temperature is critical parameters because increases in temperature and HCl concentration (decreases in pH) caused significant losses in furfural yield. It can be concluded that the reaction temperature, time and initial pH of solution are the important process variables. 3.3.2. HMF HMF which is a dehydration product of hexoses was another furan compound produced from the acid-catalyzed conversion of wheat straw by the microwave-assisted process. The surface response analysis showed that residence time and initial pH (p < 0.0001) were the main effects for HMF yield. In addition, the interactions of temperature  pH, time  pH, and temperature  time  pH were found to be highly significant (Table 2). Fig. 2(A–C) shows the response surface plots representing the combined effects of temperature, time and pH on the yield of HMF. Fig. 2(A) indicates a three dimensional graph representing the linear effect of time on the HMF yield at the range of temperature studied. The linear effect of temperature on the HMF yield was observed only at the range of 140–170 °C. This linear effect of temperature completely disappeared at higher temperatures of 170 °C, and a further increasing in temperature (>180 °C) resulted in a decrease in HMF yield. A similar linear increase in HMF yield with increasing temperature was observed at pHs higher than 0.8 (Fig. 2B). It was shown that the increase in temperature caused a decrease in HMF yield when treated at low pHs (<0.8) or in higher acidic conditions. Similarly, a linear decrease in HMF yield was observed in the range of pH 0.1–0.8 with increasing residence time (Fig. 2C). This sharp decrease in HMF yield could be attributed to the degradation of HMF

Fig. 2. Response surface plot showing the effect of temperature, time, and initial pH on the acid-catalyzed conversion of wheat straw to HMF by microwave-assisted reaction. (A) Fixed at a pH of 1.1 and L:S ratio of 105 mL/g. (B) Fixed at a residence time of 21 min and L:S ratio of 105 mL/g. (C) Fixed at a temperature of 170 °C and L:S ratio of 105 mL/g.

being accelerated at low pHs. Residence time had a positive effect on the HMF yield observed in the range of pH 1.1–2.1. The changes in HMF yield showed that the initial pH of the solution and reaction temperature were critical parameters. The positive effect of temperature (up to 170 °C) on furan yield may be associated with the lower activation energy and the increasing accessibility of acid

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ions to the sugar released by a decreasing viscosity of water. Condensation, rehydration, isomerization and fragmentation reactions that take place in the reaction media during acid-catalyzed conversion are known to cause degradation of HMF (Chedda et al., 2007). Soluble polymers, insoluble humins, levulinic acid and formic acid are possible by-products obtained from degradation of HMF. Hansen et al. (2009) reported that the yield of levulinic and formic acid obtained from rehydration of HMF increased with increasing temperature, therefore HMF yield decreased. In addition, the higher acidic conditions lead to more levulinic and formic acid formation, and this increase in yield of levulinic and formic acid paralleled the decrease in HMF yield. There is apparently no report on HCl-catalyzed conversion of wheat straw into HMF in the literature. The innovative HMF syntheses in ionic liquids have been comprehensively reviewed by Zakrzewska et al. (2011). Currently, the high yields for HMF could be achieved using only pure feedstocks such as monosaccharides (fructose, glucose), disaccharides (maltose, sucrose, cellobiose), and polysaccharides (inulin, starch, cellulose). Among studies on the direct production of HMF from raw lignocellulosic materials, Zhang and Zhao (2010) found a HMF yield of 2.1% (of hexose content) from pine wood in HCl-catalyzed ionic liquid could be obtained at 100 °C with oil bath in 60 min. They were able to achieve HMF yields of 45%, 47%, and 52% (of hexose content) from corn stalk, rice straw, and pine wood, respectively, in ionic liquid ([C4mim]Cl including CrCl36H2O) at 100 °C for 3 min by microwave heating. Binder and Raines (2009) studied on the chemical transformation of various carbohydrates and lignocellulosic biomass into furans in N,N-dimethylacetamide (DMA) containing lithium chloride (LiCl). They obtained HMF yields of 23–47% (of cellulose content) from corn stover in the presence of chromium, HCl as catalyst and 1-ethyl-3-methylimidazolium ([EMIM]Cl) as additive. Compared to these studies, the HMF yields during acidcatalyzed conversion of wheat straw were low for all processing conditions (Fig. 2A–C), however, only water with HCl as a catalyst were used for the conversion process. The high HMF yields in the literature have been achieved with conversions in special reaction media including ionic liquid, organic solvents, and metal halides. Another plausible reason for low yields of HMF during the acid-catalyzed conversion of wheat straw may be due to the temperature being limited by the maximum temperature of the microwave-assisted heating system, which was only 200 °C. Ando et al. (2000) reported that cellulose in different species of biomass only started to decompose above 230 °C. It can be concluded from these results that the temperature range used for the production of furfural in this study were not suitable for the HMF production from wheat straw by acid-catalyzed conversion. 3.3.3. Xylose Pretreatment has been considered as a major step in conversion of straw biomass into sugars (glucose and xylose). Dilute acid hydrolysis is one of the most commonly used methods for the pretreatment of lignocellulosic materials due to its low cost and effectiveness (Sun and Cheng, 2002). Xylose, derived from the hemicellulose fraction, is the main sugar for furfural production. As shown in Table 1 and Fig. 3(A–C), xylose yields from microwave-assisted process decreased with increasing temperature, time, and decreasing pH. Similar results under harsh hydrolysis conditions have been reported by Liavoga et al. (2007) for wheat straw, by Herrera et al. (2003) for sorghum straw, by Lu et al. (2009) for rapeseed straw, and by Rahman et al. (2007) for oil palm empty fruit bunch fiber. The surface response analysis showed that there was no significant interaction between variables, except for X1 X2 X3 (Table 2). Xylose yield started to decrease at temperatures above 145 °C (Fig. 3A) due to the conversion of xylose to furfural as manifested by the color change of extracts obtained after the

Fig. 3. Response surface plot showing the effect of temperature, time, and initial pH on the acid-catalyzed conversion of wheat straw to xylose by microwave-assisted reaction. (A) Fixed at a pH of 1.1 and L:S ratio of 105 mL/g. (B) Fixed at a residence time of 21 min and L:S ratio of 105 mL/g. (C) Fixed at a temperature of 170 °C and L:S ratio of 105 mL/g.

microwave process. The color of extracts obtained at 140 °C turned from yellow to brownish-yellow with increasing temperature. At a temperature of 200 °C and processing times greater than 26 min, all xylose released in the reaction media was converted to furfural

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(Fig. 3A). A negative linear effect of pH on the xylose yield was observed (Fig. 3C). Maximum xylose yield (100%) was obtained in the temperature range of 140–165 °C and a pH in the range of 2.1–1.6 (Fig. 3B). Liavoga et al. (2007) studied the H2SO4 acid-catalyzed hydrolysis of wheat straw in a batch reactor and found the maximum xylose yield of 85% was reached at an acid concentration of 0.3 M H2SO4, a temperature of 123 °C, and a processing time of 28 min. A similar result was also reported by Roberto et al. (2003), who found that the maximum xylose yield of 88% from rice straw could be achieved at an acid concentration of 1.6% H2SO4, a temperature of 121 °C, and a processing time of 27 min. In agreement with these findings, Sanchez et al. (2004) also reported that dilute-acid hydrolysis of straw at an acid concentration of 0.5% H2SO4, a temperature of 190 °C, and a processing time of oil palm empty fruit bunch fiber 5 min yielded 80% xylose. Convective–conductive heating, which involves relatively slow energy transfer rates depending on the thermal conductivity of the materials, has been commonly used for the pretreatment of lignocellulosic materials at an industrials scale. Microwave irradiation, which is an alternative method to conventional heating, provides in-core volumetric heating by direct interaction in a microwave field. The rapid and direct nature of microwave irradiation results in an efficient internal heating in reaction media. Our results have revealed that acid-catalyzed pretreatment of wheat straw by microwave-based heating provided a very effective extraction of xylose. The effect of microwave irradiation on the high extraction yield may be associated with the fact that small points with high energy density, which are commonly referred to as ‘hot spots’, are formed (Senise and Jermolovicius, 2004). Hot spots lead to a dramatic expansion in the tissue with super-heating of water inside sample matrix. This ‘explosion effect’ may cause release of the main components to interact with each other (cellulose, hemicellulose and lignin) trapped within the matrix with disrupting the recalcitrant structure of the plant cell wall (Hu and Wen, 2008).

3.3.4. Glucose Glucose was also released during acid-catalyzed conversion of wheat straw by the microwave-assisted process. The maximum level of glucose yield was 43.6% of the glucan content in wheat straw at 155 °C, pH 0.6 for 31 min, S:L ratio of 60 mL/g (run 6). Compared to xylose, glucose yields released from acid-catalyzed conversion of wheat straw were low, possibly due to only partial hydrolysis of the glucan fraction (cellulose). It was assumed that all of the glucose originated from the cellulose fraction and none from the hemicellulose fraction, which may also contain small amounts of glucose. Sanchez et al. (2004) conducted experiments to determine suitable conditions for a two-stage acid hydrolysis of the hemicellulose and cellulose parts of Bolivian straw. They reported a maximum glucose yield of about 30% of the theoretical value at a temperature of 230 °C and a residence time of 10 min. A maximum glucose yield of 46.6% of the theoretical value in two stages, which is similar to the current study, was achieved by Karimi et al. (2006). Vazquez et al. (2007) observed a similar behavior for the acid-catalyzed conversion of sorghum straw into sugars. They found that the glucose released was very low in the range of 2–6% of H3PO4 and 0–300 min of reaction time at 134 °C. Therefore, it was concluded that the glucan fraction was partially hydrolyzed during conversion. The reason for the partial hydrolysis of the glucan fraction in wheat straw is due to crystalline structure of cellulose. It is known that high temperatures and acid concentrations are needed to release glucose by depolymerization of crystalline cellulose. Ando et al. (2000) reported that the depolymerization temperature of cellulose was above 230 °C, whereas hemicelluloses decomposed easily at temperatures above 180 °C.

Fig. 4. Response surface plot showing the effect of temperature, time, and initial pH on the acid-catalyzed conversion of wheat straw to glucose by microwave-assisted reaction. (A) Fixed at a residence time of 21 min and L:S ratio of 105 mL/g. (B) Fixed at a pH of 1.1 and L:S ratio of 105 mL/g. (C) Fixed at a temperature of 170 °C and L:S ratio of 105 mL/g.

The effect of initial pH in relation to temperature on glucose yield is shown in Fig. 4(A). Two different regions of high glucose yield corresponding to a low temperature–low pH (high HCl concentration) and a high temperature–high pH (low HCl concentration) are apparent. Higher temperature with lower HCl

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concentration (higher pH) and lower temperature with higher HCl concentration (lower pH) had the same effect on glucose yield. Moreover, the residence time had a positive linear effect on glucose yield at all temperatures (Fig. 4B) and pHs (Fig. 4C). However, Sun and Cheng (2005) reported that acid concentration and pretreatment time had no significant effect on glucose yield obtained from rye straw while the glucose yield from Bermuda grass was significantly affected by acid concentration and pretreatment time. Response surface plot allows evaluating possible process conditions to achieve similar high yields. 4. Conclusions The acid-catalyzed conversion of wheat straw by microwaveassisted reaction was considerably affected by temperature, time and pH, while the effect of L:S ratio was statistically insignificant. The results of ANOVA and surface response plots revealed that initial pH of solution was the most important variable in conversion of wheat straw to furfural. Optimal microwave-assisted process conditions for the acid-catalyzed conversion of wheat straw to furfural were at a pH of 0.22, temperature of 146 °C, L:S ratio of 84 mL/g and 31 min residence time or at a pH of 1.77, temperature of 195 °C, L:S ratio of 90 mL/g and 34 min residence time. Acknowledgements The authors thank David Godfrey for technical assistance and valuable suggestions throughout this study. The authors also wish to express their gratitude to Eduardo Cacace and Dave Oomah for their valuable suggestions and discussions during RSM analysis. Oktay Yemisß thanks the Scientific and Technological and Research Council of Turkey (TUBITAK) for the scholarship (BIDEB-2219, Postdoctoral Research Scholarship). We would also like to give special thanks to Lana Fukumoto for her great help during HPLC analysis. References Amiri, H., Karimi, K., Roodpeyma, S., 2010. Production of furans from rice straw by single-phase and biphasic systems. Carbohydr. Res. 345, 2133–2138. Ando, H., Sakaki, T., Kokusho, T., Shibata, M., Uemura, Y., Hatate, Y., 2000. Decomposition behavior of plant biomass in hot compressed-water. Ind. Eng. Chem. Res. 39, 3688–3693. Binder, J.B., Raines, R.T., 2009. Simple chemical transformation of lignocellulosic biomass into furans for fuels and chemicals. J. Am. Chem. Soc. 131, 1979–1985. Buranov, A.U., Mazza, G., 2010. Extraction and characterization of hemicelluloses from flax shives by different methods. Carbohydr. Polym. 79, 17–25. Cacace, J.E., Mazza, G., 2007. Pressurized low polarity water extraction of biologically active compounds from plant products. In: Shi, J. (Ed.), Functional Food Ingredients and Nutraceuticals: Processing Technologies. Taylor & Francis Group, Baco Raton, FL, pp. 135–155. Camel, V., 2000. Microwave-assisted solvent extraction of environmental samples. Trends Anal. Chem. 19 (4), 229–248. Chedda, J.N., Roman-Leshkov, Y., Dumesic, J.A., 2007. Production of 5hydroxymethylfurfural and furfural by dehydration of biomass-derived monoand poly-saccharides. Green Chem. 9, 342–350. Dallinger, D., Kappe, C.O., 2007. Microwave-assisted synthesis in water as solvent. Chem. Rev. 107, 2563–2591. Dias, A.S., Lima, S., Pillinger, M., Valente, A.A., 2010. Furfural and furfural-based industrial chemicals. In: Pignataro, B. (Ed.), Ideas in Chemistry and Molecular Sciences: Advances in Synthetic Chemistry, vol. 8. Wiley-VCH, Weinheim, pp. 167–186. Hansen, T.H., Woodley, J.M., Riisager, A., 2009. Efficient microwave-assisted synthesis of 5-hydroxymethylfurfural from concentrated aqueous fructose. Carbohydr. Res. 344 (18), 2568–2572.

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