Initiating Highly Effective Hydrolysis of Regenerated Cellulose by

Research Article pubs.acs.org/journal/ascecg

Initiating Highly Effective Hydrolysis of Regenerated Cellulose by Controlling Transition of Crystal Form with Sulfolane under Microwave Radiation Na Teng,†,‡ Jinping Ni,† Haizhen Chen,† Qinghua Ren,‡ Haining Na,*,† Xiaoqing Liu,† Ruoyu Zhang,† and Jin Zhu*,† †

Ningbo Key Laboratory of Polymer Materials, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, Zhejiang 315201, P.R. China ‡ Department of Chemistry, College of Science, Shanghai University, Shanghai 200444, P.R. China ABSTRACT: By researching the transition of the crystal form of regenerated cellulose (RC) during hydrolysis, the key factor to achieving highly effective hydrolysis under microwave radiation is adequately recognized as reducing the formation of cellulose I during hydrolysis. With the suitable use of sulfolane, the formation of cellulose I within the process of recrystallization during hydrolysis is inhibited and thus highly promotes the reactivity of RC. Accordingly, the effectiveness of the hydrolysis of regenerated cellulose is successfully induced. The more sulfolane is used, the less cellulose I forms. As a result, a more effective hydrolysis of RC is produced. After increasing the ratio of sulfolane to 80 wt %, the conversion of cellulose is obviously improved with the highest amount over 98%. Under this condition, the yield of total reducing sugars is up to 71.9%. On the basis of controlling the transition of crystal form of regenerated cellulose, our research not only suggests an effective way to hydrolyze RC, but also provides an understanding of the critical principle of reducing the formation of cellulose I within the process of recrystallization during hydrolysis to initiate the hydrolysis of cellulose with high effectiveness. KEYWORDS: Biomass, Hydrolysis, Cellulose I, Sulfolane, Microwave radiation

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INTRODUCTION Diminishing fossil resources and increasing demand for materials (particular carbon-based polymer materials) have driven researchers to utilize abundant and renewable cellulosic biomass for producing biobased materials.1−3 In the process of utilizing cellulosic biomass, the hydrolysis of cellulose to soluble sugar is important and has attracted widespread interest.4 However, the high order crystalline structure of cellulose always makes it too recalcitrant to hydrolyze.5,6 In order to alleviate the impact of the recalcitrant structure, many attempts have been carried out during the past decades. Pretreatment7−10 was largely considered to reduce the crystallinity of cellulose. After that, although the crystallinity of cellulose is decreased to some extent as expected, most results show that it is still not enough to completely overcome the impact of crystallization and highly improve the effectiveness during hydrolysis. Specifically as noted in our previous work,11 the obtained amorphous cellulose from pretreatment is rather easily recrystallized during the following hydrolysis. Consequently, the recalcitrant structure is formed again to inhibit hydrolysis. Meanwhile, some other catalyst research was also explored to improve the reactivity of cellulose. Solid acid,12−14 heteropoly acid,15−17 metal oxide,18 acid in ionic liquid,19−21 etc. have all been tried. Continuously increasing the efficiency of cellulose © XXXX American Chemical Society

conversion and selectiveness of the sugar product have been reported. Using an assistant catalyst is another novel technique.6 It provides particular help to reduce the difficulty of hydrolysis of cellulose. In the latest report, the sugar yield is highly enhanced. Besides these progressive results, it is still hard to find the most suitable catalyst or assistant catalyst to induce hydrolysis with high effectiveness. Looking through all the progress in the hydrolysis of cellulose, there is no real practical technology that is successfully established. That is to say, the theoretical understanding regarding with the responsiveness of cellulose to hydrolyze is rather not enough. Noticing this point, we started the research to focus on the relation between the chemical or molecular structure of cellulose and its reactivity. By analyzing the transition of the crystal structure of cellulose during hydrolysis, we expect to establish the theory to improve the responsiveness of cellulose in hydrolysis and thus provide guidance to optimize the catalyst and even the assistant catalyst. In our previous work,11,22,23 on the basis of the two-step (namely, pretreatment−hydrolysis) methodology, the impact of Received: November 9, 2015 Revised: January 15, 2016

A

DOI: 10.1021/acssuschemeng.5b01464 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

technique on a freeze-dryer (LGJ-10C, Sihuan). All samples were sputter-coated with ∼10 nm gold using an anion sputter coater (E1045, Hitachi) before the test.

the molecular structure on the hydrolysis of regenerated cellulose (RC) was partially elaborated. Under microwave radiation, by controlling the crystallinity and molecular weight, the hydrolysis of RC is highly promoted. In this paper, we will extend the research to explore the effect of crystal form on the hydrolysis of RC, especially cellulose I and cellulose II. Cellulose I consists of parallel chains forming hydrogen-bonded sheets that stack on top of each other through van der Waals interactions.24 Cellulose II features an antiparallel arrangement of the strands and some intersheet hydrogen bonds.25 Finally, the key factor to induce effective hydrolysis of crystalline cellulose is expected to be recognized.

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RESULTS AND DISCUSSION Hydrolyze RC with Sulfolane under Microwave Radiation. The hydrolysis of RC with various sulfolane concentrations is performed in 0.4 wt % sulfuric acid at 160 °C for 5 min under microwave radiation. As shown in Figure 1, the

MATERIALS AND METHODS

Materials. Microcrystalline cellulose (cellulose amount, >97%) was provided by Jiangsu Longhao New Material Co. D-(+)-glucose (GR grade, >99.5%) was supplied by Aladdin Reagent and dried under vacuum at 100 °C over 6 h for further use. All the other chemicals were in chemical grade and used without further purification. Preparation of RC. RC was prepared by treating microcrystalline cellulose in 85% H3PO4 at 50 °C for 2 h, and then it was regenerated with water, filtered, washed with water until neutral, and vacuum dried at 80 °C, according to Ni.23 The degree of polymerization (DP) of RC was 43, which was determined by intrinsic viscosity measurements.26 Hydrolysis under Microwave Radiation. Hydrolysis of RC was performed in a 300 mL Teflon autoclave vessel (2 MPa), which was heated by a microwave reactor (MWave-5000, Shanghai Xinyi) with a frequency of 2450 ± 50 MHz, output power from 0 to 1000 W, and IR sensing temperature controller. The magnetic stirring speed is 600 rpm/min. Sulfuric acid as a catalyst was added into the mixture of cellulose, sulfolane, and H2O. The practical concentration of sulfuric acid during hydrolysis was controlled at 0.4 wt %. Then the vessel was heated to 160 °C within 6 min under microwave radiation, and then temperature was kept at 160 °C for a desired time. Analysis of Sugar Product. The sugar product (namely, total reducing sugar (TRS)) was measured by the 3,5-dinitrosalicylic acid (DNS) method.27 Glucose was used as the standard for the measurement of reducing sugar. The conversion of cellulose and yield of TRS was calculated as follows:

Figure 1. Conversion of cellulose and total reducing sugar yield obtained in sulfolane−H2O solvent mixtures. Condition of hydrolysis: H2SO4 (0.4 wt %) at 160 °C for 5 min.

conversion of cellulose increases slightly from 39.0% to 44.0% when the sulfolane concentration increases from 0 to 40 wt %. The TRS yield increases from 33.3% to 39.0%. No obvious difference can be observed. By increasing the ratio of sulfolane to 60−70 wt %, it shows an extreme increase in the conversion of cellulose to 57.2%−72.3%. Simultaneously, TRS yield reaches 51.3%−62.3%. The highest conversion of cellulose and TRS yield is 98% and 71.9%, respectively, with the use of 80 wt % sulfolane. It is found that the highly effective hydrolysis of cellulose is achieved only within 5 min by using sulfolane under microwave radiation. Further to observe the morphology of the residual cellulose before and after hydrolysis (Figure 2), the more sulfolane is used during hydrolysis, the more rough

Conversion of cellulose (%) = average mass of hydrolyzed cellulose/mass of loaded cellulose (1)

× 100% TRS yield (%) = average mass of TRS/mass of loaded cellulose × 0.9 × 100%

(2)

Fourier Transform Infrared (FTIR) Analysis. Fourier transform infrared (FTIR) analysis was performed using a Nicolet 6700 infrared spectrometer. The spectra (4000−400 cm−1) were recorded with a resolution of 0.9 cm−1 and 32 scans per sample. Analysis of Crystal Structure and Crystallinity. Crystal structure and crystallinity (Cr) of RC and residual cellulose after hydrolysis was determined with a Siemens D5000 X-ray Diffractometer. The diffracted intensity was measured with Cu Kα radiation at 40 kV and 40 mA in a 2θ range between 5° and 45°. The ratio of cellulose I and Cr was calculated as follows:

Ratio of cellulose I (%) = FCI/Fc × 100%

(3)

Cr = Fc/(Fc + Fa) × 100%

(4)

where FCI, Fc, and Fa are the areas of cellulose I, crystal, and amorphous region, respectively. Characterization of Morphology. The morphology of residual cellulose after hydrolysis was observed by scanning electron microscope (SEM; S4800, Hitachi). The residual cellulose was washed by use of deionized water and then dried by the vacuum freeze-drying

Figure 2. SEM photographs of regenerated cellulose before and after hydrolysis. Condition of hydrolysis: H2SO4 (0.4 wt %) and sulfolane (80, 70, 60, 40, 20, and 0 wt % in sulfolane−H2O solvent mixtures) at 160 °C for 5 min. B

DOI: 10.1021/acssuschemeng.5b01464 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering surface of cellulose is. Especially when the concentration of sulfolane is more than 60 wt %, the surface of cellulose shows a number of fragments that show that the strong hydrolysis of cellulose has happened. Effect of Crystal Form on Hydrolysis. After hydrolysis, the crystal structure of the residual cellulose is investigated. As shown in the FTIR spectra (Figure 3), all residual celluloses

Figure 4. X-ray diffraction patterns of regenerated cellulose before and after hydrolysis. Condition of hydrolysis: H2SO4 (0.4 wt %) and sulfolane (80, 70, 60, 40, 20, and 0 wt % in sulfolane−H2O solvent mixtures) at 160 °C for 5 min.

Table 1. Summary of Hydrolysis of RC by Use of Sulfolane conversion of cellulose (wt %)

Figure 3. FTIR spectra of residual cellulose. Condition of hydrolysis is H2SO4 (0.4 wt %) and sulfolane (80, 70, 60, 40 20, and 0 wt % in sulfolane−H2O solvent mixtures) at 160 °C for 5 min.

sulfolane content (wt %) 0 mina 0 20 40 60 70 80

except the one obtained by using 80 wt % sulfolane exhibit an absorption band at 1440 cm−1, which is assigned to the CH2 scissoring motion in cellulose I.28,29 Our previous work11 has carefully proved that RC only contains amorphous cellulose and a minor amount of cellulose II. Additionally, recrystallization is also proved to inevitably happen during hydrolysis and forms very fast. In other words, cellulose I existing in the residual cellulose is produced by recrystallization. With an increase in sulfolane, the absorption band at 1440 cm−1 becomes weaker and weaker (Figure 3). Especially with 80 wt % sulfolane, the absorption band shifts to 1420 cm−1. Meanwhile, the absorption band at 896 cm−1 gradually becomes stronger and sharper, which is assigned to the C− O−C stretching of the β-(1−4)-glycosidic linkage in cellulose II and amorphous cellulose. It no doubt means the reduction of cellulose I. Even partial transition of cellulose II from cellulose I by effective swelling of cellulose I initiated by sulfolane is likely to have taken place.30 From these experimental results, it can be concluded that the formation of cellulose I during hydrolysis is inhibited with the use of sulfolane. After increasing the ratio of sulfolane to 80 wt %, almost no cellulose I is produced. XRD is employed to further investigate the crystal structure of the residual cellulose in detail. As shown in Figure 4, the curve of RC only exhibits very weak peaks at 12.2°, 20.0°, and 22.1°. This is attributed to the minor amount of cellulose II existing in RC. The crystallinity of RC is rather low at 30.3%. However, after hydrolysis, due to partial hydrolysis and recrystallization of amorphous cellulose, the crystallinity of the residual celluloses are improved to 76%−80%. With the use of 0−70 wt % sulfolane, no matter whether hydrolysis proceeds more or less, the crystal proportion of the residual cellulose is composed of cellulose I and II. The more sulfolane used in hydrolysis, the less cellulose I remains (Table 1). As we know, amorphous cellulose11,23 and cellulose II31−34 are much easier to hydrolyze than cellulose I. Furthermore, with the use of sulfolane, at least over 40% cellulose (majority is amorphous and cellulose II) is hydrolyzed. Considering these results, in

20.7 −b 23.0 − − 41.6

ratio of cellulose I in residual crystalline cellulose (%)

3 min

5 min

0 min

3 min

5 min

30.5 − 34.6 − − 91.4

39.0 43.5 44.0 57.2 72.3 98.0

21.5 − 20.9 − − 7.8

27.5 − 27.1 − − 1.5

29.7 29.4 29.1 27.5 24.7 0

Time of keeping cellulose at 160 °C under microwave radiation. b“−” means not provided.

a

practice, with an increase in sulfolane, the amount of cellulose I formed during recrystallization obviously reduces. After increasing sulfolane to 80 wt %, no peak around 15.6° is exhibited in the XRD curve (Figure 4). No cellulose I is formed during hydrolysis. Consequently, after 5 min microwave radiation, the conversion of cellulose in hydrolysis reaches 98%, and sugar yield is up to 71.9%. More carefully analyzing the crystal form of the residual celluloses, the key factor to promoting the capability of RC in hydrolysis under microwave radiation is finally understood. Besides the inevitable recrystallization, inhibition of the formation of cellulose I is highly necessary to avoid hindering hydrolysis. The less cellulose I produced, the more effective hydrolysis is initiated. With the help of sulfolane, the production of cellulose I can be successfully inhibited, thus initiating hydrolysis of RC with high effectiveness. Understanding Impact of Crystal Form on Hydrolysis. In the experiment, the reaction time during hydrolysis is controlled to exhibit the impact of crystal form on hydrolysis. Figure 5 shows the XRD curves of the residual cellulose obtained from different reaction times during hydrolysis. Accordingly, the conversion of RC and sugar yield are measured as illustrated in Figure 6. All the results are summarized in Table 1. Without using sulfolane, there is no large change in the amount of cellulose I. Prolonging the time of hydrolysis only improves the conversion of cellulose to some extent. After increasing sulfolane to 40 wt %, the ratio of cellulose I is still retained at 20%−30%. A long reaction time in hydrolysis does C

DOI: 10.1021/acssuschemeng.5b01464 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

Figure 6. Conversion of cellulose and total reducing sugar yield vs reaction time. Condition of hydrolysis: H2SO4 (0.4 wt %), sulfolane (80, 70, and 60 wt % in sulfolane−H2O solvent mixtures) at 160 °C.

factor to controlling the effectiveness of hydrolysis is obtained. Inhibition of the formation of cellulose I can be recognized as the prerequisite to maintaining hydrolysis with high effectiveness. By increasing the amount of sulfolane during hydrolysis, the ratio of cellulose I is effectively reduced. Accordingly, the conversion of cellulose is obviously improved, and the highest amount reaches over 98%. The TRS yield is up to 71.9%. Our research does not only find an effective way to hydrolyze RC but also to understand the critical point to induce the hydrolysis of cellulose with high effectiveness. This process can be easily extended to the hydrolysis of other cellulosic biomass.

Figure 5. X-ray diffraction patterns of regenerated cellulose after hydrolysis. Condition of hydrolysis: H2SO4 (0.4 wt %), sulfolane (0 wt % (a), 40 wt % (b), and 80 wt % (c) in sulfolane−H2O solvent mixtures) at 160 °C for 0, 3, or 5 min.

not obviously change the conversion of cellulose and sugar yield. Until increasing the sulfolane to 80 wt %, the ratio of cellulose I decreases to lower than 10%, even short of 2%. The hydrolysis of RC is finally promoted. Although microwave radiation as a fast and energy-efficient heating technique with the ability to initiate fast hydrolysis of cellulose is utilized in the experiment, 20%−30% of cellulose I formed by recrystallization is adequate to inhibit the process of hydrolysis. The key factor to keeping the high capacity of RC to hydrolyze is directly related to reducing the formation of cellulose I of regenerated cellulose during recrystallization. In the experiment, a suitable environment of hydrolysis is required. With the help of sulfolane during hydrolysis, the conversion of cellulose and sugar yield can be effectively improved to a high level only within several minutes of microwave irradiation.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +86-574-86685283. Fax: +86-574-86685186 (H.N. Na). *E-mail: [email protected]. Tel: +86-574-86685283. Fax: +86574-86685186 (J. Zhu). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful for the financial support from Projects 21274160 and 21304104 supported by the National Natural Science Foundation of China, Ningbo Natural Science Foundation (No. 2015A610054), Ningbo Key Lab of Polymer Materials (No. 2010A22001), and Ningbo Innovation Project (No. 2015B11003).

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CONCLUSIONS In this paper, utilization of sulfolane achieves the highly effective hydrolysis of RC under microwave radiation. By observing the crystal form of the regenerated cellulose, the key D

DOI: 10.1021/acssuschemeng.5b01464 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

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DOI: 10.1021/acssuschemeng.5b01464 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX