Research Article pubs.acs.org/journal/ascecg
Extraction of Cellulose Nanocrystals with a High Yield of 88% by Simultaneous Mechanochemical Activation and Phosphotungstic Acid Hydrolysis Qilin Lu,† Zhenghan Cai,† Fengcai Lin,† Lirong Tang,† Siqun Wang,‡ and Biao Huang*,† †
College of Material Engineering, Fujian Agriculture and Forestry University, Fuzhou 350002, China Center for Renewable Carbon, The University of Tennessee, Knoxville, Tennessee 37996-4570, United States
‡
ABSTRACT: An efficient approach for extracting cellulose nanocrystals (CNCs) was put forward through phosphotungstic acid (PTA) hydrolysis of cellulose raw materials under mechanochemical activation. Response surface methodology was employed for experimental design to determine the optimum conditions of CNCs preparation with software Design Expert. The results showed that quadratic polynomial model was qualified to represent the relationship between the response and independent variables and the regression model defined well the true behavior of the system. Under the optimal conditions, a high yield of up to 88.4%, crystallinity index of 79.6%, and a higher thermal stability can be achieved by combining mechanochemical activation and phosphotungstic acid hydrolysis. This process can improve effectively the hydrolysis efficiency, avoid a lengthy separation process, and reduce the preparation time. Meanwhile, compared to other techniques, mechanochemical activation is an energy-intensive method, and the process is environment-friendly. Phosphotungstic acid hydrolysis is easier to handle than liquid acids; meanwhile, the catalyst causes fewer corrosion hazards and can readily be recycled. Thus, an efficient green high-yield approach for the preparation of CNCs was achieved in the study. KEYWORDS: Cellulose nanocrystals, Mechanochemical activation, Phosphotungstic acid, Greener process, High yield, Thermal stability
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INTRODUCTION Cellulose nanocrystals (CNCs) exhibit great potential in composite materials,1 electronic and optical devices,2 and biomedical products due to their high strength, high flexibility, biocompatibility and biodegradability.3 They have single digit nanometer in width dimension as well as hundreds to thousands of nanometers in length, and their dimensions are similar to that of carbon nanotubes.4 Cellulose nanocrystals are mainly obtained by mineral acids hydrolysis, such as sulfuric acid or hydrochloric acid.5,6 Cellulose nanocrystals obtained from sulfuric acid hydrolysis are usually sensitive to temperature due to the presence of acidic moieties at their surface,7 which limits their applications in some areas. Because of the use of high concentration acid solution, these reactions are costly and produce high amounts of effluent that need treatment. In addition, some limitations still need to be addressed, such as corrosion hazards, time-consuming production process, harsh conditions, and low yield, which limit their industrialization. It was reported that the CNCs were produced through hydrochloric acid hydrolysis, but their aqueous suspensions tended to flocculate, and only a low yield of 20% could be achieved.8 Some researchers reported a yield of 60.5% by applying a phosphotungstic acid hydrolysis method;9 however, the © XXXX American Chemical Society
hydrolysis efficiency was much lower, and the reaction process was very time-consuming. Attempts to overcome these limitations cover the utilization of a sequential one-pot route under phosphotungstic acid hydrolysis and the application of nonclassical energy sources, including ball milling, ultrasonication, or mechanochemical techniques.10 Phosphotungstic acid (PTA) is the strongest of heteropolyacids, which can be used as a solid acid catalyst in many fields because of its low toxicity, easy recycling, high acidity, and thermal stability.11 Compared with liquid mineral acids, phosphotungstic acid is easier to handle and recycle, with less waste produced. It has abundant Bronsted acid sites and therefore can be used as an alternate catalyst of liquid acids for the hydrolysis of cellulose.12 Thus, its use is effective for the hydrolysis reactions. Mechanochemical activation is induced by the direct absorption of mechanical energy, involving the combination of mechanical and chemical phenomena on a molecular scale.13 Ball milling is a well-established method of mechanical Received: December 2, 2015 Revised: January 27, 2016
A
DOI: 10.1021/acssuschemeng.5b01620 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering Table 1. Box-Behnken Design and the Response to the Yield of CNCs
yield of CNCs, Y (%)
coded levels experiment
X1 (%)
X2 (h)
X3 (h)
experimental
predicted
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
−1(10) 1(15) −1(10) 1(15) −1(10) 1(15) −1(10) 1(15) 0(12.5) 0(12.5) 0(12.5) 0(12.5) 0(12.5) 0(12.5) 0(12.5) 0(12.5) 0(12.5)
−1(4.5) −1(4.5) 1(5.5) 1(5.5) 0(5) 0(5) 0(5) 0(5) −1(4.5) −1(5.5) 1(4.5) 1(5.5) 0(5) 0(5) 0(5) 0(5) 0(5)
0(2) 0(2) 0(2) 0(2) −1(1.5) −1(1.5) 1(2.5) 1(2.5) −1(1.5) −1(1.5) 1(2.5) 1(2.5) 0(2) 0(2) 0(2) 0(2) 0(2)
52.56 79.41 65.28 74.08 43.75 52.24 48.08 71.41 48.45 71.07 80.26 57.49 85.62 84.69 88.39 84.32 93.66
53.98 79.87 64.82 72.66 43.93 53.38 46.94 71.23 46.85 71.36 79.98 59.09 87.34 87.34 87.34 87.34 87.34
supernatant fluid was extracted with diethyl ether to recycle phosphotungstic acid. The precipitate was processed by ultrasonication treatment at 20 kHz for 0.5 h in an ultrasonic reactor prior to the obtention of cellulose nanocrystals. Optimization Conditions for Preparation of Cellulose Nanocrystals. Response surface methodology was employed for experimental design to determine the optimum conditions of CNCs preparation with software Design Expert (Trial Version 7.0.0, Static Made Easy, Minneapolis, Minnesota, USA). The main factors affecting CNCs preparation include phosphotungstic acid concentration, reaction time, and ball milling time. On the basis of single-factor experiments, the proper ranges of the three independent variables were phosphotungstic acid concentration X1 (10%, 12.5%, and 15%), reaction time X2 (4.5, 5, and 5.5 h) and ball milling time X3 (1.5, 2, and 2.5 h). The response was the yield of cellulose nanocrystals, and the selection range of each variable is shown in Table 1. Determination of CNCs Yield. The total volume of the manufactured CNCs suspension was measured. A specified amount of the CNCs suspension was then transferred to a weighing bottle, followed by freeze-drying. The resulting sample was weighed. The final yield was obtained from the average of three runs of measurements and calculated according to eq 1.
activation, which is considered to be a relatively simple and environmentally friendly procedure attributed to the operations without the use of solvents and intermediate fusion.14 It has been applied in solvent-free organic synthesis,15 the pretreatment of lignocellulose and the grinding of solids into fine particles.16 The size reduction of the solid during ball milling is due to the mechanical breaking of molecular bonds and accompanied by chemical bonds distorting and bond length extending.17 When the imposed stress is beyond the chemical bonding energy, bond rupture occurs. Therefore, the mechanochemistry technique would be a potential approach to enhance production efficiency of CNCs under mild conditions. The present work describes a novel and environmentally friendly pathway for the preparation of cellulose nanocrystals. This innovative approach, combining mechanochemical activation and phosphotungstic acid hydrolysis in tandem mode, leading to an efficient, green, and economical process for cellulose nanocrystals production. Furthermore, the morphology, structure, spectroscopic, and thermal properties of the obtained CNCs were also investigated.
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Y=
EXPERIMENTAL SECTION
Materials. Bamboo pulp as cellulose raw materials for extracting CNCs was supplied by Nanping Paper Co., Ltd. (Nanping, Fujian, China), and the phosphotungstic acid (PTA) was purchased from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). All chemicals used in this work were of analytical grade without any further purification. Extraction of Cellulose Nanocrystals. The bamboo pulp was cut into pieces and beaten to form cellulose pulp with a Fiber Standard Dissociation device (GBT-A, Changchun Yueming Small Testing Machine Co., Ltd., China) for 40 min at 1500 rpm. Mechanochemical activation of cellulose pulp was performed within a ball mill equipped with two 90 mL agate jars, each of which was loaded with 20 6 mmdiameter agate balls. For each experiment, a mixture of 0.5 g of cellulose pulp, 10 g of 12.5 wt % phosphotungstic acid solution was added into the agate jar and then the mixture was subjected to milling for 1.5−2.5 h. After milling, the balls were removed from the resulting sample, which was then introduced into a round-bottomed flask equipped with a condenser and kept at 90 °C in an oil bath for 4.5−5.5 h. The resulting mixture was purified with deionized water by repetitive centrifugations at 9000 rpm for 10 min, and the collected
(m1 − m2)V1 × 100 m3V2
(1)
where m1 is the total mass of dried CNCs and weighting bottle (mg), m2 is the mass of the weighting bottle (mg), m3 is the mass of cellulose pulp (mg), V1 is the total volume of as-manufactured CNCs suspension (mL), and V2 is the volume of CNCs to be dried (mL). Morphological Analysis. The morphology and size of cellulose raw materials were observed by field emission scanning electron microscopy (FESEM) with a XL30 ESEM-FEG model FESEM (FEI Co., Ltd., USA). All the samples were sputtered and coated with gold before observation. The morphology and size of CNCs were analyzed by transmission electron microscopy (TEM) with a JEOL JEM-1010 transmission electron microscope (Japan Electronics Co., Ltd., Japan) using an accelerated voltage of 100 kV. A drop of diluted suspension of CNCs was deposited on a carbon-coated grid, followed by stained with a 2 wt % phosphotungstic acid solution. X-ray Diffraction (XRD) Analysis. The crystallinity index (CrI) of CNCs in comparison with cellulose pulp was investigated by X-ray diffraction (XRD) analysis. The X-ray diffraction (XRD) measurement was performed on an X’Pert Pro MPD X-ray diffractometer (PhilipsFEI, The Netherlands) using Cu Kα radiation at 40 kV and 30 mA. Diffractograms were collected in the range of 2θ = 6−60° at a scanning B
DOI: 10.1021/acssuschemeng.5b01620 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering rate of 0.1° s−1. The crystallinity index (CrI) was calculated according to eq 2.
CrI =
I002 − Iam × 100 I002
Optimization of Preparation Conditions. To obtain the optimal preparation conditions and provide a better visualization of the statistically significant factors derived from the statistical analysis, three-dimensional response surface plots and contour plots for the effects of independent variables on the yield of CNCs were conducted, and the results are shown in Figure 1. Figure 1a shows the effects of phosphotungstic acid concentration and reaction time on the yield of CNCs. The yield of CNCs increases with the increase of reaction time at constant phosphotungstic acid concentration, especially when the reaction time is within the range of 4.5−5 h, and the yield gradually rises with the increase of phosphotungstic acid concentration from 10% to 15%. Moreover, the contour plot is elliptical, implying that the interactive effect of phosphotungstic acid concentration and reaction time on yield is significant.20 The interaction between phosphotungstic acid concentration and ball milling time is shown in Figure 1b. At low phosphotungstic acid concentration, the phosphotungstic acid cannot provide the required acid sites to break the β-1,4glycosidic bonds in cellulose. Generally, the amorphous component of cellulose is more easily hydrolyzed, and with the increase of acid concentration, the hydrolysis degree of amorphous region rises, which promotes the production of cellulose nanocrystals. The yield of CNCs declines at higher acid concentration probably due to the synergetic effects of phosphotungstic acid catalysis and ball milling, which significantly accelerates the hydrolysis of crystalline region. The contour plot is close to circular, indicating the relatively mild interactive effects of phosphotungstic acid concentration and ball milling time.21 Mechanochemical activation plays an important role in the preparation of CNCs. The yield of CNCs increases with the extension of ball milling time from 1.5 to 2.5 h at some reaction time (Figure 1c). Much more imposed stress produced in the ball milling process was transferred to cellulose, which causes the cleavage of chemical bonds in cellulose and promotes the size reduction of cellulose. 22 With the assistance of phosphotungstic acid catalysis, the hydrolysis degree of cellulose enhances and benefits the formation of cellulose nanocrystals. Furthermore, the optimal conditions to obtain the highest yield of CNCs are determined as follows: phosphotungstic acid concentration 13.5%, reaction time 4.7 h, and ball milling time 2.2 h. Under the optimal conditions, the obtained yield of CNCs is 88.4%, which is not significantly different from the predicted value of 89.7% at 95% confidence interval. Morphology and Crystallinity of CNCs. The SEM images of bamboo pulp are shown in Figure 2a,b. The cellulose raw materials display a regular rod-like structure with a curled and flat shape, and the surface is rough with the average width of 15 μm and length of hundreds of micrometers. With the effects of mechanical ball milling and phosphotungstic acid hydrolysis, the cellulose fibrils are cleaved into small fibers, having their dimensions on the nanoscale.23 As can be seen from the TEM micrograph of CNCs (Figure 2c), short rod-like cellulose nanocrystals are obtained, and these nanocrystals form an interconnected web-like network structure that can provide higher reinforcing capability for composite materials.24 The size distributions of CNCs are calculated to find that the majority of the nanocrystals present in the dispersion are in the range of 200−300 nm in length and 25−50 nm in width (Figure 3). Meanwhile, the lower degree of agglomeration of cellulose
(2)
where I002 is the diffraction intensity of (200) plane at 2θ = 22−23° and Iam is the diffraction intensity at 2θ = 18−19°.18 FTIR Analysis. FTIR spectra of the samples were studied with a Nicolet 380 FTIR spectrometer (Thermo Electron Instruments Co., Ltd., USA) in the frequency range of 4000−400 cm−1 with a resolution of 4 cm−1. Prior to analysis, each sample was first ground with KBr and pressed into thin pellets. Thermal Gravimetric Analysis. The thermal stability of the cellulose samples was characterized with a thermal gravimetric analyzer (NETZSCH STA 449 F3 Jupiter). The samples were heated from 25 to 600 °C at a heating rate of 10 °C·min−1 in nitrogen atmosphere with a flow rate of 30 mL·min−1.
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RESULTS AND DISCUSSION Model Fitting and Statistical Analysis. By applying multiple regression analysis on the experimental data, the yield of CNCs as the response variable and the three independent variables are related by eq 3. Y = 87.34 + 8.43X1 + 0.91X 2 + 5.22X3 − 4.51X1X 2 + 3.71X1X3 − 11.35X 2X3 − 14.98X12 − 4.53X 2 2 − 18.49X32
(3)
where Y is the yield of CNCs; X1, X2, and X3 are the coded values of phosphotungstic acid concentration, reaction time, and ball milling time, respectively. The statistical significance testing of regression equation was checked by F-test, and ANOVA for the fitted quadratic polynomial model is shown in Table 2. The p-value of the model was less than 0.0001. Table 2. ANOVA for Response Surface Quadratic Model Analysis of Variance Table source
squares
df
model X1 X2 X3 X1X2 X1X3 X2X3 X12 X22 X32 residual lack of fit pure error cor total
4131.15 568.94 6.57 217.68 81.43 55.05 515.10 944.29 86.22 1439.75 72.54 12.43 60.10 4203.68
9 1 1 1 1 1 1 1 1 1 7 3 4 16
mean square 459.02 568.9 6.57 217.68 81.43 55.05 515.10 944.29 86.22 1439.75 10.36 4.14 15.03
F-value
p-value (prob > F)
44.30 54.90 0.63 21.01 7.86 5.31 49.71 91.13 8.32 138.94
