Esterification of oleic acid with methanol by immobilized lipase on wrinkled silica nanoparticles with highly ordered, radially oriented mesochannels

Esterification of oleic acid with methanol by immobilized lipase on wrinkled silica nanoparticles with highly ordered, radially oriented mesochannels

Materials Science and Engineering C 59 (2016) 35–42 Contents lists available at ScienceDirect Materials Science and Engineering C journal homepage: ...

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Materials Science and Engineering C 59 (2016) 35–42

Contents lists available at ScienceDirect

Materials Science and Engineering C journal homepage:

Esterification of oleic acid with methanol by immobilized lipase on wrinkled silica nanoparticles with highly ordered, radially oriented mesochannels Jinli Pang, Guowei Zhou ⁎, Ruirui Liu, Tianduo Li Key Laboratory of Fine Chemicals in Universities of Shandong, School of Chemistry and Pharmaceutical Engineering, Qilu University of Technology, Jinan 250353, PR China

a r t i c l e

i n f o

Article history: Received 19 March 2015 Received in revised form 16 September 2015 Accepted 23 September 2015 Available online 26 September 2015 Keywords: Wrinkled silica Radially oriented mesochannels Immobilized lipase Biocatalysis Biodiesel

a b s t r a c t Mesoporous silica nanoparticles with a wrinkled structure (wrinkled silica nanoparticles, WSNs) having highly ordered, radially oriented mesochannels were synthesized by a solvothermal method. The method used a mixture of cyclohexane, ethanol, and water as solvent, tetraethoxysilane (TEOS) as source of inorganic silica, ammonium hydroxide as hydrolysis additive, cetyltrimethylammonium bromide (CTAB) as surfactant, and polyvinylpyrrolidone (PVP) as stabilizing agent of particle growth. Particle size (240 nm to 540 nm), specific surface areas (490 m2 g−1 to 634 m2 g−1), surface morphology (radial wrinkled structures), and pore structure (radially oriented mesochannels) of WSN samples were varied using different molar ratios of CTAB to PVP. Using synthesized WSN samples with radially oriented mesochannels as support, we prepared immobilized Candida rugosa lipase (CRL) as a new biocatalyst for biodiesel production through the esterification of oleic acid with methanol. These results suggest that WSNs with highly ordered, radially oriented mesochannels have promising applications in biocatalysis, with the highest oleic acid conversion rate of about 86.4% under the optimum conditions. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Biodiesel currently attracts considerable attention because of its biodegradability, low toxicity, high security, and renewability. Biodiesel can be obtained through the catalytic esterification and transesterification of free fatty acids [1–3]. Many works have been carried out to develop a chemically catalyzed transesterification process for biodiesel production [4–7]. Chemical catalysts can cause technical problems related to biodiesel purification and separation from catalysts and by-products [8]. Thus, remarkable attention has been focused on enzyme immobilization onto solid mesoporous materials as heterogeneous catalysts for esterification because of their environment friendliness [9–12]. Su and Wei[11] reported the effects of reaction system and methanol addition mode on the immobilized lipase-catalyzed methanolysis of soapstock oil for the green production of biodiesel. They achieved a fatty acid methyl ester (FAME) yield of 95.2% with 5:1 M ratio of methanol to oil and 4% Novozym 435 at 45 °C for 10 h. The type of support influences the activity and operational stability of immobilized lipases. Numerous supports have been investigated, including macroporous and microporous polymers, silica sol–gel matrix or aerogels, ordered mesoporous silica, and other porous ceramics [13–16]. In recent years, mesoporous silica nanoparticles have received increasing attention in enzyme and protein immobilization ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (G. Zhou). 0928-4931/© 2015 Elsevier B.V. All rights reserved.

because of their tunable and uniform pore system, high specific surface area, great compatibility, and low toxicity. Mesoporous silica nanoparticles (MSNs) with special morphologies and well-defined pore structures are attracting considerable attention because of their controllable particle size, large surface area, uniform pore size, and ordered mesoporous structures [17–19]. Actually, synthesis and investigation of MSNs with a radially oriented mesochannels have become highly important topics. To further manipulate the orientation of nanochannels in MSNs, extensive research has been conducted to synthesize MSNs with different particle shapes, as well as controllable pore size and pore structures [20–24]. Zhang et al. first reported the synthesis of pure nanophases of monodisperse MSNs smaller than 130 nm with tunable porosity of stellate, raspberry, or worm-like morphologies [25]. Basset [26] formed fibrous silica nanospheres (KCC-1) using ordered fibers coming out from the center of particles and along the free radial directions in all directions, thereby producing fibrous pore morphology. Recently, Peng et al. synthesized core–shell MSNs containing controlled pore orientation (penetrating and radial pores), pore size (3.0 nm to 7.3 nm), and mesostructure (2-D hexagonal and 3D cage-like) [27]. In particular, MSNs with radiation-aligned mesopores (the pores of silica nanoparticles are mesoporous with large centerradial pore channels in random orientations) are found to have promising biomedical and catalytic applications for immobilizing some guest molecules such as enzymes, which allow active catalytic sites to disperse onto large internal surfaces and pores, which in turn improve the activity of


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catalyst system [28–30]. We demonstrated that MSNs with ordered mesochannels aligned perpendicular to the sphere surface could be used as multifunctional carriers in drug delivery, and that the unique pore configuration is more suitable for regulating diffusional loading and release from radially oriented mesochannels [31]. MSNs with a radial, wrinkled structure (wrinkled silica nanoparticles, WSNs, being formed with convex folds or lines in an untidy way in surface of silica nanoparticles) have been recently synthesized and used as supporting materials for drug delivery and catalysis [26,32,33]. To the best of our knowledge, considerable efforts have been exerted to develop pathways for WSN production, but the ability to control the silica structure and orientation of mesochannels remains very limited. Thus, WSNs with a highly ordered, radially oriented porous structure prepared using solvothermal method within a short period is unprecedented. In this paper, we report the solvothermal synthesis of WSNs by varying ratios of cetyltrimethylammonium bromide (CTAB) to polyvinylpyrrolidone (PVP). WSNs with controllable particle size (240 nm to 540 nm), Brunauer–Emmett–Teller (BET) surface areas (490 m2 g− 1 to 634 m2 g−1), and average pore size (7.4 nm to 10.1 nm) were successfully synthesized. The WSNs had highly ordered, radially oriented mesoporous channels perpendicular to the surface. The prepared materials as carriers were used to immobilize Candida rugosa lipase (CRL). WSNs with highly ordered, radially oriented mesochannels exhibited unique high loading amounts. This study mainly aimed to prepare immobilized CRL-catalyzed biodiesel from oleic acid esterification with methanol. WSNs with highly ordered, radially oriented mesochannels were found to have potential applications in biocatalysis. 2. Methodology

(P, mg g−1) immobilized onto supports and enzyme activity (Ea, U g−1) of immobilized CRL were obtained (See Supporting Information). The immobilized lipases on WSNs were denoted as WSN0-CRL, WSN1-CRL, WSN2-CRL, and WSN3-CRL. All experiments of loading amounts and activity measurement were carried out at least three times, and results were averaged. 2.4. Esterification procedure Biodiesel production from oleic acid with methanol was carried out in a 50 ml three-necked flask attached to a reflux condenser using a magnetic stirrer, and an oil bath was used to maintain reaction temperature. A typical reaction mixture in the reactor contained 8.1 ml of methanol, 3.2 ml of oleic acid, and 0.06 g of lipase. Molar ratio of methanol to oleic acid was 20:1, and quantity of lipases (free lipase or lipase content in immobilized lipases) was 2.0 wt% (wt of lipases/wt of oleic acid). After stirring for 12 h at 45 °C, catalyst was recovered by centrifugation, washed with methanol, and dried at room temperature for reuse. To determine the stability of immobilized lipase, the immobilized CRL was reused by repeating (five times) batch experiments under the same condition. To analyze the product, conversion rate (α%) of oleic acid can be evaluated according to literature [35,36]. The amount of unreacted oleic acid in the product mixture was obtained from its acid value (AV), which can be determined by titration. The conversion rate of oleic acid to methyl oleate was determined by titration with 0.1 M potassium hydroxide (KOH) standard solution, according to the following equation [35,36]: α ð%Þ ¼

AVi−AVt  100% AVi

2.1. Chemicals PVP (K30) was used as received from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). CTAB and TEOS were obtained from Shanghai Chemical Reagent Inc. of Chinese Medicine Group (Shanghai, China). Ammonium hydroxide (NH4OH) (25%), methanol, oleic acid, ethanol and cyclohexane were obtained from Tianjin Chemical Agent Co., Ltd. (Tianjin, China). C. rugosa lipase (CRL) was purchased from Sigma-Aldrich (St. Louis, MO, USA). All chemicals were of analytical grade and were used as received without any further purification. 2.2. Synthesis of WSNs In a typical synthesis, 0.5 g of tetraethoxysilane (TEOS) and 4 ml of cyclohexane were mixed and ultrasonicated with the frequency of 40 kHz (KQ3200, Kunshan Ultrasonic Instrument Co., Ltd., China) for 10 min at room temperature. The mixture was transferred into a solution composed of 25 ml deionized water, 15 ml ethanol, 0.08 g CTAB, and (0–0.8 g) PVP under vigorous stirring at 35 °C, which 0.02 mol L−1 CTAB, 0, 0.005, 0.01 and 0.02 mol L−1 PVP in the solution and then allowed to react for an hour. About 0.5 ml of ammonium hydroxide as hydrolysis additive was added to promote TEOS hydrolysis. After 4 h, the obtained microemulsion solution was transferred into a Teflon-lined stainless steel autoclave, heated to 100 °C, kept at this temperature for 12 h, filtered, washed with water, dried at room temperature for 6 h, and calcined at 550 °C for 6 h to remove organic components. The molar ratio of CTAB:PVP:cyclohexane:TEOS:ethanol:H2O:ammonium hydroxide was 1:(0, 0.023, 0.046, or 0.092):168:11:1172:6327:30. The as-prepared silica samples were denoted as WSN0, WSN1, WSN2, and WSN3 at CTAB-to-PVP molar ratios of 0, 0.023, 0.046, and 0.092, respectively.

where α is the conversion rate of oleic acid to methyl oleate, and AVi and AVt are the acid values of the feed and the products, respectively. Measurements were repeated thrice for each sample, and results were averaged. Notably, experiments with free lipase were performed by adding a desired amount of lipase. 2.5. Characterization High-resolution transmission electron microscopy (HRTEM) observations were observed on JEM-2100 electron microscope with an acceleration voltage of 200 kV. All samples were dispersed in ethanol ultrasonically and were dropped on copper grids. Field emission scanning electron microscopy (FESEM) micrographs of samples were obtained with a Nova NanoSEM 450 microscope operated at an acceleration voltage of 10.0 kV. Sample powders were dispersed in ethanol by sonication, then dropped onto the surface of a silicon wafer and were sputter-coated for two cycles with gold to avoid charging under the electron beam prior to examination. N2 adsorption–desorption experiments were measured on TriStar 3020. Samples were degassed for 6 h at 180 °C before measurements. Specific surface areas were calculated by BET (Brunauer–Emmett– Teller) method, the pore size distribution and pore volumes were calculated from the adsorption branch using BJH (Barett-Joyner-Halenda) methods. UV–vis spectra were recorded on a SHIMADZU UV-2600 spectrophotometer. Small-angle X-ray powder diffraction (SAXRD) patterns were recorded in the 2θ range 0.5°–10° at a step size of 0.02° with a Bruker D8 advance diffractometer, using CuKα radiation (30 kV, 30 mA, λ = 0.1541 nm). Fourier transformed infrared (FT-IR) spectra were collected on samples in KBr tablets using a Bruker Tensor 27 Fourier transform infrared spectrophotometer. 3. Results and discussion

2.3. Synthesis of WSNs loaded with CRL CRL immobilization onto support carriers and activity assay were performed according to our previous work [34]. The lipase amount

We investigated the effects of molar ratio of CTAB to PVP on size, surface morphology, and structure. As shown in the FESEM images of Fig. 1, all WSNs are spherical and have tunable particle size. The cationic

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Fig. 1. FESEM images of WSNs0 (a), WSNs1 (b), WSNs2(c) and WSNs3 (d).

Fig. 2. HRTEM images of WSNs0 (a), WSNs1 (b), WSNs2 (c) and WSNs3 (d). The red box indicates slightly twist arrays of radial mesochannels near the center of nanosphere.



J. Pang et al. / Materials Science and Engineering C 59 (2016) 35–42 Table 1 Physicochemical properties of the WSNs and WSNs-CRL. Sample

SBET (m2 g−1)a

V (cm3 g−1)a

D (nm)a


490 ± 2 634 ± 4 629 ± 3 603 ± 2 285 ± 5 254 ± 3 245 ± 4 281 ± 2

1.13 ± 0.01 1.31 ± 0.03 1.19 ± 0.02 1.18 ± 0.01 0.95 ± 0.04 0.70 ± 0.03 0.57 ± 0.04 0.69 ± 0.01

10.1 ± 0.1 8.36 ± 0.3 7.37 ± 0.4 7.95 ± 0.1 13.05 ± 0.5 10.82 ± 0.3 9.57 ± 0.3 9.84 ± 0.2

a D, SBET, and V stand for average BET pore diameter, surface area, and pore volume, respectively. Errors indicate the relative standard deviation for n = 3 independent experiments.

Fig. 3. SAXRD images of WSNs0 (a), WSNs1 (b), WSNs2 (c) and WSNs3 (d).

surfactant CTAB favors the synthesis of silica nanoparticles with large particle size (540 nm) and a wrinkled structure (Fig. 1a), whereas coexistence with PVP dramatically reduces the size of particles (540 nm (Fig. 1a) to 240 nm (Fig. 1d)). Remarkably, average wrinkle distances are almost identical and increased in Fig. 1b compared with WSN0. However, once ratio changes from 1:0.023 to 1:0.046, particle sizes vary from 480 nm (Fig. 1b) to 360 nm (Fig. 1c). Interestingly, in addition to spheres with radial wrinkled structures, monodisperse spherical nanoparticles are also generated (Fig. 1c). With increased amount of PVP added to mixtures at an identical mixing ratio of 1:0.092, wrinkled structures remain. Some particles have a bumpy, rough surface and a raspberry-like structure, suggesting that large particles form by the

aggregation of several smaller clusters, which is involved in the wrinkled structures. Size and wrinkled structures of WSNs are sensitive to the molar ratio of CTAB to PVP. This finding is consistent with previous results [27]. In addition to various sizes and wrinkled structures, the mesostructures of these WSN materials are also different, as determined based on HRTEM images (Figs. 2, S1, and S2). All samples exhibit highly ordered, radially oriented mesochannels. However, the orientation of mesochannels varies with increased amount of PVP introduced into reaction systems. Fig. 2a–c shows particles with highly ordered, radially oriented mesostructures (stellate-like channel) from the center to the outer surface uniformly distributed in all directions. Interestingly, WSN3 samples possess a different mesochannel orientation. Arrays of radial mesochannels near the center of nanosphere are found to slightly twist (box in Figs. 2d and S2d) and radially align toward the edge of surfaces because of restrictive power, strong selective adsorption, and molecular self-assembly of PVP molecular chains [37–39]. This property may facilitate the diffusion of the guest molecules throughout the WSNs [27].

Fig. 4. The N2 adsorption–desorption isotherms (a, c) and corresponding BJH pore size distribution curves (b, d) of WSNs0(■), WSNs1 ( ), WSNs2 ( ), WSNs3 ( ) in (a, b) and WSNs0CRL(■), WSNs1-CRL ( ), WSNs2-CRL ( ), WSNs3-CRL ( ) in (c, d).

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Fig. 5. FT-IR spectra of WSNs0 (a), WSNs1 (b), WSNs0-CRL (c), WSNs1-CRL (d) and CRL (e).

Fig. 6. CRL loading amounts of WSNs0, WSNs1, WSNs2 and WSNs3. Error bars indicate the relative standard deviation for n = 3 independent experiments.

Mesostructures of mesoporous silica spheres prepared at different ratios of CTAB to PVP can be further verified by small-angle XRD patterns (Fig. 3). All samples exhibit a single broad diffraction peak corresponding with the average mesochannel–mesochannel correlation distance in the small-angle region [40], indicating the radiationaligned mesopore structure, consistent with the above TEM observations of highly ordered, radially oriented mesochannels. The N2 adsorption–desorption isotherm of all samples before and after CRL loading exhibits a type-IV isotherm pattern similar to only one step in the middle range of a relative pressure range of 0.5–1.0, and an H3-type hysteresis loop in the relative pressure near 1.0 is observed (Fig. 4), implying the presence of slit-shaped mesopores [32]. This result indicates that the porous structure remains unchanged after CRL loading inside pore channels. In pore size curves, we see that dual pore size distributions consist of sharp peaks within 3–4 nm and wide bands within 15–35 nm, which are ascribed to the small pore mouths in the silica walls of the mesochannel structure and large radially oriented mesopores, corresponding with reported results in FESEM images of interwrinkle distances [22]. BET surface areas of these WSN materials range within 490–634 m2 g−1, and total pore volume is found to be 1.13–1.31 cm3 g−1, as shown in Table 1. These results indicate that WSNs prepared by adding PVP have the higher surface areas than those prepared without PVP. With increased PVP quantity, surface areas and pore volume decrease. Notably, the enlargement of wrinkle distance of WSN1 probably leads to increased specific surface area and pore volume, consistent with previous results [26]. Meanwhile,

surface areas and pore volume of CRL-loaded WSNs decrease compared with parent WSNs, indicating that CRL is successfully immobilized onto channels of WSNs. By contrast, a significant difference exists between FT-IR spectra of parent WSNs and CRL-loaded WSN samples. Fig. 5 presents FT-IR spectra of CRL and WSN samples before or after CRL loading. FT-IR transmittance spectra are recorded within 470–4000 cm−1. All samples show specific bands at around 1095, 806, and 472 cm−1, which conform to the typical stretching vibration of Si–O–Si in mesoporous SiO2. The band at 1547 cm−1 could be assigned to the N–H stretching vibration of –CONH– in CRL molecules, suggesting that CRL immobilized in WSN channels. A possible mechanism for the formation of mesoporous silica nanoparticles with radial wrinkle structure and CRL immobilization onto WSNs support is illustrated in Scheme 1. In this study, PVP was added into the reaction system as capping agent and steric emulsion stabilizer. The PVP molecule chains were adsorbed on CTAB cylindrical O/W (oil-in-water) microemulsion droplets, forming a continuous layer similar to a capping agent, to inhibit the growth of microemulsion droplets in all directions. The presence of the amount of PVP molecules could have stabilized the microemulsion droplets during the reaction and led to the formation of uniform smaller particles [31]. After hydrolysis reaction of TEOS and solvothermal process, WSNs with a wrinkled, highly ordered, radially oriented mesochannels can be formed. Physical adsorption is the simplest method of immobilizing enzyme onto ordered mesoporous SiO2. Possible physical adsorption forces of CRL on mesoporous silica involved here include weak van der Waals interaction, hydrogen bonding

Scheme 1. Schematic representation for formation of WSNs and CRL immobilization onto WSNs.


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Fig. 7. Ea of CRL, WSNs0-CRL, WSNs1-CRL, WSNs2-CRL and WSNs3-CRL. (pH 7.0, T = 35 °C). Error bars indicate the relative standard deviation for n = 3 independent experiments.

interaction, and electrostatic interactions between the CRL molecules and silanol groups on the internal surface of the WSNs [34]. As shown in Fig. 6, CRL loading amounts of WSN samples are apparently better than traditional SBA-15 (the abbreviation of Santa Barbara Amorphous material, a silica with well-ordered 2D hexagonal mesoporous structures), with the highest loading amount of 270 mg/g based on our previous works [34,39]. As a result, lipases may become strongly adsorbed onto highly ordered, radially oriented porous channels of WSN particles. As expected, enzyme activities of immobilized lipases are higher than those of free lipases, as shown in Fig. 7. A notable result is that the highest hydrolysis activity of immobilized CRL on WSN3 is 93 U/g, whereas that for free CRL is only 33 U/g. This result demonstrates that highly ordered, radially oriented porous structure is prone to improve lipase activity because radiation-aligned mesopores allow active catalytic sites to disperse on large internal surfaces and pores, which in turn activate enzymatic center configuration to improve the activity of catalyst system [28–30]. The operational and storage stabilities of an immobilized lipase are important parameters from which the economic viability of a biosynthetic process can be determined. The enzyme activity (Ea) of immobilized CRL

Fig. 8. Effect of reuse numbers of WSNs0-CRL (■), WSNs1-CRL (●), WSNs2-CRL (▲) and WSNs3-CRL (▼) on Ea (pH 7.0, T = 35 °C). Error bars indicate the relative standard deviation for n = 3 independent experiments.

Fig. 9. The catalytic performance of CRL, WSNs0-CRL, WSNs1-CRL, WSNs2-CRL and WSNs3-CRL in esterification reactions (8.1 ml of methanol, 3.2 ml of oleic acid, and 0.06 g of lipase, 45 °C and 12 h). Error bars indicate the relative standard deviation for n = 3 independent experiments.

after five cycles at 35 °C is shown in Fig. 8. The enzyme activities of immobilized lipase gradually decreased with increasing recycling time. The decrease in enzyme activities for the samples may be ascribed to the leakage of immobilized lipase and separation processes during successive hydrolysis reactions. The activity of MSNs3-CRL still retained 62.20 U g− 1 after the fifth reuse, which was higher than that of MSNs0-CRL, MSNs1-CRL, and MSNs2-CRL at 42.69, 50.63, and 55.69 U g− 1, respectively. The improved initial activity and operational stability of MSNs3 silica-immobilized CRL may be attributed to the arrays of radial mesochannels near the nanosphere center, which were found to slightly twist and radially align toward the edge of surfaces in WSN3 sample, resulting in slower lipase leakage during the recycling process. The storage stability of immobilized CRL was also evaluated by running the catalytic hydrolysis of triacetin once per month for 6 months at 4 °C (data not shown). After six months of storage at 4 °C, the enzyme activity of immobilized lipase was similar to those presented

Fig. 10. Effect of reuse numbers of WSNs0-CRL (■), WSNs1-CRL (●), WSNs2-CRL (▲) and WSNs3-CRL (▼) on the oleic acid conversion rate (8.1 ml of methanol, 3.2 ml of oleic acid, and 0.06 g of lipase, 45 °C and 12 h). Error bars indicate the relative standard deviation for n = 3 independent experiments.

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above, demonstrating the good long-term storage stability of immobilized lipase. The esterification of oleic acid with methanol was performed at a methanol-to-acid molar ratio of 20 (8.1 ml of methanol, 3.2 ml of oleic acid), and 0.06 g of lipase under heating to 45 °C for 12 h. The conversion rate of oleic acid is listed in Fig. 9. The immobilization of free CRL can significantly influence results. The conversion rates of oleic acid catalyzed by free CRL, WSN0-CRL, WSN1-CRL, WSN2-CRL, and WSN3-CRL are 36.1%, 67.1%, 77.7%, 80.3%, and 86.4% with the relative standard deviation 2.13%, 2.43%, 3.19% and 2.75%, respectively, which indicates that the catalytic performance of immobilized CRL is better than that of free lipase, which may be due to the better resistance of immobilized lipases to thermal denaturation than free lipase at 45 °C. This finding is in accordance with results of Kharrat et al. [41]. The improved catalytic performance of immobilized CRL is also probably due to the dramatically enlarged area of the reaction interface where biocatalytic reaction occurs [42,43]. Moreover, the conversion rates of oleic acid catalyzed by immobilized CRL after five cycles at 45 °C for 12 h are shown in Fig. 10. The conversion rate of oleic acid catalyzed by MSNs3-CRL still retained over 43% after the fifth reuse, whereas that of MSNs0-CRL, MSNs1-CRL, and MSNs2-CRL retained only 30%, 32%, and 37%, respectively. The same reason is described in Fig. 8, which illustrates the operational stability of an immobilized lipase. 4. Conclusions WSNs with a highly ordered, radially oriented porous structure were successfully synthesized by solvothermal method. Particle sizes, surface morphology, and radial mesochannels can be controlled by changing molar ratios of CTAB to PVP. Nanoparticle size decreased with increased amount of PVP from 540 nm to 240 nm. The prepared materials were applied as carriers to immobilize CRL as a new biocatalyst in the synthesis of biodiesel through the esterification of oleic acid with methanol. Enzymatic activities of CRL loaded WSN samples were better than those of free lipase, and WSN3-CRL exhibited the highest catalytic performance for the conversion of oleic acid of about 86.4%. These results indicated that immobilized CRL can be considered as an efficient catalyst for the esterification of oleic acid. Our work provides an efficient method of easily tuning both mesochannel orientations and particle sizes of WSNs, which could have potential application as catalytic support for efficient biodiesel production. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. 20976100, 51372124, 51572134) and the Program for Scientific Research Innovation Team in Colleges and Universities of Shandong Province. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. References [1] A. Talebian-Kiakalaieh, N.A.S. Ami, H. Mazaheri, A review on novel processes of biodiesel production from waste cooking oil, Appl. Energy 104 (2013) 683–710. [2] V. Brahmkhatri, A. Patel, 12-tungstophosphoric acid anchored to SBA-15: an efficient, environmentally benign reusable catalysts for biodiesel production by esterification of free fatty acids, Appl. Catal. A 403 (2011) 161–172. [3] P. Yin, W. Chen, W. Liu, H. Chen, R.J. Qu, X.G. Liu, Q.H. Tang, Q. Xu, Efficient bifunctional catalyst lipase/organophosphonic acid-functionalized silica for biodiesel synthesis by esterification of oleic acid with ethanol, Bioresour. Technol. 140 (2013) 146–151. [4] W.S. Gong, J. Lu, H.H. Wang, L.J. Liu, Q. Zhang, Biodiesel production via esterification of oleic acid catalyzed by picolinic acid modified 12-tungstophosphoric acid, Appl. Energy 134 (2014) 283–289.


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