Synthesis of Gold Nanoparticles on Rice Husk Silica for Catalysis

Article pubs.acs.org/IECR

Synthesis of Gold Nanoparticles on Rice Husk Silica for Catalysis Applications Yan Li,† Jeremy Y. Lan,† Jingjing Liu,‡ Jingfang Yu,‡ Zhiping Luo,§ Weixing Wang,∥ and Luyi Sun*,‡ †

Department of Chemistry and Biochemistry, Texas State University, San Macros, Texas 78666, United States Department of Chemical & Biomolecular Engineering and Polymer Program, Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06269, United States § Department of Chemistry and Physics, Fayetteville State University, Fayetteville, North Carolina 28301, United States ∥ School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, Guangdong 510640, China ‡

ABSTRACT: Catalysts based on gold nanoparticles (Au NPs) have received much attention during the past two decades because of their unique catalytic properties in basic and applied research. Both experimental findings and theoretical predictions show that the size of the Au NPs plays a crucial role in governing their catalytic capability, with smaller Au NPs typically exhibiting higher catalytic performances. Although mesoporous silica has been extensively used as a support for Au NPs, the diffusion of reactants and products in the pores has been a challenge. Herein, we report an alternative silica material, silica NPs from rice husks (RHs), which have a rough surface, as a potential support for Au NPs. Notably, in this study, RH, a byproduct from rice production, was used as the silica source. Silica obtained by calcining HCl-treated RHs was first modified with (3-aminopropyl)triethoxysilane (APTES), which was designed to play two roles: helping the Au precursor (AuCl4−) absorb onto the silica surface and stabilizing the resultant Au NPs obtained by reducing AuCl4− using sodium borohydride (NaBH4). Characterization of the nanostructures revealed that the Au NPs formed with a narrow size distribution of ca. 2−4 nm, which is critical for catalytic applications. The reduction of 4-nitrophenol by NaBH4 with RH-silica-supported Au NPs as a catalyst was systematically studied to demonstrate the excellent catalytic performance of the prepared catalyst.

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have been devoted to the design of supported Au catalysts.19−23 By depositing Au NPs on a solid support, one can not only effectively prevent Au NPs from aggregating, but also easily separate the catalyst from the reaction medium, facilitating recycling. Silica materials have been extensively used as catalyst supports because of their intrinsic nanoscale structures; facile modification; nontoxic nature; and negligible absorption in the ultraviolet, visible, and near-infrared regions.24 Up to now, most silica supports used for Au NPs have been mesoporous silica.25,26 Au NPs can anchor on the porous surface of mesoporous silica materials, which exhibit a high surface area and excellent thermal and chemical stability. These mesoporous-silica-supported Au NPs have been shown to exhibit high catalytic performances. However, narrow and long pores can result in part of the Au NPs being embedded within the silica framework with limited accessibility to the reagent and can hinder the transport of products, leading to a longer reaction time and lower catalytic activity.27 Alternatively, solid silica NPs have also been investigated as supports for Au NPs. However, most solid silica NPs, which typically have a smooth surface, appear to be nonideal for supporting metallic NPs in terms of loading efficiency.28 In some cases, the deposited Au NPs are too large, even forming a dense and thick Au shell on the silica NP surface, which negatively influences their catalytic

INTRODUCTION Nanoparticles (NPs) of noble metals, including Au, Pt, and Ag, have attracted extensive attention because of their unique physicochemical properties and applications in catalysis, which are markedly different from those of the corresponding bulk materials.1−3 Because of their high surface areas, noble-metal NPs can expose more active sites and, thus, exhibit a higher catalytic performance. Among these noble-metal NPs, Au-NPbased catalysts have received increasing attention in the past decade because of their excellent catalytic performance in many important industrial processes.4−6 Au NPs can catalyze many reactions, including oxidation of CO,7 oxidation of alcohols to aldehydes or acids,8,9 oxidation of cyclohexane to cyclohexanol or cyclohexanone,10 hydrogenation reaction,11 electrocatalytic reduction of CO2 to CO,12 and the water−gas shift reaction.13 The reduction of 4-nitrophenol by borohydride on noble-metal NPs is considered to be an effective and ecofriendly route to the production of 4-aminophenol in industry.14,15 This process has been applied to the manufacturing of many analgesic and antipyretic drugs.15 Both experimental findings and theoretical simulations have demonstrated that the size of the Au NPs plays a crucial role in governing their catalytic capabilities. In general, the smaller the Au NPs, the higher the catalytic activity.16,17 However, because Au NPs have a high surface area and surface energy, they tend to aggregate and form larger agglomerates, resulting in a dramatic reduction of catalytic activity.18 In addition, when Au NPs are used directly as a catalyst, it is very difficult to separate them from the reaction medium because of their very small size. To overcome these limitations, significant efforts © 2015 American Chemical Society

Received: Revised: Accepted: Published: 5656

January 16, 2015 April 18, 2015 April 25, 2015 April 26, 2015 DOI: 10.1021/acs.iecr.5b00216 Ind. Eng. Chem. Res. 2015, 54, 5656−5663

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ability, and thus, more attention has been focused on their optical properties.29,30 One of the main reasons that solid silica NPs are not ideal for supporting catalysts is probably their limited numbers of anchoring sites and the difficulty of anchoring Au NPs on their smooth external surface.28 Whereas mesoporous silica offers sufficient anchoring sites, the diffusion of reactants and products through the pores has been a challenge. One potential “compromise” between the above two scenarios is silica NPs with a rough surface, which would offer more anchoring sites and allow for the supported catalyst to be well exposed. In addition, the morphological change from a smooth to a rough surface would also improve the wetting behaviors and thus the binding capacity.28 The adsorption kinetics on a rough surface is expected to be different from that on a smooth surface because of the changed diffusion rate through a stagnant layer of solvent near the surface.31 Thus, silica NPs with a rough surface should particularly favor the immobilization of metallic NPs. The classical approach to synthesize silica NPs by the sol−gel method using silicon alkoxides (such as tetraethoxysilane, TEOS) typically produces spheres with smooth surface,25−33 and the possibility of altering the surface of TEOS silica through chemical modification to generate a rough surface has been explored.28 A variety of methods have been developed for preparing silica NPs, including a hydrothermal method,26 a microwave method,32 and flame pyrolysis,33 but most of them require special instruments. The traditional industrial method for preparing silica NPs is to treat quartz with sodium carbonate under high temperature, but this process leads to high energy consumption and severe pollution.34 Various methods for preparing silica NPs from rice husks (RHs) have also been reported.35−38 For example, Witoon et al. used RH ash as the raw material for the successful synthesis of porous silica NPs with chitosan as the template,36 and Adam et al. synthesized porous silica NPs from RHs by a template-free sol−gel approach under ambient conditions.37 Alshatwi and coworkers successfully prepared silica NPs from RHs using acid digestion under pressurized hydrothermal conditions at 120 °C, followed by a calcination process.34,38 Recently, we reported a facile method for the direct synthesis of silica NPs with tunable pore sizes from RH biomass by controlling the pretreatment and subsequent reaction conditions.39,40 The obtained silica NPs are composed of smaller primary particles, leading to a porous structure and high surface area. With their narrow size distribution and high surface area, the silica NPs might serve as an ideal support for Au NPs. In addition to the unique rough surface morphology, the route to synthesize silica NPs from RHs offers additional advantages over the conventional silicon alkoxide routes, including greener raw materials, lower costs, and higher sustainability. RHs contain ca. 15−28 wt % silica depending on the variety, origin, climate, and geographic location, offering an opportunity for the mass production of silica NPs at low cost.39−41 Harvesting silica from RHs can not only realize the full value of RH biomass, but also minimize the environmental issues associated with the current undesirable disposal of RHs, including open field burning and land-filling. In this study, the silica NPs obtained by calcining HCltreated RHs, which have a rough surface, were modified by (3-aminopropyl)triethoxysilane (APTES) and subsequently used as a support for Au NPs. The catalytic reduction of 4-nitrophenol by NaBH4 on RH-silica-supported Au NPs was studied to evaluate the catalytic ability of the obtained supported Au NPs for practical applications.

Article

MATERIALS AND METHODS

Materials. The RHs used in this study were obtained from Three H’s LLC (Memphis, TN). Hydrogen tetrachloroaurate(III) trihydrate (99.99%, HAuCl4·3H2O), concentrated hydrochloric acid (37%, HCl), 4-nitrophenol, sodium borohydride (NaBH4), TEOS, and ammonium hydroxide (28%) were purchased from VWR. (3-Aminopropyl)triethoxysilane (APTES) was obtained from Sigma-Aldrich. All of the above chemicals were used as received without further purification. Synthesis of Silica NPs from RHs (RH Silica). RH silica NPs were synthesized through the calcination of HCl-treated RHs.42 In brief, RHs were first sequentially washed with tap water and deionized (DI) water to remove dirt and then dried in an oven at 90 °C overnight. The dried RHs were refluxed in 5 wt % HCl solution for 2 h, then washed with DI water, and dried at 90 °C overnight. Silica NPs with a diameter of ca. 60−70 nm were obtained by calcining HCl-treated RHs in a furnace at 700 °C for 2 h. The yield was ca. 17 wt %. Synthesis of Silica NPs from TEOS (TEOS Silica). Another type of silica NPs with a diameter of ca. 70 nm was synthesized by hydrolysis of TEOS with aqueous ammonia according to the Stöber method.43 In a typical synthesis, 2.9 mL of TEOS, 3.8 mL of ammonium hydroxide (28%), 80 mL of ethanol, and 1.4 mL of DI water were mixed in a flask under stirring for 10 h. The obtained silica NPs were thoroughly washed using ethanol and finally dispersed in 7.5 mL of ethanol, which served as a control sample. Modification of Silica NPs by APTES. The silica NPs prepared from both RHs and TEOS were modified by APTES to obtain amino-group-capped silica NPs. Typically, 2.0 mL of APTES and 0.10 g of silica were added to 10 mL of ethanol. The mixture was stirred at room temperature for 12 h. The resultant APTES-modified silica NPs were separated by centrifugation, washed with ethanol, and then redispersed in 9.0 mL of DI water. Synthesis of Au NPs on Silica Supports. A sample of 800 mL (0.10 mol/L) of HAuCl4 ethanol solution was added to separate dispersions of modified silica NPs from RHs and TEOS under vigorous stirring. After being stirred for 24 h under ambient conditions, the mixtures were centrifuged to remove excessive AuCl4− ions and then dispersed in 9.0 mL of DI water. A sample of 1.0 mL of 50 mg/mL NaBH4 ethanol solution was added to each of the above dispersions under vigorous stirring to reduce AuCl4− to form Au NPs. RH-silicasupported and TEOS-silica-supported Au NPs were finally obtained by centrifugation. Characterization. Fourier transform infrared (FTIR) spectra were collected on a Nicolet 730 FTIR spectrophotometer. For transmission electron microscopy (TEM) observation, a drop of the NP solution was deposited on a carbon-filmsupported copper grid and air-dried, after which it was observed in a JEOL 2010F field-emission transmission electron microscope at an accelerating voltage of 200 kV. Scanning electron microscopy (SEM) images were acquired on an FEI Strata 400S field-emission scanning electron microscope. All samples were sputter-coated with a thin layer (ca. 3 nm) of Au/Pd prior to SEM imaging. X-ray diffraction (XRD) patterns were collected using a Bruker D8 diffractometer with a Bragg−Brentano θ−2θ geometry equipped with a graphite monochromator with Cu Kα (λ = 0.1540 nm) radiation. Ultraviolet−visible (UV−vis) spectra were collected on a CARY 100 Bio UV−vis spectrophotometer (Varian). 5657

DOI: 10.1021/acs.iecr.5b00216 Ind. Eng. Chem. Res. 2015, 54, 5656−5663

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Industrial & Engineering Chemistry Research Catalysis Evaluation. The reduction of 4-nitrophenol by NaBH4 was chosen as a model reaction to evaluate the catalytic activity of the supported Au NPs.44−46 First, 51 mg of NaBH4 dissolved in 3.3 mL of DI water was added to 23.0 mL of 4-nitrophenol solution (0.12 mmol/L). To this solution was added a predetermined amount of catalyst. UV−vis spectra were recorded at 1-min intervals to monitor the progress of the reaction. The absorption spectra of the solution were measured in the range of 250−550 nm. The rate constant of the reduction reaction was determined by measuring the change in absorbance at 400 nm as a function of time.

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RESULTS AND DISCUSSION Scheme 1 shows the overall strategy for the synthesis of RHsilica-supported Au NPs. In brief, silica NPs were prepared by Figure 1. SEM image of RH silica NPs.

Scheme 1. Procedure for the Preparation of RH-SilicaSupported Au NPs

silica was then obtained by calcining HCl-treated RHs in a furnace at 700 °C for 2 h. As shown in Figure 1, the diameter of the RH silica NPs was ca. 60−70 nm. Overall, the particles exhibited a very rough surface, which is probably because such NPs are formed through the agglomeration of even smaller silica NPs.51 The XRD pattern of the RH silica showed that these silica NPs were amorphous (Figure 2).

calcining HCl-treated RHs. Because RH silica NPs exhibit a rough surface (as discussed in detail in the following section), they are expected to be an excellent candidate as a support for Au NPs. To improve the interactions between the Au precursors and the RH silica support, APTES was grafted onto the silica NP surface to obtain amino-group-modified RH silica, which is expected to exhibit a high performance not only for the absorption of Au precursor AuCl4− onto the silica surface, but also for the immobilization of Au NPs after they are formed. The modified RH silica was impregnated in a solution of AuCl4− and then reduced by NaBH4, giving rise to Au NPs immobilized on the silica surface. Synthesis of Silica from RHs. Up to now, many approaches have been developed for the preparation of various nanostructured silica materials, for which silicon alkoxides (typically TEOS) are among the most popular precursors.24−33 Although the size, morphology, and porosity of the silica from alkoxide precursors can be effectively controlled, this method for synthesizing nanostructured silica still encounters several challenges, including high costs and sustainability- and environment-related issues. Recently, RHs with a high silica content (ca. 15−28 wt %) have been considered as an ideal candidate for the preparation of various silicon-based materials, especially silica.39,40,42,47,48 In this report, the silica used to support Au NPs was obtained from RHs. RHs were washed with water to remove dust and then boiled in HCl solution to remove metal cations, especially potassium ions (K+), which can lead to the melting and aggregation of silica NPs.49−51 RH

Figure 2. XRD patterns of RH silica and RH-silica-supported Au NPs.

The RH silica was modified with APTES to introduce amino groups onto the surface through the reaction between the silane groups in APTES and the hydroxyl groups on silica.52 Such amino groups are highly effective in absorbing Au precursors through coordination.53 FTIR spectroscopy was used to characterize the changes in the RH silica NPs after modification by APTES. As shown in Figure 3, the broad band between 3416 to 3554 cm−1 was due to the stretching vibration of surface silanol groups (Si−O−H) and physically adsorbed water.54 The FTIR spectrum of the RH silica sample exhibits characteristic bands of the silica framework, including the Si−O−Si asymmetric stretching vibration (1099 cm−1 with a shoulder at ca. 1220 cm−1), the Si−O−Si symmetric stretching vibration (802 cm−1), and the Si−O−Si bending vibration (467 cm−1).52 Additionally, the spectrum exhibits a band at 1635 cm−1 associated with H−O−H bending vibrations of trapped water molecules in the silica matrix.52,54,55 Compared with the FTIR spectrum of the neat RH silica, the APTES-modified silica NPs displayed several new peaks (with 5658

DOI: 10.1021/acs.iecr.5b00216 Ind. Eng. Chem. Res. 2015, 54, 5656−5663

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another broad peak in the range of 35−41° that can be ascribed to Au(111), supporting the formation of Au NPs.56 The silica NPs from TEOS were used as a control and treated according to the same procedure to support Au NPs. The results are shown in Figure 5. According to the SEM image

Figure 3. FTIR spectra of pure RH silica, pure APTES, and APTESmodified RH silica.

Figure 5. (a) SEM image of as-prepared TEOS silica and (b) TEM image of TEOS-silica-supported Au NPs. The arrows show aggregates of Au NPs.

a low intensity) in the range of 2880−2973 cm−1 and at ca. 695 cm−1, which can be assigned to the C−H stretching vibration and N−H bending vibration of the aminopropyl groups, respectively.55 The FTIR results demonstrate that the APTES molecules were successfully grafted onto the surface of the RH silica NPs. The same amount of APTES was stirred for 24 h in a roundbottom flask under the same conditions, and the solution remained transparent without any precipitates. This phenomenon supports the conclusion that the silica did not originate from APTES. APTES served only to modify the surface of the preformed RH silica. Synthesis of Au NPs on RH Silica. TEM was employed to confirm the formation of RH-silica-supported Au NPs, as shown in Figure 4. It can be clearly observed from Figure 4a

(Figure 5a), the diameter of the silica NPs synthesized by the Stöber method was ca. 70 nm, similar to that of the RH silica NPs. The former exhibited a much smoother surface, which is consistent with the literature.28,57,58 After APTES modification, the TEOS silica NPs were also treated to form Au NPs on their surfaces. The TEM image in Figure 5b shows that, although some Au NPs were supported on the silica surface, they tended to aggregate. Fewer Au NPs also appeared to be supported on the TEOS silica than on the RH silica. It was found that the amount of Au precursor AuCl4− that was absorbed by 100 mg of RH silica could reach up to ca. 25 mg, whereas the maximum absorption by 100 mg of TEOS silica was only ca. 5 mg. Catalysis Evaluation of Au NPs on RH Silica. As a model reaction, the reduction of 4-nitrophenol to 4-aminophenol by NaBH4 reducing agent was chosen to evaluate the catalytic performance of the RH-silica-supported Au NPs. As shown in Figure 6, an aqueous solution of 4-nitrophenol was found

Figure 4. (a) TEM and (b) HRTEM images of RH-silica-supported Au NPs.

that the RH-silica-supported Au NPs exhibited a narrow size distribution (ca. 2−4 nm). The small dimensions of the Au NPs are further confirmed by the high-resolution TEM (HRTEM) image in Figure 4b. As can be seen, the HRTEM image displays the crystalline lattice structure inside the NPs. The lattice planes of the Au NPs exhibited no stacking faults or twins, indicating a single-crystalline nature. The lattice fringe spacing was measured to be 0.26 nm, which is in accordance with Au(111) planes.56 XRD characterization was also conducted to confirm the formation of Au NPs. The XRD pattern of RH-silicasupported Au NPs is shown in Figure 2. Compared to RH silica, in addition to the broad peak in the range of 15−30° for amorphous SiO2, the RH-silica-supported Au NPs exhibited

Figure 6. UV−vis spectra of 4-nitrophenol and the successive reduction of 4-nitrophenol catalyzed by RH-silica-supported Au NPs (0.12 mg) at intervals of 1 min. Inset: Color change of 4-nitrophenol solution (left) before and (right) after the catalytic reaction.

to have a maximum absorption at ca. 317 nm. The peak redshifted from ca. 317 to 400 nm after the addition of NaBH4 5659

DOI: 10.1021/acs.iecr.5b00216 Ind. Eng. Chem. Res. 2015, 54, 5656−5663

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Industrial & Engineering Chemistry Research because of the formation of 4-nitrophenolate ions.59,60 However, it is difficult for the reduction to proceed further to form 4-aminophenol in the absence of a catalyst.45,61,62 After the RH-silica-supported Au NPs were added, they started to catalyze the reduction reaction by relaying electrons from the donor BH4− to the acceptor 4-nitrophenol.44 The hydrogen generated from NaBH4 purged out air, thus preventing the air oxidation of the product, 4-aminophenol. Meanwhile, the evolution of small hydrogen bubbles surrounding the catalyst particles helped in stirring the reaction system. Thus, the catalyst particles were well-distributed in the reaction medium during the course of reaction, which is favorable for the reduction reaction to occur. As a result, the absorption of 4-nitrophenolate ions at ca. 400 nm decreased gradually as the reaction proceeded; meanwhile, a new peak appeared at ca. 295 nm, which is attributed to the absorption of the generated 4-aminophenol. The isosbestic points demonstrated that 4-nitrophenol was gradually converted to 4-aminophenol and that no side reactions occurred.63 Moreover, the reduction could also be visualized by the color change of the solution. After the addition of the catalyst, the yellow color of 4-nitrophenol in aqueous solution gradually faded and ultimately bleached, as shown in the inset of Figure 6. In the catalysis evaluation, NaBH4 was present in significant excess. As such, its concentration remained virtually constant during the reaction. In this way, the reduction rate was assumed to be independent of the NaBH4 concentration. Thus, pseudofirst-order kinetics was used to evaluate the reaction rate of the catalytic reaction.64 The ratio of Ct to C0 (where Ct and C0 are the 4-nitrophenol concentrations at time t and time 0, respectively) was measured from the relative intensities of the absorbance at the corresponding times, At/A0 (where At and A0 represent the absorbances of 4-nitrophenolate ions at time t and time 0, respectively). As expected, a linear relationship between ln(Ct/C0) and reaction time was obtained, as shown in Figure 7. Thus, the

Figure 8. UV−vis spectra of 4-nitrophenol and the successive reduction of 4-nitrophenol catalyzed by TEOS-silica-supported Au NPs (0.12 mg) at intervals of 3 min.

A control experiment was conducted to evaluate the TEOS-silica-supported Au NPs under the same conditions. As shown in Figure 8, the reaction was much slower, which is believed to be due to the lower amount of Au NPs on this silica support, as well as the aggregation of the Au NPs, as shown in Figure 5b, which significantly lowered the catalytic efficiency of the Au NPs. In heterogeneous catalysis, the reaction rate generally increases linearly with increasing amount of catalyst.70 In this research, the effect of the amount of catalyst on the rate of the reduction reaction was also investigated. The amount of catalyst was varied while other parameters were kept constant to determine the influence of the former on the rate of the catalytic reaction. The results are shown in Figures 9 and 10. As expected,

Figure 9. Plot of ln(Ct/C0) vs time for the reduction of 4-nitrophenol on different amounts of RH-silica-supported Au NPs.

Figure 7. Plot of ln(Ct/C0) vs time for the reduction of 4-nitrophenol catalyzed by RH-silica-supported Au NPs (0.12 mg).

with increasing amount of catalyst, the reduction of 4-nitrophenol accelerated, and the rate constant increased virtually linearly, which is consistent with the literature.44,71 Because of the high cost of precious metals and the importance of sustainability, it is important to recover the metal catalysts after the reaction is completed.72−74 In this work, the RH-silicasupported Au NP catalyst exhibited outstanding recyclability, revealing the high stability of the catalyst. During the evaluation of catalyst recycling, the concentrations of 4-nitrophenol and

rate constant was estimated from the slope of the best-fit line as ca. 0.447 min−1. This reactivity of the RH-silica-supported Au NPs is comparable to those of a number of recently developed Au NPs supported on magnetic composite microspheres,65,66 mesoporous organic gels,67 surface-modified membranes,68 and conducting polymers.69 The very low cost of RH silica compared to the above supports is expected to bring a significant advantage for future potential commercialization of this product. 5660

DOI: 10.1021/acs.iecr.5b00216 Ind. Eng. Chem. Res. 2015, 54, 5656−5663

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Industrial & Engineering Chemistry Research Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the U.S. Environmental Protection Agency (P3 Award, SU-83529201), the National Science Foundation (DMR-1205670), the Air Force Office of Scientific Research (FA9550-12-1-0159), and a Faculty Large Grant from the University of Connecticut. W.W. acknowledges support from the National Natural Science Foundation of China (21176093).

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Figure 10. Plot of the rate of reduction vs the amount of catalyst.

NaBH4 were increased by a factor of 50 so that the reaction could be completed within 5 min. To examine the recycling performance, the supported catalyst was separated from the product at the end of each cycle of reaction and washed thoroughly with DI water. After being dried, the RH-silica-supported Au NP catalyst maintained a high activity after three cycles of reaction, exhibiting excellent stability and reusability (Figure 11).

Figure 11. Reusability of RH-silica-supported Au NPs catalyst for the reduction of 4-nitrophenol with NaBH4.

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CONCLUSIONS In summary, RH silica has been demonstrated to be an excellent support for Au NPs for catalysis applications. RH silica NPs obtained by calcining HCl-treated RHs have a rough surface, which appears to be ideal for supporting Au NPs. The generated Au NPs were very small (ca. 2−4 nm). The RH-silica-supported Au NP catalyst exhibited excellent catalytic activity, as demonstrated for the reduction of 4-nitrophenol by NaBH4 to 4-aminophenol. The Au NP catalyst also remained stable and active after each cycle of reaction and can be reused a number of times. Considering the low cost of RH silica NPs and their excellent performance as a support for Au NPs, RH silica holds great potential in industrial applications.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel.: 860-486-6895. Fax: 860-486-4745. Email: luyi.sun@ uconn.edu. 5661

DOI: 10.1021/acs.iecr.5b00216 Ind. Eng. Chem. Res. 2015, 54, 5656−5663

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DOI: 10.1021/acs.iecr.5b00216 Ind. Eng. Chem. Res. 2015, 54, 5656−5663