Acid-Functionalized Magnetic Nanoparticle as Heterogeneous

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Acid-functionalized Magnetic Nanoparticle as Heterogeneous Catalysts for Biodiesel Synthesis Hongwang Wang, Jose Covarrubias, Heidy Prock, Xiaorong Wu, Donghai Wang, and Stefan H. Bossmann J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b08743 • Publication Date (Web): 21 Oct 2015 Downloaded from http://pubs.acs.org on October 27, 2015

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The Journal of Physical Chemistry

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Acid-functionalized Magnetic Nanoparticle as Heterogeneous Catalysts for Biodiesel Synthesis

Hongwang Wang*1, Jose Covarrubias1, Heidy Prock1, Xiaorong Wu2, Donghai Wang2, Stefan H. Bossmann*1

1

Kansas State University, Department of Chemistry, CBC Building 201, Manhattan, KS 66506, USA 2 Kansas State University, Biological and Agricultural Engineering, Seaton Hall 150, Manhattan, KS 66506, USA

Keywords Biodiesel, magnetic nanocatalyst, acid catalysis, esterification, transesterification

ACS Paragon Plus Environment


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Abstract We have synthesized sulfamic acid and sulfonic acid functionalized silica coated crystalline Fe/Fe3O4 core/shell magnetic nanoparticles (MNPs), which exhibited excellent stability. We have studied their catalytic activities with respect to the esterification of oleic acid and the transesterification of glyceryl trioleate as model reactions for biodiesel production. Both acid-functionalized MNPs demonstrated excellent catalytic activities in the reaction of esterification of oleic acid. However, the sulfamic acid-functionalized MNPs showed better reactivity in the reaction of transesterification of glyceryl trioleate. Both acid-functionalized MNPs can be recovered by magneto-precipitation. The sulfamic acid-functionalized MNPs retained high reactivity (>95% conversion) throughout 5 continuous runs of reactions, indicating their potential for biodiesel production from low grade feedstock, such as waste cooking oil, containing high level of free fatty acids.


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Introduction With diminishing fossil reserves and increasing demand for energy consumption, it is imperative to find renewable and cleaner energy sources to meet the rapid population and economic growth.1,2 Biodiesel consists of long chain fatty acid esters produced by either the transesterification of triglycerides (TGs) or the esterification of free fatty acid (FFAs) with methanol or ethanol. Besides being a renewable resource, biodiesel has many advantages compared with traditional diesel fuel, such as a higher flash point, increased lubricity, and a lower emission profile. In addition, biodiesel is non-toxic and biocompatible.3,4,5 These attractive properties make biodiesel a truly environmentally friendly fuel. However, the high cost of biodiesel production must be regarded as a major

hurdle

with

respect

to

broad

commercialization.6

Base-catalyzed

transesterification is a facile process to produce biodiesel, but requires costly virgin oil as feedstock.7 Acid catalysts are capable of promoting both esterification and transesterification reactions simultaneously, thus, economical waste cooking oil can be used as feedstock for biodiesel production.8.9 However, tedious catalyst separation, serious environmental contamination, and unavoidable equipment corrosion make homogeneous acid-catalysis very unpractical for large scale biodiesel production.10,11 On the other hand, application of solid acids as heterogeneous catalysts for biodiesel production is attractive. The advantages include easy catalyst separation, simple product purification and a better environmental impact. In addition, solid acids can be easily incorporated into a packed bed continuous flow reactor, making it possible to produce biodiesel in a continuous flow system.12 Different type of solid acids, such as heteropolyacids,13 Amberlyst-15,14 zeolites,15,16 and sulfonic-acid functionalized ion



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exchange resins,17,18 etc. have been studied, however, most of the materials gave unsatisfactory results due to either quick catalyst deactivation or required critical reaction conditions. Magnetic nanoparticles have been used as solid supports to construct magnetically separable nanocatalysts.19 Because of the high surface-area-tovolume ratio, magnetic nanoparticles can carry large amount of active catalysts on their surface.19 In addition, due to the small size of the particles, the nanocatalysts are highly dispersible in solvents, making the active catalysts on the surface readily accessible to the surrounding reactants. These types of supported catalysts have been recently named as a bridge between homogeneous and heterogeneous catalysis.20 Sulfonic acid immobilized magnetic nanoparticles had been prepared, and their catalytic scope on hydrolytic and condensation reactions examined.21,22 Recently, Zillillah et al. reported that sulfonic acid grafted magnetic nanoparticles are reusable catalysts for efficient esterification of FFA in grease to produce biodiesel.23 In our previous work, we have immobilized sulfamic acid on amorphous Fe/Fe3O4 core/shell magnetic nanoparticles, and studied their catalytic activity on the ring opening of epoxidized methyl oleate.24,25 We also studied monolayer acid functionalized iron oxide nanoparticles as catalysts for carbohydrate hydrolysis.26 We further improved the stability and recyclability of magnetic nanoparticle based acid catalyst by using robust, highly magnetic body centered cubic (bcc) iron nanoparticles as solid support. Sulfonic acid and sulfamic acid were introduced on the silica coated crystalline bcc iron magnetic nanoparticles. Herein, we report the characterization of these acid functionalized nanoparticles by TEM, DLS, zeta-potential analyzer, and their catalytic activity on esterification of oleic acid and transesterification of glyceryl trioleate as model reactions for biodiesel production.


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Results and discussion Synthesis and characterization of sulfamic acid and sulfonic acid-functionalized Fe/Fe3O4 core/shell magnetic nanoparticles The synthesis of sulfonic acid and sulfamic acid-functionalized Fe/Fe3O4 core/shell magnetic nanoparticles (MNPs) is illustrated in Scheme 1. First, crystalline magnetic nanoparticles were prepared by thermal decomposition of iron pentacarbonyl, Fe(CO)5, in 1-octadecene (1-ODE) in the presence of oleylamine and hexadecylammonium chloride (HADxHCl).27 The Fe/Fe3O4 core/shell magnetic nanoparticles synthesized by this method are known for their extended stability and high magnetic moment, which we anticipate will contribute to the catalyst stability and facile magneto-separation-recycling of the catalyst after reaction. Second, silica coated MNPs (SiO2-MNPs) were prepared by hydrolyzing tetraethoxysilane (TEOS) on the surface of MNPs under basic conditions.28 This silica layer can protect the MNPs from acid erosion. Third, silanation of the SiO2-MNPs with (3-aminopropyl) triethoxysilane (APTES) and (3-mercaptopropyl) trimethoxysilane (MPTMS) produced NH2-functionalized SiO2-MNPs (NH2-SiO2-MNPs) and SH functionalized SiO2-MNPs (SH-SiO2-MNPs) respectively.29 Treating NH2-SiO2MNPs with chlorosulfuric acid created sulfamic acid-functionalized MNPs;30 treating SHSiO2-MNPs first with H2O2 and then with sulfuric acid produced sulfonic acidfunctionalized MNPs.21



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Scheme 1: Preparation of sulfonic acid-functionalized MNPs and sulfamic acidfunctionalized MNPs TEM images of the MNPs (Figure 1: 1a and 1b) show the core/shell structure of the particles, with a mean core diameter of 11 nm and a shell thickness of 2 nm. TEM images of the sulfonic acid-functionalized MNPs (Figure 1: 2a and 2b) reveal an even


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clearer core/shell structure. A distinct boundary can be discerned between the core and shell; the shell thickness increased to 3-4 nm. Similar features can be found in the TEM images of the sulfamic acid-functionalized MNPs (Figure 1: 3a and 3b). The XRD characterization of the freshly prepared Fe/Fe3O4 nanoparticles confirmed the crystalline structure: only the (110) and (200) peaks corresponding to bcc-Fe are observed.27 The XRD pattern of the acid functionalized MNPs presents the same characteristic peaks of bcc-Fe, indicating the retention of the crystalline iron(0) core. Additional, a broad peak between 20° and 30° is due to the silica coating.31 Two other peaks at 35° and 63° can be assigned to crystalline Fe3O4, which has been produced by H2O2 oxidation during the final treatment.32 The sulfonic acid-functionalized MNPs and the sulfamic acid-functionalized MNPs are highly monodisperse (polydispersity = 0.2) based on the dynamic light scattering (DLS) measurements carried out in methanol. The effective diameters are 40 nm for sulfonic acid-functionalized MNPs, and 60 mm for sulfamic acid-functionalized MNPs (Figure 2). Comparing the particle size obtained by TEM and DLS, there is very little clustering of acid-functionalized MNPs in methanol solution.33 Zeta-potential measurements in methanol show both sulfonic acid-functionalized MNPs (-18.81 mV) and the sulfamic acid-functionalized MNPs (-28.45 mV) carry negative charges on their surfaces (Figure 3).34 Acid loading of the nanoparticles, 0.32 mmol g-1 for sulfonic acid-functionalized MNPs, and 0.48 mmol g-1 for sulfamic acid-functionalized MNPs, was determined by acid-base titration according to the reported literature method.21



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Figure 1. TEM and XRD of Fe/Fe3O4, sulfonic acid-functionalized Fe/Fe3O4, and sulfamic acid-functionalized Fe/Fe3O4


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Figure 2. DLS of (A) sulfonic acid-functionalized Fe/Fe3O4 and (B) sulfamic acidfunctionalized Fe/Fe3O4

Figure 3. Zeta-potential of (A) sulfonic acid-functionalized Fe/Fe3O4 and (B) sulfamic acid-functionalized Fe/Fe3O4 Catalytic studies Glyceryl trioleate and oleic acid were used as model reagents for biodiesel production in order

to

avoid

the

complexity

of

macromolecules

in

NMR

analysis.

The

transesterification of glyceryl trioleate was carried out using sulfonic acid-functionalized MNPs or sulfamic acid-functionalized MNPs as catalysts. Glyceryl trioleate was stirred



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in methanol (8 mass equivalents) with acid-functionalized MNPs (5 mol/mol% based on loaded free acid) in a Schlenk flask at elevated temperature. The acid-functionalized MNPs were also tested with different weight percentages, specifically, 20%, 40% of oleic acid with glyceryl trioleate for their capability of simultaneously catalyzing both transesterification and esterification. The reaction process can be easily monitored by 1

H NMR. The starting material presents three sets of signals at 4.15 ppm, 4.30 ppm,

and 5.27 ppm due to the glyceryl backbone protons, with integration ratio of 2/2/1. In the product, the OCH3 of methyl ester gives a distinct signal at 3.67 ppm (Figure 4). As the reaction progresses, the signal decrease for the glyceryl backbone protons will be accompanied by a signal increase for OCH3. The conversion of the starting material can be obtained by the integration ratio between OCH3 and that of the glyceryl backbone protons. The product, methyl oleate is also confirmed with 1H-1H COSY 2-D NMR characterization (Figure 5). The catalytic performance of sulfonic acid-functionalized MNPs (catalyst I) and sulfamic acid-functionalized MNPs (catalyst II) for transesterification of glyceryl trioleate and esterification of oleic acid is summarized in Table 1. The reaction equation and the 1HNMR spectra of the starting materials and products are presented in Scheme 2. Both catalyst I and catalyst II show better performance at higher temperature for the transesterification reaction. After reacting at 100 °C for 20 hours, 88% and 100% of conversion was achieved for catalyst I and catalyst II respectively (entry 3 and 11). Esterification of oleic acid was carried out at 70 °C in methanol, and 100% conversion was achieved within 4 hours (entry 7 and 14). We also found that both catalysts are capable of catalyzing transesterification and esterification simultaneously (entry 4, 5, 12,


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and 13), with catalyst II giving better results. It is worth mentioning that throughout the esterification and transesterification reactions, both catalysts gave 100% selectivity toward the methyl oleate product. To test whether the catalytic effect is due to acid leaching from the MNPs, we stirred the reaction mixture at room temperature for 1 hour. After magnetically separating the MNPs, the reaction solution was transferred to another Schlenk flask, and stirred at the designated temperature. Only very low conversions were observed, see control experiments (entry 6, 8). This experiment demonstrated that the real catalysts are acid-functionalized MNPs. The easy separation and reusability of the catalysts are of vital importance to reduce the cost of biodiesel production. Due to the presence of the iron(0) core, the acidfunctionalized MNPs feature high magnetic moments. As demonstrated in Figure 6, acid-MNPs catalysts can be evenly dispersed in solution with magnetic stirring. Once the stirring has stopped, all the catalysts are accumulated on the stir bar within 5 seconds, and the product can simply be poured out. The recovered catalysts were used for new cycles of the transesterification reaction. The reusability of sulfonic acidfunctionalized MNPs (catalyst I) is not optimal, the conversion dropped from 88% for the first run to 62% by the fifth run. This may be due to the loss of contact sites for the substrates caused by catalyst aggregation during the reaction. The sulfamic acidfunctionalized MNPs (catalyst II) demonstrated excellent stability and reusability, maintaining over 95% conversion throughout 5 reaction cycles. In the catalyst characterization, we notice that catalyst II had higher acid loading and lower zetapotential than those of catalyst I, which may explain its superior stability and reusability.



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Figure 4. 1H NMR of glyceryl trioleate and methyl oleate

Figure 5. 1H-1H COSY spectrum of methyl oleate


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13 Table 1. Catalytic performance of sulfonic acid-functionalized Fe/Fe3O4 (Catalyst I) and sulfamic acid-functionalized Fe/Fe3O4 (catalyst II) entry

catalyst

Glyceryl trioleate(GT)/oleic acid(OA)

Temp. (°C)

Time (h)

Conversion (%)

1

I

100%GT

60

20

67

2

I

100%GT

80

20

79

3

I

100%GT

100

20

88

4

I

80%/20%

100

20

85

5

I

60%/40%

100

20

82

6

none

100%GT

100

20

Acid-Functionalized Magnetic Nanoparticle as Heterogeneous

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