Methylation of Volatile Fatty Acids with Ordered Mesoporous Carbon

Research Article pubs.acs.org/journal/ascecg

Methylation of Volatile Fatty Acids with Ordered Mesoporous Carbon and Carbon Nanotube for Renewable Energy Application Jechan Lee,† Jong-Min Jung,† Hyung Ju Kim,‡ Tae-Wan Kim,§ Ki-Hyun Kim,*,∥ and Eilhann E. Kwon*,† †

Department of Environment and Energy, Sejong University, 209 Neungdong-ro, Seoul 05006, Korea Carbon Resources Institute, Korea Research Institute of Chemical Technology, 141 Gajeong-ro, Daejeon 34114, Korea § Center for Convergent Chemical Process, Korea Research Institute of Chemical Technology, 141 Gajeong-ro, Daejeon 34114, Korea ∥ Civil and Environmental Engineering, Hanyang University, 222 Wangsimni-ro, Seoul 04763, Korea ‡

S Supporting Information *

ABSTRACT: This study introduces a new way to convert volatile fatty acids (VFAs) into fatty acid methyl esters (FAMEs) at ambient initial pressure with ordered mesoporous carbons (OMCs) and multiwalled carbon nanotube (MCNT). Activity for the methylation is contingent on the structure and geometry of the carbon materials. CMK-5 having an interconnected rod structure with hollow rod-type carbon framework exhibited the highest activity for the methylation of VFAs among the OMCs tested in this study. Reaction temperature (360 °C) and a VFA/methanol volumetric ratio (0.5) for the methylation with the CMK-5 were optimized. At the optimized conditions, the FAME yields were reached up to ∼98%. A recycling study reveals that the CMK-5 was stable after six cycles with no sign of deactivation. The CMK-5 and MCNT showed a similar FAME yield profile for the methylation of VFAs due to their similar geometry. This study suggests a new use of carbon materials for producing short chain fatty alcohols considered as an alternative gasoline. KEYWORDS: Carbon materials, Fatty alcohol, Biorefineries, Short-chain fatty acids, Esterification

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INTRODUCTION Volatile fatty acids (VFAs), also known as short-chain fatty acids, are byproducts of anaerobic digestion of biodegradable waste (biowaste).1 In Korea, approximately 5.7 × 105 tons of VFAs could be recovered from biowaste in 2012.2 VFAs have been taken into account for the raw material of value-added products such as alkanes and methyl−ethyl esters.3 Short-chain fatty alcohols (a gasoline-alternative) can be synthesized by hydrogenation of fatty acid methyl esters (FAMEs) that are produced by methylation of VFAs.4 However, homogeneouscatalyzed methylation (e.g., H2SO4, KOH, and NaOH) needs several extra steps such as catalyst separation and product neutralization.5,6 Heterogeneous catalysts have also been tried to overcome these challenges in methylation reactions. For example, polymeric resins (e.g., Amberlyst and Amberlite) and mixed oxides have been widely studied for methylation reactions.7−10 However, the catalyst stability is critical for the liquid-phase reactions.11 Even though supercritical-methylation gives a high yield of FAMEs from fatty acids without catalyst, it needs such a high pressure (20 to 40 MPa) resulting in a high operational and capital cost.12 Therefore, it is essential to develop a methylation process to synthesize FAMEs from VFAs. © 2017 American Chemical Society

Carbon has high thermal conductivity; extraordinary physical, chemical, electrical, and mechanical properties; and large surface area and pore volume. Due to the intrinsic physicochemical property, carbon materials have widely been used in various fields including environmental adsorbent,13−15 catalyst support,16−20 membrane,21−23 and supercapacitor.24−26 Furthermore, the physicochemical properties and morphology of carbon materials can be easily modified and controlled. For example, ordered mesoporous carbons (OMCs) were synthesized to have narrower pore distribution (i.e., regular arrangement of the pores) than a typical carbon material such as activated carbon,27 and the size and structure of the carbon could be precisely controlled.28,29 Herein, we introduce an advanced methylation of VFAs initiated at ambient pressure on carbon materials. To this end, the activity of the methylation of VFAs on four different OMCs and multiwalled carbon nanotube (MCNT) was compared to screen the most active OMC and investigate relationships between the structure of porous material and activity to get Received: June 15, 2017 Revised: July 10, 2017 Published: July 19, 2017 7433

DOI: 10.1021/acssuschemeng.7b01953 ACS Sustainable Chem. Eng. 2017, 5, 7433−7438

ACS Sustainable Chemistry & Engineering

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RESULTS AND DISCUSSION Properties of OMCs. The structures of OMCs were confirmed by XRD, N2 physisorption, and TEM analyses. As shown in Figure S1a, three peaks are distinct in the XRD patterns of SBA-15, CMK-3, and CMK-5, meaning that the replication of the SBA-15 template to the carbon materials was successful with ordered two-dimensional p6mm symmetry.32−34 There are two distinct XRD peaks for CMK-8 while there are six distinct XRD peaks for CMK-9 (Figure S1b). This clearly reflects a three-dimensional Ia3d space group of CMK-8 and CMK-9 from the KIT-6 template.28 The hollow structures of CMK-5 and CMK-9 interfere with diffraction between the hollow frameworks, resulting in weak XRD signals.35 Table 1 lists structural properties of the four OMCs. CMK-5 (1630 m2 g−1) and CMK-9 (1740 m2 g−1) show higher surface

insight into which pore structure might be useful for the methylation process. After screening the most active OMC, the reaction conditions with the OMC were optimized. This study provides a great venue for offering a new way to use carbon materials for renewable energy production. Furthermore, it could be justified VFAs as the raw feedstock of a renewable alternative gasoline (i.e., short-chain fatty alcohol produced via hydrogenation of the VFAs-derived FAMEs).

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Research Article

EXPERIMENTAL SECTION

Synthesis of OMCs. Four OMCs were synthesized: CMK-3 (interconnected rod structure with solid rod-type carbon framework), CMK-5 (interconnected rod structure with hollow rod-type carbon framework), CMK-8 (cubic Ia3d structure with solid-type carbon framework), and CMK-9 (cubic Ia3d structure with hollow-type carbon framework). These OMCs were synthesized using ordered mesoporous SiO2 hard templates. The templates, SBA-15 and KIT-6, were prepared as described elsewhere.27,30,31 The SBA-15 and KIT-6 were converted into their corresponding aluminosilicate forms (AlSBA-15 and Al-KIT-6) by impregnating with an ethanolic solution of AlCl3 with a Si/Al ratio of 20. The Al-SBA-15 was used as the hard template for interconnected rod structured OMCs (i.e., CMK-3 and CMK-5). The Al-KIT-6 were used as the hard template for cubic Ia3d OMCs (i.e., CMK-8 and CMK-9). For the solid-type OMCs (i.e., CMK-3 and CMK-8), 85% of the total pore volume of each corresponding SiO2 template was filled with furfuryl alcohol. The furfuryl alcohol-filled SiO2 template was dried at 35 °C for 1 h and then at 100 °C for 1 h. The dried sample was heated at 350 °C for 2 h, followed by being cooled to ambient temperature. The resultant sample was filled again with furfuryl alcohol (55% of total pore volume of each SiO2 template), followed by being dried and heated again as described above. After the second furfuryl alcohol filling step, the sample was pyrolyzed at 900 °C (heating rate: 5 °C min−1) for 2 h in N2. The SiO2 template was removed from the pyrolyzed sample using a HF/ethanol mixture. For hollow-type OMCs (i.e., CMK-5 and CMK-9), 100% of the total pore volume of each corresponding SiO2 template was filled with furfuryl alcohol at first. The filled sample was dried by freeze-drying method under vacuum. The dried sample was heated to 100 °C for 1 h and then pyrolyzed at 900 °C (heating rate: 5 °C min−1) under vacuum. The SiO2 template was removed in the same way described above. Characterization of OMCs. X-ray diffraction (XRD) analyses of the OMCs were conducted using a Rigaku Multiplex instrument with a Cu Kα source (λ = 0.15406 nm) operating at 40 kV and 40 mA (1.6 kW). The OMCs were visualized by a Philips Tecnai G220 transmission electron microscope (TEM) operating at 200 kV. N2 physisorption at −196 °C was conducted using a Micromeritics Tristar 3000 volumetric adsorption analyzer. Prior to N2 physisorption, all samples were degassed at 300 °C under vacuum for 2 h. Methylation of VFAs. VFAs, acetic acid (99.7%, Junsei Chemical), propionic acid (99%, Junsei Chemical), butyric acid (≥99%, SigmaAldrich), isobutyric acid (99%, Junsei Chemical), valeric acid (98%, Kanto Chemical), isovaleric acid (98%, Alfa Aesar), and methanol (≥99.9%, Sigma-Aldrich) were used as purchased. A total of 200 μL of fatty acid and methanol and 65 mg of an OMC were loaded in a Swagelok bulkhead union used as a batch reactor in this study (the reactants were mixed at 800 rpm). The bulkhead union was sealed with caps. After that, the sealed bulkhead was heated to a target temperature in a furnace (heating rate: 30 °C min−1) and held at the temperature for 10 s. After the reaction finished, the reactor was cooled with 4 °C water. Product samples were collected by an organic solvent such as dichloromethane (≥99.9%). An experiment was carried out three times to make ensure reproducibility. The product sample was analyzed by a Varian 450-GC equipped with an Agilent DB-Wax column. Detailed information on GC conditions, calibrations of VFAs and their corresponding methyl esters, and QA/QC in the instrument is provided in the Supporting Information (Tables S1 and S2).

Table 1. Structural Properties of the OMCs

a

material

surface areaa [m2 g−1]

total pore volumeb [cm3 g−1]

pore diameterb [nm]

CMK-3 CMK-5 CMK-8 CMK-9

1120 1630 1020 1740

1.1 1.5 1.1 1.9

3.8 3.2, 4.4 4.0 3.5, 4.8

From the BET isotherm. bFrom BJH desorption.

area than CMK-3 (1120 m2 g−1) and CMK-8 (1020 m2 g−1). This is because hollow-type OMCs are comprised of inner pores of carbon framework and intra pores between the carbon frameworks. The two types of pores of CMK-5 and CMK-9 result in bimodal mesopores as shown in Table 1.35 Figure S2 presents the typical type IV curves with sharp capillary condensation steps at P/P0 between 0.4 and 0.6 for the four OMCs. CMK-3 and CMK-8 exhibited narrower pore size distribution than CMK-5 and CMK-9, respectively. This is due to CMK-5 and CMK-9 being composed of a hollow-rod framework. The OMCs are visualized by TEM analysis as presented in Figure S3. CMK-3 and CMK-5 have a highly ordered structure comprised of two-dimensional parallel porous channels. CMK8 and CMK-9 have a typical three-dimensional cubic Ia3d mesoporous structure, showing that CMK-8 and CMK-9 have highly ordered mesoporous structures.30,36 Figure S4 describes models of the OMCs. Effects of OMC Structure on the Activity for the Methylation. It is important to estimate mass transport limitations in the OMCs for the reaction. The reactants (e.g., butyric acid and methanol) were mixed at a speed of 800 rpm, and no change in the rate of esterification on the OMCs was observed with a further increase in the mixing speed, indicating that there was no interphase transfer limitation in the reaction on the OMCs.37 The computed intraparticle transfer number (i.e., Weisz−Prater number) for the system was much less than 0.3, meaning the absence of intraparticle diffusion limitations (the Weisz−Prater criterion states that intraparticle diffusion limitations are dominant if the Weisz−Prater number is higher than 0.3).38 Therefore, the reactions in this study were conducted in the absence of internal transport limitations. As shown in Figure 1, the reaction rate was plotted against temperature in an Arrhenius plot. The apparent activation energy for the methylation reaction increased in the order: CMK-5 (29.4 kJ mol−1) < CMK-3 (36.1 kJ mol−1) < CMK-9 (37.2 kJ mol−1) < CMK-8 (42.0 kJ mol−1). The difference in 7434


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Figure 2. Reaction rates for the methylation of butyric acid. The reaction rates were calculated at less than 20% conversion of butyric acid (reaction conditions: T = 200 °C, P0 = 1 atm, butyric acid/ methanol (v/v) = 0.5). Mean values of replicates (n ≥ 3) are reported with standard deviations given as error bars.

Figure 1. Arrhenius plot for butyric acid conversion rate (μmol gmcarbon−1 s−1) for the methylation of butyric acid on the OMCs (reaction conditions: P0 = 1 atm, butyric acid/methanol (v/v) = 0.5). Mean values of replicates (n ≥ 3) are reported with standard deviations given as error bars.

Table 2. Effect of Reaction Conditions on the Methylation on the CMK-5a

the apparent activation energy suggests that the population of reactant molecules (i.e., VFA and methanol) depends on such structures of the OMCs, according to kinetic molecular theory. Considering that the pore size of the OMCs (3−5 nm; Table 1) is larger by almost 1 order of magnitude than kinetic diameter of VFAs and methanol,39,40 the pore size is large enough for the reactant molecules to collide with each other. CMK-5 showed the lowest apparent activation energy among the tested OMCs, attributed to the unique pore geometry of CMK-5. It consists of interconnected hollow rods (Figure S4), suggesting that the two-dimensional open bimodal pore system more easily allows for the reactants (VFA and methanol) to regularly enter the pores and for the product (FAME) to regularly exit the pores than the other OMCs having nonbimodal pores and/or a complex three-dimensional structure. In addition to the pore structure (i.e., confinement/ restricting mobility of methanol and acids within a pore network), surface functionalities might play a role in accelerating esterification through providing the active sites to bind and activate the alcohol and acid.41 It is known that carbons are solid acid materials for diverse transformations including methylation,42 with the OMCs to exhibit surface carboxylic acid functions which render it an effective catalyst for oxidative dehydrogenation of ethylbenzene.43 Figure 2 shows the comparison of the methylation rate on the four OMCs. It clearly indicates that the CMK-5 is most effective in the conversion of butyric acid under comparable reaction conditions. The effectiveness of the carbon materials decreased in the order: CMK-5 > CMK-3 > CMK-9 > CMK-8, consistent with the results of Figure 1. Optimization of Methylation of VFAs on OMC. From the comparison of the four OMCs, the CMK-5 is the most active carbon material for the methylation among the tested OMCs. Because of the high activity of the CMK-5, this carbon material was used for further investigations. We further investigated the effect of reaction temperature and butyric acid/methanol volumetric ratio on the esterification of butyric acid, as summarized in Table 2. Based on carbon balance before and after the reaction, we confirmed that butyric acid was only converted into methyl butyrate. As shown in Table 2, the temperature variation in the range of 200−360 °C at a butyric acid to methanol volumetric ratio of 0.5 showed an increase in butyric acid conversion from 16.4 to 92.4%. A further increase in temperature from 360 to 380 °C did not

temperature [°C]

butyric acid/ methanol (v/v)

butyric acid conversion [%]

methyl butyrate concentration [M]

200 240 270 300 320 340 360 380 360 360 360 360

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.3 1 2 3

16.4 36.5 53.6 69.2 79.8 88.8 92.4 92.4 94.5 83.3 52.2 22.2

0.5 1.4 2.0 2.4 2.8 3.2 3.3 3.4 3.3 3.1 2.7 2.3

a

Reaction conditions: P0 = 1 atm.

increase butyric acid conversion. This suggests that there is no thermal cracking of VFA that occurred in the methylation on the carbon material. The highest concentration of methyl butyrate produced through the methylation of butyric acid was ∼3.4 M at 360 °C and no further increase in the methyl butyrate concentration was observed despite increasing the temperature to 380 °C. It was also observed that butyric acid conversion (from 94.5 to 22.2%) and methyl butyrate concentration (from 3.3 to 2.3 M) decreased as the butyric acid/methanol ratio increased from 0.3 to 3. The results indicate that the butyric acid conversion and methyl butyrate concentration were not statistically different at the ratios of 0.3 and 0.5. Hence, the VFA/methanol volumetric ratio of 0.5 was chosen as an optimized reaction condition for the methylation on the CMK-5. To assess the stability of the CMK-5, we carried out a recycling study of the CMK-5 for the methylation at the optimized reaction conditions (360 °C and VFA/methanol volumetric ratio of 0.5). After each cycle, the CMK-5 was recovered by filtration and washed thoroughly with methanol followed by complete drying before being used again. As shown in Figure 3, no significant deactivation of the CMK-5, in terms of the conversion of butyric acid and the reaction rate calculated at less than 20% conversion of butyric acid, was observed after six cycles. 7435


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Figure 3. Recycling study of the CMK-5 for the methylation of butyric acid (reaction conditions: P0 = 1 atm, butyric acid/methanol (v/v) = 0.5).

Figure 5. Comparison of CMK-5 and MCNT for the methylation of butyric acid (reaction conditions: P0 = 1 atm, butyric acid/methanol (v/v) = 0.5).

In Figure 4, a mixture of six different VFAs including acetic acid, propionic acid, butyric acid, isobutyric acid, valeric acid,

most effective OMC for the methylation of VFAs. A maximum of 98% VFA yield at 360 °C and a VFA/methanol volumetric ratio of 0.5 was achieved. Importantly, the CMK-5 is stable after six consecutive reaction cycles. Also, a commercial MCNT showed a similar trend to the CMK-5, indicating a potential for industrializing the process. This study strongly suggests a new approach to develop and/or synthesize porous materials for the methylation processes and a great potential for renewable gasoline production.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01953. GC operation conditions, QA/QC in GC, XRD data, physisorption data, TEM images, and schematic models of the OMCs. (PDF)

Figure 4. Yields of methyl esters of six VFAs for the methylation on the CMK-5 (reaction conditions: T = 360 °C, P0 = 1 atm, and VFAs/ methanol (v/v) = 0.5). Mean values of replicates (n ≥ 3) are reported with standard deviations given as error bars.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

and isovaleric acid were used as the feedstock for the methylation on the CMK-5 at 360 °C and VFAs/methanol volumetric ratio of 0.5. It showed that all VFAs were successfully converted into their corresponding methyl esters such as methyl acetate, methyl propionate, methyl butyrate, methyl isobutyrate, methyl valerate, and methyl isovalerate with yields higher than 90%. VFAs that are relatively light such as acetic acid and propionic acid showed ∼98% yields of methyl acetate and methyl propionate, respectively. Even for heavier VFAs, higher than 90% yields of corresponding methyl esters were achieved. It is evident that the methylation on the CMK-5 is effective for the conversion of all types of VFAs into FAMEs. Given that the OMCs are not commercialized materials, a commercial MCNT purchased from Sigma-Aldrich was tested for the methylation of butyric acid and compared to that with the CMK-5, as shown in Figure 5. The CMK-5 and MCNT showed a similar behavior likely due to a similar structure of the CMK-5 and MCNT.

ORCID

Jechan Lee: 0000-0002-9759-361X Hyung Ju Kim: 0000-0002-3489-6488 Ki-Hyun Kim: 0000-0003-0487-4242 Eilhann E. Kwon: 0000-0001-7438-7920 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a National Research Foundation of Korea (NRF) Grant funded by the Korean Government (Ministry of Education; No. NRF-2016R1D1A1B03933027).

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REFERENCES

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CONCLUSIONS Herein, the methylation of VFAs with highly ordered mesoporous carbon materials is reported. The CMK-5 having an interconnected hollow-rod bimodal pore structure is the 7436


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Methylation of Volatile Fatty Acids with Ordered Mesoporous Carbon

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