Sulfonic Acid-Functionalized, Hyper-Cross-Linked Porous

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Kinetics, Catalysis, and Reaction Engineering

Sulfonic-acid-functionalized, hyper-crosslinked porous polyphenols as recyclable solid acid catalysts for esterification and transesterification reactions Reddi Mohan Naidu Kalla, Mi-Ra Kim, and Il Kim Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02418 • Publication Date (Web): 08 Aug 2018 Downloaded from http://pubs.acs.org on August 15, 2018

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Industrial & Engineering Chemistry Research

Sulfonic-acid-functionalized, hyper-crosslinked porous polyphenols as recyclable solid acid catalysts for esterification and transesterification reactions Reddi Mohan Naidu Kalla, Mi-Ra Kim, Il Kim* BK21 PLUS Center for Advanced Chemical Technology, Department Polymer Science and Engineering, Pusan National University, Busan 609-735, Republic of Korea

KEYWORDS. Fatty acids, Vegetable Oils, Esterification, Transesterification, Solid acid catalyst, Polyphenol, Room temperature. ABSTRACT. Easy, safe, and cost-effective hydroxyl-containing microporous hyper-crosslinked polymers based on phenol (pPhOH) and 4,4'-(propane-2,2-diyl)diphenol (or bisphenol A; BPA) were prepared by the Friedel–Crafts alkylation reaction of phenol and bisphenol A using dimethoxymethane as an external crosslinker. The pPhOH and pBPA were functionalized by chlorosulfonic acid to yield corresponding sulfonic acid-functionalized polymers, pPh-SO3H and pBPA-SO3H, respectively, with their surface areas of 210 m2/g and 324 m2/g, respectively. The physicochemical properties of pPh-SO3H and pBPA-SO3H were analyzed by Fourier transform infrared spectroscopy, X-ray diffraction, thermogravimetric analysis, scanning electron microscopy, Brunauer–Emmett–Teller analysis, and X-ray photoelectron spectroscopy. The pPh-

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SO3H and pBPA-SO3H polymers show excellent catalytic activity for the esterification of free fatty acids and transesterification of vegetable oils at room temperature, as well as excellent recyclability, signifying the potential of these porous polymers in a wide array of eco-friendly acid-promoted chemical transformations.

1. INTRODUCTION

The use of heterogeneous acid catalysts (HACs) for esterification and transesterification reactions via green and sustainable chemistry has attracted significant attention as candidates to circumvent to use conventional hazardous and corrosive homogeneous acid catalysts.1The reactions involving HACs can be performed at environmentally more friendly conditions and the catalysts are recyclable.2−12 The HACs are also utilized for both the esterification of saturated and unsaturated fatty acids and the transesterification of the lipids present in the industrial process without the formation of soap. However, in order to make HACs tangible for industrial process, the cost of catalyst must be lowered by reducing the complexity of synthesis. Tungstated zirconia,2 sulfated

zirconia,3mesoporous

materials,4sulfonated

carbon,5,6

ion

exchange

resins,7membranematerials,8 and partially sulfonated carbonized sugar, starch, or cellulose have been attempted as HACs.9−12 Sulfonic acid-functionalized covalent organic frameworks (COF),13 sulfonicacid-functionalized hybrid silica14 and metal–organic-framework (MOF)-encapsulated Keggin heteropolyacids15 have been used specifically for esterification reactions. However, majority of these catalysts employed energy consuming reaction conditions, needing more economic catalyst systems for the esterification of free fatty acids (FFAs).16 Nowadays, significant effort have been made to develop hyper-crosslinked-polymers (HCPs)because of their potential applications as catalysis,17,18 sensing,19and energy storage.20


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Since the HCPs bear reactive organic functional groups and have a high surface area, they are particularly expected to be useful as green organocatalysts, avoiding the use of toxic metals. HCPs based on aromatic hydrocarbons can be synthesized by the Friedel−Crafts alkylation reaction with an additional crosslinker.21,22 Recently, we have developed an amine-functionalized polyphenanthrene (PPhe) as a metal and acid-free catalyst for the synthesis of chromene and pyran molecules.23 The hyper-crosslinked polyaromatic spheres with unreacted bromine groups on

the

periphery

were

effective

and

reusable

catalysts

for

the

preperation

of

bis(indolyl)methanes.24 Thus, from the viewpoint of green and sustainable catalysts, these materials are highly attractive for the design of novel porous HACs-based catalysts for the successful and economic biodiesel production. In this article, we report the straightforward synthesis of the acidic HCPs via a one-pot Friedel−Crafts alkylation

followed by self-polymerization (crosslinking) of phenol and

bisphenol A using dimethoxymethane as an additional crosslinker in the presence of anhydrous FeCl3(Scheme 1). The functionalization of the phenolic hydroxyl moieties with sulfonic acid groups gave microporous HACs with good surface are a bearing tunable number of acidic sites. The resultant solid acid materials effectively catalyzed both esterification and transesterification reactions under mild reaction conditions.

2. EXPERIMENTAL SECTION

2.1. Materials and Instrumentation. Phenol (PhOH), bisphenol A (BPA), and formaldehyde dimethyl acetal (FDA; 98%) obtained from TCI, long-chain monocarboxylic and dicarboxylic acids, methanol (MeOH), and 1,2-dichloroethane (DCE) from Daejung Chem. (Seoul, Korea), anhydrous FeCl3 (98%) from Acros, and various vegetable oils obtained from local market were


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used as received. 1H NMR (400 MHz) and

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C NMR (100 MHz) spectra were taken using a

Varian INOVA 400 NMR spectrometer at 25 °C. Chemical shift values are quoted relative to Me4Si. Fourier transform infrared (FT-IR) spectra were recorded at 25 °C using a Shimadzu IR Prestige 21 spectrometer. The spectra were obtained using KBr disks and recorded in between

Scheme 1. One-pot synthesis of porous HCPs via alkylation and self-polymerization of phenol and bisphenol A and their post-functionalization using chlorosulfonic acid.


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4500 and 500 cm−1. The X-ray diffraction (XRD) study was performed by using an automatic Philips powder diffractometer with nickel-filtered Cu Kα radiation. The diffraction pattern was collected in the 2θrange of 0–80° in steps of 0.02and counting times of two seconds per step. The microstructures of the samples were examined using an S-3000 scanning electron microscope (SEM; Hitachi, Japan), and thermogravimetric analysis (TGA) was carried out using a TGA N1000 (Scinco, Seoul, and Republic of Korea). X-ray photoelectron spectroscopy (XPS) analysis was performed in a Theta Probe AR-XPS system with a monochromatic Al Kα X-ray source (1486.6 eV). The Brunauer–Emmett–Teller (BET) and density functional theory (DFT) methods (Nova 3200e system, Quantachrome Instruments, USA) were used to study the BET specific surface area and distribution of the pore size of the materials. 2.2. Synthesis of HCPs. The HCPs were synthesized using a previously reported literature method.25 Typically, anhydrous FeCl3 (1.3 g, 8 mmol) was then added to a solution of phenol (0.35 g, 4 mmol) and FDA (0.7 mL, 8 mmol) in 12 mL of DCE and the mixture was stirred for 18 h at 80 °C. After quenching the reaction with MeOH, the filtered polymer was washed with MeOH and water. The solids were finally purified by Soxhlet extraction with MeOH for 24 h and then dried under vacuum at 70 °C for 12 h. This polymer was denoted as pPhOH. The HCPs synthesized using BPA in a similar procedure, and it was denoted as pBPA. 2.3. Sulfonation of pPhOH and pBPA. The pPhOH and pBPA samples were functionalized with sulfonic acid groups following a literature procedure.26 Typically, 4.0 mL of ClSO3H in 10 mL of CH2Cl2 was added drop wise to the chilled suspension of 0.4 g of pPhOH in 10 mL of CH2Cl2and then stirred at 25 °C for 48 h. The reaction mixture was filtered and washed thoroughly with CH2Cl2 and dried to get pPh-SO3H. The pBPA-SO3H sample was also obtained in similar procedures. The concentration of acid sites of the pPh-SO3H and pBPA-SO3H catalysts


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was measured by a reverse titration with HCl (0.337 N). 10 mL of NaOH (0.130 N) was added to 0.1 g of catalyst and stirred for 30 min, and then was filtered and washed with deionized water. The additional amount of NaOH was titrated with HCl in the presence of phenolphthalein as an indicator. The concentrations of acid sites in pPh-SO3H and pBPA-SO3H were 0.85 and 1.60 mmol g−1, respectively. 2.4. Esterification of FFAs and transesterification of various vegetable oils with methanol. For the esterification myristic acid (228 mg, 1.0 mmol), pPh-SO3H or pBPA-SO3Hcatalyst (10 mg), and MeOH (50 mmol) were separately added to a 10-mL reaction flask and stirred for 6 h at room temperature (r.t.).The amount of catalyst was doubled (20 mg) for the esterification of the dicarboxylic acids, and the reactions were completed in 18 h (for pPh-SO3H) or 12 h (for pBPASO3H). For the transesterification, for example, coconut oil (100 mg), catalyst (10 mg), and methanol (4.5 g) were added to a 10-mL reaction flask and stirred for 24 h at r.t. and 10 h at 60 °C. The catalyst was separated by filtration and then 5 mL of MeOH was added to the mixture. The excess methanol was evaporated under reduced pressure to yield the desired product. 2.5. Molecular Simulations. The Materials Studio 5.0 software package (Accelrys Inc., San Diego, CA, 2005) was used for the molecular modeling of the crosslinked polymer networks with the polymer consistant force field (COMPASS II)27. Molecular simulations were carried out with the Forcite module, using a time step of 1 fs, the Nosé-Hoover thermostat with a Q ratio of 0.01, and the Andersen barostat with a time constant of 1 ps. Monomers are packed into a periodic cell using Amorphous Cell module. The enclosed script implemented for a crosslinking simulation using Forcite was modified to join monomer units through a condensation polymerization based on a set of predetermined connectivity rule with the removal of simple molecules. Packing monomers and defining their reactive atoms and crosslinking sites, the


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crosslinked organization can be generated with any degree of crosslinking. This script is composed of four steps: (a)initial packing of the simulation box (using Amorphous Cell), (b) dynamics for initial equilibration: i.e. NVT dynamics run followed by NPT run, (c) a crosslinking procedure until the target degree of cross-linking is reached, and (d) dynamics for a final energy minimization.

3. RESULTS AND DISCUSSION

3.1. Characterization. The acid-functionalization of pPhOH and pBPA was traced using the FT-IR spectra of pPhOH, pBPA, pPh-SO3H, and pBPA-SO3H (Figure 1(a)). The pPhOH and pBPA show −OH stretching frequencies at 3448 and 3421 cm−1, whereas those of pPh-SO3H, and pBPA-SO3H show the O–H stretching of the SO3H analogous frequencies at 3439 and 3431 cm−1. The O=S=O asymmetric and symmetric stretching bands were observed at 1220, 1045 and 1227, 1058, cm−1, respectively, and the S–O stretching bands were observed at 850 and 755 cm−1. These results are confirming the existence of the SO3 group linkage. Figure 1(b) shows the wide-angle powder XRD patterns of the polymers. The peaks of pPhOH and pBPA at 2θ=19.0° and 23.3°, respectively shift to 2θ = 26.7° and 26.0° for pPh-SO3H and pBPA-SO3H, respectively. All polymers are highly amorphous and the peak shifts are induced by the acid functionalization. The TGA curves of pPh-SO3H and pBPA-SO3H in Figure 1(c)show the first weight losses at 2.2% and 6.0% at 80 °C, respectively, due to the loss of physically adsorbed water molecules, followed by the loss of strongly adsorbed water molecules having a non-covalent interaction with the free sulfonic acid groups.28 The loss of sulfonic acid groups can be observed by the weight losses of 40% (for pPh-SO3H) and 55.7% (pBPA-SO3H ) in the temperature range130–270 °C. The relatively larger weight loss of pBPA-SO3H than pPh-


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SO3H is most probably due to the presence of higher concentration of sulfonic acid groups. The high thermal stability of the porous polymers could be attributed to the hyperbranched crosslinking and the rigid polymeric networks in the molecular skeletons of pPh-SO3H, and pBPA-SO3H (Figure 1(c)).

Figure 1. (a) FT-IR spectra, (b) XRD patterns, and (c) TGA thermograms of pPh-SO3H and pBPA-SO3H. For the atomic simulations29 of pPhOH and pBPA the amorphous cells were produced with 200 seed molecules comprised of 100units of 1 (Figure 2(a)), 100 units of 2, and 200 units of 3, respectively, as shown in Figure 2. The crosslinks were formed between atoms labeled R1 and R2 in fragments 1, 2, and 3. The amorphous cells for the 200-unit cluster with solvent accessible surfaces (blue) are shown in Figure 2(b) and 2(c).The simulation box size is 32.14 and 41.91 Å for pPhOH and pBPA, respectively. At low degree of crosslinking, both samples are essentially


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nonporous, since the flexible and mobile nature of growing polymer chains prevents pores from forming. As the degree of crosslinking increases to a specific degree expected micropores are formed. Figure 2(b) and (c) show the pore volumes as defined by the solvent-accessible surface30 for crosslinking degree of 100%. The solvent-accessible surface areas of pPhOH and pBPA networks are 480.5 and 548.8 m2/g, respectively, and pore volumes of pPhOH and pBPA networks are 17.8×10−3 and 7.4×10−3 cm3/g, respectively. The calculated solvent-accessible surface areas show reasonable agreement with the experimental apparent BET values as measured by N2 sorption (vide infra).

Figure 2. Fragments (a) to build networks of pPhOH (1 and 2) and pBPA (3), and snapshots of molecular simulation boxes of pPhOH (b) and pBPA (c) networks for crosslinking degree of 100% showing the solvent-accessible surface areas (blue areas). For amorphous cells construction, 100 units of 1 and 2 each were used for pPhOH and 200 units of 3 were used for pBPA, respectively. Dark blue areas represent the micropore surface and grey balls and sticks represent newly formed crosslink sites.


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The SEM images of pPhOH and pPh-SO3H (Figures 3 (a–d)) showed a tubular morphology, and, after sulfonation, most of the tubular morphology of the polymers became spherical. In contrast, in the case of pBPA and pBPA-SO3H, as shown in Figures 3 (e, f), a spherical morphology can be seen. The spherical morphology of these polymers is retained after sulfonation, and it is evident that the spherical morphology of these polymers is uniform.

Figure 3. SEM images of pPhOH (a and b), pPh-SO3H (c and d), pBPA (e), and pBPA-SO3H (f). The nitrogen adsorption, desorption isotherms and the textural properties of pPhOH, pBPA, pPh-SO3H, and pBPA-SO3H are shown in Figures 4 (a–d). The BET surface areas of pPhOH and pBPA are 440 m2/g and 635 m2/g, respectively, and those of pPh-SO3H, and pBPA-SO3H are 210 m2/g and 324 m2/g, respectively. The decreased surface area of pPh-SO3H and pBPA-SO3H


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is a result of acid functionalization, where the bulky sulfonate groups have been grafted onto the porous periphery of the HCPs. The increase in the mass of the material with respect to the weight percent of the polymer may be another factor to reduce the surface area. Together with the reduction of surface area the amount of mesoporous volumes was also dramatically reduced by functionalization. The variation of the textural properties of pPh-SO3H, and pBPA-SO3H from those of pPhOH and pBPA indicates the successful acid functionalization. The HCPs also bear micropores as expected. The pPhOH and pPh-SO3H samples have pore widths centered at about 13.72and 13.50 Å (Figure 4(b)), respectively, and pBPA and pBPA-SO3H have smaller micropores centered at 8.0 and 9.3 Å (Figure 4(d)), respectively.

Figure4. Nitrogen adsorption and desorption isotherms [(a) and (c)], and pore size distributions [(b) and (d)] of pPhOH, pPh-SO3H, pBPA, and pBPA-SO3H.


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The pPh-SO3H and pBPA-SO3H samples were subjected to XPS analysis (Figure 5). Peaks assigned to oxygen, carbon, and sulfur can be clearly observed, confirming the presence of sulfonic acid groups. The S(2p) XPS spectra of pPh-SO3H, and pBPA-SO3H show one important peak assigned to S(2p3/2)31,32due to the existence of SO3H in the pPh-SO3H, and pBPASO3Hsamples (Figure 5(d)). A reduced binding energy in the S(2p) XPS spectra for pPh-SO3H suggests an increase in the electron density on the sulfur atoms. Interestingly, a higher binding energy of pBPA-SO3H in contrast to pPh-SO3H is probably due to a greater number of electronegative oxygen atoms adjacent to the −SO3H group in pBPA-SO3H (Scheme 1).

Figure 5.XPS spectra of pPh-SO3H and pBPA-SO3H (a) and their expanded peaks [(b−d)]. 3.2. Esterification and Transesterification. The catalytic activities of the pPh-SO3H and pBPA-SO3H for the esterifications of various long alkyl chain mono and dicarboxylic FFAs with


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MeOH have been tested and the results are summarized in Table 1.The control experiments for the esterification of myristic acid (MA) with MeOH were conducted in presence of pPhOH and pBPA to assess the contribution of the non-acidic materials. Methyl myristate was not formed, even after 6 h, at room temperature (Table 1, entries 1 and 2). There are five important reaction parameters affecting the catalytic activity: i.e. temperature, reaction time, [FFA]/[MeOH] ratio, the alkyl chain length of FFAs, and the acidity of the catalyst. The esterification reaction performed at 60 °C for 3 h recorded 91% yield, based on the methyl ester peak at around 3.6 ppm from 1H NMR spectrum of the reaction mixture. The 13C NMR spectra were also collected to figure out the presence of the acidic carbon, which was used as an indicator of the completion of reaction. The reaction was usually continued under these conditions for a further 3 h, and then the catalyst was separated by centrifugation. The product yield slightly increased to 96%, as determined by proton NMR, with the isolated yield of 92%. The same reaction was also conducted at 40 °C and at r.t. for 6 h recorded 94% and 90.6% yields, respectively, with isolated yields of 90.6% and 86.5%, respectively. These results show that the reaction temperature is not a decisive factor for the catalytic activity. We also conducted esterifications of FFAs with different alkyl chain lengths such as palmitic acid (PA), stearic acid (SA), and oleic acid (OA) at r.t. The pBPA-SO3H catalyst shows high yields of 95.6%, 96.6% and 94.3% for PA, SA, and OA, respectively; however, the pPh-SO3H catalyst gives somewhat lower yields. By the extension of the reaction time to 18 h this catalyst records 91.6%, 91.3% and 90.3% yields for PA, SA, and OA, respectively. As shown in Figure 6, the esterification of PA is still ongoing even after 12 h. The product yield was 82.6% in this moment and peaks assigned to unreacted acid were clearly observed from the

13

C NMR

spectrum. The peaks assigned to unreacted acid were disappeared after 18 h of reaction.


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Considering these results and the concentration of acid sites in pPh-SO3H and pBPA-SO3H of 0.85 and 1.60 mmol g−1, respectively, the alkyl chain lengths affects the catalytic activity33 but not decisively, specifically for pBPA-SO3H catalyst. Table 1. Esterification reactions of diverse FFAs over pPh-SO3H and pBPA-SO3H.a

Entry

Fatty acid

Catalyst

Temp. (°C)

Yield (%)b

1 CH3(CH2)12COOH pPhOH r.t n.r.c 2 CH3(CH2)12COOH pBPA r.t n.r.c 3 CH3(CH2)12COOH pPh-SO3H 60 96.0 4 CH3(CH2)12COOH pPh-SO3H 40 94.0 5 CH3(CH2)12COOH pPh-SO3H r.t. 90.6 6 CH3(CH2)14COOH pPh-SO3H r.t. 91.6d 7 CH3(CH2)16COOH pPh-SO3H r.t. 91.3d 8 CH3(CH2)7CH=CH(CH2)7COOH pPh-SO3H r.t. 90.3d 9 COOH(CH2)8COOH pPh-SO3H r.t. 94.1d 10 COOH(CH2)4COOH pPh-SO3H r.t. 87.5 d 11 COOH(CH2)3COOH pPh-SO3H r.t. 86.1e 12 COOH(CH2)2COOH pPh-SO3H r.t. 84.3e 13 COOHCH2COOH pPh-SO3H r.t. 78.5 14 (COOH)2 pPh-SO3H r.t. n.r.c 15 CH3(CH2)12COOH pBPA-SO3H r.t. 95.0 16 CH3(CH2)14COOH pBPA-SO3H r.t. 95.6 17 CH3(CH2)16COOH pBPA-SO3H r.t. 96.6 18 CH3(CH2)7CH=CH(CH2)7COOH pBPA-SO3H r.t. 94.3 19 COOH(CH2)8COOH pBPA-SO3H r.t. 96.6f 20 COOH(CH2)4COOH pBPA-SO3H r.t. 95.3f 21 COOH(CH2)3COOH pBPA-SO3H r.t. 90.0f 22 COOH(CH2)2COOH pBPA-SO3H r.t. 87.6f 23 COOHCH2COOH pBPA-SO3H r.t. 86.6f 24 (COOH)2 pBPA-SO3H r.t. n.r.c a Reaction methods: FFAs (1 mmol), MeOH(50 mmol), and 10 mg of pPh-SO3H or 10 mg of pBPA-SO3H at 25 °C; reaction time =6−24 h.bYield (%) was measured from 1H NMR: [peak region of single proton close to 3.6 (for −CH3) divided by that of single proton at 2.25 (for −CH2)] × 100.cNo reaction.dReaction completed in 18 h.eReaction completed in 24 h.f Reaction completed in 12 h


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Figure 6. 13C NMR spectra of methyl palmitate after 12 h and 18 h reaction. In the case of esterifications of dicarboxylic acids, the more reaction time (18−24 h) was needed to complete the reaction because the CH2 bridge shielded the electronic effects (i.e., Meffect) and reduced the reactivity of the two –COOH groups.34 The trial to esterify oxalic acid resulted in failure even after 24 h owing to the absence of the CH2 group and the lack of Meffect. The transesterifications of oils such as coconut, olive, sesame, canola, soybean, palm, castor, linseed, and corn oils with MeOH were also conducted using the pPh-SO3H and pBPA-SO3H catalysts and the results are summarized in Table 2. Firstly, the transesterification of coconut oil was performed using the optimized conditions of esterification; however, the transesterification at r.t. for 24 h resulted in only 30% conversion, as found by 1H NMR spectroscopy. The increases of temperature to 60 °C and reaction time to 10 h, the conversions sharply increase to 94.7% for pPh-SO3H catalyst and 97.6% for pBPA-SO3H catalyst, as determined by 1H NMR spectroscopy. Table 2 reveals that all the oils were easily processed by the transesterifications with MeOH at to 60 °C for 10 h with conversions range from 77.5% to 97.6%. As expected from


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the difference of the concentration of acid sites, pBPA-SO3H is superior to pPh-SO3H at the same reaction conditions. Table 2. Transesterification of various oils over pPh-SO3H and pBPA-SO3H.a Entry Oil Catalyst Temp. (°C) Conversion (%)b 1 Coconut oil pPh-SO3H r.t. 30.0c 2 Coconut oil pPh-SO3H 50 91.0c 4 Coconut oil pPh-SO3H 60 94.7 5 Olive oil pPh-SO3H 60 71.5 6 Canola oil pPh-SO3H 60 80.4 7 Linseed oil pPh-SO3H 60 97.9 8 Palm oil pPh-SO3H 60 88.8 9 Sesame oil pPh-SO3H 60 89.4 10 Soybean oil pPh-SO3H 60 77.4 11 Corn oil pPh-SO3H 60 82.0 12 Coconut oil pBPA-SO3H 60 97.6 13 Olive oil pBPA-SO3H 60 77.5 14 Sesame oil pBPA-SO3H 60 89.4 15 Soybean oil pBPA-SO3H 60 86.4 16 Corn oil pBPA-SO3H 60 87.0 a Reaction conditions: oil (100 mg), MeOH(4.5 g ), and 10 mg of pPh-SO3H or 10 mg of pBPA-SO3H reaction period = 10 h. bConversion (%) was measured from integrated values of the glyceridic and methyl ester protons in the 1H NMR spectra using: × conversion = ×100%,where IME is the integration value of the methyl ester × ×

peak, and ITG is the integration value of the glyceridic peaks in the triglycerides of oil. c The reaction time was 24 h.

The kinetic curve of the esterification of MA at r.t. was plotted in Figure 7. The conversion increases sharply at the early period of reaction, and then show an asymptotic curve after about 3 h. After 6 h of reaction no signals indicating unreacted acid were observed. As the reaction temperature progressively increases to 40 °C and 60 °C, the unreacted acid could not be observed within 3 h and 4 h, respectively. In general, these esterifications are reversible in nature. In the early period, the rate of forward reaction is much faster than that of the reverse reaction owing to the more number of moles of the reactants and the product formation was in less number of moles. The [FFA]/[MeOH] ratio is also a significant parameter for the conversion


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of fatty acids to corresponding fatty acid esters. Theoretically, one mole of fatty acid requires one mole of methanol; however, we found that an excess of MeOH relative to FFA can shift the equilibrium toward methyl ester formation due to the reversible reaction.35 The yield of methyl esters increased from 75% to 95% after 6 h of reaction on increasing the [MeOH]/[MA]ratio 5 to 50.

Figure 7. Reaction time vs. conversion (%) curve for myristic acid at r.t. In order to get insights on the performance of the catalysts of the present study with reported catalysts we summarize the catalytic activity of the esterifications of FFAs using various catalyst systems (Table 3). Bhaumik et al.36,37 investigated the esterification of various FFAs and oils at r.t. in the presence of a sulfonated hyper-microporous polymer (HMP-1-SO3H) and a sulfonated pyrene-based porous polymer (SPPOP) catalysts. The esterification of MA with MeOH took 10 h to achieve 99% yield with HMP-1-SO3H catalyst and 90% yield with SPPOP catalyst. Fang and his coworkers38 reported the production of fatty acid methyl esters using carbon nanotube-based sulfonic acids (CNT-P-SO3H). They used 20 mg of CNT-P-SO3H catalyst for the esterification


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of OA to achieve 93% yield at 60 °C for 10 h of reaction. Zhang et al.39 also achieved 95% yield for the esterification of OA using an ordered mesoporous carbon-based solid acid catalyst (OMC) for the esterification of OA at 80 °C for 16 h of reaction. Note that it needs tedious synthesis synthetic procedure by using costly surfactants such as F127 and P123 to prepare OMCs. Baskaralingam et al.40 used the mesoporous sulfated zirconia solid acid catalyst (Zr-KIT6) for the esterification of PA at 120 °C for 3 h, achieving 75% yield. Very recently, Ding and coworkers41 achieved 90% yield for the esterification of OA at 65 °C for 8 h by using an immobilized phosphotungstic acid-based ionic liquid (IL) catalyst.

Table 3. Comparison of various porous and non-porous solid acid catalysts used for the esterification of FFAs. Entry

Catalyst (mg/wt.%)

FFA

Temp. (°C)

1 HMP-1-SO3H (10 mg) MA 2 SPPOP-3 (20 mg) MA 3 CNT-P-SO3H (20 mg) OA 4 OMCs (20 mg) OA 5 IL (15 wt.%) OA 6 Zr-KIT-6 PA 7 pBPA-SO3H (10 mg) MA a Reaction conditions: FFAs (1 mmol), MeOH temperature.

Time (h)

Yield (%)

Ref

r.t. r.t.

10 10

99 90

36 37

60 80 65 120

10 16 8 3

93 95 90 75

38 39 40 41

r.t. 6 95 This worka (50 mmol), and 10 mg of pBPA-SO3H at room

Compared to the above-mentioned catalysts, pBPA-SO3H has obvious advantages for the esterification of FFAs; that is, no need of drastic conditions and laborious workup methods. In addition, it is reusable. These features of pBPA-SO3H are attractive and associated to the hypercrosslinked microporous structure. The existence of interrelated micropores may result in (i) advantageous mass transfer from side to side, and (ii) a high surface area for the interactions of


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atoms with the reactive sites.42 These micropores present a good stage for the catalysis to follow a single route leading to the preferred products by virtue of shorter reaction times. 3.3. Leaching and Hot Filtration Tests. The loss over time of catalytic activity is one of the great and continuing concerns in the industrial catalytic processes using heterogeneous catalysts. There are a number of mechanisms of catalyst deactivation coming from both chemical and mechanical causes. Thermal degradation of the catalyst is also a factor of deactivation. Specifically leaching of active components from the solid support should be avoided in the case of polymer-supported catalysts. The acid site leaching in the pBPA-SO3H catalyst was tested for the esterification of MA. The pBPA-SO3H catalyst was separated from the reaction mixture after 1.5 h of esterification at r.t. and, at that time, the yield was 64%. Then, the reaction mixture bearing no catalyst kept stirring for 9 h and the yield slightly increased to 65.6%, indicating that there was no significant leaching of acid sites. Inspired by these results, the recyclability of the pBPA-SO3H catalyst was tested. The esterification of MA was performed six successive reaction cycles with the catalyst separated from the previous run. Figure 8 illustrates conversion vs. time plots of the six esterifications of MA. The run 2 to 6 obtained by using the recycled catalysts show very similar kinetic profiles with the first run by using the virgin catalyst, clearly suggesting that the pBPA-SO3H catalyst is recyclable and show no conspicuous deactivation. This makes the present catalyst system promising for the industrial practice together with the easiness of the preparation of catalyst.


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Figure 8. Recyclability of the pBPA-SO3H catalyst in the esterification of myristic acid.

4. CONCLUSIONS

The hyper-crosslinked polymers were obtained via the Friedel−Crafts alkylation of phenol or bisphenol A and dimethoxymethane as a crosslinker in the presence of FeCl3in 1,2dichloroethane solvent under refluxing conditions. After the straight forward sulfonation of these porous polymers, the eco-friendly, cost-effective, highly acidic, and non-hazardous sulfonated materials were utilized as solid acid catalysts for the esterification and transesterification of various long-chain FFAs and fatty oils. The catalytic activity increased with increasing acid functionalization. A significant degree of sulfonation was required, and, among the two acid catalysts, pBPA-SO3H, exhibited higher catalytic activity than pPh-SO3H owing to the higher concentration of acid sites. Furthermore, the durable stability of the sulfonated catalyst was demonstrated by the recyclability of the catalyst without leaching of acid sites. The easiness of bulk amount of catalyst, the straightforward acid functionalization, high activities for both


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esterification and transesterification, and recyclability will make the acid-functionalized HCP catalysts industrially more tangible.

ASSOCIATED CONTENT Supporting Information. 1H and 13C NMR spectra of all synthesized compounds. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail:[email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the Basic Science Research Program of the National Research Foundation of Korea (2015R1D1A1A09057372). The authors are also grateful to the BK21 PLUS Program for partial financial support. REFERENCES (1) Marchetti, J.M.; Errazu, A.F. Esterification of free fatty acids using sulfuric acid as catalyst in the presence of triglycerides. Biomass Bioenergy, 2008, 32, 892−895. (2) Furuta, S.; Matsuhashi, H.; Arata, K. Biodiesel fuel production with solid super acid catalysis in fixed bed reactor under atmospheric pressure. Catal. Commun. 2004, 5, 721−723. (3) Yadav, G.D.; Nair, J.J. Sulfated zirconia and its modified versions as promising catalysts for industrial processes. Microporous. Mesoporous. Mater. 1999, 33, 1−48.


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Table of Contents Graphic


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Sulfonic Acid-Functionalized, Hyper-Cross-Linked Porous

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