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Esterification of glycerol with acetic acid over SO3H-functionalized phenolic resin
Fuel 255 (2019) 115842
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Full Length Article
Esterification of glycerol with acetic acid over SO3H-functionalized phenolic resin
T
Yuanyuan Jianga, Xuewen Lia, Huaiyuan Zhaoa, Zhaoyin Houa,b,
⁎
a b
Key Laboratory of Biomass Chemical Engineering of Ministry of Education, Department of Chemistry, Zhejiang University, Hangzhou 310028, PR China Center of Chemistry for Frontier Technologies, Department of Chemistry, Zhejiang University, Hangzhou 310028, PR China
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Biodiesel Glycerol Esterification Phenolic resin
A series of SO3H-functionalized phenolic resin-based catalysts (PSPF-x) with different molar ratio of p-phenolsulfonic acid (PhOH-4-sa) and phenol (PhOH) were prepared via a facile polycondensation method, and employed in the esterification of glycerol with acetic acid under mild conditions. It was found that PSPF-3 (the molar ratio of PhOH-4-sa to PhOH in feed was 3:1) exhibited excellent activity and stability. The detected conversion of glycerol was 81.6% with a 98.9% selectivity of MAG and DAG at 70 °C under stoichiometric molar ratio of acetic acid to glycerol (3:1) for 7 h. The highest turnover frequency of each acid site in PSPF-3 reached 341.1 h−1 at the beginning of the reaction and it could be recycled for 5 times. Characterization results indicated that the performance of PSPF-3 could be attributed to its stable cross-linking structure, enhanced surface area and porosity. The reaction mechanism was proposed on the base of a series of experiments.
1. Introduction With the increasing global demand of energy and the decreasing supply of nonrenewable fossil fuels, many efforts have been paid to develop new sustainable fuels in the past twenty years [1,2]. And biodiesel is recommended as a promising alternative fuel for its renewable and green advantages, and high compatibility with
compression-ignition engines [3]. The productivity of biodiesel in America increased continuously to 2.6 billion gallons in 2018 [4]. Biodiesel is a mixture of long chain C10-C22 fatty acid alkyl esters, which is produced by transterification of triglycerides with methanol or ethanol [5,6]. During the transterification process, there is 10 wt% of glycerol formed as an unavoidable by-product, which leads to the drastic surplus of glycerol. It is of great importance to convert glycerol
⁎ Corresponding author at: Key Laboratory of Biomass Chemical Engineering of Ministry of Education, Department of Chemistry, Zhejiang University, Hangzhou 310028, PR China. E-mail address: [email protected] (Z. Hou).
https://doi.org/10.1016/j.fuel.2019.115842 Received 12 May 2019; Received in revised form 10 July 2019; Accepted 17 July 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Esterification of glycerol with acetic acid.
into value-added products over green catalyst [7–11]. Esterification of glycerol with acetic acid or acetic anhydride can yield monoacetin (MAG), diacetin (DAG) and triacetin (TAG), and all of them have a wide range of applications. MAG is a raw material in the production of explosives, smokeless powder and/or food additive [12]. Besides, it can react with acetone to produce solketalacetin, which is a valuable diesel additive [13]. DAG is a widely used softening agent, plasticizer and intermediate in the preparation of structural lipids [14,15]. What’s more, the mixture of MAG and DAG can be applied as raw materials to produce biodegradable polyesters, or as emulsifiers in pharmaceutical, food and cosmetics industries [16]. TAG and the mixture of DAG and TAG are effective oxygenated additives to improve the properties of conventional diesel fuel [17]. Acetic acid is a preferable acetylating agent for its low cost and nontoxic to health although its reactivity is lower than that of acetic anhydride [18]. The esterification of glycerol with acetic acid is an exothermic reversible reaction and the reaction scheme is shown in Fig. 1 [19]. This reaction performed efficiently over homogeneous acids, such as H2SO4, HCl or p-toluenesulfonic acid (PTSA). However, it is well-known that these acids are corrosive to equipment and unrecyclable, leading to the formation of wastes and pollutants [20–22]. It is an important task to find an efficient heterogeneous catalyst for this reaction. In the past decade, esterification of glycerol with acetic acid has attracted the attentions of many scientists. And the reported catalysts included commercialized ion exchanged resin [16,23,24], metal oxides [25–27], heteropolyacids [28–30], zeolites [31,32] and sulfonated activated carbon [17,33,34]. These works contributed a lot to the utilization of glycerol and provided important insight into the reaction mechanism of esterification of glycerol and/or promoted the development of solid acids. Among their works, high conversion of glycerol and controlled selectivity of MAG or mixed DAG and TAG were obtained over several catalysts, but most reactions were carried out under harsh conditions (100–180 °C with over-stoichiometric acetic acid in feed, the dosage of the catalysts varied from 2 to 10 wt% of glycerol) [35]. Besides, another drawback is the stability of the catalyst suffered from the unavoidably formed water during the esterification reaction [36]. Recently, the application of metal-free SO3H-functionalized polymers as a solid acid catalyst has become a hot topic due to its strong acidity, high moisture tolerance and easiness in preparation and separation [37–40]. In 2013, a novel macroporous polymeric acid resin (copolymer of p-phenolsulfonic acid and formaldehyde) was synthesized by Minakawa et al., and they found that this resin exhibited excellent performance for the direct esterification of carboxylic acids and alcohols without the removal of unavoidable coproduced water [41]. After that, Ikbal et al. also reported that a mesoporous p-phenolsulfonic
acid-formaldehyde resin was effective for the acetalization of glycerol for its strong Brønsted acidity, porous structure and water-tolerant property [42]. Inspired by these pioneered works, herein, we want to report a new copolymer of p-phenolsulfonic acid, phenol and formaldehyde (denoted as PSPF resin), and its excellent performance for the esterification of glycerol with acetic acid under mild conditions. The structure and property of this catalyst were characterized and discussed with its performance in the esterification of glycerol. To the best of our knowledge, the copolymer of phenol, p-phenolsulfonic acid and formaldehyde and its application in esterification of glycerol was seldom reported previously. 2. Material and methods 2.1. Catalyst preparation The preparation procedure of this PSPF resin followed the earlier report with some modifications [41]. Briefly, a certain amount of pphenolsulfonic acid (PhOH-4-sa), phenol (PhOH) and paraformaldehyde (POM) was dissolved in butanone in a 100 mL three-necked flask. Then, it was placed into an oil bath with a magnetic stirrer and a thermocouple. The polycondensation of these monomers was carried out at 120 °C for 6 h under refluxing and continuous stirring. After reaction, the flask was cooled to room temperature, and butanone was recovered via rotary evaporator under vacuum and the obtained product was washed with ethanol for several times. Finally, the solid resin in deep red color was obtained after drying in a vacuum oven at 60 °C for 12 h, which was denoted as PSPF-x (x was the molar ratio of PhOH4-sa and PhOH in feed). At the same time, phenolic resin (denoted as PR) and p-phenolsulfonic acid-formaldehyde resin (denoted as PSF) were also synthesized in the same procedures as above, and used as reference catalysts. And the synthesis procedures of PSPF-3 were shown in Fig. 2 as an example. 2.2. Characterizations Fourier transform infrared spectrometry (FTIR) analysis was performed to verify the functional groups of the synthesized samples on a Nicolet Is10 FTIR spectrometer. Thermogravimetric (TG) results were obtained on a Netzsch STA 409 thermobalance from 25 to 750 °C at a heating rate of 10 °C/min in air flow (30 mL/min). The surface morphologies of PSPF-x were observed on Leo Evo Series SEM (VP 1430, Germany) at an accelerating voltage of 3 kV. Before observation, platinum was sputtered on the surface of samples to avoid charging. N2 adsorption isotherms were detected on an ASAP 2010 analyzer after 2
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Fig. 2. Diagrammatic presentation of the synthesis routine of PSPF-3.
pretreated at 80 °C for 4 h in vacuum. The Brunner–Emmet–Teller surface area (SBET) was calculated from the adsorption branch in the p/ po range of 0.05–0.3. The acidity of PSPF-x catalysts was detected via an acid-base titration method. In a typical procedure, 0.1 g sample was dispersed in 50 mL of NaCl aqueous solution (2 mol/L) and stirred for 24 h under the protection of purified N2. Then the acid sites in this sample were neutralized with an aqueous solution of NaOH (0.01 mol/L) slowly until the pH of the suspension reached 7, which was monitored by an online pH meter. These data were summarized in Tables 1 and 2.
3. Results 3.1. Esterification of glycerol over solid acid catalysts Table 1 summarized the performance of those traditional solid acids and homogenous acids with the same amount of acid sites in added catalyst. At first, it was found that this reaction performed slowly (with a 20.6% conversion of glycerol) even without any catalyst at 70 °C and atmospheric pressure. Zeolite HBEA and HZSM-5 were less active for this reaction as the detected conversion of glycerol only increased slightly to 30.4 and 58.6%, respectively. On the other hand, Amberlyst45 was quite active for this reaction and the conversion of glycerol increased obviously to 75.3%. The conversion of glycerol (84.5%) and the selectivity of DAG (42.0%) reached their maximum over H3PW12O40, but it was regret to say that H3PW12O40 dissolved completely in the reaction mixture. It was interesting to find that the conversion of glycerol reached 81.6% over PSPF-3 under the same condition, and this value was in the same level as that obtained over homogenous p-toluenesulfonic acid and PhOH-4-sa (with the same amount of acid sites in added catalysts). Table 2 summarized the performance of PSPF-x for the esterification of glycerol with acetic acid at 70 °C. It can be found that the conversion of glycerol increased continuously with increasing the amount of PhOH-4-sa in the synthesized catalysts. Reference PR catalyst (without PhOH-4-sa) cannot promote this reaction as the detected conversion of glycerol was 25.8%. While the conversion of glycerol over PSPF-1, PSPF-3 and PSPF-9 increased to 42.0, 81.6 and 82.8%, and the selectivity of DAG also increased to 14.4, 30.9 and 34.9%, respectively. The conversion of glycerol and the selectivity of DAG over the phenolfree PSF catalyst can further increased to 87.5 and 50.2%. Regrettably, PSF was unstable during the reaction as it dissolved partially, and the reaction mixture became brown (see Fig. 3). The turnover frequency
2.3. Catalytic reaction Esterification of glycerol with acetic acid was performed in a 100 mL round-bottom flask. A typical reaction mixture included 50 mmol glycerol, 150 mmol acetic acid and 0.07 g catalyst. Then the flask was put into an oil bath and heated to desired temperature in N2 atmosphere. The reaction started under vigorously stirring (1000 rpm) at atmospheric pressure. After reaction, the flask was cooled to room temperature and solid catalyst was separated by centrifugation. Liquid product was analyzed on a gas chromatograph (Shimadzu, 14B) equipped with a flame ionization detector and a 30 m capillary column (DB-WAX 52 CB, USA). All the detected products were identified by a gas chromatography-mass spectrometry system (GC–MS, Agilent 6890) and quantified via an external calibration method. The conversion of glycerol, selectivity of products and carbon balance were calculated on the base of glycerol. Recycle experiments were carried out in the same procedures and reaction condition as above. In brief, catalyst was separated from reaction mixture by centrifuged, washed with ethanol for 3 times and dried in vacuum (60 °C, 4 h) before next recycle. Table 1 Esterification of glycerol with acetic acid over acid catalysts.a Catalyst
Blank HBEAd HZSM-5d Amberlyst-45 H3PW12O40 PSPF-3 p-toluenesulfonic acid PhOH-4-sa a b c d e f
Acidity (mmol/g)
1.8e 0.4e 3.0f 1.0e 0.8 5.8 5.7
Product selectivity (%)b
Conversion (%)
20.6 30.4 58.6 75.3 84.5 81.6 79.3 78.2
MAG
DAG
TAG
94.0 90.9 76.8 68.9 55.7 68.0 55.1 59.1
6.0 9.0 22.8 29.8 42.0 30.9 42.3 39.0
0.0 0.1 0.4 1.3 2.3 1.1 2.6 1.9
Reaction conditions: 50 mmol glycerol, 150 mmol acetic acid and 0.056 mmol acid sites in added catalysts, 70 °C, 7 h. MAG: monoacetin, DAG: diacetin, TAG: triacetin (the total amount of formed MAG + DAG + TAG + remained glycerol)/ (the amount of glycerol added) × 100% Si/Al = 25, and pretreated at 500 °C for 4 h. From [43]. From [44]. 3
Carbon balancec
99.3 99.3 99.3 98.7 99.0 98.8 99.0 99.0
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Table 2 The catalytic performance of polymer with different content of PhOH-4-sa.a Catalyst
PR PSPF-1 PSPF-3 PSPF-9 PSF a b c
Acidity (mmol/g)b
0.6 0.8 1.0 1.7
Conversion (%)
25.8 42.0 81.6 82.8 87.5
TOFc h−1
71.4 104.1 84.5 52.5
Product selectivity (%)
Carbon balance
MAG
DAG
TAG
90.7 85.0 68.0 63.7 45.9
9.1 14.4 30.9 34.9 50.2
0.1 0.5 1.1 1.4 3.9
99.0 98.8 98.8 98.0 97.5
Reaction conditions: 50 mmol glycerol, 150 mmol acetic acid and 0.07 g catalyst, 70 °C, 7 h. Measured by acid-base titration method. TOF was calculated on the basis of acidity at 7 h.
(TOF) on the basis of total acidity in added catalyst was calculated and summarized in Table 2. It was confirmed that the calculated TOF of PSPF-3 reached its maximum (104.1 h−1, average value of 7 h) under the same condition. Compared with those published works (see Table S1), these results also demonstrated that PSPF-3 is an excellent solid acid catalyst for its higher activity at lower temperature (70 °C), lower catalyst loading (1.5 wt% of glycerol) and lower acetic acid in feed.
81.6% with the increasing temperature from 50 to 70 °C, and then increased slightly at higher temperature, which might be attributed to that the esterification of glycerol with acetic acid is a reversible exothermic reaction [19]. The main products in these experiments were MAG and DAG, and the selectivity of TAG was limited (3.5%) even at 90 °C. Increasing the amount of acetic acid in the reaction mixture can improve the conversion of glycerol and the selectivity of DAG in some extent. The detected conversion of glycerol increased from 51.9 to 81.6% with the molar ratio of acetic acid to glycerol increasing from 1:1 to 3:1 (see Fig. 5), and the selectivity of DAG also increased from 16.4 to 30.9%. However, the conversion of glycerol and the production distribution changed slightly when the molar ratio of acetic acid to glycerol was higher than 6:1. These results were similar as that reported in literature [45]. Table 3 summarized the performance of PSPF-3 under varied reaction time at 70 °C. The conversion of glycerol reached 58.1% within 2 h and increased slowly to 81.6% at 7 h, and then changed slightly with the prolonged time. Meanwhile, the selectivity of DAG increased with the decreasing selectivity of MAG, and the selectivity of TAG was less than 1.2% even at 8 h. These results indicated that esterification of glycerol with acetic acid was a consecutive reaction, and MAG formed at the first, then it further reacted with acetic acid to form DAG and TAG [26]. The TOF of each acid site reached 341.1 h−1 at the beginning of the reaction (1 h).
3.2. Esterification of glycerol over PSPF-3 under varied conditions
3.3. Esterification of glycerol over recycled PSPF-3 under optimal condition
Fig. 4 showed the performance of PSPF-3 catalyst for esterification of glycerol with acetic acid at different reaction temperature. It could be found that the conversion of glycerol increased quickly from 33.9 to
The performance of recycled PSPF-3 catalyst under the optimal condition was displayed in Fig. 6. It was found that the conversion of
Fig. 4. Esterification of glycerol over PSPF-3 catalyst at different temperature.
Fig. 5. Esterification of glycerol with varied molar ratio of acetic acid/glycerol.
Fig. 3. The color of reaction mixture over (a) PSPF-3 and (b) PSF catalysts.
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conversion of glycerol increased slowly after catalyst filtration (black line) and the selectivity of products changed slightly. These results indicated that the leaching of PSPF-3 was negligible, because the esterification of glycerol with acetic acid performed slowly even without catalyst at 70 °C (see Table 1). At the same time, it was found that the reaction mixture over PSPF-3 catalyst was clear (see Fig. 3), which also confirmed that PSPF-3 was stable in the esterification of glycerol.
Table 3 Esterification of glycerol with acetic acid at varied reaction time on PSPF-3.a Time (h)
1 2 3 4 5 6 7 8
Conversion (%)
38.2 58.1 65.2 69.7 73.9 78.0 81.6 81.7
Product selectivity (%) MAG
DAG
TAG
89.1 86.5 83.6 78.8 74.9 71.6 68.0 67.1
10.8 13.3 16.1 20.7 24.5 27.6 30.9 31.7
0.1 0.2 0.3 0.5 0.6 0.9 1.1 1.2
4. Discussions Above experiments confirmed that PSPF-3 catalyst prepared via the copolymerization of PhOH-4-sa, PhOH and formaldehyde was active and stable for the esterification of glycerol with acetic acid. Under optimized conditions, the conversion of glycerol reached 81.6% with a 98.9% selectivity of MAG + DAG, and calculated TOF at the beginning of the reaction (1 h) was 341.1 h‾1 at 70 °C. In order to make the excellent performance of PSPF-3 well addressed, a series of characterizations were performed and discussed.
a Reaction conditions: 50 mmol glycerol, 150 mmol acetic acid, 0.07 g catalyst, 70 °C.
4.1. The properties of PSPF-x catalysts Fig. 8 showed the FTIR spectra of PSPF-x catalysts that prepared with different molar ratio of PhOH-4-sa and PhOH in feed. According to references, the bands at 3430 and 1360 cm−1 were attributed to the stretching and in-plane stretching of phenolic OH groups, respectively [46]. The band at around 2920 cm−1 was from the aliphatic C–H stretching [47], and the bands at 1617, 1478 cm−1 were the characteristic peaks of the aromatic C]C stretching [48]. Besides above characteristic signals from the framework of polymer, it was interesting to note that those bands at 1215 and 1165 cm‾1 (asymmetric stretching of O]S]O), 1120 and 1010 cm−1 (symmetric stretching of O]S]O), 1032 cm−1 (C–S), 609 cm−1 (OH group of SO3H) also appeared in these catalysts, which were the typical characters of SO3H-functionalized carbon materials [49–51]. At the same time, the strength of these peaks enhanced with the increasing amount of PhOH-4-sa in feed. Therefore, we think that PhOH-4-sa was introduced into the framework of synthesized catalysts successfully as shown in Fig. 2. And the detected acidity of these catalysts also confirmed that the amount of acid sites increased continuously from 0.6 (in PSPF-1) to 1.7 mmol/g (in PSF) with the increasing amount of PhOH-4-sa in catalysts (see Table 2), which was attributed to the acidity from –SO3H group. The thermal stability of these synthesized catalysts was checked by TG analysis (Fig. 9). Three mass loss stages were detected in all samples. The first stage below 100 °C might be attributed to desorption of water and/or solvent that adsorbed on the samples. And it can be found that the first mass loss enhanced obviously with the increasing amount of
Fig. 6. The reusability of PSPF-3 catalyst.
glycerol decreased gradually from 81.6 to 72.2% after five cycles, and the selectivity of DAG also decreased from 30.9 to 25.3%. The decreased activity would be attributed to the mass loss of catalyst that caused by vigorously stirring (1000 rpm), separation (centrifugation) and/or washing as only 0.06 g catalyst remained after 5 cycles. To further illustrate the good reusability of PSPF-3 catalyst, a leaching test was carried out and the result was showed in Fig. 7. Briefly, PSPF-3 was filtered off after 1 h (orange line), and the filtrate was continued to react for another 6 h at 70 °C. It was found that the
Fig. 7. The leaching test of PSPF-3 catalyst.
Fig. 8. FTIR spectra of PSPF-1, PSPF-3 and PSPF-9. 5
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of the polymer. It can be concluded that the phenol-free PSF catalyst was unstable as the decomposition of sulfonic groups occurred easily and heavily. It is interesting to note that phenol can improve the thermal stability of these catalysts and the absolute mass loss in the second stage decreased with the amount of phenol. Fig. 10 showed the N2 adsorption-desorption isotherms of these SO3H- functionalized phenolic resin-based catalysts and the calculated surface area, pore volume and pore diameter were summarized in Table 4. All catalysts showed the type I isotherms without obvious hysteresis at low p/po, the surface area and pore volume of PSPF-3 (5.5 m2/g, 0.018 cm3/g, respectively) were higher than others, which indicated that a well cross-linking structured framework (as that shown in Fig. 2) formed. Those typical SEM images also confirmed that PSPF-3 was a porous and rough material with irregular plates on its surface, which was attributed to the formation of cross-linking structure during the polycondensation process as the proper molar ratio of PhOH-4-sa and PhOH was added in feed (see Fig. 11). On the other hand, PSPF-1 and PSPF-9 were composed of tightly stacked particles with smooth surface.
Fig. 9. TG curves of PSPF-1, PSPF-3, PSPF-9 and PSF.
4.2. Possible mechanism of esterification of glycerol over PSPF-3 On the base of above results of catalytic reaction and characterization, we think that the excellent performance of PSPF-3 could be attributed to the formation of well cross-linking structured framework (see Fig. 2). And this cross-linking structure increased the surface area (see Table 4) and enhanced the thermal stability of PSPF-3 (see Fig. 9). At the same time, cross-linking structure would also improve the accessibility of acid sites. On the surface of PSPF-3, (1) the carbonyl group of acetic acid was activated by the proton from the –SO3H group to form an acylium ion and isomerized carbonium ion, and (2) the carbonium ion of acetic acid could be attacked easily by a hydroxy group of glycerol molecule to form a geminal diol with released proton. Finally, the geminal diol dehydrated to generate carbonyl group and MAG formed. And MAG can further react with the activated carbonium ion of acetic acid to DAG and TAG, successively (see Fig. 12) [25,34]. 5. Conclusions
Fig. 10. N2 adsorption-desorption isotherms of PSPF-1, PSPF-3 and PSPF-9.
In summary, a SO3H-functionalized phenolic resin-based catalyst (PSPF-3) was synthesized via a facile polycondensation routine with controlled amount of PhOH-4-sa and PhOH in feed. And this catalyst was highly active and stable for the esterification of glycerol with acetic acid under mild condition. The highest TOF value of each acid site in PSPF-3 reached 341.1 h−1 at 70 °C, and the conversion of glycerol reached 81.6% with a 98.9% selectivity of MAG and DAG at 7 h. Characterizations indicated that a well cross-linking structured framework might be formed in PSPF-3, and this cross-linking increased its surface area, thermal stability and accessibility of acid sites. These results can pave the way for the preparation of resin-based catalyst with immobilized acid sites, and also stimulate the research in catalytic synthesis of MAG and DAG from glycerol.
Table 4 Textural properties of PSPF-1, PSPF-3 and PSPF-9. Sample
Surface area (m2/g)
Pore diameter (nm)
Pore volume (cm3/g)
PSPF-1 PSPF-3 PSPF-9
1.0 5.5 1.5
22.1 17.0 15.7
0.004 0.018 0.005
PhOH-4-sa because of the hydrophilicity of sulfonic groups. The second stage in 100–300 °C would be the decomposition of sulfonic groups [52]. And the last stage above 300 °C was assigned to the decomposition
Fig. 11. SEM images of (a) PSPF-1, (b) PSPF-3 and (c) PSPF-9. 6
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Fig. 12. The mechanism of glycerol esterification with acetic acid over PSPF-3.
Acknowledgements
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Full Length Article
Esterification of glycerol with acetic acid over SO3H-functionalized phenolic resin
T
Yuanyuan Jianga, Xuewen Lia, Huaiyuan Zhaoa, Zhaoyin Houa,b,
⁎
a b
Key Laboratory of Biomass Chemical Engineering of Ministry of Education, Department of Chemistry, Zhejiang University, Hangzhou 310028, PR China Center of Chemistry for Frontier Technologies, Department of Chemistry, Zhejiang University, Hangzhou 310028, PR China
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Biodiesel Glycerol Esterification Phenolic resin
A series of SO3H-functionalized phenolic resin-based catalysts (PSPF-x) with different molar ratio of p-phenolsulfonic acid (PhOH-4-sa) and phenol (PhOH) were prepared via a facile polycondensation method, and employed in the esterification of glycerol with acetic acid under mild conditions. It was found that PSPF-3 (the molar ratio of PhOH-4-sa to PhOH in feed was 3:1) exhibited excellent activity and stability. The detected conversion of glycerol was 81.6% with a 98.9% selectivity of MAG and DAG at 70 °C under stoichiometric molar ratio of acetic acid to glycerol (3:1) for 7 h. The highest turnover frequency of each acid site in PSPF-3 reached 341.1 h−1 at the beginning of the reaction and it could be recycled for 5 times. Characterization results indicated that the performance of PSPF-3 could be attributed to its stable cross-linking structure, enhanced surface area and porosity. The reaction mechanism was proposed on the base of a series of experiments.
1. Introduction With the increasing global demand of energy and the decreasing supply of nonrenewable fossil fuels, many efforts have been paid to develop new sustainable fuels in the past twenty years [1,2]. And biodiesel is recommended as a promising alternative fuel for its renewable and green advantages, and high compatibility with
compression-ignition engines [3]. The productivity of biodiesel in America increased continuously to 2.6 billion gallons in 2018 [4]. Biodiesel is a mixture of long chain C10-C22 fatty acid alkyl esters, which is produced by transterification of triglycerides with methanol or ethanol [5,6]. During the transterification process, there is 10 wt% of glycerol formed as an unavoidable by-product, which leads to the drastic surplus of glycerol. It is of great importance to convert glycerol
⁎ Corresponding author at: Key Laboratory of Biomass Chemical Engineering of Ministry of Education, Department of Chemistry, Zhejiang University, Hangzhou 310028, PR China. E-mail address: [email protected] (Z. Hou).
https://doi.org/10.1016/j.fuel.2019.115842 Received 12 May 2019; Received in revised form 10 July 2019; Accepted 17 July 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Esterification of glycerol with acetic acid.
into value-added products over green catalyst [7–11]. Esterification of glycerol with acetic acid or acetic anhydride can yield monoacetin (MAG), diacetin (DAG) and triacetin (TAG), and all of them have a wide range of applications. MAG is a raw material in the production of explosives, smokeless powder and/or food additive [12]. Besides, it can react with acetone to produce solketalacetin, which is a valuable diesel additive [13]. DAG is a widely used softening agent, plasticizer and intermediate in the preparation of structural lipids [14,15]. What’s more, the mixture of MAG and DAG can be applied as raw materials to produce biodegradable polyesters, or as emulsifiers in pharmaceutical, food and cosmetics industries [16]. TAG and the mixture of DAG and TAG are effective oxygenated additives to improve the properties of conventional diesel fuel [17]. Acetic acid is a preferable acetylating agent for its low cost and nontoxic to health although its reactivity is lower than that of acetic anhydride [18]. The esterification of glycerol with acetic acid is an exothermic reversible reaction and the reaction scheme is shown in Fig. 1 [19]. This reaction performed efficiently over homogeneous acids, such as H2SO4, HCl or p-toluenesulfonic acid (PTSA). However, it is well-known that these acids are corrosive to equipment and unrecyclable, leading to the formation of wastes and pollutants [20–22]. It is an important task to find an efficient heterogeneous catalyst for this reaction. In the past decade, esterification of glycerol with acetic acid has attracted the attentions of many scientists. And the reported catalysts included commercialized ion exchanged resin [16,23,24], metal oxides [25–27], heteropolyacids [28–30], zeolites [31,32] and sulfonated activated carbon [17,33,34]. These works contributed a lot to the utilization of glycerol and provided important insight into the reaction mechanism of esterification of glycerol and/or promoted the development of solid acids. Among their works, high conversion of glycerol and controlled selectivity of MAG or mixed DAG and TAG were obtained over several catalysts, but most reactions were carried out under harsh conditions (100–180 °C with over-stoichiometric acetic acid in feed, the dosage of the catalysts varied from 2 to 10 wt% of glycerol) [35]. Besides, another drawback is the stability of the catalyst suffered from the unavoidably formed water during the esterification reaction [36]. Recently, the application of metal-free SO3H-functionalized polymers as a solid acid catalyst has become a hot topic due to its strong acidity, high moisture tolerance and easiness in preparation and separation [37–40]. In 2013, a novel macroporous polymeric acid resin (copolymer of p-phenolsulfonic acid and formaldehyde) was synthesized by Minakawa et al., and they found that this resin exhibited excellent performance for the direct esterification of carboxylic acids and alcohols without the removal of unavoidable coproduced water [41]. After that, Ikbal et al. also reported that a mesoporous p-phenolsulfonic
acid-formaldehyde resin was effective for the acetalization of glycerol for its strong Brønsted acidity, porous structure and water-tolerant property [42]. Inspired by these pioneered works, herein, we want to report a new copolymer of p-phenolsulfonic acid, phenol and formaldehyde (denoted as PSPF resin), and its excellent performance for the esterification of glycerol with acetic acid under mild conditions. The structure and property of this catalyst were characterized and discussed with its performance in the esterification of glycerol. To the best of our knowledge, the copolymer of phenol, p-phenolsulfonic acid and formaldehyde and its application in esterification of glycerol was seldom reported previously. 2. Material and methods 2.1. Catalyst preparation The preparation procedure of this PSPF resin followed the earlier report with some modifications [41]. Briefly, a certain amount of pphenolsulfonic acid (PhOH-4-sa), phenol (PhOH) and paraformaldehyde (POM) was dissolved in butanone in a 100 mL three-necked flask. Then, it was placed into an oil bath with a magnetic stirrer and a thermocouple. The polycondensation of these monomers was carried out at 120 °C for 6 h under refluxing and continuous stirring. After reaction, the flask was cooled to room temperature, and butanone was recovered via rotary evaporator under vacuum and the obtained product was washed with ethanol for several times. Finally, the solid resin in deep red color was obtained after drying in a vacuum oven at 60 °C for 12 h, which was denoted as PSPF-x (x was the molar ratio of PhOH4-sa and PhOH in feed). At the same time, phenolic resin (denoted as PR) and p-phenolsulfonic acid-formaldehyde resin (denoted as PSF) were also synthesized in the same procedures as above, and used as reference catalysts. And the synthesis procedures of PSPF-3 were shown in Fig. 2 as an example. 2.2. Characterizations Fourier transform infrared spectrometry (FTIR) analysis was performed to verify the functional groups of the synthesized samples on a Nicolet Is10 FTIR spectrometer. Thermogravimetric (TG) results were obtained on a Netzsch STA 409 thermobalance from 25 to 750 °C at a heating rate of 10 °C/min in air flow (30 mL/min). The surface morphologies of PSPF-x were observed on Leo Evo Series SEM (VP 1430, Germany) at an accelerating voltage of 3 kV. Before observation, platinum was sputtered on the surface of samples to avoid charging. N2 adsorption isotherms were detected on an ASAP 2010 analyzer after 2
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Fig. 2. Diagrammatic presentation of the synthesis routine of PSPF-3.
pretreated at 80 °C for 4 h in vacuum. The Brunner–Emmet–Teller surface area (SBET) was calculated from the adsorption branch in the p/ po range of 0.05–0.3. The acidity of PSPF-x catalysts was detected via an acid-base titration method. In a typical procedure, 0.1 g sample was dispersed in 50 mL of NaCl aqueous solution (2 mol/L) and stirred for 24 h under the protection of purified N2. Then the acid sites in this sample were neutralized with an aqueous solution of NaOH (0.01 mol/L) slowly until the pH of the suspension reached 7, which was monitored by an online pH meter. These data were summarized in Tables 1 and 2.
3. Results 3.1. Esterification of glycerol over solid acid catalysts Table 1 summarized the performance of those traditional solid acids and homogenous acids with the same amount of acid sites in added catalyst. At first, it was found that this reaction performed slowly (with a 20.6% conversion of glycerol) even without any catalyst at 70 °C and atmospheric pressure. Zeolite HBEA and HZSM-5 were less active for this reaction as the detected conversion of glycerol only increased slightly to 30.4 and 58.6%, respectively. On the other hand, Amberlyst45 was quite active for this reaction and the conversion of glycerol increased obviously to 75.3%. The conversion of glycerol (84.5%) and the selectivity of DAG (42.0%) reached their maximum over H3PW12O40, but it was regret to say that H3PW12O40 dissolved completely in the reaction mixture. It was interesting to find that the conversion of glycerol reached 81.6% over PSPF-3 under the same condition, and this value was in the same level as that obtained over homogenous p-toluenesulfonic acid and PhOH-4-sa (with the same amount of acid sites in added catalysts). Table 2 summarized the performance of PSPF-x for the esterification of glycerol with acetic acid at 70 °C. It can be found that the conversion of glycerol increased continuously with increasing the amount of PhOH-4-sa in the synthesized catalysts. Reference PR catalyst (without PhOH-4-sa) cannot promote this reaction as the detected conversion of glycerol was 25.8%. While the conversion of glycerol over PSPF-1, PSPF-3 and PSPF-9 increased to 42.0, 81.6 and 82.8%, and the selectivity of DAG also increased to 14.4, 30.9 and 34.9%, respectively. The conversion of glycerol and the selectivity of DAG over the phenolfree PSF catalyst can further increased to 87.5 and 50.2%. Regrettably, PSF was unstable during the reaction as it dissolved partially, and the reaction mixture became brown (see Fig. 3). The turnover frequency
2.3. Catalytic reaction Esterification of glycerol with acetic acid was performed in a 100 mL round-bottom flask. A typical reaction mixture included 50 mmol glycerol, 150 mmol acetic acid and 0.07 g catalyst. Then the flask was put into an oil bath and heated to desired temperature in N2 atmosphere. The reaction started under vigorously stirring (1000 rpm) at atmospheric pressure. After reaction, the flask was cooled to room temperature and solid catalyst was separated by centrifugation. Liquid product was analyzed on a gas chromatograph (Shimadzu, 14B) equipped with a flame ionization detector and a 30 m capillary column (DB-WAX 52 CB, USA). All the detected products were identified by a gas chromatography-mass spectrometry system (GC–MS, Agilent 6890) and quantified via an external calibration method. The conversion of glycerol, selectivity of products and carbon balance were calculated on the base of glycerol. Recycle experiments were carried out in the same procedures and reaction condition as above. In brief, catalyst was separated from reaction mixture by centrifuged, washed with ethanol for 3 times and dried in vacuum (60 °C, 4 h) before next recycle. Table 1 Esterification of glycerol with acetic acid over acid catalysts.a Catalyst
Blank HBEAd HZSM-5d Amberlyst-45 H3PW12O40 PSPF-3 p-toluenesulfonic acid PhOH-4-sa a b c d e f
Acidity (mmol/g)
1.8e 0.4e 3.0f 1.0e 0.8 5.8 5.7
Product selectivity (%)b
Conversion (%)
20.6 30.4 58.6 75.3 84.5 81.6 79.3 78.2
MAG
DAG
TAG
94.0 90.9 76.8 68.9 55.7 68.0 55.1 59.1
6.0 9.0 22.8 29.8 42.0 30.9 42.3 39.0
0.0 0.1 0.4 1.3 2.3 1.1 2.6 1.9
Reaction conditions: 50 mmol glycerol, 150 mmol acetic acid and 0.056 mmol acid sites in added catalysts, 70 °C, 7 h. MAG: monoacetin, DAG: diacetin, TAG: triacetin (the total amount of formed MAG + DAG + TAG + remained glycerol)/ (the amount of glycerol added) × 100% Si/Al = 25, and pretreated at 500 °C for 4 h. From [43]. From [44]. 3
Carbon balancec
99.3 99.3 99.3 98.7 99.0 98.8 99.0 99.0
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Table 2 The catalytic performance of polymer with different content of PhOH-4-sa.a Catalyst
PR PSPF-1 PSPF-3 PSPF-9 PSF a b c
Acidity (mmol/g)b
0.6 0.8 1.0 1.7
Conversion (%)
25.8 42.0 81.6 82.8 87.5
TOFc h−1
71.4 104.1 84.5 52.5
Product selectivity (%)
Carbon balance
MAG
DAG
TAG
90.7 85.0 68.0 63.7 45.9
9.1 14.4 30.9 34.9 50.2
0.1 0.5 1.1 1.4 3.9
99.0 98.8 98.8 98.0 97.5
Reaction conditions: 50 mmol glycerol, 150 mmol acetic acid and 0.07 g catalyst, 70 °C, 7 h. Measured by acid-base titration method. TOF was calculated on the basis of acidity at 7 h.
(TOF) on the basis of total acidity in added catalyst was calculated and summarized in Table 2. It was confirmed that the calculated TOF of PSPF-3 reached its maximum (104.1 h−1, average value of 7 h) under the same condition. Compared with those published works (see Table S1), these results also demonstrated that PSPF-3 is an excellent solid acid catalyst for its higher activity at lower temperature (70 °C), lower catalyst loading (1.5 wt% of glycerol) and lower acetic acid in feed.
81.6% with the increasing temperature from 50 to 70 °C, and then increased slightly at higher temperature, which might be attributed to that the esterification of glycerol with acetic acid is a reversible exothermic reaction [19]. The main products in these experiments were MAG and DAG, and the selectivity of TAG was limited (3.5%) even at 90 °C. Increasing the amount of acetic acid in the reaction mixture can improve the conversion of glycerol and the selectivity of DAG in some extent. The detected conversion of glycerol increased from 51.9 to 81.6% with the molar ratio of acetic acid to glycerol increasing from 1:1 to 3:1 (see Fig. 5), and the selectivity of DAG also increased from 16.4 to 30.9%. However, the conversion of glycerol and the production distribution changed slightly when the molar ratio of acetic acid to glycerol was higher than 6:1. These results were similar as that reported in literature [45]. Table 3 summarized the performance of PSPF-3 under varied reaction time at 70 °C. The conversion of glycerol reached 58.1% within 2 h and increased slowly to 81.6% at 7 h, and then changed slightly with the prolonged time. Meanwhile, the selectivity of DAG increased with the decreasing selectivity of MAG, and the selectivity of TAG was less than 1.2% even at 8 h. These results indicated that esterification of glycerol with acetic acid was a consecutive reaction, and MAG formed at the first, then it further reacted with acetic acid to form DAG and TAG [26]. The TOF of each acid site reached 341.1 h−1 at the beginning of the reaction (1 h).
3.2. Esterification of glycerol over PSPF-3 under varied conditions
3.3. Esterification of glycerol over recycled PSPF-3 under optimal condition
Fig. 4 showed the performance of PSPF-3 catalyst for esterification of glycerol with acetic acid at different reaction temperature. It could be found that the conversion of glycerol increased quickly from 33.9 to
The performance of recycled PSPF-3 catalyst under the optimal condition was displayed in Fig. 6. It was found that the conversion of
Fig. 4. Esterification of glycerol over PSPF-3 catalyst at different temperature.
Fig. 5. Esterification of glycerol with varied molar ratio of acetic acid/glycerol.
Fig. 3. The color of reaction mixture over (a) PSPF-3 and (b) PSF catalysts.
4
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conversion of glycerol increased slowly after catalyst filtration (black line) and the selectivity of products changed slightly. These results indicated that the leaching of PSPF-3 was negligible, because the esterification of glycerol with acetic acid performed slowly even without catalyst at 70 °C (see Table 1). At the same time, it was found that the reaction mixture over PSPF-3 catalyst was clear (see Fig. 3), which also confirmed that PSPF-3 was stable in the esterification of glycerol.
Table 3 Esterification of glycerol with acetic acid at varied reaction time on PSPF-3.a Time (h)
1 2 3 4 5 6 7 8
Conversion (%)
38.2 58.1 65.2 69.7 73.9 78.0 81.6 81.7
Product selectivity (%) MAG
DAG
TAG
89.1 86.5 83.6 78.8 74.9 71.6 68.0 67.1
10.8 13.3 16.1 20.7 24.5 27.6 30.9 31.7
0.1 0.2 0.3 0.5 0.6 0.9 1.1 1.2
4. Discussions Above experiments confirmed that PSPF-3 catalyst prepared via the copolymerization of PhOH-4-sa, PhOH and formaldehyde was active and stable for the esterification of glycerol with acetic acid. Under optimized conditions, the conversion of glycerol reached 81.6% with a 98.9% selectivity of MAG + DAG, and calculated TOF at the beginning of the reaction (1 h) was 341.1 h‾1 at 70 °C. In order to make the excellent performance of PSPF-3 well addressed, a series of characterizations were performed and discussed.
a Reaction conditions: 50 mmol glycerol, 150 mmol acetic acid, 0.07 g catalyst, 70 °C.
4.1. The properties of PSPF-x catalysts Fig. 8 showed the FTIR spectra of PSPF-x catalysts that prepared with different molar ratio of PhOH-4-sa and PhOH in feed. According to references, the bands at 3430 and 1360 cm−1 were attributed to the stretching and in-plane stretching of phenolic OH groups, respectively [46]. The band at around 2920 cm−1 was from the aliphatic C–H stretching [47], and the bands at 1617, 1478 cm−1 were the characteristic peaks of the aromatic C]C stretching [48]. Besides above characteristic signals from the framework of polymer, it was interesting to note that those bands at 1215 and 1165 cm‾1 (asymmetric stretching of O]S]O), 1120 and 1010 cm−1 (symmetric stretching of O]S]O), 1032 cm−1 (C–S), 609 cm−1 (OH group of SO3H) also appeared in these catalysts, which were the typical characters of SO3H-functionalized carbon materials [49–51]. At the same time, the strength of these peaks enhanced with the increasing amount of PhOH-4-sa in feed. Therefore, we think that PhOH-4-sa was introduced into the framework of synthesized catalysts successfully as shown in Fig. 2. And the detected acidity of these catalysts also confirmed that the amount of acid sites increased continuously from 0.6 (in PSPF-1) to 1.7 mmol/g (in PSF) with the increasing amount of PhOH-4-sa in catalysts (see Table 2), which was attributed to the acidity from –SO3H group. The thermal stability of these synthesized catalysts was checked by TG analysis (Fig. 9). Three mass loss stages were detected in all samples. The first stage below 100 °C might be attributed to desorption of water and/or solvent that adsorbed on the samples. And it can be found that the first mass loss enhanced obviously with the increasing amount of
Fig. 6. The reusability of PSPF-3 catalyst.
glycerol decreased gradually from 81.6 to 72.2% after five cycles, and the selectivity of DAG also decreased from 30.9 to 25.3%. The decreased activity would be attributed to the mass loss of catalyst that caused by vigorously stirring (1000 rpm), separation (centrifugation) and/or washing as only 0.06 g catalyst remained after 5 cycles. To further illustrate the good reusability of PSPF-3 catalyst, a leaching test was carried out and the result was showed in Fig. 7. Briefly, PSPF-3 was filtered off after 1 h (orange line), and the filtrate was continued to react for another 6 h at 70 °C. It was found that the
Fig. 7. The leaching test of PSPF-3 catalyst.
Fig. 8. FTIR spectra of PSPF-1, PSPF-3 and PSPF-9. 5
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of the polymer. It can be concluded that the phenol-free PSF catalyst was unstable as the decomposition of sulfonic groups occurred easily and heavily. It is interesting to note that phenol can improve the thermal stability of these catalysts and the absolute mass loss in the second stage decreased with the amount of phenol. Fig. 10 showed the N2 adsorption-desorption isotherms of these SO3H- functionalized phenolic resin-based catalysts and the calculated surface area, pore volume and pore diameter were summarized in Table 4. All catalysts showed the type I isotherms without obvious hysteresis at low p/po, the surface area and pore volume of PSPF-3 (5.5 m2/g, 0.018 cm3/g, respectively) were higher than others, which indicated that a well cross-linking structured framework (as that shown in Fig. 2) formed. Those typical SEM images also confirmed that PSPF-3 was a porous and rough material with irregular plates on its surface, which was attributed to the formation of cross-linking structure during the polycondensation process as the proper molar ratio of PhOH-4-sa and PhOH was added in feed (see Fig. 11). On the other hand, PSPF-1 and PSPF-9 were composed of tightly stacked particles with smooth surface.
Fig. 9. TG curves of PSPF-1, PSPF-3, PSPF-9 and PSF.
4.2. Possible mechanism of esterification of glycerol over PSPF-3 On the base of above results of catalytic reaction and characterization, we think that the excellent performance of PSPF-3 could be attributed to the formation of well cross-linking structured framework (see Fig. 2). And this cross-linking structure increased the surface area (see Table 4) and enhanced the thermal stability of PSPF-3 (see Fig. 9). At the same time, cross-linking structure would also improve the accessibility of acid sites. On the surface of PSPF-3, (1) the carbonyl group of acetic acid was activated by the proton from the –SO3H group to form an acylium ion and isomerized carbonium ion, and (2) the carbonium ion of acetic acid could be attacked easily by a hydroxy group of glycerol molecule to form a geminal diol with released proton. Finally, the geminal diol dehydrated to generate carbonyl group and MAG formed. And MAG can further react with the activated carbonium ion of acetic acid to DAG and TAG, successively (see Fig. 12) [25,34]. 5. Conclusions
Fig. 10. N2 adsorption-desorption isotherms of PSPF-1, PSPF-3 and PSPF-9.
In summary, a SO3H-functionalized phenolic resin-based catalyst (PSPF-3) was synthesized via a facile polycondensation routine with controlled amount of PhOH-4-sa and PhOH in feed. And this catalyst was highly active and stable for the esterification of glycerol with acetic acid under mild condition. The highest TOF value of each acid site in PSPF-3 reached 341.1 h−1 at 70 °C, and the conversion of glycerol reached 81.6% with a 98.9% selectivity of MAG and DAG at 7 h. Characterizations indicated that a well cross-linking structured framework might be formed in PSPF-3, and this cross-linking increased its surface area, thermal stability and accessibility of acid sites. These results can pave the way for the preparation of resin-based catalyst with immobilized acid sites, and also stimulate the research in catalytic synthesis of MAG and DAG from glycerol.
Table 4 Textural properties of PSPF-1, PSPF-3 and PSPF-9. Sample
Surface area (m2/g)
Pore diameter (nm)
Pore volume (cm3/g)
PSPF-1 PSPF-3 PSPF-9
1.0 5.5 1.5
22.1 17.0 15.7
0.004 0.018 0.005
PhOH-4-sa because of the hydrophilicity of sulfonic groups. The second stage in 100–300 °C would be the decomposition of sulfonic groups [52]. And the last stage above 300 °C was assigned to the decomposition
Fig. 11. SEM images of (a) PSPF-1, (b) PSPF-3 and (c) PSPF-9. 6
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Fig. 12. The mechanism of glycerol esterification with acetic acid over PSPF-3.
Acknowledgements
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