Introduction
Homogeneous liquid-acid catalysts, such as sulfuric acid, have been used extensively in acid-catalyzed reactions due to their significant catalytic properties. Use of these liquid-acid catalysts has decreased gradually in the chemical industry due to poor recyclability, significant corrosivity, and subsequent environmental pollution. Compared with liquid acids, solid acids are ‘greener’ catalysts that have attracted attention due to their ease of recycling and good catalytic properties (Okuhara, Reference Okuhara2002; Yang et al., Reference Yang, Niu, Jia, Kang, Ma and Lei2011; Ngaosuwan et al., Reference Ngaosuwan, Goodwin and Prasertdham2016). The active groups of most solid-acid catalysts leach out easily during aqueous reactions, however, resulting in the rapid loss of catalytic activity, which limits application (Okuhara, Reference Okuhara2002).
Sulfonated carbon catalysts prepared by the sulfonation of incompletely carbonized natural organic products have greater catalytic activities and stability in aqueous solutions (Toda et al., Reference Toda, Takagaki, Okamura, Kondo, Hayashi, Domen and Hara2005). The raw materials used for sulfonated carbon catalysts are very cheap and can be obtained from natural materials, including sucrose (Toda et al., Reference Toda, Takagaki, Okamura, Kondo, Hayashi, Domen and Hara2005), starch (Budarin et al., Reference Budarin, Clark, Hardy, Luque, Milkowski, Tavener and Wilson2006), cellulose (Suganuma et al., Reference Suganuma, Nakajima, Kitano, Yamaguchi, Kato, Hayashi and Hara2008), and biomass residue (Ma et al., Reference Ma, Li, Liu, Cheng, Cao, Mao and Zhu2014; Tao et al., Reference Tao, Guan, Wang, Liu and Louh2015; Guo et al., Reference Guo, Yin and Huang2017), but these sulfonated carbon catalysts often have smaller surface areas, which limits their catalytic activity (Suganuma et al., Reference Suganuma, Nakajima, Kitano, Yamaguchi, Kato, Hayashi and Hara2008; Nakajima and Hara, Reference Nakajima and Hara2012; Tao et al., Reference Tao, Guan, Wang, Liu and Louh2015). The introduction of a porous structure in sulfonated carbon catalysts during the sulfonation and carbonization process is essential, therefore (Budarin et al., Reference Budarin, Clark, Hardy, Luque, Milkowski, Tavener and Wilson2006). The loading of amorphous carbon on the surface of porous materials increases the surface area of catalysts and improves their catalytic properties (De Vyver et al., Reference De Vyver, Peng, Geboers, Schepers, de Clippel, Gommes, Goderis, Jacobs and Sels2010; Nandan et al., Reference Nandan, Sreenivasulu, Saxena and Viswanadham2011; Valle-Vigón et al., Reference Valle-Vigón, Sevilla and Fuertes2012), but the sulfonation of carbon after sucrose carbonation typically requires the use of sulfuric acid as the sulfonating agent.
One-step hydrothermal carbonization methods in aqueous solutions at low temperatures have been developed to overcome these problems. To prepare sulfonated carbon catalysts, numerous organic materials, such as furaldehyde (Xiao et al., Reference Xiao, Guo and Liang2010), water hyacinth (Laohapornchaiphan et al., Reference Laohapornchaiphan, Smith and Smith2017), and glucose (Zhang et al., Reference Zhang, Tao, Zhang, Ren, Lu and Wang2011; Zhan et al., Reference Zhan, Tao, Cai, Liua and Liu2014; Yang et al., Reference Yang, Zhang, Ao and Zhang2019) have been used as the carbon sources, while p-toluenesulfonic acid provided the -SO3H group. These carbon-based acid catalysts exhibited strong acidities and significant catalytic performances. The thermal treatment of a mixture of glucose and p-toluenesulfonic acid (TsOH) under solvent-free conditions also forms a sulfonated carbon solid-acid catalyst (Zhang et al., Reference Zhang, Ren, Liu, Guo, Guo, Lu and Wang2010a; Wang et al., Reference Wang, Xu, Ren, Liu, Lu and Wang2011a; Xu et al., Reference Xu, Li, Zhang, Wang, Liu, Qi, Zhuang, Luo and Yuan2016). Compared with hydrothermal carbonization, thermal treatment methods are simpler and more eco-friendly, and the sulfonated carbon, solid-acid catalysts obtained are typically very efficient when used in reactions such as esterification (Zhang et al., Reference Zhang, Ren, Liu, Guo, Guo, Lu and Wang2010a), fructose dehydration (Wang et al., Reference Wang, Xu, Ren, Liu, Lu and Wang2011a), and corncob hydrolysis (Xu et al., Reference Xu, Li, Zhang, Wang, Liu, Qi, Zhuang, Luo and Yuan2016).
Clay minerals are relatively inexpensive and widely available porous materials when compared with biomass-derived carbon. Palygorskite (Plg) is a nanofibrous, natural clay mineral often used as a catalyst carrier due to its porous structure and hydrothermal stability (Zhang et al., Reference Zhang, Jin, Shan, Liu, Wang and Huang2010b). In a previous work, sulfonated carbon/Plg catalyst was prepared by the carbonization of sucrose on a Plg support, followed by sulfonation by concentrated sulfuric acid. The resulting composite exhibited good catalytic performance for the synthesis of n-butyl acetate (Jiang et al., Reference Jiang, Xu, Duanmu, Gu and Chen2012).
The aim of the current study was to develop a green and economic method of preparing a sulfonated carbon, solid-acid catalyst with a large surface area by combining Plg nanofibrous support with simultaneous calcination and sulfonation of a mixture of sucrose and TsOH at low temperature without water. Three further objectives were: (1) to evaluate the catalytic activity of the sulfonated carbon/Plg solid acid catalyst with regard to the synthesis of methyl oleate: (2) to compare its performance with other solid-acid catalysts; and (3) to investigate the effects of the amounts of sucrose and TsOH on the catalytic performance and stability.
Materials and Methods
Materials
Plg was acquired from Gaojiawa, Xuyi, China, and pretreated by milling, followed by soaking in HNO3 solution (5 wt.%) for 24 h under magnetic stirring. Pretreated Plg was centrifuged, washed with distilled water, and then dried in an oven at 80°C. The sucrose and NaCl were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd., (Tianjin, China). TsOH and NaOH were supplied by Nanjing Chemical Reagent Co., Ltd. (Nanjing, China). The HNO3, oleic acid, and methanol were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All reagents were of analytical grade and used as received without further purification.
In a typical synthesis, 5 g of sucrose and 1 g of TsOH were dissolved in 15 mL of distilled water, and then pretreated Plg (5 g) was added. After stirring at room temperature for 1 h, the mixture was dried under vacuum at 40°C and then transferred into an alumina combustion boat that was subsequently placed in a horizontal tube furnace. The mixture was heated to 200°C at a heating rate of 5°C/min and held at the target temperature for 3 h under a N2 atmosphere. The product obtained was referred to as SC-Plg-n, where n denotes the mass of sucrose added. For comparison, sulfonated carbon (SC) was prepared from a mixture of sucrose and TsOH under the same conditions without Plg. The effect of Plg on the catalytic performance of SC was investigated by changing the amount of sucrose. The final products were denoted as SC-Plg-1, SC-Plg-3, SC-Plg-5, and SC-Plg-7 when the dosages of sucrose were 1, 3, 5, and 7 g, respectively.
Characterization
The X-ray diffraction (XRD) patterns of samples were collected using a D8 Advance X-ray powder diffractometer (Bruker, Karlsruhe, Germany). The sample morphologies were investigated using scanning electron microscopy (SEM, S-3000N, Hitachi Corporation, Tokyo, Japan). Elemental analysis of the catalysts was performed on an energy-dispersive X-ray spectrometer (EDX, EX-250, Horiba Corporation, Kyoto, Japan). Transmission electron microscopy (TEM) images were obtained using a JEM 2100F transmission electron microscope (JEOL Corporation, Tokyo, Japan). Fourier-transform infrared (FTIR) spectra of catalysts were recorded on a Nicolet 5700 spectrometer (Thermo Fisher Nicolet Corporation, Waltham, Massachusetts, USA) with a resolution of 4 cm–1 using KBr pellets with 1% sample content. The KBr pellets were compressed under 30 MPa after drying in the infrared oven. The elemental analysis of the catalyst was carried out with an Elementar Vario EL III element analyzer (Elementar Corporation, Hannah, Germany). N2 adsorption/desorption isotherms of the catalysts were obtained using a Micromeritics TriStar II 3020 instrument (Micromeritics Instruments Corporation, Norcross, Georgia, USA) at –196°C (77 K). Before measurement, the samples were outgassed for 12 h at 150°C in an N2 atmosphere.
The number of -SO3H groups on the surfaces of catalysts was determined by titration (Jiang et al., Reference Jiang, Xu, Duanmu, Gu and Chen2012) in which the catalyst (0.1 g) was dispersed in 20 mL of NaCl solution (2 mol/L) and then stirred magnetically for 24 h. The filtrate was titrated with NaOH solution (6 mmol/L) and phenolphthalein was used as an indicator.
Catalytic Performance
Esterification of oleic acid with methanol was used to investigate the catalytic properties of SC and SC-Plg catalysts. In a typical experiment, 1 g of oleic acid and 8 mL of methanol were added to a 25 mL round-bottom flask equipped with a reflux condenser and mechanical agitator. When the reaction mixture reached 65°C, 0.05 g of catalyst was added, and the mixture was heated at reflux for 4 h under magnetic stirring. After the reaction, the mixture was centrifuged to separate the solution from the catalyst. The supernatant (0.5 mL) was diluted with 10 mL of n-hexane and then analyzed using gas chromatography (GC – Agilent 6820 GC with a DB-1 capillary column, Agilent Technologies Inc., Palo Alto, California, USA). Heptadecane was used as an internal standard.
Recycling experiments were carried out to determine the catalyst stability. At the end of each reaction cycle, the catalyst was recycled by centrifugation, washed with acetone, and dried at 80°C.
RESULTS AND DISCUSSION
Preparation and Characterization of Catalyst
The SEM images of Plg, SC, and SC-Plg catalysts (Fig. 1) indicated that rod-shaped Plg crystals aggregated to form fibrous bundles, while SC produced larger solid particles with smoother surfaces. Carbon was produced mainly by the carbonization of sucrose, and the amount increased as the amount of sucrose increased. Rod-shaped Plg crystals firstly adsorbed the liquid mixture composed of sucrose and TsOH and then were enveloped by the thick liquid formed during the process of drying. Finally, carbon materials were formed on the surface of Plg fibers after sucrose carbonization, and dispersed Plg fibers were embedded in the black carbon solid particles and disappeared gradually with increasing sucrose content. No Plg fibers were observed in the catalyst when the sucrose dosage was 7 g, indicating that the coatings on the Plg fibers were too thick to encase the Plg crystals completely and thus form smooth, compact solids.
SEM images of a Plg, b SC, c SC-Plg-1, d SC-Plg-3, e SC-Plg-5, and f SC-Plg-7
The XRD patterns of Plg, SC, and SC-Plg catalysts (Fig. 2) revealed that Plg contained two diffraction peaks at 8.3 and 26.55°2θ, which were ascribed to Plg and quartz, respectively (Jiang et al., Reference Jiang, Xu, Duanmu, Gu and Chen2012). A weak and broad diffraction peak from 15 to 30°2θ was observed in the SC patterns due to the presence of an amorphous carbon material (Tang and Niu, Reference Tang and Niu2019). The intensity of the characteristic diffraction peaks of Plg and quartz decreased significantly after loading the carbon material, but their intensity did not change as the sucrose dosage increased. This suggested that carbon derived from sucrose carbonization was loaded uniformly on the Plg surface.
XRD patterns of a Plg, b SC, c SC-Plg-1, d SC-Plg-3, e SC-Plg-5, and f SC-Plg-7
The TEM images of Plg and SC-Plg-5 (Fig. 3) exposed rod-shaped Plg fibers 10–30 nm long, and nanosized spherical particles were also observed, which may be of quartz or another impurity. The TEM images of SC-Plg-5 (Fig. 3b) further revealed that the rod-shaped Plg fibers and spherical nanoparticles were coated with a carbon nanolayer after sucrose carbonization.
TEM images of a Plg and b SC-Plg-5
The N2 adsorption-desorption isotherms of Plg and SC-Plg catalysts showed type IV isotherms with H3 hysteresis loops (Fig. 4) (Jiang et al., Reference Jiang, Xu, Duanmu, Gu and Chen2012), indicating the presence of mesopores. Plg exhibited the largest amount of N2 adsorbed. The amount of N2 adsorbed by SC-Plg and the hysteresis loop volume decreased gradually upon increase of the sucrose dosage due to the formation of a carbonaceous coating.
N2 adsorption isotherms of a Plg, b SC-Plg-1, c SC-Plg-3, d SC-Plg-5, and e SC-Plg-7
The textural properties of Plg, SC, and Plg-SC catalysts (Table 1) included a BET surface area and pore volume of 169 m2/g and 0.324 cm3/g, respectively, for Plg. Due to the nonporous structure of SC, no BET surface area or pore volume was recorded. The introduction of Plg increased efficiently the surface area and pore volume of the resultant catalyst. The BET surface area and pore volume of SC-Plg catalysts decreased as the carbon content increased. The results indicated that carbon was deposited successfully in the mesopores and on the external surface of Plg, which decreased the BET surface area and pore volume.
Textural properties, composition analysis, acid density, and catalytic properties of catalysts
a Conversion of oleic acid
The energy-dispersive X-ray (EDX) spectrum of SC-Plg-5 (Fig. 5) showed that Si, Mg, Al, Fe, O, and K were derived from Plg (Jiang et al., Reference Jiang, Xu, Duanmu, Gu and Chen2012). S and C derived from sucrose and TsOH also appeared in Fig. 5, indicating that a S-containing carbon material was loaded onto the surface of the Plg fibers.
Energy -dispersive X-ray spectrum of SC-Plg-5
The FTIR spectra were used to reveal the identity of the S-containing organic functional groups in Plg and SC-Plg-3 (Fig. 6). The two adsorption bands at 465 and 1083 cm–1 were attributed to the Si–O–Si bands in Plg (Araújo Melo et al., Reference Araújo Melo, Ruiz, Melo, Sobrinbo and Martinelli2002), which disappeared after a carbon layer formed on the surface of Plg fibers. The bands at 3409 and 1620 cm–1 were ascribed to the O–H stretching vibrations and C=C skeletal vibrations (Xiao et al., Reference Xiao, Guo and Liang2010; Laohapornchaiphan et al., Reference Laohapornchaiphan, Smith and Smith2017). The results further illustrated that Plg fibers were covered by a carbon layer (Wang et al., Reference Wang, Jia, Li, Li, Wang, Chang, Kang and Lei2011b), corresponding to the analysis results of the TEM images of SC-Plg. Five weak bands at 1008, 1040, 1125, 1220, and 1379 cm–1 were clearly visible in the FTIR spectrum of the SC-Plg catalyst. The vibrational bands at 1008 and 1040 cm–1 were assigned to the S=O asymmetric stretching of SO3H groups (Geng et al., Reference Geng, Wang, Yu and Zhu2011), while the vibrational bands at 1125, 1220, and 1397 cm–1 were assigned to the asymmetric SO2 stretching of SO3H groups (Peng et al., Reference Peng, Philippaerts, Ke, Noyen, Clippel, Tendeloo, Jacobs and Sels2010; Geng et al., Reference Geng, Wang, Yu and Zhu2011). These results show that -SO3H groups were introduced into the carbonaceous material by adding TsOH to the synthesis mixture.
FTIR spectra of Plg and SC-Plg-5
The amount of acidic-SO3H groups on the surface of SC and SC-Plg determined by neutralization titration with NaOH are listed in Table 1. The acid content of SC was 0.345 mmol/g. The acid amount of SC-Plg decreased sharply after the introduction of Plg, but increased as the amount of sucrose increased. The acidity ranged from 0.019 to 0.051 mmol/g, less than that of sulfonated mesoporous carbon (2.3 mmol/g) (Peng et al., Reference Peng, Philippaerts, Ke, Noyen, Clippel, Tendeloo, Jacobs and Sels2010). As shown in Table 1, the S and C contents in SC and SC-Plg measured by elemental analysis also increased with the amount of sucrose. The results suggest that TsOH reacted with the carbon derived from sucrose, causing the sulfur content to increase with the amount of sucrose. The S content in the catalysts was significantly greater than that determined by neutralization titration. This illustrates that sulfur atoms in the carbon material increased with the amount of sucrose, but some sulfur atoms were encapsulated in the carbonaceous material, and were not released in the subsequent reaction.
Catalytic Performance of Catalyst
Esterification of oleic acid with methanol was used to evaluate the catalytic activity of the SC-Plg catalyst (Table 1). The conversion of oleic acid was 50.37% without a catalyst, which increased to 65.07% after adding Plg, due to the acidity of Plg. Obviously, the limited improvement in oleic acid conversion was attributed to the small number of acid sites on Plg (Araújo Melo et al., Reference Araújo Melo, Ruiz, Melo, Sobrinbo and Martinelli2002). The -SO3H groups in SC released H+ ions which reacted with ketonic oxygen of oleic acid and increased the electropositivity of the carbonyl carbon atom. Nucleophile methanol subsequently attacked the carbonyl carbon atom. Methyl oleate was formed after the removal of the H2O molecule and H+ ion. Due to its abundant -SO3H groups, the introduction of SC greatly enhanced the conversion of oleic acid to 88.87%. In contrast, Plg (50 wt.%) was added to SC by mechanical mixing and then the physical mixture of SC and Plg was used as the catalyst. The conversion of oleic acid was reduced to 77.38%. These results indicated that SC possessed greater acidity than Plg, resulting in greater catalytic activity. The conversion of oleic acid over SC-Plg decreased slightly when Plg nanofibers were added to SC by simultaneous calcination and sulfonation of a mixture of sucrose, TsOH, and Plg. The conversion of oleic acid over SC-Plg increased gradually upon increasing sucrose, but the conversion of oleic acid decreased to 81.45% when 7 g of sucrose was used. The greatest conversion (84.68%) was achieved when SC-Plg-5 was used as the catalyst because of the smaller surface area and pore volume of SC-Plg-7, despite its larger -SO3H content. This indicated that Plg could replace the expensive sucrose-derived carbon without affecting negatively the catalytic activity. The major role played by Plg was that of support of the SC catalyst and increase of the surface area of the SC catalyst. The conversion of oleic acid was determined by the releasable -SO3H groups and pore structure of the catalysts.
To investigate the effect of TsOH on the catalytic activity of SC-Plg, the dosage of TsOH was adjusted from 1 g to 1.5 and 2 g. The C content increased to 25.04% and 26.08%, and the corresponding S contents were 0.778 and 0.916 mmol/g, respectively. The results indicate that carbon materials on the surface of Plg fibers increased with TsOH. The conversion of oleic acid of SC-Plg was reduced to 82.79% in spite of the greater S content when the dosage of TsOH increased to 1.5 g. Subsequently, the conversion of oleic acid of SC-Plg increased to 85.61% when the dosage of TsOH was 2 g. The reduction of catalytic activity of SC-Plg might be caused by the increase in carbon materials despite the larger S content. The -SO3H groups existed mainly in the inner part of carbon materials, and the number of -SO3H groups released did not increase with TsOH. Excess TsOH did not improve significantly the catalytic activity of SC-Plg.
Comparison of the catalytic activities of various catalysts for the esterification of oleic acid with methanol (Table 2) revealed that the conversion of oleic acid over SC-Plg-5 catalyst was greater than over HY zeolite, mesoporous SO3H-carbon, or protonated Nafion, despite the higher reaction temperature used for the latter catalysts (Peng et al., Reference Peng, Philippaerts, Ke, Noyen, Clippel, Tendeloo, Jacobs and Sels2010; Alismaeel et al., Reference Alismaeel, Abbas, Albayati and Doyle2018). Its oleic acid conversion was also greater than that of Amberlyst-15 under the same reaction conditions but was slightly less than that of sulfonated carbon and SBA-15-C-SO3H prepared using H2SO4 as the sulfonating agent (Geng et al., Reference Geng, Yu, Wang and Zhu2012; Wang et al., Reference Wang, Wang, Tan, Jiang, Zheng, Tsubaki and Wu2015; Flores et al., Reference Flores, Omega, Cabatingan, Go, Agapay and Ju2019). The results illustrated that the naturally occurring Plg can replace the more expensive SBA-15 mesoporous molecular sieve as a catalyst carrier.
A comparison of esterification of oleic acid with methanol over various catalysts
a Conversion of oleic acid
The SC-Plg-5 catalyst retained most of its initial catalytic performance and displayed an oleic acid conversion of 68.09% after the fifth cycle (Fig. 7). This is greater than that of sugarcane bagasse-derived sulfonated carbon (66.6%) (Flores et al., Reference Flores, Omega, Cabatingan, Go, Agapay and Ju2019). The results indicated that SC-Plg possessed excellent catalytic activity and stability. The S content in SC-Plg-5 decreased from 0.547 to 0.181 mmol/g after the fifth cycle, which suggests that the decreased catalytic activity of SC-Plg-5 was caused by the loss of -SO3H groups from the carbonaceous material during the reaction (Jiang et al., Reference Jiang, Xu, Duanmu, Gu and Chen2012).
Reusability of the SC-Plg-5 catalyst
Conclusions
SC-Plg was prepared conveniently via the simultaneous carbonization-sulfonation of a mixture of sucrose, TsOH, and Plg. Plg nanofibers were used mainly as the carriers of sulfonated carbon materials. The inexpensive Plg replaced sucrose-derived carbon and increased the surface area of the catalyst. The introduction of Plg did not affect significantly the catalytic activity of the SC catalyst. SC-Plg exhibited high catalytic performance and excellent stability despite lower -SO3H content when used for the esterification of oleic acid with methanol. Excess TsOH did not improve the catalytic activity of SC-Plg because the -SO3H groups were encapsulated in the carbonaceous material, and were not completely released in the subsequent reaction. Natural Plg nanofibers were economic and efficient carriers of carbon-based, solid-acid, composite catalysts.
ACKNOWLEDGMENTS
The authors are grateful to: the National Natural Science Foundation of China (Grants Nos 51574130, 21606098 and 51908240); the Six Talent Peaks Project in Jiangsu Province (2018-JNHB-009); the Natural Science Foundation of Jiangsu province (BK20181064); the Qing Lan project of Jiangsu Province, the Natural Science Key Project of Jiangsu Higher Education Institutions (18KJA430006 and 19KJA430015); the Foundation of the Key Laboratory for Palygorskite Science and Applied Technology of Jiangsu Province (HPZ202001); and the Open National & Local Joint Engineering Research Center for Mineral Salt Deep Utilization (SF201906).
Funding
Funding sources are as stated in the Acknowledgments.
Declarations
Conflict of Interest
The authors declare that they have no conflict of interest.
