Abstract
A carbon-based solid acid catalyst was prepared from papermaking sludge via calcining followed by sulfonation with p-toluenesulfonic acid. The micromorphology of several catalysts were compared via SEM, FT-IR, N2 adsorption-desorption, XPS, and UV spectrum. It was found that the micromorphologies of biochar surface modified with concentrated H2SO4, p-toluenesulfonic acid, and sulfanilic acid were different and closely related to the binding mechanism. The biochar and p-toluenesulfonic acid exhibited π-π* stacking and hydrophobic effects. The suitable pores and gaps on the biochar surface were the key to the loading of p-toluenesulfonic acid. However, no π-π* stacking and hydrophobic effects were observed between the biochar and sulfanilic acid. The amidation grafting of sulfanilic acid to the biochar surface could lift the restriction of surface pore structure. The catalytic performance of these catalysts was evaluated via fructose, glucose, and cellulose degradation to 5-hydroxymethylfurfural (HMF). The HMF yield from fructose, glucose, and cellulose was 92.9%, 60.7%, and 28.6%, respectively, with the carbon-based solid acid catalyst. The carbon-based solid acid catalyst prepared from papermaking sludge and p-toluenesulfonic acid has the following advantages: a simple process, is environmentally friendly, and has good catalytic performance; as such, it has the prospect of industrialization.
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Loading Behavior of p-Toluenesulfonic Acid in Biochar and its Application to Prepare 5-Hydroxymethylfurfural
Liyan Xing,a Rundong Liu,a Fanchen Jing,a Ming Xu,b and Jing He a,*
A carbon-based solid acid catalyst was prepared from papermaking sludge via calcining followed by sulfonation with p-toluenesulfonic acid. The micromorphology of several catalysts were compared via SEM, FT-IR, N2 adsorption-desorption, XPS, and UV spectrum. It was found that the micromorphologies of biochar surface modified with concentrated H2SO4, p-toluenesulfonic acid, and sulfanilic acid were different and closely related to the binding mechanism. The biochar and p-toluenesulfonic acid exhibited π-π* stacking and hydrophobic effects. The suitable pores and gaps on the biochar surface were the key to the loading of p-toluenesulfonic acid. However, no π-π* stacking and hydrophobic effects were observed between the biochar and sulfanilic acid. The amidation grafting of sulfanilic acid to the biochar surface could lift the restriction of surface pore structure. The catalytic performance of these catalysts was evaluated via fructose, glucose, and cellulose degradation to 5-hydroxymethylfurfural (HMF). The HMF yield from fructose, glucose, and cellulose was 92.9%, 60.7%, and 28.6%, respectively, with the carbon-based solid acid catalyst. The carbon-based solid acid catalyst prepared from papermaking sludge and p-toluenesulfonic acid has the following advantages: a simple process, is environmentally friendly, and has good catalytic performance; as such, it has the prospect of industrialization.
DOI: 10.15376/biores.17.2.2705-2726
Keywords: p-Toluenesulfonic acid; π-π* stacking; Sulfanilic acid; Micromorphology; 5-hydroxymethylfurfural
Contact information: a: Beijing Key Laboratory of Lignocellulosic Chemistry, Engineering Research Center of Forestry Biomass Materials and Bioenergy (Ministry of Education), Beijing Forestry University, Beijing 100083 China; b: Nongyuan Technology (Beijing) Co., Ltd., Beijing 100000 China;
* Corresponding author: hejing2008@sina.com
INTRODUCTION
Biomass is a renewable resource that has the potential to partially or completely replace petrochemical resources; thus it may contribute to alleviating the energy crisis and the greenhouse effect (Corma et al. 2007). In-depth development and utilization of biomass resources is the key to solving the problems of energy exhaustion and ecological degradation. The production of high value-added chemicals and materials from biomass, e.g., ethanol, 5-hydroxymethylfurfural (HMF), oligosaccharides, lignin derivatives, and cellulosic based products, has become a hot research topic (Wang et al. 2019; Zhang et al. 2020). 5-Hydroxymethylfurfural is considered to be an important platform compound that can produce various chemicals through oxidation, hydrogenation, and condensation reactions (Gandini and Belgacem 2002).
The synthesis of HMF from carbohydrates, such as fructose, glucose, and cellulose, has attracted much attention (Iris and Tsang 2017; Thoma et al. 2020). Fructose is the most widely studied raw material because it is easier to prepare HMF and can achieve high HMF yield at high concentration, which is very important for industrial production. Glucose and cellulose have lower costs, and cellulose, in particular, is available from a wide range of sources. The efficient preparation of HMF from cellulose not only can greatly reduce the production cost of HMF, but it can also effectively solve various agricultural residues. Therefore, it is very important to study the applicability of catalysts to fructose, glucose, and cellulose.
Carbon-based solid acids are commonly used as heterogeneous catalysts for the preparation of HMF, which are usually prepared from various biomasses, e.g., starch, glucose, straw, wood chips, and leaves (Han et al. 2017; Songo et al. 2019). The catalyst is prepared by activation pretreatment, incomplete carbonization under an inert atmosphere, sulfonation, and washing. Carbon-based solid acids have the advantages of a low cost, a wide range of raw materials, and easy recovery. The most commonly used sulfonation method is to treat biochar using concentrated sulfuric acid or fuming sulfuric acid at a high temperature (180 to 250 °C) for a long time (10 to 24 h). The -SO3H is then grafted onto the surface of the biochar. However, these methods have various problems, e.g., large wastewater pollution and discharge.
Researchers have attempted to incorporate sulfonic acid into biochar via other means to prepare carbon-based solid acids. p-Toluenesulfonic acid is organic strong acid, which can cause dehydration and carbonization of paper and wood. The sulfonic acid in p-toluenesulfonic acid has a good catalytic effect on the preparation of HMF. Shen (2016) prepared a carbon-based solid acid using waste bamboo powder as the raw material and p-toluenesulfonic acid as the sulfonating agent. The catalyst was used to catalyze fructose dehydration to prepare HMF. The HMF yield was 92.1%, which was better than the commercial ion exchange resin Amberlyst-15. Wang et al. (2016) prepared a new carbon-based solid acid using glucose and p-toluenesulfonic acid as the raw materials. The product was used to catalyze the dehydration of fructose to 5-hydroxymethylfurfural (HMF) and showed good catalytic performance and recycling performance. Sulfanilic acid also contains sulfonic acid, which are acidic and commonly used for modification of carbon materials (Jia et al. 2016; Zhao et al. 2018). Therefore, in the present work, sulfanilic acid and p-toluenesulfonic acid were used in an attempt to modify biochar for the preparation of HMF.
In addition, the surface structure and composition have a very important effects on the properties of a material. Many studies have shown that mesoporous materials are widely used in the field of catalysis because of their special surface aperture structure, which is conducive to modification by loading, doping, and other means, as well as having special effects on the adsorption and desorption of raw materials and products. Mesoporous materials, e.g., carbon-based solid acids (Priadi et al. 2014), zeolites, cationic resins (Wrigstedt et al. 2016; Sun et al. 2018), and mesoporous silicon (Mérida-Morales et al. 2021) are widely used as solid acid catalysts or carriers for biomass dehydration. Cao et al. (2018) prepared a carbon-based solid acid with a high surface area using forestry wood waste and concentrated sulfuric acid. The product was used to catalyze bread waste to HMF with a yield of 30.4%. Gallo et al. (2016) studied the activity and stability of acid functionalized periodic mesoporic carbons (CMK-3 and CMK-5) for continuous HMF production from fructose. The catalysts had the same selectivity and turnover frequency (TOF) of propanesulfonic acid functionalized SBA-15. These carbon materials exhibit a high specific surface area, suitable pore size and stability, good catalytic activity, and cyclic stability. However, there are few reports on the etching effect of various sulfonating agents on the biochar surface, especially the loading behavior of p-toluenesulfonic acid on the biochar surface.
In this paper, a carbon-based solid acid was prepared from papermaking sludge and modified with p-toluenesulfonic acid, concentrated sulfuric acid, or sulfanilic acid. The relationship between the micromorphology and the preparation mechanism of the catalysts was proposed. The effects of the reaction conditions on the HMF yield from fructose, glucose, and cellulose were studied. The water consumption and cycling performance of the catalyst were investigated, which provided theoretical support for industrialization.
Materials
The papermaking sludge, which was generated at Nong Yuan Technology (Beijing, China) Co., Ltd, was collected from the waste liquid of chemi-mechanical pulp and air dried. The sludge was stored in a sealed bag after smashing and sieving through a 0.25 mm mesh.
The D-fructose (99%), glucose (98%), cellulose, 5-hydroxymethylfurfural (97%), tetrahydrofuran (THF), and dimethylsulfoxide (DMSO) were purchased from Shanghai Macleans Biochemical Technology Co., Ltd. (Shanghai, China).
Catalyst Preparation
Preparation of sludge biochar (SBC)
The sludge biochar (SBC) was prepared using a slight modification of the methodology reported by Cao et al. (2018). The samples were heated to a temperature of 300 °C at 10 °C/min and then to a temperature of 550 °C at 5 °C/min in a tube furnace under a N2 atmosphere. The retention time at a temperature of 550 °C was 1 h.
Preparation of S-SBC, T-SBC, and A-SBC by biochar treated with concentrated sulfuric acid, p-toluenesulfonic acid, or sulfanilic acid
First, 2 g of biochar was mixed with 6 g of concentrated sulfuric acid, 6 g of p-toluenesulfonic acid, or 6 g of sulfanilic acid in a 25 mL hydrothermal kettle. The three samples were heated at a temperature of 200 °C for 6 h in a muffle furnace and then transferred to crucibles for aging at a temperature of 150 °C for 4 h.
After the reaction, the mixture was washed and filtered with a large amount of deionized water (at a temperature of 80 °C) until the filtrate was neutral and the water consumption was recorded. The samples were dried at a temperature of 105 °C for 4 h and stored in sealed bags. The three samples were labeled as S-SBC, T-SBC, and A-SBC, respectively.
Preparation of A-G-SBC by biochar grafted with sulfanilic acid
First, 6 g of biochar was added to 60 mL of a 20% hydrogen peroxide solution and stirred at room temperature for 24 h. The remaining solid was filtered and fully washed with deionized water. The oxidized biochar was obtained after being dried in an oven at a temperature of 105 °C for 4 h. Then, 2 g of oxidized biochar, 4 g of sulfanilic acid, and 20 mL of deionized water were mixed in a 100 mL three-mouth flask.
The mixture was thoroughly stirred at a temperature of 100 °C for 30 min, dried and cooled to room temperature. Next, 40 mL of hydrochloric acid solution (9.6 mol/L) was added into the remaining solid and reacted at a temperature of 100 °C for 45 min. The mixture was strained while it was hot, and the biochar (A-G-SBC) grafted via sulfanilic acid was obtained after being dried at a temperature of 105 °C for 4 h (Jia et al. 2016).
Catalyst Characterization
Fourier-transform infrared spectroscopy (FTIR) (Nicolet iS 5, Thermo Fisher Scientific, Waltham, MA) was performed using the KBr method. The sample was mixed with a KBr medium, and the experiments were carried out over a scanning range of 400 to 4000 cm-1.
The microstructure and morphology of the biochar were examined via cold field emission scanning electron microscopy (FE-SEM JSM -6700F, Jeol, Tokyo, Japan).
Powder X-ray diffraction (XRD) (X’Pert PRO MPD, Panalytical, Holland, Netherlands) was performed at a 0.02° step and 5° to 90° 2θ range in continuous scanning mode; the radiation source was Cu target and calculated using the Jade software (version 6, MDI, Livermore, CA).
The N2 adsorption-desorption experiment was recorded at a temperature of -196 °C using an ASAP 2020 Plus 1.03 specific surface and pore size analyzer. The biochar samples were degassed at a temperature of 150 °C for 8 h. The specific surface area, pore volume, and mean pore diameter were obtained using the Brunauer-Emmett-Teller (BET) method and the Barrett-Joyner-Halenda (BJH) method.
The surface chemical composition and the chemical state of the functional groups on the biochar samples were determined via X-ray photon spectroscopy (XPS) (EscaLab Xi+, Thermo Fisher Scientific, Waltham, MA).
The content of the metal ions was determined via inductively coupled plasma optical emission spectrometry (ICP-OES) (iCAP Q, Thermo Fisher Scientific, Waltham, MA). The samples were digested in 5 mL of HNO3 and 3 mL of hydrofluoric acid at a temperature of 180 °C.
The total acid sites and the -SO3H content on the biochar surfaces were tested using an acid-base titration experiment (Boehm 1994; Wang et al. 2011). The total number of acid sites was measured using the following method. Briefly, 250 mg of catalyst was dispersed in 30 mL of a sodium hydroxide solution (0.05 mol/L) under ultrasound for 60 min. The supernatant solution obtained via filtration was titrated with a hydrochloric acid standard solution (0.05 mol/L) using phenolphthalein as an indicator. The -SO3H content was measured using the following method. A 250 mg sample of the catalyst was dispersed in 30 mL of a standard sodium chloride solution (0.05 mol/L) under ultrasound for 60 min to replace the H+ ions in the catalyst. Phenolphthalein was then used as an indicator, and the supernatant solution obtained via filtration was titrated with a sodium hydroxide aqueous solution (0.05 mol/L).
To analyze the UV spectrum, 0.5, 1, and 2 g biochar were mixed with 2.33 g of concentrated sulfuric acid, 2.33 g of p-toluenesulfonic acid, and 2.33 g of sulfanilic acid, respectively, in a hydrothermal reactor and calcined at a temperature of 200 °C for 6 h in a muffle furnace. Then the three samples was transferred to a crucible at a temperature of 105 °C aged for 4 h. Then, 50 mL of deionized water was added and subjected to ultrasound for 30 min. The filtrate with fine biochar particles was obtained via filtering. A small amount of filtrate was taken for full UV spectrum scanning and labeled as A, B, and C, respectively.
Effects of the reaction temperature and time on the 5-hydroxymethylfurfural (HMF) yield
First, 100 mg of fructose, 100 mg of the catalyst, and 5 mL of a H2O/DMSO (V/V, 0.5 to 4.5) solution were mixed in a thick wall glass reactor, followed by being heated at the set temperature for a set time. The reactor was cooled in an ice bath after the reaction.
Effect of the modifier dosage on the 5-hydroxymethylfurfural (HMF) yield
First, 2 g of biochar was mixed with different amounts of p-toluenesulfonic acid (1.5, 3, 6, and 1 2g), concentrated sulfuric acid (6, 12, and 24 g), or sulfanilic acid (6, 12, and 24 g) in a hydrothermal kettle and calcined at a temperature of 200 °C for 6 h. Then the samples were transferred to crucibles for aging at a temperature of 150 °C for 4 h. The samples were washed with deionized water (at a temperature of 80 °C) until neutral and then dried at a temperature of 105 °C for 4 h.
The biochar catalysts were labeled as T-SBC1, T-SBC2, T-SBC, T-SBC3, S-SBC, S-SBC1, S-SBC2, A-SBC, A-SBC1, and A-SBC2 in sequence. The HMF was prepared via the dehydration of fructose under optimal reaction conditions, and the catalytic activity of each catalyst was determined.
Effects of the calcination temperature and time on the 5-hydroxymethylfurfural (HMF) yield
First, 2 g of biochar was mixed with 6 g of p-toluenesulfonic acid and 24 g of sulfanilic acid in a hydrothermal kettle and calcined at a set temperature for the set time. The samples were washed with deionized water (at a temperature of 80 °C) until neutral and then dried at a temperature of 105 °C for 4 h. The HMF was prepared via the dehydration of fructose under optimal reaction conditions, and the catalytic activity of each catalyst was determined.
Catalytic performance for glucose and cellulose
First, 125 mg of glucose or cellulose, 100 mg of catalyst, 10 mL of H2O/THF (1 to 4 ratio, v/v), and 0.875 g of NaCl were mixed in a high-pressure reactor, and then heated at a temperature of 160 °C for 3 h. The reactor was cooled in an ice bath after the reaction.
Products Analysis
The reaction products were filtered through a 0.22 µm organic needle filter, which was composed of polypropylene shell and Teflon filter membrane (Haiyan New Oriental Plasticizing Technology Co., LTD., Jiaxing, Zhejiang Province, China). The filtrate was diluted to a suitable concentration with deionized water. Quantitative analysis of the HMF was performed via high-performance liquid chromatography (HPLC) with a UV detector and a VYDAC 214TP54 column (C18, 5 μm, 250 mm × 4.6 mm). A water and methanol solution (80 to 20 ratio, v/v) was used as the mobile phase at a flow rate of 0.6 mL/min at a column temperature of 30 °C. The standard curve line of HMF was prepared using pure HMF as the standard.
The yield of HMF was calculated according to Eq. 1,
(1)
where the moles of the starting C6 carbohydrate were fructose, glucose, or cellulose.
RESULTS AND DISCUSSION
Catalyst Characterization
SEM images
Noticeable differences in the micromorphology were observed among these solid acid catalysts (Fig. 1). The SEM image of SBC clearly showed an irregular morphology with stratified channels, pores, and ridged surfaces (Cao et al. 2018). The structure was similar to the straw fiber structure but different from the planar structure of biochar derived from sugar (Bai et al. 2015; Zhao et al. 2016).
Fig. 1. SEM images of SBC (a); T-SBC (b); S-SBC (c); A-SBC (d); and A-G-SBC (e)
The ridge structure of T-SBC was destroyed in the treatment process, and a large number of homogeneous pore structures were formed. As p-toluenesulfonic acid is a strong acid, it had a considerable corrosive effect on the surface of the biochar. The large pores of T-SBC were formed because of the large molecular structure of p-toluenesulfonic acid. The unique structure showed a larger surface area and surface active center, which can considerably improve the catalytic performance. The surface of S-SBC presented a loose gap structure with many particle structures. Probably as the molecular structure of sulfuric acid was smaller than p-toluenesulfonic acid, it can form many small particles with similar diameters on the surface of the biochar. In addition, the distribution of particles was uniform, regular, and orderly. The surface structure of A-SBC was similar to the surface structure of S-SBC. There were small particles on the surface, but no pores and gaps were formed, which may be due to the weak corrosive effect of sulfanilic acid on biochar. The surface of A-G-SBC simultaneously had small particles and pores of different sizes, which was the result of the interaction of the hydrochloric acid and sulfanilic acid.
FTIR spectra of the catalysts
In order to study the introduction of -SO3H in various catalysts, the types and structures of the functional groups of biochar were studied via the FTIR spectra (as shown in Fig. 2). The bands at 1600 cm-1 were assigned to the stretching vibration of the polycyclic aromatic hydrocarbon skeleton (Chen and Fang 2011; Xu et al. 2015). The band appearing at 799 cm-1 for the S-SBC, T-SBC, A-SBC, and A-G-SBC samples was assigned to the absorption peak of the substituent of the benzene ring, which indicated that the -SO3H groups were attached to the surface of the biochar or grafted onto the benzene ring of the biochar.
Fig. 2. The FTIR spectra of SBC, T-SBC, S-SBC, A-SBC, and A-G-SBC
The bands at 1100 cm-1 and 464 cm-1 of the T-SBC, S-SBC, A-SBC, and A-G-SBC samples represented the stretching vibration and rocking vibration of Si-O-Si from ash, which were sharper than the bands of SBC (Kim et al. 2010). The sharper peak of Si-O-Si in the S-SBC, T-SBC, A-SBC, and A-G-SBC samples was due to the dissolving of inorganic salt during sulfonation. The band at 1700 cm-1 was assigned to the S=O stretching vibration (Toda et al. 2005; Songo et al. 2019). There was no peak for SBC at 1700 cm-1 and the peak intensity of the remaining catalysts at approximately 1700 cm-1 were ranked as follows: T-SBC was greater than S-SBC, which was approximately equal to A-G-SBC, which was greater than A-SBC. This result indicated that the -SO3H group was successfully introduced to the surface of the T-SBC, S-SBC, and A-G-SBC samples, while that of A-SBC may be insufficient. The new band at 1150 cm-1 for the T-SBC, S-SBC, A-SBC, and A-G-SBC samples was assigned to the O=S=O anti-symmetric stretching vibration, which showed the same trend of peak at 1700 cm-1 (Hara et al. 2004). The peaks at 1150 cm-1 and 1700 cm-1 of T-SBC, S-SBC, A-SBC, and A-G-SBC also represent the successful introduction of -SO3H.
Textural properties of the catalysts
The specific surface area and total pore volume of a catalyst are important parameters of heterogeneous catalysis. The textural properties of various catalysts are shown in Table 1. The surface area of SBC was very small, only 0.2 m2·g-1, while the surface area and total pore volume of T-SBC, S-SBC, A-SBC, and A-G-SBC were considerably larger, which indicated that the new pores or gaps were generated after modification. The surface area of T-SBC, S-SBC, and A-G-SBC increased to more than 80 m2·g-1, while the surface area of A-SBC was only 46.1 m2·g-1. The micropores of S-SBC, A-G-SBC, and A-SBC accounted for 44.4%, 32.0%, and 59.2% of the pore volume, respectively, while they only accounted for 2.1% for T-SBC. According to the adsorption and desorption isotherms in Fig. 3 (small), all catalysts displayed a Type IV isotherm, characteristic of mesoporous materials (Thommes et al. 2015).
Fig. 3. Pore size distribution (large) and N2 adsorption-desorption isotherms (small) of T-SBC, S-SBC, A-SBC, and A-G-SBC
Most of the mesoporous aperture of S-SBC, A-G-SBC, and A-SBC was in the range of 2 to 10 nm, while that of T-SBC was 10 to 50 nm (Fig. 3-large). Both S-SBC and A-SBC were prepared by calcination and had a similar aperture structure, while -SO3H can be successfully introduced to S-SBC but was insufficient in A-SBC, according to the FTIR results. Therefore, it was speculated that sulfuric acid molecules could be adsorbed into micropores and small mesoporous (5 to 10 nm) of biochar and combined well with it. However, the amount of sulfanilic acid may be insufficiently adsorbed into the micropores and small mesopores (5 to 10 nm), resulting in insufficient introduction of -SO3H into A-SBC.
The preparation method of T-SBC was the same as that of S-SBC and A-SBC but had a larger mean pore size. The peak intensity of T-SBC was much larger than that of A-SBC according to the FTIR. Suitable pore sizes may be favorable for the loading of p-toluenesulfonic acid and was bound closely with biochar via some kind of binding force.
A G-SBC was prepared by amidation but had a similar aperture structure with S-SBC and A-SBC. The peak intensity of A-G-SBC was approximately equal to S-SBC and much larger than A-SBC. The -SO3H grafted onto the surface of biochar by amidation may remove the limitation of surface pore size and successfully introduce -SO3H to A-G-SBC.
Table 1. Textural Properties of the T-SBC, SBC, S-SBC, A-SBC, and A-G-SBC Samples
The analysis of the UV spectrum
Biochar modified with concentrated sulfuric acid and grafted with sulfanilic acid has been extensively studied, and the reaction mechanism is relatively clear. The mechanism of S-SBC is as follows: the residual H atoms in the aromatic ring are replaced by -SO3H. The mechanism of A-G-SBC is as follows: the amino group in the sulfanilic acid was aminated with carboxyl on the surface of biochar, and then grafted onto the surface of the biochar. However, the mechanism of biochar treated with p-toluenesulfonic acid is still unclear. In order to study the interaction between p-toluenesulfonic acid and biochar, the samples were tested via UV spectrum.
The electron-supplying capacity of biochar is derived from the phenolic hydroxyl structure of biochar, especially biochar prepared via cryogenic pyrolysis, which has a higher phenolic hydroxyl content. The -SO3H in p-toluenesulfonic acid and sulfanilic acid is a strong electron-withdrawing group, which causes the π electron from the benzene ring to transfer to -SO3H. Typically, π-π* stacking occurs between electron-rich and electron-poor aromatic compounds. Therefore, π-π* stacking takes place between the electron-rich biochar aromatic ring and the electron-poor benzene ring of p-toluenesulfonic acid or sulfanilic acid. Biochar contains a large number of aromatic ring structures that may increase the force of π-π* stacking. The π-π* stacking structures are shown in Fig. 4 (Peng 2013; Pang and Wang 2014).
Fig. 4. The π-π* stacking structure: S: Sandwich stack; PD: parallel stacking; T: T-type stacking; and Y: Y type stacking
Figure 5 shows the UV spectra of the suspensions of biochar and p-toluenesulfonic acid or sulfanilic acid. The peaks near 260 and 220 nm are the K-band and R-band generated by the π-π* electron transition of p-toluenesulfonic acid and sulfanilic acid, respectively. The maximum absorption wavelength of the K-band near 220 nm of p-toluenesulfonic acid was blue shifted as the addition of biochar increased (as shown in Fig 5a). The peak intensity at approximately 220 nm and 260 nm decreased perhaps because the biochar and p-toluenesulfonic acid experienced the force of π-π* stacking. The steric hindrances of the p-toluenesulfonic acid molecules increased due to the aromatic ring structure of the biochar particles, and the shape and distribution of the π electron cloud changed, which decreased the electron mobility and conjugation effect. Thus, the peak intensity decreased. However, no obvious blue shift and reduced peak intensity were observed in the UV spectra of the suspension of biochar and sulfanilic acid (as shown in Fig. 5b); therefore, no π-π* stacking occurred between the biochar and sulfanilic acid. It was speculated that most of it was present in the form of an inner salt and does not form suitable sites for the loading of sulfanilic acid, resulting in sulfanilic acid not binding to biochar in large quantities.
Fig. 5. The UV-Vis spectrum of p-toluenesulfonic acid and biochar filtrate: A-0.5: 0.5 g of biochar and 2.33 g of benzenesulfonic acid; B-1: 1 g of biochar and 2.33 g of benzenesulfonic acid; and C-2: 2 g of biochar and 2.33 g of benzenesulfonic acid
Table 2. Surface Composition (wt%) and Acidity of the Biochar
Fig. 6. The chemical state of sulfur in the SBC, S-SBC, T-SBC, A-SBC, and A-G-SBC samples
Surface composition and chemistry of SBC, S-SBC, T-SBC, A-SBC, and A-G-SBC
In order to further explore the introduction mechanism of -SO3H, the surface composition and chemical state of sulfur in the biochar were analyzed via high-resolution XPS. The content of sulfur in SBC was 0.29%, while it increased in T-SBC, S-SBC, A-SBC, and A-G-SBC, confirming the incorporation of sulfur. In addition, T-SBC was found to have the highest sulfur content, which was greater A-G-SBC, which was greater S-SBC and A-SBC. As shown in Fig. 5, in the detailed S2p track scan of SBC, the strong peak of 163.7 eV indicated that the sulfur in SBC primarily was present in the form of -SH. The peak at 164 eV for S-SBC, T-SBC, A-SBC, and A-G-SBC was weakened, while the peak at 168.7 eV and 169.2 eV increased, which indicated that the sulfur elements in S-SBC, T-SBC, A-SBC, and A-G-SBC primarily were present in the form of -SO3H. The introduction of sulfur in the sulfonation process was further confirmed.
The acid-base titration experiments of the S-SBC, T-SBC, A-SBC, and A-G-SBC samples
Acid-base titration experiments can further illustrate the introduction of -SO3H. Because the SBC was alkaline, no acid base titration was performed. According to the acid-base titration results, the total acidity of T-SBC was 3.11 mmol/g, which was slightly lower than the total acidity of A-G-SBC, and much higher than the total acidity of S-SBC and A-SBC (as shown in Table 2). The total acidity of A-G-SBC was greater than the total acidity of T-SBC because A-G-SBC contained more oxygen-containing functional groups due to H2O2 oxidization. In addition, the hydrochloric acid residue in A-G-SBC may be another reason for the increase in the total acidity. However, the acidity of -SO3H in T-SBC was 0.92 mmol/g, which was higher than the acidity in A-G-SBC (0.8 mmol/g). This indicated that the -SO3H introduced via calcination of p-toluenesulfonic acid and biochar was more than the amount introduced via grafting. The total acidity and -SO3H acidity of A-SBC were the lowest, which may be due to the weak acidity and high melting point of sulfanilic acid. Sulfanilic acid reacted with biochar in the solid form because the calcination temperature did not reach the melting point of sulfanilic acid. T-SBC, S-SBC and A-SBC had similar preparation methods. A-SBC had a similar surface pore structure with S-SBC but lower amount of -SO3H, and T-SBC had larger mean pore size but higher amount of -SO3H than A-SBC. Therefore, it was speculated that the introduction of -SO3H may be related to the surface pore structure.
Catalytic Performance
Effects of the reaction temperature and time on the 5-hydroxymethylfurfural (HMF) yield
As shown in Fig. 7a, T-SBC had the best HMF yield of 92.9% at a temperature of 140 ℃ for 80 min. The S-SBC sample had the second-best yield, with a highest HMF yield of 56.3% (Fig. 7b). However, A-SBC had almost no catalytic activity (Fig. 7c). The results were consistent with the characterization of catalysts. The high catalytic activity of T-SBC was related to its micromorphology. On the one hand, it may be that the larger mesopore (10 to 50 nm) content was conducive to the loading of p-toluenesulfonic acid. On the other hand, macroporous and mesoporous pores are favorable for the adsorption and desorption of reactants and products. There were mainly micropores on the surface of A-SBC, which was not conducive for sulfanilic acid to enter the micropores and combine with the surface. When sulfanilic acid was grafted onto the surface of biochar via amidation grafting, it was no longer limited by the surface pores, and the catalytic activity was greatly increased (Table 3). The best HMF yield of A-G-SBC was 95.2% at a temperature of 130 ℃ for 40 min.
Fig. 7. Change in the HMF yield with the reaction temperature and time with various catalysts, i.e., fructose (100mg), catalyst (100 mg), and H2O to DMSO (0.5 to 4.5, v/v).
Table 3. Surface Morphology and 5-hydroxymethylfurfural (HMF) Yield of Various Catalysts