Synthesis of Sulfonated Carbon from Discarded Masks for Effective Production of 5-Hydroxymethylfurfural

Abstract

5-hydroxymethylfurfural (HMF), as one of the top ten important platform chemicals, can be used to produce 2,5-furandicarboxylic acid (FDCA), 2,5-dimethyl furan (DMF), levulinic acid, and other chemicals. An environmentally friendly system for the synthesis of sulfonated carbon materials from discarded masks has been proposed. A series of mask-based solid acid catalysts (bMC-SO₃H) were prepared by a simple two-step process. Mechanochemical pretreatment (ball milling) of waste mask and sulfonated group precursor, followed by thermal carbonization under nitrogen gas, were used to synthesize sulfonated porous carbon. The total acid amount of the prepared bMC-SO₃H was measured by the Boehm method, which exhibited 1.2–5.3 mmol/g. The addition of the sulfonated group precursor in the mechanochemical treatment (ball milling) process caused intense structure fragmentation of the discarded masks. These sulfonated porous carbons (bMC(600)-SO₃H) as solid acid catalysts achieved fructose conversion of 100% and HMF yield of 82.1% after 120 min at 95 °C in 1-butyl-3-methylimidazolium chloride. The bMC-SO₃H could be reused five times, during which both the HMF yield and fructose conversion were stable. This work provides a strategy for the synthesis of sulfonated carbon from discarded masks and efficient catalyzed fructose upgrading to HMF.

1. Introduction

Because of the rapidly increasing consumption of petrochemicals, the reduction of carbon emissions and the search for sustainable alternative fuels have attracted attention [1]. As the only renewable carbon resource, the biomass has been converted into various chemical products by acid catalytic conversion of the biomass, such as 5-hydroxymethylfurfural (HMF) [2,3], levulinic acid [4], ethyl levulinate [4,5], and 5-ethoxymethylfurfural [6]. As one of the most important biomass-derived chemicals, HMF is a typical acid catalytic dehydration product of hexose, which can be used as a key platform compound, and compounds derived from HMF can be used as solvents and clean fuels [2,6,7]. Because of its wide applicability, it has been identified as one of the top 10 platform molecules in biorefineries of value-added chemicals and liquid transportation fuels [8]. Various acid catalyst processes have been developed for the synthesis of HMF, including homogenous acid catalysts (e.g., HCl, H₂SO₄, and NH₄Cl) [9,10,11] and heterogenous acid catalysts (e.g., S-doped porous carbon, Amberlyst-15, and Amberlyst-70) [3,12,13,14,15]. Compared to the homogeneous acid catalysts, heterogenous acid catalysts such as ionic exchange resins are recyclable, but the solid acid catalysts are normally limited to high reaction temperatures and long reaction times [16,17].

Functional carbon materials, as a promising low-cost heterogeneous acid catalyst, have been used in diverse biomass transformation processes, including hydrolysis [18], dehydration [16,19], etherification, and hydrogenation reactions [5]. Various functional carbon materials have been prepared by hydrothermal carbonization, pyrolysis carbonization, and chemical carbonization with biomass-derived materials [19,20,21], sewage sludge [22], domestic garbage [23,24,25], or polymer materials (e.g., polypropylene, waste plastics) as carbon precursors [26]. Because the COVID-19 epidemic has spread all over the world in the last three years, the production and usage of face masks have increased exponentially [27,28]. Last year, at least 1.07 billion masks were discarded, worth USD 10.76 billion [29]. The major ingredient of disposed masks is polypropylene, which is difficult to depolymerize and poses a persistent hazard to the environment [27]. Until now, most of the disposable masks have not been recycled [30], and it takes more than 400 years for the polypropylene to decompose, which results in environmental pollution [31,32]. Carbonizing waste polypropylene-based materials (e.g., disposable masks) is one of the most promising recycling and reuse methods [33]. Chao et al. [30] used microwave-driven disposable medical masks with concentrated sulfuric acid and then self-activation pyrolysis at 900 °C to create high-surface-area porous carbon materials. Yu et al., reported the synthesis of nickel-doped carbon nanomaterials from the waste mask by catalytic carbonization at 700 °C [34]. However, compared with the other works related to carbon catalysts from biomass, the direct synthesis of carbon materials from polypropylene-based materials is still challenging, and their applications are rarely reported [34], because the polypropylene-based materials are difficult to form carbon [35].

In this work, we report an effective process for preparing porous sulfonated carbon from discarded masks by thermal carbonization in nitrogen gas that had been ball milled with 5-sulfosalicylic acid. The catalytic activity of the prepared sulfonated carbon materials was evaluated for the synthesis of HMF from fructose in an ionic liquid solvent. To investigate the role of 5-sulfosalicylic acid and mechanochemical pretreatment in providing a new way to utilize discarded masks in the preparation of a solid acid catalyst, scanning electron microscopy (SEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy, and thermogravimetric analysis were used.

2. Results and Discussion

2.1. Catalyst Structural Analysis

The micromorphology of fresh mask, ball-milling treated mask (bM), mixed ball-milling treated mask (mbM), mask-based functional carbon (bMC), and sulfonated mask-based functional carbon (bMC-SO₃H) were characterized by SEM. When compared to a fresh mask (Figure 1a), the mask after ball milling had a more fibrous structure that slightly broke to produce debris (Figure 1b). As observed in Figure 1b,c, after ball-milling treatment of the mask, the fiber structure of the mask was broken. The SEM images of bMC(600) have porous, lumpy, irregular structures due to carbonization during the carbon formation process (Figure 1d), while bMC(600)-SO₃H gives rise to nanosphere carbon (Figure 1e). To further determine the porous structure and surface area of sulfonated mask-based functional carbon at 600 °C, nitrogen adsorption-desorption isotherm measurements were performed, and the results are shown in Figure 1f. The adsorption-desorption isotherm can be classified as a type IV isotherm with an H3 hysteresis loop (Figure 1f). This result reveals that the bMC(600)-SO₃H shows a typical mesoporous structure. The surface area of bMC(600)-SO₃H calculated by Brunauer–Emmett–Teller (BET) is 192.6 m²/g. The Barrett–Joyner–Halenda (BJH) pore size distributions calculated from the desorption data reveal that the average pore diameters are 9.6241 nm. TG-DTA results for fresh mask, ball-milling treated mask (bM), and mixed ball-milling treated mask (mbM) show that bM underwent an irreversible transformation at temperatures above ~430 °C under nitrogen conditions (Figure 2a). The polymer chain of polypropylene is gradually broken at about 430 °C. The mbM sample weight loss began below 100 °C, and the major weight loss occurred at temperatures higher than ~420 °C, suggesting that the ball-milling-treated mask is more thermally stable than the mixed ball-milling treated mask (mbM) (Figure 2a). TG-DTA results for bMC(600) and bMC(600)-SO₃H show that mixed ball-milling-treatment mask carbonization at 600 °C performs well in terms of thermal stability (Figure 2b). The Raman spectra of bMC(600) and bMC(600)-SO₃H show two characteristic peaks of carbon: The G band resulting from sp2 graphic nature (~1590 cm⁻¹) and the D band from sp3 disordered carbon (~1360 cm⁻¹) [20,36]. The intensity ratios of these two peaks (I_D/I_G) of the sulfonated carbon are given in Figure 2c. The I_D/I_G values of bMC(600) material (0.93) were higher than those of bMC(600)-SO₃H material (0.82), indicating that sulfonation facilitates the formation of defects (Figure 2c). It is indicated that the sulfonated carbon material (bMC(600)-SO₃H) has the higher degree of the graphitization.

In the XRD patterns, the broad peaks of the carbon at 10–30° are attributed to the graphite (002) plane [37,38]. Compared to the mask-based functional carbon (bMC), the diffraction peaks of sulfonated mask-based functional carbon (bMC-SO₃H) at 19.8° and 22.2° disappear (Figure 2d) due to the sulfonated step that promotes carbonization [39]. The FT-IR spectra were used to verify the surface functional group (Figure 2e). FT-IR spectra of bMC(600) and bMC(600)-SO₃H exhibit —OH, C=C, and C—O bands at around 3400 cm⁻¹, 1600 cm⁻¹, and 1400 cm⁻¹, respectively [14]. Compared to bMC(600), new peaks at 1220 cm⁻¹ and 1040 cm⁻¹ can be observed for bMC(600)-SO₃H, which should be attributed to —SO₃H and O=S=O symmetric stretching, respectively (Figure 2e) [14]. Ball milling and pyrolysis carbonization were used to introduce sulfonated groups into bMC(600)-SO₃H.

The surface elemental composition of the as-prepared materials was analyzed by XPS (Figure 3). XPS spectra of the as-prepared materials show C 1s and S 2p signal peaks. The C 1s spectra of the bMC(600)-SO₃H at around 286.7, 284.6, and 288.9 eV are attributed to C—OH, C—C, and COOH, respectively (Figure 3a) [31]. The C 1s spectrum of bMC(400)-SO₃H (Figure 3b) can be resolved into two carbon species at 286.9 eV and 284.6 eV, being associated with C—O and C=C/C—C groups, respectively [31,40]. In the S 2p spectra of bMC(600)-SO₃H (Figure 3c), the peaks at 163.9 eV and 165.0 eV are attributed to S—C, 165.8 eV and 167.0 eV are attributed to S=O, and 168.3 eV and 169.5 eV are attributed to -SO₃H and sulfate [30], respectively, which are in good accordance with the FT-IR analysis results and suggest that the sulfonated groups were successfully introduced in the carbon. The S 2p of the bMC(400)-SO₃H (Figure 3d) was deconvoluted into a single peak and assigned to the sulfonated group. The mass content of S for bMC(400)-SO₃H and bMC(600)-SO₃H were 0.07 and 0.24 mmol/g, respectively. It is suggested that increasing the pyrolysis temperature from 400 to 600 °C, the S mass content of the carbon material (bMC(600)-SO₃H) increased by more than three times. Moreover, the elemental constants were determined by the elemental analyzer. The atomic O/C ratio of bMC(400)-SO₃H and bMC(600)-SO₃H were 0.03 and 0.19, respectively. The H/C atomic ratios of bMC(600)-SO3H (H/C = 0.04) were lower than those of bMC(400)-SO₃H (H/C = 0.17). It is suggested that the disposed masks were difficult to carbonize at 400 °C.

2.2. Catalytic Performance

The catalytic conversion of fructose by the as-prepared, discarded, masks-derived sulfonated carbon catalyst was performed. A low HMF yield of 6.2% with 20.9% fructose conversion was obtained at 120 °C after 1 h without adding solid carbon catalyst (Entry 1,

Table 1. Conversion of fructose into 5-hydroxymethylfurfural (HMF) with different acid catalysts.

Entry	Catalyst	Acidity (mmol/g)	Conv. (%)	HMF yield (%)	Select. (%)	TOF (h−1)	Refs.
1	-	-	20.9	6.2	29.7	-	*
2	bMC(600)	1.2	22.0	8.2	37.3	2.03	*
3	bMC(400)-SO3H	3.4	40.5	25.3	62.5	1.32	*
4	bMC(600)-SO3H	4.2	90.1	79.1	87.8	2.38	*
5 a	bMC(600)-SO3H	4.2	53.2	30.1	56.6	1.76	*
6 b	Amberlyst-15	4.7	100	82.2	82.2	-	[43]
7 c	Amberlytst-70	2.55 (meq/g)	80.1	45.6	56.9	-	[15]
8 d	SBA-SO3H	0.89 (H+ loading)	96	55	57.3	-	[42]
9 e	CS-SO3H	7.9	~85	~58	~62	-	[12]

* This work, Reaction conditions: 0.1 g of fructose, 0.05 g of catalyst, 1 g of solvent, 95 °C, 1 h reaction time. ^a: 0.1 g of fructose, 0.04 g of catalyst, 1 g of solvent, 95 °C, 1 h reaction time. ^b: 0.05g of fructose, 1 g of [BMIM]Cl, preheated at 80 °C, and microwave heating at 120 °C. ^c: 10 wt% fructose in water, substrate/catalyst ratio is 80, 180 ℃ for 20 min. ^d: 44 wt% fructose and 16 wt% solid acid catalyst in water (0.5 mL), 160 °C for 70 min, 1.5 mL of methyl isobutyl ketone (MIBK)/2-butanol (70/30). ^e: 500 mg of fructose, 140 °C for 30 min, 100 mg of cellulose-derived hydrothermal carbon, 10 mL dimethyl sulfoxide (DMSO) as solvent.

). When bMC(600) was used, it gave only a fructose conversion of 22% and a low HMF yield of 8.2% (Entry 2, Table 1). The sulfonated carbon bMC(400)-SO₃H improved the fructose conversion and HMF yield to 40.5% and 25.3%, respectively. The fructose conversion and HMF yields were significantly improved by bMC(600)-SO₃H, reaching 90.1% and 79.1%, respectively. The acidity amounts of the functional carbons are given in Table 1. The sulfonated functional carbons bMC(400)-SO₃H and bMC(600)-SO₃H had significantly higher values than bMC(600). The bMC(600)-SO₃H sample had the highest acidity of 4.2 mmol/g (Table 1). It is suggested that the increase in pyrolysis temperature from 400 to 600 °C leads to an increase in the total acidity of the as-prepared, discarded, mask-derived sulfonated carbon (Table 1). Furthermore, the same acid site amount of bMC(600)-SO₃H and bMC(400)-SO₃H catalysts resulted in comparable catalytic activity in fructose dehydration into HMF (Entries 3 and 5, Table 1). The bMC(600)-SO₃H sample exhibits the highest TOF value, whereas functional carbon bMC(600) and bMC(600)-SO₃H give higher TOF values than the bMC(400)-SO₃H sample (Table 1). As a comparison, the fructose conversion was 100% with an 82.2% yield of HMF over the Amberlyst-15 catalyst in ionic liquid [BMIM]Cl (Entry 6, Table 1) [41]. It is suggested that the as-prepared carbon material (bMC(600)-SO₃H) was favorable for HMF production. It is possible due to the oxygen-containing groups (—COOH, OH) in the as-prepared carbon material (bMC(600)-SO₃H) promote the formation of HMF. Compared with many kinds of sulfonated solid catalysts in the previous reports [12,42], this work displayed comparable HMF yield at an appropriate temperature. The efficient production of HMF requires maximization of HMF selectivity and minimization of the formation of unwanted side products such as formic acid, levulinic acid, and humin [12]. The HMF selectivity for this system with bMC(600)-SO₃H as the catalyst was as high as 87.8%, which is much higher than that obtained over the other acidic catalytic reaction systems (57.3–73%) [10,13,41,42].

2.3. Effect of Reaction Temperature and Time

The effect of reaction temperature (75–115 °C) and time (10–120 min) on the reaction was studied. The conversion of fructose was only 22.5% at 75 °C for 90 min (Figure 4b). The fructose conversion reached 100% at 95 °C for the same reaction time. At 115 °C, the fructose conversion reached 100% after 60 min of reaction time. The HMF yield kept increasing along with the reaction time at 75 and 95 °C. When the reaction temperature was 115 °C, the fructose conversion quickly reached 95% with a 78.4% HMF yield within 30 min. With further prolonging of the reaction time, the HMF yield decreased to 62.5% after 120 min reaction time. The low catalytic performance of bMC(600)-SO₃H at 75 °C can be attributed to the high viscosity of [BMIM]Cl, which increases mass transfer and limits fructose dehydration. The decrease in the HMF yields suggested that the HMF was unstable under these reaction systems at high temperatures and long reaction times, which resulted in the further conversion of HMF into side-products such as levulinic acid, formic acid, and humin [3,43]. In the case of the base catalyst of fructose at 95 or 115 °C, a range of 89–95% yields of liquid products (HMF, levulinic acid, and formic acid) was obtained after 30 min of reaction time, with 5–11% yields of solid products (humins).

2.4. Kinetic Modeling Studies

The reaction kinetics of the fructose dehydration catalyzed by bMC(600)-SO₃H were studied, and the resulting experimental data was fitted with a pseudo—first—order kinetic equation (Figure 5), and the obtained kinetic parameters are given in Figure 5a. The following assumptions were determined:

(1)

All reactions are irreversible;

(2)

The main reaction is fructose to 5-HMF, which ignores other possible reactions;

(3)

All unidentified products are considered degradation products (humin);

(4)

All other intermediates had negligible concentrations.

The rate constants of fructose dehydration to HMF increased from 0.0021 to 0.1745 when the temperature was increased from 348 to 388 k. The calculated activation energy for fructose dehydration in ionic liquid with the as-prepared catalyst bMC(600)-SO₃H was calculated to be 143.7 KJ/mol (Figure 5b), which is in agreement with other previous reports for fructose dehydration [9,44,45].

2.5. Catalyst Recycling

The stability of the discarded masks-derived sulfonated carbon for the conversion of fructose to HMF was studied with bMC(600)-SO₃H as catalyst at 95 °C for 2 h (Figure 6). After each run, the used bMC(600)-SO₃H was recovered by filtration, repeatedly washed with ethanol and water, and dried at 60 °C for 12 h before its next use. The recycled catalyst was used for the next run under the same reaction conditions. As shown in Figure 6, the fructose was completely converted for all runs. The HMF yields showed only a negligible decrease during the reuse experiments, and 79.8% of the HMF yield remained after five cycles. As shown in Figure 7, the XPS spectra of the recycled catalysts of bMC(600)-SO₃H revealed the composition of C 1s (Figure 7a) and S 2p (Figure 7b) peaks. The binding energy of sulfate shifted to a low energy, which was possible due to the interaction between the sulfate groups of the catalyst and humins. It is suggested that the as-prepared catalyst, bMC(600)-SO₃H, has stability and recyclability.

3. Material and Experimental

3.1. Materials

The following compounds: 5-Sulfosalicylic acid dihydrate (5-SA, 99%); sodium bicarbonate (NaHCO₃, 99.8%); sodium carbonate anhydrous (Na₂CO₃, 99.5%); sodium hydroxide (NaOH, 98%); and sodium ethylate (C₂H₅ONa, 98%) were purchased from Aladdin (Shanghai, China). The masks were collected from daily life and bought from Beijing Kuzhikang Technology Co., Ltd. (Beijing, China), H₂O₂ (30%) was provided by Tianjin Fengchuan Chemical Reagent Co., Ltd. (Tianjin, China), and methanol (99.5%) and D-fructose (99%) were purchased from Macklin (Shanghai, China). The 5-hydroxymethylfurfural (99%) and solvent 1-butyl-3-methylimidazolium chloride ([BMIM]Cl, 98.0%) were supplied by Sigma-Aldrich (Shanghai, China). All chemicals were used as received without further treatment.

3.2. Catalyst Preparation and Characterization

The discarded masks and 5-SA were mixed at a mass ratio of 1:1 and mechanochemically pretreated by ball milling (PM100, Retsch, Germany) for 30 min at 500 rpm at room temperature as mbM. Then mbM was carbonized at 400 or 600 °C for 2 h at a muffle surface (OTF-1200X, HF-Kejing, China) under nitrogen conditions with a heating rate of 10 °C/min. bMC(400) and bMC(600) were obtained depending on the carbonization temperature. Then 1 g of carbonized material was oxidized in 5 mL of methanol with 25 mL of H₂O₂ aqueous solution at 500 rpm at room temperature. After washing with deionized water and drying at 60 °C for 12 h, the sulfonated functional carbons derived from discarded masks were obtained and denoted as bMC(400)-SO₃H and bMC(600)-SO₃H, respectively, depending on the calcination temperature. The control samples MC(400) and MC(600) were synthesized based on the synthetic method of bMC but in the absence of the acid precursor (5-SA). The bM was ball milled without 5-SA.

The characterization of the prepared sulfonated mask-derived functional carbons was performed by different methods. On a Bruker D8 ADVANCE (Bruker, Germany), X-ray diffraction (XRD) was used to analyze the crystallographic structure of catalysts (at 40 kV and 200 mA). Raman spectra analysis was conducted on a micro-Raman spectrometer (HORIB Scientific LabRAM HR Evolution).

The morphologies of the prepared functional carbons were observed by scanning electron microscopy (SEM, SU8010, HITACHI, Japan). Surface element analysis was performed with HRTEM using energy dispersive X-ray spectroscopy (EDS, Regulus 8100, HITACHI, Japan). The specific surface area was calculated by the Brunauer–Emmett–Teller (BET) method based on the measurement of the nitrogen adsorption-desorption isotherm on a BET analyzer (Micromeritics APSP 2460) at 77 K. The thermogravimetry and differential thermal analysis (TG-DTA) curves were obtained using a thermal analysis system (Rigaku TG-DTA8122).

The Fourier-transform infrared spectra in the region ranging from 4000 to 400 cm⁻¹ were obtained on an infrared spectrometer (Thermo Scientific Nicolet iS20). The XPS spectra were obtained to determine the surface elemental composition by an X-ray photoelectron spectrometer (Escalab 250xi, Thermo Fisher). The elemental constants were performed by the elemental analyzer variant EL Cube (Germany). The elements (S) in the as-prepared masks-derived sulfonated carbon were determined using an elemental analyzer (Elementar Vario EL cube (Germany)). The acidity amount of functional groups on a masks-derived sulfonated carbon surface was determined using the Boehm method [46].

3.3. Catalytic Evaluation and Product Analysis

Reaction experiments were performed in a 10 mL Teflon-lined stainless-steel reactor with a magnetic stirrer, containing 0.1 g of fructose, 0.05 g of catalyst, and 2 mL of solvent. The reaction temperature was controlled by a preheated oil bath. After the reaction, the solution was centrifuged, filtered, and analyzed by ultraperformance liquid chromatography (Waters Acquity UPLC) with a refractive index detector (RI) and a Shodex SH1011 sugar column. Fructose conversion, HMF yield, and selectivity were calculated as follows:

$$\mathrm{HMF} \mathrm{yield} \left(\%\right)=\frac{\mathrm{moles} \mathrm{of} \mathrm{HMF} \mathrm{produced}}{\mathrm{moles} \mathrm{of} \mathrm{initial} \mathrm{fructose}}\times 100\%,$$

(1)

$$\mathrm{Fructose} \mathrm{conversion} \left(\%\right)=\frac{\mathrm{moles} \mathrm{of} \mathrm{fructose} \mathrm{reacted}}{\mathrm{moles} \mathrm{of} \mathrm{initial} \mathrm{fructose}}\times 100\%,$$

(2)

$$\mathrm{Selectivity} \left(\%\right)=\frac{\mathrm{moles} \mathrm{of} \mathrm{HMF} \mathrm{produced}}{\mathrm{moles} \mathrm{of} \mathrm{fructose} \mathrm{reacted}}\times 100\%.$$

(3)

3.4. Kinetic Studies

The reaction rate constant and activation energies of fructose dehydration to HMF catalyzed by bMC(600)-SO₃H were calculated by fitting kinetic data with a pseudo-first-order kinetic equation.

The calculation of the reaction rate constants is performed according to the model as follows:

$$-\mathrm{k}·\left[\mathrm{fru}\right]=\frac{\mathrm{d}\left[\mathrm{fru}\right]}{\mathrm{dt}},$$

(4)

where [fru] represents the molar concentration of fructose and k represents the rate constant at a certain temperature. After subsequent integral research and calculations for the equation, the original equation would become:

$$\ln \frac{{\left[\mathrm{fru}\right]}_0}{{\left[\mathrm{fru}\right]}_{\mathrm{t}}}=\mathrm{kt}+\mathrm{C},$$

(5)

thereafter, we determined [fru]_t = [fru]₀ (1 − X). X denotes the rate of fructose conversion. The equation was transformed into a numerical form:

$$-\ln \left(1-\mathrm{X}\right)=\mathrm{kt}+\mathrm{C},$$

(6)

the calculated activation energy (Ea) of this reaction system was bMC(600)-SO₃H, which was obtained by fitting the results with the Arrhenius equation:

$$\mathrm{lnk}=\frac{-\mathrm{Ea}}{\mathrm{RT}}+\mathrm{lnA},$$

(7)

where Ea represents activation energy, k represents the rate constant at a certain temperature, and T represents the temperature corresponding to k. The R represented the molar gas constant, and the A represented the frequency factor.

4. Conclusions

In summary, discarded masks-derived sulfonated carbons with —COOH and —OH functional groups were synthesized via mechanochemical pretreatment and one-pot carbonization and sulfonation with 5-SA as the sulfonated group agent. Compared with functional carbon that was prepared by direct carbonization of a mask, the addition of 5-SA in the mechanochemical pretreatment caused intense structural fragmentation of the discarded mask. The as-synthesized bMC(600)-SO₃H was shown to be efficient for the synthesis of HMF from fructose, and an 82.1% yield of HMF was obtained from fructose at 95 °C after 120 min of reaction time. The activation energy for fructose dehydration was determined to be 147 kJ/mol. The discarded masks-derived sulfonated carbon catalyst was demonstrated without significant changes in HMF yields. This method for the synthesis of sulfonated carbon from discarded masks provides a new idea for the environmentally friendly and value-added resource utilization of polypropylene-based material wastes.