Introduction

5-Hydroxymethyl furfural (5-HMF) is a major important platform chemical that could be produced from cellulose and hemicellulose by hydrolysis in the acidic medium [1,2,3]. It is an intermediate in biomass-based carbohydrate chemistry and petroleum-based industrial chemistry to produce chemicals and fuels [4, 5]. Production of 5-HMF from cellulose involved 3 steps catalytic mechanism: hydrolysis of cellulose to glucose by Brønsted acid, isomerization of glucose to fructose by Lewis acid assistance, and dehydration of fructose to 5-HMF by Brønsted acid [6]. Few research groups have studied the conversion of cellulose to 5-HMF using homogeneous catalysts such as H2SO4, HCl-AlCl3, CrCl2-CrCl3, ZrOCl2/CrCl3 [7,8,9,10,11]. However, they have reported some issues such as lack of separation of the catalyst, corrosion, and toxicity. These can be overcome by the use of solid acid catalyst [12,13,14,15,16,17].

As mentioned above, the conversion of cellulose to 5-HMF is catalyzed by Brønsted acidic and Lewis acidic sites. For this purpose, bifunctional solid acid catalysts have been developed and used. Mazzotta et al. have reported the effectiveness of Ti(IV)-HSO3 catalyst for the dehydration of cellulose, glucose, and fructose. They depicted the dual role of Brønsted acidic and Lewis acidic sites for biomass conversion [18]. Similarly, Osatiashtiani et al. have used bifunctional sulfonated zirconia (S-ZrO2) catalyst for the conversion of glucose to 5-HMF [6]. The effectiveness of this catalyst was increased by impregnation on mesoporous silica, SBA-15 [19]. Mesoporous silica materials like SBA-15 and MCM-41 have been widely used as support due to high surface area 600–1200 m2/g and tunable pore size 2–50 nm [20,21,22].

Based on the above concept, in this article, we have studied the catalytic reaction of cellulose to 5-HMF in a batch reactor using sulfonated Zr-MCM-41 catalysts synthesized by in-situ method followed by sulfonation. Moreover, the role of Brønsted acidic and Lewis acidic sites presented in the synthesized catalyst useful for the catalytic reaction was also discussed.

Experimental

Materials

Analytical grade chemicals like zirconium (IV) sulfate (Zr(SO4)2), Tetraethyl orthosilicate (SiC8H20O4, TEOS), ammonium hydroxide (NH5O, 25wt%), cetyltrimethylammonium bromide (C19H42BrN, CTABr), sulfuric acid (H2SO4), cellulose ((C6H10O5)n) and 5-hydroxy methyl furfural (C6H6O3) were purchased from the M/s. Sigma Aldrich Chemicals Pvt. Ltd., Vietnam, and used without purification.

Synthesis of MCM-41 and sulfonated Zr-MCM-41

MCM-41 was synthesized by the soft template method using CTABr as a template. The desired quantities of TEOS, CTABr, and NH4OH were mixed in a glass beaker until a homogeneous solution was obtained. The mixture was transferred into a Teflon lined autoclave and kept at 100 °C for 24 h. A white precipitate was formed. It was filtered and washed with distilled water then dried at 100 °C for 12 h. Finally, it was calcined at 550 °C for 4 h in static air. We obtained MCM-41 [23]. Zr-MCM-41 was synthesized using the same procedure as that of MCM-41 with Zr/Si ratio (4, 8, 12, 15, 20 wteqauti%). To sulfonate Zr-MCM-41, it was treated with 1 M H2SO4 at room temperature for 1 h followed by filtration and washed with distilled water then dried at 100 °C for 12 h. We obtained sulfonated Zr-MCM-41 and labeled as S-xZr-MCM-41, where x represents the wt% of Zr loaded.

Characterization

The X-ray diffractions were recorded using a D8 Advance X-ray diffractometer having Ni filtered Cu Kα radiation in the range from 2θ = 0.7–70° with a scan speed of 2°/min. The N2 adsorption–desorption isotherms were measured using Micromeritics Tristar 3000 gas adsorption analyzer at 77 K. Before the isotherm measurement, 0.1 g of sample was activated at 200 °C for 3 h under vacuum to remove moisture. The surface area was calculated by the multipoint BET method, total pore volume at P/P0 = 0.99, and pore size by the BJH method. TEM images were recorded using FEI TECNAI G2 20 X-Twin high-resolution transmission electron microscopy operated at high voltage 200 kV. Energy-dispersive X-ray spectroscopy analysis was performed using Hitachi S-4700 scanning electron microscopy. FT-IR spectra were recorded on the JASCO FT-IR-4100 spectrometer in the range from 4000 – 400 cm−1 with a resolution of 4 cm−1 using the KBr disc method. Ammonia temperature-programmed desorption (NH3-TPD) was measured using Micromeritics Autochem-II 2920 analyzer from 100–600 °C with a heating rate of 10°/min.

Catalytic study of S-Zr-MCM-41

The catalytic reaction of cellulose to 5-HMF using S-Zr-MCM-41 catalysts was carried out in a Teflon-lined stainless steel reactor equipped with a mechanical stirring system. The reaction mixture, 2 g cellulose, 0.2 g catalyst, and 10 mL water were transferred into 50 mL reactor then the temperature was raised to 170 °C with a heating rate of 10 °C/min and kept at this temperature for 2 h with a rotation speed of 400 rpm/min. The reaction products were collected by centrifugation and analyzed using GC–MS Agilent 7890A with MS detector.

Results and discussion

X-ray diffraction analysis

The low and wide-angle XRD patterns of MCM-41 and sulfonated Zr-MCM-41 were shown in Fig. 1. MCM-41 has shown 3 peaks at 2θ = 2°, 3.7° and 4.4° with reflections planes (100), (110) and (200) respectively (JCPDS No. 00-049-1712) which were the main characteristic peaks of hexagonal mesoporous MCM-41 with a space group P6mm (Fig. 1a) [24]. Zr-MCM-41 catalysts have also shown a low angle XRD pattern similar to bulk MCM-41. However, a decrease in the intensity of major peaks has been observed with an increase in the amount of Zr. In wide-angle XRD of MCM-41 and Zr-MCM-41 samples, no diffraction peaks have appeared (Fig. 1b) [25]. The lattice parameters d100 and a0 for all synthesized samples were presented in Table 1. The d-spacing (d100) and unit cell parameter constant (a0) were higher for higher loadings of Zr (12, 15, and 20 wt%) compared with bare MCM-41 because of the replacement of Si4+ by Zr4+ in the framework. Consequently, a change in lattice parameters has been observed.

Fig. 1
figure 1

XRD of MCM-41 and sulfonated Zr-MCM-41: a low angle and b wide-angle

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Table 1 The lattice parameters and textural properties of MCM-41 and sulfonated Zr-MCM-41
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N2 adsorption–desorption isotherms

The N2 adsorption–desorption isotherms of MCM-41 and sulfonated Zr-MCM-41 at 77 K were shown in Fig. 2 and textural properties were presented in Table 1. For MCM-41, a hysteresis loop has been observed above the relative pressure P/P0 = 0.85 [26]. The isotherm curve of MCM-41 was similar to Type-IV with the H1 hysteresis loop of classification of the porous materials by IUPAC [27]. Therefore, it has mesopores. For sulfonated Zr-MCM-41 samples, a hysteresis loop has not appeared. It was due to the shrinkage of pore size by sulfonation. The calculated specific surface area, pore-volume, and pores size of MCM-41 was 1191 m2/g, 1.99 cm3/g, and 6.1 nm respectively. MCM-41 and S-4Zr-MCM-41 have shown surface area nearly the same. Further increase in Zr content, a change in textural properties has been observed. The surface area was reached to 874 m2/g, pore volume 0.74 cm3/g and pore size 3.6 nm. It was due to deformation effect of Zr ions incorporated into the structure of MCM-41.

Fig. 2
figure 2

N2 adsorption–desorption isotherms of MCM-41 and sulfonated Zr-MCM-41

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FT-IR analysis

FT-IR spectra of MCM-41 and sulfonated Zr-MCM-41 catalysts were shown in Fig. 3. For MCM-41, the bands appeared at 3450 cm−1 and 1640 cm−1 represented the stretching and bending vibrational bands of the O–H group of water. The symmetric and asymmetric vibrational bands of the Si–O-Si group have appeared at 1084 cm−1 and 826 cm−1 respectively. Moreover, the band appeared at 465 cm−1 represented the bending vibrational band of Si–O-Si (or) Zr–O–Si [25, 28]. In sulfonated Zr-MCM-41 samples, the major vibrational bands of MCM-41 have been replicated. Along with this, the SO2 deformation band also appeared at 550 cm−1 [29]. Hence, FT-IR analysis has confirmed that the sulfonate group has been attached to the walls of Zr-MCM-41.

Fig. 3
figure 3

FT-IR spectra of MCM-41 and S-Zr-MCM-41 samples

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TEM and EDX analysis

Fig. 4 shows the TEM images of MCM-41 and sulfonated Zr-MCM-41. Ordered hexagonal pores were obtained for MCM-41 (Fig. 4a). For sulfonated Zr-MCM-41 samples, the same hexagonal pore structure was obtained. However, the particles correspond to zirconium oxide have not appeared. It confirmed that the incorporated zirconium was interconnected with the framework of MCM-41. The TEM analysis result was correlated with XRD. The content of zirconium in sulfonated Zr-MCM-41 samples was determined using energy-dispersive X-ray spectroscopy. Table 2 shows the elemental composition of sulfonated Zr-MCM-41 samples. Experimentally obtained Zr (wt%) was near to theoretically loaded amount. The amount of Sulphur in each sample was 10–12.5 wt%. It was also confirmed the presence of sulfur in the sulfonated Zr-MCM-41 samples.

Fig. 4
figure 4

TEM images of a MCM-41, b S-4Zr-MCM-41, c S-8Zr-MCM-41, d S-12Zr-MCM-41, e S-15Zr-MCM-41, and f S-20Zr-MCM-41 (scale bar- 100 nm)

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Table 2 Elemental composition of sulfonated Zr-MCM-41 samples from EDX
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Temperature programmed desorption of NH3

Ammonia temperature-programmed desorption profile of S-8Zr-MCM-41, S-15Zr-MCM-41, and S-20Zr-MCM-41 was shown in Fig. 5. The amount of NH3 desorbed was presented in Table 3. Each sample has shown 3 desorption peaks in between the temperatures 140–170 °C, 250–270 °C and 470–570 °C which correspond to physisorbed ammonia, Brønsted acidic and Lewis acidic sites respectively [30]. By substitution of Si4+ by Zr4+ created Lewis acidic sites whereas sulfonated Zr-MCM-41 sample has generated Brønsted acidic sites (SO3H−). The total amount of NH3 desorbed was 1.807 mmol/g for S-8Zr-MCM-41, 1.809 mmol/g for S-15Zr-MCM-41, and 1.259 mmol/g for S-20Zr-MCM-41. The catalyst S-8Zr-MCM-41 has shown high Lewis acidic sites whereas S-15Zr-MCM-41 has shown high Brønsted acidic sites. The order of the ratio of Brønsted acidic site to Lewis acidic site was S-15Zr-MCM-41 > S-8Zr-MCM-41 > S-20Zr-MCM-41. The synergetic of the skeleton structure, Zr loading and Sulphur surface concentration lead to variation in the type of acidic site strength.

Fig. 5
figure 5

NH3 temperature-programmed desorption of S-Zr-MCM-41 samples

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Table 3 Amount of NH3 desorbed for S-Zr-MCM-41 samples
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Catalytic conversion of cellulose to 5-HMF

MCM-41 and sulfonated Zr-MCM-41 catalysts have been used for the catalytic conversion of cellulose to 5-HMF in the Teflon lined stainless steel reactor. The results were presented in Table 4. The conversion of cellulose without catalyst was 1.3% at 170 °C for 2 h. For MCM-41, the conversion was increased to 15.2% and S-Zr-MCM-41 catalysts 63.3–70.2%. The selectivity and yield of 5-HMF were higher with the increase in the amount of Zr. Yayati et al. have studied silica-supported tin catalyst for the catalytic reaction of glucose. It has converted glucose into fructose by isomerization because of the Lewis acidic nature of the catalyst [31]. In this article, the synthesized catalyst sulfonated Zr-MCM-41 has both Brønsted acidic and Lewis acidic sites. So, it converted cellulose into 5-HMF. Among the synthesized sulfonated Zr-MCM-41 catalysts, high conversion of cellulose and 5-HMF selectivity was obtained for S-15Zr-MCM-41 because of the high ratio of Brønsted acidic to Lewis acidic sites (NH3-TPD analysis). Therefore, the catalyst which has Brønsted acidic and Lewis acidic properties are useful for the hydrolysis of cellulose and cellulose derivatives. In the forthcoming article, we want to study the optimization of catalyst quantity, temperature, reaction time, and recyclability.

Table 4 Catalytic reaction of cellulose to 5-hydroxymethyl furfural at 170 °C for 2 h
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Conclusion

In this work, we have systematically studied the catalytic conversion of cellulose to 5-hydroxymethyl furfural using MCM-41 and sulfonated Zr-MCM-41 catalysts in a batch reactor. The characterization results have stated that replacement of Si4+ with Zr4+ in MCM-41 by in-situ synthesis, the existence of hexagonal mesopores, attachment of sulfate groups to the walls of Zr-MCM-41, and the presence of Bronsted acidic and Lewis acidic sites. The high catalytic conversion of cellulose and selectivity of 5-HMF was obtained for S-15Zr-MCM-41 at 170 °C, for 2 h because of the high ratio of Brønsted acidic to Lewis acidic sites.