Role of Brønsted and Lewis acidic sites in sulfonated Zr-MCM-41 for the catalytic reaction of cellulose into 5-hydroxymethyl furfural

A series of sulfonated Zr-MCM-41 samples were synthesized by the in-situ method followed by sulfonation using sulfuric acid for the catalytic study of cellulose to 5-hydroxymethyl furfural in batch condition. All synthesized catalysts were characterized by XRD, N2 adsorption–desorption isotherm, FT-IR, TEM, EDX, and NH3 temperature-programmed desorption analysis. The XRD and N2 adsorption–desorption isotherm results have confirmed that incorporated Zr4+ was substituted within the framework of silica MCM-41 with hexagonal pores. Similarly, the FT-IR and EDX results have proved that Zr-MCM-41 was sulfonated. The Brønsted acidic and Lewis acidic sites were identified by NH3-TPD analysis. Among the sulfonated Zr-MCM-41 catalysts, S-15Zr-MCM-41 has shown 70% cellulose conversion with 16.4% selectivity of 5-hydroxymethyl furfural at 170 °C for 2 h which was higher than other catalysts. It was attributed to the high ratio of Brønsted acidic to Lewis acidic sites.

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 H 2 SO 4 , HCl-AlCl 3 , CrCl 2 -CrCl 3 , ZrOCl 2 /CrCl 3 [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)-HSO 3 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-ZrO 2 ) 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 m 2 /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.

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 NH 4 OH 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 H 2 SO 4 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 K α radiation in the range from 2θ = 0.7-70° with a scan speed of 2°/min. The N 2 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 1 3 moisture. The surface area was calculated by the multipoint BET method, total pore volume at P/P 0 = 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 (NH 3 -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.

X-ray diffraction analysis
The  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 d 100 and a 0 for all synthesized samples were presented in Table 1. The d-spacing (d 100 ) and unit cell parameter constant (a 0 ) were higher for higher loadings of Zr (12,15, and 20 wt%) compared with bare MCM-41 because of the replacement of Si 4+ by Zr 4+ in the framework. Consequently, a change in lattice parameters has been observed.

N 2 adsorption-desorption isotherms
The N 2 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/ P 0 = 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 m 2 /g, 1.99 cm 3 /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 m 2 /g, pore volume 0.74 cm 3 /g and pore size 3.6 nm. It was due to deformation effect of Zr ions incorporated into the structure of MCM-41.    [25,28]. In sulfonated Zr-MCM-41 samples, the major vibrational bands of MCM-41 have been replicated. Along with this, the SO 2 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. 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.

Temperature programmed desorption of NH 3
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 NH 3 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].     [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 (NH 3 -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.

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 Si 4+ with Zr 4+ 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.