The depletion of fossil fuel resources and global climate change have led to more and more attention on developing a series of novel chemicals based on renewable feedstocks. Many studies have been devoted to converting renewable biomass to useful chemicals and clean fuels [1
]. Among the natural biomass, cellulosic biomass is the most abundant inedible biomass resource produced by photosynthesis from CO2
O. Recently, the conversion of cellulose to value-added chemicals has been extensively studied [4
]. Since reducing sugars (RS)—mainly glucose—are platform molecules for production of other chemicals, such as 5-hydroxymethylfurfural (HMF), levulinic acid, and alkylglucosides, depolymerization of cellulose to RS plays a critical role [7
Cellulose is a water-insoluble aggregate of long-chain β-1,4 glucan composed of glucose monomers linked by β-1,4 glycosidic bonds. Hydrolysis of β-1,4 glycosidic bonds and decomposition of hydrogen bonds linking β-1,4 glucan chains into water-soluble saccharides are the main technology to obtain RS [9
]. Up to now, several technologies in the hydrolysis of cellulose have been applied, such as enzyme hydrolysis, mineral acid hydrolysis, and supercritical water hydrolysis [13
]. Generally, enzymatic hydrolysis is the most promising approach due to its low temperature and high activity, but the high cost and reuse of the enzyme inhibit its commercialization. Mineral acids are effective for cellulose hydrolysis, however, the catalyst recovery and the waste water treatment are the problems. Supercritical water hydrolysis is also an efficient process, but it has drawbacks, such as harsh conditions and many side-reactions [16
]. On the contrary, due to the advantages of easy separation and recycling, hydrolysis of cellulose over heterogeneous catalysts has received greater attention in recent years [19
Several groups have reported that cellulose was hydrolyzed in the presence of solid acid catalysts, such as Amberlyst 15 [20
], heteropolyacids [21
], magnetic solid acid catalyst [23
], and functional carbon materials [25
]. Among them, porous solid materials modified by the -SO3
H groups are extensively investigated for their cheap and easy preparation [27
]. Although the catalysts supported by -SO3
H groups show strong acidity and high activity in the hydrolysis of cellulose, -SO3
H groups are easily leaching out and lead to environmental problems. Moreover, the hydrolysis efficiency is also low due to mass transfer resistance between solid acids and insoluble cellulose in water [29
]. Consequently, more effort should be devoted to developing the environmentally friendly and efficient solid acid catalysts for the hydrolysis of cellulose.
Clay minerals are a class of phyllosilicates, which are ubiquitous in nature [30
]. Depending on their layer structure, high specific surface area, and ion exchange capacity, a great number of new clay-based materials have been developed with fascinating functionalities [31
]. Among clay minerals, montmorillonite (Mt) is one of the most common 2:1 type cationic clay with highly hydrothermal stability and thermal stability [34
]. Functional Mt has received much attention over the past decades because it is useful in many fields, including adsorption [36
], catalysis [37
], and separation [38
]. In our previous work, acid-activated Mt had been studied for the hydrolysis of cellulose in water, which showed higher activity than that of ZSM-5, but the total reducing sugars (TRS) yield was low [39
]. In the past decade, due to the interaction between Cl−
of the ionic liquid (IL) and hydroxyl groups of the polysaccharides [40
], IL had been widely used for the dissolution of cellulose under mild conditions [41
]. Moreover, the acidic functionalization of IL can directly catalyze the hydrolysis of cellulose with the ability of dissolution. Recently, more and more attention has been focused on the hydrolysis of cellulose in the IL [43
]. However, there are still some problems, such as the high cost, high viscosity, and difficulty in separation of IL [46
]. Therefore, from the view point of green chemistry, more efficient ways should be developed for the utilization of IL.
In this work, a new heterogeneous catalyst was prepared by grafting the 1-(trimethoxy propyl silane)-3-methyl imidazolium chloride groups without acidic groups on the surface of Mt (Mt-IL). The catalytic activity and reuse of Mt-IL were examined by the hydrolysis of cellulose into RS in water. The process provided a green and efficient method for the efficient hydrolysis of cellulose catalyzed by the weakly acidic material grafted by non-acidic IL.
3. Materials and Methods
The microcrystalline cellulose powder was purchased from the ShengDeLi Synthetic Leather Material Co., Ltd., Huzhou, China, and obtained from cotton. The cellulose content was above 99 wt %. No physical or chemical pretreatments had been used to increase the non-crystalline cellulose fraction. The Ca-montmorillonite (Ca-Mt) was provided by Zhejiang ChangAn Renheng Science and Technology Co., Ltd., Hangzhou, China. The cation exchange capacity (CEC) of Mt was 80 mmol/100 g. Also, 1-methyl-1H-imidazol, mercaptopropyl trimethoxysilane (MPTMS), and 3-chloropropyl trimethoxy silane were purchased from Aladdin Chemicals Co., Ltd., Shanghai, China. All other chemicals (analytic purity) were commercially available products and were used without further purification.
3.2. Catalyst Preparation
3.2.1. Preparation of Acid-Activated Mt
The purified Ca-Mt (12 g), was dispersed in 120 mL 0.5 wt % H2SO4 and refluxed at 80 °C for 1.5 h. Then the slurry was cooled, filtered, and washed thoroughly with distilled water three times. The product was dried at 80 °C for 12 h. The activated Mt was designated as HMt.
3.2.2. Preparation of Mt-SO3H
In a typical procedure, 12 g Ca-Mt in 360 mL water was stirred for 30 min at room temperature, and then 4 mL MPTMS was added into it. Finally, the mixture was stirred at 80 °C for 2.5 h to obtain thiol functionalized Mt (Mt-SH). Then the slurry was cooled, filtered, and washed thoroughly with anhydrous ethanol two times, and the thiol group was oxidized to sulfonic acid using H2O2 in the presence of ethanol at 60 °C for 4 h (58 mL of 30% H2O2 in 174 mL ethanol for 12 g of Mt-SH). After that, the slurry was cooled, filtered, and washed thoroughly with distilled water three times. The product was dried at 80 °C for 12 h. The amount of -SO3H was adjusted by the concentration of MPTMS and the synthesized samples were designated as Mt-SO3H-1, Mt-SO3H-2, and Mt-SO3H-3, respectively.
3.2.3. Preparation of Mt-IL
Firstly, 1-(trimethoxy propyl silane)-3-methyl imidazolium chloride (Si(MeO)3PMIMCl) was synthesized. Four milliliters of 1-methyl-1H-imidazol and 10 mL of 3-chloropropyl trimethoxy silane were mixed in 100 mL of toluene and the mixture stirred at the reflux temperature of toluene (120 °C) for 12 h. Finally, the toluene was removed by rotary evaporation and an orange viscous liquid Si(MeO)3PMIMCl was obtained.
Then, the Ca-Mt was mixed with Si(MeO)3PMIMCl in anhydrous ethanol (100 mL) at 90 °C for 12 h. After silanization, the precipitate was filtered and washed with distilled water three times. The product was dried at 80 °C for 12 h. The sample was designated as Mt-IL.
3.3. Catalytic Conversion of Cellulose
The hydrolysis reaction was carried out in a Teflon-lined stainless steel autoclave (25 mL). A certain amount of distilled water, cellulose, and catalyst were introduced into the autoclave. The reaction was carried out between 170 and 220 °C under antogenetic pressures. After the reaction, the catalyst and the unreacted cellulose were removed by filtration. The liquid products were analyzed at 540 nm using 3,5-Dinitrosalicylic acid method by visible spectrophotometer manufactured by Shanghai Precision & Science Instruments Co. Ltd. Cellulose conversions were determined by the change of cellulose weight before and after the reaction, with an uncertainty of ±3%. The yield of reducing sugar was calculated from the equation: yield (%) = (weight of reducing sugar in the products)/(weight of cellulose put into the reactor) × 100%. The concentration of 5-HMF was analyzed by a Waters 2695 series HPLC equipped with an ultraviolet detector applying an InertSustain C18 (4.6 mm × 250 mm × 5 μm), and the yield of 5-HMF was calculated based on the calibration curves.
3.4. Catalyst Characterization
The X-ray diffraction (XRD) measurements were collected using a PANAlytical X’Pert PRO diffractometer between 2° and 80° (2θ) with a scanning rate of 0.1 °/s, employing Cu Kα radiation (λ = 1.54056 Å). Fourier transform infrared (FT-IR) spectra were recorded between 4000 and 400 cm−1 using a Nicolet 6700 Fourier transform spectrometer. The samples were dried at 110 °C, mixed with KBr, and exposed to infrared light. The pellets were immediately measured after preparation under ambient conditions in the mid-infrared region. The spectra were the result of averaging 32 scans at wavelengths ranging from 4000 to 400 cm−1. Thermal analysis of nanocomposites was carried out using thermogravimetric-differential thermogravimetric (TG-DTG) methods on a Mettler Toledo thermobalance. TG/DTG curves were recorded with a 10 °C/min heating rate under air atmosphere between 30 and 800 °C.
The measured process for acidic sites on the catalysts was described as follows: a catalyst (0.05 g) was treated with 0.01 mol/L of NaCl solution (20 mL) for 1 h at 20–50 °C under ultrasonic vibration. After centrifugal separation, the supernatant solution was titrated by 0.01 mol/L of NaOH solution using phenolphthalein as an indicative.