Abstract
Acicular mesoporous char sulfonic acid was prepared through a one-step method of removing the template at the same time of sulfonation using ethylene tar (ET) as the carbon source and acicular nanometer magnesium hydroxide as the hard template. This method was judged as better than the two-step method of removing the template before sulfonation because it protected the mesoporous structure from damage to a certain extent. When the mass ratio of ET to Mg(OH)2 was 1:3 and carbonization temperature was 550 °C, the catalyst prepared using the one-step method had the highest activity. The obtained catalyst had an amorphous structure with a specific surface area of 446.5 m2/g, an acid density of 4.68 mmol/g, and an average pore diameter of 3.5 nm. When the catalyst was applied in the dehydration of fructose to synthesize 5-hydroxymethylfurfural (5-HMF), 97.5% fructose conversion and 80.1% 5-HMF yield can be obtained. The activity of the catalyst did not decrease after 5 cycles, which indicated that the catalyst had good stability. This research provides a promising strategy for preparation of mesoporous char sulfonic acid and comprehensive utilization of ET.
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Preparation of Acicular Mesoporous Char Sulfonic Acid and Its Application for Conversion of Fructose to 5-Hydroxymethylfurfural
Shuang Zhang,a Xiaohui Han,b Yanjie Liu,a,* Ling Liu,a Jiajun Yang,a and Long Zhang c,*
Acicular mesoporous char sulfonic acid was prepared through a one-step method of removing the template at the same time of sulfonation using ethylene tar (ET) as the carbon source and acicular nanometer magnesium hydroxide as the hard template. This method was judged as better than the two-step method of removing the template before sulfonation because it protected the mesoporous structure from damage to a certain extent. When the mass ratio of ET to Mg(OH)2 was 1:3 and carbonization temperature was 550 °C, the catalyst prepared using the one-step method had the highest activity. The obtained catalyst had an amorphous structure with a specific surface area of 446.5 m2/g, an acid density of 4.68 mmol/g, and an average pore diameter of 3.5 nm. When the catalyst was applied in the dehydration of fructose to synthesize 5-hydroxymethylfurfural (5-HMF), 97.5% fructose conversion and 80.1% 5-HMF yield can be obtained. The activity of the catalyst did not decrease after 5 cycles, which indicated that the catalyst had good stability. This research provides a promising strategy for preparation of mesoporous char sulfonic acid and comprehensive utilization of ET.
Keywords: Fructose; 5-hydroxymethylfurfural; Ethylene tar; Acicular magnesium hydroxide; Mesoporous char sulfonic acid
Contact information: a: Institute of Petrochemical Technology, Jilin Institute of Chemical Technology, No. 45 Chengde Street, Jilin 132022, China; b: CNPC East China Design Institute Co. Ltd., Jilin Branch, Jilin 132022, China; c: School of Chemical Engineering, Changchun University of Technology, No. 2055 Yanan Street, Changchun 130012, China; *Corresponding author: zhanglongzhl@163.com
GRAPHICAL ABSTRACT
INTRODUCTION
Non-renewable petrochemical resources are facing depletion. An effective way to solve this problem is to replace the petrochemical resources with renewable biomass resources (Liguori et al. 2019). 5-hydroxymethylfurfural (5-HMF) is an important renewable biomass platform organic compound that has the structure of aromatic alcohol and aromatic aldehyde in the furan ring system (Rosatella et al. 2011). It has high reactivity and can be used to synthesize a variety of high value-added chemicals, including 2,5-furandicarboxylic acid, 2,5-dimethylfuran, and levulinic acid (Thananatthanachon and Rauchfuss 2010; Zhang et al. 2012; Hayashi et al. 2019). A large number of studies have shown that fructose can be used to synthesize 5-HMF under the action of an acid catalyst (Dutta et al. 2013; Saha and Abu-Omar 2014; Wang et al. 2019). Solid acids as heterogeneous acids have been widely used in this reaction because they can be easily separated from the reaction solution and recycled (Shimizu et al. 2009; Yang et al. 2010; Karimi et al. 2015).
One type of solid acid, carbon-based solid acid, has the advantages of high acid density, rich Brønsted acid active center, and strong binding force between the sulfonic acid functional group and matrix material (Kitano et al. 2009; Malins et al. 2015; Farabi et al. 2019). Hara et al. (2004) reported the sulfonation of naphthalene or anthracene with concentrated sulfuric acid (Hara et al. 2004). The obtained black powdered carbon-based sulfonic acid was insoluble in water, benzene, methanol, ethanol, and n-hexane. It is composed of aromatic carbon rings arranged irregularly, its acid density was found to be 4.90 mmol/g, and its specific surface area was measured as 24 m2/g. The catalyst was used in hydrolysis and esterification, its activity was higher than that of Nb2O5 and Nafion, and close to that of sulfuric acid. Later, Toda et al. (2005) reported a new type of “sugar catalyst”, which was based on sucrose or glucose. It was made through high-temperature polycondensation and cracking reactions to obtain polycyclic aromatic hydrocarbons, then sulfonation with concentrated sulfuric acid to obtain amorphous carbon-based sulfonic acid with rich phenol and carboxyl groups on the surface (Toda et al. 2005). It was applied to the deacidification of high acid value oil, and the catalytic activity was more than half that of liquid sulfuric acid. In addition, the catalyst had good thermal stability, even if it was used repeatedly at 80 to 180 °C, and there was no loss of activity. Tanemura et al. (2011, 2012) prepared condensed polynuclear aromatic resin (COPNA) through naphthalene, pyrene, or phenanthrene as raw materials, p-dimethylbenzene or p-benzaldehyde as cross-linking agent, p-toluenesulfonic acid as the catalyst, and then sulfonated the resin to obtain carbon-based solid acid (S-COPNA). Daengprasert et al. (2011) prepared naphthalene into solid sulfonic acid with the specific surface area of 1.1 m2/g and the acid density of 1.46 mmol/g, which was used for the hydrolysis of cassava residue in the mixed solvent of dimethyl sulfoxide/acetone-water. The yield of 5-HMF was 12.1% (Daengprasert et al. 2011). Carbon-based solid acids prepared from polycyclic aromatic hydrocarbons generally have low specific surface area and almost no pore structure. Increasing the specific surface area can increase the probability of collision of reactants and the internal active center of the catalyst.
Generally, silica as the hard template has been commonly used to increase the specific surface area of carbon-based solid acid. To remove it, hydrofluoric acid solution with strong corrosiveness was needed. There are still some problems in the preparation of carbon-based solid acid with silica as the template. For example, if the template is removed before sulfonation, the mesoporous structure of the catalyst will be destroyed in the process of sulfonation (Janaun and Ellis 2011). If the template is removed after sulfonation, the carbon precursor becomes filled in the pores of the silica, which makes the gap between the carbon and silica smaller. In the sulfonation reaction, it was difficult for the sulfonic acid functional groups to connect to the carbon skeleton, resulting in the lower acid density of the catalyst (Peng et al. 2010). Therefore, for the optimization of the mesoporous structure of carbon-based solid acid, it is necessary to develop hard templates that are easy to remove and to consider the removal methods of hard templates. Zhang et al. (2012) reported that acicular nanometer magnesium hydroxide can be used as a hard template to prepare mesoporous carbon materials with high specific surface area. Under high temperature calcination, magnesium hydroxide was converted into magnesium oxide, which can be removed by sulfuric acid. So, using magnesium hydroxide as a hard template to prepare carbon-based solid acid is able to simultaneously remove the template and achieve sulfonation.
In addition, most carbon-based solid acids are made from polycyclic aromatic hydrocarbons or biomass through incomplete carbonization and sulfonation (Budarin et al. 2006; Zhang et al. 2015). Due to the high price of raw materials or complex preparation of precursor, the large-scale industrial production of carbon-based solid acids is limited. Therefore, it is necessary to consider inexpensive and easily available raw materials and find a simple preparation process.
Ethylene tar (ET) is a by-product of the ethylene industry. It is cheap and rich in non-side chain and short side chain condensed aromatic hydrocarbons (Ge et al. 2016). In this work, ethylene tar was used as the carbon source, and acicular nanometer magnesium hydroxide served as the hard template. The acicular mesoporous char sulfuric acid was prepared by removing the template at the same time as sulfonation. The effects of mass ratio of ET to hard template and the effects of carbonization temperature on specific surface area, acid content, and pore structure of catalyst were evaluated. The catalyst was applied in the dehydration of fructose to synthesize 5-HMF, the activity and stability of the catalyst were investigated. Therefore, the bridge between carbohydrate biomass resources and petrochemical resources can be established with fructose as the raw material, while solving the problem of post-treatment for the ethylene industry (Fig. 1).
Fig. 1. Acicular mesoporous char sulfonic acid was used for the dehydration of fructose to synthesize 5-HMF
EXPERIMENTAL
Materials
Fructose, 5-hydroxymethylfurfural (5-HMF), polyvinyl pyrrolidone (PVP K30, average molecular weight 40000 g/mol), and polyvinyl pyrrolidone (average molecular weight 360,000 g/mol) were purchased from Sigma-Aldrich reagent Co., Ltd., Shanghai, China. Isopropyl alcohol was purchased from Aladdin reagent Co., Ltd., Shanghai, China. Concentrated sulfuric acid, sodium hydroxide, magnesium chloride hexahydrate, and potassium oleate were purchased from Beijing Chemicals Co., Ltd., Beijing, China. Ethylene tar (ET) was obtained from PetroChina Co., Ltd., Jilin, China.
Preparation of acicular nanometer magnesium hydroxide
Under the action of ultrasound, 20.0 g of magnesium chloride hexahydrate, 0.25 g of polyvinylpyrrolidone (average molecular weight was 360000), and 1.0 g of potassium oleate were dissolved in 10 mL of water, respectively. Then they were slowly mixed into a flask with three necks (Qiu et al. 2003). The flask was placed in a low temperature circulating water bath at 10 ± 0.5 °C. With violent stirring, 2.0 M NaOH was dropped into the solution with a peristaltic pump until the pH value reached 10, and the flow rate of dropping NaOH controlled at 2.0 mL/min. With violent stirring, the white suspension generated was aged for 1 h. Magnesium hydroxide was washed repeatedly with deionized water and dried in vacuum at 50 °C for 8 h.
Preparation of acicular mesoporous char sulfonic acid
The ET was vacuum distilled to remove light fraction (< 250 °C). The obtained heavy fraction and acicular nanometer magnesium hydroxide was ground evenly according to a certain mass ratio ET to Mg(OH)2. The mixture was put into a tubular furnace and heated to a certain temperature (400 to 600 °C) in nitrogen atmosphere at a heating rate of 5 °C/min and maintained for 1 h. The carbonized product was sulfonated with concentrated sulfuric acid at 150 °C in the nitrogen atmosphere for 15 h, and the hard template was removed at the same time of sulfonation (One-step method). The ratio of the carbonized product (g) and concentrated sulfuric acid (mL) was 1:10. Acicular mesoporous char sulfonic acid was washed with hot water at 80 °C until the filtrate was neutral, and dried in vacuum at 80 °C for 12 h, which was named carbonization temperature-catalyst-1.
To compare with the catalyst prepared by the two-step method of removing the template before sulfonation, the hard template was first removed from the carbonized product with 2.0 M dilute sulfuric acid, and then washed and dried. Finally, materials obtained were sulfonated with concentrated sulfuric acid, which was named carbonization temperature-catalyst-2. The remaining steps were the same as one-step method. The preparation flowchart of mesoporous char sulfonic acid is shown in Fig. 2.