Ethylene glycol (EG) is a large bulk polyol. It is widely used in the synthesis of polyester. Many big companies such as Coca Cola and Pepsi have asked their vendors to use bio EG. The conversion of cellulose directly to EG in one step is an important technology. Many companies in the world are eager to develop this process [1] - [12]. Since the first disclosure in 2008, the one-pot catalytic conversion of cellulose to EG has gained remarkable advances in scientific understanding and industrial application [12] - [17]. Catalyst and reaction conditions are the key in this process. Many researchers have reported that Ni-W₂C/carbon is a good catalyst for this reaction [1] - [17]. Kandasamy et al. [1] gave a very excellent review on this reaction. Wang and Zhang [2] reported to use multifunctional tungsten-based catalysts for one-pot conversion to convert cellulose to ethylene glycol. Xin et al. [3] reported that bimetal oxide catalyst including tungsten oxide is a good catalyst to have high selectivity to ethylene glycol. Molder et al. [4] reported the influence of lignin on the yield of ethylene glycol. Yu et al. [5] showed the synergistic Effect of Ni/W/Cu on MgAl₂O₄ for this reaction at a low H₂ pressure. Li et al. [6] reported that Ni/W/Ti is a good catalyst for this reaction and Ti-O-W linkage is beneficial for the stability of the catalyst. Sun et al. [12] reported that nickel–lanthanum oxide catalyst is a good catalyst for this reaction. Yu et al. [15] shows the reaction network and mechanism for this reaction over NiO_x-WO_y-Cu/MgAl₂O₄ catalyst. However, there is still a room to improve the yield of EG. By changing the method of ball milling, reduction method, and Ni and W₂C contents, one can improve the conversion of cellulose and selectivity of EG. The aim of this study was to develop an optimum one-pot process from cellulose to EG.

In this study, various composite Ni-W₂C/active carbon catalysts were synthesized by using different preparation methods. W₂C was prepared by carbothermal hydrogen reduction. Carbon was impregnated with metatungstate. Ni(NO₃)₂ solution was added on W₂C/carbon by incipient-wetness impregnation method. The reaction was carried out in a Parr reactor. Various preparation parameters and reaction parameters were investigated in this study.

Various composite Ni-W₂C/active carbon catalysts were synthesized by using different preparation methods. W₂C was prepared by carbothermal hydrogen reduction. The metal loading of W₂C in catalyst was 30 wt.%. Carbon was impregnated with metatungstate. It was dried at 120˚C for 12 h. Carbon was then carburized in H₂ flow, the temperature was increased from 25˚C to 450˚C, and then to 800˚C and stayed for 4 h. The W₂C/active carbon was treated with H₂SO₄ to create more function groups on the surface. Ni(NO₃)₂ aqueous solution was added in W₂C/active carbon by incipient-wetness impregnation method. After calcination and reduction processes in H₂ flow, Ni-W₂C/active carbon catalyst was obtained. The catalyst preparation method is depicted in Figure 1.

The catalysts were characterized by N₂ sorption. The bulk surface area was calculated by BET method, and the pore size distribution was obtained by BJH method. Transmission electron microscopy (JSM-F200) was used to investigate the morphology of the catalysts.

The reaction was carried out in a Parr reactor as shown in Figure 2. The

reaction con condition was 1 g cellulose, 0.3 g catalyst, 100 ml H₂O, 4 - 6 MPa H₂, 220˚C - 280˚C, and 400 rpm. Before reaction, cellulose and catalyst were grounded by ball mill for several hours.

N₂ adsorption-desorption isotherms of the catalyst are shown in Figure 3. It shows that the major adsorption was in the nanopore range. The pore size distribution of the catalyst by BJH method is shown in Figure 4. The main peak diameter is 3.5 nm.

The TEM image of the Ni-W₂C/carbon catalyst is shown in Figure 5. It shows that the catalysts had particle size around 30 nm.

The reaction results on different catalysts are displayed in Figure 6. The conversion of cellulose and EG yield increased in the order of Ni/active carbon < W₂C/active carbon < NiW₂C/carbon. High cellulose conversion and high selectivity to ethylene glycol was obtained on NiW₂C/active carbon catalyst. The sulfonated activated carbon has many functional groups on the surface of carbon. It

is believed that cellulose was converted to glucose on carbon, and then converted to EG on Ni-W₂C via hydrogenolysis reaction.

The effect of Ni content in Ni-W₂C/active carbon on the catalytic reaction of cellulose to EG is listed in Table 1. The maximum cellulose conversion was obtained for 2% Ni content on W₂C/active carbon, but the optimal EG selectivity was in the catalyst with 10% Ni content. As mentioned above, the hydrolysis of cellulose is occurred on the interaction of cellulose and active carbon. Therefore long ball milling time would increase the contact between cellulose and active carbon. However, the hydrogenolysis reaction of glucose to EG is mainly contributed to Ni-W₂C catalyst. Therefore, high Ni loading will cover the surface of carbon, block the functional groups on the surface of carbon, and decreased the conversion of cellulose. Adequate Ni loading can enhance the hydrogenolysis reaction and increasing the EG selectivity.

The effects of Ni loading on the cellulose conversion and EG yield are shown in Figure 7. The conversion of cellulose increased with an increase in Ni content at low metal loading range and reached 100% conversion at 2 wt.% Ni. It then decreased with increase of Ni content. It should be noted that when Ni loading increased, the W₂C content decreased. The EG yield followed the similar trend as cellulose conversion. It infers that the cellulose conversion is dependent on the W₂C content.

The reaction between cellulose and catalyst is a solid-solid reaction; therefore the contact between these two solid materials is important. Figure 8 shows that the longer ball milling time is, the higher cellulose conversion and EG yield are. The results show that the ball milling time and the amount of functional groups on active carbon play important roles in this reaction. Water is a good solvent. At room temperature, cellulose is not dissolved in water. At high temperature, cellulose can dissolve in water. The ionic product of water at 250˚C is 100 times

of that at room temperature.

Figure 9 shows the effect of hydrogen pressure on the cellulose conversion and EG yield. Cellulose conversion increased with an increase of hydrogen pressure. EG yield shows the same trend. As the hydrogen pressure reached 6 MPa, cellulose conversion reached 100%, but the selectivity of EG decreased, resulting in the decrease in EG yield.

In this study, various composite Ni-W₂C/active carbon catalysts were synthesized by using different preparation methods. W₂C was prepared by carbothermal hydrogen reduction. The metal loading of W₂C in catalyst was 30 wt%. Carbon was impregnated with metatungstate. It was dried at 120˚C for 12 h. Carbon was then carburized in H₂ flow, the temperature was increased from 25 to 450˚C, and then to 800˚C and stayed for 4 h. Ni(NO₃)₂ solution was added on W₂C/carbon by incipient-wetness impregnation method. The reaction was carried out in a Parr reactor under the following condition: 1 g cellulose, 0.3 g catalyst, 100 ml H₂O, 4 - 6 MPa H₂, 220˚C - 280˚C, and 400 rpm. Before reaction, cellulose and catalyst should be grounded by ball mill for several hours. High cellulose conversion and high selectivity to ethylene glycol was obtained by using sulfonated activated carbon, which has many function groups on the surface. It is concluded that cellulose was converted to glucose on carbon, and then converted to ethylene glycol on Ni-W₂C by hydrogenolysis reaction.

Financial support was provided by Ministry of Science and Technology, Taiwan.

The authors declare no conflicts of interest regarding the publication of this paper.

Chen, Y.-W. and Lee, D.-S. (2021) One Pot Synthesis of Bio-Ethylene Glycol from Cellulose. Modern Research in Catalysis, 10, 93-101. https://doi.org/10.4236/mrc.2021.104006