Highly selective hydrogenation of diesters to ethylene glycol and ethanol on aluminum-promoted CuAl/SiO2 catalysts
Graphical abstract
Introduction
The hydrogenation of diesters, including oxalate, carbonate, acetate and maleate, has drawn great attention as an efficient way to produce multiple kinds of chemicals, such as methanol, ethanol, dihydric alcohol and other pharmaceutical intermediates. However, the diesters generally possess multiple CO or CO bonds, their hydrogenation reactions generally include several cascade reactions. For instance, the dimethyl oxalate (DMO) hydrogenation reaction comprises several continuous reactions, including DMO hydrogenation to intermediate methyl glycolate (MG), MG hydrogenation to ethylene glycol (EG) and deep hydrogenation of EG to ethanol [1,2]. Therefore, it is also a significant challenge for the selective hydrogenation to obtain target products.
Copper-based catalysts were extensively investigated in the hydrogenation of esters due to their high activity toward CO and CO bonds [1]. In order to enhance hydrogenation reactivity and selectivity, the supports, such as MCM-41 [2], SBA-15 [4,9,12], KIT-6 [5] and HMS [[6], [7], [8]] with orderly channels, were introduced to fabricate Cu0 and Cu+ sites to activate H2 and polarize the CO bond of esters, respectively. For instance, Deng et al. presented a series of Cu/MCM-41 catalysts by ammonia evaporation method, which significantly enhanced the dispersion of copper species compared with impregnation method [4]. Nanoarrays Cu/SiO2 catalysts with embedded active species in monolithic channels were also prepared and exhibited excellent activity and promoted mass transfer efficiency [3,4]. Researchers also found that addition of a second metal could change the crystalline size of Cu, and improve the dispersion of Cu, such as Cu-Au [9], Cu-Ag [10], Cu-Pd [11], and Cu-Pt [12], which enhanced their hydrogenation reactivity and thermal stability. Zhao et al. have reported a Ni containing Cu–Ni bimetallic nano-catalyst, achieving an excellent selectivity toward ethanol (90 %) for the DMO hydrogenation under 2.5 MPa and 553 K [13]. Le et al. have investigated the influence of zinc doping and found that Zn makes a significant influence on the texture and structure of the catalysts [14]. Zheng et al. have found that the catalyst with 1.0 wt% La loading exhibits the highest catalytic activity, which attributed to the formation of CuOLa bond enhancing the stability of Cu2+ under reductive atmosphere [15].
The surface acidity of catalysts is also an important factor to improve the selectivity of the products in the ester hydrogenation, since the acidic sites could polarize the CO bond via the electron lone pair in oxygen, thus improving the reactivity of the ester group. The metal oxides, such as B2O3, and ZrO2, have been used as promoters to controllably tune the surface acidity of catalysts, which further improve the selectivity of the desirable product [[16], [17], [18]]. Our previous work has introduced B2O3 species by impregnation on Cu/SiO2 catalysts with improved selectivity and stability, which could be ascribed to the suitable acid sites and high dispersion of copper species by B2O3 doping [[18], [19], [20]]. Tian et al. prepared Cux–Mgy–Zrz/SiO2 catalysts, also showing better catalytic performance after doping with MgO and ZrO2 due to the change of surface acidity [21]. Alumina and zirconia supported Cu catalysts were both appeared to exhibit high activity and hydrothermal stability, which were frequently used in CO and CO2 hydrogenation [[22], [23], [24]]. For CO2 hydrogenation to methanol, the modification of Al2O3 can provide modest acidity sites as well as decreased copper cluster size [24]. The modification of aluminum on silica surface such as SBA-15 [25] and MCM-41 [26] also illuminated that it could strengthen the acidity and the interaction between copper species and the supports. Therefore, using metal oxides as promoters may controllably tune the acidity of catalysts and enhance the interaction between copper species and the supports, which further improves the catalytic performance of esters hydrogenation.
However, the reported work primarily used the surface doping or impregnation to introduce metal oxides into the catalysts. The incorporation of the metal oxides into the silica framework may facilitate the dispersion and stability of promoters. Herein, the silica precursor (silica sol) was first modified with aluminum salts to prepare CuAl/SiO2 catalysts by a hydrothermal method. The aluminum content in the silica sol was controlled to investigate its effect on hydrogenation of diesters (e.g., DMO and EC) to selectively produce EG and ethanol. All the catalysts were systematically characterized for a better understanding of their structures, surface chemistry and structure-activity relationship. The role of aluminum promotion on the catalytic performances was also discussed through correlating the surface concentration of the moderate-intensity acidic sites with the selectivity of EG, ethanol and methanol in DMO and EC hydrogenation reaction, respectively.
Section snippets
Materials
Dimethyl oxalate was purchased from Aladdin (Shanghai) Biochemical Technology Co., Ltd. Ethylene carbonate was purchased from Meryer (Shanghai) Chemical Technology Co., Ltd. Methanol, 1,4-Diocan, ammonia aqueous solution, Cu(NO3)2·3H2O (>99 %) and Al(NO3)3·9H2O (>99 %) were bought from Kelong Chemical Co., Ltd. 30 wt% colloidal silica was from Qingdao Yijida Chemical Co. Ltd.; Hydrogen gas (99.99 %) was from Tianyi Gas Co., Ltd. All the chemicals above were used as received and without any
Catalytic activity of DMO and EC hydrogenation over the Cu/SiO2 and CuAl/SiO2 catalysts
The catalytic performance over the Cu/SiO2 and CuxAl/SiO2 catalysts were investigated to understand their catalytic activity of DMO and EC hydrogenation. As shown in Fig. 1(a), all the catalysts exhibited a 100 % DMO conversion. The yield of ethanol raised while the yield of C3-C4 products (including n-propanol, ethylene glycol monomethyl ether and diethyl ether) declined with the increase of Al content on the catalysts. The optimal ethanol yield of 94.35 % was achieved over the Cu1.0Al/SiO2
Conclusions
In summary, the CuAl/SiO2 catalysts were successfully prepared using the aluminum modified silica sol as the support by a hydrothermal method. And the catalysts were investigated for catalyzing the hydrogenation of dimethyl oxalate (DMO) and ethylene carbonate (EC) to ethanol and ethylene glycol (EG), respectively. The results showed that the incorporation of Al in the silica could tune the surface acidity, reduce the Cu crystalline size and alter the textural properties of the catalysts, thus
Author contributions
G.Q.S, K.M, H.R.Y and B.L. conceived and designed the experiments, analyzed the results and participated in writing the manuscript. All authors contributed to the discussions of the results in this manuscript.
Declaration of Competing Interest
The authors declare no competing financial interest.
Acknowledgements
The authors are grateful for the support from the National Natural Science Foundation of China (21576169, 21306118). This work is also supported by outstanding young scholar fund of Sichuan University (2015SCU04A10). The authors also acknowledge the staff members of the 1W1B station in Beijing Synchrotron Radiation Facility (BSRF) for their assistance in the XAFS experiments.
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