Pure silica Beta zeolite supported copper species for efficient hydrogenolysis of glycerol to 1,2-propanediol

Highlights

• Cu nanoparticles with rich copper cation on pure silica Beta zeolite were prepared.

• The copper cation sites are resulted from Cu–O–Si–O (cupric silicate) species.

• This catalyst is highly active for hydrogenolysis of glycerol to 1,2-propanediol.

• Kinetic studies confirm that the glycerol dehydration is a key step.

Abstract

Catalytic conversion of glycerol to 1,2-propanediol is an important issue for sustainable production of fine chemicals and biodiesel. Here it is found that pure silica Beta zeolite supported copper species via hydrothermal treatment (Cu/Si-Beta-HT) are highly efficient for hydrogenolysis of glycerol to 1,2-propanediol, achieving 96.0% conversion and more than 99.9% selectivity after reaction at 240 °C for 3 h. TEM images and N₂O titration experiments confirm the high dispersion of Cu nanoparticles in the catalyst. In addition, pyridine-adsorption FT-IR spectra and NH₃-TPD profiles suggest the presence of copper species as Lewis acidic sites in the catalyst. The hydrogenolysis of glycerol to 1,2-propanediol mainly includes the dehydration of glycerol to acetol and hydrogenation of acetol into 1,2-propanediol, which are catalyzed by copper cations and Cu nanoparticles, respectively. In situ FT-IR spectra and kinetic studies suggest that the dehydration rate is slower than the hydrogenation rate, and the dehydration is a key step for the hydrogenolysis of glycerol to 1,2-propanediol.

Introduction

With increasing demand for sustainable development, catalytic production of bio-oils has drawn significant attention [[1], [2], [3]]. For example, vegetable oils and animal fats could be facilely converted into biodiesel with glycerol as a side product. However, the surplus glycerol could not be completely consumed in industrial processes, which strongly hindered the development of biodiesel production [4,5]. Therefore, catalytic conversion of glycerol into value-added products is an important issue for sustainable production of biodiesel [4,5]. Currently, catalytic hydrogenolysis of glycerol to 1,2-propanediol (1,2-PDO) is an interesting route for glycerol conversion because 1,2-PDO can be widely used in the production of polyester resins and pharmaceuticals [[6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]]. Various catalysts such as Pt, Ru, Ir, Pd, Ni, Co, and Cu catalysts are efficient for the hydrogenolysis of glycerol to 1,2-PDO [[19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29]]. Among them, the non-precious Cu catalysts are desirable due to their high selectivity for the formation of 1,2-PDO [15,29,30], but their activities are relatively low. Therefore, the development of Cu-based catalysts for hydrogenolysis of glycerol to 1,2-PDO with both excellent selectivity and high activity is strongly desirable.

It has been reported that the Cu-based catalysts followed a dehydration-hydrogenation mechanism in the hydrogenolysis of glycerol into 1,2-PDO, where adjacent copper cation sites-highly dispersed Cu nanoparticles are required [29,30]. Owing to the abundant copper cation sites and adjacent small Cu nanoparticles, Cu/SiO₂ catalyst exhibited excellent catalytic performance in the selective conversion of glycerol to 1,2-PDO [29,30]. For example, Cu/SBA-15 catalyst prepared by a simple grinding method is quite efficient for the conversion of glycerol to 1,2-PDO in a batch reactor, where the adjacent acidic sites-highly dispersed Cu nanoparticles are derived from abundant Cu–O–Si–O (cupric silicate) species in the Cu/SiO₂ catalyst [29,30]. Therefore, the construction of cupric silicate species in the Cu-based catalysts should be emphasized during the synthesis process. However, the normal impregnation is not efficient for the construction of the cupric silicate species in the Cu/SiO₂ catalyst [29]. Therefore, great efforts should be devoted to develop an unique route for the formation of Cu/SiO₂ catalyst with abundant cupric silicate species.

Recently, it has shown that zeolite supported metal catalysts are efficient for preparation of metal-based heterogeneous catalysts with abundant M-O-Si-O species [31,32]. Inspired by this strategy, we try to use pure siliceous Beta zeolite (Si-Beta) for loading Cu species for the hydrogenolysis of glycerol to 1,2-PDO via hydrothermal treatment of Cu(NO₃)₂ with Si-Beta zeolite [33,34]. The unique synthesis rendered the successful construction of abundant cupric silicate species in the Cu/Si-Beta-HT catalyst. Characterizations of TEM techniques, N₂O titration experiments, pyridine-adsorption FT-IR spectra, and NH₃-TPD profiles confirm the presence of both highly dispersed Cu nanoparticles and abundant copper cation sites. As a result, the Cu/Si-Beta-HT catalyst exhibited high activity and 1,2-PDO selectivity in the hydrogenolysis of glycerol at 240 °C.