Highly efficient synthesis of unsymmetrical organic carbonates from alcohols and diethyl carbonate over mesoporous carbon-supported MgO catalysts

Highlights

• Mesoporous carbon (MC) supported MgO catalysts were prepared by impregnation method.

• MgO are highly dispersed on the surface of MC through metal-support interaction.

• MgO/MC can efficiently catalyze transesterification of diethyl carbonates with alcohols.

• Various unsymmetrical organic carbonates can be obtained at ambient temperature.

• MgO/MC exhibit high stability and recyclability for transesterification reaction.

Abstract

Transesterification of dialkyl carbonate with various alcohols is a very attractive route for synthesizing unsymmetrical organic carbonates. Intensive efforts have been focused on developing highly active and stable heterogeneous catalysts that can efficiently catalyze the transesterification reaction at mild conditions. In this work, the synthesis of unsymmetrical organic carbonates was conducted over a series of mesoporous carbon-supported MgO (MgO/MC) catalysts. The composition-optimized MgO/MC catalyst exhibited remarkably high activity, selectivity and stability for the transesterification of diethyl carbonates with various alcohols at the commonly used reaction temperature of 125 °C, and could even work well at ambient temperature. The uniformly dispersed MgO species should be the main active sites for the transesterification reaction, while the neighboring oxygen- and nitrogen-containing groups existed on the surface of MC support may also play positive role in synergistically activating the reagents, thus leading to efficient production of the corresponding unsymmetrical organic carbonates at very mild conditions.

Introduction

Unsymmetrical organic carbonates, are very important chemical intermediates in pharmaceutical, lubricant and other fine chemicals, and have also been widely used as green solvents, additives to fuel [1], [2], [3]. In particular, compared with symmetrical carbonates, unsymmetrical carbonates with lower carbon numbers (e.g., ethyl methyl carbonate) have relatively high flash point and polarity, low viscosity, large dielectric constant and strong dissolving ability. These features make them to be used as ideal co-solvents for lithium battery, and play important role in improving the life and other performance of lithium battery. Unsymmetrical carbonates with higher carbon numbers (e.g., dodecyl ethyl carbonate) have good low-temperature resistance, viscosity-temperature property, lubricity, wear resistance and self-cleaning performance, and have been widely used as synthetic lubricant base oil in metal processing manufacturing, high-grade equipment, automobile and other lubrication.

The traditional processes for the syntheses of unsymmetrical carbonates are generally involved in the usage of toxic reagents such as phosgene, dimethyl sulfate and pyridine [4]. With the ever-growing demand for environmental protection, recent research works have been focused on developing more simple and eco-friendly routes to synthesize various organic carbonates. Methods reported in literatures include electrochemical synthesis of organic carbonates [5], [6], direct synthesis from CO₂ and alcohols [7], [8], and transesterification of dialkyl carbonates with other types of carbonic esters or with alcohols [9], [10], [11], [12], [13]. Among them, transesterification of dialkyl carbonate with various alcohols has been regarded as one of the most attractive routes to synthesize the corresponding unsymmetrical organic carbonates due to its non-toxic raw materials and relatively simple operation conditions [14], [15].

Dimethyl carbonate (DMC) and diethyl carbonate (DEC) are two common dialkyl carbonates used for the transesterification syntheses of unsymmetrical organic carbonates with alcohols. Thus far, a variety of solid compounds or hybrid composites have been investigated as heterogeneous catalysts for these transesterification reactions, including TBD (1,5,7-triazabicyclo[4.4.0]dec-5-ene) modified mesoporous MCM-41 [16], metal organic frameworks (MOFs) derived from 1,4-benzenedicarboxylic acid and Zn(NO₃)₂ [17], organotin-oxomolybdate coordinated polymer [18], Mg-La metal oxides and CsF/Al₂O₃ [19], [20], [21], [22], as well as some simple basic oxides with nanostructure, such as nanocrystalline MgO and CaO [23], [24]. However, these catalysts generally require relatively high reaction temperature, i.e., at around the boiling point of DEC (126 °C), thus leading to an energy-intensive process. In addition, the chemical stability of the hybrid composite catalysts and the basic metal oxides are commonly not very ideal, especially when a higher reaction temperature was adopted. Hence, it is still a very significant subject to develop more efficient, stable heterogeneous catalysts for the green synthesis of unsymmetrical organic carbonates under mild conditions.

Recently, porous carbon materials with large surface area, developed porosity and abundant surface functional groups have shown great advantages in fabricating highly efficient carbon-supported catalysts for the application in various important catalytic processes [25], [26], [27], [28], [29], [30], [31], [32]. For instance, Zhu et al. found that porous N-doped carbon support with more structural defects could act as a suitable stabilizer to generate supported Pd nanoparticle catalyst for selective deoxygenation of a wide variety of carbonyl compounds at room temperature and atmospheric H₂ pressure [31]. By using a residue-based activated carbon derived from passion fruit seed (ACP) as a support, Lopes Barros et al. prepared a highly stable and active HPW/ACP (HPW = 12-tungstophosphoric acid) catalyst for esterification synthesis of biodiesel from oleic acid and methanol [32].

Previously, we and coauthors reported that a kind of sol–gel-synthesized porous carbon with mesoporous characteristic and abundant surface functional groups could be used as a suitable support for preparing carbon-supported metal oxides (i.e., CaO, MgO or ZnO) with excellent catalytic activity and stability for the transesterification synthesis of biodiesel from vegetable oils and the synthesis of ethyl methyl carbonate from DEC and DMC [33], [34], [35], [36]. It was found that the abundant oxygen- and nitrogen-containing groups existed on the surface of nanoporous carbons (NC-2) or nitrogen-doped carbon materials (NCM) play critical role in achieving high dispersion of metal oxides, and in improving the chemical/structural stability of the supported metal oxide catalysts. To our knowledge, there is no report focus on the catalytic application of porous carbon supported MgO to synthesize unsymmetrical organic carbonates with DEC and alcohols.

In this work, the catalytic performance of a series of mesoporous carbon supported MgO catalysts (MgO/MC) were investigated for transesterification synthesis of unsymmetrical organic carbonates from DEC and various alcohols. It was found that very high yield of unsymmetrical organic carbonates could be achieved at very mild conditions over the condition-optimized MgO/MC catalyst. By combining a variety of characterization results, the physicochemical properties of various MgO/MC catalysts were studied, and the nature of active sites for the transesterification reaction were also discussed.