Synthesis of cyclic carbonates from epoxides and carbon dioxide over silica-supported quaternary ammonium salts under supercritical conditions
Graphical abstract
Silica-supported quaternary ammonium salt proved to be a recyclable and efficient heterogeneous catalyst for the synthesis of propylene carbonate from propylene oxide and CO2 under supercritical conditions, which requires no additional organic solvents either for the reaction or for the separation of product; the work-up procedure is straightforward and the catalyst could be reused without obvious loss of catalytic activity and selectivity. Moreover, this organic solvent-free process presented here could show much potential application in industry due to its simplicity, easy product separation from reaction medium and catalyst recycling. It could be profitably applied to the development of fix-bed continuous flow reactors, avoiding the use of solvent to isolate the products.
Introduction
Chemical fixation of CO2 is a very attractive subject from the viewpoints of better utilization of carbon resource and increased concern on our environment [1], [2], [3]. One of the most promising methodologies in this area is the synthesis of five-membered cyclic carbonates via the coupling of CO2 and epoxides (Scheme 1, Scheme 2) [4]. These organic cyclic carbonates such as ethylene carbonate and propylene carbonate (PC) are widely used for various purposes, for instance, electrolytic elements of lithium secondary batteries, polar aprotic solvents, monomers for synthesizing polycarbonates, chemical ingredients for preparing medicines or agricultural chemicals, and alkylating agents [5], [6], [7], [8], [9].
In current processes for cyclic carbonate production employed by industry, various homogeneous catalysts are used [4], [5], [6]. However, homogeneous catalysts are undesirably dissolved in a phase containing cyclic carbonates, thus it is necessary to separate the catalysts from the products through a purification process. In order to facilitate the separation of catalyst, a large number of solid catalysts have been developed, for example, polymer-supported quaternary onium salts [10], [11], magnesia [12], [13], Mg–Al mixed oxide [14], Cs-load zeolite and alumina [15], lanthanide oxychloride [16], [17]. More recently, niobium (V) oxide and other heterogeneous catalysts have been found to be efficient catalysts for the carboxylation of epoxides with CO2 [18], [19], [20], [21], [22], [23], [24], [25]. Unfortunately, these solid catalysts have insufficient activity and most of them are essentially required to contain a polar solvent as an additive for realizing activity and selectivity, which may have caused catalyst leaching and required additional processes for product separation. Solid supported catalysts have received much more attention [26], [27], because they may offer several advantages in preparative procedures [28], [29] e.g. simplifying work-up and separation and recycling of the catalyst. In heterogeneous catalysis, supercritical CO2 properties can help reduce mass and heat transfer limitations and avoid coke formation or catalyst poisoning [29]. Development of a more efficient and environmentally benign catalyst for cyclic carbonate synthesis under mild conditions still remains a challenging issue.
Quaternary ammonium salts (e.g., Et4NBr) or alkali iodides (e.g., KI) are typically used as homogeneous catalysts for cyclic carbonate synthesis [30], [31], [32]. Our strategy is that quaternary ammonium salts can be heterogenized by dispersing it on high-surface-area inorganic supports in order to get a better catalyst separation and make the catalyst recovery more facile [33]. Herein, we would like to describe a simple, efficient and recyclable catalyst of silica-supported quaternary ammonium salt for the cycloaddition reaction of CO2 and propylene oxide to produce PC, as shown in Scheme 1. In the present work, high PC yield and selectivity were obtained at 140 °C under 8 MPa in a short reaction time of 5 h, even in the absence of any additional organic solvents.
Section snippets
Reagents
Silica gel (HG/T2534-92 chromatography grade, specific surface area: 550 m2/g; pore volume: 0.70–0.90 ml/g; particle size: 400 mesh) in this study was commercially supplied by Qingdao Haiyang Chemical Reagents Co. Ltd. Epoxides were supplied from Aldrich Company and carbon dioxide with a purity of 99.99% was commercially available. Tetra-n-propylammonium bromide (n-Pr4NBr) was purified by recrystallization from methanol prior to use. Other reagents were analytical grade and were used as received.
Catalysts preparation and characterization
The counter anion effect of the silica-supported tetra-n-butyl ammonium halides on PC synthesis
Listed in Table 1 are the results for the propylene carbonate synthesis catalyzed by quaternary ammonium halides. Various silica-supported quaternary ammonium halides were screened for the cycloaddition of CO2 to propylene oxide. For comparison with the silica-supported catalysts, the catalytic activities of the corresponding lower molecular weight catalysts such as n-Bu4NF, n-Bu4NCl, n-Bu4NBr, n-Bu4NI (Table 1, entries 9–12) were also examined for the reaction of propylene oxide and
Conclusion
In conclusion, the silica-supported quaternary ammonium salt is very effective for cyclic carbonate synthesis under supercritical carbon dioxide conditions, where CO2 could act both as a reagent and a solvent. High cyclic carbonate yield together with excellent selectivity was achieved even in a short reaction time of 5 h. The silica-supported quaternary ammonium salts as heterogeneous catalysts were easily recovered by a simple filtration and reused over four times with slightly loss of its
Acknowledgements
We thank the National Science Foundation of China (Grant Nos. 20472030, 20421202), the Committee of Science and Technology of Tianjin (Grant No. 033609311), Ministry of Education of China for financial support. Thanks are due to Prof. W. Li (Institute of Catalysis, Nankai University) for kind help in measuring the BET surface area and FT-IR spectra.
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