Elsevier

Journal of CO2 Utilization

Volume 63, September 2022, 102129
Journal of CO2 Utilization

Enhancing catalytic strategy for cyclic carbonates synthesized from CO2 and epoxides by using cobaloxime-based double complex salts as catalysts

https://doi.org/10.1016/j.jcou.2022.102129Get rights and content

Highlights

  • The double and single cobaloxime salts were prepared.

  • The spectroscopic properties were achieved by different spectral techniques.

  • The cobaloxime salts have been used for the green synthesis of cyclic carbonates from CO2 under solvent-free.

Abstract

In this study, the novel neutral cobaloximes and corresponding to different cobaloxime-based double complex salts were prepared and for the first time evaluated as an efficient catalyst for the synthesis of cyclic carbonates from epoxides and CO2 without using any organic and inorganic solvents. After seeing the efficient catalytic performance of different forms of the newly synthesized neutral cobaloximes (1−3) and different cobaloxime (4−9) salts, the effect of epoxide, base, temperature, CO2 pressure, reaction time, and amount of catalyst was investigated for these catalysts. The neutral cobaloximes (1−3), double cobaloxime salts (4−6), and their single salts with simple counterions (7−9) were characterized by 1H and 13C NMR spectra, FT-IR spectra, UV-Vis spectra, LC-MS/MS spectrometers, melting point, and elemental analysis techniques. Furthermore, the compounds were utilized as catalysts in the synthesis of cyclic carbonates from epoxide and CO2. The neutral cobaloximes (1−3), double cobaloxime salts (4−6), and their single salts with simple counterions (7−9) showed high catalytic activity (at 100 °C and 1.6 MPa of CO2 pressure) for the carboxylation of epichlorohydrin. The best conversion was performed in the presence of a catalyst (6) (the double cobaloxime salt bearing ethyl) and DMAP as co-catalyst, with a 97.9% yield and 98.4% selectivities.

Introduction

As one of the main pollutant gases of the atmosphere, the continuous increase in the amount of carbon dioxide (CO2) can cause global warming, the greenhouse effect, dangerous climate change and threatens the survival of various organisms due to the acidification of the oceans [1]. Because CO2 is widely recognized as one of the greenhouse effect gases by absorbing a fraction of the radiation emitted by the Earth and then re-emitting it towards the planet [2], [3], [4]. To the avoidance of CO2 emissions, the capture and storage or utilization of CO2 would be the most promising carbon management strategy from both an industrial and an academic viewpoint [5], [6]. Among these efficient and facile strategies, the synthesis of cyclic organic carbonates via CO2 insertion in epoxide rings stands out as a great choice because of its 100 % atom efficiency and high carbon conversion. Besides, the coupling of CO2 and epoxides into cyclic carbonates under mild conditions has drawn much research attention due to its offer to extend material use with higher value [7], [8], [9], [10]. Organic cyclic carbonates are very important among all known CO2-based products and also receiving growing interest both in the academic and industrial areas because their synthesis and applications present several attractive features in the context of green chemistry and sustainability [11]. Cyclic carbonates are valuable chemicals that can be used as green solvents, in carbamates synthesis, electrolytic elements in Li-ion batteries, monomers for the production of polycarbonates, ingredients for pharmaceutical as well as fine chemical intermediates, and sustainable alternatives to toxic reactants currently employed in the chemical industry [12], [13], [14]. In addition to these fields of application, the five-membered cyclic carbonates can be applied to fields such as electrochemistry, fine chemistry, biology, etc [6].

The conversion of CO2 into valuable chemical feedstocks has also been a target of carbon capture and storage or utilization. However, the conversion of CO2 is rather challenging due to its high thermodynamic stability (ΔG° = −394 kJ/mol) and kinetically inert properties, which causes a great challenge to the transformation of CO2 into useful products. To overcome this problem, the preparation and development of active and selective catalytic systems is a central topic of research in the domain of the fixation of CO2 to fine chemicals under suitable conditions [15], [16], [17], [18]. The chemical fixation of an inexpensive, nonflammable, bio-renewable, highly abundant, easy accessibility, and non-toxic C1 feedstock CO2 benign homogeneous catalyst is of significant interest from the viewpoints of industrial application and environmentally friendly. Recently, among efficient homogeneous catalytic systems, cobalt compounds are of great interest for the cyclic carbonate formation from CO2 and epoxides under favorable conditions and without solvent due to high activity and selectivity with a wider range of substrate scope [19], [20], [21], [22], [23]. Another reason why cobalt complexes are preferred is that they are robust, easy-to-synthesize, abundant in the soil, cheap, and high air with moisture stable complexes. Our group reported the synthesis of organic cyclic carbonates by the cycloaddition reactions of CO2 with epoxides under ambient conditions by using various neutral cobaloxime catalysts [24], [25], [26], [27].

Delius and co-workers reported for the first time the serendipitous discovery of asymmetric cobaloxime-based double complex salts and their single salts with simple counterions and then these compounds were used as catalysts for the light-driven hydrogen evolution reaction [28], [29]. Inspired by this work, we synthesized a series of new neutral cobaloximes, cobaloxime double complex salts, and their single salts with simple counterions and used them for the first time in the conversion of CO2 to cyclic carbonates, as catalysts. To the best of our knowledge, the use of cobaloxime double complex salts and their single salts with simple counterions for the conversion of CO2 to organic cyclic carbonates will be the first in the literature. For this purpose, the newly synthesized different cobaloxime catalysts were characterized by 1H and 13C NMR, FT-IR, UV-Vis, LC-MS/MS spectrometers, melting point, and elemental analysis techniques. Then, these different cobaloxime catalysts have been evaluated for their catalytic activity in the coupling of CO2 and epoxides into cyclic carbonates under appropriate conditions.

Section snippets

Chemicals and measurements

All organic solvents and starting chemical reagents for the synthesis of cobaloximes and cobaloxime-based double complex salts were bought from commercial suppliers (Sigma, Agros, and Merck) and were used without further purification and chemical process. The melting point measurements were recorded in open capillary tubes under air with an Electrothermal-9100 melting point apparatus and the results are given as is. Elemental analysis (CHN) was analyzed with a thermos scientific CHNS/O

Synthesis of neutral cobaloximes (1−3) and cobaloxime salts (4−9)

To pursue both novel and efficient catalytic CO2 conversion reactions in neutral cobaloxime (1−3) complexes versus different cobaloxime salts (4−9) are prepared in their neutral form, we envisioned that the new molecular design should first integrate an active salt form in cobaloxime complexes. When the synthesized cobaloxime salts are compared with their neutral forms offer an improved catalytic activity for the synthesis of cyclic carbonates from epoxides and CO2 without using any solvent and

Conclusion

The efforts to convert CO2, the most important greenhouse gas, into useful chemicals (cyclic carbonates) continue to attract attention from an economic and environmental pollutant viewpoint with the effects of global warming. In this context, careful design and use of new catalysts to ensure high selectivity and reaction efficiency greatly assist in the conversion of CO2 into useful chemicals. In the light of this information, the novel neutral cobaloximes and the corresponding asymmetric

CRediT authorship contribution statement

Anas ALHAFEZ: Experimental studies, synthesis, and characterization studies, Emine AYTAR: Methodology and catalytic studies, Ahmet KILIC: Methodology, synthesis and characterization, writing, spectroscopic and catalytic studies.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

We acknowledge gratefully the financial support from the Research Fund of Harran University (HUBAP Projects No: 20010) Sanliurfa, Turkey.

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