Coupling Carbon Dioxide and Cyclohexane Oxide Using Metal-Free Catalyst with Tunable Selectivity of Product Under Mild Conditions

Abstract

This study introduces a metal-free binary catalytic system for coupling CO₂ with cyclohexane oxide (CHO) under mild conditions, allowing for tunable product selectivity. Using trans-cyclohexane diol (trans-CHD) and phosphazene superbase (P₄) as catalysts, the system selectively produces cyclic carbonates and oligocarbonates at 1 bar CO₂ pressure and 80 °C. By adjusting the catalyst ratio, varying proportions of cis-cyclohexane carbonate (cis-CHC), trans-cyclohexane carbonate (trans-CHC), and oligocarbonate are achieved, with 51 mol% CHO conversion and respective selectivities of 36%, 31%, and 33%. The catalytic efficiency and precise control of product outcomes underscore this system’s potential.

1. Introduction

Rising concerns over CO₂ levels have driven the development of advanced technologies to convert CO₂ into valuable products [1,2]. Beyond physical uses like supercritical fluid extraction and foaming, chemical transformation of CO₂ offers a non-toxic, abundant alternative to hazardous phosgene [3,4]. However, due to CO₂’s fully oxidized and symmetric structure, effective catalytic methods are required for its activation. Coupling CO₂ with epoxides (EPs) using metal or organic catalysts is promising, producing polycarbonates and cyclic compounds. The high ring strain of 3-membered EPs [5] enables reactions with CO₂ through copolymerization [6,7] or cycloaddition under mild conditions.

While metal-based catalysts are known for their high efficiency [8,9,10], the growing interest in metal-free catalytic systems offers an alternative for CO₂ incorporation into fine chemicals [11,12,13]. Examples include organo Lewis acids (e.g., borane) and ionic liquids [14,15]. Since the pioneering use of tetrabutylammonium bromide (TBABr) to convert CO₂ into cyclic carbonates under high temperatures and pressures [16], efforts have focused on milder conditions. For example, Zhang et al. achieved styrene carbonate synthesis with an ionic liquid catalyst at 1 bar CO₂ and 120 °C [17], and triazole derivatives enabled CO₂ and epoxide cycloaddition at ambient conditions [18].

Incorporating CO₂ into polymer main chains offers an alternative approach for utilizing this green carbon resource [19,20,21,22]. A typical metal-free catalytic system is derived from borane, with its six valence electrons enabling the acceptance of lone pairs of electrons, which activates the oxygen atom of epoxide. The first example of organocatalyzed CO₂ and epoxide copolymerization was reported using tri-methylene borane, resulting in high-molecular-weight polycarbonate [23]. Significant advancements in Lewis acid catalyst design have accelerated the generation of polycarbonate from CO₂ and epoxide [24,25]; however, the development of Lewis base catalysts for CO₂ and epoxide copolymerization remains unachieved. This may be because the rate of “back-biting” (a side reaction in copolymerization where cyclic carbonate can be formed) is much faster than the propagation of the polymer chain when using a Lewis base catalyst.

In this study, we report a metal-free binary catalyst derived from a Lewis base, capable of producing either polycarbonate (such as oligocarbonate) or cyclic carbonate by adjusting the catalyst ratio at ambient CO₂ pressure (Scheme 1). This system combines trans-cyclohexane diol (trans-CHD) and a phosphazene super base (1-tert-Butyl-4,4,4-tris(dimethylamino)-2,2-bis[tris(dimethylamino)-phosphoranylidenamino]-2λ5,4λ5-catenadi(phosphazene), P₄) to selectively couple CO₂ and cyclohexane oxide, yielding the desired cyclic carbonate or polycarbonate with high catalytic efficiency.

2. Results and Discussion

The coupling of CO₂ and cyclohexane oxide (CHO) was first attempted using a 1:1 ratio of trans-CHD and P₄ at ambient CO₂ pressure (1 bar) and 80 °C for 24 h. The conversion of CHO and the selectivity of the resulting products were analyzed by ¹H NMR spectroscopy. With 5 mol% catalyst loading, CHO was successfully coupled with CO₂ to yield multiple products, achieving a CHO conversion of 53 mol% (

Table 1. Selective coupling of CO₂ and cyclohexane oxide using P₄ and trans-CHD as the binary catalyst ^a.

Entry	[trans-CHD]:[P4]	Time/h	Conversion/mol%	trans-CHC/ mol%	cis-CHC/ mol%	Oligocarbonate	Mn SEC g/mol	Đ
1	1:1	24	53	18	33	56	600	1.45
2	1:0	24	N.A. f	N.A.	N.A.	N.A.	N.A.	N.A.
3	0:1	24	N.A.	N.A.	N.A.	N.A.	N.A.	N.A.
4 b	1:1	24	91	<1	90	<1	N.A.	N.A.
5 c	1:1	24	32	46	28	26	300	1.26
6	1:1	48	59	42	42	16	450	1.30
7	1:1	72	80	18	50	32	390	1.35
8 d	1:1	24	80	10	62	28	330	1.42
9 e	1:1	24	92	10	85	5	N.A.	N.A.
10	1:2	24	60	<1	>99	<1	N.A.	N.A.
11	16:1	24	54	<1	<1	>99	670	1.43

^a Experimental conditions: catalyst loading, 5 mol%; T = 85 °C; n_CHO = 10 mmol; the conversion of CHO and selectivity of products were characterized by ¹H NMR spectroscopy; ^b T = 105 °C; ^c T= 65 °C; ^d catalyst loading, 10 mol%; ^e catalyst loading, 20 mol%, ^f not available.

, entry 1). ¹H NMR analysis revealed 18 mol% trans-cyclohexane carbonate (trans-CHC), 33 mol% cis-cyclohexane carbonate, and 56 mol% poly(carbonate-co-ether) (Figure 1).

To investigate the cooperative effect of trans-CHD and P₄, a control experiment was conducted using either trans-CHD or P₄ individually. No product formation was detected by ¹H NMR, confirming the synergistic effect of the binary catalyst. We propose that the alkoxide, generated from the deprotonation of diol by P₄, serves as the active center for coupling CO₂ and CHO. The resulting mixture was analyzed by size-exclusion chromatography, revealing the formation of an oligomer with an average molecular weight (M_n) of 600 g/mol and a molecular weight dispersity (Đ) of 1.45 (Figure 2). This finding demonstrates that the combination of trans-CHD and P₄ effectively promotes the coupling of CO₂ and CHO under mild conditions.

To further enhance the coupling reaction, experiments were conducted under varying conditions. With an increase in temperature, the overall conversion of CHO gradually rose (from 5 mol% to 80 mol%). At a higher temperature (105 °C), the reaction exhibited high selectivity for cis-CHC (90 mol%) with a minor presence of trans isomers. In contrast, reactions at lower temperatures (45–65 °C) demonstrated relatively higher selectivity for trans-CHC and ether linkages. This may be because trans-CHC, formed from the coupling of CO₂ and CHO, is favored at lower temperatures and can undergo ring-opening to produce either oligocarbonate or cis-CHC. The ether linkage may result from either decarboxylation or copolymerization with trans-CHC.

Since oligocarbonate selectivity is highest at 85 °C with minimal ether side products, the time-dependent effect on copolymerization was examined under identical conditions (5 mol% catalyst loading, 1:1 ratio, and 1 bar CO₂). Extending the reaction time to 72 h gradually increased the overall CHO conversion (Figure 3a). After 48 h, SEC characterization revealed oligocarbonate with an M_n of 450 g/mol and Đ = 1.30, slightly lower than the M_n of 600 g/mol and Đ = 1.45 observed at 24 h. An aliquot of the mixture was analyzed by ¹H NMR spectroscopy to track product evolution. Selectivity for trans-CHC gradually decreased, while cis-CHC selectivity correspondingly increased from 30 to 50 mol%, with an overall CHO conversion of 80 mol% (Figure 3b). Extending the reaction time did not significantly affect the selectivity of ether and carbonate linkages but increased the yield of cis-CHC, suggesting that oligocarbonate selectivity is primarily governed by temperature.

Fourier-transform infrared spectroscopy (FTIR) was used to characterize the reaction mixture. An identical volume of aliquot was withdrawn from the mixture for quantitative measurement. The absorption peaks of the ether linkage were observed around 1200 cm⁻¹, while the ether derived from CHO appeared at 1250 cm⁻¹ (Figure 4, left). The region between 1700 cm⁻¹ and 1900 cm⁻¹ corresponds to carbonyl absorption, where oligocarbonate, trans-CHC, and cis-CHC can be distinctly identified. The peak at 1737 cm⁻¹ is attributed to the carbonyl group in the oligocarbonate linkage, while the peaks at 1801 and 1820 cm⁻¹ correspond to the carbonyl groups of the cis and trans isomers, respectively (Figure 4, right). Over time, the ratio between trans and cis forms gradually decreases, supporting the conclusions drawn from ¹H NMR analysis.

Extending the reaction time did not significantly increase the M_n of oligocarbonate but did enhance cyclic product formation. Therefore, we sought to evaluate the impact of the catalyst ratio on the coupling reaction. To investigate the effect of catalyst loading, various amounts of binary catalyst relative to CHO were applied under identical conditions (1 bar CO₂, 85 °C, for 24 h). Increasing the catalyst loading to 10 mol% resulted in a higher CHO conversion (80 mol%). Notably, product selectivity differed from the reaction with 5 mol%, showing a higher selectivity for cis-CHC (62 mol%) compared to trans-CHC (10 mol%) and oligocarbonate (28 mol%). This suggests that the generation rate of cis-CHC is relatively faster than that of the trans isomer and oligocarbonate. Further increasing the catalyst loading to 20 mol% supported this observation, with cis-CHC selectivity reaching 85 mol%, while trans-CHC remained around 10 mol% and oligocarbonate at 5 mol%.

To gain structural insight into the role of trans-CHD in the coupling reaction, cis-cyclohexane diol was used as a control. While the trans-CHD/P₄ system enabled moderate CHO conversion (51 mol%) and yielded various carbonates, the cis-CHD/P₄ system showed little to no CHO transformation. We hypothesize that the alkoxide generated from cyclohexane diol and P₄ interacts with CO₂ to form a complex, where the complex derived from cis-CHD forms a more stable structure, resulting in lower nucleophilicity for the reaction.

This was further investigated through theoretical study. The complex structures were optimized using density functional theory (DFT) at the B3LYP/6-311G(2d,p) level. The Mulliken charge of the optimized structures revealed that the sp³-hybridized oxygen of carbonate in the cis complex is lower than in its trans counterpart (−0.47 vs. −0.54 eV), suggesting that the cis complex has reduced nucleophilicity (Figure 5).

With an understanding of the geometry of trans-CHD, we next investigated the effect of the ratio between trans-CHD and P₄. When the trans-CHD-to-P₄ ratio was set at 1:2, allowing complete conversion of the diol to alkoxide, the coupling reaction produced cis-CHC with very high selectivity (≥99 mol%). Increasing the trans-CHD/P₄ ratio to 4:1 with 5 mol% catalyst loading relative to P₄ resulted in low selectivity for cis-CHC (approx. 15 mol%) but relatively high selectivity for trans-CHC (55 mol%) and oligocarbonate (29 mol%). Further increasing the trans-CHD/P₄ ratio to 16:1 led to a significant yield of oligocarbonate (99 mol%).

The insights from the trans-CHD/P₄-catalyzed coupling of CO₂ and CHO suggest a plausible mechanism (Scheme 2). The deprotonated trans-CHD interacts with CO₂, forming a carbonate active center that resists back-biting to yield cyclic carbonate. Instead, this active center can react with CHO to produce a dimer, leading to either trans-CHC or cis-CHC through a back-biting process. These cyclic carbonates then undergo ring-opening to generate oligocarbonate. As the trans-CHD/P₄ ratio increases, oligocarbonate selectivity is enhanced, suggesting that the in situ transformation of cis-CHC follows a transesterification process with trans-CHD, thereby forming polycarbonate oligocarbonate.

3. Materials

trans-Cyclohexane diol (≥98%) and cyclohexane oxide (≥98%) were purchased from Energy Chemical Company (Shanghai, China). Both cyclohexane diols were dried over anhydrous THF through azeotropic distillation; cyclohexane oxide was dried over CaH₂ and stored in a glovebox for further use. tert-Bu-P₄ solution (0.8 M in n-hexane) was purchased from Sigma-Aldrich (St. Louis, MO, USA). Carbon dioxide gas (≥99%) was purchased from Newradar gas company (Wuhan, China).

3.1. Instruments

¹H NMR (400 MHz) spectra were recorded on Brucker spectrometer at 273 K using chloroformed-d (CDCl₃) as solvent. Size exclusion chromatography (SEC) was performed on a Waters Alliance 2695 equipped with columns (Waters Styragel HR3, HR4, HR5, Waters, Milford, MA, USA). SEC experiments were carried out in THF at the flow rate of 1.0 mL/min at 35 °C. FTIR spectra of resulted mixture were obtained on an Alpha Bruker Platinum—ATR spectrometer in the range from 4000 cm⁻¹ to 400 cm⁻¹ at a resolution of 4 cm⁻¹ (Bruker, Billerica, MA, USA). A total of 24 scans were taken for each sample.

3.2. Experimental Procedures

Coupling CO₂ and cyclohexane oxide (CHO) catalyzed by trans-CHD and P₄ was performed in Ar-filled glovebox. Briefly, CHO (0.98 g, 10 mmol), trans-CHD (580 mg, 0.5 mmol), and P₄ (316 mg, 0.5 mmol) were charged into a flame-dried flask (20 mL). The CO₂ balloon was set at the top of flask and the flask was flushed to yield a CO₂ atmosphere. The reaction mixture was stirred at 85 °C for 24 h and quenched by benzoic acid for ¹H NMR spectroscopy analysis (see Figure S1).

4. Conclusions

A highly efficient, metal-free catalytic system for the coupling of CO₂ and cyclohexane oxide has been developed. The combination of trans-CHD and P₄ successfully enabled tunable product selectivity under mild conditions. Increasing the catalyst loading significantly improved CHO conversion and controlled the formation of cyclic carbonates and oligocarbonates. Additionally, theoretical calculations highlighted structural stability differences between cis-CHC and trans-CHC, offering insights into the observed selectivity. This study presents a green, efficient, and versatile catalytic approach for CO₂ valorization and provides a theoretical foundation for the development of more complex metal-free catalytic systems in the future.