Aluminium based binary catalytic system for the solvent free conversion of CO₂ to carbonates with high activity and selectivity

Abstract

An easily prepared and low-cost aluminium based metal complex catalyst was prepared using kojic acid (Hkoj) as a ligand, and this developed oxo-coordinated Al(koj)₃ complex showed high activity and selectivity for the CO₂ fixation reaction with epoxides under mild conditions without any organic solvents. Various cyclic carbonates were obtained in excellent yields (up to 99%). This stable catalytically active Al(koj)₃ has strong Lewis acidity for the activation of epoxides, and meanwhile the hydroxy group in Al(koj)₃ may play a role in boosting the catalytic activity through possible hydrogen bonding interactions with the epoxide.

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Introduction

Recent daily and weekly values of CO₂ concentration measured at Mauna Loa Observatory have remained above 400 parts per million.¹ This is a significant symbolic event in the global warming process, because part of global warming is attributed to increasing carbon dioxide levels in atmosphere, which is mainly caused by the combustion of huge amounts of fossil fuels (such as coal, petroleum, and natural gas). The rising CO₂ concentration may cause a pronounced change to our ecosystem because plant species are very sensitive to the increasing CO₂ concentration, as this will influence the behavior of stomata, the microscopic holes in leaves that allow leaves to take up CO₂, and as a result will cause changes to land plant photosynthesis. The existing fossil fuel reserves are limited and are being gradually depleted, and alternative resources such as CO₂ and biomass have received extensive attention.² In this context, the utilization of CO₂ as an inexpensive, ubiquitous, nontoxic, non-flammable, renewable, and green basic raw material has drawn wide attention in academia and industry.³ Many useful fine chemical products, which have previously been derived from the limited fossil fuel resources, are currently produced using CO₂ as an alternative C1 building block, such as methanol, urea, carbonate, formamide and carboxylic acid.⁴ However, just a fraction of the huge amount of CO₂ has been fixed into value-added products and the thermodynamic stability and kinetic inertness of CO₂ limits its conversion under mild conditions. A large energy input is required, such as higher CO₂ pressure and higher reaction temperatures, thus resulting in huge production costs. Therefore, development of high-efficiency, inexpensive, eco-friendly processes for CO₂ conversion and utilization are most urgently needed.⁵

One of the methods used to overcome the obstacles in CO₂ utilization is the use of high-energy small-membered ring compounds, e.g. epoxides, as the reactants with a negative Gibbs free energy for the reaction. Also, the addition of CO₂ to epoxides is a 100% atom-economical reaction, which is in accordance with the principles of green chemistry, and thus this method represents one of the most attractive routes for CO₂ utilization.⁶ Cyclic carbonates and polycarbonate products have been studied with particular interest due to their widespread application in industry, e.g. as aprotic high-boiling polar solvents for organic synthesis (replacing DMF, DMSO, NMP and acetonitrile), electrolytes for lithium-ion batteries, monomers for polymeric materials (polycarbonates, and isocyanate-free polyurethanes), and intermediates in the synthesis of fine chemicals.⁷ In the past few years, plenty of catalysts have been exploited for the synthesis of carbonates from CO₂ and epoxides, including metal-based catalysts (such as salen–metal compounds, porphyrin–metal compounds, metal oxides, metal–organic frameworks and so on), miscellaneous catalytic systems, ionic liquids, onium salts, and mono or binary metal-free catalysts.⁸ Catalysts are used to activate the epoxide or/and CO₂ and reduce the activation energy of the transformation. The epoxide, activated by the catalyst, is liable to go through ring cleavage-addition with CO₂ upon nucleophilic attack by Br⁻ or I⁻, to form an alkoxide anion, ultimately affording the corresponding cyclic carbonate (Scheme 1). Thus, electron-deficient metal catalysts are privileged for this reaction. In this respect, metal complexes based on porphyrins and metallosalen complexes have been widely investigated due to their unique Lewis acidities.⁹ In addition, oxo-coordinated metal complexes also displayed high catalytic activity in the cycloaddition reaction of CO₂ with epoxides.¹⁰ In our previous work, a tetraoxo-coordinated zinc complex, i.e. 2-hydroxypyridine N-oxide zinc(II), Zn(OPO)₂, was used and demonstrated remarkable turnover frequencies (up to 22 000 h⁻¹).¹¹ Therefore, we continued to consider highly active oxo-coordinated metal complexes for the synthesis of cyclic carbonates.

Scheme 1 Possible mechanism for the catalytic formation of cyclic carbonates.

Kojic acid (5-hydroxy-2-(hydroxymethyl)-4-pyrone, Hkoj) is considered to be one of the safest food products and is widely used as a food additive to prevent enzymatic browning.¹² Hkoj has chelating activity and its metal complexes have reasonable hydrolytic stability, neutral charge and significant lipophilicity. Thus these kojic acid based metal complexes, e.g. Al(koj)₃, may have strong Lewis acidities for the activation of epoxides, and meanwhile the hydroxy group in Al(koj)₃ may play a role in boosting the catalytic activity through the possible hydrogen bonding interactions with the epoxide (Scheme 2). Hence, in this manuscript, we conducted a thorough investigation to explore the catalytic activity of these kojic acid based metal complexes, which are easy to prepare, low cost, multifunctional for the cycloaddition of CO₂ with epoxides.

Scheme 2 Multifunctional metal complexes used for the activation of epoxides in this work.

Results and discussion

First, different kojic acid based metal complexes were prepared according to methods reported in the literatures¹³ and the structures were confirmed using FT-IR and elemental analysis. The cycloaddition of styrene oxide with CO₂ was chosen as a benchmark reaction (Table 1). TBAI (tetrabutylammonium iodide) was used as a co-catalyst for this transformation due to its excellent nucleophilicity. The initial screening experiments were conducted at 100 °C, with 1 MPa CO₂ for 10 h with a catalyst loading of 0.25 mol%. The effect of different metal centers in the complexes was evaluated for the given reaction. Zn(koj)₂ and Mn(koj)₂, which have been reported as potential radioprotective agents,^12a moderately promoted the reaction (40% and 52% yields, respectively, entries 1–2). An uncommon iron complex, Fe(koj)₃, also showed comparable reactivity (entry 3). Note that the reaction using Al(koj)₃ as a catalyst gave a good yield of 69% with the assistance of TBAI (entry 4). In the absence of Al(koj)₃, the yield dropped from 69% to 21% (entry 4 vs. 5). To clarify the role of the hydroxy group in Al(koj)₃, we also examined the reaction using Hkoj and a slightly higher yield of carbonate was obtained (entry 5 vs. 6). This result may indicate that the hydroxy group in Al(koj)₃ plays a role in boosting the catalytic activity through the possible hydrogen bonding interactions with the epoxide.^8d No reaction occurred without any catalyst (entry 7).

Table 1 Catalyst screening for the synthesis of styrene carbonate from styrene oxide with CO₂^a

Entry	Cata./mol%	TBAI/mol%	Yield^b/%
a The reaction was carried out on a 5.0 mmol scale of styrene oxide at 100 °C under 1 MPa CO2 (initial).b NMR yield using 1,1,2,2-tetrachloroethane as the internal standard.
1	Zn(koj)2/0.25	0.25	40
2	Mn(koj)2/0.25	0.25	52
3	Fe(koj)3/0.25	0.25	50
4	Al(koj)3/0.25	0.25	69
5	—	0.25	21
6	Hkoj/0.75	0.25	43
7	—	—	0

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We continued our investigations by optimizing the reaction parameters using Al(koj)₃ as the catalyst (Table 2). Remarkably, with increasing temperature from 100 °C to 120 °C, the yield of styrene carbonate increased significantly from 69% to 93% (entry 1 vs. 2). The yield decreased sharply when the reaction was conducted at 80 °C, which thus suggested that 120 °C was the appropriate temperature for the cycloaddition reaction (entry 2 vs. 3). From a thermodynamics point of view, the cycloaddition of epoxides with CO₂ is an exothermic reaction and 120 °C is an easy operation temperature for industrial production. Then, the effect of catalyst loading on the reaction was studied. A quantitative yield was obtained even after decreasing the catalyst loading from 0.25 mol% to 0.15 mol% (entry 4). Even at a loading of 0.05 mol%, a yield of 66% was achieved (entries 5–6). Thus, we defined the optimal reaction conditions for the cycloaddition as 0.15 mol% Al(koj)₃ at 120 °C with 1 MPa CO₂ under solvent-free conditions.

Table 2 Further optimization reaction conditions for the cycloaddition reaction^a

Entry	Cata./mol%	T/°C	Yield^b/%
a Reaction conditions: styrene oxide (5.0 mmol), TBAI (0.25 mol%), and 1 MPa CO2 (initial).b NMR yield with 1,1,2,2-tetrachloroethane as the internal standard.
1	Al(koj)3/0.25	120	95
2	Al(koj)3/0.25	100	69
3	Al(koj)3/0.25	80	13
4	Al(koj)3/0.15	120	94
5	Al(koj)3/0.1	120	72
6	Al(koj)3/0.05	120	66

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In order to evaluate the efficiency and general applicability of this oxo-coordinated Al(koj)₃ complex, a series of epoxides (terminal and internal) was examined for the synthesis of carbonates using CO₂ in the presence of Al(koj)₃ (Table 3). Propylene carbonate is widely used as an electrolyte and alternative green solvent in organic syntheses. An excellent yield of up to 99% was obtained using this developed catalyst system (entry 1). Using TBAB (tetrabutylammonium bromide) instead of TBAI also gave a quantitative yield of propylene carbonate. Furthermore, the reusability of the Al(koj)₃/TBAI system was examined for the synthesis of propylene carbonate, and no significant loss in its catalytic activity was observed after three successive runs (entry 1). The results achieved mark this protocol as an attractive method for potential future industrial applications. Other terminal epoxides smoothly converted to the corresponding cyclic carbonates in high yields and with excellent selectivities (entries 2–6). As for disubstituted epoxides, e.g. isobutylene oxide and cyclohexene oxide, these showed inferior activity even under harsh conditions (longer reaction times and increased amount of catalyst) compared with terminal counterparts, due to the increased steric hindrance (entries 7–8).^8c

Table 3 Conversion of CO₂ to functionalized carbonate catalyzed with Al(koj)₃/TBAI^a

Entry	Substrate	Yield^b/%
a Conditions: epoxide (5 mmol), Al(koj)3 (0.15 mol%), TBAI (0.25 mol%), 1 MPa CO2, 120 °C, and 10 h.b NMR yield with 1,1,2,2-tetrachloroethane as the internal standard. The yield in parentheses is the isolated yield.c TBAB instead of TBAI.d After three cycles using Al(koj)3/TBAI.e Al(koj)3 (0.25 mol%), 2 MPa CO2, and 15 h.
1		99(94), 94(91)^c, 96(95)^d
2		95(91)
3		91(89)
4		90(88)
5		94(91)
6		95(90)
7^e		42(40)
8^e		37(34)

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Based on previous studies¹⁴ and the above experiments, a plausible mechanism for this Al(koj)₃ catalyzed coupling of CO₂ with epoxides was proposed and depicted in Scheme 3. Lewis acid metal catalyst activates the epoxide, followed by nucleophilic attack of the iodine anion from TBAI, to form the metal-coordinated iodo alkoxide. Then, the addition of CO₂ leads to the formation of the cyclic carbonate and regeneration of the catalyst. Notably, the hydroxy group in Al(koj)₃ may also play a role in boosting the catalytic activity through possible hydrogen bonding interactions with the epoxide.

Scheme 3 Plausible reaction pathway for the coupling of epoxides with CO₂ catalyzed by Al(koj)₃/TBAI.

Conclusions

In summary, oxo-coordinated Al(koj)₃ was developed as an efficient catalyst for the synthesis of carbonates from epoxides with CO₂ under mild conditions without any solvent. A series of terminal epoxides were converted to the corresponding carbonates in yields of up to 99%. Moreover, this developed catalyst system can be reused at least three times with no significant decrease in the propylene carbonate yield. This developed catalyst system shows great potential for industrialization due to the fact that it is easily prepared, inexpensive, has stable catalytic activity and requires reaction conditions that are easily realized.

Experimental

General information

TBAI and epoxides were purchased from Alfa Aesar, Aladdin, and J&K Chemical and were used without further treatment. Kojic acid, Zn(OAc)₂, Mn(OAc)₂·2H₂O, Fe(NO₃)₃·9H₂O, and Al(NO₃)₃ were from Energy Chemical. CO₂ with a purity of 99.99% was commercially available. The products were characterized in CDCl₃ using a Varian Bruker 400 spectrometer.

Synthesis and characterization of the kojic acid metal complexes¹³

Zn(koj)₂. IR data (KBr, cm⁻¹): 1622, 1568 (C=O); 1517 (C=C); 1277, 1219 (C–O). Elemental analysis: [calcd for C₁₂H₁₀O₈Zn·1/2H₂O], calc.: C, 41.47; H, 2.90. Found: C, 41.2; H, 2.98.

Al(koj)₃. IR data (KBr, cm⁻¹): 1613, 1578 (C=O); 1527 (C=C); 1307, 1249 (C–O). Elemental analysis: [calcd for C₁₈H₁₅O₁₀Al], calc.: C, 48.01; H, 3.36. Found: C, 48.09; H, 3.30.

Mn(koj)₂. IR data (KBr, cm⁻¹): 3449 (O–H); 1613, 1576 (C=O); 1528 (C=C); 1292, 1246 (C–O). Elemental analysis: [calcd for C₁₂H₁₀O₈Mn], calc.: C, 42.75; H, 2.99. Found: C, 42.48; H, 3.05.

Fe(koj)₃. IR data (KBr, cm⁻¹): 3331 (O–H); 1610, 1561 (C=O); 1513 (C=C); 1276, 1249 (C–O). Elemental analysis: [C₁₈H₁₅O₁₀Fe·1/2H₂O], calc.: C, 45.12; H, 3.16. Found: C, 45.19; H, 3.12.

General procedure for the cycloaddition reaction of epoxides with CO₂

A mixture of the epoxide (5 mmol), metal catalyst, and TBAI was placed into a glass tube, and then the tube was placed into a 25 ml stainless steel autoclave. The reactor was charged with CO₂ to the desired pressure. Then, the autoclave was placed into an oil-bath and heated. After the reaction, the reactor was cooled in an ice-water bath, and the CO₂ was carefully vented. Diethyl ether was added to extract the carbonate products and unreacted epoxide. The yield of cyclic carbonate was determined using NMR analysis with 1,1,2,2-tetrachloroethane as the internal standard. The products were purified using column chromatography with ethyl acetate–petroleum ether as the eluent. The precipitated catalyst was washed with diethyl ether five times and dried using a pressure blowing concentrator ready to be used directly for the next reaction.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank Shaanxi University of Science and Technology for the financial support of this research (2016GBJ-20).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ra00835c

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