Atmospheric pressure conversion of carbon dioxide to cyclic carbonates using a metal-free Lewis acid-base bifunctional heterogeneous catalyst

Highlights

• Boron doped g-C₃N₄ (BGCN) has been synthesized by one step thermal polymerization method.

• CO₂ conversion was performed at atmospheric pressure.

• BGCN catalyst with optimum number of acid-base sites showed enhanced activity.

• Catalyst also showed remarkable recyclability and stability.

Abstract

Conversion of carbon dioxide (CO₂) to value-added products is imperative to combat global warming. In this regard, herein we report a metal-free heterogeneous catalyst, boron-doped graphitic carbon nitride for effective conversion of CO₂ to cyclic carbonates at atmospheric pressure under solvent-free conditions. The developed catalyst possesses acid-base dual functionality as active sites, which activate both epoxides as well as CO₂ simultaneously to carry out the cycloaddition reaction. The catalyst showed a maximum yield up to 99 % with a turnover number of 173. Detailed optimization studies have been performed to find out the best doping content of boron and best conditions by varying the reaction time, temperature and catalysts amount. Furthermore, the ease of catalyst recovery and excellent recyclability demonstrate the sustainability and versatility of the catalyst for selective and efficient conversion of CO₂ to cyclic carbonates at mild conditions.

Introduction

The persistent use of fossil fuels to fulfill the world’s energy demand is endangered by the continuous increase in the CO₂ proportion present in the atmosphere and it has major concerns over global warming [1,2]. The thermal analysis around the world shows a significant increase in earth’s surface temperature during the past century, with a tremendous increase in heating trends during the past 35 years [3]. This could bring positive effects, such as durable growing seasons and modest winters, but the negative impacts are more threatening which includes thermal expansion, extreme weather events, wildlife extinction, fluctuations in seasonal events and glacier melting. Due to the melting of glaciers in the Arctic region, the global average sea level has been rising at a rate of 3 mm per year during the past 20 years which can cause floods in low elevation areas [3]. To overcome these harmful consequences, much-needed measures have to be taken to mitigate the CO₂ amount in the atmosphere.

In this regard, several strategies have been adopted by the researcher to convert CO₂ into valuable products by employing homogenous and heterogeneous catalysis methods. Depending on the nature of the reaction and catalyst used, different products, such as dimethyl carbonate, heterocycles, formates, formic acid, methanol, α, β-unsaturated carbonyl compounds, polycarbonates, urea, urethanes, carbon monoxide, etc. can be obtained by CO₂ conversion [[4], [5], [6], [7], [8], [9], [10], [11], [12], [13]]. Currently, heterogeneous catalysis is more preferred over homogeneous catalysis reactions due to the ease of separation, reusability and more eco-friendless of used catalysts. According to the 2014 IPCC survey, it is only 0.36 % of 32.3 billion tons of global CO₂, which is used as feedstock for chemical production in industries and therefore, it is highly desirable to increase its consumption for the production of valuable products [14]. Most of the strategies adopted for CO₂ conversion involve the use of harmful solvents and harsh reaction conditions. The most simple and greener approach for utilizing CO₂ is to convert it into cyclic carbonates, which can be carried out in solvent-free conditions and with high product selectivity. These cyclic carbonates have a wide horizon of applications as polar solvents, electrolytes in Li-ion batteries, petrochemicals, the monomer for polycarbonate synthesis, and in pharmaceuticals [[15], [16], [17], [18], [19], [20], [21], [22], [23]].

The major challenge in CO₂ conversion is its inert nature (CO Bond energy =805 kJ mol⁻¹), which makes the interaction and reaction of CO₂ molecules difficult over catalyst surface [24]. To activate such a molecule with high bond energy for further reaction, it is very important to rationally design and develop efficient catalysts with high catalytic efficiency and selectivity. According to the molecular orbital theory, the CO₂ molecule has an empty low lying antibonding orbital (2π*) and incorporation of electrons in this orbital can decrease its bond order, which activates the CO₂ molecule for a chemical reaction. Therefore, it could be inferred that employing a catalyst with electron-rich moieties can effectively activate the CO₂ molecule by incorporating the electron in its antibonding orbital. Working with this strategy, researchers have used polymeric carbon nitride heterogeneous catalysts for cycloaddition of CO₂ into epoxide, including a pioneering work by Zhao et al. on boron-doped graphitic carbon nitride material. [[25], [26], [27], [28]] Carbon nitrides can effectively activate CO₂ molecules, but these materials are still not very effective to catalyze the cycloaddition reaction and require harsh conditions of high CO₂ pressure and reaction temperature. This can be attributed to the ineffective activation of the epoxides by carbon nitrides. As CO₂ is a Lewis acid and epoxides have a basic site, the development of a heterogeneous catalyst with acid-base dual functionality could be a better choice for cycloaddition of CO₂ with epoxides, wherein, both the reactants can be activated together. Following this strategy, surface-functionalized graphitic carbon nitride (GCN) catalysts have been utilized for CO₂ fixation to cyclic carbonates under mild conditions of temperature and pressure [29].

In our work, we report a series of metal-free boron-doped graphitic carbon nitride nanosheets (BGCN_x), (x denotes the boron amount in different wt%) have been synthesized and tested for thermocatalytic cycloaddition of CO₂ into epoxides at atmospheric pressure. The GCN based materials used for CO₂ conversion to cyclic carbonates are less efficient due to the presence of basic sites only. Since boron is known for its electrophilicity and it can activate epoxides [30], the doping of GCN with a suitable electrophilic element (boron) produces a heterogeneous boron-doped graphitic carbon nitride (BGCN) catalyst, which can act as a highly efficient catalyst for CO₂ cycloaddition to epoxides. The fabricated BGCN nanosheets possess dual functionality, acidic as well as basic sites which have been calculated by using temperature programmed desorption (TPD) measurements, which synergistically activate both CO₂ as well as the epoxide for the cycloaddition reaction. The reactions have been carried out at atmospheric pressure and under solvent-free (neat) conditions. The reactions showed very high selectivity as no side products were observed in the reaction mixture. Furthermore, the addition of KI as cocatalyst increases the product yield substantially. The turnover number (TON) and turnover frequency (TOF) were calculated by considering the contribution of cocatalyst KI. The activity of the catalysts was examined for several epoxides possessing different pendant groups to demonstrate the wide applicability of our catalyst. The kinetics of cycloaddition reaction are determined with respect to epichlorohydrin, which follows pseudo first-order kinetics. Thus, this work presents a unique example of dual functionalized heterogeneous catalyst operable at atmospheric pressure conditions for CO₂ conversion to cyclic carbonates.