Catalytic conversion of carbon dioxide into dimethyl carbonate using reduced copper-cerium oxide catalysts as low as 353 K and 1.3 MPa and the reaction mechanism.

Seiki Wada,Kazuki Oka,Kentaro Watanabe,Yasuo Izumi

doi:10.3389/fchem.2013.00008

Abstract

Synthesis of dimethyl carbonate (DMC) from CO2 and methanol under milder reaction conditions was performed using reduced cerium oxide catalysts and reduced copper-promoted Ce oxide catalysts. Although the conversion of methanol was low (0.005–0.11%) for 2 h of reaction, DMC was synthesized as low as 353 K and at total pressure of as low as 1.3 MPa using reduced Cu–CeO2 catalyst (0.5 wt% of Cu). The apparent activation energy was 120 kJ mol−1 and the DMC synthesis rates were proportional to the partial pressure of CO2. An optimum amount of Cu addition to CeO2 was 0.1 wt% for DMC synthesis under the conditions at 393 K and total pressure of 1.3 MPa for 2 h (conversion of methanol: 0.15%) due to the compromise of two effects of Cu: the activation of H2 during reduction prior to the kinetic tests and the block (cover) of the surface active site. The reduction effects in H2 were monitored through the reduction of Ce4+ sites to Ce3+ based on the shoulder peak intensity at 5727 eV in the Ce L3-edge X-ray absorption near-edge structure (XANES). The Ce3+ content was 10% for reduced CeO2 catalyst whereas it increased to 15% for reduced Cu–CeO2 catalyst (0.5 wt% of Cu). Moreover, the content of reduced Ce3+ sites (10%) associated with the surface O vacancy (defect sites) decreased to 5% under CO2 at 290 K for reduced Cu–CeO2 catalyst (0.1 wt% of Cu). The adsorption step of CO2 on the defect sites might be the key step in DMC synthesis and thus the DMC synthesis rate dependence on the partial pressure of CO2 was proportional. Subsequent H atom subtraction steps from methanol at the neighboring surface Lewis base sites should combine two methoxy species to the adsorbed CO2 to form DMC, water, and restore the surface O vacancy.

Highlights

Carbon dioxide is one of major green house gases
dimethyl carbonate (DMC) synthesis from CO2 and methanol was reported at a synthesis rate of 1.8–5.1 mmol h−1 gcat−1 using CeO2 at 403 K and 8.7 MPa for 2–4 h (Yoshida et al, 2006)
Because the forward reaction reduces the molar amount of materials in system from three to two and is uphill reaction (Pacheco and Marshall, 1997) (Equation 1), reaction conditions of lower reaction temperature and lower pressure are disadvantageous for the DMC synthesis reaction

Summary

Introduction

Carbon dioxide is one of major green house gases. The conversion of CO2 has been widely investigated to reduce the atmospheric concentration of CO2 (Izumi, 2013). Catalyst separation was improved for DMC synthesis using heterogeneous CeO2 (Yoshida et al, 2006), ZrO2 (Tomishige et al., 1999), solid solution of ZrO2 and CeO2 (Tomishige et al, 2001; Zhang et al, 2011b), Ga2O3/Ce0.6Zr0.4O2 (Lee et al, 2011), CexZr0.9–xY0.1O2 (Zhang et al, 2011a), SnO2–ZrO2/SiO2 (Ballivet-Tkatchenko et al, 2011), Co1.5PW12O40 (Aouissi et al, 2010), H3PW12O40/CexTi1–xO2 (La et al, 2007), Cu–KF/MgSiO (Li and Zhong, 2003), Cu–Ni–diatomite (Chen et al, 2012), Cu–Ni–graphite (Bian et al, 2009a), Cu–Ni–V2O5–active carbon (Bian et al, 2009b), and Cu–Ni–V2O5–SiO2 (Wu et al, 2006; Wang et al, 2007) at 353–453 K and 0.1–60 MPa. The conversion of methanol to DMC was as much as 7.9% for 24 h (Zhang et al., 2011b). In the viewpoint of global environment and the reduction of CO2, it is desirable to synthesize DMC from CO2 under mild reaction conditions

Methods

Results

Discussion

Conclusion