SYNTHESIS AND CHARACTERIZATION OF CATALYSTS FOR DIMETHYL CARBONATE PRODUCTION

Abstract

The need for eco-friendly fuels for transport vehicles has spurred the search for new molecules and their viable and cost-effective synthesis method. Several research groups are working on the production techniques of dimethyl carbonate (DMC) and its usage as a fuel additive for vehicles. DMC can also be used as a reactant for the synthesis of diphenyl carbonate, glycerol carbonate, etc. DMC is commercially synthesized by using various methods, such as phosgenation, oxidative carbonylation, methyl nitrite carbonylation, etc. Although each production method has its own drawbacks, transesterification of propylene carbonate (PC) with methanol is considered to be a promising and feasible method for the synthesis of DMC. This route is sustainable, clean, with no formation of any harmful or waste by-product. Recently, utilization of CO2 for the synthesis of chemicals has attracted a lot of attention. DMC can also be produced from CO2, although this route has thermodynamic limitations. An exhaustive review of the literature reveals that only a few catalysts have been tested for the DMC production via transesterification and direct CO2 conversion reactions. Considering various possibilities, the main aim of the present work was to prepare and characterize various catalysts (CeO2-based mixed oxide, hydrotalcites and supported catalysts) with different physico-chemical and textural characteristics; and to evaluate catalytic activity for the synthesized catalysts for DMC synthesis via transesterification and direct conversion of CO2 reactions. It was also aimed to optimize the operating conditions such as temperature, pressure, molar ratio, amount of catalyst and to study the kinetics and thermodynamics of the DMC synthesis; and to explore the catalytic stability and recyclability of the synthesized catalysts, and to investigate the possible mechanism, kinetics and thermodynamics of DMC synthesis with different reactions. Work of the transesterification reaction of PC for DMC production Transesterification reaction of PC with methanol for the DMC production was studied with four different sets of catalysts, namely: Ce-M (M=Co, Fe, Cu and Zn), Ceria- Lanthanum, Ceria-Zinc-support mixed metal oxide and copper-zinc-aluminium hydrotalcite catalysts. Ce–M catalysts (M=Co, Fe, Cu and Zn) were synthesized by sol-gel method and characterized by various techniques. BET surface area of CeCo, CeCu, CeFe and CeZn catalysts was found to be 40, 46, 24, and 37 m2/g, respectively. CeCu catalyst having highest basicity was found to be most effective during transesterification of PC to form DMC. Highest DMC yield of 71.9% and PC conversion of 65.4% was obtained with CeCu catalyst at the optimum reaction condition. A series of cerium-lanthanum catalysts were prepared using co-precipitation method. Catalytic activity was found to increase with an increase in the reaction temperature, reaction time and methanol/PC molar ratio and was found to be maximum in terms of DMC yield=74%, propylene glycol (PG) yield=65% and PC conversion=72% at optimum reaction conditions of reaction temperature=170°C, reaction time=6 h and methanol/PC molar ratio=10. Best catalytic activity was found for the catalyst having Ce/La molar ratio of 1/4. Ceria and zinc oxide were impregnated onto various oxide supports such as alumina (Al2O3), silica (SiO2) and titania (TiO2) in the molar ratio of 1:1:2 by depositioncoprecipitation method (CZA, CZS and CZT having supports Al2O3, TiO2 and SiO2, respectively). CZS having highest basicity and surface area showed best catalytic activity. The basic strength of SiO2 also helps in improving the highest catalytic activity. Cu-Zn-Al (CZA) hydrotalcite catalysts prepared by the co-precipitation method and calcined at 300°C, 500°C and 800°C (named as CZA300, CZA500 and CZA800) were used Abstract iv for the synthesis of DMC from methanol and PC. Pore volume distribution analysis revealed that the CZA300 and CZA500 have bimodal pore distribution with pores centered at 36±1 Å and 131±2 Å. Overall, an increase in the calcination temperature decreased the quantity of basic and acidic sites. Values of specific rate of reaction with CZA300 catalysts at 120°C, 140°C and 160°C were found to be 0.56, 0.58 and 0.65 h-1, respectively. PC conversion of 70% and DMC selectivity of 94% was observed at the optimum operating condition of reaction time=4 h, methanol/PC molar ratio=10, catalyst dose=3 wt.% of PC and temperature=160 °C. Values of the frequency factor (ko) and the activation energy (Ea) were found to be 0.375 h-1 and 2.294 kJ/mol for CeCu catalyst, whereas respective values for CZA300 catalyst were found to be 0.225 h-1 and 12.72 kJ/mol. Among all the catalysts tested, Ce-Zn/SiO2 showed highest PC conversion of 89% and DMC yield of 78%. Overall supported catalysts seem to be better option for use in the transesterification reaction of PC with methanol for DMC synthesis. Work of the direct conversion of CO2 to DMC DMC synthesis via direct conversion of CO2 was studied with different sets of catalysts namely: ceria-zirconium, ceria-manganese and ceria-calcium catalysts. The catalysts CeO2, ZrO2 and Ce0.5Zr0.5O2 were synthesized by hydrothermal method. The basic sites density and the acidic site density of synthesized catalysts were in the order: ZrO2 < CeO2 < Ce0.5Zr0.5O2. Under optimized reaction condition of reaction temperature=120 °C, reaction time=24 h, catalysts dose=1.25 g and pressure=150 bar, the optimum yield of DMC was obtained as 2.56 mmol/g-cat for Ce0.5Zr0.5O2 catalyst. Cerium-zirconium mixed oxide catalysts were also synthesized using an exo- and endo-templating method applying different Cex-Zr1-x (x=0 to 1) molar ratios. Polymer based activated carbon spheres were used as exo-template, and pluronic was used as endo-template. The Cex-Zr1-x (x=0.5) catalyst showed the highest basic sites and acidic site among all the catalysts, and giving the highest DMC yield. MnOx-CeO2 catalysts were synthesized by surfactant-template method with different ratio of Mn/Ce. The optimum yield of 2.514 mmol was found with Ce1-Mn0.125 at a pressure of 150 bar, reaction temperature 140°C, catalysts dose 1.25 g and reaction time 24 h. A series of CeO2-CaO catalysts with different Ce/Ca ratio (Ce3-Ca1, Ce1-Ca1, and Ce1-Ca3) were synthesized by surfactant-template method. Hexadecyltrimethyl ammonium bromide (CTAB) was used as the template. Ce1-Ca1 catalyst was found to possess highest activity owing to its high surface area, acidity and basicity as compared to other catalysts. Basic thermodynamic calculation shows that DMC synthesis from CO2 and methanol at 393 K becomes spontaneous at pressure ≥ 6.3×104 MPa which is very difficult to achieve. The Peng–Robinson–Stryjek–Vera equation of state (PRSV-EoS) along with the van der Waals one-fluid (1PVDW) mixing rule, were used to calculate the fugacity coefficient of species in the mixture which in turn was used to calculate equilibrium conversion at various temperatures. The values of the heat of reaction ( r ΔHo) and Gibbs free energy change ( r ΔGo) for Ce0.5Zr0.5O2 (hydrothermal method) using the data points at T=120-150oC were found to be -45.66 kJ/mol and 25.04 kJ/mol, respectively. Similarly, for Ce0.5Zr0.5O2 (templating method), the respective values were found to be -139.76 kJ/mol and 1.54 kJ/mol, respectively. Overall, Ce0.2-La0.8 and Ce-Zn/SiO2 were found to be the best catalysts for DMC production via transesterification reaction, whereas Ce1-Ca1 and Ce0.5Zr0.5O2 were the best catalysts for direct CO2 conversion reaction for DMC production.