SYNTHESIS AND CHARACTERIZATION OF PRASEODYMIUM AND ZIRCONIUM BASED CATALYSTS FOR DIMETHYL CARBONATE SYNTHESIS

Abstract

Coal, natural gas, and other fossil fuels have traditionally been used to provide the demand for fuels and additives. However, the toxic gases that release, while using these fossil fuels, into the atmosphere render them environmentally unsustainable. Consequently, it is preferable to utilize green chemicals that aid in reducing these emissions and offer a substitute for biodegradable materials. Dimethyl carbonate (DMC), a highly oxygenated chemical, with 53 wt.% oxygen content, can be employed in a variety of sectors, including those that produce paint, polymers, fuel, and cosmetics. It is biodegradable by nature and can be added as an electrolyte in lithium batteries. It is used as a raw material in carbonylation, and methylation operations and as a substitute to several hazardous compounds, including methyl chloroformate, dimethyl sulfate, phosgene, etc. The methanolysis of phosgene/urea, the oxidative carbonylation of methanol, the electrochemical method, the transesterification of methanol using propylene carbonate (PC)/ethylene carbonate (EC) with methanol, and the direct synthesis from carbon dioxide (CO2) and methanol are some of the routes through which DMC can be produced. In the present work, the transesterification route has been considered for the synthesis of DMC using methanol and propylene carbonate (PC) as raw materials, since it is environmentally friendly. The reaction can be carried out in a batch reactor in the presence of a catalyst, and the main products are DMC and propylene glycol (PG). The formed byproduct (PG) has its market value and may be used in the chemical industry. Several catalysts are used that can help in overcoming the activation energy of the reaction thus enabling this reaction to occur in the favor of the product. The heterogeneous catalysts such as bi-metallic catalysts gave better results in terms of yield as well as selectivity due to the active basic and acidic sites available for the adsorption of reactants. Several effective catalysts have been developed by various researchers for transesterification reactions, such as ZrO2, CeO2, MgOCeO2, ZnO, Fe-Zn, and ion exchange resins. There have been several attempts to combine rare-earth or transition metals to bring better performance in the reaction conditions. Praseodymium is one such element, from the family of rare earth metals where it is combined in a certain ratio with other metals and helped increase the yield and selectivity of the product in several chemical reactions. Further, zirconium-based catalysts, which are used in numerous chemical reactions, particularly organic synthesis, are in great demand. They exhibit Lewis acidic behavior, and can thus be employed in amination reactions, Michael addition reactions, oxidation reactions, transesterification reactions, and so on. In the available open literature ZrO2 has been used for the Leuckart reaction, and it has been found that minimizing the usage of noble metal catalysts and use of ZrO2, improved yield and increased product purity. In the present study, various catalysts using rare earth metal Praseodymium, zirconium, other metals, and waste such as eucalyptus leaves ash, e.g. Zr-Pr, Ti-Pr, MgPrE, ZrO2-E, and [ZrO(OH)2.(DMF)] were synthesized and evaluated for the transesterification reaction. The synthesized catalysts were analyzed through various characterization techniques, such as X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). The BET surface area and pore volume diameter have been studied through N2 adsorption-desorption using BET and BJH models respectively. The basicity was determined through the CO2-TPD for understanding the reaction mechanism. Titanium-praseodymium-based catalysts prepared through the co-precipitation method have been used to improve the yield and selectivity of DMC production. Different combinations of catalysts synthesized were referred to as Ti0.99Pr0.01, Ti0.97Pr0.03, Ti0.96Pr0.04, and Ti0.95Pr0.05, according to the molar ratio of Ti with respect to the Pr. The reaction was carried out in the batch reactor, keeping the temperature range of 160-180 °C and the molar ratio of methanol to PC in the range of 3-10. The study has also been made on oxygen vacancy concentrations in the mixed oxide catalysts due to the mixing of Pr with Ti, thereby affecting the yield and selectivity of DMC. The maximum yield of DMC (58.7%) was obtained with TiPr(3) catalyst at a temperature of 165°C and stirring speed between 600-650 rpm, which also resulted in the PC conversion of 81.7%, TOF of 0.120 h-1, and selectivity of 71.6% for DMC. The enhanced study made through various characterization techniques showed that the reaction requires a good amount of moderate and strong basic sites and appropriate oxygen vacancy on the surface of catalysts to enhance the yield and selectivity of DMC in the transesterification reaction of methanol and PC to DMC and PG. The Ti0.96Pr0.04 catalyst possesses a good number of basic sites well functional groups studied through FTIR and XPS to form suitable bonding with the methanol and PC on its sites (for adsorption) for enhancing the selectivity of DMC toward 71.6%.