STUDIES ON THE SYNTHESIS OF CATALYSTS FOR DIETHYL CARBONATE PRODUCTION

Abstract

Petroleum, petrochemical and chemical industries largely depend on fossil based carbon resources for their growth. Considering continuous depletion of these resources, there is an urgent need to develop processes which can produce alternative fuels and chemicals. Recently, many papers have been published on use of organic carbonates as alternative fuels. Organic carbonates and ethers like dimethyl carbonate (DMC), diethyl carbonate (DEC) and dimethyl ether, have attracted greater attention in the fuel and chemical industries due to their fuel additive nature and other salient applications. DEC, is a typical alkyl carbonate which possess properties of non-toxicity and biodegradability and both of them have high oxygen contents. It can be synthesized from either carbonylation or transesterification of ethanol using various agents. The oldest method known among them is the phosgenation of ethanol, however, this process is inherently toxic. A number of new non-phosgene routes have been developed in the recent years which include oxidative carbonylation of ethanol, transesterification of carbonates and alcohols, ethanolysis of urea, ethanolysis of CO2 and de-carbonylation of diethyl oxalate. Many studies have already been reported aiming towards sequestration of CO2. Carbonylation of ethanol using CO2 is directly related to sequestration of CO2 but is constrained by its less favourable thermodynamics. Transesterification of ethanol using various organic carbonates is a well-known route in which exchange of groups between alcohol and carbonate occurs. Various organic carbonates on which studies have been reported earlier include DMC, ethylene carbonate (EC) and propylene carbonate (PC). Ethanolysis of urea is also widely used because of its low-cost reactants in comparison to other reactions. Since PC itself involves CO2 for its formation, transesterification of PC and ethanol to give DEC can also be termed as the green route for the synthesis of DEC. An exhaustive review of the literature reveals that only a few catalysts have been tested for the synthesis of DEC via urea carbonylation route and PC transesterification route. Considering various possibilities, the main aim of the present work was to prepare and characterize various catalysts (cerium-zinc based oxides, Zn-Al-M (M=Ca, La, Mg and Y) ternary oxides, Mg-La based catalysts and hydroxyapatite based catalysts) with different physico-chemical and textural characteristics; and to evaluate catalytic activity for the synthesized catalysts for DEC synthesis via carbonylation of ethanol using urea route, and transesterification of ethanol and PC route. It was also aimed to perform thermodynamic analysis of various possible DEC synthesis routes. It was also aimed to optimize the operating conditions such as temperature, molar ratio of raw materials, amount of catalyst; to Abstract ii study the kinetics of DEC synthesis; to explore the catalytic stability and recyclability of the synthesized catalysts, and to investigate the possible mechanism of DEC synthesis using these reactions. Thermodynamics study of various routes for the synthesis of DEC In this section of the study, thermodynamic analysis of various possible synthesis routes of DEC was carried out and a comparative analysis has been performed. Chemical equilibrium constants at standard conditions were calculated using Gibbs free energy of the system. Benson group contribution method was used to estimate standard heat of formation and standard entropy change of some raw materials/components like DEC. Variation of heat capacity (Cp) with temperature was estimated for different components from Rozicka-Domalski model. Variation of chemical equilibrium constants with temperature and pressure was studied for various routes. CO2 route was found to be most unfavourable route for DEC synthesis due to stability of CO2 molecule. DEC synthesis through urea route was found to be best at high temperature since the equilibrium constants were found to increase exponentially. EC route was found to be best though equilibrium constant was found to gradually decrease with an increase in temperature. DMC route was also found to be moderately favourable. This study helped in comparing and choosing better DEC synthesis route and also helped in finding the optimum conditions for the DEC synthesis. Catalytic study for the synthesis of DEC from carbonylation of ethanol using urea. Ethanolysis of urea, is a two-step process. In first step, ethyl carbamate (EC) is formed which in second step reacts with ethanol to give DEC. First, cerium-zinc based oxides in various molar ratios were used for DEC synthesis from ethanol and urea. These catalysts were prepared by sol-gel self combustion process and characterized by thermogravimetric analysis (TGA), Raman spectroscopy, X-ray diffraction (XRD), N2 adsorption−desorption, scanning electron microscopic-energy dispersive atomic spectra (SEM-EDAX), Fourier transformed infrared spectroscopy (FTIR), and temperature programmed desorption (TPD) of NH3 and CO2. No additional phases except pure Ce and Zn oxide were observed in the catalyst from XRD analysis. Effects of pore surface area and surface acidity/basicity were found to be profound during the synthesis of DEC. Presence of higher amount of ZnO in CeO2-ZnO was found to enhance the basicity of the catalysts, and hence the DEC yield. FTIR and NH3-TPD confirmed high acidic density activated the first step of the reaction leading to increase the yield of EC. Ce0.1-Zn0.9 due to its fewer mild acidic sites and maximum basic sites density, performed better than other oxides. Ce0.1-Zn0.9 performed best among all the Abstract iii catalyst for which 28.8% DEC and 52.9% EC yield was obtained in 5 h at 190 °C. Raman spectroscopy and XRD pattern showed that the structure of Ce0.1-Zn0.9 oxide was intact even after 4 regenerations thereby proving the stability to the catalyst. Also, synthesis of DEC was carried out from ethanol and EC using Zn-Al-M based ternary oxides catalysts. Zn-Al-M (M=Ca, La, Mg and Y) were synthesized using two methods and their activity have been explored in depth. Pure metal oxides were observed in the XRD analysis and Al2O3 was found to be in amorphous form. Third metal oxide prepared from impregnation method was found to be present on the surface as well as in impregnated form. BET surface area of catalysts prepared by precipitation method was found to be more than that prepared by impregnation method. This may be due to the plugging of pores of Zn-Al oxide by third element during the impregnation method. All the catalysts were found to posses strong basic sites. Yield of DEC was found to in relation to the basicity of the catalysts. The ternary metal oxides prepared from precipitation method were found to be more active than the impregnation method. Zn-Al-Mg was found to be most active catalyst owing to its most basic character. The activation of EC by catalyst was studied by carrying out FTIR spectra of EC interacted catalyst. The effect of precipitants was also studied by synthesizing Zn-Al-Mg using NaOH and liquid NH3 as precipitants. Zn-Al-Mg catalyst prepared using liquid NH3 as precipitant was also found to possess activity similar to Zn-Al-Mg precipitated from NaOH, although the latter was slightly more effective. Zn-Al-Mg prepared from precipitation method was found to be reusable three times with only 5% decline in its activity. Overall, DEC yield of 40.2% and turn over frequency (TOF) of 1055 mgDEC/(gcat.h) was obtained in 5 h at 190 °C using ethanol/EC ratio of 10 and Zn-Al-Mg catalyst (10 weight% of EC). Overall, Zn-Al-Mg catalyst showed better yield and TOF than most of the studies reported on mixed oxides. Kinetics of both the steps of urea ethanolysis was studied. The activation energy of production of EC from ethanol and urea was found out to be 0.19 kJ/mol whereas the activation of energy for the second step (i.e. synthesis of DEC from EC and ethanol) was found to be 87.3 kJ/mol. Hence, for the second step, catalysts are needed while synthesis of EC needs no catalysts for its formation. Catalytic study for the synthesis of DEC from transesterification of ethanol and PC. Synthesis of DEC from transesterification of PC and ethanol is a potent non-phosgene route owing to its favorable thermodynamics. Since PC is synthesized from propylene oxide and CO2, this route involving utilization of PC, can be termed as sink of CO2. Hydroxyapatite (HAP) are naturally occurring minerals known for their excellent ion-exchange ability. In this Abstract iv study, hydroxyapatite was synthesized and further modified using Zn and Mg (Zn/HAP and Mg/HAP). Mg and Zn were found to be dispersed on the HAP surface in the amorphous form which was confirmed from XRD, FTIR and N2 adsorption desorption technique. On modifying the HAP with Mg, the crystallite structure was observed to be increased while it was observed to be decreased when it was modified with Zn. Agglomeration of particles was found since crystallite size was found to be smaller than particle size. All the catalysts were found to possess strong basic sites and the yield of DEC was found to strongly depend on the total basicity of the catalysts. HAP modified with Mg and Zn enhanced the yield of DEC and hence had a positive effect for the synthesis of DEC from PC. Mg/HAP was the best performing catalyst owing to its strong basicity. Larger pore sizes along with high BET surface area of Mg/HAP also enhanced the yield when compared with HAP and Zn/HAP. Effect of reaction conditions like temperature, time and ethanol/PC molar ratio on DEC yield was also studied. Maximum DEC yield of 52.1% was obtained in 5 hours at 160 °C using Mg/HAP catalyst. Also, Mg-La based catalysts with various Mg/La molar ratios (0.5, 1, 2, and 4) were synthesized using precipitation method. Lanthanum hydroxyl carbonate was the main component observed in Mg/La catalysts with the X-ray diffraction (XRD) analysis. Mg-La2 was found to the best performing catalyst among the synthesized catalysts. The effect of basicity was found to be profound since high correlation of PC conversion was obtained with basicity of catalysts. The reaction parameters like reaction temperature, reaction time and ethanol/PC on yield of DEC were also optimized. 46% DEC yield was obtained with 63.6% PC conversion and 72.3% selectivity at 150 °C in 4 h using 1.3% catalyst. The reaction mechanism for the synthesis of DEC from PC starts with the activation of ethanol with formation of active ethoxy group. The ethoxy group attacks the carbonyl group of PC. The cyclic chain of PC was opened and the second ethoxy group attacks the other carbonyl group followed by rearrangement of the intermediate complex to give DEC and PG. The kinetics of the reaction was also studied with the best performing catalyst. Activation energy of the reaction was calculated to be 24.7 kJ/mol which was reduced upto 15.1 kJ/mol when Mg-La2 was used. Overall, Zn-Al-Mg was found to be the best catalyst for DEC production via carbonylation of ethanol using urea, whereas Mg/HAP and MgLa2 were the best catalysts for transesterification of ethanol and PC.