Catalysis & Kinetics of Non-Phosgene Route to Synthesis of Dimethyl Carbonate (DMC)

Converting CO2 into value-added chemicals or fuels is one of the major sustainability challenges facing human society. Catalytic manufacture of dimethyl carbonate, a green chemical, using CO2 as a starting material attracts increasing attention, because it provides an alternative environmentally friendly route. However, several fundamental issues need to be investigated to improve this technology: (a) development of heterogeneous catalyst with high activity, selectivity and stability; (b) kinetics and mechanism of transesterification required to provide a basis for catalyst improvement and design of suitable reactors. In this thesis, a study on transesterification of alkyl carbonates has been presented using two types of catalysts: Metal Oxides (e.g. CaO) and Double metal cyanides. In one part, transesterification using CaO catalyst is presented to address the significant effect of catalyst pre-treatment using reactants on catalytic activity. Upon CaO pretreatment with methanol, the transesterification activity (TOF) increased significantly. In sharp contrast, pretreatment with cyclic carbonates resulted in a prolonged induction time and rate inhibition. Additionally, various characterization (SEM, CO2-TPD, XRD, FT-IR, XANES and 13C-NMR) was done on fresh CaO and treated CaO to explore the factors affecting catalytic activity. It is detected that strong basic sites have close correlation with catalytic activity. Furthermore, the formation of Ca(OCH3)2 is a key step during the pre-treatment process. Detailed investigations on catalyst recycle, effects of substrate types and reaction parameters (reactant concentrations, temperature and catalyst loading) on conversion, selectivity and initial rates are reported. The experiments revealed that with CaO as catalyst, significant contribution of the reaction is due to homogeneous catalysis from sparingly soluble CaO under reaction conditions. Therefore, an approach to analyze simultaneous homogeneous-heterogeneous catalytic transesterification has been discussed. Based on experimental concentration-time data in batch slurry reactor, detailed kinetic modeling of transesterification of propylene carbonate to DMC in both homogeneous phase as well as heterogeneous phase is reported using both empirical power law and microkinetic (based on molecular level description of catalytic cycle) models, during which corresponding rate parameters for each model were fitted and determined. In another part of the thesis, a truly heterogeneous double metal cyanide catalyst system is reported which eliminates the problems of leaching observed in metal oxide (CaO) catalysts. It is observed that transesterification of various cyclic carbonates, such as ethylene carbonate, propylene carbonate, and 1, 2-butalene carbonate to dimethyl carbonate can occur over double metal cyanide complex with high activity, selectivity and stability. Detailed investigation of the morphology and structure of the catalysts are done through different characterization techniques (BET, SEM, TEM, XRD, XPS, TGA, FT-IR and UV-Vis). Studies on different reaction parameters (catalyst loading, initial methanol/PC molar ratio, temperature and different cyclic carbonates) and surface characterization enabled the establishment of activity-performance correlation for cyclic carbonate conversion. Further, kinetic modeling using Fe-Mn double metal cyanide complex is reported in which different kinetic models based on different reaction mechanisms are discriminated to fit with the experimental data. The kinetic studies in this work provide guidance for postulating reaction mechanisms and better insight into activation mode of reactants. In the last, a brief study of transesterification of DMC with phenol for synthesis of diphenyl carbonate (a key intermediate for polycarbonates) is presented. This is an example of a highly equilibrium limited reaction which gives very low reactant conversions (< 3-5%) in batch reactors. The preliminary results presented demonstrate that with simultaneous removal of a co-product methanol, significantly higher reactant conversions can be achieved. The preliminary study suggest that reactive distillation approach can be effectively used to achieve high conversion in DPC synthesis. The methodologies developed in this work will provide insights on rational design of catalysts for ring-opening reactions as well as understanding of the reaction mechanism and catalytic cycles