CO2 is a major contributor of climate change and worldwide containment of CO2 is a major challenge. In current scenario the challenge for decreasing CO2 in atmosphere needs new ideas and technologies. Generating value added chemicals through green chemistry especially by utilizing greenhouse gas carbon dioxide as raw material is a challenging task. Present fuel oxygenates like ether compounds methyl tert, butyl ether (MTBE), ethyl tert. butyl ether (ETBE), tert. Amyl methyl ether (TAME) and alcohols like methyl alcohol (CH3OH), ethyl alcohol (C2H5OH) are being used as oxygenates. However, aforementioned oxygenates lead to many problems like corrosiveness, environmental harm, and solubility issues. In order to circumvent these problems dialkyl carbonates are alternate future fuel oxygenates exhibiting attractive environmentally benign nature like non-toxic, non-corrosive and low toxic emissions are being explored.
The dialkyl carbonates especially dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) have been widely accepted as fuel oxygenates because of excellent gasoline blending properties like high blending octane numbers, low blending reid vapour pressures (RVP) and lower amounts of toxic emissions compared to ether oxygenates. Several other applications of dialkyl carbonates were explored in green chemical industry for the preparation of polycarbonates, isocyanates, synthetic lubricants, pharmaceutical and agricultural intermediates. In addition, dialkyl carbonates are non-corrosive and highly efficient alkylating agents to replace hazardous phosgene. Due to high volatile value, they are widely used in paint industry.
Scientists have developed several processes for the synthesis of dialkyl carbonates. The conventional process involves usage of hazardous chemicals like phosgene or carbon monoxide as starting materials. The alternative and non-hazardous process involve utilization of CO2 as raw material, which is eco-friendly and cost-effective process.
The eco-friendly synthetic route involves reaction of carbon dioxide with alcohol to generate corresponding dialkyl carbonates. Amongst dialkyl carbonates, dimethyl carbonate and diethyl carbonate synthesis has been well recognised and explored. In prior art literature, number of homogeneous catalysts (e.g. tin complexes: distannoxanes) and heterogeneous catalysts (e.g. CeO2, modified Zirconia, Zeolites) have been investigated and have shown limitations of poor conversions either due to the thermodynamic limitation of reaction or due to catalyst deactivation.
Therefore, finding an alternate route for the synthesis of DMC and DEC from CO2 involving in situ transesterification of cyclic carbonate (synthesised from epoxide and CO2) with alcohol in presence of suitable catalyst would be well attempted. Significant efforts have already been devoted in efficient catalyst development for DMC & DEC synthesis from cyclic carbonates. The synthesis of DMC and DEC from epoxide through two step mechanism involving synthesis of cyclic carbonate in 1st step and then transesterifying to DMC and DEC is not economical for commercialisation.
Keeping in view the limitation associated with above processes, one-pot synthesis of DMC/DEC from CO2, alkylene oxide and alcohol was explored to make the process more viable, eco-friendly and cost-effective. Single-pot synthesis of DMC/DEC from CO2, alkylene oxide and alcohol in presence of several catalysts mainly metal oxides, inorganic bases/metal oxides, inorganic bases/phosphonium halide on polyethylene glycol, TBAB/Et3N, ionic liquid, KOH/4A° MS, biomettalic Cu—Ni/4A° MS, anion exchange resins, and Mg/Smacilite etc. have been reported in the literature.
The process conditions in single step synthesis of DEC and DMC from propylene oxide in literature references are reported to involve low catalytic activity, formation of side products, difficulty in catalyst separation and recyclability, and requirement of costly raw materials to synthesise the above catalysts.
Several patents have been published on synthesis of dialkyl carbonates from transesterification of cyclic carbonates with appropriate alcohol in presence of both homogeneous and heterogeneous catalysts. Few publications have been found on direct synthesis of dialkyl carbonates from alkylene oxide, CO2 and alcohol in single-pot reaction.
Single-pot synthesis of dialkyl carbonates is reported in U.S. Pat. No. 4,434,105 (EP 000777), which reveals the synthesis of dialkyl carbonates namely dimethyl carbonate and diethyl carbonate from ethylene oxide/propylene oxide, methanol/ethanol and carbon dioxide in the presence of homogeneous as well heterogeneous catalysts in single pot reaction. Dialkyl carbonates mainly DMC and DEC were prepared in the presence of catalysts NaI/Tl2CO3, NaI/TlOH, NaI, Tl2CO3, Imidazole, TBAB, TEAB and organic bases like guanidine, DABCO, triethanol amine and etc. Yields of DMC and DEC are high with ethylene oxide compared to propylene oxide, which may be due to sterically more feasible product formation. The main drawback of this work is expensive catalysts and halogen containing catalysts.
U.S. Pat. No. 5,218,135 has revealed two step synthesis of dialkyl carbonate from CO2, alkylene oxide and alcohol in presence of bifunctional catalysts. Initially cyclic carbonate was prepared from alkylene oxide and CO2 in presence of bifunctional catalysts then cyclic carbonate was treated with alcohol in presence of bifunctional catalyst to yield corresponding dialkyl carbonate.
Recently US 2013/0267727 A1 and also US 2006/7084292 B2 have explored the feasibility of integrated process for production of dialkyl carbonates and diols from alkylene oxide, carbon dioxide and aliphatic alcohol. In this integrated process, initially alkylene oxide was reacted with carbon dioxide to produce crude cyclic carbonate in the presence of homogeneous catalyst. Second step involve reaction between crude cyclic carbonate and alcohol in the presence of heterogeneous catalysts like metal oxides to produce dialkyl carbonates.
U.S. Pat. No. 5,218,135, U.S. Pat. No. 7,491,837 B2 and WO 03/000641 A1 also described the integrated process of making dialkyl carbonates for alkyl alcohol, carbon dioxide and alkylene oxide. They initially prepared cyclic carbonate from alkylene oxide and carbon dioxide in presence of homogeneous catalyst. In second process cyclic carbonate reacts with alcohol for producing dialkyl carbonate in the presence of transesterification catalyst.
The catalysts used in the prior art are expensive, non-renewable and non-recyclable. Formation of by-product H2O during reaction deactivates the catalyst for further use. Also low product yields are reported in references of prior art.
In prior art literature, heterogeneous catalysed single pot synthesis of dialkyl carbonates from propylene oxide, CO2 and methanol in presence of Mg-Smectite gave 32.30% of dimethyl carbonate (Green chemistry, 2003, 5, 71-75).
Present invention discloses a preparation methodology of dialkyl carbonates using wood ash catalyst; involving insertion of CO2 molecule in to an alkylene oxide and transesterification of that intermediate compound with aliphatic alcohol in to dialkyl carbonate in single pot reaction. The inherent property of wood ash catalyst of CO2 insertion along with the property of transesterification was explored for the synthesis of dialkyl carbonates.
The present invention addresses one or more such problems of the prior art as discussed above. However, it is contemplated that the invention may prove useful in addressing other problems also in a number of technical areas.