Synthesis of Dimethyl Carbonate from Urea and Methanol

There is a great concern on the utilisation of CO2 due to its increased atmospheric concentration by anthropological emissions in the last few decades. Therefore, the fixation of inexpensive and abundantly available CO2 in valuable chemicals is gaining lot of interest in recent times. Despite this, there are only a few industrial processes where CO2 is consumed directly. One such process is the synthesis of urea by the reaction of CO2 with NH3. About 70 Mt of CO2 p. a. are converted by this method worldwide [1]. Moreover, synthesis of various commercially important organic carbonates using CO2 is another suitable option of the future. Dimethyl carbonate (DMC) in particular receives huge interest from scientific community as a good substitution for phosgene in the polycarbonate synthesis. Until the 80s, DMC was produced mainly by phosgene alcoholation. Later on, this synthesis method was replaced by oxidative carbonylation of methanol (e.g. EniChem, UBE) [2]. Another synthesis method is the transesterification of ethylene carbonate with methanol. However these methods are still using toxic, corrosive und explosive substances. On the other hand, the alcoholation of urea with methanol to produce DMC is an interesting approach from both ecological and economical points of view and the urea synthesis usually occurs by the direct use of carbon dioxide. The reaction runs with methyl carbamate (MC) as an intermediate: The synthesis of MC is thermodynamically more favorable than the second step and can run even without any catalyst. Besides metal oxide catalysts (MgO, ZnO, etc.), polyphosphoric acid are used for this reaction. In this contribution, we describe the application of ZnO, MgO, CaO, TiO2, ZrO2 or Al2O3 catalysts for the synthesis of DMC from urea and methanol. In the present study, the catalytic activity of different metal oxides towards DMC synthesis was checked and additionally a comparison of achieved conversions with that of predictions made by thermodynamic calculations was also carried out. The achieved conversions are in good agreement with those of calculated ones. [1] M. Aresta, A. Dibenedetto, Dalton Trans. (2007) 2975. [2] T. Sakakura, K. Kohno, Chem. Commun. (2009) 1312.