Abstract
Dialkyl carbonates are important industrial compounds that have a low toxicity and are readily biodegrade, also replacing some highly toxic and corrosive reagents in organic chemistry. There are a number of synthetic routes towards the synthesis of dialkyl carbonates, including two commercial processes (ENIChem S.p.A. and UBE Industries, LTD). The ENIChem process involves the carbonylation of methanol in the presence of CuCl2 as a catalyst. The major drawback of this process is in the use of an explosive gas mixture (CO/O2) under certain conditions. The UBE process is a two step reaction, whereby the methanol reacts with O2 and NO in the presence of a PdCl2 catalyst to form methyl nitrite and water, followed by carbonylation of methyl nitrite to form DMC and reform NO. The major drawback associated with this process is in the combination of methanol, nitric oxide and oxygen which is also explosive under some conditions.
The transesterification reaction between a cyclic carbonate and an alcohol in the presence of a catalyst provide an alternative route towards synthesis of dialkyl carbonates, producing a glycol as by-product. This synthetic route is environmentally friendly, decreases explosion possibilities, and the reagents employed in this process are less hazardous than those of other processes. The main aim of this study was to identify and optimise the catalyst systems that could promote the transesterification reaction effectively. A number of homogeneous and heterogeneous, acidic or basic catalysts were evaluated during this study.
The study revealed that basic homogeneous catalysts such as TBD, DBU, DBN, MTBD, DABCO, and Verkade bases are effective for the transesterification reaction. The basic heterogeneous catalysts such as Amberlites® IRA 96, IRA 67 and IRA 400 showed good catalytic behaviour, but they eventually became deactivated. On the other hand, homogeneous Lewis acids such as La(OTf)3, Gd(OTf)3, and Sm(OTf)3 demonstrated good activity, even though they need high temperatures, i.e. 150 °C. The heterogeneous acidic systems such as Amberlyst® 15, Amberlyst® 36, silica, alumina, etc., showed much lower activity, if any was observed.
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Due to the fact that these reactions were carried out above room temperature and analysed at room temperature in the GC, it was important to understand the equilibrium shift under such temperature variations, and NMR studies were used here. There was no significant difference in equilibrium conversion between the NMR reactions and the autoclave reactions (analysed in a GC), indicating slight influence of temperature variation. The results obtained from the NMR study were used to calculate the reaction kinetics. The calculations indicated a direct proportional increase of the rate with respect to the catalysts pKa values.
Prof. D.B.G. Williams