(134d) Heterogeneous Catalytic Systems for the Synthesis of Organic Carbonates

We have shown that glycerol carbonate can successfully be synthesised at atmospheric pressure via the indirect carbonylation of glycerol with urea, and demonstrated that higher yields and selectivities of glycerol carbonate can be obtained through the use of newly developed heterogeneous catalysts, as opposed to previously reported homogeneous metal sulphates. The production of glycerol carbonate with this method has recently gained much popularity due to the low cost of the starting materials, and the ability of the overall process to successfully utilise a CO₂ carrier during the reaction. This environmentally friendly, ?green' process involves the reaction of glycerol and urea at mild temperatures (ca.100 ^oC). The favourability of this reaction not only lies upon its ability to synthesise glycerol carbonate from two readily available and cheap starting materials, but also the possibility of re-converting the ammonia that is released during the reaction back to urea with the Bosch-Meiser process, overall resulting in a chemical cycle that utilises CO₂.Organic carbonates are used as solvents, lubricants, plasticizes and monomers as well as in methylation and carbonylation processes for preparing isocyanates, polyurethanes and polycarbonates. Organic carbonates have been added to gasoline resulting in an improved octane value. Furthermore, their addition to diesel effectively reduces carbon particulates and their application as fuel additives potentially has a very large market. The formation of the organic carbonates via the reaction of alcohols with urea has the advantage that it can be utilised to carbonylate a wide range of alcohols as starting materials. However, current catalysts cited in the patent and commercial literature are based on metal sulfates and other catalysts that suffer from low intrinsic activity and selectivity. Although sulfate catalysts form the desired carbonate, the reaction is slow and requires very elevated quantities of the active metal, which, in addition also suffer from intense leaching, i.e. loss of active catalyst components during reaction, so that the reaction actually occurs catalytic homogeneous process instead of in a highly desired heterogeneous mechanism where the catalyst can easily be recovered as a solid. We have identified suitable heterogeneous catalysts for this reaction and also demonstrated the ease of purification for the product, with a simple vacuum-distillation yielding a product of ca. 98% purity. In this work we report the catalytic activity of heterogeneous catalysts based on gallium, zinc, silver, palladium and gold nanoparticles supported onto a range of oxides and zeolitic materials. The addition of each component to an acidic zeolite support (ZSM5) causes a dramatic increase in the selectivity of the reaction towards glycerol carbonate, while the effect on the conversion of glycerol strongly depends on the choice of oxide support. A major outcome of our research has been the unforeseen discovery that gold is a highly effective metal for the transformation of glycerol to glycerol carbonate. The postulated mechanism suggests that glycerol carbonate forms via the cyclization of a glycerol urethane intermediate.