Design of a technical Mg–Al mixed oxide catalyst for the continuous manufacture of glycerol carbonate
The availability of a heterogeneous catalyst, which contains cheap and abundant elements, has a scalable synthesis, is highly active and stable, retains its performance upon shaping into a technical form and can be operated in continuous mode, would pave the way for a more ecological and economical production of glycerol carbonate from glycerol and urea. Here, we show that a mixed oxide of Mg and Al is a promising active phase for this reaction. The solid comprises widely available and non-toxic metals, is easily obtained through the thermal decomposition of a hydrotalcite-like material and can almost match the product yield of state-of-the-art Zn-based catalysts, while displaying an outstanding resistance against leaching, which causes the rapid dissolution of the latter. In-depth characterisation uncovered that Lewis-basic centres are crucial to activate glycerol through dehydrogenation. Their concentration was maximised by optimising the composition and calcination temperature of the precursor, thus reaching up to 60% glycerol carbonate yield. Millimeter-sized extrudates featuring comparable basic properties to the powder sample, a well-developed meso- and macroporosity and high mechanical stability are obtained using a natural clay, bentonite, as a binder and thermally activating the hydrotalcite only after shaping. Upon testing in a continuous reactor under tuned conditions of temperature and pressure and in the presence of an aprotic solvent, the system attains the same glycerol yield as in the batch tests. During 100 h on stream, its activity decreases by 20% due to fouling, but can be fully restored upon burning-off of the carbonaceous deposits. This work discloses the development of a green material that exhibits high efficacy in a sustainable transformation, highlighting key parameters that should be generally taken into account in the design of an industrially relevant chemocatalytic technology.