Na-dawsonite derived aluminates for DMC production by transesterification of ethylene carbonate

摘要

The transesterification of ethylene carbonate with methanol to dimethyl carbonate, an alternative phosgene-free route to DMC, was investigated over aluminas derived from calcination at different temperatures of Na- and NH4-dawsonites. The influence of the monovalent cation (NH_4~+, Na~+) on the thermal decomposition of the starting dawsonites and derived crystallinity, morphology and porosity of the resulting aluminates was studied. The physico-chemical properties of the dawsonite precursors and the derived oxides were characterized by ICP, in situ XRD, TGA-MS, TEM, N2 adsorption, FTIR, and ~(27)Al MAS-NMR. Phase transitions during thermal decomposition are dependent on the monovalent cation in the original dawsonite, i.e. the NH4-counterpart is transformed into finely dispersed and highly amorphous aluminas with well-developed porosity in the range 473-1073 K, while Na-dawsonite exhibits an intermediate amorphous sodium-containing alumina phase at 523-773 K, which crystallizes into sodium aluminate at 773-1073 K. Tests were carried out in a parallel reactor system at 298-343 K, methanol-to-ethylene carbonate ratios of 2-12, and 0.1-10 wt.% catalyst amount. Tailoring the catalyst activation conditions is essential to optimize the transesterification performance, and consequently the production of DMC, a valuable environment-friendly chemical for many potential applications. Any of the oxides derived from NH4-dawsonite were in active in the reaction. The basicity of the Na-containing oxides, attained by calcination of Na-dawsonite at 973 K, is required to obtain an active catalyst, which rendered a maximum DMC yield ca. 65%. Recycling experiments demonstrated that activated dawsonites can be successfully reused.