Synthesis of methylamines from CO2, H2 and NH3 over Cu-Mg-Al mixed oxides

The synthesis of methylamines from carbon dioxide, hydrogen, and ammonia has been studied over catalysts derived from Cu-Mg-Al lamellar double hydroxides (LDH) with hydrotalcite-like structure. Catalysts containing 33 at% Cu(II) related to the total metal content, with different ratios of Mg(II) to AI(III), and a binary Cu-Al catalyst containing 67 at.% Cu were prepared by precipitation of metal nitrates with sodium carbonate. The effect of sample composition and of calcination temperature on the catalytic behaviour has been investigated. Catalytic tests were performed in a fixed-bed microreactor in the temperature range 473-573 K and at 0.6 MPa total pressure. Activity for methylamine formation increased for the ternary catalysts with increasing ratio x = M(III)/[M(II) + M(III)]. Maximum activity was observed for a catalyst with x = 0.33, for which XRD analysis indicated a pure hydrotalcite-like phase. Increasing the calcination temperature of this sample from 673 K to 773 K improved the activity, whereas a significant loss in activity was observed when calcined at 873 K. This behaviour could be traced to the copper surface area, which increased from 4 to 14 m(2) g(-1) (773 K) and then decreased to 9 m(2) g(-1) (873 K), depending on calcination temperature. After catalytic testing the catalysts calcined up to 773 K consisted of dispersed copper in an amorphous oxide matrix, whereas calcination at 873 K caused the additional formation of CuAl2O4. Distribution of the methylamines was similar for all catalysts with the fraction of monomethylamine being higher than 80%, besides of smaller amounts of di- and trimethylamine. Other carbon containing products observed were carbon monoxide, formed by the reverse water-gas shift reaction (RWGSR), and methanol in the absence of ammonia in the feed. No general correlation between either BET or copper surface area and catalytic activity was observed. However, for the Cu-Mg-Al catalyst with x = 0.33 calcined at different temperatures activity for methylamine formation roughly parallelled the copper surface area measured by N2O-titration. (C) 1999 Elsevier Science B.V. All rights reserved.