Heterogeneous carbonylation of dimethyl ether to methyl acetate over bifunctional catalysts containing Rh and heteropoly acids

The carbonylation of dimethyl ether to methyl acetate was carried out in a high-pressure flowed fix-bed reaction system over heterogeneous halide-free catalysts. The TG curves indicated that solid 12-tungstosilicic acid had a crystal structure (with six crystallization waters) identical to that of solid 12-tungstophosphoric acid, although they had different number of protons in molecules. The BET surface areas greatly increased by Cs substitution in H4SiW12O40, and Cs3HSiW12O40 showed a BET surface area of 206 m2 g−1. For the DME carbonylation at 473 K on 1 MPa under a CO/DME flow rate ratio of 18, Cs3HSiW12O40 showed a DME conversion (6.7 %) higher than that over Cs2HPW12O40 (4.6 %) because Cs3HSiW12O40 had a larger surface area. Cs3HSiW12O40 and Cs2HPW12O40 showed high selectivity for methyl acetate of (98.0 and 97.5 %). By adding 1.0 wt% Rh in Cs3HSiW12O40, the DME conversion greatly increased from 6.7 to 30.8 % because a multiplier effect occurred between Rh and heteropoly acid. Rh/Cs3HSiW12O40 showed a higher catalytic performance than those over the reported Rh/Cs2HPW12O40 and Rh/H-MOR for the carbonylation. The DME conversion increased with increasing Rh loading from 0 to 1.0 wt% but became almost constant when Rh loading ranged from 1.5 to 3.0 wt% over Rh/Cs3HSiW12O40. With increasing the CO/DME ratio from 1.8 to 45, the DME conversion increased from 0.9 to 99.4 % over Rh/Cs3HSiW12O40 at 473 K under 1 MPa. The 1 wt% Rh/Cs3HSiW12O40 catalyst showed an initial DME conversion of 30.8 % (after 5 min on stream) but the DME conversion decreased to 23.3 % after 5 h on stream at 473 K on 1 MPa under a CO/DME flow rate ratio of 18.