Influence of the operating conditions on the behavior and deactivation of a CuO-ZnO-ZrO₂@SAPO-11 core-shell-like catalyst in the direct synthesis of DME

The behavior of the CuO-ZnO-ZrO₂@SAPO-11 core-shell catalyst in the direct synthesis of DME from H₂ + CO + CO₂ mixtures has been assessed. The effect of the reaction conditions (temperature, pressure, space-time) and feed composition (CO₂ and H₂ concentration) on the DME yield and selectivity, CO₂ and CO_x (CO₂+CO) conversions and stability of the catalyst have been studied. The experiments have been carried out in a fixed-bed reactor in the following condition ranges: 250–325 °C; 10–50 bar; space-time, 1.25–15 g h mol_C⁻¹; CO₂/CO_x molar ratio in the feed, 0–1; and H₂/CO_x molar ratio in the feed, 2.5–4; time on stream, up to 48 h. Under mild conditions (275–300 °C range, 20–30 bar, space-time over 5 g h mol_C⁻¹, CO₂/CO_x molar ratio in the 0.5–0.75 range, and H₂/CO_x molar ratio around 3) a good compromise is reached between the yield of DME and the conversion of CO₂, with high catalyst stability. Coke deposition is the main cause of catalyst deactivation, formed by condensation of the hydrocarbon byproducts, blocking the metallic sites in the core. The formation rate of this fraction of coke is greater than that of the coke deposited in the SAPO-11 of the shell. Increasing reaction temperature favors the formation of coke, while co-feeding CO₂ attenuates this formation, due to the increase of H₂O concentration.