Study On New Catalysts For One-step Dimethyl Ether Synthesis From Syngas

Dimethyl ether (DME) is one of the important products of coal chemical industry. The one-step DME synthesis from synthesis gas (syngas) process is developed in a single reactor over bi-functional catalysts composed of copper-based methanol synthesis catalysts and dehydration catalyst. The combination of methanol synthesis and dehydration reactions results in a synergistic effect relieving the unfavorable thermodynamic for methanol synthesis and increasing single conversion of CO. So it attracted many researchers’ attention.Ta2O5 modified γ-Al2O3 samples were prepared by impregnating method, and mixed with a methanol synthesis component Cu/Zn/Al2O3 at a mass ratio of 1:1 to form the bi-functional catalysts. Through catalyst characterization and reaction evaluation on fixed-bed reactor, the impact of Ta2O5 modified e-Al2O3 as methanol dehydration catalyst on one step process of DME synthesis from syngas was studied. The results revealed that on Ta2O5 modified γ-Al2O3 samples, the weak acid strength of the γ-Al2O3 weakened but total number of acid sites increased beacuse of appearence of strong acid, which reduced the initial activation temperature of catalysts. The Ta2O5 modified γ-Al2O3 catalysts showed up a higher DME selectivity within a low temperature range of 220 to 250 ℃ in the process of producing DME from syngas. And γ-Al2O3 loading with 6wt% Ta2O5 and Cu/Zn/Al2O3 formed 6Ta-Al2O3/CZA bi-functional catalyst. Over this bi-functional catalyst, CO conversion was 82.5%, DME selectivity was 59.3%, CH3OH selectivity was 8.2% under 250℃,3MPa and 1500ml(g·h).The bi-functional catalyst was shaped by extrusion, selecting pseudo-boehmite (γ-Al2O3 precursor) as binder, adding peptizer (HNO3) and bi-functional lubricant(citric acid and sesbaria cannabina powder), and adjusting their dosage and water/powder ratio. Through catalyst strength test, reaction evaluation and characterization, the infulence of the adding and dosage of binder, peptizer, bi-functional lubricant and dosage of water/powder ratio on catalyst was studied. The results showed the optimal extrusion shaping condition is that the mass ratio of Cu/Zn/Al2O3 and pseudo-boehmite is 1:1, peptizer (HNO3) mass fraction is 4wt%, appropriate bi-functional lubricant is citric acid 5wt% and sesbania powder 2.5wt%, water/powder ratio is 0.60. The strength of bi-functional catalyst was about 153N/cm. The extrusion shaped bi-functional catalyst was tested under 250℃,3MPa and 1500 ml(g·h), CO conversion was 79.8%, DME selectivity was 49.1%, and CH3OH selectivity was 15.2%.