Oxide Modification On The Carriers Of Silver Catalyst And Its Influence On Ethylene Epoxidation Reaction

Ethylene oxide (EO) is an important organic chemical. The performance of silver catalyst used in EO production plays a decisive role in the economic benefits of EO production. Development of a silver catalyst with higher activity, selectivity and better stability is always the main goal in this field. Improving the nature of a-Al2O3 carrier is an important approach to achieve the aim.In this paper, industrialα-Al2O3 carrier was used as raw material. After impregnation with a solution or sol containing cerium, titanium or zirconium, theα-Al2O3 carrier was modified by ceria, titania or zirconia. Then a series of supported silver catalysts were prepared by soaking afore-mentioned carrier with a silver-containing solution. Carriers and silver catalysts were characterized using XRD, BET, mercury porosimetry, SEM, and O2-TPD. Catalytic performance of the catalyst in ethylene epoxidation was evaluated in a fixed bed micro-reactor.The results showed that BET surface area ofα-Al2O3 carriers modified by ceria was increased to some extent, Ag particles on the Ce/α-Al2O3 carrier was well-distributed, and thermal stability of the catalyst was improved. Ceria crystal on the carrier calcined at 900℃was relatively perfect, and silver catalyst prepared from it has a larger low-temperature oxygen desorption peak, which type of oxygen was favorable to EO selectivity. The catalyst, Ag/Ce/α-Al2O3(900) with 0.56 wt% Ce calcined at 900℃, exhibited an initial reaction temperature of 195.9℃and EO selectivity of 82.7% under the reaction conditions of 2.1MPa,7000h-1 GHSV,1.35% EO in exit and 185g EO/1 Cat./h space time yield. In comparison with the carrier without modification by Ce, reaction temperature dropped by 5℃and the EO selectivity increased by 0.7%. The catalyst performance of the silver catalyst was improved.To the titania modified sample, surface area of the carrier increased and the pore volume decreased. Also thermal stability of the supported silver catalyst increased. Compared with Ag/α-Al2O3, initial activity and selectivity of Ag/Ti/α-Al2O3 catalyst were inhibited. After the introduction of Cs as promoter, Ag/Ti/α-Al2O3(900) catalyst showed a higher selectivity but its activity was still lower.To the zirconium modified samples, an increase in SBET was observed, the Zr-modified carrier calcined at 500℃(Zr/α-Al2O3(500) showed higher SBET than the carrier Zr/α-Al2O3(900). Silver amine complex compound impregnated on the carrier decomposed more easily, and sintering of silver particles was somewhat retarded. Ag/Zr/α-Al2O3 (500) catalyst showed both poor activity and EO selectivity. After introduction of Cs, selectivity of AgCs/0.38%Zr/α-Al2O3(900) catalyst reached to 84.3%, gained 0.7% contrast to AgCs/α-Al2O3. Zr-modified catalyst has partial loss in activity.