焙烧温度对K改性Ag-Fe/ZnO-ZrO2催化剂结构和CO加氢合成低碳混合醇醚性能的影响

采用并流共沉淀法在不同焙烧温度下制备K改性Ag-Fe/ZnO-ZrO₂催化剂,考察不同焙烧温度对催化剂CO加氢合成低碳混合醇醚反应性能的影响。通过N₂物理吸附(N₂-adsorption)、X射线衍射(XRD)、氢气程序升温还原(H₂-TPR)、一氧化碳程序升温脱附(CO-TPD)等手段对催化剂进行表征。结果表明,250 ℃焙烧的催化剂,由于焙烧温度较低,表面尚未形成足够多的活性位,未能达到最佳的催化性能;300 ℃焙烧的催化剂,其CO转化率最高、醇醚选择性较高,醇醚时空产率达到最大值。随着焙烧温度进一步升高,CO转化率逐渐降低,醇选择性先降低后增大,二甲醚(DME)选择性逐渐增大,醇醚时空产率逐渐降低。催化剂性能主要与其比表面积、还原性能、所含银铁复合物分散度及CO吸脱附性能有关,即比表面积较大、易于被还原、银铁复合物分散度较高以及较多的CO吸脱附活性位,有利于催化剂CO加氢转化。催化剂表面活性位对CO的非解离吸附强度降低,有利于醇醚产物的生成;而对CO的解离吸附强度增强,则不利于烃类产物的生成。

Effect of calcination temperature on K modified Ag-Fe/ZnO-ZrO2 catalyst structure and its performance for higher alcohols and DME synthesis from CO hydrogenation

A series of K modified Ag-Fe/ZnO-ZrO₂ catalysts were prepared by co-precipitation method under different calcination temperatures. The effect of calcination temperature on the catalytic performance for higher alcohols and dimethyl ether(DME) synthesis from CO hydrogenation was investigated. The catalysts were characterized by nitrogen adsorption, XRD, H₂-TPR and CO-TPD. The results showed that the catalyst calcined at 250 ℃ could not reach the optimal performance due to insufficient active sites formed at the lower calcination temperature. The catalyst calcined at 300 ℃ exhibited highest CO conversion and higher selectivity of higher alcohols and DME and highest space time yield of higher alcohols and DME reached. As the calcination temperature increased further, the CO conversion decreased, while the selectivity of higher alcohols decreased at first and then increased, the selectivity of DME increased. The catalytic performance of the catalyst was mainly related with its specific surface area, reduction capacity, the dispersion of the σ-AgFeO₂ species and CO adsorption-desorption properties. It was proved that the catalyst with larger specific surface area, being easily reduced, higher dispersion of σ-AgFeO₂ specie and more CO adsorption active sites, would be helpful for CO hydrogenation conversion. The decrease of the non-dissociative adsorption strength for CO on the surface active sites of the catalyst is favorable for the generation of higher alcohols and DME, while the increase of the dissociative adsorption strength for CO is not favorable for the formation of hydrocarbons.