Effects of the method of active site characterization for determining structure-sensitivity in Ag-catalyzed ethylene epoxidation
Effects of the method of active site characterization for determining structure-sensitivity in Ag-catalyzed ethylene epoxidation
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DOI：
10.1016/j.jcat.2022.03.021
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发表时间：
2022-04-27
影响因子：
7.3
通讯作者：
Monnier, John R.
中科院分区：
文献类型：
--
作者：
Egelske, Benjamin T.;Xiong, Wen;Monnier, John R.
催化剂是在低表面积 α-Al2O3 载体上制备的，该载体在商业上用于上一代烯烃环氧化催化剂。低表面积氧化铝 (0.73 m(2)/g) 的预筛选表明，在 210 摄氏度的评估温度下不存在 EO 的酸催化异构化。通过初湿浸渍 AgNO3 合成的 0.1 wt% Ag 基材用作额外 Ag 化学沉积 (ED) 的基材，通过将 Ag+ 直接控制还原到预先存在的 Ag 表面上，在预先存在的 Ag 表面上形成重量负载，从而增加颗粒尺寸。 0.3、5.0重量%金属。还制备了使用 Ag2C2O4 作为 Ag 前体的 12 wt% Ag/α-Al2O3，以比较 ED 样品与更典型的工业配方催化剂的性能。通过 SEM、STEM、氢滴定对催化评估之前和之后表征的氧预覆盖的 Ag 进行表征表明，需要显微镜来准确表示 Ag 粒径的分布，但 O 预覆盖的 Ag 的 H-2 滴定可以最好地表示活性位点，因为它直接计算 Ag 表面位点的数量。 > 100 nm 的较大颗粒可抵抗银烧结和碳污垢； TOF 值对颗粒尺寸相对不敏感，表现最好和最差的样品之间只有 2.2 倍的差异。选择性不是 TOF 的函数，它显示出最显着的结构敏感性效应，其中粒径遵循 67 nm 近似至 92 nm (58% EO) < 157 nm (67% EO) < 211-542 nm (73% EO) 的趋势。较低的 EO 选择性也与较小 Ag 尺寸的污垢增加相关，这表明更强的结合 EO 前体会导致燃烧和 CO2/H2O 的形成。 (C) 2022 由爱思唯尔公司出版
Catalysts have been prepared on a low surface area alpha-Al2O3 support used commercially for previous generation olefin epoxidation catalysts. Prescreening of the low surface area alumina (0.73 m(2)/g) indicated the absence of acid catalyzed isomerization of EO at an evaluation temperature of 210 degrees C. A 0.1 wt% Ag base material synthesized by incipient wetness impregnation of AgNO3 was used as a base material for electroless deposition (ED) of additional Ag to increase particle sizes by controlled reduction of Ag+ directly onto the preexisting Ag surface to form weight loadings between 0.3 , 5.0 wt% metal. A 12 wt% Ag/alpha-Al2O3 using Ag2C2O4 as the Ag precursor was also prepared to compare performance of the ED samples with a catalyst more typical of industrial formulations. Characterization by SEM, STEM , hydrogen titration of oxygen precovered Ag characterized before and after catalytic evaluation indicated that microscopy is required to accurately represent distributions of Ag particle sizes, but H-2 titration of O-precovered Ag gives the best representation of active sites since it directly counts the number of Ag surface sites. Larger particles > 100 nm are resistant to both Ag sintering and carbon foulant; TOF values were relatively insensitive to particle size with only a 2.2 x difference between the best and worst performing samples. Selectivity, which is not a function of TOF, shows the most significant structure sensitivity effect where particle sizes follow the trend 67 nm approximate to 92 nm (58% EO) < 157 nm (67% EO) < 211-542 nm (73% EO). The lower EO selectivities were also correlated with increased fouling for the smaller Ag sizes, suggesting that more strongly bound EO precursor(s) leads to combustion and CO2/H2O formation. (C) 2022 Published by Elsevier Inc.