Kinetics of Propylene Epoxidation over Extracrystalline Gold Active Sites on AU/TS-1 Catalysts
Kinetics of Propylene Epoxidation over Extracrystalline Gold Active Sites on AU/TS-1 Catalysts
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DOI：
10.1021/acscatal.2c02213
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发表时间：
2022-08
期刊：
影响因子：
12.9
通讯作者：
Jeremy W. Arvay;Wei Hong;Christina W. Li;W. Delgass;F. Ribeiro;James W. Harris
中科院分区：
文献类型：
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作者：
Jeremy W. Arvay;Wei Hong;Christina W. Li;W. Delgass;F. Ribeiro;James W. Harris
为了确定钛硅酸盐-1（TS-1）负载的晶外和晶内金纳米粒子对丙烯直接环氧化的催化作用是否本质上不同，在气相连续搅拌釜式反应器（CSTR）中测量了沉积在TS-1上的聚乙烯吡咯烷酮（PVP）包覆的金纳米粒子（Au-PVP/TS-1）的丙烯直接环氧化动力学。TEM和TGA-DSC结果表明，即使经过一系列的预处理，原位去除配体后，PVP包覆的金纳米粒子仍然太大，无法嵌入TS-1的微孔中。活化能（51 kJ mol-1）和H2的反应级数（1.3）、O2（0.4），丙烯（0.4），环氧丙烷在Au-PVP/TS-1上测得的丙烯环氧化反应中的CO2（-0.6）、CO2（0）和水（0）与通过沉积-沉淀法制备的Au/TS-1的结果一致（Au-DP/TS-1）（52 kJ mol-1，H2，1; O2，0.4; C3H6，0.4; C3H6O，−0.6; CO2，0; H2O，0）。然而，虽然在Au-PVP/TS-1上的反应级数为氢氧化，（H2，0.8; O2，0; C3 H6，-0.3; C3 H6 O，-0.1; CO2，0; H2O，-0.2）与Au-DP/TS-1上测得的结果相似（H2，0.9; O2，0.3; C3 H6，−0.3; C3 H6 O，0; CO2，0; H2O，−0.1），活化能从Au-DP/TS-1的约30 kJ mol-1降低到Au-PVP/TS-1的4-5 kJ mol-1，表明氢氧化的限速步骤或活性位点发生了变化。此外，开发了活性位点模型，其确定了晶外Au纳米颗粒的周边的相互作用范围内的Ti的数量（即，Au-Ti活性位点对的数目）。该模型被用来估计催化营业额的频率在单独的近端Au-Ti对，假设过氧化氢不解吸和迁移从Au网站到Ti网站，而是环氧丙烷的形式在一个协调的机制，以前被称为“同时”的机制。对于含有Au-DP/TS-1和Au-PVP/TS-1的数据集，该活性位点模型估计的周转频率比先前报道的最高估计值高出20倍（473 K时为1- 5s ~（-1）比80 s ~（-1）），这表明在近端Au-单独的Ti位点不能解释所观察到的丙烯环氧化速率，并且过氧化氢的短程迁移是动力学相关的。Au-DP/TS-1和Au-PVP/TS-1上丙烯环氧化的反应级数、活化能和活性中心模型的一致性表明，在含有小的孔内金簇的催化剂和仅含有较大的晶外金纳米颗粒的催化剂上丙烯环氧化具有共同的机理。氢氧化的速率被发现成比例地变化的表面金原子的量。这也与以下假设一致，即随着金负载量的增加，观察到的氢效率和每金质量的PO位点-时间-产率的降低主要是由Au/TS-1催化剂中的金分散驱动的。
To determine whether the catalytic roles of extracrystalline and intracrystalline gold nanoparticles supported on titanosilicate-1 (TS-1) for direct propylene epoxidation are intrinsically different, the kinetics of direct propylene epoxidation were measured in a gas-phase continuous stirred tank reactor (CSTR) over polyvinylpyrrolidone-coated (PVP) gold nanoparticles (Au-PVP/TS-1) deposited on TS-1. The as-made PVP-coated gold nanoparticles were too large to fit into the micropores of TS-1, even after ligands were removedin situby a series of pretreatments, as confirmed by both TEM and TGA-DSC. The activation energy (51 kJ mol–1) and reaction orders for H2 (1.3), O2(0.4), propylene (0.4), propylene oxide (−0.6), carbon dioxide (0), and water (0) for propylene epoxidation measured on Au-PVP/TS-1 were consistent with those reported for Au/TS-1 prepared via deposition-precipitation (Au-DP/TS-1) (52 kJ mol–1, H2, 1; O2, 0.4; C3H6, 0.4; C3H6O, −0.6; CO2, 0; H2O, 0). However, while the reaction orders for hydrogen oxidation on Au-PVP/TS-1 (H2, 0.8; O2, 0; C3H6, −0.3; C3H6O, −0.1; CO2, 0; H2O, −0.2) were similar to those measured on Au-DP/TS-1 (H2, 0.9; O2, 0.3; C3H6, −0.3; C3H6O, 0; CO2, 0; H2O, −0.1), a decrease in activation energy from approximately 30 kJ mol–1for Au-DP/TS-1 to 4–5 kJ mol–1for Au-PVP/TS-1 suggests there is a change in rate-limiting step or active site for hydrogen oxidation. Additionally, an active site model was developed which determines the number of Ti within an interaction range of the perimeter of extracrystalline Au nanoparticles (i.e., the number of Au–Ti active site pairs). This model was used to estimate catalytic turnover frequencies over solely proximal Au–Ti pairs, assuming that hydrogen peroxide does not desorb and migrate from Au sites to Ti sites and instead propylene oxide forms in a concerted mechanism previously termed the “simultaneous” mechanism. Turnover frequencies estimated for this active site model for a data set containing both Au-DP/TS-1 and Au-PVP/TS-1 were ∼20× higher than the highest previous reported estimates (∼80 s–1vs 1–5 s–1at 473 K) for catalytic oxidation on noble metals, suggesting that the simultaneous mechanism occurring over proximal Au–Ti sites alone is incapable of explaining the observed rate of propylene epoxidation and that short-range migration of hydrogen peroxide is kinetically relevant. The agreement of reaction orders, activation energy, and active site model for propylene epoxidation on both Au-DP/TS-1 and Au-PVP/TS-1 suggests a common mechanism for propylene epoxidation on both catalysts containing small intraporous gold clusters and catalysts with exclusively larger extracrystalline gold nanoparticles. Rates of hydrogen oxidation were found to vary proportionally to the amount of surface gold atoms. This is also consistent with the hypothesis that the observed decrease in hydrogen efficiency and PO site-time-yield per gold mass with increasing gold loading are driven primarily by the gold dispersion in Au/TS-1 catalysts.