Selective propylene epoxidation over K-promoted Ag/CaCO₃ features promotion by surface-bound chlorine adatoms (Cl*) deposited by trace alkyl chlorides co-fed continuously in a fashion similar to ethylene epoxidation over promoted Ag/α-Al₂O₃ catalysts. Steady-state propylene epoxidation over K-Ag/CaCO₃ in the presence of gaseous promoters (NO, CO₂, and C₂H₅Cl) at varied contact times in flow reactors as well as transient, low-conversion batch epoxidation reveal allyl chloride, formed via propylene oxychlorination, to be a highly unstable intermediate. In situ allyl chloride formation and decomposition are in turn observed to control Cl* coverages and induce substantial bed-scale Cl* gradients during propylene epoxidation by (i) reducing Cl* coverage upstream in the catalyst bed via propylene oxychlorination and (ii) increasing downstream Cl* coverages by allyl chloride decomposition. This is supported by postreaction batch titrations of K-Ag/CaCO₃ beds after steady state propylene epoxidation at varying contact times which reveals bed-average Cl* coverages increasing from ∼0.03 to ∼0.2 monolayers (moles Cl per surface Ag) with propylene conversion increasing from ∼0.05% to 2.3%. Control of Cl* coverages by in situ generated allyl chloride is highly consequential for selective epoxidation with epoxide selectivity (∼25 to 55%) and site-time yield (∼20 to 54 nmol g_cat^–1 s^–1) increasing substantially with bed-averaged Cl* coverages as propylene conversion increases (∼0.05% to 2.3%). Results presented herein reveal a key difference in the mechanistic pathways dictating activity- and selectivity-salient Cl* coverages within propylene epoxidation compared with ethylene and 1,3-butadiene epoxidation directly related to the unique propensity of propylene to remove Cl* from Ag surfaces during epoxidation to generate allyl chloride.

中文翻译：

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氯化烃在 K-Ag/CaCO3 丙烯环氧化反应中的关键作用

K 促进的 Ag/CaCO ₃上的选择性丙烯环氧化的特点是通过痕量烷基氯沉积的表面结合的氯吸附原子 (Cl*) 以类似于乙烯环氧化在促进的 Ag/α-Al 2 O 3 催化剂上连续共同进_料的_方式促进. _{在气态促进剂（NO、CO 2}和 C ₂ H _5）存在下， K-Ag/CaCO ₃上的稳态丙烯环氧化Cl) 在流动反应器中的不同接触时间以及瞬态、低转化率的间歇环氧化表明，通过丙烯氧氯化形成的烯丙基氯是一种高度不稳定的中间体。反过来观察到原位烯丙基氯的形成和分解，以控制 Cl* 覆盖率并在丙烯环氧化过程中诱导大量的床层 Cl* 梯度，方法是 (i) 通过丙烯氧氯化降低催化剂床上游的 Cl* 覆盖率和 (ii) 增加下游氯代丙烯分解的 Cl* 覆盖率。_{K-Ag/CaCO 3}的反应后分批滴定支持这一点在不同接触时间的稳态丙烯环氧化后床层显示床层平均 Cl* 覆盖率从 ~ 0.03 增加到 ~ 0.2 单层（每个表面 Ag 的摩尔 Cl），丙烯转化率从 ~ 0.05% 增加到 2.3%。_{通过原位生成的烯丙基氯控制 Cl* 覆盖率对于具有环氧化物选择性（~25 至 55%）和位点时间产率（~20 至 54 nmol g cat} ^–1 s ^–1）的选择性环氧化非常重要) 随着丙烯转化率的增加（~0.05% 至 2.3%），床平均 Cl* 覆盖率大幅增加。本文提供的结果揭示了与乙烯和 1,3-丁二烯环氧化相比，丙烯环氧化中决定活性和选择性显着 Cl* 覆盖率的机制途径的关键差异，这与丙烯在过程中从 Ag 表面去除 Cl* 的独特倾向直接相关环氧化生成烯丙基氯。