A test reaction to assess the presence of Brönsted and the softness/hardness of Lewis acid sites in palladium supported catalysts
作者:Avelino Corma、Hermenegildo García、Ana Primo、Antonio Domenech
DOI:10.1039/b311513e
日期:——
Cyclic ethylene acetal of α-bromopropiophenone gives rise to three distinctive products, each one being specific to Brönsted, hard Lewis and soft Lewis sites, respectively. In the presence of Brönsted sites, hydrolysis of cyclic acetal forms the corresponding α-bromophenone. Hard or soft Lewis sites promote selectively a rearrangement of the phenyl or ethylenedioxy group to form 2-phenylpropionate ester or dioxine, respectively. This test reaction has been used to assess the influence of the support on the catalytic activity of palladium in zeolites. Three series of supports including alkali-exchanged faujasite, Beta zeolites and sepiolites have been tested. It has been observed that the nature of the zeolite plays an important role on the palladium selectivity, basic supports increasing the softness of the palladium Lewis sites. The catalytic test correlates well with electrochemical data about the reduction potential of supported palladium, XPS characterization of the oxidation state of palladium supported on the surface and activity in the Suzuki reaction.
α-溴苯丙酮的环状乙烯缩醛产生三种独特的产物,每种产物分别针对布朗斯台德、硬路易斯和软路易斯位点。在布朗斯台德位点存在下,环状缩醛水解形成相应的 α-溴苯酮。硬或软路易斯位点选择性地促进苯基或乙二氧基的重排,分别形成2-苯基丙酸酯或二恶英。该测试反应已用于评估载体对沸石中钯催化活性的影响。已经测试了三个系列的载体,包括碱交换八面沸石、β沸石和海泡石。据观察,沸石的性质对钯选择性起着重要作用,碱性载体增加了钯路易斯位点的柔软度。催化测试与负载钯还原电位的电化学数据、表面负载钯氧化态的 XPS 表征以及 Suzuki 反应的活性密切相关。