摘要
CO_2高效活化和定向转化合成高附加值化学品是催化化学领域的重要研究课题.然而,由于CO_2分子具有高度对称结构和高的碳原子氧化态,其在温和条件下的活化和转化仍然是一个挑战.众所周知, CO_2的有效吸附和活化是其转化利用的前提.因此,一种理想的CO_2催化转化材料应该具有吸附、活化和选择性转化CO_2的多功能特性.据报道,金属Pd能够催化CO_2/H2和胺反应合成甲酰胺.但是已有的催化体系通常只对脂肪族仲胺显示出高的活性,当脂肪族伯胺用作反应底物时仅低到中等的产物收率.最近, Liu和Han等报道负载型的Pd催化剂Imine-POP@Pd、Pd/LDH和Pd/NC能够催化CO_2和脂肪族伯胺反应合成甲酰胺.但是这些催化体系要求高的反应温度(140 oC)、CO_2压力(3 MPa)和Pd担载量(8.1wt%).我们小组最近发现富羟基官能团碳担载的纳米Pd能够有效催化CO_2/H2和胺反应合成甲酰胺,但是反应条件仍然比较苛刻且碳载体制备过程复杂.凹凸棒石是一种天然的一维纳米水合镁铝硅酸盐粘土矿物,不仅具有独特的链层状结构,而且含有丰富的纳米孔道和多种酸碱位点.因此,凹凸棒石有可能提供多种活性位点协同作用的反应环境,用作CO_2活化转化多功能催化材料合成的潜在载体.本论文首次以凹凸棒石为催化剂载体制备了负载型多相Pd/PAL催化剂,并将其应用于CO_2的还原胺化反应.在低于100°C、1 MPaCO_2条件下,实现了一系列不同结构仲胺和伯胺到目标产物甲酰胺的转化,并获得了较好的产物收率.催化剂重复使用性研究结果表明,催化剂Pd/PAL在反应过程中较为稳定.BET、XRD和XPS表征揭示,部分负载的金属Pd进入到了载体内部,其与载体内部酸碱位点的协同作用可能是催化剂Pd/PAL能够高效催化CO_2/H2和胺反应合成甲酰胺的重要原因.控制实验和反应机理研究表明,甲酸甲酯是甲酰胺形成的可能中间体.
For the first time, Pd supported on natural palygorskite was developed for amine formylation with CO_2 and H_2. Both secondary and primary amines with diverse structures could be converted into the desired formamides at < 100 °C, and good to excellent yields were obtained.
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