分子筛包裹Pd@PdCx团簇的设计及其制碳酸二甲酯中钯碳协同催化效应

The synthesis of dimethyl carbonate (DMC) from coal is a new efficient and clean conversion route but the process is not applied at industrial scale yet. The stability of the available catalyst for vapor-phase methyl nitrite carbonylation to DMC is not satisfactory. This is resulted from the reduction of the active sites of high-valence Pd in the presence of CO reactant. Herein, we propose a novel strategy towards preparation of efficient catalysts based on encapsulation of Pd@PdCx clusters within zeolite through one-pot synthesis and subsequent carburization. The encapsulation of Pd within zeolite will be achieved via one-pot hydrothermal synthesis of NaY with Pd precursor stabilized by ethyleneamines ligands. After that, the zeolite will be directly carburized using ethyleneamines ligands as carbon source. As a result, the PdCx will be formed as surface species (i.e., Pd@PdCx), which will lead to a significant modification of the electron properties of the Pd clusters and thus will increase the active sites of high-valence Pd. The principle of the formation of Pd clusters and subsurface carbon will be illustrated. The interactions between the subsurface carbon and high-valence Pd will be evaluated, and the synergistic effects of the subsurface carbon and Pd clusters will be investigated in the methyl nitrite carbonylation to DMC. Ultimately, the catalytic reaction mechanism of DMC reaction will be elucidated by in-situ diffuse reflectance infrared Fourier transform spectroscopy (in-situ DRIFTS) and kinetics study. The unique contribution of the study is to present a novel strategy for preparation of Pd@PdCx clusters within zeolite catalyst. Notably, the subsurface carbon will increase and stabilize the active sites of high-valence Pd for the synthesis of DMC.

煤制碳酸二甲酯（DMC）是煤炭高效清洁转化的新兴路线，但制约该过程的主要技术瓶颈是CO和亚硝酸甲酯羰基合成DMC催化剂中价态Pd活性位在还原反应气氛中稳定性较差。本项目提出了“分子筛封装-可控碳化”将Pd@PdCx团簇包裹到分子筛中的新策略，即利用廉价的乙烯胺多齿配体与Pd源形成稳定络合物将Pd一锅法封装到分子筛，然后直接以乙烯胺为碳源，可控碳化生成次表面碳修饰的Pd团簇（形成PdCx表面物种）。电子从Pd转移到次表面碳使Pd缺电子形成高活性、高稳定性的价态Pd活性位，阐明碳化过程中Pd团簇生成与碳物种演变的协同匹配规律，揭示次表面碳对价态Pd的稳定作用机制，剖析Pd团簇与次表面碳的协同催化效应，阐明Pd@PdCx上的催化反应机理，为高效催化剂的开发提供理论基础。本项目的创新之处是发展了分子筛包裹Pd@PdCx团簇的新方法，提出了利用次表面碳稳定价态Pd活性位并生成价态Pd。

项目背景：煤制碳酸二甲酯（DMC）作为现代煤化工五大示范工程中煤制乙二醇路线的拓展，可通过改变催化剂体系实现合成草酸二甲酯到碳酸二甲酯的转变，是煤炭高效清洁转化和低碳化利用的新型路线，受到了工业界和学术界的广泛关注。.研究内容：（1）催化材料：NaY分子筛包裹Pd@PdCx团簇的设计与制备；（2）反应性能：Pd/NaY催化剂上Pd团簇与次表面碳的协同催化效应；（3）反应机理：Pd/NaY催化剂上羰基合成DMC的催化反应机理。.重要结果：按照“催化材料设计→反应性能评价→反应机理研究”的技术路线开展研究，提出了“分子筛封装-可控碳化”将Pd@PdCx团簇包裹到分子筛中的新策略，阐明碳化过程中Pd团簇生成与碳物种演变的协同匹配规律，揭示了Pd团簇与次表面碳的协同催化效应，结合实验和理论计算构建完整的催化反应网络和速度控制步骤，阐明了催化反应机理。发展完善了对两个关键科学问题的认识：（1）分子筛上钯胺络合物碳化形成Pd@PdCx团簇的化学原理和基本规律；（2）如何使价态Pd活性位在羰基合成DMC的还原反应气氛中保持稳定。进而获得四项研究成果：（1）发展了一种分子筛包裹Pd@PdCx催化剂制备的新方法；（2）阐明了羰基合成DMC反应中Pd团簇与次表面碳的协同催化效应；（3）捕获羰基合成DMC的反应中间物种，阐明了合成DMC的催化反应机理；（4）以第一作者发表SCI一区论文3篇，申请发明专利3项，其中授权2项，参加国内外学术会议并做口头报告5次，培养硕博研究生6名。.关键数据：制备了高性能的Pd/NaY催化剂，转化率为89%，选择性为83%，并且能够稳定运行150小时；证实了COOCH3*物种的形成是合成DMC的速度控制步骤。.科学意义：该研究工作为羰基合成碳酸二甲酯催化剂的技术开发奠定了基础，研究成果具有一定的成果转化潜力，已经积极与企业进行技术交流，技术合作开发正在进一步的洽谈中。