负载于二氧化硅上的小尺寸氧化亚铜物种促进丙烯选择性氧化生成丙烯醛

氧化物负载的含铜材料是丙烯选择性氧化制备丙烯醛的理想催化剂,一直以来都受到人们的广泛关注.然而,对于该催化体系的结构与性能之间的关系仍不是很清楚.因此,我们以碳酸钠为沉淀剂,通过沉积沉淀法将铜负载于高比表面的二氧化硅载体上,从而得到了均匀分散且小尺寸的Cu/SiO2催化剂.另外,采用浸渍法制得了相同负载量的分散不均匀、大尺寸的Cu/SiO2催化剂.丙烯选择性氧化反应活性测试发现,沉积沉淀法制备的催化剂比浸渍法制备的更有利于丙烯醛的生成,表现出了优异的催化性能:在300℃反应时,丙烯的转化率达到25.5%,丙烯醛的选择性达到66.8%,对应的丙烯醛的生成速率高达10.5 mmol·h^-1·gcat.^-1或111.2 mmol·h^-1·gCu^-1,远远超出了浸渍法制备的催化剂性能(1.7 mmol·h^-1·gcat.^-1或17.2mmol·h^-1·gCu^-1)和文献中报道的结果.结合高角度环形暗场扫描透射电子显微镜(HAADF-STEM)和X射线吸收精细结构(XAFS)技术,对沉积沉淀法制备的催化剂进行表征,发现在反应后铜物种的结构发生了明显的变化,由小尺寸的氧化铜(CuO)团簇转变为氧化亚铜(Cu2O)团簇,并且铜物种的尺寸没有明显的增大.为了进一步探索铜物种在预处理(氢气还原)以及催化反应时(丙烯+氧气)的结构变化,对不同方法合成的两种催化剂进行了原位X射线粉末衍射测试,发现不同尺寸的铜物种在还原和反应时都经历了从氧化铜(CuO)变为金属Cu再到Cu2O的结构变化,并且Cu2O在320 min的反应过程中可以稳定存在,说明它是该催化反应的活性物种.另外,通过原位双光束傅里叶变换红外光谱追踪反应时气体分子在催化剂表面的吸脱附状态,发现丙烯可以有效地吸附在小尺寸Cu/SiO2催化剂表面,随着Cu2O的形成,检测到了烯丙基中间体(CH2=CHCH2*)的产生,该物种可以与邻近Cu2O上的一个氧发生反应,从而生成丙烯醛,因此结合N2O滴定实验,我们可认为,高度分散的小尺寸的Cu2O物种是丙烯进行高效选择性氧化反应生成丙烯醛的活性物种.

Small-sized cuprous oxide species on silica boost acrolein formation via selective oxidation of propylene

Oxide-supported copper-containing materials have attracted considerable research attention as promising candidates for acrolein formation. Nevertheless, the elucidation of the struc-ture-performance relationships for these systems remains a scientific challenge. In this work, cop-per oxide clusters deposited on a high-surface-area silica support were synthesized via a deposi-tion-precipitation approach and exhibited remarkable catalytic reactivity (up to 25.5％ conversion and 66.8％ selectivity) in the propylene-selective oxidation of acrolein at 300 ℃. Aberra-tion-corrected high-angle annular dark-field scanning transmission electron microscopy combined with X-ray absorption fine structure measurements of the catalyst before and after the reaction confirmed the transformation of the small-sized copper oxide (CuO) clusters into cuprous oxide (Cu2O) clusters. With the aid of in situ X-ray diffraction and in situ dual beam Fourier transform infrared spectroscopy (DB-FTIR), the allyl intermediate (CH2=CHCH2) was clearly observed, along with the as-formed Cu2O species. The intermediate can react with oxygen atoms from neighboring Cu2O species to form acrolein during the catalytic process, and the small-sized Cu2O clusters play a crucial role in the generation of acrolein via the selective oxidation of propylene.