Department of Chemical Engineering and Materials Science, University of Minnesota-Twin Cities, 421 Washington Avenue SE, Minneapolis, Minnesota 55455, United States.
Department of Chemistry, Chemical Theory Center, and Supercomputing Institute, University of Minnesota-Twin Cities, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States.
J Am Chem Soc. 2023 Feb 15;145(6):3408-3418. doi: 10.1021/jacs.2c10551. Epub 2023 Feb 1.
A mixed-valence oxotrimer metal-organic framework (MOF), Ni-MIL-127, with a fully coordinated nickel atom and two iron atoms in the inorganic node, generates a missing linker defect upon thermal treatment in helium (>473 K) to engender an open coordination site on nickel which catalyzes propylene oligomerization devoid of any cocatalysts or initiators. This catalyst is stable for ∼20 h on stream at 500 kPa and 473 K, unprecedented for this chemistry. The number of missing linkers on synthesized and activated Ni-MIL-127 MOFs is quantified using temperature-programmed oxidation, H nuclear magnetic resonance spectroscopy, and X-ray absorption spectroscopy to be ∼0.7 missing linkers per nickel; thus, a majority of Ni species in the MOF framework catalyze propylene oligomerization. In situ NO titrations under reaction conditions enumerate ∼62% of the nickel atoms as catalytically relevant to validate the defect density upon thermal treatment. Propylene oligomerization rates on Ni-MIL-127 measured at steady state have activation energies of 55-67 kJ mol from 448 to 493 K and are first-order in propylene pressures from 5 to 550 kPa. Density functional theory calculations on cluster models of Ni-MIL-127 are employed to validate the plausibility of the missing linker defect and the Cossee-Arlman mechanism for propylene oligomerization through comparisons between apparent activation energies from steady-state kinetics and computation. This study illustrates how MOF precatalysts engender defective Ni species which exhibit reactivity and stability characteristics that are distinct and can be engineered to improve catalytic activity for olefin oligomerization.
一种混合价态的氧三核金属有机骨架（MOF），Ni-MIL-127，在无机节点中具有完全配位的镍原子和两个铁原子，在氦气中（>473 K）热处理时会产生一个缺失的配体缺陷，从而在镍上产生一个开放的配位位，该位可以催化丙烯低聚，无需任何助催化剂或引发剂。在 500 kPa 和 473 K 的条件下，这种催化剂在连续流动状态下稳定约 20 小时，这在该化学领域是前所未有的。使用程序升温氧化、H 核磁共振波谱和 X 射线吸收光谱定量测定合成和活化的 Ni-MIL-127 MOF 中的缺失配体数量约为每个镍 0.7 个缺失配体；因此，MOF 骨架中的大多数 Ni 物种都能催化丙烯低聚。在反应条件下进行的原位 NO 滴定，在热处理后对约 62%的镍原子进行了催化相关的定量，验证了缺陷密度。在 448 到 493 K 的范围内，Ni-MIL-127 上的丙烯低聚速率在稳态下的活化能为 55-67 kJ/mol，在 5 到 550 kPa 的丙烯压力范围内呈一级反应。采用 Ni-MIL-127 团簇模型的密度泛函理论计算，通过稳态动力学和计算之间的表观活化能比较，验证了缺失配体缺陷和丙烯低聚的 Cossee-Arlman 机制的合理性。本研究说明了 MOF 前催化剂如何产生具有反应性和稳定性特征的缺陷 Ni 物种，这些特征是独特的，可以通过工程设计来提高烯烃低聚的催化活性。
