Abstract
Understanding the C-C bond activation mechanism is essential for developing the selective production of hydrocarbons in the petroleum industry and for selective polymer decomposition. In this work, ring-opening reactions of cyclopropane derivatives under hydrogen catalyzed by metal nanoparticles (NPs) in the liquid phase were studied. 40-atom rhodium (Rh) NPs, encapsulated by dendrimer molecules and supported in mesoporous silica, catalyzed the ring opening of cyclopropylbenzene at room temperature under hydrogen in benzene, and the turnover frequency (TOF) was higher than other metals or the Rh homogeneous catalyst counterparts. Comparison of reactants with various substitution groups showed that electron donation on the three-membered ring boosted the TOF of ring opening. The linear products formed with 100% selectivity for ring opening of all reactants catalyzed by the Rh NP. Surface Rh(0) acted as the active site in the NP. The capping agent played an important role in the ring-opening reaction kinetics. Larger particle size tended to show higher TOF and smaller reaction activation energy for Rh NPs encapsulated in either dendrimer or poly(vinylpyrrolidone). The generation/size of dendrimer and surface group also affected the reaction rate and activation energy.
Highlights
Carbon−carbon (C−C) bond activation in the gas phase plays an important role in the chemical industry
turnover frequency (TOF) of metal NPs was calculated by the number of molecules reacted in unit time (1 h) divided by the number of undercoordinated metal atoms, which was estimated from simple hard-sphere counting models.22 40-atom NPs of Rh, Pd, and Au encapsulated by fourth-generation polyamidoamine (PAMAM) dendrimer with hydroxyl terminal
Rh40/G4OH/SBA-15 showed a TOF of 2.24 h−1, which was higher than the homogeneous counterparts, toward the ring opening of cyclopropylbenzene at room temperature
Summary
Carbon−carbon (C−C) bond activation in the gas phase plays an important role in the chemical industry. Cracking and reforming reactions in crude oil refineries practice C−C bond activation on a large scale daily.[1] Polyethylene and polypropylene are currently the mostly widely used plastics, but their decomposition requires thermal C−C bond activation.[2] Due to thermodynamic and kinetic considerations, selective C− C bond activation is challenging.[3−6] Currently, these reactions are operated at high temperatures in industry (above 600 K). Work[7,8] showed that catalytic cyclopropane hydrogenation (ring opening) by metal single crystal or powder catalysts in the gas phase was often accompanied by hydrogenolysis, forming smaller molecules. It was posited that the reaction mechanism was metal-dependent, proceeding through different intermediates including a monoadsorbed radical,9 1,3-diadsorbed species on Ni catalysts,[10] or metallocycle intermediate on Pt.[11]
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