Mechanisms of Methylenecyclobutane Hydrogenation over Supported Metal Catalysts Studied by Parahydrogen‐Induced Polarization Technique

Abstract

In this work the mechanism of methylenecyclobutane hydrogenation over titania-supported Rh, Pt and Pd catalysts was investigated using parahydrogen-induced polarization (PHIP) technique. It was found that methylenecyclobutane hydrogenation leads to formation of a mixture of reaction products including cyclic (1-methylcyclobutene, methylcyclobutane), linear (1-pentene, cis-2-pentene, trans-2-pentene, pentane) and branched (isoprene, 2-methyl-1-butene, 2-methyl-2-butene, isopentane) compounds. Generally, at lower temperatures (150-350 °C) the major reaction product was methylcyclobutane while higher temperature of 450 °C favors the formation of branched products isoprene, 2-methyl-1-butene and 2-methyl-2-butene. PHIP effects were detected for all reaction products except methylenecyclobutane isomers 1-methylcyclobutene and isoprene implying that the corresponding compounds can incorporate two atoms from the same parahydrogen molecule in a pairwise manner in the course of the reaction in particular positions. The mechanisms were proposed for the formation of these products based on PHIP results.

Understanding Heterogeneous Catalytic Hydrogenation by Parahydrogen-Induced Polarization NMR Spectroscopy

“…Moreover, the PHIP technique is associated with chemical kinetics and spin dynamics, which enables it to be a unique tool in revealing hydrogenation reaction mechanisms, particularly on the proton addition route. HET-PHIP has been widely applied to various hydrogenation reactions, including unsaturated hydrocarbon hydrogenation, hydroformylation, hydrodesulfurization, oligomerization, and ring cleavage of cycloalkane. ,, …”

“…HET-PHIP has been widely applied to various hydrogenation reactions, including unsaturated hydrocarbon hydrogenation, hydroformylation, 124 hydrodesulfurization, 72 oligomerization, 102 and ring cleavage of cycloalkane. 81,125,126 Hydrogenation of 1,3-butadiene (BD) can produce the semihydrogenated products 1-butene (B1) and 2-butene (B2) as well as the fully reduced product butane (BA). In the reaction catalyzed by the monatomic Au/MWCNT catalyst (Figure 9a), all hydrogenation reaction products exhibited the PHIP effect, reflected by the characteristic antiphase line shape in the 1 H NMR spectra.…”

Manipulating Stereoselectivity of Parahydrogen Addition to Acetylene to Unravel Interconversion of Ethylene Nuclear Spin Isomers

“…The authors have cited additional references within the Supporting Information. [3,25,[44][45][46][47][48]27,31,32,35,[39][40][41]43]…”

Parahydrogen-Induced Polarization in Gas-Phase Heterogeneous Hydrogenation of Epoxides

“…PHIP exploits the singlet spin order of a parahydrogen (p-H 2 ) molecule by incorporating two H atoms from the same p-H 2 molecule into the same reaction product molecule . This process of pairwise addition of p-H 2 leads to dramatic amplification of the NMR signals of the corresponding protons in the reaction product up to several orders of magnitude, allowing for the detection of short-lived and low-concentrated reaction intermediates − and minor reaction products. − Moreover, these NMR signals have a characteristic antiphase line shape, which serves as a hallmark of the existence of the pairwise hydrogen addition route. − PHIP effects were discovered for the first time in homogeneous hydrogenations where the reaction proceeds at a well-defined isolated active center. , Heterogeneous supported metal catalysts were believed to be unable to provide pairwise hydrogen addition due to dissociative adsorption of H 2 and rapid migration of H atoms over the metal surface and subsurface and the support . However, in 2008, it was demonstrated that pairwise hydrogen addition to CC and CC bonds over supported metal catalysts is possible.…”

“…10–20%. − Other types of heterogeneous catalysts are also capable of providing PHIP effects, including immobilized metal complexes, , metal oxides, − carbides, and sulfides . In recent years, the PHIP technique was successfully utilized for the mechanistic studies of heterogeneous hydrogenations of various classes of compounds, e.g., unsaturated acyclic and cyclic ,− hydrocarbons, carbonyl compounds, and furan derivatives, and hydrodesulfurization of thiophene …”