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Protonation reactions involving organometallic complexes are ubiquitous in redox chemistry and often result in the generation of reactive metal hydrides. However, some organometallic species supported by η5-pentamethylcyclopentadienyl (Cp*) ligands have recently been shown to undergo ligand-centered protonation by direct proton transfer from acids or tautomerization of metal hydrides, resulting in the generation of complexes bearing the uncommon η4-pentamethylcyclopentadiene (Cp*H) ligand. Here, time-resolved pulse radiolysis (PR) and stopped-flow spectroscopic studies have been applied to examine the kinetics and atomistic details involved in the elementary electron- and proton-transfer steps leading to complexes ligated by Cp*H, using Cp*Rh(bpy) as a molecular model (where bpy is 2,2'-bipyridyl). Stopped-flow measurements coupled with infrared and UV-visible detection reveal that the sole product of initial protonation of Cp*Rh(bpy) is [Cp*Rh(H)(bpy)]+, an elusive hydride complex that has been spectroscopically and kinetically characterized here. Tautomerization of the hydride leads to the clean formation of [(Cp*H) Rh(bpy)]+. Variable-temperature and isotopic labeling experiments further confirm this assignment, providing experimental activation parameters and mechanistic insight into metal-mediated hydride-to-proton tautomerism. Spectroscopic monitoring of the second proton transfer event reveals that both the hydride and related Cp*H complex can be involved in further reactivity, showing that [(Cp*H)Rh] is not necessarily an off-cycle intermediate, but, instead, depending on the strength of the acid used to drive catalysis, an active participant in hydrogen evolution. Identification of the mechanistic roles of the protonated intermediates in the catalysis studied here could inform design of optimized catalytic systems supported by noninnocent cyclopentadienyl-type ligands. [ABSTRACT FROM AUTHOR]

Efficient transfer hydrogenolysis of benzyl phenyl ether (BPE) was accomplished over Ni 1.5 Cu 1.5 /Al 2 O 3 using isopropanol as hydrogen source. The characterization results demonstrated that the formation of NiCu alloy and electron transfer between Ni and Cu were responsible for the excellent catalytic performance. Isopropanol displayed better hydrogen-donating ability than other low-carbon alcohols because of its unique dehydrogenation pathway and small steric hindrance. The results of isotopic experiments elucidated that active hydrogen species for the C O bond cleavage originated from isopropanol rather than H 2 , while both isopropanol and H 2 were responsible for hydrogenation of phenol to cyclohexanol when reaction was conducted with mixed hydrogen sources. A mechanism study illustrated that transfer hydrogenolysis of BPE proceeded through direct cleavage of C O bond followed by phenol hydrogenation, in which the active hydrogen species were in the form of H+ and H−. [Display omitted] • Ni 1.5 Cu 1.5 /Al 2 O 3 shows excellent performance in transfer hydrogenolysis of BPE. • Isopropanol exhibits superior hydrogen-donating ability than other alcohols. • Active hydrogen for C O bond cleavage originates from isopropanol instead of H 2. • The active hydrogen species are in the form of H+ and H−. [ABSTRACT FROM AUTHOR]

Catalytic hydrodeoxygenation (HDO) of phenolic compounds is a promising strategy for bio-oil upgradation to produce high-value chemicals and fuels. Herein, a series of NiFeAlO x catalysts prepared by a co-precipitation method were applied in HDO of guaiacol as a typical model compound. The characterization results revealed that the catalyst with Ni/Fe/Al ratio of 3/0.5/2.5 possessed developed textural properties, small metal particles, abundant oxygen vacancies and acid sites. HDO of guaiacol occurred in two pathways with different hydrogen sources: the direct hydrogenation of benzene ring to 2-methoxycyclohexanone and the further hydrogenolysis and dehydration of intermediates to cyclohexane at 260 °C in n -hexane under H 2 for 6 h, while demethoxylation affording phenol followed by hydrogenation of phenol to cyclohexanol at 220 °C in isopropanol under N 2 for 2 h. The different product composition results from the type of reactive H species, where active hydrogen species are hydrogen radicals using H 2 as the hydrogen source and hydrogen ions with opposite charges using isopropanol as the H-donor solvent, respectively. The Ni 3 Fe 0.5 Al 2.5 O x catalyst maintained good performance after five recycle experiments with a slight decrease in the conversion of guaiacol. Moreover, the catalyst showed excellent universality for selective HDO of other phenolic compounds to cycloalkanes or cycloalkanols. • Ni 3 Fe 0.5 Al 2.5 O x exhibits excellent hydrodeoxygenation performance. • The Ni/Fe/Al ratio affects the physicochemical properties of catalysts. • The H-source determines the reaction pathway for guaiacol hydrodeoxygenation. [ABSTRACT FROM AUTHOR]

[Display omitted] • Fe/NC catalyst shows good activity and stability for cleaving C-O bonds in lignite. • Fe-N and Fe/Fe 3 C species in the catalyst were crucial to the catalytic activity. • Active hydrogen species were hydrogen radicals generated from H 2 and isopropanol. • The hydrocracking products have higher H/C ratio and heating values than lignite. • Catalytic hydrocracking promoted producing phenols via the cleavage of C-O bonds. Cleaving C-O bridged bonds of lignite via catalytic hydrocracking (CHC) is an effective method to produce chemicals and clean fuels. Herein, a nitrogen-doped carbon supported Fe catalyst was developed by using a one-pot method for CHC of Xilinguole lignite (XL) and its related model compounds. The doped nitrogen enhanced the interaction between Fe and N species to form Fe-N sites and favored forming smaller Fe/Fe 3 C nanocrystals, which played a crucial role in CHC performance. Toluene and phenol were produced from CHC of benzyl phenyl ether (BPE) with high yield and selectivity at 250 °C over the optimal catalyst with 4% Fe, without hydrogenation products. The Fe-based catalyst activated both H 2 and isopropanol to generate hydrogen radicals for cleaving C-O bridged bonds. The CHC of XL under different temperature and time showed that the yields of oil, asphaltene and preasphaltene were obviously promoted under 325 °C, 1 MPa, and 6 h, and they had much higher H/C ratio but remarkably lower O/C ratio, resulting in much higher heating values (28.19–36.90 MJ/kg) than XL. Phenols were the major group component with relative content of 53.6 wt% in oil from CHC, and the asphaltene and preasphaltene from CHC had obviously lower molecular weights, which could attributed to depolymerization of C-O bridged bonds in the XL macromolecular structures over the catalyst. [ABSTRACT FROM AUTHOR]

[Display omitted] • Selective hydrogenolysis of diphenyl ether C-O bond occurred over Ni/Nb 2 O 5. • The common hydrogen supply existed between H 2 and isopropanol. • Hydrodeoxygenation reaction hardly occurred due to the weak acidity of Ni/Nb 2 O 5. • The reaction route of the hydrogenolysis in DPE was presented. Selective cleavage of stable ether C-O bonds is particularly of significance for the valorization of lignin and designing high active and cheap catalysts attracts researchers to conduct a lot of investigations. The niobium species containing both Brønsted and Lewis acid sites can be applied in hydrodeoxygenation of diphenyl ether (DPE). It is important to regulate the acidity of niobium to achieve the selective cleavage of DPE without hydrodeoxygenation. Herein, the weakly acidic Ni/Nb 2 O 5 prepared by an incipient wetness impregnation method could efficiently cleave C-O bonds of DPE without hydrodeoxygenation. DPE was completely converted and the yields of target products (cyclohexane and cyclohexanol) reached 90% under the coexistence of H 2 and isopropanol. It was ascribed to the common hydrogen supply between H 2 and isopropanol which could produce more active hydrogen. Acetone was detected in the solution after the reaction, demonstrating that a dehydrogenation reaction of isopropanol provided hydrogen. In addition, the turnover frequency (TOF) and the apparent activation energy (E a) for catalytic hydrogenolysis of DPE revealed that Ni/Nb 2 O 5 possessed higher activity than Co/Nb 2 O 5 under the same conditions. The possible reaction pathways were also summarized and analyzed. [ABSTRACT FROM AUTHOR]