Mo/ZSM-5 is one of the most studied and efficient catalysts for the dehydroaromatization of methane (MDA), but the mechanism of its operation remains controversial. Here, we combine an ab initio thermodynamic analysis with a comprehensive mechanistic density functional theory study to address Mo-speciation in the zeolite and identify the active sites under the reaction conditions. We show that the exposure of Mo/ZSM-5 to the MDA conditions yields a range of reduced sites including mono- and binuclear Mo-oxo and Mo-carbide complexes. These sites can catalyze the MDA reaction via two alternative reaction channels, namely, the C-C coupling (ethylene) and the hydrocarbon-pool propagation mechanisms. Our calculations point toward the binuclear Mo-carbide species operating through the hydrocarbon-pool mechanism to be the most catalytically potent species. Although all other Mo sites in the activated catalyst can promote C-H activation in methane, they fail to provide a successful path to the desirable low-molecular-weight products.

Selective hydroamination of terminal alkynes with primary aryl amines is catalyzed by an unprecedented well-defined silica-supported tantalum complex [(≡Si-O-)Ta(η1σ-NEtMe)2(=NtBu)]. A molecular-level characterization of the surface organometallic Ta species was done with the help of characterization tech-niques including as in situ infrared, in situ elemental microanal-ysis, 1H and 13C solid-state NMR (including double and triple quanta sequence), and X-ray absorption spectroscopies. These were complemented by the state-of-the-art DNP-SENS 15N characterization. Several catalytic intermediates have been isolated in particular the 4-membered metallacycle ring inter-mediate resulting from the anti Markovnikov addition of the alkyne to the surface tantalum imido. The mechanism proposed was based on the isolation of all intermediates. A DFT calcula-tion has confirmed all the elementary steps and intermediates that were fully characterized.

ABSTRACT: Well-defined single-site silica-supported haf-niaaziridine complex [(≡Si-O-)Hf(η2-MeNCH2)(η1-NMe2)(η1-HNMe2)] was prepared using surface organometallic chemistry. Upon thermal treatment under high vacuum, the grafted spe-cies was converted into an unprecedented hafnium imido, bis-amido, complex [(≡Si-O-)Hf(=NMe)(η1-NMe2)]. The surface complexes were characterized by elemental analysis and the following spectroscopic techniques: infrared, solid-state single and multiple quantum NMR, advanced DNP-SENS, and ex-tended X-ray absorption fine structure. [(≡Si-O-)Hf(=NMe)(η1-NMe2)] catalyzed imine metathesis under mild conditions, and characterization of the reactivity showed that the imido ex-change with N-(4-phenylbenzylidene)benzylamine yielded [(≡Si-O-)Hf (η2-NMeNCH2ArCH2) (η1-NMe2)], demonstrating a kind of 2+2 mechanism involving the imine and the imido, proposed reaction mechanism is also supported by DFT calculations.

Periodic density functional theory (DFT) calculations were carried out to investigate the mechanism of methane oxidation with HO over the defined Fe sites in Fe/ZSM-5 zeolite. The initial Fe site is modeled as a [(HO)-Fe(III)-(μO)-Fe(III)-(HO)] extraframework cluster deposited in the zeolite pore and charge-compensated by two anionic lattice sites. The activation of this cluster with HO gives rise to the formation of a variety of Fe(III)-oxo and Fe(IV)-oxo complexes potentially reactive toward methane dissociation. These sites are all able to promote the first C-H bond cleavage in methane by following three possible reaction mechanisms: namely, (a) heterolytic and (b) homolytic methane dissociation as well as (c) Fenton-type reaction involving free OH radicals as the catalytic species. The C-H activation step is followed by formation of MeOH and MeOOH and regeneration of the active site. The Fenton-type path is found to proceed with the lowest activation barrier. Although the barriers for the alternative heterolytic and homolytic pathways are found to be somewhat higher, they are still quite favorable and are expected to be feasible under reaction conditions, resulting ultimately in MeOH and MeOOH products. HO oxidant competes with CH substrate for the same sites. Since the oxidation of HO to O and two [H] is energetically more favorable than the C-H oxofunctionalization, the overall efficiency of the latter target process remains low.

Cross metathesis between 2-butenes and isobutene yielding the valuable products propylene and 2-methyl-2-butene has been investigated at low pressure and temperature using WH3/Al2O3, a highly active and selective catalyst. Two parallel catalytic cycles for this reaction have been proposed where the cycle involving the less sterically hindered tungstacyclobutane intermediates is most likely favored. Moreover, it has been found that the arrangement of substituents on the least thermodynamically favored tungstacyclobutane governs the conversion rate of the cross metathesis reaction for propylene production from butenes and/or ethylene. © 2013 American Chemical Society.