Ivan I. Zakharov

A DFT quantum‐chemical study on the structures and active sites of polymethylaluminoxane

DFT quantum-chemical calculations have been performed to elucidate the geometrical and electronic structure of methylalumoxanes (-Al(Me)0-)n with different size (n=6,8,12). The three-dimensional oxo-bridged (cage) structures of methylalumoxane (MAO) have been analyzed.

An unexpected phenomenon in heterogeneous catalysis: oxidative addition of hydrogen to the sulfide catalysts

Abstract The paper summarizes experimental and theoretical evidence of an oxidative addition of hydrogen to the active metal (Ni or Co) in the active component of the sulfide HDS catalysts—a new phenomenon unknown to date in heterogeneous catalysis. Several aspects are discussed: thermodynamics of the oxidative addition of hydrogen; possible mechanism of dihydrogen activation by sulfide catalysts; electronic state of the active metals Ni(IV) and Co(III) with d6-electron configuration; reversibility of the oxidative addition and reductive elimination of hydrogen at high temperature; sites of hydrogen localization inside the active component matrix; spectroscopic evidence on the existence of “occluded” hydrogen; a strong interaction of the occluded hydrogen with the active component matrix; a new version of the catalytic cycle of thiophene hydrogenolysis.

Quantum chemical study of the electronic structure of the NiMoS2 hydrodesulfurization catalysts

Abstract The role of the electronic state of the Ni atoms in the sulfide catalysts is studied by means of ab initio molecular orbital calculations. It is shown that the Ni ion with d8 electron configuration in the square-planar sulfur surrounding is not active in the hydrodesulfurization (HDS) process. The d8-state can be transformed to the d6-state after adsorption of the H2S molecule and formation of the square-pyramidal surrounding for the nickel ion. A square-pyramidal structure of the H2S adsorption complex on Ni MoS 2 catalysts with Ni(d6) is calculated as the active center. The HDS catalytic cycle, for which the H2S adsorption complex is the initial and final state, is proposed.

A Density Functional Theory (DFT) Quantum‐Chemical Approach to the Real Structure of Poly(methylaluminoxane)

DFT quantum chemical calculations have been performed in order to optimize the geometric and electronic cage structure of poly(methylaluminoxane) (MAO) with oligomerization degree n = 9-15, and to find such structuros that fit most closely the existing experimental data on the MAO composition and structure. The following peculiarities of the MAO structure were found: i) In classic MAO (n = 9, 12, 15; Al:CH 3 :O = 1:1:1), which has a triple-layer cage structure, the inner layer contains highly reactive Al-O bonds. ii) The reaction between classic MAO and trimethylaluminium (TMA) proceeds by the concerted mechanism, with the insertion of Al-CH 3 groups into these Al-O bonds producing true MAO (Al:CH 3 :O = 1:1.5;0.75). The calculated geometric and electronic structures of true MAO with n = 6. 9, 12 are presented. iii) True MAO and classic MAO exist in equilibrum. The driving force for the formation of true MAO is the decrease in enthalpy, and of classic MAO the increase in entropy, in the equilibrium reaction between classic MAO and TMA.

A DFT Quantum-Chemical Study of Ion-Pair Formation for the Catalyst Cp2ZrMe2 /MAO

A process of ion-pair formation in the system Cp 2 ZrMe/methylaluminoxane (MAO) has been studied by means of density functional theory quantum chemical calculations for MAOs with different structures and reactive sites. An interaction of Cp 2 ZrMe 2 with a MAO of the composition (AlMeO) 6 results in the formation of a stable molecular complex of the type Al 5 Me 6 O 5 Al(Me)O-Zr(Me)Cp 2 with an equilibrium distance r(Zr-O) of 2.15 A. The interaction of Cp 2 ZrMe 2 with true MAO of the composition (Al 5 Me 12 O 6 ) proceeds with a tri-coordinated aluminum atom in the active site (OAlMe 2 ) and yields the strongly polarized molecular complex or the μ-Me-bridged contact ion pair (d) [Cp 2 (Me)Zr(μMe)Al]≡ MAO] with the distances r(Zr-μMe) = 2.38 A and r(Al-μMe) = 2.28 A. The following interaction of the μ-Me contact ion pair (d) with AlMe 3 results in a formation of the trimethylaluminum (TMA)-separated ion pair (e) [Cp 2 Zr(μMe) 2 AlMe 2 ] + -[MeMAO] with r[Zr-(MeMAO)] equal to 4.58 A. The calculated composition and structure of ion pairs (d) and (e) are consistent with the 13 C NMR data for the species detected in the Cp 2 ZrMe 2 /MAO system. An interaction of the TMA-separated ion pair (e) with ethylene results in the substitution of AlMe 3 by C 2 H 4 in a cationic part of the ion pair (e), and the following ethylene insertion into the Zr-Me bond. This reaction leads to formation of ion pair (f) of the composition [Cp 2 ZrCH 2 CH 2 CH 3 ] + -[Me-MAO] - named as the propyl-separated ion pair. Ion pair (f) exhibits distance r[Zr-(MeMAO)] = 3.88 A and strong C γ -agostic interaction of the propyl group with the Zr atom. We suppose this propyl-separated ion pair (f) to be an active center for olefin polymerization.

The molecular mechanism of low-temperature decomposition of hydrogen sulfide under conjugated chemisorption-catalysis conditions

The molecular mechanism of interaction of two hydrogen sulfide molecules with the (CoIII-Ho)2S2(SH2)4 model active center containing occluded hydrogen was studied by the density functional theory method with the B3P86 hybrid exchange-correlation functional. The reaction was found to occur in the following elementary steps: molecular adsorption of hydrogen sulfide ⇒ dissociative chemisorption ⇒ S-S bond formation in the surface intermediate {2CoIII − (μ-S2) + 2H(ads)} with the release of the first hydrogen molecule into the gas phase H2(g) ⇒ the release of the second hydrogen molecule into the gas phase H2(g) ⇒ the formation of cyclooctasulfur in the reaction 4S2(ads) → S8(ads). The first three steps occur spontaneously at room temperature, the thermodynamic driving force of the process being the stoichiometric reaction of S-S bond formation at the stage of conjugated chemisorption of two hydrogen sulfide molecules on two adjacent metal ions with the release of the first hydrogen molecule into the gas phase. The catalytic cycle is terminated by the recombination of molecular sulfur S2 into cyclooctasulfur S8 in the adsorption layer and the release of the second hydrogen molecule into the gas phase.

Oxidative addition of dihydrogen to the bimetallic sulfide catalysts: evidence by X-ray photoelectron spectroscopy

Abstract Reversible transformations Ni(IV)↔Ni(II) in alumina and Sibunit supported (Ni,Mo) sulfide catalysts were observed after in situ thermal treatment of catalysts in an X-ray photoelectron spectrometer chamber. The phenomenon is interpreted as a reductive elimination of occluded hydrogen under low pressure and high temperature, and oxidative addition of hydrogen after catalyst treatment with an (H2+H2S) mixture.

Chelate complex Ni(S2C2H2)2 as a molecular model of the hydrodesulfurization active center

Electronic states of Ni atom in a square-planar complex Ni(S2C2H2)2 and its molecular adduct with H2S were studied by means ofab initio molecular orbital calculations. H2S adsorption stabilizes the Ni(IV) state (d6) in the complex with the Ni atom shifted from the plane by 0.35 Å.

Oxidative addition of dihydrogen to Ni(II) complexes.ab initio MO/MP4 calculations of the electronic structure of NiH2Cl2(PH3)2 complex

Electronic state d6 Ni(IV) in the complex [NiH2Cl2(PH3)2] was studied by means ofab initio MO/MP4 calculations.

Structures of MAO: Experimental Data and Molecular Models According to DFT Quantum Chemical Simulations

A DFT quantum chemical calculations have been performed in order to optimize the geometric and electronic cage structure of polymethylalumoxane (MAO) with oligomerization degree n =12 and find such structures that fit most closely the existing experimental data on the MAO composition and structure. The following peculiarities of the MAO structure were found: i) In “classic” MAO (ratio A1:CH3:O = 1:1:1), which has triple-layer cage structure, the inner layer contains highly reactive bonds Al-O. ii) Reaction between “classic MAO” and trimethylaluminium (TMA) proceeds by the concerted mechanism with the insertion of A1-CH3 groups into these Al-O bonds. The reaction produces “true” MAO showing ratio A1:CH3:O = 1:1.5:0.75. Calculated geometry and electronic structures of “true” MAO with n= 12 are presented. iii) “True” MAO and “classic” MAO exist in equilibrium. Driving force for the formation of “true” MAO is the decrease of enthalpy, and of “classic” MAO is the increase of entropy in the equilibrium reaction between “classic” MAO and A1(CH3)3