Configurations of V4+ centers in the MoVO catalyst material. A systematic stability analysis of DFT results

The reactivity of a catalyst is in part determined by its geometric and electronic structure. Here we present a model that is able to describe the energy trend of the important oxidation catalyst material MoVO, as obtained from hybrid density functional calculations for various V4+/V5+ configurations. For an exemplary V/Mo occupancy, we systematically examined the universe of all V4+ distributions. The distribution of these V4+ centers, in combination with the induced lattice distortions, plays a key role in determining the stability of the material, entailing energy variations of up to ~140 kJ mol−1 per unit cell. Hence, for this kind of catalyst, it is crucial to account for the V4+ distributions. To this end, we are proposing novel predictive models based on features like the number of Mo centers with two reduced neighbors V4+ and the locations of potentially reducible centers V5+. For the V/Mo occupancy chosen, these models are able to describe the energy variation due to the V4+ distribution with root mean square errors as low as 6 kJ mol−1. Accordingly, catalytically selective sites featuring pentameric units with a single polaron center are among the most of stable configurations. Another aspect of this work is to understand energy contributions of polaron arrangements bracketing Mo centers

Computacional studies on the mechanism of the pauson-khand reaction

The application of computational methods has lead to a better understanding of the mechanism of the Pauson-Khand reaction (PKR) for the synthesis of cyclopentenone compounds. In particular the enantioselective PKR has been considered. Different methods for inducing enantioselectivity in this originally unselective reaction have been analyzed. Three cases have been considered: a) Activation of the conventional dicobalt octacarbonyl catalyst by a chiral N-oxide, b) A modification of the dicobalt catalyst by means of a substitution of two carbonyl ligands by a chiral diphosphine ligand. and c) the substitution of the dinuclear cobalt catalyst by a mononuclear rhodium catalyst with chiral ligands. The theoretical study has lead to the characterization of mechanistic aspects that would be inaccessible from a purely experimental study. The study therefore contributes to the development of more efficient catalytic methods for this important reaction.La aplicación de los métodos de la química computacional ha llevado a una mejor comprensión del mecanismo de la reacción de Pauson-Khand para la síntesis de ciclopentenonas, en especial en su variante enantioselectiva. Se han analizado tres métodos distintos para introducir enantioselectividad en esta reacción, que en su modelo original no es selectiva. Se han estudiado tres casos: a) la activación del catalizador convencional dicobalto octacarbonilo mediante un N-óxido quiral, b) la modificación de este mismo catalizador mediante la sustitución de dos ligandos carbonilo por una difosfina quiral, y c) la sustitución del catalizador dinuclear de cobalto por un catalizador mononuclear de rodio con ligandos quirales. El estudio teórico ha llevado a la caracterización de aspectos mecanísticos que son inaccesibles al estudio puramente experimental, y contribuye así al desarrollo de métodos catalíticos más eficientes para esta importante reacción

Desilication of SAPO-34: Reaction Mechanisms from Periodic DFT Calculations

With the aim of understanding the desilication of SAPO-34, we compared three different reaction mechanisms for the hydrolysis of framework silicon by use of density functional theory (DFT) calculations. All three mechanisms are characterized by stepwise hydrolyses of Si–O–Al bonds. In the most favorable mechanism water molecules adsorb strongly to the Lewis acidic Al atoms neighboring the Si atom. Furthermore, evaluation of free energies reveals that an additional water molecule may catalyze the hydrolysis of the first Si–O–Al bond

Mechanism of Si Island Formation in SAPO-34

With the aim of understanding the Si island formation in SAPO-34, we have carried out a computational mechanistic study. Briefly, the Si island formation in SAPO-34 is explained by three successive reactions. First, the framework Si atom is removed from the framework through the action of four water molecules. Second, the hydrogarnet defect generated by the desilication is healed by an available H₃PO₄ molecule. Third, the extra framework Si­(OH)₄ species inserts in the framework position of a phosphorus atom while, in a concerted fashion, “kicking out” the phosphorus atom as a H₃PO₄ extra-framework species. When these exchanges of framework and extra-framework species are repeated, the isolated Si atoms may eventually cluster into Si islands

Mechanistic Comparison of the Dealumination in SSZ-13 and the Desilication in SAPO-34

With the purpose of understanding the behavior of aluminosilicate zeolites and silicoaluminophosphates (SAPOs) in the presence of steam, we carried out a computational density functional theory (DFT) study on the desilication of SAPO-34. The mechanism studied was a stepwise hydrolysis of the four bonds to the Si heteroatom. An analogous process to the desilication of SAPO-34 is the dealumination of SSZ-13. To investigate possible mechanistic differences between the two processes, we compared the results of this study with the results of a previous study on dealumination in SSZ-13. We found that the intermediates along the dealumination path of SSZ-13 have one of the protons bonded to a bridging oxygen atom. In the corresponding intermediates of the desilication path in SAPO-34, the same proton prefers to be part of an aqua ligand coordinated to an Al atom. The principal factor determining the different proton locations is the electronic requirement of the atoms surrounding the proton. The different proton locations in SSZ-13 and SAPO-34 put clear conditions on possible mechanisms, thus causing them to be different for the two materials. We expect the principles determining the proton location also to be valid for other mechanisms of dealumination in SSZ-13 and desilication in SAPO-34

Mechanistic Comparison of the Dealumination in SSZ-13 and the Desilication in SAPO-34

With the purpose of understanding the behavior of aluminosilicate zeolites and silicoaluminophosphates (SAPOs) in the presence of steam, we carried out a computational density functional theory (DFT) study on the desilication of SAPO-34. The mechanism studied was a stepwise hydrolysis of the four bonds to the Si heteroatom. An analogous process to the desilication of SAPO-34 is the dealumination of SSZ-13. To investigate possible mechanistic differences between the two processes, we compared the results of this study with the results of a previous study on dealumination in SSZ-13. We found that the intermediates along the dealumination path of SSZ-13 have one of the protons bonded to a bridging oxygen atom. In the corresponding intermediates of the desilication path in SAPO-34, the same proton prefers to be part of an aqua ligand coordinated to an Al atom. The principal factor determining the different proton locations is the electronic requirement of the atoms surrounding the proton. The different proton locations in SSZ-13 and SAPO-34 put clear conditions on possible mechanisms, thus causing them to be different for the two materials. We expect the principles determining the proton location also to be valid for other mechanisms of dealumination in SSZ-13 and desilication in SAPO-34

Computational study of the mechanism of cyclic acetal formation via the iridium(I)-catalyzed double hydroalkoxylation of 4-pentyn-1-ol with methanol

The mechanism of Ir(I)-catalyzed double hydroalkoxylation of 4-pentyn-1-ol with methanol to form cyclic acetals has been investigated with density functional theory calculations. Using a model [Ir(PyP′)(CO) 2]+ catalyst (PyP′ = 1-[2-phosphinoethyl]pyrazole) the key steps in the first hydroalkoxylation are shown to be (i) electrophilic activation of the alkyne at the cationic Ir(I) metal center; (ii) rate-limiting C-O bond formation via intramolecular nucleophilic attack by the pendant OH group at the C4 position of the bound alkyne; and (iii) facile H+ transfer to form an Ir-bound cyclic vinyl ether intermediate. The key C-O bond forming cyclization step is greatly facilitated by the presence of an external H-bonded MeOH molecule that stabilizes the positive charge that develops at the hydroxyl proton of the bound alkyne. External MeOH also plays a key role in the H+ transfer step, for which a number of kinetically competitive pathways corresponding to either retention of the hydroxyl proton in the product or exchange with solvent were identified. The second hydroalkoxylation is initiated from the Ir-bound cyclic vinyl ether intermediate and depends on the ability of that species to access an Ir(I)-alkyl form in which the β-carbon carries a significant positive charge. Reversible C-O bond formation then occurs via nucleophilic attack of MeOH at the β-carbon and proceeds via a novel [3+2]-addition of the O-H bond over the {Ir-Cα-C β} moiety. This forms an Ir(III) hydrido-alkyl species, from which reductive elimination yields the final O,O-acetal product. This final reductive elimination is the rate-limiting step within the second hydroalkoxylation component of the cycle. The Ir(I)-alkyl intermediate can also access a MeOH-mediated C-H activation at the Cγ position that leads to exchange with external MeOH. This accounts for the experimentally observed H/D exchange at that position.</p

Computational study of the mechanism of cyclic acetal formation via the iridium(I)-catalyzed double hydroalkoxylation of 4-pentyn-1-ol with methanol

The mechanism of Ir(I)-catalyzed double hydroalkoxylation of 4-pentyn-1-ol with methanol to form cyclic acetals has been investigated with density functional theory calculations. Using a model [Ir(PyP΄)(CO)₂] + catalyst (PyP΄ = 1-[2-phosphinoethyl]pyrazole) the key steps in the first hydroalkoxylation are shown to be (i) electrophilic activation of the alkyne at the cationic Ir(I) metal center; (ii) rate-limiting C-O bond formation via intramolecular nucleophilic attack by the pendant OH group at the C4 position of the bound alkyne; and (iii) facile H⁺ transfer to form an Ir-bound cyclic vinyl ether intermediate. The key C-O bond forming cyclization step is greatly facilitated by the presence of an external H-bonded MeOH molecule that stabilizes the positive charge that develops at the hydroxyl proton of the bound alkyne. External MeOH also plays a key role in the H⁺ transfer step, for which a number of kinetically competitive pathways corresponding to either retention of the hydroxyl proton in the product or exchange with solvent were identified. The second hydroalkoxylation is initiated from the Ir-bound cyclic vinyl ether intermediate and depends on the ability of that species to access an Ir(I)-alkyl form in which the β-carbon carries a significant positive charge. Reversible C-O bond formation then occurs via nucleophilic attack of MeOH at the β-carbon and proceeds via a novel [3+2]-addition of the O-H bond over the {Ir-Cᵅ -Cᵝ } moiety. This forms an Ir(III) hydrido-alkyl species, from which reductive elimination yields the final O,O-acetal product. This final reductive elimination is the rate-limiting step within the second hydroalkoxylation component of the cycle. The Ir(I)-alkyl intermediate can also access a MeOH-mediated C-H activation at the Cˠ position that leads to exchange with external MeOH. This accounts for the experimentally observed H/D exchange at that position.9 page(s