Theoretical identification of structural heterogeneities of divalent nickel active sites in NiMCM-41 nanoporous catalysts
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- Balar, M., Azizi, Z. & Ghashghaee, M. J Nanostruct Chem (2016) 6: 365. doi:10.1007/s40097-016-0208-z
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This paper deals with the theoretical identification of the digrafted Ni species exchanged into the defect sites of MCM-41 using hybrid density functional theory. The nickel–siloxane clusters included seven 2T–6T rings. The 2MR and 5MR structures were found to be the least and most favorable sites to form thermodynamically. The Ni–O distances ranged from 1.69 to 1.79 Å with the highest asymmetry found in 5MR. The 4MR and 5MR clusters showed also interesting intertwined nickel configurations. Overall, the QTAIM calculations revealed the transient electrostatic nature of the Ni–O bonds.
Nickel ions supported on different silica, silica–alumina, natural clay, and zeolite-type porous materials are well-known catalysts for the selective dimerization [1–9] and oligomerization [1, 5, 10–19] of olefins in both gas and liquid phases. The application of silica-supported nickel catalysts for the ethylene dimerization dates back to 1980s [20–22]. Later, the large and well-ordered cavities of the Ni-exchanged MCM-41 proved highly favorable for the oligomerization of olefins . For instance, high productivities of oligomers were obtained over Ni-incorporated MCM-41 catalysts  being higher than those reported previously with silica–alumina supports . The strong interactions of Ni2+ cations residing in the mesoporous cavities with the support framework made the reduction of the nickel ions difficult . Further electron spin resonance (ESR) and Fourier transform infrared (FTIR) spectroscopic data [11, 24] served as evidence to support the role of Ni2+ cations as active sites in ethylene dimerization.
The nickel ions incorporated into the ordered mesoporous materials (OMMs) have also been suggested as efficient catalysts for the gas-phase transformation of ethylene to propylene [25–34]. For instance, the performance and durability of Ni/MCM-41 catalysts prepared from a nickel citrate precursor was ranked as auspicious with a promising potential for closing the gap between the propylene demand and supply . Ikeda et al.  proposed that layered nickel-silicate species were the main players in the conversion of ethylene on NiMCM-41 catalysts. In a similar study , the threefold coordinated Ni2+ ions situated in the 5-membered rings of the phyllosilicate pore walls of NiMCM-41 were taken as the active sites of the reaction.
Computational studies of Ni2+ binding in FER , MFI [38, 39], AFI , and silica [4, 41] have been reported earlier. Analogous reports have addressed neutral atoms [42, 43]. The present study aims at a systematic theoretical modeling of the locations of Ni2+ species in a model MCM-41 material at the fundamental level. To our knowledge, the most relevant publication to this target is that by Neiman  who considered an O3(SiO)2Ni structure unit cell with a formal charge of +2 for the nickel ion. The author did not investigate all of the defect sites available for the Ni2+ siting. This work is then the first fundamental report on the molecular-level heterogeneities of the NiMCM-41 catalysts.
The cluster model approach was employed to simulate the active sites of a nanoporous NiMCM-41 catalyst through exploring the available defect sites of MCM-41 within an extended unit cell. As a common approach [44, 45], the model nanoclusters were terminated by H atoms frozen in agreement with the geometries obtained from the crystallographic data [46–51]. The divalent nickel cation and the immediate neighbors including the O atoms from two defect-site hydroxyls were allowed to relax during the optimizations. As adopted earlier , the remaining part of the cluster was held fixed to mimic the mechanical restrictions of the matrix.
As an approximation, the structure of cristobalite is normally taken as a good representative for the amorphous silica materials in terms of type and density of the surface hydroxyl groups [51, 53–56]. Many references [42, 43, 57–60] have then applied this model to ordered silica mesoporous materials such as MCM-41. The optimizations and single-point computations were implemented using hybrid functional M06  and the Def2-TZVP basis set [62–64]. Moreover, the natural bond orbital (NBO) population  as well as the quantum theory of atoms in molecules (QTAIM) [66–71] analyses were carried out at the same level of theory.
Results and discussion
Calculated charges of selected atoms of NiMCM-41 for different cluster models at M06/Def2-TZVP level of theory
NBO partial charges
Mulliken atomic charges
The Gibbs free energy (kcal/mol), enthalpy (kcal/mol), and entropy (cal/mol/K) of the exchange reaction and the binding energy (kcal/mol) of the digrafted Ni ions in NiMCM-41 at M06/Def2-TZVP level of theory (please see the corresponding reactions in the text)
Nickel–oxygen bond length (Å) and interbond angle (in degree) for different optimized cluster models at M06/Def2-TZVP level of theory
QTAIM data for different optimized clusters at M06/Def2-TZVP level of theory
Calculated HOMO and LUMO and HOMO–LUMO energy gaps (ΔEHOMO−LUMO) for the investigated clusters at M06/Def2-TZVP level of theory
This paper investigated the diversity of the cluster models of NiMCM-41 in a systematic computational framework. Total of seven active sites (2T–6T rings) were found at the defect sites of an MCM-41 silica model. The NBO partial charge of the nickel cation was less positive on 2MR and largest on 4MR. The thermodynamic favorability of the NiMCM-41 clusters followed the order of 2MR < 3MR < 4MR < 6MR-3 < 6MR-1 < 6MR-2 < 5MR. The optimized structures indicated the Ni–O distances in the range of 1.69–1.79 Å with the highest asymmetry observed in 5MR. The highest reactivity was observed in the case of the digrafted nickel ions at 4MR and 5MR sites and the lowest one at 2MR. The 4MR and 5MR clusters showed also some intertwining features for nickel hosting. The QTAIM calculations revealed intermediate polar Ni–O bonds. Moreover, the electron densities at the BCP correlated with the Ni–O distances.
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