Correlation between Asphaltene Stability in n-Heptane and Crude Oil Composition Revealed with Chemical Imaging

Five crude oil samples with different physical properties have been studied with respect to asphaltene stability. The attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopic imaging approach of n-heptane-induced precipitation has been used to monitor crude oil behaviour under dilution with a flocculation agent. For each sample, the dynamics of asphaltene precipitation has been observed by applying this chemical imaging method. Based on these data, the stability of crude oil samples has been compared and the correlation between asphaltene stability and crude oil properties has been proposed

Property-activity relations of multifunctional reactive ensembles in cation-exchanged zeolites: a case study of methane activation on Zn²⁺-modified zeolite BEA

The reactivity theories and characterization studies for metal-containing zeolites are often focused on probing the metal sites. We present a detailed computational study of the reactivity of Zn-modified BEA zeolite towards C-H bond activation of the methane molecule as a model system that highlights the importance of representing the active site as the whole reactive ensemble integrating the extra-framework ZnEF2+ cations, framework oxygens (OF2−), and the confined space of the zeolite pores. We demonstrate that for our model system the relationship between the Lewis acidity, defined by the probe molecule adsorption energy, and the activation energy for methane C-H bond cleavage performs with a determination coefficient R2 = 0.55. This suggests that the acid properties of the localized extra-framework cations can be used only for a rough assessment of the reactivity of the cations in the metal-containing zeolites. In turn, studying the relationship between the activation energy and pyrrole adsorption energy revealed a correlation, with R2 = 0.80. This observation was accounted for by the similarity between the local geometries of the pyrrole adsorption complexes and the transition states for methane C-H bond cleavage. The inclusion of a simple descriptor for zeolite local confinement allows transferability of the obtained property-activity relations to other zeolite topologies. Our results demonstrate that the representation of the metal cationic species as a synergistically cooperating active site ensembles allows reliable detection of the relationship between the acid properties and reactivity of the metal cation in zeolite materials.ChemE/Inorganic Systems Engineerin

Nature of the Surface Intermediates Formed from Methane on Cu-ZSM-5 Zeolite: A Combined Solid-State Nuclear Magnetic Resonance and Density Functional Theory Study

The intermediates formed upon the interaction of methane with Cu-modified ZSM-5 zeolites (Cu/H-ZSM-5) have been analyzed with solid-state NMR spectroscopy and DFT methods. Methane activation by Cu/H-ZSM-5 zeolites gives rise to three distinct surface methoxy-like species (-O-CH3) detected by 13C MAS NMR spectroscopy with specific chemical shifts in the range of 53-63 ppm. DFT calculations on representative cluster models of different sites potentially present in Cu/H-ZSM-5 have been used to assign these signals to (i) methanol adsorbed on two neighboring Cu sites (Cu-(HOCH3)-Cu, 62.6 ppm), (ii) methanol adsorbed on zeolite Brønsted acid sites (52.9 ppm), and (iii) lattice-bound methoxy groups (Si-O(CH3)-Al, 58.6). The formation of these methoxy-like intermediates depends on the Cu loading and, accordingly, the type of Cu species in the Cu/H-ZSM-5 zeolite. For the sample with low (0.1 wt %) Cu loading containing exclusively mononuclear isolated Cu species, only the intermediates ii and iii have been detected. The Cu-bound intermediate (i) is formed upon methane activation by multinuclear Cu sites featuring Cu-O-Cu bridging moieties present in the materials with relatively higher Cu loading (1.38 wt %). The presented results indicate that methane activation by Cu/H-ZSM-5 can be promoted by both mono- and multinuclear Cu species confined in the zeolite matrix.Accepted Author ManuscriptChemE/Inorganic Systems EngineeringChemE/Algemee

The accuracy challenge of the DFT-based molecular assignment of ¹³C MAS NMR characterization of surface intermediates in zeolite catalysis

The influence of the model and method choice on the DFT predicted 13C NMR chemical shifts of zeolite surface methoxide species has been systematically analyzed. Twelve 13C NMR chemical shift calculation protocols on full periodic and hybrid periodic-cluster DFT calculations with varied structural relaxation procedures are examined. The primary assessment of the accuracy of the computational protocols has been carried out for the Si-O(CH3)-Al surface methoxide species in ZSM-5 zeolite with well-defined experimental NMR parameters (chemical shift, δ(13C) value) as a reference. Different configurations of these surface intermediates and their location inside the ZSM-5 pores are considered explicitly. The predicted δ value deviates by up to ±0.8 ppm from the experimental value of 59 ppm due to the varied confinement of the methoxide species at different zeolite sites (model accuracy). The choice of the exchange-correlation functional (method accuracy) introduces ±1.5 ppm uncertainty in the computed chemical shifts. The accuracy of the predicted 13C NMR chemical shifts for the computational assignment of spectral characteristics of zeolite intermediates has been further analyzed by considering the potential intermediate species formed upon methane activation by Cu/ZSM-5 zeolite. The presence of Cu species in the vicinity of surface methoxide increases the prediction uncertainty to ±2.5 ppm. The full geometry relaxation of the local environment of an active site at an appropriate level of theory is critical to ensure a good agreement between the experimental and computed NMR data. Chemical shifts (δ) calculated via full geometry relaxation of a cluster model of a relevant portion of the zeolite lattice site are in the best agreement with the experimental values. Our analysis indicates that the full geometry optimization of a cluster model at the PBE0-D3/6-311G(d,p) level of theory followed by GIAO/PBE0-D3/aug-cc-pVDZ calculations is the most suitable approach for the calculation of 13C chemical shifts of zeolite surface intermediates.ChemE/AlgemeenChemE/Inorganic Systems Engineerin

Selective Dimerization of Ethene to 2-Butene on Zn²⁺-Modified ZSM-5 Zeolite

Selective dimerization of ethene to 2-butene on Zn2+-containing ZSM-5 zeolite (Zn2+/ZSM-5) at 296-523 K has been discovered. The intermediate but-3-en-1-ylzinc species is identified with 13C CP/MAS NMR and Fourier transform infrared spectroscopy. The density functional theory study of two alternative dimerization pathways reveals that the intermediate is formed with the involvement of the saturated bridged dimeric Zn-(CH2)4-O species. It is also shown that ethene conversion to 2-butene increases with the increase in the quantity of Brønsted acid sites in Zn2+/ZSM-5 zeolite; however, the selectivity of the reaction decreases. The results obtained are of potential interest for developing industrially relevant Zn-containing zeolite catalysts for the selective conversion of ethene to 2-butene. Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.ChemE/Inorganic Systems Engineerin

Mobility of <i>tert-</i>Butyl Alcohol in MFI Framework Type Studied by Deuterium NMR

The molecular mobility of deuterated <i>tert-</i>butyl alcohol (TBA) adsorbed in MFI framework type (silicalite-1 and ZSM-5 zeolite) has been studied by use of ²H NMR spectroscopy in the range of 106–453 K. In H-ZSM-5, the reorientation of the molecule as a whole is strongly restricted (τ_C ≪ <i>Q</i>₀^–1 ≈ 10^–6 s) by hydrogen bonding to Brønsted acid sites (BAS). Being adsorbed to BAS, the motion of TBA molecules is described by intramolecular rotations around two successive <i>C</i>₃ and <i>C</i>₃′ axes (CD₃–C and C–O bonds). The activation energy for the methyl groups rotation around the C–O bond (<i>E</i>_a = 8.0 ± 1.6 kJ mol^–1) is two times lower compared to that in solid TBA (<i>J. Phys. Chem. A</i> <b>2011</b>, <i>115</i>, 7428). This shows that the motion of the butyl fragment of TBA in MFI framework is less restricted compared to the case of solid TBA. In silicalite-1, the TBA molecule is additionally involved into reorientational motions as a whole: one of the motions represents an exchange among orientations provided by the directions of four framework channels driven by the translational jump diffusion. The activation barriers for the jump diffusion between two zigzag channels (<i>E</i>_a = 7 ± 2 kJ mol^–1) and between zigzag and straight channels (<i>E</i>_a = 5 ± 1 kJ mol^–1) are of the similar values. The other motion represents a large-amplitude wobbling of the TBA molecule localized at a channel intersection site cavity. This motion is described by a fast restricted wobbling of the molecular axis in a sphere sector. The wobbling boundaries become gradually broader as temperature increases, allowing thus a larger accessible space for the TBA molecule to explore. The anisotropy of this motion persists even at 453 K, revealing the presence of a strong intracavity barrier that blocks the TBA from free rotational diffusion inside the cavity. This study demonstrates that the ²H NMR analysis of both line shape and spin–lattice relaxation represents a powerful tool to investigate the particular pore confinement effect on the molecular mobility of TBA adsorbed in the MFI framework