Use of the PIXEL method to investigate gas adsorption in metal–organic frameworks

PIXEL has been used to perform calculations of adsorbate-adsorbent interaction energies between a range of metal–organic frameworks (MOFs) and simple guest molecules. Interactions have been calculated for adsorption between MOF-5 and Ar, H(2), and N(2); Zn(2)(BDC)(2)(TED) (BDC = 1,4-benzenedicarboxylic acid, TED = triethylenediamine) and H(2); and HKUST-1 and CO(2). The locations of the adsorption sites and the calculated energies, which show differences in the Coulombic or dispersion characteristic of the interaction, compare favourably to experimental data and literature energy values calculated using density functional theory

Bronstedova kyselost v zeolitech meřená pomocí deprotonační energie

Acid forms of zeolites have been used in industry for several decades but scaling the strength of their acid centers is still an unresolved and intensely debated issue. In this paper, the Bronsted acidity strength in aluminosilicates measured by their deprotonation energy (DPE) was investigated for FAU, CHA, IFR, MOR, FER, MFI, and TON zeolites by means of periodic and cluster calculations at the density functional theory (DFT) level. The main drawback of the periodic DFT is that it does not provide reliable absolute values due to spurious errors associated with the background charge introduced in anion energy calculations. To alleviate this problem, we employed a novel approach to cluster generation to obtain accurate values of DPE. The cluster models up to 150 T atoms for the most stable Bronsted acid sites were constructed on spheres of increasing diameter as an extension of Harrison's approach to calculating Madelung constants. The averaging of DPE for clusters generated this way provides a robust estimate of DPE for investigated zeolites despite slow convergence with the cluster size. The accuracy of the cluster approach was further improved by a scaled electrostatic embedding scheme proposed in this work. The electrostatic embedding model yields the most reliable values with the average deprotonation energy of about 1245 +/- 9 kJ center dot mol(-1) for investigated acidic zeolites. The cluster calculations strongly indicate a correlation between the deprotonation energy and the zeolite framework density. The DPE results obtained with our electrostatic embedding model are highly consistent with the previously reported QM/MM and periodic calculations.Síla Bronstedovy kyselosti v aluminosilikátech byla měřena pomocí jejich deprotonační energie a DFT výpočtů. Zkoumány byly materiály na bázi FAU, CHA, IFR, MOR, FER, MFI a TON zeolitů. DFT výpočty indikují korelaci mezi deprotonační energií a hustotou zeolitické struktury

Analýza BAS OH vazeb v zeolitech

The zeolite frameworks (FER, TON, CHA, and IFR) with a large diversity of the IR band shapes in the OH stretching region were investigated by combined use of FT-IR spectroscopy and DFT calculations. The analysis of various isolated Brunsted acid sites was performed with a model based on Boltzmann weighted relative energies and OH frequencies obtained by the omega/r correlation method. This approach has recently been used to reveal the non-statistical incorporation of aluminum into the H-MOR framework. The model correctly predicts the band shapes of H-FER and H-TON zeolites in the free OH region of the FT-IR spectra. For H-CHA and H-IFR, however, the mutual BAS interactions cannot be ignored even for high Si/Al ratios, which renders the concept of isolated BAS inadequate. Thus, the periodic calculations of adsorption and catalytic properties of zeotype frameworks should always be carried out with extended models mimicking real heteroatom distribution in these materials.Zeolitické mřížky typů FER, TON, CHA a IFR s rozsáhlou diverzitou IR pásů v OH regionu byly zkoumány kombinací FTIR spektroskopie a DFT výpočtů. Aplikovaný model dobře predikoval tvary OH pásů v H-FER a H-TON zeolitech. Nicméně v případě H-CHA a H-IFR zeolitů, vzájemné BAS interakce nemohly být ignorovány z důvodu vysokého poměru Si/Al

Přehodnocení interpretace CO adsorbovaného na Lewisových kyselých polohách v MOR zeolitech s alkalickými kovy

The interaction of CO with alkali metal-exchanged mordenite has been investigated by means of IR spectroscopy and calorimetry along with theoretical calculations based on DFT corrected to coupled-cluster accuracy (DFT/CC). It has been convincingly shown that Li+ is at least partially exchanged into the constricted part of the MOR structure as manifested by the low-frequency band at 2181 cm−1 and the isosteric heat of 44 kJ/mol. In the case of Na-MOR samples, significant changes have been observed in the stabilities of high- (2177 cm−1) and low-frequency (2165 cm−1) bands upon CO desorption, with the change of the Si/Al ratio from 40 to 9. Based on the kinetic measurements, it can be concluded that a crucial role in the MOR material is played by diffusion limitations, which are significantly influenced by the Si/Al ratio and the size of the cations. Similar effects also result in the increased stability of the 2138 cm−1 band of Na/K-MOR samples with higher Al content, where the “gate” effect is observed upon N2 adsorption. The dual cationic sites are directly observed only for the K-MOR sample via the weak band around 2150 cm−1. The formation of dual cationic sites cannot be completely ruled out in the case of Na-MOR, but their presence is most likely hidden in the low-frequency band.Interakce CO s mordenity s ionty alkalických kovů byly studovány pomocí IR spektroskopie a kalorimetrie. Výsledky byly porovnány s teoretickými výpočty na bázi DFT/CC

Characterizing the dark state in thymine and uracil by double resonant spectroscopy and quantum computation.

We report on gas phase double resonant spectroscopy of both the ground state and the dark excited state in isolated uracil and thymine. We also report lifetimes of the dark state for different excitation wavelengths. In combination with ab initio calculations the results suggest that the dark state is of triplet ((3)ππ*) character

Characterizing the dark state in thymine and uracil by double resonant spectroscopy and quantum computation.

We report on gas phase double resonant spectroscopy of both the ground state and the dark excited state in isolated uracil and thymine. We also report lifetimes of the dark state for different excitation wavelengths. In combination with ab initio calculations the results suggest that the dark state is of triplet ((3)ππ*) character

Analýza BAS OH skupin vykazujících nestatistickou inkorporaci Al do MOR struktury

H-MOR zeolites with different Si/Al ratios were investigated by the combined use of FT-IR spectroscopy, calorimetry, and periodic DFT calculations. Our analysis of 14 different Br?nsted acid sites (BAS) was carried out with the ?/r correlation method parameterized to reproduce the OH band shape for the high silica H-MOR(32) sample. The estimated populations clearly show that the Al distribution exhibits non-statistical behavior. The preferential incorporation of Al into the T3 and T4 framework positions is in excellent agreement with the X-ray study of Alberti et al. on natural mordenites. This suggests that the Al distribution is very similar in both high silica and aluminum rich MOR materials. The analysis of H-MOR with lower Si/Al ratios is complicated by the interaction of the adjacent BAS. This effect leads to significant changes in relative energies that determine populations of the individual sites. Our interpretation of the experimental data is further supported by a very good agreement between calculated CO adsorption enthalpies and calorimetric measurements. An anomalously large dispersion stabilization is found for adsorption complexes located in the side pocket of the MOR framework.H-MOR zeolity s odlišnými poměry Si/Al byly zkoumány pomocí FTIR spektroskopie, kalorimetrie a DFT výpočtů. Výsledky zřetelně ukázaly nestatistické chování Al distribuce, kdy AL se preferenčně inkorporoval do T3 a T4 pozic v mřížce