Zeolite Ring Research Articles

Application of vibrational spectra in the studies of cation sorption on zeolites

The work describes results of spectroscopic studies (MIR, FIR and Raman) of zeolites on which sorption of various metal cations, namely Pb 2+, Cd 2+, Cr 3+, Zn 2+, Cu 2+ and Ag + has been carried out. Sorption has been performed on sodium forms of natural zeolites (clinoptilolites from various sources) using aqueous solutions of the metals studied. Changes in intensities and positions of the pseudolattice bands corresponding to ring vibrations have been observed in the measured spectra. Pseudolattice bands are located in the range of ca. 400–700 cm −1. The degree of changes depends on both, the type of the cation sorbed and the extent of deformation of zeolite ring units. This, in turn, is connected with the way of metal ions binding to aluminosilicate framework of the zeolite, i.e. the proportion of ion exchange and chemisorption in the process. Results of spectroscopic studies have been compared with those obtained by atomic absorption spectrometry (AAS).

Catalytic cracking of 1-butene to propene and ethene on MCM-22 zeolite

Catalytic cracking of butene to propene and ethene was investigated over HMCM-22 zeolite. The performance of HMCM-22 zeolite was markedly influenced by time-on-stream (TOS) and reaction conditions. A rapid deactivation during the first 1 h reaction, followed by a quasi-plateau in activity, was observed in the process along with significant changes in product distributions, which can be attributed to the fast coking process occurring in the large supercages of MCM-22. Properly selected reaction conditions can suppress the secondary reactions and enhance the production of propene and ethene. According to the product distribution under different butene conversion, we propose a simple reaction pathway for forming the propene, ethene and by-products from butene cracking. HMCM-22 exhibited similar product distribution with the mostly used high silica ZSM-5 zeolite under the same conversion levels. High selectivities of propene and ethene were obtained, indicating that the 10-member ring of MCM-22 zeolite played the dominant role after 1 h of TOS. However, MCM-22 exhibited lower activity and stability than that on high silica ZSM-5 zeolite with longer time-on-stream.

Selectivity of a Model Zeolite Ring over Hydrocarbons with Different Symmetry, Travelling with Different Orientations and Speeds

We explore the selectivity of a model zeolite ring over representative hydrocarbons of crude oil. The model ring consists of 7 silicon tetrahedral units and one chemically active aluminum site through which hydrocarbons with symmetries varying from almost spherically symmetric to linear chains (1D), planar (2D), and pyramidal (3D) structures diffuse. The selectivity is further investigated when the hydrocarbons travel with different orientations and speeds. The semiclassical Born-Oppenheimer molecular dynamics approximation is used to characterize the chemical dynamics, as well as to determine the energetics and reaction products. The simulations reveal noticeable differences in energy profiles and charge populations. Our results are important to understand aspects of mass transport and some of the factors that control the catalytic activity in zeolites.

Can Cu+‐Exchanged Zeolites Store Molecular Hydrogen? An ab initio Periodic Study Compared with Low‐Temperature FTIR.

Cu + -exchanged Si/Al 11:1 chabazite has been studied ab initio using the periodic CRYSTAL03 computer code with Hartree-Fock and the hybrid B3LYP Hamiltonians to characterize the structures and energetics of the Cu + ion sitting preference and its interaction with H 2 . Two sites (I and IV) have been found to be stable for Cu + ion: site I, the most stable one, envisaging coordination in a six-membered zeolite ring and site IV in which the Cu + ion sits in the largest eight-membered ring. Interaction of H 2 gives adsorption energies at B3LYP of -13 and -56 kJ/mol for sites I and IV, respectively. The B3LYP bathochromic harmonic H 2 frequency shifts are 847 and 957 cm - 1 for adsorption at sites I and IV, respectively, in good agreement with the shifts measured (1030 and 1081 cm - 1 ) in the Cu-ZSM-5 system in which Cu + ion is, respectively, three and bi-coordinated by the oxygen atoms of the zeolite framework. Analysis of the components of the adsorption energy, carried out within the cluster approach, revealed that charge transfer from the Cu(3d π ) orbital through the antibonding H 2 (σ u ) and orbital polarization play a significant role in the H 2 adsorption energy, and cause the large bathochromic H 2 frequency shift.

Can Cu+-Exchanged Zeolites Store Molecular Hydrogen? An Ab-Initio Periodic Study Compared with Low-Temperature FTIR

Cu+-exchanged Si/Al 11:1 chabazite has been studied ab initio using the periodic CRYSTAL03 computer code with Hartree−Fock and the hybrid B3LYP Hamiltonians to characterize the structures and energetics of the Cu+ ion sitting preference and its interaction with H2. Two sites (I and IV) have been found to be stable for Cu+ ion: site I, the most stable one, envisaging coordination in a six-membered zeolite ring and site IV in which the Cu+ ion sits in the largest eight-membered ring. Interaction of H2 gives adsorption energies at B3LYP of −13 and −56 kJ/mol for sites I and IV, respectively. The B3LYP bathochromic harmonic H2 frequency shifts are 847 and 957 cm-1 for adsorption at sites I and IV, respectively, in good agreement with the shifts measured (1030 and 1081 cm-1) in the Cu−ZSM-5 system in which Cu+ ion is, respectively, three and bi-coordinated by the oxygen atoms of the zeolite framework. Analysis of the components of the adsorption energy, carried out within the cluster approach, revealed that charge transfer from the Cu(3dπ) orbital through the antibonding H2(σu) and orbital polarization play a significant role in the H2 adsorption energy, and cause the large bathochromic H2 frequency shift.

Comparative energies of Zn(II) cation localization as a function of the distance between two forming cation position aluminium ions in high-silica zeolites

Periodical calculations of Zn(II) metal cation stabilization in cationic positions with distantly placed aluminium ions has been performed for high-silica ferrierite. It was found that decrease of the stabilization energy at large distances between Al ions (more than 10 Å) is about of 2 eV in comparison with nearest possible position of two Al ions in the zeolite lattice and weekly depended on following increase of the Al-Al distance. Main changes in stabilization energy occured within a 3-Å interval of these distances. Only for the localizations of both Al ions in one zeolite ring zinc cationic form is more stable than hydrogen form.

Study of the Activation of C−H and H−H Chemical Bonds by the [ZnOZn]2+ Oxycation: Influence of the Zeolite Framework Geometry

The reactivity of the [ZnOZn]2+ oxycation, located inside the zeolite micropores, has been studied by the periodical density functional method. Two different types of rings, six- and eight-membered, have been used as hosts for this cation. In addition, within each of these ring structures the arrangement of the two aluminum atoms has been explored, generating different configurations. Two different probe molecules have been used in this investigation: methane and molecular hydrogen. Both molecules prefer to chemisorb on this active site. Their dissociation reactions are strongly influenced by the surrounding zeolite environment and by the Al distribution within the zeolite ring framework. The [ZnOZn]2+ cation is able to activate both C−H and H−H bonds, particularly the latter one. These results are in line with previous calculations and recent experimental results. They also confirm that this active site is present in high-silica zeolites. However, this species seems unlikely to be the catalytic site for the alkane dehydrogenation.

Density-functional theory of potassium atoms in zeolite

We have constructed a model of a zeolite which we have optimized using density-functional theory (DFT) in local-density approximation. We have calculated the energy of the zeolite model with and without one potassium atom. From these calculations the adsorption energy of the K atom is found to be −1.057 eV. We find that the bonds in the zeolite ring show an expansion upon addition of a K atom. We have also calculated the spin-dependent bond energy for two K atoms in a zeolite, from which we predict that ferromagnetic order occurs.

Chemisorption of sodium on a six-membered ring,

The chemisorptions of a sodium atom and a cluster on a six-membered ring of zeolite are studied by using the extended Hückel method and cluster model. The six-membered ring of zeolite consists of , which is one of the basic structure units of zeolite. The site preference and the density of states are determined for both cases.

In-situ IR monitoring of the acidic and structural variations of mordenite caused by pressure and temperature

The effects of the surroundings (pressure and temperature) on the acidity and structure of mordenite zeolite have been investigated by monitoring the acidic bridging hydroxy groups, Si—OH—AI, by in-situ IR. Effects due to pressure have not been observed. However, severe effects due to temperature have been found. The frequencies of acidic bridging OH groups decreased and the acidity increased with increasing temperature. The weaker Bronsted acid sites could be turned into stronger Bronsted acid sites as the temperature was raised. In a series of mordenites, in the lower temperature region (300–473 K) the order of acidity depended on the conditions of treatment. In the higher temperature region (473–674 K), with increasing temperature the acidity of the sample decreased, reaching a minimum (Si : AI = 11.18) before increasing again (Si : AI > 11.18).The effect of temperature on the structure of mordenite has shown that the more sensitive the IR frequencies of the acid OH groups to temperature, the greater the degree of distortion of the zeolite rings and, correspondingly, the less thermostable the structure of the dealuminated mordenite. Moreover, SiOHAI fragments tend to locate in the positions at which the ∠ TOT angles are minimised.

Adsorption properties of low-silica zeolite ZK-5

Water vapor, methanol, and argon adsorption isotherms, as well as the heat of adsorption of ammonia, methanol, and carbon dioxide have been determined on zeolite types A, ZK-4, and ZK-5. The results are explained by the distribution and the bond lengths of cations in the zeolite crystal lattice. It was shown that the exchange of 10–15% of the Na+ ions with Li+ in zeolite ZK-5 leads to a deformation of the 8-membered ring impeding the diffusion of adsorbed molecules. Cations contained in the 8-membered ZK-5 zeolite ring are bound more strongly to the framework, than in zeolite NaA, resulting in their smaller adsorption capacity.