ZSM-5 Structure Research Articles

Density functional study for estimating Brønsted acid site strength in isomorphously substituted ZSM-5

We use local density functional theory to study the Bransted acid strength of a series of clusters that model a catalytic acid site within zeolite ZSM-5. We consider models of the formula (A) 3Si-OH-T(A) 3 where A = OH and OSi(OH) 3; T = Si, Al, B, Ga or Fe. We have studied the effect of replacing Si with trivalent metal cations on the structure and acid strength of the zeolite ZSM-5. Our results also show the effect of cluster size on the calculated properties. It is suggested that a more accurate descriptor of the decreased electron density around the hydroxyl hydrogen would be the electrostatic potential calculated on the total electron density surface. We studied the molecular electrostatic potential maps for the cluster models to test this fact. It is observed that the calculation results predict that acid strength increases in the order B-ZSM-5<Fe-ZSM-5<Ga-ZSM-5<Al-ZSM-5, which shows a good agreement with experimental results. The results show a prospective trend to predict properties of metal substituted zeolites which a special emphasis on Brønsted acid strength of zeolites.

Coordination of Cu+ Ions to Zeolite Frameworks Strongly Enhances Their Ability To Bind NO2. An ab Initio Density Functional Study

Comparison is made of the interaction of NO2 with Cu+ ions in the gas phase and inside zeolites using density functional theory (B3LYP functional). The zeolite is represented by a tritetrahedra model embedded in the periodic structure of zeolite ZSM-5 and by a free space cluster model. Both models yield virtually the same results. Cu+ is coordinated to two oxygen atoms of the zeolite framework only. For the complexes with NO2, several minima and transition structures on the potential energy surfaces are localized. The naked Cu+ ion preferentially binds NO2 in the η1-O trans mode, while in zeolites the Cu+ site binds NO2 in a 2η-O,O coordination. For the 2η-O,O structure the binding is three to four times stronger in the zeolite (43 kcal/mol) than in the gas phase which is due to a three-body zeolite frameworkCu+ ion−NO2 interaction. d10−s1d9 promotion leads to a more favorable orbital interaction between Cu+ and NO2 in the 2A‘‘ state and, due to reduced repulsion, to a stronger electrostatic interaction b...

Kinetic and Theoretical Study of the Effect of Molecular Sieve Structure on the Selectivity to Propylbenzenes in Alkylation of Benzene with Isopropyl Alcohol

The alkylation of benzene with isopropyl alcohol was investigated over molecular sieves of different structural types and acidity by kinetic measurements and ab initio quantum chemical calculations. The formation of n-propylbenzene occurring via bimolecular transalkylation between isopropylbenzene and benzene is enhanced by the appropriate geometry of the reaction space and the acidity of molecular sieves. n-Propylbenzene was formed over medium and large pore molecular sieves with such inner channel architecture which enables the approach of isopropylbenzene and benzene to form the reaction complex. Mesoporous sieves of MCM-41 structure provided only isopropylbenzene with some amount of diisopropylbenzenes. For the fully relaxed complex consisting of protonated isopropylbenzene and benzene, the computations verified the transfer of propyl group from one benzene molecule to another forming again isopropylbenzene. When the geometrical conditions modelling the channel intersections of ZSM-5 and ZSM-11 structures were considered n-propylbenzene was formed. The anti-Markovnikov type of proton transfer from the arenium cation to the propene part of the complex is promoted by the steric constraints present in the channel intersections of ZSM-5 and ZSM-11 zeolites.

Adsorption and catalytic destruction of trichloroethylene in hydrophobic zeolites

Several chromium exchanged ZSM-5 zeolites of varyingSiO 2/Al 2O 3 ratio were prepared and investigated ambient (23°C) adsorption and subsequent oxidative destruction (250–400°C) of gaseous trichloroethylene (TCE, Cl 2C=CHCl) in a humid air stream. With an increase in theSiO 2/Al 2O 3 ratio from 30 to 120, the TCE saturation capacity of these dual-function sorbent/catalyst (S/C) media was found to increase from 6.0 to 10.1 wt% in a humid air stream. This phenomenon was attributed to an increase in hydrophobicity coupled with reduced steric hindrance and site competition for the adsorption of TCE molecules in the competitive adsorption of TCE and water. Ambient TCE adsorption experiments carried out in dry air showed the same trend, which was attributed to increasing organophilicity of the S/C media with an increase in the SiO 2/Al 2O 3 ratio. In order to gain knowledge of physisorption sites for TCE molecules in the ZSM-5 structure, temperature-programmed desorption over a temperature range of 30–300°C and in-situ FT-IR studies at ambient conditions were also carried out. These studies revealed that in all zeolites (except for Cr-ZSM-5 withSiO 2/Al 2O 3 ratio of 120) TCE interacted with terminal silanol (SiOH) and AlOH groups. At temperatures ≥ 300°C (with the exception of Cr-ZSM-5 withSiO 2/Al 2O 3 ratio of 120), allS/C media showed >95% TCE destruction efficiency. Based on its high adsorption capacity and high activity for oxidative destruction of TCE, it is concluded that the Cr-ZSM-5S/C medium with aSiO 2/Al 2O 3 ratio of 80 gives preferred performance both as a sorbent and a catalyst.

On the nature of framework Brønsted and Lewis acid sites in ZSM-5

The acidity of aluminum in a nominally 70:1 SiO 2/Al 2O 3, ratio ZSM-5 prepared by conventional means was characterized by TPAD, 27Al and 29Si n.m.r., XRD, and FTi.r. Discrepancies observed between the amount of tetrahedral “acidic” aluminum measured by TPAD and i.r. versus Al n.m.r. are due to the presence of Lewis aluminum. Furthermore, it is suggested that this Lewis aluminum may be located in the zeolite framework. These sites are not thought to be inherent to the ZSM-5 structure but are probably generated during calcination, resulting in partial hydrolysis of AlO bonds. A small percentage of octahedral aluminum was observed in hydrated HZSM-5, but was quantitatively converted to tetrahedral aluminum by treatment with ammonium nitrate solution at pH 8.

Water-assisted ionic conductance of zeolites with ZSM-5 structure

The electrical conductivity ( σ) of compressed powders of an isostructural series of zeolites with MFI topology 1 was measured using AC impedance spectroscopy. Silicalite-1 and ZSM-5 with different Si/Al ratios containing varying numbers of Na + and/or H + were studied at 100% relative humidity and immersed in liquid water. In the latter case, σ is 1–2 orders of magnitude larger than that in saturated water vapour. The value of σ for silicalite immersed in water was found to be higher than that of water before and after the test. It was assumed that this is due to enhanced water dissociation on the silicalite surface. Both in water and at 100% RH the dependence of σ on cation content follows a curve with a maximum. A decrease in conductivity at a high concentration of charge carriers is ascribed to an increase in the repulsive interaction between ions in the conductive water layer on the zeolite channel walls. 1 According to the nomenclature of the Structure Commission of the International Zeolite Association.

Aromatization ofn-butane over Ni-ZSM-5 and Cu-ZSM-5 zeolite catalysts prepared by using Ni and Cu impregnated silica, fiber

The reaction ofn-butane aromatization was carried out over Ni-ZSM-5 and Cu-ZSM-5 zeolite catalysts prepared by using Ni and Cu impregnated silica fiber during the process of ZSM-5 zeolite synthesis. The catalysts prepared were characterized by X-ray powder diffraction, nitrogen adsorption and X-ray fluorescency. The acidic properties of the catalysts were investigated by temperature-programmed desorption of ammonia using a mass spectrometer equipped with a QTMD detector. The effect of catalyst pretreatment, reaction temperature, and time on stream on the reaction ofn-butane to aromatic hydrocarbons were investigated. The modification of ZSM-5 by Ni and Cu increased the selectivity to aromatic hydrocarbons. The state of Ni and Cu and their stabilization in the ZSM-5 structure was highly influenced by the mode of catalyst pretreatments.

Desulfurization of hydrocarbon streams

A process for desulfurizing a hydrocarbon stream which includes at least 100 ppmw sulfur in the form of organic sulfur compounds, and C4 -hydrocarbons. The hydrocarbon stream is contacted in the absence of added hydrogen with a fluidized bed of an acidic catalyst having a structure of ZSM-5, ZSM-11, ZSM-22, ZSM-23, ZSM-35, ZSM-48, MCM-22, MCM-36, MCM-49, zeolite Y, zeolite beta or mixtures thereof to convert the organic sulfur compounds to hydrogen sulfide. The catalyst contacts the hydrocarbon stream at a pressure of from 0.0 psig to about 400 psig, a temperature of from about 400° F. to about 900° F., and a weight hourly space velocity of from about 0.1 hr.-1 to about 10.0 hr.-1. Thereafter, the hydrogen sulfide is removed from the hydrocarbon stream.

The structure and electronic characteristics of metallosilicates with ZSM-5 structure

We report here the result of a computer- assisted study of metallosilicates by applying molecular dynamics (MD) and quantum chemical (QC) methods. MD calculations are used to study the local relaxation in the T12 site of the ZSM-5 structure, when Si is substituted by different metals such as Ti4+, Al3+, Ga3+, and Fe3+. QC calculations by density functional theory have been performed on the cluster models generated from the structure obtained by MD calculations. The calculation indicates that the net charge on a MO4 (where M = Ti, Al, Ga, and Fe) group and the molecular electrostatic potential values are good parameters to assess the acidic properties of metallosilicates, as shown by their correlations to the reported experimental acidity.

129Xe NMR study of ZSM-5 type zeolites Effect of cationic sites

A comprehensive 129Xe NMR spectroscopy study on H-ZSM-5 zeolites having different aluminum contents and on cation-exchanged ZSM-5 zeolites is reported. The parent H-ZSM-5 zeolites were ion-exchanged with Group I–III metal ions ( K, Ca, Sr, Ba, Al, La) to varying degrees. The chemical shift of adsorbed 129Xe is seen to be a function of the pentasil structure of ZSM-5, of the number of free Brønsted acid sites and of the number of metal cations in the framework. Differences in the chemical shift of 129Xe are seen between cations due to their different polarizing forces against xenon. The amount of cations has also an effect on the δ xe-xe term in Fraissard's equation that may be caused by changes in the diffusional characteristics of Xe atoms in the ZSM-5 framework.

Transmission electron microscopy study of zeolite TPA/ZSM-5 using a slow-scan CCD camera

Studies of high-resolution transmission electron microscopy and selected area electron diffraction are made to investigate a structure of zeolite ZSM-5 containing organic ions TPA + (tetrapropylammonium). A slow-scan CCD camera attached to a 400 kV electron microscope is used for the observation in order to make a quantitative analysis of image and diffraction patterns, and to minimize the irradiation damage of the zeolite and confined organic ions. A distribution of dark contrast attributed to the presence of TPA + ions is indicated in the observed high-resolution image, as well as in the image processed for contrast enhancement. A Fourier synthesis is made by using observed diffraction intensities along with phases calculated from a structure model. The obtained Fourier map clearly showed an existence of TPA + ions, of which concentration is approximately half of that previously reported by an X-ray diffraction study.

The status of chromium in aluminum-free chromosilicate with the ZSM-5 structure

It is shown that the hydrothermal synthesis of aluminum-free chromosilicate results in the formation of crystalline zeolite with the ZSM-5 structure and some Cr 3+ ions substitutes isomorphically for Si 4+ ions in the lattice of as-made samples. After oxidative calcination most chromium ions aggregate in α-Cr 2O 3 microcrystals on the outer surface of the zeolite. However, some chromium remains stabilized strongly in the zeolitic structure as isolated low coordinated Cr(V) ions in two discrete coordinative states. It may be supposed that Cr(V) ions chemically bonded with zeolite lattice also have out of lattice ligands and are capable of interacting specifically with different molecules.

Reduction behavior and catalytic properties of cobalt containing ZSM-5 zeolites

The reduction behaviors and catalytic properties of cobalt containing ZSM-5 zeolites prepared by various methods were compared. Temperature-programmed reduction (TPR), X-ray diffraction and IR spectroscopy were the main techniques used for the study of reduction behavior. The existence of framework cobalt and small amounts of cobalt oxide in Co-ZSM-5 samples, which were prepared by incorporation of different amounts of cobalt into the zeolite synthesis gels, and cobalt silicate-like species in the Co/ZSM-5 sample, which was prepared by precipitating impregnation of cobalt oxide on ZSM-5, were therefore established. TPR studies indicated that both framework cobalt and cobalt silicate-like species required temperatures higher than 973 K to be reduced. The structure of Co-ZSM-5 was partially destroyed after reduction at temperatures higher than 1023 K, due to the removal of framework cobalt from the ZSM-5 structure. On the other hand, the reduction of cobalt silicate-like species in Co/ZSM-5 at 1023 K did not affect the zeolitic structure. As the cobalt loading in Co-ZSM-5 was increased, increasing amounts of extra-framework cobalt oxide were formed and well dispersed in the porous structure of zeolite. The catalytic activities of these cobalt-containing zeolites in carbon monoxide hydrogenation were found to be directly proportional to the amount of cobalt metal present. The latter was obtained from the reduction of cobalt oxide under pretreatment conditions. The acidity and shape selectivity of ZSM-5 accounted for the product selectivity.

Atomic processes in the thermal destruction of zeolites as investigated by molecular dynamics and computer graphics

The destruction processes of various zeolites, such as a silicalite with the ZSM-5 structure (SiO 2-MFI), Cu-ZSM-5, and a silicalite with the faujasite structure (SiO 2-FAU), with increasing temperature were investigated by using molecular dynamics and computer graphics methods. The mobility of atoms in SiO 2-MFI was increased and the framework became unstable, as the temperature was increased. Finally the framework was destroyed to an amorphous phase. Increasing the Al content led to a decrease in the heat-resistance in Cu-ZSM-5. The change in the Si-O, Al-O, and Cu-Al distances as well as the change in the pairs of Si-O bonds are discussed to clarify the mechanism of Cu-ZSM-5 destruction. The destruction process of SiO 2-FAU was also investigated in order to understand the effect of framework structure, and it was suggested that the presence of the 4-membered ring provides one of the reasons for the inferior heat-resistance of SiO 2-FAU to that of SiO 2-MFI.

Electronic and structural properties of aluminum in the ZSM-5 framework

The relative importance of the geometry and size of clusters involved in the calculation of the electronic structure of ZSM-5 using MNDO (modified neglect of differential overlap) quantum chemical cluster calculations is reported. Calculations were carried out on Al-containing pentamer cluster models with a tetrahedral symmetry, and their geometrical parameters were systematically varied. Calculations were also performed for cluster models of varying size whose geometry was derived from the crystal structure reports of ZSM-5. The electronic properties such as density of states, energy of frontier molecular orbitals and contribution from different atomic orbitals could be derived from the results of even small cluster models, while pentamer clusters were needed to study the energetically favorable substitution sites of aluminum.

Crystallization of “inorganic” ZSM-5 in the system K2O-Al2O3-SiO2-H2O

Crystallization of high-silica zeolites from the composition y K2O : x Al2O3 : 60 SiO2 : 3000 H2O (5 <y < 20, 0 <x < 2) in stirred reactions at 150°C has been investigated. KZSM-5 and KNU-10 were the only high-silica zeolites obtained; the former could only be obtained over a very narrow composition range. One composition (y = 15, x = 0.667) was found to be a sensitive detector for traces of seed material and organic species. Thus, it was discovered that this composition, termed the “Scavenging Gel,” could be used to establish the cleanliness of a reaction vessel. In addition, it can be used to distinguish between true ZSM-5 structure directors and simple void fillers as well as give a measure of their potency. Another useful feature of this composition is that very small amounts of organic reagents can be readily screened for their ability to direct the crystallization to ZSM-5; such a test could save both time and money when examining new templates.

Pitfalls in the powder diffraction analysis of zeolites ZSM-5 and ZSM-8

Crystal structure analyses by the Rietveld method have shown that the framework structures of zeolites ZSM-5 and ZSM-8 are essentially identical. Therefore, it is difficult to distinguish the two phases especially when the template-free H forms are studied. In addition, some inconsistencies in publications on the two zeolites aggravate the correct interpretation of the powder diagrams. A powder pattern published for ZSM-8 which indicates significant differences between the lattice constants of ZSM-8 and ZSM-5 is shown to be incorrectly indexed. Correct reindexing gives lattice constants for ZSM-8 matching average ZSM-5 values. Peak splitting of a ZSM-8 reflection at 2θ≈23° (CuKα) has been used frequently to distinguish ZSM-8 from ZSM-5. However, it is also a common feature of ZSM-5 diagrams when the difference between a and b lattice constants is big enough to separate hkl and khl reflections within the instrumental resolution. Our data on two ZSM-8 samples indicate that cell dimensions of ZSM-8 do not deviate from average ZSM-5 values. It is suspected that effects in the decomposition of crystals upon calcination, and/or morphology and shape of twin individuals, and/or stacking faults account for different sorption properties of the two zeolites rather than differences in their average crystal structures.

Performance of a protonated Co-contained silicate catalyst for NO reduction in equivalent conditions to diesel engine exhaust

Effects of co-existence of H2O, CO2, and SO2 in the reaction gas on the catalytic performance of cobalt containing silicate having ZSM-5 (MFI) structure (Co-silicate) were studied. Water vapor retarded only the hydrocarbon conversion to CO2 but no effect was observed on NO conversion. Addition of CO2 or SO2 did not affect the reaction. The stability of H-Co-silicate against H2O, CO2, and SO2 was ascribed to the state of metallic active species which were stabilized by incorporation into the high siliceous MFI structure.

Platinum promoting effects in Pt/Ga zeolite Catalysts of lower alkane aromatization. II. The catalytic properties in n-butane aromatization, formation of the Catalytically active sites and reaction mechanism

Catalytic properties of a series of Ga- and Pt-Ga-containing zeolites of ZSM-5 structure in n-butane aromatization have been studied in wide range of reaction conditions. The profound Pt promotion has been well established on the basis of product selectivities analysis at different temperatures and contact times but equal conversions. The Pt and Ga synergy was also found to increase significantly selectivity to aromatics at the account of rapid dehydrogenation and suppressing such side reactions as hydrogenolysis and hydrogenation (Pt), cracking and isomerization (Ga). Both far distance effects (hydrogen spillover) and intimate contact between Pt particles and reduced Ga(I) ions seem to be responsible for the enhancement of aromatization activity and stability of Pt-Ga zeolite Catalysts. Active sites involve Pt, Ga 1+ and protonic zeolite centers which formation and structure were considered in detail. Finally, aromatization process scheme was proposed to find out most important steps controlled by Pt and Ga active sites.

Alkylation of toluene over double structure ZSM-5 type catalysts covered with a silicalite shell

Zeolite catalysts, having layered structure of ZSM-5 or borosilicate core covered with a silicalite shell, were prepared by hydrothermal crystallization of silicalite in the presence of ZSM-5 or borosilicate seeds. Characteristics of the prepared catalysts were examined by X-ray diffraction, scanning electron microscopy and temperature-programmed desorption. Comparison of these double-structured catalysts to ZSM-5 type catalysts, showed that they exhibited considerably higher para-selectivity in toluene alkylation reactions, but had slightly lower activity. The former showed higher para-selectivity at the same conversion level. This result suggests that para-selectivity could be enhanced by selectively inactivating the external acid sites without changing the pore mouth size.