Molecular simulation of adsorption and diffusion of chlorinated alkenes in ZSM-5 zeolites

In this work, adsorption and diffusion of trichloroethylene (TCE) and tetrachloroethylene (PCE) in ZSM-5-type zeolites were studied using molecular simulation methods. Grand canonical Monte Carlo technique was to calculate adsorption isotherms and heats of vaporization of TCE and PCE in zeolite. The results demonstrated that the Pnma-P2(1)2(1)2(1) symmetry transition of the zeolite framework has no significant effect on the TCE adsorption capacity of the silicalite, but it causes an increase of the PCE adsorption capacity. Simulations using a silicalite framework with Pnma symmetry showed that the adsorption capacity of the silicalite was limited to five molecules per unit cell. However, when a framework with P2(1)2(1)2(1) symmetry was used in the simulations, the capacity reached to eight molecules per unit cell, which is the actual adsorption capacity. To calculate intracrystalline diffusion coefficients of these compounds, molecular dynamics simulations were performed at different temperatures and loadings. The results show that the zeolite symmetry has a significant impact on diffusion coefficients of the sorbate molecules.     

Heats of adsorption of CO2 on high-silicon zeolites ZSM-5 and silicalite

The heat of adsorption of C0₂on NaZSM-5 at zero occupancy is 50.0 kJ/mole. The differential heats have two linearly descending segments, corresponding to the formation of two types of adsorption complexes with one or two C0₂ molecules, on the average. The heat of adsorption on silicalite coincides with the heat of adsorption of CO₂ on the noncationic segment of the NaZSM-5 zeolite structure (28–29 kJ/mole). The adsorbate-adsorbate interaction forces are not evident on the zeolites up to 1.5 mmole/g occupancy. The isotherms for the adsorption of C0₂ on zeolite NaZSM-5 and silicalite at 303 K in the occupancy region of 0–1.5 and 0–0.5 mmole/g are completely described by VMOT equations.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2636–2638, November, 1989.     

Aromatization of propene on ZSM-5 zeolites

Cu-ZSM-5, CuZn-ZSM-5 and H-ZSM-5 have been studied for propene aromatization at comparable conditions. It was established that all samples are active for aromatization (yields 50%) and demonstrate high selectivity for benzene, toluene and xylenes formation ( 90%). The role of acid centers in all stage of the aromatization is discussed.

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Cu-ZSM-5, CuZn-ZSM-5 H-ZSM-5 . ( 50%) , ( 90%). .

     

Developments and structures of mesopores in alkaline-treated ZSM-5 zeolites

ZSM-5 zeolites with SiO₂/Al₂O₃ molar ratio of 24 were treated in 0.05 M aqueous sodium hydroxide solution at 325 K in different periods. The samples were characterized by means of nitrogen adsorption at 77 K, field emission scanning electron microscopy, X-ray diffractometry, and Fourier transform infrared spectroscopy. Analysis of the experimental results showed that the alkaline treatment periods have influence on the developments and structures of mesopores in the alkaline-treated ZSM-5 zeolites. Alkaline treatment initially develops mesopores mainly from the boundary portion of MFI zeolites to the bulk, while prolonged treatment destroys the mesopores, and an optimum mesoporosity is obtained by the treatment for 1.5 h. On the other hand, crystallinities and short-range order in alkaline treated zeolites have remained virtually unchanged according to the examination from X-ray diffractometry and Fourier transform infrared spectroscopy.     

Alkylation of aromatics on B-containing ZSM-5 zeolites

The catalytic activity of ZSM-5 zeolites, modified with boric acid, has been studied in benzene and toluene alkylation by ethylene. The concentration of acid sites of different nature in these zeolites was characterized by IR spectroscopy of OH-groups and adsorbed CO. A decrease of the concentrations of both strong protic and non-protic sites was shown to occur upon modification. Secondary acid sites are assumed to appear in the zeolites, and the strength of these sites is supposed to follow the sequence: [[SiOAl]H][POH]>[BOH][MgOH]. The activity and regio selectivity of B-containing zeolites are considered in relation to the nature and concentration of the acid sites.     

Room-temperature interaction of n-hexane with ZSM-5 zeolites

In this work, room temperature interaction of n-hexane with HZSM-5 (Si/Al=20) and ion-exchanged samples containing one (CuZSM-5, FeZSM-5 and MnZSM-5) or two transition-metal cations (Fe,CuZSM-5; Cu,MnZSM-5 and Fe,MnZSM-5) was studied by microcalorimetry and TPD methods. Both differential heats and the amounts of n-hexane adsorbed per one unit cell were quantitatively determined. Higher heats of adsorption and higher amounts of adsorbed gas were found for ion-exchanged samples than for HZSM-5. The experiments of n-hexane adsorption on hydrated samples were also performed. The amounts of n-hexane adsorbed on hydrated ZSM-5 were lower in comparison with dehydrated samples, while the energies of interaction were similar.     

Alkylation of aromatics on p-containing ZSM-5 zeolites

The catalytic activity of ZSM-5 zeolites, modified with phosphorus compounds, has been studied in benzene and toluene alkylation by ethylene. Concentration of acid sites of different nature in the samples was characterized by IR spectroscopy of OH-groups and adsorbed CO. Modification was shown to decrease the concentrations of both strong protic and non-aprotic sites. New acid sites are assumed to appear in P-containing zeolites. The strength of acid sites in the zeolites is supposed to follow the sequence:[[SiOAl]H][POH]> >[MgOH. Activity and regioselectivity of P-containing zeolites are considered to depend on the nature and concentration of the acid sites.     

On the nature of acid centers in Mg-containing ZSM-5 zeolites

The catalytic activity of ZSM-5 zeolites containing Mg cations in various concentrations has been studied in the alkylation of toluene by ethylene. The concentration of acid centers of different nature in these zeolites was characterized by IR spectroscopy of OH-groups and adsorbed CO. The state of Mg cations in the samples was studied by¹⁵N NMR of the adsorbed N₂O. The increase of Mg content was shown to decrease both the concentration of non-protic acid sites of medium strength and the activity of samples, which is related to the increase of association degree of Mg cations in the zeolite.     

Modification of the acidic properties by introduction of zinc into ZSM-5 zeolites

Aromatization ofn-hexane and disproportionation of toluene over H-ZSM-5 and Zn-ZSM-5 prepared by ion-exchange with aqueous solution has been studied. Introduction of zinc into ZSM-5 does not significantly affect the total acidity determined by TPDA, but causes redistribution in the strength and type of acid sites. From the results it was concluded that the first step in the aromatization ofn-hexane on Zn-ZSM-5 is probably the formation of carbonium ions by hydride abstraction with acid sites in zeolite and that Zn species have a decisive role in oligomerization of cracking products and in dehydrogenation of cyclic intermediates.     

Influence of template on crystallization of ZSM-5 zeolites

Pentasil zeolites of ZSM-5 type are synthesised hydrothermally using triethyl-n-proplyammonium bromide (TEPA-Br) and triethyl-n-butylammonium bromide (TEBA-Br). The crystallization kinetics, followed by XRD, SEM and thermal analysis, clearly demonstrate the influence of size and molecular weight of the templating quaternary ammonium cation (QAC) species on the rates of nucleation and crystallization. The values of the apparent activation energies for nucleation and crystal growth indicate that both nucleation and crystal growth are faster when TEPA-Br rather than TEBA-Br is used as a template. The quantitative identification of intergrown phases characterizes both the phases to be ZSM-5 zeolite. Thermoanalytical curves for both these zeolites in as-synthesised forms exhibit two-step oxidative decomposition of the occluded organic species. This suggests that the quaternary ammonium cation may be located at two energetically different sites within the zeolite channels. The equilibrium sorption capacity, however, is found to increase in the order of size and molecular weight of the templating species in both the zeolites. The nature of acid site distribution, obtained from the temperature programmed desorption of ammonia is found to be independent of the templating species used during the synthesis.     

Abatement of hydrocarbons by acid ZSM-5 and BETA zeolites under cold-start conditions

Simulated cold-start tests have been carried out to evaluate the performance of H-ZSM-5 and H-BETA zeolites as hydrocarbon traps under simulated gasoline car exhaust gases, paying special attention to the effect of water on their behaviour. It is concluded that the hydrothermal treatment of the zeolites in the acidic form contributes to the better performance of these materials as hydrocarbon traps since the stabilization of the zeolites takes place. Moreover, the decrease of the surface acidity of the zeolites results in an increase of the Si/Al ratio, which contributes to the decrease of the water affinity for adsorption sites. Thus, the competition with hydrocarbon molecules in the exhaust for the adsorption sites is reduced which increases their trap efficiency. The stabilized H-ZSM-5 is the zeolite that showed the best performance with a propene offset temperature of 240 °C, which should be high enough for the three-way catalyst to carry out its role as catalytic converter.     

The kinetics of hydrocarbon conversion on ZSM-5 zeolites

The complete oxidation of hydrocarbons with various structures (methane, n and wo-pentanes, cyclohexane, and benzene) on copper-modified ZSM-5 zeolite and the aromatization of propane on H-ZSM-5, are studied to analyze diffusion effects on the reaction kinetics. Comparison of the kinetic parameters of complete oxidation shows that the process rate and the rate law strongly depend on the reactant structure. Apparently, n-pentane oxidation is controlled by kinetics; the kinetics for other hydrocarbons depends on diffusion inside zeolite channels to different degrees. In the case of propane aromatization, its dehydrogenation and cracking, as well as the formation of aromatics from olefins, occurs on different active sites. Propane conversion is supposed to occur inside the zeolite channels, and the aromatization of olefins occurs on the outer surface of the zeolite crystal Deceased.     

Adsorption, diffusion and catalysis of mesostructured zeolite HZSM-5

Adsorption and diffusion properties of n-octane in meso-structured HZSM-5 zeolites were studied by high precision intelligent gravimetric analysis (IGA) and ZLC technology between 293?K and 393?K. As expected, great increase in adsorption capacity and diffusion efficient of n-octane in the mesostructured HZSM-5 zeolites was observed compared with conventional HZSM-5. At the same time, the adsorption activation energy of n-octane in the mesostructured HZSM-5 zeolites was significantly decreased. The adsorption heats with low n-octane loading showed a clear decline with increase of mesoporosity in the zeolite samples. These results clearly indicate that introduction of mesopores into the zeolites offered a short diffusion path and high diffusion rate for reactants and products, which resulted in a high yield of fuel oil and an enhanced resistance against the catalyst deactivation in the reaction of methanol to gasoline.     

Electrochemical study of the thionine dye incorporated into ZSM-5 and HZSM-5 zeolites

The electrochemical properties of thionine dye adsorbed into ZSM-5 and HZSM-5 zeolites (TH/ZSM-5, TH/HZSM-5) are studied in 0.5 M KCl solution. The dye is strongly retained and not easily leached from the zeolites matrix. The samples are incorporated into the carbon paste electrode (TH/ZSM-5/P, TH/HZSM-5/P) for cyclic voltammetric measurements. The redox reactions of thionine incorporated into ZSM-5 zeolite contain a quasi-reversible, two-electron one proton in the pH range 1 to 10, but thionine-loaded HZSM-5 zeolite undergoes a quasi-reversible two-electron two-protons redox reaction under acidic conditions and a one proton two-electron redox reaction takes place under basic conditions. The separation of the anodic and cathodic potentials (E _p) is high in thionine-loaded zeolites (>100) with respect to the solution of thionine (E _p = 34 for ZSM-5/P and 36 mV for HZSM-5/P), indicating that there are strong interaction between thionine molecules and the zeolites. The midpoint potentials (E _m) for TH/ZSM-5/P and TH/HZSM-5/P are −0.203 and −0.381 V, respectively. However, the midpoint potentials for the solution of thionine for the electrode system of ZSM-5/P and HZSM-5/P are −0.335 and −0.407 V, respectively. Thus, thionine dye molecules incorporated into the zeolites can be reduced more easily with respect to solution of thionine. In various electrolyte solutions, the midpoint potentials remains constant, but the midpoint potential of the thionine-zeolite electrodes depends on the solution pH. Influence of the pH of the solution on the midpoint potential of an immobilized dye reveals that thionine molecules are accessible to protons. This property is ascribed to the formation of mesopores in the structure of our zeolites suffering from a calcination step. Published in Russian in Elektrokhimiya, 2007, Vol. 43, No. 7, pp. 794–800. The text was submitted by the authors in English