NaZSM-5 Zeolite Research Articles

Surface photochemistry: p-Hydroxystilbazol within nanochannels of Na + and H + ZSM-5 zeolites

This work reports the surface photochemistry study of p-hydroxystilbazol adsorbed within nanochannels of MFI zeolites, namely in a series of NaZSM-5 and HZSM-5 zeolites where the surface acidity of the host was varied systematically. Laser-induced room temperature luminescence of air equilibrated solid powdered samples of p-hydroxystilbazol adsorbed onto the two sets of zeolites, revealed the existence of multiple emissions of p-hydroxystilbazol, namely as a broad band centered at about 470 nm all cases where the host surface was highly acidic, while in other cases emissions with maxima at about 410, 530 and 660 nm were also detected. The decay times of the p-hydroxystilbazol emission vary greatly going from solution to entrapment within the nanochannels of the ZSM-5 zeolites and roughly increase by an order of magnitude upon inclusion. A lifetime distribution analysis has shown that the longest lifetime for the p-hydroxystilbazol fluorescence emission exists in the case where the 530 nm type emission was detected, i.e. whenever the hydrophobic character of the host increases. p-Hydroxystilbazol entrapped in the more acidic zeolites emits mainly in the protonated form. However, p-hydroxystilbazol may emit from neutral, protonated and zwitterionic forms in the case of the more hydrophobic zeolites. High energy (266 nm excitation and high fluences of the excitation laser (30 mJ per pulse) enabled us to evidence a 660 nm emissive form assigned to the quinoid derived from the zwitterionic form of p-hydroxystilbazol. Delayed fluorescence of p-hydroxystilbazol was also detected for the HZSM-5 and NaZSM-5 entrapment both at room temperature.

NaZSM-5-catalyzed dimethyl carbonate synthesis via the transesterification of ethylene carbonate with methanol

NaZSM-5 zeolite was found to be an efficient heterogeneous catalyst for the synthesis of dimethyl carbonate (DMC), which can serve as a building block, an additive to fuel oil, and an electrolyte in batteries, via the transesterification of ethylene carbonate (EC) with methanol. Notably, 77% DMC yield and 97% selectivity were achieved under mild reaction conditions. Furthermore, the effects of various reaction parameters such as catalyst loading, reaction time, and methanol/EC molar ratio on the catalytic performance were investigated in detail. This protocol was found to be applicable to a variety of alcohols, producing the corresponding dialkyl carbonates with moderate yields and selectivities. Moreover, the catalyst can be recovered by simple filtration with retention of catalytic activity; a stable crystal configuration and a slight alteration of its superficial structure were observed by X-ray diffraction and BET measurements.

Oxidation of methane to methanol and formaldehyde over Co–ZSM-5 molecular sieves: Tuning the reactivity and selectivity by alkaline and acid treatments of the zeolite ZSM-5 agglomerates

Alkaline treatment of templated Na–ZSM-5 zeolites with NaOH solutions resulted in the creation of intercrystalline mesopores within the zeolite agglomerates, while preserving the micropore volume and crystallinity. It was found that the zeolite external surface area increased with increasing NaOH concentration and pretreatment time. These alkaline-treated samples were loaded with cobalt and a linear relationship between the number of cobalt oxidic species and the external surface area of the zeolite could be established. This in turn leads to a linear relationship between the ZSM-5 surface area and the amount of methanol produced over Co–ZSM-5 from methane and oxygen at 423 K. Attempts to remove extra framework alumina species by an acid treatment were successful. However, this acid treatment increased, after Co-deposition, the amount of highly dispersed Co 2+ inside the ZSM-5 channels, which resulted in a higher selectivity towards formaldehyde. The nature of the charge-compensating cation also determined the cobalt speciation. Co–H–ZSM-5 samples contained more cobalt inside the channels and were more selective towards formaldehyde formation than Co–Na–ZSM-5 samples.

TiO 2/HZSM-5 nano-composite photocatalyst: HCl treatment of NaZSM-5 promotes photocatalytic degradation of methyl orange

TiO 2/HZSM-5 nano-composite photocatalysts were prepared by dispersing TiO 2 onto the surface of HCl-treated NaZSM-5 zeolite using a sol–gel method. These materials were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), BET surface area analysis and X-ray photoelectron spectroscopy (XPS) measurements. Photocatalytic activities of the supported catalysts were examined through methyl orange degradation under UV irradiation. The results showed that HCl modification of NaZSM-5 did not noticeably change the surface morphology and structure of the zeolite. Improved dispersion and restrained crystalline size of TiO 2 could be found after supporting the TiO 2 on HZSM-5. Ti–O–Si bonds did not form between the HZSM-5 bulk and the supported TiO 2. Supported TiO 2 calcinated at 400 °C presented anatase structure. The titanium was in the oxidation state of Ti 4+, and the oxygen was in the form of O 2− in both TiO 2 and SiO 2. The adsorption ability and photocatalytic degradation activity of TiO 2/HZSM-5 nano-composite were better than that of bare TiO 2, accompanied with increasing specific surface area. Meanwhile, photocatalytic degradation activity of TiO 2/HZSM-5 changed remarkably when HZSM-5 supports treated with different concentrations of HCl were used, giving the optimum HCl concentration of 0.3 mol/l. The recycling of TiO 2/HZSM-5 photocatalyst was also investigated and methyl orange degradation rate was 82.4% of the initial rate after three recycles.

Trapping the lead ion in multi-components aqueous solution by natural clinoptilolite

To study whether natural clinoptilolite could selectively capture Pb 2+ ion in the aqueous solution containing salt and glycine as well as nitrosamines, the natural zeolite was utilized in the complex solution with pH value of 1.2 in comparison with other porous materials such as zeolite NaZSM-5 and activated carbon. Clinoptilolite exhibited the highest capability in adsorbing Pb 2+ ion in the solution at 37 °C, achieving the capacity of 7 mg/g, two times more than that by other zeolites and six times over that by activated carbon, and the adsorption equilibrium could be achieved within 2 h. The impacts of solid–liquid ratio, initial concentration of Pb 2+ and the competition of other metal ions or volatile nitrosamines on the adsorption were examined, and the change in Gibbs energy for the ion-exchanged of Pb 2+ ion by clinoptilolite was discussed, through which the ion-exchange process was proven to be spontaneous. Moreover, both the selectivity of clinoptilolite toward Pb 2+ ion in complex solution and the venial toxicity of clinoptilolite on the viability of RAW264.7 cells were investigated in this article.

Dynamics of Propylene adsorbed in Na-Y and Na-ZSM5 Zeolites: A QENS and MD Simulation Study

Abstract Here we report dynamics of propylene (smallest asymmetric hydrocarbon) adsorbed in two different host systems – Na-Y and Na-ZSM5 zeolites as studied by quasielastic neutron scattering (QENS) techniques and molecular dynamics (MD) simulation techniques. MD simulation studies suggested that the rotational motion of propylene is much faster than the translation. Therefore, two different spectrometers were used to determine the translational and rotational contributions. Analysis of the QENS data (ΔE~200 μeV) revealed that the translational motion of propylene in Na-ZSM5 zeolite is a factor of 3 slower than that in Na-Y zeolite. This is consistent with the smaller void volume available in Na-ZSM5 zeolite as also the fact that the adsorption of propylene is stronger in Na-ZSM5 as compared to that in Na-Y zeolite as indicated in the Fourier Transformed Infra Red (FTIR) studies. This result is further corroborated by the MD simulation studies. It was found that the translation motion occurs at three time scales in both the systems, only one of which corresponds to that observed through the QENS measurements. To probe the faster rotational motion experiments were carried out on a Triple axis spectrometer with wider energy window (ΔE~3meV). The variation of the elastic incoherent structure factor (EISF) suggests that the observed QE broadening corresponds to isotropic rotational motion of propylene in both the hosts. Rotational diffusion coefficients of propylene molecules are found to be of the same order in both zeolites. Trajectories obtained from MD simulation corresponding to the rotational motion revealed that although on a large time scale, the rotation appears isotropic, but at short time scales the channel structure of the Na-ZSM5 restrict the rotation of propylene molecules. This is in contrast with propylene adsorbed in Na-Y zeolite where rotation remains isotropic even at short time scale. A comparison with other hydrocarbon-zeolite systems as studied by us is also presented here.

Dynamics of Adsorbed Hydrocarbon in Nanoporous Zeolite Framework

We report quasielastic neutron scattering (QENS) and molecular dynamics (MD) simulation study of the dynamics of propylene molecules adsorbed in Na-ZSM5 zeolite. MD simulation studies suggest that rotational motion is almost an order of magnitude faster than translational motion. Therefore, spectrometers having different energy resolutions were used to determine the translational and rotational contributions. Translational motion, being slower, was distinctly observed in a narrower window spectrometer while both contribute to the wider one. Diffusion of propylene in the channels of Na-ZSM5 zeolite was found to follow jump diffusion model. Dynamical parameters corresponding to translational diffusion obtained from experiment are found to be consistent with MD simulation. Variation of elastic incoherent structure factor (EISF) suggests that rotational motion of propylene is isotropic. Although at short times the rotational motion was found to be anisotropic, as indicated in the MD simulation depicting restricted channel framework, but at long time it results in isotropic rotational motion.

Diffusion of propylene adsorbed in Na-Y and Na-ZSM5 zeolites: Neutron scattering and FTIR studies

Here we report the quasielastic neutron scattering and FTIR studies on the dynamics of propylene adsorbed in Na-Y and Na-ZSM5 zeolites. QENS data show that although the mechanism of translational motion of propylene is jump diffusion in both the cases of Na-Y and Na-ZSM5 zeolites, the diffusivity is affected by the host size and is hindered in the case of Na-ZSM5. FTIR studies showed that guest-host interaction in Na-ZSM5 is stronger than that in Na-Y zeolite corroborating the QENS results.

Surface photochemistry: Benzophenone within nanochannels of H + and Na + ZSM-5 zeolites

This work reports a surface photochemistry study of benzophenone within nanochannels of MFI zeolites, in this case a series of H + and Na + ZSM-5 zeolites where the acidity of the host is varied systematically. Laser-induced room temperature luminescence of air equilibrated or argon purged solid powdered samples of benzophenone adsorbed onto the two sets of zeolites, which we will name HZSM-5 and NaZSM-5 zeolites, revealed the existence of three different benzophenone emissions. In the nanosecond time range a broad band centred at about 430 nm was detected and its origin was hydrogen bonded benzophenone. A second broad band peaking at about 470 nm and in the millisecond time range was also detected, which was the emission of the protonated benzophenone. Finally a vibrationally structured emission existed (448 nm), the normal room temperature phosphorescence emission of benzophenone. Triplet–triplet transient absorption of benzophenone was obtained for all zeolites under study, and clear evidence for the formation of hydroxybenzophenone radicals was obtained for the NaZSM-5 series (only Lewis acid sites). The more acidic the internal surface the more radical was detected. Other products resulting from α-cleavage of benzophenone were also identified. When the host was HZSM-5 (Brönsted and Lewis acid sites) benzophenone seemed to be less reactive, and no ketyl radicals from benzophenone were detected. All these findings clearly show that benzophenone is an excellent probe which is able to provide important information regarding the internal surfaces of MFI zeolites, namely the different photocatalytic activity of the different active sites of these zeolites. The present study is paradigmatic in what regards the host influence on the photochemistry of the guest.

NO x reduction by ethanol on Pd/zeolites-effect of oxygen

The reduction of NO by ethanol on palladium catalysts supported on NaZSM-5 and HZSM-5 zeolites was studied. Temperature programmed techniques, such as desorption of ethanol, NO and surface reaction (TPSR) analyses, besides NH3 (NH3–TPD), as well as, FTIR after pyridine adsorption were used to accomplish reaction products and surface properties. Results show that NaZSM5 and HZSM5 present different amounts of Lewis and Bronsted acid sites, which affect strongly the carbon product distribution, but not the nitrogen selectivity. NO reaction with ethanol in the presence of oxygen occurs only over the metal and is independent of the acid sites.

The kinetics of ethanol adsorption from the aqueous phase onto zeolite NaZSM-5

The kinetics of the isothermal adsorption of ethanol from an aqueous solution onto a hydrophobic zeolite of the NaZSM-5 type in the temperature range 298–333 K was investigated. Specific shape parameters of the adsorption degree curves were determined. The changes in the specific shape parameters of the adsorption degree curves with temperature were determined. The kinetic parameters of ethanol adsorption (Ea,ln A) were determined by the initial rate, the saturation rate and the maximum rate methods as well as from the Johnson, Mehl and Avramy equation. The kinetic model of ethanol adsorption kt=[1−(1−α)1/3] was determined by the “model fitting” method. Ethanol adsorption from aqueous solution onto NaZSM-5 is a kinetically controlled process limited by the rate of three-dimensional movement of the boundary layer of the adsorption phase. A model for the mechanism of ethanol adsorption onto NaZSM-5 is suggested on the basis of the kinetic model. Ethanol molecules in aqueous solution are associated in clusters. The activation energy of the adsorption process corresponds to the energy required for the detachment of an ethanol molecule from a cluster and its adsorption onto the zeolite.

CO 2 adsorption on silicalite-1 and cation exchanged ZSM-5 zeolites using a step change response method

The adsorption behavior of CO 2 on silicalite-1 and cation exchanged MZSM-5 zeolites (M = H, Na, and Ba) was investigated by a step change response method. Temperature programmed desorption (TPD) and in situ Fourier transform infrared (FTIR) spectroscopy were also performed to study the type of adsorbed species and their thermal stabilities. The adsorption experiments were carried out at temperatures from 323 to 473 K and CO 2 partial pressures were below atmospheric pressure. The results indicated that CO 2 was adsorbed on silicalite-1 and HZSM-5 by one type of adsorption, and on NaZSM-5 and BaZSM-5 zeolites by at least two types of adsorption over the temperature range studied. One was a weak adsorption probably due to interaction with the framework of ZSM-5, and the other was a stronger adsorption caused by interactions with the Na and Ba cations of ZSM-5. The adsorption behavior for silicalite-1 and HZSM-5 was adequately described by a single site Langmuir adsorption model but for NaZSM-5 and BaZSM-5 a dual site Langmuir model was required. FTIR spectra at various elevated desorption temperatures after adsorption of CO 2 at low temperature revealed that CO 2 was adsorbed on MZSM-5 and silicalite-1 in several ways and formed carbonate bands, which seemed to be caused by different adsorption sites. The maximum temperatures at which carbonate species were observed were 473 K for silicalite-1, 573 K for HZSM-5 and NaZSM-5 and 673 K for BaZSM-5. The TPD results indicated that the quantity of irreversibly adsorbed CO 2 on silicalite-1 and HZSM-5 was undetectable.

Microscopic and Macroscopic Characterization of Organosilane-Functionalized Nanocrystalline NaZSM-5

Nanocrystalline NaZSM-5 zeolites with systematically varied particle sizes (15, 60, and 200 nm) were functionalized with organosilanes. Through the systematic variation of particle size and therefore external surface area of NaZSM-5, the extent of functionalization and location of functional groups were spectroscopically verified. 29Si magic-angle spinning (MAS) NMR and Fourier transform infrared (FTIR) spectroscopy provided conclusive evidence that the silanol groups located on the external surface of NaZSM-5 were functionalized through reaction with the organosilanes. The 29Si NMR results provided quantitative information about the extent of functionalization on NaZSM-5. A nitroxide spin label was adsorbed on the external surface of NaZSM-5 to probe the surface properties by electron paramagnetic resonance (EPR) spectroscopy. The macroscopic and microscopic properties, such as the behavior of functionalized NaZSM-5 in different solvents, and the specific surface areas were also investigated. The hydrophobicity of the functionalized NaZSM-5 was found to increase relative to the parent NaZSM-5 as expected for an organic surface coating.

The Temperature-Dependent Adsorption Behaviour of Benzene Molecules in ZSM-5 Zeolite Pores: TPD and FT-IR Spectroscopy Studies

Temperature-programmed desorption (TPD) and in situ Fourier-transform infrared (FT-IR) spectroscopic methods were employed to investigate the effect of loading and sample temperature on the state of benzene molecules inside the channels of NaZSM-5 zeolite. TPD profiles revealed the existence of at least three distinct states of benzene adsorption, characterized by desorption peak maxima at ca. 120°C, 170°C and 220°C, respectively. Based on the growth behaviour of these bands, it is suggested that the benzene molecules occupy sinusoidal channels, straight channels and external surfaces, in that order.A reverse trend was observed during the subsequent flushing of the sample at varying temperatures. A virtually fixed amount of benzene was occluded at these three locations, depending upon the loading. The FT-IR studies revealed that the benzene molecule exists in a compressed state in the zeolitic channels, with the molecular clusters formed in the process dispersing only at temperatures above 150°C. For initial benzene loadings of up to ca. 1.5 molecules/unit cell, the spectrum obtained showed that in the O—H stretch region the bridge-bonded OH groups and hydroxyl groups associated with the internal zeolitic channels were perturbed simultaneously. The results show that even for a loading lower than necessary for saturation, a considerable amount of benzene remains condensed at the external surface of ZSM-5 zeolite.

Encapsulation of bis(ethylenediamine) copper(II) complex in zeolite matrices

Bis(ethylenediamine) copper(II) complex was encapsulated in NaY, KL, Naβ and NaZSM-5 zeolite. The redox properties of metal complexes in neat and encapsulated state were studied by cyclic voltammetry. The redox potential of metal complex is altered towards negative value upon encapsulation in various zeolites. This may be due to axial interaction with various zeolite matrix. The redox properties of biological systems are also altered (cytochrome-C, and Vitamin-B12) when they are immobilized on NaAlMCM-41 materials. The copper ethylenediamine complex in constrained environment shows higher activity for the oxidation of dimethyl sulfide compared to that of neat complex.

Adsorption of nitrosamines in acidic solution by zeolites

Adsorption of N-nitrosopyrrolidine (NPYR) by zeolite NaZSM-5, Hβ and NaZSM-11 in acidic solution with pH of 1 was investigated. The NPYR uptake process obeyed the first-rate Lagegren kinetic equation describing adsorption, and the equilibrium data were fitted to the Langmuir or Freundlich adsorption equation. Zeolite ZSM-5, Hβ and NaZSM-11 possessed a higher adsorption capacity than amorphous silica or ordered mesoporous siliceous SBA-15, implying their potential application for adsorption of nitrosamines in gastric juice to anti-cancer.

DRIFT spectroscopy study of nitrogen sorption and nitrogen–oxygen transport co-diffusion and counter-diffusion in NaLSX and NaZSM-5 zeolites

A novel DRIFT Spectroscopy approach to sorption kinetics of nitrogen, N 2, and other simple gases by microporous materials, in particular zeolites, is suggested. It utilizes time evolution of IR-stretching bands of symmetrical molecules such as N 2 due to perturbation that results from their interaction with sorption sites. The method is applied to a study of diffusion-limited sorption of N 2, both pure and in mixtures with oxygen, O 2, by zeolites NaLSX and NaZSM-5. Lower limits to transport-diffusion coefficients of N 2 in these materials were assessed at ambient temperature. Mixture experiments allowed to conclude that the rate of N 2 sorption in O 2-rich mixtures is controlled by co-diffusion of O 2 and N 2 in the secondary pore system between zeolite crystallites. They also demonstrate a weak influence of O 2 on sorption equilibrium of N 2 in NaLSX zeolite from N 2–O 2 mixtures.

Effect of alkali metal added to supported La catalysts on the catalytic activity in the gas-phase catalytic oxidation of benzyl alcohol

The oxidation activities of lanthanum (La) catalyst supported on both crystalline and amorphous oxides including NaZSM-5 zeolite and the alkali metal-added counterparts were studied using a gas-phase catalytic oxidation of benzyl alcohol. The main oxidation products were benzaldehyde and carbon dioxide, together with a trace amount of benzene and/or toluene under certain reaction conditions. The addition of alkali metal to the lanthanum catalysts supported on NaZSM-5 and SiO2 promoted the catalytic activity of the partial oxidation in the benzyl alcohol oxidation. The influences of the oxide supports, the amount of La supported on the catalyst, the added alkali metal/La atomic ratio, and the catalyst preparation on the catalytic activity in the benzyl alcohol oxidation were investigated. The amount of adsorbed ethanol on the La-supported catalyst, the O2 uptake amount, the transient response experiment of the oxidation reaction, together with the BET surface area and the X-ray powder diffraction (XRD) pattern of the catalysts, were measured in order to determine the crucial factors in the partial oxidation activity. A promotion scheme for the alkali metal added to the La/NaZSM-5 catalyst is proposed.

Selective solid state photooxidant.

Irradiation of biphenyl encapsulated in the cavities of a NaZSM-5 zeolite framework has been reported to result in the formation of an extremely long-lived radical cation. Here, we show that such zeolite encapsulated radical cations can act as irreversible one-electron oxidants for simple alkenes and dienes, in a solid-state analogue to solution-phase cosensitization. Compared to the well-known semiconductor photooxidizers, such as titanium dioxide, the NaZSM-5 zeolite-based solid photooxidants exhibit enhanced selectivity based on oxidation potential, molecular size and shape, and Lewis base character.

Equilibrium studies of adsorption of amino acids on NaZSM-5 zeolite

The adsorption of amino acids such as phenylalanine, alanine, tyrosine and tryptophan on highly hydrophobic NaZSM-5 zeolite were studied. Selective adsorption of phenylalanine and tyrosine were observed. The single component adsorption isotherms were fitted in Langmuir model. For the amino acids considered in this work, the isotherms are essentially independent of pH, but they vary significantly with the temperature. Heats of adsorption were obtained from van't Hoff plots of the Henry's law constant limit of the Langmuir isotherm and found to be higher for more hydrophobic solute.