NaZSM-5 Zeolite Research Articles

Catalytic CO oxidation over palladium supported NaZSM-5 catalysts

A new Pd supported NaZSM-5 zeolite catalyst for CO oxidation at low temperature is reported. The catalyst was prepared by impregnation method. The typical 50 and 100% conversion temperatures of the catalyst (T50 and T100) are 38 and 90°C, respectively. A 2.4% Pd/NaZSM-5 catalyst can maintain its activity for CO oxidation for more than 720h in the presence of moisture. The activities of Pd/NaZSM-5 catalysts are strongly dependent on their preparation method and pretreatment. The effects of different calcination temperature, reaction temperature, Pd loading amount and reduction atmosphere have been studied in detail. The effect of different surface Pd states on its catalytic ability has been observed. High calcination temperature results in large Pd particle dimensions, which directly leads to the decrease of activity. High reaction temperature or high Pd loading results in the high catalytic activity for CO oxidation, while H2 reduction of catalyst causes low catalytic activity. The formation of PdO2 species on Pd/NaZSM-5 catalyst surface was strongly supported by XPS and related experiments on different treatments. Characterizations of XRD and TEM show that the surface Pd species are highly dispersed as nanoparticles over NaZSM-5 zeolite, and the dimension of particles is dependent on the pretreatment temperature. Characterization of XPS proves that small PdO2 particles are easily reduced to PdO and Pd in CO or H2 atmosphere, which are related to the deactivation of the Pd/NaZSM-5 catalyst for CO oxidation.

Diffuse-Reflectance IR Spectra of Methane Adsorbed on NaZSM-5 and HZSM-5 Zeolites

Diffuse-reflectance IR spectra of methane adsorbed on high-silica NaZSM-5 and HZSM-5 zeolites point to a stronger adsorption of methane on sodium cations than on protons. For the asymmetric stretching vibration ν3, this form of adsorption is characterized by a doublet with band maxima at 2980 and 3010 cm–1. For the fully symmetric stretching vibration ν1, it is characterized by a singlet with a maximum at 2880 cm–1. Methane is also adsorbed on NaZSM-5 in a weaker form, which is characterized by absorption bands with maxima at 3002 (ν3) and 2887 (ν1) cm–1. The weaker form of methane adsorption on acidic bridging hydroxy groups of HZSM-5 is characterized by absorption bands at 3001 and 2887 cm–1 (ν3 and ν1, respectively). A difference between this form of adsorption and weak adsorption on sodium-exchanged zeolite reveals itself in the somewhat higher intensity of the band at 2887 cm–1. For methane adsorbed on NaZSM-5, the frequencies of deformational vibrations and a spectrum in the near IR region are obtained for the first time. It was found that the perturbance of adsorbed methane molecules is seen in the spectrum as in the low-frequency shifts of most of the bands that appear due to composite vibrations and overtones and as new adsorption bands that were not observed for gaseous methane.

Sorption of 4,4 ′-bipyridine in Na nZSM-5 zeolites: spectroscopic and modelling studies

The diffuse reflectance UV-visible absorption and Raman scattering spectrometries provide evidence for the sorption course of 4,4 ′-bipyridine (4,4 ′-bpy) within the porous void space of silicalite-1 [(SiO 2) 96] over several weeks at loadings of 1 and 2 molecules per unit cell. The presence of extra-framework Na + cations in the void space of Na n ZSM-5 ( n=3, 6) Na n (AlO 2) n (SiO 2) 96− n slows down the sorption process, particularly at higher loading. Raman scattering and UV-visible absorption indicate that 4,4 ′-bpy is occluded as intact molecule in the non-acidic zeolites under study. The motions of 4,4 ′-bpy within the channels of silicalite-1 approach the isotropic limit of a liquid at room temperature, whereas in NaZSM-5 zeolite, the interaction between the Na + cation and 4,4 ′-bpy hinder the dynamic motions. Monte Carlo and molecular dynamics simulations indicate that 4,4 ′-bpy is located in the straight channels of MFI zeolites. In NaZSM-5 zeolites, the extra-framework cations were found to be located in the vicinity of the N atoms outside the plane of the pyridyl groups.

Performances of rhenium oxide-encapsulated ZSM-5 catalysts in propene selective oxidation/ammoxidation

Rhenium oxide-encapsulated zeolites were prepared by chemical vapor deposition (CVD), impregnation (imp) and physical mixing (phy) methods using HZSM-5, NaZSM-5 and HY zeolites. The samples were pretreated under He at 673 for 4 h in situ before use as catalysts. Among the catalysts employed for propene selective oxidation/ammoxidation, the catalyst prepared by CVD of methyl trioxo rhenium CH 3ReO 3 (MTO) on HZSM-5 (HZ cvd) exhibited the highest selectivity (88.2%) to acrolein (main)+acrylonitrile at 648 K. The HY cvd catalyst showed less selectivity of 46%. However, the other ReOx/HZSM-5 samples prepared by impregnation and physical mixing, NaZ cvd prepared by CVD on NaZSM-5, and HZSM-5 and HY zeolites without Re exhibited very low or no activity for the selective oxidation/ammoxidation. The order of the selectivity to acrolein+acrylonitrile for the catalysts is as follows: HZ cvd>HY cvd>HZ imp=HZ phy=HY imp>NaZ cvd>HZ=HY. The results indicate that CVD of MTO complex is an effective method for Re incorporation into zeolites, that proton sites are required for interaction of Re species with the zeolite framework, and that the pore size geometry of HZSM-5 is more suitable for the formation of active sites involving Re oxide species than that of HY. The HZ cvd catalyst also exhibited the stable performance during the period of 36 h. It was found that the coexistence of ammonia was prerequisite for the performance of the ReOx/zeolite catalysts, where no selective oxidation reaction proceeded in the absence of ammonia.

FTIR Study on Molecular Motions of Benzene Adsorbed in ZSM-5 Zeolite: Role of Charge-Balancing Cations and Pore Size

Occlusion of benzene in NaZSM-5 zeolite is investigated using in situ FTIR spectroscopy as a function of substitution of Na+ with group IIA cations. At least four pairs of overlapping vibrational bands were observed in the region of out-of-plane C−H bending vibrations (2000−1800 cm-1) on adsorption of benzene in NaZSM-5 at room temperature. Whereas two pairs of these bands, e.g., one pair at around 2007 and 1986 cm-1 and the other at 1969 and 1956 cm-1, correspond to the 1960 cm-1 band of liquid benzene, the other two pairs, e.g., at 1874 and 1852 cm-1 and 1831 and 1810 cm-1, appear in place of the 1815 cm-1 band of liquid benzene in this region. No measurable difference was observed in the frequencies of these bands for adsorption in cation-exchanged samples, suggesting that any specific interaction between cations and benzene molecules is small compared to the effect of benzene−benzene interaction. These multiple bands are therefore attributed to the existence of at least two distinct clustered states o...

Catalytic reforming n-octane on Pt–Re/Al 2O 3 catalysts promoted by different additives

The effect of decationized forms of NaY, NaZSM-5 zeolites on the properties of Pt–Re/Al 2O 3 catalyst at n-octane reforming has been shown. The promoted additives increase the isomerization activity of catalysts. Activity and selectivity of the modified catalysts depend on both degree of ion exchange and (Si/Al) ratio of zeolites. One hundred percent selectivity of i-octane formation is reached at the introduction of promoter additive (element of the V group).

Adsorption of Water and Methanol on a NaZSM-5 Zeolite. A temperature-programmed desorption (TPD) study

Using temperature-programmed desorption (TPD), we have investigated the desorption behavior after subsequent co-adsorption of methanol and water and after adsorption of their mixtures on a NaZSM-5 zeolite. The course of desorption indicates that a strong mutual displacement of both components occurs. However, on the strongest adsorption sites methanol is preferentially adsorbed, and already the addition of small amounts of methanol leads to a displacement of water. Our results support the idea of a subdivision of the pore space for adsorption of water/methanol mixtures. Above all, the experiments show that in the part of the pore space where both components are adsorbed, different sites are of importance which vary significantly in their interaction strength.

Catalytic Behavior of Supported KNiCa Catalyst and Mechanistic Consideration for Carbon Dioxide Reforming of Methane

Carbon dioxide reforming of methane to synthesis gas has been investigated using a KNiCa catalyst loaded on a highly siliceous NaZSM-5 zeolite support which was promoted with alumina. The catalytic behavior of the supported KNiCa catalyst has also been compared to that of the supported Ni catalyst. Long-time catalytic measurements at 800°C show that the supported KNiCa catalyst has excellent catalyst stability for 140 h due to the promotional effect of surface carbonate species leading to surface enrichment of carbon dioxide, while the supported Ni catalyst is subjected to severe catalyst deactivation due to extensive coke deposition less than 40 h on stream. Pulse reaction, thermogravimetric analysis, isotope experiment, and X-ray absorption spectroscopy have been performed for understanding the detailed chemistry and the mechanistic aspects of the CO2 reforming. Pulse reaction and thermogravimetric analysis on the supported KNiCa catalyst indicate that methane is activated on the surface Ni species and carbon dioxide interacts with alkaline promoters to form surface carbonates which hinder the formation of inactive coke or scavenge carbon from the surface Ni species. A study of deuterium isotope effects for the reforming reaction shows that there is almost no isotope effect on the supported KNiCa catalyst, suggesting that a CH4 dissociation step is not rate determining. In this work, mechanistic investgations reveal that reaction between the adsorbed carbon species and the dissociated oxygen atoms on Ni sites of catalyst surface leads to the production of carbon monoxide and the regeneration of metallic nickel species as a consequence, which is assumed to be a rate-determining step in the CO2 reforming. It is also proposed that the oxidation step of surface carbon with surface oxygen or adsorbed CO2 as surface carbonate species on the catalyst is important for maintaining catalyst stability of the supported KNiCa by the efficient removal of surface carbon species.

Spectroscopic Identification of Adsorbed Intermediates Derived from the CO+H2O Reaction on Zeolite-Encapsulated Gold Catalysts

Identification of reaction intermediates in the water–gas shift reaction (WGSR: H2O+CO→H2+CO2) on Aun+ (1≤n<3) incorporated into NaY, Na–mordenite, and Na–ZSM-5 zeolites has been studied by means of in situ FT-IR spectroscopy. Exposure of Aun+/zeolites to a gas mixture consisting of CO+H2O at 323 K produced IR carbonyl spectra characterized by Au+–CO at 2192 cm−1 and Au0–CO at 2128 cm−1. On Aun+/NaY, a unidentate formate species (1710, 1620, and 1340 cm−1) is produced as a surface intermediate, which was very readily removed upon evacuation at 323 K. The reduction of the catalyst by H2 at 423 K prior to the admission of the reacting gas mixture caused inhibition of formate species formation. This result suggests that Au+ is the dominant species on which the reaction took place. Au+/Na–mordenite displayed unidentate formate and organic carbonate species (1936 and 1850 cm−1). The latter bands showed a significant increase in intensity with time at the expense of the adsorbed CO2 bands at 2384 and 2338 cm−1. On the other hand, Au+/Na–ZSM-5 resulted in the formation of different carbonate-like species (1936, 1850, 1730, and 1400 cm−1) as well as the rapid appearance of deformation vibrations of adsorbed water molecules at 1630 cm−1 that built up very quickly compared to that in the Au+/Na–mordenite sample. These results were consistent with the catalytic activity data that showed the highest formation rates of CO2 on Au+/NaY compared to those on Au+/Na–mordenite and Au+/Na–ZSM-5 catalysts.

Catalytic activity of Cu ion-exchanged Na·MCM-41 in the liquid-phase oxidation of 2,6-di- tert-butylphenol

The liquid-phase catalytic oxidation of 2,6-di- tert-butylphenol (BOH) was performed under mild reaction conditions using a Cu ion-exchanged Na·MCM-41 (Cu–Na·MCM-41) catalyst. 4,4′-Dihydroxy-3,3′,5,5-tetra- tert-butylbiphenyl (H 2DPQ) and 3,3′,5,5′-tetra- tert-butyl-4, 4′-diphenoquinone (DPQ) were obtained as the oxidation products. The addition of an inorganic base such as KOH onto the Cu-Na·MCM-41 catalyst or to the reaction solution was found to cause the increase in the oxidation activity. The oxidation activity of BOH in the presence of KOH in the reaction solution was greater than when using the K added Cu-Na·MCM-41 catalyst. The reaction scheme for the BOH oxidation was proposed, in which scheme the DPQ formation involves both a consecutive pathway via H 2DPQ intermediate and an oxidative dehydrogenation of a dimer formed by the C–C coupling of the corresponding phenoxy radicals, without H 2DPQ intermediate. To study the influence of the mesopore of the Na·MCM-41 on the oxidation of BOH, a comparably bulky molecule, Cu ion-exchanged NaZSM-5 (Cu-NaZSM-5) zeolite, whose pores are micro range in size, was attempted as a catalyst for the bulky phenol oxidation. The oxidation activity of the Cu-NaZSM-5 zeolite was found to be considerably lower than that of the Cu-Na·MCM-41 catalyst. The preference of the mesopore environment for the 2, 6-di- tert-butylphenol oxidation against the micropore counterpart was thus confirmed.

On the role of balancing cations in the entrapment of CO in ZSM-5 zeolite at room temperature: FTIR study

In continuation of an earlier FTIR study (Phys. Chem. Chem. Phys., 1999, 1, 191), the effect of cation exchange on encapsulation of CO in NaZSM-5 zeolite is investigated. The exchange of Na+ with a proton/deuteron or a group 2 cation gave rise to a new set of frequency-shifted ν(CO) and ν(CO2) bands on adsorption of CO. The intensity of the new bands was related approximately to the extent of cation exchange. The value of Δν was positive in the case of samples exchanged with group 2 cations and negative for those exchanged with a proton or deuteron. The frequency shift of above-mentioned bands showed a correlation with the change occurring in pore volume due to cation exchange and also with the electrostatic potential (e/r, i.e. charge/radius) associated with the balancing cation. The results suggest that pores of two different kinds may exist simultaneously in a zeolite, the original ones and the others modified by exchanged cations. These pores exhibit their independent adsorption behavior, the adsorption contribution of a particular kind of pores being proportional to their concentration. Also, the pores of larger volume are filled preferentially at lower pressures. The influence of pore size on IR bands is in conformity with the formation of weakly bonded (CO)n or (CO2)n clusters, occluded in the zeolitic cages and stabilized under the cationic field.

Preparation, characterization, and optical properties of the host-guest nanocomposite material zeolite-silver iodide

Zeolites NaZSM-5 and NaY as hosts for producing nanoscale silver iodide guest were studied with the goal of investigating the optical properties of the prepared host-guest nanocomposite materials. The host-guest nanocomposite materials of NaY–AgI and (NaZSM-5)–AgI were prepared by a heat diffusion method. The nanocomposite materials prepared were characterized by X-ray diffraction (XRD), differential thermal analysis (DTA), X-ray photoelectric spectroscopy (XPS), and adsorption. The properties of the diffuse reflectance absorption spectra and the surface photovoltage spectra of the prepared nanocomposite materials were investigated. The quantum confinement effect of the channels of zeolites made the prepared host-guest nanocomposite materials show some interesting optical properties. This makes the investigated nanomaterials suitable for use as quantum electronic and optoelectric materials.

Higher activity of CuCl 2/HZSM-5 prepared by dispersion method in selective catalytic reduction of NO by propylene (SCR-HC) at lower temperature

A series of samples have been prepared from high dispersion of CuCl 2 into NaZSM-5 and HZSM-5 zeolites, and characterized by X-ray diffraction (XRD), differential thermal analysis (DTA), hexane isotherms, and IR spectroscopy. The catalytic activity in selective reduction of NO shows that CuCl 2/HZSM-5 catalyst exhibits higher conversion at 300°C than that of CuZSM-5 prepared from an ion-exchange method. Correlation of catalytic data with infrared spectra for CuCl 2/HZSM-5, CuCl 2/NaZSM-5, and CuZSM-5 samples suggested that the catalytic sites can be assigned to synergetic effect of protons and copper species.

Adsorption of CO on NaZSM-5 zeolite under moderate temperature and pressure conditions: An FTIR investigation

Adsorption of CO on NaZSM-5 zeolite was investigated at temperatures in the range 300–470 K and at pressures of 5–500 Torr using FTIR spectroscopy. The effect of exchanging the charge balancing cation in NaZSM-5 with a proton or calcium was evaluated. Data were also collected on NaY, CaY and CaX zeolites for comparison. We detected the development of six distinct C–O stretching bands with maxima at around 2111, 2130, 2146, 2160, 2176 and 2194 cm-1 during the adsorption of CO on NaZSM-5 zeolite at ambient temperatures. This was accompanied by the appearance of a prominent band at 2356 cm-1 and weak shoulder bands at frequencies around 2336, 2340, 2370 and 2380 cm-1 in the ν3 region of CO2. All the ν(CO) bands and also the bands in the ν3 region of CO2 exhibited similar behaviour as a function of adsorbate pressure, evacuation, rise in sample temperature, and the exchange of charge balancing cation. For instance, the intensity of all the C–O stretching bands showed a similar growth behaviour with increasing adsorbate pressure, though the extent of this growth was different for the individual IR bands. Similarly, these bands were removed simultaneously on evacuation. Furthermore, while all the vibrational bands in the ν(CO) region showed a uniform isotopic shift corresponding to a frequency ratio ν(13C/12C) of ca. 0.977 and ν(18O/16O) of 0.976 for the adsorption of 13C16O and 12C18O, respectively, the bands in the ν3(CO2) region showed a red shift ν(13C/12C) of 0.972 with 13CO and an isotopic shift corresponding to 16O12C18O on 12C18O adsorption. No shift in ν(OH) bands was observed after CO adsorption under the conditions of this study. The results thus indicate that the individual zeolitic surface sites e.g., the Al3+ sites, Bronsted acid sites or the charge balancing cations, may not participate directly in the bonding of CO molecules at room temperature or above. Instead, the cage effect of zeolites plays an important role. The data are interpreted to suggest the formation of weakly bonded clusters of CO and CO2 molecules, occluded in the zeolitic cages and stabilized under the cationic field.

Oxidation activity of cerium supported NaZSM-5 zeolites with and without added alkali metals in the gas-phase catalytic oxidation of benzyl alcohol

The gas-phase catalytic oxidation of benzyl alcohol was carried out over low [CeL, ∽1×10-2 mmol (g-NaZSM-5)-1] and high [CeH, ∽1–2 mmol (g-NaZSM-5)-1] Ce supported NaZSM-5 zeolites and alkali metal added counterparts. The Ce was supported on NaZSM-5 zeolites by both ion-exchange (CeL-NaZSM-5) and impregnation (CeL- and CeH/NaZSM-5) methods. The alkali metal added to the low and high Ce supported NaZSM-5 catalysts was found to be quite different in influence on the catalytic activity of benzyl alcohol oxidation. The addition of alkali metal to the CeL- and/or CeL/NaZSM-5 zeolites was found to promote selectively catalytic activity for the partial oxidation of benzyl alcohol. The addition of the alkali metal caused an increase in the amount of O2 uptake of the prereduced CeL- and/or CeL/NaZSM-5 catalysts. A correlation was observed between the amount of O2 uptake and the amount of alkali metal added to the CeL- and/or CeL/NaZSM-5 catalysts. The alkali metal added to the CeL- and/or CeL/NaZSM-5 catalysts was suggested to promote the mobility of the oxygen species in the low Ce supported catalysts, whose oxygen species are considered to play an important role in benzaldehyde formation. The alkali metal added to the CeH/NaZSM-5 catalysts inversely tended to decrease the oxidation activity. The addition of the alkali metal to the CeH/NaZSM-5 catalyst made the particle size of CeO2 formed in the CeH/NaZSM-5 larger, and the redox ability of the oxidic cerium lower through the inhibition of the uptake of O2.

Highly Selective Photochemical and Thermal Chlorination of Benzene by Cl2 in NaZSM-5

Loading of zeolite NaZSM-5 with benzene and chlorine from the gas phase at −100 °C resulted in spontaneous reaction to form chlorobenzene and 1,4-dichlorobenzene as the sole products. Thermal react...

Dispersion of Inorganic Salts into Zeolites and Their Pore Modification

A series of samples, prepared from mechanical mixtures of inorganic salts (MoO3, CuCl2, NiCl26H2O, and Ni(NO3)26H2O) with zeolites (NaZSM-5, NaY, NaY, and MCM-41), have been investigated by X-ray diffraction (XRD), differential thermal analysis (DTA), the adsorption of probing molecules, and the determination of pore size distribution. After the heating of MoO3with zeolites at 723 K for 24 h, the samples (MoO3/NaZSM-5/NaY/MCM-41=0–0.088/0–0.24/0–0.45 g/g) show only those X-ray peaks assigned to zeolites, the characteristic peaks of MoO3having disappeared completely, which suggests that MoO3is highly dispersed in the pores of zeolites. Similar phenomena are observed for the samples of CuCl2/NaZSM-5, CuCl2/NaY, NiCl2/NaZSM-5, NiCl26H2O/NaY, and Ni(NO3)26H2O/NaY. In contrast, the inorganic salts of NiCl26H2O and Ni(NO3)26H2O cannot be dispersed into the channels of NaZSM-5 zeolite due to the limitation of the channel size. These results suggest that the dispersion only occurs under the condition that the diameter of the inorganic salts is less or similar to the pore size of the zeolites. Also, we observed that the dispersion capacity of the inorganic salts is strongly related to the pore size of the zeolites. Furthermore, the isotherms for probing molecules show that the dispersion of inorganic salts into zeolites leads to significant change in the pore size of zeolites. The MoO3/NaY samples exhibit that the pore size was selectively formed at 6.8–8.0, 6.0–6.8, 4.3–6.0, and 3.0–4.3 Å with MoO3loading at 0.08, 0.16, 0.21, and 0.24 g/g, respectively. Therefore, it is proposed that we could modify the pore sizes of zeolites to various degrees, which may be very useful to design suitable pores of zeolites for the catalysts in catalytic reactions. Catalytic data in selective reduction of NO by propylene at the temperature of 300°C show that CuCl2/HZSM-5 catalyst prepared from the dispersion method exhibits much higher catalytic conversion (39%: N2yield) than that (21%: N2yield) of CuZSM-5 catalyst prepared from the ion-exchange method.

Thermodynamics and statistical mechanics of ammonia in zeolite NaZSM5

AbstractThe adsorption isotherm and the differential heat of adsorption of ammonia in zeolite NaZSM5 (Si/Al=32) were measured over a wide range of equilibrium pressure at the temperature 303 K. The obtained data can consistently be interpreted to be due to the formation of s‐meric (s = 1 − 6) ammonia/sodium cation complexes residing in the intersections of the straight and zig‐zag channels of the zeolite structure. The energies of formation and the potential energies of the s‐mers are reported.

Two-dimensional triple-quantum 23Na MAS NMR spectroscopy of sodium cations in dehydrated zeolites

Two-dimensional triple-quantum (2D-3Q) 23Na MAS NMR spectroscopy has been applied for the investigation of sodium cations in dehydrated zeolites NaY, NaEMT, NaZSM-5 and NaMOR. The experiments have shown that the new 2D-3Q technique allows the determination of the isotropic chemical shifts and quadrupolar couplings of sodium cations with SOQE (second-order quadrupolar effect) parameters of up to ca. 4 MHz. In the present work, SOQE parameters of 1.0–1.2 MHz were found for sodium cations located at positions SI in the hexagonal prisms of dehydrated zeolites NaY and NaEMT. The sodium cations located in the 10-ring and 12-ring channels of dehydrated zeolites NaZSM-5 and NaMOR, respectively, are characterized by a SOQE parameter of 2.0 MHz while a value of 3.1 MHz was determined for sodium cations in the sidepockets of the channels in dehydrated NaMOR.

Grand canonical Monte Carlo simulation of the adsorption of CO 2 on silicalite and NaZSM-5

The adsorption of carbon dioxide in silicalite and NaZSM-5 zeolite has been studied using new Monte Carlo software. In this program, sodium cations and framework are movable during the simulation. The calculated adsorption isotherms are in good agreement with the experimental results. The energy distribution of carbon dioxide over silicalite and NaZSM-5 shows that the increase of the adsorption energy for NaZSM-5 is mainly due the electric field generated by sodium cations.