Mechanochemistry of zeolites: Part 3. Amorphization of zeolite ZSM-5 by ball milling

The samples obtained during ball milling of zeolite ZSM-5 were characterized by different methods such as X-ray diffractometry, scanning-electron microscopy, FTi.r. spectrometry, and simultaneous thermal analysis. The milling caused considerable change of morphological and particulate properties followed by loss of crystallinity of the treated zeolite. The loss of crystallinity is caused by breaking of external TOT bonds of zeolite framework. Very slow rate of amorphization of “as-synthesized” zeolite as compared with the rate of amorphization of its “activated” form is explained by the stabilizing effect of TPA⁺ ions present in the structure of the as-synthesized zeolite.     

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 Cr³⁺ ions substitutes isomorphically for Si⁴⁺ ions in the lattice of as-made samples. After oxidative calcination most chromium ions aggregate in α-Cr₂O₃ 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.     

Introduction of vanadium into Y zeolite by solid-state ion exchange

Vanadium has been introduced into Y zeolite by heating the physical mixture of V₂O₅ and HY in an airflow containing water vapor. Back ion exchange, FTi.r. and e.p.r. results suggest that ion exchange occurred between protons of zeolite and the (VO)³⁺ formed during preparation. The zeolite crystallinity is maintained, and the e.p.r. experiments show two different positions in Y zeolite for the entered vanadyl species, one in the supercage and another in the β-cage, neither of them influenced by exposure to air.     

Silver-embedded zeolite crystals as substrates for surface-enhanced Raman scattering

This paper reports the preparation of a type of Ag-embedded zeolite crystals as surface-enhanced Raman spectroscopy (SERS) substrates by chemical reduction of Ag⁺-exchanged ZSM-5. Ag⁺ ions were loaded into the zeolite framework by ion exchange. Then the exchanged-Ag⁺ ions were reduced and metallic silver clusters formed inside the zeolite channel. The resulting Ag-embedded zeolite crystals are characterized by using a number of techniques including X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy to confirm silver formed inside the crystal channel. The fabricated Ag-embedded ZSM-5 zeolite substrates displayed strong and reproducible SERS activity for different Raman probe molecules such as Tris(2,2′-bipyridyl) ruthenium(II) chloride (RuBpy) and rhodamine 6G (R6G). Since silver embedded into the zeolite channel without changing the crystal surface property, the Ag–ZSM-5 zeolite crystal can be used to prepare different SERS-active substrate (SERS-tags), in which different probe molecules may be detected. Such Ag-embedded zeolite substrate would be useful in chemical and biological sensing and in the development of SERS-based analytical devices.     

Synthesis of gallosilicates — MTW- type structure zeolites: Evidence of Ga-substituted T atoms

Gallosilicates with the ZSM-12 structure have been synthesized from the reaction mixture of sodium silicate, gallium nitrate, and methyl triethylammonium bromide at 353 K in an autoclave. XRD results associated with an i.r. study of the lattice vibrations indicated that highly crystalline materials with the ZSM-12 structure were obtained. ⁷¹Ga MAS n.m.r. studies along with ²⁹Si measurements showed that Ga is in tetrahedral positions and the Si/Ga atomic ratio = 70. T.p.d. of ammonia adsorbed on H-GaZSM-12 clearly showed the acid nature of the material, which supports strongly the presence of Ga³⁺ in the zeolite framework.     

Direct synthesis of Zn-ZSM-5 with novel morphology

Zinc-containing ZSM-5 zeolite with novel morphology was firstly synthesized in a one-step route using [Zn(NH₃)₄]²⁺ aqueous solution as zinc resource and n-butylamine as template. The materials were characterized by XRD, SEM, diffuse reflectance UV–vis spectra, FTIR for pyridine adsorption. The results showed that the formation of this novel structure might be related to the introduction of [Zn(NH₃)₄]²⁺ aqueous solution and the zinc existent state in zeolite.     

Optimization of crystal growth of hierarchical porosity ZSM-5 zeolite based on coal fly ash “waste utilization”

In this study, ZSM-5 zeolite with hierarchical porosity structure was synthesized by hydrothermal method using Al(OH)₃, SiO₂ extracted from coal fly ash as aluminum source and silicon source. The optimal synthesis parameters of ZSM-5 zeolite were determined via an orthogonal analysis table. The optimum synthesis conditions were: SiO₂/Al₂O₃/Na₂O/hexadecyltrimethylammonium bromide/tetrapropyl ammonium bromide/H₂O = 1/0.08/0.25/0.05/0.17/50, and the optimum crystallization temperature and crystallization time were 160 °C and 48 h, respectively. Scanning electron microscopy showed that the synthesized ZSM-5 zeolite exhibited a spherical shape. Nitrogen adsorption-desorption and Brunauer-Emmett-Teller measurement analysis displayed that the best sample ZSM-5 molecular sieve showed a typical type IV isotherm, and the micropores and mesopores coexisted in the sample, indicating the successful preparation of ZSM-5 zeolite with hierarchical porosity structure. The crystal growth of ZSM-5 under different hydrothermal conditions follows the “S” adjustment.     

The effect of structural properties of natural silica precursors in the mesoporogen-free synthesis of hierarchical ZSM-5 below 100 °C

The low-temperature synthesis of zeolite ZSM-5 below 100 °C is gaining new attention. This is due to the fact that such synthesis may simultaneously implement the introduction of mesopores into crystalline microporous zeolite structure. Herein, we report the use of natural silica precursors from rice husks in the mesoporogen-free synthesis of hierarchical ZSM-5 below 100 °C and their structural properties which govern the course of crystallization. Rice husks are agricultural wastes with high silica content, which should be exploited to give a positive impact, i.e. highly value-added materials. In this study, the amorphous silica from rice husks was extracted using sequential base-acid treatment. The extracted silica was similarly reactive as Ludox HS-40, even with the reduced amounts of the organic structure-directing agent (OSDA). The product was highly crystalline ZSM-5 with spherical morphology composed of small crystallites, enabling the presence of intercrystallite mesopores. The subjection of extracted silica into the calcination at 550 °C for 6 h, prior to the low-temperature synthesis, altered the silica structure via hydroxyl condensation. The distinct structural properties affected the occurring crystallization in which the resulted products were ZSM-5–disordered-mesoporous silica composites. The possible mechanisms of these two different results may involve the dual roles of tetrapropylammonium ion (TPA⁺) as zeolite OSDA and non-templating structure directing agent. These insights were based on the spectroscopic (FTIR, Raman, ²⁹Si and ²⁷Al MAS NMR spectroscopy), microscopic (TEM and HRTEM) and physicochemical characterizations (XRD and N₂ adsorption-desorption isotherm).     

Seed-induced synthesis of hierarchical architectures of ZSM-5 nanocrystalline aggregates by the solid state crystallization

A seed-induced rapid synthesis of hierarchical ZSM-5 aggregates was achieved by the solid state conversion method in an inorganic system. No secondary mesoporogen was introduced to construct supplementary pore. The physical and chemical properties of the products were characterized by using XRD, SEM, TEM, XRF and N₂ adsorption–desorption techniques. The results revealed that the synthesized samples typically exhibit the uniform spherical zeolite ZSM-5 aggregates composed of loosely packed nanocrystals. It was clearly found that the morphology, mesoporosity and particle size of synthesized samples were directly affected by the type of the seed and synthesis conditions. The well-dispersed hierarchical ZSM-5 aggregates were synthesized at NaOH/SiO₂ = 0.15, 20 ≤ SiO₂/Al₂O₃ ≤ 40 with the acid of zeolite seed solution in guiding the structure. In addition, the crystal growth of zeolite samples synthesized with zeolite seed solution was studied at 433, 453, and 473 K. The results suggested that the activation energies for the induction, transition and crystallization stages were 91.44, 104.37 and 77.68 kJ?mol^?1, respectively. The method provides a cost-effective and industrially applicable route to synthesize hierarchical ZSM-5 aggregates.     

A novel method for the growth of ZSM-5 zeolite membrane on the surface of stainless steel

Thin ordered micropore ZSM-5 zeolite membrane has formed on the stainless steel surface. The influence of three different pre-treatment methods for the growth of ZSM-5 zeolite membrane on the stainless steel was studied, including mechanical polishing, soaking in a 20 wt.% sulfuric acid aqueous solution and anodic oxidation treatment. It was found that the anodic oxidation technique was more favorable for the growth of ZSM-5 zeolite membrane on the stainless steel surface compared with the other two methods. By this appropriate pre-treatment process, ordered ZSM-5 zeolite membrane was immobilized on the stainless steel surface and it showed good interaction with the stainless steel. Energy-dispersive X-ray (EDX) spectroscopy result indicates that the ZSM-5 zeolite membrane has a Si/Al ratio of 37. X-ray diffraction (XRD), N₂ adsorption and scanning electron microscopy (SEM) were used to study the crystal structure, specific surface area and morphologies of the ZSM-5 zeolite membrane.     

Characterization of zeolites and amorphous silica—aluminas by means of aluminum-27 nuclear magnetic resonance spectroscopy: A multifield,multiparameter investigation

We have obtained the aluminum-27 nuclear magnetic resonance (n.m.r.) spectra of a large number of ZSM-5 (MFI) zeolites and amorphous silica aluminas by using static, spin-echo, and “magic-angle” sample-spinning (MAS) techniques, at various magnetic field strengths in the 2.35–11.7 Tesia range. Our results give the mean nuclear quadrupole coupling constants (QCC), the static and MAS line widths, the chemical shifts and shift distributions, the spin—spin relaxation times (T_2H), as well as the effects of hydration on T_2H. We show that such a multiparameter analysis permits an unambiguous differentiation between framework and nonframework (ZSM-5 and amorphous silica—alumina) aluminum sites, which is of use in characterizing the structures of complex commercial zeolite catalysts. We present a theoretical approach to the determination of the QCC values in zeolites and silica—aluminas based upon the field dependence of the MAS or static line widths, which also yields an approximate bond-angle distribution that is found to be in accord with that deduced from X-ray diffraction and ²⁹Si n.m.r.     

In situ e.s.r. monitoring of the coordination and oxidation states of copper in Cu-ZSM-5 up to 500°C in flowing gas mixtures: 1. Interaction with He,O2, NO,NO2, and H2O

A flow cell was designed for the monitoring of the state of copper ions in Cu-ZSM-5 in situ at high temperatures by e.s.r. In thoroughly purified He or in vacuum there is no spontaneous reduction of the cupric ions up to 500°C. The formation of adsorption complexes between isolated Cu²⁺ cations and NO molecules at 20°C leads to a measurable change in coordination. Strong adsorption complexes of Cu²⁺ with NO₂ are formed on treatment of CuH-ZSM-5 by an [NO + O₂] mixture. Bonding of Cu²⁺ ions with such strong ligands as NO₂ or H₂O attenuates the spin-lattice interaction between Cu²⁺ and the zeolitic framework. The number of Cu²⁺ ions in CuH-ZSM-5 is quantified by the use of the e.s.r. signal of frozen, dilute water solutions of CuSO₄ as a reference. In the samples used in this work the e.s.r. signal is associated with all the copper introduced into ZSM-5 by ion-exchange.     

Nonaqueous synthesis of ZSM-35 and ZSM-5

ZSM-35 and ZSM-5 were prepared from nonaqueous systems for the first time. The kinetics of crystallization was studied. The apparent activation energy of nucleation (En) and the growth of crystal (Ec) were measured. En = 7.2 Kcal/mol; Ec = 6.2 Kcal/mol. The trinary phase diagram of ZSM-35 and ZSM-5 is illustrated. During the synthesis, the molar ratio of SiO₂/Al₂O₃ does not change in the solid phase and the aluminosilicate polyanions do not exist in the liquid phase. This is evidence of a solid-phase transformation mechanism in the synthesis of the zeolites. It can be seen from i.r. spectra that the band at about 550 cm⁻¹ increases with the increase of crystallization time, which further confirms that the procedure of crystallization of zeolite in the nonaqueous system takes place in the solid phase.     

Synthesis of mesoporous ZSM-5 from rice husk ash with ultrasound assisted alkali-treatment method used in catalytic cracking of light naphtha

A series of mesoporous ZSM-5 zeolite was synthesized with ultrasonic assisted alkali-treatment technique and their catalytic activity was investigated in catalytic cracking of light naphtha. ZSM-5 zeolite was synthesized from rice husk ash without using any organic template. Effect of alkali-treatment conditions on physicochemical properties of synthesized zeolite was investigated with XRD, FESEM, TEM, N2 adsorption-desorption isotherm, NH3-TPD and TGA technique. It was found that ultrasound energy facilitates the creation of hierarchical structure of ZSM-5 during alkali-treatment. According to XRD analysis, zeolite structure was preserved after 20 min ultrasonic assisted alkaline treatment. However, prolonged dessilication time led to the destruction of MFI zeolite structure. The synthesized ZSM-5 represented highly ordered hexagonal-shape morphology. With increasing alkali-treatment time, the plough land roughness appeared on the surface of zeolite. Comparison of the textural properties samples revealed that the mesopore surface area of alkali-treated samples increased considerably with the increase of ultrasonic assisted alkali-treatment time. Results from catalytic activity tests showed that ultrasound energy has great influence on the activity of ZSM-5. The sample had the highest activity after alkali-treated for 20 min in presence of ultrasound energy which was due to their appropriate hierarchical structure.     

Synthesis and characterization of ferrisilicate zeolite of pentasil group

Synthesis of ‘ferrisilicate’ a crystalline zeolite containing iron and silicon in the zeolite lattice positions and possessing remarkable molecular sieving properties and catalytic activity has been reported. The ferrisilicate was characterized by XRD, i.r., t.g./d.t.a., SEM, XPS, EPR, chemical analysis and water and hydrocarbon sorption measurement. The infrared spectra of ferrisilicate zeolite, in the mid infrared region indicated that Fe³⁺ ions were present in the zeolite framework. The presence of a signal at g = 4.3 in the EPR spectra was assigned to Fe³⁺ ions isomorphously substituted in the tetrahedral positions. The fact that Fe³⁺ ions in ferrisilicate zeolite were in trivalent state and indeed situated at the lattice sites was confirmed by XPS study. The catalytic activity of the ferrisilicate zeolite for xylene isomerization was lower than for its aluminium analogue, probably due to low concentration as well as the strength of strong acid sites.     

Catalyst preparation through ion-exchange of zeolite Cu-, Ni-, Pd-, CuNi- and CuPd-ZSM-5

Ion-exchanged zeolite ZSM-5 is the best known catalyst for direct NO_x decomposition and a viable candidate for NO_x reduction with methane. The preparation is crucial for the efficiency of the conversion and this paper describes the ion-exchange, with Cu²⁺, Ni²⁺ or Pd²⁺, and the dual exchange, with Cu²⁺/Ni²⁺ or Cu²⁺/Pd²⁺, under appropriate pH and ion concentrations for maximum dispersion.     

Toward the rational design of zeolite synthesis: The synthesis of zeolite ZSM-18

ZSM-18 has, until now, only been made using one trisquaternary ammonium ion, 2,3,4,5,6,7,8,9-octahydro-2,2,5,5,8,8-hexamethyl-1H-benzo[1,2-c:3,4-c′:5,6-c″]tripyrrolium, 1. This trisquaternary is difficult to make and destabilizes the zeolite during calcination. We used modeling to aid the design and selection of new templates for ZSM-18. The geometry and charge distribution of these templates were chosen to best fit our estimate of “templating.” Template 2, tris-(2-trimethylamonioethyl)amine, is easy to synthesize in high yield and can be routinely calcined to produce highly sorptive ZSM-18. Template 3, tris-(2-trimethylamonioethyl)methane, produces ZSM-18 only when seeded. Si n.m.r. and Al n.m.r, indicate that aluminum is not located exclusively in the three rings as has been predicted theoretically. [H]-ZSM-18 can be steam-dealuminated. The Si n.m.r, of the dealuminated material agrees with the number of sites and relative occupancies expected.     

Effect of electrodes and zeolite cover layer on hydrocarbon sensing with p-type perovskite SrTi0.8Fe0.2O3-δ thick and thin films

Screen-printed thick film as well as pulsed laser deposited thin film sensors of the perovskite SrTi_0.8Fe_0.2O_3-δ (STF20) with gold electrodes present poor hydrocarbon selectivity when exposed to different gases (hydrocarbons, hydrogen, NO, and CO). By employing Pt-electrodes, response to H₂ and CO is eliminated. In the case of thick film devices, only NO cross interference persists. The selectivity of the thick films is further increased by applying a Pt doped zeolite (ZSM-5) as a cover layer. By adjusting the thickness of the ZSM-5 cover layer, the film selectively senses mainly saturated hydrocarbons such as propane, suppressing the response towards all the other gases. This effect is attributed to the catalytic effect of the high Pt-content of the ZSM-5 zeolite. Application of a ZSM-5 cover layer to thin films enhances the sensor output response to propane, thus reducing selectivity for unsaturated hydrocarbons. A sensor configuration having Pt electrodes on top of an STF20 thick film with an additional 50 μm cover layer of ZSM-5 was found to be the most suitable to selectively sense saturated hydrocarbons. Thin film STF20 sensors equipped with Pt-IDC electrodes and without the zeolite cover layer, were found to perform best for unsaturated hydrocarbons at 400°C.     

Synthesis of ZSM-5 zeolite composite membranes for CO2 separation

ZSM-5 type zeolites were synthesized by the hydrothermal treatment of the reaction mixture of silica sol, aluminum nitrate, sodium hydroxide, and TPABr at the temperature range of 150–180 °C in the autoclave. The shape of the ZSM-5 zeolite calcined at 450 °C was spherical or polyhedral and its crystalline size was 0.5–3 m. The synthetic ZSM-5 was found to be highly hydrophobic and active for CO₂ adsorption. ZSM-5 zeolite composite membranes supported with porous -alumina tubes have been synthesized by dip coating or pressurized coating of the reaction sol-mixture followed by hydrothermal treatment. The permeation mechanism of CO₂ through ZSM-5 membranes was a surface diffusion and the membranes prepared by the pressurized-coating hydrothermal treatment showed a fairly high CO₂/N₂ separation factor of 9.0 and a permeability of 10^–8–10^–7 mol/m² ·s·Pa at room temperature.