ZSM-5 Zeolite Crystals Research Articles

ChemInform Abstract: Dealumination of Large Crystals of Zeolite ZSM-5 by Various Methods.

ChemInform Abstract: Dealumination of Large Crystals of Zeolite ZSM-5 by Various Methods.

Simple method for preparing monoclinic single crystals of zeolite ZSM-5 and analysis of their structure

Abstract A simple method for preparing monoclinic single crystals of ZSM-5 is developed. The ZSM-5 crystals are pressed with a weight of 50 g at an angle of 45° relative to the b and c crystallographic axes of twin ZSM-5 crystals while the temperature is simultaneously increased to 573 K. The temperature is then slowly reduced to 313 K, and these steps are repeated three times. The monoclinic structure of the single crystals of ZSM-5 produced using this method is then determined.

A seed surface crystallization approach for rapid synthesis of submicron ZSM-5 zeolite with controllable crystal size and morphology

A method based on seed surface crystallization (SSC) mechanism has been proposed for facile, rapid, controllable and low-cost synthesis of ZSM-5 zeolite with submicron-size and interesting core–shell structure. The crystallization process of the ZSM-5 zeolite with all sizes can be accomplished within 2.5 h. The morphology and size of zeolite ZSM-5 products can both be facilely adjustable in the domain of 270–1100 nm depending upon the characters of silicalite-1 seeds, while the thickness of aluminum-containing zeolite ZSM-5 shells can be well tuned by changing the amount of silicalite-1 seed. More interestingly, the siliceous silicalite-1 seeds within the obtained zeolite crystals could be selectively removed by alkaline treatment. By systematically tracking the crystallization process combined with mathematical analysis, the crystallization of ZSM-5 zeolites in the reaction system is revealed as SSC mechanism with a size-independent, linear growth process. Their crystal growth rate is calculated as c.a. 0.15 μm h −1 regardless of the seed size and amount.

Template‐Synthesized Porous Silicon Carbide as an Effective Host for Zeolite Catalysts

A facile method has been developed for the fabrication of porous silicon carbide (SiC) by means of sintering a mixture of SiC powder and carbon pellets at a relatively lower temperature, that is, 1450 degrees C, in air. The pore density and the total pore volume of the resulting porous SiC could be tuned by changing the initial SiC/C weight ratio. The structure evolution and the associated property changes during the preparation were examined through X-ray diffraction, scanning electron microscopy, thermogravimetric analysis, (29)Si magic-angle spinning (MAS) NMR spectroscopy, and mercury-intrusion porosimetry analyses. Silica and SiO(x)C(y) ceramics formed in situ during the calcination process acted as binders of the porous SiC grains. The porous SiC can be used as a host for the growth of ZSM-5 zeolite crystals to form the ZSM-5/porous-SiC composite material. After loading another catalytic active component of molybdenum, a novel catalytic material, Mo-ZSM-5/porous-SiC, was obtained, which exhibited improved catalytic activity in the methane dehydroaromatization reaction.

Preparation of Binder-free Utrafine ZSM-5 Zeolite Monoliths by Vapor-phase Transformation Method

Using vapor-phase transformation method,binder-flee monoliths of ultrafine ZSM-5 zeolite crystals were prepared by conversion of the preformed bodies of diatomite and silica sol,which mixed with some zeolite seeding directors.The tiny zeolite crystals and units in the seeding directors inducted the conversion of the alumino-silicalite raw materials into intergrown ZSM-5 zeolites,which resulted in binder-free zeolite monoliths.The prepared materials possess enriched porosity,high surface area,good mechanical strength and tunable acidity which can be verified by the characterization.The products have promising applications in catalyst and adsorbent.

Intergrowth Structure of Zeolite Crystals and Pore Orientation of Individual Subunits Revealed by Electron Backscatter Diffraction/Focused Ion Beam Experiments

The two-component model of coffin-shaped intergrown crystals of zeolite ZSM-5 has been verified by a combination of electron backscattering diffraction and focused ion beam milling (see picture), which respectively revealed the orientation of the channel system in the individual subunits of the crystal and allowed these building blocks to be visualized.

High catalytic cracking activity of Al-MCM-41 type materials prepared from ZSM-5 zeolite crystals and fumed silica

Abstract Two series of mesoporous aluminosilicate materials of the Al-MCM-41 type have been prepared from pre-formed ZSM-5 crystals. The synthesis involved the alkaline attack of the zeolite alone (M(−)/ x samples) or in the presence of an external silicon source (e.g. fumed silica, M(S)/ x samples) followed by a conventional 48 h hydrothermal treatment (100 °C) with cetyltrimethylammonium chloride as the structure directing agent. The effect of sodium hydroxide concentration (0.8 M ≤ x ≤ 3 M) on the properties of the final materials was investigated by powder XRD, nitrogen sorption, pyridine sorption (IRFT), SEM, TEM, MAS and 2D MQ-MAS 27 Al NMR. It was shown that the addition of fumed silica to the synthesis gel results in the improvement of the mesopore network structure. The stability of M(−)/ x and M(S)/ x materials towards boiling water (5 days) as well as their activity in the acid-catalysed transformation of methylcyclohexane at 350 °C were compared to those of ZSM-5, Al-MCM-41 and of a Al-MCM-41//ZSM-5/H mechanical mixture. Because of the existence of a 2-fold pore system with zeolite domains, M(S)/0.8 and M(S)/1.9 composites are more efficient catalysts than amorphous Al-MCM-41 and a mechanical mixture of Al-MCM-41 and ZSM-5. The best combination in terms of hydrothermal stability and catalytic activity was found for the M(S)/1.9 sample.

ZSM-5 zeolite/porous carbon composite: Conventional- and microwave-hydrothermal synthesis from carbonized rice husk

The formation of novel ZSM-5 zeolite/porous carbon composite from carbonized rice husk has been investigated using microwave- and conventional-hydrothermal reaction at 140–160 °C. ZSM-5 zeolite crystals were formed on porous carbonized rice husk by the dissolution and in situ crystallization of silica in the carbonized rice husk. Compared to the conventional-hydrothermal (C-H) formation of ZSM-5 zeolite, the microwave-hydrothermal (M-H) reaction led to increased rate of formation by 3–4 times at 150 °C. In addition M-H reaction yielded small particles of 0.3–5 μm. The surface area of ZSM-5 zeolite (without template)/porous carbon composite is 485.4 m 2/g, and this composite has both micropores and mesopores.

Internal Transport Resistances and their Influence on Diffusion in Zeolites as Traced by Microscopic Measuring Techniques

Interference microscopy (IFM) and FTIR microscopy (IRM) are applied to study intracrystalline concentration profiles in SAPO-5, CrAPO-5 and ZSM-5 zeolite crystals. By using both techniques, the high spatial resolution of interference microscopy is complemented by the ability of FTIR spectroscopy to pinpoint adsorbates by their characteristic IR bands. In this way the experimental results are shown to unveil a number of remarkable deviations in the real structure of zeolite crystals from their textbook patterns. The influence of structural defects on the equilibrium concentration profiles as well as on sorption dynamics of guest molecules is investigated. Owing to the ability to gain direct insight into the influence of surface defects and intracrystalline defects on molecular uptake, the applied techniques give more accurate information on the molecular transport in zeolite crystals than the classical uptake methods.

Dry-Gel Synthesis of Mesoporous MCM-41 Materials with Modified Pore Structure

Abstract For the first time, the dry-gel conversion method was applied for the preparation of MCM-41 materials with partially crystalline framework. The X-ray diffraction pattern of the material obtained after a dry-gel conversion time of 36 h consists of reflections of both long-range ordered MCM-41 material and zeolite ZSM-5. The nitrogen adsorption/desorption isotherm confirms the presence of micropores and uniform mesopores with a narrow pore size distribution. By 29Si MAS NMR spectroscopy it was evidenced that a significant portion of the amorphous MCM-41 framework was transformed into a partially crystalline framework. TEM and SEM images indicate the occurrence of mesopores with disturbed arrangements, but no formation of separate zeolite ZSM-5 crystals occurred. By 1H MAS NMR spectroscopy of the above-mentioned material, the formation of hydroxyl groups with a chemical shift of ca. 4 ppm and a concentration of ca. 0.1 mmol/g was observed. The acid strength of these hydroxyl groups, as studied by adsorption of the 13C MAS NMR probe molecule 13C-2-acetone, was found to agree with that of bridging OH groups in zeolite H-ZSM-5. After conversion times of 48 h and 72 h, an extensive to total transformation of the dry gel into zeolite ZSM-5 occurred.

X-ray mapping as a tool to characterize mixtures of large zeolite ZSM-5 crystals with amorphous impurities

During the synthesis of large crystals of zeolite ZSM-5 in aluminum-rich gels, two phases formed. Instead of large crystals of aluminum-rich zeolite ZSM-5, ZSM-5 crystals with a relatively low aluminum content and amorphous silica–alumina particles crystallized simultaneously. This was demonstrated using X-ray mapping and NMR spectroscopy.

Supramolecular Interactions and Morphology Control in Microwave Synthesis of Nanoporous Materials

Application of a microwave technique to the conventional hydrothermal process is gaining importance, especially, in the synthesis of nanoporous materials. This microwave technique is regarded as a novel synthesis tool because it gives several beneficial advantages such as homogeneous nucleation, rapid synthesis, formation of uniform crystals, and small crystallites, facile morphology control, energy efficiency and so on. Recently, it was found that it offers an efficient way to control the crystal morphology, size and orientation, and even crystalline phase which are required for many emerging applications of nanoporous materials. This review summarizes recent work on the microwave effect, supramolecular interactions and control of crystal morphology upon microwave synthesis of nanoporous materials performed by the present authors. Synthesis and morphology control of nanoporous materials such as ZSM-5, zeolite beta, metallosilicates, AlPO, MCM-41, SBA-15, SBA-16, etc. have been accomplished with microwave irradiation. In particular, the rapid nucleation and crystallization of ZSM-5 zeolite under microwave irradiation made it possible to enable the continuous microwave synthesis, implying a great industrial and technological importance. The formation of nanoporous materials, especially, silicate or aluminosilicate molecular sieves was described on the basis of supramolecular interactions between organic template molecules and silicate species under microwave irradiation. Besides decreasing synthesis time, it was duly demonstrated that the microwave technique provides an effective way to control particle size distribution and macroscopic morphology in the synthesis. Moreover, for the application of these porous materials, microwave-induced nanofabrication of microporous and mesoporous materials is more important than that of simple porous materials.

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Large ZSM-5 zeolite crystals synthesized in fluoride medium show an astonishing activity, stability as well as selectivity towards light olefins in the Methanol-To-Olefins (MTO) reaction. By proper control of the synthesis parameters, ZSM-5 single crystals of unprecedented high quality are produced. The absence of usually uncontrollable variables such as structural defects, external non selective surface acid sites and extra-framework aluminium (EFAl) species was evidenced by SEM, HRTEM, CO-FTIR, 27Al and 19F MAS-NMR, Rietveld structure refinement and N2- and Ar-gas sorption measurements. Interestingly, diffusivity evaluation of different probe molecules (toluene, benzene and neopentane) has been carried out with PFG-NMR, allowing casting light on an interesting structure-diffusion-activity relationship. A “levitation” effect could be experimentally demonstrated and its impact on catalysis is highlighted in a rationalization attempt: Maxwell-Boltzmann based diffusion models properly predict product distributions for this counter-intuitively outstanding Methanol-To-Propylene (MTP) catalyst.

Effect of surface etching of ZSM-5 zeolite crystals on the rate of n-hexane sorption

Samples of the as-synthesized zeolite ZSM-5 were purified by etching the surface of crystals with an aqueous-acetone solution of hydrogen fluoride. The solution appeared to be very useful for removing by-products from the samples without affecting morphology or size of the crystals. Measurements of n-hexane uptake with a gravimetric method were performed to verify the effect of the sample treatment. The rates of the n-hexane sorption for the etched samples were up to two orders of magnitude higher than that for the non-etched one. The corrected diffusion coefficient of n-hexane was found to be only several times lower than those obtained with the methods of PFG NMR and FR as reported in the literature.

FTIR study of the interaction of some branched aliphatic molecules with the external and internal sites of H-ZSM5 zeolite

The interaction of the branched molecules pivalonitrile (2,2-dimethylpropionitrile, PN), methyl tert-butyl ether (MTBE), tert-butyl alcohol (TBA), and 2,2-dimethylbutane (DMB), as well as of methylcyclohexane, benzonitrile and benzene over different H-ZSM5 zeolite samples was investigated by FTIR spectroscopy. PN does not enter the cavities of MFI structures at room temperature and 0–25 Torr pressure, at least when Al content is quite high. Also, TBA and MTBE do not enter the cavities under these conditions although isobutene produced by their decomposition, easily enters and polymerizes in the internal acidic sites. Conversely, DMB slowly enters the MFI zeolite cavities. On the other hand, the co-presence of benzene allows PN to enter the cavities and to interact with the internal sites. This shows that the access of single molecules can be influenced by other molecules in a mixture. The experiments also confirmed that two kinds of terminal silanol groups, indistinguishable from the point of view of the OH stretching band but differing in their Bronsted acidity, are located at the external surface of the ZSM5 zeolite crystals. Additionally, it was established that the external ZSM5 zeolite surface, even for low Al content samples, contains two types of Lewis acid sites. Conversely, no evidence was found for the presence of bridging Si–(OH)–Al sites at the external surface of the ZSM5 zeolite samples. This further corroborates the proposal that the bridging “ zeolitic” Bronsted acid sites are actually formed as a consequence of the porous structure of the zeolites and do not simply arise from the substitution of Al for silicon in a silica framework.

Separation of acetic acid-water mixtures by pervaporation through silicalite membrane

Polycrystalline silicalite membranes were prepared on two kinds of porous supports by hydrothermal synthesis. The pervaporation performance of the silicalite membrane obtained was investigated using an acetic acid-water mixture as a feed. The silicalite membrane on the sintered stainless steel support selectively permeates acetic acid in the concentration of the feed acetic acid in the region of 5 to 40 vol%. However, the membrane on the porous alumina support showed no separation for the aqueous acetic acid solution. From the fact that the top layer of the membrane on the alumina support was not composed of pure silicalite but ZSM-5 zeolite crystals, which contained Brønsted acidic sites (Si(OH)Al) in the framework, it was suggested that the acidic sites associated with the framework aluminums play an important role in the separation of the acetic acid-water mixture. A long-term test of the pervaporation was also carried out to clarify the stability of the membrane.

Effect of aging temperature of alkaline aluminosilicate and silicate gels on the kinetics of crystallization of zeolite ZSM-5 and silicalite-1

The temperature dependences of nucleation of zeolite ZSM-5 and silicalite-I pass through extrema in the range from 110 °C to 120 °C. Despite having the same crystalline structures, zeolite ZSM-5 has a maximum and silicalite-1 has a minimum of nucleation in the temperature range mentioned. A method for preparation of single crystals of silicalite-1 up to 150 μm in size has been developed. The comparison of the curves of the temperature dependences of nucleation for zeolites Na-X, Na-A, and ZSM-5 shows that the positions of the maxima on these curves are determined by the type of the crystal structure of zeolite.

Silica-supported small crystals of ZSM-5 zeolite

The small crystals of the ZSM-5 zeolite were synthesized and stabilized inside pores of granulated silica gel support. The pores were filled with aqueous NaOH-Na 2Al 2O 4-TPABr. During zeolite synthesis the reactants reacted with the silica on the pores wall to form small crystals. 37 wt.-% of ZSM-5 crystals were produced. TEM measurements showed large crystals, 3-5 μm in diameter, on the outer surface of silica grains; medium crystals, 0.5-2 μm, inside the macropores and small crystals, 0.02-0.035 μm, inside the mesopores. The catalytic activity of the supported zeolite H-ZSM-5/SiO 2 measured by cracking of isobutane at 410-550°C was 2.2-7.8 times higher than for standard zeolite H-ZSM-5 with the same SiO 2/Al 2O 3 ratio and sodium content depending on the testing temperature.

Synthesis of large crystals of ZSM-5 zeolite from a smectite-type clay material

Large crystals of ZSM-5 zeolite (40×22×11 μm) were synthesized using a heat-treated clay carbonate system as the starting material. The influence of different treatments and sources of organic cations on the morphology and size of the crystals was examined. The samples obtained consist of pure crystalline ZSM-5 phases. No contamination with other zeolites or species was noticed after X-ray and microscopic examinations.

Shape-Selectivity of Large and Small Crystals of Zeolite ZSM-5

The authors have shown recently that the isomer distribution in the xylene resulting from the methylation of toluene by methanol on zeolite H-ZSM-5 is independent of conversion as long as reactant methanol is not exhausted, and that it depends inter alia on the size of the unmodified zeolite crystals. The interpretation of the observed isomer distribution as resulting from intracrystalline reaction and shape-selective diffusion led to the conclusion that only that para isomer should be generated on sufficiently large crystals of the unmodified zeolite, whereas with extremely small crystals (in the hypothetical limit consisting of only one unit cell) an isomer distribution should be observed which is not influenced by subsequent diffusion and isomerization of the primary product of alkylation. The authors now report briefly the results of experiments which have been conducted to test these conclusions. 3 refs., 2 figs., 3 tabs.