Nanocrystalline ZSM-5 Research Articles

Molecular and Meso- and Macroscopic Properties of Hierarchical Nanocrystalline ZSM-5 Zeolite Prepared by Seed Silanization

Surface-passivating silanization of protozeolitic units has been shown to be an effective strategy for the preparation of ZSM-5 nanocrystals, showing a controlled aggregation degree and a hierarchical porosity. ZSM-5 zeolite materials are thus obtained with adjustable and relatively uniform mesoporosities that have a strong influence on resulting macroscopic reaction properties, especially for macromolecular reagents. The mean sizes of the nanounits and, therefore, the textural and accessibility of these materials can be varied by changing the precrystallization conditions and the concentration of the seed-silanization agent. In addition to conventional characterization techniques, solid-state two-dimensional (2D) nuclear magnetic resonance (NMR) spectroscopy measurements and the application of the NLDFT model to the argon adsorption isotherms have allowed both the local and the mesoscopic compositions, as well as the structures of the hierarchically porous ZSM-5 materials, to be established. The resultin...

Acylation of anisole with acetic anhydride using ZSM-5 catalysts: Effect of ZSM-5 particle size in the nanoscale range

Abstract The current paper reports on the activity of nanocrystalline ZSM-5 in the liquid phase acylation of anisole with acetic anhydride. Under the optimum reaction conditions the performance of ZSM-5 catalysts having different particle sizes were tested. It was found that smaller size crystals exhibit highest activity, which is due to the increased external surface of smaller crystal.

Effect of the organic moiety nature on the synthesis of hierarchical ZSM-5 from silanized protozeolitic units

The synthesis of hierarchical ZSM-5 from silanized protozeolitic units has been accomplished following a three step procedure comprising gel precrystallization at 90 °C, grafting of the seed silanization agent (SSA) and hydrothermal crystallization at 170 °C. The following four organosilanes were tested and investigated as SSAs: 3-aminopropyltrimethoxysilane (APTMS), isobutyltriethoxysilane (IBTES), phenylaminopropyltrimethoxysilane (PHAPTMS) and octadecyltrimethoxysilane (ODTMS). The last agent became anchored to the zeolite seeds to a rather reduced extent (1.3%) and the obtained material presented properties very similar to those of a reference nanocrystalline ZSM-5 (Sext ∼ 100 m2 g−1, SBET ∼ 450 m2 g−1, crystallite size = 20–80 nm). In contrast, silanization of the protozeolitic units with APTMS or IBTES led to ZSM-5 samples having large surface areas (Sext ∼ 200 m2 g−1, SBET ∼ 500 m2 g−1) and exhibiting mesopores (Dp ∼ 8 nm). These materials are formed by 300–400 nm particles consisting of aggregates of ZSM-5 nanounits (10–20 nm). Seed silanization with PHAPTMS caused the greatest enhancement in surface area (SBET = 573 m2 g−1, Sext = 225 m2 g−1), the material so obtained being formed by the smallest nanounits (5–10 nm) within the aggregates. Moreover, this sample presented a hierarchical porosity with pores around 2.0–3.0 nm in addition to those generated by the MFI channels. The strong changes induced in the ZSM-5 crystal size and properties when using PHAPTMS as SSA have been attributed to this agent being very effective in both anchoring onto the protozeolitic units and perturbing the subsequent crystal growth step.

Synthesis and Characterization of Nanosized ZSM-5 Zeolite with Mesoporous Carbon Nanotube as Templates

Preparation of nanocrystalline ZSM-5 has been proven possible with carbon matrix by using confined space synthesis. In combination with the hydrothermal method, by the use of a mesoporous carbon nanotube as the inert support material, ZSM-5 with SiO2/Al2O3 ratios of 100 was successfully synthesized with an average crystal size of 27 nm. Their structures were preliminarily characterized by x-ray power diffraction (XRPD) and infrared spectroscopy (IR), and scanning electron microscopy (SEM), respectively. At the same time, we put forward the mechanism about the confined space synthesis in terms of the results.

Effect of Crystal Size on Physico-Chemical Properties of ZSM-5

In an attempt to produce a ZSM-5 type material with ample mesoporosity as well as micro porosity and improved external surface area, the nanocrystalline ZSM-5 (NZ) was synthesized. The material NZ was characterized by a variety of physical techniques and its properties were compared with that of micrometer range ZSM-5 (HZ). Improvement in pore volume along with mesoporosity is observed in NZ. Though there is an improvement in micropore volume, the mesopores with diameter > 100 A have mainly contributed to the improved pore volume in NZ. The organic template used in NZ synthesis decomposed at two different temperatures in DTA/TGA studies suggesting the additional role of template in mesopore formation in NZ. The NZ exhibited relatively lower acidity, but has greater amount of strong acidity when compared to HZ.

Simplified synthesis of isomorphously nickel substituted ZSM-5

Isomorphously nickel-substituted nano-crystalline ZSM-5 is synthesized in the absence of acidic aqueous fluoride medium incorporating simple and low-cost metal inorganic salt precursor NiCl2.6H2O instead of large organic cationic salt like bis (tetraethyl ammonium) tetrachloronickelate (II) with less water quantity to minimize the synthesis waste. PXRD, FT-IR, TG/DTG, XPS, UV–Vis DRS, SEM, TEM, ICP and N2 adsorption-desorption techniques were used to confirm the presence of nano-crystalline material having a MFI structure and heteroatom substitution. The unit cell dimensions increase with increasing levels of nickel substitution. The crystallite size of as synthesized samples was in the range of 60–75 nm, which increased to 60–160 nm after calcination at 550°C. Percentage crystallinity and crystallite size increases with increasing nickel substitution level up to 0.17 mol and beyond that the material becomes amorphous.

Carbon nanotube templated growth of nano-crystalline ZSM-5 and NaY zeolites

MFI-type(ZSM-5) and FAU-type(NaY) zeolites nanocrystals have been synthesized through crystallization of gel in mesoporous system of carbon nanotubes(CNTS) with an internal diameter of 20∼30 nm. Investigation by using X-ray diffraction (XRD), Fourier transform infrared (FT-IR), transmission electron microscope (TEM) and scanning electron microscope (SEM) shows that the nanocrystals possess the typical nanosized zeolites' structural characteristic which is different from those of microsized zeolites.

Synthesis, characterization, and adsorption properties of nanocrystalline ZSM-5.

Nanocrystalline ZSM-5 with a Si/Al ratio of 20 was synthesized using clear solutions and a hydrothermal synthesis procedure. The resulting ZSM-5 materials were characterized by powder X-ray diffraction, scanning electron microscopy (SEM), nitrogen adsorption isotherms, solid-state nuclear magnetic resonance, and toluene adsorption. A commercial ZSM-5 sample was similarly characterized for comparison with the synthesized materials. The particle sizes of the synthesized ZSM-5 samples were calculated using the measured external surface areas and were determined to be 15 and 60 nm. SEM images indicated that the ZSM-5 samples consist of agglomerated and possibly intergrown particles. Toluene adsorption measurements showed that the ZSM-5 sample with a particle size of 15 nm adsorbed approximately 50% more toluene than the other ZSM-5 samples, most likely due to the adsorption of toluene on the external surface. For the toluene adsorbed on the internal zeolite surface, approximately one toluene molecule was adsorbed per channel intersection for each of the ZSM-5 samples.

Comparing synthesis routes to nano-crystalline zeolite ZSM-5

Aiming at the identification of a suitable synthesis route to zeolite ZSM-5 with acidic properties and crystal diameters of about 100 nm or less, we have evaluated four different preparation methods. Three of these have been adopted from the literature, and one was developed by ourselves. The products, all of which synthesised from mixtures with a fixed Si/Al ratio of 60, were inspected by means of X-ray diffraction, scanning electron microscopy, laser-Doppler anemometry and temperature-programmed desorption of ammonia. Two methods turned out to be successful and well reproducible. The first one comprises hydrothermal crystallisation from clear solutions under autogenous pressure and has been described previously by van Grieken et al. The other approach is based on the use of colloidal silicalite-1 seed crystals in an open-vessel crystallisation at atmospheric pressure. The crystal size distribution and the Si/Al ratio of the products can be well controlled. The products from both synthesis types could be transferred into the H-form by conventional means without causing collapse of the crystal structure.

Anomalous crystallization mechanism in the synthesis of nanocrystalline ZSM-5

Nanocrystalline ZSM-5 (crystal size in the range 10–100 nm) has been synthesized in only 24 h by hydrothermal crystallization of clear supersaturated homogeneous synthesis mixtures. The influence of the aluminum source, aging time, pH, water content and presence of alkaline cations on the properties of the final nanocrystalline zeolite have been studied. Samples with different degrees of crystallinity have been prepared and characterized by X-ray fluorescence, X-ray diffraction, solid state MAS-NMR, transmission electron microscopy and nitrogen adsorption (BET) in order to investigate the crystallization mechanism. The process involves the formation of an initial amorphous solid prior to the zeolite crystallization, which is gradually transformed into nanocrystalline ZSM-5 through solid–solid transformations. Additionally, the conventional formation of ZSM-5 from crystalline nuclei generated in the remaining liquid solution is observed after 48 h.

Effects of cation exchange on the acidity and the catalytic activity of type a zeolite

Nanocrystalline ZSM-5 (∼20–50 nm) has been tested as a catalyst in the liquid phase rearrangement of 1,2-epoxyoctane, 2-methyl-2,3-epoxybutane and isophorone oxide together with a microcrystalline ZSM-5 sample (∼5 μm) employed as a reference. Whereas this last material shows a low activity for the three epoxide rearrangement reactions considered, nanocrystalline ZSM-5 leads in all cases to high epoxide conversion. This different catalytic performance is attributed to the differences in the accessibility of the reactant molecules to the catalyst active sites. The smallest crystal size of the nanocrystalline zeolite contributes favourably to both the formation of a high external surface area, with no steric constraints, and to a faster intracrystalline diffusion of the epoxide molecules towards the acid sites located within the zeolite micropores. When sterically hindered epoxides are tested, the catalytic rearrangement occurs mainly on the external surface of the nanocrystalline zeolite. Moreover, it is remarkable that for the three epoxides investigated the high catalytic activity observed over nanocrystalline ZSM-5 is accompanied by the attainment of high selectivities, typically around or over 80%, towards products with commercial applications.