Mesoporous silica with short-range MFI structure

A new type of mesoporous silica has been prepared which showed 780 m²/g of BET surface area and 0.6 ml/g of primary mesopores narrowly distributed around 4.2 nm. More importantly however, is that it showed short-range zeolite crystallinity as demonstrated by FTIR and XRD analysis, and hydrophobicity as demonstrated by water and n-hexane adsorption.

This material was synthesized via a dual-template, three-step hydrothermal–flocculation–steaming synthesis procedure developed by us recently. Briefly, MFI nanoprecursors (NPs) were first prepared by a low-temperature hydrothermal step using TPAOH as template for zeolite structure, and then flocculated using a surfactant that served as the template for the mesopores. The collected NPs are mesoporous silica exhibiting short-range MFI domains when directly calcined. However, the steaming step promoted the crystallization of the NPs and created uniform mesopores. It was found that almost every detail in these procedures affected the properties of the final product. The most important variables, however, were identified as the duration the flocculants were kept in contact with the liquid phase, and the humidity under which the steaming was conducted. By properly adjusting the procedures, the said mesoporous silica, as well as nanocrystals having high external surface area, could be produced at will.     

Ethylene epoxidation on Ag catalysts supported on non-porous, microporous and mesoporous silicates

The ethylene epoxidation activity of Ag catalysts supported on non-porous SiO₂, microporous silicalite zeolite and mesoporous MCM-41 and HMS silicates was investigated in the present study in comparison to conventional low surface area -Al₂O₃ based catalysts. The MCM-41 and HMS based catalysts exhibited similar ethylene conversion activity and ethylene oxide (EO) selectivity with the SiO₂ and -Al₂O₃ based catalysts at relatively lower temperatures (up to 230 °C), whereas their activity and selectivity decreased significantly at higher temperatures (≥300 °C). The silicalite based catalyst was highly active for a wide temperature range, similar to the SiO₂ and -Al₂O₃ based catalysts, but it was the less selective amongst all catalysts tested. High loadings of Ag particles (up to ca. 40 wt.%) with medium crystallites size (20–55 nm) could be achieved on the mesoporous materials resulting in very active epoxidation catalysts. The HMS-type silicate with the 3D network of wormhole-like framework mesopores (with average diameter of 3.5 nm), in combination with a high-textural (interparticle) porosity, appeared to be the most promising mesoporous support.     

Diffusion of ammonia in silicalite studied by QENS and PFG NMR

Ammonia diffusivities in silicalite have been studied by quasi-elastic neutron scattering (QENS) and pulsed-field gradient (PFG) NMR for temperatures between 200 and 500 K and loadings from 1.5 up to 4.3 molecules per unit cell. The diffusion coefficients obtained by both techniques increase with increasing ammonia concentration. The QENS diffusivities refer to only a certain fraction of molecules, because during the time scale of the measurement the other part of molecules is essentially immobile, in interaction with silanol groups. During the much larger time scale of the PFG NMR experiment, ammonia molecules assume both states of mobility, leading to an average diffusivity which is smaller than the diffusivity of the mobile molecules recorded by QENS. The difference between the diffusivities derived from both techniques decreases when the proportion of immobile molecules is taken into account. The residence time of ammonia in interaction with silanol groups is about two orders of magnitude longer than with oxygen atoms.     

EVALUATION OF REACTION KINETICS FOR THE DECOMPOSITION OF TETRAPROPYLAMMONIUM CATIONS IN SILICALITE

Thermogravimetric analysis was used to study the decomposition of tetrapropylammonium cations intercalated in silicalite. The decomposition was modelled using temperature-programmed desorption theory (TPD) assuming that the decomposition reaction was first-order in TPA⁺ concentration. The temperature at which the maximum decomposition rate occurred was determined as a function of heating rate. Using these data, values for the activation energy and the reaction rate constant were calculated to be 5l.4kcal/mol and 3.0 × 10¹⁵ min^-1, respectively. It was shown that the activation energy was independent of sample weight, indicating that macropore mass transfer limitations were insignificant. It was also determined that activation energy was independent of crystal size indicating that micropore diffusion limitations were also insignificant.     

Is 1-hexene epoxidation in TS-1 diffusion limited in different solvents?

PFG NMR diffusion measurements were carried out to determine the effect of solvent on intracrystalline reactant diffusivities and on 1-hexene epoxidation rates in TS-1 catalyst. Using n-hexane in silicalite as a mimic for the TS-1 system, the self-diffusivity of n-hexane in silicalite was found to be 24% higher in methanol solvent than in acetonitrile solvent and 45% higher than in acetone solvent. The presence of trace Al did not affect n-hexane diffusivity. Based on analysis of the Weisz modulus for a slab morphology, the 1-hexene epoxidation reaction in TS-1 was found to be diffusion limited only if the crystal size is at least 38 μm in the methanol system.     

AIR PURIFICATION AND VAPOR RECOVERY BY PRESSURE SWING ADSORPTION: A COMPARISON OF SILICALITE AND ACTIVATED CARBON

Air purification and vapor recovery by pressure swing adsorption (PSA) were experimentally investigated using the silicalite-DMMP-air system. The results from several cyclic steady-state PSA runs were compared at constant throughput with those from a previous study on the BPL activated carbon-DMMP-air system. The performance of BPL activated carbon was superior to that of silicalite because it demonstrated complete cleanup of the product effluent when starting from a saturated column, whereas, at similar process conditions, silicalite was able to cleanup only a portion of the product effluent. Nevertheless, both silicalite and BPL activated carbon respectively demonstrated enrichments (Y_e/Y_f) of 12 and 15 using only moderate vacuum. However, there were significant differences in the shapes of the cyclic steady-state product and exhaust profiles which were attributed to differences in the 1) mass transfer rates, 2) adsorption capacities, or 3) possibly shapes of the adsorption isotherms.     

Atomistic simulation of hydrocarbon diffusion in silicalite

Molecular dynamics simulation using a burchart-DREIDING force field has been employed to determine the self-diffusion coefficients of methane, ethane, propane, n-butane, isobutane, benzene, and pyridine in silicalite at 300 K. In the case of the alkanes, Einstein diffusion was observed early in a 500-ps simulation. Values of the diffusion coefficient for the alkanes ranged from about 9 × 10⁻⁴ cm² s⁻¹ for methane to 2 × 10⁻⁶ cm² s⁻¹ for isobutane at a loading of 0.5 molecule per unit cell (mpuc) or 2 molecules per simulation box. For the alkanes, there is good agreement between the results of simulation and experimental values of the self-diffusion coefficient obtained by microscopic techniques such as pulsed-field gradient NMR measurements. In the case of benzene and pyridine, anomalous diffusion was observed for time scales up to 9 ns and apparent diffusion coefficients (range of 10⁻¹¹ to 10⁻⁸ cm² s⁻¹) calculated form the Einstein relation increased with increased loading (up to 3.75 mpuc) and, as shown for pyridine, with increasing temperature. In the case of benzene where experimental data was available, the apparent diffusion coefficients calculated at high loadings were within an order of magnitude of experimental values.

Heats of adsorption were calculated by a canonical Monte Carlo method. For the alkanes (methane, ethane, propane, and n-butane), the magnitude of the heats of adsorption linearly increased with the number of alkane carbons in qualitative and quantitative agreement with experimental data. Heats of adsorption calculated for isobutane and benzene also agreed well with experimental values. No experimental values were available for pyridine for comparison.     

Synthesis of ZSM-5 zeolite and silicalite from rice husk ash

Local rice husk was precleaned and properly heat treated to produce high purity amorphous SiO₂ for use in the synthesis of ZSM-5 zeolite and silicalite by hydrothermal treatment (150 °C) of the precursor gels (pH 11) under autogenous pressure in a short reaction time (4–24 h). A wide range of SiO₂/Al₂O₃ molar ratios (30–2075) and a small template content were employed to fully exploit the potential of rice husk ash (RHA). The mineralogical phases, morphology, specific surface area and pore volume of the synthesized products were investigated by XRD, FT-IR, SEM and BET analyses, respectively. Under the employed conditions, it was found that the gels with a low range of SiO₂/Al₂O₃ molar ratios (<80) produced an amorphous phase to poorly crystalline ZSM-5 zeolite; those with a medium range (80–200) favored well crystalline ZSM-5 zeolite production with a large surface area; whilst those with a high range of SiO₂/Al₂O₃ molar ratios (>200) yielded silicalite. The increase in Na₂O content, which was derived from the addition of NaAlO₂ to attain the desired SiO₂/Al₂O₃ molar ratio of the gel, did not significantly enhance the crystallization rate, crystallinity, or yield of products. On the contrary, these properties were greatly affected by the increase in the SiO₂/Al₂O₃ molar ratio.     

Development of etching processes for the micropatterning of silicalite films

Silicalite (SIL-1) polycrystalline films have been synthesized on Si wafers and then micropatterned using both dry (ion milling, reactive ion etching) and wet etching processes after deploying a resin mask on the film top surfaces. The advantages and characteristics of the different etching processes are discussed and related to the orientation of the zeolite crystals in the films. Finally, the etching processes developed can also be used to release freestanding zeolite structures. This is demonstrated for the fabrication of bulk zeolite microcantilevers with a high aspect ratio.     

Synthesis of MFI-type zeolites under atmospheric pressure

Silicalite and highly silicious ZSM-5 were synthesized using two reaction mixtures with different crystal growth environments, a dispersed low density mixture and a separated high density mixture, at 93 ±3 ‡C under the atmospheric pressure. Nucleation behavior and the transformation process of two mixtures were investigated utilizing various analytical techniques such as XRD, FT-IR, TGA, SEM, and pH measurement. During the induction period, the same intermediate phase was observed in both mixtures. The presence of this phase was found to be dependent on the degree of dilution of the reaction mixture. After the induction period, a sharp increase in both the degree of crystallization and the pH of the reaction mixture was detected. This indicates that the pH change in the reaction mixture is closely related to the crystallization process. From these observations, a crystallization mechanism is proposed on the basis of the appearance of stable silicate species and the role of OH^- ions during the induction period. According to this mechanism, MFI-type zeolite grows by condensation reaction among the stable silicate species formed during the induction period.