Silicalite-1 Precursor Research Articles

Preparation of Ti-containing mesoporous zeolite using two-step hydrothermal synthesis method

Novel mesostructured titanosilicates (Ti-MST) have been successfully assembled from preformed silicalite-1 precursors using two-step hydrothermal synthesis method. The samples were characterized by XRD, FT-IR, UV–vis and N2 physisorption. The results show that Ti-MST zeolites are pure mesoporous phase possessing zeolitic secondary building units (SBU) in the pore walls. UV–vis confirms the existence of tetrahedral coordinated titanium. N2 physisorption exhibits that the pore diameter and BET surface area of Ti-MST-8 are 2.8nm and 1213m2/g. Ti-MST displays high hydrothermal stability in boiling water as compared with Ti-MCM-41 due to the incorporation of SBU fragments in the mesopore walls.

A systematic investigation of silicalite-1 precursor mixtures with varying degrees of dilution

The effect of the composition on conductivity, pH, and particle size distribution (PSD) in precursor mixtures of silicalite-1 was studied in this work. Mixtures of TPAOH, TEOS, and water spanning a broad composition range were analyzed. Conductivity and pH measurements reported are consistent with previous investigations about the conductivity and pH behavior above and below the critical aggregation concentration (CAC). Dynamic light scattering (DLS) measurements were performed to determine the PSD. ‘Dilute’ mixtures (H2O/TEOS >65) showed trimodal PSDs with nanoparticles of ∼11nm in diameter, small aggregates of ∼340nm in diameter, and aggregates larger than 1μm. The distribution of nanoparticles and aggregates changed with the mixture composition as the intensity-weighted PSDs showed more aggregates in mixtures with higher water content. However, a comparison between intensity-, volume-, and number-weighted PSDs showed that the amount of aggregates present is smaller than the amount of nanoparticles. It was also observed that TPAOH concentration slightly affected the nanoparticle size, which decreased at high TPAOH concentration. On the other hand, concentrated mixtures did not show a clear dependence of PSD or nanoparticle size on the mixture composition. Interestingly, and in contrast with ‘dilute’ mixtures that only showed one population of nanoparticles, two populations of nanoparticles of ∼2nm and ∼30nm were observed in the concentrated mixtures. The information presented in this work attempts to provide a better understanding of silicalite-1 precursors and shows the effect of the composition on the phase behavior of these synthesis mixtures. Thus, this work should help to unify previous findings on nucleation and growth among silicalite-1 mixtures with different compositions.

In Situ PFG NMR of Silicalite-1 Synthesis Mixtures

The interactions between tetrapropylammonium cations and the silica nanoparticles present in silicalite-1 precursor solutions were characterized at 70 °C using in situ pulsed field gradient (PFG) NMR experiments. In situ PFG NMR results show that the organocation diffusion coefficients at 70 °C are larger than those at 25 °C. Although the adsorption isotherms calculated from the diffusion data present similar trends at both temperatures, the adsorption isotherms at 70 °C could not be accurately described using a Lineweaver–Burk analysis. Therefore, the Freundlich equation was used to fit the adsorption data and Gibbs free energies between 6 and 15 kJ/mol were obtained. Time evolution studies show that organocation diffusion coefficients increased with the time, reached a maximum at about 6 days, and trended to a plateau towards the end of the synthesis. This maximum in the diffusion coefficient of the organocation coincides in time with the exothermic–endothermic transition that is known to occur near the...

Fast synthesis and morphology control of silicalite-1 in the presence of polyvinyl alcohol

Silicalite-1 crystals with various morphologies were prepared by addition of polyvinyl alcohol (PVA) in silicalite-1 synthesis solution at 180 °C for 6–40 h. The samples were characterized by X-ray diffraction, scanning electron microscopy, FT-IR, thermogravimetric analysis and N2 sorption. PVA in the synthesis solution promoted the fast crystallization of silicalite-1 within 6 h due to its adsorption on the surface of silicalite-1 precursors and the hydrogen bonding effect. The addition of PVA would produce silicalite-1 without aggregation because PVA served as temporary physical barriers. However, more PVA in the system (weight ratio of PVA/SiO2 = 1.75) slowed down the nucleation rate of silicalite-1 as the high viscosity of PAV hindered the selectivity between PVA and crystal facet. PVA also changed the surface energy of silicalite-1 precursors, and resulted in silicalite-1 crystals with sizes from 6 × 2 × 0.6 to 15 × 10 × 3 μm3. Silicalite-1 crystals exhibited coffin-, plate-, and rod-like structure when different weight ratios of PVA/SiO2 were applied, and the silicalite-1 crystals had high BET surface areas of 485–513 m2/g.

Micrometer-scale macroporous silica–alumina composites with spheric and lathy MFI-type crystals via seed-induced in-situ and layer-by-layer synthetic methods

Macroporous silica–alumina composites with continuous micrometer-scale pores were synthesized by seed-induced vapor-phase transport treatment. The parent silica–alumina monolith was employed as the silica and alumina source as well as the template for the three-dimensional interconnected structure. The silicalite-1 precursor solution with dispersed zeolite seeds was drawn into the pore channels of the parent monolith for modification by in-situ and layer-by-layer methods before the hydrothermal crystallization, which was believed to play a significant inducing role in the zeolite growing process. The composites synthesized are expected to display high catalytic performance on the bulky molecules since they incorporate the strong acidic advantages of the microporous zeolite and the macroporous pathways of the monolithic skeletons for mass transport.

Multi-level Modeling of Silica–Template Interactions During Initial Stages of Zeolite Synthesis

Zeolite synthesis is driven by structure-directing agents, such as tetrapropyl ammonium ions (TPA+) for Silicalite-1 and ZSM-5. However, the guiding role of these organic templates in the complex assembly to highly ordered frameworks remains unclear, limiting the prospects for advanced material synthesis. In this work, both static ab initio and dynamic classical modeling techniques are employed to provide insight into the interactions between TPA+ and Silicalite-1 precursors. We find that as soon as the typical straight 10-ring channel of Silicalite-1 or ZSM-5 is formed from smaller oligomers, the TPA+ template is partially squeezed out of the resulting cavity. Partial retention of the template in the cavity is, however, indispensable to prevent collapse of the channel and subsequent hydrolysis.

Quantitative Three-Dimensional Modeling of Zeotile Through Discrete Electron Tomography

Discrete electron tomography is a new approach for three-dimensional reconstruction of nanoscale objects. The technique exploits prior knowledge of the object to be reconstructed, which results in an improvement of the quality of the reconstructions. Through the combination of conventional transmission electron microscopy and discrete electron tomography with a model-based approach, quantitative structure determination becomes possible. In the present work, this approach is used to unravel the building scheme of Zeotile-4, a silica material with two levels of structural order. The layer sequence of slab-shaped building units could be identified. Successive layers were found to be related by a rotation of 120 degrees, resulting in a hexagonal space group. The Zeotile-4 material is a demonstration of the concept of successive structuring of silica at two levels. At the first level, the colloid chemical properties of Silicalite-1 precursors are exploited to create building units with a slablike geometry. At the second level, the slablike units are tiled using a triblock copolymer to serve as a mesoscale structuring agent.

Mesoporous material formed by acidic hydrothermal assembly of silicalite-1 precursor nanoparticles in the absence of meso-templates

Silicalite-1 precursor nanoparticle solutions are prepared from a mixture of TPAOH, H 2O and TEOS. Acidification of the clear nanoparticle solution and a consequent hydrothermal treatment results in the formation of a large pore mesoporous material. The use of meso-templates for the formation of the mesopores is no longer required. These materials will be denoted mesosil. The porous characteristics of mesosil materials can be adjusted by controlling synthesis time, temperature and pH. The maximum surface area and pore volume are up to 600 m 2/g and 1.35 cm 3/g, respectively. The porosity in these materials originates from the interparticle voids between the nanoparticles (size approximately 5 nm) in contrast to the framework porosity of M41S, SBA-15, and similar materials in which mesopores are formed through a templated synthesis route. The material is characterized by N 2-sorption, TEM, FT-Raman, DRIFT and TGA measurements.

A novel template-free sol–gel synthesis of silica materials with mesoporous structures and zeolitic walls

Hierarchical porous materials with zeolite structures show great promise in catalysis due to combining the advantages of zeolites and mesoporous materials. Here a novel template-free sol–gel method is developed to synthesize hierarchical porous silica materials. This method involves solvothermal recrystallization of the xerogel converted from uniform silicalite-1 precursor sol via vacuum drying process. The zeolite sol and the solid samples were characterized by laser light scattering, XRD, N2 adsorption/desorption isotherm, FTIR, SEM, TEM and thermal analysis technologies. The results show that we obtain two novel materials with different mesoporous structures and silicalite-1 walls by using different recrystallization media, one of which has irregular arrays of mesopores, the other possesses 3D wormhole mesoporous structure. We speculate that formation of different mesoporous structures results from different nucleation and growth process of materials

Bi-phase MOR/MFI-type zeolite core–shell composite

Core–shell zeolite composites possessing a mordenite core and a shell of intergrown silicalite-1 crystals were synthesized. A classical hydrothermal treatment of mordenite crystals with a silicalite-1 precursor mixture did not result in the formation of an extended silicalite-1 layer on the mordenite surface. The incompatibility between the core crystals and the shell precursor was circumvented by adsorption of nanoseeds on the core surface, which upon a subsequent hydrothermal treatment induced the formation of a well intergrown shell. The kinetics of shell growth was studied at 150 and 200 °C and the optimum conditions further used for the preparation of composites subjected to XRD, SEM, TG/DTA, EDS and X-ray fluorescence analyses. The integrity of the shell layer was tested by N 2 adsorption measurements on materials comprising Na- and Na,H-mordenite and a non-calcined tetrapropylammonium (TPA)-containing shell, the latter being non-permeable for the N 2 molecules. These measurements showed that 96–98% of the mordenite crystals were covered with a defect-free TPA-silicalite-1 shell after a single hydrothermal treatment. A characteristic feature of the obtained composites is the relatively large single crystal core and the very thin polycrystalline shell. Thus, the relatively large active part of the composite is covered by a tiny shell that ensures fast kinetics.

Ultra-thin zeolite films prepared by spin-coating Silicalite-1 precursor solutions

A procedure has been developed to prepare ultra-thin zeolite films supported on Si(1 0 0). Films were prepared by spin-coating a solution of Silicalite-1 zeolite precursors diluted in ethanol, followed by hydration in water vapour and heating to 60 °C. High resolution transmission electron microscopy analysis of sample cross-sections shows that the surfaces are both smooth and continuous, and films of approximately 2 and 15 nm thickness were prepared by varying the precursor dilution factor. Electron diffraction analysis shows that the 15 nm film is an amorphous arrangement of Silicalite-1 precursors. Atomic force microscopy measurements confirm that the surface is smooth over a range of several microns.

Silicalite-1 Hollow Spheres and Bodies with a Regular System of Macrocavities

Silicalite-1 macrostructures in the form of hollow silicalite-1 spheres and bodies with a regular system of macrocavities were prepared using two types of polystyrene beads as shape-directing macrotemplates. The incompatibility between the organic support and the inorganic shell was circumscribed by using negatively charged substrates, reverse of their surface charge, and adsorption of zeolite nanoseeds. These pretreated microbeads were subsequently subjected to a continuous growth in a zeolite precursor mixture. The combustion of the polystyrene core leads to the transformation of the core/shell beads into hollow silicalite-1 spheres. The mechanical properties of the latter were found to be a function of the thickness of the silicalite-1 shell, the diameter-to-wall thickness (d:t) ratio, and the calcination conditions. Silicalite-1 bodies with a regular system of macrocavities were prepared by hydrothermal treatment with a silicalite-1 precursor mixture of a layer of pretreated polystyrene beads deposited in a confined space. The obtained body comprised the polystyrene macrotemplates, which after combustion left a regular system of macrocavities. The formation of silicalite-1 macrostructures and their specific properties was studied by XRD, SEM, and TG/DTA analyses and N2 adsorption measurements.

New Evidence for Precursor Species in the Formation of MFI Zeolite in the Tetrapropylammonium Hydroxide−Tetraethyl Orthosilicate−Water System

Tetraethyl orthosilicate was hydrolyzed and polymerized in concentrated aqueous tetrapropylammonium (TPA) hydroxide solution at 0 and 10 °C and room temperature. The formation of silicate oligomers involved in the TPA-mediated self-assembly process of colloidal Silicalite-1 was investigated using 29Si NMR. A pentacyclic dodecamer with four edge-sharing five-rings on a four-ring was detected as a new intermediate in the formation of Silicalite-1 zeolite. The formation of this pentacyclic dodecamer with a curved inner hydrophobic silicon dioxide surface is a new example of the structure-directing action of TPA. The temperature was found to be a key parameter with respect to the transformation of the intermediates with 11 and 12 Si atoms. At 0 and 10 °C, the tetracyclic undecamer having three five-rings on a four-ring was converted into the pentacyclic dodecamer by insertion of one additional silicate unit. At these low temperatures, it slowly underwent an intramolecular condensation and formation of capped double five-ring not leading to Silicalite-1 formation. At room temperature, three pentacyclic dodecamers rapidly condensed into the Silicalite-1 precursor. The involvement of the pentacyclic dodecamer in the self-assembly process offers an explanation for the absence of systematically missing T-sites in colloidal Silicalite-1.

Preparation of regular macroporous structures built of intergrown silicalite-1 nanocrystals

Organized silicalite-1 bodies with a regular system of macropores were prepared. The procedure includes two stages. In the first, self-assembly of monodisperse polystyrene spheres and silicalite-1 nanocrystals is induced by the slow evaporation of the solvent. In the second stage a hydrothermal treatment of the self-assembled composite in a silicalite-1 precursor solution leads to intergrowth and closing of the mesopores between the nanocrystals building the walls of the macroporous structure. The mechanical properties of the macroporous zeolite structures obtained after removal of the polystyrene were substantially improved by this secondary growth of the zeolite crystals. The characteristic features of the self-assembled and hydrothermally treated macroporous structures were studied by XRD, FTIR, SEM, TG-DTA and nitrogen adsorption measurements.

Analysis of the nucleation and growth of TPA-silicalite-1 at elevated temperatures with the emphasis on colloidal stability

The question as to whether silicalite-1 grows via an aggregation of smaller particles or similar sized particles has been addressed by considering the fundamentals governing colloidal stability—the quantitative theory underlying colloidal stability being given by the extended Derjaguin–Landau and Verwey–Overbeek (DLVO) theory. Application of the extended DLVO theory to discrete colloidal particles in silicalite-1 precursor sols shows that a net repulsive interactive energy exists between the negatively charged particles. The thermal energy of the colloidal particles (1/2kT at 373 K, the crystallization temperature) is not sufficient to overcome the net repulsive energy barrier. The extended DLVO theory has been applied to two growth scenarios with similar results: growth by aggregation of particles of very different sizes and growth by aggregation of similar sized particles. The significance of these conclusions is that a proposed growth mechanism of MFI type zeolite (growth by aggregation of subcolloidal particles) is deemed not to be a reasonable description of molecular sieve growth. The conclusion that the colloidal crystals are stable with respect to aggregation is supported by experimental observations. The ideas presented in this study are based upon crystallization of molecular sieves from clear solutions in the presence of quaternary ammonium cations that play a significant role in the stabilization of the colloidal crystals. Extension of the ideas presented shows that the extended DLVO theory is equally applicable to wholly inorganic heterogeneous systems.

Crystallisation of silicalite-1 precursors in the amine–(C3H7)4NBr–SiO2–H2O system

The crystallisation of alkali-metal-free silicalite-1 precursors from the PIPZ–TPABr–SiO2–H2O (PIPZ = piperazine, TPA = tetrapropylammonium) system has been investigated under static and stirred conditions. In many cases large rod-like crystals of (TPA, PIPZ)-silicalite-1 were obtained. The crystallisations were accompanied by decreases in pH caused by the incorporation of base in the crystals. Smaller pH decreases were observed when the PIPZ/SiO2 molar ratio was increased.