Heterogenous events in the crystallization of zeolites

Serrano, David; Grieken, Rafael van

doi:10.1039/b100818h

Cited by 112 publications

(92 citation statements)

References 60 publications

(85 reference statements)

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“…Many of these studies employed X-ray powder diffraction. [3][4][5] These measurements were taken on product samples that were washed of excess mother liquor and dried. Such ex situ characterization techniques do not provide the direct measurement of zeolite nucleation and growth from precursor solutions or gels in real time.…”

Section: Introductionmentioning

confidence: 99%

In Situ SAXS/WAXS of Zeolite Microwave Synthesis: NaY, NaA, and Beta Zeolites

et al. 2007

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A custom waveguide apparatus is constructed to study the microwave synthesis of zeolites by in situ small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS). The WR-284 waveguide is used to heat precursor solutions using microwaves at a frequency of 2.45 GHz. The reaction vessels are designed to include sections of thin-walled glass, which permit X-rays to pass through the precursor solutions with minimal attenuation. Slots were machined into the waveguide to provide windows for X-ray energy to enter and scatter from solutions during microwave heating. The synthesis of zeolites with conventional heating is also studied using X-ray scattering in the same reactor. SAXS studies show that the crystallization of beta zeolite and NaY zeolite is preceded by a reorganization of nanosized particles in their precursor solutions or gels. The evolution of these particles during the nucleation and crystallization stages of zeolite formation depends on the properties of the precursor solution. The synthesis of NaA and NaX zeolites and sodalite from a single zeolite precursor is studied by microwave and conventional heating. Microwave heating shifts the selectivity of this synthesis in favor of NaA and NaX over sodalite; conventional heating leads to the formation of sodalite for synthesis from the same precursor. The use of microwave heating also led to a more rapid onset of NaA zeolite product crystallization compared to conventional heating. Pulsed and continuous microwave heating are compared for zeolite synthesis. The resulting rates of formation of the zeolite products, and the relative amounts of the products determined from the WAXS spectra, are similar when either pulsed or continuous microwave heating is applied in the reactor while maintaining the same synthesis temperature. The consequences of these results in terms of zeolite synthesis are discussed.

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Section: Introductionmentioning

confidence: 99%

In Situ SAXS/WAXS of Zeolite Microwave Synthesis: NaY, NaA, and Beta Zeolites

et al. 2007

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“…The simplest condensation reaction, that of the dimerization of silicic acid [Eq. (2)] is found to be favorable in 2 SiðOHÞ 4 ! ðOHÞ 3 SiOSiðOHÞ 3 þ H 2 O ð2Þ the gas phase for both neutral and charged species.…”

mentioning

confidence: 97%

“…[1,2] Whilst the general features of self-assembly through condensation polymerization are understood for siliceous systems, the conditions under which both natural and synthetic zeolites form render such a complex process extremely difficult to characterize experimentally. Thus, whilst the species initially present in silicate gels can be characterized by NMR spectroscopy with some certainty, [3,4] their charge state is more difficult to probe.…”

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confidence: 99%

Oligomerization and Cyclization Processes in the Nucleation of Microporous Silicas

2005

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The nucleation and growth of zeolites and other microporous solids continues to pose many questions. [1,2] Whilst the general features of self-assembly through condensation polymerization are understood for siliceous systems, the conditions under which both natural and synthetic zeolites form render such a complex process extremely difficult to characterize experimentally. Thus, whilst the species initially present in silicate gels can be characterized by NMR spectroscopy with some certainty, [3,4] their charge state is more difficult to probe.[5] Similarly, although scattering methods can be used to tentatively identify larger (possibly nucleation) species, [6,7] the relative stability and lifetime of the smallest clusters, significant in zeolitic structures, such as four-membered rings, remain unclear. Furthermore, in the postnucleation regime, there is much debate as to which species are responsible for crystal growth: small oligomers or larger subunits. Indeed, it has been suggested that the growth of silicalite-1 (MFI zeolite) is controlled by a unique nanocluster, [8] although there is some debate about this proposal.[9] Moreover, characterization of zeolite surfaces suggests that much smaller units are responsible for surface growth.[10]Here, we describe quantum chemical calculations that model some of the species present prior to nucleation and attempt to examine the pathways by which such key species may form and hence rationalize the assembly of zeolitic structures. In particular, we discuss the factors that drive the condensation polymerization of silicate oligomers and that favor the cyclization of such species: pH and solvent.In the past decade there have been a number of computational studies of model structures involved in the nucleation process of silicates. Structures and energies for small clusters containing up to five silicon atoms were obtained by Pereira et al. [11,12] One conclusion that can be drawn from these results is that linear silicate species are favored over cyclic structures-a situation not conducive to the formation of zeolitic structures but instead one that favors the formation of amorphous silicates.[11] These early calculations were performed using a local DFT method and considered only neutral species, although solvation effects were included using the COSMO method.[13] The roles of water and organic templates in prenucleation gels were studied by Lewis et al. [14] and Catlow et al. [15] by more approximate molecular mechan-

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“…25,26 Crystal growth occurs through an in situ, localized construction process from small, mobile species ordered by the participating cations. 27 So, it was not weird that the nucleation and crystallization of self-induction system (a) required the longest time.…”

Section: -5mentioning

confidence: 99%

Nanoseeding Effect in ZSM-5 Crystallization Process

Lü

Zhang

Tong

et al. 2013

NANO

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ZSM-5 zeolites were hydrothermally synthesized in three di®erent seeding pathways under the direction of tetrapropylammonium bromide (TPABr) template. In order to investigate the seeding e®ect in ZSM-5 crystallization process, ZSM-5 crystals and pre-fabricated MFI-type nanoseeds were added into the original self-induction system, respectively. The¯nal ZSM-5 zeolites were systematically investigated based on XRD (X-ray di®raction), SEM (scanning electron micrograph), TEM (transmission electron micrograph), nitrogen adsorption characterizations and NH 3 -TPD (ammonia-temperature programmed desorption). The self-induction system produced ca. 20 m ZSM-5 zeolite displaying hexagonally uniform prisms. After the addition of ZSM-5 crystal seeds, the crystal sizes were decreased greatly to ca. 5 m. When MFItype nanoseeds were adopted, irregular aggregate particles consisting of 20À50 nm primary particles were rapidly synthesized. The varied hydrothermal crystallization kinetics of the three synthesis system was also explored. Adjusting the seed agents alone, ZSM-5 crystals with diverse structural, morphological, textural and hydrothermal behaviors could be fabricated conveniently. The three ZSM-5 zeolites loaded by 0.05 wt.% Pt were assessed for the xylene isomerization reaction to investigate the particle size e®ect on the catalytic properties.

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Heterogenous events in the crystallization of zeolites

Cited by 112 publications

References 60 publications

In Situ SAXS/WAXS of Zeolite Microwave Synthesis: NaY, NaA, and Beta Zeolites

In Situ SAXS/WAXS of Zeolite Microwave Synthesis: NaY, NaA, and Beta Zeolites

Oligomerization and Cyclization Processes in the Nucleation of Microporous Silicas

Nanoseeding Effect in ZSM-5 Crystallization Process

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