Si-MFI Crystallization Using a “Dimer” and “Trimer” of TPA Studied with Small-Angle X-ray Scattering

Abstract

The formation and consumption of precursors and the crystallization of Si-MFI using bis(tripropylammonium) hexamethylene dihydroxide (“dimer” of tetrapropylammonium cation, TPA) and bis(tripropylammonium-N-N‘-hexamethylene)-N‘ ‘,N‘ ‘-dipropylammonium trihydroxide (“trimer” of TPA) as structure-directing agents have been investigated in situ using simultaneous, time-resolved, SAXS and WAXS and using USAXS. The formation of 2.8-nm-sized primary units is observed upon dissolution of the silica source, which is in agreement with results from earlier studies on systems with the TPA cation as a structure-directing agent. Aggregation of these nanometer-scale primary units to 10−15-nm-sized particles is found to be an essential step in nucleation of the zeolite. Crystal growth occurs via the addition of the primary units to the growing crystal. Although the size of the primary units for MFI is independent of the structure-directing agent used, the organic species does have a pronounced influence on the crystal growth step and, therefore, on the crystal growth rate, size, and morphology. The results presented here confirm a common mechanism proposed for organic-mediated crystallization of high-silica zeolites.

Oligomerization and Cyclization Processes in the Nucleation of Microporous Silicas

“…[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.…”

The role of synchrotron radiation in examining the self-assembly of crystalline nanoporous framework materials: from zeolites and aluminophosphates to metal organic hybrids

“…Unlike clear solutions the SAXS (recorded sequentially with the XANES) of the thick gel resulted in no clear shoulder or maxima. However, by subtracting the power law decay dependencies (observed to be À3 over the data) broad maxima were revealed which (like de Moor and coworkers 25,26 ) were interpreted as growing nanoparticles of a size defined by q = 2p/d. These nanoparticles increased monotonically during the initial stages of formation from B5-20 nm and then more rapidly once crystallization was observed in the WAXS.…”

“…Nanoparticle and crystal growth then occurs via addition of either elementary particles or primary units. This mechanism is effectively that described in the work of De Moor et al 25,26 and Doktor et al 29 The second scheme describes an 'aggregative growth' mechanism proposed by the group of Tsapatsis (e.g. Davis et al 37 ) as a result of an in-depth room temperature formation study of silicalite-1 (allowing TEM and AFM to be performed 'in situ').…”

“…Grizzetti et al 44 for example utilized the SAXS/WAXS combination to identify two independent populations of particles and describe a mechanism broadly similar to De Moor et al, 25,26 whilst Fan et al 34 observed similar nanometer-sized entities and structure in the SAXS data. Here the data could be fitted to a cubic form factor which suggested a relatively homogenous solution of nanoparticles.…”

Internal structure of a gel leading to NBA-ZSM-5 single crystals

“…It is noteworthy that the presence of an amorphous phase in the form of colloidal nanoparticles is observed during the synthesis of silicalite-1, the aluminum-free version of ZSM-5, suggesting that the presence 1 3 of a gel precursor is also required as an intermediate stage for crystallization from clear solutions [6]. These primary silica particles that act as precursor species for crystallization [7] are amorphous and in the range of 2-6 nm [8][9][10][11]. The primary particles evolve to larger particles in the form of aggregates, in which nucleation and subsequent growth take place [12][13][14].…”