Vapor phase transport synthesis of zeolites from sol–gel precursors

Thoma, Steven G.; Nenoff, Tina M.

doi:10.1016/s1387-1811(00)00306-1

Cited by 33 publications

(11 citation statements)

References 22 publications

(25 reference statements)

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“…Since the initial DGC dry gel contains 20–40 wt % water, it is now widely accepted that the crystallization mechanism in the DGC process is fundamentally similar to that of hydrothermal synthesis, which has a more accessible liquid phase . DGC has been demonstrated to be an effective approach to study the formation of several types of molecular sieves. − …”

Section: Introductionmentioning

confidence: 99%

New Insights into Formation of Molecular Sieve SAPO-34 for MTO Reactions

Zhang

Huang

2016

J. Phys. Chem. C

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Small-pore molecular sieve SAPO-34 is well-known as the most promising and effective catalyst for the conversion of methanol to olefins (MTO). In this paper, we have investigated the formation of SAPO-34 under different dry gel conversion (DGC) conditions using two types of synthesis gels with one containing hydrofluoric acid (HF) and the other not. Particular attention was paid to the Si incorporation and distribution in final SAPO-34 products. The results indicate that under DGC (with HF) conditions SAPO-34 is formed from the transformation of a highly crystalline, layered prephase that is held by covalent bonds. The final SAPO-34 product is a mixture of triclinic and trigonal phases containing six framework Si species. The Si distribution in triclinic and trigonal SAPO-34 is drastically different. The majority of Si species are located in the aluminosilicate domains in trigonal phase. Conversely, under DGC (without HF) conditions, the crystallization of SAPO-34 involves three transformational stages. Initially, a layered, crystalline intermediate forms and then transforms to a semicrystalline phase. The structures of both intermediates are held by weak noncovalent bonding interactions. The final SAPO-34 product is in pure trigonal form and contains two major Si species. Despite the difference in Si distribution, the SAPO-34 products prepared by using different gels have similar bulk Si contents. MTO reaction tests show that before 50 min of time-on-stream the DGC (with HF) product gives lower ethylene and propylene selectivity but higher propane selectivity than the DGC (without HF) product. After 50 min on stream, similar light olefin and propane selectivities are observed over the two SAPO-34 samples.

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Section: Introductionmentioning

confidence: 99%

New Insights into Formation of Molecular Sieve SAPO-34 for MTO Reactions

Zhang

Huang

2016

J. Phys. Chem. C

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“…The SAC requires an appropriate amount of water in the bottom of the autoclave to maintain the saturated vapor pressure, and a higher amount of steam greatly enhances the rate of crystallization. Thoma et al 18 estimated that the role of the water in the VPT solvent mixture was likely rehydration of surface siloxane groups to create hydroxyl groups that could act as adsorption sites for the structure-directing agents (SDA), and to facilitate bond cleavage required for molecular rearrangement to construct the crystalline cage of the zeolite. Serrano and van Grieken 19 took the direct conversion from gel to zeolite as evidence for the solid-solid transformation in their recent review article.…”

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confidence: 99%

Catalytic cracking of large molecules over hierarchical zeolites

Qian

Zhao

et al. 2006

Chem. Commun.

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“…It was suggested that necessary amount of water was needed for crystallization. Thomas and Nenoff [24] pointed that water readily cleaves TÀOÀT while the organo-cations adsorb onto surface acid sites. The role of water in the VPT solvent mixture is, therefore, likely twofold: rehydration of surface TO 4 groups to create hydroxyl groups that can act as adsorption sites for the organo-cations, and facilitate bond breakage required for molecular rearrangement.…”

Section: Effect Of Water Content In Synthesis Gelmentioning

confidence: 99%