Defect Sites in Zeolites: Origin and Healing

Abstract

This paper deals with the synthesis conditions–defect formation relationship in zeolites. Silicalite‐1 (MFI‐type) is used as a model material. Samples synthesized from a system with high basicity (at 100 °C), a system with moderate basicity (at 150 °C), and a fluoride‐containing system in neutral medium (at 170 °C) are compared. Well‐crystallized materials with sizes ≈0.1, 1–10, and 30–40 µm are obtained. The samples are analyzed by complementary methods providing information on the short‐ and long‐range order in the zeolite framework. A strong correlation between the number of point defects in the zeolite framework and preparation conditions is established. Silicalite‐1 synthesized under mild synthesis conditions from a highly basic system exhibits a larger number of framework defects and thus low hydrophobicity. Further, the calcined samples are subjected to aluminum and silicon incorporation by postsynthesis treatment. The Al/Si incorporation in the zeolite framework and its impact on the physicochemical properties is studied by XRD, TEM/SEM, solid‐state NMR, FTIR, and thermogravimetric analyses. The defects healing as a function of the number of point defects in the initial material and zeolite crystal size is evaluated. The results of this study will serve for fine‐tuning zeolite properties by in situ and postsynthesis methods.

Functional mother liquor reversed titanium species for the green synthesis of anatase-free hollow TS-1 with tunable titanium micro-environment via a kinetic-thermodynamic co-regulatory pathway

“…The TS130-P sample presents a more obvious signal peak of the [Si(OSi) 3 OH] group (Q 3 ) and weaker resonance of the [Si(OSi) 4 ] group (Q 4 ) compared with TS170-P sample. 38 The higher Q 3 /(Q 4 + Q 3 ) value of the TS130-P sample demonstrates its more defect sites. The FT-IR spectra (Fig.…”

“…22,31 Additionally, the abundant silanol defects in the zeolite promote the erosion of interior by OH − . 38,45 Thus, at the initial stage (within 2 hours), dissolution originates from the internal silanol defects. We reasonably believe that the dissolution of a trace amount of silicon causes the dissolution of more titanium species around silanol defects due to the inner abundant adjacent titanium hydroxyls.…”

Dealumination of small-pore zeolites through pore-opening migration process with the aid of pore-filler stabilization

“…2B ). Considering the nearly constant micropore volume during the acid treatment, the decrease in XRD peak intensity was attributed to the formation of structural defects and changes in composition, rather than to the collapse of the crystal structure ( 47 ). Removal of Al atoms from both inner and outer parts of the particles was confirmed using cross-sectional transmission electron microscopy energy dispersive x-ray spectroscopy (TEM-EDS) and SEM-EDS images (figs.…”

“…In the 1 H MAS NMR spectra of the as-synthesized samples, a large peak was observed at 3.9 ppm, attributed to the H-bonded water. After acid leaching, a small peak is observed at 4.3 ppm, which could be attributed to silanol nests ( 47 ). Although this peak was also observed after defect-healing treatment, its intensity was decreased.…”

Recent advances in solid-state NMR of zeolite catalysts

“…various types of silanols) are generated during the synthesis or post-treatment. The zeolite defect is one of the decisive parameters affecting zeolite properties including hydrophilicity/hydrophobicity, stability and catalytic activity [ 64 , 65 ]. Besides, the defects in zeolites can act as anchoring positions for the incorporation of different heteroatoms (Mo, V, B, etc.)…”