Photoluminescence Response of Terbium-Exchanged MFI-Type Materials to Si/Al Ratio, Texture, and Hydration State

Abstract

Terbium-exchanged MFI zeolite type materials, i.e., microporous-mesoporous Zeotile-1 with the Si/Al ratio in the range 33-200, Zeogrid with the Si/Al ratio of 75, and nanocrystalline MFI with the Si/Al ratio of 75, were prepared via an ion exchange procedure. All of these zeolites were investigated by means of time-resolved photoluminescence techniques in various hydration states: as-synthesized (hydrated), calcined (heated at 450 degrees C in air), and rehydrated (after a six-month exposure to the atmospheric moisture). The photoluminescence decays and spectra were analyzed by discrete exponential fitting, distribution lifetimes analysis, and area-normalized time-resolved photoluminescence spectra. The results sustained a single average terbium species coordinated to both water molecules and framework oxygens in the hydrated zeolites. The framework contribution increased with the Si/Al ratio in Zeotile-1 and was greatest for the nanocrystalline MFI zeolite. For the calcined Zeotile-1 and Zeogrid, two main terbium species of different environments were found. For the nanocrystalline Tb3+-MFI, a distinct number of species could not be inferred, indicating a more heterogeneous distribution. Rehydration further differentiated among the Tb3+-exchanged zeolites. Photoluminescence line shape and decay of Tb3+-Zeotile-1 were between those of the hydrated and calcined states indicating a slow rehydration rate in contrast with the photoluminescence properties of Tb3+-MFI, which fully recovered the values of the hydrated state. Tb3+-Zeogrid presented an intermediate case: while the PL line shape was fully restored to that measured for the hydrated sample, the decay was still longer than that measured with the hydrated sample. Terbium photoluminescence response related to zeolite texture, Si/Al ratio, and hydration state suggest different sitting and location of terbium in Zeotile-1, Zeogrid, and nanocrystalline MFI materials. In mesoporous Zeotile-1 and Zeogrid, the results sustained two types of terbium sites: one on the internal surface of mesopores, the other inside the pores, while for the nanocrystalline MFI, terbium sites inside the pores predominate.

Photoluminescence spectra and dynamics of lanthanide-doped microporous–mesoporous materials

“…Details on the synthesis and characterization of Zeotile-1 (ZT1) and Zeogrid (ZG) materials were given elsewhere [2]. Four samples of each zeolite and lanthanide type with Si-toAl ratio equal to 50, 75, 100 and 150 were obtained.…”

“…The PL properties can be improved following thermal treatment (calcination); however, they are subject to full or partial restoration to those measured for the hydrated samples upon exposure to atmospheric moisture [2]. We have recently found that silylation of the calcined terbiumexchanged BEA zeolites could maintain more or less the lanthanide luminescence properties at the values measured after the calcination [3].…”

Photoluminescence study of terbium-exchanged ultrastable Y zeolites: Number of species, photoluminescence decays, and decay-associated spectra

“…Compared to TRES, ͑peak normalized͒ TRANES has a physical significance. An isoemissive point in TRANES occurs at the wavelength ͑ iso ͒ where the ratio of the wavelength-dependent radiative rates ͓k r1 ͑ iso ͒ / k r2 ͑ iso ͔͒ is equal to the ratio of the total radiative rates ͑k r1 / k r2 ͒ of the emitting species 5,6,13 …”

“…We have recently underlined the need for caution when interpreting nonexponential decays of lanthanides in microporous/mesoporous materials in terms of a discrete distribution of the emissive species. [5][6][7] A combined information from both the spectral and the time domain as that provided by time-resolved PL spectra can bring more reliable information concerning distribution of lanthanides in these heterogeneous materials. It is acknowledged that the quantum PL efficiency obtained with lanthanides in zeolites is strongly affected by the nonradiative relaxation via the vibrational coupling between the electronic levels of the lanthanide ion with the -OH groups inherently present in a solgel matrix.…”

“…8 Albeit the PL properties can be improved following a thermal treatment at high temperatures or calcination they are subject to full or partial restoration to those measured in the hydrated state upon exposure to the moist atmosphere. 5,6 To decrease the adverse effects of water on the PL properties of the lanthanides-exchanged zeolites, the surface of zeolites can be made hydrophobic by the silylation technique. 7,[9][10][11] Here we present a detailed time-resolved PL study of nonsilylated and silylated terbium exchanged into dealuminated or ultrastable Y ͑USY͒ zeolites.…”