Langmuir 2007, 23, 6781-6787
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Investigation of the Hydrophobization Efficiency of Terbium-Exchanged BEA Zeolites by Means of FT-IR, TGA, Physical Adsorption, and Time-Resolved Photoluminescence Carmen Tiseanu,*,† Bogdan Gagea,‡ Vasile Ion Parvulescu,§ Vı´ctor Lo´renz-Fonfrı´a,| Andre´ Gessner,⊥ and Michael Uwe Kumke⊥ National Institute for Laser, Plasma and Radiation Physics, P.O. Box MG-36, RO 76900, Bucharest-Magurele, Romania, Department of Interphase Chemistry, KU LeuVen, 23 Kasteelpark Arenberg, B-3001 LeuVen (HeVerlee), Belgium, Department of Chemical Technology and Catalysis, UniVersity of Bucharest, 4-12 Regina Elisabea BVd., Bucharest 030016, Romania, Department of Materials Science and Engineering, Nagoya Institute of Technology, Showa-ku, 466-855 Nagoya, Japan, and Institute of Chemistry, Physical Chemistry, UniVersity of Potsdam, Karl-Liebknecht-Str. 24-25, 14476 Potsdam-Golm, Germany ReceiVed NoVember 12, 2006. In Final Form: February 12, 2007 Terbium-exchanged BEA zeolites were hydrophobized with phenyl-, vinyl-, and hexadecyltrimethoxysilanes by means of postsynthesis grafting. These materials were investigated using XRD, FT-IR, TGA, physical adsorption, and photoluminescence. Different methods for the analysis of the non-exponential decay of terbium photoluminescence in BEA zeolites were used ranging from discrete exponential to more complex approaches based on maximum entropy and global analysis. Two groups of decay times varying between 480 and 580 µs and 1-1.3 ms were assigned to the lifetimes of terbium exposed to water (unprotected) and protected by the organic groups, respectively. Our results showed that the preservation of terbium PL properties against detrimental effects of moisture adsorption could be ordered in the following sequence: hexadecyl > phenyl ≈ vinyl. The photoluminescence results were in good agreement with the FT-IR, TGA, and physical adsorption data.
1. Introduction Organic silylation of mesoporous materials is a major subject of research due to the possibility of tuning both chemical and physical properties.1-7 In contrast with mesoporous materials, there are few reports on the successful silylation of microporous zeolites.8-14 Several applications have been suggested for the functionalized zeolite surfaces via silylation including methods to improve the incorporation of zeolites in polyimide films,8 * To whom correspondence should be addressed. E-mail: tiseanuc@ yahoo.com. † National Institute for Laser, Plasma and Radiation Physics. ‡ KU Leuven. § University of Bucharest. | Nagoya Institute of Technology. ⊥ University of Potsdam. (1) Venuto, P. B. Microporous Mater. 1994, 2, 297. (2) Yang, W. F.; Hagaman, E. W.; Dai, S. Chem. Mater. 2004, 16, 5182. (3) Hollman, A. M.; Scherrer, N. T.; Cammers-Goodwin, A.; Bhattacharya, D. J. Membr. Sci. 2004, 239, 65. (4) Yang, C. M.; Wang, Y.; Zibrowius, B.; Schuth, F. Phys. Chem. Chem. Phys. 2004, 6, 2461. (5) Defreese, J. L.; Hwang, S. J.; Parra-Vasquez, A. N. G.; Katz, A. J. Am. Chem. Soc. 2006, 128, 5687. (6) Wang, J.; Yu, N.; Zheng, A.; Yang, J.; Wu, D.; Sun, Y.; Ye, C.; Deng, F. Microporous Mesoporous Mater. 2006, 89, 219. (7) Burkett, S. L.; Sims, S. D.; Mann, S. Chem. Commun. 1996, 11, 1367. (8) Vankelecom, I. F. J.; van den Broeck, S.; Merckx, E.; Geerts, H.; Grobet, P.; Uyterhoeven, J. B. J. Phys. Chem. B 1996, 100, 3753. (9) Cagnoli, M. V.; Casuscelli, S. G.; Alvarez, A. M.; Bengoa, J. F.; Gallegos, N. G.; Crivello, M. E.; Herrero, E. R.; Marchetti, S. G. Catal. Today 2005, 107108, 397. (10) Hong, M.; Falconer, J. L.; Noble, R. D. Ind. Eng. Chem. Res. 2005, 44, 4035. (11) Zheng, S.; Heydenrych, H. R.; Roger, H. P.; Jentys, A.; Lercher, J. A. Top. Catal. 2003, 22, 101. (12) Singh, R.; Dutta, P. K. Microporous Mesoporous Mater. 1999, 32, 29. (13) Song, W.; Marcus, D. M.; Abubakar, S. M.; Jani, E.; Haw, J. F. J. Am. Chem. Soc. 2003, 125, 13964. (14) Jeong, N. C.; Kim, H. S.; Yoon, K. B. Langmuir 2005, 21, 6038.
modification of zeolite membranes for H2 separation,10 enhanced selectivity,11 extraction of metal ions from aqueous to organic phase,12 or even investigation the nature of the surface OH.13 Surface silylation has also been used to generate a tight confinement of the semiconductors quantum dots within zeolites.14 The latter approach proved successful in retarding the moisture entrance in the modified zeolites Y for several weeks, and as result, expelling the quantum dots from the zeolite interiors to the surface. Materials that combine zeolite matrix microporosity with the luminescence properties of the exchanged- lanthanide ions were long sought for phosphor applications.15-21 It is well acknowledged that the water or moisture attack from the atmosphere has a dramatic effect on the photoluminescence properties of lanthanide dopants in terms of both intensity and lifetime.20 Although water content may be drastically reduced upon thermal treatments at high temperatures, most zeolites undergo fast and reversible rehydration upon exposure to atmospheric moisture.22-24 Hydrophobization can protect the zeolite surface against humidity and reduce the surface area as well as pore volumes, thus limiting (15) Bredol, M.; Kynast, U.; Ronda, C. AdV. Mater. 1991, 3, 361. (16) Justel, T.; Wiechert, D. U.; Lau, C.; Sendor, D.; Kynast, S. AdV. Funct. Mater. 2001, 11, 105. (17) Rocha, J.; Carlos, L. D. Curr. Opin. Solid State Mater. Sci. 2003, 7, 199. (18) Kostova, M.; Ferreia, R. A. S.; Ananias, D.; Carlos, L. D.; Rocha, J. J. Phys. Chem. B 2006, 110, 15312. (19) Sanchez, C.; Lebeau, B.; Chaput, F.; Boilot, J P. AdV. Mater. 2003, 15, 1969. (20) Maas, H.; Currao, A.; Calzaferri, G. Angew. Chem., Int. Ed. 2002, 41, 2495. (21) Rocha, J.; Ferreira, P.; Carlos, L. D.; Ferreira, A. Angew. Chem., Int. Ed. 2000, 39, 3276. (22) Olson, D. H.; Haag, W. O.; Borghard, W. S. Microporous Mesoporous Mater. 2000, 35, 435. (23) Tiseanu, C.; Kumke, M. U.; Parvulescu, V. I.; Gessner, A.; Gagea, B. C.; Martens, J. A. J. Phys. Chem. B 2006, 110, 25707. (24) Cruciani, G. Phys. Chem. Solids 2006, 67, 1973.
10.1021/la063308y CCC: $37.00 © 2007 American Chemical Society Published on Web 05/08/2007
6782 Langmuir, Vol. 23, No. 12, 2007 Scheme 1. Postgrafting of Organic Groups onto Zeolite Surfaces
the access of any poisoning species to the incorporated lanthanide exchanged species. Literature on lanthanides incorporated in modified zeolites with inert groups is relatively scarce. Studies on europium doped SBA-15, modified with inert phenyl groups have been recently reported.25 Because the silylation occurred on the external surface, it was claimed that the ion exchange took place in the SBA-15 pores and thus Eu3+ was incorporated in the channels. A structural and spectroscopic characterization of an europium complex incorporated inside the channels of MCM41 modified with phenyltriethoxysilane showed that the lanthanide complex has been incorporated inside the channels of MCM-41 by using an ion exchange method.26 Other reports include novel luminescent materials based on lanthanides exchanged into mixed zirconium phenyl- and m-sulfophenyl phosphonate materials.27 The relative small number number of the water molecules in the first coordination sphere of the lanthanide ions was explained by the hydrophobic microenvironment in the interlamellar space of the materials, due to pendent phenyl groups. Here we present a study on the organic silylation used as a tool to preserve the luminescence properties of the Tb3+exchanged BEA zeolites. Calcined terbium- exchanged BEA zeolites were functionalized with phenyl, vinyl and hexadecyl trimethoxysilanes by means of postsynthesis grafting. Characterization of these materials was carried out using adsorptiondesorption isotherms of nitrogen at 77 K, thermogravimetry (TGA), Fourier Transform Infrared Specroscopy (FT-IR), powder X-ray diffraction (XRD) and photoluminescence (PL) measurements. Different methods for terbium time-resolved luminescence data are applied in order to obtain a qualitative as well as quantitative estimation of the silylation efficiency. 2. Experimental Section 2.1. Synthesis. Tb3+-exchanged (H)BEA zeolites (CP 811 PQ Zeolites, Si/Al ) 50) were prepared following a typical ion-exchange procedure.28 Accordingly, 1 g of BEA was added to 25 mL of an aqueous solution of 0.004 M Tb3+ and stirred for 30 min at 80 °C. The samples were then recuperated, washed until free of Tb3+, and air-dried at 50 °C for 2 days. Before analysis the samples were treated following two different routes: (i) calcination of the ionexchanged zeolites at 500 °C for 4 h and (ii) the hydrophobization of the external surface of the calcined Tb3+-exchanged zeolites BEA with phenyl-, vinyl-, and hexadecyltrimethoxysilanes (C16). A conventional grafting technique was applied (see Scheme 1). Briefly, 1 g of calcined terbium-zeolite was dispersed into 100 mL of toluene (previously dried with 5 Å molecular sieves) under vigorous stirring. Next, 20 mL of trimethoxysilane derivatives were added to the toluene solution under continuous stirring at room temperature for 24 h. The grafted zeolites were then filtered, washed with toluene, and dried at 100 °C under vacuum. The zeolites obtained were denoted as with BEA/Tb3+ (parent zeolite) and BEA/Tb3+/ vinyl, BEA/Tb3+/phenyl, and BEA/Tb3+/C16, for the grafted zeolites. 2.2. Materials Characterization. Structural characterization of the investigated samples was performed using FT-IR, XRD, nitrogen adsorption-desorption isotherms at -196 °C, and thermogravimetric (25) Chuanwang, G.; Chia, P. A.; Zhao, X. S. Appl. Surf. Sci. 2004, 237, 387. (26) Guo, X.; Fu, L.; Zhang, H.; Carlos, L. D.; Peng, C.; Guo, J.; Yu, J.; Deng, R.; Sun, L. New J. Chem. 2005, 10, 1351. (27) Ferreira, R.; Pires, P.; de Castro, B.; Ferreira, R. A. S.; Carlos, L. D.; Pischel, U. New J. Chem. 2004, 28, 1506. (28) Serra, O. A.; Nassar, E. J.; Zapparolli, G.; Rosa, I. L. V. J. Alloys Compd. 1995, 225, 63.
Tiseanu et al. analysis. The specific surface areas of the samples were determined after degassing the samples in situ at 120 °C for 5 h by nitrogen adsorption isotherms collected at -196 °C using a Micromeritics ASAP2020 instrument. Powder X-ray diffraction patterns (PXRD) were collected on a Siemens D-500, using Cu KR radiation (λ ) 1.54050 Å) and quartz as an external standard. Fourier transform infrared spectra (FT-IR) were collected in a Nicolet 4700 FT-IR instrument using the KBr pellet technique; 100 scans were averaged to improve the signal-to-noise ratio, at a nominal resolution of 4 cm-1. Thermogravimetric analysis curves (thermogravimetric (TG) and heat flow curves) were collected using a SETARAM 92 16.18 apparatus. 2.3. Photoluminescence Measurements. The stationary luminescence measurements were carried out at 20 °C using a Fluoromax 3 spectrofluorometer (Jobin Yvon). For the time-resolved luminescence measurements, a nitrogen laser (LTB) operated at 10 Hz with an excitation wavelength of λex ) 337.1 nm was employed. Under the experimental conditions the Tb3+ ion was directly excited. The luminescence was detected in the range of 450 nm < λem < 650 nm using an intensified CCD camera (Andor DH720-18H-13, Andor) equipped with a spectrograph (MS257, Oriel Instruments). Photoluminescence decays at specific wavelengths (or the integrated photoluminescence decays for some wavelength interval) were analyzed using four methods: (a) Discrete fitting by a multiexponential function, n
f(t) ) B +
∑ A exp(-t/τ ) i
i
i)1
where Ai is the decay amplitude, B is a constant (the baseline offset), and τi is the decay time. (b) A lifetime distribution analysis with decay times equally spaced in ln(τ) allowing for symmetric distributions as well as Voigt profiles (Gaussian and Lorentzian mixture).29 (c) Analysis of the time-resolved PL spectra by the maximum entropy inversion of the Laplace transformation (MaxEntiLT). With use of method (c), a distribution of exponential amplitudes versus the time constant of decay (MaxEnt lifetime distribution) was obtained.30,31 For details about the particular application of MaxEnt in the analysis of multiexponential data see Brochon.30 To measure the solution entropy, either the generalized Shannon-Jaynes entropy (positive-restricted) or the generalized Shannon-Jaynes entropy for solution without sign restrictions was used.31,32 Bayesian inference was applied for an automatic and optimum choice of the regularization value, independent of the noise standard deviation knowledge.31 MaxEnt-iLT was implemented in MATLAB v7 as described.33 The number of bands recovered by MaxEnt lifetime distribution analysis directly represents the number of resolvable decay components in the data, whereas the band positions and band areas of the MaxEnt lifetime distribution give the corresponding time constants and amplitudes of the decay components. (d) Finally, time-resolved photoluminescence spectra of the full emission wavelength range of 450 nm < λem < 650 nm were analyzed by global exponential nonlinear least-squares fit (G-ExpNLLS). GExpNLLS provides global time constants of decay, and their decay associated or amplitude spectra (DAS). The DAS are the exponential amplitudes versus the wavelength for a given decay time constant.34,35 In the case of parallel photoluminescence decays, the DAS corresponds to the particular decaying species, scaled by their relative initial population. G-ExpNLLS was performed following the Levenberg-Marquard method in a program implemented in MATLAB v7, as described by Lo´renz-Fonfrı´a and Kandori.36 (29) Branco, T. J. F.; do Rego, A. M. B.; Machado, I. F.; Ferreira, L. F. V. J. Phys. Chem. B 2005, 109, 15958. (30) Brochon, J. C. Methods Enzymol. 1994, 240, 262. (31) Gull, S. F.; Skilling, J. Quantified maximum entropy MemSys5 user’s manual; Maximum Entropy Data Consultants Ltd.: Suffolk, 1990. (32) Jaynes, E. T. Proc. IEEE 1982, 70, 939. (33) Lo´renz-Fonfrı´a, V. A.; Kandori, H. Appl. Spectrosc. 2006, 60, 407. (34) Dioumaev, A. K. Biophys. Chem. 1997, 67, 1. (35) van Stokkum, I. H.; Larsen, D. S.; van Grondelle, R. Biochim. Biophys. Acta 2004, 1657, 82. (36) Lo´renz-Fonfrı´a, V. A.; Kandori, H. Submitted.
Hydrophobization Efficiency of Terbium BEA Zeolites
Figure 1. TG analysis for the parent and modified BEA/Tb3+ zeolites. (a) BEA/Tb3+; (b) BEA/Tb3+/vinyl; (c) BEA/Tb3+/phenyl; (d) BEA/ Tb3+/C16.
3. Results and Discussion 3.1. Thermogravimetric Analysis. Modified BEA/Tb3+ zeolites showed different weight loss profiles compared to those of the parent zeolite. According to Figure 1, parent zeolites lose their adsorbed water below 150 °C. Upon modification, the adsorbed water decreases, with a minimum loss measured for BEA/C16 (6.2%). The additional weight loss occurring above 200 °C is associated with complete combustion of the organic molecules. The TGA results indicate that upon modification of the calcined BEA/Tb3+ the moisture adsorption is reduced depending on the silanization organic moiety. The organic content determined from the TGA data corresponded to 0.030 mol/g for the vinyl, 0.033 mol/g for the phenyl, and 0.060 mol/g for the C16. 3.2. Textural Characterization. Table 1 compiles the textural characteristics of the investigated zeolites. Due to a partial blockage of micropores, deposition of terbium caused a decrease in both surface area and micropore volume. Further, silylation of BEA/Tb3+ with different trimethoxysilane derivatives led to an additional decrease of the surface area and micropore volume, which actually paralleled the size of the organic chain. Thus, with use of a C16 moiety, a dramatic decrease of the surface area and micropore volume was determined, which corresponds to an advanced blockage of the internal surface of this zeolite. 3.3. Structural Characterization. 3.3.1. Powder X-ray Diffraction. Figure 2 shows the PXRD patterns of the parent and modified BEA/Tb3+ zeolites. Diffraction lines associated with zeolite BEA have similar intensity in the functionalized zeolites, irrespective of the organic molecule. This confirms that, after ionic exchange with terbium and silanization of Si-OH bonds, the zeolite framework remained intact and the crystallinity was not affected by silylation. 3.3.2. Fourier Transform Infrared Spectroscopy. FT-IR bands between 800 and 400 cm-1, associated with zeolite BEA, were detected in all samples, thus confirming the results obtained with PXRD (Figures 3A and 3B). In addition, the modified zeolites showed the specific bands due to C-C or C-H bonds vibrations.
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Bands assigned to CH2 groups and Si-vinyl bonds were identified as well. BEA/Tb3+/vinyl displayed very weak bands at 1420 (assigned to Si-vinyl bonds) and 2950 and 2990 cm-1 (Csp2-H stretching vibrations), while for BEA/Tb3+/phenyl the bands at 746, 702 (not shown in figures) and 1430 cm-1 (aromatic C-C bond stretching vibration) with higher intensity were detected. The most intense bands were measured with BEA/Tb3+/C16 at 1470 (Csp3-H bending vibrations, Figure 3B) and 2850 and 2930 cm-1 (Csp3-H stretching vibrations, Figure 3A). The band intensity is much higher for BEA/Tb3+/C16 than for the vinyland phenyl-grafted zeolites due to higher organic content (see the TGA results). Although in the way it has been applied the FT-IR technique is not suited for quantitative analysis, the marked differences between band intensity points to a high content in organic molecules for zeolite BEA/Tb3+/C16. Further differences were detected in the 3700-3000 cm-1 silanol region where the parent zeolite showed the highest content in Si-OH groups while the modified zeolites have a lower content, especially in zeolites BEA/Tb3+/vinyl and BEA/Tb3+/C16 (Figure 4). The deconvolution of the large band located in the 37003100 cm-1 region accounts for the presence of three different OH groups which, according to the literature information, can be assigned to terminal silanol groups (band at 3640 cm-1), bridging OH groups with Brønsted acidity (band at 3450 cm-1), and OH groups attached to trivalent cations compensating the Al3+ charge (band at 3250 cm-1).37 The location of these bands at such low wavelengths is due to the fact that the measurements were carried out in KBr. The silylation of this zeolite caused a change in the relative ratios of these bands, indicating that the reaction presented in Scheme 1 occurs predominantly with bridging OH and OH attached to the terbium species (bands at 3450 and 3250 cm-1.) 3.4. Photoluminescence Properties. A representative timeresolved luminescence spectrum of the modified BEA/Tb3+ zeolites is illustrated in Figure 5. A direct excitation mode was established for all samples, with laser excitation wavelength at 337 nm matching one of the UV absorption levels of terbium. The possibility for the phenyl moiety to act as remote sensitizers for terbium PL was discarded based on (a) the absence of the 275 nm phenyl characteristic absorption band in terbium excitation spectrum measured at 545 nm and (b) the appearance of an even stronger emission upon direct excitation into the 5D4 state (e.g., at λex ) 487 nm, corresponding to the 5D4 f 7F6 transition). The terbium PL spectrum displays the 5D4-7F3, 5D4-7F4, 5D4-7F5, and 5D4-7F6 transitions spanning from 450 to 650 nm. The most intense PL band is located at 545 nm and corresponds to the 5D -7F transition. Three other peaks are observed at 491 nm 4 5 (5D4 f 7F6), 586 nm (5D4 f 7F4), and 624 nm (5D4 f 7F3). Within the used spectral resolution of 0.2 nm, discernible differences of the PL spectra among all investigated zeolites, related to the peak positions, relative intensities, and transitions widths, were not detected. This confirmed that silylation of BEA/ terbium zeolites did not disturb the environments at terbium sites. The terbium PL decays (λem ) 545 nm) following laser excitation at 337 nm were heterogeneous for both the parent and modified zeolites (Figure 6). A nonexponential shape of luminescence decays can be generally explained in terms of two or more terbium species or/and the presence of nonradiative energy transfer processes. The radiationless deactivation processes due to -OH coupling is a well-known contribution to the alteration of the PL quantum (37) Karge, H. G. In Verified Synthesis of Zeolitic Materials, 2nd ed.; Robson, H., Ed.; Elsevier: New York, 2001.
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Table 1. Physical Adsorption Data for the Parent and Modified BEA/Tb3+ Zeolites BET surface area (m2/g) zeolite
exp.
BEA BEA/Tb3+ BEA/Tb3+/vinyl BEA/Tb3+/phenyl BEA/Tb3+/C16
540 460 379 352 195
micropore volumea (mL/g)
theor.c
exp.
439 437 398
0.17 0.14 0.12 0.11 0.05
theor.
0.14 0.13 0.12
c
total micropore volumeb (mL/g) exp.
theor.c
0.65 0.56 0.51 0.49 0.36
0.53 0.53 0.48
a Determined using t-plot method. b Determined from micropore program. c Theoretical value calculated considering a physical mixture of exchanged zeolite and silanization derivative with compositions as determined by TGA (organic derivative was considered to have no surface area).
Figure 2. PXRD patterns of the parent and modified BEA/Tb3+zeolites: (a) BEA; (b) BEA/Tb3+; (c) BEA/Tb3+/vinyl; (d) BEA/ Tb3+/phenyl; (e) BEA/Tb3+/C16.
efficiency of lanthanides in sol-gel materials.20,38 Zeolite framework exhibits only low-energy phonons which are not expected to contribute to the nonradiative deactivation of Tb3+ photoluminescence.20 PL quenching related to ion-ion interactions located in close proximity is generally negligible for the 5D4 level of terbium and at the concentrations used.39-40 Generally, three of up to four exponentials were needed to fit the terbium PL decays using method (a) with decay times varying in the range 10-40, 100180, 480-580, and 900-1400 µs. Similar results were found using a Gaussian distribution of decay times based on method b (see Experimental Section). Unless a physical model is available, the amplitudes and decay times that resulted from the discrete exponential fit were not readily interpretable in terms of luminescent species. As the broadness of the excitation spectra of Tb3+ in BEA zeolites rendered the measurements under site-selective excitation conditions ineffective (spectra not shown), time-resolved emission spectra (TRES) at different time delays after the laser pulse were analyzed. The changes in spectral shape, spectral shift, or appearance of new peaks with time are generally used as an indication of the heterogeneity of the system.41 Such changes, though small, were found in the peak-normalized TRES (or twodimensional contour plot of TRES, Figure 7) of all terbiumBEA zeolites, in the time range of 4 < t < 16000 µs after laser excitation. However, specifying the number of terbium species that are responsible for the observed TRES behavior is not straightforward.39 In contrast with methods (a) and (b), analysis of TRES by MaxEnt is model-free, i.e., the information is obtained without pre-conditioned ideas about the number and form of the (38) Horrocks, W. D., Jr.; Sudnick, D. R. J. Am. Chem. Soc. 1979, 101, 334. (39) Tiseanu, C.; Kumke, M. U.; Parvulescu, V. I.; Koti, A. S. R.; Gagea, B. C.; Martens, J. A. J. Photochem. Photobiol. A Chem. 2007, (2-3), 299. (40) Choi, Y.; Sohn, K. S.; Park, H. D.; Choi, S. Y. Mater. Res. 2001, 16, 881. (41) Lakowicz, J. R. Principles of Fluorescence Spectrometry; Plenum Press: New York, 1983.
Figure 3. FT-IR spectra of the parent and modified BEA/Tb3+ zeolites in the region of C-H stretching vibrations (A) and C-H bending and C-C stretching vibrations (B): (a) BEA/Tb3+; (b) BEA/Tb3+/vinyl; (c) BEA/Tb3+/phenyl; (d) BEA/Tb3+/C16. With (/) are denoted the Csp3-H bending vibrations.
exponential decays present in the data. The MaxEnt lifetime distributions corresponding to the 545 nm band area decay of terbium PL in the parent and vinyl-, phenyl-, and C16-modified BEA zeolites are represented in Figure 8. Except for a short decay at 10-20 µs, up to three groups of decay times between were obtained depending on the organic moiety: 100-140 µs (phenyl and C16), 480 (parent, vinyl, and phenyl) to 580 µs (C16) and 1 (parent, vinyl, and phenyl) to 1.3 ms (C16) were obtained. Note that the lifetime distributions (Figure 8, black lines) were not forced to take positive values, but the positive values of the lifetime distributions emerged in a natural way. Using a positiverestricted entropy expression and forcing the regularization value slightly, it was possible to obtain better resolved MaxEnt lifetime distributions (Figure 8, gray lines), with both components at 480-580 µs and 1-1.3 ms nearly resolved. From the more
Hydrophobization Efficiency of Terbium BEA Zeolites
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Figure 6. Comparison between the PL decays of the parent and modified BEA/Tb3+ zeolites.
Figure 4. FT-IR spectra of the parent and modified BEA/Tb3+ zeolites in the O-H stretching vibrations. (a) BEA/Tb3+; (b) BEA/ Tb3+/vinyl; (c) BEA/Tb3+/phenyl; (d) BEA/Tb3+/C16. Figure 7. Two-dimensional contour plot of TRES spectra corresponding to the 545 nm based transition of the BEA/Tb3+/C16. The dashed line reports the observed evolution with time of the 545 nm peak transition enlarged in B.
Figure 5. PL spectrum of BEA/Tb3+/vinyl measured 200 µs after the laser pulse.
resolved lifetime distributions a straightforward estimation of the area of the 480-580 µs and 1-1.3 ms bands could be made. The area of the band at ca. 500 µs represented the 90%, 80%, 75%, and 65% of the total area for parent, vinyl-, phenyl-, and C16-modified samples, respectively. Global exponential analysis of the time-resolved luminescence data at all the wavelengths provides the decay spectrum associated to each decay-time component (DAS or decay-associated spectrum). For a system with independent decaying species, the DAS corresponds directly to the species spectra. As with the
previous methods of analysis, up to four decaying components were observed, and four DAS were obtained. The shortest decay spectra at 10-20 µs were found to be very noisy in all BEA zeolites and did not reveal any spectral features of the terbium PL. This component was noise-related and discarded from the final analysis. Although the DAS for the component at 80-140 µs (BEA/Tb3+/phenyl and BEA/Tb3+/C16 zeolites) displayed the spectral features of the terbium PL, it was still difficult to discriminate between a real terbium PL spectrum and one induced by some experimental limitations/artifacts. A quenched terbium emission with lifetimes at 80-140 µs can be induced, in principle, by an efficient nonradiative energy transfer to some defects or impurities, scattering artifacts, or a combination of these two causes. We should note here that similar short-time transients were previously observed with terbiumdoped MFI-type materials as well as USY and HMOR zeolites.39,42 We can only argue that the nature of zeolite structure on the short-time behavior of terbium PL is expected to play a secondary role. Therefore, due to the lack of unambiguous knowledge of the short component at 80-140 µs, only two terbium species (42) Tiseanu, C.; Kumke, M. U.; Gessner, A; Parvulescu, V. I.; Lorenz-Fonfria, V. A. Manuscript in preparation.
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Tiseanu et al. Scheme 2. Schematic Representation of Terbium Species in the Modified BEA Zeolites
Figure 8. Lifetime distributions recovered by MaxEnt for the parent and modified BEA/Tb+3 zeolites using 545 nm band luminescence area: (a) parent, (b) vinyl-modified, (c) phenyl-modified, and (d) C16-modified zeolites. MaxEnt lifetime distribution without signrestriction (black line) and positive-restricted MaxEnt lifetime distribution (gray line).
Figure 9. Decay-associated spectra (DAS) corresponding to the main terbium species in the modified BEA/Tb3+ zeolites (exemplified with BEA/Tb3+/C16).
with lifetimes at ∼500 and ∼1000 µs will be considered. As seen in Figure 9, the spectra of the two terbium species obtained from their corresponding DAS are relatively similar, differing mainly in their intensity. These results are in line with TRES evolution depicted in Figure 7.
Zeolite BEA has a three-dimensional interconnected channel system with 12-membered elliptical openings having mean diameters of 0.67 nm.43 The molecular sizes of the phenyl- and vinyltrimethoxysilanes are close to those of the zeolite pore openings while hexadecyltrimethoxysilane is much larger. FTIR, TG, and physical adsorption data evidenced an increased hydrophobicity of the BEA zeolite modified with the hexadecyl group relative to that of the parent zeolite, providing a more effective blocking of the pores (Figures 3 and 4, Table 1). The results show that the silylation reaction presented in Scheme 1 occurred predominantly with OH bridging and the OH of attached terbium species. The investigated BEA zeolite had a Si/Al molar ratio of 12.5 and from the used quantities (see Experimental Section) it is clear that Al is in excess to Tb3+ (1.25 × 10-3 M AlO4- versus 1 × 10-4 M Tb3+). Under these conditions it should be considered that terbium exists in zeolite only as a countercation in the close proximity to Al and not as a physically adsorbed species or as a countercation in the close proximity of silica. The nonexponential pattern of terbium PL decays in the modified zeolites was tentatively attributed to the existence of two terbium species differentiated by their accessibility for water attack. The two groups of decay times varying between 480 and 580 µs and 1-1.3 ms were assigned relative to the lifetimes of terbium exposed to water (unprotected) and protected by the organic groups, respectively (Scheme 2). Such a behavior should be again correlated with the intrinsic properties of the BEA zeolite. The used zeolite exhibited a rather small crystalline size (about 300 nm) and a very high external surface (174 m2/g from the total of 540 m2/g). Therefore, we may speculate that the two terbium species can be associated with terbium species compensating AlO4- charge of Al located on the external surface and in the close proximity to the AlO4- inside the micropores. After derivatization with organosilanes it is very probable that only Tb3+ located inside the micropores is effectively protected while Tb3+ located on the external surface is still exposed to humidity. A quantitative estimation of the silylation efficiency can be made based on the relative amplitude corresponding to terbium species with longer lifetime. Accordingly, the silylation efficiency of the BEA/terbium zeolites follows the order C16 (35%) > phenyl(25%) > vinyl (20%). If one calculates the loss (43) Newsam, J. M.; Treacy, M. M. J.; Koetsier, W. T.; De Gruyter, C. B. Proc. R. Soc. London A 1988, 420, 375.
Hydrophobization Efficiency of Terbium BEA Zeolites
of the surface area after the zeolite derivatization and normalizes to 1 g of zeolite, the results parallel the following order: C16 (51%) > phenyl (20%) > vinyl (14%).
4. Conclusions Our study presented an investigation of the functionalized terbium-exchanged zeolite BEA with hexadecyl-, phenyl-, and vinyl-trimethoxysilanes based on PXRD, FT-IR ,TGA, physical adsorption, and time-resolved luminescence spectroscopy. Structural and textural data evidenced that the hexadecyl group is the most effective in blocking zeolite BEA pores and thus blocking
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the water intrusion at the terbium sites within modified zeolites, while vinyl and phenyl gave similar but smaller effects. Timeresolved luminescence results pointed to a two species terbium distribution differentiated in terms of protection from water (moisture) attack. The quantitative estimation of silylation efficiency based on time-resolved luminescence data was in good agreement with the textural and structural data. Such results indicate that terbium luminescence can probe the extent of the silylation of BEA zeolites with various inert groups and provide quantitative estimates for its efficiency. LA063308Y
