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J. Phys. Chem. C 2010, 114, 15448–15453
Internal and External Acidity of Faujasites As Measured by a Solvatochromic Spiropyran Ingolf Kahle and Stefan Spange* Department of Polymer Chemistry, Chemnitz UniVersity of Technology, Strasse der Nationen 62, 09111 Chemnitz, Germany ReceiVed: May 26, 2010; ReVised Manuscript ReceiVed: August 11, 2010
The adsorption of the photochromic 1,3,3-trimethyl-6′-nitroindoline-2-spiro-2′-benzopyran (SP) on Y-zeolites from dichloromethane solution and the ship in the bottle reaction (SIBOR) of SP have been studied by measuring the UV/vis absorption of the resulting materials to achieve information on polarity of the internal and external surface environments. The acidity and R values (hydrogen bond donating ability) of the faujasites significantly differ whether the probe is located at the external surface or inside the Y-zeolite. Additionally, dicyanobis(1,10-phenanthroline)iron(II) [Fe(phen)2(CN)2] (1) and 4-tert-butyl-2-(dicyanomethylene)-5-[4(diethylamino)benylidene]-∆3-thiazoline (2) were used as solvatochromic probes for the external surface. The empirical Kamlet-Taft polarity parameter R (hydrogen bond donating (HBD) ability) and π* (dipolarity/ polarizability) have been determined for the faujasites surfaces using the linear solvation energy relationship (LSER). Introduction Zeolites are widely used in the petrochemical industry in which they act as size-selective catalysts for instance in fluid catalytic cracking and hydrocracking.1 The catalytic activity of zeolites is mainly associated with the internal acidity of the host.2-5 Classical zeolites are built up by corner-sharing TO4 tetrahedra (T: Si, Al) forming a three-dimensional system of channels and cavities. The substitution of a tetravalent silicon atom by a trivalent aluminum atom in a TO4 unit generates a negative charge, which is then balanced by a charge-compensating cation. These cations can be completely or partially replaced by means of ion exchange, giving rise to a large variety of cation-exchanged zeolites which have different acidity strengths. The roles of Lewis-acid sites and cation sites are not very clear. Lewis-acid sites have been reported to have a modifying effect on the strength of Brønsted-acid sites that are present or may act directly by forming charge-transfer complexes.6 However, there are several approaches to determine the strength of internal acidity of zeolites, e.g., HY, L, or ZSM-5 materials using quantum mechanical calculations, microcalorimetry, or temperature-programmed desorption.7-9 An alternative approach involves the interaction of appropriate probe molecules with solid acid catalysts which reflects adequately the key process that is responsible for the catalytic reaction.10-17 Beside NMR probes such as phosphane oxides or acetone, the use of indicator dyes has been reported.18,19 Unfortunately, common HAMMETT indicators which are useful for superacids in solution do not reflect the results which derive from a catalytic process.6,20 Fortunately, specific solvatochromic dyes are suitable to measure the external surface polarity of various solids as shown by the correlation of kinetic data of heterogeneously induced processes with surface acidity parameters.21 There are several solvatochromic probes which are well established to measure adequately surface polarity in terms of the well-accepted Kamlet-Taft empirical polarity parameters, e.g., dicyano* To whom correspondence should be addressed: e-mail stefan.spange@ chemie.tu-chemnitz.de; Fax: +49 (0) 371 531 21239; Tel: +49 (0) 371 531 21230.
SCHEME 1: Chemical Structure of Solvatochromic Probes 1 and 2
bis(1,10-phenanthroline)iron(II) [Fe(phen)2(CN)2] (1) and 4-tertbutyl-2-(dicyanomethylene)-5-[4-(diethylamino)benylidene]-∆3thiazoline (2) (Scheme 1). Kamlet-Taft solvent parameters have been established for common used solvents, ionic liquids, and solid particles, which allow a differentiated polarity assignment to be made.5,7,21-32 The Kamlet-Taft equation, in its simple form, is given in eq 1:
(XYZ) ) (XYZ)0 + aR + bβ + sπ*
(1)
(XYZ) is the result of a solvent-dependent process; (XYZ)0 is a reference system, for example, a nonpolar solvent. R describes the hydrogen bond donating (HBD) ability, β describes the hydrogen bond accepting (HBA) ability, and π* describes the dipolarity/polarizability of the solvent. a, b, and s are the solventindependent correlation coefficients, which reflect predictions of the influence of the respective parameters on the result (XYZ) of the chemical process. [Fe(phen)2(CN)2] (1) is an excellent solvatochromic probe which can be used to determine gradually hydrogen bond donating ability (R) of various silica samples and other solid acids and to estimate acceptor numbers of Lewis acid molecules.26,30c,31 Respective R values of solid acids range from about 1.00 (common silica) to 3.00 (tungsten phosphoric acid). Thus, R of solid acids is in the order of moderate and strong
10.1021/jp1048106 2010 American Chemical Society Published on Web 08/23/2010
Internal and External Acidity of Faujasites SCHEME 2: Photochromic Equilibrium between Colorless Spiro Form (I) and Colored Merocyanine (II) of SP
acidic solvents.25,27 Adsorption of Fe(phen)2(CN)2 on zeolite materials is readily possible, but only the acidity of the external surface is measurable. The molecular dimension of 1 disallows entering the window of zeolite pores of faujasites. The ship in the bottle reaction (SIBOR) toward 1 inside Y-zeolites was still not successfully accomplished due to the large size of Fe(phen)2(CN)2 and the sterically demanding ligand exchange reaction expected during its synthesis. In this work the SIBOR of the solvatochromic 1,3,3-trimethyl-6′-nitroindoline-2-spiro2′-benzopyran (SP) within faujasites and the effect of the internal surface polarity on the UV/vis absorption are central to our study. Spiropyran dyes possess the ability to change from their colorless spiro form to the intensively colored merocyanine form upon UV irradiation, influenced by temperature or acidic sides of a surrounding medium (Scheme 2).34,35 Because of the dipolar character, the merocyanine form (II) of SP is able to undergo strong interactions with the environment which has an effect upon the position and intensity of the UV/ vis absorption band. For this reason, spiropyran dyes could be used as suitable solvatochromic probes for polarity investigation on acidic surfaces. In spite of the fact that the solvatochromism of SP is already reported, the results in literature are not adequate for use in linear solvation energy relationship (LESR) relating to the number and variety of used solvents.39-42 Especially the use of strong HBD solvents (R > 1) is of importance to show whether a common acid-base reaction occurs or appropriate solvation takes place without chemical alteration of the probe. The strategy for the SIBOR to synthesize SP inside the pores in order to determine the inner surface polarity of zeolites is given in Scheme 3. The in situ synthesis of SP in faujasites by using ultrasound is a very effective method which allows a fast reaction and high dye loadings of the zeolites compared with previous results in literature.36,37 UV/vis measurements, DRIFT, and 13C-{1H}-CP-MAS NMR spectroscopy were used to confirm the formation of SP inside the zeolites and to investigate interactions between host material and the guest probe. However, there are several studies which report on solvatochromism of spiropyran compounds and ship in the bottle reaction in Y-zeolites.36-43 In contrast to these works, the special objective of this study is to show whether a correlation of surface acidity of SP with those of common solvatochromic probes is educible in order to have an access to polarity parameters of internal cavities. Additionally, the highly specific solvatochromic probes 1 and 2 were used in order to show in what extent an independent determination of polarity parameter according to Kamlet-Taft for the external surface is possible.28-32 Experimental Section Materials. 1,3,3-Trimethyl-2-methyleneindoline (98.5%; ACROS) and 2-hydroxy-5-nitrobenzaldehyde (98%; ABCR)
J. Phys. Chem. C, Vol. 114, No. 36, 2010 15449 were used without further purification. All used solvents were redistilled over appropriate drying agents prior to use. Ship in the Bottle Reaction of 1,3,3-Trimethyl-6′-nitroindoline-2-spiro-2′-benzopyran. Previously the Y-zeolites (1.00 g) were dried at 180 °C under vacuum (5 mbar) for 3 h. A solution of 1,3,3-trimethyl-2-methyleneindoline (1 mmol) in 20 mL of absolute ethanol was added, and the mixture was shaken for 5 h to ensure the diffusion of the molecules inside the cages. After that, 2-hydroxy-5-nitrobenzaldehyde (2 mmol) dissolved in 10 mL of ethanol was added, and the mixture was irradiated with ultrasound (55 W) for about 20 min at room temperature. At the end of the reaction, the solid was filtered and extracted in a Soxhlet with ethanol and dichloromethane and dried under vacuum for 3 days. The quantitative elemental analysis of the resulting powders were C: 7.48% N: 1.01% (C/N ratio: 7.42) for HY zeolite and C: 5.82% N: 0.54% (C/N ratio: 10.79) for DAY zeolite. The theoretical C/N ratio is 8.14. Quantitative Elemental Analysis. The mass of incorporated spiropyran in the faujasites was determined by quantitative elemental analysis (C, H, N) at a Vario EL from Elementar Analysensysteme GmbH Hanau. 13 C-{1H}-CP-MAS NMR Spectroscopy. Solid-state NMR spectra were recorded at 9.4 T on a Bruker Avance 400 spectrometer equipped with double-tuned probes capable of MAS (magic angle spinning). 13C-{1H}-CP-MAS NMR spectra were measured at 100.6 MHz in 4 mm standard zirconium oxide rotors (Bruker) spinning at 12.5 kHz. Cross-polarization with a contact time of 5 ms was used to enhance the sensitivity. The recycle delay was 5 s. Spectra were referenced externally to tetramethylsilane (TMS) as well as to adamantane as secondary standard (38.48 ppm for 13C). DRIFT Spectroscopy. Diffuse reflectance IR Fouriertransform (DRIFT) spectra were recorded at a BIO-RAD FTS 165 spectrometer with a praying mantis device. The host-guest materials were mixed with dry KBr. Pure potassium bromide was used as the reference. UV/vis Measurements. The UV/vis spectra of the solids were recorded in reflection technique by means of a MCS 400 diode array spectrometer from Carl Zeiss Jena GmbH, with a resolution of 1 nm. Analysis of the spectra was performed with Win-Aspect (version 1.31, Carl Zeiss Jena GmbH). For studying solvatochromism of SP, measurements were performed in solution with the transmission technique, using precision quartz cells with a light path of 2 mm. The longest wavelength UV/vis absorption maxima of the probes 1, 2, and SP were used for the LSER calculations. Results Solvatochromism of Spiropyran Dye in Well-Behaved Regular Solvents. The synthesis of SP was carried out using appropriate synthetic techniques.44 The solvatochromism of the compound was investigated in 38 organic solvents taking into account strong HBD solvents such as 2,2,2-trifluoroethanol (TFE) and 1,1,1,3,3,3-hexafluoroisopropanol (HFI). SP shows the shortest wavelength shift in HFI (λmax ) 474 nm) and exhibits the largest bathochromic shift in n-hexane (λmax ) 624 nm) which results a solvatochromic range of ∆ν ) 5071 cm-1. Two UV/vis absorption bands of SP are observed in HFI. Obviously, a portion of SP becomes protonated as shown by the acid-form showing the UV/vis absorption maximum at 384 nm. The shoulder at 474 nm relates to the solvatochromic merocyanine form of the spiropyran dye and is used for
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SCHEME 3: Strategy for the SIBOR of Nitrospiropyran within Y-Zeolite
correlation analysis. Some representative UV/vis spectra of SP in various solvents including TFE and HFI are shown in Figure 1. The results of solvatochromic measurements and multiple square correlation analyses of SP are summarized in Tables 1 and 2. As expected, νmax(SP) is a function of R and π* but is not affected by the hydrogen bond accepting ability (β) of the solvent.43 The correlation coefficient r2 is 0.94 for LSER νmax(SP) ) f(R,π*), which indicates a high validity of the multiparameter equation. The positive sign of the coefficient s correlates with a low dipole moment of the electronically excited state compared with the ground state of the merocyanine form. Protic solvents, which can act as hydrogen bond donors (HBD), strongly interact with the phenolate oxygen of the merocyanine form of SP. That interaction lowers the push-pull character of the chromophore, causing a hypsochromic shift of the UV/vis absorption band with increasing the HBD ability strength of the solvent. The correlation of νmax(SP) solely with R gives an adequate regression too which does not apply to π*. This is an important result for the intention to use SP as solvatochromic probe for determination of R. The square correlation analysis taking into account only νmax of protic solvents and R provides a LSER with a very good significance (eq 2).
ν˜ max(SP) (×10-3 cm-1) ) 16.881 + 2.131R n ) 12;
r2 ) 0.96;
sd ) 0.19;
F < 0.0001
(2)
In Figure 2 the LSER are shown in comparison with each other. Because of the excellent correlation of νmax(SP) with R, eq 2 can be directly used to determine R values of acidic surfaces as long as the unprotonated form of the spiropyran dye is detectable.
Figure 1. UV/vis absorption spectra of SP in some protic solvents with R values in the range of 0.42 (tert-butanol) to 1.96 (HFI).
TABLE 1: UV/vis Absorption Maxima of the Solvatochromic SP Measured in 38 Various Organic Solvents and the Kamlet-Taft Solvent Parameters r, β, and π* solvent
R
n-hexane tetrachloromethane benzene p-xylene toluene diethyl ether anisole 1,4-dioxane chloroform ethyl acetate dimethoxyethane 1,2-dichloroethane tetrahydrofuran pyridine dichloromethane 1,1,2,2-tetrachloroethane benzonitrile hexamethylphosphoramide tetramethylurea N,N-dimethylacetamide acetone N,N-dimethylformamide 4-butyrolactone dimethyl sulfoxide tert-butanol acetonitrile nitromethane 1-decanol N-methylformamide 2-propanol 1-butanol 1-propanol ethanol formamide methanol 1,2-ethanediol 2,2,2-trifluoroethanol 1,1,1,3,3,3-hexafluoroisopropanol
0 0 0 0 0 0 0 0 0.20 0 0 0 0 0 0.13 0 0 0 0 0 0.08 0 0 0 0.42 0.19 0.22 0.70 0.62 0.76 0.84 0.84 0.86 0.71 0.98 0.90 1.51 1.96
β
π*
0 -0.04 0.10 0.28 0.10 0.59 0.12 0.43 0.11 0.54 0.47 0.27 0.32 0.73 0.37 0.55 0.10 0.58 0.45 0.55 0.41 0.53 0.10 0.81 0.55 0.58 0.64 0.87 0.10 0.82 0 0.95 0.37 0.90 1.05 0.87 0.80 0.83 0.76 0.88 0.43 0.71 0.69 0.88 0.49 0.87 0.76 1.00 0.93 0.41 0.40 0.75 0.06 0.85 0.82 0.45 0.80 0.90 0.84 0.48 0.84 0.47 0.90 0.52 0.75 0.54 0.48 0.97 0.66 0.60 0.52 0.92 0 0.73 0 0.65
λmax(SP) νmax(SP) (nm) (×10-3 cm-1) 624 611 602 602 602 596 592 589 582 580 579 578 578 578 577 577 576 572 571 566 565 564 560 559 558 557 555 550 548 547 541 539 538 537 526 522 498 474
16.03 16.37 16.61 16.61 16.61 16.78 16.89 16.98 17.18 17.24 17.27 17.30 17.30 17.30 17.33 17.33 17.36 17.48 17.51 17.67 17.70 17.73 17.86 17.89 17.92 17.95 18.02 18.18 18.25 18.28 18.48 18.55 18.59 18.62 19.01 19.16 20.08 21.10
UV/vis Absorption Studies of 1, 2, and SP on Faujasites. The adsorption and incorporation of SP onto the external surface and inside the Y-zeolites were studied using 13C-{1H}-CP-MAS NMR, DRIFT, and UV/vis spectroscopy. The 13C-{1H}-CPMAS NMR spectroscopy was used to confirm the formation of the spiropyran dye within the Y-zeolite supercages. Beside the merocyanine form, generated by acidic ring cleavage, the 13C{1H}-CP-MAS NMR spectrum shows several signals arising from solvents as well as from starting materials which were used during the ship in the bottle reaction. For allocation of the signals and comparison with 13C NMR in solution, a spectrum of SP was recorded in acidified CDCl3 (mixture with 5% trifluoroacetic acid) to enforce acidic ring cleavage of SP (Figure 3). A complementary investigation of the solid adsorbates was the use of quantitative elemental analysis. The carbon content of dye-loaded HY and DAY zeolites were 7.48 and 5.82%,
Internal and External Acidity of Faujasites
J. Phys. Chem. C, Vol. 114, No. 36, 2010 15451
Figure 2. Relationship between calculated and measured νmax values for SP in 38 organic solvents using Kamlet-Tafts R and π* (a) and in 12 protic solvents using solely Kamlet-Tafts R value (b).
TABLE 2: Values of the Solvent-Independent Correlation Coefficients (a, b, and s of the Kamlet-Taft Parameters r, β, and π* According to Eq 1), Solute Property of a Reference System νmax,0, Correlation Coefficient (r), Standard Deviation (sd), and Significance (f) for SP in 38 Organic Solvents νmax,0
a
b
s
r2
sd
f
16.114 ((0.128) 16.195 ((0.127) 17.158 ((0.081) 16.868 ((0.482)
1.896 ((0.081) 1.908 ((0.084) 1.889 ((0.142)
0.255 ((0.123)
1.395 ((0.170) 1.443 ((0.176)
0.94 0.94 0.83 0.09
0.24 0.24 0.42 0.96
