Probing the Surface Polarity of Various Silicas and Other Moderately

J. Phys. Chem. B 2000, 104, 6417-6428

6417

Probing the Surface Polarity of Various Silicas and Other Moderately Strong Solid Acids by Means of Different Genuine Solvatochromic Dyes Stefan Spange,* Elmar Vilsmeier, and Yvonne Zimmermann Department of Polymer Chemistry, Institute of Chemistry, Chemnitz UniVersity of Technology, Strasse der Nationen 62, D-09111 Chemnitz, Germany ReceiVed: January 18, 2000; In Final Form: May 9, 2000

Reichardt’s ET(30) as well as Kamlet-Taft’s R (hydrogen-bond-donor acidity) and π* (dipolarity/polarizability) values of various solid acids, e.g., silicas, aluminas, alumosilicates, titanium dioxides, and aluminafunctionalized silica particles, are presented. The ET(30) values of the solid acids were directly measured by UV/vis spectroscopy using 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)phenolate (1a) and an eicosafluorinesubstituted derivative of 1a (1b) as surface polarity indicators. Kamlet-Taft’s R and π* parameters for the various solid acids are analyzed by means of Fe(phen)2(CN)2 [cis-dicyano-bis(1,10-phenanthroline)iron(II)] (2) and Michler’s ketone [4,4′-bis(N,N-dimethylamino)benzophenone] (3) as solvatochromic surface polarity indicators. The chemical interpretation of the R and π* parameters and the nature of the surface sites which they reflect are discussed. The correspondence of the UV/vis spectroscopic results to those of the IR-sensitive probe benzophenone and the fluorescence probe pyrene (literature data) is also discussed. It is evident that the solid surface environments observed by the various indicators are moderately strong dipolar/polarizable (π* ) 0.38 to 1.04) and are fairly strong hydrogen-bond donors (R ) 1.00 to 1.99). Theoretical ET(30) values of solid acids are calculated by applying linear solvation energy (LSE) relationships using the independently measured R and π* values of the solid acids according to ET(30) ) [ET(30)]o + aR + sπ*. The respective dependence of the R and π* terms, expressed by the coefficients a and s, upon the measured ET(30) value for solid acids is discussed in comparison to related multiple LSE correlations known for wellbehaved regular solvents and functionalized silicas. The results show in general that values of polarity parameters of strong HBD and dipolar surfaces are strongly influenced by the experimental conditions applied.

Introduction The quantification of the surface polarity and catalytic activity of solid acid catalysts is an important field of materials research.1-4 This area of research has been stimulated by the application of improved adsorption-calorimetric and solid-state NMR spectroscopic methods and techniques to well-established solid catalyst materials in examining their surface properties.4-8 These methods require special equipment and are particularly time-consuming. Recently, a number of original contributions have been published where the development and use of novel solid acids are described.9,10 Therefore appropriate methods are required which allow a rapid check of the relevant surface properties of a solid acid catalyst. For instance, the catalytic activity of solid acids can be simply estimated by Hammett indicators, which can also be applied to nonaqueous media.11-14 However, the original Hammett indicators allow only a rough estimation of the surface acidity of solid acids. They are not suitable for observing the overall surface polarity.1,13 To date, no generally agreed upon definition of the term surface polarity has emerged.15 In the broadest and most general sense the surface polarity can be viewed as the sum of all interactive forces between an adsorbed molecule and the occupied surface site or sites. This definition is based on the related interpretation of the term solvent polarity.16 Although solvent polarity is among the most widely used concepts in chemistry (see refs 16-19 for reviews), there has been little consideration of the changes in surface polarity associated with structural variations in solid acids.1-8 Most solvent polarity scales are empirical and are based * Author to whom correspondence should be addressed.

on kinetic, thermodynamic, or spectroscopic data relating to certain reference reactions.17-22 Significantly, different empirical solvent polarity scales have been shown to correlate well with each other, pointing to the existence of an underlying common feature.18,23,24 (See also the discussion of the meaning of eqs 2, 3, and 4.) Empirical solvent polarity scales based on spectroscopic measurements usually employ changes in the UV/vis absorption maximum of an indicator in different solvents (solvatochromism).17 Visible probing with suitable indicator dyes was also recommended by several authors for investigating the surface polarities of solids.25-32 Reichardt’s dye and substituted derivatives of it are very sensitive solvatochromic polarity indicators for detecting gradual differences in the polarity of various environments.17,22,33-35 We,24,30b,36 and others,31,32 recently reported that the polarity of silica, organically functionalized silica, or alumina surfaces can also be investigated by Reichardt’s dye 1a (see eqs 1a and 1b, and Chart 1).

ET(30) [kcal mol-1] ) 2.891 × 10-3 νmax (1a) [cm-1] (1a) ETN ) [ET(30) - 30.7]/32.4

(1b)

For instance, the ET(30) value is about 58 to 59 kcal mol-1 for bare silica particles36 and 62 to 65 kcal mol-1 for γ-alumina powders.32 Unfortunately, Reichardt’s standard dye 1a can be used only with weak acids because its phenolate oxygen atom becomes protonated in acidic environments with pKa values lower than

10.1021/jp000231s CCC: $19.00 © 2000 American Chemical Society Published on Web 06/21/2000

6418 J. Phys. Chem. B, Vol. 104, No. 27, 2000

Spange et al.

CHART 1: Formulas of the Probe Dyes Used

6.35 Therefore widespread use of the dye 1a for measuring the surface polarity of a variety of solid acids, e.g., aluminas, titanium oxides, alumosilicates, is not possible. However, the interaction of a surface environment with a solvatochromic dye is a composite of many effects. Acid-base, dipole-dipole, induced dipole-dipole, and dispersion forces contribute to the overall adsorption energy of a probe with an inorganic surface. This means that for each UV/vis spectrum of an adsorbed solvatochromic dye measured, surface sites of different polarity as well as different contributions of specific (acid-base) and non specific (dipolar-polarizable) interactions must be taken into account. Multiple intermolecular solute/solvent interactions can be described by the LSE (linear solvation energy) relationship of Kamlet and Taft.37 The simplified Kamlet-Taft equation applied to single solvatochromic shifts, XYZ ) νmax(probe),17 is given in eq 2:

XYZ ) (XYZ)o + aR + bβ + s(π* + dδ)

(2)

(XYZ)o is the solute property of a reference system, e.g., a nonpolar medium, R describes the HBD (hydrogen-bonddonating) acidity, β the HBA (hydrogen-bond-accepting) ability, and π* the dipolarity/polarizability of the solvents. δ is a polarizability correction term which is 1.0 for aromatic, 0.5 for polyhalogenated, and zero for aliphatic solvents; a, b, s, and d are solvent-independent correlation coefficients.37 First, Marcus has shown that the empirical ET(30) solvent polarity parameter can be expressed by an LSE relationship using the Kamlet-Taft solvent parameters R and π* [eq 3]; n is the number of solvents considered, r is the correlation coefficient, and sd is the standard deviation.38

ET(30) ) 31.2 + 15.2R + 11.5π* (a/s ) 1.32)

(3)

n ) 155, r ) 0.98, sd ) 1.1 Equation 3 demonstrates that the ET(30) value for solvents contains contributions of the hydrogen bond acidity (60%) and

dipolarity/polarizability (40%) of the solvents. It was also shown that the β term of the solvents scarcely contributes to the ET(30) value of well-behaved regular solvents.17,38 Coordination of protic solvents to the phenolate oxygen of the betaine dye causes a strong hypsochromic shift because the electron density of the phenoxide part of the dye is drastically reduced. Consequently, a similar influence of HBD groups on the ET(30) value is observed when 1a is adsorbed on surface silanol groups.36 Several authors have shown that the Kamlet-Taft solvent parameters R, β, and π* are better suited for the quantitative description of results of solvent-solid interactions, e.g. chromatographic and adsorption processes,39-42 than is the single ET(30) solvent parameter. Furthermore, the advantage of the Kamlet-Taft parameters is that they can be converted by means of multiple LSE relationships into other useful polarity scales, e.g., the acceptor number (AN) or donor number (DN) scales of Gutman and the ET(30) scale of Reichardt.18 LSE equations and results from pyrene and 4-(N,N-dimethylamino)benzonitrile (DMAB) fluorescence measurements have been also used for determining ET(30) values for cationexchanged Y-zeolites.43 The surface polarity of these solid acids was about ET(30) ) 45 to 60 kcal mol-1. However, these data are not representative, because the two indicators pyrene and DMAB, respectively, mainly measure the dipolarity/polarizability π* of an environment. Reasonable R values of zeolites were reported by Dutta and Turbeville, using salicylideneaniline as a UV/vis probe.27 The authors found that the R value of zeolites significantly depends on their Al/Si ratios. The R value was between 1.06 and 1.14 (Rav ) 1.1) for the pure siliceous form, approximately 1.5 to 1.6 for the alumina phase, and the maximum R value (R ) 1.85) was found at a Si/Al ratio of about 8. π* values for silicas and alumosilicates were also reported using 4-nitroanisole and related indicators.25 The reported values changed between π* ) 0.65 and 2.00. We are afraid that the reported π* values for the silica surface from ref 25 also contain contributions of acid-base interactions, because the calculated π* values are dependent on the basicity strength of the indicator used.30b An independent method for confirming the accuracy of the observed R and π* values of solid acids is difficult to find, because there are only a few probe molecules known which are suitable for this purpose. There are several known solventdependent processes, the results of which correlate with both, the R and π* or the β and π* term.44 For instance, the only π*-sensitive dye 5-(N,N-dimethylamino)-5′-nitro-2,2′-bisthiophene45 is not well adsorbed on silica and shows also a nonlinear UV/vis spectroscopic bathochromic shift on acidic surfaces.30b There are only two indicators which are known to be selectively responsive to the HBD term. These are pyridinium N-oxide46 and 4-nitropyridine-N-oxide.47 Unfortunately, the application of pyridinium-N-oxide to solid acids would require high-resolution solid state 13C NMR spectroscopy. Thus, a widespread use of this indicator is not possible. In strong HBD environments, the UV/vis absorption of 4-nitropyridine-N-oxide is expected at νmax > 30 000 cm-1 and can therefore not easily be utilized when the indicator is adsorbed on solid acids. In this paper, we employ a UV/vis spectroscopic method suitable for simultaneously examining the surface acidity R and the dipolarity/polarizability π* of moderately strong solid acids by means of the two surface polarity indicators Fe(phen)2(CN)2 (2) and Michler’s ketone (3).30a The transition metal complex Fe(phen)2(CN)2 [cis-dicyano-bis(1,10-phenanthroline)iron (II)]

Surface Polarity of Various Silicas and Other Solid Acids

J. Phys. Chem. B, Vol. 104, No. 27, 2000 6419

(Chart 1) has been especially recommended as a surface polarity indicator for acidic surfaces because its solvatochromic band shift in the UV/vis spectrum is significantly dependent on the acidity of the environment.48-55 The solvatochromic UV/vis absorption band of 3 is attributed to a π-π* transition which overlaps with the less intense n-π* transition.50,56 The solvatochromism of 3 has been investigated by several authors.50,56,57 On increasing both the HBD capacity and dipolarity/polarizability of a solvent, the UV/vis absorption band of 3 undergoes a significant bathochromic shift. The bathochromic effect is even significantly enhanced when a strong Lewis acid or a mobile proton interacts additionally with the carbonyl oxygen atom of 3 because a mesomerically stabilized oxenium ion is generated.58-60 In contrast, protonation of the nitrogen atoms of the dimethylamino groups by a Brønsted acid has a hypsochromic influence on the solvatochromic UV/vis absorption band of 3.50 For instance, the UV/ vis absorption band of [4-(CH3)2NC6H4]2CO-Si(CH3)3+ appears at λ ) 505 nm59 and of {[4-(CH3)2NHC6H4]2CO}2+ at λ ) 310 nm.50 Therefore, the indicator 3 seems suitable as a versatile UV/vis spectroscopic probe in order to observe different types of acidic sites and their different acidic strengths. Because the genuine solvatochromisms of the two indicators 2 and 3 can also be expressed very well in terms of the KamletTaft solvent parameters R and π*,37,50 their simultaneous use should allow the corresponding R and π* values of an unknown solvent or of a heterogeneous system to be determined. Then R and π* values of surface environments can be determined by utilizing the UV/vis absorption maxima of the respective surface polarity indicators [νmax(2) and νmax(3), respectively] when adsorbed on the solid.30,55 The mathematical equations are given in the Experimental Section.54 We have already demonstrated that the R value of organically functionalized silicas can be analyzed precisely by applying the two indicators 2 and 3.30a The R-value for alkyl-modified silicas decreases linearly with increasing the surface coverage since monolayer functionalization is accomplished.30a This result shows that the average polarity of all HBD sites was measured. In this paper, we have also presumed that the ET(30) parameter for functionalized silicas is more strongly affected by the HBD term than by the dipolarity/polarizability [eq 4 was taken from ref 37],30a compared to the LSE correlation derived for wellbehaved regular solvents [eq 3 from ref 18].

ET(30) ) 36.1 + 14.84R + 5.33π* (a/s ) 2.78)

(4)

n ) 30, r ) 0.96, sd ) 1.12 Chemists know from experience that various silica samples, obtained from several suppliers or synthesized by different experimental techniques, are often quite different in their surface properties, e.g., as supports for metallocene catalysts or in chromatographic applications. Despite the different catalytic activity of different silica samples, i.e., as supports for polar reactions, the ET(30) values of various silicas studied are rather similar and are in the range ET(30) ) 58 ( 1.5 kcal mol-1.30b This result has been reported independently by several authors.29-31,41 Furthermore, the thermal pretreatment of solid acids is also important for their surface activity. The objective of this paper is the determination of the empirical surface polarity parameters ET(30), R, and π* by means of the indicators 1a, 2, and 3 for various solid acids including different silicas, aluminas, titanium oxides, and alumosilicates. The correspondence of the R and π* values of solid acids to their ET(30) values are tested by means of

CHART 2: Two Different Structures of 3 Adsorbed on Brønsted and Lewis Acid Sites of Surfaces

correlation analyses with directly determined ET(30) values according to ET(30) ) aR + bπ*.17,30a For this purpose, an eicosafluorine-substituted phenolate betaine dye 1b61 (see Chart 1) is used. Because of the reduced basicity of the phenolate moiety, protonation of it by Brønsted acidic sites is suppressed. It is also well-known that alumina and several alumosilicates possess Brønsted as well as Lewis acid sites.62 According to well-established procedures for differentiating between these two sites, surface titration with pyridine and the sterically hindered base 2,6-di-tert-butyl pyridine (DTBP) is applied to the solvatochromic dyes when adsorbed on the solid acid in an inert liquid.63,64 It is well-established that DTBP interacts weakly with common Lewis acids (TiCl4, AlCl3, or the tert-butyl cation), but it reacts significantly with mobile protons.65 Thus, DTBP was successfully applied to trap protons in cationic isobutylene polymerization.66,67 We were able to show that DTBP trapped mobile protons on the internal surface of MCM-41.64 But we presume that DTBP is not adsorbed on Lewis acid sites (see also ref 63). For these measurements a special UV/vis technique has been successfully employed.64 For checking the order of magnitude of the R and π* values of the solid acids determined, the IR probe benzophenone (BP) is used in addition. Furthermore, this IR probe is of size similar to that of the solvatochromic indicator 3 and shows a significant shift of its carbonyl vibration when adsorbed on solids. The carbonyl stretching vibration (νmax,CO) of BP is also remarkably solvent-dependent and correlates excellently with the KamletTaft polarity parameters R and π* [ eq 5]. Equation 5 was taken from the literature.68

νmax,CO ) 1668.80 - 10.10π* - 11.00R

(5)

r ) 0.99, n ) 34 Pyrene fluorescence data from the literature should be also suitable for supporting the π* values of the solid acids.69 The intensity ratio (I1/I3) of the two UV/vis emission bands at λmax ) 354 nm (I1) and 374 nm (I3) of pyrene increases on increasing the dipolarity/polarizability π* of the solvent [eq 6]. Therefore,


Spange et al.

TABLE 1: Solid Acids Used in This Paper, Their Chemical Constitutions, and Physical Properties sample number

solid acid sample

BET-surface area [m2g-1]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

SiO2, Aerosil 200 SiO2, Aerosil 300 SiO2, Aerosil 380 SiO2, KG 60 SiO2, KG SG 432 SiO2, KG LC 1500 SiO2, 1.5% Al2O3 Cogel AlPO4, SP 8293 AlPO4, SP2-8111.09 SiO2/Al2O3, SP 18-8495 SiO2/TiO2, SP 18-8381.01 SiO2, SP 18-8470 SiO2/ZrO2, SP 18-8402.03 Al2O3, SP 18-8510 SiO2/Al2O3, SP2-8515.01 TiO2 P25 TiO2 (rutile) TiO2 (anatas) Alumosilicate, Siral 5 Alumosilicate, Siral 10 Alumosilicate, Siral 20 Alumosilicate, Siral 30 Alumosilicate, Siral 40 Alumosilicate, Siral 50 Alumosilicate, Siral 60 Alumosilicate, Siral 80 Al2O3, Pural KG 60 10% Al2O3c KG 60 30% Al2O3 KG 60 50% Al2O3 KG 60 70% Al2O3 KG 60 100% Al2O3 Al2O3 (AlEt2Cl) Al2O3 Al2O3-C

n.d.a 240 n.d.a 423 315 30 390 254 190 340 303 776 268 131 336 52 30 45 296 347 315 351 460 381 356 263 292 413 354 344 361 339 58 97 100

specific pore volume [cm3g-1]

0.63 1.42 n.d.a 0.90 0.57 0.61 1.02 1.22 0.57 1.68 0.72 0.80 0.27 0.14 0.16 0.46 0.44 0.35 0.36 0.81 0.50 0.34 0.42 0.54 0.79 0.78 0.74 0.77 0.56 0.17 0.50 0.57

av pore diameter [nm]

9.0 n.d.a 5.5 4.5 6.4 6.0 8.1 1.5 4.5 11.0 4.8 10.0 9.3 7.1 3.1 2.5 2.2 2.1 3.5 2.6 1.9 3.2 2.7 3.8 4.4 4.3 4.3 3.3 5.9 10.3 11.4

source Degussa Degussa Degussa Merck Grace Grace Grace Grace Grace Grace Grace Grace Grace Grace Grace Degussa Condea Condea Condea Condea Condea Condea Condea Condea Condea Condea Condea b b b b b b Condea Degussa

a Not determined. b See experimental part. c Grafted with diethylaluminum cloride (AlEt Cl) and hydrolyzed with water; the 10% means that 2 10% Al2O3 is produced per amount of silica.

Dong and Winnik recommended the LSE eq 6 as a tool for analyzing the value of the π* parameter of the surface environment.69

I1/I3 ) Py ) 0.64 + 1.33(π* - 0.24δ) - 0.25R

(6)

n ) 32, r ) 0.9 As seen from eq 6, for protic solvents and, accordingly, also for acidic surfaces, the knowledge of the R value is necessary in order to separate the π* value from the unit of measurement Py. Despite the wide application of pyrene for polarity measurements of solids,70,71 the separation of the π* value from the unit of measurement Py seems problematic because R and π* have an opposite influence upon Py. For silicas, a Py value of adsorbed pyrene of about 1.78 has often been reported.70 However, theoretically possible Py values can be estimated by means of eq 6 by knowledge of independently calculated R and π* values for comparison. Of course, the β term, the hydrogen-bond accepting property, also contributes to the overall surface polarity. However, the transition metal complex Cu(tmen)(acac)+, which is a suitable β indicator for functionalized silicas,72 is not adsorbed well by acidic surfaces and thus a direct examination of β values is unlikely to be possible. Suitable β indicators for acidic surfaces are still not established.72,73 We are afraid this is an intrinsic problem. However, it is expected that the β term of dried acidic surfaces should not contribute to the ET(30) value.72 We will report on this topic in a later paper.

Correlations of the examined polarity parameters R and π* with the catalytic activity of a surface-mediated polar reaction will be reported in the following paper.74 Experimental Section Chemicals. 2,6-Diphenyl-4-(2,4,6-triphenyl-1-pyridinio)phenolate (1a) and bis-2,6-(pentafluorophenyl)-4-[bis-2,6-(pentafluorophenyl)-4-phenyl-1-pyridinio]phenolate (1b) were kindly given by C. Reichardt, University of Marburg. The synthesis of 1b is still not reported in the literature.61 For supplementary information see ref 61 or contact Prof. C. Reichardt, University of Marburg, Gemany. Michler’s ketone was purchased from Merck and recrystallized twice from ethanol before use. Fe(phen)2(CN)2 was prepared and purified by the method of Schilt.75 The solvent 1,2-dichloroethane (spectroscopic grade) was purchased from Merck. It was dried over CaH2, freshly distilled before use, and stored over carefully dried anhydrous alumina. The chemical constitutions and physical properties of the solid acids used in this paper are listed in Table 1. The silicas, aluminas, alumosilicates, and titanium dioxides (samples No. 1-27 and Nos. 34, 35) were commercially available products. The producers of the solid acids are also indicated in Table 1. The alumina sample No. 33 was synthesized by a sol-gel process from diethylaluminum chloride and water. The partially alumina-functionalized silica particles No. 28-32 were synthesized by reaction of diethylaluminum chloride with silica in toluene and subsequent hydrolysis with water.

Surface Polarity of Various Silicas and Other Solid Acids Procedure for Sample Nos. 28-32. A typical procedure for synthesizing samples Nos. 28-32 is as follows: A 10-g sample of silica (KG 60®) was heated for 12 h at 400 °C in a glass flask. After the sample was cooled to room temperature under dry argon (CaH2), 100 mL of dry toluene were added. The desired amount of diethylaluminum chloride was added from a glass syringe through a septum. The reaction mixture was cooled in an ice bath during this step. After the addition was complete, the mixture was refluxed for 5 h. After cooling to room temperature the modified silica was collected in a closed glass reactor, washed with three portions of dry toluene, and dried under argon. The hydrolysis of the surface-bonded diethylaluminum chloride was carried out with a water-saturated argon gas flow over a period of 5 h. After a washing with a small portion of water, the alumina-modified silica was dried at 110 °C for 4 h. A final thermal treatment at 500 °C over 4 h yielded a white powder of alumina-functionalized silica, which was then used for the measurements. Procedure for Sample No. 33. The same apparatus was used as described before for the sample preparation of Nos. 28-32. In the glass flask, 10 mL of dry toluene were mixed with 10 mL (80.5 mmol) of diethylaluminum chloride. Then a watersaturated argon flow was passed through the solution over 4 h. To complete the hydrolysis, 20 mL of water were slowly added. After phase separation, the toluene phase was washed with water to remove residual aluminum hydroxide. The aqueous phases collected were combined and dried in a rotary evaporator under vacuum. The dried alumina powder was heated at 450 °C for 12 h and finally at 550 °C for 2 h. Sample Preparation for Determining the Aluminum Content of the Alumosilicates (Nos. 19-26) by AAS (Atomic Absorption Spectroscopy). The solid (0.1 g) was placed in a platinum crucible standing in lukewarm water and mixed with 1 g of ammonium fluoride. Under stirring, sulfuric acid was added dropwise to the mixture until a clear solution was obtained. The solution was heated for 15 min and then added to 10 mL of hydrochloric acid (2 mol/L). The resulting mixture was transferred to a glass flask which was filled with water to 100 mL. This solution (excluding sample No. 26) was diluted with water 1:1 because of the high aluminum content and then measured by AAS. The calibration for the AAS measurements was carried out with standard stock solutions containing known aluminum concentrations. UV/vis Measurements. The UV/vis absorption maxima of the dyes 1a, 1b, 2, and 3, respectively, adsorbed on the solid acid catalyst, have been recorded using a diode array spectrometer with glass fiber optics. The solid acids were heated for 12 h at 400 °C. After cooling the solid sample to room temperature under dry argon, a solution of the probe dye in 1,2-dichloroethane (DCE) is simply added to the solid material. Care must be taken to avoid overloading the surface with the indicators used, as multilayer adsorption is expected in solution at higher concentrations or interfering absorptions from the nonadsorbed dye from the solution. Therefore, the amount of dye added was restricted to 2-3 mg of 1a or 1b per 1 g of solid acid, 0.5 mg of 2 per 1 g of solid acid, and 0.1 mg of 3 per 1 g of solid acid. For the UV/vis measurement of transparent slurries, the amount of the solid acid silica must be limited in order to achieve a sufficient particle concentration in the continuously stirred slurry. An amount of 0.1 to 0.2 g silica in 15 mL of the solvent is very suitable, but a lower amount is also possible. The amount of the silica portion has no influence on the position of the UV/vis absorption maximum of the adsorbed probe dye.


Figure 1. UV/vis absorption spectra series of Fe(phen)2(CN)2 adsorbed on silica (No. 4) as a function of the probe concentration in 1,2dichloroethane, measured by the transmission technique. The corresponding spectra of the dye measured in the reflectance mode for the suspension and the dried colored silica are marked as strong solid lines. The concentration of the dye, c, for measurement with the transmission technique is given in [10-6 moldye/gsilica] (from the bottom to the top): c ) 2.01, λmax(2) ) 539 nm; c ) 4.01, λmax(2) ) 539 nm; c ) 6.02, λmax(2) ) 539 nm; c ) 8.02, λmax(2) ) 539 nm; c ) 10.00, λmax(2) ) 540 nm; c ) 14.0, λmax(2) ) 539 nm; c ) 18.1, λmax(2) ) 539 nm; c ) 20.1, λmax(2) ) 538 nm reflectance technique in suspension: 2.65 × 10-6 mol/gsilica, λmax(2) ) 542 nm (uncertain), reflectance technique without solvent: λmax(2) ) 530 nm. (The UV/vis absorption maximum is different from that in Table 3 because the silica used for this experiment was from another batch.)

The equipment employed was a UV/vis spectrometer MCS 400 connected to an immersion cuvette TSM 5A (Zeiss). The measurement of the UV/vis spectrum of the supernatant solution is possible in the same cell in which the solid is deposited. The measurements of silica particle suspensions are advantageous because they can be performed under inert conditions and reflect better the conditions suitable for liquid chromatography and catalysis in organic solvents. The reproducibility of the UV/vis spectra of the adsorbed dyes is very good. For silicas, a transparent suspension is obtained in liquid 1,2-dichloroethane. This allows one to take very good quality transmission spectra and with excellent reproducibility of less than λmax ( 1 nm. The position of the UV/vis absorption maximum of the adsorbed dye on the silica particles remains constant during 1-2 h (see later). Figure 1 shows a typical UV/vis spectra series of the dye 2 adsorbed on a silica sample (No. 4). It is clearly seen that the UV/vis absorption maximum of the adsorbed dye 2 is independent of its concentration. Because the silica slurry in 1,2dichloroethane is transparent, direct recording of reflectance spectra is not possible due to light scattering; see the terrible spectrum shown in Figure 1. However, for silica particle powders, UV/vis reflectance spectra can be readily monitored (see the second reflectance spectrum in Figure 1). Unfortunately, the reflectance technique is not well suited for excluding traces of water from the air. The hypsochromic effect observed in the UV/vis reflectance spectrum compared to the transmission spectrum (∆λ ) -10 nm) is due to moisture traces which significantly increase the surface polarity of silica. Furthermore, the influence of the suspension liquid upon the surface polarity is also of importance. Therefore we have measured the slurries of the carefully dried solid acid powders in the reflectance mode. For this purpose the UV/vis spectra of nontransparent particle suspensions of alumina, titanium dioxide, and of other solids were recorded by a special reflectance technique. A quartz plate is used as the bottom of the closed cell containing the solid sample in a suitable liquid. The sensor head for measuring the reflectance spectra is located at this quartz plate and the UV/ vis spectrum of the adsorbed dye can be monitored after the particles are deposited.


Spange et al.

This technique is very suitable for recording UV/vis spectra of particles in suspension under inert conditions. The reproducibility of the recorded UV/vis spectra is very good. The error on measuring the UV/vis absorption maxima of the dyes adsorbed on aluminas, alumosilicates, or titanium dioxides, is λmax ( 1.5 nm when applying the reflectance technique. The evidence from the measured UV/vis absorption maxima of the adsorbed dyes was tested by using an independent chemisorbed species: the triphenylmethylium ion. The positions of the UV/ vis absorption maxima of the triphenylmethylium ion at λmax ) 410 ( 2 nm and 430 ( 2 nm are independent of the solvent as well as of the counterion.76-78 In other words, they show no dependence on the environment. For each solid acid powder, the triphenylmethylium ion UV/vis absorption of chemisorbed chlorotriphenylmethane is independent of the solid acid used.48 Thus, physical influences upon the shift of the UV/vis absorption maxima of adsorbed indicator dyes are not really observed employing the reflectance UV/vis technique for the slurry. Surface titrations with the sterically hindered base 2,6-ditert-butylpyridine (DTBP) and pyridine were carried out directly in the slurry. The base was added by a microsyringe through a silicone septum. The spectra were recorded immediately after the particles were deposited. Three runs were carried out for each solid acid. The reproducibility is of the same order of magnitude as measured for the solid particles without the base. Amounts of nonadsorbed dyes, which possibly compete with the added base, can be detected in the supernatant solution by the immersion cell placed directly in the slurry. For all νmax data reported in this paper, a quantitative adsorption of the dye from the supernatant solution on the solid acid has been accomplished. Calculation of the Surface Polarity Parameters. ET(30) values can be directly calculated by using the νmax of dye 1a from the UV/vis spectrum and eq 1a. By using the νmax of the adsorbed dye 1b, the corresponding ET(1b) values are also calculated from eq 1a. The standard ET(30) values are then calculated from eq 7 according to ref 61.

ET(30) ) 1.05ET(1b) - 6.13

(7)

r ) 0.98, n ) 38, sd ) 1.33 The R and π* values were calculated by a correlation analysis of the measured absorption maxima νmax (in cm-1) of the dyes 2 and 3, using the corresponding multiple correlations of R or π* ) f[νmax(2) and νmax(3)] with the Kamlet-Taft solvent parameters as the reference system.50,54 This procedure is described in detail in ref 54. In this paper we used an extended set of absorption maxima of 2 and 3 in various solvents. The following multiple correlations, [eqs 8 and 9], were used to separate the respective property R or π* from the unit of measurement of the νmax(indicator) for 2 and 3.

R ) -7.90 + (0.45νmax(2) × 10-3) + (0.02νmax(3) × 10-3) (8) r ) 0.95, sd ) 0.17, n ) 34, F (significance) ) 0.00 π* ) 13.89 - (0.251νmax(2) × 10-3) - (0.32νmax(3) × 10-3) (9) r ) 0.57, sd ) 0.15, n ) 36, F ) 0.00 The multiple regression analyses were done with the SPSS 61 statistics program.

The quality of eq 8 is sufficient for examining accurate values of the R parameter. Despite the poor correlation coefficient of eq 9 in determining π* values, the significance is sufficient for an estimation. This must be considered by interpreting the results. IR Measurements. The infrared spectra were recorded with a BioRad spectrometer using a special device for recording the diffusion reflectance infrared Fourier technique (DRIFT) spectra. For the adsorption of benzophenone (BP) on the solid acids a cyclohexane stock solution (0.2 mol/L) was prepared. The dried solid acid (1 g) was poured into a flask under argon and was immediately covered with 10 mL cyclohexane. The desired amount of BP (10-1600 µmol) was added. After shaking for 24 h at room temperature the suspension was filtered, and the filter residue was washed with 5 mL of cyclohexane. The solid was dried under high vacuum at room temperature and stored under argon. The dried solids were measured by a DRIFT cell that allows one to work with vacuum or under argon. The IR spectra of the adsorbed BP were obtained as the difference between the IR spectrum of the sample and that of the pure solid acid. Results The UV/vis spectra of the polarity indicators 1a, 1b, 2, and 3, respectively, adsorbed on the solid acid particles were immediately recorded after the components were mixed, usually after a standard period of 5 min. The shape of the UV/vis spectrum and the position of the absorption maximum remain constant for 1 h after adsorption using the indicators 1a and 1b as well as 2. Employing these three indicators, 1,2-dichloroethane was used as solvent, because 1a and 2 are insoluble in nonpolar solvents such as cyclohexane. As already mentioned, the dye 1a becomes immediately protonated when adsorbed on several Brønsted acidic alumosilicates (Nos. 19-26) or aluminas (Nos. 28-33) which were produced by a sol-gel process from diethylaluminum chloride as well as on similarly produced alumina-functionalized silica particles (see Experimental Section). Corresponding to Drago’s results,7 the concentration of the adsorbed dye 1a or 2 did not appear to influence the position of the UV/vis maximum in the range of concentration used. See also Figure 1 and explanations in the Experimental Section. Additionally, Figure 2 parts a and b, show some typical UV/ vis spectra of the dyes 1a and 1b adsorbed on various solid acids. The typical intramolecular CT bands of the adsorbed dyes 1a and 1b are very broad in some cases, i.e., for 1a adsorbed on alumina (No. 35) and for 1b on titanium dioxide (Anatas) (No. 18). Similar broad UV/vis spectra of 1a on alumina were reported by Michels and Dorsey.32 This shows that multiple configurations of the solid surface of alumina are contributing to the ET(30) value. The measured UV/vis absorption maxima of the dyes 1a and 1b, the corresponding ET(30) values [eq 1a], and the normalized ETN values [eq 1b] for the solid acids used are given in Table 2. Because of the larger absorption coefficient of dye 2, the concentration of this dye was about one tenth of that of the dyes 1a and 1b. As discussed, the specific advantage of dye 2 is due to its sensitivity to gradual changes in the acidity of the surface environment. For example, two different bare silica samples can be clearly distinguished in their acidity when 2 is adsorbed: on SG 432 (No. 5) λmax(2) ) 532 nm and on LC 1500 (No. 6) λmax(2) ) 526 nm. This difference is evident, because the transmission technique allows a very precise determination of the UV/vis absorption maxima of adsorbed


Figure 2. (a) UV/vis reflectance absorption spectra of 1a adsorbed on alumosilicate (No. 26) 1, alumina (No. 35) 2, Aerosil 300 (No. 2) 3, and silica (No. 6) 4, from a solution in 1,2-dichloroethane. (b) UV/ vis reflectance absorption spectra of 1b adsorbed on anatas (No. 18) 1, alumina (No. 34) 2, and alumosilicate (No. 19) 3, from a solution in 1,2-dichloroethane.

indicators on silica. The probe 2 can also be used as a surface polarity indicator for alumosilicates and other quite strong solid acids where Reichardt’s dyes do not work because they become protonated. The following important observation should be noted at this point. During recent years we have investigated more than 50 different silica samples. The surface polarities change with storage time (weeks, years) depending on the kind of silica used. This effect is reproducible and can be measured for each silica sample like a fingerprint. Employing the indicator 3, different UV/vis spectroscopic behavior is observed when it is adsorbed on silica or alumina and alumosilicates, respectively. On the silicas used only the main UV/vis absorption band at about λmax(3) ) (395 ( 4) nm is observed. This corresponds well with the expected value of νmax(3) for commercial silica samples as reported in ref 37. But it must be emphasized that in pores of mesoporous siliceous MCM-41 materials, 3 shows two UV/vis absorption maxima, one at λmax(3) ) 398 nm and another at λmax(3) ) 510 nm. The latter disappears after addition of DTBP indicating Brønsted acidic sites (mobile protons) within the channel of MCM-41.64 UV/vis absorption spectra with two or three significant UV/ vis absorption maxima of the indicator 3 are also observed when it is adsorbed on aluminas, alumosilicates, or titanium dioxides. Characteristic UV/vis spectra of 3 adsorbed on an alumina and an alumosilicate sample are shown in Figure 3, parts a and b. The UV/vis absorption band of 3 at about λmax(3) ) 370400 nm relates well to weak Brønsted acidic sites, i.e., a silanol-

J. Phys. Chem. B, Vol. 104, No. 27, 2000 6423 like environment. The other one at about λmax(3) ) 500 nm is not observed with either solid acid. The absorption intensity ratio of these two bands changes with increasing adsorption time, whereas the position of the maximum wavelength remains constant during a period of 30 min. As mentioned, the two absorption maxima of 3 are attributed to two different chemical constitutions of 3. The absorption maximum at about λmax(3) ) 500 nm is caused by the oxenium ion of 3, which is produced by selectively complexing the carbonyl oxygen of 3 with either a proton or a Lewis acid.58,59 Consequently, Lewis and/or Brønsted acid sites on the surface can be responsible for the strong complexation of the carbonyl oxygen of 3. To differentiate between the two options, surface titration of adsorbed 3 with pyridine and DTBP was carried out. Typical UV/vis spectra series of those surface titration measurements are shown for an alumina (No. 34, Figure 3a and an alumosilicate (No. 23, Figure 3b) sample, respectively. For the alumina sample it is clearly seen that DTBP does not affect the position of the UV/vis absorption maximum of 3 at λmax(3) ) 390 nm and the band at about λmax(3) ) 450 nm. The overall absorption intensity of adsorbed 3 increases with the quantity of DTBP added. We think that mobile protons are trapped by DTBP which has bonded to 3 at both dimethylamino groups. It is well-known that 3 dissolved in concentrated sulfuric acid does not adsorb in the visible region of the spectrum because it is triply protonated.50 A surface titration with DTBP of 2 adsorbed on the alumina particles does not change the UV/vis spectrum. This effect is contrary to the UV/vis spectroscopic result observed for silicas.18a However, this result unambiguously shows that 2 is adsorbed on both the Brønsted and Lewis acid sites on alumina fairly strongly because an external base does not compete with 2. The UV/vis absorption band of 3 adsorbed on alumina at about λmax(3) ) 500 nm disappears when an excess of pure pyridine is added (see Figure 3c). Consequently, for alumina, the UV/vis absorption maximum of adsorbed 3 at λmax(3) ) 400 nm and the double band with UV/vis absorption maxima at λmax(3) ) 450 nm and λmax(3) ) 500 nm are caused by the Lewis acid sites on the surface. Therefore the generation of the oxenium ion of 3 is caused by the single coordination of the carbonyl oxygen atom to the acid site of the surface and the shoulder at about λmax(3) ) 450 nm by a double complexation at the carbonyl oxygen and the dimethylamino group.58,59 The situation is different when 3 is adsorbed on an alumosilicate which has Brønsted as well as Lewis acid sites. The UV/vis absorption spectrum of 3 adsorbed on alumosilicate (No. 34) is shown in Figure 3b. Two UV/vis absorption bands are clearly detectable. An excess of DTBP affects some of the Brønsted acidic sites as indicated by the apparent hypsochromic shift of the absorption band at λmax(3) ) 500 nm. But a clean differentiation between Lewis and Brønsted acidic sites on the surface of alumosilicates by DTBP titration of adsorbed 3 seems difficult. In principle, two values of the R and π* parameters of aluminas, alumosilicates, or titanium oxides can be determined using the two different λmax(3) values of adsorbed 3. However, it must be considered that the correlation equations (eqs 8 and 9) for determining the R and π* values are derived from the νmax values of 3, measured in a set of well-behaved protic and nonprotic solvents in the range of νmax(3) ) 30 000 cm-1 in n-hexane (nonpolar) to νmax(3) ) 24 960 cm-1 in 1,1,1,3,3,3hexafluoro-2-propanol (strongly polar).50 This background has to be considered using the correlation eqs 8 and 9. Consequently,


Spange et al.

TABLE 2: UV/Vis Absorption Maxima of 1a and 1b, Respectively, Adsorbed on the Solid Acids from a 1,2-Dichloroethane Solution, the Corresponding ET(30) Values Calculated from Eq 1a, and the Normalized ETN Values Calculated from Eq 1b

a

sample number

solid acid sample

νmax(1a or 1b) [10-3 cm-1]

ET(30) measured [kcal mol-1] [eq 1a]

ETN calculated from eq 1b

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

SiO2, Aerosil 200 SiO2, Aerosil 300 SiO2, Aerosil 380 SiO2, KG 60 SiO2, KG SG 432 SiO2, KG LC 1500 SiO2, 1.5% Al2O3 Cogel AlPO4, SP 8293 AlPO4, SP2-8111.09 SiO2/Al2O3, SP 18-8495 SiO2/TiO2, SP 18-8381.01 SiO2, SP 18-8470 SiO2/ZrO2, SP 18-8402.03 Al2O3. SP 18-8510 SiO2/Al2O3, SP2-8515.01 TiO2 P25 TiO2 (Rutile) TiO2 (Anatas) Siral 5 Siral 10 Siral 20 Siral 30 Siral 40 Siral 50 Siral 60 Siral 80 Al2O3. Pural KG 60 10% Al2O3 KG 60 30% Al2O3 KG 60 50% Al2O3 KG 60 70% Al2O3 KG 60 100% Al2O3 Al2O3 (AlEt2Cl) Al2O3 Al2O3-C

20.21 20.20 20.49 20.33 20.24 20.45 22.94 n.d.b n.d.b n.d.b 20.00 20.83 n.d.b 21.01 n.d.b 23.20a n.d.b 21.93a 22.97a n.d.b 22.30a n.d.b 23.20a n.d.b 21.87a 18.94 21.88 22.57a n.d.b 22.43a n.d.b 20.47a 21.73a 21.14a 20.24

57.8 58.1 58.6 58.1 57.9 58.5 62.7

0.84 0.85 0.86 0.85 0.84 0.86 0.99

57.2 59.6

0.82 0.89

60.1

0.91

63.5

1.01

59.7 62.8

0.90 0.99

60.8

0.93

63.5

1.01

59.5 54.2 62.6 61.6

0.89 0.73 0.98 0.95

61.2

0.94

55.3 59.1 57.3 57.9

0.76 0.88 0.82 0.84

Measured with 1b because 1a is not suitable. b n.d. Not determinable because the dyes become protonated.

unusually large π* values are obtained for the Lewis acidic sites of alumosilicates due to the strong bathochromically shifted π-π* transition of chemisorbed 3. However, the solvatochromic concept is based on the assumption that chemical alterations of the probe should not occur when solvatochromic shifts are interpreted in terms of reversible solvation. Therefore, the calculated R and π* parameters should be handled with caution when UV/vis absorption maxima values of chemisorbed indicators are used. Probably this bathochromicity can be similarly interpreted in terms of halochromic behavior because an ionic form of 3 is generated.60 This interpretation would even correspond to large values of the π* term for Lewis acid sites. However, both possible values of the R and π* parameters have been determined. They may be valid for the Brønsted and Lewis acid sites and will be discussed later compared with independent measurements. Utilizing the second UV/vis absorption band at λmax(3) ) 500 nm of adsorbed 3, the Kamlet-Taft parameters are assigned as R2 and π*2. The measured UV/vis absorption maxima of the indicators 2 and 3, R and π* values calculated according to eqs 6 and 7 (the first UV/vis absorption maximum of 3 was used for calculation), and the calculated ET(30) values for 35 various solid acids are listed in Table 3. Table 4 contains the νmax,2 values of 3, and the corresponding theoretically possible R2 and π*2 values. A slightly larger concentration of 3 was used for these measurements, because the shoulder at λmax(3) ) 500 nm then appears clearer as it does in Figure 3.

Testing the Surface Polarity Parameters by an IR and a Fluorescence Probe. The π* and R values of the solid acids were used to calculate theoretical values of the carbonyl stretching vibration of BP when adsorbed on the solid acids using the LSE eq 6 from ref 68 (see Introduction). BP adsorption on solid acids was investigated in particular for a silica (No. 4) [(νmax,CO,(calculated) ) 1645.5 cm-1 (BS)], for an alumina (No. 34) [(νmax,CO,(calculated) ) 1652 cm-1 (BS) and 1630 cm-1 (LS)], and for an alumosilicate (No. 23) [νmax,CO(calculated) ) 1644 cm-1 (BS) and 1628 cm-1 (LS)]. The calculated νmax,CO values are given in parentheses after the sample number; LS denotes the presumed Lewis and BS the presumed Brønsted sites. However, BP adsorbed on a solid acid shows two clearly separated IR bands for its carbonyl stretching vibration in each case. The following results are obtained: for silica [νmax,CO ) 1656 to 1659 cm-1 and νmax,CO ) 1644 to 1648 cm-1; both positions of νmax,CO are a function of the concentration of adsorbed BP (see Figure 4)], for alumina (νmax,CO ) 1656 cm-1 and νmax,CO ) 1636 to 1640 cm-1), and for the alumosilicate (νmax,CO ) 1653 cm-1 and νmax,CO ) 1643 cm-1). For silica the two IR bands indicate that BP is adsorbed on two different surface sites. However, the IR band observed at 1656 to 1659 cm-1 indicates a significantly lower polarity, similar to that of 1,2-dichloroethane or n-butanol. The interpretation of the IR data in terms of a two-state model in which adsorbed BP molecules are in a rapid equilibrium between a non-hydrogen-bonded state in which BP is physisorbed on the silica surface and a state in which BP molecules are hydrogen


J. Phys. Chem. B, Vol. 104, No. 27, 2000 6425 TABLE 3: UV/vis Absorption Maxima of the Indicator Dyes 2 and 3 Adsorbed on the Solid Acids from a 1,2-Dichloroethane Solution, Calculated r and π* Values According to Eqs 8 and 9, and ET(30) Values Calculated from Eqs 3 and 4, Respectively νmax(2) νmax(3) sample [10-3 [10-3 number cm-1] cm-1]

Figure 3. (a) UV/vis absorption spectra of 3 adsorbed on alumina (No. 34); titration with 2,6-di-tert-butylpyridine. (1: without DTBP, 2: 1.41 × 10-4, 3: 2.35 × 10-4, 4: 4.70 × 10-4 mol DTBP, 0.5042 g alumina, 6 × 10-7 mol 3, 10 mL dichloromethane); (b) UV/vis absorption spectra of 3 adsorbed on alumosilicate (No. 23); titration with 2,6-di-tert-butylpyridine. (1: without DTBP, 2: 2.35 × 10-4 mol DTBP; 0.5087 g alumosilicate, 1 × 10-6 mol 3, 10 mL dichloromethane); (c) UV/vis absorption spectra of 3 adsorbed on alumina (No. 34); titration with pyridine. (1: 0, 2: 1.24 × 10-4, 3: 2.48 × 10-4, 4: 6.19 × 10-4, 5: 1.12 × 10-3 mol pyridine; 0.5039 g alumina, 8 × 10-7 mol 3, 10 mL dichloromethane).

bonded to silanol groups of the silica surface is in accordance with 13C and 29Si NMR chemical shift and relaxation data of the acetone/silica system.79 It is presumed that BP adsorption occurs similar to acetone adsorption on silica. The agreement between calculated and measured values for the carbonyl stretching vibration is quite different. For the silica sample (No. 4), the shoulder at about νmax,CO ) 1644 to 1648 cm-1 agrees very well with the calculated νmax,CO using the measured polarity parameters from Table 3 and eq 5. For the alumina sample the agreement between calculated and measured data is poor for one of the two Kamlet-Taft parameter sets [R(2) and π*(2)] and satisfactory for the other one [R1 and π*1]. But the difference between the calculated (∆νmax,CO ) 22 cm-1) and measured (∆νmax,CO ) 20 cm-1) νmax,CO values of both parameter sets is rather similar. For the alumosilicate sample the agreement for one of the two IR bands is rather good. A precise assignment of this IR band of adsorbed BP on a specific surface site is not possible. The IR results for BP adsorption support the order of magnitude of the values of the determined Kamlet-Taft

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

18.66 18.80 18.83 18.78 18.80 19.05 20.07 20.08 20.28 19.76 18.52 18.55 19.23 19.61 20.20 19.57 19.80 19.46 19.96 20.16 19.92 20.24 20.33 19.76 19.23 19.23 19.23 19.76 19.84 19.65 19.29 19.27 19.92 19.35 18.69

25.45 25.51 26.46 25.25 25.58 25.32 25.19 26.11 26.11 26.46 a 26.46 25.84 2695 25.45 25.04 25.64 25.81 26.60 26.39 26.25 26.11 26.11 25.97 26.46 26.18 26.88 25.97 25.71 26.11 26.18 27.78 26.67 26.67 26.95

ET(30) ET(30) [kcal mol-1] [kcal mol-1] calculated calculated from eq 3 from eq 4

R

π*

1.07 1.14 1.15 1.13 1.14 1.25 1.99 1.73 1.82 1.59

1.05 1.00 0.97 1.08 0.98 1.00 0.63 0.49 0.43 0.46

59.5 59.9 59.9 60.8 59.7 61.6 68.7 63.1 63.9 60.7

57.7 58.5 58.5 58.8 58.4 60.2 69.3 64.6 65.8 62.3

1.05 1.34 1.53 1.77 1.48 1.60 1.44 1.69 1.77 1.66 1.80 1.84 1.58 1.35 1.35 1.36 1.58 1.61 1.53 1.38 1.40 1.67 1.41 1.12

0.76 0.79 0.33 0.70 0.95 0.71 0.73 0.36 0.38 0.48 0.45 0.43 0.61 0.59 0.68 0.45 0.61 0.68 0.60 0.66 0.16 0.35 0.49 0.56

55.8 60.8 58.4 65.8 64.6 63.6 61.5 63.3 64.6 63.5 65.4 64.1 62.3 58.5 59.5 57.1 62.3 63.5 61.4 59.8 54.2 60.6 58.3 54.7

55.9 60.4 60.7 66.3 63.3 63.8 61.5 63.3 64.6 63.5 65.4 65.9 63.0 59.4 59.9 58.8 63.0 63.8 62.2 60.3 57.9 62.9 59.8 55.8

a The UV/vis absorption band of 3 relating to Brønsted acid sites is not observed for this sample.

TABLE 4: Second, Long-Wavelength UV/vis Absorption Maxima of 3 Adsorbed from a 1,2-Dichloroethane Solution Probably on the Lewis Acidic Sites of the Solid Acid, Apparent r and π* Values, As Well As Theoretically Calculated ET(30) Values for These Sites from Eq 12 sample number

νmax, 2(3) [10-3 cm-1]

R2

π*2

ET(30) [kcal mol-1] calculated from eq 12

10 11 13 14 16 21 22 23 24 25 26 32 34

20.04 20.00 19.80 20.28 20.12 20.49 19.80 20.79 20.35 20.28 20.41 20.41 20.45

1.46 0.90 1.21 1.40 1.37 1.54 1.67 1.73 1.47 1.22 1.23 1.24 1.28

2.51 2.84 2.72 2.47 2.53 2.33 2.47 2.13 2.41 2.57 2.53 2.52 2.49

70.5 65.9 68.8 69.6 69.6 70.5 72.8 71.6 70.1 68.1 68.0 68.1 68.4

parameters. But the calculated νmax,CO values of adsorbed BP are too low (high polarity) compared to the values of the measured carbonyl stretching vibration (lower polarity). It should be noted that the concentration of BP was about 10 to 50 times larger than those of the solvatochromic polarity indicators 2


Figure 4. The two positions of νmax,CO of benzophenone adsorbed on silica as a function of the concentration of adsorbed benzophenone.

and 3. Therefore we assume that the solvatochromic dyes preferentially indicate the most acidic silanol groups and Lewis acid sites. The use of both values of π* and R together give, in particular, a reasonable agreement between calculated and measured data for BP adsorption. Pyrene has been recommended by several authors for the detection of the dipolarity/polarizability of an environment.65 For Aerosil 300 and Aerosil 380, Py values of 1.48 and 1.65, respectively, were estimated via eq 7 using the R and π* values from Table 3. This calculation shows that the measured Py value from the literature (Py ) 1.78) is larger than the value to be expected from eq 7. It may be that pyrene and 3 are adsorbed on quite different surface sites, because 3 is quite a strongly dipolar molecule compared to the nonpolar probe pyrene. Discussion By comparison with the Kamlet-Taft parameters of representative pure solvents, such as 2,2,2-trifluoroethanol (R ) 1.51, π* ) 0.73) and 1,1,1,3,3,3-hexafluoro-2-propanol (R ) 1.96, π* ) 0.65), it is evident that the solid surface environments observed by the various indicators are moderately strongly dipolar/polarizable (π* ) 0.38 to 1.04) and rather strong hydrogen-bond donors (R ) 1.00 to 1.99). This shows that the polarity of the solid acids studied covers only a rather small section of the Reichardt ET(30) scale. Within the framework of the Kamlet-Taft R scale, the observed differences are more significant, e.g., from the silicas with R values of about 1 to alumosilicates with R values of about 2. Hence, for solid acids the Kamlet-Taft parameters set allows a better differentiating than do the ET(30) values. Correlation Analyses of the Polarity Parameters with Each Other. Using the independently calculated R and π* values from Table 3, theoretical ET(30)values were calculated from eqs 3 and 4 and then compared with the experimentally obtained data. These calculated ET(30) values are also listed in Tables 2 and 3. A comparison of the calculated and measured ET(30) values shows a rather good agreement for 80% of the samples considered. It seems that calculated ET(30) values obtained from eq 3 agree better with those data which are measured by means of dye 1b. Dye 1b is more hydrophobic and less basic than dye 1a. Therefore it is expected that this dye is more sensitive toward the π* term than to the R term. Furthermore, the measured ET(30) values have been correlated with the independently determined R and π* values. It must be noted that only those R

Spange et al.

Figure 5. Measured versus calculated [via eq 12] ET(30) values for nineteen solid acids including silicas, aluminas, titanium dioxides, and alumosilicates used in this paper.

and π* values derived from the short wavelength UV/vis absorption band of adsorbed 3 (Table 3) have been used for the following correlation analyses [eqs 10-12]. For the ET(30) values measured by the two dyes 1a and 1b, the following multiple correlation equations were obtained.

ET(30) (via 1a) ) 52.19 + 5.12R + 0.99π* (a/s ) 5.13) (10) n ) 8, r ) 0.964, sd ) 0.235, F ) 0.0013 ET(30) (via 1b) ) 35.08 + 13.39R + 8.70π* (a/s ) 1.54) (11) n ) 11, r ) 0.922, sd ) 1.110, F ) 0.0005 The respective influence of the R and π* terms upon ET(30), expressed by the coefficient ratio a/s, is different for the two dyes. It can be clearly shown that dye 1a serves mainly as an R indicator for acidic surfaces, whereas dye 1b reflects the R and π* value of acidic surface environments similarly to the way that 1a does for solvents. The LSE equation derived for well-behaved regular solvents reflects the a/s ratio in a way similar to that in eq 11. Consequently, the basicity of a phenolate betaine dye influences the susceptibility coefficients of the Kamlet-Taft relation and accordingly the (XYZ)0 value when applied to HBD surfaces. This interpretation is in accordance to the result of the Kamlet-Taft correlation analyses for structural different solvatochromic dyes.23 In ref 23 has been shown that the coefficient a of eq 2 increases in decreasing the pKa-value of the solvatochromic indicator. This reflection shows that fluorine-substituted betaine dyes are suitable surface polarity indicators for moderately strong solid acids. Taking into account the sum of all measured ET(30), R, and π* values, eq 12 is obtained:

ET(30) ) 39.56 + 11.60R + 5.60π* (a/s ) 2.07) (12) n ) 19, r ) 0.87, sd ) 1.17, F ) 0.0000 Altogether the agreement is rather good between calculated and measured ET(30) values for the nineteen different silicas, alumosilicates, aluminas, and titanium dioxide samples considered. Figure 5 shows the plot of the measured versus calculated ET(30) values according to eq 12. One outlying point from the theoretical plot is clearly detectable: for alumina (No. 35) ET(30)(measured) - ET(30)(calculated) ) ∆ET(30) ) 2.2 kcal mol-1. However, this



Figure 6. Influence of the aluminum content of the alumosilicates used on the value of the Kamlet-Taft R parameter. The aluminum content was determined by atomic absorption spectroscopy. The arbitrarily drawn curve is a guide for the reader.

Figure 7. Influence of the aluminum content of alumosilicates on the value of the Kamlet-Taft π* parameter. The aluminum content was determined by atomic absorption spectroscopy.

outlying point is included in the individual correlation in eq 10. Hence, these correlation analyses show that the set of surface polarity indicators used is suitable for measuring the surface polarity parameters ET(30), R, and π* of solid acids. Notice that for each specific silica sample individual R and π* values are obtained. The measured ET(30) parameter observed by dye 1a and data calculated by eq 4, using the independently examined R and π* values, agree very well for silicas. This result suggests that individual surface properties of specific silica samples are caused by variations of the R and π* terms. However, a critical reflection concerning the basic concept is necessary at this point. If each of the two dyes 1a and 1b reflects the polarity of a surface in another ratio of the R and π* values, measured by means of the indicators 2 and 3, then it is not possible in principle to examine independent R and π* values of solid acids. In other words, a clean separation of the HBD term from the π* term and from the unit of measurement is not really justified. This problem exists for all measurements employing LSE relations. Using the R2 and π*2 values, a rather poor agreement between measured and calculated ET(30) values is found. This is indirect confirmation of the hypothesis that the dyes 1a and 1b have no access to Lewis acidic surface sites. Perhaps ET(30) values for Lewis acid sites should really be about 68-72 kcal mol-1. Funke and Mayr have shown by a correlation of ET(30) values of solvents with kinetic data that the ET(30) value for liquid SO2, a moderately strong Lewis acid, approaches a value of ET(30) ) 68 kcal mol-1.80 It should be mentioned that the ET(30) value of liquid SO2 cannot be measured directly using the dye 1a.81 Therefore, we believe that the reported polarity data in Tables 2 and 3 are probably representative only for the Brønsted acidic sites of solid acids. It is conceivable that the phenolate betaine dyes have no direct access to Lewis acid sites because the phenyl rings in the 2- and 6-positions of the phenoxide ring prevent this interaction. Correlation of the Kamlet-Taft Parameters of Surfaces with Structural Data. The variations of the value of the R or π* term of the silicas are obviously caused by different molecular surface structures (geminal silanols, isolated silanols, associated silanols, siloxane bridges, etc.). A classification of changes in the R or π* as a result of structure modification is still not possible, because the DRIFT spectra as well as the solid state 1H NMR and 29Si NMR spectra of the silica samples (Nos. 1-6) are virtually identical.53 This result shows that the surface polarity indicators employed are more sensitive for the detection of gradual modifications of the surface polarity than are DRIFT or solid-state NMR spectroscopy. This topic is still under study by our group. Figure 6 shows the plot of the influence of the aluminum content on the R term of the alumosilicates used in this paper. The arbitrarily drawn curve is a guide for the reader.

The surface acidity (R value) of alumosilicates (H form) as a function of the aluminum or silicon content goes through a maximum. A theoretical curve is difficult to calculate because the samples, which are available from the commercial suppliers, show significant differences concerning their Kamlet-Taft parameters for each different sample produced. The R value of each point in Figure 6 could be reproduced readily. We independently determined the corresponding aluminum content of each sample by AAS (see experimental part). When using these data as indicated by the producer, a poorly defined curve is obtained. Our results are in very good qualitative agreement with the results reported in ref 27. This is a good indication of the correctness of the determined values. Figure 7 shows the plot of the value of the π* term as function of the aluminum content of the alumosilicates used. The π* value decreases with increasing the aluminum content. For pure alumina, an increase in the π* value is observed again. Assuming that the π* values in Figure 7 are derived for the Brønsted sites, the following interpretation is possible. The decrease in the π* value is caused by a decrease in the polarizability of the alumosilicatic framework. However, the dependence shown in Figure 7 is evident and shows that the π* value is also significant and a function of the aluminum content of the alumosilicate. Conclusions Reichardt’s ET(30) as well as the Kamlet-Taft’s solvent parameters R and π* can be used as a reference system to characterize empirically the surface polarity of moderately strong solid acids, e.g., silicas, aluminas, alumosilicates, and titanium dioxides. For this purpose three structurally different types of solvatochromic UV/vis probes, N-phenolate pyridinium betaine dyes, a mixed ligand transition iron(II) complex, and an aromatic aminoketone derivative have been found to be suitable. The results generally show that the surface of solid acids exhibit solvent-like behavior and that an average polarity is seemingly measured by the indicator dyes 1a, 1b, and 2. Experimentally available ET(30) values of solid acids correlate well with independently calculated R and π* values compared to the Marcus equation for well-behaved solvents and a similar LSE relationship derived for functionalized silicas. Because the coefficients of the Kamlet-Taft LSE equation for the ET(30) value of solid acids depend on the structure of the indicator used, care must be taken in interpreting the individual parameters R and π* of solid acid surfaces. Nevertheless, the KamletTaft parameters must be handled with caution in order to be mathematically orthogonal, and they neglect any other higherorder term. Further research is necessary to evaluate exactly the individual parameters R and π* for the Lewis as well as Brønsted acidic

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