4087
J . Phys. Chem. 1991,95,4087-4092
Examination of the Soiventiike Nature of Zeolites Based on a Soivatochromic Indicator Prabir K. Dutta* and Wayne Turbeville Department of Chemistry, The Ohio State University, Columbus, Ohio 43210 (Received: October 19. 1990) Zeolites are crystalline aluminosilicates with cages and channel systems that can host a variety of organic transformations. This intracrystalline space is akin to a %olvent", and description of this space in terms of solventlike properties is appropriate. The concept of solvatochromicindicators has been successfully used to define the physicochemical properties of organic solvents. In this study, we have investigated the electronic and Raman spectroscopy of the molecule N-(2-hydroxybenzylidene)aniline and established a quantitative correlation between the spectral intensities of the benzenoid and zwitterionic forms of this molecule and the a-value of various hydroxylic solvents. The a value is a measure of the hydrogen bond donor ability of the solvent. This correlation has been used to establish an a value scale for a series of faujasitic zeolites with varying Si/AI ratios. It was found that the a value of the zeolite increased with %/AI ratio to reach a maximum around 7.8, followed by a decrease at higher Si/AI ratios. Since Na+-exchanged zeolites were examined in all cases, the interaction of the ani1 molecule in its zwitterionic form with Lewis acids (Na+) and bases (oxygen of the framework) was considered to be responsible for its formation. The Si/Al ratio of the framework determines the acid-base character of the zeolite and is reflected in a quantitative manner by the a value determined in this study.
Introduction Zeolites are crystalline aluminosilicates with internal cages and channels that are host to a variety of chemical and photochemical transformations.'*z In its role as a host, the zeolite has been described as a solvent, a solid electrolyte, and a crystalline liquid.*s Understanding the parallels between organic transformations that occur in typical solvents and zeolite matrices is of considerable interest. In order to do so, the nature of the zeolite as a solvent has to be understood. Quantification of general properties of solvents has been studied by physical organic chemists for many yearse6 Of particular interest is the solvatochromic comparison method developed by Taft and co-workers which classifies organic solvents in terms of three parameters II*, a and 87 The Il* scale is a measure of the solvents ability to stabilize a charge by virtue of its dielectric effect. The a and /3 scale relate to the H-bonding donor and acceptor ability of the solvent. For example, acetonitrile has II*, a,and @of0.75,0.31 and 0.19, respectively? The solvent effects on some measurable property of the solute (E) takes the form E = Eo + an* ba + cb. Solvatochromic methods have also been used to characterize polar surfaces such as alumina and silica.*u9 The nature of the silica surface, for example, was found to be best described by II*? There has been a study on zeolite ZSM-5, using the solvatochromic indicator, 4-nitroanisole, which provides a measure of the II* parameter.I0 The value of II* for completely dealuminated ZSM-5 (silicalite) was found to be 1.05. With increase in AI content the II* increased to 1.2 around Si/Al of 158 and then leveled off until Si/AI ratios of 36. Based on this study, it appears that, even with increasing AI content, the ability of the zeolite to solvate charges remains unchanged. It has been recognized for many years that salicylidenes (formed by condensation of salicylaldehyde with amines) give rise to a new absorption band a t -400-450 nm in polar hydroxylic solvents." The intensity of this band in a series of solvents has been used to establish a relationship with II* and a parameters of solvents.Q13 The origin of this band has been a source of controversy.
+
(1) Ward, J. W. Appl. Ind. Cafal.1984, 3, 271. (2) T w o . N . J. Pure Appl. Chem. 1986. 58. 1219. (3) Jacobs, P. A. Catal. Reo. Sci. Eng. 1982, 24, 415. (4) Kasai, P. H.; Bishop, R. J. J . Phys. Chem. 1973, 77. 2308. (5) Baphomeuf, D. J. Phys. Chem. 1979,83, 249. (6) Reichardt, C. Solvcnf Effects in Organic Chemistry; Verlag Chemic: Weinheim, Germany, 1979. (7) Kambet, M. J.; Abboud. J . L.; Abraham, M. H.; Taft, R. W. J . Org. Chem. 1983.48, 2877. (8) Michels. J. J.; Dorsey, J. G. hngmuir 1990, 6, 414. (9) Lindley, S. M.; Flowers, G. C.; Leffler, J. E. J . Org. Chem. 1985,50, 607. (10) Handreck. G. P.;Smith, T. D. J. Chem. Sa.,Faraday Trans. I 1988, 84. 1847. ( I I ) Charctte, J.; Faithairst, G.; Teyssie. Ph. Specfrochim.Acta 1964,20, 597. (12) Kazitsyna, L. A.; Mischenko, V. V. Zh. Org. Khim. 1965, I , 617.
0022-3654191 12095-4087SO2.5010 , I
,
~
Recently, we reported a spectroscopic study of the molecule salicylideneaniline (referred to as anil below) in various solvents and zeolite NaY and concluded that the 400-450 nm band could be assigned to a zwitterionic structure, as shown in the following eq~i1ibrium.l~
I
II
In this study, we develop the use of salicylideneaniline as a probe for measuring the a value of faujasitic zeolites as a function of Si/Al ratio. On the basis of both UV-visible and Raman spectroscopies, we find that Na+-exchanged faujasitic zeolites exhibit a values comparable to strong polar hydroxylic solvents. Also, there is an increase in a value with Si/Al ratio until 7 followed by a decrease. Correlation of the a value to fractional charges on atoms as calculated by the Sanderson electronegativity equalization principle is explored. Hydrocarbon transformation within faujasitic zeolites is correlated with the description of the intrazeolitic space as defined by a values. Experimental Section Salicylideneaniline was synthesized by the condensation of salicylaldehyde with aniline in methanol and then recrystallization from the same s01vent.I~ The zeolites NaY and NaX were provided by Union Carbide (Union Carbide LZY 52 and 13X, respectively). The dealuminated faujasite with Si/Al = 4.5 was provided by Katalistiks International; the faujasites with Si/AI ratio of 7.8 and 14.2 and the completely dealuminated faujasite were gifts from Dr. Jack Donohue and prepared according to the procedure described by him and his co-workers.15 The ZSM-5 used was synthesized according to the literature procedures.I6 All zeolites were ion-exchanged with 0.5 M NaCl and rinsed with distilled water prior to use. The solvents dimethyl sulfoxide (Aldrich), dimethylformamide (Fisher Scientific), hexamethylphosphoramide (Aldrich), methanol (Mallinckrodt), trifluoro-2propanol (Alfa Chemicals), trifluoroethanol (Aldrich), and hexafluoro-2-propanol (Aldrich) were not purified further before use. The adsorption of the anil into the zeolite has been described previ~usly.'~All Raman spectra were taken with a Coherent lnnova 100 krypton ion laser with the 647.1-nm laser line. The spectra were collected with a SPEX double monochromator with (13) Kamlet, M. J.; Taft, R. W. J . Chem. Soc., Perkins Trans. 2 1979, 349. (14) Turbeville, W.; Dutta, P. K. J . Phys. Chem. 1990, 94, 4060. (IS) Ray, G. J.; Nerheim, A. G.; Donohue, J. A. Zeolifes 1988,8, 458. (16) Gabelica. Z.; Blom, N.; Derouane, E. G. Appl. Carol. 1983,5,227.
0 1991 American Chemical Society
4088 The Journal of Physical Chemistry, Vol. 95, No. 10, 1991
300
400
500
600
nanometers Figure 1. Absorption spectra of ani1 (10" M) in high II* solvents (a) dimethyl sulfoxide, (b) dimethylformamide, and (c) hexamethylphosphoramide. (The lack of absorption bands below 300 nm in (c) arises from using the solvent as a reference.)
slit widths of 6 cm-I and a GaAs photomultiplier tube with photon counting. Absorption spectra were measured with a Shimadzu UV-265spectrophotometer, and diffuse reflectance spectra were measured with a Harrick diffuse reflectance apparatus with a controlled environment chamber in the spectrophotometer. Kubelka-Munk of the diffuse reflectance spectra were calculated by using BaS04 as the reference. The Raman spectra were deconvoluted in the 1500-1700-cm-' region into separate Lorentzian peaks by using the SpectraCalc data manipulation program. The areas of the curve-fit peaks a t 1577 and 1596 cm-' were determined with the same software. It has been recognized for many years that alkyl and aryl imines of salicylaldehyde gives rise to the appearance of a new absorption band at 400-450 nm upon dissolution in polar hydroxylic solvents.'I On the basis of the data of Kazitsyna and Mischenko'* for salicylidenebutylamine in a series of solvents, Kamlet and Taft derived the following equation: log 4400 nm) = 0.87 1.34II* 1 . 7 4 ~thereby ~ correlating the molar absorptivity with the parameters II* and a.I3 However, as shown in Figure 1, we do not observe the presence of the 400-450-nm band in solutions of the ani1 dissolved in solvents with high II* values and zero a such as dimethyl sulfoxide, dimethylformamide, and hexamethylphosphoramide (n*of 1.0, 0.88, and 0.87, respectively).' The necessity of hydrogen bond donor functionalities as in polar hydroxylic solvents is required for the appearance of this band. Moreover, as the hydrogen bond donor acidities as measured by the a scale of the solvent increase, there are considerable changes in the electronic spectrum of the anil. Figure 2 shows the absorption spectra in methanol, trifluoro-2-propano1, trifluoroethanol, and hexafluoro-2-propanol. The a values of methanol, trifluoroethanol, and hexafluoro-Zpropanol are 0.93, 1.5 1, and 1.96, respectively;' that of trifluoro-2-propanol is not reported in the literature, but based on its structure, the a value can be expected to be between that of methanol and trifluoroethanol. The origin of the -4400-nm band has been a source of controversy for many years and assignments to structures such as cis-o-quinone form, protonated species, and an enolic form with intermolecular H bond to solvents have been reported.I7 Recently, we concluded, based on Raman and NMR spectroscopic study, that the species giving
+
+
(17) Lewis,J.
Dutta and Turbeville
W.;Sandorfy, C . Can. J. Chem. 1982, 60, 1727.
300
500
400
600
nanometers Figure Absorption spectra of anil in (a) methanol, (b) trLoro-2propanol, (c) trifluoroethanol,and (d) hexafluoro-2-propanol (1 X IO-' M). 0.6I
0.0 0.8
1.0
1.2
1.4
1.6
1.8
2.0
ff
Figure 3. Plot of ratio of the intensities of the 400-320-nm band (Figure 2) versus a values of the solvent.
rise to the -400-450-nm band arises from the zwitterionic form of the ani1 (II).14 There exists an equilibrium between the Hbonded benzenoid form (I) and the zwitterionic form (II), which is shifted toward the ion-pair form as the solvent is changed from methanol to trifluoro-2-propanol to trifluoroethanol to hexafluoro-tpropanol. This is reflected in the electronic spectrum by the increase in intensity of the -400-nm band due to the zwitterionic form. Figure 3 is a plot of the ratio of the peak intensities of the 400-450- to the 320-nm band versus the reported a values for methanol, trifluoroethanol, and hexafluoro-2-propanol. The equation describing this correlation is R(400/320) = 0.47a - 0.44 (correlation coefficient = 0.998). On the basis of this equation and the R(400/370) observed in Figure 2 we estimate an a of 1.1 for trifluoro-2-propanol. Besides electronic spectroscopy, the other types of properties that have been correlated with solvatochromic parameters are N M R shifts, coupling constants, and emission spectra.lsJ9
The Journal of Physical Chemistry, Vol. 95, No. 10, 1991 4089
Solventlike Nature of Zeolites
0’51
,
0.0 0.8
, 1.0
,
,
,
1.2
,
,
1.4
,
1.6
,
\\ 2.0
1.8
a Figure 5. Plot of ratio of the intensities of the 1577-1596-cm-I band (Figure 4) versus a value of the solvents. I
1500
1
,
1540
I
I
1580
I
I
1620
I
16‘60
’
17b0
wavenumbers (cm-1) Figure 4. Raman spectrum in the 1500-1700-~m-~ region of anil in (a) methanol, (b) trifluoro-2-propanol, (c) trifluoroethanol, and (d) hexafluoro-2-propanol.
As we have discussed extensively in the earlier publication,14 the Raman spectra of the anil in its benzenoid and zwitterionic forms are distinct. This provides an opportunity for using Raman spectra of the anil to develop correlations with solvatochromic parameters. This is the first example, to the best of our knowledge, in using Raman bands as markers for calculation of a. Also, the Raman spectrum is more structure specific than the electronic spectrum and also provides for a comparison with this method. Finally, in the case of solid matrices such as zeolites, there are experimental advantages of a scattering technique as compared to an absorption method. The Raman spectrum of the anil in polar hydroxylic solvents exhibits bands both due to the benzenoid and zwitterionic forms. Significant changes are observed in the 1500-17Wwavenumber region as a function of solvents. The band at 1577 cm-I localized on the HOCCCN functionality is most prominent in the benzenoid form, and disappears in the zwitterionic form. The C-N stretch at 1620 cm-I shifts to 1641 cm-’in the zwitterionic form (C=N+H stretch).I4 The trends in this region are seen in Figure 4 for the anil dissolved in methanol, trifluoro-2-propano1, trifluoroethanol, and hexafluoro-2-propanol. The band at 1596 cm-’ is characteristic of the vEa vibration of ortho disubstituted benzene and remains unperturbed.I4 We chose the ratio of the integrated intensities of 1577 to 1596 cm-‘bands (obtained by deconvolution, see Experimental Section) as a normalized measure of the amount of the benzenoid form. Figure 5 shows the variation of this ratio versus the a value of the solvents. The equation describing the correlation is R( 1577/1596) = -2.21a 4.45 (correlation coefficient = 0.997). On the basis of the Raman bands observed in trifluoro-2-propanol (Figure 4b), we calculate an a value of 1.01 for this solvent. This compares reasonably well with an a value of 1.1 based on the UV-visible studies, considering the differences in the molecular properties used for these two measurements. Variations of this magnitude are not uncommon in determining solvatochromic parameters using different observables.lo It is appropriate to consider the average of the two methods
+
(18) Taft. R. W.:Kamlet. M.J. Org. Magn. Reson. 1980, / I ,485. (19) Coosemnr, L.:deSchryva, F.C.;van Dormael. A. Chcm. Phys. Lrrr. 1979, 65, 95.
300
400
500
600
nanometers Figure 6. Diffuse reflectance spectra of ani1 on dehydrated Na+-ex-
changed faujasites. The Si/AI ratios of the framework are (a) 1.3, (b) 2.6, (c) 4.5, (d) 7.8, (e) 14.2, and (f) m (completely dealuminated faujasite AI 120 ppm).
-
and define an a value of 1.OS for trifluoro-Zpropanol. The above UV-visible and Raman spectroscopic experiments of the ani1 dissolved in polar hydroxylic solvents show clearly that a measure of a can be obtained based on the equilibrium between the benzenoid and the zwitterionic form. The choice of this molecule appears to be appropriate for determining the “hydrogen bonding” donor ability of the zeolite surface. We have chosen to examine the faujasite framework for this study. Its pores are large enough to allow the ani1 molecules to penetrate into the supercages. Also, this zeolite framework has been the host for a large number of hydrocarbon based transformations and photochemical reactions.’*2 Figure 6 shows the diffuse reflectance spectra of the ani1 adsorbed into the dehydrated faujasite zeolites with different Si/AI (20) Taft, R. W.; Kamlet, M . J . J . Am. Chem. Soc. 1976, 98, 2886.
4090 The Journal of Physical Chemistry, Vol. 95, No. 10, 1991
Dutta and Turbeville
G 1.5
-
1.3
-
1.0'
'
"
0
'
4
'
"
"
'
'
12
8
_____-J 16
00
SVAI
1500
1540
1580
1620
1660
1700
wavenumbers (cm-1) Figure 7. Raman spectra of anil in the 1500-1700-cm-' region adsorbed on dehydrated Na+-exchanged faujasites. The Si/AI ratios of the framework are (a) 1.3, (b) 2.6, (c) 4.5, and (d)
*1.8
'
O
r
7
m.
ratios. For a fixed zeolitic framework, the most important structural parameter that influences the polarity of the framework is the AI content, since it introduces the presence of a negative charge, and therefore, a neutralizing cation. The size and charge of the cation are also important. We have primarily examined Na+-exchanged zeolites in this study. In Figure 6 are shown spectra of the ani1 on faujasites with Si/AI ratios of 1.3, 2.6,4.5, 7.8, 14.2, and (completely siliceous faujasite). The rise in background at shorter wavelengths arises from scattering from the zeolite particles. As the Si/AI ratio increases, the band at -400 nm typical of the zwitterionic form increases until Si/AI of 7.8 followed by a decline at higher Si/AI ratios. The spectrum of the ani1 observed on the completely siliceous zeolite (Figure 6f) resembles closely that observed in trifluoro-2-propanol (Figure 2b). Consistent with the solution studies, a values for the zeolites were determined from the ratio of the observed intensities of the -400-450 to 320-nm band and the correlation developed for the solutions (Figure 3). The a values for faujasite with Si/AI ratios of 1.3, 2.6, 4.5, 7.8, 14.2, and 0 correspond to 1.70, 1.79, 1.83, 1.90, 1.75, and 1.06, respectively. It appears that the hydrogen bonding donor ability of the dehydrated Na+-exchanged zeolite increases until the Si/AI ratio reaches a maximum of 7.8 and then decreases again. The range of a values observed for the zeolites covers the solvents from trifluoro-2-propanol to hexafluoro-2propanol, indicating a wide scale for the donor ability of the zeolite. Since the diffuse reflectance bands arc broad and on a rising background, we examined the Raman spectra of the anil adsorbed on the faujasitic zeolites. Representative spectra of the ani1 on frameworks with Si/AI ratio of 1.3, 2.6, 4.5, and m are shown in Figure 7. The trends are similar to that observed in the electronic spectrum. The a value derived from the ratio of the 1577 to I596 cm-' band and the correlation developed in Figure 4 corresponds to 1.66, 1.83, 1.88, 1.95, and 1.21 for Si/AI ratios of 1.3, 2.6, 4.5, 7.8, and a, respectively. Raman spectra could not be obtained for the framework with Si/Al ratio of 14.2, due to high background. The agreement between the a value for zeolites determined by UV-visible and Raman spectroscopy is quite reasonable. If we consider the average of the a values obtained by these two techniques as representative, then the values of a for frameworks with %/AI ratio of 1.3, 2.6, 4.5, 7.8, 14.2,
8 1.5 -
1.3
-
1 .o
0.00
0.09
0.18
0.27
0.36
0.45
AI/(Si+AI)
Figure 8. Variation of the a values of faujasites with (a) Si/AI ratios
and (b) AI/(Si
+ AI) ratios.
and are 1.68, 1.81, 1.85, 1.93, 1.75, and 1.14, respectively. There is clearly no doubt that the a values exhibits an increase with Si/AI ratio, followed by a decrease, as shown in Figure 8a. Figure 8b also clearly shows this effect as a function of framework AI composition.
Discussion ' In the only other study using the solvatochromic indicator method on zeolites, Handreck and Smith examined the adsorption of 4-nitroanisole on ZSM-5and concluded that the value of II+ (measure of solvent polarity/dipolarizability) remains unchanged if the Si/Al ratio of the framework was in the range of 36-=.1° The ability of the zeolite to stabilize a charge or a dipole by virtue of its dielectric effect did not appear to be influenced by the AI content of the framework. Since that study did not extend to ZSM-5 frameworks of lower Si/AI ratio, the value of lI* in the lower ranges is not known. The present study shows that the ability of the faujasite framework to stabilize the zwitterionic form of the ani1 can be as effective as strongly hydrogen bond donating solvents and is also markedly dependent on the Si/AI ratio of the framework. The following pertinent questions form the basis of the discussion. How and why does the zeolite stabilize the zwitterionic form? How can we account for the change in a value of zeolite framework as a function of Si/AI ratio? Are there correlations between the a value of zeolite and any intrazeolite behavior?
Solventlike Nature of Zeolites
The Journal of Physical Chemistry, Vol. 95, NO. 10, 1991 4091
ratio increases, while the conjugate base strength (framework 0) decreases. The stabilization of the zwitterionic form of the ani1 depends both on the acid and base nature of the zeolite since the D / N+H needs to interact with the base and the (2-0- with the acid. The experimentally determined a values indicate that the optimum conditions for stabilizing the zwitterion are for frameworks with Si/AI ratio around 7.8. As the ratio changes from this value in either direction, either the acidity or basicity increases. Since both acid and base functionalities are required to stabilize the zwitterion, (A- acceptor D - donor) its amount decreases as the Si/AI ratio changes from 7.8. With increasing polarity of the hydroxylic solvents, the zwitterionic Below we discuss the relevance of the a value that we measure form is stabilized by solvation. In doing so, the - O H group acts for zeolite frameworks to intrazeolitic processes. The rates of a both as an effective acceptor (ob)and donor functionality (H*+). variety of reactions such as n-hexane and cumene cracking or Therefore, a as determined by the ani1 provides a measure of both cyclopropane isomerization on protonated faujasites show a rapid the donor and acceptor properties of the solvent. In the completely increase in rate as Si/AI ratio increases to -7, followed by a Na+-exchanged zeolites (Si/AI ratios of 1.3,2.6,4.5,7.8, and 14.2) decrease a t higher Si/Al ratio^.^^-,^ there are no Bronsted type acidic sites." There are surface silanol The correlation between the rates of these reactions and the groups and such defect sites could also be present within the zeolite trends in a values as a function of the AI content of the framework framework. is excellent. In order to lend further insight into this correlation, In the case of completely siliceous faujasite (a= l.l4), the AI we examined the literature on reactions on acidic faujasites. These level is of the order of 120 ppm,I5 and the framework charge and reactions are typically catalyzed by Bronsted acid sites associated Na+ is expected to be negligible. Yet, the framework exhibits with the framework AI atoms. Various global models have been a values comparable to that of trifluoro-2-propano1, indicating developed to examine the acidic strength of zeolites as a function that resonably strong hydrogen bond donor-acceptor groups are of framework composition. For example, the zeolite has been present. Since the samples were completely dehydrated and kept considered a "polyprotic" acid based on AlOSiOAl ~ n i t s . ~This ,~ so during the spectral measurements, interference from hydration model predicts an increase in acid strength with decrease AI effects is not expected. IR and cross-polarization 29SiN M R framework content until Si/Al ratios of -7 (AI/(AI + Si) = studies have shown that dealuminated zeolites contains a variety 0.125), beyond which the AI atoms are isolated and the acidity of Si-OH groups.I5 These defects can be annealed by heating levels off. The model based on Sanderson electronegativity of the at temperatures of 700 OC. At the temperatures of activation used framework predicts a continuous increase of acidity as the A1 in this study (500 "C),spectroscopic studies indicate that a content decreases.27 The experimental data do not agree with these considerable fraction of the Si-OH groups are still resent.'^ We predictions, since the rates of acid-catalyzed reactions on faujasitic propose that these groups act as hydrogen bond donors, whereas zeolites reach a maximum for a framework composition of Si/Al the framework oxygen atoms (SiOSi) are the acceptors in staratio of ~ 7 In the . above ~ models, ~ it is implicit that the A1 atoms bilizing the zwitterionic form. The acidity of Si-OH groups in are distributed randomly. Though 29SiNMR data suggest the silica2' has been reported to be higher than that of methanol, presence of Si(2Al) species for frameworks with Si/AI > 7,29 the consistent with the a value reported for methanol (0.93) and this overwhelming evidence based on catalytic reactions and thermal study on dealuminated faujasite (1.14). It is difficult to uneanalysis is that the AI atoms are indeed distributed randomly.26*M quivocally assign the nature of the silanol groups, but structures Clearly, this issue is not settled. An alternative explanation based such as Si(OSi)30H and Si(OSi),(OH), have been proposed in on a more localized picture postulates the presence of two types the literature. Infrared bands corresponding to these groups occur of acid sites, a strong accessible site, and a hidden weaker site.27 in the 3500-3 700-cm-' region. The weakness of the latter sites arises from interactions with As the Si/AI ratio decreases AI is incorporated into the framework oxygen atoms. As the framework AI decreases, these framework along with neutralizing Na+ cations. There is conhidden sites can compete more effectively with the stronger acid siderable increase in a value for these frameworks as compared sites in interacting with reactant molecules, thereby lowering the to dealuminated faujasite. Since there are no Bronsted type acids turnover rates. This model can explain the trends observed in in these frameworks,22we propose that the negative charge of the Figure 8, if the Na+ ions are distributed between accessible and zwitterion (CO-) is interacting with Lewis acidic sites, for example, hidden sites, with differing Lewis acid character. In the absence Na+ ions. The positive charge (NH+) can interact with the of definitive structural information on these issues, we propose framework oxygen atoms. The key question here is why does the that the zeolite be considered as a solvent, with the a values ability of the zeolite framework to stabilize the zwitterionic form reflecting the global ability of the zeolite to stabilize polarized of the ani1 show an increase with Si/AI ratio, reach a maximum solute molecules within its cavities. Electrostatic fields in zeolites at Si/AI ratio of 7.8, and then decrease at higher ratios. have been proposed to play an important role in polarizing reactant In order to correlate physicochemical properties of zeolites with molecules.' The a values of zeolites, as measured in this study, framework compositions, various authors have used the Sanderson could serve as a quantitative measure of stabilization of polarized equalization principle to calculate framework charge^.^^,^ The structures via acid-base interactions with the framework. fractional charges calculated on the Na+ for the series of In the photolysis of dibenzyl ketones in ~eolites,~' it was found frameworks with Si/AI ratios of 1.2, 2.6,4.5, 7.8, and 14.2 are that the benzyl radical mobility was higher in faujasite with Si/Al 1.48, 1.65, 1.77, 1.87, and 1.94, respectively. The electrophilicity ratio of -2.5 (zeolite Y)than -1.3 (zeolite X), resulting in a lower of the cations or their acidic strength increases with Si/AI ratio. efficiency of geminate pair recombination in the higher Si/AI ratio This is consistent with titration studies on alkali-metal-exchanged framework. The possibility of interactions of the benzyl radicals f a ~ j a s i t e s . The ~ ~ fractional oxygen charges is calculated to be with Lewis acid-base sites was proposed to be a factor in the -0.40, -0.34, -0.30, -0.27, and -0.24, indicating that the nudifference in photolyzed products. It would be of interest to extend cleophilicity or basic nature of the oxygen atoms in the framework is decreasing with the Si/AI ratio. In other words, the zeolite can (25) Sohn, J . R.; DeCanio, S. J.; Fritz, P. 0.;Lunsford. J. H. J . fhys. be considered as an acid-base pair, as has been discussed in the Chem. 1986, 90, 4847. literature,24with the acidic strength (Na+) increasing as the Si/AI (26) Dwyer, J . Srud. Sur/. Sci. Carol. 1988, 37, 333.
The equilibrium that is of interest is between the benzenoid and zwitterionic form.
(27) Corma, A.; Fornca, V.; Perez-Pariente, J.; Sastre, E.; Martens, J. A,;
(21) Hair, M. L.; Hertl, W. J . fhys. Chem. 1970, 74, 91. (22) Ward, J. W. In Zeolire Chemistry and Catdysis., R a b , J., Ed.;ACS Monogr. No. 171; America1 Chemical Society: Washington, DC, 1976; p 118. (23) Mortier, W. J. J . Carol. 1978, 55, 138. (24) Barthomeuf, D. J . fhys. Chem. 1984.88, 42.
Jacobs, P. A. ACS Symp. Ser. 1988,368, 556. (28) (29) 369. (30) (31)
Mikovsky, R. J.; Marshall, J. F. J . Carol. 1976, 44, 170. Itabashi, K.; Okada. T.; Igawa. K. f r w . 7th Int. Zeolire Con/. 1986, Kerr, G. T.; Chester, A. W. Thermochim. Acfa 1971, 3, 113. Turro, N . J.; Wan, P. J . Am. Chem. Soc. 1985, 107,678.
J. Phys. Chem. 1991, 95.4092-4098
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the ani1 molecules can enter into the cages. Figure 9b shows the diffuse reflectance spectra of the ani1 in Na-ZSM-5 (Si/AI 50). From this spectrum and the correlation developed in Figure 3, we calculate an a of 1.34, making it intermediate between that of faujasite frameworks with Si/AI ratio of 14.2 and Figure 9c shows the reflectance spectrum for ani1 adsorbed on H+ form of ZSM-5.In this case, the acidic sites are strong enough to protonate the C-N group, since this spectrum resembles that of anil dissolved in HC1-saturated trifluoroethanol." This complication of reaction with the acidic groups will preclude the study of a value of H+ zeolites using the anil molecule as a probe. In conclusion, we have shown in this study that it is possible to find appropriate molecules with solvatochromic properties that can describe the "solvent like" nature of intrazeolitic space. In particular, the spectroscopic differences between the benzenoid and zwitterionic form of salicylideneaniline in hydrogen bond donating solvents have been used to quantitatively correlate electronic and Raman band intensities to the a value of the solvents. This correlation was then applied to estimate a value of a series of zeolites all with the faujasite framework but with varying Si/AI ratios. In the Na+-exchanged form of the zeolite, an a value comparable to the strongest H bond donor solvents was discovered. Moreover, the a value showed an increase with Si/A1 ratio until a maximum is reached at -7.8 and then decreased for frameworks with further dealumination. It is also being suggested that the collective property of the zeolite as determined by the a value be though of as the ability of the intrazeolitic space to stabilize polarized molecules. 05.
300
400
500
600
nanometers Figure 9. Comparison of the diffuse reflectance spectra of anil on (a) Na-A, (b) ZSM-5, and (c) H-ZSM-5.
this study to faujasite frameworks with higher Si/AI ratios and correlate the photochemical results with the a value. The anil can also be used to examine other zeolite frameworks, as long as it is able to penetrate within the cages. In the case of zeolite A, the 4-A opening to the large cages does not allow the ani1 molecule to enter the cages, and the absorption spectra resemble that of anil dissolved in a nonpolar solvent (Figure 9a). However, for Na-ZSM-5, in which the channels are -6 A in size,
Acknowledgment. We acknowledge the financial assistance provided by the Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, U.S.Department of Energy. We are grateful to Drs. Jack Donahue and Alak Bhattacharya for providing the dealuminated zeolite samples. We also thank the reviewers for helpful comments.
Relationship between Pyroelectricity and Molecular Orientation In Alternate Langmulr-Blodgett Films Toshihide Kamata, Junzo Umemura, Tohru Takenaka,* and Naokazu Koizumi Institute for Chemical Research, Kyoto University, Uji, Kyoto-fu 61 1 , Japan (Received: November I , 1990)
The relationship between pyroelectricity and molecular orientation in alternate Langmuir-Blodgett (LB) films consisting of 5-(p-dodecyloxyphenyI)pyrazine-2-carboxylicacid and deuterated stearic acid was investigated. The same studies were also performed for alternate LB films of their barium salts. Pyroelectricity of the alternate film was measured by a static method in the temperature range from -30 to 80 OC. The maximum values of the pyroelectric coefficient due to a decay of spontaneous polarization were obtained to be 1.2 p C me* K-I at 37 OC for the acid alternate film and 1.8 pC m-2 K-' at 43 OC for the barium salt alternate film. Rapid increases of positive currents due to a depolarization in the films were observed above 50 OC. The molecular orientation in the film was evaluated by Fourier-transform infrared transmission and reflection absorption spectroscopy. The temperature dependence of the orientation angles of the hydrocarbon chain axes of the constituent molecules indicates that the depolarization is due to the conformational disorder of the hydrocarbon chains. Furthermore, the temperature dependence of the COO- stretching frequencies of the barium salt alternate film suggests that the spontaneous polarization is generated by a structural modification of the carboxylate groups. It is concluded that the alternate film with the more highly oriented molecules gave the larger pyroelectricity and that ifthe molecular orientations are in the same order, the acid alternate film has greater pyroelectricity than the corresponding barium salt film. The latter finding may be due to weaker intermolecular interactions in the acid alternate film than in the barium salt film.
Introduction It has been known that Langmuir-Blodgett (LB) films are well-organid thin organic assemblies capable of various functions if molecular compositions are suitably designed. For example, X-and Z-type LB films and alternate LB films consisting of two different amphiphiles have noncentrosymmetric structure and are expected to show piezoelectric, pyroelectric, and various nonlinear optical properties.
Since the beginning of the 198Os, there have been several reports dealing with the pyroelectric effect Of noncentrosymmetric LB films. Blinov et studied pyroelectricity in x-and Z-type (1) Blinov, L. M.;(Engl. Davydova, N. ,982, N.; Lazarev, Phys.-So,id 21, ,523.V. V.; Yudin, S.G. Sou. (2) Blinov, L. M.: Mikhnev,L. V.;Wolova, E. B.; Yudin, S.G. Sw. Tech. Phys. ~ I I(Engl. . Transl.) 1983, 9, 640.
0022-3654/91/2095-4092$02.50/00 1991 American Chemical Society
