3388
B. D. MCNICOL, G. T.POTT, ANI E(. R. Loos
ectroseopie Studies of Zeolite Synthesis by B. D. McNicol,* G. T. Pott, and K. R. Loos Ko~inkl~iike/ShelI-l;aborator?r, Amsterdam, T h e Netherlands
(Received M a g SO, 1972)
Publication costs assisted by Shell Research N.V.
The synthesis of zeolites Linde A and faujasite has been studied using phosphorescence spectroscopyand laserRaman spectroscopy. The phosphorescence of tracer amounts of Fea+ was used to monitor changes in the framework of the solid gel phase during induction and crystallization periods. The liquid and solid gel phases were studied using the Raman spectra of silicate and aluminate tetrahedral anionic species and tetrarnethylammonium cationic species as probes. The results obtained were indicative of a zeolite crystallization in the solid gel phase via condensation between hydroxylated Si-A1 tetrahedra.
Previous studies of zeolite crystal growth using chemical analysis, X-ray diffraction, and electron microscopy have given rise to two main proposals for the mechanism of formation of zeolite crystals from aluminosilicate gels.lV2 On the one hand there exists a school of thought that zeolite crystallization occurs by dissolution of the amorphous gel to produce active aluminosilicate anionic species in solution. These species then grow by consumption of the gel to form the nuclei and ultimate crystals.' The second mechanism, first put forward by Breck and Flanigen,2proposes that thc crystal nuclei are formed in the solid phase of the gel and that formation of thcsc nuclei and ultimate crystal growth occurs by depolymerization of the gel by the OH- ions in the medium. The investigation of zeolite crystal growth has suffered from the lack of suitable physical techniques for the study of the complicated gel system, and as a result, hypotheses regarding the mechanism have had to rely on chemical analysis of solid and liquid phases, X-ray diffraction, and, to a lesser extent, electron microscopy. These techniques are not very sensitive in the detection of trace concentrations of crystalline material. We have skidied the crystal growth of some zeolites (Lindc A, in particular) using phosphorescence spectroscopy to study the solid phase, and laser-Raman spectroscopy to study both the liquid and the solid phase during the induction and crystallization periods of zeolite crystal growth. The phosphorescence technique has recently been applied to the study of transition metal coordination in oxides and zeolites.8 These studies showed that Fe3+ impurity ions in zeolite crystals occupy tetrahedrally coordinated AI3- framework sites. Such a coordination for Fe3+ gives rise to a phosphorescence and excitation spectrum characteristic for zeolites (Figure 1). Addition of trace amounts of Fe3+ to the gel then permits thc study of the build-up of any zeolitelike framework during the induction and crystallization periods. This technique has the advantage over X-ray diffraction and other spectroscopic techniques T h e Journal of Physzrnl Chemistry, Vol. 76,N o . 2S3 1072
that it is able to detect tetrahedrally coordinated Fe3+ in the parts per million range. Silicate, aluminate, and aluminosilicate anionic species existing in solution in significant concentrations can be studied by Raman spectroscopy and thus the liquid phase can be monitored during the reaction for changes in concentration of such species. In addition, laser-Raman spectroscopy has proved to be a useful probe for the detection of large cations such as (CH&N + occluded in zeolite ~ a v i t i e s . ~Thus by monitoring a gel system containing (CH3)JV+ we can follow any changes which may occur in the surroundings of the (CH&N+ ions.
Experimental Section A . Phosphorescence. The aluminosilicate gel for a Linde A synthesis was made by adding a solution of Fe3+-doped silicic acid in KaOH to a solution of Al(OH)8in KaOH. The gel of composition 6Naz0.A12O3+1.7SiOz.37OH20, 0.01 wt yo Fe3+ was heated to 100" and small samples of gel were extracted at diff erent time intervals during the synthesis, The gel samples were placed in 3-mm internal diameter quartz tubes and quenched in liquid nitrogen ready for measurement. Yo pretreatment of gel such as filtering or drying was required. Spectra were measured using the phosphorescence spectrometer described previously.5 B. Laser-Barnan Spectroscopy. Spectra were obtained of the centrifuged liquid phase and dried solid phase of gels designed to give Linde A or faujasite as ultimate product. Measurements were made using a (1) See, for example, S. P. Zhdanov, Advan. C h m . Ser., No. 101, 20 (1971); G. T. Kerr, J . Phys. C h m . , 70, 1047 (1966); J. Ciric, J. Colloid Interfuce Sci., 2 8 , 315 (1968). (2) D. W. Breck and E. M. Flanigen, presented at the 137th Eational Meeting of the American Chemical Society, Cleveland, Ohio, April 1960. (3) G. T. Pott and €3. D. McNicol, Chem. Plzys. Lett., 12, 62 (1971); €3. D. McNicol and G. T. Pott, J . Caatal., 2 5 , 223 (1972). (4) K. R. boos and J. F. Cole, unpublished results. (5) G. T. Pott and B. D. McNicol, J . Chem. Phys., 56, 5246 (1972).
SPECTROSCOPIC STUDTICS OF ZEOLITESYNTHESIS
3389
INTENSITY
WAVELENGTH, nm
Figure 1. Phosphorescence ( a ) and excitation (b) spectra of Fea +-doped zeolite gel.
the final crystal. These gel sites exist exclusively in the solid phase. The initial period of time during which the Fe3+ signal rises only slightly in intensity corresponds to the mll-known induction period,6 while the steeply rising part parallels the appearance of a diffraction pattern on X-ray powder photographs. This part of the kinetic curve indicates macroscopic crystallization. Crystals grown in this way were smaEIer than 5p. B. Laser-Raman Spectroscopy. Characteristic bands in the liquid phase due to A41(OH)4-at 618 crn-' and Si02(OH)22-at 772 and 915 em-' were observed during both Linde A and faujasite syntheses. Throughout the induction period, and even after crystallization had set in, the concentration of these species remained essentially constant within the error of the techniquc (15%). Furthermore, no evidence of the existence of an anionic aluminosilicate species wab noted. These observations indicate that no significant net dissolution of the solid phase of the gel occurs throughout the synthesis. I n another experiment Raman spectra were obtained of a Linde A system in which 60% of the sodium ions were replaced by (CH3)4N+ions. I n the liquid phase a spectrum identical with that of (CHJ4S+in aqueous basic solution was observed with a strong K-C symmetric stretching mode a t 752 em-]. Throughout both the induction and crystallization periods the signal did not change in shape, position, or intensity. During the induction period the centrifuged, wafihed (to pH 9)) and dried solid phase showed a weak band at 754 cm-1. Upon the onset of observable erystallization the 754-cm-' band decreased in intensity and a concomitant increase of a new band at 769 cm-l was observed. The growth in intensity of this signai paralleled the increase in Fe3+ phosphorewence after the induction period. This band at 769 em-' was similar to that found for (CH3)&+ ions occluded in sodalite4 and is therefore believed to be due to (GH3)J?+ ions occluded within the zeolitic cages of thc Linde A crystals.
. -
I/T,
I
TIME, h
Figure 2. Relative intensity of Fe3+phosphorescence of gel as a function of time.
Spex Ramalog spectrometer equipped with a Coherent Radiation Rbdel 52 argon ion laser.
Results A . Phosphorescence. The results of a typical experiment, a Linde A synthesis, are shown in Figure 2 . Similar results were obtained in the synthesis of other zeolites such as faujasite, gmelinite, and P. The phosphorescence data can be summarized as follows: (I) immediately upon the formation of the gel a weak signal similar to that shown in Figure 1 was observed; (2) the phosphorescence intensity (Figure 2 ) rises only slightly during the induction period of synthesis and then rapidly increases until finally the fully crystallized phase has a constant phosphorescence intensity; (3) the wavelength and band shape of the excitation and emission spectrum of the final crystals are essentially identical with the spectrum of the initial gel, (4 ) experinient s on Fe3+-doped neutral aluminosilicate gels showed that Fe3+ centers in such systems do not phosphoi-e~ce;(5) the liquid phase of the gel, separated by centrifugation, did not give a F e a t phosphorescence signai at any time during crystallization. These reslxlts indicate that immediately upon gel formation tetrabedrally coordinated zeolite framework sites exist which are similar to the Fe3+(A13+)sites in
Discussion The Raman and phosphorescence results do not show any observable changes occurring in the liquid phase but do show significant alteration in the solid phase, which supports the view that crystal growth occurs from the solid gel phase. Barrer and coworliers7 have recently reported the observation of amorphous laminae precuryors t o zeolite crystals. The presence of a phosphorescence signal upon gel formation is consistent with the presence of such laminae. The considerably lower intensity of (6) D. W. Breclc and E. M. Flanigen, "Molecular Sieves," SocietS of Chemical Industry, London, 1967, p 47. (7) R , Aiello, R. M, Barrer, and I, S. Kerr, Adran, Chem, Ser,, No. 101,44 (1971). The Journal of Physical Chemistry, V o l . 7 6 , ATo. 23, 197%
D. DEN ENGELSEN
3390 the phosphorescence observed throughout the induction period can be due to the presence of many more terminal hydroxylated framework Fe3+(A13+) ionic species in the gel which do not phosphoresce. The final crystals, however, having a very much higher ratio of bulk to surface framework sites would therefore have a much more intense phosphorescence signal. Similarly, the 754-cm-’ (CH3)&+ Raman band in the solid phase during the induction period is explicable as (CH3)4N+ions bound to the negatively charged AI framework sites of the amorphous gel. On transfor-
mation of the amorphous material to zeolite crystals the (CH&N+ ions become trapped in the cages of the crystals. Summarizing, it can be said that our experiments are compatible with a zeolite crystallization mechanism occurring in the solid phase where some amorphous laminae are produced upon gel. formation. During the induction period further laminar species can be formed, destroyed, and modified until the first zeolite crystals are formed, after which autocatalysis of crystal growth produces an “a~alanche”of zeolite crystals.
Transmission Ellipsometry and Polarization Spectrometry of Thin Layers
Philips Research Laboratories, Eindhoven, Netherlands
(Received M a y 19,1972)
Publication costs assisted by Philips Research Laboratories
A theoretical description is given of the transmission ellipsometry and polarization spectrometry (dichroic spectra) of thin films deposited on a transparent substrate. It is shown that transmission ellipsometry is a sensitive method of measuring the anisotropy of the optical constants. The dichroism of films thinner than the wavelength of the light used to make the measurements depends strongly on their thickness, whereas for thick films the dichroism is almost independent of the thickness. The theory about transmission ellipsometry and dichroic spectra is illustrated by Langmuir-Blodgett layers.
Introduction Ellipsometric and spectroscopic methods are a t present widely used for studying thin films. Most applications deal with isotropic thin layers on which a vast literature exists, including several well-known barndbool~s.~--~ Thus far, little attention has been paid to the transmittance and reflectance of anisotropic thin films. Anisotropy may refer to either the components of the dielectric tensor or the componente of the conductivity tensor. The absence of much information or knowledge about this subject is caused by the difficulties inherent in preparing thin anisotropic layers. For instance, evaporation techniques seldom yield anisotropic systems. An exception is formed by polymer sheets containing rod-shaped dye molecules. Stretching of such sheets gives rise to a more or less well-defined orientation of the dye molecules with their long axes in the stretch d i r e ~ t i o n . ~ However, nonrodlike molecules can also be oriented.5 The dichroic. spectra of these anisotropic systems can he interpreted in terms of the degree of orientation of thc dye molecules.6 Another class of essentially anisotropic layers BS formed by the so-called LangmuirBlodgett (LB) l a y e r ~ . ~ ,The s birefringence in nonThe Journal of Phystcal Chemistry, Vol. 76, N o . 23, 1972
absorbing layers of barium stearate has been measured by Langmuir and Blodgett by comparing the respective transmittances of the p (parallel) wave and s (“senkrecht”) wave.R The anisotropy in the optical constants of both nonabsorbing and absorbing LB layers can also be determined with (reflection) ellipsometry.g,10 I n this paper I shall present a theoretical description of the dichroic spectra of uniaxially anisotropic layers (1) A . Vasicek, “Optics of Thin Films,” North-Holland, Amsterdam,
1960. (2) 0. S. Heavens, “Optical Properties of Thin Solid Films,” Butterworths, London, 1955. (3) 0. S. Heavens, “Thin Film Physics,” Methuen, London, 1970, Chapter 6. (4) (a) E. W. Thulstrup, J. Michl, and J. H. Eggers, J . P h u s . Chem., 74, 3868 (1970); (b) H. Inoue, et al., Ber. Bunsenges P h y s . Chem., 75,441 (1971). (5) This was kindly pointed out t o me by the reviewer of the manuscript. (6) Y. Tanizaki, Btdl. Chem. SOC.J a p . , 38,1798 (1965). (7) K. B. Blodgett, J. Rmer. Chem. SOC.,57, 1007 (1935). (8) K. B. Blodgett and I. Langmuir, P h y s . Reo., 51,964 (1937). (9) D . den Engelsen, J . Opt. Soc. Amer., 61, 1460 (1971). (10) E. P. Honig, J. H . T h . Hengst, and D. den Engelsen, J . Colloid Interface Sci., in press.
