STUDIA UNIVERSITATIS BABES-BOLYAI, PHYSICA, SPECIAL ISSUE, 2003
CHARACTERIZATION BY FTIR spectroscopy OF DIFFERENT ALUMINA SAMPLES USED IN TLC
Rodica Grecu, Virginia Coman and Miuţa Filip
”Raluca Ripan”Institute for Research in Chemistry, Department of Chromatography, 30 Fântânele Street, RO-3400 Cluj-Napoca, Romania,
E-mail: r.grecu@ icrr.cj.edu.ro;
Abstract
The acidic alumina 150 T and basic alumina N used as stationary phase in TLC have been chemically modified by organosilanization reaction using the n-octadecyltrichlorosilane and 3-mercaptopropyltrimethoxysilane as trifunctional modifiers.
The study of these stationary phases by FTIR spectroscopy puts in evidence the presence of the modifier on the alumina surface. To improve the sensibility of infrared method, the difference and second derivative spectra were also analysed in order to find out the effect of the modifier on the alumina surface properties.
Key Words:
chemically modified alumina, n-octadecyl alumina, 3-mercaptopropyl alumina, FTIR spectroscopy, TLC.
INTRODUCTION
A lot of thin layer chromatographic separations are performed on chemically modified stationary phases in order to increase their selectivity and efficiency [1].
The infrared spectroscopy is a common method used to characterize the chemically modified stationary phases [1-6]. In literature, infrared studies on chromatographic aluminas have mainly concentrated on the hydroxyl groups and Al-O-Al linkages created during the dehydration process [5]. Diffuse reflectance infrared Fourier transform (DRIFT) spectra have been used to the characterization of chemically modified aluminas [6].
In this paper, n-octadecyl and 3-mercaptopropyl chemically modified aluminas (acidic alumina 150 T and basic alumina N) used as stationary phases in thin layer chromatography are studied by FTIR spectroscopy.
EXPERIMENTAL
Aluminum oxide 150 acidic type T (acidic alumina) n-octadecyl-trichlorosilane and 3-mercaptopropyltrimethoxysilane were purchased from E. Merck (Darmstadt, Germany). Aluminium oxide N (basic alumina) were supplied
by Macherey-Nagel (Düren, Germany).
STUDIA UNIVERSITATIS BABES-BOLYAI, PHYSICA, SPECIAL ISSUE, 2003
The modification of alumina surface was achieved by the organosilanization reaction of superficial O-H groups of alumina samples with n-octadecyltrichlorosilane as well as 3-mercaptopropyltrimethoxysilane [4].
Infrared spectra of samples were registered on a JASCO-610 FTIR spectrometer using KBr pellet technique. To improve the sensibility of IR method, the difference and second derivative spectra were also analyzed.
Results and discussion
The differences in the structure of acidic and basic alumina are well evidenced by their infrared spectra presented in Figure 1. The broad band centered at 3433 cm-1 assigned to n(OH) vibrations, results from the overlap of several bands due to the hydroxyl groups from the alumina surface and the adsorbed water on KBr matrix.
Figure 1. ftir spectra of acidic (top) and basic (bottom) unmodified alumina
(KBr pellet technique.
A tentatively assignment of the proeminent bands from the spectrum of unmodified aluminas is given in Table 1.
Table 1
The assignments of some vibrational modes for acidic alumina and basic alumina.
vibrational mode / Peak position [cm-1]acidic alumina / basic alumina
n (al-o-al) / 565.0 / 599.2
n (al-oh) / 698.1
735.7
818.6 / 734.2
d (oh) / ~1100 / ~1100
STUDIA UNIVERSITATIS BABES-BOLYAI, PHYSICA, SPECIAL ISSUE, 2003
Because the FTIR spectra were registered in transmission, on sample prepared using the KBr pellet method, no information on the modification (reduction) of the superficial OH groups number of alumina as effect of the silanization reaction can be obtained.
New bands noticed especially in the 2800-3000 cm-1 range of spectrum confirm the presence of the organic part of the modifier on the surface of alumina: nas(CH2) at ~ 2925 cm-1 and nasym(CH2) at ~ 2855 cm-1. The band at 2960 cm-1 is assigned to nas (CH3) and this fact confirms that, due to steric reasons, the functionality of the modifier is 2 and 3.
Due to low concentration of the modifier on the surface the intensity of the new bands is weak. the presence of modifier is better confirmed by the difference spectrum (see Figure 2). The bands observed at ~ 1470 cm-1, 950 and 1062 cm-1 were assigned to d(CH2) and ν(AlO), ν(SiO) attestig of formation of Al-O-Si bridges by attaching of silane derivative onto alumina.
Figure 2. Difference spectrum of n-octadecyl acidic alumina and acidic alumina.
Figure 3. Second derivative spectrum of
3-mercaptopropyl basic alumina (–) and basic alumina (..).
in the case of 3-mercaptopropyl modified aluminas the expected bands due to the Al-O-Si vibrations have been put in evidence comparing the second derivative FTIR spectra (Figure 3) of chemically modified and unmodified alumina. The bands assigned to CH2 groups, having a very low intensity in the spectra of 3-mercaptopropyl modified aluminas can also be easily observed using the second derivative spectra.
CONCLUSIONS
The presence of organic part of the modifier on alumina surface is evidenced by the new bands (n(CH) vibrations of CH2 and CH3 groups) observed in the 2800-3000 cm–1 range. These bands are better noticed in the difference spectrum of unmodified and chemically modified alumina.
The second derivative of FTIR spectra improves the sensibility of infrared method in the study of the chemically modified chromatographic adsorbents specially when the organic chain of modifier is short like in case of 3-mercaptopropyl.
ACKNOWLEDGEMENT
The authors thank to MEC-CNCSIS Grant Program for the financial support.
REFERENCES
1. S. Héron and A. Tchapla, Propriétés et caractérisation des phases stationnaires et phases mobiles de chromatographie liquide à polarité de phases inversée, Analusis, 1993, 21, 327-347.
2. V. Coman, Silicon Compounds Used as Stationary Phases in Liquid Chromatography, (Ph. D. Thesis), Cluj-Napoca (Romania), 1997.
3. C. Măruţoiu, M. Filip, C. Tigae, V. Coman, R. Grecu and Gh. Marcu, Synthesis and Characterization of Alumina R Chemically modified with n-Ocyl for Use as a Stationary Phase in TLC, J. Planar Chromatogr.-Mod. TLC, 2003, 16, 183-185.
4. M. Filip, V. Coman, R. Grecu and Z. moldovan, Characterization of Some Chemically Modified Acidic Alumina Samples for TLC, Proceedings of the International Symposium on Planar Separations, Budapest (Hungary), 21-23 June 2003, 231-242.
5. J.J. Pesek and M.T. Matyska, Modified aluminas as chromatographic supports for high-performance liquid Chromatography- Review, J. Chromatogr. A, 2002, 952, 111.
6. H. Azour, j. Derouault, P. Lauroua and G. Vezon, Fourier transform infrared spectroscopic characterization of grafting of 3-aminopropyl silanol onto aluminum/alumina substrate, Spectrochimica Acta. Part A, 2000, 56, 1627-1635.