Organosilicas with Covalently Bonded Groups under Thermochemical Treatment Sergei A. Alekseev and Vladimir N. Zaitsev* Chemical Faculty, KieV National Taras SheVchenko UniVersity, 64 Vladimirskaya Str., 01033 KieV, Ukraine Jacques Fraissard ESPCI, Laboratoire PMMH, UniVersite P. et M. Curie, 10 rue Vauquelin, F-75231, Paris Cedex 05, France ReceiVed December 15, 2005. ReVised Manuscript ReceiVed February 10, 2006 Silica-based hybrid materials having covalently immobilized vinyl (SiO 2 -C 2 H 3 ), chloropropyl (SiO 2 - R-Cl), trimethylsilyl (SiO 2 -SiMe 3 ), ethyl sulfonic acid (SiO 2 -R-SO 3 H), and aminopropyl (SiO 2 -R- NH 2 ) groups, as well as the salt of the latter with HNO 3 (SiO 2 -R-NH 2 ‚HNO 3 ) were studied by different thermoanalytical methods: thermogravimetry (TGA), differential thermal analysis (DTA), and temperature- programmed desorption mass spectrometry (TDP-MS). It was demonstrated that TPD MS can be successfully used for the investigation of the interfacial layer in such materials. Particularly, it was shown that a side reaction between the grafted group and aromatic solvents is possible during the preparation of SiO 2 -C 2 H 3 and SiO 2 -R-Cl. For SiO 2 -SO 3 H the formation of 2-Si-ethanesulfonic, 1-Si-ethanesulfonic, and 2,4-Si-butanesulfonic acid grafted groups with the predominance of the 2-Si isomer was found. The process of SiO 2 -NH 2 ‚HNO 3 decomposition at 500 K may be applied for the preparation of silica modified by aldehyde groups. Mechanisms of thermal transformations of bonded layer were established and the key role of the reactions of grafted groups with silanols in such processes was demonstrated. As was found for SiO 2 -R-Cl and SiO 2 -R-NH 2 , the decomposition process with participation of silanols is realized in two stages. The first one occurs in the 400-700 K range and includes the interaction between organic groups and the neighboring silanol. The second decomposition stage occurs above 700 K and includes migration of the bonded groups on the silica surface. I. Introduction Porous silicas having an immobilized organic layer (chemically modified silicas, CMS) are widely used as adsorbents, 1 chromatography phases, 2,3 and catalysts. 4,5 Use- ful properties of such hybrid materials are determined by the chemical nature of their interfacial layer and particularly by its composition, 6-8 geometry, 9 and topography (micro- heterogeneity). 10,11 Certainly, the composition of the im- mobilized layer is most crucial. Only in the simplest cases (for CMS obtained in one-step immobilization reaction) the composition of the surface layer can be determined from CMS chemical analysis. 11 Most of the CMS can only be synthesized by the multistage chemical transformations of the organic groups grafted on the silica surface (this is so- called surface assembling method). In such transformations the conversion of one grafted group to another commonly has a low yield. 11 Thus, the interfacial layer for most CMS has a multifunctional nature and its composition cannot be determined from the results of CMS chemical analysis. Spectral methods such as FTIR or MAS NMR have their own limitations if used for qualitative analysis of the CMS grafted layer. Many characteristic FTIR bands of the organic groups are located in the region of SiO 2 absorbance. Strong signals of water, adsorbed on the silica surface at ambient conditions, usually cover completely the signals of organic fragments in FTIR and 1 H MAS NMR spectra. That is why experiments on FTIR and 1 H MAS NMR of CMS require special precautions to avoid the contact of sample with water vapors. The method of mass spectroscopy is one of high selectivity and sensitivity, making it attractive for the characterization of materials with functionalized surfaces. For qualitative analysis of the immobilized layer composition, temperature- * To whom correspondence should be addressed. E-mail: zaitsev@univ.kiev.ua. (1) Papirer, E. E. Adsorption on Silica Surfaces; Marcel Dekker: New York, 2000. (2) Scott R. P. W. Silica Gel and Bonded Phases: Their Production, Properties and Use in LC; John Wiley & Sons: New York, 1993. (3) Davankov, V. A.; Navratil J. D.; Walton, H. F. Ligand Exchange Chromatography; CRC Press: Boca Raton, FL, 1988. (4) Smith, G. V.; Notheisz, F. 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