Novel Organochlorinated Xerogels: From Microporous Materials to Ordered Domains

Abstract

Hybrid silica xerogels combine the properties of organic and inorganic components in the same material, making them highly promising and versatile candidates for multiple applications. They can be tailored for specific purposes through chemical modifications, and the consequent changes in their structures warrant in-depth investigation. We describe the synthesis of three new series of organochlorinated xerogels prepared by co-condensation of tetraethyl orthosilicate (TEOS) and chloroalkyltriethoxysilane (ClRTEOS; R = methyl [M], ethyl [E], or propyl [P]) at different molar ratios. The influence of the precursors on the morphological and textural properties of the xerogels was studied using ²⁹Si NMR (Nuclear Magnetic Resonance), FTIR (Fourier-Transform Infrared Spectroscopy), N₂, and CO₂ adsorption, XRD (X-ray Diffraction), and FE-SEM (Field-Emission Scanning Electron Microscopy). The structure and morphology of these materials are closely related to the nature and amount of the precursor, and their microporosity increases proportionally to the molar percentage of ClRTEOS. In addition, the influence of the chlorine atom was investigated through comparison with their non-chlorinated analogues (RTEOS, R = M, E, or P) prepared in previous studies. The results showed that a smaller amount of precursor was needed to detect ordered domains (ladders and T₈ cages) in the local structure. The possibility of coupling self-organization with tailored porosity opens the way to novel applications for this type of organically modified silicates.

Keywords: ORMOSILs; TEOS; chloroalkyltriethoxysilane; hybrid materials; inductive effect; textural properties; xerogels.

Figure 1

(a) Silicon environments present in chloroalkyltriethoxysilane (ClRTEOS):tetraethoxysilane (TEOS) xerogels, and (b) normalized ²⁹Si NMR spectra of the hybrid xerogels synthesized with 5%, 10%, and 15% of ClRTEOS (chloromethyltriethoxysilane (ClMTEOS) (●), chloroethyltriethoxysilane (ClETEOS) (▲), and chloropropyltriethoxysilane (ClPTEOS) (■)).

Figure 2

Variation in the relative abundance of the condensed species of Si with respect to percentage ClRTEOS obtained by integrating the ²⁹Si NMR spectra for: (a) ClMTEOS, (b) ClETEOS, and (c) ClPTEOS.

Figure 3

X-ray diffraction patterns of the hybrid xerogels ClRTEOS:TEOS at different molar percentages: (a) ClMTEOS, (b) ClETEOS, and (c) ClPTEOS.

Figure 4

Skeletal density of the materials according to the precursor molar percentage for: (a) ClRTEOS series (ClMTEOS, ClETEOS, and ClPTEOS) and (b) RTEOS series (MTEOS, ETEOS, and PTEOS) in previous studies [20,21,22]. Reference material density (100%TEOS) = 1.96 g·cm⁻³.

Figure 5

N₂ isotherms (−196 °C) of chloroalkyl materials at different molar percentages of: (a) ClMTEOS, (b) ClETEOS, and (c) ClPTEOS.

Figure 6

CO₂ isotherms (0 °C) of chloroalkyl materials at different molar percentages of: (a) ClMTEOS, (b) ClETEOS, and (c) ClPTEOS.

Figure 7

Density-functional theory (DFT) porosity distribution obtained from N₂ isotherms of: (a) ClMTEOS:TEOS series, (b) ClETEOS:TEOS series, and (c) ClPTEOS:TEOS series and from CO₂ isotherms of: (d) ClMTEOS:TEOS series, (e) ClETEOS:TEOS series, and (f) ClPTEOS:TEOS series.

Figure 8

FE-SEM micrographs of: (a) reference xerogel (100% TEOS), (b) xerogel synthesized with 30% of ClMTEOS, (c) xerogel synthesized with 15% ClETEOS, and (d) xerogels synthesized with 12.5% ClPTEOS.