Chemical Modification of Metal Oxide Surfaces in Supercritical CO₂:  In Situ Infrared Studies of the Adsorption and Reaction of Organosilanes on Silica

Abstract

Infrared spectroscopy was used to probe the reaction of organosilanes with a fumed silica immersed in supercritical fluid CO2 (SCF CO2). Venting of the CO2 solvent eliminates the experimental difficulties associated with solvent absorption of the infrared radiation and enables repeated surface treatment cycles without disturbance to the amount of silica in the beam. This stability is requisite for detecting infrared bands due to adsorbed species in the spectral region containing the strong metal oxide bulk modes. SCF CO2 has been shown to extract water from silica, and we now exploit this feature for the silane treatment of silica particles. This utility of CO2 as a solvent for the reaction of organosilanes with silica is demonstrated with hexamethyldisilazane (HMDS) and octadecyltrichlorosilane (OTS). The HMDS reaction in SCF CO2 proceeds according to the conventional gas-phase process even though the ammonia generated as a byproduct reacts with the CO2 to produce ammonium carbamate. The volatile ammonium carbamate is weakly physisorbed on the surface hydroxyl groups and is easily removed with evacuation or by purging. Moreover, carbamate formation can be completely avoided by performing the reaction at relatively low CO2 pressures. Physisorption of OTS from SCF CO2 does occur via a weak interaction with the surface hydroxyl groups. Although a small amount of OTS is hydrolyzed by the residual water present in the SCF CO2 and adsorbs on the silica, the amount hydrolyzed is much lower than that found with the use of traditional nonaqueous solvents.

Synthesis of Mesoporous Organosilicate Films in Supercritical Carbon Dioxide

“…[19] The characteristic Si-O-Si stretching peaks at 1000-1250 and 813 cm -1 , and other characteristic CH peaks at 1422, 1466, and 2800-3000 cm -1 , indicate the presence of hydrolyzed OTCS. [19] In the case of NMAPTMS functional- Figure 4.…”

“…[19] The characteristic Si-O-Si stretching peaks at 1000-1250 and 813 cm -1 , and other characteristic CH peaks at 1422, 1466, and 2800-3000 cm -1 , indicate the presence of hydrolyzed OTCS. [19] In the case of NMAPTMS functional- Figure 4. TEM image and electron-diffraction pattern (inset) of a calcined mesostructured organosilicate film that was prepared using bis(triethoxysilyl)ethane, (EtO) 3 Si-C 2 H 4 -Si(OEt) 3 , as the inorganic precursor and Pluronic F108 as the structure-directing agent.…”

“…[17] The favorable transport properties of supercritical CO 2 , including low viscosity and zero surface tension, offer additional opportunities for the post-synthesis surface modification of metal oxides, including silylation and other surface modifications of porous materials. [18,19] Unlike reactions in liquid solvents, surface modification in CO 2 does not require a subsequent drying step. Moreover, the absence of surface tension in supercritical solutions prevents the collapse of nanoscale features in device structures and mesoporous films, which is a major disadvantage of liquid-based processes.…”

“…The FTIR difference spectrum shows a decrease in the OH-absorption peaks at 980 and 3746 cm -1 , an increase in the Si-CH 3 -absorption peaks at 1260 and 759 cm -1 , Si-O-Si-peaks at 450, 850, and 1000-1250 cm -1 , and the CH-stretching absorption peaks at 2964 and 2908 cm -1 . [19] The FTIR difference spectra for the capping of the surface silanols with other silane coupling agents, such as n-octyl trichlorosilane (OTCS) and N-methyl aminopropyltrimethoxysilane (NMAPTMS) in supercritical CO 2 at 60°C and 204 bar for 30 min and 1 h, respectively, are shown in Figures 5b,c, respectively. The OTCS post-treatment was followed by a thermal treatment at 200°C for 1 h in air.…”

Monolayers of derivatized poly( l -lysine)-grafted poly(ethylene glycol) on metal oxides as a class of biomolecular interfaces

“…However, these monolayers are not always straightforward to prepare, because of a complex condensation process at the interface and result very often in unpredictable molecular densities of exposed -functionalities. Therefore appropriate choice of the silane coupling chemistry and careful control of the reaction conditions (13,14) are mandatory. Other types of monolayers on oxides have yet to be investigated to avoid these disadvantages.…”