Determination of the adsorption rates of aldehydes on bare and aminopropylsilyl-modified silica gels by polynomial fitting of ultra-rapid-scanning FT-IR data

Abstract

We describe a model to determine the relative adsorption rates of aldehydes onto bare and aminopropylsilyl (APS) modified silica gels. The relative adsorption rates of three vapor-phase aldehydes (formaldehyde, acetaldehyde, and acrolein) and 1,3-butadiene were determined for two forms of silica gel and their APS counterparts. The aldehyde gases were mixed individually with nitrogen fill gas and then passed through samples of the granular silica gels or a non-adsorbing control material (usually NaCl) and into a multipass infrared (IR) gas cell. These injections, which were 100 ms in duration, were made through a solenoid valve that could be opened or closed in less than 1 ms. IR absorbance spectra were gathered at intervals of 5 ms with an ultra-rapid-scanning Fourier transform infrared (FT-IR) spectrometer. Differences in the IR absorbance at a particular wavenumber between injections through the adsorbent of interest and injections through NaCl were fit to a polynomial function. The derivative of this polynomial is proportional to the rate of adsorption. Using the ideal gas law and the Bouguer–Beer–Lambert law, a quantitative description of the rate of adsorption for each combination of adsorbate and adsorbent was calculated in units of nmol s −1 g −1.