The present work demonstrates a simple and straightforward chemical modification of cellulose nanofibril (CNF) films in order to produce CO2 adsorbent materials. The CNF films were obtained from two agricultural residues, i.e. corn husks and oat hulls. CNF from kraft pulp was used for comparison purposes. Controlled surface silylation was conducted on the preformed CNF films in aqueous media under mild conditions using three aminosilanes bearing mono, di, and triamine groups. The success of the grafting of the aminosilanes on the CNF films was demonstrated by Fourier transform infrared and X-ray photoelectron spectroscopy analyses. The results of the contact angle measurements and field emission scanning electron microscopy coupled with energy dispersive spectroscopy showed homogeneous coverage by the amino groups on the surface of the modified CNF films, particularly with the diaminosilane N-[3-(trimethoxysilyl)propyl]ethylenediamine (DAMO). The produced films were thermally stable, and when subjected to 99.9% CO2 flow at 25 °C, these modified films showed good adsorption of CO2. Indeed, after 3 h of exposure the adsorbed concentration of CO2 of the CNF films modified with DAMO was 0.90, 1.27, and 2.11 mmol CO2 g-1 polymer for CNF films from corn husks, oat hulls, and kraft pulp, respectively.