Study In Adsorption Behaviors Of Mesoporous Molecular Sieve MCM-41

Abstract:Mesoporous molecular sieve MCM-41 with designable structure has been widely studied in catalyst, host-guest chemistry and adsorption. As a typical molecular sieve, MCM-41 exhibits superiority with uniform structure, large surface area and excellent stability in adsorption aspects, and this article is dedicated to the study in adsorption behaviors of MCM-41.A series of polymers including polyvinyl alcohol (PVA), polyethylene glycol (PEG), and polystyrene (PS) were successfully adsorbed on mesoporous silica with tubular structure by adding silica particles directly into polymer solution during stirring. Presence of polymers on silica was confirmed by thermogravimetric analysis, FTIR spectroscopy and transmission electron microscopic images. Nitrogen sorption measurements were employed to investigate the variation in surface and pore properties after polymer adsorption. BET specific surface area, pore size distribution (PSD), and surface fractal analysis were calculated in traditional BET (Brunauer, Emmett and Teller) method, BJH (Barret, Joyner and Hanlenda) method, and the thermodynamic method, respectively from the isotherms. We proposed a model for conformation of polymer adsorbed based on such variations, which was further confirmed by transmission electron microscopic results. We found that PVA and PEG with high polarity (due to high content of oxygen atoms both in the chain or hydroxyl groups), could spread their chains into the tubes of molecular sieves besides the adsorption outside of the tubes via hydrogen bond, while PS with relatively low polarity could only adsorb randomly on the surface.Mesoporous sieves MCM-41 modified by trimethylchlorosilane and methyltrimethoxysilane were prepared by postsynthesis grafting (PSG) process and further characterized by thermogravimetric (TG) analysis, FTIR spectroscopy, and nitrogen sorption experiments. Samples with different degrees of modification were collected and the adsorption experiments of phenol and hydroquinone in solutions were carried out in batch experiments. Results showed that the surface properties of silica were found to be the key factors for the phenol compound adsorption in low concentration solutions, while the concentration of solutions became the dominant aspect in high concentration ones. In low concentration solutions, the modification of alkyl groups did enhance the loading amount of phenol by employing both the van der Waals force and hydrogen bonds, but it also reduced the adsorption amount of hydroquinone by covering oxygen atoms and hydroxyl groups on the surface when hydrogen bond was a dominant factor due to the specific structure of hydroquinone. According to the facts, we can achieve favorable adsorption capacities by desirable modification.