Preparation And Characterization Of Montmorillonite Based Mesoporous Materials And Their Adsorption Performance For VOCs

Volatile Organic Compounds, VOCs are kinds of environmental pollutant source which seriously affect ecological environment and threaten human health. The removal of VOCs is a more and more urgent public issue. Adsorption technology is considered as the most reliable and promising control technologies for VOCs. For now, the widely used adsorbents are activated carbon and zeolite molecular sieve. However, activated carbon has the disadvantages, such as, inflammable, difficult regeneration, easily pore clog and sensitive to moisture. Although the zeolite can overcome the above mentioned shortcomings, they have a high production cost and are hard to regenerate. Therefore, developing new porous materials with a low cost and efficient VOCs adsorption performance is a key issue in the field of environmental science and engineering. Montmorillonite(Mt) are widely used for the removal of pollutants in water for their high thermal stability, abundant reserves, low cost, environmentally-friendly characters. This research mainly involves synthesis and characterization of activated Mt(SAM) and Mt-based ordered mesoporous materials(MSC), and measurements of their adsorption-desorption properties to VOCs.Mt was activated by sulfuric acid with different concentrations. Mt based ordered mesoporous materials, a MCM-41-type mesoporous molecular sieve, was synthesized by hydrothermal treatment method while part of the layer structure of the clay were retained. Acid activated montmorillonite, was used as a precursor without any silica or aluminum regents. Cetyltrimethlammonium bromide(CTAB) was used as templating agent. The effects of various factors on the preparation of mesoporous materials from montmorillonites were investigated, including p H, amount of templating agent and Na OH,and Si/Al ratio of the precursor. Adsorption isotherms of different kinds of VOCs and consecutive cycles of toluene adsorption-desorption on the prepared samples were studied, as well as zeolite ZSM-5 and Y.The main conclusions of this work were made as follow:(1) During acidification process, cation exchange capacity of the Mt decreased with the increase of concentration of acid. BET specific surface area, pore volume increased rapidly first and then decreased gradually, which resulted in the high BET specific surface area(ca. 373.8 m2/g) and pore volume(ca. 0.5218 cm3/g). The periodic of C axis of Mt and crystallinity were diminished, but they still retain layered structure without obvious amorphous silica. The saturated capacity of toluene and dichloromethane on acid activation product were 156.6 and 119.7 mg/g, respectively, which were 1.46 and 1.62 times larger than those of raw Mt.(2) Mt-based ordered mesoporous materials(MSC), was synthesized by hydrothermal treatment using SAM as precursor. The XRD pattern of the best crystallized sample is absolutely consistent with the two dimension hexagonal p6 m pore structure(MCM-41) with the lattice parameter(a0) of 4.7 nm, and wall thickness of 1.7 nm. The TEM images exhibited ordered hexagonal arrays of mesopores with uniform size and the platy particle shape of the precursor. The results demonstrated the formation of MCM-41-type zeolite with specific surface area, pore diameter, and pore volumes of 743.1 m2/g, 3.0 nm and 0.89 cm3/g, respectively. The N2isotherms of MSC showed typical hysteresis loops of type IV isotherms, with narrow pore size distribution. SA6.0-p H10 showed the best adsorption performance for toluene, reaching a maximum of 453.0 mg/g. Adsorption capacity of toluene on SA6.0-p H10 decreased with increasing temperature, indicating exothermic nature of VOC adsorption on SA6.0-p H10.(3) At partial pressure P/P0=0.1297, adsorption capacity of toluene on zeolites of ZSM-5 and Y are 80 and 134 mg/g, respectively. Consecutive cycles of toluene adsorption-desorption on ZSM-5 and Y at 298 K revealed that both of them lost 30 mg/g capacity to toluene due to the steric effect. However, adsorption capacity of toluene on SA6.0-p H10 reached 200 mg/g while 20 cycles of toluene adsorption-desorption show the unchanged performance.(4) Adsorption isotherms of acetone, n-hexanem, cyclohexane, benzene, toluene and o-xylene on 6.0SA-p H10 and zeolite Y demonstrated that two factors had great effect on adsorption performance. For one thing, the structural characteristics of adsorbents, such as the specific surface area, pore volume and pore size of the adsorbent. For another, the nature of adsorbates, such as the size, saturated vapor pressure, boiling point, polarity and intermolecular forces.